Altered sleep regulation in leptin-deficient mice

Recent epidemiological, clinical, and experimental studies have demonstrated important links between sleep duration and architecture, circadian rhythms, and metabolism, although the genetic pathways that interconnect these processes are not well understood. Leptin is a circulating hormone and major adiposity signal involved in long-term energy homeostasis. In this study, we tested the hypothesis that leptin deficiency leads to impairments in sleep-wake regulation. Male ob/ob mice, a genetic model of leptin deficiency, had significantly disrupted sleep architecture with an elevated number of arousals from sleep [wild-type (WT) mice, 108.2 ± 7.2 vs. ob/ob mice, 148.4 ± 4.5, P < 0.001] and increased stage shifts (WT, 519.1 ± 25.2 vs. ob/ob, 748.0 ± 38.8, P < 0.001) compared with WT mice. Ob/ob mice also had more frequent, but shorter-lasting sleep bouts compared with WT mice, indicating impaired sleep consolidation. Interestingly, ob/ob mice showed changes in sleep time, with increased amounts of 24-h non-rapid eye movement (NREM) sleep (WT, 601.5 ± 10.8 vs. ob/ob, 669.2 ± 13.4 min, P < 0.001). Ob/ob mice had overall lower body temperature (WT, 35.1 ± 0.2 vs. ob/ob, 33.4 ± 0.2°C, P < 0.001) and locomotor activity counts (WT, 25125 ± 2137 vs. ob/ob, 5219 ± 1759, P < 0.001). Ob/ob mice displayed an attenuated diurnal rhythm of sleep-wake stages, NREM delta power, and locomotor activity. Following sleep deprivation, ob/ob mice had smaller amounts of NREM and REM recovery sleep, both in terms of the magnitude and the duration of the recovery response. In combination, these results indicate that leptin deficiency disrupts the regulation of sleep architecture and diurnal rhythmicity.

Obesity and metabolic syndrome in circadian Clock mutant mice

The CLOCK transcription factor is a key component of the molecular circadian clock within pacemaker neurons of the hypothalamic suprachiasmatic nucleus. We found that homozygous Clock mutant mice have a greatly attenuated diurnal feeding rhythm, are hyperphagic and obese, and develop a metabolic syndrome of hyperleptinemia, hyperlipidemia, hepatic steatosis, hyperglycemia, and hypoinsulinemia. Expression of transcripts encoding selected hypothalamic peptides associated with energy balance was attenuated in the Clock mutant mice. These results suggest that the circadian clock gene network plays an important role in mammalian energy balance.

Melatonin, sleep, and circadian rhythms: rationale for development of specific melatonin agonists

Circadian rhythm sleep disorders (CRSDs), whether chronic or transient, affect a broad range of individuals, including many elderly, those with severe visual impairments, shift workers, and jet travelers moving rapidly across many time zones. In addition, various forms of insomnia affect another large sector of the population. A feature common among CRSDs and some forms of insomnia is sensitivity to the hormone melatonin, which is secreted by the pineal gland. Accumulating evidence suggests that melatonin may regulate the circadian clock located in the suprachiasmatic nucleus (SCN) of the hypothalamus. Although the light-dark cycle is the primary signal that entrains the circadian clock to environmental cycles, exogenous melatonin has been shown to entrain the clock in individuals with no light perception and free-running circadian rhythms. Furthermore, studies have reported beneficial effects of melatonin for treatment of certain insomnias. Together, these studies suggest that melatonin may be useful for treating some insomnias and CRSDs. In these contexts, use of melatonin as a supplement has been popular in the United States. Unfortunately, the therapeutic potential of melatonin has been difficult to realize in clinical trials, possibly owing to non-specific actions of the agent and its unfavorable pharmacokinetic properties when administered orally. In an attempt to take advantage of the therapeutic opportunities available through the brain's melatonin system, researchers have developed several melatonin agonists with improved properties in comparison to melatonin. Some of these agents are now in clinical trials for treatment of insomnia or CRSDs.

Sex- and lineage-specific inheritance of depression-like behavior in the rat

The Wistar-Kyoto (WKY) rat exhibits physiological and behavioral similarities to endophenotypes of human depression. In the forced swim test (FST), a well-characterized antidepressant-reversible test for behavioral despair in rodents, WKYs express characteristics of behavioral despair; increased immobility, and decreased climbing. To map genetic loci linked to behavior in the FST, we conducted a quantitative trait loci (QTL) analysis of the segregating F2 generation of a WKY x Fisher 344 (F344) reciprocal intercross. Using linear-model-based genome scans to include covariate (sex or lineage)-by-QTL interaction effects, four significant QTL influencing climbing behavior were identified. In addition, we identified three, seven, and two suggestive QTL for climbing, immobility, and swimming, respectively. One of these loci was pleiotropic, affecting both immobility and climbing. As found in human linkage studies, several of these QTL showed sex- and/or lineage-dependent effects. A simultaneous search strategy identified three epistatic locus pairs for climbing. Multiple regression analysis was employed to characterize the joint contributions of these QTL and to clarify the sex- and lineage-dependent effects. As expected for complex traits, FST behavior is influenced by multiple QTL of small effect, each contributing 5%-10%, accounting for a total 10%-30% of the phenotypic variance. A number of loci mapped in this study share overlapping candidate regions with previously identified emotionality QTL in mice as well as with susceptibility loci recognized by linkage or genome scan analyses for major depression or bipolar disorder in humans. The presence of these loci across species suggests that these QTL may represent universal genetic factors contributing to mood disorders.

Circadian Rhythms: From the Bench to the Bedside and Falling Asleep

The discovery of the molecular core machinery underlying the generation of circadian rhythms in mammals, and the ability to alter the genes and protein products that comprise the circadian clock, has led to a new appreciation of the role of the clock in regulating many parameters of the sleep-wake cycle, beyond just the timing of sleep. The journal, Sleep, with its mission of publishing papers on basic and translational research in the field of sleep, is in an ideal position to insure that advances in the field of circadian rhythms are used to improve human sleep and in the treatment of sleep-wake disorders.

The Suprachiasmatic Nucleus Regulates Sleep Timing and Amount in Mice

CONTEXT

Sleep is regulated by circadian and homeostatic processes. The circadian pacemaker, located in the suprachiasmatic nuclei (SCN), regulates the timing and consolidation of the sleep-wake cycle, while a homeostatic mechanism governs the accumulation of sleep debt and sleep recovery. Recent studies using mice with deletions or mutations of circadian genes show that components of the circadian pacemaker can influence the total amount of baseline sleep and recovery from sleep deprivation, indicating a broader role for the SCN in sleep regulation.

OBJECTIVE

To further investigate the role of the circadian pacemaker in sleep regulation in mice, we recorded sleep in sham and SCN-lesioned mice under baseline conditions and following sleep deprivation.

RESULTS

Compared to sham controls, SCN-lesioned mice exhibited a decrease in sleep consolidation and a decrease in wakefulness during the dark phase. Following sleep deprivation, SCN-lesioned mice exhibited an attenuated increase in non-rapid eye movement sleep time but an increase in non-rapid eye movement sleep electroencephalographic delta power that was similar to that of the sham controls.

CONCLUSIONS

These findings support the hypothesis that the SCN consolidate the sleep-wake cycle by generating a signal of arousal during the subjective night (ie. the active period), thereby having the capacity to alter baseline sleep amount. Although the SCN are not involved in sleep homeostasis as defined by the increase in electroencephalographic delta power after sleep deprivation, the SCN does play a central role in the regulation of sleep and wakefulness beyond just the timing of vigilance states.

Circadian clock mutation disrupts estrous cyclicity and maintenance of pregnancy

Classic experiments have shown that ovulation and estrous cyclicity are under circadian control and that surgical ablation of the suprachiasmatic nuclei (SCN) results in estrous acyclicity in rats. Here, we characterized reproductive function in the circadian Clock mutant mouse and found that the circadian Clock mutation both disrupts estrous cyclicity and interferes with the maintenance of pregnancy. Clock mutant females have extended, irregular estrous cycles, lack a coordinated luteinizing hormone (LH) surge on the day of proestrus, exhibit increased fetal reabsorption during pregnancy, and have a high rate of full-term pregnancy failure. Clock mutants also show an unexpected decline in progesterone levels at midpregnancy and a shortened duration of pseudopregnancy, suggesting that maternal prolactin release may be abnormal. In a second set of experiments, we interrogated the function of each level of the hypothalamic-pituitary-gonadal (HPG) axis in order to determine how the Clock mutation disrupts estrous cyclicity. We report that Clock mutants fail to show an LH surge following estradiol priming in spite of the fact that hypothalamic levels of gonadotropin-releasing hormone (GnRH), pituitary release of LH, and serum levels of estradiol and progesterone are all normal in Clock/Clock females. These data suggest that Clock mutants lack an appropriate circadian daily-timing signal required to coordinate hypothalamic hormone secretion. Defining the mechanisms by which the Clock mutation disrupts reproductive function offers a model for understanding how circadian genes affect complex physiological systems.

Effects of Gamma-Hydroxybutyrate (GHB) on Vigilance States and EEG in Mice

STUDY OBJECTIVES

Gamma-hydroxybutyrate (GHB) is an endogenous neuromodulator that appears to have wide-ranging effects on vigilance and behavior. In the present study, we examined the effects of GHB on sleep-wake behavior and EEG in mice. In addition, we measured effects of GHB on body temperature and arousal or stress hormones.

DESIGN

Adult male BALB/c mice were implanted with electroencephalographic and electromyographic electrodes to record vigilance states and an intraperitoneal transmitter to record body temperature. After recovery from surgery and habituation to the recording procedure, the mice were intraperitoneally injected with saline or GHB (50, 150 or 250 mg/kg) half an hour after light onset. Blood samples to measure effects of GHB on corticosterone and prolactin levels were collected in a separate group of mice.

SETTING

N/A PATIENTS: N/A INTERVENTIONS: N/A RESULTS: At the lowest dose, GHB had no conspicuous effects on behavioral vigilance and electroencephalogram, nor on body temperature and endocrine measures. At the 2 higher doses, GHB induced a short period of electroencephalographic hypersynchrony in parallel to complete behavioral inactivity, an unnatural flat body posture, and nonresponsiveness to stimulation. After the highest dose of GHB, this state of reduced vigilance was associated with a decrease in body temperature, while prolactin and corticosterone levels were strongly increased.

CONCLUSIONS

The results do not indicate a clear sleep-promoting effect of GHB in mice, but, at higher doses, it caused electroencephalographic hypersynchronization together with a coma-like state.

Effects of age on circadian rhythms are similar in wild-type and heterozygous Clock mutant mice

The amplitudes of many circadian rhythms, at the behavioral, physiological, cellular, and biochemical levels, decrease with advanced age. Previous studies suggest that the amplitude of the central circadian pacemaker is decreased in old animals. Recently, it has been reported that expression of several circadian clock genes, including Clock, is lower in the master circadian pacemaker of old rodents. To test the hypothesis that decreased activity of a circadian clock gene renders animals more susceptible to the effects of aging, we analyzed the circadian rhythm of locomotor activity in young and old wild-type and heterozygous Clock mutant mice. We found that the effects of age and the Clock mutation were additive. These results indicate that age-related changes in circadian rhythmicity occur equally in wild-type and heterozygous Clock mutants, suggesting that the Clock mutation does not render mice more susceptible to the effects of age on the circadian pacemaker.

Daily variations of blood glucose, acid-base state and PCO2 in rats: effect of light exposure

The suprachiasmatic nuclei (SCN) of the hypothalamus are the site of the main circadian clock in mammals. Synchronization of the SCN to light is achieved by direct retinal inputs. The present study performed in rats transferred to constant darkness shows that blood glucose, pH and PCO2 display significant diurnal changes when measurements were made during the subjective day, the early subjective night or the late subjective night. The effects of a 30-min light exposure (100 lx) on these metabolic parameters at each of these circadian times were assessed. Regardless of the circadian time, light induced an increase in blood glucose, but did not affect plasma pH and PCO2. This study suggests that blood glucose, PCO2 and acid-base state are under circadian control, most likely mediated by the SCN, while the hyperglycemic response to light seems not to be gated by a circadian clock and may thus involve retinal inputs to non-SCN retino-recipient areas.

Chemosensory stimulation of luteinizing hormone secretion in male Siberian hamsters (Phodopus sungorus)

Male Siberian hamsters (Phodopus sungorus) housed in long days (LD), but not short days (SD) release luteinizing hormone (LH) when exposed to females. This study examined whether this response is specific to a female and identifies the source of a stimulus that induces LH release. Serum concentrations of LH, testosterone (T), follicle stimulating hormone (FSH), and cortisol were examined in all experiments. T concentrations mirrored the LH response; FSH and cortisol were unchanged in response to all stimuli. Exposure to an LD female, irrespective of her reproductive status, but not an SD female, elicited LH release. Exposure to another male did not trigger LH release. Males released LH when allowed physical contact with an anesthetized female, but not when separated from a normally active female, suggesting that tactile or nonvolatile chemosensory stimuli elicit LH release. Urine and secretions collected from the vagina as well as oral, midventral, perineal, and rectal glands, elicited marked behavioral responses in male P. sungorus. Despite these behavioral responses, only feces from females elicited LH release in males. Males released LH in response to feces extracted from the rectum and to cotton swabs that had been rubbed against the rectal mucosa, suggesting that a component of rectal secretions may trigger LH release in male Siberian hamsters. Taken together, these data and previous data from our laboratory indicate that both the production of and the response to a pheromone that triggers the selective release of LH is regulated by day length.

Beta-endorphin modulates the acute response to a social conflict in male mice but does not play a role in stress-induced changes in sleep

Beta-endorphin is an endogenous opioid peptide that is released during stress and has been associated with many physiological functions. In this experiment beta-endorphin deficient mice were used to study the role of endorphins in the acute physiological and behavioral responses to a social conflict, as well as their role in social stress-induced changes in sleep. Adult male beta-endorphin deficient and wild type mice were subjected to the stress of a 1 h social conflict with an aggressive dominant conspecific. After the conflict, the beta-endorphin deficient mice had higher corticosterone levels but the peak increase in body temperature was not different from that in wild type animals. In fact, body temperature returned to baseline levels faster in the beta-endorphin deficient mice. During their interaction with the aggressive conspecific several of the beta-endorphin deficient mice showed clear signs of counter aggression whereas this was not seen in any of the wild type mice. Overall, the beta-endorphin deficient mice and wild type mice had fairly similar sleep patterns under baseline conditions and also showed similar amounts of NREM sleep, REM sleep and EEG slow-wave energy after the social conflict. In addition, no differences were found in the sleep patterns of mice that showed counter aggression and mice that did not. In conclusion, the results suggest that beta-endorphin modulates the acute endocrine, thermoregulatory and behavioral response to a social conflict but the data do not support a major role for beta-endorphin in the regulation of sleep or social stress-induced alterations in sleep.

Sleep in female mice: a strain comparison across the estrous cycle

STUDY OBJECTIVE

Studying inbred strains of mice has proven useful in uncovering genetic variation in the expression of sleep patterns. However, although genetic influence on many behaviors has been shown to be gender specific, to date, sleep patterns in different strains of female mice have not been reported. In order to perform such studies in female mice, the estrous cycle must be taken into account in view of the effects of reproductive hormones on sleep. The aim of this study was thus to determine sleep patterns in female mice of different inbred strains over the estrous cycle.

DESIGN

Three strains of mice were used. Vaginal smears were performed to determine the estrous cycle stage; electroencephalographic and electromyographic activity, as well as body temperature and locomotor activity collected during a full estrous cycle, were analyzed.

MEASUREMENTS AND RESULTS

We report a major impact of the genetic background in the regulation of non-rapid eye movement sleep over a 24-hour period and clear strain differences in rapid eye movement sleep distribution over the light-dark cycle. In contrast, the estrous cycle had less influence on non-rapid eye movement sleep and rapid eye movement sleep, and these effects were dependent on the genotype of the mice.

CONCLUSIONS

Sleep regulation in female mice is influenced primarily by genetic background and, to a lesser extent, by hormonal variations associated with the estrous cycle.

Aging alters circadian and light-induced expression of clock genes in golden hamsters

Aging alters numerous aspects of circadian biology, including the amplitude of rhythms generated by the suprachiasmatic nuclei (SCN) of the hypothalamus, the site of the central circadian pacemaker in mammals, and the response of the pacemaker to environmental stimuli such as light. Although previous studies have described molecular correlates of these behavioral changes, to date only 1 study in rats has attempted to determine if there are age-related changes in the expression of genes that comprise the circadian clock itself. We used in situ hybridization to examine the effects of age on the circadian pattern of expression of a subset of the genes that comprise the molecular machinery of the circadian clock in golden hamsters. Here we report that age alters the 24-h expression profile of Clock and its binding partner Bmal1 in the hamster SCN. There is no effect of age on the 24-h profile of either Per1 or Per2 when hamsters are housed in constant darkness. We also found that light pulses, which induce smaller phase shifts in old animals than in young, lead to decreased induction of Per1, but not of Per2, in the SCN of old hamsters.

Depressive-like behavior and stress reactivity are independent traits in a Wistar Kyoto x Fisher 344 cross

Depression is a heritable disorder that is often precipitated by stress. Abnormalities of the stress-reactive hypothalamic-pituitary-adrenal (HPA) axis are also common in depressed patients. In animal models, the forced swim test (FST) is the most frequently used test of depressive-like behavior. We have used a proposed animal model of depression, the Wistar Kyoto (WKY) rat, to investigate the relationship as well as the mode of inheritance of FST behaviors and HPA measures. Through reciprocal breeding of WKY and F344 parent strains and brother-sister breeding of the F1 generation, we obtained 486 F2 animals. Parent, F1 and F2 animals were tested in the FST. Blood samples were collected for determination of basal and stress (10-min restraint) plasma corticosterone (CORT) levels, and adrenal weights were measured. We found that all measures were heritable to some extent and that this heritability was highly sex dependent. Both correlation and factor analyses of the F2 generation data demonstrate that FST behavior and HPA axis measures are not directly related. Thus, the underlying genetic components of depressive-like behavior and HPA axis abnormalities are likely to be disparate in the segregating F2 generation of a WKY x F344 cross.

Exercise elicits phase shifts and acute alterations of melatonin that vary with circadian phase

To examine the immediate phase-shifting effects of high-intensity exercise of a practical duration (1 h) on human circadian phase, five groups of healthy men 20-30 yr of age participated in studies involving no exercise or exposure to morning, afternoon, evening, or nocturnal exercise. Except during scheduled sleep/dark and exercise periods, subjects remained under modified constant routine conditions allowing a sleep period and including constant posture, knowledge of clock time, and exposure to dim light intensities averaging (+/-SD) 42 +/- 19 lx. The nocturnal onset of plasma melatonin secretion was used as a marker of circadian phase. A phase response curve was used to summarize the phase-shifting effects of exercise as a function of the timing of exercise. A significant effect of time of day on circadian phase shifts was observed (P < 0.004). Over the interval from the melatonin onset before exercise to the first onset after exercise, circadian phase was significantly advanced in the evening exercise group by 30 +/- 15 min (SE) compared with the phase delays observed in the no-exercise group (-25 +/- 14 min, P < 0.05). Phase shifts in response to evening exercise exposure were attenuated on the second day after exercise exposure and no longer significantly different from phase shifts observed in the absence of exercise. Unanticipated transient elevations of melatonin levels were observed in response to nocturnal exercise and in some evening exercise subjects. Taken together with the results from previous studies in humans and diurnal rodents, the current results suggest that 1) a longer duration of exercise exposure and/or repeated daily exposure to exercise may be necessary for reliable phase-shifting of the human circadian system and that 2) early evening exercise of high intensity may induce phase advances relevant for nonphotic entrainment of the human circadian system.