The phototransduction process of animal photoreceptors starts with the absorption of light by visual pigment, rhodopsin, resulting in the transformation of the rhodopsin into an active form, metarhodopsin. After several biochemical steps, this results in a quantum bump, a transient change in the membrane voltage. The life time of the metarhodopsin state, which triggers the phototransduction chain, determines the temporal resolution of the visual process. Metarhodopsin life time depends on the binding of arrestin molecules, which inactivate (arrest) the active metarhodopsin state. The arrestin concentration and its binding constant thus are crucial factors determining the light sensitivity, frequency response as well as the speed of light-adaptation of the photoreceptors.

We have studied the wavelength, intensity and arrestin dependence of inactivation of photoreceptors of white-eyed wild-type Drosophila and the hypomorphic arrestin mutant (w^-;arr2³) by simultaneously measuring visual pigment conversions, via metarhodopsin fluorescence, and the elicited electrophysiological responses, via the electroretinogram (ERG). We have implemented the measured data in a kinetic model of the rhodopsin-arrestin cycle, allowing us to estimate the active metarhodopsin as a function of light intensity. Arrestin reduction in the mutant increased the light sensitivity by a factor of 3.5. We present a steady-state stochastic model that quantitatively explains the different dependencies on light intensity of the prolonged depolarizing afterpotentials (PDA) in the wild type and the hypomorphic mutant. We discuss the feasibility of different experimental methods for the estimation of the PDA and arrestin content.