Mitochondrial Function in Astrocytes Is Essential for Normal Emergence from Anesthesia in Mice
Authors: Ramadasan-Nair Renjini , Hui Jessica , Itsara Leslie S. , Morgan Philip G. , Sedensky Margaret M. 
Published: February 2019

Abstract

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Background
In mice, restriction of loss of the mitochondrial complex I gene Ndufs4 to glutamatergic neurons confers a profound hypersensitivity to volatile anesthetics similar to that seen with global genetic knockout of Ndufs4. Astrocytes are crucial to glutamatergic synapse functioning during excitatory transmission. Therefore, the authors examined the role of astrocytes in the anesthetic hypersensitivity of Ndufs4(KO).

Methods
A tamoxifen-activated astrocyte-specific Ndufs4(KO) mouse was constructed. The specificity of the astrocyte-specific inducible model was confirmed by using the green fluorescent protein reporter line Ai6. Approximately 120 astrocyte-specific knockout and control mice were used for the experiments. Mice were anesthetized with varying concentrations of isoflurane or halothane; loss of righting reflex and response to a tail clamp were determined and quantified as the induction and emergence EC50s. Because norepinephrine has been implicated in emergence from anesthesia and astrocytes respond to norepinephrine to release gliotransmitters, the authors measured norepinephrine levels in the brains of control and knockout Ndufs4 animals.

Results
The induction EC50s for tail clamp in both isoflurane and halothane were similar between the control and astrocyte-specific Ndufs4(KO) mice at 3 weeks after 4-hydroxy tamoxifen injection (induction concentration, EC50(ind)—isoflurane: control = 1.27 ± 0.12, astrocyte-specific knockout = 1.21 ± 0.18, P = 0.495; halothane: control = 1.28 ± 0.05, astrocyte-specific knockout = 1.20 ± 0.05, P = 0.017). However, the emergent concentrations in both anesthetics for the astrocyte-specific Ndufs4(KO) mice were less than the controls for tail clamp; (emergence concentration, EC50(em)—isoflurane: control = 1.18 ± 0.10, astrocyte-specific knockout = 0.67 ± 0.11, P < 0.0001; halothane: control = 1.08 ± 0.09, astrocyte-specific knockout = 0.59 ± 0.12, P < 0.0001). The induction EC50s for loss of righting reflex were also similar between the control and astrocyte-specific Ndufs4(KO) mice (EC50(ind)—isoflurane: control = 1.02 ± 0.10, astrocyte-specific knockout = 0.97 ± 0.06, P = 0.264; halothane: control = 1.03 ± 0.05, astrocyte-specific knockout = 0.99 ± 0.08, P = 0.207). The emergent concentrations for loss of righting reflex in both anesthetics for the astrocyte-specific Ndufs4(KO) mice were less than the control (EC50(em)—isoflurane: control = 1.0 ± 0.07, astrocyte-specific knockout = 0.62 ± 0.12, P < 0.0001; halothane: control = 1.0 ± 0.04, astrocyte-specific KO = 0.64 ± 0.09, P < 0.0001); N ≥ 6 for control and astrocyte-specific Ndufs4(KO) mice. For all tests, similar results were seen at 7 weeks after 4-hydroxy tamoxifen injection. The total norepinephrine content of the brain in global or astrocyte-specific Ndufs4(KO) mice was unchanged compared to control mice.

Conclusions
The only phenotype of the astrocyte-specific Ndufs4(KO) mouse was a specific impairment in emergence from volatile anesthetic-induced general anesthesia. The authors conclude that normal mitochondrial function within astrocytes is essential for emergence from anesthesia.