| General anesthesia results from complex interactions involving ion channels in the brain. Halogenated inhaled general anesthetics modulate voltage-gated ion channels, but the underlying molecular mechanisms are not understood. Inhaled anesthetics may act as positive and negative allosteric gating modifiers by interacting with critical hydrophobic pockets in the ion channel. A combination of scanning mutagenesis, electrophysiology, kinetic analysis and structural modeling was used to probe halogenated general anesthetic interactions with two model voltage-gated ion channels: K-Shaw2 (K +-selective) and NaChBac (Na+-selective). In K-Shaw2, the modulation is anesthetic-specific. Halothane, isoflurane and desflurane inhibit this ion channel, while sevoflurane, the more heavily fluorinated anesthetic, activates it. In NaChBac, sevoflurane alone produces overlapping responses with opposite effects. Whereas the response to low sevoflurane concentrations (0.2 mM) is dominated by activation, at high concentrations of sevoflurane (2 mM), there is additionally evidence for open-channel block. Mutagenesis and structural modeling revealed multiple binding and allosteric sites associated with the activation and inactivation gating regions of the ion channels. Furthermore, kinetic analysis suggests that inhaled anesthetic-dependent activation results from destabilizing pre-open activated states, stabilizing the open state and/or eliminating inactivation. In contrast, inhibition might result from stabilization of pre-open closed states, more favorable inactivation and open-channel block. The novel dual allosteric modulation of voltage-gated ion channels by closely related halogenated anesthetics offers new opportunities to study the structural basis of anesthetic specificity. Moreover, these findings will help determine the inhaled anesthetic pharmacophore necessary to design general anesthetics with improved therapeutic indices. |
