Melanin-concentrating hormone receptor 1 (MCH1-R) antagonism: reduced appetite for calories and suppression of addictive-like behaviors

Behavioural and physiological effects of binge eating: A systematic review and meta-analysis of animal models

Binge eating is defined as eating abnormally large amounts in a brief period of time. Many animal studies have examined the behavioural and physiological effects of binge eating of high-fat, high-sugar foods to model the consequences of human binge eating. The present systematic review of 199 rodent studies sought to identify the behavioural and physiological consequences of binge eating and determine whether changes were specific to binge eating or to general effects of exposure to a palatable diet. A meta-analysis of 18 rodent studies revealed that binge eating produces greater anxiety-like behaviour on the Elevated Plus-Maze with a small effect size and significant funnel plot asymmetry, suggesting that the true effect size is overestimated. A history of binge-like access generally increases progressive ratio breakpoint for the binged food, without altering 'liking' as measured by lick microstructure, suggesting that dissociable effects on 'wanting' but not 'liking' accompany binge eating behaviour and contribute to its persistence. Binge eating appears to enhance compulsive food-seeking behaviour and prevent stress-induced reductions in intake but does not appear to alter depression-like behaviour or locomotor activity. Notably, binge eating may produce comparable metabolic impairments to those observed after extended continuous exposure to a palatable diet despite no overall effects on body weight outcomes in most studies.

Melanin-concentrating hormone and orexin shape social affective behavior via action in the insular cortex of rat

RATIONALE

In a social context, individuals are able to detect external information from others and coordinate behavioral responses according to the situation, a phenomenon called social decision-making. Social decision-making is multifaceted, influenced by emotional and motivational factors like stress, sickness, and hunger. However, the neurobiological basis for motivational state competition and interaction is not well known.

OBJECTIVE

We investigated possible neural mechanisms through which internal states could shape social behavior in a social affective preference (SAP) test. In the SAP test, experimental rats given a choice to interact with naïve or stressed conspecifics exhibit an age-dependent preference to interact with stressed juvenile conspecifics, but avoid stressed adult conspecifics. First, we assessed the effect of food and water deprivation on SAP behavior. Behavior in the SAP test requires the insular cortex, which receives input from the ingestion-related peptides melanin-concentrating hormone (MCH) and orexin neurons of the lateral hypothalamus (LH). This study aimed to evaluate the role of LH and insular MCH and orexin in SAP test.

METHODS

SAP tests were conducted in rats that were sated, food and water deprived or allowed 1 h of access to food and water after 14 h of deprivation (relieved condition). Separate cohorts of sated rats received cannula implants for microinjection of drugs to inhibit the LH or to block or stimulate MCH or orexin receptors in the insula prior to SAP tests or social interaction tests.

RESULTS

Food and water deprivation prior to SAP tests with juvenile rats caused a shift in preference away from the stressed rat toward the naïve juveniles. Pharmacological inhibition of LH with muscimol (100 ng/side) abolished the preference for the juvenile-stressed conspecific, as well as the preference for the adult naïve conspecific. The blockade of MCH receptor 1or orexin receptors in the insular cortex with SNAP94847 (50 μM) or TCS1102 (1 μM), respectively, also abolished the preference for the stressed juvenile conspecific, but only the antagonism of orexin receptors was able to abolish the preference for the adult naïve conspecific. Microinjection of increasing doses (50 or 500 nM) of MCH or orexin-A in the insular cortex increased the interaction time in the one-on-one social interaction test with juvenile conspecifics; however, only the microinjection of orexin-A increased the interaction time with adult naïve conspecifics.

CONCLUSIONS

Taken together, these results suggest that lateral hypothalamus peptides shape the direction of social approach or avoidance via actions MCH and orexin neurotransmission in the insular cortex.

Melanin-concentrating hormone promotes feeding through the lateral septum

Feeding is necessary for survival but can be hindered by anxiety or fear, thus neural systems that can regulate anxiety states are key to elucidating the expression of food-related behaviors. Melanin-concentrating hormone (MCH) is a neuropeptide produced in the lateral hypothalamus and zona incerta that promotes feeding and anxiogenesis. The orexigenic actions of MCH that prolong ongoing homeostatic or hedonic feeding are context-dependent and more prominent in male than female rodents, but it is not clear where MCH acts to initiate feeding. The lateral septum (LS) promotes feeding and suppresses anxiogenesis when inhibited, and it comprises the densest projections from MCH neurons. However, it is not known whether the LS is a major contributor to MCH-mediated feeding. As MCH inhibits LS cells by MCH receptor (MCHR1) activation, MCH may promote feeding via the LS. We bilaterally infused MCH into the LS and found that MCH elicited a rapid and long-lasting increase in the consumption of standard chow and a palatable, high sugar diet in male and female mice; these MCH effects were blocked by the co-administration of a MCHR1 antagonist TC- MCH 7c. Interestingly, the orexigenic effect of MCH was abolished in a novel, anxiogenic environment even when presented with a food reward, but MCH did not induce anxiety-like behaviors. These findings indicated the LS as a novel region underlying orexigenic MCH actions, which stimulated and enhanced feeding in both sexes in a context -dependent manner that was most prominent in the homecage.

Incentive motivation for palatable food blocked by intra-accumbens melanin-concentrating hormone (MCH) receptor-1 antagonist in female rats

Melanin-concentrating hormone (MCH) activity in the nucleus accumbens (Acb) has been shown to influence feeding behavior, yet this has not been characterized in terms of homeostatic vs. hedonic feeding processes. Hedonic feeding, driven by palatability rather than energy deficit, can be modeled through intra-Acb administration of the selective μ-opioid receptor agonist d-Ala2, NMe-Phe4, Glyol5-enkephalin (DAMGO), which preferentially increases consumption and incentive motivation to obtain preferred palatable food. Pharmacological activation of MCH 1 receptors (MCHR1) within Acb has been shown to promote general feeding of chow in males, but not females. However, the effects of MCH on the incentive motivation to obtain preferred palatable food have not been explored. Here, we investigated the role of MCHR1 within the Acb in DAMGO-induced incentive motivation to obtain a sucrose pellet reward. Female Sprague Dawley rats were trained and tested for operant responding under a progressive ratio (PR) breakpoint in response to concurrent intra-Acb administration of DAMGO (0 μg and 0.025 μg/.5 μl/side) immediately following intra-Acb administration of the MCHR1 antagonist (N-(3-{1-[4-(3,4-difluoro-phenoxy)-benzyl]-piperdin-4-yl}-4-methyl-phenyl)-isobutyramide (SNAP-94847; 0 μg, 1.5 μg, and 15 μg/.5 μl/side), in a counterbalanced fashion. As expected, DAMGO significantly increased PR breakpoint and overall active lever presses. SNAP-94847 did not influence PR breakpoint by itself, compared to vehicle; however, both 1.5 and 15 μg doses of SNAP-94847 significantly blocked the increased PR breakpoint produced by intra-Acb DAMGO. The results of the study demonstrate that Acb MCHR1 may play a specific role in the hedonically-driven motivation for palatable food in females.

Novel aspect of oxytocin neurons mediating parental behavior and aversive burying behavior under the control of melanin-concentrating hormone neurons

Parental behavior comprises a set of crucial actions essential for offspring survival. In this study, a double transgenic mouse model engineered to specifically express channelrhodopsin-2 (ChR2) in paraventricular hypothalamic nucleus (PVN)-oxytocin neurons and ablate lateral hypothalamic area (LHA)-melanin-concentrating hormone (MCH) neurons was used to determine the relationship between PVN-oxytocin neurons and LHA-MCH neurons associated with parental behavior. Optogenetic stimulation of ChR2-expressing PVN-oxytocin neurons induces typical parental behavior with intact LHA-MCH neurons. However, after the partial ablation of LHA-MCH neurons, even optogenetic stimulation of PVN-oxytocin neurons failed to induce parental behavior in virgin male mice, resulting in neglect rather than parental behavior. Furthermore, approximately half of the subjects exhibited burying behavior toward pups, suggesting that pups became aversive stimuli, and male mice actively performed burying behavior to avoid these aversive stimuli. This study emphasizes the novel aspect of oxytocin neurons that could result in neglect in the absence of LHA-MCH neurons regulation.

Glutamatergic signaling from melanin-concentrating hormone-producing neurons: A requirement for memory regulation, but not for metabolism control

Melanin-concentrating hormone-producing neurons (MCH neurons), found mainly in the lateral hypothalamus and surrounding areas, play essential roles in various brain functions, including sleep and wakefulness, reward, metabolism, learning, and memory. These neurons coexpress several neurotransmitters and act as glutamatergic neurons. The contribution of glutamate from MCH neurons to memory- and metabolism-related functions has not been fully investigated. In a mouse model, we conditionally knocked out Slc17a6 gene, which encodes for vesicular glutamate transporter 2 (vGlut2), in the MCH neurons exclusively by using two different methods: the Cre recombinase/loxP system and in vivo genome editing using CRISPR/Cas9. Then, we evaluated several aspects of memory and measured metabolic rates using indirect calorimetry. We found that mice with MCH neuron-exclusive vGlut2 ablation had higher discrimination ratios between novel and familiar stimuli for novel object recognition, object location, and three-chamber tests. In contrast, there was no significant change in body weight, food intake, oxygen consumption, respiratory quotient, or locomotor activity. These findings suggest that glutamatergic signaling from MCH neurons is required to regulate memory, but its role in regulating metabolic rate is negligible.

Actions of feeding-related peptides on the mesolimbic dopamine system in regulation of natural and drug rewards

The mesolimbic dopamine system is the primary neural circuit mediating motivation, reinforcement, and reward-related behavior. The activity of this system and multiple behaviors controlled by it are affected by changes in feeding and body weight, such as fasting, food restriction, or the development of obesity. Multiple different peptides and hormones that have been implicated in the control of feeding and body weight interact with the mesolimbic dopamine system to regulate many different dopamine-dependent, reward-related behaviors. In this review, we summarize the effects of a selected set of feeding-related peptides and hormones acting within the ventral tegmental area and nucleus accumbens to alter feeding, as well as food, drug, and social reward.

Multifaceted actions of melanin-concentrating hormone on mammalian energy homeostasis

Melanin-concentrating hormone (MCH) is a small cyclic peptide expressed in all mammals, mainly in the hypothalamus. MCH acts as a robust integrator of several physiological functions and has crucial roles in the regulation of sleep-wake rhythms, feeding behaviour and metabolism. MCH signalling has a very broad endocrine context and is involved in physiological functions and emotional states associated with metabolism, such as reproduction, anxiety, depression, sleep and circadian rhythms. MCH mediates its functions through two receptors (MCHR1 and MCHR2), of which only MCHR1 is common to all mammals. Owing to the wide variety of MCH downstream signalling pathways, MCHR1 agonists and antagonists have great potential as tools for the directed management of energy balance disorders and associated metabolic complications, and translational strategies using these compounds hold promise for the development of novel treatments for obesity. This Review provides an overview of the numerous roles of MCH in energy and glucose homeostasis, as well as in regulation of the mesolimbic dopaminergic circuits that encode the hedonic component of food intake.

Melanin-concentrating hormone and food intake control: Sites of action, peptide interactions, and appetition

Given the increased prevalence of obesity and its associated comorbidities, understanding the mechanisms through which the brain regulates energy balance is of critical importance. The neuropeptide melanin-concentrating hormone (MCH) is produced in the lateral hypothalamic area and the adjacent incerto-hypothalamic area and promotes both food intake and energy conservation, overall contributing to body weight gain. Decades of research into this system has provided insight into the neural pathways and mechanisms (behavioral and neurobiological) through which MCH stimulates food intake. Recent technological advancements that allow for selective manipulation of MCH neuron activity have elucidated novel mechanisms of action for the hyperphagic effects of MCH, implicating neural "volume" transmission in the cerebrospinal fluid and sex-specific effects of MCH on food intake control as understudied areas for future investigation. Highlighted here are historical and recent findings that illuminate the neurobiological mechanisms through which MCH promotes food intake, including the identification of various specific neural signaling pathways and interactions with other peptide systems. We conclude with a framework that the hyperphagic effects of MCH signaling are predominantly mediated through enhancement of an "appetition" process in which early postoral prandial signals promote further caloric consumption.

The dynamic regulation of appetitive behavior through lateral hypothalamic orexin and melanin concentrating hormone expressing cells

The lateral hypothalamic area (LHA) is a heterogeneous brain structure extensively studied for its potent role in regulating energy balance. The anatomical and molecular diversity of the LHA permits the orchestration of responses to energy sensing cues from the brain and periphery. Two of the primary cell populations within the LHA associated with integration of this information are Orexin (ORX) and Melanin Concentrating Hormone (MCH). While both of these non-overlapping populations exhibit orexigenic properties, the activities of these two systems support feeding behavior through contrasting mechanisms. We describe the anatomical and functional properties as well as interaction with other neuropeptides and brain reward and hedonic systems. Specific outputs relating to arousal, food seeking, feeding, and metabolism are coordinated through these mechanisms. We then discuss how both the ORX and MCH systems harmonize in a divergent yet overall cooperative manner to orchestrate feeding behavior through transitions between various appetitive states, and thus offer novel insights into LHA allostatic control of appetite.

Role of melanin-concentrating hormone in drug use disorders

Melanin-concentrating hormone (MCH) is a neuropeptide primarily transcribed in the lateral hypothalamus (LH), with vast projections to many areas throughout the central nervous system that play an important role in motivated behaviors and drug use. Anatomical, pharmacological and genetic studies implicate MCH in mediating the intake and reinforcement of commonly abused substances, acting by influencing several systems including the mesolimbic dopaminergic system, glutamatergic as well as GABAergic signaling and being modulated by inflammatory neuroimmune pathways. Further support for the role of MCH in controlling behavior related to drug use will be discussed as it relates to cerebral ventricular volume transmission and intracellular molecules including cocaine- and amphetamine-regulated transcript peptide, dopamine- and cAMP-regulated phosphoprotein 32 kDa. The primary goal of this review is to introduce and summarize current literature surrounding the role of MCH in mediating the intake and reinforcement of commonly abused drugs, such as alcohol, cocaine, amphetamine, nicotine and opiates.

MCH Neuron Activity Is Sufficient for Reward and Reinforces Feeding

BACKGROUND

Melanin-concentrating hormone (MCH)-expressing neurons have been implicated in regulation of energy homeostasis and reward, yet the role of their electrical activity in short-term appetite and reward modulation has not been fully understood.

OBJECTIVES

We investigated short-term behavioral and physiological effects of MCH neuron activity manipulations.

METHODS

We used optogenetic and chemogenetic approaches in Pmch-cre transgenic mice to acutely stimulate/inhibit MCH neuronal activity while probing feeding, locomotor activity, anxiety-like behaviors, glucose homeostasis, and reward.

RESULTS

MCH neuron activity is neither required nor sufficient for short-term appetite unless stimulation is temporally paired with consumption. MCH neuronal activation does not affect short-term locomotor activity, but inhibition improves glucose tolerance and is mildly anxiolytic. Finally, using two different operant tasks, we showed that activation of MCH neurons alone is sufficient to induce reward.

CONCLUSIONS

Our results confirm diverse behavioral/physiological functions of MCH neurons and suggest a direct role in reward function.

Sub-dimensions of Alcohol Use Disorder in Alcohol Preferring and Non-preferring Rats, a Comparative Study

Recent animal models of alcohol use disorder (AUD) are centered in capturing individual vulnerability differences in disease progression. Here, we used genetically selected Marchigian Sardinian alcohol-preferring (msP) and Wistars rats to apply a multidimensional model of AUD adapted from a previously described DSM-IV/DSM-5 multisymptomatic cocaine addiction model. As proof of concept, we hypothesized that msP rats, genetically selected for excessive drinking, would be more prone to develop dependence-like behavior compared to Wistars. Before exposure of animals to alcohol, we monitored basal anxiety in the elevated plus maze (EPM). Animals were then trained in prolonged operant alcohol self-administration, consisting of 30-min daily sessions for 60 days in total. Each session consisted of two 10-min periods of alcohol reinforcement separated by 10-min interval of non-reinforcement. Following training, we applied three criteria of individual vulnerability for AUD: (1) persistence of lever pressing for alcohol when it was not available; (2) motivation for alcohol in a progressive ratio (PR) schedule of reinforcement; and (3) resistance to punishment when alcohol delivery was anticipated by a foot-shock (0.3 mA). We obtained four groups corresponding to the number of criteria met (0–3 crit). Rats in the 0crit and 1crit groups were characterized as resilient, whereas rats in the 2crit and 3crit groups were characterized as prone to develop a dependent-like phenotype. As predicted, the 2–3crit groups were enriched with msP rats while the 0–1crit groups were enriched in Wistar rats. In further analysis, we calculated the global addiction score (GAS) per subject by the sum of the normalized score (z-score) of each criterion. Results showed GAS was highly correlated with animal distribution within the 3 criteria. Specifically, GAS was negative in the 0–1crit groups, and positive in the 2–3crit groups. A positive correlation between basal anxiety and quantity of alcohol intake was detected in msP rats but not Wistars. In conclusion, we demonstrated that the 0/3criteria model is a suitable approach to study individual differences in AUD and that msP rats, selected for excessive-alcohol drinking, show a higher propensity to develop AUD compared to non-preferring Wistars.

Brain Glucose-Sensing Mechanism and Energy Homeostasis

The metabolic and energy state of the organism depends largely on the availability of substrates, such as glucose for ATP production, necessary for maintaining physiological functions. Deregulation in glucose levels leads to the appearance of pathological signs that result in failures in the cardiovascular system and various diseases, such as diabetes, obesity, nephropathy, and neuropathy. Particularly, the brain relies on glucose as fuel for the normal development of neuronal activity. Regions adjacent to the cerebral ventricles, such as the hypothalamus and brainstem, exercise central control in energy homeostasis. These centers house nuclei of neurons whose excitatory activity is sensitive to changes in glucose levels. Determining the different detection mechanisms, the phenotype of neurosecretion, and neural connections involving glucose-sensitive neurons is essential to understanding the response to hypoglycemia through modulation of food intake, thermogenesis, and activation of sympathetic and parasympathetic branches, inducing glucagon and epinephrine secretion and other hypothalamic-pituitary axis-dependent counterregulatory hormones, such as glucocorticoids and growth hormone. The aim of this review focuses on integrating the current understanding of various glucose-sensing mechanisms described in the brain, thereby establishing a relationship between neuroanatomy and control of physiological processes involved in both metabolic and energy balance. This will advance the understanding of increasingly prevalent diseases in the modern world, especially diabetes, and emphasize patterns that regulate and stimulate intake, thermogenesis, and the overall synergistic effect of the neuroendocrine system.

The Melanin-Concentrating Hormone as an Integrative Peptide Driving Motivated Behaviors

The melanin-concentrating hormone (MCH) is an important peptide implicated in the control of motivated behaviors. History, however, made this peptide first known for its participation in the control of skin pigmentation, from which its name derives. In addition to this peripheral role, MCH is strongly implicated in motivated behaviors, such as feeding, drinking, mating and, more recently, maternal behavior. It is suggested that MCH acts as an integrative peptide, converging sensory information and contributing to a general arousal of the organism. In this review, we will discuss the various aspects of energy homeostasis to which MCH has been associated to, focusing on the different inputs that feed the MCH peptidergic system with information regarding the homeostatic status of the organism and the exogenous sensory information that drives this system, as well as the outputs that allow MCH to act over a wide range of homeostatic and behavioral controls, highlighting the available morphological and hodological aspects that underlie these integrative actions. Besides the well-described role of MCH in feeding behavior, a prime example of hypothalamic-mediated integration, we will also examine those functions in which the participation of MCH has not yet been extensively characterized, including sexual, maternal, and defensive behaviors. We also evaluated the available data on the distribution of MCH and its function in the context of animals in their natural environment. Finally, we briefly comment on the evidence for MCH acting as a coordinator between different modalities of motivated behaviors, highlighting the most pressing open questions that are open for investigations and that could provide us with important insights about hypothalamic-dependent homeostatic integration.

Consequences of constitutive deletion of melanin-concentrating hormone-1 receptors for feeding and foraging behaviors of mice

In order to decipher the functional involvement of melanin-concentrating hormone 1 (MCH1) receptors in the control of feeding and foraging behaviors, mice with constitutive deletion of MCH1 receptors MCH1R -/- or knockout (KO) were studied and compared to age-matched littermate control mice (MCH1R +/+ or wildtype (WT)). Several challenges to food-motivated behaviors of food-restricted WT and KO mice were implemented. There were no differences between genotypes in the acquisition of a nose-poke response that produced food or in a discrimination between a response that produced food and one that did not. There were also no genotype differences in the rate of extinction of a food-motivated response. However, during the first day of extinction, foraging behaviors were increased significantly more in KO than in WT mice. Likewise, when the response requirement to obtain food was progressively increased, KO mice made significantly more food-directed responses than WT mice. Although adulteration of food with quinine did not suppress food-directed behavior in either genotype when the mice were food-restricted, manipulation of the degree of food-deprivation resulted in suppression of behavior of WT mice without suppressing the behavior of KO mice. Although response-produced foot shock suppressed food-maintained responding of both WT and KO mice, equipotent levels of shock (based upon psychophysical thresholds) suppressed behavior of WT mice without suppressing behavior of the KO mice. Finally, under a Vogel conflict procedure, KO mice had significantly higher levels of both punished and non-punished food maintained responding. Thus, the data from challenges with both appetitive and noxious stimulus challenges support the conclusion that mice with constitutive deletion of MCH1Rs have increased food seeking motivation that is coincident with their higher metabolism. The data also highlight important differences in the biological impact of MCH1 receptor KO and MCH1 receptor antagonism.

The melanin-concentrating hormone-1 receptor modulates alcohol-induced reward and DARPP-32 phosphorylation

RATIONALE

Melanin-concentrating hormone (MCH) is involved in the regulation of food intake and has recently been associated with alcohol-related behaviors. Blockade of MCH-1 receptors (MCH1-Rs) attenuates operant alcohol self-administration and decreases cue-induced reinstatement, but the mechanism through which the MCH1-R influences these behaviors remains unknown. MCH1-Rs are highly expressed in the nucleus accumbens shell (NAcSh) where they are co-expressed with dopamine (DA) receptors. MCH has been shown to potentiate responses to dopamine and to increase phosphorylation of DARPP-32, an intracellular marker of DA receptor activation, in the NAcSh.

METHODS

In the present study, we investigated the role of the MCH1-R in alcohol reward using the conditioned place preference (CPP) paradigm. We then used immunohistochemistry (IHC) to assess activation of downstream signaling after administration of a rewarding dose of alcohol.

RESULTS

We found that alcohol-induced CPP was markedly decreased in mice with a genetic deletion of the MCH1-R as well as after pharmacological treatment with an MCH1-R antagonist, GW803430. In contrast, an isocaloric dose of dextrose did not produce CPP. The increase in DARPP-32 phosphorylation seen in wildtype (WT) mice after acute alcohol administration in the NAcSh was markedly reduced in MCH1-R knock-out (KO) mice.

CONCLUSION

Our results suggest that MCH1-Rs regulate the rewarding properties of alcohol through interactions with signaling cascades downstream of DA receptors in the NAcSh.

MCH receptor deletion does not impair glucose-conditioned flavor preferences in mice

The post-oral actions of glucose stimulate intake and condition flavor preferences in rodents. Hypothalamic melanin-concentrating hormone (MCH) neurons are implicated in sugar reward, and this study investigated their involvement in glucose preference conditioning in mice. In Exp. 1 MCH receptor 1 knockout (KO) and C57BL/6 wildtype (WT) mice learned to prefer 8% glucose over an initially more-preferred non-nutritive 0.1% sucralose+saccharin (S+S) solution. In contrast, the KO and WT mice preferred S+S to 8% fructose, which is consistent with this sugar's weak post-oral reinforcing action. In Exp. 2 KO and WT mice were trained to drink a flavored solution (CS+) paired with intragastric (IG) infusion of 16% glucose and a different flavored solution (CS-) paired with IG water. Both groups drank more CS+ than CS- in training and preferred the CS+ to CS- in a 2-bottle test. These results indicate that MCH receptor signaling is not required for flavor preferences conditioned by the post-oral actions of glucose. This contrasts with other findings implicating MCH signaling in other types of sugar reward processing.

Integration of reward signalling and appetite regulating peptide systems in the control of food-cue responses

Understanding the neurobiological substrates that encode learning about food-associated cues and how those signals are modulated is of great clinical importance especially in light of the worldwide obesity problem. Inappropriate or maladaptive responses to food-associated cues can promote over-consumption, leading to excessive energy intake and weight gain. Chronic exposure to foods rich in fat and sugar alters the reinforcing value of foods and weakens inhibitory neural control, triggering learned, but maladaptive, associations between environmental cues and food rewards. Thus, responses to food-associated cues can promote cravings and food-seeking by activating mesocorticolimbic dopamine neurocircuitry, and exert physiological effects including salivation. These responses may be analogous to the cravings experienced by abstaining drug addicts that can trigger relapse into drug self-administration. Preventing cue-triggered eating may therefore reduce the over-consumption seen in obesity and binge-eating disorder. In this review we discuss recent research examining how cues associated with palatable foods can promote reward-based feeding behaviours and the potential involvement of appetite-regulating peptides including leptin, ghrelin, orexin and melanin concentrating hormone. These peptide signals interface with mesolimbic dopaminergic regions including the ventral tegmental area to modulate reactivity to cues associated with palatable foods. Thus, a novel target for anti-obesity therapeutics is to reduce non-homeostatic, reward driven eating behaviour, which can be triggered by environmental cues associated with highly palatable, fat and sugar rich foods.

Prenatal exposure to ethanol stimulates hypothalamic CCR2 chemokine receptor system: Possible relation to increased density of orexigenic peptide neurons and ethanol drinking in adolescent offspring

Clinical and animal studies indicate that maternal consumption of ethanol during pregnancy increases alcohol drinking in the offspring. Possible underlying mechanisms may involve orexigenic peptides, which are stimulated by prenatal ethanol exposure and themselves promote drinking. Building on evidence that ethanol stimulates neuroimmune factors such as the chemokine CCL2 that in adult rats is shown to colocalize with the orexigenic peptide, melanin-concentrating hormone (MCH) in the lateral hypothalamus (LH), the present study sought to investigate the possibility that CCL2 or its receptor CCR2 in LH is stimulated by prenatal ethanol exposure, perhaps specifically within MCH neurons. Our paradigm of intraoral administration of ethanol to pregnant rats, at low-to-moderate doses (1 or 3g/kg/day) during peak hypothalamic neurogenesis, caused in adolescent male offspring twofold increase in drinking of and preference for ethanol and reinstatement of ethanol drinking in a two-bottle choice paradigm under an intermittent access schedule. This effect of prenatal ethanol exposure was associated with an increased expression of MCH and density of MCH(+) neurons in LH of preadolescent offspring. Whereas CCL2(+) cells at this age were low in density and unaffected by ethanol, CCR2(+) cells were dense in LH and increased by prenatal ethanol, with a large percentage (83-87%) identified as neurons and found to colocalize MCH. Prenatal ethanol also stimulated the genesis of CCR2(+) and MCH(+) neurons in the embryo, which co-labeled the proliferation marker, BrdU. Ethanol also increased the genesis and density of neurons that co-expressed CCR2 and MCH in LH, with triple-labeled CCR2(+)/MCH(+)/BrdU(+) neurons that were absent in control rats accounting for 35% of newly generated neurons in ethanol-exposed rats. With both the chemokine and MCH systems believed to promote ethanol consumption, this greater density of CCR2(+)/MCH(+) neurons in the LH of preadolescent rats suggests that these systems function together in promoting alcohol drinking during adolescence.

Method for determining the level of food motivation in rats

A new method for the quantitative evaluation of the level of food motivation was developed. This method takes into account not only the information, but also the energy component of operant feeding behavior of different intensity and effectiveness with a simultaneous study of metabolism by means of indirect calorimetry. Our experiments showed that an increase in the number of lever pressing episodes (from 1 to 8) to obtain one food granule during operant feeding behavior is accompanied by a progressive decrease in the level of food motivation. The level of food motivation remains practically unchanged with an increase in the ratio of pressing episodes to 16 and 32 (despite consumption of food).

Predictive models of glucose control: roles for glucose-sensing neurones

The brain can be viewed as a sophisticated control module for stabilizing blood glucose. A review of classical behavioural evidence indicates that central circuits add predictive (feedforward/anticipatory) control to the reactive (feedback/compensatory) control by peripheral organs. The brain/cephalic control is constructed and engaged, via associative learning, by sensory cues predicting energy intake or expenditure (e.g. sight, smell, taste, sound). This allows rapidly measurable sensory information (rather than slowly generated internal feedback signals, e.g. digested nutrients) to control food selection, glucose supply for fight-or-flight responses or preparedness for digestion/absorption. Predictive control is therefore useful for preventing large glucose fluctuations. We review emerging roles in predictive control of two classes of widely projecting hypothalamic neurones, orexin/hypocretin (ORX) and melanin-concentrating hormone (MCH) cells. Evidence is cited that ORX neurones (i) are activated by sensory cues (e.g. taste, sound), (ii) drive hepatic production, and muscle uptake, of glucose, via sympathetic nerves, (iii) stimulate wakefulness and exploration via global brain projections and (iv) are glucose-inhibited. MCH neurones are (i) glucose-excited, (ii) innervate learning and reward centres to promote synaptic plasticity, learning and memory and (iii) are critical for learning associations useful for predictive control (e.g. using taste to predict nutrient value of food). This evidence is unified into a model for predictive glucose control. During associative learning, inputs from some glucose-excited neurones may promote connections between the 'fast' senses and reward circuits, constructing neural shortcuts for efficient action selection. In turn, glucose-inhibited neurones may engage locomotion/exploration and coordinate the required fuel supply. Feedback inhibition of the latter neurones by glucose would ensure that glucose fluxes they stimulate (from liver, into muscle) are balanced. Estimating nutrient challenges from indirect sensory cues may become more difficult when the cues become complex and variable (e.g. like human foods today). Consequent errors of predictive glucose control may contribute to obesity and diabetes.

The role of the neurokinin-1 receptor in stress-induced reinstatement of alcohol and cocaine seeking

Neurokinin-1 receptors (NK1Rs) have been shown to mediate alcohol and opiate, but not cocaine reward in rodents. We recently reported that NK1R antagonism also blocks stress-induced reinstatement of alcohol seeking in rats, but it is presently unknown whether these antirelapse properties extend to other drug classes. Although some work has suggested that intracranial substance P (SP) infusion reinstates cocaine seeking following extinction, no studies have indicated a direct role for the NK1R in reinstatement of cocaine seeking. Here, we explored the effect of the NK1R antagonist L822429 on yohimbine-induced reinstatement of alcohol or cocaine seeking in Long-Evans rats. Consistent with our previous findings with footshock-induced reinstatement of alcohol seeking in Wistar rats, we found that L822429 attenuates yohimbine-induced reinstatement of alcohol seeking, but does not affect baseline alcohol self-administration. We observed a similar suppression of yohimbine-induced reinstatement of cocaine seeking by L822429, and found that Long-Evans rats exhibit greater sensitivity to NK1R antagonism than Wistar rats. Accordingly, Long-Evans rats exhibit differences in the expression of NK1Rs in some subcortical brain regions. Combined, our findings suggest that while NK1R antagonism differentially influences alcohol- and cocaine-related behavior, this receptor mediates stress-induced seeking of both drugs.

The use of the reinstatement model to study relapse to palatable food seeking during dieting

Excessive consumption of unhealthy foods is a major public health problem. While many people attempt to control their food intake through dieting, many relapse to unhealthy eating habits within a few months. We have begun to study this clinical condition in rats by adapting the reinstatement model, which has been used extensively to study relapse to drug seeking. In our adaptation of the relapse model, reinstatement of palatable food seeking by exposure to food-pellet priming, food-associated cues, or stress is assessed in food-restricted (to mimic dieting) rats after operant food-pellet self-administration training and subsequent extinction of the food-reinforced responding. In this review, we first outline the clinical problem and discuss a recent study in which we assessed the predictive validity of the reinstatement model for studying relapse to food seeking during dieting by using the anorexigenic drug fenfluramine. Next, we summarize results from our initial studies on the role of several stress- and feeding-related peptides (corticotropin-releasing factor, hypocretin, melanin-concentrating hormone, peptide YY3-36) in reinstatement of palatable food seeking. We then present results from our studies on the role of dopamine and medial prefrontal cortex in stress-induced reinstatement of food seeking. We conclude by discussing potential clinical implications. We offer two main conclusions: (1) the food reinstatement model is a simple, reliable, and valid model to study mechanisms of relapse to palatable food seeking during dieting, and to identify medications to prevent this relapse; (2) mechanisms of relapse to food seeking are often dissociable from mechanisms of ongoing food intake. This article is part of a Special Issue entitled 'NIDA 40th Anniversary Issue'.

The neurobiology of modafinil as an enhancer of cognitive performance and a potential treatment for substance use disorders

RATIONALE AND OBJECTIVES

Modafinil (MOD) and its R-enantiomer (R-MOD) are approved medications for narcolepsy and other sleep disorders. They have also been used, off-label, as cognitive enhancers in populations of patients with mental disorders, including substance abusers that demonstrate impaired cognitive function. A debated nonmedical use of MOD in healthy individuals to improve intellectual performance is raising questions about its potential abuse liability in this population.

RESULTS AND CONCLUSIONS

MOD has low micromolar affinity for the dopamine transporter (DAT). Inhibition of dopamine (DA) reuptake via the DAT explains the enhancement of DA levels in several brain areas, an effect shared with psychostimulants like cocaine, methylphenidate, and the amphetamines. However, its neurochemical effects and anatomical pattern of brain area activation differ from typical psychostimulants and are consistent with its beneficial effects on cognitive performance processes such as attention, learning, and memory. At variance with typical psychostimulants, MOD shows very low, if any, abuse liability, in spite of its use as a cognitive enhancer by otherwise healthy individuals. Finally, recent clinical studies have focused on the potential use of MOD as a medication for treatment of drug abuse, but have not shown consistent outcomes. However, positive trends in several result measures suggest that medications that improve cognitive function, like MOD or R-MOD, may be beneficial for the treatment of substance use disorders in certain patient populations.

The roles of melanin-concentrating hormone in energy balance and reproductive function: Are they connected?

Melanin-concentrating hormone (MCH) is an anabolic neuropeptide with multiple and diverse physiological functions including a key role in energy homoeostasis. Rodent studies have shown that the ablation of functional MCH results in a lean phenotype, increased energy expenditure and resistance to diet-induced obesity. These findings have generated interest among pharmaceutical companies vigilant for potential anti-obesity agents. Nutritional status affects reproductive physiology and behaviours, thereby optimising reproductive success and the ability to meet energetic demands. This complex control system entails the integration of direct or indirect peripheral stimuli with central effector systems and involves numerous mediators. A role for MCH in the reproductive axis has emerged, giving rise to the premise that MCH may serve as an integratory mediator between those discrete systems that regulate energy balance and reproductive function. Hence, this review focuses on published evidence concerning i) the role of MCH in energy homoeostasis and ii) the regulatory role of MCH in the reproductive axis. The question as to whether the MCH system mediates the integration of energy homoeostasis with the neuroendocrine reproductive axis and, if so, by what means has received limited coverage in the literature; evidence to date and current theories are summarised herein.

Attenuation of saccharin-seeking in rats by orexin/hypocretin receptor 1 antagonist

RATIONALE

The orexin (Orx)/hypocretin system has been implicated in reward-seeking, especially for highly salient food and drug rewards. We recently demonstrated that signaling at the OxR1 receptor is involved in sucrose reinforcement and reinstatement of sucrose-seeking elicited by sucrose-paired cues in food-restricted rats. Because sucrose reinforcement has both a hedonic and caloric component, it remains unknown what aspect of this reward drives its reinforcing value.

OBJECTIVES

The present study examined the involvement of the Orx system in operant responding for saccharin, a noncaloric, hedonic (sweet) reward, and in cue-induced reinstatement of extinguished saccharin-seeking in ad libitum-fed vs food-restricted male subjects.

METHODS

Male Sprague Dawley rats were fed ad libitum or food-restricted and trained to self-administer saccharin. We determined the effects of pretreatment with the OxR1 receptor antagonist SB-334867 (SB; 10-30 mg/kg) on fixed ratio (FR) saccharin self-administration and on cue-induced reinstatement of extinguished saccharin-seeking.

RESULTS

SB decreased responding and number of reinforcers earned during FR responding for saccharin and decreased cue-induced reinstatement of extinguished saccharin-seeking. All of these effects were obtained similarly in food-restricted and ad libitum-fed rats.

CONCLUSIONS

These results indicate that signaling at the OxR1 receptor is involved in saccharin reinforcement and reinstatement of saccharin-seeking elicited by saccharin-paired cues regardless of food restriction. These findings lead us to conclude that the Orx system contributes to the motivational effects of hedonic food rewards, independently of caloric value and homeostatic needs.

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.

Role of a genetic polymorphism in the corticotropin-releasing factor receptor 1 gene in alcohol drinking and seeking behaviors of marchigian sardinian alcohol-preferring rats

Marchigian Sardinian alcohol-preferring (msP) rats exhibit innate preference for alcohol, are highly sensitive to stress and stress-induced alcohol seeking. Genetic analysis showed that over-expression of the corticotropin-releasing factor (CRF) system of msP rats is correlated with the presence of two single nucleotide polymorphisms (SNPs) occurring in the promoter region (position -1836 and -2097) of the CRF1 receptor (CRF1-R) gene. Here we examined whether these point mutations were associated to the innate alcohol preference, stress-induced drinking, and seeking. We have recently re-derived the msP rats to obtain two distinct lines carrying the wild type (GG) and the point mutations (AA), respectively. The phenotypic characteristics of these two lines were compared with those of unselected Wistar rats. Both AA and GG rats showed similar patterns of voluntary alcohol intake and preference. Similarly, the pharmacological stressor yohimbine (0.0, 0.625, 1.25, and 2.5 mg/kg) elicited increased operant alcohol self-administration under fixed and progressive ratio reinforcement schedules in all three lines. Following extinction, yohimbine (0.0, 0.625, 1.25, and 2.5 mg/kg) significantly reinstated alcohol seeking in the three groups. However, at the highest dose this effect was no longer evident in AA rats. Treatment with the CRF1-R antagonist antalarmin (0, 5, 10, and 20 mg/kg) significantly reduced alcohol-reinforced lever pressing in the AA line (10 and 20 mg/kg) while a weaker or no effect was observed in the Wistar and the GG group, respectively. Finally, antalarmin significantly reduced yohimbine-induced increase in alcohol drinking in all three groups. In conclusion, these specific SNPs in the CRF1-R gene do not seem to play a primary role in the expression of the msP excessive drinking phenotype or stress-induced drinking but may be associated with a decreased threshold for stress-induced alcohol seeking and an increased sensitivity to the effects of pharmacological blockade of CRF1-R on alcohol drinking.