Functional Melatonin Receptors in Rat Ovaries at Various Stages of the Estrous Cycle

• Published in: The Journal of pharmacology and experimental therapeutics 2003-08, Vol.306 (2), p.694-702
• Main Authors: Soares, Jose M., Masana, Monica I., Erşahin, Çağatay, Dubocovich, Margarita L.
• Format: Article
• Language: English
• Subjects: Animals; Autoradiography; Estrous Cycle - metabolism; Female; Granulosa Cells; Guanosine 5'-O-(3-Thiotriphosphate) - metabolism; Iodine Radioisotopes; Ovary - metabolism; Rats; Rats, Sprague-Dawley; Receptors, Cell Surface - biosynthesis; Receptors, Cell Surface - genetics; Receptors, Cytoplasmic and Nuclear - biosynthesis; Receptors, Cytoplasmic and Nuclear - genetics; Receptors, Melatonin; Reverse Transcriptase Polymerase Chain Reaction; RNA, Messenger - biosynthesis; Sulfur Radioisotopes
• ISSN: 0022-3565; 1521-0103
• DOI: 10.1124/jpet.103.049916

This study investigated the receptor mechanism(s) by which the hormone melatonin directly affects ovarian function. Expression of MT 1 and MT 2 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-[ 125 I]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-[ 35 S]thio)triphosphate ([ 35 S]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 [ 35 S]GTPγS binding to ovarian sections, suggesting the presence of MT 1 constitutively active melatonin receptors. In primary cultures of ovarian granulosa cells, melatonin inhibited forskolin-stimulated cAMP accumulation through activation of G i -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 MT 2 partial agonist in vehicle-treated cells to that of an MT 1 inverse agonist in 17β-estradiol (0.1 μM)-treated granulosa cells. We conclude that MT 1 and MT 2 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.