Effects of melanocortin receptor activation and blockade on ethanol intake: a possible role for the melanocortin-4 receptor

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.

The Role of Melanocortin Plasticity in Pain-Related Outcomes After Alcohol Exposure

The global COVID-19 pandemic has shone a light on the rates and dangers of alcohol misuse in adults and adolescents in the US and globally. Alcohol exposure during adolescence causes persistent molecular, cellular, and behavioral changes that increase the risk of alcohol use disorder (AUD) into adulthood. It is established that alcohol abuse in adulthood increases the likelihood of pain hypersensitivity and the genesis of chronic pain, and humans report drinking alcohol to relieve pain symptoms. However, the longitudinal effects of alcohol exposure on pain and the underlying CNS signaling that mediates it are understudied. Specific brain regions mediate pain effects, alcohol effects, and pain-alcohol interactions, and neural signaling in those brain regions is modulated by neuropeptides. The CNS melanocortin system is sensitive to alcohol and modulates pain sensitivity, but this system is understudied in the context of pain-alcohol interactions. In this review, we focus on the role of melanocortin signaling in brain regions sensitive to alcohol and pain, in particular the amygdala. We also discuss interactions of melanocortins with other peptide systems, including the opioid system, as potential mediators of pain-alcohol interactions. Therapeutic strategies that target the melanocortin system may mitigate the negative consequences of alcohol misuse during adolescence and/or adulthood, including effects on pain-related outcomes.

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 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.

Neuronal modulation of hepatic lipid accumulation induced by bingelike drinking

Excessive alcohol consumption, including binge drinking, is a common cause of fatty liver disease. Binge drinking rapidly induces hepatic steatosis, an early step in the pathogenesis of chronic liver injury. Despite its prevalence, the process by which excessive alcohol consumption promotes hepatic lipid accumulation remains unclear. Alcohol exerts potent effects on the brain, including hypothalamic neurons crucial for metabolic regulation. However, whether or not the brain plays a role in alcohol-induced hepatic steatosis is unknown. In the brain, alcohol increases extracellular levels of adenosine, a potent neuromodulator, and previous work implicates adenosine signaling as being important for the development of alcoholic fatty liver disease. Acute alcohol exposure also increases both the activity of agouti-related protein (AgRP)-expressing neurons and AgRP immunoreactivity. Here, we show that adenosine receptor A2B signaling in the brain modulates the extent of alcohol-induced fatty liver in mice and that both the AgRP neuropeptide and the sympathetic nervous system are indispensable for hepatic steatosis induced by bingelike alcohol consumption. Together, these results indicate that the brain plays an integral role in alcohol-induced hepatic lipid accumulation and that central adenosine signaling, hypothalamic AgRP, and the sympathetic nervous system are crucial mediators of this process.

Alcohol Withdrawal and Proopiomelanocortin Neuropeptides in an Animal Model of Alcohol Dependence

BACKGROUND

Alcohol is one of the leading threats to health worldwide. Craving for alcohol makes abstinence a difficult challenge by maintaining alcohol dependence. Many studies suppose the hypothalamic-pituitary-adrenal axis, especially the proopiomelanocortin (POMC)-derived neuropeptides, to mediate craving during withdrawal in alcohol dependence. Evidence is available that the two POMC proteins, α-melanocyte-stimulating hormone (α-MSH) and β-endorphin (β-END) are altered by alcohol consumption and influence alcohol consumption, respectively.

OBJECTIVES

We investigated the dynamics of α-MSH and β-END during alcohol withdrawal and the influence of intraperitoneal administration of either α-MSH or β-END in an established rodent model (Wistar rats) for alcohol dependence.

RESULTS

After long-term alcohol self-administration over 12 months and repeated deprivation periods for 3 days, we found a significant decrease in α-MSH levels during withdrawal in rodents (p = 0.006) compared to controls, while β-END levels remained unchanged. Treatment with intraperitoneally administered α-MSH and β-END did not affect alcohol drinking behavior after deprivation.

CONCLUSION

We demonstrate the effects of alcohol deprivation on α-MSH in alcohol-dependent rodents, which appear to mimic α-MSH alteration found after fasting periods during appetite regulation. Therefore, low α-MSH levels are a possible indicator for craving in alcohol-dependent individuals and hence would be a potential target for anti-craving treatment.

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.

Bupropion, Alone and in Combination with Naltrexone, Blunts Binge-Like Ethanol Drinking and Intake Following Chronic Intermittent Access to Ethanol in Male C57BL/6J Mice

BACKGROUND

Regular binge drinking is associated with numerous adverse consequences, yet the U.S. Food and Drug Administration (FDA) has approved only 4 medications for the treatment of alcohol use disorders, and none have been specifically targeted for treating binge drinking. Here, we assessed the effectiveness of the dopamine/norepinephrine re-uptake inhibitor, bupropion (BUP), alone and in combination with naltrexone (NAL), to reduce binge-like and chronic ethanol (EtOH) intake in mice. While BUP is an FDA-approved drug that is currently used to treat depression and nicotine dependence, there has been only limited investigation to assess the ability of BUP to reduce EtOH intake.

METHODS

Male C57BL/6J mice were tested with 20% (v/v) EtOH using "drinking in the dark" (DID) procedures to model binge-like EtOH intake and following intermittent access to EtOH (IAE). In Experiment 1, mice were given intraperitoneal (i.p.) injection of 0, 20, or 40 mg/kg BUP 30 minutes before DID testing; in Experiment 2, mice were given i.p. injection of vehicle, BUP (20 mg/kg), NAL (3 mg/kg), or BUP + NAL (20 and 3 mg/kg, respectively) 30 minutes before DID testing; and in Experiment 3, mice were given i.p. injection of 0, 20, 40, or 60 mg/kg BUP 30 minutes before EtOH access after mice had 16 weeks of IAE.

RESULTS

BUP dose dependently blunted EtOH intake with DID procedures and after 16 weeks of IAE. Administration of subthreshold doses of BUP + NAL also reduced binge-like EtOH intake. Finally, BUP failed to reduce consumption of a 3% (w/v) sucrose solution.

CONCLUSIONS

BUP, alone and in combination with NAL, may represent a novel approach to treating binge EtOH intake. We are currently assessing the efficacy of BUP to curb binge drinking in a phase II clinical trial experiment.

Nucleus Accumbens MC4-R Stimulation Reduces Food and Ethanol Intake in Adult Rats Regardless of Binge-Like Ethanol Exposure during Adolescence

The melanocortin (MC) system regulates feeding and ethanol consumption. Recent evidence shows that melanocortin 4 receptor (MC4-R) stimulation within the nucleus accumbens (NAc) elicits anorectic responses and reduces ethanol consumption and ethanol palatability in adult rats. Ethanol exposure during adolescence causes long-lasting changes in neural pathways critically involved in neurobehavioral responses to ethanol. In this regard, binge-like ethanol exposure during adolescence reduces basal alpha-melanocyte-stimulating hormone (α-MSH) and alters the levels of agouti-related peptide (AgRP) in hypothalamic and limbic areas. Given the protective role of MC against excessive ethanol consumption, disturbances in the MC system induced by binge-like ethanol exposure during adolescence might contribute to excessive ethanol consumption during adulthood. In the present study, we evaluated whether binge-like ethanol exposure during adolescence leads to elevated ethanol intake and/or eating disturbance during adulthood. Toward that aim, Sprague-Dawley rats were treated with ethanol (3 g/kg i.p.; BEP group) or saline (SP group) for 14 days (PND 25 to PND 38). On PND73, all the groups were given access to 20% ethanol on an intermittent schedule. Our results showed that adult rats given intermittent access (IAE) to 20% ethanol achieved high spontaneous ethanol intake that was not significantly enhanced by binge-like ethanol pretreatment during adolescence. However, BEP group exhibited an increase in food intake without a parallel increase in body weight (BW) relative to SP group suggesting caloric efficiency disturbance. Additionally, we evaluated whether binge-like ethanol exposure during adolescence alters the expected reduction in feeding and ethanol consumption following NAc shell administration of a selective MC4-R agonist in adult rats showing high rates of ethanol consumption. For that, animals in each pretreatment condition (SP and BEP) were divided into three subgroups and given bilateral NAc infusions of the selective MC4-R agonist cyclo(NH-CH₂-CH₂-CO-His-D-Phe-Arg-Trp-Glu)-NH₂ (0, 0.75 or 1.5 μg). Results revealed that MC4-R stimulation within the NAc reduced feeding and ethanol intake in high ethanol-drinking adult rats, regardless of previous binge-like ethanol exposure during adolescence, which adds new evidence regarding the dual ability of MC compounds to control excessive ethanol and food intake.

Intra-cerebral and intra-nasal melanocortin-4 receptor antagonist blocks withdrawal hyperalgesia in alcohol-dependent rats

Humans diagnosed with alcohol use disorder are more sensitive to painful stimuli during withdrawal, which suggests that excessive alcohol drinking worsens pain outcomes. Alcohol-dependent rats exhibit increases in nociceptive sensitivity during withdrawal. Data from animal models suggest that brain melanocortin-4 receptors (MC4Rs) mediate alcohol drinking and nociception. Here we tested: (1) the effect of alcohol dependence on thermal nociception in rats, and (2) the ability of acute alcohol and (3) MC4R antagonists to reverse hyperalgesia during withdrawal in alcohol-dependent rats. Rats were trained to self-administer operant alcohol and were tested for baseline thermal nociception. Half of the rats were made dependent on alcohol, then all rats were cannulated in the lateral ventricle. We tested the effects of acute alcohol drinking, acute fixed-dose alcohol, intra-ventricular agouti-related protein (endogenous MC4R antagonist), intra-ventricular HS014 (synthetic MC4R antagonist) and intra-nasal HS014 on hyperalgesia during withdrawal in alcohol-dependent rats, relative to non-dependent drinkers and alcohol-naïve controls. Alcohol-dependent rats exhibit thermal hyperalgesia that is abolished by alcohol drinking, bolus alcohol and intra-ventricular and intra-nasal MC4R antagonists. These manipulations did not affect thermal nociception in non-dependent drinkers and alcohol-naïve controls, suggesting that alcohol dependence produces neuroadaptations in brain MC4R systems. These results suggest that brain MC4R systems may be an effective therapeutic target for reducing nociception in the alcohol-dependent organism.

Lateral hypothalamic melanocortin receptor signaling modulates binge-like ethanol drinking in C57BL/6J mice

Binge ethanol drinking is a highly pervasive and destructive behavior yet the underlying neurobiological mechanisms remain poorly understood. Recent work suggests that overlapping neurobiological mechanisms modulate feeding disorders and excessive ethanol intake, and converging evidence indicates that the melanocortin (MC) system may be a promising candidate. The aims of the present work were to examine how repeated binge-like ethanol drinking, using the 'drinking in the dark' (DID) protocol, impacts key peptides within the MC system and if site-specific manipulation of MC receptor (MCR) signaling modulates binge-like ethanol drinking. Male C57BL/6J mice were exposed to one, three or six cycles of binge-like ethanol, sucrose or water drinking, after which brain tissue was processed via immunohistochemistry (IHC) for analysis of key MC peptides, including alpha-melanocyte stimulating hormone (α-MSH) and agouti-related protein (AgRP). Results indicated that α-MSH expression was selectively decreased, while AgRP expression was selectively increased, within specific hypothalamic subregions following repeated binge-like ethanol drinking. To further explore this relationship, we used site-directed drug delivery techniques to agonize or antagonize MCRs within the lateral hypothalamus (LH). We found that the nonselective MCR agonist melanotan-II (MTII) blunted, while the nonselective MCR antagonist AgRP augmented, binge-like ethanol consumption when delivered into the LH. As these effects were region-specific, the present results suggest that a more thorough understanding of the MC neurocircuitry within the hypothalamus will help provide novel insight into the mechanisms that modulate excessive binge-like ethanol intake and may help uncover new therapeutic targets aimed at treating alcohol abuse disorders.

Genes and Alcohol Consumption: Studies with Mutant Mice

In this chapter, we review the effects of global null mutant and overexpressing transgenic mouse lines on voluntary self-administration of alcohol. We examine approximately 200 publications pertaining to the effects of 155 mouse genes on alcohol consumption in different drinking models. The targeted genes vary in function and include neurotransmitter, ion channel, neuroimmune, and neuropeptide signaling systems. The alcohol self-administration models include operant conditioning, two- and four-bottle choice continuous and intermittent access, drinking in the dark limited access, chronic intermittent ethanol, and scheduled high alcohol consumption tests. Comparisons of different drinking models using the same mutant mice are potentially the most informative, and we will highlight those examples. More mutants have been tested for continuous two-bottle choice consumption than any other test; of the 137 mouse genes examined using this model, 97 (72%) altered drinking in at least one sex. Overall, the effects of genetic manipulations on alcohol drinking often depend on the sex of the mice, alcohol concentration and time of access, genetic background, as well as the drinking test.

Evidence that Melanocortin Receptor Agonist Melanotan-II Synergistically Augments the Ability of Naltrexone to Blunt Binge-Like Ethanol Intake in Male C57BL/6J Mice

BACKGROUND

The nonselective opioid receptor antagonist, naltrexone (NAL), reduces alcohol (ethanol [EtOH]) consumption in animals and humans and is an approved medication for treating alcohol abuse disorders. Proopiomelanocortin (POMC)-derived melanocortin (MC) and opioid peptides are produced in the same neurons in the brain, and recent preclinical evidence shows that MC receptor (MCR) agonists reduce excessive EtOH drinking in animal models. Interestingly, there is a growing body of literature revealing interactions between the MC and the opioid systems in the modulation of pain, drug tolerance, and food intake.

METHODS

In the present report, a mouse model of binge EtOH drinking was employed to determine whether the MCR agonist, melanotan-II (MTII), would improve the effectiveness of NAL in reducing excessive binge-like EtOH drinking when these drugs were co-administered prior to EtOH access.

RESULTS

Both NAL and MTII blunt binge-like EtOH drinking and associated blood EtOH levels, and when administered together, a low dose of MTII (0.26 mg/kg) produces a 7.6-fold increase in the effectiveness of NAL in reducing binge-like EtOH drinking. Using isobolographic analysis, it is demonstrated that MTII increases the effectiveness of NAL in a synergistic manner.

CONCLUSIONS

The current observations suggest that activators of MC signaling may represent a new approach to treating alcohol abuse disorders and a way to potentially improve existing NAL-based therapies.

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.

Targeting central melanocortin receptors: a promising novel approach for treating alcohol abuse disorders

The melanocortin (MC) peptides are produced centrally by propiomelanocortin (POMC) neurons within the arcuate nucleus of the hypothalamus and act through five seven-transmembrane G-protein coupled melanocortin receptor (MCR) subtypes. The MC3R and MC4R subtypes, the most abundant central MCRs, are widely expressed in brain regions known to modulate neurobiological responses to ethanol, including regions of the hypothalamus and extended amygdala. Agouti-related protein (AgRP), also produced in the arcuate nucleus, is secreted in terminals expressing MCRs and functions as an endogenous MCR antagonist. This review highlights recent genetic and pharmacological findings that have implicated roles for the MC and AgRP systems in modulating ethanol consumption. Ethanol consumption is associated with significant alterations in the expression levels of various MC peptides/protein, which suggests that ethanol-induced perturbations of MC/AgRP signaling may modulate excessive ethanol intake. Consistently, MCR agonists decrease, and AgRP increases, ethanol consumption in mice. MCR agonists fail to blunt ethanol intake in mutant mice lacking the MC4R, suggesting that the protective effects of MCR agonists are modulated by the MC4R. Interestingly, recent evidence reveals that MCR agonists are more effective at blunting binge-like ethanol intake in mutant mice lacking the MC3R, suggesting that the MC3R has opposing effects on the MC4R. Finally, mutant mice lacking AgRP exhibit blunted voluntary and binge-like ethanol drinking, consistent with pharmacological studies. Collectively, these preclinical observations provide compelling evidence that compounds that target the MC system may provide therapeutic value for treating alcohol abuse disorders and that the utilization of currently available MC-targeting compounds- such as those being used to treat eating disorders- may be used as effective treatments to this end.

The protective effects of the melanocortin receptor (MCR) agonist, melanotan-II (MTII), against binge-like ethanol drinking are facilitated by deletion of the MC3 receptor in mice

Recent data have implicated the melanocortin (MC) system in modulating voluntary ethanol consumption. Administration of melanotan-II (MTII), a nonselective melanocortin receptor (MCR) agonist, reduces voluntary ethanol consumption in C57BL/6J mice. Previous studies have demonstrated that central infusion of MTII effectively reduced voluntary ethanol drinking in mutant mice lacking normal expression of MC3R (MC3R-/- mice) but failed to alter ethanol drinking in mice lacking expression of MC4R, demonstrating that central MTII administration reduces voluntary ethanol drinking by signaling through the MC4R. However, evidence shows that the neurocircuitry recruited during excessive binge-like ethanol drinking versus moderate ethanol drinking are not identical. Thus the present study sought to investigate the potential role of the MC3R in binge-like ethanol intake. To this end, the "drinking in the dark" (DID) procedure, a commonly used animal model of binge-like ethanol drinking, was employed. Wild-type MC3R+/+ and MC3R-/- mice were given intracerebroventricular (i.c.v.) infusion of MTII (0.0, 0.25, 0.50, or 1.0 μg) prior to the onset of a 4-h testing period in which mice were given access to 20% (v/v) ethanol. Immediately after the 4-h testing period, tail blood samples were collected from each animal in order to assess blood ethanol concentrations (BECs). Consistent with previous findings, central administration of MTII blunted binge-like ethanol drinking in both MC3R+/+ and MC3R-/- mice. Interestingly, all doses of MTII blunted binge-like ethanol drinking in MC3R-/- mice during the first hour of testing, while only the 1.0 μg dose reduced binge-like drinking in MC3R+/+ mice. Thus, MC3R-/- mice were more sensitive to the protective effects of MTII. These data suggest that MC3Rs oppose the protective effects of MTII against binge-like ethanol drinking, and thus selective MC3R antagonists may have potential therapeutic roles in treating excessive ethanol drinking.

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.

Adolescent binge-like ethanol exposure reduces basal α-MSH expression in the hypothalamus and the amygdala of adult rats

Melanocortins (MC) are central peptides that have been implicated in the modulation of ethanol consumption. There is experimental evidence that chronic ethanol exposure reduces α-MSH expression in the limbic and hypothalamic brain regions and alters central pro-opiomelanocortin (POMC) mRNA activity in adult rats. Adolescence is a critical developmental period of high vulnerability in which ethanol exposure alters corticotropin releasing factor, neuropeptide Y, substance P and neurokinin neuropeptide activities, all of which have key roles in ethanol consumption. Given the involvement of MC and the endogenous inverse agonist AgRP in ethanol drinking, here we evaluate whether a binge-like pattern of ethanol treatment during adolescence has a relevant impact on basal and/or ethanol-stimulated α-MSH and AgRP activities during adulthood. To this end, adolescent Sprague-Dawley rats (beginning at PND25) were pre-treated with either saline (SP group) or binge-like ethanol exposure (BEP group; 3.0 g/kg given in intraperitoneal (i.p.) injections) of one injection per day over two consecutive days, followed by 2 days without injections, repeated for a total of 8 injections. Following 25 ethanol-free days, we evaluated α-MSH and AgRP immunoreactivity (IR) in the limbic and hypothalamic nuclei of adult rats (PND63) in response to ethanol (1.5 or 3.0 g/kgi.p.) and saline. We found that binge-like ethanol exposure during adolescence significantly reduced basal α-MSH IR in the central nucleus of the amygdala (CeA), the arcuate nucleus (Arc) and the paraventricular nucleus of the hypothalamus (PVN) during adulthood. Additionally, acute ethanol elicited AgRP IR in the Arc. Rats given the adolescent ethanol treatment required higher doses of ethanol than saline-treated rats to express AgRP. In light of previous evidence that endogenous MC and AgRP regulate ethanol intake through MC-receptor signaling, we speculate that the α-MSH and AgRP disturbances induced by binge-like ethanol exposure during adolescence may contribute to excessive ethanol consumption during adulthood.

Decreased immunoreactivity of the polypeptide precursor pro-opiomelanocortin (POMC) and the prohormone convertase pc1/3 after chronic ethanol exposure in Sprague-Dawley rats

BACKGROUND

The melanocortin (MC) peptides and opioid peptide β-endorphin are cleaved from the polypeptide precursor pro-opiomelanocortin (POMC). POMC-derived peptides are generated by extensive posttranslational processing that involves several enzymes including prohormone convertase 1/3 and 2 (PC1/3 and PC2). Because ethanol (EtOH) decreases POMC mRNA levels, we determined whether the exposure to an EtOH-containing diet (ED) would significantly reduce central immunoreactivity (IR) of POMC, PC1/3, PC2, and β-endorphin.

METHODS

Male Sprague-Dawley rats were given 18 days of access to a normal rodent chow or a control diet (CD), or short-term (4 days) or long-term (18 days) access to an ED. At the end of the study, rats were perfused with 4% paraformaldehyde, and their brains were sectioned into sets for processing with POMC, PC1/3, PC2, and β-endorphin IR.

RESULTS

Rats exposed to an ED for 18 days (ED18) exhibited significant reductions of POMC and PC1/3 IR in the arcuate nucleus of the hypothalamus (Arc) relative to rats pair-fed a CD. On the other hand, rats exposed to an ED did not show any changes of central β-endorphin or PC2 IR relative to rats pair-fed a CD, regardless of length of exposure. Because there were no differences in body weights or caloric intake between the CD and ED groups, reductions of POMC and PC1/3 IR in ED-treated rats are best explained by EtOH exposure rather than altered energy balance.

CONCLUSIONS

This study shows that EtOH site-specifically reduces POMC and PC1/3 IR in rat brain. These observations are consistent with EtOH-induced reductions of α-melanocyte-stimulating hormone (α-MSH) and POMC IR that were previously reported. As MC agonists have been shown to blunt EtOH intake in rodents, exogenous MC receptor agonists, as well as targets that may increase the synthesis of endogenous α-MSH (e.g., PC1/3), may have therapeutic value for treating alcohol abuse disorders and alcoholism.

MC4-R signaling within the nucleus accumbens shell, but not the lateral hypothalamus, modulates ethanol palatability in rats

The Melanocortin (MC) system is one of the crucial neuropeptidergic systems that modulate energy balance. The roles of endogenous MC and MC-4 receptor (MC4-R) signaling within the hypothalamus in the control of homeostatic aspects of feeding are well established. Additional evidence points to a key role for the central MC system in ethanol consumption. Recently, we have shown that nucleus accumbens (NAc), but not lateral hypothalamic (LH), infusion of a selective MC4-R agonist decreases ethanol consumption. Given that MC signaling might contribute to non-homeostatic aspects of feeding within limbic circuits, we assessed here whether MC4-R signaling within the NAc and the lateral hypothalamus (LH) alters normal ingestive hedonic and/or aversive responses to ethanol in rats as measured by a taste reactivity test. Adult male Sprague-Dawley rats were given NAc- or LH- bilateral infusion of the selective MC4-R agonist cyclo (NH-CH(2)-CH(2)-CO-His-D-Phe-Arg-Trp-Glu)-NH(2) (0, 0.75 or 1.5μg/0.5μl/site) and following 30 min, the animals received 1 ml of ethanol solution (6% w/v) intraoral for 1 minute and aversive and hedonic behaviors were recorded. We found that NAc-, but not LH-administration, of a selective MC4-R agonist decreased total duration of hedonic reactions and significantly increased aversive reactions relative to saline-infused animals which support the hypothesis that MC signaling within the NAc may contribute to ethanol consumption by modulating non-homeostatic aspects (palatability) of intake.

Statistical and biological gene-lifestyle interactions of MC4R and FTO with diet and physical activity on obesity: new effects on alcohol consumption

Background

Fat mass and obesity (FTO) and melanocortin-4 receptor (MC4R) and are relevant genes associated with obesity. This could be through food intake, but results are contradictory. Modulation by diet or other lifestyle factors is also not well understood.

Objective

To investigate whether MC4R and FTO associations with body-weight are modulated by diet and physical activity (PA), and to study their association with alcohol and food intake.

Methods

Adherence to Mediterranean diet (AdMedDiet) and physical activity (PA) were assessed by validated questionnaires in 7,052 high cardiovascular risk subjects. MC4R rs17782313 and FTO rs9939609 were determined. Independent and joint associations (aggregate genetic score) as well as statistical and biological gene-lifestyle interactions were analyzed.

Results

FTO rs9939609 was associated with higher body mass index (BMI), waist circumference (WC) and obesity (P<0.05 for all). A similar, but not significant trend was found for MC4R rs17782313. Their additive effects (aggregate score) were significant and we observed a 7% per-allele increase of being obese (OR = 1.07; 95%CI 1.01–1.13). We found relevant statistical interactions (P<0.05) with PA. So, in active individuals, the associations with higher BMI, WC or obesity were not detected. A biological (non-statistical) interaction between AdMedDiet and rs9939609 and the aggregate score was found. Greater AdMedDiet in individuals carrying 4 or 3-risk alleles counterbalanced their genetic predisposition, exhibiting similar BMI (P = 0.502) than individuals with no risk alleles and lower AdMedDiet. They also had lower BMI (P = 0.021) than their counterparts with low AdMedDiet. We did not find any consistent association with energy or macronutrients, but found a novel association between these polymorphisms and lower alcohol consumption in variant-allele carriers (B+/−SE: −0.57+/−0.16 g/d per-score-allele; P = 0.001).

Conclusion

Statistical and biological interactions with PA and diet modulate the effects of FTO and MC4R polymorphisms on obesity. The novel association with alcohol consumption seems independent of their effects on BMI.

Control of food intake by MC4-R signaling in the lateral hypothalamus, nucleus accumbens shell and ventral tegmental area: interactions with ethanol

The melanocortin system is involved in animal models of obesity and anorexia-cachexia and MC4 receptors (MC4-R) are currently a target system for the development of drugs aimed to treat obesity and eating disorders in humans. Previous evidence suggest that feeding peptides might lack their orexigenic activity while stimulate ethanol intake. The present study comparatively evaluated food intake (4-h interval) in Sprague-Dawley (SD) rats drinking ethanol (6% w/v, 2 bottle choice paradigm) (EE group) and ethanol-naïve (EN) rats in response to bilateral infusion of the selective MC4-R antagonist HS014 (0, 0.02 or 0.05 μg/0.5 μl/site) or the selective MC4-R agonist cyclo(NH-CH(2)-CH(2)-CO-His-d-Phe-Arg-Trp-Glu)-NH(2) (0, 0.75 or 1.5 μg/0.5 μl/site), into the lateral hypothalamus (LH), the nucleus accumbens (NAc), or the ventral tegmental area (VTA). The main findings in the study are: (1) LH-infusions of the MC4-R antagonist increased and the agonist reduced feeding and total calories consumed, while ethanol intake remained unaltered. (2) NAc- and VTA-infusions of the selective agonist reduced food, ethanol and total calories intake. (3) NAc- and VTA-infusions of the MC4-R antagonist increased feeding in EN rats, but not in EE animals which showed a mild increase in ethanol intake, while total calories consumed remained unaltered. Present data show that having ethanol available reduces feeding elicited by NAc and VTA-MC4-R blockade. Additionally, while MC4-R signaling in the LH appears to modulate homeostatic aspects of feeding, it may contribute to non-homeostatic aspects of ingestive behaviors in the VTA and the NAc.

Neurobiology of consummatory behavior: mechanisms underlying overeating and drug use

Consummatory behavior is driven by both caloric and emotional need, and a wide variety of animal models have been useful in research on the systems that drive consumption of food and drugs. Models have included selective breeding for a specific trait, manipulation of gene expression, forced or voluntary exposure to a substance, and identification of biomarkers that predict which animals are prone to overconsuming specific substances. This research has elucidated numerous brain areas and neurochemicals that drive consummatory behavior. Although energy homeostasis is primarily mediated by the hypothalamus, reinforcement is more strongly mediated by nuclei outside the hypothalamus, in mesocorticolimbic regions. Orexigenic neurochemicals that control food intake can provide a general signal for promoting caloric intake or a more specific signal for stimulating consumption of a particular macronutrient, fat, carbohydrate, or protein. The neurochemicals involved in controlling fat ingestion--galanin, enkephalin, orexin, melanin-concentrating hormone, and the endocannabinoids--show positive feedback with this macronutrient, as these peptides both increase fat intake and are further stimulated by its intake. This positive association offers some explanation for why foods high in fat are so often overconsumed. Consumption of ethanol, a drug of abuse that also contains calories, is similarly driven by the neurochemical systems involved in fat intake, according to evidence that closely relates fat and ethanol consumption. Further understanding of the systems involved in consummatory behavior will enable the development of effective therapies for the treatment of both overeating and drug abuse.

Loss of agouti-related peptide does not significantly impact the phenotype of murine POMC deficiency

The hypothalamic melanocortin system is unique among neuropeptide systems controlling energy homeostasis, in that both anorexigenic proopiomelanocortin (POMC)-derived and orexigenic Agouti related-peptide (AgRP)-derived ligands act at the same receptors, namely melanocortin 3 and 4 receptors (MC3/4R). AgRP clearly acts as a competitive antagonist at MC3R and MC4R but may also have an inverse agonist action at these receptors. The physiological relevance of this remains uncertain. We generated a mouse lacking both POMC and AgRP [double knockout (DKO) mouse]. Phenotyping was performed in the absence and presence of glucocorticoids, and the response to central peptide administration was studied. The phenotype of DKO mice is indistinguishable from that of mice lacking Pomc alone, with both exhibiting highly similar degrees of hyperphagia and increased body length, fat, and lean mass compared with wild-type controls. After a 24-h fast, there was no difference in the refeeding response between Pomc(-/-) and DKO mice. Similarly, corticosterone supplementation caused an equivalent increase in food intake and body weight in both genotypes. Although the central administration of [Nle⁴, d-Phe⁷]-α-MSH to DKO mice caused a decrease in food intake and an increase in brown adipose tissue Ucp1 expression, both of which could be antagonized with the coadministration of AgRP, there was no effect of AgRP alone. These data suggest AgRP acts predominantly as a melanocortin antagonist. If AgRP has significant melanocortin-independent actions, these are of insufficient magnitude in vivo to impact any of the detailed phenotypes we have measured under a wide variety of conditions.

Activation of nuclear PPARγ receptors by the antidiabetic agent pioglitazone suppresses alcohol drinking and relapse to alcohol seeking

BACKGROUND

Pioglitazone and rosiglitazone belong to the class of thiazolidinediones (TZDs). They were first developed as antioxidants and then approved for the clinical treatment of insulin resistance and Type 2 diabetes. TZDs bind with high affinity and activate peroxisome proliferator-activated receptor-gamma (PPARγ) receptors, which in the brain are expressed both in neurons and in glia.

METHODS

We evaluated the effect of PPARγ activation by TZDs on alcohol drinking, relapse-like behavior, and withdrawal in the rat. We also tested the effect of TZDs on alcohol and saccharin self-administration.

RESULTS

We showed that activation of PPARγ receptors by pioglitazone (0, 10, and 30 mg/kg) and rosiglitazone (0, 10 and 30 mg/kg) given orally selectively reduced alcohol drinking. The effect was blocked by pretreatment with the selective PPARγ antagonist GW9662 (5 μg/rat) given into the lateral cerebroventricle, suggesting that this TZD's effect is mediated by PPARγ receptors in the central nervous system. Pioglitazone abolished reinstatement of alcohol seeking, a relapse-like behavior, induced by yohimbine, a pharmacologic stressor, but did not affect cue-induced relapse. In the self-administration experiments, pioglitazone reduced lever pressing for alcohol but not for saccharin. Finally, pioglitazone prevented the expression of somatic signs of alcohol withdrawal.

CONCLUSIONS

These findings provide new information about the role of brain PPARγ receptors and identify pioglitazone as candidate treatments for alcoholism and possibly other addictions.

Assessment of voluntary ethanol consumption and the effects of a melanocortin (MC) receptor agonist on ethanol intake in mutant C57BL/6J mice lacking the MC-4 receptor

BACKGROUND

The melanocortin (MC) system is composed of peptides that are cleaved from the polypeptide precursor proopiomelanocortin (POMC). Recent evidence shows that chronic exposure to ethanol significantly blunts central MC peptide immunoreactivity and MC receptor (MCR) agonists protect against high ethanol intake characteristic of C57BL/6J mice. Here, we assessed the role of the MC-4 receptor (MC4R) in voluntary ethanol intake and in modulating the effects of the nonselective MCR agonist melanotan-II (MTII) on ethanol consumption.

METHODS

To assess the role of the MC4R, MC4R knockout (Mc4r(-/-) ) and littermate wild-type (Mc4r(+/+) ) mice on a C57BL/6J background were used. Voluntary ethanol (3, 5, 8, 10, 15, and 20%, v/v) and water intake were assessed using standard two-bottle procedures. In separate experiments, Mc4r(-/-) and Mc4r(+/+) mice were given intracerebroventricular (i.c.v.) infusion of MTII (0, 0.5, or 1.0 μg/1 μl) or intraperitoneal (i.p.) injection of MTII (0 or 5 mg/kg/5 ml). The effects of MTII (0 or 0.5 μg/1 μl, i.c.v.) on 10% sucrose and 0.15% saccharin intake were assessed in C57BL/6J mice.

RESULTS

Mc4r(-/-) mice showed normal consumption of ethanol over all concentrations tested. I.c.v. infusion of MTII significantly reduced ethanol drinking in Mc4r(+/+) mice, but failed to influence ethanol intake in Mc4r(-/-) mice. When administered in an i.p. injection, MTII significantly reduced ethanol drinking in both Mc4r(-/-) and Mc4r(+/+) mice. MTII attenuated consumption of caloric (ethanol, sucrose, and food) and noncaloric (saccharin) reinforcers.

CONCLUSIONS

When given centrally, the MCR agonist MTII reduced ethanol drinking by signaling through the MC4R. On the other hand, MTII-induced reduction of ethanol drinking did not require the MC4R when administered peripherally. Together, the present observations show that the MC4R is necessary for the central actions of MCR agonists on ethanol drinking and that MTII blunts the consumption natural reinforcers, regardless of caloric content, in addition to ethanol.

Melanocortin activity in the amygdala influences alcohol intake

Melanocortins have been reported to affect alcohol intake through actions in the hypothalamus thought to be mediated by melanocortin MC4 receptors. Since these receptors are expressed in a number of amygdala regions, we have explored their role in the regulation of alcohol intake in both alcohol-preferring (P) and non-preferring (NP) rats. Injections were made at the border of the central amygdala nucleus and the basolateral amygdala. The MC3/MC4R agonist MTII reduced alcohol and food intake but increased water intake while the selective MC4R antagonist HS014 only increased food and water intake. The MC3/MC4R antagonist SHU9119 increased food and water but had little effect on alcohol intake. However, when the SHU9119 stimulation of food intake was prevented by pair-feeding, SHU9119 induced a large and prolonged decline in alcohol intake that was paralleled by an increase in water intake. These effects were only observed in P rats. We conclude that melanocortin activity in the amygdala can alter the selective preference for water and alcohol independent of effects on food intake.

The melanocortin-4 receptor: physiology, pharmacology, and pathophysiology

The melanocortin-4 receptor (MC4R) was cloned in 1993 by degenerate PCR; however, its function was unknown. Subsequent studies suggest that the MC4R might be involved in regulating energy homeostasis. This hypothesis was confirmed in 1997 by a series of seminal studies in mice. In 1998, human genetic studies demonstrated that mutations in the MC4R gene can cause monogenic obesity. We now know that mutations in the MC4R are the most common monogenic form of obesity, with more than 150 distinct mutations reported thus far. This review will summarize the studies on the MC4R, from its cloning and tissue distribution to its physiological roles in regulating energy homeostasis, cachexia, cardiovascular function, glucose and lipid homeostasis, reproduction and sexual function, drug abuse, pain perception, brain inflammation, and anxiety. I will then review the studies on the pharmacology of the receptor, including ligand binding and receptor activation, signaling pathways, as well as its regulation. Finally, the pathophysiology of the MC4R in obesity pathogenesis will be reviewed. Functional studies of the mutant MC4Rs and the therapeutic implications, including small molecules in correcting binding and signaling defect, and their potential as pharmacological chaperones in rescuing intracellularly retained mutants, will be highlighted.

CRF-1 antagonist and CRF-2 agonist decrease binge-like ethanol drinking in C57BL/6J mice independent of the HPA axis

Recent evidence suggests that corticotropin-releasing factor (CRF) receptor (CRFR) signaling is involved in modulating binge-like ethanol consumption in C57BL/6J mice. In this report, a series of experiments were performed to further characterize the role of CRFR signaling in binge-like ethanol consumption. The role of central CRFR signaling was assessed with intracerebroventricular (i.c.v.) infusion of the nonselective CRFR antagonist, alpha-helical CRF(9-41) (0, 1, 5, 10 microg/1 microl). The contribution of central CRF type 2 receptor (CRF(2)R) signaling was assessed with i.c.v. infusion of the selective CRF(2)R agonist, urocortin (Ucn) 3 (0, 0.05, 0.1, or 0.5 microg/1 microl). The role of the hypothalamic-pituitary-adrenal (HPA) axis was assessed by pretreating mice with intraperitoneal (i.p.) injection of (1) the corticosterone synthesis inhibitor, metyrapone (0, 50, 100, 150 mg/kg) or (2) the glucocorticoid receptor antagonist, mifepristone (0, 25, 50 mg/kg), and (3) by using radioimmunoassay to determine whether binge-like ethanol intake influenced plasma corticosterone levels. Finally, we determined whether the ability of the CRF(1)R antagonist, CP-154,526 (CP; 0, 10, 15 mg/kg, i.p.), to blunt binge-like drinking required normal HPA axis signaling by comparing the effectiveness of CP in adrenalectomized (ADX) and normal mice. Results showed that i.c.v. infusion of a 1 microg dose of alpha-helical CRF(9-41) significantly attenuated binge-like ethanol consumption relative to vehicle treatment, and i.c.v. infusion of Ucn 3 dose-dependently blunted binge-like drinking. On the other hand, metyrapone nonselectively reduced both ethanol and sucrose consumption, mifepristone did not alter ethanol drinking, and binge-like drinking did not correlate with plasma corticosterone levels. Finally, i.p. injection of CP significantly attenuated binge-like ethanol intake in both ADX and normal mice. Together, these results suggest that binge-like ethanol intake in C57BL/6J mice is modulated by CRF(1)R and CRF(2)R signaling, such that blockade of CRF(1)R or activation of CRF(2)R effectively reduces excessive ethanol intake. Furthermore, normal HPA axis signaling is not necessary to achieve binge-like drinking behavior.

Ethanol-induced increase of agouti-related protein (AgRP) immunoreactivity in the arcuate nucleus of the hypothalamus of C57BL/6J, but not 129/SvJ, inbred mice

BACKGROUND

The melanocortin (MC) system is composed of peptides that are cleaved from the polypeptide precursor, pro-opiomelanocortin (POMC). Previous research has shown that MC receptor (MCR) agonists reduce, and MCR antagonists increase, ethanol consumption in rats and mice. Consistently, genetic deletion of the endogenous MCR antagonist, agouti-related protein (AgRP), causes reductions of ethanol-reinforced lever pressing and binge-like ethanol drinking in C57BL/6J mice. Ethanol also has direct effects on the central MC system, as chronic exposure to an ethanol-containing diet causes significant reductions of alpha-melanocyte stimulating hormone (alpha-MSH) immunoreactivity in specific brain regions of Sprague-Dawley rats. Together, these observations suggest that the central MC system modulates neurobiological responses to ethanol. To further characterize the role of the MC system in responses to ethanol, here we compared AgRP and alpha-MSH immunoreactivity in response to an acute injection of saline or ethanol between high ethanol drinking C57BL/6J mice and moderate ethanol drinking 129/SvJ mice.

METHODS

Mice received an intraperitoneal (i.p.) injection of ethanol (1.5 g/kg or 3.5 g/kg; mixed in 0.9% saline) or an equivolume of 0.9% saline. Two hours after injection, animals were sacrificed and their brains were processed for AgRP and alpha-MSH immunoreactivity.

RESULTS

Results indicated that acute ethanol administration triggered a dose-dependent increase in AgRP immunoreactivity in the arcuate (ARC) of C57BL/6J mice, an effect that was not evident in the 129/SvJ strain. Although acute administration of ethanol did not influence alpha-MSH immunoreactivity, C57BL/6J mice had significantly greater overall alpha-MSH immunoreactivity in the ARC, dorsomedial, and lateral regions of the hypothalamus relative to the 129/SvJ strain. In contrast, C57BL/6J mice displayed significantly lower alpha-MSH immunoreactivity in the medial amygdala.

CONCLUSIONS

The results show that acute ethanol exposure has direct effects on endogenous AgRP activity in ethanol preferring C57BL/6J mice. It is suggested that ethanol-induced increases in AgRP may be part of a positive feedback system that stimulates excessive binge-like ethanol drinking in C57BL/6J mice. Inherent differences in alpha-MSH immunoreactivity may contribute to differences in neurobiological responses to ethanol that are characteristically observed between the C57BL/6J and 129/SvJ inbred strains of mice.

Genetical genomic determinants of alcohol consumption in rats and humans

BACKGROUND

We have used a genetical genomic approach, in conjunction with phenotypic analysis of alcohol consumption, to identify candidate genes that predispose to varying levels of alcohol intake by HXB/BXH recombinant inbred rat strains. In addition, in two populations of humans, we assessed genetic polymorphisms associated with alcohol consumption using a custom genotyping array for 1,350 single nucleotide polymorphisms (SNPs). Our goal was to ascertain whether our approach, which relies on statistical and informatics techniques, and non-human animal models of alcohol drinking behavior, could inform interpretation of genetic association studies with human populations.

RESULTS

In the HXB/BXH recombinant inbred (RI) rats, correlation analysis of brain gene expression levels with alcohol consumption in a two-bottle choice paradigm, and filtering based on behavioral and gene expression quantitative trait locus (QTL) analyses, generated a list of candidate genes. A literature-based, functional analysis of the interactions of the products of these candidate genes defined pathways linked to presynaptic GABA release, activation of dopamine neurons, and postsynaptic GABA receptor trafficking, in brain regions including the hypothalamus, ventral tegmentum and amygdala. The analysis also implicated energy metabolism and caloric intake control as potential influences on alcohol consumption by the recombinant inbred rats. In the human populations, polymorphisms in genes associated with GABA synthesis and GABA receptors, as well as genes related to dopaminergic transmission, were associated with alcohol consumption.

CONCLUSION

Our results emphasize the importance of the signaling pathways identified using the non-human animal models, rather than single gene products, in identifying factors responsible for complex traits such as alcohol consumption. The results suggest cross-species similarities in pathways that influence predisposition to consume alcohol by rats and humans. The importance of a well-defined phenotype is also illustrated. Our results also suggest that different genetic factors predispose alcohol dependence versus the phenotype of alcohol consumption.

Deletion of agouti-related protein blunts ethanol self-administration and binge-like drinking in mice

The melanocortin (MC) system is composed of peptides that are cleaved from the polypeptide precursor proopiomelanocortin (POMC). Recent pharmacological and genetic evidence suggests that melanocortin receptor (MCR) signaling modulates neurobiological responses to ethanol and ethanol intake. Agouti-related protein (AgRP) is synthesized by neurons in the arcuate nucleus of the hypothalamus and is a natural antagonist of MCRs. Because central administration of the functionally active AgRP fragment AgRP-(83-132) increases ethanol intake by C57BL/6 J mice, we determined if mutant mice lacking normal production of AgRP (AgRP(-/-)) and maintained on a C57BL/6 J genetic background would show reduced self-administration of ethanol relative to littermate wild-type (AgRP(+/+)) mice. AgRP(-/-) mice showed reduced 8% (v/v) ethanol-reinforced lever-pressing behavior relative to AgRP(+/+) mice in daily 2-h sessions, but normal sucrose-, saccharin- and water-reinforced lever-pressing. Similarly, AgRP(-/-) mice showed reduced consumption of 8% ethanol in a two-bottle limited access test (2 h/day), although this effect was largely sex-dependent. Using drinking-in-the-dark (DID) procedures, AgRP(-/-) mice showed blunted binge-like drinking of 20% (v/v) ethanol which was associated with lower blood ethanol levels (85 mg/dl) relative to AgRP(+/+) mice (133 mg/dl) after 4 h of intake. AgRP(-/-) mice showed normal ethanol metabolism and did not show altered sensitivity to the sedative effects of ethanol. These observations with genetically altered mice are consistent with previous pharmacological data and suggest that endogenous AgRP signaling modulates the reinforcing properties of ethanol and binge-like ethanol drinking.

Proopiomelanocortin peptides are not essential for development of ethanol-induced behavioral sensitization

BACKGROUND

Behavioral sensitization is a result of neuroadaptation to repeated drug administration and is hypothesized to reflect an increased susceptibility to drug abuse. Proopiomelanocortin (POMC) derived peptides including beta-endorphin and alpha-melanocyte stimulating hormone have been implicated in development of behavioral sensitization and the reinforcing effects of alcohol and other drugs of abuse. This study used a genetically engineered mouse strain that is deficient for neural POMC to directly determine if any POMC peptides are necessary for the development of ethanol-induced locomotor sensitization.

METHODS

Adult female mice deficient for POMC in neurons only (Pomc(-/-)Tg/Tg, KO) and wildtype (Pomc(+/+)Tg/Tg, WT) littermates were injected once daily with either saline or ethanol (i.p.) for 12 to 13 days. On ethanol test day (day 13 or 14) all mice from both treatment groups received an i.p. injection of ethanol immediately before a 15-minute analysis of locomotor activity. Blood ethanol concentration (BEC) was measured on ethanol test day immediately following the test session. Baseline locomotor activity was measured for 15 minutes after a saline injection 2 days later in both groups.

RESULTS

There was no significant difference in BEC between genotypes (WT = 2.11 +/- 0.06; KO = 2.03 +/- 0.08 mg/ml). Both WT and nPOMC-deficient mice treated repeatedly with ethanol demonstrated a significant increase in locomotor activity on test day when compared to repeated saline-treated counterparts. In addition, mice of both genotypes in the repeated saline groups showed a significant locomotor stimulant response to acute ethanol injection.

CONCLUSIONS

Central POMC peptides are not required for either the acute locomotor stimulatory effect of ethanol or the development of ethanol-induced locomotor sensitization. While these peptides may modulate other ethanol-associated behaviors, they are not essential for development of behavioral sensitization.

Oxytocin deficiency mediates hyperphagic obesity of Sim1 haploinsufficient mice

Single-minded 1 (Sim1) encodes a transcription factor essential for formation of the hypothalamic paraventricular nucleus (PVN). Sim1 haploinsufficiency is associated with hyperphagic obesity and increased linear growth in humans and mice, similar to the phenotype of melanocortin 4 receptor (Mc4r) mutations. PVN neurons in Sim1(+/-) mice are hyporesponsive to the melanocortin agonist melanotan II. PVN neuropeptides oxytocin (Oxt), TRH and CRH inhibit feeding when administered centrally. Consequently, we hypothesized that altered PVN neuropeptide expression mediates the hyperphagia of Sim1(+/-) mice. To test this hypothesis, we measured hypothalamic expression of PVN neuropeptides in Sim1(+/-) and wild-type mice. Oxt mRNA and peptide were decreased by 80% in Sim1(+/-) mice, whereas TRH, CRH, arginine vasopressin (Avp), and somatostatin mRNAs were decreased by 20-40%. Sim1(+/-) mice also showed abnormal regulation of Oxt but not CRH mRNA in response to feeding state. A selective Mc4r agonist activated PVN Oxt neurons in wild-type mice, supporting involvement of these neurons in melanocortin feeding circuits. To test whether Oxt itself regulates feeding, we measured the effects of central administration of an Oxt receptor antagonist or repeated doses of Oxt on food intake of Sim1(+/-) and wild-type mice. Sim1(+/-) mice were hypersensitive to the orexigenic effect of the Oxt receptor antagonist. Oxt decreased the food intake and weight gain of Sim1(+/-) mice at a dose that did not affect wild-type mice. Our results support the importance of Oxt neurons in feeding regulation and suggest that reduced Oxt neuropeptide is one mechanism mediating the hyperphagic obesity of Sim1(+/-) mice.

Involvement of alpha-melanocyte stimulating hormone (alpha-MSH) in differential ethanol exposure and withdrawal related depression in rat: neuroanatomical-behavioral correlates

We investigated the involvement of alpha-melanocyte stimulating hormone (alpha-MSH) following acute, chronic and withdrawal treatments of ethanol with reference to depression. The degree of depression was evaluated using Porsolt's forced swim test. While intracerebroventricular (i.c.v.) alpha-MSH (100-400 ng/rat) dose-dependently increased the immobility, opposite response was observed following administration of selective MC4 receptor antagonist HS014 (0.01-0.07 ng/rat, i.c.v.). The anti-immobility effect of acute ethanol (1-2 g/kg), injected via intra-peritoneal route (i.p.), was suppressed by central administration of alpha-MSH (100 ng/rat, i.c.v.), but was enhanced following pretreatment with HS014 (0.01 ng/rat, i.c.v.). Chronic ethanol resulted in increased immobility time, while further augmentation in immobility was noticed following ethanol withdrawal. However, concomitant HS014 (0.01 ng/rat, i.c.v.) treatment prevented tolerance as well as attenuated enhanced immobility in ethanol-withdrawn rats. Acute administration of HS014 (0.01-0.03 ng/rat, i.c.v.), at 24h post-withdrawal time point, also antagonized the ethanol withdrawal immobility in rats. The profile of alpha-MSH-immunoreactivity in the paraventricular (PVN), arcuate (ARC), paraventricular thalamic (PVT), dorsomedial hypothalamic-dorsal (DMNd) and -ventral (DMNv) nuclei, lateral hypothalamus (LH) and central nucleus of amygdala (CeA) was investigated with immunocytochemistry. Acute ethanol significantly reduced the alpha-MSH-immunoreactivity in the cells and fibers of ARC, and fibers in the PVN, DMNd, DMNv and CeA. While chronic ethanol treatment significantly increased the alpha-MSH-immunoreactivity as compared to the pair-fed control group, further augmentation was noticed following 24 h ethanol withdrawal. However, the alpha-MSH-immunoreactive profile in the PVT and LH did not respond. alpha-MSH in discrete areas may play a role in ethanol-induced antidepressant-like response and withdrawal-induced depression.

Decreased immunoreactivity of the melanocortin neuropeptide alpha-melanocyte-stimulating hormone (alpha-MSH) after chronic ethanol exposure in Sprague-Dawley rats

BACKGROUND

The melanocortin (MC) system is composed of peptides that are cleaved from the polypeptide precursor proopiomelanocortin (POMC). Recent pharmacologic and genetic evidence suggests that MC receptor (MCR) signaling modulates neurobiologic responses to ethanol and ethanol intake. Because ethanol decreases POMC mRNA levels, we determined if exposure to an ethanol-containing diet (ED) would significantly reduce central immunoreactivity of the MC peptide alpha-MSH in rats. We also determined if ethanol exposure would alter the immunoreactivity of agouti-related protein (AgRP), an endogenous MCR antagonist.

METHODS

Male Sprague-Dawley rats were given 18 days of access to normal rodent chow or a control diet (CD), or short-term (4 days) or long-term (18 days) access to an ED. At the end of the study, rats were perfused with 4% paraformaldehyde and their brains were sectioned into two sets for processing with alpha-MSH or AgRP immunohistochemistry.

RESULTS

Rats exposed to an ED showed significant reductions of central alpha-MSH immunoreactivity relative to rats exposed to a control diet (CD) or normal rodent chow. Ethanol-induced reductions of alpha-MSH immunoreactivity were site-specific and were noted in regions of the hypothalamus and extended amygdala, as well as the paraventricular nucleus of the thalamus. Because there were no differences in body weights or caloric intake between the CD and ED groups, reductions of alpha-MSH immunoreactivity in ED-treated rats are best explained by ethanol exposure rather than altered energy balance. No significant ethanol-induced alterations in hypothalamic AgRP immunoreactivity were detected.

CONCLUSIONS

The present study shows that ethanol site specifically reduces alpha-MSH immunoreactivity in rat brain. These observations, in tandem with recent pharmacologic and genetic studies, suggest that the endogenous MC system modulates neurobiologic responses to ethanol. Thus, compounds which target MCRs may prove to have therapeutic value in the treatment of excessive ethanol consumption and/or the symptoms associated with ethanol withdrawal.

Orexigenic peptides and alcohol intake: differential effects of orexin, galanin, and ghrelin

BACKGROUND

The question is which hypothalamic systems for food intake might play a role in ethanol intake and contribute to alcohol abuse. The peptide orexin was found to exhibit similar properties to galanin in its relation to dietary fat and may therefore be similar to galanin in having a stimulatory effect on alcohol intake.

METHODS

Rats were trained to drink 10% ethanol, implanted with brain cannulas, and then injected in the paraventricular nucleus (PVN), lateral hypothalamus (LH), or nucleus accumbens (NAc) with galanin, orexin-A, and for comparison, ghrelin. Ethanol, food, and water intake were measured at 1, 2, and 4 hours postinjection.

RESULTS

In the PVN, both orexin and galanin significantly increased ethanol intake, whereas ghrelin increased food intake. In the LH, orexin again induced ethanol intake, while ghrelin increased eating. In the NAc, orexin failed to influence ethanol intake but did stimulate food intake.

CONCLUSIONS

In ethanol-drinking rats, injection of orexin or galanin into the appropriate locus in the hypothalamus induced significant ethanol intake instead of food intake. Ghrelin, as a positive control, failed to influence ethanol intake at the same hypothalamic sites. In the NAc, as an anatomical control, orexin augmented eating but not ethanol intake. Thus orexin and galanin in the hypothalamus selectively stimulated ethanol intake at sites where other studies have shown that both ethanol and fat increase expression of the endogenous peptides. Thus, a neural circuit that evolved with the capability to augment food intake is apparently co-opted by ethanol and may serve as a potential positive feedback circuit for alcohol abuse.

Neurobiology of addiction. An integrative review

Evidence that psychoactive substance use disorders, bulimia nervosa, pathological gambling, and sexual addiction share an underlying biopsychological process is summarized. Definitions are offered for addiction and addictive process, the latter being the proposed designation for the underlying biopsychological process that addictive disorders are hypothesized to share. The addictive process is introduced as an interaction of impairments in three functional systems: motivation-reward, affect regulation, and behavioral inhibition. An integrative review of the literature that addresses the neurobiology of addiction is then presented, organized according to the three functional systems that constitute the addictive process. The review is directed toward identifying candidate neurochemical substrates for the impairments in motivation-reward, affect regulation, and behavioral inhibition that could contribute to an addictive process.

Alcohol-related genes: contributions from studies with genetically engineered mice

Since 1996, nearly 100 genes have been studied for their effects related to ethanol in mice using genetic modifications including gene deletion, gene overexpression, gene knock-in, and occasionally by studying existing mutants. Nearly all such studies have concentrated on genes expressed in brain, and the targeted genes range widely in their function, including most of the principal neurotransmitter systems, several neurohormones, and a number of signaling molecules. We review 141 published reports of effects (or lack thereof) of 93 genes on responses to ethanol. While most studies have focused on ethanol self-administration and reward, and/or sedative effects, other responses studied include locomotor stimulation, anxiolytic effects, and neuroadaptation (tolerance, sensitization, withdrawal). About 1/4 of the engineered mutations increase self-administration, 1/3 decrease it, and about 40% have no significant effect. In many cases, the effects on self-administration are rather modest and/or depend on the specific experimental procedures. In some cases, genes in the background strains on which the mutant is placed are important for results. Not surprisingly, review of the systems affected further supports roles for serotonin, gamma-aminobutyric acid, opioids and dopamine, all of which have long been foci of alcohol research. Novel modulatory effects of protein kinase C and G protein-activated inwardly rectifying K+ (GIRK) channels are also suggested. Some newer research with cannabinoid systems is promising, and has led to ongoing clinical trials.

Participation of alpha-melanocyte stimulating hormone in ethanol-induced anxiolysis and withdrawal anxiety in rats

Although recent reports underscore a close association between the ethanol consumption and the central melanocortin (MC) system in rats, neurobehavioral component of this association has not been explored. In this study, we investigated the role of alpha-melanocyte stimulating hormone (alpha-MSH) in ethanol (1.5-2 g/kg, i.p.) induced anxiolysis and anxiety-like behavior following withdrawal from prolonged ethanol (9% v/v ethanol, 15 days) consumption, using elevated plus maze (EPM) test in rats. While alpha-MSH (1-5 microg/rat, i.c.v.) showed dose-dependent anxiogenic-like effect, the MC4 receptor antagonist HS014 (1-10 nM/rat, i.c.v.) or antiserum against alpha-MSH (1:500-1:50 dilution, 5 microl/rat, i.c.v.) failed to produce any effect in the EPM test. The anxiolytic-like effect of ethanol was suppressed by central administration of alpha-MSH (0.5 microg/rat, i.c.v.). On the other hand, pretreatment with either HS014 (5 nM/rat, i.c.v.) or antiserum against alpha-MSH (1:100 dilution, 5 microl/rat, i.c.v.) enhanced anxiolytic action of ethanol. Moreover, ethanol withdrawal anxiety was markedly blocked by HS014 (1-10 nM/rat, i.c.v.). These results suggest that alpha-MSH may be implicated in ethanol-induced anxiolysis and withdrawal anxiety. These findings also suggest MC4 receptors as possible therapeutic target for development of drugs to address the ethanol withdrawal-related conditions.