Department of Embryology, Carnegie Institution for Science, Baltimore, Maryland, United States of America.
PLoS One. 2011;6(9):e25111. doi: 10.1371/journal.pone.0025111. Epub 2011 Sep 22.
Animals have evolved specialized photoreceptors in the retina and in extraocular tissues that allow them to measure light changes in their environment. In mammals, the retina is the only structure that detects light and relays this information to the brain. The classical photoreceptors, rods and cones, are responsible for vision through activation of rhodopsin and cone opsins. Melanopsin, another photopigment first discovered in Xenopus melanophores (Opn4x), is expressed in a small subset of retinal ganglion cells (RGCs) in the mammalian retina, where it mediates non-image forming functions such as circadian photoentrainment and sleep. While mammals have a single melanopsin gene (opn4), zebrafish show remarkable diversity with two opn4x-related and three opn4-related genes expressed in distinct patterns in multiple neuronal cell types of the developing retina, including bipolar interneurons. The intronless opn4.1 gene is transcribed in photoreceptors as well as in horizontal cells and produces functional photopigment. Four genes are also expressed in the zebrafish embryonic brain, but not in the photoreceptive pineal gland. We discovered that photoperiod length influences expression of two of the opn4-related genes in retinal layers involved in signaling light information to RGCs. Moreover, both genes are expressed in a robust diurnal rhythm but with different phases in relation to the light-dark cycle. The results suggest that melanopsin has an expanded role in modulating the retinal circuitry of fish.
动物在视网膜和眼外组织中进化出了专门的感光器，使它们能够测量环境中的光变化。在哺乳动物中，视网膜是唯一能检测光并将此信息传递给大脑的结构。经典的感光器，视杆细胞和视锥细胞，通过激活视紫红质和视锥蛋白来负责视觉。视黑素，另一种最初在非洲爪蟾黑素细胞（Opn4x）中发现的光色素，在哺乳动物视网膜的一小部分神经节细胞（RGC）中表达，介导非成像功能，如昼夜节律的光适应和睡眠。虽然哺乳动物只有一个视黑素基因（opn4），但斑马鱼表现出显著的多样性，有两个 opn4x 相关基因和三个 opn4 相关基因在发育中的视网膜的多种神经元细胞类型中以不同的模式表达，包括双极中间神经元。无内含子的 opn4.1 基因在感光器以及水平细胞中转录，并产生功能性光色素。四个基因也在斑马鱼胚胎大脑中表达，但不在感光的松果体中表达。我们发现，光周期长度影响参与向 RGCs 传递光信息的视网膜层中两个 opn4 相关基因的表达。此外，这两个基因都以昼夜节律的形式表达，但与光暗周期的相位不同。结果表明，视黑素在调节鱼类视网膜回路方面具有扩展的作用。
