Receptors may be classified according to the type of drug that they interact with or according to the specific physiologic response produced by the drug-receptor complex. By evaluating the effects of different agonists in the presence of a given antagonist, receptor sites may also be subclassified. For example, cholinergic receptors can be activated either by muscarine or nicotine. However, only the response to muscarine is antagonized by atropine, while curare will only antagonize the response to nicotine.
This evidence suggests that acetylcholine can bind to and activate at least two different receptor- subtypes, which are either muscarinic or nicotinic. Similarly, receptors and receptor subtypes exist for other agents. The number of any given receptor type or subtype on a cell may also vary. Certain disease states or drugs taken in large doses and/or chronically may increase (up-regulate) or decrease (down regulate) the number of receptors and reflect a degree of receptor adaptability in the face of a changing physiologic environment.