Dopamine D₁ receptor antagonist inhibits swallowing reflex in guinea pigs

To determine whether dopamine D₁ receptor antagonist impairs the swallowing reflex and reduces substance P (SP) in the peripheral organs, the swallowing reflex in terms of the number of swallows elicited by injections of three different volumes (0.2, 0.4, and 0.6 ml) of distilled water into the pharynx through a catheter was examined in anesthetized guinea pigs pretreated with Sch-23390. Animals were pretreated with either subcutaneous Sch-23390 (200 μg/kg) or a vehicle of Sch-23390 every 12 h for 7 days. The number of swallows was counted by submental electromyographic activity and visual observation of characteristic laryngeal movement. Injections of distilled water caused a volume-dependent increase in the number of swallows in animals without Sch-23390 treatment. Sch-23390 significantly decreased and exogenously administered SP increased the number of swallows elicited by all volumes of distilled water. FK-888 (10^-5 M, 1 ml), a specific inhibitor of the NK₁ receptor, reduced the number of swallows to a greater degree than Sch-23390. Sch-23390 significantly reduced SP content in the laryngeal and pharyngeal mucosa compared with control. These results suggest that inhibition of the dopamine D₁ receptor may impair the swallowing reflex and reduce SP content in the peripheral organs.