Anesthesia - Substrates and Molecular Theories (Lectures 8A & 8B)

Question 1

behavioural stage 1

Answer 1

Behavioural stage 1 - analgesia
From normal to unconscious
Patient is awake
Respiration, ocular movement, pupil size, and muscle tone and reflexes normal
Patient would respond to incision

Question 2

Behavioural stage 2

Answer 2

Behavioural stage 2 - excitement
Loss of consciousness to delirium (return or regular breathing)
Fighting and screaming
Respiration: jagged
Ocular movement: becoming more suppressed
Pupils: large
Muscle tone: Tenses struggle
Eyelid reflex: suppressed
Patient responds to incision
Not really conscious or unconscious → delirium

Question 3

Behavioural stage 3

Answer 3

Behavioural stage 3 - surgical anaesthesia

• Regular breathing to apnea
• Patient anaesthetised
• Pupils getting smaller from plane 1-2, larger from plane 3-4
• Muscle tone decreases
• Eye reflexes disappear gradually (corneal at plane 1, pupillary light at plane 3, no light reflex at plane 4)
• Response to incision decreases and disappears between planes 2 and 3
• Brainstem and spinal cord reflexes becoming suppressed
• Try to keep people in the upper planes of stage 3 so we can do surgery but not risk respiratory arrest - may add analgesic and muscle relaxant so we can use less anesthetic

Question 4

Behavioural stage 4

Answer 4

Behavioural stage 4 - imminent death
Apnea to stable respiratory arrest
Patient dying
Pupils large
Muscle tone and reflexes gone
No response to incision
Brain stem reflexes are fully suppressed

Question 5

EEG changes during anesthesia and their corresponding guedel’s behavioural stages

Answer 5

Level 1: low and fast beta desynchronized → wide awake (stage 1)
Level 2: rhythmic high waves, looks more like epileptic waves than sleep (stage 2)
Level 3: complex waves - high and irregular, looks more like sleep (highest cortical output - after this it goes down) (stage 3 higher planes)
Level 4-6: increasing burst suppression (later planes of stage 3)
Level 7: flat EEG (stage 4)

Question 6

What is an evoked potential - sensory

Answer 6

Sensory: change in activity seen in the somatosensory cortex measured using gross potential and EEG

Question 7

What is an evoked potential - intercranial

Answer 7

Intracranial: used in animals → put electrode in left cortex and stimulate it and see if there is activity in the right cortex → if the corpus callosum is cut there will be no activity

Question 8

What are sensory and association (non specific) cortices?

Answer 8

If we stimulate the cortex we see activity in other higher order brain areas

Question 9

the cortex in anesthesia

Answer 9

• Where consciousness is probably located → we become unconscious due a drop in activity here
• In anesthesia EPs are in specific sensory cortices but higher order EPs disappear in association cortices - the same is true in deep sleep
• Non-specific neocortex is still at low level of activation - cortical neurons are still active and they have input from the sensory cortices

Question 10

The reticular formation in anesthesia

Answer 10

• Probable that changes in the cortex reflect changes in the reticular core in the brain stem (not organized into ganglia, loose neurons)
• Reticular core supports forebrain function and is differentially suppressed by anesthetics

Question 11

Bremer’s theory of the involvement of the forebrain in sleep

Answer 11

• Bremer (1935): believed that it is sensory input that keeps the forebrain awake
• This was before reticular core theory of waking and sleeping
• Two preparations of cats

Cerveau isole: isolated the forebrain from the brainstem → cat went to sleep permanently → only have input from 2 cranial nerves

Encephale isole: isolated the brain from the spinal cord → sleeping and waking was normal in the cats → have input from many cranial nerves

Question 12

Cerveau isole cats

Answer 12

isolated the forebrain from the brainstem → cat went to sleep permanently → only have input from 2 cranial nerves

Question 13

Encephale isole cats

Answer 13

isolated the brain from the spinal cord → sleeping and waking was normal in the cats → have input from many cranial nerves

Question 14

Morruzi and Magoun (1949) classic ARAS theory

Answer 14

• believed it was arousal from the midbrain that keeps you awake not the sensory input
  believed that the core is a non specific system that works as a unit → we no longer believe this
• Receives collateral from all sensory systems (this part is true)
• Tonicallt actives cortex during waking
• Withdraws activation during sleep

Evidence
Lesions of the mesencephalic reticular formation → sleeping cat
Lesions of brainstem sensory tracts and not the reticular core → not a sleeping car

Question 15

French’s theory of ARAS in anesthesia

Answer 15

French (1953): - related ARAS theory to anesthesia
Idea: anesthetics differentially suppress the ARAS due to small neurons that are more vulnerable to suppression that larger ones
Evidence: in anesthesia cortical EPs remain and reticular EPs dissapear

Question 16

WD Winter’s classic theory of anesthesia

Answer 16

• Assumption: GABA neurons of the core are particularly responsive to anesthetics
• The reticular core is activated in stage 1 and 2 and depressed in stage 3 and 4, This means that the cortex is also excited in stage 1 and 2
• explains stage 2

Question 17

WD Winter’s theory applied to Guedel’s stages of anesthesia and EEG patterns

Answer 17

Stage 1: core is somewhat activated
- GABA neurons are inhibited
- Cortical EEG is low, fast and desynchronized

Stage 2: core is highly activated
- The little GABA neurons of the core suppressed
- Cortical EEG synchronised with spikes or waves
- Stage induced by dissociative anesthetics
- Some can progress into epileptic attack → stage 2 is a drug induced epileptic attack

Stage 3: core is getting suppressed
- Anesthetic is affecting larger excitatory neurons
- EEG is in delta - increasing burst suppression

Stage 4: core complete suppressed
- Breathing stops, flat EEG

Question 18

what stages of anesthesia does ether progress to?

Answer 18

Hypnotic
Stages 1-4 → slow onset so we see all the stages

Question 19

what stages of anesthesia does NO2 and ketamine progress to?

Answer 19

Dissociative drugs
Stages 1 and 2 → keep you in stage 2

Question 20

what stages of anesthesia does PCP and chlorolose progress to?

Answer 20

Dissociative drugs
Stages 1 and 2 and induce behavioural seizures

Question 21

what stages of anesthesia does halothane (sevoflurane, isoflurane) and barbiturates progress to?

Answer 21

Rapid hypnotics
Stages 1,3,4

Question 22

lipid theories of anesthesia

Answer 22

Meyer-overton (1900s): anesthetic potency correlates with lipid solubility → thought that anesthetics are solvents that happen to affect the brain → anything with a high Po/w will be an anesthetic (we didn’t know of membranes and channels yet)

Ferguson (1930s): believed it was fluidization of the membrane and that’s how they changed the shape of the protein (thermodynamic activity)

Mullins (1950s): it is the volume occupied that’s important not the solubility → channels

Question 23

what was the lipid theory used to explain?

Answer 23

Used to explain action of general anesthetics, local anesthetics, anticonvulsants, minor and major tranquilizers (anxiolytics and antipsychotics, hypnotics

Used to explain sedative effects of drugs → anything with a high coefficient could affect membranes at high doses and cause drowsiness and dizziness (too much of a lot of CNS drugs have hypnotic effects)

Binding assays showed that all these drugs have specific sites on proteins except for general anesthetics

Question 24

did the lipid theory address the mysteries of anesthesia

Answer 24

Consistent with:
High concentrations necessary → enough molecules have to be dissolved into the membrane so it expands
Lack of specificity → not interacting with proteins just joining the bilayer
Partition coefficient and anesthesia correlation → molecules with high coefficient join the bilayer more readily

Question 25

membrane expansion theory and evidence

Answer 25

Anesthetic dissolves into plasma membrane and associate into lipophilic areas → expand the membrane by 1% → squeezes and distorts the voltage dependent sodium channels → depresses their activity → no AP → neurons can’t fire

Evidence
All anesthetics expand plasma membranes by 1%
So why no effect on heart muscles → brain is more sensitive
Pressure reversal reverses general anesthesia because it counters expansion
Pressure forces the expanded membrane back into original state → shown with compound AP studies and animal studies
Small diameter axons are more sensitive → this is why the brain is more sensitive than peripheral nerves (general vs local anesthesia)

Question 26

did the membrane expansion theory address the mysteries of anesthesia

Answer 26

Consistent with:
High concentrations necessary → enough molecules have to be be able to dissolve into the membrane to expand it
Lack of specificity → not interacting with proteins just joining the bilayer
Partition coefficient and anesthesia correlation → molecules with high coefficient join the bilayer more readily
Pressure reversal

Question 27

LaBella protein theory and attack of lipid theories

Answer 27

Attack on lipid theories
Stereoisomers in anesthesia that does not fit lipid theory
Cut off phenomenon with partition coefficient → you increase it until a certain point where the drug doesn’t get stronger and its efficacy can actually decline because it can’t get through hard walls of fat
Association of partition coefficient fits more within anesthetic classes than between
Pressure reversal may affect proteins directly not indirectly

His theory was that anesthetics bind to only opioid receptors (that’s where the binding assays were discovered)

Question 28

Franks and Lieb 1980s
- their issues with the lipid theories
- luciferase assay

Answer 28

Issues with lipid theories
- Phospholipid bilayer isn’t perturbed by therapeutic concentrations of anesthetics → the amount of fluidization is the same as in a fever

Luciferase assay - protein whose function is correlated with strength of anesthetic
- Luciferase excites luciferin which omits light → anesthetics inhibit this reaction
- Anesthetic potency in tube parallels clinical potency → needed more of clinically weak anesthetic to inhibit light
- Anesthetics compete with luciferin for the active site on luciferase → has binding pocket that can hold one large or two small anesthetic molecules → competition could be allosteric or direct

Question 29

Protein theories in explaining anesthetic mysteries

Answer 29

1. High concentrations necessary → not well explained (gaseous anesthetics work in millimolars > micromolar > nanomolar while barbiturates can cause anesthesia in nanomolar (binding assays also work best in nanomolar))
2. Lack of specificity → not well explained, drugs with different structures can bind to the same receptors, general anesthetics don’t have a receptor
3. Partition coefficient and anesthesia correlation → well explained
4. Pressure reversal → can’t be described by protein effects