Interaction of Guanine Nucleotides with [ 3 H]Kainate and 6‐[ 3 H]Cyano‐7‐Nitroquinoxaline‐2,3‐dione Binding in Goldfish Brain

— Recent reports have suggested that a major proportion of [ 3 H]kainate binding in goldfish brain is to a novel form of G‐protein‐linked glutamate receptor. Here we confirm that guanine nucleotides decrease [ 3 H]kainate binding in goldfish brain membranes, but that binding is also reduced to a similar extent under conditions where G‐protein modulation should be minimised. Inclusion of GTPγS resulted in an approximately twofold decrease in the affinity of [ 3 H]kainate binding and a 50% reduction in the apparent B max values in both Mg 2+ /Na + and Mg 2+ /Na + ‐free buffer when assayed at 0°c. The pharmacology of [ 3 H]kainate binding is similar to that of well‐characterised ionotropic kainate receptors but unlike that of known me‐tabotropic glutamate receptors, with neither 1 S ,3 R ‐amino‐1,3‐cyclopentanedicarboxylic acid (1 S ,3 R ‐ACPD) nor ibo‐tenic acid being effective competitors. The molecular mass of the [ 3 H]kainate binding protein, as determined by radiation inactivation, was 40 kDa, similar to the subunit sizes of other lower vertebrate kainate binding proteins that are believed to comprise ligand‐gated ion channels. Furthermore, GTP‐γS also inhibited the binding of the non‐NMDA receptor‐selective antagonist 6‐[ 3 H]cyano‐7‐ni‐troquinoxaline‐2,3‐dione. These data strongly suggest that the regulatory interaction between guanine nucleotides and [ 3 H]kainate and 6‐[ 3 H]cyano‐7‐nitroquinoxaline‐2,3‐dione binding is complex and involves competition at the agonist/antagonist binding site in addition to any G‐protein‐mediated modulation.