Distinct mechanisms of light adaptation of elementary responses in photoreceptors of dipteran flies and American cockroach.

Of many light adaptation mechanisms optimizing photoreceptor functioning in the compound eyes of insects, those modifying the single-photon response, the quantum bump (QB), remain least studied. Here, by recording from photoreceptors of the blow fly , the hover fly , and the cockroach , we investigated mechanisms of rapid light adaptation by examining how properties of QBs change after light stimulation and multiquantal impulse responses during repetitive stimulation. In . , light stimulation reduced latencies, characteristic durations, and amplitudes of QBs in an intensity- and duration-dependent manner. In . , only QB amplitudes decreased consistently. In both species, time constants of QB parameters' recovery increased with the strength and duration of stimulation, reaching ∼30 s after bright prolonged 10-s pulses. In the blow fly, changes in QB amplitudes during recovery correlated with changes in half-widths but not latencies, suggesting at least two separate mechanisms of light adaptation: acceleration of QB onset by sensitizing transduction channels and acceleration of transduction channel inactivation causing QB shortening and decrease. In the cockroach, light adaptation reduced QB amplitude by apparently lowering the transduction channel availability. Impulse response data in the blow fly and cockroach were consistent with the inferences from the QB recovery experiments. However, in the hover fly , impulse response latencies and durations decreased simultaneously, whereas amplitudes decreased little, even when bright flashes were applied at high frequencies. These findings indicate the existence of dissimilar mechanisms of light adaptation in the microvilli of different species. By studying light adaptation of elementary responses in photoreceptors of the blow fly and the cockroach we found three distinct mechanisms. In the blow fly, one mechanism speeds quantum bump onset and another accelerates quantum bump inactivation, decreasing its size. In the cockroach, quantum bump amplitude decreases without changes in kinetics, indicating decreased availability of transduction channels. The findings can be explained by expression of different transduction channels in the flies and cockroaches.