1. We recently described a capsaicin‐sensitive vagal pathway mediating non‐adrenergic, non‐cholinergic (NANC) relaxations of an isolated, innervated rostral guinea‐pig tracheal preparation. These afferent fibres are carried by the superior laryngeal nerves and relaxations elicited by their activation are insensitive to autonomic ganglion blockers such as hexamethonium. In the present study this vagal relaxant pathway was further characterized. 2. Relaxations of the trachealis elicited by electrical stimulation of capsaicin‐sensitive vagal afferents were mimicked by bath application of capsaicin. Relaxations elicited by both methods were abolished when the tissue between the trachea and the adjacent oesophagus was disrupted. Indeed, separating the trachea from the oesophagus uncovered a contractile effect of capsaicin administration on the trachealis. 3. Capsaicin‐induced, oesophagus‐dependent relaxations of the trachealis were blocked by pretreatment with the fast sodium channel blocker tetrodotoxin (TTX). By contrast, capsaicin‐induced contractions of the trachealis (obtained in the absence of the oesophagus) were unaffected by tetrodotoxin. 4. Substance P, neurokinin A (NKA) and neurokinin B (NKB) also elicited NANC relaxations of precontracted trachealis that were abolished by separating the trachea from the oesophagus or by TTX pretreatment. Like capsaicin, the tachykinins elicited only contractions of the trachealis following TTX pretreatment or separation of the trachea from the adjacent oesophagus. 5. Relaxations elicited by stimulation of the capsaicin‐sensitive nerves were unaffected by a concentration of the tachykinin NK2 receptor‐selective antagonist, SR 48968, that is selective for NK2 receptor blockade and were not mimicked by the NK2 receptor‐selective agonist [beta‐Ala8]‐NKA(4‐10). This suggests that NK2 receptors are not responsible for these relaxations. By contrast, the NK3 receptor‐selective agonist, senktide analogue, and the NK1 receptor‐selective agonist, acetyl‐[Arg6, Sar9, Met (O2)11]‐SP(6‐11), elicited oesophagus‐dependent relaxations of the trachealis that were abolished by oesophagus removal. Furthermore, pretreatment with the NK1‐selective antagonists, CP 96345 and CP 99994, or pretreatment with a concentration of SR 48968 that also blocks NK3 receptors, markedly attenuated relaxations elicited by stimulation of the capsaicin‐sensitive vagal pathways. 6. The data are consistent with the hypothesis that relaxations elicited by stimulation of capsaicin‐sensitive vagal afferents involve tachykinin‐mediated activation of peripheral NANC inhibitory neurones that are in some way associated with the oesophagus. The data also indicate that airway smooth muscle tone might be regulated by peripheral reflexes initiated by activation of capsaicin‐sensitive afferent fibres.
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