ACETYLCHOLINE, THETA-RHYTHM AND ACTIVITY OF HIPPOCAMPAL-NEURONS IN THE RABBIT .1. SPONTANEOUS ACTIVITY

The background activity of hippocampal neurons was recorded extracellularly in waking rabbits in the control state and after systemic injections of physostigmine and scopolamine. Similar analysis was done in the hippocampus chronically deprived of ascending brainstem afferents. Cholinergic drugs control the number of hippocampal neurons with theta modulation and stability, but not the frequency of theta modulation. Increase of endogenous acetylcholine also resulted in regularization of the activity with suppression of delta modulation and complex spike discharges; its blockade produced the opposite changes. Both drugs changed the level of background activity in the majority of the neurons but the overall mean frequency did not vary between the states. Regression analysis demonstrated significant negative correlations with dominating decrease in the level of discharges in high-frequency neurons (> 25 spikes/s) and its increase in low-frequency ones (< 25 spikes/s) after injection of both drugs. Stability of the overall mean frequency and uniformity of its shifts during both stimulation and suppression of the cholinergic component of theta-rhythm presumably indicate that the frequency of background activity, unlike its pattern, is not directly controlled by the cholinergic septal input.