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L‐[ 3 H]Glutamate Binding to Hippocampal Synaptic Membranes: Two Binding Sites Discriminated by Their Differing Affinities for Quisqualate
Author(s) -
Werling Linda L.,
Doman Kathleen A.,
Nadler J. Victor
Publication year - 1983
Publication title -
journal of neurochemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.75
H-Index - 229
eISSN - 1471-4159
pISSN - 0022-3042
DOI - 10.1111/j.1471-4159.1983.tb04779.x
Subject(s) - affinities , glutamate receptor , hippocampal formation , synaptic membrane , chemistry , binding affinities , neuroscience , binding site , membrane , biophysics , stereochemistry , biochemistry , biology , receptor
The excitatory glutamate analogs quisqualate and ibotenate were employed to distinguish multiple binding sites for l‐[ 3 H]glutamate on freshly prepared hippocampal synaptic membranes. The fraction of bound radioligand that was displaceable by 5 μ M quisqualate was termed GLU A binding. That which persisted in the presence of 5 μ M quisqualate, but was displaceable by 100 μ M ibotenate, was termed GLU B binding. GLU A binding equilibrated within 5 min and remained unchanged for up to 80 min. GLU B binding appeared to equilibrate at least as rapidly, but incubation with ligand unmasked latent binding sites. Saturation binding curves were best fitted by single exponentials, which yielded K d values of about 200 n M (GLU A) and 1 μ M (GLU B). On the average, GLU B binding sites were about twice as abundant in these membranes as were GLU A sites. Rapid freezing of the membranes, followed by storage at ‐26°C and rapid thawing markedly diminished GLU A binding, but nearly tripled GLU B binding. Both sites bound l‐glutamate with 10–30 times the affinity of d‐glutamate. The GLU A site also bound l‐glutamate with about 10 times the affinity of l‐aspartate and discriminated poorly between l‐ and d‐aspartate. In contrast, the GLU B site bound l‐aspartate with an affinity similar to that for l‐glutamate, and with an order‐of‐magnitude greater affinity than d‐aspartate. The structural specificities of the GLU A and GLU B binding sites suggest that these sites may correspond to receptors on hippocampal pyramidal cell dendrites that are activated by iontophoretically applied l‐glutamate.