IMMEDIATE-EARLY GENE-EXPRESSION IN THE AMYGDALA FOLLOWING FOOTSHOCK STRESS AND CONTEXTUAL FEAR CONDITIONING

This study investigated the increase in expression in the amygdala of 2 immediate-early genes, c-fos and NGFI-A, following contextual fear c onditioning. The immediate-shock freezing deficit paradigm was used to compare rats that received footshock after exploring a context to rat s that received footshock immediately after placement in the chamber. The former procedure produces contextual fear conditioning while the l atter does not. Rats were either handled (handled group), placed in a test chamber without receiving footshock (context-no-footshock group), received footshock immediately upon being placed in the chamber (imme diate-footshock group), or received footshock after a 1 min delay (del ayed-footshock group). Only the delayed-footshock group displayed a fe ar response (freezing behavior). Rats were sacrificed either 15 min af ter the experience or after a retention test 24 h later. The c-fos mRN A was increased in the medial nucleus of the amygdala in all of the gr oups that were placed in the test chamber. However, rats that received footshock (immediate- and delayed-footshock groups) had greater level s of c-fos mRNA expression than rats of the context-no-footshock group . The c-fos mRNA expression in the immediate- and delayed-footshock gr oups did not differ. However, after the retention test, the expression of c-fos mRNA in the medial nucleus of the amygdala did not differ be tween groups. In contrast to c-fos, NGFI-A mRNA expression in the late ral nucleus of the amygdala was greater in the delayed-footshock group than the handled and context-no-footshock groups 15 min after the foo tshock, This elevation in NGFI-A mRNA was not seen in the immediate-fo otshock group. This suggests that NGFI-A mRNA in the lateral nucleus o f the amygdala may play a role in contextual fear conditioning. (C) 19 98 Elsevier Science B.V. All rights reserved.