Detection of an atropine-resistant component of the hippocampal theta rhythm in urethane-anesthetized rats.

An important pharmacological feature of the hippocampal theta rhythm in urethane-anesthetized animals is its apparent sensitivity to antimuscarinic drugs. This sensitivity may be partly due to a masking of the theta frequency by increases in both higher and lower frequency EEG components that are unrelated to any residual theta rhythm. The discovery of atropine-resistant, rhythmic medial septal neurons has provided a physiological trigger for averaging EEG and unit activity after large atropine doses. Such averaging has permitted the detection of an atropine-resistant component of the hippocampal theta rhythm in urethane-anesthetized rats. The postatropine theta activity recorded from both CAl (superficial to the pyramidal cell layer) and dentate (near the hippocampal fissure) in 15 rats was typically reduced in amplitude, but the recordings from the two locations maintained their phase relations to the septal units and to each other. The presence of this residual theta component after doses as large as 100 mg/kg indicates that it cannot be mediated by muscarinic cholinergic receptors. The coupling of the signal to the atropine-resistant septal cells strengthens our previous suggestion that these septo-hippocampal neurons are not cholinergic, and are therefore probably GABAergic.