Has The Mechanism of General Anesthesia Finally Been Discovered?

 

General anesthesia is one of the greatest innovations of modern medicine.  Even within  the history of that innovation there have been tremendous improvements ranging from the administration of ether in the 1960s to very closely monitored combinations of opioids, benzodiazepines, and inhaled anesthetics in more modern times. The mechanism of action of opioids and benzodiazepines at the receptor level are known, but the effects of inhaled anesthetics have been more of a source of speculation.  I first became aware of this as an undergraduate taking physical chemistry (1) when I read about Linus Pauling’s hypothesis (2).  He suggested that microcrystalline hydrates form from the reaction of anesthetic gases and water molecules at the membrane surface. Those microcrystalline hydrates then interfere with synaptic transmission leading to loss of consciousness. Pauling was a physical chemist who was awarded the Nobel Prize for his work on the hydrogen bond and wrote about many general anesthetics as not working through hydrogen bond mechanisms.  He was also very optimistic about the role of physical chemistry in biological systems. Interestingly he briefly discusses how general anesthesia and the mechanism are important for psychobiology (2):

“The progress that has been made in the field of molecular biology during this period has related in the main to somatic and genetic aspects of physiology, rather than to psychic. We may now have reached the time when a successful molecular attack on psychobiology, including the nature of encephalonic mechanisms, consciousness, memory, narcosis, sedation, and similar phenomena, can be initiated. As one of the steps in this attack I have formulated a rather detailed theory of general anesthesia, which is described in the following paragraphs.” (p. 15)

He provides an elaborate physical chemistry rationale for the hydrate-microcrystal theory of anesthesia in this paper.  Pauling’s work comes on the cusp of the era of molecular biology – a field that he is credited with creating.  In his original explanation he discussed x-ray crystallography of crystals and new biologically active protein structures continue to undergo this analysis when they are isolated and purified.

Fast forward to a paper I just read in Current Biology a few days ago (3). It is written in the context of no clear mechanism of action for volatile inhaled anesthetics since their first observed effects noted over a century ago despite numbers speculative papers including papers from the past decade in this same journal.  The authors suggest that a disruption in electron transport in the mitochondria specifically Complex I  of 4 transport proteins is the area responsible for the effects of general anesthesia. Before getting to their experiment, a few words about this system.

Electron transport, oxidative phosphorylation, and ATP synthesis are all tightly coupled processes occurring over 5 proteins known as mitochondrial complexes (Complexes I-V).  Before the era of molecular and structural biology, these processes were partially deduced using in vitro methods looking at chemical reactions in mitochondrial preparations and specific reactions that affect each step. The cofactors were determined along with the overall stoichiometry of the process. With greater emphasis on structural and molecular biology there have been additional hypotheses about the specifics of electron transfer across the complexes and how ATP synthesis occurs.  Although there is much evidence to support various hypotheses about how all of these processes occur – in all of my reading it does not appear to be settled science.  In fact, some authors talk about emergent properties of this system that cannot be defined by what is known about the current components (10).  The discussion of emergent properties is interesting on at least a couple of levels. First, that kind of discussion is routine in consciousness research. There are no clear-cut biological mechanisms that generate a conscious state and it is discussed as an emergent property of the brain. Second, the minimum requirements of a biological system to create emergent properties is never really discussed. Does the mitochondrial system of electron transport, generating a proton gradient, ATP synthesis, and tightly couple oxidation and phosphorylation qualify?

This mechanism may have implications for the science of consciousness. In humans who are in good health and have no known brain diseases - general anesthesia and non-REM (NREM) sleep are considered to the the only states of unconsciousness. During that time the thalamus appears to be inactivated (13).  There have been several studies showing that some people dream during NREM sleep so that is not a clear boundary. But in the case of this mechanism questions would include considering the synaptic mechanisms as well as the global neuroanatomical mechanisms as well as the issue of emerging properties of biological systems of varying complexity.  What does it mean if a smaller system with emergent properties can turn off a larger system with emergent properties? What is the relationship of the emergent properties between systems?  

Moving on to the paper – the authors start by pointing out that neurotransmitter recycling in neurons is dependent on ATP and endocytosis.  Further - that Complex I of the mitochondrial electron transport chain (ETC) is the rate limiting step in this process and that disrupting it causes sensitivity to volatile anesthetics (VA). Knockout mice (for a protein in Complex I) were physiologically normal but much more sensitive to VA. The authors hypothesized that VAs decrease presynaptic ATP production by the ETC (oxidative phosphorylation) leading to decreased endocytosis and neurotransmitter cycling, and that the inhibition of Complex I was the primary mechanism.  They conduct a number of experiments to illustrate the effects of VA (isoflurane) on the ETC chain looking at perturbations would increase the effect and decrease the effect and conclude that their hypotheses are supported by the data.  They conclude that Complex I inhibition may be the mechanism of action of isoflurane. If supported by other studies the mystery of the mechanism of action of VA may be solved after 170 years.

I continue to be astonished at the trajectory of brain science and all of the factors that are needed for these advancements.  Even at the level in this paper the suggestion is that the proposed hypothesis will require additional work.  This research occurs at the intersection of a series of historical hypotheses about the mitochondrial ETC and parallel hypotheses about the mechanism of action of volatile anesthetic gases. The scientific work and hypothesizing was built both on previous discovery and advances in technology.  In this area advancement was slow and is still not completely settled in either research area. A lot of science discussed in the press seems to suggest that there are arbitrary time frames or amounts of investment for advances and that is obviously not true.

George Dawson, MD, DFAPA

 

References:

1:  Moore WJ.  Physical Chemistry, Fourth Edition. Englewood Cliffs: Prentice-Hall, Inc; 1972: p. 241-243.

2:  Pauling L. A molecular theory of general anesthesia. Science. 1961 Jul 7;134(3471):15-21. doi: 10.1126/science.134.3471.15. PMID: 13733483.

3:  Jung S, Zimin PI, Woods CB, Kayser EB, Haddad D, Reczek CR, Nakamura K, Ramirez JM, Sedensky MM, Morgan PG. Isoflurane inhibition of endocytosis is an anesthetic mechanism of action. Curr Biol. 2022 Jul 25;32(14):3016-3032.e3. doi: 10.1016/j.cub.2022.05.037. Epub 2022 Jun 9. PMID: 35688155; PMCID: PMC9329204.

4:  Sharma LK, Lu J, Bai Y. Mitochondrial respiratory complex I: structure, function and implication in human diseases. Curr Med Chem. 2009;16(10):1266-77. doi: 10.2174/092986709787846578. PMID: 19355884; PMCID: PMC4706149.

5:  Wikström M, Hummer G. Stoichiometry of proton translocation by respiratory complex I and its mechanistic implications. Proc Natl Acad Sci U S A. 2012 Mar 20;109(12):4431-6. doi: 10.1073/pnas.1120949109. Epub 2012 Mar 5. PMID: 22392981; PMCID: PMC3311377.

5:  Jones AJ, Blaza JN, Varghese F, Hirst J. Respiratory Complex I in Bos taurus and Paracoccus denitrificans Pumps Four Protons across the Membrane for Every NADH Oxidized. J Biol Chem. 2017 Mar 24;292(12):4987-4995. doi: 10.1074/jbc.M116.771899. Epub 2017 Feb 7. PMID: 28174301; PMCID: PMC5377811.

7:  Toda C, Diano S. Mitochondrial UCP2 in the central regulation of metabolism. Best Pract Res Clin Endocrinol Metab. 2014 Oct;28(5):757-64. doi: 10.1016/j.beem.2014.02.006. Epub 2014 Mar 7. PMID: 25256770.

8:  Giorgio V, Fogolari F, Lippe G, Bernardi P. OSCP subunit of mitochondrial ATP synthase: role in regulation of enzyme function and of its transition to a pore. Br J Pharmacol. 2019 Nov;176(22):4247-4257. doi: 10.1111/bph.14513. Epub 2018 Nov 28. PMID: 30291799; PMCID: PMC6887684.

9:  DiMauro S, Garone C. Historical perspective on mitochondrial medicine. Dev Disabil Res Rev. 2010;16(2):106-13. doi: 10.1002/ddrr.102. PMID: 20818724; PMCID: PMC3839238.

10:  Voet D, Voet JG. Electron Transport and Oxidative Phosphorylation. In: Biochemistry, 2^nd Edition. New York: John Wiley & Sons, Inc;1995: 563-598.

11:  Kurz FT, Aon MA, O'Rourke B, Armoundas AA. Functional Implications of Cardiac Mitochondria Clustering. Adv Exp Med Biol. 2017;982:1-24. doi: 10.1007/978-3-319-55330-6_1. PMID: 28551779; PMCID: PMC7003720.

12:  Deshpande OA, Mohiuddin SS. Biochemistry, Oxidative Phosphorylation. [Updated 2021 Aug 3]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2022 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK553192/

13:  12:  Vacas S, Kurien P, Maze M. Sleep and Anesthesia - Common mechanisms of action. Sleep Med Clin. 2013 Mar;8(1):1-9. doi: 10.1016/j.jsmc.2012.11.009. PMID: 28747855; PMCID: PMC5524381.

Graphics Credit:

 Mitochondria graphic is my modification of a VisiScience slide per their user agreement for non-commercial use.

Supplementary:

1:  The Krebs cycle, citric acid cycle, or tricarboxylic acid cycle occurs only in the mitochondrial matrix basically converting chemical energy into the reducing power of NADH for ATP synthesis from electron transport.

2:  Additional review on mitochondrial dysfunction and Alzheimer's Disease:

Misrani A, Tabassum S, Yang L Mitochondrial Dysfunction and Oxidative Stress in Alzheimer’s Disease.  Front. Aging Neurosci. 2021; 13:617588. doi: 10.3389/fnagi.2021.617588