Cognitive impairment is common immediately after surgery and general anesthesia in elderly patients, with some elderly patients report lingering difficulties with concentration and memory for days or weeks postoperatively.1,2 It remains unclear, however, whether these cognitive changes are due to the effects of surgery, general anesthesia, or other perioperative confounders.1 In previous studies, we have demonstrated that sedation with 70% nitrous oxide or general anesthesia with isoflurane or isoflurane/nitrous oxide impairs the ability of aged rats to learn a spatial memory task when testing begins 2 days later.3,4 This suggests that in aged animals some general anesthetics impair cognitive performance for longer than their pharmacokinetics predict and indicates that complete loss of consciousness is not required for the development of persistent cognitive impairment.
Propofol (2,6-diisopropylphenol) is an IV drug that is widely used for induction and maintenance of general anesthesia as well as for procedural sedation. Propofol produces amnesia and hypnosis but, relative to isoflurane, is associated with a more rapid and qualitatively better recovery, with propofol-anesthetized patients experiencing fewer subjective vegetative symptoms 7 days after anesthesia and surgery than those anesthetized with isoflurane.5–9 In addition, the receptor mechanisms of action of propofol are thought to differ from those of nitrous oxide and isoflurane in that propofol acts primarily as a γ-aminobutyric acid (GABA) receptor modulator whereas nitrous oxide is primarily a N-methyl-d-aspartate (NMDA) receptor antagonist with weak GABA agonist properties and isoflurane has pleiotropic receptor effects.5 Based on these observations, we hypothesized that propofol is unlikely to produce persistent learning impairment in aged animals.
This protocol was approved by the Harvard Medical Area Standing Committee on Animals. Twenty-two 18-mo-old Fischer-344 rats were acquired from the National Institute on Aging colony at Harlan. After a 1-wk acclimation period in the laboratory, rats were food-restricted and habituated to a 12-arm radial arm maze (RAM) for 10 min daily over 5 days. During this interval, the rat was able to freely explore the maze in which food rewards (quarter pieces of Froot Loops cereal) were scattered randomly. The purpose of habituation is to acclimatize the animal to the unfamiliar environment of the maze; because food is distributed randomly, habituation does not directly assist subsequent maze learning. One rat was eliminated from the study during the period of food restriction because he was not eating food in his home cage. Thereafter, rats were randomly assigned to anesthesia or control groups. Rats randomized to the anesthesia group (n = 11) received propofol while spontaneously breathing 100% oxygen, whereas the control group (n = 10) received only 100% oxygen at identical flow rates for 2 h. To induce anesthesia, rats in the anesthesia group were placed in an anesthesia chamber flushed with 3% isoflurane in 100% oxygen with anesthetic and oxygen concentrations measured continuously (Ohmeda, Madison, WI). After loss-of-righting reflex, the isoflurane was decreased to 2% and an IV catheter was placed. Once the IV catheter was placed the isoflurane was decreased to 1% and a propofol infusion initiated at a rate of 1 mg · kg−1 · min−1. After 1 min, the isoflurane was discontinued. The propofol infusion continued at a rate of 1 mg · kg−1 · min−1 for 2 min at which time it was reduced to 0.7 mg · kg−1 · min−1 and the first mean artieral blood pressure (MAP) was taken and subsequently monitored every 5 min using a rat tail cuff (AD instruments, CO Springs, CO). If the MAP was <100, the propofol was decreased by 0.1 mg · kg−1 · min−1 whereas, if the animal moved or blinked during MAP measurement, the propofol infusion was increased by 0.1 mg · kg−1 · min−1. Temperature of the animals was monitored continuously and maintained at 37.5°C ± 0.5°C for the duration of the experiment. A venous blood gas was obtained from each animal tail vein at the end of the 2-h anesthetic. Anesthesia was terminated by discontinuing the propofol infusion and administering 100% oxygen in the anesthetizing chamber until return of righting reflex. Rats recovered for 2 days so as to avoid the confounding influence of residual anesthetic before beginning 14 days of RAM testing.
The RAM tests spatial working and reference memory and assesses the integrity of the frontal cortex, perirhinal/entorhinal cortex, and hippocampus.10–12 It can detect subtle differences in learning and memory caused by aging, sedative medications and anesthetics.3,4,13,14 The maze consists of a central platform that communicates with 12 arms, each of which is baited with a hidden food reward. The walls of the maze room display simple geometric designs providing fixed, extra-maze cues to assist spatial navigation. To ensure motivated performance, rats were food-restricted to achieve 85% of usual free-feeding body weight, but were given free access to water in the home cage. Testing consisted of a daily 15-min session in which the rat was placed on the central platform of the maze with all arms baited. The rat was allowed to choose arms in any order until all 12 arms were visited or 15 min elapsed. A correct choice was defined as one in which the rat entered a baited arm not previously explored, whereas an error was scored when the rat entered an arm it had previously visited or failed to enter the arm in 15 min. Number of correct choices before first error, total number of errors, and time to complete the maze were measured for each trial.
Performance on the RAM was analyzed with a repeated-measures ANOVA, with anesthesia group as the between-subjects factor and day of testing as the within-subjects factor. All analyses were performed in SYSTAT 7.0 for Windows. Data are expressed as mean ± sem.
Anesthesia with propofol was physiologically well tolerated in aged Fischer 344 rats using this protocol. MAP remained within a physiologically acceptable range during anesthesia (100 ± 2 mm Hg) as did the venous blood gas measurements obtained at the end of anesthesia (pH 7.37 ± 0.01, Po2 269 ± 21 mm Hg, Pco2 52 ± 1 mm Hg), with an average propofol infusion rate of 0.58 ± 0.03 mg · kg−1 · min−1.
There were no statistically significant differences between the control and propofol anesthetized rats on any measure of their performance. However, there was a significant effect of test day for time to complete the maze and number of correct choices, indicating that learning took place across the 14 days of testing. For time to complete the maze (Fig. 1), the main effect of day was significant (P < 0.001) but the main effect of group was not (P > 0.05), nor were there any group by day interactions (P > 0.05). For number of correct choices to first error (Fig. 2), the main effect of day was significant (P < 0.001) but the effect of group was not (P > 0.05), nor were there any group by day interactions (P > 0.05). In terms of total number of errors (Fig. 3), there was no significant effect of day (P > 0.05) or treatment group (P > 0.05), nor were there any group by day interactions (P > 0.05). Hence, general anesthesia with propofol does not impair acquisition of a spatial memory task administered 2 days after general anesthesia in 18-mo-old Fischer 344 rats.
This study demonstrates that spatial memory is unimpaired in aged rats when testing begins 2 days after general anesthesia with propofol. This contrasts with our previous studies performed under identical conditions demonstrating that sedation with nitrous oxide or general anesthesia with isoflurane-nitrous oxide impairs acquisition of RAM learning when testing begins 2 days later and with data indicating that the learning impairment persists for at least 2 wks after general anesthesia with 1.2% isoflurane with or without 70% nitrous oxide.3,4,15 This indicates that in aged rats memory is not persistently impaired by propofol and that the anesthetic state is neither necessary nor sufficient for development of postanesthetic memory impairment.
Clinically, recovery from propofol anesthesia is rapid. Studies demonstrate that psychomotor and memory function are impaired for only 24 h or less after surgery under propofol anesthesia16–18 and that patients experience fewer lingering subjective vegetative symptoms.6 Likewise, recovery is prompt in animals, with rats displaying no learning impairment in a visually cued water maze task immediately after a bolus of propofol.19 In that study, however, motor performance and procedural learning were tested rather than spatial learning and memory, because the escape platform was visible. The impact of advanced age on cognitive recovery from propofol anesthesia has not been studied as thoroughly. This is an important limitation of previous work because the elderly respond to and recover from general anesthesia differently. Using an experimental paradigm identical to that reported here, for example, we have shown that, although young and old rats have impaired spatial learning 2 days after isoflurane-nitrous oxide anesthesia, only the old are impaired 2 wks later. Thus, cognitive recovery in the young is not necessarily a reliable measure of recovery in the old.
The most obvious possibility for why one general anesthetic produces lingering cognitive effects, whereas another does not, is that general anesthetics have different receptor mechanisms of action. Propofol inhibits synaptic transmission in the central nervous system primarily by enhancing GABAA receptor currents while having smaller effects on sodium, potassium, cholinergic, and NMDA receptor/channels. Nitrous oxide, in contrast, inhibits agonist responses at NMDA and cholinergic receptors but has only weak actions at GABAA receptors, whereas isoflurane acts less selectively on numerous transmitter systems including those involved in GABA, potassium, sodium, cholinergic, dopaminergic, and NMDA receptors/ channels.5,20–24 These differences in affinity for specific ion channels are also associated with differences in hippocampal electrophysiology, with isoflurane blocking induction of long-term potentiation (LTP) and long-term depression (LTD), whereas propofol only partially inhibits LTP and has no effect of LTD.9,25 LTP and LTD represent electrophysiological correlates of use-dependent efficacy of the synapse and are used to study mechanisms of memory formation.26–28 As such, it appears that there are important functional differences among anesthetics with respect to memory processes. Because of these inherent differences in the molecular actions of anesthetics and because not all drugs that produce general anesthesia produce persistent changes in learning and memory in aged rats, we hypothesize that in aged rats the receptor mechanisms of the drugs, not simply the state of general anesthesia (pharmacologic coma), are important for the development of postanesthetic cognitive impairment. Specifically, based on evidence that NMDA receptor blockade produces long-lasting changes in hippocampal LTP and spatial memory,29 we speculate that the absence of persistent cognitive impairment after propofol anesthesia may reflect the fact that, unlike the other drugs we have studied previously, it has minimal direct effects on signaling through the NMDA receptor complex. It should be noted that, in a cell culture model, propofol inhibits dendritic development and in neonatal rodents it produces apoptotic neurodegeneration. The functional significance of the former changes are unclear, however, and in the latter case there was no impairment in learning and memory in adulthood despite the fact that systemic physiology was uncontrolled during the time of anesthesia.30–32
There are a number of limitations to this study. The dose of propofol was administered based upon prospectively determined hemodynamic and physiologic variables. Accordingly, the dose of propofol varied slightly from animal to animal and we did not measure serum propofol concentrations. Others have demonstrated, however, that the 50% effective concentration for propofol is 0.7 mg · kg−1 · min−1 in adult rats. The aged rats in this study received less propofol (0.58 ± 0.03 mg · kg−1 · min−1), but this difference is consistent with known differences in anesthetic sensitivity and pharmacokinetics that accompany aging.33–35 Whether a different result would be obtained at higher or lower dosages of propofol is unknown, but our previous studies have demonstrated learning impairment in aged rodents after sedation with nitrous oxide, as well as general anesthesia with isoflurane/ nitrous oxide, suggesting depth of anesthesia is not a major determinant of the impairment.3,4 It is also possible that we failed to find a persistent cognitive effect of propofol because, at 18 mo of age, Fischer rats are only late middle-age. This is unlikely, however, because we have previously shown cognitive impairment with other anesthetics in animals of the same age and under identical experimental conditions to those used here.3 In addition, recent clinical data indicate the incidence of short-term cognitive deterioration is similar among young, middle-aged, and older patients after anesthesia and surgery.1,36 In fact, using 18-mo-old Fischer 344 rats is actually a strength of the experimental design because older rats have such marked impairment at baseline that further decline is difficult to detect, potentially leading to a false negative result. Finally, it is conceivable that the use of isoflurane for induction prevented a deleterious effect of propofol. There is a report of changes in gene expression in the amygdala after just 15 min of isoflurane anesthesia, but that study used young rats and did not measure either the corresponding proteins or behaviors mediated by the amygdala.37 Moreover, although brief exposure to isoflurane can precondition the brain so it better tolerates a subsequent insult, the underlying molecular changes involved require an interval between the preconditioning stimulus and the insult. In this study, the interval from induction with isoflurane to propofol administration was ≤5 min with no intervening anesthesia recovery.38 Lastly, and perhaps more importantly, isoflurane itself produces memory impairment in this model.15
In summary, our results show that spatial working memory is intact in aged rats 2 days after IV general anesthesia with propofol. This contrasts with the persistent learning impairment we have reported previously after nitrous oxide sedation or isoflurane anesthesia in the same model. As such, we conclude that the general anesthetic state is neither a sufficient nor necessary condition for development of persistent cognitive impairment under these conditions. The choice of anesthetics may, however, play a role in late cognitive outcome in the aged.
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