An essential role for orexins in emergence from general anesthesia

Parafacial GABAergic neurone ablation induces behavioural resistance to volatile anaesthetic-induced hypnosis without reducing sleep

BACKGROUND

It is hypothesised that general anaesthetics co-opt the neural circuits regulating endogenous sleep and wakefulness to produce hypnosis. To further probe this association, we focused on the GABAergic neurones of the parafacial zone (PZGABA), a brainstem site capable of promoting non-rapid eye movement sleep.

METHODS

To determine whether PZ neurones are activated by a hypnotic dose of anaesthetics, c-Fos immunohistochemistry was performed. The behavioural and physiological contributions of PZGABA neurones to anaesthetic sensitivity were assessed in mice transfected with an adeno-associated virus (AAV)-driving expression of an mCherry fluorescent control or a caspase that irreversibly eliminates PZGABA neurones. EEG-defined sleep was measured in PZGABA-ablated and mCherry control mice, as was the homeostatic drive to sleep after sleep deprivation.

RESULTS

Consistent with anaesthetic-induced depolarisation, hypnotic doses of isoflurane significantly increased c-Fos expression three-fold in PZGABA neurones compared with oxygen-exposed mice. PZGABA-ablated mice developed significant and durable behavioural resistance to both isoflurane- and sevoflurane-induced hypnosis, with roughly 50% higher likelihood of intact righting than controls. PZGABA-ablated mice emerged from isoflurane significantly faster than mCherry controls with purposeful movements. The degree of anaesthetic resistance was inversely correlated with the number of surviving PZGABA neurones. Despite confirming that PZGABA ablation reduced the potency of two distinct volatile anaesthetics behaviourally, ablation did not alter the amount of endogenous sleep or wakefulness, nor did it affect the homeostatic sleep drive after sleep deprivation, and it did not produce EEG signatures of anaesthetic resistance during isoflurane exposure.

CONCLUSIONS

There was an unexpected dissociation in which destruction of up to 70-80% of PZGABA neurones was sufficient to alter anaesthetic susceptibility behaviourally without causing insomnia or altering sleep pressure. These findings suggest that PZGABA neurones are more critical to drug-induced hypnosis than to the regulation of natural sleep and arousal.

A ventral pallidum-locus coeruleus-lateral hypothalamus pathway modulates brain arousal in freely behaving and isoflurane-anesthetized male mice

Much progress has been made in the understanding of the neural circuits associated with sleep and anesthesia. As an important component among these circuits, the forebrain nuclei have been frequently interrogated. This study demonstrates that glutamatergic (Glu) neurons in the ventral pallidum (VP) enhance activity upon salient stimuli and state-dependently modulate brain arousal and motor activity in freely behaving male mice, and bidirectionally regulate the induction of and emergence from isoflurane general anesthesia. We delineate a neural pathway, consisting of VP Glu neurons→ noradrenergic (NA) neurons in the locus coeruleus (LC)→the lateral hypothalamus (LH) in male mice, controlling the release of noradrenaline in the LH and state-dependently modulated brain arousal, motor activity, and isoflurane general anesthesia through α2a receptors in the LH. Therefore, the VPGlu-LCNA-LH pathway and α2a receptors in the LH may be promising state-dependent regulators of brain arousal in both freely behaving and anesthetized states.

A probabilistic model of behavioural emergence from general anaesthesia in mice

BACKGROUND

Time to emergence from general anaesthesia is highly variable between individuals. This variability has been attributed to individual differences in anaesthetic sensitivity. However, this hypothesis has not been verified experimentally. We explicitly test this hypothesis by quantifying emergence from anaesthesia repeatedly in the same individuals over time.

METHODS

Genetically identical adult (12-24 weeks old) male (n=40) and female (n=20) C57BL/6J mice were exposed to 2 h of isoflurane (0.90 vol%) on 10 separate occasions. Time to emergence was measured using the return of the righting reflex. Predictions of the standard effect-site pharmacokinetic-pharmacodynamic (PK-PD) model and neuronal dynamics model of stochastic fluctuations between the awake and anaesthetised states were fit to observed emergence times. Repeated steady-state assessments of the righting reflex obtained during the last 2 h of a 4-h exposure to 0.3, 0.4, 0.6, or 0.7 vol% isoflurane (n=20 per concentration) were used to determine individual probabilities of losing the righting reflex, which was defined as an individual's anaesthetic sensitivity.

RESULTS

Emergence times varied by at least two orders of magnitude after identical anaesthetic exposure. We did not find consistent inter-individual differences in emergence times. Instead, we found that variability in emergence times across trials in each individual was as large as that between two different individuals. Emergence times were not correlated across time. Consistent with previous work, we identified large individual differences in anaesthetic sensitivity which persisted on a time scale of at least 1 week. A standard PK-PD model failed to reproduce inter-trial variability. In contrast, the neuronal dynamics model reproduced both population- and individual-level variability in emergence times.

CONCLUSIONS

Stochastic state switching contributes to inherent variability in emergence from general anaesthesia. Delayed emergence occurred in a small proportion of anaesthetic exposures in a genetically homogeneous population. The neuronal dynamics model predicts that anaesthetic emergence times will be probabilistically long, which might explain delayed emergence observed in clinical settings.

Activation of Glutamatergic Neurons in the Supramammillary Nucleus Promotes the Recovery of Consciousness under Sevoflurane Anesthesia

Volatile anesthetics have been widely applied during surgery, but the potential mechanisms by which they influence loss of consciousness (LOC), anesthesia maintenance, and recovery of consciousness (ROC) from anesthesia remain largely unknown. Recent studies have suggested that anesthesia-induced unconsciousness may be due to specific interactions between neural circuits that regulate sleep and wakefulness. Supramammillary (SuM) glutamatergic neurons are essential for sleep-wakefulness regulation. However, whether SuM glutamatergic neurons are involved in the modulation of consciousness under sevoflurane anesthesia is unclear. Here, it is shown that the activity of SuM glutamatergic neurons decreased prior to sevoflurane-induced LOC and gradually increased following ROC. Selective lesioning of SuM glutamatergic neurons promoted the induction of and delayed emergence from sevoflurane anesthesia and increased sevoflurane sensitivity. In addition, optogenetic stimulation of SuM glutamatergic neurons or the SuM-MS projection promoted behavioral arousal and cortical activation under steady-state sevoflurane anesthesia (SSSA) and reduced the depth of anesthesia and caused cortical arousal under sevoflurane-induced burst-suppression conditions. Collectively, these results provide compelling evidence that SuM glutamatergic neurons contribute to regulating states of consciousness under sevoflurane anesthesia.

Deciphering consciousness: The role of corticothalamocortical interactions in general anesthesia

General anesthesia is administered to millions of individuals each year, however, the precise mechanism by which it induces unconsciousness remains unclear. While some theories suggest that anesthesia shares similarities with natural sleep, targeting sleep-promoting areas and inhibiting arousal nuclei, recent research indicates a more complex process. Emerging evidence highlights the critical role of corticothalamocortical circuits, which are involved in higher cognitive functions, in controlling arousal states and modulating transitions between different conscious states during anesthesia. The administration of general anesthetics disrupts connectivity within these circuits, resulting in a reversible state of unconsciousness. This review elucidates how anesthetics impair corticothalamocortical interactions, thereby affecting the flow of information across various cortical layers and disrupting higher-order cognitive functions while preserving basic sensory processing. Additionally, the role of the prefrontal cortex in regulating arousal through both top-down and bottom-up pathways was examined. These findings highlight the intricate interplay between the cortical and subcortical networks in maintaining and restoring consciousness under anesthesia, offering potential therapeutic targets for enhancing anesthesia management.

Substantia Innominata Glutamatergic Neurons Modulate Sevoflurane Anesthesia in Male Mice

BACKGROUND

Accumulated evidence suggests that brain regions that promote wakefulness also facilitate emergence from general anesthesia (GA). Glutamatergic neurons in the substantia innominata (SI) regulate motivation-related aversive, depressive, and aggressive behaviors relying on heightened arousal. Here, we hypothesize that glutamatergic neurons in the SI are also involved in the regulation of the effects of sevoflurane anesthesia.

METHODS

With a combination of fiber photometry, chemogenetic and optogenetic tools, behavioral tests, and cortical electroencephalogram recordings, we investigated whether and how SI glutamatergic neurons and their projections to the lateral hypothalamus (LH) regulate sevoflurane anesthesia in adult male mice.

RESULTS

Population activity of glutamatergic neurons in the SI gradually decreased upon sevoflurane-induced loss of consciousness (LOC) and slowly returned as soon as inhalation of sevoflurane discontinued before recovery of consciousness (ROC). Chemogenetic activation of SI glutamatergic neurons dampened the animals' sensitivity to sevoflurane exposure, prolonged induction time (mean ± standard deviation [SD]; 389 ± 67 seconds vs 458 ± 53 seconds; P = .047), and shortened emergence time (305 seconds, 95% confidence interval [CI], 242-369 seconds vs 207 seconds, 95% CI, 135-279 seconds; P = .004), whereas chemogenetic inhibition of these neurons facilitated sevoflurane anesthesia. Furthermore, optogenetic activation of SI glutamatergic neurons and their terminals in LH induced cortical activation and behavioral emergence from different depths of sevoflurane anesthesia.

CONCLUSIONS

Our study shows that SI glutamatergic neuronal activity facilitates emergence from sevoflurane anesthesia and provides evidence for the involvement of the SI-LH glutamatergic pathway in the regulation of consciousness during GA.

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 (SuMVglut2 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 SuMVglut2 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 SuMVglut2 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 SuMVglut2 neurones increased sensitivity to sevoflurane or intensified cortical inhibition during sevoflurane anaesthesia. Retrograde and anterograde tracing verified monosynaptic projections from MSVglut2 neurones to SuMVglut2 neurones. The activity of MSVglut2 SuM terminals increased during loss of consciousness but recovered during recovery of consciousness. Optogenetic activation or inhibition of MSVglut2 SuM terminals induced cortical activation or inhibition, respectively, during sevoflurane anaesthesia.

CONCLUSIONS

Activation of SuMVglut2 neurones or the glutamatergic septo-supramammillary circuit induces behavioural arousal and cortical activation during sevoflurane anaesthesia.

Arousal-promoting effect of the parabrachial nucleus and the underlying mechanisms: Recent advances

The parabrachial nucleus (PBN) is responsible for integrating both internal and external sensory information and controlling/regulating a wide range of physiological processes, such as feeding, thermogenesis, nociceptive and pruritic sensations, and respiration. Recently, the PBN has been found to be involved in mediating wakefulness maintenance, sleep-wake transition, exogenous neuromodulation of awakening, and arousal-promoting process triggered by drastic changes in the internal environments, such as hypercapnia, hypoxia, and hypertension. Multiple neural pathways and subpopulations of neurons are responsible for arousal-promoting effects of the PBN. The medial PBN seems to be more important for the maintenance of physiological arousal, while the lateral PBN are more crucial in mediating interoception-driven arousal. Glutamatergic projection from the PBN to the basal forebrain (BF) and GABAergic projection from the BF to the cerebral cortex GABAergic neurons are the most pivotal neural pathways for awareness-promotion. Here, we review the relevant literature in this field in recent years and emphasize the potential prospects of PBN stimulation in translational medicine for the rehabilitation of disorders of consciousness.

VCP controls KCC2 degradation through FAF1 recruitment and accelerates emergence from anesthesia

Ubiquitin-proteasomal degradation of K+/Cl- cotransporter 2 (KCC2) in the ventral posteromedial nucleus (VPM) has been demonstrated to serve as a common mechanism by which the brain emerges from anesthesia and regains consciousness. Ubiquitin-proteasomal degradation of KCC2 during anesthesia is driven by E3 ligase Fbxl4. However, the mechanism by which ubiquitinated KCC2 is targeted to the proteasome has not been elucidated. We report in cultured neuro-2a cells that the valosin-containing protein (VCP) transported ubiquitinated KCC2 to the proteasome and in mice in vivo experiments that inhibition of VCP restored KCC2 expression in the VPM and enhanced the effects of anesthesia. In cultured neuro-2a cells, propofol-induced degradation of KCC2 was inhibited by VCP inhibitor DBeQ and VCP knockout plasmid sgRNA(VCP). Propofol-induced enhanced interaction between VCP and KCC2 was inhibited by knockout of Fbxl4 or Fas-associated factor 1 (FAF1). In in vivo studies, pharmacological or genetic inhibition of VCP in the VPM significantly prevented KCC2 degradation and enhanced propofol anesthesia; these effects were abrogated by a KCC2 antagonist VU0463271. These results demonstrate that the VCP controls ubiquitin-proteasomal degradation of KCC2 dependent on FAF1 recruitment and serves as a mechanism for the ubiquitin-proteasomal degradation of KCC2, which is responsible for the subsequent emergence from anesthesia.

Role of the ventral tegmental area in general anesthesia

The ventral tegmental area (VTA), located in the midbrain, plays a pivotal role in the regulation of many important behaviors, such as reward, addiction, aversion, memory, learning, and sleep-wakefulness cycles. The majority of VTA neurons are dopaminergic neurons, although there is a significant proportion of GABAergic neurons and few glutamatergic neurons. These neuronal types project to different brain regions, thus mediating various biological functions. Therefore, the diverse roles of the VTA might depend on its heterogeneous neuronal types and projecting circuits. General anesthesia and sleep-wakefulness cycles share the feature of reversible loss of consciousness, and several common neural mechanisms underlie these two conditions. In addition to the well-known regulatory role of VTA in sleep-wakefulness, emerging evidence has demonstrated that VTA activity is also associated with promoting emergence from general anesthesia. Herein, we reviewed the literature and summarized the evidence regarding the modulation of the VTA by general anesthesia in rodents, which will improve the understanding of the modulatory mechanism of the VTA in general anesthesia.

Molecular and cellular targets of GABAergic anesthetics in the mesopontine tegmentum that enable pain-free surgery

ABSTRACT

The mesopontine tegmental anesthesia area (MPTA) is a focal brainstem locus which, when exposed to GABAergic agents, induces brain-state transitioning from wakefulness to unconsciousness. Correspondingly, MPTA lesions render animals relatively insensitive to GABAergic anesthetics delivered systemically. Using chemogenetics, we recently identified a neuronal subpopulation within the MPTA whose excitation induces this same pro-anesthetic effect. However, very few of these "effector-neurons" express synaptic γ2-containing GABAA receptor isoforms and none express extrasynaptic δ-subunit containing receptors, suggesting that they are not the direct cellular target of GABAergic agents. Here we used pharmacological tools in rats to define the molecular target(s) of GABAergics in the MPTA. GABA microinjected into the MPTA at nanomolar concentrations, selective for GABAAδ-Rs, proved to be pro-anesthetic as was blocking GABA reuptake. Likewise, low-concentration gaboxadol/THIP, also selective for GABAAδ-Rs, was effective, whereas benzodiazepines and zolpidem, which selectively target GABAAγ2-Rs, were not. The GABAergic anesthetics pentobarbital and propofol proved pro-anesthetic when applied to the MPTA at the low concentrations present in the brain after systemic dosing. Glycinergic agonists which are inhibitory, but infective on GABAAδ-Rs, and other non-GABAergic agonists tested, were at most only marginally effective. We conclude that GABAAδ-Rs are the primary molecular target of GABAergic anesthetics in the MPTA. Immunolabeling revealed that this GABAA-R isoform is expressed exclusively by a distinct subpopulation of MPTA "δ-cells" that reside in close apposition to effector neurons. This suggests that during wakefulness, δ-cells serve as inhibitory interneurons which, when silenced by GABAergic agents, disinhibit (excite) the effector-neurons, triggering transition to unconsciousness.

The activity of cholinergic neurons in the basal forebrain interferes with anesthesia-arousal process of propofol

Previous research has demonstrated that basal forebrain (BF) regulates arousal during propofol anesthesia. However, as the BF comprises cholinergic neurons alongside two other types of neurons, the specific role of cholinergic neurons has not been definitively elucidated. In our study, calcium signal imaging was utilized to monitor the real-time activities of cholinergic neurons in the BF during propofol anesthesia. Additionally, we selectively stimulated these neurons to investigate EEG and behavioral responses during propofol anesthesia. Furthermore, we specifically lesioned cholinergic neurons in the BF to investigate the sensitivity to propofol and the induction time. The results revealed that propofol suppressed calcium signals of cholinergic neurons within the BF following intraperitoneal injection. Notably, upon recovery of the righting reflex, the calcium signals partially recovered. Spectral analysis of the EEG elucidated that optical stimulation of cholinergic neurons led to a decrease in δ power underlie propofol anesthesia. Conversely, depletion of cholinergic neurons in the BF enhanced sensitivity to propofol and shortened the induction time. These findings clarify the role of cholinergic neurons in the anesthesia-arousal process, as well as the depth and the sensitivity of propofol anesthesia.

Endogenous orexin and hyperacute autonomic responses after resuscitation in a preclinical model of cardiac arrest

Objectives

The study of autonomic responses to cardiac arrest (CA) resuscitation deserves attention due to the impact of autonomic function on survival and arousal. Orexins are known to modulate autonomic function, but the role of endogenous orexin in hyperacute recovery of autonomic function post-resuscitation is not well understood. We hypothesized that endogenous orexin facilitates hyperacute cardiovascular sympathetic activity post-resuscitation, and this response could be attenuated by suvorexant, a dual orexin receptor antagonist.

Methods

A well-established 7-min asphyxial CA rat model was studied. Heart rate (HR) and blood pressure were monitored from baseline to 90-min post-resuscitation. Autonomic function was evaluated by spectral analysis of HR variability, whereby the ratio of low- and high-frequency components (LF/HF ratio) represents the balance between sympathetic/parasympathetic activities. Plasma orexin-A levels and orexin receptors immunoreactivity in the rostral ventrolateral medulla (RVLM), the key central region for regulating sympathetic output, were measured post-resuscitation. Neurological outcome was assessed via neurologic-deficit score at 4-h post-resuscitation.

Key results

A significant increase in HR was found over 25-40 min post-resuscitation (p < 0.01 vs. baseline), which was attenuated by suvorexant significantly (p < 0.05). Increased HR (from 15-to 25-min post-resuscitation) was correlated with better neurological outcomes (rs = 0.827, p = 0.005). There was no evident increase in mean arterial pressure over 25-40 min post-resuscitation, while systolic pressure was reduced greatly by suvorexant (p < 0.05). The LF/HF ratio was higher in animals with favorable outcomes than in animals injected with suvorexant over 30-40 min post-resuscitation (p < 0.05). Plasma orexin-A levels elevated at 15-min and peaked at 30-min post-resuscitation (p < 0.01 vs. baseline). Activated orexin receptors-immunoreactive neurons were found co-stained with tyrosine hydroxylase-immunopositive cells in the RVLM at 2-h post-resuscitation.

Conclusion

Together, increased HR and elevated LF/HF ratio indicative of sympathetic arousal during a critical window (25-40 min) post-resuscitation are observed in animals with favorable outcomes. The orexin system appears to facilitate this hyperacute autonomic response post-CA.

Restoring consciousness with pharmacologic therapy: Mechanisms, targets, and future directions

Severe brain injury impairs consciousness by disrupting a broad spectrum of neurotransmitter systems. Emerging evidence suggests that pharmacologic modulation of specific neurotransmitter systems, such as dopamine, promotes recovery of consciousness. Clinical guidelines now endorse the use of amantadine in individuals with traumatic disorders of consciousness (DoC) based on level 1 evidence, and multiple neurostimulants are used off-label in clinical practice, including methylphenidate, modafinil, bromocriptine, levodopa, and zolpidem. However, the relative contributions of monoaminergic, glutamatergic, cholinergic, GABAergic, and orexinergic neurotransmitter systems to recovery of consciousness after severe brain injury are unknown, and personalized approaches to targeted therapy have yet to be developed. This review summarizes the state-of-the-science in the neurochemistry and neurobiology of neurotransmitter systems involved in conscious behaviors, followed by a discussion of how pharmacologic therapies may be used to modulate these neurotransmitter systems and promote recovery of consciousness. We consider pharmacologic modulation of consciousness at the synapse, circuit, and network levels, with a focus on the mesocircuit model that has been proposed to explain the consciousness-promoting effects of various monoaminergic, glutamatergic, and paradoxically, GABAergic therapies. Though fundamental questions remain about neurotransmitter mechanisms, target engagement and optimal therapy selection for individual patients, we propose that pharmacologic therapies hold great promise to promote recovery and improve quality of life for patients with severe brain injuries.

Distinct Neural Mechanisms Between Anesthesia Induction and Emergence: A Narrative Review

Anesthesia induction and emergence are critical periods for perioperative safety in the clinic. Traditionally, the emergence from general anesthesia has been recognized as a simple inverse process of induction resulting from the elimination of general anesthetics from the central nervous system. However, accumulated evidence has indicated that anesthesia induction and emergence are not mirror-image processes because of the occurrence of hysteresis/neural inertia in both animals and humans. An increasing number of studies have highlighted the critical role of orexinergic neurons and their involved circuits in the selective regulation of emergence but not the induction of general anesthesia. Moreover, additional brain regions have also been implicated in distinct neural mechanisms for anesthesia induction and emergence, which extends the concept that anesthetic induction and emergence are not antiparallel processes. Here, we reviewed the current literature and summarized the evidence regarding the differential mechanism of neural modulation in anesthesia induction and emergence, which will facilitate the understanding of the underlying neural mechanism for emergence from general anesthesia.

Anesthesia and the neurobiology of consciousness

In the 19th century, the discovery of general anesthesia revolutionized medical care. In the 21st century, anesthetics have become indispensable tools to study consciousness. Here, I review key aspects of the relationship between anesthesia and the neurobiology of consciousness, including interfaces of sleep and anesthetic mechanisms, anesthesia and primary sensory processing, the effects of anesthetics on large-scale functional brain networks, and mechanisms of arousal from anesthesia. I discuss the implications of the data derived from the anesthetized state for the science of consciousness and then conclude with outstanding questions, reflections, and future directions.

Neurobiological basis of emergence from anesthesia

The suppression of consciousness by anesthetics and the emergence of the brain from anesthesia are complex and elusive processes. Anesthetics may exert their inhibitory effects by binding to specific protein targets or through membrane-mediated targets, disrupting neural activity and the integrity and function of neural circuits responsible for signal transmission and conscious perception/subjective experience. Emergence from anesthesia was generally thought to depend on the elimination of the anesthetic from the body. Recently, studies have suggested that emergence from anesthesia is a dynamic and active process that can be partially controlled and is independent of the specific molecular targets of anesthetics. This article summarizes the fundamentals of anesthetics' actions in the brain and the mechanisms of emergence from anesthesia that have been recently revealed in animal studies.

Orexinergic lateral hypothalamus (LH) projections to medial septum (MS) modulate ethanol-induced sedation in male and female mice and binge-like ethanol drinking in male mice only

Orexin in both the lateral hypothalamus (LH) and medial septum (MS) is involved in sleep- and consciousness-related conditions. Since orexin modulates the intoxicating as well as rewarding effects of ethanol, this study focused on the role of orexin-projecting neurons from the LH to the MS, and this neurocircuit's role in mediating the sedative effects of alcohol. Drinking-in-the-Dark (DID) behavior was also assessed as a measure of the role of the LH-MS pathway in modulating binge-like ethanol intake, with a particular focus on sex differences in both behavioral paradigms. Male and female Hcrt-ires-cre mice received cannulation in the MS, while the LH was injected bilaterally with cre-dependent excitatory (Gq) Designer Receptor Exclusively Activated by Designer Drug (DREADD), inhibitory (Gi) DREADD or control virus. All subjects received a 3.75 g/kg dose of 20 % ethanol intraperitoneally and the sedative effect was assessed by the loss of righting reflex (LORR). After behavioral testing, brains were used for c-Fos immunohistochemistry analyses. A separate cohort of mice was used for a 2-week DID protocol using excitatory (Gq) DREADD and control virus. Gq DREADD-induced activation of the orexin neurocircuitry from the LH to the MS significantly reduced sedation time in both female and male mice. Furthermore, CNO treatment failed to alter ethanol sedation times in both animals expressing Gi DREADDs and control virus. There were no significant differences in blood ethanol concentrations (BECs) in any experimental group, suggesting that changes in sedation were not due to treatment-induced alterations of ethanol metabolism. Interestingly, in the DID study, only male mice decreased their ethanol consumption when Gq DREADDs were activated. These results provide novel evidence on the role played by this orexinergic LH to MS circuit on the sedative effects of ethanol and ethanol consumption in a sex-dependent manner. Thus, the MS should be considered further as a novel sexually dimorphic target.

Time to Wake Up! The Ongoing Search for General Anesthetic Reversal Agents

How general anesthetics work remains a topic of ongoing study. A parallel field of research has sought to identify methods to reverse general anesthesia. Reversal agents could shorten patients' recovery time and potentially reduce the risk of postoperative complications. An incomplete understanding of the mechanisms of general anesthesia has hampered the pursuit for reversal agents. Nevertheless, the search for reversal agents has furthered understanding of the mechanisms underlying general anesthesia. The study of potential reversal agents has highlighted the importance of rigorous criteria to assess recovery from general anesthesia in animal models, and has helped identify key arousal systems (e.g., cholinergic, dopaminergic, and orexinergic systems) relevant to emergence from general anesthesia. Furthermore, the effects of reversal agents have been found to be inconsistent across different general anesthetics, revealing differences in mechanisms among these drugs. The presynapse and glia probably also contribute to general anesthesia recovery alongside postsynaptic receptors. The next stage in the search for reversal agents will have to consider alternate mechanisms encompassing the tripartite synapse.

M1 cholinergic signaling in the brain modulates cytokine levels and splenic cell sub-phenotypes following cecal ligation and puncture

BACKGROUND

The contribution of the central nervous system to sepsis pathobiology is incompletely understood. In previous studies, administration of endotoxin to mice decreased activity of the vagus anti-inflammatory reflex. Treatment with the centrally-acting M1 muscarinic acetylcholine (ACh) receptor (M1AChR) attenuated this endotoxin-mediated change. We hypothesize that decreased M1AChR-mediated activity contributes to inflammation following cecal ligation and puncture (CLP), a mouse model of sepsis.

METHODS

In male C57Bl/6 mice, we quantified basal forebrain cholinergic activity (immunostaining), hippocampal neuronal activity, serum cytokine/chemokine levels (ELISA) and splenic cell subtypes (flow cytometry) at baseline, following CLP and following CLP in mice also treated with the M1AChR agonist xanomeline.

RESULTS

At 48 h. post-CLP, activity in basal forebrain cells expressing choline acetyltransferase (ChAT) was half of that observed at baseline. Lower activity was also noted in the hippocampus, which contains projections from ChAT-expressing basal forebrain neurons. Serum levels of TNFα, IL-1β, MIP-1α, IL-6, KC and G-CSF were higher post-CLP than at baseline. Post-CLP numbers of splenic macrophages and inflammatory monocytes, TNFα+ and ILβ+ neutrophils and ILβ+ monocytes were higher than baseline while numbers of central Dendritic Cells (cDCs), CD4+ and CD8+ T cells were lower. When, following CLP, mice were treated with xanomeline activity in basal forebrain ChAT-expressing neurons and in the hippocampus was significantly higher than in untreated animals. Post-CLP serum concentrations of TNFα, IL-1β, and MIP-1α, but not of IL-6, KC and G-CSF, were significantly lower in xanomeline-treated mice than in untreated mice. Post-CLP numbers of splenic neutrophils, macrophages, inflammatory monocytes and TNFα+ neutrophils also were lower in xanomeline-treated mice than in untreated animals. Percentages of IL-1β+ neutrophils, IL-1β+ monocytes, cDCs, CD4+ T cells and CD8+ T cells were similar in xanomeline-treated and untreated post-CLP mice.

CONCLUSION

Our findings indicate that M1AChR-mediated responses modulate CLP-induced alterations in serum levels of some, but not all, cytokines/chemokines and affected splenic immune response phenotypes.

One node among many: sevoflurane-induced hypnosis and the challenge of an integrative network-level view of anaesthetic action

Building on their known ability to influence sleep and arousal, Li and colleagues show that modulating the activity of glutamatergic pedunculopontine tegmental neurones also alters sevoflurane-induced hypnosis. This finding adds support for the shared sleep-anaesthesia circuit hypothesis. However, the expanding recognition of many neuronal clusters capable of modulating anaesthetic hypnosis raises the question of how disparate and anatomically distant sites ultimately interact to coordinate global changes in the state of the brain. Understanding how these individual sites work in concert to disrupt cognition and behaviour is the next challenge for anaesthetic mechanisms research.

Ketamine and the paradox of anaesthetic state transitions

Administration of subanaesthetic doses of ketamine during isoflurane anaesthesia has been shown in animals to deepen the anaesthetised state, while accelerating emergence. Duan and colleagues have now shown that the addition of subanaesthetic doses of esketamine to isoflurane has a similar effect of increasing the burst suppression ratio, while accelerating emergence. Using c-Fos expression and fibre photometry, they show that esketamine activates glutamatergic neurones in the paraventricular nucleus of the thalamus, a structure that regulates wakefulness. Chemogenetic inhibition of these neurones attenuates the arousal-promoting effects, suggesting a causal role of the paraventricular nucleus of the thalamus in esketamine-mediated acceleration of recovery from anaesthesia.

Activation of glutamatergic neurones in the pedunculopontine tegmental nucleus promotes cortical activation and behavioural emergence from sevoflurane-induced unconsciousness in mice

BACKGROUND

The neural mechanisms underlying sevoflurane-induced loss of consciousness and recovery of consciousness after anaesthesia remain unknown. We investigated whether glutamatergic pedunculopontine tegmental nucleus (PPT) neurones are involved in the regulation of states of consciousness under sevoflurane anaesthesia.

METHODS

In vivo fibre photometry combined with electroencephalography (EEG)/electromyography recording was used to record changes in the activity of glutamatergic PPT neurones under sevoflurane anaesthesia. Chemogenetic and cortical EEG recordings were used to explore their roles in the induction of and emergence from sevoflurane anaesthesia. Optogenetic methods combined with EEG recordings were used to explore the roles of glutamatergic PPT neurones and of the PPT-ventral tegmental area pathway in maintenance of anaesthesia.

RESULTS

The population activity of glutamatergic PPT neurones was reduced before sevoflurane-induced loss of righting reflex and gradually recovered after return of righting reflex. Chemogenetic inhibition of glutamatergic PPT neurones accelerated induction of anaesthesia (hM4Di-CNO vs mCherry-CNO, 76 [17] vs 121 [27] s, P<0.0001) and delayed emergence from sevoflurane anaesthesia (278 [98] vs 145 [53] s, P<0.0001) but increased sevoflurane sensitivity. Optogenetic stimulation of glutamatergic PPT neurons or of the PPT-ventral tegmental area pathway promoted cortical activation and behavioural emergence during steady-state sevoflurane anaesthesia, reduced the depth of anaesthesia, and caused cortical arousal during sevoflurane-induced EEG burst suppression.

CONCLUSIONS

Glutamatergic PPT neurones regulate induction and emergence of sevoflurane anaesthesia.

Hormonal basis of sex differences in anesthetic sensitivity

General anesthesia-a pharmacologically induced reversible state of unconsciousness-enables millions of life-saving procedures. Anesthetics induce unconsciousness in part by impinging upon sexually dimorphic and hormonally sensitive hypothalamic circuits regulating sleep and wakefulness. Thus, we hypothesized that anesthetic sensitivity should be sex-dependent and modulated by sex hormones. Using distinct behavioral measures, we show that at identical brain anesthetic concentrations, female mice are more resistant to volatile anesthetics than males. Anesthetic sensitivity is bidirectionally modulated by testosterone. Castration increases anesthetic resistance. Conversely, testosterone administration acutely increases anesthetic sensitivity. Conversion of testosterone to estradiol by aromatase is partially responsible for this effect. In contrast, oophorectomy has no effect. To identify the neuronal circuits underlying sex differences, we performed whole brain c-Fos activity mapping under anesthesia in male and female mice. Consistent with a key role of the hypothalamus, we found fewer active neurons in the ventral hypothalamic sleep-promoting regions in females than in males. In humans, we demonstrate that females regain consciousness and recover cognition faster than males after identical anesthetic exposures. Remarkably, while behavioral and neurocognitive measures in mice and humans point to increased anesthetic resistance in females, cortical activity fails to show sex differences under anesthesia in either species. Cumulatively, we demonstrate that sex differences in anesthetic sensitivity are evolutionarily conserved and not reflected in conventional electroencephalographic-based measures of anesthetic depth. This covert resistance to anesthesia may explain the higher incidence of unintended awareness under general anesthesia in females.

Propofol-induced anesthesia involves the direct inhibition of glutamatergic neurons in the lateral hypothalamus

Propofol is the most widely used intravenous general anesthetic; however, the neuronal circuits that mediate its anesthetic effects are still poorly understood. Glutamatergic neurons in the lateral hypothalamus have been reported to be involved in maintenance of arousal and consciousness. Using Vglut2-Cre transgenic mice, we recorded this group of cells specifically and found that propofol can directly inhibit the glutamatergic neurons, and enhance inhibitory synaptic inputs on these cells, thereby reducing neuronal excitability. Through chemogenetic interventions, we found that inhibition of these neurons increased the duration of propofol-induced anesthesia and reduced movement in the animals after the recovery of right reflex. In contrast, activating this group of cells reduced the duration of propofol anesthesia and increased the animals' locomotor activity after the recovery of right reflex. These results suggest that propofol-induced anesthesia involves the inhibition of glutamatergic neurons in the lateral hypothalamus.

Medial Septal Glutamatergic Neurons Modulate States of Consciousness during Sevoflurane Anesthesia in Mice

BACKGROUND

Multiple neural structures involved in maintaining wakefulness have been found to promote arousal from general anesthesia. The medial septum is a critical region that modulates arousal behavior. This study hypothesized that glutamatergic neurons in the medial septum play a crucial role in regulating states of consciousness during sevoflurane general anesthesia.

METHODS

Adult male mice were used in this study. The effects of sevoflurane anesthesia on neuronal activity were determined by fiber photometry. Lesions and chemogenetic manipulations were used to study the effects of the altered activity of medial septal glutamatergic neurons on anesthesia induction, emergence, and sensitivity to sevoflurane. Optogenetic stimulation was used to observe the role of acute activation of medial septal glutamatergic neurons on cortical activity and behavioral changes during sevoflurane-induced continuous steady state of general anesthesia and burst suppression state.

RESULTS

The authors found that medial septal glutamatergic neuronal activity decreased during sevoflurane anesthesia induction and recovered in the early period of emergence. Chemogenetic activation of medial septal glutamatergic neurons prolonged the induction time (mean ± SD, hM3Dq-clozapine N-oxide vs. hM3Dq-saline, 297.5 ± 60.1 s vs. 229.4 ± 29.9 s, P < 0.001, n = 11) and decreased the emergence time (53.2 ± 11.8 s vs. 77.5 ± 33.5 s, P = 0.025, n = 11). Lesions or chemogenetic inhibition of these neurons produced the opposite effects. During steady state of general anesthesia and deep anesthesia-induced burst suppression state, acute optogenetic activation of medial septal glutamatergic neurons induced cortical activation and behavioral emergence.

CONCLUSIONS

The study findings reveal that activation of medial septal glutamatergic neurons has arousal-promoting effects during sevoflurane anesthesia in male mice. The activation of these neurons prolongs the induction and accelerates the emergence of anesthesia.

EDITOR’S PERSPECTIVE

Dopamine D2-receptor neurons in nucleus accumbens regulate sevoflurane anesthesia in mice

INTRODUCTION

The mechanism of general anesthesia remains elusive. In recent years, numerous investigations have indicated that its mode of action is closely associated with the sleep-wake pathway. As a result, this study aimed to explore the involvement of dopamine D2 receptor (D2R) expressing neurons located in the nucleus accumbens (NAc), a critical nucleus governing sleep-wake regulation, in sevoflurane anesthesia.

METHODS

This exploration was carried out using calcium fiber photometry and optogenetics technology, while utilizing cortical electroencephalogram (EEG), loss of righting reflex (LORR), and recovery of righting reflex (RORR) as experimental indicators.

RESULTS

The findings from calcium fiber photometry revealed a decrease in the activity of NAcD2R neurons during the induction phase of sevoflurane anesthesia, with subsequent recovery observed during the anesthesia's emergence phase. Moreover, the activation of NAcD2R neurons through optogenetics technology led to a reduction in the anesthesia induction process and an extension of the arousal process in mice. Conversely, the inhibition of these neurons resulted in the opposite effect. Furthermore, the activation of NAcD2R neurons projecting into the ventral pallidum (VP) via optogenetics demonstrated a shortened induction time for mice under sevoflurane anesthesia.

DISCUSSION

In conclusion, our research outcomes suggest that NAcD2R neurons play a promotive role in the sevoflurane general anesthesia process in mice, and their activation can reduce the induction time of anesthesia via the ventral pallidum (VP).

Characterizing brain stage-dependent pupil dynamics based on lateral hypothalamic activity

Pupil dynamics presents varied correlation features with brain activity under different vigilant levels. The modulation of brain dynamic stages can arise from the lateral hypothalamus (LH), where diverse neuronal cell types contribute to arousal regulation in opposite directions via the anterior cingulate cortex (ACC). However, the relationship of the LH and pupil dynamics has seldom been investigated. Here, we performed local field potential (LFP) recordings at the LH and ACC, and whole-brain fMRI with simultaneous fiber photometry Ca2+ recording in the ACC, to evaluate their correlation with brain state-dependent pupil dynamics. Both LFP and functional magnetic resonance imaging (fMRI) data showed various correlations to pupil dynamics across trials that span negative, null, and positive correlation values, demonstrating brain state-dependent coupling features. Our results indicate that the correlation of pupil dynamics with ACC LFP and whole-brain fMRI signals depends on LH activity, suggesting a role of the latter in brain dynamic stage regulation.

Modulating anesthetic emergence with pathway-selective dopamine signaling

PURPOSE OF REVIEW

To summarize the recent preclinical findings investigating dopaminergic circuits for their involvement in reversing anesthetic-induced unconsciousness.

RECENT FINDINGS

The release of dopamine from the ventral tegmental area onto dopamine D1 receptor-expressing neurons in the nucleus accumbens promotes emergence following general anesthesia. Two relevant targets of dopamine D1 receptor-expressing neurons in the nucleus accumbens include the lateral hypothalamus and ventral pallidum. Activating mesocortical dopaminergic projections from the ventral tegmental area to the prelimbic cortex has also been shown to hasten emergence from general anesthesia. In contrast, the nigrostriatal dopamine pathway is not involved in regulating anesthetic emergence. The role of the tuberoinfundibular endocrine dopamine pathway remains to be tested; however, recent studies have identified an important function of neuroendocrine signaling on modulating general anesthesia.

SUMMARY

Potential avenues for accelerating anesthetic emergence may be found through targeting specific arousal-promoting pathways in the brain. Accumulating evidence from rodent studies manipulating cell type- and circuit-specific signaling pathways have identified dopamine as a potent modulator of general anesthesia. Specifically, dopamine signaling along the mesolimbic and mesocortical pathways plays a fundamental role in regulating consciousness.

Examining the long-term effects of traumatic brain injury on fear extinction in male rats

There is a strong association between traumatic brain injuries (TBIs) and the development of psychiatric disorders, including post-traumatic stress disorder (PTSD). Exposure-based therapy is a first-line intervention for individuals who suffer from PTSD and other anxiety-related disorders; however, up to 50% of individuals with PTSD do not respond well to this approach. Fear extinction, a core mechanism underlying exposure-based therapy, is a procedure in which a repeated presentation of a conditioned stimulus in the absence of an unconditioned stimulus leads to a decrease in fear expression, and is a useful tool to better understand exposure-based therapy. Identifying predictors of extinction would be useful in developing alternative treatments for the non-responders. We recently found that CO₂ reactivity predicts extinction phenotypes in rats, likely through the activation of orexin receptors in the lateral hypothalamus. While studies have reported mixed results in extinction of fear after TBI, none have examined the long-term durability of this phenotype in the more chronically injured brain. Here we tested the hypothesis that TBI results in a long-term deficit in fear extinction, and that CO₂ reactivity would be predictive of this extinction phenotype. Isoflurane-anesthetized adult male rats received TBI (n = 59) (produced by a controlled cortical impactor) or sham surgery (n = 29). One month post-injury or sham surgery, rats underwent a CO₂ or air challenge, followed by fear conditioning, extinction, and fear expression testing. TBI rats exposed to CO₂ (TBI-CO₂) showed no difference during extinction or fear expression relative to shams exposed to CO₂ (sham-CO₂). However, TBI-CO₂ rats, showed significantly better fear expression than TBI rats exposed to air (TBI-air). In contrast to previous findings, we observed no relationship between CO₂ reactivity and post-extinction fear expression in either the sham or TBI rats. However, compared to the previously observed naïve sample, we observed more variability in post-extinction fear expression but a very similar distribution of CO₂ reactivity in the current sample. Isoflurane anesthesia may lead to interoceptive threat habituation, possibly via action on orexin receptors in the lateral hypothalamus, and may interact with CO₂ exposure, resulting in enhanced extinction. Future work will directly test this possibility.

Topologically induced suppression of explosive synchronization

Nowadays, explosive synchronization is a well-documented phenomenon consisting in a first-order transition that may coexist with classical synchronization. Typically, explosive synchronization occurs when the network structure is represented by the classical graph Laplacian, and the node frequency and its degree are correlated. Here, we answer the question on whether this phenomenon can be observed in networks when the oscillators are coupled via degree-biased Laplacian operators. We not only observe that this is the case but also that this new representation naturally controls the transition from explosive to standard synchronization in a network. We prove analytically that explosive synchronization emerges when using this theoretical setting in star-like networks. As soon as this star-like network is topologically converted into a network containing cycles, the explosive synchronization gives rise to classical synchronization. Finally, we hypothesize that this mechanism may play a role in switching from normal to explosive states in the brain, where explosive synchronization has been proposed to be related to some pathologies like epilepsy and fibromyalgia.

Understanding the Neural Mechanisms of General Anesthesia from Interaction with Sleep-Wake State: A Decade of Discovery

The development of cutting-edge techniques to study specific brain regions and neural circuits that regulate sleep-wake brain states and general anesthesia (GA), has increased our understanding of these states that exhibit similar neurophysiologic traits. This review summarizes current knowledge focusing on cell subtypes and neural circuits that control wakefulness, rapid eye movement (REM) sleep, non-REM sleep, and GA. We also review novel insights into their interactions and raise unresolved questions and challenges in this field. Comparisons of the overlapping neural substrates of sleep-wake and GA regulation will help us to understand sleep-wake transitions and how anesthetics cause reversible loss of consciousness. SIGNIFICANCE STATEMENT: General anesthesia (GA), sharing numerous neurophysiologic traits with the process of natural sleep, is administered to millions of surgical patients annually. In the past decade, studies exploring the neural mechanisms underlying sleep-wake and GA have advanced our understanding of their interactions and how anesthetics cause reversible loss of consciousness. Pharmacotherapies targeting the neural substrates associated with sleep-wake and GA regulations have significance for clinical practice in GA and sleep medicine.

Breathe-squeeze: pharmacodynamics of a stimulus-free behavioural paradigm to track conscious states during sedation☆

BACKGROUND

Conscious states are typically inferred through responses to auditory tasks and noxious stimulation. We report the use of a stimulus-free behavioural paradigm to track state transitions in responsiveness during dexmedetomidine sedation. We hypothesised that estimated dexmedetomidine effect-site (Ce) concentrations would be higher at loss of responsiveness (LOR) compared with return of responsiveness (ROR), and both would be lower than comparable studies that used stimulus-based assessments.

METHODS

Closed-Loop Acoustic Stimulation during Sedation with Dexmedetomidine data were analysed for secondary analysis. Fourteen healthy volunteers were asked to perform the breathe-squeeze task of gripping a dynamometer when inspiring and releasing it when expiring. LOR was defined as five inspirations without accompanied squeezes; ROR was defined as the return of five inspirations accompanied by squeezes. Brain states were monitored using 64-channel EEG. Dexmedetomidine was administered as a target-controlled infusion, with Ce estimated from a pharmacokinetic model.

RESULTS

Counter to our hypothesis, mean estimated dexmedetomidine Ce was lower at LOR (0.92 ng ml-1; 95% confidence interval: 0.69-1.15) than at ROR (1.43 ng ml-1; 95% confidence interval: 1.27-1.58) (paired t-test; P=0.002). LOR was characterised by progressively increasing fronto-occipital EEG power in the 0.5-8 Hz band and loss of occipital alpha (8-12 Hz) and global beta (16-30 Hz) power. These EEG changes reverted at ROR.

CONCLUSIONS

The breathe-squeeze task can effectively track changes in responsiveness during sedation without external stimuli and might be more sensitive to state changes than stimulus-based tasks. It should be considered when perturbation of brain states is undesirable.

CLINICAL TRIAL REGISTRATION

NCT04206059.

A Sleep-Specific Midbrain Target for Sevoflurane Anesthesia

Sevoflurane has been the most widely used inhaled anesthetics with a favorable recovery profile; however, the precise mechanisms underlying its anesthetic action are still not completely understood. Here the authors show that sevoflurane activates a cluster of urocortin 1 (UCN1+ )/cocaine- and amphetamine-regulated transcript (CART+ ) neurons in the midbrain involved in its anesthesia. Furthermore, growth hormone secretagogue receptor (GHSR) is highly enriched in sevoflurane-activated UCN1+ /CART+ cells and is necessary for sleep induction. Blockade of GHSR abolishes the excitatory effect of sevoflurane on UCN1+ /CART+ neurons and attenuates its anesthetic effect. Collectively, their data suggest that anesthetic action of sevoflurane necessitates the GHSR activation in midbrain UCN1+ /CART+ neurons, which provides a novel target including the nucleus and receptor in the field of anesthesia.

Corticotropin-releasing factor neurones in the paraventricular nucleus of the hypothalamus modulate isoflurane anaesthesia and its responses to acute stress in mice

BACKGROUND

Corticotropin-releasing factor (CRF) neurones in the paraventricular nucleus (PVN) of the hypothalamus (PVN^CRF neurones) can promote wakefulness and are activated under anaesthesia. However, whether these neurones contribute to anaesthetic effects is unknown.

METHODS

With a combination of chemogenetic and molecular approaches, we examined the roles of PVN^CRF neurones in isoflurane anaesthesia in mice and further explored the underlying cellular and molecular mechanisms.

RESULTS

PVN neurones exhibited increased Fos expression during isoflurane anaesthesia (mean [standard deviation], 218 [69.3] vs 21.3 [7.3]; P<0.001), and ∼75% were PVN^CRF neurones. Chemogenetic inhibition of PVN^CRF neurones facilitated emergence from isoflurane anaesthesia (11.7 [1.1] vs 13.9 [1.2] min; P=0.001), whereas chemogenetic activation of these neurones delayed emergence from isoflurane anaesthesia (16.9 [1.2] vs 13.9 [1.3] min; P=0.002). Isoflurane exposure increased CRF protein expression in PVN (4.0 [0.1] vs 2.2 [0.3], respectively; P<0.001). Knockdown of CRF in PVN^CRF neurones mimicked the effects of chemogenetic inhibition of PVN^CRF neurones in facilitating emergence (9.6 [1.1] vs 13.0 [1.4] min; P=0.003) and also abolished the effects of chemogenetic activation of PVN^CRF neurones on delaying emergence from isoflurane anaesthesia (10.3 [1.3] vs 16.0 [2.6] min; P<0.001). Acute, but not chronic, stress delayed emergence from isoflurane anaesthesia (15.5 [1.5] vs 13.0 [1.4] min; P=0.004). This effect was reversed by chemogenetic inhibition of PVN^CRF neurones (11.7 [1.6] vs 14.7 [1.4] min; P=0.001) or knockdown of CRF in PVN^CRF neurones (12.3 [1.5] vs 15.3 [1.6] min; P=0.002).

CONCLUSIONS

CRF neurones in the PVN of the hypothalamus neurones modulate isoflurane anaesthesia and acute stress effects on anaesthesia through CRF signalling.

GABAergic neurons in the dorsomedial hypothalamus regulate states of consciousness in sevoflurane anesthesia

The neural inhibitory gamma-aminobutyric acid (GABA) system in the regulation of anesthetic consciousness is heterogeneous, and the medial hypothalamus (MH), consisting of ventromedial hypothalamus (VMH) and dorsomedial hypothalamus (DMH), plays an important role in sleep and circadian rhythm. However, the role of MH GABAergic neurons (MH^GABA) in anesthesia remains unclear. In this study, we used righting reflex, electroencephalogram (EEG), and arousal behavioral score to evaluate the sevoflurane anesthesia. Activation of MH^GABA or DMH^GABA neurons prolonged the anesthesia induction time, shortened the anesthesia emergence time, and induced EEG arousal and body movement during anesthesia; meanwhile, VMH^GABA neurons activated only induced EEG changes during 1.5% sevoflurane anesthesia. Furthermore, inhibition of DMH^GABA neurons significantly deepened sevoflurane anesthesia. Therefore, DMH^GABA neurons exert a strong emergence-promoting effect on induction, maintenance, and arousal during sevoflurane general anesthesia, which helps to reveal the mechanism of anesthesia.

Brain areas modulation in consciousness during sevoflurane anesthesia

Sevoflurane is presently one of the most used inhaled anesthetics worldwide. However, the mechanisms through which sevoflurane acts and the areas of the brain associated with changes in consciousness during anesthesia remain important and complex research questions. Sevoflurane is generally regarded as a volatile anesthetic that blindly targets neuronal (and sometimes astrocyte) GABAA receptors. This review focuses on the brain areas of sevoflurane action and their relation to changes in consciousness during anesthesia. We cover 20 years of history, from the bench to the bedside, and include perspectives on functional magnetic resonance, electroencephalogram, and pharmacological experiments. We review the interactions and neurotransmitters involved in brain circuits during sevoflurane anesthesia, improving the effectiveness and accuracy of sevoflurane's future application and shedding light on the mechanisms behind human consciousness.

General anesthesia and sleep: like and unlike

General anesthesia and sleep have long been discussed in the neurobiological context owingto their commonalities, such as unconsciousness, immobility, non-responsiveness to externalstimuli, and lack of memory upon returning to consciousness. Sleep is regulated bycomplex interactions between wake-promoting and sleep-promoting neural circuits. Anestheticsexert their effects partly by inhibiting wake-promoting neurons or activating sleep-promotingneurons. Unconscious but arousable sedation is more related to sleep-wake circuitries,whereas unconscious and unarousable anesthesia is independent of them. Generalanesthesia is notable for its ability to decrease sleep propensity. Conversely, increasedsleep propensity due to insufficient sleep potentiates anesthetic effects. Taken together, it isplausible that sleep and anesthesia are closely related phenomena but not the same ones.Further investigations on the relationship between sleep and anesthesia are warranted.

Cerebral Tissue Oxygen Saturation Correlates with Emergence from Propofol-Remifentanil Anesthesia: An Observational Cohort Study

Anesthesia emergence is accompanied by changes in cerebral circulation. It is unknown whether cerebral tissue oxygen saturation (SctO₂) could be an indicator of emergence. Changes in SctO₂, bispectral index (BIS), mean arterial pressure (MAP), and heart rate (HR) were evaluated during the emergence from propofol-remifentanil anesthesia. At the time of cessation of anesthetic delivery, SctO₂, BIS, MAP, and HR values were recorded as baseline. The changes of these parameters from the baseline were recorded as Δ SctO₂, Δ BIS, Δ MAP, and Δ HR. The behavioral signs (body movement, coughing, or eye opening) and response to commands (indicating regaining of consciousness) were used to define emergence states. Prediction probability (Pk) was used to examine the accuracy of SctO₂, BIS, MAP, and HR as indicators of emergence. SctO₂ showed an abrupt and distinctive increase when appearing behavioral signs. BIS, MAP, and HR, also increased but with a large inter-individual variability. Pk value of Δ SctO₂ was 0.97 to predict the appearance behavioral signs from 2 min before that, which was much higher than the Pk values of Δ BIS (0.81), Δ MAP (0.71) and Δ HR (0.87). The regaining of consciousness was associated with a further increase in the SctO₂ value.

Regulation of Neural Circuitry under General Anesthesia: New Methods and Findings

General anesthesia has been widely utilized since the 1840s, but its underlying neural circuits remain to be completely understood. Since both general anesthesia and sleep are reversible losses of consciousness, studies on the neural-circuit mechanisms affected by general anesthesia have mainly focused on the neural nuclei or the pathways known to regulate sleep. Three advanced technologies commonly used in neuroscience, in vivo calcium imaging, chemogenetics, and optogenetics, are used to record and modulate the activity of specific neurons or neural circuits in the brain areas of interest. Recently, they have successfully been used to study the neural nuclei and pathways of general anesthesia. This article reviews these three techniques and their applications in the brain nuclei or pathways affected by general anesthesia, to serve as a reference for further and more accurate exploration of other neural circuits under general anesthesia and to contribute to other research fields in the future.

Historical and Modern Evidence for the Role of Reward Circuitry in Emergence

Increasing evidence supports a role for brain reward circuitry in modulating arousal along with emergence from anesthesia. Emergence remains an important frontier for investigation, since no drug exists in clinical practice to initiate rapid and smooth emergence. This review discusses clinical and preclinical evidence indicating a role for two brain regions classically considered integral components of the mesolimbic brain reward circuitry, the ventral tegmental area and the nucleus accumbens, in emergence from propofol and volatile anesthesia. Then there is a description of modern systems neuroscience approaches to neural circuit investigations that will help span the large gap between preclinical and clinical investigation with the shared aim of developing therapies to promote rapid emergence without agitation or delirium. This article proposes that neuroscientists include models of whole-brain network activity in future studies to inform the translational value of preclinical investigations and foster productive dialogues with clinician anesthesiologists.

Hypocretins (orexins): The ultimate translational neuropeptides

The hypocretins (Hcrts), also known as orexins, are two neuropeptides produced exclusively in the lateral hypothalamus. They act on two specific receptors that are widely distributed across the brain and involved in a myriad of neurophysiological functions that include sleep, arousal, feeding, reward, fear, anxiety and cognition. Hcrt cell loss in humans leads to narcolepsy with cataplexy (narcolepsy type 1), a disorder characterized by intrusions of sleep into wakefulness, demonstrating that the Hcrt system is nonredundant and essential for sleep/wake stability. The causal link between Hcrts and arousal/wakefulness stabilisation has led to the development of a new class of drugs, Hcrt receptor antagonists to treat insomnia, based on the assumption that blocking orexin-induced arousal will facilitate sleep. This has been clinically validated: currently, two Hcrt receptor antagonists are approved to treat insomnia (suvorexant and lemborexant), with a New Drug Application recently submitted to the US Food and Drug Administration for a third drug (daridorexant). Other therapeutic applications under investigation include reduction of cravings in substance-use disorders and prevention of neurodegenerative disorders such as Alzheimer's disease, given the apparent bidirectional relationship between poor sleep and worsening of the disease. Circuit neuroscience findings suggest that the Hcrt system is a hub that integrates diverse inputs modulating arousal (e.g., circadian rhythms, metabolic status, positive and negative emotions) and conveys this information to multiple output regions. This neuronal architecture explains the wealth of physiological functions associated with Hcrts and highlights the potential of the Hcrt system as a therapeutic target for a number of disorders. We discuss present and future possible applications of drugs targeting the Hcrt system for the treatment of circuit-related neuropsychiatric and neurodegenerative conditions.

A Neural Circuit from the Paraventricular Thalamus to the Bed Nucleus of the Stria Terminalis for the Regulation of States of Consciousness during Sevoflurane Anesthesia in Mice

BACKGROUND

The neural circuitry underlying sevoflurane-induced modulation of consciousness is poorly understood. This study hypothesized that the paraventricular thalamus bed nucleus of the stria terminalis pathway plays an important role in regulating states of consciousness during sevoflurane anesthesia.

METHODS

Rabies virus-based transsynaptic tracing techniques were employed to reveal the neural pathway from the paraventricular thalamus to the bed nucleus of the stria terminalis. This study investigated the role of this pathway in sevoflurane anesthesia induction, maintenance, and emergence using chemogenetic and optogenetic methods combined with cortical electroencephalogram recordings. Both male and female mice were used in this study.

RESULTS

Both γ-aminobutyric acid-mediated and glutamatergic neurons in the bed nucleus of the stria terminalis receive paraventricular thalamus glutamatergic projections. Chemogenetic inhibition of paraventricular thalamus glutamatergic neurons prolonged the sevoflurane anesthesia emergence time (mean ± SD, hM4D-clozapine N-oxide vs. mCherry-clozapine N-oxide, 281 ± 88 vs. 172 ± 48 s, P < 0.001, n = 24) and decreased the induction time (101 ± 32 vs. 136 ± 34 s, P = 0.002, n = 24), as well as the EC5 0 for the loss or recovery of the righting reflex under sevoflurane anesthesia (mean [95% CI] for the concentration at which 50% of the mice lost their righting reflex, 1.16 [1.12 to 1.20] vs. 1.49 [1.46 to 1.53] vol%, P < 0.001, n = 20; and for the concentration at which 50% of the mice recovered their righting reflex, 0.95 [0.86 to 1.03] vs. 1.34 [1.29 to 1.40] vol%, P < 0.001, n = 20). Similar results were observed during suppression of the paraventricular thalamus bed nucleus-stria terminalis pathway. Optogenetic activation of this pathway produced the opposite effects. Additionally, transient stimulation of this pathway efficiently induced behavioral arousal during continuous steady-state general anesthesia with sevoflurane and reduced the depth of anesthesia during sevoflurane-induced burst suppression.

CONCLUSIONS

In mice, axonal projections from the paraventricular thalamic neurons to the bed nucleus of the stria terminalis contribute to regulating states of consciousness during sevoflurane anesthesia.

Neuroanatomical Basis for the Orexinergic Modulation of Anesthesia Arousal and Pain Control

Hypothalamic orexin (hypocretin) neurons play crucial roles in arousal control. Their involvement in anesthesia and analgesia remains to be better understood. In order to enhance our view on the neuroanatomy, we systematically mapped the projections of orexin neurons with confocal microscope and light sheet microscope. We specifically expressed optogenetic opsins tagged with fluorescence markers in orexin neurons through adeno-associated viral infection in the mouse brain. The imaging results revealed fine details and novel features of the orexin projections throughout the brain, particularly related to the nuclei regulating arousal and pain. We then optogenetically activated orexin neurons in the lateral hypothalamus to study the effects on anesthesia-related behaviors. cFos staining showed that optogenetic stimulation can activate orexin neurons in the ChR2-mCherry group, but not the control mCherry group (62.86 ± 3.923% vs. 7.9 ± 2.072%; P < 0.0001). In behavior assays, optogenetic stimulation in the ChR2-mCherry group consistently elicited robust arousal from light isoflurane anesthesia (9.429 ± 3.804 s vs. 238.2 ± 17.42 s; P < 0.0001), shortened the emergence time after deep isoflurane anesthesia (109.5 ± 13.59 s vs. 213.8 ± 21.77 s; P = 0.0023), and increased the paw withdrawal latency in a hotplate test (11.45 ± 1.185 s vs. 8.767 ± 0.7775; P = 0.0317). The structural details of orexin fibers established the neuroanatomic basis for studying the role of orexin in anesthesia and analgesia.

Dopamine transporter in the ventral tegmental area modulates recovery from propofol anesthesia in rats

OBJECTIVE(S)

To investigate the role of the dopamine transporter (DAT) in the ventral tegmental area (VTA) in the recovery from propofol anesthesia in rats.

MATERIALS AND METHODS

A total of 150 Sprague-Dawley (SD) rats were randomly split into a normal control group (NC), saline group (S), propofol anesthesia group (P), adeno-associated viral-NC-mCherry (AAV-NC) group, and AAV-DAT-RNAi (DAT-RNAi) group (n = 30 per group). In rats in the AAV intervention group, AAV was injected into the VTA nucleus via a stereotaxer. The rats in each group were continuously pumped with propofol through the tail vein at a dose of 70mg/kg/h, and the control group was infused with the same dose of saline at the same speed for 30min. Immunofluorescence staining was used to observe the expression of c-fos protein in the prefrontal cortex (PFC). The induction and recovery time of propofol anesthesia were recorded based on the time of disappearance of the righting reflex (LORR) and recovery (RORR). The anesthesia depth score was performed on all rats 10min after starting the administration and 10min after withdrawal, which represented the depth of anesthesia during anesthesia and the degree of recovery during anesthesia recovery, respectively. electroencephalogram (EEG) was recorded during propofol anesthesia and recovery.

RESULTS

Compared to the NC group, the RORR of the DAT-RNAi group was shortened, and the anesthesia depth score was higher (P < 0.05). In the DAT-RNAi group, during the period of propofol anesthesia, the β wave frequencies increased, the θ wave frequencies decreased, and the expression of c-fos protein in PFC increased and during the recovery from propofol anesthesia, the α wave and β wave frequencies were increased (P < 0.05).

CONCLUSION

Knockdown of the DAT in the VTA region can enhance the activity of PFC neurons and promote the recovery of rats from propofol anesthesia.

Electroencephalographic Evidence for Individual Neural Inertia in Mice That Decreases With Time

Previous studies have demonstrated that the brain has an intrinsic resistance to changes in arousal state. This resistance is most easily measured at the population level in the setting of general anesthesia and has been termed neural inertia. To date, no study has attempted to determine neural inertia in individuals. We hypothesize that individuals with markedly increased or decreased neural inertia might be at increased risk for complications related to state transitions, from awareness under anesthesia, to delayed emergence or confusion/impairment after emergence. Hence, an improved theoretical and practical understanding of neural inertia may have the potential to identify individuals at increased risk for these complications. This study was designed to explicitly measure neural inertia in individuals and empirically test the stochastic model of neural inertia using spectral analysis of the murine EEG. EEG was measured after induction of and emergence from isoflurane administered near the EC₅₀ dose for loss of righting in genetically inbred mice on a timescale that minimizes pharmacokinetic confounds. Neural inertia was assessed by employing classifiers constructed using linear discriminant or supervised machine learning methods to determine if features of EEG spectra reliably demonstrate path dependence at steady-state anesthesia. We also report the existence of neural inertia at the individual level, as well as the population level, and that neural inertia decreases over time, providing direct empirical evidence supporting the predictions of the stochastic model of neural inertia.

Neural Substrates for Regulation of Sleep and General Anesthesia

General anesthesia has been successfully used in clinics for over 170 years, but its mechanisms of effect remain unclear. Behaviorally, general anesthesia is similar to sleep as it produces a reversible transition between wakefulness and the state of being unaware of one’s surroundings. A discussion regarding the common circuits of sleep and general anesthesia has been ongoing as an increasing number of sleep-arousal regulatory nuclei are reported to participate in the consciousness shift occurring during general anesthesia. Recently, with progress in research technology, both positive and negative evidence for overlapping neural circuits between sleep and general anesthesia has emerged. This article provides a review of the latest evidence on the neural substrates for sleep and general anesthesia regulation by comparing the roles of pivotal nuclei in sleep and anesthesia.