DRUG CONDITIONING INDUCED BY INTRASTRIATAL APOMORPHINE ADMINISTRATION

The present study examined (1) whether the neostriatum is involved in a drug-induced conditioned locomotor response and; (2) whether this st ructure participates in the development of behavioral sensitization. M oreover, the present study addressed the question whether the developm ent of behavioral sensitization is necessary for the induction of cond itioning. Rats received injections of either apomorphine (2 mu g) or v ehicle (solution of 0.1% ascorbate/saline) into the dorsal neostriatum daily for 7 days. These treatments were performed immediately prior t o (apomorphine-paired group and vehicle group) or 30 min following (ap omorphine-unpaired group) 10-min placement in an open field which serv ed as the test environment. After a 3-day drug withdrawal period, the animals were given a 10-min non-drug vehicle test trial in the test en vironment. Three days later, a drug test with apomorphine was administ ered to the animals of the paired and unpaired treatment groups; the v ehicle group again received an injection of vehicle. The analysis of l ocomotor activity in the open field (measured as the distance traverse d) revealed that locomotor activity in the apomorphine-paired group wa s higher than in the other groups. There were no indications for behav ioral sensitization to intrastriatal apomorphine, since the locomotor response in the apomorphine-paired group did not increase, but rather decreased with daily repeated injections of apomorphine. Furthermore, only the apomorphine-paired animals showed a higher locomotor response when tested after an intrastriatal injection of vehicle in the previo usly apomorphine-paired environment, which is indicative of a conditio ned drug effect. These results suggest that the neostriatum is directl y involved in the development of drug-induced conditioning of locomoto r behavior but not in the establishment of behavioral sensitization. ( C) 1998 Elsevier Science B.V.