Effects Of The General Anesthetic Isoflurane On Voltage-Gated Sodium Channels And The Presynaptic Action Potential

The molecular mechanisms of general anesthetic action are poorly understood, especially in regard to widely used inhaled anesthetic agents. Optimization of anesthetic drug design and clinical use requires detailed understanding of the roles of specific targets involved in the therapeutic actions (unconsciousness, amnesia, immobility) and undesirable effects (cardiovascular and respiratory depression, neurotoxicity) of various anesthetics. Voltage-gated sodium channels (Nav) have been implicated as targets for anesthetic inhibition of neurotransmitter release, but block of Nav by general anesthetics was previously considered too modest at clinical concentrations to be pharmacologically relevant. However, studies in whole animals show that block of Nav by the prototypical inhaled anesthetic agent isoflurane is necessary for producing immobility and determining anesthetic potency. This dissertation addresses the electrophysiological effects of isoflurane on Nav and the presynaptic action potential in primary cultures of rat hippocampal neurons. At concentrations equivalent to those used in the clinic, isoflurane, and not the nonanesthetic molecule F6, significantly decreased spontaneous activity of hippocampal neurons and also reduced peak amplitude and upstroke velocity of the action potential. Peak current of endogenous Nav was inhibited ~10% by clinical concentrations of isoflurane, similar to previous reports. However, we found that the magnitude of Nav block depended on the state of the channel and increased at higher stimulation frequencies. Using heterologously expressed rat Nav1.2, a widely expressed neuronal isoform, we show that isoflurane stabilizes the fast-inactivated state of the channel. We propose that by stabilizing Nav in the inactivated state, isoflurane leads to accumulation of inactivated channels and inhibition of INa during trains of high frequency stimuli as would be experienced by a burst-firing neuron. This work shows that activity-dependent block contributes significantly to overall block of Nav, and supports a role for Nav inhibition in the presynaptic action of general anesthetic ethers such as isoflurane