PT - JOURNAL ARTICLE AU - Jose M. Soares, Jr. AU - Monica I. Masana AU - Çağatay Erşahin AU - Margarita L. Dubocovich TI - Functional Melatonin Receptors in Rat Ovaries at Various Stages of the Estrous Cycle AID - 10.1124/jpet.103.049916 DP - 2003 Aug 01 TA - Journal of Pharmacology and Experimental Therapeutics PG - 694--702 VI - 306 IP - 2 4099 - http://jpet.aspetjournals.org/content/306/2/694.short 4100 - http://jpet.aspetjournals.org/content/306/2/694.full SO - J Pharmacol Exp Ther2003 Aug 01; 306 AB - This study investigated the receptor mechanism(s) by which the hormone melatonin directly affects ovarian function. Expression of MT1 and MT2 melatonin receptor mRNA was detected in the rat ovaries both by reverse transcriptase-polymerase chain reaction and in situ hybridization with digoxigenin-labeled oligoprobes. Specific 2-[125I]iodomelatonin binding was significantly higher in ovarian tissue from animals sacrificed during proestrus than in metestrus, suggesting regulation of melatonin receptors by estrogens. Additionally, basal and melatonin-mediated stimulation of guanosine 5′-O-(3-[35S]thio)triphosphate ([35S]GTPγS) binding to ovarian sections was higher in proestrus compared with metestrus. During proestrus, both luzindole (0.1 μM) and 4-phenyl-2-propionamidotetraline (4P-PDOT) (0.1 μM), acting as inverse agonists, inhibited basal [35S]GTPγS binding to ovarian sections, suggesting the presence of MT1 constitutively active melatonin receptors. In primary cultures of ovarian granulosa cells, melatonin inhibited forskolin-stimulated cAMP accumulation through activation of Gi-coupled melatonin receptors. This inhibition was blocked by both, luzindole, and 4P-PDOT, acting as competitive receptor antagonists. Exposure of granulosa cells in culture to 17β-estradiol seems to alter the state of melatonin receptor coupling. Indeed, the efficacy of 4P-PDOT on forskolin-stimulated cAMP formation was reversed from an MT2 partial agonist in vehicle-treated cells to that of an MT1 inverse agonist in 17β-estradiol (0.1 μM)-treated granulosa cells. We conclude that MT1 and MT2 melatonin receptors expressed in antral follicles and corpus luteum may affect steroidogenesis through cAMP-mediated signaling. These results underscore the implications of the levels of ovarian estrogen when melatonin receptor ligands are used as therapeutic agents. The American Society for Pharmacology and Experimental Therapeutics