Revisiting anesthesia-induced preconditioning for neuroprotection in the aging brain: a narrative review
Article information
Abstract
The growing number of older adults undergoing surgery necessitates that we address the adverse effects of overt and covert perioperative stroke. Preclinical studies have suggested that anesthesia-induced preconditioning may provide neuroprotection by preserving mitochondrial function, activating cytosolic signaling pathways, and reducing neuroinflammation. However, these promising findings from animal studies have not yet translated into improved clinical outcomes. The discordance between preclinical and clinical outcomes may be due to age-related mitochondrial dysfunction and other comorbidities in older human populations, which reduce the effectiveness of anesthetic preconditioning. Mitochondria, which are central to the effectiveness of preconditioning, may be therapeutic targets to restore the neuroprotective effects of anesthetic preconditioning in the aging brain. Emerging evidence suggests that physical prehabilitation, a key component of Enhanced Recovery After Surgery programs, may influence mitochondrial function and could thus, restore anesthesia-induced preconditioning. Although further research is needed to determine the impact of physical prehabilitation on mitochondrial function and anesthetic preconditioning, incorporating physical prehabilitation into perioperative care might enhance neurological outcomes for older patients undergoing surgery.
Introduction
In recent years, increasing numbers of older adults are undergoing anesthesia and surgery. Advanced age is associated with multiple comorbidities that may impede favorable perioperative outcomes. One such condition is overt perioperative stroke, a cerebrovascular event that occurs intraoperatively or within 30-days postoperatively, which causes motor, sensory, or cognitive dysfunction that lasts at least 24 h [1,2]. Another condition leading to adverse outcomes is covert perioperative stroke, which is a silent brain infarct that can only be detected using brain imaging [3]. Although the incidence of perioperative stroke varies across patient populations and surgical procedures, the average incidences of overt and covert stroke among patients undergoing non-cardiac and non-neurological surgeries are approximately 0.3% and 7%, respectively [4]. Overt perioperative stroke is associated with severe consequences, including increased 30-day mortality and requirements for long-term care in medical facilities [5,6]. Although covert perioperative stroke does not lead to immediate life-altering consequences, it is associated with an increased risk of postoperative delirium, cognitive decline, and subsequent cerebrovascular events within the first year after surgery [3]. Therefore, identifying strategies to reduce the incidence and impact of perioperative stroke is essential.
One potential strategy for protecting the brain during surgery is the administration of a general anesthetic drug prior to the ischemic insult. This strategy is termed anesthesia-induced preconditioning. For more than two decades, numerous preclinical studies have reported that administration of a volatile anesthetic drug provides protection against ischemia and reperfusion injury [7,8]. However, in contrast to the results from animal studies, clinical studies have not yielded convincing evidence of neuroprotective effects of general anesthetic drugs. Rather, several clinical studies have suggested that volatile anesthetics may be detrimental. Two large retrospective clinical studies of older patients who underwent hip surgery reported a reduced incidence of perioperative stroke among patients who were treated with neuraxial anesthesia as compared to those treated with general anesthesia drugs. However, no definitive conclusions could be drawn from the studies because of the small proportion of patients who experienced stroke in these studies (0.7%–1.6%) [9,10]. A meta-analysis of studies involving patients who underwent hip fracture repair surgery reported no difference in the stroke incidence between participants who received general anesthesia and those who received neuraxial anesthesia, although stroke was analyzed as a secondary outcome in most of the studies [11]. Similarly, in a large-scale randomized clinical trial of 1435 patients older than 50 years (mean age, 78 years) who underwent hip surgery under either general or spinal neuraxial anesthesia, the incidence of stroke (assessed as a secondary outcome) was 0.9% and 0.6%, respectively [12]. However, these studies primarily compared postoperative outcomes between general and regional anesthesia, rather than specifically evaluating anesthesia-induced preconditioning [9–12]. Thus, the findings are difficult to interpret in the context of neuroprotection.
In this narrative review, we sought to develop an understanding of the mechanisms underlying the discordance between preclinical and clinical studies on anesthetic preconditioning. First, we summarized the anesthetic drug protocols used to induce preconditioning in preclinical studies. Next, we reviewed the literature on the molecular mechanisms underlying anesthesia-induced preconditioning, with the aim of identifying potential therapeutic targets for reducing the impact of perioperative stroke on the aging brain. To perform the review, we searched the PubMed database for recent studies (undertaken between 2019 and 2024) using the search terms “(volatile OR inhalation OR isoflurane OR sevoflurane OR desflurane OR propofol OR etomidate OR ketamine OR opioid) AND preconditioning AND brain.” The search yielded 25 studies, of which 14 were preclinical studies investigating the mechanisms of anesthesia-induced preconditioning in ischemia/reperfusion injury models (Table 1). Additionally, we reviewed previously published summaries of the mechanisms of anesthesia-induced preconditioning. Finally, through an iterative process, we reviewed the reference lists of the included studies to identify further relevant reports.
Recent Preclinical Studies that Investigated the Mechanisms of Anesthesia-induced Preconditioning in Ischemic/Reperfusion Injury Models
Necessary conditions for anesthesia-induced preconditioning in preclinical studies
The diversity of methods used to induce preconditioning and measure neurological effects hampers the comparison of the results of preclinical studies that were designed to assess the neuroprotective properties of general anesthetic drugs in ischemic injury. While studies have reported various anesthetic protocols, no consensus on the specific conditions required to induce injury or establish preconditioning in the brain has been reached. To add to this complexity, many studies have used protocols that involve multiple days of exposure to general anesthetic drugs [13–17], which does not reflect real-world clinical settings. However, a recent study used a more clinically relevant protocol for anesthetic drug treatment. In that study, rats were randomly allocated to 14 different drug-treatment groups according to the sevoflurane concentration used and the interval between the onset of ischemia and drug exposure. Specifically, rats were treated for 3 h with 1% (0.5 minimum alveolar concentration [MAC]), 2% (1.0 MAC), or 2.6% (1.3 MAC) sevoflurane, with an interval between drug treatment and the onset of ischemia of 6, 12, 24, or 48 h [18]. Focal cerebral ischemia was induced by occluding the middle cerebral artery for 2 h. Brain injury was assessed 24 h after reperfusion by using a neurological severity score based on cerebral infarct volume and brain water content. The study found that sevoflurane-induced neuroprotection was dependent on both the dose and interval duration, as improvements in the neurological severity score, infarct volume, and brain water content were observed only when ischemia was induced 24 h after exposure to 2.6% sevoflurane. Sevoflurane 2% reduced the size of the cerebral infarction and the extent of brain edema, but did not improve the neurological severity score. Importantly, sevoflurane 1% had no neuroprotective effects.
These results were inconsistent with the findings of previous studies, which reported that treatment with low concentrations of sevoflurane (between 1% and 2%) was associated with improved neurological severity scores and reduced infarct size [19,20]. In one study, rats (weight: 230–250 g) underwent an ischemic insult immediately after exposure to sevoflurane 1% for 3 h. Neurological scores at 24 h after injury were improved despite the use of a low concentration of sevoflurane. Another study used rats (8–12 weeks old; weighing 280–320 g) to evaluate the neuroprotective effects of sevoflurane 2%. Cerebral ischemia was induced 2 h after a 1-h treatment with sevoflurane 2% [20]. A substantial improvement in neurological behavioral scores was observed 24 h after the ischemic insult. However, it should be noted that these studies evaluated the immediate neuroprotective effects of sevoflurane [19,20], while a more recent study induced cerebral ischemia at least 6 h after sevoflurane exposure [18]. Variations in the timing of cerebral ischemia induction after sevoflurane exposure may have resulted in inconsistent results regarding the sevoflurane concentration required to induce neuroprotection. Based on experimental evidence, preconditioning develops in two phases: an early preconditioning phase, lasting 2–3 h after sevoflurane exposure, and a late preconditioning phase, developing 12–24 h after sevoflurane exposure, lasting for up to 72 h [21,22]. Late preconditioning may require a higher concentration of sevoflurane [18] than that required for early preconditioning [19,20].
Molecular mechanisms underlying anesthesia-induced preconditioning in the brain
The phenomenon of “preconditioning” was first discovered in 1986, when application of short periods of ischemia to the heart (i.e., ischemia-induced preconditioning) were shown to reduce the size of myocardial infarcts arising from a subsequent prolonged period of ischemia [23]. Subsequent studies have shown that volatile anesthetics provide similar protective effects in both the myocardium and brain [24–26]. Although the precise mechanisms underlying preconditioning effects remain elusive, the heart and brain may share common molecular mechanisms, as reviewed previously [27,28]. In brief, preconditioning likely activates specific cell signaling pathways, which in turn alter mitochondrial function [29]. These pathways include the reperfusion injury salvage kinase (RISK) and survivor-activating factor enhancement (SAFE) pathways. These two interacting pathways have previously been reviewed in detail [28–30] and are briefly discussed below (Fig. 1).
The SAFE and RISK pathways. Preconditioning exerts neuroprotective effects against ischemia–reperfusion injury (IRI) by activating two key signaling pathways, the SAFE and RISK pathways. These pathways promote anti-apoptotic responses and preserve mitochondrial integrity. RISK: reperfusion injury salvage kinase, GPCR: G protein–coupled receptors, PI3K: phosphatidylinositol-3-kinase, AKT: protein kinase B, PKC: protein kinase C, RAS: rat sarcoma virus, ERK1/2: extracellular signal-regulated kinase 1/2, GSK3β: glycogen synthase kinase 3 beta, BAD: BCL-2-associated death promoter, BAX: BCL-2-associated X protein, BCL-2: B-cell lymphoma 2, BCL-XL: B-cell lymphoma–extra large, mPTP: mitochondrial permeability transition pore, ROS: reactive oxygen species, Mito KATP: mitochondrial ATP-dependent K+ channel, SAFE: survivor-activating factor enhancement, TNF-α: tumor necrosis factor-alpha, JAK: Janus kinase, STAT3: signal transducer and activator of transcription 3. Created using BioRender.com.
The RISK pathway
Anesthesia-induced activation of the RISK pathway leads to activation of phosphoinositide 3-kinase (PI3K), protein kinase B (AKT), and extracellular signal-regulated kinase 1/2 (ERK1/2), thereby preserving mitochondrial integrity and function, reducing oxidative stress, and preventing cell death [29]. For example, PI3K phosphorylates AKT, which inhibits pro-apoptotic proteins (BAD and BAX) while enhancing anti-apoptotic proteins (BCL-2), thus preventing the opening of mitochondrial permeability transition pores (mPTPs). Opening of mPTPs is prevented by glycogen synthase kinase 3 beta (GSK-3β), another molecule that is phosphorylated by AKT. In addition, mPTPs, which are nonspecific channels located in the inner mitochondrial membrane, play an important role in preconditioning, as their opening is activated by ischemic/reperfusion injury. Specifically, the opening of mPTPs increases the permeability of the mitochondrial membrane to small molecules (< 1.5 kDa), leading to persistent mitochondrial depolarization, production of reactive oxygen species (ROS), mitochondrial swelling, and ultimately cell death [31]. The importance of mPTPs for preconditioning was suggested by a study in which rats underwent 2 h of focal cerebral ischemia at 24 h after exposure to sevoflurane 2.3% for 1 h [32]. In that study, sevoflurane-induced preconditioning was associated with mitochondrial protection, as indicated by increased electron transport chain complex I, III, and IV activity, reduced ROS production, and mitochondrial swelling prevention, whereas neuroprotection was blocked by intracerebroventricular injection of atractyloside, an mPTP opener.
The SAFE pathway
The SAFE pathway can be triggered by the release of cytokines (specifically, Tumour Necrosis Factor-α) which leads to activation of the JAK/STAT signaling cascade (STAT3). When phosphorylated, the transcription factor STAT3 translocates to the nucleus, increasing the expression of genes encoding proteins involved in anti-apoptotic signaling (BCL-2 and BCL-xL), anti-inflammatory responses, and cell survival and repair (stress-responsive gene expression) [33]. Although the role of STAT3 in preconditioning has been predominantly studied in the heart, a previous study showed that sevoflurane 2% can also activate the JAK2/STAT3 signaling pathway and increase BCL-2 expression in the brains of rats [34]. However, in that study, exposure to sevoflurane occurred after the ischemic insult, and further research is therefore needed to understand the role of STAT3 in sevoflurane-induced preconditioning. Interestingly, STAT3 has also been shown to provide cardiac protection through non-genetic pathways, specifically by preserving mitochondrial function via regulation of the electron transport chain, production of ROS, promotion of Ca2+ homeostasis, and inhibition of mPTP opening [30].
Cortical spreading depolarization
Although the heart and brain have been shown to share mechanisms underlying preconditioning, studies have elucidated certain mechanisms that exist only in the brain, including a process termed cortical spreading depolarization (CSD) [35]. CSD is characterized by slow propagation of self-sustaining waves of neuronal/astrocytic depolarization and metabolic changes, which is followed by a period of neuronal and electroencephalographic quiescence [35]. CSD can be induced using various methods, including electrical stimulation, cerebral cortex needling, and direct application of a solution containing potassium chloride (KCl). For example, CSD induced by the application of 1 mol/L KCl solution to the cortex for 2 h provides neuroprotection against focal cerebral ischemia in rats [36]. CSD preconditioning was likely mediated by the activation of AMPK-mediated autophagy and inhibition of apoptosis [36]. However, the role of CSD in anesthetic preconditioning has not yet been studied. Exposure to volatile anesthetics reduces CSD [37,38], which may indicate a potential neuroprotective mechanism. Given that CSD is associated with worse patient outcomes in various neurological disorders, including migraine, stroke, subarachnoid hemorrhage, and traumatic brain injury [39–42], the role of CSD in anesthetic preconditioning requires further study.
Despite decades of research, the mechanisms underlying preconditioning, particularly anesthesia-induced preconditioning, remain undetermined. In the following section, we review the recent literature focusing on the mechanisms underlying anesthetic preconditioning in the brain.
Recent updates on the mechanisms underlying anesthesia-induced preconditioning in the brain
Changes in gene expression
A recent study examined the sequential transcriptomic changes in the cortex of 3-month-old mice after exposure to isoflurane 2% for 1 h [43]. Immediate changes were detected in the expression of 33 genes (0–6 h), followed by a quiescent phase (12–24 h) and late changes in the expression of 73 genes (evaluated at 36, 48, and 72 h). Interestingly, only eight genes were affected in both the early and late phases. Although mainly genes involved in neuroplasticity were affected in the early phase, genes that were altered at later time points were involved in more diverse vital biological processes, including neuronal connectivity, the cytoskeleton, metabolism, autophagy, oxidative stress, and unfolded protein responses. These differences strongly suggest that distinct mechanisms underlie the changes that occur during the early and late phases of preconditioning. The authors of that study further suggested that late changes may reflect responses to early changes, rather than responses to the anesthetic drug itself. They based this proposal on the fact that changes in gene expression profiles were distinct in important ways and that the changes were biphasic rather than gradual in nature.
Gene expression can also be altered by microRNAs (miRNAs). MiRNAs bind to 3′-untranslated regions of the target mRNAs, thereby inhibiting the expression of multiple target genes. In particular, a recent study reported that sevoflurane preconditioning in mice (2.5%, 30 min for 4 consecutive days) prevented the reduction of miRNA-30c-5p after ischemia/reperfusion injury in the brain [44]. By upregulating miRNA-30c-5p, sevoflurane reduced the expression of homeodomain-interacting protein kinase 1 (HIPK1), a serine/threonine protein kinase that modulates the activity of transcription factors that regulate various cellular biological processes associated with inflammation and anti-stress responses. Interestingly, miRNA-30c-5p is upregulated in the myocardium after ischemia/reperfusion in rats and provides cardiac protection by reducing the expression of BTB domain and CNC homology 1 (BACH1) [45]. BACH1 is a transcription factor that is expressed in multiple tissues and that acts as a competitive inhibitor of nuclear factor-like 2 (NRF2), another transcription factor that enhances neuroprotective gene expression. By inhibiting the expression of BACH1, the expression of miRNA-30c-5p enhances NRF2 activation, leading to a reduction in ROS levels, diminished expression of CK-MB, and a reduction in the size of the cardiac infarct area after ischemic/reperfusion injury in rats [45]. As NRF2 expression is also involved in anesthesia-induced preconditioning of the brain [46,47], sevoflurane may reduce BACH1 expression through miRNA-30c-5p.
Neuroinflammation
Previous studies have shown that mitochondrial dysfunction during cerebral ischemia/reperfusion injury induces formation of nucleotide-binding domain and leucine-rich repeat protein 3 (NLRP) inflammasomes and drives inflammation and pyroptosis (programmed necrotic cell death) [48,49]. In a study using nigericin to induce the development of NLRP3 inflammasomes, treatment with sevoflurane 2.5% for 30 min had preconditioning effects as the anesthetic drug specifically inhibited formation of NLRP3 inflammasomes [50]. In another study, the neuroprotective effects of isoflurane (1.5%, 1 h) were suppressed by NLRP3 activation in the microglia of diabetic mice [51]. These results suggested that the effect of anesthetics on microglia, the innate immune cells of the brain, also plays an important role in preconditioning.
Microglia, the resident macrophages of the central nervous system, can be classified into two opposing types: pro-inflammatory, neurotoxic microglia and anti-inflammatory, neuroprotective microglia [52]. Sevoflurane may also induce preconditioning by promoting microglial polarization from a pro-inflammatory to an anti-inflammatory state [47,53]. Microglia either increase ischemic injury by releasing pro-inflammatory cytokines (pro-inflammatory state) or enhance brain recovery by removing cellular debris and secreting trophic factors (anti-inflammatory state) [54]. In one study, in which sevoflurane 2.5% was applied for 1 h on each of 5 consecutive days, changes in the phenotype of both microglia and macrophages were controlled through the GSK-3β/NRF2 pathway [47]. Specifically, exposure to sevoflurane increased GSK-3β phosphorylation, which in turn activated NRF2, leading to a shift toward the anti-inflammatory phenotype, a decrease in pro-inflammatory cytokines, and a reduction in ROS after brain ischemia. The importance of NRF2 in anesthesia-induced neuroprotection has also been shown in primary mouse cortical neurons exposed to isoflurane, where ML385 (an NRF2 inhibitor) blocked isoflurane-induced preconditioning after deprivation of oxygen and glucose [46].
Excitatory synaptic transmission
Excitotoxicity plays an important role in ischemia-associated neuronal injury [55] and exposure to anesthetics may provide neuroprotection by suppressing this effect [56,57]. Depletion of ATP during ischemia results in the inability of ion pumps in the plasma membrane to maintain electrochemical gradients in neurons, leading to depolarization due to the aberrant influx of Na+ and Ca2+ ions and efflux of K+ ions. Elevated extracellular concentrations of K+ ions activate the opening of voltage-gated Ca2+ channels in neurons, leading to increased levels of intracellular Ca2+ that trigger the release of glutamate. This cascade may further increase intracellular Ca2+ by activating glutamate receptors, including NMDA receptors, which contribute strongly to ischemia-induced excitotoxicity in both neurons and glial cells. High intracellular levels of Ca2+ activate Ca2+-dependent enzymes (specifically proteases and nitric oxide synthase), resulting in mitochondrial dysfunction, oxidative stress, and oxidation, which all contribute to apoptosis or necrosis [55].
The early neuroprotective effects of sevoflurane preconditioning (2%, 2 h) were shown to be induced by reducing or avoiding excitotoxicity through multiple mechanisms [57] including preventing phosphorylation of tyrosine residues on NMDA receptor subunits, thus inhibiting the overactivation of these receptors [58]. Furthermore, sevoflurane inhibits the interaction of NMDA receptors with postsynaptic density protein 95 (an excitatory synapse scaffold protein)-mixed lineage kinase 3 (MLK3), by inhibiting the phosphorylation of NMDA receptor subunits. This results in inhibition of the MLK3–mitogen-activated protein kinase kinase 4/7 (MKK4/7)–c-Jun NH2-terminal kinase (JNK3) pathway [57]. The MLK3–MKK7–JNK3 pathway also contributes to cerebral ischemia/reperfusion injury [59]. JNKs (JNK1, JNK2, and JNK3) are members of the serine/threonine mitogen-activated protein kinase family; however, only JNK3 is expressed in neurons [60]. In another in vitro study using acute mouse brain slices, pre-administration of sevoflurane (4%, 15 min) protected striatal medium spiny neurons from hypoxic damage by delaying membrane depolarization, thereby preventing hypoxia-induced potentiation of excitatory synaptic transmission [56]. This protection is mediated by ATP-sensitive potassium (KATP) channels, because glibenclamide, a KATP channel antagonist, blocks the neuroprotective effects of sevoflurane.
Anesthesia-induced preconditioning in the aging brain: lessons from the heart
Preclinical studies have substantially increased our knowledge of the mechanisms underlying anesthesia-induced preconditioning in the brain. Interestingly, many of these mechanisms converge on the mitochondrion. For example, anesthesia-induced activation of the RISK and SAFE pathways results in preservation of mitochondrial homeostasis [28,30,31,33,34]. Mitochondrial function is related to both neuroinflammation [61,62] and excitotoxicity [63,64]. Consequently, therapeutic methods that target the mitochondria may provide additional protection against ischemia/reperfusion injury [65,66]. The significance of mitochondrial function has been demonstrated in a previous study, in which mitochondrial dysfunction, induced by atractyloside, an mPTP opener, blocked the neuroprotective effects of sevoflurane [32].
The preclinical animal studies noted above have primarily focused on relatively young animals. In contrast, most clinical studies have been performed in older patients [9–12]. Given that comorbidities and aging are characterized by certain hallmarks, including mitochondrial dysfunction [67], the results of preclinical studies performed on young animals may have limited clinical applicability [68]. For example, a previous study suggested that mitochondrial dysfunction in diabetic myocardium leads to impaired depolarization and superoxide production, blocking preconditioning [66]. Considering that age-related mitochondrial dysfunction manifests as decreased mitochondrial respiratory capacity, increased oxidative stress, accumulation of mitochondrial DNA mutations, and altered mitochondrial dynamics [69], it is possible that preconditioning-induced protection may be reduced in aged but not younger individuals.
Anesthesia-induced preconditioning in the aged brain
Indeed, soon after the discovery that anesthesia per se might offer perioperative protection against ischemia/reperfusion insults, studies reported that aging might reduce the protective effects of anesthetic drugs. Although we were unable to identify any in vivo studies on this topic, an early in vitro study used acute hippocampal slices from young and aged rats to evaluate the neuroprotective effects of isoflurane in an oxygen and glucose deprivation model [70]. Exposure to isoflurane 1% attenuated NMDA receptor-mediated Ca2+ influx in the brain slices of 5-day-old rats, whereas Ca2+ influx increased in the brain slices of aged rats. In addition, although isoflurane reduced the death of CA1, CA3, and dentate neurons in 5-day-old rat slices, such protection did not emerge in hippocampal slices from aged mice (19–23 months). However, the viability of acute brain slices declines substantially with aging [71], which may have affected the results significantly.
Anesthesia-induced preconditioning in the aged heart
Although no studies directly investigating brain function have been conducted, several studies have evaluated the effects of aging on anesthesia-induced preconditioning of the heart. An early in vitro study, which used myocardium isolated from aged rats (20–24 months), showed that preconditioning was not induced after a 5-min exposure to 2.5% sevoflurane administered 5 min before ischemia/reperfusion injury [72]. Another in vivo study comparing young (3–5 months) and aged (20–24 months) rats showed that isoflurane exposure was not associated with an increase in ROS levels or a reduction in myocardial infarct size in aged rats [73]. A more recent study also showed the absence of sevoflurane preconditioning in aged rats (24 months), which may be ascribed to an inability to activate NFκB-regulated apoptotic genes during preconditioning [74]. However, these findings only pertained to early preconditioning. A previous study, which compared changes in gene expression 24 h after a 1-h exposure to 2.5% sevoflurane, found significantly fewer changes in the hearts of aged rats [75]. Such differences strongly suggest that, similar to early preconditioning [76], late preconditioning due to volatile anesthetics may be attenuated in aged hearts.
Ischemic preconditioning in the aged heart
In contrast to the limited studies on the effects of age on anesthesia-induced preconditioning in the heart, the effects of aging on ischemic preconditioning in the heart have been widely studied [77]. Such studies have suggested that the loss of preconditioning in aged hearts is due to an age-dependent reduction in mitochondrial function and blunted activation of the signaling pathways involved in preconditioning [68,77–79]. However, these studies have also proposed various therapeutic methods, including caloric restriction, physical exercise, and pharmacological treatment, by which ischemic preconditioning in the hearts of older individuals could be restored [80]. An early study using isolated hearts from adult (6 months) and aged (24 months) rats showed that a 40% reduction in food intake for 1 year, or regular swimming for 6 weeks, partially restored ischemic preconditioning [81]. The same study also reported complete restoration of ischemic preconditioning in aged rats in which both therapies were applied [81]. Another study involving slightly younger rats (18 months) reported that a daily (5 days per week) 1-h session of “forced running” for 6 weeks restored ischemic preconditioning to the level observed in young adult rats, by increasing intracellular polyamines and improving mitochondrial function [82]. This important effect of physical exercise on preconditioning is of great interest, and physical exercise is now regarded as an important component of prehabilitation, which is an emerging concept in Enhanced Recovery After Surgery (ERAS) programs [83].
Potential for physical prehabilitation to provide additional perioperative brain protection
Prehabilitation has three main components: nutrition, exercise, and psychosocial counseling [83]. Preoperative physical exercise enhances a patient’s cardiorespiratory capacity, improving both their physiological response to stress and their surgical outcomes [84]. Although studies to date have reported inconsistent results regarding the benefits of preoperative exercise [85–89], a multicenter randomized clinical trial of 251 patients undergoing surgery for colorectal cancer showed that a 4-week hospital supervised multimodal prehabilitation program, which included high-intensity exercise performed three times per week, was associated with a lower rate of severe complications (Comprehensive Complication Index score > 20) and fewer postoperative medical complications [88]. This approach also optimized postoperative recovery relative to standard care [88]. Similarly, a recent meta-analysis suggested that prehabilitation with an exercise component significantly reduced hospital length-of-stay and postoperative pulmonary complications, and also decreased all-cause complications, among patients undergoing upper abdominal surgery [87].
Unfortunately, the effects of preoperative physical exercise on perioperative stroke are difficult to investigate due to the low incidence of this type of stroke. However, preclinical studies have suggested that physical preconditioning directly induces preconditioning in the aged brain [90]. Most preclinical studies on exercise-induced preconditioning have used young rodents. However, one study reported that when focal cerebral infarction was induced in aged (15 months) female mice, the infarct size was reduced and neurological scores were improved in those subjected to a 9-week exercise training protocol than in those not subjected to exercise [91]. This effect was attributed to the increased expression of proteins involved in autophagy and mitophagy [91]. Notably, mitophagy is an important protective mechanism against ischemia/reperfusion injury, which involves removal of damaged mitochondria and maintenance of mitochondrial homeostasis [92,93]. Interestingly, an increased mitophagic response and improvement in mitochondrial function can restore isoflurane-induced preconditioning in the hearts of 22–24-month-old mice [93]. Although the heart and brain are two different organs with distinct metabolic properties [94], exercise may also facilitate anesthesia-induced preconditioning in the brain because these two organs share common molecular mechanisms [27,28].
A recent study investigated the effects of a 5-week physical training schedule in aged (18 months) mice before laparotomy under sevoflurane anesthesia (3%) [95]. Compared to mice that did not undergo preoperative exercise training, postoperative cognitive decline was alleviated in those that received physical training. This specifically involved the recovery of postoperative memory loss, as evidenced by behavioral tests that measured learning and memory (novel object recognition test, Y-maze test). Preoperative exercise also prevented the reduction in dendritic spines and inhibited the increase in the pro-inflammatory cytokine MCP-1 in aged mice. Importantly, the beneficial effects of physical training were associated with improved mitochondrial function. Mitochondrial density and morphology, as determined using electron microscopy, were improved in the hippocampus, and the signaling pathways involved in mitochondrial biogenesis were activated. Nevertheless, the study did not investigate the effects of exercise on anesthesia-induced preconditioning. Building on these results, future studies should investigate whether physical activity in the preoperative period could restore the neuroprotective effects of anesthetics in aged individuals.
Conclusion
Strategies to prevent and mitigate the adverse effects of perioperative stroke are increasingly needed. Anesthesia-induced preconditioning has shown promising effects in preclinical studies, mediated through the activation of specific signaling pathways and acting on neuroinflammation, excitotoxicity, and mitochondrial function. However, these findings have not yet been effectively translated into clinical practice, given that aging and comorbidities appear to attenuate the benefits observed in younger animal models. Mitochondrial dysfunction, a hallmark of aging, may account for these differences, because mitochondria play a central role in the mechanisms of preconditioning. Therapeutic approaches aimed at improving mitochondrial function could potentially restore the preconditioning effects of anesthetics in older adults. Although the number of studies is currently limited and the field is still emerging, early evidence suggests that physical prehabilitation, a key component of ERAS, may enhance mitochondrial health by upregulating proteins involved in autophagy and mitophagy, thereby promoting mitochondrial homeostasis. However, due to the lack of direct evidence, further studies are needed to clarify the role of physical preconditioning in restoring anesthesia-induced preconditioning in older populations.
Notes
Funding
This work was supported by the National Research Foundation of Korea (RS-2022-NR070193, RS-2024-00406568), the Korean Ministry of Health and Welfare (RS-2022-KH130308), and the Foundation Grant of the Canadian Institutes of Health Research (FDN-154312).
Conflicts of Interest
Beverley A. Orser served on the Board of Trustees of the International Anesthesia Research Society (San Francisco, CA, USA) and is co-director of the Perioperative Brain Health Center (Toronto, ON, Canada; http://www.perioperativebrainhealth.com). She is an inventor of one Canadian patent (number: 2,852,978) and two US patents (number: 9,517,265 and number: 10,981,954). These patents, which are held by the University of Toronto, are for new methods to prevent and treat delirium and persistent neurocognitive deficits after anesthesia and surgery, as well as to treat mood disorders. Beverley A. Orser collaborates in clinical studies supported by in-kind software donations from Cogstate Ltd. (New Haven, CT, USA). Tao Zhang and Woosuk Chung declare no conflicts of interest.
Data Availability
The datasets generated during and/or analyzed during the current study are available from the corresponding author on reasonable request.
Author Contributions
Tao Zhang (Visualization; Writing – original draft)
Woosuk Chung (Conceptualization; Writing – review & editing)
Beverley A. Orser (Resources)