Organophosphorus pesticides decrease M2 muscarinic receptor function in guinea pig airway nerves via indirect...

Area of Science:

• Environmental toxicology
• Neuroscience
• Respiratory medicine

Background:

• Organophosphorus pesticide (OP) exposure is linked to asthma.
• OPs cause airway hyperreactivity in guinea pigs via M2 muscarinic receptor dysfunction, independent of acetylcholinesterase (AChE) inhibition.
• The precise mechanism by which OPs inhibit neuronal M2 receptors in airways remains unclear.

Purpose of the Study:

• To investigate whether the OP parathion or its metabolite paraoxon directly decrease M2 receptor function on peripheral airway neurons.
• To explore potential direct mechanisms of OP neurotoxicity on M2 receptors.

Main Methods:

• Intravenous administration of paraoxon and parathion in guinea pigs.
• Measurement of vagally-induced bronchoconstriction and electrical field stimulation (EFS)-induced contractions in isolated trachea and ileum.
• Assessment of M2 receptor expression using quantitative RT-PCR and radio-ligand binding assays.
• Phosphorylation target identification using a biotin-labeled fluorophosphonate probe.

Main Results:

• Paraoxon, but not parathion, acutely potentiated vagally-induced bronchoconstriction and EFS-induced tracheal contractions, independent of AChE inhibition.
• Paraoxon did not affect vagally-induced bradycardia or EFS-induced ileum contractions.
• Paraoxon did not alter M2 receptor mRNA or protein levels, and no direct phosphorylation of M2 receptors was observed.

Conclusions:

• Neither direct pharmacologic antagonism nor downregulated M2 receptor expression explains OP inhibition of M2 function in airway nerves.
• These findings support non-cholinergic mechanisms in OP neurotoxicity.
• Further research is needed to elucidate the precise molecular targets and pathways involved in OP-induced airway hyperreactivity.