Pain - Basic Science Implications for Analgesia

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Pain - Basic Science Implications for Analgesia
Analgesics
Clifford J. Woolf

• Neural Plasticity Research Group
• Department of Anesthesia and Critical Care
• Massachusetts General Hospital andHarvard
  Medical School

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• Is there a basis for the
• separation of pain
• on the basis of
• Chronicity
• Intensity
• Mechanisms

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Pain Chronicity Acute Chronic Persistence or
Recruitment
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Pain Chronicity Acute - Transient / Recurrent
- Reversible Chronic - Long
lasting/Reversible - Persistent / Irreversible
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Pain Intensity Mild Moderate Severe Continuum
or Discrete Stimulus or Response
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Etiological Factors inflammation/tissue
damage/nerve lesions
Pain Mechanism
Pain Sydromes post-operative/arthritic/back
pain/neuropathic
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Multiple Pain Mechanisms

• Nociception
• Peripheral sensitization
• Central sensitization
• Ectopic excitability
• Decreased inhibition/
• Structural reorganization

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Multiple Pain Symptoms

• Spontaneous Pain
• Superficial/Deep
• Continuous/Intermittent
• Evoked Pain
• Thermal/Mechanical
• Allodynia
• Hyperalgesia

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Role of COX-2 selective/specific inhibitors
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Nociception
Transduction Conduction Transmission
Modulation
Noxious stimulus
Ouch Pain
primary sensory neuron central neuron
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Nociception Transduction
Nociceptor Activators
Heat H
Bradykinin
Mechanical
Cold
VR1 ASIC TRPV3
B1/B2
DRASIC/mDEG
CRM1
generator potential action potentials
COX-2 Insensitive
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Transmission/Modulation
VGCC
COX-2 Insensitive
GABAA Adensosine Opiate CB1
NMDA
Activity
Glutamate
AMPA
Sub P
mGluR
NK1
Afferent Central Terminal
Dorsal Horn Neuron
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Nociception is not COX-2Sensitive
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Peripheral Sensitization
Reduced Transduction Threshold
Primary hyperalgesia Primary heat allodynia
Innocuous/Noxious stimulus
Inflammation
primary sensory neuron central neuron
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Peripheral Sensitization
Tissue damage
Macrophage
IL1b, IL6TNFa
Mast cell

Cox-2
PGS
PG
AA
COX-2 Sensitive
VR1
EP/IP
H
Ca2
PKC
PKA
(SNS/SNS2)
Primary sensory neuron peripheral terminal
There are prostanoid and non-prostanoid
sensitizers
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Central Sensitization
Increased Pain Responsiveness
Secondary hyperalgesia Tactile allodynia
Noxious stimulus
Irritants Tissue damage Inflammation
primary sensory neuron central neuron
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Central Sensitization Central Pain
Hypersensitivity
A? fibre mechanoreceptor
Weak synapse
innocuous stimulus
non-painful sensation
Increased synaptic strength
innocuous stimulus
painful sensation
Brush-Evoked Mechanical Allodynia
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Central Sensitization - Acute Phase
COX-2 Insensitive
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COX-2 Induction in the Spinal Cord - Inflammation
24 Hrs
12 Hrs
48 Hrs
Naïve
tRNA
2 Hrs
4 Hrs
6 Hrs
1 Hr
COX-2
b-actin
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Cox-2 is not induced in the Spinal Cord by
Peripheral Nerve Injury
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Central Sensitization Late Phase (Inflammation)
COX-2 Sensitive
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There are COX-2 sensitive peripheral and central
components of inflammatory pain Cox-2 inhibitors
can only act when COX-2 is induced - time lag for
induction There are non-prostanoid contributors
to inflammatory pain - ceiling effect
Peripheral nerve injury may not be sensitive to
COX-2 inhibitors
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A
B
C
Etiology
Mechanism
Symptom
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A
B
C
Etiology
Mechanism
Symptom
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Need to differentiate Analgesic and
Anti-hypersensitivity drugs Temporal and
Intensity characteristics of pain do not reflect
mechanisms and may not be useful predictors of
analgesic action Pain Mechanisms and Drug
Mechanisms may provide the most useful input
for determining Indication and Efficacy
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Need mechanism sensitive/specific outcome
measures in addition to global pain scores Need
clinical trials that validate mechanistic
hypotheses Need to consider labeling claims in
light of action of a drug with specific pain
mechanism(s) as well as empirical clinical data
on efficacy Are there global analgesics?