Activation of corticotropin-releasing factor neurons and microglia in paraventricular nucleus precipitates visceral hypersensitivity induced by colorectal distension in rats

Research Progress of Central and Peripheral Corticotropin-Releasing Hormone in Irritable Bowel Syndrome with Comorbid Dysthymic Disorders

Irritable bowel syndrome (IBS) is considered a stress disorder characterized by psychological and gastrointestinal dysfunction. IBS patients not only suffer from intestinal symptoms such as abdominal pain, diarrhea, or constipation but also, experience dysthymic disorders such as anxiety and depression. Studies have found that corticotropin-releasing hormone plays a key role in IBS with comorbid dysthymic disorders. Next, we will summarize the effects of corticotropin-releasing hormone from the central nervous system and periphery on IBS with comorbid dysthymic disorders and relevant treatments based on published literatures in recent years.

Neural circuits regulating visceral pain

Visceral hypersensitivity, a common clinical manifestation of irritable bowel syndrome, may contribute to the development of chronic visceral pain, which is a major challenge for both patients and health providers. Neural circuits in the brain encode, store, and transfer pain information across brain regions. In this review, we focus on the anterior cingulate cortex and paraventricular nucleus of the hypothalamus to highlight the progress in identifying the neural circuits involved in visceral pain. We also discuss several neural circuit mechanisms and emphasize the importance of cross-species, multiangle approaches and the identification of specific neurons in determining the neural circuits that control visceral pain.

Butterflies in the gut: the interplay between intestinal microbiota and stress

Psychological stress is a global issue that affects at least one-third of the population worldwide and increases the risk of numerous psychiatric disorders. Accumulating evidence suggests that the gut and its inhabiting microbes may regulate stress and stress-associated behavioral abnormalities. Hence, the objective of this review is to explore the causal relationships between the gut microbiota, stress, and behavior. Dysbiosis of the microbiome after stress exposure indicated microbial adaption to stressors. Strikingly, the hyperactivated stress signaling found in microbiota-deficient rodents can be normalized by microbiota-based treatments, suggesting that gut microbiota can actively modify the stress response. Microbiota can regulate stress response via intestinal glucocorticoids or autonomic nervous system. Several studies suggest that gut bacteria are involved in the direct modulation of steroid synthesis and metabolism. This review provides recent discoveries on the pathways by which gut microbes affect stress signaling and brain circuits and ultimately impact the host's complex behavior.

Paraventricular Mast Cell-Derived Histamine Activates CRH Neurons to Mediate Adult Visceral Hypersensitivity Induced by Neonatal Maternal Separation

Neonatal maternal separation (NMS) is an early-life stress (ELS) that can result in adult visceral hypersensitivity, which is usually manifested as chronic visceral pain. Although mast cells and corticotropin-releasing hormone (CRH) neurons are involved in stress response, whether there is an interaction between mast cells and CRH neurons in hypothalamic paraventricular nucleus (PVN) during the ELS-induced visceral hypersensitivity remains elusive. Herein, we established an NMS model by separating neonatal mice from their mothers, and observed that these mice presented visceral hypersensitivity in adulthood, as indicated by elevated abdominal withdrawal reflex and lowered visceral pain threshold. The NMS-induced adult visceral hypersensitivity was accompanied by activation of mast cells and CRH neurons in PVN. Also, NMS increased the histamine content (an inflammatory mediator mainly released by mast cells) and histamine H2 receptor (H2R) expression of CRH neurons in PVN. Remarkably, intra-PVN administration with mast cell stabilizer attenuated the NMS-induced CRH neuronal activation and adult visceral pain, while histamine administration showed the opposite effects. Moreover, intra-PVN injection with H2R antagonist alleviated the NMS-induced CRH neuronal activation, PKA and CREB phosphorylation, and importantly, adult visceral pain. Together, our findings revealed a role of an interaction between paraventricular mast cells and CRH neurons in NMS-induced adult visceral hypersensitivity, thereby providing a perspective for the management of visceral pain.

Minocycline synergizes with corticosteroids in reducing colitis severity in mice via the modulation of pro-inflammatory molecules

Background: A few studies have highlighted the anti-inflammatory properties of minocycline in reducing colitis severity in mice, but its molecular mechanism is not fully understood. The aim of this study was to determine the anti-inflammatory properties of minocycline and the expression/activity profiles of molecules involved in pro-inflammatory signaling cascades, cytokines, and molecules involved in the apoptotic machinery. The synergistic effect between minocycline and corticosteroids was also evaluated. Methods: The effects of various treatment approaches were determined in mice using the dextran sulfate sodium (DSS) colitis model at gross and microscopic levels. The expression/activity profiles of various pro- or anti-inflammatory molecules were determined using Western blotting and polymerase chain reaction (PCR). Results: Minocycline treatment significantly reduced colitis severity using prophylactic and treatment approaches and produced a synergistic effect with budesonide and methylprednisolone in reducing the active state of colitis. This was mediated in part through reduced colonic expression/activity of pro-inflammatory molecules, cytokines, proteins involved in the apoptotic machinery, and increased expression of the anti-inflammatory cytokine IL-10. Conclusion: Minocycline synergizes with corticosteroids to reduce colitis severity, which could reduce their dose-dependent side effects and treatment cost. The reduction in colitis severity was achieved by modulating the expression/activity profiles of various pro- and anti-inflammatory signaling molecules, cytokines, and molecules involved in the apoptotic machinery.

Modulation of visceral pain by brain nuclei and brain circuits and the role of acupuncture: a narrative review

Visceral pain is a complex and heterogeneous pain condition that is often associated with pain-related negative emotional states, including anxiety and depression, and can exert serious effects on a patient's physical and mental health. According to modeling stimulation protocols, the current animal models of visceral pain mainly include the mechanical dilatation model, the ischemic model, and the inflammatory model. Acupuncture can exert analgesic effects by integrating and interacting input signals from acupuncture points and the sites of pain in the central nervous system. The brain nuclei involved in regulating visceral pain mainly include the nucleus of the solitary tract, parabrachial nucleus (PBN), locus coeruleus (LC), rostral ventromedial medulla (RVM), anterior cingulate cortex (ACC), paraventricular nucleus (PVN), and the amygdala. The neural circuits involved are PBN-amygdala, LC-RVM, amygdala-insula, ACC-amygdala, claustrum-ACC, bed nucleus of the stria terminalis-PVN and the PVN-ventral lateral septum circuit. Signals generated by acupuncture can modulate the central structures and interconnected neural circuits of multiple brain regions, including the medulla oblongata, cerebral cortex, thalamus, and hypothalamus. This analgesic process also involves the participation of various neurotransmitters and/or receptors, such as 5-hydroxytryptamine, glutamate, and enkephalin. In addition, acupuncture can regulate visceral pain by influencing functional connections between different brain regions and regulating glucose metabolism. However, there are still some limitations in the research efforts focusing on the specific brain mechanisms associated with the effects of acupuncture on the alleviation of visceral pain. Further animal experiments and clinical studies are now needed to improve our understanding of this area.

Distinct Effects of Non-absorbed Agents Rifaximin and Berberine on the Microbiota-Gut-Brain Axis in Dysbiosis-induced Visceral Hypersensitivity in Rats

Background/Aims

Irritable bowel syndrome (IBS) is accepted as a disorder of gut-brain interactions. Berberine and rifaximin are non-absorbed antibiotics and have been confirmed effective for IBS treatment, but there is still lack of direct comparison of their effects. This study aims to compare the effect of the 2 drugs on the alteration of gut-brain axis caused by gut microbiota from IBS patients.

Methods

Germ-free rats received fecal microbiota transplantation from screened IBS patients and healthy controls. After 14 days' colonization, rats were administrated orally with berberine, rifaximin or vehicle respectively for the next 14 days. The visceral sensitivity was evaluated, fecal microbiota profiled and microbial short chain fatty acids were determined. Immunofluorescence staining and morphological analysis were performed to evaluate microglial activation.

Results

Visceral hypersensitivity induced by IBS-fecal microbiota transplantation was relieved by berberine and rifaximin, and berberine increased sucrose preference rate. Microbial α-diversity were reduced by both drugs. Compared with rifaximin, berberine significantly changed microbial structure and enriched Lachnoclostridium. Furthermore, berberine but not rifaximin significantly increased fecal concentrations of acetate and propionate acids. Berberine restored the morphological alterations of microglia induced by dysbiosis, which may be associated with its effect on the expression of microbial gene pathways involved in peptidoglycan biosynthesis. Rifaximin affected neither the numbers of activated microglial cells nor the microglial morphological alterations.

Conclusions

Berberine enriched Lachnoclostridium, reduced the expression of peptidoglycan biosynthesis genes and increased acetate and propionate. The absence of these actions of rifaximin may explain the different effects of the drugs on microbiota-gut-brain axis.

"Sentinel or accomplice": gut microbiota and microglia crosstalk in disorders of gut-brain interaction

Abnormal brain-gut interaction is considered the core pathological mechanism behind the disorders of gut-brain interaction (DGBI), in which the intestinal microbiota plays an important role. Microglia are the "sentinels" of the central nervous system (CNS), which participate in tissue damage caused by traumatic brain injury, resist central infection and participate in neurogenesis, and are involved in the occurrence of various neurological diseases. With in-depth research on DGBI, we could find an interaction between the intestinal microbiota and microglia and that they are jointly involved in the occurrence of DGBI, especially in individuals with comorbidities of mental disorders, such as irritable bowel syndrome (IBS). This bidirectional regulation of microbiota and microglia provides a new direction for the treatment of DGBI. In this review, we focus on the role and underlying mechanism of the interaction between gut microbiota and microglia in DGBI, especially IBS, and the corresponding clinical application prospects and highlight its potential to treat DGBI in individuals with psychiatric comorbidities.

Brain Neuropeptides, Neuroinflammation, and Irritable Bowel Syndrome

BACKGROUND

Irritable bowel syndrome (IBS) is a functional bowel disorder characterized by chronic abdominal symptoms, but its pathogenesis is not fully understood.

SUMMARY

We have recently shown in rats that neuropeptides such as orexin, ghrelin, and oxytocin act in the brain to improve the intestinal barrier dysfunction, which is a major pathophysiology of IBS. We have additionally shown that the neuropeptides injected intracisternally induced a visceral antinociceptive action against colonic distension. Since it has been known that intestinal barrier dysfunction causes visceral hypersensitivity, the other main pathophysiology of IBS, the neuropeptides act centrally to reduce leaky gut, followed by improvement of visceral sensation, leading to therapeutic action on IBS. It has been recently reported that there is a bidirectional relationship between neuroinflammation in the brain and the pathophysiology of IBS. For example, activation of microglia in the brain causes visceral hypersensitivity. Accumulating evidence has suggested that orexin, ghrelin, or oxytocin could improve neuroinflammation in the CNS. All these results suggest that neuropeptides such as orexin, ghrelin, and oxytocin act in the brain to improve intestinal barrier function and visceral sensation and also induce a protective action against neuroinflammation in the brain.

KEY MESSAGES

We therefore speculated that orexin, ghrelin, or oxytocin in the brain possess dual actions, improvement of visceral sensation/leaky gut in the gut, and reduction of neuroinflammation in the brain, thereby inducing a therapeutic effect on IBS in a convergent manner.

Electroacupuncture in the treatment of IBS in rats: investigation of the mechanisms of CRH+ neurons in the paraventricular nucleus

Electroacupuncture (EA) is well documented to treat irritable bowel syndrome (IBS). However, the mechanism of the central nervous system related to IBS and acupuncture stimulation is still not well known. In this study, a rat model of IBS was established by cold-restraint comprehensive stresses for 15 days, and it was found that the levels of corticotropin-releasing hormone (CRH), corticosterone (CORT), and adrenocorticotropic hormone (ACTH) in the peripheral serum were increased; the visceral sensitivity was enhanced; and the intestinal motility was accelerated, specifically, there was an enhancement in the discharge frequency of neurons in the paraventricular nucleus (PVN). EA treatment for 3 days, 20 min/day, alleviated the increase in the levels of CRH, CORT, and ACTH in the peripheral serum of rats, reduced the visceral sensitivity of IBS rats, and inhibited colon movement and discharge frequency of the neurons in the PVN. In addition, EA could reduce the excitability of CRH neurons and the expression of corticotropin-releasing hormone receptor 1 (CRHR1) and corticotropin-releasing hormone receptor 2 (CRHR2) in PVN. At the same time, the expression of CRH, CRHR1, and CRHR2 in the peripheral colon was decreased. Taken together, EA appears to regulate intestinal functional activity through the central CRH nervous system, revealing the central regulation mechanism of EA in IBS rats, and providing a scientific research basis for the correlation among the meridians, viscera, and brain.NEW & NOTEWORTHY The purpose of this research was to determine the central regulatory mechanism of electroacupuncture (EA) in rats with irritable bowel syndrome (IBS). Our results showed that combined with the serum changes in corticotropin-releasing hormone (CRH), corticosterone (CORT), and adrenocorticotropic hormone (ACTH), the improvement of IBS by EA was related to them. Furthermore, EA could regulate intestinal functional activity through the central CRH+ nervous system.

Minocycline Protects PC12 Cells Against Cadmium-Induced Neurotoxicity by Modulating Apoptosis

Cadmium (Cd) is a well-known heavy metal and a neurotoxic agent. Minocycline (Mino) is an anti-microbial agent with a lipophilic structure that crosses the blood-brain barrier and enters the cerebral tissue. In recent studies, Mino has been introduced as an antioxidant and anti-apoptotic chemical compound, and therefore, it was examined as a protective candidate against Cd-induced neurotoxicity. In this study, PC12 cells were exposed to Cd alone, or after being pre-treated with Mino. Initially, the cell viability and oxidative stress were analyzed using the MTT assay and fluorimetry, respectively. Then, Cd-induced apoptosis and Mino anti-apoptotic effect were evaluated in both intrinsic and extrinsic pathways using western blot analysis. Exposing PC12 cells to Cd for 24 h decreased cell viability and increased production of reactive oxygen species in comparison with the control group. Cd (35 μM) also elevated the level of caspase-8, Bax/Bcl-2, and caspase-3 proteins in the cells. Mino pre-treatment for 2 h (100 nM) increased the number of viable cells and decreased the production of reactive oxygen species, and the level of all apoptotic markers in comparison to Cd-treated cells. Considering all the evidence, it appears that Mino holds promising antioxidant and anti-apoptotic activity and can protect cells against Cd-induced oxidative stress and prevent apoptotic cell death.

Site-Specific knockdown of microglia in the locus coeruleus regulates hypervigilant responses to social stress in female rats

BACKGROUND

Women are at increased risk for psychosocial stress-related anxiety disorders, yet mechanisms regulating this risk are unknown. Psychosocial stressors activate microglia, and the resulting neuroimmune responses that females exhibit heightened sensitivity to may serve as an etiological factor in their elevated risk. However, studies examining the role of microglia during stress in females are lacking.

METHODS

Microglia were manipulated in the stress-sensitive locus coeruleus (LC) of female rats in the context of social stress in two ways. First, intra-LC lipopolysaccharide (LPS; 0 or 3 μg/side, n = 5-6/group), a potent TLR4 agonist and microglial activator, was administered. One hour later, rats were exposed to control or an aggressive social defeat encounter between two males (WS, 15-min). In a separate study, females were treated with intra-LC or intra-central amygdala mannosylated liposomes containing clodronate (m-CLD; 0 or 25 μg/side, n = 13-14/group), a compound toxic to microglia. WS-evoked burying, cardiovascular responses, and sucrose preference were measured. Brain and plasma cytokines were quantified, and cardiovascular telemetry assessed autonomic balance.

RESULTS

Intra-LC LPS augmented the WS-induced burying response and increased plasma corticosterone and interleukin-1β (IL-1β). Further, the efficacy and selectivity of microinjected m-CLD was fully characterized. In the context of WS, intra-LC m-CLD attenuated the hypervigilant burying response during WS as well as the accumulation of intra-LC IL-1β. Intra-central amygdala m-CLD had no effect on WS-evoked behavior.

CONCLUSIONS

These studies highlight an innovative method for depleting microglia in a brain region specific manner and indicate that microglia in the LC differentially regulate hypervigilant WS-evoked behavioral and autonomic responses.

Targeting the endocannabinoid system for the treatment of abdominal pain in irritable bowel syndrome

The management of visceral pain in patients with disorders of gut-brain interaction, notably irritable bowel syndrome, presents a considerable clinical challenge, with few available treatment options. Patients are increasingly using cannabis and cannabinoids to control abdominal pain. Cannabis acts on receptors of the endocannabinoid system, an endogenous system of lipid mediators that regulates gastrointestinal function and pain processing pathways in health and disease. The endocannabinoid system represents a logical molecular therapeutic target for the treatment of pain in irritable bowel syndrome. Here, we review the physiological and pathophysiological functions of the endocannabinoid system with a focus on the peripheral and central regulation of gastrointestinal function and visceral nociception. We address the use of cannabinoids in pain management, comparing them to other treatment modalities, including opioids and neuromodulators. Finally, we discuss emerging therapeutic candidates targeting the endocannabinoid system for the treatment of pain in irritable bowel syndrome.

microRNA-181a contributes to gastric hypersensitivity in rats with diabetes by regulating TLR4 expression

Aim: The aim of this study is to investigate the mechanism and interaction of microRNA-181a (miR-181a), toll-like receptor 4 (TLR4) and nuclear factor-kappa B (NF-κB) in gastric hypersensitivity in diabetic rats. Methods: Diabetes was induced by a single intraperitoneal injection of streptozotocin (STZ; 65 mg/kg) in female SD rats. Gastric balloon distension technique was used to measure diabetic gastric hypersensitivity. Gastric-specific (T7-T10) dorsal root ganglion (DRG) neurons were acutely dissociated to measure excitability with patch-clamp techniques. Western blotting was employed to measure the expressions of TLR4, TRAF6 and NF-κB subunit p65 in T7-T10 DRGs. The expressions of microRNAs in T7-T10 DRGs were measured with quantitative real-time PCR and fluorescence in situ hybridization. Dual-luciferase reporter gene assay was used to detect the targeting regulation of microRNAs on TLR4. Results: (1) Diabetic rats were more sensitive to graded gastric balloon distention at 2 and 4 weeks. (2) The expression of TLR4 was significantly up-regulated in T7-T10 DRGs of diabetic rats. Intrathecal injection of CLI-095 (TLR4-selective inhibitor) attenuated diabetic gastric hypersensitivity, and markedly reversed the hyper-excitability of gastric-specific DRG neurons. (3) The expressions of miR-181a and miR-7a were significantly decreased in diabetic rats. MiR-181a could directly regulate the expression of TLR4, while miR-7a couldn't. (4) Intrathecal injection of miR-181a agomir down-regulated the expression of TLR4, reduced the hyper-excitability of gastric-specific neurons, and alleviated gastric hypersensitivity. (5) p65 and TLR4 were co-expressed in Dil-labeled DRG neurons. (6) Inhibition of p65 attenuated diabetic gastric hypersensitivity and hyper-excitability of gastric-specific DRG neurons. (7) The expression of TRAF6 was significantly up-regulated in diabetic rats. CLI-095 treatment also reduced the expression of TRAF6 and p65. Conclusion: The reduction of microRNA-181a in T7-T10 DRGs might up-regulate TLR4 expression. TLR4 activated NF-κB through MyD88-dependent signaling pathway, increased excitability of gastric-specific DRG neurons, and contributed to diabetic gastric hypersensitivity.

Olfactory Stimulation Successfully Modulates the Neurochemical, Biochemical and Behavioral Phenotypes of the Visceral Pain

Visceral pain (VP) is the organ-derived nociception in which increased inflammatory reaction and exaggerated activation of the central nucleus of the amygdala (CeA) may contribute to this deficiency. Considering the amygdala also serves as the integration center for olfaction, the present study aimed to determine whether olfactory stimulation (OS) would effectively depress over-activation and inflammatory reaction in CeA, and successfully relieve VP-induced abnormalities. Adult rats subjected to intraperitoneal injection of acetic acid inhaled lavender essential oil for 2 or 4 h. The potential benefits of OS were determined by measuring the pro-inflammatory cytokine level, intracellular potassium and the upstream small-conductance calcium-activated potassium (SK) channel expression, together with detecting the stress transmitters that participated in the modulation of CeA activity. Results indicated that in VP rats, strong potassium intensity, reduced SK channel protein level, and increased corticotropin-releasing factor, c-fos, and substance P immuno-reactivities were detected in CeA. Enhanced CeA activation corresponded well with increased inflammatory reaction and decreased locomotion, respectively. However, in rats subjected to VP and received OS, all above parameters were significantly returned to normal levels with higher change detected in treating OS of 4h. As OS successfully depresses inflammation and CeA over-activation, application of OS may serve as an alternative and effective strategy to efficiently relieve VP-induced deficiency.

Wenshen-Jianpi prescription, a Chinese herbal medicine, improves visceral hypersensitivity in a rat model of IBS-D by regulating the MEK/ERK signal pathway

The goal of the study was to analyze whether WJP can alleviate visceral hypersensitivity in IBS-D model rats. In this study, 36 Sprague–Dawley (SD) rats aged 4 weeks old were randomly divided into two groups: the model group (n = 27) and the control group (n = 9). The rat model of IBS-D was established by modified compound methods for 4 weeks. After the modification, IBS-D rats were randomly divided into three groups, namely, the IBS-D model group (n = 9), the positive drug group (n = 9), and the WJP group (n = 9), with different interventions, respectively. The control group was fed and allowed to drink water routinely. The Bristol stool scale scores were used to assess the severity of diarrhea. Abdominal withdrawal reflex (AWR) scores were used to assess visceral sensitivity. Expression of TNF-α was measured, and histopathological examinations were performed to assess colon inflammation in IBS-D model rats. Key factors of the MEK/ERK signal pathway in the tissue of the colon and hippocampus were measured to analyze the mechanism of WJP. Compared with the control group, the Bristol stool scale scores in the model group were significantly increased (p < 0.0001). The scores of the WJP group were significantly decreased compared with the model group (p = 0.0001). Compared with the control group, AWR scores in the model group at each pressure level were significantly increased (p = 0.0003, p < 0.0001, p = 0.0007, and p = 0.0009). AWR scores of the WJP group were significantly decreased compared with the model group (p = 0.0003, p = 0.0007, p = 0.0007, and p = 0.0009). Compared with the control group, the model group had significantly higher expression of TNF-α in the colon tissue (p < 0.0001). However, the WJP group had significantly lower level of TNF-α compared with the model group (p < 0.0001). Meanwhile, compared with the control group, the relative expression of the proteins of p-MEK1/2, p-ERK1, and p-ERK2 in the colon tissue was significantly increased in the model group (p < 0.0001). Compared with the model group, the relative expression of the proteins in the colon tissue were significantly decreased in the WJP group (p < 0.0001, p = 0.0019, and p = 0.0013). Compared with the control group, the relative expression of the proteins of p-MEK1/2, p-ERK1, and p-ERK2 in the hippocampus tissue were significantly increased in the model group (p < 0.0001). Compared with the model group, the relative expression of the proteins in the hippocampus tissue were significantly decreased in the WJP group (p = 0.0126, p = 0.0291, and p = 0.0145). The results indicated that WJP can alleviate visceral hypersensitivity in IBS-D model rats, possibly mediated by downregulating the expression of TNF-α, p-MEK1/2, p-ERK1, and p-ERK2 in the colon tissue. At the same time, WJP also affects downregulating the expression of p-MEK1/2, p-ERK1, and p-ERK2 in the hippocampus tissue.

Acyloxyacyl hydrolase regulates microglia-mediated pelvic pain

Chronic pelvic pain conditions such as interstitial cystitis/bladder pain syndrome (IC/BPS) remain clinical and mechanistic enigmas. Microglia are resident immune cells of the central nervous system (CNS) that respond to changes in the gut microbiome, and studies have linked microglial activation to acute and chronic pain in a variety of models, including pelvic pain. We have previously reported that mice deficient for the lipase acyloxyacyl hydrolase (AOAH) develop pelvic allodynia and exhibit symptoms, comorbidities, and gut dysbiosis mimicking IC/BPS. Here, we assessed the role of AOAH in microglial activation and pelvic pain. RNAseq analyses using the ARCHS4 database and confocal microscopy revealed that AOAH is highly expressed in wild type microglia but at low levels in astrocytes, suggesting a functional role for AOAH in microglia. Pharmacologic ablation of CNS microglia with PLX5622 resulted in decreased pelvic allodynia in AOAH-deficient mice and resurgence of pelvic pain upon drug washout. Skeletal analyses revealed that AOAH-deficient mice have an activated microglia morphology in the medial prefrontal cortex and paraventricular nucleus, brain regions associated with pain modulation. Because microglia express Toll-like receptors and respond to microbial components, we also examine the potential role of dysbiosis in microglial activation. Consistent with our hypothesis of microglia activation by leakage of gut microbes, we observed increased serum endotoxins in AOAH-deficient mice and increased activation of cultured BV2 microglial cells by stool of AOAH-deficient mice. Together, these findings demonstrate a role for AOAH in microglial modulation of pelvic pain and thus identify a novel therapeutic target for IC/BPS.

The involvement of Pellino-1 downregulation in the modulation of visceral hypersensitivity via the TLR4/NF-κB pathway in the rat fastigial nucleus

Irritable bowel syndrome (IBS) is a common functional bowel disorder whose key characteristics include chronic visceral hypersensitivity (CVH) and abnormal brain-gut interactions. Pellino-1 is an E3 ubiquitin ligase, mediating the degradation or modification of targeted proteins. Some brain regions, such as the fastigial nucleus (FN), may play important roles in CVH; however, the molecular mechanism underlying this phenomenon is not clear. In this study, we assessed the roles of Pellino-1 within the FN in modulating VH by generating a colorectal distention (CRD) model in male Sprague-Dawley rats. Our results showed that the downregulation of Pellino-1 in the fastigial nucleus (FN) was involved in the modulation of visceral hypersensitivity. The expression of Pellino-1 was downregulated in the FN of adult CRD rats compared with control rats, whereas TLR4 and NF-κB were upregulated in the CRD model. To overexpress Pellino-1, a lentivirus specifically expressing Pellino-1 and green fluorescent protein was administered into the FN. The overexpression of Pellino-1 increased the visceral sensitivity of CRD rats, and the expression of TLR4 and NF-κB increased further. After administration of TAK-242 (a specific TLR4 inhibitor), the visceral response to overexpression of Pellino-1 was reversed. Overall, the findings indicated the involvement of the FN in the development of CVH; the downregulation of Pellino-1 in the FN acted through the TLR4/NF-κB pathway to protect against CVH in a CRD rat model.

Unraveling the role of Epac1-SOCS3 signaling in the development of neonatal-CRD-induced visceral hypersensitivity in rats

Aims

Visceral hypersensitivity in irritable bowel syndrome (IBS) is widespread, but effective therapies for it remain elusive. As a canonical anti‐inflammatory protein, suppressor of cytokine signaling 3 (SOCS3) reportedly relays exchange protein 1 directly activated by cAMP (Epac1) signaling and inhibits the intracellular response to inflammatory cytokines. Despite the inhibitory effect of SOCS3 on the pro‐inflammatory response and neuroinflammation in PVN, the systematic investigation of Epac1‐SOCS3 signaling involved in visceral hypersensitivity remains unknown. This study aimed to explore Epac1‐SOCS3 signaling in the activity of hypothalamic paraventricular nucleus (PVN) corticotropin‐releasing factor (CRF) neurons and visceral hypersensitivity in adult rats experiencing neonatal colorectal distension (CRD).

Methods

Rats were subjected to neonatal CRD to simulate visceral hypersensitivity to investigate the effect of Epac1‐SOCS3 signaling on PVN CRF neurons. The expression and activity of Epac1 and SOCS3 in nociceptive hypersensitivity were determined by western blot, RT‐PCR, immunofluorescence, radioimmunoassay, electrophysiology, and pharmacology.

Results

In neonatal‐CRD‐induced visceral hypersensitivity model, Epac1 and SOCS3 expressions were downregulated and IL‐6 levels elevated in PVN. However, infusion of Epac agonist 8‐pCPT in PVN reduced CRF neuronal firing rates, and overexpression of SOCS3 in PVN by AAV‐SOCS3 inhibited the activation of PVN neurons, reduced visceral hypersensitivity, and precluded pain precipitation. Intervention with IL‐6 neutralizing antibody also alleviated the visceral hypersensitivity. In naïve rats, Epac antagonist ESI‐09 in PVN increased CRF neuronal firing. Consistently, genetic knockdown of Epac1 or SOCS3 in PVN potentiated the firing rate of CRF neurons, functionality of HPA axis, and sensitivity of visceral nociception. Moreover, pharmacological intervention with exogenous IL‐6 into PVN simulated the visceral hypersensitivity.

Conclusions

Inactivation of Epac1‐SOCS3 pathway contributed to the neuroinflammation accompanied by the sensitization of CRF neurons in PVN, precipitating visceral hypersensitivity and pain in rats experiencing neonatal CRD.

Microglia-derived TNF-α inhibiting GABAergic neurons in the anterior lateral bed nucleus of the stria terminalis precipitates visceral hypersensitivity induced by colorectal distension in rats

Irritable bowel syndrome (IBS) is a common and debilitating gastrointestinal disorder that is exacerbated by stress and characterized by abdominal pain. Although microglia in the CNS have been implicated as an important mediator of the stress response, the role of microglia and microglia-GABAergic neuron interactions in the limbic area, most notably BNST, in the development of colorectal hypersensitivity has not been determined. We established a neonatal colorectal distension-induced chronic visceral hyperalgesia model in rats. The results showed that the frequency of spontaneous discharges of alBNST GABAergic neurons and the expression of GAD65/67 were significantly decreased in rats with chronic visceral pain. Moreover, ablation of BNST GABAergic neurons significantly reduced the visceral pain threshold in normal rats. Meanwhile, the number of M1 proinflammatory microglia and the expression of the M1 proinflammatory microglia-derived cytokines IL-6 and TNF-α were increased in the alBNST of rats with chronic visceral pain. Furthermore, alBNST infusion of the microglial inhibitor minocycline or IL-6 and TNF-α neutralizing antibodies significantly increased the visceral pain threshold. The decreased frequency of spontaneous discharges of alBNST GABAergic neurons in rats with chronic visceral pain was mimicked by a bath perfusion of TNF-α, but not IL-6, and was abolished by a perfusion of the microglial inhibitor minocycline. In addition, the alBNST infusion of the microglial inhibitor minocycline upregulated the expression of GAD65/67. Moreover, ablation of BNST GABAergic neurons significantly decreased the visceral pain threshold in normal rats, which was not reversed by a subsequent infusion of the microglial inhibitor minocycline. Our findings revealed this microglia-GABAergic neuron circuit in the alBNST, and this microglia-driven disinhibitory mechanism is essential for brain and gut dysfunction in stressful condition, providing a novel potential target for treating patients with IBS presenting visceral pain that is worsened during episodes of stress.

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Changes of microglial plasticity modulated neuronal sensitivity in stressful condition.

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Inhibition of alBNST GABAergic inhibitory neurons resulted in visceral pain.

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BNST GABAergic inhibitory neurons are inhibited by microglia-derived TNF-α.

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BNST infusion of minocycline or TNF-α neutralizing antibodies alleviates pain.

Maternal Separation Induced Visceral Hypersensitivity Evaluated via Novel and Small Size Distention Balloon in Post-weaning Mice

Early life stress (ELS) disposes to functional gastrointestinal diseases in adult, such as irritable bowel syndrome (IBS). Maternal separation (MS) is a well-known animal model of IBS and has been shown to induce visceral hypersensitivity in adult rats and mice. However, to the best of our knowledge, it has not been reported whether MS induces visceral hypersensitivity in young mice, such as the post-weaning mice. Moreover, the method for evaluation of visceral sensitivity also has not been described. Accordingly, the present study aims to evaluate the visceral sensitivity caused by MS in post-weaning mice and develop a novel and small size distention balloon for assessment of visceral sensitivity of such mice. Male pups of C57BL/6 mice were randomly divided into two groups, MS (n = 12) and non-separation (NS) (n = 10). MS pups were separated from the dams through postnatal days (PND) 2 to 14, while NS pups were undisturbed. After, all pups stayed with respective dams and were weaned at PND 22. Visceral sensitivity was evaluated by colorectal distention (CRD) with a novel and small size distention balloon at PND 25. The threshold of abdominal withdrawal reflex (AWR) scores were significantly lower in MS than NS. In addition, AWR scores at different pressures of CRD were significantly higher in MS than NS. The results demonstrate that MS induced visceral hypersensitivity in post-weaning mice. The designed small size distention balloon for evaluation of visceral sensitivity is of significance to further study the pathophysiology of IBS from early life to adulthood.

Corticotropin-releasing factor receptor 1 (CRF-R1) antagonists: Promising agents to prevent visceral hypersensitivity in irritable bowel syndrome

Corticotropin-releasing factor (CRF) is a 41-amino acid polypeptide that coordinates the endocrine system, autonomic nervous system, immune system, and physiological behavior. CRF is a signaling regulator in the neuro-endocrine-immune (NEI) network that mediates visceral hypersensitivity. Rodent models to simulate changes in intestinal motility similar to those reported in the irritable bowel syndrome (IBS), demonstrate that the CRF receptor 1 (CRF-R1) mediates intestinal hypersensitivity under many conditions. However, the translation of preclinical studies into clinical trials has not been successful possibly due to the lack of sufficient understanding of the multiple variants of CRF-R1 and CRF-R1 antagonists. Investigating the sites of action of central and peripheral CRF is critical for accelerating the translation from preclinical to clinical studies.

Berberine alleviates visceral hypersensitivity in rats by altering gut microbiome and suppressing spinal microglial activation

Accumulating evidence shows that agents targeting gut dysbiosis are effective for improving symptoms of irritable bowel syndrome (IBS). However, the potential mechanisms remain unclear. In this study we investigated the effects of berberine on the microbiota-gut-brain axis in two rat models of visceral hypersensitivity, i.e., specific pathogen-free SD rats subjected to chronic water avoidance stress (WAS) and treated with berberine (200 mg· kg^-1 ·d^-1, ig, for 10 days) as well as germ-free (GF) rats subjected to fecal microbiota transplantation (FMT) from a patient with IBS (designated IBS-FMT) and treated with berberine (200 mg· kg^-1 ·d^-1, ig, for 2 weeks). Before the rats were sacrificed, visceral sensation and depressive behaviors were evaluated. Then colonic tryptase was measured and microglial activation in the dorsal lumbar spinal cord was assessed. The fecal microbiota was profiled using 16S rRNA sequencing, and short chain fatty acids (SCFAs) were measured. We showed that berberine treatment significantly alleviated chronic WAS-induced visceral hypersensitivity and activation of colonic mast cells and microglia in the dorsal lumbar spinal cord. Transfer of fecal samples from berberine-treated stressed donors to GF rats protected against acute WAS. FMT from a patient with IBS induced visceral hypersensitivity and pro-inflammatory phenotype in microglia, while berberine treatment reversed the microglial activation and altered microbial composition and function and SCFA profiles in stools of IBS-FMT rats. We demonstrated that berberine did not directly influence LPS-induced microglial activation in vitro. In both models, several SCFA-producing genera were enriched by berberine treatment, and positively correlated to the morphological parameters of microglia. In conclusion, activation of microglia in the dorsal lumbar spinal cord was involved in the pathogenesis of IBS caused by dysregulation of the microbiota-gut-brain axis, and the berberine-altered gut microbiome mediated the modulatory effects of the agent on microglial activation and visceral hypersensitivity, providing a potential option for the treatment of IBS.

Projection-specific dopamine neurons in the ventral tegmental area participated in morphine-induced hyperalgesia and anti-nociceptive tolerance in male mice

BACKGROUND

Long-term morphine use is associated with serious side effects, such as morphine-induced hyperalgesia and analgesic tolerance. Previous investigations have documented the association between dopamine (DA) neurons in the ventral tegmental area (VTA) and pain. However, whether VTA DA neurons are implicated in morphine-induced hyperalgesia and analgesic tolerance remains elusive.

METHODS

Initially, we observed behavioural effects of lidocaine administration into VTA or ablation of VTA DA neurons on morphine-induced hyperalgesia and anti-nociceptive tolerance. Subsequently, c-Fos expression in nucleus accumbens (NAc) shell-projecting and medial prefrontal cortex (mPFC)-projecting VTA DA neurons after chronic morphine treatment was respectively investigated. Afterwards, the effects of chemogenetic manipulation of NAc shell-projecting or mPFC-projecting DA neurons on morphine-induced hyperalgesia and anti-nociceptive tolerance were observed. Additionally, effects of chemogenetic manipulation of VTA GABA neurons on c-Fos expression in VTA DA neurons were investigated.

RESULTS

Lidocaine injection into VTA relieved established hyperalgesia and anti-nociceptive tolerance whereas ablation of VTA DA neurons prevented the development of morphine-induced hyperalgesia and anti-nociceptive tolerance. Chronic morphine treatment increased c-Fos expression in NAc shell-projecting DA neurons, rather than in mPFC-projecting DA neurons. Chemogenetic manipulation of NAc shell-projecting DA neurons had influence on morphine-induced hyperalgesia and tolerance. However, chemogenetic manipulation of mPFC-projecting DA neurons had no significant effects on morphine-induced hyperalgesia and anti-nociceptive tolerance. Chemogenetic manipulation of VTA GABA neurons affected the c-Fos expression in VTA DA neurons.

CONCLUSIONS

These findings revealed the involvement of NAc shell-projecting VTA DA neurons in morphine-induced hyperalgesia and anti-nociceptive tolerance, and may shed new light on the clinical management of morphine-induced hyperalgesia and analgesic tolerance.

PERSPECTIVE

This study demonstrated that NAc shell-projecting DA neurons rather than mPFC-projecting DA neurons in the VTA were implicated in morphine-induced hyperalgesia and anti-nociceptive tolerance. Our findings may pave the way for the discovery of novel therapies for morphine-induced hyperalgesia and analgesic tolerance.

Central regulatory mechanisms of visceral sensation in response to colonic distension with special reference to brain orexin

Visceral hypersensitivity is a major pathophysiology in irritable bowel syndrome (IBS). Although brain-gut interaction is considered to be involved in the regulation of visceral sensation, little had been known how brain controls visceral sensation. To improve therapeutic strategy in IBS, we should develop a novel approach to control visceral hypersensitivity. Here, we summarized recent data on central control of visceral sensation by neuropeptides in rats. Orexin, ghrelin or oxytocin in the brain is capable of inducing visceral antinociception. Dopamine, cannabinoid, adenosine, serotonin or opioid in the central nervous system (CNS) plays a role in the visceral hyposensitivity. Central ghrelin, levodopa or morphine could induce visceral antinociception via the orexinergic signaling. Orexin induces visceral antinociception through dopamine, cannabinoid, adenosine or oxytocin. Orexin nerve fibers are identified widely throughout the CNS and orexins are implicated in a number of functions. With regard to gastrointestinal functions, in addition to its visceral antinociception, orexin acts centrally to stimulate gastrointestinal motility and improve intestinal barrier function. Brain orexin is also involved in regulation of sleep/awake cycle and anti-depressive action. From these evidence, we would like to make a hypothesis that decreased orexin signaling in the brain may play a role in the pathophysiology in a part of patients with IBS who are frequently accompanied with sleep disturbance, depressive state and disturbed gut functions such as gut motility disturbance, leaky gut and visceral hypersensitivity.

Mast cells in the paraventricular nucleus participate in visceral hypersensitivity induced by neonatal maternal separation

Early-life stress (ELS) is a high-risk factor for the development of chronic visceral pain in adulthood. Emerging evidence suggests that mast cells play a key role in the development of visceral hypersensitivity through interaction with neurons. The sensitization of corticotropin-releasing factor (CRF) neurons in the hypothalamic paraventricular nucleus (PVN) plays a pivotal role in the pathogenesis of visceral pain. However, the precise mechanism by which mast cells and CRF neurons interact in the PVN in the pathogenesis of visceral hypersensitivity remains elusive. In the present study, we used neonatal maternal separation (MS), an ELS model, and observed that neonatal MS induced visceral hypersensitivity and triggered PVN mast cell activation in adult rats, which was repressed by intra-PVN infusion of the mast cell stabilizer disodium cromoglycate (cromolyn). Wild-type (WT) mice but not mast cell-deficient KitW-sh/W-sh mice that had experienced neonatal MS exhibited chronic visceral hypersensitivity. MS was associated with an increase in the expression of proinflammatory mediators, the number of CRF+ cells and CRF protein in the PVN, which was prevented by intra-PVN infusion of cromolyn. Furthermore, we demonstrated that intra-PVN infusion of the mast degranulator compound 48/80 significantly induced mast cell activation, resulting in proinflammatory mediator release, CRF neuronal sensitization, and visceral hypersensitivity, which was suppressed by cromolyn. Overall, our findings demonstrated that neonatal MS induces the activation of PVN mast cells, which secrete numerous proinflammatory mediators that may participate in neighboring CRF neuronal activity, ultimately directly inducing visceral hypersensitivity in adulthood.

BNSTAV GABA-PVNCRF Circuit Regulates Visceral Hypersensitivity Induced by Maternal Separation in Vgat-Cre Mice

Visceral hypersensitivity as a common clinical manifestation of irritable bowel syndrome (IBS) may contribute to the development of chronic visceral pain. Our prior studies authenticated that the activation of the corticotropin-releasing factor (CRF) neurons in paraventricular nucleus (PVN) contributed to visceral hypersensitivity in mice, but puzzles still remain with respect to the underlying hyperactivation of corticotropin-releasing factor neurons. Herein, we employed maternal separation (MS) to establish mouse model of visceral hypersensitivity. The neuronal circuits associated with nociceptive hypersensitivity involved paraventricular nucleus CRF neurons by means of techniques such as behavioral test, pharmacology, molecular biology, retrograde neuronal circuit tracers, electrophysiology, chemogenetics and optogenetics. MS could predispose the elevated firing frequency of CRF neurons in PVN in murine adulthood, which could be annulled via the injection of exogenous GABA (0.3mM, 0.2µl) into PVN. The PVN-projecting GABAergic neurons were mainly distributed in the anterior ventral (AV) region in the bed nucleus of stria terminalis (BNST), wherein the excitability of these GABAergic neurons was reduced. Casp3 virus was utilized to induce apoptosis of GABA neurons in BNST-AV region, resulting in the activation of CRF neurons in PVN and visceral hyperalgesia. In parallel, chemogenetic and optogenetic approaches to activate GABAergic BNST_AV-PVN circuit in MS mice abated the spontaneous firing frequency of PVN CRF neurons and prevented the development of visceral hypersensitivity. A priori, PVN^CRF-projecting GABAergic neurons in BNST-AV region participated in the occurrence of visceral hypersensitivity induced by MS. Our research may provide a new insight into the neural circuit mechanism of chronic visceral pain.

Small-Conductance Ca2+-Activated K+ Channels 2 in the Hypothalamic Paraventricular Nucleus Precipitates Visceral Hypersensitivity Induced by Neonatal Colorectal Distension in Rats

Visceral hypersensitivity is one of the pivotal pathophysiological features of visceral pain in irritable bowel syndrome (IBS). Small-conductance Ca²⁺-activated K⁺ channel (SK) is critical for a variety of functions in the central nervous system (CNS), nonetheless, whether it is involved in the pathogenesis of visceral hypersensitivity remain elusive. In this study, we examined mechanism of SK2 in hypothalamic paraventricular nucleus (PVN) in the pathogenesis of visceral hypersensitivity induced by neonatal colorectal distension (CRD). Rats undergoing neonatal CRD presented with visceral hypersensitivity as well as downregulated membrane SK2 channel and p-PKA. Intra-PVN administration of either the membrane protein transport inhibitor dynasore or the SK2 activator 1-EBIO upregulated the expression of membrane SK2 in PVN and mitigated visceral hypersensitivity. In addition, 1-EBIO administration reversed the increase in neuronal firing rates in PVN in rats undergoing neonatal CRD. On the contrary, intra-PVN administration of either the SK2 inhibitor apamin or PKA activator 8-Br-cAMP exacerbated the visceral hypersensitivity. Taken together, these findings demonstrated that visceral hypersensitivity is related to the downregulation of membrane SK2 in PVN, which may be attributed to the activation of PKA; pharmacologic activation of SK2 alleviated visceral hypersensitivity, which brings prospect of SK2 activators as a new intervention for visceral pain.

Supraspinal Mechanisms of Intestinal Hypersensitivity

Gut inflammation or injury causes intestinal hypersensitivity (IHS) and hyperalgesia, which can persist after the initiating pathology resolves, are often referred to somatic regions and exacerbated by psychological stress, anxiety or depression, suggesting the involvement of both the spinal cord and the brain. The supraspinal mechanisms of IHS remain to be fully elucidated, however, over the last decades the series of intestinal pathology-associated neuroplastic changes in the brain has been revealed, being potentially responsible for the phenomenon. This paper reviews current clinical and experimental data, including the authors' own findings, on these functional, structural, and neurochemical/molecular changes within cortical, subcortical and brainstem regions processing and modulating sensory signals from the gut. As concluded in the review, IHS can develop and maintain due to the bowel inflammation/injury-induced persistent hyperexcitability of viscerosensory brainstem and thalamic nuclei and sensitization of hypothalamic, amygdala, hippocampal, anterior insular, and anterior cingulate cortical areas implicated in the neuroendocrine, emotional and cognitive modulation of visceral sensation and pain. An additional contribution may come from the pathology-triggered dysfunction of the brainstem structures inhibiting nociception. The mechanism underlying IHS-associated regional hyperexcitability is enhanced NMDA-, AMPA- and group I metabotropic receptor-mediated glutamatergic neurotransmission in association with altered neuropeptide Y, corticotropin-releasing factor, and cannabinoid 1 receptor signaling. These alterations are at least partially mediated by brain microglia and local production of cytokines, especially tumor necrosis factor α. Studying the IHS-related brain neuroplasticity in greater depth may enable the development of new therapeutic approaches against chronic abdominal pain in inflammatory bowel disease.

Predisposition of Neonatal Maternal Separation to Visceral Hypersensitivity via Downregulation of Small-Conductance Calcium-Activated Potassium Channel Subtype 2 (SK2) in Mice

Background

Visceral hypersensitivity is a common occurrence of gastrointestinal diseases such as irritable bowel syndrome (IBS), wherein early-life stress (ELS) may have a high predisposition to the development of visceral hypersensitivity in adulthood, with the specific underlying mechanism still elusive. Herein, we assessed the potential effect of small-conductance calcium-activated potassium channel subtype 2 (SK2) in the spinal dorsal horn (DH) on the pathogenesis of visceral hypersensitivity induced by maternal separation (MS) in mice.

Methods

Neonatal mice were subjected to the MS paradigm, an established ELS model. In adulthood, the visceral pain threshold and the abdominal withdrawal reflex (AWR) were measured with an inflatable balloon. The elevated plus maze, open field test, sucrose preference test, and forced swim test were employed to evaluate the anxiety- and depression-like behaviors. The expression levels of SK2 in the spinal DH were determined by immunofluorescence and western blotting. The mRNA of SK2 and membrane palmitoylated protein 2 (MPP2) were determined by quantitative real-time polymerase chain reaction (qRT-PCR). Electrophysiology was applied to evaluate the neuronal firing rates and SK2 channel-mediated afterhyperpolarization current (I _AHP). The interaction between MPP2 and SK2 was validated by coimmunoprecipitation.

Results

In contrast to the naïve mice, ethological findings in MS mice revealed lowered visceral pain threshold, more evident anxiety- and depression-like behaviors, and downregulated expression of membrane SK2 protein and MPP2 protein. Moreover, electrophysiological results indicated increased neuronal firing rates and decreased I _AHP in the spinal DH neurons. Nonetheless, intrathecal injection of the SK2 channel activator 1-ethyl-2-benzimidazolinone (1-EBIO) in MS mice could reverse the electrophysiological alterations and elevate the visceral pain threshold. In the naïve mice, administration of the SK2 channel blocker apamin abated I _AHP and elevated spontaneous neuronal firing rates in the spinal DH neurons, reducing the visceral pain threshold. Finally, disruption of the MPP2 expression by small interfering RNA (siRNA) could amplify visceral hypersensitivity in naïve mice.

Conclusions

ELS-induced visceral pain and visceral hypersensitivity are associated with the underfunction of SK2 channels in the spinal DH.

GABAergic Neurons in the Dorsal Raphe Nucleus that Express 5-HT3A Receptors Participate in Responses to Stress Hormones

The dorsal raphe nucleus (DRN) participates in stress responses and in mood regulation via its ascending release of serotonin (5-HT) onto neural circuits within the forebrain. Although the 5-HT DRN region is easily defined via 5-HT-expressing DRN neurons, the neuroarchitecture and microcircuitry that confer its multifunctionality have remained incompletely understood and have required further investigation. In this present study, neurochemical interactions within different subregions of the rat DRN were precisely analyzed. We found that 97.5% of GABAergic neurons in the DRN expressed ionotropic 5-HT3A receptors (5-HT3ARs), whereas there were only rare parvalbumin (PV)-positive or somatostatin (SOM)-positive GABAergic neurons. Furthermore, corticosterone administration into male rats as a rodent model of depression induced significantly higher c-Fos expression in 5-HT3AR-positive GABAergic neurons compared to that in 5-HT neurons within the DRN. Taken together, our findings suggest that 5-HT3AR-positive GABAergic neurons in the DRN participate in responses to stress hormones in a rat model of depression.

Disinhibition of PVN-projecting GABAergic neurons in AV region in BNST participates in visceral hypersensitivity in rats

Ample evidence suggests that early life stress (ELS) is a high-risk factor for the development of visceral pain disorders, whereas the mechanism underlying neuronal circuit remains elusive. Herein, we employed neonatal colorectal distension (CRD) to induce visceral hypersensitivity in rats. A combination of electrophysiology, pharmacology, behavioral test, molecular biology, chemogenetics and optogenetics confirmed that CRD in neonatal rats could predispose the elevated firing frequency of the parvocellular corticotropin-releasing hormone (CRH) neurons in the paraventricular nucleus of hypothalamus (PVN) in adulthood, with the CRH neurons activated and the frequency of spontaneous inhibitory postsynaptic currents (sIPSC) diminished, both contributing to chronic visceral hypersensitivity. Moreover, following administration of exogenous GABA (300 mM/0.5 μL) and GABA_A receptor agonist muscimol (3 mM/0.5 μL) in PVN, visceral hyperalgesia was abrogated. In addition, the PVN-projecting GABAergic neurons were mainly distributed in the anterior ventral (AV) region in the bed nucleus of stria terminalis (BNST), and the excitability of these GABAergic neurons was weakened in visceral hypersensitivity. Specific depletion of the GABAergic neurons in AV region precipitated visceral hyperalgesia. Moreover, chemogenetic activation of the PVN-projecting neurons alleviated the visceral hypersensitivity. Photoactivation of PVN-projecting GABAergic neurons abated the visceral hypersensitivity in neonatal-CRD rats, whereas photoinhibition evoked visceral hyperalgesia in naïve rats. Our findings demonstrated that disinhibition of the PVN-projecting GABAergic neurons in AV region contributed to the excitation of CRH neurons, thereby mediating visceral hypersensitivity. Our study might provide a novel insight into the neuronal circuits involved in the ELS-induced visceral hypersensitivity.

Nervous mechanisms of restraint water-immersion stress-induced gastric mucosal lesion

Stress-induced gastric mucosal lesion (SGML) is one of the most common visceral complications after trauma. Exploring the nervous mechanisms of SGML has become a research hotspot. Restraint water-immersion stress (RWIS) can induce GML and has been widely used to elucidate the nervous mechanisms of SGML. It is believed that RWIS-induced GML is mainly caused by the enhanced activity of vagal parasympathetic nerves. Many central nuclei, such as the dorsal motor nucleus of the vagus, nucleus of the solitary tract, supraoptic nucleus and paraventricular nucleus of the hypothalamus, mediodorsal nucleus of the thalamus, central nucleus of the amygdala and medial prefrontal cortex, are involved in the formation of SGML in varying degrees. Neurotransmitters/neuromodulators, such as nitric oxide, hydrogen sulfide, vasoactive intestinal peptide, calcitonin gene-related peptide, substance P, enkephalin, 5-hydroxytryptamine, acetylcholine, catecholamine, glutamate, γ-aminobutyric acid, oxytocin and arginine vasopressin, can participate in the regulation of stress. However, inconsistent and even contradictory results have been obtained regarding the actual roles of each nucleus in the nervous mechanism of RWIS-induced GML, such as the involvement of different nuclei with the time of RWIS, the different levels of involvement of the sub-regions of the same nucleus, and the diverse signalling molecules, remain to be further elucidated.

Inhibition of Microglial Activation in the Amygdala Reverses Stress-Induced Abdominal Pain in the Male Rat

BACKGROUND & AIMS

Psychological stress is a trigger for the development of irritable bowel syndrome and associated symptoms including abdominal pain. Although irritable bowel syndrome patients show increased activation in the limbic brain, including the amygdala, the underlying molecular and cellular mechanisms regulating visceral nociception in the central nervous system are incompletely understood. In a rodent model of chronic stress, we explored the role of microglia in the central nucleus of the amygdala (CeA) in controlling visceral sensitivity. Microglia are activated by environmental challenges such as stress, and are able to modify neuronal activity via synaptic remodeling and inflammatory cytokine release. Inflammatory gene expression and microglial activity are regulated negatively by nuclear glucocorticoid receptors (GR), which are suppressed by the stress-activated pain mediator p38 mitogen-activated protein kinases (MAPK).

METHODS

Fisher-344 male rats were exposed to water avoidance stress (WAS) for 1 hour per day for 7 days. Microglia morphology and the expression of phospho-p38 MAPK and GR were analyzed via immunofluorescence. Microglia-mediated synaptic remodeling was investigated by quantifying the number of postsynaptic density protein 95-positive puncta. Cytokine expression levels in the CeA were assessed via quantitative polymerase chain reaction and a Luminex assay (Bio-Rad, Hercules, CA). Stereotaxic infusion into the CeA of minocycline to inhibit, or fractalkine to activate, microglia was followed by colonic sensitivity measurement via a visceromotor behavioral response to isobaric graded pressures of tonic colorectal distension.

RESULTS

WAS induced microglial deramification in the CeA. Moreover, WAS induced a 3-fold increase in the expression of phospho-p38 and decreased the ratio of nuclear GR in the microglia. The number of microglia-engulfed postsynaptic density protein 95-positive puncta in the CeA was increased 3-fold by WAS, while cytokine levels were unchanged. WAS-induced changes in microglial morphology, microglia-mediated synaptic engulfment in the CeA, and visceral hypersensitivity were reversed by minocycline whereas in stress-naïve rats, fractalkine induced microglial deramification and visceral hypersensitivity.

CONCLUSIONS

Our data show that chronic stress induces visceral hypersensitivity in male rats and is associated with microglial p38 MAPK activation, GR dysfunction, and neuronal remodeling in the CeA.

Monosynaptic Input Mapping of Diencephalic Projections to the Cerebrospinal Fluid-Contacting Nucleus in the Rat

Objective: To investigate the projections the cerebrospinal fluid-contacting (CSF-contacting) nucleus receives from the diencephalon and to speculate on the functional significance of these connections.

Methods: The retrograde tracer cholera toxin B subunit (CB) was injected into the CSF-contacting nucleus in SD rats according to the experimental formula of the stereotaxic coordinates. Animals were perfused 7–10 days after the injection, and the diencephalon was sliced at 40 μm with a freezing microtome. CB-immunofluorescence was performed on all diencephalic sections. The features of CB-positive neuron distribution in the diencephalon were observed with a fluorescence microscope.

Results: The retrograde labeled CB-positive neurons were found in the epithalamus, subthalamus, and hypothalamus. Three functional diencephalic areas including 43 sub-regions revealed projections to the CSF-contacting nucleus. The CB-positive neurons were distributed in different density ranges: sparse, moderate, and dense.

Conclusion: Based on the connectivity patterns of the CSF-contacting nucleus that receives anatomical inputs from the diencephalon, we preliminarily assume that the CSF-contacting nucleus participates in homeostasis regulation, visceral activity, stress, emotion, pain and addiction, and sleeping and arousal. The present study firstly illustrates the broad projections of the CSF-contacting nucleus from the diencephalon, which implies the complicated functions of the nucleus especially for the unique roles of coordination in neural and body fluids regulations.

The role of SOCS3 in the hypothalamic paraventricular nucleus in rat model of inflammatory pain

Background

Inflammatory molecular signals are modulated by a variety of intracellular transduction pathways, the activation of which may induce and amplify the spread of inflammatory response. Suppresser of cytokine signaling 3 (SOCS3) is an established negative feedback regulation transcription factor associated with tumor, diabetes mellitus, inflammation and anaphylaxis. Herein, we investigated whether SOCS3 in the paraventricular nucleus (PVN) can attenuate pro-inflammatory responses, and thereby relieve the inflammatory pain.

Methods

Adeno-associated virus (AAV) overexpressing SOCS3 was pre-injected into the PVN. Three weeks later, rat model of chronic inflammatory pain was established via subcutaneous injection of complete Freund's adjuvant (CFA) into the plantar center of hind paws. The therapeutic effect of SOCS3 was tested by the measurement of thermal and mechanical allodynia. In mechanistic study, the protein level of SOCS3 was evaluated by Western blotting, and the expression of c-fos and Iba-1 were assessed by immunofluorescent staining.

Results

Inflammatory pain was associated with upregulated interleukin 6 (IL-6) and SOCS3 in PVN in the acute phase. Thermal hyperalgesia can be relieved by intra-PVN injection of IL-6 neutralizing antibody (NA). Meanwhile, the upregulated c-fos and microglial activation was reversed. Furthermore, SOCS3 expression in PVN was downregulated in the chronic phase. Intra-PVN injection of AAV overexpressing SOCS3 suppressed the activation of neurons and attenuated thermal hyperalgesia and mechanical allodynia.

Conclusion

Inhibition of IL-6 signaling attenuated inflammatory hyperalgesia in the acute phase. SOCS3 overexpression in the PVN attenuated inflammatory pain in the chronic phase via suppression of neuronal activation.

Effects of (-)-Epigallocatechin Gallate (EGCG) on Energy Expenditure and Microglia-Mediated Hypothalamic Inflammation in Mice Fed a High-Fat Diet

Obesity is an escalating global epidemic caused by an imbalance between energy intake and expenditure. (−)-Epigallocatechin-3-gallate (EGCG), the major polyphenol in green tea, has been reported to be conducive to preventing obesity and alleviating obesity-related chronic diseases. However, the role of EGCG in energy metabolism disorders and central nervous system dysfunction induced by a high-fat diet (HFD) remains to be elucidated. The aim of this study was to evaluate the effects of EGCG on brown adipose tissue (BAT) thermogenesis and neuroinflammation in HFD-induced obese C57BL/6J mice. Mice were randomly divided into four groups with different diets: normal chow diet (NCD), normal chow diet supplemented with 1% EGCG (NCD + EGCG), high-fat diet (HFD), and high-fat diet supplemented with 1% EGCG (HFD + EGCG). Investigations based on a four-week experiment were carried out including the BAT activity, energy consumption, mRNA expression of major inflammatory cytokines in the hypothalamus, nuclear factor-kappa B (NF-κB) and signal transducer and activator of transcription 3 (STAT3) phosphorylation, and immunofluorescence staining of microglial marker Iba1 in hypothalamic arcuate nucleus (ARC). Experimental results demonstrated that dietary supplementation of EGCG significantly inhibited HFD-induced obesity by enhancing BAT thermogenesis, and attenuated the hypothalamic inflammation and microglia overactivation by regulating the NF-κB and STAT3 signaling pathways.

In vitro examination of microglia-neuron crosstalk with BV2 cells, and primary cultures of glia and hypothalamic neurons

Microglia respond to environmental changes by releasing cytokines that beneficially or detrimentally affect surrounding cells in addition to functioning as the resident CNS macrophages. Interactions between glia and neurons participate in many critical brain functions and diseases. We previous demonstrated that activation of microglia facilitates hypothalamic CRF neuronal activity and pain precipitation in rats. The intricate CNS environment complicates studying crosstalk between microglia and hypothalamic neurons in vivo. BV2 cells derived from raf/myc-immortalised murine neonatal microglia are the most frequently used substitute for primary cultures of microglia. In this study, we used BV2 cells and primary cultures of glia from neonatal rats to explore the interaction between microglia and hypothalamic neurons in vitro. Lipopolysaccharide (LPS) stimulated BV2 cells to adopt a microglia-like phenotype including an amoebae-like shape, Iba-1 positive staining and IL-1β secretion. Primary cultures of hypothalamic neurons treated with culture medium from LPS-treated BV2 cells increased CRF, CRFR, pCREB and cAMP levels compared to untreated neurons. Primary cultures of hypothalamic neurons incubated with culture medium from LPS-treated primary cultures of glia or exogenous IL-1β also increased CRF levels. Importantly, this increase in protein expression appeared to be IL-1β mediated and treatment with an anti-IL-1β antibody blocked the increased expression. Our data provide direct evidence that microglia can modulate hypothalamic neuronal activity and IL-1β may play a critical role in bridging the communication between microglia and neurons.

Small-Conductance Ca2+-Activated K+ Channel 2 in the Dorsal Horn of Spinal Cord Participates in Visceral Hypersensitivity in Rats

Visceral hypersensitivity is a highly complex and subjective phenomenon associated with multiple levels of the nervous system and a wide range of neurotransmission. The dorsal horn (DH) in spinal cord relays the peripheral sensory information into the brain. Small conductance Ca²⁺-activated K⁺ (SK) channels regulate neuronal excitability and firing by allowing K⁺ to efflux in response to increase in the intracellular Ca²⁺ level. In this study, we examined the influence of SK2 channels in the spinal DH on the pathogenesis of visceral hypersensitivity induced by colorectal distension (CRD) in rats. Electrophysiological results showed that rats with visceral hypersensitivity presented a decrease in the SK channel-mediated afterhyperpolarization current (I_AHP), and an increase in neuronal firing rates and c-Fos positive staining in the spinal DH. Western blot data revealed a decrease in the SK2 channel protein in the membrane fraction. Moreover, intrathecal administration of the SK2 channel activator 1-EBIO or CyPPA alleviated visceral hypersensitivity, reversed the decrease in I_AHP and the increase in neuronal firing rates in spinal DH in rats that experienced CRD. 1-EBIO or CyPPA effect could be prevented by SK2 channel blocker apamin. CRD induced an increase in c-Fos protein expression in the spinal DH, which was prevented by 1-EBIO. Together, these data suggest that visceral hypersensitivity and pain is associated with a decrease in the number and function of membrane SK2 channels in the spinal DH. Pharmacological manipulation of SK2 channels may open a new avenue for the treatment of visceral hypersensitivity and pain.

Highlights:-

Neonatal colorectal distension induced visceral hypersensitivity in rats.

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Visceral hypersensitivity rats presented a decrease in afterhyperpolarization current (I_AHP) and membrane SK2 channel protein in the spinal dorsal horn.

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Visceral hypersensitivity rats presented an increase in neuronal firing rate in the spinal dorsal horn.

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Intrathecal administration of SK2 channel activator 1-EBIO or CyPPA prevented visceral hypersensitivity and decrease in I_AHP.

Increased expression of CRF and CRF-receptors in dorsal striatum, hippocampus, and prefrontal cortex after the development of nicotine sensitization in rats

BACKGROUND

Nicotine addiction supports tobacco smoking, a main preventable cause of disease and death in Western countries. It develops through long-term neuroadaptations in the brain reward circuit by modulating intracellular pathways and regulating gene expression. This study assesses the regional expression of the transcripts of the CRF transmission in a nicotine sensitization model, since it is hypothesised that the molecular neuroadaptations that mediate the development of sensitization contribute to the development of addiction.

METHODS

Rats received intraperitoneal nicotine administrations (0.4 mg/kg) once daily for either 1 day or over 5 days. Locomotor activity was assessed to evaluate the development of sensitization. The mRNA expression of CRF and CRF1 and CRF2 receptors was measured by qPCR in the ventral mesencephalon, ventral striatum, dorsal striatum (DS), prefrontal cortex (PFCx), and hippocampus (Hip).

RESULTS

Acute nicotine administration increased locomotor activity in rats. In the sub-chronic group, locomotor activity progressively increased and reached a clear sensitization. Significant effects of sensitization on CRF mRNA levels were detected in the DS (increasing effect). Significantly higher CRF1 and CRF2 receptor levels after sensitization were detected in the Hip. Additionally, CRF2 receptor levels were augmented by sensitization in the PFCx, and treatment and time-induced increases were detected in the DS. Nicotine treatment effects were observed on CRF1R levels in the DS.

CONCLUSIONS

This study suggests that the CRF transmission, in addition to its role in increasing withdrawal-related anxiety, may be involved in the development of nicotine-habituated behaviours through reduced control of impulses and the aberrant memory plasticity characterising addiction.

Minocycline as a promising therapeutic strategy for chronic pain

Chronic pain remains to be a clinical challenge due to insufficient therapeutic strategies. Minocycline is a member of the tetracycline class of antibiotics, which has been used in clinic for decades. It is frequently reported that minocycline may has many non-antibiotic properties, among which is its anti-nociceptive effect. The results from our lab and others suggest that minocycline exerts strong analgesic effect in animal models of chronic pain including visceral pain, chemotherapy-induced periphery neuropathy, periphery injury induced neuropathic pain, diabetic neuropathic pain, spinal cord injury, inflammatory pain and bone cancer pain. In this review, we summarize the mechanisms underlying the analgesic effect of minocycline in preclinical studies. Due to a good safety record when used chronically, minocycline may become a promising therapeutic strategy for chronic pain in clinic.

IGF-1, Inflammation and Retinal Degeneration: A Close Network

Retinal degenerative diseases are a group of heterogeneous diseases that include age-related macular degeneration (AMD), retinitis pigmentosa (RP), and diabetic retinopathy (DR). The progressive degeneration of the retinal neurons results in a severe deterioration of the visual function. Neuroinflammation is an early hallmark of many neurodegenerative disorders of the retina including AMD, RP and DR. Microglial cells, key components of the retinal immune defense system, are activated in retinal degenerative diseases. In the microglia the interplay between the proinflammatory/classically activated or antiinflammatory/alternatively activated phenotypes is a complex dynamic process that occurs during the course of disease due to the different environmental signals related to pathophysiological conditions. In this regard, an adequate transition from the proinflammatory to the anti-inflammatory response is necessary to counteract retinal neurodegeneration and its subsequent damage that leads to the loss of visual function. Insulin like-growth factor-1 (IGF-1) has been considered as a pleiotropic factor in the retina under health or disease conditions and several effects of IGF-1 in retinal immune modulation have been described. In this review, we provide recent insights of inflammation as a common feature of retinal diseases (AMD, RP and RD) highlighting the role of microglia, exosomes and IGF-1 in this process.

Protective effect and mechanism of injection of glutamate into cerebellum fastigial nucleus on chronic visceral hypersensitivity in rats

AIMS

We investigated the effects of chemical stimulation of cerebellum fastigial nucleus (FN) on the chronic visceral hypersensitivity (CVH) and its possible mechanism in rats.

MAIN METHODS

We stimulated the FN by microinjecting glutamate into the FN, in order to explore whether the cerebellum fastigial nucleus played a role on CVH in rat. The model of CVH was established by colorectal distension (CRD) in neonatal rats. Abdominal withdrawal reflex (AWR) scores, pain threshold, and amplitude of electromyography (EMG) were used to assess the hyperalgesia.

KEY FINDINGS

We showed that microinjection of l-glutamate (Glu) into the FN markedly attenuated hyperalgesia. The protective effect of FN was prevented by pretreatment with the glutamate decarboxylase inhibitor, 3-mercaptopropionic acid (3-MPA) into the FN or GABA_A receptor antagonist, bicuculline (Bic) into the LHA (lateral hypothalamic area). The expressions of protein Bax, caspase-3 were decreased, but the expression of protein Bcl-2 was increased after chemical stimulation of FN. These results indicated that the FN participated in regulation of CVH, and was a specific area in the CNS for exerting protective effects on the CVH. In addition, LHA and GABA receptor may be involved in this process.

SIGNIFICANCE

Our findings might provide a new and improved understanding of the FN function, and might show an effective treatment strategy for the chronic visceral hypersensitivity.

Involvement of dopamine system in the regulation of the brain corticotropin-releasing hormone in paraventricular nucleus in a rat model of chronic visceral pain

Objective We aimed to investigate the mechanism of paraventricular nucleus (PVN) and ventral tegmental area (VTA) circuit in the pathogenesis of visceral pain-depression with a rat model induced by neonatal and adult colorectal distension (CRD). Methods Neonate male Sprague-Dayley (SD) rats underwent CRD on postnatal days 8, 10, and 12, and when matured, were tested for adult abdominal withdrawal reflex (AWR) scores to assess visceral hypersensitivity. The forced swimming test was employed to evaluate depression-like behaviors. The rats exhibiting visceral pain-depressive behaviors underwent lidocaine injection in the VTA to explore the relationship between VTA and visceral pain. Moreover, double immunofluorescence was employed to evaluate the qualitative and quantitative expression of dopamine/ c-Fos in CRD rats. After verifying the existed fiber projection from PVN to VTA, the intra-PVN microinjection of CRH-RNAi lentivirus to inhibit corticotropin-releasing hormone (CRH) expression, behavioral changes were assessed by AWR score and FST. Thereafter, with the sacrifice of the rats, the variations of TH protein in rats were evaluated by immunofluorescence and Western blot. Results Intra-VTA microinjection of lidocaine increased the pain threshold of CRD group. After intra-VTA microinjection of green retrograde tracer, immunofluorescence photomicrographs visualized the PVN with a typical green retrograde tracer. Intra-PVN microinjection of CRH-RNAi lentivirus alleviated the visceral pain-depression behaviors and decreased the TH protein expression in the VTA. Conclusion These data demonstrated that the VTA played a functional role in chronic visceral pain and depression, and the CRH-containing neurons in hypothalamic PVN may be implicated in the onset and maintenance of the chronic visceral pain and depression via the activation of dopamine in the VTA.

Microglia in health and pain: impact of noxious early life events

NEW FINDINGS

What is the topic of this review? This review discusses the origins and development of microglia, and how stress, pain or inflammation in early life disturbs microglial function during critical developmental periods, leading to altered pain sensitivity and/or increased risk of chronic pain in later life. What advances does it highlight? We highlight recent advances in understanding how disrupted microglial function impacts the developing nervous system and the consequences for pain processing and susceptibility for development of chronic pain in later life. The discovery of microglia is accredited to Pío del Río-Hortega, who recognized this 'third element' of CNS cells as being morphologically distinct from neurons and astrocytes. For decades after this finding, microglia were altogether ignored or relegated as simply being support cells. Emerging from virtual obscurity, microglia have now gained notoriety as immune cells that assume a leading role in the development, maintenance and protection of a healthy CNS. Pioneering studies have recently shed light on the origins of microglia, their role in the developing nervous system and the complex roles they play beyond the immune response. These studies reveal that altered microglial function can have a profoundly negative impact on the developing brain and may be a determinant in a range of neurodevelopmental disorders and neurodegenerative diseases. The realization that aberrant microglial function also critically underlies chronic pain, a debilitating disorder that afflicts over 1.5 billion people worldwide, was a major conceptual leap forward in the pain field. Adding to this advance is emerging evidence that early life noxious experiences can have a long-lasting impact on central pain processing and adult pain sensitivity. With microglia now coming of age, in this review we examine the association between adverse early life events, such as stress, injury or inflammation, and the influence of sex differences, on the role of microglia in pain physiology in adulthood.

Stress-Induced Chronic Visceral Pain of Gastrointestinal Origin

Visceral pain is generally poorly localized and characterized by hypersensitivity to a stimulus such as organ distension. In concert with chronic visceral pain, there is a high comorbidity with stress-related psychiatric disorders including anxiety and depression. The mechanisms linking visceral pain with these overlapping comorbidities remain to be elucidated. Evidence suggests that long term stress facilitates pain perception and sensitizes pain pathways, leading to a feed-forward cycle promoting chronic visceral pain disorders such as irritable bowel syndrome (IBS). Early life stress (ELS) is a risk-factor for the development of IBS, however the mechanisms responsible for the persistent effects of ELS on visceral perception in adulthood remain incompletely understood. In rodent models, stress in adult animals induced by restraint and water avoidance has been employed to investigate the mechanisms of stress-induce pain. ELS models such as maternal separation, limited nesting, or odor-shock conditioning, which attempt to model early childhood experiences such as neglect, poverty, or an abusive caregiver, can produce chronic, sexually dimorphic increases in visceral sensitivity in adulthood. Chronic visceral pain is a classic example of gene × environment interaction which results from maladaptive changes in neuronal circuitry leading to neuroplasticity and aberrant neuronal activity-induced signaling. One potential mechanism underlying the persistent effects of stress on visceral sensitivity could be epigenetic modulation of gene expression. While there are relatively few studies examining epigenetically mediated mechanisms involved in visceral nociception, stress-induced visceral pain has been linked to alterations in DNA methylation and histone acetylation patterns within the brain, leading to increased expression of pro-nociceptive neurotransmitters. This review will discuss the potential neuronal pathways and mechanisms responsible for stress-induced exacerbation of chronic visceral pain. Additionally, we will review the importance of specific experimental models of adult stress and ELS in enhancing our understanding of the basic molecular mechanisms of pain processing.

Oxytocin inhibited stress induced visceral hypersensitivity, enteric glial cells activation, and release of proinflammatory cytokines in maternal separated rats

Visceral hypersensitivity (VH) is a significant contributor to irritable bowel syndrome (IBS). Oxytocin (OT) possesses analgesic effects on the central nervous system (CNS) and attenuates microglial activation, however, little is known about its peripheral effects and involvement in VH of IBS. Reactive enteric glial cells (EGCs) contributes to abnormal motility in gastrointestinal (GI) diseases. The aim of this study was to evaluate the peripheral use of OT to maintain VH and activation of EGCs through involvement of the Toll-like receptor (TLR) 4/MyD88/NF-κB signaling. After assessing a baseline visceromotor response (VMR) to colorectal distension (CRD), rats were exposed to a 1h water avoidance stress (WAS) session. Before each WAS session, intraperitoneal injection of OT (1mg/kg body weight, in phosphate-buffered saline (PBS)) atosiban (0.5mg/kg body weight, in PBS) or PBS (as a vehicle control, 1ml/kg body weight) was administered. Animas are killed 24h after the last WAS session. EGCs activity, relative OT receptor expression, glial fibrillary acidic protein (GFAP) expression and TLR4/MyD88/NF-κB signaling were evaluated. Neonatal maternal separation (MS) significantly increased the OT receptor expression and enhanced VMR to CRD. WAS improved VMR to CRD only during neonatal MS. OT treatment prevented WAS-induced higher VMRs to CRD, which was reversed by an OT receptor antagonist administration. Compared to the vehicle, OT pre-treated rats reduced EGCs activation, GFAP expression and TLR4/MyD88/NF-κB signaling. We conclude that neonatal MS induces VH and visceral pain in rats. Furthermore, exogenous OT attenuated stress-induced VH and EGCs activation, which was mediated by TLR4/MyD88/NF-κB signaling.

Toll-Like Receptor 4 in Paraventricular Nucleus Mediates Visceral Hypersensitivity Induced by Maternal Separation

Neonatal maternal separation (MS) is a major early life stress that increases the risk of emotional disorders, visceral pain perception and other brain dysfunction. Elevation of toll-like receptor 4 (TLR4) signaling in the paraventricular nucleus (PVN) precipitates early life colorectal distension (CRD)-induced visceral hypersensitivity and pain in adulthood. The present study aimed to investigate the role of TLR4 signaling in the pathogenesis of postnatal MS-induced visceral hypersensitivity and pain during adulthood. The TLR4 gene was selectively knocked out in C57BL/10ScSn mice (Tlr4^-/-). MS was developed by housing the offspring alone for 6 h daily from postnatal day 2 to day 15. Visceral hypersensitivity and pain were assessed in adulthood. Tlr4^+/+, but not Tlr4^-/-, mice that had experienced neonatal MS showed chronic visceral hypersensitivity and pain. TLR4 immunoreactivity was observed predominately in microglia in the PVN, and MS was associated with an increase in the expression of protein and/or mRNA levels of TLR4, corticotropin-releasing factor (CRF), CRF receptor 1 (CRFR1), tumor necrosis factor-α, and interleukin-1β in Tlr4^+/+ mice. These alterations were not observed in Tlr4^-/- mice. Local administration of lipopolysaccharide, a TLR4 agonist, into the lateral cerebral ventricle elicited visceral hypersensitivity and TLR4 mRNA expression in the PVN, which could be prevented by NBI-35965, an antagonist to CRFR1. The present results indicate that neonatal MS induces a sensitization and upregulation of microglial TLR4 signaling activity, which facilitates the neighboring CRF neuronal activity and, eventually, precipitates visceral hypersensitivity in adulthood. Highlights (1)Neonatal MS does not induce chronic visceral hypersensitivity and pain in Tlr4^-/- mice.(2)Neonatal MS increases the expression of TLR4 mRNA, CRF protein and mRNA, CRFR1 protein, TNF-α protein, and IL-1β protein in Tlr4^+/+ mice.(3)TLR4 agonist LPS (i.c.v.) elicits visceral hypersensitivity and TLR4 mRNA expression in the PVN.

Sex differences in stress-induced visceral hypersensitivity following early life adversity: a two hit model

BACKGROUND

Early life adversity (ELA) has been indicated as a risk factor for the development of stress axis dysfunction in adulthood, specifically in females. We previously showed that unpredictable ELA induces visceral hyperalgesia in adult female rats. It remains to be determined whether ELA alters visceral nociceptive responses to stress in adulthood. The current study tested the hypothesis that following ELA, exposure to an adulthood stressor, or second hit, serves as a risk factor for exaggerated stress-induced visceral hypersensitivity that is sex-specific.

METHODS

Following ELA, adult stress was induced via a single exposure (acute) or repetitive daily exposure, 1 h/day for 7 days (chronic), to water avoidance stress (WAS).

KEY RESULTS

Acute WAS increased pain behaviors in all adult female rats, however, females that experienced unpredictable ELA exhibited significantly more pain behaviors compared to those exposed to predictable ELA or controls. Following chronic WAS, all adult females exhibited increased pain responses, however, an exaggerated response was observed in rats exposed to unpredictable or predictable ELA compared to controls. Similarly, in adult male rats exposure to acute or chronic WAS increased pain behaviors, however, there were no differences in pain behaviors between ELA groups.

CONCLUSIONS & INFERENCES

This study highlights a novel consequence of ELA on stress-induced visceral nociception in adulthood that is sex-specific. More importantly, our study suggests that ELA not only serves as a risk factor for development of chronic pain in adulthood, but also serves as a predisposition for worsening of visceral pain following adult stress in female rats.