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Shi-Jun Weng, Wen-Long Sheng, Xiong-Li Yang, Yong-Mei Zhong; Cellular localization and rhythmic expression of melatonin receptor 1 in the rat retina. Invest. Ophthalmol. Vis. Sci. 2014;55(13):2642.
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© ARVO (1962-2015); The Authors (2016-present)
Retinal melatonin modulates various visual functions via two subtypes of specific receptors, namely MT1 and MT2 receptors. The expression of these receptors is reported to be highly species- and neuron subtype-dependent, and exhibits daily fluctuation. We sought to (1) explore the cellular localization of MT1 receptor in the rat retina and (2) determine whether and how the retinal expression levels of this receptor are regulated by circadian clock and/or light.
Localization of MT1 receptor was studied in vertical retinal sections by immunofluorescence double staining, using a rabbit anti-MT1 antibody, in combination with various markers for major retinal neuron types. Diurnal and circadian variations in MT1 receptor expression levels of whole retina were assessed by Western blotting, using retinas harvested from rats entrained to a 12: 12 h LD cycle and kept in DD, respectively. To examine photic regulation of MT1 levels during the dark period, rats were given a 2-h light pulse just before retinas were collected.
Immunostaining for MT1 receptor was strong in both the INL and GCL, whereas weaker labeling was observed in the IPL and ONL. In the outer retina, horizontal cells and bipolar cells were all MT1-positive. In the inner retina, labeling for MT1 receptor was localized to glycinergic amacrine cells, including AII amacrine cells. GABAergic amacrine cells were also immunoreactive to MT1 receptor. Most cholinergic amacrine cells and a subset of dopaminergic amacrine cells were immunolabeled for MT1. Additionally, MT1 receptor immunoreactivity was seen in almost all Brn3a-labeled ganglion cells, as well as in Müller glial cells. The expression levels of retinal MT1 receptors showed evident diurnal variations, being significantly elevated during the light period. However, the free-running retinal MT1 receptor levels under DD condition seemed not to display obvious circadian changes. Furthermore, 2-h light exposure applied during the dark period acutely up-regulated retinal MT1 receptor levels.
MT1 receptor is expressed in multiple types of rat retinal neurons, suggesting that melatonin may modulate the activity of these cells. Melatonin-induced modulation may exhibit diurnal changes due to daily fluctuation of the expression levels of MT1 receptor in the retina.
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