Inhibition of neural activity depletes orexin from rat hypothalamic slice culture

Serum metabolomics study of narcolepsy type 1 based on ultra-performance liquid chromatography-tandem mass spectrometry

Narcolepsy is a chronic and underrecognized sleep disorder characterized by excessive daytime sleepiness and cataplexy. Furthermore, narcolepsy type 1 (NT1) has serious negative impacts on an individual's health, society, and the economy. Currently, many sleep centers lack the means to measure orexin levels in the cerebrospinal fluid. We aimed to analyze the characteristics of metabolite changes in patients with NT1, measured by ultra-performance liquid chromatography-tandem mass spectrometry. A principal component analysis (PCA), an orthogonal partial least square discriminant analysis (OPLS-DA), t tests, and volcano plots were used to construct a model of abnormal metabolic pathways in narcolepsy. We identified molecular changes in serum specimens from narcolepsy patients and compared them with control groups, including dehydroepiandrosterone, epinephrine, N-methyl-D-aspartic acid, and other metabolites, based on an OPLS-loading plot analysis. Nine metabolites yielded an area under the receiver operating curve > 0.75. Meanwhile, seven abnormal metabolic pathways were correlated with differential metabolites, such as metabolic pathways; neuroactive ligand‒receptor interaction; and glycine, serine, and threonine metabolism. To our knowledge, this is the first study to reveal the characteristic metabolite changes in sera from NT1 patients for the selection of potential blood biomarkers and the elucidation of NT1 pathogenesis.

[Cellular mechanisms involved in induction of selective degeneration of orexin neurons in the hypothalamus]

Narcolepsy is a kind of sleep disorder featured by selective loss of orexin neurons in the lateral hypothalamus. Several lines of evidence, including association with specific HLA haplotypes, gene polymorphism in T cell receptor and detection of autoantibodies in a subpopulation of patients, suggest that autoimmune responses play an important role in the pathogenesis of this disorder. Potential relationship with influenza virus infection has also been a matter of interest. However, these events may not be able to explain all cases of narcolepsy. Based on the structural features of orexin, in addition to the findings on the characteristics of orexin neurons obtained from studies in organotypic hypothalamic slice cultures, we proposed novel mechanisms potentially involved in selective degeneration of orexin neurons. Increase in local production of nitric oxide induced by several life style-related conditions such as shortage of sleep and intake of high fat diet leads to inactivation of protein disulfide isomerase. Consequently, abnormal aggregates of orexin and/or its precursor that possess two intra-molecular disulfide bonds accumulate within orexin neurons. In addition to the increase in endoplasmic reticulum stress, accumulation of orexin as abnormal aggregates leads to increased excitability of orexin neurons by shutdown of feedback inhibition resulting from deficits in orexin release. These mechanisms may provide a clue to understand the pathogenic mechanisms of various neurological and psychiatric disorders accompanied by a decrease of orexin.

Cystamine-mediated inhibition of protein disulfide isomerase triggers aggregation of misfolded orexin-A in the Golgi apparatus and prevents extracellular secretion of orexin-A

Orexins (orexin-A and orexin-B) are neuropeptides that are reduced in narcolepsy, a sleep disorder that is characterized by excessive daytime sleepiness, sudden sleep attacks and cataplexy. However, it remains unclear how orexins in the brain and orexin neurons are reduced in narcolepsy. Orexin-A has two closely located intramolecular disulfide bonds and is prone to misfolding due to the formation of incorrect disulfide bonds. Protein disulfide isomerase (PDI) possesses disulfide interchange activity. PDI can modify misfolded orexin-A to its native form by rearrangement of two disulfide bonds. We have previously demonstrated that sleep deprivation and a high fat diet increase nitric oxide in the brain. This increase triggers S-nitrosation and inactivation of PDI, leading to aggregation of orexin-A and reduction of orexin neurons. However, the relationship between PDI inactivation and loss of orexin neurons has not yet been fully elucidated. In the present study, we used a PDI inhibitor, cystamine, to elucidate the precise molecular mechanism by which PDI inhibition reduces the number of orexin neurons. In rat hypothalamic slice cultures, cystamine induced selective depletion of orexin-A, but not orexin-B and melanin-concentrating hormone. Moreover, cystamine triggered aggregation of orexin-A, but not orexin-B in the Golgi apparatus of hypothalamic slice cultures and in vivo mouse brains. However, cystamine did not induce endoplasmic reticulum (ER) stress, and an ER stress inducer did not trigger aggregation of orexin-A in slice cultures. Finally, we demonstrated that cystamine significantly decreased extracellular secretion of orexin-A in AD293 cells overexpressing prepro-orexin. These findings suggest that cystamine-induced PDI inhibition induces selective depletion, aggregation in the Golgi apparatus and impaired secretion of orexin-A. These effects may represent an initial step in the pathogenesis of narcolepsy.

Anti-Parkinson drugs and orexin neurons

Non-ergot-type dopamine receptor agonists such as ropinirole are used for treatment of Parkinson disease, but they frequently produce adverse actions characterized by sleepiness and sleep attacks. Because these symptoms are similar to those observed in patients with narcolepsy, a sleep disorder caused by degeneration of hypothalamic orexin neurons, involvement of orexinergic system in the adverse drug actions is suspected. We found that ropinirole and other non-ergot dopamine D₂ receptor agonists cause selective loss of orexin-immunoreactive neurons in organotypic slice culture of rat hypothalamus. The mechanism of this action is considered to involve D₂ receptor-mediated presynaptic suppression of glutamatergic excitatory inputs to orexin neurons because continuous silencing of excitatory activity of orexin neurons can deplete orexin from cell bodies. In addition, Parkinson disease itself may accompany loss of orexin neurons. Disturbance of orexinergic system may play an important role in sleep/arousal dysfunctions under these and other clinical conditions.

Orexin neurons in hypothalamic slice cultures are vulnerable to endoplasmic reticulum stress

Narcolepsy results from disruption of orexin neurons in the hypothalamus that play a key role in maintenance of the arousal state. Underlying mechanisms leading to selective loss of orexin neurons remain unknown. On the other hand, endoplasmic reticulum stress, namely, conditions associated with impairment of endoplasmic reticulum functions such as proper folding and sorting of newly synthesized proteins, is implicated in pathogenesis of several types of neurodegenerative disorders. Here we found that application of endoplasmic reticulum stress inducers such as tunicamycin (that prevents protein N-glycosylation) and thapsigargin (that inhibits Ca²⁺-ATPase) to organotypic slice cultures of the hypothalamus caused preferential loss of orexin-immunoreactive neurons, as compared to melanin-concentrating hormone- or calcitonin gene-related peptide-immunoreactive neurons. The decrease in orexin-immunoreactive neurons at early time points (6-24 h) was not accompanied by induction of cell death as indicated by the absence of caspase-3 activation and no significant change in the number of NeuN-positive cells, whereas sustained treatment with tunicamycin for 72 h induced cell death. At 24-h treatment, tunicamycin and thapsigargin did not decrease expression of prepro-orexin mRNA, suggesting that post-transcriptional mechanisms were responsible for depletion of orexin peptides. In addition, inhibition of axonal transport by colchicine and inhibition of proteasomal activity by MG132 significantly prevented the decrease in orexin immunoreactivity by tunicamycin. Comparative examinations of expression of unfolded protein response-related proteins revealed that C/EBP-homologous protein (a transcription factor that promotes induction of apoptosis) as well as phosphorylated form of RNA-dependent protein kinase-like endoplasmic reticulum kinase (a protein kinase that mediates inhibition of protein translation) was expressed more prominently in orexin neurons than in melanin-concentrating hormone neurons, in response to tunicamycin. These results indicate that orexin neurons are particularly sensitive to endoplasmic reticulum stress, which may be relevant to pathogenic events in narcolepsy.

Age-related loss of orexin/hypocretin neurons

Aging is associated with many physiological alterations-such as changes in sleep patterns, metabolism and food intake-suggestive of hypothalamic dysfunction, but the effects of senescence on specific hypothalamic nuclei and neuronal groups that mediate these alterations is unclear. The lateral hypothalamus and contiguous perifornical area (LH/PFA) contains several populations of neurons, including those that express the neuropeptides orexin (hypocretin) or melanin-concentrating hormone (MCH). Collectively, orexin and MCH neurons influence many integrative homeostatic processes related to wakefulness and energy balance. Here, we determined the effect of aging on numbers of orexin and MCH neurons in young adult (3-4 months) and old (26-28 months) Fisher 344/Brown Norway F1 hybrid rats. Aged rats exhibited a loss of greater than 40% of orexin-immunoreactive neurons in both the medial and lateral (relative to the fornix) sectors of the LH/PFA. MCH-immunoreactive neurons were also lost in aged rats, primarily in the medial LH/PFA. Neuronal loss in this area was not global as no change in cells immunoreactive for the pan-neuronal marker, NeuN, was observed in aged rats. Combined with other reports of altered receptor expression or behavioral responses to exogenously-administered neuropeptide, these data suggest that compromised orexin (and, perhaps, MCH) function is an important mediator of age-related homeostatic disturbances of hypothalamic origin. The orexin system may represent a crucial substrate linking homeostatic and cognitive dysfunction in aging, as well as a novel therapeutic target for pharmacological or genetic restoration approaches to preventing or ameliorating these disturbances.

Depolarizing stimuli cause persistent and selective loss of orexin in rat hypothalamic slice culture

A hypothalamic neuropeptide orexin (hypocretin) is a critical regulator of physiological processes including sleep/wakefulness and feeding. Using organotypic slice culture of rat hypothalamus, we found that exposure to elevated extracellular concentration of K(+) (+10-30 mM) for 24-72h led to a substantial decrease in the number of neurons immunoreactive for orexin and a co-existing neuropeptide dynorphin-A. In contrast, the same treatment affected neither the number of melanin-concentrating hormone-immunoreactive neurons nor the number of total neurons. A substantial decrease of orexin-immunoreactive neurons was also induced by 72h treatment with 1-10 microM veratridine, a Na(+) channel activator. The effect of elevated K(+) was only partially reversible, and that of veratridine was virtually irreversible, although the decrease in orexin immunoreactivity was not associated with signs of cell damage assessed by propidium iodide uptake and Hoechst 33342 nuclear staining. In addition, the level of preproorexin mRNA did not decrease during treatment with elevated K(+) or veratridine. After treatment with elevated K(+) and veratridine, c-Fos immunoreactivity appeared in orexin-immunoreactive neurons but not in melanin-concentrating hormone-immunoreactive neurons, suggesting selective excitation of orexin neurons. However, the amount of orexin released extracellularly was paradoxically decreased by treatment with elevated K(+) and veratridine. Overall, these characteristics of orexin neurons may be taken into consideration to understand the behaviors of these neurons under physiological and pathophysiological conditions.