Different tachykinin receptors mediate chloride secretion in the distal colon through activation of submucosal neurones

We investigated the role of tachykinin receptor subtypes on secretory respo nses in the guinea-pig distal colon using Ussing chamber experiments and in tracellular recordings from submucosal neurones. Choline acetyltransferase (ChAT) and vasoactive intestinal polypeptide (VIP) were demonstrated in sub mucosal neurones by immunohistochemistry. In Ussing chamber experiments substance P (SP), the NK-1-receptor agonist [ SAR(9),Met(O-2)(11)]-SP and the NK-3-receptor agonist (MePhe(7))-NKB increa sed dose-dependently short-circuit currents. The NK-2-receptor agonist (bet a Ala(8))-NKA(4-10) had no effect. Responses to 1-100 nM SP, [(SAR(9),Met(O -2)(11)]-SP and (MePhe(7))-NKB were tetrodotoxin-sensitive but hexamethoniu m-insensitive. While (MePhe(7))-NKB-responses were atropine-sensitive at al l concentrations, the atropine sensitivity of the secretory responses to SP and [SAR(9),Met(O-2)(11)]-SP dramatically decreased with increasing concen trations. [SAR(9),Met(O-2)(11)]-SP and (MePhe7)-NKB effects were blocked by the selective NK-1 and NK-3 antagonists CP-99,994-1 (1 mu M) and SR 142801 (1 mu M), respectively. Combination of both antagonists blocked the SP-res ponse. SR 142801 also suppressed the response to [SAR(9),Met(O-2)(11)]-SP. Desensitization with [SAR(9),Met(O-2)(11)]-SP significantly decreased (MePh e7)-NKB-responses but not vice versa. In intracellular recordings 90% of su bmucosal neurones were activated by both [SAR(9),Met(O-2)(11)]-SP and (MePh e(7))-NKB as indicated by membrane depolarisation and enhanced spike discha rge. These effects were tetrodotoxin-resistant and potentiated by atropine. NK-1- and NK-3-mediated responses occurred equally in ChAT-positive and in VIP-positive neurones. The results suggest the importance of NK-1- and NK-3-receptors on cholinerg ic and non-cholinergic submucosal neurones for secretory processes in the g uinea-pig distal colon.