Abstract
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 Mg 2+, while no significant differences were found in rat cortical membranes. Similar increase in high-affinity agonist binding sites by Mn 2+ 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 Mn 2+- and Mg 2+-ions was modulated through their action on different G-proteins. Results showed that efficient coupling of G-protein and 5-HT 1A receptors is crucial to modify Mg 2+ and Mn 2+ effects, whereas Mn 2+ is more potent stabilizer of agonist high-affinity binding, especially when GTPγS is present. Using Sf9 cells as model system, we have shown that G i1 proteins are required to modulate Mn 2+-dependent high-affinity agonist binding to 5-HT 1A receptors, but further studies are necessary to find the cofacors of Mn 2+ modulation to signal transduction.
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