Localization and characterization of melatonin receptors in rodent brain by in vitro autoradiography.

Little is known of the neural sites of action for the pineal hormone, melatonin. Thus, we developed an in vitro autoradiographic method using 125I-labeled melatonin (I-MEL) to study putative melatonin receptors in rodent brain. We first determined optimal in vitro labeling conditions for autoradiographic detection of I-MEL binding sites in rat median eminence, the most intensely labeled area in the rat brain. We then assessed the pharmacologic and kinetic properties of I-MEL binding sites in rat median eminence by quantitative autoradiography. These sites have high affinity for I-MEL (equilibrium dissociation constant = 43 pM). I-MEL binding was inhibited by nanomolar concentrations of melatonin or 6-chloromelatonin, but was not inhibited by serotonin, dopamine, or norepinephrine (100 microM). These results suggest that I-MEL binding sites identified by in vitro autoradiography represent specific, high-affinity melatonin receptors. Studies of the distribution of I-MEL binding in rat, Syrian hamster, and Djungarian hamster brain confirm that the median eminence and suprachiasmatic nucleus are major sites of I-MEL binding in rodent brain; other brain areas labeled in one or more of these species were the thalamus (paraventricular, anteroventral, and reuniens nuclei, nucleus of the stria medullaris, and medial part of the lateral habenular nucleus), hypothalamus (dorsomedial nucleus), subiculum, and area postrema. The presence of putative melatonin receptors in the suprachiasmatic nuclei and median eminence of these rodent species suggests that these brain regions are important loci for melatonin effects on circadian rhythms and reproduction.