Activity-dependent depression of neuronal sodium channels by the general anaesthetic isoflurane

Background The mechanisms by which volatile anaesthetics such as isoflurane alter neuronal function are poorly understood, in particular their presynaptic mechanisms. Presynaptic voltage-gated sodium channels (Na_v) have been implicated as a target for anaesthetic inhibition of neurotransmitter release. We hypothesize that state-dependent interactions of isoflurane with Na_v lead to increased inhibition of Na⁺ current (I_Na) during periods of high-frequency neuronal activity. Methods The electrophysiological effects of isoflurane, at concentrations equivalent to those used clinically, were measured on recombinant brain-type Na_v1.2 expressed in ND7/23 neuroblastoma cells and on endogenous Na_v in isolated rat neurohypophysial nerve terminals. Rate constants determined from experiments on the recombinant channel were used in a simple model of Na_v gating. Results At resting membrane potentials, isoflurane depressed peak I_Na and shifted steady-state inactivation in a hyperpolarizing direction. After membrane depolarization, isoflurane accelerated entry (τ_control=0.36 [0.03] ms compared with τ_isoflurane=0.33 [0.05] ms, P<0.05) and slowed recovery (τ_control=6.9 [1.1] ms compared with τ_isoflurane=9.0 [1.9] ms, P<0.005) from apparent fast inactivation, resulting in enhanced depression of I_Na, during high-frequency stimulation of both recombinant and endogenous nerve terminal Na_v. A simple model of Na_v gating involving stabilisation of fast inactivation, accounts for this novel form of activity-dependent block. Conclusions Isoflurane stabilises the fast-inactivated state of neuronal Na_v leading to greater depression of I_Na during high-frequency stimulation, consistent with enhanced inhibition of fast firing neurones.