EFFECTS OF INTRAVENOUS INFUSION OF CENTRAL DEPRESSANTS ON THE EVOKED POTENTIALS OF THE AUDITORY CORTEX IN THE CATS

Abstract

The effects of the infusion of the various anesthetics on the evoked potentials and the evoked rhythmic after-discharges on the auditory cortex in response to the click stimuli were investigated in the encephale isole preparations of unanesthetized cats. The evoked potential was recorded in the narrow area of the middle ectosylvian gyrus, and was easily and sensitively affected by the change of the environmental conditions and the slight dislocation of the recording electrode on the cortex. The procedures resulted usually in the progressive reduction of the amplitude. The evoked potential in the unanesthetized cat was higher in amplitude and shorter in duration than that in the anesthetized cat. The manifestation of the evoked potential in response to maximal click stimuli was followed by the evoked rhythmic after-discharge, 10 to 15 cps in frequency, 200 to 300 [mu]V in amplitude and lasting for about 1.5 seconds. The evoked rhythmic after-discharge in the auditory cortex did not synchronize with the spontaneous spindle bursts in the motor cortex. The infusion of relatively small doses of pentobarbital sodium, chloralose and chlorpromazine increased the amplitude of the evoked potential, while the infusion of urethan and ethanol even in small doses reduced the amplitude. The duration of the evoked potential was markedly decreased by the depressants except chlorpromazine. The infusion of pentobarbital sodium, chloralose and ethanol in small doses accelerated the manifestation of the evoked rhythmic after-discharge to every click stimulus. The infusion of the depressants in larger doses depressed the amplitude of the evoked potential, reduced the evoked rhythmic after-discharge in frequency and prolonged the rhythmic after-discharge in duration. Above the dose level of 20 mg/kg of pentobarbital sodium and 80 mg/kg of chloralose the evoked rhythmic after-discharge disappeared totally, while the depressed rhythmic after-discharge was still observed at the dose level of 1.0 g/kg of urethane, 800 mg/kg of ethanol and 20 mg/kg of chlorpromazine. Twenty mg/kg of morphine did not affect the evoked potential and the evoked rhythmic after-discharge. The effects of 1.0 mg/kg of methamphetamine and 100 [mu]g/kg of strychnine on the evoked potential and evoked rhythmic after-discharge on the auditory cortex did not have consistent tendencies because of much individual variation of response.