The light response of drosophila photoreceptors is accompanied by an increase in cellular calcium: Effects of specific mutations

Photoreceptors of dissociated Drosophila retinae were loaded with the fluorescent Ca²⁺ indicators, fluo-3 and Calcium Green-5N. In fluo-3-loaded, wild-type photoreceptors, a rapid increase in fluorescence (Ca²⁺ signal) accompanied the light-evoked inward current. Removal of extracellular Ca²⁺ greatly reduced the Ca²⁺ signal, indicating Ca²⁺ influx as its major cause. In Calcium Green-5N-loaded trp mutants, which lack a large fraction of the Ca²⁺ permeability underlying the light-evoked inward current, the Ca²⁺ signal was smaller relative to wild-type photoreceptors. Fluo-3-loaded norpA mutant photoreceptors, which lack a light-activated phospholipase C, generated no light-evoked inward current and no Ca²⁺ signal. The phosphoinositide pathway therefore appears necessary for both excitation and changes in cytosolic free Ca²⁺ concentration.