The contribution of the nervous system to inflammation and inflammatory disease

Recent studies have identified a major contribution of the nervous system to inflammation and to inflammatory disease. In particular, substances released from the peripheral terminals of small diameter primary afferent fibers and from sympathetic postganglionic nerve (SPGN) terminals have been implicated in several of the major components of acute inflammation (e.g., vasodilatation and plasma extravasation) as well as in the regulation of tissue injury in an inflammatory disease model, experimental arthritis in the rat. Although the release of peptides from primary afferent terminals has received the most attention, our studies have established an important contribution of mast cells and the SPGN terminals to acute inflammation. We describe studies which indicate that plasma extravasation provoked by activation of small diameter primary afferents in the knee joint of the rat involves a cascade of events in which the mast cell and then the sympathetic terminal are sequentially activated. Our studies indicate that release of prostaglandins, but neither norepinephrine nor neuropeptide Y, from the SPGN terminal contributes to increased plasma extravasation. Although activation of the SPGN terminal (via the mast cell) or more directly, via injection of bradykinin, increased plasma extravasation, surgical or pharmacological sympathectomy decreased the severity of experimental arthritis. In related studies we demonstrated that adrenal medullary-derived epinephrine can exacerbate arthritis through a beta-receptor-mediated regulation of the release of an as yet unidentified substance(s) from the SPGN terminal. Our results raise important questions as to whether acute inflammation contributes to tissue repair or to further injury in the setting of disease.