Vagotomy reverses established allergen-induced airway hyperreactivity to methacholine in the mouse

Abstract

We evaluated the role of vagal reflexes in a mouse model of allergen-induced airway hyperreactivity. Mice were actively sensitized to ovalbumin then exposed to the allergen via inhalation. Prior to ovalbumin inhalation, mice also received intratracheally-instilled particulate matter in order to boost the allergic response. In control mice, methacholine (i.v.) caused a dose-dependent increase in respiratory tract resistance (RT) that only modestly decreased if the vagi were severed bilaterally just prior to the methacholine challenge. Sensitized and challenged mice, however, manifested an airway reactivity increase that was abolished by severing the vagi prior to methacholine challenge. In an innervated ex vivo mouse lung model, methacholine selectively evoked action potential discharge in a subset of distension-sensitive A-fibers. These data support the hypothesis that the major component of the increased airway reactivity in inflamed mice is due to a vagal reflex initiated by activation of afferent fibers, even in response to a direct (i.e., smooth muscle)-acting muscarinic agonist.

Keywords: Airway hyperreactivity; Allergic inflammation; Particulate matter; Reflex; Vagotomy; Vagus nerve.

Fig. 1

Allergen and particulate matter exposure causes robust airway inflammation. Cell counts in the broncheoalveolar lavage fluid obtained from mice that were treated with vehicle control (adjuvant, adj/saline, sal) or residual oil fly ash (ROFA) and allergen (ovalbumin, OVA). Solid bars = total cells, open bars = mononuclear cells; checked bars = eosinophils; vertical hashed bars = lymphocytes, and diagonally hashed bars = neutrophils. Each bar represents the mean ± SEM of 7–12 experiments. Asterisk (*) denotes a statistically significant (P < 0.05, ANOVA) difference between cell counts in OVA/ROFA vs. adj/sal groups.

Fig. 2

Vagotomy reverses established airway hyperreactivity in inflamed mice. Change in RT evoked by intravenous administration of methacholine in saline treated animals (O) and allergically inflamed animals treated 1 day earlier with residual fly ash and ovalbumin as described in methods (●). Dose-response curves were administered to mice with vagus nerves left intact (sham, solid lines), and following bilateral vagotomy just prior to methacholine administration (dashed lines). The baseline resistance was not different between the four groups of animals. In control and OVA/ROFA animals with the vagus nerve intact the baseline resistance (cm H₂O s mL⁻¹) averaged 1.8 ± 0.1 (n = 5) and 1.9 ± 0.1 (n = 8), respectively. In control and OVA/ROFA animals with the vagus nerves cut the baseline resistance averaged 1.6 ± 0.2 (n = 5) and 1.9 ± 0.1 (n = 7). Data reflect the mean ± SEM.

Fig. 3

Methacholine activates mouse airway mechanosensitive A-fibers ex vivo. The upper trace is a single unit recording of a vagal A-fiber (conduction velocity = 9.5 m s⁻¹) responding to methacholine (1 μM) with action potential discharge (peak frequency of discharge was 11 Hz). The lower trace is the perfusion pressure (vertical line = 10 cm H₂O). More details are presented in the text.