Dynamics of memory state of the brain

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In this research project, long-term potentiation (LTP) at synapses in the rat hippocampus followed by spontaneous bursting discharges was induced and dynamical features of the responses of the hippocampal CA3 and CA1 regions to periodic stimulation were investigated, (l) LTP at afferent synapses to CA3 did not contribute to generate the spontaneous activity of CA3 but LTP at recurrent excitatory synapses in CA3 did. (2) The CA3 region where LTP was induced in recurrent excitatory circuits caused phase-locked and chaotic responses to periodic mossy fiber stimulation. (3) LTP can be induced in a local area of the CA3 region. The local area was maintained even if LTP was enhanced and resulted in spontaneous epileptiform bursts. The local area might correspond to a memory state. (4) The excitatory pathway from contralateral CA3 to CA1 via ipsilateral CA3 was augmented by tetanic stimulation of the contralateral CA3, while the direct pathway from the contralateral CA3 to CA1 was not. (5) Field potential responses of CA1 to periodic stimulation of ipsilateral CA3 depend on the stimulus frequency; the feature of the responses to 20 40 Hz stimulation changes dramatically with time. The negative field potential after an initial transient especially caused phase-locked and chaotic responses. This results from the dynamical features of CA3.The above results conclude that the CA3 region, which has rich recurrent excitatory connections, causes phase-locked and chaotic responses to periodic stimulation when LTP is induced in CA3, and those activities are projected to CA1 excitatory through Schaffer collaterals and inhibitory through interneurons.