Expression of exocytosis proteins in rat supraoptic nucleus neurons

In magnocellular neurons of the supraoptic nucleus (SON), the neuropeptides vasopressin and oxytocin are synthesized and packaged into large dense-cored vesicles (LDCVs). These vesicles undergo regulated exocytosis from nerve terminals in the posterior pituitary gland and from somata/dendrites in the SON. Regulated exocytosis of LDCVs is thought to involve the SNARE complex (comprising VAMP-2, syntaxin-1 and SNAP-25) and regulatory proteins (such as synaptotagmin 1, munc-18 and CAPS 1). Using fluorescent immunocytochemistry and confocal microscopy, in both oxytocin and vasopressin neurons, we observed VAMP-2, SNAP-25 and syntaxin-1-immunoreactivity in axon terminals. The somata and dendrites contained syntaxin-1 and other regulatory exocytosis proteins including munc-18 and CAPS 1. However, the distribution of VAMP-2 and synaptotagmin 1 in the SON was limited to putative pre-synaptic contacts as they co-localised with synaptophysin (synaptic vesicle marker) and had no co-localization with either oxytocin or vasopressin. SNAP-25 immunoreactivity in the SON was limited to glial cell processes and was not detected in oxytocin or vasopressin somata/dendrites. The present results indicate differences in expression and localization of exocytosis proteins between the axon terminals and somata/dendritic compartment. The absence of VAMP-2 and SNAP-25 immunoreactivity from the somata/dendrites suggests that there might be different SNARE protein isoforms expressed in these compartments. Alternatively, exocytosis of LDCVs from somata/dendrites may use a different mechanism from that described by the SNARE complex theory.