The prokinetic-like activity of ghrelin in rat isolated stomach is mediated via cholinergic and tachykininergic motor neurones

Ghrelin increases electrically evoked, neuronally mediated contractions of rat isolated forestomach, a prokinetic-like activity. Since the nerve type sensitive to ghrelin is unclear, we examined the activity of ghrelin in the presence of antagonists at receptors for the main gastric motor neurotransmitters. Electrical field stimulation (EFS; 5 Hz, 0.5 ms, ± 50 V, 30 s every 3 min) of circular muscle preparations evoked tetrodotoxin 1 μM-sensitive responses, consisting of a small initial contraction followed by a further contraction or more usually, by muscle relaxation. Termination of EFS evoked a large rapidly developing after-contraction. Atropine 1μM prevented contractions during EFS, increased any relaxations and prolonged the after-contractions. Nω-Nitro-l-arginine-methyl-ester-hydrochloride (L-NAME) 0.3mM prevented relaxations during EFS, changing the triphasic response into a monophasic contraction. The tachykinin NK1 and tachykinin NK2 receptor antagonists N-acetyl-l-tryptophan-3,5-bistrifluoromethyl-benzyl-ester (L-732,138 1 μM) and Cyclo[Gln–Trp–Phe–Gly–Leu–CH2N(CH3)–Leu] (MDL-29,913 1 μM) each reduced EFS-evoked relaxations; the latter also reduced the after-contractions. The tachykinin NK3 receptor antagonist (−)-(S)-N-(α-ethylbenzyl)-3-(carboxymethoxy)-2-phenylquinoline-4-carboxamide (SB-235375, 0.1 μM) had no effects. The combination of tachykinin NK1,2,3 receptor antagonists reduced the after-contractions and abolished relaxations during EFS, replacing this with a contraction. In control tissues, ghrelin 1 μM increased EFS-induced contractions and tended to reduce any relaxations. In the presence of atropine 1 μM, L-NAME 0.3 mM or the tachykinin receptor antagonists (as above), ghrelin 1 μM increased any EFS-induced contraction but in the presence of atropine had no effects on EFS-evoked relaxations. We conclude that EFS evokes responses mediated by acetylcholine, nitric oxide and tachykinins. Ghrelin facilitates both cholinergic and tachykininergic excitatory pathways, consistent with activity within the enteric nervous system and possibly the vagus nerve.