Strategies and experimental models for evaluating anesthetics: Effects on the developing nervous system

Advances in pediatric and obstetric surgery have resulted in an increase in the duration and complexity of procedures requiring anesthesia. It has been reported that anesthetic drugs cause widespread and dose-dependent apoptosis in the developing rat brain. The similarity of the physiology, pharmacology, metabolism, and reproductive systems of the nonhuman primate to that of the human especially during pregnancy, make the monkey an exceptionally good animal model for assessing potential neurotoxic effects of anesthetics. The window of vulnerability to these neuronal effects of pediatric anesthetics is restricted to the period of rapid synaptogenesis, also known as the brain growth spurt period. To minimize the risks to children resulting from the use of anesthesia, the following questions should be addressed: 1. What is the relationship between exposure and brain cell loss for drugs commonly used in the practice of pediatric anesthesia (inhaled anesthetics, midazolam, ketamine, and nitrous oxide)? 2. Are there "class effects," or does each drug need to be considered independently? 3. Are there important interactions among the drugs used as anesthetics contributing to the risk of brain cell death? 4. What is the likely period of human vulnerability? Pharmacogeneomic/system biology approaches have great potential for helping to advance the understanding of brain-related biological processes, including neuronal plasticity and neurotoxicity. Because of the complexity and temporal features of how developmental neurotoxicity is manifested, pharmacogenomic/systems biology approaches may prove to be useful tools for enhancing our understanding of the biological processes induced by anesthetics. Therefore, the main purpose of this review is to describe the application of these approaches and models, as well as protection strategies, especially as regards the issue of anesthetic-induced neuronal cell death during development. Much of the discussion that follows is based on experiments conducted with ketamine. This is due in part to the use of ketamine in the early studies and the volume of preclinical experimental work performed with this drug, as well as its use in anesthetic studies in developing rodents and nonhuman primates. Although ketamine use in pediatric anesthesia is relatively limited, the findings of the studies are sufficiently strong to merit concern about the N-methyl-D-aspartate antagonist drugs as a class. Our focus on ketamine should not be construed as implying that the risk of neurodegeneration with ketamine is greater, or less, than with other anesthetics. We are simply describing the effects where we have the most preclinical data.