Medial septal facilitation of hippocampal granule cell activity is mediated by inhibition of inhibitory interneurones.

Rats under barbiturate anaesthesia were implanted with stimulating electrodes in the medial septal nucleus and the medial perforant path. A recording electrode and cannula were implanted in the hilus of the dentate gyrus. Electrodes were positioned so that a conditioning pulse to the medial septum, although eliciting no field potential of its own, facilitated the granule cell population spike evoked by medial perforant path stimulation. In the first experiment, the infusion into the hilus of the GABA antagonist picrotoxin was found to block the facilitation. In a second experiment, it was found that a medial septal conditioning pulse blocked recurrent inhibition of the granule cells, only if it was timed to coincide with their initial activation. We suggest that these effects are mediated through an inhibitory connection from the medial septum onto inhibitory interneurones in the dentate gyrus, and that this connection may utilize the neurotransmitter GABA.