Isoflurane Preconditioning Enhances Neuronal Tolerance to Amyloid-β Toxicity in HT-22 Cells via Mild Oxidative Signaling and Akt–Nrf2 Activation

Isoflurane is a widely used volatile anesthetic with context-dependent effects on neuronal survival, particularly in neurodegenerative conditions. Increasing evidence suggests that brief, sublethal stress exposure can induce adaptive cellular responses through hormesis-based preconditioning mechanisms. In this study, we investigated whether isoflurane preconditioning enhances neuronal tolerance to amyloid-β (Aβ)-induced toxicity and explored the underlying redox-dependent molecular pathways. Using HT-22 murine hippocampal neuronal cells, we demonstrate that short-term exposure to low-dose isoflurane induces a delayed neuroprotective phenotype characterized by improved cell viability, reduced apoptotic signaling, and maintained mitochondrial membrane potential following Aβ challenge. Mechanistically, isoflurane preconditioning elicited a mild and transient increase in intracellular reactive oxygen species (ROS), which is critical for the activation of the PI3K/Akt signaling pathway. Pharmacological scavenging of reactive oxygen species abolished Akt phosphorylation and reduced the protective effects of preconditioning, supporting a hormetic signaling model rather than direct antioxidant action. Following Akt activation, isoflurane preconditioning promoted the inhibitory phosphorylation of glycogen synthase kinase-3β (GSK-3β), decreased Keap1 protein levels, and facilitated nuclear translocation and transcriptional activation of nuclear factor erythroid 2-related factor 2 (Nrf2). Consequently, the expression of Nrf2-regulated antioxidant genes, including heme oxygenase-1, NAD(P)H quinone dehydrogenase 1 (NQO1), superoxide dismutase 1 and 2 (SOD1/2), and catalase, was significantly upregulated. Collectively, these findings indicate that isoflurane preconditioning confers neuroprotection through hormesis-like mild oxidative signaling and coordinated activation of endogenous antioxidant defenses rather than via direct antioxidant scavenging.