Physiology and plasticity of putative cough fibres in the Guinea pig

Cough is initiated by activation of afferent nerve fibers with rapidly adapting receptors (RAR) that conduct action potentials in the Adelta range. In addition, various stimuli that activate airway unmylenated C-fibres evoke cough reflexes. We have used a vagally innervated, larynx-trachea-bronchus preparation, isolated from guinea pigs, to study the pharmacology of RARs and C-fibres in vitro. In this preparation afferent fibres with the RAR phenotype are exquisitely sensitive to mechanical perturbation of their receptive fields, but are unaffected by a variety of mediators (e.g. prostaglandins, histamine, bradykinin, serotonin) and by capsaicin. By contrast, C-fibres are much less sensitive to mechanical stimulation, but can be activated by capsaicin and bradykinin. Preliminary evidence supports the hypothesis that bradykinin activate C-fibre by stimulating the capsaicin (vanilloid) receptor VR1. Acids activate both C-fibres and RARs. Acids stimulate RAR fibres by a mechanism that is rapidly inactivated. C-fibres are stimulated by both a rapidly inactivating mechanism, as well as a slowly inactivating mechanism. Drugs that block VR1 inhibit the latter mechanism. Airway inflammation substantially increases the mechanical sensitivity of RAR fibres without affecting their adaptive properties. Airway inflammation also causes a phenotypic switch in neuropeptide innervation of the airways that RAR neurons begin to synthesis neurokinins and calcitonin gene related peptide. In non-inflamed animals these peptides are expressed only in C-fibre neurons. Thus, airway inflammation may not only increase the sensitivity of cough fibres, but may also qualitatively change the role played by sensory neuropeptides in cough reflexes.