Sensitization to amphetamine on the differential-reinforcement-of-low-rate 72-s schedule

The purpose of the present study is to determine whether the effect of specific intermittent injections of amphetamine (AMPH) on a differential reinforcement schedule of low rate (DRL) would result in a sensitized response to subsequent AMPH injections. Two groups of rats were trained on a DRL 72-s schedule until they reached stable baseline performance. One group (SENS, n = 8) was treated intermittently (no more than twice a week) with 1.5 mg/kg amphetamine for 3.5 weeks. The other group (CONT, n = 8) received intermittent saline (SAL) 1 ml/kg for 3.5 weeks. Acute injections of 1.5 mg/kg AMPH in the SENS group, engendered an increase in response rate, a decrease in reinforcement rate and disruption of the inter-response time (IRT) distribution profile. Acute SAL injections in the CONT group had no effect. Rats pretreated with intermittent 1.5 mg/kg AMPH, when treated with a lower dose of AMPH (0.5 mg/kg), showed an increase in response rate, a decrease in reinforcement rate and disruption of the IRT distribution profile by decreasing peak area and shifting the peak location towards a shorter IRT duration. Therefore, in rats pretreated intermittently with 1.5 mg/kg AMPH (SENS group), the dose of 0.5 mg/kg AMPH elicited a similar change in DRL 72-s response pattern, as did the acute injection of 1.5 mg/kg AMPH. In contrast, in rats pretreated with SAL (CONT group), the low dose of AMPH had either no or small effects. Thus, pretreatment with 1.5 mg/kg AMPH increases the magnitude of the response to 0.5 mg/kg AMPH. These results indicate that rats performing on the DRL 72-s schedule exhibit sensitization to AMPH, after AMPH is given intermittently over a 3-week period.