Identification of capsaicin-sensitive rectal mechanoreceptors activated by rectal distension in mice

Abstract

Rodents detect visceral pain in response to noxious levels of rectal distension. However, the mechanoreceptors that innervate the rectum and respond to noxious levels of rectal distension have not been identified. Here, we have identified the mechanoreceptors of capsaicin-sensitive rectal afferents and characterized their properties in response to circumferential stretch of the rectal wall. We have also used the lethal spotted (ls/ls) mouse to determine whether rectal mechanoreceptors that respond to capsaicin and stretch may also develop in an aganglionic rectum that is congenitally devoid of enteric ganglia. In wild type (C57BL/6) mice, graded increases in circumferential stretch applied to isolated rectal segments activated a graded increase in firing of slowly-adapting rectal mechanoreceptors. Identical stimuli applied to the aganglionic rectum of ls/ls mice also activated similar graded increases in firing of stretch-sensitive rectal afferents. In both wild type and aganglionic rectal preparations, focal compression of the serosal surface using von Frey hairs identified mechanosensitive "hot spots," that were associated with brief bursts of action potentials. Spritzing capsaicin (10 microM) selectively onto each identified mechanosensitive hot spot activated an all or none discharge of action potentials in 32 of 56 identified hot spots in wild type mice and 24 of 62 mechanosensitive hot spots in the aganglionic rectum of ls/ls mice. Each single unit activated by both capsaicin and circumferential stretch responded to low mechanical thresholds (1-2 g stretch). No high threshold rectal afferents were ever recorded in response to circumferential stretch. Anterograde labeling from recorded rectal afferents revealed two populations of capsaicin-sensitive mechanoreceptor that responded to stretch: one population terminated within myenteric ganglia, the other within the circular and longitudinal smooth muscle layers. In the aganglionic rectum of ls/ls mice, only the i.m. mechanoreceptors were identified. Both myenteric and i.m. mechanoreceptors could be identified by their immunoreactivity to the anti-TRPV1 antibody and the vesicular glutamate transporter, Vglut2. Myenteric mechanoreceptors had a unique morphology, consisting of smooth bulbous nodules that ramified within myenteric ganglia. In summary, the rectum of wild type mice is innervated by at least two populations of capsaicin-sensitive rectal mechanoreceptor, both of which respond to low mechanical thresholds within the innocuous range. These findings suggest that the visceral pain pathway activated by rectal distension is likely to involve low threshold rectal mechanoreceptors that are activated within the normal physiological range.

Differential roles of stretch-sensitive pelvic nerve afferents innervating mouse distal colon and rectum

“…HT stretch-sensitive rectal afferents were rare, consistent with a study by Spencer and colleagues (35,36). They studied mouse rectal nerve afferents (i.e., the proportion of rectal afferents distal to the major pelvic ganglion) using stretch forces up to 8 g and found no HT afferents.…”

Identification of different functional types of spinal afferent neurons innervating the mouse large intestine using a novel CGRPα transgenic reporter mouse

“…MF afferents had low thresholds to distension, fired tonically with smoothshaped action potentials, and expressed CGRP. They most closely resembled varicose intramuscular endings (53,54). Consistent with MF afferents, murine colorectal intramuscular endings were sensitive to small amounts of stretch, firing tonically (53,54).…”

“…They most closely resembled varicose intramuscular endings (53,54). Consistent with MF afferents, murine colorectal intramuscular endings were sensitive to small amounts of stretch, firing tonically (53,54). They express the vesicular glutamate transporter VGluT2 and TRPV1 and are activated by capsaicin (53).…”

“…In guinea pig, combined extracellular electrophysiological recordings and rapid neuroanatomical tracing have enabled detailed characterizations of vagal and rectal intraganglionic laminar endings (IGLEs), vascular afferents, and intramuscular arrays, as well as enteric viscerofugal neurons (7). Similar strategies have been employed for mouse colorectal afferents (53,54). However, the high density of afferents in the colorectum, which have been shown to have numerous sites of innervation in the colon, is a significant barrier to this approach in this species (55).…”

Extrinsic primary afferent signalling in the gut

“…41 In mutant mice that lack enteric ganglia in the distal bowel, stretch-sensitive mechanoreceptors that transduce mechanical stimuli from intramuscular endings are present. 74,75 In the smooth muscle of the internal anal sphincter, sacral afferents form arrays of intramuscular endings that are sensitive to both distension and to light von Frey hairs. 76 Although these endings are likely to be mechanosensitive, little physiological evidence exists to date as to whether they are likely to function as tension receptors, length receptors or a combination of the two.…”