Stress has been well-documented to affect hippocampal long-term potentiation (LTP), which is widely believed to underlie learning and memory at the cellular level. However, only few reports addressed the issue regarding the timing of presenting a stress episode in relation to LTP induction. Previously we could show that in the dentate gyrus a protein synthesis-independent early-LTP can be reinforced by swim stress into a protein synthesis-dependent late-LTP when the stress episode was applied within a time window of about 30 min around tetanization (Korz and Frey 2003). Considering the important role that the hippocampal CA1 plays in the processing of spatial and temporal information, we were now interested in studying the infl uence of swim stress on functional plasticity events in the CA1 region. Field potentials were recorded in freely moving rats subjected to a brief episode of swim 15 min before or after early-LTP induction in the CA1 by stimulating the contralateral CA3. Our results revealed that a swim episode alone exerted a transient depressing effect on baseline values of recorded fi eld potentials. Swim stress presented shortly after the induction of early-LTP resulted in its depotentiation. Furthermore, if the same stress protocol preceded early-LTP induction, it prevented the potentiation and induced a long-lasting depression of fi eld potentials. These data suggest that swim stress differentially alters synaptic plasticity in the CA1 when compared with the dentate gyrus.
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