Noradrenergic transmission in the central medial thalamic nucleus modulates the electroencephalographic activity and emergence from propofol anesthesia in rats

Zona Incerta GABAergic Neurons Facilitate Emergence from Isoflurane Anesthesia in Mice

The zona incerta (ZI) predominantly consists of gamma-aminobutyric acid (GABAergic) neurons, located adjacent to the lateral hypothalamus. GABA, acting on GABAA receptors, serves as a crucial neuromodulator in the initiation and maintenance of general anesthesia. In this study, we aimed to investigate the involvement of ZI GABAergic neurons in the general anesthesia process. Utilizing in-vivo calcium signal optical fiber recording, we observed a decrease in the activity of ZI GABAergic neurons during isoflurane anesthesia, followed by a significant increase during the recovery phase. Subsequently, we selectively ablated ZI GABAergic neurons to explore their role in general anesthesia, revealing no impact on the induction of isoflurane anesthesia but a prolonged recovery time, accompanied by a reduction in delta-band power in mice under isoflurane anesthesia. Finally, through optogenetic activation/inhibition of ZI GABAergic neurons during isoflurane anesthesia, we discovered that activation of these neurons facilitated emergence without affecting the induction process, while inhibition delayed emergence, leading to fluctuations in delta band activity. In summary, these findings highlight the involvement of ZI GABAergic neurons in modulating the emergence of isoflurane 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.

Extrasynaptic GABAA receptors in central medial thalamus mediate anesthesia in rats

Neuronal depression in the thalamus underlies anesthetic-induced loss of consciousness, while the precise sub-thalamus nuclei and molecular targets involved remain to be elucidated. The present study investigated the role of extrasynaptic GABAA receptors in the central medial thalamic nucleus (CM) in anesthesia induced by gaboxadol (THIP) and diazepam (DZP) in rats. Local lesion of the CM led to a decrease in the duration of loss of righting reflex induced by THIP and DZP. CM microinjection of THIP but not DZP induced anesthesia. The absence of righting reflex in THIP-treated rats was consistent with the increase of low frequency oscillations in the delta band in the medial prefrontal cortex. CM microinjection of GABAA receptor antagonist SR95531 significantly attenuated the anesthesia induced by systemically-administered THIP, but not DZP. Moreover, the rats with declined expression of GABAA receptor δ-subunit in the CM were less responsive to THIP or DZP. These findings explained a novel mechanism of THIP-induced loss of consciousness and highlighted the role of CM extrasynaptic GABAA receptors in mediating anesthesia.

GABAergic neurons of anterior thalamic reticular nucleus regulate states of consciousness in propofol- and isoflurane-mediated general anesthesia

BACKGROUND

The thalamus system plays critical roles in the regulation of reversible unconsciousness induced by general anesthetics, especially the arousal stage of general anesthesia (GA). But the function of thalamus in GA-induced loss of consciousness (LOC) is little known. The thalamic reticular nucleus (TRN) is the only GABAergic neurons-composed nucleus in the thalamus, which is composed of parvalbumin (PV) and somatostatin (SST)-expressing GABAergic neurons. The anterior sector of TRN (aTRN) is indicated to participate in the induction of anesthesia, but the roles remain unclear. This study aimed to reveal the role of the aTRN in propofol and isoflurane anesthesia.

METHODS

We first set up c-Fos straining to monitor the activity variation of aTRNPV and aTRNSST neurons during propofol and isoflurane anesthesia. Subsequently, optogenetic tools were utilized to activate aTRNPV and aTRNSST neurons to elucidate the roles of aTRNPV and aTRNSST neurons in propofol and isoflurane anesthesia. Electroencephalogram (EEG) recordings and behavioral tests were recorded and analyzed. Lastly, chemogenetic activation of the aTRNPV neurons was applied to confirm the function of the aTRN neurons in propofol and isoflurane anesthesia.

RESULTS

c-Fos straining showed that both aTRNPV and aTRNSST neurons are activated during the LOC period of propofol and isoflurane anesthesia. Optogenetic activation of aTRNPV and aTRNSST neurons promoted isoflurane induction and delayed the recovery of consciousness (ROC) after propofol and isoflurane anesthesia, meanwhile chemogenetic activation of the aTRNPV neurons displayed the similar effects. Moreover, optogenetic and chemogenetic activation of the aTRN neurons resulted in the accumulated burst suppression ratio (BSR) during propofol and isoflurane GA, although they represented different effects on the power distribution of EEG frequency.

CONCLUSION

Our findings reveal that the aTRN GABAergic neurons play a critical role in promoting the induction of propofol- and isoflurane-mediated GA.

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.

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.

Paraventricular thalamus controls consciousness transitions during propofol anaesthesia in mice

BACKGROUND

The neuronal mechanisms underlying propofol-induced modulation of consciousness are poorly understood. Neuroimaging studies suggest a potential role for non-specific thalamic nuclei in propofol-induced loss of consciousness. We investigated the contribution of the paraventricular thalamus (PVT), a midline thalamic nucleus that has been implicated in arousal control and general anaesthesia with inhaled anaesthetics, to loss and recovery of consciousness during propofol anaesthesia.

METHODS

Polysomnographic recordings and righting reflex test were used to determine the transitions of loss and recovery of righting reflex, used as a measure of consciousness in mice, during propofol anaesthesia in mice under conditions mimicking clinical propofol administration. PVT neuronal activities were monitored using fibre photometry and regulated using optogenetic and chemogenetic methods.

RESULTS

Population activities of PVT glutamatergic neurones began to decrease before propofol-induced loss of consciousness and rapidly increased to a peak at the onset of recovery of consciousness. Chemogenetic inhibition of PVT calretinin-expressing (PVT^CR) neurones shortened onset (from 176 [35] to 127 [26] s; P=0.001) and prolonged return (from 1568 [611] to 3126 [1616] s; P=0.002) of righting reflex. Conversely, chemogenetic activation of PVT^CR neurones exerted opposite effects. Furthermore, optogenetic silencing of PVT^CR neurones accelerated transitions to loss of consciousness (from 205 [35] to 158 [44] s; P=0.027) and slowed transitions to recovery of consciousness (from 230 [78] to 370 [99] s; P=0.041). During a steady period of unconsciousness maintained with continuous propofol infusion, brief optical activation of PVT^CR neurones restored cortical activity and arousal with a latency of about 5 s.

CONCLUSIONS

The paraventricular thalamus contributes to the control of consciousness transitions in propofol anaesthesia in mice. This provides a potential neuroanatomical target for controlling consciousness to reduce anaesthetic dose requirements and side effects.

Claustrum modulates behavioral sensitivity and EEG activity of propofol anesthesia

AIMS

The claustrum has long been regarded as a vital center for conscious control. Electrical stimulation or damage to the claustrum can result in decreased awareness or loss of consciousness, suggesting that the claustrum may be a target for the action of general anesthetics. This study aimed to determine the role of the claustrum in propofol anesthesia.

METHODS

We first applied a fiber photometry calcium signal recording system to record the claustral neuronal activity during the entire process of propofol anesthesia. Chemogenetic activation of claustral neurones was then performed to verify their role in anesthesia. Finally, muscimol (GABAa receptor agonist) and gabazine (GABAa receptor antagonist) were microinjected into the claustrum to determine whether their GABAa receptors were involved in modulating propofol anesthesia. EEG and behavioral indicators, such as anesthetic sensitivity and efficacy, were recorded and analyzed.

RESULTS

An evident anesthesia-related change in claustrum neuronal activity was suppressed during propofol-induced unconsciousness and restored following recovery from anesthesia. Chemogenetic activation of claustrum neurons results in attenuated propofol sensitivity, a shorter anesthesia duration, and an EEG shift toward wakefulness. Manipulation of GABAa receptors in the claustrum showed bidirectional control of propofol sensitivity, as activation decreases anesthesia efficiency while inactivation augments it. Additionally, inhibiting claustrum GABAa receptors increases cortical EEG slow waves.

CONCLUSIONS

Claustrum neurones and their GABAa receptors are implicated in the modulation of propofol anesthesia in both behavioral and EEG assessments. Our findings create scope to reveal the brain targets of anesthetic action further and add to the existing evidence on the consciousness-modulating role of the claustrum.

Electroacupuncture of the trigeminal nerve causes N-methyl-D-aspartate receptors to mediate blood-brain barrier opening and induces neuronal excitatory changes

The blood-brain barrier (BBB) is an important structure for maintaining environmental stability in the central nervous system (CNS). Our previous study showed that specific parameters of electroacupuncture (EA) at the head points Shuigou (GV26) and Baihui (GV20) can open the BBB; however, the mechanism by which stimulation of body surface acupuncture points on the head results in peripheral stimulation and affects the status of the central BBB and the neuronal excitatory changes has not been elucidated. We used laser spectroscopy, the In Vivo Imaging System (IVIS), immunofluorescence and immunoblotting to verified the role of the trigeminal nerve in BBB opening during EA, and we applied the central N-methyl-D-aspartate (NMDA) receptors blocker MK-801 to verify the mediating role of NMDA receptors in EA-induced BBB opening. Next, electroencephalogram (EEG) and in vivo calcium imaging techniques were applied to verify the possible electrical patterns of BBB opening promoted by different intensities of EA stimulation. The results showed that the trigeminal nerve plays an important role in the alteration of BBB permeability promoted by EA stimulation of the head acupoints. Brain NMDA receptors play a mediating role in promoting BBB permeability during EA of the trigeminal nerve, which may affect the expression of the TJ protein occludin, and thus alter BBB permeability. The analysis of the electrical mechanism showed that there was no significant change in the rhythm of local field potentials (LFP) in different brain regions across frequency bands immediately after EA of the trigeminal nerve at different intensities. However, the local primary somatosensory (S1BF) area corresponding to the trigeminal nerve showed a transient reduction in the delta rhythm of LFP with no change in the high-frequency band, and the action potential (spike) with short inter spike interval (ISI) varied with EA intensity. Meanwhile, EA of the trigeminal nerve resulted in rhythmic changes in calcium waves in the S1BF region, which were influenced by different EA intensities. This study provides a research perspective and a technical approach to further explore the mechanism of EA-induced BBB opening and its potential clinical applications.

Structural and functional organization of the midline and intralaminar nuclei of the thalamus

The midline and intralaminar nuclei of the thalamus form a major part of the "limbic thalamus;" that is, thalamic structures anatomically and functionally linked with the limbic forebrain. The midline nuclei consist of the paraventricular (PV) and paratenial nuclei, dorsally and the rhomboid and nucleus reuniens (RE), ventrally. The rostral intralaminar nuclei (ILt) consist of the central medial (CM), paracentral (PC) and central lateral (CL) nuclei. We presently concentrate on RE, PV, CM and CL nuclei of the thalamus. The nucleus reuniens receives a diverse array of input from limbic-related sites, and predominantly projects to the hippocampus and to "limbic" cortices. The RE participates in various cognitive functions including spatial working memory, executive functions (attention, behavioral flexibility) and affect/fear behavior. The PV receives significant limbic-related afferents, particularly the hypothalamus, and mainly distributes to "affective" structures of the forebrain including the bed nucleus of stria terminalis, nucleus accumbens and the amygdala. Accordingly, PV serves a critical role in "motivated behaviors" such as arousal, feeding/consummatory behavior and drug addiction. The rostral ILt receives both limbic and sensorimotor-related input and distributes widely over limbic and motor regions of the frontal cortex-and throughout the dorsal striatum. The intralaminar thalamus is critical for maintaining consciousness and directly participates in various sensorimotor functions (visuospatial or reaction time tasks) and cognitive tasks involving striatal-cortical interactions. As discussed herein, while each of the midline and intralaminar nuclei are anatomically and functionally distinct, they collectively serve a vital role in several affective, cognitive and executive behaviors - as major components of a brainstem-diencephalic-thalamocortical circuitry.

Glutamatergic neurons in paraventricular nucleus of the thalamus regulate the recovery from isoflurane anesthesia

Objectives

Previous studies have demonstrated that the paraventricular nucleus of the thalamus (PVT) is a key wakefulness-controlling nucleus in the thalamus. Therefore, PVT may also be involved in the process of general anesthesia. This study intends to explore the role of PVT in isoflurane anesthesia.

Methods

In the present study, we used the expression of c-Fos to observe the neuronal activity of PVT neurons under isoflurane anesthesia. We further recorded the effect of isoflurane anesthesia on the calcium signal of PVT glutamatergic neurons in real time with the help of calcium fiber photometry. We finally used chemogenetic technology to specifically regulate PVT glutamatergic neurons, and observed its effect on isoflurane anesthesia and cortical electroencephalography (EEG) in mice.

Results

We found that glutamatergic neurons of PVT exhibited high activity during wakefulness and low activity during isoflurane anesthesia. Activation of PVT glutamatergic neuronal caused an acceleration in emergence from isoflurane anesthesia accompanied with a decrease in EEG delta power (1–4 Hz). Whereas suppression of PVT glutamatergic neurons induced a delay recovery of isoflurane anesthesia, without affecting anesthesia induction.

Conclusions

Assuming a pharmacokinetic explanation for results can be excluded, these results demonstrate that the PVT is involved in regulating anesthesia emergence.

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.

Effect of Anesthetics on Functional Connectivity of Developing Brain

The potential anesthetic neurotoxicity on the neonate is an important focus of research investigation in the field of pediatric anesthesiology. It is essential to understand how these anesthetics may affect the development and growth of neonatal immature and vulnerable brains. Functional magnetic resonance imaging (fMRI) has suggested that using anesthetics result in reduced functional connectivity may consider as core sequence for the neurotoxicity and neurodegenerative changes in the developed brain. Anesthetics either directly impact the primary structures and functions of the brain or indirectly alter the hemodynamic parameters that contribute to cerebral blood flow (CBF) in neonatal patients. We hypothesis that anesthetic agents may either decrease the brain functional connectivity in neonatal patients or animals, which was observed by fMRI. This review will summarize the effect and mechanism of anesthesia on the rapid growth and development infant and neonate brain with fMRI through functional connectivity. It is possible to provide the new mechanism of neuronal injury induced by anesthetics and objective imaging evidence in animal developing brain.

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.

Norepinephrine modulates wakefulness via α1 adrenoceptors in paraventricular thalamic nucleus

Norepinephrine (NE) neurons in the locus coeruleus (LC) play key roles in modulating sleep and wakefulness. Recent studies have revealed that the paraventricular thalamic nucleus (PVT) is a critical wakefulness-controlling nucleus in mice. However, the effects of NE on PVT neurons remain largely unknown. Here, we investigated the mechanisms of NE modulating wakefulness in the PVT by using viral tracing, behavioral tests, slice electrophysiology, and optogenetics techniques. We found that the PVT-projecting LC neurons had few collateral projections to other brain nuclei. Behavioral tests showed that specific activation of the LC-PVT projections or microinjection of NE into the PVT accelerated emergence from general anesthesia and enhanced locomotion activity. Moreover, brain slice recording results indicated that NE increased the activity of the PVT neurons mainly by increasing the frequency of spontaneous excitatory postsynaptic currents via α1 adrenoceptors. Thus, our results demonstrate that NE modulates wakefulness via α1 adrenoceptors in the PVT.

Propofol differentially induces unconsciousness and respiratory depression through distinct interactions between GABAA receptor and GABAergic neuron in corresponding nuclei

Propofol is the most commonly used intravenous anesthetic worldwide. It can induce loss of consciousness prior to the occurrence of severe respiratory suppression, which is also a pharmacodynamic feature of all general anesthetics. However, the neural mechanisms underlying this natural phenomenon are controversial and highly related to patient safety. In the present study, we demonstrated that the pharmacodynamic effects of propofol (50 and 100 μM) on suppression of consciousness-related excitatory postsynaptic currents in the medial prefrontal cortex (mPFC) and centromedian nucleus of the thalamus (CMT) were lower than those in the kernel respiratory rhythmogenesis nucleus pre-Bötzinger complex (PrBo). Furthermore, we unexpectedly found that the GABAA receptor β3 subunit is the key target for propofol's action and that it is mutually and exclusively expressed in GABAergic neurons. It is also more abundant in the mPFC and CMT, but mainly co-localized with GABAergic neurons in the PrBo. As a result, the differentiated expression pattern should mediate more neuron suppression through the activation of GABAergic neurons in the mPFC and CMT at low doses of propofol (50 μM). However, PrBo GABAergic neurons were only activated by propofol at a high dose (100 μM). These results highlight the detailed pharmacodynamic effects of propofol on consciousness-related and respiration-related nuclei and provide the distinct interaction mechanism between the β3 subunit and GABAergic neurons in mediating the suppression of consciousness compared to the inhibition of respiration.

Effect of ketamine on voltage-gated potassium channels in rat primary sensory cortex pyramidal neurons

Ketamine is an intravenous anesthetic commonly used in clinical, which has sedative and analgesic effects. Potassium channels exert many physiological functions in excitable cells. Therefore, potassium channels may be one of the targets of ketamine. In this study, we used patch clamp to study the effects of ketamine on voltage-gated potassium channels in primary sensory cortex (S1) neurons. We recorded the outward potassium currents (IA) and delayed rectifier potassium currents (IK) separately. We found that ketamine both concentration-dependently inhibited IA currents and IK currents in S1 neurons. Ketamine (100 and 300 μM) induced a concentration-dependent hyperpolarizing shift in V1/2, without affecting the slope factor (κ) or inactivation of IA. Ketamine induced a concentration-dependent hyperpolarizing shift in V1/2 of IK, without affecting its κ. Ketamine (100 and 300 μM) did not alter the steady-state activation or its κ. Hence, ketamine inhibits IA and IK in a concentration-dependent manner in S1 pyramidal neurons. The inactivation of IA does not appear to be involved in the inhibitory effect of ketamine on IA. Ketamine inhibits IK mainly by speeding up the inactivation of IK in S1 pyramidal neurons.

Dopamine D1 Receptor in the Nucleus Accumbens Modulates the Emergence from Propofol Anesthesia in Rat

It has been reported that systemic activation of D1 receptors promotes emergence from isoflurane-induced unconsciousness, suggesting that the central dopaminergic system is involved in the process of recovering from general anesthesia. The nucleus accumbens (NAc) contains abundant GABAergic medium spiny neurons (MSNs) expressing the D1 receptor (D1R), which plays a key role in sleep-wake behavior. However, the role of NAc D1 receptors in the process of emergence from general anesthesia has not been identified. Here, using real-time in vivo fiber photometry, we found that neuronal activity in the NAc was markedly disinhibited during recovery from propofol anesthesia. Subsequently, microinjection of a D1R selective agonist (chloro-APB hydrobromide) into the NAc notably reduced the time to emerge from propofol anesthesia with a decrease in δ-band power and an increase in β-band power evident in the cortical electroencephalogram. These effects were prevented by pretreatment with a D1R antagonist (SCH-23390). Whole-cell patch clamp recordings were performed to further explore the cellular mechanism underlying the modulation of D1 receptors on MSNs under propofol anesthesia. Our data primarily demonstrated that propofol increased the frequency and prolonged the decay time of spontaneous inhibitory postsynaptic currents (sIPSCs) and miniature IPSCs (mIPSCs) of MSNs expressing D1 receptors. A D1R agonist attenuated the effect of propofol on the frequency of sIPSCs and mIPSCs, and the effects of the agonist were eliminated by preapplication of SCH-23390. Collectively, these results indicate that modulation of the D1 receptor on the activity of NAc MSNs is vital for emergence from propofol-induced unconsciousness.

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.

Propofol modulates inhibitory inputs in paraventricular thalamic nucleus of mice

The mechanisms of general anaesthetics such as propofol have drawn substantial attention. The effects of propofol on inhibitory postsynaptic currents are not exactly the same in different brain nuclei. Recent studies revealed that the paraventricular thalamic nucleus (PVT) is a critical nucleus modulating wakefulness. However, the effects of propofol on PVT neurons and the mechanisms underlying such effects remain unknown. Here, we performed the whole-cell recording of the PVT neurons in acute brain slices and bath application of propofol. We found that propofol hyperpolarized the membrane potentials of the PVT neurons and suppressed the action potentials induced by step-current injection. Propofol did not affect the spontaneous inhibitory postsynaptic currents (sIPSCs) amplitude or frequency, but prolonged the sIPSCs half-width. Besides, propofol increased miniature inhibitory synaptic currents (mIPSCs) frequency and half-width. Furthermore, propofol could induce GABA_A receptors-mediated tonic inhibitory currents dose-dependently. Thus, our results demonstrate that propofol hyperpolarizes PVT neurons by modulating inhibitory currents via GABA_A receptors in mice.

Locus Coeruleus to Paraventricular Thalamus Projections Facilitate Emergence From Isoflurane Anesthesia in Mice

Locus coeruleus (LC) sends widespread outputs to many brain regions to modulate diverse functions, including sleep/wake states, attention, and the general anesthetic state. The paraventricular thalamus (PVT) is a critical thalamic area for arousal and receives dense tyrosine-hydroxylase (TH) inputs from the LC. Although anesthesia and sleep may share a common pathway, it is important to understand the processes underlying emergence from anesthesia. In this study, we hypothesize that LC TH neurons and the TH:LC-PVT circuit may be involved in regulating emergence from anesthesia. Only male mice are used in this study. Here, using c-Fos as a marker of neural activity, we identify LC TH expressing neurons are active during anesthesia emergence. Remarkably, chemogenetic activation of LC TH neurons shortens emergence time from anesthesia and promotes cortical arousal. Moreover, enhanced c-Fos expression is observed in the PVT after LC TH neurons activation. Optogenetic activation of the TH:LC-PVT projections accelerates emergence from anesthesia, whereas, chemogenetic inhibition of the TH:LC-PVT circuit prolongs time to wakefulness. Furthermore, optogenetic activation of the TH:LC-PVT projections produces electrophysiological evidence of arousal. Together, these results demonstrate that activation of the TH:LC-PVT projections is helpful in facilitating the transition from isoflurane anesthesia to an arousal state, which may provide a new strategy in shortening the emergence time after general anesthesia.

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.

Application of quinpirole in the paraventricular thalamus facilitates emergence from isoflurane anesthesia in mice

BACKGROUND AND PURPOSE

Dopamine is well-known to contribute to emergence from anesthesia. Previous studies have demonstrated that the paraventricular thalamus (PVT) in the midline nuclei is crucial for wakefulness. Moreover, the PVT receives dopaminergic projections from the brainstem. Therefore, we hypothesize that the dopaminergic signaling in the PVT plays a role in emergence from isoflurane anesthesia.

METHODS

We used c-Fos immunohistochemistry to reveal the activity of PVT neurons in three groups: The first group (iso⁺ EM^- ) underwent the anesthesia protocol and was sacrificed before emergence. The second group (iso⁺ EM⁺ ) underwent passive emergence from the same anesthesia protocol. The last group (oxy⁺ ) received oxygen. D2-like agonist quinpirole (2 or 4 mM) or D2-like antagonist raclopride (2 or 5 mM) was microinjected into the PVT, and their effects on emergence and induction time were analyzed. Surface cortical electroencephalogram (EEG) recordings were used to explore the effects of quinpirole injection into the PVT on cortical excitability during isoflurane anesthesia. The activity of PVT neurons after quinpirole injection was assessed by c-Fos immunohistochemistry.

RESULTS

The number of c-Fos-positive nuclei for the iso⁺ EM⁺ group was significantly higher than the oxy⁺ and iso⁺ EM^- groups. Application of quinpirole (4 mM) into the PVT shortened emergence time compared with the saline group (p < .01). In contrast, administration of raclopride (2 mM) delayed emergence time (p < .05). Neither quinpirole nor raclopride exerted an effect on induction time. EEG analyses showed that quinpirole (4 mM) decreased the burst suppression ratio during isoflurane anesthesia (p < .01). The number of c-Fos-positive nuclei for the quinpirole (4 mM) group was significantly higher than saline group (p < .01).

CONCLUSIONS

Our findings suggest that the activity of PVT neurons is enhanced after emergence from anesthesia, and the dopaminergic signaling in the PVT may facilitate emergence from isoflurane anesthesia.

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.

Dopamine neurons in the ventral periaqueductal gray modulate isoflurane anesthesia in rats

Aims

General anesthesia has been applied in surgery for more than 170 years, and there is little doubt that GABA_A receptors have an important role as anesthetic molecular targets, but its neural mechanisms remain unclear. Increasing researchers have shown that dopaminergic pathways in the brain are crucial for sleep and wake. General anesthesia‐induced unconsciousness and natural sleep share some neural correlates. However, the role of GABA_A receptors in ventral periaqueductal gray (vPAG) dopamine (DA) neurons in the isoflurane‐induced unconsciousness has yet to be identified.

Methods

In the present study, we used calcium fiber photometry recording to explore that the activity of ventral periaqueductal gray (vPAG) neurons. Then, rats were unilaterally microinjected with 6‐hydroxydopamine into the vPAG area to determine the role of vPAG‐DA neurons in isoflurane‐induced‐anesthesia. Furthermore, thirty SD rats were divided into three groups: a GABA_AR agonist‐muscimol group, a GABA_AR antagonist‐gabazine group, and a control group. Finally, whole‐cell patch clamp was used to examine the effects of isoflurane and GABA_A receptor agonist/antagonist on vPAG‐DA neurons.

Results

The vPAG neurons were markedly inhibited during isoflurane anesthesia induction and that these neurons were activated during emergence from isoflurane anesthesia. Lesion to the vPAG‐DA neurons shortened the induction time and prolonged the emergence time while increasing δ power in isoflurane anesthesia. Intracerebral injection of the GABA_A receptor agonist (muscimol) into the vPAG accelerated the induction of anesthesia and delayed recovery from isoflurane anesthesia, with a decrease of δ power and an augment of β power. Injection of GABA_A receptor antagonist gabazine generated the opposite effects. Isoflurane enhanced GABAergic transmission, and GABA_A receptor agonist partly increased isoflurane‐induced inhibition of vPAG‐DA neurons, while GABA_A receptor antagonist evidently attenuated GABAergic transmission.

Conclusion

Our results suggest that vPAG‐DA neurons are involved in isoflurane anesthesia through activation of the GABA_A receptor.

Effects of ketamine on voltage-gated sodium channels in the barrel cortex and the ventral posteromedial nucleus slices of rats

Ketamine is commonly used as a dissociative anesthetic with unique actions in the central nervous system. Previous studies have found that the thalamocortical systems play an important role in general anesthetics induced unconsciousness. Whether the voltage-gated sodium channels in the thalamocortical systems are the target of ketamine remain unclear. The present study used a whole-cell patch-clamp technique to observe the effects of ketamine on voltage-gated Na channels in thalamocortical pyramidal neurons. We found that IC50 of ketamine on Na currents in the primary somatosensory barrel cortex pyramidal neurons and the thalamus ventral posteromedial nucleus pyramidal neurons was 686.72 ± 39.92 and 842.65 ± 87.28 μM, respectively. Ketamine accelerated the Na channels inactivation and slowed inactivation of Na channels after recovery but did not affect the activation. We demonstrated the detailed suppression process of neural voltage-gated Na channels by ketamine on thalamocortical slice. This may provide a new insight into the mechanical explanation for the ketamine anesthesia.

The (Un)Conscious Mouse as a Model for Human Brain Functions: Key Principles of Anesthesia and Their Impact on Translational Neuroimaging

In recent years, technical and procedural advances have brought functional magnetic resonance imaging (fMRI) to the field of murine neuroscience. Due to its unique capacity to measure functional activity non-invasively, across the entire brain, fMRI allows for the direct comparison of large-scale murine and human brain functions. This opens an avenue for bidirectional translational strategies to address fundamental questions ranging from neurological disorders to the nature of consciousness. The key challenges of murine fMRI are: (1) to generate and maintain functional brain states that approximate those of calm and relaxed human volunteers, while (2) preserving neurovascular coupling and physiological baseline conditions. Low-dose anesthetic protocols are commonly applied in murine functional brain studies to prevent stress and facilitate a calm and relaxed condition among animals. Yet, current mono-anesthesia has been shown to impair neural transmission and hemodynamic integrity. By linking the current state of murine electrophysiology, Ca2+ imaging and fMRI of anesthetic effects to findings from human studies, this systematic review proposes general principles to design, apply and monitor anesthetic protocols in a more sophisticated way. The further development of balanced multimodal anesthesia, combining two or more drugs with complementary modes of action helps to shape and maintain specific brain states and relevant aspects of murine physiology. Functional connectivity and its dynamic repertoire as assessed by fMRI can be used to make inferences about cortical states and provide additional information about whole-brain functional dynamics. Based on this, a simple and comprehensive functional neurosignature pattern can be determined for use in defining brain states and anesthetic depth in rest and in response to stimuli. Such a signature can be evaluated and shared between labs to indicate the brain state of a mouse during experiments, an important step toward translating findings across species.

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.

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.

Ketamine attenuates the glutamatergic neurotransmission in the ventral posteromedial nucleus slices of rats

Background

Ketamine is a frequently used intravenous anesthetic, which can reversibly induce loss of consciousness (LOC). Previous studies have demonstrated that thalamocortical system is critical for information transmission and integration in the brain. The ventral posteromedial nucleus (VPM) is a critical component of thalamocortical system. Glutamate is an important excitatory neurotransmitter in the brain and may be involved in ketamine-induced LOC.

Methods

The study used whole-cell patch-clamp to observe the effect of ketamine (30 μM–1000 μM) on glutamatergic neurotransmission in VPM slices.

Results

Ketamine significantly decreased the amplitude of glutamatergic spontaneous excitatory postsynaptic currents (sEPSCs), but only higher concentration of ketamine (300 μM and 1000 μM) suppressed the frequency of sEPSCs. Ketamine (100 μM–1000 μM) also decreased the amplitude of glutamatergic miniature excitatory postsynaptic currents (mEPSCs), without altering the frequency.

Conclusions

In VPM neurons, ketamine attenuates the glutamatergic neurotransmission mainly through postsynaptic mechanism and action potential may be involved in the process.

Electronic supplementary material

The online version of this article (doi:10.1186/s12871-017-0404-5) contains supplementary material, which is available to authorized users.