Upregulation of β1-adrenoceptors is involved in the formation of gastric dysmotility in the 6-hydroxydopamine rat model of Parkinson's disease

Gastrointestinal dysmotility in rodent models of Parkinson's disease

Multiple studies describe prodromal, nonmotor dysfunctions that affect the quality of life of patients who subsequently develop Parkinson's disease (PD). These prodromal dysfunctions comprise a wide array of autonomic issues, including severe gastrointestinal (GI) motility disorders such as dysphagia, delayed gastric emptying, and chronic constipation. Indeed, strong evidence from studies in humans and animal models suggests that the GI tract and its neural, mainly vagal, connection to the central nervous system (CNS) could have a major role in the etiology of PD. In fact, misfolded α-synuclein aggregates that form Lewy bodies and neurites, i.e., the histological hallmarks of PD, are detected in the enteric nervous system (ENS) before clinical diagnosis of PD. The aim of the present review is to provide novel insights into the pathogenesis of GI dysmotility in PD, focusing our attention on functional, neurochemical, and molecular alterations in animal models.

Opposing effects of β-2 and β-1 adrenergic receptor signaling on neuroinflammation and dopaminergic neuron survival in α-synuclein-mediated neurotoxicity

BACKGROUND

Noradrenergic neurons in the locus coeruleus (LC) are the primary source of norepinephrine (NE) in the brain and degeneration of these neurons is reported in the early stages of Parkinson's disease (PD), even prior to dopaminergic neuron degeneration in the substantia nigra (SN), which is a hallmark of PD pathology. NE depletion is generally associated with increased PD pathology in neurotoxin-based PD models. The effect of NE depletion in other models of PD-like α-synuclein-based models is largely unexplored. In PD models and in human patients, β-adrenergic receptors' (AR) signaling is associated with a reduction of neuroinflammation and PD pathology. However, the effect of NE depletion in the brain and the extent of NE and β-ARs signaling involvement in neuroinflammation, and dopaminergic neuron survival is poorly understood.

METHODS

Two mouse models of PD, a 6OHDA neurotoxin-based model and a human α-synuclein (hα-SYN) virus-based model of PD, were used. DSP-4 was used to deplete NE levels in the brain and its effect was confirmed by HPLC with electrochemical detection. A pharmacological approach was used to mechanistically understand the impact of DSP-4 in the hα-SYN model of PD using a norepinephrine transporter (NET) and a β-AR blocker. Epifluorescence and confocal imaging were used to study changes in microglia activation and T-cell infiltration after β1-AR and β2-AR agonist treatment in the hα-SYN virus-based model of PD.

RESULTS

Consistent with previous studies, we found that DSP-4 pretreatment increased dopaminergic neuron loss after 6OHDA injection. In contrast, DSP-4 pretreatment protected dopaminergic neurons after hα-SYN overexpression. DSP-4-mediated protection of dopaminergic neurons after hα-SYN overexpression was dependent on β-AR signaling since using a β-AR blocker prevented DSP-4-mediated dopaminergic neuron protection in this model of PD. Finally, we found that the β-2AR agonist, clenbuterol, reduced microglia activation, T-cell infiltration, and dopaminergic neuron degeneration, whereas xamoterol a β-1AR agonist showed increased neuroinflammation, blood brain barrier permeability (BBB), and dopaminergic neuron degeneration in the context of hα-SYN-mediated neurotoxicity.

CONCLUSIONS

Our data demonstrate that the effects of DSP-4 on dopaminergic neuron degeneration are model specific, and suggest that in the context of α-SYN-driven neuropathology, β2-AR specific agonists may have therapeutic benefit in PD.

6-OHDA-Induced Changes in Colonic Segment Contractility in the Rat Model of Parkinson's Disease

Background

Gastrointestinal dysfunction is one of the most common non-motor symptoms in Parkinson's disease (PD). The exact mechanisms behind these symptoms are not clearly understood. Studies in the well-established 6-hydroxydopamine (6-OHDA) lesioned rats of PD have shown altered contractility in isolated circular and longitudinal smooth muscle strips of distal colon. Contractile changes in proximal colon and distal ileum are nevertheless poorly studied. Moreover, segments may serve as better tissue preparations to understand the interplay between circular and longitudinal smooth muscle. This study aimed to compare changes in contractility between isolated full-thickness distal colon muscle strips and segments, and extend the investigation to proximal colon and distal ileum in the 6-OHDA rat model.

Methods

Spontaneous contractions and contractions induced by electrical field stimulation (EFS) and by the non-selective muscarinic agonist methacholine were investigated in strip and/or segment preparations of smooth muscle tissue from distal and proximal colon and distal ileum in an in vitro organ bath comparing 6-OHDA-lesioned rats with Sham-operated animals. Key Results. Our data showed increased contractility evoked by EFS and methacholine in segments, but not in circular and longitudinal tissue strips of distal colon after central 6-OHDA-induced dopamine denervation. Changes in proximal colon segments were also displayed in high K⁺ Krebs-induced contractility and spontaneous contractions.

Conclusions

This study further confirms changes in smooth muscle contractility in distal colon and to some extent in proximal colon, but not in distal ileum in the 6-OHDA rat model of PD. However, the changes depended on tissue preparation.

Impaired nitrergic relaxation in pyloric sphincter of the 6-OHDA Parkinson's disease rat

Patients with Parkinson's disease (PD) often suffer from delayed gastric emptying, but the underlying mechanism remains unclear. We have shown previously that a PD rat model comprising bilateral substantia nigra destruction by 6-hydroxydopamine (6-OHDA rats) exhibits gastroparesis with alteration of neural nitric oxide synthase (nNOS) and acetylcholine in gastric corpus. However, changes in pyloric motility in the 6-OHDA rats have not been characterized. Solid gastric emptying test, immunofluorescence, Western blot, and in vitro pyloric motility recordings were used to assess pyloric motor function in the 6-OHDA rats. The 6-OHDA-treated rats displayed delayed solid gastric emptying and a lower basal pyloric motility index. In the 6-OHDA rats, high K⁺-induced transient contractions were weaker in pyloric sphincters. Electric field stimulation (EFS)-induced pyloric sphincter relaxation was lower in the 6-OHDA rats. N^G-nitro-l-arginine methyl ester (l-NAME), a nonselective inhibitor of NOS, markedly inhibited the EFS-induced relaxation in both control and 6-OHDA rats. Pretreatment of tetrodotoxin abolished the effect of EFS on the pyloric motility. In addition, nNOS-positive neurons were extensively distributed in the pyloric myenteric plexus, whereas the number of nNOS-immunoreactive neurons and the protein expression of nNOS were significantly decreased in the pyloric muscularis of 6-OHDA rats. However, sodium nitroprusside-induced pyloric relaxations were similar between the control and 6-OHDA rats. These results indicate that the pyloric sphincters of 6-OHDA rats exhibit both weakened contraction and relaxation. The latter may be due to reduced nNOS in the pyloric myenteric plexus. The dysfunction of the pyloric sphincter might be involved in the delayed gastric emptying.NEW & NOTEWORTHY Reduced nitrergic neurons in pyloric myenteric plexus potently contributed to the attenuated relaxation in 6-hydroxydopamine (6-OHDA) rats, subsequently affecting gastric emptying. SNP could well improve the relaxation of pylori in 6-OHDA rats. The present study provides new insight into the diagnosis and treatment of delayed gastric emptying in patients with PD.

Activation of α7nAChR Protects Against Gastric Inflammation and Dysmotility in Parkinson's Disease Rats

The cholinergic anti-inflammatory pathway (CAIP) has been proposed to regulate gastrointestinal inflammation via acetylcholine released from the vagus nerve activating α7 nicotinic receptor (α7nAChR) on macrophages. Parkinson’s disease (PD) patients and PD rats with substantia nigra (SN) lesions exhibit gastroparesis and a decayed vagal pathway. To investigate whether activating α7nAChR could ameliorate inflammation and gastric dysmotility in PD rats, ELISA, western blot analysis, and real-time PCR were used to detect gastric inflammation. In vitro and in vivo gastric motility was investigated. Proinflammatory mediator levels and macrophage numbers were increased in the gastric muscularis of PD rats. α7nAChR was located on the gastric muscular macrophages of PD rats. The α7nAChR agonists PNU-282987 and GTS-21 decreased nuclear factor κB (NF-κB) activation and monocyte chemotactic protein-1 mRNA expression in the ex vivo gastric muscularis of PD rats, and these effects were abolished by an α7nAChR antagonist. After treatment with PNU-282987 in vivo, the PD rats showed decreased NF-κB activation, inflammatory mediator production, and contractile protein expression and improved gastric motility. The present study reveals that α7nAChR is involved in the development of gastroparesis in PD rats and provides novel insight for the treatment of gastric dysmotility in PD patients.

Probiotics: Potential novel therapeutics for microbiota-gut-brain axis dysfunction across gender and lifespan

Probiotics are live microorganisms, which when administered in adequate amounts, present a health benefit for the host. While the beneficial effects of probiotics on gastrointestinal function are generally well recognized, new animal research and clinical studies have found that alterations in gut microbial communities can have a broad range of effects throughout the body. Non-intestinal sites impacted include the immune, endocrine, cardiovascular and the central nervous system (CNS). In particular, there has been a growing interest and appreciation about the role that gut microbiota may play in affecting CNS-related function through the 'microbiota-gut-brain axis'. Emerging evidence suggests potential therapeutic benefits of probiotics in several CNS conditions, such as anxiety, depression, autism spectrum disorders and Parkinson's disease. There may also be some gender-specific variances in terms of probiotic mediated effects, with the gut microbiota shaping and being concurrently molded by the hormonal environment governing differences between the sexes. Probiotics may influence the ability of the gut microbiome to affect a variety of biological processes in the host, including neurotransmitter activity, vagal neurotransmission, generation of neuroactive metabolites and inflammatory response mediators. Some of these may engage in cross talk with host sex hormones, such as estrogens, which could be of relevance in relation to their effects on stress response and cognitive health. This raises the possibility of gender-specific variation with regards to the biological action of probiotics, including that on the endocrine and central nervous systems. In this review we aim to describe the current understanding in relation to the role and use of probiotics in microbiota-gut-brain axis-related dysfunction. Furthermore, we will address the conceptualization and classification of probiotics in the context of gender and lifespan as well as how restoring gut microbiota composition by clinical or dietary intervention can help in supporting health outcomes other than those related to the gastrointestinal tract. We also evaluate how these new learnings may impact industrial effort in probiotic research and the discovery and development of novel and more personalized, condition-specific, beneficial probiotic therapeutic agents.

MiR-17-5p promotes cellular proliferation and invasiveness by targeting RUNX3 in gastric cancer

BACKGROUND

Dysregulated microRNAs (miRNAs/miRs) directly modulate the biological functions of gastric cancer (GC) cells and contribute to the initiation and progression of GC. MiR-17-5p and runt-related transcription factor 3 (RUNX3) have been reported to be related to GC progression; however, the specific interaction between miR-17-5p and RUNX3 in GC require further investigation.

METHODS

Western blotting, real-time PCR and immunohistochemistry were used to study the expression level of miR-17-5p and RUNX3 in gastric cancer tissues and plasma. The biological function of miR-17-5p was examined by measuring cell proliferation, apoptosis and cell invasion in vitro; the target gene of miR17-5p was identified by luciferase reporter assays, RNA Binding protein immunoprecipitation (RIP) and western blotting. In vivo animal study was conducted to confirm the role of miR-17-5p during tumorigensis of gastric cancer.

RESULTS

This study showed that miR17-5p was upregulated in the plasma and tissues of patients with GC, while RUNX3 was downregulated in GC tissues. Functional experiments indicated that miR-17-5p mimics promoted the proliferation and invasion of GC via suppressing apoptosis in vitro. Furthermore, bioinformatics prediction, luciferase reporter assays, reverse transcription quantitative polymerase chain reaction assays, RIP and western blotting analysis demonstrated that RUNX3 was a direct target gene of miR-17-5p in GC. In addition, overexpression of RUNX3 suppressed the proliferation and invasiveness of GC cells. In vivo data indicated miR-17-5p agomir significantly promoted tumor growth. In contrast, miR-17-5p antagomir notably decreased tumor volume compared with control group.

CONCLUSIONS

MiR-17-5p promoted the progression of GC via directly targeting RUNX3, suggesting that miR-17-5p and RUNX3 could be considered as diagnostic and therapeutic targets for patients with GC.

Gastric smooth muscle cells manifest an abnormal phenotype in Parkinson's disease rats with gastric dysmotility

Gastroparesis is a common symptom in Parkinson's disease (PD) and whether any change occurs in gastric smooth muscle cells (SMCs) of PD patients is unclear. We previously reported that rats with bilateral substantia nigra lesions induced by 6-hydroxydopamine (6-OHDA), referred to as 6-OHDA rats, manifest typical gastroparesis. In the present study, we further investigate the underlying mechanism. By means of an organ bath system and an implantable radiotelemetry system, both a weakened contractile force of gastric circular smooth muscle and gastric myoelectric activity were detected in the 6-OHDA rats and phasic and tonic contractions elicited by carbachol or high concentration of potassium were significantly reduced in gastric circular muscle strips. A thickened smooth muscle layer was observed under a light microscope and an ultrastructure of hypertrophic SMCs, with increased caveolae and decreased dense bodies, was observed under transmission electron microscope. Furthermore, the mRNA and protein expression levels of contractile markers (myosin light chain 20, myosin heavy chain 11 and α-smooth muscle actin) and the transcription factor serum response factor (SRF) were significantly decreased, while the TNFα and IL-1β content was increased in the 6-OHDA rats. These results suggest that the decreased contractile force in 6-OHDA rats may be associated with the phenotypic abnormality observed in SMCs, which is due to downregulated contractile proteins induced by decreased SRF expression in the inflammatory muscular microenvironment.

Upregulation of gastric norepinephrine with beta-adrenoceptors and gastric dysmotility in a rat model of functional dyspepsia

Disordered motility is one of the most important pathogenic characteristics of functional dyspepsia (FD), although the underlying mechanisms remain unclear. Since the sympathetic system is important to the regulation of gastrointestinal motility, the present study aimed to investigate the role of norepinephrine (NE) and adrenoceptors in disordered gastric motility in a rat model with FD. The effect of exogenous NE on gastric motility in control and FD rats was measured through an organ bath study. The expression and distribution of beta-adrenoceptors were examined by real-time PCR, Western blotting and immunofluorescence. The results showed that endogenous gastric NE was elevated in FD rats, and hyperreactivity of gastric smooth muscle to NE and delayed gastric emptying were observed in the rat model of FD. The mRNA levels of beta1-adrenoceptor and norepinephrine transporter (NET) and the protein levels of beta2-adrenoceptor and NET were increased significantly in the gastric corpus of FD rats. All three subtypes of beta-adrenoceptors were abundantly distributed in the gastric corpus of rats. In conclusion, the enhanced NE and beta-adrenoceptors and NETs may be contributed to the disordered gastric motility in FD rats.

Critical Evaluation of Animal Models of Gastrointestinal Disorders

Preclinical research remains an important tool for discovery and validation of novel therapeutics for gastrointestinal disorders. While in vitro assays can be used to verify receptor-ligand interactions and test for structural activity of new compounds, only whole-animal studies can demonstrate drug efficacy within the gastrointestinal system. Most major gastrointestinal disorders have been modeled in animals; however the translational relevance of each model is not equal. The purpose of this chapter is to provide a critical evaluation of common animal models that are being used to develop pharmaceuticals for gastrointestinal disorders. For brevity, the models are presented for upper gastrointestinal disorders involving the esophagus, stomach, and small intestine and lower gastrointestinal disorders that focus on the colon. Particular emphasis is used to explain the face and construct validity of each model, and the limitations of each model, including data interpretation, are highlighted. This chapter does not evaluate models that rely on surgical or other non-pharmacological interventions for treatment.

Dopamine enhances duodenal epithelial permeability via the dopamine D5 receptor in rodent

AIM

The intestinal barrier is made up of epithelial cells and intercellular junctional complexes to regulate epithelial ion transport and permeability. Dopamine (DA) is able to promote duodenal epithelial ion transport through D1-like receptors, which includes subtypes of D₁ (D₁ R) and D₅ (D₅ R), but whether D1-like receptors influence the duodenal permeability is unclear.

METHODS

FITC-dextran permeability, short-circuit current (I_SC ), Western blot, immunohistochemistry and ELISA were used in human D₅ R transgenic mice and hyperendogenous enteric DA (HEnD) rats in this study.

RESULTS

Dopamine induced a downward deflection in I_SC and an increase in FITC-dextran permeability of control rat duodenum, which were inhibited by the D1-like receptor antagonist, SCH-23390. However, DA decreased duodenal transepithelial resistance (TER), an effect also reversed by SCH-23390. A strong immunofluorescence signal for D₅ R, but not D₁ R, was observed in the duodenum of control rat. In human D₅ R knock-in transgenic mice, duodenal mucosa displayed an increased basal I_SC with high FITC-dextran permeability and decreased TER with a lowered expression of tight junction proteins, suggesting attenuated duodenal barrier function in these transgenic mice. D₅ R knock-down transgenic mice manifested a decreased basal I_SC with lowered FITC-dextran permeability. Moreover, an increased FITC-dextran permeability combined with decreased TER and tight junction protein expression in duodenal mucosa were also observed in HEnD rats.

CONCLUSION

This study demonstrates, for the first time, that DA enhances duodenal permeability of control rat via D₅ R, which provides new experimental and theoretical evidence for the influence of DA on duodenal epithelial barrier function.

Gastric motor dysfunctions in Parkinson's disease: Current pre-clinical evidence

Parkinson's disease (PD) is associated with several non-motor symptoms, such as behavioral changes, urinary dysfunction, sleep disorders, fatigue and, above all, gastrointestinal (GI) dysfunction, including gastric dysmotility, constipation and anorectal dysfunction. Delayed gastric emptying, progressing to gastroparesis, is reported in up to 100% of patients with PD, and it occurs at all stages of the disease with severe consequences to the patient's quality of life. The presence of α-synuclein (α-syn) aggregates in myenteric neurons throughout the digestive tract, as well as morpho-functional alterations of the enteric nervous system (ENS), have been documented in PD. In particular, gastric dysmotility in PD has been associated with an impairment of the brain-gut axis, involving the efferent fibers of the vagal pathway projecting directly to the gastric myenteric plexus. The present review intends to provide an integrated overview of available knowledge on the possible role played by the ENS, considered as a semi-autonomous nervous network, in the pathophysiology of gastric dysmotility in PD. Particular attention has been paid review how translational evidence in humans and studies in pre-clinical models are allowing a better understanding of the functional, neurochemical and molecular alterations likely underlying gastric motor abnormalities occurring in PD.

Alteration of enteric monoamines with monoamine receptors and colonic dysmotility in 6-hydroxydopamine-induced Parkinson's disease rats

Constipation is common in Parkinson's disease (PD), in which monoamines (dopamine [DA], norepinephrine [NE], and 5-hydroxytryptamine [5-HT]) play an important role. Rats microinjected with 6-hydroxydopamine (6-OHDA) into the bilateral substantia nigra (SN) exhibit constipation, but the role of monoamines and their receptors is not clear. In the present study, colonic motility, monoamine content, and the expression of monoamine receptors were examined using strain gauge force transducers, ultraperformance liquid chromatography tandem mass spectrometry, immunofluorescence, and Western blot. The 6-OHDA rats displayed a significant reduction in dopaminergic neurons in the SN and a decreased time on rota-rod test and a marked decrease in daily fecal production and fecal water content. The amplitude of colonic spontaneous contraction was obviously decreased in 6-OHDA rats. Blocking D1-like receptor and β3-adrenoceptor (β3-AR) significantly reduced the inhibition of DA and NE on the colonic motility, respectively, whereas the 5-HT and 5-HT4 receptor agonists promoted the colonic motility. Moreover, DA content was increased in the colonic muscularis externa of 6-OHDA rats. The protein expression of β3-ARs was notably upregulated, but 5-HT4 receptors were significantly decreased in the colonic muscularis externa of 6-OHDA rats. We conclude that enhanced DA and β3-ARs and decreased 5-HT4 receptors may be contributed to the colonic dysmotility and constipation observed in 6-OHDA rats.