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Michel J. Roux, Anne Robé, Brigitte L. Kieffer; Localisation of the Delta Opioid Receptor (DOR) and Identification of Enkephalin-Expressing Cells in the Mouse Retina. Invest. Ophthalmol. Vis. Sci. 2011;52(14):4557.
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As in other regions of the Central Nervous System, a variety of neuromodulatory peptides and the corresponding receptors are expressed in the retina. However, their functions in visual processing, and even their pattern of expression, remain largely unknown, notably in mammalian retinas. This is particularly true for opioid receptors, in part due to the lack of reliable antibodies. We here take advantage of the delta opioid receptor (DOR)-eGFP knock-in mice (Scherrer et al., PNAS 2006), as well as of the PENK-eGFP transgenic mice (Gensat), to report on the localization of DOR and its agonist peptide in the mouse retina.
Retina slices or flatmount retinas were prepared from adult DOR-eGFP or PENK-eGFP mices. DOR-eGFP fluorescence was not readily detectable, and DOR localisation was detected using GFP immunostaining. The pattern of expression of eGFP under the control of the PENK promoter was compared to PENK in situ hybridization. The cell types expressing DOR-eGFP or PENK-driven eGFP were identified by immunohistochemistry.
DOR-eGFP is expressed in some OFF ganglion cells and a few amacrine cells. The dendritic processes of the ganglion cells are found in the outer part of the inner plexiform layer, in the same stratum where the processes of dopaminergic amacrine cells (also expressing DOR-eGFP) are found. These ganglion cells were expressing Brn3a and not melanopsin. In PENK-eGFP mice, GFP was expressed in a population of amacrine cells and ganglion cells, which didn’t express GAD65/67, choline-acetyl transferase, calbindin nor calretinin, and which processes were not organized in well-defined sub-strata in the inner plexiform layer.
According to GFP expression in the PENK-GFP mouse, enkephalins are expressed in some amacrine and ganglion cells, which types remain to be identified. The delta opioid receptor is expressed in dopaminergic amacrine cells and in a population of OFF ganglion cells, which from their morphology and immunohistochemical characterization, are most probably OFF delta ganglion cells.
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