Immature murine hippocampal neurones do not develop long-term structural changes after a single isoflurane exposure

Abstract

Background: Studies in developing animals show that a clinically relevant anaesthesia exposure increases neuronal death and alters brain structure. In the hippocampal dentate gyrus, the anaesthetic isoflurane induces selective apoptosis among roughly 10% of 2-week-old hippocampal granule cells in 21-day-old mice. In this work, we queried whether the 90% of granule cells surviving the exposure might be ‘injured’ and integrate abnormally into the brain. Methods: The long-term impact of isoflurane exposure on granule cell structure was studied using a transgenic mouse model fate-mapping approach to identify and label immature granule cells. Male and female mice were exposed to isoflurane for 6 h when the fate-mapped granule cells were 2 weeks old. The morphology of the fate-mapped granule cells was quantified 2 months later. Results: The gross structure of the dentate gyrus was not affected by isoflurane treatment, with granule cells present in the correct subregions. Individual isoflurane-exposed granule cells were structurally normal, exhibiting no changes in spine density, spine type, dendrite length, or presynaptic axon terminal structure (P>0.05). Granule cell axon terminals were 13% larger in female mice relative to males; however, this difference was evident regardless of treatment (difference of means=0.955; 95% confidence interval, 0.37–1.5; P=0.010). Conclusions: A single, prolonged isoflurane exposure did not impair integration of this age-specific cohort of granule cells, regardless of the animal's sex. Nonetheless, although 2-week-old cells were not affected, the results should not be extrapolated to other age cohorts, which may respond differently.