Pontine lesions attenuate physostigmine suppression of self-stimulation in the rat

In male Wistar albino rats with chronically implanted electrodes, self-stimulation behavior was compared before and after making bilateral pontine lesions involving the subcoeruleus area and adjacent tegmental field. Before lesioning, slight suppression of bar pressing after subcutaneous injection of 0.05 mg/kg physostigmine, marked stable suppression after 0.1 mg/kg, and very strong suppression after 0.2 mg/kg were observed. After making pontine lesions, the suppressive effects of physostigmine were clearly attenuated. With 0.05 and 0.1 mg/kg, no effect or only occasional slight suppression was observed in most cases. With 0.2 mg/kg, total suppression was induced in association with some peripheral effects, but the duration was obviously less than in the prelesion controls. Control saline injection did not produce a suppressive effect. From these findings, it is suggested that suppression of self-stimulation by physostigmine up to 0.1 mg/kg is due to the inhibition of motor activity through the cholinoceptive dorso-lateral pontine tegmental area and not due to a direct effect on the forebrain cholinergic process, presumably involving reinforcement or motivation.