Non-Visual Phototransduction in Non-mammalian Vertebrates
Guido ME, Valdez DJ, Nieto PS, Verra DM, Acosta MV, Contin MA.
Departamento de Química Biológica-CIQUIBIC, Facultad de Ciencias Químicas, Universidad Nacional de Córdoba-CONICET, Córdoba, ARGENTINA. mguido@mail.fcq.unc.edu.ar
A non-image forming circuitry conveys photic information from the retina to the brain regulating diverse non-visual functions such as the entrainment of activity rhythms, sleep and the pupillary light reflex (PLR). In birds, the retina, pineal gland and potentially deep brain components are photoreceptive. We previously found that retinal ganglion cells (RGCs) contain clocks synthesizing melatonin rhythmically (Garbarino et al., JBC 2004). GUCY1* chickens suffer PRC degeneration and blindness at hatch. Herein, we investigated light perception by the inner retina in the absence of functional photoreceptor cells (PRCs) by assessing the PLR and the synchronization of feeding rhythms to 12:12 h light-dark (LD) cycles in these birds. Blind chickens displayed light responses in both the PLR and the entrainment of feeding rhythms. Feeding rhythms were entrained to different LD cycles even when chicks had the head covered but not after enucleation, condition in which animals free run. Monochromatic light of 430-500 nm caused PLRs with maximal responses at 480, resembling a melanopsin-like photopigment. In addition, embryonic RGC cultures express ancestral rhabdomeric photoreceptor markers such as Pax6, Brn3, melanopsin and the G-protein q. After synchronization to a 12:12 h LD cycle, dark-maintained RGC cultures exhibited a daily variation in 3H-melatonin levels, which was significantly inhibited by light (Contin et al., FASEB J 2006). This effect was further increased by the chromophore all-trans retinal, and suppressed by specific inhibitors of the invertebrate photo-cascade involving phosphoinositide hydrolysis and Ca2+ mobilization. The non-visual photoreceptive capability of RGCs in the inner retina can be essential to temporally regulate development and physiology in response to ambient light even in the absence of formal vision.
