Opioid Receptors Mnemonic

Opioid receptors are a group of inhibitory G protein-coupled receptors with opioids as ligands.

Mu (µ) Receptor (MOP)

Mnemonic: MU CARDS

1. Miosis
2. eUphoria
3. Constipation
4. Analgesia (Supraspinal + Spinal)
5. Respiratory depression
6. Rigidity (truncal)
7. Dependency
8. Sedation

µ1 mediates supraspinal analgesia, and most of other effects including spinal analgesia is mediated by µ2.

Kappa (κ) Receptor (KOP)

Mnemonic: CAPpA

1. Constipation
2. Analgesia (Spinal)
3. Psychomimetic effects (Dysphoria)
4. Anticonvulsant

Delta (δ) Receptor (DOP)

1. Analgesia (Spinal)
2. Modulation of hormone and neurotransmitter release
3. Convulsant

Sigma (σ) Receptor

Mnemonic: S for Stimulation

1. CNS stimulation: Hallucination and Dysphoria
2. Respiratory stimulation

Other Receptors

1. Nociceptin/Orphanin FQ (N/OFQ) or Orphanin like receptors (ORL1) or κ3, i.e. NOP
   • Anxiety
   • Depression
   • Appetite
   • Development of tolerance to mu-agonists
2. Epsilon (ε) receptor
3. Zeta (ζ) receptor

Selectivity of Opioids for receptors

Strong agonists of Mu are: 6 Ms

1. Morphine
2. Methadone
3. Meperidine (Pethidine)
4. hydroMorphone
5. diaMorphine
6. Methylfentanyl

Kappa receptor agonists and Mu receptor antagonists: BNP

1. Butorphanol
2. Nalorphine
3. Nalbuphine
4. Pentazocine

Partial agonist of Mu receptor and antagonist of other:

1. Buprenorphine

Possess antimuscarinic effect: Contraindicated in MI but safe in biliary colic

Mnemonic: 2 Ps

1. Pethidine
2. Pentazocine

Management:

A. Opioid poisioning:

1. Acute: Naloxone
2. Maintenance: Naltrexone

B. Opioid de-addiction:

1. Maintenance: Methadone
2. Relapse prevention: Naltrexone
3. Withdrawal symptom treatment: Beta-blockers/clonidine

C. Opioid induced constipation: Methyl-naltrexone

Dr. Sulabh Kumar Shrestha, MS Orthopedics

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. He is currently pursuing Fellowship in Hip, Pelvi-acetabulum and Arthroplasty at B&B Hospital.