Single-vesicle imaging reveals different transport mechanisms between glutamatergic and GABAergic vesicles

Neurotransmitter uptake one vesicle at a time Despite opposing ionic gradients, synaptic vesicles are able to accumulate neurotransmitters. To resolve the mystery of how this happens, Farsi et al. made parallel measurements of pH gradients and membrane potential at the single synaptic vesicle level. Glutamatergic and GABAergic vesicles had different uptake mechanisms, revealing insights into the energetic and ionic coupling of vesicular neurotransmitter transport. Science, this issue p. 981 GABA transport into vesicles relies on the counter flow of protons, whereas glutamate uptake does not. Synaptic transmission is mediated by the release of neurotransmitters, which involves exo-endocytotic cycling of synaptic vesicles. To maintain synaptic function, synaptic vesicles are refilled with thousands of neurotransmitter molecules within seconds after endocytosis, using the energy provided by an electrochemical proton gradient. However, it is unclear how transmitter molecules carrying different net charges can be efficiently sequestered while maintaining charge neutrality and osmotic balance. We used single-vesicle imaging to monitor pH and electrical gradients and directly showed different uptake mechanisms for glutamate and γ-aminobutyric acid (GABA) operating in parallel. In contrast to glutamate, GABA was exchanged for protons, with no other ions participating in the transport cycle. Thus, only a few components are needed to guarantee reliable vesicle filling with different neurotransmitters.