Organization of brainwide inputs to discrete lateral septum projection populations.

The lateral septum (LS) is anatomically positioned to play a critical role in directing information from the hippocampus and cortex to downstream subcortical structures, such as the hypothalamus. In fact, early anatomical tracing studies investigated the organization of hippocampal inputs to the LS and its hypothalamic outputs to begin to understand how its structure might relate to its function. These studies also characterized the cellular anatomy of the LS and its organization of different molecular markers. However, little is known about the organization of non-hypothalamic projection populations within the LS and what types of input these different projection populations receive. We used retrograde tracing to determine the organization of LS projections to six brain regions that mediate various social behaviors in male mice, specifically, the basolateral amygdala (BLA), bed nucleus of the stria terminalis (BNST), nucleus accumbens (NAc), periaqueductal gray (PAG), ventromedial hypothalamus (vmH) and ventral tegmental area (VTA). We found that these projection populations occupy discrete anatomical compartments within the LS. We then used a monosynaptic rabies tracing strategy in male mice to map brainwide inputs to these six discrete LS projection populations and examine how different brain regions innervate them. We identified unique region-dependent patterns of inputs to individual LS projection populations. In particular, we observed differences in cortical, hippocampal and thalamic innervation of the six different LS projection populations, while the hypothalamic inputs were largely similar across projection populations. Thus, this study provides insight into the anatomical connectivity that may underlie the functional heterogeneity of the LS.Significance Statement The lateral septum plays a crucial role in social behavior. However, the organization of its diverse projection populations remains poorly understood. This study provides the first comprehensive brainwide mapping of inputs to lateral septum neurons projecting to six distinct downstream regions involved in social behaviors. Using retrograde and monosynaptic rabies tracing, the authors reveal that these lateral septum projection populations are anatomically organized and receive distinct sets of inputs, particularly from cortical, hippocampal, and thalamic subregions. These findings demonstrate a high level of input specificity and spatial organization within the lateral septum, suggesting a circuit-level mechanism by which the lateral septum may selectively route information to shape diverse social behaviors.