Abstract The human brain is estimated to contain over 100 billion neurons that are wired together by more than 100 trillion synapses. These synaptic connections increase the computational capabilities of neural circuits and are essential for sensation, perception, learning and memory, and the selection and expression of distinct behavioral states. While many tools now exist to activate, inhibit, or modulate specific cell types in the brain, none are currently capable of manipulating activity between user-defined pre- and postsynaptic cell types. As a result, our knowledge of the roles played by specific synaptic connections and how they contribute to information processing and behavior is still quite limited. Here we propose a fundamentally unique approach to optically control the activity of user-defined synaptic connections between specific cell types. We will develop a two-component system to enable optically reversible, synapse-selective inhibition using trans-synaptic activation of presynaptic inhibitory GPCRs (trans-OptoGi). In Aim 1, we will use high-throughput GPCR screens to engineer light-sensitive ligand: GPCR pairs not expressed in the mammalian brain. We will functionally validate these candidates using trans-cellular imaging assays, optimize their neuronal trafficking, test their activity at both excitatory and inhibitory synaptic connections in acute brain slices, and validate their efficacy in vivo to modulate mouse behavior. In Aim 2, we will build upon this approach to optically control the activity of endogenous presynaptic GPCRs in the brain using similar validation steps. Importantly, these genetically-encoded trans-synaptic tools will use common adeno-associated viral (AAV) gene delivery methods and light sources currently used for standard optogenetic experiments in many neuroscience labs. All constructs generated will be freely shared with the research community. Successful development of these tools will enable reversible, synapse-specific manipulations, which are noticeably lacking in the current neuroscience toolbox.

Project Narrative The goal of this project is to develop new light-sensitive tools to dynamically control user-defined sets of connections between brain cells called synapses. This will enable researchers to determine how specific synapses contribute to neural circuit functions in the brain in normal and disease states.