Neurogenic inflammation. A model for studying efferent actions of sensory nerves.

Several lines of evidence suggest that sensory nerves of the carotid body have an efferent function in addition to their afferent function of conducting chemoreceptive impulses to the brain. However, it has been difficult to document the release of substances from sensory nerve terminals on glomus cells and to determine whether such an efferent function plays a role in chemoreception. By comparison, the phenomenon of neurogenic inflammation has been relatively easy to study in rats and guinea pigs and has proven to be an informative model system for analyzing efferent actions of sensory nerves. The main characteristic of neurogenic inflammation is plasma leakage. Chemical irritants that activate unmyelinated sensory nerves cause plasma leakage in the skin, respiratory tract, and other organs by triggering the release of substances from sensory nerve fibers. Substance P, which is synthesized and released by some sensory neurons, appears to be the main active mediator, although other tachykinins, calcitonin gene-related peptide, and perhaps other peptides may also participate. Neurogenic inflammation results from the action of substance P on NK1 receptors, as demonstrated by selective NK1 receptor agonists and antagonists. The NK1 receptors involved in plasma leakage are located on the endothelial cells of postcapillary venules and collecting venules. Within seconds of the activation of NK1 receptors by substance P, gaps form in the endothelium of target vessels. The endothelial gaps are transient, and the leak normally ends in a few minutes. However, the magnitude of the response can increase in pathological conditions such as Mycoplasma pulmonis infection in rats, which results in a chronic inflammatory disease of the respiratory tract. The infected airway mucosa becomes abnormally vascular as a result of angiogenesis, and the endothelial cells of the newly formed vessels express increased numbers of NK1 receptors and thus are abnormally sensitive to substance P. Studies of neurogenic inflammation not only have helped to understand the efferent actions of sensory nerves but also have given insight into the mechanism and consequences of inflammatory changes in endothelial cells and in the plasma leakage that follows.