Somnogenic activities of synthetic lipid A

Tripping on the edge of consciousness

Herein the major accomplishments, trials and tribulations, and epiphanies experienced by James M. Krueger over the course of his career in sleep research are presented. They include the characterization of a) the supranormal EEG delta waves occurring during NREMS post sleep loss, b) Factor S as a muramyl peptide, c) the physiological roles of cytokines in sleep regulation, d) multiple other sleep regulatory substances, e) the dramatic changes in sleep over the course of infectious diseases, and f) sleep initiation within small neuronal/glial networks. The theory that the preservation of brain plasticity is the primordial sleep function is briefly discussed. These accomplishments resulted from collaborations with many outstanding scientists including James M. Krueger's mentors (John Pappenheimer and Manfred Karnovsky) and collaborators later in life, including Charles Dinarello, Louis Chedid, Mark Opp, Ferenc Obal jr., Dave Rector, Ping Taishi, Linda Toth, Jeannine Majde, Levente Kapas, Eva Szentirmai, Jidong Fang, Chris Davis, Sandip Roy, Tetsuya Kushikata, Fabio Garcia-Garcia, Ilia Karatsoreos, Mark Zielinski, and Alok De, plus many students, e.g. Jeremy Alt, Kathryn Jewett, Erika English, and Victor Leyva-Grado.

Role of Glia in the Regulation of Sleep in Health and Disease

Sleep is a naturally occurring physiological state that is required to sustain physical and mental health. Traditionally viewed as strictly regulated by top-down control mechanisms, sleep is now known to also originate locally. Glial cells are emerging as important contributors to the regulation of sleep-wake cycles, locally and among dedicated neural circuits. A few pioneering studies revealed that astrocytes and microglia may influence sleep pressure, duration as well as intensity, but the precise involvement of these two glial cells in the regulation of sleep remains to be fully addressed, across contexts of health and disease. In this overview article, we will first summarize the literature pertaining to the role of astrocytes and microglia in the regulation of sleep under normal physiological conditions. Afterward, we will discuss the beneficial and deleterious consequences of glia-mediated neuroinflammation, whether it is acute, or chronic and associated with brain diseases, on the regulation of sleep. Sleep disturbances are a main comorbidity in neurodegenerative diseases, and in several brain diseases that include pain, epilepsy, and cancer. Identifying the relationships between glia-mediated neuroinflammation, sleep-wake rhythm disruption and brain diseases may have important implications for the treatment of several disorders. © 2020 American Physiological Society. Compr Physiol 10:687-712, 2020.

Sleep and Microbes

Sleep is profoundly altered during the course of infectious diseases. The typical response to infection includes an initial increase in nonrapid eye movement sleep (NREMS) followed by an inhibition in NREMS. REMS is inhibited during infections. Bacterial cell wall components, such as peptidoglycan and lipopolysaccharide, macrophage digests of these components, such as muramyl peptides, and viral products, such as viral double-stranded RNA, trigger sleep responses. They do so via pathogen-associated molecular pattern recognition receptors that, in turn, enhance cytokine production. Altered sleep and associated sleep-facilitated fever responses are likely adaptive responses to infection. Normal sleep in physiological conditions may also be influenced by gut microbes because the microbiota is affected by circadian rhythms, stressors, diet, and exercise. Furthermore, sleep loss enhances translocation of viable bacteria from the intestine, which provides another means by which sleep-microbe interactions impact neurobiology.

Environmental disruption of the circadian clock leads to altered sleep and immune responses in mouse

In mammals, one of the most salient outputs of the circadian (daily) clock is the timing of the sleep-wake cycle. Modern industrialized society has led to a fundamental breakdown in the relationship between our endogenous timekeeping systems and the solar day, disrupting normal circadian rhythms. We have argued that disrupted circadian rhythms could lead to changes in allostatic load, and the capacity of organisms to respond to other environmental challenges. In this set of studies, we apply a model of circadian disruption characterized in our lab in which mice are housed in a 20h long day, with 10h of light and 10h of darkness. We explored the effects of this environmental disruption on sleep patterns, to establish if this model results in marked sleep deprivation. Given the interaction between circadian, sleep, and immune systems, we further probed if our model of circadian disruption also alters the innate immune response to peripheral bacterial endotoxin challenge. Our results demonstrate that this model of circadian disruption does not lead to marked sleep deprivation, but instead affects the timing and quality of sleep. We also show that while circadian disruption does not lead to basal changes in the immune markers we explored, the immune response is affected, both in the brain and the periphery. Together, our findings further strengthen the important role of the circadian timing system in sleep regulation and immune responses, and provide evidence that disrupting the circadian clock increases vulnerability to further environmental stressors, including immunological challenges.

Cellular immune response of pigeons in the conditions of endotoxin fever and pyrogenic tolerance

The aim of this study was to investigate changes in selected parameters of cellular immune response in the conditions of endotoxin fever and pyrogenic tolerance in pigeons. On the first day of observation the experimental birds (n = 18) were intravenously injected with Escherichia coli LPS at a dose of 10 microg/kg b.w., while the control animals (n = 6) received apyrogenic physiological saline also in the form of injection. On the second and the third day of the experiment LPS was injected additionally at 24 h intervals. Four and a half hours after the saline and pyrogen administration blood samples were collected from the control and experimental pigeons. The following immunological assays were performed: WBC, leucogram and immunophenotyping of lymphocyte subsets in peripheral blood, i.e. CD 3+ (T lymphocytes), CD 4+ (T helper lymphocytes) and CD 8+ (T suppressor/cytotoxic lymphocytes) cells. In the conditions of endotoxin fever (i.e. after the first LPS injection) leucopenia, monocytopenia, heterophilia and eosinophilia were observed. Additionally, the immunophenotyping of peripheral blood lymphocytes indicated an increase in percentage of CD 3+, CD 4+ and CD 8+ cells in response to the single injection of LPS. In contrast, the consecutive injections of LPS, which created a pyrogenic tolerance effect, caused a decrease in WBC value, heteropenia, eosinopenia and lymphocytosis. Moreover, during this state an increase in percentage of CD 3+ and CD 8+ cells was demonstrated in contrast to the percentage of CD 4+ lymphocytes. The general tendencies in cellular immune response of the affected pigeons in the conditions of endotoxin fever and pyrogenic tolerance aim at activation of defence mechanisms against LPS for its prompt elimination from the animal's organism.

Brain-immune interactions in sleep

This chapter discusses various levels of interactions between the brain and the immune system in sleep. Sleep-wake behavior and the architecture of sleep are influenced by microbial products and cytokines. On the other hand, sleep processes, and perhaps also specific sleep states, appear to promote the production and/or release of certain cytokines. The effects of immune factors such as endotoxin and cytokines on sleep reveal species specificity and usually strong dependence on parameters such as substance concentration, time relative to administration or infection with microbial products, and phase relation to sleep and/or the light-dark cycle. For instance, endotoxin increased SWS and EEG SWA in humans only at very low concentrations, whereas higher concentrations increased sleep stage 2 only, but not SWS. In animals, increases in NREM sleep and SWA were more consistent over a wide range of endotoxin doses. Also, administration of pro-inflammatory cytokines such as IL-6 and IFN-alpha in humans acutely disturbed sleep while in rats such cytokines enhanced SWS and sleep. Overall, the findings in humans indicate that strong nonspecific immune responses are acutely linked to an arousing effect. Although subjects feel subjectively tired, their sleep flattens. However, some observations indicate a delayed enhancing effect on sleep which could be related to the induction of secondary, perhaps T-cell-related factors. This would also fit with results in animals in which the T-cell-derived cytokine IL-2 enhanced sleep while cytokines with immunosuppressive functions like IL-4 and L-10 suppressed sleep. The most straightforward similarity in the cascade of events inducing sleep in both animals and humans is the enhancing effect of GHRH on SWS, and possibly the involvement of the pro-inflammatory cytokine systems of IL-1 beta and TNF-alpha. The precise mechanisms through which administered cytokines influence the central nervous system sleep processes are still unclear, although extensive research has identified the involvement of various molecular intermediates, neuropeptides, and neurotransmitters (cp. Fig. 5, Section III.B). Cytokines are not only released and found in peripheral blood mononuclear cells, but also in peripheral nerves and the brain (e.g., Hansen and Krueger, 1997; März et al., 1998). Cytokines are thereby able to influence the central nervous system sleep processes through different routes. In addition, neuronal and glial sources have been reported for various cytokines as well as for their soluble receptors (e.g., Kubota et al., 2001a). Links between the immune and endocrine systems represent a further important route through which cytokines influence sleep and, vice versa, sleep-associated processes, including variations in neurotransmitter and neuronal activity may influence cytokine levels. The ability of sleep to enhance the release and/or production of certain cytokines was also discussed. Most consistent results were found for IL-2, which may indicate a sleep-associated increase in activity of the specific immune system. Furthermore, in humans the primary response to antigens following viral challenge is enhanced by sleep. In animals results are less consistent and have focused on the secondary response. The sleep-associated modulation in cytokine levels may be mediated by endocrine parameters. Patterns of endocrine activity during sleep are probably essential for the enhancement of IL-2 and T-cell diurnal functions seen in humans: Whereas prolactin and GH release stimulate Th1-derived cytokines such as IL-2, cortisol which is decreased during the beginning of nocturnal sleep inhibits Th1-derived cytokines. The immunological function of neurotrophins, in particular NGF and BDNF, has received great interest. Effects of sleep and sleep deprivation on this cytokine family are particularly relevant in view of the effects these endogenous neurotrophins can have not only on specific immune functions and the development of immunological memories, but also on synaptic reorganization and neuronal memory formation.

Endogenous and exogenous factors on sleep-wake cycle regulation

A number of theories have proposed the involvement of different brain structures and neurotransmitters in order to explain the regulation of the sleep wake cycle. However, there is no clear consensus as to the mechanisms through which the brain structures and their various neurotransmitters interact to produce theses phases. Perhaps the problem is related to the fact sleep is a very fragile state, easily modified or influenced by a variety of substances or experimental manipulations. In this paper, we describe the evidence of two different groups of factors that induce important changes on the sleep wake cycle. The endogenous factors: neurotransmitters; hormone; peptides; and some substances of lipidic nature and exogenous factors: stress, food intake, learning, sleep deprivation, sensorial stimulation, exercise and temperature on the regulation the sleep-wake cycle. Likewise, we propose a hypothesis which attempts to reconcile the fact that endogenous and exogenous factors have similar effects.

Essential fatty acids and sleep: mini-review and hypothesis

The neurochemical basis of sleep mechanisms (onset and maintenance) is still controversial although the phenomenon itself is known to be mediated by more than a single molecule. The list of suggested endogenous sleep substances is rather long, and there is no single 'sleep center' identified in the brain. The role of fatty acids, and essential fatty acids in particular, has been ignored in sleep research. This review proposes an integration of the current knowledge about the effects of fatty acids in sleep neurochemistry, wherein fatty acids are seen to exert a direct effect on neuronal membrane structure or indirectly on the dynamics of biochemical compounds (complex lipids, prostaglandins, neurotransmitters, amino acids, interleukins) necessary for the initiation and maintenance of sleep.

Cytokines in sleep regulation

The central thesis of this essay is that the cytokine network in brain is a key element in the humoral regulation of sleep responses to infection and in the physiological regulation of sleep. We hypothesize that many cytokines, their cellular receptors, soluble receptors, and endogenous antagonists are involved in physiological sleep regulation. The expressions of some cytokines are greatly amplified by microbial challenge. This excess cytokine production during infection induces sleep responses. The excessive sleep and wakefulness that occur at different times during the course of the infectious process results from dynamic changes in various cytokines that occur during the host's response to infectious challenge. Removal of any one somnogenic cytokine inhibits normal sleep, alters the cytokine network by changing the cytokine mix, but does not completely disrupt sleep due to the redundant nature of the cytokine network. The cytokine network operates in a paracrine/autocrine fashion and is responsive to neuronal use. Finally, cytokines elicit their somnogenic actions via endocrine and neurotransmitter systems as well as having direct effects neurons and glia. Evidence in support of these postulates is reviewed in this essay.

Biological properties of bacterial peptidoglycan

Synthetic and natural muramyl peptides have a variety of biological actions in mammals, including the abilities to enhance sleep and body temperature. Although muramyl peptides can be detected constitutively in mammalian organisms, no biochemical synthetic pathways are known for muramyl peptides in mammals. However, muramyl peptides are well known as components of bacterial cell wall peptidoglycan (synonym: murein). Isolated bacterial cell walls elicit host responses similar to those produced by bacterial infections or by purified muramyl peptides. Mammalian cells which phagocytize bacteria can digest bacterial cell walls and release biologically active muramyl peptides. The released muramyl peptides then express some or all of the biological effects observed with synthetic muramyl peptides. Also, cell-free systems consisting of isolated bacterial cell walls and lysozyme produce substances with similar biological activities.

Role of biological membranes in slow-wave sleep

Two involvements of cellular membranes in slow-wave sleep (SWS) are discussed. In the first the endoplasmic reticulum (ER) is focussed upon, and in the second, the plasmalemma, where specific binding sites (receptors?) for promoters of slow-wave sleep are believed to be located. The study concerning the ER focuses on an enzyme in the brain, glucose-6-phosphatase, which, although present at low levels, manifests greatly increased activity during SWS compared to the waking state. The work on the plasmalemma has to do with the specific binding of muramyl peptides, inducers of slow-wave sleep, to various cells, and membrane preparations of various sorts, including those from brain tissue. Such cells as macrophages from mice, B-lymphocytes from human blood, and cells from a cell line (C-6 glioma) have been examined in this context.

Cytokine induction by lipopolysaccharide (LPS) corresponds to lethal toxicity and is inhibited by nontoxic Rhodobacter capsulatus LPS

Many pathological effects of gram-negative bacteria are produced by their cell wall-derived lipopolysaccharides (LPSs). Differing pathogenicity of gram-negative LPSs, however, may depend on their capacities to induce cytokines. Thus, we studied the lethal toxicity of four nonenterobacterial LPSs and compared it with their capacity to induce mononuclear cell (MNC)-derived interleukin-1 (IL-1), interleukin-6 (IL-6), and tumor necrosis factor (TNF). Unstimulated MNC did not release these cytokines. LPS from the phototrophic strain Rhodobacter capsulatus 37b4 elaborated little toxicity in galactosamine-treated mice (10 micrograms of LPS per mouse was the 100% lethal dose [LD100]) and induced IL-1 and IL-6 release only at high concentrations (10 to 50 micrograms of LPS per ml). R. capsulatus LPS failed to induce TNF activity even at the highest concentration tested (100 micrograms of LPS per ml). In contrast, LPS derived from Pseudomonas diminuta NCTC 8545 or the nodulating species Bradyrhizobium lupini DSM 30140 and Rhizobium meliloti 10406 expressed lethal toxicity (LD100, 1,000, 100, and 10 ng per mouse, respectively) and induced IL-1 or IL-6 (10 to 100, 10, and 1 ng of LPS per ml, respectively) at concentrations 1,000- to 10,000-fold lower than effective levels of R. capsulatus LPS. LPSs from P. diminuta, B. lupini, and R. meliloti also stimulated TNF production and release. MNC accumulated cell-associated IL-1 activities under circumstances in which released activity was readily detected. The cells contained only scant IL-6 activity, indicating release of this mediator rather than intracellular accumulation. Antisera to the respective cytokines inactivated biological activities of the samples selectively. The R. capsulatus LPS inhibited cytokine induction by LPS from P. diminuta, B. lupini, and R. meliloti in coincubation experiments. These results show that the in vivo lethality of the LPSs tested correlates with the induction of monocyte-derived cytokines in vitro. The results of this study suggest that the different lethality of various LPSs from gram-negative bacteria may be due to the differential ability of these LPSs to induce cytokine production.

Somnogenic activity of immune response modifiers

Sleepiness is a presenting symptom in nearly all infectious diseases. James Krueger describes how microbial products, such as muramyl peptides, lipid A and double-stranded RNA, as well as endogenous products elicited by these substances, such as interleukin 1, modulate sleep. The altered sleep during infection seems to result from an exaggerated activation of physiological sleep mechanisms, since normal sleep is controlled by a wide range of substances including many of these immune response modifiers.

Somnogenic cytokines and models concerning their effects on sleep

All the sleep-promoting substances currently identified also have other biological activities. Despite years of effort, a single specific central nervous system sleep center has not been described. These observations led us to propose a biochemical model of a sleep activational system in which the effects of several sleep factors are integrated into a regulatory scheme. These sleep factors interact by altering the metabolism, production, or activity of each other and thereby result in multiple feedback loops. This web of interactions leads to sleep stability in that minor challenges to the system will not greatly alter sleep. The system, however, is responsive to strong perturbations, such as sleep deprivation and infectious disease. The sleep-promoting effects of cytokines and their interactions with prostaglandins and the neuroendocrine system are used to illustrate the functioning of a part of the sleep activational system under normal conditions and during infectious disease. Although the actions of individuals sleep factors are not specific to sleep, their interactions at various levels of the neuraxis can mediate a specific sleep response. Such a system would also be responsive to the autonomic and environmental parameters that alter sleep.