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M.E. Guido, D.J. Valdez, E. Garbarino–Pico, L. Avalle, H. Diaz–Fajreldines, K. Cheng; Mechanisms of Circadian Photoperception in an Animal Model of Blindness . Invest. Ophthalmol. Vis. Sci. 2006;47(13):5424.
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© ARVO (1962-2015); The Authors (2016-present)
In birds, the retina, the pineal gland and extraretinal photoreceptors are responsible for photoreception of the environmental light–dark (LD) cycles that synchronize the circadian system. The retina is rhythmic itself, generating daily rhythms in diverse metabolic and physiological functions. We recently reported that chicken retinal ganglion cells (RGCs) synthesize melatonin with higher levels during the day (Garbarino et al., 2004). The GUCY1* chicken carries an autosomal recessive mutation that results in the degeneration of their photoreceptors (PRCs) cones and rods, causing blindness at hatch. We investigated, in these animals, the light responses to non–visual tasks and the circadian regulation of physiology.
We assessed in GUCY1* chickens: a) the electric responses of the retina to bright light by electroretinograms (ERG); b) the pupillary light responses (consensual reflex) to monochromatic bright light of different wavelengths; c) the feeding rhythms after synchronization to diverse LD cycles with cool white and blue fluorescent light of high intensities, and; d) the activity of the key melatonin–synthesizing enzyme, serotonin N–acetyltransferase (NAT) in preparations of RGCs and PRCs from blind and control animals at different times during the subjective day and night.
When we assessed the electric responses of GUCY1* retinas to bright light by ERG, a totally null responsiveness was obtained. However, animals were able to display detectable pupillary responses to light of different wavelengths (430, 480 and 500 nm). In addition, the GUCY1* RGCs exhibited detectable levels of NAT activity in vitro, with a differential pattern of circadian activity (p<0.001 by ANOVA) that was significantly shifted with respect to that found in RGCs from wild type animals. When we assessed the feeding activity, animals exhibited circadian rhythms of food intake that were perfectly entrained to different LD cycles (white or blue light of ∼700 lux) when their pineal gland was occluded. In LL (white light of ∼150 lux), these chickens free run with a period near 24.37 h and became arrhythmic when they have their pineal gland occluded.
In birds, photoperception takes place even in the absence of functional PRCs by a complex mechanism involving the RGCs in the inner retina that regulate retinal physiology and non–visual tasks.
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