General Anesthesia:General Anesthesia:A more complex mechanismA more complex mechanism

The Meyer-Overton correlation and new research into The Meyer-Overton correlation and new research into the mechanism of action of general anesthesia.the mechanism of action of general anesthesia.

Purposes of General Purposes of General Anesthesia:Anesthesia:

(Inhaled and Intravenous)(Inhaled and Intravenous) AmnesiaAmnesia AnalgesiaAnalgesia Immobility (muscle relaxation)Immobility (muscle relaxation) Loss of consciousnessLoss of consciousness HypnosisHypnosis Suppression of noxious reflexesSuppression of noxious reflexes

Pharmacological Manipulation Pharmacological Manipulation of the Neuronal Nexusof the Neuronal Nexus

Various areas of CNS mediate desired effectsVarious areas of CNS mediate desired effects UnconsciousnessUnconsciousness

Common mechanism with aspects of Common mechanism with aspects of consciousnessconsciousness

Cerebral cortex, thalamus, and reticular Cerebral cortex, thalamus, and reticular formationformation High density of High density of γγ-aminobutyric acid -aminobutyric acid

(GABA-A), N-methyl-D-aspartate (NMDA) (GABA-A), N-methyl-D-aspartate (NMDA) and acetylcholine (Ach) receptorsand acetylcholine (Ach) receptors

Subject to input from subcortical arousal Subject to input from subcortical arousal systemssystems

AmnesiaAmnesia Hippocampus, amygdala and prefrontal Hippocampus, amygdala and prefrontal

cortexcortex Implicit memory: recalled unconsciously Implicit memory: recalled unconsciously

(target of anesthesia)(target of anesthesia) Explicit memory: recalled consciouslyExplicit memory: recalled consciously

Use NMDA and non-NMDA receptorsUse NMDA and non-NMDA receptors Respond to NT glutamate and Respond to NT glutamate and

serotonergic interneuronsserotonergic interneurons

ImmobilityImmobility Sensory and motor neuronsSensory and motor neurons

GABA-A receptorGABA-A receptor Glutamate receptors for NMDA, alpha-Glutamate receptors for NMDA, alpha-

amino-5-methyl-3-hydroxy-4-isoxazole amino-5-methyl-3-hydroxy-4-isoxazole propionic acid (AMPA) and kainitepropionic acid (AMPA) and kainite

AnalgesiaAnalgesia NocioceptionNocioception

Blocking occurs at glutamate, GABA-A Blocking occurs at glutamate, GABA-A or (micro) receptors in spinal cordor (micro) receptors in spinal cord

Meyer-Overton CorrelationMeyer-Overton Correlation Has been used to describe the mechanism Has been used to describe the mechanism

of volatile anestheticsof volatile anesthetics Linear relationship between potency and Linear relationship between potency and

lipid solubilitylipid solubility No longer accepted universallyNo longer accepted universally Does appear in different levels of CNS Does appear in different levels of CNS

integrationintegration Molecular, subcellular and cellular Molecular, subcellular and cellular

mainlymainly

Current Views of Anesthetic Current Views of Anesthetic MechanismMechanism

Solubilization within the neuronal Solubilization within the neuronal membranemembrane Redistribution of lateral pressuresRedistribution of lateral pressures Alters conformation of membrane Alters conformation of membrane

proteins (i.e. Naproteins (i.e. Na+ + pump)pump) Anesthetics interact with many hydrophobic Anesthetics interact with many hydrophobic

sitessites Protein structures that form ion channelsProtein structures that form ion channels

Inhaled anesthetics act at lipid bilayer-Inhaled anesthetics act at lipid bilayer-protein interfaceprotein interface

Weak electrostatic forces between Weak electrostatic forces between membrane protein and anestheticmembrane protein and anesthetic

Stimulation of KStimulation of K++ leak channels (neuronal leak channels (neuronal hyperpolarization)hyperpolarization)

CaCa+2+2 sensitivity to general anesthesia sensitivity to general anesthesia

Presynaptic InhibitionPresynaptic Inhibition Three mechanisms of presynaptic inhibitionThree mechanisms of presynaptic inhibition

Mediating neuron causes CaMediating neuron causes Ca+2+2 channels channels of presynaptic neuron to close (< release of presynaptic neuron to close (< release of NT)of NT)

Ligand-gated receptors inhibit NT releaseLigand-gated receptors inhibit NT release CaCa+2 +2 independent (botulinum/tetanus)independent (botulinum/tetanus)

Activate GABA-A gated ClActivate GABA-A gated Cl-- channels channels Also evidence that background KAlso evidence that background K++ current current

(upon anesthetic induction) hyperpolarizes (upon anesthetic induction) hyperpolarizes both pre/postsynaptic neuronsboth pre/postsynaptic neurons

Postsynaptic InhibitionPostsynaptic Inhibition

Mediating neuron hyperpolarizes another Mediating neuron hyperpolarizes another neuronneuron

Agonist binds to postsynaptic GABA-A Agonist binds to postsynaptic GABA-A receptorreceptor

Inhibitory PathwaysInhibitory Pathways GABAGABA

Key inhibitory NT within the brainKey inhibitory NT within the brain Two types (A and B)Two types (A and B) GABA-A receptors increase ClGABA-A receptors increase Cl--

conductance (postsynaptic)conductance (postsynaptic) Analogous ligands (agonists) aside from Analogous ligands (agonists) aside from

GABA interact with GABA receptorsGABA interact with GABA receptors Benzodiazepines, barbiturates, Benzodiazepines, barbiturates,

anesthetic steriods, volatile anesthetic steriods, volatile anesthetics and ethanolanesthetics and ethanol

GABA-A/B/CGABA-A/B/C GABA-A: individual expression of the GABA-A: individual expression of the

GABA-A receptor subunit composition and GABA-A receptor subunit composition and subunit isoforms can modify response to subunit isoforms can modify response to anestheticanesthetic

GABA-B: linked via G proteins to KGABA-B: linked via G proteins to K++ channelschannels Activated—GABA-B receptors decrease Activated—GABA-B receptors decrease

CaCa+2+2 conductance and inhibit cAMP conductance and inhibit cAMP productionproduction

No KNOWN association with anesthesiaNo KNOWN association with anesthesia GABA-C: also ligand-gated ClGABA-C: also ligand-gated Cl-- channels channels

GABA-A ReceptorGABA-A Receptor

GABA-A receptors contain various GABA-A receptors contain various subunits within the predominate subunits within the predominate structurestructure 1-6 1-6 αα, 1-4 , 1-4 ββ, 1-4 , 1-4 γγ, , δδ, , εε, 1-2 , 1-2 ρρ

70-70 kDa glycoprotein70-70 kDa glycoprotein Contains 12 hydrophobic membrane-Contains 12 hydrophobic membrane-

spanning domainsspanning domains Two other GABA receptors (B and C)Two other GABA receptors (B and C)

GABA-A GABA-A ReceptorReceptor

GABAGABA

GABA-A ReceptorGABA-A Receptor

ClCl--

AxoplasmAxoplasm

Voet and Voet 2nd Edition

GABA-A InhibitionGABA-A Inhibition

Increase in Cl ion conductance after Increase in Cl ion conductance after activation of GABA-A receptors by anesthesiaactivation of GABA-A receptors by anesthesia

Causes localized hyperpolarization of the Causes localized hyperpolarization of the neuronal membraneneuronal membrane

Increased threshold to depolarize (to form Increased threshold to depolarize (to form AP)AP)

Increased conductance is due to an increase Increased conductance is due to an increase in the mean open time of the Cl ion channelin the mean open time of the Cl ion channel

Formation of GABAFormation of GABA Initial step utilizes Initial step utilizes αα-ketoglutarate (Krebs)-ketoglutarate (Krebs) Transamination of Transamination of αα-ketoglutarate to form -ketoglutarate to form αα--

oxoglutarate transaminase (GABA-T or glutamate)oxoglutarate transaminase (GABA-T or glutamate) Glutamate is decarboxylated to form GABA by Glutamate is decarboxylated to form GABA by

glutamate decarboxylase (GAD)glutamate decarboxylase (GAD)

Degradation of GABADegradation of GABA

Metabolized by GABA-T to form succinic Metabolized by GABA-T to form succinic semialdehyde semialdehyde

Glutamate is regenerated if in the Glutamate is regenerated if in the presence of presence of αα-ketoglutarate-ketoglutarate

If not, succinic semialdehyde is oxidized If not, succinic semialdehyde is oxidized by SSADH then succinic acid returns to by SSADH then succinic acid returns to Krebs cycle Krebs cycle

Off Topic Off Topic GAD is also present in GAD is also present in ββ cells of pancreatic cells of pancreatic

isletsislets GAD plays role in pancreatic endocrine GAD plays role in pancreatic endocrine

functionfunction Insulin and GAD coexist in the Insulin and GAD coexist in the ββ cells cells Antibodies of the 64-kDa (GAD) occur in Antibodies of the 64-kDa (GAD) occur in

almost all patients with insulin-dependent almost all patients with insulin-dependent diabetesdiabetes

Presence of GAD antibodies appear to Presence of GAD antibodies appear to precede the clinical onset of the diseaseprecede the clinical onset of the disease

GAD and development of Type-1 diabetes???GAD and development of Type-1 diabetes???

Glycine ReceptorGlycine Receptor

Ogliomeric transmembrane protein Ogliomeric transmembrane protein composed of 3 composed of 3 αα and 2 and 2 ββ subunits subunits

Agonists: Agonists: ββ-alanine and taurine as well as -alanine and taurine as well as ββ-aminobutyric acid, ethanol and -aminobutyric acid, ethanol and anesthetics as well as strychnineanesthetics as well as strychnine

Isofluorane and propofol are also allosteric Isofluorane and propofol are also allosteric effectorseffectors

Similar in structure to GABA-A receptorSimilar in structure to GABA-A receptor GLYT-1 and GLYT-2GLYT-1 and GLYT-2

Receptor consists of two polypeptide subunitsReceptor consists of two polypeptide subunits 48 kDa (48 kDa (αα) and 58 kD () and 58 kD (ββ)) Glycine binding site is located on Glycine binding site is located on αα

Each subunit has 4 hydrophobic membrane-Each subunit has 4 hydrophobic membrane-spanning sequencesspanning sequences

Garrett and Grisham 3rd Edition

Glycine Glycine αα-1 Transmembrane -1 Transmembrane DomainDomain

Protein Data Bank

Glycine ReceptorGlycine Receptor

GarGarrett and Grisham 3rd Edition

KK++ Background (Leak) Channel Background (Leak) Channel ExcitationExcitation

Leak channels influence both resting Leak channels influence both resting membrane potential and repolarizationmembrane potential and repolarization

These channels are opened by volatile These channels are opened by volatile anesthetics anesthetics Hyperpolarization of the membraneHyperpolarization of the membrane Suppresses action potential generationSuppresses action potential generation

Partially responsible for suppressing the Partially responsible for suppressing the hypoxic drive during general anesthesiahypoxic drive during general anesthesia

Hypoxic DriveHypoxic Drive Lung damageLung damage

Alveolar ventilation is inadequateAlveolar ventilation is inadequate Abnormal arterial blood gases. Abnormal arterial blood gases. Chemoreceptors become tolerant of a Chemoreceptors become tolerant of a

high pp of COhigh pp of CO2 2 ; kidneys compensate for ; kidneys compensate for the respiratory acidosis by retaining the respiratory acidosis by retaining bicarbonate (HCObicarbonate (HCO33 ) ) Keeps arterial pH normal Keeps arterial pH normal

If Too much oxygen respiratory drive will If Too much oxygen respiratory drive will be lost be lost Not breathe adequately, Not breathe adequately, Pp of COPp of CO22 in arterial blood will rise in arterial blood will rise

(loss of consciousness) (loss of consciousness)

Disruption of Ligand Diffusion Disruption of Ligand Diffusion ChreodesChreodes

A proposed mechanism of action for inhaled A proposed mechanism of action for inhaled anestheticsanesthetics

Diffusion ChreodesDiffusion ChreodesWhat the %$#* are Chreodes you ask?What the %$#* are Chreodes you ask?

Protein cavities are targeted by anesthetic Protein cavities are targeted by anesthetic moleculesmolecules

This disrupts the normal function of the proteinThis disrupts the normal function of the protein Amino acids outside the active site act as Amino acids outside the active site act as

“promoters” “promoters” These chreodes created in the landscape of the These chreodes created in the landscape of the

receptor are invoked to account for a type of receptor are invoked to account for a type of facillitated diffusion of a ligand to that receptorfacillitated diffusion of a ligand to that receptor

Exit of ligand from active site may be mediated Exit of ligand from active site may be mediated by another set of chreodesby another set of chreodes

ChreodesChreodes

It is believed that the viscosity of water near the It is believed that the viscosity of water near the protein surface is higher (due to the intermolecular protein surface is higher (due to the intermolecular forces between the amino acid side chains and the forces between the amino acid side chains and the water molecules) than the “bulk” waterwater molecules) than the “bulk” water

This ordering of “layers” of water could facilitate This ordering of “layers” of water could facilitate faster diffusion of the solute (ligand) near the faster diffusion of the solute (ligand) near the protein surfaceprotein surface

These paths for the ligand are always changing until These paths for the ligand are always changing until (over time) they continue to return to an ordering (over time) they continue to return to an ordering that promotes fastest diffusion and stabilitythat promotes fastest diffusion and stability

A molecule could potentially disrupt the ordering of A molecule could potentially disrupt the ordering of water and amino acid side chains disrupting the water and amino acid side chains disrupting the chreodeschreodes

And Finally (I know you’re And Finally (I know you’re happy) Chreodes and happy) Chreodes and

AnesthesiaAnesthesia Inhalational anesthetics (IA) are approximately Inhalational anesthetics (IA) are approximately

equal in size to the AA side chainsequal in size to the AA side chains IA have lipophilicities very close to those of IA have lipophilicities very close to those of

lipophilic side chainslipophilic side chains The presence of IA in or near a chreode could The presence of IA in or near a chreode could

alter the unique path adopted by the receptor, alter the unique path adopted by the receptor, disrupting the normal diffusion of the ligand to disrupting the normal diffusion of the ligand to the receptorthe receptor

Partition Coefficients of AA Side Partition Coefficients of AA Side Chains and Volatile Anesthetic Chains and Volatile Anesthetic

DrugsDrugs Tryptophan 2.25; Tryptophan 2.25; Sevoflurane 2.34Sevoflurane 2.34 Isoleucine 1.80; Phenylalanine 1.8; Isoleucine 1.80; Phenylalanine 1.8; Desflurane Desflurane

1.801.80 Leucine 1.70; Leucine 1.70; Halothane 1.70Halothane 1.70 Tyrosine 0.96; Tyrosine 0.96; Ether 0.89Ether 0.89