Melatonin receptor RNA expression in Xenopus retina

Melatonin is an indolamine hormone presumably synthesized by retinal photor eceptors, and may act as a paracrine signal of darkness within the retina. Previous studies have suggested that melatonin, acting through specific rec eptors, may be involved in cyclic retinal functions such as photoreceptor o uter segment disc shedding and phagocytosis, and modulation of neurotransmi tter release in the inner retina. The goal of this study was to determine i f melatonin receptor mRNA is expressed in the neural retina and retinal pig ment epithelium (RPE) of Xenopus laevis. Sheets of RPE, devoid of contamina ting cells, were obtained from Xenopus eyes, and epithelial cultures were s ubsequently established on microporous membrane fillers in a defined medium . Total RNA was isolated from whole brain, neural retina, fresh RPE sheets, and cultured RPE cells. RNA expression of the three known Xenopus melatoni n receptor subtypes (MEL1A, 1B, and 1C) was determined by reverse-transcrip tion/polymerase chain reaction (RT/PCR) amplification, followed by Southern hybridization with RNA probes. PCR-amplified cDNA encoding melatonin recep tor subtypes 1B and 1C, but not 1A, were detected in reverse-transcribed RN A obtained from brain, neural retina and RPE. RPE cells grown in culture fo r two weeks also demonstrated 1B and 1C receptor RNA expression. This study suggests that RNA encoding the 1B and 1C melatonin receptor subtypes is ex pressed in the neural retina and RPE of Xenopus retina, and the expression persists in RPE cells when grown in culture. The expression of melatonin re ceptor RNA in the RPE may reflect a regulatory role for melatonin in some d iurnal events that occur in this tissue, such as phagocytosis of photorecep tor outer segment membranes, and intracellular migration of pigment granule s. (C) 1999 Elsevier Science B.V. All rights reserved.