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Histamine is an endogenous substance that is widely distributed throughout the body. The two principal sites of storage for histamine are the mast cells in tissue and the basophils in blood.

The action of histamine is mediated through at least two receptors, H1 and H2.

H3 receptors have been reported in the brain, but for our purposes there are two classes of histamine receptors.

Intestinal and bronchial smooth muscles contain mostly H1 receptors. Gastric secretion is mediated by H2 receptors.

As you can see, the action of histamine depends on the receptors with which it interacts. Histamine itself or agonists of the histamine receptors have only minor uses in clinical medicine. Because we consider drugs that act on the gastrointestinal (GI) tract in Chapter 44, we will not consider the H2 receptor antagonists any further here.


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H1 Receptor Antagonists  
DIPHENHYDRAMINE chlorpheniramine
acrivastine dimenhydrinate
azelastine hydroxyzine
brompheniramine meclizine
clemastine promethazine
Nonsedating Antihistamines  
astemizole fexofenadine
cetirizine levocetirizine
desloratadine loratadine

As always, compare this list with the one in your textbook or class handouts and make any adjustments. Some books will divide these drugs into first- and second-generation H1 antihistamines, or into sedating versus nonsedating groups. Second-generation antihistamines do not cross into the brain as easily as the first-generation drugs and are, therefore, less likely to have central nervous system (CNS) side effects. Because many of the first-generation drugs are available over the counter, they are more recognizable by their trade names. They are inexpensive, but have significant side effects, including sedation and anticholinergic effects.

These drugs are competitive antagonists of the H1 receptor.

The H1 antagonists (antihistamines) are used to treat cases of allergic rhinitis and motion sickness, and sometimes to induce sleep.

First, note that this class of drugs is commonly referred as antihistamines. This is in spite of the fact that there is a whole group of H2 antagonists that could also be called antihistamines but aren’t.

The most common use of antihistamines is in the treatment of runny nose caused by seasonal allergies. The first-generation drugs cross the blood-brain barrier. In the CNS, they interact with histamine receptors and cause sedation. This effect is sometimes used therapeutically. A number of these drugs are used to treat motion sickness (diphenhydramine, dimenhydrinate, cyclizine, and meclizine). This action may be the result of a central antihistamine effect or a central anticholinergic action. The agents in this class vary in terms of their anticholinergic potency, the degree of sedation they induce, and their duration of action.

The nonsedating antihistamines (second ...

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