Non-sleep effects of hypocretin/orexin

Pupillometry to differentiate idiopathic hypersomnia from narcolepsy type 1

Idiopathic hypersomnia is poorly diagnosed in the absence of biomarkers to distinguish it from other central hypersomnia subtypes. Given that light plays a main role in the regulation of sleep and wake, we explored the retinal melanopsin-based pupil response in patients with idiopathic hypersomnia and narcolepsy type 1, and healthy subjects. Twenty-seven patients with narcolepsy type 1 (women 59%, 36 ± 11.5 years old), 36 patients with idiopathic hypersomnia (women 83%, 27.2 ± 7.2 years old) with long total sleep time (> 11/24 hr), and 43 controls (women 58%, 30.6 ± 9.3 years old) were included in this study. All underwent a pupillometry protocol to assess pupil diameter, and the relative post-illumination pupil response to assess melanopsin-driven pupil responses in the light non-visual input pathway. Differences between groups were assessed using logistic regressions adjusted on age and sex. We found that patients with narcolepsy type 1 had a smaller baseline pupil diameter as compared with idiopathic hypersomnia and controls (p < 0.05). In addition, both narcolepsy type 1 and idiopathic hypersomnia groups had a smaller relative post-illumination pupil response (respectively, 31.6 ± 13.9% and 33.2 ± 9.9%) as compared with controls (38.7 ± 9.7%), suggesting a reduced melanopsin-mediated pupil response in both types of central hypersomnia (p < 0.01). Both narcolepsy type 1 and idiopathic hypersomnia showed a smaller melanopsin-mediated pupil response, and narcolepsy type 1, unlike idiopathic hypersomnia, also displayed a smaller basal pupil diameter. Importantly, we found that the basal pupil size permitted to well discriminate idiopathic hypersomnia from narcolepsy type 1 with a specificity = 66.67% and a sensitivity = 72.22%. Pupillometry may aid to multi-feature differentiation of central hypersomnia subtypes.

CO2 exposure enhances Fos expression in hypothalamic neurons in rats during the light and dark phases of the diurnal cycle

Orexinergic (OX) neurons in the lateral hypothalamus (LH), perifornical area (PFA) and dorsomedial hypothalamus (DMH) play a role in the hypercapnic ventilatory response, presumably through direct inputs to central pattern generator sites and/or through interactions with other chemosensitive regions. OX neurons can produce and release orexins, excitatory neuropeptides involved in many functions, including physiological responses to changes in CO₂/pH. Thus, in the present study, we tested the hypothesis that different nuclei (LH, PFA and DMH) where the orexinergic neurons are located, show distinct activation by CO₂ during the light-dark cycle phases. For this purpose, we evaluated the Fos and OXA expression by immunohistochemistry to identify neurons that co-localize Fos + OXA in the LH, LPeF, MPeF and DMH in the light-inactive and dark-active phase in Wistar rats subjected to 3 h of normocapnia or hypercapnia (7% CO₂). Quantitative analyses of immunoreactive neurons show that hypercapnia caused an increase in the number of neurons expressing Fos in the LH, LPeF, MPeF and DMH in the light and dark phases. In addition, the number of Fos + OXA neurons increased in the LPeF and DMH independently of the phases of the diurnal cycle; whereas in the MPeF, this increase was observed exclusively in the light phase. Thus, we suggest that OX neurons are selectively activated by hypercapnia throughout the diurnal cycle, reinforcing the differential role of nuclei in the hypothalamus during central chemosensitivity.

Activating autoantibodies against G protein-coupled receptors in narcolepsy type 1

STUDY OBJECTIVES

Narcolepsy type 1 is a rare hypersomnia of central origin, which is caused by loss of hypothalamic neurons that produce the neuropeptides hypocretin-1 and -2. Hypocretin-containing nerve terminals are found in areas known to play a central role in autonomic control and in pain signaling. Cholinergic M2 receptors are found in brain areas involved with the occurrence of hallucinations and cataplexy. In addition to classical symptoms of narcolepsy, the patients suffer frequently from autonomic dysfunction, chronic pain, and hypnagogic/hypnopompic hallucinations. We aimed to test whether narcolepsy type 1 patients have autoantibodies against autonomic β2 adrenergic receptor, M2 muscarinic receptors, or nociception receptors.

METHODS

We tested the serum of ten narcolepsy type 1 patients (five female) for activating β2 adrenergic receptor autoantibodies, M2 muscarinic receptor autoantibodies, and nociception receptor autoantibodies.

RESULTS

Ten of ten patients were positive for muscarinic M2 receptor autoantibodies (P < 0.001), 9/10 were positive for autoantibodies against nociception receptors (P < 0.001), and 5/10 were positive for β2 adrenergic receptor autoantibodies (P < 0.001).

CONCLUSIONS

Narcolepsy type 1 patients harbored activating autoantibodies against M2 muscarinic receptors, nociception receptors, and β2 adrenergic receptors. M2 receptor autoantibodies may be related to the occurrence of cataplexy and, moreover, hallucinations in narcolepsy since they are found in the same brain areas that are involved with these symptoms. The occurrence of nociception receptor autoantibodies strengthens the association between narcolepsy type 1 and pain. The connection between narcolepsy type 1, autonomic complaints, and the presumed cardiovascular morbidity might be associated with the occurrence of β2 adrenergic receptor autoantibodies. On the other hand, the presence of the autoantibodies may be secondary to the destruction of the hypocretin pathways.

Hypocretin/orexin modulates body weight and the metabolism of glucose and insulin

The hypocretin/orexin (Hcrt/orexin) unit affects the functions of the nervous, cardiovascular, gastrointestinal, and reproductive systems. Hcrt/orexin ligands and receptors have been localized to different parts of the central and peripheral nervous systems, cerebrospinal fluid and blood, exocrine (pancreas, salivary, lacrimal) as well as endocrine (pancreatic islets, pituitary, adrenal) glands. Several factors including stress, glucagon-like peptide-1 agonists, glutamate, nicotine, glucose, and hypoglycaemia stimulate the expression of Hcrt/orexin system, but it is inhibited by ageing, bone morphogenetic protein, hypoxia/hypercapnia, melanocortin receptor accessory protein 2, and glucagon. Literature reports show that Hcrt/orexin can significantly increase insulin secretion from normal and diabetic rat pancreata. Hcrt/orexin decreases blood glucose concentration and reduces insulin resistance partly via increased tissue expression of glucose transporter type 4. It reduces obesity by increasing browning of fat cells and energy expenditure. Taken together, Hcrt/orexin modulates obesity and the metabolism of glucose and insulin. The Hcrt/orexin system may thus be a target in the development of new therapies for the treatment of diabetes mellitus.

The selective orexin-2 antagonist seltorexant (JNJ-42847922/MIN-202) shows antidepressant and sleep-promoting effects in patients with major depressive disorder

Excessive arousal has a role in the pathophysiology of major depressive disorder (MDD). Seltorexant (JNJ-42847922/MIN-202) is a selective antagonist of the human orexin-2 receptor (OX2R) that may normalize excessive arousal and thereby attenuate depressive symptoms. In this study, the effects of night-time arousal suppression on depressive symptoms were investigated. 47 MDD patients with a total Inventory of Depressive Symptomatology (IDS) score of ≥30 at screening were included in a randomized, double-blind, diphenhydramine-, and placebo-controlled multicentre study. Symptoms of depression were rated using the 17-item Hamilton Depression Rating Scale (HDRS₁₇). Effects on sleep were evaluated by polysomnography and by the Leeds Sleep Evaluation Questionnaire (LSEQ). To investigate the safety and tolerability of seltorexant, vital signs, suicidal ideation and adverse events were monitored. At baseline the severity of depressive symptoms correlated with sleep efficiency (SE), wake after sleep onset (WASO), duration of stage 2 sleep, and ruminations. Ten days of treatment with seltorexant (and not diphenhydramine) resulted in a significant improvement of core depressive symptoms compared to placebo; the antidepressant efficacy of seltorexant was maintained with continued treatment up to 28 days. Compared to placebo, the antidepressant efficacy of seltorexant coincided with an overall increase in (left posterior) EEG power and a relative increase in delta- and decrease in theta-, alpha- and beta power during stage 2 sleep. Treatment with seltorexant was associated with mild, self-limiting adverse drug reactions. Seltorexant affected core symptoms of depression in the absence of overt changes in the hypnogram; in contrast, diphenhydramine was not efficacious.

New therapeutic approaches for the prevention and treatment of migraine

The management of patients with migraine is often unsatisfactory because available acute and preventive therapies are either ineffective or poorly tolerated. The acute treatment of migraine attacks has been limited to the use of analgesics, combinations of analgesics with caffeine, ergotamines, and the triptans. Successful new approaches for the treatment of acute migraine target calcitonin gene-related peptide (CGRP) and serotonin (5-hydroxytryptamine, 5-HT1F) receptors. Other approaches targeting the transient receptor potential vanilloid (TRPV1) receptor, glutamate, GABAA receptors, or a combination of 5-HT1B/1D receptors and neuronal nitric oxide synthesis have been investigated but have not been successful in clinical trials thus far. In migraine prevention, the most promising new approaches are humanised antibodies against CGRP or the CGRP receptor. Non-invasive and invasive neuromodulation approaches also show promise as both acute and preventive therapies, although further studies are needed to define appropriate candidates for these therapies and optimum protocols for their use.

Effects of sleep duration and sleep quality on prevalence of type 2 diabetes mellitus: A 5-year follow-up study in China

OBJECTIVE

To explore the interactions of sleep quality and sleep duration on the development of type 2 diabetes mellitus (DM2) in Chinese adults.

RESEARCH DESIGN AND METHODS

We randomly selected 11,842 Chinese subjects from the Xuzhou community of China and obtained self-reported quality and duration of sleep by questionnaire. DM2 was assessed by fasting blood glucose. Sleep quality was categorized as good, common, or poor. Sleep duration was measured by average hours of sleep per night. We evaluated interaction, relative excess risk of interaction (RERI), the attributable proportion (AP), and the synergy index (S) using a logistic regression model.

RESULTS

The relative risk for the development of DM2 was higher in subjects with short sleep duration (1.67 [1.34-2.16]) or poor sleep quality (1.91 [1.31-2.74]) or long sleep duration (1.45 [1.02-1.77]). DM2 occurred more frequently with poor sleep quality combined with short sleep duration (odds ratio: 6.21; 95% confidence interval (CI): 2.78-11.81). RERI, AP, and S values (and their 95% CI) were 3.99 (1.41-7.76), 0.64 (0.45-0.76), and 5.15 (3.74-7.89) for the interaction between poor sleep quality and short sleep duration. In subjects with poor sleep quality combined with long sleep duration, the RERI, AP, and S values (and 95% CI) were 0.13 (-0.19 to 0.66), 0.07 (-0.35 to 0.18), and 1.19 (0.85-2.11).

CONCLUSIONS

Interactions between poor sleep quality and short sleep duration were additive. Preventive measures should focus on short sleep duration and poor sleep quality.

The Effects of Calorie Restriction in Depression and Potential Mechanisms

Depression, also called major depressive disorder, is a neuropsychiatric disorder jeopardizing an increasing number of the population worldwide. To date, a large number of studies have devoted great attention to this problematic condition and raised several hypotheses of depression. Based on these theories, many antidepressant drugs were developed for the treatment of depression. Yet, the depressed patients are often refractory to the antidepressant therapies. Recently, increasing experimental evidences demonstrated the effects of calorie restriction in neuroendocrine system and in depression. Both basic and clinical investigations indicated that short-term calorie restriction might induce an antidepressant efficacy in depression, providing a novel avenue for treatment. Molecular basis underlying the antidepressant actions of calorie restriction might involve multiple physiological processes, primarily including orexin signaling activation, increased CREB phosphorylation and neurotrophic effects, release of endorphin and ketone production. However, the effects of chronic calorie restriction were quite controversial, in the cases that it often resulted in the long-term detrimental effects via inhibiting the function of 5-HT system and decreasing leptin levels. Here we review such dual effects of calorie restriction in depression and potential molecular basis behind these effects, especially focusing on antidepressant effects.

The hypocretins/orexins: integrators of multiple physiological functions

The hypocretins (Hcrts), also known as orexins, are two peptides derived from a single precursor produced in the posterior lateral hypothalamus. Over the past decade, the orexin system has been associated with numerous physiological functions, including sleep/arousal, energy homeostasis, endocrine, visceral functions and pathological states, such as narcolepsy and drug abuse. Here, we review the discovery of Hcrt/orexins and their receptors and propose a hypothesis as to how the orexin system orchestrates these multifaceted physiological functions.

Pharmacokinetic and pharmacodynamic interactions between almorexant, a dual orexin receptor antagonist, and desipramine

Almorexant is a dual orexin receptor antagonist (DORA) with sleep-enabling effects in humans. Insomnia is often associated with mental health problems, including depression. Hence, potential interactions with antidepressants deserve attention. Desipramine was selected as a model drug because it is mainly metabolized by CYP2D6, which is inhibited by almorexant in vitro. A single-center, randomized, placebo-controlled, two-way crossover study in 20 healthy male subjects was conducted to evaluate the pharmacokinetic and pharmacodynamic interactions between almorexant and desipramine. Almorexant 200mg or matching placebo (double-blind) was administered orally once daily in the morning for 10 days, and a single oral dose of 50mg desipramine (open-label) was administered on Day 5. Almorexant increased the exposure to desipramine 3.7-fold, suggesting that almorexant is a moderate inhibitor of desipramine metabolism through inhibition of CYP2D6. Conversely, desipramine showed no relevant effects on the pharmacokinetics of almorexant. Pharmacodynamic evaluations indicated that almorexant alone reduced visuomotor coordination, postural stability, and alertness, and slightly increased calmness. Desipramine induced a reduction in subjective alertness and an increase in pupil/iris ratio. Despite the increase in exposure to desipramine, almorexant and desipramine in combination showed the same pharmacodynamic profile as almorexant alone, except for prolonging reduced alertness and preventing the miotic effect of almorexant. Co-administration also prolonged the mydriatic effect of desipramine. Overall, repeated administration of almorexant alone or with single-dose desipramine was well tolerated. The lack of a relevant interaction with antidepressants, if confirmed for other DORAs, would be a key feature for a safer class of hypnotics.

Function and dysfunction of hypocretin/orexin: an energetics point of view

The basic elements of animal behavior that are critical to survival include energy, arousal, and motivation: Energy intake and expenditure are fundamental to all organisms for the performance of any type of function; according to the Yerkes-Dodson law, an optimal level of arousal is required for animals to perform normal functions; and motivation is critical to goal-oriented behaviors in higher animals. The brain is the primary organ that controls these elements and, through evolution, has developed specialized structures to accomplish this task. The orexin/hypocretin system in the perifornical/lateral hypothalamus, which was discovered 15 years ago, is one such specialized area. This review summarizes a fast-growing body of evidence discerning how the orexin/hypocretin system integrates internal and external cues to regulate energy intake that can then be used to generate sufficient arousal for animals to perform innate and goal-oriented behaviors.

Optogenetic examination identifies a context-specific role for orexins/hypocretins in anxiety-related behavior

Maladaptation to stress is associated with psychopathology. However, our understanding of the underlying neural circuitry involved in adaptations to stress is limited. Previous work from our lab indicated the paraventricular hypothalamic neuropeptides orexins/hypocretins regulate behavioral and neuroendocrine responses to stress. To further elucidate the role of orexins in adaptation to stress, we employed optogenetic techniques to specifically examine the effects of orexin cell activation on behavior in the social interaction test and in the home cage as well as orexin receptor 1 internalization and ERK phosphorylation in brain regions receiving orexin inputs. In the social interaction test, optogenetic stimulation of orexin neurons decreased time spent in the interaction zone while increasing the frequency of entries into the interaction zone. In addition, optogenetic stimulation of orexin neurons increased the total distance traveled in the social interaction arena but had no effect on their home cage behavior. Together, these results suggest that orexin release increases anxiety in the social interaction test while increasing the salience of novel but not familiar environmental stimuli. Consistent with activation of orexin neurons, optogenetic stimulation increased orexin receptor1 internalization and ERK phosphorylation in the paraventricular thalamus (PVT) and locus coeruleus (LC), two regions heavily innervated by orexin neurons. Together these results show for the first time that elevation of orexin activity, possibly in the PVT and LC, is associated with increased anxiety, activity, and arousal in a context-specific manner.

Orexin receptor type-1 antagonist SB-334867 decreases morphine-induced antinociceptive effect in formalin test

Orexin-A and orexin-B are two neuropeptides selectively synthesized in the lateral hypothalamus (LH), a region involved in morphine induced analgesia and pain modulation. Furthermore, orexin-A has been reported to produce an analgesic effect in pain models, which was blocked by orexin-1 receptor antagonist SB-334867, but not naloxone. We studied the effects of intracerebroventricular (ICV) injection of SB-334867, a selective orexin receptor type-1 antagonist, on morphine-induced antinociceptive effect in formalin test in rats. Morphine injection at a dose of 1.5mg/kg caused a significant decrease in the formalin-induced nociceptive behaviors in phase 1, interphase, and phase 2A, whereas at doses of 3, 6, and 10mg/kg, a significant reduction in the formalin-induced nociceptive behaviors was observed in all phases. The ICV injection of SB-334867 alone had no effect on the formalin-induced nociceptive behaviors. Pre-treatment with SB-334867 at a dose of 0.5 nmol significantly attenuated the analgesia induced by morphine (at dose 1.5mg/kg of morphine; interphase and phase 2B and at dose 3mg/kg of morphine just phase 2B of formalin test). Also, pre-treatment with SB-334867 at a dose of 5 nmol considerably attenuated the morphine-induced analgesia (at dose 1.5mg/kg of morphine; phase 1, interphase, and phase 2, at dose 3 and 6 mg/kg of morphine just phase 2 of formalin test). Pre-treatment with SB-334867 at a dose of 50 nmol remarkably attenuated the morphine-induced analgesia (at dose 1.5 and 3mg/kg of morphine; in phase 1, interphase, and phase 2 and also at dose 6 mg/kg of morphine; phase 1 and phase 2B of formalin test). These data suggest that the antinociceptive effects of morphine in formalin test might be associated with orexin receptor type-1. Our findings reveal a new role for the lateral hypothalamus orexin neurons in the morphine-induced analgesia.

Reward-based behaviors and emotional processing in human with narcolepsy-cataplexy

Major advances in the past decade have led a better understanding of the pathophysiology of narcolepsy with cataplexy (NC) caused by the early loss of hypothalamic hypocretin neurons. Although a role for hypocretin in the regulation of sleep/wakefulness state is widely recognized, other functions, not necessarily related to arousal, have been identified. Hence, the hypocretin system enhances signaling in the mesolimbic pathways regulating reward processing, emotion and mood regulation, and addiction. Although studies on hypocretin-deficient mice have shown that hypocretin plays an essential role in reward-seeking, depression-like behavior and addiction, results in human narcolepsy remained subject to debate. Most of studies revealed that hypocretin-deficient narcolepsy patients either drug-free or medicated with psychostimulant had preferences toward risky choices in a decision-making task under ambiguity together with higher frequency of depressive symptoms and binge eating disorder compared to controls. However, human studies mostly reported the lack of association with pathological impulsivity and gambling, and substance and alcohol abuse in the context of narcolepsy-cataplexy. Prospective larger studies are required to confirm these findings in drug-free and medicated patients with narcolepsy. Inclusion of patients with other central hypersomnias without hypocretin deficiency will provide answer to the major question of the role of the hypocretin system in reward-based behaviors and emotional processing in humans.

[Migraine and hypothalamus]

Migraine is a complex brain disease. The "generator" of the migrainous attacks remains a subject of debate, but the hypothalamus, with its multiple connections with the other parts of the central nervous system and its controls on the pituitary gland and the autonomic nervous system, is a very serious candidate. Many of the premonitory symptoms of migraine attacks find their origin in the hypothalamus. The hormonal changes which occur during feminine genital life and which impact on the life of the migrainous women have their origin in the hypothalamus. The hypothalamus exerts control over the balance between the parasympathetic and orthosympathetic systems. Orexine, hormones originating in the hypothalamic, are involved in sleep regulation, thermoregulation and neuroendocrine and nociceptive functions. They could play a crucial role in the origin of the migrainous attack and might explain the influence of sleep, eating habits and excessive weight in the occurrence of attacks. Hypothalamic cerebral activation via H2 15OPET activity, suspected by clinical and experimental arguments as a possible trigger for migraine, has been demonstrated during spontaneous attacks. However, no conclusion can be made however as to whether this activation is the cause or the consequence of the migrainous pain.

Dual hypocretin receptor antagonism is more effective for sleep promotion than antagonism of either receptor alone

The hypocretin (orexin) system is involved in sleep/wake regulation, and antagonists of both hypocretin receptor type 1 (HCRTR1) and/or HCRTR2 are considered to be potential hypnotic medications. It is currently unclear whether blockade of either or both receptors is more effective for promoting sleep with minimal side effects. Accordingly, we compared the properties of selective HCRTR1 (SB-408124 and SB-334867) and HCRTR2 (EMPA) antagonists with that of the dual HCRTR1/R2 antagonist almorexant in the rat. All 4 antagonists bound to their respective receptors with high affinity and selectivity in vitro. Since in vivo pharmacokinetic experiments revealed poor brain penetration for SB-408124, SB-334867 was selected for subsequent in vivo studies. When injected in the mid-active phase, SB-334867 produced small increases in rapid-eye-movement (REM) and non-REM (NR) sleep. EMPA produced a significant increase in NR only at the highest dose studied. In contrast, almorexant decreased NR latency and increased both NR and REM proportionally throughout the subsequent 6 h without rebound wakefulness. The increased NR was due to a greater number of NR bouts; NR bout duration was unchanged. At the highest dose tested (100 mg/kg), almorexant fragmented sleep architecture by increasing the number of waking and REM bouts. No evidence of cataplexy was observed. HCRTR1 occupancy by almorexant declined 4–6 h post-administration while HCRTR2 occupancy was still elevated after 12 h, revealing a complex relationship between occupancy of HCRT receptors and sleep promotion. We conclude that dual HCRTR1/R2 blockade is more effective in promoting sleep than blockade of either HCRTR alone. In contrast to GABA receptor agonists which induce sleep by generalized inhibition, HCRTR antagonists seem to facilitate sleep by reducing waking “drive”.

Non-dipping blood pressure profile in narcolepsy with cataplexy

BACKGROUND

Patients with narcolepsy-cataplexy (NC) mostly exhibit undetectable hypocretin levels. Hypocretin system is one of the key players in the complex interaction between sleep and the cardiovascular system. We tested the hypothesis that hypocretin deficiency affects cardiovascular risk factors by measuring nighttime and daytime ambulatory blood pressure (BP) and the night-to-day BP ratio as well as endothelial dysfunction by the digital pulse amplitude response in drug-free patients with NC compared to controls.

METHODOLOGY

Sleep, clinical and biological cardiovascular risk factors, fingertip peripheral arterial tonometry, and 24-hour ambulatory BP monitoring were recorded in 50 drug-free patients with NC and 42 healthy control subjects, except for BP monitoring available in all controls but in 36 patients with NC.

PRINCIPAL FINDINGS

More patients than controls were smokers, obese and with dyslipidemia. One-third of patients with NC were "non-dippers" (defined as <10% drop in BP during sleep) compared to only 3% of controls. The diastolic non-dipper BP profile had up to 12-fold higher odds of being associated with NC. We noted negative correlations between mean diastolic BP fall during night, REM sleep percentage and number of sleep onset REM periods, and a positive correlation with mean sleep latency on the MSLT. The digital pulse amplitude response measured by fingertip was similar between NC and controls.

CONCLUSION

We found a high percentage of non-dippers in patients with NC with association with REM sleep dysregulation. The blunted sleep-related BP dip in NC may be of clinical relevance, as it may indicate increased risk for cardiovascular events.

Energy expenditure: role of orexin

The orexins/hypocretins are endogenous, modulatory and multifunctional neuropeptides with prominent influence on several physiological processes. The influence of orexins on energy expenditure is highlighted with focus on orexin action on individual components of energy expenditure. As orexin stabilizes and maintains normal states of arousal and the sleep/wake cycle, we also highlight orexin mediation of sleep and how sleep interacts with energy expenditure.

Neuropeptides controlling energy balance: orexins and neuromedins

In this chapter, we review the feeding and energy expenditure effects of orexin (also known as hypocretin) and neuromedin. Orexins are multifunctional neuropeptides that affect energy balance by participating in regulation of appetite, arousal, and spontaneous physical activity. Central orexin signaling for all functions originates in the lateral hypothalamus-perifornical area and is likely functionally differentiated based on site of action and on interacting neural influences. The effect of orexin on feeding is likely related to arousal in some ways but is nonetheless a separate neural process that depends on interactions with other feeding-related neuropeptides. In a pattern distinct from other neuropeptides, orexin stimulates both feeding and energy expenditure. Orexin increases in energy expenditure are mainly by increasing spontaneous physical activity, and this energy expenditure effect is more potent than the effect on feeding. Global orexin manipulations, such as in transgenic models, produce energy balance changes consistent with a dominant energy expenditure effect of orexin. Neuromedins are gut-brain peptides that reduce appetite. There are gut sources of neuromedin, but likely the key appetite-related neuromedin-producing neurons are in the hypothalamus and parallel other key anorectic neuropeptide expression in the arcuate to paraventricular hypothalamic projection. As with other hypothalamic feeding-related peptides, hindbrain sites are likely also important sources and targets of neuromedin anorectic action. Neuromedin increases physical activity in addition to reducing appetite, thus producing a consistent negative energy balance effect. Together with the other various neuropeptides, neurotransmitters, neuromodulators, and neurohormones, neuromedin and orexin act in the appetite network to produce changes in food intake and energy expenditure, which ultimately influences the regulation of body weight.

Local network regulation of orexin neurons in the lateral hypothalamus

Obesity and inadequate sleep are among the most common causes of health problems in modern society. Thus, the discovery that orexin (hypocretin) neurons play a pivotal role in sleep/wake regulation, energy balance, and consummatory behaviors has sparked immense interest in understanding the regulatory mechanisms of these neurons. The local network consisting of neurons and astrocytes within the lateral hypothalamus and perifornical area (LH/PFA), where orexin neurons reside, shapes the output of orexin neurons and the LH/PFA. Orexin neurons not only send projections to remote brain areas but also contribute to the local network where they release multiple neurotransmitters to modulate its activity. These neurotransmitters have opposing actions, whose balance is determined by the amount released and postsynaptic receptor desensitization. Modulation and negative feedback regulation of excitatory glutamatergic inputs as well as release of astrocyte-derived factors, such as lactate and ATP, can also affect the excitability of orexin neurons. Furthermore, distinct populations of LH/PFA neurons express neurotransmitters with known electrophysiological actions on orexin neurons, such as melanin-concentrating hormone, corticotropin-releasing factor, thyrotropin-releasing hormone, neurotensin, and GABA. These LH/PFA-specific mechanisms may be important for fine tuning the firing activity of orexin neurons to maintain optimal levels of prolonged output to sustain wakefulness and stimulate consummatory behaviors. Building on these exciting findings should shed further light onto the cellular mechanisms of energy balance and sleep-wake regulation.

Autonomic response to periodic leg movements during sleep in narcolepsy-cataplexy

STUDY OBJECTIVES

To test the hypothesis of autonomic nervous system dysfunction in patients with narcolepsy-cataplexy (NC) by assessing the physiologic activations associated with periodic limb movements during sleep (PLMS).

DESIGN

Sleep and heart rate (HR) were recorded during 1 night of polysomnography.

SETTING

Data were collected at the Sleep Disorders Center, Sacre-Coeur Hospital, Montreal, Canada.

PARTICIPANTS

Data from 14 patients with NC (6 men, 8 women, mean age: 52.5 ± 11.9 years) were compared with data from 14 healthy control subjects matched for age and sex.

INTERVENTIONS

NA.

MEASUREMENTS AND RESULTS

Analyses included sleep stages, PLMS, microarousals, RR intervals converted into beats per minute on segments lasting 25 heartbeats (10 RR intervals before PLMS and 15 after), and cardiac-activation amplitudes. A Group-by-Heartbeat interaction was noted for PLMS without microarousals; the patients had a tachycardia of lower amplitude and a delayed and lower-amplitude bradycardia, compared with normal control subjects. Similar significant HR modifications were observed for PLMS with microarousals between patients with NC and control subjects. Patients with NC had a reduced magnitude of cardiac activation associated with PLMS with and without microarousals, as compared with control subjects. A negative correlation was noted between cardiac-activation amplitude and age in patients with NC, but no correlation with PLMS index was found in either patients with NC or control subjects.

CONCLUSION

A significant reduction in the amplitude of PLMS-related HR responses in both tachycardia and bradycardia was found in patients with NC. These findings favor the physiologic relevance of the action of hypocretin on autonomic function that may be of clinical significance, i.e., increasing the risk of cardiovascular diseases.

Orexin/hypocretin plays a role in the response to physiological disequilibrium

In the decade since the discovery that pathology of the orexin/hypocretin system is causative for the sleep disorder narcolepsy, considerable progress has been made in understanding the functional role of the neuropeptide. Two, apparently separate functions of orexin have emerged as a consensus from studies to date. The first is the effect on vigilance state boundaries, as exemplified by narcolepsy. Thus the absence of orexin severely limits the ability to maintain prolonged periods of wakefulness or sleep and also allows the unregulated appearance of cataplexy as sudden muscle weakness during wakefulness. The second function is that orexin acts as a signaling molecule in transferring information about physiological disequilibrium to the central nervous system. Orexin activates the central arousal and motor systems during such disequilibrium and so may facilitate the necessary response and adaptation to restore equilibrium. A feasible relationship between these two functions is therefore that the maintenance of prolonged and active wakefulness is an integral part of this adaptive process. Furthermore, the limit placed on the onset of sleep by orexin suggests that these adaptive processes then continue during sleep to become integrated into the development of a coping strategy for the longer term.

Orexins/hypocretins and aminergic systems

Orexin/hypocretin neurones in the posterior hypothalamus are mutually connected with noradrenergic, serotonergic, dopaminergic, histaminergic, and cholinergic neurone systems. They activate these targets by direct post-synaptic and indirect pre-synaptic mechanisms and in turn receive inhibitory feedback and excitatory feed forward control. With respect to behavioural state control, orexin/hypocretin neurones are conducting the orchestra of biogenic amines. This review highlights the role of these players in the control of energy administration, sleep-wake architecture, cortical activation, plasticity, and memory functions in health and disease.

Childhood-onset narcolepsy, obesity and puberty in four consecutive children: a close temporal link

Narcolepsy is a rare but disabling condition that causes excessive daytime sleepiness. Interestingly, weight gain is frequent in patients with narcolepsy and it has sometimes been described very early in the course of the disease. Here, we report four consecutive obese children who were referred to our sleep laboratory for excessive daytime sleepiness and suspected sleep apnoea syndrome. They underwent nocturnal polysomnography associated with multiple sleep latency tests. Narcolepsy was diagnosed in all children with a close temporal link between the onset of narcolepsy, obesity and puberty. Scientifically, the relationship between sleep, weight, growth rate and puberty onset is striking and merits further investigation. From the clinical point of view, narcolepsy must be investigated in obese sleepy children along with obstructive sleep apnoea. Indeed, it can be controlled with appropriate treatment but the proper diagnosis relies not only upon nocturnal polysomnography but involves the systematic use of multiple sleep latency tests.

Hypocretin/orexin and energy expenditure

The hypocretins or orexins are endogenous neuropeptides synthesized in discrete lateral, perifornical and dorsal hypothalamic neurones. These multi-functional neuropeptides modulate energy homeostasis, arousal, stress, reward, reproduction and cardiovascular function. This review summarizes the role of hypocretins in modulating non-sleep-related energy expenditure with specific focus on the augmentation of whole body energy expenditure as well as hypocretin-induced physical activity and sympathetic outflow. We compare the efficacy of hypocretin-1 and 2 on energy expenditure and evaluate whether the literature implicates hypocretin signalling though the hypocretin-1 and -2 receptor as having shared and or functionally specific physiological effects. Thus far data suggest that hypocretin-1 has a more robust stimulatory effect relative to hypocretin-2. Furthermore, hypocretin-1 receptor predominantly mediates behaviours known to influence energy expenditure. Further studies on the hypocretin-2 receptor are needed.

Hypocretin/orexin increases the expression of steroidogenic enzymes in human adrenocortical NCI H295R cells

Hypocretins/orexins act through two receptor subtypes: OX(1) and OX(2). Outside the brain, orexin receptors are expressed in adrenal glands, where orexins stimulate the release of glucocorticoids. To further address the regulation of steroidogenesis, we analyzed the effect of orexins on the expression of steroidogenic enzymes in human adrenocortical National Cancer Institute (NCI) H295R cells by qPCR. In NCI H295R cells, OX(2) receptors were highly expressed, as they were in human adrenal glands. After treatment of NCI H295R cells with orexin A for 12-24 h, the cortisol synthesis rate was significantly increased, whereas 30 min of treatment showed no effect. While CYP11B1 and CYP11B2 mRNA levels were increased already at earlier time points, the expression of HSD3B2 and CYP21 mRNA was significantly up-regulated after treatment with orexin A for 12 h. Likewise, orexin B increased CYP21 and HSD3B2 mRNA levels showing, however, a lower potency compared with orexin A. The mRNA levels of CYP11A and CYP17 were unaffected by orexin A. OX(2) receptor mRNA levels were down-regulated after 12 and 24 h of orexin A treatment. Orexin A increased intracellular Ca(2+) but not cAMP concentrations in NCI H295R cells. Furthermore, inhibition of PKC and MAPK kinase/ERK kinase (MEK1/2) prevented the increase of HSD3B2 expression by orexin A. Accordingly, orexin A treatment of NCI H295R cells markedly enhanced ERK1/2 phosphorylation that was prevented by PKC and, in part, PKA inhibition. In conclusion, orexins may influence adrenal steroidogenesis by differential regulation of the expression of steroidogenic enzymes involving Ca(2+), as well as PKC-ERK1/2 signaling.

Interaction between orexins and the mesolimbic system for overriding satiety

In North American society, it is all too common for the intake of calories to outweigh an individual's energy demands. Such over-consumption where high-energy foods are readily available undoubtedly contributes to the growing problem of obesity. Palatable food stimulates brain circuits similar to those that mediate behavioral responses to drugs of abuse, which may underlie the continuation of food intake long after energy requirements are met. Among the brain areas implicated in reward and food intake, the lateral hypothalamus (LH) has long been recognized as a common region involved in both. It has been suggested that orexin neurons that are expressed exclusively within and adjacent to the LH comprise a major cellular substrate for the functioning of the LH. Here, we review the idea that the orexin neuropeptides play a key role in the rewarding aspects of food intake through interactions with both peripheral and central signals reflecting current energy stores as well as the classic reward pathway--the mesolimbic dopamine system. Furthermore, a possible heterogeneity of orexin neurons is discussed. Uncovering orexin's role in food reinforcement may provide insight into hyperphagia and obesity. In addition, the idea that food intake and substance abuse involve similar brain circuitry suggests potential for a single treatment aiding both obesity and addiction.

Sleep and the epidemic of obesity in children and adults

Sleep is an important modulator of neuroendocrine function and glucose metabolism in children as well as in adults. In recent years, sleep curtailment has become a hallmark of modern society with both children and adults having shorter bedtimes than a few decades ago. This trend for shorter sleep duration has developed over the same time period as the dramatic increase in the prevalence of obesity. There is rapidly accumulating evidence from both laboratory and epidemiological studies to indicate that chronic partial sleep loss may increase the risk of obesity and weight gain. The present article reviews laboratory evidence indicating that sleep curtailment in young adults results in a constellation of metabolic and endocrine alterations, including decreased glucose tolerance, decreased insulin sensitivity, elevated sympathovagal balance, increased evening concentrations of cortisol, increased levels of ghrelin, decreased levels of leptin, and increased hunger and appetite. We also review cross-sectional epidemiological studies associating short sleep with increased body mass index and prospective epidemiological studies that have shown an increased risk of weight gain and obesity in children and young adults who are short sleepers. Altogether, the evidence points to a possible role of decreased sleep duration in the current epidemic of obesity.

Associations between sleep loss and increased risk of obesity and diabetes

During the past few decades, sleep curtailment has become a very common in industrialized countries. This trend for shorter sleep duration has developed over the same time period as the dramatic increase in the prevalence of obesity and diabetes. Evidence is rapidly accumulating to indicate that chronic partial sleep loss may increase the risk of obesity and diabetes. Laboratory studies in healthy volunteers have shown that experimental sleep restriction is associated with an adverse impact on glucose homeostasis. Insulin sensitivity decreases rapidly and markedly without adequate compensation in beta cell function, resulting in an elevated risk of diabetes. Prospective epidemiologic studies in both children and adults are consistent with a causative role of short sleep in the increased risk of diabetes. Sleep curtailment is also associated with a dysregulation of the neuroendocrine control of appetite, with a reduction of the satiety factor, leptin, and an increase in the hunger-promoting hormone, ghrelin. Thus, sleep loss may alter the ability of leptin and ghrelin to accurately signal caloric need, acting in concert to produce an internal misperception of insufficient energy availability. The adverse impact of sleep deprivation on appetite regulation is likely to be driven by increased activity in neuronal populations expressing the excitatory peptides orexins that promote both waking and feeding. Consistent with the laboratory evidence, multiple epidemiologic studies have shown an association between short sleep and higher body mass index after controlling for a variety of possible confounders.

Hypocretin/orexin and nociceptin/orphanin FQ coordinately regulate analgesia in a mouse model of stress-induced analgesia

Stress-induced analgesia (SIA) is a key component of the defensive behavioral "fight-or-flight" response. Although the neural substrates of SIA are incompletely understood, previous studies have implicated the hypocretin/orexin (Hcrt) and nociceptin/orphanin FQ (N/OFQ) peptidergic systems in the regulation of SIA. Using immunohistochemistry in brain tissue from wild-type mice, we identified N/OFQ-containing fibers forming synaptic contacts with Hcrt neurons at both the light and electron microscopic levels. Patch clamp recordings in GFP-tagged mouse Hcrt neurons revealed that N/OFQ hyperpolarized, decreased input resistance, and blocked the firing of action potentials in Hcrt neurons. N/OFQ postsynaptic effects were consistent with opening of a G protein-regulated inwardly rectifying K+ (GIRK) channel. N/OFQ also modulated presynaptic release of GABA and glutamate onto Hcrt neurons in mouse hypothalamic slices. Orexin/ataxin-3 mice, in which the Hcrt neurons degenerate, did not exhibit SIA, although analgesia was induced by i.c.v. administration of Hcrt-1. N/OFQ blocked SIA in wild-type mice, while coadministration of Hcrt-1 overcame N/OFQ inhibition of SIA. These results establish what is, to our knowledge, a novel interaction between the N/OFQ and Hcrt systems in which the corticotropin-releasing factor and N/OFQ systems coordinately modulate the Hcrt neurons to regulate SIA.

High prevalence of eating disorders in narcolepsy with cataplexy: a case-control study

STUDY OBJECTIVES

To study the prevalence of and symptoms of eating disorders in patients with narcolepsy.

DESIGN

We performed a case-control study comparing symptoms of eating disorders in patients with narcolepsy versus healthy population controls, using the Schedules for Clinical Assessment in Neuropsychiatry (SCAN 2.1). To study whether an increased body mass index (BMI) could be responsible for symptoms of an eating disorder, we also compared patients with BMI-matched controls, using the SCAN as well as the Eating Disorder Examination-Questionnaire.

SETTING

University hospital.

PATIENTS AND PARTICIPANTS

Patients with narcolepsy/cataplexy (n = 60) were recruited from specialized sleep centers. Healthy controls (n = 120) were drawn from a population study previously performed in the Netherlands. Separately, 32 BMI-matched controls were recruited.

INTERVENTIONS

N/A.

MEASUREMENTS AND RESULTS

In total, 23.3% of the patients fulfilled the criteria for a clinical eating disorder, as opposed to none of the control subjects. Most of these were classified as Eating Disorder-Not Otherwise Specified, with an incomplete form of binge eating disorder. On the symptom level, half of the patients reported a persistent craving for food, as well as binge eating. Twenty-five percent of patients even reported binging twice a week or more often. When compared with BMI-matched controls, the significant increases persisted in symptoms of eating disorders among patients with narcolepsy. Except for a higher level of interference in daily activities due to eating problems in patients using antidepressants, medication use did not influence our findings.

CONCLUSIONS

The majority of patients with narcolepsy experience a number of symptoms of eating disorders, with an irresistible craving for food and binge eating as the most prominent features. Eating disorder symptomatology interfered with daily activities. These findings justify more attention for eating disorders in the treatment of patients with narcolepsy.

Orexin signaling mediates the antidepressant-like effect of calorie restriction

During periods of reduced food availability, animals must respond with behavioral adaptations that promote survival. Despite the fact that many psychiatric syndromes include disordered eating patterns as a component of the illness, little is known about the neurobiology underlying behavioral changes induced by short-term calorie restriction. Presently, we demonstrate that 10 d of calorie restriction, corresponding to a 20-25% weight loss, causes a marked antidepressant-like response in two rodent models of depression and that this response is dependent on the hypothalamic neuropeptide orexin (hypocretin). Wild-type mice, but not mice lacking orexin, show longer latency to immobility and less total immobility in the forced swim test after calorie restriction. In the social defeat model of chronic stress, calorie restriction reverses the behavioral deficits seen in wild-type mice but not in orexin knock-out mice. Additionally, chronic social defeat stress induces a prolonged reduction in the expression of prepro-orexin mRNA via epigenetic modification of the orexin gene promoter, whereas calorie restriction enhances the activation of orexin cells after social defeat. Together, these data indicate that orexin plays an essential role in mediating reduced depression-like symptoms induced by calorie restriction.

The Hypothalamic Orexinergic System: Pain and Primary Headaches

The primary headaches are a group of distinct individually characterized attack forms, which although varying in presentation, share some common anatomical basis responsible for the pain component of the attack. The hypothalamus is known to modulate a multitude of functions and has been shown to be involved in the pathophysiology of a variety of primary headaches including cluster headache and chronic migraine. It seems likely that it may be involved in other primary headache disorders due to their episodic nature and may underlie many of their diverse symptoms. We discuss the hypothalamic involvement in the modulation of trigeminovascular processing and examine the involvement of the hypothalamic orexinergic system as a key regulator of this function.

Involvement of hypocretin neurons in multiple system atrophy

Hypocretin/Orexin (Hcrt/Orx) neurons of the posterolateral hypothalamus have been implicated in control of sleep and autonomic function. Sleep disorders and autonomic failure are important manifestations of multiple system atrophy (MSA). We sought to determine whether Hcrt/Orx neurons were involved in this disorder. Hypothalamus was obtained from seven subjects with neuropathologically confirmed MSA, and seven age-matched controls. 50 mum sections obtained throughout the posterior hypothalamus were immunostained for Hcrt-1 and alpha-synuclein. In MSA, there was a marked reduction of the total numbers of Hcrt/Orx neurons compared to controls (1,009 +/- 190 cells in MSA vs. 3,206 +/- 185 in controls, P < 0.0001). There were abundant glial cytoplasmic inclusions in the area of distribution of Hcrt/Orx neurons in MSA. This is the first demonstration of loss of Hcrt/Orx neurons in MSA, which is consistent with a system degeneration of neurons involved in homeostatic function, including sleep and autonomic regulation, in this disorder.

Modulation of nociceptive dural input to the trigeminal nucleus caudalis via activation of the orexin 1 receptor in the rat

Migraine pathophysiology is thought to involve the trigeminal innervation of the dura mater and intracranial blood vessels. Electrical stimulation of dural blood vessels is painful in humans and causes activation of neurons in the caudal-most portion of the trigeminal nucleus in experimental animals. The hypothalamic neuropeptides orexin A and B are selectively synthesized in the lateral and posterior hypothalamus, and recent findings have implicated their involvement in nociceptive processing. To evaluate the potential for orexin receptor modulation of trigeminovascular nociceptive afferents, we examined the effects of intravenous orexin A and B on responses of neurons in the trigeminal nucleus caudalis. To dissect the receptor pharmacology of responses to stimulation we utilized the novel orexin 1 receptor (OX(1)R) antagonist N-(2-methyl-6-benzoxazolyl)-N''-1,5-naphthyridin-4-yl urea (SB-334867). Orexin A 30 microg/kg (F(1.9,9.8) = 21.93, P < 0.001) and 50 microg/kg (F(3.2,16.4) = 3.28, P < 0.045) inhibited the A-fibre responses to dural electrical stimulation over 60 min. Maximum inhibition was achieved at 25 min for both 30 microg/kg (t(5) = 19.83, n = 6, P < 0.001) and 50 microg/kg (t(5) = 7.74, n = 6, P < 0.001). The response with orexin A 30 microg/kg was reversed by pretreatment with the OX(1)R antagonist SB-334867 (F(3.5,17.5) = 0.49, P = 0.73), which had no effect when given alone. Orexin B and control vehicle administration had no significant effect on trigeminal neuronal firing. The current study demonstrates that orexin A is able to inhibit A-fibre responses to dural electrical stimulation via activation of the OX(1)R.

Narcolepsy and the hypocretins

Narcolepsy is a chronic neurologic disease characterized by excessive daytime sleepiness and one or more of three additional symptoms (cataplexy, or sudden loss of muscle tone; vivid hallucinations; and brief periods of total paralysis) related to the occurrence of rapid eye movement (REM) sleep at inappropriate times. The daytime sleepiness typically presents as a sudden overwhelming urge to sleep, followed by periods of sleep that last for seconds or minutes, or even longer. During daytime sleep episodes, patients may exhibit "automatic behavior," performing conventionalized functions (eg, taking notes), but not remembering having done so once they are awake. About 10% of narcoleptics are members of familial clusters; however, genetic factors alone are apparently insufficient to cause the disease, inasmuch as the most common genetic disorder, a mutation in chromosome 6 controlling the HLA antigen immune complex, although seen in 90% to 100% of patients, also occurs in as many as 50% of people without narcolepsy. A dog model of narcolepsy exhibits a mutation on chromosome 12 that disrupts the processing of the peptide neurotransmitter hypocretin. No such mutation characterizes human narcolepsy; however, cerebrospinal fluid (CSF) hypocretin levels are profoundly depressed in narcoleptic patients, and a specific reduction in hypocretin-containing neurons has been described. One hypothesis concerning the pathophysiology of narcolepsy proposes that the HLA subtype resulting from the mutation on chromosome 6 increases the susceptibility of hypocretin-containing brain neurons to immune attack. Because hypocretin may normally participate in the maintenance of wakefulness, the loss of neurons that release this peptide might allow REM sleep to occur at inappropriate times, ie, while the patient is awake, in contrast to its normal cyclic appearance after a period of slow-wave sleep. The cataplexy, hallucinations, and/or paralysis associated with REM episodes normally are unnoticed-or, at least, not remembered-when the transition to REM follows slow wave sleep, as is normally the case; however, they are remembered when, in people with narcolepsy, the REM episode starts during a period of wakefulness. The association of narcolepsy with a deficiency in a specific neurotransmitter, in this case, hypocretin, is reminiscent of the associations between Parkinson disease and dopamine, or early Alzheimer disease and acetylcholine.

GABA(B) receptor-mediated modulation of hypocretin/orexin neurones in mouse hypothalamus

Hypocretin/orexin (Hcrt) is a critical neurotransmitter for the maintenance of wakefulness and has been implicated in several other functions, including energy metabolism and reward. Using whole-cell patch-clamp recordings from transgenic mice in which enhanced green fluorescent protein was linked to the Hcrt promoter, we investigated GABAergic control of the Hcrt neurones in hypothalamic slices. Bath application of GABA or muscimol caused an early hyperpolarization mediated by Cl(-) and a late depolarization mediated by the efflux of bicarbonate. These GABA(A) receptor-mediated responses were blocked by picrotoxin and bicuculline. Under the GABA(A) blockade condition, GABA produced consistent hyperpolarization, decreased firing rate and input resistance. The selective GABA(B) agonist (R)-baclofen caused a similar response with an EC(50) of 7.1 mum. The effects of (R)-baclofen were blocked by the GABA(B) antagonist CGP 52432 but persisted in the presence of tetrodotoxin, suggesting direct postsynaptic effects. The existence of GABA(B) modulation was supported by GABA(B(1)) subunit immunoreactivity on Hcrt cells colabelled with antisera to the Hcrt-2 peptide. Furthermore, GABA(B) receptor activation inhibited the presynaptic release of both glutamate and GABA. (R)-Baclofen depressed the amplitude of evoked excitatory postsynaptic currents (EPSCs) and inhibitory synaptic currents (IPSCs), and also decreased the frequency of both spontaneous and miniature EPSCs and IPSCs with a modest effect on their amplitudes. These data suggest that GABA(B) receptors modulate Hcrt neuronal activity via both pre- and postsynaptic mechanisms, which may underlie the promotion of non-rapid eye movement sleep and have implications for the use of GABA(B) agonists in the treatment of substance addiction through direct interaction with the Hcrt system.

Metabolic state signalling through central hypocretin/orexin neurons

Hypothalamic neurons that produce the peptide transmitters hypocretins/orexins have attracted much recent attention. They provide direct and predominantly excitatory inputs to all major brain areas except the cerebellum, with the net effect of stimulating wakefulness and arousal. These inputs are essential for generating sustained wakefulness in mammals, and defects in hypocretin signalling result in narcolepsy. In addition, new roles for hypocretins/orexins are emerging in reward-seeking, learning, and memory. Recent studies also indicate that hypocretin/orexin neurons can alter their intrinsic electrical activity according to ambient fluctuations in the levels of nutrients and appetite-regulating hormones. These intriguing electrical responses are perhaps the strongest candidates to date for the elusive neural correlates of after-meal sleepiness and hunger-induced wakefulness. Hypocretin/orexin neurons may thus directly translate rises and falls in body energy levels into different states of consciousness.

Orexin 1 Receptor Activation Attenuates Neurogenic Dural Vasodilation in an Animal Model of Trigeminovascular Nociception

The pathophysiology underlying the pulsating quality of the pain of a migraine attack is not fully understood, although trigeminal vascular afferents containing the sensory neuropeptide calcitonin gene-related peptide (CGRP) must have a role. Antimigraine drugs, such as triptans, serotonin 5-hydroxytryptamine(1B/1D) receptor agonists, reproducibly block neurogenic vasodilation associated with CGRP release. We examined the effects of the hypothalamic neuropeptides orexin A and orexin B on neurogenic dural vasodilation, dissecting out the receptor pharmacology with the novel orexin 1 (OX1) receptor antagonist N-(2-methyl-6-benzoxazolyl)-N''-1,5-naphthyridin-4-yl urea (SB-334867). Electrical stimulation of dural afferents (50-300 microA) resulted in reproducible dural vasodilation of 136 +/- 9%. Orexin A 30 microg kg(-1), but not 3 and 10 microg kg(-1), inhibited the dilation brought about by electrical stimulation over 60 min and maximally after 15 min by 60% (t7= 7.138; P < 0.001; n = 8). This response was reversed by pretreatment with the OX1 receptor antagonist SB-334867. Addition of CGRP(8-37) at the point of maximal effect of orexin A produced a further significant decrease in neurogenic dural vasodilation compared with orexin A only. CGRP administration (1 microg kg(-1)) produced a reproducible dural blood vessel dilation of 145 +/- 7% that was not inhibited by intravenous administration of orexin A (30 microg kg(-1)). Orexin B had no significant effect even at the highest dose. The current study demonstrates that orexin A is able to inhibit neurogenic dural vasodilation via activation of the OX1 receptor, resulting in inhibition of prejunctional release of CGRP from trigeminal neurons.