Brain-penetrant microtubule-stabilizer epothilone B delays isoflurane-induced unconsciousness in mice

Inhalational anesthetics are currently believed to cause unconsciousness by acting on multiple molecular targets including neural ion channels, receptors, mitochondria, synaptic proteins, and cytoskeletal proteins. Inhalational anesthetics including isoflurane bind to cytoskeletal microtubules (MTs), potentially contributing to causing unconsciousness. This possibility is supported by our demonstration of isoflurane resistance in rats treated once with the brain-penetrant MT-stabilizing drug epothilone B (epoB), and by a recent study in mice using a similar drug given daily over two weeks, which found increased sensitivity to isoflurane. To further characterize the contribution of MTs as functionally relevant targets of volatile anesthetics in mice, we measured latencies to loss of righting reflex (LORR) under isoflurane in mice injected once subcutaneously with vehicle or epoB. We found significantly increased LORR latencies (i.e., anesthetic resistance) in 8 mg/kg epoB-treated mice on the day following injection, with reduced effects on subsequent days. The 29-s within-subject increase in LORR latencies is not large compared to the variability among different animals, but it represents a statistically large within-subject effect as represented by a Cohen's d of 0.8. The effect could not be accounted for by tolerance from repeated exposure to isoflurane. Our results support that binding of the inhalational anesthetic isoflurane to MTs contributes to LORR in mice, as it does in rats. Our findings support the Orchestrated Objective Reduction (Orch OR) model that posits consciousness as a property of a quantum physical state of neural MTs. We also discuss possible sex differences in anesthetic mechanisms suggested by our data.