Regional differences in nerve terminal Na+ channel subtype expression and Na+ channel-dependent glutamate and GABA release in rat CNS.

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.

Isoflurane inhibits NaChBac, a prokaryotic voltage-gated sodium channel.

Volatile anesthetics inhibit mammalian voltage-gated Na(+) 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(+) 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 -60 mV or higher with a peak voltage of activation of 0 mV (from a holding potential of -80 mV) or -10 mV (from a holding potential of -100 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 -80 mV (IC(50) = 0.35 mM) than from -100 mV (IC(50) = 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(+) channel, like mammalian voltage-gated Na(+) 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.