Evidence for the involvement of ATP, but not of VIP/PACAP or nitric oxide, in the excitatory effect of capsaicin in the small intestine

The contractile effect of capsaicin in the guinea-pig small intestine involves an activation of enteric cholinergic neurons. Our present data show that the P 2 purinoceptor antagonist pyridoxal-phosphate-6-azophenyl-2′,4′-disulphonic acid (PPADS, 30 μM) significantly reduces the contractile response to capsaicin (2 μM) in the presence, but not in the absence, of the tachykinin receptor antagonists [O-Pro 9, (Spiro-γ-lactam)Leu 10, Trp 11]physalaemin (1–11) (GR 82334; 3 μM) and ( S)-( N)-(1-(3-(1-benzoyl-3-(3,4-dichlorophenyl)piperidin-3-yl)propyl)-4-phenylpiperidine-4-yl)- N-methylacetamide (SR 142801; 100 nM) (for blocking tachykinin NK 1 and NK 3 receptors, respectively). PPADS (30 μM) fails to influence submaximal cholinergic contractions evoked by cholecystokinin octapeptide (CCK-8; 2–3 nM) or senktide (1 nM), or the direct smooth muscle-contracting effect of histamine (100–200 nM). A higher concentration (300 μM) of PPADS is also without effect against the stimulatory action of cholecystokinin octapeptide. This means that PPADS can probably be safely used as a purinoceptor antagonist in intestinal preparations. The putative pituitary adenylate cyclase activating peptide (PACAP) receptor antagonist PACAP-(6–38) (3 μM) significantly reduces the contractile effect of PACAP-(1–38) (10 nM) and abolishes that of vasoactive intestinal polypeptide (VIP; 10 nM). PACAP-(6–38) (3 μM) fails to influence the effect of capsaicin (2 μM) both in the absence and in the presence of tachykinin receptor antagonists. The nitric oxide (NO) synthase inhibitor N G-nitro- l-arginine ( l-NOARG; 100 μM) also fails to inhibit the capsaicin-induced motor response. We conclude that an endogenous ligand of PPADS-sensitive P 2 purinoceptors (possibly ATP), but not a VIP/PACAP-like peptide or NO, is involved in the nontachykininergic activation of cholinergic neurons in the course of the capsaicin-induced contraction.