Subsets of Nanometer Vesicles in the Fly Release Differential Fractions of Vesicular Serotonin Content during Exocytosis.

Serotonin, a monoamine neurotransmitter, is important in both the central nervous system (CNS) and the peripheral nervous system. Malfunction of serotonin signaling leads to various disorders. We studied serotonin signaling from serotonergic neurons inside the ventral nerve cord of Drosophila melanogaster. Serotonergic neurons and stimulated release were visualized and achieved with mCherry and channelrhodopsin-2 (an optogenetically transfected ion channel), respectively, and two electrochemical techniques quantified serotonin release and vesicular content. Mean vesicular serotonin content released during exocytosis from these neurons was 84 %, considerably higher than reported in previous studies regarding octopamine (4.5 %) and glutamate release (31 %). Serotonin content within all vesicles is uniformly changed when serotonin concentration is inhibited or enhanced. However, serotonin release exhibits two Gaussian distributions: higher frequency of small release events, and similar or slightly higher frequency of large events, resulting in differential release fractions ranging from partial (13-18 %) to full (100 %) release after treatment with agents to either enhance or diminish release. This is the first example of consistent full exocytotic release events we have observed in any system. We suggest one pool of vesicles can release significantly diverse fractions of transmitter load during exocytosis, a potentially novel pathway to regulate exocytosis and neuronal signaling.