Sources of noise in photoreceptor transduction.

Photoreceptors are surprisingly noisy, and the properties of this noise are providing clues to the molecular mechanisms underlying phototransduction. Three distinct components of noise arise from excitation of the rhodopsin photopigment molecule: the first is activation by photons, and the second is thermal activation in darkness. The third component, induced following intense bleaching lights, probably reflects reversibility in the reactions that inactivate isomerized rhodopsin; in this way a small degree of reactivation of the excited form of rhodopsin is generated from the vast amounts of bleached photoproduct that exist in the photoreceptor following intense exposures. A fourth component of noise, the continuous component present in darkness, probably arises from stochastic fluctuations in the number of excited molecules of a biochemical intermediate, either transducin (G-protein) or phosphodiesterase. This fourth component of noise appears to be much more prominent in cones than in rods. A fifth (and smaller) noise component is caused by the random opening and closing of light-sensitive channels in the outer segment.