Virus- and interferon-induced loss of inhibitory M₂ muscarinic receptor function and gene expression in cultured airway parasympathetic neurons

Viral infections increase vagally mediated reflex broncho-constriction. Decreased function of inhibitory M₂ muscarinic receptors on the parasympathetic nerve endings is likely to contribute to increased acetylcholine release. In this study, we used cultured airway parasympathetic neurons to determine the effects of parainfluenza virus and of interferon (IFN)-γ on acetylcholine release, inhibitory M₂ receptor function, and M₂ receptor gene expression. In control cultures, electrically stimulated acetylcholine release increased when the inhibitory M₂ receptors were blocked using atropine (10^-5 M) and decreased when these receptors were stimulated using methacholine (10^-5 M). Acetylcholine release was increased by viral infection and by treatment with IFN-γ (300 U/ml). In these cells, atropine did not further potentiate, nor did methacholine inhibit, acetylcholine release, suggesting decreased inhibitory M₂ receptor function and/or expression. Using a competitive reverse transcription-polymerase chain reaction method, we demonstrated that M₂ receptor gene expression was decreased by more that an order of magnitude both by virus infection and by treatment with IFN. Thus, viral infections may increase vagally mediated bronchoconstriction both by directly inhibiting M₂ receptor gene expression and by causing release of IFN-γ which inhibits M₂ receptor gene expression.