Sex differences in behavioral circadian rhythms in laboratory rodents

Sex differences in spontaneous behavior and cognition in mice using an automated behavior monitoring system

Isolation of sex differences as a key characteristic underlying neurobehavioral differentiation is an essential component of studies in neuroscience. The current study sought to address this concern by observing behavioral differences using an automated home cage system for neurobehavioral assessment, a method rapidly increasing in use due to advances in technology and advantages such as reduced handling stress and cross-lab variability. Sex differences in C57BL/6 mice arose for motor activity and circadian-linked behavior, with females being more active compared to males, and males having a stronger anticipatory increase in activity leading up to the onset of the light phase compared to females. These activity differences were observed not only across the lifespan, but also in different genetic background mouse strains across different testing sites showing the generalizability and robustness of these observed effects. Activity differences were also observed in performance on a spatial learning and reversal task with females making more responses and receiving a corresponding elevation in reward pellets. Notably, there were no sex differences in learning nor achieved accuracy, suggesting these observed effects were predominantly in activity. The outcomes of this study align with previous reports showcasing differences in activity between males and females. The comparison across strains and testing sites showed robust and reproducible differences in behavior between female and male mice that are relevant to consider when designing behavioral studies. Furthermore, the observed sex differences in performance on the learning and reversal procedure raise concern for interpretation of behavior differences between sexes due to the attribution of these differences to motor activity rather than cognition.

Increased susceptibility to kainate-induced seizures in a mouse model of tuberous sclerosis complex: Importance of sex and circadian cycle

OBJECTIVE

Comorbidity of epilepsy and autism in tuberous sclerosis complex 2 (TSC2) is very frequent, but the link between these conditions is still poorly understood. To study neurological problems related to autism, the scientific community has been using an animal model of TSC2, Tsc2+/- mice. However, it is still unknown whether this model has the propensity to exhibit increased seizure susceptibility. Further, the importance of sex and/or the circadian cycle in this biological process has never been addressed. This research aimed to determine whether male and female Tsc2+/- mice have altered seizure susceptibility at light and dark phases.

METHODS

We assessed seizure susceptibility and progression in a Tsc2+/- mouse model using the chemical convulsant kainic acid (KA), a potent agonist of the AMPA/kainate class of glutamate receptors. Both male and female animals at adult age were evaluated during non-active and active periods. Seizure severity was determined by integrating individual scores per mouse according to a modified Racine scale. Locomotor behavior was monitored during control and after KA administration.

RESULTS

We found increased seizure susceptibility in Tsc2+/- mice with a significant influence of sex and circadian cycle on seizure onset, progression, and behavioral outcomes. While, compared to controls, Tsc2+/- males overall exhibited higher susceptibility independently of circadian cycle, Tsc2+/- females were more susceptible during the dark and post-ovulatory phase. Interestingly, sexual dimorphisms related to KA susceptibility were always reported during light phase independently of the genetic background, with females being the most vulnerable.

SIGNIFICANCE

The enhanced susceptibility in the Tsc2 mouse model suggests that other neurological alterations, beside brain lesions, may be involved in seizure occurrence for TSC. Importantly, our work highlighted the importance of considering biological sex and circadian cycle for further studies of TSC-related epilepsy research.

PLAIN LANGUAGE SUMMARY

Tuberous sclerosis complex (TSC) is a rare genetic disorder. It causes brain lesions and is linked to epilepsy, intellectual disability, and autism. We wanted to investigate epilepsy in this model. We found that these mice have more induced seizures than control animals. Our results show that these mice can be used in future epilepsy research for this disorder. We also found that sex and time of day can influence the results. This must be considered in this type of research.

Circadian neurogenetics and its implications in neurophysiology, behavior, and chronomedicine

The circadian rhythm affects multiple physiological processes, and disruption of the circadian system can be involved in a range of disease-related pathways. The genetic underpinnings of the circadian rhythm have been well-studied in model organisms. Significant progress has been made in understanding how clock genes affect the physiological functions of the nervous system. In addition, circadian timing is becoming a key factor in improving drug efficacy and reducing drug toxicity. The circadian biology of the target cell determines how the organ responds to the drug at a specific time of day, thus regulating pharmacodynamics. The current review brings together recent advances that have begun to unravel the molecular mechanisms of how the circadian clock affects neurophysiological and behavioral processes associated with human brain diseases. We start with a brief description of how the ubiquitous circadian rhythms are regulated at the genetic, cellular, and neural circuit levels, based on knowledge derived from extensive research on model organisms. We then summarize the latest findings from genetic studies of human brain disorders, focusing on the role of human clock gene variants in these diseases. Lastly, we discuss the impact of common dietary factors and medications on human circadian rhythms and advocate for a broader application of the concept of chronomedicine.

Gender perspective on the association between liver enzyme markers and non-alcoholic fatty liver disease: insights from the general population

Objective

Every distinct liver enzyme biomarker exhibits a strong correlation with non-alcoholic fatty liver disease (NAFLD). This study aims to comprehensively analyze and compare the associations of alanine aminotransferase (ALT), aspartate aminotransferase (AST), and gamma-glutamyl transferase (GGT) with NAFLD from a gender perspective.

Methods

This study was conducted on 6,840 females and 7,411 males from the NAGALA cohort. Multivariable logistic regression analysis was used to compare the associations between liver enzyme markers and NAFLD in both genders, recording the corresponding adjusted odds ratios (ORs) and 95% confidence intervals (CIs). Receiver operating characteristic (ROC) curves were used to evaluate the accuracy of individual liver enzyme markers and different combinations of them in identifying NAFLD.

Results

Liver enzyme markers ALT, AST, and GGT were all independently associated with NAFLD and exhibited significant gender differences (All P-interaction<0.05). In both genders, ALT exhibited the most significant association with NAFLD, with adjusted standardized ORs of 2.19 (95% CI: 2.01-2.39) in males and 1.60 (95% CI: 1.35-1.89) in females. Additionally, ROC analysis showed that ALT had significantly higher accuracy in identifying NAFLD than AST and GGT in both genders (Delong P-value < 0.05), and the accuracy of ALT in identifying NAFLD in males was higher than that in females [Area under the ROC curve (AUC): male 0.79, female 0.77]. Furthermore, out of the various combinations of liver enzymes, ALT+GGT showed the highest accuracy in identifying NAFLD in both genders, with AUCs of 0.77 (95% CI: 0.75-0.79) in females and 0.79 (95% CI: 0.78-0.81) in males.

Conclusion

Our study revealed significant gender differences in the associations of the three commonly used liver enzyme markers with NAFLD. In both genders, the use of ALT alone may be the simplest and most effective tool for screening NAFLD, especially in males.

Sex Differences in Long-term Outcome of Prenatal Exposure to Excess Glucocorticoids-Implications for Development of Psychiatric Disorders

Exposure to prenatal insults, such as excess glucocorticoids (GC), may lead to pathological outcomes, including neuropsychiatric disorders. The aim of the present study was to investigate the long-term effects of in utero exposure to the synthetic GC analog dexamethasone (Dex) in adult female offspring. We monitored spontaneous activity in the home cage under a constant 12 h/12 h light/dark cycle, as well as the changes following a 6-h advance of dark onset (phase shift). For comparison, we re-analysed data previously recorded in males. Dex-exposed females were spontaneously more active, and the activity onset re-entrained slower than in controls. In contrast, Dex-exposed males were less active, and the activity onset re-entrained faster than in controls. Following the phase shift, control females displayed a transient reorganisation of behaviour in light and virtually no change in dark, while Dex-exposed females showed limited variations from baseline in both light and dark, suggesting weaker photic entrainment. Next, we ran bulk RNA-sequencing in the suprachiasmatic nucleus (SCN) of Dex and control females. SPIA pathway analysis of ~ 2300 differentially expressed genes identified significantly downregulated dopamine signalling, and upregulated glutamate and GABA signalling. We selected a set of candidate genes matching the behaviour alterations and found consistent differential regulation for ~ 73% of tested genes in SCN and hippocampus tissue samples. Taken together, our data highlight sex differences in the outcome of prenatal exposure to excess GC in adult mice: in contrast to depression-like behaviour in males, the phenotype in females, defined by behaviour and differential gene expression, is consistent with ADHD models.

Accelerometer-based predictions of behaviour elucidate factors affecting the daily activity patterns of spotted hyenas

Animal activity patterns are highly variable and influenced by internal and external factors, including social processes. Quantifying activity patterns in natural settings can be challenging, as it is difficult to monitor animals over long time periods. Here, we developed and validated a machine-learning-based classifier to identify behavioural states from accelerometer data of wild spotted hyenas (Crocuta crocuta), social carnivores that live in large fission-fusion societies. By combining this classifier with continuous collar-based accelerometer data from five hyenas, we generated a complete record of activity patterns over more than one month. We used these continuous behavioural sequences to investigate how past activity, individual idiosyncrasies, and social synchronization influence hyena activity patterns. We found that hyenas exhibit characteristic crepuscular-nocturnal daily activity patterns. Time spent active was independent of activity level on previous days, suggesting that hyenas do not show activity compensation. We also found limited evidence for an effect of individual identity on activity, and showed that pairs of hyenas who synchronized their activity patterns must have spent more time together. This study sheds light on the patterns and drivers of activity in spotted hyena societies, and also provides a useful tool for quantifying behavioural sequences from accelerometer data.

Differential locomotor activity responses to day-time light intensity in juvenile and adult solitary Cape mole-rats, Georychus capensis (Rodentia: Bathyergidae)

The Cape mole-rat (Georychus capensis) is a solitary, strictly subterranean rodent that is responsive to light and entrains to photic cues despite having a reduced visual system. Circadian entrainment is maintained throughout life, but age can alter the amplitude of the response and re-entrainment time. Mole-rats are long-lived for their size which raises questions regarding the robustness of their circadian rhythms and how impacts their locomotor activity rhythms. The locomotor activity rhythms of juvenile and adult Cape mole-rats were investigated. They were exposed to pre-experimental and post-experimental control cycles under fluorescent lights, six 12 h light:12 h dark cycles of decreasing intensities and a constant dark cycle (DD). All animals exhibited more activity during the dark phases of all light regimes. Juveniles were more active than adults and displayed more variable activity during both the light and dark phases. Adults exhibited relatively stable levels of activity under all experimental conditions, whereas juvenile activity decreased as the light intensity was reduced. The amplitude of Cape mole-rat rhythms was consistently low, but similar across light regimes and between adults and juveniles. Cape mole-rats have functional circadian systems, are primarily nocturnal and respond differentially to light intensity depending on their age. Light intensity does not affect the locomotor activity responses of Cape mole-rats in a predictable manner, and could indicate more complex interactions with light wavelengths. The circadian systems of juveniles appear to be more sensitive than those of adults, although the mechanism of the light response remains unclear.

Risk taking for potential losses but not gains increases with time of day

Humans exhibit distinct risk preferences when facing choices involving potential gains and losses. These preferences are believed to be subject to neuromodulatory influence, particularly from dopamine and serotonin. As neuromodulators manifest circadian rhythms, this suggests decision making under risk might be affected by time of day. Here, in a large subject sample collected using a smartphone application, we found that risky options with potential losses were increasingly chosen over the course of the day. We observed this result in both a within-subjects design (N = 2599) comparing risky options chosen earlier and later in the day in the same individuals, and in a between-subjects design (N = 26,720) showing our effect generalizes across ages and genders. Using computational modelling, we show this diurnal change in risk preference reflects a decrease in sensitivity to increasing losses, but no change was observed in the relative impacts of gains and losses on choice (i.e., loss aversion). Thus, our findings reveal a striking diurnal modulation in human decision making, a pattern with potential importance for real-life decisions that include voting, medical decisions, and financial investments.

Sex-specific role of the circadian transcription factor NPAS2 in opioid tolerance, withdrawal and analgesia

Opioids like fentanyl remain the mainstay treatment for chronic pain. Unfortunately, opioid's high dependence liability has led to the current opioid crisis, in part, because of side-effects that develop during long-term use, including analgesic tolerance and physical dependence. Both tolerance and dependence to opioids may lead to escalation of required doses to achieve previous therapeutic efficacy. Additionally, altered sleep and circadian rhythms are common in people on opioid therapy. Opioids impact sleep and circadian rhythms, while disruptions to sleep and circadian rhythms likely mediate the effects of opioids. However, the mechanisms underlying these bidirectional relationships between circadian rhythms and opioids remain largely unknown. The circadian protein, neuronal PAS domain protein 2 (NPAS2), regulates circadian-dependent gene transcription in structure of the central nervous system that modulate opioids and pain. Here, male and female wild-type and NPAS2-deficient (NPAS2-/-) mice were used to investigate the role of NPAS2 in fentanyl analgesia, tolerance, hyperalgesia and physical dependence. Overall, thermal pain thresholds, acute analgesia and tolerance to a fixed dose of fentanyl were largely similar between wild-type and NPAS2-/- mice. However, female NPAS2-/- exhibited augmented analgesic tolerance and significantly more behavioral symptoms of physical dependence to fentanyl. Only male NPAS2-/- mice had increased fentanyl-induced hypersensitivity, when compared with wild-type males. Together, our findings suggest sex-specific effects of NPAS2 signaling in the regulation of fentanyl-induced tolerance, hyperalgesia and dependence.

Atypical behavioral and thermoregulatory circadian rhythms in mice lacking a microbiome

Trillions of microbial oscillators reside throughout the mammalian body, yet their contributions toward fundamental features of host circadian rhythms (CRs) have not been characterized. Here, we demonstrate that the microbiome contributes to host CRs in activity and thermoregulation. Mice devoid of microbes (germ-free, GF) exhibited higher-amplitude CRs in a light-dark cycle and longer circadian periods in constant darkness. Circadian entrainment to food was greater in GF mice, but resetting responses to simulated jet-lag were unaffected. Microbial transplantation with cecal contents of conventionally-raised mice normalized CRs of GF mice, indicating that the concurrent activity of gut microbes modulates host circadian networks. Obesogenic effects of high-fat diet were absent in GF mice, but some circadian-disruptive effects persisted. Transkingdom (host-microbe) interactions affect circadian period and entrainment of CRs in diverse traits, and microbes alter interactions among light- and food-entrainable circadian processes in the face of environmental (light, diet) perturbations.

Sex Differences in Circadian Rhythms

Sex as a biological variable is the focus of much literature and has been emphasized by the National Institutes of Health, in part, to remedy a long history of male-dominated studies in preclinical and clinical research. We propose that time-of-day is also a crucial biological variable in biomedical research. In common with sex differences, time-of-day should be considered in analyses and reported to improve reproducibility of studies and to provide the appropriate context to the conclusions. Endogenous circadian rhythms are present in virtually all living organisms, including bacteria, plants, invertebrates, and vertebrates. Virtually all physiological and behavioral processes display daily fluctuations in optimal performance that are driven by these endogenous circadian clocks; importantly, many of those circadian rhythms also show sex differences. In this review, we describe some of the documented sex differences in circadian rhythms.

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.

Relationships between endogenous circadian period, physiological and cognitive parameters and sex in aged gray mouse lemurs (Microcebus murinus)

The biological clock generates circadian rhythms, with an endogenous period tau close to 24 h. The circadian resonance theory proposes that lifespan is reduced when endogenous period goes far from 24 h. It has been suggested that daily resetting of the circadian clock to the 24 h external photoperiod might induce marginal costs that would accumulate over time and forward accelerate aging and affect fitness. In this study, we aimed to evaluate the link between the endogenous period and biomarkers of aging in order to investigate the mechanisms of the circadian resonance theory. We studied 39 middle-aged and aged Microcebus murinus, a nocturnal non-human primate whose endogenous period is about 23.1 h, measuring the endogenous period of locomotor activity, as well as several physiological and behavioral parameters (rhythm fragmentation and amplitude, energetic expenditure, oxidative stress, insulin-like growth factor-1 (IGF-1) concentrations and cognitive performances) in both males and females. We found that aged males with tau far from 24 h displayed increased oxidative stress. We also demonstrated a positive correlation between tau and IGF-1 concentrations, as well as learning performances, in males and females. Together these results suggest that a great deviation of tau from 24 h leads to increased biomarkers of age-related impairments.

New integrative approaches to discovery of pathophysiological mechanisms triggered by night shift work

Synchronization to periodic cues such as food/water availability and light/dark cycles is crucial for living organisms' homeostasis. Both factors have been heavily influenced by human activity, with artificial light at night (ALAN) being an evolutionary challenge imposed over roughly the last century. Evidence from studies in humans and animal models shows that overt circadian misalignment, such as that imposed to about 20% of the workforce by night shift work (NSW), negatively impinges on the internal temporal order of endocrinology, physiology, metabolism, and behavior. Moreover, NSW is often associated to mistimed feeding, with both unnatural behaviors being known to increase the risk of chronic diseases, such as eating disorders, overweight, obesity, cardiovascular, metabolic (particularly type 2 diabetes mellitus) and gastrointestinal disorders, some types of cancer, as well as mental disease including sleep disturbances, cognitive disorders, and depression. Regarding deleterious effects of ALAN on reproduction, increased risk of miscarriage, preterm delivery and low birth weight have been reported in shift-worker women. These mounting lines of evidence prompt further efforts to advance our understanding of the effects of long-term NSW on health. Emerging data suggest that NSW with or without mistimed feeding modify gene expression and functional readouts in different tissues/organs, which seem to translate into persistent cardiometabolic and endocrine dysfunction. However, this research avenue still faces multiple challenges, such as functional characterization of new experimental models more closely resembling human long-term NSW and mistimed feeding in males versus females; studying further target organs; identifying molecular changes by means of deep multi-omics analyses; and exploring biomarkers of NSW with translational medicine potential. Using high-throughput and systems biology is a relatively new approach to study NSW, aimed to generate experiments addressing new biological factors, pathways, and mechanisms, going beyond the boundaries of the circadian clock molecular machinery.

Circadian-Dependent and Sex-Dependent Increases in Intravenous Cocaine Self-Administration in Npas2 Mutant Mice

Substance use disorder (SUD) is associated with disruptions in circadian rhythms. The circadian transcription factor neuronal PAS domain protein 2 (NPAS2) is enriched in reward-related brain regions and regulates reward, but its role in SU is unclear. To examine the role of NPAS2 in drug taking, we measured intravenous cocaine self-administration (acquisition, dose-response, progressive ratio, extinction, cue-induced reinstatement) in wild-type (WT) and Npas2 mutant mice at different times of day. In the light (inactive) phase, cocaine self-administration, reinforcement, motivation and extinction responding were increased in all Npas2 mutants. Sex differences emerged during the dark (active) phase with Npas2 mutation increasing self-administration, extinction responding, and reinstatement only in females as well as reinforcement and motivation in males and females. To determine whether circulating hormones are driving these sex differences, we ovariectomized WT and Npas2 mutant females and confirmed that unlike sham controls, ovariectomized mutant mice showed no increase in self-administration. To identify whether striatal brain regions are activated in Npas2 mutant females, we measured cocaine-induced ΔFosB expression. Relative to WT, ΔFosB expression was increased in D1+ neurons in the nucleus accumbens (NAc) core and dorsolateral (DLS) striatum in Npas2 mutant females after dark phase self-administration. We also identified potential target genes that may underlie the behavioral responses to cocaine in Npas2 mutant females. These results suggest NPAS2 regulates reward and activity in specific striatal regions in a sex and time of day (TOD)-specific manner. Striatal activation could be augmented by circulating sex hormones, leading to an increased effect of Npas2 mutation in females.SIGNIFICANCE STATEMENT Circadian disruptions are a common symptom of substance use disorders (SUDs) and chronic exposure to drugs of abuse alters circadian rhythms, which may contribute to subsequent SU. Diurnal rhythms are commonly found in behavioral responses to drugs of abuse with drug sensitivity and motivation peaking during the dark (active) phase in nocturnal rodents. Emerging evidence links disrupted circadian genes to SU vulnerability and drug-induced alterations to these genes may augment drug-seeking. The circadian transcription factor neuronal PAS domain protein 2 (NPAS2) is enriched in reward-related brain regions and regulates reward, but its role in SU is unclear. To examine the role of NPAS2 in drug taking, we measured intravenous cocaine self-administration in wild-type (WT) and Npas2 mutant mice at different times of day.

Cross-species physiological interactions of endocrine disrupting chemicals with the circadian clock

Endocrine disrupting chemicals (EDCs) are endocrine-active chemical pollutants that disrupt reproductive, neuroendocrine, cardiovascular and metabolic health across species. The circadian clock is a transcriptional oscillator responsible for entraining 24-hour rhythms of physiology, behavior and metabolism. Extensive bidirectional cross talk exists between circadian and endocrine systems and circadian rhythmicity is present at all levels of endocrine control, from synthesis and release of hormones, to sensitivity of target tissues to hormone action. In mammals, a range of hormones directly alter clock gene expression and circadian physiology via nuclear receptor (NR) binding and subsequent genomic action, modulating physiological processes such as nutrient and energy metabolism, stress response, reproductive physiology and circadian behavioral rhythms. The potential for EDCs to perturb circadian clocks or circadian-driven physiology is not well characterized. For this reason, we explore evidence for parallel endocrine and circadian disruption following EDC exposure across species. In the reviewed studies, EDCs dysregulated core clock and circadian rhythm network gene expression in brain and peripheral organs, and altered circadian reproductive, behavioral and metabolic rhythms. Circadian impacts occurred in parallel to endocrine and metabolic alterations such as impaired fertility and dysregulated metabolic and energetic homeostasis. Further research is warranted to understand the nature of interaction between circadian and endocrine systems in mediating physiological effects of EDC exposure at environmental levels.

Vasopressin regulates daily rhythms and circadian clock circuits in a manner influenced by sex

Arginine vasopressin (AVP) is a neurohormone that alters cellular physiology through both endocrine and synaptic signaling. Circadian rhythms in AVP release and other biological processes are driven by the suprachiasmatic nucleus (SCN) of the anterior hypothalamus. Loss of vasopressin signaling alters circadian behavior, but the basis of these effects remains unclear. Here we investigate the role of AVP signaling in circadian timekeeping by analyzing behavior and SCN function in a novel AVP-deficient mouse model. Consistent with previous work, loss of AVP signaling increases water consumption and accelerates recovery to simulated jetlag. We expand on these results to show that loss of AVP increases period, imprecision and plasticity of behavioral rhythms under constant darkness. Interestingly, the effect of AVP deficiency on circadian period was influenced by sex, with loss of AVP lengthening period in females but not males. Examining SCN function directly with ex vivo bioluminescence imaging of clock protein expression, we demonstrate that loss of AVP signaling modulates the period, precision, and phase relationships of SCN neurons in both sexes. This pattern of results suggests that there are likely sex differences in downstream targets of the SCN. Collectively, this work indicates that AVP signaling modulates circadian circuits in a manner influenced by sex, which provides new insight into sexual dimorphisms in the regulation of daily rhythms.

Crosstalk between circadian rhythms and the microbiota

Circadian rhythms influence daily molecular oscillations in gene/protein expression and aspects of biology and physiology, including behaviour, body temperature and sleep–wake cycles. These circadian rhythms have been associated with a number of metabolic, immune and microbial changes that correlate with health and susceptibility to disease, including infection. While light is the main inducer of circadian rhythms, other factors, including the microbiota, can have important effects on peripheral rhythms. The microbiota have been of significant interest to many investigators over the past decade, with the development of molecular techniques to identify large numbers of species and their function. These studies have shown microbial associations with disease susceptibility, and some of these have demonstrated that alterations in microbiota cause disease. Microbial circadian oscillations impact host metabolism and immunity directly and indirectly. Interestingly, microbial oscillations also regulate host circadian rhythms, and the host circadian rhythms in turn modulate microbial composition. Thus, it is of considerable interest and importance to understand the crosstalk between circadian rhythms and microbiota and especially the microbial influences on the host. In this review, we aim to discuss the role of circadian microbial oscillations and how they influence host immunity. In addition, we discuss how host circadian rhythms can also modulate microbial rhythms. We also discuss potential connections between microbes and circadian rhythms and how these may be used therapeutically to maximize clinical success.

Survival is reduced when endogenous period deviates from 24 h in a non-human primate, supporting the circadian resonance theory

Circadian rhythms are ubiquitous attributes across living organisms and allow the coordination of internal biological functions with optimal phases of the environment, suggesting a significant adaptive advantage. The endogenous period called tau lies close to 24 h and is thought to be implicated in individuals' fitness: according to the circadian resonance theory, fitness is reduced when tau gets far from 24 h. In this study, we measured the endogenous period of 142 mouse lemurs (Microcebus murinus), and analyzed how it is related to their survival. We found different effects according to sex and season. No impact of tau on mortality was found in females. However, in males, the deviation of tau from 24 h substantially correlates with an increase in mortality, particularly during the inactive season (winter). These results, comparable to other observations in mice or drosophila, show that captive gray mouse lemurs enjoy better fitness when their circadian period closely matches the environmental periodicity. In addition to their deep implications in health and aging research, these results raise further ecological and evolutionary issues regarding the relationships between fitness and circadian clock.

Type 1 equilibrative nucleoside transporter (ENT1) regulates sex-specific ethanol drinking during disruption of circadian rhythms

Disruptions in circadian rhythms are risk factors for excessive alcohol drinking. The ethanol-sensitive adenosine equilibrative nucleoside transporter type 1 (ENT1, slc29a1) regulates ethanol-related behaviors, sleep, and entrainment of circadian rhythms. However, the mechanism underlying the increased ethanol consumption in ENT1 knockout (KO) mice in constant light (LL) and whether there are sex differences in ethanol consumption in ENT1 mice are less studied. Here, we investigated the effects of loss of ENT1, LL, and sex on ethanol drinking using two-bottle choice. In addition, we monitored the locomotor activity rhythms. We found that LL increased ethanol drinking and reduced accumbal ENT1 expression and adenosine levels in male but not female mice, compared with control mice. Interestingly, only LL-exposed male, not female, ENT1 KO mice exhibited higher ethanol drinking and a longer circadian period with a higher amplitude compared with wild-type (WT) mice. Furthermore, viral-mediated rescue of ENT1 expression in the NAc of ENT1 KO mice reduced ethanol drinking, demonstrating a possible causal link between ENT1 expression and ethanol drinking in males. Together, our findings indicate that deficiency of ENT1 expression contributes to excessive ethanol drinking in a sex-dependent manner.

Perinatal exposure to nicotine disrupts circadian locomotor and learning efficiency rhythms in juvenile mice

The increased rates of nicotine exposure by electronic nicotine delivery systems (vaping), ingestion, or patches during pregnancy as an alternative to the smoking of tobacco arise concerns about the neurodevelopmental, cognitive, and behavioral long-term consequences in the juvenile offspring. Nowadays, the use of electronic cigarettes as supposed a safer smoking alternative has been increased mainly in young females at reproductive age, due to the "safety" misconception. However, previous studies suggest that exposure to nicotine during pregnancy and prenatal development may lead to detrimental effects in the postnatal lifespan. Nicotine, as an alkaloid, alters the reward system acting as acetylcholine (ACh) agonist on nicotinic cholinergic receptors (nAChRs). In early brain development, the cholinergic system is also involved in neurite outgrowth, cell survival, proliferation, differentiation, neurogenesis, and many other critical processes being considered as a developmental signal marker. The nicotine noxious effect at those early stages may impact the system programming and plasticity in the long-term postnatal life. In this study, we analyze the circadian locomotor activity and learning efficiency rhythms in the juvenile male offspring of mice exposed to nicotine through pregnancy and lactation. Attenuated rhythm amplitude and relative power of the circadian component were found in the nicotine exposed offspring (pN). The acrophase (the best performance during a 24-h cycle) of learning efficiency was delayed and the long-term memory consolidation task failed after 8 days of learning experience. The aforementioned results suggest nicotine exposure in uterus modifies the circadian modulation related to the memory consolidation and locomotor systems as well as its environmental temporal synchronization.

Sexual Dimorphism in Stress-induced Hyperthermia in SNAP25Δ3 mice, a mouse model with disabled Gβγ regulation of the exocytotic fusion apparatus

Behavioral assays in the mouse can show marked differences between male and female animals of a given genotype. These differences identified in such preclinical studies may have important clinical implications. We recently made a mouse model with impaired presynaptic inhibition through Gβγ-SNARE signaling. Here, we examine the role of sexual dimorphism in the severity of the phenotypes of this model, the SNAP25Δ3 mouse. In males, we already reported that SNAP25Δ3 homozygotes demonstrated phenotypes in motor coordination, nociception, spatial memory and stress processing. We now report that while minimal sexually dimorphic effects were observed for the nociceptive, motor or memory phenotypes, large differences were observed in the stress-induced hyperthermia paradigm, with male SNAP25Δ3 homozygotes exhibiting an increase in body temperature subsequent to handling relative to wild-type littermates, while no such genotype-dependent effect was observed in females. This suggests sexually dimorphic mechanisms of Gβγ-SNARE signaling for stress processing or thermoregulation within the mouse. Second, we examined the effects of heterozygosity with respect to the SNAP25Δ3 mutation. Heterozygote SNAP25Δ3 animals were tested alongside homozygote and wild-type littermates in all of the aforementioned paradigms and displayed phenotypes similar to wild-type animals or an intermediate state. From this, we conclude that the SNAP25Δ3 mutation does not behave in an autosomal dominant manner, but rather displays incomplete dominance for many phenotypes.

A standardized method to assess the endogenous activity and the light-response of the retinal clock in mammals

Purpose

The bioluminescence reporter PER2::Luciferase (PER2::Luc) provides a powerful tool to study the regulation of biological clocks in explant tissues, including the retinal clock. However, the establishment of a standardized procedure to replicate experimental conditions and to enable meaningful comparisons between findings from different studies is still lacking. In addition, different parameters may affect the retinal circadian bioluminescence signal and its dynamic in in vitro assays. In the present study, we first evaluated the effect of sex and age on the main parameters of the mouse retinal clock. We then examined the impact of medium change on PER2::Luc rhythm and compared two light stimulation protocols of the retinal clock.

Methods

In a first set of experiments, retinal explants from both male and female Per2^Luc mice of different ages (1 to 8 months) are cultured and the period, phase, amplitude, and rhythmic power of PER2::Luc oscillations are analyzed. In a second set of experiments, we quantified the effect of a medium change done after 4, 6, 8, 9, or 10 days of culture on the phase and period of retinal explants. Finally, we compared the phase shift and the period change resulting from two methods of light stimulations of retinal explants: the first involved the transfer of the cultured tissues from the Lumicycle into a light stimulation chamber, while the second used a light delivery apparatus embedded in the Lumicycle.

Results

We do not observe any sex-dependent effects on the amplitude, period, phase, and rhythmic power of the in vitro retinal PER2::Luc oscillations in animals aged of 2 to 3 months. The most remarkable effect of age is on the amplitude of PER2::Luc oscillations that significantly decrease from 1 to 4-5 months, whereas the endogenous period and rhythmic power increase slightly until 2 to 3 months and then do not change until 8 months. The phase is not affected by age. We then show that a medium change occurring after 4 days of culture does not alter the phase of PER2::Luc rhythm by comparison with day 0, whereas a medium change done after 6, 8, 9, or 10 days in culture advances the phase and lengthens the period. Finally, we observe that the physical displacement of the culture dishes containing retinal explants, even in complete darkness, induces a strong phase shift of PER2::Luc oscillations.

Conclusions

Our work shows that the retina cultures are particularly sensitive to some aspects of the culture procedure, and it provides an accurate standard protocol to avoid biases due to artifactually induced phase shifts resulting from the medium change or physical displacement.

Sex differences in circadian endocrine rhythms: Clinical implications

Organisms have developed a highly conserved and tightly regulated circadian system, to adjust their daily activities to day/night cycles. This system consists of a central clock, which is located in the hypothalamic suprachiasmatic nucleus, and the peripheral clocks that are ubiquitously expressed in all tissues. Both the central and peripheral clocks communicate with each other and achieve circadian oscillations of gene expression through transcriptional/translational loops mediated by clock transcription factors. It is worth mentioning that circadian non-transcriptional/non-translational rhythms also occur in non-nucleated cells. Interestingly, sex has been identified as an important factor influencing the activity of the circadian system. Indeed, several sex differences have been documented in the anatomy, physiology and pathophysiology that pertain to circadian rhythms. In this review, we present the historical milestones of understanding circadian rhythms, describe the central and peripheral components of the circadian clock system, discuss representative examples of sexual dimorphism of circadian rhythms, and present the most relevant clinical implications.

Population-, sex- and individual level divergence in life-history and activity patterns in an annual killifish

Variation in life-history strategies along a slow-fast continuum is largely governed by life-history trade-offs. The pace-of-life syndrome hypothesis (POLS) expands on this idea and suggests coevolution of these traits with personality and physiology at different levels of biological organization. However, it remains unclear to what extent covariation at different levels aligns and if also behavioral patterns such as diurnal activity changes should be incorporated. Here, we investigate variation in life-history traits as well as behavioral variation at the individual, sex and population level in the Turquoise killifish Nothobranchius furzeri. We performed a common garden laboratory experiment with four populations that differ in pond permanence and scored life-history and behavioral (co-) variation at the individual and population level for both males and females. In addition, we focused on diurnal activity change as a behavioral trait that remains understudied in ecology. Our results demonstrate sex-specific variation in adult body size and diurnal activity change among populations that originate from ponds with differences in permanence. However, there was no pond permanence-dependent divergence in maturation time, juvenile growth rate, fecundity and average activity level. With regard to behavior, individuals differed consistently in locomotor activity and diurnal activity change while, in contrast with POLS predictions, we found no indications for life-history and behavioral covariation at any level. Overall, this study illustrates that diurnal activity change differs consistently between individuals, sexes and populations although this variation does not appear to match POLS predictions.

Reproductive and Environmental Drivers of Time and Activity Budgets of Striped Skunks

The regulation of daily and circannual activity patterns is an important mechanism by which animals may balance energetic requirements associated with both abiotic and biotic variables. Using collar-mounted accelerometers, we assess the relative importance of reproductive stage and environmental conditions on the overall dynamic body acceleration (ODBA) of free-living striped skunks (Mephitis mephitis). We found that activity timing relative to photoperiod varied across seasonal stages for both sexes. Surprisingly, male skunks did not commence activity earlier than females during the mating interval. Moreover, while female skunks began activity before dusk and terminated activity after dawn during mid- through late summer (lactation period), the duration of activity bouts in females during this period was not different from other seasons. Both male and female skunks exhibited high variability and fragmentation in daily activity rhythms except during the lactation period, when females appear to switch to prolonged bouts of nocturnal activity. Overall, ODBA varied by season and sex, with changes in ODBA indicative of seasonal reproductive requirements such as conspecific competition for mates in males and lactation in females. Weather conditions had little effect on skunk activity levels except during the winter season, when snow cover and temperature negatively influenced daily ODBA. Taken together, the activity patterns of striped skunks appear to be primarily driven by seasonal investment in reproduction and secondarily by thermoregulatory constraints during the non-winter months. Our results highlight the importance of considering how environmental and reproductive drivers may interact to affect activity across both the daily and seasonal cycle.

Effects of Sodium Lighting On Circadian Rhythms in Rats

Rodent studies often must be conducted during an animal's active phase (that is, in darkness) yet also during a typical day shift for staff. Low-pressure sodium lighting (LPSL), to which human retinas are more sensitive than rodents' at low intensity, has been used to facilitate study conduct in dark phase. The assumption was that LPSL would be equivalent to total darkness due to low rodent retinal sensitivity but provide enough lighting for safe technical manipulations due to higher human retinal sensitivity. Unlike other light sources, LPSL has been tested for effects on circadian rhythm specific to locomotive activities in albino mice. Whether LPSL affects circadian rhythms in rats is unknown. In this study, circadian endpoints were derived from body temperature and locomotor activity via telemeters in 8 adult male Wistar rats. When moved from a 12:12-h white-light (that is, cold white fluorescent light):dark (LD) cycle to a 12:12-h white-light:sodium-light cycle, rats demonstrated free-running and disrupted circadian rhythms (that is, lengthened circadian period and reduced circadian robustness and amplitude). Body temperature and locomotor activity were significantly lower in the LPSL phase as compared with dark phase under the baseline condition. When exposed to a 12:12 h sodium-light:dark (SD) cycle, rats entrained with a circadian period similar to 12:12-h white-light:dark (LD), but significantly different from the period under constant darkness (DD). Circadian onset and acrophase were delayed under SD compared with LD. When illuminated with a LPSL pulse under DD, rats showed phase shifts similar to white-light pulse effects, consistent with the phase response curve. To determine whether the image-forming photoreceptors are involved in this process, we used electroretinography. Compared with white light, 589-nm light generated during electroretinography elicited rod photoreceptors responses with longer latency and cone photoreceptor responses with lower amplitude. These results indicate that LPSL is a weaker zeitgeber than white light and may alter the circadian system in rats. Furthermore, because LPSL appeared to be visible to rats, it may not be an appropriate substitute for actual darkness.

Brain Microdialysate Monoamines in Relation to Circadian Rhythms, Sleep, and Sleep Deprivation - a Systematic Review, Network Meta-analysis, and New Primary Data

Disruption of the monoaminergic system, e.g. by sleep deprivation (SD), seems to promote certain diseases. Assessment of monoamine levels over the circadian cycle, during different sleep stages and during SD is instrumental to understand the molecular dynamics during and after SD. To provide a complete overview of all available evidence, we performed a systematic review. A comprehensive search was performed for microdialysis and certain monoamines (dopamine, serotonin, noradrenaline, adrenaline), certain monoamine metabolites (3,4-dihydroxyphenylacetic acid (DOPAC), 5-hydroxyindoleacetic acid (5-HIAA)) and a precursor (5-hydroxytryptophan (5-HTP)) in PubMed and EMBASE. After screening of the search results by two independent reviewers, 94 publications were included. All results were tabulated and described qualitatively. Network-meta analyses (NMAs) were performed to compare noradrenaline and serotonin concentrations between sleep stages. We further present experimental monoamine data from the medial prefrontal cortical (mPFC). Monoamine levels varied with brain region and circadian cycle. During sleep, monoamine levels generally decreased compared to wake. These qualitative observations were supported by the NMAs: noradrenaline and serotonin levels decreased from wakefulness to slow wave sleep and decreased further during Rapid Eye Movement sleep. In contrast, monoamine levels generally increased during SD, and sometimes remained high even during subsequent recovery. Decreases during or after SD were only reported for serotonin. In our experiment, SD did not affect any of the mPFC monoamine levels. Concluding, monoamine levels vary over the light-dark cycle and between sleep stages. SD modifies the patterns, with effects sometimes lasting beyond the SD period.

Alterations in glutamatergic signaling contribute to the decline of circadian photoentrainment in aged mice

Robust physiological circadian rhythms form an integral part of well-being. The aging process has been found to negatively impact systems that drive circadian physiology, typically manifesting as symptoms associated with abnormal/disrupted sleeping patterns. Here, we investigated the age-related decline in light-driven circadian entrainment in male C57BL/6J mice. We compared light-driven resetting of circadian behavioral activity in young (1-2 months) and old (14-18 months) mice and explored alterations in the glutamatergic pathway at the level of the circadian pacemaker, the suprachiasmatic nucleus (SCN). Aged animals showed a significant reduction in sensitivity to behavioral phase resetting by light. We show that this change was through alterations in N-Methyl-D-aspartate (NMDA) signaling at the SCN, where NMDA, a glutamatergic agonist, was less potent in inducing clock resetting. Finally, we show that this shift in NMDA sensitivity was through the reduced SCN expression of this receptor's NR2B subunit. Only in young animals did an NR2B antagonist attenuate behavioral resetting. These results can help target treatments that aim to improve both physiological and behavioral circadian entrainment in aged populations.

Calorie restriction effects on circadian rhythms in gene expression are sex dependent

The rhythms in the expression of circadian clock genes are affected by calorie restriction (CR), a dietary paradigm known to increase lifespan. Many physiological effects of CR differ between males and females; here we investigated if the sex of animals affects the CR induced changes in the circadian rhythms. The liver expression of some circadian clock genes such as Bmal1 and three Periods (Per1, Per2 and Per3) and the effect of CR on the expression of these genes were sex independent, while the expression of Rev-Erb alpha, Ror gamma and both Cryptochome (Cry1 and Cry2) genes was different between males and females. The effect of CR on Rev-Erb alpha, Ror gamma and Cry1 gene expression was sex dependent. The expression and the effects of CR were sex-specific for several genes previously reported to be regulated by CR: Fmo3, Mup4, Serpina12 and Cyp4a12, while the expression of Cyp4a14a was sex independent. IGF signaling plays an important role in aging and CR effects. Igf-1 expression is regulated by CR and by the circadian clock, we found that rhythms in Igf-1 expression have sexual dimorphism. Our data provide molecular evidence that the sex of animals is an important modulator of circadian rhythms in gene expression and their response to CR.

A Memory of Early Life Physical Activity Is Retained in Bone Marrow of Male Rats Fed a High-Fat Diet

Studies have reported opposing effects of high-fat (HF) diet and mechanical stimulation on lineage commitment of the bone marrow stem cells. Yet, how bone marrow modulates its gene expression in response to the combined effects of mechanical loading and a HF diet has not been addressed. We investigated whether early-life (before onset of sexual maturity at 6 weeks of age) voluntary physical activity can modulate the effects of a HF diet on male Sprague Dawley rats. In the bone marrow, early-life HF diet resulted in adipocyte hypertrophy and a pro-inflammatory and pro-adipogenic gene expression profile. The bone marrow of the rats that undertook wheel exercise while on a HF diet retained a memory of the early-life exercise. This memory lasted at least 60 days after the cessation of the voluntary exercise. Our results are consistent with the marrow adipose tissue having a unique response to HF feeding in the presence or absence of exercise.

Persistent Effects of Developmental Exposure to 17α-Ethinylestradiol on the Zebrafish (Danio rerio) Brain Transcriptome and Behavior

The synthetic estrogen 17α-ethinylestradiol (EE₂) is an endocrine disrupting compound of concern due to its persistence and widespread presence in the aquatic environment. Effects of developmental exposure to low concentrations of EE₂ in fish on reproduction and behavior not only persisted to adulthood, but have also been observed to be transmitted to several generations of unexposed progeny. To investigate the possible biological mechanisms of the persistent anxiogenic phenotype, we exposed zebrafish embryos for 80 days post fertilization to 0, 3, and 10 ng/L EE₂ (measured concentrations 2.14 and 7.34 ng/L). After discontinued exposure, the animals were allowed to recover for 120 days in clean water. Adult males and females were later tested for changes in stress response and shoal cohesion, and whole-brain gene expression was analyzed with RNA sequencing. The results show increased anxiety in the novel tank and scototaxis tests, and increased shoal cohesion in fish exposed during development to EE₂. RNA sequencing revealed 34 coding genes differentially expressed in male brains and 62 in female brains as a result of EE₂ exposure. Several differences were observed between males and females in differential gene expression, with only one gene, sv2b, coding for a synaptic vesicle protein, that was affected by EE₂ in both sexes. Functional analyses showed that in female brains, EE₂ had significant effects on pathways connected to the circadian rhythm, cytoskeleton and motor proteins and synaptic proteins. A large number of non-coding sequences including 19 novel miRNAs were also differentially expressed in the female brain. The largest treatment effect in male brains was observed in pathways related to cholesterol biosynthesis and synaptic proteins. Circadian rhythm and cholesterol biosynthesis, previously implicated in anxiety behavior, might represent possible candidate pathways connecting the transcriptome changes to the alterations to behavior. Further the observed alteration in expression of genes involved in synaptogenesis and synaptic function may be important for the developmental modulations resulting in an anxiety phenotype. This study represents an initial survey of the fish brain transcriptome by RNA sequencing after long-term recovery from developmental exposure to an estrogenic compound.

Gender and the circadian pattern of body temperature in normoxia and hypoxia

Circadian patterns are at the core of many physiological processes, and their disruption can have short- and long-term consequences. This essay focuses on one of the best known patterns, the daily oscillation of body temperature (Tb), and the possibility of its difference between genders. From human and animal studies globally considered, the tentative conclusion is reached that differences in Tb circadian pattern between genders are very small and probably limited to the timing of the rhythm, not to its amplitude. Such similarity between genders, despite the differences in hormonal systems, presumably testifies to the importance that the Tb circadian pattern plays in the economy of the organism and its survival against environmental challenges. The second part of the article presents some previously unpublished experimental data from behaving male and female rats during hypoxia in synchronized conditions. In adult rats hypoxia (10.5% O₂ for three days) caused a profound drop of the Tb daily oscillations; by day 3 they were 55% (♀) and 22% (♂) of the normoxic amplitudes, with a statistically significant gender difference. In pre-puberty rats (26-day old) hypoxia caused a major disruption of the circadian pattern qualitatively similar to the adults but not different between genders. Hence, on the basis of this preliminary set of data, it seems that sex-hormones may be a factor in how the Tb daily pattern responds to hypoxia. The implications of the effects of hypoxia on the circadian patterns, and the possibility that such effects may differ between genders, are matters that could have biological and clinical implications and deserve further investigations.

Neuroendocrine underpinnings of sex differences in circadian timing systems

There are compelling reasons to study the role of steroids and sex differences in the circadian timing system. A solid history of research demonstrates the ubiquity of circadian changes that impact virtually all behavioral and biological responses. Furthermore, steroid hormones can modulate every attribute of circadian responses including the period, amplitude and phase. Finally, desynchronization of circadian rhythmicity, and either enhancing or damping amplitude of various circadian responses can produce different effects in the sexes. Studies of the neuroendocrine underpinnings of circadian timing systems and underlying sex differences have paralleled the overall development of the field as a whole. Early experimental studies established the ubiquity of circadian rhythms by cataloging daily and seasonal changes in whole organism responses. The next generation of experiments demonstrated that daily changes are not a result of environmental synchronizing cues, and are internally orchestrated, and that these differ in the sexes. This work was followed by the revelation of molecular circadian rhythms within individual cells. At present, there is a proliferation of work on the consequences of these daily oscillations in health and in disease, and awareness that these may differ in the sexes. In the present discourse we describe the paradigms used to examine circadian oscillation, to characterize how these internal timing signals are synchronized to local environmental conditions, and how hormones of gonadal and/or adrenal origin modulate circadian responses. Evidence pointing to endocrinologically and genetically mediated sex differences in circadian timing systems can be seen at many levels of the neuroendocrine and endocrine systems, from the cell, the gland and organ, and to whole animal behavior, including sleep/wake or rest/activity cycles, responses to external stimuli, and responses to drugs. We review evidence indicating that the analysis of the circadian timing system is amenable to experimental analysis at many levels of the neuraxis, and on several different time scales, rendering it especially useful for the exploration of mechanisms associated with sex differences.

A Multi-Oscillatory Circadian System Times Female Reproduction

Rhythms in female reproduction are critical to insure that timing of ovulation coincides with oocyte maturation and optimal sexual arousal. This fine tuning of female reproduction involves both the estradiol feedback as an indicator of oocyte maturation, and the master circadian clock of the suprachiasmatic nuclei (SCN) as an indicator of the time of the day. Herein, we are providing an overview of the state of knowledge regarding the differential inhibitory and stimulatory effects of estradiol at different stages of the reproductive axis, and the mechanisms through which the two main neurotransmitters of the SCN, arginine vasopressin, and vasoactive intestinal peptide, convey daily time cues to the reproductive axis. In addition, we will report the most recent findings on the putative functions of peripheral clocks located throughout the reproductive axis [kisspeptin (Kp) neurons, gonadotropin-releasing hormone neurons, gonadotropic cells, the ovary, and the uterus]. This review will point to the critical position of the Kp neurons of the anteroventral periventricular nucleus, which integrate both the stimulatory estradiol signal, and the daily arginine vasopressinergic signal, while displaying a circadian clock. Finally, given the critical role of the light/dark cycle in the synchronization of female reproduction, we will discuss the impact of circadian disruptions observed during shift-work conditions on female reproductive performance and fertility in both animal model and humans.