1997 Pharmacology Semester 1 page 1 of 3
By Duy Thai: www.geocities.com/d.thai
• Agents do not act unless they are bound
• A drug has to find a site (a discreet surface molecule) and bind to it in order for it to do something.
• Either an endogenously produced chemical or an artificial chemical can bind to a receptor.
• There are things such as drug families
• Drugs in the same family have a chemical link
• Ligand - locus interaction:
• The ligand (drug) must bind to a specific locus (receptive molecule)
• However, the ligand may or may not evoke a response. The binding of ligand to locus is not sufficient to
say that it will evoke a response. The binding is only the first in a whole series of interactions, finally
leading to a response. i.e. Just because a drug binds to a molecule, it does not mean that the drug will have
• A receptor is a binding site for a drug which leads to a response. Otherwise, it cannot be called a
receptor. A receptor is only called a receptor if it has functional consequences.
• When binding leads to a response, the binding molecule is called a receptor.
• e.g. You can block the effects of Ach on a frogs heart by washing it in methylene blue. When the heart is washed, it
still has a blue appearance since the dye has penetrated into the cells, however, the dye has been washed from the
receptors and so Ach can work again.
• A chemical can bind with great affinity to a receptor.
• A high affinity means that even with low concentrations of the chemical, it is able to bind to many of the
• Chemical specificity means that the chemical structure of the drug is specific for a particular receptor and
it also governs the drugs affinity. If the chemical structure is altered slightly, we may be able to change its
Types of receptors
• Intracellular receptor
• The drug needs to pass through the cell membrane first to bind to an intracellular receptor. This receptor
could be cytosolic or nuclear.
• Nuclear receptors tend to alter gene tra