Neurotransmitter release: the dark side of the vacuolar-H+ATPase.

Vacuolar-H+ATPase (V-ATPase) is a complex enzyme with numerous subunits organized in two domains. The membrane domain V0 contains a proteolipid hexameric ring that translocates protons when ATP is hydrolysed by the catalytic cytoplasmic sector (V1). In nerve terminals, V-ATPase generates an electrochemical proton gradient that is acid and positive inside synaptic vesicles. It is used by specific neurotransmitter-proton antiporters to accumulate neurotransmitters inside their storage organelles. During synaptic activity, neurotransmitters are released from synaptic vesicles docked at specialized portions of the presynaptic plasma membrane, the active zones. A fusion pore opens that allows the neurotransmitter to be released from the synaptic vesicle lumen into the synaptic cleft. We briefly review experimental data suggesting that the membrane domain of V-ATPase could be such a fusion pore. We also discuss the functional implications for quantal neurotransmitter release of the sequential use of the same V-ATPase membrane domain in two different events, neurotransmitter accumulation in synaptic vesicles first, and then release from these organelles during synaptic activity.