EFFECTS OF PERFORANT PATH PROCAINE ON HIPPOCAMPAL TYPE-2 RHYTHMICAL SLOW-WAVE ACTIVITY-(THETA) IN THE URETHANE-ANESTHETIZED RAT

Previous research has suggested that the entorhinal cortex plays a maj or 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 positione d in the perforant pathway of one or both hemispheres. Recording elect rodes 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 o f both hemispheres. Concomitant with this change in RSA, there was a l oss of perforant path evoked responses, although commissural/associati onal control potentials remained unaltered. A greater reduction in RSA amplitude was observed following bilateral procaine microinfusion. RS A phase reversal also occurred more dorsally in microelectrode depth p rofiles conducted through the hippocampus during perforant path inacti vation. In current source density analyses performed under baseline co nditions, large rhythmic sinks were observed in stratum oriens, in str atum radiatum, and in strata adjacent to the hippocampal fissure. A rh ythmic source was often observed in stratum pyramidale. Following perf orant path inactivation decreases in the magnitude of the phasic sinks located near the fissure and stratum radiatum were observed. In contr ast to the reduction in RSA amplitude observed in the stratum oriens r egion, the sink in this region and the source in stratum pyramidale re mained relatively unaltered. These results demonstrate that the entorh inal region contributes to the production of RSA observed under uretha ne anesthesia. Furthermore, the CSD and amplitude changes following pe rforant path inactivation suggest that a substantial portion of RSA re corded in stratum oriens may result from ventrally located RSA dipoles . (C) 1994 Wiley-Liss, Inc.