Effect of fentanyl on the number of citric acid-induced coughs in mice. The mice were exposed for 3 min to citric acid (0.1 M) 30 min before the i.v. injection of fentanyl (5 μg/kg) to determine the frequency of control coughs (pre-value: open column). The mice were also exposed for 3 min to citric acid 30 min after the i.v. injection of fentanyl. The number of coughs produced per 3-min period of exposure to citric acid 30 min after the i.v. injection of fentanyl (post-value: hatched column) was counted. Each column represents the mean with S.E.M (n = 10). *P < 0.05 vs. respective pre-value. #P < 0.05 vs. respective vehicle (saline)-treated group.