Behavioral arousal blocks light-induced phase advances in locomotor rhythmicity but not light-induced Per1 and Fos expression in the hamster suprachiasmatic nucleus

Vasopressin Resets the Central Circadian Clock in a Manner Influenced by Sex and Vasoactive Intestinal Polypeptide Signaling

BACKGROUND/AIMS

Circadian rhythms in behavior and physiology are programmed by the suprachiasmatic nucleus (SCN) of the hypothalamus. A subset of SCN neurons produce the neuropeptide arginine vasopressin (AVP), but it remains unclear whether AVP signaling influences the SCN clock directly.

METHODS

Here, we test that AVP signaling acting through V1A and V1B receptors influences molecular rhythms in SCN neurons. V1 receptor agonists were applied ex vivo to PERIOD2::LUCIFERASE SCN slices, allowing for real-time monitoring of changes in molecular clock function.

RESULTS

V1A/B agonists reset the phase of the SCN molecular clock in a time-dependent manner, with larger magnitude responses by the female SCN. Further, we found evidence that both Gαq and Gαs signaling pathways interact with V1A/B-induced SCN resetting, and that this response requires vasoactive intestinal polypeptide (VIP) signaling.

CONCLUSIONS

Collectively, this work indicates that AVP signaling resets SCN molecular rhythms in conjunction with VIP signaling and in a manner influenced by sex. This highlights the utility of studying clock function in both sexes and suggests that signal integration in central clock circuits regulates emergent properties important for the control of daily rhythms in behavior and physiology.

Light Effects on Behavioural Performance Depend on the Individual State of Vigilance

Research has shown that exposure to bright white light or blue-enriched light enhances alertness, but this effect is not consistently observed in tasks demanding high-level cognition (e.g., Sustained Attention to Response Task-SART, which measures inhibitory control). Individual differences in sensitivity to light effects might be mediated by variations in the basal level of arousal. We tested this hypothesis by measuring the participants' behavioural state of vigilance before light exposure, through the Psychomotor Vigilance Task. Then we compared the effects of a blue-enriched vs. dim light at nighttime on the performance of the auditory SART, by controlling for individual differences in basal arousal. The results replicated the alerting effects of blue-enriched light, as indexed by lower values of both proximal temperature and distal-proximal gradient. The main finding was that lighting effects on SART performance were highly variable across individuals and depended on their prior state of vigilance. Specifically, participants with higher levels of basal vigilance before light exposure benefited most from blue-enriched lighting, responding faster in the SART. These results highlight the importance of considering basal vigilance to define the boundary conditions of light effects on cognitive performance. Our study adds to current research delineating the complex and reciprocal interactions between lighting effects, arousal, cognitive task demands and behavioural performance.

Neurobiology of the Sleep-Wake Cycle: Sleep Architecture, Circadian Regulation, and Regulatory Feedback

This mini-review article presents the remarkable progress that has been made in the past decade in our understanding of the neural circuitry underlying the regulation of sleep-wake states and circadian control of behaviors. Following a brief introduction to sleep architecture and physiology, the authors describe the neural circuitry and neurotransmitters that regulate sleep and cortical arousal (i.e., wakefulness). They next examine how sleep and wakefulness are regulated by mutual inhibition between sleep-and arousal-promoting circuitry and how this interaction functions analogously to an electronic “flip-flop” switch that ensures behavioral state stability. The authors then discuss the role of circadian and homeostatic processes in the consolidation of sleep, including the physiologic basis of homeostatic sleep drive (i.e., wake-dependent increase in sleep propensity) and the role of the SCN in the circadian regulation of sleep-wake cycles. Finally, they describe the hypothalamic circuitry for the integration of photic and nonphotic environmental time cues and how this integration allows organisms to sculpt patterns of rest-activity and sleep-wake cycles that are optimally adaptive.

N-nitrosomelatonin enhances photic synchronization of mammalian circadian rhythms

Most physiological processes in mammals are synchronized to the daily light:dark cycle by a circadian clock located in the hypothalamic suprachiasmatic nucleus. Signal transduction of light-induced phase advances of the clock is mediated through a neuronal nitric oxide synthase-guanilyl cyclase pathway. We have employed a novel nitric oxide-donor, N-nitrosomelatonin, to enhance the photic synchronization of circadian rhythms in hamsters. The intraperitoneal administration of this drug before a sub-saturating light pulse at circadian time 18 generated a twofold increase of locomotor rhythm phase-advances, having no effect over saturating light pulses. This potentiation was also obtained even when inhibiting suprachiasmatic nitric oxide synthase activity. However, N-nitrosomelatonin had no effect on light-induced phase delays at circadian time 14. The photic-enhancing effects were correlated with an increased suprachiasmatic immunoreactivity of FBJ murine osteosarcoma viral oncogene and period1. Moreover, in vivo nitric oxide release by N-nitrosomelatonin was verified by measuring nitrate and nitrite levels in suprachiasmatic nuclei homogenates. The compound also accelerated resynchronization to an abrupt 6-h advance in the light:dark cycle (but not resynchronization to a 6-h delay). Here, we demonstrate the chronobiotic properties of N-nitrosomelatonin, emphasizing the importance of nitric oxide-mediated transduction for circadian phase advances.

Drugs that prevent mouse sleep also block light-induced locomotor suppression, circadian rhythm phase shifts and the drop in core temperature

Exposure of mice to a brief light stimulus during their nocturnal active phase induces several simultaneous behavioral or physiological responses, including circadian rhythm phase shifts, a drop in core body temperature (Tc), suppression of locomotor activity and sleep. Each response is triggered by light, endures for a relatively fixed interval and does not require additional light for expression. The present studies address the ability of the psychostimulant drugs, methamphetamine (MA), modafinil (MOD) or caffeine (CAF), to modify the light-induced responses. Drug or vehicle (VEH) was injected at CT11 into constant dark-housed mice then exposed to 5-min 100μW/cm(2) light or no light at CT13. Controls (VEH/Light) showed approximately 60-min phase delays. In contrast, response was substantially attenuated by each drug (only 12-15min delays). Under a 12-h light:12-h dark (LD12:12) photoperiod, VEH/light-treated mice experienced a Tc drop of about 1.3°C coincident with locomotor suppression and both effects were abolished by drug pre-treatment. Each drug elevated activity during the post-injection interval, but there was also evidence for CAF-induced hypoactivity in the dark prior to the photic test stimulus. CAF acutely elevated Tc; MA acutely lowered it, but both drugs reduced Tc during the early dark (ZT12.5-ZT13). The ability of the psychostimulant drugs to block the several effects of light exposure is not the result of drug-induced hyperactivity. The results raise questions concerning the manner in which drugs, activity, sleep and Tc influence behavioral and physiological responses to light.

Nocturnal Light and Nocturnal Rodents

Investigators typically study one function of the circadian visual system at a time, be it photoreception, transmission of photic information to the suprachiasmatic nucleus (SCN), light control of rhythm phase, locomotor activity, or gene expression. There are good reasons for such a focused approach, but sometimes it is advantageous to look at the broader picture, asking how all the parts and functions complete the whole. Here, several seemingly disparate functions of the circadian visual system are examined. They share common characteristics with respect to regulation by light and, to the extent known, share a common input neuroanatomy. The argument presented is that the 3 hypothalamically mediated effects of light for which there are the most data, circadian clock phase shifts, suppression of nocturnal locomotion (“negative masking”), and suppression of nocturnal pineal function, are regulated by a common photic input pathway terminating in the SCN. For each, light triggers a relatively fixed interval response that is irradiance-dependent, the effective stimulus can be very brief light exposure, and the response continues to completion in the absence of additional light. The presence of a triggered, fixed-length response interval is of particular importance to the understanding of the circuitry and mechanisms regulating circadian rhythm phase shifts because it implies that the SCN clock response to light is not instantaneous. It also may explain why certain stimuli (neuropeptide Y or novel wheel running) administered many minutes after light exposure are able to block light-induced phase shifts. The understanding of negative masking is complicated by the fact that it can be represented as a positive change, that is, light-induced sleep, not just as a reduction in locomotion. Acute nocturnal light exposure also induces adrenal hormone secretion and a rapid drop in body temperature, physiological responses that appear to be regulated similarly to the other light effects. The likelihood of a common regulatory basis for the several responses suggests that additional light-induced responses will be forthcoming and raises questions about the relationships between light, SCN cellular anatomy, the molecular clockworks of SCN neurons, and SCN throughput mechanisms for regulating disparate downstream activities.

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).

cGMP-phosphodiesterase inhibition enhances photic responses and synchronization of the biological circadian clock in rodents

The master circadian clock in mammals is located in the hypothalamic suprachiasmatic nuclei (SCN) and is synchronized by several environmental stimuli, mainly the light-dark (LD) cycle. Light pulses in the late subjective night induce phase advances in locomotor circadian rhythms and the expression of clock genes (such as Per1-2). The mechanism responsible for light-induced phase advances involves the activation of guanylyl cyclase (GC), cGMP and its related protein kinase (PKG). Pharmacological manipulation of cGMP by phosphodiesterase (PDE) inhibition (e.g., sildenafil) increases low-intensity light-induced circadian responses, which could reflect the ability of the cGMP-dependent pathway to directly affect the photic sensitivity of the master circadian clock within the SCN. Indeed, sildenafil is also able to increase the phase-shifting effect of saturating (1200 lux) light pulses leading to phase advances of about 9 hours, as well as in C57 a mouse strain that shows reduced phase advances. In addition, sildenafil was effective in both male and female hamsters, as well as after oral administration. Other PDE inhibitors (such as vardenafil and tadalafil) also increased light-induced phase advances of locomotor activity rhythms and accelerated reentrainment after a phase advance in the LD cycle. Pharmacological inhibition of the main downstream target of cGMP, PKG, blocked light-induced expression of Per1. Our results indicate that the cGMP-dependent pathway can directly modulate the light-induced expression of clock-genes within the SCN and the magnitude of light-induced phase advances of overt rhythms, and provide promising tools to design treatments for human circadian disruptions.

Scheduled Food Hastens Re-Entrainment More Than Melatonin Does after a 6-h Phase Advance of the Light-Dark Cycle in Rats

Circadian desynchrony occurs when individuals are exposed to abrupt phase shifts of the light-dark cycle, as in jet lag. For reducing symptoms and for speeding up resynchronization, several strategies have been suggested, including scheduled exercise, exposure to bright light, drugs, and especially exogenous melatonin administration. Restricted feeding schedules have shown to be powerful entraining signals for metabolic and hormonal daily cycles, as well as for clock genes in tissues and organs of the periphery. This study explored in a rat model of jet lag the contribution of exogenous melatonin or scheduled feeding on the re-entrainment speed of spontaneous general activity and core temperature after a 6-h phase advance of the light-dark cycle. In a first phase, the treatment was scheduled for 5 days prior to the phase shift, while in a second stage, the treatment was simultaneous with the phase advance of the light-dark cycle. Melatonin administration and especially scheduled feeding simultaneous with the phase shift improved significantly the re-entrainment speed. The evaluation of the free-running activity and temperature following the 5-day treatment proved that both exogenous melatonin and specially scheduled feeding accelerated re-entrainment of the SCN-driven general activity and core temperature, respectively, with 7, 5 days ( p < 0.01) and 3, 3 days ( p < 0.001). The present results show the relevance of feeding schedules as entraining signals for the circadian system and highlight the importance of using them as a strategy for preventing internal desynchrony.

On the intrinsic regulation of neuropeptide Y release in the mammalian suprachiasmatic nucleus circadian clock

Timing of the circadian clock of the suprachiasmatic nucleus (SCN) is regulated by photic and non-photic inputs. Of these, neuropeptide Y (NPY) signaling from the intergeniculate leaflet (IGL) to the SCN plays a prominent role. Although NPY is critical to clock regulation, neither the mechanisms modulating IGL NPY neuronal activity nor the nature of regulatory NPY signaling in the SCN clock are understood, as NPY release in the SCN has never been measured. Here, microdialysis procedures for in vivo measurement of NPY were used in complementary experiments to address these questions. First, neuronal release of NPY in the hamster SCN was rhythmic under a 14L : 10D photocycle, with the acrophase soon after lights-on and the nadir at midday. No rhythmic fluctuation in NPY occurred under constant darkness. Second, a behavioral phase-resetting stimulus (wheel-running at midday that induces IGL serotonin release) acutely stimulated SCN NPY release. Third, bilateral IGL microinjection of the serotonin agonist, (+/-)-2-dipropyl-amino-8-hydroxyl-1,2,3,4-tetrahydronapthalene (8-OH-DPAT) (another non-photic phase-resetting stimulant), at midday enhanced SCN NPY release. Conversely, similar application of the serotonin antagonist, metergoline, abolished wheel-running-induced SCN NPY release. IGL microinjection of the GABA agonist, muscimol, suppressed SCN NPY release. These results support an intra-IGL mechanism whereby behavior-induced serotonergic activity suppresses inhibitory GABAergic transmission, promoting NPY activity and subsequent phase resetting. Collectively, these results confirm IGL-mediated NPY release in the SCN and verify that its daily rhythm of release is dependent upon the 14L : 10D photocycle, and that it is modulated by appropriately-timed phase-resetting behavior, probably mediated by serotonergic activation of NPY units in the IGL.

"Time sweet time": circadian characterization of galectin-1 null mice

BACKGROUND

Recent evidence suggests a two-way interaction between the immune and circadian systems. Circadian control of immune factors, as well as the effect of immunological variables on circadian rhythms, might be key elements in both physiological and pathological responses to the environment. Among these relevant factors, galectin-1 is a member of a family of evolutionarily-conserved glycan-binding proteins with both extracellular and intracellular effects, playing important roles in immune cell processes and inflammatory responses. Many of these actions have been studied through the use of mice with a null mutation in the galectin-1 (Lgals1) gene. To further analyze the role of endogenous galectin-1 in vivo, we aimed to characterize the circadian behavior of galectin-1 null (Lgals1-/-) mice.

METHODS

We analyzed wheel-running activity in light-dark conditions, constant darkness, phase responses to light pulses (LP) at circadian time 15, and reentrainment to 6 hour shifts in light-dark schedule in wild-type (WT) and Lgals1-/- mice.

RESULTS

We found significant differences in free-running period, which was longer in mutant than in WT mice (24.02 vs 23.57 h, p < 0.005), phase delays in response to LP (2.92 vs 1.90 circadian h, p < 0.05), and also in alpha (14.88 vs. 12.35 circadian h, p < 0.05).

CONCLUSIONS

Given the effect of a null mutation on circadian period and entrainment, we indicate that galectin-1 could be involved in the regulation of murine circadian rhythmicity. This is the first study implicating galectin-1 in the mammalian circadian system.

New light on the serotonergic paradox in the rat circadian system

The main mammalian circadian clock, localized in the suprachiasmatic nuclei can be synchronized not only with light, but also with serotonergic activation. Serotonergic agonists and serotonin reuptake inhibitors (e.g., fluoxetine) have a non-photic influence (shifting effects during daytime and attenuation of photic resetting during nighttime) on hamsters' and mice' main clock. Surprisingly, in rats serotonergic modulation of the clock shows essentially photic-like features in vivo (shifting effects during nighttime). To delineate this apparent paradox, we analyzed the effects of fluoxetine and serotonin agonists on rats' clock. First, fluoxetine induced behavioral phase-advances associated with down-regulated expression of the clock genes Per1 and Rorbeta and up-regulated expression of Rev-erbalpha during daytime. Moreover, fluoxetine produced an attenuation of light-induced phase-advances in association with altered expression of Per1, Per2 and Rorbeta during nighttime. Second, we showed that 5-HT(1A) receptors -maybe with co-activation of 5-HT(7) receptors- were implicated in non-photic effects on the main clock. By contrast, 5-HT(3) and 5-HT(2C) receptors were involved in photic-like effects and, for 5-HT(2C) subtype only, in potentiation of photic resetting. Thus this study demonstrates that as for other nocturnal rodents, a global activation of the serotonergic system induces non-photic effects in the rats' clock during daytime and nighttime.

High-fat feeding alters the clock synchronization to light

High-fat feeding in rodents leads to metabolic abnormalities mimicking the human metabolic syndrome, including obesity and insulin resistance. These metabolic diseases are associated with altered temporal organization of many physiological functions. The master circadian clock located in the suprachiasmatic nuclei controls most physiological functions and metabolic processes. Furthermore, under certain conditions of feeding (hypocaloric diet), metabolic cues are capable of altering the suprachiasmatic clock's responses to light. To determine whether high-fat feeding (hypercaloric diet) can also affect resetting properties of the suprachiasmatic clock, we investigated photic synchronization in mice fed a high-fat or chow (low-fat) diet for 3 months, using wheel-running activity and body temperature rhythms as daily phase markers (i.e. suprachiasmatic clock's hands). Compared with the control diet, mice fed with the high-fat diet exhibited increased body mass index, hyperleptinaemia, higher blood glucose, and increased insulinaemia. Concomitantly, high-fat feeding led to impaired adjustment to local time by photic resetting. At the behavioural and physiological levels, these alterations include slower rate of re-entrainment of behavioural and body temperature rhythms after 'jet-lag' test (6 h advanced light-dark cycle) and reduced phase-advancing responses to light. At a molecular level, light-induced phase shifts have been correlated, within suprachiasmatic cells, with a high induction of c-FOS, the protein product of immediate early gene c-fos, and phosphorylation of the extracellular signal-regulated kinases I/II (P-ERK). In mice fed a high-fat diet, photic induction of both c-FOS and P-ERK in the suprachiasmatic nuclei was markedly reduced. Taken together, the present data demonstrate that high-fat feeding modifies circadian synchronization to light.

Interactions of GABA A receptor activation and light on period mRNA expression in the suprachiasmatic nucleus

Activation of gamma-aminobutyric acid (GABA) A receptors in the suprachiasmatic nucleus (SCN) resets the circadian clock during the day and inhibits the ability of light to reset the clock at night. Light in turn acts during the day to inhibit the phase-resetting effects of GABA. Some evidence suggests that Period mRNA changes in the SCN are responsible for these interactions between light and GABA. Here, the hypothesis that light and the GABA A receptor interact by altering the expression of Period 1 and/or Period 2 mRNA in the SCN is tested. The GABA A agonist muscimol was injected near the SCN just prior to a light pulse, during the mid-subjective day and the early and late subjective night. Changes in Period 1 and Period 2 mRNA were measured in the SCN by in situ hybridization. Light-induced Period 1 mRNA was inhibited by GABA A receptor activation in the early and late subjective night, while Period 2 mRNA was only inhibited during the late night. During the subjective day, light had no effect on the ability of muscimol to suppress Period 1 mRNA hybridization signal. Thus, light and GABA A receptor activation inhibit each other's ability to induce behavioral phase shifts throughout the subjective day and night. However, only in the late night are these behavioral effects correlated with changes in Period gene expression. Together, our data support the hypothesis that the interacting effects of light and GABA are the result of the opposing actions of these stimuli on Period mRNA, but only during the subjective night.

Identification of novel light-induced genes in the suprachiasmatic nucleus

Background

The transmission of information about the photic environment to the circadian clock involves a complex array of neurotransmitters, receptors, and second messenger systems. Exposure of an animal to light during the subjective night initiates rapid transcription of a number of immediate-early genes in the suprachiasmatic nucleus of the hypothalamus. Some of these genes have known roles in entraining the circadian clock, while others have unknown functions. Using laser capture microscopy, microarray analysis, and quantitative real-time PCR, we performed a comprehensive screen for changes in gene expression immediately following a 30 minute light pulse in suprachiasmatic nucleus of mice.

Results

The results of the microarray screen successfully identified previously known light-induced genes as well as several novel genes that may be important in the circadian clock. Newly identified light-induced genes include early growth response 2, proviral integration site 3, growth-arrest and DNA-damage-inducible 45 beta, and TCDD-inducible poly(ADP-ribose) polymerase. Comparative analysis of promoter sequences revealed the presence of evolutionarily conserved CRE and associated TATA box elements in most of the light-induced genes, while other core clock genes generally lack this combination of promoter elements.

Conclusion

The photic signalling cascade in the suprachiasmatic nucleus activates an array of immediate-early genes, most of which have unknown functions in the circadian clock. Detected evolutionary conservation of CRE and TATA box elements in promoters of light-induced genes suggest that the functional role of these elements has likely remained the same over evolutionary time across mammalian orders.

Sildenafil accelerates reentrainment of circadian rhythms after advancing light schedules

Mammalian circadian rhythms are generated by a master clock located in the suprachiasmatic nuclei and entrained by light-activated signaling pathways. In hamsters, the mechanism responsible for light-induced phase advances involves the activation of guanylyl cyclase, cGMP and its related kinase (PKG). It is not completely known whether interference with this pathway affects entrainment of the clock, including adaptation to changing light schedules. Here we report that cGMP-specific phosphodiesterase 5 is present in the hamster suprachiasmatic nuclei, and administration of the inhibitor sildenafil (3.5 mg/kg, i.p.) enhances circadian responses to light and decreases the amount of time necessary for reentrainment after phase advances of the light-dark cycle. These results suggest that sildenafil may be useful for treatment of circadian adaptation to environmental changes, including transmeridian eastbound flight schedules.

Light and GABAAreceptor activation alterPeriodmRNA levels in the SCN of diurnal Nile grass rats

We examined Period (Per) mRNA rhythms in the suprachiasmatic nucleus (SCN) of a diurnal rodent and assessed how phase-shifting stimuli acutely affect SCN Per mRNA using semiquantitative in situ hybridization. First, Per1 and Per2 varied rhythmically in the SCN over the course of one circadian cycle in constant darkness: Per1 mRNA was highest in the early to mid-subjective day, while Per2 mRNA levels peaked in the late subjective day. Second, acute light exposure in the early subjective night significantly increased both Per1 and Per2 mRNA. Third, Per2 but not Per1 levels decreased 1 and 2 h after injection of the gamma-aminobutyric acid (GABA)(A) receptor agonist muscimol into the SCN during the subjective day. Fourth, muscimol also reduced the light-induced Per2 in the early subjective night, but Per1 induction by light was not significantly affected. Consistent with previous studies, these data demonstrate that diurnal and nocturnal animals show very similar daily patterns of Per mRNA and light-induced Per increases in the SCN. As with light, muscimol alters circadian phase, and daytime phase alterations induced by muscimol are associated with significant decreases in Per2 mRNA. In diurnal animals, muscimol-induced decreases in Per are associated with phase delays rather than advances. The direction of the daytime phase shift may be determined by the relative suppression of Per1 vs. Per2 in SCN cells. As in nocturnal animals, changes in Per1 and Per2 mRNA by photic and non-photic stimuli appear to be associated with circadian phase alteration.

The circadian visual system, 2005

The primary mammalian circadian clock resides in the suprachiasmatic nucleus (SCN), a recipient of dense retinohypothalamic innervation. In its most basic form, the circadian rhythm system is part of the greater visual system. A secondary component of the circadian visual system is the retinorecipient intergeniculate leaflet (IGL) which has connections to many parts of the brain, including efferents converging on targets of the SCN. The IGL also provides a major input to the SCN, with a third major SCN afferent projection arriving from the median raphe nucleus. The last decade has seen a blossoming of research into the anatomy and function of the visual, geniculohypothalamic and midbrain serotonergic systems modulating circadian rhythmicity in a variety of species. There has also been a substantial and simultaneous elaboration of knowledge about the intrinsic structure of the SCN. Many of the developments have been driven by molecular biological investigation of the circadian clock and the molecular tools are enabling novel understanding of regional function within the SCN. The present discussion is an extension of the material covered by the 1994 review, "The Circadian Visual System."

Illuminating the impact of habitual behaviors in depression

Researchers have hypothesized that habitual behaviors are zeitgebers for the circadian clock. However, few studies have examined the relationship between habitual behaviors and light, the strongest zeitgeber. Depression is an ideal model in which to explore this relationship because depression is a disorder associated with disruptions in circadian biological activity, sleep, and social rhythms (or patterns of habitual behaviors). We hypothesized that individuals with fewer habitual behaviors have less average exposure to light from morning rise time to evening bedtime and that a reduction in light exposure increases the likelihood of depression. Thirty-nine depressed and 39 never-depressed participants wore an ambulatory light monitor and completed the Social Rhythm Metric over the course of 2 weeks. Linear and logistic regression techniques were used to calculate regression coefficients, and confidence limits based on the distribution of the product of two normal random variables were computed to test the significance of the mediation effect. Infrequent habitual behaviors were associated with a decrease in average levels of light exposure, and low levels of light increased the likelihood of depression. This mediation effect was partial; the overall number of habitual behaviors had a direct relationship with depression above and beyond the association with light exposure. Longitudinal studies are needed to empirically demonstrate the direction of relationships between each of the variables tested.

The opioid fentanyl affects light input, electrical activity andPergene expression in the hamster suprachiasmatic nuclei

The suprachiasmatic nuclei (SCN) contain a major circadian pacemaker, which is regulated by photic and nonphotic stimuli. Although enkephalins are present in the SCN, their role in phase regulation of the pacemaker is largely unknown. The opioid agonist fentanyl, a homologue of morphine, is an addictive drug that induces phase shifts of circadian rhythms in hamsters. We observed that these phase shifts are blocked by naloxone, which is a critical test for true opioid receptor involvement, and conclude that opioid receptors are the sole mediators of the actions of fentanyl on the circadian timing system. A strong interaction between opioids and light input was shown by the ability of fentanyl and light to completely block each other's phase shifts of behavioural activity rhythms. Neuronal ensemble recordings in vitro provide first evidence that SCN cells show direct responses to fentanyl and react with a suppression of firing rate. Moreover, we show that fentanyl induces a strong attenuation of light-induced Syrian hamster Period 1 (shPer1) gene expression during the night. During the subjective day, we found no evidence for a role of shPer1 in mediation of fentanyl-induced phase shifts. Based on the present results, however, we cannot exclude the involvement of shPer2. Our data indicate that opioids can strongly modify the photic responsiveness of the circadian pacemaker and may do so via direct effects on SCN electrical activity and regulation of Per genes. This suggests that the pathways regulating addictive behaviour and the circadian clock intersect.

Let there be “more” light: enhancement of light actions on the circadian system through non-photic pathways

Circadian rhythms are internally generated circa 24 h rhythms. The phase of the circadian pacemaker in mammals can be adjusted by external stimuli such as the daily cycle of light, as well as by internal stimuli such as information related to the physiological and behavioral status of the organism, collectively called "non-photic stimuli". We review a large number of studies regarding photic-non-photic interactions on the circadian system, with special focus on two widely described neurotransmitters associated with non-photic input pathways: neuropeptide Y (NPY) and serotonin 5-HT. Both neurotransmitters are capable of phase advancing the master pacemaker oscillation when applied during the subjective day, as do several behavioral manipulations. Also, both are capable of inhibiting light-induced phase shifts during the subjective night, suggesting a dynamic interaction between photic and non-photic stimuli in the fine-tuning of the pacemaker function. Suppression of the NPYergic and/or serotonergic non-photic input pathways can in turn potentiate the phase-shifting effects of light. These findings pose new questions about the possibility of a physiological role for the dynamic interaction between photic and non-photic inputs. This might be particularly important in the case of circadian system adjustments under certain conditions, such as depression, shift work or jet lag.

Forced Desynchronization of Dual Circadian Oscillators within the Rat Suprachiasmatic Nucleus

The circadian clock in the suprachiasmatic nucleus of the hypothalamus (SCN) contains multiple autonomous single-cell circadian oscillators and their basic intracellular oscillatory mechanism is beginning to be identified. Less well understood is how individual SCN cells create an integrated tissue pacemaker that produces a coherent read-out to the rest of the organism. Intercellular coupling mechanisms must coordinate individual cellular periods to generate the averaged, genotype-specific circadian period of whole animals. To noninvasively dissociate this circadian oscillatory network in vivo, we (T.C. and A.D.-N.) have developed an experimental paradigm that exposes animals to exotic light-dark (LD) cycles with periods close to the limits of circadian entrainment. If individual oscillators with different periods are loosely coupled within the network, perhaps some of them would be synchronized to the external cycle while others remain unentrained. In fact, rats exposed to an artificially short 22 hr LD cycle express two stable circadian motor activity rhythms with different period lengths in individual animals. Our analysis of SCN gene expression under such conditions suggests that these two motor activity rhythms reflect the separate activities of two oscillators in the anatomically defined ventrolateral and dorsomedial SCN subdivisions. Our "forced desychronization" protocol has allowed the first stable separation of these two regional oscillators in vivo, correlating their activities to distinct behavioral outputs, and providing a powerful approach for understanding SCN tissue organization and signaling mechanisms in behaving animals.