Perivagal antagonist treatment in rats selectively blocks the reflex and afferent responses of vagal lung C fibers to intravenous agonists

Abstract

The terminals of vagal lung C fibers (VLCFs) express various types of pharmacological receptors that are important to the elicitation of airway reflexes and the development of airway hypersensitivity. We investigated the blockade of the reflex and afferent responses of VLCFs to intravenous injections of agonists using perivagal treatment with antagonists (PAT) targeting the transient receptor potential vanilloid 1, P2X, and 5-HT(3) receptors in anesthetized rats. Blockading these responses via perivagal capsaicin treatment (PCT), which blocks the neural conduction of C fibers, was also studied. We used capsaicin, α,β-methylene-ATP, and phenylbiguanide as the agonists, and capsazepine, iso-pyridoxalphosphate-6-azophenyl-2',5'-disulfonate, and tropisetron as the antagonists of transient receptor potential vanilloid 1, P2X, and 5-HT(3) receptors, respectively. We found that each of the PATs abolished the VLCF-mediated reflex apnea evoked by the corresponding agonist, while having no effect on the response to other agonists. Perivagal vehicle treatment failed to produce any such blockade. These blockades had partially recovered at 3 h after removal of the PATs. In contrast, PCT abolished the reflex apneic response to all three agonists. Both PATs and PCT did not affect the myelinated afferent-mediated apneic response to lung inflation. Consistently, our electrophysiological studies revealed that each of the PATs prevented the VLCF responses to the corresponding agonist, but not to any other agonist. PCT inevitably prevented the VLCF responses to all three agonists. Thus these PATs selectively blocked the stimulatory action of corresponding agonists on the VLCF terminals via mechanisms that are distinct from those of PCT. PAT may become a novel intervention for studying the pharmacological modulation of VLCFs.

Sensitization by Pulmonary Reactive Oxygen Species of Rat Vagal Lung C-Fibers: The Roles of the TRPV1, TRPA1, and P2X Receptors

“…Notably, the potentiating effect observed at 1 min following inhalation of 0.05% H 2 O 2 had disappeared at 40 min following inhalation (Figure 2A, right panel). Similar results were obtained when α,β-meATP (Figure 2B), a P2X receptor agonist [22], and phenylbiguanide (Figure 2C), a 5-HT 3 receptor agonist [17], were used individually as the VLCF stimulants.…”

“…In this context, our findings, which show that all three receptors, TRPV1, TRPA1, and P2X, take part in the development of this afferent sensitization, suggest that the process is Ca 2+ -dependent; this is because these receptors are non-selective cation channels that are mainly permeable to Ca 2+ [10], [24]. Since capsaicin, α,β-meATP, and phenylbiguanide are TRPV1, P2X, 5-HT 3 receptor agonists, respectively [17], [21], [22], it appears that increased lung ROS produces a nonspecific increase in the electrical excitability of the VLCFs. Alternatively, activation of these receptors by H 2 O 2 may possibly promote a common cellular mechanism that sensitizes the functioning of these ionotropic receptors.…”

“…For stimulation of the VLCFs, capsaicin (0.5 µg/kg; a TRPV1 receptor agonist) [21], α,β-methylene-ATP (α,β-meATP; 10 µg/kg; a P2X receptor agonist) [22], and phenylbiguanide (5 µg/kg; a 5-HT 3 receptor agonist) [17] with a volume of 0.1 ml were injected as a bolus into the right atrium; these drugs were first injected into the catheter (dead space = ∼0.2 ml) and then flushed into the circulation by an injection of 0.4 of ml saline. Receptor antagonists for TRPV1 (capsazepine; 3 mg/kg) [21], TRPA1 (HC-030031; 3 mg/kg;) [18], and P2X receptors [ iso -pyridoxalphosphate-6-azophenyl-2′,5′-disulphonate ( iso -PPADS); 15 mg/kg] [18], as well as dimethylthiourea (a •OH scavenger; 1 g/kg) [36], with a volume of 0.35 ml, were slowly injected into the jugular vein over a 1-min duration.…”

“…An aerosol of catalase or heat-inactivated catalase was generated and inhaled into the lower airways for a period of 5 min using the nebulizer and the circuit for delivery of the H 2 O 2 aerosol [21]. The doses and treatment times of the drugs used in this study in order to be effective were adopted from previous studies [17], [18], [20], [21], [23] or determined in our preliminary study (Supporting Information Table S1). The vehicle for phenylbiguanide, α,β-meATP, iso -PPADS, and dimethylthiourea was saline.…”

“…Investigations targeting the sensory responses of VLCFs to agonists have revealed that their nerve terminals may express a range of pharmacological receptors [9], [10]; these include the transient receptor potential vanilloid 1 (TRPV1), the transient receptor potential ankyrin 1 (TRPA1), and the P2X receptors [17]–[21]. In this context, a direct increase in the lung level of ROS by airway challenge with aerosolized H 2 O 2 has been shown to stimulate VLCFs via activation of the TRPV1, TRPA1, and P2X receptors [20]–[23].…”