Enhancement of agonist binding to 5-HT1A receptors in rat brain membranes by millimolar Mn2+

Manganese in millimolar concentration caused increase in specific binding of [3H]8-OH-DPAT to rat hippocampal membranes up to 44% in comparison with experiments in the presence of Mg2+, while no significant differences were found in rat cortical membranes. Similar increase in high-affinity agonist binding sites by Mn2+ was found in displacement curves of 8-OH-DPAT, where antagonist [3H]WAY100635 was used as reporter ligand. The removal of bivalent ions with EDTA caused full loss of high-affinity binding of agonists, but not for antagonists. Therefore it was hypothesized, that the effect of Mn2+- and Mg2+-ions was modulated through their action on different G-proteins. Results showed that efficient coupling of G-protein and 5-HT1A receptors is crucial to modify Mg2+ and Mn2+ effects, whereas Mn2+ is more potent stabilizer of agonist high-affinity binding, especially when GTPγS is present. Using Sf9 cells as model system, we have shown that Gi1 proteins are required to modulate Mn2+-dependent high-affinity agonist binding to 5-HT1A receptors, but further studies are necessary to find the cofacors of Mn2+ modulation to signal transduction.