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

PHB-10889; No of Pages 6 Physiology & Behavior xxx (2015) xxx–xxx

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Deletion of Melanin Concentrating Hormone Receptor-1 disrupts overeating in the presence of food cues Andrew Sherwood a, Peter C. Holland a, Antoine Adamantidis b, Alexander W. Johnson c,⁎ a b c

Department of Psychological and Brain Sciences, Johns Hopkins University, Baltimore, MD 21218, United States Douglas Hospital Research Centre, Department of Psychiatry, McGill University, 6875 LaSalle Boulevard, Montreal, Canada Department of Psychology and Neuroscience Program, Michigan State University, East Lansing, MI 48824, United States

H I G H L I G H T S • • • •

Overeating in the presence of food cues was examined. MCH-1R knockout mice were tested for cue potentiated feeding. Deletion of MCH-1R disrupted cue potentiated feeding. This disruption reflected reduction in size and number of licking bursts.

a r t i c l e

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Article history: Received 13 March 2015 Received in revised form 9 May 2015 Accepted 29 May 2015 Available online xxxx Keywords: Lateral hypothalamus Licking microstructure Binge eating Obesity Glucose sensing Allostasis

a b s t r a c t Exposure to environmental cues associated with food can evoke eating behavior in the absence of hunger. This capacity for reward cues to promote feeding behaviors under sated conditions can be examined in the laboratory using cue-potentiated feeding (CPF). The orexigenic neuropeptide Melanin Concentrating Hormone (MCH) is expressed throughout brain circuitry critical for CPF. We examined whether deletion of the MCH receptor, MCH-1R, would in KO mice disrupt overeating in the presence of a Pavlovian CS+ associated with sucrose delivery. While both wild-type controls and KO mice showed comparable food magazine approach responses during the CPF test, MCH-1R deletion significantly impaired the ability of the CS+ to evoke overeating of sucrose under satiety. Through the use of a refined analysis of meal intake, it was revealed that this disruption to overeating behavior in KO mice reflected a reduction in the capacity for the CS+ to initiate and maintain bursts of licking behavior. These findings suggest that overeating during CPF requires intact MCH-1R signaling and may be due to an influence of the CS+ on the palatability of food and on regulatory mechanisms of peripheral control. Thus, disruptions to MCH-1R signaling may be a useful pharmacological tool to inhibit this form of overeating behavior. © 2015 Elsevier Inc. All rights reserved.

1. Introduction The obesogenic environment is characterized by a sedentary lifestyle and the availability of energy dense foods that can be acquired at little cost [1]. However, the detrimental effects of this environment are proving costly for our society as it contributes to weight gain, obesity and associated co-morbidities (e.g., heart disease and diabetes) [2]. This affects not only obese individuals whose quality of life is severely reduced, but also society in general, where in the US alone associated annual healthcare costs are estimated to be in excess of $190 billion [3]. At the same time, there is a lack of available pharmacotherapeutic strategies to aid in reducing body weight in obese individuals [4]. Thus,

⁎ Corresponding author at: Department of Psychology, Michigan State University, East Lansing, MI 48824, United States. E-mail address: [email protected] (A.W. Johnson).

there is a critical need to identify the variables that influence overeating of food and the underlying brain mechanisms controlling this behavior. Food-associated cues (e.g., television advertisements, radio jingles, and catchy signboards) likely contribute to eating by altering food preferences and enhancing consumption [5], which may promote weight gain and obesity. In the laboratory it is possible to examine the influence of food cues on eating behaviors using cue-potentiated feeding (CPF), where external cues paired with food delivery lead to significant overeating behavior under non-deprived conditions [5,6]. This learned overeating response has been revealed in mice [7], rats [6,8], and humans [9, 10], with foods of varying degrees of nutrition and palatability. CPF has been shown to depend on limbic and prefrontal circuitry that includes the lateral hypothalamus (LH) [11], basolateral amygdala (BLA) [6], ventral hippocampus (VH) [12] and ventromedial prefrontal cortex (vmPFC) [13]. Due to its synthesis in the LH [14,15] and projections through both CPF and classical reward circuits [16], the central feeding peptide Melanin

http://dx.doi.org/10.1016/j.physbeh.2015.05.037 0031-9384/© 2015 Elsevier Inc. All rights reserved.

Please cite this article as: A. Sherwood, et al., Deletion of Melanin Concentrating Hormone Receptor-1 disrupts overeating in the presence of food cues, Physiol Behav (2015), http://dx.doi.org/10.1016/j.physbeh.2015.05.037

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A. Sherwood et al. / Physiology & Behavior xxx (2015) xxx–xxx

Concentrating Hormone (MCH) may play a critical role in influencing eating in the presence of reward cues. MCH exerts its physiological effects by binding to and activating the G protein-coupled MCH receptors, MCH-1R and MCH-2R. While in several species (e.g., primates, dogs and ferrets) the action of MCH-2R is preserved [17], in rodents it is either absent or non-functional [18]. MCH is upregulated during periods of food withdrawal or in hypoleptinemic ob/ob mice [19,20], and elicits food intake when infused centrally [21,22]. Transgenic overexpression of MCH also leads to hyperphagia and weight gain [23], whereas deletion or antagonism of MCH-1R suppresses intake [24,25]. With respect to reward learning, MCH influences both food-seeking and cocaine-seeking [26–28], and deletion of MCH-1R disrupts conditioning of incentive motivation to a reward-paired auditory cue, leading to reductions in its ability to promote novel instrumental nose-poke responding [29]. Given this expression in CPF circuitry and its role in the regulation of food intake and reward learning, we hypothesized that MCH would play a significant role in CPF. Here we used a lack-of-function approach through MCH-1R gene knockout (KO) mice [18]. Under food-deprived conditions, mice were trained to acquire a simple Pavlovian discrimination followed by ad-libitum access to lab chow for a period of ≥3 days. After this satiety treatment, we examined the ability of food cues to promote overeating under non-deprived conditions. Furthermore, we used microstructure analyses to examine the variables that may underlie any changes in consummatory behavior (e.g., orosensory positive feedback and/or conditioned negative feedback) [30–32]. 2. Methods 2.1. Subjects The inactivation of the MCH-1R allele and the generation of KO animals and the genotyping method have been previously described [33]. Heterozygous MCH-1R+/− mice were backcrossed a minimum of eight times to the C57BL/6J strain (Jackson Laboratory, Bar Harbor, ME, USA). Seventeen WT and thirteen KO mice were used and were tested at approximately 3 months old, and were housed three or four to a cage under a 12 h light/dark cycle (lights on at 07:00–19:00 h). Food deprivation began at least 2 days prior to the start of the experiment by restricting access to two daily meal pellets. Behavioral training and testing were completed in the light cycle between 09:00 and 17:00 h and were conducted under the auspices of the Johns Hopkins University Institutional Animal Care and Use Committee. 2.2. Apparatus All behavioral procedures were conducted in six individual chambers (53 × 35 × 35 cm LWH) with aluminum front and back walls, clear polycarbonate sides, and a floor made of 17.8-mm stainless steel rods spaced 0.5 cm apart (Med Associates, St Albans, VT, USA). Each chamber was contained in a custom-built sound attenuating box with tubing connecting to solenoids located on the outside of the sound attenuating box. Fluid rewards could be delivered, via solenoid activation, to a 50 μL food well that was housed in a food magazine and recessed in the center of one end of the chamber. The food well contained a custombuilt lickometer through which fiber optics was used to introduce a light beam through the fluid–air interface of a fluid bolus. Through this approach, individual time-stamped licks were detected as disturbances in the light surface at the interface when the fluid was contacted. A vacuum was attached to the bottom of the food well, which could be released via an attached solenoid. An infrared photocell placed inside the magazine monitored the time spent and number of head entries made into it. An audio generator, which could be programmed to emit a 3 kHz tone or white noise (each 80 dB), was mounted on the outside of the chamber on the wall opposite the magazine. Chamber illumination was provided by a 28 V, 100 mA house light mounted on the inside wall of the sound-attenuating chamber. An IBM-compatible computer

equipped with Med-PC software (Med Associates, St. Albans, VT, USA) controlled and recorded all stimuli and responses. 2.3. Procedure Prior to training mice received at least two days of food deprivation, and once they reached ~ 90% of their baseline weights the magazine training commenced. Mice were placed in their assigned testing chambers with a 10% sucrose (w/v) reward immediately available in the food well. Upon entering the food magazine the mouse was free to consume its reward, initiating the first of 60 trials. The inter-trial interval between reward deliveries varied randomly on a random-time 30 s schedule with a full session of 60 trials taking approximately 30–45 min. At the conclusion of the second day of the magazine training, any subject that did not have at least 10 s of time in the magazine with the reward present (US time) was given additional training in order to reach this criterion. Following the food magazine training, mice received 16 days of Pavlovian conditioning. During each Pavlovian conditioning session, mice received a total of 20 trials: 10 reinforced trials and 10 non-reinforced trials. Each trial consisted of a 20 second presentation of either a tone or white noise stimulus divided into four 5 s epochs. For the reinforced CS+ trials, a 10% sucrose reward was delivered twice during two of the four epochs, with the constraint that for the first 5 s epoch, sucrose delivery could occur on a maximum of three and a minimum of two (out of the ten) trials. Subsequently, CS + responses from the seven trials where sucrose was not delivered during the first 5-s epoch were used as our measure of conditioning. Given this measure was uncontaminated by US delivery, it more appropriately reflected conditioning to the CS. The pseudo-random reward schedule for the epochs was changed every third day. Any reward not consumed was removed at the end of the trial via the vacuum situated at the bottom of the well. During a CS− trial, no reward was delivered. For data presentation, CS− responses during the first 5-s epoch, from seven of the ten trials (chosen at random) were used. Which stimulus was reinforced, tone or noise, was counterbalanced across the two groups. Initially, a dummy solenoid was paired with the reward delivery to facilitate consumption of the reward and acquisition of the stimulus–reward pairing, but this additional cue was removed after the third day of training. At the completion of 16 Pavlovian conditioning sessions, subjects were given 2 days (3 nights) of ad-libitum access to their standard laboratory diet with the goal of restoring subjects to at least 100% of their original baseline weight. Subjects were then tested for CPF with the CS + and CS −, each tested individually on separate days, with the order of tests fully counterbalanced across and within groups of mice. The two potentiated feeding test sessions examined the effects of each stimulus on consumption: the sucrose reward was available for consumption at all times for both tests. At the start of the session, 50 μL of sucrose was available in the food cup, and additional 50 μL deliveries occurred every 40 licks as mice consumed the liquid. Test sessions began with a 2-min baseline period. This was followed by one of 4 test trials during which either the tone or the noise stimulus was presented for 20 s. Each trial was then followed by a 2-min interval before the onset of the next 20 s trial, or the end of the test session. 2.4. Data analyses The Pavlovian training data (magazine responses/min) were analyzed with a mixed ANOVA with within subject variables of cue (CS+, CS-) and session block (1-4), and between-subject variable of group (WT, KO). For the CPF test the rate of licking (licks/min) and the size and number of licking bursts were also examined. A licking burst was defined as two or more consecutive licks, with pauses greater than 1 s determining the licking burst termination. The burst number reflected the initiation of licking behavior following a 1 s pause. We previously conducted an extensive parametric study supporting the suitability of the

Please cite this article as: A. Sherwood, et al., Deletion of Melanin Concentrating Hormone Receptor-1 disrupts overeating in the presence of food cues, Physiol Behav (2015), http://dx.doi.org/10.1016/j.physbeh.2015.05.037

A. Sherwood et al. / Physiology & Behavior xxx (2015) xxx–xxx

1 s pause criterion in C57/BL6J mice [31]. For the CPF test, the patterns of licking behavior were examined using a four-way group × test (CS+, CS−) × period (ITI, CS) × trial (1–4) mixed ANOVA. Finally, food magazine responses/min and licking microstructure (burst size and burst number) were analyzed using separate two-way group (WT, KO) × cue (CS+, CS−) mixed ANOVAs. Significant interactions were followed by planned orthogonal individual comparisons designed to evaluate differences in the pattern of licking or food magazine behavior associated with the CPF test conditions. 3. Results 3.1. Pavlovian training and prefeeding During Pavlovian training both WT and KO mice showed similar acquisition of the simple discrimination (Fig. 1). Three-way ANOVA revealed a main effect of cue (F(1,27) = 52.94, p b 0.001), block (F(3,81) = 7.06, p b 0.001) and an interaction between the two variables (F(1,27) = 20.58, p b 0.001). No effect of group nor its interaction among the variables was revealed (F's b 1, p's N 0.89). In addition to comparable acquisition of the Pavlovian discrimination, both groups of mice had similar weights throughout the study: in the final two sessions of training WT mice weighed 23.0 g ± 0.52 and KO mice weighed 22.28 g ± 0.43 (F(1,27) = 1.10, p = 0.30). Following ad-libitum access to lab chow for 3 days, both WT (mean percent increase in body weight ± SEM = 13.38 ± 0.86) and KO (16.73 ± 1.79) mice showed comparable weight gain (F(1,27) = 3.20, p = 0.08). Finally, there were no differences in mean weight for the two test sessions (WT = 26.1 g ± 0.65; KO = 26.0 ± 0.64; F b 1).

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pattern of consumption for KO mice (Fig. 2b). A four-way group (WT, KO) × test (CS+, CS−) × period (ITI, CS) × trial (1–4) ANOVA revealed no main effect of group (F b 1) and test (F b 1), a main effect of period (F(1,27) = 12.35, p b 0.001) but no effect of trial (F(1,27) = 1.01, p = 0.39). There were, however, a number of significant interactions including test × group (F(1,27) = 7.84, p b 0.001), test × period (F(1,27) = 9.49, p b 0.01), and, importantly, a three-way test × period × group interaction (F(1,27) = 8.36, p b 0.01). To examine the nature of this three-way interaction, two-way test × period ANOVAs were conducted separately for WT and KO mice. For WT mice this analysis revealed a main effect of test (F(1,15) = 6.46, p b 0.05), period (F(1,15) = 8.09, p = 0.01) and a significant interaction between the two variables (F(1,15) = 14.64, p = 0.001). This interaction reflected a significant increase in sucrose intake during the CS+ compared to the ITI period (F(1,15) = 12.12, p b 0.01), and when CS+ intake was compared to CS− intake (F(1,15) = 10.16, p b 0.01). In contrast, no differences in intake were revealed between CS − and ITI periods (F b 1). A similar analysis for KO mice revealed a main effect of period (F(1,15) = 6.08, p b 0.05), but no effect of test (F(1,15) = 2.11, p = 0.17), nor interaction between the two variables (F b 1). Thus, MCH1R deletion significantly disrupted the capacity for the CS + to evoke feeding behavior. In contrast to the effects on learned overeating behavior, both WT and KO mice showed similar response rates/min to the food magazine during the CS+ (WT = 8.78 ± 1.59; KO = 9.00 ± 1.06) and during the CS − (WT = 2.01 ± 0.96; KO = 2.88 ± 0.94): two-way group × cue ANOVA revealed a main effect of cue only (F(1,27) = 49.34, p b 0.001).

3.3. Licking microstructure 3.2. Cue-potentiated feeding test During the baseline period, both groups of mice showed a similar pattern of intake. For WT mice the mean lick rate/min for the CS+ test was 47.67 ± 10.42 and 50.61 ± 9.21 for KO mice. One-way ANOVA revealed no differences between the groups (F b 1). The data of most interest are sucrose intake during cue presentations, presented in Fig. 2. In WT mice presentation of the CS+ led to an increase in sucrose consumption as evidenced by the increased lick rate compared to the preceding ITI period (Fig. 2a). By comparison, cue presentation had a negligible effect on the

Fig. 1. Pavlovian conditioning of MCH-1R wild-type (WT) and knockout (KO) mice. WT and KO mice showed a comparable increase in food cup responding during CS+ cue relative to the CS−. Data are presented in food magazine responses/min during the initial 5 s of CS presentation—pre-CS (5 s prior to CS). Error bars indicate standard error of the mean (SEM).

By testing in our consummatory chambers, we were able to accurately time-stamp individual licks and subsequently conduct a microstructural analysis on the time-stamped licking data to determine the nature of the CS+ evoked overeating response in WT mice. First, we examined the mean burst size of licking that each mouse made during the CPF test session—that is the average number of licks between 1 s pauses—as this measure is thought to index reward palatability (Fig. 3a). In WT mice, presentation of the CS+ led to a large burst of licking behavior that was not evident when the mice consumed during the CS− control cue. By comparison, KO mice showed on average relatively small bursts of licking during both CS+ consumption and CS− consumption. Twoway ANOVA conducted on these data revealed no main effect of group (F(1,27) = 1.77, p = 0.19) or cue (F b 1); however, the interaction was close to significance (F(1,27) = 3.85, p = 0.06) due to a tendency for WT (F(1,27) = 4.08, p = 0.05) but not KO (F b 1) mice to show enhanced burst size for CS+ compared to CS− consumption. In addition, burst size differed significantly between the groups during CS+ (F(1,27) = 4.25, p b 0.05) but not CS− (F b 1) consumption. These data suggest that the CS+ prolonged feeding behavior in WT mice (but not KO) by enhancing the palatability of the sucrose. Second, we examined the number of licking bursts that the mice initiated during CS + and CS − consumption—that is the number of times that the subject initiated a new feeding episode after a pause greater than 1 s—as this measure is thought to reflect post-ingestive negative feedback. For both groups burst number was greater during the CS +, however, during the CS +, WT mice appeared to initiate more bursts of licking compared to KO mice (Fig. 3b). These impressions were confirmed by ANOVA, which revealed no main effect of group (F(1,27) = 2.81, p = 0.1), a main effect of cue (F(1,27) = 28.0, p b 0.001) and a group × cue interaction (F(1,27) = 4.27, p b 0.05). Individual planned comparisons revealed a greater number of bursts during the CS+ compared to CS− for both WT (F(1,27) = 30.2, p b 0.0001) and KO (F(1,27) = 4.71, p b 0.05) mice; however, the number of bursts was significantly greater for WT compared to KO during the CS+ (F(1,27) = 4.43, p b 0.05) but not CS− (F b 1). Thus, although the

Please cite this article as: A. Sherwood, et al., Deletion of Melanin Concentrating Hormone Receptor-1 disrupts overeating in the presence of food cues, Physiol Behav (2015), http://dx.doi.org/10.1016/j.physbeh.2015.05.037

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Fig. 2. CPF test of MCH-1R wild-type (WT) and knockout (KO) mice. (a) WT mice increased their licks/min during presentations of the CS+, but not the CS−, when compared to the intertrial intervals (ITIs). (b) By comparison, KO mice demonstrated similar lick rates throughout the CPF test. Error bars indicate SEM.

CS+ was effective in influencing this post-ingestive measure in both groups of mice, its effect was more salient in WT mice. 4. Discussion Deletion of MCH-1R significantly reduced the ability of a Pavlovian CS+ to evoke overeating under non-deprived conditions. Despite this effect on CPF, KO mice showed equivalent lick rates during baseline, CS− and ITI periods. Moreover, during the CPF test the CS+ elicited similar approach behavior to the food magazine in both WT and KO mice. Thus, MCH-1R KO mice entered the magazine during presentations of the CS+; however, whilst there, they failed to engage in prolonged cueevoked consumption of the sucrose reward compared to their WT controls. These results suggest a critical role for MCH in eating behavior in the presence of reward cues. Through an analysis of licking microstructure, our findings in WT mice also suggest that the CS+ is capable of increasing the palatability of food and influencing the potency of postingestive negative feedback mechanisms [31], as reflected by an increase in both the mean size and the number of licking bursts, respectively. The

mean size of licking bursts that occur during each bout of licking is thought to reflect the potency of orosensory feedback [34] as they show a monotonic relationship with increasing tastant palatability [31,32] and are unaffected by sham feeding conditions [30] where ingested fluid is prevented from reaching the gastrointestinal tract. Conversely, the number of bursts in a meal is critically affected by disruptions in gastrointestinal signaling [30]. Under normal feeding conditions, this measure displays an inverted U-shaped function following increases in sucrose concentration [31,32], which reflects post-ingestive negative feedback due to the increasing caloric value of sucrose. In KO mice, the significant reduction in conditioned potentiation of feeding behavior reflected a reduction in CS+ evoked burst size and number, which may suggest that MCH-1R has relatively broad effects on learned overeating. This may include food-specific palatability effects, and perhaps more general effects on endogenous peripheral signals typically used in the regulation of food intake [5,35]. The disruptions in CPF reported here are consistent with previous lesion data and the known distributions of MCH neurons (LH) [14,15] and MCH-1R (e.g. in BLA and vmPFC) [16]. Lesions to the BLA abolish

Fig. 3. Licking microstructure during CPF trials in MCH-1R wild-type (WT) and knockout (KO) mice. (a) The mean burst size (defined as the average number of licks before a pause greater than 1 s) was greater during CS+ trials compared to CS− trials in the WT, but not the KO group. # indicates group × cue interaction, p = 0.06; * indicates main effect of cue, p = 0.05. (b) The number of bursts (defined as the number of bursts initiated following a pause greater than 1 s) increased during CS+ trials relative to CS− trials in both groups; however, the number of burst during the CS+ was significantly greater in the WT group than in the KO group (see text for statistical analyses). # indicates significant group interaction, p b 0.05; * indicates main effect of cue, p's b 0.05. Error bars indicate SEM.

Please cite this article as: A. Sherwood, et al., Deletion of Melanin Concentrating Hormone Receptor-1 disrupts overeating in the presence of food cues, Physiol Behav (2015), http://dx.doi.org/10.1016/j.physbeh.2015.05.037

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CPF [6], as does functional disconnection of BLA from LH [36], accomplished by making unilateral lesions of BLA in one hemisphere and of LH in the other. Furthermore, bilateral lesions of the vmPFC severely reduced CPF produced by a chamber context uniquely associated with food pellets [13], whereas previous anatomical tract-tracing and immediate-early gene (IEG) activation studies have suggested that inputs from BLA and vmPFC to LH are important for CPF [37]. The current results extend these findings to suggest that MCH projections from LH also play a critical role. In addition to this expression throughout CPF circuitry, it is notable that MCH-immunoreactive fibers also project to a number of brain areas associated with gustatory function, which may underlie the CS+ evoked increase in sucrose palatability. This includes the gustatory cortex, the ventroposteromedial thalamus and the parabrachial nucleus [16]. In addition, using retrograde viral tracers, descending projections from LH MCH neurons to peripheral sites critical for orosensory stimulation (masseter muscle and submandibular salivary gland) have been revealed [38]. Moreover, MCH-1R is densely expressed within the nucleus accumbens shell (ACBS), a reward area thought to influence hedonic eating or so-called ‘liking’ of food [39]. In this region, manipulations of opioid activity in medium spiny neurons influence taste reactivity (TR) responses (such as lateral tongue protrusions) thought to index sensory-hedonic features of food [40,41]. Consistent with expression of MCH-1R in this hedonic hotspot [16,42], direct ACBS infusion of MCH evokes TR responses to sucrose, whereas lateral ventricle injections of μ-, δ- or κ-opioid antagonists prevent the elicitation of these hedonic responses [43]. These findings suggest that MCH mediates hedonic eating of food through interactions with opioids in the ventral striatum, and may suggest that the actions of MCH-1R in ACBS underlie the cueevoked palatability responses that promote overeating, and perhaps other tasks that rely on sensory associations of reward value. However, this interpretation should be treated with caution, as anatomical tracttracing and IEG findings suggest that at least projections to the LH from ACB did not contain neurons that were active during CPF [37]. Furthermore, communication between ACB and BLA was unnecessary for CPF as revealed by contralateral disconnection procedures [35]. During CPF, CS + presentations also promoted the initiation of bursts of licking behavior in WT mice, an effect that was disrupted by MCH-1R deletion. Although the specific MCH mechanisms underlying this finding remain to be determined, it may reflect an influence of cue presentation on peripheral signaling [30,34]. Consistent with this account, previous studies suggest that CSs can evoke the peripheral release of metabolic signals (e.g., insulin and ghrelin) [44–46]. MCH is ideally suited for mediating these effects of reward cues on enteric nervous system function due to its expression in gastrointestinal circuitry, including the duodenum, stomach [47] and pancreas [48], where it regulates metabolic signals, such as insulin secretion [49] and islet secretory function [48]. Consistent with this role in glucose metabolism, LH MCH neurons are depolarized in the presence of glucose leading to the production of ATP and subsequent cessation of plasmalemmal ATPsensitive K+ channels [50]. In addition, when optical stimulation of LH-MCH is combined with consumption of the non-caloric sweetener sucralose, MCH neuronal activation can substitute for glucose metabolism, reversing the normal preference for sucrose over sucralose and resulting in evoked dopamine responses in the striatum [51]. These findings suggest that MCH neurons form part of a complex network that integrates sensory, reward, and nutritional information from the periphery and central nervous system [51,52]. Based on this multifaceted role in the control of food intake, MCH neurons may function within a framework of allostasis [53], in which, through learning, animals come to anticipate their needs rather than simply reacting to them. For example, MCH may respond to sucrose-associated CSs leading to the release of insulin [48,49,52] in anticipation of glucose consumption, enhancing the efficiency of glucose metabolism [53]. It is tempting to speculate, however, that this efficient design for predictive regulation [52,53] may also lead to vulnerabilities to other mechanisms of learning that influence food intake such as CPF. Future studies using foods of varying

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degrees of nutrition and palatability, along with specific targeting of MCH within the central and enteric nervous systems, will be informative for elucidating the complex role played by this neuropeptide on both the regulation and dysregulation of appetite control. Nevertheless, the current findings highlight the importance in understanding neuropeptide mechanisms that influence cue-evoked feeding. It is notable that the gastric feeding signal ghrelin also plays a critical role in CPF [7,12], which further suggests a complex interplay between peripheral and central systems underlying vulnerability to overeating in the presence of reward cues. 5. Conclusions The media advertisements and other stimuli that we are exposed to on a daily basis are often associated with food and may be triggering feeding in the absence of a need for calories [5]. CPF provides a reliable model to examine overeating behavior across species [6–10]. Furthermore, these effects are typically uncompensated for by internal regulatory mechanisms [54]. CPF may also be relevant for studies examining eating disorders, as the rapid consumption of large amounts of food in a short period of time characterizes binge-eating, which is a core symptom across virtually all eating disorder diagnoses (e.g., bulimia nervosa and binge eating disorder) [55] and is associated with elevated rates of obesity (e.g., [56]). MCH-1R deletion significantly disrupted CPF, by reducing the ability of the CS + to initiate and prolong overeating behavior. Our findings suggest that the initiation of eating behavior may reflect the capacity of reward cues to disrupt peripheral mechanisms of inhibitory control [30], whereas cue-evoked increases in the palatability of food may prolong overeating behavior [34]. Thus, deletion of MCH-1R appears under these circumstances to be advantageous as it significantly reduces vulnerability to both the initiation and maintenance of feeding behaviors in the presence of external stimuli associated with palatable food. Accordingly, drugs targeting MCH-1R signaling may be useful pharmacological tools to treat obesity and eating disorders in humans [57–59]. Acknowledgments These studies was supported by NIDDK grant R21-DK84415 to A.W.J. References [1] H.-R. Berthoud, The neurobiology of food intake in an obesogenic environment, Proc. Nutr. Soc. 71 (2012) 478–487. [2] J.B. Dixon, The effect of obesity on health outcomes, Mol. Cell. Endocrinol. 316 (2010) 104–108. [3] J. Cawley, C. Meyerhoefer, The medical care costs of obesity: an instrumental variables approach, J. Health Econ. 31 (2012) 219–230. [4] R.A.H. Adan, Mechanisms underlying current and future anti-obesity drugs, Trends Neurosci. 32 (2013) 133–140. [5] A.W. Johnson, Eating beyond metabolic need: how environmental cues influence feeding behavior, Trends Neurosci. 36 (2013) 101–109. [6] P.C. Holland, T. Hatfield, M. Gallagher, Rats with basolateral amygdala lesions show normal increases in conditioned stimulus processing but reduced conditioned potentiation of eating, Behav. Neurosci. 115 (2001) 945–950. [7] A.K. Walker, I.E. Ibia, J.M. Zigman, Disruption of cue-potentiated feeding in mice with blocked ghrelin signaling, Physiol. Behav. 108 (2012) 34–43. [8] E. Zamble, Augmentation of eating following a signal for feeding in rats, Learn. Motiv. 4 (1973) 138–147. [9] L.L. Birch, L. Mcphee, S. Sullivan, S. Johnson, Conditioned meal initiation in young children, Appetite 13 (1989) 105–113. [10] C.E. Cornell, J. Rodin, H. Weingarten, Stimulus-induced eating when satiated, Physiol. Behav. 45 (1989) 695–704. [11] G.D. Petrovich, Amygdalar and prefrontal pathways to the lateral hypothalamus are activated by a learned cue that stimulates eating, J. Neurosci. 25 (2005) 8295–8302. [12] S.E. Kanoski, S.M. Fortin, K.M. Ricks, H.J. Grill, Ghrelin signaling in the ventral hippocampus stimulates learned and motivational aspects of feeding via PI3K-Akt signaling, Biol. Psychiatry 73 (9) (2013) 915–923. [13] G.D. Petrovich, C.A. Ross, P.C. Holland, M. Gallagher, Medial prefrontal cortex is necessary for an appetitive contextual conditioned stimulus to promote eating in sated rats, J. Neurosci. 27 (2007) 6436–6441.

Please cite this article as: A. Sherwood, et al., Deletion of Melanin Concentrating Hormone Receptor-1 disrupts overeating in the presence of food cues, Physiol Behav (2015), http://dx.doi.org/10.1016/j.physbeh.2015.05.037

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Please cite this article as: A. Sherwood, et al., Deletion of Melanin Concentrating Hormone Receptor-1 disrupts overeating in the presence of food cues, Physiol Behav (2015), http://dx.doi.org/10.1016/j.physbeh.2015.05.037