Abstract Uterine contractility is regulated and controlled by a complex system such that pregnancy is maintained and parturition occurs at full term. Coordinated uterine contractions occurring prior to 37 weeks gestation, i.e., preterm labor (PTL), could lead to preterm birth (PTB) which affects 15 million newborns and causes 1 million neonatal deaths worldwide annually. Given that contractions are a central feature of PTL, tocolytics are used in PTB management, yet current tocolytics are not sufficiently effective. We recently found that uterine smooth muscle (USM) cells from mouse and human express bitter taste receptors (TAS2Rs) and their canonical signaling components (i.e., G-protein gustducin and PLCβ2). Also bitter tastants (e.g., phenanthroline (PHEN), and chloroquine, a FDA-approved antimalarial drug) relax uterine strips pre-contracted by uterotonics (e.g., oxytocin and prostaglandin F2α) more completely than current commonly used tocolytics (i.e., nifedipine, indomethacin and MgSO4). Moreover, bitter tastants (e.g., chloroquine) can prevent mouse PTB induced by bacterial endotoxin lipopolysaccharide (LPS) and nuclear progesterone receptor antagonist RU486 more often than commonly used tocolytics and in a gustducin dependent manner. In our preliminary studies, we further found that (1) PHEN can stop pregnant mouse uterine contractions induced by serotonin, endothelin-1, neuromedin S and U-46619, all pathophysiological mediators in the uterus, (2) Tas2r expression in USM is decreased when pregnant mice approach parturition and is restored after labor, and (3) the simultaneous deletion of the three main Tas2rs expressed in USM increases the probability of PTB in mice. We therefore propose that (1) TAS2Rs are a class of proteins regulating uterine contraction and gestational duration, and (2) TAS2R agonists are broad spectrum tocolytics potentially suitable for PTB management. To test these hypotheses, we will directly study how bitter tastants activate the TAS2R signaling pathway to relax human myometrium with pharmacological approaches and siRNA lentiviral expression technology (Aim 1). We will then use Tas2r deletion mice to determine whether bitter tastants activate TAS2Rs to cause USM relaxation, and whether the TAS2Rs are critical for maintaining uterine quiescence during pregnancy, setting gestational duration, in mice (Aim 2). Finally, we will study the effectiveness of bitter tastants in preventing mouse PTB induced by LPS and RU486, and determine the role of TAS2Rs in bitter tastants' prevention of LPS- or RU486- induced PTB (Aim 3). This study will uncover the molecular mechanisms by which bitter tastants relax mouse and human uteri, determine whether the TAS2R family plays a major role in uterine quiescence during pregnancy and parturition, establish whether the TAS2R family is an attractive therapeutic target for treating PTL in human pregnancy, and help determine whether bitter tastants can be developed as new and more effect...