Electrophysiological and pharmacological characteristics of facilitation of hippocampal population spikes by stimulation of the medial septum.

In rats under urethane anesthesia, single shock or tetanic stimulation of the medial septum--which evoked only minimal field potentials--sharply enhanced population spikes evoked in area CA1 by commissural stimulation. An enhancement of population spikes was observed only (a) in areas CA1 and CA2 (adjacent to CA1 in the dorsal hippocampus), but not in the fascia dentata or the deep pyramidal layers CA3 or CA4; (b) in a narrow range of depth, close to the stratum pyramidale; (c) when the intensity of commissural stimulation was of adequate intensity. A comparable facilitation of population spikes was produced at the same sites by microiontophoretic release of acetylcholine. The septal facilitatory action increased in effectiveness with the number of tetanic pulses (up to 10-12) at a given frequency, and it had a maximum at frequencies of 50-100 Hz. It reached a maximum 20-50 ms after the end of septal stimulation, and then decayed slowly, the overall duration being up to 300 ms. The cholinergic nature of the facilitation induced by septal stimulation was confirmed by the parallel potentiation of septal action and that of acetylcholine by physostigmine and their depression by atropine and scopolamine.