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A peptide is defined as low molecular weight polymers of less than 40 amino acids joined together by amide linkages. Oxytocin, often abbreviated OXT, is included in this group and is a nonapeptide, meaning that it contains a sequence of nine peptides. The sequence is cysteine, tyrosine, isoleucine, glutamine, asparagine, cysteine, proline, leucine, and glycine [CYIQNCPLG]. One molecule of Oxytocin has a molecular mass of 1,007 daltons, and its molecular sequence is C43H66N12O12S2. One international unit of Oxytocin is the equivalent of about two micrograms of pure peptide.


Oxytocin is made in magnocellular neurosecretory cells and superoptic nucleus and paraventricular nucleus of the hypothalamus, and is stored or released into the blood by the posterior lobe of the pitituary gland. Oxytocin is also made by neurons in the paraventricular nucleus that projects to other parts of the brain and to the spinal cord. While males and females have different amounts of the peptide in their bodies, males synthesize Oxytocin in the same regions of the hypothalamus as in females, and also within the testes and perhaps other reproductive tissues. In the pitituary gland, Oxytocin is packaged in large dense core vesicles where it is bound to neurophysin I. Neurophysin is a large peptide fragment, the giant precourser molecule, from which Oxytocin is derived by enzymatic cleavage.


Secretion of Oxytocin from the neurosecretory nerve endings is regulated by the electrical activity of Oxytocin in the cells and the hypothalamus. These cells generate action potentials that propogate the axons to the nerve endings in the pitituary; the endings contain large numbers of Oxytocin-containing vesicles, which are released by exocytosis when the nerve terminals are depolarized.

Both the production of Oxytocin and the response to Oxytocin are modulated by circulating levels of sex steroids in the bloodstream. Pregnant women have large amounts of Oxytocin that are released at the birth of their child, mostly due to movement of the fetus in the uterus. However, this increase in production can also be attributed to abruptly declining concentrations of progesterone, because as progesterone decreases, Oxytocin concentrations increase.

The most important stimulus for release of hypothalamic Oxytocin is initiated by physical stimulation. (The act of nursing or suckling is relayed within a few milliseconds via a spinal reflex arc) These symbols depend on Oxytocin secreting neurons, leading to its release. A number of factors that can inhibit Oxytocin release include acute stress. For example, Oxytocin’s neurons are repressed by catecholamines, which are released by the adrenal gland in response to many types of stress, including fright.

Continued: Part 2, Uses and Side Effects of Oxytocin

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Last updated October 2013