Effects of light and melatonin treatment on body temperature and melatonin secretion daily rhythms in a diurnal rodent, the fat sand rat

Fundamentals of sleep regulation: Model and benchmark values for fractal and oscillatory neurodynamics

Homeostatic, circadian and ultradian mechanisms play crucial roles in the regulation of sleep. Evidence suggests that ratios of low-to-high frequency power in the electroencephalogram (EEG) spectrum indicate the instantaneous level of sleep pressure, influenced by factors such as individual sleep-wake history, current sleep stage, age-related differences and brain topography characteristics. These effects are well captured and reflected in the spectral exponent, a composite measure of the constant low-to-high frequency ratio in the periodogram, which is scale-free and exhibits lower interindividual variability compared to slow wave activity, potentially serving as a suitable standardization and reference measure. Here we propose an index of sleep homeostasis based on the spectral exponent, reflecting the level of membrane hyperpolarization and/or network bistability in the central nervous system in humans. In addition, we advance the idea that the U-shaped overnight deceleration of oscillatory slow and fast sleep spindle frequencies marks the biological night, providing somnologists with an EEG-index of circadian sleep regulation. Evidence supporting this assertion comes from studies based on sleep replacement, forced desynchrony protocols and high-resolution analyses of sleep spindles. Finally, ultradian sleep regulatory mechanisms are indicated by the recurrent, abrupt shifts in dominant oscillatory frequencies, with spindle ranges signifying non-rapid eye movement and non-spindle oscillations - rapid eye movement phases of the sleep cycles. Reconsidering the indicators of fundamental sleep regulatory processes in the framework of the new Fractal and Oscillatory Adjustment Model (FOAM) offers an appealing opportunity to bridge the gap between the two-process model of sleep regulation and clinical somnology.

Dim Light at Night Promotes Circadian Disruption in Female Rats, at the Metabolic, Reproductive, and Behavioral Level

Inhabitants of urban areas are constantly exposed to light at night, which is an important environmental factor leading to circadian disruption. Streetlights filtering light through the windows and night dim light lamps are common sources of dim light at night (DLAN). The female population is susceptible to circadian disruption. The present study is aimed to determine the impact of DLAN on female Wistar rats circadian rhythms, metabolism, reproductive physiology, and behavior. After 5 weeks of DLAN exposure daily, oscillations in activity and body temperature of female rats are abolished. DLAN also decreases nocturnal food ingestion, which results in a diminishment in total food consumption. These alterations in the temporal organization of the body are associated with a significant decrease in melatonin plasmatic levels, reproductive disruptions, decreased exploration times, and marked anhedonia. This study highlights the importance of avoiding exposure to light at night, even at low intensities, to maintain the circadian organization of physiology, and denotes the great necessity of increasing the studies in females since the sexual dimorphism within the effects of desynchronizing protocols has been poorly studied.

Melatonin as an Antioxidant Agent in Stroke: An Updated Review

Stroke is a devastating disease associated with high mortality and disability worldwide, and is generally classified as ischemic or hemorrhagic, which share certain similar pathophysiological processes. Oxidative stress is a critical factor involved in stroke-induced injury, which not only directly damages brain tissue, but also enhances a series of pathological signaling cascades, contributing to inflammation, brain edema, and neuronal death. To alleviate these serious secondary brain injuries, neuroprotective agents targeting oxidative stress inhibition may serve as a promising treatment strategy. Melatonin is a hormone secreted by the pineal gland, and has various properties, such as antioxidation, anti-inflammation, circadian rhythm modulation, and promotion of tissue regeneration. Numerous animal experiments studying stroke have confirmed that melatonin exerts considerable neuroprotective effects, partially via anti-oxidative stress. In this review, we introduce the possible role of melatonin as an antioxidant in the treatment of stroke based on the latest published studies of animal experiments and clinical research.

Light Pollution, Circadian Photoreception, and Melatonin in Vertebrates

Artificial light at night (ALAN) is increasing exponentially worldwide, accelerated by the transition to new efficient lighting technologies. However, ALAN and resulting light pollution can cause unintended physiological consequences. In vertebrates, production of melatonin—the “hormone of darkness” and a key player in circadian regulation—can be suppressed by ALAN. In this paper, we provide an overview of research on melatonin and ALAN in vertebrates. We discuss how ALAN disrupts natural photic environments, its effect on melatonin and circadian rhythms, and different photoreceptor systems across vertebrate taxa. We then present the results of a systematic review in which we identified studies on melatonin under typical light-polluted conditions in fishes, amphibians, reptiles, birds, and mammals, including humans. Melatonin is suppressed by extremely low light intensities in many vertebrates, ranging from 0.01–0.03 lx for fishes and rodents to 6 lx for sensitive humans. Even lower, wavelength-dependent intensities are implied by some studies and require rigorous testing in ecological contexts. In many studies, melatonin suppression occurs at the minimum light levels tested, and, in better-studied groups, melatonin suppression is reported to occur at lower light levels. We identify major research gaps and conclude that, for most groups, crucial information is lacking. No studies were identified for amphibians and reptiles and long-term impacts of low-level ALAN exposure are unknown. Given the high sensitivity of vertebrate melatonin production to ALAN and the paucity of available information, it is crucial to research impacts of ALAN further in order to inform effective mitigation strategies for human health and the wellbeing and fitness of vertebrates in natural ecosystems.

Melatonin MT1 and MT2 Receptors Exhibit Distinct Effects in the Modulation of Body Temperature across the Light/Dark Cycle

Melatonin (MLT) is a neurohormone that regulates many physiological functions including sleep, pain, thermoregulation, and circadian rhythms. MLT acts mainly through two G-protein-coupled receptors named MT₁ and MT₂, but also through an MLT type-3 receptor (MT₃). However, the role of MLT receptor subtypes in thermoregulation is still unknown. We have thus investigated the effects of selective and non-selective MLT receptor agonists/antagonists on body temperature (T_b) in rats across the 12/12-h light-dark cycle. Rectal temperature was measured every 15 min from 4:00 a.m. to 9:30 a.m. and from 4:00 p.m. to 9:30 p.m., following subcutaneous injection of each compound at either 5:00 a.m. or 5:00 p.m. MLT (40 mg/kg) had no effect when injected at 5 a.m., whereas it decreased T_b during the light phase only when injected at 5:00 p.m. This effect was blocked by the selective MT₂ receptor antagonist 4P-PDOT and the non-selective MT₁/MT₂ receptor antagonist, luzindole, but not by the α₁/MT₃ receptors antagonist prazosin. However, unlike MLT, neither the selective MT₁ receptor partial agonist UCM871 (14 mg/kg) nor the selective MT₂ partial agonist UCM924 (40 mg/kg) altered T_b during the light phase. In contrast, UCM871 injected at 5:00 p.m. increased T_b at the beginning of the dark phase, whereas UCM924 injected at 5:00 a.m. decreased T_b at the end of the dark phase. These effects were blocked by luzindole and 4P-PDOT, respectively. The MT₃ receptor agonist GR135531 (10 mg/kg) did not affect T_b. These data suggest that the simultaneous activation of both MT₁ and MT₂ receptors is necessary to regulate T_b during the light phase, whereas in a complex but yet unknown manner, they regulate T_b differently during the dark phase. Overall, MT₁ and MT₂ receptors display complementary but also distinct roles in modulating circadian fluctuations of T_b.

The transition from day-to-night activity is a risk factor for the development of CNS oxygen toxicity in the diurnal fat sand rat (Psammomys obesus)

Performance and safety are impaired in employees engaged in shift work. Combat divers who use closed-circuit oxygen diving apparatus undergo part of their training during the night hours. The greatest risk involved in diving with such apparatus is the development of central nervous system oxygen toxicity (CNS-OT). We investigated whether the switch from day-to-night activity may be a risk factor for the development of CNS-OT using a diurnal animal model, the fat sand rat (Psammomys obesus). Animals were kept on a 12:12 light-dark schedule (6 a.m. to 6 p.m. at 500 lx). The study included two groups: (1) Control group: animals were kept awake and active during the day, between 09:00 and 15:00. (2) Experimental group: animals were kept awake and active during the night, between 21:00 and 03:00, when they were exposed to dim light in order to simulate the conditions prevalent during combat diver training. This continued for a period of 3 weeks, 5 days a week. On completion of this phase, 6-sulphatoxymelatonin (6-SMT) levels in urine were determined over a period of 24 h. Animals were then exposed to hyperbaric oxygen (HBO). To investigate the effect of acute melatonin administration, melatonin (50 mg/kg) or its vehicle was administered to the animals in both groups 20 min prior to HBO exposure. After the exposure, the activity of superoxide dismutase, catalase and glutathione peroxidase was measured, as were the levels of neuronal nitric oxide synthase (nNOS) and overall nitrotyrosylation in the cortex and hippocampus. Latency to CNS-OT was significantly reduced after the transition from day-to-night activity. This was associated with alterations in the level of melatonin metabolites secreted in the urine. Acute melatonin administration had no effect on latency to CNS-OT in either of the groups. Nevertheless, the activity of superoxide dismutase and catalase, as well as nitrotyrosine and nNOS levels, were altered in the hippocampus following melatonin administration. On the basis of these results, we suggest that a switch from diurnal to nocturnal activity may represent an additional risk factor for the development of CNS-OT. Utilizing a diurnal animal model may contribute to our understanding of the heightened risk of developing CNS-OT when diving with closed-circuit oxygen apparatus at night.

Plasticity of circadian clocks and consequences for metabolism

The increased prevalence of metabolic disorders and obesity in modern society, together with the widespread use of artificial light at night, have led researchers to investigate whether altered patterns of light exposure contribute to metabolic disorders. This article discusses the experimental evidence that perturbed environmental cycles induce rhythm disorders in the circadian system, thus leading to metabolic disorders. This notion is generally supported by animal studies. Distorted environmental cycles, including continuous exposure to light, affect the neuronal organization of the central circadian pacemaker in the suprachiasmatic nucleus (SCN), its waveform and amplitude of the rhythm in electrical activity. Moreover, repeated exposure to a shifted light cycle or the application of dim light at night are environmental cues that cause a change in SCN function. The effects on the SCN waveform are the result of changes in synchronization among the SCN's neuronal cell population, which lead consistently to metabolic disturbances. Furthermore, we discuss the effects of sleep deprivation and the time of feeding on metabolism, as these factors are associated with exposure to disturbed environmental cycles. Finally, we suggest that these experimental studies reveal a causal relationship between the rhythm disorders and the metabolic disorders observed in epidemiological studies performed in humans.

Non-alcoholic fatty liver disease: the role of nuclear receptors and circadian rhythmicity

Non-alcoholic fatty liver disease (NAFLD) is the accumulation of triglycerides in the hepatocytes in the absence of excess alcohol intake, and is caused by an imbalance between hepatic synthesis and breakdown of fats, as well as fatty acid storage and disposal. Liver metabolic pathways are driven by circadian biological clocks, and hepatic health is maintained by proper timing of circadian patterns of metabolic gene expression with the alternation of anabolic processes corresponding to feeding/activity during wake times, and catabolic processes characterizing fasting/resting during sleep. A number of nuclear receptors in the liver are expressed rhythmically, bind hormones and metabolites, sense energy flux and expenditure, and connect the metabolic pathways to the molecular clockwork throughout the 24-h day. In this review, we describe the role played by the nuclear receptors in the genesis of NAFLD in relationship with the circadian clock circuitry.

The effects of light at night on circadian clocks and metabolism

Most organisms display endogenously produced ∼ 24-hour fluctuations in physiology and behavior, termed circadian rhythms. Circadian rhythms are driven by a transcriptional-translational feedback loop that is hierarchically expressed throughout the brain and body, with the suprachiasmatic nucleus of the hypothalamus serving as the master circadian oscillator at the top of the hierarchy. Appropriate circadian regulation is important for many homeostatic functions including energy regulation. Multiple genes involved in nutrient metabolism display rhythmic oscillations, and metabolically related hormones such as glucagon, insulin, ghrelin, leptin, and corticosterone are released in a circadian fashion. Mice harboring mutations in circadian clock genes alter feeding behavior, endocrine signaling, and dietary fat absorption. Moreover, misalignment between behavioral and molecular circadian clocks can result in obesity in both rodents and humans. Importantly, circadian rhythms are most potently synchronized to the external environment by light information and exposure to light at night potentially disrupts circadian system function. Since the advent of electric lights around the turn of the 20th century, exposure to artificial and irregular light schedules has become commonplace. The increase in exposure to light at night parallels the global increase in the prevalence of obesity and metabolic disorders. In this review, we propose that exposure to light at night alters metabolic function through disruption of the circadian system. We first provide an introduction to the circadian system, with a specific emphasis on the effects of light on circadian rhythms. Next we address interactions between the circadian system and metabolism. Finally, we review current experimental and epidemiological work directly associating exposure to light at night and metabolism.

Molecular bases of circadian rhythmicity in renal physiology and pathology

The physiological processes that maintain body homeostasis oscillate during the day. Diurnal changes characterize kidney functions, comprising regulation of hydro-electrolytic and acid-base balance, reabsorption of small solutes and hormone production. Renal physiology is characterized by 24-h periodicity and contributes to circadian variability of blood pressure levels, related as well to nychthemeral changes of sodium sensitivity, physical activity, vascular tone, autonomic function and neurotransmitter release from sympathetic innervations. The circadian rhythmicity of body physiology is driven by central and peripheral biological clockworks and entrained by the geophysical light/dark cycle. Chronodisruption, defined as the mismatch between environmental-social cues and physiological-behavioral patterns, causes internal desynchronization of periodic functions, leading to pathophysiological mechanisms underlying degenerative, immune related, metabolic and neoplastic diseases. In this review we will address the genetic, molecular and anatomical elements that hardwire circadian rhythmicity in renal physiology and subtend disarray of time-dependent changes in renal pathology.

Effects of circadian phase and melatonin injection on anxiety-like behavior in nocturnal and diurnal rodents

Animals show daily rhythms in most bodily functions, resulting from the integration of information from an endogenous circadian clock and external stimuli. These rhythms are adaptive and are expected to be related to activity patterns, i.e., to be opposite in diurnal and nocturnal species. Melatonin is secreted during the night in all mammalian species, regardless of their activity patterns. Consequently, in diurnal species the nocturnal secretion of melatonin is concurrent with the resting phase, whereas in nocturnal species it is related to an increase in activity. In this research, we examined in three diurnal and three nocturnal rodent species whether a daily rhythm in anxiety-like behavior exists; whether it differs between nocturnal and diurnal species; and how melatonin affects anxiety-like behavior in species with different activity patterns. Anxiety-like behavior levels were analyzed using the elevated plus-maze. We found a daily rhythm in anxiety-like behavior and a significant response to daytime melatonin administration in all three nocturnal species, which showed significantly lower levels of anxiety during the dark phase, and after melatonin administration. The diurnal species showed either an inverse pattern to that of the nocturnal species in anxiety-like behavior rhythm and in response to daytime melatonin injection, or no rhythm and, accordingly, no response to melatonin.

Clock genes and clock-controlled genes in the regulation of metabolic rhythms

Daily rotation of the Earth on its axis and yearly revolution around the Sun impose to living organisms adaptation to nyctohemeral and seasonal periodicity. Terrestrial life forms have developed endogenous molecular circadian clocks to synchronize their behavioral, biological, and metabolic rhythms to environmental cues, with the aim to perform at their best over a 24-h span. The coordinated circadian regulation of sleep/wake, rest/activity, fasting/feeding, and catabolic/anabolic cycles is crucial for optimal health. Circadian rhythms in gene expression synchronize biochemical processes and metabolic fluxes with the external environment, allowing the organism to function effectively in response to predictable physiological challenges. In mammals, this daily timekeeping is driven by the biological clocks of the circadian timing system, composed of master molecular oscillators within the suprachiasmatic nuclei of the hypothalamus, pacing self-sustained and cell-autonomous molecular oscillators in peripheral tissues through neural and humoral signals. Nutritional status is sensed by nuclear receptors and coreceptors, transcriptional regulatory proteins, and protein kinases, which synchronize metabolic gene expression and epigenetic modification, as well as energy production and expenditure, with behavioral and light-dark alternance. Physiological rhythmicity characterizes these biological processes and body functions, and multiple rhythms coexist presenting different phases, which may determine different ways of coordination among the circadian patterns, at both the cellular and whole-body levels. A complete loss of rhythmicity or a change of phase may alter the physiological array of rhythms, with the onset of chronodisruption or internal desynchronization, leading to metabolic derangement and disease, i.e., chronopathology.

Seasonal patterns of body temperature daily rhythms in group-living Cape ground squirrels Xerus inauris

Organisms respond to cyclical environmental conditions by entraining their endogenous biological rhythms. Such physiological responses are expected to be substantial for species inhabiting arid environments which incur large variations in daily and seasonal ambient temperature (T_a). We measured core body temperature (T_b) daily rhythms of Cape ground squirrels Xerus inauris inhabiting an area of Kalahari grassland for six months from the Austral winter through to the summer. Squirrels inhabited two different areas: an exposed flood plain and a nearby wooded, shady area, and occurred in different social group sizes, defined by the number of individuals that shared a sleeping burrow. Of a suite of environmental variables measured, maximal daily T_a provided the greatest explanatory power for mean T_b whereas sunrise had greatest power for T_b acrophase. There were significant changes in mean T_b and T_b acrophase over time with mean T_b increasing and T_b acrophase becoming earlier as the season progressed. Squirrels also emerged from their burrows earlier and returned to them later over the measurement period. Greater increases in T_b, sometimes in excess of 5°C, were noted during the first hour post emergence, after which T_b remained relatively constant. This is consistent with observations that squirrels entered their burrows during the day to ‘offload’ heat. In addition, greater T_b amplitude values were noted in individuals inhabiting the flood plain compared with the woodland suggesting that squirrels dealt with increased environmental variability by attempting to reduce their T_a-T_b gradient. Finally, there were significant effects of age and group size on T_b with a lower and less variable T_b in younger individuals and those from larger group sizes. These data indicate that Cape ground squirrels have a labile T_b which is sensitive to a number of abiotic and biotic factors and which enables them to be active in a harsh and variable environment.

ARNTL2 and SERPINE1: potential biomarkers for tumor aggressiveness in colorectal cancer

PURPOSE

Cathepsin and plasmin may favor cancer cell invasion degrading extracellular matrix. Plasmin formation from plasminogen is regulated by plasminogen activator inhibitor type-1 (PAI-1). ARNTL2 activates the promoters of the PAI-1 gene, officially called SERPINE1, driving the circadian variation in circulating PAI-1 levels.

METHODS

We evaluated ARNTL2 and SERPINE1 expression in 50 colorectal cancer specimens and adjacent normal tissue and in colon cancer cell lines.

RESULTS

We found up-regulation of ARNTL2 (P = 0.004) and SERPINE1 (P = 0.002) in tumor tissue. A statistically significant association was found between high ARNTL2 mRNA levels and vascular invasion (P < 0.0001), and between high SERPINE1 mRNA levels and microsatellite instability (MSI-H and MSI-L, P = 0.025). Sorting the subjects into quartile groups, a statistically significant association was found between high ARNTL2 expression and lymph node involvement (P < 0.001), between high SERPINE1 expression and grading (P < 0.001) and between high SERPINE1 expression and MSI H-L (P < 0.0001). In SW480 cells, a more proliferative model compared to CaCo2 cells, there were higher mRNA levels of ARNTL2 (P < 0.001) and SERPINE1 (P = 0.001).

CONCLUSION

ARNTL2 and SERPINE1 expression is increased in colorectal cancer and in a highly proliferative colon cancer cell line and is related to tumor invasiveness and aggressiveness.