Body temperature and sleep: Are they controlled by the same mechanism?

Breathing Rhythm and Pattern and Their Influence on Emotion

Breathing is a vital rhythmic motor behavior with a surprisingly broad influence on the brain and body. The apparent simplicity of breathing belies a complex neural control system, the breathing central pattern generator (bCPG), that exhibits diverse operational modes to regulate gas exchange and coordinate breathing with an array of behaviors. In this review, we focus on selected advances in our understanding of the bCPG. At the core of the bCPG is the preBötzinger complex (preBötC), which drives inspiratory rhythm via an unexpectedly sophisticated emergent mechanism. Synchronization dynamics underlying preBötC rhythmogenesis imbue the system with robustness and lability. These dynamics are modulated by inputs from throughout the brain and generate rhythmic, patterned activity that is widely distributed. The connectivity and an emerging literature support a link between breathing, emotion, and cognition that is becoming experimentally tractable. These advances bring great potential for elucidating function and dysfunction in breathing and other mammalian neural circuits.

Monosynaptic Projections to Excitatory and Inhibitory preBötzinger Complex Neurons

The key driver of breathing rhythm is the preBötzinger Complex (preBötC) whose activity is modulated by various functional inputs, e.g., volitional, physiological, and emotional. While the preBötC is highly interconnected with other regions of the breathing central pattern generator (bCPG) in the brainstem, there is no data about the direct projections to either excitatory and inhibitory preBötC subpopulations from other elements of the bCPG or from suprapontine regions. Using modified rabies tracing, we identified neurons throughout the brain that send monosynaptic projections to identified excitatory and inhibitory preBötC neurons in mice. Within the brainstem, neurons from sites in the bCPG, including the contralateral preBötC, Bötzinger Complex, the nucleus of the solitary tract (NTS), parafacial region (pF L /pF V ), and parabrachial nuclei (PB), send direct projections to both excitatory and inhibitory preBötC neurons. Suprapontine inputs to the excitatory and inhibitory preBötC neurons include the superior colliculus, red nucleus, amygdala, hypothalamus, and cortex; these projections represent potential direct pathways for volitional, emotional, and physiological control of breathing.

Relationship between the Daily Rhythm of Distal Skin Temperature and Brown Adipose Tissue 18F-FDG Uptake in Young Sedentary Adults

The present study examines whether the daily rhythm of distal skin temperature (DST) is associated with brown adipose tissue (BAT) metabolism as determined by ¹⁸F-fluorodeoxyglucose (¹⁸F-FDG) uptake in young adults. Using a wireless thermometer (iButton) worn on the nondominant wrist, DST was measured in 77 subjects (26% male; age 22 ± 2 years; body mass index 25.2 ± 4.8 kg/m²) for 7 consecutive days. The temperatures to which they were habitually exposed over the day were also recorded. The interday stability of DST was calculated from the collected data, along with the intraday variability and relative amplitude; the mean temperature of the 5 and 10 consecutive hours with the maximum and minimum DST values, respectively; and when these hours occurred. Following exposure to cold, BAT volume and mean and peak standardized ¹⁸F-FDG uptake (SUV_mean and SUV_peak) were determined for each subject via static ¹⁸F-FDG positron emission tomography/computed tomography scanning. Relative amplitude and the time at which the 10 consecutive hours of minimum DST values occurred were positively associated with BAT volume, SUV_mean, and SUV_peak (p ≤ 0.02), whereas the mean DST of that period was inversely associated with the latter BAT variables (p ≤ 0.01). The interday stability and intraday variability of the DST were also associated (directly and inversely, respectively) with BAT SUV_peak (p ≤ 0.02 for both). All of these associations disappeared, however, when the analyses were adjusted for the ambient temperature to which the subjects were habitually exposed. Thus, the relationship between the daily rhythm of DST and BAT activity estimated by ¹⁸F-FDG uptake is masked by environmental and likely behavioral factors. Of note is that those participants exposed to the lowest ambient temperature showed 3 to 5 times more BAT volume and activity compared with subjects who were exposed to a warmer ambient temperature.

Neuronal noise as an origin of sleep arousals and its role in sudden infant death syndrome

In addition to regular sleep/wake cycles, humans and animals exhibit brief arousals from sleep. Although much is known about consolidated sleep and wakefulness, the mechanism that triggers arousals remains enigmatic. Here, we argue that arousals are caused by the intrinsic neuronal noise of wake-promoting neurons. We propose a model that simulates the superposition of the noise from a group of neurons, and show that, occasionally, the superposed noise exceeds the excitability threshold and provokes an arousal. Because neuronal noise decreases with increasing temperature, our model predicts arousal frequency to decrease as well. To test this prediction, we perform experiments on the sleep/wake behavior of zebrafish larvae and find that increasing water temperatures lead to fewer and shorter arousals, as predicted by our analytic derivations and model simulations. Our findings indicate a previously unrecognized neurophysiological mechanism that links sleep arousals with temperature regulation, and may explain the origin of the clinically observed higher risk for sudden infant death syndrome with increased ambient temperature.

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.

Sincronização não-fótica: o efeito do exercício físico aeróbio

As principais alterações, agudas e crônicas, provocadas pelo exercício físico aeróbio (EF) sobre o organismo são, de maneira geral, bem conhecidas. No entanto, existe um efeito em particular do EF que começou a ser elucidado no começo da década de 90, em humanos, que tem a capacidade de alterar a relação temporal do organismo com o meio. A modificação da expressão dos ritmos circadianos, causada pelo EF, qualifica-o como sincronizador dos osciladores biológicos. O principal sincronizador da ritmicidade biológica é o ciclo geofísico claro/escuro. A alternância do dia e da noite, através de diferenças nos níveis de luminosidade, é percebida por meio de vias fóticas pelo sistema de temporização circadiana (STC). Esses estímulos, chamados fóticos, fornecem informações temporais para o STC sincronizando os osciladores biológicos com esse ciclo ambiental. Outros estímulos também são capazes de sincronizá-los e são chamados de sincronizadores não-fóticos. Esta revisão aborda o efeito do EF sobre o sistema de temporização e, ao mesmo tempo, discute as possíveis e prováveis aplicações cronobiológicas dos conhecimentos abordados. O EF pode afetar o STC através de vias não-fóticas, podendo beneficiar a saúde de indivíduos em diversas situações, tais como vôos transmeridianos, trabalhos noturnos e distúrbios do sono. Ressalta-se, também, que devem ser realizados mais estudos no cotidiano das pessoas para compreender melhor a relação entre, e a contribuição dos, diferentes sincronizadores em um contexto real.

The human sleep–wake cycle reconsidered from a thermoregulatory point of view

Sleep is typically initiated on the declining portion of the circadian rhythm of core body temperature (CBT) when its rate of change, and body heat loss, is maximal. Distal vasodilatation plays a primary role in the circadian regulation of body heat loss and is strongly associated with sleepiness and sleep induction. In contrast, sleep (i.e. non-REM sleep and slow-wave activity, SWA) has no or only a minor thermoregulatory function. Two lines of evidence support this statement. First, detailed analyses of thermoregulatory changes before and after lights off show clearly that they start before stage 2 sleep begins. Second, accumulation of sleep pressure with increasing time awake, increases subjective sleepiness and SWA during the succeeding recovery night, but does not influence the thermoregulatory system. Taken together, the circadian modulation of sleepiness and sleep induction is clearly associated with thermoregulatory changes, but the thermoregulatory system seems to be independent of the sleepiness/sleep regulatory system. A simplified model is presented which attempts to explain the relationship between these two systems. It is based on the main hypothesis that all thermoregulatory effects which lead to an increase in the core/shell ratio (e.g. a reduced shell by increased distal skin temperature) lead to increased sleepiness and, as a consequence, to increased sleep propensity. However, the sleepiness/sleep regulatory system feeds back onto the thermoregulatory system only indirectly via sleep-related behaviors (e.g. relaxation, lying down).

Thermoregulatory effects of melatonin in relation to sleepiness

Thermoregulatory processes have long been implicated in the initiation of human sleep. In this paper, we review our own studies conducted over the last decade showing a crucial role for melatonin as a mediator between the thermoregulatory and arousal system in humans. Distal heat loss, via increased skin temperature, seems to be intimately coupled with increased sleepiness and sleep induction. Exogenous melatonin administration during the day when melatonin is essentially absent mimics the endogenous thermophysiological processes occurring in the evening and induces sleepiness. Using a cold thermic challenge test, it was shown that melatonin-induced sleepiness occurs in parallel with reduction in the thermoregulatory set-point (threshold); thus, melatonin may act as a circadian modulator of the thermoregulatory set-point. In addition, an orthostatic challenge can partially block the melatonin-induced effects, suggesting an important role of the sympathetic nervous system as a link between the thermoregulatory and arousal systems. A topographical analysis of finger skin temperature with infrared thermometry revealed that the most distal parts of the fingers, i.e., fingertips, represent the important skin regions for heat loss regulation, most probably via opening the arteriovenous anastomoses, and this is clearly potentiated by melatonin. Taken together, melatonin is involved in the fine-tuning of vascular tone in selective vascular beds, as circulating melatonin levels rise and fall throughout the night. Besides the role of melatonin as "nature's soporific", it can also serve as nature's nocturnal vascular modulator.

Thermophysiologic aspects of the three-process-model of sleepiness regulation

The following overview reconsiders the three-process model of sleepiness regulation (homeostatic, circadian, and sleep inertia) from a thermophysiologic point of view. Our results gathered over the last decade indicate that the homeostatic aspect of sleepiness regulation (ie, buildup of sleepiness during wakefulness and its decay during sleep) is not related to the thermoregulatory system, whereas the two other processes of sleepiness regulation (ie, circadian and sleep inertia process) are clearly related to thermoregulation in humans. Distal skin temperature of hands and feet seems to be the crucial variable for the association between thermophysiology, sleepiness, and sleep. Increased distal skin temperature before a nocturnal sleep episode is a good predictor for short sleep-onset latency. The disappearance of sleep inertia after sleep or a nap episode shows very similar kinetics as distal vasoconstriction. Furthermore, relaxation-induced sleepiness (eg, after lying down, at lights-off, with thermal biofeedback training) also evokes an increase in distal skin temperatures. The reverse effect occurs at lights-on or a posture change from supine to standing, Therefore, in terms of thermophysiology, sleep inertia can be explained as the reverse of a relaxation process (ie, decrease in distal skin temperatures). Our results reinterpret the so-called "sleep-evoked" reduction of core body temperature as a consequence of relaxation-induced vasodilatation after lights-off. Sleep per se has no further thermoregulatory effect. Taken together, a thermophysiologic approach may provide a successful strategy to treat sleep-onset insomnia and alleviate sleep inertia.

Sleep induction and temperature lowering by medial preoptic α1 adrenergic receptors

Changes in sleep-wakefulness (S-W) and body temperature (T(b)) on administration of alpha(1) agonist (methoxamine) and antagonist (prazosin) into the medial preoptic area (mPOA) were studied in rats. Presynaptic catecholaminergic terminals of the mPOA were destroyed by injecting 6-hydroxydopamine at the ventral noradrenergic bundle (VNA), before administration of the drugs. Microinjection of 0.05 microg methoxamine induced sleep, though 0.1 microg prazosin produced no change in S-W. On the other hand, in normal rats, the same dose of methoxamine produced no change, while prazosin produced arousal. Denervation hypersensitivity may be responsible for the appearance of hypnogenic response on methoxamine administration, in the VNA-lesioned rats. The VNA-lesioned animals (before administration of any drug) had higher pre-injection values of wake period than the normal rats. A reduction in the tonic activity of noradrenergic fibers to the mPOA, and resulting reduced activity of alpha(1) receptors, may be responsible for increased wake period in the VNA-lesioned rats. The action of prazosin was probably abolished in the absence of tonic activity of alpha(1) receptor in the VNA-lesioned rats. Reduction and increase in T(b) produced by methoxamine and prazosin, respectively, confirm the involvement of alpha(1) receptors in the thermal changes. Methoxamine was less effective, than in normal rats, in reducing T(b). So, the possibility of involvement of presynaptic receptors in the thermal response is suggested. The results suggest the involvement of separate sets of alpha(1) receptors (and neurons) in hypnogenesis and in lowering T(b). As sleep is associated with fall in T(b), the alpha(1) adrenergic receptors may be involved in interlinking sleep regulation and thermoregulation.