A Melatonin Rhythm Persists in Rat Pineals in Darkness1

Abstract

Male rats that were maintained in continuous darkness or blinded by enucleation exhibited circadian rhythms of wheel-running activity. Pineal bodies taken from rats that were killed during the phase of greater locomotor activity contained significantly more melatonin than the pineal bodies of rats killed during the phase of lesser activity. Thus, a rhythm of pineal melatonin content persists and is phase-locked with the free-running, circadian rhythm of locomotor activity. However, the melatonin cycle was suppressed in intact rats that were maintained in continuous light. (Endocrinology89: 1361, 1971)