Delta power, a measure of EEG activity in the 1-4 Hz range, in slow-wave sleep (SWS) is in a quantitative and predictive relationship with prior wakefulness. Thus, sleep loss evokes a proportional increase in delta power, and excess sleep a decrease. Therefore, delta power is thought to reflect SWS need and its underlying homeostatically regulated recovery process. The neurophysiological substrate of this process is unknown and forward genetics might help elucidate the nature of what is depleted during wakefulness and recovered during SWS. We applied a mathematical method that quantifies the relationship between the sleep-wake distribution and delta power to sleep data of six inbred mouse strains. The results demonstrated that the rate at which SWS need accumulated varied greatly with genotype. This conclusion was confirmed in a "dose-response" study of sleep loss and changes in delta power; delta power strongly depended on both the duration of prior wakefulness and genotype. We followed the segregation of the rebound of delta power after sleep deprivation in 25 BXD recombinant inbred strains by quantitative trait loci (QTL) analysis. One "significant" QTL was identified on chromosome 13 that accounted for 49% of the genetic variance in this trait. Interestingly, the rate at which SWS need decreases did not vary with genotype in any of the 31 inbred strains studied. These results demonstrate, for the first time, that the increase of SWS need is under a strong genetic control, and they provide a basis for identifying genes underlying SWS homeostasis. Key words: EEG delta power; slow-wave activity; sleep deprivation; homeostatic regulation of non-REM sleep; simulation of Process S; BXD recombinant-inbred mouse strains; QTL;Dps1; Dps2; Dps3; forward genetics Slow oscillations in the delta frequency range (1-4 Hz) are characteristic of the EEG during slow-wave sleep (SWS) (i.e., non-REM sleep in humans). Delta oscillations reflect synchronized burst-pause firing patterns of hyperpolarized thalamocortical and corticothalamic neurons (Steriade et al., 1993;McCormick and Bal, 1997;Steriade, 1999). Activity in the delta frequency range can be quantified as delta power by Fourier analysis. Delta power is negatively correlated with the response to arousing stimuli (Neckelmann and Ursin, 1993) and SWS fragmentation (Franken et al., 1991a) and thus can be seen as a measure of SWS intensity. Delta power is also in a quantitative and predictive relationship with prior sleep and wakefulness in mammals, including humans. Sleep loss evokes an increase in delta power during subsequent SWS that is proportional to the loss (Tobler and Borbély, 1986;Dijk et al., 1987), excess sleep results in an attenuation of delta power (Werth et al., 1996), and delta power decreases over the course of a sleep period, independent of the circadian phase at which sleep is initiated (Dijk and Czeisler, 1995). These and other observations have been interpreted as evidence that SWS is a restorative and homeostatically regulated behavior and that delta ...

Melanopsin has been proposed as an important photoreceptive molecule for the mammalian circadian system. Its importance in this role was tested in melanopsin knockout mice. These mice entrained to a light/dark cycle, phase-shifted after a light pulse, and increased circadian period when light intensity increased. Induction of the immediate-early gene c-fos was observed after a nighttime light pulse in both wild-type and knockout mice. However, the magnitude of these behavioral responses in knockout mice was 40% lower than in wild-type mice. Although melanopsin is not essential for the circadian clock to receive photic input, it contributes significantly to the magnitude of photic responses.

Sleep is regulated by a homeostatic process that determines its need and by a circadian process that determines its timing. By using sleep deprivation and transcriptome profiling in inbred mouse strains, we show that genetic background affects susceptibility to sleep loss at the transcriptional level in a tissue-dependent manner. In the brain, Homer1a expression best reflects the response to sleep loss. Timecourse gene expression analysis suggests that 2,032 brain transcripts are under circadian control. However, only 391 remain rhythmic when mice are sleep-deprived at four time points around the clock, suggesting that most diurnal changes in gene transcription are, in fact, sleep-wake-dependent. By generating a transgenic mouse line, we show that in Homer1-expressing cells specifically, apart from Homer1a, three other activity-induced genes (Ptgs2, Jph3, and Nptx2) are overexpressed after sleep loss. All four genes play a role in recovery from glutamate-induced neuronal hyperactivity. The consistent activation of Homer1a suggests a role for sleep in intracellular calcium homeostasis for protecting and recovering from the neuronal activation imposed by wakefulness.homeostasis ͉ microarray ͉ mRNA tagging ͉ sleep deprivation ͉ sleep function T wo main processes regulate sleep. A homeostatic process regulates sleep need and intensity according to the time spent awake or asleep. A circadian process regulates the appropriate timing of sleep and wakefulness across the 24-h day. A highly reliable index of the homeostatic process is provided by the amplitude and prevalence of delta (1-to 4-Hz) oscillations in the electroencephalogram (EEG) of nonrapid eye movement (NREM) sleep (hereafter, ''delta power''). Delta power is high at sleep onset and decreases during sleep, in parallel with sleep depth. Sleep deprivations and naps induce a predictable increase or decrease, respectively, in delta power during subsequent sleep. The interaction between homeostatic and circadian processes is mathematically described in the two-process model of sleep regulation, which provides a framework for prediction and interpretation of a large body of experimental data (1).Among hypotheses concerning the physiological function of waking-induced changes in sleep, the most compelling suggests that sleep plays a key role in synaptic plasticity (2, 3). More specifically, EEG delta power during NREM sleep has been shown to play a critical role in learning-induced plasticity (4-6). In general, the prediction is that local neural activation due to specific behavioral (cognitive) demands imposes a burden on the brain which necessitates sleep and which is reflected by the EEG delta power.On the basis of mathematical modeling and experimental data, we have shown that sleep need, as indexed by the EEG delta power, is under genetic control (7), which is of direct relevance for explaining the interindividual vulnerability to sleep loss in human subjects (8, 9). However, deciphering the molecular bases of sleep need is rendered difficult because the contr...

Vigilance states, electroencephalogram (EEG) power spectra (0.25-25.0 Hz), and cortical temperature (TCRT) of 10 rats were obtained during a baseline day, a 24-h sleep deprivation (SD) period, and 2 days of recovery (recoveries 1 and 2). EEG power density in waking gradually increased in most frequencies during the SD period. Non-rapid-eye-movement (NREM) sleep was enhanced on both recovery days, and rapid-eye-movement sleep was enhanced only on recovery 1. In the initial 4 h of recovery 1, EEG slow-wave activity (SWA; mean power density 0.75-4.0 Hz) in NREM sleep was elevated relative to baseline, and the number of brief awakenings (nBA) was reduced. In the dark period of recovery 1 and the light period of recovery 2, SWA was below baseline, and nBA was increased. During the entire recovery period, SWA and nBA, both expressed as deviation from baseline values, were negatively correlated. During the SD period, TCRT was above baseline, and in the initial 16 h of recovery 1 it was below baseline. Whereas TCRT was negatively correlated with NREM sleep, no significant correlation was found between TCRT and SWA within NREM sleep. It is concluded that SD causes a short-lasting intensification of sleep, as indicated by the enhanced SWA and the reduced nBA, and a long-lasting increase in sleep duration. The different time courses of SWA and TCRT suggest that variations in NREM sleep intensity are not directly related to changes in TCRT.

Animal behavior is synchronized to the 24-hour light:dark (LD) cycle by regulatory programs that produce circadian fluctuations in gene expression throughout the body. In mammals, the transcription factor CLOCK controls circadian oscillation in the suprachiasmatic nucleus of the brain; its paralog, neuronal PAS domain protein 2 (NPAS2), performs a similar function in other forebrain sites. To investigate the role of NPAS2 in behavioral manifestations of circadian rhythm, we studied locomotor activity, sleep patterns, and adaptability to both light- and restricted food-driven entrainment in NPAS2-deficient mice. Our results indicate that NPAS2 plays a substantive role in maintaining circadian behaviors in normal LD and feeding conditions and that NPAS2 is critical for adaptability to food restriction.

The genetic variation in spontaneous rhythmic electroencephalographic (EEG) activity was assessed by the quantitative analysis of the EEG in six inbred mice strains. Mean spectral EEG profiles (0–25 Hz) over 24 h were obtained for paradoxical sleep (PS), slow-wave sleep (SWS), and wakefulness. A highly significant genotype-specific variation was found for theta peak frequency during both PS and SWS, which strongly suggests the presence of a gene with a major effect. The strain distribution of theta peak frequency during exploratory behavior differed from that during sleep. In SWS, the relative contributions of delta (1–4 Hz) and sigma (11–15) power to the EEG varied with genotype and power in both frequency bands was negatively correlated. In addition, the EEG dynamics at state transitions were analyzed with a 4-s resolution. The onset of PS, but not that of wakefulness, was preceded by a pronounced peak in high-frequency (>11 Hz) power. These findings are discussed in terms of the neurophysiological mechanisms underlying rhythm generation and their control and modulation by the brain stem reticular-activating system.

Low-threshold (T-type) Ca2+ channels encoded by the Ca V 3 genes endow neurons with oscillatory properties that underlie slow waves characteristic of the non-rapid eye movement (NREM) sleep EEG. Three Ca V 3 channel subtypes are expressed in the thalamocortical (TC) system, but their respective roles for the sleep EEG are unclear. Ca V 3.3 protein is expressed abundantly in the nucleus reticularis thalami (nRt), an essential oscillatory burst generator. We report the characterization of a transgenic Ca V 3.3 −/− mouse line and demonstrate that Ca V 3.3 channels are indispensable for nRt function and for sleep spindles, a hallmark of natural sleep. The absence of Ca V 3.3 channels prevented oscillatory bursting in the lowfrequency (4-10 Hz) range in nRt cells but spared tonic discharge. In contrast, adjacent TC neurons expressing Ca V 3.1 channels retained low-threshold bursts. Nevertheless, the generation of synchronized thalamic network oscillations underlying sleep-spindle waves was weakened markedly because of the reduced inhibition of TC neurons via nRt cells. T currents in Ca V 3.3 −/− mice were <30% compared with those in WT mice, and the remaining current, carried by Ca V 3.2 channels, generated dendritic [Ca 2+ ] i signals insufficient to provoke oscillatory bursting that arises from interplay with Ca 2+ -dependent small conductance-type 2 K + channels. Finally, naturally sleeping Ca V 3.3 −/− mice showed a selective reduction in the power density of the σ frequency band (10-12 Hz) at transitions from NREM to REM sleep, with other EEG waves remaining unaltered. Together, these data identify a central role for Ca V 3.3 channels in the rhythmogenic properties of the sleep-spindle generator and provide a molecular target to elucidate the roles of sleep spindles for brain function and development.2+ channels encoded by the Ca V 3 genes activate near resting membrane potentials and generate low-threshold Ca 2+ spikes leading to burst firing and low-frequency oscillatory discharge that are prominent in some thalamic, olivary, and cerebellar neurons (1). Among the low-threshold Ca 2+ currents carried by Ca V 3 channels, those mediated by Ca V 3.3 channels are unique in that they display the slowest time course, the fastest recovery from inactivation, and often the most depolarized activation voltages (2, 3). Moreover, Ca V 3.3 mRNA is expressed predominantly in brain and shows highest regional specificity (3-5). To date, identification of specific physiological roles for Ca V 3.3 channels has been hampered for several reasons. First, these channels typically are coexpressed with Ca V 3.1 and/or Ca V 3.2 channels (4, 5), and specific pharmacological tools are not available (1). Second, Ca V 3.3 channels often are found in distal dendrites, limiting accessibility for electrophysiological characterization (6, 7). Finally, Ca V 3.3 −/− mice have not been reported, whereas Ca V 3.1 −/− and Ca V 3.2 knockdown mice have helped address the roles of Ca V 3.1 and Ca V 3.2 channels in sleep and pain, respectively (8-10)...

Analyses in mice deficient for the blue-light-sensitive photopigment melanopsin show that direct effects of light on behavior and EEG depend on the time of day. The data further suggest an unexpected role for melanopsin in sleep homeostasis.