Slow brain oscillations of sleep, resting state, and vigilance

The most important quest of cognitive neuroscience may be to unravel the mechanisms by which the brain selects, links, consolidates, and integrates new information into its neuronal network, while preventing saturation to occur. During the past decade, neuroscientists working within several disciplines have observed an important involvement of the specific types of brain oscillations that occur during sleep--the cortical slow oscillations; during the resting state--the fMRI resting state networks including the default-mode network (DMN); and during task performance--the performance modulations that link as well to modulations in electroencephalography or magnetoencephalography frequency content. Understanding the role of these slow oscillations thus appears to be essential for our fundamental understanding of brain function. Brain activity is characterized by oscillations occurring in spike frequency, field potentials or blood oxygen level-dependent functional magnetic resonance imaging signals. Environmental stimuli, reaching the brain through our senses, activate or inactivate neuronal populations and modulate ongoing activity. The effect they sort is to a large extent determined by the momentary state of the slow endogenous oscillations of the brain. In the absence of sensory input, as is the case during rest or sleep, brain activity does not cease. Rather, its oscillations continue and change with respect to their dominant frequencies and coupling topography. This chapter briefly introduces the topics that will be addressed in this dedicated volume of Progress in Brain Research on slow oscillations and sets the stage for excellent papers discussing their molecular, cellular, network physiological and cognitive performance aspects. Getting to know about slow oscillations is essential for our understanding of plasticity, memory, brain structure from synapse to DMN, cognition, consciousness, and ultimately for our understanding of the mechanisms and functions of sleep and vigilance.

Manipulation of skin temperature improves nocturnal sleep in narcolepsy

OBJECTIVE

Besides excessive daytime sleepiness, disturbed nocturnal sleep is a major complaint of patients with narcolepsy. Previously, alterations in skin temperature regulation in narcoleptic patients have been shown to be related to increased sleepiness. This study tests the hypothesis that direct control of nocturnal skin temperature might be applied to improve the disturbed sleep of narcoleptic patients.

METHODS

Participants were eight patients (five males) diagnosed as having narcolepsy with cataplexy according to the ICSD-2 criteria, mean (SD) age 28.6 (6.4) years, range 18-35 years. During two nights, sleep was recorded polysomnographically while proximal and distal skin temperature were manipulated using a comfortable thermosuit that induced skin temperature to cycle slowly with an amplitude of only 0.4 degrees C within the comfortable range normally observed during sleep. Logistic regression was used to evaluate the effect of skin temperature manipulation on the probability of occurrence of different sleep stages and nocturnal wakefulness.

RESULTS

Proximal skin warming significantly suppressed wakefulness and enhanced slow wave sleep (SWS). In contrast, distal skin warming enhanced wakefulness and stage 1 sleep at the cost of SWS and REM sleep. The optimal combination of proximal skin warming and distal skin cooling led to a 160% increase in SWS, a 50% increase in REM sleep and a 68% decrease in wakefulness, compared with the least beneficial combination of proximal skin cooling and distal skin warming.

INTERPRETATION

Subtle skin temperature manipulations under controlled conditions significantly improved the typical nocturnal sleep problems in narcolepsy.

Correlating the alpha rhythm to BOLD using simultaneous EEG/fMRI: Inter-subject variability

Simultaneous recording of electroencephalogram/functional magnetic resonance images (EEG/fMRI) was applied to identify blood oxygenation level-dependent (BOLD) changes associated with spontaneous variations of the alpha rhythm, which is considered the hallmark of the brain resting state. The analysis was focused on inter-subject variability associated with the resting state. Data from 7 normal subjects are presented. Confirming earlier findings, three subjects showed a negative correlation between the BOLD signal and the average power time series within the alpha band (8--12 Hz) in extensive areas of the occipital, parietal and frontal lobes. In small thalamic areas, the BOLD signal was positively correlated with the alpha power. For subjects 3 and 4, who displayed two different states during the data acquisition time, it was shown that the corresponding correlation patterns were different, thus demonstrating the state dependency of the results. In subject 5, the changes in BOLD were observed mainly in the frontal and temporal lobes. Subject 6 only showed positive correlations, thus contradicting the negative BOLD alpha power cortical correlations that were found in most subjects. Results suggest that the resting state varies over subjects and, sometimes, even within one subject. As the resting state plays an important role in many fMRI experiments, the inter-subject variability of this state should be addressed when comparing fMRI results from different subjects.

Shortened seasonal photoperiodic cycles accelerate aging of the diurnal and circadian locomotor activity rhythms in a primate

The gray mouse lemur (Microcebus murinus), a prosimian primate, exhibits seasonal rhythms strictly controlled by photoperiodic variations. Previous studies indicated that longevity can be altered by long-term acceleration of seasonal rhythms, providing a model for assessing various aspects of aging. To assess the effect of aging and accelerated aging on the circadian system of this primate, we compared the circadian rhythm of the locomotor activity in adult mouse lemurs (2-4.5 years, n = 9), aged mouse lemurs (5-9 years, n = 10), and adult mouse lemurs that had been exposed from birth to a shortened seasonal photoperiodic cycle (2-4.5 years, n = 7). Compared to adult animals, aged mouse lemurs showed a significant increase in intradaily variability and an advanced activity onset. Aging was characterized by a decrease in amplitude, with both a decrease in nocturnal activity and an increase in daytime activity. When maintained in constant dim red light, aged animals exhibited a shortening of the free-running period (22.8 +/- 0.1 h) compared to adult animals (23.5 +/- 0.1 h). A 3- to 5-year exposure to an accelerated seasonal photoperiodic rhythm ("annual" duration of 5 months) in accelerated mouse lemurs produced disturbances of the locomotor activity rhythm that resembled those of aged mouse lemurs, whether animals were studied in entrained or in free-running conditions. The present study demonstrated a weakened and fragmented locomotor activity rhythm during normal aging in this primate. Increasing the number of expressed seasonal cycles accelerated aging of parameters related to circadian rhythmicity in adult animals.

Environmental light and suprachiasmatic nucleus interact in the regulation of body temperature

The mammalian biological clock, located in the suprachiasmatic nucleus (SCN), is crucial for circadian rhythms in physiology and behavior. However, equivocal findings have been reported on its role in the circadian regulation of body temperature. The goal of the present studies was to investigate the interaction between the SCN and environmental light in the regulation of body temperature. All recordings were performed by telemetry in free moving male Wistar rats. Firstly, we demonstrated an endogenous circadian rhythm in body temperature independent of locomotor activity. This rhythm was abolished by stereotactic lesioning of the SCN. Secondly, we demonstrated a circadian phase-dependent suppressive effect of light ('negative masking') on body temperature. Light suppressed body temperature more at the end of the subjective night (circadian time [CT] 22) than in the middle (CT 6) and at the end (CT 10) of the subjective day. This circadian-phase dependent suppression was not demonstrated in SCN-lesioned animals. Surprisingly, after half a year of recovery from lesioning of the SCN, light regained its suppressing action on body temperature, resulting in a daily body temperature rhythm only under light-dark conditions. In contrast to body temperature, light could not substantially mimic a daytime inhibitory SCN-output in the regulation of heart rate and locomotor activity. The present results suggest that, after lesioning of the SCN as main relay station for the immediate body temperature-inhibition by light, secondary relay nuclei can fully take over this function of the SCN. These findings provide a possible explanation for the controversy in literature over the question whether the SCN is required for the diurnal rhythm in body temperature. Furthermore, they show that light may have an acute effect on behavior and physiology of the organism via the SCN, which extends beyond the generally acknowledged effect on melatonin secretion.

Long-term effects of cranial irradiation for childhood malignancy on sleep in adulthood

BACKGROUND

Cranial radiation therapy (CRT) is required for successful treatment of a variety of brain tumours in childhood.

OBJECTIVE

To investigate whether childhood CRT leads to altered sleep-wakefulness organization in adulthood, and to identify the determinants of such alterations.

SUBJECTS AND METHODS

Subjective (questionnaires) and objective (actigraphy) measures of circadian rhythmicity and sleep were assessed in 25 individuals, 8-29 years after CRT for medulloblastoma (n=17) or other intracranial tumours (n=8), and in a group of 34 age-matched healthy individuals. Serum GH peak during insulin-induced hypoglycaemia and serum concentrations of prolactin and leptin (expressed per fat mass) were determined in the CRT group.

RESULTS

The CRT group showed a markedly increased sleep duration (8.66 h, compared with 7.66 h in controls). In addition, the sleep-wake rhythm showed greater amplitude and less fragmentation, and less tolerance for alterations in the timing of sleep. Regression analysis showed both radiation dosage and neuroendocrine status to be determinants of sleep changes, suggesting that some of the alterations may be normalized with hormone supplementation.

CONCLUSION

The present study shows that high-dose cranial radiation therapy in childhood is associated with objective and subjective changes in the sleep-wake rhythm in adulthood.