Key points• Neural-mediated relaxation occurs in the gastrointestinal tract. To accomplish this function, two neurotransmitters, ATP or a related purine and nitric oxide, are released by inhibitory motorneurons.• The type of purinergic receptor is still under debate but previous data using a classical pharmacological approach (receptor agonists and antagonists) suggested that P2Y 1 receptors are responsible for purinergic neurotransmission in the gastrointestinal tract.• In the present study we used a genetically modified mouse in which P2Y 1 receptors had been knocked out.• P2Y 1 -deficient mice had functional nitrergic neurotransmission but purinergic neurotransmission was absent.• The present work confirms the hypothesis demonstrating that P2Y 1 receptors mediate the purinergic component of the smooth muscle relaxation in the gastrointestinal tract.Abstract Purinergic and nitrergic co-transmission is the dominant mechanism responsible for neural-mediated smooth muscle relaxation in the gastrointestinal tract. The aim of the present paper was to test whether or not P2Y 1 receptors are involved in purinergic neurotransmission using P2Y 1 −/− knock-out mice. Tension and microelectrode recordings were performed on colonic strips. In wild type (WT) animals, electrical field stimulation (EFS) caused an inhibitory junction potential (IJP) that consisted of a fast IJP (MRS2500 sensitive, 1 μM) followed by a sustained IJP (N ω -nitro-L-arginine (L-NNA) sensitive, 1 mM). The fast component of the IJP was absent in P2Y 1 −/− mice whereas the sustained IJP (L-NNA sensitive) was recorded. In WT animals, EFS-induced inhibition of spontaneous motility was blocked by the consecutive addition of L-NNA and MRS2500. In P2Y 1 −/− mice, EFS responses were completely blocked by L-NNA. In WT and P2Y 1 −/− animals, L-NNA induced a smooth muscle depolarization but 'spontaneous' IJP (MRS2500 sensitive) could be recorded in WT but not in P2Y 1 −/− animals. Finally, in WT animals, 1 μM MRS2365 caused a smooth muscle hyperpolarization that was blocked by 1 μM MRS2500. In contrast, 1 μM MRS2365 did not modify smooth muscle resting membrane potential in P2Y 1 −/− mice. β-Nicotinamide adenine dinucleotide (β-NAD, 1 mM) partially mimicked the effect of MRS2365. We conclude that P2Y 1 receptors mediate purinergic neurotransmission in the gastrointestinal tract and β-NAD partially fulfils the criteria to participate in rodent purinergic neurotransmission. The P2Y 1 −/− mouse is a useful animal model to study the selective loss of purinergic neurotransmission.

Inhibitory purinergic neuromuscular transmission in the human colon was pharmacologically assessed by the use of new P2Y(1) receptor antagonists MRS2179, MRS2279, and MRS2500. The rank order of potency of the P2Y(1) antagonists is MRS2500 > MRS2279 > MRS2179. We found that β-NAD partially fulfills the criteria to be considered an inhibitory neurotransmitter in the human colon, but the relative contribution of each purine (ATP/ADP vsβ-NAD) requires further studies.

1- P2Y(1) receptor is the general mechanism of purinergic inhibition in the gastrointestinal tract, 2- P2Y(1)(-/-) mouse is a useful animal model to study selective impairment of purinergic neurotransmission and 3- P2Y(1)(-/-) mouse might help in the identification of purinergic neurotransmitter(s).

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