How Do Drugs Work & What Are Receptors?

How Do Drugs Work?

Did you ever wonder how aspirin knows to go to your head when you have a headache and to your elbow when you have "Tennis Elbow"? Or how one or two small aspirins containing only 325-650 mg of active drug can relieve a headache or ease the inflammation of a strained muscle or tendon in a 195 lb. athlete?
The answer to the first question is that drugs are distributed throughout the body by the blood and other fluids of distribution (see distribution below). Once they arrive at the proper site of action, they act by binding to receptors, usually located on the outer membrane of cells, or on enzymes located within the cell.

What Are Receptors?

Receptors are like biological "light switches" which turn on and off when stimulated by a drug which binds to the receptor and activates it. For example, narcotic pain relievers like morphine bind to receptors in the brain that sense pain and decrease the intensity of that perception. Non-narcotic pain relievers like aspirin, Motrin (ibuprofen) or Tylenol (acetaminophen) bind to an enzyme located in cells outside of the brain close to where the pain is localized (e.g., hand, foot, low back, but not in the brain) and decrease the formation of biologically-active substances known as prostaglandins, which cause pain and inflammation.
These "peripherally-acting" (act outside of the central nervous system (CNS)) analgesics may also decrease the sensitivity of the local pain nerves causing fewer pain impulses to be sensed and transmitted to the brain for appreciation.
In some instances, a drug's site of action or "receptor" may actually be something which resides within the body, but is not anatomically a part of the body. For example, when you take an antacid like Tums or Rolaids, the site of action is the acid in the stomach which is chemically neutralized. However, if you take an over-the-counter (OTC) medication which inhibits stomach acid production instead of just neutralizing it (e.g., Tagamet (cimetidine) or Pepsid-AC (famotidine)), these compounds bind to and inhibit recptors in the stomach wall responsible for producing acid.
Another example of drugs which bind to a receptor that is not part of your body are antibiotics. Antibiotics bind to portions of a bacterium that is living in your body and making you sick. Most antibiotics inhibit an enzyme inside the bacteria which causes the bacteria to either stop reproducing or to die from inhibition of a vital biochemical process.
In many instances, the enzyme in the bacteria does not exist in humans, or the human form of the enzyme does not bind the inhibiting drug to the same extent that the bacterial enzyme does, thus providing what pharmacologists call a "Selective Toxicity". Selective toxicity means that the drug is far more toxic to the sensitive bacteria than it is to humans thus providing sick patients with a benefit that far outweighs any risks of direct toxicity. Of course, this does not mean that certain patients won't be allergic to certain drugs.
Penicillin is a good example to discuss. Although penicillin inhibits an enzyme found in sensitive bacteria which helps to "build" part of the cell wall around the outside of the bacteria, and this enzymatic process does not occur in human cells, some patients develop an allergy to penicillin (and related cepahlosporin) antibiotics. This allergy is different from a direct toxicity and demonstrates that certain people's immune system become "sensitized" to some foreign drug molecules (xenobiotics) which are not generally found in the body.
As medical science has learned more about how drugs act, pharmacologists have discovered that the body is full of different types of receptors which respond to many different types of drugs. Some receptors are very selective and specific, while others lack such specificity and respond to several different types of drug molecules.
To date, receptors have been identified for the following common drugs, or neurotransmitters* found in the body: narcotics (morphine), benzodiazepines (Valium, Xanax), acetylcholine* (nicotinic and muscarinic cholinergic receptors), dopamine*, serotonin* (5-hydroxytryptamine; 5-HT), epinephrine (adrenalin) and norepinephrine* (a and b adrenergic receptors), and many others.
Neurotransmitters* are chemicals released from the end of one neuron (nerve cell) which diffuse across the space between neurons called the synaptic cleft and stimulate an adjacent neuron to signal the transmission of information.
The rest of this section is designed to explain the complicated journey of a drug through the body, which pharmacologists call pharmacokinetics.

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