Effects of small amygdala lesions on fear, but not aggression, in the rat

The effects of amygdala lesions on passive avoidance of drinking (dPA) and social interactions in a resident-intruder test were examined in two experiments that utilized different lines of Long-Evans hooded rats. The lesions were fairly well restricted to the rostral half of the central nucleus (rACe), or the cholinergically richly innervated basolateral nucleus (ABL) or the medial nucleus (AMe) of the amygdala. In both experiments, dPA deficits indicating disturbances in fear conditioning or fear expression were found with ABL and rACe lesions. The rACe lesions produced a greater deficit. AMe lesions caused no dPA deficit at all, which contrasts with the mild PA deficits reported by others employing larger lesions extending to the cortical nucleus and, perhaps, damaging the central nucleus. Social behavior was not affected by the lesions in any clear manner. In rats from a long-standing home colony, rACe lesions increased a behavior of plowing and kicking the wood-chip cage bedding during social encounters, and AMe lesions increased lateral defense behaviors. Both effects are paradoxical, suggesting increased anxiety in the fear-deficient rACe rats and increased defense with AMe lesions, despite several previous reports of decreased defense. In the second experiment, rats purchased from a supplier showed no lesion effects during social interactions; like the control group, all three lesion groups exhibited increases in offense associated with cohabitation with a female. The ABL lesions, particularly, had no effect comparable to the decreased offense recently reported to occur following neurotoxin lesions.