Long-term potentiation in the rat hippocampal CA1 region is inhibited selectively 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 whether the induction of LTP in the hippocampal regions was altered with the 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 sessions 24 h before. LTP of the amplitude of population spikes and the slope 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 training session or in those which had received overtraining. The inhibition of LTP in the CA1 was neither due to stress accompanied with training 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.