Melatonin Synthesis: Acetylserotonin O-Methyltransferase (ASMT) Is Strongly Expressed in a Subpopulation of Pinealocytes in the Male Rat Pineal Gland

Marked cell-to-cell differences in melatonin synthesis were observed in the rat pineal gland.

• The last enzyme in melatonin synthesis, ASMT, showed variable immunostaining intensity across different cells.
• Differences were noted in the abundance of transcripts for ASMT, TPH-1, and another enzyme, suggesting varying levels of melatonin synthesis.
• ASMT was localized in the cytoplasm of pinealocytes in the superficial pineal gland, with additional presence in the pineal stalk and deep pineal gland.
• Inconsistent colocalization of ASMT was found with known pinealocyte markers, indicating variability in melatonin synthesis among individual cells.
• No ASMT protein was detected in other parts of the central nervous system or peripheral tissues.

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