Effects of midbrain raphe stimulation and lesion on EEG activity in rats.

In order to clarify whether the dorsal raphe (d-R) and medial raphe nuclei (m-R) of the midbrain play a different role in EEG activity, the effects of either electrical stimulation or lesion of each raphe nucleus were examined in rats. After m-R stimulation, EEG activity changed to a marked drowsy pattern. D-R stimulation, however, failed to produce such effects. M-R lesion elicited a significant arousal in EEG. The change in hippocampal EEG was the most characteristic, i.e., synchronized waves with an extreme increase in voltage. D-R lesion did not produce such a change. The hippocampal EEG arousal was easily abolished by L-5-hydroxytryptophan without any change in the cortical EEG or behavior. These results suggest that the m-R plays a more important role than the d-R in regulating the hippocampal EEG activity, especially in inhibiting the appearance of high voltage synchronized waves in the hippocampus.