LONG-TERM POTENTIATION IN THE RAT HIPPOCAMPAL CA1 REGION IS INHIBITEDSELECTIVELY AT THE ACQUISITION STAGE OF DISCRIMINATORY AVOIDANCE-LEARNING

Long-term potentiation (LTP) of hippocampal synaptic efficacy has been regarded as a synaptic model of learning and memory. We examined whet her the induction of LTP in the hippocampal regions was altered with t he advance of discriminatory avoidance learning. Evoked potentials in the CA1 region or dentate gyrus were recorded before and after tetanic stimulation in anesthetized rats which had been given training sessio ns 24 h before. LTP of the amplitude of population spikes and the slop e of excitatory postsynaptic potentials recorded in the CA1 region was smaller in rats at a learning stage 24 h after the avoidance rate had first been significantly increased than in naive rats. The magnitude of LTP was not altered in rats which had been exposed to the first tra ining session or in those which had received overtraining. The inhibit ion of LTP in the CA1 was neither due to stress accompanied with train ing nor liberation from the stress by learning avoidance response. In contrast, LTP induced in the dentate gyrus was rather enhanced at the learning stage when LTP in the CA1 was inhibited. The results suggest that the acquisition of discriminatory avoidance learning selectively inhibits LTP in the hippocampal CA1 region.