A true enigma of modern medicine has persisted for over 150 years; the mechanism(s) by which volatile anesthetics (VAs) produce reversible loss of consciousness remains an unsolved mystery. Using genetic approaches, we demonstrated that mitochondrial complex I, an entry point of the mitochondrial electron transport chain, specifically controls the sensitivity of multiple species, including worms and humans, to VAs. These broad phylogenetic effects indicate that an ancient mechanism is at hand, linking mitochondrial function to synaptic silencing in the presence of VAs. We began mechanistic studies in mice by exploiting Ndufs4(KO), a mouse defective in complex I function and extremely hypersensitive to VAs. Testing cell-specific Ndufs4(KO) mice, we found that VA sensitivity was fully controlled by glutamatergic KO, with no effect of loss of NDUFS4 from GABAergic or cholinergic neurons. Surprisingly, an astrocytic-specific KO of NDUFS4 was defective only in arousal from VAs. Preliminary data indicate that neurons in the locus coeruleus mediate this effect. This novel role of astrocytes offers a new approach to investigate crucial arousal pathways. In addition, we are exploring the mechanisms underlying anesthetic induced neurotoxicity (AIN). From our work in nematodes, we have identified new candidate molecules that can be tested as AIN therapies in mice. We showed that inhibition of the unfolded protein response in the endoplasmic reticulum, or inhibition of mTOR, a cellular metabolic switch, alleviated AIN in worms. We are exploring the roles of these pathways in AIN, and relating them to exciting new data which indicate that VAs themselves produce metabolic changes specific to neonatal mice. Many questions remain unanswered. 1. How do complex I defects control VA sensitivity. We showed that excitatory neurotransmission in Ndufs4(KO) was hypersensitive to isoflurane inhibition compared to WT. Our recent data suggest that isoflurane inhibits synaptic endocytosis in both WT and KO animals and that this inhibition results from a decrease in ATP production.
Our aims are to characterize the mechanism underlying inhibition of neurotransmitter endocytosis by VAs. 2. What pathways transduce AIN in neonatal mice; how can those pathways be inhibited? We are extending C. elegans studies to test exciting new small molecule candidates that may alleviate AIN in mice. We are also exploring ER-stress and mTOR activity as potential signaling pathways mediating AIN in mice. 3. How does mitochondrial function in astrocytes control arousal from the anesthetized state? We are studying astrocyte signaling to determine how astrocytes affect synaptic function during and following VA exposure. Astrocyte/neural pathways necessary for emergence from the anesthetized state will be investigated. Mitochondrial function is linked to behavior in VAs in worms, mice, and man. Our proposed studies are aimed to identify the basic, molecular mechanisms of action of VAs.
The goal of our laboratory is to understand the mechanisms of action of volatile anesthetics (VAs) in order to optimize clinical care and understand control of consciousness. We discovered that VAs inhibit mitochondrial function, producing specific defects in excitatory neurotransmission. We are extending the studies of VAs and mitochondria to understand arousal from the anesthetic state, and how VAs produce undesired long-term effects.