Green C B, Cahill G M, Besharse J C
Department of Anatomy and Cell Biology, University of Kansas Medical Center 66160-7400, USA.
Vis Neurosci. 1995 Jul-Aug;12(4):663-70. doi: 10.1017/s0952523800008956.
Serotonin has important roles, both as a neurotransmitter and as a precursor for melatonin synthesis. In the vertebrate retina, the role and the localization of serotonin have been controversial. Studies examining serotonin immunoreactivity and uptake of radiolabeled serotonin have localized serotonin to inner retinal neurons, particularly populations of amacrine cells, and have proposed that these cells are the sites of serotonin synthesis. However, other reports identify other cells, such as bipolars and photoreceptors, as serotonergic neurons. Tryptophan hydroxylase (TPH), the rate-limiting enzyme in the serotonin synthetic pathway, was recently cloned from Xenopus laevis retina, providing a specific probe for localization of serotonin synthesis. Here we demonstrate that the majority of retinal mRNA encoding TPH is present in photoreceptor cells in Xenopus laevis retina. These cells also contain TPH enzyme activity. Therefore, in addition to being the site of melatonin synthesis, the photoreceptor cells also synthesize serotonin, providing a supply of the substrate needed for the production of melatonin.
血清素具有重要作用，既是一种神经递质，也是褪黑素合成的前体。在脊椎动物视网膜中，血清素的作用和定位一直存在争议。研究血清素免疫反应性和放射性标记血清素摄取的实验已将血清素定位到视网膜内层神经元，特别是无长突细胞群体，并提出这些细胞是血清素合成的场所。然而，其他报告将双极细胞和光感受器等其他细胞鉴定为血清素能神经元。色氨酸羟化酶（TPH）是血清素合成途径中的限速酶，最近从非洲爪蟾视网膜中克隆出来，为血清素合成的定位提供了一种特异性探针。在这里，我们证明非洲爪蟾视网膜中编码TPH的大部分视网膜mRNA存在于光感受器细胞中。这些细胞还含有TPH酶活性。因此，除了作为褪黑素合成的场所外，光感受器细胞还合成血清素，为褪黑素的产生提供所需的底物供应。
