General Anesthesia: A more complex mechanism

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General AnesthesiaA more complex mechanism
The Meyer-Overton correlation and new research
into the mechanism of action of general
anesthesia.
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Purposes of General Anesthesia(Inhaled and
Intravenous)

• Amnesia
• Analgesia
• Immobility (muscle relaxation)
• Loss of consciousness
• Hypnosis
• Suppression of noxious reflexes

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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 ?-aminobutyric acid (GABA-A),
  N-methyl-D-aspartate (NMDA) and acetylcholine
  (Ach) receptors
• Subject to input from subcortical arousal systems

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• 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

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• 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

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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

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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

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• Inhaled anesthetics act at lipid bilayer-protein
  interface
• Weak electrostatic forces between membrane
  protein and anesthetic
• Stimulation of K leak channels (neuronal
  hyperpolarization)
• Ca2 sensitivity to general anesthesia

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Presynaptic Inhibition

• Three mechanisms of presynaptic inhibition
• Mediating neuron causes Ca2 channels of
  presynaptic neuron to close (lt release of NT)
• Ligand-gated receptors inhibit NT release
• Ca2 independent (botulinum/tetanus)
• Activate GABA-A gated Cl- channels
• Also evidence that background K current (upon
  anesthetic induction) hyperpolarizes both
  pre/postsynaptic neurons

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Postsynaptic Inhibition

• Mediating neuron hyperpolarizes another neuron
• Agonist binds to postsynaptic GABA-A receptor

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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

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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
• ActivatedGABA-B receptors decrease Ca2
  conductance and inhibit cAMP production
• No KNOWN association with anesthesia
• GABA-C also ligand-gated Cl- channels

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GABA-A Receptor

• GABA-A receptors contain various subunits within
  the predominate structure
• 1-6 a, 1-4 ß, 1-4 ?, d, e, 1-2 ?
• 70-70 kDa glycoprotein
• Contains 12 hydrophobic membrane-spanning domains
• Two other GABA receptors (B and C)

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GABA-A Receptor
Voet and Voet 2nd Edition
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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

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Formation of GABA

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

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Degradation of GABA

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

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Off Topic ?

• GAD is also present in ß cells of pancreatic
  islets
• GAD plays role in pancreatic endocrine function
• Insulin and GAD coexist in the ß 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???

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Glycine Receptor

• Ogliomeric transmembrane protein composed of 3 a
  and 2 ß subunits
• Agonists ß-alanine and taurine as well as
  ß-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

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• Receptor consists of two polypeptide subunits
• 48 kDa (a) and 58 kD (ß)
• Glycine binding site is located on a
• Each subunit has 4 hydrophobic membrane-spanning
  sequences

Garrett and Grisham 3rd Edition
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Glycine a-1 Transmembrane Domain
Protein Data Bank
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Glycine Receptor
Gar
Garrett and Grisham 3rd Edition
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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

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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)

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Disruption of Ligand Diffusion Chreodes

• A proposed mechanism of action for inhaled
  anesthetics

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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

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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

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And Finally (I know youre 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

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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