Influence of the Glutamatergic MS-LH Circuit Activation on Emergence From Sevoflurane Anesthesia in Mice

BACKGROUND: Glutamatergic neurons in the medial septum (MS) are identified to promote emergence from sevoflurane general anesthesia (GA), with the potential downstream neural circuit remaining to be explored. METHODS: Rabies virus (RV)-mediated monosynaptic retrograde tracing and anterograde tracing were first used to identify the projection from glutamatergic MS neurons (MS Glu ) to glutamatergic neurons in the lateral hypothalamus (LH, LH Glu ). In vivo fiber photometry, optogenetic bidirectionally manipulations, electroencephalogram/electromyogram (EEG/EMG), and behavioral tests were further employed to investigate the role of the circuit from MS Glu neurons to the LH (MS Glu -LH circuit) in regulating states of consciousness under two different states of sevoflurane GA: continuous, steady-state general anesthesia (CSSGA) and burst-suppression (BS) oscillations. RESULTS: The retrogradely labeled upstream neurons of LH Glu neurons were extensively detected in the MS, and most RV-infected neurons in the MS were co-labeled by Vesicular glutamate transporter 2 (Vglut2, mean ± standard error of the mean [SEM], 86.3% ± 1.5%, n = 4 mice). And the MS Glu -LH Glu circuit constitutes the highest proportion among the three downstream LH neuronal populations (presynaptic boutons co-localized ratio: glutamatergic, 77.0% ± 2.2%; γ-aminobutyric acid-ergic, 59.6% ± 0.9%; orexinergic, 28.5% ± 2.0%; n = 4 mice). The calcium activity of the MS Glu -LH circuit was inhibited concurrently as the process of loss of consciousness during 2.4% sevoflurane induction. Optogenetic activation of the MS Glu -LH circuit promoted behavioral arousal and increased β power of EEG (stimulation vs pre-stimulation, 16.8% ± 2.3% vs 9.7% ± 1.7%, P =.0065; n = 8 mice) during CSSGA. In contrast, during CSSGA, optogenetic inhibition of the MS Glu -LH projection deepened cortical inhibition, characterized by increased δ power and decreased power of β and γ (inhibition vs pre-inhibition, δ: 62.4% ± 4.5% vs 55.3% ± 4.2%, P =.0404; β: 6.7% ± 0.8% vs 9.4% ± 1.3%, P =.0069; γ: 3.3% ± 0.6% vs 4.8% ± 0.8%, P =.0076; n = 8 mice). Optogenetic bidirectionally manipulations of the MS Glu -LH circuit induced similar effects during BS: activation of this projection resulted in cortical activation with decreased burst-suppression ratio (BSR; median [25%–75% percentiles], stim vs pre, 59.0% [43.8%–64.5%] vs 77.5% [74.0%–84.3%], P =.0121; n = 8 mice), while inhibition of this projection led to cortical inhibition with increased BSR (inhib vs pre, 76.1% ± 7.0% vs 64.5% ± 8.4%, P =.0382; n = 8 mice). CONCLUSIONS: This study reveals that activation of the glutamatergic MS-LH circuit promotes emergence from sevoflurane GA.