2. Outline
• Why we need to understand mechanisms
• Key principles in neuronal development and neurotoxicity
• What neurotoxic effects have been described
• Possible mechanisms
4. Bradford Hill’s
guidelines for causation
• Strength of association
• Consistency
• Specificity
• Temporality
• Biologic gradient
• Plausibility
• Coherence
• Experiment
• Analogy
5. Bradford Hill’s
guidelines for causation
• Strength of association
• Consistency
• Specificity
• Temporality
• Biologic gradient
• Plausibility
• Coherence
• Experiment
• Analogy
6. Anaesthetic exposure Clinical effect
Clinical studies
Biological variability in population studies
Effects are usually small with considerable degrees of uncertainty
?
9. Anaesthetic exposure Clinical effect
?
Clinical studies
Different population
Clinical studies constrained by defining the population
10. Bradford Hill’s
guidelines for causation
• Strength of association
• Consistency
• Specificity
• Temporality
• Biologic gradient
• Plausibility
• Coherence
• Experiment
• Analogy
11. Anaesthetic exposure Clinical effect
A
B
C
D
E
Mechanisms
Tighter control of experimental conditions –
greater certainty in each prediction
Deductive logic to predict the phenomenon
21. Normal development
• Stages of normal development
• Neurogenesis & proliferation
• Migration
• Differentiation
• Synapse formation
• Myelination
• Receptor function changes during development
• Both number of neurons and number of synapse halves during
development
• Neural density greatest in foetus; trimmed in neonatal period and infancy
• Number of synapses greatest in infancy; trimmed during later childhood
22. Normal development
• GABA & NMDA receptors directly involved in
• Cell proliferation
• Migration
• Cell survival
• Dendritic maturation
• GABA & NMDA receptors indirectly involved in
balance of activity and hence generation of trophic
factors, differentiation and growth
25. Apoptosis
• Ketamine, isoflurane, midazolam, propofol, sevoflurane
• Dose effect
• Combination worse
• Window of vulnerability day 7 in a rat
• Also seen in monkey, guinea pig, mouse, lamb and pig
• Some evidence for long term neurobehavioural effect
26. • 7-day old rat
Ikonomidou et al. Blockade of NMDA Receptors and Apoptotic
Neurodegeneration in the Developing Brain. Science 1999; 283, 5398
Same effect with
Ketamine (20 mg/kg x7)
Saline Treatment MK-801 (0.5 mg/kg)
30. Neurogenesis
• Isoflurane and sevoflurane
• Hippocampal neural precursor
cells
• No death of neural progenitors
• Decrease neuronal
proliferation
31. Dendritic architecture
• First two weeks: decrease in synaptic and dendritic
spine density
• Older: increase in number of dendrites
32. • Day 15 rat pups
• 5hrs anaesthesia: propofol,
ketamine, midazolam
• Increased dendritic spine density
Control KetaminePropofol
33. • Day 16 rat pups
• Isoflurane, desflurane,
sevoflurane
• 30, 60, 120 minutes
• No cell death
• Increased spine density
Control 120 min60 min30 min
40. • GABA activation leads to neuronal quiescence and
because neuronal development is activity dependent,
this leads to cell death
• Upregulation of NMDA receptors during blockade leads
to subsequent excitotoxic cell death
41. Up regulation of NMDA
• Up regulation during blockade, exicitotoxic upon withdrawal
• BUT
• Apoptosis occurs during delivery
• Occurs with agents that have no NMDA activity – e.g. propofol
• Some NMDA antagonists are protective – e.g. xenon
42. • S ketamine and racemic
ketamine have same toxicity
for same dose on mg/kg
• BUT, High concentration
with direct application to
tumour cells, not neurons
43. Use it or lose it
• During development we have an oversupply of neurons and
synapses
• More than half degenerate via apoptosis
• Inactivity triggers cell death via a loss of trophic factors
• Synapse and dendritic development are also activity dependent
47. Abnormal neuroinhibition
• All volatile agents probably have similar effects at equivalent
MAC; implying a pharmacodynamic effect
• BUT,
• GABA is excitatory in developing neurons
• Xenon and dexmedetomidine cause inhibition but no apoptosis
• GABA antagonists do not reverse the toxic effect
• Tetrodotoxin does not change dendritic morphology
51. Abnormal neuroinhibition
• Cannot be directly related to GABA or NMDA
• May be related to the final “balance of activity”
• But cannot explain all observations
• Perhaps the uncertainty in neurotoxic mechanism is
not unexpected as we don’t really understand the
mechanism for anaesthetic action!
• And general anaesthetics are “dirty” drugs that act on
multiple receptors
52. Summary
• There are multiple effects
• Likely to be multiple mechanisms
• Mechanisms are still poorly understood
• Essential that more mechanistic research is done
• A long way from translation of mechanistic research
to human clinical practice
