Organophosphorus Pesticides Decrease M2 Muscarinic Receptor Function in Guinea Pig Airway Nerves via Indirect Mechanisms

Becky J Proskocil,Pamela J Lein,Allison D Fryer,Donald A Bruun,Charles M Thompson,Carol Feghali-Bostwick

doi:10.1371/journal.pone.0010562

Becky J Proskocil, Pamela J Lein + Show 4 more

Open Access

https://doi.org/10.1371/journal.pone.0010562

Copy DOI

Abstract

BackgroundEpidemiological studies link organophosphorus pesticide (OP) exposures to asthma, and we have shown that the OPs chlorpyrifos, diazinon and parathion cause airway hyperreactivity in guinea pigs 24 hr after a single subcutaneous injection. OP-induced airway hyperreactivity involves M2 muscarinic receptor dysfunction on airway nerves independent of acetylcholinesterase (AChE) inhibition, but how OPs inhibit neuronal M2 receptors in airways is not known. In the central nervous system, OPs interact directly with neurons to alter muscarinic receptor function or expression; therefore, in this study we tested whether the OP parathion or its oxon metabolite, paraoxon, might decrease M2 receptor function on peripheral neurons via similar direct mechanisms.Methodology/Principal FindingsIntravenous administration of paraoxon, but not parathion, caused acute frequency-dependent potentiation of vagally-induced bronchoconstriction and increased electrical field stimulation (EFS)-induced contractions in isolated trachea independent of AChE inhibition. However, paraoxon had no effect on vagally-induced bradycardia in intact guinea pigs or EFS-induced contractions in isolated ileum, suggesting mechanisms other than pharmacologic antagonism of M2 receptors. Paraoxon did not alter M2 receptor expression in cultured cells at the mRNA or protein level as determined by quantitative RT-PCR and radio-ligand binding assays, respectively. Additionally, a biotin-labeled fluorophosphonate, which was used as a probe to identify molecular targets phosphorylated by OPs, did not phosphorylate proteins in guinea pig cardiac membranes that were recognized by M2 receptor antibodies.Conclusions/SignificanceThese data indicate that neither direct pharmacologic antagonism nor downregulated expression of M2 receptors contributes to OP inhibition of M2 function in airway nerves, adding to the growing evidence of non-cholinergic mechanisms of OP neurotoxicity.

Highlights

Asthma prevalence and severity has increased over the past two decades, with the greatest increase occurring in children and adolescents living in urban environments [1,2]
organophosphorus pesticide (OP)-induced inhibition of M2 receptor function in airway nerves and the consequent airway hyperreactivity is consistent with previous studies demonstrating that neuronal M2 receptors are dysfunctional in animal models of antigen, virus- or ozoneinduced airway hyperreactivity [36,37,38], and in patients with asthma [39]
Electrical stimulation of both vagus nerves induced reproducible bronchconstrictions that were frequency dependent and atropine sensitive, demonstrating that this effect was mediated by release of ACh onto postjunctional muscarinic receptors

Summary

Introduction

Asthma prevalence and severity has increased over the past two decades, with the greatest increase occurring in children and adolescents living in urban environments [1,2]. Epidemiological and clinical studies have linked OP exposures to symptoms associated with asthma including airway hyperreactivity and wheezing [15,16,17,18,19,20,21,22,23] Consistent with these findings from human studies, it has been reported that OPs induce bronchospasm in a variety of animals [24,25], and we have recently established that the OPs chlorpyrifos, parathion and diazinon induce airway hyperreactivity in a guinea pig model [26,27]. The mechanism involved decreased function of autoinhibitory M2 muscarinic receptors on the parasympathetic nerves supplying airway smooth muscle [26,27] Activation of these M2 receptors functions as a negative feedback mechanism, decreasing ACh release from prejunctional parasympathetic nerves to limit vagally-induced bronchoconstriction [31,34,35]. In the central nervous system, OPs interact directly with neurons to alter muscarinic receptor function or expression; in this study we tested whether the OP parathion or its oxon metabolite, paraoxon, might decrease M2 receptor function on peripheral neurons via similar direct mechanisms

Methods

Results

Conclusion