Melatonin synthesis and clock gene regulation in the pineal organ of teleost fish compared to mammals: Similarities and differences

Teleost fish possess three aanat genes, while mammals and birds express only one.

• The pineal organ in vertebrates synthesizes and secretes melatonin in a rhythmic pattern.
• AANAT activity, the key enzyme in melatonin production, increases at night due to the expression of the aanat gene.
• In fish, the aanat1a and aanat1b genes are found in the retina and nervous system, while aanat2 is restricted to the pineal organ.
• All core clock genes and the aanat2 gene are expressed in the pineal organ of several fish species.
• Unlike mammals, the teleostean pineal organ has a well-developed indigenous clock that regulates aanat2 expression and melatonin synthesis.
• The fish pineal organ also contains molecular components for direct responses to light and temperature.

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