Selective Anesthesia-induced Neuroinflammation in Developing Mouse Brain and Cognitive Impairment

Abstract

AN estimated 6 million children undergo surgical care each year in the United States alone.1The widespread and prevalent use of anesthesia in children makes its safety a major health issue of interest2(reviewed in Sun3). Recent population studies have suggested that anesthesia and surgery could be risk factors for subsequent cognitive impairment (reviewed in Sun3). Specifically, children who have multiple exposures (e.g., three times) to anesthesia and surgery at an early age (e.g. , before age 4) are at an increased risk of developing learning disabilities4,5(reviewed in Sun3). These data suggest that children may not reach cognitive potentials compared with their peers who have not undergone anesthesia and surgery. These findings have become a major public health issue.2However, not every child develops cognitive impairment after having anesthesia and surgery, and older children may be less susceptible to this phenomenon.4Therefore, we have hypothesized that there is a multifactorial model of the cognitive impairment such that the combination of an environmental insult (precipitating factors, e.g. , selective anesthesia) with an age vulnerability (predisposing factors, e.g. , certain age groups) is needed to cause the cognitive impairment. In the present studies, we have tested this hypothesis by identifying the selective effects of anesthetics (sevoflurane vs. desflurane) and anesthesia regimen (one vs. three times) on cognitive impairment in different age groups (6 vs. 60 days) of mice.Neuroinflammation, including microglia activation and increases in the levels of proinflammatory cytokines in the brain, may lead to cognitive impairment.6–10Specifically, proinflammatory cytokines, particularly tumor necrosis factor (TNF)-α and interleukin (IL)-6, can be released by the microglia during its activation, fueling brain inflammation and leading to cognitive impairment in humans11–14and in animals.15–17We therefore assessed the effects of different anesthetics and anesthesia regimens on the levels of IL-6 and TNF-α, and ionized calcium-binding adaptor molecule 1 (IBA1), the marker of microglia activation,18,19in the brain tissues of mice.Finally, enriched environment (EE) has been shown to improve brain function. Delayed EE mitigates anesthesia-induced learning and memory impairment in rats.20We therefore determined whether EE, which occurred immediately after the anesthesia, could attenuate the anesthesia-induced cognitive impairment in mice in the present experiments.The animal protocol was approved by the Standing Committee on Animals at Massachusetts General Hospital, Boston, Massachusetts. Postnatal day (P) 6 or P60 C57BL/6J (The Jackson Laboratory, Bar Harbor, ME) mice (both male and female) received either anesthetic sevoflurane or desflurane plus 60% oxygen (balanced with nitrogen) to maintain sufficient partial pressure of oxygen (PO2) levels during anesthesia, as performed in our previous studies.21Control groups received 60% oxygen at an identical flow rate in similar chambers. There was no significant difference in learning and memory function between the mice that received 60% oxygen and the mice that received 21% oxygen (data not shown). The anesthetic and oxygen concentrations were measured continuously (GE Datex-Ohmeda, Tewksbury, MA). The temperature of the anesthetizing chamber was controlled to maintain a 37° ± 0.5°C rectal temperature in the mice. We determined pH, and partial pressure of oxygen (PO2) and carbon dioxide (PCO2) in the neonatal mice using the methods described by Satomoto et al .22Specifically, the young mice had a quick arterial blood sampling from the femoral artery at the end of 2 h of anesthesia, and the samples were transferred into heparinized glass capillary tubes. A single sample (100 μl) was analyzed immediately after blood collection by using a blood gas analyzer (ITC, Edison, NJ). Anesthesia with 3% sevoflurane21,22for 2 h did not significantly change the values of pH, PO2, or PCO2as compared with the control group. Anesthesia with 9% desflurane for 2 h did not significantly change the values of pH, PO2, or PCO2as compared with the control group (pH, 7.33 ± 0.05 vs. 7.41 ± 0.14 for control vs. desflurane anesthesia; PO2, 174 ± 12.4 mmHg vs. 142 ± 27.0 mmHg for control vs. desflurane anesthesia; and PCO2, 48 ± 5.1 mmHg vs. 41 ± 9.9 mmHg for control vs. desflurane anesthesia). Furthermore, as compared with the control mice, the anesthetized mice did not show significant changes in behavior (e.g. , eating, drinking, general activity, and body weight) after the anesthesia. Mortality rate of mice in these studies was less than 1%. For the intervention studies, ketorolac (1 mg/kg),23one of the nonsteroidal anti-inflammatory drugs, was given to mice 30 min by means of intraperitoneal injection before each of the 3-day sevoflurane anesthesia sessions.A round steel pool, 150 cm in diameter and 60 cm in height, was filled with water to a height of 1.0 cm above the top of a