Abstract

The pineal hormone melatonin regulates reproductive function in seasonally breeding mammals. Recent studies using 125I-labeled 2-iodomelatonin (I-MEL) reveal that the distribution of putative melatonin receptors is species-specific; only the hypophysial pars tuberalis (PT) is a consistent site of I-MEL binding in all photoperiodic species examined. In the present study, we used in vitro autoradiography to examine the distribution of I-MEL binding in the ferret brain and pituitary. We report that I-MEL binding is restricted to the PT and pars distalis (PD) of the pituitary; I-MEL binding is absent from brain. I-MEL binds in the PT and PD with high affinity (Kd values ca. 40 pM) and the rank order of potency for inhibition of I-MEL binding (6-chloromelatonin = melatonin greater than 6-hydroxymelatonin greater than N-acetylserotonin greater than serotonin) is the same as that observed for high-affinity melatonin receptors from other species. The consistent presence of high affinity melatonin receptors in the PT of a variety of photoperiodic species suggests that the PT plays a major role in mediating the effects of melatonin on neuroendocrine function.