Éva Szentirmai

Obestatin alters sleep in rats

Obestatin is a recently identified peptide derived from the ghrelin gene. Ghrelin stimulates food intake whereas obestatin has an opposite effect in rats. Previous experiments in our laboratory revealed that ghrelin also induces wakefulness in rats. The aim of the present experiments was to study the effect of obestatin on sleep. Rats received intraperitoneal (n = 7; 16 or 64 microg/kg) or intracerebroventricular (i.c.v.; n = 11) injection of pyrogen-free isotonic NaCl or obestatin (1, 4 and 16 microg in a volume of 4 microl) at dark onset. Sleep-wake activity was recorded for 23 h. I.c.v. administration of 16 microg obestatin induced a significant increase (approximately 58%) in the time spent in non-rapid-eye-movement sleep (NREMS) in the first hour after the injection. This resulted from an increase in the number of NREMS episodes and shortened sleep latency. Electroencephalographic (EEG) slow-wave activity (0.5-4 Hz) was reduced by obestatin treatment. The initial increase in NREMS time was followed by a decrease in both NREMS and REMS in the second hour after the injection. Peripheral injection of obestatin did not induce significant changes in sleep in any post-injection hours. Results suggest that the sleep-promoting effect of centrally administered obestatin may be part of the behavioral manifestation of satiety elicited by the peptide. Current results confirm the finding that two regulatory peptides derived from the same gene have opposite actions in the same species.

The preproghrelin gene is required for the normal integration of thermoregulation and sleep in mice

Peptidergic mechanisms controlling feeding, metabolism, thermoregulation, and sleep overlap in the hypothalamus. Low ambient temperatures and food restriction induce hypothermic (torpor) bouts and characteristic metabolic and sleep changes in mice. We report that mice lacking the preproghrelin gene, but not those lacking the ghrelin receptor, have impaired abilities to manifest and integrate normal sleep and thermoregulatory responses to metabolic challenges. In response to fasting at 17 °C (a subthermoneutral ambient temperature), preproghrelin knockout mice enter hypothermic bouts associated with reduced sleep, culminating in a marked drop in body temperature to near-ambient levels. Prior treatment with obestatin, another preproghrelin gene product, attenuates the hypothermic response of preproghrelin knockout mice. Results suggest that obestatin is a component in the coordinated regulation of metabolism and sleep during torpor.

Spontaneous and influenza virus-induced sleep are altered in TNF-α double-receptor deficient mice

Tumor necrosis factor-alpha (TNF-alpha) is associated with sleep regulation in health and disease. Previous studies assessed sleep in mice genetically deficient in the TNF-alpha 55-kDa receptor. In this study, spontaneous and influenza virus-induced sleep profiles were assessed in mice deficient in both the 55-kDa and 75-kDa TNF-alpha receptors [TNF-2R knockouts (KO)] and wild-type (WT) strain controls. Under baseline conditions the TNF-2R KO mice had less non-rapid eye movement sleep (NREMS) than WTs during the nighttime and more rapid eye movement sleep (REMS) than controls during the daytime. The differences between nighttime maximum and daytime minimum values of electroencephalogram (EEG) delta power during NREMS were greater in the TNF-2R KO mice than in WTs. Viral challenge (mouse-adapted influenza X-31) enhanced NREMS and decreased REMS in both strains roughly to the same extent. EEG delta power responses to viral challenge differed substantially between strains; the WT animals increased, whereas the TNF-2R KO mice decreased their EEG delta wave power during NREMS. There were no differences between strains in body temperatures or locomotor activity in uninfected mice or after viral challenge. Analyses of cortical mRNAs confirmed that the TNF-2R KO mice lacked both TNF-alpha receptors; these mice also had higher levels of orexin mRNA and reduced levels of the purine P2X7 receptor compared with WTs. Results reinforce the hypothesis that TNF-alpha is involved in physiological sleep regulation but plays a limited role in the acute-phase response induced by influenza virus.

Detection of mouse-adapted human influenza virus in the olfactory bulbs of mice within hours after intranasal infection

Influenza pneumonitis causes severe systemic symptoms in mice, including hypothermia and excess sleep. The association of extrapulmonary virus, particularly virus in the brain, with the onset of such disease symptoms has not been investigated. Mature C57BL/6 male mice were infected intranasally with mouse-adapted human influenza viruses (PR8 or X-31) under inhalation, systemic, or no anesthesia. Core body temperatures were monitored continuously by radiotelemetry, and tissues (lung, brain, olfactory bulb, spleen, blood) were harvested at the time of onset of hypothermia (13 to 24 h post infection [PI]) or at 4 or 7 h PI. Whole RNA from all tissues was examined by one or more of three reverse transcriptase-polymerase chain reaction (RT-PCR) procedures using H1N1 nucleoprotein (NP) primers for minus polarity RNA (genomic or vRNA) or plus polarity RNA (replication intermediates). Selected cytokines were assayed at 4, 7, and 15 h in the olfactory bulb (OB). Minus and plus RNA strands were readily detected in OBs as early as 4 h PI by nested RT-PCR. Anesthesia was not required for viral invasion of the OB. Cytokine mRNAs were also significantly elevated in the OB at 7 and 15 h PI in infected mice. Controls receiving boiled virus expressed only input vRNA and that only in lung. Immunohistochemistry demonstrated localization of H1N1 and NP antigens in olfactory nerves and the glomerular layer of the OB. Therefore a mouse-adapted human influenza virus strain, not known to be neurotropic, was detected in the mouse OB within 4 h PI where it appeared to induce replication intermediates and cytokines.

Intact brown adipose tissue thermogenesis is required for restorative sleep responses after sleep loss

Metabolic signals related to feeding and body temperature regulation have profound effects on vigilance. Brown adipose tissue (BAT) is a key effector organ in the regulation of metabolism in several species, including rats and mice. Significant amounts of active BAT are also present throughout adulthood in humans. The metabolic activity of BAT is due to the tissue‐specific presence of the uncoupling protein‐1 (UCP‐1). To test the involvement of BAT thermogenesis in sleep regulation, we investigated the effects of two sleep‐promoting stimuli in UCP‐1‐deficient mice. Sleep deprivation by gentle handling increased UCP‐1 mRNA expression in BAT and elicited rebound increases in non‐rapid‐eye‐movement sleep and rapid‐eye‐movement sleep accompanied by elevated slow‐wave activity of the electroencephalogram. The rebound sleep increases were significantly attenuated, by ~ 35–45%, in UCP‐1‐knockout (KO) mice. Wild‐type (WT) mice with capsaicin‐induced sensory denervation of the interscapular BAT pads showed similar impairments in restorative sleep responses after sleep deprivation, suggesting a role of neuronal sleep‐promoting signaling from the BAT. Exposure of WT mice to 35 °C ambient temperature for 5 days led to increased sleep and body temperature and suppressed feeding and energy expenditure. Sleep increases in the warm environment were significantly suppressed, by ~ 50%, in UCP‐1‐KO animals while their food intake and energy expenditure did not differ from those of the WTs. These results suggest that the metabolic activity of the BAT plays a role in generating a metabolic environment that is permissive for optimal sleep. Impaired BAT function may be a common underlying cause of sleep insufficiency and metabolic disorders.

Sickness behaviour after lipopolysaccharide treatment in ghrelin deficient mice.

Ghrelin is an orexigenic hormone produced mainly by the gastrointestinal system and the brain. Much evidence also indicates a role for ghrelin in sleep and thermoregulation. Further, ghrelin was recently implicated in immune system modulation. Administration of bacterial lipopolysaccharide (LPS) induces fever, anorexia, and increased non-rapid-eye movement sleep (NREMS) and these actions are mediated primarily by proinflammatory cytokines. Ghrelin reduces LPS-induced fever, suppresses circulating levels of proinflammatory cytokines and reduces the severity and mortality of various models of experimental endotoxemia. In the present study, we determined the role of intact ghrelin signaling in LPS-induced sleep, feeding, and thermoregulatory responses in mice. Sleep-wake activity was determined after intraperitoneal, dark onset administration of 0.4, 2 and 10 μg LPS in preproghrelin knockout (KO) and wild-type (WT) mice. In addition, body temperature, motor activity and changes in 24-h food intake and body weight were measured. LPS induced dose-dependent increases in NREMS, and suppressed rapid-eye movement sleep, electroencephalographic slow-wave activity, motor activity, food intake and body weight in both Ppg KO and WT mice. Body temperature changes showed a biphasic pattern with a decrease during the dark period followed by an increase in the light phase. The effects of the low and middle doses of LPS were indistinguishable between the two genotypes. Administration of 10 μg LPS, however, induced significantly larger changes in NREMS and wakefulness amounts, body temperature, food intake and body weight in the Ppg KO mice. These findings support a role for ghrelin as an endogenous modulator of inflammatory responses and a central component of arousal and feeding circuits.

Energy homeostasis regulatory peptides in hibernating grizzly bears

Grizzly bears (Ursus arctos horribilis) are inactive for up to 6 months during hibernation. They undergo profound seasonal changes in food intake, body mass, and energy expenditure. The circa-annual regulation of metabolism is poorly understood. In this study, we measured plasma ghrelin, leptin, obestatin, and neuropeptide-Y (NPY) levels, hormones known to be involved in the regulation of energy homeostasis, in ten grizzly bears. Blood samples were collected during the active summer period, early hibernation and late hibernation. Plasma levels of leptin, obestatin, and NPY did not change between the active and the hibernation periods. Plasma total ghrelin and desacyl-ghrelin concentrations significantly decreased during the inactive winter period compared to summer levels. The elevated ghrelin levels may help enhance body mass during pre-hibernation, while the low plasma ghrelin concentrations during hibernation season may contribute to the maintenance of hypophagia, low energy utilization and behavioral inactivity. Our results suggest that ghrelin plays a potential role in the regulation of metabolic changes and energy homeostasis during hibernation in grizzly bears.

Brown adipose tissue at the intersection of sleep and temperature regulation.

The article demonstrates the importance of brown fat in creating and maintaining a metabolic environment which is permissive for optimal restorative sleep after sleep loss. The authors propose that impaired brown fat function could be a common underlying cause of poor sleep and metabolic disorders.

Ghrelin Regulation of Sleep, Circadian Clock, and Body Temperature

The regulation of sleep, body temperature, and metabolism is intertwined on functional, structural, and behavioral levels. The hypothalamus emerges as a key brain region that coordinates these functions by integrating central and peripheral signals. Increasing evidence suggests that products of the preproghrelin gene provide important contribution to this integrative function. In this review, we present evidence from human and animal studies supporting the role of ghrelin in the regulation of (1) sleep–wake activity, (2) circadian rhythms, and (3) metabolism and body temperature. Central ghrelinergic mechanisms—as part of the hypothalamic ghrelin–orexin–neuropeptide Y circuit—play a role in promoting wakefulness and feeding. Ghrelin modulates the activity of the suprachiasmatic nucleus but it is not a crucial component of or a key input signal to the food-entrainable oscillator. Products of the preproghrelin gene, particularly obestatin, are involved in maintaining normal body temperature and metabolism under conditions when increased metabolic heat production is required. Our understanding of the physiological role of the preproghrelin gene products expanded remarkably in the last decade, yet, full comprehension of their role in the regulation of vigilance, circadian clocks, and body temperature remains incomplete.

Ghrelin and Sleep Regulation

Classic models of sleep regulation posit that the timing and amount of sleep are determined by the duration of prior wakefulness and whether or not the circadian phase is favorable for sleep. Growing body of evidence indicates, however, that in addition to these factors, other signals from the external and internal environment also play a key role in sleep regulation. Changes in metabolic environment, such as positive and negative energy balance, adiposity, postprandial state, and shifts in lipolytic activity all have fundamental effects on sleep. The signaling mechanisms that connect metabolism to sleep regulation include hormones of the gastrointestinal tract and the adipose tissue. Ghrelin signaling in the brain has emerged as one of the key components of the arousal system that is activated in negative energy states and possibly under other physiological conditions. We review recent human and animals studies on the role of ghrelin in sleep regulation and in the function of biological clocks.