METAL-ION AND GUANINE-NUCLEOTIDE MODULATIONS OF AGONIST INTERACTION IN G-PROTEIN-COUPLED SEROTONIN(1A) RECEPTORS FROM BOVINE HIPPOCAMPUS

1. The serotonin type 1A (5-HT1A) receptors are members of a superfami ly of seven transmembrane domain receptors that couple to GTP-binding regulatory proteins (G-proteins). We have studied the modulation of ag onist binding to 5-HT1A receptors from bovine hippocampus by metal ion s and guanine nucleotide. 2. Bovine hippocampal membranes containing t he 5-HT1A receptor were isolated. These membranes exhibited high-affin ity binding sites for the specific agonist [H-3]OH-DPAT. 3. The agonis t binding is inhibited by monovalent cations Na+, K+, and Li+ in a con centration-dependent manner. Divalent cations such as Ca2+, Mg2+, and Mn2+, on the other hand, show more complex behavior and induce enhance ment of agonist binding up to a certain concentration. The effect of t he metal ions on agonist binding is strongly modulated in the presence of GTP-gamma-S, a nonhydrolyzable analogue of GTP, indicating that th ese receptors are coupled to G-proteins. 4. To gain further insight in to the mechanisms of agonist binding to bovine hippocampal 5-HT1A rece ptors under these conditions, the binding affinities and binding sites have been analyzed by Scatchard analysis of saturation binding data. Our results are relevant to ongoing analyses of the overall regulation of receptor activity for G-protein-coupled seven transmembrane domain receptors.