Photic Entrainment of the Circadian System

Circadian rhythms are essential for the survival of all organisms, enabling them to predict daily changes in the environment and time their behaviour appropriately. The molecular basis of such rhythms is the circadian clock, a self-sustaining molecular oscillator comprising a transcriptional–translational feedback loop. This must be continually readjusted to remain in alignment with the external world through a process termed entrainment, in which the phase of the master circadian clock in the suprachiasmatic nuclei (SCN) is adjusted in response to external time cues. In mammals, the primary time cue, or “zeitgeber”, is light, which inputs directly to the SCN where it is integrated with additional non-photic zeitgebers. The molecular mechanisms underlying photic entrainment are complex, comprising a number of regulatory factors. This review will outline the photoreception pathways mediating photic entrainment, and our current understanding of the molecular pathways that drive it in the SCN.

Nested calcium dynamics support daily cell unity and diversity in the suprachiasmatic nuclei of free-behaving mice

The suprachiasmatic nuclei (SCN) of the anterior hypothalamus host the circadian pacemaker that synchronizes mammalian rhythms with the day-night cycle. SCN neurons are intrinsically rhythmic, thanks to a conserved cell-autonomous clock mechanism. In addition, circuit-level emergent properties confer a unique degree of precision and robustness to SCN neuronal rhythmicity. However, the multicellular functional organization of the SCN is not yet fully understood. Although SCN neurons are well coordinated, experimental evidences indicate that some neurons oscillate out of phase in SCN explants, and possibly to a larger extent in vivo. Here, we used microendoscopic Ca2+i imaging to investigate SCN rhythmicity at a single cell resolution in free-behaving mice. We found that SCN neurons in vivo exhibited fast Ca2+i spikes superimposed upon slow changes in baseline Ca2+i levels. Both spikes and baseline followed a time-of-day modulation in many neurons, but independently from each other. Daily rhythms in basal Ca2+i were well coordinated, while spike activity from the same neurons peaked at multiple times of the light cycle, and unveiled clock-independent interactions at the multicellular level. Hence, fast Ca2+i spikes and slow changes in baseline Ca2+i levels highlighted how diverse activity patterns could articulate within the temporal network unity of the SCN in vivo, and provided support for a multiplex neuronal code in the circadian pacemaker.

The sleep-wake distribution contributes to the peripheral rhythms in PERIOD-2

In the mouse, Period-2 (Per2) expression in tissues peripheral to the suprachiasmatic nuclei (SCN) increases during sleep deprivation and at times of the day when animals are predominantly awake spontaneously, suggesting that the circadian sleep-wake distribution directly contributes to the daily rhythms in Per2. We found support for this hypothesis by recording sleep-wake state alongside PER2 bioluminescence in freely behaving mice, demonstrating that PER2 bioluminescence increases during spontaneous waking and decreases during sleep. The temporary reinstatement of PER2-bioluminescence rhythmicity in behaviorally arrhythmic SCN-lesioned mice submitted to daily recurring sleep deprivations substantiates our hypothesis. Mathematical modelling revealed that PER2 dynamics can be described by a damped harmonic oscillator driven by two forces: a sleep-wake-dependent force and a SCN-independent circadian force. Our work underscores the notion that in peripheral tissues the clock gene circuitry integrates sleep-wake information and could thereby contribute to behavioral adaptability to respond to homeostatic requirements.

Daily electrical activity in the master circadian clock of a diurnal mammal

Circadian rhythms in mammals are orchestrated by a central clock within the suprachiasmatic nuclei (SCN). Our understanding of the electrophysiological basis of SCN activity comes overwhelmingly from a small number of nocturnal rodent species, and the extent to which these are retained in day-active animals remains unclear. Here, we recorded the spontaneous and evoked electrical activity of single SCN neurons in the diurnal rodent Rhabdomys pumilio, and developed cutting-edge data assimilation and mathematical modeling approaches to uncover the underlying ionic mechanisms. As in nocturnal rodents, R. pumilio SCN neurons were more excited during daytime hours. By contrast, the evoked activity of R. pumilio neurons included a prominent suppressive response that is not present in the SCN of nocturnal rodents. Our modeling revealed and subsequent experiments confirmed transient subthreshold A-type potassium channels as the primary determinant of this response, and suggest a key role for this ionic mechanism in optimizing SCN function to accommodate R. pumilio’s diurnal niche.

Altered circadian rhythms and sleep in a new Angelman Syndrome mouse model

Normal neurodevelopment requires precise expression of the key ubiquitin ligase gene Ube3a. Comparing newly generated mouse models for Ube3a down-regulation (models of Angelman syndrome) vs. Ube3a up-regulation (models for autism), we find reciprocal effects of Ube3a gene dosage on phenotypes associated with circadian rhythmicity, including the amount of locomotor activity. In contrast to previous reports, we find that Ube3a is imprinted in neurons of the suprachiasmatic nuclei, the pacemaking circadian brain locus. In addition, Ube3a-deficient mice lack the typical drop in wake late in the dark period and have blunted responses to sleep deprivation. Suppression of physical activity by light in Ube3a-deficient mice is not due to anxiety as measured by behavioral tests and stress hormones; quantification of stress hormones may serve as an easily measurable biomarker for evaluating potential therapeutic treatments for Angelman syndrome. We conclude that reduced Ube3a gene dosage affects not only neurodevelopment but also sleep patterns and circadian rhythms.

The Circadian Rhythms of STAT3 in the Rat Pineal Gland and Its Involvement in Arylalkylamine-N-Acetyltransferase Regulation

In rodents, the melatonin production by the pineal gland is controlled through adrenergic signaling from the suprachiasmatic nuclei and regulation of the principal enzyme in its synthesis, arylalkylamine-N-acetyltransferase (AANAT). In the present study, we identified increased isoprenaline-induced aa-nat expression and nocturnal AANAT activity in the pineal glands in response to the silencing of the signal transducer and activator of transcription 3 (STAT3) with siRNA or STAT3 inhibitors WP1066 and AZD1480. This AANAT activity enhancement in vivo did not interfere with light-induced AANAT suppression. Systemic or in vitro lipopolysaccharide (LPS) administration markedly increased Stat3 expression and STAT3 phosphorylation, but it did not significantly affect AANAT expression or activity. Simultaneous LPS administration and Stat3 silencing enhanced the aa-nat transcription and AANAT activity to a similar extent as Stat3 inhibition without LPS co-administration. Furthermore, we describe the circadian rhythmicity in Stat3 expression and the phosphorylated form of STAT3 protein in the rat pineal gland. Our data suggest that the higher nocturnal endogenous level of STAT3 in the pineal gland decelerates or hampers the process of NA-induced AANAT activation or affects the AANAT enzyme stability.

Effects of Sleep Deprivation by Olfactorily Induced Sexual Arousal Compared to Immobilization Stress and Manual Sleep Deprivation on Neuromessengers and Time Keeping Genes in the Suprachiasmatic Nuclei and Other Cerebral Entities of Syrian Hamsters—An Immunohistochemical Study

We investigated the effects of sexual arousal induced by olfactory stimuli on the expression of neuromodulators, neurotransmitters and sexual steroid receptors in the suprachiasmatic nucleus (SCN, the circadian pacemaker of mammals) and other cerebral entities of Syrian hamsters (Mesocricetus auratus) compared to manual sleep deprivation and immobilization stress. The hamsters kept under a 12:12 hours (h) light:dark cycle were deprived of sleep by sexual stimulation, gentle manual handling or immobilization stress for 1 h at the beginning of the light phase and subsequently sacrificed at zeitgeber time 01:00, respectively; for comparison, hamsters were manually sleep deprived for 6 or 20 h or sacrificed after completing a full sleep phase. As demonstrated by immunohistochemistry, apart from various alterations after manual sleep deprivation, sexual stimulation caused down-regulation of arginine-vasopressin (AVP), vasointestinal peptide (VIP), serotonin (5-HT), substance P (SP), and met-enkephalin (ME) in the SCN. Somatostatin (SOM) was diminished in the medial periventricular nucleus (MPVN). In contrast, an increase in AVP was observed in the PVN, that of oxytocin (OXY) in the supraoptic nucleus (SON), of tyrosine-hydroxylase (TH) in the infundibular nucleus (IN), and dopamine beta-hydroxylase (DBH) in the A7 neuron population of the brain stem (A7), respectively. Testosterone in plasma was increased. The results indicate that sexual arousal extensively influences the neuropeptide systems of the SCN, suggesting an involvement of the SCN in reproductive behavior.

Dissecting and modeling photic and melanopsin effects to predict sleep disturbances induced by irregular light exposure in mice

Artificial lighting, day-length changes, shift work, and transmeridian travel all lead to sleep–wake disturbances. The nychthemeral sleep–wake cycle (SWc) is known to be controlled by output from the central circadian clock in the suprachiasmatic nuclei (SCN), which is entrained to the light–dark cycle. Additionally, via intrinsically photosensitive retinal ganglion cells containing the photopigment melanopsin (Opn4), short-term light–dark alternations exert direct and acute influences on sleep and waking. However, the extent to which longer exposures typically experienced across the 24-h day exert such an effect has never been clarified or quantified, as disentangling sustained direct light effects (SDLE) from circadian effects is difficult. Recording sleep in mice lacking a circadian pacemaker, either through transgenesis (Syt10cre/creBmal1fl/-) or SCN lesioning and/or melanopsin-based phototransduction (Opn4−/−), we uncovered, contrary to prevailing assumptions, that the contribution of SDLE is as important as circadian-driven input in determining SWc amplitude. Specifically, SDLE were primarily mediated (>80%) through melanopsin, of which half were then relayed through the SCN, revealing an ancillary purpose for this structure, independent of its clock function in organizing SWc. Based on these findings, we designed a model to estimate the effect of atypical light–dark cycles on SWc. This model predicted SWc amplitude in mice exposed to simulated transequatorial or transmeridian paradigms. Taken together, we demonstrate this SDLE is a crucial mechanism influencing behavior on par with the circadian system. In a broader context, these findings mandate considering SDLE, in addition to circadian drive, for coping with health consequences of atypical light exposure in our society.

Preoperative oral melatonin can reduce preoperative anxiety and postoperative analgesia in a dose-dependent manner

Background Preoperative anxiety has deleterious effects on patients’ outcome through its influence on intraoperative requirements of anesthetics and analgesics (Bayrak et al., J Coll Physicians Surg Pak 29:868–873, 2019), postoperative (PO) pain intensity, and analgesia requirement, and may even increase PO morbidity and mortality after certain types of surgery. Melatonin is a methoxyindole synthesized and secreted principally by the pineal gland at night under control of an endogenous rhythm of secretion generated by the suprachiasmatic nuclei. The current study hypothesized that preoperative melatonin could reduce patients’ anxiety and reduce intraoperative (IO) and postoperative (PO) analgesic in a dose-dependent manner. Results Preoperative consultation was, to some extent, effective in reducing patients’ anxiety and apprehension. At 1 h after receiving premedication, Anxiety Specific to Surgery Questionnaire (ASSQ) scores were significantly lower in study groups in comparison to baseline scores and at 1 h scores of P group patients (patients who received 3 ml of plain distilled water), and this significant effect extended for 3-h PO. The reported ∆∆ASSQ between study groups was 25.9% between M2 (melatonin) and Z (midazolam) groups and 36.9% between groups M1 (received melatonin in a dose of 3 mg) and M2 (received melatonin in a dose of 6 mg). Preoperative anxiolytic therapy allowed reduction of PO pain scores and analgesia consumption with prolongation of duration till 1st request of rescue analgesia, and these effects were more pronounced with melatonin 6 mg in comparison to placebo, melatonin 3mg, or midazolam. Conclusion Preoperative melatonin is an appropriate policy for reduction of preoperative anxiety and provided reduction of PO anxiety, pain scores, and consumption of analgesia thus promoting early recovery and short PO hospital stay. Dose dependency was evident, and preoperative melatonin 6-mg dose provided satisfactory effect.