Role of Guanine Nucleotides as Endogenous Ligands of a Kainic Acid Binding Site Population in the Mammalian Cerebellum

An endogenous inhibitor of the membrane binding of kainic acid was extracted from pig brain tissue and purified. The substance was identified as GMP by structural analysis: Most likely it corresponds to an inhibitor previously extracted from the rat brain. The nucleotide is active as an inhibitor for kainate binding on goldfish brain synaptosomes, probably owing to direct displacement on receptor sites; it is also active on a low-affinity kainate site population in membranes from rat cerebellum. The interaction of GMP with the latter sites leads to a concentration-dependent kainate binding increase or inhibition, thus demonstrating that these sites can bind the nucleotide and cooperatively increase their affinity. Other guanine nucleotides show interaction with these sites, by either an increase (GTP) or inhibition (cyclic GMP or GDP) of kainate binding. These findings support the view that a guanine nucleotide is the endogenous ligand of a receptor in the mammalian cerebellum similar to the kainate binding protein present with high density in the cerebellum of lower vertebrates, whose function is probably connected to the role of the glial cells in this zone.