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 -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 -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.