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S. Hattar, J. L. Ecker, O. N. Dumitrescu, S.-K. Chen, K. Y. Wong, N. M. Alam, G. T. Prusky, D. M. Berson; Functions and Target Innervations of Distinct Subtypes of Melanopsin Cells. Invest. Ophthalmol. Vis. Sci. 2009;50(13):5027.
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© ARVO (1962-2015); The Authors (2016-present)
Intrinsically photosensitive retinal ganglion cells (ipRGCs) use the photopigment melanopsin and drive non-image-forming visual behaviors such as circadian photoentrainment. There is increasing evidence that the melanopsin-containing RGCs represent several different subtypes in the retina. Characterizing the different subtypes of melanopsin cells and their brain targets will provide insight into the diverse functions of this newly discovered photoreceptive system.
Using the traditional and conditional Cre-lox system in mice, melanopsin-containing RGCs were made to express either alkaline phosphatase (to visualize their form and projections), or EGFP (to target them for whole cell recording and dye-filling). We have also mated the melanopsin-Cre animals with the Brainbow mice to label the cells with fluorescent proteins of different colors. Melanopsin-based photoresponses were recorded under synaptic blockade in heterozygotes.
Three morphological types of melanopsin-expressing RGC photoreceptors were identified. "M1" cells correspond to the established ipRGC type, with relatively small somas and sparse, disordered dendritic arbors in the OFF sublamina of the inner plexiform layer (IPL) with the fastest and largest intrinsic photoresponses. Two novel types were found. "M2" cells have medium-sized somas. Their dendritic arbors stratify in the ON sublamina and generate slower and smaller intrinsic light responses than M1s. "M3" cells resemble ON alpha cells, with very large somas, and large radiate arbors of thick dendrites stratifying in the inner IPL and generate the slowest and weakest photoresponses. In addition to the well-identified brain targets of ipRGCs, we showed substantial innervation in the dorsal lateral geniculate nucleus, an area important for image formation. Indeed, we found that low-resolution pattern vision persisted in mice in which melanopsin cells were the only functional photoreceptors. Some rods and cones were labeled in traditional cre animals, but not when cre was induced in adults. This implies transient melanopsin expression in the outer retina. The conditional cre model also allowed us to label few cells per retina, to trace the axonal projections of few ipRGCs to specific brain regions.
We have identified distinct subtypes of melanopsin cells with distinct morphological and electrophysiological properties as well as distinct patterns of target innervation and a role in low-resolution pattern discrimination.
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