S9: opioids & respiratory pharmacology Flashcards
Compare 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
Describe the pain pathway
Nociceptors stimulated Release of substance P and glutamate Afferent nerve stimulated Fibres decussate Action potential ascends Synapse in thalamus Project to post central gyrus
List the different ways pain is modulated
Have modulators in peripheral system and in central system
Peripherally: substantia gelatinosa
Centrally: peri-aqueductal grey
Outline how pain is modulated peripherally
Tissue damage stimulates A delta & C afferent nerves, which enter the dorsal horn (substantia gelatinosa (SG) is found here) and project to the thalamus, causing pain and sensation
Laminae 1+5 are where pain fibres transmit
Tissue damage also sends inhibitory signals to the SG
Rubbing the painful area -> sends A beta fibres which stimulate the SG -> send inhibitory signals to laminae in dorsal horn, which reduces pain going to thalamus
Outline how pain is modulated centrally
Tissue damage stimulates A delta & C afferent nerves, which enter the dorsal horn and project to the thalamus, causing pain and sensation
Thalamus stimulates cortex & periaqueductal grey matter
Periaqueductal grey matter sends inhibitory signals to the dorsal horn via endogenous opioids -> reduces pain being sent to the thalamus
Describe endogenous opioids
Enkephalins, endorphins and dynorphins Opioid receptors = G protein receptors Three receptor subtypes: 1) MOP/u 2) DOP/delta 3) KOP/K
Describe the MOP receptor
Most important clinically & causes most side-effects
Found predominantly in the brainstem and thalamus
GPCR
Describe the phosphorylation and uncoupling mechanism of opioid tolerance
Opioid binds to GPCR -> decreased cAMP -> decreased pain
ALSO: intracellular phosphorylation occurs, which changes the u receptors -> arrestin can bind and displace the G protein OR opioid may not have the same effect -> don’t get the same decrease in pain
Describe cAMP production mechanism of opioid tolerance
When opioid is removed: massive increase in cAMP
Causes neuronal excitability
Causes withdrawal symptoms – cramping, sweating, vomiting, diarrhoea -> common cause of death in addicts
Describe opioids
Exploit natural opioid receptors, either agonise or antagonise
Main therapeutic effects via u-receptors
Aim to modulate pain
Also indicated in: cough, diarrhoea & palliation
List examples of opioids
Strong agonists – morphine, fentanyl
Moderate agonists – codeine
Mixed agonist-antagonist/partial agonist – buprenorphine
Antagonists – naloxone
Describe the WHO analgesic ladder
Used for chronic pain management Simple analgesia – paracetamol, NSAIDs Weak opioid – codeine Strong opioid – morphine, fentanyl (NB: for neuropathic pain use different drugs – anticonvulsants, tricyclics etc)
Describe morphine
Strong affinity to u receptors
Complete activation of u
Actions: analgesia, euphoria
Side effects: respiratory depression, emesis, GI tract, CVS, miosis & histamine release – caution in asthmatics
Describe fentanyl
100x potency compared to morphine, higher affinity for u receptor
Less histamine release, sedation & constipation
Actions: analgesia, anaesthetic
Side effects: respiratory depression, constipation & vomiting
Describe codeine
Approx. 1/10 potency of morphine
Actions: mild-moderate analgesia, cough depressant
Side effects: constipation, respiratory depression (worse in children)
NB: active metabolite of codeine requires CYP2D6 oxidation activity -> deficiency in this CYP enzyme means less active metabolite being produced & less analgesia
Describe buprenorphine
Very high affinity for u receptor, long duration of action
Actions: moderate to severe pain, opioid addiction treatment
Side effects: respiratory depression, low BP, nausea & dizziness