Influence of medial septal and entorhinal cortex lesions on theta activity recorded from the hippocampus and median raphe nucleus.

The electrophysiological relationship between the hippocampus (HP) and the median raphe nucleus (MRN) was examined in the freely moving rat. Bilateral HP recording electrodes were implanted in all animals (n = 15). In the first group of animals, placement of a monopolar recording electrode in the MRN allowed for simultaneous recording of field activity from the HP and MRN, both prior to and following electrolytic destruction of the medial septum (MS) (n = 10). Immediately following MS lesions, theta activity recorded from both sites was abolished. These data suggest that the MS may serve as a pacing input to both the HP and MRN. In a second group of animals, field activity was recorded from the HP and MRN, both prior to and following electrolytic destruction of the entorhinal cortex (EC) (n = 5). Type 1 theta activity recorded from both the HP and MRN was attenuated by EC lesions; however, detomidine-induced immobility-related theta activity was unaffected by destruction of the EC. In light of these findings, the plausibility of EC inputs to the MRN was discussed. The present results suggest that theta activity recorded from the MRN is dependent on the same neural mechanisms that control theta production in the HP.