Frequency-Dependent Inhibition of Sodium Channels by the General Anesthetic Isoflurane

Abstract

Voltage-gated sodium channels (Nav) are important for initiation and propagation of the action potential in excitable cells, thus contributing to cell-to-cell communication via neurotransmitter release at the synapse. Nav are potential presynaptic targets for inhaled volatile anaesthetics (VAs), which despite their widespread use have poorly understood mechanisms of action. Many drugs that act on Nav, including local anesthetics, show state-dependent effects, that is affinity of the drug depends on whether the channel is open, closed or inactivated. Upon action potential firing, Nav open and rapidly transition into a non-conducting, inactivated state. During high-frequency firing, inactivated channels accumulate as there is insufficient time to return to a closed state before subsequent action potential firing, resulting in frequency-dependent reduction of INa. Therefore, drugs that affect the rates of channel state transitions can have a profound effect on neurotransmitter release and neuronal communication. We investigated the state-dependent effects of the prototypical VA isoflurane on recombinant and native Nav during high-frequency firing. We expressed Nav1.2a, a major isoform found in the central nervous system, in the neuronal mammalian cell line ND7/23 and found that isoflurane enhanced frequency-dependent reduction of INa by enhancing apparent inactivation. To confirm these results in a more physiological preparation we studied the effects of isoflurane on native Nav in rat neurohypophysial nerve terminals and show similar frequency-dependent findings. These data led us to propose a simple pharmacological model of open channel block to account for the effects of isoflurane on Nav1.2. Our data indicate that high-frequency neuronal firing potentiates isoflurane effects, which might contribute to selective modulation of fast firing neuronal networks.Supported by NIH grant GM 58055.