INVOLVEMENT OF M2-MUSCARINIC AND NON-M2-MUSCARINIC RECEPTORS IN HIPPOCAMPAL THETA-RHYTHM INDUCED BY CARBACHOL INFUSION INTO THE SEPTUM OF THE RAT

Binding and autoradiographic studies have shown the presence of a rath er high density of M2 muscarinic subtype receptors and the apparent ab sence or low density of the M1 subtype in the septum. We tested the hy pothesis that, in the urethane-anesthetized rat, septal M2 receptors a re involved in the generation of the hippocampal theta (theta) rhythm induced by intraseptal administration of carbachol, a potent cholinomi metic agent. Carbachol-induced theta was blocked by local infusion of the unspecific muscarinic antagonist agent, atropine (20 mu g (29.55 n M)), given 10 min prior to carbachol. The intraseptal administration o f low to high doses of gallamine (range: 20-180 mu g (22.43-201.90 nM) ), a specific M2 antagonist which displays high affinity for the septa l region, resulted in significant changes in the electrophysiological characteristics of carbachol-induced theta but failed to abolish this rhythm. It is suggested that the latter may have resulted from a combi ned activation of both M2 and non-M2 receptors at septal level.