Septal modulation of the population spike in the fascia dentata produced by perforant path stimulation in the rat.

Electrical stimulation of the perforant path, which originates in the entorhinal cortex, produces a characteristic excitatory postsynaptic field potential (extracellular EPSP) which can be recorded in the fascia dentata. This evoked response may include a population spike, if stimulation is sufficient. In the anaesthetized rat, stimulation of the medial septum, when paired with perforant path stimulation, was found to augment the population spike component of the evoked field potential. Stimulation of the septum alone produced no apparent field potential. The augmentation effect was found to have a rapid onset (4 ms), which is sufficient for the participation of interneurons, and a relatively long time course (150 ms). Presynaptic mechanisms of facilitation were ruled out as there was no concurrent alteration of the extracellular EPSP. A change in population spike threshold, compatible with a postsynaptic mechanism, was observed and some possible models of action discussed. Augmentation survived depletion of hippocampal norepinephrine caused by injections of 6-hydroxydopamine into the dorsal noradrenergic bundle, indicating that the facilitation was not due to an activation of the ascending noradrenergic fibres of passage originating from the locus coeruleus. The cholinergic septo-hippocampal pathway was ruled out as a likely candidate for the modulation as the augmentation survived injections of the muscarinic antagonists atropine and scopolamine and the nicotinic antagonists tubocurarine and dihydro-beta-erythroidine. A relationship between the septal modulation and hippocampal theta was suggested.