ИСТИНА

BACKGROUND. The primary event in vision is the 11-cis → all-trans isomerization of the visual-pigment chromophore retinal upon absorption of a photon. Recovery of the maximal light sensitivity of photoreceptor cells after illumination requires restoration of the full complement of visual pigment (rhodopsin) with 11-cis retinal. The chain of reactions whereby this is achieved in darkness is called the visual cycle. It has been much studied in vertebrates (having c-opsins), but much less in invertebrates (having r-opsins, which largely rely on light-driven all-trans ↔ 11-cis interconversion). PURPOSE. To study the visual cycle in the crustacean Mysis relicta, with special attention to possible differences between two populations representing different degrees of ecological adaptation to dark environments: one (Lp) from a very dark lake, the other (Sp) from the more variably lit Baltic Sea. METHODS. We measured changes in the content of 11-cis and all-trans (as well as 13-cis) retinal and retinol in hexane eye extracts by high-performance liquid chromatography (HPLC) before and immediately after briefly exposing initially dark-adapted animals to a strong white light, and at fixed post-exposure time points over a period of ca. 2 weeks spent in darkness at 4 ºC. RESULTS. Thoroughly dark-adapted, unexposed controls indicated practically no difference in the percentages of 11-cis and all-trans retinal between the two populations (65% and 28% for Sp; 60% and 33% for Lp, whereby 13-cis retinal accounts for the rest). The percentages obtained just after the strong light exposure were also similar in the two populations, as 11-cis retinal decreased to 6% (Sp) and 8% (Lp), and all-trans increased to 78% (Sp) and 82% (Lp). In terms of absolute amounts, however, there was a 13-15-fold decrease in 11-cis retinal that was not balanced by the modest increase (about 1.5-fold) in all-trans retinal. This implies that most of the all-trans retinal arising from photoisomerization of 11-cis must have been quickly removed, presumably transformed into all-trans retinol. Accordingly, the total amount of retinals in the sample was reduced by more than 40% within minutes after the light exposure. Dark-adaptation entailed a slow increase in 11-cis towards its initial value. In Lp, this took (at least) the entire duration of the experiment (16 days), in Sp about 10 days. All-trans and 13-cis retinal remained almost constant throughout; thus recovery of total retinal paralleled that of 11-cis. Free retinol, on the other hand, increased significantly after the light exposure in both populations, but much more in Lp than in Sp. In Lp, where we measured the isomers separately, the immediate increase in 11-cis retinol was 15-fold and in all-trans 7-fold. In darkness retinol fell back towards the pre-exposure values over ca. two weeks, consistent with the idea that this decline is related to the synthesis of 11-cis retinal. CONCLUSIONS. Our results, together with published literature, suggest that dark regeneration of rhodopsin in Mysis relies on an enzymatic visual cycle in pigment cells, where all-trans retinol is converted to 11-cis retinol and then to 11-cis retinal, resembling the situation in vertebrate retina. In the Lp population (living in an especially dark environment) this involves a larger pool of free retinol with larger relative as well as absolute changes compared with the Sp population.