Evidence for arylalkylamine N-acetyltransferase (AANAT2) expression in rainbow trout peripheral tissues with emphasis in the gastrointestinal tract.

In fish, melatonin is reported to be produced mainly in the pineal organ, but there is also evidence for the presence of melatonin in a number of extrapineal sites where it could act as an intracellular mediator or paracrine signal. The present study use the reverse transcription-polymerase chain reaction (RT-PCR) to evaluate the expression of the enzyme arylalkylamine-N-acetyltransferase (AANAT2), which catalyzes the limiting step for melatonin synthesis, in different peripheral tissues of rainbow trout, with emphasis in the gastrointestinal tract (GIT). The results show AANAT2 gene expression in almost all peripheral tissues tested, including gills, kidney, muscle, skin, liver, Brockmann bodies, gall bladder, spleen and GIT, but not in adipose tissue. Furthermore, in trout GIT we observed that AANAT2 is expressed only in the muscular layer of all segments tested (esophagus, stomach, pyloric ceca, foregut, midgut and hindgut), but not in the mucosal layer. No significant differences were obtained among the different GIT segments evaluated. These results support an almost ubiquitous synthesis of melatonin in peripheral organs of rainbow trout, which can be related with a local role of the hormone as autocrine or paracrine factor. In addition, our data support the existence of a local synthesis of melatonin in trout GIT, which is discussed and could be involved in some sort of endocrine regulation of feeding and digestive activity, acting as an antioxidant or contributing to maintain melatonin levels in plasma.