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es neuronal apoptosis and results in learning deficits in rodents [1,2]. We showed that exposure to 2% sevoflurane for 6 h resulted in blood–brain barrier (BBB) disruption in the hippocampus of postnatal day 6 (POD6) mice [3]. There are no reports regarding whether neonatal sevoflurane exposure-induced BBB disruption are dependent on exposure time and whether they are reversible. The present study was performed to examine the length of time required for neonatal sevoflurane exposure to cause BBB disruption in mice. Furthermore, we investigated whether these changes were reversible. The Animal Care and Use Committee of Tokyo Medical and Dental University approved the protocol in the current study (0170118A). C57BL/6NCr strain mice (male and female) at POD6 were used for the study. To reduce variability, the same number of pups from each litter were used for the same experiment. Sevoflurane anesthesia was performed as previously described [2,3]. Briefly, POD6 mice were placed in a humid and warm chamber immediately after removal from the maternal cage. Sevoflurane (2%) was administered with 40% oxygen as the carrier gas for 6 h. Total gas flow was 1 L/min. To study the effects of BBB disruption, POD6 mice were randomly divided into control or sevoflurane groups and exposed to carrier gas or sevoflurane, respectively (total number of pups used = 18). BBB disruption in the hippocampal CA1 region were examined using a transmission electron microscope (TEM) (H-7100; Hitachi, Japan) at 2, 4, and 6 h during sevoflurane exposure, and 24 and 48 h after sevoflurane exposure. Furthermore, we evaluated POD16 mice and 8-week-old mice that were exposed to carrier gas or 2% sevoflurane for 6 h. Brain tissue were perfused with freshly prepared 4% paraformaldehyde and 2.5% glutaraldehyde. After dehydration, the hippocampal CA1 region was embedded in Epon 812, cut with a microtome, stained with uranyl acetate and lead citrate, and examined using TEM. We observed perivascular spaces because in our previous investigation [3] sevoflurane disrupted the perivascular spaces. We picked 15 capillaries in the hippocampal CA1 region for each animal sampled at each time point. The number of capillaries with destroyed perivascular space was counted and presented as a percentage of the number of capillaries with destroyed ultrastructure (%). Values are expressed as mean ± SD. In the control group, the ultrastructure of capillaries in the CA1 region of the hippocampus was continuous, and the perivascular spaces were normal (Figs. 1A and 1F). However, the ultrastructural integrity was locally collapsed after 2 h of sevoflurane exposure (Figs. 1B and 1G). The perivascular spaces gradually became enlarged and collapsed depending on the sevoflurane exposure time (Figs. 1B–1D, 1G and 1H). To clarify Letter to the Editor