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Abstract:
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Pre-clinical and clinical data suggest that the amygdala plays a role in the detection of emotional events and in the production of fear responses. The amygdala is composed of distinct nuclei that may serve different functional roles in the modulation of fear. The present study examined the roles of the medial (MeA) and central (CeA) nucleus of the amygdala in unconditioned and conditioned fear. Following bilateral ibotenic acid lesions of the MeA or CeA, rats were exposed to cat odor, an unconditioned fear stimulus. In comparison with sham-operated controls, rats with MeA lesions exhibited significant deficits in cat odor-induced unconditioned fear as indicated by a significant reduction in the duration of freezing and avoidance and an increase in the frequency of contact with the cat odor stimulus. In contrast, excitotoxic lesions of the CeA had no significant effects on cat odor-induced unconditioned fear. To examine the role of the MeA and CeA in conditioned fear, rats with similar fiber-sparing lesions of the MeA and CeA were exposed to foot-shock. Conditioned freezing was measured in the immediate post-shock period and a retention test administered after 24-h. Results indicated that MeA lesions had no reliable effects on contextual fear conditioning as indicated by no significant differences in freezing between lesion and control groups in the immediate post-shock period and in the retention test. In contrast, CeA lesions produced significant deficits in freezing occurring in the post-Shock interval and in the retention test. Together, these results suggest that the MeA, but not the CeA, plays a role in the mediation of predator odor-induced unconditioned fear. In contrast, the CeA, but not the MeA, appears to playa role in fear conditioning to a context paired with electric foot-shock. |
