Overlapping Peptide Control of Alcohol Self-Administration and Feeding

Neural circuit mechanisms of the cholecystokinin (CCK) neuropeptide system in addiction

Given historical focus on the roles for cholecystokinin (CCK) as a peripheral hormone controlling gastrointestinal processes and a brainstem peptide regulating food intake, the study of CCK as a limbic neuromodulator coordinating reward-seeking and emotional behavior remains underappreciated. Furthermore, localization of CCK to specialized interneurons throughout the hippocampus and cortex relegated CCK to being examined primarily as a static cell type marker rather than a dynamic functional neuromodulator. Yet, over three decades of literature have been generated by efforts to delineate the central mechanisms of addiction-related behaviors mediated by the CCK system across the striatum, amygdala, hypothalamus, and midbrain. Here, we cover fundamental findings that implicate CCK neuron activity and CCK receptor signaling in modulating drug intake and drug-seeking (focusing on psychostimulants, opioids, and alcohol). In doing so, we highlight the few studies that indicate sex differences in CCK expression and corresponding drug effects, emphasizing the importance of examining hormonal influences and sex as a biological variable in translating basic science discoveries to effective treatments for substance use disorders in human patients. Finally, we point toward understudied subcortical sources of endogenous CCK and describe how continued neurotechnology advancements can be leveraged to modernize understanding of the neural circuit mechanisms underlying CCK release and signaling in addiction-relevant behaviors.

Does endogenous cholecystokinin modulate alcohol intake?

Alcohol use disorder or alcoholism is characterized by uncontrollable alcohol use and intoxication, as well as a heightened state of anxiety after alcohol withdrawal. Ethanol-associated stimuli also drive the urge to drink by means of classical conditioning. Alcoholism has been considered a dopamine (DA) dysregulation syndrome that involves the activity of the central amygdala circuitry of anxiety. Cholecystokinin (CCK) is the most abundant neuropeptide in the mammal brain, where it activates two receptors, CCK₁ and CCK₂. Genetic evidence relates CCK₁ receptors to alcoholism in humans. CCK₂ activity has been associated with the onset of human anxiety. CCK modulates DA release in the nucleus accumbens (NAc) and it is expressed in the γ-aminobutyric acid (GABA)-expressing basket interneurons in the cerebral cortex. CCK interacts with serotonin (5-HT) neurotransmission through 5-HT₃ receptors to regulate mesocorticolimbic pathways and with GABA to attenuate anxiety in the amygdala. Finally, CCK stimulates the release of orexins and oxytocin in the hypothalamus, two relevant hypothalamic neuropeptides involved in signaling satiety for ethanol and well-being respectively. Given the "dimmer-switch" function of endogenous CCK in the neurotransmission by 5-HT, DA, GABA, and glutamate in normal and pathological behaviors (Ballaz and Bourin, 2020), we hypothesize that CCK adjusts functioning of the reward and anxiety circuitries altered by ethanol. This review gathers data supporting this hypothesis, and suggests mechanisms underlying a role for endogenous CCK in alcoholism.

Gut-brain axis and addictive disorders: A review with focus on alcohol and drugs of abuse

Due to the limited efficacy of existing medications for addictive disorders including alcohol use disorder (AUD), the need for additional medications is substantial. Potential new medications for addiction can be identified through investigation of the neurochemical substrates mediating the ability of drugs of abuse such as alcohol to activate the mesolimbic dopamine system. Interestingly, recent studies implicate neuropeptides of the gut-brain axis as modulators of reward and addiction processes. The present review therefore summarizes the current studies investigating the ability of the gut-brain peptides ghrelin, glucagon-like peptide-1 (GLP-1), amylin and neuromedin U (NMU) to modulate alcohol- and drug-related behaviors in rodents and humans. Extensive literature demonstrates that ghrelin, the only known orexigenic neuropeptide to date, enhances reward as well as the intake of alcohol, and other drugs of abuse, while ghrelin receptor antagonism has the opposite effects. On the other hand, the anorexigenic peptides GLP-1, amylin and NMU independently inhibits reward from alcohol and drugs of abuse in rodents. Collectively, these rodent and human studies imply that central ghrelin, GLP-1, amylin and NMU signaling may contribute to addiction processes. Therefore, the need for randomized clinical trials investigating the effects of agents targeting these aforementioned systems on drug/alcohol use is substantial.

Central administration of the anorexigenic peptide neuromedin U decreases alcohol intake and attenuates alcohol-induced reward in rodents

By investigating the neurochemical mechanisms through which alcohol activates the brain reward systems, novel treatment strategies for alcohol use disorder (AUD), a chronic relapsing disease, can be developed. In contrast to the common view of the function of gut-brain peptides, such as neuromedin U (NMU), to regulate food intake and appetite, a novel role in reinforcement mediation has been implied. The anorexigenic effects of NMU are mediated via NMU2 receptors, preferably in the arcuate nucleus and paraventricular nucleus. The expression of NMU2 receptors is also expressed in several reward-related areas in the brain, suggesting a role in reward regulation. The present experiments were therefore set up to investigate the effect of intracerebroventricular administration of NMU on alcohol-mediated behaviors in rodents. We found that central administration of NMU attenuated alcohol-induced locomotor stimulation, accumbal dopamine release and the expression of conditioned place preference in mice. In addition, NMU dose dependently decreased alcohol intake in high, but not in low, alcohol-consuming rats. Central NMU administration did not alter the blood alcohol concentrations nor change the corticosterone levels in rodents. Given that AUD is a major health-care challenge causing an enormous cost to society and novel treatment strategies are warranted, our data suggest that NMU analogues deserve to be evaluated as novel treatment of AUD in humans.

Ghrelin in psychiatric disorders - A review

Ghrelin is a 28-amino-acid peptide hormone, first described in 1999 and broadly expressed in the organism. As the only known orexigenic hormone secreted in the periphery, it increases hunger and appetite, promoting food intake. Ghrelin has also been shown to be involved in various physiological processes being regulated in the central nervous system such as sleep, mood, memory and reward. Accordingly, it has been implicated in a series of psychiatric disorders, making it subject of increasing investigation, with knowledge rapidly accumulating. This review aims at providing a concise yet comprehensive overview of the role of ghrelin in psychiatric disorders. Ghrelin was consistently shown to exert neuroprotective and memory-enhancing effects and alleviated psychopathology in animal models of dementia. Few human studies show a disruption of the ghrelin system in dementia. It was also shown to play a crucial role in the pathophysiology of addictive disorders, promoting drug reward, enhancing drug seeking behavior and increasing craving in both animals and humans. Ghrelin's exact role in depression and anxiety is still being debated, as it was shown to both promote and alleviate depressive and anxiety-behavior in animal studies, with an overweight of evidence suggesting antidepressant effects. Not surprisingly, the ghrelin system is also implicated in eating disorders, however its exact role remains to be elucidated. Its widespread involvement has made the ghrelin system a promising target for future therapies, with encouraging findings in recent literature.

Role of appetite-regulating peptides in the pathophysiology of addiction: implications for pharmacotherapy

Food intake and appetite are regulated by various circulating hormones including ghrelin and glucagon-like-peptide 1 (GLP-1). Ghrelin, mainly released from the stomach, increases food intake, induces appetite, enhances adiposity as well as releases growth hormone. Hypothalamic "ghrelin receptors" (GHS-R1A) have a critical role in food intake regulation, but GHS-R1A are also expressed in reward related areas. GLP-1 is produced in the intestinal mucosa as well as in the hindbrain in response to nutrient ingestion. This gut-brain hormone reduces food intake as well as regulates glucose homeostasis, foremost via GLP-1 receptors in hypothalamus and brain stem. However, GLP-1 receptors are expressed in areas intimately associated with reward regulation. Given that regulation of food and drug intake share common neurobiological substrates, the possibility that ghrelin and GLP-1 play an important role in reward regulation should be considered. Indeed, this leading article describes that the orexigenic peptide ghrelin activates the cholinergic-dopaminergic reward link, an important part of the reward systems in the brain associated with reinforcement and thereby increases the incentive salience for motivated behaviors via this system. We also review the role of ghrelin signaling for reward induced by alcohol and addictive drugs from a preclinical, clinical and human genetic perspective. In addition, the recent findings showing that GLP-1 controls reward induced by alcohol, amphetamine, cocaine and nicotine in rodents are overviewed herein. Finally, the role of several other appetite regulatory hormones for reward and addiction is briefly discussed. Collectively, these data suggest that ghrelin and GLP-1 receptors may be novel targets for development of pharmacological treatments of alcohol and drug dependence.

The glucagon-like peptide 1 analogue Exendin-4 attenuates alcohol mediated behaviors in rodents

Development of alcohol use disorders largely depends on the effects of alcohol on the brain reward systems. Emerging evidence indicate that common mechanisms regulate food and alcohol intake and raise the possibility that endocrine signals from the gut may play an important role for alcohol consumption, alcohol-induced reward and the motivation to consume alcohol. Glucagon-like peptide 1 (GLP-1), a gastrointestinal peptide regulating food intake and glucose homeostasis, has recently been shown to target central brain areas involved in reward and motivation, including the ventral tegmental area and nucleus accumbens. Herein we investigated the effects of the GLP-1 receptor agonist, Exendin-4 (Ex4), on various measures of alcohol-induced reward as well as on alcohol intake and alcohol seeking behavior in rodents. Treatment with Ex4, at a dose with no effect per se, attenuated alcohol-induced locomotor stimulation and accumbal dopamine release in mice. Furthermore, conditioned place preference for alcohol was abolished by both acute and chronic treatment with Ex4 in mice. Finally we found that Ex4 treatment decreased alcohol intake, using the intermittent access 20% alcohol two-bottle-choice model, as well as alcohol seeking behavior, using the progressive ratio test in the operant self-administration model, in rats. These novel findings indicate that GLP-1 signaling attenuates the reinforcing properties of alcohol implying that the physiological role of GLP-1 extends beyond glucose homeostasis and food intake regulation. Collectively these findings implicate that the GLP-1 receptor may be a potential target for the development of novel treatment strategies for alcohol use disorders.

Quantitative trait locus for body weight identified on rat chromosome 4 in inbred alcohol-preferring and -nonpreferring rats: potential implications for neuropeptide Y and corticotrophin releasing hormone 2

The alcohol-preferring (P) and -nonpreferring (NP) rat lines were developed using bidirectional selective breeding for alcohol consumption (g/kg/day) and alcohol preference (water:ethanol ratio). During a preliminary study, we detected a difference in body weight between inbred P (iP) and inbred NP (iNP) rats that appeared to be associated with the transfer of the Chromosome 4 quantitative trait locus (QTL) seen in the P.NP and NP.P congenic strains. After the initial confirmation that iP rats displayed lower body weight when compared to iNP rats (data not shown), body weight and growth rates of each chromosome 4 reciprocal congenic rat strain (P.NP and NP.P) were measured, and their body weight was consistent with their respective donor strain phenotype, confirming that a quantitative trait locus for body weight mapped to the chromosome 4 interval. Utilizing the newly developed interval-specific congenic strains (ISCS-A and ISCS-B), the QTL interval was further narrowed identifying the following candidate genes of interest: neuropeptide Y (Npy), juxtaposed with another zinc finger gene 1 (Jazf1), corticotrophin releasing factor receptor 2 (Crfr2) and LanC lantibiotic synthetase component C-like 2 (Lancl2). These findings indicate that a biologically active variant(s) regulates body weight on rat chromosome 4 in iP and iNP rats. This QTL for body weight was successfully captured in the P.NP and NP.P congenic strains, and interval-specific congenic strains (ISCSs) were subsequently employed to fine-map the QTL interval identifying the following candidate genes of interest: Npy, Jazf1, Crfr2 and Lancl2. Both Npy and Crfr2 have been previously identified as candidate genes of interest underlying the chromosome 4 QTL for alcohol consumption in iP and iNP rats.

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.

Role of melanin-concentrating hormone in the control of ethanol consumption: Region-specific effects revealed by expression and injection studies

The peptide melanin-concentrating hormone (MCH), produced mainly by cells in the lateral hypothalamus (LH), perifornical area (PF) and zona incerta (ZI), is suggested to have a role in the consumption of rewarding substances, such as ethanol, sucrose and palatable food. However, there is limited information on the specific brain sites where MCH acts to stimulate intake of these rewarding substances and on the feedback effects that their consumption has on the expression of endogenous MCH. The current study investigated MCH in relation to ethanol consumption, in Sprague-Dawley rats. In Experiment 1, chronic consumption of ethanol (from 0.70 to 2.7 g/kg/day) dose-dependently reduced MCH gene expression in the LH. In Experiments 2-4, the opposite effect was observed with acute oral ethanol, which stimulated MCH expression specifically in the LH but not the ZI. In Experiment 5, the effect of MCH injection in brain-cannulated rats on ethanol consumption was examined. Compared to saline, MCH injected in the paraventricular nucleus (PVN) and nucleus accumbens (NAc) selectively stimulated ethanol consumption without affecting food or water intake. In contrast, it reduced ethanol intake when administered into the LH, while having no effect in the ZI. These results demonstrate that voluntary, chronic consumption of ethanol leads to local negative feedback control of MCH expression in the LH. However, with a brief exposure, ethanol stimulates MCH-expressing neurons in this region, which through projections to the feeding-related PVN and reward-related NAc can promote further drinking behavior.

Alpha-conotoxin MII-sensitive nicotinic acetylcholine receptors are involved in mediating the ghrelin-induced locomotor stimulation and dopamine overflow in nucleus accumbens

Previously, we have reported that the orexigenic peptide ghrelin activates the cholinergic-dopaminergic reward link, involving nicotinic acetylcholine receptors (nAChR). The alpha(3)-alpha(7) and beta(2)-beta(4) subunits of the nAChR can be combined into pentameric nAChRs, with different functional roles. The present experiments show that the locomotor stimulatory effects of ghrelin, either into laterodorsal tegmental area (LDTg) or ventral tegmental area (VTA), are mediated via ventral tegmental nAChR, but neither the alpha(4)beta(2) (using dihydro-beta-erythroidine) nor the alpha(7) (using methyllycaconitine) subtypes appears to be involved. On the other hand, the alpha(3)beta(2), beta(3) and/or alpha(6) (using alpha-conotoxin MII) subtypes in the VTA mediate the stimulatory and DA-enhancing effects of ghrelin, a pattern that ghrelin shares with ethanol (n=5-8). Radioligand-binding experiments shown that ghrelin does not interfere directly with nAChRs (n=26). We therefore suggest that the alpha(3)beta(2), beta(3) and/or alpha(6) subtypes might be pharmacological targets for treatment of addictive behaviours including compulsive overeating and alcoholism.

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.

Getting beneath the phenotype of anorexia nervosa: the search for viable endophenotypes and genotypes

Treatment for anorexia nervosa has changed little from that described by Gull over a century ago. To focus merely on symptomatic relief from "not eating," as occurs with some forms of hospital care, is primitive. The evidence base to guide treatment is thin. Nevertheless, there is hope that better understanding of the causes and maintaining factors may translate into more sophisticated treatments. This review aims to look beyond the overt and startling "not eating" phenotype of anorexia nervosa and consider eating disorder endophenotypes. The first part of the review sets the eating behaviour, clinical, and psychopathological features into the context of what is now understood about the central control of appetite. The evidence base for a framework of potential eating disorder endophenotypes follows. Finally, ideas about how to translate endophenotypes into treatment are introduced.

Ghrelin administration into tegmental areas stimulates locomotor activity and increases extracellular concentration of dopamine in the nucleus accumbens

Ghrelin stimulates appetite, increases food intake and causes adiposity by mechanisms that include direct actions on the brain. Previously, we showed that intracerebroventricular administration of ghrelin has stimulatory and dopamine-enhancing properties. These effects of ghrelin are mediated via central nicotine receptors, suggesting that ghrelin can activate the acetylcholine-dopamine reward link. This reward link consists of cholinergic input from the laterodorsal tegmental area (LDTg) to the mesolimbic dopamine system that originates in the ventral tegmental area (VTA) and projects to the nucleus accumbens. Given that growth hormone secretagogue receptors (GHSR-1A) are expressed in the VTA and LDTg, brain areas involved in reward, the present series of experiments were undertaken to examine the hypothesis that these regions may mediate the stimulatory and dopamine-enhancing effects of ghrelin, by means of locomotor activity and in vivo microdialysis in freely moving mice. We found that local administration of ghrelin into the VTA (1 microg in 1 microl) induced an increase in locomotor activity and in the extracellular concentration of accumbal dopamine. In addition, local administration of ghrelin into the LDTg (1 microg in 1 microl) caused a locomotor stimulation and an increase in the extracellular levels of accumbal dopamine. Taken together, this indicates that ghrelin might, via activation of GHSR-1A in the VTA and LDTg, stimulate the acetylcholine-dopamine reward link, implicating that ghrelin is a part of the neurochemical overlap between the reward systems and those that regulate energy balance.

The Syntaxin Binding Protein 1 Gene (Stxbp1 ) Is a Candidate for an Ethanol Preference Drinking Locus on Mouse Chromosome 2

BACKGROUND

We previously mapped a quantitative trait locus (QTL) for ethanol preference drinking to mouse chromosome 2 (mapped with high confidence, LOD = 15.5, p = 3 x 10(-16)). The specific gene(s) in the QTL interval responsible for phenotypic variation in ethanol preference drinking has not been identified.

METHODS

In the current study, we investigated the association of the syntaxin binding protein 1 gene (Stxbp1) with ethanol preference drinking and other ethanol traits using a panel of B6 x D2 (BXD) recombinant inbred (RI) strains derived from the C57BL/6J (B6) and DBA/2J (D2) inbred mouse strains. Confirmation analyses for ethanol consumption and withdrawal were performed using a large B6D2 F2 cross, short-term selected lines derived from the B6 and D2 progenitor strains, and standard inbred strains.

RESULTS

BXD RI strain analysis detected provisional associations between Stxbp1 molecular variants and ethanol consumption, as well as severity of acute ethanol withdrawal, ethanol-conditioned taste aversion, and ethanol-induced hypothermia. Confirmation analyses using three independent genetic models supported the involvement of Stxbp1 in ethanol preference drinking but not in ethanol withdrawal.

CONCLUSIONS

Stxbp1 encodes a Sec1/Munc18-type protein essential for vesicular neurotransmitter release. The present study provides supporting evidence for the involvement of Stxbp1 in ethanol preference drinking.

Ghrelin stimulates locomotor activity and accumbal dopamine-overflow via central cholinergic systems in mice: implications for its involvement in brain reward

It is becoming increasingly apparent that there is a degree of neurochemical overlap between the reward systems and those regulating energy balance. We therefore investigated whether ghrelin, a stomach-derived and centrally derived orexigenic peptide, might act on the reward systems. Central ghrelin administration (1 microg/microL, to the third ventricle) induced an acute increase in locomotor activity as well as dopamine-overflow in the nucleus accumbens, suggesting that ghrelin can activate the mesoaccumbal dopamine system originating in the ventral tegmental area, a system associated with reward and motivated behaviour. The cholinergic afferents to the ventral tegmental area have been implicated in natural reward and in regulating mesoaccumbal dopamine neurons. The possibility that nicotinic receptors are involved in mediating the stimulatory and dopamine-enhancing effects of ghrelin is supported by the findings that peripheral injection of the unselective nicotinic antagonist mecamylamine (2.0 mg/kg) blocked these ghrelin-induced effects. Tentatively, ghrelin may, via activation of the acetylcholine-dopamine reward link, increase the incentive values of signals associated with motivated behaviours of importance for survival such as feeding behaviour. It will be important to discover whether this has therapeutic implications for compulsive addictive behaviours, such as eating behaviour disorders and drug dependence.

Association of galanin haplotypes with alcoholism and anxiety in two ethnically distinct populations

The neuropeptide galanin (GAL) is widely expressed in the central nervous system. Animal studies have implicated GAL in alcohol abuse and anxiety: chronic ethanol intake increases hypothalamic GAL mRNA; high levels of stress increase GAL release in the central amygdala. The coding sequence of the galanin gene, GAL, is highly conserved and a functional polymorphism has not yet been found. The aim of our study was, for the first time, to identify GAL haplotypes and investigate associations with alcoholism and anxiety. Seven single-nucleotide polymorphisms (SNPs) spanning GAL were genotyped in 65 controls from five populations: US and Finnish Caucasians, African Americans, Plains and Southwestern Indians. A single haplotype block with little evidence of historical recombination was observed for each population. Four tag SNPs were then genotyped in DSM-III-R lifetime alcoholics and nonalcoholics from two population isolates: 514 Finnish Caucasian men and 331 Plains Indian men and women. Tridimensional Personality Questionnaire harm avoidance (HA) scores, a dimensional measure of anxiety, were obtained. There was a haplotype association with alcoholism in both the Finnish (P=0.001) and Plains Indian (P=0.004) men. The SNPs were also significantly associated. Alcoholics were divided into high and low HA groups (>or= and Collapse

Key Words

• single-nucleotide polymorphism
• linkage disequilibrium
• haplotype
• galanin
• alcoholism
• anxiety

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MESH Headings

• Alcoholism/genetics
• Anxiety/genetics
• Base Sequence
• Black People/genetics
• DNA Primers
• Ethnicity/genetics
• Female
• Finland
• Galanin/genetics
• Genotype
• Harm Reduction
• Humans
• Indians, North American/genetics
• Male
• Surveys and Questionnaires
• United States
• White People/genetics
• Black or African American

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Grants

• Z99 AA999999 Intramural NIH HHS

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Affiliation(s)

I Belfer Pain and Neurosensory Mechanisms Branch, National Institute of Dental and Craniofacial Research, Bethesda, MD 20892, USA.
H Hipp
C McKnight
C Evans
B Buzas
A Bollettino
B Albaugh
M Virkkunen
Q Yuan
M B Max
D Goldman
M A Enoch

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NIH symposium series: ingestive mechanisms in obesity, substance abuse and mental disorders

This report summarizes the background and specific objectives for a symposium on the neurobiology of nonhomeostatic eating and drug abuse that was held at the 2004 Annual Meeting of the Society for the Study of Ingestive Behavior (SSIB). The symposium was the first of a series funded by a conference grant from four institutes of the National Institutes of Health. The encompassing goal of the series is to analyze the roles for the biological mechanisms of ingestion in obesity, eating disorders and other theoretically related areas including addiction, depression and schizophrenia. The symptoms and treatments of these diverse pathologies routinely involve aberrations in the mechanisms regulating eating and body weight. The presentations and discussion from this symposium (1) identified changes in neurotransmitter dynamics and gene expression in brain "reward circuits" accompanying learning of behaviors to obtain palatable foods or drugs of abuse; (2) analyzed behavioral findings in animals and humans, and neuroimaging data in humans, supporting treatment with GABA(B) agonists to reduce craving for drugs of abuse and possibly for highly rewarding foods; and (3) used neuroimaging data in humans to establish novel serotonergic targets for normalizing reward processes and impulse control in anorexia nervosa and bulimia. Overall, the symposium clearly revealed our rapidly broadening understanding of the alterations in the brain at the molecular, cellular and systems levels that are associated with craving and nonhomeostatic consumption of food and drugs of abuse. This knowledge gained largely in animal models translates to novel and better strategies for treating human patients.

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

BACKGROUND

The melanocortin (MC) system is composed of peptides that are cleaved from the polypeptide precursor pro-opiomelanocortin. A growing body of literature suggests that the MC system modulates neurobiological responses to drugs of abuse. Because ethanol has direct effects on central pro-opiomelanocortin activity, it is possible that MC neuropeptides participate in the control of voluntary ethanol consumption. Here we assessed the possibility that MC receptor (MCR) agonists modulate ethanol intake via the MC3 receptor (MC3R) and/or the MC4 receptor (MC4R) and whether the MCR antagonist AgRP-(83-132) controls ethanol consumption.

METHODS

Mc3r-deficient (Mc3r) and wild-type (Mc3r) littermate mice were given intraperitoneal (10 mg/kg) and intracerebroventricular (1.0 microg ICV) doses of melanotan II (MTII), a nonselective MCR agonist. To assess the role of MC4R, C57BL/6J mice were given an ICV infusion of the highly selective MC4R agonist cyclo(NH-CH2-CH2-CO-His-d-Phe-Arg-Trp-Glu)-NH2 (1.0 or 3.0 microg). Finally, naïve C57BL/6J mice were given an ICV infusion of AgRP-(83-132) (0.05 and 1.0 microg).

RESULTS

MTII was similarly effective at reducing ethanol drinking in Mc3r-deficient (Mc3r) and wild-type (Mc3r) littermate mice. Furthermore, ICV infusion of the MC4R agonist significantly reduced ethanol drinking, whereas ICV infusion of AgRP-(83-132) significantly increased ethanol drinking in C57BL/6J mice. Neither MTII nor AgRP-(83-132) altered blood ethanol levels at doses that modulated ethanol drinking.

CONCLUSIONS

The present results suggest that MC4R, and not MC3R, modulates MCR agonist-induced reduction of ethanol consumption and that ethanol intake is increased by the antagonistic actions of AgRP-(83-132). These findings strengthen the argument that MCR signaling controls ethanol consumption and that compounds directed at MCR may represent promising targets for treating alcohol abuse disorders in addition to obesity.

Development of medications for alcohol use disorders: recent advances and ongoing challenges

During the past decade, efforts to develop medications for alcoholism have burgeoned. Three agents, disulfiram, naltrexone and acamprosate, are now approved in a large number of countries. Although many patients have benefited from existing medications, their effects are moderate, and some alcoholics fail to respond to them. A host of new agents are currently under active investigation. Critical barriers must be overcome to ensure that future efforts in the development of medications for alcohol use disorders reach full fruition. These challenges include: establishing key targets for drug discovery; validating animal and human screening models; and developing biomarkers to help predict treatment success. In addition, it is important to formulate methodological and statistical strategies to efficiently conduct alcohol pharmacotherapy trials; to specify genetic and phenotypic patient characteristics associated with efficacy and safety for lead compounds; to forge productive alliances among governmental agencies, the pharmaceutical industry and academic researchers to further drug development; and, ultimately and perhaps most difficult, to engage the practitioner community to incorporate medications into the alcohol treatment process.