Identification of functional intramuscular rectal mechanoreceptors in aganglionic rectal smooth muscle from piebald lethal mice

Abstract

The mechanosensitive endings of low-threshold, slowly adapting pelvic afferents that innervate the rectum have been previously identified as rectal intraganglionic laminar endings (rIGLEs) that lie within myenteric ganglia. We tested whether the aganglionic rectum of piebald-lethal (s(l)/s(l)) mice lacks rIGLEs and whether this could explain impaired distension-evoked reflexes from this region. Extracellular recordings were made from fine rectal nerves in C57BL/6 wild-type and s(l)/s(l) mice, combined with anterograde labeling. In C57BL/6 mice, graded circumferential stretch applied to the rectum activated graded increases in firing of slowly adapting rectal mechanoreceptors. In s(l)/s(l) mice, graded stretch of the aganglionic rectum activated similar graded increases in rectal afferent firing. Stretch-sensitive afferents responded at low mechanical thresholds and fired more intensely at noxious levels of stretch. They could also be activated by probing their receptive fields with von Frey hairs and by muscle contraction. Anterograde labeling from recorded rectal nerves identified the mechanoreceptors of muscular afferents in the aganglionic rectal smooth muscle. A population of afferents were also recorded in both C57BL/6 and s(l)/s(l) mice that were activated by von Frey hair probing, but not stretch. In summary, the aganglionic rectum is innervated by a population of stretch-sensitive rectal afferent mechanoreceptor which develops and functions in the absence of any enteric ganglia. These results suggest that in patients with Hirschsprung's disease the inability to activate extrinsic distension reflexes from the aganglionic rectum is unlikely to be due to the absence of stretch-sensitive extrinsic mechanoreceptors.

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.…”