Postsynaptic excitation of prefrontal cortical pyramidal neurons by hypocretin-1/orexin A through the inhibition of potassium currents

The Contribution of HCN Channelopathies in Different Epileptic Syndromes, Mechanisms, Modulators, and Potential Treatment Targets: A Systematic Review

Background

Hyperpolarization-activated cyclic nucleotide-gated (HCN) current reduces dendritic summation, suppresses dendritic calcium spikes, and enables inhibitory GABA-mediated postsynaptic potentials, thereby suppressing epilepsy. However, it is unclear whether increased HCN current can produce epilepsy. We hypothesized that gain-of-function (GOF) and loss-of-function (LOF) variants of HCN channel genes may cause epilepsy.

Objectives

This systematic review aims to summarize the role of HCN channelopathies in epilepsy, update genetic findings in patients, create genotype–phenotype correlations, and discuss animal models, GOF and LOF mechanisms, and potential treatment targets.

Methods

The review was conducted in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses statement, for all years until August 2021.

Results

We identified pathogenic variants of HCN1 (n = 24), HCN2 (n = 8), HCN3 (n = 2), and HCN4 (n = 6) that were associated with epilepsy in 74 cases (43 HCN1, 20 HCN2, 2 HCN3, and 9 HCN4). Epilepsy was associated with GOF and LOF variants, and the mechanisms were indeterminate. Less than half of the cases became seizure-free and some developed drug-resistant epilepsy. Of the 74 cases, 12 (16.2%) died, comprising HCN1 (n = 4), HCN2 (n = 2), HCN3 (n = 2), and HCN4 (n = 4). Of the deceased cases, 10 (83%) had a sudden unexpected death in epilepsy (SUDEP) and 2 (16.7%) due to cardiopulmonary failure. SUDEP affected more adults (n = 10) than children (n = 2). HCN1 variants p.M234R, p.C329S, p.V414M, p.M153I, and p.M305L, as well as HCN2 variants p.S632W and delPPP (p.719–721), were associated with different phenotypes. HCN1 p.L157V and HCN4 p.R550C were associated with genetic generalized epilepsy. There are several HCN animal models, pharmacological targets, and modulators, but precise drugs have not been developed. Currently, there are no HCN channel openers.

Conclusion

We recommend clinicians to include HCN genes in epilepsy gene panels. Researchers should explore the possible underlying mechanisms for GOF and LOF variants by identifying the specific neuronal subtypes and neuroanatomical locations of each identified pathogenic variant. Researchers should identify specific HCN channel openers and blockers with high binding affinity. Such information will give clarity to the involvement of HCN channelopathies in epilepsy and provide the opportunity to develop targeted treatments.

Early Sociability and Social Memory Impairment in the A53T Mouse Model of Parkinson's Disease Are Ameliorated by Chemogenetic Modulation of Orexin Neuron Activity

Parkinson's disease (PD) is a multi-layered progressive neurodegenerative disease. Signature motor system impairments are accompanied by a variety of other symptoms such as mood, sleep, metabolic, and cognitive disorders. Interestingly, social cognition impairments can be observed from the earliest stages of the disease, prior to the onset of the motor symptoms. In this study, we investigated age-related reductions in sociability and social memory in the A53T mouse model of PD. Since inflammation and astrogliosis are an integral part of PD pathology and impair proper neuronal function, we examined astrogliosis and inflammation markers and parvalbumin expression in medial pre-frontal cortex (mPFC), part of the brain responsible for social cognition regulation. Finally, we used DREADDs (Designer Receptors Exclusively Activated by Designer Drugs) for the stimulation and inhibition of orexin neuronal activity to modulate sociability and social memory in A53T mice. We observed that social cognition impairment in A53T mice is accompanied by an increase in astrogliosis and inflammation markers, in addition to loss of parvalbumin neurons and inhibitory pre-synaptic terminals in the mPFC. Moreover, DREADD-induced activation of orexin neurons restores social cognition in the A53T mouse model of PD. SIGNIFICANCE STATEMENT: Social cognition is severely affected in the early stages of Parkinson's disease. In this study, we identified the A53T mouse as a model of social cognitive impairment in PD. Observed alterations in sociability and social memory are accompanied by loss of parvalbumin positive neurons and loss of inhibitory input to mPFC. Stimulating orexin neurons using a chemogenetic approach (DREADDs) ameliorated social cognitive impairment. This study identifies a role for orexin neurons in social cognition in PD and suggests potential therapeutic targets for PD-related social cognition impairments.

[Chronic alcoholism influences the mRNA level of the orexin receptor type 1 (OX1R) in emotiogenic structures of the rat brain]

Orexin and its receptors were shown to be involved into mechanisms of pathological craving to alcohol. This paper demonstrates that the orexin receptor type 1 (OX1R) mRNA level significantly decreased in the prefrontal cortex of rats chronically (during 6 months) consuming ethanol compared with intact control. The same results were observed on day 1 and day 7 of alcohol withdrawal after chronic alcoholization. On the contrary, in the hippocampus, the OX1R mRNA level increased on day 1 and day 7 of alcohol withdrawal. In the ventral tegmental area, the OX1R mRNA level did not change on the day 1 and day 7 of alcohol withdrawal compared with the groups of chronic alcoholization and intact control. These findings point out involvement of the prefrontal cortex and hippocampus first of all in mechanisms mediating chronic alcohol intoxication. The ventral tegmental area is described as a typical dopaminergic structure providing the executive mechanism of emotion reactions connected with alcohol abuse in particular. It is possibe, that the modulating action of orexins on dopaminergic neurons in this structure does not provide a significant effect on control of emotion reactions in alcoholism.

Orexin/Hypocretin Signaling

Orexin/hypocretin peptide (orexin-A and orexin-B) signaling is believed to take place via the two G-protein-coupled receptors (GPCRs), named OX1 and OX2 orexin receptors, as described in the previous chapters. Signaling of orexin peptides has been investigated in diverse endogenously orexin receptor-expressing cells - mainly neurons but also other types of cells - and in recombinant cells expressing the receptors in a heterologous manner. Findings in the different systems are partially convergent but also indicate cellular background-specific signaling. The general picture suggests an inherently high degree of diversity in orexin receptor signaling.In the current chapter, I present orexin signaling on the cellular and molecular levels. Discussion of the connection to (potential) physiological orexin responses is only brief since these are in focus of other chapters in this book. The same goes for the post-synaptic signaling mechanisms, which are dealt with in Burdakov: Postsynaptic actions of orexin. The current chapter is organized according to the tissue type, starting from the central nervous system. Finally, receptor signaling pathways are discussed across tissues, cell types, and even species.

Orexin Directly Enhances the Excitability of Globus Pallidus Internus Neurons in Rat by Co-activating OX1 and OX2 Receptors

Orexin, released from the hypothalamus, has been implicated in various basic non-somatic functions including feeding, the sleep-wakefulness cycle, emotion, and cognition. However, the role of orexin in somatic motor control is still little known. Here, using whole-cell patch clamp recording and immunostaining, we investigated the effect and the underlying receptor mechanism of orexin-A on neurons in the globus pallidus internus (GPi), a critical structure in the basal ganglia and an effective target for deep brain stimulation therapy. Our results showed that orexin-A induced direct postsynaptic excitation of GPi neurons in a concentration-dependent manner. The orexin-A-induced excitation was mediated via co-activation of both OX1 and OX2 receptors. Furthermore, the immunostaining results showed that OX1 and OX2 receptors were co-localized in the same GPi neurons. These results suggest that the central orexinergic system actively modulates the motor functions of the basal ganglia via direct innervation on GPi neurons and presumably participates in somatic-non-somatic integration.

Cellular activation of hypothalamic hypocretin/orexin neurons facilitates short-term spatial memory in mice

The hypothalamic hypocretin/orexin (HO) system holds a central role in the regulation of several physiological functions critical for food-seeking behavior including mnemonic processes for effective foraging behavior. It is unclear however whether physiological increases in HO neuronal activity can support such processes. Using a designer rM3Ds receptor activation approach increasing HO neuronal activity resulted in improved short-term memory for novel locations. When tested on a non-spatial novelty object recognition task no significant difference was detected between groups indicating that hypothalamic HO neuronal activation can selectively facilitate short-term spatial memory for potentially supporting memory for locations during active exploration.

Orexin-1 receptor signalling in the prelimbic cortex and ventral tegmental area regulates cue-induced reinstatement of ethanol-seeking in iP rats

Orexins (hypocretins) are hypothalamic neuropeptides that innervate the entire neuraxis, including the prelimbic cortex and ventral tegmental area and have been implicated in ethanol-seeking behaviour. The present study aimed to use the orexin-1 (OX1 ) receptor antagonist SB-334867 to examine the role of prelimbic cortex and ventral tegmental area OX1 receptors in cue-induced reinstatement of ethanol-seeking. Ethanol-preferring rats (iP) rats were trained to self-administer ethanol (10 percent v/v, FR3) or sucrose (0.2-1 percent w/v, FR3) in the presence of reward-associated cues before being implanted with indwelling guide cannulae. Rats then underwent extinction training for 11 days. On test days, rats were given a microinjection of vehicle or SB-334867 (3 μg/side) and presented with reward-associated cues to precipitate reinstatement. Results show SB-334867 infused into the prelimbic cortex attenuated cue-induced reinstatement of ethanol-seeking, but not sucrose-seeking. OX1 antagonism in the ventral tegmental area also attenuated cue-induced reinstatement of ethanol-seeking. These findings suggest that OX1 receptors located in the prelimbic cortex and ventral tegmental area are part of a circuit driving cue-mediated ethanol-seeking behaviour.

Functional inactivation of hypocretin 1 receptors in the medial prefrontal cortex affects the pyramidal neuron activity and gamma oscillations: An in vivo multiple-channel single-unit recording study

The hypocretin signaling is thought to play a critical role in maintaining wakefulness via stimulating the subcortical arousal pathways. Although the cortical areas, including the medial prefrontal cortex (mPFC), receive dense hypocretinergic fibers and express its receptors, it remains unclear whether the hypocretins can directly regulate the neural activity of the mPFC in vivo. In the present study, using multiple-channel single-unit recording study, we found that infusion of the SB-334867, a blocker for the Hcrtr1, beside the recording sites within the mPFC substantially exerted an inhibitory effect on the putative pyramidal neuron (PPN) activity in naturally behaving rats. In addition, functional blockade of the Hcrtr1 also selectively reduced the power of the gamma oscillations. The PPN activity and the power of the neural oscillations were not affected after microinjection of the TCS-OX2-29, a blocker for the Hcrtr2, within the mPFC. Together, these data indicate that endogenous hypocretins acting on the Hcrtr1 are required for the normal neural activity in the mPFC in vivo, and thus might directly contribute cortical arousal and mPFC-dependent cognitive processes.

Substance P Activates Ca2+-Permeable Nonselective Cation Channels through a Phosphatidylcholine-Specific Phospholipase C Signaling Pathway in nNOS-Expressing GABAergic Neurons in Visual Cortex

To understand the functions of the neocortex, it is essential to characterize the properties of neurons constituting cortical circuits. Here, we focused on a distinct group of GABAergic neurons that are defined by a specific colocalization of intense labeling for both neuronal nitric oxide synthase (nNOS) and substance P (SP) receptor [neurokinin 1 (NK1) receptors]. We investigated the mechanisms of the SP actions on these neurons in visual cortical slices obtained from young glutamate decarboxylase 67-green fluorescent protein knock-in mice. Bath application of SP induced a nonselective cation current leading to depolarization that was inhibited by the NK1 antagonists in nNOS-immunopositive neurons. Ruthenium red and La(3+), transient receptor potential (TRP) channel blockers, suppressed the SP-induced current. The SP-induced current was mediated by G proteins and suppressed by D609, an inhibitor of phosphatidylcholine-specific phospholipase C (PC-PLC), but not by inhibitors of phosphatidylinositol-specific PLC, adenylate cyclase or Src tyrosine kinases. Ca(2+) imaging experiments under voltage clamp showed that SP induced a rise in intracellular Ca(2+) that was abolished by removal of extracellular Ca(2+) but not by depletion of intracellular Ca(2+) stores. These results suggest that SP regulates nNOS neurons by activating TRP-like Ca(2+)-permeable nonselective cation channels through a PC-PLC-dependent signaling pathway.

Orexin A attenuates the sleep-promoting effect of adenosine in the lateral hypothalamus of rats

Orexin neurons within the lateral hypothalamus play a crucial role in the promotion and maintenance of arousal. Studies have strongly suggested that orexin neurons are an important target in endogenous adenosine-regulated sleep homeostasis. Orexin A induces a robust increase in the firing activity of orexin neurons, while adenosine has an inhibitory effect. Whether the excitatory action of orexins in the lateral hypothalamus actually promotes wakefulness and reverses the sleep-producing effect of adenosine in vivo is less clear. In this study, electroencephalographic and electromyographic recordings were used to investigate the effects of orexin A and adenosine on sleep and wakefulness in rats. We found that microinjection of orexin A into the lateral hypothalamus increased wakefulness with a concomitant reduction of sleep during the first 3 h of post-injection recording, and this was completely blocked by a selective antagonist for orexin receptor 1, SB 334867. The enhancement of wakefulness also occurred after application of the excitatory neurotransmitter glutamate in the first 3 h post-injection. However, in the presence of the NMDA receptor antagonist APV, orexin A did not induce any change of sleep and wakefulness in the first 3 h. Further, exogenous application of adenosine into the lateral hypothalamus induced a marked increase of sleep in the first 3-h post-injection. No significant change in sleep and wakefulness was detected after adenosine application followed by orexin A administration into the same brain area. These findings suggest that the sleep-promoting action of adenosine can be reversed by orexin A applied to the lateral hypothalamus, perhaps by exciting glutamatergic input to orexin neurons via the action of orexin receptor 1.

The hypocretins/orexins: integrators of multiple physiological functions

The hypocretins (Hcrts), also known as orexins, are two peptides derived from a single precursor produced in the posterior lateral hypothalamus. Over the past decade, the orexin system has been associated with numerous physiological functions, including sleep/arousal, energy homeostasis, endocrine, visceral functions and pathological states, such as narcolepsy and drug abuse. Here, we review the discovery of Hcrt/orexins and their receptors and propose a hypothesis as to how the orexin system orchestrates these multifaceted physiological functions.

The hypocretin/orexin system: an increasingly important role in neuropsychiatry

Hypocretins, also named as orexins, are excitatory neuropeptides secreted by neurons specifically located in lateral hypothalamus and perifornical areas. Orexinergic fibers are extensively distributed in various brain regions and involved in a number of physiological functions, such as arousal, cognition, stress, appetite, and metabolism. Arousal is the most important function of orexin system as dysfunction of orexin signaling leads to narcolepsy. In addition to narcolepsy, orexin dysfunction is associated with serious neural disorders, including addiction, depression, and anxiety. However, some results linking orexin with these disorders are still contradictory, which may result from differences of detection methods or the precision of tools used in measurements; strategies targeted to orexin system (e.g., antagonists to orexin receptors, gene delivery, and cell transplantation) are promising new tools for treatment of neuropsychiatric disorders, though studies are still in a stage of preclinical or clinical research.

Orexin/hypocretin receptor signalling: a functional perspective

Multiple homeostatic systems are regulated by orexin (hypocretin) peptides and their two known GPCRs. Activation of orexin receptors promotes waking and is essential for expression of normal sleep and waking behaviour, with the sleep disorder narcolepsy resulting from the absence of orexin signalling. Orexin receptors also influence systems regulating appetite/metabolism, stress and reward, and are found in several peripheral tissues. Nevertheless, much remains unknown about the signalling pathways and targets engaged by native receptors. In this review, we integrate knowledge about the orexin receptor signalling capabilities obtained from studies in expression systems and various native cell types (as presented in Kukkonen and Leonard, this issue of British Journal of Pharmacology) with knowledge of orexin signalling in different tissues. The tissues reviewed include the CNS, the gastrointestinal tract, the pituitary gland, pancreas, adrenal gland, adipose tissue and the male reproductive system. We also summarize the findings in different native and recombinant cell lines, especially focusing on the different cascades in CHO cells, which is the most investigated cell line. This reveals that while a substantial gap exists between what is known about orexin receptor signalling and effectors in recombinant systems and native systems, mounting evidence suggests that orexin receptor signalling is more diverse than originally thought. Moreover, rather than being restricted to orexin receptor 'overexpressing' cells, this signalling diversity may be utilized by native receptors in a site-specific manner.

Orexinergic input to dopaminergic neurons of the human ventral tegmental area

The mesolimbic reward pathway arising from dopaminergic (DA) neurons of the ventral tegmental area (VTA) has been strongly implicated in reward processing and drug abuse. In rodents, behaviors associated with this projection are profoundly influenced by an orexinergic input from the lateral hypothalamus to the VTA. Because the existence and significance of an analogous orexigenic regulatory mechanism acting in the human VTA have been elusive, here we addressed the possibility that orexinergic neurons provide direct input to DA neurons of the human VTA. Dual-label immunohistochemistry was used and orexinergic projections to the VTA and to DA neurons of the neighboring substantia nigra (SN) were analyzed comparatively in adult male humans and rats. Orexin B-immunoreactive (IR) axons apposed to tyrosine hydroxylase (TH)-IR DA and to non-DA neurons were scarce in the VTA and SN of both species. In the VTA, 15.0±2.8% of TH-IR perikarya in humans and 3.2±0.3% in rats received orexin B-IR afferent contacts. On average, 0.24±0.05 and 0.05±0.005 orexinergic appositions per TH-IR perikaryon were detected in humans and rats, respectively. The majority (86–88%) of randomly encountered orexinergic contacts targeted the dendritic compartment of DA neurons. Finally, DA neurons of the SN also received orexinergic innervation in both species. Based on the observation of five times heavier orexinergic input to TH-IR neurons of the human, compared with the rat, VTA, we propose that orexinergic mechanism acting in the VTA may play just as important roles in reward processing and drug abuse in humans, as already established well in rodents.

Hypocretin (orexin) regulates glutamate input to fast-spiking interneurons in layer V of the Fr2 region of the murine prefrontal cortex

We studied the effect of hypocretin 1 (orexin A) in the frontal area 2 (Fr2) of the murine neocortex, implicated in the motivation-dependent goal-directed tasks. In layer V, hypocretin stimulated the spontaneous excitatory postsynaptic currents (EPSCs) on fast-spiking (FS) interneurons. The effect was accompanied by increased frequency of miniature EPSCs, indicating that hypocretin can target the glutamatergic terminals. Moreover, hypocretin stimulated the spontaneous inhibitory postsynaptic currents (IPSCs) on pyramidal neurons, with no effect on miniature IPSCs. This action was prevented by blocking 1) the ionotropic glutamatergic receptors; 2) the hypocretin receptor type 1 (HCRTR-1), with SB-334867. Finally, hypocretin increased the firing frequency in FS cells, and the effect was blocked when the ionotropic glutamate transmission was inhibited. Immunolocalization confirmed that HCRTR-1 is highly expressed in Fr2, particularly in layer V-VI. Conspicuous labeling was observed in pyramidal neuron somata and in VGLUT1+ glutamatergic terminals, but not in VGLUT2+ fibers (mainly thalamocortical afferents). The expression of HCRTR-1 in GABAergic structures was scarce. We conclude that 1) hypocretin regulates glutamate release in Fr2; 2) the effect presents a presynaptic component; 3) the peptide control of FS cells is indirect, and probably mediated by the regulation of glutamatergic input onto these cells.

Ascending orexinergic pathways and alcohol-seeking

Orexin (hypocretin) containing neurons reside in discrete regions of the lateral hypothalamus from where they innervate the entire neuroaxis. Via actions upon orexin receptors (OX1 and OX2), the orexin peptides (orexin A and orexin B) are thought to play a role in ethanol consumption and seeking. While a role for OX1 receptors in these behaviours is established, the case for OX2 receptors is less clear at present, although recent data certainly support an involvement of OX2 receptors in ethanol consumption. In terms of circuitry, orexin receptors the ventral tegmental area appear to contribute to ethanol consumption. Other loci remain to be characterised, and we suggest prefrontal cortical orexin receptors deserve attention in this respect.

Physiology of the orexinergic/hypocretinergic system: a revisit in 2012

The neuropeptides orexins and their G protein-coupled receptors, OX(1) and OX(2), were discovered in 1998, and since then, their role has been investigated in many functions mediated by the central nervous system, including sleep and wakefulness, appetite/metabolism, stress response, reward/addiction, and analgesia. Orexins also have peripheral actions of less clear physiological significance still. Cellular responses to the orexin receptor activity are highly diverse. The receptors couple to at least three families of heterotrimeric G proteins and other proteins that ultimately regulate entities such as phospholipases and kinases, which impact on neuronal excitation, synaptic plasticity, and cell death. This article is a 10-year update of my previous review on the physiology of the orexinergic/hypocretinergic system. I seek to provide a comprehensive update of orexin physiology that spans from the molecular players in orexin receptor signaling to the systemic responses yet emphasizing the cellular physiological aspects of this system.

Orexin-A excites pyramidal neurons in layer 2/3 of the rat prefrontal cortex

The arousal peptides, orexins, play an important role in regulating the function of the prefrontal cortex (PFC). Although orexins have been shown to increase the excitability of deep-layer neurons in the medial prefrontal cortex (mPFC), little is known about their effect on layer 2/3, the main intracortical processing layer. In this study, we investigated the effect of orexin-A on pyramidal neurons in layer 2/3 of the mPFC using whole-cell recordings in rat brain slices. We observed that orexin-A reversibly depolarized layer 2/3 pyramidal neurons through a postsynaptic action. This depolarization was concentration-dependent and mediated via orexin receptor 1. In voltage-clamp recordings, the orexin-A-induced current was reduced by the replacement of internal K(+) with Cs(+), removal of external Na(+), or an application of flufenamic acid (an inhibitor of nonselective cation channels). A blocker of Na(+)/Ca(2+) exchangers (SN-6) did not influence the excitatory effect of orexin-A. Moreover, the current induced by orexin-A reversed near E(k) when the external solution contained low levels of Na(+). When recording with Cs(+)-containing pipettes in normal external solution, the reversal potential of the current was approximately -25 mV. These data suggest an involvement of both K(+) channels and nonselective cation channels in the effect of orexin-A. The direct excitatory action of orexin-A on layer 2/3 mPFC neurons may contribute to the modulation of PFC activity, and play a role in cognitive arousal.

Arousal effects of orexin A on acute alcohol intoxication-induced coma in rats

The key role of the hypothalamic neuropeptides orexins in maintenance and promotion of arousal has been well established in normal mammalian animals, but whether orexins exert arousal effects under pathological condition such as coma was little studied. In this study, a model of unconscious rats induced by acute alcohol intoxication was used to examine the effects of orexins through intracerebroventricular injection. The results revealed that either orexin A or orexin B induced decrease of duration of loss of right reflex in alcohol-induced unconscious rats. In the presence of the selective orexin receptor 1 antagonist SB 334867 and orexin receptor 2 antagonist TCS OX2 29, the excitatory action of orexin A was completely blocked. Our data further presented that orexin A also induced reduction of delta power in EEG in these rats. Single-unit recording experiment in vivo demonstrated that orexin A could evoke increase of firing activity of prefrontal cortex neurons in unconscious rats. This excitation was completely inhibited by an H(1) receptor antagonist, pyrilamine, whereas application of α(1)-adrenoreceptor antagonist prazosin or 5-HT(2) selective receptor antagonist ritanserin partially attenuated the excitatory effects of orexin A on these neurons. Consistently, the results of EEG recordings showed that microinjection of pyrilamine, prazosin, or ritanserin suppressed reduction of delta power in EEG induced by orexin A on unconscious rats. Thus, these data suggest that orexins exert arousal effects on alcohol-induced unconscious rats by the promotion of cortical activity through activation of histaminergic, noradrenergic and serotonergic systems. This article is part of a Special Issue entitled 'Post-Traumatic Stress Disorder'.

Discrete cue-conditioned alcohol-seeking after protracted abstinence: pattern of neural activation and involvement of orexin₁ receptors

BACKGROUND AND PURPOSE

The enduring propensity for alcoholics to relapse even following years of abstinence presents a major hurdle for treatment. Here we report a model of relapse following protracted abstinence and investigate the pattern of neuronal activation following cue-induced reinstatement and administration of the orexin₁ receptor antagonist SB-334867 in inbred alcohol-preferring rats.

EXPERIMENTAL APPROACH

Rats were trained to self-administer alcohol under operant conditions and divided into two groups: immediate (reinstated immediately following extinction) and delayed (extinguished and then housed for 5 months before reinstatement). Prior to reinstatement, animals were treated with vehicle (immediate n= 11, delayed n= 11) or SB-334867 (20 mg·kg⁻¹ i.p.; immediate n= 6, delayed n= 11). Fos expression was compared between each group and to animals that underwent extinction only.

KEY RESULTS

SB-334867 significantly attenuated cue-induced reinstatement in both groups. Immediate reinstatement increased Fos expression in the nucleus accumbens (NAc), infra-limbic (IL), pre-limbic (PrL), orbitofrontal (OFC) and piriform cortices, the lateral and dorsomedial hypothalamus, central amygdala and basolateral amygdala (BLA), and the bed nucleus of the stria terminalis. Following delayed reinstatement, Fos expression was further elevated in cortical structures. Concurrent with preventing reinstatement, SB-334867 decreased Fos in NAc core, PrL and OFC following immediate reinstatement. Following protracted abstinence, SB-334867 treatment decreased reinstatement-induced Fos in the PrL, OFC and piriform cortices.

CONCLUSIONS AND IMPLICATIONS

Cue-induced alcohol seeking can be triggered following protracted abstinence in rats. The effects of SB-334867 on both behaviour and Fos expression suggest that the orexin system is implicated in cue-induced reinstatement, although some loci may shift following protracted abstinence.

Cytosolic calcium elevation induced by orexin/hypocretin in granule cell domain cells of the rat cochlear nucleus in vitro

Using rat brain slice preparations, we examined the effect of orexin on cytosolic Ca(2+) concentrations ([Ca(2+)](i)) in the granule cell domain (GCD) cells of the cochlear nucleus that carry non-auditory information to the dorsal cochlear nucleus. Application of orexin concentration-dependently increased [Ca(2+)](i), and in two thirds of GCD cells these increases persisted in the presence of tetrodotoxin. There was no significant difference between the dose-response curve for orexin-A and that for orexin-B. Extracellular Ca(2+) removal abolished the [Ca(2+)](i) elevation induced by orexin-B, whereas depletion of intracellular Ca(2+) stores had no effect. The orexin-B-induced elevation of [Ca(2+)](i) was not blocked by inhibitors of reverse-mode Na(+)/Ca(2+) exchanger (NCX) and nonselective cation channel, whereas it was blocked by lowering the extracellular Na(+) or by applying inhibitors of forward-mode NCX and voltage-gated R- and T-type Ca(2+) channels. The ORX-B-induced increase in [Ca(2+)](i) was also blocked by inhibitors of adenylcyclase (AC) and protein kinase A (PKA), but not by inhibitors of phosphatidylcholine-specific and phosphatidylinositol-specific phospholipase C. In electrophysiological experiments using whole-cell patch clamp recordings, half of GCD cells were depolarized by orexin-B, and the depolarization was abolished by a forward-mode NCX inhibitor. These results suggest that orexin increases [Ca(2+)](i) postsynaptically via orexin 2 receptors, and the increase in [Ca(2+)](i) is induced via the AC-PKA-forward-mode NCX-membrane depolarization-mediated activation of voltage-gated R- and T-type Ca(2+) channels. The results further support the hypothesis that the orexin system participates in integrating neural systems that are involved in arousal, sensory processing, energy homeostasis and autonomic function.

Orexin A-induced extracellular calcium influx in prefrontal cortex neurons involves L-type calcium channels

Orexins, novel excitatory neuropeptides from the lateral hypothalamus, have been strongly implicated in the regulation of sleep and wakefulness. In this study, we explored the effects and mechanisms of orexin A on intracellular free Ca2+ concentration ([Ca2+]i) of freshly dissociated neurons from layers V and VI in prefrontal cortex (PFC). Changes in [Ca2+]i were measured with fluo-4/AM using confocal laser scanning microscopy. The results revealed that application of orexin A (0.1-1 microM) induced increase of [Ca2+]i in a dose-dependent manner. This elevation of [Ca2+]i was completely blocked by pretreatment with selective orexin receptor 1 antagonist SB 334867. While depletion of intracellular Ca2+ stores by the endoplasmic reticulum inhibitor thapsigargin (2 pM), [Ca2+]i in PFC neurons showed no increase in response to orexin A. Under extracellular Ca2+-free condition, orexin A failed to induce any changes of Ca2+ fluorescence intensity in these acutely dissociated cells. Our data further demonstrated that the orexin A-induced increase of [Ca2+]i was completely abolished by the inhibition of intracellular protein kinase C or phospholipase C activities using specific inhibitors, BIS II (1 microM) and D609 (10 microM), respectively. Selective blockade of L-type Ca2+ channels by nifedipine (5 microM) significantly suppressed the elevation of [Ca2+]i induced by orexin A. Therefore, these findings suggest that exposure to orexin A could induce increase of [Ca2+]i in neurons from deep layers of PFC, which depends on extracellular Ca2+ influx via L-type Ca2+ channels through activation of intracellular PLC-PKC signaling pathway by binding orexin receptor 1.

The modulation of orexin A on HCN currents of pyramidal neurons in mouse prelimbic cortex

The hyperpolarization-activated/cyclic nucleotide (HCN)-gated channels make important contributions to neural excitability. In prefrontal cortex, HCN channels are localized on the distal dendrites of layer V pyramidal neurons and decrease neural excitability when they are open. In the present study, using whole-cell voltage clamp recordings, the effect of an arousal peptide, orexin A, on HCN currents in layer V pyramidal neurons from mouse prelimbic cortex (PL), the homolog of the prefrontal cortex was investigated. The results demonstrated that orexin A suppressed HCN currents and shifted their activation curve to a more negative direction. This action of orexin A was blocked by SB334867, an orexin receptor 1 (OXR1) blocker and bisindolylmaleimide, a protein kinase C (PKC) inhibitor, indicating the involvement of OXR1 and PKC. The excitatory effect of orexin A on PL pyramidal neurons was enhanced when HCN currents were diminished, while attenuated when HCN currents were enlarged. In summary, orexin A inhibits HCN currents and enhances excitability of pyramidal neurons in PL, which may contribute to arousal and cognition.

Hypocretin mechanisms in nicotine addiction: evidence and speculation

BACKGROUND

The hypocretin/orexin system has been implicated in arousal mechanisms, sleep, and sleep disorders, including narcolepsy, and more recently in drug addiction. Theoretically, hypocretin (hcrt) mechanisms appear to be potential substrates for nicotine addiction: arousal and attentional mechanisms influence use and withdrawal symptoms, and hcrt systems overlap anatomically with a number of brain regions associated with nicotine addiction.

OBJECTIVE

This review summarizes the studies that have examined hcrt mechanisms in the effects of nicotine and describes hcrt innervation of, and effects in, several brain regions implicated in nicotine addiction. The review speculates on the possible mechanisms by which hcrt may contribute to nicotine addiction in these regions, with the objective of encouraging research in this area.

RESULTS

In a small literature, both experimenter-administered and self-administered nicotine have been shown to elicit or depend on hcrt signaling. However, although untested in experimental designs, there is compelling evidence that hcrt mechanisms in the ventral tegmental area, the pontine region, thalamocortical circuits, the prefrontal cortex, and the amygdala could have a broad influence on nicotine addiction.

CONCLUSIONS

Evidence reviewed leads to the conclusion that hcrt mechanisms could mediate several dimensions of nicotine addiction, including a multi-faceted regulation of mesocorticolimbic dopaminergic function, but beyond dopaminergic mechanisms, hcrt could influence nicotine use and relapse during abstinence through broadly based arousal/attentional effects. These speculative ideas need to be examined experimentally; the potential gains are a more thorough understanding of the pathophysiology of nicotine addiction, and the discovery of novel targets for the development of pharmacotherapeutics.

Role of orexin/hypocretin in dependence and addiction

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

Hypothalamic regulation of sleep and arousal

Normal waking is associated with neuronal activity in several chemically defined ascending arousal systems. These include monoaminergic neurons in the brainstem and posterior hypothalamus, cholinergic neurons in the brainstem and basal forebrain, and hypocretin (orexin) neurons in the lateral hypothalamus. Collectively, these systems impart tonic activation to their neuronal targets in the diencephalon and neocortex that is reflected in the low-voltage fast-frequency electroencephalogram patterns of wakefulness. Neuronal discharge in these arousal systems declines rapidly at sleep onset. Transitions from waking to sleep, therefore, involve coordinated inhibition of multiple arousal systems. An important source of sleep-related inhibition of arousal arises from neurons located in the preoptic hypothalamus. These preoptic neurons are strongly activated during sleep, exhibiting sleep/waking state-dependent discharge patterns that are the reciprocal of that observed in the arousal systems. The majority of preoptic sleep regulatory neurons synthesize the inhibitory neurotransmitter GABA. Anatomical and functional evidence supports the hypothesis that GABAergic neurons in the median preoptic nucleus (MnPN) and ventrolateral preoptic area (VLPO) exert inhibitory control over the monoaminergic systems and the hypocretin system during sleep. Recent findings indicate that MnPN and VLPO neurons integrate homeostatic aspects of sleep regulation and are important targets for endogenous sleep factors, such as adenosine and growth hormone releasing hormone.

Hypocretin-1 potentiates NMDA receptor-mediated somatodendritic secretion from locus ceruleus neurons

Our previous observations showed that several stimuli, including high-K(+) solution, glutamate, and voltage pulses, induce somatic noradrenaline (NA) secretion from locus ceruleus (LC) neurons. Hypocretin (orexin), a hypothalamic peptide critical for normal wakefulness, has been shown to evoke NA release from the axon terminals of LC neurons. Here, we used amperometry to test the effect of hypocretin-1 (HCRT) on NMDA receptor-mediated somatodendritic release in LC neurons. Either HCRT or NMDA applied alone dose-dependently induced somatodendritic secretion. Bath application of HCRT notably potentiated NMDA receptor-mediated somatodendritic NA release. This potentiation was blocked by SB 334867, a selective HCRT receptor (Hcrtr 1) antagonist, or bisindolylmaleimide, a specific protein kinase C (PKC) inhibitor, indicating the involvement of Hcrtr 1 and PKC. Consistent with this, phorbol 12-myristate 13-acetate, a PKC activator, mimicked the HCRT-induced potentiation. Furthermore, HCRT enhanced NMDA-induced intracellular Ca(2+) elevation via activation of Hcrtr 1 and PKC, which may contribute to HCRT-potentiated somatodendritic secretion. These results suggest that HCRT modulates LC activity not only by regulating noradrenergic input to its targets, but also by affecting noradrenergic communication in the soma and dendrites.

Modulatory effects of hypocretin-1/orexin-A with glutamate and γ-aminobutyric acid on freshly isolated pyramidal neurons from the rat prefrontal cortex

It is widely known that hypocretins are essential for the regulation of wakefulness. Our recent reports have found that hypocretin-1 shows a direct postsynaptic excitatory effect on rat prefrontal cortex (PFC) pyramidal neurons. It remains unclear whether hypocretin-1 may interact with two classical neurotransmitter systems, glutamate and gamma-aminobutyric acid (GABA) in rat PFC. For this reason, we here investigated the modulatory actions of hypocretin-1 with these two transmitters on freshly isolated PFC pyramidal neurons using whole-cell patch-clamp recordings. We found that coadministration of hypocretin-1 and glutamate showed a synergistic effect on the recorded cells, and hypocretin-1 could excite the neurons even if GABA was present. Thus, our data suggest that there may be hypocretin-glutamate and hypocretin-GABA interactions in the PFC.