ABSTRACT:We investigated the extent of isoflurane-induced neurodegeneration on the fetuses of pregnant rats exposed in utero. Pregnant rats at gestational d 21 were divided into three experimental groups. Rats in the control group spontaneously breathed 100% oxygen for 1 h. Rats in the treatment groups breathed either 1.3 or 3% isoflurane in 100% oxygen through an endotracheal tube, with mechanical ventilation for 1 h. Rat pups were delivered by cesarian section 6 h after treatment, and fetal blood was sampled from the left ventricle of each fetal heart and evaluated for S100␤. Fetal brains were then evaluated for apoptosis, using caspase-3 immunohistochemistry in the CA1 region of the hippocampus and the retrosplenial cortex (RS). The 3% isoflurane treatment group showed significantly higher levels of S100␤ levels and significantly increased average densities of total caspase-3-positive cells in the CA1 hippocampus and RS cortex compared with the control and the 1.3% isoflurane groups. There were no differences in S100␤ levels or densities of caspase-3-positive cells between the control and 1.3% isoflurane groups. Isoflurane at a concentration of 3% for 1 h increased neurodegeneration in the hippocampal CA1 area and the retrosplenial cortex in the developing brain of fetal rats. Increasing evidence suggests that commonly used inhalational anesthetics, especially isoflurane, dose-and timedependently induce damage in various types of tissues and cells, including hippocampal slices (1), lymphocytes (2,3), neuroglioma (4), liver cells (5), gingival fibroblasts (6), neurosecretory PC12 cells (7,8), and primary cortical and striatal neurons (2,7,8). In animal studies, isoflurane, alone or in combination with midazolam or nitrous oxide, caused widespread neurodegeneration in the 7-d-old rodent brain and was associated with subsequent cognitive impairment (9,10). In the elderly rodent, isoflurane exposure also caused persistent memory impairment (11). Recent clinical studies suggest a greater incidence of learning disabilities in children exposed to anesthesia during surgery before the age of 3, especially in children over the course of multiple surgeries (12,13).Pregnant women sometimes require general anesthesia for various surgeries, such as cesarian sections and fetal surgeries.Because anesthetics easily cross the placenta, these interventions expose developing fetal brains to inhalational anesthetics as well. Because neurons in the developing brains are especially vulnerable to anesthesia-mediated neurodegeneration (9), it is possible that inhalational anesthetics administered to pregnant mothers can harm the developing fetal brains (14). We have previously shown that 1.3% isoflurane administered during late pregnancy did not produce detectable injury to the rat fetal brains (15). However, in the case of fetal surgery used to correct various congenital malformations during midgestation (18 -25 wk) (16), the fetal brain can be exposed to 2-3 times (2.5-3 minimal alveolar concentration [MAC]) the normally used clin...