Behavior-dependent evoked potentials in the hippocampal CA1 region of the rat. II. Effect of eserine, atropine, ether and pentobarbital.

The correlations of the rat's behaviors and the hippocampal EEG with the averaged evoked potentials (AEPs) evoked by the Schaffer collaterals in the hippocampal CA1 region of the rat were studied after intraperitoneal injections of several drugs known to affect hippocampal EEG. Ether and eserine induced continuous train of rhythmical slow activity (RSA) of 5-6/sec in the hippocampal EEG, during which the AEPs showed waveforms intermediate between those elicited during large irregular activity (LIA) in the awake, immobile control and those elicited during walking in the control. Low dose nembutal and atropine induced high amplitude LIA during immobility, resembling EEG during slow-wave sleep (SWS). The AEPs during these LIA states, and that during LIA of wake-immobility were of similar waveforms. The AEP waveforms are proposed to form a continuum which corresponds to the continuum of EEG from high amplitude LIA to RSA of increasing frequency. AEP waveforms do not depend only on walking or similar movements which correlate with high frequency RSA. Atropine sulfate (25-50 mg/kg i.p.) severely dampened the oscillations in the AEPs of rats during walking or similar movements, even though the high frequency RSA was essentially unaffected. The difference between AEPs during immobility and those during walking was markedly reduced after atropine, even though the EEG-behavior relationship persisted. The effect of atropine on AEPs may be interpreted as a direct effect on the hippocampus which is apparently inconsistent with present knowledge. If the effect was on inputs from the brain stem or the septum to the hippocampus, the hypothesis that there are two pharmacological types of RSA (atropine-sensitive and -resistant) requires re-definition and re-examination.