Opioids have rich receptor pharmacology, complex tolerance mechanisms, and high clinical relevance — a reliable exam topic.
Receptor subtypes
- Mu (μ): analgesia, euphoria, respiratory depression, constipation, miosis
- Kappa (κ): spinal analgesia, sedation, dysphoria
- Delta (δ): analgesia, mood modulation
Classification
- Full agonists: morphine, fentanyl, methadone, tramadol (atypical)
- Partial agonist: buprenorphine
- Mixed agonist-antagonist: pentazocine
- Antagonists: naloxone, naltrexone
Buprenorphine’s high receptor affinity can displace full agonists and precipitate withdrawal — a common vignette.
Tolerance develops to analgesic, euphoric and respiratory effects — but not to constipation and miosis.