Mechanisms of Non-Visual Phototransduction in Vertebrates.
Mario E. Guido. Dpto. Qca. Biol.-CIQUIBIC, Facultad de Ciencias Químicas, Universidad Nacional de Córdoba-CONICET, Córdoba, ARGENTINA. mguido@mail.fcq.unc.edu.ar
In mammals, a non-image forming circuitry conveys photic information from the retina to the brain regulating diverse non-visual functions. In birds, the retina, pineal gland and likely deep brain components are photoreceptive. We investigated light perception, in the absence of functional photoreceptor cells (PRCs), in GUCY1* chickens that suffer PRC degeneration and blindness at hatch, by assessing the pupillary light reflex (PLR) and the synchronization of feeding rhythms to various 12:12 h light-dark (LD) cycles. Blind birds displayed light responses in both the PLR and the entrainment of feeding rhythms. Feeding rhythms were entrained to different LD cycles even when blind chickens had the head covered. Also, we observed PLRs to monochromatic light of 430, 480 and 500 nm in GUCY1* chickens with maximal responses at 480, resembling a melanopsin-like photopigment. We previously found that retinal ganglion cells (RGCs) contain clocks synthesizing melatonin rhythmically (Garbarino et al., JBC 2004). Now, we found that 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.
