Key definitions:

  • Opiates are all naturally-occurring substances with morphine-like properties
  • Opioids is a general term for substances with an affinity for opioid receptors
  • Opium is a mixture of alkaloids from the poppy plant

Classification of Opioids

  • Naturally occurring
    • Endogenous opioids
      • Endorphins
      • Enkephalins
      • Dynorphins
    • Opium derivatives
    • Phenanthrenes
      • Morphine
      • Codeine
  • Semisynthetic
    Simple modifications to morphine.
    • Diacetylmorphine
    • Buprenorphine
    • Oxycodone
  • Synthetic
    • Phenylpiperidines
      • Fentanyl
      • Alfentanil
      • Remifentanil
      • Pethidine
    • Diphenylpropylamines
      • Methadone

Opioid Receptor Classification

All opioid receptors are Gi receptors. Activation:

  • Inhibits adenylyl cyclase, reducing cAMP
    • Pre-synaptically inhibits voltage-gated Ca2+ channels
      • Decreases Ca2+ influx
      • Reduces neurotransmitter release
    • Post-synaptically stimulates activates K+ channels
      • Causes K+ efflux
      • Leads to membrane hyperpolarisation
Receptor Actions Notable Properties
MOP Analgesia (spinal and brain), euphoria, meiosis (via stimulation of the Edinger-Westphal nucleus), nausea and vomiting (via CTZ), sedation, bradycardia, inhibition of gut motility, urinary retention, physical dependence Only opioid receptor to cause nausea/vomiting
KOP Analgesia (predominantly spinal), sedation, meiosis, dysphoria Less respiratory depression
DOP Analgesia, respiratory depression, urinary retention, physical dependence Minimal constipation
NOP Anxiety, depression, change in appetite Hyperalgesia at low doses, analgesic at high doses

Mechanism of effects:

  • Respiratory depression
    Decreases central chemoreceptor sensitivity to CO2.
  • Constipation
    Stimulation of opioid receptors in the gut.
    • Normally activated by local endogenous opioids (used as neurotransmitters)
    • Agonism of these receptors (µ, k, and to a smaller extent, δ) reduces GIT secretions and peristalsis


  1. Peck TE, Hill SA. Pharmacology for Anaesthesia and Intensive Care. 4th Ed. Cambridge University Press. 2014.
  2. Katzung BG, Trevor AJ. Basic and Clinical Pharmacology. 13th Ed. McGraw-Hill Education Europe. 2015.
  3. Petkov V. Essential Pharmacology For The ANZCA Primary Examination. Vesselin Petkov. 2012.
Last updated 2019-07-18

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