SUPPRESSION OF PAIR-PULSE DEPRESSION OF THE POPULATION SPIKE IN THE DENTATE GYRUS DURING CARBACHOL-INDUCED THETA-LIKE ACTIVITY IN GUINEA-PIG HIPPOCAMPAL SLICES

In pair-pulse stimulation experiments, pair-pulse depression (PPD) of the population spike (PS) occurred at intervals shorter than 20 ms in the dentate gyrus in guinea pig hippocampal slices. Application of 50 mu M carbachol resulted in an increase in the test PS amplitude and ca used suppression of PPD. This suppression was antagonized by atropine sulfate, a muscarinic receptor antagonist. Carbachol at 50 mu M induce d intermittent bursts of theta-like activity (TLA). We compared the pa ir-pulse index (PPI) during TLA with that in a rest period between bur sts of TLA. The PPI was defined as the ratio of the amplitude of the t est PS to that of the conditioning PS. The PPI during TLA were signifi cantly larger than that during the rest period, although there were no significant differences in the conditioning PS amplitude and the test pEPSP slope. When TLA was induced, the PPI during the rest period was increased by bicuculline. The PPI during TLA did not change significa ntly with the drug. The increase by bicuculline in the PPI during the rest period was caused by increase in the test PS amplitude. PPD can o ccur due to inhibition of granule cell activity by inhibitory neurons. Our findings suggest that the action of inhibitory neurons on granule cell activity is suppressed by activation of muscarinic receptors, wi th stronger suppression during TLA than during the rest period between bursts of TLA. (C) 1997 Elsevier Science Ireland Ltd.