Putative ClC-2 chloride channel mediates inward rectification in Drosophila retinal photoreceptors

We report that Drosophila retinal photoreceptors express inwardly rectifying chloride channels that seem to be orthologous to mammalian ClC-2 inward rectifier channels. We measured inwardly rectifying Cl(-) currents in photoreceptor plasma membranes: Hyperpolarization under whole-cell tight-seal voltage clamp induced inward Cl(-) currents; and hyperpolarization of voltage-clamped inside-out patches excised from plasma membrane induced Cl(-) currents that have a unitary channel conductance of approximately 3.7 pS. The channel was inhibited by 1 mM: Zn(2+) and by 1 mM: 9-anthracene, but was insensitive to DIDS. Its anion permeability sequence is Cl(-) = SCN(-)> Br(-)>> I(-), characteristic of ClC-2 channels. Exogenous polyunsaturated fatty acid, linolenic acid, enhanced or activated the inward rectifier Cl(-) currents in both whole-cell and excised patch-clamp recordings. Using RT-PCR, we found expression in Drosophila retina of a ClC-2 gene orthologous to mammalian ClC-2 channels. Antibodies to rat ClC-2 channels labeled Drosophila photoreceptor plasma membranes and synaptic regions. Our results provide evidence that the inward rectification in Drosophila retinal photoreceptors is mediated by ClC-2-like channels in the non-transducing (extra-rhabdomeral) plasma membrane, and that this inward rectification can be modulated by polyunsaturated fatty acid.