The hippocampus is involved in the detection of novelty and is essential for certain forms of learning about environmental events and relationships. The cellular and molecular mechanisms of one form of hippocampal synaptic plasticity, long-term potentiation (LTP), are thought to overlap significantly with the neural mechanisms of learning. In this study changes in hippocampal synaptic efficacy were measured in awake, freely behaving rats during exploration of novel environments. Field potentials were recorded in the dentate gyrus following single-pulse electrical stimulation of the perforant pathway. Because hippocampal physiology is modulated by ongoing behavior, evoked potentials collected during Type 1 vs. Type 2 behavior were evaluated separately. The effect of prior LTP induction at perforant path-dentate synapses on exploration-induced changes was evaluated. Effects of shifts in basal body temperature, activity, and general arousal were controlled by measuring evoked potentials bilaterally, with prior LTP induced only in one hippocampus. The results show that exploration causes an increase in population spike amplitude with no change in excitatory postsynaptic potential during Type 1 behavior that last longer than 5 minutes. Prior induction of hippocampal LTP occludes the change induced by exploration. Thus, the modification of hippocampal synaptic function induced by exploration is not due to general factors such as temperature change and at least partly overlaps with the mechanisms of LTP maintenance and Type 1 behavioral modulation.
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