Orexin-A (hypocretin-1) is possibly involved in generation of anxiety-like behavior

Involvement of orexin in the regulation of stress, depression and reward in alcohol dependence

There is growing evidence from preclinical studies for an involvement of orexins (ORX) in the regulation of stress, affectivity and addictive behavior. The aim of our study was to gather corresponding clinical data and to elucidate the relationships between alcohol withdrawal stress, ORX plasma concentration and psychopathology. A consecutive sample of thirty-four alcohol-dependent inpatients was included in the study. Blood was drawn at onset of withdrawal and following 2 weeks of controlled abstinence in order to assess ORX, ACTH and cortisol plasma concentrations. In parallel, we assessed clinically relevant psychological distress symptoms applying the Brief Symptom Inventory (BSI). We found a significant positive correlation between ORX and global distress indices of the BSI (p ≤ 0.05). In a regression model, ORX concentration during acute withdrawal explained 24% of the variance of symptom severity (p<0.01). No association with craving, ACTH or cortisol plasma concentration was detected. Our results suggest an involvement of ORX in the affective dysregulation seen commonly in alcohol dependent patients during alcohol withdrawal. Moreover, the effects on global distress indices as well as the earlier studied effects on reinstatement of drug seeking behaviors may point on an involvement of ORX in impaired brain stress systems.

Dichotomous cellular properties of mouse orexin/hypocretin neurons

Hypothalamic hypocretin/orexin (Hcrt/Orx) neurons recently emerged as critical regulators of sleep–wake cycles, reward seeking and body energy balance. However, at the level of cellular and network properties, it remains unclear whether Hcrt/Orx neurons are one homogeneous population, or whether there are several distinct types of Hcrt/Orx cells. Here, we collated diverse structural and functional information about individual Hcrt/Orx neurons in mouse brain slices, by combining patch-clamp analysis of spike firing, membrane currents and synaptic inputs with confocal imaging of cell shape and subsequent 3-dimensional Sholl analysis of dendritic architecture. Statistical cluster analysis of intrinsic firing properties revealed that Hcrt/Orx neurons fall into two distinct types. These two cell types also differ in the complexity of their dendritic arbour, the strength of AMPA and GABAA receptor-mediated synaptic drive that they receive, and the density of low-threshold, 4-aminopyridine-sensitive, transient K+ current. Our results provide quantitative evidence that, at the cellular level, the mouse Hcrt/Orx system is composed of two classes of neurons with different firing properties, morphologies and synaptic input organization.

Central orexin-A increases colonic motility in conscious rats

Increasing evidence has indicated that brain orexin plays a vital role in the regulation of gastrointestinal physiology such as gastric secretion, gastric motility and pancreatic secretion. However, little is known whether orexin in the brain is involved in the physiology of the lower gastrointestinal tract. The aim of this study was therefore to elucidate whether orexin-A in the brain is involved in the regulation of colonic motility. In this study, we measured fecal pellet output and recorded intraluminal colonic pressure waves in freely moving conscious rats to evaluate the effects of central orexin-A on colonic motor functions. Intracisternal but not intraperitoneal injection of orexin-A dose-dependently (1-10 μg) increased fecal pellet output. Findings obtained from manometric recordings revealed that intracisternal administration of orexin-A at a dose of 10 μg significantly enhanced colonic motor contractions. These results suggest for the first time that orexin-A acts centrally in the brain to enhance fecal pellet output and stimulate colonic motility in conscious rats. The present study would furthermore support our hypothesis that orexin-A in the brain may be an important candidate as a mediator of the cephalic phase gut stimulation including stimulated colonic motility in addition to well known physiological response such as stimulation of gastric acid and pancreatic acid secretion, and gastric motility.

Direct and indirect control of orexin/hypocretin neurons by glycine receptors

Hypothalamic hypocretin/orexin (hcrt/orx) neurons promote arousal and reward seeking, while reduction in their activity has been linked to narcolepsy, obesity and depression. However, the mechanisms influencing the activity of hcrt/orx networks in situ are not fully understood. Here we show that glycine, a neurotransmitter best known for its actions in the brainstem and spinal cord, elicits dose dependent postsynaptic Cl⁻ currents in hcrt/orx cells in acute mouse brain slices. The effect was blocked by the glycine receptor (GLyR) antagonist strychnine and mimicked by the GlyR agonist alanine. Postsynaptic GlyRs on hcrt/orx cells remained functional during both early postnatal and adult periods, and gramicidin-perforated patch-clamp recordings revealed that they progressively switch from excitatory to inhibitory during the first two postnatal weeks. The pharmacological profile of the glycine response suggested that developed hcrt/orx neurons contain α/β-heteromeric GlyRs that lack α2-subunits, whereas α2-subunits, whereas α2-subunits are present in early postnatal hcrt/orx neurons. All postsynaptic currents (PSCs) in developed hcrt/orx cells were blocked by inhibitors of GABA and glutamate receptors, with no evidence of GlyR-mediated PSCs. However, the frequency but not amplitude of miniature PSCs was reduced by strychnine and increased by glycine in ~50% of hcrt/orx neurons. Together, these results provide the first evidence for functional GlyRs in identified hcrt/orx circuits and suggest that the activity of developed hcrt/orx cells is regulated by two GlyR pools: inhibitory extrasynaptic GlyRs located on all hcrt/orx cells and excitatory GlyRs located on presynaptic terminals contacting some hcrt/orx cells.

Multiple hypothalamic circuits sense and regulate glucose levels

The hypothalamus monitors body energy status in part through specialized glucose sensing neurons that comprise both glucose-excited and glucose-inhibited cells. Here we discuss recent work on the elucidation of neurochemical identities and physiological significance of these hypothalamic cells, including caveats resulting from the currently imprecise functional and molecular definitions of glucose sensing and differences in glucose-sensing responses obtained with different experimental techniques. We discuss the recently observed adaptive glucose-sensing responses of orexin/hypocretin-containing neurons, which allow these cells to sense changes in glucose levels rather than its absolute concentration, as well as the glucose-sensing abilities of melanin-concentrating hormone, neuropeptide Y, and proopiomelanocortin-containing neurons and the recent data on the role of ventromedial hypothalamic steroidogenic factor-1 (SF-1)/glutamate-containing cells in glucose homeostasis. We propose a model where orexin/hypocretin and SF-1/glutamate neurons cooperate in stimulating the sympathetic outflow to the liver and pancreas to increase blood glucose, which in turn provides negative feedback inhibition to these cells. Orexin/hypocretin neurons also stimulate feeding and reward seeking and are activated by hunger and stress, thereby providing a potential link between glucose sensing and goal-oriented behavior. The cell-type-specific neuromodulatory actions of glucose in several neurochemically distinct hypothalamic circuits are thus likely to be involved in coordinating higher brain function and behavior with autonomic adjustments in blood glucose levels.

Increased orexin and melanin-concentrating hormone expression in the perifornical lateral hypothalamus of rats prone to overconsuming a fat-rich diet

The goal of this study is to examine the expression pattern of orexigenic peptides, orexin (OX) and melanin-concentrating hormone (MCH), in the perifornical lateral hypothalamus (PFLH) in subpopulations of Sprague-Dawley rats differing in their propensity to overconsume a high-fat diet. Immediately after an initial 5-day screening test that predicts long-term consumption, rats identified as high-fat consumers (HFC), ingesting 35% more calories of a high-fat relative to low-fat chow diet, had significantly elevated mRNA expression of OX in the perifornical but not lateral hypothalamic area and of MCH mRNA in both areas, when compared to control rats that consume similar amounts of these diets. This same OX and MCH expression pattern was seen in HFC rats maintained for two weeks on a low-fat chow diet, indicating that increased expression of these orexigenic peptides, occurring independently of the high-fat diet, may be an inherent characteristic of these rats. These HFC rats were also more active and slightly more anxious than controls, as measured by line crossings and time spent in the periphery or middle segments of an open field. Together, these results demonstrate that animals prone to overeating a high-fat diet show a baseline increase in orexigenic peptide expression in the PFLH along with higher behavioral arousal, which together may contribute to their increased consummatory behavior.

Low cerebrospinal fluid and plasma orexin-A (hypocretin-1) concentrations in combat-related posttraumatic stress disorder

The hypothalamic neuropeptide, orexin-A has a number of regulatory effects in humans and pre-clinical evidence suggests a link to neuroendocrine systems known to be pathophysiologically related to posttraumatic stress disorder (PTSD). However, there are no reports of central nervous system (CNS) or peripheral orexin-A concentrations in patients with PTSD, or any anxiety disorder. Cerebrospinal fluid (CSF) and plasma levels of orexin-A were serially determined in patients with PTSD and healthy comparison subjects to characterize the relationships between orexin-A (in the CNS and peripheral circulation) and central indices of monoaminergic neurotransmission and to determine the degree to which CNS orexin-A concentrations reflect those in the circulating blood. CSF and plasma samples were obtained serially over a 6-h period in 10 male combat veterans with chronic PTSD and 10 healthy male subjects through an indwelling subarachnoid catheter. Orexin-A concentrations were determined in plasma and CSF and CSF levels of the serotonin metabolite, 5-hydroxyindolacetic acid (5-HIAA), and the dopamine metabolite, homovanillic acid (HVA), were determined over the sampling period. CSF and plasma orexin-A concentrations were significantly lower in the patients with PTSD as compared with healthy comparison subjects at all time points. In addition, CSF orexin-A concentrations strongly and negatively correlated with PTSD severity as measured by the Clinician-Administered PTSD Scale (CAPS) in patients with PTSD. Peripheral and CNS concentrations of orexin-A were correlated in the healthy comparison subjects and peripheral orexin-A also correlated with CNS serotonergic tone. These findings suggest low central and peripheral orexin-A activity in patients with chronic PTSD are related to symptom severity and raise the possibility that orexin-A is part of the pathophysiological mechanisms of combat-related PTSD.

Orexin mediates initiation of sexual behavior in sexually naive male rats, but is not critical for sexual performance

The hypothalamic neuropeptide orexin mediates arousal, sleep, and naturally rewarding behaviors, including food intake. Male sexual behavior is altered by orexin receptor-1 agonists or antagonists, suggesting a role for orexin-A in this naturally rewarding behavior. However, the specific role of endogenous orexin-A or B in different elements of male sexual behavior is currently unclear. Therefore, the current studies utilized markers for neural activation and orexin cell-specific lesions to test the hypothesis that orexin is critical for sexual motivation and performance in male rats. First, cFos expression in orexin neurons was demonstrated following presentation of a receptive or non-receptive female without further activation by different elements of mating. Next, the functional role of orexin was tested utilizing orexin-B conjugated saporin, resulting in orexin cell body lesions in the hypothalamus. Lesions were conducted in sexually naive males and subsequent sexual behavior was recorded during four mating trials. Lesion males showed shortened latencies to mount and intromit during the first, but not subsequent mating trials, suggesting lesions facilitated initiation of sexual behavior in sexually naive, but not experienced males. Likewise, lesions did not affect sexual motivation in experienced males, determined by runway tests. Finally, elevated plus maze tests demonstrated reduced anxiety-like behaviors in lesioned males, supporting a role for orexin in anxiety associated with initial exposure to the female in naive animals. Overall, these findings show that orexin is not critical for male sexual performance or motivation, but may play a role in arousal and anxiety related to sexual behavior in naive animals.

Orexin receptor subtype activation and locomotor behaviour in the rat

AIM

Orexin-producing neurones, located primarily in the perifornical region of the lateral hypothalamus, project to a wide spectrum of brain sites where they influence numerous behaviours as well as modulating the neuroendocrine and autonomic responses to stress. While some of the actions of orexin appear to be mediated via the type 1 receptor, some are not, including its action on the release of one stress hormone, prolactin. We describe here the ability of orexin to increase locomotor behaviours and identify the importance of both receptor subtypes in these actions.

METHODS

Rats were tested for their behavioural responses to the central activation of both the type 1 (OX(1)R) and type 2 (OX(2)R) receptor (ICV orexin A), compared to OX(2)R activation using a relatively selective OX(2)R agonist in the absence or presence of an orexin receptor antagonist that possesses highest affinity for OX(1)R.

RESULTS

Increases in locomotor activity were observed, effects which were expressed by not only orexin A, which binds to both the OX(1)R and the OX(2)R receptors, but also by the relatively selective OX(2)R agonist [(Ala(11), Leu(15))-orexin B]. Furthermore, the OX(1)R selective antagonist only partially blocked the action of orexin A on most locomotor behaviours and did not block the actions of [(Ala(11), Leu(15))-orexin B].

CONCLUSION

We conclude that orexin A exerts its effects on locomotor behaviour via both the OX(1)R and OX(2)R and that agonism or antagonism of only one of these receptors for therapeutic purposes (i.e. sleep disorders) would not provide selectivity in terms of associated behavioural side effects.

Changes in emotional behavior produced by orexin microinjections in the paraventricular nucleus of the thalamus

The paraventricular nucleus of the midline thalamus (PVT) innervates areas of the extended amygdala known to play a key role in the expression of emotional behaviors. In this study, microinjections of orexins (hypocretins), which have excitatory actions on neurons in the PVT, in the midline thalamus were used to investigate if the PVT modulates the expression of emotional behavior in the open field. First, the approach-avoidance tendency (number and duration of visit to the center area) associated with novelty was examined in orexin treated rats before and after placing a novel object in the center of the open field. Second, the expression of ethological behaviors (rearing, locomotion, freezing, and grooming) in the open field was used to determine the effects of orexins on emotionality. Microinjections of orexin-A (OXA) or orexin-B (OXB) in the PVT decreased exploration of the center area and the novel object indicating that the center area and the object had more aversive properties in orexin treated rats. Both OXA and OXB microinjections in the PVT increased the expression of freezing and grooming behaviors which are indicative of a negative emotional state. The results indicate that microinjections of orexins in the PVT made the test situation more aversive and produced avoidance behaviors. This suggests that orexins may act at the PVT to modulate behaviors associated with a negative emotional state.

Physiological functions of glucose-inhibited neurones

Glucose-inhibited neurones are an integral part of neurocircuits regulating cognitive arousal, body weight and vital adaptive behaviours. Their firing is directly suppressed by extracellular glucose through poorly understood signalling cascades culminating in opening of post-synaptic K(+) or possibly Cl(-) channels. In mammalian brains, two groups of glucose-inhibited neurones are best understood at present: neurones of the hypothalamic arcuate nucleus (ARC) that express peptide transmitters NPY and agouti-related peptide (AgRP) and neurones of the lateral hypothalamus (LH) that express peptide transmitters orexins/hypocretins. The activity of ARC NPY/AgRP neurones promotes food intake and suppresses energy expenditure, and their destruction causes a severe reduction in food intake and body weight. The physiological actions of ARC NPY/AgRP cells are mediated by projections to numerous hypothalamic areas, as well as extrahypothalamic sites such as the thalamus and ventral tegmental area. Orexin/hypocretin neurones of the LH are critical for normal wakefulness, energy expenditure and reward-seeking, and their destruction causes narcolepsy. Orexin actions are mediated by highly widespread central projections to virtually all brain areas except the cerebellum, including monosynaptic innervation of the cerebral cortex and autonomic pre-ganglionic neurones. There, orexins act on two specific G-protein-coupled receptors generally linked to neuronal excitation. In addition to sensing physiological changes in sugar levels, the firing of both NPY/AgRP and orexin neurones is inhibited by the 'satiety' hormone leptin and stimulated by the 'hunger' hormone ghrelin. Glucose-inhibited neurones are thus well placed to coordinate diverse brain states and behaviours based on energy levels.

Orexin A attenuates unconditioned sexual motivation in male rats

Orexins are neuropeptides involved in multiple neurophysiological functions such as reward and motivation. However, it is not clear whether orexins are implicated in sexual motivation. This study aims to evaluate the effects of orexin A and the OX(1)R antagonist SB334867 on unconditioned sexual motivation. Forty-five male Wistar rats are divided into four groups. The four groups are respectively administered intracerebroventricularly with saline, orexin A (1, 10 microg), 10% DMSO (cyclodextrin) and SB334867 (5, 15 microg) 10-15 min before sexual motivation tests. The preference for a receptive female to a male in an open arena with two tethered animals is designated as unconditioned sexual motivation. The results show that orexin A reduces the female preference (reducing time in the female zone and/or increasing time in the male zone), the number of visits for the female zone and the total distance traveled in sexually high-motivated males. SB334867 has no effect on the female preference, the number of visits and the distance traveled in either sexually high-motivated or low-motivated males. Our experiments reveal that centrally administered orexin A attenuates sexual motivation in high-motivated males although endogenous orexin A might not play an important role in the expression of unconditioned sexual motivation.

Recent advances in the neurobiology of anxiety disorders: implications for novel therapeutics

Anxiety disorders are a highly prevalent and disabling class of psychiatric disorders. This review focuses on new directions in neurobiological research and implications for the development of novel psychopharmacological treatments. Neuroanatomical and neuroimaging research in anxiety disorders has centered on the role of the amygdala, reciprocal connections between the amygdala and the prefrontal cortex, and, most recently, alterations in interoceptive processing by the anterior insula. Anxiety disorders are characterized by alterations in a diverse range of neurochemical systems, suggesting ample novel targets for drug therapies. Corticotropin-releasing factor (CRF) concentrations are elevated in a subset of anxiety disorders, which suggests the potential utility of CRF receptor antagonists. Pharmacological blockade of the memory-enhancing effects of stress hormones such as glucocorticoids and noradrenaline holds promise as a preventative approach for trauma-related anxiety. The glutamatergic system has been largely overlooked as a potential pharmacological target, although convergent preclinical, neuroimaging, and early clinical findings suggest that glutamate receptor antagonists may have potent anxiolytic effects. Glutamatergic receptor agonists (e.g., D-cycloserine) also have an emerging role in the treatment of anxiety as facilitators of fear extinction during concurrent behavioral interventions. The neuropeptides substance P, neuropeptide Y, oxytocin, orexin, and galanin are each implicated in anxiety pathways, and neuropeptide analogs or antagonists show early promise as anxiolytics in preclinical and/or clinical research. Each of these active areas of research holds promise for expanding and improving evidence-based treatment options for individuals suffering with clinical anxiety.

Neuropeptide S: a novel modulator of stress and arousal

Neuropeptide S (NPS) is a recently identified bioactive peptide that modulates stress and arousal. NPS is expressed in a few discrete nuclei in the brainstem, such as the pericoerulear (locus coeruleus (LC)) area and the parabrachial nucleus. NPS activates its cognate G protein-coupled receptor at low nanomolar agonist concentrations and induces elevation of intracellular Ca2+ and cAMP, therefore acting as an excitatory transmitter. The NPS receptor is widely expressed in the brain, including regions known to regulate stress responses such as hypothalamus, thalamus, amygdala and limbic cortex. We have recently reported that the NPS system can modulate stress responses and induce wakefulness based on a battery of behavioral tests. Activation of NPS receptors induces arousal and reduces all sleep stages. At the same time, NPS produces anxiolytic-like effects in rodents. These studies indicate that the NPS system has a unique pharmacological profile to promote both anxiolytic and arousal effects. NPS might interact with other hypothalamic neuropeptide systems that are known to be involved in stress and appetite control and thus might be a valuable target for development of a new class of drugs to treat anxiety disorders.

Control of hypothalamic orexin neurons by acid and CO2

Hypothalamic orexin/hypocretin neurons recently emerged as key orchestrators of brain states and adaptive behaviors. They are critical for normal stimulation of wakefulness and breathing: Orexin loss causes narcolepsy and compromises vital ventilatory adaptations. However, it is unclear how orexin neurons generate appropriate adjustments in their activity during changes in physiological circumstances. Extracellular levels of acid and CO2 are fundamental physicochemical signals controlling wakefulness and breathing, but their effects on the firing of orexin neurons are unknown. Here we show that the spontaneous firing rate of identified orexin neurons is profoundly affected by physiological fluctuations in ambient levels of H+ and CO2. These responses resemble those of known chemosensory neurons both qualitatively (acidification is excitatory, alkalinization is inhibitory) and quantitatively (approximately 100% change in firing rate per 0.1 unit change in pHe). Evoked firing of orexin cells is similarly modified by physiologically relevant changes in pHe: Acidification increases intrinsic excitability, whereas alkalinization depresses it. The effects of pHe involve acid-induced closure of leak-like K+ channels in the orexin cell membrane. These results suggest a new mechanism of how orexin/hypocretin networks generate homeostatically appropriate firing patterns.

Intracisternal injection of orexin-A prevents ethanol-induced gastric mucosal damage in rats

BACKGROUND

Accumulating evidence indicates that orexin-A in the brain stimulates vagal flow projecting to the stomach. Since the vagal system plays an important role in gastric mucosal integrity, we hypothesized that orexin-A in the brain might have a gastroprotective action.

METHODS

We examined the effect of centrally administered orexin-A on the development of gastric mucosal damage evoked by ethanol and its possible mechanism of action in rats.

RESULTS

Intracisternal but not intraperitoneal injection of orexin-A significantly inhibited the severity of gastric mucosal damage by 70% ethanol in a dose-dependent manner, suggesting that orexin-A acts in the brain to prevent ethanol-induced gastric mucosal damage. The antiulcer action was observed in rats administered with orexin-A centrally but not orexin-B, indicating that the action is mediated through orexin 1 receptors. The gastroprotective action of centrally administered orexin-A was blocked by pretreatment with atropine, Nomega-nitro-L-arginine methylester, or indomethacin.

CONCLUSIONS

These results suggest that orexin-A acts on orexin 1 receptors in the brain to exert a gastroprotective action against ethanol. The vagal muscarinic system, nitric oxide, and prostaglandins may mediate the cytoprotective action of centrally administered orexin-A.

K+ channels stimulated by glucose: a new energy-sensing pathway

Insights into how sugar can turn off cell activity are emerging from studies of hypothalamic neurons. Brain states are coordinated by hypothalamic orexin/hypocretin neurons, whose loss leads to narcoleptic instability of consciousness and inability to rouse when hungry. Recent studies indicate that glucose blocks the electrical activity of orexin cells by opening K+ channels in their membrane. This new energy-sensing mechanism is so sensitive that even small changes in glucose levels, of the type occurring between meals, can turn orexin cells on and off. Glucose-stimulated K+ channels share biophysical properties with "leak" (two-pore domain) K+ channels, the newest and least understood K+ channel family. A hypothesis is outlined whereby the stimulation of brain K+ channels by sugar could relieve stress and enhance reward, although probably at a cost of increased sleepiness.

CCK and NPY as anti-anxiety treatment targets: promises, pitfalls, and strategies

Short CCK peptides elicit panic attacks in humans and anxiogenic-like effects in some animal models, but CCK receptor antagonists have not been found clinically effective. Yet CCK overactivity appears to be involved in submissive behaviour, and CCKB receptor expression and binding are increased in suicide victims and animal models of anxiety. Preliminary data suggest that involvement of CCK and its receptor subtypes in anxiety can be better described when focusing on distinct endophenotypes, and considering environmental contingencies and confounds originating from interactions with dopamin-, opioid- and glutamatergic neurotransmission. In contrast, NPY is an anti-anxiety peptide with robust effects in various animal models when administrated into several brain regions. Studies with non-peptide antagonists selective for receptor subtypes have revealed the role of endogenous NPY in active coping. At least Y1, Y2 and Y5 receptors in various brain regions are involved, with the strongest evidence for contribution of Y1.