Differential effects of anesthetic and nonanesthetic cyclobutanes on neuronal voltage-gated sodium channels

Background: Despite their key role in the generation and propagation of act ion potentials in excitable cells, voltage-gated sodium (Na+) channels have been considered to be insensitive to general anesthetics. The authors test ed the sensitivity of neuronal Na+ channels to structurally similar anesthe tic (1-chloro-1,2,2-trifluorocyclobutane; F3) and nonanesthetic (1,2-dichlo rohexafluorocyclobutane; F6) polyhalogenated cyclobutanes by neurochemical and electrophysiologic methods. Methods: Synaptosomes (pinched-off nerve terminals) from adult rat cerebral cortex were used to determine the effects of F3 and F6 on 4-aminopyridine- or veratridine-evoked (Na+ channel-dependent) glutamate release (using an enzyme-coupled spectrofluorimetric assay) and increases in intracellular Ca 2+ ([Ca2+](i)) (using ion-specific spectrofluorimetry). Effects of F3 and F 6 on Na+ currents were evaluated directly in rat lumbar dorsal root ganglio n neurons by whole-cell patch-clamp recording. Results: F3 inhibited glutamate release evoked by 4-aminopyridine (inhibito ry concentration of 50% [IC50] = 0.77 mM [similar to 0.8 minimum alveolar c oncentration (MAC)] or veratridine (IC50 = 0.42 mM [similar to 0.4 MAC]), a nd veratridine-evoked increases in [Ca2+](i), (IC50 = 0.5 mM [similar to 0. 5 MAC]) in synaptosomes; F6 had no significant effects up to 0.05 mM (appro ximately twice predicted MAC). F3 caused reversible membrane potential-inde pendent inhibition of peak Na+ currents (70 +/- 9% block at 0.6 mM [similar to 0.6 MAC]), and a hyperpolarizing shift in the voltage dependence of ste ady state inactivation in dorsal root ganglion neurons (-21 +/- 9.3 mV at 0 .6 mM). F6 inhibited peak Na+ currents to a lesser extent (F6 +/- 2% block at 0.018 mM [predicted MAC]) and had minimal effects on steady state inacti vation. Conclusions: The anesthetic cyclobutane F3 significantly inhibited Na+ chan nel-mediated glutamate release and increases in [Ca2+](i). In contrast, the nonanesthetic cyclobutane F6 had no significant effects at predicted anest hetic concentrations. F3 inhibited dorsal root ganglion neuron Na+ channels with a potency and by mechanisms similar to those of conventional volatile anesthetics; F6 was less effective and did not produce voltage-dependent b lock, This concordance between anesthetic activity and Na+ channel inhibiti on supports a role for presynaptic Na+ channels as targets for general anes thetic effects and suggests that shifting the voltage-dependence of Na+ cha nnel inactivation is an important property of volatile anesthetic compounds .