A true enigma of modern medicine has persisted for over 150 years. How can volatile anesthetics, as a class of drugs, simultaneously cause a patient to reversibly lose consciousness, feel no pain, and have no memory of surgery? The mechanism by which volatile anesthetics (VAs) produce reversible loss of consciousness and render an organism insensate to a surgical incision remains an unsolved mystery. Our laboratory has exploited a very simple animal model, the nematode C. elegans, to investigate the molecular mechanisms of volatile anesthetic action. Previously, we demonstrated that mitochondrial complex I, an entry point of the electron transport chain, specifically controls the sensitivity of C. elegans to volatile anesthetics. We also found that complex I defects control anesthetic sensitivity in humans. We now have an exciting opportunity to extend those findings into mice using the knockout animal, Ndufs4. The Ndufs4 KO mice lack a subunit of complex I, which results in complex I dysfunction. We anesthetized young (PN23-27) Ndusf4 and wild-type (WT) mice with isoflurane or halothane. For either VA, the KO mice became unresponsive to a tail pinch at a dose 2.8-fold lower than for WT controls. These KO mice display the greatest change in VA sensitivity ever described in a mammal. We also measured the EC50s for loss of righting reflex (LORR) of the KO mice for anesthetics whose targets are well characterized. Surprisingly, the animals were actually resistant to the effects of ketamine. The effects of the complex I KO are specific in terms of anesthetic and not simply the result of generalized CNS depression. Complex I dysfunction causes hypersensitivity to volatile anesthetics across phylogeny. However, the question remains, how do complex I defects affect VA sensitivity. We hypothesize that complex I depression (i.e. energy depletion) leads to defective synaptic function in specific cell types, making them more susceptible to neuronal silencing by VAs. We will knock out Ndufs4 in GABAergic, cholinergic or glutamatergic neurons, in neuronal support cells known as astrocytes, as well as in specific regions of the brain. We will also perform electrophysiologic measurements of WT and mutant mice with and without volatile anesthetics. Our initial results indicate that the VA hypersensitivity in Ndufs4 mice is mediated through glutamatergic neurons.
Our aims are to characterize which neurons and regions of the brain are important for the anesthetic phenotype of these animals, as well as to discover how basic neuronal physiology is altered in the KO animals. Our overarching goal is to understand how the VAs functions. We have linked mitochondrial function to behavior in VAs in worms, mice, and man, a finding the field must consider. Our proposed studies will tease out which cell types mediate the anesthetic response, and will give important insights into the basic mechanisms of action of VAs.
We have discovered a link between mitochondrial complex I and volatile anesthetic sensitivity that holds true for multiple species. We will determine which nervous system cell types and regions are important for determining anesthetic sensitivity, and test the electrophysiologic effects of this defect on neuronal function. Our studies are intended to uncover basic mechanisms by which volatile anesthetics exert their effects and eliminate their adverse side effects.
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