Protective effects of endotoxin in a rat model of chronic inflammation are accompanied by suppressed secretion of pro-inflammatory cytokines and biphasic alteration in hypothalamo-pituitary-adrenal axis activity

Decoding the role of gut microbiota in Alzheimer's pathogenesis and envisioning future therapeutic avenues

Alzheimer's disease (AD) emerges as a perturbing neurodegenerative malady, with a profound comprehension of its underlying pathogenic mechanisms continuing to evade our intellectual grasp. Within the intricate tapestry of human health and affliction, the enteric microbial consortium, ensconced within the milieu of the human gastrointestinal tract, assumes a role of cardinal significance. Recent epochs have borne witness to investigations that posit marked divergences in the composition of the gut microbiota between individuals grappling with AD and those favored by robust health. The composite vicissitudes in the configuration of the enteric microbial assembly are posited to choreograph a participatory role in the inception and progression of AD, facilitated by the intricate conduit acknowledged as the gut-brain axis. Notwithstanding, the precise nature of this interlaced relationship remains enshrouded within the recesses of obscurity, poised for an exhaustive revelation. This review embarks upon the endeavor to focalize meticulously upon the mechanistic sway exerted by the enteric microbiota upon AD, plunging profoundly into the execution of interventions that govern the milieu of enteric microorganisms. In doing so, it bestows relevance upon the therapeutic stratagems that form the bedrock of AD's management, all whilst casting a prospective gaze into the horizon of medical advancements.

Early Life Stress Activates Glial Cells in the Hippocampus but Attenuates Cytokine Secretion in Response to an Immune Challenge in Rat Pups

OBJECTIVE

Early life stress (ELS) increases the vulnerability to developing psychopathological disorders in adulthood that are accompanied by brain inflammatory processes. However, it is not known how a combined double hit (stress and immune) at an early age affects the response of the neuroimmune system. Here we investigated the effect of periodic maternal separation (MS) followed by administration of lipopolysaccharide (LPS) on glial cells in the CA3 region and hilus of the hippocampus and on cytokine release on postnatal day (PN) 15.

METHODS

Male rat pups were subjected to MS (3 h/day, PN1-14). MS and control pups received a single LPS injection (1 mg/kg of body weight) on PN14. They were subjected to an open field test 1 h later. The pups were sacrificed 90 min after LPS injection (PN14) or on PN15 for cytokine or immunohistological analyses, respectively.

RESULTS

LPS reduced the locomotion and induced high corticosterone levels in treated pups. MS or LPS reduced microglial density and activated microglial cells in the hippocampal CA3 and hilus regions. Microglial activation was highest in MS-LPS pups. The astrocyte density was mildly reduced by MS or LPS in the CA3 region and hilus, but the reduction was maximal in MS-LPS pups. LPS increased the secretion of plasmatic interleukin (IL)-1β, tumor necrosis factor-α, and IL-6, and of hippocampal IL-1β protein, but these were attenuated in MS-LPS pups.

CONCLUSION

Although MS and LPS activate neuroimmune cells, stress attenuates the hippocampal and peripheral cytokine response to LPS through an as-yet unidentified adaptive mechanism. These results provide information regarding the neurobiology of stress and inflammation.

The reduction of CSF tumor necrosis factor alpha levels in schizophrenia: no correlations with psychopathology and coincident metabolic characteristics

AIM

The aim of this study was to assess whether tumor necrosis factor alpha (TNF-α) levels are correlated with the behavioral syndrome of schizophrenia and/or metabolic abnormalities.

METHODS

Sixty patients with first-onset schizophrenia were recruited. The concentrations of TNF-α in the cerebrospinal fluid (CSF) were determined in 22 schizophrenia patients and ten patients with nonsuppurative appendicitis using a radioimmunoassay. Physiological characteristics such as fasting blood glucose, fasting insulin, triglycerides, corrected QT interval, waist circumference, and body mass index were measured prior to CSF collection. Subjects were screened for insulin resistance using the homeostasis model assessment. The extent of positive and negative behavioral symptoms was scored using the Positive and Negative Syndrome Scale.

RESULTS

The CSF TNF-α levels in schizophrenic patients were significantly lower than those in the control group. The age of disease onset was positively correlated with the CSF TNF-α level using Pearson correlation analysis (r=0.37, P<0.05). There were no significant differences in CSF TNF-α levels in terms of age, duration of schizophrenia, or systolic and diastolic blood pressure. Furthermore, the CSF TNF-α levels were not significantly correlated with fasting blood glucose, fasting insulin, insulin resistance index, triglycerides, corrected QT interval, waist circumference, or body mass index. No significant correlation was found between CSF TNF-α levels and the Positive and Negative Syndrome Scale total scores or other factors scores. There were also no significant differences in CSF TNF-α levels between patients with schizophrenia types I and II.

CONCLUSION

CSF TNF-α levels are decreased in schizophrenia, although this reduction does not correlate with the psychopathology or coincident metabolic characteristics of this disease.

Stress-Induced Glucocorticoid Release Upregulates Uncoupling Protein-2 Expression and Enhances Resistance to Endotoxin-Induced Lethality

OBJECTIVE

Although psychological and/or physiological stress has been well documented to influence immune responses, the precise mechanism for immunomodulation remains to be elucidated. The present work describes the role of the hypothalamic-pituitary-adrenal (HPA) axis in the mechanism of stress-mediated enhanced-resistance to lethality after lipopolysaccharide (LPS) injection.

METHODS/RESULTS

Preconditioning with restraint stress (RS) resulted in enhanced activation of the HPA axis in response to LPS injection and suppressed LPS-induced release of proinflammatory cytokines and nitric oxide metabolites. Melanocortin 2 receptor-deficient mice (MC2R(-/-)) failed to increase plasma levels of glucocorticoids in response to LPS injection, and exhibited high sensitivity to LPS-induced lethality with enhanced release of proinflammatory cytokines as compared with MC2R(+/-) mice. Real-time PCR analysis revealed that RS induced upregulation of uncoupling protein-2 (UCP2) in macrophages in the lung and the liver of MC2R(+/-), but not of MC2R(-/-), mice. In addition, RS increased UCP2-dependent uncoupling activity of isolated mitochondria from the liver of MC2R(+/-), but not of MC2R(-/-), mice. In vitro study revealed that corticosterone and dexamethasone directly increased UCP2 expression in mouse RAW 264.7 macrophages and suppressed the generation of LPS-induced mitochondrial reactive oxygen species (ROS) and TNF-α production. Knockdown of UCP2 by small interfering RNA blunted the dexamethasone action for suppressing LPS-induced mitochondrial ROS and TNF-α production.

CONCLUSION

The present work suggests that RS enhances activation of the HPA axis to release glucocorticoids and upregulation of UCP2 in macrophages, thereby increasing the resistance to endotoxin-induced systemic inflammation and death.

Hypothalamic-pituitary-adrenal axis activity and upper respiratory tract infection in young children transitioning to primary school

RATIONALE

We have previously reported an increase in salivary cortisol in a cohort of 4-year-old children transitioning to primary school. We hypothesised that increased cortisol in response to this acute naturalistic stress in early development may be immunostimulatory and associated with positive health outcomes.

OBJECTIVES

We tested this hypothesis by measuring upper respiratory tract infection (URI) across the first 6 months of school, in relation to salivary cortisol at the end of the second week following school transition

METHODS

Seventy children supplied morning and evening saliva samples for cortisol assay. Children were psychologically assessed for temperament and behavioural adaptation. Symptoms of URI were recorded in diary form, and variables relating to URI occurrence, duration and severity were assessed.

RESULTS

Children with higher evening cortisol at school transition experienced significantly fewer episodes of URI over the following 6 months. Diurnal cortisol change was negatively correlated with number of illnesses across the 6 months, indicating an association between a greater decline in cortisol across the day and a greater number of colds. URI severity was associated with the greatest resistance to URI infection in children who were less socially isolated and who had a smaller diurnal change in cortisol across the day.

CONCLUSIONS

Our results showing that higher cortisol is associated with lower URI may be explained by proposing that increased cortisol in response to the naturalistic stress of school transition may prime the immune system to develop resistance to URI at this critical stage of a child's development.

Evidence from knockout mice that neuropeptide-Y Y2 and Y4 receptor signalling prevents long-term depression-like behaviour caused by immune challenge

Neuropeptide Y participates in the acute behavioural responses to immune challenge, since Y2 receptor knockout (Y2⁻/⁻) mice are particularly sensitive to the short-term anxiogenic-like effect of bacterial lipopolysaccharide. The present exploratory study addressed the involvement of Y2 and Y4 receptors in the long-term behavioural responses to immune challenge. A single intraperitoneal injection of lipopolysaccharide (0.83 mg/kg) to control mice did not affect open field behaviour 3 h post-treatment but enhanced anxiety-like behaviour in Y2⁻/⁻ as well as Y4⁻/⁻ mice. Four weeks post-treatment this behavioural effect of lipopolysaccharide persisted in Y4⁻/⁻ mice but had gone in Y2⁻/⁻ mice. Depression-related behaviour in the forced swim test was enhanced 1 day post-lipopolysaccharide in control and Y2⁻/⁻ mice, but not in Y4⁻/⁻ mice. Four weeks post-treatment, the depressogenic-like effect of lipopolysaccharide had waned in control mice, persisted in Y2⁻/⁻ mice and was first observed in Y4⁻/⁻ mice. In summary, knockout of Y2 and/or Y4 receptors unmasks the ability of a single lipopolysaccharide injection to cause a delayed and prolonged increase in anxiety- and/or depression-like behaviour. These findings suggest that neuropeptide Y acting via Y2 and Y4 receptors prevents the development of long-term anxiety- and depression-like behaviour caused by acute immune challenge.

New insights into cytokine gene expression in the rat hypothalamus following endotoxin challenge

Peripheral injection of the endotoxin LPS in rats 3 weeks prior to a second injection of LPS derived from another bacterial strain results in elevated corticosterone and decreased pro-inflammatory cytokines in the blood. We further investigated this model by measuring cytokine expression in the hypothalamus and spleen. In LPS-pretreated rats, hypothalamic expression of a range of cytokines was attenuated in response to the second injection of LPS while splenic expression was elevated. This is the first demonstration that prior exposure to an endotoxin can differentially affect cytokine expression in the brain and peripheral tissues when a host is confronted with a second, acute, pro-inflammatory stimulus. Changes in hypothalamic cytokine expression in endotoxin pretreated rats may provide new evidence for the involvement of central cytokine pathways in modulating peripheral inflammation and mediating psychopathological alterations associated with inflammatory diseases.

Re-exposure to endotoxin induces differential cytokine gene expression in the rat hypothalamus and spleen

This study was designed to investigate whether the pattern of hypothalamic and splenic cytokine expression induced by peripheral administration of a bacterial lipopolysaccharide (LPS) is affected by prior exposure to LPS derived from another bacterial strain. Injection of LPS from Salmonella enteritidis (LPS(2)) alone resulted in increased hypothalamic gene expression of IL-1beta, IL-6, TNFalpha, IL-1ra and IL-10. However, pre-exposure to LPS derived from Escherichia coli (LPS(1)) 3 weeks before, significantly attenuated hypothalamic IL-1ra, IL-6 and IL-10 expression. IL-1beta expression also tended to be lower. This pattern contrasted with the robust cytokine expression in the spleen of LPS(2)-treated rats previously exposed to LPS(1), since pre-treatment with endotoxin resulted in a significantly greater response of IL-1beta and IL-1ra to LPS(2). Expression of TNFalpha and IL-10 also tended to be higher. Pre-treatment with LPS(1) did not significantly affect the marked increase in corticosterone and adrenaline blood levels induced by LPS(2). Thus, while endotoxin pre-exposure seemed not to induce a "tolerant" state in the periphery as judged by the immune and endocrine parameters evaluated upon re-stimulation, expression of four of the six cytokines measured was decreased in the hypothalamus. This is the first demonstration that endotoxin priming can differentially affect cytokine expression in the central nervous system and peripheral tissues when a host is confronted with a second, acute, pro-inflammatory stimulus. These results may provide new evidence for the involvement of cytokine pathways in the central nervous system in modulating peripheral inflammation and mediating cognitive and behavioural alterations during inflammatory diseases.