Opposing actions of prostaglandins and oxytocin determine the onset of murine labor

Abstract

Prostaglandins (PGs) have been recently proven essential for parturition in mice. To dissect the contributions of the two cyclooxygenase (COX) isoforms to the synthesis of PGs during pregnancy, we have characterized the parturition phenotype of COX-1-deficient mice. We find that mice with targeted disruption of the COX-1 gene have delayed parturition resulting in neonatal death. Results of matings of COX-1-deficient females with COX-1 intact males, and blastocyst transfer of COX-1-deficient or -intact embryos into wild-type foster mothers, proved necessity and sufficiency of maternal COX-1 for the normal onset of labor. COX-1 expression is induced in gravid murine uterus and by in situ hybridization; this induction is localized to the decidua. Measurement of uterine PGs further confirmed that COX-1 accounted for the majority of PGF2alpha production. To evaluate the interaction of PGs with oxytocin during murine labor, we generated mice deficient in both oxytocin and COX-1. Surprisingly, the combined oxytocin and COX-1-deficient mice initiated labor at the normal time. COX-1-deficient mice demonstrated impaired luteolysis, as evidenced by elevated serum progesterone concentration and ovarian histology late in gestation, and delayed induction of uterine oxytocin receptors. In contrast, simultaneous oxytocin and COX-1 deficiency restored the normal onset of labor by allowing luteolysis in the absence of elevated PGF2alpha production. These findings demonstrate that COX-1 is essential for normal labor in the mouse, with a critical function being to overcome the luteotrophic action of oxytocin in late gestation.

Figure 1

Induction of COX-1 expression during gestation. (A) Northern blot analyses of total uterine RNA during pregnancy. Samples from nongravid (NG) or pregnant females of the gestational age in days given above the corresponding lanes were hybridized to COX-1 or cyclophilin A- (CYC) radiolabeled probes. (B) Decidual (endometrial) localization of COX-1 in gravid uterus at 18.5 d of gestation. Histologic sections of intact uterine implantation sites including uterus, fetus, placenta, and extra-embryonic membranes were hybridized to a radiolabeled COX-1 RNA probe. Shown is a representative bright field (Upper), nuclear fast red counterstained, region of uterus and amnion, along with the corresponding dark field photograph of the section (Lower), after exposure to emulsion and development. High level COX-1 expression is detected primarily within the decidua (D), as opposed to myometrium (M) or amnion, as demonstrated by deposition of silver grains.

Figure 2

Impaired OT receptor mRNA induction in COX-1 KO, but not COX-1 KO/OT KO mice. Total uterine RNA from nongravid (NG) or pregnant females of the gestational age (in days) and genotype given above the corresponding lanes were hybridized to OT receptor (OTR) or 18S ribosomal RNA (18S) radiolabeled probes.

Figure 3

Plasma progesterone concentration in COX-1 KO, OT KO, and COX-1 KO/OT KO mice. Blood was collected by retroorbital phlebotomy in n = 4–6 mice per genotype group, and plasma progesterone concentration was determined. Data are presented as mean ± SEM. ∗, P < 0.01 vs. wt, OT KO, and COX-1 KO/OT KO at day 19.

Figure 4

Corpus luteum histology at day 19 of gestation in wt, COX-1 KO, and COX-1 KO/OT KO mice. Luteal architecture and cell morphology in wt and COX-1 KO/OT KO mice appear similar and differ from COX-1 KO mice. (Hematoxylin/eosin-stained sections were analyzed by light microscopy at ×200 magnification.)

Figure 5

Impaired uterine PGF2α production in COX-1 KO and COX-1 KO/OT KO mice. Prostaglandins were extracted from uterus isolated from the indicated genotypes, and PGF2α concentration was determined by ELISA. Assay specificity was confirmed by administration of indomethacin (Indo) to a subset of wt mice at day 15.5 of gestation for suppression of prostaglandin production. Data are presented as mean, with day 19 samples ± SEM. ∗, P < 0.05 vs. wt at day 19.