Neural aspects of allergic rhinitis.
PURPOSE OF REVIEW
Recent advances have helped to clarify the role of nerves in allergic rhinitis.


RECENT FINDINGS
Mast cell histamine release activates histamine H1 receptors on a subset of nonmyelinated Type C afferent trigeminal neurons to convey the sensation of itch. The itch nerves may be distinct from those responsible for burning pain and the dull ache of congestion. Activation of brain stem centers leads to cognition of nasal pruritus, generation of the systemic sneeze reflex and recruitment of local nasal parasympathetic reflexes. These cholinergic reflexes stimulate M3 muscarinic receptors on the submucosal glands to cause exocytosis and the thick mucous discharge of allergic rhinitis. The importance of these neural pathways is demonstrated by the benefits of antihistamines to block itch and sneeze, and anticholinergic drugs to block glandular secretion. One open question remains the role of mediators of allergic inflammation on the sensitivity and reactivity of afferent neurons and these secretory reflexes.


SUMMARY
The neurology of the nose helps to explain the sensations encountered in allergic rhinitis and opens new frontiers for drug discovery.