S9 L1 Opioids Flashcards
What is the difference between nociception and pain?
Nociception → non conscious neural traffic due to trauma or potential trauma to tissue
Pain → complex, unpleasant awareness of sensation modified by experience, expectation, immediate context and culture
How is pain ‘felt’?
1- Damage to tissue - release bradykinin, prostaglandins and serotonins
2- Nociceptors stimulated
3- Release of substance P and glutamate
4- Afferent nerve stimulated - Aδ fibres or Ac fibres
→ Aδ- sharp pain
→ Ac- unmyelinated, dull pain, more stimuli require to generate same response
5- Enters dorsal horn and synapses on lamina 1 or 5 on 2nd order neurones
6- Fibres decussate
7- Action potential ascends in spinothalamic tract
8- Synapse in the thalamus on 3rd order neurones
9- Project to post central gyrus (primary sensory cortex- pain felt)
How can we modulate pain?
Modulators in the central and peripheral system
Peripherally → Substantia gelatinosa
Centrally → Peri aqueductal grey
How is pain modulated peripherally?
- Tissue damage → Aδ and Ac fibres to lamina 1 and 5 → project to thalamus etc…
- Inhibitory signals also sent to the substantia gelatinosa (in the dorsal horn)
- ‘Rub it better’ → Activates Aβ fibres which stimulate the substantia gelatinosa → Inhibitory projections to lamina 1 and 5 in dorsal horn - reduced amount of pain to the thalamus
How is pain modulated centrally?
- Tissue damage → Aδ and Ac fibres….. DH…. thalamus…. cortex
- Periaqueductal grey found in midbrain- thalamus and cortex act on PAG- Stimulate it
- PAG sends inhibitory signals via nucleus raphe magnus in the medulla to the dorsal horn to reduce the amount of pain from the dorsal horn going to the thalamus
→ Inhibitory signals through endogenous opioids and 5-HT
What are the endogenous opioids?
Enkephalins, Endorphins and Dynorphins
Act on opioid receptors = G protein coupled receptors
Three receptor subtypes - MOP/µ, DOP/δ and KOP/K
Which is the most important opioid receptor clinically?
MOP/µ
Found in the brainstem and thalamus
Responsible for therapeutic and adverse effects
How do the opioid receptors work?
GPCR
- Agonist binds
- Activate inhibitory G protein
- Leads to decrease in cAMP, via inhibition of adenylate cyclase
- Efflux of potassium
- Hyperpolarisation of the membrane (→ less AP)
- Decrease substance P and GABA release
- Increase dopamine release
What is opioid tolerance?
- Phosphorylation and uncoupling
2. cAMP production
How does phosphorylation and uncoupling lead to tolerance?
- Normally → opioid binds to µ receptor attached to G protein → ↓cAMP → ↓pain
- Opioid binding also leads to intracellular phosphorylation by intracellular kinase → begin to modulate µ receptors
→ Facilitate Arrestin binding → displacement of G protein
→ Opioid unable to bind as effectively so doesn’t have the same effect - Therefore opioid binding → don’t get same ↓cAMP → don’t get same ↓pain
How does cAMP production lead to tolerance?
- Normally → opioid binds to µ receptor attached to G protein → ↓cAMP → ↓pain
- Over period of time → remove opioid? → rebound effect → ↑↑↑cAMP inside cells
- Therefore
→ ↓amount of time between opioid doses
→ ↑dose- require more opioid to get the same ↓cAMP and ↓pain
Also ↑cAMP leads to ↑neuronal excitability - withdrawal symptoms (cramping, sweating, diarrhoea vomiting, extreme agitation) → important to treat to prevent death
What are the general principles of opioid receptors?
- Exploit natural opioid receptors either by agonising or antagonising them
- Main therapeutic effects via µ-receptors
- Aim to modulate pain
- Also indicated in cough, diarrhoea and palliation
What are the different classes of opioids?
Strong agonists Moderate agonist Mixed agonist/antagonists/ partial agonists Antagonist Others
What is the WHO analgesic ladder?
For chronic pain
Start with simple analgesia gradually increase strength
Simple analgesia → weak opioid → strong opioid
(neuropathic pain treated slightly differently)
What is the pharmacokinetics of morphine?
Strong agonist
→ Absorption
- Taken PO, IV, IM, SC, PR
- Gut absorption erratic
- Significant first pass effect - 40% oral bioavailability
→ Distribution
- Lipophilic - rapidly enters al tissues including foetal
- Struggles to cross the BBB - no protein binding
→ Metabolism
- Morphine + glucaronic acid → M6G (therapeutic effect) + M3G (neuronal excitability)
→ Elimination
- Renally
What is the mechanism of action of morphine?
Strong affinity to µ-receptors
Complete activation of µ-receptors
What does morphine result in?
Analgesia → inability to feel pain
Euphoria → feeling or state of intense excitement
