Molecular aspects of neurohypophysial hormone release.

The secretion of hormone from the nerve term inals in the infundibular process of the neurohypophysis is mediated by the arrival of impulses generated in cell bodies of neurons in the anterior hypothalam us and propagated along the fibres of the hypothalamo-hypophysial tract. Influx of Ca2+ into the secretory cells is believed to play a part in hormone release. I t has been proposed th a t Ca2+ may act either by causing dissociation of the hormones from complexes formed with the protein neurophysin, or by promoting extracellular extrusion of the contents of the neurosecretory particles (exocytosis). Evidence is presented th a t allo­ steric transitions are involved in the interaction between neurophysin and neurohypophysial hormone and the possibility th a t these m ight determine some of the changes in the u ltra ­ structure of nerve term inals in stim ulated neurohypophyses is discussed. The occurrence of neurophysin-like proteins in extracts from porcine kidney, mammary gland, uterus and serum has been dem onstrated and m ust be considered when critical experimental tests for the exocytosis theory are evaluated.