We have developed two new tools for tracing neural connections. The first is a means of identifying cells that project to a region of interest within the brain, illuminating these cells with intense fluorescence and allowing their finest morphological details to be seen. This permits either highly detailed reconstruction of the cells' anatomy or their targeting, in live tissue, on the basis of their morphology for physiological study. The second tool is a means of identifying, on a large scale, neurons which are directly presynaptic to either a targeted cell type or indeed a single neuron. Because this technique also results in intensely fluorescent cellular details, this too could be used either for detailed anatomical studies or for physiological ones. This should allow a significantly more precise understanding of the organization of nervous systems than has previously been possible

A comprehensive characterization of neuronal cell types, their distributions, and patterns of connectivity is critical for understanding the properties of neural circuits and how they generate behaviors. Here we review the experiences of the BRAIN Initiative Cell Census Consortium, ten pilot projects funded by the U.S. BRAIN Initiative, in developing, validating, and scaling up emerging genomic and anatomical mapping technologies for creating a complete inventory of neuronal cell types and their connections in multiple species and during development. These projects lay the foundation for a larger and longer-term effort to generate whole-brain cell atlases in species including mice and humans.

The basolateral amygdalar complex (BLA) is implicated in behaviors ranging from fear acquisition to addiction. Optogenetic methods have enabled the association of circuit-specific functions to uniquely connected BLA cell types. Thus, a systematic and detailed connectivity profile of BLA projection neurons to inform granular, cell type-specific interrogations is warranted. Here, we apply machine-learning based computational and informatics analysis techniques to the results of circuit-tracing experiments to create a foundational, comprehensive BLA connectivity map. The analyses identify three distinct domains within the anterior BLA (BLAa) that house target-specific projection neurons with distinguishable morphological features. We identify brain-wide targets of projection neurons in the three BLAa domains, as well as in the posterior BLA, ventral BLA, posterior basomedial, and lateral amygdalar nuclei. Inputs to each nucleus also are identified via retrograde tracing. The data suggests that connectionally unique, domain-specific BLAa neurons are associated with distinct behavior networks.