Locomotor activity and dopamine synthesis following 1 and 15 days of withdrawal from repeated apomorphine treatments

In two experiments, the effects of repeated apomorphine treatments on locomotor activity and terminal field dopamine synthesis was assessed after either a 1- or 15-day withdrawal period. In the first experiment, rats (n = 11/group) were treated with apomorphine (1.0 mg/kg, s.c.) or vehicle and tested for locomotor activity daily for 10 days. Fifteen days after the last repeated treatment, all rats received 1.0 mg/kg apomorphine and were tested for locomotor activity. Locomotor sensitization developed over the 10 day period and was still evident after the 15-day withdrawal period. In the second experiment, rats (n = 11/group) were treated with apomorphine (1.0 mg/kg, s.c.) or vehicle and tested for locomotor activity daily for 10 days. Dopamine synthesis was assessed following 1 or 15 days of withdrawal by measuring dihydroxyphenylalanine (DOPA) accumulation (after DOPA decarboxylase inhibition with NSD-1015) in striatum and nucleus accumbens- olfactory tubercle. As in the first experiment, rats treated with repeated apomorphine showed locomotor sensitization over the 10 days, relative to controls. Dopamine synthesis was reliably enhanced in the striatum, but not nucleus accumbens-olfactory tubercle, following both 1- and 15-day withdrawal periods. These results indicate that enhanced basal dopamine synthesis following repeated apomorphine treatments, similar to locomotor sensitization, is a persistent phenomenon.