Identification of spinal afferent nerve endings in the colonic mucosa and submucosa that communicate directly with the spinal cord: The gut-brain axis

The major sensory nerve pathway between the colon and central nervous system (spinal cord and brain) that underlies the gut-brain axis, is via spinal afferent neurons, with cell bodies in dorsal root ganglia (DRG). Our aim was to identify the sensory nerve endings in the colon that arise from single colorectal-projecting DRG neurons. C57BL/6 mice were anesthetized and lumbosacral L6-S1 DRG injected with dextran biotin. Mice recovered for 7 days. The whole colon was then removed and stained to visualize single axons and nerve endings immunoreactive to calcitonin gene-related peptide (CGRP). Single axons arising from DRG were identified in the distal colon and their morphological features and CGRP immunoreactivity characterized. After entering the colon, single axons ramified rostrally or caudally along many rows of myenteric ganglia with little circumferential displacement, giving off varicose endings in multiple ganglia. Nerve endings arising from two classes of colorectal-projecting DRG neuron were identified. One class was peptidergic neurons that had nerve endings in circular muscle, myenteric ganglia, and submucosa. Another class of nonpeptidergic neurons innervated mucosal crypts, myenteric ganglia, and submucosa. Different morphological types of nerve endings which innervate different anatomical layers of colon can arise from the same axon and sensory neuron in DRG. These findings suggest single peptidergic and nonpeptidergic sensory neurons in DRG are potentially capable of detecting sensory stimuli from different anatomical layers of the colon, via different types of nerve endings.