Light adaptation of the microvillus refractory period in blow fly photoreceptors.

Analysis of flash-induced quantum bumps (QB) recorded in vivo from photoreceptors of the blow fly Protophormia terraenovae, which were light-adapted by stimuli of varying duration and intensity, revealed that a subset of trials contained a secondary QB-like transient occurring after the primary QB. By averaging traces with onset-aligned light-induced QBs, we discovered secondary signals with the following properties: (1) The relative magnitude of the secondary signal increased with both the intensity and the duration of the adapting stimulus. The secondary signal was absent in dark-adapted photoreceptors, but following bright 10 s adapting pulses its size approached   40% of the primary QB. (2) The interval between the peaks of the primary QB and the secondary signal decreased with increasing adaptation: from  100 ms in minimally light-adapted photoreceptors to  40 ms after the strongest stimulation. (3) The larger was the secondary signal, the sooner it appeared after the primary QB. (4) Following strong light adaptation, in five experiments we observed two and in one experiment three evenly spaced secondary peaks with progressively decreasing amplitudes. Together, these observations indicate that the secondary signal likely represents a QB arising from incomplete termination of phototransduction in some microvilli during strong light stimulation. The systematic, strong shortening of the secondary peak interval with increased adaptation implies that the microvillus refractory period in P. terraenovae is itself subject to light adaptation. Because refractory period shortening increases the pool of microvilli available for transduction, this effect would raise the effective signal-to-noise ratio of the photoreceptor in bright light.