Monitoring single-synapse glutamate release and presynaptic calcium concentration in organised brain tissue.

Brain function relies in large part on Ca2+-dependent release of the excitatory neurotransmitter glutamate from neuronal axons. Establishing the causal relationship between presynaptic Ca2+ dynamics and probabilistic glutamate release is therefore a fundamental quest across neurosciences. Its progress, however, has hitherto depended primarily on the exploration of either cultured nerve cells or giant central synapses accessible to direct experimental probing in situ. Here we show that combining patch-clamp with time-resolved imaging of Ca2+ -sensitive fluorescence lifetime of Oregon Green BAPTA-1 (Tornado-FLIM) enables readout of single spike-evoked presynaptic Ca2+ concentration dynamics, with nanomolar sensitivity, in individual neuronal axons in acute brain slices. In parallel, intensity Tornado imaging of a locally expressed extracellular optical glutamate sensor iGluSnFr provides direct monitoring of single-quantum, single-synapse glutamate releases in situ. These two methods pave the way for simultaneous registration of presynaptic Ca2+ dynamics and transmitter release in an intact brain at the level of individual synapses.