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 (GABA^BNST) 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 GABA^BNST 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 GABA^BNST terminals in the DpMe during NREM sleep elicited rapid transitions to wakefulness and increased activity of glutamatergic DpMe (GLUT^DpMe) neurons, as assessed by <i>c-fos</i> mRNA expression and calcium imaging. Similarly, an aversive air-puff activated GLUT^DpMe neurons, suggesting engagement by emotionally salient stimuli. Ablation of GLUT^DpMe neurons markedly attenuated arousal responses triggered by GABA^BNST stimulation, underscoring their essential role in this circuit. While monosynaptic rabies tracing revealed local input neurons to GLUT^DpMe cells, in situ hybridization identified few <i>Vgat-</i>positive interneurons among them. These findings suggest that GABA^BNST neurons may influence GLUT^DpMe 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.