In addition to being produced in the hypothalamus, CRH is also synthesized in peripheral tissues, such as T lymphocytes, and is highly expressed in the placenta. In the placenta, CRH is a marker that determines the length of gestation and the timing of parturition and delivery. A rapid increase in circulating levels of CRH occurs at the onset of parturition, suggesting that, in addition to its metabolic functions, CRH may act as a trigger for parturition.
In the short term, CRH can suppress appetite, increase subjective feelings of anxiety, and perform other functions like boosting attention. Although the distal action of CRH is immunosuppression via the action of cortisol, CRH itself can actually heighten inflammation, a process being investigated in multiple sclerosis research.
The CRH-1 receptor antagonist pexacerfont is currently under
investigation for the treatment of generalized anxiety disorder. Another CRH-1 antagonist antalarmin has been researched in animal studies for the treatment of anxiety, depression and other conditions, but no human trials with this compound have been carried out.
Also, abnormally high levels of CRH have been found in the cerebrospinal fluid of people that have committed suicide.
Levels rise towards the end of pregnancy just before birth and current theory suggests three roles of CRH in parturition:
Increases levels of dehydroepiandrosterone (DHEA) directly by action on the fetal adrenal gland, and indirectly via the mother's pituitary gland. DHEA has a role in preparing for and stimulating cervical contractions.
Increases prostaglandin availability in uteroplacental tissues. Prostaglandins activate cervical contractions.
Prior to parturition it may have a role inhibiting contractions, through increasing cAMP levels in the myometrium.
In culture, trophoblast CRH is inhibited by progesterone, which remains high throughout pregnancy. Its release is stimulated by glucocorticoids and catecholamines, which increase prior to parturition lifting this progesterone block.
The 41-amino acid sequence of CRH was first discovered in sheep by Vale et al. in 1981. Its full sequence is:
The rat and human peptides are identical and differ from the ovine sequence only by 7 amino acids.
Role in non-mammalian vertebrates
In mammals, studies suggest that CRH has no significant thyrotropic effect. However, in representatives of all non-mammalian vertebrates, it has been found that, in addition to its corticotropic function, CRH has a potent thyrotropic function, acting with TRH to control the thyroid axis (TRH has been found to be less potent than CRH in some species).
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