Orexin-A facilitates emergence of the rat from isoflurane anesthesia via mediation of the basal forebrain

Ventral Tegmental Area Glutamatergic Neurons Facilitated Emergence From Isoflurane Anesthesia Involves Excitation of Lateral Septum GABA-ergic Neurons in Mice

BACKGROUND

Ventral tegmental area (VTA) glutamatergic neurons promote wakefulness in the sleep-wake cycle; however, their roles and neural circuit mechanisms during isoflurane (ISO) anesthesia remain unclear.

METHODS

Fiber photometry and in vivo electrophysiology were used to observe the changes in neuronal or terminal activity during ISO anesthesia and arousal processes. Optogenetic and anesthesia behaviors were used to investigate the effects of VTA glutamatergic neurons and their projections to the lateral septum (LS) during ISO anesthesia and arousal. Anterograde and retrograde tracings were performed to identify the connections between VTA glutamatergic neurons and the LS.

RESULTS

Population activity and firing rates of VTA glutamatergic neurons decreased during ISO anesthesia (ISO: 95% confidence interval [CI], 0.83-2.06 Spikes.s -1 vs wake: 95% CI, 3.53-7.83 Spikes.s -1 ; P =.0001; n = 34 from 4 mice). Optogenetic activation of VTA glutamatergic neurons reduced the burst-suppression ratio in electroencephalography (laser: 95% CI, 13.09%-28.76% vs pre: 95% CI, 52.85%-71.59%; P =.0009; n = 6) and facilitated emergence (ChR2: 95% CI, 343.3-388.0 seconds vs mCherry: 95% CI, 447.6-509.8 seconds; P < .0001; n = 11/12) from ISO anesthesia. VTA glutamatergic neurons monosynaptically innervated LS γ-aminobutyric acid (GABA)-ergic neurons. The activity of VTA glutamatergic terminals in the LS decreased during ISO anesthesia, and optogenetic activation of the VTA glutamatergic terminals in the LS facilitated emergence from ISO anesthesia. Furthermore, optogenetic activation of VTA glutamatergic terminals increased the firing rates of LS γ-aminobutyric acid-ergic (GABAergic) neurons (laser: 95% CI, 0.85-4.03 Spikes.s -1 vs pre: 95% CI, 0.24-0.78 Spikes.s -1 ; P =.008; n = 23 from 4 mice) during ISO anesthesia.

CONCLUSIONS

VTA glutamatergic neurons facilitated emergence from ISO anesthesia involving excitation of LS GABAergic neurons.

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.

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.

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.

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.

Selective optogenetic modulation of the PBN terminals in the lateral hypothalamic area and basal forebrain regulates emergence from isoflurane anesthesia in mice

While the mechanism of general anesthesia has been extensively studied, the underlying neural circuitry has yet to be fully understood. The parabrachial nucleus (PBN) plays a crucial role in modulating wakefulness and promoting arousal from general anesthesia. However, the specific role of PBN projections in the process of general anesthesia remains unclear. In this study, we bilaterally injected AAV-associated viruses encoding excitatory or inhibitory optogenetic probes into the PBN and implanted optical fibers in the LH or BF area. After four weeks, we optogenetically activated or inhibited the PBN-LH and PBN-BF pathways under 1.5 vol% isoflurane. We calculated the time it took for anesthesia induction and emergence, simultaneously monitoring changes in the burst-suppression ratio using electroencephalogram recording. Our findings indicate that optogenetic activation of the PBN-LH and PBN-BF projections plays a significant role in promoting both cortical and behavioral emergence from isoflurane inhalation, without significantly affecting the induction time. Conversely, photoinhibition of these pathways prolonged the recovery time, with no notable difference observed during the induction phase.In summary, our results demonstrate that the PBN-LH and PBN-BF pathways are crucial for promoting arousal from isoflurane general anesthesia, but do not have a pronounced impact on the induction phase.

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.

The role of the basal forebrain in general anesthesia

The basal forebrain is a group of nerve nuclei on the ventral side of the ventral ganglion, composed of γ-aminobutyric acid neurons, glutamatergic neurons, cholinergic neurons, and orexigenic neurons. Previous studies have focused on the involvement of the basal forebrain in regulating reward, learning, movement, sleep-awakening, and other neurobiological behaviors, but its role in the regulation of general anesthesia has not been systematically elucidated. Therefore, the different neuronal subtypes in the basal forebrain and projection pathways in general anesthesia will be discussed in this paper. In this paper, we aim to determine and elaborate on the role of the basal forebrain in general anesthesia and the development of theoretical research and provide a new theory.

Inactivation of Prefrontal Cortex Attenuates Behavioral Arousal Induced by Stimulation of Basal Forebrain During Sevoflurane Anesthesia

Cholinergic stimulation of prefrontal cortex (PFC) can reverse anesthesia. Conversely, inactivation of PFC can delay emergence from anesthesia. PFC receives cholinergic projections from basal forebrain, which contains wake-promoting neurons. However, the role of basal forebrain cholinergic neurons in arousal from the anesthetized state requires refinement, and it is currently unknown whether the arousal-promoting effect of basal forebrain is mediated through PFC. To address these gaps in knowledge, we implemented a novel approach to the use of chemogenetic stimulation and tested the role of basal forebrain cholinergic neurons in behavioral arousal during sevoflurane anesthesia. Next, we investigated the effect of tetrodotoxin-mediated inactivation of PFC on behavioral arousal produced by electrical stimulation of basal forebrain during sevoflurane anesthesia.

Roles of Neuropeptides in Sleep-Wake Regulation

Sleep and wakefulness are basic behavioral states that require coordination between several brain regions, and they involve multiple neurochemical systems, including neuropeptides. Neuropeptides are a group of peptides produced by neurons and neuroendocrine cells of the central nervous system. Like traditional neurotransmitters, neuropeptides can bind to specific surface receptors and subsequently regulate neuronal activities. For example, orexin is a crucial component for the maintenance of wakefulness and the suppression of rapid eye movement (REM) sleep. In addition to orexin, melanin-concentrating hormone, and galanin may promote REM sleep. These results suggest that neuropeptides play an important role in sleep–wake regulation. These neuropeptides can be divided into three categories according to their effects on sleep–wake behaviors in rodents and humans. (i) Galanin, melanin-concentrating hormone, and vasoactive intestinal polypeptide are sleep-promoting peptides. It is also noticeable that vasoactive intestinal polypeptide particularly increases REM sleep. (ii) Orexin and neuropeptide S have been shown to induce wakefulness. (iii) Neuropeptide Y and substance P may have a bidirectional function as they can produce both arousal and sleep-inducing effects. This review will introduce the distribution of various neuropeptides in the brain and summarize the roles of different neuropeptides in sleep–wake regulation. We aim to lay the foundation for future studies to uncover the mechanisms that underlie the initiation, maintenance, and end of sleep–wake states.

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.

Activation of Orexinergic Neurons Inhibits the Anesthetic Effect of Desflurane on Consciousness State via Paraventricular Thalamic Nucleus in Rats

BACKGROUND

Orexin, a neuropeptide derived from the perifornical area of the hypothalamus (PeFLH), promotes the recovery of propofol, isoflurane, and sevoflurane anesthesias, without influencing the induction time. However, whether the orexinergic system also plays a similar role in desflurane anesthesia, which is widely applied in clinical practice owing to its most rapid onset and offset time among all volatile anesthetics, has not yet been studied. In the present study, we explored the effect of the orexinergic system on the consciousness state induced by desflurane anesthesia.

METHODS

The c-Fos staining was used to observe the activity changes of orexinergic neurons in the PeFLH and their efferent projection regions under desflurane anesthesia. Chemogenetic and optogenetic techniques were applied to compare the effect of PeFLH orexinergic neurons on the induction, emergence, and maintenance states between desflurane and isoflurane anesthesias. Orexinergic terminals in the paraventricular thalamic nucleus (PVT) were manipulated with pharmacologic, chemogenetic, and optogenetic techniques to assess the effect of orexinergic circuitry on desflurane anesthesia.

RESULTS

Desflurane anesthesia inhibited the activity of orexinergic neurons in the PeFLH, as well as the neuronal activity in PVT, basal forebrain, dorsal raphe nucleus, and ventral tegmental area, as demonstrated by c-Fos staining. Activation of PeFLH orexinergic neurons prolonged the induction time and accelerated emergence from desflurane anesthesia but only influenced the emergence in isoflurane anesthesia, as demonstrated by chemogenetic and pharmacologic techniques. Meanwhile, optical activation of orexinergic neurons exhibited a long-lasting inhibitory effect on burst-suppression ratio (BSR) under desflurane anesthesia, and the effect may be contributed by the orexinergic PeFLH-PVT circuitry. The orexin-2 receptor (OX2R), but not orexin-1 receptor (OX1R), in the PVT, which had been inhibited most significantly by desflurane, mediated the proemergence effect of desflurane anesthesia.

CONCLUSIONS

We discovered, for the first time, that orexinergic neurons in the PeFLH could not only influence the maintenance and emergence from isoflurane and desflurane anesthesias but also affect the induction under desflurane anesthesia. Furthermore, this specific effect is probably mediated by orexinergic PeFLH-PVT circuitry, especially OX2Rs in the PVT.

Activation of Dopamine Signals in the Olfactory Tubercle Facilitates Emergence from Isoflurane Anesthesia in Mice

Activation of dopamine (DA) neurons is essential for the transition from sleep to wakefulness and maintenance of awakening, and sufficient to accelerate the emergence from general anesthesia in animals. Dopamine receptors (DR) are involve in arousal mediation. In the present study, we showed that the olfactory tubercle (OT) was active during emergence from isoflurane anesthesia, local injection of dopamine D1 receptor (D1R) agonist chloro-APB (1 mg/mL) and D2 receptor (D2R) agonist quinpirole (1 mg/mL) into OT enhanced behavioural and cortical arousal from isoflurane anesthesia, while D1R antagonist SCH-23390 (1 mg/mL) and D2R antagonist raclopride (2.5 mg/mL) prolonged recovery time. Optogenetic activation of DAergic terminals in OT also promoted behavioural and cortical arousal from isoflurane anesthesia. However, neither D1R/D2R agonists nor D1R/D2R antagonists microinjection had influences on the induction of isoflurane anesthesia. Optogenetic stimulation on DAergic terminals in OT also had no impact on the anesthesia induction. Our results indicated that DA signals in OT accelerated emergence from isoflurane anesthesia. Furthermore, the induction of general anesthesia, different from the emergence process, was not mediated by the OT DAergic pathways.

The Neural Circuits Underlying General Anesthesia and Sleep

General anesthesia is characterized by loss of consciousness, amnesia, analgesia, and immobility. Important molecular targets of general anesthetics have been identified, but the neural circuits underlying the discrete end points of general anesthesia remain incompletely understood. General anesthesia and natural sleep share the common feature of reversible unconsciousness, and recent developments in neuroscience have enabled elegant studies that investigate the brain nuclei and neural circuits underlying this important end point. A common approach to measure cortical activity across the brain is electroencephalogram (EEG), which can reflect local neuronal activity as well as connectivity among brain regions. The EEG oscillations observed during general anesthesia depend greatly on the anesthetic agent as well as dosing, and only some resemble those observed during sleep. For example, the EEG oscillations during dexmedetomidine sedation are similar to those of stage 2 nonrapid eye movement (NREM) sleep, but high doses of propofol and ether anesthetics produce burst suppression, a pattern that is never observed during natural sleep. Sleep is primarily driven by withdrawal of subcortical excitation to the cortex, but anesthetics can directly act at both subcortical and cortical targets. While some anesthetics appear to activate specific sleep-active regions to induce unconsciousness, not all sleep-active regions play a significant role in anesthesia. Anesthetics also inhibit cortical neurons, and it is likely that each class of anesthetic drugs produces a distinct combination of subcortical and cortical effects that lead to unconsciousness. Conversely, arousal circuits that promote wakefulness are involved in anesthetic emergence and activating them can induce emergence and accelerate recovery of consciousness. Modern neuroscience techniques that enable the manipulation of specific neural circuits have led to new insights into the neural circuitry underlying general anesthesia and sleep. In the coming years, we will continue to better understand the mechanisms that generate these distinct states of reversible unconsciousness.

Dorsal raphe serotonergic neurons promote arousal from isoflurane anesthesia

Aims

General anesthesia has been widely applied in surgical or nonsurgical medical procedures, but the mechanism behind remains elusive. Because of shared neural circuits of sleep and anesthesia, whether serotonergic system, which is highly implicated in modulation of sleep and wakefulness, regulates general anesthesia as well is worth investigating.

Methods

Immunostaining and fiber photometry were used to assess the neuronal activities. Electroencephalography spectra and burst‐suppression ratio (BSR) were used to measure anesthetic depth and loss or recovery of righting reflex to indicate the induction or emergence time of general anesthesia. Regulation of serotonergic system was achieved through optogenetic, chemogenetic, or pharmacological methods.

Results

We found that both Fos expression and calcium activity were significantly decreased during general anesthesia. Activation of 5‐HT neurons in the dorsal raphe nucleus (DRN) decreased the depth of anesthesia and facilitated the emergence from anesthesia, and inhibition deepened the anesthesia and prolonged the emergence time. Furthermore, agonism or antagonism of 5‐HT 1A or 2C receptors mimicked the effect of manipulating DRN serotonergic neurons.

Conclusion

Our results demonstrate that 5‐HT neurons in the DRN play a regulative role of general anesthesia, and activation of serotonergic neurons could facilitate emergence from general anesthesia partly through 5‐HT 1A and 2C receptors.

Lateral Hypothalamic Area Glutamatergic Neurons and Their Projections to the Lateral Habenula Modulate the Anesthetic Potency of Isoflurane in Mice

The lateral hypothalamic area (LHA) plays a pivotal role in regulating consciousness transition, in which orexinergic neurons, GABAergic neurons, and melanin-concentrating hormone neurons are involved. Glutamatergic neurons have a large population in the LHA, but their anesthesia-related effect has not been explored. Here, we found that genetic ablation of LHA glutamatergic neurons shortened the induction time and prolonged the recovery time of isoflurane anesthesia in mice. In contrast, chemogenetic activation of LHA glutamatergic neurons increased the time to anesthesia and decreased the time to recovery. Optogenetic activation of LHA glutamatergic neurons during the maintenance of anesthesia reduced the burst suppression pattern of the electroencephalogram (EEG) and shifted EEG features to an arousal pattern. Photostimulation of LHA glutamatergic projections to the lateral habenula (LHb) also facilitated the emergence from anesthesia and the transition of anesthesia depth to a lighter level. Collectively, LHA glutamatergic neurons and their projections to the LHb regulate anesthetic potency and EEG features.

The Inert Brain: Explaining Neural Inertia as Post-anaesthetic Sleep Inertia

"Neural inertia" is the brain's tendency to resist changes in its arousal state: it is manifested as emergence from anaesthesia occurring at lower drug doses than those required for anaesthetic induction, a phenomenon observed across very different species, from invertebrates to mammals. However, the brain is also subject to another form of inertia, familiar to most people: sleep inertia, the feeling of grogginess, confusion and impaired performance that typically follows awakening. Here, we propose a novel account of neural inertia, as the result of sleep inertia taking place after the artificial sleep induced by anaesthetics. We argue that the orexinergic and noradrenergic systems may be key mechanisms for the control of these transition states, with the orexinergic system exerting a stabilising effect through the noradrenergic system. This effect may be reflected at the macroscale in terms of altered functional anticorrelations between default mode and executive control networks of the human brain. The hypothesised link between neural inertia and sleep inertia could explain why different anaesthetic drugs induce different levels of neural inertia, and why elderly individuals and narcoleptic patients are more susceptible to neural inertia. This novel hypothesis also enables us to generate several empirically testable predictions at both the behavioural and neural levels, with potential implications for clinical practice.

Preoptic Area Modulation of Arousal in Natural and Drug Induced Unconscious States

The role of the hypothalamic preoptic area (POA) in arousal state regulation has been studied since Constantin von Economo first recognized its importance in the early twentieth century. Over the intervening decades, the POA has been shown to modulate arousal in both natural (sleep and wake) as well as drug-induced (anesthetic-induced unconsciousness) states. While the POA is well known for its role in sleep promotion, populations of wake-promoting neurons within the region have also been identified. However, the complexity and molecular heterogeneity of the POA has made distinguishing these two populations difficult. Though multiple lines of evidence demonstrate that general anesthetics modulate the activity of the POA, the region's heterogeneity has also made it challenging to determine whether the same neurons involved in sleep/wake regulation also modulate arousal in response to general anesthetics. While a number of studies show that sleep-promoting POA neurons are activated by various anesthetics, recent work suggests this is not universal to all arousal-regulating POA neurons. Technical innovations are making it increasingly possible to classify and distinguish the molecular identities of neurons involved in sleep/wake regulation as well as anesthetic-induced unconsciousness. Here, we review the current understanding of the POA's role in arousal state regulation of both natural and drug-induced forms of unconsciousness, including its molecular organization and connectivity to other known sleep and wake promoting regions. Further insights into the molecular identities and connectivity of arousal-regulating POA neurons will be critical in fully understanding how this complex region regulates arousal states.

The Inert Brain: Explaining Neural Inertia as Post-anaesthetic Sleep Inertia

"Neural inertia" is the brain's tendency to resist changes in its arousal state: it is manifested as emergence from anaesthesia occurring at lower drug doses than those required for anaesthetic induction, a phenomenon observed across very different species, from invertebrates to mammals. However, the brain is also subject to another form of inertia, familiar to most people: sleep inertia, the feeling of grogginess, confusion and impaired performance that typically follows awakening. Here, we propose a novel account of neural inertia, as the result of sleep inertia taking place after the artificial sleep induced by anaesthetics. We argue that the orexinergic and noradrenergic systems may be key mechanisms for the control of these transition states, with the orexinergic system exerting a stabilising effect through the noradrenergic system. This effect may be reflected at the macroscale in terms of altered functional anticorrelations between default mode and executive control networks of the human brain. The hypothesised link between neural inertia and sleep inertia could explain why different anaesthetic drugs induce different levels of neural inertia, and why elderly individuals and narcoleptic patients are more susceptible to neural inertia. This novel hypothesis also enables us to generate several empirically testable predictions at both the behavioural and neural levels, with potential implications for clinical practice.

GABAA receptors in the basal forebrain mediates emergence from propofol anaesthesia in rats

PURPOSE

The aim of the current study was to explore the role of the basal forebrain (BF) in propofol anaesthesia.

METHODS

In the present study, we observed the neural activities of the BF during propofol anaesthesia using calcium fibre photometry recording. Subsequently, ibotenic acid was injected into the BF to verify the role of the BF in propofol anaesthesia. Finally, to test whether GABA_A receptors in the BF were involved in modulating propofol anaesthesia, muscimol (GABA_A receptor agonist) and gabazine (GABA_A receptor antagonist) were microinjected into the BF. Cortical electroencephalogram (EEG), time to loss of righting reflex (LORR), and recovery of righting reflex (RORR) under propofol anaesthesia were recorded and analysed.

RESULTS

The activity of BF neurons was inhibited during induction of propofol anaesthesia and activated during emergence from propofol anaesthesia. In addition, non-specifical lesion of BF neurons significantly prolonged the time to RORR and increased delta power in the frontal cortex under propofol anaesthesia. Next, microinjection of muscimol into the BF delayed emergence from propofol anaesthesia, increased delta power of the frontal cortex, and decreased gamma power under propofol anaesthesia. Conversely, infusion of gabazine accelerated emergence times and decreased EEG delta power.

CONCLUSIONS

The basal forebrain is involved in modulating frontal cortex delta activity and emergence from propofol anaesthesia. Additionally, the GABA_A receptors in the basal forebrain are involved in regulating emergence propofol anaesthesia.

Selective optogenetic activation of orexinergic terminals in the basal forebrain and locus coeruleus promotes emergence from isoflurane anaesthesia in rats

BACKGROUND

The neuropeptide orexin promotes arousal from general anaesthesia, however the neuronal circuits that mediate this effect have not been defined. We investigated whether orexinergic neurones modulate the basal forebrain (BF) and locus coeruleus (LC) in emergence from anaesthesia.

METHODS

Hcrt^cre rats were generated using a CRISPR/Cas9-based approach. Viruses encoding optogenetic probes were injected into the perifornical lateral hypothalamic (PeFLH) area, optogenetic fibres were embedded in the PeFLH, BF, or LC, and changes in anaesthesia state under 1.4 vol% or 0.8 vol% isoflurane were determined.

RESULTS

In the PeFLH, 98.8% (0.4%) of orexin-A-positive cells expressed tdTomato, and 91.9% (2.2%) of tdTomato cells were orexin-A-positive. Under 1.4 vol% isoflurane anaesthesia, compared with control groups, burst suppression ratio was less, and emergence time was shorter in groups with optogenetic activation of orexinergic cell bodies in the PeFLH (923 [162] vs 493 [68] s, P=0.0003) or orexinergic terminals in the BF (937 (122) vs 674 (108) s, P=0.0049) or LC (913 [128] vs 742 [76] s, P=0.022). Optical stimulation of orexinergic terminals in the BF and LC also improved the movement scores of rats under 0.8 vol% isoflurane anaesthesia.

CONCLUSIONS

Activation of orexinergic terminals in the FB or LC mediates facilitation of emergence from anaesthesia by orexinergic neurones during isoflurane anaesthesia.

Neural Circuits for Sleep-Wake Regulation

The neural mechanisms of sleep, a fundamental biological behavior from invertebrates to humans, have been a long-standing mystery and present an enormous challenge. Gradually, perspectives on the neurobiology of sleep have been more various with the technical innovations over the recent decades, and studies have now identified many specific neural circuits that selectively regulate the initiation and maintenance of wake, rapid eye movement (REM) sleep, and non-REM (NREM) sleep. The cholinergic system in basal forebrain (BF) that fire maximally during waking and REM sleep is one of the key neuromodulation systems related to waking and REM sleep. Here we outline the recent progress of the BF cholinergic system in sleep-wake cycle. The intricate local connectivity and multiple projections to other cortical and subcortical regions of the BF cholinergic system elaborately presented here form a conceptual framework for understanding the coordinating effects with the dissecting regions. This framework also provides evidences regarding the relationships between the general anesthesia and wakefulness/sleep cycle focusing on the neural circuitry of unconsciousness induced by anesthetic drugs.

Escape From Oblivion: Neural Mechanisms of Emergence From General Anesthesia

The question of how general anesthetics suppress consciousness has persisted since the mid-19th century, but it is only relatively recently that the field has turned its focus to a systematic understanding of emergence. Once assumed to be a purely passive process, spontaneously occurring as residual levels of anesthetics dwindle below a critical value, emergence from general anesthesia has been reconsidered as an active and controllable process. Emergence is driven by mechanisms that can be distinct from entry to the anesthetized state. In this narrative review, we focus on the burgeoning scientific understanding of anesthetic emergence, summarizing current knowledge of the neurotransmitter, neuromodulators, and neuronal groups that prime the brain as it prepares for its journey back from oblivion. We also review evidence for possible strategies that may actively bias the brain back toward the wakeful state.

Optogenetic/Chemogenetic Activation of GABAergic Neurons in the Ventral Tegmental Area Facilitates General Anesthesia via Projections to the Lateral Hypothalamus in Mice

The ventral tegmental area (VTA) reportedly regulates sleep and wakefulness through communication with the lateral hypothalamus (LH). It has also been suggested that adequate anesthesia produced by administration of chloral hydrate, ketamine, or halothane significantly reduces the GABAergic neuronal firing rate within the VTA. However, the exact effects on GABAergic neurons in the VTA and the mechanisms through which these neurons modulate anesthesia through associated neural circuits is still unclear. Here, we used optogenetic and chemogenetic methods to specifically activate or inhibit GABAergic neuronal perikarya in the VTA or their projections to the LH in Vgat-Cre mice. Electroencephalogram (EEG) spectral analyses and burst suppression ratio (BSR) calculations were conducted following administration of 0.8 or 1.0% isoflurane, respectively; and loss of righting reflex (LORR), recovery of righting reflex (RORR), and anesthesia sensitivity were assessed under 1.4% isoflurane anesthesia. The results showed that activation of GABAergic neurons in the VTA increased delta wave power from 40.0 to 46.4% (P = 0.006) and decreased gamma wave power from 15.2 to 11.5% (P = 0.017) during anesthesia maintenance. BSR was increased from 51.8 to 68.3% (P = 0.017). Induction time (LORR) was reduced from 333 to 290 s (P = 0.019), whereas arousal time (RORR) was prolonged from 498 to 661 s (P = 0.007). Conversely, inhibition of VTA GABAergic neurons led to opposite effects. In contrast, optical activation of VTA-LH GABAergic projection neurons increased power of slow delta waves from 44.2 to 48.8% (P = 0.014) and decreased that of gamma oscillations from 10.2 to 8.0%. BSR was increased from 39.9 to 60.2% (P = 0.0002). LORR was reduced from 330 to 232 s (P = 0.002), and RORR increased from 396 to 565 s (P = 0.007). Optical inhibition of the projection neurons caused opposite effects in terms of both the EEG spectrum and the BSR, except that inhibition of this projection did not accelerate arousal time. These results indicate that VTA GABAergic neurons could facilitate the anesthetic effects of isoflurane during induction and maintenance while postponing anesthetic recovery, at least partially, through modulation of their projections to the LH.

Serotonergic neurons in the dorsal raphe nucleus mediate the arousal-promoting effect of orexin during isoflurane anesthesia in male rats

Previous studies have demonstrated that the activation of orexinergic neurons facilitates the recovery of animals from general anesthesia. Moreover, serotonergic neurons that receive projections from orexin neurons have also been shown to participate in sleep-wakefulness regulation. In the present study, we aimed to explore whether orexinergic neurons facilitate emergence from isoflurane anesthesia in rats by activating serotonergic neurons. Orexin A (30 or 100 pmol), orexin B (30 or 100 pmol), and their respective antagonists SB-334867 and TCS-OX2-29 (5 or 20 μg) were microinjected into the dorsal raphe nucleus (DRN) of rats, and their effects on induction and emergence times were analyzed. Electroencephalogram (EEG) changes were also recorded and analyzed to illuminate the effect of orexin injection into the DRN on cortical excitability under isoflurane anesthesia. Activation of serotonergic neurons was detected via immunohistochemical analysis of c-Fos expression following orexin administration. Our results indicated that injection of neither orexins nor orexin antagonists into the rat DRN exerted an impact on induction time, whereas orexin-A injection (100 pmol) enhanced arousal when compared with the saline group. In contrast, administration of orexin receptor type 1 antagonist SB-334867 (20 μg) prolonged emergence time from isoflurane anesthesia. Microinjection of orexin-A induced an arousal pattern on EEG, and decreased the burst suppression ratio under isoflurane anesthesia. Isoflurane anesthesia inhibited the activity of serotonergic neurons, as shown by decrease in the number of c-Fos-immunoreactive serotonergic neurons when compared with the sham group. This inhibitory effect was partially reversed by administration of orexin-A. Taken together, our findings suggest that orexinergic signals facilitate emergence from isoflurane anesthesia, at least partially, by reversing the effects of isoflurane on serotonergic neurons of the DRN.

Multimodal assessment of recovery from coma in a rat model of diffuse brainstem tegmentum injury

Despite the association between brainstem lesions and coma, a mechanistic understanding of coma pathogenesis and recovery is lacking. We developed a coma model in the rat mimicking human brainstem coma, which allowed multimodal analysis of a brainstem tegmentum lesion's effects on behavior, cortical electrophysiology, and global brain functional connectivity. After coma induction, we observed a transient period (∼1h) of unresponsiveness accompanied by cortical burst-suppression. Comatose rats then gradually regained behavioral responsiveness concurrent with emergence of delta/theta-predominant cortical rhythms in primary somatosensory cortex. During the acute stage of coma recovery (∼1-8h), longitudinal resting-state functional MRI revealed an increase in functional connectivity between subcortical arousal nuclei in the thalamus, basal forebrain, and basal ganglia and cortical regions implicated in awareness. This rat coma model provides an experimental platform to systematically study network-based mechanisms of coma pathogenesis and recovery, as well as to test targeted therapies aimed at promoting recovery of consciousness after coma.

Parabrachial Neurons Promote Behavior and Electroencephalographic Arousal From General Anesthesia

General anesthesia has been used clinically for more than 170 years, yet its underlying mechanisms are still not fully understood. The parabrachial nucleus (PBN) in the brainstem has been known to be crucial for regulating wakefulness and signs of arousal on the cortical electroencephalogram (EEG). Lesions of the parabrachial complex lead to unresponsiveness and a monotonous high-voltage, and a slow-wave EEG, which are the two main features of general anesthesia. However, it is unclear whether and how the PBN functions in the process of general anesthesia. By recording the levels of calcium in vivo in real-time, we found that the neural activity in PBN is suppressed during anesthesia, while it is robustly activated during recovery from propofol and isoflurane anesthesia. The activation of PBN neurons by “designer receptors exclusively activated by designer drugs” (DREADDs) shortened the recovery time but did not change the induction time. Cortical EEG recordings revealed that the neural activation of PBN specifically affected the recovery period, with a decrease of δ-band power or an increase in β-band power; no EEG changes were seen in the anesthesia period. Furthermore, the activation of PBN elicited neural activation in the prefrontal cortex, basal forebrain, lateral hypothalamus, thalamus, and supramammillary nucleus. Thus, PBN is critical for behavioral and electroencephalographic arousal without affecting the induction of general anesthesia.

Towards a better understanding of anesthesia emergence mechanisms: Research and clinical implications

Emergence from anesthesia (AE) is the ending stage of anesthesia featuring the transition from unconsciousness to complete wakefulness and recovery of consciousness (RoC). A wide range of undesirable complications, including coughing, respiratory/cardiovascular events, and mental status changes such as emergence delirium, and delayed RoC, may occur during this critical phase. In general anesthesia processes, induction and AE represent a neurobiological example of “hysteresis”. Indeed, AE mechanisms should not be simply considered as reverse events of those occurring in the induction phase. Anesthesia-induced loss of consciousness (LoC) and AE until RoC are quite distinct phenomena with, in part, a distinct neurobiology. Althoughanaesthetics produce LoC mostly by affecting cortical connectivity, arousal processes at the end of anesthesia are triggered by structures deep in the brain, rather than being induced within the neocortex. This work aimed to provide an overview on AE processes research, in terms of mechanisms, and EEG findings. Because most of the research in this field concerns preclinical investigations, translational suggestions and research perspectives are proposed. However, little is known about the relationship between AE neurobiology, and potential complications occurring during the emergence, and after the RoC. Thus, another scope of this review is to underline why a better understanding of AE mechanisms could have significant clinical implications, such as improving the patients’ quality of recovery, and avoiding early and late postoperative complications.

The integrative role of orexin/hypocretin neurons in nociceptive perception and analgesic regulation

The level of wakefulness is one of the major factors affecting nociception and pain. Stress-induced analgesia supports an animal’s survival via prompt defensive responses against predators or competitors. Previous studies have shown the pharmacological effects of orexin peptides on analgesia. However, orexin neurons contain not only orexin but also other co-transmitters such as dynorphin, neurotensin and glutamate. Thus, the physiological importance of orexin neuronal activity in nociception is unknown. Here we show that adult-stage selective ablation of orexin neurons enhances pain-related behaviors, while pharmacogenetic activation of orexin neurons induces analgesia. Additionally, we found correlative activation of orexin neurons during nociception using fiber photometry recordings of orexin neurons in conscious animals. These findings suggest an integrative role for orexin neurons in nociceptive perception and pain regulation.