Stone man syndrome is a rare disorder that affects the connective tissues of the muscles. Stone man syndrome or Fibrodysplasia ossificans progressiva is a mutation caused in the repair mechanism of the body that affects the fibrous tissue including muscles, ligament and tendon. When the fibrous tissue is damaged, it gets ossified i.e. muscles undergo changes to form bones. To be more specific, muscles are replaced by bones. Bones are formed as if there is an extra skeletal structure is formed other than the original skeleton. Formation of bones by replacing muscles restricts the movement of the person who is affected with this disorder. This disorder is only found in 1 in 2 million people in the world.
Symptoms of stone man syndrome
Fibrodysplasia ossificans progressiva is generally found during early childhood. The progression of the disorders starts from the neck to shoulders, and gradually proceeds to lower parts of the body and finally to the legs. Body movements will be restricted progressively because the joints get affected with the disorder. The patient finds it difficult to open mouth, which in turn causes trouble while eating and speaking. Gradually, the people affected with this disorder will experience lack of nutrition due to the difficulty in eating. Stone man syndrome also causes breathing difficulties because of the additional bone formation surrounding the rib cage, which obstructs the lungs capacity to expand.
Muscle injuries or trauma caused in an individual suffering from this syndrome may trigger inflammation and swelling in the muscles, which is followed by the ossification in the area of injury. Injury caused by surgical procedures and fall triggers the formation of bones. Individuals suffering from stone man syndrome generally have abnormal toes, which helps to differentiate stone man syndrome from other muscle and bone problems. These individuals also suffer from other abnormalities in skeleton and have small thumbs. Most patients experience sudden appearance of lumps in their body.
As this disorder is affected very rarely, the symptoms of this disorder can be diagnosed as fibrosis or cancer. Misdiagnosis will lead to biopsies and other related tests of cancer.
How mutation is caused
In patients with fibrodysplasia ossificans progressiva, the gene ACVR1 will get mutated. This genes provide instructions for the production of a certain protein known as BMP or bone morphogenetic protein type 1 receptors. This protein can be found in several body tissues such as cartilage and skeletal muscles. It controls the development of muscles and bones, which includes the replacement of cartilages into bones. Replacement of cartilages into bones is known as ossification and it occurs as a normal process from birth to teenage.
According to researchers, the gene that is mutated will change the shape and functions of the receptor at certain circumstances. This results in the constant activation of the receptor causing the overgrowth of cartilage and bone, and fusing of joints. This results in the symptoms of stone man syndrome. In most cases, people without any history of this disorder in the family are affected with this disorder. Rarely, individuals will inherit the mutated gene from the parent.
Safer to eat than to breathe.
Adding a touch of cinnamon or vanilla to your e-cigarette might ease your stress levels, but the same can’t be said for your white blood cells.
That’s the take-home message from a study conducted by a team of US researchers who examined the impact vaporised flavour additives had on our health. It turns out they aren’t as innocent as we’d thought.
We’ve happily swallowed them in our food for generations with no evidence of ill effects, so few have questioned whether inhaling the same compounds as a vapour might be a cause for concern.
Yet suspected links between flavouring chemicals such as diacetyl and respiratory conditions such as bronchitis have hinted at a potential for serious health problems.
To gain a better understanding of what was going on, scientists from the University of Rochester Medical Centre in the United States exposed white blood cells called monocytes to seven common e-cigarette flavourings, and looked for signs of oxidative stress.
The results showed there’s a clear need to have a rethink on just how safe flavoured e-cigarettes might be.
The researchers found flavourings produced compounds called reactive oxygen species– substances that are known to damage cell structures in high enough concentrations.
White cells exposed to the flavourings also pumped out a signalling protein called interleukin 8 in concentrations that went up in accordance with the dose.
This protein acts as a chemical siren for other immune cells, calling them to a site of infection or cellular damage. In other words, the cells were responding to the presence of flavourings as if they recognised them as hazardous.
Lastly, they looked at the general health or viability of the cells as they were exposed to different additives, to get some idea of the flavourings’ general toxicity.
Some of the additives were more toxic than others, but it was in combination that they really did some damage.
“Cinnamon, vanilla, and butter flavouring chemicals were the most toxic but our research showed that mixing flavours of e-liquids caused by far the most toxicity to white blood cells,” says toxicologist Thivanka Muthumalage.
Putting the results into perspective, there’s a big leap between white blood cells screaming for help in a petri dish and a physiological reaction in an otherwise healthy human being.
But it does add a new level of complexity to the debate over how we might want to deal with the growing fad of swapping old fashioned cigarettes for vaping.
“Our scientific findings show that e-liquid flavours can, and should, be regulated and that e-juice bottles must have a descriptive listing of all ingredients. We urge regulatory agencies to act to protect public health.”
A couple of years ago, the US Food and Drug Administration (FDA) announced it would use the same set of rules to regulate both electronic and non-electronic cigarettes.
This was overturned last year, when the FDA released a new set of regulations, focussing on beating nicotine addiction by relaxing laws surrounding the vaporising of tobacco compounds.
Whether this kind of research impacts on future changes to regulations is anybody’s guess. More research will no doubt fill in the missing details on exactly how vaping in all its forms affects our bodies.
In the meantime, if e-cigarettes are your way to beat an addiction, you might want to consider how badly you want that cinnamon puff.
Here’s what the science actually says.
They may even have to pay fines if they don’t warn customers about the risks of chemicals in coffee.
The Council for Education and Research on Toxics – the group behind the lawsuit – wants to penalise companies that don’t warn customers that coffee contains acrylamide, a chemical that California lists as one “known to cause cancer.”
Acrylamide naturally forms when plants and grains are cooked at high temperatures. It’s created in the process known as the maillard reaction, in which high heat transforms sugars and amino acids in ways that change flavour and tend to brown food. When potatoes, bread, biscuits, or coffee are heated, acrylamide forms.
But there’s no conclusive reason to believe that coffee or other foods expose humans to dangerous levels of acrylamide. And there’s no known way to make coffee without acrylamide.
Acrylamide and cancer risk
The chemical in question here was first discovered by Swedish scientists in 2002, according to the American Cancer Society.
Data suggests that in large quantities, acrylamide is carcinogenic to some animals. Animal studies have shown that putting acrylamide in drinking water can give rats and mice cancer.
According to officials at the European Food Safety Authority, “it is likely that it has been present in food since cooking began,” since frying, baking, and roasting all create acrylamide. (This is the case for foods derived from plants including grains, however, not necessarily for meat or fish.)
Acrylamide does pose risks to humans, as industrial accidents when people have inhaled large quantities of it have shown.
It’s also one of the many chemicals produced in cigarette smoke, though in higher amounts than from making coffee or toast.
It’s important to remember that the dose of a chemical generally determines whether it’s harmful. Caffeine, which is also found in coffee, can be deadly at high doses – but that doesn’t mean all caffeine is bad.
And if you eat cooked food, acrylamide can’t be avoided. It’s present in about one-third of the calories the average US or European person consumes.
Humans also metabolise the chemical differently than animals do and so far, studies haven’t found any harmful connection between consumption of foods containing acrylamide and various common cancers.
If anything, most of the data that we have so far indicates that people who consume a lot of coffee have a lower risk for diseases including liver disease, heart disease, Type 2 diabetes, depression, and – especially important in this case – cancer.
According to the evidence, drinking coffee isn’t unhealthy.
At least one major review of studies found that the more coffee people drink, the lower their risk for liver cirrhosis or liver cancer.
A review of more than 200 studies found that people who drank three or four cups of coffee per day were 19 percent less likely to die from cardiovascular disease.
Heavy coffee drinkers have an 18 percent reduced risk for cancer overall, according to one large study, and some data indicates that coffee drinkers may be less likely to suffer from oral or pharyngeal cancer and advanced prostate cancer.
A number of studies have also found that coffee drinkers are less likely to suffer from Alzheimer’s or dementia.
As far as the risks go, the International Agency for Research on Cancer has studied coffee and warned that extremely hot drinks may pose cancer risk if they burn the esophagus. But they have concluded that the drink itself is unlikely to cause certain cancers.
Most of the health benefits researchers have found have been in observational studies, meaning we don’t know that drinking coffee is responsible for the reductions in disease risk.
But in most of those cases, researchers have concluded that drinking coffee probably isn’t causing any harm.
1 billion light years in a supercomputer.
Saying our Universe is a big place is a bit of an understatement. And with recent advances in our understanding of the effects of dark matter and black holes on the formation of galaxies, it’s surprisingly dynamic.
Already it’s making some pretty accurate predictions, promising insights that no doubt will lead to a bunch of exciting discoveries.
Titled Illustris: The Next Generation (or IllustrisTNG for short), it’s clear that the simulation is expected to expand the frontiers of space discovery.
“When we observe galaxies using a telescope, we can only measure certain quantities,” says astrophysicist Shy Genel from the Flatiron Institute’s Center for Computational Astrophysics (CCA).
“With the simulation, we can track all the properties for all these galaxies. And not just how the galaxy looks now, but its entire formation history.”
IllustrisTNG is the successor to Illustris, and based on the contributions of researchers from CCA, MIT, Harvard University, University of Cambridge, Heidelberg Institute of Theoretical Studies, and the Institute for Advanced Study.
In effect, TNG actually consists of 18 simulations covering various scales – each a cubic mock-up of space up to 1 billion light years wide – tracing the evolution of the Universe from just after the Big Bang into the future.
With more resolution and new physics, the improved simulations not only cover more ground, they can now accurately model the patterns of clustering galaxies, and how this web shifts over time.
Keeping track of a billion square light years of galactic material, magnetic fields, and dark matter requires some impressive hardware.
So the project uses Germany’s fastest supercomputer, the Hazel Hen, to crunch the numbers.
It still took two months to run through the calculations for just one of the larger simulations, amassing 500 terabytes of data.
That said, it’s a lot quicker than waiting for a Universe to grow.
“Analysing this huge mountain of data will keep us busy for years to come, and it promises many exciting new insights into different astrophysical processes,” says Volker Springel from the Heidelberg Institute for Theoretical Studies.
The level of accuracy being achieved so far is exciting. Already, three papers have been published describing the large scale clustering of matter, helping us understand how galaxies form.
Of course, anything found in the simulations are simply flags that require closer inspection by astronomers.
One piece of research published in the Monthly Notices of the Royal Astronomical Societydemonstrated the nature of faint haloes of stars that form as galaxies collide and merge; normally too faint to see, the simulation shows what signature to hunt for.
“It is particularly fascinating that we can accurately predict the influence of supermassive black holes on the distribution of matter out to large scales,” says Springel.
“This is crucial for reliably interpreting forthcoming cosmological measurements.”
Such discoveries will no doubt feed back into the models and fine tune them even further, promising further iterations to come.
It’s exciting to imagine what secrets a third generation of Illustris might one day reveal.
Anybody who has grappled with weight loss knows that even once the kilos come off, it can be hard to keep them off in the long term.
A study on the hormones that govern feelings of hunger and fullness has shown why it’s such a struggle for some individuals, revealing that changes in body chemistry two years after shedding weight can have an impact on our desire to go back for second helpings.
Researchers from Norway and Denmark made the discovery after tracking the fitness, body mass index (BMI), hormone concentrations, and reported hunger levels of 35 adults over two years.
Each volunteer entered the program as severely obese before undergoing a rigorous weight loss program involving a calorie-restricted diet, exercise, and therapy.
Following a three week residential session, the participants went home and continued to exercise and eat healthier.
Two years later all of the subjects successfully lost significant amounts of weight and had better cardiovascular fitness. But their hormones and reported feelings of fullness and hunger told an interesting story.
A month into the weight loss program, the volunteers reported feeling fuller after meals, and experienced no significant change in their hunger levels while they fasted.
Coinciding with this sensation were higher levels of a hormone called peptide YY, which when released in the gut shuts down appetite and impedes our gastric functions.
All good news.
But one year later, those feelings of fullness went south as hunger levels and a desire to eat steadily climbed. Worse still, they remained high after another twelve months.
These hunger sensations were reflected in their levels of a hormone called ghrelin, which slowly crept up from day one.
Meanwhile those appetite-satisfying peptide YY levels stayed steady after the four week mark, not shrinking but not climbing any higher either.
In other words, their initial efforts in improving activity and changing diet were rewarded with a depressed appetite, but pangs of hunger quickly took over any sensations of a satisfied belly.
The study can’t tell us much about the exact mechanisms behind these hormone changes, and the researchers admit in their report that they don’t have enough information to comment on the energy balance of the participants at the time measurements were taken.
Knowing details about the metabolisms of those losing weight might help us better understand what’s going on below the surface.
But the research does show how losing weight isn’t always a simple case of setting up good habits and watching the fat fade.
For many in the overweight and obese category who want to lose weight, it’s not enough to just muster the willpower to get up more often and eat smaller, healthier portions.
We are only just beginning to understand the effects weight gain has on how we store fat, and the difficulties involved in reversing the impact of being obese in the first place.
Research like this shows how complex weight loss can be, while also providing hope that with such knowledge there could be solutions that might just make it easier in the future.
This research was published in American Journal of Physiology-Endocrinology and Metabolism.
While in graduate school in mathematics at the University of Wisconsin-Madison, I took a logic course from David Griffeath. The class was fun. Griffeath brought a playfulness and openness to problems. Much to my delight, about a decade later, I ran into him at a conference on traffic models. During a presentation on computational models of traffic jams, his hand went up. I wondered what Griffeath – a mathematical logician – would have to say about traffic jams. He did not disappoint. Without even a hint of excitement in his voice, he said: ‘If you are modelling a traffic jam, you should just keep track of the non-cars.’
The collective response followed the familiar pattern when someone drops an unexpected, but once stated, obvious idea: a puzzled silence, giving way to a roomful of nodding heads and smiles. Nothing else needed to be said.
Griffeath had made a brilliant observation. During a traffic jam, most of the spaces on the road are filled with cars. Modelling each car takes up an enormous amount of memory. Keeping track of the empty spaces instead would use less memory – in fact almost none. Furthermore, the dynamics of the non-cars might be more amenable to analysis.
Versions of this story occur routinely at academic conferences, in research laboratories or policy meetings, within design groups, and in strategic brainstorming sessions. They share three characteristics. First, the problems are complex: they concern high-dimensional contexts that are difficult to explain, engineer, evolve or predict. Second, the breakthrough ideas do not arise by magic, nor are they constructed anew from whole cloth. They take an existing idea, insight, trick or rule, and apply it in a novel way, or they combine ideas – like Apple’s breakthrough repurposing of the touchscreen technology. In Griffeath’s case, he applied a concept from information theory: minimum description length. Fewer words are required to say ‘No-L’ than to list ‘ABCDEFGHIJKMNOPQRSTUVWXYZ’. I should add that these new ideas typically produce modest gains. But, collectively, they can have large effects. Progress occurs as much through sequences of small steps as through giant leaps.
Third, these ideas are birthed in group settings. One person presents her perspective on a problem, describes an approach to finding a solution or identifies a sticking point, and a second person makes a suggestion or knows a workaround. The late computer scientist John Holland commonly asked: ‘Have you thought about this as a Markov process, with a set of states and transition between those states?’ That query would force the presenter to define states. That simple act would often lead to an insight.
The burgeoning of teams – most academic research is now done in teams, as is most investing and even most songwriting (at least for the good songs) – tracks the growing complexity of our world. We used to build roads from A to B. Now we construct transportation infrastructure with environmental, social, economic and political impacts.
The complexity of modern problems often precludes any one person from fully understanding them. Factors contributing to rising obesity levels, for example, include transportation systems and infrastructure, media, convenience foods, changing social norms, human biology and psychological factors. Designing an aircraft carrier, to take another example, requires knowledge of nuclear engineering, naval architecture, metallurgy, hydrodynamics, information systems, military protocols, the exercise of modern warfare and, given the long building time, the ability to predict trends in weapon systems.
The multidimensional or layered character of complex problems also undermines the principle of meritocracy: the idea that the ‘best person’ should be hired. There is no best person. When putting together an oncological research team, a biotech company such as Gilead or Genentech would not construct a multiple-choice test and hire the top scorers, or hire people whose resumes score highest according to some performance criteria. Instead, they would seek diversity. They would build a team of people who bring diverse knowledge bases, tools and analytic skills. That team would more likely than not include mathematicians (though not logicians such as Griffeath). And the mathematicians would likely study dynamical systems and differential equations.
Believers in a meritocracy might grant that teams ought to be diverse but then argue that meritocratic principles should apply within each category. Thus the team should consist of the ‘best’ mathematicians, the ‘best’ oncologists, and the ‘best’ biostatisticians from within the pool.
That position suffers from a similar flaw. Even with a knowledge domain, no test or criteria applied to individuals will produce the best team. Each of these domains possesses such depth and breadth, that no test can exist. Consider the field of neuroscience. Upwards of 50,000 papers were published last year covering various techniques, domains of enquiry and levels of analysis, ranging from molecules and synapses up through networks of neurons. Given that complexity, any attempt to rank a collection of neuroscientists from best to worst, as if they were competitors in the 50-metre butterfly, must fail. What could be true is that given a specific task and the composition of a particular team, one scientist would be more likely to contribute than another. Optimal hiring depends on context. Optimal teams will be diverse.
Evidence for this claim can be seen in the way that papers and patents that combine diverse ideas tend to rank as high-impact. It can also be found in the structure of the so-called random decision forest, a state-of-the-art machine-learning algorithm. Random forests consist of ensembles of decision trees. If classifying pictures, each tree makes a vote: is that a picture of a fox or a dog? A weighted majority rules. Random forests can serve many ends. They can identify bank fraud and diseases, recommend ceiling fans and predict online dating behaviour.
When building a forest, you do not select the best trees as they tend to make similar classifications. You want diversity. Programmers achieve that diversity by training each tree on different data, a technique known as bagging. They also boost the forest ‘cognitively’ by training trees on the hardest cases – those that the current forest gets wrong. This ensures even more diversity and accurate forests.
Yet the fallacy of meritocracy persists. Corporations, non-profits, governments, universities and even preschools test, score and hire the ‘best’. This all but guarantees not creating the best team. Ranking people by common criteria produces homogeneity. And when biases creep in, it results in people who look like those making the decisions. That’s not likely to lead to breakthroughs. As Astro Teller, CEO of X, the ‘moonshoot factory’ at Alphabet, Google’s parent company, has said: ‘Having people who have different mental perspectives is what’s important. If you want to explore things you haven’t explored, having people who look just like you and think just like you is not the best way.’ We must see the forest.
In the summer of 1935, the physicists Albert Einstein and Erwin Schrödinger engaged in a rich, multifaceted and sometimes fretful correspondence about the implications of the new theory of quantum mechanics. The focus of their worry was what Schrödinger later dubbed entanglement: the inability to describe two quantum systems or particles independently, after they have interacted.
Until his death, Einstein remained convinced that entanglement showed how quantum mechanics was incomplete. Schrödinger thought that entanglement was the defining feature of the new physics, but this didn’t mean that he accepted it lightly. ‘I know of course how the hocus pocus works mathematically,’ he wrote to Einstein on 13 July 1935. ‘But I do not like such a theory.’ Schrödinger’s famous cat, suspended between life and death, first appeared in these letters, a byproduct of the struggle to articulate what bothered the pair.
The problem is that entanglement violates how the world ought to work. Information can’t travel faster than the speed of light, for one. But in a 1935 paper, Einstein and his co-authors showed how entanglement leads to what’s now called quantum nonlocality, the eerie link that appears to exist between entangled particles. If two quantum systems meet and then separate, even across a distance of thousands of lightyears, it becomes impossible to measure the features of one system (such as its position, momentum and polarity) without instantly steering the other into a corresponding state.
Up to today, most experiments have tested entanglement over spatial gaps. The assumption is that the ‘nonlocal’ part of quantum nonlocality refers to the entanglement of properties across space. But what if entanglement also occurs across time? Is there such a thing as temporal nonlocality?
The answer, as it turns out, is yes. Just when you thought quantum mechanics couldn’t get any weirder, a team of physicists at the Hebrew University of Jerusalem reported in 2013 that they had successfully entangled photons that never coexisted. Previous experiments involving a technique called ‘entanglement swapping’ had already showed quantum correlations across time, by delaying the measurement of one of the coexisting entangled particles; but Eli Megidish and his collaborators were the first to show entanglement between photons whose lifespans did not overlap at all.
Here’s how they did it. First, they created an entangled pair of photons, ‘1-2’ (step I in the diagram below). Soon after, they measured the polarisation of photon 1 (a property describing the direction of light’s oscillation) – thus ‘killing’ it (step II). Photon 2 was sent on a wild goose chase while a new entangled pair, ‘3-4’, was created (step III). Photon 3 was then measured along with the itinerant photon 2 in such a way that the entanglement relation was ‘swapped’ from the old pairs (‘1-2’ and ‘3-4’) onto the new ‘2-3’ combo (step IV). Some time later (step V), the polarisation of the lone survivor, photon 4, is measured, and the results are compared with those of the long-dead photon 1 (back at step II).
The upshot? The data revealed the existence of quantum correlations between ‘temporally nonlocal’ photons 1 and 4. That is, entanglement can occur across two quantum systems that never coexisted.
What on Earth can this mean? Prima facie, it seems as troubling as saying that the polarity of starlight in the far-distant past – say, greater than twice Earth’s lifetime – nevertheless influenced the polarity of starlight falling through your amateur telescope this winter. Even more bizarrely: maybe it implies that the measurements carried out by your eye upon starlight falling through your telescope this winter somehow dictated the polarity of photons more than 9 billion years old.
Lest this scenario strike you as too outlandish, Megidish and his colleagues can’t resist speculating on possible and rather spooky interpretations of their results. Perhaps the measurement of photon 1’s polarisation at step II somehow steers the future polarisation of 4, or the measurement of photon 4’s polarisation at step V somehow rewrites the past polarisation state of photon 1. In both forward and backward directions, quantum correlations span the causal void between the death of one photon and the birth of the other.
Just a spoonful of relativity helps the spookiness go down, though. In developing his theory of special relativity, Einstein deposed the concept of simultaneity from its Newtonian pedestal. As a consequence, simultaneity went from being an absolute property to being a relative one. There is no single timekeeper for the Universe; precisely when something is occurring depends on your precise location relative to what you are observing, known as your frame of reference. So the key to avoiding strange causal behaviour (steering the future or rewriting the past) in instances of temporal separation is to accept that calling events ‘simultaneous’ carries little metaphysical weight. It is only a frame-specific property, a choice among many alternative but equally viable ones – a matter of convention, or record-keeping.
The lesson carries over directly to both spatial and temporal quantum nonlocality. Mysteries regarding entangled pairs of particles amount to disagreements about labelling, brought about by relativity. Einstein showed that no sequence of events can be metaphysically privileged – can be considered more real – than any other. Only by accepting this insight can one make headway on such quantum puzzles.
The various frames of reference in the Hebrew University experiment (the lab’s frame, photon 1’s frame, photon 4’s frame, and so on) have their own ‘historians’, so to speak. While these historians will disagree about how things went down, not one of them can claim a corner on truth. A different sequence of events unfolds within each one, according to that spatiotemporal point of view. Clearly, then, any attempt at assigning frame-specific properties generally, or tying general properties to one particular frame, will cause disputes among the historians. But here’s the thing: while there might be legitimate disagreement about which properties should be assigned to which particles and when, there shouldn’t be disagreement about the very existence of these properties, particles, and events.
These findings drive yet another wedge between our beloved classical intuitions and the empirical realities of quantum mechanics. As was true for Schrödinger and his contemporaries, scientific progress is going to involve investigating the limitations of certain metaphysical views. Schrödinger’s cat, half-alive and half-dead, was created to illustrate how the entanglement of systems leads to macroscopic phenomena that defy our usual understanding of the relations between objects and their properties: an organism such as a cat is either dead or alive. No middle ground there.
Most contemporary philosophical accounts of the relationship between objects and their properties embrace entanglement solely from the perspective of spatial nonlocality. But there’s still significant work to be done on incorporating temporal nonlocality – not only in object-property discussions, but also in debates over material composition (such as the relation between a lump of clay and the statue it forms), and part-whole relations (such as how a hand relates to a limb, or a limb to a person). For example, the ‘puzzle’ of how parts fit with an overall whole presumes clear-cut spatial boundaries among underlying components, yet spatial nonlocality cautions against this view. Temporal nonlocality further complicates this picture: how does one describe an entity whose constituent parts are not even coexistent?
Discerning the nature of entanglement might at times be an uncomfortable project. It’s not clear what substantive metaphysics might emerge from scrutiny of fascinating new research by the likes of Megidish and other physicists. In a letter to Einstein, Schrödinger notes wryly (and deploying an odd metaphor): ‘One has the feeling that it is precisely the most important statements of the new theory that can really be squeezed into these Spanish boots – but only with difficulty.’ We cannot afford to ignore spatial ortemporal nonlocality in future metaphysics: whether or not the boots fit, we’ll have to wear ’em.
Sexual behavior is essential for the survival of many species. In female rodents, mate preference and copulatory behavior depend on pheromones and are synchronized with ovulation to ensure reproductive success. The neural circuits driving this orchestration in the brain have, however, remained elusive. Here, we demonstrate that neurons controlling ovulation in the mammalian brain are at the core of a branching neural circuit governing both mate preference and copulatory behavior. We show that male odors detected in the vomeronasal organ activate kisspeptin neurons in female mice. Classical kisspeptin/Kiss1R signaling subsequently triggers olfactory-driven mate preference. In contrast, copulatory behavior is elicited by kisspeptin neurons in a parallel circuit independent of Kiss1R involving nitric oxide signaling. Consistent with this, we find that kisspeptin neurons impinge onto nitric oxide-synthesizing neurons in the ventromedial hypothalamus. Our data establish kisspeptin neurons as a central regulatory hub orchestrating sexual behavior in the female mouse brain.
Female copulatory behaviors are exquisitely orchestrated by sex hormones in order to coincide with ovulation and thus to ensure the highest possible chance of fertilization1. Female rodents typically control the initiation and timing of copulatory contacts with males2. This is achieved by a succession of precopulatory behaviors bringing the female in contact with males and is driven by sexual motivation and mate preferences. Once in direct contact with the male, the female will display receptive behaviors, such as the lordosis posture, which is necessary for intromission. The neural circuits and the individual neurons underlying the coordination of sexual motivation and mate preference with ovulation have, however, remained elusive.
Mice are nocturnal animals and heavily rely on olfactory cues, such as pheromones, to identify potential mates3. Pheromones are detected and processed by a highly specialized neural circuit, the accessory olfactory system, which initiates in the vomeronasal organ (VNO) in the nasal septum. The vomeronasal pathway runs in parallel to the main olfactory system originating in the main olfactory epithelium (MOE). The accessory olfactory system has been functionally linked to a range of innate behaviors including reproduction4,5,6,7,8. How exactly pheromones impinge on both reproductive physiology and behavior has only recently begun to emerge. Signals triggered by pheromones in the neuroepithelium of the VNO are transmitted to gonadotropin-releasing hormone (GnRH) neurons in the reproductive center of the neuroendocrine brain9. GnRH neurons are located in the preoptic area of the anterior hypothalamus and control the hypothalamus–pituitary gonadal axis by releasing the GnRH peptide from axon terminals in the median eminence10. GnRH then acts on gonadotrope cells in the anterior pituitary to trigger secretion of the gonadotropins; luteinizing hormone (LH) and follicle-stimulating hormone (FSH). LH and FSH in turn regulate sex hormone production (i.e., estradiol in females and testosterone in males) and the maturation of the gametes. In rodents, pheromones are known to modulate GnRH neuronal activity in a sex-dependent manner11,12. Exposure to female pheromones activates GnRH neurons in male mice consequently leading to a LH/testosterone surge, and vice versa male pheromones induce LH release in female mice13.
Sex hormones provide gonadal feedback to the brain; however, GnRH neurons do not express the appropriate steroid hormone receptors and are thus not direct targets of this feedback regulation10. Instead, gonadal sex steroid feedback is indirectly relayed to GnRH neurons. One critical neuronal population relaying sex steroid feedback to GnRH neurons lies in the rostral periventricular area of the third ventricle (RP3V) of the hypothalamus and produces the neuropeptide kisspeptin. Kisspeptin is a potent stimulator of GnRH neuronal activity (acting via its receptor Kiss1R, also known as GPR54)14,15 and controls ovulation by driving the LH surge16,17. RP3V kisspeptin neurons display a profound sexual dimorphism with greater neuron numbers in female than in male mice18 consistent with a pivotal role of these cells in female reproduction.
We recently found that the RP3V kisspeptin neuronal population in female mice is specifically activated by male but not by female pheromones19, raising the possibility that these neurons serve reproductive functions in addition to providing estradiol feedback to GnRH neurons. To dissect the functional role of RP3V kisspeptin neurons in reproduction, we applied a combination of complementary genetic strategies. We demonstrate that the RP3V kisspeptin neuronal population is necessary for the expression of both male-directed mate preference and lordosis behavior using specific viral-based cell ablation techniques and optogenetic stimulation. We then analyzed the neural circuitry downstream of RP3V kisspeptin neurons and found that mutant mice lacking GnRH secretion in adulthood also failed to show any male-directed preference. These data suggest that kisspeptin neurons act through GnRH neurons to trigger olfactory-driven mate preferences in female mice. In contrast, lordosis behavior did not depend on GnRH neurons downstream of RP3V kisspeptin neurons. To dissect the downstream neural circuitry-mediating lordosis in female mice, we then employed a combination of genetic transsynaptic tracing and viral tract tracing from RP3V kisspeptin neurons. We found that a subset of neurons in the ventrolateral part of the ventromedial hypothalamus (VMHvl) that express nitric oxide synthase (nNOS) and are communicating with kisspeptin neurons. Consistent with the idea that nitric oxide (NO) is a key neurotransmitter downstream of kisspeptin neurons, female mice deficient in nNOS showed a strong decrease in lordosis behavior. Taken together, our results demonstrate that kisspeptin governs both mate preference and sexual motivation in female mice, indicating that sexual behavior and ovulation are coordinated by the same neuropeptide.
Male odors activate RP3V kisspeptin neurons
We previously found that RP3V kisspeptin neurons are specifically activated by male odors derived from either urine19 or soiled bedding in female mice. To determine the olfactory input pathway impinging onto these cells, we selectively ablated the vomeronasal organ (VNO) by surgical removal and/or the main olfactory epithelium (MOE) by intranasal infusion with a zinc sulfate (ZnSO4) solution in female C57Bl/6J mice (see general remarks on behavioral tests in “Methods” section and Supplementary Table 1 for details on hormonal treatments prior to testing). We then tested male odor-triggered kisspeptin neuron activation in these animals by using c-Fos as a marker. Removal of the VNO (VNOx), but not ablation of the MOE (MOEx), completely eliminated the ability of male odors contained in soiled bedding to activate RP3V kisspeptin neurons in ovariectomized female mice supplemented with estradiol and progesterone (OVX+E+P) (VNOi group vs. control group, P < 0.001; MOEx group vs. control group, P = 0.001; VNOx group vs. control group, P > 0.99; Dunn’s multiple comparison test; Fig. 1a). These results demonstrate that pheromonal input triggers c-Fos expression in kisspeptin neurons via the vomeronasal pathway. The number of kisspeptin neurons was not affected by either VNOx or zinc sulfate treatment.
RP3V kisspeptin neurons trigger mate preferences
Since pheromonal cues are critical for mate recognition20,21, we next asked whether the kisspeptin peptide plays a role in olfactory mate preference. To address this question, we analyzed mice lacking a functional Kiss1 gene22. Kisspeptin knockout (Kiss−/−; OVX+E+P) mice failed to show any male-directed preference (one-sample t test (H0: mean equals 0); P = 0.42; Fig. 1b), whereas control littermates (OVX+E+P) displayed robust preferences for the male (one-sample t test (H0: mean equals 0); P < 0.001; Fig. 1b). A single subcutaneous (sc) injection of kisspeptin (Kp-10; 0.52 µg kg−1) triggered a male-directed preference in Kiss−/− (OVX+E+P) females (one-sample t test (H0: mean equals 0); P < 0.001; Fig. 1b). These data implicate the kisspeptin neuropeptide in olfactory-driven partner preference. Kisspeptin is, however, not only expressed in the RP3V, but also in neurons located in the arcuate nucleus of the hypothalamus (ARC) in the adult rodent brain18. To analyze the specific role of the RP3V kisspeptin neuron population in olfactory mate preference, we ablated these cells by injecting an adeno-associated virus (AAV) encoding a Cre recombinase-dependent caspase 323 bilaterally into the RP3V of mice expressing Cre in kisspeptin neurons (KissIC). Caspase 3 kills the Cre-expressing cells by inducing apoptosis23. Stereotaxic viral delivery into the RP3V led to a 71% decrease in kisspeptin-immunoreactive cells in Cre+ females compared to control Cre− animals (unpaired t test; P < 0.001; Fig. 1c, d), suggesting efficient ablation of this neuronal population. In contrast, kisspeptin immunoreactivity in the ARC was not affected in this experimental paradigm (unpaired t test; P = 0.81, Supplementary Fig. 1). Strikingly, Cre+ females failed to show any male-directed preferences after acute ablation of the RP3V kisspeptin neuron population (one-sample t test (H0: mean equals 0); P = 0.73; Fig. 1e), whereas a single sc injection of Kp-10 was sufficient to induce a male-directed preference in these female (OVX+E+P) mice (one-sample t test (H0: mean equals 0); P = 0.041; Fig. 1e). Taken together, these data demonstrate that RP3V kisspeptin neurons could be an essential component of the neural circuits downstream of the vomeronasal organ mediating pheromone-driven mate preference in female mice.
RP3V kisspeptin neurons are essential for lordosis
Pheromone-triggered mate preference ultimately leads to the display of copulatory behaviors. We therefore next investigated the role of the RP3V kisspeptin neurons in lordosis behavior, which is characterized by an arching of the back and an immobile posture by the female in response to male mounting. We found consistent activation of hypothalamic neurons upon mating using c-Fos immunoreactivity as a marker (Supplementary Fig. 2) in ovary intact, proestrous female mice. Specifically, ~30% of RP3V kisspeptin neurons displayed c-Fos immunoreactivity in intact females (unpaired t test; P = 0.022; Fig. 2aand Supplementary Fig. 3) following this experimental paradigm. We next asked whether a single injection with Kp-10 is sufficient to stimulate lordosis behavior in female mice. We found that a sc Kp-10 injection in OVX+E females robustly stimulated lordosis behavior (paired t test; P < 0.001; Fig. 2b). Likewise, an intracerebroventricular (icv) injection of Kp-10 (paired t test; P = 0.012; Supplementary Fig. 4) also stimulated lordosis behavior in C57Bl6 (OVX+E+P) female mice (for details on hormone treatments, see Supplementary Table 1). Next, we analyzed lordosis behavior in Kiss−/− (OVX+E+P) females and observed strong deficits (Mann–Whitney U test; P = 0.029; Fig. 2c), which could be reversed by a single sc injection of Kp-10 (paired t test; P ≤ 0.01; Fig. 2d). Consistent with this, acute ablation of ~70% of kisspeptin neurons by bilateral injection of an AAV encoding a Cre recombinase-dependent caspase 3 into the RP3V of adult female (OVX+E+P) KissICmice also led to profound deficits in lordosis behavior (unpaired t test; P = 0.006; Fig. 2e), which were reversible upon a single sc Kp-10 injection (paired t test; P = 0.03; Fig. 2f).
To test whether an activation of RP3V kisspeptin neurons is sufficient to trigger lordosis behavior, we stereotaxically injected an AAV encoding a Cre-dependent channelrhodopsin (ChR2) bilaterally into the RP3V of female KissIC mice (Fig. 2g). Blue light photostimulation (1–15 s at 10 Hz) elicited robust firing in virally transduced kisspeptin neurons with spike fidelity of 99% (Fig. 2h) in brain slice preparations. Photostimulation of RP3V kisspeptin neurons in vivo at 10 Hz for <15 s per male mount also was successful in enhancing lordosis behavior in Cre+ (OVX+E) female mice (paired t test; P = 0.025; Fig. 2i) without enhancing lordosis expression in Cre− (OVX+E) female mice injected with the AAV-ChR2 virus (paired t test; P = 0.72; Fig. 2i). Taken together, our experiments demonstrate that RP3V kisspeptin neurons are an integral part of the neural network involved in both mate preference and lordosis behavior in female mice.
Olfactory control of lordosis behavior
Next, we determined the individual contribution of the vomeronasal pathway and the main olfactory system on RP3V kisspeptin neuron activation during sexual interaction with a male. VNO removal (Dunn’s multiple comparison test; P = 0.006; Fig. 3a) but not the ablation of the MOE (Dunn’s multiple comparison test following Kruskal–Wallis test; P > 0.99; Fig. 3a) disrupted lordosis behavior in C57Bl/6J (OVX+E+P) females (without affecting the male behavior toward the female; Dunn’s multiple comparison test; VNOi/ZnSO4 vs. VNOi/Saline P > 0.99; VNOx/Saline vs. VNOi/Saline P > 0.99; VNOx/ZnSO4 vs. VNOi/Saline P > 0.99; Supplementary Fig. 5) indicating that lordosis mainly depends on a functional vomeronasal pathway. While ablation of the VNO resulted in a dramatic attenuation in lordosis behavior, c-Fos expression in RP3V kisspeptin neurons remained significant (Dunn’s multiple comparison test; P = 0.01 compared to controls). This might reflect activation through the MOE by specific volatile odors only secreted by the male when in direct contact with the female, since ablation of both the VNO and the MOE in mice completely eliminated RP3V kisspeptin neuron activation (Fig. 3b). Regardless, activation of RP3V kisspeptin neurons exclusively through the main olfactory system when in direct contact with the male seems to be insufficient to trigger lordosis. Taken together, our data reveal that RP3V kisspeptin neurons are an essential part of a motivational neural pathway that is triggered by male olfactory cues detected and processed predominantly through the vomeronsal pathway, ultimately leading to the female adapting a specific mating posture facilitating intromission.
Mate preference, but not lordosis, is GnRH dependent
RP3V kisspeptin neurons directly innervate GnRH neurons24,25 and are implicated in generating the preovulatory LH surge16,17,26,27,28. Kisspeptin can activate GnRH neurons via its canonical receptor Kiss1R, which is expressed in ∼95% of these cells14,15,29. To test whether RP3V kisspeptin neurons act on GnRH neurons to drive olfactory mate preference and lordosis behavior, we used GnRH::Cre; DicerloxP/loxPfemales, which are incapable of synthesizing and secreting GnRH in adulthood30. GnRH::Cre; DicerloxP/loxP (OVX+E+P) females failed to show male-directed preferences and actually showed a preference for the female (one-sample t test (H0: mean equals 0); P = 0.049; Fig. 4a). A single sc injection of GnRH (0.025 mg kg−1) restored this behavior in these females supplemented with only estradiol (OVX+E) (one-sample ttest (H0: mean equals 0); P = 0.01; Fig. 4a), whereas an sc injection of Kp-10 failed to elicit a male-directed preference in female (OVX+E+P) GnRH::Cre; DicerloxP/loxP mice (one-sample t test (H0: mean equals 0); P = 0.38; Fig. 4a). We found that lordosis behavior was not affected in (OVX+E+P) GnRH::Cre; DicerloxP/loxP females (Mann–Whitney U test; P = 0.79; Fig. 4b). These results suggest that although GnRH neurons are required for mate preference, they might not be essential for the expression of lordosis behavior. Consistent with this, GnRH injection into Kiss−/− females failed to stimulate lordosis behavior in (OVX+E) mice (paired t test; P = 0.65; Fig. 4c).
Kisspeptin control of lordosis is mediated by nitric oxide
To identify potential candidate neurons downstream of RP3V kisspeptin neurons other than GnRH neurons, we used ovary-intact female KissIC/R26-BIZ mice which express the transsynaptic tracer barley lectin (BL) exclusively in kisspeptin neurons31. Because BL is also expressed in ARC kisseptin neurons in these animals, we injected a Cre-dependent mCherry adeno-associated virus bilaterally into the RP3V to delineate the projections from RP3V kisspeptin neurons. We observed a cluster of BL+ cells in the ventrolateral part of the ventromedial hypothalamus (VMHvl), a brain area previously implicated in reproductive behaviors32. Subsequent immunohistochemical analyses (Fig. 5a–g) showed that a major subpopulation of the BL+ neurons in the VMHvl express neuronal nitric oxide synthase (nNOS), previously implicated in reproductive behaviors33,34. Furthermore, our tract tracing suggests that RP3V kisspeptin neurons project to the VMHvl and therefore, these VMHvl nNOS neurons may be downstream of RP3V kisspeptin neurons. We observed at proestrus that 31.27 ± 6.22% of nNOS neurons in the VMHvl were BL+, whereas at diestrus 20.25 ± 1.94% contained the tracer, however this was not found to be statistically significant (P > 0.05, Bonferroni multiple comparison test; Fig. 5h). Taken together, these data indicate that nNOS neurons within the VMHvl are part of a neural pathway containing kisspeptin neurons.
To further dissect the functional role of NO signaling in this neural circuit important for sexual behavior, we next analyzed mate preference and lordosis behavior in mice deficient in nNOS. We found that nNOS knockout (nNOS−/−) (OVX+E+P) females actually showed a small, albeit significant, preference for the female (one-sample t test (H0: mean equals 0); P = 0.01; Fig. 6a). However, a male-directed preference comparable to control littermates could be induced when injected sc with a cocktail of the nitric oxide donor SNAP and the guanylate cyclase agonist BAY 41-2272 (one-sample t test (H0: mean equals 0); P = 0.03; Fig. 6a). Accordingly, nNOS−/− (OVX+E+P) females also showed a strong decrease in lordosis behavior compared to control littermates (Tukey’s multiple comparison test; P = 0.002; Fig. 6b), which was restored by an sc injection of the cocktail SNAP+BAY 41-2272 (Tukey’s multiple comparison test; P = 0.04; Fig. 6b). By contrast, an sc injection of either KP-10 into female (OVX+E+P) mice or GnRH into (OVX+E) mice failed to stimulate lordosis behavior in nNOS−/− females (Tukey’s multiple comparison test; respectively P = 0.56; P = 0.19; Fig. 6c). In a final experiment, Kiss−/− (OVX+E+P) females were injected sc with the cocktail SNAP+BAY 41-2272 and WT-like levels of lordosis behavior were observed (paired t test; P = 0.02; Fig. 6d).
Taken together, these data demonstrate that NO is a key neurotransmitter downstream of kisspeptin neurons mediating both mate preference and lordosis behavior. The nNOS neuron population in the VMHvl might be a potential important downstream relay of RP3V kisspeptin neurons in governing lordosis behavior but also mate preference consistent with previous studies35.
The kisspeptin peptide is a well-established potent activator of the hypothalamic-pituitary-gonadal axis. The present study provides the first evidence that kisspeptin neurons in the RP3V are an essential part of a motivational neural circuit that is triggered by male olfactory cues detected and processed predominantly through the vomeronasal pathway, ultimately leading to the female adapting the lordosis posture facilitating intromission and consequently fertilization. Our data establish these cells as a central hub in the neural network governing the orchestration of sexual behavior in female mice.
Mate preferences are sexually differentiated: when in breeding condition, males and females seek out and mate with opposite sex conspecifics20. Many mammalian species use olfactory cues to identify potential mates3. It has therefore been postulated that sex differences in the circuits detecting and processing these olfactory cues might provide an entry point into understanding the neural basis of mate preferences. Indeed, numerous studies (reviewed in ref. 20) using c-Fos as neuronal marker of activation have reported important sex differences along both the main and accessory olfactory pathways when animals are exposed to olfactory cues. However, how these olfactory stimuli are ultimately integrated in the neuroendocrine brain to trigger the appropriate behavioral and hormonal responses has not yet been elucidated.
Intriguingly, GnRH neurons receive direct inputs from both the main and accessory olfactory pathways, but none of these inputs were found to differ between the two sexes9. This suggests that additional neuronal populations must be involved in transferring chemosensory information to GnRH neurons in female mice. Our previous observations19 of a sex-specific activation of the RP3V kisspeptin neuronal population by opposite sex odor cues suggest that this particular subset of neurons might present the missing link in how olfactory cues can elicit sexually differentiated behavioral and neuroendocrine responses. The present experiments now confirm that kisspeptin neurons in the RP3V indeed present a central hub in transferring olfactory information perceived through the vomeronasal pathway to the reproductive center of the neuroendocrine brain leading to sex-specific mate preferences and mating behaviors.
Kisspeptin acts through its canonical receptor Kiss1R expressed by ~95% of GnRH neurons to control GnRH neuronal activity and secretion. Previous studies have provided evidence for a direct role of GnRH in sexual behavior. Systemic or brain injections with GnRH can stimulate sexual behavior in both male and female rodents36,37,38,39, suggesting that GnRH neurons are part of the neural circuits regulating these behaviors. Consistent with this, it was shown that GnRH neurons have synaptic connections with subsets of neurons in several brain areas previously implicated in sexual behavior using a mouse model in which the transneuronal tracer barley lectin was specifically expressed in GnRH neurons9. These include the medial amygdala and the posteromedial part of the bed nucleus of the stria terminalis, both known to relay pheromone signals, as well as the medial preoptic area, the ventrolateral part of the ventromedial hypothalamus (VMHvl) and the ventral part of the premammilary nucleus (PMV). Using a viral-mediated tract tracing approach, it was demonstrated that kisspeptin neurons in the RP3V directly contact GnRH neuron cell bodies and proximal dendrites in the rostral preoptic area24. It thus seemed reasonable to assume that kisspeptin’s effects on sexual behavior are mediated through Kiss1R on GnRH neurons. Intriguingly, GnRH::Cre; DicerloxP/loxP females showed normal levels of lordosis behavior upon priming with estradiol and progesterone. These data are consistent with previous studies in mice harboring a deletion in the GnRH gene (and therefore fail to synthesize GnRH), which also appear (after OVX and priming with estradiol and progesterone) to have no deficits in sexual behavior40.
RP3V kisspeptin neurons, in contrast, seem to act through GnRH signaling in modulating mate preferences, since mate preferences were clearly disrupted in GnRH::Cre; DicerloxP/loxP females. Furthermore, a peripheral injection with GnRH, but not with Kp-10, restored a male-directed preference in GnRH::Cre; DicerloxP/loxP females, suggesting that GnRH neurons must be downstream of kisspeptin neurons in this neural circuit. We found that lordosis behavior was normal in GnRH::Cre; DicerloxP/loxP females. This result is in line with a previous study41 showing normal lordosis behavior in female Kiss1R knockout mice. Several studies42,43 have demonstrated that kisspeptin can activate neuropeptide FF receptors (NPFFR1 and NPFFR2). Herbison and colleagues44 have demonstrated that kisspeptin can modulate arcuate neuron excitability at least partially via NPFF receptors independently of Kiss1R. As NPFFR2 and (to a lesser extent) NPFFR1 have been detected in the VMH of mice45, it is tempting to speculate that these receptors may play a key role in the facilitation of lordosis behavior. Future studies using NPFF receptor mutant mice may provide new evidence of the involvement of these receptors in the kisspeptidergic modulation of reproductive behavior.
Taken together, our genetic analyses challenge the idea that GnRH plays a prominent role in lordosis behavior under normal physiological conditions. There are some caveats that we must consider however regarding the GnRH::Cre; DicerloxP/loxP mouse model used in the present studies. Although there is no longer any GnRH synthesis and secretion in adult GnRH::Cre; DicerloxP/loxP mice30, these neurons do not die and remain electrically active and could thus presumably still respond to kisspeptin through Kiss1R. Nevertheless, the present results suggest that GnRH neurons rather play a facilitatory than a critical role in lordosis behavior. Finally, it is quite likely that specialized subpopulations of kisspeptin neurons exist within the RP3V since transynaptic tracing has indicated that the majority of kisspeptin neurons within the RP3V does not communicate with GnRH neurons46. This suggests that RP3V kisspeptin neurons may have other functions in the adult female mouse brain that do not directly involve GnRH neurons and this could well be mediating olfactory-driven sexual behavior.
We detected BL+ cells in the VMHvl, a brain area necessary for the expression of lordosis behavior in KissIC/R26-BlZ mice, in which BL is specifically expressed in kisspeptin neurons31. Furthermore, we identified a major subpopulation of these neurons as nNOS+. While no significant difference was found between the percentage of BL+ nNOS neurons at proestrus compared to metestrus/diestrus, we noticed that the BL+/nNOS+ cluster in the VMHvl was more clearly detected when females were in proestrus. Previous studies have indicated that WGA (which has ~98% sequence homology with BL) is activity dependent47, however the half-life of BL within neurons remains unknown. Therefore, it is unclear whether this genetic model provides sufficient temporal resolution to accurately detect changes in synaptic activity across the limited time frame of the estrous cycle.
The trend toward estrous cycle-dependent changes in BL/nNOS immunoreactivity could, however, be potentially interesting in light of the recent study by Dey and colleagues48 showing that female mouse vomeronasal sensory neurons (VSN) were temporarily silenced to a subset of male pheromones during diestrus, but were fully responsive to the same odors when in estrus. It was proposed that the silencing of these VSN occurred under the influence of progesterone, which temporarily rises during diestrus. In this respect, it is interesting to note that RP3V kisspeptin neurons also express progestin receptors and thus are sensitive to the actions of progesterone. Furthermore, progestin receptors are also present in the lateral part of the VMH and its expression is highly dependent on estrogens49,50,51,52. Finally, a recent study53 showed that close to 100% of neurons expressing estradiol receptors in the VMHvl are actually nNOS-expressing neurons. Taken together, these findings reveal that kisspeptin neurons communicate with nNOS neurons in the VMHvl, a central hub driving female sexual behavior. Further specific ablation or stimulation of the VMHvl nNOS population will be necessary to decipher its precise role in female reproductive behaviors.
Several studies have implicated the neurotransmitter NO in sexual behavior33,34. The present studies show that nNOS neurons are most likely downstream of kisspeptin and GnRH neurons since neither GnRH nor Kp-10 was able to induce lordosis behavior in nNOS−/− females. Possibly the nNOS neuron population in the VMHvl plays an important role because of its synaptic connection with kisspeptin neurons. However, it cannot be ruled out that other nNOS neuron populations are also involved, for instance, in the rostral preoptic area, where nNOS neurons are part of a neural circuitry mediating ovarian cyclicity and ovulation54, and in the paraventricular nucleus (PVN), where nNOS co-localizes with oxytocin. The latter has also been implicated in sexual behavior in female rodents: mating activates PVN oxytocin neurons and increases local oxytocin concentrations, in particular following a pace mating paradigm in which the female controls the mating, thereby linking oxytocin in particular with sexual motivation55,56. Therefore, further experiments are needed to dissect the precise role of the different nNOS populations in the brain in controlling reproductive behaviors.
Taken together, our data strongly suggest that RP3V kisspeptin neurons mediate sexual behavior and that hypothalamic NO signaling could be an essential mechanism downstream of kisspeptin neurons in governing sexual behavior.
The discovery that a neuropeptide that governs ovulation is also able to elicit sexual behavior opens new avenues of research for the treatment of low sexual desire in women, including hypoactive sexual desire disorder (HSDD). HSDD represents the most severe form of low sexual desire. Although the causes of low sexual desire and HSDD are multi-factorial, there is convincing evidence that sex hormones play an important role since the prevalence of HSDD increases after natural or surgically induced menopause57. At present, there is no good treatment available for low sex drive including HSDD in women. Testosterone has been used in postmenopausal women but also has strong adverse effects leading to virilization. For premenopausal women, there is currently no treatment available. A recent study from Comninos et al.58used functional magnetic resonance imaging to show that kisspeptin enhances limbic brain activity specifically in response to sexual and couple-bonding visual stimuli in men. Taken together, therapeutics targeting kisspeptin signaling in the brain may hold the potential to provide novel treatment options for low sexual desire.
Kisspeptin knockout (Kiss−/−)22, Kisspeptin-IRES-Cre (KissIC)26, GnRH::Cre12; DicerloxP/loxP59, R26-BIZ31, and nNOS knockout (nNOS−/−)54,60 mouse strains have been previously described and validated, and are on different genetic backgrounds (see Supplementary Table 1). All experiments were performed on adult (>8 weeks of age) female mice unless otherwise stated.
Animal care and experimental procedures were performed in accordance with the guidelines established by the institutional animal care and use committee of the Royal Netherlands Academy of Arts and Science and by the National Institutes of Health “Guide for the Care and Use of Research Animals, Eight Edition,” and were approved by the Ethical Committee for Animal Use of the Universities of Liege (Belgium), Saarland (Germany) and of Otago (New Zealand). Female mice were placed into individual cages under a reversed light/dark cycle (12 h:12 h light/dark; 21.00 h lights on and 9.00 lights off) with food and water ad libitum.
Ovariectomy and hormone supplementation
Unless otherwise stated, females were ovariectomized in adulthood (>8 weeks of age) under general anesthesia after either subcutaneous (sc) injections of ketamine (80 mg kg−1 per mouse) and medetomidine (Domitor, Pfizer, 1 mg kg−1 per mouse) or under 5% isoflurane, in order to control for endogenous hormone concentrations and to prevent pregnancies upon repeated testing. At the same time, all females received a 5-mm-long silastic capsule (inner diameter: 1.57 mm; outer diameter: 2.41 mm) containing crystalline 17β-estradiol (diluted 1:1 with cholesterol) subcutaneously in the neck. The dose of E2 (E8875, Sigma) was based on a previous study61 showing that this treatment leads to estradiol levels similar to mice in estrus. At the end of surgery, females under ketamine/medetomidine anesthesia received an sc injection of atipamezole (Antisedan, Pfizer, 4 mg kg−1 per mouse) to antagonize medetomidine-induced effects and accelerate recovery. In order to induce sexual receptivity at the day of testing, all females received a subcutaneous injection with progesterone (500 µg, P0130, Sigma) 3 h before the onset of the behavioral test, unless stated otherwise (for overview of all the different hormone treatments, see Supplementary Table 1).
Viruses and stereotaxic injections for behavioral testing
The AAV5-flex-taCasp3-TEVp virus (abbreviated to “AAV-Casp3” (Vector Core, University of North Carolina)) was stereotaxically injected bilateral into the RP3V in KissIC mice (total: Cre−: n = 14; Cre+: n = 14). AAV-Casp3 uses the T2A peptide encoding sequence to ensure bicistronic expression of pro-taCasp3 and TEVp after Cre-mediated recombination. taCasp3 triggers cell autonomous apoptosis, thereby minimizing toxicity to adjacent Cre− cells23.
AAV5-EF1a-DIO-hChR2(H134R)-mCherry-WPRE-pA virus (abbreviated to “AAV-ChR2” (Vector Core, University of North Carolina)) was stereotaxically (see below) injected bilaterally into the RP3V in KissICmice (Cre−: n = 8; Cre+: n = 8) to selectively express the light-activated cation channel channelrhodopsin-2 and mCherry62 in kisspeptin neurons.
Mice were placed in a motorized stereotaxic frame (Neurostar, Germany) under 5% isoflurane anesthesia. The skull was exposed by a midline scalp incision, and the stereotaxic frame was aligned at Bregma using visual landmarks. After alignment of the head of the mice, a drill was placed above the skull at coordinates (according to the Paxinos Brain Atlas63) corresponding to the rostral periventricular area of the third ventricle (RP3V; rostrocaudal, 0.2 mm; mediolateral, ±0.1 mm) and a hole drilled through the skull bone to expose the brain. A 33-gauge steel needle loaded with virus (AAV-Casp3 or AAV-ChR2) was slowly inserted through the hole until it penetrated to a depth of 5.8 mm. Virus (1 μl per brain site injected) was delivered at 100 nl min−1 through a Hamilton syringe using a syringe pump (Harvard Apparatus). The needle was left in place for an additional 10 min to allow diffusion of the virus before being slowly removed. Following AAV-Casp3 injection, the hole was filled with dental cement and the skin was sutured. Following AAV-ChR2 injection, a bilateral cannula (200 µm core diameter; Doric Lenses) holding optical fibers with 45°-oriented mirror tips was inserted into the RP3V at a distance of 0.25 mm (mediolateral) from the center of injection and further fixed to the skull with dental cement. Mice were allowed to recover on a heating pad and returned to their home cage after waking up. All mice received an sc injection with Caprofen (5 mg kg−1) for post-operative analgesia.
Viruses and stereotaxic injections for electrophysiology
Adult female (>2 months old) heterozygous KissIC mice were group-housed under conditions of controlled temperature (22 ± 2 °C) and lighting (12-h light, 12-h dark cycles) with ad libitum access to food and water. Mice were anesthetized, placed in a stereotaxic apparatus, and given simultaneous bilateral 0.5 µl injections of AAV9-EF1-DIO-hChR2-(H134R)-mCherry-WPRE-hGH (2.2 × 1013 GC ml−1; Penn Vector Core) into the RP3V (coordinates according to the Paxinos Brain Atlas63), 0.2 mm anterior to Bregma and 5.8 mm in depth) at a rate of 100 nl min−1. The syringes were left in situ for 3 min before and 10 min after the injections. Following a recovery period, mice were bilaterally ovariectomized under anesthesia and, after >2 weeks, received subcutaneous silastic implants containing 17-β-estradiol (1 µg per 20 g body weight) according to Bronson64. Implants were made of 17-β-estradiol dissolved in ethanol and mixed with medical grade adhesive (0.1 mg ml−1 adhesive), which is then injected into 1 mm internal diameter silastic tubing. Six days later, mice received a subcutaneous injection of estradiol benzoate (1 µg per 20 g body weight) in the morning and were used for electrophysiology the following day
Cannula implantation for ICV kisspeptin administration
Mice were placed in a motorized stereotaxic frame (Neurostar, Germany) under 5% isoflurane anesthesia. The skull was exposed by a midline scalp incision, and the stereotaxic frame was aligned at Bregma using visual landmarks. After alignment of the head of the mice, a drill was placed above the skull at coordinates corresponding to the lateral ventricle (lateral+1, anterior–posterior: −0.34; dorsoventral: −2.5) and a hole drilled through the skull bone to expose the brain. Then, a 26-gauge cannula cut at 2 mm from pedestal was implanted and fixed to the skull with dental cement. A dummy was inserted to close the cannula until the behavioral experiment. Mice were allowed to recover on a heating pad and returned to their home cage after waking up. All mice received an sc injection with Temsegic (0.05 mg kg−1) for post-operative analgesia.
Removal of the VNO
One week after ovariectomy, subjects underwent either bilateral removal of the VNO or sham surgery (VNOx or VNOi groups)65. Briefly, animals were placed on their back and the lower jaw was gently opened after general anesthesia. A midline incision was made in the soft palate extending rostrally from behind the first palatal ridge to the incisors, and the underlying bone was exposed by blunt dissection. In VNOi animals, the incision was then closed with reabsorbable sutures. For VNOx animals, the rostral end of the VNO was exposed by drilling, the caudal end of the vomer bone was cut, and the VNO was removed bilaterally with a gentle twisting motion. Bleeding was controlled using a blunted 18-gauge needle attached to a vacuum. Animals were carefully monitored after surgery for bleeding and or breathing difficulties.
Ablation of the MOE
Two weeks after the removal of the VNO (VNOx females) or sham surgery (VNOi females), mice received an intranasal application of 10% ZnSO4 to lesion the main olfactory epithelium (MOEx) or saline solution (MOEi) under general anesthesia66.
Kisspeptin-10 (Kp-10) was synthesized in Strasbourg, France (Sequence: Tyr-Asn-Trp-Asn-Ser-Phe-Gly-Leu-Arg-Tyr-NH2, NeoMPS; weight = 25.7 mg). To determine whether kisspeptin-stimulated female sexual behavior in wild-type C57BL/6J mice, females received an sc injection of Kp-10 at the dose of 0.52 µg kg−1 (injection volume 100 µl) 2 h before the lordosis test. Each animal was used as its own control, i.e., the animal was injected 1 day with Kp-10 and the other day with saline. Females were not injected with progesterone in this particular experiment. Injections were separated by at least 3 days.
When injected intracerebroventricularly, females were injected with 10.4 ng kg−1 of Kp-10 (injection volume 2 µl), 1 h before the lordosis test through a cannula inserted into the lateral ventricle.
Two hours before mate preference test, female mice received a single sc injection of GnRH (0.025 mg kg−1, Polypeptide Laboratories France SAS, SC087).
S-nitroso-N-acetyl-DL-Penicillamine (SNAP) (N398-Sigma) is a NO donor. In order to ascertain the most efficient activity of SNAP (8 mg kg−1) was combined with the guanylate cyclase agonist BAY-41-2272 (10 mg kg−1; B8810—Sigma) 1 h before injection. One hour before behavioral tests, female mice received a single subcutaneous injection (100 µl) of the cocktail SNAP+BAY 41-2272.
Brain slice preparation for electrophysiology
Mice were killed by cervical dislocation, decapitated and brains quickly removed. Coronal brain slices (200–250 μm) containing the rostral periventricular area of the third ventricle (RP3V) were cut with a vibratome (VT1000S; Leica) in an ice-cold solution containing (in mM): NaCl 87, KCl 2.5, NaHCO3 25, NaH2PO4 1.25, CaCl2 0.5, MgCl2 6, glucose 25, and sucrose 75. Slices were then incubated at 30 °C for at least 1 h in artificial cerebrospinal fluid (aCSF; in mM): NaCl 120, KCl 3, NaHCO3 26, NaH2PO4 1, CaCl2 2.5, MgCl2 1.2, and glucose 10. All solutions were equilibrated with 95%O2/5%CO2.
Cell-attached recordings and light stimulation
Slices were placed under an upright microscope fitted for epifluorescence (Olympus, Tokyo, Japan) and constantly perfused (1.5 ml min−1) with warm (~30 °C) aCSF. mCherry-expressing RP3V neurons were first visualized by brief fluorescence illumination and subsequently approached using infrared differential interference contrast optics. Action potential firing was recorded in voltage clamp mode in the cell-attached loose patch configuration. Recording electrodes (3–5 MΩ) pulled from borosilicate capillaries (Warner Instruments, Hamden, CT) with a horizontal puller (Sutter Instruments, Navato, CA) were filled with aCSF including 10 mM HEPES. Low resistance seals (10–30 MΩ) were achieved by applying either no suction or the lowest amount of suction required to detect spikes. For ChR2 activation, blue light was delivered to the slice through a ×40 immersion objective (0.8 NA, Olympus) via a 470 nm light-emitting diode (LED, CoolLED) connected to the vertical illumination port of the microscope. Stimulation consisted of 1–15 s trains of blue light pulses (2 ms duration; ~0.25 mW) delivered at 10 Hz, repeated ten times every 60 s in each cell. Electrophysiological signals were recorded using a Multiclamp 700B amplifier (Molecular Devices, Sunnyvale, CA) connected to a Digidata 1440A digitizer (Molecular Devices). Signals were low-pass filtered at 3 kHz before being digitized at a rate of 10 kHz and stored on a personal computer. Signal acquisition and analysis was carried out with pClamp 10 (Molecular Devices). Spikes were detected using the threshold crossing method. In each cell, spike fidelity was calculated by dividing the number of light-evoked spikes by the number of blue light stimuli and expressed as a percentage.
All experimental females were brought into behavioral estrus by ovariectomy (OVX) in adulthood and combined treatment with estradiol (E) through a silastic capsule and an acute injection (3 h before testing) with progesterone (P), unless stated otherwise (for all details on hormone treatments, see Supplementary Table 1). Females were always tested during the dark phase of the light/dark cycle. Finally, levels of female sexual behavior displayed by the control (wild type) females vary as function of the background strain with 129SvJ females (Kiss+/+) showing relatively low levels compared to C57BL6/j females.
Assessment of the MOE lesion
Anosmia was assessed by submitting females to the hidden cookie test66. Briefly, female mice were food-deprived overnight. A small piece of a chocolate chip cookie was buried (~1 cm deep) at a random location in a clean Plexiglas aquarium (35 cm long × 25 cm high × 19 cm wide) containing fresh sawdust. The time it took each mouse to find the cookie was recorded. The test lasted until the mouse had located the cookie or 10 min if the cookie was not found. All mice treated with ZnSO4 failed to find the hidden cookie and were thus considered to be anosmic.
Exposure to odors in bedding
Four groups of gonadally intact males (n = 5 each) were placed in clean cages containing fresh sawdust. Bedding was collected 12 h later and directly used as olfactory stimulus for the experimental females. Thirty-six hours before bedding exposure, all experimental females (singly housed) were placed on clean sawdust in two separate housing units to separate females, which were going to be exposed to male bedding or to clean bedding as control (and thus to prevent the controls from being exposed to male odors). On the day of testing, females were injected with P (500 µg) to induce behavioral estrus. This hormonal treatment made the experimental females behaviorally receptive at the time of odor exposure. Three hours after P injection, 15 g of fresh male-soiled or clean bedding was placed into the subject’s own cage. Ninety minutes after bedding exposure, females were perfused with paraformaldehyde and brains were collected.
Mate preference tests
To assess mate preferences shown in response to auditory and olfactory stimuli, we used a box (60 cm long × 30 cm high × 13 cm wide) that was divided into three compartments using perforated opaque partitions. The partitions contained perforated holes at a height of 8 cm to facilitate the diffusion of odors from the two-side compartments to the middle compartment. Tests were performed during the dark phase of the light cycle (5 h after lights out). Animals were habituated to the three compartment box only once on the day before the behavioral experiments by placing them in the middle compartment for 10 min (with no stimulus animals placed in the two-side compartments). On the day of testing, an intact male stimulus and an estrous female stimulus were placed in the lateral compartments with their own bedding to make the stimuli as odorous as possible. Three hours after receiving a progesterone injection (500 µg), the female subject was introduced into the middle compartment, and was observed for 10 min. The time the subject spent poking her nose through the holes of the partition or actively sniffing the bottom of the partition in front of the female vs. male stimulus animal was recorded. A preference score was calculated by dividing the time spent investigating the male compartment minus the time spent investigating the female compartment by the total time spent investigating both compartments. A positive value of the preference score indicates a mate preference directed toward the stimulus male, whereas a negative value indicates a mate preference directed toward the stimulus female (for details, see ref. 67).
Lordosis tests without photostimulation
Females were subjected to weekly lordosis tests in a Plexiglas aquarium (37 cm long × 17 cm high × 21 cm wide). A sexually experienced male was placed alone in the aquarium and allowed to adapt for 15 min. Subsequently, 3 h after receiving a subcutaneous progesterone injection (500 µg, P0130, Sigma) to induce behavioral estrus, the lordosis responses of the female to the mounts of the stimulus male were recorded. The test lasted until the female received 10 mounts or 10 min had elapsed. For the first experiment (mating-induced Fos activation), ovary intact females were paired with males during 30 min. A lordosis quotient (LQ) was calculated by dividing the number of lordosis responses displayed by the female subjects by the number of mounts received (x100). Before each experimental condition (drug injection, cell ablation, and optogenetic stimulation), all females were subjected to at least three lordosis tests (with progesterone) in order to acquire sufficient sexual experience and thus a significant LQ. Tests were performed during the dark phase of the light cycle (5 h after lights out; for details, see ref. 67). For all details on the different hormone treatments (estradiol vs. estradiol+progesterone), see Supplementary Table 1.
Lordosis tests with photostimulation
Prior to the lordosis test, the cannula was connected to an optical fiber (Doric Lenses), which in turn was connected to a blue laser (wavelength = 473 nm) via an optical rotatory join allowing free movements of the animal. The optic fiber was flexible and long enough to allow the female to freely behave and interact with the male. After three pretests, KissICfemales (Cre− and Cre+) were then divided in two groups. On test 4, half of the females received optogenetic stimulation (stimulated), whereas the other half did not (unstimulated). On test 5 (conducted 1 week later), unstimulated females received an optogenetic stimulation while previously stimulated females did not, thus each female acted as her own control. Blue light was delivered through the optic cable at 10 Hz as soon as the male approached the female (sniffing and showing mount attempt). The duration of the stimulation varied as a function of the male, i.e., the time it took him to mount the female, however this was never longer than 15 s. Tests were performed over 10 min and the number of mounts was recorded as well as the number of female lordosis responses. Importantly, in order to observe possible stimulatory effects of photostimulation on lordosis behavior, females were not injected with progesterone before the lordosis test, and were thus only on estradiol treatment (by silastic capsule, previously described).
Transcardial perfusion and OCT embedding for histology
Female mice were anesthetized and perfused transcardially with saline followed immediately by 4% ice-cold paraformaldehyde. Brains were removed and postfixed in 4% paraformaldehyde for 2 h. Brains were then cryoprotected in 30% sucrose68 in PBS and when sunken, were embedded in Optimal Cutting Temperature compound (OCT, Tissue-Tek). A glass box was placed in a slurry of ethanol and dry ice. The glass box was then partially filled with isopentane. The tissue was placed in a plastic cuvette filled with OCT, placed into the isopentane bath and rapidly frozen. Brains were subsequently stored at −80 °C prior to sectioning.
Histological assessment of VNO removal
As previously described65, snouts were removed immediately after perfusion, cleared of all soft tissue, and soaked for 30 min in rapid decalcifier (Apex Engineering Products). Decalcified snouts were then soaked overnight in 30% sucrose68 at which time a 1:1 mixture of 30% sucrose and OCT was suctioned into the nasal passages. Snouts were then incubated for 4 h in the 1:1 solution and were finally frozen in OCT and stored at −80 °C. Snouts were sectioned at 10 µm thickness on a cryostat. One section every 150 µm was transferred directly onto Superfrost Plus glass slides and dried overnight. Sections were rinsed and stained with hemotoxylin and eosin to assess the presence of blood clots in the nasal sinuses and to determine whether the VNO was completely removed. A total of 32 mice were used and upon examination, 8 were excluded because the VNO was not completely removed. No blood clots were detected in any of the animals.
Immunohistochemical detection of c-Fos or kisspeptin
Brain sections (30 µm thick) were cut on a Leica CM3050S cryostat. Forebrains were cut coronally from the rostral telencephalon to the posterior hypothalamus. Sections were saved in four different series, placed in antifreeze solution, and stored at −20 °C.
Immunostaining was carried out on free-floating sections. All incubations were carried out at room temperature, and all washes of brain tissue sections were performed using Tris-buffered saline (TBS 0.05 M) or Tris-buffered saline containing 0.1% Triton X-100 (TBST). Briefly, sections were rinsed and endogenous peroxidase activity was quenched by incubating the sections for 30 min with 0.3% hydrogen peroxide. Non-specific-binding sites were then blocked by incubating sections for 30 min with 5% normal goat serum (NGS) (Dako Cytomation, Denmark). Sections were then incubated either with a rabbit polyclonal antibody (1/5000 in TBST-NGS 5%; anti-kisspeptin-10, AB9754, Chemicon, Millipore) raised against the decapeptide kisspeptin-10 (derived from the Kiss-1 gene product) for 48 h at 4 °C or with a rabbit polyclonal anti-c-Fos antibody (1/2000 in TBST-NGS 5%; c-Fos (4): sc-52R, Santa Cruz Inc.) raised against the N terminus of c-Fos of human origin. Sections were then incubated for 1 h in avidin-biotin complex (1/800, ABC, Vector Laboratory, Burlingam, CA) and then reacted for 5 min with 3,3′-diaminobenzidine tetrahydrochloride (DAB Kit, Vector Laboratory). Sections were then washed, dried overnight, left in xylene (Sigma) for 15 min, and coverslipped using Eukit (Fluka, Steinheim, Germany).
Immunohistochemical detection of c-Fos and kisspeptin
To determine the distribution of c-Fos and kisspeptin double-labeled neurons, ovary intact females in proestrus (activation of kisspeptin cells following mating) or ovariectomized females (VNOx/MOEx experiment) were perfused with 4% paraformaldehyde in 0.1 M PBS 90 min after the introduction of the male to the female (onset of behavioral testing) or 90 min after being placed in the empty testing arena for the unmated controls. Regarding male bedding exposure (VNOx/MOEx), females were perfused 90 min after the onset of odor exposure (male or clean bedding). For the dual immunohistochemistry, sections were first washed in 0.1 M PBS pH 7.4 (PBS), peroxidase activity was blocked in PBS solution with 0.3% H2O2, and then permeabilized in PBS-0.1% Triton X-100 (PBST) and saturated in 5% NGS in PBST. Immediately after this step, sections were incubated in diluted anti-c-Fos antibody overnight. On the following day, sections were washed in PBST and incubated in a goat anti-rabbit biotinylated secondary antibody (Dako, Prod. Ref. B0432, 0.75 µg ml−1 PBST). Sections were then washed in PBST and incubated in the Vectastain Elite ABC Kit (Vector, Prod. Ref. PK6100). After development with the DAB Substrate Kit (Vector, SK-4100), in a black precipitate (3,3′-diaminobenzidine (DAB) plus Ni2+), sections were washed thoroughly in PBS, and residual peroxidase activity blocked in PBS solution with 0.3% H2O2. Sections were then permeabilized and blocked in 5% NGS-PBST and incubated in anti-kisspeptin-10 antibody for 72 h. Similar secondary antibody and ABC incubation steps were then performed. The developing reaction used in this step was a DAB brown precipitate, using the same kit. Following this, sections were mounted in Eukitt after being air-dried.
Immunohistochemical detection of barley lectin and nNOS
In order to identify cells that are synaptically connected to kisspeptin neurons, KissIC/R26-BIZ mice were transcardially perfused with 4% paraformaldehyde at proestrus or metestrus/diestrus (determined via vaginal cytology). Brains were sectioned at 14 μm and collected in series of five on SuperFrost Plus slides (Roth) and stored at −80 °C. The transsynaptic tracer BL was immunohistologically detected using goat anti-wheat germ agglutinin (1:1000, Vector Laboratories) and has been described previously31. A Tyramide Signal Amplification Plus Biotin kit was used for signal amplification. Briefly, slides were washed 3× for 5 min in PBS, incubated in ice-cold methanol with 0.3% H2O2 for 30 min, washed 3× for 5 min in TNT (0.1 M Tris, 0.15 M NaCl, 0.05% Tween 20), incubated for 10 min in 0.5% Triton X-100 in PBS, washed 3× for 5 min in TNT, blocked with TNB for 30 min, and incubated with anti-wheat germ agglutinin (1:1000) in TNB overnight at 4 °C in a humidified chamber. The next morning, slides were brought to RT for 2 h, washed 3× for 5 min in TNT and incubated with biotinylated anti-goat IgG (1:500) in TNB for 1 h at RT. Slides were then washed 3× for 5 min in TNT and incubated with streptavidin-conjugated horseradish peroxidase (1:100) in TNB for 30 min at RT. Slides were then washed 3× for 5 min in TNT and incubated for 10 min in biotin plus amplification reagent (1:50) in 1× plus amplification diluent at RT. Slides were then washed 3× for 5 min in TNT followed by incubation in streptavidin-conjugated Cy5 (1:500) in TNB for 30 min at RT. Slides were then washed 3× for 5 min with TNT. The slides were incubated overnight at 4 °C followed by 2 h at RT with rabbit anti-nNOS (1:300) in 0.1 M PBS containing 0.5% λ-carrageenan (Sigma) and 0.02% sodium azide. The sections were then treated with Cy3-conjugated donkey anti-rabbit (1:500) in PBS containing 0.5% λ-carrageenan (Sigma) and 0.02% sodium azide in PBS for 1 h at RT. Nuclei were stained with bisbenzimide solution (1:2000 in 0.1 M PBS, 5 min at RT) and coverslipped with Fluoromount-G (Southern Biotech). Images were taken using either a Zeiss Axioskop or a Zeiss Axio Scan Z1 epifluorescence microscope.
Immunohistochemical detection of mCherry
To immunohistochemically detect mCherry, slides were washed 3× for 5 min in PBS, incubated in 0.3% Triton X-100, 5% donkey serum, and 0.02% sodium azide in PBS for 1 h at RT followed by incubation with anti-ds-red (1:1000, recognizes mCherry) in PBS containing 0.5% λ-carrageenan (Sigma) and 0.02% sodium azide overnight at 4 °C. The next day slides were washed 3× for 5 min with PBS containing 0.5% Tween 20 (PBS+Tween), incubated with Cy3-conjugated donkey anti-rabbit in PBS containing 0.5% λ-carrageenan (Sigma) and 0.02% sodium azide and washed 3× for 5 min in PBS+Tween. Nuclei were stained with bisbenzimide solution (1:2000 in 0.1 M PBS, 5 min at RT) and coverslipped with Fluoromount-G (Southern Biotech). Images were taken using either a Zeiss Axioskop or a Zeiss Axio Scan Z1 epifluorescence microscope.
Quantification and statistical analysis
Kisspeptin-immunoreactive (-ir) cell bodies were counted manually and bilaterally in three to four adjacent brain sections (with an interval of 120 µm between them) delineating the RP3V (anteroventral periventricular area+Periventricular preoptic zone) using a Zeiss Axioskop microscope (×40 objective). Cell counts are expressed as mean number per section for each experimental condition. Analysis of kisspeptin-ir density in the ARC was performed as previously described69. To analyze kisspeptin/c-Fos double labeling, three to four sections were selected from the RP3V and the total number of kisspeptin and kisspeptin/c-Fos co-labeled cells were counted in order to obtain the percentage of kisspeptin cells expressing c-Fos immunoreactivity per section.
nNOS-ir and BL+ nNOS-ir cell bodies were counted unilaterally in eight to ten sections containing the VMHvl (Bregma −1.34 to −1.94 according to ref. 63. Cell counts are expressed as mean number per section for each experimental condition. Statistical significance was determined using Bonferroni’s multiple comparison test.
Randomization was not used in this study and no statistical methods were used to predetermine sample size. Investigators were blinded to the group allocation during experiments or data analysis. Data were analyzed using the GraphPad Prism 7 software. For all statistical comparisons, we first analyzed the data distribution with the Shapiro–Wilk test for normality. Preference score data were analyzed by comparison to hypothesized mean (H0: mean equal 0) using a non-directional one-sample two-tailed t test (Figs. 1b, e, 4a and 6a)70. For comparison of paired samples comparing two groups, statistical analysis was performed by using a paired-sample two-tailed t test (Figs. 2b, d, f, i, 4c and 6d). Comparison of unpaired samples comparing two groups was then performed using an unpaired-sample two-tailed ttest (Figs. 1c and 2a, e). For comparison of more than two groups, an ANOVA test followed by a Tukey’s (Figs. 1e and 6b, c) or Bonferroni’s (Fig. 5) two-tailed multiple comparisons test was used. Comparison of more than two data sets violating the normal distribution, the Kruskal–Wallis ANOVA two-tailed test followed by a Dunn’s multiple comparison two-tailed test was used (Figs. 1a and 3a, b). Comparison of two data sets violating the normal distribution, a one-sample Wilcoxon two-tailed test was used (Figs. 2c and 4b).
Scientists made a significant advancement in figuring out how the brain is controlling sex.
By studying female mice, the researchers discovered that a brain hormone called kisspeptin (not surprisingly also known as the “kiss hormone”) is the driving force in our attraction to the opposite sex as well as sexual behavior. The scientists found that a male mouse secretes pheromones to activate the kisspeptin neurons which send this signal along to other neurons. These, in turn, release the gonadotropin hormones that control attraction to the opposite sex. The kisspeptins also transmit signals to cells that make the neurotransmitter nitric oxide, responsible for triggering our sexual behavior.
The research was led by Professor Julie Bakker, who heads the Laboratory of Neuroendocrinology at Liège University in Belgium, and Professor Ulrich Boehm, who teaches in the department of Experimental and Clinical Pharmacology and Toxicology of Saarland University in Germany.
“This work has provided new insight into how the brain decodes signals from the outside world and then translates these environmental cues into behavior, “ explained Boehm. “In many animals, sexual behavior is timed to occur with ovulation to ensure the highest possible chance of fertilization and therefore, continuation of the species. Until now, little was known about how the brain ties together ovulation, attraction and sex. Now we know that a single molecule – kisspeptin – controls all of these aspects through different brain circuits running in parallel with one another.”
The study suggests kisspeptin is the one molecule that rules them all, controlling puberty, fertility, attraction and sex. Professor Julie Bakker thinks one application of their discovery is creating therapies for women with low sexual desire, a condition estimated to affect up to 40% of women at some time in their lives.
“There are currently no good treatments available for women suffering from low sexual desire, “ said Bakker. “The discovery that kisspeptin controls both attraction and sexual desire opens up exciting new possibilities for the development of treatments for low sexual desire.”