Differential modulation of 5-hydroxytryptamine1D binding sites by GTP and GTPgammaS

Incubation of bovine caudate membranes with 10(-4) M GTP results in a competitive inhibition of 5-hydroxytryptamine1D (5-HT1D) receptor binding sites labeled by [3H]5-HT. By contrast, incubation of bovine caudate membranes with 10(-4) M GTPgammaS, a non-hydrolyzable analog of GTP, results in a decrease in both the apparent affinity (Kd) and density (Bmax) of 5-HT1D binding sites. The Kd is increased by 67 +/- 8%, (P less than 0.01, n = 12) while the Bmax is reduced by 52 +/- 10% in comparison with control values (P less than 0.05, n = 12). Pre-incubation studies indicate that GTPgammaS-induced changes in 5-HT1D receptors are persistent and time-dependent. These results indicate that GTPgammaS induces non-competitive changes in 5-HT1D receptor-G-protein interactions that are reflected as a decrease in the affinity and density of binding sites labeled by [3H]5-HT.