Neurotoxic lesions of the lateral nucleus of the amygdala decrease conditioned fear but not unconditioned fear of a predator odor: Comparison with electrolytic lesions

Considerable evidence suggests that the lateral (LA) and basal (BA) nuclei of the amygdala are sites of plasticity and storage of emotional memory. Re cent arguments, however, have seriously challenged this view, suggesting th at the effects of amygdala lesions are attributable to interference with pe rformance of fear behavior and not learning and memory. One way to address this controversy is to measure the same behavioral response during both con ditioned and unconditioned fear. This is done in the present study by measu ring fear- related freezing behavior after electrolytic and neurotoxic lesi ons of the LA or LA/ BA nuclei in rats in a contextual fear conditioning pa radigm and unconditioned fear to a predator odor. Electrolytic LA lesions a ttenuated post- shock freezing, retention test freezing, and freezing to th e predator odor trimethylthiazoline (TMT). In contrast, excitotoxic NMDA le sions of the LA had no effect on post- shock freezing but significantly att enuated retention test freezing. Furthermore, excitotoxic LA lesions did no t diminish freezing to TMT. Larger excitotoxic lesions that included the BA significantly reduced freezing in both the post- shock and retention tests but did not appreciably decrease freezing to TMT. The results suggest that the LA is important for memory of learned fear but not for generation of f reezing behavior. In addition, the BA plays a role in freezing in condition ed fear situations but not in unconditioned fear. The studies suggest that the LA and BA play different roles in fear conditioning, but neither of the m has a significant role in unconditioned freezing to a predator odor.