Imaging of evoked dense-core-vesicle exocytosis in hippocampal neurons reveals long latencies and kiss-and-run fusion events

Abstract

Evoked neuropeptide secretion in the central nervous system occurs slowly, but the basis for slow release is not fully understood. Whereas exocytosis of single synaptic vesicles in neurons and of dense-core vesicles (DCVs) in endocrine cells have been directly visualized, single DCV exocytic events in neurons of the central nervous system have not been previously studied. We imaged DCV exocytosis in primary cultured hippocampal neurons using fluorescent propeptide cargo and total internal reflectance fluorescence microscopy. The majority of Ca2+-triggered exocytic events occurred from immobile plasma-membrane-proximal DCVs in the cell soma, whereas there were few events in the neurites. Strikingly, DCVs in the cell soma exhibited 50-fold greater release probabilities than those in neurites. Latencies to depolarization-evoked fusion for DCVs were surprisingly long, occurring with an average time constant (τ) of 16 seconds for DCVs in the soma and even longer for DCVs in neurites. All of the single DCV release events exhibited rapid fusion-pore openings and closures, the kinetics of which were highly dependent upon Ca2+ levels. These 'kiss-and-run' events were associated with limited cargo secretion. Thus, the slow evoked release of neuropeptides could be attributed to very prolonged latencies from stimulation to fusion and transient fusion-pore openings that might limit cargo secretion.