EXAMPLES OF CLASSIC RECEPTORS

We saw that drugs will equilibrate, go into the blood, and finally will hit the sensitive structure which will give the pharmacological effect—this is usually referred to as the receptor.

The idea of receptors comes from physiology, and was not always understood. When we give epinephrine or some other endogenous agonist (things that we know are natural transmitters), they will have effects by binding on something that becomes saturated with this agonist, or drug. Pharmacology started putting antagonists, or blockers, which would bind to the receptor instead of the agonist. When molecular biology came around, cloning of the cDNA, (the gene encoding different receptors), made it possible to recognize receptors through amplifying them (naturally occurring receptors are so sparse that you couldn’t isolate the protein).

For example, with acetylcholine receptors there are nicotinic (in ganglionic transmission) and muscarenic (in post ganglionic nerve fibers going to muscles or nerves, etc.) subtypes with acetylcholine as the transmitter. Pharmacologists have invented other compounds that can mimic acetylcholine but can be more specific for one subtype of receptor.

Receptors bind the endogenous transmitter, but must still give a signal… how? With respect to the nicotinic receptor, for example, there is a channel with an electrical current that is elicited by binding of acetylcholine. In other cases, there is no channel but instead a complex of enzymes or proteins that are connected to the receptors and they start a biochemical cascade, which will create the effects (2^nd messenger).

In adrenergic receptors, there are no channels (they are all linked to some protein mechanism or enzyme). Epinephrine and norepinephrine are the regular transmitters, but in pharmacology we have analogs and blockers of these transmitters that can be more specific for the alpha or beta subtype.

For example, a nose spray to un-clog your nose is an analog of epinephrine, which effects only the alpha receptors and so constricts your nasal vessels.

There are many other types of receptors for different transmitters.

Classical receptors are those that are usually on the cell membrane and are there to receive an endogenous agonist, however we use them to put pharmacological agonists or blockers.

Category: Pharmacology Notes