Interactions of ethanol with nicotine, dizocilpine, CGP 40116, and 1-(m-chlorophenyl)-biguanide in rats

Nicotine causes opposite effects on alcohol intake: Evidence in an animal experimental model of abstinence and relapse from alcohol

INTRODUCTION

Tobacco and alcohol are frequently co-abused, but the mechanism underlying this interaction is not well understood. Experimental data on the influence of nicotine upon alcohol consumption are not conclusive.

METHODS

To elucidate the role of nicotine in alcohol consumption, alcohol-experienced rats were submitted to consecutive phases of forced abstinence from alcohol, followed by relapses, in which their alcohol consumption was measured in a 2-bottle choice test. Rats were assigned to one of 4 groups: (a) "Control," which received daily saline injections during both the abstinence and relapse phases, (b) "Nic. All," which received nicotine injections during both phases, (c) "Nic. Abst.," which received nicotine during the abstinence phase only, and (d) "Nic. Rel.," which received nicotine during the relapse phase only. The nicotine doses (0.4, 0.6, and 0.8 mg/kg) were administered in an escalating fashion. Alcohol consumption was measured 3 times per day.

RESULTS

Overall, the rats treated daily with nicotine during both the abstinence and relapse phases (Nic. All) significantly increased their alcohol intake compared with the rats treated daily with vehicle alone (Control). Similarly, rats treated with nicotine during the alcohol abstinence phase only (Nic. Abst.) also increased their alcohol consumption. However, rats treated with nicotine during the alcohol relapse phase only (Nic. Rel.) decreased their alcohol intake. In addition, a more exhaustive analysis showed critical differences in patterns of alcohol consumption during the first hour and the first day of alcohol access.

DISCUSSION

Taken together, we provide evidence that depending on the timing of exposure, the same dose of nicotine can have opposite effects on alcohol consumption.

The role of 5-HT3 receptors in drug abuse and as a target for pharmacotherapy

Alcohol and drug abuse continue to be a major public health problem in the United States and other industrialized nations. Extensive preclinical research indicates the mesolimbic dopamine (DA) pathway and associated regions mediate the rewarding and reinforcing effects of drugs of abuse and natural rewards, such as food and sex. The serotonergic (5-HT) system, in concert with others neurotransmitter systems, plays a key role in modulating neuronal systems within the mesolimbic pathway. A substantial portion of this modulation is mediated by activity at the 5-HT3 receptor. The 5-HT3 receptor is unique among the 5-HT receptors in that it directly gates an ion channel inducing rapid depolarization that, in turn, causes the release of neurotransmitters and/or peptides. Preclinical findings indicate that antagonism of the 5-HT3 receptor in the ventral tegmental area, nucleus accumbens or amygdala reduces alcohol self-administration and/or alcohol-associated effects. Less is known about the effects of 5-HT3 receptor activity on the self-administration of other drugs of abuse or their associated effects. Clinical findings parallel the preclinical findings such that antagonism of the 5-HT3 receptor reduces alcohol consumption and some of its subjective effects. This review provides an overview of the structure, function, and pharmacology of 5-HT3 receptors, the role of these receptors in regulating DA neurotransmission in mesolimbic brain areas, and discusses data from animal and human studies implicating 5-HT3 receptors as targets for the development of new pharmacological agents to treat addictions.

Glutamatergic substrates of drug addiction and alcoholism

The past two decades have witnessed a dramatic accumulation of evidence indicating that the excitatory amino acid glutamate plays an important role in drug addiction and alcoholism. The purpose of this review is to summarize findings on glutamatergic substrates of addiction, surveying data from both human and animal studies. The effects of various drugs of abuse on glutamatergic neurotransmission are discussed, as are the effects of pharmacological or genetic manipulation of various components of glutamate transmission on drug reinforcement, conditioned reward, extinction, and relapse-like behavior. In addition, glutamatergic agents that are currently in use or are undergoing testing in clinical trials for the treatment of addiction are discussed, including acamprosate, N-acetylcysteine, modafinil, topiramate, lamotrigine, gabapentin and memantine. All drugs of abuse appear to modulate glutamatergic transmission, albeit by different mechanisms, and this modulation of glutamate transmission is believed to result in long-lasting neuroplastic changes in the brain that may contribute to the perseveration of drug-seeking behavior and drug-associated memories. In general, attenuation of glutamatergic transmission reduces drug reward, reinforcement, and relapse-like behavior. On the other hand, potentiation of glutamatergic transmission appears to facilitate the extinction of drug-seeking behavior. However, attempts at identifying genetic polymorphisms in components of glutamate transmission in humans have yielded only a limited number of candidate genes that may serve as risk factors for the development of addiction. Nonetheless, manipulation of glutamatergic neurotransmission appears to be a promising avenue of research in developing improved therapeutic agents for the treatment of drug addiction and alcoholism.

Understanding how the brain perceives alcohol: neurobiological basis of ethanol discrimination

Understanding the neurobiological mechanisms that regulate how the brain perceives the intoxicating effects of alcohol is highly relevant to understanding the development and maintenance of alcohol addiction. The basis for the subjective effects of intoxication can be studied in drug discrimination procedures in which animals are trained to differentiate the presence of internal stimulus effects of a given dose of ethanol (EtOH) from its absence. Research on the discriminative stimulus effects of psychoactive drugs has shown that these effects are mediated by specific receptor systems. In the case of alcohol, action mediated through ionotropic glutamate, gamma-aminobutyric acid, and serotonergic receptors concurrently produce complex, or multiple, basis for the discriminative stimulus effects of EtOH. These receptor systems may contribute differentially to the discriminative stimulus effects of EtOH based on the EtOH dose, species differences, physiological states, and genetic composition of the individual. An understanding of the receptor mechanisms that mediate the discriminative stimulus effects of EtOH can be used to develop medications aimed at decreasing the subjective effects associated with repeated intoxication. The goal of this symposium was to present an overview of recent findings that highlight the neurobiological mechanisms of EtOH's subjective effects and to suggest the relevance of these discoveries to both basic and clinical alcohol research.

Nicotine-induced conditioned taste aversion in the rat: Effects of ethanol

It has been shown that small doses of ethanol antagonise the discriminative stimulus properties of nicotine in the rat. The aim of the present study was to evaluate whether ethanol could antagonise the aversive stimulus effects of nicotine. Wistar rats were trained to associate nicotine injections with a novel tasting fluid (0.1% saccharin) in the conditioned taste aversion procedure. Nicotine (0.3 mg/kg, s.c.) was injected 5 min after the end of a 20-min exposure to the saccharin solution. Ethanol (0.25-0.5 g/kg, i.p.) was administered 5 or 50 min before nicotine. In general, ethanol did not inhibit nicotine-induced conditioned taste aversion. Contrary to the findings in drug discrimination studies, a slight but significant enhancement of nicotine-induced taste aversion conditioning was observed after ethanol pre-treatment. Blood ethanol levels were measured in a separate group of rats. Maximal blood ethanol levels after i.p. administration of 0.25 or 0.5 g/kg ethanol exceeded 20 and 80 mg%, respectively. Concluding, the present results may indicate that ethanol does not attenuate nicotine-induced conditioned taste aversion in the rat.

Extended blockade of the discriminative stimulus effects of nicotine with low doses of ethanol

The aim of the present study was to further evaluate effects of ethanol on nicotine discrimination and to correlate these effects with blood ethanol levels. Rats were trained to discriminate 0.3 mg/kg nicotine from its vehicle in the standard two-lever operant procedure. In antagonism tests, small doses of ethanol (0.25-0.5 g/kg) were injected either 5 or 50 min before nicotine. Both doses of ethanol partially antagonized the nicotine cue regardless of the pre-treatment time. Ethanol attenuated also inhibitory effects of nicotine on the rate of responding. Suppression of the cueing effects of nicotine was noted even 60 min after the injection of 0.25 g/kg ethanol, i.e. at the time point when the blood ethanol level was close to zero. Ethanol-induced antagonism of the nicotine cue disappeared when longer time (110 min) was allowed to elapse between the ethanol (0.5 g/kg) and nicotine injection. Concluding, the present results may indicate that the effects of ethanol on nicotine discrimination are not primarily related to blood ethanol levels.

Effect of 5-HT3 receptor over-expression on the discriminative stimulus effects of ethanol

BACKGROUND

Drug discrimination studies using selective antagonists and agonists have suggested that 5-HT3 receptors may modulate ethanol's discriminative stimulus effects. However, conflicting data between laboratories leaves the issue of 5-HT3 receptor involvement in ethanol's discriminative stimulus effects in question. The present study utilized transgenic mice that over-express 5-HT3 receptors in conjunction with traditional pharmacological techniques to examine the contribution of 5-HT3 receptors to ethanol's discriminative stimulus.

METHODS

Ten 5-HT3 over-expressing (5-HT3 OE) and 18 B6SJL wild-type (WT) mice were trained to discriminate 1.5 g/kg ethanol from saline in daily 15 min, milk reinforced operant sessions. After training, ethanol substitution and response-rate suppression dose response curves were determined for ethanol, midazolam, dizocilpine, cocaine, mCPP, MD-354, YC-30 and MDL-72222. Antagonism tests combining ethanol with MDL-72222 and ondansetron were also conducted.

RESULTS

The 5-HT3 OE and WT mice learned the ethanol discrimination in a comparable number of training sessions. Similar patterns of substitution were generated in both groups of mice for most test drugs. 5-HT3 OE mice were more sensitive to the rate suppressing effects of dizocilpine and MDL-72222 than were WT mice. Neither of the 5-HT3 antagonist tested significantly attenuated ethanol's discriminative stimulus effects in either 5-HT3 OE or WT mice.

CONCLUSIONS

The results of the present study are consistent with a minimal role of 5-HT3 receptors in transducing ethanol's discriminative stimulus effects. Over-expression of 5-HT3 receptors does not alter the relative efficacy of GABAA positive modulators or NMDA antagonists for producing ethanol-like discriminative stimulus effects. However, 5-HT3 receptor over-expression does appear to modulate the response-rate altering effects of the uncompetitive NMDA antagonist, dizocilpine, and the 5-HT3 antagonist, MDL-72222.

Effects of a novel uncompetitive NMDA receptor antagonist, MRZ 2/579 on ethanol self-administration and ethanol withdrawal seizures in the rat

It has been repeatedly reported that NMDA receptors may contribute to ethanol-induced discriminative stimulus effects and withdrawal syndrome. However, the role of NMDA receptors in the reinforcing properties of ethanol remains unclear. The aim of the present study was to evaluate effects of the novel low-affinity, uncompetitive NMDA receptor antagonist, 1-amino-1,3,3,5,5-pentamethyl-cyclohexane hydrochloride (MRZ 2/579), on ethanol self-administration and ethanol withdrawal-associated seizures in rats. Both an operant (lever pressing for ethanol) and non-operant two-bottle choice setups were employed to initiate ethanol self-administration. In another procedure, forced treatment with high doses (9--15 g/kg/day) was used to induce physical dependence on ethanol. MRZ 2/579 delivered chronically by osmotic minipumps (9.6 mg/day, s.c.) did not alter either operant or non-operant ethanol drinking behaviour in a maintenance phase of ethanol self-administration. In contrast, repeated daily injections of the drug (5 mg/kg, i.p.) led to a progressive decrease in operant responding for ethanol. MRZ 2/579 (0.5--7.5 mg/kg, i.p.) and another low-affinity NMDA receptor antagonist, memantine (1--10 mg/kg, i.p.) dose-dependently suppressed ethanol withdrawal seizures with efficacies comparable with that of a standard benzodiazepine derivative, diazepam. The results of the present study indicate that: (i) intermittent administration of MRZ 2/579 may lead to a gradual decrease of operant responding for ethanol; and (ii) the group of low-affinity uncompetitive NMDA receptor antagonists may be an interesting alternative to benzodiazepines in the treatment of alcohol withdrawal.

Effects of N-methyl-D-aspartate receptor antagonists on reinforced and nonreinforced responding for ethanol in rats

Results of several recent studies indicate that the discriminative stimulus effects of ethanol are related, at least partially, to ethanol-induced decrease in the N-Methyl-D-aspartate (NMDA) receptor function. The role of NMDA receptors in ethanol reinforcement remains still unclear. The aim of the present study was to evaluate the effects of two novel NMDA receptor antagonists in rats lever pressing for 8% ethanol in the oral self-administration procedure. In addition, the effects of the drugs on intensity of nonreinforced responding for ethanol (i.e., "experimental craving") were examined in the extinction procedure. To assess selectivity of the drugs' actions the same range of doses was tested in rats lever pressing for water (control experiments). A low-affinity, uncompetitive NMDA receptor antagonist, MRZ 2/579 (2.5-7.5 mg/kg) selectively and dose-dependently decreased ethanol self-administration. This compound exerted also selective effects on nonreinforced responding for ethanol with lower dose (2.5 mg/kg) increasing and higher dose (5 mg/kg) suppressing operant behavior in the extinction procedure. MRZ 2/579 (5 mg/kg) did not alter open field activity when given in combination with either saline or ethanol (0.5-1 g/kg). In contrast, a glycineB site antagonist, MRZ 2/576 (2.5-7.5 mg/kg) did not produce any selective effects on either reinforced or nonreinforced lever pressing for ethanol. The present results suggest that MRZ 2/579 may selectively suppress both ethanol self-administration and experimental ethanol craving.