We report the existence of a 'placental clock', which is active from an early stage in human pregnancy and determines the length of gestation and the timing of parturition and delivery. Using a prospective, longitudinal cohort study of 485 pregnant women we have demonstrated that placental secretion of corticotropin-releasing hormone (CRH) is a marker of this process and that measurement of the maternal plasma CRH concentration as early as 16-20 weeks of gestation identifies groups of women who are destined to experience normal term, preterm or post-term delivery. Further, we report that the exponential rise in maternal plasma CRH concentrations with advancing pregnancy is associated with a concomitant fall in concentrations of the specific CRH binding protein in late pregnancy, leading to a rapid increase in circulating levels of bioavailable CRH at a time that coincides with the onset of parturition, suggesting that CRH may act directly as a trigger for parturition in humans.

Initially the hypothalamic factor responsible for the release of corticotropin (CRF), was thought to be a simple peptide. More recent work has led to the conclusion that CRF is a multifactorial complex. In 1979 we proposed that vasopressin, much disputed as a CRF candidate, was a major constituent of the complex, interacting with a potentiating the CRF activity of the other component(s). The recent characterization of a 41 residue ovine hypothalamic peptide capable of releasing adrenocorticotropic hormone (ACTH) in a dose-related manner has allowed us to compare its CRF bioactivity with that of vasopressin and simple extracts of the hypothalamus, and to investigate any interaction it may have with vasopressin and other hypothalamic factors in the release of ACTH. We report here that the new CRF is more potent than vasopressin in releasing ACTH. When given simultaneously with vasopressin a fourfold potentiation of CRF activity with steep dose-response characteristics were observed. It also potentiated vasopressin-free hypothalamic extracts, suggesting that a new CRF does not account for all the nonvasopressin portion of the CRF complex.

No abstract

In recent studies to clone and characterize genes coding for the corticotropin-releasing factor-binding protein (CRF-BP), analysis of the tissue distribution of the CRF-BP gene indicated a high level of expression in the rat brain. We have now characterized by immunohistochemical and hybridization histochemical means the cellular localization of CRF-BP protein and mRNA expression, respectively. Results from both approaches converged to indicate that CRF-BP is expressed predominantly in the cerebral cortex, including all major archi-, paleo-, and neocortical fields. Other prominent sites of mRNA and protein expression include subcortical limbic system structures (amygdala, bed nucleus of the stria terminalis), sensory relays associated with the auditory, olfactory, vestibular, and trigeminal systems, several raphe nuclei, and a number of cell groups in the brainstem reticular core. Expression in the hypothalamus appears largely limited to the ventral premammillary and dorsomedial nuclei; only isolated CRF-BP-stained cells are apparent in neurosecretory cell groups. Dual immunstaining for CRF and CRF-BP revealed a partial colocalization in some of these regions. In addition, prominent CRF-BP-stained terminal fields have been identified in association with CRF-expressing cell groups in circumscribed hypothalamic and limbic structures. In the anterior pituitary, CRF-BP mRNA and immunoreactivity were colocalized with corticotropin-immunoreactivity in a majority of corticotropes. Thus, CRF-BP could serve to modify the actions of CRF by intra-and intercellular mechanisms, in CRF-related pathways in the central nervous system and pituitary.

Corticotropin-releasing factor (CRF), is a potent stimulator of synthesis and secretion of preopiomelanocortin-derived peptides. Although CRF concentrations in the human peripheral circulation are normally low, they increase throughout pregnancy and fall rapidly after parturition. Maternal plasma CRF probably originates from the placenta, which responds to the bioactive peptide and produces the peptide and its messenger RNA. Even though CRF concentrations in late gestational maternal plasma are similar to those in rat hypothalamic portal blood and to those that can stimulate release of adrenocorticotropic hormone (ACTH) in vitro, maternal plasma ACTH concentrations increase only slightly with advancing gestation and remain within the normal range. Several groups have now reported the existence of a CRF-binding protein in human plasma which inactivates CRF and which has been proposed to prevent inappropriate pituitary-adrenal stimulation in pregnancy. The binding protein was recently purified from human plasma. We have now isolated and partially sequenced the binding protein, allowing us to clone and characterize its complementary DNA from human liver and rat brain. Expression of the cDNAs for human and rat binding protein in COS7 cells showed that these proteins bind CRF with the same affinity as the native human protein. Both rat and human recombinant binding proteins inhibit CRF binding to a CRF antibody and inhibit CRF-induced ACTH release by pituitary cells in vitro.