Alcohol Self-Administration: Role of Mesolimbic Dopamine

Operant alcohol self-administration targets GluA2-containing AMPAR expression and synaptic activity in the nucleus accumbens in a manner that drives the reinforcing properties of the drug

Rationale

The positive reinforcing effects of alcohol (ethanol) drive its repetitive use and contribute to alcohol use disorder (AUD). Ethanol alters the expression of glutamate AMPA receptor (AMPAR) subunits in reward-related brain regions, but the extent to which this molecular effect regulates ethanol's reinforcing properties is unclear.

Objective

This study investigates whether ethanol self-administration changes AMPAR subunit expression and synaptic activity in the nucleus accumbens core (AcbC) to regulate ethanol's reinforcing effects in male C57BL/6J mice.

Results

Sucrose-sweetened ethanol self-administration (0.81±0.11 g/kg/day) increased AMPAR GluA2 protein expression in the AcbC, without effect on GluA1, compared to sucrose-only controls. Infusion of myristoylated Pep2m in the AcbC, which blocks GluA2 binding to N-ethylmaleimide-sensitive fusion protein (NSF) and reduces GluA2-containing AMPAR activity, reduced ethanol-reinforced responding without affecting sucrose-only self-administration or motor activity. Antagonizing GluA2-lacking AMPARs, through AcbC infusion of NASPM, had no effect on ethanol self-administration. AcbC neurons receiving projections from the basolateral amygdala (BLA) showed increased sEPSC frequency and GluA2-like decay kinetics in ethanol self-administering mice as compared to sucrose. Optogenetic activation of these neurons revealed an ethanol-enhanced AMPA/NMDA ratio and reduced paired-pulse ratio, indicating elevated AMPAR activity and glutamate release specifically at AcbC terminals of BLA projecting neurons.

Conclusions

Ethanol use upregulates GluA2 protein expression in the AcbC and AMPAR synaptic activity in AcbC neurons receiving BLA projections. GluA2-containing AMPAR activity in the AcbC regulates the positive reinforcing effects of ethanol through an NSF-dependent mechanism. This highlights a potential target for therapeutic interventions in AUD.

Ethanol antagonizes P2X4 receptors in ventral tegmental area neurons

P2X4 receptors are found throughout the central nervous system, and studies have shown that these purinergic receptors are important regulators of alcohol intake. The ventral tegmental area (VTA) is an important region for the rewarding and reinforcing properties of alcohol, but the role of P2X4 receptors in this region is unknown. Using both immunohistochemical and electrophysiological methods, we examined the interaction between P2X4 receptors and alcohol on VTA neurons. Incubation of brain slices containing the VTA for 2 h with siRNA targeting P2X4 receptors resulted in about a 25% reduction in P2X4 immunoreactivity in tyrosine hydroxylase positive VTA neurons. In electrophysiological experiments, ATP (0.5-3 mM) produced a reduction in the spontaneous firing rate, and ethanol significantly reduced this inhibition. Exposure to siP2X4 for 2 h via the recording micropipette resulted in a suppression of the response of VTA neurons to ATP, but no significant reduction in the ethanol inhibition of the ATP response was observed after this P2X4 downregulation. These results support the idea that VTA neurons are inhibited by ATP, ethanol antagonizes this inhibition, and the ethanol-sensitive component of ATP inhibition is mediated by P2X4 receptors. This interaction of ethanol with P2X4 receptors may be an important regulator of the rewarding effects of ethanol, making P2X4 receptors an intriguing target for the development of agents to treat alcohol use disorders.

Inhibition of AMPA receptors (AMPARs) containing transmembrane AMPAR regulatory protein γ-8 with JNJ-55511118 shows preclinical efficacy in reducing chronic repetitive alcohol self-administration

BACKGROUND

A prominent therapeutic indication for alcohol use disorder (AUD) is reduction in chronic repetitive alcohol use. Glutamate α-Amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors (AMPARs) regulate chronic alcohol self-administration in preclinical models. Recent evidence indicates that the expression and function of AMPARs require the transmembrane AMPAR regulatory protein γ-8 (TARP γ-8). This study evaluated the preclinical efficacy of JNJ-55511118, a novel, selective, high-affinity inhibitor of TARP γ-8-bound AMPARs, in reducing chronic operant alcohol self-administration.

METHODS

Separate groups of male and female C57BL/6J mice (n = 8/sex/group) were trained to lever press for sweetened alcohol (9% v/v + sucrose 2% w/v) or sucrose only (2% w/v) in operant conditioning chambers using an FR-4 schedule of reinforcement. After a 40-day baseline, JNJ-55511118 (0, 1, and 10 mg/kg, p.o.) was administered in randomized order 1 h before self-administration sessions. Parameters of operant behavior including response rate, total reinforcers, and head entries in the drinking troughs were computer recorded.

RESULTS

During baseline, responding to alcohol, but not sucrose, was greater in female than male mice. In male mice, both doses of JNJ-55511118 decreased multiple parameters of alcohol self-administration but did not reduce behavior-matched sucrose-only self-administration. JNJ-55511118 had no effect on sweetened alcohol or sucrose self-administration in female mice. Subsequent tests of motor function showed that the lowest effective dose of JNJ-55511118 (1 mg/kg) had no effect on open-field activity in male mice.

CONCLUSIONS

This study shows for the first time that TARP γ-8-bound AMPARs regulate a behavioral pathology associated with addiction. The preclinical efficacy of JNJ-55511118 in reducing alcohol self-administration in male mice suggests that inhibition of TARP γ-8-bound AMPARs is a novel and highly significant neural target for developing medications to treat AUD and other forms of addiction.

Intranasal Oxytocin Does Not Modulate Responses to Alcohol in Social Drinkers

BACKGROUND

Preclinical and clinical evidence suggest that the neuropeptide oxytocin may be of value in treating alcohol use disorder, by either reducing the rewarding effects of alcohol or reducing negative affect induced by alcohol withdrawal. However, the effect of a single dose of oxytocin on subjective and psychomotor responses to alcohol in social drinkers is not known.

METHODS

This study examined the effect of intranasal oxytocin on subjective, behavioral, and physiological responses to a moderate dose of alcohol (0.8 g/kg) in young adult social drinkers. Participants (N = 35) completed 2 study sessions at which they consumed beverages containing alcohol (ALC; N = 20) or placebo (NoALC; N = 15) in combination with intranasal oxytocin (40 IU with a 20 IU booster) or placebo. They received oxytocin at one session and placebo at the other session (order counterbalanced) 20 minutes before consuming beverages. Subjective mood and drug effects ratings, heart rate and blood pressure, and 4 behavioral tasks (flanker task, digit span, go/no-go, and pursuit rotor) were the primary outcome measures.

RESULTS

ALC produced its expected subjective and behavioral effects; including feeling intoxicated and impaired performance on the digit span and go/no-go tasks. Oxytocin alone had no significant subjective or physiological effects, and it did not affect responses to alcohol on any measure.

CONCLUSIONS

We can conclude that, under these conditions, a single dose of intranasal oxytocin does not alter the effects of acute alcohol in healthy young adult social drinkers. Further research is needed to determine whether oxytocin alters responses to alcohol under different conditions, and to determine its potential as an aid in treatment for substance use disorders.

Enhanced Striatal Dopamine Release to Expectation of Alcohol: A Potential Risk Factor for Alcohol Use Disorder

BACKGROUND

We used positron emission tomography imaging with [¹¹C]raclopride to examine the effects of consumption of alcohol or placebo beverage by participants with alcohol use disorder (AUD) compared with healthy participants with and without family history of AUD. We sought to assess dopamine release following alcohol exposure in relation to AUD risk.

METHODS

Three groups were enrolled: participants with AUD (n = 15) and healthy participants with family history negative (n = 34) or positive (n = 16) for AUD. Participants consumed a placebo (n = 65) or alcohol (n = 63) beverage in counterbalanced order before positron emission tomography scanning (128 scans). Binding potential (BP_ND) in the two drink conditions and the percent change in BP_ND between conditions were evaluated across striatal subregions. Subjective effects of beverage consumption were rated. Effects of group, drink order, and sex were evaluated.

RESULTS

Alcohol resulted in greater dopamine release than did placebo in the ventral striatum (p < .001). There were no main effects of group, drink order, or sex on ventral striatum BP_ND or percent change in BP_ND. However, there was a drink order-by-group interaction (p = .02) whereby family history-positive participants who received placebo first had both lower placebo BP_ND and less difference between placebo and alcohol BP_ND than all other groups, consistent with expectation of alcohol powerfully evoking dopamine release in this group. Subjective responses showed the same order-by-group interaction.

CONCLUSIONS

Hyper-responsivity of the dopaminergic system in family history-positive participants to expectation of alcohol may contribute to the expression of familial risk for AUD.

Alcohol and the Brain: Neuronal Molecular Targets, Synapses, and Circuits

Ethanol is one of the most commonly abused drugs. Although environmental and genetic factors contribute to the etiology of alcohol use disorders, it is ethanol's actions in the brain that explain (1) acute ethanol-related behavioral changes, such as stimulant followed by depressant effects, and (2) chronic changes in behavior, including escalated use, tolerance, compulsive seeking, and dependence. Our knowledge of ethanol use and abuse thus relies on understanding its effects on the brain. Scientists have employed both bottom-up and top-down approaches, building from molecular targets to behavioral analyses and vice versa, respectively. This review highlights current progress in the field, focusing on recent and emerging molecular, cellular, and circuit effects of the drug that impact ethanol-related behaviors. The focus of the field is now on pinpointing which molecular effects in specific neurons within a brain region contribute to behavioral changes across the course of acute and chronic ethanol exposure.

Cue-induced reinstatement of alcohol-seeking behavior is associated with increased CaMKII T286 phosphorylation in the reward pathway of mice

Cue-induced reinstatement of alcohol-seeking is a hallmark behavioral pathology of addiction. Evidence suggests that reinstatement (e.g., relapse), may be regulated by cell signaling systems that underlie neuroplasticity. A variety of plasticity events require activation of calcium calmodulin-dependent protein kinase II (CaMKII) in components of the reward pathway, such as the nucleus accumbens and amygdala. We sought to determine if cue-induced reinstatement of alcohol-seeking behavior is associated with changes in the activation state (e.g., phosphorylation) of CaMKII-T286. Male C57BL/6J mice (n=14) were trained to lever press on a fixed-ratio-4 schedule of sweetened alcohol (2% sucrose+9% EtOH) reinforcement. After 14-d of extinction (no cues or reinforcers), mice underwent a response-contingent reinstatement (n=7) vs. an additional day of extinction (n=7). Brains were removed immediately after the test and processed for evaluation of pCaMKII-T286 immunoreactivity (IR). Number of pCaMKII-T286 positive cells/mm² was quantified from coronal brain sections using Bioquant Image Analysis software. Mice emitted significantly more responses on the alcohol vs. the inactive lever throughout the baseline phase with average alcohol intake of 1.1±0.03g/kg/1-h. During extinction, responses on the alcohol lever decreased to inactive lever levels by day 7. Significant cue-induced reinstatement of alcohol-seeking was observed during a single test with no effects on the inactive lever. Reinstatement was associated with increased pCaMKII-T286 IR specifically in amygdala (LA and BLA), nucleus accumbens (AcbSh), lateral septum, mediodorsal thalamus, and piriform cortex as compared to extinction control. Brain regions showing no change included the dorsal striatum, medial septum, cingulate cortex, habenula, paraventricular thalamus, and ventral hypothalamus. These results show response contingent cue-induced reinstatement of alcohol-seeking behavior is associated with selective increases in pCaMKII-T286 in specific reward- and memory-related brain regions of male C57BL/6J mice. Primary molecular mechanisms of associative learning and memory may regulate relapse in alcohol addiction.

Varenicline Reduces Alcohol Intake During Repeated Cycles of Alcohol Reaccess Following Deprivation in Alcohol-Preferring (P) Rats

BACKGROUND

Most alcoholics experience periods of voluntary alcohol abstinence or imposed alcohol deprivation followed by a return to alcohol drinking. This study examined whether varenicline (VAR) reduces alcohol intake during a return to drinking after periods of alcohol deprivation in rats selectively bred for high alcohol drinking (the alcohol preferring or "P" rats).

METHODS

Alcohol-experienced P rats were given 24-hour access to food and water and scheduled access to alcohol (15% and 30% v/v) for 2 h/d. After 4 weeks, rats were deprived of alcohol for 2 weeks, followed by reaccess to alcohol for 2 weeks, and this pattern was repeated for a total of 3 cycles. Rats were fed either vehicle (VEH) or VAR, in doses of 0.5, 1.0, or 2.0 mg/kg BW, at 1 hour prior to onset of the daily alcohol reaccess period for the first 5 days of each of the 3 alcohol reaccess cycles.

RESULTS

Low-dose VAR (0.5 mg/kg BW) reduced alcohol intake during the 5 days of drug treatment in alcohol reaccess cycles 1 and 2. Higher doses of VAR (1.0 mg/kg BW and 2.0 mg/kg BW) reduced alcohol intake during the 5 days of treatment in all 3 alcohol reaccess cycles. The decrease in alcohol intake disappeared with termination of VAR treatment in all alcohol reaccess cycles.

CONCLUSIONS

The results demonstrate that VAR decreases alcohol intake during multiple cycles of alcohol reaccess following alcohol deprivation in rats and suggests that it may prevent a return to heavy alcohol drinking during a lapse from alcohol abstinence in humans with alcohol use disorder.

Animated images in the analysis of zebrafish behavior

This invited review is based upon a recent oral paper I presented at the Virtual Reality Symposium of the 34th International Ethological Conference (2015, Cairns, Australia), and as such it describes studies conducted mainly in my own laboratory. It reviews how we utilized visual stimuli for inducing behavioral responses in the zebrafish with a focus on shoaling, group forming behavior. The zebrafish is gaining increasing popularity in neuroscience. With this interest, its behavior is also more frequently studied. One of the many advantages of the zebrafish over traditional laboratory rodents is that this species is diurnal, and it relies heavily upon its visual system. Thus, similarly to our own species, zebrafish respond to visual stimuli in a robust and easily quantifiable manner. For the past decade, we have been exploring how to use such visual stimuli, and have developed numerous paradigms with which we can induce and quantify a variety of behavioral responses, including shoaling. This review summarizes some of these studies, and discusses questions including whether one should use live fish as stimulus, whether and how one could present animated (moving images) of fish, and how one could optimize a range of stimulus presentation parameters to elicit the most robust responses in zebrafish. Although the zebrafish is a relative newcomer in ethology and behavioral neuroscience, and although many of our findings only represent the first steps in this research, our results suggest that the behavioral analysis of the zebrafish will have an important place in biomedical research.

Combining Varenicline (Chantix) with Naltrexone Decreases Alcohol Drinking More Effectively Than Does Either Drug Alone in a Rodent Model of Alcoholism

BACKGROUND

This study examined whether varenicline (VAR), or naltrexone (NTX), alone or in combination, reduces alcohol drinking in alcohol-preferring (P) rats with a genetic predisposition toward high voluntary alcohol intake.

METHODS

Alcohol-experienced P rats that had been drinking alcohol (15% v/v) for 2 h/d for 4 weeks were fed either vehicle (VEH), VAR alone (0.5, 1.0, or 2.0 mg/kg body weight [BW]), NTX alone (10.0, 15.0, or 20.0 mg/kg BW), or VAR + NTX in 1 of 4 dose combinations (0.5 VAR + 10.0 NTX, 0.5 VAR + 15.0 NTX, 1.0 VAR + 10.0 NTX, or 1.0 VAR + 15.0 NTX) at 1 hour prior to alcohol access for 10 consecutive days, and the effects on alcohol intake were assessed.

RESULTS

When administered alone, VAR in doses of 0.5 or 1.0 mg/kg BW did not alter alcohol intake but a dose of 2.0 mg/kg BW decreased alcohol intake. This effect disappeared when drug treatment was terminated. NTX in doses of 10.0 and 15.0 mg/kg BW did not alter alcohol intake but a dose of 20.0 mg/kg BW decreased alcohol intake. Combining low doses of VAR and NTX into a single medication reduced alcohol intake as well as did high doses of each drug alone. Reduced alcohol intake occurred immediately after onset of treatment with the combined medication and continued throughout prolonged treatment.

CONCLUSIONS

Low doses of VAR and NTX, when combined in a single medication, reduce alcohol intake in a rodent model of alcoholism. This approach has the advantage of reducing potential side effects associated with each drug. Lowering the dose of NTX and VAR in a combined treatment approach that maintains efficacy while reducing the incidence of negative side effects may increase patient compliance and improve clinical outcomes for alcoholics and heavy drinkers who want to reduce their alcohol intake.

Critical needs in drug discovery for cessation of alcohol and nicotine polysubstance abuse

Polysubstance abuse of alcohol and nicotine has been overlooked in our understanding of the neurobiology of addiction and especially in the development of novel therapeutics for its treatment. Estimates show that as many as 92% of people with alcohol use disorders also smoke tobacco. The health risks associated with both excessive alcohol consumption and tobacco smoking create an urgent biomedical need for the discovery of effective cessation treatments, as opposed to current approaches that attempt to independently treat each abused agent. The lack of treatment approaches for alcohol and nicotine abuse/dependence mirrors a similar lack of research in the neurobiology of polysubstance abuse. This review discusses three critical needs in medications development for alcohol and nicotine co-abuse: (1) the need for a better understanding of the clinical condition (i.e. alcohol and nicotine polysubstance abuse), (2) the need to better understand how these drugs interact in order to identify new targets for therapeutic development and (3) the need for animal models that better mimic this human condition. Current and emerging treatments available for the cessation of each drug and their mechanisms of action are discussed within this context followed by what is known about the pharmacological interactions of alcohol and nicotine. Much has been and will continue to be gained from studying comorbid alcohol and nicotine exposure.

Lack of effect of nucleus accumbens dopamine D1 receptor blockade on consumption during the first two days of operant self-administration of sweetened ethanol in adult Long-Evans rats

The mechanisms underlying ethanol self-administration are not fully understood; however, it is clear that ethanol self-administration stimulates nucleus accumbens dopamine release in well-trained animals. During operant sweetened ethanol self-administration behavior, an adaptation in the nucleus accumbens dopamine system occurs between the first and second exposure, paralleling a dramatic increase in sweetened ethanol intake, which suggests a single exposure to sweetened ethanol may be sufficient to learn the association between sweetened ethanol cues and its reinforcing properties. In the present experiment, we test the effects of blockade of nucleus accumbens dopamine D1 receptors on operant sweetened ethanol self-administration behavior during the first 2 days of exposure. Adult male Long-Evans rats were first trained to self-administer 10% sucrose (10S) across 6 days in an appetitive and consummatory operant model (appetitive interval: 10-min pre-drinking wait period and a lever response requirement of 4; consummatory interval: 20-min access to the drinking solution). After training on 10S, the drinking solution was switched to 10% sucrose plus 10% ethanol (10S10E); control rats continued drinking 10S throughout the experiment. Bilateral nucleus accumbens microinjections of the dopamine D1 antagonist, SCH-23390 (0, 1.0, or 3.0 μg/side), immediately preceded the first two sessions of drinking 10S10E. Results show that blocking nucleus accumbens dopamine D1 receptors has little or no influence on consumption during the first 2 days of exposure to the sweetened ethanol solution or maintenance of sucrose-only drinking. Furthermore, the high dose of SCH-23390, 3.0 μg/side, reduced open-field locomotor activity. In conclusion, we found no evidence to suggest that nucleus accumbens D1 receptor activation is involved in consumption of a sweetened ethanol solution during the first 2 days of exposure or maintenance of sucrose drinking, but rather D1 receptors seem necessary for general locomotor activity that contributes to initiation of appetitive behavior.

Drug effects on multiple and concurrent schedules of ethanol- and food-maintained behaviour: context-dependent selectivity

BACKGROUND AND PURPOSE

Drugs that more potently or effectively reduce ethanol-maintained behaviour versus an alternative are considered selective and are considered promising pharmacotherapies for alcoholism. Such results are often obtained using separate groups or multiple schedules where ethanol and the alternative are available alone or sequentially. Recently, we observed that when ethanol and food were available sequentially under a multiple schedule, fluvoxamine and varenicline were selective; yet this selectivity disappeared when ethanol and food were concurrently available.

EXPERIMENTAL APPROACH

We examined the generality of these findings by comparing doses of several drugs required to decrease ethanol- and food-maintained responding under a multiple schedule and under a concurrent schedule. Effects were determined for chlordiazepoxide, 2,5-dimethoxy-4-iodoamphetamine (DOI), meta-chlorophenylpiperazine (mCPP), morphine, naltrexone and d-amphetamine.

KEY RESULTS

Under the multiple schedule, ED50 values for decreases in ethanol-maintained responding were significantly different and lower than ED50 s for decreases in food-maintained responding (demonstrating selectivity) for each drug except for chlordiazepoxide (which was equipotent) and naltrexone (which did not affect responding). However, this selectivity vanished or even inverted under the concurrent schedule, such that ED50 values for decreasing ethanol- and food-maintained responding were not different (or, following DOI, the ED50 for food-maintained responding was lower than for ethanol-maintained responding).

CONCLUSIONS AND IMPLICATIONS

Results are consistent with those seen following fluvoxamine and varenicline administration, and suggest that selectivity is assay-dependent. These results indicate the need for careful interpretation of selective drug effects, especially when obtained in situations where ethanol or the alternative is the only programmed reinforcement available.

Neuronal nicotinic acetylcholine receptors: common molecular substrates of nicotine and alcohol dependence

Alcohol and nicotine are often co-abused. As many as 80-95% of alcoholics are also smokers, suggesting that ethanol and nicotine, the primary addictive component of tobacco smoke, may functionally interact in the central nervous system and/or share a common mechanism of action. While nicotine initiates dependence by binding to and activating neuronal nicotinic acetylcholine receptors (nAChRs), ligand-gated cation channels normally activated by endogenous acetylcholine (ACh), ethanol is much less specific with the ability to modulate multiple gene products including those encoding voltage-gated ion channels, and excitatory/inhibitory neurotransmitter receptors. However, emerging data indicate that ethanol interacts with nAChRs, both directly and indirectly, in the mesocorticolimbic dopaminergic (DAergic) reward circuitry to affect brain reward systems. Like nicotine, ethanol activates DAergic neurons of the ventral tegmental area (VTA) which project to the nucleus accumbens (NAc). Blockade of VTA nAChRs reduces ethanol-mediated activation of DAergic neurons, NAc DA release, consumption, and operant responding for ethanol in rodents. Thus, ethanol may increase ACh release into the VTA driving activation of DAergic neurons through nAChRs. In addition, ethanol potentiates distinct nAChR subtype responses to ACh and nicotine in vitro and in DAergic neurons. The smoking cessation therapeutic and nAChR partial agonist, varenicline, reduces alcohol consumption in heavy drinking smokers and rodent models of alcohol consumption. Finally, single nucleotide polymorphisms in nAChR subunit genes are associated with alcohol dependence phenotypes and smoking behaviors in human populations. Together, results from pre-clinical, clinical, and genetic studies indicate that nAChRs may have an inherent role in the abusive properties of ethanol, as well as in nicotine and alcohol co-dependence.

Chronic self-administration of alcohol results in elevated ΔFosB: comparison of hybrid mice with distinct drinking patterns

Background

The inability to reduce or regulate alcohol intake is a hallmark symptom for alcohol use disorders. Research on novel behavioral and genetic models of experience-induced changes in drinking will further our knowledge on alcohol use disorders. Distinct alcohol self-administration behaviors were previously observed when comparing two F1 hybrid strains of mice: C57BL/6J x NZB/B1NJ (BxN) show reduced alcohol preference after experience with high concentrations of alcohol and periods of abstinence while C57BL/6J x FVB/NJ (BxF) show sustained alcohol preference. These phenotypes are interesting because these hybrids demonstrate the occurrence of genetic additivity (BxN) and overdominance (BxF) in ethanol intake in an experience dependent manner. Specifically, BxF exhibit sustained alcohol preference and BxN exhibit reduced alcohol preference after experience with high ethanol concentrations; however, experience with low ethanol concentrations produce sustained alcohol preference for both hybrids. In the present study, we tested the hypothesis that these phenotypes are represented by differential production of the inducible transcription factor, ΔFosB, in reward, aversion, and stress related brain regions.

Results

Changes in neuronal plasticity (as measured by ΔFosB levels) were experience dependent, as well as brain region and genotype specific, further supporting that neuronal circuitry underlies motivational aspects of ethanol consumption. BxN mice exhibiting reduced alcohol preference had lower ΔFosB levels in the Edinger-Westphal nucleus than mice exhibiting sustained alcohol preference, and increased ΔFosB levels in central medial amygdala as compared with control mice. BxN mice showing sustained alcohol preference exhibited higher ΔFosB levels in the ventral tegmental area, Edinger-Westphal nucleus, and amygdala (central and lateral divisions). Moreover, in BxN mice ΔFosB levels in the Edinger-Westphal nucleus and ventral tegmental regions significantly positively correlated with ethanol preference and intake. Additionally, hierarchical clustering analysis revealed that many ethanol-naïve mice with overall low ΔFosB levels are in a cluster, whereas many mice displaying sustained alcohol preference with overall high ΔFosB levels are in a cluster together.

Conclusions

By comparing and contrasting two alcohol phenotypes, this study demonstrates that the reward- and stress-related circuits (including the Edinger-Westphal nucleus, ventral tegmental area, amygdala) undergo significant plasticity that manifests as reduced alcohol preference.

The dual orexin/hypocretin receptor antagonist, almorexant, in the ventral tegmental area attenuates ethanol self-administration

Recent studies have implicated the hypocretin/orexinergic system in reward-seeking behavior. Almorexant, a dual orexin/hypocretin R₁ and R₂ receptor antagonist, has proven effective in preclinical studies in promoting sleep in animal models and was in Phase III clinical trials for sleep disorders. The present study combines behavioral assays with in vitro biochemical and electrophysiological techniques to elucidate the role of almorexant in ethanol and sucrose intake. Using an operant self-administration paradigm, we demonstrate that systemic administration of almorexant decreased operant self-administration of both 20% ethanol and 5% sucrose. We further demonstrate that intra-ventral tegmental area (VTA) infusions, but not intra-substantia nigra infusions, of almorexant reduced ethanol self-administration. Extracellular recordings performed in VTA neurons revealed that orexin-A increased firing and this enhancement of firing was blocked by almorexant. The results demonstrate that orexin/hypocretin receptors in distinct brain regions regulate ethanol and sucrose mediated behaviors.

Dopamine and serotonin interactively modulate prefrontal cortex neurons in vitro

BACKGROUND

Dopamine (DA) and serotonin (5-HT) are released in cortex under similar circumstances, and many psychiatric drugs bind to both types of receptors, yet little is known about how they interact.

METHODS

To characterize the nature of these interactions, the current study used in vitro patch-clamp recordings to measure the effects of DA and/or 5-HT on pyramidal cells in layer V of the medial prefrontal cortex.

RESULTS

Either DA or 5-HT applied in isolation increased the evoked excitability of prefrontal cortex neurons, as shown previously. Coapplication of DA and 5-HT produced either a larger increase in excitability than when either was given alone or a significant decrease that was never observed when either was given alone. Dopamine or 5-HT also "primed" neurons to respond in an exaggerated manner to the subsequent application of the other monoamine.

CONCLUSIONS

These data reveal the unappreciated interactive nature of neuromodulation in cortex by showing that the combined effects of DA and 5-HT can be different from their effects recorded in isolation. On the basis of these findings, we present a theory of how DA and 5-HT might synergistically modulate cortical circuits during various tasks.

Sex differences in striatal dopamine release in young adults after oral alcohol challenge: a positron emission tomography imaging study with [¹¹C]raclopride

BACKGROUND

We used a positron emission tomography paradigm with the D2/3 radiotracer [¹¹C]raclopride and an alcohol challenge to examine the magnitude of alcohol-induced dopamine release and compare it between young men and women.

METHODS

Twenty-one nonalcohol-dependent young social drinkers completed two positron emission tomography scans on separate days following ingestion of a juice mix containing either ethanol (.75 mg/kg body water) or trace ethanol only. The extent of dopamine released after alcohol was estimated by the percentage difference in [¹¹C]raclopride binding potential (ΔBP(ND)) between days.

RESULTS

Alcohol administration significantly displaced [¹¹C]raclopride in all striatal subregions, indicating dopamine release, with the largest effect observed in the ventral striatum. Linear mixed model analysis across all striatal subregions of regional ΔBP(ND) with region of interest as repeated measure showed a highly significant effect of sex (p < .001). Ventrostriatal dopamine release in men, but not in women, showed a significant positive correlation to alcohol-induced measures of subjective activation. Furthermore, we found a significant negative correlation between the frequency of maximum alcohol consumption per 24 hours and ventrostriatal ΔBP(ND) (r = .739, p = .009) in men.

CONCLUSIONS

This study provides definitive evidence that oral alcohol induces dopamine release in nonalcoholic human subjects and shows sex differences in the magnitude of this effect. The ability of alcohol to stimulate dopamine release may contribute to its rewarding effects and, thereby, to its abuse liability in humans. Our report further suggests several biological mechanisms that may mediate the difference in vulnerability for alcoholism between men and women.

Alcoholism: A Systems Approach From Molecular Physiology to Addictive Behavior

Alcohol consumption is an integral part of daily life in many societies. The benefits associated with the production, sale, and use of alcoholic beverages come at an enormous cost to these societies. The World Health Organization ranks alcohol as one of the primary causes of the global burden of disease in industrialized countries. Alcohol-related diseases, especially alcoholism, are the result of cumulative responses to alcohol exposure, the genetic make-up of an individual, and the environmental perturbations over time. This complex gene × environment interaction, which has to be seen in a life-span perspective, leads to a large heterogeneity among alcohol-dependent patients, in terms of both the symptom dimensions and the severity of this disorder. Therefore, a reductionistic approach is not very practical if a better understanding of the pathological processes leading to an addictive behavior is to be achieved. Instead, a systems-oriented perspective in which the interactions and dynamics of all endogenous and environmental factors involved are centrally integrated, will lead to further progress in alcohol research. This review adheres to a systems biology perspective such that the interaction of alcohol with primary and secondary targets within the brain is described in relation to the behavioral consequences. As a result of the interaction of alcohol with these targets, alterations in gene expression and synaptic plasticity take place that lead to long-lasting alteration in neuronal network activity. As a subsequent consequence, alcohol-seeking responses ensue that can finally lead via complex environmental interactions to an addictive behavior.

Ethanol regulation of D(1) dopamine receptor signaling is mediated by protein kinase C in an isozyme-specific manner

Ethanol consumption potentiates dopaminergic signaling that is partially mediated by the D(1) dopamine receptor; however, the mechanism(s) underlying ethanol-dependent modulation of D(1) signaling is unclear. We now show that ethanol treatment of D(1) receptor-expressing cells decreases D(1) receptor phosphorylation and concurrently potentiates dopamine-stimulated cAMP accumulation. Protein kinase C (PKC) inhibitors mimic the effects of ethanol on D(1) receptor phosphorylation and dopamine-stimulated cAMP levels in a manner that is non-additive with ethanol treatment. Ethanol was also found to modulate specific PKC activities as demonstrated using in vitro kinase assays where ethanol treatment attenuated the activities of lipid-stimulated PKCgamma and PKCdelta in membrane fractions, but did not affect the activities of PKCalpha, PKCbeta(1), or PKCvarepsilon. Importantly, ethanol treatment potentiated D(1) receptor-mediated DARPP-32 phosphorylation in rat striatal slices, supporting the notion that ethanol enhances D(1) receptor signaling in vivo. These findings suggest that ethanol inhibits the activities of specific PKC isozymes, resulting in decreased D(1) receptor phosphorylation and enhanced dopaminergic signaling.

The role of dopamine in alcohol self-administration in humans: individual differences

OBJECTIVE

To clarify dopamine's role in alcohol self-administration in a heterogeneous sample of drinkers using acute phenylalanine/tyrosine depletion (APTD).

METHODS

Sixteen men with variable drinking histories were characterized on their ethanol-induced cardiac response, a marker previously proposed to index dopamine system reactivity and vulnerability to alcohol abuse. During separate sessions participants were administered (i) a nutritionally balanced (BAL) amino acid (AA) mixture, (ii) a mixture lacking the dopamine precursors, phenylalanine and tyrosine, and (iii) APTD followed by the dopamine precursor, L-DOPA. Five hours after AA administration, participants could earn units of alcohol using a progressive ratio breakpoint task.

RESULTS

Alcohol self-administration was reduced in the APTD and APTD+L-DOPA conditions relative to the BAL condition. In both cases the changes were predicted by ethanol-induced cardiac change.

CONCLUSIONS

The motivation to drink is likely regulated by more than one neurobiological mechanism. Individual differences in cardiac responsivity to ethanol might provide a peripheral marker of responsiveness to pharmacological manipulations of dopamine.

Downregulation of mu opioid receptor by RNA interference in the ventral tegmental area reduces ethanol consumption in mice

Pharmacological and genetic studies have implicated the mu opioid receptor (MOR) in the regulation of ethanol intake in animal models and humans. Non-specific antagonists of opioid receptors have been shown to affect ethanol consumption when infused directly into the ventral tegmental area (VTA) of rats. However, administration of MOR-selective antagonists into the VTA has yielded mixed results. We used RNA interference (RNAi) to specifically decrease levels of MOR messenger RNA in the VTA of mice and examined the effect on ethanol consumption in a two-bottle choice paradigm. Mice were injected in the VTA with lentivirus expressing either a small hairpin RNA (shRNA) targeting MOR or a control shRNA. One week after virus injection, mice were examined for ethanol consumption in a two-bottle choice experiment with increasing concentrations of ethanol over the course of 1 month. Expression of an shRNA targeting MOR in the VTA led to a significant reduction in ethanol consumption. These results strengthen the hypothesis that MOR in the VTA is one of the key brain substrates mediating alcohol consumption. The RNAi combined with lentiviral delivery can be used successfully in brain to effect a sustained reduction in expression of specific genes for behavioral analysis.

Blockade of GABA(A) receptors within the extended amygdala attenuates D(2) regulation of alcohol-motivated behaviors in the ventral tegmental area of alcohol-preferring (P) rats

The dopamine (DA) mesolimbic pathway, which originates from DA cell bodies within the ventral tegmental area (VTA), has been shown by various studies to play a role in the mediation of various drugs of abuse including alcohol (EtOH). It has been suggested that the VTA's control of EtOH reward is mediated in part by the D2 receptors within the VTA. These receptors may be under the regulation of reciprocal GABAergic inputs from forebrain components of the mesolimbic path such as the nucleus accumbens (NAcc), a classic EtOH reward substrate, and the bed nucleus of the stria terminalis, a substrate recently implicated in EtOH reinforcement, forming a self-regulating feedback loop. To test this hypothesis, D2 regulation of EtOH self-administration (SA) was evaluated by the microinfusion of the D2 antagonist eticlopride into the VTA of P rats, which produced profound reductions in EtOH SA in the highest (20.0 and 40.0microg) doses tested in both BST/VTA and NAcc/VTA implanted P rats. To determine the role of GABA in the mediation of EtOH SA, a 32.0ng dose the non-selective GABA antagonist SR 95531 was microinfused into the BST producing no effect on responding for EtOH and into the NAcc which lead to a reduction in EtOH responding. Finally, the hypothesis that GABA innervation of the VTA from the mesolimbic forebrain may influence EtOH SA was examined by the simultaneous infusion of eticlopride (40.0microg) into the VTA and SR 95531 (32.0ng) into either the BST or NAcc. This combination infusion completely attenuated the reduction in EtOH SA observed with the 40.0microg dose of eticlopride alone in both groups of animals. These results suggest that while the D2 receptors within the VTA regulate EtOH-motivated behaviors, this is modulated by GABAergic input from the mesolimbic forebrain, specifically from the BST and NAcc.

The urocortin 1 neurocircuit: Ethanol-sensitivity and potential involvement in alcohol consumption

One of the hallmarks of alcoholism is continued excessive consumption of alcohol-containing beverages despite the negative consequences of such behavior. The neurocircuitry regulating alcohol consumption is not well understood. Recent studies have shown that the neuropeptide urocortin 1 (Ucn1), a member of the corticotropin-releasing factor (CRF) family of peptides, could be an important player in the regulation of alcohol consumption. This evidence is accumulated along three directions of research: (1) Ucn 1-containing neurons are extremely sensitive to alcohol; (2) the Ucn1 neurocircuit may contribute to the genetic predisposition to high alcohol intake in mice and rats; (3) manipulation of the Ucn1 system alters alcohol consumption and sensitivity. This paper reviews the current knowledge of the Ucn1 neurocircuit and the evidence for its involvement in alcohol-related behaviors, and proposes a mechanism for its involvement in the regulation of alcohol consumption.

Involvement of accumbal glycine receptors in the regulation of voluntary ethanol intake in the rat

BACKGROUND

Extracellular dopamine (DA) levels in the nucleus accumbens (nAc) increase after ethanol (EtOH) administration in the rat, a response that may be involved in the positive reinforcing effects of EtOH. The mechanisms underlying this DA activation and how they relate to EtOH reinforcement remain to be elucidated, but recent data indicate that glycine receptors (GlyRs) in the nAc may be involved. Here this hypothesis was further challenged by examining the influence of bilateral accumbal application of glycine (a GlyR agonist), strychnine (a GlyR competitive antagonist), or Ringer on EtOH intake and preference, as well as on the concomitant DA output in the nAc, in EtOH high-preferring male Wistar rats.

METHODS

EtOH high-preferring male Wistar rats [EtOH preference >60% during continuous access to a bottle of EtOH (6% v/v) and a bottle of water] were limited to drink 1 hr/day (limited access drinking). Thereafter, the animals were equipped bilaterally with microdialysis probes aimed at the mAc, and were subjected to in vivo microdialysis (coupled to high-pressure liquid chromatography with electrochemical detection) and reversed microdialysis (for drug application) during two experimental days (balanced study), during which the animals were allowed a choice between EtOH and water.

RESULTS

The EtOH consumption in rats that were perfused with Ringer in the nAc was approximately 0.9 g/kg/hr and associated with a significant increase in extracellular accumbal DA levels. In a subpopulation of rats, bilateral accumbal glycine (100 microM) perfusion produced a significant increase in accumbal DA output and a decrease in EtOH preference and intake. In these glycine responders, the EtOH consumed (approximately 0.7 g/kg/hr) did not produce a further increase of DA levels. In other rats, bilateral glycine perfusion did not change the accumbal DA output, and voluntary EtOH intake was not altered. In these glycine nonresponders, EtOH tended to increase accumbal DA levels. Bilateral accumbal strychnine (20 microM) perfusion significantly decreased DA output in the nAc, and the DA levels remained decreased despite a statistically significant increase of EtOH intake. Finally, the increase in accumbal DA levels observed after EtOH consumption in Ringer-treated rats was significantly larger in glycine responders than in glycine nonresponders.

CONCLUSIONS

The present findings suggest that glycine and strychnine alter extracellular DA levels in the nAc, probably via GlyR stimulation and blockade, respectively, and concomitantly glycine and strychnine reciprocally alter also EtOH consumption in EtOH high-preferring male Wistar rats. The possibility of developing selective GlyR agonists and/or antagonists should be explored. Such agents could prove of value in the treatment of alcoholism.

A study of the association of (Val66Met) polymorphism in the brain-derived neurotrophic factor gene with alcohol dependence and extreme violence in Chinese males

From studies of genetic-knockout animals, brain-derived neurotrophic factor (BDNF), a member of the neurotrophin growth-factor family, has been implicated in both alcohol preference and aggressive behaviour. To test whether a BDNF genetic variant may be associated with alcohol-dependent and violent behaviours, we studied Val66Met polymorphism of the BDNF-gene in 110 cases of alcohol-dependence, in 134 extremely violent convicts, and in 149 individuals without psychosis or mood disorders. We also examined the association of this polymorphism with antisocial personality disorder comorbidity in the extremely violent convicts. The results showed that the genotype and allele frequencies for Val66Met polymorphism at the BDNF-gene site did not differ among the three groups. Furthermore, it was not demonstrated that this polymorphism is associated with antisocial personality disorder comorbidity in the extremely violent convicts. Based on these findings, it seems reasonable to suggest that this BDNF-gene Val66Met polymorphism is unlikely to play a major role in the genetic susceptibility to the traits of alcohol-dependence or violence proneness.

Dopamine and alcoholism: neurobiological basis of ethanol abuse

The role of the dopamine (DA) system in brain reward mechanisms and the development of substance abuse has been well established. We review earlier animal and human studies on DA and alcoholism with some relevant issues relating to those studies. The present animal and human data suggest several alterations in the DA system in the context of alcoholism. Receptor studies imply that DA D(2) receptor density and function are lower at least among type 1 alcoholics, which suggests that they could benefit from drugs that enhance DAergic activity, such as partial DA agonists. These drugs could help to restore suboptimal levels of DAergic activity by reducing both the craving for alcohol in abstinence and the euphoria subsequent to alcohol's release of DA in the nucleus accumbens (NAC), thus providing negative reinforcement for relapse.

The Dopamine D1 Antagonist Reduces Ethanol Reward for C57BL/6 Mice

BACKGROUND

Dopamine D1 antagonist effects on behaviors related to obtaining and consuming ethanol remain unclear. The highly selective D1 antagonist ecopipam (SCH 39166), which has no effect on the serotonin system, was used to evaluate the role of D1 receptors in ethanol reward and its potential for treating alcohol abuse by determining its effect on several measures of ethanol reward in C57BL/6 (B6) mice.

METHODS

Ecopipam (0.025-0.2 mg/kg) effects on instrumental and contingent consummatory responses and on noncontingent consummatory responses for ethanol and water reward were determined in food-restricted male mice trained to lever-respond for 12% ethanol delivered on a fixed ratio-4 reinforcement schedule. The mice were tested for 15-min sessions under preprandial (high-hunger and low-thirst) and postprandial (low-hunger and high-thirst) test conditions.

RESULTS

Ecopipam dose-dependently reduced instrumental and consummatory responses for ethanol and ethanol intake when tested under hunger- or thirst-motivated conditions with free access to water. Under thirst motivation with no access to an alternate fluid source, lever responses for ethanol and water were similar; however, ecopipam reduced responding for ethanol more than responding for water reward. When given concurrent free access to the same fluid delivered for lever pressing, animals made more contacts for ethanol than for water; ecopipam reduced free ethanol but not water contacts.

CONCLUSIONS

Ecopipam attenuated ethanol reward at doses that did not affect water reward, indicating an effect independent of reductions in motor system function or general motivation and arousal. Ecopipam also reduced ethanol reward to the same degree under hunger, thirst, or sated conditions, again indicating that it affected ethanol reward at doses that did not grossly affect general motivational states. These data suggest that ecopipam may reduce ethanol reward with few side effects and that it warrants further investigation as a pharmacological tool for treating alcohol abuse.

Long-lasting alterations of the mesolimbic dopamine system after periadolescent ethanol drinking by alcohol-preferring rats

BACKGROUND

This study tested the hypothesis that ethanol consumption by alcohol-preferring (P) rats during the periadolescent period causes persistent alterations in the mesolimbic dopamine (DA) system. After ethanol drinking during periadolescence, P rats were examined for alterations in basal locomotor activity, changes in extracellular DA levels and extraction fraction in the nucleus accumbens (NAc) by using no-net-flux (NNF) microdialysis, and changes in the response of the mesolimbic DA system to ethanol.

METHODS

Male P rat pups were given 24-hr free-choice access to 15% (v/v) ethanol from postnatal day (PD) 30 through PD 60. On PD 70, rats were assessed for locomotor activity. On PD 70 to 80, rats were implanted with bilateral guide cannulas aimed above the NAc. After at least 5 days, microdialysis probes were inserted bilaterally; on the following day, NNF microdialysis experiments were conducted. On the day after the NNF experiment, conventional microdialysis experiments were conducted to measure extracellular levels of DA in response to intraperitoneal injection of saline or ethanol 2.5 g/kg.

RESULTS

Compared with the ethanol-naive group, ethanol drinking by P rats during periadolescence did not alter basal locomotor activity, nor did it alter the basal extracellular concentration of DA. There was, however, a significant increase in the extraction fraction of DA of ethanol-drinking animals relative to the controls (57.4 +/- 2.7% and 45.8 +/- 2.3%, respectively). Additionally, compared with controls, P rats with exposure to ethanol during the periadolescent period showed a prolonged increase in the extracellular levels of DA after a challenge dose of ethanol.

CONCLUSIONS

The results of the microdialysis experiments suggest that periadolescent ethanol drinking by P rats increases basal DA neurotransmission (as indicated by higher DA clearance while maintaining the same extracellular DA concentrations) and prolongs the response of DA neurotransmission to ethanol.

Biological and behavioral markers of alcohol sensitivity

This article summarizes a symposium that was organized by Dr. Kim Fromme and presented at the 2003 annual meeting of the Research Society on Alcoholism in Ft. Lauderdale, Florida. The four presentations illustrate the emerging technologies and methods that are now being used to investigate the genetic basis of differential sensitivity to alcohol and their behavioral manifestations. Combining human genotyping with laboratory measures of behavior and subjective reports, these presentations represent state-of-the-art approaches to crossing the bridge from the Decade of the Brain to the Decade of Behavior. Dr. De Wit's paper describes her research on the neurobiological basis for individual differences in sensitivity to the stimulant and sedative effects of alcohol. Evidence suggests that activity of the dopaminergic and GABAergic neurotransmitters underlie these stimulant and sedative effects, respectively. Both Drs. Hutchison's and Corbin's papers describe their research on polymorphisms for the serotonin transporter (SLC6A4) as a determinant of the subjective effects of alcohol challenge. Dr. Hutchinson's and Ms. Ray's findings indicate that individuals with the short form of the SLC6A4 alleles (S) demonstrated a low level of response to alcohol, thus supporting previous research that the S allele may be associated with increased risk for alcohol dependence. In contrast, Dr. Corbin did not find a reliable association between the SLC6A4 genotype and subjective response to alcohol. Mr. Cook's and Dr. Wall's paper adds another dimension to this article by presenting research on both the aldehyde dehydrogenase (ALDH2) and alcohol dehydrogenase (ADH2) genetic variants and their association with the alcohol-related flushing response that is prevalent in Asian populations. Dr. David Goldman provides concluding remarks.

Ethanol increases extracellular dopamine concentration in the ventral striatum in C57BL/6 mice

BACKGROUND

Mesolimbic dopamine is thought to play a role in the reinforcing properties of ethanol, but ethanol-induced changes in extracellular dopamine in the ventral striatum have not been well characterized in mouse models.

METHODS

Two experiments were used to characterize the pharmacodynamic response of ethanol in the ventral striatum in C57BL/6 mice. The first experiment determined the effect of ethanol on ventral striatal dopamine in male and female mice after intraperitoneal injection of either 2.0 g/kg ethanol or saline. The second experiment was a replication in males, except that the mice were habituated to intraperitoneal injections before the dialysis experiment.

RESULTS

Distinct patterns of dopamine activity in response to ethanol were demonstrated in male and female C57BL/6 mice. A significant increase in dialysate dopamine relative to saline injection was observed in females but not in males. With habituation to intraperitoneal injection before the dialysis experiment, ethanol administration caused a significant dopamine response in males as well. A linear decline was observed in dialysate ethanol concentrations after the peak concentration was reached. Concurrent analysis of the time course of dopamine and ethanol content showed that the dopamine response declined significantly faster than the ethanol concentrations.

CONCLUSIONS

The C57BL/6 mouse strain is a justifiable model system for studying the mechanisms involved in ethanol regulation of mesolimbic dopamine activity. Habituation to intraperitoneal injection should be used in male C57BL/6 mice for experiments in which the dopamine response is measured after intraperitoneal injection of a drug. The dissociation between dopamine and ethanol may indicate an acute neural adaptation to ethanol-induced dopamine response in the ventral striatum after a single ethanol injection.

Alcohol-induced memory impairment in trace fear conditioning: a hippocampus-specific effect

It has been hypothesized that the amnesic effects of alcohol are through selective disruption of hippocampal function. Delay and trace fear conditioning are useful paradigms to investigate hippocampal-dependent and independent forms of memory. With delay fear conditioning, learning of explicit cues does not depend on normal hippocampal function, whereas learning explicit cues in trace fear conditioning does. In both delay and trace fear conditioning, the hippocampus is involved in learning to contextual cues, but it may not be entirely necessary. The present study investigates the effects of alcohol on the acquisition of delay and trace fear conditioning in mice, using freezing as a measure of learning. Male C57BL/6J mice were injected with 0.8 or 1.6 g/kg of 20% v/v alcohol and were immediately exposed to eight tone-footshock pairings in which the conditional stimulus (CS) either coterminated with a footshock unconditional stimulus (US) (delay conditioning) or was separated from the footshock by a 30-s trace interval (trace conditioning). During trace, but not delay fear conditioning, 0.8 g/kg alcohol impaired learning to a tone CS. This dose also impaired context-dependent learning in both procedures (although only slightly for trace fear conditioning). The 1.6 g/kg alcohol exerted a nonselective impairment on learning. The impairment by alcohol of learning to a tone CS when it is hippocampus-dependent, but not when it is hippocampus-independent provides further support for the hypothesis that alcohol exerts a selective effect on hippocampus-dependent learning.

The acquisition and maintenance of voluntary ethanol drinking in the rat: effects of dopaminergic lesions and naloxone

Wistar male rats were microinfused bilaterally with 6-hydroxydopamine or vehicle into the ventral tegmental area. After recovery, ethanol drinking was established using a sucrose-fading paradigm, i.e. rats were given twice a day access to drinks containing increasing amounts of ethanol and decreasing amounts of sucrose. Mean daily intakes at each ethanol/sucrose concentration were similar irrespective of the level of dopamine depletion that, in some animals, reached 80-90%. The percentage of rats testing as ethanol preferers in a two-bottle choice test also appeared similar in both the lesioned and control groups. After completing the sucrose-fading protocol, all rats were switched to one access per day during which they were presented with a drink containing 10% ethanol with 5% sucrose. Naloxone administration (15 min before the daily access period) decreased ethanol beverage consumption by about 50%, irrespective of the level of dopamine depletion. Total daily water intake was not altered by naloxone. In a two-bottle choice situation, naloxone suppressed intake of an ethanol drink (10% ethanol/5% sucrose), but not the intake of 5% sucrose alone. Thus, a lesion of the dopaminergic cell bodies that results in extensive depletion of dopamine in mesolimbic target regions produced no measurable effect on intake of the sweetened ethanol drinks during the acquisition phase of the sucrose-fading paradigm. Furthermore, during the maintenance phase of drinking, the marked effect of naloxone in inhibiting ethanol beverage ingestion (but not water ingestion or sucrose alone solutions) occurred despite extensive loss of dopaminergic innervation to telencephalic target regions. A preliminary account of these experiments appeared in an abstract form and as an Internet publication. (Supported by NIAAA grants P50-03510 and T32-0720).

Muscimol injected into the medial prefrontal cortex of the rat alters ethanol self-administration

The role of the rodent prefrontal cortex in the regulation of ethanol self-administration has not been widely explored. Understanding the role of GABAergic transmission in this area in relation to ethanol self-administration is important, as the GABA system may be one of several targets for alcohol's actions in the brain. Rats were initiated to drink 10% ethanol from a dipper using a sucrose-substitution procedure. When baseline behavior was stable, bilateral microinjections of muscimol (a GABA(A) agonist) into the prefrontal cortex were tested at doses of 17.5, 30, 100 and 300 ng/microl. Ethanol self-administration was decreased by approximately 40% at the 30-ng dose and 30% at the 100-ng dose. No effects were observed at either the 17.5- or 300-ng dose. The effect on the pattern of self-administration was to shorten the size of the first run of drinking without affecting the rate of drinking. The hypothesis is put forward that the injections increased glutamatergic output to the nucleus accumbens (nAcc) that in turn increased accumbens output. This increased output is proposed as similar to the effects of dopaminergic (DA) manipulations within this system.

Regional central nervous system densities of delta-opioid receptors in alcohol-preferring P, alcohol-nonpreferring NP, and unselected Wistar rats

The densities of delta-opioid receptors in the central nervous system of alcohol-naive, adult, male, alcohol-preferring P, alcohol-nonpreferring NP, and Wistar rats were examined with the use of quantitative autoradiography. Slides with coronal 20-microm sections through the regions of interest were incubated in 5 nM [3H]-[D-Pen(2),D-Pen(5)]enkephalin (DPDPE) to label delta(1)-opioid receptor sites. Nonspecific binding was determined in the presence of 10 microM naloxone. Significant differences between the P and the NP rat lines were found in numerous cortical regions, the basolateral amygdala, and the posterior hippocampus, with 10%-20% lower [3H]-DPDPE binding found in the P line. In most regions examined, binding levels in the Wistar rats were intermediate between those of the P and the NP rats. Significantly lower [3H]-DPDPE binding levels in the P rat may indicate fewer delta(1)-opioid receptors or decreased binding affinity. The lower binding in certain limbic regions, such as the basolateral amygdala and posterior hippocampus, as well as cortical differences in the P rat may be associated with the divergent alcohol drinking behaviors found between the P and the NP lines.

A cocaine analog, 2beta-propanoyl-3beta-(4-tolyl)-tropane (PTT), reduces tyrosine hydroxylase in the mesolimbic dopamine pathway

Tyrosine hydroxylase (TH) is the rate-limiting enzyme in catecholamine biosynthesis. Previously published results have established that chronic cocaine administration (30-45 mg/kg per day, 10-14 days) resulted in an upregulation of TH gene expression in dopaminergic pathways of rats. The present studies tested the effects of a tropane analog, PTT (2beta-propanoyl-3beta-(4-tolyl)-tropane), on TH expression. This drug has similar actions to cocaine, but possesses markedly different pharmacokinetics (20 times more potent at binding the dopamine transporter, markedly increased metabolic stability, and 10-20 times more potent in behavioral measures). Moreover, PTT demonstrates an increased selectivity for the dopamine (DA) and norepinephrine (NE) transporters compared with cocaine. In direct contrast to the previously reported effects of cocaine, 10 days of PTT administration (3.0 mg/kg per day, i.p.) produced a uniform downregulation of TH protein and activity gene expression. TH activity and immunoreactive protein where decreased by 54 and 69%, respectively in the nucleus accumbens. Within the ventral tegmental area, TH activity and protein were decreased by 33 and 19%, respectively. The underlying mechanisms for these fundamental differences are unclear, but likely reflect varying and selective affinities and lengths of occupancy at biogenic amine transporters.

Reduced operant ethanol self-administration and in vivo mesolimbic dopamine responses to ethanol in PKCepsilon-deficient mice

There is increasing evidence that individual protein kinase C (PKC) isozymes mediate specific effects of ethanol on the nervous system. In addition, multiple lines of evidence suggest that the mesoaccumbens dopamine reward system is critically involved in the rewarding and reinforcing effects of ethanol. Yet little is known about the role of individual PKC isozymes in ethanol reinforcement processes or in regulation of mesolimbic systems. In this study, we report that mice lacking the epsilon isoform of PKC (PKCepsilon) show reduced operant ethanol self-administration and an absence of ethanol-induced increase in extracellular dopamine levels in the nucleus accumbens. PKCepsilon null mice exhibited a 53% decrease in alcohol-reinforced operant responses under basal conditions, as well as following ethanol deprivation. Behavioural analysis revealed that while both genotypes had the same number of drinking bouts following deprivation, PKCepsilon null mice demonstrated a 61% reduction in number of ethanol reinforcers per bout and a 57% reduction in ethanol-reinforced response rate. In vivo microdialysis experiments showed that, in contrast to wild-type mice, PKCepsilon null mice exhibited no change in extracellular levels of dopamine in the nucleus accumbens following acute administration of ethanol (1 and 2 g/kg i.p.), while mesolimbic dopamine responses to cocaine (20 mg/kg i.p.) or high potassium (100 mM) in these mice were comparable with that of wild-types. These data provide further evidence that increases in extracellular mesolimbic dopamine levels contribute to the reinforcing effects of ethanol, and indicate that pharmacological agents inhibiting PKCepsilon may be useful in the treatment of alcohol dependence.

Ethanol-induced increases in dopamine extracellular concentration in rat nucleus accumbens are accounted for by increased release and not uptake inhibition

We performed a quantitative microdialysis study to determine whether the increase in dialysate dopamine from the nucleus accumbens caused by intraperitoneal administration of ethanol (1 g/kg) was due to enhanced dopamine release or inhibition of dopamine uptake. The Lönnroth method (no net flux), adapted for transient conditions, was used to follow the time course of true extracellular dopamine concentrations in the nucleus accumbens simultaneously with the in vivo recovery of dopamine across the microdialysis probe. Separate groups of rats were perfused with artificial cerebral spinal fluid containing 0, 4, 8, or 12 nM dopamine for the entire experiment. Samples were taken every 10 min. Each rat received a saline or an ethanol injection. The concentration of dopamine gained by or lost from the probe was plotted as a function of the concentration of dopamine perfused into the probe for each time point. Linear regression was used to determine the slope of the line (in vivo recovery) and the x-intercept (point of no net flux) for each plot. The in vivo recovery did not significantly change over time for the saline- or the ethanol-injected rats. However, the point of no net flux (true extracellular concentration of dopamine) significantly increased after the ethanol injection from 9.4+/-0.4 nM (mean of six basal samples) to 13.2+/-1.8 nM, at the maximum, but did not change after the saline injection. On the basis of these results, it is suggested that the primary mechanism by which ethanol increases dialysate dopamine levels in the nucleus accumbens after intraperitoneal administration is by increasing dopamine release from the terminals, rather than by inhibiting the dopamine transporter.

Developmental considerations of neurotoxic exposures

In this article, the authors provide a conceptual framework in which to consider alternative approaches to identify the developmental consequences of exposing the developing brain to neurotoxic substances. Concepts underlying brain development and issues regarding neurobehavioral testing in children are reviewed. In addition, the authors selectively review preclinical data identifying mechanisms contributing to neurobehavioral compromise, and clinical data identifying deficits resulting from exposure to two classes of neurotoxins: exposure to drugs of abuse, including alcohol, nicotine, and cocaine; and exposure to environmental agents, including lead, methyl-mercury, PCBs, and organophosphorus compounds.

Microinjections of dopaminergic agents in the nucleus accumbens affect ethanol consumption but not palatability

It was determined whether ethanol palatability in rats could be changed by manipulating the reinforcement experienced during limited access consumption. During the first 3 days of the experiment, initial taste reactivity (TR) testing to distilled water (1 day) and 10% alcohol (2 days) was performed. Following the establishment of baseline TR, separate groups of animals received bilateral microinjections (0.5 microl/side) into the nucleus accumbens of either the nonspecific dopamine agonist d-amphetamine sulfate (20 microg, n = 10), the D(2) antagonist raclopride (1.0 microg, n = 8), or physiological saline (n = 5). The injections occurred at the same time each day for 5 consecutive days. Five minutes after the microinjection, the fluid-deprived rats were given 30-min access to 10% ethanol. Over the 3 days following drug administration, TR to distilled water and 10% alcohol was repeated. After this, the rats were once again given 30 min of access to 10% ethanol for 5 consecutive days, but without drug microinjection prior to alcohol access. A final TR exposure (the same as the others) was performed over the final 3 days of the study. Both raclopride and d-amphetamine administration produced reductions in ethanol consumption (in comparison to saline treatment). However, treatment with d-amphetamine and raclopride during ethanol consumption did not cause significant, conditioned changes in palatability as measured by the taste reactivity procedure. These results suggest that dopamine plays a role in the motivation to consume ethanol but this neurotransmitter is not involved in evaluating its incentive value.

The dopamine hypothesis of reward: past and current status

Mesolimbic dopaminergic neurons are thought to serve as a final common neural pathway for mediating reinforcement processes. However, several recent findings have challenged the view that mesolimbic dopamine has a crucial role in the maintenance of reinforcement processes, or the subjective rewarding actions of natural rewards and drugs of abuse. Instead, there is growing evidence that dopamine is involved in the formation of associations between salient contextual stimuli and internal rewarding or aversive events. This evidence suggests that dopaminergic-neuron activation aids the organism in learning to recognize stimuli associated with such events. Thus, mesolimbic dopaminergic neurons have an important function in the acquisition of behavior reinforced by natural reward and drug stimuli. Furthermore, long-lasting neuroadaptive changes in mesolimbic dopamine-mediated transmission that develop during chronic drug use might contribute to compulsive drug-seeking behavior and relapse.

The role of opioid-dopamine interactions in the induction and maintenance of ethanol consumption

1. Alcohol is one of the most widely used recreational drugs, but also one of the most widely abused, causing vast economic, social and personal damage. 2. Several animal models are available to study the reinforcing mechanisms that are the basis of the abuse liability of ethanol. Innate differences in opioid or dopamine neurotransmission may enhance the abuse liability of ethanol, as indicated by animal and human studies. 3. Opioid antagonists have been shown to be effective, both experimentally and clinically, in decreasing ethanol consumption, presumably since ethanol induces the release of endogenous opioid peptides in vivo. However, ethanol may also stimulate the formation of opiate-like compounds, which could interact with opioid (or dopamine) receptors. Ethanol may cause changes in neurotransmission mediated via opioid receptors that determines whether alcohol abuse is more or less likely. 4. Ethanol appears to facilitate dopamine release by increasing opioidergic activity, disinhibiting dopaminergic neurons (by inhibition of GABAergic neurotransmission) via mu-opioid receptors in the ventral tegmental area (VTA) and delta-opioid receptors in the nucleus accumbens (NAcc). The effects of ethanol would be antagonised by presynaptic kappa-opioid receptors present on dopaminergic terminals in the NAcc. 5. Mesolimbic dopamine release induced by ethanol consumption seems to indicate ethanol-related stimuli are important, focussing attention on and enabling learning of the stimuli. However, studies indicate that there are redundant pathways, and neural pathways 'downstream' of the mesolimbic dopamine system, which also enable the reinforcing properties of ethanol to be mediated.