Mark R. Opp

How (and why) the immune system makes us sleep

Good sleep is necessary for physical and mental health. For example, sleep loss impairs immune function, and sleep is altered during infection. Immune signalling molecules are present in the healthy brain, where they interact with neurochemical systems to contribute to the regulation of normal sleep. Animal studies have shown that interactions between immune signalling molecules (such as the cytokine interleukin 1) and brain neurochemical systems (such as the serotonin system) are amplified during infection, indicating that these interactions might underlie the changes in sleep that occur during infection. Why should the immune system cause us to sleep differently when we are sick? We propose that the alterations in sleep architecture during infection are exquisitely tailored to support the generation of fever, which in turn imparts survival value.

Wnt10b Inhibits Development of White and Brown Adipose Tissues

Wnt is a family of secreted signaling proteins that regulate diverse developmental processes. Activation of canonical Wnt signaling by Wnt10b inhibits differentiation of preadipocytes in vitro. To determine whether Wnt signaling blocks adipogenesis in vivo, we created transgenic mice in which Wnt10b is expressed from the FABP4 promoter. Expression of Wnt10b in adipose impairs development of this tissue throughout the body, with a decline of ∼50% in total body fat and a reduction of ∼60% in weight of epididymal and perirenal depots. FABP4-Wnt10b mice resist accumulation of adipose tissue when fed a high fat diet. Furthermore, transgenic mice are more glucose-tolerant and insulin-sensitive than wild type mice. Expression of Wnt10b from the FABP4 promoter also blocks development of brown adipose tissue. Interscapular tissue of FABP4-Wnt10b mice has the visual appearance of white adipose tissue but expresses neither brown (e.g. uncoupling protein 1) nor white adipocyte markers. Transgenic mice are unable to maintain a core body temperature when placed in a cold environment, providing further evidence that Wnt10b inhibits development of brown adipose tissue. Although food intake is not altered in FABP4-Wnt10b mice, oxygen consumption is decreased. Thus, FABP4-Wnt10b mice on a chow diet gain more weight than controls, largely because of an increase in weight of skin. In summary, inhibition by Wnt10b of white and brown adipose tissue development results in lean mice without lipodystrophic diabetes.

Rat Strain Differences Suggest a Role for Corticotropin-Releasing Hormone in Modulating Sleep

Corticotropin-releasing hormone (CRH) mediates many of the hormonal, behavioral, and autonomic responses to a variety of stressors. There is also evidence suggesting that CRH may be involved in the modulation of physiologic waking. Lewis (LEW) rats possess a hypothalamic gene defect that results in reduced synthesis and secretion of CRH relative to genetically related Fischer 344 (F344) and Sprague-Dawley (Sp-D) rat strains. We therefore hypothesized that LEW rats would spend less time awake, and more time asleep, than either F344 or Sp-D rats. Adult male LEW, F344, and Sp-D rats were surgically provided with electroencephalograph (EEG) recording electrodes, and a thermistor to measure cortical brain temperature [T(cort)]. Additional rats were also provided with a chronic guide cannula directed into a lateral cerebral ventricle. Spontaneous sleep-wake behavior was determined from 48-h recordings of the EEG, T(cort), and body movements from freely behaving, undisturbed rats. Analyses of 48-h recordings from undisturbed animals indicate that LEW rats spend less time awake and more time in slow-wave sleep, relative to the other strains tested. Rapid eye movement sleep did not differ consistently between rat strains. LEW and Sp-D rats exhibit the same degree of waking in response to intracerebroventricular administration of CRH, indicating central mechanisms mediating behavioral responses to exogenously administered CRH are intact in LEW rats. These data provide support for the hypothesis that CRH may be a modulator of waking and sleep.

IL-10 as a mediator in the HPA axis and brain.

Certain functional interactions between the nervous, endocrine, and immune systems are mediated by cytokines. The pro-inflammatory cytokines, interleukin-1 (IL-1) and tumor necrosis factor (TNF) were among the first to be recognized in this regard. A modulator of these cytokines, IL-10, has been shown to have a wide range of activities in the immune system; in this review, we describe its production and actions in the hypothalamic-pituitary-adrenal (HPA) axis. IL-10 is produced in pituitary, hypothalamic, and neural tissues in addition to lymphocytes. IL-10 enhances corticotropin releasing factor (CRF) and corticotropin (ACTH) production in hypothalamic and pituitary tissues, respectively. Further downstream in the HPA axis endogenous IL-10 has the potential to contribute to regulation of glucocorticosteroid production both tonically and following stressors. Our studies and those of others reviewed here indicate that IL-10 may be an important endogenous regulator in HPA axis activity and in CNS pathologies such as multiple sclerosis. Thus, in addition to its more widely recognized role in immunity, IL-10s neuroendocrine activities described here point to its role as an important regulator in communication between the immune and neuroendocrine systems.

Blockade of corticotropin-releasing hormone receptors reduces spontaneous waking in the rat

We have previously hypothesized that corticotropin-releasing hormone (CRH) is involved in the regulation of physiological waking. To further elucidate this role for CRH, we administered intracerebroventricularly into rats two specific CRH-receptor antagonists, α-helical CRH-(9-41) (α-hCRH) or astressin, and determined changes in electroencephalogram-defined waking and sleep. Our results indicate that both of these receptor antagonists reduce the amount of time spent awake in a dose-related manner when administered before the dark period of the light-dark cycle. However, the time courses for these effects differ between antagonists; effective doses of α-hCRH reduce waking during the first 2 h postinjection, whereas effective doses of astressin reduce waking during postinjection hours 7-12. In contrast to dark-onset administrations, the amount of waking is not altered by either CRH-receptor antagonist when administered before the light period. These results support our hypothesis that CRH contributes to the regulation of physiological waking, since interfering with the binding of CRH to its receptor reduces spontaneous waking.

Interleukin-6 is pyrogenic but not somnogenic.

Interleukin-6 (IL6) induces acute phase protein production and is hypothesized to mediate systemic and central effects of IL1. To determine whether IL6 possesses somnogenic properties, rabbits were injected intracerebroventricularly with IL6; sleep-wake activity was determined and brain temperatures recorded for 6 hr. IL6 induced fever in a dose-related manner with no effect on sleep-wake activity. IL6, therefore, is the first cytokine reported to elicit fever without promoting sleep. We conclude that the somnogenic action of IL1 is not mediated through IL6.

Cytokines and normal sleep.

Purpose of review Cytokines are mediators of immune system responses with multiple biologic actions on several target tissues. Over the past two decades, research has explored the interactions between cytokines and sleep mechanisms of the brain. This short review highlights selected findings that have advanced our understanding of the relation between cytokines and sleep. Recent findings A complex network of cytokines and their receptors exists in brain. Cytokines may either promote or inhibit sleep. Of cytokines studied thus far, evidence indicates that interleukin-1 and tumor necrosis factor play a role in the regulation of non-rapid eye movement sleep. Their sites of action for regulating such sleep likely include the hypothalamic preoptic area and the basal forebrain. Mechanisms of action include direct receptor-mediated effects on neurons and the synthesis and release of numerous transmitters, peptides, and hormones that lead to subsequent changes in sleep. Among others, the cascade of responses induced by cytokines that may lead to subsequent alterations in sleep includes alterations in nitric oxide synthesis and effects on neurohormonal systems such as growth hormone releasing hormone. The activation by cytokines of the hypothalamic-pituitary-adrenal axis also influences sleep. Studies suggest that there is a significant overlap between neurohormonal systems such as the somatotropic and hypothalamic-pituitary-adrenal axes and cytokines, particularly with regard to their effects on sleep-wake regulation. Summary There is increasing evidence of a role for cytokines in regulating spontaneous non-rapid eye movement sleep. The somatotropic hormonal system and hypothalamic-pituitary-adrenal axis mediate, in part, the effects of cytokines on sleep.

Inhibition of growth hormone-releasing factor suppresses both sleep and growth hormone secretion in the rat.

To study the possible involvement of hypothalamic growth hormone-releasing factor (GRF) in sleep regulation, a competitive GRF-antagonist, the peptide (N-Ac-Tyr1,D-Arg2)-GRF(1-29)-NH2, was intracerebroventricularly injected into rats (0.003, 0.3, and 14 nmol), and the EEG and brain temperature were recorded for 12 h during the light cycle of the day. Growth hormone (GH) concentrations were determined from plasma samples taken at 20-min intervals for 3 h after 14 nmol GRF-antagonist. The onset of non-rapid eye movement sleep (NREMS) was delayed in response to 0.3 and 14 nmol GRF-antagonist, the duration of NREMS was decreased for one or more hours and after 14 nmol EEG slow wave amplitudes were decreased during NREMS in postinjection hour 1. The high dose of GRF-antagonist also suppressed REMS for 4 h, inhibited GH secretion, and elicited a slight biphasic variation in brain temperature. These findings, together with previous observations indicating a sleep-promoting effect for GRF, support the hypothesis that hypothalamic GRF is involved in sleep regulation and might be responsible for the correlation between NREMS and GH secretion reported in various species.

Interleukin-10 (cytokine synthesis inhibitory factor) acts in the central nervous system of rats to reduce sleep

Interleukin-10 (IL-10), originally designated a cytokine synthesis inhibitory factor, inhibits the synthesis of the pro-inflammatory cytokines IL-1 and tumor necrosis factor by stimulated human and mouse monocytes/macrophages; these cytokines are involved in the regulation of sleep. To determine if IL-10 reduces spontaneous sleep, we injected murine recombinant IL-10 intracerebroventricularly into rats prior to light onset. Non-rapid eye movements sleep was reduced. The behavioral responses to IL-10 were abolished by heat-inactivation of this cytokine. We believe these to be the first observations of central nervous system actions for this cytokine. These results further support the hypothesis that cytokines are involved in the regulation of sleep, and suggest an additional mechanism whereby sleep may be altered in response to an activated immune system.

Somnogenic and pyrogenic effects of interleukin-1β and lipopolysaccharide in intact and vagotomized rats

The vagus nerve appears to serve a role in mediating peripheral immunologic influences on CNS processes. Previous work demonstrates that subdiaphragmatic vagotomy prevents or attenuates many of the behavioral and physiological responses to exogenous interleukin-1 (IL-1) or lipopolysaccharide (LPS). We determined whether the somnogenic effects of IL-1 and LPS were altered in vagotomized rats, and whether the effects of vagotomy on IL-1- and LPS-induced alterations in sleep would vary as a function of circadian phase. The data indicate that vagotomy does not influence the normal circadian patterns of sleep and wakefulness in untreated rats, or modify the pyrogenic or somnogenic effects of intracerebroventricular administration of IL-1. However, in unchallenged animals vagotomy reduces basal brain temperatures, increases delta wave amplitudes during slow-wave sleep (SWS), and induces a reduced rate of weight gain, gastric distension, and adrenal hypoplasia. Vagotomy attenuates the febrile effects of IL-1 during both light and dark phases, attenuated IL-1-induced sleep enhancement during the dark phase, and attenuated IL-1-induced increases in delta wave amplitudes within SWS during the light period. In LPS-treated rats, vagotomy attenuates the febrile and SWS responses to LPS after administration at light onset, but not after administration at dark onset. These results indicate that subdiaphragmatic vagotomy attenuates several of the somnogenic and pyrogenic effects of IL-1beta and LPS, although the effectiveness of the vagal transection in modulating these responses is influenced by circadian factors.