β-Nicotinamide Adenine Dinucleotide Is an Enteric Inhibitory Neurotransmitter in Human and Nonhuman Primate Colons

β-NAD meets the criteria for a neurotransmitter better than ATP in human and monkey colons and therefore may contribute to neural regulation of colonic motility. SMCs are unlikely targets for inhibitory purine neurotransmitters because dominant responses of SMCs were activation of net inward, rather than outward, current.