A study of inhibitory antagonism in cat visual cortex.

Since there seems to be good evidence that GABA may act as an inhibitory neurotransmitter in the mammalian cortex, we tested the effects of an antagonist of GABA, namely the alkaloid bicuculine, on the response properties of visual cortex neurons, using a computer-controlled stimulus presentation system to assess quantitatively the changes in receptive field organization after the drug. Complex cells were most affected, increasing both evoked and spontaneous activity and losing some of their specificities for stimulus parameters such as orientation and direction. Hyper-complex cells lost their inhibitory flanks, responding equally well to long and short bars after the drug. Simple cells were the least affected, usually becoming somewhat depressed after the drug. Preliminary tests with another inhibitory amino acid antagonist, strychnine, showed that it excited simple cells, indicating that possibly more than one inhibitory transmitter is at work in the cortex. The results are discussed with relation to the synaptic anatomy of the cortex, and it is concluded that a class of stellate cells, using GABA, is a likely candidate for the transmitter of some intracortical inhibition.