Interpreting the identification of glutamatergic inputs to the sublaterodorsal tegmental nucleus and their role in REM sleep control: insights and limitations

Dear Editor, We read with interest the recent report by Ngo et al. [1] mapping glutamatergic afferents to the sublaterodorsal tegmental nucleus (SLD) and examining their role in REM sleep. This study employs an impressive combination of pathway tracing, calcium fiber photometry, and optogenetics to chart input sources to SLD glutamate neurons and to probe their activity during sleep—wake cycles. Identifying widespread inputs—from cortex as well as from pontine and medullary regions—significantly advances our understanding of the REM-generating circuitry. In particular, the finding that ventrolateral PAG (vlPAG) glutamatergic neurons increase activity just before REM onset and decrease at REM offset suggests a role in gating state transitions. Such mapping of brain-wide glutamate pathways to the SLD is novel and lays important groundwork for dissecting how REM is initiated and terminated. Moreover, the use of optogenetic manipulation (presumably stimulation/inhibition of specific inputs) provides initial functional evidence that these pathways can modulate REM probability. Together, these findings place SLD glutamatergic neurons in a broader network context and hint at how brainstem and subcortical excitatory inputs drive REM sleep onset.