EXPOSURE TO A CONDITIONED AVERSIVE ENVIRONMENT INTERFERES WITH LONG-TERM POTENTIATION INDUCTION IN THE FIMBRIA-CA3 PATHWAY

The effect of re-exposure of rats to an aversive environment on the in duction of long-term potentiation was investigated in the CA3 region 3 and 12 h after contextual conditioning. Electrophysiological recordin gs showed that re-exposure of rats to the conditioning chamber produce d a significant and long-lasting decrease in population spike amplitud e al both post-conditioning delays. High-frequency stimulation of the fimbria induced a large and persistent increase in CA3 population spik e amplitude (about 400% of baseline) in animals of control groups and shocked animals that were not re-exposed to the conditioning environme nt. However, high-frequency stimulation applied during re-exposure of shocked subjects 3 h after the initial exposure resulted in a small an d transient increase in population spike amplitude (about 140% of base line); when applied 12 h after the initial exposure, it produced a per sistent depression of the response (-30% of baseline). Behavioural tes ting indicated that re-exposure of shocked animals to the conditioning environment elicited a qualitatively and quantitatively similar freez ing behaviour al both post-conditioning delays (3 or 12 h). In contras t to the long-lasting decrease in CA3 population spike amplitude produ ced by re-exposure to the aversive environment, the level of freezing behaviour diminished rapidly within 10 min of exposure. These results suggest that, during exposure to a conditioned aversive environment, a lterations in fimbria-CA3 neural processing may be dissociated from co ntextual fear-induced freezing behaviour. In addition, processes under lying long-term potentiation induction in fimbria-CA3 pathway may be o pposite to those taking place during hippocampal processing of conditi oned aversive contexts. (C) 1997 IBRO. Published by Elsevier Science L td.