Imaging glutamate with genetically encoded fluorescent sensors

Superimposed on the vast and complex synaptic network is a largely invisible set of chemical inputs, such as neurotransmitters and neuromodulators, that exert a profound influence on brain function across many structures and temporal scales. Thus, the determination of the spatiotemporal relationships between these chemical signals with synaptic resolution in the intact brain is essential to decipher the codes for transferring information across circuitry and systems. Recent advances in imaging technology have been employed to determine the extent of spatial and temporal neurotransmitter dynamics in the brain, especially glutamate, the most abundant excitatory neurotransmitter. Here, we discuss recent imaging approaches, particularly with a focus on the design and application of genetically encoded indicator iGluSnFR, in analyzing glutamate transients in vitro, ex vivo, and in vivo.