Regulation of states of consciousness by supramammillary nucleus glutamatergic neurones during sevoflurane anaesthesia in mice.

BACKGROUND

The supramammillary nucleus (SuM), located in the caudal hypothalamus, includes wake-promoting glutamatergic neurones. Their potential role in regulating states of consciousness during general anaesthesia remains unknown.

METHODS

We used in vivo fibre photometry, c-Fos staining, chemogenetic and optogenetic manipulations, and electroencephalography/electromyography to explore the roles of glutamatergic SuM neurones (SuM neurones) at different phases of sevoflurane anaesthesia. Rabies-mediated retrograde and anterograde tract tracing were used to investigate the monosynaptic glutamatergic inputs from the medial septum (MS) to SuM. Their roles in sevoflurane anaesthesia were investigated by in vivo fibre photometry and optogenetic manipulations.

RESULTS

The population activity of SuM neurones decreased at loss of consciousness but increased during recovery of consciousness under sevoflurane anaesthesia. Their activity also decreased during suppression but increased during bursts in sevoflurane-induced burst-suppression oscillations. Activating SuM neurones chemogenetically or optogenetically decreased sensitivity to sevoflurane, induced behavioural arousal and cortical activation during continuous steady-state anaesthesia, and stable burst-suppression oscillations under sevoflurane. In contrast, chemogenetic or optogenetic inhibition of SuM neurones increased sensitivity to sevoflurane or intensified cortical inhibition during sevoflurane anaesthesia. Retrograde and anterograde tracing verified monosynaptic projections from MS neurones to SuM neurones. The activity of MS SuM terminals increased during loss of consciousness but recovered during recovery of consciousness. Optogenetic activation or inhibition of MS SuM terminals induced cortical activation or inhibition, respectively, during sevoflurane anaesthesia.

CONCLUSIONS

Activation of SuM neurones or the glutamatergic septo-supramammillary circuit induces behavioural arousal and cortical activation during sevoflurane anaesthesia.