Diurnal and circadian regulation of a melatonin receptor, MT1, in the golden rabbitfish, Siganus guttatus

Oct 19, 2006General and comparative endocrinology

Daily and internal clock control of melatonin receptor MT1 in the golden rabbitfish

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Abstract

The full-length melatonin receptor (MT1) cDNA in the golden rabbitfish is 1257 bp long and encodes a protein of 350 amino acids.

MT1 is highly homologous to melatonin receptors in other nonmammalian species.
High levels of MT1 mRNA expression were observed in the brain, retina, liver, and kidney, with notable day-night differences.
In the retina and whole brain, MT1 mRNA expression increased at nighttime under various light conditions, indicating circadian regulation.
The cultured pineal gland also exhibited diurnal variations in MT1 mRNA expression, suggesting a contribution to increased expression in the whole brain.
Alterations in light conditions affected both melatonin release and MT1 mRNA expression in cultured pineal gland samples.

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The golden rabbitfish Siganus guttatus is a reef fish with a restricted lunar-synchronized spawning rhythmicity and releases gametes simultaneously around the first quarter moon period during the spawning season. In order to understand the molecular aspects of the "circa" rhythms in this species, the full-length melatonin receptor (MT1) cDNA was cloned, and its diurnal/circadian regulation was examined. The full-length MT1 cDNA (1257 bp) contained an open reading frame that encodes a protein of 350 amino acids; this protein is highly homologous to MT1 of nonmammalian species. A high expression of MT1 mRNA with a day-night difference was observed in the whole brain, retina, liver, and kidney. When diurnal variations in MT1 mRNA expression in the retina and whole brain were examined using real-time quantitative RT-PCR, an increase in the mRNA expression was observed during nighttime in both tissues under conditions of light/dark, constant darkness, and constant light. This suggests that MT1 mRNA expression is under circadian regulation. The expression of MT1 mRNA in the cultured pineal gland also showed diurnal variations with high expression levels during nighttime; this suggests that the increased expression level observed in the whole brain is partially of pineal origin. Alternation of light conditions in the pineal gland cultures resulted in the changes in melatonin release into the culture medium as well as MT1 mRNA expression in the pineal gland. The present results suggest that melatonin and its receptors play an important role in the exertion of daily and circadian variations in the neural tissues.

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Diurnal and circadian regulation of a melatonin receptor, MT1, in the golden rabbitfish, Siganus guttatus

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