The effect of clomipramine on wake/sleep and orexinergic expression in rats

Cerebrospinal Fluid Orexin A Levels and Autonomic Function in Kleine-Levin Syndrome

STUDY OBJECTIVES

Kleine-Levin syndrome (KLS) is a rare disorder of relapsing sleepiness. The hypothesis was that the syndrome is related to a change in the vigilance peptide orexin A.

METHODS

From 2002 to 2013, 57 patients with relapsing hypersomnolence were clinically assessed in a referral academic center in Beijing, China, and 44 (28 males and 16 females; mean age 18.3 ± 8.9 y (mean ± standard deviation, range 9-57 y) were determined to have clinical and behavioral criteria consistent with KLS. Cerebrospinal fluid orexin A levels and diurnal blood pressure were measured in relapse versus remission in a subgroup of patients.

RESULTS

Presenting symptoms included relapsing or remitting excessive sleepiness-associated parallel complaints of cognitive changes (82%), eating disorders (84%); depression (45%); irritability (36%); hypersexuality (18%); and compulsions (11%). Episodes were 8.2 ± 3.3 days in duration. In relapse, diurnal values for blood pressure and heart rate were lower (P < 0.001). In a subgroup (n = 34), cerebrospinal fluid orexin A levels were ∼31% lower in a relapse versus remission (215.7 ± 81.5 versus 319.2 ± 95.92 pg/ml, P < 0.001); in three patients a pattern of lower levels during subsequent relapses was documented.

CONCLUSIONS

There are lower orexin A levels in the symptomatic phase than in remission and a fall and rise in blood pressure and heart rate, suggesting a role for orexin dysregulation in KLS pathophysiology.

Chromosome 1 replacement increases brain orexins and antidepressive measures without increasing locomotor activity

Decreased orexin level has been well demonstrated in patients suffering from narcolepsy, depression accompanied with suicide attempt; obstructive sleep apnea and comorbidity were also demonstrated in these diseases. As C57BL/6J (B6) mice are more "depressed" and have lower brain orexins than A/J mice, B6 mice having chromosome 1 replacement (B6A1 mice) might have restored orexin levels and less depressive behavior. We studied the behavior of 4-6 month old B6, A/J and B6A1 mice with forced swim, tail suspension, and locomotor activity tests. The animals were then sacrificed and hypothalamus and medullas dissected from brain tissue. Orexins-A and -B were determined by radioimmunoassay. Compared with A/J mice, B6 mice displayed several signs of depression, including increased immobility, increased locomotors activity, and decreased orexin A and -B levels in both the hypothalamus and medulla. Compared to B6 mice, B6A1 mice exhibited significantly higher levels of orexins-A and -B in both brain regions. B6A1 mice also exhibited antidepressive features in most of measured variables, including decreased locomotor activity, decreased immobility and increased swim in tail suspension test; compared with B6 mice, however. B6A1 mice also reversed immobility in the early phase of the swim test. In summary, B6 mice exhibited depressive attributes compared with A/J mice, including increased locomotor activity, greater immobility, and decreased brain orexins, these were largely reversed in B6A1 mice. We conclude that orexin levels modulate these B6 behaviors, likely due to expression of A/J alleles on Chromosome 1.

Hypothalamic orexin-A (hypocretin-1) neuronal projections to the vestibular complex and cerebellum in the rat

Immunohistochemistry combined with retrograde tract-tracing techniques were used to investigate the distribution of orexin-A (OX-A)- and OX-A receptor-like (OX1) immunoreactivity within the vestibular complex and cerebellum, and the location of hypothalamic OX-A neurons sending axonal projections to these regions in the Wistar rat. OX-A immunoreactive fibers and presumptive terminals were found throughout the medial (MVe) and lateral (LVe) vestibular nuclei. Light fiber labeling was also observed in the spinal and superior vestibular nuclei. Within the cerebellum, dense fiber and presumptive terminal labeling was observed in the medial cerebellar nucleus (Med; fastigial nucleus), with less dense labeling in the interposed (Int) and lateral cerebellar nuclei (Lat; dentate nucleus). A few scattered OX-A immunoreactive fibers were also observed throughout the cortex of the paraflocculus. OX1-like immunoreactivity was found densely concentrated within LVe, moderate in MVe, and scattered within the spinal and superior vestibular nuclei. Within the cerebellum, OX1-like immunoreactivity was also observed densely within Med and in the dorsolateral aspects of Int. Additionally, OX1 like-labeling was found in Lat, and within the granular layer of the caudal paraflocculus cerebellar cortex. Fluorogold (FG) microinjected into these vestibular and cerebellar regions resulted in retrogradely labeled neurons throughout the ipsilateral hypothalamus. Retrogradely labeled neurons containing OX-A like immunoreactivity were observed dorsal and caudal to the anterior hypothalamic nucleus and extending laterally into the lateral hypothalamic area, with the largest number clustered around the dorsal aspects of the fornix in the perifornical area. A few FG OX-A like-immunoreactive neurons were also observed scattered throughout the dorsomedial, and posterior hypothalamic nuclei. These data indicate that axons from OX-A neurons terminate within the vestibular complex and deep cerebellar nuclei of the cerebellum and although the function of these pathways is unknown, they likely represent pathways by which hypothalamic OX-A containing neurons co-ordinate vestibulo-cerebellar motor and autonomic functions associated with ingestive behaviors.

Role of orexin in the pathophysiology of depression: potential for pharmacological intervention

Depression is a devastating mental disorder with an increasing impact throughout the world, whereas the efficacy of currently available pharmacological treatment is still limited. Growing evidence from preclinical and clinical studies suggests that orexins (neuropeptides that are also known as hypocretins) and their receptors are involved in the physiopathology of depression. Indeed, the orexinergic system regulates functions that are disturbed in depressive states such as sleep, reward system, feeding behavior, the stress response and monoaminergic neurotransmission. Nevertheless, the precise role of orexins in behavioral and neurophysiological impairments observed in depression is still unclear. Both hypoactivity and hyperactivity of orexin signaling pathways have been found to be associated with depression. These discrepancies in the literature prompted the necessity for additional investigations, as the orexinergic system appears to be a promising target to treat the symptoms of depression. This assumption is underlined by recent data suggesting that pharmacological blockade of orexin receptors induces a robust antidepressant-like effect in an animal model of depression. Further preclinical and clinical studies are needed to progress the overall understanding of the orexinergic alterations in depression, which will eventually translate preliminary observations into real therapeutic potential. The aim of this paper is to provide an overview of human and animal research dedicated to the study of the specific involvement of orexins in depression, and to propose a framework in which disturbances of the orexinergic system are regarded as an integral component of the etiology of depression.

Activation of orexin neurons in dorsomedial/perifornical hypothalamus and antidepressant reversal in a rodent model of depression

Chronic stressful life events are risk factors for depression often accompanied by homeostatic disturbances. Hypothalamic neuropeptides, such as orexins (OXs) and melanin-concentrating hormone (MCH), are involved in regulation of several autonomic functions that are altered in depression. However, little is known about the link between orexinergic or MCH-ergic systems and depression. Using double immunohistochemical labeling for OX- or MCH-containing neurons and Fos protein, we studied the effects of a chronic selective serotonin reuptake inhibitor antidepressant treatment (fluoxetine) on the OX and MCH neuronal activation in mice exposed to unpredictable chronic mild stress (UCMS), a rodent model of depression. Western blot was also performed to assess OX and MCH receptor expression in various brain areas. Finally, almorexant, a dual OX receptor antagonist, was assessed in the tail suspension test. UCMS induced physical and behavioral disturbances in mice reversed by 6-week fluoxetine treatment. Orexinergic neurons were more activated in the dorsomedial and perifornical hypothalamic area (DMH-PFA) of UCMS-subjected mice compared to the lateral hypothalamus (LH), and this increase was reversed by 6-week fluoxetine treatment. UCMS also reduced expression of OX-receptor 2 in the thalamus and hypothalamus, but not in animals chronically treated with fluoxetine. MCH neurons were neither affected by UCMS nor by antidepressant treatment, while UCMS modulated MCH receptor 1 expression in thalamus and hippocampus. Finally, chronic but not acute administration of almorexant, induced antidepressant-like effect in the tail suspension test. These data suggest that OX neurons in the DMH-PFA and MCH-ergic system may contribute to the pathophysiology of depressive disorders.