Oxytocin-neurophysin I mRNA abundance in equine uterine endometrium.

A positive-feedback loop between luteal oxytocin and uterine prostaglandin F2 alpha (PGF) is a major signal for luteolysis in ruminants. Likewise, uterine PGF causes luteolysis in mares, but the involvement of oxytocin in this process is unclear. We wanted: 1) to determine if the oxytocin-neurophysin I (OT-NP I) gene is transcribed into mRNA in the endometrium of mares; and, if so, 2) to analyze relative changes in abundance of endometrial OT-NP I mRNA throughout the estrous cycle and during early stages of pregnancy. Endometrial biopsies were obtained from nonbred mares during estrus, and 5, 10, and 15 d after ovulation (n = 3/d). Biopsies were also obtained from pregnant mares 10, 15, and 20 d after ovulation (n = 3/d). Relative amounts of OT-NP I and glyceraldehyde-3-phosphate dehydrogenase mRNA in endometrium were assessed by reverse transcription-polymerase chain reaction and Southern blotting. Endometrial OT-NP I mRNA abundance changed with day of the cycle or pregnancy, and levels at estrus were higher than at any other days examined. The OT-NP I mRNA levels were negatively correlated with serum progesterone across all days examined and positively correlated with serum estradiol in nonbred mares. The reverse transcription-polymerase chain reaction products for both OT-NP I and glyceraldehyde-3-phosphate dehydrogenase were cloned into vectors and sequenced. Each shared greater than 89% nucleotide and predicted amino acid identities with the respective human, bovine, ovine, and rat products. Uterine oxytocin may be involved in regulation of reproductive tract function during the estrous cycle and/or establishment of pregnancy in horses.