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F. R. Postma, H. Topley, M. Hou, D. L. Paul; Connexin45 Is Expressed in Off Cone Bipolar Cells. Invest. Ophthalmol. Vis. Sci. 2009;50(13):4563.
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Many retinal neurons establish electrical synapses or gap junctions. Although intercellular channels in most neuronal gap junctions are composed of connexin36 (Cx36), some retinal cells express Cx45. It has been hypothesized that Cx45 is expressed by ON cone bipolar cells and is involved in gap junctions with AII amacrines, which in turn express Cx36. To test this hypothesis, we used double immunocytochemistry to establish the identity of retinal neurons expressing Cx45 and to assess Cx45-positive cells for glycine, which infers the presence of gap junctions with glycinergic AII amacrines.
We used two mouse lines in which the Cx45 gene was replaced by a LacZ reporter gene; one in which recombination of a Cx45flx allele was accomplished by crossing with a nestin-cre transgenic line and one in which Cx45 was constitutively deleted in the germ line (Cx45KO). Immunocytochemistry was performed on cryostat sections of adult retinas
The expression pattern of β-gal, reporting Cx45, was identical in heterozygous nestin-cre/Cx45flx and heterozygous Cx45KO mice. β-gal was detected in subsets of ganglion cells, amacrine cells and bipolar cells. Double staining for β-gal and calbindin, calretinin, PKC, recoverin or CaB5 suggested Cx45 expression in a subset of OFF cone bipolars but not rod bipolars. Double labeling for β-gal and glycine revealed no detectable overlap in staining. In addition, homozygous and heterozygous nestin-Cre/Cx45flx retinas displayed identical glycine diffusion patterns.
Cx45-positive bipolar cells in the mouse retina most likely do not couple to AII amacrine cells arguing against the existence of a functional Cx36-Cx45 heterotypic junction. Cx45 positive bipolar cells form a specific class some of which positive for recoverin corresponding to OFF cone bipolar cells (type 2).
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