Distribution, projections, and association with calbindin baskets of motor neurons, interneurons, and sensory neurons in guinea-pig distal colon

Normal gut function relies on the activity of the enteric nervous system (ENS) found within the wall of the gastrointestinal tract. The structural and functional organization of the ENS has been extensively studied in the guinea pig small intestine, but less is known about colonic circuitry. Given that there are significant differences between these regions in function, observed motor patterns and pathology, it would be valuable to have a better understanding of the colonic ENS. Furthermore, disorders of colonic motor function, such as irritable bowel syndrome, are much more common. We have recently reported specialized basket-like structures, immunoreactive for calbindin, that likely underlie synaptic inputs to specific types of calretinin-immunoreactive neurons in the guinea-pig colon. Based on detailed immunohistochemical analysis, we postulated the recipient neurons may be excitatory motor neurons and ascending interneurons. In the present study, we combined retrograde tracing and immunohistochemistry to examine the projections of circular muscle motor neurons, myenteric interneurons, and putative sensory neurons. We focused on neurons with immunoreactivity for calbindin, calretinin and nitric oxide synthase and their relationship with calbindin baskets. Retrograde tracing using indocarbocyanine dye (DiI) revealed that many of the nerve cell bodies surrounded by calbindin baskets belong to motor neurons and ascending interneurons. Unique functional classes of myenteric neurons were identified based on morphology, neuronal markers and polarity of projection. We provide evidence for three groups of ascending motor neurons based on immunoreactivity and association with calbindin baskets, a finding that may have significant functional implications.