Oxytocin release, control

Clarke G, Lincoln DW, Merrick LP (1979) Dopaminergic control of oxytocin release in lactating rats. J Endocrinol 55 409-420. 

If increase in y causes a change in x, which tends to increase y, feedback is said to be positive in other words, a further signal output is evoked by the response it induces or provokes. This is intrinsically an unstable system, but there are physiologic situations where such control is valuable. In the positive feedback situation, the signal output will continue until no further response is required. Suckling provides an example stimulation of nipple receptors by the suckling child provokes an increased oxytocin release from the posterior pituitary with a corresponding increase in milk flow. Removal of the stimulus causes cessation of oxytocin release. 

These messengers also play a role in regulating contraction of myometrium, which consists of smooth muscle fibres. Contraction is controlled by increases in the concentration of cytosolic Ca ions. Prostaglandins activate Ca ion channels in the plasma membrane of the fibres oxytocin activates release of Ca from intracellular stores. The increase in concentration of Ca ions leads to activation of myosin light-chain kinase which leads to crossbridge cycling and contraction (as described in Chapter 22 Figure 22.12). 

G-protein-coupled receptors.79 A defect in the type 2 vasopressin receptor leads to the condition of nephrogenic diabetes insipidus in which the body fails to concentrate the urine.77 80 Oxytocin acts on smooth muscles of the uterus during childbirth and triggers the release of milk from the mammary glands.81 The latter response is partially controlled by the suckling of the infant, which induces the nervous system to release oxytocin into the bloodstream. 

The hypothalamic control of the posterior pituitary is quite different than that of the anterior and intermediate lobes. Specific neurons have their cell bodies in certain hypothalamic nuclei. Cell bodies in the paraventricular nuclei manufacture oxytocin, whereas the supraoptic nuclei contain cell bodies that synthesize ADH. The axons from these cells extend downward through the infundibulum to terminate in the posterior pituitary. Hormones synthesized in the hypothalamic cell bodies are transported down the axon to be stored in neurosecretory granules in their respective nerve terminals (located in the posterior pituitary). When an appropriate stimulus is present, these neurons fire an action potential, which causes the hormones to release from their pituitary nerve terminals. The hormones are ultimately picked up by the systemic circulation and transported to their target tissues. 

Oxytocin is a hormone involved in controlling the onset of labour in women and the subsequent release of milk. It was the first peptide hormone to be synthesized, in 1953, and the synthetic version of the hormone, Syntooinon (identical, of course, with the natural version isolated... 

Vasopressin plays a role in the control of blood pressure by regulating contraction of smooth muscle. Like oxytocin, vasopressin is released by the action of the hypothalamus on the posterior pituitary and is transported by the blood to specific receptors. 

Vasopressin and oxytocin or one or several precursors of them are produced in the pericarya of cells in certain nuclei of the hypothalamus and maybe also within the axons leading from these cell bodies to the neural lobe of the pituitary. The hormones or their precursors are transported in neurosecretory granules from the perikarya of the cells to the neural lobe where they are stored and from where the controlled release takes place. The appropriate (or inappropriate) stimuli for release produce nerve signals, the last pathway of which is the nerve membrane surrounding the terminal swellings of the axon. There is no information on the speed with which the neurosecretory granules move within the axons. 

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