Opioid Receptors Mnemonic

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

opioid receptors

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

 

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

  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

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