Relaxant innervation of the guinea‐pig trachealis: demonstration of capsaicin‐sensitive and ‐insensitive vagal pathways.

Abstract

SUMMARY1. The guinea-pig trachea was isolated with its extrinsic innervation intact and placed in a water-jacketed dissecting dish containing warmed, oxygenated Krebs solution. The trachea was not separated from the oesophagus. Two adjacent cartilage rings of the rostral portion of the trachea were cut open opposite the trachealis and prepared for isometric tension measurements.2. Following the addition of atropine and contraction of the trachealis with prostaglandin F2a (PGF2.), stimulation of the cervical sympathetic trunks elicited relaxations that were abolished by propranolol or hexamethonium. Stimulation of the vagus nerves caudal to the nodose ganglia also elicited relaxations. These vagally mediated relaxations were unaffected by propranolol but were abolished by hexamethonium or by cutting the recurrent laryngeal nerves. 3. After cutting the vagi caudal to the nodose ganglia, stimulation of the vagi rostral to the nodose ganglia elicited relaxations of the trachealis that were not significantly affected by either propranolol or hexamethonium but were abolished by cutting the superior laryngeal nerves. Stimulation of right vagi which had undergone supranodose vagotomy 14 days prior to experimentation was without effect on the smooth muscle of the guinea-pig trachea while the response to stimulation of the left vagus was unchanged.4. Acute capsaicin desensitization abolished relaxations of the guinea-pig trachealis elicited by stimulation of the vagal fibres carried by the superior laryngeal nerves. In contrast, capsaicin desensitization only modestly inhibited relaxations elicited by stimulation of the preganglionic parasympathetic fibres carried by the recurrent laryngeal nerves and had no effect on sympathetic nerve-induced relaxations.5. Removing the oesophagus selectively abolished relaxations elicited by stimulation of both vagal pathways of non-adrenergic relaxant innervation. Nonadrenergic relaxations of the trachealis elicited by electrical field stimulation were unaffected by removing the oesophagus. Oesophagus removal also had no effect on the parasympathetic-cholinergic contractile innervation or the sympathetic relaxant innervation of the trachealis.* To whom correspondence should be addressed. MS 1153B. J. CANVING AND B. J. UTNDEM 6. The results indicate that the guinea-pig trachealis receives non-adrenergic relaxant innervation from both parasympathetic and capsaicin-sensitive vagal pathways. The results also suggest that the neurones mediating non-adrenergic relaxations of the trachea are sensitive to oesophagus removal. The observation that oesophagus removal abolishes parasympathetic relaxations of the trachealis while having no effect on parasympathetic contractions supports the hypothesis that the guinea-pig trachealis receives excitatory and inhibitory innervation from distinct vagal parasympathetic pathways.

Evidence that antidromically stimulated vagal afferents activate inhibitory neurones innervating guinea‐pig trachealis.

“…The trachea and oesophagus of each animal were isolated with their extrinsic innervation intact and pinned dorsal-side down to the bottom of a water-jacketed dissecting dish continuously overfilled (20 ml min-') with warmed (37°C), oxygenated Krebs solution of the following composition (mM): NaCl, 118; KCl, 5-4; NaH2PO4, 1; MgSO4, 1 2; CaCl2, 1-9; NaHCO3, 25; dextrose, 11-1. In some preparations two adjacent cartilage rings of the rostral trachea (rings 6 and 7 caudal to the larynx) were cut open opposite the trachealis and prepared for isometric tension measurements (see Canning & Undem, 1993a). In other preparations, isometric tension was measured in a rostral segment of intact tracheal tubes as previously described (Canning & Undem, 1993b).…”

“…We previously reported that the capsaicin-sensitive fibres mediating NANC relaxations of the trachealis are carried by the superior laryngeal nerves (Canning & Undem, 1993a). Accordingly, this pathway (which we will henceforth call the capsaicin-sensitive vagal pathway) was selectively stimulated by cutting the vagus nerves caudal to the nodose ganglia and stimulating them bilaterally rostral to the ganglia with suction electrodes (World Precision Instruments, Sarasota, FL, USA).…”

“…The electrodes were attached to a Grass stimulator (Model S44) that delivered rectangular pulses of current at supramaximal stimulus intensities (150 V, 1 ms pulse duration) at a maximal frequency (24 Hz) for 10 s (see Canning & Undem, 1993b for rationale behind stimulus intensities). In some experiments parasympathetic relaxations of the trachealis were elicited by stimulating the vagi caudal to the nodose ganglia as previously described (Canning & Undem, 1993a, b).…”

“…Using an isolated, innervated guinea-pig tracheal preparation we have demonstrated that vagally mediated, parasympathetic nerve-induced relaxations of guinea-pig trachealis are dependent upon the integrity of the tissue between the trachea and the adjacent oesophagus (Canning & Undem, 1993a). Disrupting the tissue between the trachea and oesophagus in any way renders the vagi incapable of mediating NANC relaxations of the trachealis.…”

“…This pathway is also anatomically distinguishable from the parasympathetic pathway. Thus, the preganglionic parasympathetic fibres are carried by the recurrent laryngeal nerves, whereas the capsaicin-sensitive pathways form part of the superior laryngeal nerves (Canning & Undem, 1993a). Like the parasympathetic relaxant innervation, however, the capsaicin-sensitive relaxant innervation is disrupted when the trachea is separated from the adjacent oesophagus.…”