Filling the gut activates paradoxical sleep in suckling rats

Lifestyle mediators of associations among siestas, obesity, and metabolic health

OBJECTIVE

The aim of this study was to determine the association between siestas/no siestas and obesity, considering siesta duration (long: >30 minutes, short: ≤30 minutes), and test whether siesta traits and/or lifestyle factors mediate the association of siestas with obesity and metabolic syndrome (MetS).

METHODS

This was a cross-sectional study of 3275 adults from a Mediterranean population (the Obesity, Nutrigenetics, TIming, and MEditerranean [ONTIME] study) who had the opportunity of taking siestas because it is culturally embedded.

RESULTS

Thirty-five percent of participants usually took siestas (16% long siestas). Compared with the no-siesta group, long siestas were associated with higher values of BMI, waist circumference, fasting glucose, systolic blood pressure, and diastolic blood pressure, as well as with a higher prevalence of MetS (41%; p = 0.015). In contrast, the probability of having elevated SBP was lower in the short-siesta group (21%; p = 0.044) than in the no-siesta group. Smoking a higher number of cigarettes per day mediated the association of long siestas with higher BMI (by 12%, percentage of association mediated by smoking; p < 0.05). Similarly, delays in nighttime sleep and eating schedules and higher energy intake at lunch (the meal preceding siestas) mediated the association between higher BMI and long siestas by 8%, 4%, and 5% (all p < 0.05). Napping in bed (vs. sofa/armchair) showed a trend to mediate the association between long siestas and higher SBP (by 6%; p = 0.055).

CONCLUSIONS

Siesta duration is relevant in obesity/MetS. Timing of nighttime sleep and eating, energy intake at lunch, cigarette smoking, and siesta location mediated this association.

Postprandial sleep mechanics in Drosophila

Food consumption is thought to induce sleepiness. However, little is known about how postprandial sleep is regulated. Here, we simultaneously measured sleep and food intake of individual flies and found a transient rise in sleep following meals. Depending on the amount consumed, the effect ranged from slightly arousing to strongly sleep inducing. Postprandial sleep was positively correlated with ingested volume, protein, and salt—but not sucrose—revealing meal property-specific regulation. Silencing of leucokinin receptor (Lkr) neurons specifically reduced sleep induced by protein consumption. Thermogenetic stimulation of leucokinin (Lk) neurons decreased whereas Lk downregulation by RNAi increased postprandial sleep, suggestive of an inhibitory connection in the Lk-Lkr circuit. We further identified a subset of non-leucokininergic cells proximal to Lkr neurons that rhythmically increased postprandial sleep when silenced, suggesting that these cells are cyclically gated inhibitory inputs to Lkr neurons. Together, these findings reveal the dynamic nature of postprandial sleep.

DOI:http://dx.doi.org/10.7554/eLife.19334.001

Many of us have experienced feelings of sleepiness after a large meal. However, there is little scientific evidence that this “food coma” effect is real. If it is, it may vary between individuals, or depend on the type of food consumed. This variability makes it difficult to study the causes of post-meal sleepiness.

Murphy et al. have now developed a system that can measure fruit fly sleep and feeding behavior at the same time. Recordings using this system reveal that after a meal, flies sleep more for a short period before returning to a normal state of wakefulness. The sleep period lasts around 20-40 minutes, with flies that ate more generally sleeping more.

Further investigation revealed that salty or protein-rich foods promote sleep, whereas sugary foods do not. By using genetic tools to turn on and off neurons in the fly brain, Murphy et al. identified a number of brain circuits that play a role in controlling post-meal sleepiness. Some of these respond specifically to the consumption of protein. Others are sensitive to the fruit fly’s internal clock, reducing post-meal sleepiness only around dusk. Thus, post-meal sleepiness can be regulated in a number of different ways.

Future experiments are now needed to explore the genes and circuits that enable meal size and the protein or salt content of food to drive sleep. In nature, sleep is likely a vulnerable state for animals. Thus, another challenge will be to uncover why post-meal sleep is important. Does sleeping after a meal boost digestion? Or might it help animals to form memories about a food source, making it easier to find similar food in the future?

DOI:http://dx.doi.org/10.7554/eLife.19334.002

Rem sleep, early experience, and the development of reproductive strategies

We hypothesize that rapid eye movement or REM sleep evolved, in part, to mediate sexual/reproductive behaviors and strategies. Because development of sexual and mating strategies depends crucially on early attachment experiences, we further hypothesize that REM functions to mediate attachment processes early in life. Evidence for these hypotheses comes from (1) the correlation of REM variables with both attachment and sexual/reproductive variables; (2) attachment-related and sex-related hormonal release during REM; (3) selective activation during REM of brain sites implicated in attachment and sexual processes; (4) effects of maternal deprivation on REM; (5) effects of REM deprivation on sexual behaviors; and (6) the REM-associated sexual excitation. To explain why we find associations among REM sleep, attachment, and adult reproductive strategies, we rely on recent extensions of parent-offspring conflict theory. Using data from recent findings on genomic imprinting, Haig (2000) and others suggest that paternally expressed genes are selected to promote growth of the developing fetus/child at the expense of the mother, while maternally expressed genes counter these effects. Because developmental REM facilitates attachment-related outcomes in the child, developmental REM may be regulated by paternally expressed genes. In that case, REM may have evolved to support the "aims" of paternal genes at the expense of maternal genes.

Thermal and nutritional modulation of sleep in infant rats

Infant rats cycle rapidly between periods of high muscle tone (indicative of wakefulness) and periods of atonia (indicative of sleep). Here, the influence of air temperature on sleep in 8-day-old rats was examined by testing pups at thermoneutrality (35 degrees C) and during moderate (28 degrees C) and extreme (20 degrees C) cold challenge; also, pups were tested 1, 4, and 8 hr after infusion of milk to assess the effects of food deprivation on sleep. Whereas moderate cooling slightly reduced sleep durations and altered the temporal patterning of myoclonic twitching, extreme cooling substantially decreased sleep durations and inhibited twitching. In contrast, food deprivation had little effect. Therefore, thermoregulatory mechanisms engaged during moderate cooling sustain sleep, whereas extreme cooling overwhelms these mechanisms, thereby promoting arousal.

A full belly and colostrum: Two major determinants of filial love

The establishment of filial bonding in sheep relies on the success of the first suckling bouts. Previous work has shown that colostrum ingestion facilitates the development of a preference for the mother. We conducted a series of experiments to investigate the influence of quantitative and qualitative factors linked to fluid ingestion. The first experiment demonstrated that the procedure was not detrimental to the mother-young interaction and did not cause any harm to the lamb. In a second experiment, lambs could interact with their dam, but were deprived of suckling during the first 12 hr after birth (The udder was covered.) and fed via a nasogastric tube in the presence of their mother on seven occasions. When tested in a two-choice test at 12 hr of age, lambs that received either colostrum or saline (total amount = 10% birth weight) spent more time near their own dam than near an alien ewe. When smaller volumes of fluids were used (5% birth weight), lambs that received colostrum showed a preference for their mother while those ingesting saline did not. The differences persisted at 24 hr even though all the lambs had access to the maternal udder from 12 hr onward, and these differences were not related to weight variations or body temperature. In the choice test, the motor skills and vocal activity of the lambs were not affected by the treatment nor were the behaviors of the ewes. The third experiment showed that colostrum, and saline to a lesser extent, reduces the number of vocalizations emitted by lambs compared to sham-intubated lambs, but did not otherwise influence mother-young interactions. This suggests that post-ingestive factors may facilitate the development of mother preference through a satiated or comforted state. This is the first evidence that depending on the amount of liquids ingested, a preferential relationship with the mother can be established via nutritional and nonnutritional signals originating from the gastrointestinal region, pointing out the major role played by the gut-brain axis in the development of filial bonding.

Hippocampal theta in the newborn rat is revealed under conditions that promote REM sleep

Hippocampal theta activity, a high-amplitude, slow (4-12 Hz) oscillation that occurs in a variety of behavioral contexts, is thought to emerge in infant rats only after 1 week of age. However, we report here that unanesthetized 2- and 4-d-old rats with electrodes implanted in the CA1 field of the hippocampus and tested in thermoneutral conditions exhibit theta activity. Moreover, this infant theta is characterized by the same neuronal bursting pattern and power spectrum that characterize theta in adults. Simultaneous measures of behavior and neck muscle tone indicated that bouts of theta occurred predominantly during periods of muscle atonia (with or without concurrent myoclonic twitching), indicative of REM sleep. In contrast, sharp waves were accompanied by startles (i.e., simultaneous and vigorous movement of all four limbs). These findings underscore the need for comprehensive in vivo investigations of the pharmacology, neural substrates, and behavioral correlates of hippocampal field activity in neonates.

Gastric saline infusion reduces ultrasonic vocalizations and brown fat activity in suckling rat pups

Under standard conditions involving isolation and cooling, it has been documented that intraoral infusion of milk and injection of the intestinal peptide cholecystokinin (CCK) result in an attenuation in ultrasonic vocalizations (USV) emitted by infant rat pups. One of the most effective stimuli in inhibiting ingestion in suckling rat pups is gastric distension, but the effect of gastric distension on USV production has not been reported. In this experiment, we subjected infant rats to intragastric infusion of isotonic saline (2% body weight) to produce a natural level of gastric distension and hydration. We found that this stimulus resulted in a powerful reduction in USV emissions in isolated 10-day-old rats. In a subsequent experiment, we found that gastric saline infusion also diminished brown adipose tissue (BAT) thermogenesis. There were different time courses of the gastric saline infusion effects on BAT thermogenesis and on USV emissions, however, suggesting that these processes may be independently regulated. We hypothesize that this stimulus induces a transient activation of the parasympathetic nervous system, which overrides the sympathetic control of BAT and USV production.

CCK and development of mother preference in sheep: a neonatal time course study

We investigated the effects of a CCK-A receptor antagonist on the development of mother preference. Lambs received 2-NAP either at birth or 6, or 12 h later. Controls were given saline. When tested at 24 h of age, lambs receiving 2-NAP at birth or 6 h later did not display mother preference unlike controls and lambs which were given 2-NAP at 12 h. The effect of the antagonist persisted at 48 h of age in lambs treated at birth. This suggests that endogenous CCK participates in the development of mother preference only in the first few hours following birth.

Mesopontine contribution to the expression of active 'twitch' sleep in decerebrate week-old rats

Myoclonic twitching is a ubiquitous feature of infant behavior that has been used as an index of active sleep. Although the active sleep of infants differs in some ways from the REM sleep of adults, their marked similarities have led many to view them them as homologous behavioral states. Recently, however, this view has been challenged. One avenue for resolving this issue entails examination of the neural substrates of active sleep. If the neural substrates of active sleep were found to be similar to those of REM sleep, then this would support the view that the two states are homologous. Therefore, in the present study, decerebrations were performed in the pons and midbrain to determine whether the mesopontine region is important for the expression of active sleep in infants, just as it is for the expression of REM sleep in adults. It was found that, in comparison to controls, caudal pontine decerebrations reduced myoclonic twitching by 76%, rostral pontine decerebrations reduced twitching by 40%, and midbrain transections had no significant effect on twitching. Moreover, analysis of the temporal organization of twitching indicated that pontine decerebrations predominantly affected high-frequency twitching while leaving unaffected the low-frequency twitching that is thought to be contributed by local spinal circuits at this age. These results indicate that the mesopontine region plays a central role in the expression of active sleep in infant rats.