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 responses in the guinea-pig distal colon using Ussing chamber experiments and intracellular recordings from submucosal neurones. Choline acetyltransferase (ChAT) and vasoactive intestinal polypeptide (VIP) were demonstrated in submucosal neurones by immunohistochemistry. In Ussing chamber experiments substance P (SP), the NK-1-receptor agonist [SAR⁹,Met(O₂)¹¹]-SP and the NK-3-receptor agonist (MePhe⁷)-NKB increased dose-dependently short-circuit currents. The NK-2-receptor agonist (βAla⁸)NKA(4-10) had no effect. Responses to 1-100 nM SP, [(SAR⁹,Met(O₂)¹¹]-SP and (MePhe⁷)-NKB were tetrodotoxin-sensitive but hexamethonium-insensitive. While (MePhe⁷)-NKB- responses were atropine-sensitive at all concentrations, the atropine sensitivity of the secretory responses to SP and [SAR⁹,Met(O₂)¹¹]-SP dramatically decreased with increasing concentrations. [SAR⁹,Met(O₂)¹¹]SP and (MePhe⁷)-NKB effects were blocked by the selective NK-1 and NK-3 antagonists CP-99,994-1 (1 μM) and SR 142801 (1 μM), respectively. Combination of both antagonists blocked the SP-response. SR 142801 also suppressed the response to [SAR⁹,Met(O₂)¹¹]-SP. Desensitization with [SAR⁹,Met(O₂)¹¹]-SP significantly decreased (MePhe⁷)-NKB-responses but not vice versa. In intracellular recordings 90% of submucosal neurones were activated by both [SAR⁹,Met(O₂)¹¹]-SP and (MePhe⁷)-NKB as indicated by membrane depolarisation and enhanced spike discharge. 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 cholinergic and non-cholinergic submucosal neurones for secretory processes in the guinea- pig distal colon.