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 mu M 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+.
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