Immunohistochemical localization of vanilloid receptor subtype 1 (TRPV1) in the guinea pig respiratory system

Transient receptor potential vanilloid-1 (TRPV1) containing nerves are implicated in cough and bronchoconstriction although the significance of their documentation on non-neuronal cells is unclear. We have investigated the anatomical distribution and location of TRPV1 in an animal species often utilized in models of cough and airway inflammation. The distribution and localization of TRPV1 immunoreactivity in the lung was studied using confocal microscopy. Double labelling were carried out using the panaxonal marker, protein gene product 9.5 (PGP) and the neuropeptide substance P. TRPV1 was localized to fine axons within the epithelium of the trachea, however this represented only a fraction of the total axonal innervation of the epithelium. TRPV1 immunoreactive axons were also found in and around subepithelial regions of the airways, including smooth muscle and blood vessels and within the lower airways, found in the vicinity of bronchi and bronchioles, and in and around alveolar tissue. TRPV1 in the epithelium of the trachea was co-localized with substance P containing axons, although TRPV1 immunoreactive neuropeptide negative axons were also discernible. We found evidence for TRPV1 localization to axons throughout the respiratory tract. The distribution was heterogeneous and represented a fraction of the total neuronal innervation of the airways. No TRPV1 was found localized to airway epithelial cells. TRPV1 was often co-localized with the sensory neuropeptide substance P but there was evidence of TRPV1 positive neurones that did not express substance P. This suggests a role for TRPV1 in the airway that is independent of sensory neuropeptides.