LPS-induced endotoxic shock does not cause early brain edema formation – An MRI study in rats

Application of magnetic resonance imaging-related techniques in the diagnosis of sepsis-associated encephalopathy: present status and prospect

Sepsis-associated encephalopathy (SAE) refers to diffuse brain dysfunction secondary to systemic infection without central nervous system infection. The early diagnosis of SAE remains a major clinical problem, and its diagnosis is still exclusionary. Magnetic resonance imaging (MRI) related techniques, such as magnetic resonance spectroscopy (MRS), molecular MRI (mMRI), arterial spin-labeling (ASL), fluid-attenuated inversion recovery (FLAIR), and diffusion-weighted imaging (DWI), currently provide new options for the early identification of SAE. This review collected clinical and basic research and case reports related to SAE and MRI-related techniques in recent years, summarized and analyzed the basic principles and applications of MRI technology in diagnosing SAE, and provided a basis for diagnosing SAE by MRI-related techniques.

A Toolbox to Investigate the Impact of Impaired Oxygen Delivery in Experimental Disease Models

Impaired oxygen utilization is the underlying pathophysiological process in different shock states. Clinically most important are septic and hemorrhagic shock, which comprise more than 75% of all clinical cases of shock. Both forms lead to severe dysfunction of the microcirculation and the mitochondria that can cause or further aggravate tissue damage and inflammation. However, the detailed mechanisms of acute and long-term effects of impaired oxygen utilization are still elusive. Importantly, a defective oxygen exploitation can impact multiple organs simultaneously and organ damage can be aggravated due to intense organ cross-talk or the presence of a systemic inflammatory response. Complexity is further increased through a large heterogeneity in the human population, differences in genetics, age and gender, comorbidities or disease history. To gain a deeper understanding of the principles, mechanisms, interconnections and consequences of impaired oxygen delivery and utilization, interdisciplinary preclinical as well as clinical research is required. In this review, we provide a "tool-box" that covers widely used animal disease models for septic and hemorrhagic shock and methods to determine the structure and function of the microcirculation as well as mitochondrial function. Furthermore, we suggest magnetic resonance imaging as a multimodal imaging platform to noninvasively assess the consequences of impaired oxygen delivery on organ function, cell metabolism, alterations in tissue textures or inflammation. Combining structural and functional analyses of oxygen delivery and utilization in animal models with additional data obtained by multiparametric MRI-based techniques can help to unravel mechanisms underlying immediate effects as well as long-term consequences of impaired oxygen delivery on multiple organs and may narrow the gap between experimental preclinical research and the human patient.

Structural and biochemical imaging reveals systemic LPS-induced changes in the rat brain

Despite mounting evidence for the role of inflammation in Major Depressive Disorder (MDD), in vivo preclinical investigations of inflammation-induced negative affect using whole brain imaging modalities are scarce, precluding a valid model within which to evaluate pharmacological interventions. Here we used an E. coli lipopolysaccharide (LPS)-based model of inflammation-induced depressive signs in rats to explore brain changes using multimodal neuroimaging methods. During the acute phase of the LPS response (2 h post injection), prior to the emergence of a task-quantifiable depressive phenotype, striatal glutamine levels and splenial, retrosplenial, and peri-callosal hippocampal cortex volumes were greater than at baseline. LPS-induced depressive behaviors observed at 24 h, however, occurred concurrently with lower than control levels of striatal glutamine and a reversibility of volume expansion (i.e., shrinkage of splenial, retrosplenial, and peri-callosal hippocampal cortex to baseline volumes). In both striatum and hippocampus at 24 h, mRNA expression in LPS relative to control animals demonstrated alterations in enzymes and transporters regulating glutamine homeostasis. Collectively, the observed behavioral, in vivo structural and metabolic, and mRNA expression alterations suggest a critical role for astrocytic regulation of inflammation-induced depressive behaviors.

Volumetric response of the adult brain to seizures depends on the developmental stage when systemic inflammation was induced

Inflammation has detrimental influences on the developing brain including triggering the epileptogenesis. On the other hand, seizure episodes may induce inflammatory processes and further increase of brain excitability. The present study focuses on the problem whether transitory systemic inflammation during developmental period may have critical importance to functional and/or structural features of the adult brain. An inflammatory status was induced with lipopolysaccharide (LPS) in 6- or 30-day-old rats. Two-month-old rats which experienced the inflammation and untreated controls received injections of pilocarpine, and the intensity of their seizure behavior was rated during a 6-hour period. Three days thereafter, the animals were perfused; their brains were postfixed and subjected to magnetic resonance imaging (MRI) scans. Then, volumes of the brain and of its main regions were assessed. LPS injections alone performed at different developmental stages led to different changes in the volume of adult brain and also to different susceptibility to seizures induced in adulthood. Moreover, the LPS pretreatments modified different volumetric responses of the brain and of its regions to seizures. The responses showed strong inverse correlations with the intensity of seizures but exclusively in rats treated with LPS on postnatal day 30. It could be concluded that generalized inflammation elicited at developmental stages may have strong age-dependent effects on the adult brain regarding not only its susceptibility to action of a seizuregenic agent but also its volumetric reactivity to seizures.

Modulatory effects of vagal stimulation on neurophysiological parameters and the cellular immune response in the rat brain during systemic inflammation

Background

Stimulation of the vagus nerve has modulating, anti-inflammatory effects on the cellular immune response in the blood and the spleen, stabilizing brain function. Here, we aimed to investigate its potential effects on immune-to-brain communication focusing on neurophysiological readouts and leukocyte migration to the brain during severe sepsis-like endotoxemia.

Methods

Systemic inflammation was induced by intravenous administration of lipopolysaccharide (LPS; 5 mg/kg). Animals received either no manipulation of the vagus nerve, vagotomy, or vagotomy plus vagus nerve stimulation of the distal trunk. Somatosensory evoked potentials and evoked flow velocity response were measured for 4.5 h as indicators of brain function and neurovascular coupling, respectively. In addition, brain areas with (cortex) and without (hypothalamus) tight blood-brain barrier were studied separately using immunohistochemistry and RT-PCR. Moreover, plasma cytokine and leptin levels were analyzed by ELISA.

Results

LPS induced a decline of both neurophysiological parameters, which was prevented by vagus nerve stimulation. As for peripheral organs, LPS-stimulated neutrophil counts increased in the brain and colocalized in the brain with endothelial intercellular adhesion molecule (ICAM)-1. Interestingly, vagal stimulation reduced this colocalization and decreased nuclear translocation of the brain cell activation marker nuclear factor interleukin 6 (NF-IL6). Furthermore, it reduced the gene expression of inflammatory markers and extravasation signals (IL-6, CXCL-1, ICAM-1) in the hypothalamus but not the cortex linked to a moderate decrease in circulating cytokine levels (interleukin 6, tumor necrosis factor alpha) as well as lower plasma leptin concentration.

Conclusions

Our data suggest beneficial effects of anti-inflammatory vagus nerve stimulation on brain function by reducing the interaction of neurotrophil granulocytes with the brain endothelium as well as attenuating inflammatory responses in brain areas lacking a blood-brain barrier.

Electronic supplementary material

The online version of this article (doi:10.1186/s40635-016-0091-4) contains supplementary material, which is available to authorized users.

Neurohistological abnormalities during early porcine endotoxemia

BACKGROUND

Brain dysfunction is common in sepsis. We aimed to assess whether cerebral perfusion, oxygenation, and/or metabolism are abnormal during early endotoxemia, and how they may relate to potential neurohistological changes.

METHODS

In this prospective animal study, we included 12 pigs (weight: 42 ± 4 kg; mean ± SD) that were exposed to Escherichia coli lipopolysaccharide (E. coli LPS B0111 : B4, 0.4 μg/kg/h) or saline infusion (n = 6, each) for 10 h. Systemic hemodynamics, cerebral blood flow, intracranial pressure, and brain tissue oxygen tension were continuously measured. At the end of the experiment, formalin-fixed brains were cut in coronal sections and embedded in paraffin. Afterwards, the sections were cut at 5 microns and stained with hematoxylin and eosin.

RESULTS

Stable systemic hemodynamics in both groups were associated with higher carotid arterial blood flow after 10 h of endotoxemia (9.0 ± 2.2 ml/kg/min) compared to controls (6.6 ± 1.2 ml/kg/min; time-group interaction: P = 0.014). Intracranial pressure, cerebral perfusion pressure, brain oxygen consumption, and brain tissue oxygen tension were similar in both groups. In four of the six endotoxemic animals but in none of the controls, cerebral tissue lesions were found (encephalomalacia with spongy degeneration of white matter, axonal swelling, and ischemic neuronal thalamic necrosis), including significant venous vascular alterations, predominantly in the brainstem, in three of the four animals.

CONCLUSIONS

Early endotoxemia seems to be associated with histological signs of brain damage unrelated to systemic or cerebral hemodynamics or oxygenation.

Magnetic resonance imaging characterization of microbial infections

The investigation of microbial infections relies to a large part on animal models of infection, if host pathogen interactions or the host response are considered. Especially for the assessment of novel therapeutic agents, animal models are required. Non-invasive imaging methods to study such models have gained increasing importance over the recent years. In particular, magnetic resonance imaging (MRI) affords a variety of diagnostic options, and can be used for longitudinal studies. In this review, we introduce the most important MRI modalities that show how MRI has been used for the investigation of animal models of infection previously and how it may be applied in the future.

Imaging in sepsis-associated encephalopathy--insights and opportunities

Sepsis-associated encephalopathy (SAE) refers to a clinical spectrum of acute neurological dysfunction that arises in the context of sepsis. Although the pathophysiology of SAE is incompletely understood, it is thought to involve endothelial activation, blood-brain barrier leakage, inflammatory cell migration, and neuronal loss with neurotransmitter imbalance. SAE is associated with a high risk of mortality. Imaging studies using MRI and CT have demonstrated changes in the brains of patients with SAE that are also seen in disorders such as stroke. Next-generation imaging techniques such as magnetic resonance spectroscopy, diffusion tensor imaging and PET, as well as experimental imaging modalities, provide options for early identification of patients with SAE, and could aid in identification of pathophysiological processes that represent possible therapeutic targets. In this Review, we explore the recent literature on imaging in SAE, relating the findings of these studies to pathological data and experimental studies to obtain insights into the pathophysiology of sepsis-associated neurological dysfunction. Furthermore, we suggest how novel imaging technologies can be used for early-stage proof-of-concept and proof-of-mechanism translational studies, which may help to improve diagnosis in SAE.

Systemic inflammation alters central 5-HT function as determined by pharmacological MRI

Considerable evidence indicates a link between systemic inflammation and central 5-HT function. This study used pharmacological magnetic resonance imaging (phMRI) to study the effects of systemic inflammatory events on central 5-HT function. Changes in blood oxygenation level dependent (BOLD) contrast were detected in selected brain regions of anaesthetised rats in response to intravenous administration of the 5-HT-releasing agent, fenfluramine (10 mg/kg). Further groups of rats were pre-treated with the bacterial lipopolysaccharide (LPS; 0.5 mg/kg), to induce systemic inflammation, or the selective 5-HT2A receptor antagonist MDL100907 prior to fenfluramine. The resultant phMRI data were investigated further through measurements of cortical 5-HT release (microdialysis), and vascular responsivity, as well as a more thorough investigation of the role of the 5-HT2A receptor in sickness behaviour. Fenfluramine evoked a positive BOLD response in the motor cortex (+15.9±2%) and a negative BOLD response in the dorsal raphe nucleus (-9.9±4.2%) and nucleus accumbens (-7.7±5.3%). In all regions, BOLD responses to fenfluramine were significantly attenuated by pre-treatment with LPS (p<0.0001), but neurovascular coupling remained intact, and fenfluramine-evoked 5-HT release was not affected. However, increased expression of the 5-HT2A receptor mRNA and decreased 5-HT2A-dependent behaviour (wet-dog shakes) was a feature of the LPS treatment and may underpin the altered phMRI signal. MDL100907 (0.5 mg/kg), 5-HT2A antagonist, significantly reduced the BOLD responses to fenfluramine in all three regions (p<0.0001) in a similar manner to LPS. Together these results suggest that systemic inflammation decreases brain 5-HT activity as assessed by phMRI. However, these effects do not appear to be mediated by changes in 5-HT release, but are associated with changes in 5-HT2A-receptor-mediated downstream signalling pathways.

Activated protein C improves pial microcirculation in experimental endotoxemia in rats

INTRODUCTION

The brain is one of the first organs affected clinically in sepsis. Microcirculatory alterations are suggested to be a critical component in the pathophysiology of sepsis. The aim of this study was to investigate the effects of recombinant human activated protein C (rhAPC) on the pial microcirculation in experimental endotoxemia using intravital microscopy. Our hypothesis is rhAPC protects pial microcirculation in endotoxemia.

METHODS

Endotoxemia was generated in Lewis rats with intravenous injection of lipopolysaccharide (LPS, 5 mg/kg i.v.). Dura mater was removed through a cranial window to expose pial vessels on the brain surface. The microcirculation, including leukocyte-endothelial interaction, functional capillary density (FCD) and plasma extravasation of pial vessels was examined by fluorescent intravital microscopy (IVM) 2 h after administration of LPS, LPS and rhAPC or equivalent amount of saline (used as Control group). Plasma cytokine levels of interleukin 1 alpha (IL1-α), tumor necrosis factor-α (TNF-α), interferon γ (IFN-γ), Monocyte chemotactic protein-1 (MCP-1) and Granulocyte-macrophage colony-stimulating factor (GM-CSF) were evaluated after IVM.

RESULTS

LPS challenge significantly increased leukocyte adhesion (773±190 vs. 592±152 n/mm(2) Control), decreased FCD (218±54 vs. 418±74 cm/cm(2) Control) and increased proinflammatory cytokine levels (IL-1α: 5032±1502 vs. 8±21 pg/ml; TNF-α: 1823±1007 vs. 168±228 pg/ml; IFN-γ: 785±434 vs. 0 pg/ml; GM-CSF: 54±52 vs. 1±3 pg/ml) compared to control animals. rhAPC treatment significantly reduced leukocyte adhesion (599±111 n/mm(2)), increased FCD (516±118 cm/cm(2)) and reduced IL-1α levels (2134±937 pg/ml) in the endotoxemic rats.

CONCLUSION

APC treatment significantly improves pial microcirculation by reducing leukocyte adhesion and increasing FCD.

Experimental Endotoxemia Induces Leukocyte Adherence and Plasma Extravasation Within the Rat Pial Microcirculation

Disturbance of capillary perfusions due to leukocyte adhesion, disseminated intravascular coagulation, tissue edema is critical components in the pathophysiology of sepsis. Alterations in brain microcirculation during sepsis are not clearly understood. The aim of this study is to gain an improved understanding of alterations through direct visualization of brain microcirculations in an experimental endotoxemia using intravital microscopy (IVM). Endotoxemia was induced in Lewis rats with Lipopolysaccharide (LPS, 15 mg/kg i.v.). The dura mater was removed via a cranial window to expose the pial vessels on the brain surface. Using fluorescence dyes, plasma extravasation of pial venous vessels and leukocyte-endothelial interaction were visualized by intravital microscopy 4 h after LPS administration. Plasma cytokine levels of IL1-β, IL-6, IFN-γ, TNF-α and KC/GRO were evaluated after IVM. A significant plasma extravasation of the pial venous vessels was found in endotoxemia rats compared to control animals. In addition, a significantly increased number of leukocytes adherent to the pial venous endothelium was observed in septic animals. Endotoxemia also induced a significant elevation of plasma cytokine levels of IL1-β, IL-6, IFN-γ, TNF-α and KC/GRO. Endotoxemia increased permeability in the brain pial vessels accompanied by an increase of leukocyte-endothelium interactions and an increase of inflammatory cytokines in the plasma.

Cerebral microcirculation is impaired during sepsis: an experimental study

Introduction

Pathophysiology of brain dysfunction due to sepsis remains poorly understood. Cerebral microcirculatory alterations may play a role; however, experimental data are scarce. This study sought to investigate whether the cerebral microcirculation is altered in a clinically relevant animal model of septic shock.

Methods

Fifteen anesthetized, invasively monitored, and mechanically ventilated female sheep were allocated to a sham procedure (n = 5) or sepsis (n = 10), in which peritonitis was induced by intra-abdominal injection of autologous faeces. Animals were observed until spontaneous death or for a maximum of 20 hours. In addition to global hemodynamic assessment, the microcirculation of the cerebral cortex was evaluated using Sidestream Dark-Field (SDF) videomicroscopy at baseline, 6 hours, 12 hours and at shock onset. At least five images of 20 seconds each from separate areas were recorded at each time point and stored under a random number to be analyzed, using a semi-quantitative method, by an investigator blinded to time and condition.

Results

All septic animals developed a hyperdynamic state associated with organ dysfunction and, ultimately, septic shock. In the septic animals, there was a progressive decrease in cerebral total perfused vessel density (from 5.9 ± 0.9 at baseline to 4.8 ± 0.7 n/mm at shock onset, P = 0.009), functional capillary density (from 2.8 ± 0.4 to 2.1 ± 0.7 n/mm, P = 0.049), the proportion of small perfused vessels (from 95 ± 3 to 85 ± 8%, P = 0.02), and the total number of perfused capillaries (from 22.7 ± 2.7 to 17.5 ± 5.2 n/mm, P = 0.04). There were no significant changes in microcirculatory flow index over time. In sham animals, the cerebral microcirculation was unaltered during the study period.

Conclusions

In this model of peritonitis, the cerebral microcirculation was impaired during sepsis, with a significant reduction in perfused small vessels at the onset of septic shock. These alterations may play a role in the pathogenesis of septic encephalopathy.

Sepsis-associated encephalopathy: a magnetic resonance imaging and spectroscopy study

Brain dysfunction is frequently observed in sepsis as a consequence of changes in cerebral structure and metabolism, resulting in worse outcome and reduced life-quality of surviving patients. However, the mechanisms of sepsis-associated encephalopathy development and a better characterization of this syndrome in vivo are lacking. Here, we used magnetic resonance imaging (MRI) techniques to assess brain morphology and metabolism in a murine sepsis model (cecal ligation and puncture, CLP). Sham-operated and CLP mice were subjected to a complete MRI session at baseline, 6 and 24 h after surgery. Accumulation of vasogenic edematic fluid at the base of the brain was observed in T(2)-weighted image at 6 and 24 h after CLP. Also, the water apparent diffusion coefficients in both hippocampus and cortex were decreased, suggesting a cytotoxic edema in brains of nonsurvival septic animals. Moreover, the N-acetylaspartate/choline ratio was reduced in brains of septic mice, indicating neuronal damage. In conclusion, noninvasive assessment by MRI allowed the identification of new aspects of brain damage in sepsis, including cytotoxic and vasogenic edema as well as neuronal damage. These findings highlight the potential applications of MRI techniques for the diagnostic and therapeutic studies in sepsis.

Effects of inducible nitric oxide synthase inhibition or norepinephrine on the neurovascular coupling in an endotoxic rat shock model

Introduction

The inducible nitric oxide synthase (iNOS) plays a crucial role in early sepsis-related microcirculatory dysfunction. Compared to a catecholamine therapy we tested effects of a specific iNOS-inhibitor (1400W) on the microcirculatory function in the brain.

Methods

Seventy SD-rats (280-310 g) were divided into 1 control and 6 sepsis groups. Sepsis groups received 1 or 5 mg/kg lipopolysaccharide (LPS) intravenously to induce a moderate or severe sepsis syndrome. Thirty minutes later rats were further randomized into subgroups receiving moderate volume therapy alone or additionally continuous norepinephrine (NE) or 1400W infusion. Separately, effects of 1400W on neurofunctional parameters were investigated in 3 rats without sepsis induction. Performing electric forepaw-stimulation evoked potentials (N2-P1 amplitude, P1-latency) and local hemodynamic responses were recorded with surface electrodes and laser Doppler over the somatosensory cortex at baseline and repeatedly after LPS administration. Cytokine levels (tumor necrosis factor-alpha (TNFα), interleukin-6 (IL6), interferon-gamma (IFNγ)) and cell destruction markers (neuron-specific enolase (NSE), S-100 calcium binding protein B (S100B)) were obtained at the end of experiments.

Results

During sepsis progression resting cerebral blood flow increased and functionally activated hemodynamic responses decreased in a dose-dependent manner. Whereas 1400W and NE improved blood pressure, only 1400W stabilized resting flow levels. However, both regimens were ineffective on the functionally coupled flow responses and destruction markers were similar between groups.

Conclusions

NE and 1400W appeared to be ineffective in mitigating the effects of sepsis on the neurovascular coupling. Other regimens are needed to protect the cerebral microcirculation under septic conditions.