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

 

	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