Plasma corticosterone levels during extinction of a lever-press response in hippocampectomized rats.

Abstract

Abstract Male Long-Evans rats with bilateral lesions of the hippocampus or of the cortex overlying the hippocampus and unoperated rats were trained to lever press for water reinforcement on a continuous reinforcement schedule, and then extinguished. The hippocampectomized animals responded at higher rates than controls during early acquisition sessions, the difference decreasing over training and increasing again during extinction. Plasma levels of corticosterone were determined in the resting (non-stressed) state on ad lib and deprivation watering schedules, and following ether anesthesia, exposure to a novel environment, a reinforced operant session and the first extinction operant session. The results indicated that hippocampal lesions produced by aspiration cause no essential deficit in pituitary-adrenal function. However, the hippocampectomized rats failed to exhibit the normal elevation of plasma corticosterone during the first extinction session. This finding contradicts proposals that hippocampectomy increases the frustrative-emotional response to unmet reward expectancies. It is supportive of attentional process theories of hippocampal function.