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. Our experiments in awake and sleeping rabbits demonstrated that waves of excitation can return to hippocampal field CA1 and the dentate gyrus via fibers of the perforant path, these waves having initially entered field CA1 via potentiated synapses of Schäffer collaterals; during sleep, re-entrant waves of excitation reach a maximum and have a high probability of evoking discharges of dentate gyrus neurons. Thus, the new stimulus, potentiating synaptic connections in the hippocampus and, probably, the entorhinal cortex during waking, create conditions for reactivation of the corresponding hippocampal neuron populations during sleep by waves of excitation returning via the entorhinal cortex.
Studies on living slices of hippocampus-entorhinal cortex formation from adult rats were performed to investigate changes in responses in field CA3 to stimulation of mossy fibers in conditions of perforant path tetanization with different parameters. Tetanization of the perforant path at frequencies of 10 and 100 Hz induced depression of responses in CA3 on testing of this same path. Tetanization of the perforant path at a frequency of 10 Hz and an amplitude subthreshold for potentiating mossy fiber synapses in CA3 became threshold if preceded by tetanization of the perforant path at a frequency of 100 Hz. Tetanization of mossy fibers at 10 Hz resulted in potentiation of the input to CA3, while tetanization at 100 Hz induced depression. High-frequency tetanization of the perforant path (100 Hz) delivered in trains following at the frequency of the theta rhythm, led mainly to depression of field CA3 responses to stimulation of mossy fibers.
The possibility of the restoration of long-term potentiation in the CA1 region and the dentate gyrus of the hippocampus during stimulation respectively of the dorsal raphé nuclei and locus coeruleus, with stimulus parameters inducing behavioral reactions, was investigated in freely-behaving rats. It was demonstrated that stimulation of the locus coeruleus, which was ineffective prior to the tetanization of the perforant path, led to the restoration of extinguished long-term posttetanic potentiation in the dentate gyrus of the hippocampus induced by tetanization of the perforant path. Stimulation of the dorsal raphé nucleus, which was ineffective prior to the tetanization of the Schaffer collaterals, led to the restoration of long-term posttetanic potentiation in the CA1 region of the hippocampus induced by tetanization of the Schaffer collaterals. A mathematical model is proposed which has made it possible to describe the restoration of long-term posttetanic potentiation on the basis of the notion of the existence of several states of calcium/calmodulin-dependent protein kinase. The restoration of long-term potentiation during stimulation of emotiogenic zones was examined as a model of the phenomenon of emotional reminding.
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