In vivo imaging of intersynaptic vesicle exchange using VGLUT1 Venus knock-in mice.

The vesicular glutamate transporter VGLUT1 loads synaptic vesicles with the neurotransmitter glutamate and thereby determines glutamate release at many synapses in the mammalian brain. Due to its function and selective localization, VGLUT1 is one of the most specific markers for glutamatergic synaptic vesicles. It has been used widely to identify glutamatergic synapses, and its expression levels are tightly correlated with changes in quantal size, modulations of synaptic plasticity, and corresponding behaviors. We generated a fluorescent VGLUT1(Venus) knock-in mouse for the analysis of VGLUT1 and glutamatergic synaptic vesicle trafficking. The mutation does not affect glutamatergic synapse function, and thus the new mouse model represents a universal tool for the analysis of glutamatergic transmitter systems in the forebrain. Previous studies demonstrated synaptic vesicle exchange between terminals in vitro. Using the VGLUT1(Venus) knock-in, we show that synaptic vesicles are dynamically shared among boutons in the cortex of mice in vivo. We provide a detailed analysis of synaptic vesicle sharing in vitro, and show that network homeostasis leads to dynamic scaling of synaptic VGLUT1 levels.