Autoradiographic characterization of [3H]6-cyano-7-nitroquinoxaline-2,3-dione binding sites in adult chick brain.

The non-N-methyl-D-aspartate binding sites have been characterized in chick brain using quantitative autoradiography, and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) as a radioligand. [3H]CNQX binding sites were densely localized in the molecular layer of cerebellum. Other areas of prominent binding were the superficial layers of optic tectum, one of the isthmic nuclei, the hippocampus, the hyperstriatum accessorium and the archistriatum ventrale. Analysis of equilibrium binding data in the cerebellar molecular layer indicated that [3H]CNQX bound to a single class of sites (Kd = 78.9 +/- 11.8 nM and Bmax = 41.2 +/- 3.0 pmol/mg protein). Competition experiments in six different regions of chick brain gave the Ki and Bmax values for the inhibition of [3H]CNQX binding by various standard compounds and indicated that: (i) [3H]CNQX labelled non-N-methyl-D-aspartate binding sites with high affinity of (RS)-alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) and kainate, (ii) the displacement curves for AMPA and kainate were biphasic in all regions studied, and (iii) the potencies of AMPA and kainate in displacing [3H]CNQX binding were different in the regions studied. Our results indicate that [3H]CNQX labelled non-N-methyl-D-aspartate binding sites in chick brain. In the cerebellar molecular layer, these sites were more sensitive to kainate than AMPA, as were the binding sites in the superficial layers of optic tectum and nucleus isthmi magnocellularis. However, non-N-methyl-D-aspartate binding sites in forebrain regions such as hippocampus and hyperstriatum ventrale appeared to be different in being equally sensitive to AMPA and kainate.