Opiate Receptors
The distribution of the opiate receptors in the brain of a guinea pig.  Red areas represent the highest density of opiate receptors.  Yellow areas contain a  moderate density, while blue, purple and white represent low density.  
Image from  Snyder, 1996.  Used with permission by W.H. Freeman and Company.
 
 
 
 
 		 Scientists have identified four types of opiate receptors: mu1, mu2, kappa and delta receptors.  All opioids exert their effects by activating one or more of these receptors.  Analgesia involves activation of mu1 receptors in the brain and kappa receptors in the spinal cord.  Mu2 receptors are are involved in respiratory depression and intestinal constipation. The contribution of delta receptors to analgesia in unclear, and may be more closely related to euphoria (Hollt, 1983). 

The actions of opioids on receptors can vary depending on the location within the body (Kandel and Schwartz, 1985).  For example, a particular opioid may act as an antagonist at the kappa receptors in the brain, but as an agonist at the same type of receptors in the large  intestines. 

Activation of Mu1, Mu2, and delta receptors close potassium channels, while kappa receptors are linked to calcium channels.  Humans that have become tolerant to activation of one receptor type are not necessarily tolerant to the others.  Sometimes analgesic medication is prescribed to activate only certain receptors (Leonard, 1992).


    Main Page   History 1    History 2    Ligands & Receptors   Endorphins    Analgesia 1    Analgesia 2   Euphoria    Tolerance & Dependency    Common Opioids    Current Research    Acknowledgments   Literature Cited

Animal Physiology HOT TOPICS
   This site was created during the fall semester of 1999 by Wes Self, a student at 
  Davidson College, as part of an assignment in Biology 312 (Animal Physiology). 

  Questions, comments and suggestions are appreciated at weself@davidson.edu