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A. F. Wiechmann, D. M. Sherry; Distribution of Melatonin Receptors in Retinal Bipolar Cells of Xenopus laevis. Invest. Ophthalmol. Vis. Sci. 2010;51(13):4132.
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© ARVO (1962-2015); The Authors (2016-present)
Melatonin receptors have been identified in several retinal cell types, including photoreceptors, horizontal cells, amacrine cells, and ganglion cells. Recent reports show that melatonin potentiates rod signals to ON-type bipolar cells. However, melatonin receptor expression in bipolar cells has not been reported to date. To investigate the potential for direct melatonin receptor signaling in bipolar cells, double-label confocal immunohistochemistry was performed on the retina of Xenopus laevis.
Sections of adult Xenopus laevis were processed for immunohistochemistry using antibodies specific for each of the three melatonin receptor subtypes (Mel1a, Mel1b, and Mel1c). Double-label immunohistochemistry was performed using either Mel1a or Mel1b antibody in combination with an antibody against Goα, a specific marker for ON-type bipolar cells, or antibodies against markers for other retinal cell types. Immunolabeled retinal sections were analyzed by laser scanning confocal microscopy.
Mel1b was localized to photoreceptor inner segment membranes and to bipolar cells that contacted cone pedicles. Mel1b-immunoreactive bipolar cell somas were located in the distal portion of the inner nuclear layer (INL), and had axons that ramified in the ON and OFF sublaminae of the inner plexiform layer (IPL), indicating that Mel1b was expressed by both ON- and OFF-bipolar cell types. The Mel1b receptor co-localized with Goα in some, but not all, ON-bipolar cell somas in the INL and dendrites in the OPL. Mel1a also was localized to the OPL, but did not appear to be associated specifically with bipolar cells, suggesting that Mel1a in the OPL may be associated with horizontal cell processes. There did not appear to be any association of Mel1a or Mel1b immunoreactivity with peanut agglutinin labeling at the flat contacts between cones and OFF-bipolar cells.
This study shows that both Mel1a and Mel1b are expressed in the OPL and could modulate transmission from photoreceptors to second-order neurons in the Xenopus laevis retina, although Mel1b is more likely to modulate transmission to bipolar cells directly. Labeling patterns suggest that Mel1b is expressed by bipolar cells in a cell type selective manner that includes both ON- and OFF-cell types. Neither Mel1a nor Mel1b appear to be localized to the flat contacts between OFF-type bipolar cells and photoreceptors, suggesting that any melatonin modulation of transmission to OFF bipolar cells occurs away from the site of contact with the photoreceptor terminal.
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