Releasing hormones and inhibiting hormones are hormones whose main purpose is to control the release of other hormones, either by stimulating or inhibiting their release. They are also called liberins and statins , or releasing factors and inhibiting factors. The examples are hypothalamic-pituitary hormones that can be classified from several viewpoints: they are hypothalamic hormones, they are hypophysiotropic hormones, and they are tropic hormones. For example, thyrotropin-releasing hormone is released from the hypothalamus in response to low levels of secretion of thyroid-stimulating hormone from the pituitary gland. The TSH in turn is under feedback control by the thyroid hormonesT4 and T3. When the level of TSH is too high, they feed back on the brain to shut down the secretion of TRH. Synthetic TRH is also used by physicians as a test of TSH reserve in the pituitary gland as it should stimulate the release of TSH and prolactin from this gland. The main releasing hormones are as follows:
The hypothalamus uses thyrotropin-releasing hormone to tell the pituitary to release thyrotropin.
There are various other inhibiting factors that also have tropic endocrine inhibition activity. Such activity is only one of many functions that they have, and they are not always called hormones, although many are neuropeptides or neurosteroids. They include the following:
The hypothalamus uses dopamine as a prolactostatin to tell the pituitary to inhibit prolactin; it is also created elsewhere in the brain and the adrenal cortex as a neurotransmitter to affect many other systems.
Releasing hormones increase the intracellular concentration of calcium, resulting in vesicle fusion of the respective primary hormone. For GnRH, TRH and GHRH the increase in Ca2+ is achieved by the releasing hormone coupling and activating G protein coupled receptors coupled to the Gqalpha subunit, activating the IP3/DAG pathway to increase Ca2+. For GHRH, however, this is a minor pathway, the main one being the cAMP dependent pathway.
Notable researchers
and Andrew W. Schally were awarded the Nobel Prize in Physiology and Medicine in 1977 for their contributions to understanding "the peptide hormone production of the brain"; these scientists independently first isolated TRH and GnRH and then identified their structures.
