Effects of sex, thyro-parathyroidectomy, and light regime on levels and circadian rhythms of wheel-running in rats

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.

•

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

•

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

•

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

•

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

•

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

Circadian Responses to Light in the BTBR Mouse

Animals with altered freerunning periods are valuable in understanding properties of the circadian clock. Understanding the relationship between endogenous clock properties, entrainment, and influence of light in terms of parametric and non-parametric models can help us better understand how different populations adapt to external light cycles. Many clinical populations often show significant changes in circadian properties that in turn cause sleep and circadian problems, possibly exacerbating their underlying clinical condition. BTBR T⁺Itpr3^tf/J (BTBR) mice are a model commonly used for the study of autism spectrum disorders (ASD). Adults and adolescents with ASD frequently exhibit profound sleep and circadian disruptions, including increased latency to sleep, insomnia, advanced and delayed sleep phase disorders, and sleep fragmentation. Here, we investigated the circadian phenotype of BTBR mice in freerunning and light-entrained conditions and found that this strain of mice showed noticeably short freerunning periods (~22.75 h). In addition, when compared to C57BL/6J controls, BTBR mice also showed higher levels of activity even though this activity was compressed into a shorter active phase. Phase delays and phase advances to light were significantly larger in BTBR mice. Despite the short freerunning period, BTBR mice exhibited normal entrainment in light-dark cycles and accelerated entrainment to both advanced and delayed light cycles. Their ability to entrain to skeleton photoperiods of 1 min suggests that this entrainment cannot be attributed to masking. Period differences were also correlated with differences in the number of vasoactive intestinal polypeptide–expressing cells in the suprachiasmatic nucleus (SCN). Overall, the BTBR model, with their unique freerunning and entrainment properties, makes an interesting model to understand the underlying circadian clock.

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.

In utero Exposure to Valproic-Acid Alters Circadian Organisation and Clock-Gene Expression: Implications for Autism Spectrum Disorders

Autism Spectrum Disorder (ASD) is a pervasive neurodevelopmental disorder characterised by restrictive patterns of behaviour and alterations in social interaction and communication. Up to 80% of children with ASD exhibit sleep-wake cycle disturbances, emphasising the pressing need for novel approaches in the treatment of ASD-associated comorbidities. While sleep disturbances have been identified in ASD individuals, little has been done to assess the contribution of the circadian system to these findings. The objective of this study is to characterise circadian behaviour and clock-gene expression in a valproic acid (VPA)-induced animal model of autism to highlight perturbations potentially contributing to these disturbances. Male and female VPA-exposed offspring underwent circadian challenges, including baseline light-dark cycles, constant dark/light and light pulse protocols. Baseline analysis showed that VPA-exposed males, but not females, had a greater distribution of wheel-running behaviour across light-dark phases and a later activity offset (p < 0.0001), while controls showed greater activity confinement to the dark phase (p = 0.0256). Constant light analysis indicated an attenuated masking response and an increase in the number of days to reach arrhythmicity (p < 0.0001). A 1-h light pulse (150 lux) at CT 15 after 6 days of constant dark showed that both sexes exposed to VPA exhibited a lesser phase-shift when compared to controls (p = 0.0043). Immunohistochemical and western-blot assays reveal no alterations in retinal organisation or function. However, immunohistochemical assay of the SCN revealed altered expression of BMAL1 expression in VPA-exposed males (p = 0.0016), and in females (p = 0.0053). These findings suggest alterations within the core clockwork of the SCN and reduced photic-entrainment capacity, independent of retinal dysfunction. The results of this study shed light on the nature of circadian dysregulation in VPA-exposed animals and highlights the urgent need for novel perspectives in the treatment of ASD-associated comorbidities.

A review of the current state of knowledge on sex differences in sleep and circadian phenotypes in rodents

Sleep is a vital part of our lives as it is required to maintain health and optimal cognition. In humans, sex differences are relatively well-established for many sleep phenotypes. However, precise differences in sleep phenotypes between male and female rodents are less documented. The main goal of this article is to review sex differences in sleep architecture and electroencephalographic (EEG) activity during wakefulness and sleep in rodents. The effects of acute sleep deprivation on sleep duration and EEG activity in male and female rodents will also be covered, in addition to sex differences in specific circadian phenotypes. When possible, the contribution of the female estrous cycle to the observed differences between males and females will be described. In general, male rodents spend more time in non-rapid eye movement sleep (NREMS) in comparison to females, while other differences between sexes in sleep phenotypes are species- and estrous cycle phase-dependent. Altogether, the review illustrates the need for a sex-based perspective in basic sleep and circadian research, including the consideration of sex chromosomes and gonadal hormones in sleep and circadian phenotypes.

•

In rodents, males spend less time awake, and more time in NREMS than females.

•

The recovery from sleep deprivation is also dependent on biological sex.

•

Gonadal hormones modulate sleep and circadian phenotypes in rodents.

•

A more systematic comparison of sex in basic sleep/circadian research is needed.

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.

Effect of acute total sleep deprivation on plasma melatonin, cortisol and metabolite rhythms in females

Disruption to sleep and circadian rhythms can impact on metabolism. The study aimed to investigate the effect of acute sleep deprivation on plasma melatonin, cortisol and metabolites, to increase understanding of the metabolic pathways involved in sleep/wake regulation processes. Twelve healthy young female participants remained in controlled laboratory conditions for ~92 hr with respect to posture, meals and environmental light (18:00-23:00 hr and 07:00-09:00 hr <8 lux; 23:00-07:00 hr 0 lux (sleep opportunity) or <8 lux (continuous wakefulness); 09:00-18:00 hr ~90 lux). Regular blood samples were collected for 70 hr for plasma melatonin and cortisol, and targeted liquid chromatography-mass spectrometry metabolomics. Timepoints between 00:00 and 06:00 hr for day 1 (baseline sleep), day 2 (sleep deprivation) and day 3 (recovery sleep) were analysed. Cosinor analysis and MetaCycle analysis were performed for detection of rhythmicity. Night-time melatonin levels were significantly increased during sleep deprivation and returned to baseline levels during recovery sleep. No significant differences were observed in cortisol levels. Of 130 plasma metabolites quantified, 41 metabolites were significantly altered across the study nights, with the majority decreasing during sleep deprivation, most notably phosphatidylcholines. In cosinor analysis, 58 metabolites maintained their rhythmicity across the study days, with the majority showing a phase advance during acute sleep deprivation. This observation differs to that previously reported for males. Our study is the first of metabolic profiling in females during sleep deprivation and recovery sleep, and offers a novel view of human sleep/wake regulation and sex differences.

Motor deficits, impaired response inhibition, and blunted response to methylphenidate following neonatal exposure to decabromodiphenyl ether

Decabromodiphenyl ether (decaBDE) is an applied brominated flame retardant that is widely-used in electronic equipment. After decades of use, decaBDE and other members of its polybrominated diphenyl ether class have become globally-distributed environmental contaminants that can be measured in the atmosphere, water bodies, wildlife, food staples and human breastmilk. Although it has been banned in Europe and voluntarily withdrawn from the U.S. market, it is still used in Asian countries. Evidence from epidemiological and animal studies indicate that decaBDE exposure targets brain development and produces behavioral impairments. The current study examined an array of motor and learning behaviors in a C57BL6/J mouse model to determine the breadth of the developmental neurotoxicity produced by decaBDE. Mouse pups were given a single daily oral dose of 0 or 20mg/kg decaBDE from postnatal day 1 to 21 and were tested in adulthood. Exposed male mice had impaired forelimb grip strength, altered motor output in a circadian wheel-running procedure, increased response errors during an operant differential reinforcement of low rates (DRL) procedure and a blunted response to an acute methylphenidate challenge administered before DRL testing. With the exception of altered wheel-running output, exposed females were not affected. Neither sex had altered somatic growth, motor coordination impairments on the Rotarod, gross learning deficits during operant lever-press acquisition, or impaired food motivation. The overall pattern of effects suggests that males are more sensitive to developmental decaBDE exposure, especially when performing behaviors that require effortful motor output or when learning tasks that require sufficient response inhibition for their successful completion.

Sex and ancestry determine the free-running circadian period

The endogenous, free-running circadian period (τ) determines the phase relationship that an organism assumes when entrained to the 24-h day. We found a shorter circadian period in African Americans compared to non-Hispanic European Americans (24.07 versus 24.33 h). We speculate that a short circadian period, closer to 24 h, was advantageous to humans living around the equator, but when humans migrated North out of Africa, where the photoperiod changes with seasons, natural selection favoured people with longer circadian periods. Recently, in evolutionary terms, immigrants came from Europe and Africa to America ('the New World'). The Europeans were descendents of people who had lived in Europe for thousands of years with changing photoperiods (and presumably longer periods), whereas Africans had ancestors who had always lived around the equator (with shorter periods). It may have been advantageous to have a longer circadian period while living in Europe early in the evolution of humans. In our modern world, however, it is better to have a shorter period, because it helps make our circadian rhythms earlier, which is adaptive in our early-bird-dominated society. European American women had a shorter circadian period than men (24.24 versus 24.41), but there was no sex difference in African Americans (24.07 for both men and women). We speculate that selection pressures in Europe made men develop a slightly longer period than women to help them track dawn which could be useful for hunters, but less important for women as gatherers.

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.

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.

Impact of Sleep and Circadian Disruption on Energy Balance and Diabetes: A Summary of Workshop Discussions

A workshop was held at the National Institute for Diabetes and Digestive and Kidney Diseases with a focus on the impact of sleep and circadian disruption on energy balance and diabetes. The workshop identified a number of key principles for research in this area and a number of specific opportunities. Studies in this area would be facilitated by active collaboration between investigators in sleep/circadian research and investigators in metabolism/diabetes. There is a need to translate the elegant findings from basic research into improving the metabolic health of the American public. There is also a need for investigators studying the impact of sleep/circadian disruption in humans to move beyond measurements of insulin and glucose and conduct more in-depth phenotyping. There is also a need for the assessments of sleep and circadian rhythms as well as assessments for sleep-disordered breathing to be incorporated into all ongoing cohort studies related to diabetes risk. Studies in humans need to complement the elegant short-term laboratory-based human studies of simulated short sleep and shift work etc. with studies in subjects in the general population with these disorders. It is conceivable that chronic adaptations occur, and if so, the mechanisms by which they occur needs to be identified and understood. Particular areas of opportunity that are ready for translation are studies to address whether CPAP treatment of patients with pre-diabetes and obstructive sleep apnea (OSA) prevents or delays the onset of diabetes and whether temporal restricted feeding has the same impact on obesity rates in humans as it does in mice.

Energy regulation in context: Free-living female arctic ground squirrels modulate the relationship between thyroid hormones and activity among life history stages

Thyroid hormones (THs), key regulators of lipid and carbohydrate metabolism, are likely modulators of energy allocation within and among animal life history stages. Despite their role in modulating metabolism, few studies have investigated whether THs vary among life history stages in free-living animals or if they exhibit stage-specific relationships to total energy expenditure and activity levels. We measured plasma total triiodothyronine (tT3) and thyroxine (tT4) at four, discrete life history stages of female arctic ground squirrels from two different populations in northern Alaska to test whether plasma THs correlate with life history stage-specific changes in metabolic rate and energy demand. We also tested whether THs explained individual variation in aboveground activity levels within life history stages. T3 peaked during lactation and was lowest during pre-hibernation fattening, consistent with known changes in basal metabolism and core body temperature. In contrast, T4 was elevated shortly after terminating hibernation but remained low and stable across other life-history stages in the active season. THs were consistently higher in the population that spent more time above-ground but the relationship between THs and activity varied among life history stages. T3 was positively correlated with activity only during lactation (r(2)=0.50) whereas T4 was positively correlated with activity immediately following lactation (r(2)=0.48) and during fattening (r(2)=0.53). Our results support the hypothesis that THs are an important modulator of basal metabolism but also suggest that the relationship between THs and activity varies among life history stages.

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.

Sex-related difference in food-anticipatory activity of mice

The expression of food-anticipatory activity (FAA) is induced by restricted feeding (RF), and its entrainment requires food-entrainable oscillators, the neuroanatomical basis of which is currently unclear. Although RF impacts various hormones, sex-related differences in FAA are unclear. 'Here, we report significantly more food-anticipatory wheel-running activity in male than in female mice during RF. In parallel with the sex-related difference in FAA, male and female mice display different food intake and body weight in response to RF. Since gonadal hormones could be involved in the sex-specific difference in FAA, we compared sham and gonadectomized male and female wild-type mice. In gonadectomized mice, the sex difference in FAA was abolished, indicating a role for gonadal hormones in FAA. Further, plasma concentrations of the hormone ghrelin were higher in female than in male mice during ad libitum (AL) feeding, and RF induced a temporal advance in its peak in both sexes. RF also shifted the expression peak of the circadian gene mPer1 in the hippocampus and liver, although no sex difference was found in either the level or the cyclic phase of its expression. Per1(Brdm1) mutant mice were still sexually dimorphic for FAA, but diminished FAA was noted in both male and female Per2(Brdm1) mutant mice. In summary, our results imply that gonadal hormones contribute to the sex difference in FAA, possibly through modulating ghrelin activity.

Different short-term mild exercise modalities lead to differential effects on body composition in healthy prepubertal male rats

Physical activity has a vital role in regulating and improving bone strength. Responsiveness of bone mass to exercise is age dependent with the prepubertal period suggested to be the most effective stage for interventions. There is a paucity of data on the effects of exercise on bone architecture and body composition when studied within the prepubertal period. We examined the effect of two forms of low-impact exercise on prepubertal changes in body composition and bone architecture. Weanling male rats were assigned to control (CON), bipedal stance (BPS), or wheel exercise (WEX) groups for 15 days until the onset of puberty. Distance travelled via WEX was recorded, food intake measured, and body composition quantified. Trabecular and cortical microarchitecture of the femur were determined by microcomputed tomography. WEX led to a higher lean mass and reduced fat mass compared to CON. WEX animals had greater femoral cortical cross-sectional thickness and closed porosity compared to CON. The different exercise modalities had no effect on body weight or food intake, but WEX significantly altered body composition and femoral microarchitecture. These data suggest that short-term mild voluntary exercise in normal prepubertal rats can alter body composition dependent upon the exercise modality.

ESR1 and ESR2 differentially regulate daily and circadian activity rhythms in female mice

Estrogenic signaling shapes and modifies daily and circadian rhythms, the disruption of which has been implicated in psychiatric, neurologic, cardiovascular, and metabolic disease, among others. However, the activational mechanisms contributing to these effects remain poorly characterized. To determine the activational impact of estrogen on daily behavior patterns and differentiate between the contributions of the estrogen receptors ESR1 and ESR2, ovariectomized adult female mice were administered estradiol, the ESR1 agonist propylpyrazole triol, the ESR2 agonist diarylpropionitrile, or cholesterol (control). Animals were singly housed with running wheels in a 12-hour light, 12-hour dark cycle or total darkness. Estradiol increased total activity and amplitude, consolidated activity to the dark phase, delayed the time of peak activity (acrophase of wheel running), advanced the time of activity onset, and shortened the free running period (τ), but did not alter the duration of activity (α). Importantly, activation of ESR1 or ESR2 differentially impacted daily and circadian rhythms. ESR1 stimulation increased total wheel running and amplitude and reduced the proportion of activity in the light vs the dark. Conversely, ESR2 activation modified the distribution of activity across the day, delayed acrophase of wheel running, and advanced the time of activity onset. Interestingly, τ was shortened by estradiol or either estrogen receptor agonist. Finally, estradiol-treated animals administered a light pulse in the early subjective night, but no other time, had an attenuated response compared with controls. This decreased phase response was mirrored by animals treated with diarylpropionitrile, but not propylpyrazole triol. To conclude, estradiol has strong activational effects on the temporal patterning and expression of daily and circadian behavior, and these effects are due to distinct mechanisms elicited by ESR1 and ESR2 activation.

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.

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.

Behavioral daily rhythmic activity pattern of adolescent female rat is modulated by acute and chronic cocaine

Cocaine is one of well-known drugs of abuse, and many children experience early exposure to cocaine. Because of an immature neuronal system in adolescents, they may react differently to repeated cocaine administration compared to adults. Most of the published papers report the effect of cocaine on adult male rats and this paper focused on the effects of cocaine on the 24 h locomotor activity rhythm patterns activity of adolescent Sprague Dawley (SD) female rats. Changes in the locomotor activity rhythm patterns could indicate that cocaine elicits long-term changes in the clock genes of the body that regulate different physiological processes. The objective of this study was to investigate whether cocaine in adolescent female rats modulated their daily activity pattern. Animals were divided into control (saline), 3.0, 7.5, 15.0 mg/kg cocaine groups. On experimental day 1 (ED 1), all groups were given saline injection. From ED 2 to ED 7, either saline or cocaine (3.0, 7.5, or 15.0 mg/kg) was given daily. ED 8 to ED 10 were the washout days, where no injection was given. On ED 11, the animals were injected with saline or with the same dose of cocaine as they were treated on ED 2 to ED 7. Each animal's locomotor activities was recorded nonstop following saline or cocaine injection for 11 consecutive days using the open field assay. In conclusion, it was observed that all three groups receiving repeated cocaine administration (3.0, 7.5, and 15.0 mg/kg) displayed significantly altered locomotor activity rhythm patterns.

Effects of forward and backward transitions in light intensities in tau-illuminance curves of the rat motor activity rhythm under constant dim light

Circadian rhythms are strongly influenced by light intensity, the effects of which may persist beyond the duration of light exposure (aftereffects). Here, the authors constructed period-illuminance curves for the motor activity circadian rhythm of male and female rats by recording the effects of a series of small upward and downward steps in light intensity (illuminance ranging between .01 lux of dim red light and 1 lux of white light) on their activity. In all cases, stepwise changes were made in five logarithmic steps (irradiance: dim red light: .692 µW/cm(2) and white light: .006, .016, .044, .12, and .315 µW/cm(2), corresponding, respectively, to .02, .05, .14, .13, and 1 lux measured at cage level), with changes in intensity every 2 wks. One group of rats (DLD) started in dim red light, moved up to 1 lux white light, and then back down to the original light intensity. Another group (LDL) started at 1 lux, moved down to .01 lux, and then back up to the original intensity. Motor activity data were recorded throughout the experiment and tau values, the percentage of variance explained by the rhythm, and the mean motor activity for each stage and group were calculated. The results show differences in the dynamics of tau values between the DLD and LDL groups and between males and females. In the LDL group, the tau values of both males and females were dependent on light intensity, and were similar for the forward and backward transitions. In other words, no aftereffects were found, and no differences were detected between males and females. In the DLD group, however, differences were found between males and females. Males had a tau value of 24 h 20 min under dim red light, 25 h 40 min under 1 lux, and 24 h 50 min on return to dim red light. It is noticeable that the tau values of the backward branch of the illuminance curve contradicted classical predictions, since at .38 and .14 lux the tau values were shorter than those found under the same intensities after exposure to 1 lux. Females became arrhythmic at 1 lux, and only one half of them recovered their circadian rhythm at .02 lux. The other one half remained arrhythmic even under dim red light. Thus, some of the results of this paper contradict the predictions of standard descriptions of the functioning of the circadian clock, possibly due to the effects of dim light.

Sex difference in the near-24-hour intrinsic period of the human circadian timing system

The circadian rhythms of melatonin and body temperature are set to an earlier hour in women than in men, even when the women and men maintain nearly identical and consistent bedtimes and wake times. Moreover, women tend to wake up earlier than men and exhibit a greater preference for morning activities than men. Although the neurobiological mechanism underlying this sex difference in circadian alignment is unknown, multiple studies in nonhuman animals have demonstrated a sex difference in circadian period that could account for such a difference in circadian alignment between women and men. Whether a sex difference in intrinsic circadian period in humans underlies the difference in circadian alignment between men and women is unknown. We analyzed precise estimates of intrinsic circadian period collected from 157 individuals (52 women, 105 men; aged 18-74 y) studied in a month-long inpatient protocol designed to minimize confounding influences on circadian period estimation. Overall, the average intrinsic period of the melatonin and temperature rhythms in this population was very close to 24 h [24.15 ± 0.2 h (24 h 9 min ± 12 min)]. We further found that the intrinsic circadian period was significantly shorter in women [24.09 ± 0.2 h (24 h 5 min ± 12 min)] than in men [24.19 ± 0.2 h (24 h 11 min ± 12 min); P < 0.01] and that a significantly greater proportion of women have intrinsic circadian periods shorter than 24.0 h (35% vs. 14%; P < 0.01). The shorter average intrinsic circadian period observed in women may have implications for understanding sex differences in habitual sleep duration and insomnia prevalence.

Voluntary exercise does not enhance long-term potentiation in the adolescent female dentate gyrus

The hippocampus is a dynamic brain structure involved with learning and memory. Long-term potentiation (LTP) is a neuronal model of learning and memory and, in adult rodents, is enhanced by voluntary exercise (VEx). The current study sought to elucidate whether synaptic plasticity in the male and female adolescent hippocampus is augmented by VEx. Consistent with previous studies, VEx significantly enhanced LTP in adolescent males following weak and strong theta-burst stimulation. Despite running the same amount as males, however, VEx did not enhance LTP in females above non-runner females. Surprisingly, the exercise-induced enhancement to LTP in males was seen in the absence of a change in brain derived neurotrophic factor in the dentate gyrus (DG). These findings indicate that adolescent males and females are differentially sensitive to the potentiating effect of exercise on hippocampal synaptic plasticity.

Sex differences in phase angle of entrainment and melatonin amplitude in humans

Studies of sex differences in the timing of human circadian rhythms have reported conflicting results. This may be because the studies conducted to date have not controlled for the masking effects of the rest activity cycle on the circadian rhythms being assessed. In the present analysis of data collected under controlled conditions, we examined sex differences in the timing of circadian rhythms while minimizing masking from behavioral and environmental factors using a constant routine (CR) protocol. All participants (28 women and 28 men paired by habitual wake time; age range, 18 30 years) maintained a regular self selected sleep wake schedule at home prior to the study. After 3 baseline days in the laboratory, participants began a CR. Women were found to have a significantly higher melatonin amplitude and lower temperature amplitude than men. While sleep timing was the same between the 2 groups, the timing of the circadian rhythms of core body temperature and pineal melatonin secretion was earlier relative to sleep time in women as compared to men. Sleep therefore occurred at a later biological time for women than men, despite being at the same clock time. Given that sleep propensity and structure vary with circadian phase and are impacted by circulating melatonin, these findings may have important implications for understanding sex differences in sleep timing and duration, diurnal preference, and the prevalence of sleep disorders such as insomnia.

Chronic forced exercise during adolescence decreases cocaine conditioned place preference in Lewis rats

Chronic physical activity (exercise) may be beneficial in the prevention of substance use disorders; however, the extent to which physical activity can interfere with the reinforcing effects of drugs during the adolescent period, which is one of great vulnerability for drug experimentation, has not been fully evaluated. Here, we assess the effects of chronic forced exercise during adolescence on preference for cocaine using the conditioned place preference (CPP) paradigm in male and female Lewis rats. The group of rats exposed to exercise ran on a treadmill for 6 weeks on a progressive time-increased schedule for up to 1h of exercise per day, while the groups of sedentary rats remained in their home cage. Following the 6 weeks of exercise exposure, rats were tested for cocaine CPP. Results showed that chronic exercise significantly attenuated cocaine CPP in both males and females compared to a sedentary environment. Furthermore, male exercise rats failed to show significant cocaine CPP. In contrast, female exercise rats still showed cocaine CPP but it was significantly reduced compared to the female sedentary rats. Females also exhibited greater cocaine CPP than males overall. These findings suggest that strategies to promote physical activity during adolescence may be protective against cocaine abuse in both males and females, and these findings merit further investigation. We also corroborate a gender-specific sensitivity to the reinforcing effects of cocaine, highlighting the need to consider gender-tailored exercise interventions for drug abuse prevention.

Effects of repeated light-dark phase shifts on voluntary ethanol and water intake in male and female Fischer and Lewis rats

Several lines of evidence implicate reciprocal interactions between excessive alcohol (ethanol) intake and dysregulation of circadian biological rhythms. Thus, chronic alcohol intake leads to widespread circadian disruption in both humans and experimental animals, while in turn, chronobiological disruption has been hypothesized to promote or sustain excessive alcohol intake. Nevertheless, the effects of circadian disruption on voluntary ethanol intake have not been investigated extensively, and prior studies have reported both increased and decreased ethanol intake in rats maintained under "shift-lag" lighting regimens mimicking those experienced by shift workers and transmeridian travelers. In the present study, male and female inbred Fischer and Lewis rats were housed in running wheel cages with continuous free-choice access to both water and 10% (vol/vol) ethanol solution and exposed to repeated 6-h phase advances of the daily light-dark (LD) cycle, whereas controls were kept under standard LD 12:12 conditions. Shift-lag lighting reduced overall ethanol and water intake, and reduced ethanol preference in Fischer rats. Although contrary to the hypothesis that circadian disruption would increase voluntary ethanol intake, these results are consistent with our previous report of reduced ethanol intake in selectively bred high-alcohol-drinking (HAD1) rats housed under a similar lighting regimen. We conclude that chronic circadian disruption is a form of chronobiological stressor that, like other stressors, can either increase or decrease ethanol intake, depending on a variety of poorly understood variables.

Fentanyl, but not haloperidol, entrains persisting circadian activity episodes when administered at 24- and 31-h intervals

Administration of several drugs of abuse on a 24-h schedule has been shown to entrain both pre-drug (anticipatory) and post-drug (evoked) circadian activity episodes that persist for several days when the drug is withheld. The present study tested the entrainment effects of fentanyl, an opioid agonist with a noted abuse liability, and haloperidol, an anti-psychotic dopamine antagonist without apparent abuse liability. Adult female Sprague-Dawley rats housed under constant light in cages with attached running wheels received repeated low, medium, or high doses of either fentanyl or haloperidol on a 24-h administration schedule followed by a 31-h schedule (Experiment 1) or solely on a 31-h schedule (Experiment 2). The results showed that all three doses of fentanyl entrained both pre-drug and post-drug episodes of wheel running when administered every 24h, and the combined pre- and post-fentanyl activity episodes persisted for at least 3 days when the drug was withheld during test days. On the 31-h schedule, fentanyl produced an "ensuing" activity episode approximately 24h post-administration, but failed to produce an anticipatory episode 29-31h post-administration. In contrast, haloperidol injections failed to produce both pre-drug episodes on the 24-h schedule and circadian ensuing episodes on the 31-h schedule, and post-haloperidol suppression of activity appeared to mask the free-running activity rhythm. Taken together, these results provide additional evidence that drugs of abuse share a common ability to entrain circadian activity episodes.

Sex, season and melatonin administration affects daily activity rhythms in a marsupial, the brown antechinus, Antechinus stuartii

The carnivorous marsupial Antechinus stuartii relies on photoperiodic changes to time reproductive activities, including behaviour, in spring. Similar to other mammals, the administration of the hormone melatonin is known to affect the synchronisation of reproduction in A. stuartii. The present study sought to explore the alterations in locomotor activity from the winter solstice in both males (body mass 35 g) and females (body mass 20 g) as a result of the influences of the changes in the natural photocycle and also of melatonin administration while under the natural photocycle. The total daily activity was found to differ between sexes, with males more active than females, irrespective of melatonin or control treatments. Daily activity patterns were significantly different between male groups but not female treatment groups. Activity patterns were also found to differ between males and females. The significance of these differences is discussed with relation to the profound physiological differences between the sexes, in this mammal where an irreversible stress response is part of the complete post-mating mortality of all males, but not females.

Pre- and post-nicotine circadian activity rhythms can be differentiated by a paired environmental cue

Previous studies have shown that addictive drugs presented daily at fixed times produce circadian (oscillator-driven) anticipatory and evoked activity rhythms in rats. Other studies have shown that environmental cues paired with addictive drugs produce tolerance to drug effects and elicit craving behavior when presented without the drug. The present study tested these circadian entrainment and paired-cue conditioning effects together. This study compared the ability of daily nicotine and saline injections at different fixed times to entrain pre-injection (anticipatory) and post-injection (evoked) circadian activity rhythms in two groups of female Sprague-Dawley rats. One group (Paired) had an environmental cue (a tone) paired with the effects of the nicotine injection, and the second group (Unpaired) had the tone paired with the effects of the saline injection. The rats were housed singly for 56 days in chambers with attached wheels under constant dim light and rate-limited food access. During three separate injection phases, nicotine and saline were administered daily at different fixed times, and the tone was presented at the second injection time. Three multi-day test phases examined circadian activity (a) without injections or tone, (b) with the tone alone at normal and novel times, and (c) with the tone absent and with injections occurring at normal and at novel times. The results showed that nicotine entrained both pre- and post-injection circadian oscillators, and the nicotine-paired tone interfered with pre-injection anticipatory activity.

Sex differences in adult suprachiasmatic nucleus neurons emerging late prenatally in rats

The suprachiasmatic nucleus (SCN) is implicated in the control of circadian rhythms of gonadal function. Although several structures surrounding the SCN are sensitive to the effects of gonadal steroids, similar effects in the SCN remain unclear. For example, there are conflicting data on whether the SCN is sexually differentiated. This study attempted to determine sex differences in the number of SCN cells generated during late gestation, and if testosterone mediates these differences. Pregnant female rats were treated with 5-bromo-2'-deoxyuridine (BrdU; 50 mg/kg) on gestational day 18 (E18), the day when aromatase activity peaks in the developing rat fetus. These animals were also given injections of oil or testosterone propionate (10 mg/0.1 mL peanut oil) from E15 until parturition. Litters were allowed to survive until adulthood and were killed on postnatal day 60 (PN60). Following fixation, brain sections containing the SCN from these rats were processed for BrdU immunocytochemistry. A second set of SCN sections was processed for immunocytochemistry detecting BrdU and some of the cell groups prevalent within the SCN. Data showed that female rats have a higher number of cells labeled with BrdU in the SCN, particularly in the medial and caudal SCN. This sex difference was abolished in animals treated with testosterone during late gestation. Double immunocytochemistry revealed that BrdU-labeled cells were neurons expressing calbindin-D28K, vasoactive intestinal peptide and, to a lesser degree, vasopressin. Our results unveiled a previously unknown effect of gonadal steroids on the developing SCN, which may contribute to the emergence of gender-specific circadian rhythms.

Effects of chronic dysthyroidism on activity and exploration

The aim of the present study was to determine the influence of thyroid function on the activity and exploratory behaviour of male Wistar rats. Dysthyroidism was induced by adding drugs to their drinking water from the ninth day of gestation. This method is not as stressful as daily thyroxine injections or thyroidectomy, and therefore did not affect the analysed behavioural patterns. After weaning, the drugs were administered to the young rats until the end of the experiment. Activity and exploration were measured using the Boissier test, a light-darkness test and an open-field test when they were 77 days old. In order to verify that the animals' motor capacity had not been impaired, a psychomotor battery was used. Chronic hyperthyroidism produced a significant increase in activity, but did not affect exploration. On the other hand, hypothyroidism did not affect activity, but did increase exploration. This increase in exploration was observed in activity-independent behavioural parameters, such as head dipping and glancing.

Circadian locomotor activity rhythms of the diurnal Indian palm squirrel in constant light

The parametric or tonic effects of light were studied in a recently established diurnal circadian model-the Indian palm squirrel, Funambulus pennanti. Sixteen squirrels (7 female, 9 male) were housed individually under varying constant light conditions (0.1 lux to 46 lux) with gross locomotor activity continuously monitored. Free-running period (tau), amplitude, mesor and day-to-day stability of the activity rhythm were determined using modified periodogram and iterative harmonic analyses, while the ratio of activity to rest time was estimated by eye-fit. The main findings were as follows: 1) tau did not vary between sexes or between light conditions, although a trend for tau to lengthen when light intensity was increased was noted; 2) amplitude and mesor did not show sex differences, but both sexes showed a decrease in amplitude and mesor when light intensity was decreased; 3) the stability of the activity rhythm was greater in males than in females, and a trend was observed for rhythm stability to decrease when light intensity was reduced. These descriptive data contribute to the growing literature on this diurnal species.

Testicular hormones modulate circadian rhythms of the diurnal rodent, Octodon degus

Sex differences have been identified in a variety of circadian rhythms, including free-running rhythms, light-induced phase shifts, sleep patterns, hormonal fluctuations, and rates of reentrainment. In the precocial, diurnal rodent Octodon degus, sex differences have been found in length of free-running rhythm (tau), phase response curves, rates of reentrainment, and in the use of social cues to facilitate reentrainment. Although gonadal hormones primarily organize circadian rhythms during early development, adult gonadal hormones have activational properties on various aspects of circadian rhythms in a number of species examined. Gonadectomy of adult female O. degus did not influence tau, phase angle of entrainment, or activity patterns in previous experiments. The present experiment examined the role of gonadal hormones in adult male degus' circadian wheel-running rhythms. We predicted that male gonadal hormones would have an activational effect on some aspects of circadian rhythms, particularly those in which we see sex differences. Phase angles of entrainment, tau, length of the active period (alpha), maximum and mean activity levels, and activity amplitude were examined for intact and castrated males housed in LD 12:12. Responses to light pulses while housed in constant darkness (DD) were also compared. Castration had no significant effect on tau or light-induced phase shifts. However, castration significantly increased phase angle of entrainment and decreased activity levels. The data indicate that adult gonadal steroids are not responsible for the sex differences in endogenous circadian mechanisms of O. degus (tau, PRC), although they influence activity level and phase angle of entrainment. This is most likely due to masking properties of testosterone, similar to the activity-increasing effects of estrogen during estrus in O. degus females.

Differential control of morning and evening components in the activity rhythm of Drosophila melanogaster--sex-specific differences suggest a different quality of activity

The rhythms of locomotor activity of male and virgin or mated female flies were compared in the Drosophila melanogaster wild-type strains CantonS, Berlin, and OregonR. Under light-dark conditions, most flies showed a bimodal activity pattern with a morning peak around lights-on and an evening peak before lights-off. For all strains, a distinct sexual dimorphism was observed in the phase of the morning peak. Males had a significantly earlier morning peak than females and consequently a larger phase angle between morning and evening peak (psi(m, e)). Under constant dark conditions, the morning component merged with the evening component to a unimodal activity band in about half of the flies. In those flies who maintained bimodality, the sex-specific difference in psi(m, e) disappeared. Other sex-specific differences were now apparent: Males showed a shorter free-running period than females, and in two of the three strains, females were more active than males. Morning and evening components seem to contribute to the free-running period. Spontaneous or externally provoked change in psi(m, e) were correlated with period changes. In some flies, the morning and the evening components showed splitting, indicating that they are the output of two different oscillators. The sexual dimorphism in the phase of the morning peak under LD-conditions suggests that the function of activity during morning and evening peak might be different, for example, during the morning peak, males are active to find females. Overall, the results underline the multioscillatory nature of Drosophila's circadian system.

Voluntary wheel running: a review and novel interpretation

Voluntary wheel running by animals is an activity that has been observed and recorded in great detail for almost a century. This review shows that it is performed, often with startling intensity and coordination, by a wide variety of wild, laboratory and domestic species with diverse evolutionary histories. However, despite the plethora of published studies on wheel running, there is considerable disagreement between many findings, thus leading to a lack of consensus on explanations of the causality and function. In the initial part of this review, I discuss the internal and external factors that may be involved in the causality of this behaviour, with an emphasis on disparities in both the factual and theoretical development of the subject. I then address the various proposed functions of wheel running, again highlighting evidence to the contrary. This leads to the conclusion that any single theory on the basis of wheel running is likely to be simplistic with little generality. I then present a novel, behaviour-based interpretation in which it is argued that wheel running has no directly analogous naturally occurring behaviour, it is (sometimes) performed for its own sake per se rather than as a redirected or substitute activity, and studies on motivation show that wheel running is self-reinforcing and perceived by animals as 'important'. This review proposes that wheel running may be an artefact of captive environments or of the running-wheel itself, possibly resulting from feedback dysfunction. I also discuss the ubiquity and intensity of its performance, along with its great plasticity and maladaptiveness, all indicating that if it is an artefact, it is nevertheless one of great interest to behavioural science. Copyright 1998 The Association for the Study of Animal Behaviour.

Circadian rhythms and the pharmacology of affective illness

The chronic effects of antidepressant drugs (ADs) on circadian rhythms of behavior, physiology and endocrinology are reviewed. The timekeeping properties of several classes of ADs, including tricyclic antidepressants, selective serotonin reuptake inhibitors, monoamine oxidase inhibitors, serotonin agonists and antagonists, benzodiazepines, and melatonin are reviewed. Pharmacological effects on the circadian amplitude and phase, as well as effects on day-night measurements of motor activity, sleep-wake, body temperature (Tb), 3-methoxy-4-hydroxyphenylglycol, cortisol, thyroid hormone, prolactin, growth hormone and melatonin are examined. ADs often lower nocturnal Tb and affect the homeostatic regulation of sleep. ADs often advance the timing and decrease the amount of slow wave sleep, reduce rapid eye movement sleep and increase or decrease arousal. Together, AD effects on nocturnal Tb and sleep may be related to their therapeutic properties. ADs sometimes delay nocturnal cortisol timing and increase nocturnal melatonin, thyroid hormone and prolactin levels; these effects often vary with diagnosis, and clinical state. The effects of ADs on the coupling of the central circadian pacemaker to photic and nonphotic zeitgebers are discussed.

Sex differences and effects of social cues on daily rhythms following phase advances in Octodon degus

Two experiments were designed to determine whether social cues could enhance the rate of resynchronization in body temperature and general activity rhythms in male or female Octodon degus following a 6 h phase advance. The first experiment examined average resynchronization rates for animals in each condition. The second experiment examined resynchronization rates for a smaller group of animals, each treated as its own control. Female phase-shifters resynchronized temperature and activity rhythms significantly faster when housed with an entrained (donor) female than those females housed with another phase-shifting female or housed alone. Females housed with entrained males resynchronized their temperature rhythms significantly slower than females housed with entrained females. No differences in resynchronization rate for phase-shifting males existed between test conditions. However, activity rhythms of male controls (housed alone) reentrained significantly faster than those of female controls. These experiments demonstrate a sex difference in (i) reentrainment rate by photic cues alone; (ii) donors' effect on female phase-shifters' resynchronization; and (iii) phase-shifters' resynchronization response to donor cues. In these studies, resynchronization in the presence of another animal could either have been achieved by entrainment of the pacemaker or by masking of the circadian rhythms.

The interaction of thyroid state, MAOI drug treatment, and light on the level and circadian pattern of wheel-running in rats

In order to examine the relationship between thyroid status, the circadian system, and antidepressant drug response, the antidepressant drug clorgyline, a monoamine oxidase inhibitor (MAOI), was administered chronically to sham-operated or thyroparathyroidectomized rats. Wheel-running was monitored continuously in a light-dark (LD) cycle, and then in constant dim light. In LD, MAOI treatment increased levels of running. This effect was delayed in hypothyroid rats relative to euthyroid rats. In constant light, the MAOI-induced increase in running was diminished in euthyroid but not hypothyroid animals. Hypothyroid animals were less responsive to the change in lighting than were euthyroid animals, and this was more apparent in hypothyroid rats given MAOI. The daily pattern of running differed with lighting condition as well as with treatment group. MAOI-treatment of hypothyroid animals phase-advanced the pattern of wheel-running. MAOI-treatment of control animals increased the amplitude of wheel-running particularly in the LD cycle. These results indicate that thyroid status, lighting, and MAOI treatment interact to alter the behavioral response to chronic drug treatment.

Disruption of body temperature and behavior rhythms during reproduction in dwarf hamsters (Phodopus)

The internal gestation and subsequent lactation of mammalian reproduction represent a considerable physiological challenge. The extent of disruption in the daily rhythm of four parameters, core body temperature, nest attendance, activity, and wheel running, was monitored in Djungarian hamster (Phodopus campbelli) and Siberian hamster (P. sungorus) females implanted intraperitoneally with biotelemetric thermistors. The amplitude of each rhythm decreased during late gestation, culminating in a substantial disruption at parturition, and did not begin a recovery until the latter third of lactation. In each species, the change in the core body temperature rhythm was primarily the result of an elevation in light phase body temperature to approximate the normally occurring dark phase temperature, although the disruption was more extensive in P. sungorus than in P. campbelli. As this maternal hyperthermia is associated with the provision of essential heat to the altricial liter, these species differences in the vulnerability to hyperthermia may constrain the reproductive success of these extreme cold adapted small mammals.

Effects of thyroxine and thyroparathyroidectomy on circadian wheel running in rats

Thyroparathyroidectomized (TPX) and thyroidectomized male rats display shorter free-running activity periods and enhanced activity levels. These experiments were designed to determine whether this effect is due to the loss of thyroid hormones. The running wheel activity of 36 male rats, 19 TPX and 17 sham operated, was studied. The animals were kept in constant conditions for 7 weeks to obtain baseline data. Half the rats were then injected SC with capsules containing T4, while the other half were injected with blanks. All animals were then allowed to free-run undisturbed for another 8-9 weeks. TPX rats displayed significantly shorter baseline periods (average difference: 0.26 h) and heightened activity. Thyroxine treatment significantly lengthened TPX animals' cycles (average increase: 0.28 h) but did not affect intact rats' circadian rhythms. Thyroxine did, however, significantly decreased the activity levels of both TPX and sham-operated rats. These findings indicate that changes in TPX rats' activity cycles are caused by a reduction in thyroid hormones and that thyroxine acts on activity rhythms and levels by different mechanisms.

Thyroid and thyroxine effects on adrenoreceptors in relation to circadian activity

Experiments were conducted to ascertain if changes in central adrenergic receptors could be associated with altered circadian activity patterns induced by thyroparathyroidectomy (TPX) and thyroxine. An initial experiment used TPX and sham-operated rats that had been exposed to dim red light for 7 months. The alpha and beta receptor densities were compared in the suprachiasmatic nuclei (SCN), preoptic (PO), septum, and caudate-putamen. TPX animals showed significant reductions in beta 1 and beta 2 receptor densities in SCN and PO, and alpha 1 densities in SCN, but no other changes. A second experiment, lasting 4 months, examined the effects of thyroxine, which has been shown to reverse the period-shortening effects of TPX surgery. Thyroxine significantly increased beta 1 receptors in both the SCN and ventromedial hypothalamus (VMH), the only regions that displayed significant reductions in TPXs during the second experiment. Increases of sevenfold and threefold were observed in the SCNs of TPXs and shams, respectively, but thyroxine's action in the VMH was limited to TPX animals, an effect that mimics thyroxine's action on circadian activity rhythms.

Effects of hypothyroidism on rat circadian activity and temperature rhythms and their response to light

Male rats made hypothyroid by administration of propylthiouracil plus sodium ipodate in drinking water were compared to controls in terms of period of circadian activity and temperature rhythms, amount of gross motor activity, and mean temperature. Animals were studied under entrainment, constant darkness (DD), and constant dim light (LL). There was no difference in the period of the circadian activity rhythm between groups in DD. However, hypothyroid rats showed significant blunting of the period-lengthening response to increasing ambient illumination. As expected, the period of the circadian temperature rhythm increased in controls with increasing ambient illumination. In contrast, the period of the circadian temperature rhythm in hypothyroid animals actually shortened under LL compared to DD. This blunting of the period-lengthening response to increasing ambient illumination of both activity and temperature rhythms in hypothyroid animals could not be explained by differences in activity level or mean temperature between the groups.

Effects of exercise on ethanol-induced hypothermia and loss of righting response in C57BL/6J mice

C57BL/6J mice were given 5 weeks of voluntary wheel running and then studied for ethanol (EtOH) sensitivity as indicated by EtOH-induced hypothermia and loss of righting response (LORR) after 3.8 g/kg EtOH (20% w/v). Mice were assigned to wheel (free access to a running wheel in the home cage) or no wheel conditions, and wheel counts were monitored by a computer at 5-min intervals around the clock. In Experiment 1, duration of EtOH-induced LORR was assessed as amount of time required for the animal to right itself three times in a 30-s period, and body temperature was assessed by rectal probe. Wheel animals showed significantly shorter LORR and significantly less hypothermia at regaining the righting response than no wheel controls. In Experiment 2, temperature was assessed at 45 and 90 min after EtOH challenge. Baseline temperatures for wheel and no wheel animals did not differ, but wheel animals showed dramatic resistance to EtOH-induced hypothermia at both time points. Together with our earlier work, these results provide evidence that prior exercise can offset the effects of EtOH intoxication in several domains of EtOH sensitivity.

Thyroparathyroidectomy produces a progressive escape deficit in rats

Abnormal thyroid status and affective disorders have been associated in the human clinical literature. It has recently been shown that pretreatment with thyroid hormone can prevent escape deficits produced by inescapable shock in an animal analogue of depression. In this report we provide evidence that hypothyroid status can produce an escape deficit in rats. While sham-operated rats improved their performance on a simple escape task over three days of testing, thyroparathyroidectomized rats showed a pronounced decrease in their responses. Markov transition analysis was used to obtain conditional probabilities of escaping given a prior escape or failure to escape for the two groups. This analysis shows that the structure of the data set may be similar for the two groups. These results suggest that if intact rats learn to escape, then hypothyroid rats may learn not to escape.