Melatonin secretion in vitro from the pineal complex of the lamprey Petromyzon marinus.

Melatonin secretion from cultured pineal complexes of the lamprey, Petromyzon marinus, was measured by radioimmunoassay (RIA) for 6 days under either a 12:12 light:dark cycle (L:D) or continuous darkness (D:D) at either 10 degrees or 20 degrees. Under the L:D cycle, melatonin release was completely suppressed during daytime and high during night-time, with larger amplitude at 20 degrees than at 10 degrees. Under D:D conditions, no significant circadian rhythmicity in secretion could be revealed by periodogram analysis at either 10 degrees or 20 degrees. This is the first direct evidence of melatonin secretion in a lamprey, suggesting that both light and temperature can affect in vitro melatonin secretion.