Targeting of green fluorescent protein to secretory granules in oxytocin magnocellular neurons and its secretion from neurohypophysial nerve terminals in transgenic mice.

Oxytocin (OT) is a hypothalamic nonapeptide that is synthesized as part of a larger precursor protein that also contains an approximately 10-kDa protein called neurophysin at its C-terminus. This precursor protein is trafficked through the regulated secretory pathway into secretory granules and then axonally transported to and secreted from nerve terminals in the neural lobe of the pituitary. In this paper, we show that the AI-03 transgene that contains enhanced green fluorescent protein (EGFP) fused to the end of the neurophysin at the C-terminus of the OT pre-prohormone, is expressed selectively in OT-magnocellular neurons and is trafficked to secretory granules in transgenic mice. The EGFP-containing secretory granules are then transported to OT-neurosecretory terminals in the neurohypophysis, where the EGFP fluorescence undergoes depolarization-induced calcium-dependent secretion. The endogenous fluorescence in the neural lobes is sufficiently intense to image secretory events in individual OT nerve terminals (neurosecretosomes) isolated from the posterior pituitaries in these transgenic mice.