Indoles in the photoreceptor cells of the lamprey pineal complex.

In the pineal complex of the brook lamprey, the hydroxyindole-0-methyltransferase (involved in the biosynthesis of 5-methoxyindoles) displays a daily rhythm of activity with a nocturnal peak, during a light/dark cycle and in constant darkness. Immunoreactive indoles were found in both varieties of photoreceptor cells (P1 and P2): serotonin and N-acetylserotonin-like substances in P2 and a melatonin-like compound in P1. It is suggested that P1 and P2 are involved in indole metabolism.