Opioids Flashcards
How is pain formed?
1) stimulation of nociceptors
2) release of substance P and glutamate
3) stimulation of afferent nerves
4) decussation of fibres
5) action potential ascends
6) synapse in thalamus
7) project to primary sensory cortex
What are the pain modulators?
Peripherally:
Substantia gelatinosa
Centrally:
Peri aqueductal grey
What are examples of endogenous opioids?
Enkephalins
Dynorphins
B-endorphins
What receptors do opioids act on?
μ - enkephalins, B-endorphins
δ - enkephalins
κ - dynorphins
All GPCRs
What is the WHO analgesic ladder?
Simple analgesia - paracetamol, NSAIDs
Weak opioid - codeine
Strong opioid - morphine, fentanyl
Typically used for chronic pain
If acute pain, can go straight to strong opiate
What is the general principles of opioid use?
Exploit natural opioid receptors - agonist or antagonism
Mainly act via μ receptors
Aim to modulate pain
Also indicated in cough, diarrhoea and palliation
Describe pharmacokinetics of morphine
Absorption:
PO, IV, IM, SC, PR
Erratic gut absorption
Significant first pass
Distribution:
Rapidly enters all tissues, inc foetal
Metabolism:
Morphine + glucuronic acid => M6G + M3G
Elimination:
Renal
What are the actions of morphine?
Strong agonist
Bind to μ receptors
Causes analgesia and euphoria
What are some side effects of morphine?
Respiratory depression - medullary resp centre made less responsive to CO2
Emesis - stimulates CTZ
Decreased GI motility => constipation
Miosis
Mast cell degranulation => histamine release - be careful w/ asthmatics
Describe pharmacokinetics of fentanyl
Absorption:
IV, epidural, intrathecal, nasal
80-100% bioavailability
Distribution:
Highly lipophilic and protein bound
High level of CNS membrane
Metabolism:
CYP 3A4
Elimination:
Half life 6 mins
Renally excreted
What are the actions of fentanyl?
Strong agonist
Analgesia
Anaesthetic
What are the side effects of fentanyl?
Respiratory depression
Constipation
Vomiting
Describe the pharmacokinetics of codeine
Administration:
PO, SC
Metabolism:
Codeine => morphine via CYP 2D6
Elimination:
Glucoronidation of morphine
Renal excretion
What are the actions of codeine?
Moderate antagonist
Mild - moderate analgesia
Cough depressant
What are the side effects of codeine?
Constipation
Respiratory distress