General Anesthesia: A more complex mechanism

1. General Anesthesia:A more complex mechanism The Meyer-Overton correlation and new research into the mechanism of action of general anesthesia.

2. Purposes of General Anesthesia:(Inhaled and Intravenous) • Amnesia • Analgesia • Immobility (muscle relaxation) • Loss of consciousness • Hypnosis • Suppression of noxious reflexes

3. Pharmacological Manipulation of the Neuronal Nexus • Various areas of CNS mediate desired effects • Unconsciousness • Common mechanism with aspects of consciousness • Cerebral cortex, thalamus, and reticular formation • High density of γ-aminobutyric acid (GABA-A), N-methyl-D-aspartate (NMDA) and acetylcholine (Ach) receptors • Subject to input from subcortical arousal systems

4. Amnesia • Hippocampus, amygdala and prefrontal cortex • Implicit memory: recalled unconsciously (target of anesthesia) • Explicit memory: recalled consciously • Use NMDA and non-NMDA receptors • Respond to NT glutamate and serotonergic interneurons

5. Immobility • Sensory and motor neurons • GABA-A receptor • Glutamate receptors for NMDA, alpha-amino-5-methyl-3-hydroxy-4-isoxazole propionic acid (AMPA) and kainite • Analgesia • Nocioception • Blocking occurs at glutamate, GABA-A or (micro) receptors in spinal cord

6. Meyer-Overton Correlation • Has been used to describe the mechanism of volatile anesthetics • Linear relationship between potency and lipid solubility • No longer accepted universally • Does appear in different levels of CNS integration • Molecular, subcellular and cellular mainly

7. Current Views of Anesthetic Mechanism • Solubilization within the neuronal membrane • Redistribution of lateral pressures • Alters conformation of membrane proteins (i.e. Na+ pump) • Anesthetics interact with many hydrophobic sites • Protein structures that form ion channels

8. Inhaled anesthetics act at lipid bilayer-protein interface • Weak electrostatic forces between membrane protein and anesthetic • Stimulation of K+ leak channels (neuronal hyperpolarization) • Ca+2 sensitivity to general anesthesia

9. Presynaptic Inhibition • Three mechanisms of presynaptic inhibition • Mediating neuron causes Ca+2 channels of presynaptic neuron to close (< release of NT) • Ligand-gated receptors inhibit NT release • Ca+2 independent (botulinum/tetanus) • Activate GABA-A gated Cl- channels • Also evidence that background K+ current (upon anesthetic induction) hyperpolarizes both pre/postsynaptic neurons

10. Postsynaptic Inhibition • Mediating neuron hyperpolarizes another neuron • Agonist binds to postsynaptic GABA-A receptor

11. Inhibitory Pathways • GABA • Key inhibitory NT within the brain • Two types (A and B) • GABA-A receptors increase Cl- conductance (postsynaptic) • Analogous ligands (agonists) aside from GABA interact with GABA receptors • Benzodiazepines, barbiturates, anesthetic steriods, volatile anesthetics and ethanol

12. GABA-A/B/C • GABA-A: individual expression of the GABA-A receptor subunit composition and subunit isoforms can modify response to anesthetic • GABA-B: linked via G proteins to K+ channels • Activated—GABA-B receptors decrease Ca+2 conductance and inhibit cAMP production • No KNOWN association with anesthesia • GABA-C: also ligand-gated Cl- channels

13. GABA-A Receptor • GABA-A receptors contain various subunits within the predominate structure • 1-6 α, 1-4 β, 1-4 γ, δ, ε, 1-2 ρ • 70-70 kDa glycoprotein • Contains 12 hydrophobic membrane-spanning domains • Two other GABA receptors (B and C)

14. GABA Cl- Axoplasm GABA-A Receptor Voet and Voet 2nd Edition GABA-A Receptor

15. GABA-A Inhibition • Increase in Cl ion conductance after activation of GABA-A receptors by anesthesia • Causes localized hyperpolarization of the neuronal membrane • Increased threshold to depolarize (to form AP) • Increased conductance is due to an increase in the mean open time of the Cl ion channel

16. Formation of GABA • Initial step utilizes α-ketoglutarate (Krebs) • Transamination of α-ketoglutarate to form α-oxoglutarate transaminase (GABA-T or glutamate) • Glutamate is decarboxylated to form GABA by glutamate decarboxylase (GAD)

17. Degradation of GABA • Metabolized by GABA-T to form succinic semialdehyde • Glutamate is regenerated if in the presence of α-ketoglutarate • If not, succinic semialdehyde is oxidized by SSADH then succinic acid returns to Krebs cycle

18. Off Topic  • GAD is also present in β cells of pancreatic islets • GAD plays role in pancreatic endocrine function • Insulin and GAD coexist in the β cells • Antibodies of the 64-kDa (GAD) occur in almost all patients with insulin-dependent diabetes • Presence of GAD antibodies appear to precede the clinical onset of the disease • GAD and development of Type-1 diabetes???

19. Glycine Receptor • Ogliomeric transmembrane protein composed of 3 α and 2 β subunits • Agonists: β-alanine and taurine as well as β-aminobutyric acid, ethanol and anesthetics as well as strychnine • Isofluorane and propofol are also allosteric effectors • Similar in structure to GABA-A receptor • GLYT-1 and GLYT-2

20. Receptor consists of two polypeptide subunits • 48 kDa (α) and 58 kD (β) • Glycine binding site is located on α • Each subunit has 4 hydrophobic membrane-spanning sequences Garrett and Grisham 3rd Edition

21. Glycine α-1 Transmembrane Domain Protein Data Bank

22. Glycine Receptor Gar Garrett and Grisham 3rd Edition

23. K+ Background (Leak) Channel Excitation • Leak channels influence both resting membrane potential and repolarization • These channels are opened by volatile anesthetics • Hyperpolarization of the membrane • Suppresses action potential generation • Partially responsible for suppressing the hypoxic drive during general anesthesia

24. Hypoxic Drive • Lung damage • Alveolar ventilation is inadequate • Abnormal arterial blood gases. • Chemoreceptors become tolerant of a high pp of CO2 ; kidneys compensate for the respiratory acidosis by retaining bicarbonate (HCO3 ) • Keeps arterial pH normal • If Too much oxygen respiratory drive will be lost • Not breathe adequately, • Pp of CO2 in arterial blood will rise (loss of consciousness)

25. Disruption of Ligand Diffusion Chreodes A proposed mechanism of action for inhaled anesthetics

26. Diffusion ChreodesWhat the %$#* are Chreodes you ask? • Protein cavities are targeted by anesthetic molecules • This disrupts the normal function of the protein • Amino acids outside the active site act as “promoters” • These chreodes created in the landscape of the receptor are invoked to account for a type of facillitated diffusion of a ligand to that receptor • Exit of ligand from active site may be mediated by another set of chreodes

27. Chreodes • It is believed that the viscosity of water near the protein surface is higher (due to the intermolecular forces between the amino acid side chains and the water molecules) than the “bulk” water • This ordering of “layers” of water could facilitate faster diffusion of the solute (ligand) near the protein surface • These paths for the ligand are always changing until (over time) they continue to return to an ordering that promotes fastest diffusion and stability • A molecule could potentially disrupt the ordering of water and amino acid side chains disrupting the chreodes

28. And Finally (I know you’re happy) Chreodes and Anesthesia • Inhalational anesthetics (IA) are approximately equal in size to the AA side chains • IA have lipophilicities very close to those of lipophilic side chains • The presence of IA in or near a chreode could alter the unique path adopted by the receptor, disrupting the normal diffusion of the ligand to the receptor

29. Partition Coefficients of AA Side Chains and Volatile Anesthetic Drugs • Tryptophan 2.25; Sevoflurane 2.34 • Isoleucine 1.80; Phenylalanine 1.8; Desflurane 1.80 • Leucine 1.70; Halothane 1.70 • Tyrosine 0.96; Ether 0.89