DIFFERENT PHOTORECEPTORS WITHIN THE SAME RETINA EXPRESS UNIQUE COMBINATIONS OF POTASSIUM CHANNELS

Single electrode current and voltage clamp recordings in Calliphora a, and whole-cell voltage clamp recordings in Drosophila were used to ch aracterise the voltage-gated K channels in both major classes of photo receptors, R7/8 (long visual fibres, LVFs) and R1-6 (short visual fibr es, SVFs). R7/8 were identified by their unique spectral properties, c a. 3-4 fold higher input resistances and 3-4 fold lower cell capacitan ce. In Calliphora SVFs possess both fast and slow activating delayed r ectifier potassium conductances. Drosophila SVFs possess a slowly inac tivating delayed rectifier (I-Ks), a very rapidly inactivating A chann el encoded by the Shaker gene (I-A), and, in a minority of cells, a th ird K conductance with intermediate kinetics (I-Kf). In both species t he LVFs lack the slowest component, but exhibit the faster K conductan ce(s) with properties indistinguishable from those in the SVFs. These findings add to established evidence demonstrating the significant rol e played by potassium channels in tuning the photoreceptor membrane. T he results also suggest that R1-6 photoreceptors and R7/8 form inputs to visual subsystems tuned to different temporal frequencies.