Effect of light on 5-methoxyindole-3-acetic acid in the rat.

This study aims to determine the effects of light on the levels of 5-MIAA to provide further information on this indoleamine, using a sensitive and specific radioimmunoassay (RIA) for immunoreactive 5-methoxyindole-3-acetic acid (5-MIAA) developed in our laboratories using a specific antibody and tritiated label. Significant differences were found in the immunoreactive 5-MIAA levels between mid-light and mid-dark pineal glands in rats adapted to 12/12 hrs light/dark and in constant darkness. Under constant light, the circadian rhythm was abolished. The rat serum displayed no diurnal variations in 5-MIAA levels under aux photic conditions. The persistence of rhythms found in constant darkness but abolished in constant light suggests that the pineal 5-MIAA is endogenous and uses light as an environmental cue. In addition to melatonin, 5-MIAA could possibly be another pineal photoperiodic signal in animals.