Dendrites of hypothalamic magnocellular neurons release neurohypophysial peptides by exocytosis.

Exocytosis of neurosecretory granules from dendrites of magnocellular neurons can be visualized electron microscopically after incubation of hypothalamic brain slices in media containing 1.2 mM tannic acid, which stabilizes extracellular peptidergic granule cores, and permits their immunocytochemical identification. Morphometric analysis of stimulated slices demonstrates that exocytosis of neurosecretory granules from the dendrites of magnocellular neurons can account for the vasopressin and oxytocin known to be released into the hypothalamus. Exocytosis from cell bodies of magnocellular neurons was not observed in stimulated slices from normal rats but, when granules had been caused to accumulate in the neuronal somata by prior administration of colchicine, exocytosis of granules from the somata was unambiguously identified. These data demonstrate exocytosis from dendrites and cell bodies of a mammalian peptidergic neuron, and show that all parts of their plasmalemma are competent for exocytosis of granules.