The role of tachykinins on bacterial infections.

Tachykinins represent a family of neuropeptides sharing similar C-terminus sequences, but exhibiting preferential binding to one of three receptors called neurokinin receptors (NK-R). While known for its role in contracting smooth muscle or acting as a pain signal neurotransmitter, substance P (SP) and other tachykinins can directly influence immune responses. Studies from the early 1980s revealed that human lymphocytes bore NK-R, but it remains unclear, even to-date, why such receptors are expressed on leukocytes. Nerve tracing studies have provided some speculation that the nervous system can assist the immune system in stimulating an immune response dependent upon which neuropeptide-bearing fibers infiltrate specific lymphoid structures. Such observations have important implications for regulating mucosal responses given that tachykinin-bearing nerve fibers extensively innervate the gut, and SP concentrations in the gut are second only to the brain. Such evidence suggests that SP and related neuropeptides may be important in controlling bacterial infections of the gut. This is shown by blocking SP action in which mice show increased susceptibility to Salmonella infections since induction of IFN-gamma is significantly reduced. In addition, the absence or its presence of SP's or the newly discovered lymphocyte-derived neurokinin called hemokinin's action can modify host IgA responses. Thus, tachykinins introduce new circuits to immune regulation suggesting that these neuropeptides exhibit cytokine- and chemokine-like action.