Activation of brain stem neurons by irritant chemical stimulation of the throat assessed by c-fos immunohistochemistry.

The method of c-fos immunohistochemistry was used to identify the brain stem distribution of neurons activated following irritant chemical stimulation of the laryngopharyngeal mucosa. In pentobarbital-anesthetized rats, either water (control), nicotine (600 mM, 1 ml) or capsaicin (330 microM, 1 ml) was applied to the pharynx via a cannula placed posterior to the soft palate. Following nicotine and capsaicin, there was a significant increase in fos-like immunoreactivity (FLI) compared with controls in the following areas: nucleus of the solitary tract from the level of the pyramidal decussation caudally to the level of the area postrema rostrally; dorsomedial aspect of trigeminal subnucleus caudalis (Vc); and paratrigeminal islands interspersed in the spinal trigeminal tract. There was significantly more FLI in Vc and paratrigeminal nuclei following capsaicin than following nicotine, while the reverse was true for NTS. In addition, there was a significant increase in FLI in area postrema and the ventrolateral medullary region dorsal to the lateral reticular nucleus following nicotine but not capsaicin. The distributions of FLI in NTS, area postrema, Vc, and paratrigeminal nuclei are consistent with prior anatomical tract-tracing studies and suggest roles for these brain stem regions in mediating sensory and reflex responses to irritant chemical stimulation of the upper respiratory mucosa.