Objectives: 

The endocrine basis of stress in fish is still poorly understood. Some studies describe a direct relationship between cortisol and melatonin, with cortisol modulating plasma melatonin levels. Glucocorticoid receptors have been described in trout pineal organ and seem to modulate melatonin secretion from cultured organs. The aim of the present study was to determine how pineal melatonin synthesis is affected when animals are exposed to different stressors, and the role of cortisol in medianting such effects.

Materials: 

Immature trout were exposed to different stressors, hypoxia, chasing and high stocking (5 days) during day- and night-time and immediately sampled. In a second experiment, groups of animals were IP administered with cortisol implants (50 ug/g bw) and sampled at 5-h or 48-h post-injection during the day or night. For each fish, plasma melatonin levels, pineal organ content of melatonin and serotonin, aanat2 mRNA abundance and AANAT2 enzyme activity were measured.

Results: 

Day-night variations were found for plasma and pineal organ melatonin content with the highest values being measured at night-time. Exposing animals to stress did significantly affect this pattern at both day and night, but the effect was more prominent at night. The decrease of melatonin at night-time in stressed animals was accompanied by an increase of pineal organ serotonin content (from which melatonin is synthesized) and decreased aanat2 mRNA abundance and enzymatic activity. Such effect was also observed in cortisol IP administered fish.

Conclusions: 

Our results indicate that stress is influencing negatively the synthesis of melatonin in the pineal organ and also the day-night variations of circulating melatonin.