Summary Measuring the activity of many individual neurons at once while knowing their wiring diagrams would provide exciting information on how the components of a network interact. Knowledge of wiring diagrams has rapidly improved due to advances in the field of connectomics, and capabilities for simultaneous measurement of many individual neurons has increased exponentially with large-scale recording techniques. However, it is still difficult to combine such measurements. Registering high-resolution imaging for tracing neural projections with electrophysiological measurements, such as electrode arrays, is extremely difficult. With optical imaging, such tracing is possible, but neural activity measurements are often limited to particular geometries, most commonly a single plane in z. Although new imaging advances for volumetric imaging have eased this limitation somewhat, complicated instrumentation puts such technologies out of reach for most labs. This proposal addresses this challenge by using multicolor aequorin-fluorescent proteins (Aeq-FPs) as both fluorescent structural tracers and functional indicators for recording calcium activity. Aeq-FPs are bioluminescent indicators of calcium concentration that emit light from the entire cell including the dendritic and axonal arbors. In the proposed scheme, each neuron will express a unique combination of Aeq-FP colors so that it is color-coded to have its own spectral signature. The activity of individual neurons can be distinguished from the spectrum of the emitted bioluminescence without resolving the spatial position of the origin of the light. This enables simultaneous recording of the activity of many cells in arbitrary spatial arrangements including from different layers in the cortex. Connected networks are identified by limiting expression of the Aeq-FPs to neurons that are one synapse away from “starter” cells using transsynaptic viral vectors (modified rabies for retrograde transport and adeno- associated viruses (AAVs) for anterograde transport). The unique color combinations expressed in each cell also facilitate structural tracing. With these combined technologies, the network of microcircuits defined by connectivity to a single “starter” cell will be traced in three dimensions and correlated to measurements of activity in a single trial. In Aim 1, the starter cell is postsynaptic from the network, so this data will show how the presynaptic network involving multiple different types of cells from across cortical layers affects starter cell activity. In Aim 2, the starter cell is presynaptic to the labeled network and will express channelrhodopsin. Optically stimulating the starter cell will show how the network activity is affected by the modulation of the single cell. Such measurement capabilities will enable new types of experiments relating structure and activity and could be readily adopted by many labs.

Narrative Neurons are connected through intricately branched axons and dendrites into complex networks. The capability to measure the activity of multiple neurons at once and also map out the connections of those networks would be a powerful tool for learning how these structures govern activity patterns. Bioluminescent aequorin-fluorescent proteins are used to make both of these measurements in the same network.