5-HT7 receptor activation inhibits substance-p-induced but not carbachol-induced contractions in guinea-pig ileum: a possible cross-talk between 5-HT7 and NK receptors?

Michael J. Etty1,2, Gareth J. Sanger1 & Selim Cellek1 1Neurology and Gastrointestinal Centre of Excellence in Drug Discovery, GlaxoSmithKline, New Frontiers Science Park North, Third Avenue, Harlow, Essex, CM19 5AW, UK, 2School of Biomedical Sciences, University of Nottingham, NG7 2UH, UK.

5-HT7 receptor activation is known to mediate relaxation of gastrointestinal smooth muscle. Previous studies have demonstrated that activation of 5-HT7 receptors inhibits contractions induced by substance-P in the guinea-pig ileum (Carter et al., 1995), carbachol in the human colon (Prins et al., 1999) and prostaglandin-F2α in dog stomach (Janssen et al., 2005). We have investigated the effect of a non-selective 5-HT receptor agonist 5-carboxamidotryptamine (5-CT) on substance-P and carbachol-induced contractions in the guinea-pig ileum, in the absence or presence of a cocktail of non-5-HT7 receptor antagonists, in order to elucidate whether or not the relaxation response in this particular tissue is due to direct activation of the 5-HT7 receptor.

Whole guinea pig ileal segments (~10 mm) were dissected from adult male Dunkin Hartley guinea pigs (300-350g) 10 cm distal to the ileocaecal junction and suspended in 5 ml tissue baths containing Krebs’ buffer (NaCl 121.5, CaCl2 2.5, KH2PO4 1.2, KCl 4.7, MgSO4 1.2, NaHCO3 25.0, glucose 5.6 mM) at 37°C, oxygenated with 5% CO2 in O2 under 1g tension for isometric recording. The tissues were contracted either with carbachol (300 nM; EC80; 1.9±0.2 g) or substance-P (100 nM; EC80; 2.1±0.2 g) in the absence or presence of an antagonist cocktail (WAY100635 [0.1 µM; 5-HT1A receptor antagonist], GR127935 [1 µM; 5-HT1B and 5-HT1D receptor antagonist], ketanserin [1 µM; 5-HT2A receptor antagonist], SB204741 [1 µM; 5-HT2B receptor antagonist], ondansetron [1 µM; 5-HT3 receptor antagonist], SB204070A [0.1 µM; 5-HT4 receptor antagonist] and scopolamine [10µM only for substance-P experiments]).

5-CT (0.01-100 µM) inhibited substance-P-induced contractions (maximum inhibition of 26.9±8.4% (n=7) and 39.4±6.6% (n=8) at 3 µM 5-CT in the absence or presence of antagonist cocktail respectively) but had no effect on carbachol-induced contractions (3.1±1.7% vs 7.8±3.9% inhibition, vehicle vs 5-CT at 10µM respectively, n=4). The inhibitory effect of 5-CT on substance-P-induced contractions was fully reversed by the selective 5-HT7 receptor antagonist SB269970 (1µM).

These results suggest that, at least in the guinea-pig ileum, the inhibitory action of 5-HT7 receptor activation is specific to the receptor or pathways involved in mediating substance-P-induced contraction and further suggest a possible cross-talk between 5-HT7 and NK receptors.

Carter D. et al. (1995) Eur. J. Pharmacol., 280, 243-250.
Janssen P. et al. (2005) Am. J. Physiol., 289, G108-G115.
Prins N.H. et al. (1999) Br. J. Pharmacol., 128, 849-852.