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

SIMPLE SUMMARY: Oxytocinase is an enzyme that breaks down oxytocin in serum and tissues. The presence of oxytocin and oxytocinase mRNA and protein and their relationship to other genes in the horse embryo/conceptus that regulate pregnancy recognition and embryo/conceptus fixation have not been investigated. Our objective was to characterize the expression of oxytocin, oxytocinase and other related genes in horse embryos. Oxytocinase was increased at D15 compared to D10 and was identified in the horse embryo membranes. Oxytocin expression was highest at D21, and lowest on D12 and D14; no protein was identified, while its receptor (OXTR) was highest on D14 and D21. Oxytocinase was highest at D15 compared to D10 and high expression of prostaglandin E synthase was found on D14, D15 and D21. Oxytocinase may have a role in embryonic fixation. Further investigation of the role of oxytocinase during pregnancy is warranted. ABSTRACT: Leucyl and cystinyl aminopeptidase (LNPEP/oxytocinase) is an enzyme that metabolizes oxytocin in serum and tissues. The presence of oxytocin/neurophysin I (OXT), oxytocin and LNPEP and their relationship to other genes is unknown in the equine conceptus. Our objective was to characterize gene expression of LNPEP and OXT on D8, 10, 12, 14, 15, 16 and 21 conceptuses in relationship to other genes. Immunohistochemistry, western blot and liquid chromatography with tandem mass spectrometry (LC-MS/MS) were used for identification of oxytocin and LNPEP in D15, 16 and 18 conceptuses. LNPEP was increased at D15 compared to D10, was immunolocalized in the equine trophectoderm and endoderm, and protein was confirmed by LC-MS/MS. Maximal abundance of OXT was at D21, and lowest on D12 and D14, but no protein was identified. OXTR abundance was highest on D14 and D21. LNPEP was correlated with PTGFR and PTGES on D12 and D14–D15, and high expression of PTGES, PTGS2 was found on D14, D15 and D21; PTGFR was found on D8 and D12–21. LNPEP may have a role in prostaglandin regulation and conceptus fixation by decreasing the availability of oxytocin. Further investigation on the role embryonic LNPEP during pregnancy is warranted.