Haloperidol produces within-session increments in operant response duration in rats.

On the basis of previously reported observations that haloperidol induces within-session decrements in operant response rate in rats, it was suspected that other measures of operant behavior may also display within-session changes after treatment with this neuroleptic. Accordingly, haloperidol (0.02, 0.04, 0.08, 0.16 mg/kg) was administered to six rats trained on a fixed-ratio 20 schedule of liquid food reinforcement, and response duration was recorded as a measure of drug effects independent of response rate. Significant within-session decrements in response rate and increments in response duration were observed as responding ensued. At the 0.08 mg/kg dose, 300% increases in response duration were seen during the last third of the responses made in a session. The progressive slowing of individual motor acts were interpreted as neuroleptic-induced Parkinsonism in the rat, and within-session decrements in response rate were likewise seen as a manifestation of the same pharmacological effects that increased response duration.