Iodinated melatonin mimics melatonin action and reveals discrete binding sites in fetal brain

Iodinated melatonin was used to study melatonin sites of action in brain. Iodomelatonin mimicked the effects of melatonin on reproductive development in Djungarian hamster fetuses. 125I-melatonin injected into the dam was recovered from fetal brain. In vitro autoradiographic studies revealed a remarkably discrete distribution of competitive 125I-melatonin-binding sites in the fetal brain, with binding in median eminence/arcuate nucleus area greater than suprachiasmatic nucleus greater than pineal gland much greater than anterior pituitary gland much greater than preoptic area. 125I-melatonin promises to be a useful tool for understanding the sites and mechanism of action of melatonin.