The effects of amygdala lesions on hippocampal activity and classical eyeblink conditioning in rats

The hippocampus and the amygdala have long been associated with memory, emotion, and motivated behaviors. Although the role of these two brain areas in learning a simple, discrete motor response has been well studied, a definitive theory concerning their functions remains elusive. The present experiment involved selective lesions of the central nucleus (CE) or the basolateral nucleus (BA) of the amygdala in rats followed by single-unit analyses of hippocampal CA1 subfield activity during classical eye blink conditioning. Removal of CE or BA adversely affected the development of conditioned responding. Differences between groups in the patterns of hippocampal activity were observed. Similar to previous rabbit studies, hippocampal activity recorded from sham rats showed that CA1 cells became active during the CS-US period as conditioning progressed with activity especially prevalent just prior to US onset. Increased activity over training was seen during the CS-US interval in CE-lesioned rats, but the pattern differed from control rats-uniform excitation was seen across the entire CS-US period. BA-lesioned rats initially showed uniform CS-US period activation in early phases of training, but then showed patterns of hippocampal activity that resembled control rats in later stages of conditioning. The data suggest that the amygdala may play a modulatory role in the acquisition of conditioned eye blink responses and also in the formation of learning-related activity in the hippocampus.