Effect of the mGluR5 antagonist 6-methyl-2-(phenylethynyl)pyridine (MPEP) on the acute locomotor stimulant properties of cocaine, D-amphetamine, and the dopamine reuptake inhibitor GBR12909 in mice

Behavioral sensitization due to social defeat stress in mice: antagonism at mGluR5 and NMDA receptors

RATIONALE

Repeated administration of psychostimulants progressively augments the behavioral response to and increases self-administration behavior of these drugs. Experience of repeated intermittent social defeat stress episodes also leads to a sensitized locomotor response following psychostimulant challenge. Both metabotropic and ionotropic glutamate receptors have been shown to be critical in the induction and expression of stimulant sensitization, but their role in sensitization due to social defeat stress remains unclear.

OBJECTIVE

We evaluated the role of mGluR5 and NMDA glutamate receptors in the development of amphetamine-induced and social defeat stress-induced sensitization, using the non-competitive mGluR5 antagonist, MPEP, and the non-competitive NMDA antagonist, dizocilpine (MK-801).

METHODS

In adult, male CFW mice, sensitization was induced by either ten daily injections of D-amphetamine (1 mg/kg) or ten daily brief episodes of social defeat. Mice were pretreated with MPEP (3 mg/kg or 10 mg/kg) or dizocilpine (0.1 mg/kg) prior to amphetamine injections. Mice subjected to social defeat were pretreated with MPEP (10 mg/kg) or dizocilpine (0.1 mg/kg). Ten days after induction, the expression of locomotor sensitization to amphetamine was determined.

RESULTS

The induction of sensitization due to social defeat stress was prevented by MPEP, yet MPEP did not inhibit the development of behavioral sensitization to amphetamine. Confirming and extending earlier results, dizocilpine pretreatment blocked both amphetamine-induced and stress-induced sensitization.

CONCLUSIONS

These data indicate that behavioral sensitization to social defeat stress is dependent on mGluR5 receptors, whereas low-dose amphetamine sensitization may not be.

Effects of MPEP on locomotion, sensitization and conditioned reward induced by cocaine or morphine

Exposure to environmental cues is considered a major cause of relapse in detoxified addicts. Recent findings showed an involvement of glutamate in cue-induced relapse and suggest that subtype 5 of metabotropic glutamate receptors (mGluR5) is involved in conditioned drug-reward. The present study applied the conditioned place preference (CPP) paradigm to examine the involvement of mGluR5 in cocaine- and morphine-induced behaviours. Results of previous mice-studies were extended into rats by using the selective mGluR5 antagonist 2-methyl-6-(phenylethynyl)pyridine (MPEP). As a result, the evaluated behavioural parameters were dose-relatedly affected by MPEP. Low-dosed MPEP (10 mg/kg, i.p.) did not affect spontaneous locomotion, reduced cocaine-induced hyperlocomotion and produced sensitized locomotion, while showing no effect on sensitized locomotion induced by repeated cocaine or morphine. Low-dosed MPEP did not genuinely block development of cocaine- and morphine-CPP, but rendered CPP expression state-dependent. The medium MPEP-dose (30 mg/kg) was most effective in reducing spontaneous locomotion. The high MPEP-dose (50 mg/kg) was most effective in reducing both body-weight and morphine-CPP expression. Cocaine-CPP expression was not affected by any MPEP-dose. In conclusion, mGluR5 are involved in modulation of spontaneous and cocaine-induced locomotion, in state-dependent learning and in expression of morphine-CPP. Thus, MPEP may be beneficial for relapse prevention in morphine-addicts.

Genetic and genomic approaches to reward and addiction

Drug addiction is recognized as a mental disease affecting the brain's natural reward system. Drugs of abuse strongly activate reward structures in the brain and induce lasting changes in behavior that reflect changes in neuron physiology and biochemistry. With the ultimate goal of developing therapeutic interventions, it is of interest to determine the molecular and cellular components of motivation and reward, and identify those gene products that contribute to the process of drug addiction. Our laboratory has chosen three general genetic approaches to examine reward and addiction: reverse genetics to assess the role of candidate genes in drug responsiveness, forward genetics to discover novel regulators of dopamine transmission, and gene expression profiling to define gene sets in different brain structures that contribute to the molecular and neurobiological basis of reward.