Opioid receptors

Key facts about opioid receptors

  • Inhibitory G-protein coupled receptors
  • Activation causes opening of potassium channels and closure of voltage-gated calcium channels – causes hyperpolarisation
  • Activation leads to reduced neuronal excitability and signal transmission
  • Widely distributed in the brain, spinal cord, digestive tract and peripheral sensory neurons
  • 3 types
    • Mu
    • Delta
    • Kappa

Endogenous opioids

There are a range of endogenous opioids, which include:

  • endorphins
  • enkephalins
  • endomorphins
  • dynorphins
  • nociceptin

Comparison of receptor types


While found throughout the CNS, mu receptors are densely concentrated in some areas:

  • Basal ganglia
  • Limbic structures
  • Dorsal horn of the spinal cord
  • Regions associated with nociception and sensorimotor regulation e.g. periaqueductal grey matter, some thalamic nuclei

Kappa receptors have a similar distribution to mu receptors.

Delta receptors have a more limited distribution but are also more densely located in basal ganglia and limbic structures.

Other similar receptors

Sigma receptors were previously thought to be a fourth class of opioid receptor, but this has been discounted as their effects are not reversed by naloxone.

There is also another receptor found in the brain and spinal cord called the nociceptin/orphanin FQ peptide (NOP) receptor, or orphan receptor, but this also is not sensitive to naloxone, so is not considered a class of opioid receptor.

Useful Sources

  1. Yentis, SM et al. (2013) Anaesthesia and Intensive Care A-Z: An Encyclopedia of Principles and Practice, 5e (FRCA Study Guides). Edinburgh: Elsevier.
  2. Ikezu, T and Gendelman, HE (2008) Neuroimmune PharmacologyNew York: Springer.


What’s Next?


Pain transmission


Test Me

Link to questions on this topic


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