An endorphin molecule.    Morphine, an active ingredient   in opium. The boxed area of  both  molecules binds to opiate   receptors. Notice the structural   similarity of these areas.

Receptors and ligands carry out a great deal of physiology on the subcellular level.  Receptors are sites on the surface of cells  that allow specific molecules to bind to them.  Ligands are these specific molecules.  An agonist is a ligand that binds to the active site of a receptor and initiates some change in body function.  An antagonist is a ligand that does not bring about a functional change when it binds to receptors. (Campbell, 1996).  Endorphins are the body's natural agonists at the opiate receptors.  Activation of opiate receptors by endorphins regulate many functions within the body, including pain perception, internal temperature, appetite, sexual behavior and blood pressure (Hawkes, 1992).  Because of structural similarities to endorphins, opioids also act as agonists at opioid receptors.  However, opioids and endorphins do not trigger identical responses. [See Endorphins]  Naloxone is an antagonist at the opiate receptors. Without activating the receptors, naloxone binds very tightly and prevents agonists from reaching the receptors.  Naloxone is used as a treatment for opioid overdose.  Naloxone out-competes opioids for access to the active sites, and therefore blocks opiate receptor activation, even if large amounts of opioids are present (Snyder, 1978).

 

A diagram of an opiate receptor. The protruding portion of opioids (boxed in the diagrams above) enter the "cavity" on the receptor's surface.  The "flat surface" serves as a landing pad for the benzene ring of opioid molecules.  Meanwhile, the anionic (negatively charged) region binds to positively charge nitrogen parts on opioids.   Images on this page from  Snyder, 1996. Used with permission from  W.H. Freeman and Company.

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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