  Since the early work of Bernard Katz, the process of cellular chemical
communication via exocytosis, quantal release, has been considered to be all or
none. Recent evidence has shown exocytosis to be partial or 'subquantal' at
single-cell model systems, but there is a need to understand this at
communicating nerve cells. Partial release allows nerve cells to control the
signal at the site of release during individual events, where the smaller the
fraction released, the greater the range of regulation. Here we show that the
fraction of the vesicular octopamine content released from a living Drosophila
larval neuromuscular neuron is very small. The percentage of released molecules
was found to be only 4.5% for simple events and 10.7% for complex (i.e.,
oscillating or flickering) events. This large content, combined with partial
release controlled by fluctuations of the fusion pore, offers presynaptic
plasticity that can be widely regulated. Two works published in 2010 suggested
that the Katz principle, [1] was incorrect for all-or-none release and that
only part of the chemical load of vesicles was released during exocytosis, at
least as measured as a full spike during amperometry. [2] The combination of
electrochemical methods to measure both release and vesicle content in 2015
added a wealth of information to support the concept of partial release in
exocytosis. [3] Additionally, this has recently been supported by work with
TIRF microscopy showing 'subquantal' release from vesicles in adrenal
chromaffin cells and using super-resolution STED microscopy. [4] It appears
that the full event generally involves release of only part of the load of
chemical messenger in single-cell model systems like adrenal chromaffin and
PC12 cells. Is this also true at living neurons in a nervous system and to what
extent? To answer this critical question, we quantified the number of
octopamine molecules in the neuromuscular neurons of Drosophila larvae by
adapting an amperometric technique developed in our
