Asthmatic Augmentation of Airway Vagal Activity Involves Decreased Central Expression and Activity of CD73 in Rats

Endothelin-1-mediated Brainstem Glial Activation Produces Asthmatic Airway Vagal Hypertonia Via Enhanced ATP-P2X4 Receptor Signaling in Sprague-Dawley Rats

The occurrence of major asthma symptoms is largely attributed to airway vagal hypertonia, of which the central mechanisms remain unclear. This study tests the hypotheses that endothelin-1-mediated brainstem glial activation produces asthmatic airway vagal hypertonia via enhanced action of adenosine 5'-triphosphate on neuronal purinergic P2X4 receptors. A rat model of asthma was prepared using ovalbumin. Airway vagal tone was evaluated by the recurrent laryngeal discharge and plethysmographic measurement of pulmonary function. The changes in the brainstem were examined using ELISA, Western blot, luciferin-luciferase, quantitative reverse transcription-polymerase chain reaction, enzyme activity assay and immunofluorescent staining, respectively. The results showed that in the medulla of rats, endothelin receptor type B and P2X4 receptors were primarily expressed in astrocytes and neurons, respectively, and both of which, along with endothelin-1 content, were significantly increased after ovalbumin sensitization. Ovalbumin sensitization significantly increased recurrent laryngeal discharge, which was blocked by acute intracisternal injection of P2X4 receptor antagonist 5-BDBD, knockdown of brainstem P2X4 receptors, and chronic intraperitoneal injection of endothelin receptor type B antagonist BQ788, respectively. Ovalbumin sensitization activated microglia and astrocytes and significantly decreased ecto-5'-nucleotidase activity in the medulla, and all of which, together with the increase of medullary P2X4 receptor expression and decrease of pulmonary function, were reversed by chronic BQ788 treatment. These results demonstrated that in rats, allergic airway challenge activates both microglia and astrocytes in the medulla via enhanced endothelin-1/endothelin receptor type B signaling, which subsequently causes airway vagal hypertonia via augmented adenosine 5'-triphosphate/P2X4 receptor signaling in central neurons of airway vagal reflex.

Moxonidine inhibits excitatory inputs to airway vagal preganglionic neurons via activation of both α2-adrenoceptors and imidazoline I1 receptors

As an imidazoline I1 receptor agonist with very weak binding affinity for α₂-adrenoceptors, moxonidine is commonly used in the treatment of hypertension. Moxonidine also has been implicated to act centrally to reduce airway vagal outflow. However, it is unknown at which central sites moxonidine acts to affect airway vagal activity, and how moxonidine takes effect at synaptic and receptor levels. In this study, airway vagal preganglionic neurons (AVPNs) were retrogradely labeled in neonatal rats from the intrathoracic trachea; retrogradely labeled AVPNs in the external formation of the nucleus ambiguus (NA) were identified in rhythmically active medullary slices using whole-cell patch-clamp techniques; and the effects of moxonidine on the spontaneous excitatory postsynaptic currents (EPSCs) of AVPNs were observed at synaptic level. The results show that moxonidine (10 μmol·L^-1) significantly inhibited the frequency of spontaneous EPSCs in both inspiratory-activated and inspiratory-inhibited AVPNs. This effect was partially blocked by SKF-86466 (10 μmol·L^-1), a highly selective antagonist of α₂-adrenoceptors, or AGN-192403, a selective antagonist of imidazoline I1 receptors, and was completely blocked by efaroxan (10 μmol·L^-1), an antagonist of both α₂-adrenoceptors and imidazoline I1 receptors. These results demonstrate that moxonidine inhibits the excitatory inputs to AVPNs via activation of both α₂-adrenoceptors and imidazoline I1 receptors, and suggest that physiologically both of these two types of receptors are involved in the central regulation of airway vagal activity at preganglionic level. Moxonidine might be potentially useful in diseases with aberrant airway vagal activity such as asthma and chronic obstructive diseases.

Asthmatic Airway Vagal Hypertonia Involves Chloride Dyshomeostasis of Preganglionic Neurons in Rats

Airway vagal hypertonia is closely related to the severity of asthma; however, the mechanisms of its genesis are unclear. This study aims to prove that asthmatic airway vagal hypertonia involves neuronal Cl^– dyshomeostasis. The experimental airway allergy model was prepared with ovalbumin in male adult Sprague-Dawley rats. Plethysmography was used to evaluate airway vagal response to intracisternally injected γ-aminobutyric acid (GABA). Immunofluorescent staining and Western-blot assay were used to examine the expression of microglia-specific proteins, Na⁺-K⁺-2Cl^– co-transporter 1 (NKCC1), K⁺-Cl^– co-transporter 2 (KCC2) and brain-derived nerve growth factor (BDNF) in airway vagal centers. Pulmonary inflammatory changes were examined with hematoxylin and eosin staining of lung sections and ELISA assay of ovalbumin-specific IgE in bronchoalveolar lavage fluid (BALF). The results showed that histochemically, experimental airway allergy activated microglia, upregulated NKCC1, downregulated KCC2, and increased the content of BDNF in airway vagal centers. Functionally, experimental airway allergy augmented the excitatory airway vagal response to intracisternally injected GABA, which was attenuated by intracisternally pre-injected NKCC1 inhibitor bumetanide. All of the changes induced by experimental airway allergy were prevented or mitigated by chronic intracerebroventricular or intraperitoneal injection of minocycline, an inhibitor of microglia activation. These results demonstrate that experimental airway allergy augments the excitatory response of airway vagal centers to GABA, which might be the result of neuronal Cl^– dyshomeostasis subsequent to microglia activation, increased BDNF release and altered expression of Cl^– transporters. Cl^– dyshomeostasis in airway vagal centers might contribute to the genesis of airway vagal hypertonia in asthma.