Abstract
Melatonin receptor gene expression as well as melatonin synthesis and secretion activities were examined in the pineal gland of the grass puffer, which exhibits unique lunar/tidal cycle-synchronized mass spawing: spawning occurs before high tide on the day of spring tide during spawing season. Melatonin synthesizing activity was assessed by the abundance of arylalkylamine N-acetyltransferase 2 (AANAT2) mRNA. The amount of aanat2 mRNA was low during light phase and initiated to increase after the light was turned off. The secretion of melatonin from primary pineal organ culture was stimulated after the light was turned off and ceased immediately after the light was turned on. The expression levels of four melatonin receptor subtype genes (mel1a1.4, mel1a1.7, mel1b, and mel1c) showed synchronous variations, and the levels tended to be high during the dark phase under light/dark conditions. These results suggest that the action of melatonin on the pineal gland is highly dependent on light and photoperiod, possibly with stronger action during night time. Under constant darkness, the expression of four melatonin receptor subtype genes showed unique ultradian oscillations with the period of 14.0–15.4 h, suggesting the presence of a circatidal oscillator in the pineal gland. The present results indicate that melatonin may serve local chronobiological functions in the pineal gland. These cyclic expressions of melatonin receptor genes in the pineal gland may be important in the control of the lunar/tidal cycle-synchronized mass spawning in the grass puffer.
Highlights
Melatonin is produced mainly in the pineal gland and retina in fish, and its plasma concentration is higher during nighttime than daytime
In the present study, melatonin receptor gene expression as well as melatonin synthesis and secretion activites were examined in the pineal gland of grass puffer, a semilunar-synchronized spawner
Melatonin synthesizing activity was assessed by the abundance of aanat2 mRNA, which encodes a rate-limiting enzyme in the conversion of serotonin to melatonin
Summary
Melatonin is produced mainly in the pineal gland and retina in fish, and its plasma concentration is higher during nighttime than daytime. This daily rhythm of circulating melatonin informs the organism about the time within a day, whereas the duration of the nocturnal elevation of melatonin that corresponds to photoperiod informs the organism about the season within a year (Reiter, 1993). There are three types of melatonin receptors, Mel1a (MT1), Mel1b (MT2), and Mel1c. Mel1a and Mel1b have been identified in all vertebrate species investigated, whereas Mel1c has been found only in non-mammalian species (Ebisawa et al, 1994; Reppert et al, 1995). Phylogenetic analyses have shown that there are four subtypes of melatonin receptor genes in fish (Reppert et al, 1995)
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