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

Long-lasting effect of NMDA receptor antagonist memantine on ethanol-cue association and relapse

It is well known that the glutamatergic system plays a crucial role in alcohol addiction and especially in relapse-like behaviour. However, results of clinical studies on compounds that influence the activity of the glutamatergic system have been disappointing so far. The aim of our study was to establish treatment conditions under which the N-methyl-d-aspartate receptor (NMDAR) antagonist memantine may produce more reliable treatment effect with respect to alcohol relapse-like behaviour. For this purpose, male Wistar rats were trained to associate several discrete stimuli with ethanol delivery. Thereafter, half of the animals received a brief memory reactivation session followed by two administrations of 20 mg/kg of memantine, while the other half received the same treatment without memory reactivation. Afterwards, a cue-induced ethanol-seeking behaviour test was performed followed by repeated extinction sessions and a reacquisition test. Our data show that administration of memantine reduced responding on the ethanol-associated lever in a cue-induced ethanol-seeking test. This reduction did not depend on whether or not a memory reactivation session was introduced prior to memantine administration. Following extinction, however, reacquisition of ethanol self-administration was only impaired in the group where memantine was given after a short memory reactivation session, showing that this schedule of drug administration produced a long-lasting disruption of the association between the conditioned stimuli and the delivery of ethanol. In conclusion, we show that memantine disrupted the drug-cue association, which consequently interfered with relapse-like behaviour supporting the possibility that memantine is a treatment option for alcoholism. Our data supports the possibility that memantine is a treatment option for alcoholism. However, the effectiveness of this drug seems to lie in its ability to disrupt conditioned behaviours and should be given in conjunction with exposure to conditioned drug stimuli.

Paternal alcohol exposure reduces alcohol drinking and increases behavioral sensitivity to alcohol selectively in male offspring

Alcohol use disorder (AUD) is heritable, but the genetic basis for this disease remains poorly understood. Although numerous gene variants have been associated with AUD, these variants account for only a small fraction of the total risk. The idea of inheritance of acquired characteristics, i.e. “epigenetic inheritance,” is re-emerging as a proven adjunct to traditional modes of genetic inheritance. We hypothesized that alcohol drinking and neurobiological sensitivity to alcohol are influenced by ancestral alcohol exposure. To test this hypothesis, we exposed male mice to chronic vapor ethanol or control conditions, mated them to ethanol-naïve females, and tested adult offspring for ethanol drinking, ethanol-induced behaviors, gene expression, and DNA methylation. We found that ethanol-sired male offspring had reduced ethanol preference and consumption, enhanced sensitivity to the anxiolytic and motor-enhancing effects of ethanol, and increased Bdnf expression in the ventral tegmental area (VTA) compared to control-sired male offspring. There were no differences among ethanol- and control-sired female offspring on these assays. Ethanol exposure also decreased DNA methylation at the BdnfÆpromoter of sire's germ cells and hypomethylation was maintained in the VTA of both male and female ethanol-sired offspring. Our findings show that paternal alcohol exposure is a previously unrecognized regulator of alcohol drinking and behavioral sensitivity to alcohol in male, but not female, offspring. Paternal alcohol exposure also induces epigenetic alterations (DNA hypomethylation) and gene expression changes that persist in the VTA of offspring. These results provide new insight into the inheritance and development of alcohol drinking behaviors.

Pharmacological approaches to reducing craving in patients with alcohol use disorders

Research on the concept of craving may lead to better understanding of the biobehavioural circuitries that contribute to the complexity of alcohol use disorders (AUDs). The experiences described as craving or desire to drink are often associated with physical responses such as increased salivation and heart rate, and alteration of stress hormones, as well as psychological responses such as anxiety and depression. Greater craving has been associated with an increased probability of alcohol relapse. Reversal of craving, which is understood as a symptom of protracted abstinence, offers the possibility of preventing relapses and treating alcoholism. Various medications have been studied to establish whether they are able to reduce craving; however, the results obtained from clinical studies have been inconsistent. Here, we review the interdisciplinary models developed to evaluate craving, then the different approaches used to assess and measure craving and, finally, the medications utilized and tested to lessen craving in patients suffering from AUDs.

Enhanced AMPA receptor activity increases operant alcohol self-administration and cue-induced reinstatement

Long-term alcohol exposure produces neuroadaptations that contribute to the progression of alcohol abuse disorders. Chronic alcohol consumption results in strengthened excitatory neurotransmission and increased α-amino-3-hydroxyl-5-methyl-4-isoxazole-propionate receptors (AMPA) receptor signaling in animal models. However, the mechanistic role of enhanced AMPA receptor activity in alcohol-reinforcement and alcohol-seeking behavior remains unclear. This study examined the role of enhanced AMPA receptor function using the selective positive allosteric modulator, aniracetam, in modulating operant alcohol self-administration and cue-induced reinstatement. Male alcohol-preferring (P-) rats, trained to self-administer alcohol (15%, v/v) versus water were pre-treated with aniracetam to assess effects on maintenance of alcohol self-administration. To determine reinforcer specificity, P-rats were trained to self-administer sucrose (0.8%, w/v) versus water, and effects of aniracetam were tested. The role of aniracetam in modulating relapse of alcohol-seeking was assessed using a response contingent cue-induced reinstatement procedure in P-rats trained to self-administer 15% alcohol. Aniracetam pre-treatment significantly increased alcohol-reinforced responses relative to vehicle treatment. This increase was not attributed to aniracetam-induced hyperactivity as aniracetam pre-treatment did not alter locomotor activity. AMPA receptor involvement was confirmed because 6,7-dinitroquinoxaline-2,3-dione (AMPA receptor antagonist) blocked the aniracetam-induced increase in alcohol self-administration. Aniracetam did not alter sucrose-reinforced responses in sucrose-trained P-rats, suggesting that enhanced AMPA receptor activity is selective in modulating the reinforcing function of alcohol. Finally, aniracetam pre-treatment potentiated cue-induced reinstatement of alcohol-seeking behavior versus vehicle-treated P-rats. These data suggest that enhanced glutamate activity at AMPA receptors may be key in facilitating alcohol consumption and seeking behavior, which could ultimately contribute to the development of alcohol abuse disorders.

Timing-dependent reduction in ethanol sedation and drinking preference by NMDA receptor co-agonist d-serine

NMDA receptors become a major contributor to acute ethanol intoxication effects at high concentrations as ethanol binds to a unique site on the receptor and inhibits glutamatergic activity in multiple brain areas. Although a convincing body of literature exists on the ability of NMDA receptor antagonists to mimic and worsen cellular and behavioral ethanol effects, receptor agonists have been less well-studied. In addition to a primary agonist site for glutamate, the NMDA receptor contains a separate co-agonist site that responds to endogenous amino acids glycine and d-serine. d-serine is both selective for this co-agonist site and potent in boosting NMDA dependent activity even after systemic administration. In this study, we hypothesized that exogenous d-serine might ameliorate some acute ethanol behaviors by opposing NMDA receptor inhibition. We injected adult male C57 mice with a high concentration of d-serine at various time windows relative to ethanol administration and monitored sedation, motor coordination and voluntary ethanol drinking. d-serine (2.7 g/kg, ip) prolonged latency to a loss of righting reflex (LoRR) and shortened LoRR duration when given 15 min before ethanol (3 g/kg) but not when it was injected with or shortly after ethanol. Blood samples taken at sedative recovery and at fixed time intervals revealed no effect of d-serine on ethanol concentration but an ethanol-induced decrease in l-serine and glycine content was prevented by acute d-serine pre-administration. d-serine had no effect on ethanol-induced (2 g/kg) rotarod deficits in young adult animals but independently and interactively degraded motor performance in a subset of older mice. Finally, a week-long series of daily ip injections resulted in a 50% decrease in free choice ethanol preference for d-serine treated animals compared to saline-injected controls in a two-bottle choice experiment.

Alternative splicing of AMPA subunits in prefrontal cortical fields of cynomolgus monkeys following chronic ethanol self-administration

Functional impairment of the orbital and medial prefrontal cortex underlies deficits in executive control that characterize addictive disorders, including alcohol addiction. Previous studies indicate that alcohol alters glutamate neurotransmission and one substrate of these effects may be through the reconfiguration of the subunits constituting ionotropic glutamate receptor (iGluR) complexes. Glutamatergic transmission is integral to cortico-cortical and cortico-subcortical communication and alcohol-induced changes in the abundance of the receptor subunits and/or their splice variants may result in critical functional impairments of prefrontal cortex in alcohol dependence. To this end, the effects of chronic ethanol self-administration on glutamate receptor ionotropic AMPA (GRIA) subunit variant and kainate (GRIK) subunit mRNA expression were studied in the orbitofrontal cortex (OFC), dorsolateral prefrontal cortex (DLPFC), and anterior cingulate cortex (ACC) of male cynomolgus monkeys. In DLPFC, total AMPA splice variant expression and total kainate receptor subunit expression were significantly decreased in alcohol drinking monkeys. Expression levels of GRIA3 flip and flop and GRIA4 flop mRNAs in this region were positively correlated with daily ethanol intake and blood ethanol concentrations (BEC) averaged over the 6 months prior to necropsy. In OFC, AMPA subunit splice variant expression was reduced in the alcohol treated group. GRIA2 flop mRNA levels in this region were positively correlated with daily ethanol intake and BEC averaged over the 6 months prior to necropsy. Results from these studies provide further evidence of transcriptional regulation of iGluR subunits in the primate brain following chronic alcohol self-administration. Additional studies examining the cellular localization of such effects in the framework of primate prefrontal cortical circuitry are warranted.

Preclinical evaluation of riluzole: assessments of ethanol self-administration and ethanol withdrawal symptoms

BACKGROUND

Many of the neurobehavioral effects of ethanol are mediated by inhibition of excitatory N-methyl-D-aspartate (NMDA) and enhancement of inhibitory gamma-amino-butyric-acid (GABA) receptor systems. There is growing interest in drugs that alter these systems as potential medications for problems associated with alcoholism. The drug riluzole, approved for treatment of amyotrophic lateral sclerosis (ALS), inhibits NMDA and enhances GABA(A) receptor system activity. This study was designed to determine the preclinical efficacy of riluzole to modulate ethanol self-administration and withdrawal.

METHODS

Male C57BL/6J mice were trained to lever press on a concurrent fixed-ratio 1 schedule of ethanol (10% v/v) versus water reinforcement during daily 16-hour sessions. Riluzole (1 to 40 mg/kg, IP) was evaluated on ethanol self-administration after acute and chronic (2 week) treatment. To determine if riluzole influences ethanol withdrawal-associated seizures, mice were fed an ethanol-containing or control liquid diet for 18 days. The effects of a single injection of riluzole (30 mg/kg) were examined on handling-induced convulsions after ethanol withdrawal.

RESULTS

Acute riluzole (30 and 40 mg/kg) reduced ethanol self-administration during the first 4 hours of the session, which corresponds to the known pharmacokinetics of this drug. Ethanol self-administration was also reduced by riluzole after chronic treatment. Riluzole (30 mg/kg) significantly decreased the severity of ethanol-induced convulsions 2 hours after ethanol withdrawal.

CONCLUSIONS

These results demonstrate that riluzole decreases ethanol self-administration and may reduce ethanol withdrawal severity in mice. Thus, riluzole may have utility in the treatment of problems associated with alcoholism.

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.

Neuropharmacology of alcohol addiction

Despite the generally held view that alcohol is an unspecific pharmacological agent, recent molecular pharmacology studies demonstrated that alcohol has only a few known primary targets. These are the NMDA, GABA(A), glycine, 5-hydroxytryptamine 3 (serotonin) and nicotinic ACh receptors as well as L-type Ca(2+) channels and G-protein-activated inwardly rectifying K(+) channels. Following this first hit of alcohol on specific targets in the brain, a second wave of indirect effects on a variety of neurotransmitter/neuropeptide systems is initiated that leads subsequently to the typical acute behavioural effects of alcohol, ranging from disinhibition to sedation and even hypnosis, with increasing concentrations of alcohol. Besides these acute pharmacodynamic aspects of alcohol, we discuss the neurochemical substrates that are involved in the initiation and maintenance phase of an alcohol drinking behaviour. Finally, addictive behaviour towards alcohol as measured by alcohol-seeking and relapse behaviour is reviewed in the context of specific neurotransmitter/neuropeptide systems and their signalling pathways. The activity of the mesolimbic dopaminergic system plays a crucial role during the initiation phase of alcohol consumption. Following long-term, chronic alcohol consumption virtually all brain neurotransmission seems to be affected, making it difficult to define which of the systems contributes the most to the transition from controlled to compulsive alcohol use. However, compulsive alcohol drinking is characterized by a decrease in the function of the reward neurocircuitry and a recruitment of antireward/stress mechanisms comes into place, with a hypertrophic corticotropin-releasing factor system and a hyperfunctional glutamatergic system being the most important ones.

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.

A pilot double-blind treatment trial of memantine for alcohol dependence

BACKGROUND

There is growing evidence that N-methyl-d-aspartate (NMDA) receptor antagonists may have potential for the treatment of alcohol disorders. Memantine is a selective noncompetitive NMDA receptor antagonist that has been shown to decrease alcohol craving in moderate drinkers. This 16-week double-blind outpatient pilot clinical trial determined if memantine was more effective than placebo at reducing alcohol use in actively drinking alcohol-dependent patients.

METHODS

Forty-four treatment-seeking alcohol-dependent individuals were enrolled, with 34 patients stratified to either the memantine group (n=19; maximum dose of 40 mg/d) or the placebo (PBO; n=15) group. The primary outcome measures were related to alcohol use (average drinks per day, average drinks per drinking day, percentage of heavy drinking days, and percentage of days abstinent) based on the timeline follow-back (TLFB). Secondary outcome measures included the Obsessive Compulsive Drinking Scale, Clinical Global Impression ratings, and gamma-glutamyltransferase (GGT), a biomarker of recent alcohol use. To enhance retention, patients received voucher incentives for clinic attendance.

RESULTS

Of those randomized, approximately 80% (27) completed the entire 16-week trial. Longitudinal analysis of drinks per day and drinks per drinking day showed a significant reduction in alcohol use, but no difference between the 2 groups. Further, the percentage of heavy drinking days indicated that both groups showed a significant decrease in drinking behavior, but there was significant treatment effect in favor of the PBO group. Similarly, for the percentage of days abstinent, the PBO group achieved a significantly greater percentage of days abstinent at a faster rate than the memantine group. Lastly, the memantine group reported a greater number of side effects compared with the PBO group, such that 26% of patients had their drug dose decreased or discontinued due to memantine-related side effects.

CONCLUSIONS

The results of this double-blind placebo-controlled pilot trial do not support the use of memantine for the treatment of actively drinking alcohol-dependent patients. However, voucher incentives did facilitate retention.

Effect of glycine site/NMDA receptor antagonist MRZ2/576 on the conditioned place preference and locomotor activity induced by morphine in mice

OBJECTIVE

To study the effect of glycine site/NMDA (N-methyl-D-aspartate) receptor antagonist MRZ2/576 on the conditioned place preference (CPP) and locomotor activity induced by morphine in mice.

METHODS

Different doses (1.25, 2.5 and 5 mg/kg, i.p.) of MRZ2/576 were used to evaluate the effect of MRZ2/576 on the acquisition and expression of CPP induced by morphine (5 mg/kg) in mice. In addition, we examined the locomotor activity of mice in conditioning and testing phase of CPP paradigm.

RESULTS

MRZ2/576 alone could not establish place preference, but a 5 mg/kg dose of MRZ2/576 could block both acquisition and expression of morphine-induced CPP. In testing phase of CPP, there was no statistical difference for locomotor activity between the groups; injection of MRZ2/576 showed a dose-dependent decrease of locomotor activity on both control and morphine-treated mice, especially 5 mg/kg of MRZ2/576 significantly suppressed the locomotor activity of mice.

CONCLUSION

Based on the present results, we assume that MRZ2/576 can antagonize the rewarding effect of morphine, suggesting that this glycine site/NMDA receptor antagonist could be used to treat addictions due to its light side effect profile.

Resurgence of alcohol seeking produced by discontinuing non-drug reinforcement as an animal model of drug relapse

Findings from basic behavioral research suggest that simply discontinuing reinforcement for a recently reinforced operant response can cause the recurrence (i.e. resurgence) of a different previously reinforced response. The present experiment examined resurgence as an animal model of drug relapse. Initially, rats pressed levers to self-administer alcohol during baseline conditions. Next, alcohol self-administration was discontinued and non-drug reinforcers (food pellets) were presented contingent on an alternative response (chain pulling). Finally, when the non-drug reinforcer was discontinued, alcohol seeking recurred even though alcohol was still unavailable for lever pressing. These results suggest that simply discontinuing non-drug reinforcement for a behavior may be sufficient to produce relapse to drug seeking. The resurgence procedure could provide a method to examine environmental, pharmacological, and neurobiological factors that lead to relapse following the loss of a non-drug source of reinforcement.

Ethanol-related behaviors in mice lacking the NMDA receptor NR2A subunit

RATIONALE

The ionotropic NMDA glutamate receptor is composed of NR1 and NR2 (NR2A-D) subunits. While there is compelling evidence that NMDA receptors modulate behavioral effects of ethanol, there is little understanding of how the subunit composition of the NMDA receptor mediates these effects.

OBJECTIVES

In the current study, we assessed the relative roles of NMDA subunits via phenotypic assessment of ethanol-related behaviors in NR2A knockout (KO) mice.

RESULTS

Results demonstrated that NR2A KO and heterozygous mice failed to show evidence of ethanol-induced conditioned place preference. As compared to wild-type (WT) controls, KO mice showed impaired motor coordination at baseline and, in some instances, following ethanol treatment on the accelerating rotarod, balance beam, and wire-hang tests. By contrast, open field locomotor-stimulant, sedative/hypnotic, and hypothermic responses to ethanol were not different between genotypes, nor was voluntary ethanol consumption and preference in a two-bottle choice paradigm. Blood ethanol concentrations were lower in KO than WT mice following intraperitoneal ethanol injection.

CONCLUSIONS

Results suggest that the loss of NR2A subunit-containing NMDA receptors impairs the ability to form or express learned reward-related responses to ethanol and causes deficits in motor coordination. However, the loss of NR2A does not alter other measures of acute ethanol intoxication or ethanol consumption, possibly implicating other NMDA subunits in these effects. These data provide novel insight into the role of NMDA receptors in modulating the behavioral effects of ethanol.

N-methyl-D-aspartate and group I metabotropic glutamate receptors are involved in the expression of ethanol-induced sensitization in mice

Effects of acamprosate and ionotropic uncompetitive N-methyl-D-aspartate receptor antagonists and group I metabotropic glutamatergic receptor antagonists on the expression of ethanol-induced sensitization were investigated in mice. The results indicated that acamprosate (200 and 400 mg/kg) and N-methyl-D-aspartate receptor antagonists, neramexane (10 and 20 mg/kg) and MK-801 (0.1 and 0.2 mg/kg), inhibited the expression of ethanol-induced sensitization. Acamprosate, but not the other compounds tested, also blocked the stimulant effect of acute injections of ethanol. Among the group I metabotropic glutamatergic receptor antagonists, only the metabotropic glutamatergic receptor 5 antagonist, MTEP (5, 10, and 20 mg/kg) showed an effect similar to the N-methyl-D-aspartate receptor antagonists. The metabotropic glutamatergic receptor 1 antagonist, EMQMCM (5, 10, and 20 mg/kg), however, potentiated the inhibitory effect of MK-801 on the expression of ethanol-induced sensitization. The findings indicate that glutamatergic neurotransmission is important in the ethanol-induced sensitization process, and suggest that co-administration of metabotropic glutamatergic receptor 1 antagonists and N-methyl-D-aspartate receptor antagonists may be useful in therapy for alcoholism.

The role of the NMDA receptor in alcohol relapse: a pharmacological mapping study using the alcohol deprivation effect

Modulators of glutamate receptors especially of the N-methyl-D-aspartate receptors (NMDARs) have recently been suggested as putative pharmacotherapeutic agents in the treatment of alcohol relapse. However, at present it is not clear, which binding and modulatory sites of the NMDAR are involved in relapse behavior. We, therefore, performed a pharmacological mapping study in long-term alcohol drinking rats using the alcohol deprivation effect (ADE) as a model for relapse behavior. In a comprehensive fashion, we studied dose-response curves, employing the following selective pharmacological agents: the NMDAR competitive antagonist CGP37849, the glycine binding site antagonist L-701.324, the NR2B subunit selective antagonist ifenprodil, which acts at the polyamine binding site, the NMDAR channel blocker neramexane, and ethanol, which acts as a functional antagonist at the NMDAR. Our data show that the animals' alcohol consumption inversely correlates with the dose of ethanol administered intraperitoneally. This indicates that under the present experimental conditions alcohol intake during an ADE is an entirely pharmacologically driven behavior that is not under the control of other factors such as taste or novelty of alcohol re-exposure. The effects of the administration of the aforementioned compounds were comparable to those of ethanol, suggesting a similar pharmacological impact on relapse behavior. Repeated administration of both competitive and uncompetitive NMDAR antagonist dose-dependently suppressed alcohol consumption during ADE. In addition, ifenprodil and L-701.324 dose-dependently reduced the expression of an ADE as well. In summary, the results suggest that an inhibition of NMDAR function in general, rather than a particular interference with a specific binding site of this receptor, is sufficient for the reduction of relapse behavior.

The effects of acamprosate and neramexane on cue-induced reinstatement of ethanol-seeking behavior in rat

This study examines, for the first time, the effects of acamprosate and the non-competitive N-methyl-D-aspartate (NMDA) receptor antagonist neramexane on ethanol-seeking induced by alcohol-related environmental stimuli in an animal model of relapse. Wistar rats were trained to operantly self-administer ethanol (10% w/v) or water on a fixed-ratio 1 schedule in a 30-min daily session. Ethanol availability was signaled by an olfactory discriminative stimulus of orange extract (S+). In addition, each lever press was accompanied by a 5-s illumination of the operant chamber's house light (CS+). Water availability was signaled by anise odor (S-) and 5-s white noise stimulus (CS-). After completion of the conditioning phase, indicated by stable levels of responding, operant behaviors were extinguished. Prior to reinstatement tests, animals were divided into groups according to either treatment with acamprosate (100, 200 mg/kg given twice), neramexane (1.0, 2.0, 4.0 mg/kg), or vehicle. In vehicle-treated rats, re-exposure to the S+/CS+ in the absence of further ethanol availability elicited strong recovery of responding. No effect was observed following presentation of water-paired cues (S-/CS-). Acamprosate dose-dependently attenuated recovery of responding elicited by ethanol-paired cues (S+/CS+), whereas responding under S-/CS- was not modified by drug administration. Treatment with 1.0 and 2.0 mg/kg of neramexane did not significantly modify responding under both S+/CS+ and S-/CS- conditions. However, a slight reduction of cue-induced reinstatement of alcohol seeking was observed. At the dose of 4.0 mg/kg, neramexane elicited a marked inhibition of responding following presentation of both ethanol- and water-paired cues. In conclusion, acamprosate significantly and selectively reduced alcohol-seeking elicited by environmental stimuli predictive of alcohol availability. Treatment with neramexane that shares part of the pharmacological effects of acamprosate on NMDA receptors, however, resulted in a nonselective reduction of lever responding.

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.

Effect of neramexane on ethanol dependence and reinforcement

It has been suggested that drugs modulating the glutamate/N-methyl-D-aspartate (NMDA) receptor system may be useful in the treatment of alcohol dependence. The effect of neramexane, a low-to-moderate affinity uncompetitive NMDA receptor antagonist, was examined on the development and expression of ethanol dependence (withdrawal-associated audiogenic seizures) and ethanol-induced conditioned place preference. Neramexane hydrochloride (3.5 mg/kg and higher) inhibited both the development and expression of ethanol dependence. Neramexane hydrochloride also inhibited the acquisition (1.75 mg/kg and higher) and expression (3.5 mg/kg and higher) of ethanol-induced place preference. Our data support therapeutic potential of neramexane as a treatment for alcohol abuse.

Acute effects of memantine in combination with alcohol in moderate drinkers

RATIONALE

Alcohol effects in humans involve N-methyl- d-aspartate (NMDA) receptor-mediated glutamatergic neurotransmission. It has been proposed that NMDA receptor antagonists may be effective in the treatment of alcohol dependence.

OBJECTIVE

This study evaluated the acute effects of memantine, an NMDA receptor antagonist, on the subjective, physiological, and performance effects of alcohol in moderate (10-30 drinks per week) alcohol drinkers.

METHODS

Eighteen volunteers without alcohol dependence were tested using a double-blind design with three 3-day long inpatient phases separated by at least a 2-week wash-out period. Memantine (0, 15, and 30 mg) was administered 4 h before alcohol (1.5 g/l body water), which was given in four divided doses every 20 min.

RESULTS

Pretreatment with memantine attenuated the craving for alcohol before alcohol administration, but not after alcohol was given. Memantine increased the dissociative effects of alcohol, without altering its sedative, stimulant, and overall intoxicating effects. Memantine also did not affect alcohol-induced impairment in performance, physiological changes, or pharmacokinetics. Memantine increased subjective reports of dissociation, confusion, and stimulation, and impaired motor coordination on the balance task.

CONCLUSIONS

Memantine was well tolerated in combination with alcohol. The findings suggest that NMDA receptor neurotransmission may be involved in alcohol craving and alcohol-induced subjective dissociative effects.

The Role of NMDA Receptor Binding Sites in Ethanol Place Conditioning

Little is known about the specific role of glutamate, in particular its actions at N-methyl-D-aspartate (NMDA) receptors, in ethanol reward. Pretreatment with channel blockers MK-801 and ketamine, NMDA NR2B receptor subunit antagonists ifenprodil and CP-101,606, and the glycine(B) partial agonist (+)-HA-966 did not alter acquisition of ethanol-induced conditioned place preference (CPP) in mice. However, pretreatment with the competitive antagonist CGP-37849 attenuated acquisition of ethanol-induced CPP. Follow-up experiments indicated that CGP-37849 also blocked acquisition of ethanol-induced and lithium chloride-induced conditioned place aversion but did not produce rewarding or aversive effects on its own. These results suggest that the NMDA receptor glutamate binding site is important for ethanol place conditioning. Moreover, these results suggest CGP-37849 modulates ethanol place conditioning by impairing the ability to learn these tasks.

Acute effects of memantine in combination with alcohol in moderate drinkers

RATIONALE

Alcohol effects in humans involve N-methyl- d-aspartate (NMDA) receptor-mediated glutamatergic neurotransmission. It has been proposed that NMDA receptor antagonists may be effective in the treatment of alcohol dependence.

OBJECTIVE

This study evaluated the acute effects of memantine, an NMDA receptor antagonist, on the subjective, physiological, and performance effects of alcohol in moderate (10-30 drinks per week) alcohol drinkers.

METHODS

Eighteen volunteers without alcohol dependence were tested using a double-blind design with three 3-day long inpatient phases separated by at least a 2-week wash-out period. Memantine (0, 15, and 30 mg) was administered 4 h before alcohol (1.5 g/l body water), which was given in four divided doses every 20 min.

RESULTS

Pretreatment with memantine attenuated the craving for alcohol before alcohol administration, but not after alcohol was given. Memantine increased the dissociative effects of alcohol, without altering its sedative, stimulant, and overall intoxicating effects. Memantine also did not affect alcohol-induced impairment in performance, physiological changes, or pharmacokinetics. Memantine increased subjective reports of dissociation, confusion, and stimulation, and impaired motor coordination on the balance task.

CONCLUSIONS

Memantine was well tolerated in combination with alcohol. The findings suggest that NMDA receptor neurotransmission may be involved in alcohol craving and alcohol-induced subjective dissociative effects.

Toxic cocaine- and convulsant-induced modification of forced swimming behaviors and their interaction with ethanol: comparison with immobilization stress

BACKGROUND

Swimming behaviors in the forced swimming test have been reported to be depressed by stressors. Since toxic convulsion-inducing drugs related to dopamine [cocaine (COC)], benzodiazepine [methyl 6,7-dimethoxy-4-ethyl-beta-carboline-carboxylate (DMCM)], gamma-aminobutyric acid (GABA) [bicuculline (BIC)], and glutamate [N-methyl-D-aspartate (NMDA)] receptors can function as stressors, the present study compared their effects on the forced swimming behaviors with the effects of immobilization stress (IM) in rats. Their interactions with ethanol (EtOH), the most frequently coabused drug with COC which also induces convulsions as withdrawal symptoms but interferes with the convulsions caused by other drugs, were also investigated.

RESULTS

Similar to the IM (10 min) group, depressed swimming behaviors (attenuated time until immobility and activity counts) were observed in the BIC (5 mg/kg IP) and DMCM (10 mg/kg IP) groups at the 5 h time point, after which no toxic behavioral symptoms were observed. However, they were normalized to the control levels at the 12 h point, with or without EtOH (1.5 g/kg IP). In the COC (60 mg/kg IP) and NMDA (200 mg/kg IP) groups, the depression occurred late (12 h point), and was normalized by the EtOH cotreatment. At the 5 h point, the COC treatment enhanced the swimming behaviors above the control level.

CONCLUSIONS

Although the physiological stress (IM), BIC, and DMCM also depressed the swimming behaviors, a delayed occurrence and EtOH-induced recovery of depressed swimming were observed only in the COC and NMDA groups. This might be correlated with the previously-reported delayed responses of DA and NMDA neurons rather than direct effects of the drugs, which could be suppressed by EtOH. Furthermore, the characteristic psychostimulant effects of COC seemed to be correlated with an early enhancement of swimming behaviors.

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.

Therapeutic potential of NMDA receptor antagonists in the treatment of alcohol and substance use disorders

Despite the fact that the use of alcohol, nicotine and other drugs is the major external factor contributing to mortality in industrialised countries, there are few medications available to treat alcohol and substance use disorders. In recent years, major advances have been made in the understanding of the neurobiological basis for these disorders and these advances should lead to the development of new pharmacotherapeutics. A substantial amount of the research suggests that N-methyl-D-aspartate (NMDA) receptor neurotransmission contributes to mediating the behavioural effects of alcohol and other drugs of abuse. This research supports the therapeutic potential of NMDA receptor antagonists in alcohol and substance use disorders. In this paper the authors present their opinion on the goals and stages of pharmacological treatment of these complex psychiatric disorders. Available preclinical research using designs that model aspects of alcohol and substance use disorders is summarised, with an emphasis on research published in the last two years. In animal models, NMDA antagonists inhibit physical dependence and the reinforcing effects of a variety of abused substances. The ability of NMDA antagonists to inhibit tolerance to drug effects and contribute possible antidepressant and anxiolytic effects are also important from the perspective of drug development. This review summarises the relevant clinical laboratory and treatment data. Finally, it presents the status of the current development of NMDA receptor antagonists and discusses candidates with the greatest potential for clinical development.

Reinstatement of ethanol seeking in rats: behavioral analysis

The reinstatement model has been repeatedly used to study relapse to heroin- or cocaine-seeking behaviour in rats. The aim of the present study was to evaluate basic behavioral parameters of cue-induced reinstatement of ethanol seeking in a within-session paradigm. Rats were trained to respond for ethanol in an oral self-administration procedure where each lever press resulted in presentation of 0.1 ml of 8% ethanol from a liquid dipper. In the reinstatement paradigm operant behaviour was first extinguished for 20 or 60 min by switching the dipper off. Then, ethanol-associated stimuli were noncontingently delivered and reinstatement of responding was assessed. Deliveries of the empty dipper, i.e., visual/auditory cues only, did not result in any reinstatement. In contrast, 15 random presentations of the dipper containing either ethanol (4-8%; v/v) or water significantly reinstated ethanol seeking. In a control self-administration experiment responding dropped to nonsignificant levels when water was substituted for ethanol. The magnitude of reinstatement did not depend on the duration of the extinction phase. These results seem to indicate that in the present paradigm reinstatement of ethanol seeking is driven by a compound stimulus including the visual/auditory cues and some nonspecific sensory properties of liquid available in the dipper.

Involvement of nicotinic receptors in alcohol self-administration

BACKGROUND

Alcohol and nicotine, in the form of tobacco, are commonly co-abused. Nicotinic receptors also have been implicated in alcohol action. We designed the present study to examine the possible involvement of nicotinic receptors in alcohol self-administration.

METHODS AND RESULTS

Pretreatment with lower doses (0.1-0.4 mg/kg) of nicotine, administered acutely or chronically, did not affect alcohol consumption, whereas a higher dose (0.8 mg/kg) initially suppressed alcohol consumption but stimulated alcohol consumption on repeated treatment. We observed the same pattern of nicotine effects on alcohol self-administration using an operant procedure. A dose of 0.8 mg/kg of nicotine initially suppressed operant responding for alcohol. Such suppression of alcohol self-administration was more pronounced during the first 20 min of the 60 min operant session. Responding for alcohol in the nicotine treated group, however, was significantly increased above the saline treated group by the 5th day of treatment. Mecamylamine, a noncompetitive nicotinic receptor antagonist, reduced alcohol consumption, whereas dihydro-beta-erythroidine (DHbetaE), a competitive nicotinic receptor antagonist, did not modify alcohol consumption.

CONCLUSIONS

The stimulation of alcohol intake induced by nicotine treatment and the suppression of alcohol intake induced by mecamylamine provide evidence for the involvement of nicotinic receptors in alcohol consumption and/or self-administration. The failure of DHbetaE to reduce alcohol consumption, however, suggests that ethanol-nicotine interaction is mediated by other nicotinic receptor subtypes rather than alpha4beta2 receptor subtype, or that mecamylamine acts through a nonnicotinic mechanism.