Complex Binding of l‐[3H]Glutamate to Hippocampal Synaptic Membranes in the Absence of Sodium

Document Type

Journal Article

Publication Date



Journal of Neurochemistry








Glutamate; Hippocampus; Neurotrans‐mitter receptors


Abstract: Specific binding of L‐[3H]glutamate was investigated with a thoroughly washed synaptic membrane preparation from rat hippocampal formation, a region of brain densely innervated by putatively glutamatergic fibers. L‐[3H]Glutamate bound rapidly, saturably, and reversibly to these membranes in the absence of Na+. Specific binding was greatest around 38°C and at a slightly acidic pH. Saturation isotherms fit a model of two independent binding sites with dissociation constants of 11 and 570 nM and corresponding densities of 2.5 and 47 pmol/mg protein. All potent amino acid excitants, except N‐methyl‐D‐aspartate and kainate, and several excitatory amino acid antagonists inhibited specific radioligand binding with IC50 values between 10−1 M and 10−4 M. In contrast, weak amino acid excitants and an inhibitor of glutamate uptake were nearly inactive. Displacement curves were analyzed with a computer program that assumed the simultaneous contributions of two independent sites at which each compound competitively inhibited the binding of L‐[3H]glutamate. According to this analysis, ibotenate and the L‐ and D‐isomers of glutamate and aspartate bind preferentially to the high‐affinity site, whereas quisqualate, L‐α‐aminoadipate, and the L‐ and D‐isomers of homocysteate bind preferentially to the low‐affinity site. With the notable exception of γ‐D‐glutamylglycine, all of the more potent antagonists appear to bind preferentially to the low‐affinity site. Both sites exhibit marked stereoselectivity for L‐glutamate. D‐ and L‐Homocysteate and most excitatory amino acid antagonists increased specific binding at concentrations below those required to demonstrate inhibition. Some properties of the low‐affinity binding site resemble those of junctional glutamate receptors on insect muscle, but neither site appears to correspond to the “N‐methyl‐D‐aspartate receptor” or the “quisqualate receptor.” Copyright © 1982, Wiley Blackwell. All rights reserved

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