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

Circadian regulation of developmental synaptogenesis via the hypocretinergic system

The circadian clock orchestrates a wide variety of physiological and behavioral processes, enabling animals to adapt to daily environmental changes, particularly the day-night cycle. However, the circadian clock's role in the developmental processes remains unclear. Here, we employ the in vivo long-term time-lapse imaging of retinotectal synapses in the optic tectum of larval zebrafish and reveal that synaptogenesis, a fundamental developmental process for neural circuit formation, exhibits circadian rhythm. This rhythmicity arises primarily from the synapse formation rather than elimination and requires the hypocretinergic neural system. Disruption of this synaptogenic rhythm, by impairing either the circadian clock or the hypocretinergic system, affects the arrangement of the retinotectal synapses on axon arbors and the refinement of the postsynaptic tectal neuron's receptive field. Thus, our findings demonstrate that the developmental synaptogenesis is under hypocretin-dependent circadian regulation, suggesting an important role of the circadian clock in neural development.

Orexin Reserve: A Mechanistic Framework for the Role of Orexins (Hypocretins) in Addiction

In 2014, we proposed that orexin signaling transformed motivationally relevant states into adaptive behavior directed toward exploiting an opportunity or managing a threat, a process we referred to as motivational activation. Advancements in animal models since then have permitted higher-resolution measurements of motivational states; in particular, the behavioral economics approach for studying drug demand characterizes conditions that lead to the enhanced motivation that underlies addiction. This motivational plasticity is paralleled by persistently increased orexin expression in a topographically specific manner-a finding confirmed across species, including in humans. Normalization of orexin levels also reduces drug motivation in addiction models. These new advancements lead us to update our proposed framework for the orexin function. We now propose that the capacity of orexin neurons to exhibit dynamic shifts in peptide production contributes to their role in adaptive motivational regulation and that this is achieved via a pool of reserve orexin neurons. This reserve is normally bidirectionally recruited to permit motivational plasticity that promotes flexible, adaptive behavior. In pathological states such as addiction, however, we propose that the orexin system loses capacity to adaptively adjust peptide production, resulting in focused hypermotivation for drug, driven by aberrantly and persistently high expression in the orexin reserve pool. This mechanistic framework has implications for the understanding and treatment of several psychiatric disorders beyond addiction, particularly those characterized by motivational dysfunction.

Neural correlates and potential targets for the contribution of orexin to addiction in cortical and subcortical areas

The orexin (hypocretin) is one of the hypothalamic neuropeptides that plays a critical role in some behaviors including feeding, sleep, arousal, reward processing, and drug addiction. This variety of functions can be described by a united function for orexins in translating states of heightened motivation, for example during physiological requirement states or following exposure to reward opportunities, into planned goal-directed behaviors. An addicted state is characterized by robust activation of orexin neurons from the environment, which triggers downstream circuits to facilitate behavior directed towards obtaining the drug. Two orexin receptors 1 (OX1R) and 2 (OX2R) are widely distributed in the brain. Here, we will introduce and describe the cortical and subcortical brain areas involved in addictive-like behaviors and the impact of orexin on addiction.

Acute orexin antagonism selectively modulates anticipatory anxiety in humans: implications for addiction and anxiety

Research indicates that heightened anticipatory anxiety underlies several forms of psychopathology. Anticipatory anxiety can be reliably and objectively measured in the laboratory using the No-Predictable-Unpredictable (NPU) threat paradigm. The NPU paradigm is an ideal research tool for the NIH 'Fast-Fail' approach of screening promising compounds and testing human target engagement. Evidence from preclinical studies suggests that the hypocretin/orexin (ORX) hypothalamic neuropeptide system is a potential means for modulating anticipatory anxiety and disrupting stress-related alcohol use. The current study tested this question using a psychophysiological probe of the ORX system in humans. We examined whether a single dose of suvorexant (SUV; 10 mg; dual ORX receptor antagonist) can effectively and selectively target a well-validated human laboratory index of exaggerated anticipatory anxiety using a within-subjects placebo-controlled design. A total of twenty-one volunteers completed two laboratory sessions during acute administration of 10 mg SUV or placebo. Across sessions, we administered the NPU paradigm probing sustained anticipatory anxiety and fear while startle eyeblink was recorded as an index of aversive reactivity. Questionnaires assessing mood states and subjective drug effects were also collected. Results indicated SUV was well-tolerated. Compared with placebo, SUV was associated with decreased startle reactivity during anticipatory anxiety but not fear or no-threat conditions. Therefore, SUV selectively and effectively reduced objective indicators of anticipatory anxiety in humans and engaged our laboratory target of psychopathology. ORX antagonism may be a promising strategy for modulating human anxiety and potentially, stress-related alcohol use.

Blockade of corticotropin-releasing factor-1 receptors in the infralimbic cortex prevents stress-induced reinstatement of alcohol seeking in male Wistar rats: Evidence of interaction between CRF1 and orexin receptor signaling

Alcohol use dysregulates responsivity to stress, which is mediated by corticotropin-releasing factor (CRF). With repeated cycles of alcohol use, the hypothalamic-pituitary-adrenal axis becomes hyporesponsive, rendering individuals vulnerable to the reinstatement of alcohol-seeking behavior during stressful episodes. Orexin (Orx; also called hypocretin) plays a well-established role in regulating diverse physiological processes, including stress, and interacts with CRF. The infralimbic cortex (IL) is a CRF-rich region. Anatomical evidence suggests that CRF and Orx interact in this area. To test the behavioral implication of CRF and Orx transmission in the IL during the stress-induced reinstatement of alcohol-seeking behavior, male Wistar rats were trained to self-administer 10% alcohol for 3 weeks. The rats then underwent two weeks of extinction training (identical to the alcohol self-administration sessions, but alcohol was withheld). The day after the last extinction session, the rats received a bilateral intra-IL injection of the CRF1 receptor antagonist CP154,526 (0.6 μg/0.5 μl/side), the dual Orx receptor antagonist TCS1102 (15 μg/0.5 μl/side), or their combination and then were tested for the footshock stress-induced reinstatement of alcohol-seeking behavior. CP154,526 significantly prevented reinstatement, but TCS1102 did not produce such an effect. Interestingly, the co-administration of TCS1102 and CP154,526 reversed the effect of CP154,526 alone, and footshock stress induced a significant increase in Crhr1 and Hcrtr2 mRNA expression in the IL. These results demonstrate a functional interaction between Orx receptor and CRF1 receptor signaling and suggest that CRF1 receptor antagonism may ameliorate stress-induced alcohol-seeking behavior.

Sleep-Wake and Arousal Dysfunctions in Post-Traumatic Stress Disorder:Role of Orexin Systems

Post-traumatic stress disorder (PTSD) is a trauma-related condition that produces distressing fear memory intrusions, avoidance behaviors, hyperarousal/startle, stress responses and insomnia. This review focuses on the importance of the orexin neural system as a novel mechanism related to the pathophysiology of PTSD. Orexinergic neurons originate in the lateral hypothalamus and project widely to key neurotransmitter system neurons, autonomic neurons, the hypothalamic-pituitaryadrenal (HPA) axis, and fear-related neural circuits. After trauma or stress, the basolateral amygdala (BLA) transmits sensory information to the central nucleus of the amygdala (CeA) and in turn to the hypothalamus and other subcortical and brainstem regions to promote fear and threat. Orexin receptors have a prominent role in this circuit as fear conditioned orexin receptor knockout mice show decreased fear expression while dual orexin receptor antagonists (DORAs) inhibit fear acquisition and expression. Orexin activation of an infralimbic-amygdala circuit impedes fear extinction while DORA treatments enhance it. Increased orexin signaling to the amygdalocortical- hippocampal circuit promotes avoidance behaviors. Orexin has an important role in activating sympathetic nervous system (SNS) activity and the HPA axis stress responses. Blockade of orexin receptors reduces fear-conditioned startle responses. In PTSD models, individuals demonstrate sleep disturbances such as increased sleep latency and more transitions to wakefulness. Increased orexin activity impairs sleep by promoting wakefulness and reducing total sleep time while DORA treatments enhance sleep onset and maintenance. The orexinergic neural system provides important mechanisms for understanding multiple PTSD behaviors and provides new medication targets to treat this often persistent and debilitating illness.

Intra-CA1 injection of orexin receptors antagonism attenuates the stress-induced analgesia in a rat acute pain model

Orexins or hypocretins are excitatory neuropeptides predominantly produced by neuronal clusters in the lateral hypothalamus. The orexinergic system's involvement in pain modulation makes it a candidate for pain control alternative to the opioid system. Moreover, orexin-1 and orexin -2 receptors (OX1r and OX2r, respectively) play a role in responsiveness to stressful stimuli. Some evidence indicates that the Cornu Ammonis 1 (CA1) region of the hippocampus potentially participates in the modulation of both pain and stress. In quest of better understanding the interaction between orexin receptors and stress-induced analgesia (SIA), The present study examined the involvement of OX1r and OX2r within the CA1 in response to acute pain after exposure to forced swim stress (FSS) for a 6-min period. Adult male Wistar rats received different doses of OX1r antagonist (SB334867; 1, 3, 10, and 30 nmol), OX2r antagonist (TCS OX2 29; 3, 10, 30 and 100 nmol), or vehicle (0.5 μl DMSO) through an implanted cannula. After that, animals individually experienced acute pain by performing the tail-flick test. Results indicated that FSS produces antinociceptive responses in the tail-flick test. Blockade of both orexin receptors within the CA1 region attenuated the analgesic effect of FSS. The antinociceptive effect of swim stress was prevented by lower doses of SB334867 than TCS OX2 29. These findings show that the orexinergic system might be partially involved in the SIA via the OX1 and OX2 receptors in the hippocampal CA1 region.

Appetitive Behavior in the Social Transmission of Food Preference Paradigm Predicts Activation of Orexin-A producing Neurons in a Sex-Dependent Manner

Orexin-producing cells in the lateral hypothalamic area have been shown to be involved in a wide variety of behavioral and cognitive functions, including the recall of appetitive associations and a variety of social behaviors. Here, we investigated the role of orexin in the acquisition and recall of socially transmitted food preferences in the rat. Rats were euthanized following either acquisition, short-term recall, or long-term recall of a socially transmitted food preference and their brains were processed for orexin-A and c-Fos expression. We found that while there were no significant differences in c-Fos expression between control and experimental subjects at any of the tested timepoints, females displayed significantly more activity in both orexinergic and non-orexinergic cells in the lateral hypothalamus. In the infralimbic cortex, we found that social behavior was significantly predictive of c-Fos expression, with social behaviors related to olfactory exploration appearing to be particularly influential. We additionally found that appetitive behavior was significantly predictive of orexin-A activity in a sex-dependent matter, with the total amount eaten correlating negatively with orexin-A/c-Fos colocalization in male rats but not female rats. These findings suggest a potential sex-specific role for the orexin system in balancing the stimulation of feeding behavior with the sleep/wake cycle.

Do orexin/hypocretin neurons signal stress or reward?

Hypothalamic neurons that produce the peptide transmitters orexins/hypocretins (HONs) broadcast their predominantly neuroexcitatory outputs to the entire brain via their extremely wide axonal projections. HONs were originally reported to be activated by food deprivation, and to stimulate arousal, energy expenditure, and eating. This led to extensive studies of HONs in the context of nutrient-sensing and energy balance control. While activation of HONs by body energy depletion continues to be supported by experimental evidence, it has also become clear that HONs are robustly activated not only by nutrient depletion, but also by diverse sensory stimuli (both neutral and those associated with rewarding or aversive events), seemingly unrelated to each other or to energy balance. One theory that could unify these findings is that all these stimuli signal "stress" - defined either as a potentially harmful state, or an awareness of reward deficiency. If HON activity is conceptualized as a cumulative representation of stress, then many of the reported HONs outputs - including EEG arousal, sympathetic activation, place avoidance, and exploratory behaviours - could be viewed as logical stress-counteracting responses. We discuss evidence for and against this unifying theory of HON function, including the alterations in HON activity observed in anxiety and depression disorders. We propose that, in order to orchestrate stress-countering responses, HONs need to coactivate motivation and aversion brain systems, and the impact of HON stimulation on affective states may be perceived as rewarding or aversive depending on the baseline HON activity.

Orexin 1 and 2 Receptors in the Prelimbic Cortex Modulate Threat Valuation

The ability to distinguish between threatening (repulsors), neutral and appetitive stimuli (attractors) stimuli is essential for survival. The orexinergic neurons of hypothalamus send projections to the limbic structures, such as different subregions of the medial prefrontal cortex (mPFC), suggesting that the orexinergic mechanism in the prelimbic cortex (PL) is involved in the processing of fear and anxiety. We investigated the role of orexin receptors type 1 (OX₁R) and type 2 (OX₂R) in the PL in such processes upon confrontation with an erratically moving robo-beetle in mice. The selective blockade of OX₁R and OX₂R in the PL with SB 334867 (3, 30, 300 nM) and TCS OX2 29 (3, 30, 300 nM), respectively, did not affect general exploratory behavior or reactive fear such as avoidance, jumping or freezing, but significantly enhances tolerance and approach behavior at the highest dose of each antagonist tested (300 nM). We interpret these findings as evidence for an altered cognitive appraisal of the potential threatening stimulus. Consequently, the orexin system seems to bias the perception of stimuli towards danger or threat via OX₁R and OX₂R in the PL.

Orexin Modulation of VTA Dopamine Neuron Activity: Relevance to Schizophrenia

BACKGROUND

The hippocampus is a region consistently implicated in schizophrenia and has been advanced as a therapeutic target for positive, negative, and cognitive deficits associated with the disease. Recently, we reported that the paraventricular nucleus of the thalamus (PVT) works in concert with the ventral hippocampus to regulate dopamine system function; however, the PVT has yet to be investigated as target for the treatment of the disease. Given the dense expression of orexin receptors in the thalamus, we believe these to be a possible target for pharmacological regulation of PVT activity.

METHODS

Here we used the methylazoxymethanol acetate (MAM) rodent model, which displays pathological alterations consistent with schizophrenia to determine whether orexin receptor blockade can restore ventral tegmental area dopamine system function. We measured dopamine neuron population activity, using in vivo electrophysiology, following administration of the dual orexin antagonist, TCS 1102 (both intraperitoneal and intracranial into the PVT in MAM- and saline-treated rats), and orexin A and B peptides (intracranial into the PVT in naïve rats).

RESULTS

Aberrant dopamine system function in MAM-treated rats was normalized by the systemic administration of TCS 1102. To investigate the potential site of action, the orexin peptides A and B were administered directly into the PVT, where they significantly increased ventral tegmental area dopamine neuron population activity in control rats. In addition, the direct administration of TCS 1102 into the PVT reproduced the beneficial effects seen with the systemic administration in MAM-treated rats.

CONCLUSION

Taken together, these data suggest the orexin system may represent a novel site of therapeutic intervention for psychosis via an action in the PVT.

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.

A projection from the paraventricular nucleus of the thalamus to the shell of the nucleus accumbens contributes to footshock stress-induced social avoidance

The paraventricular nucleus of the thalamus (PVT) is an area of the dorsal midline thalamus that contributes to footshock induced anxiety. The PVT sends a dense projection to the shell of the nucleus accumbens (NAcSh) and the present study explored if this projection is involved in the behavioral changes produced by a single exposure of rats to inescapable footshocks. The inhibitory Designer Receptors Exclusively Activated by Designer Drugs (DREADDs) hM4Di was transduced in PVT neurons that project to the NAcSh. Rats were exposed to an episode of moderately intense footshock (1.5 mA × 2 s × 5) and assigned to either high-responder (HR) or low-responder groups (LR) according to their level of fear generalization 24 h later. The effect of chemogenetic inhibition of the PVT-NAcSh projection on anxiety- and fear-like behaviors was assessed at approximately 2 weeks post-footshock. HR showed a higher level of social avoidance compared to non-shocked animals and LR. The elevated level of social avoidance was attenuated in the HR treated with the hM4Di agonist clozapine (0.01 mg/kg, i.p.) or clozapine N-oxide (CNO) administrations in the NAcSh while avoidance of open spaces and contextual fear expression were not affected. Analysis of protein product of the early to immediate gene cfos indicated that these effects were mediated by dynorphin neurons in the NAcSh. This study provides evidence for a role of a projection from the PVT to the NAcSh in stress-induced social avoidance independent of anxiety to non-social stimuli and contextual fear mechanisms.

The orexin (hypocretin) neuropeptide system is a target for novel therapeutics to treat cocaine use disorder with alcohol coabuse

An estimated 50-90% of individuals with cocaine use disorder (CUD) also report using alcohol. Cocaine users report coabusing alcohol to 'self-medicate' against the negative emotional side effects of the cocaine 'crash', including the onset of anxiety. Thus, pharmaceutical strategies to treat CUD would ideally reduce the motivational properties of cocaine, alcohol, and their combination, as well as reduce the onset of anxiety during drug withdrawal. The hypothalamic orexin (hypocretin) neuropeptide system offers a promising target, as orexin neurons are critically involved in activating behavioral and physiological states to respond to both positive and negative motivators. Here, we seek to describe studies demonstrating efficacy of orexin receptor antagonists in reducing cocaine, alcohol- and stress-related behaviors, but note that these studies have largely focused on each of these phenomena in isolation. For orexin-based compounds to be viable in the clinical setting, we argue that it is imperative that their efficacy be tested in animal models that account for polysubstance use patterns. To begin to examine this, we present new data showing that rats' preferred level of cocaine intake is significantly increased following chronic homecage access to alcohol. We also report that cocaine intake and motivation are reduced by a selective orexin-1 receptor antagonist when rats have a history of cocaine + alcohol, but not a limited history of cocaine alone. In light of these proof-of-principle data, we outline what we believe to be the key priorities going forward with respect to further examining the orexin system in models of polysubstance use. This article is part of the special issue on Neurocircuitry Modulating Drug and Alcohol Abuse.

Sex- and Stress-Dependent Effects on Dendritic Morphology and Spine Densities in Putative Orexin Neurons

We recently found that non-stressed female rats have higher basal prepro-orexin expression and activation of orexinergic neurons compared to non-stressed males, which lead to impaired habituation to repeated restraint stress at the behavioral, neural, and endocrine level. Here, we extended our study of sex differences in the orexin system by examining spine densities and dendritic morphology in putative orexin neurons in adult male and female rats that were exposed to 5 consecutive days of 30-min restraint. Analysis of spine distribution and density indicated that putative orexinergic neurons in control non-stressed females had significantly more dendritic spines than those in control males, and the majority of these were mushroom spines. This morphological finding may suggest more excitatory input onto orexin neurons in female rats. As orexin neurons are known to promote the hypothalamic-pituitary-adrenal response, this morphological change in orexin neurons could underlie the impaired habituation to repeated stress in female rats. Dendritic complexity did not differ between non-stressed males and females, however repeated restraint stress decreased total dendritic length, nodes, and branching primarily in males. Thus, reduced dendritic complexity of putative orexinergic neurons is observed in males but not in females after 5days of repeated restraint stress. This morphological change might be reflective of decreased orexin system function, which may allow males to habituate more fully to repeated restraint than females. These results extend our understanding of the role of orexin neurons in regulating habituation and demonstrate changes in putative orexin cell morphology and spines that may underlie sex differences in habituation.

Orexin Depolarizes Central Amygdala Neurons via Orexin Receptor 1, Phospholipase C and Sodium-Calcium Exchanger and Modulates Conditioned Fear

Orexins (OX), also known as hypocretins, are excitatory neuropeptides with well-described roles in regulation of wakefulness, arousal, energy homeostasis, and anxiety. An additional and recently recognized role of OX is modulation of fear responses. We studied the OX neurons of the perifornical hypothalamus (PeF) which send projections to the amygdala, a region critical in fear learning and fear expression. Within the amygdala, the highest density of OX-positive fibers was detected in the central nucleus (CeA). The specific mechanisms underlying OX neurotransmission within the CeA were explored utilizing rat brain slice electrophysiology, pharmacology, and chemogenetic stimulation. We show that OX induces postsynaptic depolarization of medial CeA neurons that is mediated by OX receptor 1 (OXR1) but not OX receptor 2 (OXR2). We further characterized the mechanism of CeA depolarization by OX as phospholipase C (PLC)- and sodium-calcium exchanger (NCX)- dependent. Selective chemogenetic stimulation of OX PeF fibers recapitulated OXR1 dependent depolarization of CeA neurons. We also observed that OXR1 activity modified presynaptic release of glutamate within the CeA. Finally, either systemic or intra-CeA perfusion of OXR1 antagonist reduced the expression of conditioned fear. Together, these data suggest the PeF-CeA orexinergic pathway can modulate conditioned fear through a signal transduction mechanism involving PLC and NCX activity and that selective OXR1 antagonism may be a putative treatment for fear-related disorders.

Orexin as a modulator of fear-related behavior: Hypothalamic control of noradrenaline circuit

Fear is an important physiological function for survival. It appears when animals or humans are confronted with an environmental threat. The amygdala has been shown to play a highly important role in emergence of fear. Hypothalamic orexin neurons are activated by fearful stimuli to evoke a 'defense reaction' with an increase in arousal level and sympathetic outflow to deal with the imminent danger. However, how this system contributes to the emergence of fear-related behavior is not well understood. Orexin neurons in the hypothalamus send excitatory innervations to noradrenergic neurons in the locus coeruleus (NA^LC) which express orexin receptor 1 (OX1R) and send projections to the lateral amygdala (LA). Inhibition of this di-synaptic orexin → NA^LC → LA pathway by pharmacological or opto/chemogenetic methods reduces cue-induced fear expression. Excitatory manipulation of this pathway induces freezing, a fear-related behavior that only occurs when the environment contains some elements suggestive of danger. Although, fear memory helps animals respond to a context or cue previously paired with an aversive stimulus, fear-related behavior is sometimes evoked even in a distinct context containing some similar elements, which is known as fear generalization. Our recent observation suggests that the orexin → NA^LC → LA pathway might contribute to this response. This review focuses on recent advances regarding the role of hypothalamic orexin neurons in behavioral fear expression. We also discuss the potential effectiveness of orexin receptor antagonists for treating excessive fear response or overgeneralization seen in anxiety disorder and post-traumatic stress disorder (PTSD).

Orexins and stress

The neuropeptides orexins are important in regulating the neurobiological systems that respond to stressful stimuli. Furthermore, orexins are known to play a role many of the phenotypes associated with stress-related mental illness such as changes in cognition, sleep-wake states, and appetite. Interestingly, orexins are altered in stress-related psychiatric disorders such as Major Depressive Disorder and Anxiety Disorders. Thus, orexins may be a potential target for treatment of these disorders. In this review, we will focus on what is known about the role of orexins in acute and repeated stress, in stress-induced phenotypes relevant to psychiatric illness in preclinical models, and in stress-related psychiatric illness in humans. We will also briefly discuss how orexins may contribute to sex differences in the stress response and subsequent phenotypes relevant to mental health, as many stress-related psychiatric disorders are twice as prevalent in women.

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'.

The contribution of orexins to sex differences in the stress response

Women are twice as likely as men to suffer from stress-related psychiatric disorders, such as post-traumatic stress disorder (PTSD) and Major Depressive Disorder (MDD), however, the biological basis of these sex differences is not fully understood. Interestingly, orexins are known to be dysregulated in these disorders. This review first discusses the important role of orexins regulating the response to stress. Next, we review the evidence for sex differences in the orexin system, in which the majority of both preclinical and clinical studies have reported higher orexin system expression in females. Finally, we discuss the functional consequences of these sex differences in orexin expression. Most importantly, the preclinical literature reveals that higher orexin system activity in females contributes to exaggerated neuroendocrine and behavioral responses to stress. In sum, the available data suggests that orexins may be important in the etiology of stress-related psychiatric disorders that present differently in men and women. Thus, targeting orexins could potentially ameliorate many phenotypes of stress-related illness in a sex-specific way.

Orexin A Differentially Influences the Extinction Retention of Recent and Remote Fear Memory

Recently the role of the orexin system in the learning and memory, especially orexin A, which could enhance fear memory through regulating the activity of amygdala, has drawn considerable attention. However, the relationship between orexin A and extinction memory remains unclear. To investigate the effect of orexin A on extinction memory in humans, we recruited 43 male subjects and divided them into a recent group and remote group. After acquiring Pavlovian fear conditioning, individuals in recent group experienced fear extinction 24 h after acquisition, and remote group underwent extinction 2 weeks later. Meanwhile, plasma orexin A levels before extinction were measured by enzyme-linked immunosorbent assay. Both groups received memory test 24 h after fear extinction. The results showed that both recent and remote groups successfully acquired fear conditioning and had spontaneous recovery at test. In particular, the correlational analysis indicated that orexin A levels before extinction were negatively associated with fear responses during test only in recent group, but not in remote group. Moreover, individuals with high orexin A levels still kept low fear responses after extinction in recent group by subgroup analyses. The results suggest that orexin A could influence the retention of recent fear memory extinction, without affecting remote fear extinction. These findings remind us the orexin system can be a potential treatment target for fear-related disorders, and the mechanisms of recent and remote fear extinction may be different.

Sex- and Age-dependent Effects of Orexin 1 Receptor Blockade on Open-Field Behavior and Neuronal Activity

Adolescence is a sensitive and critical period in brain development where psychiatric disorders such as anxiety, depression and post-traumatic stress disorder are more likely to emerge following a stressful life event. Females are two times more likely to suffer from psychiatric disorders than males. Patients with these disorders show alterations in orexins (also called hypocretins), important neuropeptides that regulate arousal, wakefulness and the hypothalamic-pituitary-adrenal axis activity. Little is known on the role of orexins in mediating arousal behaviors in male and female rats during adolescence or adulthood. Here, we examine the influence of orexin 1 receptor blockade by SB334867 in open-field behavior in male and female rats during early adolescence (PND 31-33) or adulthood (PND 75-77). Animals were injected with 0 (vehicle), 1, 10, or 30 mg/kg SB334867 (i.p.). Thirty minutes later, they were placed in an open field, and behavior and neuronal activity (c-Fos) were assessed. In adolescent males, SB334867 significantly increased immobility in the 10 mg/kg group compared to vehicle. However, this increase in immobility in adolescent males was not observed in adolescent females. In contrast to adolescent males, adult males in the 10 mg/kg dose group showed the opposite effect on immobility compared to vehicle. These results indicate that 10 mg/kg dose of SB334867 has opposing effects in adolescent and adult males, but few effects in adolescent and adult females. Differences in functional networks between limbic regions may underlie these effects of orexin receptor blockade that are sex- and age-dependent in rats.

Reduced Orexin System Function Contributes to Resilience to Repeated Social Stress

Exposure to stress increases the risk of developing affective disorders such as depression and post-traumatic stress disorder (PTSD). However, these disorders occur in only a subset of individuals, those that are more vulnerable to the effects of stress, whereas others remain resilient. The coping style adopted to deal with the stressor, either passive or active coping, is related to vulnerability or resilience, respectively. Important neural substrates that mediate responses to a stressor are the orexins. These neuropeptides are altered in the cerebrospinal fluid of patients with stress-related illnesses such as depression and PTSD. The present experiments used a rodent social defeat model that generates actively coping rats and passively coping rats, which we have previously shown exhibit resilient and vulnerable profiles, respectively, to examine if orexins play a role in these stress-induced phenotypes. In situ radiolabeling and qPCR revealed that actively coping rats expressed significantly lower prepro-orexin mRNA compared with passively coping rats. This led to the hypothesis that lower levels of orexins contribute to resilience to repeated social stress. To test this hypothesis, rats first underwent 5 d of social defeat to establish active and passive coping phenotypes. Then, orexin neurons were inhibited before each social defeat for three additional days using designer receptors exclusively activated by designer drugs (DREADDs). Inhibition of orexins increased social interaction behavior and decreased depressive-like behavior in the vulnerable population of rats. Indeed, these data suggest that lowering orexins promoted resilience to social defeat and may be an important target for treatment of stress-related disorders.

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 2 receptor regulation of the hypothalamic-pituitary-adrenal (HPA) response to acute and repeated stress

Orexins are hypothalamic neuropeptides that have a documented role in mediating the acute stress response. However, their role in habituation to repeated stress, and the role of orexin receptors (OX₁R and OX₂R) in the stress response, has yet to be defined. Orexin neuronal activation and levels in the cerebrospinal fluid (CSF) were found to be stimulated with acute restraint, but were significantly reduced by day five of repeated restraint. As certain disease states such as panic disorder are associated with increased central orexin levels and failure to habituate to repeated stress, the effect of activating orexin signaling via Designer Receptors Exclusively Activated by Designer Drugs (DREADDs) on the hypothalamic-pituitary-adrenal (HPA) response was evaluated after repeated restraint. While vehicle-treated rats displayed habituation of Adrenocorticotropic Hormone (ACTH) from day 1 to day 5 of restraint, stimulating orexins did not further increase ACTH beyond vehicle levels for either acute or repeated restraint. We delineated the roles of orexin receptors in acute and repeated stress using a selective OX₂R antagonist (MK-1064). Pretreatment with MK-1064 reduced day 1 ACTH levels, but did not allow further habituation on day 5 compared with vehicle-treated rats, indicating that endogenous OX₂R activity plays a role in acute stress, but not in habituation to repeated stress. However, in restrained rats with further stimulated orexins by DREADDs, MK-1064 decreased ACTH levels on day 5. Collectively, these results indicate that the OX₂R plays a role in acute stress, and can prevent habituation to repeated stress under conditions of high orexin release.

Role of the Orexin/Hypocretin System in Stress-Related Psychiatric Disorders

Orexins (hypocretins) are critically involved in coordinating appropriate physiological and behavioral responses to aversive and threatening stimuli. Acute stressors engage orexin neurons via direct projections from stress-sensitive brain regions. Orexin neurons, in turn, facilitate adaptive behavior via reciprocal connections as well as via direct projections to the hypophysiotropic neurons that coordinate the hypothalamic-pituitary-adrenal (HPA) axis response to stress. Consequently, hyperactivity of the orexin system is associated with increased motivated arousal and anxiety, and is emerging as a key feature of panic disorder. Accordingly, there has been significant interest in the therapeutic potential of pharmacological agents that antagonize orexin signaling at their receptors for the treatment of anxiety disorders. In contrast, disorders characterized by inappropriately low levels of motivated arousal, such as depression, generally appear to be associated with hypoactivity of the orexin system. This includes narcolepsy with cataplexy, a disorder characterized by the progressive loss of orexin neurons and increased rates of moderate/severe depression symptomology. Here, we provide a comprehensive overview of both clinical and preclinical evidence highlighting the role of orexin signaling in stress reactivity, as well as how perturbations to this system can result in dysregulated behavioral phenotypes.

A Decade of Orexin/Hypocretin and Addiction: Where Are We Now?

One decade ago, our laboratory provided the first direct evidence linking orexin/hypocretin signaling with drug seeking by showing that activation of these neurons promotes conditioned morphine-seeking behavior. In the years since, contributions from many investigators have revealed roles for orexins in addiction for all drugs of abuse tested, but only under select circumstances. We recently proposed that orexins play a fundamentally unified role in coordinating "motivational activation" under numerous behavioral conditions, and here we unpack this hypothesis as it applies to drug addiction. We describe evidence collected over the past 10 years that elaborates the role of orexin in drug seeking under circumstances where high levels of effort are required to obtain the drug, or when motivation for drug reward is augmented by the presence of external stimuli like drug-associated cues/contexts or stressors. Evidence from studies using traditional self-administration and reinstatement models, as well as behavioral economic analyses of drug demand elasticity, clearly delineates a role for orexin in modulating motivational, rather than the primary reinforcing aspects of drug reward. We also discuss the anatomical interconnectedness of the orexin system with wider motivation and reward circuits, with a particular focus on how orexin modulates prefrontal and other glutamatergic inputs onto ventral tegmental area dopamine neurons. Last, we look ahead to the next decade of the research in this area, highlighting the recent FDA approval of the dual orexin receptor antagonist suvorexant (Belsomra®) for the treatment of insomnia as a promising sign of the potential clinical utility of orexin-based therapies for the treatment of addiction.

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.

Changes in Galanin Systems in a Rat Model of Post-Traumatic Stress Disorder (PTSD)

Post-traumatic stress disorder (PTSD) is a chronic syndrome triggered by exposure to trauma and a failure to recover from a normal negative emotional reaction to traumatic stress. The neurobiology of PTSD and the participation of neuropeptides in the neural systems and circuits that control fear and anxiety are not fully understood. The long-term dysregulation of neuropeptide systems contributes to the development of anxiety disorders, including PTSD. The neuropeptide galanin (Gal) and its receptors participate in anxiety-like and depression-related behaviors via the modulation of neuroendocrine and monoaminergic systems. The objective of this research was to investigate how Gal expression changes in the brain of rats 2 weeks after exposure to footshock. Rats exposed to footshocks were subdivided into high responders (HR; immobility>60%) and low responders (LR; immobility<40%) based on immobility elicited by a novel tone one day after exposure. On day 14, rats were anesthetized, and the amygdala, hypothalamus, pituitary and adrenal glands were removed for analysis using real-time polymerase chain reaction (RT-PCR). Gal mRNA levels were increased in the amygdala and hypothalamus of HR compared with the control and LR. In contrast, Gal mRNA levels were decreased in the adrenal and pituitary glands of HR compared with the control and LR. Thus, the differential regulation (dysregulation) of the neuropeptide Gal in these tissues may contribute to anxiety and PTSD development.

Orexin Neurons Respond Differentially to Auditory Cues Associated with Appetitive versus Aversive Outcomes

Orexin (Orx) neurons are known to be involved in the promotion and maintenance of waking because they discharge in association with cortical activation and muscle tone during waking and because, in their absence, waking with muscle tone cannot be maintained and narcolepsy with cataplexy ensues. Whether Orx neurons discharge during waking in association with particular conditions, notably with appetitive versus aversive stimuli or positive versus negative emotions, is debated and considered important in understanding their role in supporting particular waking behaviors. Here, we used the technique of juxtacellular recording and labeling in head-fixed rats to characterize the discharge of Orx neurons during the performance of an associative discrimination task with auditory cues for appetitive versus aversive outcomes. Of 57 active, recorded, and neurobiotin-labeled neurons in the lateral hypothalamus, 11 were immunohistochemically identified as Orx-positive (Orx(+)), whereas none were identified as melanin-concentrating hormone-positive. Orx(+) neurons discharged at significantly higher rates during the tone associated with sucrose than during the tone associated with quinine delivered upon licking. They also discharged at high rates after the tone associated with sucrose. Across periods and outcomes, their discharge was positively correlated with EEG gamma activity and EMG activity, which is indicative of cortical activation and behavioral arousal. These results suggest that Orx neurons discharge in a manner characteristic of reward neurons yet also characteristic of arousal neurons. Accordingly, the Orx neurons may respond to and participate in reward processes while modulating cortical activity and muscle tone to promote and maintain arousal along with learned adaptive behavioral responses.

SIGNIFICANCE STATEMENT

Orexin neurons play a critical role in promoting and maintaining a waking state because, in their absence, narcolepsy with cataplexy ensues. Known to discharge during waking and not during sleep, they have also been proposed to be selectively active during appetitive behaviors. Here, we recorded and labeled neurons in rats to determine the discharge of immunohistochemically identified orexin neurons during performance of an associative discrimination task. Orexin neurons responded differentially to auditory cues associated with appetitive sucrose versus aversive quinine, indicating that they behave like reward neurons. However, correlated discharge with cortical and muscle activity indicates that they also behave like arousal neurons and can thus promote cortical activation with behavioral arousal and muscle tone during adaptive waking behaviors.

An augmented CO2 chemoreflex and overactive orexin system are linked with hypertension in young and adult spontaneously hypertensive rats

KEY POINTS

Activation of central chemoreceptors by CO2 increases sympathetic nerve activity (SNA), arterial blood pressure (ABP) and breathing. These effects are exaggerated in spontaneously hypertensive rats (SHRs), resulting in an augmented CO2 chemoreflex that affects both breathing and ABP. The augmented CO2 chemoreflex and the high ABP are measureable in young SHRs (postnatal day 30-58) and become greater in adult SHRs. Blockade of orexin receptors can normalize the augmented CO2 chemoreflex and the high ABP in young SHRs and normalize the augmented CO2 chemoreflex and significantly lower the high ABP in adult SHRs. In the hypothalamus, SHRs have more orexin neurons, and a greater proportion of them increase their activity with CO2 . The orexin system is overactive in SHRs and contributes to the augmented CO2 chemoreflex and hypertension. Modulation of the orexin system may be beneficial in the treatment of neurogenic hypertension.

ABSTRACT

Activation of central chemoreceptors by CO2 increases arterial blood pressure (ABP), sympathetic nerve activity and breathing. In spontaneously hypertensive rats (SHRs), high ABP is associated with enhanced sympathetic nerve activity and peripheral chemoreflexes. We hypothesized that an augmented CO2 chemoreflex and overactive orexin system are linked with high ABP in both young (postnatal day 30-58) and adult SHRs (4-6 months). Our main findings are as follows. (i) An augmented CO2 chemoreflex and higher ABP in SHRs are measureable at a young age and increase in adulthood. In wakefulness, the ventilatory response to normoxic hypercapnia is higher in young SHRs (mean ± SEM: 179 ± 11% increase) than in age-matched normotensive Wistar-Kyoto rats (114 ± 9% increase), but lower than in adult SHRs (226 ± 10% increase; P < 0.05). The resting ABP is higher in young SHRs (122 ± 5 mmHg) than in age-matched Wistar-Kyoto rats (99 ± 5 mmHg), but lower than in adult SHRs (152 ± 4 mmHg; P < 0.05). (ii) Spontaneously hypertensive rats have more orexin neurons and more CO2 -activated orexin neurons in the hypothalamus. (iii) Antagonism of orexin receptors with a dual orexin receptor antagonist, almorexant, normalizes the augmented CO2 chemoreflex in young and adult SHRs and the high ABP in young SHRs and significantly lowers ABP in adult SHRs. (iv) Attenuation of peripheral chemoreflexes by hyperoxia does not abolish the augmented CO2 chemoreflex (breathing and ABP) in SHRs, which indicates an important role for the central chemoreflex. We suggest that an overactive orexin system may play an important role in the augmented central CO2 chemoreflex and in the development of hypertension in SHRs.

Effects of footshocks on anxiety-like behavior and mRNA levels of precursor peptides for corticotropin releasing factor and opioids in the forebrain of the rat

Corticotropin releasing factor (CRF) and dynorphin are neuropeptides that are associated with the negative emotional states. Experimental evidence indicates that dynorphin neurons located in the nucleus accumbens and CRF neurons in the bed nucleus of the stria terminalis (BST) and the central nucleus of the amygdala (CeA) mediate anxiety-like behaviors immediately after the stressful experience (24-48h). The present study was done to evaluate if changes in the levels of the mRNA for these peptides in the striatum, BST, and CeA were associated with the long-lasting avoidance of novelty, a measure of an anxiety-like state, in a subset of rats exposed to unpredictable and moderately intense footshocks (5×2s of 1.5mA). Shocked rats with enhanced fear to a novel tone 24h after the footshocks (high responders; HR) displayed long-lasting avoidance in the elevated T-maze whereas shocked rats with low levels of acute fear (low responders; LR) had low levels of avoidance similar to nonshocked rats. An increase in the level of proCRF mRNA was detected in the CeA of the HR compared to LR and nonshocked rats but not in other areas of the brain sampled. In contrast, prodynorphin and proenkephalin mRNA levels in the striatum, BST and CeA were not different between HR, LR and nonshocked rats. This study provides evidence that CRF neurons in the CeA may play a role in the anxiety-like state produced in a subset of rats exposed to footshocks.

Blocking of orexin receptors in the paraventricular nucleus of the thalamus has no effect on the expression of conditioned fear in rats

The paraventricular nucleus of the thalamus (PVT) projects to the central nucleus of the amygdala and recent experimental evidence indicates a role for the PVT in conditioned fear. Furthermore, the PVT contains a high density of orexin receptors and fibers and acute injections of orexin antagonist into the PVT produce anxiolytic effects. The present study was done to determine if administration of a dual orexin receptor antagonist (DORA) in the region of the PVT interferes with the expression of conditioned fear in rats exposed to cued and contextual conditioning paradigms. Infusion of 0.5 μl of the DORA N-biphenyl-2-yl-1-[(1-methyl-1H-benzimidazol-2yl) sulfanyl] acetyl-L-prolinamide at a concentration of 0.1, 1.0, and 10 nmol had no effect on the freezing produced by exposing rats to an auditory cue or the context associated with foot shock. In contrast, the 1.0 and 10 nmol doses were anxiolytic in the social interaction test. The results of the present study do not support a role for orexin receptors in the PVT in the expression of learned fear. The finding that the 1.0 and 10 nmol doses of DORA in the PVT region were anxiolytic in the social interaction test is consistent with other studies indicating a role for orexins in the PVT in anxiety-like behaviors.

Orexin neuropeptides contribute to the development and persistence of generalized avoidance behavior in the rat

RATIONALE

Avoidance of contexts directly associated with fearful experiences represents an adaptive behavioral survival strategy. Over-interpretation of contextual cues leading to generalized avoidance of situations that are only remotely similar to the original fear context represents a pathologic process that contributes to anxiety disorders. Orexin neuropeptides modulate anxiety-like behavioral and physiological responses.

OBJECTIVE

The objective of this paper was to investigate the impact of pharmacological orexin receptor blockade on generalized avoidance behavior.

METHODS

Rats received a single electric foot-shock in the dark side of a two-compartment shuttle box followed by situational context reminders. After shock, rats were treated chronically (3 weeks) with the orexin receptor antagonist almorexant or with the selective serotonin reuptake inhibitor sertraline, used as positive anxiolytic control. In week 3, avoidance behavior was measured under conditions of high (dark-light (DL)-box) and low (elevated plus maze (EPM)) similarity to the original shock context. Avoidance behavior was re-assessed 5 and 17 weeks after treatment termination.

RESULTS

Avoidance in the DL box (contextual fear memory) remained unaffected by any treatment and lasted 20 weeks post-shock exposure. Avoidance in the EPM (neophobic fear generalization) was partially attenuated during treatment with almorexant and sertraline at week 3. Following 5 and 17 weeks of drug washout, avoidance in the EPM was significantly reduced in almorexant- but not in sertraline-treated rats. Almorexant also reduced persistent avoidance in the EPM upon treatment initiation 3 weeks after shock exposure.

CONCLUSION

Chronic orexin receptor blockade in rats reduces both the development and persistence of generalized avoidance in situations with low similarity to the initial shock context.

Spontaneously hypertensive rats have more orexin neurons in their medial hypothalamus than normotensive rats

NEW FINDINGS

What is the central question of this study? Blockade of orexin receptors reduces blood pressure in spontaneously hypertensive rats (SHRs) but not in normotensive Wistar-Kyoto (WKY) rats, suggesting that upregulation of orexin signalling underlies the hypertensive phenotype of the SHR. However, it is not known what causes this upregulation. What is the main finding and its importance? Using orexin immunolabelling, we show that SHRs have 20% more orexin neurons than normotensive WKY and Wistar rats in the medial hypothalamus, which is a good match to their blood pressure phenotype. In contrast, there is no such match for the orexin neurons of the lateral hypothalamus. Essential hypertension may be linked to an increase in orexin neurons in the medial hypothalamus. The neuropeptide orexin contributes to the regulation of blood pressure as part of its role in the control of arousal during wakefulness and motivated behaviour (including responses to psychological stress). Recent work shows that pharmacological blockade of orexin receptors reduces blood pressure in spontaneously hypertensive rats (SHRs) but not in normotensive Wistar-Kyoto (WKY) rats. It is not clear why orexin signalling is upregulated in the SHR, but one possibility is that these animals have more orexin neurons than their normotensive WKY and Wistar relatives. To test this possibility, SHRs, WKY and Wistar male rats (6-16 weeks old) were killed, perfused and their brains sectioned and immunolabelled for orexin A. Labelled neurons were plotted and counted in the six best labelled hemisections (120 μm apart) of each brain. There were significantly more orexin neurons (+20%) in the medial hypothalamus (medial to fornix) of SHRs compared with WKY and Wistar rats (126 ± 4 versus 106 ± 5 and 104 ± 5 per hemisection, respectively, P < 0.05), which matches well the blood pressure phenotypes. In contrast, counts in the lateral hypothalamus did not match the blood pressure phenotypes (69 ± 2 versus 50 ± 3 and 76 ± 4, respectively). The results support the idea that orexin signalling is upregulated in the SHR and suggest that this is due, at least in part, to a greater number of orexin neurons in the medial hypothalamus. These medial orexin neurons, which are also involved in hyperarousal and stress responses, may contribute to the development of essential hypertension.

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

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

LINKED ARTICLES

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

Orexin antagonists for neuropsychiatric disease: progress and potential pitfalls

The tight regulation of sleep/wake states is critical for mental and physiological wellbeing. For example, dysregulation of sleep/wake systems predisposes individuals to metabolic disorders such as obesity and psychiatric problems, including depression. Contributing to this understanding, the last decade has seen significant advances in our appreciation of the complex interactions between brain systems that control the transition between sleep and wake states. Pivotal to our increased understanding of this pathway was the description of a group of neurons in the lateral hypothalamus (LH) that express the neuropeptides orexin A and B (hypocretin, Hcrt-1 and Hcrt-2). Orexin neurons were quickly placed at center stage with the demonstration that loss of normal orexin function is associated with the development of narcolepsy—a condition in which sufferers fail to maintain normal levels of daytime wakefulness. Since these initial seminal findings, much progress has been made in our understanding of the physiology and function of the orexin system. For example, the orexin system has been identified as a key modulator of autonomic and neuroendocrine function, arousal, reward and attention. Notably, studies in animals suggest that dysregulation of orexin function is associated with neuropsychiatric states such as addiction and mood disorders including depression and anxiety. This review discusses the progress associated with therapeutic attempts to restore orexin system function and treat neuropsychiatric conditions such as addiction, depression and anxiety. We also highlight potential pitfalls and challenges associated with targeting this system to treat these neuropsychiatric states.

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.