Isoform-specific exocytotic protein mRNA expression in hypothalamic magnocellular neurons: Regulation after osmotic challenge

Exocytosis is regulated by proteins which interact to promote docking and f usion of secretory granules with the plasma membrane. We have used in situ hybridization to study the mRNA expression for vesicle-associated membrane protein (VAMP) isoforms VAMP-1 and VAMP-2, synaptosomal-associated protein of 25-kDa (SNAP-25) isoforms SNAP-25a and SNAP-25b, mammalian homologue of unc18 (munc-18) and Hrs-2 in neurosecretory neurons of the magnocellular pa raventricular (PVN) and supraoptic (SON) nuclei of normal and osmotically c hallenged animals. In PVN and SON neurons of normal animals, strong labelin g was demonstrated for VAMP-2 and SNAP-25a mRNA, whereas VAMP-1 or SNAP-25b mRNA could not be detected. Salt-loading (2% NaCl as drinking water), an a nimal model which increases the expression and secretion of hormones from h ypothalamic magnocellular neurons, resulted in significantly increased mRNA levels for VAMP-2 (36%, 28%), munc-18 (74%, 68%) and SNAP-25a (59%, 77%) i n the PVN and SON, respectively. There was no significant increase in Hrs-2 mRNA levels in the PVN, whereas a significant increase (22%) was observed in the SON. In the posterior pituitary, immunohistochemistry showed a marke d decrease in numbers and intensity of vasopressin-immunoreactive (-IR) ner ve endings after salt-loading. There were no obvious changes in numbers or intensity of VAMP-2-, munc-18-, Hrs-2- or SNAP-25-IR fibers. Large varicosi ties containing VAMP-2- and Hrs-2 immunocreactivity were seen in salt-loade d animals. The results show isoform-specific mRNA expression in neurosecret ory neurons and an increased mRNA expression of proteins participating in t he molecular regulation of exocytosis during an experimental situation char acterized by increased secretion. Copyright (C) 1999 S. Karger AG, Basel.