Opsin1-2, Gqα and arrestin levels at Limulus rhabdoms are controlled by diurnal light and a circadian clock

Abstract

Summary Dark and light adaptation in photoreceptors involve multiple processes including those which change protein concentrations at photosensitive membranes. Light and dark-adaptive changes in protein levels at rhabdoms have been described in detail in white eyed Drosophila maintained under artificial light. Here we tested whether protein levels at rhabdoms change significantly in the highly pigmented lateral eyes of wild caught Limulus maintained in natural diurnal illumination and whether these changes are under circadian control. We found that rhabdomeral levels of opsins (Ops 1-2), the G protein activated by rhodopsin (Gqα) and arrestin change significantly day to night and that nighttime levels of each protein at rhabdoms are significantly influenced by signals from the animal's central circadian clock. Clock input at night increases Ops1-2 and Gqα and decreases arrestin levels at rhabdoms. Clock input is also required for a rapid decrease in rhabdomeral Ops1-2 beginning at sunrise. We found further that dark-adaptation during the day and night are not equivalent. During daytime dark-adaptation, when clock input is silent, the increase of Ops1-2 at rhabdoms is small and Gqα levels do not increase. However, increases in Ops1-2 and Gqα at rhabdoms are enhanced during daytime dark-adaption by treatments which elevate cAMP in photoreceptors suggesting the clock influences dark-adaptive increases in Ops1-2 and Gqα at Limulus rhabdoms by activating cAMP-dependent processes. The circadian regulation of Ops1-2 and Gqα levels at rhabdoms probably has a dual role: to increase retinal sensitivity at night and to protect photoreceptors from light damage during the day.