Interaction between the corticotropin-releasing factor system and hypocretins (orexins): a novel circuit mediating stress response

Distinct populations of presympathetic-premotor neurons express orexin or melanin-concentrating hormone in the rat lateral hypothalamus

Orexin and melanin-concentrating hormone (MCH) have been implicated in mediating a variety of different behaviors. These include sleep and wakefulness, locomotion, ingestive behaviors, and fight-or-flight response, as well as anxiety- and panic-like behaviors in rodents. Despite such diversity, all these processes require coordinated recruitment of the autonomic and somatomotor efferents. We have previously mapped the locations of presympathetic-premotor neurons (PSPMNs) in the rat brain. These putative dual-function neurons send trans-synaptic projections to somatomotor and sympathetic targets and likely participate in somatomotor-sympathetic integration. A significant portion of these neurons is found within the dorsomedial (DMH) and lateral hypothalamus (LH), areas of the brain that contain MCH- and orexin- synthesizing neurons in the central nervous system. Thus, we hypothesized that hypothalamic PSPMNs utilize MCH or orexin as their neurotransmitter. To test this hypothesis, we identified PSPMNs by using recombinant strains of the pseudorabies virus (PRV) for trans-synaptic tract tracing. PRV-152, a strain that expresses enhanced green fluorescent protein, was injected into sympathectomized gastrocnemius muscle, whereas PRV-BaBlu, which expresses beta-galactosidase, was injected into the adrenal gland in the same animals. By using immunofluorescent methods, we determined whether co-infected neurons express MCH or orexin. Our findings demonstrate that PSPMNs synthesizing either MCH or orexin are present within LH, where they form two separate populations. PSPMNs located around the fornix express orexin, whereas those located around the cerebral peduncle are more likely to express MCH. These two clusters of PSPMNs within LH likely play distinct functional roles in autonomic homeostasis and stress coping mechanisms.

Prolonged wakefulness induces experience-dependent synaptic plasticity in mouse hypocretin/orexin neurons

Sleep is a natural process that preserves energy, facilitates development, and restores the nervous system in higher animals. Sleep loss resulting from physiological and pathological conditions exerts tremendous pressure on neuronal circuitry responsible for sleep-wake regulation. It is not yet clear how acute and chronic sleep loss modify neuronal activities and lead to adaptive changes in animals. Here, we show that acute and chronic prolonged wakefulness in mice induced by modafinil treatment produced long-term potentiation (LTP) of glutamatergic synapses on hypocretin/orexin neurons in the lateral hypothalamus, a well-established arousal/wake-promoting center. A similar potentiation of synaptic strength at glutamatergic synapses on hypocretin/orexin neurons was also seen when mice were sleep deprived for 4 hours by gentle handling. Blockade of dopamine D1 receptors attenuated prolonged wakefulness and synaptic plasticity in these neurons, suggesting that modafinil functions through activation of the dopamine system. Also, activation of the cAMP pathway was not able to further induce LTP at glutamatergic synapses in brain slices from mice treated with modafinil. These results indicate that synaptic plasticity due to prolonged wakefulness occurs in circuits responsible for arousal and may contribute to changes in the brain and body of animals experiencing sleep loss.

Autoantibodies against appetite-regulating peptide hormones and neuropeptides: putative modulation by gut microflora

Objective

Peptide hormones synthesized in gastrointestinal and adipose tissues in addition to neuropeptides regulate appetite and body weight. Previously, autoantibodies directed against melanocortin peptides were found in patients with eating disorders; however, it remains unknown whether autoantibodies directed against other appetite-regulating peptides are present in human sera and whether their levels are influenced by gut-related antigens.

Methods

Healthy women were studied for the presence of immunoglobulin (Ig) G and IgA autoantibodies directed against 14 key appetite-regulating peptides. The concept of molecular mimicry was applied to search in silico whether bacteria, viruses, or fungi contain proteins with amino acid sequences identical to appetite-regulating peptides. In addition, autoantibodies serum levels were studied in germ-free and specific pathogen-free rats.

Results

We found these IgG and IgA autoantibodies directed against leptin, ghrelin, peptide YY, neuropeptide Y, and other appetite-regulating peptides are present in human sera at levels of 100–900 ng/mL. Numerous cases of sequence homology with these peptides were identified among commensal and pathogenic micro-organisms including Lactobacilli, bacteroides, Helicobacter pylori, Escherichia coli, and Candida species. Decreased levels of IgA autoantibodies directed against several appetite-regulating peptides and increased levels of antighrelin IgG were found in germ-free rats compared with specific pathogen-free rats.

Conclusion

Healthy humans and rats display autoantibodies directed against appetite-regulating peptide hormones and neuropeptides, suggesting that these autoantibodies may have physiologic implications in hunger and satiety pathways. Gut-related antigens including the intestinal microflora may influence production of theses autoantibodies, suggesting a new link between the gut and appetite control.

Stress blunts serotonin- and hypocretin-evoked EPSCs in prefrontal cortex: role of corticosterone-mediated apical dendritic atrophy

Morphological studies show that repeated restraint stress leads to selective atrophy in the apical dendritic field of pyramidal cells in the medial prefrontal cortex (mPFC). However, the functional consequence of this selectivity remains unclear. The apical dendrite of layer V pyramidal neurons in the mPFC is a selective locus for the generation of increased excitatory postsynaptic currents (EPSCs) by serotonin (5-HT) and hypocretin (orexin). On that basis, we hypothesized that apical dendritic atrophy might result in a blunting of 5-HT- and hypocretin-induced excitatory responses. Using a combination of whole-cell recording and two-photon imaging in rat mPFC slices, we were able to correlate electrophysiological and morphological changes in the same layer V pyramidal neurons. Repeated mild restraint stress produced a decrement in both 5-HT- and hypocretin-induced EPSCs, an effect that was correlated with a decrease in apical tuft dendritic branch length and spine density in the distal tuft branches. Chronic treatment with the stress hormone corticosterone, while reducing 5-HT responses and generally mimicking the morphological effects of stress, failed to produce a significant decrease in hypocretin-induced EPSCs. Accentuating this difference, pretreatment of stressed animals with the glucocorticoid receptor antagonist RU486 blocked reductions in 5-HT-induced EPSCs but not hypocretin-induced EPSCs. We conclude: (i) stress-induced apical dendritic atrophy results in diminished responses to apically targeted excitatory inputs and (ii) corticosterone plays a greater role in stress-induced reductions in EPSCs evoked by 5-HT as compared with hypocretin, possibly reflecting the different pathways activated by the two transmitters.

Orexins and orexin receptors: from molecules to integrative physiology

Recent studies have implicated the orexin system as a critical regulator of sleep/wake states, feeding behavior, and reward processes. Orexin deficiency results in narcolepsy-cataplexy in humans, dogs, and rodents, suggesting that the orexin system is particularly important for maintenance of wakefulness. Orexin agonists and antagonists are thought to be promising avenues toward the treatment of sleep disorders, eating disorders, and drug addiction. In this chapter, we discuss the current understanding of the physiological roles of orexins in regulation of arousal, sleep/wake states, energy homeostasis, and reward systems.

Orexin neuronal circuitry: role in the regulation of sleep and wakefulness

Orexin A and orexin B were initially identified as endogenous ligands for two orphan G protein-coupled receptors [104]. They were initially recognized as regulators of feeding behavior in view of their exclusive production in the lateral hypothalamic area (LHA), a region known as the feeding center, and their pharmacological activity [104,30,49,107]. Subsequently, the finding that orexin deficiency causes narcolepsy in humans and animals suggested that these hypothalamic neuropeptides play a critical role in regulating sleep/wake cycle [22,46,71,95,117]. These peptides activate waking-active monoaminergic and cholinergic neurons in the hypothalamus/brain stem regions to maintain a long, consolidated awake period. Recent studies on efferent and afferent systems of orexin neurons, and phenotypic characterization of genetically modified mice in the orexin system further suggested roles of orexin in the coordination of emotion, energy homeostasis, reward system, and arousal [3,80,106,137]. A link between the limbic system and orexin neurons might be important for increasing vigilance during emotional stimuli. Orexin neurons are also regulated by peripheral metabolic cues, including ghrelin, leptin, and glucose, suggesting that they might have important roles as a link between energy homeostasis and vigilance states [137]. Recent research has also implicated orexins in reward systems and the mechanisms of drug addiction [13,48,91]. These observations suggest that orexin neurons sense the outer and inner environment of the body, and maintain proper wakefulness 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.

Central neural and endocrine mechanisms of non-exercise activity thermogenesis and their potential impact on obesity

The rise in obesity is associated with a decline in the amount of physical activity in which people engage. The energy expended through everyday non-exercise activity, called non-exercise activity thermogenesis (NEAT), has a considerable potential impact on energy balance and weight gain. Comparatively little attention has been paid to the central mechanisms of energy expenditure and how decreases in NEAT might contribute to obesity. In this review, we first examine the sensory and endocrine mechanisms through which energy availability and energy balance are detected that may influence NEAT. Second, we describe the neural pathways that integrate these signals. Lastly, we consider the effector mechanisms that modulate NEAT through the alteration of activity levels as well as through changes in the energy efficiency of movement. Systems that regulate NEAT according to energy balance may be linked to neural circuits that modulate sleep, addiction and the stress response. The neural and endocrine systems that control NEAT are potential targets for the treatment of obesity.

Addiction and arousal: the hypocretin connection

The hypocretins, also known as orexins, are two neuropeptides now commonly described as critical components to maintain and regulate the stability of arousal. Several lines of evidence have raised the hypothesis that hypocretin-producing neurons are part of the circuitries that mediate the hypothalamic response to acute stress. Intracerebral administration of hypocretin leads to a dose-related reinstatement of drug and food seeking behaviors. Furthermore, stress-induced reinstatement can be blocked with hypocretin receptor 1 antagonism. These results, together with recent data showing that hypocretin is critically involved in cocaine sensitization through the recruitment of NMDA receptors in the ventral tegmental area, strongly suggest that activation of hypocretin neurons play a critical role in the development of the addiction process. The activity of hypocretin neurons may affect addictive behavior by contributing to brain sensitization or by modulating the brain reward system. Hypocretinergic cells, in coordination with brain stress systems may lead to a vulnerable state that facilitates the resumption of drug seeking behavior. Hence, the hypocretinergic system is a new drug target that may be used to prevent relapse of drug seeking.

Midline and intralaminar thalamic connections with the orbital and medial prefrontal networks in macaque monkeys

Although the midline and intralaminar thalamic nuclei (MITN) were long believed to project "nonspecifically," they are now known from rat studies to have restricted connections to the prefrontal cortex. This has not been studied thoroughly in primates, however, and it is not known how MITN are associated with the "orbital" and "medial" prefrontal networks. This study examined the connections of MITN in cynomolgus monkeys (Macaca fascicularis). Experiments with retrograde and anterograde tracer injections into the orbital and medial prefrontal cortex (OMPFC) showed that MITN are strongly connected with the medial prefrontal network. The dorsal nuclei of the midline thalamus, including the paraventricular (Pa) and parataenial nuclei (Pt), had heavy connections with medial network areas 25, 32, and 14c in the subgenual region. Areas 13a and 12o, which are associated with both networks, were strongly connected with the Pt and the central intermedial nucleus, respectively. Otherwise, orbital network areas had weak connections with MITN. Anterograde tracer injections into the dorsal midline thalamus resulted in heavy terminal labeling in the medial prefrontal network, most notably in areas ventral to the genu of the corpus callosum (25, 32, and 14c), but also in adjacent areas (13a and 13b). Retrograde tracer injection into the dorsal midline labeled similar areas. The medial network, particularly the subgenual region, is involved in visceral and emotional control and has been implicated in mood disorders. The strong connections between the subgenual cortex and the Pa provide a pathway through which stress signals from the Pa may influence these prefrontal circuits.

Effects of rapid eye movement sleep deprivation on hypocretin neurons in the hypothalamus of a rat model of depression

Hypocretin (Hcrt, also known as orexin) is a hypothalamic neuropeptide linked to narcolepsy, a disorder diagnosed by the appearance of rapid eye-movement sleep (REMS)-state characteristics during waking. Major targets of Hcrt-containing fibers include the locus coeruleus and the raphe nucleus, areas with important roles in regulation of mood and sleep. A relationship between REMS and mood is suggested by studies demonstrating that REMS-deprivation (REMSD) ameliorates depressive symptoms in humans. Additional support is found in animal studies where antidepressants and REMSD have similar effects on monoamiergic systems thought to be involved in major depression. Recently, we have reported that Wistar-Kyoto (WKY) rats, an animal model of depression, have reduced number and size of hypothalamic cells expressing Hcrt-immunoractivity compared to the parent, Wistar (WIS) strain, suggesting the possibility that the depressive-like attributes of the WKY rat may be determined by this relative reduction in Hcrt cells [Allard, J.S., Tizabi, Y., Shaffery, J.P., Trouth, C.O., Manaye, K., 2004. Stereological analysis of the hypothalamic hypocretin/orexin neurons in an animal model of depression. Neuropeptides 38, 311-315]. In this study, we sought to test the hypothesis that REMSD would result in a greater increase in the number and/or size of hypothalamic, Hcrt-immunoreactive (Hcrt-ir) neurons in WKY, compared to WIS rats. The effect of REMSD, using the multiple-small-platforms-over-water (SPRD) method, on size and number of Hcrt-ir cells were compared within and across strains of rats that experienced multiple-large-platforms-over-water (LPC) as well as to those in a normal, home-cage-control (CC) setting. In accord with previous findings, the number of Hcrt-ir cells was larger in all three WIS groups compared to the respective WKY groups. REMSD produced a 20% increase (p<0.02) in the number of hypothalamic Hcrt-ir neurons in WKY rats compared to cage control WKY (WKY-CC) animals. However, an unexpected higher increase in number of Hcrt-ir cells was also observed in the WKY-LPC group compared to both WKY-CC (31%, p<0.001) and WKY-SPRD (20%, p<0.002) rats. A similar, smaller, but non-significant, pattern of change was noted in WIS-LPC group. Overall the data indicate a differential response to environmental manipulations where WKY rats appear to be more reactive than WIS rats. Moreover, the findings do not support direct antidepressant-like activity for REMSD on hypothalamic Hcrt neurons in WKY rats.

Reduced orexin levels in the cerebrospinal fluid of suicidal patients with major depressive disorder

Orexins are neuropeptides selectively expressed in a small number of neurons in the lateral-posterior hypothalamus. We measured orexin-A in the cerebrospinal fluid (CSF) of 66 patients with major depressive disorder (MDD), dysthymia and adjustment disorder after a suicide attempt. Blood samples confirmed that the patients were free from antidepressive and neuroleptic medication at the time of the lumbar punctures. CSF levels of orexin-A were significantly lower in patients with MDD than in patients with adjustment disorder and dysthymia. Orexin correlated significantly with CSF levels of somatostatin, delta sleep inducing peptide-like immunoreactivity (DSIP-LI) and corticotrophin releasing factor (CRF), but not with leptin or vasopressin. Plasma levels of thyroid-stimulating hormone (TSH) were not reduced in MDD patients, and did not correlate with CSF-orexin. Our results suggest that suicidal patients with MDD have distinct neurobiological features, involving compromised levels of hypothalamic peptides regulating the state of arousal.

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.

The hypothalamic peptidergic system, hypocretin/orexin and vigilance control

Using forward and reverse genetics, the genes (hypocretin/orexin ligand and its receptor) involved in the pathogenesis of the sleep disorder, narcolepsy, in animals, have been identified. Mutations in hypocretin related-genes are extremely rare in humans, but hypocretin-ligand deficiency is found in most narcolepsy-cataplexy cases. Hypocretin deficiency in humans can be clinically detected by CSF hypocretin-1 measures, and undetectably low CSF hypocretin-1 is now included in the revised international diagnostic criteria of narcolepsy. Since hypocretin-ligand deficiency is the major pathophysiology in human narcolepsy, hypocretin replacements (using hypocretin agonists or gene therapy) are promising future therapeutic options. New insights into the roles of hypocretin system on sleep physiology have also rapidly increased. Hypocretins are involved in various fundamental hypothalamic functions such as feeding, energy homeostasis and neuroendocrine regulation. Hypocretin neurons project to most ascending arousal systems (including monoaminergic and cholinergic systems), and generally exhibit excitatory inputs. Together with the recent finding of the sleep promoting system in the hypothalamus (especially in the GABA/galanin ventrolateral preoptic area which exhibits inhibitory inputs to these ascending systems), the hypothalamus is now recognized as the most important brain site for the sleep switch, and other peptidergic systems may also participate in this regulation. Meanwhile, narcolepsy now appears to be a more complex condition than previously thought. The pathophysiology of the disease is involved in the abnormalities of sleep and various hypothalamic functions due to hypocretin deficiency, such as the changes in energy homeostasis, stress reactions and rewarding. Narcolepsy is therefore, an important model to study the link between sleep regulation and other fundamental hypothalamic functions.

Brain orexins and wake regulation in rats exposed to maternal deprivation

Maternal deprivation (MD) is a neonatal stressor that leads to behavioral and molecular manifestations of chronic stress in adulthood. Recent evidence has suggested that stress may impact wake regulation through corticotropin-releasing hormone (CRH) and the orexinergic system. We studied the wake/sleep features and brain levels of orexin and orexin receptors in adult rats neonatally subjected to either ten days of MD or a control procedure from postnatal day 4. At 3 months of age, one set of rats from both groups underwent 48 h of polysomnographic recording. All rats (including those that did not undergo surgery) were subsequently sacrificed for ELISA, radioimmunoassay and western blot measurement of orexins, orexin receptors and CRH in multiple brain regions. Neonatal MD induced an increase of total wake time (decreased total sleep) during the light period, which corresponds to human night time. This increase was specifically composed of quiet wake, while a small but significant decrease of active wake was observed during the dark period. At the molecular level, MD led to increased hypothalamic CRH and orexin A, and frontal cortical orexin 1 receptors (OX1R). However, hippocampal orexin B was reduced in the MD group. Our study discovered for the first time that the adult MD rat has sleep and neurobiological features of hyperarousal, which is typical in human insomnia. We concluded that neonatal MD produces adult hyperarousal in sleep physiology and neurobiology, and that the adult MD rat could be a model of insomnia with an orexinergic mechanism.

Hypoglycemia activates arousal-related neurons and increases wake time in adult rats

Hypoglycemia resulting from excess of exogenous or endogenous insulin elicits central nervous system activation that contributes to counterregulatory hormone secretion. In adult humans without diabetes, hypoglycemia occurring during sleep usually produces cortical activation with awakening. However, in adult humans with type 1 diabetes, hypoglycemic arousal appears blunted or absent. We hypothesized that insulin injection sufficient to produce hypoglycemia would induce awakening in adult male rats. Polysomnographic studies were carried out to characterize the effect of insulin injection on measures of sleep and waking during a circadian time of increased sleep. Compared to a baseline day, insulin treatment more than doubled the time spent awake, from 18.4+/-2.6% after saline injection to 48.0+/-5.5% after insulin. Insulin injection also reduced rapid eye movement sleep (REMS) from 27.3+/-1.8% to 5.6+/-1.3%. The percent of time in non-REM sleep (NREMS) sleep was not different between saline and insulin days, however, NREMS after insulin was fragmented, with increased number and decreased duration of episodes. These electrophysiological data indicate that insulin-induced hypoglycemia is an arousing stimulus in rats, as in nondiabetic adult humans. We also studied the effect of insulin on activation of selected arousal-related neurons using immunohistochemical detection of Fos. Fos-immunoreactivity increased in orexin (OX) neurons after insulin, from 8.7+/-4.9% after saline injection to 37+/-9% after insulin. Basal forebrain cholinergic nuclei also showed increased Fos-immunoreactivity after insulin. These correlated behavioral and histological data provide targets for future studies of the neural pathways underlying hypoglycemic arousal.

The neural circuit of orexin (hypocretin): maintaining sleep and wakefulness

Sleep and wakefulness are regulated to occur at appropriate times that are in accordance with our internal and external environments. Avoiding danger and finding food, which are life-essential activities that are regulated by emotion, reward and energy balance, require vigilance and therefore, by definition, wakefulness. The orexin (hypocretin) system regulates sleep and wakefulness through interactions with systems that regulate emotion, reward and energy homeostasis.

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.

Predator threat induces behavioral inhibition, pituitary-adrenal activation and changes in amygdala CRF-binding protein gene expression

Behavioral inhibition (BI) is an adaptive defensive response to threat; however, extreme BI is associated with anxiety-related psychopathology. When rats are exposed to a natural predator they display stress- and anxiety-related behavioral alterations and physiological activation. To develop a preclinical rodent model to study mechanisms underlying human BI and anxiety, we examined the extent to which ferret exposure elicits anxiety-related BI and HPA and amygdala activation of the CRF system. In the first experiment, BI and other behaviors were assessed in the presence or absence of a ferret. In the second experiment, ferret-induced corticosterone release and changes in brain c-fos expression were assessed. In the final experiment, gene chip and quantitative real time-PCR analyses were performed on amygdala tissue from control and ferret-exposed rats. Ferret exposure increased BI and submissive posturing, as well as plasma corticosterone and the number of Fos-positive cells in several brain regions including the amygdala. Gene expression analysis revealed increased amygdalar mRNA for CRF-binding protein, but not the CRF1 receptor, CRF2 receptor or CRF. In rodents, ferret exposure can be used to elicit anxiety-related BI, which is associated with HPA and amygdala activation. Since the amygdala and the CRF system have been implicated in adaptive and maladaptive anxiety responses in humans, these data support use of our rodent model to further investigate mechanisms underlying anxiety-related psychopathology in humans.

Noradrenergic inputs to the bed nucleus of the stria terminalis and paraventricular nucleus of the hypothalamus underlie hypothalamic-pituitary-adrenal axis but not hypophagic or conditioned avoidance responses to systemic yohimbine

The alpha2 adrenoceptor antagonist yohimbine (YO) increases transmitter release from adrenergic/noradrenergic (NA) neurons. Systemic YO activates the hypothalamic-pituitary-adrenal (HPA) axis, inhibits feeding, and supports conditioned flavor avoidance (CFA) in rats. To determine whether these effects require NA inputs to the bed nucleus of the stria terminalis (BNST), vehicle or saporin toxin conjugated to an antibody against dopamine beta hydroxylase (DSAP) was microinjected bilaterally into the BNST to remove its NA inputs. Subsequent tests failed to reveal any lesion effect on the ability of YO (5.0 mg/kg, i.p.) to inhibit food intake or to support CFA. Conversely, HPA axis responses to YO were significantly blunted in DSAP rats. In a terminal experiment, DSAP and control rats were perfused 90-120 min after intraperitoneal injection of YO or vehicle. Brains were processed to reveal Fos immunolabeling and lesion extent. NA fibers were markedly depleted in the BNST and medial parvocellular paraventricular hypothalamus (PVNmp) in DSAP rats, evidence for collateralized NA inputs to these regions. DSAP rats displayed significant loss of caudal medullary NA neurons, and markedly blunted Fos activation in the BNST and in corticotropin-releasing hormone-positive PVNmp neurons after YO. We conclude that a population of medullary NA neurons provides collateral inputs to the BNST and PVNmp, and that these inputs contribute importantly to Fos expression and HPA axis activation after YO treatment. Conversely, NA-mediated activation of BNST and PVNmp neurons is unnecessary for YO to inhibit food intake or support CFA, evidence for the sufficiency of other intact neural pathways in mediating those effects.

Addiction and arousal: alternative roles of hypothalamic peptides

The importance of the lateral hypothalamus in the regulation of reward and motivation has long been recognized. However, the neuronal network involved in such a hypothalamic regulation of reward remains essentially unknown. Recently, hypocretin-containing neurons, a group of hypothalamic neurons known to be associated with the stability of arousal, have emerged as important structures in the control of brain reward function. This review summarizes a Mini-Symposium presented at the 2006 Annual Meeting of the Society for Neuroscience.

Excitatory action of hypocretin/orexin on neurons of the central medial amygdala

The neurons of the lateral hypothalamus that contain hypocretin/orexin (hcrt/orx) are thought to promote arousal through the excitatory action they exert on the multiple areas to which they project within the CNS. We show here that the hcrt/orx peptides can also exert a strong action on the amygdala, a structure known for its implication in emotional aspects of behavior. Indeed, the hcrt/orx peptides, applied in acute rat brain slices, excite a specific class of "low threshold burst" neurons in the central medial (CeM) nucleus which is considered as a major output of the amygdala. These excitatory effects are postsynaptic, mediated by Hcrt2/OX2 receptors and result from the closure of a potassium conductance. They occur on a class of neurons that are also excited by vasopressin acting through V1a receptors. These results suggest that the hcrt/orx system can act through the amygdala to augment arousal and evoke the autonomic and behavioral responses associated with fear, stress or emotion.

Orexin-induced apoptosis: the key role of the seven-transmembrane domain orexin type 2 receptor

Orexin-A and orexin-B are regulatory peptides involved in the control of feeding, sleep-wakefulness, and exerting various endocrine and metabolic actions. Recently we demonstrated that orexins, acting at OX(1) receptor (OX(1)R), are proapoptotic peptides. The aim of this study was to investigate the role of the receptor subtype OX(2)R in the control of apoptosis. Orexins caused a caspase-dependent cell death by apoptosis and a drastic cell growth inhibition in Chinese hamster ovary cells transfected with OX(2)R cDNA. On addition of either orexin (10(-6) m) for 48 h, apoptosis was demonstrated by DNA fragmentation, chromatin condensation, annexin-V binding, and activation of caspase-3 and caspase-9. Orexins were active on apoptosis and cell growth inhibition in the range of concentrations between 10(-10) and 10(-5) m with an EC(50) of 5 x 10(-8) m peptides. No effect of orexins could be detected in parental Chinese hamster ovary cells. A rat pancreatic acinar cell line, AR42J, which expresses OX(2)R but not OX(1)R, also underwent growth suppression and apoptosis on treatment with orexins. Suppression of AR42J cell growth by 10(-6) m orexin was more than 75% after 24 h. Induction of annexin-V-labeled AR42J cell number was dose dependent, with EC(50) of 5.1 x 10(-8) m orexin-A and 9.8 x 10(-8) m orexin-B. The OX(2)R agonist [Ala (11), d-Leu (15)]orexin-B promoted effects on cell growth and apoptosis, which were similar to those elicited by orexins. The OX(1)R antagonist SB33487 did not alter orexin-induced inhibition of growth or orexin-induced stimulation of apoptosis in AR42J cells. For the first time, we provide functional and pharmacological evidence for a role of the OX(2)R in orexin-induced apoptosis.

Arousal and reward: a dichotomy in orexin function

The orexins (or hypocretins) are neuropeptide transmitters made exclusively in hypothalamic neurons that have extensive CNS projections. Previous studies reported that this system is most strongly associated with feeding, arousal and the maintenance of waking. We review here recent studies that reveal a novel and important role for the orexin/hypocretin neuronal system in reward processing and addiction. We propose that the current evidence indicates a dichotomy in orexin function, such that orexin neurons in the lateral hypothalamus regulate reward processing for both food and abused drugs, whereas those in the perifornical and dorsomedial hypothalamus regulate arousal and response to stress. Evidence also indicates roles for lateral hypothalamus orexin neurons and ventral tegmental orexin receptors in reward-based learning and memory.

Orexin neuronal changes in the locus coeruleus of the aging rhesus macaque

Orexin neuropeptides regulate arousal state and excite the noradrenergic locus coeruleus (LC), so it is plausible that an age-related loss of orexin neurons and projections to the LC contributes to poor sleep quality in elderly humans and nonhuman primates. To test this hypothesis we examined orexin B-immunoreactivity in the lateral hypothalamic area (LHA) and the LC of male rhesus macaques (Macaca mulatta) throughout the life span. Orexin perikarya, localized predominantly in the LHA, showed identical distribution patterns irrespective of age. Similarly, orexin neuron number and serum orexin B concentrations did not differ with age. In contrast, orexin B-immunoreactive axon density in the LC of old animals was significantly lower than that observed in the young or adult animals. Furthermore, the age-related decline was associated with a significant decrease in tyrosine hydroxylase (TH) mRNA in the LC, despite no change in TH-immunoreactive neuron number. Taken together, these data suggest that age-related decreases in excitatory orexin innervation to the noradrenergic LC may contribute to the etiology of poor sleep quality in the elderly.

Integration of feeding and spontaneous physical activity: role for orexin

Spontaneous physical activity is activity that is non-volitional, or subconscious, such as fidgeting and shifting in one's seat, and time spent moving (standing and ambulating). Recent evidence indicates that spontaneous physical activity, and the resulting thermogenesis (non-exercise activity thermogenesis) may be regulated by brain systems. A large number of brain areas, with their associated neurotransmitter populations and connectivity, participate in the regulation of feeding behavior by acting as energy sensing and modulating centers. Although less well characterized, it is likely that a multitude of neurotransmitters and brain areas act to mediate spontaneous physical activity. These two behaviors, feeding and spontaneous physical activity, affect energy intake and expenditure and thus are important to body weight. Interestingly, often the two behaviors are affected simultaneously; when feeding is affected, so too is spontaneous physical activity, and both food intake and physical activity (whether spontaneous or volitional) influence activity of brain areas important to both. Several brain areas and neuropeptides are important to feeding and spontaneous physical activity. The lateral hypothalamus is one area that appears important to both behaviors, as stimulation or lesion of this region produces alterations in feeding behavior and spontaneous physical activity. Orexin neurons, with their central location in the lateral hypothalamus, widespread projections and connectivity to other brain areas important to energy homeostasis, are well situated to perform an integrative function. This review focuses on how hypothalamic orexins participate in both feeding and spontaneous physical activity, and provides potential models for the integration of signals important to both.

The orexin system regulates alcohol-seeking in rats

1. Orexin-containing neurons have been implicated in feeding, sleep-wake cycles and more recently in drug-seeking behaviour. 2. Pretreatment of alcohol-preferring (iP) rats with an orexin1 receptor antagonist (SB-334867, 20 mg kg(-1), intraperitoneally) completely abolished an olfactory cue-induced reinstatement of alcohol-seeking behaviour, and also attenuated alcohol responding under an operant fixed ratio regimen without affecting water responding. 3. The mRNA encoding orexin within the hypothalamus was expressed at a similar density in iP and non-preferring (NP) rats; chronic consumption of ethanol in iP rats did not significantly regulate the density of this expression, but did increase the area of expression within the lateral, but not medial, hypothalamus. 4. These data indicate that while orexin may not be implicated in the development of an alcohol preference, re-exposure of cues previously associated with alcohol availability is sufficient and adequate to activate orexin-containing neurons and drive reinstatement of alcohol-seeking.

Multiple components of the defense response depend on orexin: Evidence from orexin knockout mice and orexin neuron-ablated mice

Stressor induces not only cognitive, emotional and behavioral changes but also autonomic changes. Although research on the neural circuits underlying such autonomic changes has implicated the hypothalamus in the defense response against stressors, neurotransmitters in this multifaceted and coordinated response have not been revealed. In this brief review, here we summarize our recent discovery using orexin knockout mice and orexin neuron-ablated mice of possible contribution of orexin in the defense response and discuss future directions.

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

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

Corticotropin-releasing factor neurons in the hypothalamic paraventricular nucleus are involved in arousal/yawning response of rats

Our previous studies have suggested that activation of the hypothalamic paraventricular (PVN) descending oxytocinergic projections is involved in the induction of yawning accompanied by an arousal response, but the possibility that neural systems other than the oxytocinergic system in the PVN also mediate the arousal/yawning response cannot be ruled out. We assessed the activity of corticotropin-releasing factor (CRF) neurons during yawning induced by the PVN stimulation in anesthetized, spontaneously breathing rats using double-staining for c-Fos and CRF. Yawning response was evaluated by monitoring an intercostals electromyogram as an index of inspiratory activity and a digastric electromyogram as an indicator of mouth opening. We also recorded the electrocorticogram (ECoG) to determine the arousal response during yawning. Microinjection of l-glutamate (2-5 nmol) into the PVN produced a frequent yawning accompanied by an arousal shift in the ECoG, and these behavioral effects were associated with a significant increase of c-Fos positive CRF neurons in the medial parvocellular subdivision of the PVN. In addition, a marked enhancement in the c-Fos expression was found in the both locus coeruleus (LC) and global area in the cortex when the frequency of yawning response was increased by the PVN stimulation, suggesting that the arousal response during yawning might be mediated by the activation of LC neurons. The present study suggests that an activation of CRF neurons in the PVN is responsible for the arousal response accompanied by yawning behavior.

New approaches to antidepressant drug discovery: beyond monoamines

All available antidepressant medications are based on serendipitous discoveries of the clinical efficacy of two classes of antidepressants more than 50 years ago. These tricyclic and monoamine oxidase inhibitor antidepressants were subsequently found to promote serotonin or noradrenaline function in the brain. Newer agents are more specific but have the same core mechanisms of action in promoting these monoamine neurotransmitters. This is unfortunate, because only approximately 50% of individuals with depression show full remission in response to these mechanisms. This review summarizes the obstacles that have hindered the development of non-monoamine-based antidepressants, and provides a progress report on some of the most promising current strategies.

Cholecystokinin activates orexin/hypocretin neurons through the cholecystokinin A receptor

Orexin A and B are neuropeptides implicated in the regulation of sleep/wakefulness and energy homeostasis. The regulatory mechanism of the activity of orexin neurons is not precisely understood. Using transgenic mice in which orexin neurons specifically express yellow cameleon 2.1, we screened for factors that affect the activity of orexin neurons (a total of 21 peptides and six other factors were examined) and found that a sulfated octapeptide form of cholecystokinin (CCK-8S), neurotensin, oxytocin, and vasopressin activate orexin neurons. The mechanisms that underlie CCK-8S-induced activation of orexin neurons were studied by both calcium imaging and slice patch-clamp recording. CCK-8S induced inward current in the orexin neurons. The CCKA receptor antagonist lorglumide inhibited CCK-8S-induced activation of orexin neurons, whereas the CCKB receptor agonists CCK-4 (a tetrapeptide form of cholecystokinin) and nonsulfated CCK-8 had little effect. The CCK-8S-induced increase in intracellular calcium concentration was eliminated by removing extracellular calcium but not by an addition of thapsigargin. Nifedipine, omega-conotoxin, omega-agatoxin, 4-ethylphenylamino-1,2-dimethyl-6-methylaminopyrimidinium chloride, and SNX-482 had little effect, but La3+, Gd3+, and 2-aminoethoxydiphenylborate inhibited CCK-8S-induced calcium influx. Additionally, the CCK-8S-induced inward current was dramatically enhanced in the calcium-free solution and was inhibited by the cation channel blocker SKF96365, suggesting an involvement of extracellular calcium-sensitive cation channels. CCK-8S did not induce an increase in intracellular calcium concentration when membrane potential was clamped at -60 mV, suggesting that the calcium increase is induced by depolarization. The evidence presented here expands our understanding of the regulation of orexin neurons and the physiological role of CCK in the CNS.

Afferents to the orexin neurons of the rat brain

Emotions, stress, hunger, and circadian rhythms all promote wakefulness and behavioral arousal. Little is known about the pathways mediating these influences, but the orexin-producing neurons of the hypothalamus may play an essential role. These cells heavily innervate many wake-promoting brain regions, and mice lacking the orexin neurons have narcolepsy and fail to rouse in response to hunger (Yamanaka et al. [2003] Neuron 38:701-713). To identify the afferents to the orexin neurons, we first injected a retrograde tracer into the orexin neuron field of rats. Retrogradely labeled neurons were abundant in the allocortex, claustrum, lateral septum, bed nucleus of the stria terminalis, and in many hypothalamic regions including the preoptic area, dorsomedial nucleus, lateral hypothalamus, and posterior hypothalamus. Retrograde labeling in the brainstem was generally more modest, but labeling was strong in the periaqueductal gray matter, dorsal raphe nucleus, and lateral parabrachial nucleus. Injection of an anterograde tracer confirmed that most of these regions directly innervate the orexin neurons, with some of the heaviest input coming from the lateral septum, preoptic area, and posterior hypothalamus. In addition, hypothalamic regions preferentially innervate orexin neurons in the medial and perifornical parts of the field, but most projections from the brainstem target the lateral part of the field. Inputs from the suprachiasmatic nucleus are mainly relayed via the subparaventricular zone and dorsomedial nucleus. These observations suggest that the orexin neurons may integrate a variety of interoceptive and homeostatic signals to increase behavioral arousal in response to hunger, stress, circadian signals, and autonomic challenges.

Stress and arousal: the corticotrophin-releasing factor/hypocretin circuitry

The hypocretins (also know as orexins) are two neuropeptides now commonly described as critical components for maintaining and regulating the stability of arousal. Several lines of evidence have raised the hypothesis that hypocretin-producing neurons are part of the circuitries that mediate the hypothalamic response to acute stress. New data indicate that the corticotrophin-releasing factor (CRF) peptidergic system directly innervates hypocretin-expressing neurons. CRF depolarizes hypocretin neurons, and this effect is blocked by a CRF-R1 antagonist. Furthermore, activation of hypocretinergic neurons by stress is impaired in CRF-R1 knockout mice. These data suggest that CRF-R1 receptor mediates the stress-induced activation of the hypocretinergic system. A significant amount of evidence also indicates that hypocretin cells connect reciprocally to the CRF system. We propose that upon stressor stimuli, CRF activates the hypocretin system, which relays these signals to brain stem nuclei involved in the modulation of arousal as well as to the extended amygdala, a structure involved in the negative motivational state that drives addiction.

Sleep disturbances, psychiatric disorders, and psychotropic drugs

Brain neurotransmitter dysfunctions involved in the pathophysiological processes of psychiatric disorders are likely to be reflected by concomitant alterations in sleep continuity and architecture. Since the corrective effects of psychotropic drugs on dysfunctional neurotransmission systems can be evidenced through polysomnographic recordings, one may consider sleep as a kind of “window” on the neurobiology of psychiatric disorders. During the last 10 years, major breakthroughs in our understanding of sleep-wake mechanisms have provided some indications on how psychotropic drugs could influence the sleep-wake cycle. In this review, recent inroads into the understanding of sleep regulatory neural mechanisms are introduced and discussed in terms of the effects of psychotropic drugs. The relationship between the patho-physiological process of a disease, its consequence on sleep, and the corrective effect of a psychotropic drug are exemplified by two psychopathological states: substance withdrawal and major depression. One may conclude that polysomnographic recordings are a unique noninvasive tool to analyze brain functioning, and are particularly well suited to evaluating the objective effects of new psychotropic drugs.

Role for hypocretin in mediating stress-induced reinstatement of cocaine-seeking behavior

Hypocretin-1 and -2 (Hcrt-1 and Hcrt-2), also referred to as orexin-A and -B, are neuropeptides synthesized by a few thousand neurons in the lateral hypothalamus. Hypocretin-containing neurons project throughout the brain, with a prominent input to basal forebrain structures involved in motivation, reward, and stress. However, the role of hypocretins in addiction-related behaviors remains largely unexplored. Here we show that intracerebroventricular infusions of Hcrt-1 lead to a dose-related reinstatement of cocaine seeking without altering cocaine intake in rats. Hcrt-1 also dramatically elevates intracranial self-stimulation thresholds, indicating that, unlike treatments with reinforcing properties such as cocaine, Hcrt-1 negatively regulates the activity of brain reward circuitries. Hypocretin-induced reinstatement of cocaine seeking was prevented by blockade of noradrenergic and corticotropin-releasing factor systems, suggesting that Hcrt-1 reinstated drug seeking through induction of a stress-like state. Consistent with this interpretation, the selective Hcrt-1 receptor antagonist SB-334867 blocked footshock-induced reinstatement of previously extinguished cocaine-seeking behavior. These findings reveal a previously unidentified role for hypocretins in driving drug seeking through activation of stress pathways in the brain.

The hypocretins and sleep

The hypocretins (also called the orexins) are two neuropeptides derived from the same precursor whose expression is restricted to a few thousand neurons of the lateral hypothalamus. Two G-protein coupled receptors for the hypocretins have been identified, and these show different distributions within the central nervous system and differential affinities for the two hypocretins. Hypocretin fibers project throughout the brain, including several areas implicated in regulation of the sleep/wakefulness cycle. Central administration of synthetic hypocretin-1 affects blood pressure, hormone secretion and locomotor activity, and increases wakefulness while suppressing rapid eye movement sleep. Most human patients with narcolepsy have greatly reduced levels of hypocretin peptides in their cerebral spinal fluid and no or barely detectable hypocretin-containing neurons in their hypothalamus. Multiple lines of evidence suggest that the hypocretinergic system integrates homeostatic, metabolic and limbic information and provides a coherent output that results in stability of the states of vigilance.

Hypocretins: waking, arousal, or action?

The role of hypocretin (orexin) neurotransmission in waking and arousal, though of intense interest, is poorly understood. In this issue of Neuron, demonstrate that, in general, hypocretin neurons are minimally active during both sleep and quiet waking. In contrast, these neurons display robust activity during periods of alert and/or active waking.

Injection of neuropeptide W into paraventricular nucleus of hypothalamus increases food intake

Neuropeptide W (NPW) is an endogenous ligand for G protein-coupled receptor 7 (GPR7). There are two forms of the peptide, designated as neuropeptide W-23 (NPW23) and neuropeptide W-30 (NPW30). In the current study we found that intracerebroventricular administration of NPW23 increased c-Fos immunoreactivity (IR) in a variety of brain sites, many of which are involved in the regulation of feeding. In particular, we noted that c-Fos IR levels were increased in hypocretin-expressing neurons in the perifornical region of the lateral hypothalamus (LH). We then studied whether injection of NPW23 into the paraventricular nucleus of the hypothalamus (PVN) and the LH increased food intake over a 24-h time period. Intra-PVN injection of NPW23 at doses ranging from 0.1 to 3 nmol increased feeding for up to 4 h, and doses ranging from 0.3 to 3 nmol increased feeding for up to 24 h. In contrast, only the 3-nmol dose of NPW23 increased feeding after administration into the LH. Together, these data suggest a modulatory role for NPW in the control of food intake.

Input organization and plasticity of hypocretin neurons: possible clues to obesity's association with insomnia

The lateral hypothalamic hypocretin (also called orexin) neurons have emerged as instrumental in triggering arousal and regulating energy metabolism. The lack of hypocretin signaling is the cause of narcolepsy while elevated hypocretin levels induce arousal, elevated food intake, and adiposity. Here, we report an unorthodox synaptic organization on the hypocretin neurons in which excitatory synaptic currents and asymmetric synapses exert control on the cell bodies of these long-projective neurons with minimal inhibitory input. Overnight food deprivation promotes the formation of more excitatory synapses and synaptic currents onto hypocretin cells; this is reversed by re-feeding and blocked by leptin administration. This unique wiring and acute stress-induced plasticity of the hypocretin neurons correlates well with their being involved in the control of arousal and alertness that are so vital to survival, but this circuitry may also be an underlying cause of insomnia and associated metabolic disturbances, including obesity.

Sleep disturbances, psychiatric disorders, and psychotropic drugs

Brain neurotransmitter dysfunctions involved in the pathophysiological processes of psychiatric disorders are likely to be reflected by concomitant alterations in sleep continuity and architecture. Since the corrective effects of psychotropic drugs on dysfunctional neurotransmission systems can be evidenced through polysomnographic recordings, one may consider sleep as a kind of "window" on the neurobiology of psychiatric disorders. During the last 10 years, major breakthroughs in our understanding of sleep-wake mechanisms have provided some indications on how psychotropic drugs could influence the sleep-wake cycle. In this review, recent inroads into the understanding of sleep regulatory neural mechanisms are introduced and discussed in terms of the effects of psychotropic drugs. The relationship between the pathophysiological process of a disease, its consequence on sleep, and the corrective effect of a psychotropic drug are exemplified by two psychopathological states: substance withdrawal and major depression. One may conclude that polysomnographic recordings are a unique noninvasive tool to analyze brain functioning, and are particularly well suited to evaluating the objective effects of new psychotropic drugs.

Brain structures and receptors involved in alertness

Transitions between sleep and wakefulness are regulated by complex neurobiological mechanisms, which ultimately can be delineated as oscillations between two opponent processes--one promoting sleep and the other promoting wakefulness. The suprachiasmatic nuclei (SCN) provide temporal organization to the sleep-wake cycle through arousal mechanisms that oppose homeostatic drive or sleep. Assuming that individual cells in the SCN are competent circadian oscillators, it is important to understand how these cells communicate and remain synchronized with each other. Examination of the brain structures and receptors that are involved in alertness and the complex phenomena involved in regulation of the circadian sleep-wake cycle has provided evidence for an important role for the noradrenergic locus coeruleus (LC) system in the circadian regulation of alertness and performance. However, the broad interest in mechanisms underlying alertness is not solely to understand wakefulness but also to gain insight into how to maintain alertness and cognitive performance while awake. Few studies have attempted to link the role of a brain system in sleep-wake regulation with a role in cognitive performance during waking. We hypothesize that the dorsomedial hypothalamic nucleus (DMH) modulates the circadian rhythm of sleep and waking via projections to the LC. We propose a SCN-DMH-LC signalling pathway that may influence the activity of the LC and thereby a variety of central nervous system functions related to noradrenergic innervations, including alertness, vigilance, attention, learning and memory. The influence of sleep drive on the LC system may be important for our understanding of the deleterious effects of sleep loss on performance, and presents a logical target for developing new treatments to counteract impairments in alertness and performance due to poor quality sleep.