Differential effects of imipramine in rats as a function of DRL schedule value

Imipramine, a tricyclic anti-depressant, has selective effects on differential-reinforcement-of-low-response-rate (DRL) schedule performance as a function of schedule value. On a DRL 9-sec schedule of water reinforcement, imipramine at doses of 2.5 to 20.0 mg/kg resulted in no significant change in the number of reinforcements or responses. A dose of 5.0 mg/kg imipramine, given to rats performing on a DRL > 72-sec schedule, produced a significant increase in the number of reinforcements and a concomittant decrease in responses. This effect was associated with a shift in the inter-response time (IRT) distribution to longer IRTs. Higher doses (20.0 and 40.0 mg/kg) decreased both response rate and reinforcement rate on the DRL 72-sec schedule. These findings demonstrate the importance of schedule parameters in determining the behavioral effects of imipramine.