The effect of experimentally induced focal epilepsy on operant responding in the rat.

Dose-response studies were performed on freely moving rats bearing chronically implanted electrodes. The effects of intracortical injections of penicillin, bicuculline, and conjugated estrogens (Premarin) were determined on EEG activity and motor performance. Bicuculline in doses of 0.025 and 0.05% regularly produced spike and afterdischarge (AD) patterns of rapid onset (less than 10 sec,) brief duration (less than 20 min), and marked intensity. Lower doses had little or no effect. Premarin 1% and penicillin 100 international units produced spike and AD patterns of slower onset (5 min), longer duration (up to 2 hr), and mild intensity. Smaller doses generally produced no AD activity, whereas larger doses produced epileptic patterns severe enough to preclude their use in operant studies. Penicillin and Premarin both produced a significant decrease in operant-response rate in animals trained to bar press on a FR 20 schedule. A marked variability was noted in the behavioral responses of individual rats to the disruptive effects of the epileptogens.