Modulation of GABA_C Response by Ca²⁺ and Other Divalent Cations in Horizontal Cells of the Catfish Retina

GABA_C responses were recorded in cultured cone-driven horizontal cells from the catfish retina using the patch clamp technique. At a holding potential of −49 mV, a bicuculline-resistant inward current (I_GABA) was observed when 10 μM GABA was applied. The amplitude of I_GABA increased as the extracellular Ca²⁺ ([Ca²⁺]_o) was increased. Concentration–response curves of I_GABA at 2.5 and 10 mM [Ca²⁺]_o had similar EC₅₀ (3.0 and 3.1 μM) and Hill coefficients (1.54 and 1.24). However, the maximal response estimated at 10 mM [Ca²⁺]_o was larger than the maximal response at 2.5 mM [Ca²⁺]_o. Increasing Ca influx through voltage-gated Ca channels and the resulting rise in the intracellular Ca²⁺ concentration had no effects on I_GABA. However, I_GABA was inhibited by extracellular divalent cations, with the following order of the inhibitory potency: Zn²⁺ > Ni²⁺ > Cd²⁺ > Co²⁺. The inhibitory action of Zn²⁺ on the [Ca²⁺]_o-dependent I_GABA increase was noncompetitive. The action of [Ca²⁺]_o on I_GABA was mimicked by Ba²⁺ or Sr²⁺. These results demonstrate that the extracellular domain of GABA_C receptors has two functionally distinct binding sites represented by Ca²⁺ (facilitation) and Zn²⁺ (inhibition). Since [Ca²⁺]_o and [Zn²⁺]_o change into the opposite direction by light, it seems likely that they modify cooperatively the efficacy of the positive feedback consisting of the GABA_C receptor.