NMDA receptor antagonists prevent conditioned activation of intracranial self-stimulation in rats

The uncompetitive NMDA receptor antagonists ketamine and memantine preferentially increase the choice for a small, immediate reward in low-impulsive rats

RATIONALE

Impulsive behavior is categorically differentiated between impulsive action, the inability to withhold from acting out a response, and impulsive choice, the greater preference for an immediate and smaller reward over a delayed but more advantageous reward. While the effects of N-methyl-D-aspartic acid (NMDA) receptor antagonists on impulsive action have been extensively characterized, there are very few and conflicting reports on the effects of this class of drugs on impulsive choice.

OBJECTIVES

Using a modified adjusting delay task, we investigated the effects of uncompetitive and competitive blockade of NMDA receptors on impulsive choice.

METHODS

Male Wistar rats were trained in a modified adjusting delay task, which involved repeated choice between a low reinforcing solution delivered immediately and a highly reinforcing solution delivered after a variable delay. Rats were then administered either the NMDA receptor uncompetitive antagonists ketamine or memantine, or the competitive antagonists D-AP-5 or CGS 19755.

RESULTS

Ketamine treatment dose-dependently increased impulsive choice, and this effect was selective for low-impulsive but not high-impulsive rats. Similarly, memantine treatment dose-dependently increased impulsive choice with a preferential effect for low-impulsive rats. While D-AP-5 treatment did not affect impulsive choice, CGS 19755 increased impulsivity, however, at the same doses at which it caused a marked response inhibition.

CONCLUSIONS

NMDA receptor uncompetitive, but not competitive, antagonists significantly increased impulsive choice, preferentially in low-impulsive rats. These findings demonstrate that the effects of NMDA receptor blockade on impulsive choice are not generalizable and depend on the specific mechanism of action of the antagonist used.

Effects of NMDA receptor channel blockers, MK-801 and memantine, on locomotor activity and tolerance to delay of reward in Wistar-Kyoto and spontaneously hypertensive rats

N-Methyl-D-aspartate (NMDA) receptor blockade enhances motor activity and stimulates dopamine metabolism, effects shared with classical psychostimulant drugs. The present study aimed to characterize behavioral effects of two NMDA receptor channel blockers, MK-801 and memantine, in both Wistar-Kyoto (WKY) and spontaneously hypertensive (SHR) rats. In Experiment 1, SHR rats demonstrated higher spontaneous locomotor activity and spent more time in the central area of the open field apparatus compared with WKY rats. Rats of both strains pre-treated with MK-801 (0.01-0.3 mg/kg) or memantine (1-32 mg/kg) demonstrated dose-dependent increases in the total distance traveled and time spent in the central area. Experiment 2 was based on the two-lever discrete-trial delayed reinforcement task in which rats could press one lever to obtain one pellet immediately or another lever for four pellets delivered after a variable delay (0-60 s). Tolerance to delay of reward did not differ between strains. MK-801 (0.03-0.3 mg/kg) and memantine (1-10 mg/kg) produced small, but significant, facilitation of the large-reward lever responding and markedly impaired operant performance at higher dose levels (increased number of missed trials). For both experiments, effects of MK-801 and memantine were more pronounced in WKY compared with SHR rats. Additional studies are needed to address the utility of noncompetitive NMDA receptor blockers in the treatment of attention deficit and hyperactivity disorder.

The NMDA receptor channel blocker memantine and opioid receptor antagonist naltrexone inhibit the saccharin deprivation effect in rats

Several drugs, such as N-methyl-D-aspartate (NMDA) receptor channel blockers (memantine), naltrexone (but not naloxone) and acamprosate, have previously been reported to attenuate the expression of the alcohol deprivation effect, a phenomenon seen as an increase in post-deprivation alcohol consumption. The present study aimed to evaluate the effects of these drugs on the development and expression of the saccharin deprivation effect in adult male Wistar rats. Memantine (13 mg/kg per day) and naltrexone (5 mg/kg, twice daily), but not naloxone (24 mg/kg per day) or acamprosate (200 mg/kg, twice daily), prevented the increase in the consumption of saccharin after a 1-week deprivation from free-choice, unlimited access to saccharin (0.1%, w/v). Taken together with the results of previous studies, these results suggest that naltrexone and memantine attenuate the expression of both the alcohol and saccharin deprivation effects.

Conditioned facilitation of brain reward function after repeated cocaine administration

Cocaine lowers brain reward thresholds, reflecting increased brain reward function. The authors investigated whether, similar to acute cocaine administration, cocaine-predictive conditioned stimuli would lower intracranial self-stimulation (ICSS) thresholds. Rats received a saline injection for 5 days, a cocaine injection (10 mg/kg) for 20 consecutive days, then saline for 5 additional days. Thresholds were measured immediately before and 10 min after each injection. The initial 5 saline injections had no effect on thresholds, whereas cocaine significantly lowered thresholds for 20 days. There was no tolerance or sensitization to this effect of cocaine over days. During the last 5 days when cocaine administration was substituted with saline, rats demonstrated a conditioned lowering of thresholds during the 2nd daily ICSS session. These data demonstrate that cocaine-predictive conditioned stimuli induce a conditioned facilitation of brain reward function.

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.

In search of a new pharmacological treatment for drug and alcohol addiction: N-methyl-D-aspartate (NMDA) antagonists

The most challenging aspect of treating alcohol and drug addiction is the relapsing course of these disorders. Although substitution therapies for nicotine and opioid dependence have proven to be relatively effective, there is a need for new pharmacotherapies designed to decrease the frequency and severity of relapse. The aim of this paper is to provide an overview of the potential utility of N-methyl-D-aspartate (NMDA) receptor antagonists as treatments for substance abuse as shown in preclinical models and preliminary clinical trials. It is hypothesized that NMDA receptors mediate the common adaptive processes that are involved the development, maintenance, and expression of drug and alcohol addiction. Modulation of glutamatergic neurotransmission with NMDA receptor antagonists offers a novel treatment approach. It is proposed that NMDA antagonists may have multiple functions in treating addictions, including an attenuation of withdrawal effects, normalization of the affective changes following initiation of abstinence which arise from neurochemical changes resulting from chronic addiction, and an attenuation of conditioned responses arising from drug-related stimuli.

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.

Ethanol-reinforced behaviour in the rat: effects of uncompetitive NMDA receptor antagonist, memantine

Ethanol has been reported to alter NMDA receptor-mediated biochemical and electrophysiological responses in vitro. The aim of the present study was to evaluate the effects of an uncompetitive NMDA receptor antagonist memantine, in animal models of alcoholism. Male Wistar rats were trained to drink 8% ethanol in a free-choice, limited access procedure. A separate group of animals was trained to lever press for 8% ethanol in an operant procedure where ethanol was introduced in the presence of sucrose. The selectivity of memantine's actions was assessed by studying its effects on food or water consumption in separate control experiments. Memantine (4.5-24 mg/kg) significantly, but not dose dependently, affected ethanol drinking in the limited access procedure. However, only 6 mg/kg memantine selectively decreased ethanol drinking. Memantine did not alter ethanol intake in rats trained to lever press for ethanol in the operant procedure. Only 9 mg/kg memantine reduced operant responding in the extinction procedure in the rats trained to lever press for ethanol. The same dose of memantine significantly reduced the operant behaviour of rats trained to respond for water. These results indicate that: (i) single doses of memantine only moderately and not dose dependently reduce alcohol drinking in the limited access procedure; (ii) memantine produces non-selective effects on operant behaviour in rats trained to lever press for ethanol in an oral self-administration procedure.