Deletion of Melanin Concentrating Hormone Receptor-1 disrupts overeating in the presence of food cues

Melanin concentrating hormone projections to the nucleus accumbens enhance the reward value of food consumption and do not induce feeding or REM sleep

Regulation of food intake and energy balance is critical to survival. Hunger develops as a response to energy deficit and drives food-seeking and consumption. However, motivations to eat are varied in nature, and promoted by factors other than energy deficit. When dysregulated, non-homeostatic drives to consume can contribute to disorders of food intake, adding to the increasing prevalence of restrictive eating disorders and obesity. Melanin-concentrating hormone (MCH) neurons have been implicated in the regulation of feeding behavior, in addition to a number of other fundamental behaviors including sleep, anxiety, and maternal behavior. Several studies suggest that MCH peptide increases food consumption, while studies of MCH neurons show effects only on cued feeding, and others show no effect of MCH neuron manipulation on feeding. MCH neurons have widespread projections to diverse downstream brain regions yet few studies have investigated the function of specific projections or differentiated the behaviors they regulate. Here we use optogenetics, in combination with different behavioral paradigms, to elucidate the role of MCH projections to the nucleus accumbens (NAc) in sleep and feeding behavior. We show that MCH neurons projecting to the NAc do not induce changes in baseline feeding or REM sleep, but do enhance the preference for a food paired with optogenetic stimulation. Furthermore, this effect is diminished in female mice relative to males, in line with previous results suggesting sex differences in the functional role of MCH neurons. These results suggest that MCH projections to the NAc can enhance the rewarding value of consumed food.

Cue-potentiated feeding in rodents: Implications for weight regulation in obesogenic environments

Cue-potentiated feeding (CPF) describes instances where food intake is increased by exposure to conditioned cues associated with food, often in the absence of hunger. CPF effects have been reported in a range of experimental protocols developed by researchers working across diverse fields spanning behavioural neuroscience, social psychology and ecology. Here we review the evolution of research on cue-potentiated feeding in animal models to identify important behavioural parameters and key neural circuits and pharmacological systems underlying the effect. Overall, evidence indicates that social, discrete and contextual stimuli can be used to elicit CPF effects across multiple species, though effects are often subtle and sensitive to procedural variables. While regular exposure to food cues is thought to be a key risk factor for overeating in so-called 'obesogenic' environments, further work is needed to identify whether CPF promotes positive energy balance and weight gain over the longer term. We suggest several methodological and conceptual areas for inquiry to elucidate the contribution of CPF to the regulation of food choice and energy intake.

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.

Hypothalamic melanin-concentrating hormone neurons integrate food-motivated appetitive and consummatory processes in rats

The lateral hypothalamic area (LHA) integrates homeostatic processes and reward-motivated behaviors. Here we show that LHA neurons that produce melanin-concentrating hormone (MCH) are dynamically responsive to both food-directed appetitive and consummatory processes in male rats. Specifically, results reveal that MCH neuron Ca2+ activity increases in response to both discrete and contextual food-predictive cues and is correlated with food-motivated responses. MCH neuron activity also increases during eating, and this response is highly predictive of caloric consumption and declines throughout a meal, thus supporting a role for MCH neurons in the positive feedback consummatory process known as appetition. These physiological MCH neural responses are functionally relevant as chemogenetic MCH neuron activation promotes appetitive behavioral responses to food-predictive cues and increases meal size. Finally, MCH neuron activation enhances preference for a noncaloric flavor paired with intragastric glucose. Collectively, these data identify a hypothalamic neural population that orchestrates both food-motivated appetitive and intake-promoting consummatory processes.

The melanin-concentrating hormone system as a target for the treatment of sleep disorders

Given the widespread prevalence of sleep disorders and their impacts on health, it is critical that researchers continue to identify and evaluate novel avenues of treatment. Recently the melanin-concentrating hormone (MCH) system has attracted commercial and scientific interest as a potential target of pharmacotherapy for sleep disorders. This interest emerges from basic scientific research demonstrating a role for MCH in regulating sleep, and particularly REM sleep. In addition to this role in sleep regulation, the MCH system and the MCH receptor 1 (MCHR1) have been implicated in a wide variety of other physiological functions and behaviors, including feeding/metabolism, reward, anxiety, depression, and learning. The basic research literature on sleep and the MCH system, and the history of MCH drug development, provide cause for both skepticism and cautious optimism about the prospects of MCH-targeting drugs in sleep disorders. Extensive efforts have focused on developing MCHR1 antagonists for use in obesity, however, few of these drugs have advanced to clinical trials, and none have gained regulatory approval. Additional basic research will be needed to fully characterize the MCH system’s role in sleep regulation, for example, to fully differentiate between MCH-neuron and peptide/receptor-mediated functions. Additionally, a number of issues relating to drug design will continue to pose a practical challenge for novel pharmacotherapies targeting the MCH system.

Food cue reactivity: Neurobiological and behavioral underpinnings

The modern obesogenic environment contains an abundance of food cues (e.g., sight, smell of food) as well cues that are associated with food through learning and memory processes. Food cue exposure can lead to food seeking and excessive consumption in otherwise food-sated individuals, and a high level of food cue responsivity is a risk factor for overweight and obesity. Similar food cue responses are observed in experimental rodent models, and these models are therefore useful for mechanistically identifying the neural circuits mediating food cue responsivity. This review draws from both experimental rodent models and human data to characterize the behavioral and biological processes through which food-associated stimuli contribute to overeating and weight gain. Two rodent models are emphasized - cue-potentiated feeding and Pavlovian-instrumental transfer - that provide insight in the neural circuits and peptide systems underlying food cue responsivity. Data from humans are highlighted that reveal physiological, psychological, and neural mechanisms that connect food cue responsivity with overeating and weight gain. The collective literature identifies connections between heightened food cue responsivity and obesity in both rodents and humans, and identifies underlying brain regions (nucleus accumbens, amygdala, orbitofrontal cortex, hippocampus) and endocrine systems (ghrelin) that regulate food cue responsivity in both species. These species similarities are encouraging for the possibility of mechanistic rodent model research and further human research leading to novel treatments for excessive food cue responsivity in humans.

The physiological control of eating: signals, neurons, and networks

During the past 30 yr, investigating the physiology of eating behaviors has generated a truly vast literature. This is fueled in part by a dramatic increase in obesity and its comorbidities that has coincided with an ever increasing sophistication of genetically based manipulations. These techniques have produced results with a remarkable degree of cell specificity, particularly at the cell signaling level, and have played a lead role in advancing the field. However, putting these findings into a brain-wide context that connects physiological signals and neurons to behavior and somatic physiology requires a thorough consideration of neuronal connections: a field that has also seen an extraordinary technological revolution. Our goal is to present a comprehensive and balanced assessment of how physiological signals associated with energy homeostasis interact at many brain levels to control eating behaviors. A major theme is that these signals engage sets of interacting neural networks throughout the brain that are defined by specific neural connections. We begin by discussing some fundamental concepts, including ones that still engender vigorous debate, that provide the necessary frameworks for understanding how the brain controls meal initiation and termination. These include key word definitions, ATP availability as the pivotal regulated variable in energy homeostasis, neuropeptide signaling, homeostatic and hedonic eating, and meal structure. Within this context, we discuss network models of how key regions in the endbrain (or telencephalon), hypothalamus, hindbrain, medulla, vagus nerve, and spinal cord work together with the gastrointestinal tract to enable the complex motor events that permit animals to eat in diverse situations.

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.

Oxytocin and Food Intake Control: Neural, Behavioral, and Signaling Mechanisms

The neuropeptide oxytocin is produced in the paraventricular hypothalamic nucleus and the supraoptic nucleus of the hypothalamus. In addition to its extensively studied influence on social behavior and reproductive function, central oxytocin signaling potently reduces food intake in both humans and animal models and has potential therapeutic use for obesity treatment. In this review, we highlight rodent model research that illuminates various neural, behavioral, and signaling mechanisms through which oxytocin’s anorexigenic effects occur. The research supports a framework through which oxytocin reduces food intake via amplification of within-meal physiological satiation signals rather than by altering between-meal interoceptive hunger and satiety states. We also emphasize the distributed neural sites of action for oxytocin’s effects on food intake and review evidence supporting the notion that central oxytocin is communicated throughout the brain, at least in part, through humoral-like volume transmission. Finally, we highlight mechanisms through which oxytocin interacts with various energy balance-associated neuropeptide and endocrine systems (e.g., agouti-related peptide, melanin-concentrating hormone, leptin), as well as the behavioral mechanisms through which oxytocin inhibits food intake, including effects on nutrient-specific ingestion, meal size control, food reward-motivated responses, and competing motivations.

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.

Melanin-concentrating hormone in rat nucleus accumbens or lateral hypothalamus differentially impacts morphine and food seeking behaviors

BACKGROUND

Identifying neural substrates that are differentially affected by drugs of abuse and natural rewards is key to finding a target for an efficacious treatment for substance abuse. Melanin-concentrating hormone is a polypeptide with an inhibitory effect on the mesolimbic dopamine system. Here we test the hypothesis that melanin-concentrating hormone in the lateral hypothalamus and nucleus accumbens shell is differentially involved in the regulation of morphine and food-rewarded behaviors.

METHODS

Male Sprague-Dawley rats were trained with morphine (5.0 mg/kg, subcutaneously) or food pellets (standard chow, 10-14 g) to induce a conditioned place preference, immediately followed by extinction training. Melanin-concentrating hormone (1.0 µg/side) or saline was infused into the nucleus accumbens shell or lateral hypothalamus before the reinstatement primed by morphine or food, and locomotor activity was simultaneously monitored. As the comparison, melanin-concentrating hormone was also microinjected into the nucleus accumbens shell or lateral hypothalamus before the expression of food or morphine-induced conditioned place preference.

RESULTS

Microinfusion of melanin-concentrating hormone into the nucleus accumbens shell (but not into the lateral hypothalamus) prevented the reinstatement of morphine conditioned place preference but had no effect on the reinstatement of food conditioned place preference. In contrast, microinfusion of melanin-concentrating hormone into the lateral hypothalamus (but not in the nucleus accumbens shell) inhibited the reinstatement of food conditioned place preference but had no effect on the reinstatement of morphine conditioned place preference.

CONCLUSIONS

These results suggest a clear double dissociation of melanin-concentrating hormone in morphine/food rewarding behaviors and melanin-concentrating hormone in the nucleus accumbens shell. Melanin-concentrating hormone could be a potential target for therapeutic intervention for morphine abuse without affecting natural rewards.

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.

Neural substrates of fear-induced hypophagia in male and female rats

Cessation of eating under fear is an adaptive response that aids survival by prioritizing the expression of defensive behaviors over feeding behavior. However, this response can become maladaptive when persistent. Thus, accurate mediation of the competition between fear and feeding is important in health and disease; yet, the underlying neural substrates are largely unknown. The current study identified brain regions that were recruited when a fear cue inhibited feeding in male and female rats. We used a previously established behavioral paradigm to elicit hypophagia with a conditioned cue for footshocks, and Fos imaging to map activation patterns during this behavior. We found that distinct patterns of recruitment were associated with feeding and fear expression, and that these patterns were similar in males and females except within the medial prefrontal cortex (mPFC). In both sexes, food consumption was associated with activation of cell groups in the central amygdalar nucleus, hypothalamus, and dorsal vagal complex, and exposure to food cues was associated with activation of the anterior basolateral amygdalar nucleus. In contrast, fear expression was associated with activation of the lateral and posterior basomedial amygdalar nuclei. Interestingly, selective recruitment of the mPFC in females, but not in males, was associated with both feeding and freezing behavior, suggesting sex differences in the neuronal processing underlying the competition between feeding and fear. This study provided the first evidence of the neural network mediating fear-induced hypophagia, and important functional activation maps for future interrogation of the underlying neural substrates.

Lateral Hypothalamus as a Motivation-Cognition Interface in the Control of Feeding Behavior

Converging evidence for an essential function of the lateral hypothalamus (LHA) in the control of feeding behavior has been accumulating since the classic work conducted almost 80 years ago. The LHA is also important in reward and reinforcement processes and behavioral state control. A unifying function for the LHA across these processes has not been fully established. Nonetheless, it is considered to integrate motivation with behavior. More recent work has demonstrated that the LHA is also required when cognitive processes, such as associative learning and memory control feeding behavior, suggesting it may serve as a motivation-cognition interface. Structurally, the LHA is well positioned within the cerebral hemisphere, with its extensive connectional network across the forebrain-brainstem axis, to link motivational and behavioral systems with cognitive processes. Studies that examined how learned cues control food seeking and consumption have implicated the LHA, but due to methodological limitations could not determine whether it underlies motivation, learning, or the integration of these processes. Furthermore, the identification of specific substrates has been limited by the LHA's extraordinary complexity and heterogeneity. Recent methodological advancements with chemo-and opto-genetic approaches have enabled unprecedented specificity in interrogations of distinct neurons and their pathways in behaving animals, including manipulations during temporally distinct events. These approaches have revealed novel insights about the LHA structure and function. Recent findings that the GABA LHA neurons control feeding and food-reward learning and memory will be reviewed together with past work within the context of the LHA function as an interface between cognition and motivation.

Contributions of Pavlovian incentive motivation to cue-potentiated feeding

Cues signaling the availability of palatable food acquire the ability to potentiate food seeking and consumption. The current study employed a combination of behavioral, pharmacological, and analytical techniques to probe the role of Pavlovian incentive motivation in cue-potentiated feeding. We show that a cue paired with sucrose solution (CS+) can transfer its control over feeding to stimulate sucrose consumption at a new receptacle, and that this effect depends on activation of D1 dopamine receptors, which is known to modulate other forms of cue-motivated behavior but not taste palatability. Microstructural analyses of sucrose-licking behavior revealed that the CS+ tended to increase the frequency with which rats engaged in active bouts of licking behavior without having a reliable effect on the duration of those licking bouts, a measure that was instead associated with sucrose palatability. Furthermore, we found that individual differences in CS+ elicited increases in bout frequency were associated with total sucrose intake at test, supporting the view that this process was related to meaningful dysregulation of eating behavior. The current study, therefore, (1) demonstrates that a dopamine-dependent Pavlovian incentive motivational process can mediate cue-potentiated feeding, and (2) lays out an experimental and analytical approach for parsing this aspect of behavior.

Examining the influence of CS duration and US density on cue-potentiated feeding through analyses of licking microstructure

In the current study, groups of mice were trained with either short (20 s) or long (120 s) conditioned stimulus (CS) durations associated with different rates of sucrose unconditioned stimulus (US) delivery, to examine whether different behavioral forms of cue-potentiated feeding in sated mice would be evoked. In training mice received presentations of an auditory CS for 20 s during which a sucrose US was delivered at a density of 1/9 s (Group-20-s). A second group of mice received an auditory CS for 120 s and a US density of 1/49 s (Group-120-s). During training, a shorter CS duration and higher rate of US delivery resulted in greater acquisition of food cup responding, and during the test stage Group-20-s mice also displayed higher CS evoked lick rates, though all mice showed cue-potentiated feeding. An analysis of licking microstructure also revealed that Group-120-s mice displayed CS evoked licking behavior that reflected an increase in the perceived palatability of the sucrose US. These findings are discussed with respect to the influence of CS interval and US density on associatively activated sensory and affective representations of a US, and contrast mediated effects resulting from presentation of excitatory and inhibitory conditioned stimuli.

Characterizing ingestive behavior through licking microstructure: Underlying neurobiology and its use in the study of obesity in animal models

Ingestive behavior is controlled by multiple distinct peripheral and central physiological mechanisms that ultimately determine whether a particular food should be accepted or avoided. As rodents consume a fluid they display stereotyped rhythmic tongue movements, and by analyzing the temporal distribution of pauses of licking, it is possible through analyses of licking microstructure to uncover dissociable evaluative and motivational variables that contribute to ingestive behavior. The mean number of licks occurring within each burst of licking (burst and cluster size) reflects the palatability of the consumed solution, whereas the frequency of initiating novel bouts of licking behavior (burst and cluster number) is dependent upon the degree of gastrointestinal inhibition that accrues through continued fluid ingestion. This review describes the analysis of these measures within a context of the behavioral variables that come to influence the acceptance or avoidance of a fluid, and the neurobiological mechanisms that underlie alterations in the temporal distribution of pauses of licks. The application of these studies to models of obesity in animals is also described.

Obesity: Current and potential pharmacotherapeutics and targets

Obesity is a global epidemic that contributes to a number of health complications including cardiovascular disease, type 2 diabetes, cancer and neuropsychiatric disorders. Pharmacotherapeutic strategies to treat obesity are urgently needed. Research over the past two decades has increased substantially our knowledge of central and peripheral mechanisms underlying homeostatic energy balance. Homeostatic mechanisms involve multiple components including neuronal circuits, some originating in hypothalamus and brain stem, as well as peripherally-derived satiety, hunger and adiposity signals that modulate neural activity and regulate eating behavior. Dysregulation of one or more of these homeostatic components results in obesity. Coincident with obesity, reward mechanisms that regulate hedonic aspects of food intake override the homeostatic regulation of eating. In addition to functional interactions between homeostatic and reward systems in the regulation of food intake, homeostatic signals have the ability to alter vulnerability to drug abuse. Regarding the treatment of obesity, pharmacological monotherapies primarily focus on a single protein target. FDA-approved monotherapy options include phentermine (Adipex-P®), orlistat (Xenical®), lorcaserin (Belviq®) and liraglutide (Saxenda®). However, monotherapies have limited efficacy, in part due to the recruitment of alternate and counter-regulatory pathways. Consequently, a multi-target approach may provide greater benefit. Recently, two combination products have been approved by the FDA to treat obesity, including phentermine/topiramate (Qsymia®) and naltrexone/bupropion (Contrave®). The current review provides an overview of homeostatic and reward mechanisms that regulate energy balance, potential therapeutic targets for obesity and current treatment options, including some candidate therapeutics in clinical development. Finally, challenges in anti-obesity drug development are discussed.

Sleep & metabolism: The multitasking ability of lateral hypothalamic inhibitory circuitries

The anatomical and functional mapping of lateral hypothalamic circuits has been limited by the numerous cell types and complex, yet unclear, connectivity. Recent advances in functional dissection of input-output neurons in the lateral hypothalamus have identified subset of inhibitory cells as crucial modulators of both sleep-wake states and metabolism. Here, we summarize these recent studies and discuss the multi-tasking functions of hypothalamic circuitries in integrating sleep and metabolism in the mammalian brain.

Disruptions in effort-based decision-making and consummatory behavior following antagonism of the dopamine D2 receptor

Dopamine is known to influence motivational processes, however the precise role of this neurotransmitter remains a contentious issue. In the current study we sought to further characterize dopamine signaling in reward-based decision-making and consummatory behavior in mice, via lateral ventricle infusion of the dopamine D2 receptor antagonist eticlopride. In Experiment 1, we examined effort-based decision-making, in which mice had a choice between one lever, where a single response led to the delivery of a low value reward (2% sucrose); and a second lever, which led to a higher value reward (20% sucrose) that gradually required more effort to obtain. As the response schedule for the high value reward became more strict, low dose (4μg in 0.5μl) central infusions of eticlopride biased preference away from the high value reward, and toward the lever that led to the low value reward. Similarly, a higher dose of eticlopride (8μg in 0.5μl) also disrupted choice responding for the high value reward, however it did so by increasing omissions. In Experiment 2, we assessed the effects of eticlopride on consumption of 20% sucrose. The antagonist led to a dose-dependent reduction in intake, and through an analysis of licking microstructure, it was revealed that this in part reflected a reduction in the motivation to engage in consummatory behavior, rather than alterations in the evaluation of the reward. These results suggest that disruptions in D2 receptor signaling reduce the willingness to engage in effortful operant responding and consumption of a desirable outcome.

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.

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.

Genome-Wide Mapping of Growth-Related Quantitative Trait Loci in Orange-Spotted Grouper (Epinephelus coioides) Using Double Digest Restriction-Site Associated DNA Sequencing (ddRADseq)

Mapping of quantitative trait loci (QTL) is essential for the discovery of genetic structures that related to complex quantitative traits. In this study, we identified 264,072 raw SNPs (single-nucleotide polymorphisms) by double digest restriction site associated DNA sequencing (ddRADseq), and utilized 3029 of these SNPs to construct a genetic linkage map in orange-spotted grouper (Epinephelus coioides) using a regression mapping algorithm. The genetic map contained 24 linkage groups (LGs) spanning a total genetic distance of 1231.98 cM. Twenty-seven significant growth-related QTLs were identified. Furthermore, we identified 17 genes (fez2, alg3, ece2, arvcf, sla27a4, sgk223, camk2, prrc2b, mchr1, sardh, pappa, syk, tert, wdrcp91, ftz-f1, mate1 and notch1) including three (tert, ftz-f1 and notch1) that have been reported to be involved in fish growth. To summarize, we mapped growth-related QTLs in the orange-spotted grouper. These QTLs will be useful in marker-assisted selection (MAS) efforts to improve growth-related traits in this economically important fish.

The antagonism of ghrelin alters the appetitive response to learned cues associated with food

The rapid increase in obesity may be partly mediated by an increase in the exposure to cues for food. Food-paired cues play a role in food procurement and intake under conditions of satiety. The mechanism by which this occurs requires characterization, but may involve ghrelin. This orexigenic peptide alters the response to food-paired conditioned stimuli, and neural responses to food images in reward nuclei. Therefore, we tested whether a ghrelin receptor antagonist alters the influence of food-paired cues on the performance of instrumental responses that earn food and the consumption of food itself using tests of Pavlovian-to-instrumental transfer (PIT) and cue potentiated feeding (CPF), respectively. Food-deprived rats received Pavlovian conditioning where an auditory cue was paired with delivery of sucrose solution followed by instrumental conditioning to lever press for sucrose. Following training, rats were given ad libitum access to chow. On test day, rats were injected with the ghrelin receptor antagonist GHRP-6 [D-Lys3] and then tested for PIT or CPF. Disrupting ghrelin signaling enhanced expression of PIT. In addition, GHRP-6 [D-Lys3] impaired the initiation of feeding behavior in CPF without influencing overall intake of sucrose. Finally, in PIT tested rats, enhanced FOS immunoreactivity was revealed following the antagonist in regions thought to underlie PIT; however, the antagonist had no effect on FOS immunoreactivity in CPF tested rats.

Organization of connections between the amygdala, medial prefrontal cortex, and lateral hypothalamus: a single and double retrograde tracing study in rats

The amygdala and medial prefrontal cortex (mPFC) are highly interconnected telencephalic areas critical for cognitive processes, including associative learning and decision making. Both structures strongly innervate the lateral hypothalamus (LHA), an important component of the networks underlying the control of feeding and other motivated behaviors. The amygdala-prefrontal-lateral hypothalamic system is therefore well positioned to exert cognitive control over behavior. However, the organization of this system is not well defined, particularly the topography of specific circuitries between distinct cell groups within these complex, heterogeneous regions. This study used two retrograde tracers to map the connections from the amygdala (central and basolateral area nuclei) and mPFC to the LHA in detail, and to determine whether amygdalar pathways to the mPFC and to LHA originate from the same or different neurons. One tracer was placed into a distinct mPFC area (dorsal anterior cingulate, prelimbic, infralimbic, or rostromedial orbital), and the other into dorsal or ventral LHA. We report that the central nucleus and basolateral area of the amygdala send projections to distinct LHA regions, dorsal and ventral, respectively. The basolateral area, but not central nucleus, also sends substantial projections to the mPFC, topographically organized rostrocaudal to dorsoventral. The entire mPFC, in turn, projects to the LHA, providing a separate route for potential amygdalar influence following mPFC processing. Nearly all amygdalar projections to the mPFC and to the LHA originated from different neurons suggesting amygdala and amygdala-mPFC processing influence the LHA independently, and the balance of these parallel pathways ultimately controls motivated behaviors.