Functional melatonin receptors in rat ovaries at various stages of the estrous cycle

This study investigated the receptor mechanism(s) by which the hormone melatonin directly affects ovarian function. Expres-sion 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 oli-goprobes. Specific 2-[125I]iodomelatonin binding was signifi-cantly higher in ovarian tissue from animals sacrificed during proestrus than in metestrus, suggesting regulation of melatonin receptors by estrogens. Additionally, basal and melatonin-me-diated stimulation of guanosine 5-O-(3-[35S]thio)triphosphate ([35S]GTPS) binding to ovarian sections was higher in proestrus compared with metestrus. During proestrus, both luzindole (0.1 M) and 4-phenyl-2-propionamidotetraline (4P-PDOT) (0.1 M), acting as inverse agonists, inhibited basa