Department of Cell Biology, University of Oklahoma Health Sciences Center, Oklahoma City, Oklahoma 73104, USA.
J Comp Neurol. 2012 Apr 15;520(6):1115-27. doi: 10.1002/cne.22783.
Melatonin receptors have been identified in several retinal cell types, including photoreceptors, horizontal cells, amacrine cells, and ganglion cells. Recent reports suggest that melatonin potentiates signaling from rods to inner retinal neurons. However, the organization of the melatonin receptors mediating this action in the outer plexiform layer (OPL) is not clear. To assess melatonin receptor localization in the OPL, double-label confocal immunohistochemistry for Mel1a or Mel1b melatonin receptors was performed in combination with markers for cone photoreceptors (calbindin, XAP-1) and ON bipolar cells (guanine nucleotide binding protein alpha, Goα) on the retina of Xenopus laevis. Both Mel1a and Mel1b receptors were specifically associated with processes contacting the pedicles of cones, but localized to processes from different sets of second-order neurons. Mel1a receptors localized to the large axonal processes of horizontal cells, while Mel1b receptors localized to the dendrites of OFF bipolar cells. Both receptors also localized to third-order amacrine and ganglion cells and their processes in the inner plexiform layer. This study indicates that Mel1a and Mel1b melatonin receptors are expressed specifically in the Xenopus OPL to modulate transmission from cones to horizontal cells and OFF bipolar cells, respectively; they are second-order neurons that predominantly contact ribbon synapses and display OFF responses to light. When combined with results from recent physiological studies, the current results suggest a conserved function for melatonin in enhancing transmission from rods to second-order neurons across species, although the precise mechanisms by which melatonin enhances this transmission are likely to vary in a species-dependent manner.
褪黑素受体已在多种视网膜细胞类型中被鉴定出来，包括光感受器、水平细胞、无长突细胞和节细胞。最近的报告表明，褪黑素增强了杆状细胞向视网膜内层神经元的信号传递。然而，介导这种作用的褪黑素受体在外丛状层（OPL）中的组织方式尚不清楚。为了评估褪黑素受体在 OPL 中的定位，在非洲爪蟾的视网膜上进行了 Mel1a 或 Mel1b 褪黑素受体的双重标记共聚焦免疫组织化学与视锥细胞（钙结合蛋白，XAP-1）和 ON 双极细胞（鸟嘌呤核苷酸结合蛋白α，Goα）标志物的组合检测。Mel1a 和 Mel1b 受体都与接触视锥细胞柄的突起特异性相关，但定位于来自不同第二级神经元的突起。Mel1a 受体定位于水平细胞的大轴突突起，而 Mel1b 受体定位于 OFF 双极细胞的树突。两种受体也定位于内丛状层中的第三级无长突细胞和节细胞及其突起。本研究表明，Mel1a 和 Mel1b 褪黑素受体在 Xenopus OPL 中特异性表达，以分别调节来自视锥细胞到水平细胞和 OFF 双极细胞的传递；它们是第二级神经元，主要与带状突触接触，并对光产生 OFF 反应。当与最近的生理研究结果相结合时，目前的结果表明，褪黑素在增强从视杆细胞到第二级神经元的传递方面在物种间具有保守功能，尽管褪黑素增强这种传递的确切机制可能因物种而异。
