PRESYNAPTIC INHIBITION IN THE LATERAL GENICULATE BODY INDUCED BY STIMU LATION OF THE CEREBRAL CORTEX

Abstract

Effects of conditioning stimulation of the cerebral cortex upon excitability of the terminals of the optic tract (OT) were examined in unanesthetized cats with immobilization. The cortical stimulation was made with a single shock or repetitive pulses at 100-300/sec.1. The antidromically conducted action potential of the OT, produced by stimulation of the OT terminals within the lateral geniculate body (LGB) and recorded from the optic chiasma, was enhanced following the cortical conditioning stimulation. This facilitatory effect appeared with a latency not less than 10 msec, reached the maximum at about 60-90 msec and continued sometimes beyond 400 msec. The facilitation increased with increasing the number of pulses for the cortical conditioning stimulation and was seen with varying the intensity of the testing stimulation in a wide range.2. The action potential of the terminals of the OT, produced by stimulation of the optic chiasma and recorded from within the LGB, was suppressed by cortical conditioning stimulation. The time course of this suppression was about the same as that of facilitation of the antidromic response of the OT.3. Combining the above-mentioned findings, it was concluded that the effects of cortical stimulation were manifestation of the presynaptic inhibition in the LGB.4. In the cortex ipsilateral to the OT terminals examined, a posterior half of the lateral gyrus was the area from which facilitation of the antidromic OT response was most easily elicited. Next to this, stimulation of the middle supra-and ectosylvian gyri was effective. The active spots were found distributed also on the contralateral cortex in about the same way as in the ipsilateral side.5. Nembutal was effective to suppress the cortical facilitation of the antidromic OT response. Attempts to see the effects of picrotoxin failed to yield conclusive evidence.