Tissues hypothalamus

Although mast cells and basophils probably account for >90% of stored histamine in the body, histamine is also present in platelets, enterochromaffin-like cells, endothelial cells, and neurons. Histamine can act as a neurotransmitter in the brain. Histaminergic nerves have their cell bodies within a very small area of the brain (the magnocellular nuclei of the posterior hypothalamus) but have axons in most areas of the forebrain. There is also evidence for axons projecting into the spinal (Fig. 1) cord. Finally, there is evidence that histamine synthesis can be induced in tissues undergoing rapid tissue growth and repair. In certain neonatal tissues (e.g. liver), the rate of synthesis of this unstored diffusable histamine (termed nascent histamine) is profound and may point to a role for histamine is cell proliferation. 

High concentrations of KOP have been found in the cerebral cortex and hypothalamus KOP is also present in the gastrointestinal tract, in immune cells as well as in other peripheral tissues. KOPs have been implicated in the regulation of nociception, diuresis, feeding, neuroendocrine and immune system functions [2]. 

Prolactin is peptide hormone secreted by the pituitary gland. It acts on prolactin receptors in breast tissue where it stimulates production of casein and lactalbu-min. It also acts on the testes and ovaries to inhibit the effects of gonadotrophins. Since the secretion of prolactin is under tonic dopaminergic inhibition by the hypothalamus, dopamine D2-receptor antagonists... 

Support for the limited metabolism of IMPA to MPA with resultant retention of the MPA is provided by distribution studies of Sarin (GB). Twenty-four hours after intravenous injection of [3H]-labeled Sarin, unextractable label was present in all tissues except the plasma and kidney. At least some unextractable label was presumed by the study authors to be protein-bound MPA (Little et al. 1986, 1988). The concentration of bound MPA was found to be 20-65% of bound IMPA in different mouse brain areas, with the highest concentration found in the hypothalamus. 

In permissiveness, one hormone enhances the responsiveness of the target tissue to a second hormone in other words, the first hormone increases the activity of the second. For example, the normal maturation of the reproductive system requires reproductive hormones from the hypothalamus, pituitary, and gonads as well as the presence of thyroid hormone. Although thyroid hormone by itself has no effect on the reproductive system, if it is absent the development of this system is delayed. Therefore, thyroid hormone is considered to have a permissive effect on the reproductive hormones, facilitating their actions causing sexual maturation. 

As its name implies, the neurohypophysis is derived embryonically from nervous tissue. It is essentially an outgrowth of the hypothalamus and is composed of bundles of axons, or neural tracts, of neurosecretory cells originating in two hypothalamic nuclei. These neurons are referred to as neurosecretory cells because they generate action potentials as well as synthesize hormones. The cell bodies of the neurosecretory cells in the supraoptic nuclei produce primarily antidiuretic hormone (ADH) and the cell bodies of the paraventricular nuclei produce primarily oxytocin. These hormones are then transported down the axons to the neurohypophysis and stored in membrane-bound vesicles in the neuron terminals. Much like neurotransmitters, the hormones are released in response to the arrival of action potentials at the neuron terminal. 

The adenohypophysis is derived embryonically from glandular tissue, specifically, Rathke s pouch. This tissue originates from the oropharynx, or the roof of the mouth. It then migrates toward the embryonic nervous tissue destined to form the neurohypophysis. When these two tissues come into contact, Rathke s pouch loses its connection with the roof of the mouth and the pituitary gland is formed. Unlike the neurohypophysis, which releases hormones originally synthesized in the hypothalamus, the adenohypophysis synthesizes its own hormones in specialized groups of cells. Similar to the neurohypophysis, however, the release of these hormones into the blood is regulated by the hypothalamus. 

Located in close proximity to the primary capillary plexus in the hypothalamus are specialized neurosecretory cells. In fact, the axons of these cells terminate on the capillaries. The neurosecretory cells synthesize two types of hormones releasing hormones and inhibiting hormones (see Table 10.2). Each of these hormones helps to regulate the release of a particular hormone from the adenohypophysis. For example, thyrotropin-releasing hormone produced by the neurosecretory cells of the hypothalamus stimulates secretion of thyrotropin from the thyrotrope cells of the adenohypophysis. The hypo-thalamic-releasing hormone is picked up by the primary capillary plexus travels through the hypothalamic-hypophyseal portal veins to the anterior pituitary leaves the blood by way of the secondary capillary plexus and exerts its effect on the appropriate cells of the adenohypophysis. The hypophyseal hormone, in this case, thyrotropin, is then picked up by the secondary capillary plexus, removed from the pituitary by the venous blood, and delivered to its target tissue. 

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