β-Nicotinamide adenine dinucleotide is an inhibitory neurotransmitter in visceral smooth muscle

• DOI: 10.1073/pnas.0705510104
• Publication Date: 2007-10-04T00:53:32Z
• Authors: Violeta N. Mutafo...; Sung Jin Hwang; Xuemei Hao; Hui Chen; Michael X. Zhu; Jackie D. Wood; Sean M. Ward; Kenton M. Sanders
• Keywords: 0301 basic medicine; Neurotransmitter Agents; Receptors, Purinergic P2; Muscle, Smooth; In Vitro Techniques; NAD; Electric Stimulation; Recombinant Proteins; Cell Line; 3. Good health; Adenosine Diphosphate; Mice, Inbred C57BL; Mice; Receptors, Purinergic P2Y1; 03 medical and health sciences; Adenosine Triphosphate; Purinergic P2 Receptor Antagonists; Animals; Humans

Peripheral inhibitory nerves are physiological regulators of the contractile behavior of visceral smooth muscles. One of the transmitters responsible for inhibitory neurotransmission has been reputed to be a purine, possibly ATP. However, the exact identity of this substance has never been verified. Here we show that β-nicotinamide adenine dinucleotide (β-NAD), an inhibitory neurotransmitter candidate, is released by stimulation of enteric nerves in gastrointestinal muscles, and the pharmacological profile of β-NAD mimics the endogenous neurotransmitter better than ATP. Levels of β-NAD in superfusates of muscles after nerve stimulation exceed ATP by at least 30-fold; unlike ATP, the release of β-NAD depends on the frequency of nerve stimulation. β-NAD is released from enteric neurons, and release was blocked by tetrodotoxin or ω-conotoxin GVIA. β-NAD is an agonist for P2Y1 receptors, as demonstrated by receptor-mediated responses in HEK293 cells expressing P2Y1 receptors. Exogenous β-NAD mimics the effects of the enteric inhibitory neurotransmitter. Responses to β-NAD and inhibitory junction potentials are blocked by the P2Y1-selective antagonist, MRS2179, and the nonselective P2 receptor antagonists, pyridoxal phosphate 6-azophenyl-2′,4′-disulfonic acid and suramin. Responses to ATP are not blocked by these P2Y receptor inhibitors. The expression of CD38 in gastrointestinal muscles, and specifically in interstitial cells of Cajal, provides a means of transmitter disposal after stimulation. β-NAD meets the traditional criteria for a neurotransmitter that contributes to enteric inhibitory regulation of visceral smooth muscles.