Voltage-sensitive potassium channels in Drosophila photoreceptors

A preparation of dissociated Drosophila ommatidia is described that allows single-channel and whole-cell patch-clamp analysis of currents in identified sensory neurons. Three distinct classes of voltage-sensitive potassium conductances are characterized; all were detected in distal parts of ommatidia from sevenless mutants and hence in one cell class (R1-6 photoreceptors). Rapidly inactivating A-channels (IA), coded by the Shaker gene, were isolated in multichannel patches from adult flies. While showing similar kinetics to muscle A-channels, they differ from previously characterized wild-type Shaker channels in having a much more negative voltage operating range, being half-inactivated at approximately -70 mV. Two delayed rectifier conductances were characterized in whole-cell recordings from pupal photoreceptors. The most commonly encountered class (IKs) is similar to previously reported delayed rectifier conductances in Drosophila. It inactivates slowly (time constant, approximately 500 msec) and is half-inactivated at approximately -40 mV. A more rapidly inactivating delayed rectifier (IKf) was detected in approximately 30% of cells; it is half-inactivated at approximately -80 mV. Both IKs and IKf are blocked by 100 microM quinidine. Neither are greatly affected by 4-aminopyridine, which blocks IA at 1-5 mM. None of the three conductances was calcium dependent, nor were they obviously affected by the eag mutation, which affects K channels in muscle. The developmental profile of the channels is the inverse of that described in muscle. Both IKs and IKf are present at the earliest pupal stages examined (approximately 60 hr), but IA was only first detected at approximately 76 hr.(ABSTRACT TRUNCATED AT 250 WORDS)