The Bed Nucleus of the Stria Terminalis-Deep Mesencephalic Nucleus Circuit Linking Emotion and Wakefulness.

The bed nucleus of the stria terminalis (BNST), a part of the extended amygdala, integrates emotional and arousal-related signals. While GABAergic BNST (GABABNST) neurons have been implicated in promoting transitions from non-rapid eye movement (NREM) sleep to wakefulness, their downstream mechanisms remain unclear. Here, we identify a neuronal circuit through which GABABNST neurons promote arousal via projections to a midbrain region known as the deep mesencephalic nucleus (DpMe), located within the broader mesencephalic reticular formation. In male mice, we used a combination of optogenetics, fiber photometry, neural ablation, and tracing approaches to dissect this circuit. Optogenetic stimulation of GABABNST terminals in the DpMe during NREM sleep elicited rapid transitions to wakefulness and increased activity of glutamatergic DpMe (GLUTDpMe) neurons, as assessed by c-fos mRNA expression and calcium imaging. Similarly, an aversive air-puff activated GLUTDpMe neurons, suggesting engagement by emotionally salient stimuli. Ablation of GLUTDpMe neurons markedly attenuated arousal responses triggered by GABABNST stimulation, underscoring their essential role in this circuit. While monosynaptic rabies tracing revealed local input neurons to GLUTDpMe cells, in situ hybridization identified few Vgat-positive interneurons among them. These findings suggest that GABABNST neurons may influence GLUTDpMe neurons through noncanonical GABAergic mechanisms or via more complex local circuits beyond a simple disinhibition model. Together, these findings delineate a previously uncharacterized BNST-DpMe circuit that allows emotionally relevant stimuli to override sleep and promote arousal. This pathway may contribute to stress-related sleep disturbances and represents a potential target for therapeutic treatments for sleep disorders associated with emotional dysregulation.