CURRENT SOURCE DENSITY ANALYSIS OF THE HIPPOCAMPAL THETA RHYTHM - ASSOCIATED SUSTAINED POTENTIALS AND CANDIDATE SYNAPTIC GENERATORS

Single-electrode depth profiles of the hippocampal EEG were made in ur ethane-anesthetized rats and rats trained in an alternating running/dr inking task. Current source density (CSD) was computed from the voltag e as a function of depth. A problem inherent to AC-coupled profiles wa s eliminated by incorporating sustained potential components of the EE G. 'AC' profiles force phasic current sinks to alternate with current sources at each lamina, changing the magnitude and even the sign of th e computed membrane current. It was possible to include DC potentials in the profiles from anesthetized rats by using glass micropipettes fo r recording. A method of 'subtracting' profiles of the non-theta EEG f rom theta profiles was developed as an approach to including sustained potentials in recordings from freely-moving animals implanted with pl atinum electrodes. 'DC' profiles are superior to 'AC' profiles for ana lysis of EEG activity because 'DC'-CSD values can be considered correc t in sign and more closely represent the actual membrane current magni tudes. Since hippocampal inputs are laminated, CSD analysis leads to s traightforward predictions of the afferents involved. Theta-related ac tivity in afferents from entorhinal neurons, hippocampal interneurons and ipsi- and contralateral hippocampal pyramids all appear to contrib ute to sources and sinks in CA1 and the dentate area. The largest thet a-related generator was a sink at the fissure, having both phasic and tonic components. This sink may reflect activity in afferents from the lateral entorhinal cortex. The phase of the dentate mid-molecular sin k suggests that medial entorhinal afferents drive the theta-related gr anule and pyramidal cell firing. The sustained components may be simpl y due to different average rates of firing during theta rhythm than du ring non-theta EEG in afferents whose firing rates are also phasically modulated.