Measurement of thermal contribution to photoreceptor sensitivity

Activation of a visual pigment molecule to initiate phototransduction requires a minimum energy, E-a, that need not be wholly derived from a photon, but may be supplemented by heat(1), Theory(2,3) predicts that absorbance at very long wavelengths declines with the fraction of molecules that have a sufficient complement of thermal energy, and that E-a is inversely related to the wavelength of maximum absorbance (lambda(max)) of the pigment, Consistent with the first of these predictions, warming increases relative visual sensitivity to long wavelengths(4-8). Here we measure this effect in amphibian photoreceptors with different pigments to estimate E-a (refs 2, 5-7) and test experimentally the predictions of an inverse relation between E-a and lambda(max). For rods and 'red' cones in the adult frog retina, we find no significant difference in E-a between the two pigments involved, although their lambda(max) values are very different. We also determined E-a for the rhodopsin in toad retinal rods--spectrally similar to frog rhodopsin hut differing in amino-acid sequence-and found that it was significantly higher. In addition, we estimated E-a for two pigments whose lambda(max) difference was due only to a chromophore difference (A1 and A2 pigment, in adult and larval bag cones), Here E-a for A2 was lower than for Al. Our results refute the idea of a necessary relation between lambda(max) and E-a but show that the A1 --> A2 chromophore substitution decreases E-a.