Abstract
Abstract: :
Purpose: The single photon response of retinal rods is highly reproducible. This reproducibility requires timely deactivation of rhodopsin. We sought to test whether reproducibility is affected by reduced levels of rhodopsin kinase (RK) or arrestin (Arr). Methods: Heterozygotic knockout mice (RK+/–, Arr+/–) were generated and characterized as previously described (Xu et al 1997; Chen et al 1999). Levels of RK and Arr in purified rod outer segments were determined by quantitative western blot. Suction electrodes were used to record from individual rod outer segments protruding from small pieces of dark–adapted retina. Reproducibility was assessed by measuring the normalized residual variance after scaling the time–dependent variance to the mean squared response (Mendez et al 2000) as well as by calculating the coefficient of variation of the elementary response amplitudes (Baylor, Lamb and Yau, 1979). Results: Reduced RK expression produced dim flash responses that recovered more slowly than those of wildtype rods across a wide range of flash strengths. Saturating flash responses of RK+/– rods recovered with a dominant time constant of 0.28 ms, whereas those of wildtype rods had a dominant time constant of 0.20 ms. Nevertheless, reproducibility of the RK+/– rods was not significantly different from that measured in wildtype rods. In contrast, Arr+/– rods showed no change in the dim flash kinetics or the dominant time constant of recovery (0.21 ms), yet the single photon responses were more variable. Conclusions: Our results suggest that in normal mouse rods, Arr binding does not rate–limit the recovery of the flash response, but is required for reproducible rhodopsin deactivation. In contrast, reduced expression of RK can apparently slow rhodopsin deactivation and thus the flash response kinetics, but without affecting reproducibility. We conclude that the mechanism(s) that confer reproducibility to the single photon response occur downstream of RK binding and require timely arrestin binding.
Keywords: photoreceptors • signal transduction • electrophysiology: non–clinical