Dissociation between changes in glutamate receptor binding sites and their coupling to phosphatidylinositol metabolism following intrahippocampal colchicine injection

Abstract Intrahippocampal colchicine injection produces a rapid death of granule cells and pyramidal neurons in the hippocampus in the rat. Under the appropriate assay conditions, [3H]glutamate labels the N-methyl- d -aspartate type of glutamate receptors while [3H]α-amino-3-hydroxy-5-methylisoxazole-4-propionate labels the quisqualate type. Unilateral injection of colchicine (15 μg) in the dorsal hippocampus did not produce any change in [3H]glutamate and [3H]α-amino-3-hydroxy-5-methylisoxazole-4-propionate binding in membrane fractions from the dentate gyrus or CA1 field contralateral to the injection side, at least up to 12 days after the injection. However, it produced a progressive decrease in the binding of both ligands in dentate gyrus and CA1 of the injected hippocampus. In the dentate gyrus the changes in binding as a function of time after the injection were biphasic with a rapid exponential decrease (t1/2) about 8 days for both [3H]glutamate and [3H]α-amino-3-hydroxy-5-methylisoxazole-4-propionate) until 12 days after the injection followed by a much slower decrease afterwards. A similar pattern was observed in CA1 although the changes in binding were smaller and delayed by about three days as compared to the dentate gyrus. Kinetic analyses of the binding at equilibrium were performed seven days after the injection and indicated that the changes in [3H]glutamate binding were due to a change in the maximum number of sites but not in affinity for the ligand. For [3H]α-amino-3-hydroxy-5-methylisoxazole-4-propionate Scatchard plots indicated the existence of both high and low affinity sites and the effect of colchicine injection appeared to be a decrease in the maximum number of low affinity sites. Basal and quisqualate-stimulated conversion of phosphatidylinositol to inositol phosphate derivatives were also determined in hippocampal slices at various times following colchicine injection. The changes in basal phosphatidylinositol hydrolysis were biphasic with a marked increase occurring one to two days after the injection followed by a marked decrease in hydrolysis and a recovery to control values by 30 days. Quisqualate-stimulated phosphatidylinositol hydrolysis was markedly enhanced between four and eight days after the injection and was still slightly increased by 30 days. No effect of N-methyl- d -aspartate was observed at any time after colchicine injection. These results indicate that the majority of the sites labeled by [3H]glutamate and [3H]α-amino-3-hydroxy-5-methylisoxazole-4-propionate are localized mainly postsynaptically and on neuronal structures. The exponential decay of the receptor binding suggests that the number of binding sites directly reflects the number of surviving neurons on the hippocampal subfields. Finally the changes in quisqualate-stimulated phosphatidylinositol hydrolysis suggest that colchicine treatment modifies the coupling between certain types of glutamate receptors to the phosphatidylinositol system possibly as a result of the modification of cytoskeletal proteins.