On the molecular origin of photoreceptor noise

Retinal photoreceptors are noisy. They generate discrete electrical events in the dark indistinguishable from those evoked by light and thereby limit visual sensitivity at low levels of illumination. The random spontaneous events are strongly temperature-dependent and have been attributed to thermal isomerizations of the vitamin A chromophore of rhodopsin, the light-sensitive molecule in photoreceptors. But thermal generation of dark events in both vertebrate and invertebrate photoreceptors requires activation energies in the range of 23 to 27 kcal mol-1, which are significantly less than the energy barrier of 45 kcal mol-1, for photoisomerization of the chromophore of native rhodopsin. We propose that photoreceptor noise results from the thermal isomerization of a relatively unstable form of rhodopsin, one in which the Schiff-base linkage between the chromophore and protein is unprotonated. This molecular mechanism is supported by both theoretical calculations of the properties of rhodopsin and experimental measurements of the properties of photoreceptor noise.