Successful reproduction in female mammals is precisely timed and must be able to withstand the metabolic demand of pregnancy and lactation. We show that kisspeptin-expressing neurons in the arcuate hypothalamus (Kiss1 ARH ) of female mice control the daily timing of food intake, along with the circadian regulation of locomotor activity, sleep, and core body temperature. Toxin-induced silencing of Kiss1 ARH neurons shifts wakefulness and food consumption to the light phase and induces weight gain. Toxin-silenced mice are less physically active and have attenuated temperature rhythms. Because the rhythm of the master clock in the suprachiasmatic nucleus (SCN) appears to be intact, we hypothesize that Kiss1 ARH neurons signal to neurons downstream of the master clock to modulate the output of the SCN. We conclude that, in addition to their wellestablished role in regulating fertility, Kiss1 ARH neurons are a critical component of the hypothalamic circadian oscillator network that times overt rhythms of physiology and behavior.

Temporal perception is fundamental to environmental adaptation in humans and other animals. To deal with timing and time perception, organisms have developed multiple systems that are active over a broad range of order of magnitude, the most important being circadian timing, interval timing and millisecond timing. The circadian pacemaker is located in the suprachiasmatic nuclei (SCN) of the hypothalamus, and is driven by a selfsustaining oscillator with a period close to 24 h. Time estimation in the second-to-minutes range -known as interval timing -involves the interaction of the basal ganglia and the prefrontal cortex. In this work we tested the hypothesis that interval timing in mice is sensitive to circadian modulations. Animals were trained following the peak-interval (PI) procedure. Results show significant differences in the estimation of 24-second intervals at different times of day, with a higher accuracy in the group trained at night, which were maintained under constant dark (DD) conditions. Interval timing was also studied in animals under constant light (LL) conditions, which abolish circadian rhythmicity. Mice under LL conditions were unable to acquire temporal control in the peak interval procedure.Moreover, short time estimation in animals subjected to circadian desynchronizations (modeling jet lag-like situations) was also affected. Taken together, our results indicate that short-time estimation is modulated by the circadian clock.© 2010 Elsevier B.V. All rights reserved. Keywords:Circadian rhythms Interval timingBasal ganglia Suprachiasmatic nuclei IntroductionTiming and time perception are fundamental to survival and goal reaching in humans and other animals. Organisms have developed diverse mechanisms for timing across different scales, the most important being circadian timing, interval timing and millisecond timing . The circadian pacemaker -which is driven by a self-sustaining oscillator with a period close to 24 h -is located in the suprachiasmatic nuclei (SCN) of the hypothalamus (Dunlap et al., 2004), and the principal signal that adjusts its activity is the light-dark cycle (Morin and Allen, 2006;Golombek and Rosenstein, 2010). The molecular mechanism of the endogenous circadian clock is comprised by interlocked transcription-translation feedback loops (Reppert and Weaver, 2002). On the other hand, the perception of shorter durations in the seconds-to-minutes range, known as interval timing, is crucial to learning, memory, decision making and other cognitive

Duration discrimination within the seconds-to-minutes range, known as interval timing, involves the interaction of cortico-striatal circuits via dopaminergic-glutamatergic pathways. Besides interval timing, most (if not all) organisms exhibit circadian rhythms in physiological, metabolic and behavioral functions with periods close to 24 h. We have previously reported that both circadian disruption and desynchronization impaired interval timing in mice. In this work we studied the involvement of dopamine (DA) signaling in the interaction between circadian and interval timing. We report that daily injections of levodopa improved timing performance in the peak-interval procedure in C57BL/6 mice with circadian disruptions, suggesting that a daily increase of DA is necessary for an accurate performance in the timing task. Moreover, striatal DA levels measured by reverse-phase high-pressure liquid chromatography indicated a daily rhythm under light/dark conditions. This daily variation was affected by inducing circadian disruption under constant light (LL). We also demonstrated a daily oscillation in tyrosine hydroxylase levels, DA turnover (3,4-dihydroxyphenylacetic acid/DA levels), and both mRNA and protein levels of the circadian component Period2 (Per2) in the striatum and substantia nigra, two brain areas relevant for interval timing. None of these oscillations persisted under LL conditions. We suggest that the lack of DA rhythmicity in the striatum under LL - probably regulated by Per2 - could be responsible for impaired performance in the timing task. Our findings add further support to the notion that circadian and interval timing share some common processes, interacting at the level of the dopaminergic system.

Biological clocks are genetically encoded oscillators that allow organisms to keep track of their environment. Among them, the circadian system is a highly conserved timing structure that regulates several physiological, metabolic and behavioural functions with periods close to 24 h. Time is also crucial for everyday activities that involve conscious time estimation. Timing behaviour in the second-to-minutes range, known as interval timing, involves the interaction of cortico-striatal circuits. In this review, we summarize current findings on the neurobiological basis of the circadian system, both at the genetic and behavioural level, and also focus on its interactions with interval timing and seasonal rhythms, in order to construct a multi-level biological clock.

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Study Objectives Sleep disturbances are common co-morbidities of epileptic disorders. Dravet syndrome (DS) is an intractable epilepsy accompanied by disturbed sleep. While there is evidence that daily sleep timing is disrupted in DS, the difficulty of chronically recording polysomnographic sleep from patients has left our understanding of the effect of DS on circadian sleep regulation incomplete. We aim to characterize circadian sleep regulation in a mouse model of DS. Methods Here we exploit long-term electrocorticographic recordings of sleep in a mouse model of DS in which one copy of the Scn1a gene is deleted. This model both genocopies and phenocopies the disease in humans. We test the hypothesis that the deletion of Scn1a in DS mice is associated with impaired circadian regulation of sleep. Results We find that DS mice show impairments in circadian sleep regulation, including a fragmented rhythm of non-rapid eye movement (NREM) sleep and an elongated circadian period of sleep. Next, we characterize re-entrainment of sleep stages and siesta following jet lag in the mouse. Strikingly, we find that re-entrainment of sleep following jet lag is normal in DS mice, in contrast to previous demonstrations of slowed re-entrainment of wheel-running activity. Finally, we report that DS mice are more likely to have an absent or altered daily “siesta”. Conclusions Our findings support the hypothesis that the circadian regulation of sleep is altered in DS and highlight the value of long-term chronic polysomnographic recording in studying the role of the circadian clock on sleep/wake cycles in pre-clinical models of disease.

Introduction: At night, the pineal gland produces the indoleamines, melatonin, N-acetylserotonin (NAS), and N-acetyltryptamine (NAT). Melatonin is accepted as a hormone of night. Could NAS and NAT serve that role too? Methods: Concentrationresponse measurements with overexpressed human melatonin receptors MT 1 and MT 2 ; mass spectrometry analysis of norepinephrine-stimulated secretions from isolated rat pineal glands; analysis of 24-hour periodic samples of rat blood. Results:We show that NAT and NAS do activate melatonin receptors MT 1 and MT 2 , although with lower potency than melatonin, and that in vitro, melatonin and NAS are secreted from stimulated, isolated pineal glands in roughly equimolar amounts, but secretion of NAT was much less. All three were found at roughly equal concentrations in blood during the night. However, during the day, serum melatonin fell to very low values creating a high-amplitude circadian rhythm that was absent after pinealectomy, whereas NAS and NAT showed only small or no circadian variation.Conclusion: Blood levels of NAS and NAT were insufficient to activate peripheral melatonin receptors, and they were invariant, so they could not serve as circulating hormones of night. However, they could instead act in paracrine circadian fashion near the pineal gland or via other higher-affinity receptors. K E Y W O R D Smelatonin, melatonin receptor, N-acetylserotonin, N-acetyltryptamine, pinealectomy 2 of 14 | LEE Et aL. microdialysis, NAS and melatonin levels showed parallel sustained nocturnal elevation within the rat pineal gland. 7 In other work, NAT levels became a few-fold higher at night than in day in blood plasma of rhesus macaque. 6 Could NAT and NAS also serve as signals of night? We evaluate that question in the rat.Melatonin regulates sleep and seasonal breeding. 8-10 The actions on sleep involve the widely distributed G protein-coupled melatonin receptors, MT 1 (MTNR1A), and MT 2 (MTNR1B). 11-14 These receptors with ~55% sequence identity 15 couple primarily to signaling by G i proteins, 16 but MT 1 may also couple to G q/11 . 17 The two melatonin receptors can form homo-and heterodimers in HEK293 cells. 18,19 Our experiments test whether NAT and NAS could act as night hormones that activate melatonin receptors in a circadian rhythm using four criteria: They must (a) be agonists of melatonin receptors, (b) be secreted when NE stimulates the pineal gland, (c) show large amplitude circadian fluctuations in blood, and (d) have blood concentrations sufficient to stimulate melatonin receptors. We show that NAT and NAS satisfy the first two criteria well but fail in the last two. These experiments are critical for understanding the activation of melatonin receptors by indoleamines in vivo. They bear on chronobiology and problems of shift work, jet lag, sleeplessness, and seasonal depression.

Nested within the hypothalamus, the suprachiasmatic nuclei (SCN) represent a central biological clock that regulates daily and circadian (i.e., close to 24 h) rhythms in mammals. Besides the SCN, a number of peripheral oscillators throughout the body control local rhythms and are usually kept in pace by the central clock. In order to represent an adaptive value, circadian rhythms must be entrained by environmental signals or zeitgebers, the main one being the daily light-dark (LD) cycle. The SCN adopt a stable phase relationship with the LD cycle that, when challenged, results in abrupt or chronic changes in overt rhythms and, in turn, in physiological, behavioral, and metabolic variables. Changes in entrainment, both acute and chronic, may have severe consequences in human performance and pathological outcome. Indeed, animal models of desynchronization have become a useful tool to understand such changes and to evaluate potential treatments in human subjects. Here we review a number of alterations in circadian entrainment, including jet lag, social jet lag (i.e., desynchronization between body rhythms and normal time schedules), shift work, and exposure to nocturnal light, both in human subjects and in laboratory animals. Finally, we focus on the health consequences related to circadian/entrainment disorders and propose a number of approaches for the management of circadian desynchronization. Circadian EntrainmentThe circadian clock located in the hypothalamic suprachiasmatic nucleus (SCN) is the central oscillator that regulates daily rhythms in mammals. Peripheral oscillators coupled to the SCN are synchronized by neural and humoral pathways and are able