Connectivity and phenotype of vasopressin 1a receptor cells in the lateral septum

The neuropeptide arginine-vasopressin (AVP) regulates sexually-differentiated social behaviors, including sexual behavior, aggression, and social communication. Much of AVP's effect on social behavior is mediated by the most widely-expressed AVP receptor in the central nervous system: the vasopressin 1a receptor (V1aR). Dense expression of V1aR is found in males and females in the lateral septum (LS), which also receives heavy input from sexually-differentiated populations of AVP cells in the extended amygdala. In order to access and interrogate the structure and function of V1aR cells, we developed and validated a V1aR-Cre knockin mouse line (Avpr1a-P2A-iCre). V1aR-Cre mice did not differ from their Cre-negative littermates in their health, sensorimotor function, anxiety/motivational behavioral responses, or in their V1aR binding levels in the LS. The distribution of Cre in the brain, as assessed by crosses with Cre-reporter mouse lines and in situ hybridization (ISH), was highly similar to patterns of V1aR binding and demonstrated strong colocalization between Cre and V1aR expression within LS cells. We used this V1aR-Cre mouse to identify the inputs and outputs of V1aR+ cells in the LS, via a monosynaptic retrograde rabies virus as well as anterograde synaptophysin-mRuby AAV tracing approaches. Monosynaptic inputs to V1aR LS cells were observed from the medial preoptic area (MPA), lateral hypothalamus (LH), hippocampus (mostly ventral), medial septum, diagonal band of Broca (DBB), and the supramammillary nucleus (SuM). Synaptophysin labeling revealed outputs to some of these same structures (MS, LH, DBB, SuM) as well as parts of the lateral preoptic area (LPO), anterior hypothalamic area (AHA), and ventral pallidum. Notably, most structures (except for hippocampus) bidirectionally connected to V1aR LS cells also contain populations of V1aR+ cells and are targets for BNST/MeA AVP cells, suggesting an integrated AVP/V1aR circuit. Finally, using ISH, we measured levels of V1aR mRNA expression in subregions of the LS and colocalization of V1aR with oxytocin receptor (OTR) mRNA, which also has a high affinity for AVP, within the LS. We found similar high percentages of cells containing V1aR+ puncta across dorsal and intermediate LS in both males and females. In contrast, the ventral LS contained fewer V1aR+ cells in both males and females. The highest level of co-expression of V1aR and OTR was found in the intermediate LS in both sexes, suggesting the possible location of functional interactions between AVP and oxytocin within the LS. We also sought to identify other phenotypic aspects of V1aR cells using ISH and confirmed that all LS V1aR cells are GABAergic and that most also express corticotropin-releasing hormone receptor 2, suggesting a substrate by which AVP could interact with stress-related systems in the LS. Our characterized V1aR-Cre mouse will be a valuable resource for understanding the role of AVP/V1aR in behavioral and physiological systems. Using this mouse, we have characterized the connectional architecture of V1aR cells in the LS and revealed an interlocking set of structures that may be the substrate through which AVP regulates social and emotional behavior.