Jan-Sebastian Grigoleit

Dose-Dependent Effects of Endotoxin on Neurobehavioral Functions in Humans

Clinical and experimental evidence document that inflammation and increased peripheral cytokine levels are associated with depression-like symptoms and neuropsychological disturbances in humans. However, it remains unclear whether and to what extent cognitive functions like memory and attention are affected by and related to the dose of the inflammatory stimulus. Thus, in a cross-over, double-blind, experimental approach, healthy male volunteers were administered with either placebo or bacterial lipopolysaccharide (LPS) at doses of 0.4 (n = 18) or 0.8 ng/kg of body weight (n = 16). Pro- and anti-inflammatory cytokines, norephinephrine and cortisol concentrations were analyzed before and 1, 1.75, 3, 4, 6, and 24 h after injection. In addition, changes in mood and anxiety levels were determined together with working memory (n-back task) and long term memory performance (recall of emotional and neutral pictures of the International Affective Picture System). Endotoxin administration caused a profound transient physiological response with dose-related elevations in body temperature and heart rate, increases in plasma interleukin (IL)-6, IL-10, tumor necrosis factor (TNF)-α and IL-1 receptor antagonist (IL-1ra), salivary and plasma cortisol, and plasma norepinephrine. These changes were accompanied by dose-related decreased mood and increased anxiety levels. LPS administration did not affect accuracy in working memory performance but improved reaction time in the high-dose LPS condition compared to the control conditon. In contrast, long-term memory performance was impaired selectively for emotional stimuli after administration of the lower but not of the higher dose of LPS. These data suggest the existence of at least two counter-acting mechanisms, one promoting and one inhibiting cognitive performance during acute systemic inflammation.

Endotoxin-induced experimental systemic inflammation in humans: a model to disentangle immune-to-brain communication.

Systemic inflammation is among the most prominent and most frequently observed responses of the immune system. Over the past decades, it has become clear that inflammatory cytokines not only affect immune and metabolic functions but also cause a wide range of behavioral and mood changes. Based on experimental findings in animals and observations in clinical populations it has been hypothesized that inflammation-induced neurocognitive changes contribute to the pathophysiology of neuropsychiatric diseases. However, since certain aspects of human behavior cannot be modeled in laboratory animals, there is a need for human models of systemic inflammation. In this review, we summarize recent studies employing administration of endotoxin as a model to induce transient systemic inflammation in healthy human subjects.

Acute experimental endotoxemia induces visceral hypersensitivity and altered pain evaluation in healthy humans

Summary A systemic, endotoxin‐induced immune activation leads to decreased visceral sensory and pain thresholds and altered subjective pain ratings in healthy humans. Abstract Growing evidence suggests that systemic immune activation plays a role in the pathophysiology of pain in functional bowel disorders. By implementing a randomized crossover study with an injection of endotoxin or saline, we aimed to test the hypothesis that endotoxin‐induced systemic inflammation increases visceral pain sensitivity in humans. Eleven healthy men (mean ± standard error of the mean age 26.6 ± 1.1 years) received an intravenous injection of either lipopolysaccharide (LPS; 0.4 ng/kg) or saline on 2 otherwise identical study days. Blood samples were collected 15 min before and 1, 2, 3, 4, and 6 h after injection to characterize changes in immune parameters including proinflammatory cytokines. Rectal sensory and pain thresholds and subjective pain ratings were assessed with barostat rectal distensions 2 h after injection. LPS administration induced an acute inflammatory response indicated by transient increases in tumor necrosis factor alpha, interleukin 6, and body temperature (all P < .001). The LPS‐induced immune activation increased sensitivity to rectal distensions as reflected by significantly decreased visceral sensory and pain thresholds (both P < .05) compared to saline control. Visceral stimuli were rated as more unpleasant (P < .05) and inducing increased urge to defecate (P < .01). Pain thresholds correlated with interleukin 6 at +1 h (r = 0.60, P < .05) and +3 h (r = 0.67, P < .05) within the LPS condition. This report is novel in that it demonstrates that a transient systemic immune activation results in decreased visceral sensory and pain thresholds and altered subjective pain ratings. Our results support the relevance of inflammatory processes in the pathophysiology of visceral hyperalgesia and underscore the need for studies to further elucidate immune‐to‐brain communication pathways in gastrointestinal disorders.

Neural response to emotional stimuli during experimental human endotoxemia

Increases in peripheral cytokines during acute inflammation may affect various neuropsychological functions. The aim of this functional magnetic resonance imaging (fMRI) study was to investigate the effects of acute endotoxemia on mood and the neural response to emotionally aversive visual stimuli in healthy human subjects. In a double‐blind, randomized crossover study, 18 healthy males received a bolus injection of bacterial lipopolysaccharide (LPS; 0.4 ng/kg) or saline. Plasma levels of pro‐ and anti‐inflammatory cytokines and cortisol as well as mood ratings were analyzed together with the blood‐oxygen‐level dependent (BOLD) response during the presentation of aversive versus neutral pictures. Endotoxin administration induced pronounced transient increases in plasma levels of TNF‐α, IL‐1ra, IL‐6, IL‐10, and cortisol. Positive mood was decreased and state anxiety increased. In addition, activation of right inferior orbitofrontal cortex (OFC) in response to emotional visual stimuli was significantly increased in the LPS condition. Increased prefrontal activation during the presentation of emotional material may reflect enhanced cognitive regulation of emotions as an adaptive response during an acute inflammation. These findings may have implications for the putative role of inflammatory processes in the pathophysiology of depression. Hum Brain Mapp 34:2217–2227, 2013.

Inflammation-induced hyperalgesia: effects of timing, dosage, and negative affect on somatic pain sensitivity in human experimental endotoxemia.

BACKGROUND Inflammation-induced pain amplification and hypersensitivity play a role in the pathophysiology of numerous clinical conditions. Experimental endotoxemia has recently been implemented as model to analyze immune-mediated processes in human pain. In this study, we aimed to analyze dose- and time-dependent effects of lipopolysaccharide (LPS) on clinically-relevant pain models for musculoskeletal and neuropathic pain as well as the interaction among LPS-induced changes in inflammatory markers, pain sensitivity and negative affect. METHODS In this randomized, double-blind, placebo-controlled study, healthy male subjects received an intravenous injection of either a moderate dose of LPS (0.8 ng/kg Escherichiacoli), low-dose LPS (0.4 ng/kg), or saline (placebo control group). Pressure pain thresholds (PPT), mechanical pain sensitivity (MPS), and cold pain sensitivity (CP) were assessed before and 1, 3, and 6h post injection to assess time-dependent LPS effects on pain sensitivity. Plasma cytokines (TNF-α, IL-6, IL-8, IL-10) and state anxiety were repeatedly measured before, and 1, 2, 3, 4, and 6h after injection of LPS or placebo. RESULTS LPS administration induced a systemic immune activation, reflected by significant increases in cytokine levels, body temperature, and negative mood with pronounced effects to the higher LPS dose. Significant decreases of PPTs were observed only 3h after injection of the moderate dose of LPS (0.8 ng/kg). MPS and CP were not affected by LPS-induced immune activation. Correlation analyses revealed that decreased PPTs were associated with peak IL-6 increases and negative mood. CONCLUSIONS Our results revealed widespread increases in musculoskeletal pain sensitivity in response to a moderate dose of LPS (0.8 ng/kg), which correlate both with changes in IL-6 and negative mood. These data extend and refine existing knowledge about immune mechanisms mediating hyperalgesia with implications for the pathophysiology of chronic pain and neuropsychiatric conditions.

Lipopolysaccharide-induced experimental immune activation does not impair memory functions in humans

Systemic immune activation occurring together with release of peripheral cytokines can affect behavior and the functioning of the central nervous system (CNS). However, it remains unknown whether and to what extent cognitive functions like memory and attention are affected during transient immune activation. We employed a human endotoxemia model and standardized neuropsychological tests to assess the cognitive effects of an experimental inflammation in two groups of 12 healthy young men before and after intravenous injection of lipopolysaccharide (LPS, Escherichia coli, 0.4 ng/kg) or physiological saline. Endotoxin administration caused a profound transient physiological response with elevations in body temperature, number of circulating neutrophils, and increases in plasma cytokine levels [interleukin (IL)-6, IL-10, tumor necrosis factor (TNF)-α], and concentrations of norepinephrine, ACTH and cortisol. However, these changes in immune and neuroendocrine parameters were not associated with alterations of memory performance, selective attention or executive functions.

Mood disturbance during experimental endotoxemia: Predictors of state anxiety as a psychological component of sickness behavior

Lipopolysaccharide (LPS) administration is a well-established model to assess afferent immune-to-brain communication and behavioral aspects of inflammation. Nevertheless, only few studies in comparatively small samples have assessed state anxiety as a psychological component of sickness behavior despite possible clinical implications for the pathophysiology of neuropsychiatric conditions. Thus, the goal of the present analyses carried out in a large, pooled dataset from two independent study sites was to analyze the state anxiety response to LPS administration and to investigate predictors (i.e., cytokine changes; pre-existing anxiety and depression symptoms assessed with the Hospital Anxiety and Depression Scale) of the LPS-induced state anxiety changes at different time points after LPS administration. Data from 186 healthy volunteers who participated in one of six randomized, placebo-controlled human studies involving intravenous administration of LPS at doses of 0.4-0.8ng/kg body weight were combined. State anxiety as well as circulating interleukin (IL)-6, tumor necrosis factor (TNF)-α and IL-10 concentrations were significantly increased 2h and 3h after LPS administration, with a peak at 2h, and returned to baseline 6h after administration. Greater changes in IL-6 from baseline to 3h after LPS administration significantly and independently predicted a more pronounced LPS-induced state anxiety response. In addition, higher pre-existing subclinical anxiety symptoms significantly predicted a lower increase in state anxiety 3h and 6h after LPS-administration, which was mediated by TNF-α changes. In conclusion, our findings give additional support for a putative role of inflammatory mechanisms in the pathophysiology of stress-related and anxiety disorders and give new insight on the potential role of pre-existing subclinical affective symptoms.

Experimental human endotoxemia enhances brain activity during social cognition

Acute peripheral inflammation with corresponding increases in peripheral cytokines affects neuropsychological functions and induces depression-like symptoms. However, possible effects of increased immune responses on social cognition remain unknown. Therefore, this study investigated the effects of experimentally induced acute inflammation on performance and neural responses during a social cognition task assessing Theory of Mind (ToM) ability. In this double-blind randomized crossover functional magnetic resonance imaging study, 18 healthy right-handed male volunteers received an injection of bacterial lipopolysaccharide (LPS; 0.4 ng/kg) or saline, respectively. Plasma levels of pro- and anti-inflammatory cytokines as well as mood ratings were analyzed together with brain activation during a validated ToM task (i.e. Reading the Mind in the Eyes Test). LPS administration induced pronounced transient increases in pro- (IL-6, TNF-α) and anti-inflammatory (IL-10, IL-1ra) cytokines as well as decreases in mood. Social cognition performance was not affected by acute inflammation. However, altered neural activity was observed during the ToM task after LPS administration, reflected by increased responses in the fusiform gyrus, temporo-parietal junction, superior temporal gyrus and precuneus. The increased task-related neural responses in the LPS condition may reflect a compensatory strategy or a greater social cognitive processing as a function of sickness.

Single-trial conditioning in a human taste-endotoxin paradigm induces conditioned odor aversion but not cytokine responses.

Immunological responses to bacterial endotoxin can be behaviorally conditioned in rodents. However, it is unclear whether an acute systemic inflammatory response can be behaviorally conditioned in humans. Thus, in a double-blind placebo-controlled study, 20 healthy, male subjects received either a single injection of lipopolysaccharide (LPS) or saline together with a novel tasting beverage (conditioned stimulus, CS). Five days later, all subjects received a saline injection and were re-exposed to the CS. Blood was drawn prior to as well as 0.5, 1.5, 3, 4, 6, and 24 h after LPS administration or CS re-exposure. Endotoxin administration led to transient increases in plasma concentrations of interleukin (IL)-6, IL-10, and tumor necrosis factor (TNF)-α and to a significant rise in body temperature. Sole presentation of the CS during evocation did induce neither alterations in body temperature nor changes in plasma cytokine levels. However, subjects in the experimental group rated the smell of the CS significantly more aversive compared to the control group. Employing endotoxin as a US in a single trial taste-immune conditioning paradigm in humans shows a behaviorally conditioned smell aversion but no learned alterations in cytokine levels.

Biological and psychological predictors of visceral pain sensitivity in healthy premenopausal women.

Factors that are associated with pain perception remain incompletely understood, especially in the visceral pain field. Therefore, the current study aimed to investigate possible psychological and biological predictors of visceral pain sensitivity in healthy subjects.