Anesthesia/Surgery Induces Cognitive Impairment in Female Alzheimer’s Disease Transgenic Mice

Abstract

Anesthesia and/or surgery may promote Alzheimer’s disease (AD) by accelerating its neuropathogenesis. Other studies showed different findings. However, the potential sex difference among these studies has not been well considered, and it is unknown whether male or female AD patients are more vulnerable to develop postoperative cognitive dysfunction. We therefore set out to perform a proof of concept study to determine whether anesthesia and surgery can have different effects in male and female AD transgenic (Tg) mice, and in female AD Tg plus Cyclophilin D knockout (CypD KO) mice. The mice received an abdominal surgery under sevoflurane anesthesia (anesthesia/surgery). Fear Conditioning System (FCS) was used to assess the cognitive function. Hippocampal levels of synaptic marker postsynaptic density 95 (PSD-95) and synaptophysin (SVP) were measured using western blot analysis. Here we showed that the anesthesia/surgery decreased the freezing time in context test of FCS at 7 days after the anesthesia/surgery in female, but not male, mice. The anesthesia/surgery reduced hippocampus levels of synaptic marker PSD-95 and SVP in female, but not male, mice. The anesthesia/surgery induced neither reduction in freezing time in FCS nor decreased hippocampus levels of PSD-95 and SVP in the AD Tg plus CypD KO mice. These data suggest that the anesthesia/surgery induced a sex-dependent cognitive impairment and reduction in hippocampus levels of synaptic markers in AD Tg mice, potentially via a mitochondria-associated mechanism. These findings could promote clinical investigations to determine whether female AD patients are more vulnerable to the development of postoperative cognitive dysfunction.

Keywords

Alzheimer’s disease cognitive function cyclophilin D sevoflurane surgery synaptic marker

INTRODUCTION

Alzheimer’s disease (AD) is the most common form of dementia and is one of the greatest public health problems in the world. Moreover, the impact of AD will only increase with the anticipated demographic changes in the coming decades. Several studies have shown the potential association between anesthesia/surgery and AD. Specifically, Chen et al. found that previous exposure to surgery under general anesthesia, but not sedation, might increase the risk of AD [1]. Another study found that surgery under sevoflurane anesthesia could promote progression of cognitive dysfunction in patients with mild cognitive impairment [2]. Another retrospective study with 24,901 patients in the anesthesia/surgery group and 110,972 participants in the control group also showed a hazard ratio of 1.99 that anesthesia/surgery could be a risk factor of dementia [3]. Moreover, anesthesia and surgery have been shown to lead to long-term cognitive dysfunction [4 –7]. Finally, anesthesia/surgery may cause cognitive dysfunction, which AD patients are susceptible to develop ([8], reviewed in [9 –11]). However, there are studies which demonstrate no association between anesthesia/surgery and dementia [12 –16]. Interestingly, the majority of these studies did not specifically determine the role of sex in the potential association between anesthesia/surgery and AD, dementia, and cognitive dysfunction.

Several studies showed that women are more vulnerable to develop AD and have severer symptoms of AD [17 –19]. Whether anesthesia/surgery can induce a sex-dependent neurotoxicity and neurobehavioral deficits associated with AD neuropathogenesis remains unknown. Answering this question may ultimately suggest that female AD patients may be more vulnerable to anesthesia/surgery-induced neurotoxicity and to develop postoperative cognitive dysfunction.

It is therefore important to embark such studies in rodents and to assess whether anesthesia/surgery can have different effects in female and male AD transgenic (Tg) mice. Our previous studies [20, 21] have shown that abdominal surgery under isoflurane anesthesia can induce behavioral changes associated with cognitive impairment and delirium. Postsynaptic density 95 (PSD-95) [22 –27] and synaptophysin (SVP) [28, 29] are the markers of synapse in the hippocampus. Sevoflurane is one of the most commonly used inhalation anesthetics in patients; we therefore set out to determine the effects of abdominal surgery under sevoflurane on cognitive function and levels of PSD-95 and SVP in hippocampus in both female and male AD Tg mice. The hypothesis was that the anesthesia/surgery only induced cognitive impairment in female, but not male, AD Tg mice.

Zhu et al. reported that mitochondrial dysfunction is associated with AD neuropathogenesis (reviewed in [30, 31]). Cyclophilin D (CypD) is the component of mitochondrial permeability transition pore (mPTP) [32]. CypD knockout (KO) can stabilize mitochondrial function [33 –35]. Our previous studies also showed that mPTP could be involved in the anesthesia neurotoxicity [36]. Taken together, we employed the AD Tg plus CypD KO mice (AD Tg + CypD KO mice) in the studies to illustrate the potential role of CypD in the sex difference of anesthesia/surgery neurotoxicity in AD Tg mice.

MATERIALS AND METHODS

Mice anesthesia/surgery

All experiments were performed in accordance with the National Institutes of Health guidelines and regulations. The animal protocol was approved (Protocol number: 2006N000219, Boston, MA) by the Massachusetts General Hospital (Boston, MA) Standing Committee on the Use of Animals in Research and Teaching. Efforts were made to minimize the number of animals used. The AD transgenic mice were purchased from Jackson Lab (Bar Harbor, ME) (B6SJL Tg(APPSwFlLon,PSEN1*M146L*L286V)6799Vas/Mmjax; Stock Number: 006554), and maintained in our own laboratory. Standard genotype technique was used to confirm the condition of AD Tg mice. CypD KO mice were also purchased from Jackson laboratory (B6;129-Ppiftm1Jmol/J; Stock Number: 009071). We then crossed the AD Tg mice with the CypD KO mice to generate the AD Tg + CypD KO mice. The mice were housed in a controlled environment (20–22°C; 12 h of light/dark on a reversed light cycle) for seven days prior to the studies. We employed both female and male AD Tg mice, and female AD Tg + CypD KO mice in the studies. The mice were randomly assigned to anesthesia/surgery group or control condition group. The anesthesia/surgery was performed between 9:00 am and 12:00 pm. A simple laparotomy was made under 3% sevoflurane anesthesia (anesthesia/surgery). Specifically, anesthesia was induced and maintained with 3% sevoflurane in 60% oxygen in a transparent acrylic chamber. 15 min after the induction, each of the mice was moved out of the chamber, and sevoflurane anesthesia was maintained via a cone device. One 16-gauge needle was inserted into the cone near the nose of the mouse to monitor the concentration of sevoflurane. A longitudinal midline incision was made from the xiphoid to the 0.5 cm proximal pubic symphysis on the skin, abdominal muscles, and peritoneum. Then, the incision was sutured layer by layer with 5-0 Vicryl thread. At the end of the procedure, EMLA cream (2.5% lidocaine and 2.5% prilocaine) was applied to the incision, and then every 8 h for three days to treat the pain associated with the incision. Our previous studies [37] have shown that there was no significant difference of pain threshold between the control and surgery mice at 24 h after the surgery. The procedure for each mouse lasted about 10 min, and the mouse was put back into the anesthesia chamber for up to 2 h to receive the rest of the anesthesia consisting of 3% sevoflurane in 60% oxygen. We used this combination of anesthesia and surgery in the studies because our previous studies showed that the anesthetic could induce cognitive impairment [36 , 39] and surgery might potentiate the anesthesia-induced neurotoxicity and neurobehavioral deficits. Moreover, the combination of anesthesia and surgery was shown to induce cognitive impairment in our previous studies [20, 21]. The temperature of the anesthetizing chamber was controlled (DC Temperature Control System; FHC, Bowdoinham, ME) to maintain the rectal temperature of the mice at 37±0.5°C during the anesthesia/surgery. After recovering from the anesthesia, each mouse was returned to a home cage with food and water available ad libitum. The mice in the control group (food and water available ad libitum) were placed in their home cages with 60% oxygen for 2 h, which was consistent with the condition of non-surgery in humans. Our previous studies found that neither this type of surgery [37, 40] nor anesthesia with 3% sevoflurane [38, 41] significantly disturbed the values of blood pressure and blood gas in the mice. EMLA was able to treat the pain associated with the surgery in the mice [37, 42].

Fear conditioning system (FCS)

The FCS was performed as those described by Saab et al. [43], Zhang et al. [36], and Xu et al. [37] with modifications. Specifically, the pairing (two pairing methods) in FCS (Stoelting Co., Wood Dale, IL) was performed at one day after the anesthesia/surgery. Each mouse was allowed to explore the FCS chamber for 180 s before presentation of a 2-Hz pulsating tone (80 dB, 3,600 Hz) that persisted for 60 s. The tone was followed immediately by a mild foot shock (0.8 mA for 0.5 s). The context test was performed at 3 and then 7 days after the anesthesia/surgery, respectively. Each mouse was allowed to stay in the chamber for a total of 390 s. Function of learning and memory in the context test was assessed by measuring the amount of time the mouse demonstrated “freezing behavior”, which is defined as a completely immobile posture except for respiratory efforts during the second 180 s. The tone test was also performed at 3 and 7 days after the anesthesia/surgery. Each mouse was allowed to stay in the chamber for a total of 390 s. The same tone was presented for the second 180 s without the foot shock. Function of learning and memory in the tone test was assessed by measuring the amount of time the mouse demonstrated “freezing behavior”, defined as a completely immobile posture except for respiratory efforts, during the second 180 s. The “freezing behavior” was analyzed by Any-Maze (freezing on threshold: 10; freezing off threshold: 20; minimum freezing duration: 1 s) (Stoelting). The freezing time was represented in seconds.

Brain tissue harvest, lysis, and protein amount quantification

Different group of mice were used for the biochemistry studies. The mouse hippocampus was harvested at 7 days after the anesthesia/surgery. The harvested hippocampus tissues were homogenized on ice using immunoprecipitation buffer (10 mM Tris-HCl, pH 7.4, 150 mM NaCl, 2 mM EDTA, 0.5% Nonidet P-40) plus protease inhibitors (1 μg/ml aprotinin, 1 μg/ml leupeptin, 1 μg/ml pepstatin A). The lysates were collected, centrifuged at 13,000 rpm for 15 min, and quantified for total proteins by bicinchoninic acid protein assay kit (Pierce, Iselin, NJ). The hippocampus tissues were then subjected to western blot analysis.

Western blot analysis

We used postsynaptic density 95 (PSD-95) antibody (Cell Signaling, Danvers, MA, 1:1,000) to detect PSD-95 (molecular weight of 95 kDa). We used synaptophysin (SVP) antibody (Cell Signaling, 1:1,000) to detect SVP (molecular weight of 38 kDa). We used β-Actin antibody (Sigma, St. Louis, MO, 1:5,000) to detect non-targeted protein β-Actin (molecular weight of 42 kDa). The quantification of western blot was accomplished as described in our previous studies [20]. Briefly, we analyzed the signal intensity via Quantity One image analysis program (Bio-Rad, Hercules, CA). We used two steps to analyze the western blots. At the first step, we used β-Actin levels to standardize protein amounts (e.g., calculating the ratio of PSD-95 to β-Actin) and to limit the disparities in the protein quantity loaded. At the second step, we expressed the protein levels obtained from the anesthesia/surgery mice as a percentage in relation to the control condition.

Statistical analysis

Data were expressed as mean±standard deviation. The number of samples was six per group in the biochemistry studies, and varied from 11 to 14 per group in the behavioral studies, and the samples were normally distributed (tested by normality test). For the FCS test, the percentage of freezing time for both context and tone tests of the FCS was used to determine the function of learning and memory. The changes in the percentage of freezing time in both treated and control mice were presented. Student’s t-test was used to compare the difference from the control group. The nature of the hypothesis testing was two tailed. p values less than 0.05 were considered statistically significant. Prism 6 software (GraphPad Software, Inc., La Jolla, CA) was used to analyze the data.

RESULTS

Anesthesia/surgery induced cognitive impairment in female, but not male, AD Tg mice

The major goal of the current study was to determine whether anesthesia/surgery had different effects in female and male AD Tg mice. We found that the anesthesia/surgery (black bar) did not significantly alter the freezing time, observed in both context test (Fig. 1A) and tone test (Fig. 1B) of FCS, as compared to the control condition (white bar) in female AD Tg mice at 3 days after the anesthesia/surgery. However, at 7 days after the anesthesia/surgery, the anesthesia/surgery (black bar) significantly decreased the freezing time in the context test of FCS as compared to the control condition (white bar) in the female AD Tg mice (Fig. 1C, Student’s t-test, p = 0.0159). The anesthesia/surgery (black bar) did not significantly change the freezing time in the tone test of FCS as compared to the control condition (white bar) in the female AD Tg mice (Fig. 1D) at 7 days after the anesthesia/surgery. These data suggest that the anesthesia/surgery could induce hippocampus-dependent cognitive impairment at 7, but not 3, days after the anesthesia/surgery in the female AD Tg mice.

In the male AD Tg mice, however, we found that the anesthesia/surgery (black bar) did not significantly change the freezing time of both context (Fig. 2A, C) and tone (Fig. 2B, D) test of FCS at 3 (Fig. 2A, B) or 7 (Fig. 2C, D) days after the anesthesia/surgery as compared to the control condition (white bar). These data suggest that the anesthesia/surgery did not induce cognitive impairment as compared to control condition in the male AD Tg mice.

Anesthesia/surgery decreased the levels of PSD-95 and SVP in the hippocampus of female, but not male, AD Tg mice

Given the findings that the anesthesia/surgery had different effects on cognitive function between female and male AD Tg mice, next, we determined the potential cellular mechanisms. Postsynaptic density 95 (PSD-95) [22 –27] and synaptophysin (SVP) [28, 29] are the markers of synapse in the hippocampus, we therefore determined the effects of the anesthesia/surgery on the levels of PSD-95 and SVP in the hippocampus of both female and male AD Tg mice. The immunoblotting of PSD-95 showed that the anesthesia/surgery (lanes 4 to 6) decreased the density of the bands representing PSD-95 in the hippocampus of female AD Tg mice as compared to the control condition (lanes 1 to 3) (Fig. 3A). The anesthesia/surgery did not cause visible reductions in the bands representing β-Actin in the hippocampus of female AD Tg mice as compared to the control condition (Fig. 3A). The quantification of the western blot, based on the ratio of the levels of PSD-95 to the levels of β-Actin, showed that the anesthesia/surgery (black bar) decreased the levels of PSD-95 in the hippocampus of the female AD Tg mice as compared to the control condition (white bar): 65% versus 100% (Fig. 3B, p = 0.0071, Student’s t-test). Similarly, the anesthesia/surgery (lanes 3 and 4 in Fig. 3C, black bar in Fig. 3D) decreased the levels of SVP in the hippocampus of the female AD mice as compared to the control condition (lanes 1 and 2 in Fig. 3C, white bar in Fig. 3D): 78% versus 100%, p = 0.0061, Student’s t-test.

However, the anesthesia/surgery (lane 2 in Fig. 4A and 4C, black bar in Fig. 4B and 4D) did not significantly change the levels of PSD-95 (Fig. 4A, B) and SVP (Fig. 4C, D) in the hippocampus of the male AD Tg mice as compared to the control condition (lane 1 in Fig. 4A and 4C, white bar in Fig. 4B and 4D), respectively. Taken together, these results showed that the anesthesia/surgery might induce hippocampus-dependent cognitive impairment and reduction in the levels of synaptic markers (PSD-95 and SVP) in hippocampus of female, but not male, AD Tg mice. These findings suggest the sex difference on the cognitive function and hippocampus levels of synaptic markers in AD Tg mice in the response to anesthesia/surgery.

Anesthesia/surgery induced neither cognitive impairment nor reduction in levels of PSD-95 and SVP in hippocampus of AD Tg + CypD KO female mice

Anesthesia/surgery could have different effects on cognitive function and hippocampus level of synaptic marker between female and male AD Tg mice (Figs. 1–4). Moreover, previous studies showed that there was sex difference in mitochondrial function, which could be responsible for the sex difference observed in neurodegenerative disorders [44]. Finally, CypD is a component of the mitochondrial permeability transition pore [33] and mitochondria dysfunction contributes to the anesthesia-induced cognitive impairment [36]. We therefore asked whether the knockout of CypD was able to rescue the anesthesia/surgery-induced cognitive impairment and reduction in hippocampus levels of synaptic makers in female AD Tg mice. We investigated the effects of the anesthesia/surgery on cognitive function and hippocampus levels of PSD-95 and SVP in AD Tg + CypD KO mice. We found that the anesthesia/surgery induced neither cognitive impairment in the AD Tg + CypD KO mice (Fig. 5) nor significant changes in the hippocampus levels of PSD-95 and SVP in the mice (Fig. 6). These data suggest that knockout of CypD might prevent the anesthesia/surgery-induced cognitive impairment by stabilizing mitochondria function in AD Tg female mice.

DISCUSSION

The major goal of the current study was to determine whether the anesthesia/surgery was able to induce different effects on cognitive function and levels of synaptic markers in hippocampus in female and male AD Tg mice. We found that the anesthesia/surgery induced hippocampus-dependent cognitive impairment as evidenced by reduction in freezing time of context test of FCS in the female AD Tg mice (Fig. 1). The same anesthesia/surgery, however, did not cause cognitive impairment in the male AD Tg mice (Fig. 2). These results demonstrate the sex difference on cognitive function in AD Tg mice following the anesthesia/surgery. Pending further investigations, these results suggest that female AD patients could be more vulnerable to develop postoperative cognitive dysfunction than male AD patients. These findings in AD Tg mice are consistent with the outcomes obtained from the previous studies that women are more vulnerable to develop AD and have severer symptoms of AD [17 –19]. Specifically, a recent study confirmed the previous findings that AD and other dementia occur more in women [17]. Moreover, women who have ɛ4 allele of apolipoprotein E gene (APOE ɛ4) have a greater risk to develop AD as compared to men who have the APOE ɛ4 [18]. In moderate to severe AD, women who have APOE ɛ4 show more prevalent and severer neuropsychiatric symptoms, and demonstrate higher frequency and greater degree of irritability as compared to men [19]. Finally, hippocampus atrophy, a hallmark of AD, develops faster in women than that in men, and women are quicker to develop to AD as compared to men [45].

These findings in current animal studies could guide us to design clinical investigation to determine whether anesthesia/surgery may induce a sex-dependent cognitive impairment in AD patients. Specifically, we should consider the impact of sex on the outcomes when we investigate the potential association between anesthesia/surgery and AD development in the future.

Next, we found that the anesthesia/surgery decreased the levels of synaptic marker PSD-95 [22 –27] and SVP [28, 29] in the hippocampus of female (Fig. 3), but not male (Fig. 4), AD Tg mice. These data are consistent with the data that the anesthesia/surgery induced hippocampus-dependent cognitive impairment only in female, but not male, AD Tg mice (Figs. 1 and 2). The reason by which the anesthesia/surgery caused different effects on the levels of synaptic markers in hippocampus between female and male AD mice is not known at the current time. However, the current findings have established a system and proved a concept that the anesthesia/surgery may have different effects on cognitive function and levels of synaptic markers in hippocampus between female and male AD Tg mice. We will use the established system to further determine why female AD Tg mice are more vulnerable to develop cognitive impairment, which could lead to further investigation of AD neuropathogenesis and to the development of preventing and treating strategy of AD.

Note that the Fear Conditioning learning and memory is highly associated with emotion and amygdala is responsible for such behaviors (reviewed in [46]). Therefore, the potential cellular changes induced by the anesthesia/surgery in amygdala could also be responsible for the anesthesia/surgery-induced cognitive impairment in the female AD Tg mice, in addition to the cellular changes (e.g., reduction in synaptic markers) in the hippocampus. The future studies should test this hypothesis by comparing the effects of the anesthesia/surgery on cellular changes in amygdala between female and male AD Tg mice.

Estrogen has been shown to have neuroprotective effects (reviewed in [47]). Specifically, estrogen can attenuate Aβ neurotoxicity [48, 49] and oxidative stress-induced neuronal death [50 –52]. Yue et al. showed that brain estrogen deficiency enhanced Aβ deposition in brain tissues of mice, increased Aβ generation and impaired Aβ clearance and degradation [53]. However, the effects of estrogen on the anesthesia/surgery-induced cognitive impairment have not been reported. Instead, the outcomes from the current studies showed that the anesthesia/surgery was able to induce cognitive impairment in female AD Tg mice, but not in male AD Tg mice. These data suggest that male hormone androgen, but not female hormone estrogen, may attenuate the anesthesia/surgery-induced cognitive impairment in the AD Tg mice.

Clinical investigations suggest that both estrogen and androgen have neuroprotective effects in adult brain and can attenuate the AD neuropathogenesis (reviewed in [54]). Specifically, childbearing women have increased risk to develop dementia and cognitive impairment as compared to nulliparous [55 –58], given the fact that pregnancy leads to reduction in women’s life time exposure to estrogen [59]. Dementia has been reported to increase in patients with oophorectomy and/or hysterectomy [60 –63]. Men have increased risk of AD following normal and age-related loss of testosterone [64 –68]. The facts that estrogen can attenuate the AD neuropathogenesis and dementia may not be able to explain the outcomes from the current studies that the anesthesia/surgery induced cognitive impairment in female, but not male, AD Tg mice.

However, the sex hormone can have both activational effects, which are transient action, and organizational effects, which are the permanent action, on brain function [69]. The sex hormone can have long-time influence on brain development [70, 71]. Carroll et al. showed that adult female 3xAD Tg mice had more Aβ accumulation and cognitive impairment than male 3xAD Tg mice [72]. In the next experiment, male and female 3xAD Tg mice were demasculinized and defeminized by treatment with androgen receptor antagonist flutamide and testosterone propionate, respectively, during postnatal days 1–21. At 7 months of age, the feminized male 3xAD Tg mice demonstrated Aβ accumulation and cognitive impairment, and the masculinized female 3xAD Tg mice had reduced Aβ levels [72]. These data illustrate the long-term (organizational actions) effects of sex hormone on AD neuropathogenesis and that the female AD Tg mice are more vulnerable to the development of AD neuropathogenesis and neurobehavioral deficit. Consistently, our current studies showed that female AD Tg mice were more vulnerable to the development of cognitive impairment and reductions in the hippocampus levels of synaptic markers following the anesthesia/surgery.

Mitochondrial dysfunction has been reported to contribute to AD neuropathogenesis (reviewed in [30, 31]). A recent study demonstrated sex difference in brain mitochondrial function [44]. Specifically, young female mice have higher levels of mitochondrial antioxidant glutathione and greater production of reactive oxygen species as compared to young male mice [44]. This difference disappears with aging and ovariectomy but not orchidectomy in the mice [44]. We therefore hypothesized that the sex difference in mitochondrial function may account for, at least partially, the behavioral difference between female and male AD Tg mice following the anesthesia/surgery. As the first step, we combined the AD Tg mice with CypD KO mice to generate AD Tg + CypD KO mice and to determine the effects of the anesthesia/surgery on cognitive function and hippocampus levels of PSD-95 and SVP in the female AD Tg + CypD KO mice. We found that the anesthesia/surgery induced neither cognitive impairment (Fig. 5) nor reduction in hippocampus levels of synaptic marker PSD-95 and SVP (Fig. 6) in the female AD Tg + CypD KO mice. CypD, a member of the family of peptidyl-prolyl cis–transisomerases and the component of mPTP, regulates the opening of mPTP and mitochondrial function [32]. CypD KO (using CypD KO mice) and treatment with CypD inhibitor Cyclosporin A have been shown to improve the mitochondrial function and increase mitochondria stability against calcium stress [33]. Specifically, Du et al. showed that decrease in CypD levels could prevent the Aβ- and calcium-induced mitochondrial dysfunction and cell death. Moreover, CypD deficiency improved cognitive function and synaptic function in AD Tg mice [34]. Finally, these beneficial effects of CypD-deficiency could last up to 12 months in the AD Tg mice [35]. In addition, our previous studies have shown that anesthesia and/or surgery can induce mitochondrial dysfunction and cognitive impairment [20 , 73]. Taken together, the findings that CypD KO prevented the anesthesia/surgery-induced cognitive impairment and reduction in hippocampus levels of synaptic markers in female AD Tg mice suggest that the sex difference in cognitive function and levels of hippocampus synaptic markers following the anesthesia/surgery could be owing, at least partially, to the sex difference in mitochondrial function, or specifically the levels of CypD, in the mice. The future studies should test this hypothesis by assessing the levels of CypD and mitochondrial function between female and male AD Tg mice with and without the insult of anesthesia/surgery. The outcomes of such studies could promote the further mechanistic research of the neuropathogenesis of AD, postoperative cognitive dysfunction and postoperative delirium.

The current studies have several limitations. First, we did not systematically determine the potential difference in mitochondrial function between the female and male AD Tg mice, which could be responsible for the differences observed in female and male AD Tg mice following the anesthesia/surgery. However, the major purpose of the current study was to establish a system and to prove a concept that anesthesia/surgery caused different effects in cognitive function and hippocampus levels of synaptic marker in male and female AD Tg mice. We should be able to use the established system to further determine underlying mechanisms of the sex difference in the AD Tg mice following the anesthesia/surgery insult. Second, we did not compare the difference of anesthesia/surgery-induced changes in cognitive function and the hippocampus levels of synaptic markers between AD Tg mice and wild-type mice. However, such studies do not belong to the scope of the current studies, which focus on determining the potential sex difference in AD Tg mice only. We will use the established system to perform such studies in the future. Finally, we did not check the mouse estrous cycle in the current studies. The mouse in different estrous cycle could have different behaviors following the anesthesia/surgery, which may need to be considered in the future investigations.

In conclusion, we found that abdominal surgery under sevoflurane (anesthesia/surgery) was able to induce hippocampus-dependent cognitive impairment and reduction in hippocampus levels of synaptic markers in female, but not male, AD Tg mice. The anesthesia/surgery did not induce the cognitive impairment and reduction in hippocampus levels of synaptic markers in female AD Tg + CypD KO mice. Pending further investigation, these results suggest that female AD patients could be more vulnerable to develop postoperative cognitive dysfunction than male AD patients. Moreover, the difference between female and male AD Tg mice following anesthesia/surgery could be due to the difference in brain mitochondrial function (e.g., CypD level) between the female and male AD Tg mice. Given the fact that there are about 8.5 million AD patients who may need surgery under anesthesia every year, such findings should guild us how to better provide anesthesia and surgery care for AD patients, ultimately leading to the development of treating and preventing strategy of AD.

Footnotes

ACKNOWLEDGMENTS

This research was supported by R01GM088801, R01AG041274, and R01HD 086977 from National Institutes of Health, Bethesda, Maryland (to Z. X.). Dr. Yuan Shen was partially supported by 81571034 from the National Natural Science Foundation of China. The costs of sevoflurane and EMLA cream (2.5% lidocaine and 2.5% prilocaine) were generously provided by the Department of Anesthesia, Critical Care and Pain Medicine at Massachusetts General Hospital. The studies were performed in the Geriatric Anesthesia Research Unit in the Department of Anesthesia, Critical Care and Pain Medicine at Massachusetts General Hospital, Boston, MA. These works should be attributed to the Department of Anesthesia, Critical Care and Pain Medicine at Massachusetts General Hospital and Harvard Medical School.

Authors’ disclosures available online ().

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