General and comparative endocrinologyFeb 16, 2008

How Fasting and Refeeding Affect Melatonin Production and Related Chemicals in Rainbow Trout

Circadian Biology Weekly Brief ↗PubMed ↗DOI ↗

Updated Apr 3, 2026

Abstract

Fasted rainbow trout exhibited reduced melatonin levels throughout a 24-hour cycle, associated with decreased enzyme activity.

Pineal melatonin levels and the activity of the enzyme responsible for its production showed daily variations, peaking at night.
Food deprivation for 7 days led to lower melatonin content and decreased enzyme activity in trout.
In fasted fish, levels of serotonin and its metabolite, 5-HIAA, increased at night but did not change significantly during the day.
Circulating melatonin levels were lower at midday in food-deprived trout, but not at night.
Refeeding for 5 days generally counteracted the effects of fasting on melatonin levels.
Cortisol levels in plasma decreased after food deprivation and remained low after refeeding.

Simplified

The effects of food deprivation and refeeding on daily rhythms of serotonin (5-HT), 5-hydroxyindoleacetic acid (5-HIAA) and melatonin contents, as well as on arylalkylamine N-acetyltransferase (AANAT) activity were evaluated in the pineal organ of rainbow trout. In addition, changes in circulating melatonin and cortisol levels were tested at one single point at day and night. Immature rainbow trout were distributed in 3 experimental groups: fish fed, fish fasted (7 days), and fish fasted for 7 days and refed for 5 days. All fish were sampled from each treatment group at different times of the day-night cycle. Pineal melatonin levels and AANAT activity showed daily variations in either fed, fasted and refed trout, displaying highest values at night. Fasted trout showed reduced melatonin content throughout the 24-h cycle, which was associated with decreased AANAT activity. Rhythms of pineal 5-HT and 5-HIAA levels were evident in all groups and were negatively correlated to melatonin in fed fish groups, but not in fasted fish. A higher content of 5-HT and 5-HIAA was observed in fasted fish during the night with no apparent changes during daytime for 5-HT and increased 5-HIAA levels. Furthermore, decreased circulating levels of melatonin were observed at midday, but not at night, in food deprived trout. Refeeding for 5 days generally counteracted the effects of food deprivation. Cortisol levels in plasma were reduced after food deprivation and remained low in refed fish. The results show that food deprivation impairs daily rhythms of melatonin content in trout pineal organ by affecting the activity of melatonin synthesizing enzymes rather than by a deficiency in substrate availability.

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How Fasting and Refeeding Affect Melatonin Production and Related Chemicals in Rainbow Trout

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