Orexins/hypocretins regulate drinking behaviour

Inbreeding depression and runs of homozygosity islands in Asturiana de los Valles cattle breed after 30 years of selection

Inbreeding depression results in a decrease in the average phenotypic values of affected traits. It has been traditionally estimated from pedigree-based inbreeding coefficients. However, with the development of single-nucleotide polymorphism arrays, novel methods were developed for calculating the inbreeding coefficient, and consequently, inbreeding depression. The aim of the study was to analyse inbreeding depression in 6 growth and 2 reproductive traits in the Asturiana de los Valles cattle breed using both genealogical and molecular information. The pedigree group comprised 225,848 records and an average equivalent number of complete generations of 2.3. The molecular data comprised genotypes of 2693 animals using the Affymetrix medium-density chip. Using the pedigree information, three different inbreeding coefficients were estimated for the genotyped animals: the full pedigree coefficient (FPED ), and the recent and ancient inbreeding coefficients based on the information of the last three generations (FPED<3G ) and until the last three generations (FPED>3G ), respectively. Using the molecular data, seven inbreeding coefficients were calculated. Four of them were estimated based on runs of homozygosity (ROH), considering (1) the total length (FROH ), (2) segments shorter than 4 megabases (FROH<4 ), (3) between 4 and 17 megabases (FROH4-17 ), and (4) longer than 17 Mb (FROH>17 ). Additionally, the three inbreeding coefficients implemented in the Plink software (FHAT1-3 ) were estimated. Inbreeding depression was estimated using linear mixed-effects model with inbreeding coefficients used as covariates. All analysed traits (birth weight, preweaning average daily gain, weaning weight adjusted at 180 days, carcass weight, calving ease, age at first calving, calving interval) showed a statistically significant non-zero effect of inbreeding depression estimated from the pedigree group, except for the Postweaning Average Daily Gain trait. When inbreeding coefficients were based on the genomic group, statistically significant inbreeding depression was observed for two traits, Preweaning Average Daily Gain and Weaning Weight based on FROH , FROH>17 , and FHAT3 inbreeding coefficients. Nevertheless, similar to inbreeding depression estimated based on pedigree information, estimates of inbreeding depression based on genomic information had no relevant economic impact. Despite this, from a long-term perspective, genotyped data could be included to maximize genetic progress in genetic programs following an optimal genetic contribution strategy and to consider individual inbreeding load instead global inbreeding. ROH islands were identified on chromosomes 2, 3, 8, 10, and 16. Such regions contain several candidate genes for growth development, intramuscular fat, body weight and lipid metabolism that are related to production traits selected in Asturiana de los Valles breed.

Orexin deficiency modulates the dipsogenic effects of angiotensin II in a sex-dependent manner

The orexin (hypocretin) neuropeptide system is an important regulator of ingestive behaviors, i.e., it promotes food and water intake. Here, we investigated the role of orexin in drinking induced by the potent dipsogen angiotensin II (ANG II). Specifically, male and female orexin-deficient mice received intracerebroventricular (ICV) injections of ANG II, followed by measuring their water intake within 15 min. We found that lower doses of ANG II (100 ng) significantly stimulated drinking in males but not in females, indicating a general sex-dependent effect that was not affected by orexin deficiency. However, higher doses of ANG II (500 ng) were sufficient to induce drinking in female wild-type mice, while female orexin-deficient mice still did not respond to the dipsogenic properties of ANG II. In conclusion, these results suggest sex-dependent effects in ANG II-induced drinking and further support the sexual dimorphism of orexin system functions.

Dual orexin/hypocretin receptor antagonism attenuates NMDA receptor hypofunction-induced attentional impairments in a rat model of schizophrenia

Schizophrenia is a neuropsychiatric condition that is associated with impaired attentional processing and performance. Failure to support increasing attentional load may result, in part, from inhibitory failure in attention-relevant cortical regions, and available antipsychotics often fail to address this issue. Orexin/hypocretin receptors are found throughout the brain and are expressed on neurons relevant to both attention and schizophrenia, highlighting them as a potential target to treat schizophrenia-associated attentional dysfunction. In the present experiment, rats (N = 14) trained in a visual sustained attention task that required discrimination of trials which presented a visual signal from trials during which no signal was presented. Once trained, rats were then co-administered the psychotomimetic N-methyl-D-aspartate (NMDA) receptor antagonist dizocilpine (MK-801: 0 or 0.1mg/kg, intraperitoneal injections) and the dual orexin receptor antagonist filorexant (MK-6096: 0, 0.1, or 1mM, intracerebroventricular infusions) prior to task performance across six sessions. Dizocilpine impaired overall accuracy during signal trials, slowed reaction times for correctly-responded trials, and increased the number of omitted trials throughout the task. Dizocilpine-induced increases in signal trial deficits, correct response latencies, and errors of omission were reduced following infusions of the 0.1mM, but not 1mM, dose of filorexant. As such, orexin receptor blockade may improve attentional deficits in a state of NMDA receptor hypofunction.

Cellular Localization of Orexin 1 Receptor in Human Hypothalamus and Morphological Analysis of Neurons Expressing the Receptor

The orexin system is related to food behavior, energy balance, wakefulness and the reward system. It consists of the neuropeptides orexin A and B, and their receptors, orexin 1 receptor (OX1R) and orexin 2 receptor (OX2R). OX1R has selective affinity for orexin A, and is implicated in multiple functions, such as reward, emotions, and autonomic regulation. This study provides information about the OX1R distribution in human hypothalamus. The human hypothalamus, despite its small size, demonstrates a remarkable complexity in terms of cell populations and cellular morphology. Numerous studies have focused on various neurotransmitters and neuropeptides in the hypothalamus, both in animals and humans, however, there is limited experimental data on the morphological characteristics of neurons. The immunohistochemical analysis of the human hypothalamus revealed that OX1R is mainly found in the lateral hypothalamic area, the lateral preoptic nucleus, the supraoptic nucleus, the dorsomedial nucleus, the ventromedial nucleus, and the paraventricular nucleus. The rest of the hypothalamic nuclei do not express the receptor, except for a very low number of neurons in the mammillary bodies. After identifying the nuclei and neuronal groups that were immunopositive for OX1R, a morphological and morphometric analysis of those neurons was conducted using the Golgi method. The analysis revealed that the neurons in the lateral hypothalamic area were uniform in terms of their morphological characteristics, often forming small groups of three to four neurons. A high proportion of neurons in this area (over 80%) expressed the OX1R, with particularly high expression in the lateral tuberal nucleus (over 95% of neurons). These results were analyzed, and shown to represent, at the cellular level, the distribution of OX1R, and we discuss the regulatory role of orexin A in the intra-hypothalamic areas, such as its special role in the plasticity of neurons, as well as in neuronal networks of the human hypothalamus.

Dual orexin/hypocretin receptor antagonism attenuates attentional impairments in an NMDA receptor hypofunction model of schizophrenia

Schizophrenia is a neuropsychiatric condition that is associated with impaired attentional processing and performance. Failure to support increasing attentional load may result, in part, from abnormally overactive basal forebrain projections to the prefrontal cortex, and available antipsychotics often fail to address this issue. Orexin/hypocretin receptors are expressed on corticopetal cholinergic neurons, and their blockade has been shown to decrease the activity of cortical basal forebrain outputs and prefrontal cortical cholinergic neurotransmission. In the present experiment, rats (N = 14) trained in a visual sustained attention task that required discrimination of trials which presented a visual signal from trials during which no signal was presented. Once trained, rats were then co-administered the psychotomimetic N-methyl-D-aspartate (NMDA) receptor antagonist dizocilpine (MK-801: 0 or 0.1 mg/kg, intraperitoneal injections) and the dual orexin receptor antagonist filorexant (MK-6096: 0, 0.1, or 1 mM, intracerebroventricular infusions) prior to task performance across six sessions. Dizocilpine impaired overall accuracy during signal trials, slowed reaction times for correctly-responded trials, and increased the number of omitted trials throughout the task. Dizocilpine-induced increases in signal trial deficits, correct response latencies, and errors of omission were reduced following infusions of the 0.1 mM, but not 1 mM, dose of filorexant. Orexin receptor blockade, perhaps through anticholinergic mechanisms, may improve attentional deficits in a state of NMDA receptor hypofunction.

Highlights

Schizophrenia is associated with attentional deficits that may stem from abnormally reactive BF projections to the prefrontal cortexOrexin receptor antagonists decrease acetylcholine release and reduce prefrontal cortical activityThe dual orexin receptor antagonist filorexant alleviated impairments of attention following NMDA receptor blockade.

Actions of feeding-related peptides on the mesolimbic dopamine system in regulation of natural and drug rewards

The mesolimbic dopamine system is the primary neural circuit mediating motivation, reinforcement, and reward-related behavior. The activity of this system and multiple behaviors controlled by it are affected by changes in feeding and body weight, such as fasting, food restriction, or the development of obesity. Multiple different peptides and hormones that have been implicated in the control of feeding and body weight interact with the mesolimbic dopamine system to regulate many different dopamine-dependent, reward-related behaviors. In this review, we summarize the effects of a selected set of feeding-related peptides and hormones acting within the ventral tegmental area and nucleus accumbens to alter feeding, as well as food, drug, and social reward.

Orexin receptors in GtoPdb v.2021.3

Orexin receptors (nomenclature as agreed by the NC-IUPHAR Subcommittee on Orexin receptors [42]) are activated by the endogenous polypeptides orexin-A and orexin-B (also known as hypocretin-1 and -2; 33 and 28 aa) derived from a common precursor, preproorexin or orexin precursor, by proteolytic cleavage and some typical peptide modifications [109]. Currently the only orexin receptor ligands in clinical use are suvorexant and lemborexant, which are used as hypnotics. Orexin receptor crystal structures have been solved [134, 133, 54, 117, 46].

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.

Roles of central orexinergic system on cardiovascular function and acupuncture on intervention of cardiovascular risk: Orexinergic system mediate the role of acupuncture?

Central orexinergic system contributes to the regulation of cardiovascular function. Orexinergic neurons receiving projections of nerve fibers from multiple structures of brain which involved in control and regulation of cardiovascular function locate in hypothalamus, and their axon terminals widely project to various central structures where orexins receptors are expressed. Here, we summarize the present knowledge that describes the influence of central orexinergic system on cardiovascular activity, the relevance of dysfunction in central orexinergic system with hypertension and psychological stress induced cardiovascular reactivity which are serious risk factors for cardiovascular disease and cardiovascular death. We propose that central orexinergic system may be potentially important targets for the prevention of cardiovascular disease and cardiovascular death, and different orexinergic system involved neuronal circuits may be involved in distinct cardiovascular functions. Acupuncture having bidirectional regulatory ability and a much lower incidence of side effects can prevent disease. We review the improvement of acupuncture on hypertension and psychological stress induced cardiovascular reactivity. We think that acupuncture intervenes hypertension and psychological stress induced cardiovascular reactivity to prevent cardiovascular disease and cardiovascular death. We also summarize relation between acupuncture and central orexinergic system. We propose a hypothesis that acupuncture improve hypertension and psychological stress induced cardiovascular reactivity through regulating central orexinergic system. The knowledge is beneficial for the development of potential therapeutic targets and methods to prevent cardiovascular disease and cardiovascular death.

Orexin neurons couple neural systems mediating fluid balance with motivation-related circuits

During extracellular dehydration, neural systems that sense deficits in body fluid homeostasis operate in tandem with those that mediate motivation and reward in order to promote ingestive behaviors that restore fluid balance. We hypothesized that hypothalamic orexin (Ox) neurons act as an interface to couple brain regions sensing and processing information about body fluid status with central nervous system motivation and reward systems. An initial set of anterograde and retrograde tracing experiments suggested that structures along the lamina terminalis (LT), a region of the forebrain that serves to monitor and integrate information reflecting body fluid balance, project to hypothalamic Ox neurons that, in turn, project to dopamine neurons in the ventral tegmental area (VTA). A second set of experiments determined whether Ox neuron activation is associated with extracellular dehydration and the seeking out and consumption of water and saline. An elevation of Fos-like immunoreactivity in Ox neurons was observed in fluid-depleted rats that were allowed to ingest water and sodium. A final experiment was conducted to determine whether Ox release in the VTA promotes thirst and salt appetite. Bilateral microinjection of the Ox Type I receptor antagonist SB-408124 into the VTA prior to acute extracellular dehydration attenuated fluid intake. Together, these studies support the hypothesis that structures along the LT modulate activity in the VTA through actions of orexinergic neurons that have cell bodies in the hypothalamus. This pathway may function to facilitate sustained consumption of fluids necessary for restoration of fluid balance. (PsycINFO Database Record

Potential role of orexin A binding the receptor 1 for orexins in normal and cryptorchid dogs

Background

Cryptorchidism is one of the most common birth disorders of the male reproductive system identified in dogs and other mammals. This condition is characterised by the absence of one (unilateral) or both (bilateral) gonads from the scrotum. The peptides orexin A (OxA) and B (OxB) were obtained by post-transcriptional proteolytic cleavage of a precursor molecule, called prepro-orexin. These substances bind two types of G-coupled receptors called receptor 1 (OX1R) and 2 (OX2R) for orexins. OX1R is specific to OxA while OX2R binds the two peptides with equal affinity. Orexins modulate a great variety of body functions, such as the reproductive mechanism. The purpose of the present research was to study the presence of OxA and its receptor 1 and their possible involvement in the canine testis under healthy and pathological conditions.

Methods

This study was performed using adult male normal dogs and male dogs affected by unilateral cryptorchidism. Tissue samples were collected from testes and were divided into three groups: normal, contralateral and cryptic. The samples were used for immunohistochemistry, Western blot and in vitro tests for testosterone evaluation in normal and pathological conditions.

Results

OxA-immunoreactivity (IR) was described in interstitial Leydig cells of the normal gonad, and Leydig, Sertoli cells and gonocytes in the cryptic gonad. In the normal testis, OX1R-IR was described in Leydig cells, in pachytene and second spermatocytes and in immature and mature spermatids throughout the stages of the germ developing cycle of the male gonad. In the cryptic testis OX1R-IR was distributed in Leydig and Sertoli cells. The presence of prepro-orexin and OX1R was demonstrated by Western blot analysis. The incubation of fresh testis slices with OxA caused the stimulation of testosterone synthesis in the normal and cryptic gonad while the steroidogenic OxA-induced effect was cancelled by adding the selective OX1R antagonist SB-408124.

Conclusions

These results led us to hypothesise that OxA binding OX1R might be involved in the modulation of spermatogenesis and steroidogenesis in canine testis in healthy and pathological conditions.

Contribution of Dynorphin and Orexin Neuropeptide Systems to the Motivational Effects of Alcohol

Understanding the neural systems that drive alcohol motivation and are disrupted in alcohol use disorders is of critical importance in developing novel treatments. The dynorphin and orexin/hypocretin neuropeptide systems are particularly relevant with respect to alcohol use and misuse. Both systems are strongly associated with alcohol-seeking behaviors, particularly in cases of high levels of alcohol use as seen in dependence. Furthermore, both systems also play a role in stress and anxiety, indicating that disruption of these systems may underlie long-term homeostatic dysregulation seen in alcohol use disorders. These systems are also closely interrelated with one another - dynorphin/kappa opioid receptors and orexin/hypocretin receptors are found in similar regions and hypocretin/orexin neurons also express dynorphin - suggesting that these two systems may work together in the regulation of alcohol seeking and may be mutually disrupted in alcohol use disorders. This chapter reviews studies demonstrating a role for each of these systems in motivated behavior, with a focus on their roles in regulating alcohol-seeking and self-administration behaviors. Consideration is also given to evidence indicating that these neuropeptide systems may be viable targets for the development of potential treatments for alcohol use disorders.

Hypocretin/Orexin and Plastic Adaptations Associated with Drug Abuse

Dopamine neurons in the ventral tegmental area (VTA) are a critical part of the neural circuits that underlie reward learning and motivation. Dopamine neurons send dense projections throughout the brain and recent observations suggest that both the intrinsic properties and the functional output of dopamine neurons are dependent on projection target and are subject to neuromodulatory influences. Lateral hypothalamic hypocretin (also termed orexin) neurons project to the VTA and contain both hypocretin and dynorphin peptides in the same dense core vesicles suggesting they may be co-released. Hypocretin peptides act at excitatory Gαq protein-coupled receptors and dynorphin acts at inhibitory Gαi/o protein-coupled receptors, which are both expressed on subpopulations of dopamine neurons. This review describes a role for neuromodulation of dopamine neurons and the influence on motivated behaviour in response to natural and drug rewards.

Increased activity of the orexin system in the paraventricular nucleus contributes to salt-sensitive hypertension

The orexin system is involved in arginine vasopressin (AVP) regulation, and its overactivation has been implicated in hypertension. However, its role in salt-sensitive hypertension (SSHTN) is unknown. Here, we tested the hypothesis that hyperactivity of the orexin system in the paraventricular nucleus (PVN) contributes to SSHTN via enhancing AVP signaling. Eight-week-old male Dahl salt-sensitive (Dahl S) and age- and sex-matched Sprague-Dawley (SD) rats were placed on a high-salt (HS; 8% NaCl) or normal-salt (NS; 0.4% NaCl) diet for 4 wk. HS intake did not alter mean arterial pressure (MAP), PVN mRNA levels of orexin receptor 1 (OX1R), or OX2R but slightly increased PVN AVP mRNA expression in SD rats. HS diet induced significant increases in MAP and PVN mRNA levels of OX1R, OX2R, and AVP in Dahl S rats. Intracerebroventricular infusion of orexin A (0.2 nmol) dramatically increased AVP mRNA levels and immunoreactivity in the PVN of SD rats. Incubation of cultured hypothalamus neurons from newborn SD rats with orexin A increased AVP mRNA expression, which was attenuated by OX1R blockade. In addition, increased cerebrospinal fluid Na⁺ concentration through intracerebroventricular infusion of NaCl solution (4 µmol) increased PVN OX1R and AVP mRNA levels and immunoreactivity in SD rats. Furthermore, bilateral PVN microinjection of the OX1R antagonist SB-408124 resulted in a greater reduction in MAP in HS intake (-16 ± 5 mmHg) compared with NS-fed (-4 ± 4 mmHg) anesthetized Dahl S rats. These results suggest that elevated PVN OX1R activation may contribute to SSHTN by enhancing AVP signaling.NEW & NOTEWORTHY To our best knowledge, this study is the first to investigate the involvement of the orexin system in salt-sensitive hypertension. Our results suggest that the orexin system may contribute to the Dahl model of salt-sensitive hypertension by enhancing vasopressin signaling in the hypothalamic paraventricular nucleus.

Hypothalamic regulation of the sleep/wake cycle

Sleep is one of the most important physiological functions in mammals. It is regulated by not only homeostatic regulation but also circadian clock. Several neuropeptide-producing neurons located in the hypothalamus are implicated in the regulation of sleep/wakefulness. Among them, orexin/hypocretin-producing neurons (orexin neurons) are a crucial component for maintenance of wakefulness, because lack of orexin function results in narcolepsy, which is a sleep disorder. Recent findings have identified substances that excite or inhibit neural activity of orexin neurons. Furthermore neural projections of the neurons which release these substances have been revealed. In addition to orexin, melanin concentrating hormone (MCH)-producing neurons in the lateral hypothalamic area (LHA) are also implicated in the regulation of sleep/wakefulness. MCH neurons are active during sleep but become silent during wakefulness. Recently developed innovative methods including optogenetics and pharmacogenetics have provided substantial insights into the regulation of sleep/wakefulness. In vivo optical recordings and retrograde and anterograde tracing methods will allow us to understand additional details regarding important interactions between these two types of neurons in the LHA and other neurons in the brain. Finally we discuss the circadian clock and sleep/wake cycle. Understanding of the neural networks and its circadian modulation of sleep/wake cycles remain to be investigated.

The Orexin System and Hypertension

In this review, we focus on the role of orexin signaling in blood pressure control and its potential link to hypertension by summarizing evidence from several experimental animal models of hypertension. Studies using the spontaneously hypertensive rat (SHR) animal model of human essential hypertension show that pharmacological blockade of orexin receptors reduces blood pressure in SHRs but not in Wistar-Kyoto rats. In addition, increased activity of the orexin system contributes to elevated blood pressure and sympathetic nerve activity (SNA) in dark-active period Schlager hypertensive (BPH/2J) mice, another genetic model of neurogenic hypertension. Similar to these two models, Sprague-Dawley rats with stress-induced hypertension display an overactive central orexin system. Furthermore, upregulation of the orexin receptor 1 increases firing of hypothalamic paraventricular nucleus neurons, augments SNA, and contributes to hypertension in the obese Zucker rat, an animal model of obesity-related hypertension. Finally, we propose a hypothesis for the implication of the orexin system in salt-sensitive hypertension. All of this evidence, coupled with the important role of elevated SNA in increasing blood pressure, strongly suggests that hyperactivity of the orexin system contributes to hypertension.

The genome landscape of indigenous African cattle

BACKGROUND

The history of African indigenous cattle and their adaptation to environmental and human selection pressure is at the root of their remarkable diversity. Characterization of this diversity is an essential step towards understanding the genomic basis of productivity and adaptation to survival under African farming systems.

RESULTS

We analyze patterns of African cattle genetic variation by sequencing 48 genomes from five indigenous populations and comparing them to the genomes of 53 commercial taurine breeds. We find the highest genetic diversity among African zebu and sanga cattle. Our search for genomic regions under selection reveals signatures of selection for environmental adaptive traits. In particular, we identify signatures of selection including genes and/or pathways controlling anemia and feeding behavior in the trypanotolerant N'Dama, coat color and horn development in Ankole, and heat tolerance and tick resistance across African cattle especially in zebu breeds.

CONCLUSIONS

Our findings unravel at the genome-wide level, the unique adaptive diversity of African cattle while emphasizing the opportunities for sustainable improvement of livestock productivity on the continent.

Orexins (hypocretins) and energy balance: More than feeding

Initially implicated in the regulation of feeding, orexins/hypocretins are now acknowledged to play a major role in the control of a wide variety of biological processes, such as sleep, energy expenditure, pain, cardiovascular function and neuroendocrine regulation, a feature that makes them one of the most pleiotropic families of hypothalamic neuropeptides. While the orexigenic effect of orexins is well described, their central effects on energy expenditure and particularly on brown adipose tissue (BAT) thermogenesis are not totally unraveled. Better understanding of these actions and their possible interrelationship with other hypothalamic systems controlling thermogenesis, such as AMP-activated protein kinase (AMPK) and endoplasmic reticulum (ER) stress, will help to clarify the exact role and pathophysiological relevance of these neuropeptides have on energy balance.

Lateral hypothalamic orexin and melanin-concentrating hormone neurons provide direct input to gonadotropin-releasing hormone neurons in the human

Hypophysiotropic projections of gonadotropin-releasing hormone (GnRH)-synthesizing neurons form the final common output way of the hypothalamus in the neuroendocrine control of reproduction. Several peptidergic neuronal systems of the medial hypothalamus innervate human GnRH cells and mediate crucially important hormonal and metabolic signals to the reproductive axis, whereas much less is known about the contribution of the lateral hypothalamic area to the afferent control of human GnRH neurons. Orexin (ORX)- and melanin-concentrating hormone (MCH)-synthesizing neurons of this region have been implicated in diverse behavioral and autonomic processes, including sleep and wakefulness, feeding and other functions. In the present immunohistochemical study, we addressed the anatomical connectivity of these neurons to human GnRH cells in post-mortem hypothalamic samples obtained from autopsies. We found that 38.9 ± 10.3% and 17.7 ± 3.3% of GnRH-immunoreactive (IR) perikarya in the infundibular nucleus of human male subjects received ORX-IR and MCH-IR contacts, respectively. On average, each 1 mm segment of GnRH dendrites received 7.3 ± 1.1 ORX-IR and 3.7 ± 0.5 MCH-IR axo-dendritic appositions. Overall, the axo-dendritic contacts dominated over the axo-somatic contacts and represented 80.5 ± 6.4% of ORX-IR and 76.7 ± 4.6% of MCH-IR inputs to GnRH cells. Based on functional evidence from studies of laboratory animals, the direct axo-somatic and axo-dendritic input from ORX and MCH neurons to the human GnRH neuronal system may convey critical metabolic and other homeostatic signals to the reproducive axis. In this study, we also report the generation and characterization of new antibodies for immunohistochemical detection of GnRH neurons in histological sections.

The role of brain somatostatin receptor 2 in the regulation of feeding and drinking behavior

Somatostatin was discovered four decades ago as hypothalamic factor inhibiting growth hormone release. Subsequently, somatostatin was found to be widely distributed throughout the brain and to exert pleiotropic actions via interaction with five somatostatin receptors (sst1-5) that are also widely expressed throughout the brain. Interestingly, in contrast to the predominantly inhibitory actions of peripheral somatostatin, the activation of brain sst2 signaling by intracerebroventricular injection of stable somatostatin agonists potently stimulates food intake and independently, drinking behavior in rodents. The orexigenic response involves downstream orexin-1, neuropeptide Y1 and μ receptor signaling while the dipsogenic effect is mediated through the activation of the brain angiotensin 1 receptor. Brain sst2 activation is part of mechanisms underlying the stimulation of feeding and more prominently water intake in the dark phase and is able to counteract the anorexic response to visceral stressors.

To ingest or rest? Specialized roles of lateral hypothalamic area neurons in coordinating energy balance

Survival depends on an organism’s ability to sense nutrient status and accordingly regulate intake and energy expenditure behaviors. Uncoupling of energy sensing and behavior, however, underlies energy balance disorders such as anorexia or obesity. The hypothalamus regulates energy balance, and in particular the lateral hypothalamic area (LHA) is poised to coordinate peripheral cues of energy status and behaviors that impact weight, such as drinking, locomotor behavior, arousal/sleep and autonomic output. There are several populations of LHA neurons that are defined by their neuropeptide content and contribute to energy balance. LHA neurons that express the neuropeptides melanin-concentrating hormone (MCH) or orexins/hypocretins (OX) are best characterized and these neurons play important roles in regulating ingestion, arousal, locomotor behavior and autonomic function via distinct neuronal circuits. Recently, another population of LHA neurons containing the neuropeptide Neurotensin (Nts) has been implicated in coordinating anorectic stimuli and behavior to regulate hydration and energy balance. Understanding the specific roles of MCH, OX and Nts neurons in harmonizing energy sensing and behavior thus has the potential to inform pharmacological strategies to modify behaviors and treat energy balance disorders.

Orexin/hypocretin role in reward: implications for opioid and other addictions

Addiction is a devastating disorder that affects 15.3 million people worldwide. While prevalent, few effective treatments exist. Orexin receptors have been proposed as a potential target for anti-craving medications. Orexins, also known as hypocretins, are neuropeptides produced in neurons of the lateral and dorsomedial hypothalamus and perifornical area, which project widely throughout the brain. The absence of orexins in rodents and humans leads to narcolepsy. However, orexins also have an established role in reward seeking. This review will discuss some of the original studies describing the roles of the orexins in reward seeking as well as specific works that were presented at the 2013 International Narcotics Research Conference. Orexin signalling can promote drug-induced plasticity of glutamatergic synapses onto dopamine neurons of the ventral tegmental area (VTA), a brain region implicated in motivated behaviour. Additional evidence suggests that orexin signalling can also promote drug seeking by initiating an endocannabinoid-mediated synaptic depression of GABAergic inputs to the VTA, and thereby disinhibiting dopaminergic neurons. Orexin neurons co-express the inhibitory opioid peptide dynorphin. It has been proposed that orexin in the VTA may not mediate reward per se, but rather occludes the 'anti-reward' effects of dynorphin. Finally, orexin signalling in the prefrontal cortex and the central amygdala is implicated in reinstatement of reward seeking. This review will highlight recent work describing the role of orexin signalling in cellular processes underlying addiction-related behaviours and propose novel hypotheses for the mechanisms by which orexin signalling may impart drug seeking.

LINKED ARTICLES

This article is part of a themed section on Opioids: New Pathways to Functional Selectivity. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2015.172.issue-2.

The role of the lateral hypothalamus and orexin in ingestive behavior: a model for the translation of past experience and sensed deficits into motivated behaviors

The hypothalamus has been recognized for its involvement in both maintaining homeostasis and mediating motivated behaviors. The present article discusses a region of the hypothalamus known as the lateral hypothalamic area (LHA). It is proposed that brain nuclei within the LHA including the dorsal region of the lateral hypothalamus (LHAd) and perifornical area (PeF) provide a link between neural systems that regulate homeostasis and those that mediate appetitive motivated behaviors. Functional and immunohistochemical data indicate that the LHA promotes many motivated behaviors including food intake, water intake, salt intake, and sexual behavior. Anatomical tracing experiments demonstrate that the LHA is positioned to receive inputs from brain areas involved in regulating body fluid and energy homeostasis. Regions within the LHA send dense projections to the ventral tegmental area (VTA), providing a pathway for the LHA to influence dopaminergic systems generally recognized to be involved in motivated behaviors and their reinforcement. Furthermore, the LHA contains neurons that synthesize orexin/hypocretin, a neuropeptide that promotes many appetitive motivated behaviors. The LHA also receives inputs from brain areas involved in reward-related learning and orexin neuron activation can become conditioned to environmental stimuli that are associated with rewards. Therefore, it is hypothesized that the LHA integrates signaling from areas that regulate body fluid and energy balance and reward-related learning. In turn, this information is “fed into” mesolimbic circuitry to influence the performance of motivated behaviors. This hypothesis may foster experiments that will result in an improved understanding of LHA function. An improved understanding of LHA function may aid in treating disorders that are associated with an excess or impairment in the expression of ingestive behavior including obesity, anorexia, impairments in thirst, salt gluttony, and salt deficiency.

Orexin-1 receptor antagonist in central nucleus of the amygdala attenuates the acquisition of flavor-taste preference in rats

Previous studies demonstrated that the intracerebroventricular administration of SB-334867-A, a selective antagonist of orexin OX1R receptors, blocks the acquisition of saccharin-induced conditioned flavor preference (CFP) but not LiCl-induced taste aversion learning (TAL). Orexinergic fibers from the lateral hypothalamus end in the central nucleus of the amygdala (CeA), which expresses orexin OX1R receptors. Taste and sensory inputs also are present in CeA, which may contribute to the development of taste learning. This study analyzed the effect of two doses (1.5 and 6μg/0.5μl) of SB-334867-A administered into the CeA on flavor-taste preference induced by saccharin and on TAL induced by a single administration of LiCl (0.15M, 20ml/kg, i.p.). Outcomes indicate that inactivation of orexinergic receptors in the CeA attenuates flavor-taste preference in a two-bottle test (saccharin vs. water). Intra-amygdalar SB-334867-A does not affect gustatory processing or the preference for the sweet taste of saccharin given that SB-334867-A- and DMSO-treated groups (control animals) increased the intake of the saccharin-associated flavor across training acquisition sessions. Furthermore, SB-334867-A in the CeA does not block TAL acquisition ruling out the possibility that functional inactivation of OX1R receptors interferes with taste processing. Orexin receptors in the CeA appear to intervene in the association of a flavor with orosensory stimuli, e.g., a sweet and pleasant taste, but could be unnecessary when the association is established with visceral stimuli, e.g., lithium chloride. These data suggest that orexinergic projections to the CeA may contribute to the reinforcing signals facilitating the acquisition of taste learning and the change in hedonic evaluation of the taste, which would have important implications for the OX1R-targeted pharmacological treatment of eating disorders.

The orexin system in the enteric nervous system of the bottlenose dolphin (Tursiops truncatus)

This study provides a general approach to the presence and possible role of orexins and their receptors in the gut (three gastric chambers and intestine) of confined environment bottlenose dolphin. The expression of prepro-orexin, orexin A and B and orexin 1 and 2 receptors were investigated by single immunostaining and western blot analysis. The co-localization of vasoactive intestinal peptide and orexin 1 receptor in the enteric nervous system was examined by double immunostaining. Also, orexin A concentration were measured in plasma samples to assess the possible diurnal variation of the plasma level of peptide in this species. Our results showed that the orexin system is widely distributed in bottlenose dolphin enteric nervous system of the all gastrointestinal tract examined. They are very peculiar and partially differs from that of terrestrial mammals. Orexin peptides and prepro-orexin were expressed in the main stomach, pyloric stomach and proximal intestine; while orexin receptors were expressed in the all examined tracts, with the exception of main stomach where found no evidence of orexin 2 receptor. Co-localization of vasoactive intestinal peptide and orexin 1 receptor were more evident in the pyloric stomach and proximal intestine. These data could suggest a possible role of orexin system on the contractility of bottlenose dolphin gastrointestinal districts. Finally, in agreement with several reports, bottlenose dolphin orexin A plasma level was higher in the morning during fasting. Our results emphasize some common features between bottlenose dolphin and terrestrial mammals. Certainly, further functional investigations may help to better explain the role of the orexin system in the energy balance of bottlenose dolphin and the complex interaction between feeding and digestive physiology.

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

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

Orexin A-induced anxiety-like behavior is mediated through GABA-ergic, α- and β-adrenergic neurotransmissions in mice

Orexins are hypothalamic neuropeptides, which are involved in several physiological functions of the central nervous system, including anxiety and stress. Several studies provide biochemical and behavioral evidence about the anxiogenic action of orexin A. However, we have little evidence about the underlying neuromodulation. Therefore, the aim of the present study was to investigate the involvement of neurotransmitters in the orexin A-induced anxiety-like behavior in elevated plus maze (EPM) test in mice. Accordingly, mice were pretreated with a non-selective muscarinic cholinergic antagonist, atropine; a γ-aminobutyric acid subunit A (GABA-A) receptor antagonist, bicuculline; a D2, D3, D4 dopamine receptor antagonist, haloperidol; a non-specific nitric oxide synthase (NOS) inhibitor, nitro-l-arginine; a nonselective α-adrenergic receptor antagonist, phenoxybenzamine and a β-adrenergic receptor antagonist, propranolol 30min prior to the intracerebroventricular administration of orexin A. The EPM test started 30min after the i.c.v. injection of the neuropeptide. Our results show that orexin A decreases significantly the time spent in the arms (open/open+closed) and this action is reversed by bicuculline, phenoxybenzamine and propranolol, but not by atropine, haloperidol or nitro-l-arginine. Our results provide evidence for the first time that the orexin A-induced anxiety-like behavior is mediated through GABA-A-ergic, α- and β-adrenergic neurotransmissions, whereas muscarinic cholinergic, dopaminergic and nitrergic neurotransmissions may not be implicated.

Concurrent and robust regulation of feeding behaviors and metabolism by orexin neurons

Orexin neurons in the hypothalamus regulate energy homeostasis by coordinating various physiological responses. Past studies have shown the role of the orexin peptide itself; however, orexin neurons contain not only orexin but also other neurotransmitters such as glutamate and dynorphin. In this study, we examined the physiological role of orexin neurons in feeding behavior and metabolism by pharmacogenetic activation and chronic ablation. We generated novel orexin-Cre mice and utilized Cre-dependent adeno-associated virus vectors to express Gq-coupled modified GPCR, hM3Dq or diphtheria toxin fragment A in orexin neurons. By intraperitoneal injection of clozapine-N oxide in orexin-Cre mice expressing hM3Dq in orexin neurons, we could selectively manipulate the activity of orexin neurons. Pharmacogenetic stimulation of orexin neurons simultaneously increased locomotive activity, food intake, water intake and the respiratory exchange ratio (RER). Elevation of blood glucose levels and RER persisted even after locomotion and feeding behaviors returned to basal levels. Accordantly, 83% ablation of orexin neurons resulted in decreased food and water intake, while 70% ablation had almost no effect on these parameters. Our results indicate that orexin neurons play an integral role in regulation of both feeding behavior and metabolism. This regulation is so robust that greater than 80% of orexin neurons were ablated before significant changes in feeding behavior emerged.

Conditional ablation of orexin/hypocretin neurons: a new mouse model for the study of narcolepsy and orexin system function

The sleep disorder narcolepsy results from loss of hypothalamic orexin/hypocretin neurons. Although narcolepsy onset is usually postpubertal, current mouse models involve loss of either orexin peptides or orexin neurons from birth. To create a model of orexin/hypocretin deficiency with closer fidelity to human narcolepsy, diphtheria toxin A (DTA) was expressed in orexin neurons under control of the Tet-off system. Upon doxycycline removal from the diet of postpubertal orexin-tTA;TetO DTA mice, orexin neurodegeneration was rapid, with 80% cell loss within 7 d, and resulted in disrupted sleep architecture. Cataplexy, the pathognomic symptom of narcolepsy, occurred by 14 d when ∼5% of the orexin neurons remained. Cataplexy frequency increased for at least 11 weeks after doxycycline. Temporary doxycycline removal followed by reintroduction after several days enabled partial lesion of orexin neurons. DTA-induced orexin neurodegeneration caused a body weight increase without a change in food consumption, mimicking metabolic aspects of human narcolepsy. Because the orexin/hypocretin system has been implicated in the control of metabolism and addiction as well as sleep/wake regulation, orexin-tTA; TetO DTA mice are a novel model in which to study these functions, for pharmacological studies of cataplexy, and to study network reorganization as orexin input is lost.

The action of orexin B on passive avoidance learning. Involvement of neurotransmitters

The extensive projection of orexigenic neurons and the diffuse expression of orexin receptors suggest that endogenous orexins are involved in several physiological functions of the central nervous system, including learning and memory. Our previous study demonstrated that orexin A improves learning, consolidation and retrieval processes, which involves α- and β-adrenergic, cholinergic, dopaminergic, GABA-A-ergic, opiate and nitrergic neurotransmissions. However, we have little evidence about the action of orexin B on memory processes and the underlying neuromodulation. Therefore, the aim of the present study was to investigate the action of orexin B on passive avoidance learning and the involvement of neurotransmitters in this action in rats. Accordingly, rats were pretreated with the selective orexin 2 receptor (OX2R) antagonist, EMPA; the γ-aminobutyric acid subunit A (GABA-A) receptor antagonist, the bicuculline; a D2, D3, D4 dopamine receptor antagonist, haloperidol; the nonselective opioid receptor antagonist, naloxone; the non-specific nitric oxide synthase (NOS) inhibitor, nitro-l-arginine; the nonselective α-adrenergic receptor antagonist, phenoxybenzamine and the β-adrenergic receptor antagonist, propranolol. Our results demonstrate that orexin B can improve learning, consolidation of memory and retrieval. EMPA reversed completely the action of orexin B on memory consolidation. Bicuculline blocked fully; naloxone, nitro-l-arginine, phenoxybenzamine and propranolol attenuated the orexin B-induced memory consolidation, whereas haloperidol was ineffective. These data suggest that orexin B improves memory functions through OX2R and GABA-ergic, opiate, nitrergic, α- and β-adrenergic neurotransmissions are also involved in this action.

Orexin-1 receptor mediates the increased food and water intake induced by intracerebroventricular injection of the stable somatostatin pan-agonist, ODT8-SST in rats

Intracerebroventricular (icv) injection of the stable somatostatin pan-agonist, ODT8-SST induces a somatostatin 2 receptor (sst2) mediated robust feeding response that involves neuropeptide Y and opioid systems in rats. We investigated whether the orexigenic system driven by orexin also plays a role. Food and water intake after icv injection was measured concomitantly in non-fasted and non-water deprived rats during the light phase. In vehicle treated rats (100% DMSO, icv), ODT8-SST (1μg/rat, icv) significantly increased the 2-h food and water intake compared to icv vehicle plus saline (5.1±1.0g vs. 1.2±0.4g and 11.3±1.9mL vs. 2.5±1.2mL, respectively). The orexin-1 receptor antagonist, SB-334867 (16μg/rat, icv) completely inhibited the 2-h food and water intake induced by icv ODT8-SST. In contrast, the icv pretreatment with the selective somatostatin sst2 antagonist, S-406-028, established to block the orexigenic effect of icv ODT8-SST, did not modify the increased food and water intake induced by icv orexin-A (10.7μg/rat). These data indicate that orexin-1 receptor signaling system is part of the brain neurocircuitry contributing to the orexigenic and dipsogenic responses induced by icv ODT8-SST and that orexin-A stimulates food intake independently from brain sst2 activation.

The orexinergic system influences conditioned odor aversion learning in the rat: a theory on the processes and hypothesis on the circuit involved

A large variety of behaviors that are essential for animal survival depend on the perception and processing of surrounding smells present in the natural environment. In particular, food-search behavior, which is conditioned by hunger, is directly driven by the perception of odors associated with food, and feeding status modulates olfactory sensitivity. The orexinergic hypothalamic peptide orexin A (OXA), one of the central and peripheral hormones that triggers food intake, has been shown to increase olfactory sensitivity in various experimental conditions including the conditioned odor aversion learning paradigm (COA). COA is an associative task that corresponds to the association between an olfactory conditioned stimulus (CS) and a delayed gastric malaise. Previous studies have shown that this association is formed only if the delay separating the CS presentation from the malaise is short, suggesting that the memory trace of the odor is relatively unstable. To test the selectivity of the OXA system in olfactory sensitivity, a recent study compared the effects of fasting and of central infusion of OXA during the acquisition of COA. Results showed that the increased olfactory sensitivity induced by fasting and by OXA infusion was accompanied by enhanced COA learning performances. In reference to the duration of action of OXA, the present work details the results obtained during the successive COA extinction tests and suggests a hypothesis concerning the role of the OXA component of fasting on the memory processes underlying CS-malaise association during COA. Moreover, referring to previous data in the literature we suggest a functional circuit model where fasting modulates olfactory memory processes through direct and/or indirect activation of particular OXA brain targets including the olfactory bulb, the locus coeruleus (LC) and the amygdala.

OX1 and OX2 orexin/hypocretin receptor pharmacogenetics

Orexin/hypocretin peptide mutations are rare in humans. Even though human narcolepsy is associated with orexin deficiency, this is only extremely rarely due to mutations in the gene coding prepro-orexin, the precursor for both orexin peptides. In contrast, coding and non-coding variants of the OX₁ and OX₂ orexin receptors have been identified in many human populations; sometimes, these have been associated with disease phenotype, although most confer a relatively low risk. In most cases, these studies have been based on a candidate gene hypothesis that predicts the involvement of orexins in the relevant pathophysiological processes. In the current review, the known human OX₁/HCRTR1 and OX₂/HCRTR2 genetic variants/polymorphisms as well as studies concerning their involvement in disorders such as narcolepsy, excessive daytime sleepiness, cluster headache, polydipsia-hyponatremia in schizophrenia, and affective disorders are discussed. In most cases, the functional cellular or pharmacological correlates of orexin variants have not been investigated—with the exception of the possible impact of an amino acid 10 Pro/Ser variant of OX₂ on orexin potency—leaving conclusions on the nature of the receptor variant effects speculative. Nevertheless, we present perspectives that could shape the basis for further studies. The pharmacology and other properties of the orexin receptor variants are discussed in the context of GPCR signaling. Since orexinergic therapeutics are emerging, the impact of receptor variants on the affinity or potency of ligands deserves consideration. This perspective (pharmacogenetics) is also discussed in the review.

Inflammation-sleep interface in brain disease: TNF, insulin, orexin

The depth, pattern, timing and duration of unconsciousness, including sleep, vary greatly in inflammatory disease, and are regarded as reliable indicators of disease severity. Similarly, these indicators are applicable to the encephalopathies of sepsis, malaria, and trypanosomiasis, and to viral diseases such as influenza and AIDS. They are also applicable to sterile neuroinflammatory states, including Alzheimer’s disease, Parkinson’s disease, traumatic brain injury, stroke and type-2 diabetes, as well as in iatrogenic brain states following brain irradiation and chemotherapy. Here we make the case that the cycles of unconsciousness that constitute normal sleep, as well as its aberrations, which range from sickness behavior through daytime sleepiness to the coma of inflammatory disease states, have common origins that involve increased inflammatory cytokines and consequent insulin resistance and loss of appetite due to reduction in orexigenic activity. Orexin reduction has broad implications, which are as yet little appreciated in the chronic inflammatory conditions listed, whether they be infectious or sterile in origin. Not only is reduction in orexin levels characterized by loss of appetite, it is associated with inappropriate and excessive sleep and, when dramatic and chronic, leads to coma. Moreover, such reduction is associated with impaired cognition and a reduction in motor control. We propose that advanced understanding and appreciation of the importance of orexin as a key regulator of pathways involved in the maintenance of normal appetite, sleep patterns, cognition, and motor control may afford novel treatment opportunities.

The orexin component of fasting triggers memory processes underlying conditioned food selection in the rat

To test the selectivity of the orexin A (OXA) system in olfactory sensitivity, the present study compared the effects of fasting and of central infusion of OXA on the memory processes underlying odor–malaise association during the conditioned odor aversion (COA) paradigm. Animals implanted with a cannula in the left ventricle received ICV infusion of OXA or artificial cerebrospinal fluid (ACSF) 1 h before COA acquisition. An additional group of intact rats were food-deprived for 24 h before acquisition. Results showed that the increased olfactory sensitivity induced by fasting and by OXA infusion was accompanied by enhanced COA performance. The present results suggest that fasting-induced central OXA release influenced COA learning by increasing not only olfactory sensitivity, but also the memory processes underlying the odor–malaise association.

Effects of hypocretin and norepinephrine interaction in bed nucleus of the stria terminalis on arterial pressure

Forebrain neuronal circuits containing hypocretin-1 (hcrt-1) and norepinephrine (NE) are important components of central arousal-related processes. Recently, these two systems have been shown to have an overlapping distribution within the bed nucleus of the stria terminalis (BST), a limbic structure activated by stressful challenges, and which functions to adjust arterial pressure (AP) and heart rate (HR) to the stressor. However, whether hcrt-1 and NE interact in BST to alter cardiovascular function is unknown. Experiments were done in urethane-α-chloralose anesthetized, paralyzed, and artificially ventilated male Wistar rats to investigate the effect of hcrt-1 and NE on the cardiovascular responses elicited by l-glutamate (Glu) stimulation of BST neurons. Microinjections of hcrt-1, NE or tyramine into BST attenuated the decrease in AP and HR to Glu stimulation of BST. Additionally, combined injections of hcrt-1 with NE or tyramine did not elicit a greater attenuation than either compound alone. Furthermore, injections into BST of the α2-adrenergic receptor (α2-AR) antagonist yohimbine, but not the α1-AR antagonist 2-{[β-(4-hydroxyphenyl)ethyl]aminomethyl}-1-tetralone hydrochloride, blocked both the hcrt-1 and NE-induced inhibition of the BST cardiovascular depressors responses. Finally, injections into BST of the GABAA receptor antagonist bicuculline, but not the GABAB receptor antagonist phaclofen, blocked the hcrt-1 and NE attenuation of the BST Glu-induced depressor and bradycardia responses. These data suggest that hcrt-1 effects in BST are mediated by NE neurons, and hcrt-1 likely acts to facilitate the synaptic release of NE. NE neurons, acting through α2-AR may activate Gabaergic neurons in BST, which in turn through the activation of GABAA receptors inhibit a BST sympathoinhibitory pathway. Taken together, these data suggest that hcrt-1 pathways to BST through their interaction with NE and Gabaergic neurons may function in the coordination of cardiovascular responses associated with different behavioral states.

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.

The hypocretins and the reward function: what have we learned so far?

A general consensus acknowledges that drug consumption (including alcohol, tobacco, and illicit drugs) constitutes the leading cause of preventable death worldwide. But the global burden of drug abuse extends the mortality statistics. Indeed, the comorbid long-term debilitating effects of the disease also significantly deteriorate the quality of life of individuals suffering from addiction disorders. Despite the large body of evidence delineating the cellular and molecular adaptations induced by chronic drug consumption, the brain mechanisms responsible for drug craving and relapse remain insufficiently understood, and even the most recent developments in the field have not brought significant improvement in the management of drug dependence. Though, recent preclinical evidence suggests that disrupting the hypocretin (orexin) system may serve as an anticraving medication therapy. Here, we discuss how the hypocretins, which orchestrate normal wakefulness, metabolic health and the execution of goal-oriented behaviors, may be compromised and contribute to elicit compulsive drug seeking. We propose an overview on the most recent studies demonstrating an important role for the hypocretin neuropeptide system in the regulation of drug reward and the prevention of drug relapse, and we question the relevance of disrupting the hypocretin system to alleviate symptoms of drug addiction.

The physiological role of orexin/hypocretin neurons in the regulation of sleep/wakefulness and neuroendocrine functions

The hypothalamus monitors body homeostasis and regulates various behaviors such as feeding, thermogenesis, and sleeping. Orexins (also known as hypocretins) were identified as endogenous ligands for two orphan G-protein-coupled receptors in the lateral hypothalamic area. They were initially recognized as regulators of feeding behavior, but they are mainly regarded as key modulators of the sleep/wakefulness cycle. Orexins activate orexin neurons, monoaminergic and cholinergic neurons in the hypothalamus/brainstem regions, to maintain a long, consolidated awake period. Anatomical studies of neural projections from/to orexin neurons and phenotypic characterization of transgenic mice revealed various roles for orexin neurons in the coordination of emotion, energy homeostasis, reward system, and arousal. For example, orexin neurons are regulated by peripheral metabolic cues, including ghrelin, leptin, and glucose concentration. This suggests that they may provide a link between energy homeostasis and arousal states. A link between the limbic system and orexin neurons might be important for increasing vigilance during emotional stimuli. Orexins are also involved in reward systems and the mechanisms of drug addiction. These findings suggest that orexin neurons sense the outer and inner environment of the body and maintain the proper wakefulness level of animals for survival. This review discusses the mechanism by which orexins maintain sleep/wakefulness states and how this mechanism relates to other systems that regulate emotion, reward, and energy homeostasis.

Orexin-B-like immunoreactivity localizes in both luteinizing-hormone-containing cells and melanin-concentrating hormone-containing fibers in the red-bellied piranha (Pygocentrus nattereri) pituitary

We examined orexin-like immunoreactivity in the pituitary of the red-bellied piranha (Pygocentrus nattereri). Orexin-B-immunoreactive (IR) cells corresponded to luteinizing hormone (LH)-containing cells in the pars distalis, and orexin-B-IR fibers corresponded to melanin-concentrating hormone (MCH)-containing fibers in the pars nervosa. In the pars distalis, orexin-B-IR puncta that were also immunoreactive for MCH were observed around the orexin-B-IR cells. In the ventral hypothalamus, orexin-B-IR and MCH-IR neurons were found in the nucleus lateralis tuberis. Immunoelectron-microscopic analysis revealed that the orexin-B-like substance co-localized with LH in secretory granules and with MCH in MCH-containing neurons. Some of the MCH secreted in the pituitary might participate in the modulation of LH secretion from the gonadotrophs, together with orexin-B, leading to food intake by the stimulation of growth hormone secretion from the somatotrophs.

Orexins cause epileptic activity

Orexins have been implicated in the regulation of sleep-wake cycle, energy homeostasis, drinking behavior, analgesia, attention, learning and memory but their effects on epileptic activity are controversial. We investigated whether intracortical injections of orexin A (100 pmol) and B (100 pmol) cause epileptic activity in rats. We observed epileptic seizure findings on these two groups rats. Orexin A and B also significantly increased total EEG power spectrum. Our findings indicate that orexins cause epileptic activity.

Glucocorticoids are required for meal-induced changes in the expression of hypothalamic neuropeptides

Glucocorticoid deficiency is associated with a decrease of food intake. Orexigenic peptides, neuropeptide Y (NPY) and agouti related protein (AgRP), and the anorexigenic peptide proopiomelanocortin (POMC), expressed in the arcuate nucleus of the hypothalamus (ARC), are regulated by meal-induced signals. Orexigenic neuropeptides, melanin-concentrating hormone (MCH) and orexin, expressed in the lateral hypothalamic area (LHA), also control food intake. Thus, the present study was designed to test the hypothesis that glucocorticoids are required for changes in the expression of hypothalamic neuropeptides induced by feeding. Male Wistar rats (230-280 g) were subjected to ADX or sham surgery. ADX animals received 0.9% NaCl in the drinking water, and half of them received corticosterone in the drinking water (B: 25 mg/L, ADX+B). Six days after surgery, animals were fasted for 16 h and they were decapitated before or 2 h after refeeding for brain tissue and blood collections. Adrenalectomy decreased NPY/AgRP and POMC expression in the ARC in fasted and refed animals, respectively. Refeeding decreased NPY/AgRP and increased POMC mRNA expression in the ARC of sham and ADX+B groups, with no effects in ADX animals. The expression of MCH and orexin mRNA expression in the LHA was increased in ADX and ADX+B groups in fasted condition, however there was no effect of refeeding on the expression of MCH and orexin in the LHA in the three experimental groups. Refeeding increased plasma leptin and insulin levels in sham and ADX+B animals, with no changes in leptin concentrations in ADX group, and insulin response to feeding was lower in this group. Taken together, these data demonstrated that circulating glucocorticoids are required for meal-induced changes in NPY, AgRP and POMC mRNA expression in the ARC. The lower leptin and insulin responses to feeding may contribute to the altered hypothalamic neuropeptide expression after adrenalectomy.

Orexins increase penicillin-induced epileptic activity

Orexins have been implicated with physiological function including sleep-wake cycle, energy homeostasis, drinking behavior, analgesia, attention, learning and memory but their effects on excitability are controversial. We investigated the effects of intracortical injections of orexin A (100 pmol) and B (100 pmol) on the electrophysiological manifestation of epileptic seizures induced by cortical penicillin application in adult male rats. In comparison to saline, orexin A and B enhanced significantly the spike number, spike amplitude and spectral power values induced by cortical penicillin. Our findings indicates that orexins enhances the hyperexcitable and hypersyncronic cortical epileptic activity induced by focal application of penicillin-G.