Deposition of sperm in the reproductive tract of a mare triggers a transient inflammatory response that is normally resolved within 36h. In mares susceptible to persistent breeding-induced endometritis (PBIE), however, the inflammation is not cleared within this time span resulting in alterations in the uterine environment that can interfere with corpus luteum maintenance and/or conceptus survival and thereby compromise fertility. Recent attention has focused on suppressing the uterine inflammatory response in susceptible mares using systemic corticosteroids or non-steroidal anti-inflammatory drugs. In this respect, administration of corticosteroids at the time of insemination has been reported to improve pregnancy rates in susceptible mares (Bucca et al., 2008), and has been widely adopted in clinical practice. The aim of the current study was to investigate the effects of a single dexamethasone administration at the time of breeding on the endometrial inflammatory reaction.
In this study, five mares known to be susceptible to PBIE were monitored during two estrous cycles. During the first cycle, mares were randomly assigned to being treated or not with corticosteroids. In the second cycle, mares were crossed-over, such that each served as its own control. When the mares were in oestrus with an ovarian follicle ≥35mm, they were inseminated and simultaneously injected with hCG to induce ovulation. During the treated cycle, mares were injected with 50 mg dexamethasone intravenously 1 hour before insemination. Twenty-four hours after artificial insemination, the mares were examined by transrectal ultrasonography and the amount of oedema and intra-uterine fluid were recorded. Next, a low volume intra-uterine flush was performed. The recovered fluid was then centrifuged, the sediment was used for cytological examination and the supernatant was used for Nitric Oxide (NO) determination. After the lavage, an endometrial biopsy was recovered for rtPCR and histology and immunohistochemistry analysis. Quantitative real-time PCR analysis was used to examine mRNA expression for inflammatory cascade enzymes such as COX2, iNOS, LOX5 and LOX5 activating protein (FLAP). Histology and immunohistochemistry was used to examine the presence of PMNs and CD3 positive T cells and the expression of LOX5 and COX2.
Ultrasound finding showed no difference in uterine fluid accumulation after treatment, while the uterine edema was significant decreased by dexamethasone (p < 0.05). No decrease in PMN number were found after treatment. Though, the absolute PMN number was increased in the superficial layer of the endometrium after treatment. The gene expression of lipoxygenase 5 (LOX5), LOX5 activating protein (FLAP), cyclooxygenase2 (COX2) and inducible nitric oxide (iNOS) were not significant reduced after treatment. Although the LOX5 positive cell infiltration was significantly reduced in the superficial and middle layer (p<0.05) after treatment. There was no change in the NO concentration and the CD3 and COX2 positive cell infiltration after treatment. In conclusion, the results of this study suggest that while treating mares susceptible to PBIE with dexamethasone shortly before mating does reduce post-insemination endometrial oedema but does not appear to prevent intra-uterine accumulation of fluid or neutrophils. Neither does dexamethasone treatment appears to suppress uterine production of various inflammatory mediators, nor the endometrial expression of mRNA for major inflammatory enzymes. Moreover, since pre-breeding dexamethasone treatment fails to suppress key elements of the major inflammatory pathways 24 h after AI, there is no clear explanation for how pre-AI corticosteroid treatment could improve pregnancy rates in susceptible mares.