What We Already Know about This Topic:
- Oligodendrocyte proliferation and maturation are prerequisites for myelination in the central nervous system. Disruption of these processes can lead to long-term impairment of neural function.
- Laboratory models demonstrate a variety of effects of anesthetics on the immature brain, but the consequences of early life anesthesia exposure on oligodendrocyte development have not been previously reported.
What This Article Tells Us That Is New:
- Exposure of 7-day-old mouse pups to isoflurane (1.5%, 4 h) results in lasting impairments of oligodendrocyte proliferation and differentiation.
- These effects lead to defects in myelinations and are associated with cognitive dysfunction.
- The underlying molecular mechanisms involve the isoflurane-induced activation of the mammalian target of rapamycin pathway and a related decrease in DNA methylation in oligodendrocyte progenitors.
Early postnatal exposure to general anesthetics may interfere with brain development. We tested the hypothesis that isoflurane causes a lasting disruption in myelin development via actions on the mammalian target of rapamycin pathway.
Mice were exposed to 1.5% isoflurane for 4 h at postnatal day 7. The mammalian target of rapamycin inhibitor, rapamycin, or the promyelination drug, clemastine, were administered on days 21 to 35. Mice underwent Y-maze and novel object position recognition tests (n = 12 per group) on days 56 to 62 or were euthanized for either immunohistochemistry (n = 8 per group) or Western blotting (n = 8 per group) at day 35 or were euthanized for electron microscopy at day 63.
Isoflurane exposure increased the percentage of phospho-S6–positive oligodendrocytes in fimbria of hippocampus from 22 ± 7% to 51 ± 6% (P < 0.0001). In Y-maze testing, isoflurane-exposed mice did not discriminate normally between old and novel arms, spending equal time in both (50 ± 5% old:50 ± 5% novel; P = 0.999), indicating impaired spatial learning. Treatment with clemastine restored discrimination, as evidenced by increased time spent in the novel arm (43 ± 6% old:57 ± 6% novel; P < 0.001), and rapamycin had a similar effect (44 ± 8% old:56 ± 8% novel; P < 0.001). Electron microscopy shows a reduction in myelin thickness as measured by an increase in g-ratio from 0.76 ± 0.06 for controls to 0.79 ± 0.06 for the isoflurane group (P < 0.001). Isoflurane exposure followed by rapamycin treatment resulted in a g-ratio (0.75 ± 0.05) that did not differ significantly from the control value (P = 0.426). Immunohistochemistry and Western blotting show that isoflurane acts on oligodendrocyte precursor cells to inhibit both proliferation and differentiation. DNA methylation and expression of a DNA methyl transferase 1 are reduced in oligodendrocyte precursor cells after isoflurane treatment. Effects of isoflurane on oligodendrocyte precursor cells were abolished by treatment with rapamycin.
Early postnatal exposure to isoflurane in mice causes lasting disruptions of oligodendrocyte development in the hippocampus via actions on the mammalian target of rapamycin pathway.