Yawning, a thermoregulatory mechanism during fever? A study of yawning frequency and its predictors during experimentally induced sickness

Back to the future of psychoneuroimmunology: Studying inflammation-induced sickness behavior

What do we know about sickness behavior? In this article, I guide you through some of the complexity of sickness behavior occurring after an immune challenge. I highlight the many features of behavioral and affective changes induced during experimental endotoxemia in humans, and describe how little we know about many of these features. I argue that we need to dismantle the components of inflammation-induced sickness behavior, and study each component in detail. I also point out the large inter-individual differences in inflammation-induced behavioral and affective changes, and the fact that psychosocial factors likely interact with inflammation to shape inflammation-induced sickness behavior. PNI clearly lacks investigations of the vulnerability and resilient factors underlying the inter-individual variability in sickness behavior. Throughout the article, I base my argument on my published articles, and provide concrete examples from my experience and the data that I have collected over the past 10 years. Given the relevance of inflammation-induced sickness behavior for inflammation-associated depression and for how people react to infections, I encourage current and future psychoneuroimmunologists to return towards basic science of sickness behavior.

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Inflammation-related sickness behavior is relevant for inflammation-associated depression

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The many features of sickness behavior should be investigated in detail

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There are large inter-individual variability in sickness behavior

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Vulnerability and resilient factors predicting sickness responses are little known

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I call for a return towards basic science of sickness behavior.

Sick for science: experimental endotoxemia as a translational tool to develop and test new therapies for inflammation-associated depression

Depression is one of the global leading causes of disability, but treatments remain limited and classical antidepressants were found to be ineffective in a substantial proportion of patients. Thus, novel effective therapies for the treatment of depression are urgently needed. Given the emerging role of inflammation in the etiology and pathophysiology of affective disorders, we herein illustrate how experimental endotoxemia, a translational model of systemic inflammation, could be used as a tool to develop and test new therapeutic options against depression. Our concept is based on the striking overlap of inflammatory, neural, and affective characteristics in patients with inflammation-associated depression and in endotoxin-challenged healthy subjects. Experimental administration of endotoxin in healthy volunteers is safe, well-tolerated, and without known long-term health risks. It offers a highly standardized translational approach to characterize potential targets of therapies against inflammation-associated depression, as well as to identify characteristics of patients that would benefit from these interventions, and, therefore, could contribute to improve personalization of treatment and to increase the overall rate of responders.

An automated pressure-swing absorption system to administer low oxygen therapy for persons with spinal cord injury

Mild episodes of breathing low oxygen (O2) (i.e., acute intermittent hypoxia, AIH) elicits rapid mechanisms of neural plasticity that enhance respiratory and non-respiratory motor function after spinal cord injury (SCI). Despite promising outcomes in humans and rodents with SCI, the translational potential of AIH as a clinical therapy remains dependent on a safer and more reliable air delivery system. The purpose of this study is to investigate the performance of a novel AIH delivery system to overcome inconsistencies in human AIH protocols using a hand-operated (manual) delivery system. Specifically, we characterized system performance of AIH delivery in terms of flow rate, O2 concentration, dose timing, and air temperature. Our data show that a novel 'automated' delivery system: i) produces reliable AIH with a goodness-of-fit at 98.1% of 'ideal'; ii) eliminates dose timing errors via programmable solenoid switches; iii) reduces fluctuations in O2 to less than 0.01%; and iv) delivers 62.7% more air flow than the 'manual' delivery method. Automated physiological recordings, threshold detection, and visual feedback of the participant's blood O2 saturation, heart rate, and blood pressure ensures real-time user safety. In summary, the 'automated' system outperformed the 'manual' delivery method in terms of accuracy, reliability, and safety. The 'automated' system offers several design features that move the technology closer to a medically approved treatment for clinical and home use.

Comparison of bacterial lipopolysaccharide-induced sickness behavior in rodents and humans: Relevance for symptoms of anxiety and depression

Increasing evidence from animal and human studies suggests that inflammation may be involved in mood disorders. Sickness behavior and emotional changes induced by experimental inflammatory stimuli have been extensively studied in humans and rodents to better understand the mechanisms underlying inflammation-driven mood alterations. However, research in animals and humans have remained compartmentalized and a comprehensive comparison of inflammation-induced sickness and depressive-like behavior between rodents and humans is lacking. Thus, here, we highlight similarities and differences in the effects of bacterial lipopolysaccharide administration on the physiological (fever and cytokines), behavioral and emotional components of the sickness response in rodents and humans, and discuss the translational challenges involved. We also emphasize the differences between observable sickness behavior and subjective sickness reports, and advocate for the need to obtain both subjective reports and objective measurements of sickness behavior in humans. We aim to provide complementary insights for translational clinical and experimental research on inflammation-induced behavioral and emotional changes, and their relevance for mood disorders such as depression.

Properties of the Sickness Questionnaire in an Australian sample with chronic medically unexplained symptoms

Sickness behavior including malaise, fatigue and increased pain sensitivity is thought to be adaptive and facilitate recovery from disease. However, it may also reduce functioning and health if symptoms persists, which is why validated instruments for its assessment are needed. We evaluated the English translation of the Sickness Questionnaire (SicknessQ) in an Australian population of 156 participants with high level of persistent musculoskeletal pain and/or gastrointestinal symptoms without an organic explanation. The SicknessQ total score had an adequate model fit and no other models were found to fit data better. The SicknessQ correlated most strongly with fatigue, stress, anxiety and depression, which explained 62% of the variance in SicknessQ, but not with physical functioning. The mean score (8.9; 95 %CI: 8.0–9.8) was in between those previously reported in a general population sample and in primary care patients. In conclusion, the evaluation of the English version of the SicknessQ in an Australian sample with significant, chronic unexplained medical symptoms supports the use of the English version of the total SicknessQ score as an overall measure of sickness behavior.

Biological motion during inflammation in humans

Biological motion is a powerful perceptual cue that can reveal important information about the inner state of an individual. Activation of inflammatory processes likely leads to changes in gait, posture, and mobility patterns, but the specific characteristics of inflammation-related biological motion have not been characterized. The aim of this study was to determine the effect of inflammation on gait and motion in humans. Systemic inflammation was induced in 19 healthy volunteers with an intravenous injection of lipopolysaccharide (2 ng/kg body weight). Biological motion parameters (walking speed, stride length and time, arm, leg, head, and shoulder angles) were assessed during a walking paradigm and the timed-up-and-go test. Cytokine concentrations, body temperature, and sickness symptoms were measured. During inflammation, compared to placebo, participants exhibited shorter, slower, and wider strides, less arm extension, less knee flexion, and a more downward-tilting head while walking. They were also slower and took a shorter first step in the timed-up-and-go test. Higher interleukin-6 concentrations, stronger sickness symptoms, and lower body temperature predicted the inflammation-related alterations in biological motion. These findings show that biological motion contains clear information about the inflammatory status of an individual, and may be used by peers or artificial intelligence to recognize that someone is sick or contagious.

Manipulating neck temperature alters contagious yawning in humans

The existence of yawning across a diverse array of species has led many researchers to postulate its neurological significance. One hypothesis, which has garnered recent support, posits that yawns function to cool the brain by flushing hyperthermic blood away from the skull while simultaneously introducing a cooler arterial supply. The current study tested this hypothesis by examining how manipulations aimed at modifying carotid artery temperature, which in turn directly alters cranial temperature, influences contagious yawning in humans. Participants held either a warm (46 °C), cold (4 °C) or room temperature (22 °C) pack firmly to their neck, just over their carotid arteries, for a period of five minutes prior to watching a contagious yawning stimulus. Thermographic imaging verified that these manipulations produced predicted changes in temperature at the superomedial orbital area, a region previously used as a noninvasive measure of brain temperature (i.e., the brain temperature tunnel). As predicted by past research, both the urge to yawn and overall yawn frequency significantly diminished in the cooling condition (p < .05). Less than half (48.5%) of the participants in the cooling condition reported the urge to yawn, while this urge was expressed by the vast majority of participants in the warming condition (84.8%). Moreover, there was a threefold difference in the mean number of yawns per participant between the cooling and warming conditions (0.364 compared to 1.121). These findings are consistent with previous research indicating that yawns function as a compensatory brain cooling mechanism.

The effect of lipopolysaccharide (LPS) on inflammatory markers in blood and brain and on behavior in individually-housed pigs

Most of us have experienced deterioration of mood while ill. In humans, immune activation is associated with lethargy and social withdrawal, irritability and aggression; changes in social motivation could, in theory, lead to less functional interactions. This might also be the case for animals housed in close confinement. Tail biting in pigs is an example of damaging social behavior, and sickness is thought to be a risk factor for tail biting outbreaks. One possible mechanism whereby sickness may influence behavior is through cytokines. To identify possible mediators between immune activation and behavioral change, we injected 16 gilts with lipopolysaccharide (LPS; O111:B4; 1.5 μg kg^-1 IV through a permanent catheter). In LPS-treated pigs, a significant increase in cortisol, TNF-α, IL-1 receptor antagonist, IL-6, and IL-8 was observed alongside decreased activity within the first 6 h after the injection. CRP was elevated at 12 and 24 h after injection, and food intake was reduced for the first 24 h after injection. Three days post-injection, LPS pigs had lower levels of noradrenaline in their hypothalamus, hippocampus and frontal cortex compared to saline-injected pigs. Pigs injected with LPS also had higher levels of the pro-inflammatory cytokine IFN-γ in their frontal cortex compared to saline-injected pigs. Thus, a low dose of LPS can induce changes in brain cytokine levels and neurotransmitter levels that persist after inflammatory and stress markers in the periphery have returned to baseline levels.

Sex differences in how inflammation affects behavior: What we can learn from experimental inflammatory models in humans

Human models demonstrate that experimental activation of the innate immune system has profound effects on brain activation and behavior, inducing fatigue, worsened mood and pain sensitivity. It has been proposed that inflammation is a mechanism involved in the etiology and maintenance of depression, chronic pain and long-term fatigue. These diseases show a strong female overrepresentation, suggesting that a better understanding of sex differences in how inflammation drives behavior could help the development of individualized treatment interventions. For this purpose, we here review sex differences in studies using experimental inflammatory models to investigate changes in brain activity and behavior. We suggest a model in which inflammation accentuates sex differences in brain networks and pre-existing vulnerability factors. This effect could render women more vulnerable to the detrimental effects of immune-to-brain communication over time. We call for systematic and large scale investigations of vulnerability factors for women in the behavioral response to inflammation.

Communication of health in experimentally sick men and women: A pilot study

The way people communicate their ill-health and the factors involved in ill-health communication remain poorly known. In the present study, we tested how men and women communicate their sickness and assessed whether sickness-related variables (i.e., body temperature, immune response, subjective sickness symptoms) predicted communicative behaviors. Twenty-two participants were filmed during experimentally induced sickness, triggered by lipopolysaccharide administration (2ng/kg body weight), and after placebo administration, in presence of female care providers. Two trained raters scored participants' communicative behaviors (verbal complaints, moaning and sighs/deep breaths). The physiological and subjective sickness responses were similar in both sexes. Participants were more likely to moan and complain when sick, although the frequency of these behaviors remained low and no clear sex differences was observed. Nevertheless, frequency of sighs/deep breaths was increased amongst sick men but not in women. Sickness-related variables did not predict sigh/deep breath frequency. In this setting, sick men appear to display a lower threshold of expressing their malaise as compared to similarly sick women.