Identifying the diversity of neuronal cell types of the nervous system is one of the main objectives of the BRAIN Initiative, with the vision that distinct neuronal identities will allow for their selective manipulation and reveal their functional contributions in health and disease. However, identifying all the cell types of the mammalian brain is not a trivial undertaking and is hindered by the fact that a satisfactory definition of neuronal cell type is nonexistent, with terms like “class”, “subclass”, “type”, and “subtype” often used interchangeably without proper definition. As part of the BICCN (BRAIN Initiative Cell Census Network), we have developed a practical, robust, and multi-pronged workflow to systematically classify projection neuron types based on their precise anatomical location, long-range connectivity, input/output organization, and detailed neuronal morphology. These connectivity-based neuronal classification strategies can be integrated or validated with other cell type specific information, such as molecular identities (mRNA expression, epigenomics, or genetic labeling), electrophysiological properties, and functional specificities. In this panel presentation, Dr. Hong-Wei Dong will present our recent progress of constructing a whole mouse brain wiring diagram, which lays an anatomical frame for characterizing neuronal cell types. Dr. Giorgio Ascoli will present our progress of developing informatics tools to reconstruct, analyze, and present cell type-specific morphology and a novel shotgun projectome strategy. Dr. Byungkook Lim will present our effort of developing cutting edge tracing technologies to dissect cell type- and projection- specific neural circuits.

Learning Objectives:

1. Constructing a whole-brain wiring diagram

2. Anatomical characterization of neuron cell types based on anatomical locations, projection targets, and neuronal morphology

3. Genetically dissect cell type-specific neural circuits