Current Biology
Volume 22, Issue 21, 6 November 2012, Pages 2008-2016
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Article
Direct Activation of Sleep-Promoting VLPO Neurons by Volatile Anesthetics Contributes to Anesthetic Hypnosis

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Summary

Background

Despite seventeen decades of continuous clinical use, the neuronal mechanisms through which volatile anesthetics act to produce unconsciousness remain obscure. One emerging possibility is that anesthetics exert their hypnotic effects by hijacking endogenous arousal circuits. A key sleep-promoting component of this circuitry is the ventrolateral preoptic nucleus (VLPO), a hypothalamic region containing both state-independent neurons and neurons that preferentially fire during natural sleep.

Results

Using c-Fos immunohistochemistry as a biomarker for antecedent neuronal activity, we show that isoflurane and halothane increase the number of active neurons in the VLPO, but only when mice are sedated or unconscious. Destroying VLPO neurons produces an acute resistance to isoflurane-induced hypnosis. Electrophysiological studies prove that the neurons depolarized by isoflurane belong to the subpopulation of VLPO neurons responsible for promoting natural sleep, whereas neighboring non-sleep-active VLPO neurons are unaffected by isoflurane. Finally, we show that this anesthetic-induced depolarization is not solely due to a presynaptic inhibition of wake-active neurons as previously hypothesized but rather is due to a direct postsynaptic effect on VLPO neurons themselves arising from the closing of a background potassium conductance.

Conclusions

Cumulatively, this work demonstrates that anesthetics are capable of directly activating endogenous sleep-promoting networks and that such actions contribute to their hypnotic properties.

Highlights

► Hypnotic doses of isoflurane directly depolarize a subset of hypothalamic neurons ► This subset in VLPO is electrophysiologically identical to those that regulate sleep ► Lesioning VLPO produces acute resistance to induction with isoflurane ► Systems for promoting natural sleep and anesthesia are mechanistically entwined

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