The neural substrate of the startle response

Fifteen rats were exposed to various intensities of startle-eliciting auditory stimulation and footshock, and measures of response probability and amplitude were obtained. Subjects then received (1) lesions directed at the nucleus reticularis gigantocellularis (RGI), (2) lesions directed at the nucleus reticularis pontis caudalis (RPC) or (3) no lesions. After surgery, all subjects were tested for the same measures of startle responsiveness as had been collected in the pre-operative series. In addition, sensitivity to heat-induced pain and running wheel activity were also evaluated. Subjects with lesions of RPC showed a significant decrease in both the probability and amplitude of response to footshock as well as to acoustic signals. Subjects in the other two groups did not show significant decreases in responsiveness. No difference in pain sensitivity was found between groups. Subjects with lesions of RPC, however, were more (rather than less) active than the unoperated subjects. These data support the hypothesis that the mammalian startle response is elaborated through the nucleus reticularis pontis caudalis.