Passive avoidance after ibotenic acid and radio frequency lesions in the rat amygdala.

Ibotenic acid (IBO) is assumed to lesion neurons and to spare fibers of passage. The effects of such lesions in the amygdaloid complex of rats were compared to those of radio frequency lesions (RF) on a passive avoidance task. Both lesions impaired the acquisition of the passive avoidance response. Plasma corticosterone levels were measured after a retention test in which no electrical shocks were applied. None of the lesion groups showed reduced corticosterone levels, the IBO lesioned rats actually showed significantly higher corticosterone levels than both RF lesioned and control animals. The corticosterone results are not consistent with a general reduction in fear. The slower avoidance learning may be a conditioning deficit due to impaired sensory information processing in the amygdaloid complex.