PATCH-CLAMP experiments have shown that fusion of secretory granules w
ith the plasma membrane does not always occur as an all-or-none event,
but can develop slowly in a fluctuating manner or can be transient1-5
. These observations suggested that release could be detected during s
uch incomplete fusion events. To test this hypothesis we have combined
patch-clamp measurements of the activity of single exocytotic fusion
pores in beige mouse mast cells with the electrochemical detection of
serotonin released during the exocytotic events. We report here that o
n fusion pore opening there is a small release of serotonin which is d
irectly proportional to the pore conductance. We also show that a sign
ificant release occurs during transient fusion events. These results d
emonstrate, to our knowledge for the first time, release of a neurotra
nsmitter from a secretory vesicle that did not undergo complete fusion
.