Delayed environmental enrichment reverses sevoflurane-induced memory impairment in rats.

BACKGROUND: Anesthesia given to immature rodents causes cognitive decline, raising the possibility that the same might be true for millions of children undergoing surgical procedures under general anesthesia each year. We tested the hypothesis that anesthesia-induced cognitive decline in rats is treatable. We also tested if anesthesia-induced cognitive decline is aggravated by tissue injury. METHODS: Seven-day old rats underwent sevoflurane anesthesia (1 minimum alveolar concentration, 4 h) with or without tail clamping. At 4 weeks, rats were randomized to environmental enrichment or normal housing. At 8 weeks rats underwent neurocognitive testing, which consisted of fear conditioning, spatial reference memory, and water maze-based memory consolidation tests, and interrogated working memory, short-term memory, and early long-term memory. RESULTS: Sevoflurane-treated rats had a greater escape latency when the delay between memory acquisition and memory retrieval was increased from 1 min to 1 h, indicating that short-term memory was impaired. Delayed environmental enrichment reversed the effects of sevoflurane on short-term memory and generally improved many tested aspects of cognitive function, both in sevoflurane-treated and control animals. The performance of tail-clamped rats did not differ from those rats receiving anesthesia alone. CONCLUSION: Sevoflurane-induced cognitive decline in rats is treatable. Delayed environmental enrichment rescued the sevoflurane-induced impairment in short-term memory. Tissue injury did not worsen the anesthesia-induced memory impairment. These findings may have relevance to neonatal and pediatric anesthesia.

Isoflurane does not affect brain cell death, hippocampal neurogenesis, or long-term neurocognitive outcome in aged rats.

BACKGROUND: Roughly, 10% of elderly patients develop postoperative cognitive dysfunction. General anesthesia impairs spatial memory in aged rats, but the mechanism is not known. Hippocampal neurogenesis affects spatial learning and memory in rats, and isoflurane affects neurogenesis in neonatal and young adult rats. We tested the hypothesis that isoflurane impairs neurogenesis and hippocampal function in aged rats. METHODS: Isoflurane was administered to 16-month-old rats at one minimum alveolar concentration for 4 h. FluoroJade staining was performed to assess brain cell death 16 h after isoflurane administration. Dentate gyrus progenitor proliferation was assessed by bromodeoxyuridine injection 4 days after anesthesia and quantification of bromodeoxyuridine+ cells 12 h later. Neuronal differentiation was studied by determining colocalization of bromodeoxyuridine with the immature neuronal marker NeuroD 5 days after anesthesia. New neuronal survival was assessed by quantifying cells coexpressing bromodeoxyuridine and the mature neuronal marker NeuN 5 weeks after anesthesia. Four months after anesthesia, associative learning was assessed by fear conditioning. Spatial reference memory acquisition and retention was tested in the Morris Water Maze. RESULTS: Cell death was sporadic and not different between groups. We did not detect any differences in hippocampal progenitor proliferation, neuronal differentiation, new neuronal survival, or in any of the tests of long-term hippocampal function. CONCLUSION: In aged rats, isoflurane does not affect brain cell death, hippocampal neurogenesis, or long-term neurocognitive outcome.

Beyond anesthetic properties: the effects of isoflurane on brain cell death, neurogenesis, and long-term neurocognitive function.

Anesthetic drugs cause brain cell death and long-term neurocognitive dysfunction in neonatal rats. Recently, human data also suggest that anesthesia early in life may cause cognitive impairment. The connection between cell death and neurocognitive decline is uncertain. It is conceivable that mechanisms other than brain cell death contribute to neurocognitive outcome of neonatal anesthesia. In a series of experiments, we demonstrate that isoflurane exposure causes significant hypercarbia in postnatal day 7 rats and that exposure to isoflurane or carbon dioxide for 4 h provoked brain cell death. However, 1 h of isoflurane exposure was not sufficient to cause brain cell death. Moreover, only 4 h of isoflurane exposure, but not 1 or 2 h of exposure or 4 h of carbon dioxide, led to impaired hippocampal function,questioning the association between anesthesia-induced brain cell death and neurocognitive dysfunction. Neurogenesis both in the developing and adult dentate gyrus is important for hippocampal function, specifically learning and memory. γ-Amino-butyric-acid regulates proliferation and neuronal differentiation both in the developing and the adult brain. Inhaled anesthetics are γ-amino-butyric-acid-ergic and may therefore affect neurogenesis, which could be an alternative mechanism mediating anesthesia-induced neurocognitive decline in immature rats. Understanding the mechanism will help guide clinical trials aiming to define the scope of the problem in humans and may lead to preventive and therapeutic strategies.

Beyond anesthetic properties: the effects of isoflurane on brain cell death, neurogenesis, and long-term neurocognitive function.

Anesthetic drugs cause brain cell death and long-term neurocognitive dysfunction in neonatal rats. Recently, human data also suggest that anesthesia early in life may cause cognitive impairment. The connection between cell death and neurocognitive decline is uncertain. It is conceivable that mechanisms other than brain cell death contribute to neurocognitive outcome of neonatal anesthesia. In a series of experiments, we demonstrate that isoflurane exposure causes significant hypercarbia in postnatal day 7 rats and that exposure to isoflurane or carbon dioxide for 4 h provoked brain cell death. However, 1 h of isoflurane exposure was not sufficient to cause brain cell death. Moreover, only 4 h of isoflurane exposure, but not 1 or 2 h of exposure or 4 h of carbon dioxide, led to impaired hippocampal function,questioning the association between anesthesia-induced brain cell death and neurocognitive dysfunction. Neurogenesis both in the developing and adult dentate gyrus is important for hippocampal function, specifically learning and memory. γ-Amino-butyric-acid regulates proliferation and neuronal differentiation both in the developing and the adult brain. Inhaled anesthetics are γ-amino-butyric-acid-ergic and may therefore affect neurogenesis, which could be an alternative mechanism mediating anesthesia-induced neurocognitive decline in immature rats. Understanding the mechanism will help guide clinical trials aiming to define the scope of the problem in humans and may lead to preventive and therapeutic strategies.

Isoflurane differentially affects neurogenesis and long-term neurocognitive function in 60-day-old and 7-day-old rats.

BACKGROUND: Anesthetic agents cause cell death in the developing rodent brain and long-term, mostly hippocampal-dependent, neurocognitive dysfunction. However, a causal link between these findings has not been shown. Postnatal hippocampal neurogenesis affects hippocampal function into adulthood; therefore, the authors tested the hypothesis that isoflurane affects long-term neurocognitive function via an effect on dentate gyrus neurogenesis. METHODS: The S-phase marker 5-bromodeoxyuridine was administered at various times before, during, and after 4 h of isoflurane given to postnatal day (P)60 and P7 rats to assess dentate gyrus progenitor proliferation, early neuronal lineage selection, and long-term survival of new granule cell neurons. Fear conditioning and spatial reference memory was tested at various intervals from 2 weeks until 8 months after anesthesia. RESULTS: In P60 rats, isoflurane increased early neuronal differentiation as assessed by BrdU/NeuroD costaining, decreased progenitor proliferation for 1 day, and subsequently increased progenitor proliferation 5-10 days after anesthesia. In P7 rats, isoflurane did not induce neuronal lineage selection but decreased progenitor proliferation until at least 5 days after anesthesia. Isoflurane improved spatial reference memory of P60 rats long-term, but it caused a delayed-onset, progressive, persistent hippocampal deficit in P7 rats in fear conditioning and spatial reference memory tasks. CONCLUSION: The authors conclude that isoflurane differentially affects both neurogenesis and long-term neurocognitive function in P60 and P7 rats. Neurogenesis might mediate the long-term neurocognitive outcome after isoflurane at different ages.

Effect of hypercarbia and isoflurane on brain cell death and neurocognitive dysfunction in 7-day-old rats.

BACKGROUND: Millions of neonates undergo anesthesia each year. Certain anesthetic agents cause brain cell death and long-term neurocognitive dysfunction in postnatal day (P)7 rats. Despite its intuitive appeal, a causal link between cell death and neurocognitive decline after anesthesia has not been established. If one existed, the degree of cell death would be expected to correlate with the degree of neurocognitive dysfunction caused by anesthesia. The authors therefore tested if cell death caused by various durations of isoflurane at 1 minimum alveolar concentration causes duration-dependent long-term neurocognitive dysfunction. METHODS: Isoflurane was administered to P7 rats at 1 minimum alveolar concentration for 0, 1, 2, or 4 h. To control for the respiratory depressant effects of anesthesia, a group of rats was treated with 4 h of carbon dioxide. Cell death was assessed by FluoroJade staining 12 h after the end of each intervention, and neurocognitive outcome was assessed 8 weeks later by using fear conditioning, spatial reference memory, and spatial working memory tasks. RESULTS: Widespread brain cell death was caused by 2 h and 4 h of isoflurane and by 4 h of carbon dioxide. The degree and distribution of thalamic cell death was similar in 4 h isoflurane-treated and 4-h carbon dioxide-treated rats. Only 4 h of isoflurane caused a long-term neurocognitive deficit affecting both spatial reference memory and spatial working memory. Working memory was improved in carbon dioxide-treated rats. CONCLUSION: Isoflurane-induced brain cell death may be partly caused by hypercarbia. The inconsistencies between cell death and neurocognitive outcome suggest that additional or alternative mechanisms may mediate anesthesia-induced long-term neurocognitive dysfunction.

Increasing the duration of isoflurane anesthesia decreases the minimum alveolar anesthetic concentration in 7-day-old but not in 60-day-old rats.

BACKGROUND: While studying neurotoxicity in rats, we observed that the anesthetic minimum alveolar anesthetic concentration (MAC) of isoflurane decreases with increasing duration of anesthesia in 7-day-old but not in 60-day-old rats. After 15 min of anesthesia in 7-day-old rats, MAC was 3.5% compared with 1.3% at 4 h. We investigated whether kinetic or dynamic factors mediated this decrease. METHODS: In 7-day-old rats, we measured inspired and cerebral partial pressures of isoflurane at MAC as a function of duration of anesthesia. In 60-day-old rats, we measured inspired partial pressures of isoflurane at MAC as a function of duration of anesthesia. Finally, we determined the effect of administering 1 mg/kg naloxone and of delaying the initiation of the MAC determination (pinching the tail) on MAC in 7-day-old rats. RESULTS: In 7-day-old rats, both inspired and cerebral measures of MAC decreased from 1 to 4 h. The inspired MAC decreased 56%, whereas the cerebral MAC decreased 33%. At 4 h, the inspired MAC approximated the cerebral MAC (i.e., the partial pressures did not differ appreciably). Neither administration of 1 mg/kg naloxone nor delaying tail clamping until 3 h reversed the decrease in MAC. In 60-day-old rats, inspired MAC of isoflurane was stable from 1 to 4 h of anesthesia. CONCLUSIONS: MAC of isoflurane decreases over 1-4 h of anesthesia in 7-day-old but not in 60-day-old rats. Both pharmacodynamic and a pharmacokinetic components contribute to the decrease in MAC in 7-day-old rats. Neither endorphins nor sensory desensitization mediate the pharmacodynamic component.

Effect of combining anesthetics in neonates on long-term cognitive function.

BACKGROUND: With growing evidence that anesthesia exposure in infancy affects cognitive development, it is important to understand how distinct anesthetic agents and combinations can alter long-term memory. Investigations of neuronal death suggest that combining anesthetic agents increases the extent of neuronal injury. However, it is unclear how the use of simultaneously combined anesthetics affects cognitive outcome relative to the use of a single agent. METHODS: Postnatal day 7 (P7) male rats were administered either sevoflurane as a single agent or the combined delivery of sevoflurane with nitrous oxide at 1 Minimum Alveolar Concentration for 4 h. Behavior was assessed in adulthood using the forced alternating T-maze, social recognition, and context-specific object recognition tasks. RESULTS: Animals exposed to either anesthetic were unimpaired in the forced alternating T-maze test and had intact social recognition. Subjects treated with the combined anesthetic displayed a deficit, however, in the object recognition task, while those treated with sevoflurane alone were unaffected. CONCLUSION: A combined sevoflurane and nitrous oxide anesthetic led to a distinct behavioral outcome compared with sevoflurane alone, suggesting that the simultaneous use of multiple agents may uniquely influence early neural and cognitive development and potentially impacts associative memory.