| Abstract: | BACKGROUND: Histamine is a key mediator of the allergic immediate reaction. Antihistamines belong to the most frequently used treatment modalities in allergic rhinitis. METHODS: The National Libraray of Medicine was searched for current data of the effects of histamine and antihistamines in allergic rhinitis. RESULTS: Histamine acts on 4 different histamine receptors. Activation of H1-receptors on nasal trigeminal nerve fibers transmits nasal itch and sneezing. Nasal hypersecretion is mainly mediated by an trigeminal-parasympathetic reflex. Activation of H1-receptors results in contraction of nasal endothelial cells with consecutive plasma extravasation and edema formation. Histamine also activates H2-receptors on smooth muscle cells surrounding nasal capacitance vessels. They transmit muscle relaxation, increased blood content and an enlarged volume of nasal mucosa. Via peripheral H3-receptors, histamine modulates neurogenic inflammation and via H4-receptors functions of immune cells. Oral second generation antihistamines inhibit histamine dependent activation of nasal H1-receptors. They mainly reduce nasal itch, sneezing, and hypersecretion. In addition, allergy related activity impairment is reduced resulting in improved physical and mental performance. Second generation antihistamines reduce proinflammatory effects mediated by H1-receptors, however, drug concentrations necessary for mast cell stabilization as observed in vitro are not reached in vivo. Oral second generation antihistamines are readily absorbed and reduce allergy symptoms for approximately 24 hours, allowing convenient once daily medication. Modern antihistamines are generally safe; tachyphylaxis, tolerance or rebound has not been observed. CONCLUSION: Due to their minimal adverse effects and efficient symptom reduction oral second generation antihistamines are particularly useful for the treatment of less severe intermittent forms of nasal allergy. |
