Reward summation and the effects of pimozide, clonidine, and amphetamine on fixed-interval responding for brain stimulation.

Two models of reward summation were examined in 16 rats lever pressing for intracranial stimulation under fixed-interval (FI) reinforcement. The first model examined rate-frequency functions and the second model traded off frequency and train duration. The second model was selected to assess the effects of three drugs on reward summation. Both clonidine and pimozide inhibited FI self-stimulation, but pimozide's effect could not be distinguished from a performance deficit. Two amphetamine isomers facilitated self-stimulation in a manner suggesting enhanced reinforcement. The dextro isomer was four times more effective than the levo isomer to facilitate self-stimulation. This study shows that the combination of the FI schedule with a reward-summation model is well suited for evaluating the effects of drugs on self-stimulation. The advantages of this model are that inter-reinforcement intervals are separated, which minimizes priming and stimulation aftereffects, and more responding does not increase stimulation availability, thus eliminating rate-dependency effects.