Evidence for peripheral mechanisms mediating the antitussive actions of opioids in the guinea pig

1. Comparisons were made between the doses required of aerosol and intraperitoneally administered morphine, dextromethorphan, codeine and the specific peripherally acting μ-receptor agonist DALDA (H-Tyr-D-Arg-Phe-Lys-NH 2) to suppress citric acid-induced coughing in conscious guinea pigs. 2. Estimated ID 50s for inhibition of numbers of coughs induced by an aerosol of 5% citric acid were 1.0 and 2.4 mg/kg for intraperitoneally administered morphine and dextromethorphan, respectively. 3. The estimated ID 50s after inhalation of morphine and dextromethorphan as aerosols were ≈ 2.2 and ≈ 12 μg/kg, respectively. 4. Aerosilized codeine (≈ 72 μg/kg, n = 5) significantly inhibited coughing by 62 ± 23% whereas 3 mg/kg, i.p. was required to significantly reduce coughing by a similar degree (60 ± 6%, n = 7). 5. Inhalation of DALDA (≈ 7.2 μg/kg, n = 7) also significantly inhibited coughing. 6. The antitussive effect of inhaled morphine (≈ 7.2 μg/kg, n = 11) was inhibited after administration of 3 mg/kg of either naloxone hydrochloride or naloxone methylbromide intraperitoneally. 7. The results support the hypothesis that effects at a peripheral site can make a major contribution to the antitussive actions of these drugs.