Latency of Repeated Responses to Single-Pulse Stimulation of Schaffer Collaterals Recorded in Hippocampal Field CA1 in Rats during Sleep.

It has been suggested that hippocampal neurons activated via the neocortex by a novel stimulus during waking and storing memories of the stimulus must undergo repeated activation during sleep in order for the neocortex to form the corresponding permanent memory trace. This impelled us to study the possibility that reverberation of excitation occurs in neural networks connecting the hippocampus and the entorhinal cortex. Double and triple responses to single-pulse stimuli applied to Schaffer collaterals with previously potentiated synapses were recorded in the proximal part of field CA1 in the dorsal hippocampus in sleeping rats. Analysis of the latent periods of repeated responses suggested that the wave of excitation initiated in hippocampal field CA1, returning directly from the entorhinal cortex to the recording zone in field CA1 via fibers of the perforant pathway and also via field CA2, may again elicit discharges in field CA1 neurons, though not those which had been activated initially on stimulation of Schaffer collaterals. Thus, the neuronal circuits connecting the hippocampus and entorhinal cortex do not mediate reactivation of 'trained' neurons during subsequent sleep periods. [ABSTRACT FROM AUTHOR]