D1 dopamine receptor blockade prevents the facilitation of amphetamine self-administration induced by prior exposure to the drug.

Prior exposure to amphetamine leads to sensitized locomotor responding to subsequent injections and an enhanced predisposition to self-administer low doses of the drug. Because D1 dopamine (DA) receptors have been shown to play an important role in the development of sensitized locomotor responding to amphetamine, the present experiment assessed their contribution to the development of facilitated amphetamine self-administration produced by prior exposure to the drug. During a pre-exposure phase, rats were administered two injections on each of 10 consecutive days. The first injection (saline, 1 ml/kg, i.p., or the D1 DA receptor antagonist SCH23390, 0.04 mg/kg, s.c.) preceded the second (saline or amphetamine, 1.5 mg/kg, i.p.) by 30 min. Starting 10 days after the last injection, animals were given the opportunity to lever press for a low dose of amphetamine (10 microg/kg per infusion) in a two-lever (active versus inactive) continuous reinforcement operant task, in each of seven daily sessions. Consistent with previous reports, prior exposure to amphetamine resulted in an increase in active versus inactive lever pressing. Blocking D1 DA receptors with SCH23390 prior to each of the amphetamine pre-exposure injections prevented the development of this enhanced self-administration of amphetamine. When animals were grouped according to their locomotor response to a novel environment (assessed prior to the experiment), it was found, again in agreement with previous reports, that enhanced drug self-administration (as indicated by increased active versus inactive lever pressing as well as shorter latencies to emit the first active lever press, shorter inter-response times and more time-out responses on this lever) was observed only in amphetamine pre-exposed rats that had shown a locomotor response to novelty above the median of the subject sample (high responders). Preceding the amphetamine pre-exposure injections with SCH23390 blocked the development of enhanced drug self-administration observed in these animals. These findings, indicating that manipulations known to block the induction of locomotor and dopaminergic sensitization by amphetamine also block the facilitation of drug self-administration, suggest an important and common role for D1 DA receptor activation in the development of enhanced responding to and for drug.