Oxytocin antagonist [1-D(CH2)5,Tyr(ME)2,Thr4,Tyr-NH2(9)]ornithine vasotocin inhibits oxytocin-induced prostaglandin F2alpha release in late-pregnant cows

Oxytocin receptor binding sites in the periphery of the neonatal mouse

Oxytocin (OXT) is a pleiotropic regulator of physiology and behavior. An emerging body of evidence demonstrates a role for OXT in the transition to postnatal life of the infant. To identify potential sites of OXT action via the OXT receptor (OXTR) in the newborn mouse, we performed receptor autoradiography on 20 μm sagittal sections of whole postnatal day 0 male and female mice on a C57BL/6J background using the ¹²⁵iodinated ornithine vasotocin analog ([¹²⁵I]-OVTA) radioligand. A competitive binding assay on both wild-type (WT) and OXTR knockout (OXTR KO) tissue was used to assess the selectivity of [¹²⁵I]-OVTA for neonatal OXTR. Radioactive ligand (0.05 nM [¹²⁵I]-OVTA) was competed against concentrations of 0 nM, 10 nM, and 1000 nM excess unlabeled OXT. Autoradiographs demonstrated the high selectivity of the radioligand for infant peripheral OXTR. Specific ligand binding activity for OXTR was observed in the oronasal cavity, the eye, whisker pads, adrenal gland, and anogenital region in the neonatal OXTR WT mouse, but was absent in neonatal OXTR KO. Nonspecific binding was observed in areas with a high lipid content such as the scapular brown adipose tissue and the liver: in these regions, binding was present in both OXTR WT and KO mice, and could not be competed away with OXT in either WT or KO mice. Collectively, these data confirm novel OXT targets in the periphery of the neonate. These peripheral OXTR sites, coupled with the immaturity of the neonate’s own OXT system, suggest a role for exogenous OXT in modulating peripheral physiology and development.

Oxytocin stimulated release of PGF2α and its inhibition by a cyclooxygenase inhibitor and an oxytocin receptor antagonist from equine endometrial cultures

Uterine inflammation results in a poor uterine environment and early embryonic loss in the mare due to an inhibition of maternal recognition of pregnancy caused from increased prostaglandin F2α (PGF2α). Oxytocin binds to endometrial cell receptors to activate prostaglandin synthesis. An oxytocin receptor antagonist (Atosiban) and a cyclooxygenase inhibitor (indomethacin) both decrease PGF2α production. The aim of this study was to evaluate the in vitro effects of Atosiban and indomethacin on equine uterine prostaglandin secretion. Equine endometrial explants were harvested on day two of behavioral estrus. Endometrial explant cultures were challenged with oxytocin (250nM) and PGF2α concentrations were measured over time. Explants were also cultured with Atosiban and indomethacin for 6h to determine the influence on PGF2α secretion. When endometrial explants were challenged with oxytocin, PGF2α concentrations were greater (P<0.0001) at each time point over the 24h of culture as compared to controls. Oxytocin failed (P<0.001) to elicit PGF2α release in explants cultured with either Atosiban or indomethacin. These findings show equine endometrial explants can be stimulated with oxytocin to increase secretion of PGF2α and this secretion can be inhibited through an oxytocin receptor antagonist and a Cox inhibitor, suggesting that this response to oxytocin involves an oxytocin receptor mediated event that activates the prostaglandin synthesis cascade through cyclooxygenase. Furthermore, this data suggests a role for the use of these inhibitors in vivo to decrease uterine PGF2α secretion and prevent early luteal regression and embryonic loss.

The effect of two different routes of administration of oxytocin on peripheral plasma prostaglandin F(2α) metabolite levels in early post-partum dairy cows

Various parenteral treatment forms of oxytocin, as often used under praxis circumstances, may act differently on contractility of the uterus during the first days of the puerperium. Various patterns of such induced uterotonic responses may lead to alterations in the emptying characteristics of the uterine lumen, thus influencing, as a late consequence, the process of involution. Therefore, this study was designed to test whether two different parenteral administration forms of oxytocin induce changes in peripheral plasma concentrations of 15-ketodihydro-prostaglandin F(2α) (PGF(2α) metabolite) in early post-partum cows. Between 13 and 15 h after uncomplicated calving, healthy dairy cows without retained foetal membranes were treated with 50 IU oxytocin, either intramuscularly (OT-IM group; n = 15) or intravenously (OT-IV group; n = 16). Saline solution was administered intramuscularly as controls (CON group; n = 15). Jugular blood samples were taken at 10-min intervals from 1 h before to 2 h after treatment. Plasma PGF(2α) metabolite levels were measured by radioimmunoassay. No significant differences in peripheral plasma PGF(2α) metabolite concentrations occurred in the OT-IM and CON groups, but mean values significantly increased in the OT-IV group, peaking at 20 min after treatment and reaching pre-treatment baseline values again at 120 min. Although the source of prostaglandins was not investigated in this study, our results suggest that exogenous oxytocin may enhance secretion of prostaglandins by the uterus during the first day after normal calving. These prostaglandins might contribute, by an endocrine or paracrine route, to the stimulation of myometrial contractility when exogenous oxytocin is given during this early post-partum stage.

Prostaglandins and reproduction in female farm animals

Prostaglandins impact on ovarian, uterine, placental, and pituitary function to regulate reproduction in female livestock. They play important roles in ovulation, luteal function, maternal recognition of pregnancy, implantation, maintenance of gestation, microbial-induced abortion, parturition, postpartum uterine and ovarian infections, and resumption of postpartum ovarian cyclicity. Prostaglandins have both positive and negative effects on reproduction; they are used to synchronize oestrus, terminate pseudopregnancy in mares, induce parturition, and treat retained placenta, luteinized cysts, pyometra, and chronic endometritis. Improved therapeutic uses for prostaglandins will be developed when we understand better their involvement in implantation, maintenance of luteal function, and establishment and maintenance of pregnancy.

Oxytocin induces PGE2 release from bovine cervical mucosa in vivo

Oxytocin receptor (OTR) concentrations in bovine cervical mucosa rise steeply a few days before estrus to high concentrations and fall rapidly after estrus. To study the physiological role of these OTR, the effect of OT on the release of PGE, from the cervical mucosa of periestrous cows in vivo was determined by inserting bags made of dialysis tubing containing isooncotic saline solution in the endocervix for two 2-h periods, a fresh bag for each period. During the first period no treatment was given, during the second period OT (100 IU) or saline was injected i.m. PGE2 content in the second bag was significantly greater in OT-treated cows than in saline-treated cows. In a second experiment cervical resistance to stretch, achieved by distention of a balloon inside the cervical canal, was measured in periestrous cows before and 10 h after i.m. injection of OT, or endocervical application of 2.5mg PGE1 in a jelly, or the inactive jelly. A significant reduction in the resistance was achieved with both OT and PGE1; in the doses given the effect of PGE1 was longer lasting than that of OT.

Receptor blockers - general aspects with respect to their use in domestic animal reproduction

Receptor blockers compete with the respective agonist for binding to a given receptor without inducing complete signal transduction. In recent years, major interest has focused on sex-steroid hormone receptor blockers (antagonists). Indications have been obtained that inadequate changes in receptor conformation and subsequent failure of transcriptional activation are major events preventing hormonal activity. However, various subtypes and variants of receptors and receptor mutations have also been identified. Expression of antihormonal effects may vary depending on the type of receptor the blocker is bound to. Hence, receptor blockers may also have an inherent agonistic activity. Aglepristone is the first antiprogestin registered for veterinary use with the indication "interruption or prevention of pregnancy"; similarly, these types of compounds were successfully used for induction of parturition in the dog and cat and for conservative treatment of pyometra in the dog. Moreover, application of antiprogestins has clearly demonstrated the role of progesterone as a major factor controlling overt pseudopregnancy in dogs. With respect to farm animals, parturition was induced in cows without an increased incidence of retained fetal membranes. Other than antiprogestins, antioestrogens and antiandrogens are still in a more experimental phase. In particular for use in humans, high-affinity blockers binding to the oxytocin/vasopressin receptor are in development; they exert distinct tocolytic activities. Also, the release of GnRH can be inhibited by respective antagonists; however, their use in reproduction is still hampered by the high dose requirement and the side effects observed.