Hypocretins and Arousal

Alterations of pleiotropic neuropeptide-receptor gene couples in Cetacea

BACKGROUND

Habitat transitions have considerable consequences in organism homeostasis, as they require the adjustment of several concurrent physiological compartments to maintain stability and adapt to a changing environment. Within the range of molecules with a crucial role in the regulation of different physiological processes, neuropeptides are key agents. Here, we examined the coding status of several neuropeptides and their receptors with pleiotropic activity in Cetacea.

RESULTS

Analysis of 202 mammalian genomes, including 41 species of Cetacea, exposed an intricate mutational landscape compatible with gene sequence modification and loss. Specifically for Cetacea, in the 12 genes analysed we have determined patterns of loss ranging from species-specific disruptive mutations (e.g. neuropeptide FF-amide peptide precursor; NPFF) to complete erosion of the gene across the cetacean stem lineage (e.g. somatostatin receptor 4; SSTR4).

CONCLUSIONS

Impairment of some of these neuromodulators may have contributed to the unique energetic metabolism, circadian rhythmicity and diving response displayed by this group of iconic mammals.

A cluster of neuropeptide S neurons regulates breathing and arousal

Neuropeptide S (NPS) is a highly conserved peptide found in all tetrapods that functions in the brain to promote heightened arousal; however, the subpopulations mediating these phenomena remain unknown. We generated mice expressing Cre recombinase from the Nps gene locus (NpsCre) and examined populations of NPS+ neurons in the lateral parabrachial area (LPBA), the peri-locus coeruleus (peri-LC) region of the pons, and the dorsomedial thalamus (DMT). We performed brain-wide mapping of input and output regions of NPS+ clusters and characterized expression patterns of the NPS receptor 1 (NPSR1). While the activity of all three NPS+ subpopulations tracked with vigilance state, only NPS+ neurons of the LPBA exhibited both increased activity prior to wakefulness and decreased activity during REM sleep, similar to the behavioral phenotype observed upon NPSR1 activation. Accordingly, we found that activation of the LPBA but not the peri-LC NPS+ neurons increased wake and reduced REM sleep. Furthermore, given the extended role of the LPBA in respiration and the link between behavioral arousal and breathing rate, we demonstrated that the LPBA but not the peri-LC NPS+ neuronal activation increased respiratory rate. Together, our data suggest that NPS+ neurons of the LPBA represent an unexplored subpopulation regulating breathing, and they are sufficient to recapitulate the sleep/wake phenotypes observed with broad NPS system activation.

Azulene as a biphenyl mimetic in orexin/hypocretin receptor agonists

Azulene is a rare ring structure in drugs, and we investigated whether it could be used as a biphenyl mimetic in known orexin receptor agonist Nag 26, which is binding to both orexin receptors OX₁ and OX₂ with preference towards OX₂. The most potent azulene-based compound was identified as an OX₁ orexin receptor agonist (pEC₅₀ = 5.79 ± 0.07, maximum response = 81 ± 8% (s.e.m. of five independent experiments) of the maximum response to orexin-A in Ca²⁺ elevation assay). However, the azulene ring and the biphenyl scaffold are not identical in their spatial shape and electron distribution, and their derivatives may adopt different binding modes in the binding site.

Narcolepsy: Mending a broken neural circuit that controls arousal

Narcolepsy is a sleep disorder caused by insufficient levels of orexins, with current treatment options addressing symptoms rather than etiology. New research reveals that transplantation of orexin cells in a mouse model of narcolepsy reduces cataplexy.

Amygdala neurocircuitry at the interface between emotional regulation and narcolepsy with cataplexy

Narcolepsy is a sleep disorder characterized by chronic and excessive daytime sleepiness, and sudden intrusion of sleep during wakefulness that can fall into two categories: type 1 and type 2. Type 1 narcolepsy in humans is widely believed to be caused as a result of loss of neurons in the brain that contain the key arousal neuropeptide Orexin (Orx; also known as Hypocretin). Patients with type 1 narcolepsy often also present with cataplexy, the sudden paralysis of voluntary muscles which is triggered by strong emotions (e.g., laughter in humans, social play in dogs, and chocolate in rodents). The amygdala is a crucial emotion-processing center of the brain; however, little is known about the role of the amygdala in sleep/wake and narcolepsy with cataplexy. A collection of reports across human functional neuroimaging analyses and rodent behavioral paradigms points toward the amygdala as a critical node linking emotional regulation to cataplexy. Here, we review the existing evidence suggesting a functional role for the amygdala network in narcolepsy, and build upon a framework that describes relevant contributions from the central nucleus of the amygdala (CeA), basolateral amygdala (BLA), and the extended amygdala, including the bed nucleus of stria terminalis (BNST). We propose that detailed examinations of amygdala neurocircuitry controlling transitions between emotional arousal states may substantially advance progress in understanding the etiology of narcolepsy with cataplexy, leading to enhanced treatment opportunities.

Dysregulation of striatal dopamine D2/D3 receptor-mediated by hypocretin induces depressive behaviors in rats

BACKGROUND

The dysregulation of the dopamine system contributes to depressive-like behaviors in rats, and the neurological functions regulated by hypocretin are severely affected in depression. However, whether suvorexant plays a role in alleviating depression by affecting the dopamine system is unclear.

METHODS

To preliminarily explore the mechanism of suvorexant (10 mg/kg) in the treatment of depression, the mRNA and protein expression of TH, Drd2, Drd3, GluN2A, DAT, and GluN2B in the striatum of rats was quantified by qPCR and western blotting. The plasma hypocretin-1 and dopamine levels and the striatal dopamine levels were determined by ELISA.

RESULTS

i) Compared to those of the control group, chronic unpredictable mild stress (CUMS) rats showed depressive-like behaviors, which were subsequently reversed by treatment with suvorexant. ii) The mRNA and protein expressions of TH, Drd2, Drd3, GluN2A, and GluN2B in the striatum of CUMS were significantly increased compared with those in the controls, but decreased after suvorexant treatment. iii) Compared with those in the control group, the plasma and striatal dopamine levels of CUMS decreased while plasma hypocretin-1 levels increased, which was reversed after suvorexant treatment.

LIMITATIONS

i) The suvorexant is a dual hypocretin receptor antagonist; however, the responsible receptor is unclear. ii) We only focused on related factors in the striatum but did not explore other brain regions, nor did we directly explore the relationship among these factors.

CONCLUSION

Depressive-like behaviors induced by CUMS can be reversed by suvorexant, and the therapeutic effects of suvorexant may be mediated by affecting the dopamine system.

Optogenetic and pharmacological interventions link hypocretin neurons to impulsivity in mice

Neurons in the lateral hypothalamus expressing the neuropeptide Hypocretin, also known as orexin, are known critical modulators of arousal stability. However, their role in the different components of the arousal construct such as attention and decision making is poorly understood. Here we study Hypocretin neuronal circuit dynamics during stop action impulsivity in a Go/NoGo task in mice. We show that Hypocretin neuronal activity correlates with anticipation of reward. We then assessed the causal role of Hypocretin neuronal activity using optogenetics in a Go/NoGo task. We show that stimulation of Hypocretin neurons during the cue period dramatically increases the number of premature responses. These effects are mimicked by amphetamine, reduced by atomoxetine, a norepinephrine uptake inhibitor, and blocked by a Hypocretin receptor 1 selective antagonist. We conclude that Hypocretin neurons have a key role in the integration of salient stimuli during wakefulness to produce appropriate and timely responses to rewarding and aversive cues.

Adolescent sleep shapes social novelty preference in mice

Sleep disturbances frequently occur in neurodevelopmental disorders such as autism, but the developmental role of sleep is largely unexplored, and a causal relationship between developmental sleep defects and behavioral consequences in adulthood remains elusive. Here, we show that in mice, sleep disruption (SD) in adolescence, but not in adulthood, causes long-lasting impairment in social novelty preference. Furthermore, adolescent SD alters the activation and release patterns of dopaminergic neurons in the ventral tegmental area (VTA) in response to social novelty. This developmental sleep function is mediated by balanced VTA activity during adolescence; chemogenetic excitation mimics, whereas silencing rescues, the social deficits of adolescent SD. Finally, we show that in Shank3-mutant mice, improving sleep or rectifying VTA activity during adolescence ameliorates adult social deficits. Together, our results identify a critical role of sleep and dopaminergic activity in the development of social interaction behavior.

Extended Amygdala Neuropeptide Circuitry of Emotional Arousal: Waking Up on the Wrong Side of the Bed Nuclei of Stria Terminalis

Sleep is fundamental to life, and poor sleep quality is linked to the suboptimal function of the neural circuits that process and respond to emotional stimuli. Wakefulness ("arousal") is chiefly regulated by circadian and homeostatic forces, but affective mood states also strongly impact the balance between sleep and wake. Considering the bidirectional relationships between sleep/wake changes and emotional dynamics, we use the term "emotional arousal" as a representative characteristic of the profound overlap between brain pathways that: (1) modulate wakefulness; (2) interpret emotional information; and (3) calibrate motivated behaviors. Interestingly, many emotional arousal circuits communicate using specialized signaling molecules called neuropeptides to broadly modify neural network activities. One major neuropeptide-enriched brain region that is critical for emotional processing and has been recently implicated in sleep regulation is the bed nuclei of stria terminalis (BNST), a core component of the extended amygdala (an anatomical term that also includes the central and medial amygdalae, nucleus accumbens shell, and transition zones betwixt). The BNST encompasses an astonishing diversity of cell types that differ across many features including spatial organization, molecular signature, biological sex and hormonal milieu, synaptic input, axonal output, neurophysiological communication mode, and functional role. Given this tremendous complexity, comprehensive elucidation of the BNST neuropeptide circuit mechanisms underlying emotional arousal presents an ambitious set of challenges. In this review, we describe how rigorous investigation of these unresolved questions may reveal key insights to enhancing psychiatric treatments and global psychological wellbeing.

Hypocretin (Orexin) Replacement Therapies

Twenty-two years after their discovery, the hypocretins (Hcrts), also known as orexins, are two of the most studied peptidergic systems, involved in myriad physiological systems that range from sleep, arousal, motivation, homeostatic regulation, fear, anxiety and learning. A causal relationship between activity of Hcrt and arousal stability was established shortly after their discovery and have led to the development of a new class of drugs to treat insomnia. In this review we discuss the many faces of the Hcrt system and examine recent findings that implicate decreased Hcrt function in the pathogenesis of a number of neuropsychiatric conditions. We also discuss future therapeutic strategies to replace or enhance Hcrt function as a treatment option for these neuropsychiatric conditions.

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.

A common neuronal mechanism of hypertension and sleep disturbances in spontaneously hypertensive rats: Role of orexinergic neurons

Epidemiologic studies have shown that sleep disorders are associated with the development of hypertension. The present study investigated dynamic changes in sleep patterns during the development of hypertension across the lifespan in spontaneously hypertensive rats (SHRs) and the neural mechanism that underlies these comorbidities, with a focus on the orexinergic system. Blood pressure in rats was measured using a noninvasive blood pressure tail cuff. Sleep was monitored by electroencephalographic and electromyographic recordings. Immunohistochemistry was used to detect the density and activity of orexinergic neurons in the perifornical nucleus. Hcrt2-SAP (400 or 800 ng) was microinjected in the lateral hypothalamus to lesion orexinergic neurons. Compared with Wistar-Kyoto rats, SHRs exhibited various patterns of sleep disturbances. In SHRs, dynamic changes in hypersomnia in the rats' active phase was not synchronized with the development of hypertension, but hyperarousal in the inactive phase and difficulties in falling asleep were observed concurrently with the development of hypertension. Furthermore, the density and activity of orexinergic neurons in the perifornical nucleus were significantly higher in SHRs than in age-matched Wistar-Kyoto rats. The reduction of orexinergic neurons in the lateral hypothalamus partially ameliorated the development of hypertension and prevented difficulties in falling asleep in SHRs. These results indicate that although the correlation between sleep disturbances and hypertension is very complex, common mechanisms may underlie these comorbidities in SHRs. Overactivity of the orexin system may be one such common mechanism.

Orexin-1 Receptor Signaling in Ventral Pallidum Regulates Motivation for the Opioid Remifentanil

Signaling at the orexin-1 receptor (OxR1) is important for motivated drug taking. Using a within-session behavioral economics (BE) procedure, we previously found that pharmacologic blockade of the OxR1 decreased motivation (increased demand elasticity) for the potent and short-acting opioid remifentanil and reduced low-effort remifentanil consumption. However, the mechanism through which orexin regulates remifentanil demand is currently unknown. Previous work implicated OxR1 signaling within ventral pallidum (VP) as a potential target. VP is densely innervated by orexin fibers and is known to regulate opioid reward. Accordingly, this study sought to determine the role of VP OxR1 signaling in remifentanil demand and cue-induced reinstatement of remifentanil seeking in male rats. Intra-VP microinjections of the OxR1 antagonist SB-334867 (SB) decreased motivation (increased demand elasticity; α) for remifentanil without affecting remifentanil consumption at low effort. Baseline α values predicted the degree of cue-induced remifentanil seeking, and microinjection of SB into VP attenuated this behavior without affecting extinction responding. Baseline α values also predicted SB efficacy, such that SB was most effective in attenuating reinstatement behavior in highly motivated rats. Together, these findings support a selective role for VP OxR1 signaling in motivation for the opioid remifentanil. Our findings also highlight the utility of BE in predicting relapse propensity and efficacy of treatment with OxR1 antagonists.SIGNIFICANCE STATEMENT Abuse of opioids has risen rapidly and continues to be a major health crisis. Thus, there is an urgent need to better understand the neurobiological and behavioral mechanisms underlying opioid addiction. Here, we investigate the role of orexin-1 receptor signaling (OxR1) within ventral pallidum (VP) in remifentanil demand and cue-induced reinstatement of remifentanil seeking. Using a within-session behavioral economics procedure, we show that intra-VP microinjections of the OxR1 antagonist SB-334867 decreased motivation (increased demand elasticity) without affecting remifentanil consumption at low effort. We also found that SB microinjected intra-VP attenuated cue-induced reinstatement of remifentanil seeking. Together, our results support a role for VP OxR1 signaling in opioid reward.

Transgenic Archaerhodopsin-3 Expression in Hypocretin/Orexin Neurons Engenders Cellular Dysfunction and Features of Type 2 Narcolepsy

Narcolepsy, characterized by excessive daytime sleepiness, is associated with dysfunction of the hypothalamic hypocretin/orexin (Hcrt) system, either due to extensive loss of Hcrt cells (Type 1, NT1) or hypothesized Hcrt signaling impairment (Type 2, NT2). Accordingly, efforts to recapitulate narcolepsy-like symptoms in mice have involved ablating these cells or interrupting Hcrt signaling. Here, we describe orexin/Arch mice, in which a modified archaerhodopsin-3 gene was inserted downstream of the prepro-orexin promoter, resulting in expression of the yellow light-sensitive Arch-3 proton pump specifically within Hcrt neurons. Histological examination along with ex vivo and in vivo electrophysiological recordings of male and female orexin/Arch mice demonstrated silencing of Hcrt neurons when these cells were photoilluminated. However, high expression of the Arch transgene affected cellular and physiological parameters independent of photoillumination. The excitability of Hcrt neurons was reduced, and both circadian and metabolic parameters were perturbed in a subset of orexin/Arch mice that exhibited high levels of Arch expression. Orexin/Arch mice also had increased REM sleep under baseline conditions but did not exhibit cataplexy, a sudden loss of muscle tone during wakefulness characteristic of NT1. These aberrations resembled some aspects of mouse models with Hcrt neuron ablation, yet the number of Hcrt neurons in orexin/Arch mice was not reduced. Thus, orexin/Arch mice may be useful to investigate Hcrt system dysfunction when these neurons are intact, as is thought to occur in narcolepsy without cataplexy (NT2). These results also demonstrate the utility of extended phenotypic screening of transgenic models when specific neural circuits have been manipulated.SIGNIFICANCE STATEMENT Optogenetics has become an invaluable tool for functional dissection of neural circuitry. While opsin expression is often achieved by viral injection, stably integrated transgenes offer some practical advantages. Here, we demonstrate successful transgenic expression of an inhibitory opsin in hypocretin/orexin neurons, which are thought to promote or maintain wakefulness. Both brief and prolonged illumination resulted in inhibition of these neurons and induced sleep. However, even in the absence of illumination, these cells exhibited altered electrical characteristics, particularly when transgene expression was high. These aberrant properties affected metabolism and sleep, resulting in a phenotype reminiscent of the narcolepsy Type 2, a sleep disorder for which no good animal model currently exists.

Sleep: Short Sleepers Should Keep Count of Their Hypocretin Neurons

Sleep durations vary greatly across animals from 2 to 20 hours with no clear explanation. A small Mexican cavefish reveals how the brain can adapt to increase its wake-stabilizing hypocretin circuit and dramatically reduce sleep, likely to allow adaptive foraging.

Optical probing of orexin/hypocretin receptor antagonists

Study Objectives

The present study investigated the function of Hypocretin (Hcrt or Orexin/OX) receptor antagonists in sleep modulation and memory function with optical methods in transgenic mice.

Methods

We used Hcrt-IRES-Cre knock-in mice and AAV vectors expressing channelrhodopsin-2 (ChR2) to render Hcrt neurons sensitive to blue light stimulation. We optogenetically stimulated Hcrt neurons and measured latencies to wakefulness in the presence or absence of OX1/2R antagonists and Zolpidem. We also examined endogenous Hcrt neuronal activity with fiber photometry. Changes in memory after optogenetic sleep disruption were evaluated by the novel object recognition test (NOR) and compared for groups treated with vehicle, OX1/2R antagonists, or Zolpidem. We also analyzed electroencephalogram (EEG) power spectra of wakefulness, rapid eye movement (REM) sleep, and non-REM (NREM) sleep following the injections of vehicle, OX1/2R antagonists, and Zolpidem in young adult mice.

Results

Acute optogenetic stimulation of Hcrt neurons at different frequencies resulted in wakefulness. Treatment with dual OX1/2R antagonists (DORAs) DORA12 and MK6096, as well as selective OX2R antagonist MK1064 and Zolpidem, but not selective OX1R antagonist 1SORA1, significantly reduced the bout length of optogenetic stimulation-evoked wakefulness episode. Fiber photometry recordings of GCaMP6f signals showed that Hcrt neurons are active during wakefulness, even in the presence of OXR antagonists. Treatment with dual OX1/2R antagonists improved memory function despite optogenetic sleep fragmentation caused impaired memory function in a NOR test.

Conclusions

Our results show DORAs and selective OX2R antagonists stabilize sleep and improve sleep-dependent cognitive processes even when challenged by optogenetic stimulation mimicking highly arousing stimuli.

Parallel circuits from the bed nuclei of stria terminalis to the lateral hypothalamus drive opposing emotional states

Lateral hypothalamus (LH) neurons containing the neuropeptide hypocretin (HCRT; orexin) modulate affective components of arousal, but their relevant synaptic inputs remain poorly defined. Here we identified inputs onto LH neurons that originate from neuronal populations in the bed nuclei of stria terminalis (BNST; a heterogeneous region of extended amygdala). We characterized two non-overlapping LH-projecting GABAergic BNST subpopulations that express distinct neuropeptides (corticotropin-releasing factor, CRF, and cholecystokinin, CCK). To functionally interrogate BNST→LH circuitry, we used tools for monitoring and manipulating neural activity with cell-type-specific resolution in freely behaving mice. We found that Crf-BNST and Cck-BNST neurons respectively provide abundant and sparse inputs onto Hcrt-LH neurons, display discrete physiological responses to salient stimuli, drive opposite emotionally valenced behaviors, and receive different proportions of inputs from upstream networks. Together, our data provide an advanced model for how parallel BNST→LH pathways promote divergent emotional states via connectivity patterns of genetically defined, circuit-specific neuronal subpopulations.

Dynorphin Counteracts Orexin in the Paraventricular Nucleus of the Thalamus: Cellular and Behavioral Evidence

The orexin (Orx) system plays a critical role in drug addiction and reward-related behaviors. The dynorphin (Dyn) system promotes depressive-like behavior and plays a key role in the aversive effects of stress. Orx and Dyn are co-released and have opposing functions in reward and motivation in the ventral tegmental area (VTA). Previous studies suggested that OrxA transmission in the posterior paraventricular nucleus of the thalamus (pPVT) participates in cocaine-seeking behavior. This study determined whether Orx and Dyn interact in the pPVT. Using the brain slice preparation for cellular recordings, superfusion of DynA onto pPVT neurons decreased the frequency of spontaneous and miniature excitatory postsynaptic currents (s/mEPSCs). OrxA increased the frequency of sEPSCs but had no effect on mEPSCs, suggesting a network-driven effect of OrxA. The amplitudes of s/mEPSCs were unaffected by the peptides, indicating a presynaptic action on glutamate release. Augmentation of OrxA-induced glutamate release was reversed by DynA. Utilizing a behavioral approach, separate groups of male Wistar rats were trained to self-administer cocaine or sweetened condensed milk (SCM). After extinction, rats received intra-pPVT administration of OrxA±DynA±the κ-opioid receptor antagonist nor-binaltorphimine (NorBNI) under extinction conditions. OrxA reinstated cocaine- and SCM-seeking behavior, with a greater effect in cocaine animals. DynA blocked OrxA-induced cocaine seeking but not SCM seeking. NorBNI did not induce or potentiate cocaine-seeking behavior induced by OrxA but reversed DynA effect. This indicates that the κ-opioid system in the pPVT counteracts OrxA-induced cocaine seeking, suggesting a novel therapeutic target to prevent cocaine relapse.

The Subthalamic Neurons are Activated by Both Orexin-A and Orexin-B

The subthalamic nucleus is an important nucleus in the indirect pathway of the basal ganglia circuit and therefore is involved in motor control under both normal and pathological conditions. Morphological studies reveal that the subthalamic nucleus receives relatively dense orexinergic projections originating from the hypothalamus. Both orexin-1 (OX₁) and orexin-2 (OX₂) receptors are expressed in the subthalamic nucleus. To explore the functions of orexinergic system in the subthalamic nucleus, extracellular electrophysiological recordings and behavioral tests were performed in the present study. Exogenous application of orexin-A significantly increased the spontaneous firing rate from 5.70 ± 0.66 Hz to 9.87 ± 1.18 Hz in 64.00% subthalamic neurons recorded. OX₁ receptors are involved in orexin-A-induced excitation. Application of orexin-B increased the firing rate from 7.47 ± 0.92 Hz to 11.85 ± 1.39 Hz in 80.95% subthalamic neurons recorded, entirely through OX₂ receptors. Both OX₁ and OX₂ receptor antagonists decreased the firing rate in 43.75% and 62.50% subthalamic neurons recorded respectively, suggesting the involvement of endogenous orexinergic system in the control of spontaneous firing activity. Further elevated body swing test revealed that microinjection of orexins and the receptor antagonists into the subthalamic nucleus induced contralateral-biased swing and ipsilateral-biased swing, respectively. Taken together, the present study suggests that orexins play important roles in the subthalamic nucleus which may provide further evidence for the involvement of subthalamic orexinergic tone in Parkinson's disease.

SIGNIFICANCE

Previous morphological studies indicate that the subthalamic nucleus receives orexinergic innervation and expresses both OX₁ and OX₂ receptors. Using in vivo multibarrel electrophysiological recordings, the present study revealed that exogenous application of orexin-A and orexin-B increased the spontaneous firing rate of the subthalamic neurons through OX₁ and OX₂ receptors. Endogenous orexinergic system was involved in the control of spontaneous firing of the subthalamic neurons. Further behavioral test revealed that intrasubthalamic application of orexins and the receptor antagonists induced biased swing behavior. The present study may provide further evidence for the involvement of subthalamic orexinergic tone in Parkinson's disease.