Characterization of two [3H]glutamate binding sites in rat hippocampal membranes.
The specific binding of L-[3H]glutamate was investigated in the presence and the absence of sodium ions in freshly prepared membranes from rat hippocampus. Sodium ions were found to have a biphasic effect; low concentrations induced a marked inhibition of the binding (in the range 0.5-5.0 mM), whereas higher concentrations resulted in a dose-dependent stimulation of binding (in the range 10-150 mM). These results permit the discrimination of two binding sites in hippocampal membranes. Both Na+-independent and Na+-dependent binding sites were saturable, exhibiting dissociation constants at 30 degrees C of 750 nM and 2.4 microM, respectively, with Hill coefficients not significantly different from unity, and maximal number of sites of 6.5 and 75 pmol/mg protein, respectively. [3H]Glutamate binding to both sites reached equilibrium between 5 and 10 min and was reversible. The relative potencies of a wide range of compounds, with known pharmacological activities, to inhibit [3H]glutamate binding were very different for the Na+-independent and Na+-dependent binding and suggested that the former sites were related to post-synaptic glutamate receptors, whereas the latter were related to high-affinity uptake sites. This conclusion was also supported by the considerable variation in the regional distribution of the Na+-dependent binding site, which paralleled that of the high-affinity glutamate uptake; the Na+-independent binding exhibited less regional variation.
D000818:Animals; D001921:Brain / Q000378:metabolism; D005971:Glutamates / Q000378:metabolism; D006624:Hippocampus / Q000378:metabolism*; D066298:In Vitro Techniques; D007700:Kinetics; D008566:Membranes / Q000378:metabolism; D051381:Rats; D011956:Receptors, Cell Surface / Q000378:metabolism*; D017470:Receptors, Glutamate; D012964:Sodium / Q000502:physiology
D005971:Glutamates; D011956:Receptors, Cell Surface; D017470:Receptors, Glutamate; D012964:Sodium