Neurochemical phenotypes of myenteric neurons in the rhesus monkey

Understanding the neurochemical composition of the enteric nervous system (ENS) is critical for elucidating neurological function in the gastrointestinal (GI) tract in health and disease. Despite their status as the closest models of human neurological systems, relatively little is known about enteric neurochemistry in nonhuman primates. We describe neurochemical coding of the enteric nervous system, specifically the myenteric plexus, of the rhesus monkey (Macaca mulatta) by immunohistochemistry and directly compare it to human tissues. There are considerable differences in the myenteric plexus along different segments of the monkey GI tract. While acetylcholine neurons make up the majority of myenteric neurons in the stomach (70%), they are a minority in the rectum (47%). Conversely, only 22% of gastric myenteric neurons express nitric oxide synthase (NOS) compared to 52% in the rectum. Vasoactive intestinal peptide (VIP) is more prominent in the stomach (37%) versus the rest of the GI tract (≈10%), and catecholamine neurons are rare (≈1%). There is significant coexpression of NOS and VIP in myenteric neurons that is more prominent in the proximal GI tract. Taken as a whole, these data provide insight into the neurochemical anatomy underlying GI motility. While overall similarity to other mammalian species is clear, there are some notable differences between the ENS of rhesus monkeys, humans, and other species that will be important to take into account when evaluating models of human diseases in animals.