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

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.

Rabies virus (RV)-mediated monosynaptic retrograde tracing and anterograde tracing were first used to identify the projection from glutamatergic MS neurons (MSGlu) to glutamatergic neurons in the lateral hypothalamus (LH, LHGlu). 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 MSGlu neurons to the LH (MSGlu-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.

The retrogradely labeled upstream neurons of LHGlu 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 MSGlu-LHGlu 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 MSGlu-LH circuit was inhibited concurrently as the process of loss of consciousness during 2.4% sevoflurane induction. Optogenetic activation of the MSGlu-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 MSGlu-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 MSGlu-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).

This study reveals that activation of the glutamatergic MS-LH circuit promotes emergence from sevoflurane GA.