Figure 6. Neurokinin receptor antagonists have no effect on cough evoked electrically or by citric acid in control animals.

Cough was evoked from the tracheal mucosa electrically (16 Hz, 10 s train, 1 ms pulse duration, 1–10 V) (A) and subsequently by citric acid (0.001–2 m) applied topically to the tracheal mucosa (B). Animals were first pretreated intraperitoneally either with vehicle (dimethyl sulfoxide; n = 6) or with a combination of three neurokinin receptor antagonists CP99994, SR48968 and SB223412 (1 mg kg⁻¹ each, n = 5). The antagonists (administered at doses known to abolish C-fibre-mediated reflexes in guinea pigs; Bolser et al. 1997; Canning et al. 2001; Mazzone & Canning, 2002a) had no effect on the percentage of animals coughing in response to any stimulation voltage or citric acid challenge and had no effect on the tussigenic potency or efficacy of citric acid (P > 0.1). Moreover, in these animals, which were not challenged with capsaicin prior to evoking cough (see Figs 7 and 8), the antagonists had no effect on the peak expiratory pressures during cough evoked either electrically or by citric acid (the peak expiratory pressures of coughs evoked following antagonist administration averaged 106 ± 13% of that evoked in stimulation (voltage or citric acid concentration) matched controls; P > 0.1).