Blockade of melanocortin transmission inhibits cocaine reward

A molecularly defined orbitofrontal cortical neuron population controls compulsive-like behavior, but not inflexible choice or habit

Habits are familiar behaviors triggered by cues, not outcome predictability, and are insensitive to changes in the environment. They are adaptive under many circumstances but can be considered antecedent to compulsions and intrusive thoughts that drive persistent, potentially maladaptive behavior. Whether compulsive-like and habit-like behaviors share neural substrates is still being determined. Here, we investigated mice bred to display inflexible reward-seeking behaviors that are insensitive to action consequences. We found that these mice demonstrate habitual response biases and compulsive-like grooming behavior that was reversible by fluoxetine and ketamine. They also suffer dendritic spine attrition on excitatory neurons in the orbitofrontal cortex (OFC). Nevertheless, synaptic melanocortin 4 receptor (MC4R), a factor implicated in compulsive behavior, is preserved, leading to the hypothesis that Mc4r+ OFC neurons may drive aberrant behaviors. Repeated chemogenetic stimulation of Mc4r+ OFC neurons triggered compulsive and not inflexible or habitual response biases in otherwise typical mice. Thus, Mc4r+ neurons within the OFC appear to drive compulsive-like behavior that is dissociable from habitual behavior. Understanding which neuron populations trigger distinct behaviors may advance efforts to mitigate harmful compulsions.

Control of goal-directed and inflexible actions by dorsal striatal melanocortin systems, in coordination with the central nucleus of the amygdala

The dorsomedial striatum (DMS) is associated with flexible goal seeking, as opposed to routinized habits. Whether local mechanisms brake this function, for instance when habits may be adaptive, is incompletely understood. We find that a sub-population of dopamine D1 receptor-containing striatal neurons express the melanocortin-4 receptor (MC4R) for α-melanocyte stimulating hormone. These neurons within the DMS are necessary and sufficient for controlling the capacity of mice to flexibly adjust actions based on the likelihood that they will be rewarded. In investigating MC4R function, we found that it suppresses immediate-early gene levels in the DMS and concurrently, flexible goal seeking. MC4R+ neurons receive input from the central nucleus of the amygdala, and behavioral experiments indicate that they are functionally integrated into an amygdalo-striatal circuit that suppresses action flexibility in favor of routine. Publicly available spatial transcriptomics datasets were analyzed for gene transcript correlates of Mc4r expression across the striatal subregions, revealing considerable co-variation in dorsal structures. This insight led to the discovery that the function of MC4R in the dorsolateral striatum complements that in the DMS, in this case suppressing habit-like behavior. Altogether, our findings suggest that striatal MC4R controls the capacity for goal-directed and inflexible actions alike.

Taste dysfunction as a predictor of depression in schizophrenia: A systematic review and meta-analysis

OBJECTIVE

This study aims to investigate the relationship between taste dysfunction and depression among patients with schizophrenia, to achieve early detection of depression in clinical practice.

METHODS

Following PRISMA guidance, a comprehensive literature search was conducted globally, covering papers published from 1961 to June 2023. A total of 17 manuscripts were selected through meta-analysis and sensitivity analysis after examining available materials from seven databases to determine the correlation between depression and taste dysfunction.

RESULTS

The comparison of the 17 selected manuscripts revealed that individuals with gustatory dysfunction may be more likely to experience depressive symptoms (SMD, 0.51, 95% CI, 0.08 to 0.93, p = 0.02). Depression is associated with taste dysfunction in certain aspects, as indicated by the pleasantness ratings of sucrose solutions (SMD, -0.53, 95% confidence interval [CI] -1.11 to 0.05, p = 0.08), gustatory identification ability (SMD, 0.96, 95% CI, 0.03 to 1.89, p = 0.04), and the perception threshold of sweet taste (MD, 0.80, 95% CI, 0.79 to 0.81, p < 0.00001).

CONCLUSIONS

Due to variations in the methods, designs, and selection criteria employed in the included studies, it is necessary to establish a feasible framework. Future research using detailed and targeted approaches can provide clearer and more unified conclusions on the relationship between taste dysfunction and depression. Moreover, further high-quality research is needed to obtain clearer conclusions and explore the potential of taste dysfunction as an effective tool for early screening of depression.

TRIAL REGISTRATION

This review has been registered in the PROSPERO on April 2022 with the identifier CRD42023400172.

Chronic Nicotine Consumption and Withdrawal Regulate Melanocortin Receptor, CRF, and CRF Receptor mRNA Levels in the Rat Brain

Alterations in the various neuropeptide systems in the mesocorticolimbic circuitry have been implicated in negative effects associated with drug withdrawal. The corticotropin-releasing factor (CRF) and α-melanocyte-stimulating hormone are two peptides that may be involved. This study investigated the regulatory effects of chronic nicotine exposure and withdrawal on the mRNA levels of melanocortin receptors (MC3R, MC4R), CRF, and CRF receptors (CRFR1 and CRFR2) expressed in the mesocorticolimbic system. Rats were given drinking water with nicotine or without nicotine (control group) for 12 weeks, after which they continued receiving nicotine (chronic exposure) or were withdrawn from nicotine for 24 or 48 h. The animals were decapitated following behavioral testing for withdrawal signs. Quantitative real-time PCR analysis demonstrated that nicotine exposure (with or without withdrawal) increased levels of CRF and CRFR1 mRNA in the amygdala, CRF mRNA in the medial prefrontal cortex, and CRFR1 mRNA in the septum. Nicotine withdrawal also enhanced MC3R and MC4R mRNA levels in different brain regions, while chronic nicotine exposure was associated with increased MC4R mRNA levels in the nucleus accumbens. These results suggest that chronic nicotine exposure and withdrawal regulate CRF and melanocortin signaling in the mesocorticolimbic system, possibly contributing to negative affective state and nicotine addiction.

Proopiomelanocortin projections to the nucleus accumbens modulate acquisition and maintenance of operant palatable pellet administration in mice

Obesity is a crisis in the United States, producing many co-morbid diseases that can drastically decrease quality of life. While diet is a major focus for therapeutic intervention, the need to understand underlying appetitive neurocircuitry persists. Proopiomelanocortin (POMC) peptides are well-known for their anorexigenic activity, but also mediate reward and learning. The nucleus accumbens (NAcc) is best known for its role in reward-based learning, but the contribution of POMC projections to NAcc on feeding are controversial since the two major POMC-derived peptides (β-endorphin and α-MSH) have opposite effects on food intake. Our objective was to determine the effect of stimulating POMC projections in the NAcc on acquisition and maintenance of operant self-administration of a palatable food. Adult POMCCre mice were microinjected into the NAcc with a Cre-dependent retrograde adeno-associated viral vector expressing Gq Designer Receptors Exclusively Activated by Designer Drugs (DREADDs). Mice were trained to self-administer palatable 20-mg pellets in daily operant sessions. Acquisition of self-administration (fixed ratio 30) and baseline self-administration were measured in daily sessions, with mice receiving injections of either JHU37152 (DREADD agonist) or saline (i.p.) 15 min prior to the sessions. POMC neuron stimulation (JHU injection) before training sessions produced a significant increase in rate of acquisition and accuracy compared to the saline treated group, with no significant effect on rewards earned. Removal of POMC neuron stimulation before sessions initially reduced consumption with a gradual increase in responding for reinforcer over 3 days of saline injections. Reinstatement of POMC neuron stimulation (JHU) before the session resulted in a significant decrease in responding and rewards earned. These results suggest a complex role of POMC peptides within the NAcc that increase reward learning for a novel palatable food while decreasing consumption of the reinforcer following experience with it.

Neuropeptides Modulate Feeding via the Dopamine Reward Pathway

Dopamine (DA) is a catecholamine neurotransmitter widely distributed in the central nervous system. It participates in various physiological functions, such as feeding, anxiety, fear, sleeping and arousal. The regulation of feeding is exceptionally complex, involving energy homeostasis and reward motivation. The reward system comprises the ventral tegmental area (VTA), nucleus accumbens (NAc), hypothalamus, and limbic system. This paper illustrates the detailed mechanisms of eight typical orexigenic and anorexic neuropeptides that regulate food intake through the reward system. According to recent literature, neuropeptides released from the hypothalamus and other brain regions regulate reward feeding predominantly through dopaminergic neurons projecting from the VTA to the NAc. In addition, their effect on the dopaminergic system is mediated by the prefrontal cortex, paraventricular thalamus, laterodorsal tegmental area, amygdala, and complex neural circuits. Research on neuropeptides involved in reward feeding can help identify more targets to treat diseases with metabolic disorders, such as obesity.

Melanocortin receptor agonist melanotan-II microinjected in the nucleus accumbens decreases appetitive and consumptive responding for food

RATIONALE

Obesity is a major health problem worldwide. An understanding of the factors that drive feeding behaviors is key to the development of pharmaceuticals to decrease appetite and consumption. Proopiomelanocortin (POMC), the melanocortin peptide precursor, is essential in the regulation of body weight and ingestive behaviors. Deletion of POMC or impairment of melanocortin signaling in the brain results in hyperphagic obesity. Neurons in the hypothalamic arcuate nucleus produce POMC and project to many areas including the nucleus accumbens (NAcc), which is well established in the rewarding and reinforcing effects of both food and drugs of abuse.

OBJECTIVE

These studies sought to determine the role of melanocortins in the NAcc on consumption of and motivation to obtain access to standard rodent chow.

METHODS

Male, C57BL/6J mice were microinjected bilaterally into the NAcc (100 nl/side) with the melanocortin receptor 3/4 agonist melanotan-II (MT-II; 0.1, 0.3, and 1 nmol), and ingestive behaviors were examined in both home cage and operant food self-administration experiments. In addition, the ability of MT-II in the NAcc to produce aversive properties or affect metabolic rate were tested.

RESULTS

MT-II injected into the NAcc significantly decreased consumption in both home cage and operant paradigms, and furthermore decreased appetitive responding to gain access to food. There was no development of conditioned taste avoidance or change in metabolic parameters following anorexic doses of MT-II.

CONCLUSIONS

MT-II in the NAcc decreased both the motivation to eat and the amount of food consumed without inducing an aversive state or affecting metabolic rate, suggesting a role for melanocortin signaling in the NAcc that is selective for appetite and satiety without affecting metabolism or producing an aversive state.

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.

Inter-individual variability amplified through breeding reveals control of reward-related action strategies by Melanocortin-4 Receptor in the dorsomedial striatum

In day-to-day life, we often must choose between pursuing familiar behaviors or adjusting behaviors when new strategies might be more fruitful. The dorsomedial striatum (DMS) is indispensable for arbitrating between old and new action strategies. To uncover molecular mechanisms, we trained mice to generate nose poke responses for food, then uncoupled the predictive relationship between one action and its outcome. We then bred the mice that failed to rapidly modify responding. This breeding created offspring with the same tendencies, failing to inhibit behaviors that were not reinforced. These mice had less post-synaptic density protein 95 in the DMS. Also, densities of the melanocortin-4 receptor (MC4R), a high-affinity receptor for α-melanocyte-stimulating hormone, predicted individuals' response strategies. Specifically, high MC4R levels were associated with poor response inhibition. We next found that reducing Mc4r in the DMS in otherwise typical mice expedited response inhibition, allowing mice to modify behavior when rewards were unavailable or lost value. This process required inputs from the orbitofrontal cortex, a brain region canonically associated with response strategy switching. Thus, MC4R in the DMS appears to propel reward-seeking behavior, even when it is not fruitful, while moderating MC4R presence increases the capacity of mice to inhibit such behaviors.

TRH in the nucleus accumbens acts downstream to α-MSH to decrease food intake in rats

Feeding-regulatory peptides such as thyrotropin-releasing hormone (TRH), α-melanocyte-stimulating hormone (α-MSH) and their receptors are expressed in brain regions involved in the homeostatic and hedonic control of food intake, such as the hypothalamus and the mesolimbic system, respectively. The nucleus accumbens (NAc) is part of the latter, a brain circuit involved in processing reward stimuli and the appetitive motivation of feeding. When TRH or α-MSH are administered in the NAc, both decrease food intake, through activating their respective receptors, TRH-R1 and MC4R. The actions of α-MSH as a homeostatic feeding-regulator involves the increase of hypothalamic TRH expression, thus, we aimed to identify whether TRH signaling in the NAc was also participating in α-MSH-induced reduction of food intake. α-MSH administration in the NAc of 48 h fasted rats reduced their food intake during the 2-h period of refeeding, increased accumbal TRH mRNA expression and decreased that of MC4R. Such downregulated MC4R mRNA levels implied a compensatory decrease of α-MSH actions in the NAc after the previous pathway stimulation. The co-administration of α-MSH along with an antisense oligonucleotide directed against pro-TRH mRNA in the NAc impaired the α-MSH-induced feeding reduction, supporting that the accumbal TRHergic pathway is downstream of α-MSH actions to inhibit feeding. Our results suggested that TRH in the NAc mediates some effects of α-MSH on inhibition of food intake; this supports the role of TRH not only as a homeostatic regulator but also as modulating the motivational aspects of feeding.

Stimulation of Melanocortin Receptor-4 (MC4R) Prevents Mitochondrial Damage Induced by Binge Ethanol Protocol in Adolescent Rat Hippocampus

Binge drinking is a common pattern of adolescent alcohol consumption characterized by a high alcohol intake within a short period of time; which may seriously affect brain function, triggering in some cases an addictive behavior. Current evidence indicates that alcohol addictive conduct is related to the impairment of the Melanocortin System (MCS). This system participates in the regulation of food intake and promotes anti-inflammatory response in the brain. However, the cellular mechanisms involved in the protective effects induced by MCS against binge-alcohol intoxication are still unknown. Here, we studied the effects of MCS activation on mitochondrial and oxidative damage induced by a binge-like protocol in the hippocampus of adolescent rats. We used a pharmacological activator of MC4R (RO27-3225) and evaluated its effects against oxidative injury, mitochondrial failure, and bioenergetics impairment induced by binge ethanol protocol in the hippocampus of adolescent's rats. Our results indicate that MC4R agonist reduces hippocampal oxidative damage promoting antioxidant (Nrf-2) and mitochondrial biogenesis (PGC1-alpha) pathways in animals subjected to the binge-like protocol. Additionally, MC4R activation prevented mitochondrial potential loss and increased mitochondrial mass that were significantly reduced by binge ethanol protocol. Finally, RO27-3225 treatment increased ATP production and mitochondrial respiratory complex expression in adolescent rats exposed to ethanol. Altogether, these findings show that activation of the MCS pathway through MC4R prevents these negative effects of binge ethanol protocol, suggesting a possible role of the MCS in the reduction of the neurotoxic effects induced by alcohol intoxication in adolescents.

Maternal Diet Influences the Reinstatement of Cocaine-Seeking Behavior and the Expression of Melanocortin-4 Receptors in Female Offspring of Rats

Recent studies have emphasized the role of the maternal diet in the development of mental disorders in offspring. Substance use disorder is a major global health and economic burden. Therefore, the search for predisposing factors for the development of this disease can contribute to reducing the health and social damage associated with addiction. In this study, we focused on the impact of the maternal diet on changes in melanocortin-4 (MC-4) receptors as well as on behavioral changes related to cocaine addiction. Rat dams consumed a high-fat diet (HFD), high-sugar diet (HSD, rich in sucrose), or mixed diet (MD) during pregnancy and lactation. Using an intravenous cocaine self-administration model, the susceptibility of female offspring to cocaine reward and cocaine-seeking propensities was evaluated. In addition, the level of MC-4 receptors in the rat brain structures related to cocaine reward and relapse was assessed. Modified maternal diets did not affect cocaine self-administration in offspring. However, the maternal HSD enhanced cocaine-seeking behavior in female offspring. In addition, we observed that the maternal HSD and MD led to increased expression of MC-4 receptors in the nucleus accumbens, while increased MC-4 receptor levels in the dorsal striatum were observed after exposure to the maternal HSD and HFD. Taken together, it can be concluded that a maternal HSD is an important factor that triggers cocaine-seeking behavior in female offspring and the expression of MC-4 receptors.

Maternal high-sugar diet changes offspring vulnerability to reinstatement of cocaine-seeking behavior: Role of melanocortin-4 receptors

Maternal diet significantly influences the proper development of offspring in utero. Modifications of diet composition may lead to metabolic and mental disorders that may predispose offspring to a substance use disorder. We assessed the impact of a maternal high-sugar diet (HSD, rich in sucrose) consumed during pregnancy and lactation on the offspring phenotype in the context of the rewarding and motivational effects of cocaine and changes within the central melanocortin (MC) system. Using an intravenous cocaine self-administration model, we showed that maternal HSD leads to increased relapse of cocaine-seeking behavior in male offspring. In addition, we demonstrated that cocaine induces changes in the level of MC-4 receptors in the offspring brain, and these changes depend on maternal diet. These studies also reveal that an MC-4 receptor antagonist reduces the reinstatement of cocaine-seeking behavior, and offspring exposed to maternal HSD are more sensitive to its effects than offspring exposed to the maternal control diet. Taken together, the results suggest that a maternal HSD and MC-4 receptors play an important role in cocaine relapse.

Nicotine-induced Brain Stimulation Reward is Modulated by Melanocortin-4 Receptors in Ovariectomized Rats

Apart from reproduction, estrogen influences a multitude of processes. Increase in estrogen levels in women is known to promote reward probably mediated via the melanocortin and dopamine systems. Reduced estrogen in post-menopausal women attenuates reward, evoking the need for stimulation with greater rewarding salience. This is reflected in the well-recognized phenomena of difficulty in quitting and increased craving for nicotine in women following the onset of menopause. The present study aims at understanding the role of melanocortin receptors (MC-R) in nicotine-induced reward behavior following ovariectomy in rats. The MC4-R mRNA level was increased in ipsilateral nucleus accumbens (Acb) of the intact rats implanted with electrode in medial forebrain bundle and trained in intracranial self-stimulation (ICSS) paradigm. Additional groups of ICSS trained rats were ovariectomized (OVX) and subjected to reward evaluation. Trained OVX rats revealed a significant increase in threshold frequency and rightward shift in rate frequency curve, suggesting reward deficit behavior. However, pre-administration with nicotine, alpha-melanocyte stimulating hormone (α-MSH) or NDP-MSH (MC4-R agonist) to OVX animals restored the rewarding activity in ICSS protocol; HS014 (MC4-R antagonist) suppressed the lever press activity. Prior treatment with sub-effective doses of α-MSH or NDP-MSH potentiated the reward effect of nicotine, but was attenuated by HS014. Alpha-MSH-immunoreactivity was decreased in the Acb shell, arcuate and paraventricular nucleus of hypothalamus, and ventral bed nucleus of stria terminalis in the OVX rats, while nicotine treatment restored the same. We suggest a role for the endogenous MC system, perhaps acting via MC4-R, in the nicotine-induced reward in OVX rats.

The melanocortin system as a potential target for treating alcohol use disorders: A review of pre-clinical data

The melanocortin (MC) system consists of neuropeptides that are cleaved from the polypeptide precursor proopiomelanocortin (POMC). In the brain, MC neuropeptides signal primarily through the MC-3 and MC-4 receptors, which are widely expressed throughout the brain. While the MC system has been largely studied for its role in food intake and body weight regulation, converging evidence has emerged over approximately the last 20-years showing that alcohol (ethanol), and other drugs of abuse influence the central MC system, and that manipulating MC receptor signalling modulates ethanol intake. Although there is divergent evidence, the wealth of data appears to suggest that activating MC signalling, primarily through the MC-4 receptor, is protective against excessive ethanol consumption. In the present review, we first describe the MC system and then detail how ethanol exposure and consumption alters central MC and MC-receptor expression and levels. This is followed by a review of the data, from pharmacological and genetic studies, which show that manipulations of MC receptor activity alter ethanol intake. We then briefly highlight studies implicating a role for the MC system in modulating neurobiological responses and intake of other drugs of abuse, including amphetamine, cocaine and opioids. Finally, we introduce relatively new observations that the drug, bupropion (BUP), a drug that activates central MC activity, significantly reduces ethanol intake in rodent models when administered alone and in combination with the non-selective opioid receptor antagonist, naltrexone. Phase II clinical trials are currently underway to assess the efficacy of BUP as a treatment for alcohol use disorders.

A Motivational and Neuropeptidergic Hub: Anatomical and Functional Diversity within the Nucleus Accumbens Shell

The mesocorticolimbic pathway is canonically known as the "reward pathway." Embedded within the center of this circuit is the striatum, a massive and complex network hub that synthesizes motivation, affect, learning, cognition, stress, and sensorimotor information. Although striatal subregions collectively share many anatomical and functional similarities, it has become increasingly clear that it is an extraordinarily heterogeneous region. In particular, the nucleus accumbens (NAc) medial shell has repeatedly demonstrated that the rules dictated by more dorsal aspects of the striatum do not apply or are even reversed in functional logic. These discrepancies are perhaps most easily captured when isolating the functions of various neuromodulatory peptide systems within the striatum. Endogenous peptides are thought to play a critical role in modulating striatal signals to either amplify or dampen evoked behaviors. Here we describe the anatomical-functional backdrop upon which several neuropeptides act within the NAc to modulate behavior, with a specific emphasis on nucleus accumbens medial shell and stress responsivity. Additionally, we propose that, as the field continues to dissect fast neurotransmitter systems within the NAc, we must also provide considerable contextual weight to the roles local peptides play in modulating these circuits to more comprehensively understand how this important subregion gates motivated behaviors.

Glucagon-Like Peptide-1 (GLP-1) and 5-Hydroxytryptamine 2c (5-HT2c) Receptor Agonists in the Ventral Tegmental Area (VTA) Inhibit Ghrelin-Stimulated Appetitive Reward

Current literature indicates that the orexigenic peptide ghrelin increases appetitive motivation via signaling in the mesolimbic reward system. Another gastric peptide, glucagon-like peptide-1 (GLP-1), and the neurotransmitter 5-hydroxytryptamine (5-HT), are both known to suppress operant responding for food by acting on key mesolimbic nuclei, including the ventral tegmental area (VTA). In order to investigate the interaction effects of ghrelin, GLP-1, and 5-HT within the VTA, we measured operant responding for sucrose pellets after the administration of ghrelin, the GLP-1 receptor agonist exendin-4 (Ex-4), and the 5-HT_2c receptor agonist Ro60-0175 in male Sprague-Dawley rats. Following training on a progressive ratio 3 (PR3) schedule, animals were first injected with ghrelin into the VTA at doses of 3 to 300 pmol. In subsequent testing, separate rats were administered intraperitoneal (IP) Ex-4 (0.1–1.0 µg/kg) or VTA Ex-4 (0.01–0.1 µg) paired with 300 pmol ghrelin. In a final group of rats, the 5-HT_2c agonist Ro60-0175 was injected IP (0.25–1.0 mg/kg) or into the VTA (1.5–3.0 µg), and under both conditions paired with 300 pmol ghrelin delivered into the VTA. Our results indicated that ghrelin administration increased operant responding for food reward and that this effect was attenuated by IP and VTA Ex-4 pretreatment as well as pre-administration of IP or VTA Ro60-0175. These data provide compelling evidence that mesolimbic GLP-1 and serotonergic circuitry interact with the ghrelinergic system to suppress ghrelin’s effects on the mediation of food reinforcement.

Small Molecule Neuropeptide S and Melanocortin 4 Receptor Ligands as Potential Treatments for Substance Use Disorders

There is a vital need for novel approaches and biological targets for drug discovery and development. Treatment strategies for substance use disorders (SUDs) to date have been mostly ineffective other than substitution-like therapeutics. Two such targets are the peptide G-protein-coupled receptors neuropeptide S (NPS) and melanocortin 4 (MC4). Preclinical evidence suggests that antagonists, inverse agonists, or negative allosteric modulators of these receptors might be novel therapeutics for SUDs. NPS is a relatively unexplored receptor with high potential for treating SUD. MC4 has a strong link to early-onset obesity, and emerging evidence suggests significant overlap between food-maintained and drug-maintained behaviors making MC4 an intriguing target for SUD. This chapter provides an overview of the literature in relation to the roles of NPS and MC4 in drug-seeking behaviors and then provides a medicinal chemistry-based survey of the small molecule ligands for each receptor.

Acupuncture – Self-Appraisal and the Reward System

Acupuncture is an ancient therapy with a variety of different explanatory models. A cascade of physiological effects has been reported, both in the peripheral and the central nervous system, following the insertion of a needle or light tapping of the skin. Clinical trials testing the specific claims of acupuncture have generally tried to focus on testing the efficacy of applying specific techniques and/or specified points. However, different conditions may respond differently to different modes of stimulation.

Recently, it was demonstrated that both superficial and deep needling (with de qi/Hibiki) resulted in amelioration of patellofemoral pain and unpleasantness. The pleasurable aspect of the acupuncture experience has largely been ignored as it has been considered secondary to its pain alleviating effects. This aspect of acupuncture treatment is likely to be related to activation of self-appraisal and the reward system.

When a patient seeks a therapist there are expectations of a specific effect. These expectations are partly based on self-relevant phenomena and self-referentia introspection and constitute the preference. Also, when asked about the effect of the treatment, processes that orientate pre-attentive anticipatory or mnemonic information and processes that mediate self-reflection and recollection are integrated together with sensory detection to enable a decision about the patient's perception of the effect of acupuncture treatment. These ‘self-appraisal’ processes are dependent on two integrated networks: a ventral medial prefrontal cortex paralimbic limbic ‘affective’ pathway and a dorsal medial prefrontal cortex cortical hippocampal ‘cognitive’ pathway.

The limbic structures are implicated in the reward system and play a key role in most diseases and illness responses including chronic pain and depression, regulating mood and neuromodulatory responses (eg sensory, autonomic, and endocrine). The pleasurable and neuromodulatory aspects of acupuncture as well as ‘placebo needling’ may partly be explained by the activation or deactivation of limbic structures including the hippocampus, amygdala, and their connections with the hypothalamus.

In patients with patellofemoral pain, the effects of superficial and deep needling remained for six months. These long term pain-alleviating effects have been attributed to activation of pain inhibiting systems in cortical and subcortical pathways. When considering long term effects the cortical cerebellar system needs to be taken into account. The cortical cerebellar system is probably central to the development of neural models that learn and eventually stimulate routinely executed (eg motor skills) and long term (eg pain alleviation) cognitive processes. These higher order cognitive processes are initially mediated in prefrontal cortical loci but later shift control iteratively to internal cerebellar representations of these processes. Possibly part of the long term healing effects of acupuncture may be attributed to changes in the cerebellar system thereby sparing processing load in cortical and subcortical areas.

As cortical and subcortical structures are activated and/or de-activated following stimulation of receptors in the skin, disregarding site, ‘placebo or sham needling’ does not exist and conclusions drawn on the basis that it is an inert control are invalid.

‘Self’ may be seen as a shifting illusion, ceaselessly constructed and deconstructed, and the effect of acupuncture may reflect its status (as well as that of the therapist).

[Hunger and satiety factors in the regulation of pleasure associated with feeding behavior]

Feeding is an instinctive behavior accompanied by rewarding feeling of pleasure during obtaining and ingesting food, corresponding to the preparatory and consummatory phases of motivated behavior, respectively. Perception of this emotional state together with alternating feelings of hunger and satiety drives the feeding behavior. Because alterations of feeding behavior including either overeating or anorexia may lead to obesity and cachexia, respectively, understanding the neurochemical mechanisms of regulation of feeding pleasure may help to develop new therapies of these diseases. The dopamine (DA) system of the mesolimbic projections plays a key role in behavioral reward in general and is also involved in regulating feeding-associated pleasure in the forebrain including the nucleus accumbens (NAc) and the lateral hypothalamic area (LHA). It suggests that this DA system can be selectively activated by factors specific to different types of motivated behavior including hunger- and satiety- related hormones. Indeed, central administrations of either orexigenic ghrelin or anorexigenic α-melanocyte-stimulating hormone (α-MSH) increase DA release in the NAc. However, DA has also been shown to inhibit food intake when injected into the LHA, historically known as a « hunger center », indicating DA functional involvement in regulation of both appetite and feeding pleasure. Although both NAc and LHA contain neurons expressing melanocortin receptors, only the LHA receives the α-MSH containing nerve terminals from the α-MSH producing neurons of the hypothalamic arcuate nucleus, the main relay of the peripheral hunger and satiety signals to the brain. A recent study showed that α-MSH in the LHA enhances satiety and inhibits feeding pleasure while potently stimulating DA release in this area during both preparatory and consummatory phases of feeding. It suggests that altered signaling by α-MSH to the DA system in the LHA may be involved in the pathophysiology of obesity and anorexia and the possible underlying mechanisms are discussed.

Hypothalamic-specific proopiomelanocortin deficiency reduces alcohol drinking in male and female mice

Opioid receptor antagonist naltrexone reduces alcohol consumption and relapse in both humans and rodents. This study investigated whether hypothalamic proopiomelanocortin (POMC) neurons (producing beta-endorphin and melanocortins) play a role in alcohol drinking behaviors. Both male and female mice with targeted deletion of two neuronal Pomc enhancers nPE1 and nPE2 (nPE-/-), resulting in hypothalamic-specific POMC deficiency, were studied in short-access (4-h/day) drinking-in-the-dark (DID, alcohol in one bottle, intermittent access (IA, 24-h cycles of alcohol access every other day, alcohol vs. water in a two-bottle choice) and alcohol deprivation effect (ADE) models. Wild-type nPE+/+ exposed to 1-week DID rapidly established stable alcohol drinking behavior with more intake in females, whereas nPE-/- mice of both sexes had less intake and less preference. Although nPE-/- showed less saccharin intake and preference than nPE+/+, there was no genotype difference in sucrose intake or preference in the DID paradigm. After 3-week IA, nPE+/+ gradually escalated to high alcohol intake and preference, with more intake in females, whereas nPE-/- showed less escalation. Pharmacological blockade of mu-opioid receptors with naltrexone reduced intake in nPE+/+ in a dose-dependent manner, but had blunted effects in nPE-/- of both sexes. When alcohol was presented again after 1-week abstinence from IA, nPE+/+ of both sexes displayed significant increases in alcohol intake (ADE or relapse-like drinking), with more pronounced ADE in females, whereas nPE-/- did not show ADE in either sex. Our results suggest that neuronal POMC is involved in modulation of alcohol 'binge' drinking, escalation and 'relapse', probably via hypothalamic-mediated mechanisms, with sex differences.

The Role of the Melanocortin System in Metabolic Disease: New Developments and Advances

Obesity is increasing in prevalence across all sectors of society, and with it a constellation of associated ailments including hypertension, type 2 diabetes, and eating disorders. The melanocortin system is a critical neural system underlying the control of body weight and other functions. Deficits in the melanocortin system may promote or exacerbate the comorbidities of obesity. This system has therefore generated great interest as a potential target for treatment of obesity. However, drugs targeting melanocortin receptors are plagued by problematic side effects, including undesirable increases in sympathetic nervous system activity, heart rate, and blood pressure. Circumnavigating this roadblock will require a clearer picture of the precise neural circuits that mediate the functions of melanocortins. Recent, novel experimental approaches have significantly advanced our understanding of these pathways. We here review the latest advances in our understanding of the role of melanocortins in food intake, reward pathways, blood pressure, glucose control, and energy expenditure. The evidence suggests that downstream melanocortin-responsive circuits responsible for different physiological actions do diverge. Ultimately, a more complete understanding of melanocortin pathways and their myriad roles should allow treatments tailored to the mix of metabolic disorders in the individual patient.

Hunger and Satiety Gauge Reward Sensitivity

Many of the neurocircuits and hormones known to underlie the sensations of hunger and satiety also substantially alter the activity of the dopaminergic reward system. Much interest lies in the ways that hunger, satiety, and reward tie together, as the epidemic of obesity seems tied to the recent development and mass availability of highly palatable foods. In this review, we will first discuss the basic neurocircuitry of the midbrain and basal forebrain reward system. We will elaborate how several important mediators of hunger-the agouti-related protein neurons of the arcuate nucleus, the lateral hypothalamic nucleus, and ghrelin-enhance the sensitivity of the dopaminergic reward system. Then, we will elaborate how mediators of satiety-the nucleus tractus solitarius, pro-opiomelanocortin neurons of the arcuate nucleus, and its peripheral hormonal influences such as leptin-reduce the reward system sensitivity. We hope to provide a template by which future research may identify the ways in which highly rewarding foods bypass this balanced system to produce excessive food consumption.

Genetic Similarities between Compulsive Overeating and Addiction Phenotypes: A Case for "Food Addiction"?

There exists a continuous spectrum of overeating, where at the extremes there are casual overindulgences and at the other a 'pathological' drive to consume palatable foods. It has been proposed that pathological eating behaviors may be the result of addictive appetitive behavior and loss of ability to regulate the consumption of highly processed foods containing refined carbohydrates, fats, salt, and caffeine. In this review, we highlight the genetic similarities underlying substance addiction phenotypes and overeating compulsions seen in individuals with binge eating disorder. We relate these similarities to findings from neuroimaging studies on reward processing and clinical diagnostic criteria based on addiction phenotypes. The abundance of similarities between compulsive overeating and substance addictions puts forth a case for a 'food addiction' phenotype as a valid, diagnosable disorder.

Melanocortin 4 Receptor and Dopamine D2 Receptor Expression in Brain Areas Involved in Food Intake

BACKGROUND

The melanocortin 4 receptor (MC4R) is involved in the regulation of homeostatic energy balance by the hypothalamus. Recent reports showed that MC4R can also control the motivation for food in association with a brain reward system, such as dopamine. We investigated the expression levels of MC4R and the dopamine D2 receptor (D2R), which is known to be related to food rewards, in both the hypothalamus and brain regions involved in food rewards.

METHODS

We examined the expression levels of D2R and MC4R by dual immunofluorescence histochemistry in hypothalamic regions and in the bed nucleus of the stria terminalis (BNST), the central amygdala, and the ventral tegmental area of transgenic mice expressing enhanced green fluorescent protein under the control of the D2R gene.

RESULTS

In the hypothalamic area, significant coexpression of MC4R and D2R was observed in the arcuate nucleus. We observed a significant coexpression of D2R and MC4R in the BNST, which has been suggested to be an important site for food reward.

CONCLUSION

We suggest that MC4R and D2R function in the hypothalamus for control of energy homeostasis and that within the brain regions related with rewards, such as the BNST, the melanocortin system works synergistically with dopamine for the integration of food motivation in the control of feeding behaviors.

Moderate voluntary exercise attenuates the metabolic syndrome in melanocortin-4 receptor-deficient rats showing central dopaminergic dysregulation

Objective

Melanocortin-4 receptors (MC4Rs) are highly expressed by dopamine-secreting neurons of the mesolimbic tract, but their functional role has not been fully resolved. Voluntary wheel running (VWR) induces adaptations in the mesolimbic dopamine system and has a myriad of long-term beneficial effects on health. In the present experiments we asked whether MC4R function regulates the effects of VWR, and whether VWR ameliorates MC4R-associated symptoms of the metabolic syndrome.

Methods

Electrically evoked dopamine release was measured in slice preparations from sedentary wild-type and MC4R-deficient Mc4r^K314X (HOM) rats. VWR was assessed in wild-type and HOM rats, and in MC4R-deficient loxTB^Mc4r mice, wild-type mice body weight-matched to loxTB^Mc4r mice, and wild-type mice with intracerebroventricular administration of the MC4R antagonist SHU9119. Mesolimbic dopamine system function (gene/protein expression) and metabolic parameters were examined in wheel-running and sedentary wild-type and HOM rats.

Results

Sedentary obese HOM rats had increased electrically evoked dopamine release in several ventral tegmental area (VTA) projection sites compared to wild-type controls. MC4R loss-of-function decreased VWR, and this was partially independent of body weight. HOM wheel-runners had attenuated markers of intracellular D1-type dopamine receptor signaling despite increased dopamine flux in the VTA. VWR increased and decreased ΔFosB levels in the nucleus accumbens (NAc) of wild-type and HOM runners, respectively. VWR improved metabolic parameters in wild-type wheel-runners. Finally, moderate voluntary exercise corrected many aspects of the metabolic syndrome in HOM runners.

Conclusions

Central dopamine dysregulation during VWR reinforces the link between MC4R function and molecular and behavioral responding to rewards. The data also suggest that exercise can be a successful lifestyle intervention in MC4R-haploinsufficient individuals despite reduced positive reinforcement during exercise training.

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MC4R-deficiency causes metabolic syndrome.

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Loss of MC4R signaling decreases voluntary wheel running (VWR).

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Despite moderate amounts of VWR, MC4R-associated metabolic syndrome is severely attenuated.

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MC4R-deficiency is associated with mesolimbic dopamine dysregulation during VWR.

Dopamine release in the lateral hypothalamus is stimulated by α-MSH in both the anticipatory and consummatory phases of feeding

α-Melanocyte-stimulating hormone (α-MSH), is a hypothalamic neuropeptide signaling satiation, but it is not known if α-MSH may stimulate dopamine release in a feeding control brain region of the lateral hypothalamic area (LHA), during the anticipatory and consummatory phases of feeding behavior. To address these questions, dynamics of dopamine release were measured in 15 min microdialysis samples simultaneously from the LHA and the nucleus accumbens (NAc) during consecutive exposure and provision of food and 1% sucrose in Wistar rats after overnight food deprivation. α-MSH was infused via the microdialysis probe either into the LHA or NAc starting before food exposure. Food, sucrose and water intakes were automatically monitored and analyzed concomitantly with microdialysis samples. We found that LHA-α-MSH-infused rats stopped eating earlier and consumed less food and sucrose as compared to control and NAc-α-MSH-infused rats. Exposure to food produced a peak of LHA dopamine in both LHA-α-MSH and NAc-α-MSH-infused rats but not in the controls. During food provision, LHA dopamine levels were strongly elevated in LHA-α-MSH infused rats, while delivery of α-MSH into the NAc induced a less intense increase of dopamine in both NAc and LHA. In all rats, LHA dopamine levels correlated inversely with sucrose intake. In conclusion, our study showed that α-MSH stimulates dopamine release in the LHA during both the anticipatory and consummatory phases of feeding, decreases food intake and inhibits sucrose intake. These data suggest that LHA dopamine release can be involved in α-MSH anorexigenic effects.

Central melanocortins regulate the motivation for sucrose reward

The role of the melanocortin (MC) system in feeding behavior is well established. Food intake is potently suppressed by central infusion of the MC 3/4 receptor agonist α-melanocyte stimulating hormone (α-MSH), whereas the MC 3/4 receptor inverse-agonist Agouti Related Peptide (AGRP) has the opposite effect. MC receptors are widely expressed in both hypothalamic and extra-hypothalamic brain regions, including nuclei involved in food reward and motivation, such as the nucleus accumbens (NAc) and the ventral tegmental area. This suggests that MCs modulate motivational aspects of food intake. To test this hypothesis, rats were injected intracerebroventricularly with α-MSH or AGRP and their motivation for sucrose was tested under a progressive ratio schedule of reinforcement. Food motivated behavior was dose-dependently decreased by α-MSH. Conversely, AGRP increased responding for sucrose, an effect that was blocked by pretreatment with the dopamine receptor antagonist α-flupenthixol. In contrast to progressive ratio responding, free intake of sucrose remained unaltered upon α-MSH or AGRP infusion. In addition, we investigated whether the effects of α-MSH and AGRP on food motivation were mediated by the NAc shell. In situ hybridization of MC3 and MC4 receptor expression confirmed that the MC4 receptor was expressed throughout the NAc, and injection of α-MSH and AGRP into the NAc shell caused a decrease and an increase in motivation for sucrose, respectively. These data show that the motivation for palatable food is modulated by MC4 receptors in the NAc shell, and demonstrate cross-talk between the MC and dopamine system in the modulation of food motivation.

Alpha-melanocyte stimulating hormone modulates ethanol self-administration in posterior ventral tegmental area through melanocortin-4 receptors

Although the role of alpha-melanocyte stimulating hormone (α-MSH) in alcohol seeking behaviour in rats has been demonstrated, the underlying mechanisms are not understood. Herein, we test the hypothesis that α-MSH might have a permissive effect in promoting the reward action of ethanol. Rats were implanted with cannulae targeted at the posterior ventral tegmental area (pVTA), because the site is sensitive to reinforcing effects of ethanol. These rats were trained to self-administer ethanol in standard two-lever (active/inactive) operant chamber test. Each active lever press resulted in self-administration of 100 nl of ethanol (100-300 mg%) containing solution. Over a period of 7 days, ethanol significantly increased the number of lever presses, which was considered as a measure of reward. Because ethanol at 200 mg% resulted in maximum number of lever presses (∼18-20 lever presses/30-minute session), the dose was employed in further studies. While prior administration of melanocortin (MC) agonists, α-MSH or [Nle4,D-Phe7]-alpha-MSH into pVTA, resulted in an 89% increase in lever presses, the response was attenuated following pre-treatment with MC4 receptors (MC4R) antagonist, HS014. In an immunohistochemical study, the brains of rats that were trained to self-infuse ethanol showed significantly increased α-MSH immunoreactivity in the nucleus accumbens shell, bed nucleus of stria terminalis and arcuate nucleus of the hypothalamus. In the pVTA, α-MSH fibres were found to run close to the dopamine cells, labelled with tyrosine hydroxylase antibodies. We suggest that α-MSH-MC4R system in the pVTA might be a part of the neuroadaptive mechanism underlying ethanol addiction.

Persistent increases in rat hypothalamic POMC gene expression following chronic withdrawal from chronic "binge" pattern escalating-dose, but not steady-dose, cocaine

Recent research suggests an involvement of pro-opiomelanocortin (POMC) gene products (e.g., beta-endorphin) in modulating cocaine-induced reward and addiction-like behaviors in rodents. In this study, we investigated whether chronic "binge" cocaine and its withdrawal altered POMC gene expression in the brain of rats. Male Fischer rats were treated with two different chronic (14-day) "binge" pattern cocaine administration regimens (three injections at 1-h intervals, i.p.): steady-dose (45mg/kg/day) and escalating-dose (90mg/kg on the last day). Although there was no POMC mRNA alteration after chronic steady-dose cocaine, a significant decrease in POMC mRNA levels in the hypothalamus was found after chronic escalating-dose cocaine. In contrast, after acute (1-day) withdrawal from chronic "binge" escalating-dose regimen, but not steady-dose regimen, there were increased hypothalamic POMC mRNA levels that persisted into 14days of protracted withdrawal. To study the role of the endogenous opioid systems in the cocaine withdrawal effects, we administered a single naloxone injection (1mg/kg) that caused elevated POMC mRNA levels observed 24h later in cocaine naïve rats, but it did not lead to further increases in cocaine-withdrawn rats. Our results suggest that during withdrawal from chronic "binge" escalating-dose cocaine: (1) there was a persistent increase in hypothalamic POMC gene expression; and (2) hyposensitivity of the POMC gene expression to naloxone indicates altered opioidergic tone at or above the hypothalamic level.

Decreased consumption of rewarding sucrose solutions after injection of melanocortins into the ventral tegmental area of rats

RATIONALE

The mesolimbic dopamine system is an important component of the neural circuitry controlling reward-related behavior. We have recently shown that the melanocortin peptides decrease normal homeostatic feeding through actions in the ventral tegmental area. It is unknown, however, whether melanocortin peptides can also act on dopamine pathways to regulate hedonic, reward-related aspects of feeding.

OBJECTIVES

In these studies, we tested whether injection of melanocortin receptor agonists directly into the ventral tegmental area (VTA) affected the intake of appetizing and rewarding sugar solutions in two-bottle choice tests.

METHODS

Varying doses of the melanocortin receptor agonist, MTII, were injected into the VTA, and the intake of different sugar solutions was measured in two-bottle choice tests to distinguish between potential effects on homeostatic versus hedonic aspects of feeding. In addition, 24-h food intake was measured throughout the experiments.

RESULTS

Injection of MTII into the VTA dose dependently decreased the intake of 1 and 2 % sucrose solutions and 0.2 % saccharin solutions and decreased 24-h food intake in each study. Although MTII also decreased the intake of a 10 % sucrose solution, MTII appeared to be less potent in rats exposed to 10 % sucrose, as only the highest dose of MTII tested was effective at reducing 10 % sucrose intake and food intake in these rats.

CONCLUSIONS

These studies demonstrate that melanocortins can act directly in the VTA to control reward-related feeding. Thus, these studies add to the growing body of evidence showing that melanocortins can interact with the mesolimbic dopamine system to regulate multiple reward-related behaviors.

A molecular profile of cocaine abuse includes the differential expression of genes that regulate transcription, chromatin, and dopamine cell phenotype

Chronic drug abuse, craving, and relapse are thought to be linked to long-lasting changes in neural gene expression arising through transcriptional and chromatin-related mechanisms. The key contributions of midbrain dopamine (DA)-synthesizing neurons throughout the addiction process provide a compelling rationale for determining the drug-induced molecular changes that occur in these cells. Yet our understanding of these processes remains rudimentary. The postmortem human brain constitutes a unique resource that can be exploited to gain insights into the pathophysiology of complex disorders such as drug addiction. In this study, we analyzed the profiles of midbrain gene expression in chronic cocaine abusers and well-matched drug-free control subjects using microarray and quantitative PCR. A small number of genes exhibited robust differential expression; many of these are involved in the regulation of transcription, chromatin, or DA cell phenotype. Transcript abundances for approximately half of these differentially expressed genes were diagnostic for assigning subjects to the cocaine-abusing vs control cohort. Identification of a molecular signature associated with pathophysiological changes occurring in cocaine abusers' midbrains should contribute to the development of biomarkers and novel therapeutic targets for drug addiction.

Neonatal melanocortin receptor agonist treatment reduces play fighting and promotes adult attachment in prairie voles in a sex-dependent manner

The melanocortin receptor (MCR) system has been studied extensively for its role in feeding and sexual behavior, but effects on social behavior have received little attention. α-MSH interacts with neural systems involved in sociality, including oxytocin, dopamine, and opioid systems. Acute melanotan-II (MTII), an MC3/4R agonist, potentiates brain oxytocin (OT) release and facilitates OT-dependent partner preference formation in socially monogamous prairie voles. Here we examined the long-term impact of early-life MCR stimulation on hypothalamic neuronal activity and social development in prairie voles. Male and female voles were given daily subcutaneous injections of 10 mg/kg MTII or saline between postnatal days (PND) 1-7. Neonatally-treated males displayed a reduction in initiated play fighting bouts as juveniles compared to control males. Neonatal exposure to MTII facilitated partner preference formation in adult females, but not males, after a brief cohabitation with an opposite-sex partner. Acute MTII injection elicited a significant burst of the immediate early gene EGR-1 immunoreactivity in hypothalamic OT, vasopressin, and corticotrophin releasing factor neurons, when tested in PND 6-7 animals. Daily neonatal treatment with 1 mg/kg of a more selective, brain penetrant MC4R agonist, PF44687, promoted adult partner preferences in both females and males compared with vehicle controls. Thus, developmental exposure to MCR agonists lead to a persistent change in social behavior, suggestive of structural or functional changes in the neural circuits involved in the formation of social relationships.

Gender-specific roles for the melanocortin-3 receptor in the regulation of the mesolimbic dopamine system in mice

The melanocortin-3 receptor (MC3R) and MC4R are known to play critical roles in energy homeostasis. However, the physiological functions of the MC3R remain poorly understood. Earlier reports indicated that the ventral tegmental area (VTA) is one of the highest sites of MC3R expression, and we sought to determine the function of the receptor in this brain region. A MC3R-green-fluorescent protein transgenic mouse and a MC3R knockout mouse strain were used to characterize the neurochemical identity of the MC3R neurons in the VTA and to determine the effects of global MC3R deletion on VTA dopamine (DA) homeostasis. We demonstrate that the MC3R, but not MC4R, is expressed in up to a third of dopaminergic neurons of the VTA. Global deletion of the MC3R increases total dopamine by 42% in the VTA and decreases sucrose intake and preference in female but not male mice. Ovariectomy restores dopamine levels to normal, but aberrant decreased VTA dopamine levels are also observed in prepubertal female mice. Because arcuate Agouti-related peptide/neuropeptide Y neurons are known to innervate and regulate VTA signaling, the MC3R in dopaminergic neurons provides a specific input for communication of nutritional state within the mesolimbic dopamine system. Data provided here suggest that this input may be highly sexually dimorphic, functioning as a specific circuit regulating effects of estrogen on VTA dopamine levels and on sucrose preference. Overall, this data support a sexually dimorphic function of MC3R in regulation of the mesolimbic dopaminergic system and reward.

Cannabinoid, melanocortin and opioid receptor expression on DRD1 and DRD2 subpopulations in rat striatum

The striatum harbors two neuronal populations that enable action selection. One population represents the striatonigral pathway, expresses the dopamine receptor D1 (DRD1) and promotes the execution of motor programs, while the other population represents the striatopallidal pathway, expresses the dopamine receptor D2 (DRD2) and suppresses voluntary activity. The two populations integrate distinct sensorimotor, cognitive, and emotional information streams and their combined activity enables the selection of adaptive behaviors. Characterization of these populations is critical to the understanding of their role in action selection, because it aids the identification of the molecular mechanisms that separate them. To that end, we used fluorescent in situ hybridization to quantify the percentage of striatal cells that (co)express dopaminergic receptors and receptors of the cannabinoid, melanocortin or opioid neurotransmitters systems. Our main findings are that the cannabinoid 1 receptor is equally expressed on both populations with a gradient from dorsal to ventral striatum, that the opioid receptors have a preference for expression with either the DRD1 or DRD2 and that the melanocortin 4 receptor (MC4R) is predominantly expressed in ventral parts of the striatum. In addition, we find that the level of MC4R expression determines its localization to either the DRD1 or the DRD2 population. Thereby, we provide insight into the sensitivity of the two dopaminoceptive populations to these neurotransmitters and progress the understanding of the mechanisms that enable action selection.

Synaptic changes induced by melanocortin signalling

The melanocortin system has a well-established role in the regulation of energy homeostasis, but there is growing evidence of its involvement in memory, nociception, mood disorders and addiction. In this Review, we focus on the role of the melanocortin 4 receptor and provide an integrative view of the molecular mechanisms that lead to melanocortin-induced changes in synaptic plasticity within these diverse physiological systems. We also highlight the importance of melanocortin peptides and receptors in chronic pain syndromes, memory impairments, depression and drug abuse, and the possibility of targeting them for therapeutic purposes.

Age-specific locomotor response to nicotine in yellow and mottled yellow A(vy)/a mice

Background

Most Agouti viable yellow (A^vy) mice display constitutive expression of agouti protein, which acts as an inverse agonist at the melanocortin receptor 4 (Mc4r), resulting in adult-onset obesity as well as an altered sensitivity to some drugs of abuse. We investigated the influence of excessive agouti expression on open-field locomotor response to daily 0.5 mg/kg (-)-freebase nicotine injections in 27 early adolescent and 27 young adult male A^vy/a and a/a mice, and assessed the effects of nicotine administration (0.5 mg/kg) followed by open-field testing on serum corticosterone levels in a separate group of 25 young adult male A^vy/a and a/a mice.

Findings

Young adult A^vy/a mice displayed pronounced nicotine-induced hypolocomotion (a 24% reduction in distance traveled) compared to their a/a littermates. Early adolescent A^vy/a mice did not differ from their a/a littermates or saline-matched controls in locomotion following nicotine administration. Young adult A^vy/a mice also displayed increased thigmotaxis (a 5% increase in time spent outside the center of the apparatus) on the first day of nicotine administration as compared to saline-matched controls, while a/a mice did not. An increase in serum corticosterone levels 20 minutes after nicotine injection in a separate group of young adult male mice (n = 25) was proportionally similar between A^vy/a and a/a mice.

Conclusions

Overall, the results suggest an age- and epigenotype- or genotype-specific response to nicotine administration in young adult male A^vy/a mice. It appears the A^vy/a locomotor and thigmotaxic responses to acute nicotine administration are not mediated solely by hypothalamic-pituitary-adrenal (HPA) axis stimulation.

The central GLP-1: implications for food and drug reward

Glucagon-like-peptide-1 (GLP-1) and its long acting analogs comprise a novel class of type 2 diabetes (T2D) treatment. What makes them unique among other T2D drugs is their concurrent ability to reduce food intake, a great benefit considering the frequent comorbidity of T2D and obesity. The precise neural site of action underlying this beneficial effect is vigorously researched. In accordance with the classical model of food intake control GLP-1 action on feeding has been primarily ascribed to receptor populations in the hypothalamus and the hindbrain. In contrast to this common view, relevant GLP-1 receptor populations are distributed more widely, with a prominent mesolimbic complement emerging. The physiological relevance of the mesolimbic GLP-1 is suggested by the demonstration that similar anorexic effects can be obtained by independent stimulation of the mesolimbic and hypothalamic GLP-1 receptors (GLP-1R). Results reviewed here support the idea that mesolimbic GLP-1R are sufficient to reduce hunger-driven feeding, the hedonic value of food and food-motivation. In parallel, emerging evidence suggests that the range of action of GLP-1 on reward behavior is not limited to food-derived reward but extends to cocaine, amphetamine, and alcohol reward. The new discoveries concerning GLP-1 action on the mesolimbic reward system significantly extend the potential therapeutic range of this drug target.

Addiction and the adrenal cortex

Substantial evidence shows that the hypophyseal–pituitary–adrenal (HPA) axis and corticosteroids are involved in the process of addiction to a variety of agents, and the adrenal cortex has a key role. In general, plasma concentrations of cortisol (or corticosterone in rats or mice) increase on drug withdrawal in a manner that suggests correlation with the behavioural and symptomatic sequelae both in man and in experimental animals. Corticosteroid levels fall back to normal values in resumption of drug intake. The possible interactions between brain corticotrophin releasing hormone (CRH) and proopiomelanocortin (POMC) products and the systemic HPA, and additionally with the local CRH–POMC system in the adrenal gland itself, are complex. Nevertheless, the evidence increasingly suggests that all may be interlinked and that CRH in the brain and brain POMC products interact with the blood-borne HPA directly or indirectly. Corticosteroids themselves are known to affect mood profoundly and may themselves be addictive. Additionally, there is a heightened susceptibility for addicted subjects to relapse in conditions that are associated with change in HPA activity, such as in stress, or at different times of the day. Recent studies give compelling evidence that a significant part of the array of addictive symptoms is directly attributable to the secretory activity of the adrenal cortex and the actions of corticosteroids. Additionally, sex differences in addiction may also be attributable to adrenocortical function: in humans, males may be protected through higher secretion of DHEA (and DHEAS), and in rats, females may be more susceptible because of higher corticosterone secretion.

Repeated agouti related peptide (83-132) injections inhibit cocaine-induced locomotor sensitisation, but not via the nucleus accumbens

Drug addiction is a chronic relapsing brain disease for which many of the underlying neuronal mechanisms are yet to be unravelled. There seems to be an interaction between the melanocortin system and drugs of abuse. For instance, infusion of the melanocortin MC4 receptor antagonist SHU9119 (Ac-Nle-cyclo(-Asp-His-D-2-Nal-Arg-Trp-Lys)-NH2) into the nucleus accumbens results in conditioned place avoidance, reduces the amount of lever presses for cocaine and blocks development of cocaine-induced locomotor sensitisation. The aim of this study is to determine whether the induction of locomotor sensitisation to repeated cocaine is inhibited by the melanocortin MC4 receptor inverse agonist Agouti Related Peptide (AgRP83-132). Rats were sensitised to daily cocaine injections for 5 consecutive days and 30 min prior to every daily cocaine injection, rats received an intracerebroventricular (i.c.v.) or intra nucleus accumbens injection with AgRP(83-132) or saline, to determine whether we could inhibit cocaine-induced locomotor sensitisation. We show that i.c.v. injections of AgRP(83-132) inhibit cocaine-induced locomotor sensitisation. This effect is not regulated via the nucleus accumbens, since injecting the melanocortin receptor inverse agonist AgRP(83-132) directly into the nucleus accumbens was unable to inhibit the cocaine-induced locomotor sensitisation. This implicates that the nucleus accumbens is an unlikely site to inhibit the induction of locomotor sensitisation via the melanocortin MC4 receptor. This is in contrast to other studies that show an effect of the melanocortin MC4 receptor antagonist SHU9119 on locomotor sensitisation when injected into the nucleus accumbens.

The role of melanocortins and Neuropeptide Y in food reward

The Neuropeptide Y and the melanocortin peptides are two well-described hypothalamic feeding peptides regulating energy balance. Predominantly expressed within the arcuate nucleus, these neurons project to different brain areas and modulate various aspects of feeding. Hedonic feeding, where one overindulges in palatable food consumption beyond one's nutritional necessities, is one such aspect regulated by NPY/melanocortin signaling. Research suggests that NPY/melanocortin regulate hedonic aspects of feeding through its projections to the brain reward circuitry (ventral tegmental area, lateral hypothalamus, nucleus accumbens etc.), however, exact target areas have not yet been identified. The current work explores literature to provide a mechanistic explanation for the effects of these peptides on food reward.

The expression of MC4Rs in D1R neurons regulates food intake and locomotor sensitization to cocaine

While it is known that mice lacking melanocortin 4 receptor (MC4R) expression develop hyperphagia resulting in early-onset obesity, the specific neural circuits that mediate this process remain unclear. Here, we report that selective restoration of MC4R expression within dopamine-1 receptor-expressing neurons [MC4R/dopamine 1 receptor (D1R) mice] partially blunts the severe obesity seen in MC4R-null mice by decreasing meal size, but not meal frequency, in the dark cycle. We also report that both acute cocaine-induced anorexia and the development of locomotor sensitization to repeated administration of cocaine are blunted in MC4R-null mice and normalized in MC4R/D1R mice. Neuronal retrograde tracing identifies the lateral hypothalamic area as the primary target of MC4R-expressing neurons in the nucleus accumbens. Biochemical studies in the ventral striatum show that phosphorylation of DARPP-32(Thr) (-34) and GluR1(Ser) (-845) is diminished in MC4R-null mice after chronic cocaine administration but rescued in MC4R/D1R mice. These findings highlight a physiological role of MC4R-mediated signaling within D1R neurons in the long-term regulation of energy balance and behavioral responses to cocaine.