Role of corticotropin-releasing hormone in onset of labour.

Corticotropin-releasing hormone (CRH) derived from the placenta is secreted into the maternal circulation in large amounts during the third trimester of human pregnancy and may have an important role in the onset of labour. Although the biological role of CRH remains enigmatic, the presence of functional CRH receptors in the myometrium suggests that CRH may modulate myometrial contractility and hence parturition. CRH action is mediated via multiple receptor subtypes and pregnancy results in differential receptor expression. These receptors are primarily linked to the adenylate cyclase second messenger system, which would help the intracellular microenvironment to maintain the required myometrial quiescence. CRH can exert further actions such as inhibition of prostaglandin production to prevent contractions. At term under the influence of oxytocin there is a modification in the coupling mechanisms that leads to a decrease in the biological activity of the CRH receptor and in the generation of cyclic adenosine monophosphate which favours myometrial contractions. CRH, via distinct receptor subtypes, is then able to enhance the contractile response of the myometrium. This hypothesis places CRH in a central role in coordinating the smooth transition from a state of relaxation to one of contraction.