Diurnal expressions of four subtypes of melatonin receptor genes in the optic tectum and retina of goldfish

Four subtypes of melatonin receptor genes (Mel_1a 1.4, Mel_1a 1.7, Mel_1b, and Mel_1c) are considered to be expressed to mediate various physiological functions of melatonin in goldfish (Carassius auratus). To examine their tissue distribution and diurnal changes in expression levels, we cloned partial gene fragments for these melatonin receptor subtypes, and established specific RT-PCR and quantitative real-time PCR systems. Mel_1a 1.4 and Mel_1b were predominantly expressed in various neuronal and peripheral tissues, while Mel_1a 1.7 and Mel_1c were expressed in the restricted tissues. All subtype genes were expressed in the optic tectum, diencephalon, mesencephalon, vagal lobe, retina and spleen. The real-time PCR analyses showed that significant differences among time were observed for Mel_1a 1.4 in the optic tectum and for Mel_1a 1.7 and Mel_1b in the retina. In the retina, the levels of Mel_1a 1.7 and Mel_1b mRNAs showed diurnal changes with one peak at ZT24. The present results show differential distribution of four subtypes of melatonin receptor mRNAs in the neuronal and peripheral tissues. However, the expressions of all subtype genes in the retinorecipient brain regions and retina reinforce the role of the melatonin receptor in processing visual information. Furthermore, the present study demonstrates diurnal expressions of the major subtype genes, i.e. Mel_1a 1.4 in the optic tectum and Mel_1a 1.7 in the retina.