Methods for anterograde tracing of active circuits

Project Summary A central yet unrealized goal of modern neuroscience is to map all the neural circuits within the brain and the changes that occur during development, learning, aging, and in disease states. Towards this end, we recently developed a rationally designed method for anterograde transsynaptic tracing from excitatory presynaptic neurons that is monosynaptic, activity-dependent, and non-toxic and provides genetic access to postsynaptic cells. This rational approach has many advantages over traditional methods based on viruses' intrinsic transsynaptic labeling capabilities. We have shown that in our method, the tracer is released presynaptically and binds to a postsynaptic protein before being taken up by the postsynaptic cell. The tracer can be fused to Cre or other recombinases, allowing genetic labeling and access to the postsynaptic cell. Here, using a similar approach, we will develop a new method for anterograde tracing from inhibitory neurons. This will represent the first method capable of tracing from genetically determined inhibitory starter cells in an anterograde direction, monosynaptically, and without retrograde transmission or toxicity. This method will be particularly valuable because other methods of anterograde tracing that depend on bulk labeling or electrophysiological activation of postsynaptic cells cannot be used for tracing circuits that originate from inhibitory neurons. In addition, we will develop an analogous tracer for cholinergic circuits, which cannot currently be efficiently traced by viruses either in the anterograde or retrograde directions. Finally, we will develop a method for labeling and providing genetic access to cells that have been either excited or inhibited by specific presynaptic neurons over a prescribed time period. This application takes advantage of the unique activity dependence of the transsynaptic labeling techniques that we have developed.