Characterization of serum and tissue oxytocinase and tissue oxytocin in the pregnant and non-pregnant mare

Abstract

Oxytocin is a hormone with functions in: reproduction, maternal bonding, milk ejection, and feeding/social behavior, and is reported to be present in a variety of tissues. Our goal is to characterize oxytocin and leucyl and cystinyl aminopeptidase (LNPEP/oxytocinase), a key regulator of oxytocin in mares. We measured serum and tissue LNPEP by ELISA from ovulation (D0) until D21–22 in non-pregnant (n = 5) and pregnant mares (n = 6); and in periparturient and postpartum mares (n = 18). Placenta (n = 7) and homogenized tissue of diestrus mares (n = 6) were evaluated using protein determinations and LNPEP ELISAs. Identification of LNPEP and OXT protein in tissues was also performed via western blot, immunohistochemistry and liquid chromatography-mass spectrometry (LC-MS/MS). Furthermore, in situ hybridization was performed for LNPEP and OXT on endometrium, myometrium, pituitary and corpus luteum (CL). Serum LNPEP concentration were similar. Placental LNPEP U/mg protein was highest in the body and pregnant horn. The highest to lowest LNPEP U/mg protein by tissue were: myometrium > follicle wall > endometrium > kidney > CL > liver. Oxytocin was identified in the equine pituitary, CL and placenta and is likely to act in autocrine or paracrine manner, while LNPEP may act systemically and locally to regulate the availability of OXT.

The Timing of the Maternal Recognition of Pregnancy Is Specific to Individual Mares

“…Although the equine endometrium produces oxytocin [16][17][18], until recently the consensus of opinion has been that the equine CL does not do likewise [19,20]. Hence, in equids, unlike ruminants [21], CL-derived oxytocin was not thought to stimulate the luteolytic pulses of endometrial PGF2α to lyse the CL at the end of dioestrus.…”

“…Hence, in equids, unlike ruminants [21], CL-derived oxytocin was not thought to stimulate the luteolytic pulses of endometrial PGF2α to lyse the CL at the end of dioestrus. However, recent work by Diel de Amorin [18] localised oxytocin to the CL, although the role it may play in luteolysis or luteostasis has not been determined.…”