3.1Effects of anesthetics on GABAA receptors
GABAA receptor is the main target of modern anesthetics and sedatives.GABAAR is a GABA gated Cl channel. During neuronal development, Na + - K + - 2Cl combined transporter (NKCC1) is mainly regulated by postsynaptic membrane. The expression of NKCC1 in cortical neurons was the highest within 2 weeks after birth and decreased rapidly after 2 weeks.The main effects of sevoflurane on the brain include the activation of various two pore domain K+ channels,depression of glutamate release, inhibit nicotinic acetylcholine receptor and enhance a variety of γ-aminobutyric acid (GABA)A and strychnine-sensitive glycine receptors(Alkire and McReynolds et al., 2007; Solt and Forman, 2007; Ishizeki and Nishikawa et al., 2008).All of these effects, which lead to inhibition and induction of the anesthetic state, should not cause hyperexcitability by themselves and can be observed during and immediately after sevoflurane anesthesia.On the other hand, as the charge carrier of GABAA and strychn-sensitive glycine receptor channels, the change of the transmembrane gradient of Cl- can reverse the output activation of these receptors from inhibition to excitation, so it may play an important role in the hyperexcitability induced by sevoflurane anesthesia(Bormann and Hamill et al., 1987; Staley and Soldo et al., 1995). This possibility is indirectly supported by the observation that all general anesthetics with gaba-acting components may induce episodes of hyperactivity(Mohanram and Kumar et al., 2007; Zeiler and Kaplan, 2008).
3.2Effects of anesthetics on NMDA receptors
NMDARs (N-methyl-D-aspartate receptor) are glutamate-gated ionic receptors (ligand-gated ion channels), which are widely distributed in the central nervous system. NMDARs mediate many neuronal functions during neuronal development as glutamate plays an important role by regulating neuronal survival and migration, axonal and dendritic structure, synaptogenesis and synaptic plasticity(McDonald and Johnston, 1990; Komuro and Rakic, 1993; Reiprich and Kilb et al., 2005). Clinically, NMDARs are targets of several anesthetics. Ketamine is a dissociative anesthetic, mainly as a non-competitive antagonist, blocking NMDA ion channel, commonly used for analgesia and anesthesia in various pediatric surgeries because of its rapid onset and short acting followed by rapid recovery(Staley and Soldo et al., 1995; Kohrs and Durieux, 1998; Lunardi and Ori et al., 2010). Recent studies have further shown that repeated exposure to ketamine during brain development can lead to subsequent long-term cognitive deficits.It has been suggested that ketamine-induced apoptosis is associated with compensatory upregulation of NMDARs, which is then overstimulated by endogenous glutamate, leading to Ca2+ signaling dysregulation(Shi and Guo et al., 2010; Liu and Paule et al., 2011; Sinner and Friedrich et al., 2011).
3.3 Effects of anesthetics on voltage-gated Na+ channels
There is evidence that volatile anesthesia play an important role in presynaptic voltage-gated Na+ channels(Herold and Prosch et al., 2014).In the late 1970s, the effects of inhaled volatile anesthetic ether and haloalkane on sodium ion currents were reported for the first time in clinical use(Nikonorov and Blanck et al., 1998; Herold and Hemmings, 2012).During steady-state inactivation, all anesthetics produce a typical reversible hyperpolarized shift, with a (not always reversible) drop in peak Na+ current, resulting in impaired axon conduction. The results showed that the inhibition of Na+ conductivity and conduction of action potential resulted from the significant increase of membrane thickness after anesthesia adsorption. Brain specific subtype Nav1.2 is one of the earliest subtypes sensitive to clinical volatile anesthetics concentration(Kendig and Courtney et al., 1979).The Nav family consists of nine homologous pores forming a subunit (NaV1.1-NaV1.9) with a different cellular and subcellular distributions(Bean and Shrager et al., 1981).
To date, the effects of various volatile anesthetics on Nav1.4, Nav1.5, Nav1.6, Nav1.7 and Nav1.8 have been reported(Ratnakumari and Hemmings, 1998; Catterall, 2000; OuYang and Hemmings, 2007; Yokoyama, 2011). Nav1.6 and Nav1.2 were highly expressed in most excitatory neurons(Herold and Prosch et al., 2014; Purtell and Gingrich et al., 2015),while Nav1.1 was preferentially expressed in inhibitory GABAergic interneurons(Whitaker and Clare et al., 2000; Tian and Wang et al., 2014). Volatile anesthetics mainly inhibit Nav by stabilizing Nav inactivation and delay recovery from steady-state inactivation(Lorincz and Nusser, 2008).Although there were small pharmaco-specific quantitative differences in their effects, all volatile anesthetics inhibited Na+ current peaks in a volt-dependent manner, leading to steady-state inactivation of volt-dependent hyperpolarization. Compared with isoflurane, low concentrations of sevoflurane resulted in increased Na+ peak current and increased inactivation recovery rate constant.This acceleration is in contrast to the electrophysiological effects of volatile anesthetics on NaChBac and Nav in mammals, where the recovery time of inactivation is prolonged by sevoflurane at multiple binding sites, affecting activation gating (e.g., s4-S5 linker or activation gating) and inactivation gating. (O to 5 MAC) Accelerates current attenuation and slows down channel port congestion.Studies have shown that isoflurane inhibits Na+ current and action potential amplitudes in isolated rat neuropituitary nerve endings by inhibiting voltage-gated sodium channels (Nav)(Ogiwara and Miyamoto et al., 2007).Due to differences in expression of specific Nav subtypes between and within neurons, Nav1.2 and Nav1.6 are more inhibited than Nav1.1 and may contribute to the selective effects of volatile anesthetics on synaptic transmission regions and neurotransmitters.Volatile anesthetics inhibit the release of glutamate by blocking Na + channels(Ouyang and Wang et al., 2003; Zhou and Johnson et al., 2019).Propofol has also been shown to inhibit Nav and glutamate release in synaptosomes in a dosedependent and reversible manner, contributing to its anesthetic anticonvulsant and neuroprotective effects(Ouyang and Wang et al., 2003) .The results showed that propofol can directly regulate neuronal excitability by inhibiting Nav and persistent Nav current(Ratnakumari and Hemmings, 1996).
3.4 Effects of anesthetics on K+ channels
Potassium ion channels are the most widely distributed ion channel among many ion channels. Potassium channels play an important role in controlling neuronal excitability and neurotransmitter release in the central nervous system by determining the membrane potential of neurons(Ratnakumari and Vysotskaya et al., 2000; Gutman and Chandy et al., 2003).They are voltage-gated (Kv)channels, the background/leak or tandem 2-pore (K2P)families, inwardly-rectifying (Kir) channels, and Ca2+-activated (KCa) channels. Kv has been shown to be a molecular target of general anaesthesia.The Kv channel family of voltage switch is composed of 40 coding hole recombination subunits, which are further divided into 12 subunits according to sequence homology (Kv1-Kv12).The mechanism of sevoflurane action on Kv channel can be interpreted as that sevoflurane is beneficial to the opening state of Kv1.3 and Kv1.5 channels, but accelerates their inactivation at high membrane potential(Greene and Hoshi, 2017; Huang and Chacron, 2017).
K+ channels belong to the Kv1x family and are important targets of volatile anesthetics. Changing the activity of Kv channel in CMT plays an important role in the change of awakening state under general anesthesia.Kulkarni et al reported that halothane and ketamine restrained the Kv2.1 channel in a dose-dependent manner(Lujan and Aguado, 2015).Propofol inhibits its activity by decreasing the expression level of Kv2.1 channels in cortical neurons(Cooper, 2011).Similar to K+channels associated with Shab, the voltage-gated Kv3.2 channel, a homolog of the K-Shaw2 channel, is inhibited by alkanes and halogenated inhalation anesthetics such as halothane, isoflurane, chloroform and desflurane(Misonou, 2010).Interestingly, the Kv3.2 channel was activated by sevoflurane(Woll and Peng et al., 2017).In addition, many intravenous anesthesia drugs such as thiopental, methoxyl, propofol, midazolam, and haloperidol (but not barbiturates or ketamine) also inhibit K+ currents similar to Shaw voltage dependence(Liang and Anderson et al., 2015).Among other Kv channels, sevoflurane inhibited the activity of Kv7.1 channel, but the mechanism is not clear(Rajan and Wischmeyer et al., 2001). KV7.2/7.3, Kv3.4 and Kv4.2 channels are resistant to volatile general anesthetics(Kulkarni and Zorn et al., 1996).
K2P channel conduction time and voltage independent background current or leakage current, generate negative membrane potential in activated and non-activated cells.Based on coding genes and biophysics, the K2P channel can be divided into six subgroups (TREK TASK TWIK TALK THIK and TRESK).TASK and TREK channel subsets are the most concerned among K+ channels in the neuronal background activated by anesthesia(Du G and Chen et al., 2011).TREK-1 and its functional homologue TREK-2 are sensitive to volatile anesthetics such as chloroform, ether, halothane and isoflurane(Lee and Lolicato et al., 2021). While TRAAK, which is related to the structure and function of the K2P channel, even forms a functional heterodimer channel with TREK-1 and is insensitive to volatile anesthetics(Laigle and Confort-Gouny et al., 2012; Djillani and Mazella et al., 2019). This difference may be due to the special structural characteristics of the transmembrane helical straightening buckling of the M4 section of the TRAAK channel (KCNK4)(Wang and Shi et al., 2019).Another general anesthetic gases that open the TREK-1 channel are nitrous oxide and xenon(Gruss and Bushell et al., 2004).Otherwise, several pharmacologically relevant intravenous anesthetics concentrations (chloral hydrate and trichloroethanol, but not phenobarbital) have been shown to open TREK-1 and TRAAK channels(Viatchenko-Karpinski and Ling et al., 2018).Volatile anesthetics (of which chloroform is the strongest) activate TASK-2 channels, but not as effectively as TASK and TREK channels (Bista and Pawlowski et al., 2015). Although in vitro studies have shown that currents in TALK-1 and TALk-2 channels are inhibited by chloroform, halothane, and isoflurane, since they are expressed primarily in the pancreas, they are unlikely to play a role in general anesthesia and altered wakeiness(Kawasaki and Kawasaki et al., 2001) .THIK subfamily channels include THIK-1 andTHIK-2,the function of THIK-2 channel is weak, and the current of THIK-1 channel is inhibited by inhaled anesthetics such as halothane(Enyedi and Czirjak, 2015).TRESK-1 and TRESK-2 belong to the TRESK channel subfamily.TRESK channel protein expression was detected in the dorsal root and trigeminal ganglion, cerebellum, brainstem, sympathetic ganglion and parasympathetic ganglion of the nervous system, suggesting that TRESK may play a role in pain transmission and anesthesia(Chae and Zhang et al., 2010).
TRESKKO mice were also used to study the role of the TRESK channel in volatile general anesthesia. The results showed that the MAC values of isoflurane increased slightly (8%), while the MAC values of other volatile anesthetics did not change significantly(Wei and Gutman et al., 2005).The KCa family can be divided into three categories:BK(high conductance), IK(medium conductance) and SK(small conductance) channels(Salkoff and Butler et al., 2006).BK channels are tetramer.They consist of 6/7 subunits that span the diaphragmatic segment and are encoded by the Slo1-3 gene(Schreiber and Wei et al., 1998).But Slo1 and Slo2 are expressed in the brain, while Slo3 is expressed specifically in the testes and sperm(Begenisich and Nakamoto et al., 2004).Slo1 channels are activated by voltage and intracellular Ca2+, while IK and SK channels are widely expressed in almost the entire brain only by intracellular Ca2+ gated BK channels, whereas IK channels are mainly expressed in T-lymphocytic red blood cells and parotid glands(Hong and Puil et al., 1994). Slo2 channels are activated by Na+, not Ca2+. The clinically relevant dose (0.5 mmol/L) of volatile anesthetic halothane reduced the probability of BK channel opening without changing the single channel conductance.However, this effect is blocked by increasing the cytoplasmic free Ca2+ concentration from 1 lmol/L to 100 lmol/L, suggesting that halothane is a channel-blocking blocker(Pancrazio and Park et al., 1992). In addition to halothane, isoflurane and enflurane also inhibit the BK channel at clinically relevant doses(Pancrazio and Park et al., 1993; Namba and Ishii et al., 2000). Otherwise, intravenous anesthesia ketamine selectively blocked THE BK channel in a dose-dependent manner. Many volatile anesthetics (e.g., halothane, isoflurane, enflurane, and sevoflurane) quickly and reversibly inhibit the IK channel current with an EC50 of 0.4 to 1mmol /L, whereas SK channels are not inhibited by volatile anesthetics(Dreixler and Jenkins et al., 2000). However, intravenous administration of ketamine, barbiturates and methoxamine can block SK channel(Dreixler and Jenkins et al., 2000).Patch clamp brain section experiments further confirmed that propofol enhanced inhibition of posterior trapezoidal neuronal discharge by blocking SK channels(Ying and Goldstein, 2005). Anesthetics can induce apoptosis of nerve cells through their effect on K+ channels. Although the effect of general anaesthesia on Ca2+-activated K+ channels has been studied, its underlying mechanisms and overall role in general anaesthesia remain to be explored(Fig2).
3.5 Effects of anesthetics on Ca2+ channels
Voltage-gated calcium channels can be divided into 5 subtypes, known as L, N, P/Q, R and T subtypes(Catterall, 2011).According to the degree of membrane depolarization required for activation, the two main subtypes of voltage-gated calcium channels (Cav) are divided into low-voltage activation (LVA, also known as T-type) channels and high-voltage activation (HVA, also known as L-type) channels(Simms and Zamponi, 2014).According to different SUBunits of Cav, T-type Cav is divided into subtypes Cav3.1,Cav3.2, and Cav3.3, L-type Cav is divided into Cav1.1 to 1.4, and for other Cav subtypes,Cav2.1, Cav2.2, and Cav2.3 are Type NP/Q and R caVs are involved in regulating neurotransmitter release and dendritic Ca2+ transients(Catterall, 2011).Inhibition of Cav to reduce transmitter release has long been considered a reasonable mechanism for general anesthesia(Miao and Frazer et al., 1995). Voltage-gated calcium channels (Cav) in the hippocampus are relatively insensitive to isoflurane compared to Nav(Hall and Lieb et al., 1994; Wu and Sun et al., 2004).Changes in intracellular calcium concentration induced by general anesthesia can increase potassium conductance and lead to excessive cell polarization, thereby reducing cell excitability(Kress and Tas, 1993; Orestes and Todorovic, 2010).The effect of volatile anesthetics on the excitability and neurotransmission of nervous system cells, in addition to the heart L-type channel, neuronal calcium channel is considered to be an important target of general anesthesia(Charlesworth and Pocock et al., 1994; Miao and Frazer et al., 1995; McDowell and Pancrazio et al., 1996).
Isoflurane can inhibit the T-type calcium current peak of the natural thalamic cortical neurons in acute rat brain tablets and reduce the activity of thalamic cells, which may be the key to inducing loss of consciousness and general anesthesia hypnosis(Eckle and Digruccio et al., 2012). T-type calcium channel currents in thalamic neurons play a key role in the production of low-amplitude oscillatory bursts and are associated with arousal and sleep states, as well as epileptic seizures. It has reported that enflurane can block the T-type Cav current of thalamic reticular nucleus neurons(Joksovic and Bayliss et al., 2005).Recent studies have found that isoflurane inhibits extracellular secretion of dopaminergic neurons through a different mechanism than non-dopaminergic neurons, which involves reducing Ca2+ entry through Cav2.1 and/or Cav2.2(Torturo and Zhou et al., 2019).Propofol inhibited glutamate release at clinically relevant concentrations mainly due to direct inhibition of P/ Q type voltage-sensitive calcium channel currents(Kitayama and Hirota et al., 2002).The sensitivity of knockout mice without N-type Cav to propofol was significantly lower than that of wild-type mice, suggesting that inhibition of n-type channels could counteract the anesthetic effect of propofol[95].Takei et al. also demonstrated the role of r-type (Cav2.3) calcium channels, finding that Cav2.3 knockout reduced the anesthetic sensitivity of propofol(Takei and Saegusa et al., 2003). Both L-type and T-type Cavs are possible targets of etomidate (Hirota and Lambert, 1996; Todorovic and Perez-Reyes et al., 2000).Clinically relevant concentrations of the anesthetic barbiturate sodium thiopental directly inhibit P/Qtype calcium(Joksovic and Bayliss et al., 2005).Etomidate and thiopental interact with α -1, 4-dihydropyridine subunit binding site of L-channel in rat cortical membrane, but not with verapamil binding site(Hirota and Lambert, 1996; Hirota and Lambert, 1996).Neuronal calcium channel subtypes are less sensitive to intravenous general anesthesia, although volt-gated calcium channels may be the putative site of the effects of intravenous anesthesia(Fig3).