Hormones pituitary hormone secretion

Adrenocorticotropic hormone A hormone secreted by the anterior pituitary that controls secretion of cortisol from the adrenal glands also referred to as corticotropin. 

Knigge, U. and Warberg, J. The role of histamine in the neuroendocrine regulation of pituitary hormone secretion. Acta Endocrinol. (Copenh.) 124 609-619,1991. 

In addition to its central effects on blood pressure, Ang II acts on the central nervous system to stimulate drinking (dipsogenic effect) and increase the secretion of vasopressin and adrenocorticotropic hormone (ACTH). The physiologic significance of the effects of Ang II on drinking and pituitary hormone secretion is not known. 

The neurotransmitter histamine (HA) exerts several functions in the hypothalamus [1-2] including an involvement in the neuroendocrine regulation of pituitary hormone secretion [3]. HA has no effect directly at the level of the pituitary gland, but influences the secretion of anterior pituitary hormones either by an exerted e.g. in the paraventricular nucleus (PVN) on other central transmitters or hypothalamic regulating factors, which subsequently regulate the release of anterior pituitary hormones. In addition, HA acts on the supraoptic nucleus (SON) in the hypothalamus where the posterior pituitary hormones are synthesized and thereby exerts a direct effect on the release of the posterior pituitary hormones. Immunohistochemical studies have revealed that the histaminergic neurons, which originate in the tuberomammillary nuclei of the posterior hypothalamus, densely innervate most of the hypothalamic areas involved in the neuroendocrine control of pituitary hormone secretion [4-5]. Within the last two decades the effect of HA on pituitary hormone secretion have been explored in several studies and it has been... 

Autoradiographic studies have revealed that H3 receptors are located in most areas of the brain with the highest content in the basal ganglia and the cortex [9]. H3 receptors are distributed with moderate density in the hypothalamus, where the highest density of histaminergic nerve fibers are found [4-5], and were demonstrated in the anterior, medial and posterior hypothalamus [8]. However, their localization in hypothalamic areas primarily involved in the regulation of pituitary hormone secretion were not studied in detail. In the pituitary gland the H3 receptors were scarce. 

When studying the role of various H3 receptor compounds on pituitary hormone secretion some points have to be considered a) are the actions of the compounds related to H3 receptor binding or to a non-specific action, b) are the actions related to an auto- or heteroreceptor or eventually postsynaptic receptor effect of the compounds, and c) are the actions related to an effect known to involve histaminergic neurons. In the following, we will give a short description of the effect of HA in the neuroendocrine regulation of pituitary hormone secretion (Table 1) and focus on the role of H3 receptors in modulation of the secretion of the individual pituitary hormones based on previously published investigations and our own recent experiments. 

THE H3 RECEPTOR AND POSTERIOR PITUITARY HORMONE SECRETION 3.1. Vasopressin... 

However, receptor autoradiography and in vitro studies have suggested that H3 receptors are located on other aminergic neurons in the brain. Since amines such as serotonin and catecholamines are involved in the regulation of pituitary hormone secretion, it is obvious that an action of the H3 receptor compounds may be exerted via these H3 heteroreceptors. Only few studies have evaluated this heteroreceptor action. It has been excluded that the effect of the H3 receptor agonists is due to an effect on H3 receptors located on serotonergic neurons, while an effect on catecholaminergic neurons has yet not been excluded. 

Effect of H3 receptor compounds on pituitary hormone secretion. 

The suggestion of the existence of postsynaptic H3 receptors have not been clarified in relation to pituitary hormone secretion, although the finding in a single study of a stimulatory effect of the H3 receptor agonist RmHA infused centrally on PRL may suggest such an effect - in as much as the effect was prevented by THIOP. 

In summary, the H3 receptor agonists are useful tools to investigate pituitary hormone secretion when a reduction of the activity of histaminergic neurons is required. Conversely, H3 receptor antagonists may be useful to study hormone secretion during increased histaminergic activity. However, an increased activity induced by THIOP have not been shown to have any marked effect on either basal or stimulated hormone secretion. 

Barreca T, Picciotto A, Franceschini R, et al. Effects of acute administration of recombinant interferon alpha 2b on pituitary hormone secretion in patients with chronic active hepatitis. Curr Ther Res 1992 52 695-701. 

The direct actions of DA on pituitary hormone secretion are largely inhibitory, maintaining basal secretion of prolactin from anterior pituitary lactotrophs and POMC-derived peptide hormones from intermediate lobe melanotrophs via inhibitory D2 receptors located on these cells. Episodic surges of these hormones are associated with... 

DA control of anterior pituitary hormone secretion also mediated through transynaptic regulation of hypothalamic neurosecretory neurons. This occurs via axonal-somatic/ dendritic interactions in hypothalamic regions containing neurosecretory neuron perikarya and/or through axonal-axonal interactions on their terminals in the median eminence. Diencephalic DA neurons may regulate neuropeptide release directly via stimulatory Di or inhibitory D2 receptors located on hypothalamic neurosecretory neurons, or they may act indirectly through stimulatory and/or inhibitory interneurons. 

A variety of chemically-identified neurons within the ARC receive both indirect (extrinsic) and direct (intrinsic) enkephalinergic neuronal input (Magoul et al., 1993) suggesting a role for enkephalin in the neuroendocrine regulation of pituitary hormone secretion. Enkephalin-IR perikarya in the bed nucleus of the stria terminalis, medial preoptic nucleus, periventricular nucleus and dorsomedial nucleus all provide extrinsic input to the rostral ARC, whereas intrinsic enkephalin neurons connect the rostral and caudal portions of the ARC (Magoul et al., 1993). Enkephalin-IR neurons innervate TH-IR neurons (perikarya and dendrites) in the DM-ARC (but not in the VL-ARC), (5-endorphin neurons in the VL-ARC, and NPY neurons in the ventromedial ARC (Magoul et al., 1994). There are symmetrical synaptic connections between enkephalin axon terminals and POMC perikarya in the ARC (Zhang et al., 1987), and reciprocal synaptic associations with NPY neurons in the ventromedial ARC (Li et al., 1993). 

Frohman LA, Maeda K, Berelowitz M, Szabo M, Thominet J (1982) Effects of neurotensin on hypothalamic and pituitary hormone secretion. Ann New York Acad Sci 00 172-182. 

Racagni G, Apud JA, Cocchi D, Locatelli V, Muller EE (1982) GABAergic control of anterior pituitary hormone secretion. Life Sci 32 823-838. 

Figure 13.2 The hierarchical control of hormone release. Releasing hormones from the hypothalamus travel directly to the anterior pituitary hormones secreted by the anterior pituitary control most somatic endocrine glands. 

Tsigos C, Papanicolaou DA, Defensor R, Mitsiadis CS, Kyrou I, Chrousos GP (1997) Dose effects of recombinant human inter-leukm-6 on pituitary hormone secretion and energy expenditure. Neuroendocrinology 66 54-62. 

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