Effect of raphe lesions on the development of chronic tolerance to pentobarbital and cross-tolerance to ethanol.

Sham and electrolytic lesions of the dorsal, median, and median + dorsal raphe nuclei were made in different groups of rats, and the differential patterns of regional 5-HT depletion were verified chemically. One week later, an initial dose-response curve for the motor impairment effect (moving belt test) of pentobarbital was obtained. Matched subgroups of the animals in each lesioned group received daily gavage with either pentobarbital (50 mg/kg) or water for 36 days. Tolerance to the motor impairment effect of pentobarbital was measured at 4-day intervals. Lesions of the dorsal raphe nucleus had no influence on the development of tolerance, whereas median and median + dorsal raphe lesions resulted in slower development of tolerance, though plasma pentobarbital levels were unaltered. The effect of the combined lesion was similar to that of the median raphe lesion alone. A separate study revealed a similar differential effect of median versus dorsal raphe lesions on the development of cross-tolerance to ethanol.