Lesions of orexin neurons block conditioned place preference for sexual behavior in male rats

The Orexin/Hypocretin System, the Peptidergic Regulator of Vigilance, Orchestrates Adaptation to Stress

The orexin/hypocretin neuropeptide family has emerged as a focal point of neuroscientific research following the discovery that this family plays a crucial role in a variety of physiological and behavioral processes. These neuropeptides serve as powerful neuromodulators, intricately shaping autonomic, endocrine, and behavioral responses across species. Notably, they serve as master regulators of vigilance and stress responses; however, their roles in food intake, metabolism, and thermoregulation appear complementary and warrant further investigation. This narrative review provides a journey through the evolution of our understanding of the orexin system, from its initial discovery to the promising progress made in developing orexin derivatives. It goes beyond conventional boundaries, striving to synthesize the multifaceted activities of orexins. Special emphasis is placed on domains such as stress response, fear, anxiety, and learning, in which the authors have contributed to the literature with original publications. This paper also overviews the advancement of orexin pharmacology, which has already yielded some promising successes, particularly in the treatment of sleep disorders.

Orexin neuron activity in mating mice - a pilot study

Mating behaviours affect hypothalamic orexin/hypocretin neurons and vice versa. However, activity of orexin neurons has not been recorded during mating before. We report an anecdotal dataset of freely-moving miniature microscope recordings of orexin neuron activity during mating behaviours, as well as an oral sexual encounter previously undocumented in mice. Across the orexin neuron population in the male, firing rates were maximally diverse during ejaculation, similarly diverse though weaker during intromission, and inverse to this during anterior thrusting. In the female mouse, orexin neurons tended to decrease firing during intromission after a transient increase. We provide this brief dataset for re-use, to enable further studies of these rare behaviours with challenging surgical preparations.

Instrumental and Pavlovian Mechanisms in Alcohol Use Disorder

Purpose of Review

Current theories of alcohol use disorders (AUD) highlight the importance of Pavlovian and instrumental learning processes mainly based on preclinical animal studies. Here, we summarize available evidence for alterations of those processes in human participants with AUD with a focus on habitual versus goal-directed instrumental learning, Pavlovian conditioning, and Pavlovian-to-instrumental transfer (PIT) paradigms.

Recent Findings

The balance between habitual and goal-directed control in AUD participants has been studied using outcome devaluation or sequential decision-making procedures, which have found some evidence of reduced goal-directed/model-based control, but little evidence for stronger habitual responding. The employed Pavlovian learning and PIT paradigms have shown considerable differences regarding experimental procedures, e.g., alcohol-related or conventional reinforcers or stimuli.

Summary

While studies of basic learning processes in human participants with AUD support a role of Pavlovian and instrumental learning mechanisms in the development and maintenance of drug addiction, current studies are characterized by large variability regarding methodology, sample characteristics, and results, and translation from animal paradigms to human research remains challenging. Longitudinal approaches with reliable and ecologically valid paradigms of Pavlovian and instrumental processes, including alcohol-related cues and outcomes, are warranted and should be combined with state-of-the-art imaging techniques, computational approaches, and ecological momentary assessment methods.

Involvement of orexin/hypocretin in the expression of social play behaviour in juvenile rats

Social play is a highly rewarding and motivated behaviour displayed by juveniles of many mammalian species. We hypothesized that the orexin/hypocretin (ORX) system is involved in the expression of juvenile social play behaviour because this system is interconnected with brain regions that comprise the social behaviour and mesocorticolimbic reward networks. We found that exposure to social play increased recruitment of ORX-A neurons in juvenile rats. Furthermore, central administration of ORX-A decreased social play duration, while central blockade of ORX-1 receptors differentially altered social play duration in juvenile rats with low versus high baseline levels of social play (increasing social play in low baseline social play individuals and decreasing social play in high baseline social play individuals). Together, our results provided the first evidence of a role for the ORX system in the modulation of juvenile social play behaviour.

Orexins and stress

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

Hypocretin as a Hub for Arousal and Motivation

The lateral hypothalamus is comprised of a heterogeneous mix of neurons that serve to integrate and regulate sleep, feeding, stress, energy balance, reward, and motivated behavior. Within these populations, the hypocretin/orexin neurons are among the most well studied. Here, we provide an overview on how these neurons act as a central hub integrating sensory and physiological information to tune arousal and motivated behavior accordingly. We give special attention to their role in sleep-wake states and conditions of hyper-arousal, as is the case with stress-induced anxiety. We further discuss their roles in feeding, drug-seeking, and sexual behavior, which are all dependent on the motivational state of the animal. We further emphasize the application of powerful techniques, such as optogenetics, chemogenetics, and fiber photometry, to delineate the role these neurons play in lateral hypothalamic functions.

Hypocretin/orexin knock-out mice display disrupted behavioral and dopamine responses to cocaine

The hypocretin/orexin (HCRT) system is implicated in reward and reinforcement processes through actions on the mesolimbic dopamine (DA) system. Here we provide evidence for the relationship between HCRT and DA in vivo in anesthetized and freely moving mice. The ability of cocaine to elicit reward-related behaviors in mice lacking the HCRT prepro-peptide (HCRT knock-out; KO) and wild-type controls was determined using conditioned place preference. Using a combination of microdialysis and in vivo fast scan cyclic voltammetry in anesthetized and freely moving mice, we investigated the underlying role of HCRT in the regulation of DA release and uptake. We show that, unlike wild-type mice, HCRT KO mice fail to develop characteristic conditioned place preference for cocaine. These mice also demonstrated reduced DA release and uptake under baseline conditions in both anesthetized and freely moving experiments. Further, diminished DA signaling in HCRT KO mice persists following administration of cocaine. These findings indicate that HCRT is essential for the expression of behaviors associated with the rewarding effects of cocaine, and suggest that HCRT regulation of reward and reinforcement may be related to disruptions to DA neurotransmission.

Orexin/Hypocretin System: Role in Food and Drug Overconsumption

The neuropeptide orexin/hypocretin (OX), while largely transcribed within the hypothalamus, is released throughout the brain to affect complex behaviors. Primarily through the hypothalamus itself, OX homeostatically regulates adaptive behaviors needed for survival, including food intake, sleep-wake regulation, mating, and maternal behavior. However, through extrahypothalamic limbic brain regions, OX promotes seeking and intake of rewarding substances of abuse, like palatable food, alcohol, nicotine, and cocaine. This neuropeptide, in turn, is stimulated by the intake of or early life exposure to these substances, forming a nonhomeostatic, positive feedback loop. The specific OX receptor involved in these behaviors, whether adaptive behavior or substance seeking and intake, is dependent on the particular brain region that contributes to them. Thus, we propose that, while the primary function of OX is to maintain arousal for the performance of adaptive behaviors, this neuropeptide system is readily co-opted by rewarding substances that involve positive feedback, ultimately promoting their abuse.

The role of ghrelin signalling for sexual behaviour in male mice

Ghrelin, a gut-brain signal, is well known to regulate energy homeostasis, food intake and appetite foremost via hypothalamic ghrelin receptors (GHS-R1A). In addition, ghrelin activates the reward systems in the brain, namely the mesolimbic dopamine system, and regulates thereby the rewarding properties of addictive drugs as well as of palatable foods. Given that the mesolimbic dopamine system mandates the reinforcing properties of addictive drugs and natural rewards, such as sexual behaviour, we hypothesize that ghrelin plays an important role for male sexual behaviour, a subject for the present studies. Herein we show that ghrelin treatment increases, whereas pharmacological suppression (using the GHSR-1A antagonist JMV2959) or genetic deletion of the GHS-R1A in male mice decreases the sexual motivation for as well as sexual behaviour with female mice in oestrus. Pre-treatment with L-dopa (a dopamine precursor) prior to treatment with JMV2959 significantly increased the preference for female mouse compared with vehicle treatment. On the contrary, treatment with 5-hydroxythyptohan (a precursor for serotonin) prior to treatment with JMV2959 decreased the sexual motivation compared to vehicle. In separate experiments, we show that ghrelin and GHS-R1A antagonism do not affect the time spent over female bedding as measured in the androgen-dependent bedding test. Collectively, these data show that the hunger hormone ghrelin and its receptor are required for normal sexual behaviour in male mice and that the effects of the ghrelin signalling system on sexual behaviour involve dopamine neurotransmission.

Hypothalamic neuropeptide signaling in alcohol addiction

The hypothalamus is now known to regulate alcohol intake in addition to its established role in food intake, in part through neuromodulatory neurochemicals termed neuropeptides. Certain orexigenic neuropeptides act in the hypothalamus to promote alcohol drinking, although they affect different aspects of the drinking response. These neuropeptides, which include galanin, the endogenous opioid enkephalin, and orexin/hypocretin, appear to stimulate alcohol intake not only through mechanisms that promote food intake but also by enhancing reward and reinforcement from alcohol. Moreover, these neuropeptides participate in a positive feedback relationship with alcohol, whereby they are upregulated by alcohol intake to promote even further consumption. They contrast with other orexigenic neuropeptides, such as melanin-concentrating hormone and neuropeptide Y, which promote alcohol intake under limited circumstances, are not consistently stimulated by alcohol, and do not enhance reward. They also contrast with neuropeptides that can be anorexigenic, including the endogenous opioid dynorphin, corticotropin-releasing factor, and melanocortins, which act in the hypothalamus to inhibit alcohol drinking as well as reward and therefore counter the ingestive drive promoted by orexigenic neuropeptides. Thus, while multiple hypothalamic neuropeptides may work together to regulate different aspects of the alcohol drinking response, excessive signaling from orexigenic neuropeptides or inadequate signaling from anorexigenic neuropeptides can therefore allow alcohol drinking to become dysregulated.

Understanding how discrete populations of hypothalamic neurons orchestrate complicated behavioral states

A major question in systems neuroscience is how a single population of neurons can interact with the rest of the brain to orchestrate complex behavioral states. The hypothalamus contains many such discrete neuronal populations that individually regulate arousal, feeding, and drinking. For example, hypothalamic neurons that express hypocretin (Hcrt) neuropeptides can sense homeostatic and metabolic factors affecting wakefulness and orchestrate organismal arousal. Neurons that express agouti-related protein (AgRP) can sense the metabolic needs of the body and orchestrate a state of hunger. The organum vasculosum of the lamina terminalis (OVLT) can detect the hypertonicity of blood and orchestrate a state of thirst. Each hypothalamic population is sufficient to generate complicated behavioral states through the combined efforts of distinct efferent projections. The principal challenge to understanding these brain systems is therefore to determine the individual roles of each downstream projection for each behavioral state. In recent years, the development and application of temporally precise, genetically encoded tools has greatly improved our understanding of the structure and function of these neural systems. This review will survey recent advances in our understanding of how these individual hypothalamic populations can orchestrate complicated behavioral states due to the combined efforts of individual downstream projections.

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

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

LINKED ARTICLES

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

The role of the lateral hypothalamus and orexin in ingestive behavior: a model for the translation of past experience and sensed deficits into motivated behaviors

The hypothalamus has been recognized for its involvement in both maintaining homeostasis and mediating motivated behaviors. The present article discusses a region of the hypothalamus known as the lateral hypothalamic area (LHA). It is proposed that brain nuclei within the LHA including the dorsal region of the lateral hypothalamus (LHAd) and perifornical area (PeF) provide a link between neural systems that regulate homeostasis and those that mediate appetitive motivated behaviors. Functional and immunohistochemical data indicate that the LHA promotes many motivated behaviors including food intake, water intake, salt intake, and sexual behavior. Anatomical tracing experiments demonstrate that the LHA is positioned to receive inputs from brain areas involved in regulating body fluid and energy homeostasis. Regions within the LHA send dense projections to the ventral tegmental area (VTA), providing a pathway for the LHA to influence dopaminergic systems generally recognized to be involved in motivated behaviors and their reinforcement. Furthermore, the LHA contains neurons that synthesize orexin/hypocretin, a neuropeptide that promotes many appetitive motivated behaviors. The LHA also receives inputs from brain areas involved in reward-related learning and orexin neuron activation can become conditioned to environmental stimuli that are associated with rewards. Therefore, it is hypothesized that the LHA integrates signaling from areas that regulate body fluid and energy balance and reward-related learning. In turn, this information is “fed into” mesolimbic circuitry to influence the performance of motivated behaviors. This hypothesis may foster experiments that will result in an improved understanding of LHA function. An improved understanding of LHA function may aid in treating disorders that are associated with an excess or impairment in the expression of ingestive behavior including obesity, anorexia, impairments in thirst, salt gluttony, and salt deficiency.

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.

Effects of sex and reproductive experience on the number of orexin A-immunoreactive cells in the prairie vole brain

Large populations of cells synthesizing the neuropeptide orexin (OX) exist in the caudal hypothalamus of all species examined and are implicated in physiological and behavioral processes including arousal, stress, anxiety and depression, reproduction, and goal-directed behaviors. Hypothalamic OX expression is sexually dimorphic in different directions in laboratory rats (F>M) and mice (M>F), suggesting different roles in male and female physiology and behavior that are species-specific. We here examined if the number of hypothalamic cells immunoreactive for orexin A (OXA) differs between male and female prairie voles (Microtus ochrogaster), a socially monogamous species that pairbonds after mating and in which both sexes care for offspring, and if reproductive experience influences their number of OXA-immunoreactive (OXA-ir) cells. It was found that the total number of OXA-ir cells did not differ between the sexes, but females had more OXA-ir cells than males in anterior levels of the caudal hypothalamus, while males had more OXA-ir cells posteriorly. Sexually experienced females sacrificed 12 days after the birth of their first litter, or one day after birth of a second litter, had more OXA-ir cells in anterior levels but not posterior levels of the caudal hypothalamus compared to females housed with a brother (incest avoidance prevents sibling mating). Male prairie voles showed no effect of reproductive experience but showed an unexpected effect of cohabitation duration regardless of mating. The sex difference in the distribution of OXA-ir cells, and their increased number in anterior levels of the caudal hypothalamus of reproductively experienced female prairie voles, may reflect a sex-specific mechanism involved in pairbonding, parenting, or lactation in this species.

The orexinergic system influences conditioned odor aversion learning in the rat: a theory on the processes and hypothesis on the circuit involved

A large variety of behaviors that are essential for animal survival depend on the perception and processing of surrounding smells present in the natural environment. In particular, food-search behavior, which is conditioned by hunger, is directly driven by the perception of odors associated with food, and feeding status modulates olfactory sensitivity. The orexinergic hypothalamic peptide orexin A (OXA), one of the central and peripheral hormones that triggers food intake, has been shown to increase olfactory sensitivity in various experimental conditions including the conditioned odor aversion learning paradigm (COA). COA is an associative task that corresponds to the association between an olfactory conditioned stimulus (CS) and a delayed gastric malaise. Previous studies have shown that this association is formed only if the delay separating the CS presentation from the malaise is short, suggesting that the memory trace of the odor is relatively unstable. To test the selectivity of the OXA system in olfactory sensitivity, a recent study compared the effects of fasting and of central infusion of OXA during the acquisition of COA. Results showed that the increased olfactory sensitivity induced by fasting and by OXA infusion was accompanied by enhanced COA learning performances. In reference to the duration of action of OXA, the present work details the results obtained during the successive COA extinction tests and suggests a hypothesis concerning the role of the OXA component of fasting on the memory processes underlying CS-malaise association during COA. Moreover, referring to previous data in the literature we suggest a functional circuit model where fasting modulates olfactory memory processes through direct and/or indirect activation of particular OXA brain targets including the olfactory bulb, the locus coeruleus (LC) and the amygdala.

Involvements of stress hormones in the restraint-induced conditioned place preference

The conditioned place preference (CPP) paradigm is widely used when examining the reinforcing effects of drugs. Some previous studies have shown that an acute stressor, such as restraint could also induce CPP. Although the modulating effects of stress hormones on various forms of learning are well known, the finding that a stressor has a potentially direct role in the reinforcement mechanism is novel. This study focused on the function of stress hormones in restraint-induced CPP in Wistar rats administered agonist or antagonist of 2 critical stress hormones prior to conditioning. Results showed that peripheral applications of corticosterone (CORT, 1, 3, 5, and 10 mg/kg, subcutaneously) failed to induce CPP. Furthermore, a glucocorticoid (GC) antagonist (mifepristone, 10, 40, or 100 mg/kg, sc) failed to block the restraint-induced CPP. Intracerebroventricular injection of a selective corticotropin-releasing factor receptor 1 (CRFR1) antagonist antalarmin (1 μg/5 μl), on the contrary, completely blocked the restraint-induced CPP. We concluded that CRFR1 plays an essential role in the neural mechanism of restraint-induced CPP. Negative feedback of CORT from peripheral sources may not be involved in this phenomenon.

Neuropeptides and central control of sexual behaviour from the past to the present: a review

Of the numerous neuropeptides identified in the central nervous system, only a few are involved in the control of sexual behaviour. Among these, the most studied are oxytocin, adrenocorticotropin, α-melanocyte stimulating hormone and opioid peptides. While opioid peptides inhibit sexual performance, the others facilitate sexual behaviour in most of the species studied so far (rats, mice, monkeys and humans). However, evidence for a sexual role of gonadotropin-releasing hormone, corticotropin releasing factor, neuropeptide Y, galanin and galanin-like peptide, cholecystokinin, substance P, vasoactive intestinal peptide, vasopressin, angiotensin II, hypocretins/orexins and VGF-derived peptides are also available. Corticotropin releasing factor, neuropeptide Y, cholecystokinin, vasopressin and angiotensin II inhibit, while substance P, vasoactive intestinal peptide, hypocretins/orexins and some VGF-derived peptide facilitate sexual behaviour. Neuropeptides influence sexual behaviour by acting mainly in the hypothalamic nuclei (i.e., lateral hypothalamus, paraventricular nucleus, ventromedial nucleus, arcuate nucleus), in the medial preoptic area and in the spinal cord. However, it is often unclear whether neuropeptides influence the anticipatory phase (sexual arousal and/or motivation) or the consummatory phase (performance) of sexual behaviour, except in a few cases (e.g., opioid peptides and oxytocin). Unfortunately, scarce information has been added in the last 15 years on the neural mechanisms by which neuropeptides influence sexual behaviour, most studied neuropeptides apart. This may be due to a decreased interest of researchers on neuropeptides and sexual behaviour or on sexual behaviour in general. Such a decrease may be related to the discovery of orally effective, locally acting type V phosphodiesterase inhibitors for the therapy of erectile dysfunction.

The hypocretins and the reward function: what have we learned so far?

A general consensus acknowledges that drug consumption (including alcohol, tobacco, and illicit drugs) constitutes the leading cause of preventable death worldwide. But the global burden of drug abuse extends the mortality statistics. Indeed, the comorbid long-term debilitating effects of the disease also significantly deteriorate the quality of life of individuals suffering from addiction disorders. Despite the large body of evidence delineating the cellular and molecular adaptations induced by chronic drug consumption, the brain mechanisms responsible for drug craving and relapse remain insufficiently understood, and even the most recent developments in the field have not brought significant improvement in the management of drug dependence. Though, recent preclinical evidence suggests that disrupting the hypocretin (orexin) system may serve as an anticraving medication therapy. Here, we discuss how the hypocretins, which orchestrate normal wakefulness, metabolic health and the execution of goal-oriented behaviors, may be compromised and contribute to elicit compulsive drug seeking. We propose an overview on the most recent studies demonstrating an important role for the hypocretin neuropeptide system in the regulation of drug reward and the prevention of drug relapse, and we question the relevance of disrupting the hypocretin system to alleviate symptoms of drug addiction.

Role of orexin in the pathophysiology of depression: potential for pharmacological intervention

Depression is a devastating mental disorder with an increasing impact throughout the world, whereas the efficacy of currently available pharmacological treatment is still limited. Growing evidence from preclinical and clinical studies suggests that orexins (neuropeptides that are also known as hypocretins) and their receptors are involved in the physiopathology of depression. Indeed, the orexinergic system regulates functions that are disturbed in depressive states such as sleep, reward system, feeding behavior, the stress response and monoaminergic neurotransmission. Nevertheless, the precise role of orexins in behavioral and neurophysiological impairments observed in depression is still unclear. Both hypoactivity and hyperactivity of orexin signaling pathways have been found to be associated with depression. These discrepancies in the literature prompted the necessity for additional investigations, as the orexinergic system appears to be a promising target to treat the symptoms of depression. This assumption is underlined by recent data suggesting that pharmacological blockade of orexin receptors induces a robust antidepressant-like effect in an animal model of depression. Further preclinical and clinical studies are needed to progress the overall understanding of the orexinergic alterations in depression, which will eventually translate preliminary observations into real therapeutic potential. The aim of this paper is to provide an overview of human and animal research dedicated to the study of the specific involvement of orexins in depression, and to propose a framework in which disturbances of the orexinergic system are regarded as an integral component of the etiology of depression.

Complementary roles of orexin and melanin-concentrating hormone in feeding behavior

Transcribed within the lateral hypothalamus, the neuropeptides orexin/hypocretin (OX) and melanin-concentrating hormone (MCH) both promote palatable food intake and are stimulated by palatable food. While these two neuropeptides share this similar positive relationship with food, recent evidence suggests that this occurs through different albeit complementary effects on behavior, with OX promoting food seeking and motivation for palatable food and MCH functioning during ongoing food intake, reinforcing the consumption of calorically dense foods. Further differences are evident in their effects on physiological processes, which are largely opposite in nature. For example, activation of OX receptors, which is neuronally excitatory, promotes waking, increases energy expenditure, and enhances limbic dopamine levels and reward. In contrast, activation of MCH receptors, which is neuronally inhibitory, promotes paradoxical sleep, enhances energy conservation, reduces limbic dopamine, and increases depressive behavior. This review describes these different effects of the neuropeptides, developing the hypothesis that they stimulate the consumption of palatable food through excessive seeking in the case of OX and through excessive energy conservation in the case of MCH. It proposes that OX initiates food intake and subsequently stimulates MCH which then acts to prolong the consumption of palatable, energy-dense food.

Adolescent gain in positive valence of a socially relevant stimulus: engagement of the mesocorticolimbic reward circuitry

A successful transition from childhood to adulthood requires adolescent maturation of social information processing. The neurobiological underpinnings of this maturational process remain elusive. This research employed the male Syrian hamster as a tractable animal model for investigating the neural circuitry involved in this critical transition. In this species, adult and juvenile males display different behavioral and neural responses to vaginal secretions, which contain pheromones essential for expression of sexual behavior in adulthood. These studies tested the hypothesis that vaginal secretions acquire positive valence over adolescent development via remodeling of neural circuits underlying sexual reward. Sexually naïve adult, but not juvenile, hamsters showed a conditioned place preference for vaginal secretions. Differences in behavioral response to vaginal secretions between juveniles and adults correlated with a difference in the vaginal secretion-induced neural activation pattern in mesocorticolimbic reward circuitry. Fos immunoreactivity increased in response to vaginal secretions in the medial amygdala and ventral tegmental dopaminergic cells of both juvenile and adult males. However, only in adults was there a Fos response to vaginal secretions in non-dopaminergic cells in interfascicular ventral tegmental area, nucleus accumbens core and infralimbic medial prefrontal cortex. These results demonstrate that a socially relevant chemosensory stimulus acquires the status of an unconditioned reward during adolescence, and that this adolescent gain in social reward is correlated with experience-independent engagement of specific cell groups in reward circuitry.

Effect of orexin-B-saporin-induced lesions of the lateral hypothalamus on performance on a progressive ratio schedule

It has been suggested that a sub-population of orexinergic neurones whose somata lie in the lateral hypothalamic area (LHA) play an important role in regulating the reinforcing value of both food and drugs. This experiment examined the effect of disruption of orexinergic mechanisms in the LHA on performance on the progressive ratio schedule of reinforcement, in which the response requirement increases progressively for successive reinforcers. The data were analysed using a mathematical model which yields a quantitative index of reinforcer value and dissociates effects of interventions on motor and motivational processes. Rats were trained under a progressive ratio schedule using food-pellet reinforcement. They received bilateral injections of conjugated orexin-B-saporin (OxSap) into the LHA or sham lesions. Training continued for a further 40 sessions after surgery. Equations were fitted to the response rate data from each rat, and the parameters of the model were derived for successive blocks of 10 sessions. The OxSap lesion reduced the number of orexin-containing neurones in the LHA by approximately 50% compared with the sham-lesioned group. The parameter expressing the incentive value of the reinforcer was not significantly altered by the lesion. However, the parameter related to the maximum response rate was significantly affected, suggesting that motor capacity was diminished in the OxSap-lesioned group. The results indicate that OxSap lesions of the LHA disrupted food-reinforced responding on the progressive ratio schedule. It is suggested that this disruption was brought about by a change in non-motivational (motor) processes.

Multiple roles for orexin/hypocretin in addiction

Orexins/hypocretins are hypothalamic peptides involved in arousal and wakefulness, but also play a critical role in drug addiction and reward-related behaviors. Here, we review the roles played by orexins in a variety of animal models of drug addiction, emphasizing both commonalities and differences for orexin's involvement in seeking of the major classes of abused drugs, as well as food. One common theme that emerges is an involvement of orexins in drug seeking triggered by external stimuli (e.g., cues, contexts or stressors). We also discuss the functional neuronal circuits in which orexins are embedded, and how these circuits mediate addiction-related behaviors, with particular focus on the role of orexin and glutamate interactions within the ventral tegmental area. Finally, we attempt to contextualize the role of orexins in reward by discussing ways in which these peptides, expressed in only a few thousand neurons in the brain, can have such wide-ranging effects on behavior.

Hypocretinergic neurons are activated in conjunction with goal-oriented survival-related motor behaviors

Hypocretinergic neurons are located in the area of the lateral hypothalamus which is responsible for mediating goal-directed, survival-related behaviors. Consequently, we hypothesize that the hypocretinergic system functions to promote these behaviors including those patterns of somatomotor activation upon which they are based. Further, we hypothesize that the hypocretinergic system is not involved with repetitive motor activities unless they occur in conjunction with the goal-oriented behaviors that are governed by the lateral hypothalamus. In order to determine the veracity of these hypotheses, we examined Fos immunoreactivity (as a marker of neuronal activity) in hypocretinergic neurons in the cat during: a) Exploratory Motor Activity; b) Locomotion without Reward; c) Locomotion with Reward; and d) Wakefulness without Motor Activity. Significantly greater numbers of hypocretinergic neurons expressed c-fos when the animals were exploring an unknown environment during Exploratory Motor Activity compared with all other paradigms. In addition, a larger number of Hcrt+Fos+neurons were activated during Locomotion with Reward than during Wakefulness without Motor Activity. Finally, very few hypocretinergic neurons were activated during Locomotion without Reward and Wakefulness without Motor Activity, wherein there was an absence of goal-directed activities. We conclude that the hypocretinergic system does not promote wakefulness per se or motor activity per se but is responsible for mediating specific goal-oriented behaviors that take place during wakefulness. Accordingly, we suggest that the hypocretinergic system is responsible for controlling the somatomotor system and coordinating its activity with other systems in order to produce successful goal-oriented survival-related behaviors that are controlled by the lateral hypothalamus.