Possible involvement of somatotrophic hormones in the postpartum regulation of ovarian activity in mares

Abstract

In horses, resumption of ovarian function after parturition occurs early and the majority of mares ovulate within two weeks after parturition. Previous research suggests that early postpartum increases in the peripheral concentration of prolactin and somatotrophic hormones are the major stimulus for resumption of ovarian activity and ovulation. In the present study, we investigated if placental tissue may be a source of somatotrophic hormones and thus influence ovarian activity. Ovarian follicular development and concentrations of prolactin, insulin like growth factor-1 (IGF1) and LH were determined in 14 healthy Haflinger mares (9.2±3.8 years). From D15 before until D15 after parturition (pp), transrectal ultrasonography of ovarian activity was performed and blood samples were collected for determination of hormone concentrations (prolactin, IGF1 by ELISA; LH by RIA). All foals were healthy, mature and stayed with their dams. Samples from the allantochorion and amnion were obtained immediately after delivery. The relative abundance of mRNA for IGF1 and the prolactin receptor (PRLR) was analyzed by qPCR. Formalin fixed samples were analyzed for protein expression of IGF1 and PRLR by immunohistochemistry. Statistical analysis (IBM SPSS 27.0) was done by analysis of variance (for repeated measures) comparing data from mares that ovulated(group ov; n=5) and did not ovulate until D15 pp (group non-ov, n=9). Values are mean ± SD. Mares from group ov foaled later in the year than non-ov mares (day of year 84±29 vs. 50±20, p<0.05), but no differences in gestation length, placental weight or size were detected. Already on D15 before parturition, ovarian follicles <2 cm diameter were present in all mares, but growth to preovulatory size occurred only pp with a higher growth rate in ov mares (day x group p<0.05). Prolactin and IGF1 concentrations increased from D5 before parturition, peaked at D1 pp (ov vs. non-ov: prolactin 13.0±6.3 vs. 7.9±4.4 ng/ml, p<0.05; IGF1 409±71 vs. 350±65 ng/ml, p<0.05) and declined thereafter. The LH concentration increased from D1 pp in group ov (D12 pp: 23.82±15.96 ng/ml) but not in non-ov (D12 pp: 10.82±5.26 ng/ml; day x group p<0.001). There was a high abundance of mRNA and protein for IGF1 and PRLR in placental tissue irrespective of group. In conclusion, ovarian follicles are detectable before foaling, but develop to preovulatory size only pp when prolactin and LH concentrations increase. In agreement with the situation in cattle, IGF1 stimulates ovarian activity in lactating mares. Placental tissue produces IGF1 that may be under the influence of prolactin as suggested by the presence of PRLR in both amnion and allantochorion.