Glutamatergic activation of the medial septum complex: an enhancement of the dentate gyrus population spike and accompanying EEG and unit changes

A large number of cells from the medial septum complex (RISC) innervate the dentate gyrus of the hippocampus. Electrical prestimulation of the MSC enh ances perforant path-dentate gyrus evoked field potentials. Considering the large number of fibres that pass through this region, thr effects glutamat ergic stimulation of the MSC had on dentate gyrus field potentials, and acc ompanying changes in units, and EEG, was investigated in urethane-anaesthet ized rats. The perforant path was stimulated at a rate of 0.1 Hz, evoking a n EPSP and a population spike recorded in the dentate gyrus granule cell la yer. L-glutamate was delivered by pressure ejection. Glutamate ejection to the MSC produced a significant enhancement of the population spike. The dur ation of enhancement ranged from 1 to 49 min (similar to = 10.5 min). A con sistent, but relatively short increase in the EPSP slope was also demonstra ted. MSC activation induced a theta rhythm in 7 of 10 animals (duration = 2 0-112 s). Theta rhythm induction preceded spike enhancement and occurred fo r a shorter duration than the enhancement. The effects on spontaneous unit activity were mixed. However, all changes in firing rate preceded spike enh ancement, and their duration rarely coincided with the duration of the spik e enhancement. The population spike enhancement usually occurred without ev idence of a change in paired-pulse inhibition. (C) 2000 Published by Elsevi er Science B.V. All rights reserved.