Mg²⁺ enhances high affinity [³H]8-HYDROXY-2-(DI-N-PROPYLAMINO) tetralin binding and guanine nucleotide modulation of serotonin-1a receptors

Abstract

The effects of MgCl₂ on the binding of the serotonin 1a (5HT1a) receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin ([³H]8-OH-DPAT) to bovine hippocampal membranes were investigated. MgCl₂ was found to enhance both [³H]8-OH-DPAT binding and guanine nucleotide modulation of high affinity binding. The effect of Mg²⁺ on promoting high affinity agonist binding was due to a 3.3 fold decrease in the dissociation constant rather than an increase in the number of binding sites. Mg²⁺ acted to increase the efficacy of the nonhydrolyzable GTP analog 5′ guanylylimidodiphosphate (GppNHp) in promoting the interconversion of high and low affinity states. Addition of MgCl₂ significantly increased the sensitivity of [³H]8-OH-DPAT binding to GppNHp, decreasing the concentration needed for half-maximal inhibition of binding from greater than 50 µM to 300 nM. Our findings that Mg²⁺ enhances high affinity [³H]8-OH-DPAT binding without change in number of binding sites and that guanine nucleotide modulation of binding can occur in the absence of Mg²⁺ suggests that ternary complex formation between receptor, ligand and G-protein can occur in the absence of Mg²⁺.