Oxytocin in the Initiation of Labor

  • Rosemary D. Leake


A single cell (magnocellular neuron) is responsible for synthesis, transport, and release of oxytocin. Oxytocin prohormone is synthesized in single neuronal cells of the supraoptic and paraventricular portions of the hypothalamus. The prohormone consists of the nonapeptide oxytocin, glycine-lysine-arginine, and a specific 93- to 95-amino-acid carrier protein, the neurophysin. The prohormone is synthesized in the rough endoplasmic reticulum of the neurosecretory neurons. It is then packaged into secretory granules for transport along the magnocellular neurons terminating in the posterior pituitary and median eminence. Collateral projections also extend to the brainstem and/or spinal cord. During axonal transport the prohormone is enzymatically converted to the nonapeptide hormone and its neurophysin. Ultimately, granules are stored in terminal bulbs in the posterior pituitary gland. Polarization of the neurosecretory neurons results in exocytic release of oxytocin and its neurophysin from the granules into the circulation. Functionally, circulating oxytocin is uterotonic and affects milk ejection. Oxytocin is also found in the corpus luteum testis, and spinal cord; its function in these sites is unclear. The function of the oxytocin-specific neurophysin is also unclear. However, the oxytocin neurophysin, like the vasopressin- specific neurophysin, may be involved in the regulation of prolactin secretion.1 This possibility is supported by the observation that prolactin immunoreactivity has been noted in the oxytocinergic portion of the magnocellular neurosecretory system.2


Corpus Luteum Diabetes Insipidus Nonpregnant Woman Uterine Contraction Magnocellular Neuron 
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Copyright information

© Plenum Press, New York 1990

Authors and Affiliations

  • Rosemary D. Leake
    • 1
  1. 1.Department of Pediatrics, School of MedicineUniversity of California Los Angeles, Harbor/UCLA Medical CenterTorranceUSA

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