TRYPTOPHAN-HYDROXYLASE IS EXPRESSED BY PHOTORECEPTORS IN XENOPUS-LAEVIS RETINA

Serotonin has important roles, both as a neurotransmitter and as a pre cursor for melatonin synthesis. In the vertebrate retina, the role and the localization of serotonin have been controversial. Studies examin ing serotonin immunoreactivity and uptake of radiolabeled serotonin ha ve localized serotonin to inner retinal neurons, particularly populati ons of amacrine cells, and have proposed that these cells are the site s of serotonin synthesis. However, other reports identify other cells, such as bipolars and photoreceptors, as serotonergic neurons. Tryptop han hydroxylase (TPH), the rate-limiting enzyme in the serotonin synth etic pathway, was recently cloned from Xenopus laevis retina, providin g a specific probe for localization of serotonin synthesis. Here we de monstrate that the majority of retinal mRNA encoding TPH is present in photoreceptor cells in Xenopus laevis retina. These cells also contai n TPH enzyme activity. Therefore, in addition to being the site of mel atonin synthesis, the photoreceptor cells also synthesize serotonin, p roviding a supply of the substrate needed for the production of melato nin.