Neurokinin-induced salivation in the anesthetized rat: a three receptor hypothesis

Substance P (3 micrograms/kg), neurokinin A (20 micrograms/kg), neurokinin B (6 micrograms/kg) and acetylcholine (875 micrograms/kg) all produced salivation upon i.v. infusion in the anesthetized rat. Against single equivalent agonist doses, atropine (135 micrograms/kg i.v.) blocked both acetylcholine- and neurokinin B-, but not substance P- or neurokinin A-induced salivation. [D-Pro2,D-Trp7,9]-substance P (1 mg/kg i.v.), a putative substance P antagonist, reduced responses to mammalian neurokinins but caused a 2-fold potentiation of acetylcholine-induced salivation. [D-Pro2,D-Trp6,8,Nle10]-Neurokinin B (1 mg/kg i.v.), a novel putative neurokinin B antagonist, significantly reduced substance P- and neurokinin B- but not acetylcholine- or neurokinin A-induced salivation. The three agonists (at doses that produced salivation) and [D-Pro2,D-Trp6,8,Nle10]-neurokinin B (1 mg/kg i.v.) lowered blood pressure in anesthetized rats by 35 to 40%. [D-Pro2,D-Trp7,9]-Substance P (1 mg/kg i.v.) had no significant effect on blood pressure. Hydralazine at 0.60 mg/kg (i.v.), a dose which lowered blood pressure by 47%, did not reduce substance P-induced salivation. Thus, blockade of neurokinin-induced salivation by [D-Pro2,D-Trp6,8,Nle10]-neurokinin B was probably not due to hypotension. Based on the differential effects of the three antagonists on neurokinin- and acetylcholine-induced salivation, we hypothesize the existence of three distinct neurokinin receptors in rat salivary gland, and suggest that neurokinin B receptors reside presynaptically.