| Abstract: | Focal potentials (FP) elicited by stimulation of collateral-commissural fibers were recorded in the radial and pyramidal layers of the CA1 area in surviving mouse hippocampal slices. The influence of conditioning stimulation on responses in the tested neuronal pathway (the "priming" effect) at 50–1000 msec intervals between the conditioning and test stimuli and variable stimulation strengths was investigated. The relationship of the duration of the FP of the radial layer to the strength of the test stimulation at a 200 msec interval between the conditioning and test stimuli was studied in the first series of experiments. Three different regions of like relationship were distinguished. In region I (weak stimulation) the duration of the FP did not depend on the stimulus strength or on conditioning (i.e., the "priming" effect was not observed). In region II the duration of the FP in the control was shorter as compared with that observed in region I, which is associated with the triggering of the process of feed-forward inhibition. Conditioning led to the partial restoration of the duration of the FP (the "priming" effect, which evidently develops as the result of the suppression of feed-forward inhibition). In region III, by contrast with region II, the stimulation strength was sufficient for the suprathreshold excitation of the pyramidal neurons, which conditioned the development not only of feed-forward, but of feed-backward inhibition as well. The form of the FP in the radial layer is distorted in the process, and their duration cannot serve as an index of "priming." The influence of conditioning on the effect of paired-pulse depression of population peaks in the pyramidal layer was studied in the second series of experiments in order to identify possible changes in feed-backward inhibition. The principal effect consisted in a decrease in paired-pulse depression in the circuits tested; from this the conclusion was drawn of a suppression of feed-backward inhibition. It was concluded that both feed-forward and feed-backward inhibition are suppressed in "priming."Institute of the Brain, Russian Academy of Medical Sciences, Moscow. Translated from Neirofiziologiya, Vol. 24, No. 2, pp. 178–185, March–April, 1992. |
