Circadian Oscillators and Non-Visual Photoreceptors in the Chicken Retinal Ganglion Cells Mario E. Guido, María Ana Contin and Daniela Verra
Departamento de Química Biológica-CIQUIBIC (CONICET), Facultad de Ciencias Químicas, Universidad Nacional de Córdoba (UNC), Córdoba, Argentina
In vertebrates, retinal ganglion cells (RGCs) convey photic information to the brain regarding ambient illumination conditions whereas a subset of these cells may act as non-visual photoreceptors regulating a number of non-image forming functions. In blind birds lacking functional visual photoreceptors, light still regulates pupil responses and the entrainment of daily rhythms (Valdez et al., 2009); remarkably, photic synchronization is lost after enucleation. RGCs also contain oscillators displaying daily rhythms in gene expression, and lipid and melatonin biosynthesis under constant conditions (Garbarino et al., 2004a-b). Moreover, primary cultures of immunopurified embryonic RGCs are able to generate self-sustained rhythms in gene expression and melatonin biosynthesis. Rhythms in melatonin synthesis were synchronized to the LD cycle. Thus, some RGCs in the cultures were intrinsically photosensitive (ip) acting through a cascade similar to that of rhabdomeric photoreceptors involving phospholipase C activation (Contin et al., 2006). Cultures of RGCs expressed the mRNAs for the Gq protein, two isoforms of the photopigment melanopsin (Opn4x and Opn4m) and the photoisomerases RGR and peropsin. Around 15-20% of the cells expressed the Opn4x protein and exhibited significant changes in intracellular Ca+2 mobilization after light exposure. Besides, light stimulation causes the activation of the phosphoinositide (PIP) cycle with the increase in intracellular levels of inositol 1,4,5-trisphosphate (IP3) and the differential activation of PIP kinase activities. Changes in IP3 levels and Ca+2 mobilization by light were reverted by the administration of the PLC inhibitor U73122 (10 µM). Results indicate that oscillators and photoreceptors are present in the same population of embryonic RGCs, and that these features appeared very early in development even before the rest of cell types have became totally differentiated.
