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

The role of the neuropeptide S system in addiction: focus on its interaction with the CRF and hypocretin/orexin neurotransmission

Recent behavioral, pharmacological and molecular findings have linked the NPS system to drug dependence. Most of the evidence supports the possibility that increased NPS activity may contribute to shaping vulnerability to addiction, especially relapse. However, data suggesting that the anxiolytic-like properties of NPS may have protective effects on addiction have been also published. In addition, evidence from conditioned place preference experiments, though not unequivocal, suggests that NPS per se is devoid of motivational properties. Intriguingly, several effects of NPS on drugs of abuse appear to be mediated by downstream activation of brain corticotrophin releasing factor (CRF) and hypocretin-1/orexin-A (Hcrt-1/Ox-A) systems. The major objective of the present article is to review the existing work on NPS and addiction. Particular attention is devoted to the interpretation of findings revealing complex neuroanatomical and functional interactions between NPS, CRF, and the Hcrt-1/Ox-A systems. Original data aimed at shedding light on the role of NPS in reward processing are also shown. Finally, existing findings are discussed within the framework of addiction theories, and the potential of the NPS system as a treatment target for addiction is analyzed.

The Orexin-1 Receptor Antagonist SB-334867 Reduces Alcohol Relapse Drinking, but not Alcohol-Seeking, in Alcohol-Preferring (P) Rats

PRINCIPLE

The orexin system has been hypothesized to regulate drug-seeking and drug self-administration behaviors, including ethanol (EtOH) seeking and consumption. However, studies on the effects of orexin receptor antagonists have not been conducted on robust alcohol-relapse behavior.

OBJECTIVES

This study assessed the effects of the orexin-1 receptor antagonist, SB-334867, on alcohol-seeking behavior and responding for alcohol under relapse conditions.

METHODS

Adult alcohol-preferring (P) rats self-trained in 2-lever operant chambers to administer 15% EtOH (vol/vol) on a fixed-ratio-5 and water on a fixed-ratio-1 schedule of reinforcement. After 10 weeks, rats underwent extinction training for 7 sessions. Animals were then maintained in their home cages for 2 weeks before being tested for Pavlovian Spontaneous Recovery (PSR; a measure of alcohol seeking) for 4 sessions. Rats were then allowed a week in their home cages before being returned to the operant chamber with access to EtOH and water (relapse). Thirty minutes before the PSR and relapse test sessions, rats received 0, 10, or 20 mg/kg SB-334867.

RESULTS

Responses on the EtOH lever during the 1st PSR test session were ~70 presses/session (3-fold higher than baseline); SB-334867 did not alter responses on the EtOH lever. Under relapse conditions, P rats increased responding on the EtOH lever from 250 (at baseline) to 350 responses/session; both doses of SD-334867 prevented this increase.

CONCLUSIONS

The results of this study suggest that activation of orexin-1 receptors is not involved in intrinsically initiated EtOH seeking, but may regulate the consummatory behavior of EtOH consumption.

Putative genes mediating the effects of orexins in the posterior paraventricular thalamus on neuroendocrine and behavioral adaptations to repeated stress

Exposure to repeated stress is often associated with psychopathology. However, our understanding of the underlying neural circuitry that regulates responses to repeated stress is limited. The posterior paraventricular thalamus (pPVT) is a brain region responsible for transmission of multimodal sensory information to limbic structures that regulate responses to both acute and repeated stress. Orexin-containing cells originating in the hypothalamus heavily innervate the pPVT. Our previous work has shown that activation of orexin1 receptors in the pPVT during repeated swim stress is important for facilitation of the hypothalamic-pituitary-adrenal (HPA) axis response to subsequent novel restraint. However, the genes responsible for these orexin-mediated adaptations to repeated stress are not known. Using a custom PCR array we examined the expression of 186 specific mRNAs in the pPVT of animals exposed to repeated swim stress (4 days of 15min swim/day) with or without direct pPVT microinfusion of the orexin1 receptor antagonist SB334867 prior to each daily swim stress. Tissue was collected the next morning under basal non stressed conditions. Repeated stress and/or orexin receptor blockade significantly altered expression of only 9 specific genes including growth factors (Vegfa, Bax and Mt3), G-protein coupled receptors (Adora2a, Grm2 and Crhr1), immune-related genes (Ptgs2 and Cx3cr1) and an epigenetic-related gene (Hdac5). These genes represent potential targets for further characterization of orexin-mediated adaptations to repeated stress in the pPVT.

Mechanism for Hypocretin-mediated sleep-to-wake transitions

Current models of sleep/wake regulation posit that Hypocretin (Hcrt)-expressing neurons in the lateral hypothalamus promote and stabilize wakefulness by projecting to subcortical arousal centers. However, the critical downstream effectors of Hcrt neurons are unknown. Here we use optogenetic, pharmacological, and computational tools to investigate the functional connectivity between Hcrt neurons and downstream noradrenergic neurons in the locus coeruleus (LC) during nonrapid eye movement (NREM) sleep. We found that photoinhibiting LC neurons during Hcrt stimulation blocked Hcrt-mediated sleep-to-wake transitions. In contrast, when LC neurons were optically stimulated to increase membrane excitability, concomitant photostimulation of Hcrt neurons significantly increased the probability of sleep-to-wake transitions compared with Hcrt stimulation alone. We also built a conductance-based computational model of Hcrt-LC circuitry that recapitulates our behavioral results using LC neurons as the main effectors of Hcrt signaling. These results establish the Hcrt-LC connection as a critical integrator-effector circuit that regulates NREM sleep/wake behavior during the inactive period. This coupling of distinct neuronal systems can be generalized to other hypothalamic integrator nuclei with downstream effector/output populations in the brain.

Neurogenesis-independent antidepressant-like effects on behavior and stress axis response of a dual orexin receptor antagonist in a rodent model of depression

Growing evidence indicates that an increase of orexin (or hypocretin) signaling is involved in the pathophysiology of major depression, but little is known regarding the causal link between the orexinergic system and depressive-like states. Here we blocked orexin receptors in mice subjected to unpredictable chronic mild stress (UCMS) to investigate putative antidepressant-like effects of this treatment, as well as the underlying mechanisms. BALB/c mice were exposed to 9 weeks of UCMS and from the third week onward treated daily with fluoxetine (20 mg/kg per day, per os) or with the dual orexin receptor antagonist almorexant (100 mg/kg per day, per os). The effects of UCMS regimen and pharmacological treatments were assessed by physical measures and behavioral testing. The dexamethasone suppression test was performed to examine the integrity of the negative feedback of the hypothalamic-pituitary-adrenal (HPA) axis, and immunohistochemical markers were used to assess cell proliferation (Ki-67), immature newborn neurons (doublecortin), and mature newborn neurons (5-bromo-2'-deoxyuridine/NeuN) in the dorsal and ventral parts of the hippocampus. Our results show that 7 weeks of fluoxetine or almorexant treatments counteract the UCMS-induced physical and behavioral alterations. Both treatments prevented the HPA axis dysregulation caused by UCMS, but only fluoxetine reversed the UCMS-induced decrease of hippocampal cell proliferation and neurogenesis, while chronic almorexant treatment decreased cell proliferation and neurogenesis specifically in the ventral hippocampus. Taken together, this is the first evidence that pharmacological blockade of the orexinergic system induces a robust antidepressant-like effect and the restoration of stress-related HPA axis defect independently from a neurogenic action.

Somatostatin varicosities contain the vesicular GABA transporter and contact orexin neurons in the hypothalamus

Somatostatin (SST) is a neuropeptide with known inhibitory actions in the hypothalamus, where it inhibits release of growth hormone-releasing hormone (GHRH), while also influencing the sleep-wake cycle. Here we investigated in the rat whether SST neurons might additionally release GABA (gamma-aminobutyric acid) or glutamate in different regions and whether they might contact orexin neurons that play an important role in the maintenance of wakefulness. In dual-immunostained sections viewed by epifluorescence microscopy, we examined if SST varicosities were immunopositive for the vesicular transporter for GABA (VGAT) or glutamate (VGLUT2) in the posterolateral hypothalamus and neighboring arcuate nucleus and median eminence. Of the SST varicosities in the posterolateral hypothalamus, 18% were immunopositive for VGAT, whereas ≤ 1% were immunopositive for VGLUT2. In the arcuate and median eminence, 26 and 64% were VGAT+ and < 3% VGLUT2 + , respectively. In triple-immunostained sections viewed by epifluorescence and confocal microscopy, SST varicosities were seen in contact with orexin somata, and of these varicosities, a significant proportion (23%) contained VGAT along with synaptophysin, the presynaptic marker for small synaptic vesicles, and a similar proportion (25%) abutted puncta that were immunostained for gephyrin, the postsynaptic marker for GABAergic synapses. Our results indicate that a significant proportion of SST varicosities in the hypothalamus have the capacity to release GABA, to form inhibitory synapses upon orexin neurons, and accordingly through their peptide and/or amino acid, to inhibit orexin neurons, as well as GHRH neurons. Thus while regulating GHRH release, SST neurons could serve to attenuate arousal and permit progression through the sleep cycle.

Diversity and noise effects in a model of homeostatic regulation of the sleep-wake cycle

Recent advances in sleep neurobiology have allowed development of physiologically based mathematical models of sleep regulation that account for the neuronal dynamics responsible for the regulation of sleep-wake cycles and allow detailed examination of the underlying mechanisms. Neuronal systems in general, and those involved in sleep regulation in particular, are noisy and heterogeneous by their nature. It has been shown in various systems that certain levels of noise and diversity can significantly improve signal encoding. However, these phenomena, especially the effects of diversity, are rarely considered in the models of sleep regulation. The present paper is focused on a neuron-based physiologically motivated model of sleep-wake cycles that proposes a novel mechanism of the homeostatic regulation of sleep based on the dynamics of a wake-promoting neuropeptide orexin. Here this model is generalized by the introduction of intrinsic diversity and noise in the orexin-producing neurons, in order to study the effect of their presence on the sleep-wake cycle. A simple quantitative measure of the quality of a sleep-wake cycle is introduced and used to systematically study the generalized model for different levels of noise and diversity. The model is shown to exhibit a clear diversity-induced resonance: that is, the best wake-sleep cycle turns out to correspond to an intermediate level of diversity at the synapses of the orexin-producing neurons. On the other hand, only a mild evidence of stochastic resonance is found, when the level of noise is varied. These results show that disorder, especially in the form of quenched diversity, can be a key-element for an efficient or optimal functioning of the homeostatic regulation of the sleep-wake cycle. Furthermore, this study provides an example of a constructive role of diversity in a neuronal system that can be extended beyond the system studied here.

Antagonism of orexin-1 receptors attenuates swim- and restraint stress-induced antinociceptive behaviors in formalin test

Orexin (ORX) plays an important role in pain modulation. ORX receptors have been found in many brain structures and are known to be involved in pain processing. It is well-established that the acute and chronic forms of stress could induce hormonal and neuronal changes that affect both pain threshold and nociceptive behaviors. The role of OX1R receptors in stress-induced analgesia (SIA) has not been fully elucidated. In the present study, using the formalin test, attempts were made to evaluate the effects of acute immobilization restraint stress and swimming stress on pain behavioral responses following OX1R antagonist administration in rats. Animals received OX1R antagonist (SB-334867), vehicle, or naloxone before exposure to acute restraint stress (30min) or swimming stress test (6min, 20±1°C), and immediately submitted to hind paw formalin injection (50μl, 2%). Acute 30-min exposure to restraint stress as well as 6-min exposure to swim stress could significantly reduce the formalin-induced nociceptive behaviors in rats. This antinociceptive effect with either restraint stress or swim stress was fully prevented by OX1R antagonist (SB-334867), while the SB-334867 alone had no effect. However, the opioid receptor antagonist naloxone could not totally reverse the antinociception effect with either form of stress. It is suggested that OX1R might be involved in antinociception behaviors induced by these two forms of stress. These data highlight the significant role of OX1R as a novel target for treatment of stress-related disorders.

Brain orexin promotes obesity resistance

Resistance to obesity is becoming an exception rather than the norm, and understanding mechanisms that lead some to remain lean in spite of an obesigenic environment is critical if we are to find new ways to reverse this trend. Levels of energy intake and physical activity both contribute to body weight management, but it is challenging for most to adopt major long-term changes in either factor. Physical activity outside of formal exercise, also referred to as activity of daily living, and in stricter form, spontaneous physical activity (SPA), may be an attractive modifiable variable for obesity prevention. In this review, we discuss individual variability in SPA and NEAT (nonexercise thermogenesis, or the energy expended by SPA) and its relationship to obesity resistance. The hypothalamic neuropeptide orexin (hypocretin) may play a key role in regulating SPA and NEAT. We discuss how elevated orexin signaling capacity, in the context of a brain network modulating SPA, may play a major role in defining individual variability in SPA and NEAT. Greater activation of this SPA network leads to a lower propensity for fat mass gain and therefore may be an attractive target for obesity prevention and therapy.

Dual hypocretin receptor antagonism is more effective for sleep promotion than antagonism of either receptor alone

The hypocretin (orexin) system is involved in sleep/wake regulation, and antagonists of both hypocretin receptor type 1 (HCRTR1) and/or HCRTR2 are considered to be potential hypnotic medications. It is currently unclear whether blockade of either or both receptors is more effective for promoting sleep with minimal side effects. Accordingly, we compared the properties of selective HCRTR1 (SB-408124 and SB-334867) and HCRTR2 (EMPA) antagonists with that of the dual HCRTR1/R2 antagonist almorexant in the rat. All 4 antagonists bound to their respective receptors with high affinity and selectivity in vitro. Since in vivo pharmacokinetic experiments revealed poor brain penetration for SB-408124, SB-334867 was selected for subsequent in vivo studies. When injected in the mid-active phase, SB-334867 produced small increases in rapid-eye-movement (REM) and non-REM (NR) sleep. EMPA produced a significant increase in NR only at the highest dose studied. In contrast, almorexant decreased NR latency and increased both NR and REM proportionally throughout the subsequent 6 h without rebound wakefulness. The increased NR was due to a greater number of NR bouts; NR bout duration was unchanged. At the highest dose tested (100 mg/kg), almorexant fragmented sleep architecture by increasing the number of waking and REM bouts. No evidence of cataplexy was observed. HCRTR1 occupancy by almorexant declined 4–6 h post-administration while HCRTR2 occupancy was still elevated after 12 h, revealing a complex relationship between occupancy of HCRT receptors and sleep promotion. We conclude that dual HCRTR1/R2 blockade is more effective in promoting sleep than blockade of either HCRTR alone. In contrast to GABA receptor agonists which induce sleep by generalized inhibition, HCRTR antagonists seem to facilitate sleep by reducing waking “drive”.

Acute pain and a motivational pathway in adult rats: influence of early life pain experience

Background

The importance of neonatal experience upon behaviour in later life is increasingly recognised. The overlap between pain and reward pathways led us to hypothesise that neonatal pain experience influences reward-related pathways and behaviours in adulthood.

Methodology/Principal Findings

Rat pups received repeat plantar skin incisions (neonatal IN) or control procedures (neonatal anesthesia only, AN) at postnatal days (P)3, 10 and 17. When adult, rats with neonatal ‘pain history’ showed greater sensory sensitivity than control rats following acute plantar skin incision. Motivational behaviour in the two groups of rats was tested in a novelty-induced hypophagia (NIH) paradigm. The sensitivity of this paradigm to pain-induced changes in motivational behaviour was shown by significant increases in the time spent in the central zone of the arena (43.7±5.9% vs. 22.5±6.7%, p<0.05), close to centrally placed food treats, and decreased number of rears (9.5±1.4 vs. 19.2±2.3, p<0.001) in rats with acute plantar skin incision compared to naive, uninjured animals. Rats with a neonatal ‘pain history’ showed the same pain-induced behaviour in the novelty-induced hypophagia paradigm as controls. However, differences were observed in reward-related neural activity between the two groups. Two hours after behavioural testing, brains were harvested and neuronal activity mapped using c-Fos expression in lateral hypothalamic orexin neurons, part of a specific reward seeking pathway. Pain-induced activity in orexin neurons of control rats (18.4±2.8%) was the same as in uninjured naive animals (15.5±2.6%), but in those rats with a ‘pain history’, orexinergic activity was significantly increased (27.2±4.1%, p<0.01). Furthermore the extent of orexin neuron activation in individual rats with a ‘pain history’ was highly correlated with their motivational behaviour (r = −0.86, p = 0.01).

Conclusions/Significance

These results show that acute pain alters motivational behaviour and that neonatal pain experience causes long-term changes in brain motivational orexinergic pathways, known to modulate mesolimbic dopaminergic reward circuitry.

The hypocretin/orexin system: implications for drug reward and relapse

Hypocretins (also known as orexins) are hypothalamic neuropeptides involved in the regulation of sleep/wake states and feeding behavior. Recent studies have also demonstrated an important role for the hypocretin/orexin system in the addictive properties of drugs of abuse, consistent with the reciprocal innervations between hypocretin neurons and brain areas involved in reward processing. This system participates in the primary reinforcing effects of opioids, nicotine, and alcohol. Hypocretins are also involved in the neurobiological mechanisms underlying relapse to drug-seeking behavior induced by drug-related environmental stimuli and stress, as mainly described in the case of psychostimulants. Based on these preclinical studies, the use of selective ligands targeting hypocretin receptors could represent a new therapeutical strategy for the treatment of substance abuse disorders. In this review, we discuss and update the current knowledge about the participation of the hypocretin system in drug addiction and the possible neurobiological mechanisms involved in these processes regulated by hypocretin transmission.

The orexin-1 receptor antagonist SB-334867 decreases sympathetic responses to a moderate dose of methamphetamine and stress

We recently discovered that inhibiting neurons in the dorsomedial hypothalamus (DMH) attenuated hyperthermia, tachycardia, hypertension, and hyperactivity evoked by the substituted amphetamine 3, 4-methylenedioxymethamphetamine (MDMA). Neurons that synthesize orexin are also found in the region of the DMH. As orexin and its receptors are involved in the regulation of heart rate and temperature, they would seem to be logical candidates as mediators of the effects evoked by amphetamines. The goal of this study was to determine if blockade of orexin-1 receptors in conscious rats would suppress cardiovascular and thermogenic responses evoked by a range of methamphetamine (METH) doses. Male Sprague-Dawley rats (n=6 per group) were implanted with telemetric transmitters measuring body temperature, heart rate, and mean arterial pressure. Animals were randomized to receive pretreatment with either the orexin-1 receptor antagonist SB-334867 (10mg/kg) or an equal volume of vehicle. Thirty minutes later animals were given intraperitoneal (i.p.) injections of either saline, a low (1mg/kg), moderate (5mg/kg) or high (10mg/kg) dose of METH. Pretreatment with SB-334867 significantly attenuated increases in body temperature and mean arterial pressure evoked by the moderate but not the low or high dose of METH. Furthermore, animals treated with SB-334867, compared to vehicle, had lower temperature and heart rate increases after the stress of an i.p. injection. In conclusion, temperature and cardiovascular responses to a moderate dose of METH and to stress appear to involve orexin-1 receptors. The failure to affect a low and a high dose of METH suggests a complex pharmacology dependent on dose. A better understanding of this may lead to the knowledge of how monoamines influence the orexin system and vice versa.

Involvement of NOX1/NADPH oxidase in morphine-induced analgesia and tolerance

The involvement of reactive oxygen species (ROS) in morphine-induced analgesia and tolerance has been suggested, yet how and where ROS take part in these processes remains largely unknown. Here, we report a novel role for the superoxide-generating enzyme NOX1/NADPH oxidase in the regulation of analgesia and acute analgesic tolerance. In mice lacking Nox1 (Nox1(-/Y)), the magnitude of the analgesia induced by morphine was significantly augmented. More importantly, analgesic tolerance induced by repeated administration of morphine was significantly suppressed compared with that in the littermates, wild-type Nox1(+/Y). In a membrane fraction obtained from the dorsal spinal cord, no difference was observed in morphine-induced [(35)S]GTPγS-binding between the genotypes, whereas morphine-stimulated GTPase activity was significantly attenuated in Nox1(-/Y). At 2 h after morphine administration, a significant decline in [(35)S]GTPγS-binding was observed in Nox1(+/Y) but not in Nox1(-/Y). No difference in the maximal binding and affinity of [(3)H]DAMGO was observed between the genotypes, but the translocation of protein kinase C isoforms to the membrane fraction following morphine administration was almost completely abolished in Nox1(-/Y). Finally, the phosphorylation of RGS9-2 and formation of a complex by Gαi2/RGS9-2 with 14-3-3 found in morphine-treated Nox1(+/Y) were significantly suppressed in Nox1(-/Y). Together, these results suggest that NOX1/NADPH oxidase attenuates the pharmacological effects of opioids by regulating GTPase activity and the phosphorylation of RGS9-2 by protein kinase C. NOX1/NADPH oxidase may thus be a novel target for the development of adjuvant therapy to retain the beneficial effects of morphine.

Activation of central orexin/hypocretin neurons by dietary amino acids

Hypothalamic orexin/hypocretin (orx/hcrt) neurons regulate energy balance, wakefulness, and reward; their loss produces narcolepsy and weight gain. Glucose can lower the activity of orx/hcrt cells, but whether other dietary macronutrients have similar effects is unclear. We show that orx/hcrt cells are stimulated by nutritionally relevant mixtures of amino acids (AAs), both in brain slice patch-clamp experiments, and in c-Fos expression assays following central or peripheral administration of AAs to mice in vivo. Physiological mixtures of AAs electrically excited orx/hcrt cells through a dual mechanism involving inhibition of K(ATP) channels and activation of system-A amino acid transporters. Nonessential AAs were more potent in activating orx/hcrt cells than essential AAs. Moreover, the presence of physiological concentrations of AAs suppressed the glucose responses of orx/hcrt cells. These results suggest a new mechanism of hypothalamic integration of macronutrient signals and imply that orx/hcrt cells sense macronutrient balance, rather than net energy value, in extracellular fluid.

[Relationships between sleep and addiction]

While it is well known that there is an interaction between sleep disorders and substance abuse, it is certainly more complex than was previously thought. There is a positive relationship both between having a substance use disorder and suffering from a sleep disorder, and vice versa. The effects on sleep depend on the substance used, but it has been shown that both during use and in withdrawal periods consumers have various sleep problems, and basically more fragmented sleep. We know that sleep problems must be taken into account to prevent addiction relapses. Recent research shows that the hypocretinergic system defined by neuropeptide hypocretin / orexin (Hcrt / ox), located in the lateral hypothalamus and involved in, among other things, the regulation of the sleep-wake cycle, may play an important role in addictive behaviors. Different studies have demonstrated interactions between the hypocretinergic system, acute response to stress circuits and reward systems. We also know that selective optogenetic activation of the hypocretinergic system increases the probability of transition from sleep to wakefulness, and is sufficient for initiating an addictive compulsive behavior relapse. Hypocretinergic system activation could explain the hyperarousal associated with stress and addiction. Improved knowledge of this interaction would help us to understand better the mechanisms of addiction and find new strategies for the treatment of addictions.

Orexin neurons and emotional stress

Stress increases cardiac function, ventilation, and body temperature and induces analgesia. These changes, which result in an increase in metabolic rate, oxygen supply, and the conduction velocity of nerve impulses, prepare the body for a fight-or-flight response. A part of the hypothalamus called the defense area has long been known to play a key role in these responses, but the precise mechanisms are largely unknown. Our recent findings suggest that orexin (hypocretin) neurons act as a master switch of the fight-or-flight response. In addition, our results, as well as those from other researchers, suggest that orexin neurons do not modulate specific behaviors such as the fight-or-flight responses but rather integrate the autonomic functions and behaviors in a broad sense or in a vigilance state-dependent manner. The orexin system seems to be a pivotal link between the subconscious and the conscious brain functions.

Orexin, stress, and anxiety/panic states

A panic response is an adaptive response to deal with an imminent threat and consists of an integrated pattern of behavioral (aggression, fleeing, or freezing) and increased cardiorespiratory and endocrine responses that are highly conserved across vertebrate species. In the 1920s and 1940s, Philip Bard and Walter Hess, respectively, determined that the posterior regions of the hypothalamus are critical for a "fight-or-flight" reaction to deal with an imminent threat. Since the 1940s it was determined that the posterior hypothalamic panic area was located dorsal (perifornical hypothalamus: PeF) and dorsomedial (dorsomedial hypothalamus: DMH) to the fornix. This area is also critical for regulating circadian rhythms and in 1998, a novel wake-promoting neuropeptide called orexin (ORX)/hypocretin was discovered and determined to be almost exclusively synthesized in the DMH/PeF perifornical hypothalamus and adjacent lateral hypothalamus. The most proximally emergent role of ORX is in regulation of wakefulness through interactions with efferent systems that mediate arousal and energy homeostasis. A hypoactive ORX system is also linked to narcolepsy. However, ORX role in more complex emotional responses is emerging in more recent studies where ORX is linked to depression and anxiety states. Here, we review data that demonstrates ORX ability to mobilize a coordinated adaptive panic/defense response (anxiety, cardiorespiratory, and endocrine components), and summarize the evidence that supports a hyperactive ORX system being linked to pathological panic and anxiety states.

Evidence for a role of inhibition of orexinergic neurons in the anxiolytic and sedative effects of diazepam: A c-Fos study

The classical benzodiazepine diazepam (DZ) induces anxiolysis at low doses and sedation and hypnosis at higher doses. Different brain areas and neuronal populations most likely mediate these different behavioral effects. We used c-Fos immunohistochemistry as an indirect way to study neuronal activation or inhibition induced by DZ at anxiolytic and sedative doses (0.5 and 5mg/kg, respectively) in various brain areas involved in anxiety, arousal, sedation and addiction in C57BL/6J mice. We also focused on the two neuronal populations, orexinergic and dopaminergic neuronal populations, with the help of double-immunohistochemistry using c-Fos and orexin-A antibodies and c-Fos and tyrosine hydroxylase antibodies. We found that different brain areas of unhabituated mice reacted differently to the mild stress induced by vehicle injection. Also the response to anxiolytic or sedative doses of DZ differed between the areas, suggesting that distinct brain areas mediate the behavioral effects of low and high DZ doses. Our findings propose a role for inhibition of orexin neurons in the anxiolytic and sleep-promoting effects of DZ. In addition, the activation of central amygdala neurons by DZ treatment was associated with anxiolytic and sedative effects. On the other hand, the ventral hippocampus, basolateral amygdala, ventral tegmental area and prefrontal cortex were sensitive even to the mild injection stress, but not to the anxiolytic dose of DZ.

Highly specific role of hypocretin (orexin) neurons: differential activation as a function of diurnal phase, operant reinforcement versus operant avoidance and light level

Hypocretin (Hcrt) cell loss is responsible for narcolepsy, but Hcrt's role in normal behavior is unclear. We found that Hcrt knock-out mice were unable to work for food or water reward during the light phase. However, they were unimpaired relative to wild-type (WT) mice when working for reward during the dark phase or when working to avoid shock in the light or dark phase. In WT mice, expression of Fos in Hcrt neurons occurs only in the light phase when working for positive reinforcement. Expression was seen throughout the mediolateral extent of the Hcrt field. Fos was not expressed when expected or unexpected unearned rewards were presented, when working to avoid negative reinforcement, or when given or expecting shock, even though these conditions elicit maximal electroencephalogram (EEG) arousal. Fos was not expressed in the light phase when light was removed. This may explain the lack of light-induced arousal in narcoleptics and its presence in normal individuals. This is the first demonstration of such specificity of arousal system function and has implications for understanding the motivational and circadian consequences of arousal system dysfunction. The current results also indicate that comparable and complementary specificities must exist in other arousal systems.

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.

Relevance of both type-1 and type-2 corticotropin releasing factor receptors in stress-induced relapse to cocaine seeking behaviour

The essential role of corticotropin releasing factor (CRF) and its type-1 receptor (CRF1) in stress-induced relapse to drug seeking has been demonstrated. The bed nucleus of the stria terminalis is the major anatomical substrate for this CRF/CRF1 receptor action. More recently, the role of type-2 CRF (CRF2) receptors in stress-induced relapse to cocaine seeking has also has been documented. The ventral tegmental area is the anatomical substrate for this CRF/CRF2 receptor action. The new information involving CRF2 receptors in stress-induced relapse to cocaine seeking has generated a need for a reappraisal of the existing anatomical and pharmacological evidence that have been used to support the critical role of CRF1 receptors. The role of CRF2 receptors in stress-induced relapse to drug seeking also opens the question of the putative role of the other peptides of the CRH family (urocotin-1, urocortin-2 and urocortin-3) that have high affinity for CRF2 receptors. In this commentary, the available evidence supporting the role of both CRF1 and CRF2 receptors in stress-induced relapse to drug seeking is reviewed.

Local network regulation of orexin neurons in the lateral hypothalamus

Obesity and inadequate sleep are among the most common causes of health problems in modern society. Thus, the discovery that orexin (hypocretin) neurons play a pivotal role in sleep/wake regulation, energy balance, and consummatory behaviors has sparked immense interest in understanding the regulatory mechanisms of these neurons. The local network consisting of neurons and astrocytes within the lateral hypothalamus and perifornical area (LH/PFA), where orexin neurons reside, shapes the output of orexin neurons and the LH/PFA. Orexin neurons not only send projections to remote brain areas but also contribute to the local network where they release multiple neurotransmitters to modulate its activity. These neurotransmitters have opposing actions, whose balance is determined by the amount released and postsynaptic receptor desensitization. Modulation and negative feedback regulation of excitatory glutamatergic inputs as well as release of astrocyte-derived factors, such as lactate and ATP, can also affect the excitability of orexin neurons. Furthermore, distinct populations of LH/PFA neurons express neurotransmitters with known electrophysiological actions on orexin neurons, such as melanin-concentrating hormone, corticotropin-releasing factor, thyrotropin-releasing hormone, neurotensin, and GABA. These LH/PFA-specific mechanisms may be important for fine tuning the firing activity of orexin neurons to maintain optimal levels of prolonged output to sustain wakefulness and stimulate consummatory behaviors. Building on these exciting findings should shed further light onto the cellular mechanisms of energy balance and sleep-wake regulation.

Connectomics of orexin-producing neurons: interface of systems of emotion, energy homeostasis and arousal

Avoiding danger and finding food, which are life-sustaining activities that are regulated by emotion, reward and energy balance, require proper wakefulness. The orexin system controls sleep and wakefulness through interactions with systems that regulate emotion, reward and energy homeostasis. Recent findings have brought about the possibility of novel therapies targeting the orexin system for sleep disorders, including insomnia and narcolepsy-cataplexy, as well as other pathological conditions such as obesity and drug addiction [1]. In this review, we will discuss the current understanding of the integrative physiology and clinical perspectives of the orexin system. We will briefly review signaling through orexin A and B receptors and discuss the role of orexins in the pathophysiology of narcolepsy. We will also examine connections between orexin neurons and other brain areas involved in feeding behavior, reward and emotion. Finally, we will consider the therapeutic potential of drugs that target orexin receptors.

Lateral thinking about leptin: a review of leptin action via the lateral hypothalamus

The lateral hypothalamic area (LHA) was initially described as a "feeding center" but we are now beginning to understand that the LHA contributes to other aspects of physiology as well. Indeed, the best-characterized neuronal populations of the LHA (which contain melanin-concentrating hormone (MCH) or the hypocretins/orexins (OX)) are not strictly orexigenic, but also have roles in regulation of the autonomic and sympathetic nervous systems as well as in modulating motivated behavior. Leptin is an anorectic hormone that regulates energy homeostasis and the mesolimbic DA system (which transduces the wanting of food, drugs of abuse, and sex) in part, via actions at the LHA. At least three populations of LHA neurons are regulated by leptin: those containing MCH, OX or the long form of the leptin receptor, LepRb. The emerging picture of leptin interaction with these LHA populations suggests that the LHA is not merely regulating feeding, but is a crucial integrator of energy balance and motivated behavior.

The lateral hypothalamus as integrator of metabolic and environmental needs: from electrical self-stimulation to opto-genetics

As one of the evolutionary oldest parts of the brain, the diencephalon evolved to harmonize changing environmental conditions with the internal state for survival of the individual and the species. The pioneering work of physiologists and psychologists around the middle of the last century clearly demonstrated that the hypothalamus is crucial for the display of motivated behaviors, culminating in the discovery of electrical self-stimulation behavior and providing the first neurological hint accounting for the concepts of reinforcement and reward. Here we review recent progress in understanding the role of the lateral hypothalamic area in the control of ingestive behavior and the regulation of energy balance. With its vast array of interoceptive and exteroceptive afferent inputs and its equally rich efferent connectivity, the lateral hypothalamic area is in an ideal position to integrate large amounts of information and orchestrate adaptive responses. Most important for energy homeostasis, it receives metabolic state information through both neural and humoral routes and can affect energy assimilation and energy expenditure through direct access to behavioral, autonomic, and endocrine effector pathways. The complex interplays of classical and peptide neurotransmitters such as orexin carrying out these integrative functions are just beginning to be understood. Exciting new techniques allowing selective stimulation or inhibition of specific neuronal phenotypes will greatly facilitate the functional mapping of both input and output pathways.

Activation of orexin neurons in dorsomedial/perifornical hypothalamus and antidepressant reversal in a rodent model of depression

Chronic stressful life events are risk factors for depression often accompanied by homeostatic disturbances. Hypothalamic neuropeptides, such as orexins (OXs) and melanin-concentrating hormone (MCH), are involved in regulation of several autonomic functions that are altered in depression. However, little is known about the link between orexinergic or MCH-ergic systems and depression. Using double immunohistochemical labeling for OX- or MCH-containing neurons and Fos protein, we studied the effects of a chronic selective serotonin reuptake inhibitor antidepressant treatment (fluoxetine) on the OX and MCH neuronal activation in mice exposed to unpredictable chronic mild stress (UCMS), a rodent model of depression. Western blot was also performed to assess OX and MCH receptor expression in various brain areas. Finally, almorexant, a dual OX receptor antagonist, was assessed in the tail suspension test. UCMS induced physical and behavioral disturbances in mice reversed by 6-week fluoxetine treatment. Orexinergic neurons were more activated in the dorsomedial and perifornical hypothalamic area (DMH-PFA) of UCMS-subjected mice compared to the lateral hypothalamus (LH), and this increase was reversed by 6-week fluoxetine treatment. UCMS also reduced expression of OX-receptor 2 in the thalamus and hypothalamus, but not in animals chronically treated with fluoxetine. MCH neurons were neither affected by UCMS nor by antidepressant treatment, while UCMS modulated MCH receptor 1 expression in thalamus and hippocampus. Finally, chronic but not acute administration of almorexant, induced antidepressant-like effect in the tail suspension test. These data suggest that OX neurons in the DMH-PFA and MCH-ergic system may contribute to the pathophysiology of depressive disorders.

Orexin receptors: pharmacology and therapeutic opportunities

Orexin-A and -B (also known as hypocretin-1 and -2) are neuropeptides produced in the lateral hypothalamus that promote many aspects of arousal through the OX1 and OX2 receptors. In fact, they are necessary for normal wakefulness, as loss of the orexin-producing neurons causes narcolepsy in humans and rodents. This has generated considerable interest in developing small-molecule orexin receptor antagonists as a novel therapy for the treatment of insomnia. Orexin antagonists, especially those that block OX2 or both OX1 and OX2 receptors, clearly promote sleep in animals, and clinical results are encouraging: Several compounds are in Phase III trials. As the orexin system mainly promotes arousal, these new compounds will likely improve insomnia without incurring many of the side effects encountered with current medications.

A selective, non-peptide CRF receptor 1 antagonist prevents sodium lactate-induced acute panic-like responses

Corticotropin releasing factor (CRF) is implicated in a variety of stress-related disorders such as depression and anxiety, and blocking CRF receptors is a putative strategy for treating such disorders. Using a well-studied animal model of panic, we tested the efficacy of JNJ19567470/CRA5626, a selective, non-peptidergic CRF type 1 receptor (CRF1) antagonist (3, 10 and 40 mg/kg intraperitoneal injection), in preventing the sodium lactate (NaLac)-induced panic-like behavioural and cardiovascular responses. Adult male rats with chronic reduction of GABA levels (by inhibition of GABA synthesis with l-allyglycine, a glutamic acid decarboxylase inhibitor) in the dorsomedial/perifornical hypothalamus are highly anxious and exhibit physiological and behavioural responses to intravenous NaLac infusions similar to patients with panic disorder. These 'panic-prone' rats pre-treated with vehicle injections displayed NaLac-induced increases in autonomic responses (i.e. tachycardia and hypertensive responses), anxiety-like behaviour in the social interaction test, and flight-like increases in locomotor activity. However, systemically injecting such panic-prone rats with the highest dose of CRF1 receptor antagonist prior to NaLac infusions blocked all NaLac-induced behaviour and cardiovascular responses. These data suggest that selective CRF1 receptor antagonists could be a novel target for developing anti-panic drugs that are as effective as benzodiazepines in acute treatment of a panic attack without the deleterious side-effects (e.g. sedation and cognitive impairment) associated with benzodiazepines.

Developmental programming of energy balance and its hypothalamic regulation

Developmental programming is an important physiological process that allows different phenotypes to originate from a single genotype. Through plasticity in early life, the developing organism can adopt a phenotype (within the limits of its genetic background) that is best suited to its expected environment. In humans, together with the relative irreversibility of the phenomenon, the low predictive value of the fetal environment for later conditions in affluent countries makes it a potential contributor to the obesity epidemic of recent decades. Here, we review the current evidence for developmental programming of energy balance. For a proper understanding of the subject, knowledge about energy balance is indispensable. Therefore, we first present an overview of the major hypothalamic routes through which energy balance is regulated and their ontogeny. With this background, we then turn to the available evidence for programming of energy balance by the early nutritional environment, in both man and rodent models. A wealth of studies suggest that energy balance can indeed be permanently affected by the early-life environment. However, the direction of the effects of programming appears to vary considerably, both between and within different animal models. Because of these inconsistencies, a comprehensive picture is still elusive. More standardization between studies seems essential to reach veritable conclusions about the role of developmental programming in adult energy balance and obesity.

Orexins/hypocretins increase the promoter activity of selective steroidogenic enzymes

Orexins (hypocretins) regulate multiple physiological functions, including central regulation of energy homeostasis and sleep-wake behavior but also peripheral hormonal actions. Recent data suggest specific effects of orexins at adrenal glands. To further assess the mechanism by which orexins regulate steroidogenesis we analyzed the effect of orexin A and B on the transcriptional activity of the luciferase reporter gene driven by the human steroid 21-hydroxylase (CYP21), 3β-hydroxysteroid dehydrogenase (HSD3B2), 11β-hydroxylase (CYP11B1), and aldosterone synthase (CYP11B2) gene promoter regions. After transient transfection of the reporter gene constructs into human NCI H295R cells, treatment with orexin A and B for 6 and 12h increased the promoter activity of the CYP11B2, HSD3B2 and, to a lesser extend, CYP21 genes. The activity of the CYP11B1 was increased by both orexins after 3h of treatment. Compared to the effects of forskolin or angiotensin II, however, the effect of orexins on the transcriptional activity of the steroidogenic enzyme genes was moderate. Our results suggest that orexins increase the expression of steroidogenic enzymes at the transcriptional level and that orexins play a role in the long term regulation of adrenal steroid production.

Wake-promoting effects of orexin: Its independent actions against the background of an impaired corticotropine-releasing hormone receptor system

It is widely accepted that orexin (hypocretin) bears wake-promoting effects. While under normal conditions the circadian rhythm of orexin release has a clear circadian distribution, the amplitude of orexin fluctuation is dampened in depression. Interestingly, clinical symptoms of depression include several sleep disturbances. In this disease, corticotropin-releasing hormone (CRH) seems to be another factor influencing sleep. As neurophysiological interactions and anatomical connections between the orexinergic and the CRH system point to mutual influences of these two neuropeptides, we examined whether a dysfunctional CRH-receptor system in two different CRH receptor knock out models alters general wake-promoting effects of orexin applied exogenously. Orexin was injected intracerebroventricularlly into CNS-restricted CRH-receptor type 1 knockout mice (CRH-R1 KO) and CRH-receptor type 2 knockout mice (CRH-R2 KO) and baseline sleep was recorded from the freely behaving mice. A third experiment included antisauvagine-30 injections (CRH-R2 antagonist) into CRH-R1 KO animals. Orexin had similar wake-promoting effects in CRH-R1KO mice, in CRH-R2 KO animals and in CRH-R1KO mice treated with antisauvagine-30. Consistent results were obtained from all corresponding control littermate experiments. According to our results we conclude that the wake-promoting effects of orexin are not influenced by a possible contribution of CRH.

Direct inhibition of hypocretin/orexin neurons in the lateral hypothalamus by nociceptin/orphanin FQ blocks stress-induced analgesia in rats

We recently demonstrated that hypocretin/orexin (Hcrt) and nociceptin/orphanin FQ (N/OFQ) systems coordinately regulate nociception in a mouse model of stress-induced analgesia (SIA). However, the site of N/OFQ action on modulation of SIA was elusive, since N/OFQ was administered via intracerebroventricular (i.c.v.) injection acting on widely distributed N/OFQ receptors (NOP) in the brain. In the present study, we tested the hypothesis that N/OFQ modulates the SIA directly via the inhibition of the Hcrt neurons in the lateral hypothalamus. Using both fluorescent and electron microscopy, we found that N/OFQ-containing neurons are located in the lateral hypothalamus and the N/OFQ-containing fibers make direct contacts with the Hcrt neurons. Paw thermal nociceptive test revealed that the immobilization restraint of the rat increased the thermal pain threshold by 20.5 ± 7.6%. Bilateral microinjection of N/OFQ (9 μg/side) into the rat perifornical area of the lateral hypothalamus, the brain area in which the Hcrt neurons are exclusively located, abolished the SIA. Activity of Hcrt neurons in the same animals was assessed using Fos immunohistochemistry. Percentage of Fos(+)/Hcrt neurons was lower in rats injected with N/OFQ than rats injected with saline, with the difference between groups stronger in the Hcrt neurons located medially to the fornix than in Hcrt neurons located laterally to the fornix. These results suggest that N/OFQ modulation of SIA is mediated by direct inhibition of Hcrt neuronal activity in the perifornical area. The uncovered peptidergic interaction circuitry may have broad implication in coordinated modulation by Hcrt and N/OFQ on other stress adaptive responses.

Tobacco dependence, the insular cortex and the hypocretin connection

Tobacco use is a major cause of disease and premature death in the United States. Nicotine is considered the key component of tobacco responsible for addiction in human smokers. Accumulating evidence supports an important role for the hypocretin (orexin) neuropeptide system in regulating the reinforcing properties of most major drugs of abuse, including nicotine. Here, data showing that nicotine activates hypocretin-producing neurons in the lateral hypothalamus, and that disruption of hypocretin transmission decreases nicotine self-administration behavior in rats will be reviewed. Recent findings suggesting that plasma hypocretin levels may be related to the magnitude of cigarette craving in abstinent smokers will be discussed. Finally, the data suggesting that hypocretin transmission in the insular cortex may play an important role in regulating nicotine self-administration behavior in rats will be reviewed. This latter finding may provide mechanistic insight into the apparent disruption of tobacco addiction reported in human smokers with stroke-associated damage to the insular cortex.

Central orexin-A increases gastric motility in rats

Orexin receptor type-1 (OX1R) is expressed in the dorsal motor nucleus of vagi (DMV). Although orexin-A (OXA) plays an important role in mediating stress responses, it remains unclear how central OXA regulates gastric dysmotility induced by stress. Acute restraint stress (ARS) delays solid gastric emptying via the central corticotropin releasing factor (CRF) and peripheral autonomic neural pathways. We have previously shown that ARS impairs postprandial antro-pyloric coordination and delays solid gastric emptying in rats. We also showed that postprandial gastric contractions were augmented in response to ARS in rats. However, the mechanism of augmented postprandial gastric contractions induced by ARS remains unclear. We tested the hypothesis that augmented gastric motility induced by ARS is mediated via the central OX1R. We also assessed the role of endogenous OXA in the mediation of gastric motility under non-stressed conditions in conscious rats. A strain gauge transducer was implanted on the antrum to record postprandial gastric motility. To investigate whether endogenous OXA is involved in ARS-induced augmented gastric motility, selective OX1R antagonist, SB-334867 (16 μg), was administered intracerebroventricularly (icv). Icv-injection of SB-334867 abolished the augmented gastric contractions induced by ARS. Spontaneous postprandial gastric motility was enhanced by icv-injection of OXA (10 μg), while it was attenuated by icv-injection of SB-3334867. It is suggested that central OXA mediates augmented gastric motility induced by ARS in rats. Central OXA also modulates postprandial gastric contractions in non-stressed conditions.

Orexins in the paraventricular nucleus of the thalamus mediate anxiety-like responses in rats

RATIONALE

Anatomical studies have shown that the paraventricular nucleus of the thalamus (PVT) innervates areas of the forebrain involved in the expression and regulation of emotional behaviors including fear and anxiety. In addition, the PVT is densely innervated by fibers containing orexin-A (OXA) and orexin-B (OXB), peptides that are well-known for their arousal effects on behavior.

OBJECTIVES

In this study, we investigate whether microinjections of orexin receptor agonists and antagonists in the PVT region alter expression of anxiety-like behaviors in the rat as measured in the elevated plus maze.

RESULTS

We report that microinjections of OXA and OXB in the PVT region elicited anxiety-like response as indicated by a reduction in open arm time and entries. In addition, OXA and OXB produced changes in ethological measures indicative of an anxiety state. Central administrations of antagonists for corticotropin releasing factor (CRF) or the opioid kappa receptors attenuated the anxiogenic effects produced by microinjections of OXA in the PVT region. We also provide evidence that endogenously released orexins act at the PVT to produce anxiety by showing that microinjections of TCSOX229, an orexin-2 receptor antagonist, in the PVT region attenuated the anxiogenic effects produced by a previous exposure to footshock stress.

CONCLUSIONS

This study indicates that endogenously released orexins act on the PVT to regulate anxiety levels through mechanisms involving the brain kappa and CRF receptors.

Sleep and metabolism: role of hypothalamic neuronal circuitry

Sleep and metabolism are intertwined physiologically and behaviorally, but the neural systems underlying their coordination are still poorly understood. The hypothalamus is likely to play a major role in the regulation sleep, metabolism, and their interaction. And increasing evidence suggests that hypocretin cells in the lateral hypothalamus may provide particularly important contributions. Here we review: 1) direct interactions between biological arousal and metabolic systems in the hypothalamus, and 2) indirect interactions between these two systems mediated by stress or reward, emphasizing the role of hypocretins. An increased understanding of the mechanisms underlying these interactions may provide novel approaches for the treatment of patients with sleep disorders and obesity, as well as suggest new therapeutic strategies for symptoms of aging, stress, or addiction.