Pain and analgesic pathways
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Pain and Analgesic Pathways. Robert B. Raffa, Ph.D. Professor of Pharmacology Temple University School of Pharmacy & School of Medicine. Current Knowledge. Different ‘types’ of pain, not just different degrees of pain Multiple chemical mediators of pain

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Pain and analgesic pathways

Pain and Analgesic Pathways

Robert B. Raffa, Ph.D.

Professor of Pharmacology

Temple University

School of Pharmacy & School of Medicine

Current knowledge
Current Knowledge

  • Different ‘types’ of pain, not just different degrees of pain

  • Multiple chemical mediators of pain

  • Optimal therapy matches the analgesic(s) with the type(s) of pain

Types of pain
Types of Pain


  • normal physiology (mechanisms known)

  • beneficial

  • treated with conventional analgesics (NSAIDs, acetaminophen, opioids)

  • unrelieved, it becomes deleterious


  • aberrant physiology (mechanisms unknown)

  • poor quality of life

  • difficult to treat

Normal pain pathways


Normal Pain Pathways


  • Tissue injury

  • histamine

  • bradykinin

  • etc.

lateral spino-

thalamic tract

dorsal root




Fig 3-25

Normal pain pathways1

Normal Pain Pathways


(periaqueductal gray)

locus ceruleus

raphe nuclei



Fig 3-25

Sharp localized

Primary (1o) Afferents





Review question

Which is/are TRUE?

  • Pain can be beneficial

  • Pain can be harmful

  • Nociception is normal

  • C-fibers transmit sharp, localized pain

Multiple types in injury

Multiple types in injury

In many injuries and chronic disorders (e.g., arthritis, cancer), there are multiple sources and types of tissue injury and, thus, multiple sources and causes (‘types’) of pain.

Current analgesics options
Current Analgesics Options

  • NSAIDs: 1970’s

  • opioids: 1970’s

  • tramadol: 1980’s & 1990’s

  • COX-2 inhibitors: 1990’s

  • acetaminophen: unknown

  • combinations

  • adjuncts

Who analgesic ladder
WHO Analgesic ‘Ladder’

Step 3

Strong opioids (e.g., morphine)

with or without non-opioids




Step 2

Weak opioids (e.g., codeine)

with or without non-opioids

Step 1

Non-opioids (e.g., NSAIDs,

acetaminophen = paracetamol)

Sites of action


Aa PGs


Sites of Action

Sites of action1

Sites of Action




Na+ channels)

Sites of action2

Sites of Action


(m, d, k)

Sites of action3

Sites of Action

Acetaminophen ?

Review question1

The WHO analgesic ladder is a guide to the proper dose of analgesic:

  • True

  • False

Resistant pains
‘Resistant’ Pains

  • Migraine

  • Neuropathic pain

  • Sickle cell pain

  • etc.


  • Periodic, pulsatile headaches. Familial disorder that usually begins in childhood or early adulthood and tends to decrease in frequency in later life.

  • Possible causes: (1) humoral disturbance that alters vascular responsiveness which in turn elicits pain, or (2) a neurological disturbance in the meninges, from which pain and vasomotor changes result. More specifically, could be due to vascular changes triggered by 5-HT release, to a neuronal abnormality, or excess activity of peptidergic nerve terminals in meningeal vessels. The release of 5-HT also leads to local inflammatory response and the release of other mediators (e.g., bradykinin and prostaglandins) that act on nociceptive nerve terminals, causing pain and also releasing neuropeptides which further reinforce and prolong the pain. Afferent nerve terminals in blood vessel walls may become hypersensitive to vascular distension, thus accounting for the fact that many anti-migraine drugs are vasoconstrictors.


Pharmacologic management

  • Acute attack

    • analgesics (e.g., NSAIDs, APAP)

    • ‘triptans’ (5-HT agonists)

  • Prophylaxis

    • ß blockers

    • anticonvulsants

    • Ca2+ channel blockers

    • etc.

Neuropathic pain
Neuropathic Pain

Common types

  • diabetic neuropathy

  • post-herpetic neuralgia

  • ‘phantom limb’

  • etc.

    Pharmacologic management

  • opioids

  • tramadol

  • topical anesthetics

  • antidepressants

  • anticonvulsants

Possible mechanisms
Possible Mechanisms

  • ‘Central sensitization’. Overactivity of a 2o neuron in the dorsal horn leads to enhanced pain transmission characterized by a lowered threshold for activation and expanded receptive fields, leading to the activation of key excitatory amino acid receptors such as the N-methyl-D-aspartate (NMDA) receptor.

  • Disinhibition. Reduced activation of central inhibitory inputs from endogenous opioid, 5-HT, and norepinephrine pathways.

  • Sympathetic activation. Sympathetic nerve endings sprout from a nearby blood vessel toward the site of injury and can enhance signal transmission in the DRG. Catecholamine release and up-regulation of adrenoceptors on free nerve endings also contribute to sympathetically mediated pain.

  • Peripheral sensitization. Injury to peripheral nerves may lead to hyperexcitability of peripheral nerve terminals (nociceptors). This may be due to altered expression of Na+ channels, Ca2+ channels, or adrenoceptors in peripheral nerves and DRG.

Mechanisms of pain
Mechanisms of Pain


Mechanisms of pain1
Mechanisms of Pain

Central Sensitization

Mechanisms of pain2
Mechanisms of Pain


Mechanisms of pain3
Mechanisms of Pain


Combination analgesics
Combination Analgesics

Possible rationales

  • No single perfect analgesic

  • Complementary PK

  • Multiple sites/mechanisms of action target multiple pain pathways

  • Potentially synergistic analgesia

  • Comparable efficacy, but reduced AE profile

Raffa, RB. J Clin Pharm Ther. 2001;26:257-64.

Review question2

‘Triptans’ are most associated with:

  • Diabetic neuropathy

  • Migraine headache

  • Neuropathic pain

Review question3

Central sensitization and up-regulation are the same thing:

  • True

  • False

Review question4

Almost everyone experiences:

  • Hyperalgesia

  • Allodynia