Excitatory amino acid transmitters in neuronal circuits of the cat visual cortex

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Abstract
1. To test a possibility that glutamate (Glu) and aspartate (Asp) are transmitters in the visual cortex and to locate their operating sites in the cortical circuitry, we studied effects of microionophoretic application of Glu/Asp antagonists on visual responses of cortical neurons in the cat. The antagonists tested were kynurenic acid (KYNA), cis-2,3-piperidine dicarboxylic acid, and .gamma.-D-glutamylglycine. 2. Among these antagonists, KYNA was most effective in blocking visual responses of cortical neurones; it eliminated visual responses in 156 of the 188 cells tested. Usually the maximal suppressive effect appeared 20-30 s after starting KYNA application and recovery of cell''s responsiveness 30-60s after stopping the application. 3. KYNA antagonized excitations induced by ionophoretic application of Glu and Asp but did not block those by acetylcholine, suggesting that KYNA is a selective antagonist of Glu/Asp, and its action is not due to general depressant effects. This suggestion was further supported by the observation that in corticogeniculate cells the latency and probability of invasion of antidromic spikes into the somatodendritic part following electrical stimulation of the lateral geniculate were not changed while visual responses were completely suppressed by KYNA. 4. In terms of actions of the three agonists which give the basis for classifying excitatory amino acid receptors into at least three types, KYNA antagonized excitations by N-methyl-D-aspartic acid (NMDA) and kainate in almost all the cells tested but did not block those by quisqualate in about half of the cells. These results suggest that KYNA reacts more preferentially with NMDA and kainate receptors than with quisqualate receptors. 5. Effectiveness of KYNA was related to types of receptive fields of cells and to their laminar locations. In 79 of the 104 simple cells tested, KYNA completely suppressed their visual responses, while such a complete block was seen in only 18 of the 68 complex and 3 of the 16 special complex cells. The great majority of the cells in layers IVab, IVc and the upper part of layer VI were completely suppressed by KYNA, whereas most of the cells in the other layers were incompletely suppressed or not suppressed at all. 6. In some of these insensitive cells, KYNA was further tested using the specially assembled micropipettes with two drug barrels; tips of the drug barrels were apart 25 and 100 .mu.m from that of the recording electrode. Simultaneous application of KYNA from the two drug barrels also turned out to be ineffective. This indicates that their main visual inputs may not be transmitted by Glu/Asp. 7. In the light of recent understandings of the cortical circuitry, these findings suggest that Glu/Asp may be the excitatory transmitters of the geniculate afferents to first-order neurones in the visual cortex.