Opiates and Opioids


Morphine and the opiates, drugs found in the opium poppy or their chemical derivatives, are some of the most valuable drugs in medicine due to their ability to suppress pain. They act on systems in the brain, spinal cord and body that are more weakly regulated by endogenous opioids, natural transmitters. The effects of opioids can now be explained in terms of their acting as agonists at one of the four opiate receptors found in the brain, spinal cord and peripheral nervous system. All four receptors are inhibitory but the major one is the μ‐opioid receptor which mediates the wanted (analgesia and relief of anxiety) and unwanted effects of opioids (respiratory depression, nausea, constipation and the potential for dependence). These receptor events underlie the medical and nonmedical use of opioid drugs.

Key Concepts:

  • Opiates and opioids refer to drugs that are based on the structure or actions of morphine and codeine found naturally in the opium poppy.

  • These exogenous agents modulate the function of the central nervous system and body systems through activation of inhibitory opioid receptors, of which there are four.

  • The most important is the μ‐opioid receptor, named after morphine.

  • The receptors are not present just in case we wander into a poppy field but because there are endogenous opioid peptides that function as inhibitory neurotransmitters through activation of these receptors.

  • Opioids are valuable medical drugs – the analgesia and relief of anxiety are important effects but there are side‐effects that include respiratory depression, nausea, constipation, sedation and the potential for dependence.

  • Drugs such as heroin have dangers due to these side‐effects.

Keywords: opiates; opioids; pain; analgesia; dependence; receptor

Figure 1.

Spinal opiate analgesia. Action potentials in peripheral sensory neurons arrive in the central terminals of C‐fibres and produce the release of glutamate and substance P, among others. By activation of postsynaptic receptors on spinal neurons, ‘pain messages’ are sent to the brain. Opiates, by acting on inhibitory presynaptic receptors, reduce release of these transmitters. Furthermore, postsynaptic opioid receptors inhibit the excitability of the spinal neurons directly and actions of opiates in the brain trigger descending inhibitions. The ensemble of these actions leads to analgesia.


Further Reading

Dickenson AH (1995) Spinal cord pharmacology of pain. British Journal of Anaesthesia 75: 193–200.

Drugscope. http://www.drugscope.org.uk/

Tyler A (1988) Street Drugs. London: Hodder and Stoughton.

Uhl GR, Childers S and Pasternak G (1994) An opiate receptor gene family reunion. Trends in Neurosciences 17: 89–93.

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Dickenson, Tony(Nov 2012) Opiates and Opioids. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0000251.pub2]