Effects of perforant path procaine on hippocampal type 2 rhythmical slow-wave activity (theta) in the urethane-anesthetized rat

Abstract

Previous research has suggested that the entorhinal cortex plays a major role in the production of type 1 rhythmical slow-wave activity (RSA) recorded in the hippocampus of the freely moving preparation. In the present experiment we investigated the contribution of the entorhinal cortex to the type 2 fields recorded under urethane anesthesia. Rats had stimulating electrodes and cannulae filled with procaine positioned in the perforant pathway of one or both hemispheres. Recording electrodes were positioned in the dorsal hippocampus of each hemisphere to record perforant path and commissural/associational evoked potentials and RSA fields. Following unilateral procaine blockade, a decrease in RSA amplitude was observed in the stratum oriens and fissure regions of both hemispheres. Concomitant with this change in RSA, there was a loss of perforant path evoked responses, although commissural/associational control potentials remained unaltered. A greater reduction in RSA amplitude was observed following bilateral procaine microinfusion. RSA phase reversal also occurred more dorsally in microelectrode depth profiles conducted through the hippocampus during perforant path inactivation. In current source density analyses performed under baseline conditions, large rhythmic sinks were observed in stratum oriens, in stratum radiatum, and in strata adjacent to the hippocampal fissure. A rhythmic source was often observed in stratum pyramidale. Following perforant path inactivation decreases in the magnitude of the phasic sinks located near the fissure and stratum radiatum were observed. In contrast to the reduction in RSA amplitude observed in the stratum oriens region, the sink in this region and the source in stratum pyramidale remained relatively unaltered. These results demonstrate that the entorhinal region contributes to the production of RSA observed under urethane anesthesia. Furthermore, the CSD and amplitude changes following perforant path inactivation suggest that a substantial portion of RSA recorded in stratum oriens may result from ventrally located RSA dipoles.