Neuromodulators inflammatory mediators

Histamine (B). Histamine is stored in basophils and tissue mast cells. It plays a role in inflammatory and allergic reactions (p. 72, 326) and produces bronchoconstriction, increased intestinal peristalsis, and dilation and increased permeability of small blood vessels. In the gastric mucosa, it is released from enterochromaffin-like cells and stimulates acid secretion by the parietal cells. In the CNS, it acts as a neuromodulator. Two receptor subtypes (G-pro-tein-coupled), H and H2. are of therapeutic importance both mediate vascular responses. Prejunctional H3 receptors exist in brain and the periphery. 

Histamine was synthesized in 1907 and later isolated from mammalian tissues. Early hypotheses concerning the possible physiologic roles of tissue histamine were based on similarities between the effects of intravenously administered histamine and the symptoms of anaphylactic shock and tissue injury. Marked species variation is observed, but in humans histamine is an important mediator of immediate allergic (such as urticaria) and inflammatory reactions, although it plays only a modest role in anaphylaxis. Histamine plays an important role in gastric acid secretion (see Chapter 62) and functions as a neurotransmitter and neuromodulator (see Chapters 6 and 21). Newer evidence indicates that histamine also plays a role in chemotaxis of white blood cells. 

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