Involvement of nitric oxide (NO) in cough reflex sensitivity between non-sensitized and OVA-sensitized guinea pigs

BACKGROUND Exhaled nitric oxide (ENO) is elevated in bronchial asthma patients, and inhaled corticosteroid therapy lowers the elevated ENO levels in such patients. ENO appears to be an inflammatory marker, but its role in the pathophysiology of cough remains unclear. This study aimed to elucidate the relationship between NO and increased cough reflex sensitivity induced by allergic airway reactions. METHODS Cough reflex sensitivity to inhaled capsaicin was observed under NO depletion caused by NO synthase (NOS) inhibitors in non-sensitized and ovalbumin (OVA)-sensitized guinea pigs. The bronchoalveolar lavage fluid (BALF) was analyzed in an NO depletion setting using the inducible NOS (iNOS) inhibitor ONO1714 in OVA-sensitized guinea pigs. RESULTS NO depletion by the non-selective NOS inhibitor L-NAME suppressed cough reflex sensitivity in non-sensitized guinea pigs and OVA-induced increase in cough reflex sensitivity in sensitized guinea pigs; however, iNOS inhibition caused by ONO1714 partially suppressed the OVA-induced increase in cough reflex sensitivity, but not the normal cough response in non-sensitized guinea pigs. ONO1714 did not change BAL cell components in OVA-sensitized guinea pigs. CONCLUSIONS The results suggest that NO may be involved not only in the normal cough reflex circuit, but also in the OVA-induced increase in cough reflex sensitivity, possibly via a different mechanism of action. Further studies are needed to clarify the precise mechanism.