Journal of Neuroscience, Vol 6, 355-363, Copyright © 1986 by Society for Neuroscience
Induction of glutamate binding sites in hippocampal membranes by transient exposure to high concentrations of glutamate or glutamate analogs
M Kessler, M Baudry, JT Cummins, S Way and G Lynch
The number of Na+-independent, Cl--dependent glutamate binding sites in rat
hippocampal membranes is increased two- to fourfold after pre- exposing
isolated membranes or hippocampal slices to high concentrations (0.1-10 mM)
of L-glutamate or of glutamate analogs with high affinity for this binding
site, such as quisqualate, homocysteate, or aminoadipate. N-Methylaspartate
and kainate are ineffective. A similar binding increase is induced by
transient exposure to the dipeptide tyrosylglutamate. The newly induced
binding sites appear to be identical with pre-existing Cl--dependent
binding sites by several criteria: They have a similar pharmacological
profile, they are sensitive to low concentrations of Na+, and the number of
sites can be further increased by transient exposure to micromolar calcium
concentrations. Moreover, binding of [3H]APB, a ligand selective for the
Cl--dependent glutamate binding sites, is also increased after glutamate
preincubation. The induction of binding sites by high glutamate
concentrations, described herein, is calcium-independent, not inhibited by
leupeptin and, therefore, different from the previously described
activation of binding sites by a calcium-sensitive protease. The high
concentration of ligand needed to induce increased binding suggests the
presence in hippocampal membranes of a binding site with low, millimolar
affinity that is functionally related to the known high- affinity binding
sites. Several interpretations of the observed effects and their
implications for the possible relationship between the binding site and the
synaptic receptor are discussed.