[Latencies of Repeated Responses to Single Pulse Stimulation of the Schaffer Collaterals Registered in Hippocampal Field CA1 in Rats During Sleep].

It is assumed that hippocampal neurons which were activated via the neocortex by new stimulus during the wakefulness and after that maintain its transient memory trace must be reactivated during the sleep to consolidate corresponding permanent memory trace in the neocortex. So we investigated the possibility of reverberation of excitation in the neuronal circuits connecting the hippocampus and entorhinal cortex. In sleeping rats within proximal part of the field CA1 of dorsal hippocampus we recorded double and triple responses to single pulse stimulation of Schaffer collaterals with previously potentiated synapses.

[The excitation wave returning to the hippocampus through the entorhinal cortex can reactivate the populations of "trained" CA1 neurons during deep sleep].

It is suggested that the information about a new stimulus from the neocortex is transferred to the hippocampus and forms there a transient trace in the form of a distributed pattern of modified synapses. During sleep, the neuronal populations which store this trace are reactivated and return to the neocortex the information necessary for consolidation of the permanent memory trace. A possible mechanism of the reactivation of the "learned" hippocampal neurons during memory consolidation is the reverberation of excitation in the neuronal circuits connecting the hippocampus and the entorhinal cortex.

Return of excitatory waves from field CA1 to the hippocampal formation is facilitated after tetanization of Schäffer collaterals during sleep.

Current concepts hold that during learning in waking animals, new information is transmitted from the neocortex to the hippocampus, where it leaves a temporary trace in the form of a mosaic of modified synapses. During sleep, reactivation of the neuron population initially activated by the new stimulus has the result that this information is returned to the neocortex, ensuring consolidation of a permanent memory trace. Exchange of information between the neocortex and hippocampal formation is mediated mainly by the entorhinal cortex, whose internal connections, in principle, allow "messages" from the output of the hippocampal formation to return to its inputs.

[Return of excitatory waves from the field CA1 to the hippocampal formation is facilitated after Schaffer collateral tetanization and during sleep].

In agreement with existing notion, in a waking animal, novel information during learning transfers from the neocortex to the hippocampus where creates a transient trace in the form of a distributed pattern of modified synapses. During sleep, due to reactivation of neuronal populations initially activated by novel stimulus, this information flows back to the neocortex thus facilitating permanent memory trace consolidation. Information transfer between the neocortex and hippocampal formation is realized, in general, via the entorhinal cortex whose intrinsic connections basically permit "messages" from the exit of the hippocampal formation to return to its entrances.

Conditions required for the appearance of double responses in hippocampal field CA1 to application of single stimuli to Shäffer collaterals in freely moving rats.

Stimulation of Shäffer collaterals with single current impulses could evoke double responses in hippocampal field CA1 in freely moving rats. The late response - the population excitatory postsynaptic potential with a preceding transient potential, often biphasic - occurred only after an early population spike and was time-locked to it. The shape characteristics of the late response, its polarity, and its latent period relative to the early population spike suggest that stimulation of Shäffer collaterals gives rise, in CA1, to a wave of excitation which passes through the entorhinal cortex and returns to CA1 directly via fibers of the perforant path.