Orexin (hypocretin)-like immunoreactivity in the cat hypothalamus: a light and electron microscopic study

Orexinergic neurons in the hypothalami of an Asiatic lion, an African lion, and a Southeast African cheetah

Employing orexin-A immunohistochemistry, we describe the distribution, morphology, and nuclear parcellation of orexinergic neurons within the hypothalami of an Asiatic lion (Panthera leo subsp. persica), an African lion (Panthera leo subsp. melanochaita), and a Southeast African cheetah (Acinonyx jubatus subsp. jubatus). In all three felids, the clustering of large, bipolar, and multipolar hypothalamic orexinergic neurons primarily follows the pattern observed in other mammals. The orexinergic neurons were found, primarily, to form three distinct clusters-the main, zona incerta, and optic tract clusters. In addition, large orexinergic neurons were observed in the ventromedial supraoptic region of the hypothalamus, where they are not typically observed in other species. As has been observed in cetartiodactyls and the African elephant, a cluster of small, multipolar orexinergic neurons, the parvocellular cluster, was observed in the medial zone of the hypothalamus in all three felids, although this parvocellular cluster has not been reported in other carnivores. In both subspecies of lions, but not the cheetah, potential orexin-immunopositive neurons were observed in the paraventricular hypothalamic nucleus, supraoptic nucleus, the lateral part of the retrochiasmatic area, and the inner layer of the median eminence. The distribution and parcellation of orexinergic neurons in the hypothalami of the three felids studied appear to be more complex than observed in many other mammals and for the two subspecies of lion may be even more complex. These findings are discussed in terms of potential technical concerns, phylogenetic variations of this system, and potentially associated functional aspects of the orexinergic system.

Hypothalamic orexinergic neuron changes during the hibernation of the Syrian hamster

Hibernation in small mammals is a highly regulated process with periods of torpor involving drops in body temperature and metabolic rate, as well as a general decrease in neural activity, all of which proceed alongside complex brain adaptive changes that appear to protect the brain from extreme hypoxia and low temperatures. All these changes are rapidly reversed, with no apparent brain damage occurring, during the short periods of arousal, interspersed during torpor—characterized by transitory and partial rewarming and activity, including sleep activation, and feeding in some species. The orexins are neuropeptides synthesized in hypothalamic neurons that project to multiple brain regions and are known to participate in the regulation of a variety of processes including feeding behavior, the sleep-wake cycle, and autonomic functions such as brown adipose tissue thermogenesis. Using multiple immunohistochemical techniques and quantitative analysis, we have characterized the orexinergic system in the brain of the Syrian hamster—a facultative hibernator. Our results revealed that orexinergic neurons in this species consisted of a neuronal population restricted to the lateral hypothalamic area, whereas orexinergic fibers distribute throughout the rostrocaudal extent of the brain, particularly innervating catecholaminergic and serotonergic neuronal populations. We characterized the changes of orexinergic cells in the different phases of hibernation based on the intensity of immunostaining for the neuronal activity marker C-Fos and orexin A (OXA). During torpor, we found an increase in C-Fos immunostaining intensity in orexinergic neurons, accompanied by a decrease in OXA immunostaining. These changes were accompanied by a volume reduction and a fragmentation of the Golgi apparatus (GA) as well as a decrease in the colocalization of OXA and the GA marker GM-130. Importantly, during arousal, C-Fos and OXA expression in orexinergic neurons was highest and the structural appearance and the volume of the GA along with the colocalization of OXA/GM-130 reverted to euthermic levels. We discuss the involvement of orexinergic cells in the regulation of mammalian hibernation and, in particular, the possibility that the high activation of orexinergic cells during the arousal stage guides the rewarming as well as the feeding and sleep behaviors characteristic of this phase.

An overview of the orexinergic system in different animal species

Orexin (hypocretin), is a neuropeptide produced by a subset of neurons in the lateral hypothalamus. From the lateral hypothalamus, the orexin-containing neurons project their fibres extensively to other brain structures, and the spinal cord constituting the central orexinergic system. Generally, the term ''orexinergic system'' usually refers to the orexin peptides and their receptors, as well as to the orexin neurons and their projections to different parts of the central nervous system. The extensive networks of orexin axonal fibres and their terminals allow these neuropeptidergic neurons to exert great influence on their target regions. The hypothalamic neurons containing the orexin neuropeptides have been implicated in diverse functions, especially related to the control of a variety of homeostatic functions including feeding behaviour, arousal, wakefulness stability and energy expenditure. The broad range of functions regulated by the orexinergic system has led to its description as ''physiological integrator''. In the last two decades, the orexinergic system has been a topic of great interest to the scientific community with many reports in the public domain. From the documentations, variations exist in the neuroanatomical profile of the orexinergic neuron soma, fibres and their receptors from animal to animal. Hence, this review highlights the distinct variabilities in the morphophysiological aspects of the orexinergic system in the vertebrate animals, mammals and non-mammals, its presence in other brain-related structures, including its involvement in ageing and neurodegenerative diseases. The presence of the neuropeptide in the cerebrospinal fluid and peripheral tissues, as well as its alteration in different animal models and conditions are also reviewed.

Hypocretin (Orexin) Replacement Therapies

Twenty-two years after their discovery, the hypocretins (Hcrts), also known as orexins, are two of the most studied peptidergic systems, involved in myriad physiological systems that range from sleep, arousal, motivation, homeostatic regulation, fear, anxiety and learning. A causal relationship between activity of Hcrt and arousal stability was established shortly after their discovery and have led to the development of a new class of drugs to treat insomnia. In this review we discuss the many faces of the Hcrt system and examine recent findings that implicate decreased Hcrt function in the pathogenesis of a number of neuropsychiatric conditions. We also discuss future therapeutic strategies to replace or enhance Hcrt function as a treatment option for these neuropsychiatric conditions.

Hypocretin (orexin) immunoreactivity in the feline midbrain: Relevance for the generation of wakefulness

Hypocretins (Hcrt) 1 and 2 are two neuropeptides synthesized from neurons that are located in the perifornical area of the lateral hypothalamus. These neurons project diffusely throughout the central nervous system, and have been implicated in the generation and maintenance of wakefulness, as well as in critical physiological processes that occur during this behavioral state, such as motivation. The hypocretinergic projections towards the feline midbrain have not been studied before. Therefore, the aim of the present study was to analyze their relationship to the midbrain neurons, that are critically involved in the control of sleep and wakefulness. With this purpose, we examined the distribution of Hcrt1-positive fibers in the midbrain and pontomesencephalic area of the domestic cat (Felis catus), and their relationship with catecholaminergic and cholinergic neurons by means of single and double immunohistochemistry. Hcrtergic axons with distinctive varicosities and buttons were heterogeneously distributed, exhibiting different densities in distinct regions of the midbrain. High Hcrtergic fiber densities were observed in the periaqueductal gray, interpeduncular nucleus, locus coeruleus and cholinergic mesopontine regions. In addition, we studied in detail the Hcrtergic projection towards the dopaminergic nuclei of the midbrain. While very few Hcrt + fibers were observed in the substantia nigra pars compacta, the highest density of Hcrtergic fibers was found in the dopaminergic ventral periaqueductal gray area (also called A10dc area); appositions between Hcrtergic terminals and dopaminergic somata and dendrites were observed within this area. Because this dopaminergic area has been involved in the control of wakefulness, the present anatomical data provides relevant support about the role of the Hcrtergic system in the generation of this behavioral state.

Orexinergic fibers are in contact with Kölliker-Fuse nucleus neurons projecting to the respiration-related nuclei in the medulla oblongata and spinal cord of the rat

The neural pathways underlying the respiratory variation dependent on vigilance states remain unsettled. In the present study, we examined the orexinergic innervation of Kölliker-Fuse nucleus (KFN) neurons sending their axons to the rostral ventral respiratory group (rVRG) and phrenic nucleus (PhN) as well as to the hypoglossal nucleus (HGN) by using a combined retrograde tracing and immunohistochemistry. After injection of cholera toxin B subunit (CTb) into the KFN, CTb-labeled neurons that are also immunoreactive for orexin (ORX) were found prominently in the perifornical and medial regions and additionally in the lateral region of the hypothalamic ORX field. After injection of fluorogold (FG) into the rVRG, PhN or HGN, we found an overlapping distribution of ORX-immunoreactive axon terminals and FG-labeled neurons in the KFN. Within the neuropil of the KFN, asymmetrical synaptic contacts were made between these terminals and neurons. We further demonstrated that many neurons labeled with FG injected into the rVRG, PhN, or HGN are immunoreactive for ORX receptor 2. Present data suggest that rVRG-, PhN- and HGN-projecting KFN neurons may be under the excitatory influence of the ORXergic neurons for the state-dependent regulation of respiration.

Decreased orexin (hypocretin) immunoreactivity in the hypothalamus and pontine nuclei in sudden infant death syndrome

Infants at risk of sudden infant death syndrome (SIDS) have been shown to have dysfunctional sleep and poor arousal thresholds. In animal studies, both these attributes have been linked to impaired signalling of the neuropeptide orexin. This study examined the immunoreactivity of orexin (OxA and OxB) in the tuberal hypothalamus (n = 27) and the pons (n = 15) of infants (1-10 months) who died from SIDS compared to age-matched non-SIDS infants. The percentage of orexin immunoreactive neurons and the total number of neurons were quantified in the dorsomedial, perifornical and lateral hypothalamus at three levels of the tuberal hypothalamus. In the pons, the area of orexin immunoreactive fibres were quantified in the locus coeruleus (LC), dorsal raphe (DR), laterodorsal tegmental (LDT), medial parabrachial, dorsal tegmental (DTg) and pontine nuclei (Pn) using automated methods. OxA and OxB were co-expressed in all hypothalamic and pontine nuclei examined. In SIDS infants, orexin immunoreactivity was decreased by up to 21 % within each of the three levels of the hypothalamus compared to non-SIDS (p ≤ 0.050). In the pons, a 40-50 % decrease in OxA occurred in the all pontine nuclei, while a similar decrease in OxB immunoreactivity was observed in the LC, LDT, DTg and Pn (p ≤ 0.025). No correlations were found between the decreased orexin immunoreactivity and previously identified risk factors for SIDS, including prone sleeping position and cigarette smoke exposure. This finding of reduced orexin immunoreactivity in SIDS infants may be associated with sleep dysfunction and impaired arousal.

Organization of the orexin/hypocretin system in the brain of two basal actinopterygian fishes, the cladistians Polypterus senegalus and Erpetoichthys calabaricus

Cladistians are primitive actinopterygian fishes mostly neglected in neuroanatomical studies. In the present study, the detailed neuroanatomical distribution of orexin (hypocretin)-like immunoreactive (OX-ir) cell bodies and fibers was analyzed in the brain of two species representative of the two extant genera of cladistians. Antibodies against mammalian orexin-A and orexin-B peptides were used. Simultaneous detection of orexins with neuropeptide Y (NPY), tyrosine hydroxylase (TH), and serotonin (5-HT) was used to establish accurately the topography of the orexin system and to evaluate the possible interactions with NPY and monoaminergic systems. A largely common pattern of OX-ir distribution in the two cladistian species was observed. Most OX-ir cells were located in the suprachiasmatic nucleus and tuberal hypothalamus, whereas scarce cells were observed in the posterior tubercle. In addition, a population of OX-ir cells was found in the preoptic area only in Polypterus and some cells also contained TH. The observed widespread distribution of OX-ir fibers was especially abundant in the retrobulbar area, subpallial areas, preoptic area, suprachiasmatic nucleus, tuberal hypothalamic area, prethalamus, thalamus, pretectum, optic tectum, and tegmentum. Low innervation was found in relation to monoaminergic cell groups, whereas a high NPY innervation was observed in all OX-ir cell groups. These relationships would represent the anatomical substrate for the functional interdependence between these systems. The organization of the orexin system in cladistians revealed a pattern largely consistent with those reported for all studied groups of vertebrates, suggesting that the primitive organization of this peptidergic system occurred in the common ancestor of gnathostome vertebrates.

The hypocretins (orexins) mediate the "phasic" components of REM sleep: A new hypothesis

In 1998, a group of phenotypically distinct neurons were discovered in the postero-lateral hypothalamus which contained the neuropeptides hypocretin 1 and hypocretin 2 (also called orexin A and orexin B), which are excitatory neuromodulators. Hypocretinergic neurons project throughout the central nervous system and have been involved in the generation and maintenance of wakefulness. The sleep disorder narcolepsy, characterized by hypersomnia and cataplexy, is produced by degeneration of these neurons. The hypocretinergic neurons are active during wakefulness in conjunction with the presence of motor activity that occurs during survival-related behaviors. These neurons decrease their firing rate during non-REM sleep; however there is still controversy upon the activity and role of these neurons during REM sleep. Hence, in the present report we conducted a critical review of the literature of the hypocretinergic system during REM sleep, and hypothesize a possible role of this system in the generation of REM sleep.

Co-localization of hypocretin-1 and leucine-enkephalin in hypothalamic neurons projecting to the nucleus of the solitary tract and their effect on arterial pressure

Experiments were done to investigate whether hypothalamic hypocretin-1 (hcrt-1; orexin-A) neurons that sent axonal projections to cardiovascular responsive sites in the nucleus of the solitary tract (NTS) co-expressed leucine-enkephalin (L-Enk), and to determine the effects of co-administration of hcrt-1 and D-Ala2,D-Leu5-Enkephalin (DADL) into NTS on mean arterial pressure (MAP) and heart rate. In the first series, in the Wistar rat the retrograde tract-tracer fluorogold (FG) was microinjected (50nl) into caudal NTS sites at which L-glutamate (0.25 M; 10 nl) elicited decreases in MAP and where fibers hcrt-1 immunoreactive fibers were observed that also contained L-Enk immunoreactivity. Of the number of hypothalamic hcrt-1 immunoreactive neurons identified ipsilateral to the NTS injection site (1207 ± 78), 32.3 ± 2.3% co-expressed L-Enk immunoreactivity and of these, 2.6 ± 1.1% were retrogradely labeled with FG. Hcrt-1/L-Enk neurons projecting to NTS were found mainly within the perifornical region. In the second series, the region of caudal NTS found to contain axons that co-expressed hcrt-1 and L-Enk immunoreactivity was microinjected with a combination of hcrt-1 and DADL in α-chloralose anesthetized Wistar rats. Microinjection of DADL into NTS elicited depressor and bradycardia responses similar to those elicited by microinjection of hcrt-1. An hcrt-1 injection immediately after the DADL injection elicited an almost twofold increase in the magnitude of the depressor and bradycardia responses compared to those elicited by hcrt-1 alone. Prior injections of the non-specific opioid receptor antagonist naloxone or the specific opioid δ-receptor antagonist ICI 154,129 significantly attenuated the cardiovascular responses to the combined hcrt-1-DADL injections. Taken together, these data suggest that activation of hypothalamic-opioidergic neuronal systems contribute to the NTS hcrt-1 induced cardiovascular responses, and that this descending hypothalamo-medullary pathway may represent the anatomical substrate by which hcrt-1/L-Enk neurons function in the coordination of autonomic-cardiovascular responses during different behavioral states.

A unified survival theory of the functioning of the hypocretinergic system

This article advances the theory that the hypocretinergic (orexinergic) system initiates, coordinates, and maintains survival behaviors and survival-related processes (i.e., the Unified Survival Theory of the Functioning of the Hypocretinergic System or "Unified Hypocretinergic Survival Theory"). A priori presumptive support for the Unified Hypocretinergic Survival Theory emanates from the fact that neurons that contain hypocretin are located in the key executive central nervous system (CNS) site, the lateral hypothalamus, that for decades has been well-documented to govern core survival behaviors such as fight, flight, and food consumption. In addition, the hypocretinergic system exhibits the requisite morphological and electrophysiological capabilities to control survival behaviors and related processes. Complementary behavioral data demonstrate that all facets of "survival" are coordinated by the hypocretinergic system and that hypocretinergic directives are not promulgated except during survival behaviors. Importantly, it has been shown that survival behaviors are selectively impacted when the hypocretinergic system is impaired or rendered nonfunctional, whereas other behaviors are relatively unaffected. The Unified Hypocretinergic Survival Theory resolves the disparate, perplexing, and often paradoxical-appearing results of previous studies; it also provides a foundation for future hypothesis-driven basic science and clinical explorations of the hypocretinergic system.

Role of orexin receptors in obesity: from cellular to behavioral evidence

The orexin peptides and their two receptors are involved in multiple physiological processes, including energy homeostasis, arousal, stress and reward. Higher signaling of the orexin peptides at the orexin receptors (OXR) protects against obesity, but it is less clear how their activation in different brain regions contributes to this behavioral output. This review summarizes the evidence available for a role of central OXR in energy homeostasis and their contribution to obesity. A detailed analysis of anatomical, cellular and behavioral evidence shows that modulation of energy homeostasis by the OXR is largely dependent upon anatomical and cellular context. It also shows that obesity resistance provided by activation of the OXR is distributed across multiple brain sites with site-specific actions. We suggest that understanding the role of the OXR in the development of obesity requires considering both specific mechanisms within brain regions and interactions of orexinergic input between multiple sites.

Molecular and neuroanatomical characterization of single neurons in the mouse medullary gigantocellular reticular nucleus

Medullary gigantocellular reticular nucleus (mGi) neurons have been ascribed a variety of behaviors, many of which may fall under the concepts of either arousal or motivation. Despite this, many details of the connectivity of mGi neurons, particularly in reference to those neurons with ascending axons, remain unknown. To provide a neuroanatomical and molecular characterization of these cells, with reference to arousal and level-setting systems, large medullary reticular neurons were characterized with retrograde dye techniques and with real-time reverse transcriptase PCR (RT-PCR) analyses of single-neuron mRNA expression in the mouse. We have shown that receptors consistent with participation in generalized arousal are expressed by single mGi neurons and that receptors from different families of arousal-related neurotransmitters are rarely coexpressed. Through retrograde labeling, we have shown that neurons with ascending axons and neurons with descending axons tend to form like-with-like clusters, a finding that is consistent across age and gender. In comparing the two groups of retrogradely labeled neurons in neonatal animals, those neurons with axons that ascend to the midbrain show markers for GABAergic or coincident GABAergic and glutamatergic function; in contrast, approximately 60% of the neurons with axons that descend to the spinal cord are glutamatergic. We discuss the mGi's relationship to the voluntary and emotional motor systems and speculate that neurons in the mGi may represent a mammalian analogue to Mauthner cells, with a separation of function for neurons with ascending and descending axons.

Hypocretinergic neurons are activated in conjunction with goal-oriented survival-related motor behaviors

Hypocretinergic neurons are located in the area of the lateral hypothalamus which is responsible for mediating goal-directed, survival-related behaviors. Consequently, we hypothesize that the hypocretinergic system functions to promote these behaviors including those patterns of somatomotor activation upon which they are based. Further, we hypothesize that the hypocretinergic system is not involved with repetitive motor activities unless they occur in conjunction with the goal-oriented behaviors that are governed by the lateral hypothalamus. In order to determine the veracity of these hypotheses, we examined Fos immunoreactivity (as a marker of neuronal activity) in hypocretinergic neurons in the cat during: a) Exploratory Motor Activity; b) Locomotion without Reward; c) Locomotion with Reward; and d) Wakefulness without Motor Activity. Significantly greater numbers of hypocretinergic neurons expressed c-fos when the animals were exploring an unknown environment during Exploratory Motor Activity compared with all other paradigms. In addition, a larger number of Hcrt+Fos+neurons were activated during Locomotion with Reward than during Wakefulness without Motor Activity. Finally, very few hypocretinergic neurons were activated during Locomotion without Reward and Wakefulness without Motor Activity, wherein there was an absence of goal-directed activities. We conclude that the hypocretinergic system does not promote wakefulness per se or motor activity per se but is responsible for mediating specific goal-oriented behaviors that take place during wakefulness. Accordingly, we suggest that the hypocretinergic system is responsible for controlling the somatomotor system and coordinating its activity with other systems in order to produce successful goal-oriented survival-related behaviors that are controlled by the lateral hypothalamus.

The injection of hypocretin-1 into the nucleus pontis oralis induces either active sleep or wakefulness depending on the behavioral state when it is administered

STUDY OBJECTIVES

We previously reported that the microinjection of hypocretin (orexin) into the nucleus pontis oralis (NPO) induces a behavioral state that is comparable to naturally occurring active (rapid eye movement) sleep. However, other laboratories have found that wakefulness occurs following injections of hypocretin into the NPO. The present study tested the hypothesis that the discrepancy in behavioral state responses to hypocretin injections is due to the fact that hypocretin was not administered during the same states of sleep or wakefulness.

DESIGN

Adult cats were implanted with electrodes to record sleep and waking states. Hypocretin-1 (0.25 microL, 500microM) was microinjected into the NPO while the animals were awake or in quiet (non-rapid eye movement) sleep.

MEASUREMENTS AND RESULTS

When hyprocretin-1 was microinjected into the NPO during quiet sleep, active sleep occurred with a short latency. In addition, there was a significant increase in the time spent in active sleep and in the number of episodes of this state. On the other hand, the injection of hyprocretin-1 during wakefulness resulted not only in a significant increase in wakefulness, but also in a decrease in the percentage and frequency of episodes of active sleep.

CONCLUSIONS

The present data demonstrate that the behavioral state of the animal dictates whether active sleep or wakefulness is induced following the injection of hypocretin. Therefore, we suggest that hypocretin-1 enhances ongoing states of wakefulness and their accompanying patterns of physiologic activity and that hypocretin-1 is also capable of promoting active sleep and the changes in various processes that occur during this state.

Distribution of orexin-A immunoreactive neurons and their terminal networks in the brain of the rock hyrax, Procavia capensis

The present study describes the distribution of orexin-A immunoreactive neurons and terminal networks in relation to the previously described catecholaminergic, cholinergic and serotonergic systems within the brain of the rock hyrax, Procavia capensis. Adult female rock hyrax brains were sectioned and immunohistochemically stained with an antibody to orexin-A. The staining revealed that the neurons were mainly located within the hypothalamus as with other mammals. The orexinergic terminal network distribution also resembled the typical mammalian plan. High-density orexinergic terminal networks were located within regions of the diencephalon (e.g. paraventricular nuclei), midbrain (e.g. serotonergic nuclei) and pons (locus coeruleus), while medium density orexinergic terminal networks were evident in the telencephalic (e.g. basal forebrain), diencephalic (e.g. hypothalamus), midbrain (e.g. periaqueductal gray matter), pontine (e.g. serotonergic nuclei) and medullary regions (e.g. serotonergic and catecholaminergic nuclei). Although the distribution of the orexinergic terminal networks was typically mammalian, the rock hyrax did show one atypical feature, the presence of a high-density orexinergic terminal network within the anterodorsal nucleus of the dorsal thalamus (AD). The dense orexinergic innervation of the AD nucleus has only been reported previously in the Nile grass rat, Arvicanthis niloticus and Syrian hamster, Mesocricetus auratus, both diurnal mammals. It is possible that orexinergic innervation of the AD nucleus might be a unique feature associated with diurnal mammals. It was also noted that the dense orexinergic innervation of the AD nucleus coincided with previously identified cholinergic neurons and terminal networks in this particular nucleus of the rock hyrax brain. It is possible that this dense orexinergic innervation of the AD nucleus in the brain of the rock hyrax may act in concert with the cholinergic neurons and/or the cholinergic axonal terminals, which in turn may influence arousal states and motivational processing.

Dual projections of tuberomammillary neurons to whisker-related, sensory and motor regions of the rat

The primary goal of this study was to examine in the rat whether neurons in the tuberomammillary nucleus (TMN) provide axon collaterals to whisker-related, sensorimotor regions at cortical and brainstem levels, using two different retrograde tracers. When injections were made at primary sensory (S1) barrel field/primary whisker motor (M1) cortices, dual-projecting TMN neurons were observed mainly in the ventrolateral subdivision; the projection was almost exclusively ipsilateral. On the other hand, following injections of tracers into whisker-related, principal sensory trigeminal (Pr5)/lateral facial motor (Mo7) nuclei, dual-projecting cells were observed mainly in the dorsomedial subdivision; the projection was bilateral with a slight contralateral dominance. Taken together, the present observation demonstrated that each subdivision of the TMN possessed a differential functional organization with respect to its collateral projection to whisker-related sensorimotor targets, suggesting that the histaminergic projection might play a modulatory role in vibrissal sensorimotor integration, which allows the guidance of behavioral action essential for the survival of the animal.

Hypocretin/Orexin neuropeptides: participation in the control of sleep-wakefulness cycle and energy homeostasis

Hypocretins or orexins (Hcrt/Orx) are hypothalamic neuropeptides that are synthesized by neurons located mainly in the perifornical area of the posterolateral hypothalamus. These hypothalamic neurons are the origin of an extensive and divergent projection system innervating numerous structures of the central nervous system. In recent years it has become clear that these neuropeptides are involved in the regulation of many organic functions, such as feeding, thermoregulation and neuroendocrine and cardiovascular control, as well as in the control of the sleep-wakefulness cycle. In this respect, Hcrt/Orx activate two subtypes of G protein-coupled receptors (Hcrt/Orx1R and Hcrt/Orx2R) that show a partly segregated and prominent distribution in neural structures involved in sleep-wakefulness regulation. Wakefulness-enhancing and/or sleep-suppressing actions of Hcrt/Orx have been reported in specific areas of the brainstem. Moreover, presently there are animal models of human narcolepsy consisting in modifications of Hcrt/Orx receptors or absence of these peptides. This strongly suggests that narcolepsy is the direct consequence of a hypofunction of the Hcrt/Orx system, which is most likely due to Hcrt/Orx neurons degeneration.

The main focus of this review is to update and illustrate the available data on the actions of Hcrt/Orx neuropeptides with special interest in their participation in the control of the sleep-wakefulness cycle and the regulation of energy homeostasis. Current pharmacological treatment of narcolepsy is also discussed.

Cross-talk between orexins (hypocretins) and the neuroendocrine axes (hypothalamic-pituitary axes)

Lesioning and electrical stimulation experiments carried out during the first half of the twentieth century showed that the lateral hypothalamic area (LHA) is involved in the neuroendocrine control of hormone secretion. However, the molecular basis of this phenomenon remained unclear until fifty years later when in 1998, two different laboratories discovered a new family of hypothalamic neuropeptides, the orexins or hypocretins (OX-A/Hcrt1 and OX-B/Hcrt2). Since then, remarkable evidence has revealed that orexins/hypocretins play a prominent role in regulating virtually all the neuroendocrine axes, acting as pivotal signals in the coordination of endocrine responses with regards to sleep, arousal and energy homeostasis. The clinical relevance of these actions is supported by human data showing impairment of virtually all the neuroendocrine axes in orexin/hypocretin-deficient narcoleptic patients. Here, we summarize more than ten years of knowledge about the orexins/hypocretins with particular focus on their role as neuroendocrine regulators. Understanding this aspect of orexin/hypocretin physiology could open new therapeutic possibilities in the treatment of sleep, energy homeostasis and endocrine pathologies.

Distribution of hypothalamic neurons with orexin (hypocretin) or melanin concentrating hormone (MCH) immunoreactivity and multisynaptic connections with diaphragm motoneurons

Prior work showed that neurons in the lateral, dorsal, and perifornical regions of the tuberal and mammillary levels of the hypothalamus participate in the control of breathing. The same areas also contain large numbers of neurons that produce either orexins (hypocretins) or melanin concentrating hormone (MCH). These peptides have been implicated in regulating energy balance and physiological changes that occur in transitions between sleep and wakefulness, amongst other functions. The goal of this study was to determine if hypothalamic neurons involved in respiratory control, which were identified in cats by the retrograde transneuronal transport of rabies virus from the diaphragm, were immunopositive for either orexin-A or MCH. In animals with limited rabies infection of the hypothalamus (<10 infected cells/section), where the neurons with the most direct influences on diaphragm motoneurons were presumably labeled, a large fraction (28-75%) of the infected hypothalamic neurons contained orexin-A. In the same cases, 6-33% of rabies-infected hypothalamic cells contained MCH. However, in animals with more extensive infection, where rabies had presumably passed transneuronally through more synapses, the fraction of infected cells that contained orexin-A was lower. The findings from these experiments thus support the notion that hypothalamic influences on breathing are substantially mediated through orexins or MCH.

Orexin-A inputs onto visuomotor cell groups in the monkey brainstem

Orexin-A, synthesized by neurons of the lateral hypothalamus helps to maintain wakefulness through excitatory projections to nuclei involved in arousal. Obvious changes in eye movements, eyelid position and pupil reactions seen in the transition to sleep led to the investigation of orexin-A projections to visuomotor cell groups to determine whether direct pathways exist that may modify visuomotor behaviors during the sleep-wake cycle. Histological markers were used to define these specific visuomotor cell groups in monkey brainstem sections and combined with orexin-A immunostaining. The dense supply by orexin-A boutons around adjacent neurons in the dorsal raphe nucleus served as a control standard for a strong orexin-A input. The quantitative analysis assessing various functional cell groups of the oculomotor system revealed that almost no input from orexin-A terminals reached motoneurons supplying the singly-innervated muscle fibers of the extraocular muscles in the oculomotor nucleus, the omnipause neurons in the nucleus raphe interpositus and the premotor neurons in the rostral interstitial nucleus of the medial longitudinal fasciculus. In contrast, the motoneurons supplying the multiply-innervated muscle fibers of the extraocular muscles, the motoneurons of the levator palpebrae muscle in the central caudal nucleus, and especially the preganglionic neurons supplying the ciliary ganglion received a strong orexin input. We interpret these results as evidence that orexin-A does modulate pupil size, lid position, and possibly convergence and eye alignment via the motoneurons of multiply-innervated muscle fibres. However orexin-A does not directly modulate premotor pathways for saccades or the singly-innervated muscle fibre motoneurons.

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

We have previously found that neonatal treatment with clomipramine (CLI) induced a decrease in brain orexins during the juvenile period and that these changes were reversed at adulthood. This study investigated the effect of CLI on the orexinergic component and sleep/wake states. Two groups of adult male rats were conducted for 48-h polysomnographic recording. One group of rats was treated with CLI (20 mg/kg every 12 h), and a second group was treated with equivolume of saline (SAL) simultaneously after the first 24 h of polysomnographic recording. Rats were killed 2 h after the third dose of treatment. Brain tissues were collected for radioimmunoassay quantification of orexins and real-time PCR analysis of prepro-orexin and orexin receptor mRNA. The CLI group had significantly shorter rapid eye movement (REM) sleep and longer REM latency compared with both the baseline day and the SAL group and had significantly less active wake and more quiet wake. Compared with the control rats, the CLI rats had significantly higher mRNA expression of prepro-orexin in the hypothalamus and the frontal cortex, but not in the hippocampus. The CLI rats also had significantly less orexin B in the hypothalamus than the control rats. These results suggest that suppression of active wake and orexin B by CLI may be a factor responsible for CLI-induced depression and that the increase of prepro-orexin mRNA may be a sign of increased brain orexins found in this model.

Comparative immunohistochemical analysis of the distribution of orexins (hypocretins) in the brain of amphibians

The orexins (hypocretins) are peptides found primarily in neurons of the hypothalamus of all vertebrates. Many differences were reported about the precise location of orexin containing cells and their projections throughout the brain in different species. However, there are few direct cross-species comparisons. Previous studies in anuran amphibians have also reported notable species differences. We examined and directly compared the distribution of orexinergic neurons and fibers within the brains of representatives of the three amphibian orders, anurans, urodeles and gymnophionans. Simultaneous detection of orexins and tyrosine hydroxylase was used to assess the precise location of the orexins in the brain and to evaluate the possible influence of the orexin system on the catecholaminergic cell groups. Although some differences were noted, a common pattern for the distribution of orexins in amphibians was observed. In all species, most immunoreactive neurons were observed in the suprachiasmatic nucleus, whereas the cells in the preoptic area and the tuberal region were more variable. Orexin immunoreactive fibers in the brain of all species included abundant fibers throughout the preoptic area and hypothalamus, whereas moderate amounts of fibers were present in the pallium, striatum, septum, thalamus, optic tectum, torus semicircularis, rhombencephalon and spinal cord. The use of double immunohistochemistry in amphibians revealed orexinergic innervation in dopaminergic and noradrenergic cell groups, such as the midbrain tegmentum, locus coeruleus and nucleus of the solitary tract, as was previously reported in mammals.

Neuronal mechanisms of active (rapid eye movement) sleep induced by microinjections of hypocretin into the nucleus pontis oralis of the cat

Hypocretinergic (orexinergic) neurons in the hypothalamus project to the nucleus pontis oralis, a nucleus which plays a crucial role in the generation of active (rapid eye movement) sleep. We recently reported that the microinjection of hypocretin into the nucleus pontis oralis of chronically-instrumented, unanesthetized cats induces a behavioral state that is comparable to naturally-occurring active sleep. The present study examined the intracellular signaling pathways underlying the active sleep-inducing effects of hypocretin. Accordingly, hypocretin-1, a protein kinase C inhibitor and a protein kinase A inhibitor were injected into the nucleus pontis oralis in selected combinations in order to determine their effects on sleep and waking states of chronically instrumented, unanesthetized cats. Microinjections of hypocretin-1 into the nucleus pontis oralis elicited active sleep with a short latency. However, a pre-injection of bisindolylmaleimide-I, a protein kinase C-specific inhibitor, completely blocked the active sleep-inducing effects of hypocretin-1. The combined injection of bisindolylmaleimide-I and hypocretin-1 significantly increased the latency to active sleep induced by hypocretin-1; it also abolished the increase in the time spent in active sleep induced by hypocretin-1. On the other hand, the injection of 2'5'-dideoxyadenosine, an adenylyl cyclase inhibitor, did not block the occurrence of active sleep by hypocretin-1. We conclude that the active sleep-inducing effect of hypocretin in the nucleus pontis oralis is mediated by intracellular signaling pathways that act via G-protein stimulation of protein kinase C.

Age-related changes of hypocretin in basal forebrain of guinea pig

Hypocretin-1 (hcrt-1) and hypocretin-2 (hcrt-2) have been implicated in a wide variety of functions including sleep and wakefulness as well as related behaviors. Many of these functions of the hypocretins involve the activation of cholinergic neurons in the basal forebrain (BF). These neurons have been shown to exhibit age-related changes in a variety of species. In the present experiment, in adult and aged guinea pigs, we compared hypocretin immunoreactivity in regions of the BF that include the medial septal nucleus (MS), the vertical and horizontal limbs of the diagonal band of Broca (VDB and HDB) and the magocellular preoptic nucleus (MCPO). In adult guinea pigs (3-5 months of age), all of the preceding BF regions contained dense hypocretin fibers with varicosities. On the contrary, in old guinea pigs (27-28 months), although the MS exhibited a similar intensity of hypocretin immunoreactivity compared with the adult guinea pig, there was a significant decrease in the intensity of immunoreactivity of hypocretinergic fibers in the VDB, HDB and MCPO. These data indicate that the hypocretinergic innervation of specific nuclei of the BF is compromised during the aging process. We suggest that the reduction in hypocretinergic innervation of the BF nuclei may contribute to the age-related changes in the states of sleep and wakefulness as well as deficits in related systems that occur in old age.

Age-related ultrastructural changes in hypocretinergic terminals in the brainstem and spinal cord of cats

In a previous study, we noted the presence of enlarged "spot-like" structures that are immunoreactive to hypocretin in specific regions of brainstem of aged cats; similar structures were not seen in the same regions of adult cats. In the present study, electron microscopy was combined with hypocretin immunohistochemistry to examine the ultrastructure of these enlarged "spot-like" structures, which were found to consist of enlarged axonal terminals. The terminals were comprised of a large pale core with a dark peripheral rim. Many granules were present within the core; most of these granular structures were not immunostained. On the other hand, the rim contained a high density of hypocretin immunoreactivity; in this peripheral region, large dense-core vesicles and small synaptic vesicles without dense cores were observed. In addition, mitochondria in the peripheral rim region exhibited high electron density, which is indicative of the presence of age-related degenerative changes. Synaptic contacts were observed between the enlarged terminals and adjacent dendrites. Most of these synapses were asymmetric (Gray type I), although a few of them were symmetric (Gray type II). These data suggest that hypocretin transmission is altered during aging, which would be expected to result in age-related changes in the functioning of the hypocretinergic system.

Afferent projections to the pontine micturition center in the cat

The pontine micturition center (PMC) or Barrington's nucleus controls micturition by way of its descending projections to the sacral spinal cord. However, little is known about the afferents to the PMC that control its function and may be responsible for dysfunction in patients with urge-incontinence and overactive bladder. In five female cats, wheatgerm agglutinin-conjugated horseradish peroxidase (WGA-HRP) injections were made in the PMC and adjoining dorsolateral pontine tegmentum. Retrogradely labeled neurons were found in a large area, including the medullary and pontine medial and lateral tegmental field; dorsomedial, lateral, and ventrolateral periaqueductal gray matter (PAG); posterior hypothalamus; medial preoptic area (MPO); bed nucleus of the stria terminalis; central nucleus of the amygdala; and infralimbic, prelimbic, and insular cortices. To verify whether these areas indeed project specifically to the PMC or perhaps only to adjacent structures in the pontine tegmentum, in 67 cats (3)H-leucine or WGA-HRP injections were made in each of these regions. Five cell groups appeared to have direct connections to the PMC, the ventromedial pontomedullary tegmental field, the ventrolateral and dorsomedial PAG, the MPO, and the posterior hypothalamus. The possible functions of these projections are discussed. These results indicate that all other parts of the brain that influence micturition have no direct connection with the PMC.

Hypocretinergic control of spinal cord motoneurons

Hypocretinergic (orexinergic) neurons in the lateral hypothalamus project to motor columns in the lumbar spinal cord. Consequently, we sought to determine whether the hypocretinergic system modulates the electrical activity of motoneurons. Using in vivo intracellular recording techniques, we examined the response of spinal motoneurons in the cat to electrical stimulation of the lateral hypothalamus. In addition, we examined the membrane potential response to orthodromic stimulation and intracellular current injection before and after both hypothalamic stimulation and the juxtacellular application of hypocretin-1. It was found that (1) hypothalamic stimulation produced a complex sequence of depolarizing- hyperpolarizing potentials in spinal motoneurons; (2) the depolarizing potentials decreased in amplitude after the application of SB-334867, a hypocretin type 1 receptor antagonist; (3) the EPSP induced by dorsal root stimulation was not affected by the application of SB-334867; (4) subthreshold stimulation of dorsal roots and intracellular depolarizing current steps produced spike potentials when applied in concert to stimulation of the hypothalamus or after the local application of hypocretin-1; (5) the juxtacellular application of hypocretin-1 induced motoneuron depolarization and, frequently, high-frequency discharge; (6) hypocretin-1 produced a significant decrease in rheobase (36%), membrane time constant (16.4%), and the equalizing time constant (23.3%); (7) in a small number of motoneurons, hypocretin-1 produced an increase in the synaptic noise; and (8) the input resistance was not affected after hypocretin-1. The juxtacellular application of vehicle (saline) and denatured hypocretin-1 did not produce changes in the preceding electrophysiological properties. We conclude that hypothalamic hypocretinergic neurons are capable of modulating the activity of lumbar motoneurons through presynaptic and postsynaptic mechanisms. The lack of hypocretin-induced facilitation of motoneurons may be a critical component of the pathophysiology of cataplexy.

Distribution of hypocretin (orexin) immunoreactivity in the feline pons and medulla

The distribution of hypocretin-1 (hcrt-1) and hypocretin-2 (hcrt-2) immunoreactivities in the cat brainstem was examined using immunohistochemical techniques. Hcrt-1- and hcrt-2-positive fibers with varicosities were detected in almost all brainstem regions. However, no hcrt-1- or hcrt-2-immunoreactive neuronal somata were observed in the cat brainstem. Both hcrt-1- and hcrt-2-labeled fibers exhibited different densities in distinct regions of the brainstem. In most brainstem regions, the intensity of hcrt-1 immunoreactivity was higher than that of hcrt-2 immunoreactivity. The highest densities of hcrt-1- and hcrt-2-positive fibers were found in the nucleus raphe dorsalis (RD), the laterodorsal tegmental nucleus (LDT) and the locus coeruleus (LC), suggesting an important role for these peptides in functions related to sleep-wake behavior.

Cardiovascular responses to hypocretin-1 in nucleus ambiguus of the ovariectomized female rat

Experiments were done to investigate the effect of chronic estrogen (E; 30 pg/ml plasma) treatment (15-25 days) in the ovariectomized (OVX) female Wistar rat on the cardiovascular responses to hypocretin-1 (hcrt-1) in the nucleus ambiguus (Amb). Microinjections of hcrt-1 (0.5-2.5 pmol) into the external formation of Amb (Ambe) in the urethane anaesthetized, E treated OVX animal or OVX only animal, elicited a dose-related decrease in heart rate (HR). On the other hand, hcrt-1 injections into Ambe did not elicit consistent changes in mean arterial pressure (MAP). The HR response was mediated by vagal excitation as ipsilateral vagotomy abolished the bradycardia response. The bradycardia responses were consistently of greater magnitude and longer duration in the OVX+E animals compared to the OVX only female animals. Finally, it was found that the reflex bradycardia to activation of arterial baroreceptors, as a result of increasing systemic arterial pressure with phenylephrine, was only significantly potentiated in the OVX+E animals. These data suggest that hcrt-1 in the Ambe of the female elicits an increase in vagal cardiomotor neuronal activity to the heart, and that the circulating level of E alters not only the sensitivity of Ambe neurons to hcrt-1 but also the sensitivity of these neurons during activation of baroreceptor afferent inputs.

Immunoelectron microscopic examination of orexin-like immunoreactive fibers in the dorsal horn of the rat spinal cord

The ultrastructure and synaptic relationships of orexin A-like immunoreactive neuronal fibers in the dorsal horn of the rat cervical spinal cord were examined at both the light and electron microscopic levels. At the light microscopic level, many intensely immunostained orexin A-like fibers were found, while at the electron microscopic level, immunoreactivity in these fibers was mostly confined to axon terminals. Most of the axon terminals contained dense-cored vesicles. Immunoreactive and immunonegative dense-cored vesicles were occasionally found within the same orexin A-like immunoreactive axon terminals, which were often found making synapses with immunonegative dendrites. These synapses were both asymmetric and symmetric, with the asymmetric ones predominant. Orexin A-like immunoreactive processes that contained no synaptic vesicles were also found with less frequency. These processes were also observed receiving synaptic inputs from immunonegative axon terminals, but the synapses were mostly asymmetric. Sometimes, such processes were found to receive multiple synaptic inputs for which the presynaptic immunonegative axon terminals could make synapses on other immunonegative dendrites simultaneously. Occasionally, synapses between the orexin A-like immunoreactive axon terminals and orexin A-like immunoreactive processes containing no synaptic vesicles were also found. The present results provide solid morphological evidence that orexin A may be involved in pain-inhibition mechanisms in the spinal cord and suggest that this function may be complex and occur in conjunction with the regulatory effects of other neurotransmitters.

Waking selective neurons in the posterior hypothalamus and their response to histamine H3-receptor ligands: an electrophysiological study in freely moving cats

Neurons which discharge selectively during waking (waking selective) have been found in the tuberomamillary nucleus (TM) and adjacent areas of the posterior hypothalamus. Although they share some electrophysiological properties with aminergic neurons, there is no direct evidence that they are histaminergic. We have recorded from posterior hypothalamic neurons during the sleep-wake cycle in freely moving cats, and investigated the effects on waking selective neurons of specific ligands of histaminergic H3-receptors, which autoregulate the activity of histaminergic neurons. Two types of neurons were seen. Waking selective neurons, termed "waking-on (W-on)," were located exclusively within the TM and adjacent areas, and discharged at a low regular rate during waking (1.71-2.97 Hz), decreased firing during light slow wave sleep (SWS), became silent during deep SWS and paradoxical sleep (PS) and resumed their activity on, or a few seconds before, awakening. "Waking-related" neurons, located in an area dorsal to the TM, displayed a similar, although less regular, low rate of firing (1.74-5.41 Hz) and a similar discharge profile during the sleep-wake cycle; however, unlike "W-on" neurons, they did not completely stop firing during deep SWS and PS. Intramuscular (i.m.) injection of ciproxifan (an H3-receptor antagonist, 1mg/kg), significantly increased the discharge rate of W-on neurons and induced c-fos expression in histamine-immunoreactive neurons, whereas i.m. injection of imetit (an H3-receptor agonist, 1mg/kg) or microinjection of alpha-methylhistamine (another H3-receptor agonist, 0.025-0.1 microg/0.2 microl) in the vicinity of these cells significantly decreased their discharge rate. Moreover, the effect of the antagonist was reversed by the agonists and vice versa. In contrast, "waking-related" neurons were unaffected by these H3-receptor ligands. These data provide evidence for the histaminergic nature of "W-on" neurons and their role in cortical desynchronization during waking, and highlight the heterogeneity of posterior hypothalamic neuronal populations, which might serve different functions during the wakefulness.

Estrogen alters the bradycardia response to hypocretin-1 in the nucleus tractus solitarius of the ovariectomized female

Experiments were performed to investigate the effect of 17beta-estradiol (E; 30 pg/ml plasma) treatment (15-25 days) in the ovariectomized (OVX) female Wistar rat on the cardiovascular responses to hypocretin-1 (hcrt-1) in the nucleus tractus solitarius (NTS). In an initial series of experiments, the distribution of hcrt-1-like immunoreactivity within the region of the NTS was mapped in both OVX only and OVX+E animals. Hcrt-1 immunoreactivity was found throughout the NTS region in both groups of females, predominantly within the caudal interstitial, commissural, medial and lateral subnuclei of the NTS. The relative density of hcrt-1 immunoreactivity in all NTS subnuclei was similar in both female groups. Microinjections of hcrt-1 (0.5-10 pmol) into the caudal lateral and medial subnuclei of the NTS complex of the alpha-chloralose of the urethane-anaesthetized E-treated OVX rat elicited a dose-related decrease in heart rate (HR). On the other hand, although a dose-response effect on arterial pressure was evident, significant arterial pressure responses were observed only at the higher dose of hcrt-1 (>2.5 pmol). In the OVX only female rat, microinjection of hcrt-1 into similar NTS sites elicited a bradycardia and depressor response only at the highest dose of hcrt-1, and these responses were significantly smaller in magnitude than those elicited in the OVX+E animal. In addition, in the OVX only animals, a few sites within the caudal commissural subnucleus of the NTS complex were found at which hcrt-1 elicited tachycardia and pressor responses. Finally, it was found that the reflex bradycardia to the activation of arterial baroreceptors as a result of increasing systemic arterial pressure with phenylephrine (2-4 microg/kg) was significantly potentiated in the OVX+E animals only. These data suggest that hcrt-1 in the NTS of the female activates a neuronal circuit that controls the circulation and that the circulating level of E alters the sensitivity of these cardiovascular circuits to hcrt-1.

Hypocretinergic facilitation of synaptic activity of neurons in the nucleus pontis oralis of the cat

The present study was undertaken to explore the neuronal mechanisms of hypocretin actions on neurons in the nucleus pontis oralis (NPO), a nucleus which plays a key role in the generation of active (REM) sleep. Specifically, we sought to determine whether excitatory postsynaptic potentials (EPSPs) evoked by stimulation of the laterodorsal tegmental nucleus (LDT) and spontaneous EPSPs in NPO neurons are modulated by hypocretin. Accordingly, recordings were obtained from NPO neurons in the cat in conjunction with the juxtacellular microinjection of hypocretin-1 onto intracellularly recorded cells. The application of hypocretin-1 significantly increased the mean amplitude of LDT-evoked EPSPs of NPO neurons. In addition, the frequency and the amplitude of spontaneous EPSPs in NPO neurons increased following hypocretin-1 administration. These data suggest that hypocretinergic processes in the NPO are capable of modulating the activity of NPO neurons that receive excitatory cholinergic inputs from neurons in the LDT.

Electron microscopic examination of the orexin immunoreactivity in the dorsal raphe nucleus

The ultrastructure and the synaptic relationships of the orexin-A-like immunoreactive fibers in the dorsal raphe nucleus were examined with an immunoelectron microscopic method. At the electron microscopic level, most of the immunoreactive fibers, a varicosity appearance at the light microscopic level, were found as axon terminals. The large dense-cored vesicles contained in the immunoreactive axon terminals were the most intensely immunostained organellae. These axon terminals were often found to make synapses. While the axo-dendritic synapses were usually asymmetric in appearance, the axo-somatic synapses were symmetric. Orexin-A-like immunoreactive processes with no synaptic vesicles were also found. These processes often received asymmetric synapses. With less frequency, the synapses were found between the orexin-like immunoreactive processes. The results suggest that the orexin peptides are stored in the large dense-cored vesicles; the orexin-containing fibers may have influences on the physiological activities of the dorsal raphe nucleus through direct synaptic relationships.

Morphological study of orexin neurons in the hypothalamus of the Long-Evans rat, with special reference to co-expression of orexin and NADPH-diaphorase or nitric oxide synthase activities

Orexins, novel neuropeptides, are exclusively localized in the hypothalamus and implicated in the regulation of a variety of activities, including food intake and energy balance. Nitric oxide (NO), an unconventional neurotransmitter, is widely present in numerous brain regions including the hypothalamus, and has similar physiological roles to those of the orexins. The present study was undertaken to examine the distribution of orexin neurons and the presence of neuronal nitric oxide synthase (nNOS) in the orexin neurons to clarify whether NO interacts with the orexins in the neuronal regulation activities in the Long-Evans rat. We used two double-labeling methods: nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d) histochemistry in combination with orexin immunohistochemistry, and double-labeling fluorescent immunohistochemistry for orexin and nNOS. The majority of the orexin immunoreactive neurons were localized mainly in the areas of the dorsomedial hypothalamic nucleus (DMN), the dorsal part of the perifornical nucleus (PEF) and lateral hypothalamic area. The orexin immunoreactive cell bodies were medium in size, and triangular, round, elliptic, and fusiform in shape. The sizes and shapes of orexin neurons in the different parts were similar. Cell bodies coexpressing the orexin and nNOS or NADPH-d were present in the areas of the DMN and the PEF, and the nerve fibers containing orexin and nNOS were distributed in the DMN and PEF, arcuate nucleus (ARN) and ventromedial hypothalamic nucleus (VMH). These results provide morphological evidence that there exists a population of nNOS- or NADPH-d-/orexin-coexpressing neurons in the orexinergic cell group in the hypothalamus, and taken together with previous findings, suggest that NO may play a role in the mechanisms by which orexin neurons regulate food intake and energy balance.

Cardiovascular effects of hypocretin-1 in nucleus of the solitary tract

Experiments were done in male Wistar rats to investigate the effects of microinjection of hypocretin-1 (Hcrt-1) into the nucleus of the solitary tract (NTS) on mean arterial pressure (MAP), heart rate (HR), and the baroreflex. In the first series, the distribution of Hcrt-1-like immunoreactivity (Ir) was mapped within the region of NTS. Hcrt-1 Ir was found throughout the NTS region, predominantly within the caudal dorsolateral (Slt), medial (Sm), and interstitial subnuclei of the NTS. In the second series, in alpha-chloralose or urethane-anesthetized rats, microinjection of Hcrt-1 (0.5-5 pmol) into the caudal NTS elicited a dose-dependent decrease in MAP and HR. A mapping of the caudal NTS region showed that the largest depressor and bradycardia responses elicited by Hcrt-1 were from sites in the Slt and Sm. In addition, doses >2.5 pmol at a small number of sites localized to the caudal commissural nucleus of NTS elicited pressor and tachycardia responses. Intravenous administration of the muscarinic receptor blocker atropine methyl bromide abolished the bradycardia response and attenuated the depressor response, whereas subsequent administration of the nicotinic receptor blocker hexamethonium bromide abolished the remaining MAP response. Finally, microinjection of Hcrt-1 into the NTS significantly potentiated the reflex bradycardia to activation of arterial baroreceptors as a result of increasing MAP by systemic injections of phenylephrine (2-4 microg/kg). These results suggest that Hcrt-1 in the NTS activates neuronal circuits that increases vagal activity to the heart, inhibits sympathetic activity to the heart and vasculature, and alters the excitability of NTS neuronal circuits that reflexly control the circulation.

Reciprocal synaptic relationships between orexin- and melanin-concentrating hormone-containing neurons in the rat lateral hypothalamus: a novel circuit implicated in feeding regulation

OBJECTIVE

Both orexin (ORX)- and melanin-concentrating hormone (MCH) are expressed in different neurons in the lateral hypothalamic area (LH), and are considered to have common effects on stimulating food intake. There are no reports to demonstrate neural interactions between them at the ultrastructural level. We observed these neurons in the LH to evaluate the relationships between them.

DESIGN

We used two different types of double immunostaining to reveal the ultrastructure of both the ORX- and MCH-containing neurons. A preembedding double immunostaining technique was used to study the synaptic relationships between the two kinds of neuron.

RESULTS

The main new findings are as follows: 1) Both ORX- and MCH-containing neurons received other synaptic input and made synaptic input to other neurons; 2) Reciprocal synaptic relationships were observed between the ORX- and MCH-containing neurons.

CONCLUSION

The ORX- and MCH-containing neurons in the lateral hypothalamic area may influence food intake through synapse with each other.

Ultrastructural localization of orexin-1 receptor in pre- and post-synaptic neurons in the rat arcuate nucleus

The distribution and ultrastructural localization of the orexin-1 receptor (OX(1)R) in the rat arcuate nucleus were studied using immunocytochemical techniques. OX(1)R-containing neurons were found throughout the nucleus, but were concentrated in its posterior region. Both OX(1)R-positive perikarya and dendrites were found to receive synapses from other unknown axon terminals. In addition, a small number of OX(1)R-positive axon terminals were observed. These OX(1)R-immunoreactive axon terminals were often found to make synapses on unidentified immunonegative dendritic processes. The present results indicate that orexin may act on food intake regulating neurons through both pre- and post-synaptic OX(1)R.

Co-localization of hypocretin-1 and hypocretin-2 in the cat hypothalamus and brainstem

Hypocretin-1 (hcrt-1) and hypocretin-2 (hcrt-2) are two recently discovered hypothalamic neuropeptides. In the present study, using double immunofluorescent techniques, the co-localization of hcrt-1 and hcrt-2 was examined in neuronal soma and fibers/terminals located, respectively, in the cat hypothalamus and brainstem. In the hypothalamus, all hcrt-1 positive neuronal soma also displayed hcrt-2 immunoreactivity. In the brainstem, both hcrt-1 and hcrt-2 antibodies labeled the same fibers/terminals, indicating that hcrt-1 and hcrt-2 co-localize not only in the neuronal soma (hypothalamus) but also in their fibers/terminals (brainstem). If both peptides are released following neuronal activity, then the distinct effects of these peptides in the brain are likely to depend on the types of postsynaptic receptors that are activated.

Induction of active (REM) sleep and motor inhibition by hypocretin in the nucleus pontis oralis of the cat

Hypocretin (orexin)-containing neurons in the hypothalamus, which have been implicated in the pathology of narcolepsy, project to nuclei in the brain stem reticular formation that are involved in the control of the behavioral states of sleep and wakefulness. Among these nuclei is the nucleus pontis oralis (NPO). Consequently, the present study was undertaken to determine if the hypocretinergic system provides regulatory input to neurons in the NPO with respect to the generation of the states of sleep and wakefulness. Accordingly, polygraphic recordings and behavioral observations were obtained before and after hypocretin-1 and -2 were microinjected into the NPO in chronic, unanesthetized cats. Microinjections of either hypocretin-1 or -2 elicited, with a short latency, a state of active [rapid eye movement (REM)] sleep that appeared identical to naturally occurring active sleep. The percentage of time spent in active sleep was significantly increased. Dissociated states, which are characterized by the presence of muscle atonia without one or more of the electrophysiological correlates of active sleep, also arose following the injection. The effect of juxtacellular application of hypocretin-1 on the electrical activity of intracellularly recorded NPO neurons was then examined in the anesthetized cat. In this preparation, the application of hypocretin-1 resulted in the depolarization of NPO neurons, an increase in the frequency of their discharge and an increase in their excitability. These latter data represent the first description of the in vivo action of hypocretin on intracellularly recorded neuronal activity and provide evidence that the active sleep-inducing effects of hypocretin are due to a direct excitatory action on NPO neurons. Therefore we suggest that hypocretinergic processes in the NPO may play a role in the generation of active sleep, particularly muscle atonia and therefore are likely to be involved in the pathology of narcolepsy.

Orexin B immunoreactive fibers and terminals innervate the sensory and motor neurons of jaw-elevator muscles in the rat

The relationship between orexinergic terminals and the sensory and motor neurons of jaw-elevator muscles was examined by means of anti-orexin B (OXB) immunohistochemistry combined with horseradish peroxidase (HRP) retrograde tracing in the rat. HRP was initially injected into the jaw-elevator muscles; 48 h later the animals were sacrificed and fixed for HRP reaction and anti-OXB immunohistochemistry. OXB-like terminals were observed to distribute in the trigeminal mesencephalic nucleus (Vme) and the trigeminal motor nucleus (Vmo) where they closely contact the Vme neuronal somata and the Vmo neuronal somata and dendrites retrogradely labeled with HRP. The results of this study provide anatomical evidence of a direct OXB orexinergic innervation of the sensory and motor neurons controlling jaw-elevator muscles involved in mastication. Its functional significance related to the feeding behavior and bruxism is discussed.

Age-related changes in hypocretin (orexin) immunoreactivity in the cat brainstem

Terminals of hypothalamic hypocretin-containing neurons are observed within brainstem nuclei involved in the control of sleep and wakefulness. Because aged humans, cats and other species exhibit changes in sleep and wakefulness in old age, we were interested in examining age-related changes in hypocretin/orexin projections to the following brainstem regions which are associated with the regulation of sleep and wakefulness: the dorsal raphe nucleus, the laterodorsal tegmental nucleus, the pedunculo-pontine tegmental nucleus and the locus coeruleus. Based upon the results of immunohistochemical determinations, in all the regions examined, round or oval "spot-like" structures were observed in aged cats. Many of these "spot-like" structures resembled enlarged varicosities of a nature that would be expected to disrupt hypocretin neurotransmission. In addition, a site-specific decrease in immunostaining was observed in the locus coeruleus in old cats compared with adult controls; this result likely reflects a decrease in the number of labeled fibers, which indicates that there occurs a degeneration of hypocretinergic function in conjunction with old age. The proceeding changes may account for some of sleep-wake disturbance which are observed in aged animals as well as elderly humans.

Effects of hypocretin-saporin injections into the medial septum on sleep and hippocampal theta

Neurons containing the peptide hypocretin, also known as orexin, were recently implicated in the human sleep disorder narcolepsy. Hypocretin neurons are located only in the lateral hypothalamus from where they innervate virtually the entire brain and spinal cord. This peptide is believed to be involved in regulating feeding and wakefulness. However, to fully understand what other behaviors are regulated by this peptide it is necessary to investigate each hypocretin target site. In the present study, we focus on one hypocretin target site, the medial septum, where there is a dense collection of hypocretin-2 receptor-containing cells, and degenerating axons are present here in canines with narcolepsy [J. Neurosci. 19 (1999) 248]. We utilize a saporin toxin conjugated to the hypocretin receptor binding ligand, hypocretin-2, and find that when this toxin is injected into the medial septum, it lesions the parvalbumin and cholinergic neurons. We contrast the effects of the hypocretin-saporin with another saporin conjugated toxin, 192 IgG-saporin, that lesions only the cholinergic neurons in the basal forebrain. 192 IgG-saporin reduced theta activity, a finding consistent with previous reports [J. Neurophysiol. 79 (1998) 1633; Neurodegeneration 4 (1995) 61; Neuroscience 62 (1994) 1033]. However, hypocretin-saporin completely eliminated hippocampal theta activity by day 12, indicating that parvalbumin-containing cells in the medial septum generate theta. The daily amount of sleep and wakefulness were not different between hypocretin-saporin, 192 IgG-saporin, or saline-treated rats. The homeostatic response to 12 h prolonged wakefulness was also not affected in hypocretin-saporin lesioned rats. These findings suggest that hypocretin neurons could facilitate theta generation during episodes of purposeful behavior by activating GABAergic neurons in the MS/VDB. In this way, hypocretin, which is implicated in feeding, energy metabolism and wakefulness, serves to influence cognitive processes critical for the animal's survival.