Both neuropeptide Y and serotonin are necessary for entrainment of circadian rhythms in mice by daily treadmill running schedules

Biological Sex Influences Daily Locomotor Rhythms in Mice Held Under Different Housing Conditions

Daily rhythms are programmed by a central circadian clock that is modulated by photoperiod. Here, we recorded locomotor activity rhythms in C57Bl/6 or mPer2Luc mice of both sexes held under different housing conditions. First, we confirm that the structure of locomotor activity rhythms differs between male and female mice in both genetic backgrounds. Male mice exhibit a nightly "siesta," whereas female mice fluctuate between nights with and without a nightly siesta, which corresponds with changes in locomotor activity levels, circadian period, and vaginal cytology. The nightly siesta is modulated by the presence of a running wheel in both sexes but is not required for the infradian patterning of locomotor rhythms in females. Finally, photoperiodic changes in locomotor rhythms differed by sex, and females displayed phase-jumping responses earlier than males under a parametric photoentrainment assay simulating increasing day length. Collectively, these results highlight that sex and sex hormones influence daily locomotor rhythms under a variety of different environmental conditions.

Nonphotic entrainment of central and peripheral circadian clocks in mice by scheduled voluntary exercise under constant darkness

In mammals, the central circadian pacemaker in the suprachiasmatic nucleus (SCN) entrains to an environmental light-dark (LD) cycle and organizes the temporal order of circadian rhythms in physiology and behavior. Previously, some studies have demonstrated that scheduled exercise could entrain the free-running behavior rhythm in nocturnal rodents. However, it remains unknown whether entrainment by scheduled exercise alters the internal temporal order of the behavioral circadian rhythms or clock gene expression in the SCN, extra-SCN brain regions, and peripheral organs when mice are entrained to the scheduled exercise under constant darkness (DD). In the present study, we examined circadian rhythms in locomotor activity and clock gene Per1 expression by bioluminescence reporter (Per1-luc) in the SCN, arcuate nucleus (ARC), liver, and skeletal muscle of mice entrained to an LD cycle, mice free-running under DD, and mice entrained to daily exposure to a new cage with a running wheel (NCRW) under DD. All mice showed a steady-state entrainment of behavioral circadian rhythms to NCRW exposure under DD in parallel with shortening of the α when compared with that under DD. The temporal order of behavioral circadian rhythms and the Per1-luc rhythms in the SCN and peripheral tissues but not in the ARC were maintained in the mice entrained to the NCRW and LD cycles; in contrast, the temporal order was altered in the mice under DD. The present findings reveal that the SCN entrains to daily exercise, and daily exercise reorganizes the internal temporal order of behavioral circadian rhythms and clock gene expression in the SCN and peripheral tissues.

A Role for Thalamic Projection GABAergic Neurons in Circadian Responses to Light

The thalamus is an important hub for sensory information and participates in sensory perception, regulation of attention, arousal and sleep. These functions are executed primarily by glutamatergic thalamocortical neurons that extend axons to the cortex and initiate cortico-thalamocortical connectional loops. However, the thalamus also contains projection GABAergic neurons that do not extend axons toward the cortex. Here, we have harnessed recent insight into the development of the intergeniculate leaflet (IGL) and the ventral lateral geniculate nucleus (LGv) to specifically target and manipulate thalamic projection GABAergic neurons in female and male mice. Our results show that thalamic GABAergic neurons of the IGL and LGv receive retinal input from diverse classes of retinal ganglion cells (RGCs) but not from the M1 intrinsically photosensitive retinal ganglion cell (ipRGC) type. We describe the synergistic role of the photoreceptor melanopsin and the thalamic neurons of the IGL/LGv in circadian entrainment to dim light. We identify a requirement for the thalamic IGL/LGv neurons in the rapid changes in vigilance states associated with circadian light transitions.SIGNIFICANCE STATEMENT The intergeniculate leaflet (IGL) and ventral lateral geniculate nucleus (LGv) are part of the extended circadian system and mediate some nonimage-forming visual functions. Here, we show that each of these structures has a thalamic (dorsal) as well as prethalamic (ventral) developmental origin. We map the retinal input to thalamus-derived cells in the IGL/LGv complex and discover that while RGC input is dominant, this is not likely to originate from M1ipRGCs. We implicate thalamic cells in the IGL/LGv in vigilance state transitions at circadian light changes and in overt behavioral entrainment to dim light, the latter exacerbated by concomitant loss of melanopsin expression.

Effects of prenatal exposure to fluoxetine on circadian rhythmicity in the locomotor activity and neuropeptide Y and 5-HT expression in male and female adult Wistar rats

Serotonin (5-HT) reuptake inhibitors, such as fluoxetine, are the most prescribed antidepressant for maternal depression. In this sense, it exposes mothers and the brains of infants to increased modulatory and trophic effects of serotonergic neurotransmission. 5-HT promotes essential brain changes throughout its development, which include neuron migration, differentiation, and organization of neural circuitries related to emotional, cognitive, and circadian behavior. Early exposure to the SSRIs induces long-term effects on behavioral and neural serotonergic signalization. We have aimed to evaluate the circadian rhythm of locomotor activity and the neurochemical content, neuropeptide Y (NPY) and 5-HT in three brain areas: intergeniculate leaflet (IGL), suprachiasmatic nuclei (SCN) and raphe nuclei (RN), at two zeitgebers (ZT6 and ZT18), in male and female rat's offspring early exposed (developmental period GD13-GD21) to fluoxetine (20mg/kg). First, we have conducted daily records of the locomotor activity rhythm using activity sensors coupled to individual cages over four weeks. We have lastly evaluated the immunoreactivity of NPY in both SCN and IGL, and as well the 5-HT expression in the dorsal and medial RN. In summary, our results showed that (1) prenatal fluoxetine affects phase entrainment of the rest/activity rhythm at ZT6 and ZT18, more in male than female specimens, and (2) modulates the NPY and 5-HT expression. Here, we show male rats are more susceptible to phase entrainment and the NPY and 5-HT misexpression compared to female ones. The sex differences induced by early exposure to fluoxetine in both the circadian rhythm of locomotor activity and the neurochemical expression into SCN, IGL, and midbrain raphe are an important highlight in the present work. Thus, our results may help to improve the knowledge on neurobiological mechanisms of circadian rhythms and are relevant to understanding the "broken brains" and behavioral abnormalities of offspring early exposed to antidepressants.

Systematic review of drugs that modify the circadian system's phase-shifting responses to light exposure

We searched PubMed for primary research quantifying drug modification of light-induced circadian phase-shifting in rodents. This search, conducted for work published between 1960 and 2018, yielded a total of 146 papers reporting results from 901 studies. Relevant articles were those with any extractable data on phase resetting in wildtype (non-trait selected) rodents administered a drug, alongside a vehicle/control group, near or at the time of exposure. Most circadian pharmacology experiments were done using drugs thought to act directly on either the brain's central pacemaker, the suprachiasmatic nucleus (SCN), the SCN's primary relay, the retinohypothalamic tract, secondary pathways originating from the medial/dorsal raphe nuclei and intergeniculate leaflet, or the brain's sleep-arousal centers. While the neurotransmitter systems underlying these circuits were of particular interest, including those involving glutamate, gamma-aminobutyric acid, serotonin, and acetylcholine, other signaling modalities have also been assessed, including agonists and antagonists of receptors linked to dopamine, histamine, endocannabinoids, adenosine, opioids, and second-messenger pathways downstream of glutamate receptor activation. In an effort to identify drugs that unduly influence circadian responses to light, we quantified the net effects of each drug class by ratioing the size of the phase-shift observed after administration to that observed with vehicle in a given experiment. This allowed us to organize data across the literature, compare the relative efficacy of one mechanism versus another, and clarify which drugs might best suppress or potentiate phase resetting. Aggregation of the available data in this manner suggested that several candidates might be clinically relevant as auxiliary treatments to suppress ectopic light responses during shiftwork or amplify the circadian effects of timed bright light therapy. Future empirical research will be necessary to validate these possibilities.

Food reward induction of rhythmic clock gene expression in the prefrontal cortex of rats is accompanied by changes in miR-34a-5p expression

The current study is focused on mechanisms by which the peripheral circadian oscillator in the prefrontal cortex (PFC) participates in food reward-induced activity. The experimental group of male Wistar rats was trained to receive a food reward with a low hedonic and caloric value. Afterwards, animals were exposed to a 5 h phase advance. Experimental animals could access a small food reward as they had been accustomed to, while control rats were exposed to the same phase shift without access to a food reward. When synchronisation to a new light:dark cycle was accompanied by intake of food reward, animals exerted more exact phase shift compared to the controls. In rats with access to a food reward, a rhythm in dopamine receptors types 1 and 2 in the PFC was detected. Rhythmic clock gene expression was induced in the PFC of rats when a food reward was provided together with a phase shift. The per2 and clock genes are predicted targets of miR-34a-5p. The precursor form of miR-34a-5p (pre-miR-34a-5p) showed a daily rhythm in expression in the PFC of the control and experimental groups. On the other hand, the mature form of miR-34a-5p exerted an inverted rhythm compared to pre-miR-34a-5p and negative correlation with per and clock genes expression only in the PFC of rewarded rats. A difference in the pattern of mature and pre-miR-34a-5p values was not related to expression of enzymes drosha, dicer and dgcr8. A role of the clock genes and miR-34a-5p in reward-facilitated synchronisation has been hypothesised.

Timed daily exercise remodels circadian rhythms in mice

Regular exercise is important for physical and mental health. An underexplored and intriguing property of exercise is its actions on the body’s 24 h or circadian rhythms. Molecular clock cells in the brain’s suprachiasmatic nuclei (SCN) use electrical and chemical signals to orchestrate their activity and convey time of day information to the rest of the brain and body. To date, the long-lasting effects of regular physical exercise on SCN clock cell coordination and communication remain unresolved. Utilizing mouse models in which SCN intercellular neuropeptide signaling is impaired as well as those with intact SCN neurochemical signaling, we examined how daily scheduled voluntary exercise (SVE) influenced behavioral rhythms and SCN molecular and neuronal activities. We show that in mice with disrupted neuropeptide signaling, SVE promotes SCN clock cell synchrony and robust 24 h rhythms in behavior. Interestingly, in both intact and neuropeptide signaling deficient animals, SVE reduces SCN neural activity and alters GABAergic signaling. These findings illustrate the potential utility of regular exercise as a long-lasting and effective non-invasive intervention in the elderly or mentally ill where circadian rhythms can be blunted and poorly aligned to the external world.

Using mice with disrupted neuropeptide signaling, Hughes et al. show that daily scheduled voluntary exercise (SVE) promotes suprachiasmatic nuclei (SCN) clock cell synchrony and robust 24 h rhythms in behavior. This study suggests the potential utility of regular exercise as a non-invasive intervention for the elderly or mentally ill, where circadian rhythms can be poorly aligned to the external world.

Blunted diurnal firing in lateral habenula projections to dorsal raphe nucleus and delayed photoentrainment in stress-susceptible mice

Daily rhythms are disrupted in patients with mood disorders. The lateral habenula (LHb) and dorsal raphe nucleus (DRN) contribute to circadian timekeeping and regulate mood. Thus, pathophysiology in these nuclei may be responsible for aberrations in daily rhythms during mood disorders. Using the 15-day chronic social defeat stress (CSDS) paradigm and in vitro slice electrophysiology, we measured the effects of stress on diurnal rhythms in firing of LHb cells projecting to the DRN (cells^LHb→DRN) and unlabeled DRN cells. We also performed optogenetic experiments to investigate if increased firing in cells^LHb→DRN during exposure to a weak 7-day social defeat stress (SDS) paradigm induces stress-susceptibility. Last, we investigated whether exposure to CSDS affected the ability of mice to photoentrain to a new light–dark (LD) cycle. The cells^LHb→DRN and unlabeled DRN cells of stress-susceptible mice express greater blunted diurnal firing compared to stress-näive (control) and stress-resilient mice. Daytime optogenetic activation of cells^LHb→DRN during SDS induces stress-susceptibility which shows the direct correlation between increased activity in this circuit and putative mood disorders. Finally, we found that stress-susceptible mice are slower, while stress-resilient mice are faster, at photoentraining to a new LD cycle. Our findings suggest that exposure to strong stressors induces blunted daily rhythms in firing in cells^LHb→DRN, DRN cells and decreases the initial rate of photoentrainment in susceptible-mice. In contrast, resilient-mice may undergo homeostatic adaptations that maintain daily rhythms in firing in cells^LHb→DRN and also show rapid photoentrainment to a new LD cycle.

Daily rhythms are disrupted in patients suffering from mood disorders, and it is known that the lateral habenula and dorsal raphe nucleus contribute to circadian timekeeping and regulate mood. This study shows that stress-susceptible mice have blunted and inverted diurnal firing rhythms in lateral habenula cells that project to the dorsal raphe nucleus, and have a slow rate of photoentrainment to a new light cycle.

Vestibular stimulation by 2G hypergravity modifies resynchronization in temperature rhythm in rats

Input from the light/dark (LD) cycle constitutes the primary synchronizing stimulus for the suprachiasmatic nucleus (SCN) circadian clock. However, the SCN can also be synchronized by non-photic inputs. Here, we hypothesized that the vestibular system, which detects head motion and orientation relative to gravity, may provide sensory inputs to synchronize circadian rhythmicity. We investigated the resynchronization of core temperature (Tc) circadian rhythm to a six-hour phase advance of the LD cycle (LD + 6) using hypergravity (2 G) as a vestibular stimulation in control and bilateral vestibular loss (BVL) rats. Three conditions were tested: an LD + 6 exposure alone, a series of seven 2 G pulses without LD + 6, and a series of seven one-hour 2 G pulses (once a day) following LD + 6. First, following LD + 6, sham rats exposed to 2 G pulses resynchronized earlier than BVL rats (p = 0.01), and earlier than sham rats exposed to LD + 6 alone (p = 0.002). Each 2 G pulse caused an acute drop of Tc in sham rats (-2.8 ± 0.3 °C; p < 0.001), while BVL rats remained unaffected. This confirms that the vestibular system influences chronobiological regulation and supports the hypothesis that vestibular input, like physical activity, should be considered as a potent time cue for biological rhythm synchronization, acting in synergy with the visual system.

Blunted Diurnal Firing in Lateral Habenula Projections to Dorsal Raphe Nucleus and Delayed Photoentrainment in Stress-Susceptible Mice.. [DOI: 10.1101/2020.03.19.998732] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

Daily rhythms are disrupted in patients suffering from mood disorders. The lateral habenula (LHb) and dorsal raphe nucleus (DRN) contribute to circadian timekeeping and regulate mood. Thus, pathophysiology in these nuclei may be responsible for aberrations in daily rhythms during mood disorders. Using the chronic social defeat stress (CSDS) paradigm and in-vitro slice electrophysiology we measured the effects of stress on diurnal rhythms in firing of LHb cells projecting to the DRN (cellsLHb→DRN) and DRN cells alone. We also performed optogenetic experiments to investigate if increased firing in cellsLHb→DRN during exposure to subthreshold social defeat stress (SSDS), induces stress-susceptibility. Last we investigated whether exposure to CSDS affected the ability of mice to phototentrain to a new LD cycle. The cellsLHb→DRN and DRN cells alone of stress-susceptible mice express greater blunted diurnal firing compared to stress-naive (control) and stress-resilient mice. Day-time optogenetic activation of cellsLHb→DRN during SSDS induces stress-susceptibility which shows the direct correlation between increased activity in this circuit and putative mood disorders. Finally, we found that stress-susceptible mice are slower, while stress-resilient mice are faster, at photoentraining to a new LD cycle. Our findings suggest that CSDS induces blunted daily rhythms in firing in cellsLHb→DRN and slow rate of photoentrainment in susceptible-mice. In contrast, resilientmice may undergo homeostatic adaptations that maintain daily rhythms in firing in cellsLHb→DRN and also show rapid photoentrainment to a new LD-cycle.

Neurogenetic basis for circadian regulation of metabolism by the hypothalamus

Circadian rhythms are driven by a transcription-translation feedback loop that separates anabolic and catabolic processes across the Earth's 24-h light-dark cycle. Central pacemaker neurons that perceive light entrain a distributed clock network and are closely juxtaposed with hypothalamic neurons involved in regulation of sleep/wake and fast/feeding states. Gaps remain in identifying how pacemaker and extrapacemaker neurons communicate with energy-sensing neurons and the distinct role of circuit interactions versus transcriptionally driven cell-autonomous clocks in the timing of organismal bioenergetics. In this review, we discuss the reciprocal relationship through which the central clock drives appetitive behavior and metabolic homeostasis and the pathways through which nutrient state and sleep/wake behavior affect central clock function.

Human circadian phase-response curves for exercise

KEY POINTS

Exercise elicits circadian phase-shifting effects, but additional information is needed. The phase-response curve describing the magnitude and direction of circadian rhythm phase shifts, depending on the time of the zeigeber (time cue) stimulus, is the most fundamental chronobiological tool for alleviating circadian misalignment and related morbidity. Fifty-one older and 48 young adults followed a circadian rhythms measurement protocol for up to 5.5 days, and performed 1 h of moderate treadmill exercise for 3 consecutive days at one of eight times of the day/night. Temporal changes in the phase of 6-sulphatoxymelatonin (aMT6s) were measured from evening onset, cosine acrophase, morning offset and duration of excretion. Significant phase-response curves were established for aMT6 onset and acrophase with large phase delays from 7:00 pm to 10:00 pm and large phase advances at both 7:00 am and from 1:00 pm to 4:00 pm. Delays or advances would be desired, for example, for adjustment to westward or eastward air travel, respectively. Along with known synergism with bright light, the above PRCs with a second phase advance region (afternoon) could support both practical and clinical applications.

ABSTRACT

Although bright light is regarded as the primary circadian zeitgeber, its limitations support exploring alternative zeitgebers. Exercise elicits significant circadian phase-shifting effects, but fundamental information regarding these effects is needed. The primary aim of the present study was to establish phase-response curves (PRCs) documenting the size and direction of phase shifts in relation to the circadian time of exercise. Aerobically fit older (n = 51; 59-75 years) and young adults (n = 48; 18-30 years) followed a 90 min laboratory ultrashort sleep-wake cycle (60 min wake/30 min sleep) for up to 5½ days. At the same clock time on three consecutive days, each participant performed 60 min of moderate treadmill exercise (65-75% of heart rate reserve) at one of eight times of day/night. To describe PRCs, phase shifts were measured for the cosine-fitted acrophase of urinary 6-sulphatoxymelatonin (aMT6s), as well as for the evening rise, morning decline and change in duration of aMT6s excretion. Significant PRCs were found for aMT6s acrophase, onset and duration, with peak phase advances corresponding to clock times of 7:00 am and from 1:00 pm to 4:00 pm, delays from 7:00 pm to 10:00 pm, and minimal shifts around 4:00 pm and 2:00 am. There were no significant age or sex differences. The amplitudes of the aMT6s onset and acrophase PRCs are comparable to expectations for bright light of equal duration. The phase advance to afternoon exercise and the exercise-induced PRC for change in aMT6s duration are novel findings. The results support further research exploring additive phase-shifting effects of bright light and exercise and health benefits.

Entrainment of the mouse circadian clock: Effects of stress, exercise, and nutrition

The circadian clock system in mammals plays a fundamental role in maintaining homeostasis. Entrainment is an important characteristic of the internal clock, by which appropriate timing is maintained according to external daily stimuli, such as light, stress, exercise, and/or food. Disorganized entrainment or a misaligned clock time, such as jet lag, increases health disturbances. The central clock in the suprachiasmatic nuclei, located in the hypothalamus, receives information about arousal stimuli, such as physical stress or exercise, and changes the clock time by modifying neural activity or the expression of circadian clock genes. Although feeding stimuli cannot entrain the central clock in a normal light-dark cycle, the central clock can partially detect the metabolic status. Local clocks in the peripheral tissues, including liver and kidney, have a strong direct response to the external stimuli of stress, exercise, and/or food that is independent of the central clock. The mechanism underlying entrainment by stress/exercise is mediated by glucocorticoids, sympathetic nerves, oxidative stress, hypoxia, pH, cytokines, and temperature. Food/nutrition-induced entrainment is mediated by fasting-induced hormonal or metabolic changes and re-feeding-induced insulin or oxyntomodulin secretion. Chrono-nutrition is a clinical application based on chronobiology research. Future studies are required to elucidate the effects of eating and nutrient composition on the human circadian clock. Here, we focus on the central and peripheral clocks mostly in rodents' studies and review the findings of recent investigations of the effects of stress, exercise, and food on the entrainment system.

Using light to tell the time of day: sensory coding in the mammalian circadian visual network

Circadian clocks are a near-ubiquitous feature of biology, allowing organisms to optimise their physiology to make the most efficient use of resources and adjust behaviour to maximise survival over the solar day. To fulfil this role, circadian clocks require information about time in the external world. This is most reliably obtained by measuring the pronounced changes in illumination associated with the earth's rotation. In mammals, these changes are exclusively detected in the retina and are relayed by direct and indirect neural pathways to the master circadian clock in the hypothalamic suprachiasmatic nuclei. Recent work reveals a surprising level of complexity in this sensory control of the circadian system, including the participation of multiple photoreceptive pathways conveying distinct aspects of visual and/or time-of-day information. In this Review, I summarise these important recent advances, present hypotheses as to the functions and neural origins of these sensory signals, highlight key challenges for future research and discuss the implications of our current knowledge for animals and humans in the modern world.

Geniculohypothalamic GABAergic projections gate suprachiasmatic nucleus responses to retinal input

KEY POINTS

Visual input to the suprachiasmatic nucleus circadian clock is critical for animals to adapt their physiology and behaviour in line with the solar day. In addition to direct retinal projections, the clock receives input from the visual thalamus, although the role of this geniculohypothalamic pathway in circadian photoreception is poorly understood. In the present study, we develop a novel brain slice preparation that preserves the geniculohypothalamic pathway to show that GABAergic thalamic neurons inhibit retinally-driven activity in the central clock in a circadian time-dependent manner. We also show that in vivo manipulation of thalamic signalling adjusts specific features of the hypothalamic light response, indicating that the geniculohypothalamic pathway is primarily activated by crossed retinal inputs. Our data provide a mechanism by which geniculohypothalamic signals can adjust the magnitude of circadian and more acute hypothalamic light responses according to time-of-day and establish an important new model for future investigations of the circadian visual system.

ABSTRACT

Sensory input to the master mammalian circadian clock, the suprachiasmatic nucleus (SCN), is vital in allowing animals to optimize physiology and behaviour alongside daily changes in the environment. Retinal inputs encoding changes in external illumination provide the principle source of such information. The SCN also receives input from other retinorecipient brain regions, primarily via the geniculohypothalamic tract (GHT), although the contribution of these indirect projections to circadian photoreception is currently poorly understood. To address this deficit, in the present study, we established an in vitro mouse brain slice preparation that retains connectivity across the extended circadian system. Using multi-electrode recordings, we first confirm that this preparation retains intact optic projections to the SCN, thalamus and pretectum and a functional GHT. We next show that optogenetic activation of GHT neurons selectively suppresses SCN responses to retinal input, and also that this effect exhibits a pronounced day/night variation and involves a GABAergic mechanism. This inhibitory action was not associated with overt circadian rhythmicity in GHT output, indicating modulation at the SCN level. Finally, we use in vivo electrophysiological recordings alongside pharmacological inactivation or optogenetic excitation to show that GHT signalling actively modulates specific features of the SCN light response, indicating that GHT cells are primarily activated by crossed retinal projections. Taken together, our data establish a new model for studying network communication in the extended circadian system and provide novel insight into the roles of GHT-signalling, revealing a mechanism by which thalamic activity can help gate retinal input to the SCN according to time of day.

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.

Non-Photic Modulation of Phase Shifts to Long Light Pulses

Circadian rhythms can be reset by both photic and non-photic stimuli. Recent studies have used long light exposure to produce photic phase shifts or to enhance non-photic phase shifts. The presence or absence of light can also influence the expression of locomotor rhythms through masking; light during the night attenuates locomotor activity, while darkness during the day induces locomotor activity in nocturnal animals. Given this dual role of light, the current study was designed to examine the relative contributions of photic and non-photic components present in a long light pulse paradigm. Mice entrained to a light/dark cycle were exposed to light pulses of various durations (0, 3, 6, 9, or 12 h) starting at the time of lights-off. After the light exposure, animals were placed in DD and were either left undisturbed in their home cages or had their wheels locked for the remainder of the subjective night and subsequent subjective day. Light treatments of 6, 9, and 12 h produced large phase delays. These treatments were associated with decreased activity during the nocturnal light and increased activity during the initial hours of darkness following light exposure. When the wheels were locked to prevent high-amplitude activity, the resulting phase delays to the light were significantly attenuated, suggesting that the activity following the light exposure may have contributed to the overall phase shift. In a second experiment, telemetry probes were used to assess what effect permanently locking the wheels had on the phase shift to the long light pulses. These animals had phase shifts fully as large as animals without any form of wheel lock, suggesting that while non-photic events can modulate photic phase shifts, they do not play a role in the full phase-shift response observed in animals exposed to long light pulses. This paradigm will facilitate investigations into non-photic responses of the mouse circadian system.

Forced rather than voluntary exercise entrains peripheral clocks via a corticosterone/noradrenaline increase in PER2::LUC mice

Exercise during the inactive period can entrain locomotor activity and peripheral circadian clock rhythm in mice; however, mechanisms underlying this entrainment are yet to be elucidated. Here, we showed that the bioluminescence rhythm of peripheral clocks in PER2::LUC mice was strongly entrained by forced treadmill and forced wheel-running exercise rather than by voluntary wheel-running exercise at middle time during the inactivity period. Exercise-induced entrainment was accompanied by increased levels of serum corticosterone and norepinephrine in peripheral tissues, similar to the physical stress-induced response. Adrenalectomy with norepinephrine receptor blockers completely blocked the treadmill exercise-induced entrainment. The entrainment of the peripheral clock by exercise is independent of the suprachiasmatic nucleus clock, the main oscillator in mammals. The present results suggest that the response of forced exercise, but not voluntary exercise, may be similar to that of stress, and possesses the entrainment ability of peripheral clocks through the activation of the adrenal gland and the sympathetic nervous system.

Activation of M1/4 receptors phase advances the hamster circadian clock during the day

The mammalian circadian clock in the suprachiasmatic nucleus (SCN) can be reset by the cholinergic agonist carbachol. In hamsters, intraSCN carbachol produces phase advances during the day. This phenomenon has previously been attributed to the muscarinic receptors, as carbachol-induced phase shifts are blocked by pretreatment with the muscarinic antagonist atropine. The SCN contains all five muscarinic receptors, leaving open the question as to which muscarinic receptors mediate these shifts. Here we test two selective muscarinic agonists, the M1/4 agonist McN-A-343 and the M2/3 agonist bethanechol, in addition to the non-selective cholinergic agonist carbachol. Consistent with previous reports, carbachol produced significant phase advances when injected to the SCN during the mid-subjective day. At the doses used here, McN-A-343, but not bethanechol, also produced significant phase shifts when injected to the SCN during the mid-subjective day. Phase shifts to McN-A-343 were as large as those produced by carbachol, suggesting that activation of the M1/4 receptors alone can fully account for the daytime phase advances produced by cholinergic agonists. Given acetylcholine's role in arousal, and the similarity between phase advances to carbachol/McN-A-343 and to exercise and arousal manipulations, it is possible that acetylcholine may contribute to non-photic resetting of the circadian clock.

Synchronization of Biological Clock Neurons by Light and Peripheral Feedback Systems Promotes Circadian Rhythms and Health

In mammals, the suprachiasmatic nucleus (SCN) functions as a circadian clock that drives 24-h rhythms in both physiology and behavior. The SCN is a multicellular oscillator in which individual neurons function as cell-autonomous oscillators. The production of a coherent output rhythm is dependent upon mutual synchronization among single cells and requires both synaptic communication and gap junctions. Changes in phase-synchronization between individual cells have consequences on the amplitude of the SCN’s electrical activity rhythm, and these changes play a major role in the ability to adapt to seasonal changes. Both aging and sleep deprivation negatively affect the circadian amplitude of the SCN, whereas behavioral activity (i.e., exercise) has a positive effect on amplitude. Given that the amplitude of the SCN’s electrical activity rhythm is essential for achieving robust rhythmicity in physiology and behavior, the mechanisms that underlie neuronal synchronization warrant further study. A growing body of evidence suggests that the functional integrity of the SCN contributes to health, well-being, cognitive performance, and alertness; in contrast, deterioration of the 24-h rhythm is a risk factor for neurodegenerative disease, cancer, depression, and sleep disorders.

Effects of lighting condition on circadian behavior in 5-HT1A receptor knockout mice

Serotonin (5-HT) is an important regulator of the mammalian circadian system, and has been implicated in modulating entrained and free-running rhythms, as well as photic and non-photic phase shifting. In general, 5-HT appears to oppose the actions of light on the circadian system of nocturnal rodents. As well, 5-HT mediates, at least in part, some non-photic responses. The 5-HT1A, 1B and 7 receptors regulate these acute responses to zeitgebers. 5-HT also regulates some entrained and free-running properties of the circadian clock. The receptors that contribute to these phenomena have not been fully examined. Here, we use 5-HT1A receptor knockout (KO) mice to examine the response of the mouse circadian system to a variety of lighting conditions, including a normal light-dark cycle (LD), T-cycles, phase advanced LD cycles, constant darkness (DD), constant light (LL) and a 6 hour dark pulse starting at CT5. Relative to wildtype mice, the 5-HT1A receptor KO mice have lower levels of activity during the first 8h of the night/subjective night in LD and LL, later activity onsets on transient days during re-entrainment, shorter free-running periods in LL when housed with wheels, and smaller phase shifts to dark pulses. No differences were noted in activity levels during DD, alpha under any light condition, free-running period in DD, or phase angle of entrainment in LD. While the 5-HT1A receptor plays an important role in regulating photic and non-photic phase shifting, its contribution to entrained and free-running properties of the circadian clock is relatively minor.

Pet-1 deficiency alters the circadian clock and its temporal organization of behavior

The serotonin and circadian systems are two important interactive regulatory networks in the mammalian brain that regulate behavior and physiology in ways that are known to impact human mental health. Previous work on the interaction between these two systems suggests that serotonin modulates photic input to the central circadian clock (the suprachiasmatic nuclei; SCN) from the retina and serves as a signal for locomotor activity, novelty, and arousal to shift the SCN clock, but effects of disruption of serotonergic signaling from the raphe nuclei on circadian behavior and on SCN function are not fully characterized. In this study, we examined the effects on diurnal and circadian behavior, and on ex vivo molecular rhythms of the SCN, of genetic deficiency in Pet-1, an ETS transcription factor that is necessary to establish and maintain the serotonergic phenotype of raphe neurons. Pet-1^−/− mice exhibit loss of rhythmic behavioral coherence and an extended daily activity duration, as well as changes in the molecular rhythms expressed by the clock, such that ex vivo SCN from Pet-1^−/− mice exhibit period lengthening and sex-dependent changes in rhythmic amplitude. Together, our results indicate that Pet-1 regulation of raphe neuron serotonin phenotype contributes to the period, precision and light/dark partitioning of locomotor behavioral rhythms by the circadian clock through direct actions on the SCN clock itself, as well as through non-clock effects.

Relaxin-3/RXFP3 networks: an emerging target for the treatment of depression and other neuropsychiatric diseases?

Animal and clinical studies of gene-environment interactions have helped elucidate the mechanisms involved in the pathophysiology of several mental illnesses including anxiety, depression, and schizophrenia; and have led to the discovery of improved treatments. The study of neuropeptides and their receptors is a parallel frontier of neuropsychopharmacology research and has revealed the involvement of several peptide systems in mental illnesses and identified novel targets for their treatment. Relaxin-3 is a newly discovered neuropeptide that binds, and activates the G-protein coupled receptor, RXFP3. Existing anatomical and functional evidence suggests relaxin-3 is an arousal transmitter which is highly responsive to environmental stimuli, particularly neurogenic stressors, and in turn modulates behavioral responses to these stressors and alters key neural processes, including hippocampal theta rhythm and associated learning and memory. Here, we review published experimental data on relaxin-3/RXFP3 systems in rodents, and attempt to highlight aspects that are relevant and/or potentially translatable to the etiology and treatment of major depression and anxiety. Evidence pertinent to autism spectrum and metabolism/eating disorders, or related psychiatric conditions, is also discussed. We also nominate some key experimental studies required to better establish the therapeutic potential of this intriguing neuromodulatory signaling system, including an examination of the impact of RXFP3 agonists and antagonists on the overall activity of distinct or common neural substrates and circuitry that are identified as dysfunctional in these debilitating brain diseases.

Involvement of 5-HT₃ and 5-HT₄ receptors in the regulation of circadian clock gene expression in mouse small intestine

Several lines of evidence suggest that 5-HT receptors play a critical role in the expression of clock genes in the suprachiasmatic nucleus, the main circadian oscillator in hamsters. The contributions of 5-HT-receptor subtypes in the intestine, where they are expressed at high concentrations, are however not yet clarified. The 5-HT synthesis inhibitor, p-chlorophenylalanine, attenuated the daily rhythm of Per1 and Per2 gene expression in the intestine. Injection of 5-HT and agonists of the 5-HT3 and 5-HT4 receptors increased Per1/Per2 expression and decreased Bmal1 expression in a dose-dependent manner. Although treatment with antagonists of 5-HT3 and 5-HT4 alone did not affect clock gene expression, co-injection of these antagonists with 5-HT blocked the 5-HT-induced changes in clock gene expression. Increased tissue levels of 5-HT due to treatment with the antidepressants clomipramine and fluvoxamine did not affect clock gene expression. The present results suggest that the 5-HT system in the small intestine may play a critical role in regulating circadian rhythms through 5-HT3/5-HT4-receptor activation.

De novo assembly and characterization of two transcriptomes reveal multiple light-mediated functions in the scallop eye (Bivalvia: Pectinidae)

Background

The eye has evolved across 13 separate lineages of molluscs. Yet, there have been very few studies examining the molecular machinary underlying eye function of this group, which is due, in part, to a lack of genomic resources. The scallop (Bivalvia: Pectinidae) represents a compeling molluscan model to study photoreception due to its morphologically novel and separately evolved mirror-type eye. We sequenced the adult eye transcriptome of two scallop species to: 1) identify the phototransduction pathway components; 2) identify any additional light detection functions; and 3) test the hypothesis that molluscs possess genes not found in other animal lineages.

Results

A total of 3,039 contigs from the bay scallop, Argopecten irradians and 26,395 contigs from the sea scallop, Placopecten magellanicus were produced by 454 sequencing. Targeted BLAST searches and functional annotation using Gene Ontology (GO) terms and KEGG pathways identified transcripts from three light detection systems: two phototransduction pathways and the circadian clock, a previously unrecognized function of the scallop eye. By comparing the scallop transcriptomes to molluscan and non-molluscan genomes, we discovered that a large proportion of the transcripts (7,776 sequences) may be specific to the scallop lineage. Nearly one-third of these contain transmembrane protein domains, suggesting these unannotated transcripts may be sensory receptors.

Conclusions

Our data provide the most comprehensive transcriptomic resource currently available from a single molluscan eye type. Candidate genes potentially involved in sensory reception were identified, and are worthy of further investigation. This resource, combined with recent phylogenetic and genomic data, provides a strong foundation for future investigations of the function and evolution of molluscan photosensory systems in this morphologically and taxonomically diverse phylum.

The effects of perinatal fluoxetine treatment on the circadian system of the adult mouse

RATIONAL

Depression is prevalent among women of childbearing age and is frequently treated with selective serotonin reuptake inhibitors (SSRIs). As some SSRIs, such as fluoxetine (Flx), can cross the placenta, it is possible that the neurodevelopment of the fetus may be affected, leading to altered behavior in adulthood.

OBJECTIVES

In this study, we examined the effects of perinatal Flx exposure on the subsequent expression of circadian rhythms in adult mice.

METHODS

Dams were treated with 25 mg/kg/day Flx in their drinking water from embryonic day 15 to postnatal day 12. Circadian organization of wheel running rhythms and phase shifts to photic and non-photic stimuli were assessed in the offspring starting at 6 weeks of age.

RESULTS

We found that perinatal Flx exposure led to larger light-induced phase advances (1.19 ± 0.51 vs. 0.55 ± 0.25 h), smaller phase advances to the serotonin agonist 8-OH-DPAT during the mid-subjective day (0.44 ± 0.15 vs. 0.70 ± 0.17 h), and a shorter free-running period in constant darkness (23.47 ± 0.13 vs. 23.64 ± 0.13 h).

CONCLUSIONS

These results suggest that perinatal exposure to SSRIs may have consequences for the functioning of the circadian system later in life.

Neuroanatomy of the extended circadian rhythm system

The suprachiasmatic nucleus (SCN), site of the primary clock in the circadian rhythm system, has three major afferent connections. The most important consists of a retinohypothalamic projection through which photic information, received by classical rod/cone photoreceptors and intrinsically photoreceptive retinal ganglion cells, gains access to the clock. This information influences phase and period of circadian rhythms. The two other robust afferent projections are the median raphe serotonergic pathway and the geniculohypothalamic (GHT), NPY-containing pathway from the thalamic intergeniculate leaflet (IGL). Beyond this simple framework, the number of anatomical routes that could theoretically be involved in rhythm regulation is enormous, with the SCN projecting to 15 regions and being directly innervated by about 35. If multisynaptic afferents to the SCN are included, the number expands to approximately brain 85 areas providing input to the SCN. The IGL, a known contributor to circadian rhythm regulation, has a still greater level of complexity. This nucleus connects abundantly throughout the brain (to approximately 100 regions) by pathways that are largely bilateral and reciprocal. Few of these sites have been evaluated for their contributions to circadian rhythm regulation, although most have a theoretical possibility of doing so via the GHT. The anatomy of IGL connections suggests that one of its functions may be regulation of eye movements during sleep. Together, neural circuits of the SCN and IGL are complex and interconnected. As yet, few have been tested with respect to their involvement in rhythm regulation.

Setting the main circadian clock of a diurnal mammal by hypocaloric feeding

Caloric restriction attenuates the onset of a number of pathologies related to ageing. In mammals, circadian rhythms, controlled by the hypothalamic suprachiasmatic (SCN) clock, are altered with ageing. Although light is the main synchronizer for the clock, a daily hypocaloric feeding (HF) may also modulate the SCN activity in nocturnal rodents. Here we report that a HF also affects behavioural, physiological and molecular circadian rhythms of the diurnal rodent Arvicanthis ansorgei. Under constant darkness HF, but not normocaloric feeding (NF), entrains circadian behaviour. Under a light–dark cycle, HF at midnight led to phase delays of the rhythms of locomotor activity and plasma corticosterone. Furthermore, Per2 and vasopressin gene oscillations in the SCN were phase delayed in HF Arvicanthis compared with animals fed ad libitum. Moreover, light-induced expression of Per genes in the SCN was modified in HF Arvicanthis, despite a non-significant effect on light-induced behavioural phase delays. Together, our data show that HF affects the circadian system of the diurnal rodent Arvicanthis ansorgei differentially from nocturnal rodents. The Arvicanthis model has relevance for the potential use of HF to manipulate circadian rhythms in diurnal species including humans.

Cocaine modulates pathways for photic and nonphotic entrainment of the mammalian SCN circadian clock

Cocaine abuse is highly disruptive to circadian physiological and behavioral rhythms. The present study was undertaken to determine whether such effects are manifest through actions on critical photic and nonphotic regulatory pathways in the master circadian clock of the mouse suprachiasmatic nucleus (SCN). Impairment of SCN photic signaling by systemic (intraperitoneal) cocaine injection was evidenced by strong (60%) attenuation of light-induced phase-delay shifts of circadian locomotor activity during the early night. A nonphotic action of cocaine was apparent from its induction of 1-h circadian phase-advance shifts at midday. The serotonin receptor antagonist, metergoline, blocked shifting by 80%, implicating a serotonergic mechanism. Reverse microdialysis perfusion of the SCN with cocaine at midday induced 3.7 h phase-advance shifts. Control perfusions with lidocaine and artificial cerebrospinal fluid had little shifting effect. In complementary in vitro experiments, photic-like phase-delay shifts of the SCN circadian neuronal activity rhythm induced by glutamate application to the SCN were completely blocked by cocaine. Cocaine treatment of SCN slices alone at subjective midday, but not the subjective night, induced 3-h phase-advance shifts. Lidocaine had no shifting effect. Cocaine-induced phase shifts were completely blocked by metergoline, but not by the dopamine receptor antagonist, fluphenazine. Finally, pretreatment of SCN slices for 2 h with a low concentration of serotonin agonist (to block subsequent serotonergic phase resetting) abolished cocaine-induced phase shifts at subjective midday. These results reveal multiple effects of cocaine on adult circadian clock regulation that are registered within the SCN and involve enhanced serotonergic transmission.

Feedback actions of locomotor activity to the circadian clock

The phase of the mammalian circadian system can be entrained to a range of environmental stimuli, or zeitgebers, including food availability and light. Further, locomotor activity can act as an entraining signal and represents a mechanism for an endogenous behavior to feedback and influence subsequent circadian function. This process involves a number of nuclei distributed across the brain stem, thalamus, and hypothalamus and ultimately alters SCN electrical and molecular function to induce phase shifts in the master circadian pacemaker. Locomotor activity feedback to the circadian system is effective across both nocturnal and diurnal species, including humans, and has recently been shown to improve circadian function in a mouse model with a weakened circadian system. This raises the possibility that exercise may be useful as a noninvasive treatment in cases of human circadian dysfunction including aging, shift work, transmeridian travel, and the blind.

Entrainment of circadian clocks in mammals by arousal and food

Circadian rhythms in mammals are regulated by a system of endogenous circadian oscillators (clock cells) in the brain and in most peripheral organs and tissues. One group of clock cells in the hypothalamic SCN (suprachiasmatic nuclei) functions as a pacemaker for co-ordinating the timing of oscillators elsewhere in the brain and body. This master clock can be reset and entrained by daily LD (light–dark) cycles and thereby also serves to interface internal with external time, ensuring an appropriate alignment of behavioural and physiological rhythms with the solar day. Two features of the mammalian circadian system provide flexibility in circadian programming to exploit temporal regularities of social stimuli or food availability. One feature is the sensitivity of the SCN pacemaker to behavioural arousal stimulated during the usual sleep period, which can reset its phase and modulate its response to LD stimuli. Neural pathways from the brainstem and thalamus mediate these effects by releasing neurochemicals that inhibit retinal inputs to the SCN clock or that alter clock-gene expression in SCN clock cells. A second feature is the sensitivity of circadian oscillators outside of the SCN to stimuli associated with food intake, which enables animals to uncouple rhythms of behaviour and physiology from LD cycles and align these with predictable daily mealtimes. The location of oscillators necessary for food-entrained behavioural rhythms is not yet certain. Persistence of these rhythms in mice with clock-gene mutations that disable the SCN pacemaker suggests diversity in the molecular basis of light- and food-entrainable clocks.

Glucocorticoid-mediated nycthemeral and photoperiodic regulation of tph2 expression

In the Syrian hamster dorsal and median raphé nuclei, the tryptophan hydroxylase 2 gene (tph2), which codes the rate-limiting enzyme of serotonin synthesis, displays daily variations in its expression in animals entrained to a long but not to a short photoperiod. The present study aimed to assess the role of glucocorticoids in the nycthemeral and photoperiodic regulation of daily tph2 expression. In hamsters held in long photoperiod from birth, after adrenalectomy and glucocorticoid implants the suppression of glucocorticoid rhythms induced an abolition of the daily variations in tph2-mRNA concentrations, a decrease in the amplitude of body temperature rhythms and an increase in testosterone levels. All these effects were reversed after experimental restoration of a clear daily rhythm in the plasma glucocorticoid concentrations. We conclude that the photoperiod-dependent rhythm of glucocorticoids is the main regulator of tph2 daily expression.

Roles of light and serotonin in the regulation of gastrin-releasing peptide and arginine vasopressin output in the hamster SCN circadian clock

Daily timing of the mammalian circadian clock of the suprachiasmatic nucleus (SCN) is regulated by photic input from the retina via the retinohypothalamic tract. This signaling is mediated by glutamate, which activates SCN retinorecipient units communicating to pacemaker cells in part through the release of gastrin-releasing peptide (GRP). Efferent signaling from the SCN involves another SCN-containing peptide, arginine vasopressin (AVP). Little is known regarding the mechanisms regulating these peptides, as literature on in vivo peptide release in the SCN is sparse. Here, microdialysis-radioimmunoassay procedures were used to characterize mechanisms controlling GRP and AVP release in the hamster SCN. In animals housed under a 14/10-h light-dark cycle both peptides exhibited daily fluctuations of release, with levels increasing during the morning to peak around midday. Under constant darkness, this pattern persisted for AVP, but rhythmicity was altered for GRP, characterized by a broad plateau throughout the subjective night and early subjective day. Neuronal release of the peptides was confirmed by their suppression with reverse-microdialysis perfusion of calcium blockers and stimulation with depolarizing agents. Reverse-microdialysis perfusion with the 5-HT(1A,7) agonist 8-OH-DPAT ((±)-8-hydroxydipropylaminotetralin hydrobromide) during the day significantly suppressed GRP but had little effect on AVP. Also, perfusion with the glutamate agonist NMDA, or exposure to light at night, increased GRP but did not affect AVP. These analyses reveal distinct daily rhythms of SCN peptidergic activity, with GRP but not AVP release attenuated by serotonergic activation that inhibits photic phase-resetting, and activated by glutamatergic and photic stimulation that mediate this phase-resetting.

Physiology of circadian entrainment

Mammalian circadian rhythms are controlled by endogenous biological oscillators, including a master clock located in the hypothalamic suprachiasmatic nuclei (SCN). Since the period of this oscillation is of approximately 24 h, to keep synchrony with the environment, circadian rhythms need to be entrained daily by means of Zeitgeber ("time giver") signals, such as the light-dark cycle. Recent advances in the neurophysiology and molecular biology of circadian rhythmicity allow a better understanding of synchronization. In this review we cover several aspects of the mechanisms for photic entrainment of mammalian circadian rhythms, including retinal sensitivity to light by means of novel photopigments as well as circadian variations in the retina that contribute to the regulation of retinal physiology. Downstream from the retina, we examine retinohypothalamic communication through neurotransmitter (glutamate, aspartate, pituitary adenylate cyclase-activating polypeptide) interaction with SCN receptors and the resulting signal transduction pathways in suprachiasmatic neurons, as well as putative neuron-glia interactions. Finally, we describe and analyze clock gene expression and its importance in entrainment mechanisms, as well as circadian disorders or retinal diseases related to entrainment deficits, including experimental and clinical treatments.

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.

Environmental modulation of alcohol intake in hamsters: effects of wheel running and constant light exposure

BACKGROUND

Alcohol abuse leads to marked disruptions of circadian rhythms, and these disturbances in themselves can increase the drive to drink. Circadian clock timing is regulated by light, as well as by nonphotic influences such as food, social interactions, and wheel running. We previously reported that alcohol markedly disrupts photic and nonphotic modes of circadian rhythm regulation in Syrian hamsters. As an extension of this work, we characterize the hedonic interrelationship between wheel running and ethanol (EtOH) intake and the effects of environmental circadian disruption (long-term exposure to constant light [LL]) on the drive to drink.

METHODS

First, we tested the effect of wheel running on chronic free-choice consumption of a 20% (v/v) EtOH solution and water. Second, the effect of this alcohol drinking on wheel running in alcohol-naive animals was investigated. Third, we assessed the influence of LL, known to suppress locomotor activity and cause circadian rhythm disruption, on EtOH consumption and wheel-running behavior.

RESULTS

Inhibitory effects of wheel running on EtOH intake and vice versa were observed. Exposure to LL, while not affecting EtOH intake, induced rhythm splitting in 75% of the animals. Notably, the splitting phenotype was associated with lower levels of EtOH consumption and preference prior to, and throughout, the period of LL exposure.

CONCLUSIONS

These results are evidence that exercise may offer an efficacious clinical approach to reducing EtOH intake. Also, predisposition for light-induced (or other) forms of circadian disruption may modulate the drive to drink.

Phenotype and function of raphe projections to the suprachiasmatic nucleus

The circadian clock, located in the suprachiasmatic nucleus (SCN), receives a major afferent from the median raphe nucleus (MRN). In the Syrian hamster, only about 50% of the cells giving rise to this afferent contain serotonin. There is mixed evidence as to whether the serotonergic portion of this projection is involved in non-photic phase shifting of circadian locomotor rhythms. In order to better characterize the non-serotonergic projections, we conducted retrograde tract tracing using the beta subunit of cholera toxin combined with multi-label immunohistochemistry. Similar to previous findings, almost half of the retrogradely labeled cells contained serotonin. Additionally, approximately 30% of the retrogradely labeled cells contained vesicular glutamate transporter 3 (VGLUT3), but not serotonin. Surprisingly, some dorsal raphe cholera toxin labeling was also noted, particularly in animals with central-SCN injections. To determine if the non-serotonergic projections were important for non-photic phase shifts elicited by MRN stimulation, the MRN was electrically stimulated in animals pretreated with SCN injection of either the serotonin neurotoxin 5,7-dihydroxytryptamine or vehicle control. Intact animals phase advanced to midday electrical stimulation of the raphe while lesioned animals did not. Together, these results show that although some of the non-serotonergic raphe projections to the SCN contain VGLUT3, it is the serotonergic raphe innervation of the SCN that is critical for non-photic phase shifting elicited by MRN stimulation.

Circadian clock resetting by behavioral arousal: neural correlates in the midbrain raphe nuclei and locus coeruleus

Some procedures for stimulating arousal in the usual daily rest period (e.g., gentle handling, novel wheel-induced running) can phase shift circadian rhythms in Syrian hamsters, while other arousal procedures are ineffective (inescapable stress, caffeine, modafinil). The dorsal and median raphe nuclei (DRN, MnR) have been implicated in clock resetting by arousal and, in rats and mice, exhibit strong regionally specific responses to inescapable stress and anxiogenic drugs. To examine a possible role for the midbrain raphe nuclei in the differential effects of arousal procedures on circadian rhythms, hamsters were aroused for 3 h in the mid-rest period by confinement to a novel running wheel, gentle handling (with minimal activity) or physical restraint (with intermittent, loud compressed air stimulation) and sacrificed immediately thereafter. Regional expression of c-fos and tryptophan hydroxylase (TrpOH) were quantified immunocytochemically in the DRN, MnR and locus coeruleus (LC). Neither gentle handling nor wheel running had a large impact on c-fos expression in these areas, although the manipulations were associated with a small increase in c-Fos in TrpOH-like and TrpOH-negative cells, respectively, in the caudal interfascicular DRN region. By contrast, restraint stress significantly increased c-Fos in both TrpOH-like and TrpOH-negative cells in the rostral DRN and LC. c-Fos-positive cells in the DRN did not express tyrosine hydroxylase. These results reveal regionally specific monoaminergic correlates of arousal-induced circadian clock resetting, and suggest a hypothesis that strong activation of some DRN and LC neurons by inescapable stress may oppose clock resetting in response to arousal during the daily sleep period. More generally, these results complement evidence from other rodent species for functional topographic organization of the DRN.

Functional neuroanatomy of sleep and circadian rhythms

The daily sleep-wake cycle is perhaps the most dramatic overt manifestation of the circadian timing system, and this is especially true for the monophasic sleep-wake cycle of humans. Considerable recent progress has been made in elucidating the neurobiological mechanisms underlying sleep and arousal, and more generally, of circadian rhythmicity in behavioral and physiological systems. This paper broadly reviews these mechanisms from a functional neuroanatomical and neurochemical perspective, highlighting both historical and recent advances. In particular, I focus on the neural pathways underlying reciprocal interactions between the sleep-regulatory and circadian timing systems, and the functional implications of these interactions. While these two regulatory systems have often been considered in isolation, sleep-wake and circadian regulation are closely intertwined processes controlled by extensively integrated neurobiological mechanisms.

Interspecific contact affects phase response and activity in Desert hamsters

Circadian rhythms enhance survival and reproductive fitness of animals by promoting optimal timing of behavior and physiology with reference to geophysical changes in environment. Although light is considered the dominant stimulus for entraining circadian rhythms, social stimuli can also act as zeitgebers. The aim of this study was to analyze how Desert hamsters (Phodopus roborovskii) coordinate their behavior in time with that of animals of another competing species (Mongolian gerbils, Meriones unguiculatus). First, the behavior of hamsters was analyzed during a step-wise avoidance test. Two effects were observed: a) spatial separation if it was possible or b) shortening of the activity period due to contact without chance for avoidance. The latter finding was now further analyzed using a phase response curve (PRC). Here, phase shifts of Desert hamsters caused by single social interactions with Mongolian gerbils were quantified. Phase advances during the rest period were found at CT3 and CT9, a similar tendency was observed at CT6. A second phase advance was determined at CT18, coinciding with the end of the activity period. Then, it was tested whether additional activity during the stimulus was a trigger for the phase response. Although an increase in activity occurred especially when stimuli were applied during the rest period, there was no general relation between additional activity measured and the phase response shown. Overall, relevance of interspecific contact as nonphotic zeitgeber was indicated by phase shifts in a phase response curve. The shape of it can be explained by two behavioral adaptations; stress and contact avoidance.

Brief constant light accelerates serotonergic re-entrainment to large shifts of the daily light/dark cycle

Brief ( approximately 2 day) constant light exposure (LL(b)) in hamsters dramatically enhances circadian phase-resetting induced by the 5-HT receptor agonist, (+/-)-2-dipropyl-amino-8-hydroxyl-1,2,3,4-tetrahydronapthalene (8-OH-DPAT) and other nonphotic stimuli. The present study was undertaken to determine if LL(b) can also amplify phase-resetting responses to endogenous 5-HT and accelerate re-entrainment to large-magnitude advance and delay shifts of the light/dark (LD) cycle. First, central serotonergic activity was increased by i.p. injection of L-tryptophan+/-the 5-HT reuptake inhibitor fluoxetine. Hamsters under LD or exposed to LL(b) received vehicle or drugs during the early morning, and phase-shifts of the locomotor activity rhythm were measured after release to constant darkness. Neither drug phase-shifted animals not exposed to LL(b) (P>0.5 vs. vehicle); however in animals receiving LL(b,)L-tryptophan with and without fluoxetine produced large phase-advance shifts (means=2.5+/-0.4 h and 2.6+/-0.2 h, respectively; both P<0.035 vs. vehicle). Next, the effects of LL(b) combined with 8-OH-DPAT or L-tryptophan+fluoxetine on serotonergic re-entrainment to 10 h phase-advance and phase-delay shifts of the LD cycle were assessed. In groups not exposed to LL(b), vehicle controls re-entrained slowly to the advance and delay shifts (means=16+/-1 and 24+/-4 days, respectively), but those treated with 8-OH-DPAT re-entrained faster (means=11+/-2 and 9+/-2 days, respectively; both P<0.05 vs. vehicle). In groups exposed to LL(b), vehicle controls re-entrained slowly to the advance and delay shifts (means=15+/-2 and 25+/-3 days, respectively); however those receiving 8-OH-DPAT rapidly re-entrained to the delay and advance shifts, with the majority (75%) requiring only 1-2 days (means=2+/-1 and 4+/-2 days, respectively; both P<0.05 vs. vehicle). Animals exposed to LL(b) and treated with L-tryptophan+fluoxetine also exhibited accelerated re-entrainment to a 10 h advance shift (mean=5+/-2 days; P<0.05 vs. vehicle). Thus through enhancing serotonergic phase-resetting, LL(b) facilitates rapid re-entrainment to large shifts of the LD cycle which offers a potential approach for treating circadian-related desynchronies.

Circadian and dark-pulse activation of orexin/hypocretin neurons

Temporal control of brain and behavioral states emerges as a consequence of the interaction between circadian and homeostatic neural circuits. This interaction permits the daily rhythm of sleep and wake, regulated in parallel by circadian cues originating from the suprachiasmatic nuclei (SCN) and arousal-promoting signals arising from the orexin-containing neurons in the tuberal hypothalamus (TH). Intriguingly, the SCN circadian clock can be reset by arousal-promoting stimuli while activation of orexin/hypocretin neurons is believed to be under circadian control, suggesting the existence of a reciprocal relationship. Unfortunately, since orexin neurons are themselves activated by locomotor promoting cues, it is unclear how these two systems interact to regulate behavioral rhythms. Here mice were placed in conditions of constant light, which suppressed locomotor activity, but also revealed a highly pronounced circadian pattern in orexin neuronal activation. Significantly, activation of orexin neurons in the medial and lateral TH occurred prior to the onset of sustained wheel-running activity. Moreover, exposure to a 6 h dark pulse during the subjective day, a stimulus that promotes arousal and phase advances behavioral rhythms, activated neurons in the medial and lateral TH including those containing orexin. Concurrently, this stimulus suppressed SCN activity while activating cells in the median raphe. In contrast, dark pulse exposure during the subjective night did not reset SCN-controlled behavioral rhythms and caused a transient suppression of neuronal activation in the TH. Collectively these results demonstrate, for the first time, pronounced circadian control of orexin neuron activation and implicate recruitment of orexin cells in dark pulse resetting of the SCN circadian clock.