Measurement of thermal contribution to photoreceptor sensitivity

Activation of a visual pigment molecule to initiate phototransduction requires a minimum energy, Ea, that need not be wholly derived from a photon, but may be supplemented by heat1. Theory2,3 predicts that absorbance at very long wavelengths declines with the fraction of molecules that have a sufficient complement of thermal energy, and that Ea is inversely related to the wavelength of maximum absorbance (λmax) of the pigment. Consistent with the first of these predictions, warming increases relative visual sensitivity to long wavelengths4,5,6,7,8. Here we measure this effect in amphibian photoreceptors with different pigments to estimate Ea (refs 2, 5,6,7) and test experimentally the predictions of an inverse relation between Ea and λmax. For rods and ‘red’ cones in the adult frog retina, we find no significant difference in Ea between the two pigments involved, although their λmax values are very different. We also determined Ea for the rhodopsin in toad retinal rods—spectrally similar to frog rhodopsin but differing in amino-acid sequence—and found that it was significantly higher. In addition, we estimated Ea for two pigments whose λmax difference was due only to a chromophore difference (A1 and A2 pigment, in adult and larval frog cones). Here Ea for A2 was lower than for A1. Our results refute the idea of a necessary relation between λmax and Ea, but show that the A1 → A2 chromophore substitution decreases Ea.