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Oxytocin
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Oxytocin was the first hormone to have its structure identified and to be synthesized in the laboratory. This was achieved in 1953 by two groups, one working in France and the other in the United States. The hormone plays an important role in birth and in feeding the infant. It has two major actions, the first being to promote contractions of the uterus, an action which is used in obstetric practice when the hormone is infused if labour is protracted. The second action is to cause contraction of the muscular elements surrounding the alveoli and milk ducts in the breasts, thereby helping to expel the milk (milk let-down). Oxytocin is ... found in men, but its role is not clear, although it has been suggested that it aids sperm transport during mating.
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Oxytocin is a peptide, which is produced in the paraventricular nuclei of the hypothalamus and is stored in the posterior pituitary. Oxytocin consists of nine amino acids linked with a [1-6] disulfide bond and a carboxyamidated tail. A hormone once thought to be limited to female smooth muscle reproductive physiology, more current findings have determined that oxytocin ... functions as a neurotransmitter, may be involved in neuropsychiatric disorders, social/sexual behavior and is important in male reproductive physiology. Oxytocin and the related peptide, Arg8-Vasopressin, maintain renal water and sodium balance. Highly conserved across species boundaries, oxytocin-like neurohypophysial peptides are substituted primarily at residues 4 and/or 8. Acting in classical endocrine fashion, Oxytocin elicits regulatory effects by binding specific cell surface receptors which in turn initiate a secondary intracellular response cascade via a phosphoinositide signaling pathway.
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Oxytocin ... has activities of cytokines. Oxytocin is capable of replacing the IL2 requirement for induction of IFN-gamma in mouse spleen cell cultures (Johnson et al, 1985). Oxytocin plays a role in mediating ACTH secretion in the rat induced by IL1-beta (Watanobe et al, 1995). IL6 up-regulates the Oxytocin receptor in cultured uterine smooth muscle cells and cultured myocytes (Rauk et al, 1989; Schmid et al, 2001).
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Oxytocin's physical effects on humans include uterine contractions just prior to birth that enable cervical dilation, and the contractions during the second and third stages of labor. It is ... involved in the letdown reflex in breastfeeding that releases milk from the myoepithelial cells into the lactation ducts. When breastfeeding for the first few weeks, this dual action of oxytocin can cause mildly painful contractions in the uterus. It also plays a little-understood role in orgasm; oxytocin levels in the blood of both males and females are significantly elevated during sexual arousal and orgasm. It may facilitate sperm transport in ejaculation through the same mechanism it uses to induce the letdown reflex.
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Oxytocin appears to stimulate the growth of myoepthelial cells both in vitro and in vivo. When administered with estrogen and progesterone via implantable pellets into a virgin moue mammary gland it ... enhanced myoepthelial differentiation of the cap cells surrounding the terminal end bud (Sapino et al., 1993). However, in the mammary gland its more characteristic and physiological action is to promote contraction of the myoepithelial cells. In an elegant study Olins and Bremel (Olins & Bremel, 1984) found that oxytocin stimulated the phosphorylation of the myosin light chain in rat mammary myoepithelial cells within 30 seconds of binding. Influx of extracellular calcium ions regulated the duration of the response. There is also evidence for involvement of intracellular calcium stores from studies of the effects of oxytocin on tissue pieces (Da Costa et al., 1995).
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Oxytocin is syntheized in the hypothalamus in specific nuclei, the paraventricular nucleus and the supraoptic nucleus in the hypothalamus. [A cluster of nerve cells in the brain is often called a nucleus. This is different from the nucleus of a single cell.] Neurons in these hypothalamic nuclei synthesize the oxytocin precursor and package it into vesicles. Oxytocin is initially synthesized as a large molecular weight precursor which ... consists of the oxytocin-carrier peptide neurophysin. The precursor is proteolytically cleaved in the neuron in the oxytocin-containing vesicle to yield oxytocin bound to neurophysin. The oxytocin-neurophysin complex is the intracellular storage form of oxytocin.
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Oxytocin