Modulation of inhibitory glycine receptors in cultured embryonic mouse hippocampal neurons by zinc, thiol containing redox agents and carnosine

Modulation of inhibitory glycine receptors by zinc (Zn²⁺) and endogenous redox agents such as glutathione may alter inhibition in the mammalian brain. Despite the abundance of Zn²⁺ in the hippocampus and its ability to modulate glycine receptors, few studies have examined Zn²⁺ modulation of hippocampal glycine receptors. Whether redox agents modulate hippocampal glycine receptors also remains unknown. This study examined Zn²⁺ and redox modulation of glycine receptor-mediated currents in cultured embryonic mouse hippocampal neurons using whole-cell recordings. Zn²⁺ concentrations below 10 μM potentiated currents elicited by low glycine, β-alanine, and taurine concentrations by 300-400%. Zn²⁺ concentrations above 300 μM produced nearly complete inhibition. Potentiating Zn²⁺ concentrations shifted the dose-response curves for the three agonists to the left and decreased the Hill coefficient for glycine and β-alanine but not taurine. Inhibiting Zn²⁺ concentrations shifted the dose-response curves for glycine and β-alanine to the right but reduced the maximum taurine response. Histidine residues may participate in potentiation because diethyl pyrocarbonate and pH 5.4 diminished Zn²⁺ enhancement of glycine currents. pH 5.4 diminished Zn²⁺ block of glycine currents, but diethyl pyrocarbonate did not. These findings indicate that separate sites mediate Zn²⁺ potentiation and inhibition. The redox agents glutathione, dithiothreitol, tris(2-carboxyethyl)phosphine, and 5,5′-dithiobis(2-nitrobenzoic acid) did not alter glycine currents by a redox mechanism. However, glutathione and dithiothreitol interfered with the effects of Zn²⁺ on glycine currents by chelating it. Carnosine had similar effects. Thus, Zn²⁺ and thiol containing redox agents that chelate Zn²⁺ modulate hippocampal glycine receptors with the mechanism of Zn²⁺ modulation being agonist dependent.