Naltrexone reverses ethanol-induced dopamine release in the nucleus accumbens in awake, freely moving rats

[3H]DPDPE binding to delta opioid receptors in the rat mesocorticolimbic and nigrostriatal pathways is transiently increased by acute ethanol administration

Dopaminergic transmission in the mesolimbic and nigrostriatal pathways plays a key role in the reinforcement mechanisms and brain sensitivity to ethanol, respectively. Ethanol reinforcement and high alcohol drinking behaviour have been postulated to be partially mediated by a neurobiological mechanism involving the ethanol-induced activation of the endogenous opioid system. Activation of opioid neural pathways by ethanol may include alterations in the processing, release and/or the receptor binding of opioid peptides. The aim of this work was to investigate the effects of acute ethanol administration on delta opioid receptors in the rat mesocortical, meso-accumbens and nigrostriatal pathways by quantitative receptor autoradiography, using [(3)H] (2-D-penicillamine, 5-D-penicillamine)-enkephalin as radioligand. A significant increase in [(3)H] (2-D-penicillamine, 5-D-penicillamine)-enkephalin binding was observed in the substantia nigra pars reticulata 1 h after ethanol treatment. Two hours after drug exposure, ligand binding was significantly increased in the frontal and prefrontal cortices, the core and shell regions of the nucleus accumbens, and in the anterior-medial and medial-posterior regions of the caudate-putamen. In contrast, ligand binding was significantly decreased in the posterior region of the caudate-putamen 30 min after ethanol administration. The observed effects may reflect ethanol-induced changes in ligand binding affinity and/or in receptor density. Our results suggest that transitory changes in delta opioid receptors with different kinetic patterns may be involved in ethanol reinforcement and brain sensitivity to the drug. Ethanol-induced delta receptor up- and down-regulation mechanisms may participate in modulation of dopaminergic transmission in the mesocorticolimbic and nigrostriatal pathways.

The role of mesolimbic dopamine in the development and maintenance of ethanol reinforcement

The neurobiological processes by which ethanol seeking and consumption are established and maintained are thought to involve areas of the brain that mediate motivated behavior, such as the mesolimbic dopamine system. The mesolimbic dopamine system is comprised of cells that originate in the ventral tegmental area (VTA) and project to several forebrain regions, including a prominent terminal area, the nucleus accumbens (NAcc). The NAcc has been subdivided into core and shell subregions. Both areas receive converging excitatory input from the cortex and amygdala and dopamine input from the VTA, with the accumbal medium spiny neuron situated to integrate the signals. Although forced ethanol administration enhances dopamine activity in the NAcc, conclusions regarding the role of mesolimbic dopamine in ethanol reinforcement cannot be made from these experiments. Behavioral experiments consistently show that pharmacological manipulations of the dopamine transmission in the NAcc alter responding for ethanol, although ethanol reinforcement is maintained after lesions of the accumbal dopamine system. Additionally, extracellular dopamine increases in the NAcc during operant self-administration of ethanol, which is consistent with a role of dopamine in ethanol reinforcement. Behavioral studies that distinguish appetitive responding from ethanol consumption show that dopamine is important in ethanol-seeking behavior, whereas neurochemical studies suggest that accumbal dopamine is also important during ethanol consumption before pharmacological effects occur. Cellular studies suggest that ethanol alters synaptic plasticity in the mesolimbic system, possibly through dopaminergic mechanisms, and this may underlie the development of ethanol reinforcement. Thus, anatomical, pharmacological, neurochemical, cellular, and behavioral studies are more clearly defining the role of mesolimbic dopamine in ethanol reinforcement.

Accumbal Strychnine-Sensitive Glycine Receptors: An Access Point for Ethanol to the Brain Reward System

BACKGROUND

Ethanol (EtOH), like other drugs of abuse, increases extracellular dopamine (DA) levels in the nucleus accumbens (nAc) of the brain reward system, an effect that may be of importance for alcohol addiction. How this DA increase is produced is not fully understood, although previous studies from the present laboratories indicate that nicotinic acetylcholine receptors in the ventral tegmental area play an important role in mediating this effect. Furthermore, activation of these receptors may be secondary to some priming effect produced by EtOH in the nAc. We recently demonstrated that strychnine-sensitive glycine receptors (GlyRs) are present in the nAc and that they are involved in regulating extracellular DA levels. Here we examine the tentative role of these accumbal GlyRs in the above-mentioned priming mechanism of EtOH.

METHOD

In vivo microdialysis (coupled to high pressure liquid chromatography with electrochemical detection) and reversed microdialysis, in awake, freely moving adult male Wistar rats.

RESULTS

Local perfusion of strychnine decreased accumbal DA levels per se and completely prevented the increase of accumbal DA levels after both local and systemic EtOH administration. Accumbal perfusion of the GlyR agonist glycine instead increased DA levels in a subpopulation of rats and prevented the EtOH-induced increase after local but not systemic EtOH in all animals.

CONCLUSION

The present results suggest that GlyRs in the nAc might constitute targets for EtOH in its mesolimbic DA-activating effect. Gene polymorphism and drug developmental studies that focus on this receptor population and its relation to alcohol dependence are warranted.

INTERACTIONS BETWEEN CANNABINOID AND OPIOID RECEPTOR SYSTEMS IN THE MEDIATION OF ETHANOL EFFECTS

Over the past few years, advances in the investigation of the neurochemical circuits involved in the development and treatment of alcohol dependence have identified peptides and receptors as potential key targets in the treatment of problems related to alcohol consumption. The endogenous opioid system is modified by alcohol intake in areas of the brain related to reward systems, and differential basal levels of opioid gene expression are found in rodents with a high preference for ethanol. This suggests a greater vulnerability to alcohol consumption in relation to differences in genetic background. Further evidence of the involvement of opioid peptides in alcohol dependence is the ability of the opioid antagonist naltrexone to reduce alcohol intake in animal models of dependence and in alcohol-dependent patients. Abundant evidence indicates that the activation of cannabinoid receptors stimulates the release of opioid peptides, therefore the cannabinoid receptor antagonists may presumably alter opioid peptide release, thus facilitating the reduction of ethanol consumption. However, little is known about the effects of ethanol on the endogenous cannabinoid system, the vulnerability of cannabinoid receptors to alcohol intake or their neurochemical implications in reducing consumption of alcohol. In this paper, we review the role of opioid and cannabinoid receptor systems, their vulnerability to alcohol intake and the development of dependence, and the targeting of these systems in the treatment of alcoholism.

A ONE-YEAR PRAGMATIC TRIAL OF NALTREXONE VS DISULFIRAM IN THE TREATMENT OF ALCOHOL DEPENDENCE

AIMS

To compare the efficacy of naltrexone and disulfiram in preventing an alcoholic relapse in routine clinical practice in an Indian metropolis.

METHODS

Hundred alcohol-dependent men, for whom a family member would accompany the patient to follow-up appointments, were randomly allocated to a year of treatment with either naltrexone or disulfiram. Patients, the accompanying family member and the treating psychiatrist were aware of the nature of treatment given. Alcohol consumption, craving and adverse events were recorded weekly for the first three months, then fortnightly for the rest of the year, by the treating psychiatrist. Serum gamma-glutamyl transferase (GGT) was measured at the start and the end of the study.

RESULTS

At the end of the year, 97 patients were still in contact. Relapse, the consumption of >5 drinks (40 g of ethanol) in a 24 h period, occurred at a mean of 119 days with disulfiram and at 63 days with naltrexone (P = 0.020). Mean serum GGT, which had not differed between the two groups initially, was 117 U/l with naltrexone and 85 U/l with disulfiram (P = 0.038) at the end of the study. Eighty-six per cent of the patients remained abstinent throughout the study with disulfiram compared to 44% with naltrexone (P = 0.0009). However, patients allocated to naltrexone had significantly lower craving than those allocated to disulfiram.

CONCLUSIONS

Disulfiram is superior to naltrexone in preventing a relapse among alcohol-dependent men with family support. Comparison between these treatments in other settings and in different types of alcoholics is warranted.

Effects of Naltrexone and Nalmefene on Subjective Response to Alcohol Among Non-Treatment-Seeking Alcoholics and Social Drinkers

BACKGROUND

Despite the relative success of opiate antagonist medication within controlled clinical trials for alcoholism, laboratory studies have not fully examined potential mechanisms for their efficacy in alcohol-dependent persons. The present study evaluated the impact of naltrexone and nalmefene on craving and subjective effects after a moderate alcohol dose among non-treatment-seeking alcoholics (n = 125) and social drinkers (n = 90).

METHODS

Participants were randomly assigned to receive placebo, naltrexone (titrated to 50 mg/day), or nalmefene (titrated to 40 mg/day) for seven days before an alcohol challenge clinical laboratory session. During the clinical laboratory session, a drink of alcohol (0.4 mg/kg for men, 0.34 mg/kg for women) was provided in a bar-like setting. The effects of the alcohol dose on subjective craving, stimulation, and sedation were measured before having free access to alcohol.

RESULTS

Alcoholics reported higher levels of craving than social drinkers before and after the drink as well as higher levels of alcohol-induced stimulation. Both opiate antagonist medications suppressed initial increases in craving and stimulation.

CONCLUSIONS

These findings demonstrate that both naltrexone and nalmefene are associated with reduced alcohol-induced craving and stimulation among alcoholics who are not actively attempting to reduce drinking. These data provide insights into potential mechanisms that may underlie opiate antagonists' effects in the context of treatment.

Naltrexone attenuates cue- but not drug-induced methamphetamine seeking: a possible mechanism for the dissociation of primary and secondary reward

The present study was aimed to clarify the role of the opioid system in the reinstatement of methamphetamine (METH)-seeking behavior in METH self-administering rats. Following 12 days of self-administration of METH, the replacement of METH with saline resulted in a gradual decrease in lever press responses (extinction). Under extinction conditions, METH-priming or re-exposure to cues previously paired with METH infusion markedly increased the responses (reinstatement of drug-seeking). Naltrexone administered 30 min before re-exposure to METH-associated cues attenuated reinstatement of drug-seeking behavior. On the other hand, administration of this antagonist had no effect on the reinstatement induced by METH-priming. We discussed these findings in relation with the dissociation of primary and secondary reward, suggesting that an opioid mechanism is responsible for this dissociation. Further, these results indicate the possibility of using naltrexone as an anti-relapse agent.

Pharmacotherapy of alcohol dependence: a review of the clinical data

Over the last 20 years, the role of adjuvant pharmacotherapy in optimising outcome in rehabilitation programmes for alcohol-dependent patients has become increasingly evident. New avenues for rational drug treatment have arisen from better understanding of the neurobiological substrates of alcohol dependence, including adaptive changes in amino acid neurotransmitter systems, stimulation of dopamine and opioid peptide systems, and, possibly, changes in serotonergic activity. Disulfiram, naltrexone and acamprosate are currently the only treatments approved for the management of alcohol dependence. However, there is still no unequivocal evidence from randomised controlled clinical trials that disulfiram improves abstinence rates over the long term. Aversive therapy with disulfiram is not without risk for certain patients, and should be closely supervised. Both naltrexone and acamprosate improve outcome in rehabilitation of alcohol-dependent patients, but seem to act on different aspects of drinking pathology. Naltrexone is thought to decrease relapse to heavy drinking by attenuating the rewarding effects of alcohol. However, data from the naltrexone clinical trial programme are somewhat inconsistent, with several large studies being negative. Acamprosate is believed to maintain abstinence by blocking the negative craving that alcohol-dependent patients experience in the absence of alcohol. The clinical development programme has involved a large number of patients and studies, of which the vast majority have shown a beneficial effect of acamprosate on increasing abstinence rates. Both drugs are generally well tolerated; nausea is reported by around 10% of patients treated with naltrexone, while the most frequent adverse effect reported with acamprosate is diarrhoea. Another opioid receptor antagonist, nalmefene, has shown promising activity in pilot studies, and may have a similar profile to naltrexone. Data from studies of SSRIs in alcohol dependence are somewhat heterogeneous, but it appears that these drugs may indirectly improve outcome by treating underlying depression rather than affecting drinking behaviour per se. Similarly, the anxiolytic buspirone may act by ameliorating underlying psychiatric pathology. Dopaminergic neuroleptics, benzodiazepines and antimanic drugs have not yet demonstrated evidence of activity in large controlled clinical trials. Trials with drugs acting at serotonin receptors have yielded disappointing results, with the possible exception of ondansetron. Because the biological basis of alcohol dependence appears to be multifactorial, the future of management of alcoholism may be combination therapy, using drugs acting on different neuronal pathways, such as acamprosate and naltrexone. Pharmacotherapy should be used in association with appropriate psychosocial support and specific treatment provided for any underlying psychiatric comorbidities.

Opioids and alcoholism

Although far from conclusive, evidence implicating the endogenous opioid system in the development and maintenance of alcoholism is growing. Currently available data suggest that ethanol increases opioid neurotransmission and that this activation is part of the mechanism responsible for its reinforcing effects. Findings from preclinical research indicate that ethanol consumption and ethanol-induced dopamine (DA) release are both reduced by opioid antagonists. Individual differences in endogenous opioid activity have been linked to inherited risks for alcoholism in studies comparing ethanol-preferring and nonpreferring rats, as well as in studies using targeted gene mutation (knockout) strategies. To a large extent, findings from human studies have paralleled those from the preclinical work. Persons who differ in family history of alcoholism have been shown to also differ in basal beta-endorphin activity, beta-endorphin response to alcohol, and subjective and HPA axis hormonal response to opioid antagonists. Findings from clinical trials indicate that opioid antagonists may reduce ethanol consumption in alcoholics, particularly in persons who have resumed drinking. Nevertheless, many questions remain unanswered about the use of opioid antagonists in alcoholism treatment and about the exact role of the opioid system in ethanol preference and reward. The progression of knowledge in this field suggests that many of these questions are imminently answerable, as our ability to characterize relationships between opioid activity and human behavior continues to develop. This paper summarizes both the progress that has been made and the gaps that remain in our understanding of the interactions between the endogenous opioid system and risk for alcoholism.

Effect of the combination of naltrexone and acamprosate on alcohol intake in mice

Both naltrexone and acamprosate have been utilized clinically in recovering alcoholics with varying success. In the experiment reported here the combination of naltrexone and acamprosate was examined in a limited access alcohol model using C57BL/6 mice to determine if there was evidence of additive or synergistic effects. The results of this experiment demonstrate that naltrexone, at the higher dose but not the lower dose, significantly reduced alcohol consumption. When combined with naltrexone, acamprosate reduced alcohol consumption across both doses of naltrexone. This effect was sensitive to both dose and number of days of exposure to the naltrexone/acamprosate combination.

Central opioid receptors differentially regulate the nalmefene-induced suppression of ethanol- and saccharin-reinforced behaviors in alcohol-preferring (P) rats

The exact opioid-sensitive receptors participating in EtOH-seeking behaviors remains unclear. Previous studies have reported higher densities of micro-opioid receptor binding in the nucleus accumbens (NACC) of P relative to NP rats; however, no differences were seen in delta-receptor binding. In contrast to the NACC, substantially lower levels of micro-receptor binding have been observed in the ventral tegmental area (VTA) of both P and NP rats, albeit no line differences have been observed. In the present study, opioid receptors in the NACC, VTA, and hippocampus were evaluated for their capacity to regulate both EtOH- and saccharin-motivated behaviors in the genetically selected alcohol-preferring (P) rat. To accomplish this, nalmefene, an opiate antagonist with preferential binding affinity for the micro-opioid receptor was unilaterally or bilaterally infused during concurrent availability of 1 h daily EtOH (10% v/v) and saccharin (0.025 or 0.050% w/v) solutions. Rats performed under a two-lever fixed ratio (FR) schedule in which four responses on one lever produced the EtOH solution, and four on a second lever produced the saccharin solution. The results demonstrated that when responding maintained by both EtOH and saccharin are matched at basal levels, unilateral (1-60 microg) or bilateral (0.5-10 microg) microinjections of nalmefene into the NACC produced selective dose-dependent reductions on responding maintained by EtOH. Unilateral (40, 60 microg) and bilateral (10 microg) VTA infusions were also observed to selectively reduced EtOH responding; however, greater nalmefene doses were required and the magnitude of suppression on EtOH responding was markedly less compared with the NACC. The greater sensitivity of nalmefene to suppress EtOH responding in the NACC is likely due to the greater number of opioid receptors in the NACC relative to the VTA. Only bilateral infusion of the 40 microg dose in the NACC and VTA suppressed responding maintained by both EtOH and saccharin. In contrast, intrahippocampal infusions dose dependently suppressed EtOH- and saccharin-maintained responding over a range of doses (1-20 microg). The present study provides evidence that nalmefene suppresses EtOH-motivated behaviors via blockade of opioid receptors within the NACC and VTA, and under various dose conditions both reinforcer and neuroanatomical specificity can be observed.

IDRD2 TaqIA polymorphism is associated with urinary homovanillic acid levels in a sample of Spanish male alcoholic patients

The TaqIA1 allele of the dopamine receptor gene D2 (DRD2) has been associated with alcoholism, as well as with other addictive behaviours. The exact nature of how the presence of this allele can be a vulnerability factor in the development of alcoholism remains unclear. In this study we found that the presence in the DRD2 genotype of the TaqIA1 allele in Spanish alcoholics is associated with higher levels of urine homovanillic acid (HVA) when compared to patients homozygous for the TaqIA2 allele. A sample of 142 Spanish male alcoholic patients was split into 2 groups on the basis of the presence or absence of the A1 allele in their genotype. The urine sample was analyzed by high performance liquid cromatography (HPLC), and the concentration of homovanillic acid (HVA), 5-hydroxyindoleacetic acid (5-HIAA) and vanilylmandelic acid (VMA) was determined. We found a statistical difference in the concentration of HVA between the groups, that suggests this polymorphism could be related to the variance of urine HVA levels.

Neurobiologic processes in drug reward and addiction

Neurophysiologic processes underlie the uncontrolled, compulsive behaviors defining the addicted state. These"hard-wired"changes in the brain are considered critical for the transition from casual to addictive drug use. This review of preclinical and clinical (primarily neuroimaging) studies will describe how the delineation between pleasure, reward, and addiction has evolved as our understanding of the biologic mechanisms underlying these processes has progressed. Although the mesolimbic dopaminergic efflux associated with drug reward was previously considered the biologic equivalent of pleasure, dopaminergic activation occurs in the presence of unexpected and novel stimuli (either pleasurable or aversive) and appears to determine the motivational state of wanting or expectation. The persistent release of dopamine during chronic drug use progressively recruits limbic brain regions and the prefrontal cortex, embedding drug cues into the amygdala (through glutaminergic mechanisms) and involving the amygdala, anterior cingulate, orbitofrontal cortex, and dorsolateral prefrontal cortex in the obsessive craving for drugs. The abstinent, addicted brain is subsequently primed to return to drug use when triggered by a single use of drug, contextual drug cues, craving, or stress, with each process defined by a relatively distinct brain region or neural pathway. The compulsive drive toward drug use is complemented by deficits in impulse control and decision making, which are also mediated by the orbitofrontal cortex and anterior cingulate. Within this framework, future targets for pharmacologic treatment are suggested.

Reverse Selection for Differential Response to the Locomotor Stimulant Effects of Ethanol Provides Evidence for Pleiotropic Genetic Influence on Locomotor Response to Other Drugs of Abuse

BACKGROUND

Addictive drugs share the ability to induce euphoria, which may be associated with their potential for abuse. Replicate mouse lines with high (FAST-1, FAST-2) and low (SLOW-1, SLOW-2) sensitivity to ethanol-induced psychomotor stimulation (a possible animal model for the euphoria experienced by humans) have provided evidence for common genetic influences (pleiotropy) on sensitivity to the effects of ethanol and of GABA-A receptor acting compounds on locomotor activity. Differences between FAST and SLOW mice in locomotor response to certain other drugs were found later in selection. Reverse selection produced lines (r-FAST-1, r-FAST-2, r-SLOW-1, r-SLOW-2) with similar locomotor responses to ethanol. These lines are well suited for asking whether the same alleles that influence sensitivity to ethanol are also responsible for these later arising differences in drug sensitivity.

METHODS

Two replicate sets of forward- and reverse-selected FAST and SLOW lines were tested for the effects of multiple doses of morphine, cocaine, methamphetamine, nicotine, and scopolamine on their locomotor behavior. We predicted that differences in drug sensitivity between the FAST and SLOW lines would be reduced or eliminated in the reverse-selected lines.

RESULTS

Differences in sensitivity to morphine, cocaine, methamphetamine, and nicotine that arose in earlier generations of the FAST-1 and SLOW-1 lines ultimately also appeared in the FAST-2 and SLOW-2 lines. However, some differences between the FAST-2 and SLOW-2 lines (those in response to cocaine and methamphetamine) were not seen until several generations after selection had been relaxed. In lines reverse-selected for sensitivity to ethanol, differences in sensitivity to the other drugs were decreased, eliminated, or even reversed. No differences in scopolamine response were found in the replicate 1 forward- or reverse-selected lines. However, a small difference in scopolamine response in the replicate 2 lines was reversed.

CONCLUSIONS

Genes that influence the locomotor response to ethanol also influence locomotor response to other drugs with stimulant effects in the FAST and SLOW mice. The current data most strongly support this conclusion for sensitivity to morphine and nicotine.

Alcohol promotes dopamine release in the human nucleus accumbens

Microdialysis experiments in rodents indicate that ethanol promotes dopamine release predominantly in the nucleus accumbens, a phenomenon that is implicated in the reinforcing effects of drugs of abuse. The aim of the present study was to test the hypothesis in humans that an oral dose of ethanol would lead to dopamine release in the ventral striatum, including the nucleus accumbens. Six healthy subjects underwent two [(11)C]raclopride PET scans following either alcohol (1 ml/kg) in orange juice or orange juice alone. Subjective mood changes, heart rate, and blood-alcohol levels were monitored throughout the procedure. Personality traits were evaluated using the tridimensional personality questionnaire. PET images were co-registered with MRI and transformed into stereotaxic space. Statistical parametric maps of [(11)C]raclopride binding potential change were generated. There was a significant reduction in [(11)C]raclopride binding potential bilaterally in the ventral striatum/nucleus accumbens in the alcohol condition compared to the orange juice condition, indicative of increased extracellular dopamine. Moreover, the magnitude of the change in [(11)C]raclopride binding correlated with the alcohol-induced increase in heart rate, which is thought to be a marker of the psychostimulant effects of the drug, and with the personality dimension of impulsiveness. The present study is the first report that, in humans, alcohol promotes dopamine release in the brain, with a preferential effect in the ventral striatum. These findings support the hypothesis that mesolimbic dopamine activation is a common property of abused substances, possibly mediating their reinforcing effects.

The nociceptin/orphanin FQ/NOP receptor system as a target for treatment of alcohol abuse: a review of recent work in alcohol-preferring rats

The intracerebroventricular administration of the 17 amino acid peptide nociceptin/orphanin FQ (N/OFQ), the endogenous ligand of the NOP receptor (previously referred to as ORL-1 or OP4 receptor), reduces voluntary 10% ethanol intake in genetically selected Marchigian Sardinian alcohol-preferring (msP) rats. Studies aimed at the pharmacological characterization of the receptor, which mediates the effect, have shown that the C-terminal 13 amino acid sequence is crucial for activity and that the selective NOP receptor antagonist [Nphe(1)]N/OFQ(1-13)NH(2) blocks the effect of N/OFQ on ethanol drinking. In place conditioning studies, N/OFQ abolishes the conditioned place preference (CPP) induced by ethanol in msP rats, or by morphine in nonselected Wistar rats; these findings suggest that N/OFQ is able to abolish the rewarding properties of ethanol and morphine. Moreover, N/OFQ inhibits reinstatement of alcohol-seeking behavior induced to electric footshock stress, as well as reinstatement of alcohol-seeking behavior induced by ethanol-paired cues. Together, these findings suggest that N/OFQ and its receptor may represent an interesting target for pharmacological treatment of alcohol abuse.

Current Pharmacotherapies of Alcoholism: A U.S. Perspective

Advances in the neurobiology of addiction and improved clinical trial methodology have accelerated the evaluation of medication for alcoholism. While psychosocial interventions have been useful to reduce consumption and support abstinence, considerable improvement in treatment is needed. Medication can play a crucial role in the reduction of craving and drinking and the maintenance of abstinence. This article reviews pharmacotherapy for alcoholism with an emphasis on the perspective of the United States. The opiate antagonist naltrexone, the glutamate modulator acamprosate, and serotonergic agents will be highlighted in this review. In general, both naltrexone and acamprosate have been found in a number of studies to be efficacious agents for the treatment of alcohol dependence. Serotonin reuptake inhibitors have not consistently shown to be efficacious but may be useful in certain subgroups of alcoholics. The serotonin type-3 antagonist, ondansetron, has shown promise in early-onset alcoholics but needs more extensive study.

Effect of naltrexone on oral consumption of concurrently available ethanol and cocaine in the rat

Comorbid abuse of and dependency on multiple drugs is a common occurrence clinically. We have developed an animal model that provides rats with the opportunity to choose, through oral consumption, between concurrently available ethanol and cocaine with water also available. This provides the ability to screen for the effectiveness of potential pharmacotherapeutic agents on the baseline consumption of both drugs. We used this animal model to evaluate the effects of naltrexone, at doses of 0, 1.0, 3.0, and 10.0 mg/kg, on concurrent oral consumption of ethanol and cocaine solutions. Naltrexone at all doses significantly reduced both consumption of and preference for ethanol. Consumption of both cocaine and water was unaffected by naltrexone, supporting the suggestion that the effects of naltrexone were selective for ethanol. These findings support the suggestion that ethanol and cocaine act on different central reward pathways. The implications of these findings for the clinical use of naltrexone in populations with comorbid ethanol and cocaine abuse are discussed.

Effects of melanocortin receptor ligands on ethanol intake and opioid peptide levels in alcohol-preferring AA rats

Melanocortin (MC) peptides are suggested to play a role in opiate dependence, where they antagonise the addictive properties of opiates. To further study the involvement of the MCs in drug dependence, we analysed the effects of the MC(4)-receptor antagonist HS014 (1 nmol/rat), and the non-selective MC-receptor agonist MTII (1 nmol/rat), using i.c.v. administration, on ethanol intake in alcohol-preferring AA rats. The rats had access to ethanol during 40 days, resulting in a mean ethanol intake of 6.6 g/kg/day, before treatment. One group received only artificial cerebrospinal fluid solution. MTII caused a reduction in ethanol intake and ethanol preference, whereas HS014 was without effect. No effect on water intake was observed. A decrease in food intake was detected after MTII, whereas HS014 induced an increase in food intake. Analysis of dynorphin B and Met-enkephalin-Arg(6)Phe(7) immunoreactive levels revealed that MTII and HS014 altered opioid peptide levels in several brain areas and the pituitary gland of the rats with an established ethanol intake. This is the first report showing that manipulation of the MC-receptor system changes ethanol intake in chronically ethanol-drinking AA rats. In addition, manipulation of the MC system modulates ethanol-induced changes in opioid peptide levels.

Behavioral neurobiology of alcohol addiction: recent advances and challenges

Addictive behavior associated with alcoholism is characterized by compulsive preoccupation with obtaining alcohol, loss of control over consumption, and development of tolerance and dependence, as well as impaired social and occupational functioning. Like other addictive disorders, alcoholism is characterized by chronic vulnerability to relapse after cessation of drinking. To understand the factors that compel some individuals to drink excessively, alcohol research has focused on the identification of brain mechanisms that support the reinforcing actions of alcohol and the progression of changes in neural function induced by chronic ethanol consumption that lead to the development of dependence. More recently, increasing attention has been directed toward the understanding of neurobiological and environmental factors in susceptibility to relapse.

Effects of acute acamprosate and homotaurine on ethanol intake and ethanol-stimulated mesolimbic dopamine release

The purpose of the present study was to determine the acute effects of the anticraving compound acamprosate (calcium acetylhomotaurinate) and the closely related compound homotaurine on ethanol intake and ethanol-stimulated dopamine release in the nucleus accumbens. Male rats were treated with acamprosate (200 or 400 mg/kg intraperitoneally, i.p.) or homotaurine (10, 50, or 100 mg/kg i.p.) 15 min prior to access to 10% ethanol and water for 1 h in a two-bottle choice restricted access paradigm. A separate group of rats was implanted with microdialysis probes in the nucleus accumbens and given an acute injection of ethanol (1.5 g/kg i.p.) that was preceded by saline, acamprosate, or homotaurine. Acamprosate and homotaurine dose-dependently reduced ethanol intake and preference. These compounds also delayed or suppressed ethanol-stimulated increases in nucleus accumbens dopamine release, suggesting that acamprosate and homotaurine may reduce ethanol intake by interfering with the ability of ethanol to activate the mesolimbic dopamine reward system.

Acute ethanol administration differentially modulates mu opioid receptors in the rat meso-accumbens and mesocortical pathways

Biochemical and pharmacological evidence suggest that the dopaminergic mesolimbic system plays a key role in mediating the reinforcing properties of alcohol and other drugs of abuse. Alcohol reinforcement and high alcohol drinking behavior have been postulated to be partially mediated by a neurobiological mechanism involving the alcohol-induced activation of the endogenous opioid system. The aim of this work was to study the effect of the in vivo acute administration of ethanol on mu (mu) opioid receptors in the rat dopaminergic meso-accumbens and mesocortical pathways by quantitative receptor autoradiography. [(3)H]DAMGO binding was significantly decreased in the ventral tegmental area (VTA) 30 min after ethanol administration. A small ethanol-induced reduction was observed in the shell region of the nucleus accumbens 1 h after exposure. In contrast, 2 h after ethanol administration, [(3)H]DAMGO binding was significantly increased in the frontal and prefrontal cortices. The observed changes correlated well with high ethanol plasma levels. Our results suggest that the reinforcing properties of ethanol may be partially mediated by mechanisms involving the ethanol-induced down- and up-regulation of mu receptors in the dopaminergic mesolimbic system. Mu receptors in the VTA and the frontal and prefrontal cortices may be involved in the in vivo acute responses to ethanol and could play a key role in modulating the dopaminergic activity of the mesocortical pathway in response to the drug. In contrast, the contribution of both mu and delta receptors in the nucleus accumbens might be relevant in these processes.

Locomotor-stimulating effects of indirect dopamine agonists are attenuated in Fawn hooded rats independent of postweaning social experience

The effects of the indirect dopamine (DA) agonists cocaine and D-amphetamine on locomotor activity were examined in Fawn hooded (FH) rats and Wistar rats. The effect of isolation rearing was also examined to determine if it might have different effects in these two strains. Contrary to previous findings in other rat strains, only small increases in locomotor-stimulating responses to low doses of cocaine were observed in the present study as a result of isolation rearing. However, at higher cocaine doses, locomotor activity was substantially attenuated in FH rats relative to Wistar rats. A similar pattern of effects was observed for amphetamine in FH rats but only at the intermediate dose. The effects of strain and rearing were independent. There was no evidence for interactions between these factors.

Genetic analysis of the mu-opioid receptor in alcohol-dependent individuals

On the basis of various study results, it is suggested that the ethanol-induced activation of the endogenous opioid system may play an important role in mediating the reinforcing effects of ethanol. The mesolimbic dopamine reward system is activated by both ethanol and opioids, and genetic differences in the sensitivity of the endogenous opioid system to alcohol may be an important factor determining the risk for the development of excessive alcohol consumption. Thus, variants of the mu-opioid receptor (muOR) gene may confer vulnerability to alcohol dependence. Five exon 1 variants of the muOR were investigated in 327 alcohol-dependent and 340 healthy control subjects. The Val6 variant of the +17C/T polymorphism and the Asp40 variant of the +118A/G polymorphism showed a trend to an increased allele frequency in alcohol-dependent subjects. The latter polymorphism was investigated in more detail. The dopamine receptor agonist apomorphine causes an increase in growth hormone (GH) levels in the blood by stimulating the release of growth hormone-releasing hormone. beta-endorphin also activates this regulatory circuit. We found a blunted response in intoxicated alcohol-dependent subjects, but no difference in GH response between the groups of alcohol-dependent subjects with and without the variant Asp allele. However, alcohol-dependent subjects with the Asp allele showed a significantly higher GH response at day 7 after alcohol withdrawal and a tendency to lower novelty seeking. These results suggest to us that there is reduced dopaminergic neuronal activity in alcohol-dependent subjects with the muOR Asp40 allele, along with a compensating increase in dopamine receptor activity. The difference between the two groups of alcohol-dependent subjects can be demonstrated only under certain conditions such as alcohol withdrawal, which necessitates the adaptation of the neurones to a new homeostasis.

The selective kappa-opioid receptor agonist U50,488H attenuates voluntary ethanol intake in the rat

Non-selective opioid receptor antagonists are increasingly used in the treatment of alcohol dependence. The clinical effects are significant but the effect size is rather small and unpleasant side effects may limit the benefits of the compounds. Ligands acting at mu- and/or delta- receptors can alter the voluntary intake of ethanol in various animal models. Therefore, the attenuating effects of selective opioid receptor ligands on ethanol intake may be of clinical interest in the treatment of alcoholism. The objective of this study was to examine the effects of a selective kappa-receptor agonist, U50,488H on voluntary ethanol intake in the rat. We used a restricted access model with a free choice between an ethanol solution (10% v/v) and water. During the 3-days baseline period, the rats received a daily saline injection (1 ml/kg, i.p.) 15 min before the 2 h access to ethanol. The animals had free access to water at all times. The control group received a daily saline injection during the 4-days treatment-period, whereas the treatment groups received a daily dose of U50,488H (2.5, 5.0 or 10 mg/kg per day). Animals treated with U50,488H dose-dependently decreased their ethanol intake. The effect of the highest dose of U50,488H was reduced by pre-treatment with the selective kappa-antagonist nor-binaltorphimine (nor-BNI). These results demonstrate that activation of kappa-opioid receptors can attenuate voluntary ethanol intake in the rat, and the data suggest that the brain dynorphin/kappa-receptor systems may represent a novel target for pharmacotherapy in the treatment of alcohol dependence.

Effect of acamprosate and naltrexone, alone or in combination, on ethanol consumption

Both acamprosate and naltrexone have demonstrated clinical utility in reducing relapse to alcohol use in recovering alcoholics. The present experiments examined the effects of acamprosate and naltrexone, either alone or in combination, on basal ethanol consumption in a limited-access model with the use of outbred Wistar rats. Naltrexone, 0.1 mg/kg, significantly reduced ethanol consumption as previously reported. Acamprosate, 50 mg/kg, did not significantly reduce ethanol consumption when administered alone and provided no evidence of additive or synergistic effects when combined with naltrexone. Acamprosate, 200 mg/kg, produced a modest reduction in ethanol consumption when administered alone but no evidence of additive or synergistic effects when combined with naltrexone. From these findings, it is suggested that a combination approach of these drugs may not be any more effective than monotherapy.

Suppression of ethanol responding by centrally administered CTOP and naltrindole in AA and Wistar rats

BACKGROUND

Both mu- and delta-opioid receptors have been implicated in the reinforcing actions of ethanol. However, selective opioid receptor antagonists have not altered ethanol intake in all rodent strains consistently, which suggests that genotype may modulate their suppressive effects. Therefore, we tested the effects of the selective mu-antagonist D-Pen-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2 (CTOP) and the selective delta-antagonist naltrindole in both high-drinking AA (Alko, Alcohol) and heterogeneous Wistar rats.

METHODS

AA and Wistar rats were trained to respond for ethanol (10% w/v) in a two-lever operant condition by using a saccharin fading procedure. After stable baseline responding was established, rats were implanted stereotaxically either with a guide cannula above the lateral ventricle or with bilateral cannulas above the nucleus accumbens, basolateral amygdala, or ventral tegmental area. After postoperative recovery, AA and Wistar animals were tested after intracerebroventricular microinjections of either CTOP (0-3 microg) or naltrindole (0-30 microg) or subcutaneous injections of naloxone (0-1 g/kg), which was used as a reference antagonist. Effects of intracerebral microinjections of CTOP and naltrindole (both 0-500 ng) were tested only in Wistar rats.

RESULTS

Subcutaneous naloxone and intracerebroventricular CTOP and naltrindole suppressed ethanol self-administration in a similar manner in AA and Wistar rats. Cumulative response patterns indicated that naloxone and naltrindole had no effect on the initiation of responding but suppressed it later during the session, whereas CTOP also affected initiation. In Wistar rats, naltrindole microinjections into both the nucleus accumbens and basolateral amygdala decreased ethanol responding, whereas CTOP was effective only in the amygdala. Injections of these antagonists into the ventral tegmental area had little effect on ethanol intake.

CONCLUSIONS

The results confirm previous results which showed that both mu- and delta-opioid receptors are involved in the regulation of ethanol self-administration and indicate that genetic differences between AA and Wistar rats produced by selection do not modify the effects of opioid antagonists. The nucleus accumbens and the basolateral amygdala may be important central sites for the mediation of their suppressive effects.

Acute ethanol administration induces changes in TRH and proenkephalin expression in hypothalamic and limbic regions of rat brain

Thyrotropin releasing hormone (TRH) present in several brain areas has been proposed as a neuromodulator. Its administration produces opposite effects to those observed with acute ethanol consumption. Opioid peptides, in contrast, have been proposed to mediate some of the effects of alcohol intoxication. We measured TRH content and the levels of its mRNA in hypothalamic and limbic zones 1-24 h after acute ethanol injection. We report here fast and transient changes in the content of TRH and its mRNA in these areas. The levels of proenkephalin mRNA varied differently from those of proTRH mRNA, depending on the time and region studied. Wistar rats were administered one dose of ethanol (intraperitoneal, 3 g/kg body weight) and brains dissected in hypothalamus, hippocampus, amygdala, n. accumbens and frontal cortex, for TRH quantification by radioimmunoassay or for proTRH mRNA measurement by RT-PCR. After 1 h injection, TRH levels were increased in hippocampus and decreased in n. accumbens; after 4 h, it decreased in the hypothalamus, frontal cortex and amygdala, recovering to control values in all regions at 24 h. ProTRH mRNA levels increased at 1 h post-injection in total hypothalamus and hippocampus, while they decreased in the frontal cortex. The effect of ethanol was also studied in primary culture of hypothalamic cells; a fast and transient increase in proTRH mRNA was observed at 1 h of incubation (0.001% final ethanol concentration). Changes in the mRNA levels of proTRH and proenkephalin were quantified by in situ hybridization in rats administered ethanol intragastrically (2.5 g/kg). Opposite alterations were observed for these two mRNAs in hippocampus and frontal cortex, while in n. accumbens and the paraventricular nucleus of the hypothalamus, both mRNA levels were increased but with different kinetics. These results give support for TRH and enkephalin neurons as targets of ethanol and, as possible mediators of some of its observed behavioral effects.

Effects of naltrexone on the accumulation of L-3, 4-dihydroxyphenylalanine and 5-hydroxy-L-tryptophan and on the firing rate induced by acute ethanol administration

In order to characterize the effects of naltrexone, a mu-opioid receptor antagonist, on acute ethanol-induced functional modification of dopaminergic neurons in the nigrastriatal and mesolimbic dopamine systems, the accumulation of L-3, 4-dihydroxyphenylalanine (L-DOPA) in the cerebral cortex, dorsal striatum and nucleus accumbens and of 5-hydroxy-L-tryptophan (5-HTP) in the hippocampus was measured in normal rats using the mu-hydroxybenzylhydrazine dihydrochloride (NSD-1015) enzymatic inhibition method. In addition, the firing rates of dopaminergic neurons were recorded in the substantia nigra and ventral tegmental area. Naltrexone resulted in a decrease in the dopaminergic neuronal firing rates activated by ethanol and eventually in a reduction of the dopamine synthesis induced by ethanol in the dorsal striatum and nucleus accumbens, but not in the cerebral cortex. Mesolimbic dopamine neurons were slightly more sensitive to ethanol and naltrexone than were nigrostriatal dopamine neurons. The widespread inhibitory action of naltrexone also decreased the ethanol-induced stimulation of hippocampal serotonin synthesis.

Basal levels and alcohol-induced changes in nociceptin/orphanin FQ, dynorphin, and enkephalin levels in C57BL/6J mice

In order to investigate the involvement of the opioid and nociceptin/orphanin FQ (N/OFQ) system in alcohol drinking behaviour, N/OFQ and the opioid peptides dynorphin B (DYNB) and Met-enkephalin-Arg(6) Phe(7) (MEAP) were examined in the alcohol-preferring C57BL/6J mice. Basal peptide levels were compared in the brain and the pituitary gland with basal levels in the alcohol-avoiding DBA/2J mice. Furthermore, the effects of chronic alcohol self-administration on peptides were studied in the C57BL/6J mice. Compared to the DBA/2J mice, C57BL/6J mice had low immunoreactive (ir) levels of DYNB and MEAP in the nucleus accumbens, the hippocampus, and the substantia nigra, low ir-DYNB levels in the striatum and low ir-MEAP levels in the frontal cortex. Higher ir-DYNB levels in the pituitary gland and in the periaqueductal gray (PAG) and higher ir-N/OFQ levels in the frontal cortex and in the hippocampus were detected in C57BL/6J mice compared to the DBA/2J mice. After 4 weeks of voluntary alcohol consumption, only minor changes in steady-state peptide levels were identified. However, 5 days after the alcohol-drinking period, lower levels of all peptides were detected in the ventral tegmental area and ir-DYNB levels were also lower in the amygdala and in the substantia nigra. Twenty-one days after cessation of alcohol self-administration, the opioid peptides in alcohol-consuming C57BL/6J mice were lower in the PAG, the N/OFQ was lower in the frontal cortex and DYNB was higher in the amygdala and substantia nigra as compared to control C57BL/6J mice. This study demonstrates strain differences between C57BL/6J mice and DBA/2J mice that could contribute to divergent drug-taking behaviour, and it also demonstrates time- and structure-specific changes in neuropeptide levels after alcohol self-administration in the C57BL/6J mice.

Ethanol enhances Nicotine's effects on DRL performance in rats

The present experiment examined effects of nicotine (0.0, 0.3, 0.56, and 1.0 mg/kg; IP) and ethanol (0.0, 0.5, 1.5, and 3.0 g/kg; IG) on operant behavior using a differential reinforcement of low response rate (DRL) schedule in rats. DRL schedules are sensitive to effects of nicotine and provide an assessment of the subject's ability to accurately estimate time and to inhibit schedule-controlled responding. When administered alone, nicotine shifted the mode of the interresponse time distribution to the left and reduced the percentage of reinforced responses. Nicotine also had an inverted U-shaped dose effect on the number of "bursting" responses. When administered after pretreatment with ethanol, nicotine's effects on the distribution of interresponse times and bursting were potentiated. These effects are consistent with previous reports and with the suggestion that ethanol pretreatment can potentiate effects of subsequently administered nicotine. Published by Elsevier Science Inc.

Combination pharmacotherapy in alcoholism: a novel treatment approach

Combination pharmacotherapy has proven effective in a number of psychiatric disorders, including depression and schizophrenia. However, compared with other affective disorders, few studies have explored the use of combination therapy in alcoholism, and the majority have been limited to animal models. There is evidence to support a role for combination therapy in alcoholism. For example, several neurochemical systems, including the dopaminergic, serotonergic, and opioidergic, appear to affect alcohol intake. Studies in several different types of alcohol-preferring rats have suggested that coadministration of agents to target more than one of these systems simultaneously may produce beneficial effects on alcohol intake, while avoiding problematic effects, such as alterations in food or water intake. Data from preliminary clinical studies have shown trends toward combination therapy reducing alcohol intake in humans. While such findings are encouraging, they must be explored further in larger, randomized, double-blind trials.

Opioid antagonists and alcoholism treatment

The increasing evidence for a neurobiologic basis for alcoholism has spurred the search for pharmacologic agents to treat alcohol abuse. The complex set of symptoms and behaviors that characterizes alcoholism has been linked to dopaminergic and opioid neurotransmitter systems, suggesting that opioid antagonists, such as naltrexone, may alter the positive reinforcement effects and craving involved in alcoholism. Laboratory and clinical investigations of naltrexone have demonstrated the potential for this agent to reduce craving, increase the aversive effects of alcohol, decrease drinking days, and increase abstinence. While naltrexone and other opioid antagonists, such as nalmefene, may be effective components of an alcohol-treatment program, they should only be used in combination with psychosocial interventions, such as support groups and psychotherapy.

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.

Mmu and D2 receptor antisense oligonucleotides injected in nucleus accumbens suppress high alcohol intake in genetic drinking HEP rats

Numerous pharmacological and other studies have implicated both Mmu and dopamine receptor subtypes in alcohol consumption. In the genetic drinking rat as well as those chemically induced to drink, evidence has accrued that the abnormal intake of alcohol is underpined by these receptors in the brain. The purpose of this investigation was to demonstrate unequivocally that a biological impairment by antisense oligodeoxynucleotide (ODN) targeted specifically to these two receptor subtypes would disrupt ongoing alcohol drinking. In this project, a new strain of female and male high-ethanol preferring (HEP) rats was used that had free access to preferred concentrations of alcohol over water in a two choice paradigm. A guide cannula for a microinjection needle was first implanted bilaterally above the nucleus accumbens (NAC) of each rat. Following recovery, a dose of either 250 or 500 ng of the Mmu ODN or 500 ng D2ODN was microinjected into the NAC of the rat in a volume of 0.8-1.0 microl. A standard temporal sequence was used in which microinjections were given four times at successive 12-h intervals over a 2-day interval. The control mismatch ODNs corresponding to both the Mmu or D2 receptor antisense were microinjected identically at homologous sites in the NAC. Following the experiments, the brain of each rat was removed and sectioned in the coronal plane for histological analysis so that each microinjection site was identified. The results showed that the Mmu receptor antisense caused a significant dose dependent fall in free access alcohol drinking within 12 to 24 h following the initial microinjection. This decline often persisted for 1 to 2 days in terms of both g/kg intake and proportion of alcohol to water consumed. Similarly, the D2 receptor ODN likewise induced an intense and significant decline in both g/kg and proportion measures of alcohol intake. Since the corresponding mismatch ODN for both Mmu and D2 receptors exerted no effect on either of these measures of alcohol consumption, the specificity of molecular action of the respective antisense molecules on drinking behavior of the HEP rats was confirmed. Thus, these results provide the first unequivocal evidence that the genes for D2 and Mmu receptors are fundamentally involved in abnormal alcohol drinking in the genetically predisposed individual. Finally, important new anatomical evidence is introduced for the critical role of the NAC in the genetic basis of aberrant drinking of alcohol.

Tetrahydropapaveroline injected in the ventral tegmental area shifts dopamine efflux differentially in the shell and core of nucleus accumbens in high-ethanol-preferring (HEP) rats

Since the 1970s tetrahydropapaveroline (THP) and other tetrahydroisoquinoline alkaloids have been implicated in the etiology of alcoholism. When injected into the cerebral ventricle or at specific sites in the mesolimbic system such as the ventral tegmental area (VTA), THP evokes spontaneous and intense intake of alcohol in the nondrinking animal. Further, THP evokes the extracellular efflux of dopamine in the nucleus accumbens (NAC), which comprises, in part, the postulated alcohol drinking "circuit" of neurons. The purpose of this study was to characterize the action of a THP reactive structure, the VTA, on the activity of dopamine and its metabolism in the NAC. In the anesthetized high-ethanol-preferring (HEP) rat, artificial CSF was perfused for 10 min at a rate of 10 microl per min specifically in either the core or shell of the NAC. A microbore push-pull cannula system was selected over a microdialysis probe because of its superior recovery of neurotransmitters and tip exposure of less than 1.0 mm. After a series of 5-min perfusions, a single microinjection of 5.0 microg/microl of THP was made in the ipsilateral VTA while the NAC was perfused simultaneously. Sequential samples of the NAC perfusate were assayed by an HPLC coulometric system to quantitate the concentrations of dopamine and its metabolites, DOPAC and HVA, as well as the 5-HT metabolite, 5-HIAA. The results showed that THP injected in the VTA caused a significant increase by 94 +/- 23% in the efflux of dopamine from the core of the NAC. Conversely, the THP injected identically in the VTA suppressed the efflux of dopamine within the shell of the NAC by 51 +/- 10%. The levels of DOPAC, HVA and 5-HIAA within the core and shell of the NAC generally paralleled the increase and decrease in efflux, respectively, of dopamine. CSF control injections in the VTA as well as injections outside of the VTA failed to alter dopamine or metabolite activity in the NAC. These results demonstrate that the presence of THP in the VTA alters directly the function of the pathway of mesolimbic neurons generally and the dopaminergic system specifically. That such a perturbation could account for the induction of alcohol preference is proposed in relation to a reinforcing mechanism involving opioidergic and dopaminergic elements.

Left and right 6-hydroxydopamine lesions of the medial prefrontal cortex differentially affect voluntary ethanol consumption

Dopaminergic projections to the medial prefrontal cortex (mPFC) were unilaterally lesioned with 6-hydroxydopamine (6-OHDA) to examine how dopamine (DA) asymmetry in the mPFC influences voluntary ethanol consumption. Differences in nucleus accumbens (NAS) DA neurotransmission have been related to individual differences in locomotor activity and in the rewarding efficacy of ethanol. Therefore, differences in locomotor activity were used to further characterize the effects of unilateral mPFC 6-OHDA lesions on ethanol consumption. Male Long Evans rats were assessed for high versus low levels of spontaneous locomotor activity. DA terminals in the left or right mPFC were unilaterally lesioned with 6-OHDA, resulting in an average DA depletion of 54% and 50%, respectively. After a minimum seven-day recovery period, preference for a 10% ethanol solution vs. water was determined in a 24-h 2-bottle home-cage free-choice paradigm. Left mPFC 6-OHDA lesions increased and right lesions decreased ethanol consumption. These differential effects of left and right lesions were primarily attributable to rats exhibiting low locomotor activity prior to surgery. The present data suggest that right greater than left cortical DA asymmetry in combination with low endogenous NAS DA (predicted by low locomotor activity levels) may increase the vulnerability to abuse ethanol.

Ethanol induced differences in medial prefrontal cortex dopamine asymmetry and in nucleus accumbens dopamine metabolism in left- and right-turning rats

Ethanol (0.5 g/kg i.p.) 15 min prior to sacrifice increased homovanillic acid (HVA) levels in the left medial prefrontal cortex (mPFC) of left-turning rats and in the right mPFC of right-turning rats. In the nucleus accumbens (NAS), ethanol decreased dopamine (DA), 3,4-dihydroxyphenylacetic acid (DOPAC), and HVA levels in rats that exhibited low levels of locomotor activity but not in rats that exhibited high levels of locomotor activity. This laboratory has previously shown that rats exhibiting differences in turning and locomotor activity behavior display different preferences for ethanol. The present results suggest that ethanol-induced differences in mPFC and NAS DA activity may be related to individual differences in the susceptibility to abuse ethanol.

Suppression of ethanol-reinforced behavior by naltrexone is associated with attenuation of the ethanol-induced increase in dialysate dopamine levels in the nucleus accumbens

The opiate antagonist naltrexone suppresses ethanol-reinforced behavior in animals and decreases ethanol intake in humans. However, the mechanisms underlying these actions are not well understood. Experiments were designed to test the hypothesis that naltrexone attenuates the rewarding properties of ethanol by interfering with ethanol-induced stimulation of dopamine activity in the nucleus accumbens (NAcc). Simultaneous measures of the effects of naltrexone on dialysate dopamine levels in the NAcc and on operant responding for oral ethanol were used. Male Wistar rats were trained to self-administer ethanol (10-15%, w/v) in 0.2% (w/v) saccharin during daily 30 min sessions and were surgically prepared for intracranial microdialysis. Experiments began after reliable self-administration was established. Rats were injected with naltrexone (0.25 mg/kg, s.c.) or saline and 10 min later were placed inside the operant chamber for a 20 min waiting period with no ethanol available, followed by 30 min of access to ethanol. A transient rise in dialysate dopamine levels was observed during the waiting period, and this effect was not altered by naltrexone. Ethanol self-administration reliably increased dopamine levels in controls. Naltrexone significantly suppressed ethanol self-administration and prevented ethanol-induced increases in dialysate dopamine levels. Subsequent dose-effect analyses established that the latter effect was not merely a function of reduced ethanol intake but that naltrexone attenuated the efficacy of ethanol to elevate dialysate dopamine levels. These results suggest that suppression of ethanol self-administration by opiate antagonists is the result of interference with dopamine-dependent aspects of ethanol reinforcement, although possible additional effects via nondopaminergic mechanisms cannot be eliminated as a factor in opiate antagonist-induced reduction of ethanol intake.

The effects of the naltrexone implant on rodent social interactions and cocaine-induced conditioned place preference

Two experiments were conducted to determine the behavioral properties of the naltrexone implant on: 1) rodent social interactions; and 2) the appetitive properties of cocaine. Rats were surgically implanted with a naltrexone implant (placebo, 10 or 30 mg) and placed into an open field for the recording of social interactions. The naltrexone implants increased latency to initiate contact and decreased pinning, bouts of grooming, and crawl unders on all 7 days. Other rats were surgically implanted with naltrexone (60, 120, or 240 mg) and habituated to a two-chambered conditioned place preference apparatus. After 6 days of conditioning, place preference was computer recorded. Cocaine produced a dose-dependent conditioned place preference in the rats implanted with placebo or 60 mg of naltrexone. The 120 and 240 mg naltrexone implants blocked the emergence of cocaine-induced place preference. The results indicate that naltrexone implants produce significant social behavioral effects within 1 day, and are effective at attenuating the conditioned place preference produced by cocaine.