Further evidence demonstrating that N-methyl-D-aspartate and kainate activate distinct ion channels

Several excitatory amino acid receptors encoded by rat brain mRNA were expressed in Xenopus oocytes. Experimental protocols using an open channel blocker (MK-801) were designed to test the common receptor-channel hypothesis in which N-methyl-D-aspartate (NMDA) and kainate activate the same ion channel but induce different open channel conformations with different ionic permeabilities. The present data demonstrate that NMDA exposes previously trapped MK-801 molecules to the transmembrane field and accelerates their dissociation from the channel at positive potentials, while kainate lacks this effect. Therefore, kainate does not activate the same ion channel as NMDA does. Furthermore, differential inhibition of the NMDA response or the kainate response by the competitive antagonists D-2-amino-5-phosphonopentanoic acid (D-AP5) and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) indicates that NMDA and kainate do not share the same binding site. Thus, these several lines of evidence demonstrate that two distinct receptor-channels are activated by NMDA and kainate, respectively.