The limit of photoreceptor sensitivity: Molecular mechanisms of dark noise in retinal cones

Detection threshold in cone photoreceptors requires the simultaneous absorption of several photons because single photon current does not exceed in amplitude intrinsic fluctuations in the outer segment dark current (dark noise). To understand the mechanisms that limit light sensitivity we characterized the molecular origin of dark noise in intact, isolated bass single cones. Dark noise arises from fluctuations in cytoplasmic concentrations of cGMP and Ca due to activity in darkness of both phosphodiesterase (PDE) the enzyme that synthesizes cGMP and guanylate cyclase (GC) the enzyme that hydrolyzes it. In cones loaded with high concentration Ca buffering agents, we demonstrate that variation in cGMP level arise from fluctuations in the mean enzymatic activity of phosphodiesterase (PDE). The rates of PDE activation and inactivation determine the quantitative characteristics of the dark noise power spectrum. We develop a mathematical model that accurately predicts this power spectrum and analysis of the experimental data with the theoretical model allowed us to determine the rates of PDE activation and deactivation in the intact cell. In fish cones the mean lifetime of active PDE at room temperature is about 55 msec. In non-mammalian rods, in contrast, active PDE lifetime is about 555 msec. This remarkable difference helps explain why cones are noisier than rods and why cone photocurrents are smaller in peak amplitude and faster in time to peak than those in rods. Both these features make cones less light sensitive than rods.