Diversity and connectivity of principal neurons in the lateral and basal nuclei of the mouse amygdala

The basolateral amygdala is a non-layered cortical structure playing a role in various cognitive processes. Despite many studies focusing on local information processing within the circuits of the basolateral amygdala, the characteristics of excitatory principal neurons (PNs) are still not fully revealed. Here, we combined neuroanatomical, electrophysiological, and tracing techniques to determine the single-cell features, dendritic and axonal projections of PNs within the lateral (LA) and basal amygdala (BA). Using a mouse reporter line, we found that cholecystokinin (CCK) expression defines two spatially and functionally segregated groups of PNs both in the LA and BA. PNs in CCK-positive (CCK+) areas of the LA (LAa) had small somata and short dendrites which matched their single-cell electrophysiological properties. PNs in CCK-negative (CCK-) areas of the LA (LAp) and all BA had similarly ramified dendrites and single-cell features with some differences. Importantly, the dendritic arbors of PNs were restricted to the subnuclei defined by CCK expression, which corresponded to various extra-amygdalar afferents indicating specific inputs on distinct PN groups. Axonal arborization patterns of PNs within the basolateral amygdala and surrounding areas showed consistency to their soma location. For instance, BA PNs that projected to the medial prefrontal cortex but not to the lateral nucleus of the central amygdala were present in CCK+ areas. In contrast, those BA PNs that projected to the lateral part of the central nucleus were found in the subnucleus lacking CCK. Our study revealed that the basolateral amygdala is composed of functionally different subnuclei with specific inputs and outputs. This structural arrangement may empower the LA and BA to flexibly channel processed information toward their downstream regions, which can be a key requirement for diverse amygdala functions in cognitive operation.