EXCITATORY AMINO-ACID RECEPTORS PARTICIPATE IN SYNAPTIC TRANSMISSION OF VISUAL RESPONSES IN THE SUPERFICIAL LAYERS OF THE CAT SUPERIOR COLLICULUS

The contribution of NMDA and non-NMDA receptors to visual synaptic tra nsmission in the superficial layers of the superior colliculus of the cat has been studied using extracellular recording and iontophoretic t echniques. Neuronal responses to natural visual stimulation and the ej ection of NMDA and AMPA were observed in the absence and presence of t he antagonists CNQX, CPP and AP5. CNQX routinely reduced the responses to visual stimulation at ejection currents which selectively blocked the responses to AMPA but not those to NMDA. Agonist selective ejectio n currents of CPP and AP5 also reduced visual responses of most SC neu rons, but there was a substantial majority whose visual responses were resistant to these antagonists. Neurons with CPP/AP5 resistant visual responses were more commonly found 750 - 1000 mu m from the dorsal su rface of the SC. The data indicate that, while non-NMDA receptors are heavily involved in visual synaptic transmission in the superficial SC , the involvement of NMDA receptors Varies with recording depth.