Hippocampal EEG responses induced by carbachol and atropine infusions into the septum and the hippocampus in the urethane-anaesthetized rat.

Infusion of 1 microgram of carbachol, a potent cholinergic agonist, into the lateral septum of the urethane-anaesthetized rat systematically caused the induction of clear-cut hippocampal theta (theta). However, infusion of an equivalent amount of the drug into the hippocampus, close to the recording electrode, failed to induce theta in 50% of the animals and produced a mixture of theta waves and desynchronized activity, resulting in atypical EEG patterns, in the remaining subjects. Both carbachol EEG effects were blocked by intraseptal infusion of the antimuscarinic agent, atropine. Our data demonstrate that muscarinic receptors in the septum are predominant sites for cholinergic agonist-antagonist action capable of generating or suppressing hippocampal theta in the rat. They also indicate that intraseptal cholinergic mechanisms play an important role in the initiation and generation of this rhythm.