Pharmacology · Analgesics

Opioid Receptor Pharmacology

Mechanisms, clinical effects, toxicity, and management — a comprehensive guide to μ, κ, and δ opioid signaling.

🧬 Opioid Receptors & Signal Transduction

  • Three canonical receptor families
    • μ (mu) — primary target for most analgesic opioids
    • κ (kappa) — spinal analgesia, dysphoria, diuresis
    • δ (delta) — less well-characterized; analgesia, antidepressant-like effects
  • Endogenous ligands
    • β-endorphin — μ & δ affinity
    • Enkephalins — δ & μ affinity
    • Dynorphins — κ affinity
  • Signal transduction
    • All three receptors couple to Gi/o proteins
    • ↓ adenylate cyclase → ↓ cAMP
    • ↓ voltage-gated Ca²⁺ channels (presynaptic)
    • ↑ K⁺ conductance (postsynaptic) → hyperpolarization
  • Net effect
    • Presynaptic inhibition — reduced neurotransmitter release
    • Postsynaptic inhibition — reduced neuronal excitability
  • μ receptor pharmacology
    • Highest analgesic efficacy
    • Mediates respiratory depression, euphoria, and physical dependence
    • Prototype agonist: morphine
μ receptor activation is responsible for the classic opioid triad: analgesia, euphoria, and respiratory depression.

💊 Morphine — Prototype μ-Agonist

  • Pharmacodynamic effects
    • Analgesia — raises pain threshold; alters pain perception and emotional response
    • Sedation — drowsiness, mental clouding
    • Respiratory depression — blunts chemoreceptor response to CO₂
    • Cardiovascular — minimal direct cardiac effect; peripheral vasodilation (histamine release)
    • Smooth muscle — relaxes longitudinal fibers; constricts circular fibers
  • Organ-system effects
    • GI — ↓ peristalsis → constipation, cramping
    • GU — urinary retention, urgency
    • Biliary — ↑ sphincter of Oddi pressure
    • Pupils — miosis (pinpoint pupils)
    • CTZ — nausea and vomiting (area postrema stimulation)
    • Histamine — mast cell release → pruritus, vasodilation, bronchospasm
  • Antitussive action
    • Suppresses cough reflex via medullary cough center
    • Independent of analgesia and respiratory depression
  • Pharmacokinetics
    • Extensive first-pass metabolism → oral bioavailability ~25%
    • Hepatic glucuronidation → morphine-3-glucuronide (inactive) and morphine-6-glucuronide (active)
    • Morphine-6-glucuronide — potent analgesic, accumulates in renal impairment
    • Renal excretion; dose adjustment required in CKD
  • Contraindications
    • Head trauma — increased ICP risk (hypercapnia, vasodilation)
    • Pulmonary dysfunction — except pulmonary edema
    • Hepatic or renal dysfunction — accumulation risk
    • Adrenal or thyroid insufficiency — exaggerated responses
    • Pregnancy — neonatal depression/dependence (meperidine preferred in labor)
In respiratory depression from morphine, do not give O₂ alone — give naloxone. Oxygen may mask the hypoxic drive.

⚗️ Other Full μ-Agonists

Drug Key Features Clinical Use / Notes
Meperidine Antimuscarinic Tachycardia
No miosis, no GI/GU/biliary spasm 		Metabolized to normeperidine (CYP450) → serotonin reuptake inhibition; risk of serotonin syndrome and seizures
Methadone Long-acting
NMDA receptor antagonist activity 		Maintenance therapy for opioid dependence; also used for chronic pain
Codeine Prodrug
CYP2D6 → morphine 		Mild to moderate analgesia; combination with NSAIDs; antitussive
Hydrocodone Combination with acetaminophen Oral analgesic; abuse potential
Oxycodone High potency Oral and parenteral formulations; extended-release available
Fentanyl High potency Lipophilic Transdermal, IV, transmucosal; rapid onset; used in anesthesia and chronic pain
  • Meperidine — unique considerations
    • Seizures from normeperidine accumulation — treat with benzodiazepines, NOT naloxone
    • Avoid in elderly and renal impairment
    • Serotonin syndrome risk with SSRIs, MAOIs
  • Codeine — CYP2D6 polymorphism
    • Poor metabolizers → reduced analgesia
    • Ultrarapid metabolizers → toxicity risk
    • Inhibitors (fluoxetine, paroxetine) → ↓ activation, ↓ effect

🔄 Partial Agonists & Mixed Agonist-Antagonists

  • Partial μ-agonist: Buprenorphine
    • High μ-receptor affinity, low intrinsic activity
    • Analgesia with ceiling effect — less respiratory depression than full agonists
    • Can precipitate withdrawal in opioid-dependent patients
    • Suboxone® — buprenorphine + naloxone (prevents IV abuse)
  • Mixed agonist-antagonists
    • Pentazocine — κ agonist, μ antagonist
    • Nalbuphine — κ agonist, μ antagonist
    • Butorphanol — κ agonist, μ antagonist
  • κ-mediated effects
    • Spinal analgesia (via κ receptors in dorsal horn)
    • Dysphoria, psychotomimetic effects
    • Diuresis (antidiuretic hormone inhibition)
    • Sedation, miosis (less than μ)
  • Clinical pearls
    • Mixed agonists are useful for pain in patients with mild opioid dependence
    • Precipitate withdrawal if given to patients on full μ-agonists
    • Ceiling effect for respiratory depression — safer in overdose
Pentazocine and nalbuphine can precipitate acute withdrawal in patients physically dependent on morphine or heroin.

🚫 Opioid Antagonists

Drug Route Primary Use Key Feature
Naloxone IV, IM, intranasal Reversal of opioid-induced respiratory depression Rapid onset (1–2 min IV); short duration (30–60 min)
Naltrexone Oral, IM (depot) Opioid dependence, alcohol craving reduction Long-acting; blocks euphoric effects
Methylnaltrexone Subcutaneous, oral Opioid-induced constipation Does NOT cross the BBB — no withdrawal precipitation
  • Naloxone — reversal protocol
    • Initial dose: 0.4–2 mg IV; repeat every 2–3 min as needed
    • Duration shorter than many opioids — monitor for re-sedation
    • May precipitate acute withdrawal (tachycardia, hypertension, vomiting)
  • Naltrexone — maintenance
    • Blocks μ, κ, δ receptors
    • Reduces alcohol craving via unclear mechanism
    • Patient must be opioid-free for 7–10 days before initiation
  • Methylnaltrexone — peripheral selectivity
    • Quaternary ammonium compound — cannot cross BBB
    • Relieves constipation without affecting central analgesia
    • Does not precipitate withdrawal in opioid-dependent patients

🫀 Clinical Pharmacology & Adverse Effects

  • Adverse effect profile (shared across μ-agonists)
    • Constipation — most common; does NOT develop tolerance
    • Nausea/vomiting — CTZ stimulation; tolerance develops
    • Miosis — pinpoint pupils; does NOT develop tolerance
    • Urinary retention — detrusor muscle relaxation, sphincter spasm
    • Biliary colic — sphincter of Oddi spasm (avoid in biliary disease)
    • Pruritus — histamine release (especially morphine)
    • Sedation and cognitive impairment
  • Respiratory effects
    • ↓ sensitivity to CO₂ → ↓ respiratory rate and tidal volume
    • Sleep apnea exacerbation
    • Pulmonary edema — rare, but can occur (naloxone is treatment)
  • Cardiovascular effects
    • Bradycardia (vagal stimulation)
    • Orthostatic hypotension — vasodilation, histamine release
    • Minimal direct myocardial depression
  • Endocrine effects
    • ↓ gonadotropins → hypogonadism, decreased libido
    • ↓ cortisol, ↓ TSH (chronic use)
  • Immunomodulation
    • μ agonists suppress immune function (NK cell activity, antibody production)
    • Clinical significance in chronic opioid therapy
Constipation and miosis are the only opioid effects that do NOT develop tolerance with chronic use.

⚠️ Acute Toxicity & Overdose

  • Classic opioid toxicity triad
    • Pinpoint pupils (miosis)
    • Respiratory depression (↓ rate, ↓ depth)
    • Coma (depressed consciousness)
  • Additional features
    • Hypotension, bradycardia
    • Hypothermia
    • Pulmonary edema (non-cardiogenic)
    • Rhabdomyolysis (prolonged immobilization)
  • Management algorithm
    Airway & Breathing → IV Naloxone → Monitor & Repeat → Supportive Care
    • Naloxone 0.4–2 mg IV/IM; may need higher doses for synthetic opioids (fentanyl)
    • Repeat every 2–3 min; may need continuous infusion
    • Observe for re-sedation — naloxone duration shorter than most opioids
    • Do NOT give oxygen alone — it may mask hypoxic drive; naloxone is the antidote
  • Special considerations
    • Meperidine overdose — seizures from normeperidine; treat with benzodiazepines
    • Buprenorphine overdose — naloxone may be less effective (high receptor affinity)
    • Mixed agonist-antagonist overdose — respiratory depression is less severe (ceiling effect)
In opioid overdose, naloxone is the definitive treatment — but always maintain airway support and monitor for re-sedation.

🔄 Tolerance, Dependence & Withdrawal

  • Tolerance
    • Pharmacodynamic adaptation — receptor desensitization, ↓ G-protein coupling
    • Develops to analgesia, euphoria, sedation, respiratory depression
    • Does NOT develop to miosis or constipation
    • Cross-tolerance among μ-agonists
  • Physical dependence
    • Adaptive state requiring drug to maintain homeostasis
    • Withdrawal symptoms upon abrupt cessation or antagonist administration
  • Withdrawal syndrome — timeline and features
    • Onset: 6–12 hours after last dose (short-acting) / 24–48 hours (methadone)
    • Peak: 36–72 hours
    • Duration: 5–10 days
    • Early signs: yawning, lacrimation, rhinorrhea, sweating
    • Progressive: anxiety, restlessness, piloerection (goosebumps), mydriasis
    • Late: muscle cramps, bone pain, GI distress (diarrhea, vomiting), tachycardia, hypertension
  • Management of withdrawal
    • Supportive care — hydration, antiemetics, antidiarrheals
    • Methadone — μ-agonist maintenance; gradual taper
    • Clonidine — α₂-agonist; reduces autonomic symptoms (but not cravings)
    • Buprenorphine — partial agonist; reduces withdrawal severity
    • Naltrexone — for maintenance after detoxification
Withdrawal is not life-threatening in healthy adults, but can be severe and is best managed with methadone or buprenorphine under supervision.

🩺 Special Clinical Applications

  • Opioid-induced constipation (OIC)
    • Methylnaltrexone — peripheral μ-antagonist; does not cross BBB
    • Naloxegol — PEGylated naloxol; peripheral μ-antagonist
    • Lubiprostone — chloride channel activator; not opioid-specific
  • Antidiarrheal — Loperamide
    • μ-agonist that does NOT cross the BBB (P-glycoprotein substrate)
    • Slows GI transit, reduces fluid loss
    • No analgesic or euphoric effects at therapeutic doses
    • High doses can cross BBB → cardiotoxicity (QT prolongation)
  • Antitussive — Dextromethorphan
    • μ-agonist with central cough suppression
    • NMDA antagonist at higher doses
    • No significant analgesic or respiratory depressant effects at antitussive doses
    • Commonly combined with guaifenesin or antihistamines
  • Opioid use disorder — medication-assisted treatment (MAT)
    • Methadone — full μ-agonist; daily supervised dosing
    • Buprenorphine — partial μ-agonist; office-based prescribing
    • Naltrexone — μ-antagonist; blocks euphoria; requires detoxification first
  • Pain in renal impairment
    • Fentanyl — safest due to non-renal clearance
    • Morphine — avoid (M6G accumulates)
    • Hydromorphone — use with caution; dose reduction needed
    • Meperidine — contraindicated (normeperidine accumulates → seizures)
Loperamide is not an analgesic — it acts peripherally. High doses used for euphoria can cause fatal ventricular arrhythmias.