Neurophysiological and behavioral processes regulated by hypocretin (orexin) are severely affected in depression. However, alterations in hypocretin have so far not been studied in the human brain. We explored the hypocretin system changes in the hypothalamus and cortex in depression from male and female subjects.
We quantified the differences between depression patients and well-matched controls, in terms of hypothalamic hypocretin-1 immunoreactivity (ir) and hypocretin receptors (Hcrtr-receptors)-mRNA in the anterior cingulate cortex (ACC) and dorsolateral prefrontal cortex. In addition, we determined the alterations in the hypocretin system in a frequently used model for depression, the chronic unpredictable mild stress (CUMS) rat.
i) Compared to control subjects, the amount of hypocretin-immunoreactivity (ir) was significantly increased in female but not in male depression patients; ii) hypothalamic hypocretin-ir showed a clear diurnal fluctuation, which was absent in depression; iii) male depressive patients who had committed suicide showed significantly increased ACC Hcrt-receptor-2-mRNA expression compared to male controls; and iv) female but not male CUMS rats showed a highly significant positive correlation between the mRNA levels of corticotropin-releasing hormone and prepro-hypocretin in the hypothalamus, and a significantly increased Hcrt-receptor-1-mRNA expression in the frontal cortex compared to female control rats.
The clear sex-related change found in the hypothalamic hypocretin-1-ir in depression should be taken into account in the development of hypocretin-targeted therapeutic strategies.
Our study shows, for the first time in postmortem human brain, that hypothalamic hypocretin/orexin is increased in female – but not in male – depressive patients. In addition, there was a diurnal fluctuation in hypothalamic hypocretin-1-ir in the control subjects, which was absent in depression. Moreover, we observed that Hcrt-receptor-mRNA expression showed differences in the ACC and DLPFC depending on age. Male depressive patients who had committed suicide had significantly increased ACC Hcrt-receptor-2-mRNA expression. Our data thus indicate sex-, brain area-, age-, and potentially suicide-related changes in the hypocretin/orexin system in depression. Finally, a significant positive correlation between hypothalamic CRH-mRNA and prepro-hypocretin-mRNA and a significant increase in Hcrt-receptor-1-mRNA expression in the frontal cortex in female – but not male – CUMS rats strengthen the presence of sexually dimorphic hypocretin/orexin system changes in mood disorder.
4.1. Hypocretin-ir in the Hypothalamus in Depression
It should be noted that the increased IOD of hypocretin-ir may indicate an increase in either hypocretin-expressing neuron number (related to the area stained) and/or staining intensity (measured as OD). An increase of either of these parameters indicates an increased expression of hypocretin protein levels. The significantly increased hypocretin-ir in female depressive patients indicates that hypocretin may play a key role in the etiology of depression, which is more prevalent in females than in males (Piccinelli and Wilkinson, 2000). As we have indicated in the Introduction section, studies have found that Hcrt-receptor-1 gene, or a linked locus, may modulate the risk for mood disorders (Rainero et al., 2011) and Hcrt-receptor-1 gene knockout mice showed increased anxiety-like behavior and altered depression-like behaviors (Abbas et al., 2015). One may thus speculate that the higher levels of hypocretin-1-ir in female patients may enhance depressive symptoms. It should be noted that since all the female subjects studied in our study were in their postmenopausal stage, one would not expect to see the hot flash-related hypocretin changes reported earlier (Federici et al., 2016).
The sex difference in the alterations of hypocretin, which happens in the framework of the stress-hypothesis, was further supported by our animal study, showing a significant positive correlation between hypothalamic prepro-hypocretin-mRNA and CRH-mRNA only in female CUMS rats. In view of this, it is of interest to note that the dual Hcrt-receptor antagonist almorexant was found to prevent HPA axis dysregulation caused by CUMS and offers evidence for the possibility that pharmacological blockade of the hypocretin system induces a robust antidepressant-like effect as well as the restoration of the stress-related HPA axis defect (Nollet et al., 2012). It should be noted, however, that other animal models of depression showed different results in terms of changes in the hypocretin system, such as the genetically depressed Wistar-Kyoto male rats, which showed a lower number and size of hypocretin-1 neurons than its control Wistar male rats (Allard et al., 2004). In another genetically depressed rat model, i.e. the Flinders Sensitive Line (FSL), the number of hypothalamic hypocretin-positive neurons was higher in female FSL rats than in the female control rats, i.e. Flinders Resistant Line (FRL) although this publication by Mikrouli et al. offers no data on male rats (Mikrouli et al., 2011). It is a frequent phenomenon that animal models tend to mimic “just a few symptoms rather than a complete psychiatric disorder”. This is a reason to validate the data obtained in animal models on human postmortem material.
We found a clear day-night fluctuation in hypothalamic hypocretin-1-ir in the control subjects, with higher levels at night, which is similar to the pattern reported for lumbar puncture CSF hypocretin-1 levels obtained by continuous in vivo sampling (Salomon et al., 2003). These findings are in agreement with the concept that hypocretin neurons may play a key role in sleep-wake regulation (Saper, 2013), and the absence of the day-night hypocretin-1-ir fluctuation in depression may thus relate to the frequently occurring symptoms of sleep disorders in this condition. It should be noted that in our earlier research we demonstrated a clear diurnal rhythm in the biological clock, the suprachiasmatic nucleus (SCN), for its main neuropeptide, vasopressin, both on the protein level (Hofman et al., 1993) and on the mRNA level (Zhou et al., 2001) in postmortem material, when the patients were grouped according to time of death. Interestingly, a direct projection from the SCN onto the hypocretin neurons was observed in the brains of rat and human (Abrahamson et al., 2001), which indicates that the SCN may directly regulate the function of hypocretin-immunoreactive neurons. Our finding of the absence of a diurnal hypocretin rhythm in depression agrees with our earlier observation of a diminished SCN function in depression (Zhou et al., 2001). Our data (Zhou et al., 2001) and those of others (Li et al., 2013) thus also show that postmortem studies can indeed reflect the day-night fluctuations during life.
It is of interest to note that in rats the maximal activity of the hypocretin system takes place in their active period, i.e. at night (Mileykovskiy et al., 2005 ;Yoshida et al., 2001) Surprisingly, we found the highest hypocretin-1-ir levels in the human hypothalamus at night. However, this observation agrees with two human studies that reported the lowest CSF hypocretin levels during the daytime (Salomon et al., 2003 ;Grady et al., 2006). This means that the nocturnal elevation of hypocretin-1-ir in the hypothalamus is not simply due to a lack of transport or secretion. A similar phenomenon was observed for melatonin, which is also involved in sleep-wake control: in rats, melatonin levels increase during the dark period (their active phase) and decrease during the light period (rest phase) (Gutjahr et al., 2004), while in humans its levels increase during the dark period (rest phase) and decrease during the light period (active phase) (Zeitzer et al., 2007). The possibility that the diurnal regulation systems act in a different way in a diurnal and a nocturnal species (such as rodents) warrants further investigation.
A recent study showed an age-related decline in the number of hypothalamic hypocretin neurons in the range from 0 to 60 years of age in control subjects (Hunt et al., 2015). We did not find such a correlation, but it should be noted that, unlike in the study by others, the control subjects we studied did not contain very young ages.
4.2. PFC Hcrt-receptors in Depression and in Relation to Suicide
Earlier, our group found, with NBB cortex samples (depression patients without suicide), that the ACC seems to be more vulnerable than the DLPFC to alterations in depression-related molecules, such as nitric oxide synthase, gamma-aminobutyric acid and glutamate (Gao et al., 2013 ;Zhao et al., 2012). In our study, with NBB cortex samples, again we observed a trend for lower Hcrt-receptor-1-mRNA expression in the ACC in depression but no changes in Hcrt-receptors in the DLPFC. The novel finding with SMRI cortex samples (containing depressive patients who committed suicide or died of causes other than suicide) that there was a significantly increased Hcrt-receptor-2-mRNA expression in the ACC, but not in the DLPFC, in male MDD patients who had committed suicide is in agreement with previous findings that the ACC is more vulnerable to suicide than the DLPFC (Zhao et al., 2012 ; Drevets et al., 2008) and that there is a sex difference in the prevalence of suicide (Maguen et al., 2015). Our data concerning increased ACC Hcrt-receptor-2-mRNA expression in male suicide patients are thus interesting, although too limited for a final conclusion. They do represent, however, a strong rationale for further studies on this topic. Finally, the decreased Hcrt-receptor-1-mRNA expression with aging we observed in the DLPFC may at least partly explain the findings that in SMRI cortex samples (younger) both Hcrt-receptors were detectable in ACC and DLPFC, while in the NBB series (older) Hcrt-receptor-1-mRNA expression was only detectable in ACC and Hcrt-receptor-2-mRNA expression was only detectable in the DLPFC.
Some concerns of the present postmortem brain material study should be mentioned. We did not find significant differences in the hypothalamic hypocretin-1-ir expression between MDD and BD patients, which is in accordance with our previous findings for CRH, AVP and OXT and for receptors in the hypothalamus (Bao et al., 2005 ; Wang et al., 2008), although a final conclusion on this phenomenon should be based upon a larger sample size. Secondly, one of the inherent potential confounding factors in a postmortem study is medication use. However, we do not think that our main conclusions are cofounded by antidepressants, since increased hypothalamic hypocretin-1-ir was only observed in female depressive patients and increased expression of Hcrt-receptor-2-mRNA in the ACC was only observed in male depressive suicides, although all the depressive groups had been on antidepressants. In addition, animal studies showed that benzodiazepines (Panhelainen and Korpi, 2012), haloperidol (Dalal et al., 2003) and fluoxetine (Nollet et al., 2011), taken by the depressive patients in the present study, may inhibit hypocretin neurons and/or decrease hypocretin levels. Therefore, had antidepressants interfered with our measurements, this would have led to an underestimation of the increased hypocretin-1 levels observed in female depressive patients. It is noted that Calegare et al. found that sub-chronic treatment of adult malerats with fluoxetine increased the levels of prepro-hypocretin mRNA in the hypothalamus without affecting the hypocretin-1 CSF levels ( Calegare et al., 2016). In our study, there were 2 out of 10 male depression patients who had taken fluoxetine, while their hypothalamic hypocretin-1-ir levels (IOD: 0.128 and 0.134) were fully within the range of the other male depression patients (IOD range from 0.103 to 0.248, median value 0.166). Finally, it should also be noted that the Hcrt-receptor data are based upon mRNA measurements and have yet to be confirmed on the protein level.
A clear sex-related change was found in the hypothalamic hypocretin-1-ir in depression. The CUMS rat depression model did not replicate changes found in the hypocretin systems in the human brain in depression. Since sex-related changes in hypothalamic hypocretin-1-ir expression were observed in depression, this factor should be taken into account in the development of hypocretin-targeted therapeutic strategies.