The signs of airway
inflammation and hyperresponsiveness that occur in animals exposed to
air pollutants are often strain- and species-specific. To investigate the underlying causes of this phenomenon, BALB/c and C57bl/6 mice were exposed intratracheally to residual
oil fly ash (ROFA, 3 mg/kg) and examined after 24 h for signs of airway
inflammation. BALB/c showed significantly higher numbers of neutrophils and increased
airway hyperresponsiveness in response to
methacholine challenge, whereas B6 mice showed no significant change in either inflammatory endpoint. To determine the underlying cause of this strain specificity, cultures of dorsal root ganglion (DRG) sensory neurons, which innervate the upper airways in situ, were explanted from both BALB/c and B6 fetal mice. After 5-7 days in culture, they were exposed to ROFA, other urban and industrial
particulate matter (PM; e.g.,
oil fly ash, woodstove, Mt. St. Helen, St. Louis, Ottawa,
coal fly ash) or to prototype irritants (e.g.,
capsaicin 3-10 microM, pH 5.0 and 6.5). In all instances (except for woodstove), DRG neurons from BALB/c mice released significantly higher levels of the pro-inflammatory
cytokine IL-6 into their nutrient media relative to neurons from B6 mice. This
cytokine release could be significantly reduced for all PM treated cultures (except woodstove) by pretreatment of cultures with
capsazepine (CPZ), a competitive antagonist of
vanilloid receptors. DRG neurons, cultured from BALB/c and B6 neonates, were loaded with
Fluo-3 AM and exposed to the prototype irritants,
acid pH (5.0, 6.5), or
capsaicin (3, 10 microM). Analysis of their increases in intracellular
calcium showed that significantly higher numbers of BALB/c neurons responded to these prototype irritants, relative to B6 neurons. Morphometric analysis of BALB/c neurons, histochemically stained with
cobalt to label neurons bearing
capsaicin-sensitive receptors, showed a significantly higher level of stained neurons relative to B6 neurons. Finally, semiquantitative RT-PCR showed a higher expression of
VR1 receptor mRNA in DRG and spinal cord taken from neonatal BALB/c mice relative to B6 mice. Taken together, these data suggest that
capsaicin and
acid-sensitive
irritant receptors, located on somatosensory cell bodies and their nerve fiber terminals, subserve PM-induced airway
inflammation and are quantitatively different in responsive and nonresponsive mouse strains.