Title: Mechanisms of Action of General Anesthetics
1Mechanisms of Action of General Anesthetics
- BCP 401 - Principles of Pharmacology
- JP Dilger
- November 26, 2008
2Outline
- What is general anesthesia?
- MAC - Minimum Alveolar Concentration
- Chemical structures of general anesthetics
- Meyer-Overton correlation exceptions
- Possible anesthetic binding sites
- Molecular approaches to understanding anesthesia
- In vivo approaches to understanding anesthesia
3What is general anesthesia?
Do you think I should give him 50 or 100?
4Analgesia
5Amnesia
6Hypnosis/Sedation
7Immobility
8General Anesthesia
- Analgesia
- Amnesia
- Hypnosis/sedation
- Immobility
9How is anesthesia measured?
- Lack of purposeful response to painful stimulus
- surgical incision (humans, other mammals)
- tail clamp (rat, mouse)
- heat (rat, mouse, fruit fly)
- Loss of righting reflex (rat, mouse)
- Loss of spontaneous movement
- Swimming (goldfish, tadpole)
- flying (fruit fly)
- crawling (nematode)
10MAC Minimum Alveolar Concentration
- 1.0 MAC is the concentration of inhalational
anesthetic required to blunt the muscular
response to surgical skin incision of 50 of a
population of unparalyzed patients.
11MAC the population response to anesthetics
12Halothane MAC in the Animal Kingdom
13MAC is not the whole story
MAC for isoflurane in rats Control
1.300.25 Decerebrated 1.260.14
MAC for isoflurane in goats Control
1.20.3 CNS bypass 2.90.7 Recovery 1.30.1
14Anesthetics - Halogenated ethers
MW
1
2
3
4
5
6
7
8
Diethyl ether
74
H
H
CH
H
H
H
H
H
3
Fluroxene
126
H
H
CH
H
F
F
F
2
Methoxyflurane
165
F
H
H
H
F
Cl
H
Cl
Desflurane
168
H
F
H
F
F
F
F
F
Isoflurane
184
H
F
H
F
Cl
F
F
F
Enflurane
184
F
F
H
F
F
Cl
H
F
Sevoflurane
200
H
H
F
H
CF
F
F
F
3
15Other inhalational anesthetics
16Intravenous anesthetics
- Propofol
- Etomidate
- Alphaxalone
Thiopental Midazolam Ketamine
17Anesthetic Properties
18Partition Coefficients
19Meyer Overton Correlation
HAL Halothane ISO Isoflurane ENF Enflurane SEV Sev
oflurane FLU Fluroxene DES Desflurane CYC Cyclopro
pane MOF Methoxyflurane DEE Diethylether CHL Chlor
oform DDM Dichlorodiflurormethane PFE Perfluoroeth
ane CTF Carbon tetrafluroide SHF Sulfur
hexafluoride CYC Cyclopropane ETH Ethylene NO2 Nit
rous oxide XEN Xenon NIT Nitrogen KRY Krypton ARG
Argon
20Non-anesthetics (non-immobilizers)
Correlation of 22
anesthetics in mouse
10000
1000
100
10
1
0.1
Olive Oil Gas Partition Coefficient
21Awake! Awareness during anesthesia
- 25 incidents in 19,575 cases
- 1-2 per 1000
- 26,000 per year in US
Alba Christensen, 2007
22Lipid-Based Theories of Anesthetic Action
- Direct action on lipids leads to Indirect action
on proteins - Thickness and/or Volume Fluidity
- Curvature Dielectric properties
- Phase Transitions Ionic Permeability
23Status of Lipid Theories
- Currently in disfavor
- little physical evidence - effects on bilayer
properties are small - anesthetic to lipid ratio is low
- 50 mM anesthetic distributed uniformly through
50Å thick membrane corresponds to - 1 anesthetic molecule per 60 lipids
- 1.5 of the molecules
- 1 of the volume
- specificity problem - lipids are ubiquitous
- what about nonimmobilizers?
24Fireflies and Anesthetics
25Protein-based theories of Anesthetic Action
- Anesthetics bind to amphipathic sites on
proteins. - induce/prevent conformational change
- alter kinetics of conformational changes
- compete with ligands
26Status of Protein Theories
- Currently favored by many investigators
- anesthetic binding sites have been identified on
several proteins - some of these sites are stereospecific
- effects on some proteins seen at relevant
concentrations of anesthetics - the binding of 1 anesthetic molecule to 1 protein
could produce a significant effect if the binding
occurred at a functionally important part of the
protein
27Status of Protein Theories
- Problems
- lack of a chemical anesthetic antagonist
- specificity problem - proteins are ubiquitous
- which protein(s)?
28Where do anesthetics act in the CNS?
CNS
Neurons
Glia
Axons
Synapses
Channels
Receptors Pumps Second Messengers Intracellular
Enzymes ??
29Synaptic Effects of Anesthetics
Without anesthetic
With anesthetic
30Direct effects of anesthetics on ion channels
- Channel blocking effects of general anesthetics
on nicotinic AChR.
(Dilger et al 1994)
31Nicotine Reversal of Sedation?
- Implant cannula for microinjection of nicotine
into rat thalamus (?4?2 AChR) - After 1 week, induce loss of righting reflex with
sevoflurane - Inject nicotine look for signs of arousal
- Partial eye or other purposeful movements
- Complete turning over ambulation
(Alkire et al 2007)
32Effects of Nicotine
(Alkire et al 2007)
33What about nicotine antagonists?
- They do NOT induce LORR themselves
- Consider nicotinic activity in thalamus as
regulating an on switch but not an off switch
34An Anesthetic Binding Site on the GABAA Receptor?
Krasowski et al 1998
35Model of Binding Site
Mascia et al 2000
36Genetics
- Jurd et al 2003
- Knock-in mice b3(N265M) GABAA receptors
- Molecular biology
- Electrophysiology
- Behavioral
- Anesthetics etomidate, propofol, enflurane,
alphaxolone
37Jurd et al. - etomidate
38Jurd et al. enflurane
39Working Hypothesis for Anesthesia
Eger et al, 1997 Solt Forman, 2007
40Milestones in Anesthetic Mechanisms
- Meyer-Overton Correlation
- Anesthetics may interact directly with proteins
- Brain vs. spinal cord
- Role of GABAA receptor for propofol and etomidate
effects on awareness
41Summary?
- Progress in anesthesia research is severely
hindered by - our inadequate understanding of the brain
- the low affinity of anesthetics for their site of
action - the lack of a graded measure of depth of
anesthesia - the lack of an agent that reverses anesthesia
- many nonspecific effects
42Summary!
- Progress in anesthesia research is attainable
because of - the powerful tools of molecular biology,
electrophysiology and genetics - many people are studying many possible targets
for anesthetics
43Bibliography
- Alkire MT, McReynolds JR, Hahn EL, Trivedi AN.
Thalamic microinjection of nicotine reverses
sevoflurane-induced loss of righting reflex in
the rat. Anesthesiology 107 264-672, 2007 - Antognini JF, Schwartz K. Exaggerated anesthetic
requirements in the preferentially anesthetized
brain. Anesthesiology 79 1244-1249 , 1993 - Dilger JP, Vidal AM, Mody HI, Liu Y Evidence for
direct actions of general anesthetics on an ion
channel protein - a new look at a unified
mechanism of action. Anesthesiology 81
431-442, 1994 - Eger EI, Koblin DD, Harris RA, Kendig JJ,
Pohorille A, Halsey MJ, Trudell JR Hypothesis -
inhaled anesthetics produce immobility and
amnesia by different mechanisms at different
sites. Anesthesia Analgesia. 84915-918, 1997 - Franks NP, Lieb WR Molecular and cellular
mechanisms of general anaesthesia. Nature 367
607-614 , 1994 - Franks NP, Lieb WR Do general anaesthetics act by
competitive binding to specific receptors? Nature
310 599, 1984 - Jurd R, Arras M, Lambert S, Drexler B, Siegwart
R, Crestani F, Zaugg M, Vogt KE, Ledermann B,
Antkowiak B, Rudolph U. General anesthetic
actions in vivo strongly attenuated by a point
mutation in the GABAA receptor b3 subunit. FASEB
J 17, 250, 2003 - Koblin DD, Chortkoff BS, Laster MJ, Eger EI,
Halsey MJ, Ionescu P Polyhalogenated and
perfluorinated compounds that disobey the
Meyer-Overton hypothesis. Anesthesia and
Analgesia 79 1043-1048, 1994 - Krasowski MD, Koltchine VV, Rick CE, Ye Q, Finn
SE, Harrison NL. Propofol and other intravenous
anesthetics have sites of action on the
g-aminobutyric acid type A receptor distinct from
that for isoflurane. Molecular Pharmacology 53
530-538,1998 - Mascia MP, Trudell JR, Harris RA. Specific
binding sites for alcohols and anesthetics on
ligand-gated ion channels. Proc Natl Acad Sci U S
A. 979305-10, 2000 - Rampil IJ, Mason P, Singh H Anesthetic potency
(MAC) is independent of forebrain structures in
the rat. Anesthesiology 78 707-712 , 1993 - Sebel PS, et al. The incidence of awareness
during anesthesia A multicenter United States
study. Anesth Analg 99833-9, 2004 - Solt K, Forman SA. Correlating the clinical
actions and molecular mechanisms of general
anesthetics. Curr Opin Anaesth 20300-6, 2007 - Sonner JM, Antognini JF, Dutton RC, Flood P, Gray
AT, Harris RA, Homanics GE, Kendig J, Orser B,
Raines DE, Rampil IJ, Trudell J, Vissel B, Eger
EI. Inhaled anesthetics and immobility
mechanisms, mysteries, and minimum alveolar
anesthetic concentration. Anesth Analg.
97718-40, 2003