Biochemical and behavioral evidence for an interaction between ethanol and calcium channel antagonists

Separate mechanisms regulating accumbal taurine levels during baseline conditions and following ethanol exposure in the rat

Ethanol-induced dopamine release in the nucleus accumbens (nAc) is associated with reward and reinforcement, and for ethanol to elevate nAc dopamine levels, a simultaneous increase in endogenous taurine is required within the same brain region. By employing in vivo microdialysis in male Wistar rats combined with pharmacological, chemogenetic and metabolic approaches, our aim with this study was to identify mechanisms underlying ethanol-induced taurine release. Our results demonstrate that the taurine elevation, elicited by either systemic or local ethanol administration, occurs both in presence and absence of action potential firing or NMDA receptor blockade. Inhibition of volume regulated anion channels did not alter the ethanol-induced taurine levels, while inhibition of the taurine transporter occluded the ethanol-induced taurine increase, putatively due to a ceiling effect. Selective manipulation of nAc astrocytes using Gq-coupled designer receptors exclusively activated by designer drugs (DREADDs) did not affect ethanol-induced taurine release. However, activation of Gi-coupled DREADDs, or metabolic inhibition using fluorocitrate, rather enhanced than depressed taurine elevation. Finally, ethanol-induced taurine increase was fully blocked in rats pre-treated with the L-type Ca2+-channel blocker nicardipine, suggesting that the release is Ca2+ dependent. In conclusion, while astrocytes appear to be important regulators of basal taurine levels in the nAc, they do not appear to be the main cells underlying ethanol-induced taurine release.

The influence of drug class on reward in substance use disorders

In the United States, the societal costs associated with drug use surpass $500 billion annually. The rewarding and reinforcing properties that drive the use of these addictive substances are typically examined concerning the neurobiological effects responsible for their abuse potential. In this review, terms such as "abuse potential," "drug," and "addictive properties" are used due to their relevance to the methodological, theoretical, and conceptual framework for understanding the phenomenon of drug-taking behavior and the associated body of preclinical and clinical literature. The use of these terms is not intended to cast aspersions on individuals with substance use disorders (SUD). Understanding what motivates substance use has been a focus of SUD research for decades. Much of this corpus of work has focused on the shared effects of each drug class to increase dopaminergic transmission within the central reward pathways of the brain, or the "reward center." However, the precise influence of each drug class on dopamine signaling, and the extent thereof, differs considerably. Furthermore, the aforementioned substances have effects on several neurobiological targets that mediate and modulate their addictive properties. The current manuscript sought to review the influence of drug class on the rewarding effects of each of the major pharmacological classes of addictive drugs (i.e., psychostimulants, opioids, nicotine, alcohol, and cannabinoids). Our review suggests that even subtle differences in drug effects can result in significant variability in the subjective experience of the drug, altering rewarding and other reinforcing effects. Additionally, this review will argue that reward (i.e., the attractive and motivational property of a stimulus) alone is not sufficient to explain the abuse liability of these substances. Instead, abuse potential is best examined as a function of both positive and negative reinforcing drug effects (i.e., stimuli that the subject will work to attain and stimuli that the subject will work to end or avoid, respectively). Though reward is central to drug use, the factors that motivate and maintain drug taking are varied and complex, with much to be elucidated.

Semaglutide reduces alcohol intake and relapse-like drinking in male and female rats

BACKGROUND

Glucagon-like peptide1 receptor (GLP-1R) agonists have been found to reduce alcohol drinking in rodents and overweight patients with alcohol use disorder (AUD). However, the probability of low semaglutide doses, an agonist with higher potency and affinity for GLP-1R, to attenuate alcohol-related responses in rodents and the underlying neuronal mechanisms is unknown.

METHODS

In the intermittent access model, we examined the ability of semaglutide to decrease alcohol intake and block relapse-like drinking, as well as imaging the binding of fluorescently marked semaglutide to nucleus accumbens (NAc) in both male and female rats. The suppressive effect of semaglutide on alcohol-induced locomotor stimulation and in vivo dopamine release in NAc was tested in male mice. We evaluated effect of semaglutide on the in vivo release of dopamine metabolites (DOPAC and HVA) and gene expression of enzymes metabolising dopamine (MAOA and COMT) in male mice.

FINDINGS

In male and female rats, acute and repeated semaglutide administration reduced alcohol intake and prevented relapse-like drinking. Moreover, fluorescently labelled semaglutide was detected in NAc of alcohol-drinking male and female rats. Further, semaglutide attenuated the ability of alcohol to cause hyperlocomotion and to elevate dopamine in NAc in male mice. As further shown in male mice, semaglutide enhanced DOPAC and HVA in NAc when alcohol was onboard and increased the gene expression of COMT and MAOA.

INTERPRETATION

Altogether, this indicates that semaglutide reduces alcohol drinking behaviours, possibly via a reduction in alcohol-induced reward and NAc dependent mechanisms. As semaglutide also decreased body weight of alcohol-drinking rats of both sexes, upcoming clinical studies should test the plausibility that semaglutide reduces alcohol intake and body weight in overweight AUD patients.

FUNDING

Swedish Research Council (2019-01676), LUA/ALF (723941) from the Sahlgrenska University Hospital and the Swedish brain foundation.

The glycine-containing dipeptide leucine-glycine raises accumbal dopamine levels in a subpopulation of rats presenting a lower endogenous dopamine tone

Interventions that elevate glycine levels and target the glycine receptor (GlyR) in the nucleus Accumbens (nAc) reduce ethanol intake in rats, supposedly by acting on the brain reward system via increased basal and attenuated ethanol-induced nAc dopamine release. Glycine transport across the blood brain barrier (BBB) appears inefficient, but glycine-containing dipeptides elevate whole brain tissue dopamine levels in mice. This study explores whether treatment with the glycine-containing dipeptides leucine-glycine (Leu-Gly) and glycine-leucine (Gly-Leu) by means of a hypothesized, facilitated BBB passage, alter nAc glycine and dopamine levels and locomotor activity in two rodent models. The acute effects of Leu-Gly and Gly-Leu (1–1000 mg/kg, i.p.) alone or Leu-Gly in combination with ethanol on locomotion in male NMRI mice were examined in locomotor activity boxes. Striatal and brainstem slices were obtained for ex vivo HPLC analyses of tissue levels of glycine and dopamine. Furthermore, the effects of Leu-Gly i.p. (1–1000 mg/kg) on glycine and dopamine output in the nAc were examined using in vivo microdialysis coupled to HPLC in freely moving male Wistar rats. Leu-Gly and Gly-Leu did not significantly alter locomotion, ethanol-induced hyperlocomotor activity or tissue levels of glycine or dopamine, apart from Gly-Leu 10 mg/kg that slightly raised nAc dopamine. Microdialysis revealed no significant alterations in nAc glycine or dopamine levels when regarding all rats as a homogenous group. In a subgroup of rats defined as dopamine responders, a significant elevation of nAc dopamine (20%) was seen following Leu-Gly 10–1000 mg/kg i.p, and this group of animals presented lower baseline dopamine levels compared to dopamine non-responders. To conclude, peripheral injection of glycine-containing dipeptides appears inefficient in elevating central glycine levels but raises accumbal dopamine levels in a subgroup of rats with a lower endogenous dopamine tone. The tentative relationship between dopamine baseline and ensuing response to glycinergic treatment and presumptive direct interactions between glycine-containing dipeptides and the GlyR bear insights for refinement of the glycinergic treatment concept for alcohol use disorder (AUD).

L-Type Calcium Channel Blockers: A Potential Novel Therapeutic Approach to Drug Dependence

This review describes interactions between compounds, primarily dihydropyridines, that block L-type calcium channels and drugs that cause dependence, and the potential importance of these interactions. The main dependence-inducing drugs covered are alcohol, psychostimulants, opioids, and nicotine. In preclinical studies, L-type calcium channel blockers prevent or reduce important components of dependence on these drugs, particularly their reinforcing actions and the withdrawal syndromes. The channel blockers also reduce the development of tolerance and/or sensitization, and they have no intrinsic dependence liability. In some instances, their effects include reversal of brain changes established during drug dependence. Prolonged treatment with alcohol, opioids, psychostimulant drugs, or nicotine causes upregulation of dihydropyridine binding sites. Few clinical studies have been carried out so far, and reports are conflicting, although there is some evidence of effectiveness of L-channel blockers in opioid withdrawal. However, the doses of L-type channel blockers used clinically so far have necessarily been limited by potential cardiovascular problems and may not have provided sufficient central levels of the drugs to affect neuronal dihydropyridine binding sites. New L-type calcium channel blocking compounds are being developed with more selective actions on subtypes of L-channel. The preclinical evidence suggests that L-type calcium channels may play a crucial role in the development of dependence to different types of drugs. Mechanisms for this are proposed, including changes in the activity of mesolimbic dopamine neurons, genomic effects, and alterations in synaptic plasticity. Newly developed, more selective L-type calcium channel blockers could be of considerable value in the treatment of drug dependence. SIGNIFICANCE STATEMENT: Dependence on drugs is a very serious health problem with little effective treatment. Preclinical evidence shows drugs that block particular calcium channels, the L-type, reduce dependence-related effects of alcohol, opioids, psychostimulants, and nicotine. Clinical studies have been restricted by potential cardiovascular side effects, but new, more selective L-channel blockers are becoming available. L-channel blockers have no intrinsic dependence liability, and laboratory evidence suggests they reverse previously developed effects of dependence-inducing drugs. They could provide a novel approach to addiction treatment.

Long-term treatment with a glucagon-like peptide-1 receptor agonist reduces ethanol intake in male and female rats

Given the limited efficacy of available pharmacotherapies for treatment of alcohol use disorder (AUD), the need for new medications is substantial. Preclinical studies have shown that acute administration of glucagon-like peptide-1 receptor (GLP-1R) agonists inhibits various ethanol-related behaviours, indicating this system as a potential target for AUD. However, the effects of long-term systemic treatment of GLP-1R agonists on ethanol intake in male and female rodents are to date unknown. Therefore, we investigated the effects of 9 or 5 weeks of once weekly administration of dulaglutide, a long-acting GLP-1R agonist, on ethanol intake in male and female rats. The ethanol intake during treatment discontinuation was also monitored. In an initial attempt to identify preliminary underlying mechanisms, the effects of 9 weeks of once weekly dulaglutide treatment on monoaminergic signalling in reward-related areas were explored in both sexes. We found that 9 or 5 weeks of once weekly dulaglutide treatment reduced ethanol intake and preference in male and female rats. Following discontinuation of dulaglutide treatment, the decrease in ethanol consumption was prolonged in males, but not females. We demonstrated that 9 weeks of dulaglutide treatment differentially influenced monoaminergic signalling in reward-related areas of male and female rats. Collectively, these data imply that the GLP-1R attracts interest as a potential molecular target in the medical treatment of AUD in humans: more specifically, dulaglutide should be evaluated as a potential medication for treatment thereof.

Brain region-specific neuromedin U signalling regulates alcohol-related behaviours and food intake in rodents

Albeit neuromedin U (NMU) attenuates alcohol‐mediated behaviours, its mechanisms of action are poorly defined. Providing that the behavioural effects of alcohol are processed within the nucleus accumbens (NAc) shell, anterior ventral tegmental area (aVTA), and laterodorsal tegmental area (LDTg), we assessed the involvement of NMU signalling in the aforementioned areas on alcohol‐mediated behaviours in rodents. We further examined the expression of NMU and NMU receptor 2 (NMUR2) in NAc and the dorsal striatum of high compared with low alcohol‐consuming rats, as this area is of importance in the maintenance of alcohol use disorder (AUD). Finally, we investigated the involvement of NAc shell, aVTA and LDTg in the consumption of chow and palatable peanut butter, to expand the link between NMU and reward‐related behaviours. We demonstrated here, that NMU into the NAc shell, but not aVTA or LDTg, blocked the ability of acute alcohol to cause locomotor stimulation and to induce memory retrieval of alcohol reward, as well as reduced peanut butter in mice. In addition, NMU into NAc shell decreased alcohol intake in rats. On a molecular level, we found increased NMU and decreased NMUR2 expression in the dorsal striatum in high compared with low alcohol‐consuming rats. Both aVTA and LDTg, rather than NAc shell, were identified as novel sites of action for NMU's anorexigenic properties in mice based on NMU's ability to selectively reduce chow intake when injected to these areas. Collectively, these data indicate that NMU signalling in different brain areas selectively modulates different behaviours.

Energy drink constituents (caffeine and taurine) selectively potentiate ethanol-induced locomotion in mice

Mixing alcohol with energy drinks has emerged as a popular trend over the last decade. However, epidemiological studies have found this consumption to be associated with increased hazards, such as binge drinking, increased alcohol-related harm and risk of developing alcohol use disorder. The mechanisms underlying these effects are not clear, but much attention has been attributed to caffeine. However, taurine, another common ingredient in energy drinks, has also been associated with the dopamine elevating properties of ethanol, and may in this respect contribute to the increased liability associated with the mixture of alcohol and energy drinks. In the present study we measured locomotor activity, a phenomenon previously linked to the dopamine activating and reinforcing properties of the drug, following acute systemic administration with caffeine (1, 5, 15, 30 mg/kg), taurine (30, 60, 300, 600 mg/kg) and ethanol (1.75, 2.5, 3.25 g/kg), alone or in combination. We found that ethanol and caffeine, but not taurine, increased locomotion compared to vehicle. In addition, when combined with ethanol, caffeine, but not taurine, increased the locomotor stimulatory effect of ethanol. Furthermore, the combination of caffeine and taurine were able to further enhance the ethanol-induced locomotor response. Eleven days of intermittent caffeine exposure produced a sensitized response to the caffeine-induced locomotion, but did not alter the additive effect produced by the combination of caffeine and taurine on ethanol-induced locomotion. Based on the present study we suggest that the combination of caffeine and taurine, at a specific dose range, enhances the locomotor stimulatory properties of ethanol, a phenomenon previously linked to the reinforcing properties of the drug.

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.

GLP-1 signaling and alcohol-mediated behaviors; preclinical and clinical evidence

Alcohol addiction, affecting approximately four percent of the population, contributes significantly to the global burden of diseases and is a substantial cost to the society. The neurochemical mechanisms regulating alcohol mediated behaviors is complex and in more recent years a new physiological role of the gut-brain peptides, traditionally known to regulate appetite and food intake, have been suggested. Indeed, regulators of alcohol-mediated behaviors. One of these gut-brain peptides is the annorexigenic peptide glucagon-like peptide-1 (GLP-1), Preclinical studies show that GLP-1 receptor activation, either by GLP-1 or analogues, attenuate the ability of alcohol to activate the mesolimbic dopamine system as well as decrease alcohol consumption and operant self-administration. In further support for the endogenous GLP-1 system in addiction processes are the experimental data showing that a GLP-1 receptor antagonist increases alcohol intake. Moreover, GLP-1 receptor agonists prevent the ability of other addictive drugs to activate the mesolimbic dopamine system. The number of clinical studies is limited, but show i) that genetic variation in the GLP-1 receptor gene is associated with alcohol addiction as well as increased alcohol infusion in humans, ii) that plasma levels of GLP-1 are associated with the subjective experience of cocaine and iii) that a GLP-1 receptor agonist reduces alcohol intake in patients with type-2 diabetes mellitus. These experimental and clinical studies raises the concern that clinically available GLP-1 receptor agonists deserves to be tested as potential treatments of patients with addictive disorders including alcohol addiction. This article is part of the Special Issue entitled 'Metabolic Impairment as Risk Factors for Neurodegenerative Disorders.'

A cannabinoid receptor antagonist attenuates ghrelin-induced activation of the mesolimbic dopamine system in mice

Ghrelin has been attributed various physiological processes including food intake and reward regulation, through activation of the mesolimbic dopamine system. Reward modulation involves the mesolimbic dopamine system, consisting of the ventral tegmental area (VTA) dopamine neurons targeting nucleus accumbens (NAc), a system that ghrelin activates through VTA-dependent mechanisms. In the first study, we found that systemic intraperitoneal (ip) administration of rimonabant attenuated intracerebroventricular (icv) ghrelin's ability to cause locomotor stimulation and NAc dopamine release in mice. Ghrelin-induced (icv) chow intake was not altered by rimonabant administration (ip). Finally, we showed that bilateral VTA administration of rimonabant blocks the ability of intra-VTA administered ghrelin to increase locomotor activity, but does not affect food intake in mice. Collectively, these data indicate clear dissociation between regulation of food intake and activation of the mesolimbic dopamine system.

From Gene to Behavior: L-Type Calcium Channel Mechanisms Underlying Neuropsychiatric Symptoms

The L-type calcium channels (LTCCs) Cav1.2 and Cav1.3, encoded by the CACNA1C and CACNA1D genes, respectively, are important regulators of calcium influx into cells and are critical for normal brain development and plasticity. In humans, CACNA1C has emerged as one of the most widely reproduced and prominent candidate risk genes for a range of neuropsychiatric disorders, including bipolar disorder (BD), schizophrenia (SCZ), major depressive disorder, autism spectrum disorder, and attention deficit hyperactivity disorder. Separately, CACNA1D has been found to be associated with BD and autism spectrum disorder, as well as cocaine dependence, a comorbid feature associated with psychiatric disorders. Despite growing evidence of a significant link between CACNA1C and CACNA1D and psychiatric disorders, our understanding of the biological mechanisms by which these LTCCs mediate neuropsychiatric-associated endophenotypes, many of which are shared across the different disorders, remains rudimentary. Clinical studies with LTCC blockers testing their efficacy to alleviate symptoms associated with BD, SCZ, and drug dependence have provided mixed results, underscoring the importance of further exploring the neurobiological consequences of dysregulated Cav1.2 and Cav1.3. Here, we provide a review of clinical studies that have evaluated LTCC blockers for BD, SCZ, and drug dependence-associated symptoms, as well as rodent studies that have identified Cav1.2- and Cav1.3-specific molecular and cellular cascades that underlie mood (anxiety, depression), social behavior, cognition, and addiction.

The glucagon-like peptide 1 receptor agonist liraglutide attenuates the reinforcing properties of alcohol in rodents

The incretin hormone, glucagon-like peptide 1 (GLP-1), regulates gastric emptying, glucose-dependent stimulation of insulin secretion and glucagon release, and GLP-1 analogs are therefore approved for treatment of type II diabetes. GLP-1 receptors are expressed in reward-related areas such as the ventral tegmental area and nucleus accumbens, and GLP-1 was recently shown to regulate several alcohol-mediated behaviors as well as amphetamine-induced, cocaine-induced and nicotine-induced reward. The present series of experiments were undertaken to investigate the effect of the GLP-1 receptor agonist, liraglutide, on several alcohol-related behaviors in rats that model different aspects of alcohol use disorder in humans. Acute liraglutide treatment suppressed the well-documented effects of alcohol on the mesolimbic dopamine system, namely alcohol-induced accumbal dopamine release and conditioned place preference in mice. In addition, acute administration of liraglutide prevented the alcohol deprivation effect and reduced alcohol intake in outbred rats, while repeated treatment of liraglutide decreased alcohol intake in outbred rats as well as reduced operant self-administration of alcohol in selectively bred Sardinian alcohol-preferring rats. Collectively, these data suggest that GLP-1 receptor agonists could be tested for treatment of alcohol dependence in humans.

Genetic variation of the growth hormone secretagogue receptor gene is associated with alcohol use disorders identification test scores and smoking

The multifaceted gut‐brain peptide ghrelin and its receptor (GHSR‐1a) are implicated in mechanisms regulating not only the energy balance but also the reward circuitry. In our pre‐clinical models, we have shown that ghrelin increases whereas GHSR‐1a antagonists decrease alcohol consumption and the motivation to consume alcohol in rodents. Moreover, ghrelin signaling is required for the rewarding properties of addictive drugs including alcohol and nicotine in rodents. Given the hereditary component underlying addictive behaviors and disorders, we sought to investigate whether single nucleotide polymorphisms (SNPs) located in the pre‐proghrelin gene (GHRL) and GHSR‐1a gene (GHSR) are associated with alcohol use, measured by the alcohol use disorders identification test (AUDIT) and smoking. Two SNPs located in GHRL, rs4684677 (Gln90Leu) and rs696217 (Leu72Met), and one in GHSR, rs2948694, were genotyped in a subset (n = 4161) of a Finnish population‐based cohort, the Genetics of Sexuality and Aggression project. The effect of these SNPs on AUDIT scores and smoking was investigated using linear and logistic regressions, respectively. We found that the minor allele of the rs2948694 SNP was nominally associated with higher AUDIT scores (P = 0.0204, recessive model) and smoking (P = 0.0002, dominant model). Furthermore, post hoc analyses showed that this risk allele was also associated with increased likelihood of having high level of alcohol problems as determined by AUDIT scores ≥ 16 (P = 0.0043, recessive model). These convergent findings lend further support for the hypothesized involvement of ghrelin signaling in addictive disorders.

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 the nicotine-induced locomotor stimulation, accumbal dopamine release, conditioned place preference as well as the expression of locomotor sensitization in mice

The gastrointestinal peptide glucagon-like peptide 1 (GLP-1) is known to regulate consummatory behavior and is released in response to nutrient ingestion. Analogues of this peptide recently emerged as novel pharmacotherapies for treatment of type II diabetes since they reduce gastric emptying, glucagon secretion as well as enhance glucose-dependent insulin secretion. The findings that GLP-1 targets reward related areas including mesolimbic dopamine areas indicate that the physiological role of GLP-1 extends beyond food intake and glucose homeostasis control to include reward regulation. The present series of experiments was therefore designed to investigate the effects of the GLP-1 receptor agonist, Exendin-4 (Ex4), on established nicotine-induced effects on the mesolimbic dopamine system in mice. Specifically, we show that treatment with Ex4, at a dose with no effect per se, attenuate nicotine-induced locomotor stimulation, accumbal dopamine release as well as the expression of conditioned place preference in mice. In accordance, Ex4 also blocks nicotine-induced expression of locomotor sensitization in mice. Given that development of nicotine addiction largely depends on the effects of nicotine on the mesolimbic dopamine system these findings indicate that the GLP-1 receptor may be a potential target for the development of novel treatment strategies for nicotine cessations in humans.

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.

Genetic variation of the ghrelin signalling system in individuals with amphetamine dependence

The development of amphetamine dependence largely depends on the effects of amphetamine in the brain reward systems. Ghrelin, an orexigenic peptide, activates the reward systems and is required for reward induced by alcohol, nicotine, cocaine and amphetamine in mice. Human genetic studies have shown that polymorphisms in the pre-proghrelin (GHRL) as well as GHS-R1A (GHSR) genes are associated with high alcohol consumption, increased weight and smoking in males. Since the heritability factor underlying drug dependence is shared between different drugs of abuse, we here examine the association between single nucleotide polymorphisms (SNPs) and haplotypes in the GHRL and GHSR, and amphetamine dependence. GHRL and GHSR SNPs were genotyped in Swedish amphetamine dependent individuals (n = 104) and controls from the general population (n = 310). A case-control analysis was performed and SNPs and haplotypes were additionally tested for association against Addiction Severity Interview (ASI) composite score of drug use. The minor G-allele of the GHSR SNP rs2948694, was more common among amphetamine dependent individuals when compared to controls (p_c = 0.02). A significant association between the GHRL SNP rs4684677 and ASI composite score of drug use was also reported (p_c = 0.03). The haplotype analysis did not add to the information given by the individual polymorphisms. Although genetic variability of the ghrelin signalling system is not a diagnostic marker for amphetamine dependence and problem severity of drug use, the present results strengthen the notion that ghrelin and its receptor may be involved in the development of addictive behaviours and may thus serve as suitable targets for new treatments of such disorders.

Concomitant release of ventral tegmental acetylcholine and accumbal dopamine by ghrelin in rats

Ghrelin, an orexigenic peptide, regulates energy balance specifically via hypothalamic circuits. Growing evidence suggest that ghrelin increases the incentive value of motivated behaviours via activation of the cholinergic-dopaminergic reward link. It encompasses the cholinergic afferent projection from the laterodorsal tegmental area (LDTg) to the dopaminergic cells of the ventral tegmental area (VTA) and the mesolimbic dopamine system projecting from the VTA to nucleus accumbens (N.Acc.). Ghrelin receptors (GHS-R1A) are expressed in these reward nodes and ghrelin administration into the LDTg increases accumbal dopamine, an effect involving nicotinic acetylcholine receptors in the VTA. The present series of experiments were undertaken directly to test this hypothesis. Here we show that ghrelin, administered peripherally or locally into the LDTg concomitantly increases ventral tegmental acetylcholine as well as accumbal dopamine release. A GHS-R1A antagonist blocks this synchronous neurotransmitter release induced by peripheral ghrelin. In addition, local perfusion of the unselective nicotinic antagonist mecamylamine into the VTA blocks the ability of ghrelin (administered into the LDTg) to increase N.Acc.-dopamine, but not VTA-acetylcholine. Collectively our data indicate that ghrelin activates the LDTg causing a release of acetylcholine in the VTA, which in turn activates local nicotinic acetylcholine receptors causing a release of accumbal dopamine. Given that a dysfunction in the cholinergic-dopaminergic reward system is involved in addictive behaviours, including compulsive overeating and alcohol use disorder, and that hyperghrelinemia is associated with such addictive behaviours, ghrelin-responsive circuits may serve as a novel pharmacological target for treatment of alcohol use disorder as well as binge eating.

Dantrolene blockade of ryanodine receptor impairs ethanol-induced behavioral stimulation, ethanol intake and loss of righting reflex

Calcium has been characterized as one of the most ubiquitous, universal and versatile intracellular signals. Among other substances with the ability to alter intracellular calcium levels, ethanol has been described as particularly relevant because of its social and economic impact. Ethanol effects on calcium distribution and flux in vitro have been widely studied, showing that acute ethanol administration can modulate intracellular calcium concentrations in a dose dependent manner. Intracellular calcium released from the endoplasmic reticulum plays a determinant role in several cellular processes. In this study, we aim to assess the effect of dantrolene, a ryanodine receptor antagonist, on three different ethanol-elicited behaviors: locomotor activity, loss of righting reflex and ethanol intake. Mice were challenged with an injection of dantrolene (0-5 mg/kg, i.p.) 30 min before ethanol (0-4 g/kg, i.p.) administration. Animals were immediately placed in an open field cylinder to monitor distance travelled horizontally or in a V-shaped trough to measure righting reflex recovery time. For ethanol intake, dantrolene (0-5mg/kg, i.p.) was administered 30 min before ethanol (20%, v/v) exposure, following a drinking in the dark paradigm. Our results showed that dantrolene selectively reduces ethanol-induced stimulation, loss of righting reflex, and ethanol intake in a dose dependent manner. Together, these data suggest that intracellular calcium released from the endoplasmic reticulum may play a critical role in behavioral effects caused by ethanol, and point to a calcium-dependent pathway as a possible cellular mechanism of action for ethanol.

Behavioral effects of non-viral mediated RNA interference of synapsin II in the medial prefrontal cortex of the rat

Synapsin II is a synaptic vesicle-associated phosphoprotein that has been implicated in the pathophysiology of schizophrenia. Researchers have demonstrated reductions in synapsin II mRNA and protein in post-mortem prefrontal cortex and hippocampus samples from patients with schizophrenia. Synapsin II protein expression has been shown to be regulated by dopamine D(1) and D(2) receptor activation. Furthermore, behavioral testing of the synapsin II knockout mouse has revealed a schizophrenic-like behavioral phenotype in this mutant strain, suggesting a relationship between dysregulated and/or reduced synapsin II and schizophrenia. However, it remains unknown the specific regions of the brain of which perturbations in synapsin II play a role in the pathophysiology of this disease. The aim of this project was to evaluate animals with a selective knock-down of synapsin II in the medial prefrontal cortex through the use of siRNA technology. Two weeks after continuous infusion of synapsin II siRNAs, animals were examined for the presence of a schizophrenic-like behavioral phenotype. Our results reveal that rats with selective reductions in medial prefrontal cortical synapsin II demonstrate deficits in sensorimotor gating (prepulse inhibition), hyperlocomotion, and reduced social behavior. These results implicate a role for decreased medial prefrontal cortical synapsin II levels in the pathophysiology of schizophrenia and the mechanisms of aberrant prefrontal cortical circuitry, and suggest that increasing synapsin II levels in the medial prefrontal cortex may potentially serve as a novel therapeutic target for this devastating disorder.

Intracellular calcium chelation with BAPTA-AM modulates ethanol-induced behavioral effects in mice

Calcium (Ca(2+)) has been characterized as one of the most ubiquitous, universal and versatile intracellular signaling molecules responsible for controlling numerous cellular processes. Ethanol-induced effects on Ca(2+) distribution and flux have been widely studied in vitro, showing that acute ethanol administration can modulate intracellular Ca(2+) concentrations in a dose dependent manner. In vivo, the relationship between Ca(2+) manipulation and the corresponding ethanol-induced behavioral effects have focused on Ca(2+) flux through voltage-gated Ca(2+) channels. The present study investigated the role of inward Ca(2+) currents in ethanol-induced psychomotor effects (stimulation and sedation) and ethanol intake. We studied the effects of the fast Ca(2+) chelator, BAPTA-AM, on ethanol-induced locomotor activity and the sedative effects of ethanol. Swiss (RjOrl) mice were pretreated with BAPTA-AM (0-10 mg/kg) 30 min before an ethanol (0-4 g/kg) challenge. Our results revealed that pretreatment with BAPTA-AM prevented locomotor stimulation produced by ethanol without altering basal locomotion. In contrast, BAPTA-AM reversed ethanol-induced hypnotic effects. In a second set of experiments, we investigated the effects of intracellular Ca(2+) chelation on ethanol intake. Following a drinking-in-the-dark methodology, male C57BL/6J mice were offered 20% v/v ethanol, tap water, or 0.1% sweetened water. The results of these experiments revealed that BAPTA-AM pretreatment (0-5 mg/kg) reduced ethanol consumption in a dose-dependent manner while leaving water and sweetened water intake unaffected. Our findings support the role of inward Ca(2+) currents in mediating different behavioral responses induced by ethanol. Our results are discussed together with data indicating that ethanol appears to be more sensitive to intracellular Ca(2+) manipulations than other psychoactive drugs.

Novel therapeutic strategies for alcohol and drug addiction: focus on GABA, ion channels and transcranial magnetic stimulation

Drug addiction represents a major social problem where addicts and alcoholics continue to seek and take drugs despite adverse social, personal, emotional, and legal consequences. A number of pharmacological compounds have been tested in human addicts with the goal of reducing the level or frequency of intake, but these pharmacotherapies have often been of only moderate efficacy or act in a sub-population of humans. Thus, there is a tremendous need for new therapeutic interventions to treat addiction. Here, we review recent interesting studies focusing on gamma-aminobutyric acid receptors, voltage-gated ion channels, and transcranial magnetic stimulation. Some of these treatments show considerable promise to reduce addictive behaviors, or the early clinical studies or pre-clinical rationale suggest that a promising avenue could be developed. Thus, it is likely that within a decade or so, we could have important new and effective treatments to achieve the goal of reducing the burden of human addiction and alcoholism.

Ghrelin receptor antagonism attenuates nicotine-induced locomotor stimulation, accumbal dopamine release and conditioned place preference in mice

BACKGROUND

The orexigenic peptide ghrelin activates the reward systems, specifically the cholinergic-dopaminergic reward link, suggesting that ghrelin may increase the incentive salience of motivated behaviours such as food seeking. Moreover, central ghrelin signalling, involving the growth hormone secretagogue receptor 1A (GHS-R1A), is required for the rewarding properties, as measured by locomotor stimulation, accumbal dopamine release and conditioned place preference, of alcohol, cocaine as well as amphetamine. As the target circuits for other drugs of abuse, including nicotine, in the brain includes this reward link, we sought to determine whether the central ghrelin signalling system is involved in nicotine's activation of this system.

METHODS

This was investigated by studying the effects of peripheral administration of a GHS-R1A antagonist (JMV2959) on the nicotine-induced locomotor simulation, accumbal dopamine release and conditioned place preference.

RESULTS

In the present study we found that the ability of nicotine to increase the locomotor activity, accumbal dopamine release and to condition place preference were reduced in mice treated with a GHS-R1A antagonist.

CONCLUSION

Thus GHS-R1A appears to be required not only for alcohol, cocaine and amphetamine-induced reward, but also for reward induced by nicotine. Our data suggest that the central ghrelin signalling system may constitute a novel potential target for treatment of drug dependence.

The alcohol-induced locomotor stimulation and accumbal dopamine release is suppressed in ghrelin knockout mice

Ghrelin, the first endogenous ligand for the type 1A growth hormone secretagogue receptor (GHS-R1A), plays a role in energy balance, feeding behavior, and reward. Previously, we showed that pharmacologic and genetic suppression of the GHS-R1A attenuates the alcohol-induced stimulation, accumbal dopamine release, and conditioned place preference as well as alcohol consumption in mice, implying that the GHS-R1A is required for alcohol reward. The present study further elucidates the role of ghrelin for alcohol-induced dopamine release in nucleus accumbens and locomotor stimulation by means of ghrelin knockout mice. We found that the ability of alcohol to increase accumbal dopamine release in wild-type mice is not observed in ghrelin knockout mice. Furthermore, alcohol induced a locomotor stimulation in the wild-type mice and ghrelin knockout mice; however, the locomotor stimulation in homozygote mice was significantly lower than in the wild-type mice. The present series of experiments suggest that endogenous ghrelin may be required for the ability of alcohol to activate the mesolimbic dopamine system.

Expression of the gene encoding the ghrelin receptor in rats selected for differential alcohol preference

The mechanisms involved in alcohol use disorder, a chronic relapsing brain disorder, are complex and involve various signalling systems in the brain. Recently, the orexigenic peptide ghrelin was shown to be required for alcohol-induced reward, an effect mediated via ghrelin receptors, GHS-R1A, at the level of the cholinergic-dopaminergic reward link. Moreover, ghrelin increases and GHR-R1A antagonists reduce moderate alcohol consumption in mice, and a single nucleotide polymorphism in the GHS-R1A gene has been associated with high alcohol consumption in humans. Therefore, GHS-R1A gene expression and alcohol intake were investigated in high, AA (Alko, Alcohol), versus low, ANA (Alko, Non-Alcohol), alcohol consuming rats as well as in Wistar rats. In the AA and ANA rats plasma ghrelin levels were also measured. GHS-R1A gene expression was increased in AA compared to ANA rats in nucleus accumbens, ventral tegmental area, amygdala, prefrontal cortex and hippocampus. A similar trend was observed in the ventral tegmental area of Wistar rats consuming high amounts of alcohol. Furthermore, the AA rats had significantly smaller reduction of plasma ghrelin levels over time, after several weeks of alcohol exposure, than had the ANA rats. The present study provides further evidence for that the ghrelin signalling system, in particular at the level of the mesocortocolimbic dopamine system, is involved in alcohol consumption, and thus possibly contributes to alcohol use disorder. Therefore the GHS-R1A may constitute a novel candidate for development of new treatment strategies for alcohol dependence.

Glutamatergic regulation of ghrelin-induced activation of the mesolimbic dopamine system

Recently, we demonstrated that the central ghrelin signalling system, involving the ghrelin receptor (GHS-R1A), is important for alcohol reinforcement. Ghrelin targets a key mesolimbic circuit involved in natural as well as drug-induced reinforcement, that includes a dopamine projection from the ventral tegmental area (VTA) to the nucleus accumbens. The aim of the present study was to determine whether it is possible to suppress ghrelin's effects on this mesolimbic dopaminergic pathway can be suppressed, by interrupting afferent inputs to the VTA dopaminergic cells, as shown previously for cholinergic afferents. Thus, the effects of pharmacological suppression of glutamatergic, orexin A and opioid neurotransmitter systems on ghrelin-induced activation of the mesolimbic dopamine system were investigated. We found in the present study that ghrelin-induced locomotor stimulation was attenuated by VTA administration of the N-methyl-D-aspartic acid receptor antagonist (AP5) but not by VTA administration of an orexin A receptor antagonist (SB334867) or by peripheral administration of an opioid receptor antagonist (naltrexone). Intra-VTA administration of AP5 also suppressed the ghrelin-induced dopamine release in the nucleus accumbens. Finally the effects of peripheral ghrelin on locomotor stimulation and accumbal dopamine release were blocked by intra-VTA administration of a GHS-R1A antagonist (BIM28163), indicating that GHS-R1A signalling within the VTA is required for the ghrelin-induced activation of the mesolimbic dopamine system. Given the clinical knowledge that hyperghrelinemia is associated with addictive behaviours (such as compulsive overeating and alcohol use disorder) our finding highlights a potential therapeutic strategy involving glutamatergic control of ghrelin action at the level of the mesolimbic dopamine system.

L-type calcium channels and psychiatric disorders: A brief review

Emerging evidence from genome-wide association studies (GWAS) support the association of polymorphisms in the alpha 1C subunit of the L-type voltage-gated calcium channel gene (CACNA1C) with bipolar disorder. These studies extend a rich prior literature implicating dysfunction of L-type calcium channels (LTCCs) in the pathophysiology of neuropsychiatric disorders. Moreover, calcium channel blockers reduce Ca(2+) flux by binding to the α1 subunit of the LTCC and are used extensively for treating hypertension, preventing angina, cardiac arrhythmias and stroke. Calcium channel blockers have also been studied clinically in psychiatric conditions such as mood disorders and substance abuse/dependence, yielding conflicting results. In this review, we begin with a summary of LTCC pharmacology. For each category of disorder, this article then provides a review of animal and human data. In particular, we extensively focus on animal models of depression and clinical trials in mood disorders and substance abuse/dependence. Through examining rationale and study design of published clinical trials, we provide some of the possible reasons why we still do not have definitive evidence of efficacy of calcium-channel antagonists for mood disorders. Refinement of genetic results and target phenotypes, enrollment of adequate sample sizes in clinical trials and progress in physiologic and pharmacologic studies to synthesize tissue and isoform specific calcium channel antagonists, are all future challenges of research in this promising field. © 2010 Wiley-Liss, Inc.

Participation of L-type calcium channels in ethanol-induced behavioral stimulation and motor incoordination: effects of diltiazem and verapamil

Calcium flux through voltage gate calcium channels (VGCC) is involved in many neuronal processes such as membrane depolarization, gene expression, hormone secretion, and neurotransmitter release. Several studies have shown that either acute or chronic exposure to ethanol modifies calcium influx through high voltage activated channels. Of special relevance is the L-type VGCC. Pharmacological manipulation of L-type calcium channels affects ethanol intake, ethanol discrimination and manifestations of withdrawal syndrome. The present study investigates the role of L-type channels on the psychomotor effects (stimulation and sedation/ataxia) of ethanol by testing the effects of different L-type calcium channel blockers (CCB) on such behaviors. Mice were pretreated intraperitoneally with the CCB, diltiazem (0-40 mg/kg) or verapamil (0-30 mg/kg) 30 min before ethanol (0-3.5 g/kg). Locomotion was measured in an open field chamber for 20 min immediately after ethanol. The two CCB tested prevented locomotor stimulation, but not locomotor suppression produced by ethanol. Doses of the two CCB which reduced ethanol stimulation, did not alter spontaneous locomotion. The ataxic effects of ethanol (1.25 g/kg), measured with an accelerating rotarod task, were not affected by diltiazem (20mg/kg) or verapamil (15 mg/kg). In addition, our results indicated that ethanol is more sensitive to the antagonism of L-type calcium channels than other drugs with stimulant properties; doses of the two CCB that reduced ethanol stimulation did not reduce the psychomotor effects of amphetamine, caffeine or cocaine. In conclusion, these data provide further evidence of the important involvement of L-type calcium channels in the behavioral effects produced by ethanol.

Ghrelin receptor antagonism attenuates cocaine- and amphetamine-induced locomotor stimulation, accumbal dopamine release, and conditioned place preference

INTRODUCTION

Recently we demonstrated that genetic or pharmacological suppression of the central ghrelin signaling system, involving the growth hormone secretagogue receptor 1A (GHS-R1A), lead to a reduced reward profile from alcohol. As the target circuits for ghrelin in the brain include a mesolimbic reward pathway that is intimately associated with reward-seeking behaviour, we sought to determine whether the central ghrelin signaling system is required for reward from drugs of abuse other than alcohol, namely cocaine or amphetamine.

RESULTS

We found that amphetamine-as well as cocaine-induced locomotor stimulation and accumbal dopamine release were reduced in mice treated with a GHS-R1A antagonist. Moreover, the ability of these drugs to condition a place preference was also attenuated by the GHS-R1A antagonist.

CONCLUSIONS

Thus GHS-R1A appears to be required not only for alcohol-induced reward, but also for reward induced by psychostimulant drugs. Our data suggest that the central ghrelin signaling system constitutes a novel potential target for treatment of addictive behaviours such as drug dependence.

Requirement of central ghrelin signaling for alcohol reward

The stomach-derived hormone ghrelin interacts with key CNS circuits regulating energy balance and body weight. Here we provide evidence that the central ghrelin signaling system is required for alcohol reward. Central ghrelin administration (to brain ventricles or to tegmental areas involved in reward) increased alcohol intake in a 2-bottle (alcohol/water) free choice limited access paradigm in mice. By contrast, central or peripheral administration of ghrelin receptor (GHS-R1A) antagonists suppressed alcohol intake in this model. Alcohol-induced locomotor stimulation, accumbal dopamine release and conditioned place preference were abolished in models of suppressed central ghrelin signaling: GHS-R1A knockout mice and mice treated with 2 different GHS-R1A antagonists. Thus, central ghrelin signaling, via GHS-R1A, not only stimulates the reward system, but is also required for stimulation of that system by alcohol. Our data suggest that central ghrelin signaling constitutes a potential target for treatment of alcohol-related disorders.

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.

Voluntary ethanol intake increases extracellular acetylcholine levels in the ventral tegmental area in the rat

AIMS

Concurrent use of ethanol and nicotine (tobacco) is often seen in human beings. In previous animal experiments, we have demonstrated that nicotinic acetylcholine receptors, especially alpha-conotoxin MII and mecamylamine sensitive receptors located in the ventral tegmental area may be involved in the stimulatory, dopamine enhancing, and rewarding effects of ethanol in rodents. Ethanol may exert these effects via direct interaction with nicotinic acetylcholine receptors and/or indirectly via enhancement of extracellular acetylcholine levels in the ventral tegmental area. The present experiments investigated a possible indirect effect of ethanol in stimulating the mesoaccumbal dopamine system.

METHODS

Neurochemical effects of voluntary ethanol intake on extracellular ventral tegmental acetylcholine and accumbal dopamine levels were measured by means of in vivo microdialysis with a two-probe approach in freely moving rats.

RESULTS

Obtained data indicate that voluntary ethanol intake ( approximately 0.7 g/kg/h) leads to an increase of extracellular acetylcholine levels in the ventral tegmental area, and an almost time-locked increase of dopamine levels in the nucleus accumbens. A positive correlation between the ventral tegmental acetylcholine levels and ethanol intake as well as preference was also observed.

CONCLUSION

The present results suggest that voluntary ethanol intake enhances extracellular ventral tegmental acetylcholine that may interact with nicotinic acetylcholine receptors, possibly alpha-conotoxin MII sensitive receptors, localized in the ventral tegmental area that subsequently may stimulate dopamine overflow in the nucleus accumbens.

Is an alpha-conotoxin MII-sensitive mechanism involved in the neurochemical, stimulatory, and rewarding effects of ethanol?

Ethanol and nicotine are the most commonly abused drugs among human beings, and a large body of evidence, from both epidemiologic and preclinical studies, indicates that there is a positive correlation between intake of both drugs. Findings of studies from our research group have demonstrated that nicotinic acetylcholine receptors, especially those located in the ventral tegmental area, are important for the stimulatory, rewarding, and dopamine-enhancing effects of ethanol. Furthermore, results of recent work indicate that the alpha4beta2* and the alpha7* receptor subunits of the nicotinic acetylcholine receptors do not seem to be involved in the neurochemical and behavioral effects of ethanol in rodents. The aim of the current study was to investigate further the role of different nicotinic acetylcholine receptor subunits in the stimulatory, dopamine-enhancing, and rewarding effects of ethanol in rodents by using the peptide alpha-conotoxin MII (5 nmol; an antagonist of the alpha3beta2*, beta3*, and alpha6* subunits of the nicotinic acetylcholine receptor) administered locally into the ventral tegmental area. A significant reduction of ethanol-induced accumbal dopamine overflow, measured by means of in vivo microdialysis, and of locomotor stimulation was observed in mice. Furthermore, alpha-conotoxin MII was demonstrated to reduce voluntary ethanol intake significantly in both rats and mice. These results indicate that alpha-conotoxin MU-sensitive receptors may be important in mediating the stimulatory, dopamine-enhancing, and rewarding effects of ethanol, and that alpha-conotoxin MII-sensitive receptors may constitute targets for development of new adjuvant for treatment of ethanol dependence.

Neurochemical and behavioral studies on ethanol and nicotine interactions

The most commonly abused drugs, alcohol and nicotine, are likely also the most costly drugs in terms of health and societal costs. A large body of evidence from epidemiological studies indicate that smoking and alcohol-intake are positively correlated. The mesocorticolimbic dopamine system has been implicated in mediating some of the reinforcing effects of ethanol, however, the mechanism(s) of action remains to be elucidated; consideration as to ethanol's ability to interact with ligand-gated ion channels should be considered. Accumulating evidence from electrophysiological, pharmacological and neurochemical studies suggest that ethanol may interact with the nicotinic acetylcholine receptor (nAChR). Thus, it has been shown that the ethanol-induced stimulation of the mesolimbic dopamine system and of locomotor activity as well as ethanol intake and preference in rodents may involve central nicotinic acetylcholine receptors. Additionally, data has been presented that nAChRs located in the ventral tegmental area may be of particular importance for these effects of ethanol. Studies aimed at defining the nAChR subpopulation(s) involved in mediating ethanol-induced locomotor stimulation and accumbal dopamine overflow as well as ethanol-intake have revealed that alpha(3)beta(2) or alpha(6) (using alpha-Conotoxin MII) but not alpha(4)beta(2) (using dihydro-beta-erythroidine) or alpha(7) (using methyllycaconitine), could represent targets for developing new drugs in the treatment of alcoholism. These results do not allow any conclusion as to whether the involvement nAChRs in mediating the effects of ethanol is direct and/or indirect. With regard to an indirect effect, evidence has accumulated indicating that the cholinergic excitatory input to the dopaminergic neurons in the ventral tegmental area may be an important part of the neuronal circuits mediating natural as well as drug-rewarded behavior. The possibility may thus be considered that ethanol activates the cholinergic afferents causing a release of acetylcholine in the ventral tegemental area leading to a stimulation of nAChRs and thereby excite the mesocorticolimbic dopamine system.

Ethanol elevates accumbal dopamine levels via indirect activation of ventral tegmental nicotinic acetylcholine receptors

It was previously demonstrated that the central nicotinic acetylcholine receptor antagonist mecamylamine perfused in the ventral tegmental area (VTA) counteracts the elevation of extracellular dopamine levels in the nucleus accumbens after systemic ethanol, as measured by in vivo microdialysis. In the present study we investigated the effect of different concentrations of ethanol perfused locally in the VTA or in the nucleus accumbens on extracellular accumbal dopamine levels. Ethanol (10-1000 mM) perfused in the VTA did not influence dopamine output in the nucleus accumbens. However, ethanol (300 mM) perfused in the nucleus accumbens increased accumbal dopamine levels to approximately the same extent (30%) as observed after systemic ethanol, whereas ethanol (1000 mM) decreased the dopamine output by approximately 50%. Next, the hypothesis that endogenous acetylcholine is required for the increased accumbal dopamine levels after ethanol was challenged. It was shown that in animals pre-treated with vesamicol, a potent inhibitor of vesicular acetylcholine storage, ethanol (300 mM) in the nucleus accumbens failed to elevate extracellular accumbal dopamine levels. Similarly, in animals perfused with mecamylamine in the VTA, but not in the nucleus accumbens, ethanol in the nucleus accumbens (300 mM) failed to increase accumbal dopamine levels. However, whereas dihydro-beta-erythroidine (antagonist for the nicotinic receptor subtype alpha4beta2) perfused in the VTA prevented the increase in accumbal dopamine after systemic nicotine, the antagonist was unable to prevent the dopamine elevating effects of ethanol. Finally, to investigate whether mecamylamine exerts its antagonizing effect of ethanol induced accumbal dopamine levels through an interaction with the NMDA receptor MK-801, the effects of the prototypic NMDA receptor antagonist were examined and compared to those of mecamylamine. After perfusion in the VTA, MK-801 enhanced accumbal dopamine levels by itself but did not antagonize the enhancing effect of ethanol. The present set of experiments indicate that the mesolimbic dopamine activating effects of ethanol may be due to an indirect rather than direct activation of ventral tegmental nicotinic acetylcholine receptors of a subtype composition different from the alpha4beta2. Furthermore, it is argued that the primary site of action of ethanol in its accumbal dopamine elevating effect may be located to the nucleus accumbens or nearby regions.

Role of different nicotinic acetylcholine receptors in mediating behavioral and neurochemical effects of ethanol in mice

Ethanol and nicotine are the most abused drugs, and it is well known that co-abuse of ethanol and nicotine is frequent in human beings. We have previously obtained results indicating that the ethanol-induced stimulation of both the mesolimbic dopamine system and locomotor activity may involve activation of central nicotinic acetylcholine receptors (nAChRs), especially those located in the ventral tegmental area. Different subpopulations of nAChRs have been identified, and, in the present series of experiments, we have studied the effects of various nAChR antagonists on the stimulation of dopamine overflow in the nucleus accumbens and on locomotor activity induced by ethanol in male mice. Ethanol (2.0 g/kg, i.p.) enhanced dopamine overflow in the nucleus accumbens by approximately 40%, measured by means of in vivo microdialysis in awake, freely moving mice. Mecamylamine (negative allosteric modulator of nAChR; 2.0 mg/kg, i.p.) blocked the ethanol-induced stimulation of both locomotor activity and accumbal dopamine overflow. Methyllycaconitine citrate (alpha(7) antagonist; 2.0 mg/kg, i.p.) and dihydro-beta-erythroidine (competitive and selective alpha(4)beta(2) antagonist; 0.5 mg/kg, s.c.), in doses that had no marked effects per se, did not significantly reduce the behavioral and neurochemical stimulation caused by ethanol. The present results support the suggestion that the stimulatory effects of ethanol on locomotor activity and dopamine release do not involve the alpha(4)beta(2) or alpha(7) subunit compositions of the nAChR and that the effects of mecamylamine are mediated through a site not directly associated with the alpha(4)beta(2) or alpha(7) nAChR subunits.

Studies on a model of long term alcohol drinking

This study investigated the effects of a dihydropyridine calcium channel antagonist in a free drinking model previously reported to show increased and 'uncontrolled' drinking. The results showed, rather than the previously reported increase in consumption, a gradual decrease in alcohol intake over 9-18 months. When the alcohol was withdrawn from one group of rats after 55 weeks free choice, the animals showed no behavioural signs of physical withdrawal, but they did demonstrate the expected elevated ethanol intake on reintroduction to ethanol after 2 weeks abstinence. A second group of rats were given 62 weeks free choice access to ethanol in groups of four, then transferred to single housing and baseline drinking levels established. Intraperitoneal injections of nimodipine 5 mg/kg, 20 mg/kg or Tween vehicle were then given once daily. Nimodipine had no effect on ethanol intake of animals with continuous access to ethanol, or of those animals withdrawn from ethanol and then reintroduced after 2 weeks of abstinence. However the unexpectedly low alcohol intake may have prevented any effects of nimodipine being seen.

Nicotinic mechanisms involved in the dopamine activating and reinforcing properties of ethanol

Ethanol shares with all major dependence producing drugs the ability to activate brain mesocorticolimbic dopamine neurons, an important part of the brain reward systems. This dopamine activation may be involved in mediating the positive reinforcing effects of ethanol. The mechanisms of action of ethanol in its activation of this dopamine system remain, however, to be elucidated. A selective pharmacological interference with these mechanisms may offer a possibility to reduce the reinforcing properties of ethanol without simultaneously interfering with the reinforcing properties of natural rewards. Ethanol has been shown to directly influence the function of various ligand-gated ion-channels. Several of these are located on or nearby mesocorticolimbic dopamine neurons. One such receptor is the nicotinic acetylcholine receptor (nAChR). The present article reviews a series of investigations aimed at investigating whether nAChRs are involved in the dopamine activating and reinforcing properties of ethanol. To this end acute and chronic behavioral and neurochemical experiments were performed in mice and rats. The results obtained indicate that central nAChRs in the ventral tegmental area are involved in mediating the mesolimbic dopamine activating and reinforcing effects of ethanol. Furthermore, the ethanol-induced activation of these receptors is probably indirect, subsequent to a primary interference of ethanol in the nucleus accumbens. Moreover, subchronic nicotine treatment enhances the reinforcing and dopamine activating properties of ethanol. This long-term effect may, however, derive from autonomic adaptations in response to intermittent blockade of peripheral nAChRs (rather than from intermittent stimulation of central receptors), and appears to be associated with development of a disinhibitory behavior that could involve also other neurotransmitters, e.g. serotonin. Taken together, these findings could provide a neurobiological explanation to the often observed co-abuse of nicotine and ethanol in man. Furthermore, since the behavioral models applied previously have predicted therapeutic drug effects in the clinic, the results suggest that selective blockade of the ventral tegmental nAChRs that are involved in the above effects may provide a new pharmacological alternative in the treatment of alcoholism.

Neonatal nicotine administration influences ethanol-induced behaviors

Neonatal mice were administered nicotine (66 microg (-)-nicotine base/kg body weight (bw) s.c. twice daily at 0800 and 1700 h on postnatal days 10 and 14) and control mice received saline (10 ml 0. 9% NaCl/kg bw s.c.) on the same occasions. Behavioral testing was initiated 3 months after birth. In Experiment 1, neonatal nicotine administration did not affect spontaneous motor activity but altered the peak dose stimulatory effect of ethanol upon locomotion and rearing activity from 3.0 mg/kg, in the control mice, to 1.5 mg/kg. Administration of the nicotine antagonist, mecamylamine (MEC, 2.0 mg/kg), had no effect upon the peak dose stimulatory effect (i.e., 1. 5 mg/kg) evidenced in the nicotine-treated mice, but attenuated the stimulatory effect of the 3.0 mg/kg dose of ethanol in the control mice. In Experiment 2, the effects of neonatal nicotine administration upon ethanol intake and preference were assessed. In the single fluid access (one-bottle) test, nicotine-treated mice consumed both more ethanol (2%, 4%, or 6% concentrations) and more tap water than control mice. In the two-bottle ethanol preference test, nicotine-treated mice consumed more ethanol and tap water. Further analysis of the high-preferring (HP) ethanol mice indicated higher ethanol intake and preference in the nicotine-treated mice but no differences in tap water or total fluid intake. The present findings are considered together with prevailing notions of nicotine receptor alterations and possible cross-sensitization effects modulating substance abuse.

Discriminative and affective stimulus effects of dihydropyridine calcium channel modulators: relationship to antialcohol effects

Voltage-operated calcium channels (VOCCs) have been implicated in alcoholism. Thus, dihydropyridine (DHP) VOCC antagonists, such as nimodipine, reduce ethanol (EtOH) intake and preference in a variety of animal models of alcoholism. Paradoxically, the DHP VOCC agonist BAY k 8644 also demonstrates antialcohol effects in such models. The antialcohol effects of BAY k 8644 are stereoselective [the "agonistic" (-)-enantiomer being more potent than the "antagonistic" (+)-enantiomer], and are not blocked by pretreatment with nimodipine. The present review summarizes studies on the effects of DHPs in drug discrimination (DD), conditioned taste aversion (CTA), and conditioned place preference (CPP) paradigms, and discusses the possibility that the apparent antialcohol effect of these compounds is related to their discriminative and/or affective stimulus effects. In rats trained to discriminate nimodipine from vehicle, (-)-BAY k 8644 completely generalizes to the nimodipine cue; whereas, in rats trained to discriminate (-)-BAY k 8644, nimodipine completely generalizes to, and is unable to block, the (-)-BAY k 8644 cue. The same stereoselectivity is obtained for BAY k 8644 in DD paradigms and models of alcoholism. The apparent similarity of these profiles of activity suggests that a common neurobiological mechanism underlies the discriminative stimulus and antialcohol effects of DHPs. It appears unlikely, however, that the antialcohol effects of DHPs are based on substitution for, or blockade of, the EtOH cue, as these compounds were not found to generalize to, or block, the EtOH cue. Comparison of the effects of DHPs in CTA and CPP paradigms suggests that the affective stimulus effects of these compounds are dissimilar, and that the mechanism underlying the latter effects is probably not related to the mechanism underlying the antialcohol effects of DHP VOCC modulators.

Effects of nimodipine on extracellular dopamine levels in the rat nucleus accumbens in ethanol withdrawal

Withdrawal from chronic ethanol intoxication is associated with a reduction of dopamine neurotransmission. However, the mechanisms of dopamine depletion, a putative neurochemical correlate of the dysphoric symptomatology, are not yet understood. To assess the role of L-type calcium channels in the inhibition of the dopaminergic system in the withdrawal state, the effects of the dihydropyridine calcium channel antagonist nimodipine on the extracellular levels of dopamine were studied in the nucleus accumbens shell of awake rats 10 h after withdrawal from chronic ethanol intoxication. In control, chronic sucrose-withdrawn rats, nimodipine did not change extracellular dopamine levels. However, in ethanol-withdrawn rats nimodipine (5 or 10 mg/kg s.c.) increased extracellular dopamine to 136 +/- 16 and 305 +/- 19% of pre-administration values, respectively, the latter dose elevating levels above those of controls. The elevations of extracellular DA by nimodipine (10 mg/kg) were associated with a significant reduction (-17%) of the overall behavioural score of the withdrawal symptomatology, as evaluated for 11 behavioural items. Significant reductions of the score for convulsions (-47%) and, to a lesser extent, for catatonia (-30%) and tremors (-15%) contributed to the overall effect. It is suggested that overactivity of L-type calcium channels is involved in the mechanisms of dopamine depletion as well as in certain behavioural/neurological signs associated with ethanol withdrawal. By restoring depleted dopamine levels, dihydropyridines might ameliorate the dysphoric symptoms of ethanol abstinence.

Effects of calcium channel blockade on intravenous self-administration of ethanol in rats

In the present study the involvement of voltage-operated calcium channels (VOCCs) in the acquisition and maintenance of operant i.v. ethanol (EtOH) self-administration was investigated in rats. Rats readily learned to self-administer EtOH (unit dose range: 0.5-4% v/v) within five daily 2-h sessions, when infusions were made contingent upon nose-poking in a hole containing infrared sensors. Response rate was related to the EtOH concentration in an inverted U-shaped manner, the maximal rate and intake being observed at a unit dose of 1% v/v (0.27 mg EtOH/infusion). Self-administration of EtOH appeared to be behaviorally specific, as responding in the reinforced hole did not coincide with increased responding in a nonreinforced hole. Daily treatment with the dihydropyridine VOCC blocker nimodipine (2.5-20 mg/kg, i.p., t-15 min) dose-dependently attenuated acquisition of EtOH self-administration; the 5 mg/kg dose resulting in a partial, and the 10 and 20 mg/kg doses in a complete prevention of i.v. self-administration behavior. The effects of nimodipine (2.5-5.0 mg/kg) were considered to be relatively specific, as an inhibition of the reinforced responding could be demonstrated in the absence of a significant effect on nonreinforced responding. When tested in rats showing stable self-administration behavior (unit dose: 1% v/v EtOH), nimodipine showed biphasic dose-response effects; with 2.5 and 5 mg/kg resulting in a mild increase, and 10 and 20 mg/kg resulting in a decrease of self-administration behavior, respectively. The present study suggests that blockade of VOCCs attenuates the reinforcing stimulus effects of EtOH; and, as such, the data may offer an explanation for the previously reported EtOH intake-reducing effects of dihydropyridine calcium channel ligands obtained in two-bottle choice paradigms. Dihydropyridine derivatives, such as nimodipine, may therefore offer an interesting approach to the pharmacotherapy of alcoholism.

Differing acute interactions of ethanol with two structurally related dihydropyridines, nitrendipine and felodipine

Previous work showed that while several dihydropyridine calcium channel antagonists have protective effects against the ethanol withdrawal syndrome, felodipine differed in lacking this action. Dihydropyridine calcium channel antagonists have also been shown to potentiate the acute behavioral actions of ethanol. The present study compares the effects of felodipine on the acute effects of ethanol, with those of nitrendipine, a dihydropyridine previously shown to be effective against the ethanol withdrawal syndrome. Comparison was made at doses of the compounds that have previously been shown to produce similar displacement of dihydropyridine binding in central nervous system (CNS) tissue. Felodipine had a small potentiating effect on the general anesthetic effects of ethanol, but was considerably less effective in this respect than nitrendipine. Some potentiation of the ataxic effect of ethanol was seen after concurrent administration of felodipine, but this was less than that seen after nitrendipine. In the locomotor studies, both felodipine and nitrendipine significantly decreased the locomotor stimulation produced by ethanol; the effects of the two compounds were similar, but dose-dependency was not seen at the doses tested. Chronic administration of felodipine for 2 weeks did not produce tolerance to the sedative effect of felodipine or cross-tolerance to nitrendipine. After chronic administration of the felodipine, administration of an acute dose of ethanol resulted in an increase in locomotor activity, but this was not seen after chronic administration of nitrendipine or vehicle. The results, therefore, suggest that felodipine was considerably less effective in potentiating the acute effects of ethanol than nitrendipine at doses that were equi-effective in displacing central dihydropyridine binding. The interactions of these two calcium channel antagonists with ethanol, therefore, did not parallel their effects on central dihydropyridine binding.