Pregnanolone appearance coincides with the period of implantation following embryonic diapause in the roe deer

Roe deer (Capreolus capreolus) are seasonal animals with fertilisation taking place in summer (July/August). Thereafter, the embryo enters a 4-5-month period of embryonic diapause. This reproductive strategy, displaying a reduced embryonic developmental velocity at the blastocyst stage, leads to reactivation and implantation in late December-January and parturition in spring (May/June). The endocrine mechanisms governing reactivation, implantation and placentation remain poorly understood. Here, we applied ultra-high-performance liquid chromatography coupled to high-resolution mass spectrometry (UHPLC-HRMS) to profile circulating progestogens from reactivation through late gestation (December-April) and in placental and luteal tissue. Notably, the neurosteroid pregnanolone (3α,5β-THP) appeared exclusively in pregnant does from late December/early January onwards, coinciding with the period of implantation. Pregnanolone was detected in placental but not luteal tissue, suggesting the placenta as a potential source of origin. Our results indicate a previously unrecognised shift in progestogens from reactivation/early placentation onwards, suggesting that non-classical progestogens may contribute to the endocrine regulation of implantation and pregnancy maintenance. Further investigation into the origin and functional roles of pregnanolone are needed to elucidate its contribution to pregnancy and reproductive success in roe deer.