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 rather high density of M2 muscarinic subtype receptors and the apparent absence or low density of the M1 subtype in the septum. We tested the hypothesis that, in the urethane-anesthetized rat, septal M2 receptors are involved in the generation of the hippocampal theta (theta) rhythm induced by intraseptal administration of carbachol, a potent cholinomimetic agent. Carbachol-induced theta was blocked by local infusion of the unspecific muscarinic antagonist agent, atropine (20 mu g (29.55 nM)), given 10 min prior to carbachol. The intraseptal administration of 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 septal 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 combined activation of both M2 and non-M2 receptors at septal level.