Orexin-containing Neurons Research Articles

Tandem-Pore K+ Channels Mediate Inhibition of Orexin Neurons by Glucose

Glucose-inhibited neurons orchestrate behavior and metabolism according to body energy levels, but how glucose inhibits these cells is unknown. We studied glucose inhibition of orexin/hypocretin neurons, which promote wakefulness (their loss causes narcolepsy) and also regulate metabolism and reward. Here we demonstrate that their inhibition by glucose is mediated by ion channels not previously implicated in central or peripheral glucose sensing: tandem-pore K(+) (K(2P)) channels. Importantly, we show that this electrical mechanism is sufficiently sensitive to encode variations in glucose levels reflecting those occurring physiologically between normal meals. Moreover, we provide evidence that glucose acts at an extracellular site on orexin neurons, and this information is transmitted to the channels by an intracellular intermediary that is not ATP, Ca(2+), or glucose itself. These results reveal an unexpected energy-sensing pathway in neurons that regulate states of consciousness and energy balance.

Drugs for sleep disorders: mechanisms and therapeutic prospects

The past decade has witnessed an explosion of knowledge about the neural mechanisms that control sleep and arousal, triggered by two discoveries relating to the sleep disorder narcolepsy. Narcolepsy is caused by the loss of orexin-containing neurons in the hypothalamus, and a novel nonstimulant wakefulness-promoting drug, modafinil, alleviates excessive day-time sleepiness associated with the disorder. The level of arousal is controlled by an intricate interplay between distinct wakefulness- and sleep-promoting nuclei situated in the hypothalamus and brainstem and the interconnections between the nuclei and the neurotransmitters involved have been mapped. Wakefulness-promoting nuclei include the orexinergic lateral hypothalamic/perifornical area, the histaminergic tuberomammillary nucleus, the cholinergic pedunculopontine tegmental nucleus, the noradrenergic locus coeruleus, the 5-hydroxytryptaminergic raphe nuclei and possibly the dopaminergic ventral tegmental area. The major sleep-promoting nucleus is the GABAergic ventrolateral preoptic nucleus of the hypothalamus. Currently available and future drugs exert their therapeutic effects in the three major classes of sleep disorder (insomnia, hypersomnia, parasomnia) by modifying neurotransmission at distinct sites within the arousal-controlling neuronal network. This enables classification of therapeutic drugs for sleep disorders on the basis of their modes of action: drugs that interact with the GABAergic sleep-promoting system, drugs that interact with different wakefulness-promoting systems and drugs that modulate the level of arousal by mechanisms that do not initially involve the basic network (e.g. melatonin, adenosine). The development of novel therapeutic drugs for sleep disorders is based on the synthesis of molecular/cellular mechanisms and the sites of action within the arousal-controlling neuronal network.

Ghrelin-induced sleep responses in ad libitum fed and food-restricted rats

Ghrelin is an endogenous ligand for the growth hormone secretagogue receptor and a well-characterized food intake regulatory peptide. Hypothalamic ghrelin-, neuropeptide Y (NPY)-, and orexin-containing neurons form a feeding regulatory circuit. Orexins and NPY are also implicated in sleep–wake regulation. Sleep responses and motor activity after central administration of 0.2, 1, or 5 μg ghrelin in free-feeding rats as well as in feeding-restricted rats (1 μg dose) were determined. Food and water intake and behavioral responses after the light onset injection of saline or 1 μg ghrelin were also recorded. Light onset injection of ghrelin suppressed non-rapid-eye-movement sleep (NREMS) and rapid-eye-movement sleep (REMS) for 2 h. In the first hour, ghrelin induced increases in behavioral activity including feeding, exploring, and grooming and stimulated food and water intake. Ghrelin administration at dark onset also elicited NREMS and REMS suppression in hours 1 and 2, but the effect was not as marked as that, which occurred in the light period. In hours 3–12, a secondary NREMS increase was observed after some doses of ghrelin. In the feeding-restricted rats, ghrelin suppressed NREMS in hours 1 and 2 and REMS in hours 3–12. Data are consistent with the notion that ghrelin has a role in the integration of feeding, metabolism, and sleep regulation.

Sleep Circuitry and the Hypnotic Mechanism of GABA<SUB>A</SUB> Drugs

Early in the twentieth century, von Economo provided the first evidence linking the hypothalamus with sleep-wake behavior. His studies concluded that the anterior hypothalamus was associated with sleep, whereas the posterior hypothalamus was associated with waking. In the decades following these observations, a wealth of research has shown that an elaborate circuitry comprising a number of brain regions, cell types, and extracellular messengers underlies sleep-wake behavior. In this review, we discuss data generated in the past 10 years that highlight the role of the hypothalamus in sleep-wake behavior and control. In particular, we will focus on the identification of the ventrolateral preoptic nucleus (VLPO) as a sleep center and the hypocretin/orexin cells in the perifornical region of the hypothalamus as constituting a waking center; these two centers are critical for the maintenance of normal sleep-wake architecture, and provide a foundation for our understanding of sleep-wake behavior and its underlying physiology. The data from these and other regions traditionally associated with the sleep-wake cycle have led to a flip-flop switch model of sleep-wake control. The switch is composed of two sets of mutually inhibitory groups of neurons: a sleep group and an arousal group, with the latter modulated by orexin-containing neurons in the lateral hypothalamus. The sleep-promoting GABA (gamma-amino-butyric acid) receptor agonists are a diverse class of drugs, which include barbiturates, benzodiazepines, chloral hydrate, ethanol, and gaseous anesthetics, that have been used to study sleep physiology for many years. Recent studies suggest that these drugs may exert their hypnotic effects in a regionally specific manner. For example, some GABAA agonists appear to promote sleep by inhibiting the histaminergic cells in the tuberomammillary nucleus and weakly activating the VLPO via agonist binding to the alpha1 subunit of GABAA receptors; whereas, gaboxadol (THIP; 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol) binds to the alpha4delta-subunits, potentially promoting sleep by activation of the VLPO. The integration of these data into the flip-flop switch model can be used to better understand sleep-wake control and augment existing therapeutic treatments for sleep disorders.

Orexin neuron-mediated skeletal muscle vasodilation and shift of baroreflex during defense response in mice

We have previously shown that some features of the defense response, such as increases in arterial blood pressure (AP), heart rate (HR), and ventilation were attenuated in prepro-orexin knockout (ORX-KO) mice. Here, we examined whether the same was true in orexin neuron-ablated [orexin/ataxin-3 transgenic mice (ORX/ATX-Tg)] mice. In addition, we examined other features of the defense response: skeletal muscular vasodilation and shift of baroreceptor reflex. In both anesthetized and conscious conditions, basal AP in ORX/ATX-Tg mice was significantly lower by approximately 20 mmHg than in wild-type (WT) controls, as was the case in ORX-KO mice. The difference in AP disappeared after treatment with an alpha-blocker but not with a beta-blocker, indicating lower sympathetic vasoconstrictor outflow. Stimulation of the perifornical area (PFA) in urethane-anesthetized ORX/ATX-Tg mice elicited smaller and shorter-lasting increases in AP, HR, and ventilation, and skeletal muscle vasodilation than in WT controls. In addition, air jet stress-induced elevations of AP and HR were attenuated in conscious ORX/ATX-Tg mice. After pretreatment with a beta-blocker, atenolol, stimulation of PFA suppressed phenylephrine (50 microg/kg iv)-induced bradycardia (DeltaHR=-360+/-29 beats/min without PFA stimulation vs. -166+/-26 during stimulation) in WT. This demonstrated the resetting of the baroreflex. In ORX/ATX-Tg mice, however, no significant suppression was observed (-355+/-16 without stimulation vs. -300+/-30 during stimulation). The present study provided further support for our hypothesis that orexin-containing neurons in PFA play a role as a master switch to activate multiple efferent pathways of the defense response and also operate as a regulator of basal AP.

Direct Involvement of Orexinergic Systems in the Activation of the Mesolimbic Dopamine Pathway and Related Behaviors Induced by Morphine

In this study, we investigated the role of orexinergic systems in dopamine-related behaviors induced by the mu-opioid receptor agonist morphine in rodents. Extensive coexpression of tyrosine hydroxylase with orexin receptors was observed in the mouse ventral tegmental area (VTA). The levels of dopamine and its major metabolites in the nucleus accumbens were markedly increased by the microinjection of orexin A and orexin B into the VTA. The subcutaneous morphine-induced place preference and hyperlocomotion observed in wild-type mice were abolished in mice that lacked the prepro-orexin gene. An intra-VTA injection of a selective orexin receptor antagonist SB334867A [1-(2-methylbenzoxazol-6-yl)-3-[1.5]naphthyridin-4-yl urea] significantly suppressed the morphine-induced place preference in rats. Furthermore, the increased level of dialysate dopamine produced by morphine in the mouse brain was significantly decreased by deletion of the prepro-orexin gene. These findings provide new evidence that orexin-containing neurons in the VTA are directly implicated in the rewarding effect and hyperlocomotion induced by morphine through activation of the mesolimbic dopamine pathway in rodents.

Multi-Transcriptional Profiling of Melanin-Concentrating Hormone and Orexin-Containing Neurons

1.Melanin-concentrating hormone (MCH) and orexin-containing neurons participate in hypothalamic circuits that control energy homeostasis. While these two systems have projections to widespread target areas within the central nervous system, little is known about intrinsic characteristics and the molecular composition of both the MCH and orexin neurons themselves. 2. By a combinatory approach of quantitative immunocytochemical identification and analysis with laser microdissection and semi-quantitative Real-time RT-PCR, here we present multi-transcriptional profiling of MCH and orexin neurons in the rat lateral hypothalamus. 3. Immunocytochemical analysis showed that orexin peptide expression was increased after fasting both during the activity and resting period of rats, whereas MCH peptide content was only clearly upregulated at resting phase. Subsequent transcriptional profiling showed distinct expression patterns of MCH, orexin and cocaine-amphetamine regulated transcript (CART) between MCH and orexin neurons. A low expression level of dynorphin was found both in MCH and orexin neurons. Receptor expression profiles, reflecting interaction with neuropeptide Y, melanocortins, leptin, glucocorticoids and GABA, showed approximately similar expression patterns among the MCH and orexin neuronal systems. Expression of glutamate- and GABA-markers revealed a possible contributory role of both glutamate and GABA in functional output of MCH and orexin neurons. 4. This method allowed differential screening at mRNA level after immunocytochemical neuron identification and analysis in heterogeneous brain regions, which can further specify functioning of the individual neurons. With respect to MCH and orexin neurons, this study emphasizes that these neurons are targets for stimulatory and inhibitory signals from other brain regions including the arcuate nucleus and the general circulation. Additionally, both glutamate and GABA appear to be involved in MCH and orexin neuronal functioning related to feeding and regulation of the energy balance.

Preclinical Pharmacology of FMPD [6-Fluoro-10-[3-(2-methoxyethyl)-4-methyl-piperazin-1-yl]-2-methyl-4H-3-thia-4,9-diaza-benzo[f]azulene]: A Potential Novel Antipsychotic with Lower Histamine H1Receptor Affinity Than Olanzapine

FMPD [6-fluoro-10-[3-(2-methoxyethyl)-4-methyl-piperazin-1-yl]-2-methyl-4H-3-thia-4,9-diaza-benzo[f]azulene] is a potential novel antipsychotic with high affinity for dopamine D2 (Ki= 6.3 nM), 5-HT(2A) (Ki= 7.3 nM), and 5-HT6 (Ki= 8.0 nM) human recombinant receptors and lower affinity for histamine H1 (Ki= 30 nM) and 5-HT2C (Ki= 102 nM) human recombinant receptors than olanzapine. Oral administration of FMPD increased rat nucleus accumbens 3,4-dihyroxyphenylacetic acid concentrations (ED200 = 6 mg/kg), blocked 5-HT2A agonist-induced increases in rat serum corticosterone levels (ED50= 1.8 mg/kg), and inhibited the ex vivo binding of [125I]SB-258585 [4-iodo-N-[4-methoxy-3-(4-methyl-piperazin-1-yl)-phenyl]-benzenesulfonamide] to striatal 5-HT6 receptors (ED50= 10 mg/kg) but failed to inhibit ex vivo binding of [3H]pyrilamine to hypothalamic histamine H1 receptors at doses of up to 30 mg/kg. In electrophysiology studies, acute administration of FMPD selectively elevated the number of spontaneously active A10 (versus A9) dopamine neurons and chronic administration selectively decreased the number of spontaneously active A10 (versus A9) dopamine neurons. FMPD did not produce catalepsy at doses lower than 25 mg/kg p.o. In Fos-induction studies, FMPD had an atypical antipsychotic profile in the striatum and nucleus accumbens and increased Fos expression in orexin-containing neurons of the hypothalamus. FMPD produced only a transient elevation of prolactin levels. These data indicate that FMPD is an orally available potent antagonist of dopamine D2, 5-HT2A, and 5-HT6 receptors and a weak antagonist of H1 and 5-HT2C receptors. FMPD has the potential to have efficacy in treating schizophrenia and bipolar mania with a low risk of treatment-emergent extrapyramidal symptoms, prolactin elevation, and weight gain. Clinical trials are needed to test these hypotheses.

Orexin-saporin lesions of the medial septum impair spatial memory

The medial septum and diagonal band of Broca (MSDB) provide a major input to the hippocampus and are important for spatial learning and memory. Although electrolytic MSDB lesions have prominent memory impairing effects, selective lesions of either cholinergic or GABAergic MSDB neurons do not or only mildly impair spatial memory. MSDB neurons are targets of orexin-containing neurons from the hypothalamus. At present, the functional significance of orexin afferents to MSDB is unclear, and the present study investigated a possible involvement of orexin innervation of the MSDB in spatial memory. Orexin-saporin, a toxin that damages neurons containing the hypocretin-2 receptor, was administered into the MSDB of rats. Rats were subsequently tested on a water maze to assess spatial reference memory and a plus maze to assess spatial working memory. At 100 ng/μl, orexin-saporin destroyed primarily GABAergic septohippocampal neurons, sparing the majority of cholinergic neurons. At 200 ng/μl, orexin-saporin almost totally eliminated GABAergic septohippocampal neurons and destroyed many cholinergic neurons. Spatial reference memory was impaired at both concentrations of orexin-saporin with a dramatic impairment observed for 24-h retention. Short-term reference memory was also impaired at both concentrations. Rats treated with 200 ng/μl, but not 100 ng/μl, of orexin-saporin were also impaired on a spontaneous alternation task, showing a deficit in spatial working memory. Our results, together with previous studies, suggest that orexin innervation of the MSDB may modulate spatial memory by acting on both GABAergic and cholinergic septohippocampal neurons.

Orexin stimulates breathing via medullary and spinal pathways

A central neuronal network that regulates respiration may include hypothalamic neurons that produce orexin, a peptide that influences sleep and arousal. In these experiments, we investigated 1) projections of orexin-containing neurons to the pre-Botzinger region of the rostral ventrolateral medulla that regulates rhythmic breathing and to phrenic motoneurons that innervate the diaphragm; 2) the presence of orexin A receptors in the pre-Botzinger region and in phrenic motoneurons; and 3) physiological effects of orexin administered into the pre-Botzinger region and phrenic nuclei at the C3-C4 levels. We found orexin-containing fibers within the pre-Botzinger complex. However, only 0.5% of orexin-containing neurons projected to the pre-Botzinger region, whereas 2.9% of orexin-containing neurons innervated the phrenic nucleus. Neurons of the pre-Botzinger region and phrenic nucleus stained for orexin receptors, and activation of orexin receptors by microperfusion of orexin in either site produced a dose-dependent, significant (P <0.05) increase in diaphragm electromyographic activity. These data indicate that orexin regulates respiratory activity and may have a role in the pathophysiology of sleep-related respiratory disorders.

Hypothalamic orexin (hypocretin) neurons express vesicular glutamate transporters VGLUT1 or VGLUT2.

Initially recognized for their importance in control of appetite, orexins (also called hypocretins) are neuropeptides that are also involved in regulating sleep, arousal, and cardiovascular function. Loss of orexin appears to be the primary cause of narcolepsy. Cells expressing the orexins are restricted to a discrete region of the hypothalamus, but their terminal projections are widely distributed throughout the brain. With the diversity of function and broad distribution of orexin terminals, it is not known whether the orexin cells constitute a homogeneous population. Because orexins produce neuroexcitatory effects, we hypothesized that orexin-containing neurons are glutamatergic. In the present study we used digoxigenin-labeled cRNA probes for the vesicular glutamate transporters, VGLUT1 and VGLUT2, for in situ hybridization studies in combination with immunohistochemical detection of orexin cell bodies in the hypothalamus. In general, cells in the hypothalamus expressed low levels of the vesicular glutamate transporters relative to other areas of the forebrain, such as the cortex and thalamus. Light labeling for VGLUT2 mRNA was detected in about 50% of the orexin-immunoreactive neurons, and a much smaller percentage (approximately 13%) of orexin-immunoreactive cells was found to express VGLUT1. Despite the fact that intense labeling for GAD67 mRNA was found in a large number of cells throughout the hypothalamus, none of the orexin-immunoreactive cells was found to be GABAergic. These findings, showing that many of the orexin neurons are glutamatergic, are consistent with the neuroexcitatory effects of orexin but suggest that another neurochemical phenotype may define the remaining subset of orexin neurons.

A Separate Path to Your Dreams

The neurobehavioral states of wakefulness, rapid eye movement (REM) sleep, and non-REM sleep are characterized by distinct patterns of neuronal activity, as well as by differences in consciousness, responsiveness to stimuli, and muscle tone. The narcolepsy-cataplexy syndrome, which involves the loss of orexin-containing hypothalamic neurons, is associated with disturbances of the normal pattern of sleep and wakefulness, including overwhelming sleepiness, non-REM "sleep attacks," sleep-onset hallucinations, and cataplexy--a sudden, emotionally triggered loss of postural muscle tone. Cataplexy and sleep-onset hallucinations represent an intrusion of the REM state into wakefulness and are believed to reflect a neurological disturbance distinct from that underlying the inability to stay awake. Willie et al. investigated mutant mice lacking either orexins or the type 2 orexin receptor (OX2R) and discovered that, whereas both experienced similar non-REM sleep disturbances, the mice lacking orexin were more severely cataplectic. The authors used behavioral, pharmacological, and electrophysiological (electroencephalograms and electromyograms) analysis to characterize two classes of "behavioral arrests": abrupt arrests, analogous to human cataplectic attacks, and gradual arrests, equivalent to human sleep attacks. Both mutants exhibited disruptions of wakefulness and similar susceptibility to gradual arrests. The orexin -knockout mice, however, were more susceptible to abrupt arrests. Calcium imaging experiments in brain slices from wild-type mice and mice lacking OX2R indicated that OX2R was required for orexin-mediated stimulation of neurons in the tuberomammillary nucleus, a structure implicated in promoting wakefulness and suppressing non-REM sleep. Thus, OX2R seems to be critical to narcoleptic disruption of patterns of non-REM sleep and wakefulness, whereas OX2R-independent pathways play an important role in dysregulation of REM sleep. J. T. Willie, R. M. Chemelli, C. M. Sinton, S. Tokita, S. C. Williams, Y. Y. Kisanuki, J. N. Marcus, C. Lee, J. K. Elmquist, K. A. Kohlmeier, C. S. Leonard, J. A. Richardson, R. E. Hammer, M. Yanagisawa, Distinct narcolepsy syndromes in orexin receptor-2 and orexin null mice: Molecular genetic dissection of non-REM and REM sleep regulatory processes. Neuron 38 , 715-730 (2003). [Online Journal]

Attenuated defense response and low basal blood pressure in orexin knockout mice.

The perifornical area of the hypothalamus has been known as the center for the defense response, or "fight or flight" response, which is characterized by a concomitant rise in arterial blood pressure (AP), heart rate (HR), and respiratory frequency (Rf). We examined whether orexin, a recently identified hypothalamic neuropeptide, contributes to the defense response and basal cardiovascular regulation using orexin knockout mice. Microinjection of a GABA-A receptor antagonist, bicuculline methiodide (0.1-1 mM in 20 nl), to the perifornical area in urethane-anesthetized wild-type mice elicited dose-dependent increases in AP, HR, and Rf. Although similar changes were observed in orexin knockout mice, intensities were smaller and duration was shorter than those in wild-type mice. Moreover, in an awake and freely moving condition, telemeter-indwelling orexin knockout mice showed diminished cardiovascular and behavioral responses to emotional stress in the resident-intruder test. We also found that basal AP in orexin knockout mice was significantly lower in both anesthetized (117 +/- 8 mmHg in wild type and 92 +/- 3 in knockout) and conscious (125 +/- 6 mmHg in wild type and 109 +/- 2 in knockout) conditions. alpha-Adrenergic blockade with prazosin or ganglion blockade with hexamethonium canceled the difference in basal AP. HR and cardiac contractile parameters by echocardiography did not differ between the two strains of mice. These results indicate lower sympathetic vasoconstrictor tone in knockout mice. The present study suggests that orexin-containing neurons in the perifornical area play a role as one of the efferent pathways of defense response and also operate as a regulator of AP at basal condition by activating sympathetic outflow.

Orexin neurons project to diverse sympathetic outflow systems

The viral transneuronal labeling method was used to demonstrate that orexin-containing neurons of the lateral hypothalamic area (LHA) are linked via multisynaptic connections to different sympathetic outflow systems. Two different types of transneuronal tracing experiments were performed: single- and double-virus studies. In the first series of experiments, Bartha pseudorabies virus (PRV), a retrograde transneuronal tracer, was injected into single sympathetic targets, viz., stellate ganglion, adrenal gland, celiac ganglion, and kidney. Six to 7 days post-injection, orexin (hypocretin) neurons were transneuronally labeled. In a second set of experiments, the double-virus tracing method was used to determine whether single orexin LHA neurons are linked to two different sympathetic outflow systems. Two isogenic forms of Bartha PRV were used that differed by a single gene. β-Galactosidase Bartha PRV was injected into the stellate ganglion and green fluorescent protein Bartha PRV into the adrenal gland of the same rat. The reverse placement of viral injections was made in another set of rats. In both paradigms, some orexin LHA neurons were transneuronally labeled with both viruses, indicating that they are capable of modulating multiple sympathetic outflow systems. These findings raise the possibility that orexin LHA neurons regulate general sympathetic functions, such as those that occur during arousal or the fight-or-flight response.

Roles of orexins in the regulation of feeding and arousal

Maintenance of energy homeostasis requires the coordination of systems that regulate feeding, autonomic and endocrine functions with those that govern an appropriate state of arousal. The hypothalamus play a critical role in maintaining energy homeostasis by integrating and coordinating these systems. Recent studies have suggested that orexin-containing neurons in the lateral hypothalamic area constitute an important central pathway that promotes adaptive behavioral and arousal responses to metabolic and environmental signals. This article review recent studies on orexin to suggest that orexin neurons play a significant role in feeding and arousal regulation, possibly by coordinating the complex behavioral and physiological responses of these complementary homeostatic functions.

Selective Action of Orexin (Hypocretin) on Nonspecific Thalamocortical Projection Neurons

As is evident from the pathological consequences of its absence in narcolepsy, orexin (hypocretin) appears to be critical for the maintenance of wakefulness. Via diffuse projections through the brain, orexin-containing neurons in the hypothalamus may act on a number of wake-promoting systems. Among these are the intralaminar and midline thalamic nuclei, which project in turn in a widespread manner to the cerebral cortex within the nonspecific thalamocortical projection system. Testing the effect of orexin in rat brain slices, in two nuclei of this system, centromedial (CM) nuclei and rhomboid nuclei, we found that it depolarized and excited all neurons tested through a direct postsynaptic action. An additional analysis of this effect in CM neurons indicates that it results from the decrease of a potassium conductance. By a detailed comparison of the effects of orexin A and B, we established that orexin B was more potent than orexin A, indicating the probable mediation by orexin type 2 receptors. In contrast to its effect on the nonspecific thalamocortical projection neurons, orexin had no effect on the specific sensory relay neurons of the somatic, ventral posterolateral, and visual dorsal lateral geniculate nuclei. Orexin differs in this regard from norepinephrine and acetylcholine, to which neurons in the specific and nonspecific systems are sensitive. Orexin may thus act in the thalamus to promote wakefulness by exciting neurons of the nonspecific thalamocortical projection system, which, through widespread projections to the cerebral cortex, stimulate and maintain cortical activation.

Roles of orexins in regulation of feeding and wakefulness.

Maintenance of energy homeostasis requires the coordination of systems that regulate feeding, body temperature, autonomic and endocrine functions with those that govern an appropriate state of arousal (wakefulness). Historically, the hypothalamus has been recognized to play a critical role in maintaining energy homeostasis by integrating these factors and coordinating metabolic, neuroendocrine and behavioral responses and arousal states. Recent studies have suggested that orexin-containing neurons in the lateral hypothalamic area (LHA) constitute an important central pathway that promotes adaptive behavioral and arousal responses to metabolic and environmental signals. Orexins, also called hypocretins, are neuropeptides originally identified as endogenous ligands for two orphan G-protein-coupled receptors termed orexin receptors -1 and -2. Orexin-A and -B are expressed by a specific population of neurons in the LHA. These neurons project to numerous brain regions, with monoaminergic and cholinergic nuclei of the hypothalamus, midbrain, and pons receiving particularly strong innervations. The orexinergic system is anatomically well-placed to coordinate the metabolic, motivational, motor, autonomic, and arousal processes necessary to elicit environmentally appropriate behaviors. Recent studies on orexin suggest that the orexinergic system plays a significant role in feeding and sleep-wakefulness regulation, possibly by coordinating the complex behavioral and physiological responses of these complementary homeostatic functions. Orexin neurons may provide an integrative link between peripheral metabolism and central regulation of behaviors required for an adaptive response to homeostatic challenges.

Orexin neurons of the hypothalamus express adenosine A1 receptors

Adenosine is a putative sleep factor with effects mainly mediated by the A1 receptor. Recent studies have implicated the hypothalamic orexin/hypocretin-containing neurons in the control of sleep–wakefulness. To help determine if adenosine might play a role in the control of orexin neurons, immunohistochemistry was used to characterize the distribution of adenosine A1 receptor protein on the orexinergic neurons. About 30% of orexin-containing neurons were labeled. The data supports the presence of adenosine A1 receptors on orexinergic neurons and suggests a possible substrate for a functional role of adenosine in the regulation of orexinergic activity.

5-HT1A receptor immunoreactivity in hypothalamic neurons involved in body weight control.

Serotonin (5-hydroxytryptamine; 5-HT) is a regulator of feeding behavior. The effect of serotonin on food intake is believed to be primarily mediated via 5-HT(1A) and 5-HT(2C) receptors, which both are expressed in hypothalamic regions implicated in regulation of feeding behavior. Using an antiserum to the 5-HT(1A) receptor, immunoreactive neurons were observed in the rat supraoptic, paraventricular, arcuate and ventromedial nuclei and lateral hypothalamic area. 5-HT(1A) receptor immunoreactivity was demonstrated in neuropeptide Y-, agouti-related peptide-, proopiomelanocortin- and cocaine- and amphetamine-regulated transcript-containing neurons of the arcuate nucleus. In the lateral hypothalamus, 5-HT(1A) receptor immunoreactivity was observed in melanin-concentrating hormone- and orexin-containing neurons. The results suggest that serotonin via postsynaptic 5-HT(1A) receptors affects the release of peptides regulating food intake.

Developmental changes in the orexin 2 receptor mRNA in hypoglossal motoneurons.

Hypothalamic orexin-containing neurons project to many CNS targets, including motoneurons. We assessed developmental changes in the expression of the orexin type 2 receptor (ORX2r) mRNA in hypoglossal (XII) motoneurons. Identified motoneurons were dissociated from 4- to 33-day-old rats and subjected to single-cell reverse transcription and PCR; nearly all contained the ORX2r mRNA. In 40 motoneurons studied using semi-nested PCR, and in another 39 subjected to quantitative, real-time PCR, the number of reverse-transcribed mRNA copies per cell was significantly higher around day 20 postnatally than at any other age. Thus, ORX may postsynaptically excite XII motoneurons, with the ORX2r mRNA production increased during the critical period for development of the rapid eye movement sleep and its disorder narcolepsy/cataplexy.