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Jan Verweij, Shawntay Chaney, David R Copenhagen; Melanopsin ganglion cells in fetal retinas are light responsive. Invest. Ophthalmol. Vis. Sci. 2016;57(12):4661.
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© ARVO (1962-2015); The Authors (2016-present)
Regression of hyaloid vasculature and formation of retinal vasculature in postnatal mice are thought to be stimulated by light activation of melanopsin cells, presumably ganglion cells, in the fetal retina. We directly tested the hypothesis that melanopsin ganglion cells in the fetal retina are light responsive by using calcium indicators.
We used cre/lox genetic system to liberate the calcium indicators GCaMP3 or GCaMP6 in melanopsin cells (Opn4-cre::Ai38/Ai96 mice). Timed pregnant mice and their e16.5 embryos were euthanized. Fetal retinas were isolated and simultaneously stimulated and imaged with 480 nm light. Some experiments were done using postnatal day 7 (p7) retinas.
Several dozen GCaMP positive cells could be visualized in each field of view (660x660 µm). GCaMP positive cells in e16.5 and p7 retinas responded transiently to the onset of light in pups genotyped as Opn4-cre +/- (expressing melanopsin). GCaMP positive cells did not respond in pups genotyped as Opn4-cre +/+ (effectively melanopsin knockout retinas). Light responsive cells displayed a refractory period after bright light. The relative refractory period was on the order of 100 s. Light responses in e16.5 and p7 retinas were differently affected by Tetrodotoxin (TTX, 1 µM). The average response was reduced by more than 70% in e16.5 and by about 30% in p7 pups. Retinal waves (spontaneous excitation) propagated through e16.5 and p8 retinas at 100-150 µm/s and a rate of roughly 1/minute.
Retinal ganglion cells are light responsive as early as e16.5. Also, there are spontaneous periodic bursts of propagated activity well before birth. The differential effects in TTX suggest that voltage activated sodium channels play a more significant role in visual signal transduction in fetal retinas. Both light-evoked and spontaneous excitation in the fetal retina could influence activity dependent maturation of the visual system as well as the vascular system in the eye.
This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.
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