Molecular and cellular analyses of melatonin receptor-mediated cAMP signaling in rat corpus epididymis

By using 2-[I-125]iodomelatonin receptor binding studies, we have previousl y demonstrated high affinity melatonin receptors, the binding activities of which are regulated by testosterone, in the corpus epididymis of rats. In this report, some of the basic molecular and cellular characteristics of th ese high affinity melatonin receptors in rat corpus epididymis were analyze d. MEL1A and MEL1B receptor mRNAs were expressed by rat corpus epididymal e pithelial cells as revealed by in situ hybridization. Functionally, these h igh affinity melatonin receptors are negatively coupled to adenylyl cyclase via pertussis toxin (PTX) sensitive G(i) protein and the inhibitory effect s of melatonin on forskolin-stimulated cAMP accumulation were enhanced by 5 alpha-dihydrotestosterone (5 alpha-DHT). Interestingly, opposing interacti ons between melatonin and beta-adrenergic receptor signaling in rat epididy mal epithelial cells were observed with melatonin inhibiting norepinephrine - and isoproterenol-stimulated cAMP accumulation. In conclusion, our data s upport a modulatory action of melatonin, mediated via pertussis toxin-sensi tive G(i)-coupled MEL1A and MEL1B receptors, in androgenic and adrenergic r egulation of rat corpus epididymal epithelial cell functions.