Direct Activation of Sleep-Promoting VLPO Neurons by Volatile Anesthetics Contributes to Anesthetic Hypnosis

Abstract

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, while 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.

Designer receptor manipulations reveal a role of the locus coeruleus noradrenergic system in isoflurane general anesthesia

“…Blockade of either of these receptors inhibited CNO-facilitated emergence here, reciprocally validating that enhanced NE from LC-hM3Dq activation mediates accelerated emergence. Hypnotic doses of isoflurane have been shown to activate putative sleep-promoting ventrolateral preoptic (VLPO) area neurons; complementary to its excitatory actions in other regions, NE specifically hyperpolarizes the isoflurane-sensitive population of VLPO neurons (38). In the present study, we have shown that LC-NE activation also antagonizes induction of isoflurane anesthesia.…”

Serotonin differentially modulates excitatory and inhibitory synaptic inputs to putative sleep-promoting neurons of the ventrolateral preoptic nucleus

“…In accordance with previous reports, the VLPO PSP neurons were identified by their inhibitory response to bath application of NA (50 mM for 30 s) and were named as NA (-) neurons, whereas the neighboring neurons were excited by NA application (i.e. NA (þ) neurons) (Gallopin et al, 2005(Gallopin et al, , 2000Liu et al, 2013;Moore et al, 2012). The VLPO neurons were then recorded in whole-cell patch-clamp configuration, and their electrophysiological properties were analyzed in currentclamp mode.…”

“…The inhibition of these VLPO neurons by wake-promoting neurotransmitters is in agreement with their inactivity during wakefulness, and suggests that they should correspond to the sleep-active cells recorded in vivo and that are involved in the inhibition of arousal systems (Saper et al, 2001;Szymusiak et al, 1998). Henceforth, sleep-promoting neurons have been identified according to their inhibitory response to bath application of noradrenaline (NA) (Gallopin et al, 2005(Gallopin et al, , 2000Liu et al, 2013;Moore et al, 2012;Saint-Mleux et al, 2004;Varin et al, 2015). In contrast to other neurotransmitters that promote wakefulness, 5-HT was shown to induce opposite responses, as revealed by ex vivo electrophysiological recordings of presumed sleep-promoting (PSP) neurons in the rat VLPO.…”

What is unconsciousness in a fly or a worm? A review of general anesthesia in different animal models

“…Yet these lesions resulted in chronic sleep deprivation, suggesting the VLPO is still important for sleep homeostasis. Further electrophysiological studies have shown that isoflurane can directly activate the sleep-promoting VLPO neurons in the mammalian brain (Moore et al, 2012), and in the fly, acute activation of sleep-promoting neurons increases sleep and produces isoflurane hypersensitivity, whereas acutely blocking these fly neurons has the opposite effect . This indicates that there is a connection between sleep and general anesthesia in mammals and flies, yet it is clear that sleep pathways are not the sole mechanism for general anesthesia.…”