Higher Rate of Thermal Activation of Red Cone Pigments With 11–Cis A2 Compared to 11–Cis A1 Retinal as Chromophore

Abstract

Abstract: : Purpose: Red cone pigment with 11–cis A₂ retinal as chromophore has a relatively high rate of thermal isomerization. This sustained spontaneous activity desensitizes the cells and accelerates their response kinetics in darkness. However, most vertebrate species, including mammals, use 11–cis A₁ retinal as chromophore for their visual pigments. To investigate whether the difference in chromophore affects the rate of cone pigment thermal activation, we studied photoreceptors with 11–cis A₁ red cone pigment. Methods: Using a suction electrode, we recorded membrane current from single larval salamander red cones before and after pigment bleach and regeneration with 11–cis A₁ retinal, as well as from transgenic mouse rods expressing 11–cis A₁ human red cone opsin. We estimated the rate of spontaneous thermal activation of the cone pigment in each case from the power spectrum of the dark current noise. Results:In larval salamander red cones, bleaching of the native pigment (which contains a mixture of 11–cis A₁ and 11–cis A₂ retinal) and regeneration with exogenous 11–cis A₁ retinal resulted in a six–fold decrease in the rate of pigment thermal activation. The lower rate of thermal activation of 11–cis A₁ cone pigment was confirmed in transgenic mouse rods expressing human red cone opsin, where its presence did not cause detectable increase in dark noise. In contrast, as we have previously shown, transgenic expression of the same opsin in Xenopus rods, where it is combined with 11–cis A₂ retinal, produces a significant increase in dark noise. Conclusions: We find that 11–cis A₁ red cone pigments have a significantly lower rate of thermal activation than the corresponding 11–cis A₂ pigments. Using independently derived mean levels of 48% 11–cis A₁ and 52% 11–cis A₂ retinal in larval salamander red cones, we calculate that the 11–cis A₁ red cone pigment has a rate of thermal activation eleven–fold lower than the corresponding rate for 11–cis A₂ cone pigment. A similar ratio has been previously found for amphibian 11–cis A₁ and 11–cis A₂ rod pigments (Donner et al., 1990, J Physiol 428, 673–92), even though the absolute rates in this case are significantly lower. The spontaneous thermal activation of cone pigment would not be expected to be a significant factor for the low sensitivity and fast response kinetics of mammalian cones, which use 11–cis A₁ retinal, though that remains to be directly tested.

Keywords: photoreceptors • opsins • electrophysiology: non-clinical