In mammals, sensory stimuli in visceral organs, including those that underlie pain perception, are detected by spinal afferent neurons, whose cell bodies lie in dorsal root ganglia (DRG). One of the major challenges in visceral organs has been how to identify the different types of nerve endings of spinal afferents that transduce sensory stimuli into action potentials. The reason why spinal afferent nerve endings have been so challenging to identify is because no techniques have been available, until now, that can selectively label only spinal afferents, in high resolution. We have utilized an anterograde tracing technique, recently developed in our laboratory, which facilitates selective labeling of only spinal afferent axons and their nerve endings in visceral organs. Mice were anesthetized, lumbosacral DRGs surgically exposed, then injected with dextran-amine. Seven days post-surgery, the large intestine was removed. The characteristics of thirteen types of spinal afferent nerve endings were identified in detail. The greatest proportion of nerve endings was in submucosa (32%), circular muscle (25%) and myenteric ganglia (22%). Two morphologically distinct classes innervated myenteric ganglia. These were most commonly a novel class of intraganglionic varicose endings (IGVEs) and occasionally rectal intraganglionic laminar endings (rIGLEs). Three distinct classes of varicose nerve endings were found to innervate the submucosa and circular muscle, while one class innervated internodal strands, blood vessels, crypts of lieberkuhn, the mucosa and the longitudinal muscle. Distinct populations of sensory endings were CGRP-positive. We present the first complete characterization of the different types of spinal afferent nerve endings in a mammalian visceral organ. The findings reveal an unexpectedly complex array of different types of primary afferent endings that innervate specific layers of the large intestine. Some of the novel classes of nerve endings identified must underlie the transduction of noxious and/or innocuous stimuli from the large intestine.
The mechanisms underlying distension-evoked peristalsis in the colon are incompletely understood. It is well known that, following colonic distension, 5-hydroxytryptamine (5-HT) is released from enterochromaffin (EC) cells in the intestinal mucosa. It is also known that exogenous 5-HT can stimulate peristalsis. These observations have led some investigators to propose that endogenous 5-HT release from EC cells might be involved in the initiation of colonic peristalsis, following distension. However, because no direct evidence exists to support this hypothesis, the aim of this study was to determine directly whether release of 5-HT from EC cells was required for distension-evoked colonic peristalsis. Real-time amperometric recordings of 5-HT release and video imaging of colonic wall movements were performed on isolated segments of guinea pig distal colon, during distension-evoked peristalsis. Amperometric recordings revealed basal and transient release of 5-HT from EC cells before and during the initiation of peristalsis, respectively. However, removal of mucosa (and submucosal plexus) abolished 5-HT release but did not inhibit the initiation of peristalsis nor prevent the propagation of fecal pellets or intraluminal fluid. Maintained colonic distension by fecal pellets induced repetitive peristaltic waves, whose intrinsic frequency was also unaffected by removal of the submucosal plexus and mucosa, although their propagation velocities were slower. In conclusion, the mechanoreceptors and sensory neurons activated by radial distension to initiate peristalsis lie in the myenteric plexus and/or muscularis externa, and their activation does not require the submucosal plexus, release of 5-HT from EC cells, nor the presence of the mucosa. The propagation of peristalsis and propulsion of liquid or solid content along the colon is entrained by activity within the myenteric plexus and/or muscularis externa and does not require sensory feedback from the mucosa, nor neural inputs arising from submucosal ganglia.
In patients with irritable bowel syndrome, visceral pain is evoked more readily following distension of the colorectum. However, the identity of extrinsic afferent nerve pathway that detects and transmits visceral pain from the colorectum to the spinal cord is unclear. In this study, we identified which extrinsic nerve pathway(s) underlies nociception from the colorectum to the spinal cord of rodents. Electromyogram recordings were made from the transverse oblique abdominal muscles in anesthetized wild type (C57BL/6) mice and acute noxious intraluminal distension stimuli (100–120 mmHg) were applied to the terminal 15 mm of colorectum to activate visceromotor responses (VMRs). Lesioning the lumbar colonic nerves in vivo had no detectable effect on the VMRs evoked by colorectal distension. Also, lesions applied to the right or left hypogastric nerves failed to reduce VMRs. However, lesions applied to both left and right branches of the rectal nerves abolished VMRs, regardless of whether the lumbar colonic or hypogastric nerves were severed. Electrical stimulation applied to either the lumbar colonic or hypogastric nerves in vivo, failed to elicit a VMR. In contrast, electrical stimulation (2–5 Hz, 0.4 ms, 60 V) applied to the rectum reliably elicited VMRs, which were abolished by selective lesioning of the rectal nerves. DiI retrograde labeling from the colorectum (injection sites 9–15 mm from the anus, measured in unstretched preparations) labeled sensory neurons primarily in dorsal root ganglia (DRG) of the lumbosacral region of the spinal cord (L6-S1). In contrast, injection of DiI into the mid to proximal colon (injection sites 30–75 mm from the anus, measured in unstretched preparations) labeled sensory neurons in DRG primarily of the lower thoracic level (T6-L2) of the spinal cord. The visceral pain pathway activated by acute noxious distension of the terminal 15 mm of mouse colorectum is transmitted predominantly, if not solely, through rectal/pelvic afferent nerve fibers to the spinal cord. The sensory neurons of this spinal afferent pathway lie primarily in the lumbosacral region of the spinal cord, between L6 and S1.
Spencer NJ, Kyloh M, Wattchow DA, Thomas A, Sia TC, Brookes SJ, Nicholas SJ. Characterization of motor patterns in isolated human colon: are there differences in patients with slow-transit constipation?
Non-technical summary The endothelin-3 (ET-3) gene is essential for the development of the enteric nervous system in the gastrointestinal tract of mammals, including humans and mice. Loss of the ET-3 gene leads to the formation of an aganglionic colorectum and impaired bowel function. Endogenous endothelin peptides and their receptors also play a major role in nociception in a variety of organs and species, including humans. However, whether nociception is altered in the aganglionic region of the colorectum is unknown. We show that in ET-3 deficient mice, there is a loss of nociception from the aganglionic rectum, but not other visceral organs. This loss of nociception is due to a reduction in spinal afferent innervation and a selective deficiency in specific classes of rectal afferent nerve fibres, which are necessary for detection of noxious stimuli from this region.Abstract Endothelin peptides and their endogenous receptors play a major role in nociception in a variety of different organs. They also play an essential role in the development of the enteric nervous system. Mice with deletions of the endothelin-3 gene (lethal spotted mice, ls/ls) develop congenital aganglionosis. However, little is known about how nociception might be affected in the aganglionic rectum of mice deficient in endothelin-3. In this study we investigated changes in spinal afferent innervation and visceral pain transmission from the aganglionic rectum in ls/ls mice. Electromyogram recordings from anaesthetized ls/ls mice revealed a deficit in visceromotor responses arising from the aganglionic colorectum in response to noxious colorectal distension. Loss of visceromotor responses (VMRs) in ls/ls mice was selective, as no reduction in VMRs was detected after stimulation of the bladder or somatic organs. Calcitonin gene related peptide (CGRP) immunoreactivity, retrograde neuronal tracing and extracellular afferent recordings from the aganglionic rectum revealed decreased colorectal spinal innervation, combined with a reduction in mechanosensitivity of rectal afferents. The sensory defect in ls/ls mice is primarily associated with changes in low threshold wide dynamic range rectal afferents. In conclusion, disruption of endothelin 3 gene expression not only affects development and function of the enteric nervous system, but also specific classes of spinal rectal mechanoreceptors, which are required for visceral nociception from the colorectum.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.