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 (MSC) innervate the dentate gyrus of the hippocampus. Electrical prestimulation of the MSC enhances perforant path-dentate gyrus evoked field potentials. Considering the large number of fibres that pass through this region, the effects glutamatergic stimulation of the MSC had on dentate gyrus field potentials, and accompanying changes in units, and EEG, was investigated in urethane-anaesthetized rats. The perforant path was stimulated at a rate of 0.1 Hz, evoking an EPSP and a population spike recorded in the dentate gyrus granule cell layer. L-glutamate was delivered by pressure ejection. Glutamate ejection to the MSC produced a significant enhancement of the population spike. The duration of enhancement ranged from 1 to 49 min ( approximately =10.5 min). A consistent, but relatively short increase in the EPSP slope was also demonstrated. MSC activation induced a theta rhythm in 7 of 10 animals (duration=20-112 s). Theta rhythm induction preceded spike enhancement and occurred for a shorter duration than the enhancement. The effects on spontaneous unit activity were mixed. However, all changes in firing rate preceded spike enhancement, and their duration rarely coincided with the duration of the spike enhancement. The population spike enhancement usually occurred without evidence of a change in paired-pulse inhibition.