Distribution of transient receptor potential vanilloid 1 channel-expressing nerve fibers in mouse rectal and colonic enteric nervous system: relationship to peptidergic and nitrergic neurons.

In the gut, transient receptor potential vanilloid (TRPV) 1 activation leads to release of neurotransmitters such as neuropeptides and nitric oxide. However, the distribution of TRPV1 nerve fibers and neurotransmitters released form sensory nerve endings in the enteric nervous system are currently not well understood. The present study investigated the immunohistochemical distribution of TRPV1 channels, sensory neuropeptides, and nitric oxide and their co-localization in mouse large intestine. Numerous TRPV1 and calcitonin gene-related peptide (CGRP) immunoreactivities were detected, mainly in the mucosa, submucosal layer, and myenteric plexus. Abundant substance P (SP), neurokinin A (NKA), and neuronal nitric oxide synthase (nNOS)-immunoreactivity were revealed in muscle layers. Motor function studies of circular and longitudinal muscles found that contractile responses to capsaicin in the rectum were most sensitive among the rectum, and distal, transverse, and proximal colon. Double labeling studies were carried out in horizontal sections of mouse rectum. TRPV1/protein gene product (PGP)9.5 double labeled axons were observed, but PGP9.5 and neuronal nuclear protein immunopositive cell bodies did not express TRPV1 immunoreactivity in the myenteric plexus. In the mucosa, submucosal layer, deep muscular plexus, circular muscle, myenteric plexus and longitudinal muscle layer, TRPV1 nerve fibers were found to contain CGRP, SP and nNOS. SP and NKA were almost entirely colocalized at the axons and cell bodies in all layers. Double labeling with c-Kit revealed that TRPV1 nerve fibers localized adjacent to the interstitial cells of Cajal (ICC). These results suggest that the TRPV1-expressing nerve and its neurotransmitters regulate various functions of the large intestine.