2019B04 Outline the theories, both current and discredited, as to how volatile anaesthetics
cause loss of consciousness.

 

List:

·     Consciousness

·     Lipid hypothesis

·     Protein hypothesis

·     Other hypotheses

 

Consciousness:

Definition

·      Awareness of one’s physical state, motivational state, emotions and thoughts

Mechanism

·      Poorly understood

·      Dependent upon precisely organized interactions between

o Ascending reticular activating system (ARAS) in brainstem

o Thalamus

o Cerebral cortex

·      Disruption of these interactions causes unconsciousness

Key mediators

·      Glutamate

·      Noradrenaline

·      Acetylcholine

 

Lipid hypothesis (main discredited theory):

Theory

·      Accumulation of volatile agent in CNS bilayer causes distortion of membrane function

o   Critical volume hypothesis

o   Lateral phase separation hypothesis

Rationale

·      Myer-Overton correlation: between anaesthetic potency and solubility in olive oil

Problem

·      Imperfect correlation

·      Exceptions: existence of inert anaesthetic-like compounds

·      Carbon chain length cut-off: molecules beyond a certain length lose anaesthetic effect

·      Temperature: minor changes cause membrane distortion comparable to anaesthetics

·      Structural isomeric differences: e.g. isoflurane MAC 1.2%, enflurane MAC 1.7%

·      Stereoisomeric differences: e.g. R-etomidate 10x more potent than S-etomidate

 

Protein hypothesis (currently accepted)

Theory

·      Interaction with hydrophobic areas of key membrane proteins including ion channels

Rationale

·      Correlation between anaesthetic potency and solubility in amphipathic substances
(closer correlation than with lipid solubility)

·      Correlation between anaesthetic potency and inhibition of firefly luciferase
(a protein-only structure)

Subsequent validation

·      Effect of volatile agents at ion channels

o ↑Activity of GABA, glycine, 2PK, 5-HT

o ↓Activity at nnAChR, NMDA, AMPA

 

Other hypotheses:

Sleep

·      Hypothesis: volatiles agents activate sleep circuitry

·      Rationale: EEG signature of volatile agents is similar to slow wave sleep

·      Flaw: volatile agents induce unrousable unconsciousness and immobility

Cellular targets

·      Hypothesis: volatile agents impair neuronal function specifically

·      Rationale: relative preservation of other body functions

·      Flaw: proven significant effects on many tissues, e.g. skeletal muscle relaxation

Subcellular targets

·      E.g. ↑↓ function of second messengers

 

 

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