Gut Melatonin in Vertebrates: Chronobiology and Physiology

Area of Science:

• Chronobiology
• Endocrinology
• Gastroenterology

Background:

• Melatonin, a hormone discovered in the pineal gland, is also found in extra-pineal sites like the gut.
• Arylalkylamine N-acetyltransferase (AANAT) is the key enzyme regulating melatonin biosynthesis.
• Pineal melatonin production is synchronized with the light-dark cycle, exhibiting a nocturnal peak.

Purpose of the Study:

• To review current data on the chronobiology and physiology of gut melatonin in vertebrates, including fish.
• To compare the temporal patterns and regulatory mechanisms of gut melatonin with pineal melatonin.
• To highlight the unique physiological roles of gut-derived melatonin.

Main Methods:

• Review of experimental studies on melatonin and AANAT protein levels in fish gut segments.
• Analysis of daily variations in gut melatonin and AANAT intensity across different seasons.
• Experimental investigations to identify synchronizing cues for the gut melatoninergic system.

Main Results:

• Gut melatonin levels and AANAT protein intensity in carp exhibit daily variations, peaking at midday across all seasons.
• Unlike pineal melatonin, gut melatonin's rhythm is not directly regulated by light-dark cycles.
• Environmental non-photic cues are suggested as potential synchronizers for the gut melatoninergic system.

Conclusions:

• The gut melatoninergic system in carp displays a unique circadian rhythm, distinct from the pineal system.
• Environmental non-photic cues likely play a crucial role in synchronizing gut melatonin production.
• Gut-derived melatonin may have unique paracrine and/or autocrine functions, differing from pineal melatonin's systemic actions.