The effect of pentobarbital on the auditory evoked response in the brainstem of the rat.

The effect of pentobarbital on the auditory evoked response in the brainstem was studied in 20 male rats. Auditory evoked responses were recorded at regular intervals from the onset of a surgical level of anaesthesia until the return of spontaneous movement. There were small (less than 0.1 msec) but significant increases in the mean latency of each of the four major waves of the auditory evoked response under barbiturate anaesthesia. This is consistent with the effect of pentobarbital on far-field potentials generated from subthalamic locations in the somatosensory system. The mean transmission time in the brainstem, measured as the wave I-IV interpeak latency, increased significantly from 2.11 msec when the animals were awake to 2.15 msec when they were anaesthetized. The effect of pentobarbital on the amplitudes of the four waves was more complex than that on the latencies. Overall, there was an increase in the amplitude of wave I under barbiturate anaesthesia while wave II remained constant and wave III decreased. Wave IV was the component most affected by pentobarbital. In most animals there was either a marked reduction in amplitude or a complete abolition of this wave for one or more intervals during the period of anaesthesia.