Part 1: Image-Mapped Tutorial
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1 - Synthesis of Vasopressin and Oxytocin
The cells bodies located within the supraoptic and paraventricular nuclei of the hypothalamus produce peptide (short chain protein) hormones called vasopressin and oxytocin. The supraoptic nuclei lie in a pair just above the optic chiasm whereas the paraventricular nuclei lie to either side of the third ventricle. Hence, these nuclei are named based on their location./p>
2 - Transport of Vasopressin and Oxytocin
Following the production of vasopressin and oxytocin in the neurosecretory cells of the hypothalamus, these hormones are packaged in vesicles and transported down the axons of the neurons to the posterior pituitary.
3 - Release of Vasopressin and Oxytocin
Oxytocin and vasopressin are transported down the axons of the same neurons that synthesize them and accumulate in the neurons' terminal buttons located in the posterior pituitary. They are stored in the axonal endings until their release into the bloodstream is triggered by an action potential. Upon release, oxytocin stimulates contraction of smooth muscle in the uterus during labor and contraction of the secretory ducts of the mammary glands during nursing inducing the ejection of milk. Vasopressin or antidiuretic hormone helps to regulate blood pressure by increasing the reabsorption of water in the kidneys.
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The release of vasopressin is determined primarily by osmoreceptors in the hypothalamus (Blaser, 1999). These receptors sense the osmotic pressure of blood causing an increase in vasopressin secretion in response to increased osmolarity (increased particle, decreased plasma fluid in the blood). Blood plasma osmolality is normally maintained at 285 mosmol/kg. When osmolality reaches 290, vasopressin is released. Vasopressin is also released in response to stimulation of baroreceptors located in the circulatory vessels. A decrease in blood pressure triggers an increase in secretion of vasopressin. Once released, vasopressin acts on the renal collecting tubule cells of the kidney through a membrane receptor that activates adenyl cyclase. Adenyl cyclase produces adenosine monophosphate (cAMP), which acts as a second messenger to activate intracellular kinases. These enzymes (kinases) cause the intracellular water channels of the kidneys to fuse with the membrane of the tubule lumen. This allows the free flow of water in the cell from the luminal fluid. This free water is then quickly transferred to the normally hyperosmolar extracellular fluid of the renal medulla. When vasopressin is absent, the channels remain in the intracellular space and the luminal membrane is impermeable to water. In addition to its effect on kidney function, vasopressin causes constriction of blood vessel walls, thereby raising blood pressure when needed.
Until the isolation of oxytocin in the early 1950's, all hormones were thought to be steroid molecules. Oxytocin, perhaps the phylogenetically oldest of the neuropeptides, was the neuropeptide to be identified. Oxytocin and vasopressin are members of a nine amino acid peptide family that appear to have evolved with the emergence of mammals. In addition to being released during childbirth and nursing, oxytocin is released during sexual orgasm and is involved in the modulation of some parental, caregiving behaviors (Insel, 1992). A very recent series of experiments indicates that the release of oxytocin from hypothalamic neurons is also triggered by emotional stress. The temporal and spatial pattern of this stress response may be controlled by the integration of activity at multiple brain levels including the hypothalamus and posterior pituitary. Furthermore, recent research indicates the interactive role of histamine and norepinephrine in the control of secretion of both vasopressin and oxytocin from the supraoptic and paraventricular nuclei of the hypothalamus.
| References |
Blaser, B. (1999). Introduction to internal medicine - The pituitary gland [Hebrew University Medical School course support materials]. Retrieved May 15, 2000 from the World Wide Web: http://www.md.huji.ac.il/md/pituitary/
Insel, T.R. (1992). Oxytocin- a neuropeptide for affiliation: evidence from behavioral, receptor autoradiographic, and comparative studies. Psychoneuroendocrinology, 17(1), 3-35.