Volatile anesthetics inhibit mammalian voltage-gated Na ϩ channels, an action that contributes to their presynaptic inhibition of neurotransmitter release. We measured the effects of isoflurane, a prototypical halogenated ether volatile anesthetic, on the prokaryotic voltage-gated Na ϩ channel from Bacillus halodurans (NaChBac). Using whole-cell patch-clamp recording, human embryonic kidney 293 cells transfected with NaChBac displayed large inward currents (I Na ) that activated at potentials of Ϫ60 mV or higher with a peak voltage of activation of 0 mV (from a holding potential of Ϫ80 mV) or Ϫ10 mV (from a holding potential of Ϫ100 mV). Isoflurane inhibited I Na in a concentration-dependent manner over a clinically relevant concentration range; inhibition was significantly more potent from a holding potential of Ϫ80 mV (IC 50 ϭ 0.35 mM) than from Ϫ100 mV (IC 50 ϭ 0.48 mM). Isoflurane positively shifted the voltage dependence of peak activation, and it negatively shifted the voltage dependence of end steady-state activation. The voltage dependence of inactivation was negatively shifted with no change in slope factor. Enhanced inactivation of I Na was 8-fold more sensitive to isoflurane than reduction of channel opening. In addition to tonic block of closed and/or open channels, isoflurane enhanced use-dependent block by delaying recovery from inactivation. These results indicate that a prokaryotic voltage-gated Na ϩ channel, like mammalian voltage-gated Na ϩ channels, is inhibited by clinical concentrations of isoflurane involving multiple state-dependent mechanisms. NaChBac should provide a useful model for structure-function studies of volatile anesthetic actions on voltage-gated ion channels.

We tested the hypothesis that expression of pre-synaptic voltage-gated sodium channel (Na v ) subtypes coupled to neurotransmitter release differs between transmitter types and CNS regions in a nerve terminal-specific manner. Na v coupling to transmitter release was determined by measuring the sensitivity of 4-aminopyridine (4AP)-evoked [3 H]glutamate and [ 14 C]GABA release to the specific Na v blocker tetrodotoxin (TTX) for nerve terminals isolated from rat cerebral cortex, hippocampus, striatum and spinal cord. Expression of various Na v subtypes was measured by immunoblotting using subtype-specific antibodies. Potencies of TTX for inhibition of glutamate and GABA release varied between CNS regions. However, the efficacies of TTX for inhibition of 4AP-evoked glutamate release were greater than for inhibition of GABA release in all regions except spinal cord. The relative nerve terminal expression of total Na v subtypes as well as of specific subtypes varied considerably between CNS regions. The region-specific potencies of TTX for inhibition of 4AP-evoked glutamate release correlated with greater relative expression of total nerve terminal Na v and Na v 1.2. Nerve terminal-specific differences in the expression of specific Na v subtypes contribute to transmitter-specific and regional differences in pharmacological sensitivities of transmitter release.