Imaging chemical neurotransmission with genetically encoded fluorescent sensors

A major challenge in neuroscience is to decipher the logic of neural circuitry and to link it to learning, memory, and behavior. Synaptic transmission is a critical event underlying information processing within neural circuitry. In the extracellular space, the concentrations and distributions of excitatory, inhibitory, and modulatory neurotransmitters impact signal integration, which in turn shapes and refines the function of neural networks. 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. Here, we review approaches and probes that have been employed to determine the spatial and temporal extent of neurotransmitter dynamics in the brain. We specifically focus on the design, screening, characterization, and application of genetically encoded indicators directly probing glutamate, the most abundant excitatory neurotransmitter. These indicators provide synaptic resolution of glutamate dynamics with cell-type specificity. We also discuss strategies for developing a suite of genetically encoded probes for a variety of neurotransmitters and neuromodulators.