Nitric Oxide Regulates Estrus Cycle Dependent Colonic Motility in Mice

Enteric Neuronal Substrates Underlying Spontaneous and Evoked Neurogenic Contractions in Mouse Colon

BACKGROUND & AIMS

Gastrointestinal motility persists when peripheral cholinergic signaling is blocked genetically or pharmacologically, and a recent study suggests nitric oxide drives propagating neurogenic contractions.

METHODS

To determine the neuronal substrates that underlie these contractions, we measured contractile-associated movements together with calcium responses of cholinergic or nitrergic myenteric neurons in unparalyzed ex vivo preparations of whole mouse colon. We chose to look at these 2 subpopulations because they encompass nearly all myenteric neurons.

RESULTS

Many, but not all, cholinergic neurons of the middle colon exhibited contractile-associated calcium responses with distinct features. By contrast, a large population of nitrergic neurons of the middle colon shut their activity off just before contraction onset, whereas another population of nitrergic neurons initiated a response just after contraction onset. When contractions were evoked by a variety of stimuli to the proximal and distal colon, the same neuronal subtypes exhibited the same activity patterns during the contraction. However, stimulation of proximal colon produced a transient, stimulation-locked response before the ensuing contraction in a subpopulation of cholinergic neurons and in nearly all nitrergic neurons, suggesting that distinct neuronal activity patterns underlie specific stimuli. Finally, although blockade of nitric oxide failed to arrest the generation or propagation of neurogenic contractions, chemogenetic elimination of nitrergic activity impaired their propagation to middle and distal colon.

CONCLUSIONS

Genetic approaches were used to study the activity patterns of enteric neurons underlying spontaneous and evoked neurogenic contractions in unparalyzed colon. These approaches can be combined with a variety of other approaches to identify the neuronal subtypes and subclasses that coordinate colonic motility.

Sex specific effects of environmental toxin-derived alpha synuclein on enteric neuronal-epithelial interactions

Background

Parkinson's Disease (PD) is a neurodegenerative disorder with prodromal gastrointestinal (GI) issues often emerging decades before motor symptoms. Pathologically, PD can be driven by accumulation of misfolded alpha synuclein (aSyn) protein in the brain and periphery, including the GI tract. Disease epidemiology differs by sex, with men twice as likely to develop PD. Women, however, experience faster disease progression, higher mortality, and more severe GI symptoms. Gut calcitonin gene related peptide (CGRP) is a key regulator of intestinal contractions and visceral pain. The current study tests the hypothesis that sex differences in GI symptomology in PD are the result of aSyn aggregation altering enteric CGRP signaling pathways.

Methods

To facilitate peripheral aSyn aggregation, the pesticide rotenone was administered intraperitoneally once daily for two weeks to male and female mice. Mice were sacrificed two weeks after the last rotenone injection and immunohistochemistry was performed on sections of proximal colon.

Key Results

Levels of aSyn were heightened in myenteric plexus neurons and a subset of neurons immunoreactive to CGRP in rotenone treated mice. Female mice exhibited 153% more myenteric aSyn, 26% more apical CGRP immunoreactivity, and 66.7% more aSyn in apical CGRP + fibers after rotenone when compared to males. Goblet cell numbers were diminished but the individual cells were larger in the apical regions of crypts in the colons of rotenone treated mice.

Conclusions

This study used a mouse model of PD to uncover sex specific alterations in enteric neuronal and epithelial populations, underscoring the importance of considering sex as a biological variable while investigating prodromal GI symptoms.

KEY POINTS

Mouse model of Parkinson's Disease (PD) was used to investigate sex specific impact of enteric alpha synuclein (aSyn) on colonic goblet cells and CGRP + neurons and fibers. Sex specific alterations in intestinal neuronal and epithelial signaling pathways in response to aSyn provides insight into sex differences in PD etiology and prodromal gastrointestinal symptoms.

West Nile virus triggers intestinal dysmotility via T cell-mediated enteric nervous system injury

Intestinal dysmotility syndromes have been epidemiologically associated with several antecedent bacterial and viral infections. To model this phenotype, we previously infected mice with the neurotropic flavivirus West Nile virus (WNV) and demonstrated intestinal transit defects. Here, we found that within 1 week of WNV infection, enteric neurons and glia became damaged, resulting in sustained reductions of neuronal cells and their networks of connecting fibers. Using cell-depleting antibodies, adoptive transfer experiments, and mice lacking specific immune cells or immune functions, we show that infiltrating WNV-specific CD4+ and CD8+ T cells damaged the enteric nervous system (ENS) and glia, which led to intestinal dysmotility; these T cells used multiple and redundant effector molecules including perforin and Fas ligand. In comparison, WNV-triggered ENS injury and intestinal dysmotility appeared to not require infiltrating monocytes, and damage may have been limited by resident muscularis macrophages. Overall, our experiments support a model in which antigen-specific T cell subsets and their effector molecules responding to WNV infection direct immune pathology against enteric neurons and supporting glia that results in intestinal dysmotility.

Faster Gastrointestinal Transit, Reduced Small Intestinal Smooth Muscle Tone and Dysmotility in the Nlgn3R451C Mouse Model of Autism

Individuals with autism often experience gastrointestinal issues but the cause is unknown. Many gene mutations that modify neuronal synapse function are associated with autism and therefore may impact the enteric nervous system that regulates gastrointestinal function. A missense mutation in the Nlgn3 gene encoding the cell adhesion protein Neuroligin-3 was identified in two brothers with autism who both experienced severe gastrointestinal dysfunction. Mice expressing this mutation (Nlgn3R451C mice) are a well-studied preclinical model of autism and show autism-relevant characteristics, including impaired social interaction and communication, as well as repetitive behaviour. We previously showed colonic dysmotility in response to GABAergic inhibition and increased myenteric neuronal numbers in the small intestine in Nlgn3R451C mice bred on a mixed genetic background. Here, we show that gut dysfunction is a persistent phenotype of the Nlgn3 R451C mutation in mice backcrossed onto a C57BL/6 background. We report that Nlgn3R451C mice show a 30.9% faster gastrointestinal transit (p = 0.0004) in vivo and have 6% longer small intestines (p = 0.04) compared to wild-types due to a reduction in smooth muscle tone. In Nlgn3R451C mice, we observed a decrease in resting jejunal diameter (proximal jejunum: 10.6% decrease, p = 0.02; mid: 9.8%, p = 0.04; distal: 11.5%, p = 0.009) and neurally regulated dysmotility as well as shorter durations of contractile complexes (mid: 25.6% reduction in duration, p = 0.009; distal: 30.5%, p = 0.004) in the ileum. In Nlgn3R451C mouse colons, short contractions were inhibited to a greater extent (57.2% by the GABAA antagonist, gabazine, compared to 40.6% in wild-type mice (p = 0.007). The inhibition of nitric oxide synthesis decreased the frequency of contractile complexes in the jejunum (WT p = 0.0006, Nlgn3R451C p = 0.002), but not the ileum, in both wild-type and Nlgn3R451C mice. These findings demonstrate that changes in enteric nervous system function contribute to gastrointestinal dysmotility in mice expressing the autism-associated R451C missense mutation in the Neuroligin-3 protein.

The substantia nigra modulates proximal colon tone and motility in a vagally-dependent manner in the rat

A monosynaptic pathway connects the substantia nigra pars compacta (SNpc) to neurons of the dorsal motor nucleus of the vagus (DMV). This monosynaptic pathway modulates the vagal control of gastric motility. It is not known, however, whether this nigro-vagal pathway also modulates the tone and motility of the proximal colon. In rats, microinjection of retrograde tracers in the proximal colon and of anterograde tracers in SNpc showed that bilaterally labelled colonic-projecting neurons in the DMV received inputs from SNpc neurons. Microinjections of the ionotropic glutamate receptor agonist, NMDA, in the SNpc increased proximal colonic motility and tone, as measured via a strain gauge aligned with the colonic circular smooth muscle; the motility increase was inhibited by acute subdiaphragmatic vagotomy. Upon transfection of SNpc with pAAV-hSyn-hM3D(Gq)-mCherry, chemogenetic activation of nigro-vagal nerve terminals by brainstem application of clozapine-N-oxide increased the firing rate of DMV neurons and proximal colon motility; both responses were abolished by brainstem pretreatment with the dopaminergic D1-like antagonist SCH23390. Chemogenetic inhibition of nigro-vagal nerve terminals following SNpc transfection with pAAV-hSyn-hM4D(Gi)-mCherry decreased the firing rate of DMV neurons and inhibited proximal colon motility. These data suggest that a nigro-vagal pathway modulates activity of the proximal colon motility tonically via a discrete dopaminergic synapse in a manner dependent on vagal efferent nerve activity. Impairment of this nigro-vagal pathway may contribute to the severely reduced colonic transit and prominent constipation observed in both patients and animal models of parkinsonism. KEY POINTS: Substantia nigra pars compacta (SNpc) neurons are connected to the dorsal motor nucleus of the vagus (DMV) neurons via a presumed direct pathway. Brainstem neurons in the lateral DMV innervate the proximal colon. Colonic-projecting DMV neurons receive inputs from neurons of the SNpc. The nigro-vagal pathway modulates tone and motility of the proximal colon via D1-like receptors in the DMV. The present study provides the mechanistic basis for explaining how SNpc alterations may lead to a high rate of constipation in patients with Parkinson's Disease.

Postnatal development of the enteric neurons expressing neuronal nitric oxide synthase

Neurons, expressing neuronal nitric oxide synthase (nNOS) in the enteric ganglia are inhibitory motor neurons or interneurons. The aim of the study was to identify the percentage, cross-sectional area of nNOS-immunoreactive (IR) neurons and their colocalization with choline acetyltransferase (ChAT), vasoactive intestinal polypeptide (VIP), and neuropeptide Y in the intramural ganglia of the myenteric (MP) and submucous plexus (SP) of the small intestine (SI) and large intestine (LI) of rats of different age groups using immunohistochemical methods. In the intramural ganglia of the MP, the largest percentage of nNOS-IR neurons was detected in newborn rats in the LI (81 ± 0.9%) and SI (48 ± 4.1%). Subsequently, it decreased in ontogenesis up to 60 days of life (26 ± 0.9% LI, 29 ± 3.2% SI), and did not change until senescence. In the SP, abundant nNOS-IR neurons were also detected in newborns (82 ± 7.0% SI, 85 ± 3.2% LI), while their percentage decreased significantly in the next 20 days. Furthermore, a very small number of nNOS-IR neurons was detected in 30-day- and 2-month-old animals, but they again appeared in large numbers in aged rats. In the MP, the highest percentage of nNOS+/ChAT+ neurons was in 1-day-old, 10-day-old, and 2-year-old rats. In the SP, the largest number of nNOS-IR neurons colocalized ChAT regardless of age. In the MP of all rats, many nNOS-IR neurons colocalized VIP, and the maximal percentage of nNOS+/VIP+ neurons was found in 2-year-old rats, minimal-in newborns. In conclusion, nNOS expression in neurons of the gut is decreased in early postnatal ontogenesis and subsequently increased in aged rats.

Arctiin alleviates functional constipation by enhancing intestinal motility in mice

Functional constipation (FC), a common symptom that is primarily associated with intestinal motility dysfunction, is a common problem worldwide. Arctiin (Arc) is a lignan glycoside isolated from the Chinese herbal medicine Arctium lappa L., which is a health food in China. The present study aimed to evaluate the laxative effects of Arc against FC in mice. A model of FC induced by loperamide (5 mg/kg) was established in male Institute of Cancer Research (ICR) mice. Arc was administered at a dose of 100 mg/kg as a protective agent. The faecal status, intestinal motility and histological analyses were evaluated. Furthermore, the levels of gastrointestinal motility-associated neurotransmitters, such as motilin (MTL), nitric oxide (NO), and brain-derived neurotrophic factor (BDNF) and the protective effect of Arc on interstitial cells of Cajal (ICC) were assessed. Arc treatment reversed the loperamide-induced reduction in faecal number and water content and the intestinal transit ratio in ICR mice. Histological analysis confirmed that Arc administration mitigated colonic injury. Moreover, Arc treatment increased levels of motilin and brain-derived neurotrophic factor while decreasing nitric oxide levels and ICC injury in the colon of FC mice. Arc decreased inflammation induction and aquaporin expression levels. Owing to its pro-intestinal motility property, Arc was shown to have a protective effect against FC and may thus serve as a promising therapeutic strategy for the management of FC.

Luteolin ameliorates loperamide-induced functional constipation in mice

Functional constipation (FC) is one of the most common gastrointestinal disorders characterized by hard stools and infrequent bowel movements, which is associated with dysfunction of the enteric nervous system and intestinal motility. Luteolin, a naturally occurring flavone, was reported to possess potential pharmacological activities on intestinal inflammation and nerve injury. This study aimed to explore the role of luteolin and its functional mechanism in loperamide-induced FC mice. Our results showed that luteolin treatment reversed the reduction in defecation frequency, fecal water content, and intestinal transit ratio, and the elevation in transit time of FC models. Consistently, luteolin increased the thickness of the muscular layer and lessened colonic histopathological injury induced by loperamide. Furthermore, we revealed that luteolin treatment increased the expression of neuronal protein HuC/D and the levels of intestinal motility-related biomarkers, including substance P (SP), vasoactive intestinal polypeptide (VIP), and acetylcholine (ACh), as well as interstitial cells of Cajal (ICC) biomarker KIT proto-oncogene, receptor tyrosine kinase (C-Kit), and anoctamin-1 (ANO1), implying that luteolin mediated enhancement of colonic function and contributed to the anti-intestinal dysmotility against loperamide-induced FC. Additionally, luteolin decreased the upregulation of aquaporin (AQP)-3, AQP-4, and AQP-8 in the colon of FC mice. In summary, our data showed that luteolin might be an attractive option for developing FC-relieving medications.

The Comparison of the Influence of Bisphenol A (BPA) and Its Analogue Bisphenol S (BPS) on the Enteric Nervous System of the Distal Colon in Mice

Bisphenol A (BPA), commonly used as a plasticizer in various branches of industry has a strong negative effect on living organisms. Therefore, more and more often it is replaced in production of plastics by other substances. One of them is bisphenol S (BPS). This study for the first time compares the impact of BPA and BPS on the enteric neurons using double immunofluorescence technique. It has been shown that both BPA and BPS affect the number of enteric neurons containing substance P (SP), galanin (GAL), vasoactive intestinal polypeptide (VIP), neuronal isoform of nitric oxide synthase (nNOS-a marker of nitrergic neurons) and/or vesicular acetylcholine transporter (VAChT- a marker of cholinergic neurons). The changes noted under the impact of both bisphenols are similar and consisted of an increase in the number of enteric neurons immunoreactive to all neuronal factors studied. The impact of BPS on some populations of neurons was stronger than that noted under the influence of BPA. The obtained results clearly show that BPS (similarly to BPA) administered for long time is not neutral for the enteric neurons even in relatively low doses and may be more potent than BPA for certain neuronal populations.

Examining enteric nervous system function in rat and mouse: an interspecies comparison of colonic motility

Gastrointestinal motility is crucial to gut health and has been associated with different disorders such as inflammatory bowel diseases and postoperative ileus. Despite rat and mouse being the two animal models most widely used in gastrointestinal research, minimal studies in rats have investigated gastrointestinal motility. Therefore, our study provides a comparison of colonic motility in the mouse and rat to clarify species differences and assess the relative effectiveness of each animal model for colonic motility research. We describe the protocol modifications and optimization undertaken to enable video imaging of colonic motility in the rat. Apart from the broad difference in terms of gastrointestinal diameter and length, we identified differences in the fundamental histology of the proximal colon such that the rat had larger villus height-to-width and villus height-to-crypt depth ratios compared with mouse. Since gut motility is tightly regulated by the enteric nervous system (ENS), we investigated how colonic contractile activity within each rodent species responds to modulation of the ENS inhibitory neuronal network. Here we used N^ω-nitro-l-arginine (l-NNA), an inhibitor of nitric oxide synthase (NOS) to assess proximal colon responses to the stimulatory effect of blocking the major inhibitory neurotransmitter, nitric oxide (NO). In rats, the frequency of proximal colonic contractions increased in the presence of l-NNA (vs. control levels) to a greater extent than in mice. This is despite a similar number of NOS-expressing neurons in the myenteric plexus across species. Given this increase in colonic contraction frequency, the rat represents another relevant animal model for investigating how gastrointestinal motility is regulated by the inhibitory neuronal network of the ENS.NEW & NOTEWORTHY Mice and rats are widely used in gastrointestinal research but have fundamental differences that make them important as different models for different questions. We found that mice have a higher villi length-to-width and villi length-to-crypt depth ratio than rat in proximal colon. Using the ex vivo video imaging technique, we observed that rat colon has more prominent response to blockade of major inhibitory neurotransmitter (nitric oxide) in myenteric plexus than mouse colon.

Neonatal antibiotics have long term sex-dependent effects on the enteric nervous system

Infants and young children receive the highest exposures to antibiotics globally. Although there is building evidence that early life exposure to antibiotics increases susceptibility to various diseases including gut disorders later in life, the lasting impact of early life antibiotics on the physiology of the gut and its enteric nervous system (ENS) remains unclear. We treated neonatal mice with the antibiotic vancomycin during their first 10 postnatal days, then examined potential lasting effects of the antibiotic treatment on their colons during young adulthood (6 weeks old). We found that neonatal vancomycin treatment disrupted the gut functions of young adult female and male mice differently. Antibiotic-exposed females had significantly longer whole gut transit while antibiotic-treated males had significantly lower faecal weights compared to controls. Both male and female antibiotic-treated mice had greater percentages of faecal water content. Neonatal vancomycin treatment also had sexually dimorphic impacts on the neurochemistry and Ca2+ activity of young adult myenteric and submucosal neurons. Myenteric neurons of male mice were more disrupted than those of females, while opposing changes in submucosal neurons were seen in each sex. Neonatal vancomycin also induced sustained changes in colonic microbiota and lasting depletion of mucosal serotonin (5-HT) levels. Antibiotic impacts on microbiota and mucosal 5-HT were not sex-dependent, but we propose that the responses of the host to these changes are sex-specific. This first demonstration of long-term impacts of neonatal antibiotics on the ENS, gut microbiota and mucosal 5-HT has important implications for gut function and other physiological systems of the host. KEY POINTS: Early life exposure to antibiotics can increase susceptibility to diseases including functional gastrointestinal (GI) disorders later in life. Yet, the lasting impact of this common therapy on the gut and its enteric nervous system (ENS) remains unclear. We investigated the long-term impact of neonatal antibiotic treatment by treating mice with the antibiotic vancomycin during their neonatal period, then examining their colons during young adulthood. Adolescent female mice given neonatal vancomycin treatment had significantly longer whole gut transit times, while adolescent male and female mice treated with neonatal antibiotics had significantly wetter stools. Effects of neonatal vancomycin treatment on the neurochemistry and Ca2+ activity of myenteric and submucosal neurons were sexually dimorphic. Neonatal vancomycin also had lasting effects on the colonic microbiome and mucosal serotonin biosynthesis that were not sex-dependent. Different male and female responses to antibiotic-induced disruptions of the ENS, microbiota and mucosal serotonin biosynthesis can lead to sex-specific impacts on gut function.

Mechanisms underlying initiation of propulsion in guinea pig distal colon

Colonic motor complexes (CMCs) are a major neurogenic activity in guineapig distal colon. The identity of the enteric neurons that initiate this activity is not established. Specialized intrinsic primary afferent neurons (IPANs) are a major candidate. We aimed to test this hypothesis. To do this, segments of guineapig distal colon were suspended vertically in heated organ baths and propulsive forces acting on a pellet inside the lumen were recorded by isometric force transducer while pharmacological agents were applied to affect IPAN function. In the absence of drugs, CMCs acted periodically on the pellet, generating peak propulsive forces of 12.7 ± 5 g at 0.56 ± 0.22 cpm, lasting 49 ± 17 s (215 preparations; n = 60). Most but not all CMCs were abolished by nicotinic receptor blockade to inhibit fast excitatory synaptic transmission (50/62 preparations; n = 25). Remarkably, CMCs inhibited by hexamethonium were restored by a pharmacological strategy that aimed to enhance IPAN excitability. Thus, CMCs were restored by increased smooth muscle tension (using BAY K8644, bethanechol or carbachol) and by IPAN excitation using phorbol dibutyrate; NK3 receptor agonist, senktide; and partially by αCGRP. The IPAN inhibitor, 5,6-dichloro-1-ethyl-1,3-dihydro-2H-benzimidazole-2-one (DCEBIO), decreased CMC frequency. CGRP, but not NK3-receptor antagonists, decreased CMC frequency in naive preparations. Finally, CMCs were blocked by tetrodotoxin, and this was not reversed by any drugs listed above. These results support a major role for IPANs that does not require fast synaptic transmission, in the periodic initiation of neurogenic propulsive contractions. Endogenous CGRP plays a role in determining CMC frequency, whereas further unidentified signaling pathways may determine their amplitude and duration.NEW & NOTEWORTHY The colonic motor complex (CMC) initiates propulsion in guinea pig colon. Here, CMCs evoked by an intraluminal pellet were restored during nicotinic receptor blockade by pharmacological agents that directly or indirectly enhance intrinsic primary afferent neuron (IPAN) excitability. IPANs are the only enteric neuron in colon that contain CGRP. Blocking CGRP receptors decreased CMC frequency, implicating their role in CMC initiation. The results support a role for IPANs in the initiation of CMCs.

Tamoxifen administration alters gastrointestinal motility in mice

BACKGROUND

Tamoxifen is widely used for Cre-estrogen receptor-mediated genomic recombination in transgenic mouse models to mark cells for lineage tracing and to study gene function. However, recent studies have highlighted off-target effects of tamoxifen in various tissues and cell types when used for induction of Cre recombination. Despite the widespread use of these transgenic Cre models to assess gastrointestinal (GI) function, the effect of tamoxifen exposure on GI motility has not been described.

METHODS

We examined the effects of tamoxifen on GI motility by measuring total GI transit, gastric emptying, small intestinal transit, and colonic contractility in wild-type adult mice.

KEY RESULTS

We observed a significant delay in total GI transit in tamoxifen-treated mice, with unaltered gastric emptying, accelerated small intestinal transit, and abnormal colonic motility.

CONCLUSION

Our findings highlight the importance of considering GI motility alterations induced by tamoxifen when designing protocols that utilize tamoxifen as a Cre-driver for studying GI function.