Cingulate cell discharge patterns related to hippocampal EEG and their modulation by muscarinic and nicotinic agents

Surface to depth recordings of slow wave θ activity were made through the posterior cingulate cortex (area 29). In other experiments the discharge patterns of neurons located throughout this region were recorded during the occurrence of large amplitude irregular activity (LIA) and slow wave θ activity (type 2) in the hippocampal formation of the urethane-anesthetized rat. The response of these neurons to the systemic administration of the cholinergic agonists, eserine and nicotine, was also tested. The majority of cells (80%) related to hippocampal EEG states were classified as tonic θ-on cells (non-rhythmic). These cells increased their discharge rates significantly, when the slow wave activity in the hippocampus changed from LIA to θ. The administration of eserine and nicotine induced slow wave θ in the hippocampus accompanied by the increased discharge rate of tonic θ-on cingulate cells. The excitatory action of nicotine on the discharges of tonic θ-on cingulate cortical cells is in direct contrast to its inhibitory action on phasic and tonic hippocampal θ-on cell discharges. The observation that phase reversals did not occur in area 29, together with the low incidence of phasic (rhythmic) θ-on cells, suggests that the posterior cingulate cortex does not independently generate type 2 θ. Possible significance of these findings for the interactive functioning of the hippocampal formation and posterior cingulate cortex is discussed.