Addiction and arousal: the hypocretin connection

Optogenetic deconstruction of sleep-wake circuitry in the brain

How does the brain regulate the sleep–wake cycle? What are the temporal codes of sleep and wake-promoting neural circuits? How do these circuits interact with each other across the light/dark cycle? Over the past few decades, many studies from a variety of disciplines have made substantial progress in answering these fundamental questions. For example, neurobiologists have identified multiple, redundant wake-promoting circuits in the brainstem, hypothalamus, and basal forebrain. Sleep-promoting circuits have been found in the preoptic area and hypothalamus. One of the greatest challenges in recent years has been to selectively record and manipulate these sleep–wake centers in vivo with high spatial and temporal resolution. Recent developments in microbial opsin-based neuromodulation tools, collectively referred to as “optogenetics,” have provided a novel method to demonstrate causal links between neural activity and specific behaviors. Here, we propose to use optogenetics as a fundamental tool to probe the necessity, sufficiency, and connectivity of defined neural circuits in the regulation of sleep and wakefulness.

Discovery of a functional immunoreceptor tyrosine-based switch motif in a 7-transmembrane-spanning receptor: role in the orexin receptor OX1R-driven apoptosis

The orexin neuropeptides promote robust apoptosis in cancer cells. We have recently shown that the 7-transmembrane-spanning orexin receptor OX1R mediates apoptosis through an original mechanism. OX1R is equipped with a tyrosine-based inhibitory motif ITIM, which is tyrosine-phosphorylated on receptor activation, allowing the recruitment and activation of the tyrosine phosphatase SHP-2, leading to apoptosis. We show here that another motif, immunoreceptor tyrosine-based switch motif (ITSM), is present in OX1R and is mandatory for OX1R-mediated apoptosis. This conclusion is based on the following observations: 1) a canonical ITSM sequence is present in the first intracellular loop of OX1R; 2) mutation of Y(83) to F within ITSM abolished OX1R-mediated apoptosis but did not alter orexin-induced inositol phosphate formation or calcium transient via coupling of OX1R to G(q) protein; 3) mutation of Y(83) to F further abolished orexin-induced tyrosine phosphorylation in ITSM and subsequent recruitment of SHP-2 by the receptor. Finally, we developed a structural model of OX1R showing that the spatial localization of phosphotyrosines in ITSM and ITIM in OX1R is compatible with their interaction with the two SH2 domains of SHP-2. These data represent the first evidence for a functional role of an ITSM in a 7-transmembrane-spanning receptor.

The role of hypocretin in driving arousal and goal-oriented behaviors

The hypocretins (Hcrts), also called orexins, are two neuropeptides secreted by a few thousand neurons restricted to the lateral hypothalamus. The Hcrt peptides bind to two receptors located in nuclei associated with diverse cognitive and physiological functions. Experimental evidence has demonstrated that the physiological roles of hypocretins extend far beyond its initial role in food consumption and has emerged as a key system in the fields of sleep disorders and drug addiction. Here, we discuss recent evidence demonstrating a key role of hypocretin in the motivation for reward seeking in general, and drug taking in particular, and we delineate a physiological framework for this peptidergic system in orchestrating the appropriate levels of alertness required for the elaboration and the execution of goal-oriented behaviors. We propose a general role for hypocretins in mediating arousal, especially when an organism must respond to unexpected stressors and environmental challenges, which serve to shape survival behaviors. We also discuss the limit of the current experimental paradigms to address the question of how a system normally involved in the regulation of vigilance states and hyperarousal may promote a pathological state that elicits compulsive craving and relapse to drug seeking.

Lateral hypothalamic orexin/hypocretin neurons: A role in reward-seeking and addiction

Orexins (synonymous with hypocretins) are recently discovered neuropeptides made exclusively in hypothalamus. Behavioral, anatomical, and neurophysiological studies show that a subset of these cells, specifically those in lateral hypothalamus (LH), are involved in reward processing and addictive behaviors. Fos expression in LH orexin neurons varied in proportion to conditioned place preference (CPP) for morphine, cocaine, or food. This relationship occurred both in drug-naïve rats and in animals during protracted morphine withdrawal, when drug preference was elevated but food preference was decreased. Inputs to the LH orexin cell field from lateral septum and bed nucleus of the stria terminalis were Fos-activated during cocaine CPP in proportion to the preference expressed in each animal. This implies that these inputs may be involved in driving the conditioned responses in LH orexin neurons. Related studies showed that LH orexin neurons that project to ventral tegmental area (VTA) had 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, stimulation of LH orexin neurons, or microinjection of orexin into VTA, reinstated an extinguished morphine preference. In self-administration studies, the orexin 1 receptor antagonist SB-334867 (SB) blocked cocaine-seeking induced by discrete or contextual cues previously associated with cocaine, but not by a priming injection of cocaine. There was no effect of SB on cocaine self-administration itself, indicating that it did not interfere with the drug's reinforcing properties. 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. This LH-to-VTA orexin pathway was found to be necessary for learning a morphine place preference. These findings are consistent with results 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.

Hypocretin/orexin in arousal and stress

Multiple lines of evidence indicate that hypocretin/orexin (HCRT) participates in the regulation of arousal and arousal-related process. For example, HCRT axons and receptors are found within a variety of arousal-related systems. Moreover, when administered centrally, HCRT exerts robust wake-promoting actions. Finally, a dysregulation of HCRT neurotransmission is associated with the sleep/arousal disorder, narcolepsy. Combined, these observations suggested that HCRT might be a key transmitter system in the regulation of waking. Nonetheless, subsequent evidence indicates that HCRT may not play a prominent role in the initiation of normal waking. Instead HCRT may participate in a variety of processes such as consolidation of waking and/or coupling metabolic state with behavioral state. Additionally, substantial evidence suggests a potential involvement of HCRT in high-arousal conditions, including stress. Thus, HCRT neurotransmission is closely linked to high-arousal conditions, including stress, and HCRT administration exerts a variety of stress-like physiological and behavioral effects that are superimposed on HCRT-induced increases in arousal. Combined, this evidence suggests the hypothesis that HCRT may participate in behavioral responding under high-arousal aversive conditions. Importantly, these actions of HCRT may not be limited to stress. Like stress, appetitive conditions are associated with elevated arousal levels and a stress-like activation of various physiological systems. These and other observations suggest that HCRT may, at least in part, exert affectively neutral actions that are important under high-arousal conditions associated with elevated motivation and/or need for action.

Orexin A/hypocretin-1 selectively promotes motivation for positive reinforcers

Orexin A/hypocretin-1 (oxA/hcrt-1) is known to be a modulator of dopamine-dependent neuronal activity and behaviors. However, the role of this system in driving motivated behaviors remains poorly understood. Here, we show that orexin/hypocretin receptor-1 (ox/hcrt-1R) signaling is important for motivation for highly salient, positive reinforcement. Blockade of ox/hcrt-1R selectively reduced work to self-administer cocaine or high fat food pellets. Moreover, oxA/hcrt-1 strengthened presynaptic glutamatergic inputs to the ventral tegmental area (VTA) only in cocaine or high fat self-administering rats. Finally, oxA/hcrt-1-mediated excitatory synaptic transmission onto VTA neurons was not potentiated following an arousing, aversive stimulus, suggesting that oxA/hcrt-1-mediated glutamatergic synaptic transmission was potentiated selectively with highly salient positive reinforcers. These experiments provide evidence for a selective role of oxA/hcrt-1 signaling in motivation for highly salient reinforcers and may represent a unique opportunity to design novel therapies that selectively reduce excessive drive to consume positive reinforcers of high salience.

The neuropharmacology of relapse to food seeking: methodology, main findings, and comparison with relapse to drug seeking

Relapse to old, unhealthy eating habits is a major problem in human dietary treatments. The mechanisms underlying this relapse are unknown. Surprisingly, until recently this clinical problem has not been systematically studied in animal models. Here, we review results from recent studies in which a reinstatement model (commonly used to study relapse to abused drugs) was employed to characterize the effect of pharmacological agents on relapse to food seeking induced by either food priming (non-contingent exposure to small amounts of food), cues previously associated with food, or injections of the pharmacological stressor yohimbine. We also address methodological issues related to the use of the reinstatement model to study relapse to food seeking, similarities and differences in mechanisms underlying reinstatement of food seeking versus drug seeking, and the degree to which the reinstatement procedure provides a suitable model for studying relapse in humans. We conclude by discussing implications for medication development and future research. We offer three tentative conclusions: (1)The neuronal mechanisms of food-priming- and cue-induced reinstatement are likely different from those of reinstatement induced by the pharmacological stressor yohimbine. (2)The neuronal mechanisms of reinstatement of food seeking are possibly different from those of ongoing food-reinforced operant responding. (3)The neuronal mechanisms underlying reinstatement of food seeking overlap to some degree with those of reinstatement of drug seeking.

Regulation of alcohol-seeking by orexin (hypocretin) neurons

Orexins (hypocretins) are found primarily within a restricted portion of neurons within the hypothalamus, but provide innervation across the neuraxis. Orexin A (hypocretin 1) has been implicated in drug and food reward. Not surprisingly therefore, interest has come to bear on whether orexins are implicated in aspects of alcohol consumption and/or seeking. This mini-review provides a concise, but timely, discussion on this issue. The evidence to date would suggest a role for orexins in alcohol use, and integration of orexin-containing neurons in reward-seeking circuitry. There are however still many unanswered questions, some of which are canvassed herein.

Orexin-A acts on the paraventricular nucleus of the midline thalamus to inhibit locomotor activity in rats

Orexins (hypocretins) are novel peptides that have been shown to play a role in control of behavioral arousal. The paraventricular nucleus of the midline thalamus (PVT) is one area of the brain that is the most densely innervated by orexin fibers. In addition, the PVT sends a dense projection to the nucleus accumbens, an area of the striatum involved in the regulation of locomotion. This study was done to determine the effect of microinjections of orexin-A (OXA) or the orexin receptor antagonist SB334867 in the PVT on locomotor activity (LA) in morphine-naïve and morphine-sensitized rats. Microinjections of OXA (3 microg/500 nl) in or near the PVT inhibited LA in rats tested in a novel and familiar environment as well as in rats expressing behavioral sensitization to repeated injections of morphine. In contrast, microinjections of SB334867 had no effect on LA in any of the test situations. Using an approach involving experimenter based analysis of ethological behaviors; we found that microinjections of OXA in the midline thalamus decreased LA while at the same time increasing the expression of grooming and freezing. These results suggest that OXA can act on the PVT and the midline thalamus to produce arousal independent of LA.

Influenza H1N1 and the world wide economic crisis--a model of coherence?

A recent published model described the phenomenon of a global panic reaction (GPR) on the stock markets based on two remarkable stock market crashes in the months of January and March [Sperling W, Bleich S, Reulbach U, Black Monday on stock markets throughout the world - a new phenomenon of collective panic disorder? A psychiatric approach. Med Hypotheses; 2008]. This model was completed by a therapeutic approach following typical elements of cognitive behavioural therapy (CBT) [Sperling W, Biermann T, Maler JM, Global panic reaction - a therapeutic approach to a world-wide economic crisis. Med Hypotheses; 2009]. The phenomenon of a global panic reaction due to economic crises seems to have even larger implications on human health as well. It is well known that acute and chronic distress is competent to suppress the immune system by various mechanisms that are discussed in detail. This global panic reaction - that has also been observed in former times - might therefore be responsible for the new variation of recent influenza pandemic coming from Mexico.

Reinstatement of cocaine seeking by hypocretin (orexin) in the ventral tegmental area: independence from the local corticotropin-releasing factor network

BACKGROUND

Hypocretin (Hcrt), an arousal- and feeding-associated peptide, is expressed in lateral hypothalamic neurons that project to the ventral tegmental area (VTA). Intra-VTA Hcrt reinstates morphine-conditioned place preferences, and intracerebroventricular and intra-VTA corticotropin-releasing factor (CRF) reinstate cocaine seeking. Each is presumed to act, at least in part, through actions local to the VTA. Here, we examined the possibility that VTA perfusion of Hcrt reinstates cocaine seeking and, if so, whether it does so through the VTA mechanism that is implicated in reinstatement by CRF.

METHODS

Rats were trained to lever-press for intravenous cocaine (2 weeks) and then underwent extinction training (saline substituted for cocaine: 3 weeks). Reinstatement behavior was tested and VTA dialysates were collected and assayed for glutamate or dopamine following footshock or perfusion of Hcrt or CRF, with or without Hcrt or CRF antagonists, into the VTA.

RESULTS

Ventral tegmental area perfusion of Hcrt-1 or footshock stress reinstated cocaine seeking and caused release of VTA glutamate and dopamine. The effects of Hcrt-1 were blocked by a selective Hcrt-1 antagonist, but not a CRF antagonist, and were not mimicked by Hcrt-2. The Hcrt-1 antagonist did not block CRF-dependent footshock-induced reinstatement or glutamate or dopamine release. The behavioral and neurochemical effects of Hcrt-1 were attenuated but not blocked by kynurenic acid, an ionotropic glutamate antagonist that blocks footshock-induced reinstatement and glutamate release.

CONCLUSIONS

While Hcrt and CRF are known to interact in some area of the brain, in the VTA proper they appear to have largely independent actions on the mesolimbic dopamine mechanisms of cocaine seeking.

The hypocretins as sensors for metabolism and arousal

Sleep disturbances are associated with hormonal imbalances and may result in metabolic disorders including obesity and diabetes. Therefore, circuits controlling both sleep and metabolism are likely to play a role in these physiopathological conditions. The hypocretin (Hcrt) system is a strong candidate for mediating both sleep and metabolic imbalances because Hcrt neurons are sensitive to metabolic hormones, including leptin and ghrelin, and modulate arousal and goal-orientated behaviours. This review discusses the role of Hcrt neurons as a sensors of energy balance and arousal and proposes new ways of probing local hypothalamic circuits regulating sleep and metabolism with unprecedented cellular specificity and temporal resolution.

Sleep and metabolism: shared circuits, new connections

Association between sleep disturbances and hormonal imbalances can result in metabolic disorders, including obesity and diabetes. The hypothalamus is likely to play a part in these pathophysiological conditions because it contains sleep-wake circuits that are sensitive to metabolic hormones, including leptin and ghrelin. Thus, shared hypothalamic circuits such as the hypocretin and melanin-concentrating hormone systems are strong candidates for mediating both sleep and metabolic imbalances. This review reveals new roles for these systems as sensors and effectors of sleep and wakefulness, and discusses their plasticity in regulating sleep and energy balance. New optical tools that remotely control neuronal circuit activity provide an effective means to understand the cooperativity of shared circuits in regulating hypothalamic functions such as sleep and metabolism.

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.