Melatonin is produced and released by pineal gland into the bloodstream in response to a hypothalamic circadian clock. All vertebrates produce and secrete melatonin almost exclusively during the nighttime period. Melatonin regulates biological rhythms and is believed to be important for a successful pregnancy (Tamura et al., 2008). To date, two melatonin receptors, MT1 and MT2 receptors, have been identified (Dubocovich and Markowska, 2005) in mammals, including humans; both are G protein-coupled receptors (Slominski et al., 2012). Both isoforms have been confirmed in non-pregnant and pregnant uteri (Schlabritz-Loutsevitch et al., 2003).

Serum levels of melatonin at night fluctuate throughout gestation, increasing throughout the first half of pregnancy, decreasing between 20 and 36 weeks of gestation, and increasing again before delivery. Twin pregnancies increase and pre-eclampsia decreases nocturnal secretion of melatonin, suggesting a possible link between pregnancy maintenance and serum melatonin levels in human (Nakamura et al., 2001). In rodents, melatonin is a crucial circadian "gating" signal for delivery. Female rats with pinealectomy-induced melatonin deficiency could conceive and maintain their pregnancies, but they delivered their offspring not only during normal daytime (as the control group did) but also during the night. (Takayama et al., 2003).

There are limited and contradictory data on the effect of melatonin on uterine contractions, especially in pregnancy. It is known that pregnant women with a low plasma level of melatonin and a non-rhythmic melatonin profile have a shorter gestational period and a higher risk of preterm birth (McCarthy et al., 2020). The melatonin receptor agonist agomelatine inhibited non-pregnant and pregnant myometrial contractions isolated from rats, suggesting that melatonin receptors are involved in uterine contractions. (Kacar et al., 2023). Some data suggest that melatonin analogues may inhibit uterine contractions by activating the high-conductance calcium-dependent potassium channel (BKCa) in pregnant uterine myocytes (Steffens et al., 2003). Among non-uterine smooth muscles, melatonin dilates vascular muscles through the opening of BKCa (Zhao et al., 2017).

As pharmacotherapy for preterm birth is limited and inadequate (Rath and Kehl, 2018), new targets and mechanisms of action are needed to expand the potential therapeutic arsenal for tocolysis. Therefore, our objective was to investigate the effects of the melatonin agonist ramelteon on KCl-induced uterine contractions in non-pregnant and pregnant rats in vitro, as well as to detect the mRNA and protein expression of melatonin receptors and to determine their localization in the uterus.