Abstract

The effects of halothane upon the central auditory nuclei were studied in cats bearing permanently implanted electrodes by simultaneously analyzing with computer techniques clickevoked responses at the auditory cortex, medial geniculate and inferior colliculus. At the auditory cortex and medial geniculate, halothane in concentrations of 0.9 and 1.5% exerted a dose-related depressant effect on the two-click recovery curve and increased the latency and amplitude of the evoked responses. At the inferior colliculus halothane had no effect on recovery curve or latency but tended to decrease amplitude of the evoked responses. These auditory recovery functions provided a more complete and reliable index of neuronal excitability than measurement of the amplitude of the evoked responses alone. The functional depressions at the level of the medial geniculate and at the auditory cortex were similar and parallel, suggesting that the cortex might have merely passively reflected geniculate depression induced by halothane. The immunity of the inferior colliculus, in contrast to the extreme sensitivity displayed by the next nuclear mass in the ascending auditory pathways, localized the level of drug action. Pentobarbital had effects similar to those of halothane.