Growing evidence indicates that an increase of orexin (or hypocretin) signaling is involved in the pathophysiology of major depression, but little is known regarding the causal link between the orexinergic system and depressive-like states. Here we blocked orexin receptors in mice subjected to unpredictable chronic mild stress (UCMS) to investigate putative antidepressant-like effects of this treatment, as well as the underlying mechanisms. BALB/c mice were exposed to 9 weeks of UCMS and from the third week onward treated daily with fluoxetine (20 mg/kg per day, per os) or with the dual orexin receptor antagonist almorexant (100 mg/kg per day, per os). The effects of UCMS regimen and pharmacological treatments were assessed by physical measures and behavioral testing. The dexamethasone suppression test was performed to examine the integrity of the negative feedback of the hypothalamic-pituitary-adrenal (HPA) axis, and immunohistochemical markers were used to assess cell proliferation (Ki-67), immature newborn neurons (doublecortin), and mature newborn neurons (5-bromo-2 0-deoxyuridine/NeuN) in the dorsal and ventral parts of the hippocampus. Our results show that 7 weeks of fluoxetine or almorexant treatments counteract the UCMS-induced physical and behavioral alterations. Both treatments prevented the HPA axis dysregulation caused by UCMS, but only fluoxetine reversed the UCMS-induced decrease of hippocampal cell proliferation and neurogenesis, while chronic almorexant treatment decreased cell proliferation and neurogenesis specifically in the ventral hippocampus. Taken together, this is the first evidence that pharmacological blockade of the orexinergic system induces a robust antidepressant-like effect and the restoration of stress-related HPA axis defect independently from a neurogenic action.

Chronic stress and depression are associated with decreased levels of hippocampal neurogenesis. On the other hand, antidepressants as well as environmental enrichment may rely in part on their pro-neurogenic effects to improve cognition and mood. Because a functional heterogeneity has been consistently reported along the septo-temporal axis of the hippocampus, regional changes in neurogenesis could differentially contribute to these effects and affect distinct hippocampal functions. Mapping these regional changes could therefore provide a better understanding of the function of newborn neurons. While some studies report region-specific effects of stress and antidepressants on neurogenesis, it is unclear whether these changes affect distinct populations of newborn neurons according to their developmental stage in a region-specific manner. By using endogenous markers and BrdU labeling we quantified the regional changes in cell proliferation and survival as well as in the number of neuronal progenitors and immature neurons following unpredictable chronic mild stress (UCMS), environmental enrichment (EE) and chronic fluoxetine (20 mg/kg/day) treatment along the septo-temporal axis of the hippocampus. EE promoted cell proliferation and survival of 4-week-old newborn cells as well as increased the number and proportion of post-mitotic immature neurons specifically within the septal hippocampus. By contrast, UCMS uniformly decreased cell proliferation, survival and immature newborn neurons but differentially affected progenitor cells with a decrease restricted to the temporal regions of the hippocampus. Whereas fluoxetine treatment in control mice affected proliferation and survival specifically in the temporal hippocampus, it reversed most of the UCMS-induced alterations all along the septo-temporal axis. These results highlight that different factors known for exerting a mood improving effect differentially regulate neurogenesis along the septo-temporal axis of the hippocampus. Such region and stage specific effects may correlate to distinct functional properties of newborn neurons along the septo-temporal axis of the hippocampus which may contribute differently to the pathophysiology of affective disorders.