Waking and sleeping in the rat made obese through a high-fat hypercaloric diet

Ultrasonic vocalisations during rapid eye movement sleep in the rat

Rats are known to use a 22-kHz ultrasonic vocalisation as a distress call to warn of danger to other members of their group. We monitored 22-kHz ultrasonic vocalisation emissions in rats (lean and obese) as part of a sleep deprivation study to detect the eventual presence of stress during the procedure. Unexpectedly, we detected ultrasonic vocalisation emission during rapid eye movement (REM) sleep, but not during non-REM (NREM) sleep, in all the rats. The event occurs during the expiratory phase and can take place singularly or as a train. No difference was detected in the number or duration of these events in lean versus obese rats, during the light versus the dark period, and after sleep deprivation. As far as we know, this is the first report showing that rats can vocalise during REM sleep.

High-fat diet-induced dopaminergic dysregulation induces REM sleep fragmentation and ADHD-like behaviors

Consumption of a high-fat diet (HFD) has been associated with reduced wakefulness and various behavioral deficits, including anxiety, depression, and anhedonia. The dopaminergic system, which plays a crucial role in sleep and ADHD, is known to be vulnerable to chronic HFD. However, the association between HFD-induced behavioral and molecular changes remains unclear. Therefore, we investigated the effects of a HFD on the dopaminergic system and its association with behavioral deficits in male mice. The mice were divided into normal diet and HFD groups and were analyzed for sleep patterns, behavior tests, and transcription levels of dopamine-related genes in the brain. The HFD group showed decreased wakefulness, increased REM sleep with fragmented patterns, decreased time spent in the center zone of the open field test, shorter immobile time in the tail suspension test, impaired visuospatial memory, and reduced sucrose preference. Additionally, the HFD group had decreased mRNA levels of D1R, COMT, and DAT in the nucleus accumbens, which negatively correlated with REM sleep proportion and REM sleep bout count. The results suggest that HFD-induced behavioral deficits were resemblance to ADHD-like behavioral phenotypes and disturbs REM sleep by dysregulating the dopaminergic system.

Prostaglandin E2 activates melanin-concentrating hormone neurons to drive diet-induced obesity

Hypothalamic inflammation reduces appetite and body weight during inflammatory diseases, while promoting weight gain when induced by high-fat diet (HFD). How hypothalamic inflammation can induce opposite energy balance outcomes remains unclear. We found that prostaglandin E2 (PGE2), a key hypothalamic inflammatory mediator of sickness, also mediates diet-induced obesity (DIO) by activating appetite-promoting melanin-concentrating hormone (MCH) neurons in the hypothalamus in rats and mice. The effect of PGE2 on MCH neurons is excitatory at low concentrations while inhibitory at high concentrations, indicating that these neurons can bidirectionally respond to varying levels of inflammation. During prolonged HFD, endogenous PGE2 depolarizes MCH neurons through an EP2 receptor-mediated inhibition of the electrogenic Na+/K+-ATPase. Disrupting this mechanism by genetic deletion of EP2 receptors on MCH neurons is protective against DIO and liver steatosis in male and female mice. Thus, an inflammatory mediator can directly stimulate appetite-promoting neurons to exacerbate DIO and fatty liver.

Many faces of sleep regulation: beyond the time of day and prior wake time

The two-process model of sleep regulation posits two main processes regulating sleep: the circadian process controlled by the circadian clock and the homeostatic process that depends on the history of sleep and wakefulness. The model has provided a dominant conceptual framework for sleep research since its publication ~ 40 years ago. The time of day and prior wake time are the primary factors affecting the circadian and homeostatic processes, respectively. However, it is critical to consider other factors influencing sleep. Since sleep is incompatible with other behaviors, it is affected by the need for essential behaviors such as eating, foraging, mating, caring for offspring, and avoiding predators. Sleep is also affected by sensory inputs, sickness, increased need for memory consolidation after learning, and other factors. Here, we review multiple factors influencing sleep and discuss recent insights into the mechanisms balancing competing needs.

Be cool to be far: Exploiting hibernation for space exploration

In mammals, torpor/hibernation is a state that is characterized by an active reduction in metabolic rate followed by a progressive decrease in body temperature. Torpor was successfully mimicked in non-hibernators by inhibiting the activity of neurons within the brainstem region of the Raphe Pallidus, or by activating the adenosine A1 receptors in the brain. This state, called synthetic torpor, may be exploited for many medical applications, and for space exploration, providing many benefits for biological adaptation to the space environment, among which an enhanced protection from cosmic rays. As regards the use of synthetic torpor in space, to fully evaluate the degree of physiological advantage provided by this state, it is strongly advisable to move from Earth-based experiments to 'in the field' tests, possibly on board the International Space Station.

Chronic high-caloric diet accentuates age-induced sleep alterations in mice

Obesity and sleep disturbances comprise major health problems which are likely interrelated. Diet-induced obesity in young mice has been demonstrated to lead towards an altered sleep homeostasis. In the current study, we investigated the effect of chronic (12 weeks) high-caloric diet (HCD, 45% fat) consumption on sleep and the sleep electroencephalogram (EEG) in young and older mice (6-month-old, n = 9; 18-month-old, n = 8 and 24-month-old, n = 4) and compared with age-matched controls on normal chow (n = 11, n = 9 and n = 9 respectively). Half of the 24-month-old mice did not cope well with HCD, therefore this group has a lower n and limited statistical power. We recorded EEG and the electromyogram for continuous 48-h and performed a 6-h sleep deprivation during the second day. In aged HCD fed mice (18 months old) compared to young, an aging effect was still evident, characterized by decreased waking and increased NREM sleep in the dark period, decreased REM sleep during the light period, as well as increased slow-wave-activity (SWA, EEG power in NREM sleep in 0.5-4.0 Hz). Additionally, aged HCD treated mice showed increased NREM sleep and decreased waking, compared to age-matched controls, denoting an enhanced aging phenotype in the sleep architecture. Notably, an overall increase was found in the slow component of SWA (0.5-2.5 Hz) in aged HCD fed mice compared to age-matched controls. Our data suggest that the effect of aging is the dominant variable irrespective of diet. However, a synergistic effect of aging and diet is noted indicating that chronic HCD consumption exacerbates age-associated sleep alterations.

Chronic high-caloric diet modifies sleep homeostasis in mice

Obesity prevalence and sleep habit changes are commonplace nowadays, due to modern lifestyle. A bidirectional relationship likely exists between sleep quality and metabolic disruptions, which could impact quality of life. In our study, we investigated the effects of a chronic high-caloric diet on sleep architecture and sleep regulation in mice. We studied the effect of 3 months high-caloric diet (HCD, 45% fat) on sleep and the sleep electroencephalogram (EEG) in C57BL/6J mice during 24-hr baseline (BL) recordings, and after 6-hr sleep deprivation (SD). We examined the effect of HCD on sleep homeostasis, by performing parameter estimation analysis and simulations of the sleep homeostatic Process S, a measure of sleep pressure, which is reflected in the non-rapid-eye-movement (NREM) sleep slow-wave-activity (SWA, EEG power density between 0.5 and 4.0 Hz). Compared to controls (n = 11, 30.7 ± 0.8 g), mice fed with HCD (n = 9, 47.6 ± 0.8 g) showed an increased likelihood of consecutive NREM-REM sleep cycles, increased REM sleep and decreased NREM sleep EEG SWA. After SD, these effects were more pronounced. The simulation resulted in a close fit between the time course of SWA and Process S in both groups. HCD fed mice had a slower time constant (T_i  = 15.98 hr) for the increase in homeostatic sleep pressure compared with controls (5.95 hr) indicating a reduced effect of waking on the increase in sleep pressure. Our results suggest that chronic HCD consumption impacts sleep regulation.

Neural and Molecular Mechanisms Involved in Controlling the Quality of Feeding Behavior: Diet Selection and Feeding Patterns

We are what we eat. There are three aspects of feeding: what, when, and how much. These aspects represent the quantity (how much) and quality (what and when) of feeding. The quantitative aspect of feeding has been studied extensively, because weight is primarily determined by the balance between caloric intake and expenditure. In contrast, less is known about the mechanisms that regulate the qualitative aspects of feeding, although they also significantly impact the control of weight and health. However, two aspects of feeding quality relevant to weight loss and weight regain are discussed in this review: macronutrient-based diet selection (what) and feeding pattern (when). This review covers the importance of these two factors in controlling weight and health, and the central mechanisms that regulate them. The relatively limited and fragmented knowledge on these topics indicates that we lack an integrated understanding of the qualitative aspects of feeding behavior. To promote better understanding of weight control, research efforts must focus more on the mechanisms that control the quality and quantity of feeding behavior. This understanding will contribute to improving dietary interventions for achieving weight control and for preventing weight regain following weight loss.

Objective Measurements of Energy Balance Are Associated With Sleep Architecture in Healthy Adults

Study Objectives

We objectively measured body composition, energy expenditure, caloric intake, and sleep in a large, diverse sample of healthy men and women and determined how energy balance and diet associated with sleep physiology.

Methods

Healthy adults (n = 50; 21-50 years) participated in an in-laboratory study involving two baseline sleep nights (BL1-2, 10 hours time-in-bed/night, 2200-0800 hours). Polysomnography was recorded on BL2. Demographic information, body composition, and energy expenditure measurements were collected at study admittance and on BL1. Daily food/drink intake was recorded both before (on BL1) and after (on BL2) the sleep measurement. Partial Pearson's correlations assessed the relationship between energy balance and sleep physiology variables.

Results

At baseline, greater fat-free mass associated with lower total sleep time (r = -0.52, p = .030), lower sleep efficiency (r = -0.53, p = .004), and greater wake after sleep onset (r = 0.55, p = .002). Higher body fat percentage (r = 0.39, p = .038) and being overweight (Body Mass Index [BMI] 25-30; p = .026) associated with more rapid eye movement (REM) sleep. Higher protein intake (r's = 0.46-0.52; p's < .001-.002) and lower carbohydrate intake (r's = -0.31 to -0.34; p's = .027-.046) on BL1 and BL2 associated with more REM sleep. Greater fiber consumption on BL1 and BL2 associated with more slow-wave sleep (SWS; r's = 0.33-0.35; p's = .02-.03). More SWS related to increased carbohydrate intake the following day (BL2, r = 0.32, p = .037).

Conclusions

Body composition and diet were related to baseline sleep characteristics, including SWS and REM sleep duration and sleep maintenance. Future studies should further evaluate the influence of energy balance measures on sleep physiology, since dietary interventions may be useful in treating insufficient sleep, poor sleep quality, excessive sleepiness or other sleep disorders.

Wake-sleep and cardiovascular regulatory changes in rats made obese by a high-fat diet

Obesity is known to be associated with alterations in wake-sleep (WS) architecture and cardiovascular parameters. This study was aimed at assessing the possible influence of diet-induced obesity (DIO) on sleep homeostasis and on the WS state-dependent levels of arterial pressure (AP) and heart rate in the rat. Two groups of age-matched Sprague-Dawley rats were fed either a high-fat hypercaloric diet, leading to DIO, or a normocaloric standard diet (lean controls) for 8 weeks. While under general anesthesia, animals were implanted with instrumentation for the recording of electroencephalogram, electromyogram, arterial pressure, and deep brain temperature. The experimental protocol consisted of 48h of baseline, 12h of gentle handling, enhancing wake and depressing sleep, and 36-h post-handling recovery. Compared to lean controls, DIO rats showed: i) the same amount of rapid-eye movement (REM) and non-REM (NREM) sleep in the rest period, although the latter was characterized by more fragmented episodes; ii) an increase in both REM sleep and NREM sleep in the activity period; iii) a comparable post-handling sleep homeostatic response, in terms of either the degree of Delta power increase during NREM sleep or the quantitative compensation of the REM sleep loss at the end of the 36-h recovery period; iv) significantly higher levels of AP, irrespectively of the different WS states and of the changes in their intensity throughout the experimental protocol. Overall, these changes may be the reflection of a modification in the activity of the hypothalamic areas where WS, autonomic, and metabolic regulations are known to interact.

Diet-Induced Obesity and Circadian Disruption of Feeding Behavior

Feeding behavior shows a rhythmic daily pattern, which in nocturnal rodents is observed mainly during the dark period. This rhythmicity is under the influence of the hypothalamic suprachiasmatic nucleus (SCN), the main biological clock. Nevertheless, various studies have shown that in rodent models of obesity, using high-energy diets, the general locomotor activity and feeding rhythms can be disrupted. Here, we review the data on the effects of diet-induced obesity (DIO) on locomotor activity and feeding patterns, as well as the effect on the brain sites within the neural circuitry involved in metabolic and rewarding feeding behavior. In general, DIO may alter locomotor activity by decreasing total activity. On the other hand, DIO largely alters eating patterns, producing increased overall ingestion and number of eating bouts that can extend to the resting period. Furthermore, within the hypothalamic areas, little effect has been reported on the molecular circadian mechanism in DIO animals with ad libitum hypercaloric diets and little or no data exist so far on its effects on the reward system areas. We further discuss the possibility of an uncoupling of metabolic and reward systems in DIO and highlight a gap of circadian and metabolic research that may help to better understand the implications of obesity.

Diet/Energy Balance Affect Sleep and Wakefulness Independent of Body Weight

STUDY OBJECTIVES

Excessive daytime sleepiness commonly affects obese people, even in those without sleep apnea, yet its causes remain uncertain. We sought to determine whether acute dietary changes could induce or rescue wake impairments independent of body weight.

DESIGN

We implemented a novel feeding paradigm that generates two groups of mice with equal body weight but opposing energetic balance. Two subsets of mice consuming either regular chow (RC) or high-fat diet (HFD) for 8 w were switched to the opposite diet for 1 w. Sleep recordings were conducted at Week 0 (baseline), Week 8 (pre-diet switch), and Week 9 (post-diet switch) for all groups. Sleep homeostasis was measured at Week 8 and Week 9.

PARTICIPANTS

Young adult, male C57BL/6J mice.

MEASUREMENTS AND RESULTS

Differences in total wake, nonrapid eye movement (NREM), and rapid eye movement (REM) time were quantified, in addition to changes in bout fragmentation/consolidation. At Week 9, the two diet switch groups had similar body weight. However, animals switched to HFD (and thus gaining weight) had decreased wake time, increased NREM sleep time, and worsened sleep/wake fragmentation compared to mice switched to RC (which were in weight loss). These effects were driven by significant sleep/wake changes induced by acute dietary manipulations (Week 8 → Week 9). Sleep homeostasis, as measured by delta power increase following sleep deprivation, was unaffected by our feeding paradigm.

CONCLUSIONS

Acute dietary manipulations are sufficient to alter sleep and wakefulness independent of body weight and without effects on sleep homeostasis.