Involvement of M2 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 urethaneanesthetized rat, septal M2 receptors are involved in the generation of the hippocampla theta (ϑ) rhythm induced by itraseptal administration of carbachol, a potent cholinomimetic agent. Carbachol-induced ϑ was blocked by local infusion of the unspecific muscarinic antagonist agent, atropine (20 μg (29.55 nM)), given 10 min prior to carbachol. The intraseptal administration of low to high doses of gallamine (range: 20–180 μg (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 ϑ 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.