Hippocampal rhythmical slow activity following ibotenic acid lesions of the septal region. I. Relations to behavior and effects of atropine and urethane.

The effects of intraseptal injections of various concentrations of ibotenic acid on hippocampal electrical activity were studied in freely moving and urethane-anesthetized rats. Ibotenic acid selectively abolished the atropine-sensitive form of hippocampal rhythmical slow activity (RSA) normally seen during urethane anesthesia. Large amplitude irregular activity (LIA) and RSA in the waking state were somewhat depressed as well. Despite this, clear RSA persisted in the waking state in association with locomotion or struggling (Type 1 behavior). As in normal rats, such RSA was resistant to systemic administration of atropine. Analysis of brain sections stained with gallocyanin or for acetylcholinesterase showed that ibotenic acid produced cell loss in the dorsal lateral septal nucleus and the septohippocampal nucleus. Cells in the medial septal and diagonal band nuclei were resistant to ibotenic acid. The results suggest that intrinsic septal circuitry is critically involved in the generation of the atropine-sensitive (presumably cholinergic) form of RSA. The mechanisms by which LIA and the two forms of RSA are generated in the hippocampus is discussed.