Respiratory tract infections increase susceptibility to neurogenic inflammation in the rat trachea

This study reveals that respiratory tract infections make the tracheal mucosa of rats more susceptible to neurogenic inflammation, which is a type of inflammation mediated by neuropeptides released from sensory nerves. Neurogenic inflammation was produced in the tracheas of 2 groups of Long-Evans rats by electrical stimulation of the vagus nerve (5 V, 1 ms, 20 Hz for 5 min) or by an injection of capsaicin (15 to 200 micrograms/kg i.v.) or substance P (0.05 to 5.0 micrograms/kg i.v.). Rats of one group were pathogen-free; the others had serologic evidence of naturally occurring airway infections caused by Sendai virus, coronavirus, and Mycoplasma pulmonis. The stimuli produced neurogenic inflammation in both groups of rats, but the magnitude of this inflammation was much greater in the infected rats. The susceptibility of the infected rats to neurogenic inflammation was manifested by a 2.0 to 3.1 times larger increase in vascular permeability to Monastral blue, 5 times larger increase in number of neutrophils adhering to the endothelium of venules, and conspicuous morphologic changes in the tracheal epithelium. When pathogen-free rats acquired respiratory tract infections, they too became susceptible to neurogenic inflammation. Other experiments showed that infection by Sendai virus was essential for the change, although infection by M. pulmonis or coronavirus may also be necessary. The susceptibility to neurogenic inflammation outlasted the transient pathologic changes caused in the airway mucosa by the viral infections and may have been permanent.