Anatomical Connectivity of the Intercalated Cells of the Amygdala.

The Intercalated Cells of the Amygdala (ITCs) are a fundamental processing structure in the amygdala that remain relatively understudied. They are phylogenetically conserved from insectivores through primates, inhibitory, and project to several of the main processing and output stations of the amygdala and basal forebrain. Through these connections, the ITCs are best known for their role in conditioned fear, where they are required for fear extinction learning and recall. Prior work on ITC connectivity is limited, and thus holistic characterization of their afferent and efferent connectivity in a genetically defined manner is incomplete. The ITCs express the FoxP2 transcription factor, affording genetic access to these neurons for viral input-output mapping. To fully characterize the anatomical connectivity of the ITCs, we used cre-dependent viral strategies in FoxP2-cre mice to reveal the projections of the main (mITC), caudal (cITC), and lateral (lITC) clusters along with their presynaptic sources of innervation. Broadly, the results confirm many known pathways, reveal previously unknown ones, and demonstrate important novel insights about each nucleus's unique connectivity profile and relative distributions. We show that the ITCs receive information from a wide range of cortical, subcortical, basal, amygdalar, hippocampal and thalamic structures, and project broadly to areas of the basal forebrain, hypothalamus, and entire extent of the amygdala. The results provide a comprehensive map of their connectivity and suggest that the ITCs could potentially influence a broad range of behaviors by integrating information from a wide array of sources throughout the brain. Significance Statement The Intercalated Cells of the Amygdala (ITCs) are a fundamental processing structure in the amygdala, yet their anatomy has not been fully characterized. We present a comprehensive input-output mapping of the different clusters using cell-type specific viral approaches combined with quantification of anatomical strength. We confirm the limited existing anatomical data, and importantly, identify previously unknown connectivity with novel implications for function. Moreover, we find that different clusters have unique connectivity, suggesting they serve different processing functions. Thus, we provide for the first time, a full view of the anatomical connectivity of these different ITC clusters. This work should provide a foundation for future studies of this processing center in the amygdala.