Histamine has many actions in both the periphery and in the central nervous system.
For example, the effects of hayfever can be reduced in many people if they take anti-histamines. These are typically antagonists of the H1 class of histamine receptor. This is an effect of histamine in the lungs, acting in particular on the smooth muscle of the smaller bronchioles to cause constriction, and reduced air flow. A distinct effect of histamine is seen in the stomach, where histamine is involved in acid secretion. Other drugs can be used to block these receptors without any effect on the airways.
This subtype is chiefly linked to stimulation of phospholipase C activity, and the subsequent contraction of smooth muscle, for example, in the lungs.
This subtype is chiefly linked to stimulation of adenylyl cyclase activity, and in the stomach leads to release of acid into the gastric lumen.
This subtype of histamine receptor has yet to be cloned, and it's signal transduction pattern also remains to be uneqivocally identified. It functions however, to reduce the release of many neurotransmitters, including histamine itself.
Steve Alexander (Monday 12 August 1996)