Molecular Logic of Cell Diversity and Circuit Connectivity in the REM Sleep Hub

The complexity of the brain arises from the diversity of its circuits and the molecular heterogeneity of the cells that compose them. A mechanistic understanding therefore requires mapping cellular identity and connectivity at single-cell resolution. Here we define the cellular taxonomy of the murine sublaterodorsal tegmental nucleus (SLD), a critical hub for REM sleep, using single-nucleus RNA sequencing. We identified all major brain cell classes, with oligodendrocytes as the most abundant, and resolved seventeen transcriptionally distinct neuronal groups defined by neurotransmitters, neuromodulators, and neuropeptides, each with unique molecular signatures. Projection-specific analysis further revealed that glutamatergic subpopulations targeting the ventrolateral periaqueductal gray (vlPAG) and ventral medulla are molecularly distinct, marked by characteristic receptor motifs. Strikingly, we provide the first direct evidence that SLD ^GLUT neurons innervate the vlPAG. This newly uncovered SLD ^GLUT→vlPAG pathway represents a previously unrecognized circuit node for REM sleep regulation, with the potential to act as a REM-OFF population suppressing it. Together, these findings establish a transcriptionally resolved atlas of the SLD, reveal the molecular logic of its circuit connectivity, and nominate candidate molecular actuators of REM sleep control, opening new avenues for dissecting how brainstem circuits orchestrate REM state and its transitions.