Advanced Circadian Rhythms Research Articles

Resveratrol Induces Myotube Development by Altering Circadian Metabolism via the SIRT1-AMPK-PP2A Axis.

Resveratrol is a polyphenol known to have metabolic as well as circadian effects. However, there is little information regarding the metabolic and circadian effect of resveratrol on muscle cells. We sought to investigate the metabolic impact of resveratrol throughout the circadian cycle to clarify the associated signaling pathways. C2C12 myotubes were incubated with resveratrol in the presence of increasing concentrations of glucose, and metabolic and clock proteins were measured for 24 h. Resveratrol led to SIRT1, AMPK and PP2A activation. Myotubes treated with increasing glucose concentrations showed higher activation of the mTOR signaling pathway. However, resveratrol did not activate the mTOR signaling pathway, except for P70S6K and S6. In accordance with the reduced mTOR activity, resveratrol led to advanced circadian rhythms and reduced levels of pBMAL1 and CRY1. Resveratrol increased myogenin expression and advanced its rhythms. In conclusion, resveratrol activates the SIRT1-AMPK-PP2A axis, advances circadian rhythms and induces muscle development.

β-Hydroxy-β-methylbutyrate (HMB) leads to phospholipase D2 (PLD2) activation and alters circadian rhythms in myotubes.

β-Hydroxy-β-methylbutyrate (HMB) is a breakdown product of leucine, which promotes muscle growth. Although some studies indicate that HMB activates AKT and mTOR, others show activation of the downstream effectors, P70S6K and S6, independent of mTOR. Our aim was to study the metabolic effect of HMB around the circadian clock in order to determine more accurately the signaling pathway involved. C2C12 myotubes were treated with HMB and clock, metabolic and myogenic markers were measured around the clock. HMB-treated C2C12 myotubes showed no activation of AKT and mTOR, but did show activation of P70S6K and S6. Activation of P70S6K and S6 was also found when myotubes were treated with HMB combined with metformin, an indirect mTOR inhibitor, or rapamycin, a direct mTOR inhibitor. The activation of the P70S6K and S6 independent of AKT and mTOR, was accompanied by increased activation of phospholipase D2 (PLD). In addition, HMB led to high amplitude and advanced circadian rhythms. In conclusion, HMB induces myogenesis in C2C12 by activating P70S6K and S6 via PLD2, rather than AKT and mTOR, leading to high amplitude advanced rhythms.

Cardiorespiratory Fitness Is Associated with Phase Advanced Circadian Rhythms in Healthy Adolescents

Cardiorespiratory Fitness Is Associated with Phase Advanced Circadian Rhythms in Healthy Adolescents

Evening light environments can be designed to consolidate and increase the duration of REM-sleep

Evening exposure to short-wavelength light has disruptive effects on circadian rhythms and sleep. These effects can be mitigated by blocking short-wavelength (blue) frequencies, which has led to the development of evening blue-depleted light environments (BDLEs). We have previously reported that residing 5 days in an evening BDLE, compared with residing in a normal indoor light environment of similar photopic lux, advances circadian rhythms and increases the duration of rapid eye movement (REM) sleep in a randomized cross-over trial with twelve healthy participants. The current study extends these findings by testing whether residing in the evening BDLE affects the consolidation and microstructure of REM sleep in the same sample. Evening BDLE significantly reduces the fragmentation of REM sleep (p = 0.0003), and REM sleep microarousals in (p = 0.0493) without significantly changing REM density or the latency to first REM sleep episode. Moreover, the increased accumulation of REM sleep is not at the expense of NREM stage 3 sleep. BDLE further has a unique effect on REM sleep fragmentation (p = 0.0479) over and above that of circadian rhythms phase-shift, indicating a non-circadian effect of BDLE. If these effects can be replicated in clinical populations, this may have a therapeutic potential in disorders characterized by fragmented REM sleep.

145 Aerobic Fitness Is Associated with Advanced Circadian Rhythms in Adolescents

Abstract Introduction Aerobic fitness facilitates brain synaptic plasticity, which influences global and local sleep expression. While it is known that sleep patterns/behavior and non-rapid eye movement (NREM) sleep slow wave activity (SWA) tracks brain maturation, little is known about how aerobic fitness and sleep interact during development in youth. The aim of this pilot was to characterize relationships among aerobic fitness, measures of global/local sleep expression, and habitual sleep patterns in children and adolescents. We hypothesized that greater aerobic fitness would be associated with better sleep quality, indicated by increased SWA. Methods 20 adolescents (mean age=14.6±2.3 years old, range 11-17, 11 females) were evaluated for AF (peak VO2 assessed by ramp-type progressive cycle ergometry in the laboratory), habitual sleep duration and efficiency (continuous 7-14 day actigraphy with sleep diary), and topographic patterns of spectral power in slow wave, theta, and sleep spindle frequency ranges in non-rapid eye movement (NREM) sleep using overnight polysomnography with high-density electroencephalography (hdEEG, 128 channels). Results Significant relationships were observed between peak VO2 and habitual bedtime (r=-0.604, p=0.013) and wake-up time (r=-0.644, p=0.007), with greater fitness associated with an earlier sleep schedule (going to bed and waking up earlier). Peak VO2 was a significant predictor of slow oscillations (0.5-1Hz, p=0.018) and theta activity (4.5-7.5Hz, p=0.002) over anterior frontal and central derivations (p<0.001 and p=0.001, respectively) after adjusting for sex and pubertal development stage. Similar associations were detected for fast sleep spindle activity (13-16Hz, p=0.006), which was greater over temporo-parietal derivations. Conclusion Greater AF was associated with earlier habitual sleep times and with enhanced expression of developmentally-relevant sleep oscillations during NREM sleep. These data suggest that AF may 1) minimize the behavioral sleep delay commonly seen during adolescence, and 2) impact topographically-specific features of sleep physiology known to mechanistically support neuroplasticity and cognitive processes which are dependent on prefrontal cortex and hippocampal function in adolescents and adults. Support (if any) NCATS grant #UL1TR001414 & PERC Systems Biology Fund

Selective MT 2 Melatonin Receptor Antagonist Modulates Circadian Activity via Inhibition of the Endogenous Melatonin Signal in the East Bound Jet Lag Model

Activation of melatonin receptors facilitates re-entrainment of circadian activity rhythms following an advance of the dark onset. In an eastbound jet lag model, administration of exogenous melatonin, at the new dark onset, accelerates re-entrainment via activation of the MT1 receptor (Dubocovich et al., 2005). Conversely, melatonin proficient mice (C3H/HeN) with a targeted deletion of the MT2 receptor (MT2KO), but not the MT1 receptor (MT1KO), experience a deceleration in re-entrainment, probably due to impairment of the endogenous melatonin facilitated signaling mediated by activation of the MT2 receptor (Pfeffer et al., 2012). To further elucidate the functional modulatory role of melatonin receptors on the synchronization of the circadian clock, we assessed locomotor activity rhythms in C3H/HeN mice in response to pharmacological intervention to determine the ability of the MT2 selective antagonist, ATBT-23 (hMT1/hMT2 Ki=190), to decelerate re-entrainment by blocking the MT2 receptor, thus mimicking the MT2KO. Male C3H/HeN mice in a 12/12 L/D cycle were subjected to an eastbound jet lag paradigm in which Vehicle (30% ethanol/saline, s.c) or ATBT-23 (1 mg/kg in Vehicle, s.c) were administered on three consecutive days at the new dark onset following an abrupt 6-hour advance of the L/D cycle. The number of days to re-entrain and re-entrainment rates (hrs/day) were analyzed by two-way ANOVA (Tukey's post hoc test) and a mixed effect two-way repeated measure ANOVA (Sidak's post hoc test), respectively. WT (7.54±0.33 days, n=28) and MT1KO (7.36±0.54 days, n=14) mice treated with vehicle did not show a significant difference in the number of days necessary to attain re-entrainment. However, MT2KO (11.92±0.40 days, n=13) mice, treated with vehicle, took significantly longer to re-entrain when compared to WT and MT1KO mice (p<0.0001). This deceleration of re-entrainment in the MT2KO was reproduced by ATBT-23, which decelerated the rate of re-entrainment in both WT (p<0.0001, n=25) and MT1KO (p<0.01, n=12) males, increasing the number of days to re-entrain to 9.76±0.46 (p<0.01) and 10.00±0.63 (p<0.05) days, respectively. ATBT-23 (n=14) did not significantly affect the number of days to re-entrain in MT2KO mice, suggesting deceleration of re-entrainment is not mediated by the MT1 receptor. The present study suggests that ATBT-23 decelerates re-entrainment of circadian activity rhythms by blocking the potent acceleration of re-entrainment by endogenous melatonin via action on the MT2 receptor. Therefore, selective MT2 melatonin receptor antagonists can potentially be used to delay advanced circadian rhythms, as observed in depression and advance sleep phase disorders, when administered at proper circadian times.

Circadian Phase Advances in Response to Weekend Morning Light in Adolescents With Short Sleep and Late Bedtimes on School Nights.

Many adolescents fall asleep too late to get enough sleep (8–10 h) on school nights. Morning bright light advances circadian rhythms and could help adolescents fall asleep earlier. Morning bright light treatment before school, however, is difficult to fit into their morning schedule; weekends are more feasible. We examined phase advances in response to morning light treatment delivered over one weekend. Thirty-seven adolescents (16 males; 14.7–18.0 years) who reported short school-night sleep (≤7 h) and late bedtimes (school-nights ≥23:00; weekend/non-school nights ≥24:00) slept as usual at home for ∼2 weeks (“baseline”) and then kept a fixed sleep schedule (baseline school-night bed and wake-up times ±30 min) for ∼1 week before living in the lab for one weekend. Sleep behavior was measured with wrist actigraphy and sleep diary. On Saturday morning, we woke each participant 1 h after his/her midpoint of baseline weekend/non-school night sleep and 1 h earlier on Sunday. They remained in dim room light (∼20 lux) or received 1.5 or 2.5 h of intermittent morning bright light (∼6000 lux) on both mornings. The dim light melatonin onset (DLMO), a phase marker of the circadian timing system, was measured on Friday and Sunday evenings to compute the weekend circadian phase shift. The dim room light and 1.5-h bright light groups advanced the same amount (0.6 ± 0.4 and 0.6 ± 0.5 h). The 2.5-h bright light group advanced 1.0 ± 0.4 h, which was significantly more than the other groups. These data suggest that it is possible to phase advance the circadian clock of adolescents who have late bedtimes and short school-night sleep in one weekend using light that begins shortly after their sleep midpoint.

Correcting delayed circadian phase with bright light therapy predicts improvement in ADHD symptoms: A pilot study

Attention-deficit/hyperactivity disorder (ADHD) is a common condition with comorbid insomnia reported in >70% of children and adults. These patients demonstrate delays in sleep-wake rhythms, nocturnal rise in melatonin, and early morning rise in cortisol. Given that standard psychopharmacologic treatments for ADHD often do not completely control symptoms in participants with circadian rhythm delay, we sought to test whether bright light therapy (BLT) advances circadian rhythms and further reduces ADHD symptoms over standard treatments. In addition to standard of care, participants with ADHD diagnosis underwent 1 week of baseline assessment followed by 2-weeks of 30-min morning 10,000-lux BLT beginning 3 h after mid-sleep time. Participants minimized overhead light after 4 p.m., wore an actigraphy watch, and recorded BLT time on daily sleep logs. Dim Light Melatonin Onset (DLMO) was assessed at baseline and after 2-week treatment. ADHD symptoms were measured by the ADHD-Rating Scales (ADHD-RS). BLT significantly advanced the phase of DLMO by 31 min [mean time (SEM), 20:36 (0:21) advanced to 20:05 (0:20)] and mid-sleep time by 57 min [4:37 (0:22) advanced to 3:40 (0:16); paired t-tests, p = 0.002 and 0.004, respectively). Phase advances (in DLMO or mid-sleep time) were significantly correlated with decreased ADHD-RS total scores (p = 0.027 and 0.044) and Hyperactive-Impulsive sub-scores (p = 0.014 and 0.013, respectively). Actigraphy analysis for a subset of 8 participants with significant DLMO phase advance revealed no significant changes in total sleep time, sleep efficiency, wake after sleep onset, or percent wake during sleep interval. This is the first successful use of BLT for advancing melatonin phase and improving ADHD symptoms in adults. BLT may be a complementary treatment for both delayed sleep timing and ADHD symptoms in adults.

Evaluation of the Effect of Melatonin On Improvment of Sleep Quality in Children with Attentin Deficit/hyperactivity Disorder Whom Received Ritalin

Attention-deficit/hyperactivity disorder (ADHD) is one of the most common psychiatric disorders in childhood. Around 25-50% of these children suffered from some kind of sleep disorder especially with chronic form of insomnia. Nowadays, we know that there is a noticeable relationship between ADHD and sleep disorders and by improving these children's sleep, not only the daily functions of them improve, but also the symptoms of ADHD maybe become better. This research evaluates the effects of melatonin on improvement of Sleep quality in children with ADHD whom received Ritalin Participants included 22 children suffered ADHD and concomitant sleep problems patients who underwent melatonin prescription and placebo at night in two sequencial periods. The effects of the melatonin were compared with placebo on the sleep problem improvements in each one. Although sleep onset, total sleep time and sleep quality advanced with melatonin, wake up time and the time to go to bed had not significant difference between the melatonin and placebo group. In addition, there were significant effects on attention, hyper activity index and impulsivity of the patients in the melatonin group. Melatonin advanced circadian rhythms of sleep-wake and not only enhanced total time asleep in children with ADHD and chronic sleep onset insomnia, but also can improve the quality of life of these patients.

Phase Resetting of the Mammalian Circadian Clock by DNA Damage

To anticipate the momentum of the day, most organisms have developed an internal clock that drives circadian rhythms in metabolism, physiology, and behavior [1]. Recent studies indicate that cell-cycle progression and DNA-damage-response pathways are under circadian control [2-4]. Because circadian output processes can feed back into the clock, we investigated whether DNA damage affects the mammalian circadian clock. By using Rat-1 fibroblasts expressing an mPer2 promoter-driven luciferase reporter, we show that ionizing radiation exclusively phase advances circadian rhythms in a dose- and time-dependent manner. Notably, this in vitro finding translates to the living animal, because ionizing radiation also phase advanced behavioral rhythms in mice. The underlying mechanism involves ATM-mediated damage signaling as radiation-induced phase shifting was suppressed in fibroblasts from cancer-predisposed ataxia telangiectasia and Nijmegen breakage syndrome patients. Ionizing radiation-induced phase shifting depends on neither upregulation or downregulation of clock gene expression nor on de novo protein synthesis and, thus, differs mechanistically from dexamethasone- and forskolin-provoked clock resetting [5]. Interestingly, ultraviolet light and tert-butyl hydroperoxide also elicited a phase-advancing effect. Taken together, our data provide evidence that the mammalian circadian clock, like that of the lower eukaryote Neurospora[6], responds to DNA damage and suggest that clock resetting is a universal property of DNA damage.

Effect of Melatonin on Sleep, Behavior, and Cognition in ADHD and Chronic Sleep-Onset Insomnia

To investigate the effect of melatonin treatment on sleep, behavior, cognition, and quality of life in children with attention-deficit/hyperactivity disorder (ADHD) and chronic sleep onset insomnia. A total of 105 medication-free children, ages 6 to 12 years, with rigorously diagnosed ADHD and chronic sleep onset insomnia participated in a randomized, double-blind, placebo-controlled trial using 3 or 6 mg melatonin (depending on body weight), or placebo for 4 weeks. Primary outcome parameters were actigraphy-derived sleep onset, total time asleep, and salivary dim light melatonin onset. Sleep onset advanced by 26.9 +/- 47.8 minutes with melatonin and delayed by 10.5 +/- 37.4 minutes with placebo (p < .0001). There was an advance in dim light melatonin onset of 44.4 +/- 67.9 minutes in melatonin and a delay of 12.8 +/- 60.0 minutes in placebo (p < .0001). Total time asleep increased with melatonin (19.8 +/- 61.9 minutes) as compared to placebo (-13.6 +/- 50.6 minutes; p = .01). There was no significant effect on behavior, cognition, and quality of life, and significant adverse events did not occur. Melatonin advanced circadian rhythms of sleep-wake and endogenous melatonin and enhanced total time asleep in children with ADHD and chronic sleep onset insomnia; however, no effect was found on problem behavior, cognitive performance, or quality of life.

Ramelteon Phase Advanced Circadian Rhythms of Neuronal Firing in The Suprachiasmatic Nucleus (SCN) Brain Slice by Activation of Both MT 1 and MT 2 Melatonin (MLT) Receptors

Ramelteon, a high affinity MT1 (Ki: 14 pM) and MT2 (Ki: 104 pM) MLT receptor agonist promotes sleep and phase shifts circadian rhythms (Curr Opinion Inv Drugs 1:–121, 2005). This study determined the MLT receptor type(s) involved in ramelteon-mediated phase shifts of spontaneous circadian rhythms of neuronal firing in SCN brain slices from C3H/HeN mice using single-unit recordings. SCN brain slices from wild type (WT) mice showed a peak of neuronal firing at circadian time (CT) 6.1 ± 0.1 h (n=5) [CT12: activity onset for host mice], which was advanced by ramelteon (10 pM, 10 min) application on day 1 at CT 10 to CT 1.2 ± 0.7 h (n=4, p < 0.0001) on day 2. Ramelteon significantly advanced the peak of neuronal firing in SCN from WT (4.9 ± 0.7 h, n=4), MT1KO (3.4 ± 0.15 h, n=5) and MT2KO (1.6 ± 0.4 h, n=3), but not from MT1MT2KO mice. Ramelteon-mediated phase advance in SCN brain slices from WT mice was MLT receptor specific and resulted from activation of MT2 receptors with a smaller contribution from MT1 receptors. MLT (10 pM), however, induced phase advances of identical magnitude via activation of MT2 receptors in SCN from WT (2.7 ± 0.15h, n=4) and MT1 KO (3 ± 0.1h, n=4) mice, but did not affect phase in SCN from MT2KO or MT1MT2KO mice (Hudson et al., Neuropshychopharmacol 30:, 2005). We conclude that ramelteon uncovered a functional MT1 melatonin receptor in the SCN also involved in phase shifting circadian rhythms. Supported by a Takeda Investigator-Sponsored Research Grant 06-016R.

An arousing, musically enhanced bird song stimulus mediates circadian rhythm phase advances in dim light

A musically enhanced bird song stimulus presented in the early subjective night phase delays human circadian rhythms. This study determined the phase-shifting effects of the same stimulus in the early subjective day. Eleven subjects (ages 18-63 yr; mean +/- SD: 28.0 +/- 16.6 yr) completed two 4-day laboratory sessions in constant dim light (<20 lux). They received two consecutive presentations of either a 2-h musically enhanced bird song or control stimulus from 0600 to 0800 on the second and third mornings while awake. The 4-day sessions employing either the stimulus or control were counterbalanced. Core body temperature (CBT) was collected throughout the study, and salivary melatonin was obtained every 30 min from 1900 to 2330 on the baseline and poststimulus/postcontrol nights. Dim light melatonin onset and CBT minimum circadian phase before and after stimulus or control presentation was assessed. The musically enhanced bird song stimulus produced significantly larger phase advances of the circadian melatonin (mean +/- SD: 0.87 +/- 0.36 vs. 0.24 +/- 0.22 h) and CBT (1.08 +/- 0.50 vs. 0.43 +/- 0.37 h) rhythms than the control. The stimulus also decreased fatigue and total mood disturbance, suggesting arousing effects. This study shows that a musically enhanced bird song stimulus presented during the early subjective day phase advances circadian rhythms. However, it remains unclear whether the phase shifts are due directly to effects of the stimulus on the clock or are arousal- or dim light-mediated effects. This nonphotic stimulus mediates circadian resynchronization in either the phase advance or delay direction.

Advancing Human Circadian Rhythms with Afternoon Melatonin and Morning Intermittent Bright Light

Both light and melatonin can be used to phase shift the human circadian clock, but the phase-advancing effect of the combination has not been extensively investigated. The objective of the study was to determine whether phase advances induced by morning intermittent bright light and a gradually advancing sleep schedule could be increased with afternoon melatonin. Healthy adults (25 males, 19 females, between the ages of 19 and 45 yr) participated in the study. There were 3 d of a gradually advancing sleep/dark period (wake time 1 h earlier each morning), bright light on awakening [four 30-min bright-light pulses (approximately 5000 lux) alternating with 30 min room light < 60 lux] and afternoon melatonin, either 0.5 or 3.0 mg melatonin timed to induce maximal phase advances, or matching placebo. The dim light melatonin onset was measured before and after the treatment to determine the phase advance. There were significantly larger phase advances with 0.5 mg (2.5 h, n = 16) and 3.0 mg melatonin (2.6 h, n = 13), compared with placebo (1.7 h, n = 15), but there was no difference between the two melatonin doses. Subjects did not experience jet lag-type symptoms during the 3-d treatment Afternoon melatonin, morning intermittent bright light, and a gradually advancing sleep schedule advanced circadian rhythms almost 1 h/d and thus produced very little circadian misalignment. This treatment could be used in any situation in which people need to phase advance their circadian clock, such as before eastward jet travel or for delayed sleep phase syndrome.

The Treatment of Early-Morning Awakening Insomnia With 2 Evenings of Bright Light

To assess the effectiveness of brief bright-light therapy for the treatment of early-morning awakening insomnia. Twenty-four healthy adults with early-morning awakening insomnia were assigned to either the bright-light condition (2,500-lux white light) or the control (dim red light) condition. The circadian phase of rectal temperature and urinary melatonin rhythms were assessed with 26-hour constant routines before and after 2 evenings of light therapy. Sleep and daytime functioning were monitored using sleep diaries, activity monitors, and mood scales before light therapy and for 4 weeks during the follow-up period. While there were no significant circadian phase changes in the dim-light control group, the bright-light group had significant 2-hour phase delays of circadian temperature and melatonin rhythm. Compared to pretreatment measures, over the 4-week follow-up period, the bright-light group had a greater reduction of time awake after sleep onset, showed a trend toward waking later, and had a greater increase of total sleep time. Participants in the bright-light condition also tended to report greater reductions of negative daytime symptoms, including significantly fewer days of feeling depressed at the 4-week follow-up, as compared with the control group. Two evenings of bright-light exposure phase delayed the circadian rhythms of early-morning awakening insomniacs. It also improved diary and actigraphy sleep measures and improved some indexes of daytime functioning for up to 1 month after light exposure. The study suggests that a brief course of evening bright-light therapy can be an effective treatment for early-morning awakening insomniacs who have relatively phase advanced circadian rhythms.

Naps and circadian rhythms in postmenopausal women.

Napping patterns and relationships among naps, circadian rhythms, and nocturnal sleep were evaluated in postmenopausal women. Naps and nocturnal sleep were compared between depressed women and healthy volunteers. The study included 436 postmenopausal women between the ages of 50 and 81 years. Psychiatric interviews were conducted using the Structured Clinical Interview for DSM-IV Axis I Disorders. Each participant wore a wrist-activity monitor and kept a daily sleep log at home for 1 week. The major urinary melatonin metabolite (6-sulfatoxymelatonin [aMT6s]) was measured for two 24-hour periods at home. There was no significant difference in naps and nocturnal sleep patterns between depressed participants (n = 30) and those with no history of mental disorder (n = 222). Three peaks occurred in the timing of napping: approximately 1 hour before bedtime, 8 to 9 hours after wake-up time, and 2 hours after waking, in descending order of magnitude. Significant inverse correlations were observed between evening nap duration and both wake-up time and aMT6s acrophase. The onset time of aMT6s excretion was advanced by 32 minutes in evening nappers. Evening napping, a characteristic of these postmenopausal women, may be related to an advance of circadian rhythms in this age group. Relationships between evening napping and earlier wake-up time may be a common manifestation of advanced circadian rhythms or may be a homeostatic effect.

24-h Monitoring of plasma norepinephrine, MHPG, cortisol, growth hormone and prolactin in depression

Depression is associated with alterations in hormone and catecholamine circadian rhythms. Analysis of these alterations has the potential to distinguish between three neurobiological models of depression, the catecholamine model, the phase advance model and the dysregulation model. Although a number of studies of 24-h rhythms have been reported, inconsistencies among the findings have complicated efforts to model the chronobiology of depression. The present study takes advantage of frequent plasma sampling over the 24-h period and a multioscillator cosinor model to fit the 24-h rhythms. Method: Plasma levels of norepinephrine, cortisol, prolacatin and growth hormone were sampled at 30-min intervals, and MHPG at 60-min intervals, over a 24-h period in 22 patients with major depressive disorder and 20 healthy control volunteers. Results: The depressed patients had phase advanced circadian rhythms for cortisol, norepinephrine and MHPG, phase advanced hemicircadian rhythms for cortisol and prolactin, and a phase advanced ultradian rhythm for prolactin compared to healthy control subjects. In addition, the rhythm-corrected 24-h mean value (mesor) of norepinephrine was lower in the depressed patients compared to the healthy controls. There also was a poorer goodness-of-fit for norepinephrine to the circadian oscillator in the depressed patients relative to the healthy controls. Conclusions: These findings provide partial support for the dysregulation model of depression and are consistent with those studies that have found phase advances in cortisol, norepinephrine and MHPG rhythms in depression.

Photic phase response curve in Octodon degus: assessment as a function of activity phase preference.

Light exposure during the early and late subjective night generally phase delays and advances circadian rhythms, respectively. However, this generality was recently questioned in a photic entrainment study in Octodon degus. Because degus can invert their activity phase preference from diurnal to nocturnal as a function of activity level, assessment of phase preference is critical for computations of phase reference [circadian time (CT) 0] toward the development of a photic phase response curve. After determining activity phase preference in a 24-h light-dark cycle (LD 12:12), degus were released in constant darkness. In this study, diurnal (n = 5) and nocturnal (n = 7) degus were randomly subjected to 1-h light pulses (30-35 lx) at many circadian phases (CT 1-6: n = 7; CT 7-12: n = 8; CT 13-18: n = 8; and CT 19-24: n = 7). The circadian phase of body temperature (Tb) onset was defined as CT 12 in nocturnal animals. In diurnal animals, CT 0 was determined as Tb onset + 1 h. Light phase delayed and advanced circadian rhythms when delivered during the early (CT 13-16) and late (CT 20-23) subjective night, respectively. No significant phase shifts were observed during the middle of the subjective day (CT 3-10). Thus, regardless of activity phase preference, photic entrainment of the circadian pacemaker in Octodon degus is similar to most other diurnal and nocturnal species, suggesting that entrainment mechanisms do not determine overt diurnal and nocturnal behavior.

Selective MT2 melatonin receptor antagonists block melatonin-mediated phase advances of circadian rhythms.

This study demonstrates the involvement of the MT2 (Mel1b) melatonin receptor in mediating phase advances of circadian activity rhythms by melatonin. In situ hybridization histochemistry with digoxigenin-labeled oligonucleotide probes revealed for the first time the expression of mt1 and MT2 melatonin receptor mRNA within the suprachiasmatic nucleus of the C3H/HeN mouse. Melatonin (0.9 to 30 microg/mouse, s.c.) administration during 3 days at the end of the subjective day (CT 10) to C3H/HeN mice kept in constant dark phase advanced circadian rhythms of wheel running activity in a dose-dependent manner [EC50=0.72 microg/mouse; 0.98+/-0.08 h (n=15) maximal advance at 9 microg/mouse]. Neither the selective MT2 melatonin receptor antagonists 4P-ADOT and 4P-PDOT (90 microg/mouse, s.c.) nor luzindole (300 microg/mouse, s.c.), which shows 25-fold higher affinity for the MT2 than the mt1 subtype, affected the phase of circadian activity rhythms when given alone at CT 10. All three antagonists, however, shifted to the right the dose-response curve to melatonin, as they significantly reduced the phase shifting effects of 0.9 and 3 microg melatonin. This is the first study to demonstrate that melatonin phase advances circadian rhythms by activation of a membrane-bound melatonin receptor and strongly suggests that this effect is mediated through the MT2 melatonin receptor subtype within the circadian timing system. We conclude that the MT2 melatonin receptor subtype is a novel therapeutic target for the development of subtype-selective analogs for the treatment of circadian sleep and mood-related disorders.

The aged suprachiasmatic nucleus is phase-shifted by cAMP in vitro

The cyclic adenosine monophosphate (cAMP) analog, 8-bromo-cAMP, phase advanced circadian neuronal rhythms in both aged and adult rat suprachiasmatic nuclei (SCN) by ≈2 h in vitro. Rhythm amplitude was 20% lower in aged compared to adult SCN. The diminished efficacy of serotonergic agonists to phase shift behavioral rhythms of aged animals may be due to decrements in signal transduction mechanisms proximal to cAMP.