Cell-type specific parallel circuits in the bed nucleus of the stria terminalis and the central nucleus of the amygdala of the mouse

The central extended amygdala (EAc) is a forebrain macrosystem which has been widely implicated in fear, anxiety and pain. Its two key structures, the lateral bed nucleus of the stria terminalis (STL) and the central nucleus of amygdala (CeA), share similar mesoscale connectivity. However, it is not known whether they also share similar cell-specific neuronal circuits. We addressed this question using tract-tracing and immunofluorescence to reveal the connectivity of two neuronal populations expressing either protein kinase C delta (PKCδ) or somatostatin (SOM). PKCδ and SOM are expressed predominantly in the dorsal part of STL (STLD) and in the lateral/capsular parts of CeA (CeL/C). We found that, in both STLD and CeL/C, PKCδ+ cells are the main recipient of extra-EAc inputs from the external lateral part of the parabrachial nucleus (LPBE), while SOM+ cells are the sources of long-range projections to extra-EAc targets, including LPBE and periaqueductal gray. PKCδ+ cells can also integrate inputs from posterior basolateral nucleus of amygdala or insular cortex. Within EAc, PKCδ+, but not SOM+ neurons, serve as the major source of projections to the ventral part of STL and to the medial part of CeA. However, both cell types mediate interconnections between STLD and CeL/C. These results unveil the pivotal positions of PKCδ and SOM neurons in organizing parallel cell-specific neuronal circuits of CeA and STL, which further support the idea of EAc as a structural and functional macrostructure.