The Receptor Most psychoactive drugs exert their action at a receptor. Some receptors are broken down further into subtypes as in the k1 and k2 receptors. A substance that binds to a receptor is called a ligand, thus endorphins are the natural ligand for the opiate receptor. Receptors have several properties. This kind of binding to a receptor is referred to as specific.
Affinity refers to the strength that a substance binds to a receptor.
Some chemical bonds are stronger than others resulting in some substances having a greater affinity than others for a receptor. In respect to opiate receptors and opioid analgesics the stronger the affinity, the stronger the analgesic properties of the substance. Therefore, morphine which is a strong analgesic has a stronger affinity for the opiate receptor than codeine which is a weaker analgesic. Opiate receptors have been found in every vertebrate and even in some invertebrate species. Therefore, opiate receptors and the endogenous opioids are basic within the scheme of evolution .