Opioid receptors are a group of inhibitory G protein-coupled receptors with opioids as ligands.
Mu (µ) Receptor (MOP)
Mnemonic: MU CARDS
- Miosis
- eUphoria
- Constipation
- Analgesia (Supraspinal + Spinal)
- Respiratory depression
- Rigidity (truncal)
- Dependency
- Sedation
µ1 mediates supraspinal analgesia, and most of other effects including spinal analgesia is mediated by µ2.
Kappa (κ) Receptor (KOP)
Mnemonic: CAPpA
- Constipation
- Analgesia (Spinal)
- Psychomimetic effects (Dysphoria)
- Anticonvulsant
Delta (δ) Receptor (DOP)
- Analgesia (Spinal)
- Modulation of hormone and neurotransmitter release
- Convulsant
Sigma (σ) Receptor
Mnemonic: S for Stimulation
- CNS stimulation: Hallucination and Dysphoria
- Respiratory stimulation
Other Receptors
- Nociceptin/Orphanin FQ (N/OFQ) or Orphanin like receptors (ORL1) or κ3, i.e. NOP
- Anxiety
- Depression
- Appetite
- Development of tolerance to mu-agonists
- Epsilon (ε) receptor
- Zeta (ζ) receptor
Selectivity of Opioids for receptors
RECEPTOR TYPE | ||||
Opioid | MOP | KOP | DOP | NOP |
Endogenous | ||||
Endorphin | +++ | +++ | +++ | – |
Enkaphalin | + | – | +++ | – |
Dynorphin | ++ | +++ | + | + |
N/OFQ | – | – | – | +++ |
Drugs | ||||
Agonists | ||||
Morphine | +++ | + | + | – |
Pethidine | +++ | + | + | – |
Diamorphine | +++ | + | + | – |
Fentanyl | +++ | + | – | – |
Tramadol | Weak agonist | Weak agonist | Weak agonist | – |
Codeine | Weak agonist | + | Weak agonist | – |
Partial agonists | ||||
Buprenorphine | Partial agonist | Antagonistic | Antagonistic | – |
Pentazocine, Nalbuphine | Antagonist | Agonist | – | – |
Butorphanol, Nalorphine | Antagonist | Agonist/Parital-agonist | – | – |
Antagonists | ||||
Naloxone (parenteral) – nalmefene is long acting parenteral | Antagonist | Antagonist | Weak antagonist | – |
Naltrexone (oral) | Antagonist | Antagonist | Weak antagonist | – |
Strong agonists of Mu are: 6 Ms
- Morphine
- Methadone
- Meperidine (Pethidine)
- hydroMorphone
- diaMorphine
- Methylfentanyl
Kappa receptor agonists and Mu receptor antagonists: BNP
- Butorphanol
- Nalorphine
- Nalbuphine
- Pentazocine
Partial agonist of Mu receptor and antagonist of other:
- Buprenorphine
Possess antimuscarinic effect: Contraindicated in MI but safe in biliary colic
Mnemonic: 2 Ps
- Pethidine
- Pentazocine
Management:
A. Opioid poisioning:
- Acute: Naloxone
- Maintenance: Naltrexone
B. Opioid de-addiction:
- Maintenance: Methadone
- Relapse prevention: Naltrexone
- Withdrawal symptom treatment: Beta-blockers/clonidine
C. Opioid induced constipation: Methyl-naltrexone
![dr. sulabh kumar shrestha](https://epomedicine.com/wp-content/uploads/2020/07/profile.jpg)
He is the section editor of Orthopedics in Epomedicine. He searches for and share simpler ways to make complicated medical topics simple. He also loves writing poetry, listening and playing music.
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