Characterization of melatonin synthesis in the gastrointestinal tract of rainbow trout (Oncorhynchus mykiss): distribution, relation with serotonin, daily rhythms and photoperiod regulation.

Melatonin is synthesized in peripheral locations of vertebrates, including the gastrointestinal tract (GIT). In teleost, information regarding this topic is scarce. Here we studied the presence and synthesis of melatonin at the rainbow trout GIT. Different sections of trout GIT (from esophagus to hindgut) were dissected out and assayed for contents of melatonin, serotonin (5-HT) and its metabolite, 5-hydroxyindole acetic acid, as well as for aanat1, aanat2 and hiomt mRNA abundance. A trout group was pinealectomized to evaluate changes in plasma and gut melatonin content. Finally, the daily profile of melatonin and 5-HT content, and aanat1, aanat2 and hiomt mRNA abundance were analyzed in gut of trout kept under normal lighting, and then under constant darkness. Melatonin was detected in all GIT regions with higher concentrations in the muscular wall than in the mucosa, a similar trend to that of 5-HT. In contrast, transcripts of melatonin synthesis enzymes were more abundant in the mucosa. Pinealectomy did not affect melatonin levels in midgut and hindgut either at day or at night. Additionally, no daily rhythms could be defined for melatonin content in gut tissues but increases during late light phase and at midnight occurred. However, aanat1, aanat2 and hiomt mRNA abundance showed clear daily rhythms with peaks at night. These rhythms remained with a 3-h phase advanced peak in fish exposed to constant darkness. Our results provide clear evidence for a local synthesis of melatonin in trout GIT that might be influenced by the content of 5-HT in the tissue. The process is affected by environmental light cycle and is likely to be under circadian regulation.