Anatomical substrates of orexin-dopamine interactions: lateral hypothalamic projections to the ventral tegmental area

Activation of orexin/hypocretin projections to basal forebrain and paraventricular thalamus by acute nicotine

Orexin/hypocretin neurons of the lateral hypothalamus/perifornical area project to a diverse array of brain regions and are responsive to a variety of psychostimulant drugs. It has been shown that orexin neurons are activated by systemic nicotine administration suggesting a possible orexinergic contribution to the effects of this drug on arousal and cognitive function. The basal forebrain and paraventricular nucleus of the dorsal thalamus (PVT) both receive orexin inputs and have been implicated in arousal, attention and psychostimulant drug responses. However, it is unknown whether orexin inputs to these areas are activated by psychostimulant drugs such as nicotine. Here, we infused the retrograde tract tracer cholera toxin B subunit (CTb) into either the basal forebrain or PVT of adult male rats. Seven to 10 days later, animals received an acute systemic administration of (-) nicotine hydrogen tartrate or vehicle and were euthanized 2h later. Triple-label immunohistochemistry/immunofluorescence was used to detect Fos expression in retrogradely-labeled orexin neurons. Nicotine increased Fos expression in orexin neurons projecting to both basal forebrain and PVT. The relative activation in lateral and medial banks of retrogradely-labeled orexin neurons was similar following basal forebrain CTb deposits, but was more pronounced in the medial bank following PVT deposits of CTb. Our findings suggest that orexin inputs to the basal forebrain and PVT may contribute to nicotine effects on arousal and cognition and provide further support for the existence of functional heterogeneity across the medial-lateral distribution of orexin neurons.

Nucleus accumbens shell and core involvement in drug context-induced reinstatement of cocaine seeking in rats

RATIONALE

The nucleus accumbens (NAC) is a functionally heterogeneous brain region with respect to its involvement in cocaine-seeking behavior triggered by drug-associated explicit conditioned stimuli, foot shock stress, or cocaine itself in the reinstatement animal model of drug relapse. However, it is not known whether the NAC or its subregions are critical for reinstatement of cocaine-seeking behavior produced by re-exposure to a previously cocaine-paired environmental context.

OBJECTIVES

The present study was designed to evaluate potentially unique contributions of the NAC core and shell to this behavior.

MATERIALS AND METHODS

Rats were trained to lever press for unsignaled cocaine infusions (0.15 mg/infusion, intravenous) in a distinct environmental context. Lever responding was then extinguished in a distinctly different environmental context (extinction context) during a minimum of seven daily training sessions. Subsequently, using a counterbalanced testing design, rats were re-exposed to the cocaine-paired context or the extinction context while cocaine seeking (i.e., responding on the previously cocaine-reinforced lever) was assessed. Before each test session, neural activity was inhibited selectively in the NAC core or shell using bilateral microinfusions of the gamma-aminobutyric acid agonists, baclofen and muscimol (0/0 or 1.0/0.1 mM; 0.3 microl per hemisphere).

RESULTS

Neural inactivation of the NAC shell or core attenuated responding in the cocaine context and, interestingly, increased responding in the extinction context. Control experiments indicated no effects on general activity or food-reinforced instrumental behavior.

CONCLUSIONS

These findings suggest that both subregions of the NAC may promote context-induced reinstatement by facilitating drug context-induced motivation for cocaine and context discrimination.

Insulin, leptin, and food reward: update 2008

The hormones insulin and leptin have been demonstrated to act in the central nervous system (CNS) as regulators of energy homeostasis at medial hypothalamic sites. In a previous review, we described new research demonstrating that, in addition to these direct homeostatic actions at the hypothalamus, CNS circuitry that subserves reward and motivation is also a direct and an indirect target for insulin and leptin action. Specifically, insulin and leptin can decrease food reward behaviors and modulate the function of neurotransmitter systems and neural circuitry that mediate food reward, i.e., midbrain dopamine and opioidergic pathways. Here we summarize new behavioral, systems, and cellular evidence in support of this hypothesis and in the context of research into the homeostatic roles of both hormones in the CNS. We discuss some current issues in the field that should provide additional insight into this hypothetical model. The understanding of neuroendocrine modulation of food reward, as well as food reward modulation by diet and obesity, may point to new directions for therapeutic approaches to overeating or eating disorders.

Orexin A attenuates unconditioned sexual motivation in male rats

Orexins are neuropeptides involved in multiple neurophysiological functions such as reward and motivation. However, it is not clear whether orexins are implicated in sexual motivation. This study aims to evaluate the effects of orexin A and the OX(1)R antagonist SB334867 on unconditioned sexual motivation. Forty-five male Wistar rats are divided into four groups. The four groups are respectively administered intracerebroventricularly with saline, orexin A (1, 10 microg), 10% DMSO (cyclodextrin) and SB334867 (5, 15 microg) 10-15 min before sexual motivation tests. The preference for a receptive female to a male in an open arena with two tethered animals is designated as unconditioned sexual motivation. The results show that orexin A reduces the female preference (reducing time in the female zone and/or increasing time in the male zone), the number of visits for the female zone and the total distance traveled in sexually high-motivated males. SB334867 has no effect on the female preference, the number of visits and the distance traveled in either sexually high-motivated or low-motivated males. Our experiments reveal that centrally administered orexin A attenuates sexual motivation in high-motivated males although endogenous orexin A might not play an important role in the expression of unconditioned sexual motivation.

Orexin B/hypocretin 2 increases glutamatergic transmission to ventral tegmental area neurons

The orexins (hypocretins) play a crucial role in arousal, feeding and reward. Highly relevant to these functions, orexin-containing neurons from the lateral hypothalamus project densely to the ventral tegmental area (VTA), which is the origin of dopamine projections implicated in motivation and reward. Orexin A/hypocretin 1 (oxA/hcrt-1) can enable long-term changes associated with drugs of abuse; however, the effects of orexin B/hypocretin 2 (oxB/hcrt-2) on excitatory synaptic transmission in the VTA are unknown. We used whole-cell patch-clamp electrophysiology in rat horizontal midbrain slices to examine the effects of oxB/hcrt-2 on excitatory synaptic transmission. We observed that oxB/hcrt-2 has distinct effects from oxA/hcrt-1 in the VTA. oxB/Hcrt-2 (100 nM) increased presynaptic glutamate release in addition to a postsynaptic potentiation of NMDA receptors (NMDARs). The oxB/hcrt-2-mediated postsynaptic potentiation of NMDARs was mediated via activation of orexin/hypocretin 2 (OX2/Hcrt-2) receptors and protein kinase C (PKC). Furthermore, the increase in transmitter release probability was also PKC-dependent, but not through activation of orexin/hypocretin 1 (OX1/Hcrt-1) or OX2/Hcrt-2 receptors. Finally, oxB/hcrt-2 or the selective OX2/Hcrt-2 receptor agonist ala(11)-D-leu(15)-orexin B, significantly reduced spike-timing-induced long-term potentiation. Taken together, these results support a dual role for oxB/hcrt-2 in mediating enhanced glutamatergic transmission in the VTA, and suggest that oxA/hcrt-1 and oxB/hcrt-2 exert different functional roles in modulating the enhancement of the motivational components of arousal and feeding.

Novel functions of orexigenic hypothalamic peptides: from genes to behavior

The regulation of energy balance depends on the precise co-ordination of multiple peripheral and central systems. Much recent research has highlighted the importance of behavioral mechanisms is this control and suggested that the regulation of body weight shares central nervous system pathways in common with other complex behaviors, including learning and drug addiction. We present a brief review of some of this work and highlight the novel functions for central orexigenic neuropeptides. We review evidence that organisms engage in critical regulatory behaviors before and after ingestion has occurred. Additional evidence supports the idea that appetitive mechanisms are engaged that are critical for the regulation of intake during the act of ingestion. We briefly discuss the recent work on the potential role for central nervous system reward centers, how those might be critically linked to the central regulation of food intake, and how they may be dysregulated by the abundance of highly palatable, energy-dense foods.

Abnormal response of melanin-concentrating hormone deficient mice to fasting: hyperactivity and rapid eye movement sleep suppression

Melanin-concentrating hormone (MCH) is a hypothalamic neuropeptide that has been implicated in energy homeostasis. Pharmacological studies with MCH and its receptor antagonists have suggested additional behavioral roles for the neuropeptide in the control of mood and vigilance states. These suggestions have been supported by a report of modified sleep in the MCH-1 receptor knockout mouse. Here we found that MCH knockout (MCH(-)(/)(-)) mice slept less during both the light and dark phases under baseline conditions. In response to fasting, MCH(-)(/)(-) mice exhibited marked hyperactivity, accelerated weight loss and an exaggerated decrease in rapid eye movement (REM) sleep. Following a 6-h period of sleep deprivation, however, the sleep rebound in MCH(-)(/)(-) mice was normal. Thus MCH(-)(/)(-) mice adapt poorly to fasting, and their loss of bodyweight under this condition is associated with behavioral hyperactivity and abnormal expression of REM sleep. These results support a role for MCH in vigilance state regulation in response to changes in energy homeostasis and may relate to a recent report of initial clinical trials with a novel MCH-1 receptor antagonist. When combined with caloric restriction, the treatment of healthy, obese subjects with this compound resulted in some subjects experiencing vivid dreams and sleep disturbances.

Orexin mediates the expression of precipitated morphine withdrawal and concurrent activation of the nucleus accumbens shell

BACKGROUND

The lateral hypothalamic neuropeptide orexin (or hypocretin) is implicated in drug addiction. Although a role for orexin has been shown in reward and dependence, the molecular and neural mechanisms are unclear. We investigated the mechanism and neuroanatomic basis of orexin's role in morphine withdrawal.

METHODS

C57BL/6J mice received chronic morphine followed by naloxone (0 or 1 mg/kg, subcutaneous) to precipitate withdrawal. Before naloxone, mice received SB-334867 (0 or 20 mg/kg, intraperitoneal), an orexin 1 receptor (Ox1r) antagonist. Using immunohistochemistry, c-Fos, a marker of cell activation, was quantified in the nucleus accumbens (Acb), lateral hypothalamus (LH), ventral tegmental area (VTA), and locus coeruleus (LC). Retrograde tracing with fluorogold (FG) was performed to determine whether orexin neurons project directly to the Acb.

RESULTS

SB-334867 before naloxone significantly attenuated withdrawal symptoms. Withdrawal was accompanied by an increase in c-Fos expression in the Acb shell (AcbSh), which was reduced by SB-334867 but had no effect on the VTA or the LC. Morphine withdrawal increased c-Fos expression in the dorsomedial (DMH) and perifornical (PFA) regions but not in the lateral region of the LH (LLH). Orexin neurons do not appear to form direct connections with Acb neurons.

CONCLUSIONS

Altogether, these data demonstrate that orexin, acting via Ox1r, is critical for the expression of morphine withdrawal. AcbSh activation during withdrawal is dependent on Ox1r function and is likely mediated by indirect action of LH orexin neurons.

Nucleus paragigantocellularis afferents in male and female rats: organization, gonadal steroid receptor expression, and activation during sexual behavior

The supraspinal regulation of genital reflexes is poorly understood. The brainstem nucleus paragigantocellularis (nPGi) of rats is a well-established source of tonic inhibition of genital reflexes. However, the organization, gonadal steroid receptor expression, and activity of nPGi afferents during sex have not been fully characterized in male and female rats. To delineate the anatomical and physiological organization of nPGi afferents, the retrograde tracer Fluoro-Gold (FG) was injected into the nPGi of sexually experienced male and female rats. Animals engaged in sexual behavior 1 hour before sacrifice. Cells containing FG, estrogen receptor-alpha (ER(alpha)), androgen receptor (AR), and the immediate-early gene product Fos were identified immunocytochemically. Retrograde labeling from the nPGi was prominent in the bed nucleus of the stria terminalis, paraventricular nucleus (PVN), posterior hypothalamus, precommissural nucleus, deep mesencephalic nucleus, and periaqueductal gray (PAG) of both sexes. Sex differences were observed in the caudal medial preoptic area (MPO), with significantly more FG+ cells observed in males, and in the PAG and inferior colliculus, where significantly more FG+ cells were observed in females. The majority of regions that contained FG+ cells also contained ER(alpha) or AR, indicating sensitivity to gonadal steroids. The proportions of FG+ cells that co-localized with sex-induced Fos was high in the PVN of both sexes and high in the MPO of males but low in the PAG of both sexes despite the large number of PAG-nPGi output neurons and Fos+ cells in both sexes. The characterization of these afferents will lead to a further understanding of the neural regulation of genital reflexes.

Role of lateral hypothalamic orexin neurons in reward processing and addiction

Orexins (also known as hypocretins) are recently discovered neuropeptides made exclusively in hypothalamic neurons that have been shown to be important in narcolepsy/cataplexy and arousal. Here, we conducted behavioral, anatomical and neurophysiological studies that show that a subset of these cells, located specifically in lateral hypothalamus (LH), are involved in reward processing and addictive behaviors. We found that Fos expression in LH orexin neurons varied in proportion to preference for morphine, cocaine or food. This relationship obtained both in drug naïve rats and in animals during protracted morphine withdrawal, when drug preference was elevated but food preference was decreased. Recent studies showed that LH orexin neurons that project to ventral tegmental area (VTA) have greater Fos induction in association with elevated morphine preference during protracted withdrawal than non-VTA-projecting orexin neurons, indicating that the VTA is an important site of action for orexin's role in reward processing. In addition, we found that stimulation of LH orexin neurons, or microinjection of orexin into VTA, reinstated an extinguished morphine preference. Most recently, using a self-administration paradigm we discovered that the Ox1 receptor antagonist SB-334867 (SB) blocks cocaine-seeking induced by discrete or contextual cues, but not by a priming injection of cocaine. Neurophysiological studies revealed that locally applied orexin often augmented responses of VTA dopamine (DA) neurons to activation of the medial prefrontal cortex (mPFC), consistent with the view that orexin facilitates activation of VTA DA neurons by stimulus-reward associations. We also recently showed that orexin in VTA is necessary for learning a morphine place preference. These findings are consistent with results from others showing that orexin facilitates glutamate-mediated responses, and is necessary for glutamate-dependent long-term potentiation, in VTA DA neurons. We surmise from these studies that LH orexin neurons play an important role in reward processing and addiction, and that LH orexin cells are an important input to VTA for behavioral effects associated with reward-paired stimuli.

Involvement of Corticotrophin-Releasing Factor and Orexin-A in Chronic Migraine and Medication-Overuse Headache: Findings From Cerebrospinal Fluid

The study set out to investigate the role of corticotrophin-releasing factor (CRF) and orexin-A in chronic migraine (CM) and medication-overuse headache (MOH). Twenty-seven patients affected by CM and 30 with MOH were enrolled. Control CSF specimens were obtained from 20 age-matched subjects who underwent lumbar puncture for diagnostic purposes, and in all of them CSF and blood tests excluded central nervous system or systemic diseases. Orexin-A and CRF were determined by radioimmunoassay methods. Significantly higher levels of orexin-A and CRF were found in the CSF of MOH and to a lesser extent in patients with CM compared with control subjects (orexin-A: P < 0.001 and P < 0.02; CRF: P < 0.002 and P < 0.0003). A significant positive correlation was also found between CSF orexin-A values and those of CRF ( R = 0.71; P < 0.0008), monthly drug intake group ( R = 0.39; P < 0.03) and scores of a self-completion 10-item instrument to measure dependence upon a variety of substances, the Leeds Dependence Questionnaire (LDQ) in the MOH group ( R = 0.68; P < 0.0003). The significantly higher orexin-A levels found in CM and MOH can be interpreted as a compensatory response to chronic head pain or, alternatively, as an expression of hypothalamic response to stress due to chronic pain. A potential role for orexin-A in driving drug seeking in MOH patients through activation of stress pathways in the brain can also be hypothesized.

Food deprivation-like effects of neuropeptide Y on heroin self-administration and reinstatement of heroin seeking in rats

Numerous findings suggest that drug seeking and ingestive behaviors share common neurobiological mechanisms, but the relevant pathways are unknown. Dietary manipulations result in changes in endocrine the and/or neuropeptide signals, such as the hormones leptin and ghrelin, which are dynamically linked to energy balance and the regulation of feeding behavior. We have recently demonstrated that food deprivation-induced reinstatement of heroin seeking can be blocked with leptin, and others have suggested a role for ghrelin in drug-related behaviors. The feeding-relevant effects of leptin and ghrelin involve the inhibition or activation, respectively, of neuropeptide Y/agouti-related peptide (NPY/AGRP) neurons in the hypothalamus. However, the effects of NPY, a highly potent orexigenic peptide, on drug-related behaviors have not been thoroughly studied. Here we examined the effect of acute NPY administration on the rate of heroin self-administration and the reinstatement of extinguished heroin-seeking behavior. Heroin intake (0.05mg/kg/infusion) was tested using a self-administration procedure (FR-1), 10-min post-NPY injections (0.0, 4.0, and 10microg/rat, ICV). In a different group of rats, NPY-induced reinstatement (0.0, 4.0, and 10microg/rat, ICV) of extinguished heroin seeking was assessed. NPY injections increased on-going heroin self-administration, and induced a reinstatement of extinguished heroin-seeking behavior. These findings suggest that NPY can modulate the rewarding and conditioned reinforcing effects of drugs of abuse.

Repeated amphetamine administration induces Fos in prefrontal cortical neurons that project to the lateral hypothalamus but not the nucleus accumbens or basolateral amygdala

RATIONALE

The development of sensitization to amphetamine (AMPH) is dependent on increases in excitatory outflow from the medial prefrontal cortex (mPFC) to subcortical centers. These projections are clearly important for the progressive enhancement of the behavioral response during drug administration that persists through withdrawal.

OBJECTIVES

The objective of this study was to identify the mPFC subcortical pathway(s) activated by a sensitizing regimen of AMPH.

MATERIALS AND METHODS

Using retrograde labeling techniques, Fos activation was evaluated in the predominant projection pathways of the mPFC of sensitized rats after a challenge injection of AMPH.

RESULTS

There was a significant increase in Fos-immunoreactive cells in the mPFC, nucleus accumbens (NAc), basolateral amygdala (BLA), and lateral hypothalamus (LH) of rats treated repeatedly with AMPH when compared to vehicle-treated controls. The mPFC pyramidal neurons that project to the LH but not the NAc or BLA show a significant induction of Fos after repeated AMPH treatment. In addition, we found a dramatic increase in Fos-activated orexin neurons.

CONCLUSIONS

The LH, a region implicated in natural and drug reward processes, may play a role in the development and persistence of sensitization to repeated AMPH through its connections with the mPFC and possibly through its orexin neurons.

Rat meningeal and brain microvasculature pericytes co-express the vesicular glutamate transporters 2 and 3

Pericytes are small cells that are apposed to brain and meningeal microvasculature and control capillary contraction, thereby regulating local cerebral perfusion. Pericytes respond to exogenously applied glutamate in vitro and express metabotropic glutamate receptors. However, it is unclear if pericytes have the capacity to release glutamate. We therefore determined whether pericytes express vesicular glutamate transporters (VGLUTs), which are considered to be unambiguous markers of cells that use glutamate as an intercellular signaling molecule. Leptomeningeal and brain microvasculature-associated pericytes of the adult rat, as defined by the presence of NG2 proteoglycan, expressed both VGLUT2- and VGLUT3-immunoreactivity, but did not express VGLUT1. Consistent with the hypothesis that pericytes release glutamate, VGLUT2- and VGLUT3-immunoreactivities appeared to be localized to secretory vesicles. These results suggest that glutamate is released from pericytes of the leptomeninges and brain microvasculature, and demonstrate for the first time the co-localization of VGLUT2 and VGLUT3.

Systemic and nasal delivery of orexin-A (Hypocretin-1) reduces the effects of sleep deprivation on cognitive performance in nonhuman primates

Hypocretin-1 (orexin-A) was administered to sleep-deprived (30-36 h) rhesus monkeys immediately preceding testing on a multi-image delayed match-to-sample (DMS) short-term memory task. The DMS task used multiple delays and stimulus images and effectively measures cognitive defects produced by sleep deprivation (Porrino et al., 2005). Two methods of administration of orexin-A were tested, intravenous injections (2.5-10.0 microg/kg, i.v.) and a novel method developed for nasal delivery via an atomizer spray mist to the nostrils (dose estimated 1.0 microg/kg). Results showed that orexin-A delivered via the intravenous and nasal routes significantly improved performance in sleep-deprived monkeys; however, the nasal delivery method was significantly more effective than the highest dose (10 microg/kg) of intravenous orexin-A tested. The improvement in performance by orexin-A was specific to trials classified as high versus low cognitive load as determined by performance difficulty under normal testing conditions. Except for the maximum intravenous dose (10 microg/kg), neither delivery method affected task performance in alert non-sleep-deprived animals. The improved performance in sleep-deprived animals was accompanied by orexin-A related alterations in local cerebral glucose metabolism (CMRglc) in specific brain regions shown previously to be engaged by the task and impaired by sleep deprivation (Porrino et al., 2005). Consistent with the differential effects on performance, nasal delivered orexin-A produced a more pronounced reversal of sleep deprivation induced changes in brain metabolic activity (CMRglc) than intravenous orexin-A. These findings provide strong evidence for the effectiveness of intranasal orexin-A in alleviating cognitive deficits produced by loss of sleep.

Orexin/hypocretin modulation of the basal forebrain cholinergic system: insights from in vivo microdialysis studies

Since its discovery less than a decade ago, interest in the hypothalamic orexin/hypocretin system has blossomed due to the diversity and importance of the roles played by these neuropeptides. Orexin neurons have widespread projections throughout the central nervous system and intense research has focused on elucidating the pathways and mechanisms by which orexins exert their diverse array of functions. Our group has recently focused on orexin inputs to the basal forebrain cholinergic system, which plays a crucial role in cognitive--particularly attentional--function. Orexin cells provide a robust input to cholinergic neurons in the basal forebrain and act here to modulate cortical acetylcholine release. Orexin A also increases local glutamate release within the basal forebrain, suggesting an additional, indirect effect of orexins on basal forebrain cholinergic activity. Orexin activation of the basal forebrain cholinergic system appears to be especially relevant in the context of homeostatic challenges, such as food deprivation. Thus, orexins can stimulate cortical cholinergic transmission which, in turn, may promote the detection and selection of stimuli related to physiological needs. In this manner, orexin interactions with the basal forebrain cholinergic system are likely to form a link between arousal and attention in support of the cognitive components of motivated behavior.

Functional implications of glutamatergic projections to the ventral tegmental area

Glutamatergic afferents of the ventral tegmental area (VTA) play an important role in the functioning of the VTA and are involved in the pathophysiology of drug addiction. It has recently been demonstrated that the VTA is densely innervated by glutamatergic axons and that glutamatergic neurons projecting to the VTA are situated in almost all structures that project there. While the projection from the prefrontal cortex is essentially entirely glutamatergic, subcortical glutamatergic neurons innervating the VTA intermingle with non-glutamatergic, most likely GABAergic and/or peptidergic VTA-projecting neurons. The first part of this review focuses on the origins and putative functional implications of various glutamatergic projections to the VTA. In the second part we consider how different neuropeptides via different mechanisms modulate glutamatergic actions in the VTA. We conclude by developing a model of how the glutamatergic afferents might together contribute to the functions of the VTA.

Neuropeptidergic mediators of spontaneous physical activity and non-exercise activity thermogenesis

Lean individuals have high levels of spontaneous physical activity (SPA) and the energy expenditure derived from that activity, termed non-exercise activity thermogenesis or NEAT, appears to protect them from obesity. Conversely, obesity in different human populations is characterized by low levels of SPA and NEAT. Like in humans, elevated SPA in rats appears to protect against obesity: obesity-resistant rats have significantly greater SPA and NEAT than obesity-prone rats. We review the literature on brain mechanisms important in mediating SPA and NEAT. The focus is on neuropeptides, including cholecystokinin, corticotropin-releasing hormone (also known as corticotropin-releasing factor), neuromedin U, neuropeptide Y, leptin, agouti-related protein, orexin-A (also known as hypocretin-1), and ghrelin. We also review information regarding interactions between these neuropeptides and dopamine, a neurotransmitter important in mediating motor function. Finally, we present evidence that elevated signaling of pathways mediating SPA and NEAT may protect against weight gain and obesity.

Orexins and orexin receptors: from molecules to integrative physiology

Recent studies have implicated the orexin system as a critical regulator of sleep/wake states, feeding behavior, and reward processes. Orexin deficiency results in narcolepsy-cataplexy in humans, dogs, and rodents, suggesting that the orexin system is particularly important for maintenance of wakefulness. Orexin agonists and antagonists are thought to be promising avenues toward the treatment of sleep disorders, eating disorders, and drug addiction. In this chapter, we discuss the current understanding of the physiological roles of orexins in regulation of arousal, sleep/wake states, energy homeostasis, and reward systems.

Orexin neuronal circuitry: role in the regulation of sleep and wakefulness

Orexin A and orexin B were initially identified as endogenous ligands for two orphan G protein-coupled receptors [104]. They were initially recognized as regulators of feeding behavior in view of their exclusive production in the lateral hypothalamic area (LHA), a region known as the feeding center, and their pharmacological activity [104,30,49,107]. Subsequently, the finding that orexin deficiency causes narcolepsy in humans and animals suggested that these hypothalamic neuropeptides play a critical role in regulating sleep/wake cycle [22,46,71,95,117]. These peptides activate waking-active monoaminergic and cholinergic neurons in the hypothalamus/brain stem regions to maintain a long, consolidated awake period. Recent studies on efferent and afferent systems of orexin neurons, and phenotypic characterization of genetically modified mice in the orexin system further suggested roles of orexin in the coordination of emotion, energy homeostasis, reward system, and arousal [3,80,106,137]. A link between the limbic system and orexin neurons might be important for increasing vigilance during emotional stimuli. Orexin neurons are also regulated by peripheral metabolic cues, including ghrelin, leptin, and glucose, suggesting that they might have important roles as a link between energy homeostasis and vigilance states [137]. Recent research has also implicated orexins in reward systems and the mechanisms of drug addiction [13,48,91]. These observations suggest that orexin neurons sense the outer and inner environment of the body, and maintain proper wakefulness of animals for survival. This review discusses the mechanism by which orexins maintain sleep/wakefulness states, and how this mechanism relates to other systems that regulate emotion, reward, and energy homeostasis.

A selective role for neuronal activity regulated pentraxin in the processing of sensory-specific incentive value

Neuronal activity regulated pentraxin (Narp) is a secreted neuronal product which clusters AMPA receptors and regulates excitatory synaptogenesis. Although Narp is selectively enriched in brain, its role in behavior is not known. As Narp is expressed prominently in limbic regions, we examined whether Narp deletion affects performance on tasks used to assess motivational consequences of food-rewarded learning. Narp knock-out (KO) mice were unimpaired in learning simple pavlovian discriminations, instrumental lever pressing, and in acquisition of at least two aspects of pavlovian incentive learning, conditioned reinforcement and pavlovian-instrumental transfer. In contrast, Narp deletion resulted in a substantial deficit in the ability to use specific outcome expectancies to modulate instrumental performance in a devaluation task. In this task, mice were trained to respond on two levers for two different rewards. After training, mice were prefed with one of the two rewards, devaluing it. Responding on both levers was then assessed in extinction. Whereas control mice showed a significant preference in responding on the lever associated with the nondevalued reward, Narp KO mice responded equally on both levers, failing to suppress responding on the lever associated with the devalued reward. Both groups consumed more of the nondevalued reward in a subsequent choice test, indicating Narp KO mice could distinguish between the rewards themselves. These data suggest Narp has a selective role in processing sensory-specific information necessary for appropriate devaluation performance, but not in general motivational effects of reward-predictive cues on performance.

The endocannabinoid system in the regulation of cardiometabolic risk factors

Obesity has increased at a striking rate over the last 3 decades in the Western world. This negative trend dramatically affects physical health and, ultimately, cardiovascular risks. In fact, particularly at the visceral level, obesity is strongly associated with an increased risk for life-threatening conditions, such as type 2 diabetes mellitus, hypertension, dyslipidemia, and cardiovascular disease. Although nutritional changes and physical activity are commonly thought of as the core treatments for obesity, it is necessary to further support obese patients with a pharmacologic approach for 2 reasons: to reduce the metabolic risk profile, and to avoid the regaining of weight. Among the various pharmacologic targets explored in recent years, the endocannabinoid (EC) system now constitutes the most promising proposal so far. In this review, after focusing on the central and peripheral signaling pathways that preserve energy homeostasis, we review the role of the EC system in regulating food's rewarding properties, controlling caloric intake by acting in hypothalamic pathways, and in modulating metabolic functions of several peripheral organs. In addition, we provide evidence that supports the recently proposed hypothesis that a close association exists between obesity and overactivation of the EC system.

Role of orexin receptors in the nucleus accumbens in dopamine-dependent turning behaviour of rats

The role of orexin receptors in the nucleus accumbens shell in rat turning behaviour of rats was studied. Unilateral injection of neither the orexin 1 and 2 receptor agonist orexin A (2 microg) nor the orexin 1 receptor antagonist SB 334867 (20 ng) into the nucleus accumbens shell elicited turning behaviour. Unilateral injection of a mixture of dopamine D(1) (SKF 38393) and D2 (quinpirole) receptor agonists into the nucleus accumbens shell has been found to elicit contraversive pivoting. Orexin A (1 and 2 microg) dose-dependently potentiated the contraversive pivoting induced by a mixture of SKF 38393 (1 microg) and quinpirole (10 microg) injected into the nucleus accumbens shell whereas SB 334867 (10 and 20 ng) did not significantly affect the pivoting. The potentiating effect of orexin A (2 microg) on the dopaminergic pivoting was not significantly inhibited by SB 334867 (10 and 20 ng) injected into the nucleus accumbens shell. The contraversive pivoting induced by a mixture of SKF 38393 (1 microg) and quinpirole (10 microg) injected into the nucleus accumbens shell was also potentiated by the orexin 2 receptor agonist orexin B (0.5, 1 and 2 microg), which alone did not elicit turning behaviour. These results suggest that orexin 2 receptors in the nucleus accumbens shell play a modulatory role in rat turning behaviour.

Addiction and arousal: the hypocretin connection

The hypocretins, also known as orexins, are two neuropeptides now commonly described as critical components to maintain and regulate the stability of arousal. Several lines of evidence have raised the hypothesis that hypocretin-producing neurons are part of the circuitries that mediate the hypothalamic response to acute stress. Intracerebral administration of hypocretin leads to a dose-related reinstatement of drug and food seeking behaviors. Furthermore, stress-induced reinstatement can be blocked with hypocretin receptor 1 antagonism. These results, together with recent data showing that hypocretin is critically involved in cocaine sensitization through the recruitment of NMDA receptors in the ventral tegmental area, strongly suggest that activation of hypocretin neurons play a critical role in the development of the addiction process. The activity of hypocretin neurons may affect addictive behavior by contributing to brain sensitization or by modulating the brain reward system. Hypocretinergic cells, in coordination with brain stress systems may lead to a vulnerable state that facilitates the resumption of drug seeking behavior. Hence, the hypocretinergic system is a new drug target that may be used to prevent relapse of drug seeking.

Synaptic plasticity and addiction

Addiction is caused, in part, by powerful and long-lasting memories of the drug experience. Relapse caused by exposure to cues associated with the drug experience is a major clinical problem that contributes to the persistence of addiction. Here we present the accumulated evidence that drugs of abuse can hijack synaptic plasticity mechanisms in key brain circuits, most importantly in the mesolimbic dopamine system, which is central to reward processing in the brain. Reversing or preventing these drug-induced synaptic modifications may prove beneficial in the treatment of one of society's most intractable health problems.

The orexins/hypocretins and schizophrenia

Advances in molecular biology have led to new peptides and proteins being discovered on a regular basis, including the isolation of a number of neurotransmitter candidates. Rarely, however, do these immediately capture the attention of the scientific community. The isolation and characterization of the orexin/hypocretin peptides a decade ago resulted in a slew of studies that have helped clarified their diverse functions, including prominent roles in arousal and appetitive behavior. A number of recent studies have detailed the role of the orexins/hypocretins in attention and cognition and uncovered an involvement in schizophrenia and the mechanisms of action of antipsychotic drugs (APDs). This issue of Schizophrenia Bulletin presents several articles that review our current understanding and point to future directions for the study of the orexins/hypocretins in schizophrenia and APD actions.

Schizophrenia, hypocretin (orexin), and the thalamocortical activating system

Diminished connectivity between midline-intralaminar thalamic nuclei and prefrontal cortex has been suggested to contribute to cognitive deficits that are detectable even in early stages of schizophrenia. The midline-intralaminar relay cells comprise the final link in the ascending arousal pathway and are selectively excited by the wake-promoting peptides hypocretin 1 and 2 (orexin A and B). This excitation occurs both at the level of the relay cell bodies and their axon terminals within prefrontal cortex. In rat brain slices, the release of glutamate from midline-intralaminar thalamocortical terminals induces excitatory postsynaptic currents (EPSCs) in layer V pyramidal cells in prefrontal cortex. When hypocretin is infused into medial prefrontal cortex of behaving animals, it improves performance in a complex cognitive task requiring divided attention. Chronic restraint stress causes atrophy of the apical dendritic arbors in layer V prefrontal pyramidal cells and leads to a reduction in hypocretin-induced EPSCs, indicating impairment in excitatory thalamocortical transmission. Thus, taken together with evidence for an underlying loss of excitatory thalamocortical connectivity in schizophrenia, stress in this illness could further exacerbate a breakdown in cortical processing of incoming information from the ascending arousal system.

The orexin-1 antagonist SB-334867 blocks antipsychotic treatment emergent catalepsy: implications for the treatment of extrapyramidal symptoms

We have previously shown that the orexin-1 antagonist SB-334867 blocks the electrophysiological effects of haloperidol and olanzapine on the activity of A9 and A10 dopamine neurons. To evaluate if orexin-1 antagonists might block other effects of antipsychotic drugs in animals, we examined the effects of SB-334867 on behavioral, neurochemical, and neuroendocrine effects of antipsychotic drugs. Pretreatment with SB-334867 (0.01-10 mg/kg, intraperitoneal [IP]) significantly decreased the catalepsy produced by the administration of haloperidol (1 mg/kg, subcutaneous [SC]), risperidone (2 mg/kg, SC), and olanzapine (10 mg/kg, SC). Administration of SB-334467 also reversed catalepsy after it had been established in animals pretreated 2 hours earlier with haloperidol. However, pretreatment with SB-334867 (1-10 mg/kg, IP) did not block the decreases in exploratory locomotor activity produced by administration of haloperidol (0.1 mg/kg, SC) or risperidone (0.3 mg/kg, SC). In addition, pretreatment with SB-334867 (1-10 mg/kg, IP) neither blocked the increased levels of dihydroxyphenylacetic acid (DOPAC) in the nucleus accumbens or striatum nor the elevation in serum prolactin produced by administration of haloperidol (0.1 mg/kg, SC) and risperidone (1 mg/kg, SC). Administration of SB-334867 alone neither changed locomotor activity and DOPAC or prolactin levels nor produced catalepsy. These results show that orexin-1 antagonists block the catoleptogenic effects of antipsychotics but do not block other locomotor, neurochemical, or neuroendocrine effects of antipsychotics. Because catalepsy is thought to be a good predictor of extrapyramidal symptoms in humans, treatment with orexin-1 antagonists might decrease the occurrence or severity of antipsychotic treatment-emergent extrapyramidal symptoms in humans.

Pro-opiomelanocortin modulates the thermogenic and physical activity responses to high-fat feeding and markedly influences dietary fat preference

Complete proopiomelanocortin (POMC) deficiency causes a human syndrome of hypoadrenalism, altered skin and hair pigmentation, and severe hyperphagic obesity. Heterozygote carriers of nonsense mutations are strongly predisposed to obesity. Pomc(+/-) mice have normal body weight on a chow diet but increase food intake and become more obese than wild-type littermates when placed on a high-fat diet. To further explore the mechanisms whereby dietary fat interacts with Pomc genotype to produce obesity, we examined Pomc-null, Pomc(+/-), and wild-type mice for changes in the components of energy balance in response to provision of a high-fat diet and macronutrient preference when presented with a selection of dietary choices. In contrast to wild-type mice, Pomc null mice did not increase their resting energy expenditure or their spontaneous physical activity when given a high-fat diet. Pomc(+/-) mice increased resting energy expenditure similarly to wild types, but their increase in physical activity was significantly less than that seen in wild-type mice. In two independent experimental tests of macronutrient preference, Pomc genotype was a strong predictor of dietary fat preference with Pomc null animals choosing to eat approximately twice as much fat, but similar amounts of carbohydrate and protein, as wild-type animals. Pomc(+/-) mice showed an intermediate response. In summary, POMC-derived peptides have influences on multiple aspects of the organism's response to the presentation of high-fat diet. This includes a major influence, readily discernible even in heterozygote animals, on the dietary preference for fat.

Activation of afferents to the ventral tegmental area in response to acute amphetamine: a double-labelling study

The ventral tegmental area (VTA), primary source of the mesocorticolimbic dopaminergic system, is regarded as a critical site for initiation of behavioural sensitization to psychostimulants. The present study was undertaken to identify the neural pathways converging on the VTA that are potentially implicated in this process. Rats were sensitized by a single exposure to amphetamine (5 mg/kg, s.c.). The distribution of VTA-projecting neurons activated by amphetamine was examined by combining retrograde transport of the cholera toxin beta subunit (CTb), injected into the VTA, with immunodetection of Fos. The quantitative analysis of CTb-Fos double labelling demonstrates that amphetamine induced a rapid activation of Fos in a large number of brain areas projecting to the VTA. More than half of the CTb-Fos double-labelled neurons were located in the prefrontal cortex, lateral preoptic area-lateral hypothalamus, pontomesencephalic tegmentum, dorsal raphe nucleus, ventral pallidum and nucleus accumbens. In addition, scattered CTb-Fos double-labelled cells were observed in many other VTA afferent structures, such as claustrum, lateral septum, diagonal band-magnocellular preoptic nucleus, deep mesencephalic nucleus, oral part of pontine reticular nucleus and dorsomedial tegmental area. This suggests that systemic amphetamine activates a wide population of neurons projecting to the VTA that may be important for the modulation of neurobehavioural plasticity produced by this psychostimulant.

Orexin axons in the rat ventral tegmental area synapse infrequently onto dopamine and gamma-aminobutyric acid neurons

Cells in the ventral tegmental area (VTA) facilitate motivated behaviors, and the activity of VTA neurons is regulated by dense projections from the lateral hypothalamic area (LHA). Orexin (Orx) neurons in the lateral and perifornical hypothalamus play important roles in arousal, feeding, and energy metabolism. Orx cells contribute substantially to the LHA projection to the rat midbrain. However, the morphological features of Orx fibers in the VTA and whether they synapse onto dopamine (DA) or gamma-aminobutyric acid (GABA) neurons have not yet been investigated. We utilized immunoperoxidase and immunogold-silver staining to examine the morphological features and synaptic incidence of Orx-labeled axons in the VTA. We then combined immunoperoxidase labeling for Orx with immunogold-silver labeling for GABA or for tyrosine hydroxylase (TH) in DA neurons. Electron microscopic analysis revealed that most Orx-labeled axons in the VTA were passing fibers. The less common Orx varicosities were occasionally apposed to TH- or GABA-labeled dendrites without synapsing. Only a small proportion of Orx-positive axons synapsed onto dendrites or soma. The synapses included both asymmetric and symmetric types and targeted TH- and GABA-labeled profiles with equal frequency. These findings suggest that most Orx fibers in the VTA are axons passing to caudal brainstem structures. However, Orx does mediate some direct synaptic influence on VTA DA and GABA neurons. Additional nonsynaptic effects are suggested by the presence of numerous dense-cored vesicles. These studies have important implications for understanding the mechanisms whereby Orx can alter behavior through regulating VTA DA and GABA cell activity.

Orexin signaling in the ventral tegmental area is required for high-fat appetite induced by opioid stimulation of the nucleus accumbens

The overriding of satiety and homeostatic control mechanisms by cognitive, rewarding, and emotional aspects of palatable foods may contribute to the evolving obesity crisis, but little is known about neural pathways and mechanisms responsible for crosstalk between the "cognitive" and "metabolic" brain in the control of appetite. Here we show that neural connections between the nucleus accumbens and hypothalamus might be part of this link. Using the well known model of selective stimulation of high-fat intake induced by intra-accumbens injection of the mu-opioid receptor agonist D-Ala2-N-Me-Phe4-gly5-ol-enkephalin (DAMGO), we demonstrate that orexin signaling in the ventral tegmental area is important for this reward-driven appetite to override metabolic repletion signals in presatiated rats. We further show that accumbens DAMGO in the absence of food selectively increases the proportion of orexin neurons expressing c-Fos in parts of the perifornical hypothalamus and that neural projections originating in DAMGO-responsive sites of the nucleus accumbens make close anatomical contacts with hypothalamic orexin neurons. These findings suggest that direct accumbens-hypothalamic projections can stimulate hypothalamic orexin neurons, which in turn through orexin-1 receptor signaling in the ventral tegmental area and possibly other sites interfaces with the motivational and motor systems to increase intake of palatable food.

Ventral tegmental area neurons in learned appetitive behavior and positive reinforcement

Ventral tegmental area (VTA) neuron firing precedes behaviors elicited by reward-predictive sensory cues and scales with the magnitude and unpredictability of received rewards. These patterns are consistent with roles in the performance of learned appetitive behaviors and in positive reinforcement, respectively. The VTA includes subpopulations of neurons with different afferent connections, neurotransmitter content, and projection targets. Because the VTA and substantia nigra pars compacta are the sole sources of striatal and limbic forebrain dopamine, measurements of dopamine release and manipulations of dopamine function have provided critical evidence supporting a VTA contribution to these functions. However, the VTA also sends GABAergic and glutamatergic projections to the nucleus accumbens and prefrontal cortex. Furthermore, VTA-mediated but dopamine-independent positive reinforcement has been demonstrated. Consequently, identifying the neurotransmitter content and projection target of VTA neurons recorded in vivo will be critical for determining their contribution to learned appetitive behaviors.

Neural mechanisms underlying obesity and drug addiction

Increasing rates of obesity have alarmed health officials and prompted much public dialogue. While the factors leading to obesity are numerous, an inability to control intake of freely available food is central to the problem. In order to understand this, we need to better define the mechanisms by which the brain regulates food intake, and why it is often difficult to control consumption. From this point of view, it seems valuable to consider the commonalities between food intake and drug abuse. While research in the two fields has historically emphasized different neural substrates, recent data have increased interest in better defining elements that may underlie both drug addiction and obesity. Here we discuss some of these shared elements with an emphasis on emerging areas of research that better define common mechanisms leading to overconsumption.

Long-lasting up-regulation of orexin receptor type 2 protein levels in the rat nucleus accumbens after chronic cocaine administration

Hypothalamic orexin (hypocretin) neurons project to the key structures of the limbic system and orexin receptors, both orexin receptor type 1 (OXR1) and type 2 (OXR2), are expressed in most limbic regions. Emerging evidence suggests that orexin is among important neurotransmitters that regulate addictive properties of drugs of abuse. In this study, we examined the effect of psychostimulant cocaine on orexin receptor protein abundance in the rat limbic system in vivo. Intermittent administration of cocaine (20 mg/kg, i.p., once daily for 5 days) caused a typical behavioral sensitization response to a challenge cocaine injection at a 14-day withdrawal period. Repeated cocaine administration at the same withdrawal time also increased OXR2 protein levels in the nucleus accumbens while repeated cocaine had no effect on OXR1 and orexin neuropeptide (both orexin-A and orexin-B) levels in this region. In contrast to the nucleus accumbens, OXR2 levels in the frontal cortex, the ventral tegmental area, the hippocampus, and the dorsal striatum (caudate putamen) were not altered by cocaine. Remarkably, the up-regulated OXR2 levels in the nucleus accumbens showed a long-lasting nature as it persisted up to 60 days after the discontinuation of repeated cocaine treatments. In contrast to chronic cocaine administration, an acute cocaine injection was insufficient to modify levels of any orexin receptor and peptide. Our data identify the up-regulation of OXR2 in the nucleus accumbens as an enduring molecular event that is correlated well with behavioral plasticity in response to chronic psychostimulant administration. This OXR2 up-regulation may reflect a key adaptation of limbic orexinergic transmission to chronic drug exposure and may thus be critical for the expression of motor plasticity.

The hypothalamic peptidergic system, hypocretin/orexin and vigilance control

Using forward and reverse genetics, the genes (hypocretin/orexin ligand and its receptor) involved in the pathogenesis of the sleep disorder, narcolepsy, in animals, have been identified. Mutations in hypocretin related-genes are extremely rare in humans, but hypocretin-ligand deficiency is found in most narcolepsy-cataplexy cases. Hypocretin deficiency in humans can be clinically detected by CSF hypocretin-1 measures, and undetectably low CSF hypocretin-1 is now included in the revised international diagnostic criteria of narcolepsy. Since hypocretin-ligand deficiency is the major pathophysiology in human narcolepsy, hypocretin replacements (using hypocretin agonists or gene therapy) are promising future therapeutic options. New insights into the roles of hypocretin system on sleep physiology have also rapidly increased. Hypocretins are involved in various fundamental hypothalamic functions such as feeding, energy homeostasis and neuroendocrine regulation. Hypocretin neurons project to most ascending arousal systems (including monoaminergic and cholinergic systems), and generally exhibit excitatory inputs. Together with the recent finding of the sleep promoting system in the hypothalamus (especially in the GABA/galanin ventrolateral preoptic area which exhibits inhibitory inputs to these ascending systems), the hypothalamus is now recognized as the most important brain site for the sleep switch, and other peptidergic systems may also participate in this regulation. Meanwhile, narcolepsy now appears to be a more complex condition than previously thought. The pathophysiology of the disease is involved in the abnormalities of sleep and various hypothalamic functions due to hypocretin deficiency, such as the changes in energy homeostasis, stress reactions and rewarding. Narcolepsy is therefore, an important model to study the link between sleep regulation and other fundamental hypothalamic functions.

Glutamate synaptic inputs to ventral tegmental area neurons in the rat derive primarily from subcortical sources

Dopamine and GABA neurons in the ventral tegmental area project to the nucleus accumbens and prefrontal cortex and modulate locomotor and reward behaviors as well as cognitive and affective processes. Both midbrain cell types receive synapses from glutamate afferents that provide an essential control of behaviorally-linked activity patterns, although the sources of glutamate inputs have not yet been completely characterized. We used antibodies against the vesicular glutamate transporter subtypes 1 and 2 (VGlut1 and VGlut2) to investigate the morphology and synaptic organization of axons containing these proteins as putative markers of glutamate afferents from cortical versus subcortical sites, respectively, in rats. We also characterized the ventral tegmental area cell populations receiving VGlut1+ or VGlut2+ synapses according to their transmitter phenotype (dopamine or GABA) and major projection target (nucleus accumbens or prefrontal cortex). By light and electron microscopic examination, VGlut2+ as opposed to VGlut1+ axon terminals were more numerous, had a larger average size, synapsed more proximally, and were more likely to form convergent synapses onto the same target. Both axon types formed predominantly asymmetric synapses, although VGlut2+ terminals more often formed synapses with symmetric morphology. No absolute selectivity was observed for VGlut1+ or VGlut2+ axons to target any particular cell population. However, the synapses onto mesoaccumbens neurons more often involved VGlut2+ terminals, whereas mesoprefrontal neurons received relatively equal synaptic inputs from VGlut1+ and VGlut2+ profiles. The distinct morphological features of VGlut1 and VGlut2 positive axons suggest that glutamate inputs from presumed cortical and subcortical sources, respectively, differ in the nature and intensity of their physiological actions on midbrain neurons. More specifically, our findings imply that subcortical glutamate inputs to the ventral tegmental area expressing VGlut2 predominate over cortical sources of excitation expressing VGlut1 and are more likely to drive the behaviorally-linked bursts in dopamine cells that signal future expectancy or attentional shifting.

Lateral hypothalamic orexin neurons are critically involved in learning to associate an environment with morphine reward

Previously, we reported that lateral hypothalamic (LH) orexin neurons are stimulated in proportion to the preference shown for reward-associated cues during conditioned place preference (CPP) testing. Here, we examine for the first time the role of these neurons in the acquisition of morphine CPP. Results show that LH orexin neurons, but not those in the perifornical area (PFA), are stimulated during conditioning when morphine is given in a novel drug-paired environment (CPP compartment) but not when given in the home cage, nor when saline was given in the CPP environment. Furthermore, bilateral excitotoxic lesions of the LH orexin area completely blocked the acquisition of morphine CPP. Lesions that spared LH orexin neurons had no effect. Orexin neurons in the LH project to the ventral tegmental area (VTA), an area important in the acquisition of morphine CPP. Therefore, we investigated the importance of the LH orexin connection to the VTA in the acquisition of a morphine CPP using a disconnection technique involving a unilateral excitotoxic lesion of LH orexin neurons and contralateral blockade of VTA orexin receptors. Results indicated that a unilateral LH orexin lesion together with a microinjection of the orexin A antagonist (SB 334867) into the contralateral VTA prior to each morphine-pairing session was sufficient to block the development of a morphine CPP. Either of these treatments by themselves was not sufficient to block CPP development. These results demonstrate the importance of LH orexin neurons and their projections to the VTA in the formation of associations between environmental cues and drug reward.

Implication of protein kinase C in the orexin-induced elevation of extracellular dopamine levels and its rewarding effect

In the present study, we investigated the role of orexinergic systems in the activation of midbrain dopamine neurons. In an in vitro study, exposure to either orexin A or orexin B under superfusion conditions produced a transient increase in the intracellular Ca(2+) concentration through the phospholipase C (PLC)/protein kinase C (PKC) pathway via G(q11)alpha or Gbetagamma subunits in midbrain cultured neurons, which were shown to be tyrosine hydroxylase (TH)-positive cells, but not in purified midbrain astrocytes. Here we show that in vivo injection with a selective PKC inhibitor chelerythrine chloride or 2-{8-[(dimethylamino)methyl]-6,7,8,9-tetrahydropyrido[1,2-a]indol-3-yl}-3-1-methyl-1H-indol-3-ylmaleimide HCl (Ro-32-0432) into the ventral tegmental area (VTA) significantly suppressed the place preference and increased levels of dopamine in the nucleus accumbens (NAcc) induced by intra-VTA injection of orexins. These results strongly support the idea that activation of the orexin-containing neuron in the VTA leads to the direct activation of mesolimbic dopamine neurons through the activation of the PLC/PKC pathway via G(q11)alpha or Gbetagamma-subunit activation, which could be associated with the development of its rewarding effect.

The orexin-1 receptor antagonist SB-334867 blocks the effects of antipsychotics on the activity of A9 and A10 dopamine neurons: implications for antipsychotic therapy

Antipsychotic drugs alter the activity of dopamine neurons in the ventral tegmental area (A10) and substantia nigra pars compacta (A9). As there is a dense projection of orexin neurons from the lateral hypothalamus to A10 dopaminergic neurons, and some antipsychotics have been shown to increase the expression of c-fos in orexin-containing cells in the hypothalamus, we hypothesized that stimulation of orexin receptors plays a role in the effects of antipsychotics on the activity of A9 and A10 dopamine cells. Single-unit recordings in anesthetized rats demonstrated the central effects of the selective orexin-1 receptor antagonist SB-334867 (2 mg/kg, intravenous), as it reversed the excitatory effects of orexin-A administration (6 microg, intracerebroventricular) on the activity of locus coeruleus (LC) cells. Recordings from midbrain dopamine neurons showed that acute administration of SB-334867 alone did not alter the number of spontaneously active A9 or A10 cells, but did reverse: (1) the increase in the number of spontaneously active A9 and/or A10 dopamine cells caused by the acute administration of haloperidol (1 mg/kg, subcutaneous) or olanzapine (10 mg/kg, s.c.) and (2) the decrease in the number of spontaneously active A9 and/or A10 dopamine cells caused by the chronic administration of haloperidol (1 mg/kg/day x 21 days, s.c.) or olanzapine (10 mg/kg/day x 21 days, s.c.). However, SB-334867 did not block a different electrophysiological effect of olanzapine, as it did not block the olanzapine-induced activation of LC cells. These results indicate that activation of orexin-1 receptors plays an important role on the effects of antipsychotic drugs on dopamine neuronal activity and may play an important role in the clinical effects of antipsychotic drugs.

A role for hypocretin (orexin) in male sexual behavior

The role of hypocretin (orexin; hcrt/orx) neurons in regulation of arousal is well established. Recently, hcrt/orx has been implicated in food reward and drug-seeking behavior. We report here that in male rats, Fos immunoreactivity (ir) in hcrt/orx neurons increases markedly during copulation, whereas castration produces decreases in hcrt/orx neuron cell counts and protein levels in a time course consistent with postcastration impairments in copulatory behavior. This effect was reversed by estradiol replacement. Immunolabeling for androgen (AR) and estrogen (ER alpha) receptors revealed no colocalization of hcrt/orx with AR and few hcrt/orx neurons expressing ER alpha, suggesting that hormonal regulation of hcrt/orx expression is via afferents from neurons containing those receptors. We also demonstrate that systemic administration of the orexin-1 receptor antagonist SB 334867 [N-(2-methyl-6-benzoxazolyl)-N''-1,5-naphthyridin-4-yl urea] impairs copulatory behavior. One locus for the prosexual effects of hcrt/orx may be the ventral tegmental area (VTA). We show here that hcrt-1/orx-A produces dose-dependent increases in firing rate and population activity of VTA dopamine (DA) neurons in vivo. Activation of hcrt/orx during copulation, and in turn, excitation of VTA DA neurons by hcrt/orx, may contribute to the robust increases in nucleus accumbens DA previously observed during male sexual behavior. Subsequent triple immunolabeling in anterior VTA showed that Fos-ir in tyrosine hydroxylase-positive neurons apposed to hcrt/orx fibers increases during copulation. Together, these data support the view that hcrt/orx peptides may act in a steroid-sensitive manner to facilitate the energized pursuit of natural rewards like sex via activation of the mesolimbic DA system.

Body weight in patients with Parkinson's disease

There is some evidence suggesting that Parkinson's disease (PD) patients exhibit lower body weight when compared to age-matched healthy subjects. Low body mass index (BMI) is correlated with low bone mineral density, both of which are major risk factors for hip fractures. Possible determinants of weight loss in PD patients include hyposmia, impaired hand-mouth coordination, difficulty chewing, dysphagia, intestinal hypomotility, depression, decreased reward processing of dopaminergic mesolimbic regions, nausea, and anorexia as the side effects of medication, and increased energy requirements due to muscular rigidity and involuntary movements. It is unclear whether PD patients in general, or only a subgroup of those affected, definitely show lower BMI in the advanced stages of the disease. We therefore recommend that the body weight of PD patients be monitored monthly as the disease progresses, and that a patient's nutrition should be supplemented with sufficient amounts of vitamin D and calcium to reduce the risk of hip fractures and strengthen bone density. Because meal times may coincide with unpredictable off periods associated with akinesia and impaired hand-mouth coordination, PD patients also need flexible food schedules that accommodate the associated symptoms of this disease.

The involvement of dopamine in the modulation of sleep and waking

Dopamine (DA)-containing neurons involved in the regulation of sleep and waking (W) arise in the ventral tegmental area (VTA) and the substantia nigra pars compacta (SNc). The VTA and SNc cells have efferent and afferent connections with the dorsal raphe nucleus (DRN), the pedunculopontine and laterodorsal tegmental nuclei (PPT/LDT), the locus coeruleus (LC), the lateral and posterior hypothalamus (LH), the basal forebrain (BFB), and the thalamus. Molecular cloning techniques have enabled the characterization of two distinct groups of DA receptors, D(1)-like and D(2)-like receptors. The D(1) subfamily includes the D(1) and D(5) receptors, whereas the D(2) subfamily comprises the D(2), D(3), and D(4) receptors. Systemic administration of a selective D(1) receptor agonist induces behavioral arousal, together with an increase of W and a reduction of slow wave sleep (SWS) and REM sleep (REMS). Systemic injection of a DA D(2) receptor agonist induces biphasic effects, such that low doses reduce W and increase SWS and REMS (predominant activation of the D(2) autoreceptor), whereas large doses induce the opposite effect (predominant facilitation of the D(2) postsynaptic receptor). Compounds with DA D(1) or D(2) receptor blocking properties augment non-REMS and reduce W. Preliminary findings tend to indicate that the administration of a DA D(3)-preferring agonist induces somnolence and sleep in laboratory animals and man. DA neurons in the VTA and the SNc do not change their mean firing rate across the sleep-wake cycle. It has been proposed that DA cells in the midbrain show a change in temporal pattern rather than firing rate during the sleep-wake cycle. The available evidence tends to indicate that during W there occurs an increase of burst firing activity of DA neurons, and an enhanced release of DA in the VTA, the nucleus accumbens (NAc), and a number of forebrain structures. A series of structures relevant for the regulation of the behavioral state, including the DRN, LDT/PPT, LC, and LH, could be partly responsible for the changes in the temporal pattern of activity of DA neurons.

Bidirectional dopaminergic modulation of excitatory synaptic transmission in orexin neurons

Orexin neurons in the lateral hypothalamus (LH)/perifornical area (PFA) are known to promote food intake as well as provide excitatory influence on the dopaminergic reward pathway. Dopamine (DA), in turn, inhibits the reward pathway and food intake through its action in the LH/PFA. However, the cellular mechanism by which DA modulates orexin neurons remains largely unknown. Therefore, we examined the effect of DA on the excitatory neurotransmission to orexin neurons. Whole-cell patch-clamp recordings were performed using acute rat hypothalamic slices, and orexin neurons were identified by their electrophysiological and immunohistochemical characteristics. Pharmacologically isolated action potential-independent miniature EPSCs (mEPSCs) were monitored. Bath application of DA induced a bidirectional effect on the excitatory synaptic transmission dose dependently. A low dose of DA (1 microM) increased mEPSC frequency, which was blocked by the D1-like receptor antagonist SCH 23390, and mimicked by the D1-like receptor agonist SKF 81297. In contrast, higher doses of DA (10-100 microM) decreased mEPSC frequency, which could be blocked with the D2-like receptor antagonist, sulpiride. Quinpirole, the D2-like receptor agonist, also reduced mEPSC frequency. None of these compounds affected the mEPSCs amplitude, suggesting the locus of action was presynaptic. Furthermore, DA (1 microM) induced an increase in the action potential firing, whereas DA (100 microM) hyperpolarized and ceased the firing of orexin neurons, indicating the effect of DA on excitatory synaptic transmission may influence the activity of the postsynaptic cell. In conclusion, our results suggest that D1- and D2-like receptors have opposing effects on the excitatory presynaptic terminals impinging onto orexin neurons.

Addiction and arousal: alternative roles of hypothalamic peptides

The importance of the lateral hypothalamus in the regulation of reward and motivation has long been recognized. However, the neuronal network involved in such a hypothalamic regulation of reward remains essentially unknown. Recently, hypocretin-containing neurons, a group of hypothalamic neurons known to be associated with the stability of arousal, have emerged as important structures in the control of brain reward function. This review summarizes a Mini-Symposium presented at the 2006 Annual Meeting of the Society for Neuroscience.

Food intake and reward mechanisms in patients with schizophrenia: implications for metabolic disturbances and treatment with second-generation antipsychotic agents

Obesity is highly prevalent among patients with schizophrenia and is associated with detrimental health consequences. Although excessive consumption of fast food and pharmacotherapy with such second-generation antipsychotic agents (SGAs) as clozapine and olanzapine has been implicated in the schizophrenia/obesity comorbidity, the pathophysiology of this link remains unclear. Here, we propose a mechanism based on brain reward function, a relevant etiologic factor in both schizophrenia and overeating. A comprehensive literature search on neurobiology of schizophrenia and of eating behavior was performed. The collected articles were critically reviewed and relevant data were extracted and summarized within four key areas: (1) energy homeostasis, (2) food reward and hedonics, (3) reward function in schizophrenia, and (4) metabolic effects of the SGAs. A mesolimbic hyperdopaminergic state may render motivational/incentive reward system insensitive to low salience/palatability food. This, together with poor cognitive control from hypofunctional prefrontal cortex and enhanced hedonic impact of food, owing to exaggerated opioidergic drive (clinically manifested as pain insensitivity), may underlie unhealthy eating habits in patients with schizophrenia. Treatment with SGAs purportedly improves dopamine-mediated reward aspects, but at the cost of increased appetite and worsened or at least not improved opiodergic capacity. These effects can further deteriorate eating patterns. Pathophysiological and therapeutic implications of these insights need further validation via prospective clinical trials and neuroimaging studies.