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M. A. Contin, D. M. Verra, G. A. Salvador, M. G. Ilincheta, N. M. Giusto, M. E. Guido; Intrinsically Photoreceptive Retinal Ganglion Cells: Involvement of a Phosphoinositide Cycle in the Phototransduction Cascade. Invest. Ophthalmol. Vis. Sci. 2009;50(13):5033.
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In Vertebrates, non-visual photoperception provides a measure of environmental lighting conditions that synchronizes biological clocks through retinal ganglion cells (RGCs) that send photic information to the brain. A subset of RGCs expressing the photopigment melanopsin (Opn4) is intrinsically photosensitive (ipRGC) acting as circadian photoreceptors. Our studies and others showed that the nature of the biochemical events in ipRGC phototransduction may resemble that in rhabdomeric (Rb) photoreceptors. Light activates Gq/11-class G proteins, which increase phospholipase C activity leading to the enhancement of Ca+2, and membrane depolarization. However, it remains unknown the complete mechanism of phototransduction and involvement of PIPs in ipRGCs. To achieve this goal we examined in chick RGCs primary culture model, exposed to light at different times: a) the formation of inositol-P3 b) the activity of Diacylglycerol kinase, Phosphatidylinositol k and Phosphatidylinositol phosphate K (DAGK, PIK and PIPK respectively).
Primary cultures of chicken RGCs were obtained at embryonic day 8 (E8) byanti-Thy-1 immunopanning and synchronized at time 0 by medium exchange. At the third day the cells were exposed to bright light (1200 lux) during 5, 30, 60, 90, 300 and 600 second, or kept in the dark. Then we measure inositol-P3 by Dowex chromatography columns and kinases activity in homogenates cell samples with 32P-ATP.
Our results showed a significant and rapid increase IP3 levels (1.3 fold) and the activation of PIK, PIPK and DAGK activities after a brief light pulse (5 sec-1.5 min) as compared to dark-maintained RGC cultures.
These results indicate that chicken ipRGC phototransduction involves a PIP cascade with higher activity of phosphoinositide cycle enzyme during the first 5 seconds of stimulation, suggesting the involvement of phosphoinositide cycle like phototransduction mechanism in rhabdomeric photoreceptor cells.
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