Rustam Yukhananov

Long-term impairment of acquisition of a spatial memory task following isoflurane-nitrous oxide anesthesia in rats

BackgroundThe authors demonstrated previously that isoflurane–nitrous oxide anesthesia attenuates performance improvement on an already-learned spatial memory task and that the effect persists for weeks. This experiment was designed to test the hypothesis that learning of new information is particularly susceptible to prolonged disruption after general anesthesia. MethodsSix- (n = 5) and 20- (n = 5) month-old male Fischer 344 rats were anesthetized for 2 h with 1.2% isoflurane, 70% nitrous oxide, and 30% oxygen. Age-matched control rats received 30% oxygen and 70% nitrogen (n = 5 per group). Rats breathed spontaneously, and anesthetic and oxygen concentrations were measured. Spatial learning was assessed daily for 21 days on a 12-arm radial maze (RAM) beginning 48 h after anesthesia. In a post hoc experiment to examine locomotion, swim speed was assessed in a separate group of identically treated rats (n = 3 per group) for 4 days beginning 48 h after anesthesia. ResultsAged rats were slower to complete the maze, made fewer correct choices before first error, and made more errors at baseline than young rats (P < 0.05). Anesthesia worsened maze performance in both age groups, as evidenced by increased time to complete the maze and a decreased number of correct choices before first error (P < 0.05), but there were no statistically significant differences in total number of errors. Interestingly, there were no age-by-anesthesia interactions. Aged rats swam slower than adult rats (P < 0.001), but there were no differences between the control and anesthesia groups. ConclusionsIsoflurane–nitrous oxide anesthesia is associated with a persistent deficit in RAM performance that is not explained by impaired locomotion. This impairment occurs in adult and aged rats, indicating that it is not an age-specific phenomenon. Thus, RAM performance is altered after general anesthesia for longer than predicted by the pharmacology of the drugs used, which, by inference, suggests a long-term deficit in learning/memory.

Impaired acquisition of spatial memory 2 weeks after isoflurane and isoflurane-nitrous oxide anesthesia in aged rats.

Aged rats are impaired on a spatial memory task for at least 24–48 h after isoflurane-nitrous oxide anesthesia. In this study, we tested how long the impairment lasts and investigated the role of nitrous oxide. Eighteen-month-old rats were randomized to anesthesia for 2 h with 1.2% isoflurane with or without 70% nitrous oxide or a control group (30% oxygen). Two weeks later, rats were tested daily for 14 days on a 12-arm radial maze. The number of correct choices to first error, total errors, and time to complete the maze were recorded. Rats anesthetized with 1.2% isoflurane with 70% nitrous oxide made fewer correct choices before first error (P ≤ 0.05). Trends toward similar results were noted for error rate and time to complete the maze, but these did not achieve statistical significance. Post hoc analysis comparing all anesthetized rats to controls demonstrated that anesthetized rats made fewer correct choices to first error (P ≤ 0.05) and took longer to complete the maze (P ≤ 0.05). There were no differences in total number of errors (P ≤ 0.06). Thus, spatial memory is impaired for 2 wk after general anesthesia in aged rats independent of whether nitrous oxide is used.

Spatial memory performance 2 weeks after general anesthesia in adult rats

We have previously demonstrated that general anesthesia with 1.2% isoflurane-70% nitrous oxide impairs acquisition of a radial arm maze task in both young and aged rats when testing begins 2 days after anesthesia and in aged rats when testing begins 2 wk later. We designed this study to examine whether postanesthesia learning impairment is persistent in young rats. Six-month-old rats were randomized to anesthesia for 2 h with 1.2% isoflurane-70% nitrous oxide, 1.8% isoflurane, or a control group that received 30% oxygen (n = 10 per group). Rats recovered for 2 wk and were then tested daily on a radial arm maze for 14 days. There were no differences between the controls and anesthesia groups in number of correct choices to first error or time to complete the maze. There was no main effect of group in terms of total number of errors (P > 0.05) but the group by day interaction was significant (P < 0.05), reflecting improved performance in the 1.2% isoflurane-70% nitrous oxide group relative to controls during the later days of testing (P < 0.005). Hence, in adult rats, previous general anesthesia is not associated with impaired learning 2 wk later. In fact, previous 1.2% isoflurane-70% nitrous oxide improves maze performance 2 wk later.

Nitrous oxide decreases cortical methionine synthase transiently but produces lasting memory impairment in aged rats

BACKGROUND: Nitrous oxide is a commonly used anesthetic that inhibits the activity of methionine synthase, an enzyme involved in methylation reactions and DNA synthesis and repair. This inhibition triggers vacuole formation and degeneration of neurons in areas of the developing and mature brain that are important for spatial memory, raising the possibility that nitrous oxide might have sustained effects on learning. METHODS: To test this possibility, we randomized 18-month-old Fischer 344 rats (n = 13 per group) to 4 h of 70% nitrous oxide + 30% oxygen or 70% nitrogen + 30% oxygen (control) and assessed memory using a 12-arm radial maze for 14 days beginning 2 days after nitrous oxide inhalation. In separate, identically treated groups of rats, we measured methionine synthase activity in the cortex and liver at the end of nitrous oxide exposure and 2 days later (n = 3 rats per group per time point) using a standard assay. RESULTS: Liver and cortical methionine synthase was inhibited during nitrous oxide inhalation (6% and 23% of control in liver and cortex, respectively; P < 0.01). Liver enzyme activity remained depressed 2 days later, whereas cortical enzyme activity recovered. There was no difference in error rate between control and nitrous oxide treated rats. However, those exposed to nitrous oxide took more time to complete the maze and made fewer correct choices before first error (P < 0.05). CONCLUSIONS: Sedation with 70% nitrous oxide profoundly, but transiently, reduces the activity of cortical methionine synthase but produces lasting impairment in spatial working memory in aged rats.

NMDA-R1 antisense oligodeoxynucleotides modify formalin-induced nociception and spinal c-Fos expression in rat spinal cord

Noxious peripheral stimuli (thermal, mechanical, or chemical) produce long-term adaptations in the sensitivity of central nociceptive neurons to subsequent noxious stimuli. The mechanisms responsible for this central sensitization are multifactorial, but the activation of spinal N-methyl-D-aspartate (NMDA) receptors plays a pivotal role. Using antisense oligodeoxynucleotides, we tested the role of the NR1 subunit of the NMDA receptor in the nociception and expression of the immediate early gene c-fos following formalin-induced pain. Rats received NMDA-R1 antisense, sense, or missense oligodeoxynucleotides intrathecally three times over a 48-h interval. The day after the last injection of the oligodeoxynucleotide, the formalin test was performed. Pain-related behavior was quantified by counting the incidence of flinching of the injected paw for 60 min, and the animals were perfused and the spinal cord removed for c-Fos immunohistochemistry 60 min later. Immunopositive cells were counted in the laminae I/II(0) and V of the lumbar enlargement. Treatment with NR1 antisense oligodeoxynucleotide resulted in a marked decrease in flinching. Similarly, the antisense oligodeoxynucleotide virtually abolished formalin-induced expression of c-Fos-like immunoreactivity (Fos-IR) in the spinal cord dorsal horn ipsilateral to injection. In contrast, the corresponding sense or missense oligodeoxynucleotides had no effect on either formalin-evoked behavior or c-Fos immunoreactivity. We conclude that an NR1 antisense oligodeoxynucleotide inhibits both nociceptive behavior and c-fos expression following formalin injection in rats, demonstrating that NR1 plays an important role in the development of noxious stimulation induced c-fos expression in this model.

General anesthesia does not reduce life expectancy in aged rats.

A recent clinical study demonstrated that deep anesthesia, as measured by Bispectral index monitoring, was associated with increased 1-yr mortality among middle-aged and elderly surgical patients. We have previously demonstrated impaired cognitive performance in aged rats for weeks after general anesthesia with 1.2% isoflurane-70% nitrous oxide-30% oxygen. However, the effects of 2 h of anesthesia with 1.2% isoflurane-70% nitrous oxide-30% oxygen on rodent life expectancy are unknown and may have confounded our results. Accordingly, we designed this study to determine if general anesthesia alters life expectancy in aged rats. Sixteen 22-mo-old Fischer 344 rats were randomized to anesthesia for 2 h with 1.2% isoflurane-70% nitrous oxide-30% oxygen or a control group that received 30% oxygen (n = 8 per group). Rats recovered in an enriched oxygen environment and then were placed in their home cage under routine conditions. The number of days between anesthesia administration and death were recorded and Kaplan-Meier survival curves generated and compared statistically using the log-rank test and bootstrap method. There was no difference in long-term survival between the control and anesthesia groups. Hence, general anesthesia with 1.2% isoflurane-70% nitrous oxide-30% oxygen does not reduce life expectancy in aged Fischer 344 rats.