EXPLORING MELANOPSIN FUNCTION IN MAMMALS WITH DIVERSE PHOTORECEPTOR ARCHITECTURE

Abstract

The properties of visual pigments and retinal architecture in mammals are adapted to suit diverse light environments. Intrinsically photosensitive retinal ganglion cells (ipRGCs), which express the visual pigment melanopsin, integrate both their intrinsic photoresponse and input from rods and cones to mediate both image and non-image forming visual functions. Molecular modeling and genetic analyses suggest that the absorption spectra of all mammalian melanopsins are nearly identical. However, melanopsin expression varies significantly among mammals with different photoreceptor architectures, suggesting potential variability in melanopsin signaling kinetics and melanopsin-mediated behavior. This study investigates the impact of the evolutionary gain or loss of cone photoreceptor types on melanopsin signaling kinetics. This study also investigates the expression of melanopsin the nine-banded armadillo, a terrestrial monochromat. Detailed analysis of phosphorylation at the C-terminus of mouse melanopsin has identified key residues that are important for the kinetics of ipRGC-regulated behavior. Using an in vitro fluorescent calcium signaling assay, our results demonstrate a correlation between delayed melanopsin activation and deactivation kinetics in rod monochromats. Furthermore, mammals with activity patterns during the day (trichromats and dichromats) have similar or faster melanopsin signaling kinetics than wildtype mouse melanopsin. In addition, we confirmed the expression of melanopsin in the nine-banded armadillo. These findings suggest that the presence or absence of photopic vision (i.e., the possession of cone opsins) is a key evolutionary factor driving rapid melanopsin signaling in ipRGCs.?