"We frequently perform surgical procedures on children, including premature infants, and those procedures have become increasingly more complex and take longer to perform," says the study's lead author Vesna Jevtovic-Todorovic, M.D., associate professor of anesthesiology at the University of Virginia Health System. "That means many pediatric patients are being exposed to anesthetic drugs more frequently and for longer periods of time. Our results would suggest that might be problematic."
Previously, Jevtovic-Todorovic was at Washington University School of Medicine in St. Louis, where the rest of the research team is located. The investigators anesthetized 7-day-old rats with a combination of three drugs -- midazolam, nitrous oxide and isoflurane -- commonly used in pediatric surgery.
As the animals recovered from the anesthesia, the researchers divided them into three groups: One group was sacrificed the next day and their brains examined, a second group grew to be about a month old and a third group grew into adulthood. The latter two groups were tested for effects of anesthesia on learning and memory. Members of the research team also recorded electrical activity in the hippocampus, a brain structure known to be important in learning and memory.
"These infant rats were anesthetized during the brain growth spurt period called synaptogenesis, which lasts for the first few weeks of life in rats, but in humans it extends from the third trimester of pregnancy until about age 3," says senior investigator John W. Olney, M.D., the John P. Feighner Professor of Neuropsychopharmacology at Washington University School of Medicine in St. Louis. "During this period, nerve cells in the brain make connections with one another and form large networks. But if something interferes with that process, the cells are programmed to kill themselves."
In this study, the team found moderately severe cell death had occurred in several brain regions in every brain examined. This included brain regions involved in learning and memory such as hippocampus.
In addition, the rats exposed to anesthesia in infancy had significant learning and memory deficits, both at 1 month of age and in adulthood. Rats were tested in several kinds of mazes that behavioral scientists commonly use to evaluate learning and memory. In all of these tests, rats that had been anesthetized in infancy were significantly worse than those that had not been given the standard anesthesia drug combination.
The researchers also examined brain slices from the hippocampus of month-old rats. They ran electrical currents through those slices to induce a process known as long-term potentiation (LTP), which is thought to occur during learning and memory formation. Brain slices from rats who had been anesthetized with the three drug "cocktail" had far less LTP activity than normal.
"In each part of this study, we found essentially what we expected," Jevtovic-Todorovic says. "Once we had confirmed cell death, we would have expected behavioral deficits, and we found those as the rats grew into adulthood. In the electrophysiological experiments, we also found evidence of disturbances in the neural circuits of the hippocampus, the brain region which, through those circuits, plays an important role in learning and memory."
The team also found that the rats appeared to behave normally in most other ways, and there were no outward signs of brain damage.
"That's important because if similar brain damage had occurred in a human infant, it appears there would not be any overt signs that would alert you to it," Olney says.
This study fits together with a line of research that has repeatedly identified a relationship between certain classes of drugs that inhibit nerve cell activity and damage to the developing brain. Anesthetic drugs work in one of two ways, both of which inhibit nerve cell activity: Either they inhibit excitatory neurotransmission in the brain or they enhance inhibitory neurotransmission.
The excitatory system that stimulates nerve cells is what scientists call the NMDA glutamate transmitter system. In 1998, Jevtovic-Todorovic discovered that the drug nitrous oxide, or laughing gas, work by inhibiting the NMDA glutamate system. Another anesthetic drug known as Ketamine, also works by inhibiting the NMDA glutamate system.
Other anesthetic drugs work by enhancing the inhibitory activity of GABA (Gamma Amino Butyric Acid). GABA is the primary inhibitory transmitter in the brain.
In related research, Olney and colleagues in Germany demonstrated that when the developing brain is exposed to drugs that block NMDA glutamate activity, nerve cells in the brain commit suicide. They also found that drugs that enhance GABA activity can cause nerve cells in the developing brain to self-destruct.
The above findings prompted them to study alcohol, which is known to block NMDA glutamate activity and also to enhance GABA activity. They found that alcohol powerfully triggers nerve cell suicide in the developing brain, providing a likely explanation for the learning and memory disturbances associated with the human fetal alcohol syndrome. More recently, Olney and colleagues demonstrated that sodium channel blocking drugs used in pediatric medicine to manage epilepsy also cause nerve cell suicide in the infant rat brain.
"In all of these studies, we have found that drugs that enhance GABA inhibition or that inhibit glutamate excitation can trigger massive cell suicide in the developing brain," Olney says. "If you put nerve cells to sleep when they are supposed to be making connections, it interferes with their timing, and nerve cells are programmed to kill themselves if they don't make their connections on time."
Part of the reason cells are programmed to self-destruct is that there is redundancy built into the system. An infant is born with an excess number of nerve cells, and some cell death is normal in the developing brain. But Olney's team has found that when drugs interfere with the cell and put it to sleep when it is trying to make connections, the suicide rate rises to abnormally high proportions.
Previous studies by these researchers have helped explain how abuse of certain drugs, including alcohol, can damage the developing brain. But in the present study by Jevtovic-Todorovic and colleagues, the investigators found that drugs used commonly in pediatric anesthesia also can damage the developing brain.
According to Olney, this is a serious dilemma because anesthesia is required to do surgery, and surgery is the only option for some infants with life-threatening problems.
"But some pediatric surgery is elective," Olney says. "In light of these findings, I would recommend that if surgery really does not have to be performed early in life, it would be prudent to postpone it."
The investigators also suggest that some surgical procedures might not require general anesthesia, or in some cases the duration of general anesthesia could be reduced. They also say that the common practice of keeping newborns continuously sedated in pediatric intensive care units should carefully be evaluated in order to minimize potential damage from the sedating drugs.
Jevtovic-Todorovic V, Hartman RE, Izumi Y, Benshoff ND, Dikranian K, Zorumski CF, Olney JW, Wozniak DF. Early exposure to common anesthetic agents causes widespread neurodegeneration in the developing rat brain and persistent learning deficits. Journal of Neuroscience, vol. 23:3, pp. 876-882, Feb. 1, 2003.
This research was supported by grants from the National Institute on Alcohol Abuse and Alcoholism (NIAAA), the National Institute on Aging (NIA), The National Institute on Drug Abuse (NIDA), the National Institute of Mental Health (NIMH) and the National Institute of Child Health and Human Development (NICHD) of the National Institutes of Health.