Reset of hippocampal rhythmical activities by afferent stimulation.

Physostigmine induced theta rhythm and unit activity were recorded from the dorsal hippocampus in immobilized locally anesthetized rats. Correlations between theta and rhythmical unit activity and their modifications by hippocampal afferent stimulation were studied. The principal finding was that electrical stimulation of afferents reset theta and rhythmical unit activity in phase. Poststimulus theta displayed a variable frequency which depended upon the structure stimulated. Lower frequencies were evoked by septal, higher frequencies by entorhinal and reticular formation stimulations. When theta rhythms were absent either by spontaneous disappearance or as a consequence of lesions in the fornix superior of septum, the reset was not observed. The reset of the theta rhythms and unit activity by afferent stimulation, suggests that the hippocampus may participate in timing mechanisms.