Preoptic/anterior hypothalamic warming increases EEG delta frequency activity within non-rapid eye movement sleep

Sleep and Core Body Temperature Alterations Induced by Space Radiation in Rats

Sleep problems in astronauts can arise from mission demands and stress and can impact both their health and ability to accomplish mission objectives. In addition to mission-related physical and psychological stressors, the long durations of the proposed Mars missions will expose astronauts to space radiation (SR), which has a significant impact on the brain and may also alter sleep and physiological functions. Therefore, in this study, we assessed sleep, EEG spectra, activity, and core body temperature (CBT) in rats exposed to SR and compared them to age-matched nonirradiated rats. Male outbred Wistar rats (8-9 months old at the time of the study) received SR (15 cGy GCRsim, n = 15) or served as age- and time-matched controls (CTRL, n = 15) without irradiation. At least 90 days after SR and 3 weeks prior to recording, all rats were implanted with telemetry transmitters for recording EEG, activity, and CBT. Sleep, EEG spectra (delta, 0.5-4 Hz; theta, 4-8 Hz; alpha, 8-12 Hz; sigma, 12-16 Hz; beta, 16-24 Hz), activity, and CBT were examined during light and dark periods and during waking and sleeping states. When compared to the CTRLs, SR produced significant reductions in the amounts of dark period total sleep time, total nonrapid eye movement sleep (NREM), and total rapid eye movement sleep (REM), with significant decreases in light and dark period NREM deltas and dark period REM thetas as well as increases in alpha and sigma in NREM and REM during either light or dark periods. The SR animals showed modest increases in some measures of activity. CBT was significantly reduced during waking and sleeping in the light period. These data demonstrate that SR alone can produce alterations to sleep and temperature control that could have consequences for astronauts and their ability to meet mission demands.

How Temperature Influences Sleep

Sleep is a fundamental, evolutionarily conserved, plastic behavior that is regulated by circadian and homeostatic mechanisms as well as genetic factors and environmental factors, such as light, humidity, and temperature. Among environmental cues, temperature plays an important role in the regulation of sleep. This review presents an overview of thermoreception in animals and the neural circuits that link this process to sleep. Understanding the influence of temperature on sleep can provide insight into basic physiologic processes that are required for survival and guide strategies to manage sleep disorders.

Role of the Preoptic Area in Sleep and Thermoregulation

Sleep and body temperature are tightly interconnected in mammals: warming up our body helps to fall asleep and the body temperature in turn drops while falling asleep. The preoptic area of the hypothalamus (POA) serves as an essential brain region to coordinate sleep and body temperature. Understanding how these two behaviors are controlled within the POA requires the molecular identification of the involved circuits and mapping their local and brain-wide connectivity. Here, we review our current understanding of how sleep and body temperature are regulated with a focus on recently discovered sleep- and thermo-regulatory POA neurons. We further discuss unresolved key questions including the anatomical and functional overlap of sleep- and thermo-regulatory neurons, their pathways and the role of various signaling molecules. We suggest that analysis of genetically defined circuits will provide novel insights into the mechanisms underlying the coordinated regulation of sleep and body temperature in health and disease.

Thermoregulation and Sleep: Functional Interaction and Central Nervous Control

Each of the wake-sleep states is characterized by specific changes in autonomic activity and bodily functions. The goal of such changes is not always clear. During non-rapid eye movement (NREM) sleep, the autonomic outflow and the activity of the endocrine system, the respiratory system, the cardiovascular system, and the thermoregulatory system seem to be directed at increasing energy saving. During rapid eye movement (REM) sleep, the goal of the specific autonomic and regulatory changes is unclear, since a large instability of autonomic activity and cardiorespiratory function is observed in concomitance with thermoregulatory changes, which are apparently non-functional to thermal homeostasis. Reciprocally, the activation of thermoregulatory responses under thermal challenges interferes with sleep occurrence. Such a double-edged and reciprocal interaction between sleep and thermoregulation may be favored by the fact that the central network controlling sleep overlaps in several parts with the central network controlling thermoregulation. The understanding of the central mechanism behind the interaction between sleep and thermoregulation may help to understand the functionality of thermoregulatory sleep-related changes and, ultimately, the function(s) of sleep. © 2021 American Physiological Society. Compr Physiol 11:1591-1604, 2021.

Sleep and thermoregulation

In homeothermic animals sleep preparatory behaviours often promote thermal efficiency, including warmth-seeking, adopting particular postures (curling up, head tucking) and nest building, all promoting warmer skin microclimates. Skin warmth induces NREM sleep and body cooling via circuitry that connects skin sensation to the preoptic hypothalamus. Coupling sleep induction and lower body temperature could serve to minimise energy expenditure or allow energy reallocation. Cooling during NREM sleep may also induce transcriptional changes in genes whose products facilitate housekeeping functions or measure the time spent sleeping.

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.

Body temperature and sleep

Sleep in mammals is accompanied by a decrease in core body temperature (CBT). The circadian clock in the hypothalamic suprachiasmatic nucleus regulates daily rhythms in both CBT and arousal states, and these rhythms are normally coupled. Reductions in metabolic heat production resulting from behavioral quiescence and reduced muscle tone along with changes in autonomic nervous system activity and thermoeffector activity contribute to the sleep-related fall in CBT. Reductions in sympathetic tone to the peripheral vasculature resulting in heat loss through the skin are reflected in a sleep-related increase in distal skin temperature that is a prominent feature of sleep onset in humans. Within a sleep episode, patterns of autonomic nervous system and thermoeffector activity and the ability to defend against heat and cold exposure differ during nonrapid eye movement (NREM) and rapid eye movement sleep. Anatomic and functional integration of the control of arousal states and thermoregulation occur in the preoptic/anterior hypothalamus. Subsets or warm-sensing neurons in the preoptic/anterior hypothalamus implicated in CBT regulation are spontaneously activated during sleep onset and NREM sleep compared to waking and may underlie sleep-related changes in autonomic nervous system and thermoeffector activity.

Wheel running improves REM sleep and attenuates stress-induced flattening of diurnal rhythms in F344 rats

Regular physical activity produces resistance to the negative health consequences of stressor exposure. One way that exercise may confer stress resistance is by reducing the impact of stress on diurnal rhythms and sleep; disruptions of which contribute to stress-related disease including mood disorders. Given the link between diurnal rhythm disruptions and stress-related disorders and that exercise both promotes stress resistance and is a powerful non-photic biological entrainment cue, we tested if wheel running could reduce stress-induced disruptions of sleep/wake behavior and diurnal rhythms. Adult, male F344 rats with or without access to running wheels were instrumented for biotelemetric recording of diurnal rhythms of locomotor activity, heart rate, core body temperature (CBT), and sleep (i.e. REM, NREM, and WAKE) in the presence of a 12 h light/dark cycle. Following 6 weeks of sedentary or exercise conditions, rats were exposed to an acute stressor known to disrupt diurnal rhythms and produce behaviors associated with mood disorders. Prior to stressor exposure, exercise rats had higher CBT, more locomotor activity during the dark cycle, and greater %REM during the light cycle relative to sedentary rats. NREM and REM sleep were consolidated immediately following peak running to a greater extent in exercise, compared to sedentary rats. In response to stressor exposure, exercise rats expressed higher stress-induced hyperthermia than sedentary rats. Stressor exposure disrupted diurnal rhythms in sedentary rats; and wheel running reduced these effects. Improvements in sleep and reduced diurnal rhythm disruptions following stress could contribute to the health promoting and stress protective effects of exercise.

A Path to Sleep Is through the Eye

Sleep is expressed as a circadian rhythm and the two phenomena exist in a poorly understood relationship. Light affects each, simultaneously influencing rhythm phase and rapidly inducing sleep.

Light has long been known to modulate sleep, but recent discoveries support its use as an effective nocturnal stimulus for eliciting sleep in certain rodents. “Photosomnolence” is mediated by classical and ganglion cell photoreceptors and occurs despite the ongoing high levels of locomotion at the time of stimulus onset. Brief photic stimuli trigger rapid locomotor suppression, sleep, and a large drop in core body temperature (Tc; Phase 1), followed by a relatively fixed duration interval of sleep (Phase 2) and recovery (Phase 3) to pre-sleep activity levels. Additional light can lengthen Phase 2. Potential retinal pathways through which the sleep system might be light-activated are described and the potential roles of orexin (hypocretin) and melanin-concentrating hormone are discussed. The visual input route is a practical avenue to follow in pursuit of the neural circuitry and mechanisms governing sleep and arousal in small nocturnal mammals and the organizational principles may be similar in diurnal humans. Photosomnolence studies are likely to be particularly advantageous because the timing of sleep is largely under experimenter control. Sleep can now be effectively studied using uncomplicated, nonintrusive methods with behavior evaluation software tools; surgery for EEG electrode placement is avoidable. The research protocol for light-induced sleep is easily implemented and useful for assessing the effects of experimental manipulations on the sleep induction pathway. Moreover, the experimental designs and associated results benefit from a substantial amount of existing neuroanatomical and pharmacological literature that provides a solid framework guiding the conduct and interpretation of future investigations.

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.

Brief light stimulation during the mouse nocturnal activity phase simultaneously induces a decline in core temperature and locomotor activity followed by EEG-determined sleep

Light exerts a variety of effects on mammals. Unexpectedly, one of these effects is the cessation of nocturnal locomotion and the induction of behavioral sleep (photosomnolence). Here, we extend the initial observations in several ways, including the fundamental demonstration that core body temperature (T(c)) drops substantially (about 1.5°C) in response to the light stimulation at CT15 or CT18 in a manner suggesting that the change is a direct response to light rather than simply a result of the locomotor suppression. The results show that 1) the decline of locomotion and T(c) begin soon after nocturnal light stimulation; 2) the variability in the magnitude and onset of light-induced locomotor suppression is very large, whereas the variability in T(c) is very small; 3) T(c) recovers from the light-induced decline in advance of the recovery of locomotion; 4) under entrained and freerunning conditions, the daily late afternoon T(c) increase occurs in advance of the corresponding increase in wheel running; and 5) toward the end of the subjective night, the nocturnally elevated T(c) persists longer than does locomotor activity. Finally, EEG measurements confirm light-induced sleep and, when T(c) or locomotion was measured, show their temporal association with sleep onset. Both EEG- and immobility-based sleep detection methods confirm rapid induction of light-induced sleep. The similarities between light-induced loss of locomotion and drop in T(c) suggest a common cause for parallel responses. The photosomnolence response may be contingent upon both the absence of locomotion and a simultaneous low T(c).

Control of sleep and wakefulness in health and disease

Sleep and wake are actively promoted states of consciousness that are dependent on a network of state-modulating neurons arising from both the brain stem and hypothalamus. This network helps to coordinate the occurrence of a sleep state in billions of cortical neurons. In many neurological diseases, there is a specific disruption to one of the components of this network. Under conditions of such disruptions, we often gain an improved understanding of the underlying function of the specific component under nonpathological conditions. The loss or dysfunction of one of the hypothalamic or brain stem regions that are responsible for promotion of sleep or wake can lead to disruptions in sleep and wake states that are often subtle, but sometime quite pronounced and of significant medical importance. By understanding the neural substrate and its pathophysiology, one can more appropriately target therapies that might help the specific sleep disruption. This chapter reviews what is currently understood about the neurobiological underpinnings of sleep and wake regulation and how various pathologies evoke changes in these regulatory mechanisms.

EEG delta oscillations as a correlate of basic homeostatic and motivational processes

Functional significance of delta oscillations is not fully understood. One way to approach this question would be from an evolutionary perspective. Delta oscillations dominate the EEG of waking reptiles. In humans, they are prominent only in early developmental stages and during slow-wave sleep. Increase of delta power has been documented in a wide array of developmental disorders and pathological conditions. Considerable evidence on the association between delta waves and autonomic and metabolic processes hints that they may be involved in integration of cerebral activity with homeostatic processes. Much evidence suggests the involvement of delta oscillations in motivation. They increase during hunger, sexual arousal, and in substance users. They also increase during panic attacks and sustained pain. In cognitive domain, they are implicated in attention, salience detection, and subliminal perception. This evidence shows that delta oscillations are associated with evolutionary old basic processes, which in waking adults are overshadowed by more advanced processes associated with higher frequency oscillations. The former processes rise in activity, however, when the latter are dysfunctional.

Interleukin-1 inhibits putative cholinergic neurons in vitro and REM sleep when microinjected into the rat laterodorsal tegmental nucleus

STUDY OBJECTIVES

REM sleep is suppressed during infection, an effect mimicked by the administration of cytokines such as interleukin-1 (IL-1). In spite of this observation, brain sites and neurochemical systems mediating IL-1-induced suppression of REM sleep have not been identified. Cholinergic neurons in the brainstem laterodorsal tegmental nucleus (LDT) are part of the neuronal circuitry responsible for REM sleep generation. Since IL-1 inhibits acetylcholine synthesis and release, the aim of this study was to test the two different, but related hypotheses. We hypothesized that IL-1 inhibits LDT cholinergic neurons, and that, as a result of this inhibition, IL-1 suppresses REM sleep.

DESIGN, MEASUREMENT, AND RESULTS

To test these hypotheses, the electrophysiological activity of putative cholinergic LDT neurons was recorded in a rat brainstem slice preparation. Interleukin-1 significantly inhibited the firing rate of 76% of recorded putative cholinergic LDT neurons and reduced the amplitude of glutamatergic evoked potentials in 60% of recorded neurons. When IL-1 (1 ng) was microinjected into the LDT of freely behaving rats, REM sleep was reduced by about 50% (from 12.7% +/- 1.5% of recording time [after vehicle] to 6.1% +/- 1.4% following IL-1 administration) during post-injection hours 3-4.

CONCLUSIONS

Results of this study support the hypothesis that IL-1 can suppress REM sleep by acting at the level of the LDT nucleus. Furthermore this effect may result from the inhibition of evoked glutamatergic responses and of spontaneous firing of putative cholinergic LDT neurons.

Hypothalamic regulation of sleep and arousal

Normal waking is associated with neuronal activity in several chemically defined ascending arousal systems. These include monoaminergic neurons in the brainstem and posterior hypothalamus, cholinergic neurons in the brainstem and basal forebrain, and hypocretin (orexin) neurons in the lateral hypothalamus. Collectively, these systems impart tonic activation to their neuronal targets in the diencephalon and neocortex that is reflected in the low-voltage fast-frequency electroencephalogram patterns of wakefulness. Neuronal discharge in these arousal systems declines rapidly at sleep onset. Transitions from waking to sleep, therefore, involve coordinated inhibition of multiple arousal systems. An important source of sleep-related inhibition of arousal arises from neurons located in the preoptic hypothalamus. These preoptic neurons are strongly activated during sleep, exhibiting sleep/waking state-dependent discharge patterns that are the reciprocal of that observed in the arousal systems. The majority of preoptic sleep regulatory neurons synthesize the inhibitory neurotransmitter GABA. Anatomical and functional evidence supports the hypothesis that GABAergic neurons in the median preoptic nucleus (MnPN) and ventrolateral preoptic area (VLPO) exert inhibitory control over the monoaminergic systems and the hypocretin system during sleep. Recent findings indicate that MnPN and VLPO neurons integrate homeostatic aspects of sleep regulation and are important targets for endogenous sleep factors, such as adenosine and growth hormone releasing hormone.

Persistence of sleep-temperature coupling after suprachiasmatic nuclei lesions in rats

The suprachiasmatic nucleus (SCN) regulates the circadian rhythms of body temperature (Tb) and vigilance states in mammals. We studied rats in which circadian rhythmicity was abolished after SCN lesions (SCNx rats) to investigate the association between the ultradian rhythms of sleep-wake states and brain temperature (Tbr), which are exposed after lesions. Ultradian rhythms of Tbr (mean period: 3.6 h) and sleep were closely associated in SCNx rats. Within each ultradian cycle, nonrapid eye movement (NREM) sleep was initiated 5 ± 1 min after Tbr peaks, after which temperature continued a slow decline (0.02 ± 0.006°C/min) until it reached a minimum. Sleep and slow wave activity (SWA), an index of sleep intensity, were associated with declining temperature. Cross-correlation analysis revealed that the rhythm of Tbr preceded that of SWA by 2–10 min. We also investigated the thermoregulatory and sleep-wake responses of SCNx rats and controls to mild ambient cooling (18°C) and warming (30°C) over 24-h periods. SCNx rats and controls responded similarly to changes in ambient temperature. Cooling decreased REM sleep and increased wake. Warming increased Tbr, blunted the amplitude of ultradian Tbr rhythms, and increased the number of transitions into NREM sleep. SCNx rats and controls had similar percentages of NREM sleep, REM sleep, and wake, as well as the same average Tb within each 24-h period. Our results suggest that, in rats, the SCN modulates the timing but not the amount of sleep or the homeostatic control of sleep-wake states or Tb during deviations in ambient temperature.

Retrograde double-labeling study of common afferent projections to the dorsal raphe and the nuclear core of the locus coeruleus in the rat

Common afferent projections to the dorsal raphe (DR) and locus coeruleus (LC) nuclei were analyzed in the rat by making paired injections of retrograde tracers, gold-conjugated and inactivated wheatgerm agglutinin-horseradish peroxidase (WGA-apo-HRP-gold) and Fluorogold (FG), into the DR and the nuclear core of the LC. Our results demonstrate that the largest number of double-labeled neurons was located at various preoptic regions including medial preoptic area, lateral preoptic nucleus, and ventrolateral preoptic nucleus. The majority of labeled cells were also observed at the lateral hypothalamus, where the number of labeled cells was comparable to that of neurons at the medial preoptic area or lateral preoptic nucleus. A few double-labeled cells were observed at various hypothalamic regions including anterior, medial tuberal, posterior, and arcuate nuclei, as well as mesencephalic areas including substantia nigra compacta and ventrolateral/lateral periaqueductal gray matter. Cells were also observed at prelimbic/infralimbic prefrontal cortices, diagonal band of Broca, bed nucleus of stria terminalis, and pontine/medullary regions including various raphe nuclei, Barrington's nucleus, gigantocellularis, paragigantocellularis, prepositus hypoglossi, subcoeruleus, and dorsomedial tegmental area. Although electrophysiological studies need to be performed, a large number of double-labeled neurons located at preoptic regions as well as lateral hypothalamus might have their major role in simultaneous control over these monoaminergic nuclei as a means of influencing various sleep and arousal states of the animal. Double-labeled cells at the other locations might be positioned to influence a variety of other functions such as analgesia, cognition, and stress responses.

Interleukin-1beta modulates state-dependent discharge activity of preoptic area and basal forebrain neurons: role in sleep regulation

Interleukin-1beta (IL-1) is a pro-inflammatory cytokine that has been implicated in the regulation of nonrapid eye movement (nonREM) sleep. IL-1, IL-1 receptors and the IL-1 receptor antagonist (ra) are present normally in discrete brain regions, including the preoptic area (POA) of the hypothalamus and the adjoining magnocellular basal forebrain (BF). The POA/BF have been implicated in the regulation of sleep-wakefulness. We hypothesized that IL-1 promotes nonREM sleep, in part by altering the state-dependent discharge activity of POA/BF neurons. We recorded the sleep-wake discharge profiles of 83 neurons in the lateral POA/BF and assessed the effects of IL-1, IL-1ra, and IL-ra + IL-1 delivered through a microdialysis probe on state-dependent neuronal discharge activity. IL-1 decreased the discharge rate of POA/BF neurons as a group (n = 55) but wake-related and sleep-related neurons responded differently. IL-1 significantly decreased the discharge rate of wake-related neurons. Of 24 wake-related neurons studied, 19 (79%) neurons exhibited a greater than 20% change in their discharge in the presence of IL-1 during waking. IL-1 suppressed the discharge activity of 18 of 19 responsive neurons. Of 13 sleep-related neurons studied, IL-1 increased the discharge activity of five and suppressed the discharge activity of four neurons. IL-1ra increased the discharge activity of four of nine neurons and significantly attenuated IL-1-induced effects on neuronal activity of POA/BF neurons (n = 19). These results suggest that the sleep-promoting effects of IL-1 may be mediated, in part, via the suppression of wake-related neurons and the activation of a subpopulation of sleep-related neurons in the POA/BF.

Preoptic area warming inhibits wake-active neurons in the perifornical lateral hypothalamus

Activation of the preoptic area (POA) warm sensitive neurons is known to promote non-REM (NREM) sleep and inhibit neuronal discharge in arousal-related brain structures. The perifornical area of the lateral hypothalamus (PF/LH) was recently recognized to be an additional important arousal promoting region. We studied the behavior of PF/LH neurons in rats during the normal sleep-wake cycle and in response to local POA warming. Most PF/LH neurons were wake-active, and exhibited low discharge throughout NREM. Seventy four percent of these wake-active neurons exhibited moderate or strong activation in REM sleep compared to NREM sleep. A substantial group (26%) exhibited very low discharge in REM as well as NREM sleep. Fifty two percent of units in the PF/LH area were responsive to POA warming; 90% of responsive neurons exhibited a significant reduction (-26.47+/-2.16% for 1 degrees C of POA warming) in their discharge rate. The inhibitory effect of POA warming on PF/LH neurons was not associated with EEG slowing. This study supports the hypothesis that sleep induction by POA warm sensitive neurons is mediated through the inhibition of multiple arousal-related structures.

Afferents to the ventrolateral preoptic nucleus

Sleep is influenced by diverse factors such as circadian time, affective states, ambient temperature, pain, etc., but pathways mediating these influences are unknown. To identify pathways that may influence sleep, we examined afferents to the ventrolateral preoptic nucleus (VLPO), an area critically implicated in promoting sleep. Injections of the retrograde tracer cholera toxin B subunit (CTB) into the VLPO produced modest numbers of CTB-labeled monoaminergic neurons in the tuberomammillary nucleus, raphe nuclei, and ventrolateral medulla, as well as a few neurons in the locus coeruleus. Immunohistochemistry for monoaminergic markers showed dense innervation of the VLPO by histaminergic, noradrenergic, and serotonergic fibers. Along with previous findings, these results suggest that the VLPO and monoaminergic nuclei may be reciprocally connected. Retrograde and anterograde tracing showed moderate or heavy inputs to the VLPO from hypothalamic regions including the median preoptic nucleus, lateral hypothalamic area, and dorsomedial hypothalamic nucleus (DMH), autonomic regions including the infralimbic cortex and parabrachial nucleus, and limbic regions including the lateral septal nucleus and ventral subiculum. Light to moderate inputs arose from orexin and melanin concentrating hormone neurons, but cholinergic or dopaminergic inputs were extremely sparse. Suprachiasmatic nucleus (SCN) projections to the VLPO were sparse, but the heavy input to the VLPO from the DMH, which receives direct and indirect SCN inputs, could provide an alternate pathway regulating the circadian timing of sleep. These robust pathways suggest candidate mechanisms by which sleep may be influenced by brain systems regulating arousal, autonomic, limbic, and circadian functions.

Band-specific electroencephalogram and brain cooling abnormalities during NREM sleep in patients with winter depression

BACKGROUND

We previously reported that delta wave activity and facial skin temperatures, an index of brain cooling activity, were both abnormal during sleep in patients with winter depression (SAD). Because other electroencephalographic (EEG) frequencies may also convey relevant thermal, homeostatic, and circadian information, we sought to spectrally analyze delta, theta, alpha, and sigma frequencies during sleep from 23 patients with SAD and 23 healthy control subjects.

METHODS

We computed means for delta, theta, alpha, and sigma power during both NREM and REM sleep. We also generated 22 cross-correlation functions for each group by crossing facial and rectal temperature with each other, as well as with delta, theta, alpha, and sigma frequencies.

RESULTS

We found that delta, theta, and alpha frequency activities were all increased during NREM, but not REM sleep, in patients with SAD. In addition, there were significant and abnormal cross-correlations between facial temperatures and delta and theta frequencies during NREM sleep in patients with SAD.

CONCLUSIONS

Patients with winter depression exhibit correlated abnormalities of sleep homeostasis and brain cooling during NREM sleep. Their EEG profiles during NREM sleep resemble the EEG profiles of subjects who have been sleep deprived. Further studies of NREM sleep homeostasis in patients with SAD seem warranted.

Brain structures and mechanisms involved in the generation of NREM sleep: focus on the preoptic hypothalamus

Four lines of research have greatly increased our understanding of the hypothalamic preoptic area (POA) sleep-promoting system. First, sleep-active neurons within the POA have been identified using both electrophysiological recording and immediate early gene protein (c-Fos) staining methods. Segregated sleep-active neurons were found in ventrolateral and median POA (VLPO and MnPN). Additional sleep-active neurons may be intermixed with non-sleep specific neurons in other POA regions and the adjacent basal forebrain. Second, the putative sleep factors, adenosine and prostaglandin D2, were found to excite sleep-active neurons. Other sleep factors may also modulate these sleep-active populations. Third, many sleep-active neurons are warm-sensitive neurons (WSNs). WSNs are identified by excitatory responses to small increases in local POA temperature. The same local POA thermal stimuli strongly modulate sleep propensity and EEG delta activity within sleep. Interactions between sleep regulation and thermoregulation are consistent with studies of circadian sleep propensity, prolonged sleep deprivation in rats, and species differences in sleep amounts. Fourth, sleep-active neurons were found to co-localize the inhibitory neurotransmitter, gamma-aminobutyric acid and to have projections to arousal-related neuronal subgroups in the posterior hypothalamus and midbrain. Sleep-active and arousal-related neurons exhibit reciprocal changes in discharge across the wake-NREM-REM cycle, and activation of WSNs suppresses the neuronal activity of some arousal-related neuronal groups. These studies establish mechanisms by which POA hypnogenic neurons can inhibit EEG and behavioral arousal. In addition, there is evidence that arousal-related neurotransmitters inhibit VLPO sleep-active neurons. Mutually inhibitory interactions between sleep-promoting and the arousal system provide a substrate for a <<<<sleep-wake switch>>>>. 2001 Harcourt Publishers Ltd

Sleep-related c-Fos protein expression in the preoptic hypothalamus: effects of ambient warming

Preoptic area (POA) neuronal activity promotes sleep, but the localization of critical sleep-active neurons is not completely known. Thermal stimulation of the POA also facilitates sleep. This study used the c-Fos protein immunostaining method to localize POA sleep-active neurons at control (22 degrees C) and mildly elevated (31.5 degrees C) ambient temperatures. At 22 degrees C, after sleep, but not after waking, we found increased numbers of c-Fos immunoreactive neurons (IRNs) in both rostral and caudal parts of the median preoptic nucleus (MnPN) and in the ventrolateral preoptic area (VLPO). In animals sleeping at 31.5 degrees C, significantly more Fos IRNs were found in the rostral MnPN compared with animals sleeping at 22 degrees C. In VLPO, Fos IRN counts were no longer increased over waking levels after sleep at the elevated ambient temperature. Sleep-associated Fos IRNs were also found diffusely in the POA, but counts were lower than those made after waking. This study supports a hypothesis that the MnPN, as well as the VLPO, is part of the POA sleep-facilitating system and that the rostral MnPN may facilitate sleep, particularly at elevated ambient temperatures.

Discharge patterns of neurons in cholinergic regions of the basal forebrain during waking and sleep

A subset of neurons recorded in the magnocellular basal forebrain (mBF) of cats and rats exhibit elevated discharge rates during waking and REM sleep, and diminished discharge during sleep with cortical EEG synchrony (nonREM sleep). This pattern is observed in mBF neurons in cats with identified ascending projections, and in neurons located in cholinergic regions of the rat mBF. However, the cholinergic versus noncholinergic nature of recorded cells could not be determined with the extracellular recording method employed. During waking, discharge of mBF neurons is strongly movement-related. Peak discharge rates occur during a variety of head and limb movements. Discharge rates during waking immobility are reduced by >50% compared to rates during waking movement. The absence of movement accounts for more of the variance in discharge across the sleep-wake cycle than does the presence of cortical EEG synchronization. Several factors participate in the regulation of mBF neuronal activity across arousal states. Tonic inhibition mediated by adenosine appears to be present during both waking and sleep. In some mBF neurons, increased GABAergic inhibition contributes to nonREM sleep-related reductions in discharge rate. Fluctuations in mBF cell activity during waking behaviors may reflect changing excitatory input from neurons in the pontine and midbrain tegmentum.

Discharge modulation of rat dorsal raphe neurons during sleep and waking: effects of preoptic/basal forebrain warming

In cats, putative serotonergic neurons (PSNs) recorded from the dorsal raphe nucleus (DRN) across the sleep-wake cycle exhibit the so-called rapid eye movement sleep-off (REM-off) discharge pattern. Since, the sleep-wake discharge patterns of DRN neurons in behaving rats is poorly known, the present study examined this neuronal populations. The PSNs recorded in this study exhibited: (1) progressive decrease in discharge rate from waking to NREM to REM sleep; (2) long action potential duration, and (3) reduction of discharge rate after systemic administration of a selective 5-HT(1A) agonist, (+/-)-8-hydroxy-2-(di-n-propylamino) tetralin hydrobromide (8-OH-DPAT). Evidence supports the hypothesis that NREM sleep is modulated by thermoregulatory mechanisms localized in the preoptic area and adjacent basal forebrain (POA/BF). We previously reported that POA/BF warming suppresses the discharge of wake-promoting neurons in the posterior hypothalamus and the basal forebrain. Since the DRN is one component of the brainstem arousal system and receives projections from POA/BF, we examined the effects of local POA/BF warming by 1.5-2.0 degrees C during waking on the discharge of DRN neurons. POA/BF warming reduced the discharge in 14 of 19 PSNs and in 12 of 17 other wake-related neurons in the DRN. DRN neuronal discharge reduction occurred without accompanying EEG frequency or behavioral changes. These results suggest that PSNs recorded in DRN in unrestrained and unanesthetized rats exhibit a "wake-active REM-off" discharge pattern and further support the hypothesis that the POA/BF warm-sensitive hypnogenic system induces sleep by a coordinated inhibition of multiple arousal systems including that modulated by the DRN.

Ultradian oscillations in cranial thermoregulation and electroencephalographic slow-wave activity during sleep are abnormal in humans with annual winter depression

The level of core body, and presumably brain temperature during sleep varies with clinical state in patients with seasonal affective disorder (SAD), becoming elevated during winter depression and lowered during clinical remission induced by either light treatment or summer. During sleep, brain temperatures are in part determined by the level of brain cooling activity, which may be reflected by facial skin temperatures. In many animals, the level of brain cooling activity oscillates across the NREM-REM sleep cycle. Facial skin temperatures during sleep in patients with winter depression are abnormally low and uncorrelated with rectal temperatures, although their relationship to EEG-defined sleep stages remains unknown. We therefore measured the sleep EEG, core body and facial skin temperatures in 23 patients with winter depression and 23 healthy controls, and tested the hypothesis that ultradian oscillations in facial skin temperatures exist in humans and are abnormal in patients with winter depression. We found that facial skin temperatures oscillated significantly across the NREM-REM sleep cycle, and were again significantly lower and uncorrelated with rectal temperatures in patients with winter depression. Mean slow-wave activity and NREM episode duration were significantly greater in patients with winter depression, whereas the intraepisodic dynamics of slow-wave activity were normal in patients with winter depression. These results suggest that brain cooling activity oscillates in an ultradian manner during sleep in humans and is reduced during winter depression, and provide additional support for the hypothesis that brain temperatures are elevated during winter depression.

Effect of lesions of the ventrolateral preoptic nucleus on NREM and REM sleep

Neurons in the ventrolateral preoptic nucleus (VLPO) in rats show c-fos activation after sleep and provide GABAergic innervation of the major monoamine arousal systems, suggesting that they may be a necessary part of the brain circuitry that produces sleep. We examined the effects on sleep behavior in rats of cell-specific damage to the VLPO by microinjection of ibotenic acid. Severe lesions of the central cell cluster of the VLPO ( approximately 80-90% cell loss bilaterally) caused a 60-70% decrease in delta power and a 50-60% decrease in nonrapid-eye-movement (NREM) sleep time (p < 0.001). The number of remaining Fos-immunoreactive neurons in the VLPO cell cluster was linearly related to NREM sleep time (r = 0.77; p < 0.001) and total electroencephalogram delta power (r = 0. 79; p < 0.001) but not to rapid-eye-movement (REM) sleep (r = 0.35; p > 0.10). Lesions in the region containing scattered VLPO neurons medial or dorsal to the cell cluster caused smaller changes in NREM sleep time (24.5 or 15%, respectively) but were more closely associated with loss of REM sleep (r = 0.74; p < 0.01). The insomnia caused by bilateral VLPO lesions persisted for at least 3 weeks. Lesions of the VLPO caused no change in mean body temperature or its circadian variation; after small lesions of the ventromedial preoptic nucleus, body temperature showed normal circadian variation but a wider temperature range, and sleep behavior was not affected. These experiments delineate distinct preoptic sites with primary effects on the regulation of NREM sleep, REM sleep, and body temperature.

Central administration of the somatostatin analog octreotide induces captopril-insensitive sleep responses

The effects of intracerebroventricular injections of the long-lasting somatostatin analog octreotide (Oct) were studied on sleep and behavior in rats. Pyrogen-free physiological saline and Oct (0.001, 0.01, 0.1 microgram) or vehicle were administered at light onset, and the electroencephalogram (EEG), motor activity, and cortical brain temperature were recorded during the 12-h light period. Plasma growth hormone (GH) concentrations were measured in samples taken at 30-min intervals after Oct. Oct (0.01 and 0.1 microgram) suppressed non-rapid eye movement sleep (NREMS) for 1-2 h. NREMS intensity (delta EEG activity during NREMS) dose dependently increased in hour 3 postinjection and thereafter (0.1 microgram). Plasma GH concentrations were suppressed after Oct (0.01 and 0.1 microgram), but pulses of GH secretions occurred 90-120 min postinjection in each rat. Oct (0.1 microgram) enhanced behavioral activity, including prompt drinking followed by grooming, scratching, and feeding. Intracerebroventricular injection of the angiotensin-converting enzyme inhibitor captopril (30 microgram, 10 min before Oct), blocked these behavioral responses but not the Oct-induced sleep alterations. The changes in sleep after intracerebroventricular Oct suggest an intracerebral action site and might result from Oct-induced variations in the sleep-promoting activity of GH-releasing hormone.

GABAA receptor modulation of temperature sensitive neurons in the diagonal band of Broca in vitro

Several regions of the brain, including the horizontal limb of the diagonal band of Broca (HDB), contain neurons that are responsive to changes in local temperature. These neurons are hypothesized to participate in thermoregulation and sleep-wake control. The HDB contains a large number of gamma-aminobutyric acid (GABA) terminals, and it has many neurons that utilize GABA as a neurotransmitter. Therefore, in this study we characterized the in vitro effects of the GABAA receptor agonist muscimol (0.5, 0.25, 0.1 and 0.0625 microM doses) and the GABAA receptor antagonist bicuculline (3.0 and 1.0 microM doses) on the firing rate and thermosensitivity of HDB neurons. Of the 51 neurons recorded in a submerged slice chamber, 53% were warm sensitive, 45% were temperature insensitive and 2% were cold sensitive. All neurons exposed to bath applied muscimol exhibited reductions in both firing rate and thermosensitivity. Muscimol induced reductions were maintained for at least 20 min after washout. Neurons exposed to bicuculline had no change in firing rate or thermosensitivity. However, after bicuculline washout there were reductions in both firing rate and thermosensitivity. These findings support the hypothesis that GABAA receptor induced inhibition of HDB thermosensitive neurons can modulate both thermoregulation and sleep-wake control.

Region-dependent difference in the sleep-promoting potency of an adenosine A2A receptor agonist

The present study has demonstrated that the sleep-promoting potency of 2-[p-(2-carboxyethyl)phenethylamino]-5'-N-ethylcarboxamido adenosine (CGS21680), a selective agonist for the adenosine A2A receptor, varies depending on the location of the administration. CGS21680 was continuously administered to rats through a chronically implanted cannula for 6 h during their active phase. The tip of the cannula was located in the subarachnoid space or the brain ventricle neighbouring the established brain areas implicated in the regulation of sleep-wake phenomena, i.e. rostral basal forebrain, medial preoptic area, lateral preoptic area, posterior hypothalamus, and dorsal tegmentum of the pons and medulla. At an infusion rate of 2.0 pmol/min, the magnitude of increase in non-rapid eye movement sleep varied from 14 min (a 15% increase) to 96 min (a 103% increase), and those of rapid eye movement sleep varied from 6 min (a 40% increase) to 28 min (a 264% increase) from the respective baseline values. The largest increases in both types of sleep occurred when CGS21680 was administered to the subarachnoid space underlying the rostral basal forebrain. These findings were interpreted to mean that the major, if not the only, site responsible for the CGS21680-inducing sleep was located in or near the rostral basal forebrain. This interpretation was supported by the findings that the administration of CGS21680 to the rostral basal forebrain produced predominant expression of Fos within the shell of the nucleus accumbens and the medial portion of the olfactory tubercle, and that the microdialysis perfusion of CGS21680 into the shell of the nucleus accumbens also exhibited a sleep-promoting effect.

Identification of two novel diurnal genes by screening of a rat brain cDNA library

While the hypothalamus is fundamental for sleep and circadian regulation, the molecular mechanisms involved are poorly understood. We have used a differential gene expression technique to identify hypothalamic genes which have altered expression in rat sleep periods. Complex cDNA probes from rat hypothalami removed at Zeitgeber times 4 and 15 were hybridised to rat brain cDNA library girds. From 30 differentially expressed clones, six were further analysed and two were confirmed to exhibit increased expression at Zeitgeber time 4. A Northern blot hybridization of brain, heart, kidney, lung, testis and skin mRNA showed that both clones were brain specific. Therefore, we have identified two novel brain specific diurnally expressed hypothalamic genes. Both genes may have roles in sleep or circadian regulation.

Daily rhythms in Fos activity in the rat ventrolateral preoptic area and midline thalamic nuclei

The present experiment investigated the expression of the nuclear phosphoprotein Fos over the 24-h light-dark cycle in regions of the rat brain related to sleep and vigilance, including the ventrolateral preoptic area (VLPO), the paraventricular thalamic nucleus (PVT), and the central medial thalamic nucleus (CMT). Immunocytochemistry for Fos, an immediate-early gene product used as an index of neuronal activity, was carried out on brain sections from rats perfused at zeitgeber time (ZT) 1, ZT 5, ZT 12.5, and ZT 17 (lights on ZT 0-ZT 12). The number of Fos-immunopositive (Fos+) cells in the VLPO was elevated at ZT 5 and 12.5 (i.e., during or just after the rest phase of the cycle). Fos+ cell number increased at ZT 17 and ZT 1 in the PVT and CMT, 180 degrees out of phase with the VLPO. A positive correlation was found between the numbers of Fos+ cells in the PVT and CMT, and Fos expression in each thalamic nucleus was negatively correlated with VLPO Fos+ cell number. The VLPO, PVT, and CMT may integrate circadian and homeostatic influences to regulate the sleep-wake cycle.

Sleep-waking discharge patterns of ventrolateral preoptic/anterior hypothalamic neurons in rats

Numerous lesion, stimulation and recording studies in experimental animals demonstrate the importance of neurons within the preoptic/anterior hypothalamic area (POA) in the regulation of sleep induction and sleep maintenance. Recently, a discrete cluster of cells in the ventrolateral POA (vlPOA) of rats was found to exhibit elevated c-fos gene expression during sleep, indicating that these neurons are strongly activated during nonREM and/or REM sleep stages. We examined neuronal discharge during wakefulness and sleep throughout the dorsal to ventral extent of the lateral POA in rats, using chronic microwire technique. We found that neurons with elevated discharge rates during sleep, compared to waking, were localized to the vlPOA. As a group, vlPOA neurons displayed elevated discharge rates during both nonREM and REM sleep. Discharge of vlPOA neurons reflected the depth of sleep, i.e., discharge rates increased significantly from light to deep nonREM sleep. During recovery sleep following 12-14 h of sleep deprivation, vlPOA neurons displayed increased sleep-related discharge, compared to baseline sleep. Neurons in the vlPOA displaying increased neuronal discharge during sleep were located in the same area where neurons exhibit increased c-fos gene expression during sleep. Such neurons are likely components of a rostral hypothalamic mechanism that regulates sleep onset and sleep maintenance.

Effects of temperature on sleep in the developing rat

In altricial species, such as humans and rats, much of the development of autonomic systems occurs postnatally. Consequently, vulnerabilities exist early in postnatal development when immature autonomic functions are challenged by external factors such as variations in ambient temperature (Ta). Ta profoundly influences sleep/wake state structure in adult animals and humans, and exposure to excessive warmth has been implicated as a risk factor in sudden infant death syndrome. To better understand the relationship between temperature and sleep during development, we investigated the effect of Ta variation on sleep/wake state structure and sleep intensity in developing rats. In this experiment, sleep intensity was measured by the intensity of slow-wave activity during slow-wave sleep. Neonatal Long-Evans hooded rat pups were surgically prepared for chronic sleep/wake state and brain temperature (Tbr) recording. Two-hour recordings of sleep/wake state and Tbr were obtained from rats on postnatal day 12 (P12), P14, P16, P18, and P20 at a Ta of either 28.0-30.0, 33.0-35.0, or 38.0-40.0 degrees C. Ta significantly influenced sleep/wake state structure but had little, if any, effect on sleep intensity in developing rats.

Electrophysiology of excitatory and inhibitory afferents to rat histaminergic tuberomammillary nucleus neurons from hypothalamic and forebrain sites

Anatomical evidence exists for projections to the tuberomammillary nucleus (TM) from the nucleus of the diagonal band of Broca (DBB) and the lateral preoptic area (LPO). The physiological effects of activating these inputs were studied by recording postsynaptic responses intracellularly from TM cells during both electrical stimulation and local nanodrop application of glutamate in horizontally cut brain slices. Electrical stimulation of the DBB, LPO and anterior lateral hypothalamic area (LH) usually evoked fast IPSPs (approximately 75% of responses) which were blocked by bicuculline or picrotoxin, suggesting GABA(A) mediation. The remaining excitatory responses evoked by stimulation of the LPO and LH were blocked by non-NMDA receptor antagonists (CNQX or NBQX) and the NMDA receptor antagonist, AP-5. Glutamate applied to the above areas induced postsynaptic responses in TM cells similar to those seen with electrical stimulation. Spontaneous firing in TM cells was suppressed by glutamate applied in the DBB. Glutamate applied in the LPO or LH evoked both inhibitory and excitatory responses. Changes in PSPs and firing rates were interpreted to result from glutamate activation of the neurons in the DBB, LPO and LH areas with inhibitory or excitatory connections to recorded TM neurons. These results support previous anatomical findings and suggest that inhibitory and excitatory synaptic control of TM activity is exerted by the DBB, LPO and LH areas.

Thermosensitive neurons of the diagonal band in rats: relation to wakefulness and non-rapid eye movement sleep

The thermosensitivity of the neurons in the diagonal band of Broca (DBB) was studied in 12 freely moving rats by determining responses to local cooling or warming with a water perfused thermode. Of 151 neurons studied, 37 (25%) neurons met the criterion for thermosensitivity including 17 warm-sensitive (WSNs) and 20 cold-sensitive neurons (CSNs). The spontaneous discharge rates of WSNs and CSNs were recorded through 1-3 sleep-waking cycles. The discharge of WSNs and CSNs during waking and non-rapid eye movement (NREM) sleep were different. Of 17 WSNs, 10 exhibited increased discharge rates during NREM sleep as compared with waking (NREM/Wake discharge ratio, > 1.2). Of 20 CSNs, 14 discharged more slowly during NREM sleep as compared with waking (NREM/Wake discharge ratio, < 0.8). In both WSNs and CSNs, changes in discharge rate preceded EEG changes at the waking-NREM transition. These results support a hypothesis that the activation of sleep-related WSNs and the deactivation of wake-related CSNs play a role in the onset and regulation of NREM sleep.

Sleep induction by microinjection of pentobarbital into the medial preoptic area in rats

In a previous study we have shown that microinjection of the benzodiazepine hypnotic triazolam into the medial preoptic area increases sleep in rats. In order to determine whether this effect is specific to benzodiazepines, or whether it occurs with hypnotic medications from other pharmacologic classes, we have microinjected pentobarbital (1 and 100 micrograms) and vehicle in random sequence into rats and performed two hour sleep studies in the daytime with the lights on. Both doses significantly decreased sleep latency and increased nonREM and total sleep. The amount of REM sleep, REM latency, and intermittent waking time were not significantly altered. These data are consistent with the hypothesis that the medial preoptic area may be involved in sleep induction by both benzodiazepine and barbiturate hypnotic medications.

Preoptic/anterior hypothalamic neurons: thermosensitivity in wakefulness and non rapid eye movement sleep

Thermosensitive neurons of the preoptic/anterior hypothalamic area (POAH) have been implicated in the regulation of both body temperature and non rapid eye movement (NREM) sleep. During NREM sleep, a majority of POAH warm-sensitive neurons (WSN) exhibit increased discharge compared to wakefulness. Cold-sensitive neurons (CSN) exhibit reduced discharge in NREM sleep compared to wakefulness. To further study the mechanism underlying these processes, the present study compared discharge rate and thermosensitivity (discharge rate change/degree C) of WSNs and CSNs in NREM sleep and wakefulness in freely moving adult cats. The thermosensitivity of 24 WSNs and 31 CSNs from the medial POAH was determined from responses to local POAH warming and cooling. WSNs with increased discharge in NREM sleep exhibited increased thermosensitivity during NREM sleep compared to wakefulness. CSNs with decreased discharge during NREM sleep exhibited decreased thermosensitivity in NREM sleep. The change in thermosensitivity from wakefulness to NREM sleep was correlated with the change in discharge rate in WSNs but not in CSNs. In addition, 9 of 47 neurons that were thermo-insensitive during wakefulness became warm-sensitive during NREM sleep. Changes in POAH neuronal thermosensitivity could be a component of the mechanism for stabilization of state after state transition.

Activation of ventrolateral preoptic neurons during sleep

The rostral hypothalamus and adjacent basal forebrain participate in the generation of sleep, but the neuronal circuitry involved in this process remains poorly characterized. Immunocytochemistry was used to identify the FOS protein, an immediate-early gene product, in a group of ventrolateral preoptic neurons that is specifically activated during sleep. The retrograde tracer cholera toxin B, in combination with FOS immunocytochemistry, was used to show that sleep-activated ventrolateral preoptic neurons innervate the tuberomammillary nucleus, a posterior hypothalamic cell group thought to participate in the modulation of arousal. This monosynaptic pathway in the hypothalamus may play a key role in determining sleep-wake states.

Local preoptic/anterior hypothalamic warming alters spontaneous and evoked neuronal activity in the magno-cellular basal forebrain

Local warming of the medial preoptic/anterior hypothalamus (POAH) promotes sleep, enhances EEG slow-wave activity during sleep, and suppresses arousal-related discharge in neurons of the midbrain reticular formation (MRF) and the posterior lateral hypothalamic area (PLHa). Another important site of sleep and arousal regulation, and a potential site of POAH thermal modulation, is the magnocellular basal forebrain (BF). We examined the ability of local POAH warming during wakefulness to influence the spontaneous and evoked discharge of neurons recorded in the BF of unanesthetized, unrestrained cats. Seventy of 174 BF neurons responded to 60-90 s periods of POAH warming with either increases or decreases in discharge rate. Forty-one of the 70 responsive cells displayed suppression of waking discharge during warming. Discharge rate in these cells declined by an average of 26.04 +/- 2.76%/degrees C of POAH temperature increase. The majority of warming-suppressed BF cells (73%) displayed higher rates of discharge during periods of wakefulness compared to periods of sleep. Twenty-nine of 70 responsive cells responded to POAH warming with an average increase in discharge rate of 43.81 +/- 6.26%/degrees C. A majority of these neurons (62%) exhibited higher spontaneous discharge rates during sleep compared to waking. Orthodromic excitatory responses were evoked in 29 BF cells by electrical stimulation of the MRF or PLHa. Thirteen of 29 cells displayed a waking-related discharge pattern, and responded to POAH warming with a significant suppression of evoked excitation. For a group of 15 behavioral state-indifferent cells (i.e., cells displaying no modulation of spontaneous discharge rate across the sleep-waking cycle), POAH warming had no effect on evoked excitatory responses. These results support the hypothesis that thermosensitive neurons of the POAH exert control of sleep-waking state, in part, via modulation of arousal- and sleep-regulating cell types within the magnocellular BF.

Promotion of sleep by heat in young rats

The aim of the experiments was to study the effects of a moderate heat load on sleep in young (26-day-old) rats and to determine whether the sleep-promoting effect of heat results from stimulation of the homeostatic sleep process. The changes in sleep-wake activity, electroencephalogram slow wave activity (SWA) during non-rapid eye movement sleep (NREMS) and cortical temperature (Tcrt) were determined during and after long (24-h) and short (2.5-h) heat loads (elevation of ambient temperature from 26 degrees C to 32 degrees C), and after total sleep deprivation (SD) combined with a short-term heat load. The heat exposures elicited increases in Tcrt and rectal temperature (2 and 1.7 degrees C respectively). The long-term heat load induced persistent, albeit slight enhancements in NREMS. Rapid eye movement sleep (REMS) increased with a 12-h delay during the 24-h heat load. Heat elicited an immediate large increase in SWA. After this initial increase, SWA declined and tended to fall below the baseline level during the last 12 h of the 24-h heat load. SWA and REMS were significantly suppressed after termination of 24-h heat loading. The increased SWA during the short-term heat load was not followed by subsequent alterations in sleep when the ambient temperature had returned to normal. However, after the combination of SD with the short-term heat load the durations of NREMS and SWA were significantly enhanced compared with those found after SD at 26 degrees C. The results are interpreted as suggesting that heat increases NREMS in the young rat by the same mechanism as is involved in the enhancement of NREMS after SD: a stimulation of sleep drive.