Airway hyperresponsiveness (AHR) is present in almost all patients with symptomatic
asthma, yet its mechanism is not well understood. Airway
inflammation is thought to be an important underlying mechanism involved in causing AHR. Recent studies indicate release of
neuropeptides from C-fiber endings plays a pivotal role in airway
inflammation.
Substance P (SP) is a critical
neurotransmitter of sensory C-fiber and a well-known effector of inflammatory response. However, roles of other
neuropeptides and interaction among these
neuropeptides in airway
inflammation and AHR were largely unknown.
Calcitonin gene-related peptide (CGRP), another intrapulmonary
neuropeptide that functions as a potent
vasodilator and neutrophils activator, is released from the same C-fiber ending as SP is released. By using an
ozone-stressing animal model, previously we had demonstrated that CGRP might be involved in the development of AHR in rabbits. To extend the functional roles of SP, and to explore the possible interactive roles of SP and CGRP in airway
inflammation, we examined expressions of SP, SP receptor (neurokinin 1, or NK-1R) and CGRP in vivo and ex vivo. We exposed guinea pigs at intervals to inhalation of
ozone to induce airway
inflammation. Animals were sacrificed at different time points; SP, SP receptor and CGRP expression were determined during the onset and progression of airway
inflammation by radioimmunoassay, immunohistochemistry and in situ hybridization. Our data showed that after exposure to
ozone, the concentration of SP in lung homogenate and the number of SP-immunoreactive cell bodies in lung slides increased within 24h, peaked on day 2, and then decreased slowly. Interestingly, CGRP expressions exhibited a similar temporal and spatial pattern, and there was a strong correlation between SP expression and CGRP expression, indicating a possible cooperative action of these two
neuropeptides. We also noted an increased expression of SP receptor NK-1R in the development of airway
inflammation. In order to test the hypothesis that CGRP as a coexisting
neurotransmitter with SP can regulate the expression of NK-1R in the lung, and contribute to the SP-mediated inflammatory response, we used in vitro lung tissue culture to determine the effect of CGRP on NK-1R expression. We found that NK-1R expression was induced by CGRP incubation at both
mRNA and
protein levels, and the induction was attenuated by additions of the inhibitors of
Protein Kinase A (PKA) pathway,
Calmodulin-dependent
Kinase pathway, and
Tyrosine Protein Kinase pathway. In conclusion, our data provide compelling evidence that SP and CGRP are involved in the development of airway
inflammation. The interaction between SP and CGRP is likely to contribute to the pathogenesis of AHR and other lung inflammatory diseases.