SDZ-216525

Epinastine is an antihistamine drug with binding affinities at 5-hydroxytryptamine (5-HT) receptors. The current study was performed to investigate whether epinastine could modulate the cholinergic contraction in guinea pig and human airways in vitro. Isolated guinea pig and human airway preparations were suspended in organ baths containing modified Krebs-Henseleit solution. Electrical field stimulation was applied to elicit cholinergic contractions. Epinastine produced a concentration-dependent inhibition of the cholinergic contraction in guinea pig airways and pretreatment with methysergide (5-HT1/2/7 antagonist) significantly attenuated these inhibitory effects of epinastine. Pretreatment with tropisetron (5-HT3/4 antagonist), ketanserin (5-HT2 antagonist), SDZ216-525 (5-HT1A antagonist) or phentolamine (alpha-adrenergic antagonist) had no effect. Epinastine did not displace the concentration-response curve to acetylcholine. These results suggest that epinastine inhibits the cholinergic contraction in guinea pig airways through stimulation of prejunctional 5-hydroxytryptamine receptors, located to postganglionic cholinergic nerves. Inhibitory effects of epinastine on the cholinergic contraction in human airways in vitro were also demonstrated, which suggests that a similar mechanism might be present in human airways. The pharmacological profile of epinastine, which shows binding affinity at the 5-hydroxytryptamine7 receptor but not at the 5-hydroxytryptamine1 receptor subtypes corroborates the hypothesis that the inhibitory prejunctional 5-hydroxytryptamine receptor on cholinergic nerves is of the 5-hydroxytryptamine7 subtype.

Electrical field stimulation of guinea pig tracheal strips and human bronchial rings, in vitro, evokes a cholinergic contraction mediated by the release of acetylcholine. 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) is a 5-HT1A and 5-HT7 agonist. In this study, we have investigated whether 8-OH-DPAT could modulate the cholinergic contraction in guinea pig and human airways in vitro. 8-OH-DPAT (1 to 30 μ M) produced a concentration-dependent inhibition of the cholinergic contraction in guinea pig tracheal strips with a maximal inhibition of 75.8% ± 4.7% (30 μ M, 0.5 Hz). Pretreatment of the tissues with the 5- HT1/2/7 antagonist methysergide (10 to 30 μ M) significantly attenuated the inhibitory effects of 8-OH-DPAT (10 to 30 μ M) on the cholinergic contraction. Pretreatment with ketanserin (10 μ M), a 5-HT2 antagonist, tropisetron (1 μ M), a 5-HT3/4 antagonist, SDZ 216-525 (1 to 10 μ M) and pindobind (10 μ M), both selective 5-HT1A antagonists, or capsaicin (10 μ M), which depletes sensory nerves from neuropeptides, had no effect on the inhibition of the cholinergic contraction by 8-OH-DPAT (10 to 30 μ M). 5-carboxamidotryptamine (5-CT) (10 to 100 μ M), a 5-HT1/2/7 agonist, partially mimicked the inhibitory effects of 8-OH-DPAT on the cholinergic contraction. 8-OH-DPAT (10 to 30 μ M) also inhibited the cholinergic contraction in human bronchial rings in vitro with a maximal inhibition of 46.2% ± 7.2% (30 μ M, 1 Hz). SDZ 216-525 (10 μ M) had no effect, whereas methysergide (30 μ M) partially prevented the effect of 8-OH-DPAT in human airways. 8-OH-DPAT (30 μ M) did not displace the concentration–response curve to acetylcholine (10 nM-30 mM) in guinea pig and human airways in vitro. These results suggest that 8-OH-DPAT inhibits the cholinergic contraction in guinea pig and human airways in vitro through stimulation of prejunctional atypical 5-HT receptors, possibly of the 5-HT7 subtype, located on postganglionic cholinergic nerves.