The oxytocin-prostaglandins pathways in the horse (Equus caballus) placenta during pregnancy, physiological parturition, and parturition with fetal membrane retention

Abstract

Despite their importance in mammalian reproduction, substances in the oxytocin-prostaglandins pathways have not been investigated in the horse placenta during most of pregnancy and parturition. Therefore, we quantified placental content of oxytocin (OXT), oxytocin receptor (OXTR), and prostaglandin E2 and F2 alpha during days 90-240 of pregnancy (PREG), physiological parturition (PHYS), and parturition with fetal membrane retention (FMR) in heavy draft horses (PREG = 13, pHYS = 11, FMR = 10). We also quantified OXTR and prostaglandin endoperoxide synthase-2 (PTGS2) mRNA expression and determined the immunolocalization of OXT, OXTR, and PTGS2. For relative quantification of OXT and OXTR, we used western blotting with densitometry. To quantify the prostaglandins, we used enzyme immunoassays. For relative quantification of OXTR and PTGS2, we used RT-qPCR. For immunolocalization of OXT, OXTR, and PTGS2, we used immunohistochemistry. We found that OXT was present in cells of the allantochorion and endometrium in all groups. PTGS2 expression in the allantochorion was 14.7-fold lower in FMR than in PHYS (p = 0.007). These results suggest that OXT is synthesized in the horse placenta. As PTGS2 synthesis is induced by inflammation, they also suggest that FMR in heavy draft horses may be associated with dysregulation of inflammatory processes. In the placenta, a delicate balance of hormones helps to support pregnancy and initiate parturition 1-3. Parturition is a proinflammatory event 4-8 , and delivery of the fetus and the placenta are accompanied by coordinated contractions, pain, and swelling, which are mostly mediated by prostaglandins 9. Exposure of the placenta to endogenous or exogenous oxytocin (OXT) causes it to release prostaglandin E2 (PGE 2) and prostaglandin F2 alpha (PGF 2ɑ) within a few hours 10-21 (see Supplementary Fig S1 for the pathways that start with OXT release and end with the prostaglandins binding to their receptors). OXT is released from the posterior pituitary gland 22 , and during pregnancy and parturition in humans, from the placenta 23. Given that, of all domestic and laboratory species, horses may be the most similar to humans in terms of the endocrinology of pregnancy and parturition 24 , it seems likely that horses also release OXT from the placenta. Moreover, human placental OXT is not believed to play a role in myometrial contractions, unlike pituitary OXT 13. Thus, it seems likely that horse placental OXT would be a signal for prostaglandin release in the placenta itself, and not a signal for myometrial contractions. However, the presence of OXT in horse placental cells needs to be verified.

Environmental constraints and pathologies that modulate equine placental genes and development

“…Gene expression of IL-1β and IL-6 (as well as protein expression of IL-6) was also increased in retained allantochorions (Jaworska & Janowski 2019). Gene expression of prostaglandin endoperoxide synthase-2 (PTGS2), involved in prostaglandin production, was however decreased in retained placenta which could contribute to the observed imbalance in inflammatory processes and tissue remodelling in this pathology (Rapacz-Leonard et al 2020). Finally, protein expression of oxytocin receptors (OXTR) was greatly reduced in retained placentas in case of secondary atony, but this difference was not observed in retained allantochorion without secondary atony (Rapacz-Leonard et al 2015b.…”

The role of placental immunological, pro-inflammatory, and proteolytic factors in the process of fetal membrane expulsion in mares

“…Podczas ciąży u klaczy ekspresja tego enzymu hamowana jest poprzez wysoki poziom pregnanów, głównie 5α-dihydroprogesteronu (DHP) (44). Podczas porodu, wraz ze spadkiem syntezy pregnanów (8), enzym ten ulega wzmożonej ekspresji (54,66). Zgodnie z najnowszymi wynikami, ekspresja PTGS2 w momencie wydalenia płodu jest istotnie niższa u klaczy, u których w ciągu najbliższych 3 godzin rozwinie się zatrzymanie błon płodowych (ryc.…”

“…Dane użyte do przygotowania ryciny zostały opublikowane (54) i zdeponowane w publicznym repozytorium danych pod adresem https://doi.org/10.5061/dryad.fbg79cnr4 jak i same regulują jej przebieg (6,40,59,67). Jest to zgodne z innymi doniesieniami (26,27,54) o zaburzeniach szlaku syntezy prozapalnej u klaczy z zatrzymaniem błon płodowych (ryc. 3).…”

Expression of Oxytocin/Neurophysin I and Oxytocinase in the Equine Conceptus from Day 8 to Day 21 Post-Ovulation

“…In mares, there is a positive feedback loop during luteolysis and parturition involving oxytocin, OXTR and endometrial PGF2α secretion [ 18 , 19 , 20 ]. De Ruijter-Villani and co-authors (2015) reported in D14 pregnant mare endometria that downregulation of the PGF receptor intercepted the oxytocin-prostaglandin F feedback loop [ 21 ].…”