Circulating methylglyoxal enhances capsaicin-sensitive lung vagal afferent-mediated apneic reflex sensitivity in rats

Methylglyoxal (MG), a reactive metabolic byproduct of glycolysis, is linked to painful diabetic neuropathy. Diabetic patients are accompanied by more frequent exacerbations of asthma. Sensitization of capsaicin-sensitive lung vagal afferents (CSLVAs), the nociceptive-like nerve endings in the respiratory tract, contributes to the pathogenesis of airway hypersensitivity diseases such as asthma. However, whether MG sensitizes CSLVAs and the underlying mechanisms remain unknown. This study was undertaken in male adult Sprague-Dawley rats to investigate 1) whether intravenous administration of MG enhances the apneic reflex response to CSLVA stimulants, and if so, 2) whether CSLVAs play a role in the MG-induced enhancement of apneic reflex response; 3) whether the transient receptor potential ankyrin subtype 1 protein (TRPA1), reactive oxygen species (ROS), and activation of NADPH oxidase contribute to this MG-induced airway reflex hypersensitivity. Our results showed that intravenous administration of MG enhanced apneic response to right-atrial bolus injection of capsaicin in anesthetized, spontaneously breathing rats. This sensitizing effect was found 10min after MG administration and persisted at 90min. Similarly, the apneic reflex response to α,β-methylene-ATP (another CSLVA stimulant) was enhanced by MG. In contrast, saline administration did not augment the apneic reflex responses to these two stimulants. The sensitizing effect of MG on apneic response to stimulants was abolished by PCT of vagi, suggesting the involvement of CSLVAs. Additionally, the sensitizing effect of MG on the CSLVAs-mediated apneic reflex responses to stimulants was significantly attenuated by pretreatment with TRPA1 antagonists (HC030031 and AP18), an antioxidant (N-acetyl-L-cysteine), or an inhibitor of NADPH oxidase (apocynin), whereas their vehicle failed to do so. These results suggest that MG may sensitize CSLVAs and augment airway reflex, at least in part, through the involvement of TRPA1, ROS, and activation of NADPH oxidase.