Brain-wide mapping of efferent projections of glutamatergic (Onecut3⁺ ) neurons in the lateral mouse hypothalamus

Aim: This study mapped the spatiotemporal positions and connectivity of Onecut3⁺ neuronal populations in the developing and adult mouse brain.

Methods: We generated fluorescent reporter mice to chart Onecut3⁺ neurons for brain-wide analysis. Moreover, we crossed Onecut3-iCre and Mapt-mGFP (Tau-mGFP) mice to visualize axonal projections. A dual Cre/Flp-dependent AAV construct in Onecut3-iCre cross-bred with Slc17a6-FLPo mice was used in an intersectional strategy to map the connectivity of glutamatergic lateral hypothalamic neurons in the adult mouse.

Results: We first found that Onecut3 marks a hitherto undescribed Slc17a6⁺ /Vglut2⁺ neuronal cohort in the lateral hypothalamus, with the majority expressing thyrotropin-releasing hormone. In the adult, Onecut3⁺ /Vglut2⁺ neurons of the lateral hypothalamus had both intra- and extrahypothalamic efferents, particularly to the septal complex and habenula, where they targeted other cohorts of Onecut3⁺ neurons and additionally to the neocortex and hippocampus. This arrangement suggests that intrinsic reinforcement loops could exist for Onecut3⁺ neurons to coordinate their activity along the brain's midline axis.

Conclusion: We present both a toolbox to manipulate novel subtypes of hypothalamic neurons and an anatomical arrangement by which extrahypothalamic targets can be simultaneously entrained.