Expression of tachykinins in nonnociceptive vagal afferent neurons during respiratory viral infection in guinea pigs

Immunohistochemistry was combined with retrograde labeling to characterize the effect of respiratory infection with Sendai virus on the number of Substance P/Neurokinin A-containing vagal afferent neurons whose cell bodies resided in the nodose ganglia and whose receptive fields were located in guinea pig trachea. Of the neurons labeled from the trachea of vehicle-inoculated guinea pigs, few stained positively for Substance P/Neurokinin A (∼ 3% of total labeled neurons). These neurons had small diameter cell bodies (mode = 16-20 μm), a feature of nociceptive-like C-fibers. Viral infection (Day 4 after inoculation) was associated with a significantly greater number of labeled neurons containing Substance P/Neurokinin A (∼ 20% of total labeled neurons). The majority of these had a relatively large cell body diameter (mode = 36-40 μm), a feature of nonnociceptive afferent neurons. This induction appeared to be reversible as there were significantly fewer Substance P/Neurokinin A positive neurons in nodose ganglia from virus-inoculated guinea pigs at Day 28 after inoculation, a time point when virus-induced airway inflammation had all but resolved. These findings support the hypothesis that viral infection leads to a qualitative change in the vagal afferent innervation of guinea pig airways such that both small diameter nociceptive-like neurons and large diameter nonnociceptive neurons express tachykinins.