Response to electric shock in rats: effects of selective midbrain raphe lesions.

The forebrain serotonin (5-HT) concentrations of rats with lesions in the median (M; n equal to 5), dorsal (D; n equal to 5), and both (DM; n equal to 6) midbrain raphe nuclei were, respectively, 22, 48, and 70% lower than in control animals (n equal to 10). The lesion and control groups, however, did not evidence differences in pain sensitivity as measured by the flinch-jump technique. On the other hand, of the animals tested, those with M (n equal to 3) and DM (n equal to 4) lesions required more trials than controls (n equal to 6) to acquire a one-way avoidance response. D lesion rats (n equal to 2) did not differ from controls in one-way avoidance learning, except in terms of prolonged escape latencies during the first three trials. The previously reported increased sensitivity to painful stimuli subsequent to medial forebrain bundle lesions or para-chlorophenylalanine administration, therefore, does not appear to be due exclusively to disruption of ascending 5-HT fibers originating in the dorsal and median raphe nuclei. The effects of midbrain raphe lesions of avoidance learning, furthermore, depend on lesion locus, and are not due to either hypo- or hyperalgesia.