Sleep deprivation in the rat: XII. Effect on ambient temperature choice

Muscle temperature is least altered during total sleep deprivation in rats

It has often been said that the brain is mostly benefitted from sleep. To understand the importance of sleep, extensive studies on other organs are too required. One such unexplored area is the understanding of muscle physiology during the sleep-wake cycle. Changes in muscle tone with different sleep phases are evident from the rapid eye movement sleep muscle atonia. There is variation in brain and body temperature during sleep stages, the brain temperature being higher during rapid eye movement sleep than slow-wave sleep. However, the change in muscle temperature with different sleep stages is not known. In this study, we have implanted pre-calibrated K-type thermocouples in the hypothalamus and the dorsal nuchal muscle, and a peritoneal transmitter to monitor the hypothalamic, muscle, and body temperature respectively in rats during 24 h sleep-wake cycle. The changes in muscle, body, and hypothalamic temperature during total sleep deprivation were also monitored. During normal sleep-wake stages, the temperature in the decreasing order was that of the hypothalamus, body, and muscle. Total sleep deprivation by gentle handling caused a significant increase in hypothalamic and body temperature, while there was least change in the muscle temperature. The circadian rhythm of the hypothalamic and body temperature in the sleep-deprived rats was disrupted, while the same was preserved in the muscle temperature. The results of our study show that muscle atonia during rapid eye movement sleep is a physiologically regulated thermally quiescent muscle state offering a conducive environment for muscle rest and repair.

Exposure to total 36-hr sleep deprivation reduces physiological and psychological thermal strain to whole-body uncompensable passive heat stress in young adult men

Total sleep deprivation (TSD) is associated with endothelial dysfunction and a consequent decrease in vascular reactivity and increase in peripheral vascular resistance. These effectors compromise the body's ability to thermoregulate in hot and cold stress conditions. We investigated heat-unacclimated young adult men (26 ± 2 years) to determine whether 36 hr of TSD compared to an 8 or 4-hr sleep condition, would suppress the responses of the autonomic system (body rectal temperature [T_re ], heart rate [HR], root mean square of successive interbeat intervals, physiological strain, blood pressure [BP], circulating blood catecholamines, sweating rate and subjective sensations) to whole-body uncompensable passive heat stress in traditional Finnish sauna heat (T_air  = 80-90°C, rh = 30%). Sauna bathing that induced whole-body hyperthermia had a residual effect on reducing BP in the 8-hr and 4-hr sleep per night conditions according to BP measurements. By contrast, 36 hr of total wakefulness led to an increase in BP. These observed sleep deprivation-dependent differences in BP modifications were not accompanied by changes in the blood plasma epinephrine and norepinephrine concentrations. However, during sauna bathing, an increase in BP following 36 hr of TSD was accompanied by significant decreases in body T_re , HR and physiological strain, together with a diminished sweating rate, enhanced vagus-mediated autonomic control of HR variability, and improved thermal perception by the subjects. Our results suggest the impaired ability of the body to accumulate external heat in the body's core under uncompensable passive heat conditions following 36 hr of TSD, because of the TSD-attenuated autonomic system response to acute heat stress.

Acute sleep deprivation elevates brain and body temperature in rats

Available sleep deprivation studies lack data on simultaneous changes in hypothalamic, cortical and body temperature during sleep deprivation and recovery. Ten adult male Wistar rats chronically implanted with electroencephalogram, electro-oculogram and electromyogram electrodes for recording sleep were used in this study. Hypothalamic and cortical temperatures were measured by pre-implanted thermocouples. A radio transmitter (TA10TAF-40, DSI USA) was implanted intraperitoneally to measure body temperature. All the temperatures were measured simultaneously at 15-s intervals during baseline conditions, sleep deprivation and recovery sleep. Sleep deprivation was carried out for 24 hr by the gentle handling method; however, sleep and temperature were only recorded during the first 12 hr of deprivation. During sleep deprivation the body, hypothalamic and cortical temperatures increased significantly as compared to baseline. During recovery sleep, body and cortical temperature recovered earlier than the hypothalamic temperature. Hypothalamic temperature remained higher than the baseline values throughout 12 hr of recovery sleep. In the recovery sleep, cortical temperature decreased immediately and reached near baseline by 4 hr. We observed a quicker return of cortical temperature towards control temperature during recovery sleep compared with hypothalamic and body temperature. The results of the present study show that acute sleep deprivation results in a rise in both cortical and hypothalamic temperature, along with body temperature. A rise in cortical temperature may be a contributing factor for cognitive dysfunction resulting from sleep deprivation.

The Temperature Dependence of Sleep

Mammals have evolved a range of behavioural and neurological mechanisms that coordinate cycles of thermoregulation and sleep. Whether diurnal or nocturnal, sleep onset and a reduction in core temperature occur together. Non-rapid eye movement (NREM) sleep episodes are also accompanied by core and brain cooling. Thermoregulatory behaviours, like nest building and curling up, accompany this circadian temperature decline in preparation for sleeping. This could be a matter of simply comfort as animals seek warmth to compensate for lower temperatures. However, in both humans and other mammals, direct skin warming can shorten sleep-latency and promote NREM sleep. We discuss the evidence that body cooling and sleep are more fundamentally connected and that thermoregulatory behaviours, prior to sleep, form warm microclimates that accelerate NREM directly through neuronal circuits. Paradoxically, this warmth might also induce vasodilation and body cooling. In this way, warmth seeking and nesting behaviour might enhance the circadian cycle by activating specific circuits that link NREM initiation to body cooling. We suggest that these circuits explain why NREM onset is most likely when core temperature is at its steepest rate of decline and why transitions to NREM are accompanied by a decrease in brain temperature. This connection may have implications for energy homeostasis and the function of sleep.

REM Sleep and Endothermy: Potential Sites and Mechanism of a Reciprocal Interference

Numerous data show a reciprocal interaction between REM sleep and thermoregulation. During REM sleep, the function of thermoregulation appears to be impaired; from the other hand, the tonic activation of thermogenesis, such as during cold exposure, suppresses REM sleep occurrence. Recently, both the central neural network controlling REM sleep and the central neural network controlling thermoregulation have been progressively unraveled. Thermoregulation was shown to be controlled by a central “core” circuit, responsible for the maintenance of body temperature, modulated by a set of accessory areas. REM sleep was suggested to be controlled by a group of hypothalamic neurons overlooking at the REM sleep generating circuits within the brainstem. The two networks overlap in a few areas, and in this review, we will suggest that in such overlap may reside the explanation of the reciprocal interaction between REM sleep and thermoregulation. Considering the peculiar modulation of thermoregulation by REM sleep the result of their coincidental evolution, REM sleep may therefore be seen as a period of transient heterothermy.

Insomnia Caused by Serotonin Depletion is Due to Hypothermia

STUDY OBJECTIVE

Serotonin (5-hydroxytryptamine, 5-HT) neurons are now thought to promote wakefulness. Early experiments using the tryptophan hydroxylase inhibitor para-chlorophenylalanine (PCPA) had led to the opposite conclusion, that 5-HT causes sleep, but those studies were subsequently contradicted by electrophysiological and behavioral data. Here we tested the hypothesis that the difference in conclusions was due to failure of early PCPA experiments to control for the recently recognized role of 5-HT in thermoregulation.

DESIGN

Adult male C57BL/6N mice were treated with PCPA (800 mg/kg intraperitoneally for 5 d; n = 15) or saline (n = 15), and housed at 20 °C (normal room temperature) or at 33 °C (thermoneutral for mice) for 24 h. In a separate set of experiments, mice were exposed to 4 °C for 4 h to characterize their ability to thermoregulate.

MEASUREMENTS AND RESULTS

PCPA treatment reduced brain 5-HT to less than 12% of that of controls. PCPA-treated mice housed at 20 °C spent significantly more time awake than controls. However, core body temperature decreased from 36.5 °C to 35.1 °C. When housed at 33 °C, body temperature remained normal, and total sleep duration, sleep architecture, and time in each vigilance state were the same as controls. When challenged with 4 °C, PCPA-treated mice experienced a precipitous drop in body temperature, whereas control mice maintained a normal body temperature.

CONCLUSIONS

These results indicate that early experiments using para-chlorophenylalanine that led to the conclusion that 5-hydroxytryptamine (5-HT) causes sleep were likely confounded by hypothermia. Temperature controls should be considered in experiments using 5-HT depletion.

Prostaglandin involvement in hyperthermia induced by sleep deprivation: a pharmacological and autoradiographic study

AIMS

Hyperthermia is a characteristic functional effect of sleep deprivation (SD). We hypothesize here that prostaglandin E2 (PGE2) could be involved in hyperthermia induced by sleep deprivation.

MAIN METHODS

To address this issue we examined the effects of a selective cyclo-oxygenase-2 inhibitor (COX-2) agent on hyperthermia induced by SD in rats. We also investigated binding to PGE2 receptors in hypothalamic brain areas of sleep-deprived rats using in vitro autoradiography. Male Wistar rats were deprived of sleep for 96 h using the platform technique. Sleep deprived and control groups received saline or Celecoxib (20, 30 and 40 mg/kg; p.o.) daily during the SD period. Colonic temperature was measured daily.

KEY FINDINGS

Results indicated that core temperature of sleep-deprived rats that receiving saline increased from the first to the fourth day of SD compared to baseline and to the respective control group. However, the hyperthermia induced by SD was not blocked by COX-2 inhibitor at any dose. [(3)H]PGE2 binding did not differ significantly among the groups in any of a number of hypothalamic areas examined.

SIGNIFICANCE

Although SD rats showed no response to the COX-2 inhibitor and no alterations in [(3)H]PGE2 binding, the possibility remains that other prostaglandin system and/or receptor subtypes may be altered by SD.

Increases in amino-cupric-silver staining of the supraoptic nucleus after sleep deprivation

Sleep deprived rats undergo a predictable sequence of physiological changes, including changes in skin condition, increased energy expenditure, and altered thermoregulation. Amino-cupric-silver staining was used to identify sleep deprivation related changes in the brain. A significant increase in staining was observed in the supraoptic nucleus (SON) of the hypothalamus of rats with high sleep loss (>45 h) vs. their yoked controls. Follow-up experiments showed that staining was not significantly different in rats sleep deprived for less than 45 h, suggesting that injurious sleep deprivation-related processes occur above a threshold quantity of sleep loss. These anatomical changes suggest that the effects of sleep deprivation may be related to protein metabolism in certain brain regions.

Insomnia and ageing: Implications for healthcare practice and policy

Although ageing itself does not lead to insomnia, changes in sleep architecture (the ‘typical’ physiological progression from wakefulness to deep sleep) and health status create a vulnerability to the development of insomnia, which can be precipitated by a trigger event. This review highlights some of the problems associated with insomnia in older people and offers insights into the possible approaches to stop insomnia from becoming a ‘rite of passage’. The main conclusion from this review however, is that sleep research focusing specifically on the ageing population is badly needed, alongside a unified diagnostic system and research structure (Leger, 2000). These findings are also discussed in relation to both healthcare policy and practice.

Sleep deprivation in rats with preoptic/anterior hypothalamic lesions

Chronic total sleep deprivation (TSD) of rats by the disk-over-water method reliably produces initial increases and subsequent decreases in waking intraperitoneal (Tip) and hypothalamic (Thy) temperatures, progressive increases in energy expenditure, skin lesions on the tail and plantar surfaces, debilitated appearance, and eventual death. We investigated the possible role of the preoptic/anterior hypothalamus (POAH) in the mediation of the TSD effects by comparing these effects in POAH-lesioned and unlesioned rats. Bilateral POAH lesions sufficient in size to impair homeothermic responses to changes in ambient temperature did not produce TSD-like temperature changes under baseline ambient temperatures of 28-29 degrees C, implying that the thermoregulatory changes produced by TSD do not result from impairment of the lesioned area. However, the possibility remains that the TSD effects are mediated by damage to POAH areas that were not lesioned. During TSD, lesioned and unlesioned rats showed similar progressive increases in energy expenditure, but the lesioned rats showed earlier, steeper, and eventually greater declines in Tip and Thy. This result suggests that in unlesioned rats the POAH may counter-regulate against, and thereby attenuate, the reduction in heat retention caused by TSD. This failure of regulation in lesioned rats is consistent with their impaired response to ambient temperature change and implies that, in unlesioned rats, some POAH thermoregulatory mechanisms continue to function normally during TSD. Lesioned rats did not show the characteristic TSD-induced early increases in Tip and Thy. This result could imply either that heat retention was so compromised that body temperatures did not rise in spite of a TSD-induced increases in thermoregulatory setpoint, or that the setpoint increase in unlesioned rats is POAH-mediated. Notwithstanding the greater Tip and Thy declines in lesioned rats, they survived the TSD procedure longer than the unlesioned rats, thus supporting previous indications that death did not result from hypothermia.

Sleep deprivation in the rat by the disk-over-water method

Chronic sleep deprivation may be required to reveal the most serious physiological consequences of sleep loss, but it usually requires strong stimulation which can obscure the interpretation of effects. The disk-over-water method permits chronic sleep deprivation of rats with gentle physical stimulation that can be equally applied to yoked control rats. A series of studies with this method has revealed little or no pathology in the control rats. The deprived rats show a reliable syndrome that includes temperature changes (which vary with the sleep stages that are lost); heat seeking behavior; increased food intake; weight loss; increased metabolic rate; increased plasma norepinephrine; decreased plasma thyroxine; an increased triiodothyronine-thyroxine ratio; and an increase of an enzyme which mediates thermogenesis by brown adipose tissue. The temperature changes are attributable to excessive heat loss and an elevated thermoregulatory setpoint, both of which increase thermoregulatory load, and the other changes are interpretable as responses to this increased load. This pattern indicates that sleep serves a thermoregulatory function in the rat. The sleep deprived rats also show stereotypic ulcerative and hyperkeratotic lesions localized to the tail and plantar surfaces of the paws, and they die within a matter of weeks; the mediation of these changes is unresolved.

Functional consequences of sustained sleep deprivation in the rat

Sleep deprivation disrupts vital biological processes that are necessary for cognitive ability and physical health, but the physiological changes that underlie these outward effects are largely unknown. The purpose of the present studies in the laboratory rat is to prolong sleep deprivation to delineate the pathophysiology and to determine its mediation. In the rat, the course of prolonged sleep deprivation has a syndromic nature and eventuates in a life-threatening state. An early and central symptom of sleep deprivation is a progressive increase in peripheral energy expenditure to nearly double normal levels. An attempt to alleviate this negative energy balance by feeding rats a balanced diet that is high in its efficiency of utilization prolongs survival and attenuates or delays development of malnutrition-like symptoms, indicating that several symptoms can be manipulated to some extent by energy and nutrient consumption. Most changes in neuroendocrine parameters appear to be responses to metabolic demands, such as increased plasma catecholamines indicating sympathetic activation. Plasma total thyroid hormones, however, decline to severely low levels; a metabolic complication that is associated with other sleep deprivation-induced symptoms, such as a decline in body temperature to hypothermic levels despite increased energy expenditure. Metabolic mapping of the brain revealed a dissociation between the energy metabolism of the brain and that of the body. Sleep deprivation's effects on cerebral structures are heterogeneous and unidirectional toward decreased functional activity. The hypometabolic brain structures are concentrated in the hypothalamus, thalamus and limbic systems, whereas few regions in the rest of the brain and none in the medulla, are affected. Correspondence can be found between some of the affected cerebral structures and several of the peripheral symptoms, such as hyperphagia and possible heat retention problems. The factor predisposing to mortality is a decreased resistance to infection. Lethal opportunistic organisms are permitted to infect the bloodstream, which presumably results in a cascade of toxic-like reactions. Host defense is thus the first system to fail. There is neither fever nor marked tissue inflammatory reactions typical of infectious disease states, suggesting that sleep deprivation is immunosuppressive. Each of the four abnormalities identified--(1) a deep negative energy balance and associated malnutrition; (2) heterogeneous decreases in cerebral function; (3) low thyroid hormone concentrations; and (4) decrease resistance to infection--can be viewed as having an early origin during the sleep deprivation process to signify the foremost pathogenic situation to which the other abnormalities might be secondarily related.(ABSTRACT TRUNCATED AT 400 WORDS)

Sleep and waking have a major effect on the 24-hr rhythm of cortical temperature in the rat

The relationship between the time course of cortical temperature (TCRT) and sleep-wake alternation was investigated by correlation analyses and a computer simulation. The data for these analyses were collected in 10 rats in a 4-day experiment (LD 12:12), during which vigilance states and TCRT were determined for consecutive 8-sec epochs. On day 1 baseline recordings were obtained; on day 2 the animals were sleep-deprived; and days 3 and 4 served as recovery days. The correlation analyses revealed that the alternation of sleep and waking accounted for 84% of the variance of TCRT when analyzed for hourly intervals. The residual variance displayed a 24-hr periodicity with an amplitude of 0.15 degrees C. Similar results were obtained in a separate data set of a 2-day experiment, which consisted of a baseline day (LD 12:12) and a day with constant darkness. The periodicity of the residual variance of TCRT can therefore be considered to represent the circadian temperature rhythm not masked by the vigilance states. In the computer simulation, the time course of TCRT was simulated on the basis of the sequence of the vigilance states with an 8-sec time resolution. It was assumed that TCRT increases during waking and rapid-eye-movement (REM) sleep according to an exponential saturating function, and decreases exponentially during non-REM sleep. The simulations could account for 88-93% of the variance of TCRT. We conclude that in the rat, the major part of the variation of TCRT is accounted for by vigilance states, whereas a minor part can be attributed to a direct effect of the circadian pacemaker.

Cortical temperature and EEG slow-wave activity in the rat: analysis of vigilance state related changes

Vigilance states, cortical temperature (TCRT), and electroencephalograph (EEG) slow-wave-activity (SWA, mean power density in the 0.75-4.0 Hz range) of ten rats were recorded continuously during a baseline day, and two recovery days (Recovery 1 and 2) after 24 h of sleep deprivation (SD). The short term changes of TCRT were analysed within episodes of nonrapid eye movement sleep (NREMS), REM sleep (REMS) and waking (W), and at transitions between vigilance states. SWA was analysed within NREMS episodes and at W to NREMS (WN) transitions. TCRT increased during episodes of W and REMS, and decreased during NREMS episodes. These changes were a function of episode duration, and, for W and NREMS, of TCRT at episode onset. In Recovery 1 the increase in TCRT at NREMS to REMS (NR) and NREMS to W (NW) transitions tended to be attenuated. SWA within NREMS episodes was enhanced after SD. Over all experimental days, the increase of SWA and the decrease of TCRT in NREMS episodes were not correlated. It is concluded that during recovery from SD the changes in TCRT at state transitions were little affected. The lack of a relationship between changes in TCRT and SWA indicates that separate mechanisms underlie the regulation of brain temperature and sleep intensity.