Changes in plasma melatonin levels and pineal organ melatonin synthesis following acclimation of rainbow trout (Oncorhynchus mykiss) to different water salinities

A salinity-dependent increase in melatonin content was found in both plasma and pineal organs of rainbow trout.

• Exposure to higher salinity levels resulted in increased plasma osmolalities in rainbow trout compared to freshwater controls.
• Nighttime melatonin levels in plasma and pineal organs increased with salinity, linked to higher expression of the aanat2 gene and AANAT2 enzyme activity.
• Daytime increases in melatonin synthesis were also observed, suggesting a consistent response to salinity regardless of light conditions.
• Levels of serotonin and its metabolite in the pineal organ decreased with rising salinity, indicating a potential shift in substrate use for melatonin production.
• These findings suggest that external salinity may enhance melatonin synthesis in the rainbow trout pineal organ, possibly affecting osmoregulatory processes.

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