Monoamine-depleting doses of methamphetamine in enriched and isolated rats: consequences for subsequent methamphetamine-induced hyperactivity and reward.

The current study examined whether environmental enrichment alters the effects of monoamine-depleting doses of methamphetamine on subsequent methamphetamine-induced hyperactivity and conditioned place preference. Rats were raised in either an enriched or isolated condition from 21 to 55 days of age and then were treated with monoamine-depleting doses of methamphetamine (10 mg/kg, four injections at 2 h intervals) or saline. Eight days later, rats were assessed for methamphetamine-induced (0.3 or 1.0 mg/kg) hyperactivity and conditioned place preference. Results indicated that the monoamine-depleting dose regimen produced a similar hyperthermic response in enriched and isolated rats. Enriched and isolated rats also displayed a similar depletion of dopamine in the striatum and serotonin in nucleus accumbens. The monoamine-depleting dose regimen, however, enhanced methamphetamine hyperactivity across repeated conditioning sessions in enriched rats, but not in isolated rats. In contrast to isolated rats, enriched rats failed to display significant conditioned place preference to the low dose of methamphetamine (0.3 mg/kg) following the monoamine-depleting dose regimen, suggesting that the rewarding effect of methamphetamine was blunted by the combined effect of enrichment and methamphetamine treatment. Thus, environmental enrichment may exacerbate the behavioral consequences of monoamine-depleting doses of methamphetamine.