Behavioral effects of selective midbrain raphe lesions in the rat

Lesions were produced in the median (n = 8), dorsal (n = 7) or both (n = 7) midbrain raphe nuclei and their effects on behavior (days 16-54 postoperatively) compared to that of controls (n = 9). In addition, forebrain 5-hydroxytryptamine (5-HT) concentration were determined. Only the median and combined lesion groups showed increased running wheel and open field activity, as well as enhanced reactivity to novel stimuli and environmental change. None of the lesion groups, however, showed changes in home cage activity on postoperative day 21. Although all lesion groups were deficient in the acquisition and retention of one-way avoidance, the deficits were of a greater magnitude in the median and combined lesion groups. The latter two groups, furthermore, were impaired in forced extinction of the one-way avoidance response, but only the combined lesion group evidenced facilitation of two-way avoidance acquistion. Thus, in contrast to the effects of median or combined raphe lesions, lesions in the dorsal raphe nucleus affected few of the behavioral parameters studied. These results suggest that the dorsal raphe nucleus plays a different behavioral role than the median raphe nucleus. The median nucleus appears to be involved in the regulation of activity level, the reaction to novelty and environmental change, and the response to aversive stimuli. Possible mechanisms for the observed behavioral changes are discussed, as well as their apparent similarity to the effects of other mesencephalic and limbic lesions. Lastly, the median, dorsal and combined raphe lesions lowered forebraine 5-HT but 26, 65, and 77%, respectively, versus controls. These reductions differed significantly from each other, and with previously reported data indicate that the dorsal raphe nucleus in the principal origin of forebrain 5-HT. It is suggested, furthermore, that the behavioral effects of midbrain raphe lesions are not due primarily to their associated reduction in forebrain 5-HT.