The mechanism by which volatile anesthetics (VAs) produce reversible loss of consciousness and render an organism insensate to pain remains an unsolved mystery of medicine for over 150 years. We demonstrated that mitochondrial complex I, an entry point and rate limiting component of the mitochondrial electron transport chain, controls anesthetic sensitivity across the animal kingdom, from worms to mice to humans. The implication is that an ancient mechanism is at hand, linking mitochondrial function to synaptic silencing by VAs. Ndufs4(KO) mice lack a subunit of mitochondrial complex I, which increases sensitivity of the complex to isoflurane inhibition. For either isoflurane or halothane, the KO mice became unresponsive to a tail pinch at a dose ~3-fold lower than for controls. Ndufs4(KO) was also hypersensitive to VAs using loss of righting reflex as the endpoint. These KO mice display the greatest change in VA sensitivity described in a mammal. We also discovered that VA sensitivity was fully controlled by glutamatergic expression of the mutation, with no effect from GABAergic, cholinergic or astrocyte expression. We measured the EC50s for loss of righting reflex (LORR) of the KO mice for other anesthetics whose targets are well characterized. Surprisingly, the animals were actually resistant to the effects of ketamine. The effects of the Ndufs4(KO) are specific in terms of anesthetic and not simply the result of generalized CNS depression. We wish to understand, in this proposal how complex I defects in the spinal cord cause hypersensitivity to VAs. We discovered that TREK-1 channels in spinal cord slices from Ndufs4(KO) are hypersensitive to isoflurane, in agreement with others who have shown that VA sensitivity of mice is dependent on TREK-1 function. We hypothesize that the VA hypersensitivity is mediated through a mitochondrial effect on TREK-1 channels in ventral horn neurons, and aim to characterize the mechanisms underlying mitochondrially induced changes in TREK-1sensitivity to VAs. We will move from cellular and molecular targets to whole animal behaviors in 3 specific aims: 1) investigating which cells must be defective in mitochondrial function in order to produce TREK-I hypersensitivity; 2) discover the effects of Ndufs4(KO) on phosphorylation sites within TREK-1 channels in the spinal cord, and 3) since we predict that TREK-1 activation underlies the VA hypersensitivity of our KO response to tail clamp, construct an Ndufs4(KO) that lacks TREK-1, and test its behavior in VAs. Our overarching goal is to understand the molecular targets of VAs. We have linked mitochondrial function to behavior in VAs in worms, mice, and man, and propose that mitochondria metabolism is a novel but very plausible mechanism underlying effects of VAs.
The mechanisms of action of volatile anesthetics (VAs) is currently unknown. From invertebrates to mammals, defects in mitochondrial metabolism produce profound hypersensitivity to VAs. Our data are consistent with inhibition of mitochondrial function, with downstream effects on potassium channels, as a novel mechanism of VA action.
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