A competitive inhibitor of kainic acid binding from the goldfish nervous tissue.

An inhibitor of the receptor binding of the neuroexcitant kainic acid was extracted from the nervous tissue of the goldfish and purified. The substance acts as a competitive inhibitor (displacer) on the kainate binding sites in membranes from the fish nervous system; this action is selective since the substance does not affect the membrane binding of glutamate, the common ligand for the excitatory amino acid binding sites. The interaction of the substance with the fish kainate binding sites displays a positive cooperativity, similar to that measured for kainic acid itself. Thus the endogenous kainate binding inhibitor (KBI) can be assumed as a candidate for the role of physiological ligand of receptors for kainic acid in the fish. The substance, at the tested concentration, does not significantly affect the binding of kainic acid in membranes from rat brain while it is active on the sites from the pigeon cerebellum. The relevance of these findings for the understanding of the functional heterogeneity of the kainate receptors in different species is discussed.