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V. Matos-Cruz, S. Hattar, M. E. Halpern; Diversity of Melanopsin-Expressing Cells in the Zebrafish Retina. Invest. Ophthalmol. Vis. Sci. 2010;51(13):681.
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Melanopsin (Opn4) is a photopigment involved in mediating circadian photoentrainment and other non-image forming functions. This opsin sublass has evolved into two groups, Opn4m and Opn4x. In mammals, the single opn4m gene is expressed in a small subset of intrinsically photosensitive retinal ganglion cells. However, non-mammalian vertebrates contain at least 1 gene of each group and they have widespread expression. We are interested in understanding the complexity and function of the two subclasses of melanopsin proteins in image and non-image forming visual functions.
Five zebrafish DNA sequences were identified by bioinformatics analyses and opn4 full-length cDNAs were cloned. RNA in situ hybridization analyses was performed during different developmental stages. Samples were plastic embedded and sectioned to determine the precise cellular localization. Colocalization studies were also performed by RNA in situ hybridization double labeling.
The zebrafish melanopsin-expressing cells are widely expressed in retinal neurons and some brain regions. These melanopsin genes are regulated developmentally in zebrafish. Interestingly, 2 zebrafish melanopsin genes are expressed at 24 hours post-fertilization (hpf) in the developing brain before retinogenesis occurs. All genes start to be expressed at 72 hpf in the retina, with the exception of opn4x2, which is expressed at 2 days post-fertilization (dpf). Expression expands during retinal development and by 5 dpf opn4-related genes are expressed throughout the retina in bipolar, amacrine, classical photoreceptors, retinal ganglion cells and horizontal cells in largely non-overlapping patterns. Semi-thin sections and colocalization with classical retinal cell type-specific markers confirmed cell identity. Expression of these genes persists during adulthood.
The development and identity of the melanopsin-expressing cells suggest that melanopsin may not only be involved in non-image forming functions, but may also play a role in image-forming responses. We are currently developing tools to address the function of the melanopsin-expressing cells in zebrafish.
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