INTERACTION OF GUANINE-NUCLEOTIDES WITH [H-3] KAINATE AND 6-[H-3]CYANO-7-NITROQUINOXALINE-2,3-DIONE BINDING IN GOLDFISH BRAIN

Recent reports have suggested that a major proportion of [H-3]kainate binding in goldfish brain is to a novel form of G-protein-linked gluta mate receptor. Here we confirm that guanine nucleotides decrease [H-3] kainate binding in goldfish brain membranes, but that binding is also reduced to a similar extent under conditions where G-protein modulatio n should be minimised. Inclusion of GTPgammaS resulted in an approxima tely twofold decrease in the affinity of [H-3]kainate binding and a 50 % reduction in the apparent B(max) values in both Mg2+/Na+ and Mg2+/Na +-free buffer when assayed at 0-degrees-C. The pharmacology of [H-3]ka inate binding is similar to that of well-characterised ionotropic kain ate receptors but unlike that of known metabotropic glutamate receptor s, with neither 1S,3R-amino-1,3-cyclopentanedicarboxylic acid (1S,3R-A CPD) nor ibotenic acid being effective competitors. The molecular mass of the [H-3]kainate binding protein, as determined by radiation inact ivation, was 40 kDa, similar to the subunit sizes of other lower verte brate kainate binding proteins that are believed to comprise ligand-ga ted ion channels. Furthermore, GTPgammaS also inhibited the binding of the non-NMDA receptor-selective antagonist 6-[H-3]cyano-7-nitroquinox aline-2,3-dione. These data strongly suggest that the regulatory inter action between guanine nucleotides and [H-3]kainate and 6-[H-3]cyano-7 -nitroquinoxaline-2,3-dione binding is complex and involves competitio n at the agonist/antagonist binding site in addition to any G-protein- mediated modulation.