Activation of orexin neurones after noxious but not conditioned fear stimuli in rats

International Union of Basic and Clinical Pharmacology CXIV: Orexin Receptor Function, Nomenclature and Pharmacology

The orexin system consists of the peptide transmitters orexin-A and -B and the G protein-coupled orexin receptors OX1 and OX2 Orexin receptors are capable of coupling to all four families of heterotrimeric G proteins, and there are also other complex features of the orexin receptor signaling. The system was discovered 25 years ago and was immediately identified as a central regulator of sleep and wakefulness; this is exemplified by the symptomatology of the disorder narcolepsy with cataplexy, in which orexinergic neurons degenerate. Subsequent translation of these findings into drug discovery and development has resulted to date in three clinically used orexin receptor antagonists to treat insomnia. In addition to sleep and wakefulness, the orexin system appears to be a central player at least in addiction and reward, and has a role in depression, anxiety and pain gating. Additional antagonists and agonists are in development to treat, for instance, insomnia, narcolepsy with or without cataplexy and other disorders with excessive daytime sleepiness, depression with insomnia, anxiety, schizophrenia, as well as eating and substance use disorders. The orexin system has thus proved an important regulator of numerous neural functions and a valuable drug target. Orexin prepro-peptide and orexin receptors are also expressed outside the central nervous system, but their potential physiological roles there remain unknown. SIGNIFICANCE STATEMENT: The orexin system was discovered 25 years ago and immediately emerged as an essential sleep-wakefulness regulator. This discovery has tremendously increased the understanding of these processes and has thus far resulted in the market approval of three orexin receptor antagonists, which promote more physiological aspects of sleep than previous hypnotics. Further, orexin receptor agonists and antagonists with different pharmacodynamic properties are in development since research has revealed additional potential therapeutic indications. Orexin receptor signaling is complex and may represent novel features.

The opposite roles of orexin neurons in pain and itch neural processing

Pain and itch are antagonistically regulated sensations; pain suppresses itch, and inhibition of pain enhances itch. Understanding the central neural circuit of antagonistic regulation between pain and itch is required to develop new therapeutics better to manage these two feelings in a clinical situation. However, evidence of the neural mechanism underlying the pain-itch interaction in the central nervous system (CNS) is still insufficient. To pave the way for this research area, our laboratory has focused on orexin (ORX) producing neurons in the hypothalamus, which is known as a master switch that induces various defense responses when animals face a stressful environment. This review article summarized the previous evidence and our latest findings to argue the neural regulation between pain and itch and the bidirectional roles of ORX neurons in processing these two sensations. i.e., pain relief and itch exacerbation. Further, we discussed the possible neural circuit mechanism for the opposite controlling of pain and itch by ORX neurons. Focusing on the roles of ORX neurons would provide a new perspective to understand the antagonistic regulation of pain and itch in CNS.

Hypothalamic orexinergic neurons modulate pain and itch in an opposite way: pain relief and itch exacerbation

Pain and itch are recognized as antagonistic sensations; pain suppresses itch and inhibition of pain generates itch. There is still a lack of evidence about the neural mechanism of the interaction between pain and itch in the central nervous system. In this study, we focused on the orexin (ORX) neurons in the lateral hypothalamus (LH), which mediate various "defense responses" when animals confront stressors. We found that the scratching behaviors induced by the pruritogen were significantly suppressed in ORX-neuron-ablated (ORX-abl) mice. The exaggerated pain behavior and attenuated itch behavior observed in ORX-abl mice indicated that ORX neurons modulate pain and itch in an opposite way, i.e., pain relief and itch exacerbation. In addition, most of the ORX neurons responded to both pain and itch input. Our results suggest that ORX neurons inversely regulate pain- and itch-related behaviors, which could be understood as a defense response to cope with stress environment.

The Paraventricular Nucleus of the Thalamus as an Integrating and Relay Node in the Brain Anxiety Network

The brain anxiety network is composed of a number of interconnected cortical regions that detect threats and execute appropriate defensive responses via projections to the shell of the nucleus accumbens (NAcSh), dorsolateral region of the bed nucleus of the stria terminalis (BSTDL) and lateral region of the central nucleus of the amygdala (CeL). The paraventricular nucleus of the thalamus (PVT) is anatomically positioned to integrate threat- and arousal-related signals from cortex and hypothalamus and then relay these signals to neural circuits in the NAcSh, BSTDL, and CeL that mediate defensive responses. This review describes the anatomical connections of the PVT that support the view that the PVT may be a critical node in the brain anxiety network. Experimental findings are reviewed showing that the arousal peptides orexins (hypocretins) act at the PVT to promote avoidance of potential threats especially following exposure of rats to a single episode of footshocks. Recent anatomical and experimental findings are discussed which show that neurons in the PVT provide divergent projections to subcortical regions that mediate defensive behaviors and that the projection to the NAcSh is critical for the enhanced social avoidance displayed in rats exposed to footshocks. A theoretical model is proposed for how the PVT integrates cortical and hypothalamic signals to modulate the behavioral responses associated with anxiety and other challenging situations.

The role of the orexin system in stress response

Orexins are neuropeptides that are exclusively produced by hypothalamic neurons, which project throughout the entire brain. Orexin, also known as hypocretins, were initially identified to play a fundamental role in food intake, arousal and the regulation of sleep and wakefulness. Recent studies identified orexins to be critical for diverse physiological processes including motivation, reward, attention, emotional regulation, stress and anxiety. Here, I review recent findings that indicate orexin has an important role in acute and chronic stress. I also summarize the recent optogenetic and chemogenetic studies that have advanced our understanding of the orexin system. I will conclude by discussing clinical studies that implicate orexins in mental health disorders. This article is part of the Special Issue entitled 'Hypothalamic Control of Homeostasis'.

Cardiovascular pressor effects of orexins in the dorsomedial hypothalamus

Orexins are important regulators of cardiovascular functions in various physiological and pathological conditions. The dorsomedial hypothalamus (DMH), an essential mediator of cardiovascular responses to stress, contains dense orexinergic innervations and receptors. We examined whether orexins can regulate cardiovascular functions through their actions in the DMH in anesthetized rats. An intra-DMH injection of orexin A (30pmol) produced elevation of arterial pressure and heart rate. Orexin A-sensitive sites were located within or immediately adjacent to the DMH and larger responses were induced at the compact part of the dorsomedial hypothalamic nucleus. Orexin A-induced responses were attenuated by intra-DMH pretreatment with an orexin receptor 1 (OX1R) antagonist, SB-334867 (15nmol) (17.7 ± 2.8 vs. 5.2 ± 1.0mmHg; 54.6 ± 10.0 vs. 22.8 ± 7.4 beats/min). Intra-DMH applied [Ala¹¹,D-Leu¹⁵]-orexin B (300 pmol), an orexin receptor 2 (OX2R) agonist, elicited cardiovascular responses mimicking the responses of orexin A, except for a smaller pressor response (7.4 ± 1.7 vs. 16.4 ± 1.8mmHg). In a series of experiment, effects of orexin B (100pmol) and then orexin A (30pmol), were examined at a same site. Two patterns of responses were observed in 12 intra-DMH sites: (1) both orexin A and B (9 sites), and (2) only orexin A (3 sites) induced cardiovascular responses, respectively suggesting OX1R/OX2R-mediated and OX1R-predominant mechanisms. In conclusion, orexins regulated cardiovascular functions through OX1R/OX2R- or OX1R-mediated mechanisms at different locations in the DMH.

Hypocretin receptor 1 blockade produces bimodal modulation of cocaine-associated mesolimbic dopamine signaling

RATIONALE

Cocaine addiction is a chronic psychiatric disorder characterized by pathological motivation to obtain cocaine and behavioral and neurochemical hypersensitivity to cocaine-associated cues. These features of cocaine addiction are thought to be driven by aberrant phasic dopamine signaling. We previously demonstrated that blockade of the hypocretin receptor 1 (HCRTr1) attenuates cocaine self-administration and reduces cocaine-induced enhancement of dopamine signaling. Despite this evidence, the effects of HCRTr1 blockade on endogenous phasic dopamine release are unknown.

OBJECTIVE

In the current studies, we assessed whether blockade of HCRTr1 alters spontaneous and cue-evoked dopamine release in the nucleus accumbens core of freely moving rats.

METHODS

We first validated the behavioral and neurochemical effects of the novel, highly selective, HCRTr1 antagonist RTIOX-276 using cocaine self-administration and fast-scan cyclic voltammetry (FSCV) in anesthetized rats. We then used FSCV in freely moving rats to examine whether RTIOX-276 impacts spontaneous and cue-evoked dopamine release. Finally, we used ex vivo slice FSCV to determine whether the effects of RTIOX-276 on dopamine signaling involve dopamine terminal adaptations.

RESULTS

Doses of RTIOX-276 that attenuate the motivation for cocaine reduce spontaneous dopamine transient amplitude and cue-evoked dopamine release. Further, these doses attenuated cocaine-induced dopamine uptake inhibition at the level of dopamine terminals.

CONCLUSION

Our results provide support for the standing hypothesis that HCRTr1 blockade suppresses endogenous phasic dopamine signals, likely via actions at dopamine cell bodies. These results also elucidate a second process through which HCRTr1 blockade attenuates the effects of cocaine by reducing cocaine sensitivity at dopamine terminals.

Activation of orexin/hypocretin neurons is associated with individual differences in cued fear extinction

Identifying the neurobiological mechanisms that underlie differential sensitivity to stress is critical for understanding the development and expression of stress-induced disorders, such as post-traumatic stress disorder (PTSD). Preclinical studies have suggested that rodents display different phenotypes associated with extinction of Pavlovian conditioned fear responses, with some rodent populations being resistant to extinction. An emerging literature also suggests a role for orexins in the consolidation processes associated with fear learning and extinction. To examine the possibility that the orexin system might be involved in individual differences in fear extinction, we used a Pavlovian conditioning paradigm in outbred Long-Evans rats. Rats showed significant variability in the extinction of cue-conditioned freezing and extinction recall, and animals were divided into groups based on their extinction profiles based on a median split of percent freezing behavior during repeated exposure to the conditioned cue. Animals resistant to extinction (high freezers) showed more freezing during repeated cue presentations during the within trial and between trial extinction sessions compared with the group showing significant extinction (low freezers), although there were no differences between these groups in freezing upon return to the conditioned context or during the conditioning session. Following the extinction recall session, activation of orexin neurons was determined using dual label immunohistochemistry for cFos in orexin positive neurons in the hypothalamus. Individual differences in the extinction of cue conditioned fear were associated with differential activation of hypothalamic orexin neurons. Animals showing poor extinction of cue-induced freezing (high freezers) had significantly greater percentage of orexin neurons with Fos in the medial hypothalamus than animals displaying significant extinction and good extinction recall (low freezers). Further, the freezing during extinction learning was positively correlated with the percentage of activated orexin neurons in both the lateral and medial hypothalamic regions. No differences in the overall density of orexin neurons or Fos activation were seen between extinction phenotypes. Although correlative, our results support other studies implicating a role of the orexinergic system in regulating extinction of conditioned responses to threat.

Orexin and Central Modulation of Cardiovascular and Respiratory Function

Orexin makes an important contribution to the regulation of cardiorespiratory function. When injected centrally under anesthesia, orexin increases blood pressure, heart rate, sympathetic nerve activity, and the amplitude and frequency of respiration. This is consistent with the location of orexin neurons in the hypothalamus and the distribution of orexin terminals at all levels of the central autonomic and respiratory network. These cardiorespiratory responses are components of arousal and are necessary to allow the expression of motivated behaviors. Thus, orexin contributes to the cardiorespiratory response to acute stressors, especially those of a psychogenic nature. Consequently, upregulation of orexin signaling, whether it is spontaneous or environmentally induced, can increase blood pressure and lead to hypertension, as is the case for the spontaneously hypertensive rat and the hypertensive BPH/2J Schlager mouse. Blockade of orexin receptors will reduce blood pressure in these animals, which could be a new pharmacological approach for the treatment of some forms of hypertension. Orexin can also magnify the respiratory reflex to hypercapnia in order to maintain respiratory homeostasis, and this may be in part why it is upregulated during obstructive sleep apnea. In this pathological condition, blockade of orexin receptors would make the apnea worse. To summarize, orexin is an important modulator of cardiorespiratory function. Acting on orexin signaling may help in the treatment of some cardiovascular and respiratory disorders.

The integrative role of orexin/hypocretin neurons in nociceptive perception and analgesic regulation

The level of wakefulness is one of the major factors affecting nociception and pain. Stress-induced analgesia supports an animal’s survival via prompt defensive responses against predators or competitors. Previous studies have shown the pharmacological effects of orexin peptides on analgesia. However, orexin neurons contain not only orexin but also other co-transmitters such as dynorphin, neurotensin and glutamate. Thus, the physiological importance of orexin neuronal activity in nociception is unknown. Here we show that adult-stage selective ablation of orexin neurons enhances pain-related behaviors, while pharmacogenetic activation of orexin neurons induces analgesia. Additionally, we found correlative activation of orexin neurons during nociception using fiber photometry recordings of orexin neurons in conscious animals. These findings suggest an integrative role for orexin neurons in nociceptive perception and pain regulation.

The Dichotomous Effect of Chronic Stress on Obesity

Obesity and metabolic diseases are linked to chronic stress and low socioeconomic status. The mechanistic link between stress and obesity has not been clarified, partly due to the inherent complexity exemplified by the bidirectional effect of stress on eating and body weight. Recent studies focusing on adaptive thermogenesis and brown adipose tissue (BAT) function support a dichotomous relation to explain the impact of stress on obesity: stress promotes obesity in the presence of hyperphagia and unchanged BAT function; stress results in weight loss and/or obesity resistance in the presence of hypophagia, or when hyperphagia is associated with BAT recruitment and enhanced thermogenesis. Mechanistically dissecting the bidirectional effects of stress on metabolic outcomes might open new avenues for innovative pharmacotherapies for the treatment of obesity-associated diseases.

Stress induces analgesia via orexin 1 receptor-initiated endocannabinoid/CB1 signaling in the mouse periaqueductal gray

The orexin system consists of orexin A/hypocretin 1 and orexin B/hypocretin 2, and OX1 and OX2 receptors. Our previous electrophysiological study showed that orexin A in the rat ventrolateral periaqueductal gray (vlPAG) induced antinociception via an OX1 receptor-initiated and endocannabinoid-mediated disinhibition mechanism. Here, we further characterized antinociceptive effects of orexins in the mouse vlPAG and investigated whether this mechanism in the vlPAG can contribute to stress-induced analgesia (SIA) in mice. Intra-vlPAG (i.pag.) microinjection of orexin A in the mouse vlPAG increased the hot-plate latency. This effect was mimicked by i.pag. injection of WIN 55,212-2, a CB1 agonist, and antagonized by i.pag. injection of the antagonist of OX1 (SB 334867) or CB1 (AM 251), but not OX2 (TCS-OX2-29) or opioid (naloxone), receptors. [Ala(11), D-Leu(15)]-orexin B (i.pag.), an OX2 selective agonist, also induced antinociception in a manner blocked by i.pag. injection of TCS-OX2-29, but not SB 334867 or AM 251. Mice receiving restraint stress for 30 min showed significantly longer hot-plate latency, more c-Fos-expressing orexin neurons in the lateral hypothalamus and higher orexin levels in the vlPAG than unrestrained mice. Restraint SIA in mice was prevented by i.pag. or intraperitoneal injection of SB 334867 or AM 251, but not TCS-OX2-29 or naloxone. These results suggest that during stress, hypothalamic orexin neurons are activated, releasing orexins into the vlPAG to induce analgesia, possibly via the OX1 receptor-initiated, endocannabinoid-mediated disinhibition mechanism previously reported. Although activating either OX1 or OX2 receptors in the vlPAG can lead to antinociception, only OX1 receptor-initiated antinociception is endocannabinoid-dependent.

Thermoregulation under pressure: a role for orexin neurons

In the past, studies on stress responses and sleep/wake regulation were performed separately. The discovery of orexin (hypocretin) in 1998, however, dramatically changed the course of research and new findings regarding its role in these complex processes provided a better insight into their interactions and intricacies. Orexin-containing neuronal activity has been found to be minimal during sleep. It increases during the waking period and further increases during the active waking period, which includes stress responses and exploratory behaviors. Autonomic regulation of the body, which includes body temperature, blood flow, and ventilation, is also activated along with the change in vigilance states. Our recent findings suggest that orexin neurons act as a conductor of orchestration for vigilance states, behaviors, and autonomic functions. Body temperature regulation by orexin neurons seems to be mediated by one of its cotransmitters while cardiovascular and respiratory regulation are mediated by orexin itself.

The Deakin/Graeff hypothesis: focus on serotonergic inhibition of panic

The Deakin/Graeff hypothesis proposes that different subpopulations of serotonergic neurons through topographically organized projections to forebrain and brainstem structures modulate the response to acute and chronic stressors, and that dysfunction of these neurons increases vulnerability to affective and anxiety disorders, including panic disorder. We outline evidence supporting the existence of a serotonergic system originally discussed by Deakin/Graeff that is implicated in the inhibition of panic-like behavioral and physiological responses. Evidence supporting this panic inhibition system comes from the following observations: (1) serotonergic neurons located in the 'ventrolateral dorsal raphe nucleus' (DRVL) as well as the ventrolateral periaqueductal gray (VLPAG) inhibit dorsal periaqueductal gray-elicited panic-like responses; (2) chronic, but not acute, antidepressant treatment potentiates serotonin's panicolytic effect; (3) contextual fear activates a central nucleus of the amygdala-DRVL/VLPAG circuit implicated in mediating freezing and inhibiting panic-like escape behaviors; (4) DRVL/VLPAG serotonergic neurons are central chemoreceptors and modulate the behavioral and cardiorespiratory response to panicogenic agents such as sodium lactate and CO2. Implications of the panic inhibition system are discussed.

Blocking of corticotrophin releasing factor receptor-1 during footshock attenuates context fear but not the upregulation of prepro-orexin mRNA in rats

Hypothalamic neuropeptides called orexins (hypocretins) are well known for their roles in promoting arousal. Orexins have also been shown to play a role in fear and anxiety produced by the exposure of rats to an acute episode of moderately intense footshocks. Recent evidence indicates that stress activates orexin neurons through a corticotropin releasing factor (CRF) mechanism. In this study, we examined the effect of a CRF receptor-1 (CRF-R1) antagonist antalarmin (20mg/kg, i.p.) given before shock exposure on subsequent expression of contextual fear and the levels of prepro-orexin (ppOX) mRNA in the hypothalamus. Antalarmin decreased fear and ultrasonic vocalization expression to the shock context at 2 and 10 days after shock exposure. However, antalarmin did not prevent the increases in ppOX mRNA produced by the shock experience. This study provides evidence that blocking of CRF-R1 at the time of footshocks attenuates contextual fear. While an increase in the activity of the orexin system may contribute to fear, this activation does not appear to be sufficient for fear expression.

Orexins (hypocretins) contribute to fear and avoidance in rats exposed to a single episode of footshocks

Orexins (hypocretins) are peptides that have been shown to regulate behavioral arousal and wakefulness. Recent evidence indicates that orexin neurons are activated by stress and that orexins play a role in anxiety. The present paper describes a series of experiments that examined whether orexins are involved in the anxiety that resulted from exposing rats to an acute episode of footshocks (5 × 2 s of 1.5 mA shocks). We found that prepro-orexin (ppOX) mRNA was elevated in rats at 6 and 14 days after exposure to footshock and that ppOX mRNA levels were correlated with fear at 14 days post-shock. Systemic injections of the non-selective dual orexin receptor antagonist TCS-1102 (10 and 20 mg/kg, i.p.) were found to decrease fear and anxiety in rats 14 days after exposure to footshock. We also found that rats that exhibited a high level of immobility to a novel tone the day after the footshock episode (high responders, HR) showed significantly elevated levels of ppOX mRNA at 14 days post-shock compared to control rats. Furthermore, TCS-1102 (10 mg/kg, i.p.) was found to have anxiolytic effects that were specific for HR when tested in the elevated T-maze. This study provides evidence linking the orexin system to the anxiety produced by exposure of rats to a single episode of footshocks. It also provides preclinical evidence in support of the use of orexin antagonists for the treatment of anxiety in response to an acute episode of stress.

A unified survival theory of the functioning of the hypocretinergic system

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

Orexin-1 receptor antagonism fails to reduce anxiety-like behaviour in either plus-maze-naïve or plus-maze-experienced mice

Although several lines of evidence have recently implicated orexins and their receptors in fear and anxiety, there is also a growing number of apparently inconsistent and/or negative findings. In the present study, we have used ethological methods to comprehensively profile the behavioural effects of the orexin-1 receptor antagonist SB-334867 (3-30 mg/kg) in mice exposed to the elevated plus-maze. Two experiments were performed, the first involving test-naïve animals and the second using prior undrugged experience of the maze to induce a qualitatively different emotional response to that seen on first exposure. In Experiment 1, a reference benzodiazepine (chlordiazepoxide, CDP, 15 mg/kg) produced a robust anxioselective profile comprising substantial increases in open arm exploration and reduced risk assessment without any signiifcant change in general activity levels. In contrast, SB-334867 failed to produce any behavioural effects over the dose range tested. In Experiment 2, 5 min undrugged experience of the maze 24h prior to testing increased open arm avoidance and abolished the anxiolytic efficacy of CDP. Despite this altered baseline, SB-334867 again failed to alter plus-maze behaviour. These findings agree with several recent reports that orexin receptor antagonists, such as SB-334867 and almorexant, do not alter basal anxiety levels in rats but markedly contrast with the anxiolytic-like effects of the same agents when anxiety levels have been exacerbated by fear conditioning, drug challenge or hypercapnia. This unique pattern of activity suggests that orexin receptor antagonists may have therapeutic value in those clinical anxiety disorders characterised by intense emotional arousal.

Activation of GABAergic pathway by hypocretin in the median raphe nucleus (MRN) mediates stress-induced theta rhythm in rats

The frequency of electroencephalograms (EEGs) is predominant in theta rhythm during stress (e.g., footshock) in rats. Median raphe nucleus (MRN) desynchronizes hippocampal theta waves via activation of GABAergic neurons in the medial septum-diagonal band of Broca (MS-DBB), a theta rhythm pacemaker. Increased hypocretin mediates stress responses in addition to the maintenance of wakefulness. Hypocretin receptors are abundant in the MRN, suggesting a possible role of hypocretin in modulating stress-induced theta rhythm. Our results indicated that the intensity of theta waves was enhanced by footshock and that a hypocretin receptor antagonist (TCS1102) suppressed the footshock-induced theta waves. Administration of hypocretin-1 (1 and 10 μg) and hypocretin-2 (10 μg) directly into the MRN simulated the effect of footshock and significantly increased theta waves. Co-administration of GABA(A) receptor antagonist, bicuculline, into the MRN blocked the increase of theta waves induced by hypocretins or footshock. These results suggested that stress enhances the release of hypocretins, activates GABAergic neurons in the MRN, blocks the ability of MRN to desynchronize theta waves, and subsequently increases the intensity of theta rhythm.

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.

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.

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.

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.

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.

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

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

Activation of orexin neurons through non-NMDA glutamate receptors evidenced by c-Fos immunohistochemistry

Orexin neuropeptides participate in the regulation of feeding as well as the regulation and maintenance of wakefulness and the cognitive functions. Orexin A and B share a common precursor, prepro-orexin and neurons are localized in the lateral hypothalamus. Physiological studies showed that these neurons are regulated by glutamatergic innervations. We aimed to assess the effects of kainic acid as a potent agonist for non-NMDA glutamate receptors in the activation of orexin neurons. We also analyzed the effect of glutamate antagonist CNQX, injected prior to kainic acid, on this activation. Expression of c-Fos protein was used as a marker for neuronal activation. Dual immunohistochemical labeling was performed for prepro-orexin and c-Fos and the percentages of c-Fos-expressing orexin neurons were obtained for control, kainic acid, and CNQX groups. Kainic acid injection caused statistically significant increase in the number of c-Fos-positive neurons when compared to control group (62.69 and 36.31%, respectively). Activation of orexin neurons was blocked, in part, by CNQX (43.36%). In the light of these results, it is concluded that glutamate takes part in the regulation of orexin neurons and partially exerts its effects through non-NMDA glutamate receptors and that orexin neurons express functional non-NMDA receptors.

Hypocretin/orexin in arousal and stress

Multiple lines of evidence indicate that hypocretin/orexin (HCRT) participates in the regulation of arousal and arousal-related process. For example, HCRT axons and receptors are found within a variety of arousal-related systems. Moreover, when administered centrally, HCRT exerts robust wake-promoting actions. Finally, a dysregulation of HCRT neurotransmission is associated with the sleep/arousal disorder, narcolepsy. Combined, these observations suggested that HCRT might be a key transmitter system in the regulation of waking. Nonetheless, subsequent evidence indicates that HCRT may not play a prominent role in the initiation of normal waking. Instead HCRT may participate in a variety of processes such as consolidation of waking and/or coupling metabolic state with behavioral state. Additionally, substantial evidence suggests a potential involvement of HCRT in high-arousal conditions, including stress. Thus, HCRT neurotransmission is closely linked to high-arousal conditions, including stress, and HCRT administration exerts a variety of stress-like physiological and behavioral effects that are superimposed on HCRT-induced increases in arousal. Combined, this evidence suggests the hypothesis that HCRT may participate in behavioral responding under high-arousal aversive conditions. Importantly, these actions of HCRT may not be limited to stress. Like stress, appetitive conditions are associated with elevated arousal levels and a stress-like activation of various physiological systems. These and other observations suggest that HCRT may, at least in part, exert affectively neutral actions that are important under high-arousal conditions associated with elevated motivation and/or need for action.

Role of orexin/hypocretin in dependence and addiction

The orexins (or hypocretins) are hypothalamic neuropeptides that have been implicated in a variety of behaviors ranging from feeding to sleep and arousal. Evidence from animal models suggests a role for orexins in reward processing and drug addiction. In this review, we discuss orexin's interaction with the mesocorticolimbic reward pathway and the effects of drugs of abuse on the orexin system. We further review models of drug dependence and addiction and describe behavioral alterations that are seen when the orexin system is manipulated both pharmacologically and genetically. Based on the findings reported in the literature thus far, we posit that orexin functioning contributes to both drug reward and drug-related stress/aversive responsiveness; however, diverse anatomical substrates, and perhaps receptor specificity, contribute differentially to reward and stress components.

Orexin-A acts on the paraventricular nucleus of the midline thalamus to inhibit locomotor activity in rats

Orexins (hypocretins) are novel peptides that have been shown to play a role in control of behavioral arousal. The paraventricular nucleus of the midline thalamus (PVT) is one area of the brain that is the most densely innervated by orexin fibers. In addition, the PVT sends a dense projection to the nucleus accumbens, an area of the striatum involved in the regulation of locomotion. This study was done to determine the effect of microinjections of orexin-A (OXA) or the orexin receptor antagonist SB334867 in the PVT on locomotor activity (LA) in morphine-naïve and morphine-sensitized rats. Microinjections of OXA (3 microg/500 nl) in or near the PVT inhibited LA in rats tested in a novel and familiar environment as well as in rats expressing behavioral sensitization to repeated injections of morphine. In contrast, microinjections of SB334867 had no effect on LA in any of the test situations. Using an approach involving experimenter based analysis of ethological behaviors; we found that microinjections of OXA in the midline thalamus decreased LA while at the same time increasing the expression of grooming and freezing. These results suggest that OXA can act on the PVT and the midline thalamus to produce arousal independent of LA.

Orexin neurons in the hypothalamus mediate cardiorespiratory responses induced by disinhibition of the amygdala and bed nucleus of the stria terminalis

We previously showed that the defense response elicited by stressors was attenuated in prepro-orexin knockout mice and in orexin neuron-ablated mice, and we proposed that orexin serves as a master switch within multiple efferent pathways that mediate the defense response. In this study we sought to determine whether excitation of the amygdala (AMG) or the bed nucleus of stria terminalis (BNST) activates orexin-containing neurons and whether those neurons are essential in eliciting cardiorespiratory responses to the stimulus. In urethane-anesthetized mice, the GABA-A receptor antagonist bicuculline was microinjected into the AMG or BNST and blood pressure, heart rate, and respiration were measured. Injection of bicuculline in either site induced long-lasting dose-dependent cardiorespiratory excitation in wild-type mice. In contrast, mice in which orexin neurons had been ablated demonstrated no such response after activation of the AMG and an attenuated response after activation of the BNST. Double immunohistochemical staining for orexin and c-Fos, an indicator of neural activation, revealed that an injection of bicuculline induced significantly larger numbers of orexin positive neurons that expressed c-Fos in the perifornical/dorsomedial hypothalamus (58.2+/-6.4% into AMG and 66.4+/-6.6% into BNST, n=3 each) than did vehicle (18.2+/-4.4% into AMG and 28.3+/-2.1% into BNST). Disinhibition to the BNST induced widespread expression of c-Fos not only in orexin-containing neurons but also other neurons in the hypothalamus. We conclude that orexin-containing neurons in the medial hypothalamus mediate at least a part of AMG- and BNST-induced cardiorespiratory responses.

Inhibitory effects of an orexin-2 receptor antagonist on orexin A- and stress-induced ACTH responses in conscious rats

Orexins, recognized for their diverse functions in sleep/wakefulness/arousal and appetite regulation, may play provocative roles in stress response. Although the PVN of the hypothalamus expresses an abundance of orexin-2 receptor (OX-2R), the involvement of OX-2R in regulating ACTH response to stress remains unclear. To address this, we examined effects of a selective antagonist to OX-2R (N-{(1S)-1-[6,7-dimethoxy-3,4-dihydro-2(1H)-isoquinolinyl]carbonyl}-2,2-dimethylpropyl)-N-{4-pyridinylmethyl}amine upon plasma ACTH concentrations after administration of orexin A and swimming stress. Increases in ACTH levels with orexin A or swimming stress were attenuated with prior administration of an OX-2R antagonist. These results suggest that swimming stress facilitates ACTH release, at least in part via activation of OX-2R.

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.

Centrally administered orexin-A activates corticotropin-releasing factor-containing neurons in the hypothalamic paraventricular nucleus and central amygdaloid nucleus of rats: possible involvement of central orexins on stress-activated central CRF neurons

We examined the effects of centrally administered orexin-A on corticotropin-releasing factor (CRF)-containing neurons in the hypothalamic paraventricular nucleus (PVN) and the central amygdaloid nucleus (CeA) of rats, using dual immunostaining for CRF and Fos. Ninety minutes after intracerebroventricular administration of orexin-A, approximately 96% and 45% of CRF-containing neurons expressed Fos-like immunoreactivity (LI) in the PVN and the CeA, respectively. We also examined the effects of immobilized stress and cold exposure on orexin-A-containing neurons in the rat hypothalamus using dual immunostaining for orexin-A and Fos. After immobilized stress for 20 min and cold exposure at 4 degrees C for 30 min, approximately 24% and 15% of orexin-A-containing neurons expressed Fos-LI, respectively. These results suggest that orexins in the central nervous system may be involved in the activation of central CRF neurons induced by stress.

Locomotor-dependent and -independent components to hypocretin-1 (orexin A) regulation in sleep-wake consolidating monkeys

The hypocretin system is involved in the integration of hypothalamic functions with sleep and wake. Hypocretin-1 release peaks at the end of the active period in both diurnal and nocturnal species. A role for hypocretin-1 in the generation of locomotor activity has been suggested by electrophysiological and neurochemical studies in rodents, dogs and cats. These species, however, do not consolidate wake into a single, daily bout and manipulations of locomotion elicit changes in wakefulness, making it difficult to parse the relative contribution of these two factors. We have examined the relationship between locomotion and hypocretin-1 in a wake-consolidating animal, the squirrel monkey (Saimiri sciureus). Strikingly, we found that restricting locomotion to 17% of usual activity had no significant effect on the normal diurnal rise in cerebrospinal fluid (CSF) hypocretin-1, despite an associated increase in CSF cortisol. Increasing locomotion to greater than baseline activity did not significantly increase CSF hypocretin-1 concentrations, but did appear to have a positive modulatory effect on CSF hypocretin-1. In this wake-consolidating animal, locomotion is not necessary for CSF hypocretin-1 to increase throughout the daytime, but high levels of locomotion are likely to provide a small positive feedback onto the hypocretin system.

Cardiovascular effects of leptin and orexins

Leptin, the product of the ob gene, is a satiety factor secreted mainly in adipose tissue and is part of a signaling mechanism regulating the content of body fat. It acts on leptin receptors, most of which are located in the hypothalamus, a region of the brain known to control body homeostasis. The fastest and strongest hypothalamic response to leptin in ob/ob mice occurs in the paraventricular nucleus, which is involved in neuroendocrine and autonomic functions. On the other hand, orexins (orexin-A and -B) or hypocretins (hypocretin-1 and -2) were recently discovered in the hypothalamus, in which a number of neuropeptides are known to stimulate or suppress food intake. These substances are considered important for the regulation of appetite and energy homeostasis. Orexins were initially thought to function in the hypothalamic regulation of feeding behavior, but orexin-containing fibers and their receptors are also distributed in parts of the brain closely associated with the regulation of cardiovascular and autonomic functions. Functional studies have shown that these peptides are involved in cardiovascular and sympathetic regulation. The objective of this article is to summarize evidence on the effects of leptin and orexins on cardiovascular function in vivo and in vitro and to discuss the pathophysiological relevance of these peptides and possible interactions.