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V. Laurent-Gydé, D. Hicks; Role of Melatonin in Photoreceptor Phagocytosis in the Rodent Retina. Invest. Ophthalmol. Vis. Sci. 2010;51(13):892.
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© ARVO (1962-2015); The Authors (2016-present)
Melatonin (mel) a zeitgeber hormone, for which Aryl-alkylamine-N acetyl transferase (Aa-nat) is the limiting enzyme of synthesis, is thought to play roles in retinal photoreceptor phagocytosis (PP), critical for photoreceptor survival. In order to investigate mel impact on mammalian retinal physiology, we first studied PP in rat retinas treated or not with mel. We then evaluated Aa-nat gene and protein productionin rodent retinas under different lighting conditions.
Adult rats that had received a melatonin implant were kept in constant darkness (DD) and examined for retinal immunohistochemistry using an anti-rhodopsin antibody to assess PP by fluorescence microscopy. In parallel, retinas of adult Arvicanthis ansorgei a cone-rich diurnal rodent, kept in a 12-hour light-dark cycle (LD) were processed for non-radioactive in situ hybridization (ISH) using an Aa-nat riboprobe. Also, circadian retinal Aa-nat gene expression was studied by real time PCR in animals housed under different lighting conditions (LD, DD, and constant light (LL) cycles). Finally, retinas of adult Arvicanthis maintained under DD cycles for 72h were processed for western-blotting using a specific anti-AA-NAT antibody in order to investigate circadian AA-NAT expression.
Rat PP displays a marked circadian expression with amaximumat CT2 in a DD cycle. Aa-nat gene expression revealed by ISH reached a maximum at night (ZT19) and a trough during the day (ZT6). Aa-nat expression was restricted to cone photoreceptors in both species. Arvicanthis retinal Aa-nat showed robust circadian expression, maximumat ZT19 in a 12h LD cycle and conserved in DD conditions. 36h LL exposure altered the slope and time of maximum Aa-nat mRNA expression. Finally, AA-NAT protein expression in Arvicanthis retinas was actually maximal at CT7, in the middle of the subjective day.
Mel implants do not modify PP in DD rat retina. Aa-nat expression occurs uniquely in cones of Arvicanthis retina. However, LL conditions induce marked alterations in the expression profile, and protein presence in subjective daytime under DD conditions appears paradoxical. Use of cone-rich mammalian species such as Arvicanthis ansorgei may help understand melatonin regulation and action on retinal physiology.
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