Changes in circadian rhythms during puberty in Rattus norvegicus: developmental time course and gonadal dependency

Continuous body temperature as a window into adolescent development

Continuous body temperature is a rich source of information on hormonal status, biological rhythms, and metabolism, all of which undergo stereotyped change across adolescence. Due to the direct actions of these dynamic systems on body temperature regulation, continuous temperature may be uniquely suited to monitoring adolescent development and the impacts of exogenous reproductive hormones or peptides (e.g., hormonal contraception, puberty blockers, gender affirming hormone treatment). This mini-review outlines how traditional methods for monitoring the timing and tempo of puberty may be augmented by markers derived from continuous body temperature. These features may provide greater temporal precision, scalability, and reduce reliance on self-report, particularly in females. Continuous body temperature data can now be gathered with ease across a variety of wearable form factors, providing the opportunity to develop tools that aid in individual, parental, clinical, and researcher awareness and education.

Modified Wavelet Analyses Permit Quantification of Dynamic Interactions Between Ultradian and Circadian Rhythms

Circadian rhythms provide daily temporal structure to cellular and organismal biological processes, ranging from gene expression to cognition. Higher-frequency (intradaily) ultradian rhythms are similarly ubiquitous but have garnered far less empirical study, in part because of the properties that define them-multimodal periods, non-stationarity, circadian harmonics, and diurnal modulation-pose challenges to their accurate and precise quantification. Wavelet analyses are ideally suited to address these challenges, but wavelet-based measurement of ultradian rhythms has remained largely idiographic. Here, we describe novel analytical approaches, based on discrete and continuous wavelet transforms, which permit quantification of rhythmic power distribution across a broad ultradian spectrum, as well as precise identification of period within empirically determined ultradian bands. Moreover, the aggregation of normalized wavelet matrices allows group-level analyses of experimental treatments, thereby circumventing limitations of idiographic approaches. The accuracy and precision of these wavelet analyses were validated using in silico and in vivo models with known ultradian features. Experiments in male and female mice yielded robust and repeatable measures of ultradian period and power in home cage locomotor activity, confirming and extending reports of ultradian rhythm modulation by sex, gonadal hormones, and circadian entrainment. Seasonal changes in day length modulated ultradian period and power, and exerted opposite effects in the light and dark phases of the 24 h day, underscoring the importance of evaluating ultradian rhythms with attention to circadian phase. Sex differences in ultradian rhythms were more prominent at night and depended on gonadal hormones in male mice. Thus, relatively straightforward modifications to the wavelet procedure allowed quantification of ultradian rhythms with appropriate time-frequency resolution, generating accurate, and repeatable measures of period and power which are suitable for group-level analyses. These analytical tools may afford deeper understanding of how ultradian rhythms are generated and respond to interoceptive and exteroceptive cues.

Sex differences in the diathetic effects of shift work schedules on circulating cytokine levels and pathological outcomes of ischemic stroke during middle age

Shift work is associated with increased risk for vascular disease, including stroke- and cardiovascular-related mortality. However, evidence from these studies is inadequate to distinguish the effect of altered circadian rhythms in isolation from other risk factors for stroke associated with shift work (e.g., smoking, poor diet, lower socioeconomic status). Thus, the present study examined the diathetic effects of exposure to shifted LD cycles during early adulthood on circadian rhythmicity, inflammatory signaling and ischemic stroke pathology during middle age, when stroke risk is high and outcomes are more severe. Entrainment of circadian activity was stable in all animals maintained on a fixed light:dark 12:12 cycle but was severely disrupted during exposure to shifted LD cycles (12hr advance/5d). Following treatment, circadian entrainment in the shifted LD group was distinguished by increased daytime activity and decreased rhythm amplitude that persisted into middle-age. Circadian rhythm desynchronization in shifted LD males and females was accompanied by significant elevations in circulating levels of the inflammatory cytokine IL-17A and gut-derived inflammatory mediator lipopolysaccharide (LPS) during the post-treatment period. Middle-cerebral artery occlusion, 3 months after exposure to shifted LD cycles, resulted in greater post-stroke mortality in shifted LD females. In surviving subjects, sensorimotor performance, assessed 2- and 5-days post-stroke, was impaired in males of both treatment groups, whereas in females, recovery of function was observed in fixed but not shifted LD rats. Overall, these results indicate that early exposure to shifted LD cycles promotes an inflammatory phenotype that amplifies stroke impairments, specifically in females, later in life.

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Early exposure to shifted LD cycles alters circadian entrainment of the activity rhythm that persists into middle age.

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In conjunction with circadian dysregulation, shift work-like schedules promote the induction of key inflammatory mediators.

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In females, exposure to shift work-like schedules amplifies functional impairments caused by strokes arising later in life.

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Circadian dysregulation during shift work is a hysteretic risk factor in the overall severity of ischemic strokes.

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Shift work-related circadian dysregulation affects stroke outcomes independent of lifestyle vascular disease risk factors.

Constitutional delay of growth and puberty in female mice is induced by circadian rhythm disruption in utero

Constitutional delay of growth and puberty (CDGP) refers to the late onset of puberty. CDGP is associated with poor psychosocial outcomes and elevated risk of cardiovascular and osteoporotic diseases, especially in women. The environmental factors that contribute to CDGP are poorly understood. Here, we investigated the effects of chronic circadian disturbance (CCD) during the fetal stage on the pubertal development of female mice. Compared to non-stressed female (NS-F) mice that were not exposed to CCD in utero, adolescent CCD female (CCD-F) mice exhibited phenotypes that were consistent with CDGP, including lower body weight, reduced levels of circulating gonadal hormones, decreased expression of gonadal hormones and steroid synthesis-related enzymes in the ovary and hypothalamus, irregular estrus cycles, and tardive vaginal introitus initial opening (VO) days (equivalent to the menarche). Phenotypic differences in the above-noted parameters were not observed in CCD-F mice once they had reached adulthood. The expression of genes involved in fatty acid metabolism was perturbed in the ovary and hypothalamus of CCD-F mice. In addition, the ovaries of these animals exhibited altered diurnal expression profiles of circadian clock genes. Together, our findings not only suggest that CCD during fetal development may result in delayed puberty in female mice, they also offer insights on potential mechanisms that underlie CDGP.

Sex Differences in Pubertal Circadian and Ultradian Rhythmic Development Under Semi-naturalistic Conditions

Biological rhythms in core body temperature (CBT) provide informative markers of adolescent development under controlled laboratory conditions. However, it is unknown whether these markers are preserved under more variable, semi-naturalistic conditions, and whether CBT may therefore prove useful in a real-world setting. To evaluate this possibility, we examined fecal steroid concentrations and CBT rhythms from pre-adolescence (p26) through early adulthood (p76) in intact male and female Wistar rats under natural light and climate at the Stephen Glickman Field Station for the Study of Behavior, Ecology and Reproduction. Despite greater environmental variability, CBT markers of pubertal onset and its rhythmic progression were comparable with those previously reported in laboratory conditions in female rats and extend actigraphy-based findings in males. Specifically, sex differences emerged in CBT circadian rhythm (CR) power and amplitude prior to pubertal onset and persisted into early adulthood, with females exhibiting elevated CBT and decreased CR power compared with males. Within-day (ultradian rhythm [UR]) patterns also exhibited a pronounced sex difference associated with estrous cyclicity. Pubertal onset, defined by vaginal opening, preputial separation, and sex steroid concentrations, occurred later than previously reported under lab conditions for both sexes. Vaginal opening and increased fecal estradiol concentrations were closely tied to the commencement of 4-day oscillations in CBT and UR power. By contrast, preputial separation and the first rise in testosterone concentration were not associated with adolescent changes to CBT rhythms in male rats. Together, males and females exhibited unique temporal patterning of CBT and sex steroids across pubertal development, with tractable associations between hormonal concentrations, external development, and temporal structure in females. The preservation of these features outside the laboratory supports CBT as a strong candidate for translational pubertal monitoring under semi-naturalistic conditions in females.

Central precocious puberty during COVID-19 pandemic and sleep disturbance: an exploratory study

Background

Increased incidence of central precocious puberty (CPP) after coronavirus infectious disease-19 lockdown has been reported. Our study aims in investigating changes in CPP rates and in sleep patterns in CPP and healthy controls.

Methods

CPP were retrospectively evaluated from April 2020 to April 2021. Parents of girls diagnosed with CPP during lockdown and of matched healthy controls filled out a questionnaire about sleep disturbances (SDSC questionnaire) and sleep schedules.

Results

Thirty-five CPP and 37 controls completed the survey. Incidence of new CPP cases significantly increased in 2020–2021 compared to 2017–2020 (5:100 vs 2:100, p = 0.02). Sleep disturbance rates did not differ between CPP and healthy controls before lockdown. During lockdown, CPP reported higher rates of sleep disturbs for total score (p = 0.005), excessive somnolence (p = 0.049), sleep breathing disorders (p = 0.049), and sleep–wake transition disorders (p = 0.005). Moreover, CPP group more frequently shifted toward later bedtime (p = 0.03) during lockdown compared to controls. Hours of sleep and smartphone exposure around bedtime did not differ between groups.

Conclusions

Our study confirms the observation of increased incidence of CPP after lockdown measures. Additionally, CPP showed higher rates of sleep disturbances and later bedtime compared to controls. The causality link between sleep disturbances and CPP should be further investigated to gain knowledge in this association.

Sex Hormone Suppression and Physical Activity: Possible Implications for Transgender Individuals

Purpose

Transgender individuals tend to be less physically active than cisgender individuals, and the primary focus of these physical activity barriers have been psychosocial in nature. Very little attention has been given to the role that changes in the sex hormone milieu (such as that occurring during gender-affirming hormone therapy) play on physical activity. The purpose of this study was to explore the effects of sex hormone suppression using a gonadotropin-releasing hormone agonist (GnRHa) on physical activity levels and patterns.

Methods

Female and male rats received 4 weeks of sex hormone suppression using the GnRHa goserelin acetate (GA) or received a placebo as a control (CON). Animals were then allowed free access to voluntary running wheels, and activity was recorded throughout the treatment period.

Results

Female rats receiving GA (F GA) had a significantly lower total wheel running distance than female CON (F CON, 53±11 km vs. 113±28 km, respectively, p=0.042), and male rats receiving GA (M GA) had a significantly lower total wheel running distance when compared with male CON (M CON, 31±7 km vs. 69±18 km, respectively, p=0.037). Differences in daily wheel running distances were first observed at day 18 between F GA and F CON (p=0.037) and at day 2 between M GA and M CON (p=0.021).

Conclusion

Reduced sex hormone availability reduced wheel running activity in female and male rats. Understanding the role that sex hormone manipulation has on physical activity may be an important consideration in promoting physical activity in transgender individuals receiving treatments that reduce sex hormone availability.

Long-term consequences of peri-adolescent social isolation on social preference, anxiety-like behaviour, and vasopressin neural circuitry of male and female rats

Social isolation during the juvenile and adolescent stages (peri-adolescent social isolation) can have long-term consequences for behavioural and neural development. Most of this research, however, has relied on data from males, and very few studies have included both sexes. The present study investigated the impact of peri-adolescent social isolation on social preference, anxiety-like behaviour, and vasopressin neural circuitry of male and female Long Evans rats. Rats were either housed alone for 3 weeks beginning at weaning (Isolated) or in groups (Group-housed). In adulthood, rats were tested in social preference, open field, marble burying, and light/dark box tests, and brains were processed for vasopressin immunohistochemistry. Isolated males exhibited a lower social preference score and spent more time in the light zone of the light/dark box than their group-housed counterparts. Isolated and Group-housed females did not differ in these measures. Peri-adolescent social isolation did not alter vasopressin fibre density in target areas known to influence social and anxiety-like behaviours (the lateral septum or lateral habenula), but increased fibre density in an output pathway of the circadian pacemaker (projections to the paraventricular nucleus of the thalamus); an effect detected across both sexes. A previously unreported sex difference was also detected for vasopressin fibre density in the paraventricular nucleus of the thalamus (females > males). These findings demonstrate long-term consequences of peri-adolescent social isolation on social preference, anxiety-like behaviour, and the circadian vasopressin pathway and suggest that socio-affective development of males is more vulnerable to social stressors during the juvenile and adolescent stages.

Adolescent Development of Biological Rhythms in Female Rats: Estradiol Dependence and Effects of Combined Contraceptives

Adolescence is a period of continuous development, including the maturation of endogenous rhythms across systems and timescales. Although, these dynamic changes are well-recognized, their continuous structure and hormonal dependence have not been systematically characterized. Given the well-established link between core body temperature (CBT) and reproductive hormones in adults, we hypothesized that high-resolution CBT can be applied to passively monitor pubertal development and disruption with high fidelity. To examine this possibility, we used signal processing to investigate the trajectory of CBT rhythms at the within-day (ultradian), daily (circadian), and ovulatory timescales, their dependence on estradiol (E2), and the effects of hormonal contraceptives. Puberty onset was marked by a rise in fecal estradiol (fE2), followed by an elevation in CBT and circadian power. This time period marked the commencement of 4-day rhythmicity in fE2, CBT, and ultradian power marking the onset of the estrous cycle. The rise in circadian amplitude was accelerated by E2 treatment, indicating a role for this hormone in rhythmic development. Contraceptive administration in later adolescence reduced CBT and circadian power and resulted in disruption to 4-day cycles that persisted after discontinuation. Our data reveal with precise temporal resolution how biological rhythms change across adolescence and demonstrate a role for E2 in the emergence and preservation of multiscale rhythmicity. These findings also demonstrate how hormones delivered exogenously in a non-rhythmic pattern can disrupt rhythmic development. These data lay the groundwork for a future in which temperature metrics provide an inexpensive, convenient method for monitoring pubertal maturation and support the development of hormone therapies that better mimic and support human chronobiology.

Methods and Challenges in Investigating Sex-Specific Consequences of Social Stressors in Adolescence in Rats: Is It the Stress or the Social or the Stage of Development?

Adolescence is a time of social learning and social restructuring that is accompanied by changes in both the hypothalamic-pituitary-gonadal axis and the hypothalamic-pituitary-adrenal (HPA) axis. The activation of these axes by puberty and stressors, respectively, shapes adolescent development. Models of social stress in rats are used to understand the consequences of perturbations of the social environment for ongoing brain development. This paper reviews the challenges in investigating the sex-specific consequences of social stressors, sex differences in the models of social stress used in rats and the sex-specific effects on behaviour and provides an overview of sex differences in HPA responding to stressors, the variability in pubertal development and in strains of rats that require consideration in conducting such research, and directions for future research.

Sex differences in daily timekeeping and circadian clock circuits

The circadian system regulates behavior and physiology in many ways important for health. Circadian rhythms are expressed by nearly every cell in the body, and this large system is coordinated by a central clock in the suprachiasmatic nucleus (SCN). Sex differences in daily rhythms are evident in humans and understanding how circadian function is modulated by biological sex is an important goal. This review highlights work examining effects of sex and gonadal hormones on daily rhythms, with a focus on behavior and SCN circuitry in animal models commonly used in pre-clinical studies. Many questions remain in this area of the field, which would benefit from further work investigating this topic.

Juvenile hormone affects the development and strength of circadian rhythms in young bumble bee (Bombus terrestris) workers

The circadian and endocrine systems influence many physiological processes in animals, but little is known on the ways they interact in insects. We tested the hypothesis that juvenile hormone (JH) influences circadian rhythms in the social bumble bee Bombus terrestris. JH is the major gonadotropin in this species coordinating processes such as vitellogenesis, oogenesis, wax production, and behaviors associated with reproduction. It is unknown however, whether it also influences circadian processes. We topically treated newly-emerged bees with the allatoxin Precocene-I (P-I) to reduce circulating JH titers and applied the natural JH (JH-III) for replacement therapy. We repeated this experiment in three trials, each with bees from different source colonies. Measurements of ovarian activity suggest that our JH manipulations were effective; bees treated with P-I had inactive ovaries, and this effect was fully recovered by subsequent JH treatment. We found that JH augments the strength of circadian rhythms and the pace of rhythm development in individually isolated newly emerged worker bees. JH manipulation did not affect the free-running circadian period, overall level of locomotor activity, sleep amount, or sleep structure. Given that acute manipulation at an early age produced relatively long-lasting effects, we propose that JH effects on circadian rhythms are mostly organizational, accelerating the development or integration of the circadian system.

Pregnancy Induces an Earlier Chronotype in Both Mice and Women

Daily rhythms generated by endogenous circadian mechanisms and synchronized to the light-dark cycle have been implicated in the timing of birth in a wide variety of species. Although chronodisruption (e.g., shift work or clock gene mutations) is associated with poor reproductive outcomes, little is known about circadian timing during pregnancy. This study tested whether daily rhythms change during full-term pregnancies in mice and women. We compared running wheel activity continuously in both nonpregnant ( n = 14) and pregnant ( n = 13) 12- to 24-week-old C57BL/6NJ mice. We also monitored wrist actigraphy in women ( N = 39) for 2 weeks before conception and then throughout pregnancy and measured daily times of sleep onset. We found that on the third day of pregnancy, mice shift their activity to an earlier time compared with nonpregnant dams. Their time of daily activity onset was maximally advanced by almost 4 h around day 7 of pregnancy and then shifted back to the nonpregnant state approximately 1 week before delivery. Mice also showed reduced levels of locomotor activity during their last week of pregnancy. Similarly, in women, the timing of sleep onset was earlier during the first and second trimesters (gestational weeks 4-13 and 14-27) than before pregnancy and returned to the prepregnant state during the third trimester (weeks 28 until delivery). Women also showed reduced levels of locomotor activity throughout pregnancy. These results indicate that pregnancy induces changes in daily rhythms, altering both time of onset and amount of activity. These changes are conserved between mice and women.

Sleep Timing in Patients with Precocious and Delayed Pubertal Development

Previous studies have reported a shift in the timing of sleep during adolescence toward a later time. To date, it is unclear whether hormonal changes during puberty might contribute to this change in sleeping behavior. We systematically assessed pubertal development and sleep timing in a cross-sectional case-control study in girls with precocious (n = 42) and boys with delayed pubertal development (n = 19). We used the Munich ChronoType Questionnaire and the Children’s ChronoType Questionnaire to assess sleep timing in patients and age- and sex-matched controls (n = 309) and used the midpoint of sleep on free days, corrected for potential sleep debt accumulated during the school week, as a marker for sleep timing. Compared to the controls, girls with central precocious puberty showed a delay in sleep timing of 54 min, and girls with premature pubarche slept on average 30 min later. Male adolescents with delayed pubertal development showed an average sleep midpoint that was 40 min earlier compared to the control group. The results of this pilot study suggest an association between pubertal onset and shifts in sleep timing, which is a novel finding in human sleep behavior. Prospective studies in larger cohorts will be needed to examine the robustness and generalizability of the findings.

Short-term succinic acid treatment mitigates cerebellar mitochondrial OXPHOS dysfunction, neurodegeneration and ataxia in a Purkinje-specific spinocerebellar ataxia type 1 (SCA1) mouse model

Mitochondrial dysfunction plays a significant role in neurodegenerative disease including ataxias and other movement disorders, particularly those marked by progressive degeneration in the cerebellum. In this study, we investigate the role of mitochondrial oxidative phosphorylation (OXPHOS) deficits in cerebellar tissue of a Purkinje cell-driven spinocerebellar ataxia type 1 (SCA1) mouse. Using RNA sequencing transcriptomics, OXPHOS complex assembly analysis and oxygen consumption assays, we report that in the presence of mutant polyglutamine-expanded ataxin-1, SCA1 mice display deficits in cerebellar OXPHOS complex I (NADH-coenzyme Q oxidoreductase). Complex I genes are upregulated at the time of symptom onset and upregulation persists into late stage disease; yet, functional assembly of complex I macromolecules are diminished and oxygen respiration through complex I is reduced. Acute treatment of postsymptomatic SCA1 mice with succinic acid, a complex II (succinate dehydrogenase) electron donor to bypass complex I dysfunction, ameliorated cerebellar OXPHOS dysfunction, reduced cerebellar pathology and improved motor behavior. Thus, exploration of mitochondrial dysfunction and its role in neurodegenerative ataxias, and warrants further investigation.

The Relationship Between Estrogen and the Decline in Delta Power During Adolescence

Study Objectives

During adolescence, there is a precipitous decrease in slow-wave sleep (SWS) and its spectral correlate, delta power, which may reflect cortical reorganization. The temporal association between the decrease in delta power and puberty suggests that sex steroids may initiate these changes. This association has not been previously investigated.

Methods

To determine whether estrogen triggers the adolescent decline in delta power, we compared delta power in 14 girls with central precocious puberty (CPP) and 6 age-matched, prepubertal controls. Five CPP participants were re-studied 7-14 months after pubertal suppression to determine if the changes in delta power are reversible after restoring a prepubertal hormonal milieu. The change in delta power was also compared between CPP participants and five historic controls from a longitudinal polysomnographic study.

Results

CPP participants (6.7-10.5 years) spent 30% of the night in SWS. Delta power (3.7 × 106 ± 2.7 × 105 µV2) predominated in the first 2 non-rapid eye movement episodes and decayed exponentially (tau 0.006 minutes). Age-matched controls demonstrated similar sleep staging (24% SWS) and delta dynamics (3.3 × 106 ± 5.1 × 105 µV2, tau 0.004 minutes). Four out of 5 CPP participants had a significant decrease (26%) in delta power after hormone suppression (p < .05), similar to historic controls.

Conclusion

Using an innovative model of girls with CPP studied before and after estrogen suppression, the effects of puberty on the decline in delta power were dissociated from those of chronologic age. The current studies suggest that increased estrogen does not cause the adolescent decline in delta power and indicate that neurodevelopmental changes per se or other factors associated with puberty drive these sleep changes.

The bear circadian clock doesn't 'sleep' during winter dormancy

Background

Most biological functions are synchronized to the environmental light:dark cycle via a circadian timekeeping system. Bears exhibit shallow torpor combined with metabolic suppression during winter dormancy. We sought to confirm that free-running circadian rhythms of body temperature (Tb) and activity were expressed in torpid grizzly (brown) bears and that they were functionally responsive to environmental light. We also measured activity and ambient light exposures in denning wild bears to determine if rhythms were evident and what the photic conditions of their natural dens were. Lastly, we used cultured skin fibroblasts obtained from captive torpid bears to assess molecular clock operation in peripheral tissues. Circadian parameters were estimated using robust wavelet transforms and maximum entropy spectral analyses.

Results

Captive grizzly bears housed in constant darkness during winter dormancy expressed circadian rhythms of activity and Tb. The rhythm period of juvenile bears was significantly shorter than that of adult bears. However, the period of activity rhythms in adult captive bears was virtually identical to that of adult wild denning bears as was the strength of the activity rhythms. Similar to what has been found in other mammals, a single light exposure during the bear’s active period delayed subsequent activity onsets whereas these were advanced when light was applied during the bear’s inactive period. Lastly, in vitro studies confirmed the expression of molecular circadian rhythms with a period comparable to the bear’s own behavioral rhythms.

Conclusions

Based on these findings we conclude that the circadian system is functional in torpid bears and their peripheral tissues even when housed in constant darkness, is responsive to phase-shifting effects of light, and therefore, is a normal facet of torpid bear physiology.

Electronic supplementary material

The online version of this article (doi:10.1186/s12983-016-0173-x) contains supplementary material, which is available to authorized users.

Daily timing of the adolescent sleep phase: Insights from a cross-species comparison

Adolescence is a time of tremendous adjustment and includes changes in cognition, emotion, independence, social environment, and physiology. One of the most consistent changes exhibited by human adolescents is a dramatic delay in the daily timing of the sleep-wake cycle. This delay is strongly correlated with pubertal maturation and is believed to be influenced by gonadal hormone-induced changes in the neural mechanisms regulating sleep and/or circadian timing. Data from both human and non-human animals indicate that developmental changes in the intrinsic period of the circadian mechanism or its sensitivity to light are not adequate to explain adolescent changes in the daily timing of sleep and wakefulness. Rather, current evidence suggests that pubertal changes in the homeostatic drive to sleep and/or behaviorally induced changes in the amount and/or timing of light exposure permit adolescents to stay up later in the evening and cause them to wake up later in the morning.

Sex Differences in the Impact of Shift Work Schedules on Pathological Outcomes in an Animal Model of Ischemic Stroke

Circadian clock desynchronization has been implicated in the pathophysiology of cardiovascular disease and related risk factors (eg, obesity, diabetes). Thus, we examined the extent to which circadian desynchronization exacerbates ischemic stroke outcomes and whether its detrimental effects on stroke severity and functional impairments are further modified by biological sex. Circadian entrainment of activity rhythms in all male and female rats was observed during exposure to a fixed light-dark (LD) 12:12 cycle but was severely disrupted when this LD cycle was routinely shifted (12 h advance/5 d) for approximately 7 weeks. In contrast to the regular estrous cycles in fixed LD animals, cyclicity was abolished and persistent estrus was evident in all shifted LD females. The disruption of estrous cyclicity in shifted LD females was associated with a significant increase in serum estradiol levels relative to that observed in fixed LD controls. Circadian rhythm disruption exacerbated stroke outcomes in both shifted LD male and female rats and further amplified sex differences in stroke impairments. In males, but not females, circadian disruption after exposure to the shifted LD cycle was marked by high rates of mortality. In surviving females, circadian desynchronization after exposure to shifted LD cycles produced significant increases in stroke-induced infarct volume and sensorimotor deficits with corresponding decreases in serum IGF-1 levels. These results suggest that circadian rhythm disruption associated with shift work schedules or the irregular nature of our everyday work and/or social environments may interact with other nonmodifiable risk factors such as biological sex to modulate the pathological effects of stroke.

Circadian activity rhythm in pre-pubertal and pubertal marmosets (Callithrix jacchus) living in family groups

In marmosets, a phase advance was observed in activity onset in pubertal animals living in captivity under semi-natural conditions which had stronger correlation with the times of sunrise over the course of the year than the age of the animal. In order to evaluate the effect of puberty on the circadian activity rhythm in male and female marmosets living in family groups in controlled lighting conditions, the activity of 5 dyads of twins (4 ♀/♂ and 1 ♂/♂) and their respective parents was continuously monitored by actiwatches between the 4th and 12th months of age. The families were kept under LD 12:12 h with constant humidity and temperature. The onset of puberty was identified by monitoring fecal steroids. Juveniles showed higher totals of daily activity and differences in the daily distribution of activity in relation to parents, in which the bimodal profile was characterized by higher levels in evening activity in relation to morning activity. Regarding the phase, the activity onset and offset, occurred later in relation to parents. After entering puberty, the activity onset and offset occurred later and there was an increase in total daily activity. On the other hand, when assessing the effect of sex, only females showed a delay in the activity offset and an increase in total daily activity. Therefore, the circadian activity rhythm in marmosets has peculiar characteristics in the juvenile stage in relation to the total of daily activity, the onset and offset of the active phase, and the distribution of activity during this phase. Besides, the entering puberty was associated with a phase delay and increase on total daily activity, with differences between sexes, possibly due to hormonal influences and/or social modulation on rhythm.

Opening the Debate: How to Fulfill the Need for Physicians' Training in Circadian-Related Topics in a Full Medical School Curriculum

BACKGROUND

Circadian rhythms are daily changes in our physiology and behavior that are manifested as patterns of brain wave activity, periodic hormone production, recurring cell regeneration, and other oscillatory biological activities. Their importance to human health is becoming apparent; they are deranged by shift work and jet-lag and in disparate conditions such as insomnia, sleep syndromes, coronary heart attacks, and depression, and are endogenous factors that contribute to cancer development and progression.

DISCUSSION

As evidence of the circadian connection to human health has grown, so has the number of Americans experiencing disruption of circadian rhythms due to the demands of an industrialized society. Today, there is a growing work force that experiences night shift work and time-zone shifts shaping the demands on physicians to best meet the needs of patients exposed to chronic circadian disruptions. The diverse range of illness associated with altered rhythms suggests that physicians in various fields will see its impact in their patients. However, medical education, with an already full curriculum, struggles to address this issue.

SUMMARY

Here, we emphasize the need for incorporating the topic of circadian rhythms in the medical curriculum and propose strategies to accomplish this goal.

Suprachiasmatic nucleus as the site of androgen action on circadian rhythms

Androgens act widely in the body in both central and peripheral sites. Prior studies indicate that in the mouse, suprachiasmatic nucleus (SCN) cells bear androgen receptors (ARs). The SCN of the hypothalamus in mammals is the locus of a brain clock that regulates circadian rhythms in physiology and behavior. Gonadectomy results in reduced AR expression in the SCN and in marked lengthening of the period of free-running activity rhythms. Both responses are restored by systemic administration of androgens, but the site of action remains unknown. Our goal was to determine whether intracranial androgen implants targeted to the SCN are sufficient to restore the characteristic free-running period in gonadectomized male mice. The results indicate that hypothalamic implants of testosterone propionate in or very near the SCN produce both anatomical and behavioral effects, namely increased AR expression in the SCN and restored period of free-running locomotor activity. The effect of the implant on the period of the free-running locomotor rhythm is positively correlated with the amount of AR expression in the SCN. There is no such correlation of period change with amount of AR expression in other brain regions examined, namely the preoptic area, bed nucleus of the stria terminalis and premammillary nucleus. We conclude that the SCN is the site of action of androgen effects on the period of circadian activity rhythmicity.

Developmental treatment with ethinyl estradiol, but not bisphenol A, causes alterations in sexually dimorphic behaviors in male and female Sprague Dawley rats

The developing central nervous system may be particularly sensitive to bisphenol A (BPA)-induced alterations. Here, pregnant Sprague Dawley rats (n = 11-12/group) were gavaged daily with vehicle, 2.5 or 25.0 μg/kg BPA, or 5.0 or 10.0 μg/kg ethinyl estradiol (EE2) on gestational days 6-21. The BPA doses were selected to be below the no-observed-adverse-effect level (NOAEL) of 5 mg/kg/day. On postnatal days 1-21, all offspring/litter were orally treated with the same dose. A naïve control group was not gavaged. Body weight, pubertal age, estrous cyclicity, and adult serum hormone levels were measured. Adolescent play, running wheel activity, flavored solution intake, female sex behavior, and manually elicited lordosis were assessed. No significant differences existed between the vehicle and naïve control groups. Vehicle controls exhibited significant sexual dimorphism for most behaviors, indicating these evaluations were sensitive to sex differences. However, only EE2 treatment caused significant effects. Relative to female controls, EE2-treated females were heavier, exhibited delayed vaginal opening, aberrant estrous cyclicity, increased play behavior, decreased running wheel activity, and increased aggression toward the stimulus male during sexual behavior assessments. Relative to male controls, EE2-treated males were older at testes descent and preputial separation and had lower testosterone levels. These results suggest EE2-induced masculinization/defeminization of females and are consistent with increased volume of the sexually dimorphic nucleus of the preoptic area (SDN-POA) at weaning in female siblings of these subjects (He, Z., Paule, M. G. and Ferguson, S. A. (2012) Low oral doses of bisphenol A increase volume of the sexually dimorphic nucleus of the preoptic area in male, but not female, rats at postnatal day 21. Neurotoxicol. Teratol. 34, 331-337). Although EE2 treatment caused pubertal delays and decreased testosterone levels in males, their behaviors were within the range of control males. Conversely, BPA treatment did not alter any measured endpoint. Similar to our previous reports (Ferguson, S. A., Law, C. D. Jr and Abshire, J. S. (2011) Developmental treatment with bisphenol A or ethinyl estradiol causes few alterations on early preweaning measures. Toxicol. Sci. 124, 149-160; Ferguson, S. A., Law, C. D. and Abshire, J. S. (2012) Developmental treatment with bisphenol A causes few alterations on measures of postweaning activity and learning. Neurotoxicol. Teratol. 34, 598-606), the BPA doses and design used here produced few alterations.

Gonadal steroid modulation of sleep and wakefulness in male and female rats is sexually differentiated and neonatally organized by steroid exposure

The paucity of clinical and preclinical studies investigating sex differences in sleep has resulted in mixed findings as to the exact nature of these differences. Although gonadal steroids are known to modulate sleep in females, less is known about males. Moreover, little evidence exists concerning the origin of these sex differences in sleep behavior. Thus, the goal of this study was to directly compare the sensitivity of sleep behavior in male and female Sprague Dawley rats to changes in the gonadal steroid milieu and to test whether the sex differences in sleep are the result of brain sexual differentiation or differences in circulating gonadal steroids. Here we report the magnitude of change in sleep behavior induced by either estradiol (E2) or testosterone (T) was greater in females compared with males, suggesting that sleep behavior in females is more sensitive to the suppressive effects of gonadal steroids. Furthermore, we demonstrated that the organizational effects of early gonadal steroid exposure result in male-like responsivity to gonadal steroids and directly alter the activity of the ventrolateral preoptic area (VLPO), an established sleep-promoting nucleus, in adult masculinized females. Moreover, the nonaromatizable androgen dihydrotestosterone did not suppress sleep in either males or females, suggesting that the T-mediated effect in females was due to the aromatization of T into E2. Together our data suggest that, like sex behavior, sex differences in sleep follow the classical organizational/activational effects of gonadal steroids.

Sex differences in behavioral circadian rhythms in laboratory rodents

There is a strong bias in basic research on circadian rhythms toward the use of only male animals in studies. Furthermore, of the studies that use female subjects, many use only females and do not compare results between males and females. This review focuses on behavioral aspects of circadian rhythms that differ between the sexes. Differences exist in the timing of daily onset of activity, responses to both photic and non-photic stimuli, and in changes across the lifespan. These differences may reflect biologically important traits that are ecologically relevant and impact on a variety of responses to behavioral and physiological challenges. Overall, more work needs to be done to investigate differences between males and females as well as differences that are the result of hormonal changes across the lifespan.

Sex differences in circadian timing systems: implications for disease

Virtually every eukaryotic cell has an endogenous circadian clock and a biological sex. These cell-based clocks have been conceptualized as oscillators whose phase can be reset by internal signals such as hormones, and external cues such as light. The present review highlights the inter-relationship between circadian clocks and sex differences. In mammals, the suprachiasmatic nucleus (SCN) serves as a master clock synchronizing the phase of clocks throughout the body. Gonadal steroid receptors are expressed in almost every site that receives direct SCN input. Here we review sex differences in the circadian timing system in the hypothalamic-pituitary-gonadal axis (HPG), the hypothalamic-adrenal-pituitary (HPA) axis, and sleep-arousal systems. We also point to ways in which disruption of circadian rhythms within these systems differs in the sexes and is associated with dysfunction and disease. Understanding sex differentiated circadian timing systems can lead to improved treatment strategies for these conditions.

Adolescent sleep patterns in humans and laboratory animals

This article is part of a Special Issue "Puberty and Adolescence". One of the defining characteristics of adolescence in humans is a large shift in the timing and structure of sleep. Some of these changes are easily observable at the behavioral level, such as a shift in sleep patterns from a relatively morning to a relatively evening chronotype. However, there are equally large changes in the underlying architecture of sleep, including a >60% decrease in slow brain wave activity, which may reflect cortical pruning. In this review we examine the developmental forces driving adolescent sleep patterns using a cross-species comparison. We find that behavioral and physiological sleep parameters change during adolescence in non-human mammalian species, ranging from primates to rodents, in a manner that is often hormone-dependent. However, the overt appearance of these changes is species-specific, with polyphasic sleepers, such as rodents, showing a phase-advance in sleep timing and consolidation of daily sleep/wake rhythms. Using the classic two-process model of sleep regulation, we demonstrate via a series of simulations that many of the species-specific characteristics of adolescent sleep patterns can be explained by a universal decrease in the build-up and dissipation of sleep pressure. Moreover, and counterintuitively, we find that these changes do not necessitate a large decrease in overall sleep need, fitting the adolescent sleep literature. We compare these results to our previous review detailing evidence for adolescent changes in the regulation of sleep by the circadian timekeeping system (Hagenauer and Lee, 2012), and suggest that both processes may be responsible for adolescent sleep patterns.

Different adaptation of the motor activity rhythm to chronic phase shifts between adolescent and adult rats

Chronic phase shifts is a common feature in modern societies, which may induce sleep alterations and other health problems. The effects of phase shift on the circadian rhythms have been described to be more pronounced in old than in young animals. However, few works address the effects of chronic phase shifts during adolescence. Here we tested the development of the motor activity circadian rhythm of young rats under chronic phase shifts, which consisted on 6-h advances (A), 6h delays (D) or 6h advances and delays alternated every 5 days (AD) during the first 60 days after weaning. Moreover, the rhythmic pattern was compared to that of adult rats under the same lighting conditions. Results indicate that adolescent rats, independently on the lighting environment, developed a clear circadian rhythm, whose amplitude increased the first 50 days after weaning and showed a more stable circadian rhythm than adults under the same lighting conditions. In the case of A and AD groups, circadian disruption was observed only in adult rats. In all groups, the offset of activity correlated with light pattern better than the onset, and this correlation was always higher in the case of the rhythm of the pubertal rats. When AD groups were transferred to constant darkness, the group submitted to this condition during adolescence showed shorter period than that submitted in their adulthood. In conclusion, differently from adult rats, adolescent rats submitted to chronic phase shifts did not show circadian disruption and developed a single circadian rhythm, suggesting permanent changes in the circadian system.

Gonadal- and sex-chromosome-dependent sex differences in the circadian system

Compelling reasons to study the role of sex in the circadian system include the higher rates of sleep disorders in women than in men and evidence that sex steroids modulate circadian control of locomotor activity. To address the issue of sex differences in the circadian system, we examined daily and circadian rhythms in wheel-running activity, electrical activity within the suprachiasmatic nucleus, and PER2::LUC-driven bioluminescence of gonadally-intact adult male and female C57BL/6J mice. We observed greater precision of activity onset in 12-hour light, 12-hour dark cycle for male mice, longer activity duration in 24 hours of constant darkness for female mice, and phase-delayed PER2::LUC bioluminescence rhythm in female pituitary and liver. Next, in order to investigate whether sex differences in behavior are sex chromosome or gonadal sex dependent, we used the 4 core genotypes (FCG) mouse model, in which sex chromosome complement is independent of gonadal phenotype. Gonadal males had more androgen receptor expression in the suprachiasmatic nucleus and behaviorally reduced photic phase shift response compared with gonadal female FCG mice. Removal of circulating gonadal hormones in adults, to test activational vs organizational effects of sex revealed that XX animals have longer activity duration than XY animals regardless of gonadal phenotype. Additionally, we observed that the activational effects of gonadal hormones were more important for regulating activity levels in gonadal male mice than in gonadal female FCG mice. Taken together, sex differences in the circadian rhythms of activity, neuronal physiology, and gene expression were subtle but provide important clues for understanding the pathophysiology of the circadian system.

Chronotype but not sleep length is related to salivary testosterone in young adult men

Sex hormones, including testosterone, are hypothesized to have an influence on the human circadian system. We sampled male students in the period after adolescence. We used the Composite Scale of Morningness (CSM) to assess chronotype and saliva testosterone sampling in 106 University students (23.87±3.56 years; range 19-37) between 26.4.2011 and 6.5.2011, always between 0800h and 0900h. There was a significant negative relationship between CSM scores and saliva testosterone (r(s)=-0.220, p=0.023, two-tailed test) but not between testosterone and average sleep length. Age and testosterone did not correlate with each other nor did age and CSM scores. Our data suggest that chronotype in men might be influenced by testosterone and that high testosterone levels lead to a stronger evening-orientation. Sleep duration was uncorrelated with testosterone, suggesting that timing of sleep - rather than sleep length itself - is influenced by testosterone.

The neuroendocrine control of the circadian system: adolescent chronotype

Scientists, public health and school officials are paying growing attention to the mechanism underlying the delayed sleep patterns common in human adolescents. Data suggest that a propensity towards evening chronotype develops during puberty, and may be caused by developmental alterations in internal daily timekeeping. New support for this theory has emerged from recent studies which show that pubertal changes in chronotype occur in many laboratory species similar to human adolescents. Using these species as models, we find that pubertal changes in chronotype differ by sex, are internally generated, and driven by reproductive hormones. These chronotype changes are accompanied by alterations in the fundamental properties of the circadian timekeeping system, including endogenous rhythm period and sensitivity to environmental time cues. After comparing the developmental progression of chronotype in different species, we propose a theory regarding the ecological relevance of adolescent chronotype, and provide suggestions for improving the sleep of human adolescents.

The Changes They are A-Timed: Metabolism, Endogenous Clocks, and the Timing of Puberty

Childhood obesity has increased dramatically over the last several decades, particularly in industrialized countries, often accompanied by acceleration of pubertal progression and associated reproductive abnormalities (Biro et al., 2006; Rosenfield et al., 2009). The timing of pubertal initiation and progression in mammals is likely influenced by nutritional and metabolic state, leading to the hypothesis that deviations from normal metabolic rate, such as those seen in obesity, may contribute to observed alterations in the rate of pubertal progression. While several recent reviews have addressed the effects of metabolic disorders on reproductive function in general, this review will explore previous and current models of pubertal timing, outlining a potential role of endogenous timing mechanisms such as cellular circadian clocks in the initiation of puberty, and how these clocks might be altered by metabolic factors. Additionally, we will examine recently elucidated neuroendocrine regulators of pubertal progression such as kisspeptin, explore models detailing how the mammalian reproductive axis is silenced during the juvenile period and reactivated at appropriate developmental times, and emphasize how metabolic dysfunction such as childhood obesity may alter timing cues that advance or delay pubertal progression, resulting in diminished reproductive capacity.

Chronotype changes during puberty depend on gonadal hormones in the slow-developing rodent, Octodon degus

During puberty, human adolescents develop a later chronotype, exhibiting a delay in the timing of rest and activity as well as other daily physiological rhythms. The purpose of this study was to determine whether similar changes in chronotype occur during puberty in a laboratory rodent species, and, if so, to determine whether they are due to pubertal hormones acting on the circadian timekeeping system. To test this hypothesis, we carefully tracked daily activity rhythms across puberty in the slow-developing rodent Octodon degus. We confirmed that male degus showed a large reorganization of activity rhythms that correlated with secondary sex development during puberty, including a loss of bimodality and a 3-5 h phase-advance. Similar to humans, this circadian reorganization showed distinct sex differences, with females showing little change during puberty in two separate experiments. Prepubertal gonadectomy (GDX) eliminated the changes, whereas SHAM gonadectomy had little impact. Therefore, gonadal hormones are likely to play a role in pubertal changes in chronotype in this rodent species. Using evidence from a variety of species, including our recent studies in the rat, we conclude that chronotype changes during puberty are a well-demonstrated phenomenon in mammals.