Modulation of the influences of cortical input on hippocampal neurons by cholinergic substances.

The cholinergic modulation of responses of individual neurons and of the focal potentials of the hippocampus, induced by electrical stimulation of the perforant path or mossy fibers were studied in two groups of unanesthetized rabbits, one with an intact septal region (IS), and one with its basal undercutting (BU). In all of the animals the responses to stimulation were blocked or markedly suppressed in a substantial portion of the neurons (50% in IS, 69% in BU) against the background of the administration of eserine. Facilitation of the responses was observed in 10 and 8% of cases, respectively. Scopolamine restored the initial reactivity of hippocampal neurons and intensified responses to stimulation of the perforant path. The effect of eserine was reproduced by stimulation of the medial septal region (MS-DB). The depressive effect of stimulation of the MS-DB was intensified by the administration of eserine and blocked by scopolamine. Brief conditioning stimulation of the MS-DB which imitates a theta salvo facilitated responses to test stimulation of the MS-DB with delays of 70-150 msec, but suppressed them at smaller and greater intervals. Focal potentials in response to stimulation of the perforant path in CA1 were suppressed to an equal extent (by 43%) during sensory stimulation inducing natural theta rhythm, during the action of eserine, and with stimulation of the MS-DB In the BU group, these effects led to the complete suppression of focal potentials; scopolamine restored them. It is hypothesized that the principal function of the septohippocampal cholinergic input resided in the negative filtration of signals arriving against the background of theta rhythm that has been turned on by another, preceding influence, as a result of which their interference with the processing and recording of received information is prevented.