Prostaglandins and the regulation of parturition in mares.

Prostaglandins play an essential role during the perinatal period in the mare. Prostaglandin concentrations are low for the majority of pregnancy due to the regulatory action of progestagens on those enzymes responsible for metabolism of prostaglandins. Towards term, prostaglandin concentrations gradually increase, closely associated with upregulation of the fetal hypothalamo-pituitary-adrenal axis, stimulation of the prostaglandin synthesising enzyme PGHS-2 and changes in the ratio of progestagens and oestrogens. Recent evidence in the mare indicates that proinflammatory cytokines are key mediators of prostaglandin synthesis both at term parturition in healthy mares and at preterm parturition associated with placental infection. Prostaglandin concentrations rise substantially during active labour and decline after birth, associated with delivery of the placenta. During induced labour, prostaglandin concentrations are variable depending on the proximity to spontaneous parturition at term. Once the proinflammatory endocrine cascade is initiated, it is difficult to prevent active labour by administration of drugs that reduce prostaglandin concentrations in peripheral plasma. Further work is needed to establish the inter-relationships between prostaglandin production and other endocrine changes associated with labour at term and preterm in the mare.