Tachykinin NK2 receptors and enhancement of cholinergic transmission in the inflamed rat colon:: an in vivo motility study

1 In the gastrointestinal tract, tachykinin NK2 receptors are localized both on smooth muscle and nerve fibres. NK2 receptor antagonists reduce exaggerated intestinal motility in various diarrhoea models but the site of action contributing to this effect is unknown. In this study we investigated the effects of atropine (1.4 mu mol kg(-1), i.v.), hexamethonium (13.5 mu mol kg(-1), i.v.), and nepadutant (0.1 mu mol kg(-1), i.v.), a selective tachykinin NK2 receptor antagonist, on distension (0.5 and 1 ml)-, or irritation (acetic acid, 0.5 ml of 7.5% v v(-1))-induced motility in the rat distal colon in vivo. The effects of atropine, hexamethonium or N-(' ())-nitro-L-argininemethylester (L-NAME, 1.85 mu mol kg(-1), i.v.) on [beta Ala(8)]NKA(4-10) (10 nmol kg(-1), i.v.)-induced colonic contractions were also investigated. 2 When the colonic balloon was filled with a subthreshold volume (0.5 ml), the intraluminal instillation of acetic acid triggered a high-amplitude phasic colonic motility which was partially reduced by nepadutant and suppressed by either hexamethonium or atropine. Filling of the balloon with 1 ml evoked reflex (hexamethonium-sensitive), atropine-sensitive phasic colonic motility: nepadutant had no significant effect on the distension-evoked motility. 3 Neither hexamethonium nor atropine significantly reduced [beta Ala(8)]NKA(4-10)-induced colonic contractions, whereas nepadutant suppressed them. Following L-NAME pretreatment, [beta Ala(8)]NKA(4-10)-induced colonic contractions were inhibited by both atropine and hexamethonium. In hexamethonium-pretreated animals, an atropine-sensitive component of [beta Ala(8)]NKA(4-10)-induced colonic contractions was also evident. 4 These results indicate that the application of irritants onto the colonic mucosa induces the release of endogenous tachykinins which enhance excitatory cholinergic mechanisms through the stimulation of NK2 receptors.