This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. A new rabies virus-based method for mapping monosynaptic connections in neuronal circuits will be used to (1) label neurons and pre-synaptic terminals associated with those neurons and (2) determine if all pre-synaptic terminals impinging on the identified postsynaptic neuron are trans-synaptically marked by this approach. NCMIR developments in labeling chemistry for correlated light microscopy (LM) and electron microscopy (EM) will be extended to work with this system enabling high-resolution assessment of labeled terminals and associated structures in well-preserved specimens. The use of the new method being developed to produce [unreadable][unreadable][unreadable]multicolor EM[unreadable][unreadable][unreadable] images will also be explored to enable layering of these methods to address questions regarding detailed distribution of multiple molecular constituents in identified synaptic complexes.