Changes in the innervation of the mouse internal anal sphincter during aging

Decreases in mucosally-evoked tachykinin signaling pathways can explain age-related reductions in murine colonic motility patterns

BACKGROUND

Increasing age increases the incidence of chronic constipation and fecal impaction. The contribution of the natural aging process to this phenotype is unclear. This study explored the effects of age on key motility patterns in the murine colon and determined the contribution that altered neurokinin 2 (NK2) -mediated signaling made to the aging phenotype.

METHODS

Mucosal reflexes, colonic migrating motor complexes (CMMCs) and colonic motility assays were explored in isolated ex vivo colons from 3, 12-14, 18- and 24-months old mice and the NK2-mediated response determined. Electrical field stimulation (EFS) or exogenous drug application were used to explore the role of the mucosa in colonic segments.

KEY RESULTS

Aging reduced the force of contraction of the distal colon mucosal reflex, the frequency and force of contraction of CMMCs and the NK2-mediated component of both motility patterns. Ondansetron, a 5-HT3 receptor antagonist, blocked a component of both motility patterns in full thickness but not in mucosa-free segments of the distal colon. 5, hydroxytryptamine (5-HT) and EFS-evoked NK2-dependent contractions were reduced with increasing age. Smooth muscle sensitivity to 5-HT or neurokinin A (NKA) was not altered with age. In isolated colon motility assays application of NKA decreased transit time in 24-months colon and the NK2 antagonist GR159897 increased transit times in both 3- and 24-months old colons.

CONCLUSIONS AND INFERENCES

Aging impairs key motility patterns in the murine colon. These changes involve a decrease in mucosally-evoked NK2-mediated signaling. Targeting NK2-mediated signaling may provide a novel approach to treating age-related motility disorders in the lower bowel.

Age and Sex-Dependent Differences in the Neurochemical Characterization of Calcitonin Gene-Related Peptide-Like Immunoreactive (CGRP-LI) Nervous Structures in the Porcine Descending Colon

Neurons of the enteric nervous system (ENS) may undergo changes during maturation and aging, but knowledge of physiological stimuli-dependent changes in the ENS is still fragmentary. On the other hand, the frequency of many ENS-related intestinal illnesses depends on age and/or sex. The double immunofluorescence technique was used to study the influence of both of these factors on calcitonin gene-related peptide (CGRP)—positive enteric nervous structures—in the descending colon in young and adult female and castrated male pigs. The influence of age and gender on the number and neurochemical characterization (i.e., co-localization of CGRP with substance P, nitric oxide synthase, galanin, cocaine- and amphetamine-regulated transcript peptide and vesicular acetylcholine transporter) of CGRP-positive nerve structures in the colonic wall has been shown. These observations strongly suggest the participation of CGRP in adaptive processes in the ENS during GI tract maturation. Moreover, although the castration of males may mask some aspects of sex-dependent influences on the ENS, the sex-specific differences in CGRP-positive nervous structures were mainly visible in adult animals. This may suggest that the distribution and exact role of this substance in the ENS depend on the sex hormones.

Changes in murine anorectum signaling across the life course

BACKGROUND

Increasing age is associated with an increase in the incidence of chronic constipation and fecal impaction. The contribution of the natural aging process to these conditions is not fully understood. This study examined the effects of increasing age on the function of the murine anorectum.

METHODS

The effects of increasing age on cholinergic, nitrergic, and purinergic signaling pathways in the murine anorectum were examined using classical organ bath assays to examine tissue function and electrochemical sensing to determine age-related changes in nitric oxide and acetylcholine release.

KEY RESULTS

Nitrergic relaxation increased between 3 and 6 months, peaked at 12 months and declined in the 18 and 24 months groups. These changes were in part explained by an age-related decrease in nitric oxide (NO) release. Cholinergic signaling was maintained with age by an increase in acetylcholine (ACh) release and a compensatory decrease in cholinesterase activity. Age-related changes in purinergic relaxation were qualitatively similar to nitrergic relaxation although the relaxations were much smaller. Increasing age did not alter the response of the anorectum smooth muscle to exogenously applied ACh, ATP, sodium nitroprusside or KCl. Similarly, there was no change in basal tension developed by the anorectum.

CONCLUSIONS AND INFERENCES

The decrease in nitrergic signaling with increasing age may contribute to the age-related fecal impaction and constipation previously described in this model by partially obstructing defecation.

Differences in innervated neurons of the internal anal sphincter based on age and sex: A histological study

AIM

Previous studies have shown sex and age differences in anal sphincter function, but few morphological studies have focused on the quality and quantity of the nerves that control the sphincter muscles. The present study aimed to determine whether there are morphological and quantitative sex and age differences in the nerves in the conjoined longitudinal muscle.

METHODS

This was a single-center, retrospective study using surgical specimens from 44 patients who underwent abdominoperineal resection between 2003 and 2012. Hematoxylin-eosin- and S-100-stained peripheral nerves (nerve fibers and ganglion cells) in the conjoined longitudinal muscle beneath the dentate line were observed microscopically. A qualitative examination assessed the degeneration score, which was based on the presence or absence of karyopyknosis, vacuolar degeneration, acidophilic degeneration of the cytoplasm, denucleation and adventitial neuronal changes. For quantitative examinations, each neuronal and muscular area was traced to calculate the neuronal area ratio in S-100-immunostained photomicrographs at the observation site.

RESULTS

Women had a significantly lower quantity of nerves than men. Older individuals (aged ≥80 years) had a significantly lower quantity of nerves than younger individuals. Furthermore, older individuals tended to show greater morphological changes that appeared to be a result of degeneration.

CONCLUSIONS

The present findings suggest that anal hypofunction in women and older individuals might result from differences in the quantity and quality of the neurons controlling the anal sphincter muscle. Geriatr Gerontol Int 2018; 18: 495-500.

Morphological and neurochemical differences in peptidergic nerve fibers of the mouse vagina

The vagina is innervated by a complex arrangement of sensory, sympathetic, and parasympathetic nerve fibers that contain classical transmitters plus an array of neuropeptides and enzymes known to regulate diverse processes including blood flow and nociception. The neurochemical characteristics and distributions of peptide-containing nerves in the mouse vagina are unknown. This study used multiple labeling immunohistochemistry, confocal maging and analysis to investigate the presence and colocalization of the peptides vasoactive intestinal polypeptide (VIP), calcitonin-gene related peptide (CGRP), substance P (SP), neuropeptide tyrosine (NPY), and the nitric oxide synthesizing enzyme neuronal nitric oxide synthase (nNOS) in nerve fibers of the murine vaginal wall. We compared cervical and vulvar areas of the vagina in young nullipara and older multipara C57Bl/6 mice, and identified differences including that small ganglia were restricted to cervical segments, epithelial fibers were mainly present in vulvar segments and most nerve fibers were found in the lamina propria of the cervical region of the vagina, where a higher number of fibers containing immunoreactivity for VIP, CGRP, SP, or nNOS were found. Two populations of VIP-containing fibers were identified: fibers containing CGRP and fibers containing VIP but not CGRP. Differences between young and older mice were present in multiple layers of the vaginal wall, with older mice showing overall loss of innervation of epithelium of the proximal vagina and reduced proportions of VIP, CGRP, and SP containing nerve fibers in the distal epithelium. The distal vagina also showed increased vascularization and perivascular fibers containing NPY. Immunolabeling of ganglia associated with the vagina indicated the likely origin of some peptidergic fibers. Our results reveal regional differences and age- or parity-related changes in innervation of the mouse vagina, effecting the distribution of neuropeptides with diverse roles in function of the female genital tract.

Gastrointestinal Dysmotility in the Elderly

The number of persons 60 years and older has increased 3-fold between 1950 and 2000. Aging alone does not greatly impact the gastrointestinal (GI) tract. Digestive dysfunction, including esophageal reflux, achalasia, dysphagia, dyspepsia, delayed gastric emptying, constipation, fecal incontinence, and fecal impaction, is a result of the highly prevalent comorbid conditions and the medications with which those conditions are treated. A multidisciplinary approach with the expertise of a geriatrician, gastroenterologist, neurologist, speech pathologist, and physical therapist ensures a comprehensive functional and neurological assessment of the older patient. Radiographic and endoscopic evaluation may be warranted in the evaluation of the symptomatic older patient with consideration given to the risks and benefits of the test being used. Treatment of the digestive dysfunction is aimed at improving health-related quality of life if cure cannot be achieved. Promotion of healthy aging, treatment of comorbid conditions, and avoidance of polypharmacy may prevent some of these digestive disorders. The age-related changes in GI motility, clinical presentation of GI dysmotility, and therapeutic principles in the symptomatic older patient are reviewed here.

Bimodal effect of oxidative stress in internal anal sphincter smooth muscle

Changes in oxidative stress may affect basal tone and relaxation of the internal anal sphincter (IAS) smooth muscle in aging. We examined this issue by investigating the effects of the oxidative stress inducer 6-anilino-5,8-quinolinedione (LY-83583) in basal as well as U-46619-stimulated tone, and nonadrenergic, noncholinergic (NANC) relaxation in rat IAS. LY-83583, which works via generation of reactive oxygen species in living cells, produced a bimodal effect in IAS tone: lower concentrations (0.1 nM to 10 μM) produced a concentration-dependent increase, while higher concentrations (50-100 μM) produced a decrease in IAS tone. An increase in IAS tone by lower concentrations was associated with an increase in RhoA/Rho kinase (ROCK) activity. This was evident by the increase in RhoA/ROCK in the particulate fractions, in ROCK activity, and in the levels of phosphorylated (p) (Thr696)-myosin phosphatase target subunit 1 and p(Thr18/Ser19)-20-kDa myosin light chain. Conversely, higher concentrations of LY-83583 produced inhibitory effects on RhoA/ROCK. Interestingly, both the excitatory and inhibitory effects of LY-83583 in the IAS were reversed by superoxide dismutase. The excitatory effects of LY-83583 were found to resemble those with neuronal nitric oxide synthase (nNOS) inhibition by l-NNA, since it produced a significant increase in the IAS tone and attenuated NANC relaxation. These effects of LY-83583 and l-NNA were reversible by l-arginine. This suggests the role of nNOS inhibition and RhoA/ROCK activation in the increase in IAS tone by LY-83583. These data have important implications in the pathophysiology and therapeutic targeting of rectoanal disorders, especially associated with IAS dysfunction.

Neurogenic mechanisms in bladder and bowel ageing

The prevalence of both urinary and faecal incontinence, and also chronic constipation, increases with ageing and these conditions have a major impact on the quality of life of the elderly. Management of bladder and bowel dysfunction in the elderly is currently far from ideal and also carries a significant financial burden. Understanding how these changes occur is thus a major priority in biogerontology. The functions of the bladder and terminal bowel are regulated by complex neuronal networks. In particular neurons of the spinal cord and peripheral ganglia play a key role in regulating micturition and defaecation reflexes as well as promoting continence. In this review we discuss the evidence for ageing-induced neuronal dysfunction that might predispose to neurogenic forms of incontinence in the elderly.

Aging-associated oxidative stress leads to decrease in IAS tone via RhoA/ROCK downregulation

Internal anal sphincter (IAS) tone plays an important role in rectoanal incontinence (RI). IAS tone may be compromised during aging, leading to RI in certain patients. We examined the influence of oxidative stress in the aging-associated decrease in IAS tone (AADI). Using adult (4-6 mo old) and aging (24-30 mo old) rats, we determined the effect of oxidative stress on IAS tone and the regulatory RhoA/ROCK signal transduction cascade. We determined the effect of the oxidative stress inducer LY83583, which produces superoxide anions (O2 (·-)), on basal and stimulated IAS tone before and after treatment of intact smooth muscle strips and smooth muscle cells with the O2 (·-) scavenger SOD. Our data showed that AADI was associated with a decrease in RhoA/ROCK expression at the transcriptional and translational levels. Oxidative stress with a LY83583-mediated decrease in IAS tone and relaxation of IAS smooth muscle cells was associated with a decrease in RhoA/ROCK signal transduction, which was reversible by SOD. In addition, LY83583 caused a significant decrease in IAS contraction produced by the RhoA activator and a known RhoA/ROCK agonist, U46619, that was also reversible by SOD. The inhibitory effects of LY83583 and the ROCK inhibitor Y27632 on the U46619-induced increase in IAS tone were similar. We conclude that an increase in oxidative stress plays an important role in AADI in the elderly and may be one of the underlying mechanisms of RI in certain aging patients.

Aging of the mammalian gastrointestinal tract: a complex organ system

Gastrointestinal disorders are a major cause of morbidity in the elderly population. The gastrointestinal tract is the most complex organ system; its diverse cells perform a range of functions essential to life, not only secretion, digestion, absorption and excretion, but also, very importantly, defence. The gastrointestinal tract acts not only as a barrier to harmful materials and pathogens but also contains the vast number of beneficial bacterial populations that make up the microbiota. Communication between the cells of the gastrointestinal tract and the central nervous and endocrine systems modifies behaviour; the organisms of the microbiota also contribute to this brain-gut-enteric microbiota axis. Age-related physiological changes in the gut are not only common, but also variable, and likely to be influenced by external factors as well as intrinsic aging of the cells involved. The cellular and molecular changes exhibited by the aging gut cells also vary. Aging intestinal smooth muscle cells exhibit a number of changes in the signalling pathways that regulate contraction. There is some evidence for age-associated degeneration of neurons and glia of the enteric nervous system, although enteric neuronal losses are likely not to be nearly as extensive as previously believed. Aging enteric neurons have been shown to exhibit a senescence-associated phenotype. Epithelial stem cells exhibit increased mitochondrial mutation in aging that affects their progeny in the mucosal epithelium. Changes to the microbiota and intestinal immune system during aging are likely to contribute to wider aging of the organism and are increasingly important areas of analysis. How changes of the different cell types of the gut during aging affect the numerous cellular interactions that are essential for normal gut functions will be important areas for future aging research.

Anorectal physiology and pathophysiology in the elderly

Anorectal medical disorders facing the elderly include fecal incontinence, fecal impaction with overflow fecal incontinence, chronic constipation, dyssynergic defecation, hemorrhoids, anal fissure, and pelvic floor disorders. This article discusses the latest advances in age-related changes in morphology and function of anal sphincter, changes in cellular and molecular biology, alterations in neurotransmitters and reflexes, and their impact on functional changes of the anorectum in the elderly. These biophysiologic changes have implications for the pathophysiology of anorectal disorders. A clear understanding and working knowledge of the functional anatomy and pathophysiology will enable appropriate diagnosis and treatment of these disorders.