Two arylalkylamine N-acetyltransferase genes mediate melatonin synthesis in fish

Serotonin N-acetyltransferase (arylalkylamine N-acetyltransferase, AANAT, EC 2.3.1.87) is the first enzyme in the conversion of serotonin to melatonin. Large changes in AANAT activity play an important role in the daily rhythms in melatonin production. Although a single AANAT gene has been found in mammals and the chicken, we have now identified two AANAT genes in fish. These genes are designated AANAT-1 and AANAT-2; all known AANATs belong to the AANAT-1 subfamily. Pike AANAT-1 is nearly exclusively expressed in the retina and AANAT-2 in the pineal gland. The abundance of each mRNA changes on a circadian basis, with retinal AANAT-1 mRNA peaking in late afternoon and pineal AANAT-2 mRNA peaking 6 h later. The pike AANAT-1 and AANAT-2 enzymes (66% identical amino acids) exhibit marked differences in their affinity for serotonin, relative affinity for indoleethylamines versus phenylethylamines and temperature-activity relationships. Two AANAT genes also exist in another fish, the trout. The evolution of two AANATs may represent a strategy to optimally meet tissue-related requirements for synthesis of melatonin: pineal melatonin serves an endocrine role and retinal melatonin plays a paracrine role.     

Differential daily rhythms of melatonin in the pineal gland and gut of goldfish Carassius auratus in response to light

The objectives of this study were to test the effects of light on melatonin rhythms in the pineal gland and gut of goldfish Carassius auratus and to investigate whether melatonin function differed in these two tissues, which are photosensitive and non-photosensitive respectively. Rhythms were evaluated by measuring arylalkylamine N-acetyltransferase (AANAT2) and melatonin receptor 1 (MT-R1) mRNA expression and melatonin concentration in the pineal gland, gut (in vivo), and cell cultures of the two tissues (in vitro). Compared to control, pineal gland melatonin secretion was higher at night, whereas the 24-h dark and ophthalmectomy groups maintained higher AANAT2 and MT-R1 mRNA expression during the day. Melatonin levels and AANAT2 and MT-R1 mRNA expression in the gut were also the highest at night, but the 24-h light, dark, and ophthalmectomy groups did not significantly differ from control. Furthermore, we measured AANAT2 and MT-R1 mRNA expression in high temperature water (30 °C) to investigate differences in the antioxidant capacity of pineal gland vs. gut melatonin. Melatonin and H₂O₂ levels, as well as AANAT2 and MT-R1 mRNA expression, were all higher in the two tissues under thermal stress, compared with their levels at 22 °C. Taken together, our results suggest that light has no effect on melatonin patterns in the gut, which appears to exhibit its own circadian rhythm, but both gut and pineal gland melatonin exhibit similar antioxidant function.