Mescaline increases startle responding equally in normal and raphe-lesioned rats

Electrolytic lesions targeting either the dorsal or median raphe nucleus in rats were used to test whether serotonin-containing midbrain cells mediate the effects of mescaline on startle responses. Lesion effectiveness was confirmed by decreased tryptophan hydroxylase activity in the striatum or hippocampus. Median, but not dorsal, raphe lesions increased startle magnitudes to air-puff stimuli. Despite baseline differences, mescaline (10 mg/kg) produced comparable 25% increases in startle magnitudes in both sham- and raphe-lesioned animals. This result does not support the hypothesis that mescaline's effect on startle is mediated by the midbrain raphe nuclei.

Study at a glance

Abstract

To test the possible involvement of serotonin-containing cells of the midbrain in mediating the effects of mescaline on startle responding, electrolytic lesions were made in either the dorsal or median raphe nucleus in rats. Decreases in either striatal or hippocampal tryptophan hydroxylase activity confirmed the effectiveness of the lesions. One week later, startle was measured in response to 30 air-puff stimuli for each rat. Median, but not dorsal, raphe lesions increased startle magnitudes throughout the test session. The following day each group was divided into matched halves and were given 60 trials, 30 minutes after intraperitoneal injection of either saline or 10 mg/kg mescaline. Despite the large differences in baseline startle among the groups, mescaline produced comparable 25% increases in startle magnitudes in both sham- and raphe-lesioned animals. This result fails to support the hypothesis that increased startle responding produced by mescaline is mediated by the midbrain raphe nuclei.