The Glutamatergic Projection from the Substantia Nigra Pars Reticulata to the Dorsal Raphe Nucleus Facilitates Social Hierarchy in Mice

Social hierarchy constitutes a fundamental organizational characteristic among various social species, significantly influencing individual survival, health, and reproductive success within these societies. Neurons in the substantia nigra pars reticulata (SNr) exhibit extensive connectivity with the dorsal raphe nucleus (DRN), a critical structure implicated in social interaction, reward processing, and the establishment of social rank. However, the specific neuronal types within the SNr, as well as the associated neural circuits that regulate social dominance, remain inadequately characterized. This study aims to elucidate the crucial role of SNr glutamatergic neurons in the establishment and maintenance of social hierarchy in mice. Employing fiber photometry, we observed that the activation of SNr glutamatergic neurons increased during the initiation of effortful behaviors in the tube test. Further investigations revealed that optogenetic activation or chemogenetic inhibition of the glutamatergic neural circuit connecting the SNr to the DRN induced upward or downward shifts in social ranks, respectively. Additionally, our findings indicate that the activation of SNr glutamatergic terminals in the DRN reduces anxiety levels in mice. Collectively, these results underscore the critical role of the glutamatergic pathway from the SNr to the DRN in regulating social hierarchy. This work enhances our understanding of the functions of SNr glutamatergic neurons in both physiological contexts and neurological disorders.