Activating metabotropic glutamate receptor‑7 attenuates visceral hypersensitivity in neonatal maternally separated rats

Early life adverse exposures in irritable bowel syndrome: new insights and opportunities

Irritable bowel syndrome (IBS) is a prevalent functional gastrointestinal disorder worldwide. Extensive research has identified multiple factors contributing to its development, including genetic predisposition, chronic infection, gut dysbiosis, aberrant serotonin metabolism, and brain dysfunction. Recent studies have emphasized the critical role of the early life stage as a susceptibility window for IBS. Current evidence suggests that diet can heighten the risk of IBS in offspring by influencing the microbiota composition, intestinal epithelium structure, gene expression, and brain-gut axis. The use of antibiotics during pregnancy and the neonatal period disrupts the normal gut microbiota structure, aligning it with the characteristics observed in IBS patients. Additionally, early life stress impacts susceptibility to IBS by modulating TLR4, NK1, and the hypothalamic-pituitary-adrenal (HPA) axis while compromising the offspring's immune system. Formula feeding facilitates the colonization of pathogenic bacteria in the intestines, concurrently reducing the presence of probiotics. This disruption of the Th1 and Th2 cell balance in the immune system weakens the intestinal epithelial barrier. Furthermore, studies suggest that delivery mode influences the occurrence of IBS by altering the composition of gut microbes. This review aims to provide a comprehensive summary of the existing evidence regarding the impact of adverse early life exposures on IBS during pregnancy, intrapartum, and neonatal period. By consolidating this knowledge, the review enhances our understanding of the direct and indirect mechanisms underlying early life-related IBS and offers new insights and research directions from childhood to adulthood.

Metabotropic glutamate receptor-8 relieves neonatal maternal separation-induced visceral hypersensitivity in rats by regulating expression of TNF-α

Background

Visceral hypersensitivity (VH) is one of the most common causes of irritable bowel syndrome (IBS). The anti-hyperalgesic effects of metabotropic glutamate receptor 8 (mGluR8) has been identified in the central nervous system (CNS). However, whether this receptor has a similar function in the gastrointestinal tract has not been well studied. The present study aimed to explore the role of this receptor in a visceral hypersensitivity-related IBS rat model.

Methods

Neonatal rats were separated from their mothers for 3 hours daily from postnatal day 2 to day 14 to establish neonatal maternal separation (NMS) models. The mGluR8 agonist (S)-3,4-DCPG (10 mg/kg) and the mGluR8 antagonist (RS)-α- methylserine-O-phosphate (MSOP) (10 mg/kg) were used to examine the role of mGLuR8 in the NMS rats. The expression of mGluR8, related inflammatory factors, and inflammatory signal pathways were assessed in colon tissues.

Results

Our data showed that mGluR8 expression was increased in the colonic mucosa of NMS rats compared to controls. In addition, selective activation of mGluR8 ameliorated visceral hypersensitivity, whereas antagonization of mGluR8 aggravated visceral hypersensitivity. Treatment with (S)-3,4-DCPG (10 mg/kg) reduced the expression of myeloperoxidase (MPO) in intestinal mucosa of NMS rats. Furthermore, activating mGluR8 reduced the expression of tumor necrosis factor-α (TNF-α), whereas antagonizing mGluR8 promoted that. The expressions of toll-like receptor 4 (TLR4) and nuclear factor-κB (NF-κB) did not significantly change upon activation or antagonization of mGluR8 receptor.

Conclusions

The activation of mGluR8 receptor ameliorates visceral hypersensitivity in NMS rats, and the underlying mechanisms may be associated with the inhibition of TNF-α and the suppression of colonic inflammatory response.

Neurotransmitter and Intestinal Interactions: Focus on the Microbiota-Gut-Brain Axis in Irritable Bowel Syndrome

Irritable bowel syndrome (IBS) is a functional gastrointestinal disorder of unknown etiology. IBS is caused by a disruption in the gut-brain axis. Given the importance of the gut microbiota in maintaining local and systemic homeostasis of immunity, endocrine, and other physiological processes, the microbiota-gut-brain axis has been proposed as a key regulator in IBS. Neurotransmitters have been shown to affect blood flow regulation, intestinal motility, nutrient absorption, the gastrointestinal immune system, and the microbiota in recent studies. It has the potential role to play a function in the pathophysiology of the gastrointestinal and neurological systems. Transmitters and their receptors, including 5-hydroxytryptamine, dopamine, γ-aminobutyric acid, and histamine, play an important role in IBS, especially in visceral sensitivity and gastrointestinal motility. Studies in this field have shed light on revealing the mechanism by which neurotransmitters act in the pathogenesis of IBS and discovering new therapeutic strategies based on traditional pharmacological approaches that target the nervous system or novel therapies that target the microbiota.

Neurotransmitter Dysfunction in Irritable Bowel Syndrome: Emerging Approaches for Management

Irritable bowel syndrome (IBS) is a functional gastrointestinal disorder whose aetiology is still unknown. Most hypotheses point out the gut-brain axis as a key factor for IBS. The axis is composed of different anatomic and functional structures intercommunicated through neurotransmitters. However, the implications of key neurotransmitters such as norepinephrine, serotonin, glutamate, GABA or acetylcholine in IBS are poorly studied. The aim of this review is to evaluate the current evidence about neurotransmitter dysfunction in IBS and explore the potential therapeutic approaches. IBS patients with altered colorectal motility show augmented norepinephrine and acetylcholine levels in plasma and an increased sensitivity of central serotonin receptors. A decrease of colonic mucosal serotonin transporter and a downregulation of α2 adrenoceptors are also correlated with visceral hypersensitivity and an increase of 5-hydroxyindole acetic acid levels, enhanced expression of high affinity choline transporter and lower levels of GABA. Given these neurotransmitter dysfunctions, novel pharmacological approaches such as 5-HT₃ receptor antagonists and 5-HT₄ receptor agonists are being explored for IBS management, for their antiemetic and prokinetic effects. GABA-analogous medications are being considered to reduce visceral pain. Moreover, agonists and antagonists of muscarinic receptors are under clinical trials. Targeting neurotransmitter dysfunction could provide promising new approaches for IBS management.

Pathophysiologic Role of Neurotransmitters in Digestive Diseases

Neurotransmitters are special molecules that serve as messengers in chemical synapses between neurons, cells, or receptors, including catecholamines, serotonin, dopamine, and other neurotransmitters, which play an important role in both human physiology and pathology. Compelling evidence has indicated that neurotransmitters have an important physiological role in various digestive diseases. They act as ligands in combination with central or peripheral receptors, and transmits signals through chemical synapses, which are involved in regulating the physiological and pathological processes of the digestive tract organs. For instance, neurotransmitters regulate blood circulation and affect intestinal movement, nutrient absorption, the gastrointestinal innate immune system, and the microbiome. In this review, we will focus on the role of neurotransmitters in the pathogenesis of digestive tract diseases to provide novel therapeutic targets for new drug development in digestive diseases.

Electroacupuncture Improves IBS Visceral Hypersensitivity by Inhibiting the Activation of Astrocytes in the Medial Thalamus and Anterior Cingulate Cortex

Objective

To explore whether the effect of electroacupuncture (EA) on visceral hypersensitivity (VH) in rats with irritable bowel syndrome (IBS) is related to the changes of astrocyte activation in the medial thalamus (MT) and anterior cingulate cortex (ACC).

Method

Male Sprague-Dawley rats were randomly divided into the normal control (NC) group, model control (MC) group, electroacupuncture (EA) group, and fluorocitrate (FCA) group. A model of visceral hypersensitivity was established by neonatal colorectal irritation. In the EA group, needles were inserted into the skin at the Tianshu (ST25) and Shangjuxu (ST37) acupoints, once a day for 7 days. The FCA group received intrathecal injection of FCA on the 1st, 4th, and 7th days. Visceral hypersensitivity was evaluated by the abdominal withdrawal reflex (AWR), and glial fibrillary acidic protein (GFAP) mRNA and protein levels in the MT and ACC were detected by real-time PCR, immunohistochemistry, and western blots.

Results

The AWR score in the MC group was significantly higher than in the NC group, and EA and FCA reduced the AWR score of VH rats. GFAP mRNA and protein levels in the MT and ACC of rats in the MC group were significantly increased compared with the NC group. After either electroacupuncture or fluorocitrate, GFAP mRNA and protein levels in the MT and ACC were both clearly reduced.

Conclusion

Electroacupuncture alleviates IBS visceral hypersensitivity by inhibiting the activation of astrocytes in the MT and ACC.

Early-life stress and inflammation: A systematic review of a key experimental approach in rodents

Repeated maternal separation is the most widely used pre-clinical approach to investigate the relationship between early-life chronic stress and its neuropsychiatric and physical consequences. In this systematic review, we identified 46 studies that conducted repeated maternal separation or single-episode maternal separation and reported measurements of interleukin-1b, interleukin-6, interleukin-10, tumour necrosis factor-alpha, or microglia activation and density. We report that in the short-term and in the context of later-life stress, repeated maternal separation has pro-inflammatory immune consequences in diverse tissues. Repeated maternal separation animals exhibit greater microglial activation and elevated pro-inflammatory cytokine signalling in key brain regions implicated in human psychiatric disorders. Notably, repeated maternal separation generally has no long-term effect on cytokine expression in any tissue in the absence of later-life stress. These observations suggest that the elevated inflammatory signalling that has been reported in humans with a history of early-life stress may be the joint consequence of ongoing stressor exposure together with potentiated neural and/or immune responsiveness to stressors. Finally, our findings provide detailed guidance for future studies interrogating the causal roles of early-life stress and inflammation in disorders such as major depression.