12. Pharmacology of pain: Opoid receptor agonists Flashcards
Nociception
Mechanism:
- Peripheral stimuli → CNS
Polymodal nociceptors (PMN)
Main peripheral sense neurons that respond to noxious stimuli
- Sensitised to prostaglandins
- Release glutamate (fast transmitter) & various peptides
Nociceptive fibres terminate in…
Superficial layers of the dorsal horn
- Forming synaptic connections with transmission neurons running to the thalamus
Transmission of nociceptive fibres to the dorsal horn is subjected to…
Modulatory influences constituting “Gate control” mechanism
Descending pathways from the midbrain & brainstem exert…
Strong inhibitory effect on dorsal horn transmission
Opidoids cause analgesia by…
- By activating the descending pathways
- By inhibiting transmission in the dorsal horn
Opioids mimic actions of…
Endogenous opioids (Endorhpin & encephalin)
- Peptides produced in nervous & endocrine systems
- Stimulate opioid receptors
List the opioid receptors
- μ (mu)
- κ (kappa)
- δ (delta)
All opioid receptors function by…
Exerting inhibitory modulation of synaptic transmission in both CNS & peripheral nerve cells
Indications of opioid agonist use
- Premedication before general anaesthesia
- Neuroleptanalgesia
- Combination in balance anaesthesia
- Analgesia
- Diarrhoea
- Antitussive (cough prevention)
Side effects of opioid agonist use
- Respiratory depression
- Bradycardia
- Vasodilation, hypotension
- Vomiting
- Constipation
- Urinary retention
List some opioid agonists
- Morphine
- Butorphanol
- Buprenorphine
- Fentanyl
- Tramadol
- Methadone
Morphine
- Agonist, Schedule 2
- Analgesic
- Depression of respiratory, vasomotor & cough centres
- Stimulation of chemoreceptor trigger zone & vomiting centre
- Myosis (constriction of pupil)
- Inhibition of peristaltic movements
- Good somatic & visceral analgesia
- Treatment of diarrhoea, cough suppression
Butorphanol
- Synthetic mixed agonist
- Similar to morphine
- Slow onset, relatively long duration (3-4hr)
- Used alone or in combination with α2 agonists
Dog/cat: Analgesic, premedication, ↓non-productive cough
Buprenorphine
- Semisynthetic mixed agonist
- More potent than morphine with less side effects
- Onset 15 minutes after IV
- Long duration (12hr)
- Not to be used in bone surgery
- Slight sedation
- Primary drug for pancreatitis
Fentanyl
- Synthetic agonist, Schedule 2
- Somatic analgesia
- Respiratory depression, bradycardia
- Few minutes duration
- Transdermal patches
- Peak action at 24 hours
- Used for 3 days