The transition metal Zn2+ is differentially distributed in the central nervous system, where it is proposed to be a neuromodulator. One of the documented effects of Zn2+ is the antagonism of γ-aminobutyric acid (GABA)-mediated synaptic inhibition. This antagonism is presumed to result from a direct interaction of Zn2+ with the GABA receptor/ionophore complex, although the characteristics of Zn2+ sensitivity are dependent on the particular GABA subunit combination. In this study, we examined the effects of Zn2+ on homomeric ρ1 GABA-activated channels expressed in Xenopus oocytes. Zn2+ was found to be a mixed antagonist of these recombinant ρ1 GABA receptors. The antagonism was predominately competitive at low Zn2+ concentrations (≤100 μM), whereas at high Zn2+ concentrations (>100 μM) a noncompetitive antagonism was apparent. Evidence is presented showing that the antagonism was not due to an interaction of GABA and Zn2+ in solution but, rather, resulted from interactions of these two ligands with the GABA- activated channel. A mechanism is proposed for Zn2+mediated antagonism in which GABA and Zn2+ bind to distinct sites on the GABA complex. The apparent mixed antagonism may arise from different K(i) values for the binding of Zn2+ to non-agonist-bound or agonist-bound receptors. However, two distinct Zn2+ binding sites, one competitive and one noncompetitive, could also give rise to the dual antagonism.
|Original language||English (US)|
|Number of pages||8|
|State||Published - Jan 1 1995|
ASJC Scopus subject areas
- Molecular Medicine