Neuronal circuitry between the inferior mesenteric ganglion and lower intestine of the dog.

Neuronal circuitry between the inferior mesenteric ganglion (IMG) and the distal colon as well as the rectum, forming the intestino-intestinal reflex pathway, was investigated in the dog using immunohistochemistry combined with retrograde tract tracing and denervation experiments. Virtually all IMG neurons were tyrosine hydroxylase (TH)-immunoreactive. Of these ganglionic neurons, about 64% were also immunoreactive for calbindin (Calb), some 35% for neuropeptide Y (NPY), and 2% for vasoactive intestinal peptide (VIP). The retrograde tracer experiments revealed that both Calb/TH neurons and NPY/TH neurons projected to the distal colon and the rectum. In these intestinal walls, Calb/TH positive varicose fibers were found in the myenteric and submucous ganglia as well as in the longitudinal muscle layer, while NPY/TH positive fibers were mainly distributed around the vascular walls. Around Calb/TH neurons of the IMG, abundant varicose nerve fibers immunoreactive for VIP, dynorphin (DYN), calcitonin gene-related peptide (CGRP), enkephalin (ENK), substance P (SP) and bombesin (BOM) were distributed. These immunoreactive fibers disappeared after the total denervation of the IMG. After the application of Fast Blue into the IMG or distal stumps of transected lumbar colonic and hypogastric nerves, retrogradely labeled neurons occurred in the myenteric plexus with increasing density along the distal colon and rectum, and were immunoreactive for VIP, DYN, CGRP, ENK, SP or BOM. Double immunostaining of nerve fibers in the distal stumps of the ligated colonic and hypogastric nerves revealed the presence of viscerofugal fibers containing VIP with DYN and/or CGRP and those containing ENK with SP and/or BOM. These results demonstrate for the first time that the efferent limb of the canine intestino-intestinal reflex arch via the IMG consists of Calb-immunoreactive ganglion neurons projecting to the longitudinal muscles in addition to the enteric plexus of the lower intestine and also of NPY-immunoreactive ganglion neurons projecting to the intestinal blood vessels, and that the afferent limb is composed of at least two discrete groups with different peptide contents, i.e., myenteric neurons containing VIP with DYN and/or CGRP and those containing ENK with SP and/or BOM.