Abstract

The effects of various physical or chemical treatments which cause modifications of protein-protein or protein-lipid interactions on basal and calcium-stimulated [³H]glutamate receptor binding have been studied in rat hippocampal membranes. Increasing temperature increased the number of binding sites without changing their affinity for [³H]glutamate; in addition, no significant stimulation by calcium of this binding was observed below 15-20°. An Arrhenius-like plot of the data indicated a marked discontinuity at 21° and yielded activation energies of 20-60 kcal·mole^-1. Treatment of membranes with the detergents Triton X-100 or sodium deoxycholate resulted in a dose-dependent reduction in [³H]glutamate binding accompanied by a loss of the stimulatory effect of calcium ions. Treatment with phospholipase C decreased basal binding without altering the percentage increase in binding produced by calcium, whereas treatment with phospholipase D did not affect basal binding, but instead reduced the effect of calcium. Trypsin and chymotrypsin caused an increased basal binding and did not change the stimulatory effect of calcium. These data support the hypothesis that calcium increases the number of [³H]glutamate binding sites by inducing local changes in membrane fluidity.