S7-1.5 Sunday, Jan. 6 What the clock tells the eye: Lessons from an ancient arthropod BATTELLE, Barbara-Anne; Univ, of FL, Whitney Laboratory for Marine Bioscience Battelle@whitney.ufl.edu
Eyes are major targets for regulation by circadian clocks, but effects of circadian clocks on vision are not fully understood in any system. Among invertebrates, effects of circadian rhythms on eyes are perhaps best understood in the American horseshoe crab Limulus polyphemus. This animal uses its compound lateral eyes (LEs) to find mates, and it spawns at night and during the day. Behavioral studies suggest Limulus see at night nearly as well as during the day, and electrophysiological studies show that its LEs are dramatically more sensitive to light at night than during the day. Half the nighttime increase in LE sensitivity can be attributed to signals from central circadian clocks. Circadian signals reach the eyes via axons from central, clock-driven, efferent neurons that project through the optic nerves. These efferent neurons are active at night and silent during the day. When active, they release the biogenic amine octopamine which elevates cAMP in post synaptic cells. The effects of clock input on LEs are diverse. Clock input at night drives changes in LE structure that increase photon catch and electrophysiological properties of photoreceptors such that their signal to noise ratio increases. Recent evidence shows that clock input also influences the dark-adaptive biochemistry of photoreceptors. Rhabdomeral concentrations of several proteins critical for the photoresponse change significantly day to night. At night, the levels of opsin (Ops), the protein moiety of visual pigment, and the alpha subunit of the G protein activated by the visual pigment (Gqalpha), increase, and arrestin, the protein that quenches the photoresponse, decreases. Clock input is required for normal nighttime increases in rhabdomeral concentrations of Ops and Gqalpha and these effects are mediated by octopamine and activation of the cAMP cascade.