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

Abstract

We report that Drosohhila retinal photoreceptors express inwardly rectifying chloride channels that seem to be orthologous to mammalian CIC-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 CI- currents that have a unitary channel conductance of similar to 3.7 pS. The channel was inhibited by 1 mM Zn2+ and by 1 mM 9-anthracene, but was insensitive to DIDS. Its anion permeability sequence is Cl- = SCN- > Br- > > I-, characteristic of CIC-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 CIC-2 gene orthologous to mammalian CIC-2 channels. Antibodies to rat CIC-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 CIC-2-like channels in the non-transducing (extra-rhabdomeral) plasma membrane, and that this inward rectification can be modulated by polyunsatturated fatty acid.