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C. J. Sheely, D. S. McNeill, J. L. Ecker, D. Morhardt, W. Guido, R. L. Brown, S. Hattar; Development and Functional Connections of Melanopsin Expressing Retinal Ganglion Cells in the Mouse. Invest. Ophthalmol. Vis. Sci. 2009;50(13):5029.
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The mammalian eye detects light for the purposes of forming images and modulating physiological processes. All light signals are sent to the brain by the retinal ganglion cells (RGCs), which are the sole output neurons of the retina. RGCs receive visual input from rod and cone photoreceptors; however, 1-2% of ganglion cells contain the photopigment melanopsin and are intrinsically photosensitive (mRGCs). The mRGCs play a crucial role in signaling light information to the suprachiasmatic nucleus (SCN), which is the circadian pacemaker, and olivary pretectal nucleus (OPN) which controls pupil constriction. Although the development and targeting of retinal ganglion cells is well studied, the mRGCs are a novel population of photoreceptors whose developmental hallmarks are unknown. We have investigated development of mRGCs in mice, including the onset of melanopsin expression, mRGC birthdates, and their targeting and functional connections.
Immunostaining was performed using rabbit -melanopsin, rat -BrdU, and chicken -β-gal antibodies. To trace axonal projections of the RGCs, we used fluorescent cholera toxin B subunit. To specifically label projections of the mRGCs we used genetic reporters: a Tau-LacZ allele of melanopsin (Melanopsin Tau-LacZ) and a Melanopsin-Cre animal mated to an alkaline phosphatase reporter, Z/AP.
Melanopsin is expressed embryonically at e15 and mRGC birth begins at e14. The mRGCs begin SCN innervation at P0, continuing throughout the first two weeks of postnatal development. The Melanopsin-Cre-Z/AP system reveals OPN innervation at P0, while the Tau-LacZ reporter does not show innervation until P7. To determine the functionality of these connections, we measured pupillary light response across postnatal development and first detect a response at P7.
The mRGCs follow a slightly delayed course of development in comparison to the general RGC population. The discrepancy seen in OPN innervation reveals that mRGCs may be composed of two different populations that vary spatially and temporally in development.
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