The hippocampal formation is thought to be involved in spatial learning. A role as a more general, intermediate-term memory store has also been suggested. In this report, we address a moderately persistent change in propagation of excitation through hippocampal circuitry which resulted from exposure of animals to environments in which they were free to move and explore. Rats were prepared for chronic recording of perforant path-evoked dentate granule-cell population responses. A large increase in the synaptic component of this response developed over the first several minutes after the animals were transferred to a different but not necessarily novel environment, and decayed with a time constant of about 5 min. This apparent growth in synaptic strength was remarkably well correlated with the animal’s recent history of exploratory behavior, and was not due simply to handling or to the electrical stimulation. The amplitude of the population-spike component also varied over time, but was not obviously correlated with changes in the synaptic component. These response changes were different in both apparent mechanism and time course from previously reported, longer lasting, environmentally induced changes in the population-spike component. They also differed from the behavioral state-dependent gating effect reported by Winson and Abzug (1977) to the extent that the present effects long outlasted the behaviors that produced them. Although further analysis is required, it is possible that this phenomenon may reflect one mode of information storage in the hippocampal formation.
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