The use of a running wheel to measure activity in rodents: relationship to energy balance, general activity, and reward

The Running Wheel Enhances Food Anticipatory Activity: An Exploratory Study

Rodents anticipate rewarding stimuli such as daily meals, mates, and stimulant drugs. When a single meal is provided daily at a fixed time of day, an increase in activity, known as food anticipatory activity (FAA), occurs several hours before feeding time. The factors affecting the expression of FAA have not been well-studied. Understanding these factors may provide clues to the undiscovered anatomical substrates of food entrainment. In this study we determined whether wheel-running activity, which is also rewarding to rodents, modulated the robustness of FAA. We found that access to a freely rotating wheel enhanced the robustness of FAA. This enhancement was lost when the wheel was removed. In addition, while prior exposure to a running wheel alone did not enhance FAA, the presence of a locked wheel did enhance FAA as long as mice had previously run in the wheel. Together, these data suggest that FAA, like wheel-running activity, is influenced by reward signaling.

A murine model of peripheral irradiation-induced fatigue

PURPOSE

Fatigue is the most ubiquitous side effect of cancer treatment, but its etiology remains elusive. Further investigations into cancer-related fatigue pathobiology necessitate the expanded use of animal models. This study describes the development of a murine model of radiation-induced fatigue.

METHODS

Voluntary wheel running activity measured fatigue in 5-8 week-old, male C57BL/6 mice before and after γ irradiation totaling 2400cGy (3 consecutive days×800cGy daily fractionated doses) to the lower abdominal areas. Three trials confirmed fatigue behavior at this dose. Anhedonia, body weight, and hemoglobin were also measured. Gastrointestinal, skeletal muscle, and bone marrow tissue samples were evaluated for signs of damage.

RESULTS

In two validation trials, irradiated mice (trial 1, n=8; trial 2, n=8) covered less cumulative distance in kilometers post-irradiation (trial 1, mean=115.3±12.3; trial 2, mean=113.6±21.8) than sham controls (trial 1, n=5, mean=126.3±5.7, p=0.05; trial 2, n=8, mean=140.9±25.4, p=0.02). Decreased mean daily running distance and speed were observed during the last four hours of the dark cycle in irradiated mice compared to controls for two weeks post-irradiation. There were no differences in saccharin preference or hemoglobin levels between groups, no effect of changes in body weight or hemoglobin on wheel running distance, additionally, histology showed no damage to muscle, bone marrow, or gastrointestinal integrity, with the latter confirmed by ELISA.

CONCLUSION

We characterized a novel mouse model of fatigue caused by peripheral radiation and not associated with anemia, weight changes, or anhedonia. This model provides opportunities for detailed study of the mechanisms of radiation-induced fatigue.

Endurance exercise and selective breeding for longevity extend Drosophila healthspan by overlapping mechanisms

Endurance exercise has emerged as a powerful intervention that promotes healthy aging by maintaining the functional capacity of critical organ systems. In addition, long-term exercise reduces the incidence of age-related diseases in humans and in model organisms. Despite these evident benefits, the genetic pathways required for exercise interventions to achieve these effects are still relatively poorly understood. Here, we compare gene expression changes during endurance training in Drosophila melanogaster to gene expression changes during selective breeding for longevity. Microarrays indicate that 65% of gene expression changes found in flies selectively bred for longevity are also found in flies subjected to three weeks of exercise training. We find that both selective breeding and endurance training increase endurance, cardiac performance, running speed, flying height, and levels of autophagy in adipose tissue. Both interventions generally upregulate stress defense, folate metabolism, and lipase activity, while downregulating carbohydrate metabolism and odorant receptor expression. Several members of the methuselah-like (mthl) gene family are downregulated by both interventions. Knockdown of mthl-3 was sufficient to provide extension of negative geotaxis behavior, endurance and cardiac stress resistance. These results provide support for endurance exercise as a broadly acting anti-aging intervention and confirm that exercise training acts in part by targeting longevity assurance pathways.

A dopamine and noradrenaline reuptake inhibitor (bupropion) does not alter exercise performance of bank voles

Physical performance is determined both by biophysical and physiological limitations and behavioral characteristic, specifically motivation. We applied an experimental evolution approach combined with pharmacological manipulation to test the hypothesis that evolution of increased aerobic exercise performance can be triggered by evolution of motivation to undertake physical activity. We used a unique model system: bank voles from A lines, selected for high swim-induced aerobic metabolism (VO₂swim), which achieved a 61% higher mass-adjusted VO₂swim than those from unselected C lines. Because the voles could float on the water surface with only a minimum activity, the maximum rate of metabolism achieved in that test depended not only on their aerobic capacity, but also on motivation to undertake intensive activity. Therefore, we hypothesized that signaling of neurotransmitters putatively involved in regulating physical activity (dopamine and noradrenaline) had changed in response to selection. We measured VO₂swim after intraperitoneal injections of saline or the norepinephrine and dopamine reuptake inhibitor bupropion (20 mg/kg or 30 mg/kg). Additionally, we measured forced-exercise VO₂ (VO₂max). In C lines, VO₂swim (mass-adjusted mean ± standard error (SE): 4.0 ± 0.1 mLO₂/min) was lower than VO₂max (5.0 ± 0.1 mLO₂/min), but in A lines VO2swim (6.0 ± 0.1 mLO₂/min) was as high as VO₂max (6.0 ± 0.1 mLO₂/min). Thus, the selection effectively changed both the physiological-physical performance limit and mechanisms responsible for the willingness to undertake vigorous locomotor activity. Surprisingly, the drug had no effect on the achieved level of VO₂swim. Thus, the results did not allow firm conclusions concerning involvement of these neurotransmitters in evolution of increased aerobic exercise performance in the experimental evolution model system.

Corticosterone and dopamine D2/D3 receptors mediate the motivation for voluntary wheel running in C57BL/6J mice

Physical exercise can improve cognition but whether this is related to motivation levels is unknown. Voluntary wheel running is a rewarding activity proposed as a model of motivation to exercise. To question the potential effects of exercise motivation on subsequent behaviour, we used a pharmacological approach targeting some reward mechanisms. The stress hormone corticosterone has rewarding effects mediated by activation of low affinity glucocorticoid receptors (GR). To investigate whether corticosterone synthesis motivates exercise via activation of GRs and subsequently, impacts on behaviour, we treated C57BL/6J mice acutely with the inhibitor of corticosterone synthesis metyrapone (35mg/kg) or repeatedly with the GR antagonist mifepristone (30mg/kg) prior to 1-h running wheel sessions. To investigate whether reducing motivation to exercise impacts on behaviour, we antagonised running-induced dopamine D2/D3 receptors activation with sulpiride (25 or 50mg/kg) and assessed locomotor, anxiety-related and memory performance after 20 running sessions over 4 weeks. We found that corticosterone synthesis contributes to running levels, but the maintenance of running behaviour was not mediated by activation of GRs. Intermittent exercise was not associated with changes in behavioural or cognitive performance. The persistent reduction in exercise levels triggered by sulpiride also had limited impact on behavioural performance, although the level of performance for some behaviours was related to the level of exercise. Altogether, these findings indicate that corticosterone and dopamine D2/D3 receptor activation contribute to the motivation for wheel running, but suggest that motivation for exercise is not a sufficient factor to alter behaviour in healthy mice.

Short-term feeding at the wrong time is sufficient to desynchronize peripheral clocks and induce obesity with hyperphagia, physical inactivity and metabolic disorders in mice

BACKGROUND

The circadian clock regulates various physiological and behavioral rhythms such as feeding and locomotor activity. Feeding at unusual times of the day (inactive phase) is thought to be associated with obesity and metabolic disorders in experimental animals and in humans.

OBJECTIVE

The present study aimed to determine the underlying mechanisms through which time-of-day-dependent feeding influences metabolic homeostasis.

METHODS

We compared food consumption, wheel-running activity, core body temperature, hormonal and metabolic variables in blood, lipid accumulation in the liver, circadian expression of clock and metabolic genes in peripheral tissues, and body weight gain between mice fed only during the sleep phase (DF, daytime feeding) and those fed only during the active phase (NF, nighttime feeding). All mice were fed with the same high-fat high-sucrose diet throughout the experiment. To the best of our knowledge, this is the first study to examine the metabolic effects of time-imposed restricted feeding (RF) in mice with free access to a running wheel.

RESULTS

After one week of RF, DF mice gained more weight and developed hyperphagia, higher feed efficiency and more adiposity than NF mice. The daily amount of running on the wheel was rapidly and obviously reduced by DF, which might have been the result of time-of-day-dependent hypothermia. The amount of daily food consumption and hypothalamic mRNA expression of orexigenic neuropeptide Y and agouti-related protein were significantly higher in DF, than in NF mice, although levels of plasma leptin that fluctuate in an RF-dependent circadian manner, were significantly higher in DF mice. These findings suggested that the DF induced leptin resistance. The circadian phases of plasma insulin and ghrelin were synchronized to RF, although the corticosterone phase was unaffected. Peak levels of plasma insulin were remarkably higher in DF mice, although HOMA-IR was identical between the two groups. Significantly more free fatty acids, triglycerides and cholesterol accumulated in the livers of DF, than NF mice, which resulted from the increased expression of lipogenic genes such as Scd1, Acaca, and Fasn. Temporal expression of circadian clock genes became synchronized to RF in the liver but not in skeletal muscle, suggesting that uncoupling metabolic rhythms between the liver and skeletal muscle also contribute to DF-induced adiposity.

CONCLUSION

Feeding at an unusual time of day (inactive phase) desynchronizes peripheral clocks and causes obesity and metabolic disorders by inducing leptin resistance, hyperphagia, physical inactivity, hepatic fat accumulation and adiposity.

Deletion of G-protein-coupled receptor 55 promotes obesity by reducing physical activity

BACKGROUND/OBJECTIVES

Cannabinoid receptor 1 (CB1) is the best-characterized cannabinoid receptor, and CB1 antagonists are used in clinical trials to treat obesity. Because of the wide range of CB1 functions, the side effects of CB1 antagonists pose serious concerns. G-protein-coupled receptor 55 (GPR55) is an atypical cannabinoid receptor, and its pharmacology and functions are distinct from CB1. GPR55 regulates neuropathic pain, gut, bone, immune functions and motor coordination. GPR55 is expressed in various brain regions and peripheral tissues. However, the roles of GPR55 in energy and glucose homeostasis are unknown. Here we have investigated the roles of GPR55 in energy balance and insulin sensitivity using GPR55-null mice (GPR55(-/-)).

METHODS

Body composition of the mice was measured by EchoMRI. Food intake, feeding behavior, energy expenditure and physical activity of GPR55(-/-) mice were determined by indirect calorimetry. Muscle function was assessed by forced treadmill running test. Insulin sensitivity was evaluated by glucose and insulin tolerance tests. Adipose inflammation was assessed by flow cytometry analysis of adipose tissue macrophages. The expression of inflammatory markers in adipose tissues and orexigenic/anorexigenic peptides in the hypothalamus was also analyzed by real-time PCR.

RESULTS

GPR55(-/-) mice had normal total energy intake and feeding pattern (i.e., no changes in meal size, meal number or feeding frequency). Intriguingly, whereas adult GPR55(-/-) mice only showed a modest increase in overall body weight, they exhibited significantly increased fat mass and insulin resistance. The spontaneous locomotor activity of GPR55(-/-) mice was dramatically decreased, whereas resting metabolic rate and non-shivering thermogenesis were unchanged. Moreover, GPR55(-/-) mice exhibited significantly decreased voluntary physical activity, showing reduced running distance on the running wheels, whereas muscle function appeared to be normal.

CONCLUSIONS

GPR55 has an important role in energy homeostasis. GPR55 ablation increases adiposity and insulin resistance by selectively decreasing physical activity, but not by altering feeding behavior as CB1.

Period-independent novel circadian oscillators revealed by timed exercise and palatable meals

The mammalian circadian system is a hierarchical network of oscillators organized to optimally coordinate behavior and physiology with daily environmental cycles. The suprachiasmatic nucleus (SCN) of the hypothalamus is at the top of this hierarchy, synchronizing to the environmental light-dark cycle, and coordinates the phases of peripheral clocks. The Period genes are critical components of the molecular timekeeping mechanism of these clocks. Circadian clocks are disabled in Period1/2/3 triple mutant mice, resulting in arrhythmic behavior in constant conditions. We uncovered rhythmic behavior in this mutant by simply exposing the mice to timed access to a palatable meal or running wheel. The emergent circadian behavior rhythms free-ran for many cycles under constant conditions without cyclic environmental cues. Together, these data demonstrate that the palatable meal-inducible circadian oscillator (PICO) and wheel-inducible circadian oscillator (WICO) are generated by non-canonical circadian clocks. Entrainment of these novel oscillators by palatable snacks and timed exercise could become novel therapeutics for human conditions caused by disruptions of the circadian clocks.

Mad men, women and steroid cocktails: a review of the impact of sex and other factors on anabolic androgenic steroids effects on affective behaviors

RATIONALE

For several decades, elite athletes and a growing number of recreational consumers have used anabolic androgenic steroids (AAS) as performance enhancing drugs. Despite mounting evidence that illicit use of these synthetic steroids has detrimental effects on affective states, information available on sex-specific actions of these drugs is lacking.

OBJECTIVES

The focus of this review is to assess information to date on the importance of sex and its interaction with other environmental factors on affective behaviors, with an emphasis on data derived from non-human studies.

METHODS

The PubMed database was searched for relevant studies in both sexes.

RESULTS

Studies examining AAS use in females are limited, reflecting the lower prevalence of use in this sex. Data, however, indicate significant sex-specific differences in AAS effects on anxiety-like and aggressive behaviors, interactions with other drugs of abuse, and the interplay of AAS with other environmental factors such as diet and exercise.

CONCLUSIONS

Current methods for assessing AAS use have limitations that suggest biases of both under- and over-reporting, which may be amplified for females who are poorly represented in self-report studies of human subjects and are rarely used in animal studies. Data from animal literature suggest that there are significant sex-specific differences in the impact of AAS on aggression, anxiety, and concomitant use of other abused substances. These results have relevance for human females who take these drugs as performance-enhancing substances and for transgender XX individuals who may illicitly self-administer AAS as they transition to a male gender identity.

Voluntary Running-Wheel Activity, Arterial Blood Gases, and Thermal Antinociception in Rats after 3 Buprenorphine Formulations

Buprenorphine HCl (BUP) is a μ-opioid agonist used in laboratory rodents. New formulations of buprenorphine (for example, sustained-released buprenorphine [BUP SR], extended-release buprenorphine [BUP ER]) have been developed to extend the analgesic duration. In a crossover design, 8 adult rats were injected subcutaneously with either BUP, BUP SR, BUP ER, or saline, after which voluntary running-wheel activity, arterial blood gases, and thermal withdrawal latency were assessed. Wheel running was decreased at 24 h compared with baseline in all treatment groups but returned to baseline by 48 h. Arterial pH, HCO3(-), and CO2 were not changed between groups or over time. However, arterial oxygen was lower than baseline in the BUP (-8 ± 2 mm Hg), BUP SR (-7 ± 1 mm Hg), and BUP ER (-17 ± 2 mm Hg) groups compared with saline controls (3 ± 2 mm Hg); the BUP ER group showed the greatest decrease when all time points were combined. BUP increased the withdrawal latency at 1 h (15% ± 3%), whereas BUP ER increased latencies at 4, 8, 12, and 48 h (35% ± 11%, 21% ± 7%, 26% ± 7%, and 22% ± 9%, respectively) and BUP SR prolonged latencies at 24, 48, and 72 h (15% ± 6%, 18% ± 5%, and 20% ± 8%, respectively). The duration of thermal analgesia varied between buprenorphine formulations, but all 3 formulations reduced voluntary-running activity at 24 h after injection and might cause hypoxemia in normal adult rats.

Effect of environment on the long-term consequences of chronic pain

Much evidence from pain patients and animal models shows that chronic pain does not exist in a vacuum but has varied comorbidities and far-reaching consequences. Patients with long-term pain often develop anxiety and depression and can manifest changes in cognitive functioning, particularly with working memory. Longitudinal studies in rodent models also show the development of anxiety-like behavior and cognitive changes weeks to months after an injury causing long-term pain. Brain imaging studies in pain patients and rodent models find that chronic pain is associated with anatomical and functional alterations in the brain. Nevertheless, studies in humans reveal that lifestyle choices, such as the practice of meditation or yoga, can reduce pain perception and have the opposite effect on the brain as does chronic pain. In rodent models, studies show that physical activity and a socially enriched environment reduce pain behavior and normalize brain function. Together, these studies suggest that the burden of chronic pain can be reduced by nonpharmacological interventions.

Voluntary exercise during extinction of auditory fear conditioning reduces the relapse of fear associated with potentiated activity of striatal direct pathway neurons

Relapse of previously extinguished fear presents a significant, pervasive obstacle to the successful long-term treatment of anxiety and trauma-related disorders. Thus, identification of a novel means to enhance fear extinction to stand the passage of time and generalize across contexts is of the utmost importance. Acute bouts of exercise can be used as inexpensive, noninvasive treatment strategies to reduce anxiety, and have been shown to enhance memory for extinction when performed in close temporal proximity to the extinction session. However, it is unclear whether acute exercise can be used to prevent relapse of fear, and the neural mechanisms underlying this potential effect are unknown. The current study therefore examined whether acute exercise during extinction of auditory fear can protect against the later relapse of fear. Male F344 rats lacking an extended history of wheel running were conditioned to fear a tone CS and subsequently extinguished within either a freely mobile running wheel, a locked wheel, or a control context lacking a wheel. Rats exposed to fear extinction within a freely mobile wheel ran during fear extinction, and demonstrated reduced fear as well as attenuated corticosterone levels during re-exposure to the extinguished CS during the relapse test in a novel context 1week later. Examination of cfos mRNA patterns elicited by re-exposure to the extinguished CS during the relapse test revealed that acute exercise during extinction decreased activation of brain circuits classically involved in driving fear expression and interestingly, increased activity within neurons of the direct striatal pathway involved in reward signaling. These data suggest that exercise during extinction reduces relapse through a mechanism involving the direct pathway of the striatum. It is suggested that a positive affective state could become associated with the CS during exercise during extinction, thus resulting in a relapse-resistant extinction memory.

Cognitive and behavior deficits in sickle cell mice are associated with profound neuropathologic changes in hippocampus and cerebellum

Strokes are perhaps the most serious complications of sickle cell disease (SCD) and by the fifth decade occur in approximately 25% of patients. While most patients do not develop strokes, mounting evidence indicates that even without brain abnormalities on imaging studies, SCD patients can present profound neurocognitive dysfunction. We sought to evaluate the neurocognitive behavior profile of humanized SCD mice (Townes, BERK) and to identify hematologic and neuropathologic abnormalities associated with the behavioral alterations observed in these mice. Heterozygous and homozygous Townes mice displayed severe cognitive deficits shown by significant delays in spatial learning compared to controls. Homozygous Townes also had increased depression- and anxiety-like behaviors as well as reduced performance on voluntary wheel running compared to controls. Behavior deficits observed in Townes were also seen in BERKs. Interestingly, most deficits in homozygotes were observed in older mice and were associated with worsening anemia. Further, neuropathologic abnormalities including the presence of large bands of dark/pyknotic (shrunken) neurons in CA1 and CA3 fields of hippocampus and evidence of neuronal dropout in cerebellum were present in homozygotes but not control Townes. These observations suggest that cognitive and behavioral deficits in SCD mice mirror those described in SCD patients and that aging, anemia, and profound neuropathologic changes in hippocampus and cerebellum are possible biologic correlates of those deficits. These findings support using SCD mice for studies of cognitive deficits in SCD and point to vulnerable brain areas with susceptibility to neuronal injury in SCD and to mechanisms that potentially underlie those deficits.

Nocturnal to Diurnal Switches with Spontaneous Suppression of Wheel-Running Behavior in a Subterranean Rodent

Several rodent species that are diurnal in the field become nocturnal in the lab. It has been suggested that the use of running-wheels in the lab might contribute to this timing switch. This proposition is based on studies that indicate feed-back of vigorous wheel-running on the period and phase of circadian clocks that time daily activity rhythms. Tuco-tucos (Ctenomys aff. knighti) are subterranean rodents that are diurnal in the field but are robustly nocturnal in laboratory, with or without access to running wheels. We assessed their energy metabolism by continuously and simultaneously monitoring rates of oxygen consumption, body temperature, general motor and wheel running activity for several days in the presence and absence of wheels. Surprisingly, some individuals spontaneously suppressed running-wheel activity and switched to diurnality in the respirometry chamber, whereas the remaining animals continued to be nocturnal even after wheel removal. This is the first report of timing switches that occur with spontaneous wheel-running suppression and which are not replicated by removal of the wheel.

How the Body Talks to the Brain; Peripheral Mediators of Physical Activity-Induced Proliferation in the Adult Hippocampus

In the hippocampal dentate gyrus, stem cells maintain the capacity to produce new neurons into adulthood. These adult-generated neurons become fully functional and are incorporated into the existing hippocampal circuit. The process of adult neurogenesis contributes to hippocampal functioning and is influenced by various environmental, hormonal and disease-related factors. One of the most potent stimuli of neurogenesis is physical activity (PA). While the bodily and peripheral changes of PA are well known, e.g. in relation to diet or cardiovascular conditions, little is known about which of these also exert central effects on the brain. Here, we discuss PA-induced changes in peripheral mediators that can modify hippocampal proliferation, and address changes with age, sex or PA duration/intensity. Of the many peripheral factors known to be triggered by PA, serotonin, FGF-2, IGF-1, VEGF, β-endorphin and adiponectin are best known for their stimulatory effects on hippocampal proliferation. Interestingly, while age negatively affects hippocampal proliferation per se, also the PA-induced response to most of these peripheral mediators is reduced and particularly the response to IGF-1 and NPY strongly declines with age. Sex differences per se have generally little effects on PA-induced neurogenesis. Compared to short term exercise, long term PA may negatively affect proliferation, due to a parallel decline in FGF-2 and the β-endorphin receptor, and an activation of the stress system particularly during conditions of prolonged exercise but this depends on other variables as well and remains a matter of discussion. Taken together, of many possible mediators, serotonin, FGF-2, IGF-1, VEGF, β-endorphin and adiponectin are the ones that most strongly contribute to the central effects of PA on the hippocampus. For a subgroup of these factors, brain sensitivity and responsivity is reduced with age.

Mouse Models of Frailty: an Emerging Field

Frailty is highly prevalent in the elderly, increasing the risk of poor outcomes that include falls, incident disability, hospitalization, and mortality. Thus, a great need exists to characterize the underlying mechanisms and ultimately identify strategies that prevent, delay, and even reverse frailty. Mouse models can provide insight into molecular mechanisms of frailty by reducing variability in lifestyle and genetic factors that can complicate interpretation of human clinical data. Frailty, generally recognized as a syndrome involving reduced homeostatic reserve in response to physiologic challenges and increasing susceptibility to poor health outcomes, is predominantly assessed using two independent strategies, integrated phenotype and deficit accumulation. The integrated phenotype defines frailty by the presentation of factors affecting functional capacity such as weight loss, exhaustion, low activity levels, slow gait, and grip strength. The deficit accumulation paradigm draws parameters from a greater range of physiological systems, such as the ability to perform daily activities, coordination and gait, mental components, physiological problems, and history and presence of medical morbidities. This strategic division also applies within the emerging field of mouse frailty models, with both methodologies showing usefulness in providing insight into physiologic mechanisms and testing interventions. Our review will explore the strategies used, caveats in methodology, and future directions in the application of animal models for the study of the frailty syndrome.

Investigation of pre-pubertal sex differences in wheel running and social behavior in three mouse strains

Sex differences in social behaviors exist in mammals during adulthood, and further evidence suggests that sex differences in behavior are present before sexual maturity. In order to model behavioral disorders in animals, it is important to assess baseline sex-related behavioral differences, especially when studying disorders for which sex-related behavioral effects are expected. We investigated the effect of sex on behavior in 3 strains of pre-pubertal mice (C57BL/6, CFW, and CF1) using a wheel-running assay. We found no significant sex differences in latency to run on the wheel or total duration of wheel running within each strain. During the social interaction test, there were no differences between sexes in latency or total duration of contact or following between a subject and novel mouse. We also evaluated behavioral patterns of wheel running and stereotypical behaviors, such as burrowing and grooming. Both sexes showed characteristic wheel running behavior, spending the majority of each trial interacting with the wheel when it was free and more time performing other activities (e.g., stereotypical behaviors, general locomotion) when it was jammed. These results provide evidence that, among various strains of pre-pubertal mice, baseline sex-related behavioral differences are not strong enough to influence the measured behaviors.

Obesity-related changes in bone structural and material properties in hyperphagic OLETF rats and protection by voluntary wheel running

PURPOSE

To examine how the development of obesity and the associated insulin resistance affect bone structural and material properties, and bone formation and resorption markers in the Otsuka Long-Evans Tokushima Fatty (OLETF) rat model.

METHODS

This was a 36-week study of sedentary, hyperphagic, male OLETF rats (OLETF-SED), exercise-treated OLETF rats (OLETF-EX) and sedentary non-hyperphagic controls (LETO-SED) with data collection at 13, 20, and 40 weeks of age (n = 5-8 animals per group per timepoint).

RESULTS

Body mass and fat (%) were significantly greater in OLETF-SED versus controls. OLETF-SED were insulin resistant at 13 and 20 weeks, with overt diabetes by 40 weeks. At 13weeks, OLETF-SED had lower total body BMC and BMD and serum P1NP compared with LETO-SED. Differences in total body BMC and BMD between OLETF-SED and LETO-SED persisted at 20 weeks, with reductions in total and cortical BMD of the tibia. OLETF-SED also had lesser femur diameter, cross-sectional area, polar moment of area, and torque at fracture than LETO-SED. By 40 weeks, OLETF-SED had elevated bone resorption and reduced intrinsic bone strength. OLETF-EX did not show the excessive weight gain, obesity, insulin resistance or diabetes observed in OLETF-SED. OLETF-EX had greater BMD than OLETF-SED, and structural and material properties of the femur were significantly increased in OLETF-EX relative to OLETF-SED and LETO-SED.

CONCLUSIONS

The negative skeletal effects of excessive adiposity and insulin resistance were evident early in the progressive obesity with lasting negative impacts on intrinsic and extrinsic bone strength. Exercise protected against obesity-associated skeletal changes with marked benefits on the biomechanical properties of bone.

SCORHE: a novel and practical approach to video monitoring of laboratory mice housed in vivarium cage racks

The System for Continuous Observation of Rodents in Home-cage Environment (SCORHE) was developed to demonstrate the viability of compact and scalable designs for quantifying activity levels and behavior patterns for mice housed within a commercial ventilated cage rack. The SCORHE in-rack design provides day- and night-time monitoring with the consistency and convenience of the home-cage environment. The dual-video camera custom hardware design makes efficient use of space, does not require home-cage modification, and is animal-facility user-friendly. Given the system's low cost and suitability for use in existing vivariums without modification to the animal husbandry procedures or housing setup, SCORHE opens up the potential for the wider use of automated video monitoring in animal facilities. SCORHE's potential uses include day-to-day health monitoring, as well as advanced behavioral screening and ethology experiments, ranging from the assessment of the short- and long-term effects of experimental cancer treatments to the evaluation of mouse models. When used for phenotyping and animal model studies, SCORHE aims to eliminate the concerns often associated with many mouse-monitoring methods, such as circadian rhythm disruption, acclimation periods, lack of night-time measurements, and short monitoring periods. Custom software integrates two video streams to extract several mouse activity and behavior measures. Studies comparing the activity levels of ABCB5 knockout and HMGN1 overexpresser mice with their respective C57BL parental strains demonstrate SCORHE's efficacy in characterizing the activity profiles for singly- and doubly-housed mice. Another study was conducted to demonstrate the ability of SCORHE to detect a change in activity resulting from administering a sedative.

Mu opioid receptor modulation in the nucleus accumbens lowers voluntary wheel running in rats bred for high running motivation

The exact role of opioid receptor signaling in mediating voluntary wheel running is unclear. To provide additional understanding, female rats selectively bred for motivation of low (LVR) versus high voluntary running (HVR) behaviors were used. Aims of this study were 1) to identify intrinsic differences in nucleus accumbens (NAc) mRNA expression of opioid-related transcripts and 2) to determine if nightly wheel running is differently influenced by bilateral NAc injections of either the mu-opioid receptor agonist D-Ala2, NMe-Phe4, Glyo5-enkephalin (DAMGO) (0.25, 2.5 μg/side), or its antagonist, naltrexone (5, 10, 20 μg/side). In Experiment 1, intrinsic expression of Oprm1 and Pdyn mRNAs were higher in HVR compared to LVR. Thus, the data imply that line differences in opioidergic mRNA in the NAc could partially contribute to differences in wheel running behavior. In Experiment 2, a significant decrease in running distance was present in HVR rats treated with 2.5 μg DAMGO, or with 10 μg and 20 μg naltrexone between hours 0-1 of the dark cycle. Neither DAMGO nor naltrexone had a significant effect on running distance in LVR rats. Taken together, the data suggest that the high nightly voluntary running distance expressed by HVR rats is mediated by increased endogenous mu-opioid receptor signaling in the NAc, that is disturbed by either agonism or antagonism. In summary, our findings on NAc opioidergic mRNA expression and mu-opioid receptor modulations suggest HVR rats, compared to LVR rats, express higher running levels mediated by an increase in motivation driven, in part, by elevated NAc opioidergic signaling.

Effects of voluntary exercise on spontaneous physical activity and food consumption in mice: Results from an artificial selection experiment

We evaluated the effect of voluntary exercise on spontaneous physical activity (SPA) and food consumption in mice from 4 replicate lines bred for 57 generations for high voluntary wheel running (HR) and from 4 non-selected control (C) lines. Beginning at ~24 days of age, mice were housed in standard cages or in cages with attached wheels. Wheel activity and SPA were monitored in 1-min intervals. Data from the 8th week of the experiment were analyzed because mice were sexually mature and had plateaued in body mass, weekly wheel running distance, SPA, and food consumption. Body mass, length, and masses of the retroperitoneal fat pad, liver, and heart were recorded after the 13th week. SPA of both HR and C mice decreased with wheel access, due to reductions in both duration and average intensity of SPA. However, total activity duration (SPA+wheel running; min/day) was ~1/3 greater when mice were housed with wheels, and food consumption was significantly increased. Overall, food consumption in both HR and C mice was more strongly affected by wheel running than by SPA. Duration of wheel running had a stronger effect than average speed, but the opposite was true for SPA. With body mass as a covariate, chronic wheel access significantly reduced fat pad mass and increased heart mass in both HR and C mice. Given that both HR and C mice housed with wheels had increased food consumption, the energetic cost of wheel running was not fully compensated by concomitant reductions in SPA. The experiment demonstrates that both duration and intensity of both wheel running and SPA were significant predictors of food consumption. This sort of detailed analysis of the effects of different aspects of physical activity on food consumption has not previously been reported for a non-human animal, and it sets the stage for longitudinal examination of energy balance and its components in rodent models.

Food-anticipatory activity in Syrian hamsters: behavioral and molecular responses in the hypothalamus according to photoperiodic conditions

When food availability is restricted, animals adjust their behavior according to the timing of food access. Most rodents, such as rats and mice, and a wide number of other animals express before timed food access a bout of activity, defined as food-anticipatory activity (FAA). One notable exception amongst rodents is the Syrian hamster, a photoperiodic species that is not prone to express FAA. The present study was designed to understand the reasons for the low FAA in that species. First, we used both wheel-running activity and general cage activity to assess locomotor behavior. Second, the possible effects of photoperiod was tested by challenging hamsters with restricted feeding under long (LP) or short (SP) photoperiods. Third, because daytime light may inhibit voluntary activity, hamsters were also exposed to successive steps of full and skeleton photoperiods (two 1-h light pulses simulating dawn and dusk). When hamsters were exposed to skeleton photoperiods, not full photoperiod, they expressed FAA in the wheel independently of daylength, indicating that FAA in the wheel is masked by daytime light under full photoperiods. During FAA under skeleton photoperiods, c-Fos expression was increased in the arcuate nuclei independently of the photoperiod, but differentially increased in the ventromedial and dorsomedial hypothalamic nuclei according to the photoperiod. FAA in general activity was hardly modulated by daytime light, but was reduced under SP. Together, these findings show that food-restricted Syrian hamsters are not prone to display FAA under common laboratory conditions, because of the presence of light during daytime that suppresses FAA expression in the wheel.

Deletion of Metabotropic Glutamate Receptors 2 and 3 (mGlu2 & mGlu3) in Mice Disrupts Sleep and Wheel-Running Activity, and Increases the Sensitivity of the Circadian System to Light

Sleep and/or circadian rhythm disruption (SCRD) is seen in up to 80% of schizophrenia patients. The co-morbidity of schizophrenia and SCRD may in part stem from dysfunction in common brain mechanisms, which include the glutamate system, and in particular, the group II metabotropic glutamate receptors mGlu2 and mGlu3 (encoded by the genes Grm2 and Grm3). These receptors are relevant to the pathophysiology and potential treatment of schizophrenia, and have also been implicated in sleep and circadian function. In the present study, we characterised the sleep and circadian rhythms of Grm2/3 double knockout (Grm2/3^-/-) mice, to provide further evidence for the involvement of group II metabotropic glutamate receptors in the regulation of sleep and circadian rhythms. We report several novel findings. Firstly, Grm2/3^-/- mice demonstrated a decrease in immobility-determined sleep time and an increase in immobility-determined sleep fragmentation. Secondly, Grm2/3^-/- mice showed heightened sensitivity to the circadian effects of light, manifested as increased period lengthening in constant light, and greater phase delays in response to nocturnal light pulses. Greater light-induced phase delays were also exhibited by wildtype C57Bl/6J mice following administration of the mGlu2/3 negative allosteric modulator RO4432717. These results confirm the involvement of group II metabotropic glutamate receptors in photic entrainment and sleep regulation pathways. Finally, the diurnal wheel-running rhythms of Grm2/3^-/- mice were perturbed under a standard light/dark cycle, but their diurnal rest-activity rhythms were unaltered in cages lacking running wheels, as determined with passive infrared motion detectors. Hence, when assessing the diurnal rest-activity rhythms of mice, the choice of assay can have a major bearing on the results obtained.

Mechanisms of chemotherapy-induced behavioral toxicities

While chemotherapeutic agents have yielded relative success in the treatment of cancer, patients are often plagued with unwanted and even debilitating side-effects from the treatment which can lead to dose reduction or even cessation of treatment. Common side effects (symptoms) of chemotherapy include (i) cognitive deficiencies such as problems with attention, memory and executive functioning; (ii) fatigue and motivational deficit; and (iii) neuropathy. These symptoms often develop during treatment but can remain even after cessation of chemotherapy, severely impacting long-term quality of life. Little is known about the underlying mechanisms responsible for the development of these behavioral toxicities, however, neuroinflammation is widely considered to be one of the major mechanisms responsible for chemotherapy-induced symptoms. Here, we critically assess what is known in regards to the role of neuroinflammation in chemotherapy-induced symptoms. We also argue that, based on the available evidence, neuroinflammation is unlikely the only mechanism involved in the pathogenesis of chemotherapy-induced behavioral toxicities. We evaluate two other putative candidate mechanisms. To this end we discuss the mediating role of damage-associated molecular patterns (DAMPs) activated in response to chemotherapy-induced cellular damage. We also review the literature with respect to possible alternative mechanisms such as a chemotherapy-induced change in the bioenergetic status of the tissue involving changes in mitochondrial function in relation to chemotherapy-induced behavioral toxicities. Understanding the mechanisms that underlie the emergence of fatigue, neuropathy, and cognitive difficulties is vital to better treatment and long-term survival of cancer patients.

Molecular and metabolomic effects of voluntary running wheel activity on skeletal muscle in late middle-aged rats

We examined the molecular and metabolomic effects of voluntary running wheel activity in late middle-aged male Sprague Dawley rats (16–17 months). Rats were assigned either continuous voluntary running wheel access for 8 weeks (RW+) or cage-matched without running wheel access (RW−). The 9 RW+ rats averaged 83 m/day (range: 8–163 m), yet exhibited both 84% reduced individual body weight gain (4.3 g vs. 26.3 g, P = 0.02) and 6.5% reduced individual average daily food intake (20.6 g vs. 22.0 g, P = 0.09) over the 8 weeks. Hindlimb muscles were harvested following an overnight fast. Muscle weights and myofiber cross-sectional area showed no difference between groups. Western blots of gastrocnemius muscle lysates with a panel of antibodies suggest that running wheel activity improved oxidative metabolism (53% increase in PGC1α, P = 0.03), increased autophagy (36% increase in LC3B-II/-I ratio, P = 0.03), and modulated growth signaling (26% increase in myostatin, P = 0.04). RW+ muscle also showed 43% increased glycogen phosphorylase expression (P = 0.04) and 45% increased glycogen content (P = 0.04). Metabolomic profiling of plantaris and soleus muscles indicated that even low-volume voluntary running wheel activity is associated with decreases in many long-chain fatty acids (e.g., palmitoleate, myristoleate, and eicosatrienoate) relative to RW− rats. Relative increases in acylcarnitines and acyl glycerophospholipids were also observed in RW+ plantaris. These data establish that even modest amounts of physical activity during late middle-age promote extensive metabolic remodeling of skeletal muscle.

Free access to a running-wheel advances the phase of behavioral and physiological circadian rhythms and peripheral molecular clocks in mice

Behavioral and physiological circadian rhythms are controlled by endogenous oscillators in animals. Voluntary wheel-running in rodents is thought to be an appropriate model of aerobic exercise in humans. We evaluated the effects of chronic voluntary exercise on the circadian system by analyzing temporal profiles of feeding, core body temperature, plasma hormone concentrations and peripheral expression of clock and clock-controlled genes in mice housed under sedentary (SED) conditions or given free access to a running-wheel (RW) for four weeks. Voluntary wheel-running activity advanced the circadian phases of increases in body temperature, food intake and corticosterone secretion in the mice. The circadian expression of clock and clock-controlled genes was tissue- and gene-specifically affected in the RW mice. The temporal expression of E-box-dependent circadian clock genes such as Per1, Per2, Nr1d1 and Dbp were slightly, but significantly phase-advanced in the liver and white adipose tissue, but not in brown adipose tissue and skeletal muscle. Peak levels of Per1, Per2 and Nr1d1 expression were significantly increased in the skeletal muscle of RW mice. The circadian phase and levels of hepatic mRNA expression of the clock-controlled genes that are involved in cholesterol and fatty acid metabolism significantly differed between SED and RW mice. These findings indicated that endogenous clock-governed voluntary wheel-running activity provides feedback to the central circadian clock that systemically governs behavioral and physiological rhythms.

Tumor growth increases neuroinflammation, fatigue and depressive-like behavior prior to alterations in muscle function

Cancer patients frequently suffer from fatigue, a complex syndrome associated with loss of muscle mass, weakness, and depressed mood. Cancer-related fatigue (CRF) can be present at the time of diagnosis, during treatment, and persists for years after treatment. CRF negatively influences quality of life, limits functional independence, and is associated with decreased survival in patients with incurable disease. Currently there are no effective treatments to reduce CRF. The aim of this study was to use a mouse model of tumor growth and discriminate between two main components of fatigue: loss of muscle mass/function and altered mood/motivation. Here we show that tumor growth increased fatigue- and depressive-like behaviors, and reduced body and muscle mass. Decreased voluntary wheel running activity (VWRA) and increased depressive-like behavior in the forced swim and sucrose preference tests were evident in tumor-bearing mice within the first two weeks of tumor growth and preceded the loss of body and muscle mass. At three weeks, tumor-bearing mice had reduced grip strength but this was not associated with altered expression of myosin isoforms or impaired contractile properties of muscles. These increases in fatigue and depressive-like behaviors were paralleled by increased expression of IL-1β mRNA in the cortex and hippocampus. Minocycline administration reduced tumor-induced expression of IL-1β in the brain, reduced depressive-like behavior, and improved grip strength without altering muscle mass. Taken together, these results indicate that neuroinflammation and depressed mood, rather than muscle wasting, contribute to decreased voluntary activity and precede major changes in muscle contractile properties with tumor growth.

Sleep and circadian rhythm disruption and recognition memory in schizophrenia

Schizophrenia patients often show irregularities in sleep and circadian rhythms and deficits in recognition memory. Similar phenotypes are seen in schizophrenia-relevant genetic mouse models, such as synaptosomal associated protein of 25 kDa (Snap-25) point mutant mice, vasoactive intestinal peptide receptor 2 (Vipr2) knockout mice, and neuregulin 1 (Nrg1)-deficient mice. Sleep and circadian abnormalities and impaired recognition memory may be causally related in both schizophrenia patients and schizophrenia-relevant mouse models, since sleep deprivation, abnormal photic input, and the manipulation of core clock genes (cryptochrome 1/2) can all disrupt object recognition memory in rodent models. The recognition deficits observed in patients and mouse models (both schizophrenia-related and -unrelated) are discussed here in terms of the dual-process theory of recognition, which postulates that there are two recognition mechanisms-recollection versus familiarity-that can be selectively impaired by brain lesions, neuropsychiatric conditions, and putatively, sleep and circadian rhythm disruption. However, based on this view, the findings from patient studies and studies using genetic mouse models (Nrg1 deficiency) seem to be inconsistent with each other. Schizophrenia patients are impaired at recollection (and to a lesser extent, familiarity judgments), but Nrg1-deficient mice are impaired at familiarity-based object recognition, raising concerns regarding the validity of using these genetically modified mice to model recognition phenotypes observed in patients. This issue can be resolved in future animal studies by examining performance in different variants of the spontaneous recognition task-the standard, perirhinal cortex-dependent, object recognition task versus the hippocampus-dependent object-place recognition task-in order to see which of the two recognition mechanisms is more disrupted.

Role of the dorsal medial habenula in the regulation of voluntary activity, motor function, hedonic state, and primary reinforcement

The habenular complex in the epithalamus consists of distinct regions with diverse neuronal populations. Past studies have suggested a role for the habenula in voluntary exercise motivation and reinforcement of intracranial self-stimulation but have not assigned these effects to specific habenula subnuclei. Here, we have developed a genetic model in which neurons of the dorsal medial habenula (dMHb) are developmentally eliminated, via tissue-specific deletion of the transcription factor Pou4f1 (Brn3a). Mice with dMHb lesions perform poorly in motivation-based locomotor behaviors, such as voluntary wheel running and the accelerating rotarod, but show only minor abnormalities in gait and balance and exhibit normal levels of basal locomotion. These mice also show deficits in sucrose preference, but not in the forced swim test, two measures of depression-related phenotypes in rodents. We have also used Cre recombinase-mediated expression of channelrhodopsin-2 and halorhodopsin to activate dMHb neurons or silence their output in freely moving mice, respectively. Optical activation of the dMHb in vivo supports intracranial self-stimulation, showing that dMHb activity is intrinsically reinforcing, whereas optical silencing of dMHb outputs is aversive. Together, our findings demonstrate that the dMHb is involved in exercise motivation and the regulation of hedonic state, and is part of an intrinsic reinforcement circuit.

Clinically relevant frailty index for mice

Frailty is a clinical syndrome associated with the aging process and adverse outcomes. The purpose of this short report was to initiate the development of a Frailty Index in 27- to 28-month-old C57BL/6 mice that matched the clinical criteria used in humans (weakness, slow walking speed, low activity level, poor endurance). The selected criteria included grip strength, walking speed, physical activity, and endurance. The criteria in mice were evaluated by the inverted-cling grip test, rotarod test, voluntary wheel running, and derived endurance scores. Each criterion had a designated cutoff point (1.5 SD below the cohort mean) to identify the mice with the lowest performance. If a mouse presented with three of the criteria scores below the cutoff points, it was identified as frail. Mild frailty was designated if two criteria were below the cutoff points. In this mouse cohort, one mouse was identified as frail and one was mildly frail. This prevalence of 9% frailty is consistent with the prevalence of frailty in humans at the same survival age. Collectively, our selected criterion, cutoff point, and Frailty Index provide a potential standardized definition for frailty in mice that is consistent with the operational definition of frailty in humans.

Physically active rats lose more weight during calorie restriction

Daily physical activity shows substantial inter-individual variation, and low physical activity is associated with obesity and weight gain. Elevated physical activity is also associated with high intrinsic aerobic capacity, which confers considerable metabolic health benefits. Rats artificially selected for high intrinsic aerobic capacity (high-capacity runners, HCR) are more physically active than their low-capacity counterparts (low-capacity runners, LCR). To test the hypothesis that physical activity counters metabolic thriftiness, we measured physical activity and weight loss during three weeks of 50% calorie restriction (CR) in the HCR and LCR rat lines. At baseline, HCR ate more and were more active than LCR; this was seen in male rats, where LCR are considerably heavier than HCR, as well as in a set of female rats where body weight did not differ between the lines, demonstrating that this effect is consistent across sex and not secondary to body weight. We show for the first time that HCR lose more weight than LCR relative to baseline. Physical activity levels declined throughout CR, and this was more pronounced in HCR than in LCR, yet some aspects of activity remained elevated in HCR relative to LCR even during CR. This is consistent with the idea that low physical activity contributes to metabolic thriftiness during food restriction, allowing LCR to defend body mass, particularly lean mass. This has implications for physical activity during diet-induced weight loss, the genetic underpinnings of individual differences in weight loss during a diet, and the potential evolutionary opposition between metabolic thriftiness and aerobic capacity.

Compensatory elevation of voluntary activity in mouse mutants with impaired mitochondrial energy metabolism

Mitochondria play a crucial role in determining whole‐body metabolism and exercise capacity. Genetic mouse models of mild mitochondrial dysfunction provide an opportunity to understand how mitochondrial function affects these parameters. MCLK1 (a.k.a. Coq7) is an enzyme implicated in the biosynthesis of ubiquinone (UQ; Coenzyme Q). Low levels of MCLK1 in Mclk1^+/− heterozygous mutants lead to abnormal sub‐mitochondrial distribution of UQ, impaired mitochondrial function, elevated mitochondrial oxidative stress, and increased lifespan. Here, we report that young Mclk1^+/− males, but not females, show a significant decrease in whole‐body metabolic rate as measured by indirect calorimetry. Such a sex‐specific effect of mitochondrial dysfunction on energy metabolism has also been reported for heterozygous mice carrying a mutation for the gene encoding the “Rieske” protein of mitochondrial complex III (RISP^+/P224S). We find that both Mclk1^+/− and RISP^+/P224S males are capable of restoring their defective metabolic rates by making significantly more voluntary use of a running wheel compared to wild type. However, this increase in voluntary activity does not reflect their exercise capacity, which we found to be impaired as revealed by a shorter treadmill distance run before exhaustion. In contrast to what is observed in Mclk1^+/− and RISP^+/P224S mutants, Sod2^+/− mice with elevated oxidative stress and major mitochondrial dysfunction did not increase voluntary activity. Our study reveals a sex‐specific effect on how impaired mitochondrial function impacts whole‐body energy metabolism and locomotory behavior, and contributes to the understanding of the metabolic and behavioral consequences of mitochondrial disorders.

Mitochondria play a crucial role in determining whole‐body metabolism, lifespan and exercise capacity. This study reports sex‐specific effects of mitochondrial dysfunction, resulting in increased spontaneous activity in response to impaired metabolic rates. These findings contribute to the understanding of the metabolic and behavioral consequences of mitochondrial disorders.

Dopamine D1/D2 receptors do not mediate the expression of conditioned place preference induced by the aftereffect of wheel running

BACKGROUND

Rats lever-press for access to running wheels suggesting that wheel running by itself is reinforcing. Furthermore, pairings of an episode of wheel running and subsequent confinement in a specific environment can establish a conditioned place preference (CPP). This finding implies that the reinforcing effects of wheel running outlast the actual occurrence of physical activity, a phenomenon referred to as aftereffect of wheel running. Aftereffect-induced CPP involves Pavlovian conditioning, i.e. repeated pairings of the aftereffect of wheel running with a specific environment creates a learned association between aftereffect and environment and, in turn, a preference for that environment. Given the involvement of dopamine systems in mediating effects of Pavlovian stimuli on appetitive behavior, a role of dopamine in mediating aftereffect-induced CPP seems plausible. Here we assessed whether the mixed D1/D2 receptor antagonist flupenthixol (0.25 mg/kg, i.p.) can block the expression of an aftereffect-induced CPP.

RESULTS

In line with earlier studies, our results demonstrate that rats displayed a conditioned preference for environments paired with the aftereffect of wheel running and further show that the magnitude of CPP was not related to the wheel running rate. Furthermore, we found that flupenthixol (0.25 mg/kg, i.p.) reduced locomotor activity but did not attenuate the expression of an aftereffect-induced CPP.

CONCLUSION

The expression of a CPP produced by the aftereffect of wheel running seems not to depend on dopamine D1/D2 receptor activation.

Diet and sex modify exercise and cardiac adaptation in the mouse

The heart adapts to exercise stimuli in a sex-dimorphic manner when mice are fed the traditional soy-based chow. Females undergo more voluntary exercise (4 wk) than males and exhibit more cardiac hypertrophy per kilometer run (18, 32). We have found that diet plays a critical role in cage wheel exercise and cardiac adaptation to the exercise stimulus in this sex dimorphism. Specifically, feeding male mice a casein-based, soy-free diet increases daily running distance over soy-fed counterparts to equal that of females. Moreover, casein-fed males have a greater capacity to increase their cardiac mass in response to exercise compared with soy-fed males. To further explore the biochemical mechanisms for these differences, we performed a candidate-based RT-PCR screen on genes previously implicated in diet- or exercise-based cardiac hypertrophy. Of the genes screened, many exhibit significant exercise, diet, or sex effects but only transforming growth factor-β1 shows a significant three-way interaction with no genes showing a two-way interaction. Finally, we show that the expression and activity of adenosine monophosphate-activated kinase-α2 and acetyl-CoA carboxylase is dependent on exercise, diet, and sex.

Relaxin-3 receptor (Rxfp3) gene knockout mice display reduced running wheel activity: implications for role of relaxin-3/RXFP3 signalling in sustained arousal

Anatomical and pharmacological evidence suggests the neuropeptide, relaxin-3, is the preferred endogenous ligand for the relaxin family peptide-3 receptor (RXFP3) and suggests a number of putative stress- and arousal-related roles for RXFP3 signalling. However, in vitro and in vivo evidence demonstrates exogenous relaxin-3 can activate other relaxin peptide family receptors, and the role of relaxin-3/RXFP3 signalling in specific brain circuits and associated behaviours in mice is not well described. In this study, we characterised the behaviour of cohorts of male and female Rxfp3 gene knockout (KO) mice (C57/B6J(RXFP3TM1/DGen)), relative to wild-type (WT) littermates to determine if this receptor KO strain has a similar phenotype to its ligand KO equivalent. Rxfp3 KO mice displayed similar performance to WT littermates in several acute behavioural paradigms designed to gauge motor coordination (rotarod test), spatial memory (Y-maze), depressive-like behaviour (repeat forced-swim test) and sensorimotor gating (prepulse inhibition of acoustic startle). Notably however, male and female Rxfp3 KO mice displayed robust and consistent (dark phase) hypoactivity on voluntary home-cage running wheels (∼20-60% less activity/h), and a small but significant decrease in anxiety-like behavioural traits in the elevated plus maze and light/dark box paradigms. Importantly, this phenotype is near identical to that observed in two independent lines of relaxin-3 KO mice, suggesting these phenotypes are due to the elimination of ligand or receptor and RXFP3-linked signalling. Furthermore, this behavioural characterisation of Rxfp3 KO mice identifies them as a useful experimental model for studying RXFP3-linked signalling and assessing the selectivity and/or potential off-target actions of RXFP3 agonists and antagonists, which could lead to an improved understanding of dysfunctional arousal in mental health disorders, including depression, anxiety, insomnia and neurodegenerative diseases.

Disrupted daily light-dark cycles induce physical inactivity and enhance weight gain in mice depending on dietary fat intake

We evaluated associations between obesity induced by a high-fat diet (HFD) and the environmental light-dark (LD) cycle that entrains the master circadian clock located in the suprachiasmatic nucleus of mammals. Mice were fed normal diet or HFD for 6 weeks in individual cages with running wheels under a normal 12 h light-12 h dark cycle (LD 12 : 12) or an ultradian 3 h light-3 h dark cycle (LD 3 : 3) that might perturb the central clock. Circadian behavioral rhythms in mice fed both diets were disrupted by light-induced direct suppression of the behavior (masking effect) under LD 3 : 3. The ultradian LD cycle reduced the total daily activity of wheel running and enhanced body weight gain in the mice fed the HFD. Secondary effects such as obesity are probably not associated with inactivity induced under these circumstances because wheel-running activity decreased markedly within a few days of transfer from LD 12 : 12 to LD 3 : 3. Food consumption was significantly suppressed under LD 3 : 3 in mice fed the HFD. These findings suggest that the aberrant LD cycle induced physical inactivity and enhanced weight gain depending on dietary fat consumption. This might help to explain the higher incidence of obesity among shift workers.

Dietary heat-killed Lactobacillus brevis SBC8803 promotes voluntary wheel-running and affects sleep rhythms in mice

AIMS

We previously reported that heat-killed Lactobacillus brevis SBC8803 enhances appetite via changes in autonomic neurotransmission. Here we assessed whether a diet supplemented with heat-killed SBC8803 affects circadian locomotor rhythmicity and sleep architecture.

MAIN METHODS AND KEY FINDINGS

Daily total activity gradually increased in mice over 4 weeks and supplementation with heat-killed SBC8803 significantly intensified the increase, which reached saturation at 25 days. Electroencephalography revealed that SBC8803 supplementation significantly reduced the total amount of time spent in non-rapid eye movement (NREM) sleep and increased the amount of time spent being awake during the latter half of the nighttime, but tended to increase the total amount of time spent in NREM sleep during the daytime. Dietary supplementation with SBC8803 can extend the duration of activity during the nighttime and of sleep during the daytime. Daily voluntary wheel-running and sleep rhythmicity become intensified when heat-killed SBC8803 is added to the diet.

SIGNIFICANCE

Dietary heat-killed SBC8803 can modulate circadian locomotion and sleep rhythms, which might benefit individuals with circadian rhythms that have been disrupted by stress or ageing.

Wheel running in the wild

The importance of exercise for health and neurogenesis is becoming increasingly clear. Wheel running is often used in the laboratory for triggering enhanced activity levels, despite the common objection that this behaviour is an artefact of captivity and merely signifies neurosis or stereotypy. If wheel running is indeed caused by captive housing, wild mice are not expected to use a running wheel in nature. This however, to our knowledge, has never been tested. Here, we show that when running wheels are placed in nature, they are frequently used by wild mice, also when no extrinsic reward is provided. Bout lengths of running wheel behaviour in the wild match those for captive mice. This finding falsifies one criterion for stereotypic behaviour, and suggests that running wheel activity is an elective behaviour. In a time when lifestyle in general and lack of exercise in particular are a major cause of disease in the modern world, research into physical activity is of utmost importance. Our findings may help alleviate the main concern regarding the use of running wheels in research on exercise.

Sex and exercise interact to alter the expression of anabolic androgenic steroid-induced anxiety-like behaviors in the mouse

Anabolic androgenic steroids (AAS) are taken by both sexes to enhance athletic performance and body image, nearly always in conjunction with an exercise regime. Although taken to improve physical attributes, chronic AAS use can promote negative behavior, including anxiety. Few studies have directly compared the impact of AAS use in males versus females or assessed the interaction of exercise and AAS. We show that AAS increase anxiety-like behaviors in female but not male mice and that voluntary exercise accentuates these sex-specific differences. We also show that levels of the anxiogenic peptide corticotrophin releasing factor (CRF) are significantly greater in males, but that AAS selectively increase CRF levels in females, thus abrogating this sex-specific difference. Exercise did not ameliorate AAS-induced anxiety or alter CRF levels in females. Exercise was anxiolytic in males, but this behavioral outcome did not correlate with CRF levels. Brain-derived neurotrophic factor (BDNF) has also been implicated in the expression of anxiety. As with CRF, levels of hippocampal BDNF mRNA were significantly greater in males than females. AAS and exercise were without effect on BDNF mRNA in females. In males, anxiolytic effects of exercise correlated with increased BDNF mRNA, however AAS-induced changes in BDNF mRNA and anxiety did not. In sum, we find that AAS elicit sex-specific differences in anxiety and that voluntary exercise accentuates these differences. In addition, our data suggest that these behavioral outcomes may reflect convergent actions of AAS and exercise on a sexually differentiated CRF signaling system within the extended amygdala.

Behavioral Masking and cFos Responses to Light in Day- and Night-Active Grass Rats

Light not only entrains the circadian system but also has acute effects on physiology and behavior, a phenomenon known as masking. Behavioral masking responses to bright light differ in diurnal and nocturnal species, such that light increases arousal in the former and decreases it in the latter. Comparisons made within a species that displays both diurnal and nocturnal patterns of behavior may provide insight into how masking differs between chronotypes and the association between mechanisms controlling masking and the circadian drive for activity. Nile grass rats ( Arvicanthis niloticus) provide a useful model for studying such issues because when these animals are housed with running wheels, some run primarily during day, while others run at night. Here we compared behavioral masking responses to 2-h pulses of light and darkness given across a 12:12 light/dark cycle in day-active (DA) and night-active (NA) grass rats. Both wheel-running activity (WRA) and general activity (GA) were monitored. Light pulses at night tended to increase both WRA and GA overall in the DA grass rats, while in NA grass rats, light pulses significantly reduced WRA but had no effect on GA. Dark pulses during the day tended to decrease both WRA and GA in the DA grass rats, while in the NA grass rats, they tended to increase WRA in the early day but had no effect on GA overall. Next, we measured cFos expression within 2 brain areas potentially involved in masking, the intergeniculate leaflet (IGL) and the olivary pretectal area (OPT), of DA and NA grass rats either sacrificed on a control night or after a 1-h light pulse at ZT14. In DA grass rats, light at ZT14 induced cFos in the IGL and OPT, whereas in NA grass rats, cFos levels in both structures were high at ZT14 and were not altered by a 1-h light pulse. Overall, these results suggest that masking responses to light and darkness are dependent on the chronotype of the individual and that the responsiveness of the IGL and OPT to light may depend on or contribute to the behavioral response of these animals.

Estrous cycle fluctuations in sex and ingestive behavior are accentuated by exercise or cold ambient temperatures

This article is part of a Special Issue "Energy Balance". In female Syrian hamsters (Mesocricetus auratus), low circulating levels of ovarian steroids are associated with increased food hoarding and decreased sexual motivation, but these effects are exaggerated in food-restricted females. To determine whether cold ambient temperature has the same effects as food restriction, groups of hamsters were fed ad libitum while they were housed at either 5 °C or 22 °C, and then tested for behavior for 90 min on each day of the estrous cycle. In females housed at 22 °C, high levels of sexual motivation and low levels of food hoarding were seen every day of the estrous cycle. In females housed at 5 °C, high levels of sexual motivation were restricted to the periovulatory day. On the three nonestrous days, these females showed high levels of food hoarding, but not food intake. A separate cohort of females were provided with access to running wheels and housed at 22 °C. They showed high levels of sexual motivation restricted to the periovulatory day, similar to the pattern of sexual motivation seen in cold-housed females. Unlike cold-housed females, those with running wheels showed low levels of food hoarding and high levels of food intake. Food restriction, cold housing, and access to wheels had no significant effect on plasma estradiol or progesterone concentrations, but significantly decreased plasma leptin concentrations. All three energetic challenges unmask estrous cycle fluctuations in sexual motivation that are obscured in laboratory conditions, i.e., isolation in a small cage with an overabundance of food.

Voluntary exercise adapts the hypothalamus-pituitary-thyroid axis in male rats

The hypothalamic-pituitary thyroid (HPT) axis modulates energy homeostasis. Its activity decreases in conditions of negative energy balance but the effects of chronic exercise on the axis are controversial and unknown at hypothalamic level. Wistar male rats were exposed for up to 14 days to voluntary wheel running (WR), or pair-feeding (PF; 18% food restriction), or to repeated restraint (RR), a mild stressor. WR and RR diminished food intake; body weight gain decreased in the 3 experimental groups, but WAT mass and serum leptin more intensely in the WR group. WR, but not RR, produced a delayed inhibition of central markers of HPT axis activity. At day 14, in WR rats paraventricular nucleus-pro-TRH mRNA and serum TSH levels decreased, anterior pituitary TRH-receptor 1 mRNA levels increased, but serum thyroid hormone levels were unaltered, which is consistent with decreased secretion of TRH and clearance of thyroid hormones. A similar pattern was observed if WR animals were euthanized during their activity phase. In contrast, in PF animals the profound drop of HPT axis activity included decreased serum T3 levels and hepatic deiodinase 1 activity; these changes were correlated with an intense increase in serum corticosterone levels. WR effects on HPT axis were not associated with changes in the activity of the hypothalamic-pituitary adrenal axis, but correlated positively with serum leptin levels. These data demonstrate that voluntary WR adapts the status of the HPT axis, through pathways that are distinct from those observed during food restriction or repeated stress.

Methodological considerations for measuring spontaneous physical activity in rodents

When exploring biological determinants of spontaneous physical activity (SPA), it is critical to consider whether methodological factors differentially affect rodents and the measured SPA. We determined whether acclimation time, sensory stimulation, vendor, or chamber size affected measures in rodents with varying propensity for SPA. We used principal component analysis to determine which SPA components (ambulatory and vertical counts, time in SPA, and distance traveled) best described the variability in SPA measurements. We compared radiotelemetry and infrared photobeams used to measure SPA and exploratory activity. Acclimation time, sensory stimulation, vendor, and chamber size independently influenced SPA, and the effect was moderated by the propensity for SPA. A 24-h acclimation period prior to SPA measurement was sufficient for habituation. Principal component analysis showed that ambulatory and vertical measurements of SPA describe different dimensions of the rodent's SPA behavior. Smaller testing chambers and a sensory attenuation cubicle around the chamber reduced SPA. SPA varies between rodents purchased from different vendors. Radiotelemetry and infrared photobeams differ in their sensitivity to detect phenotypic differences in SPA and exploratory activity. These data highlight methodological considerations in rodent SPA measurement and a need to standardize SPA methodology.

Leanness and heightened nonresting energy expenditure: role of skeletal muscle activity thermogenesis

A high-calorie diet accompanied by low levels of physical activity (PA) accounts for the widespread prevalence of obesity today, and yet some people remain lean even in this obesogenic environment. Here, we investigate the cause for this exception. A key trait that predicts high PA in both humans and laboratory rodents is intrinsic aerobic capacity. Rats artificially selected as high-capacity runners (HCR) are lean and consistently more physically active than their low-capacity runner (LCR) counterparts; this applies to both males and females. Here, we demonstrate that HCR show heightened total energy expenditure (TEE) and hypothesize that this is due to higher nonresting energy expenditure (NREE; includes activity EE). After matching for body weight and lean mass, female HCR consistently had heightened nonresting EE, but not resting EE, compared with female LCR. Because of the dominant role of skeletal muscle in nonresting EE, we examined muscle energy use. We found that lean female HCR had higher muscle heat dissipation during activity, explaining their low economy of activity and high activity EE. This may be due to the amplified skeletal muscle expression levels of proteins involved in EE and reduced expression levels of proteins involved in energy conservation in HCR relative to LCR. This is also associated with an increased sympathetic drive to skeletal muscle in HCR compared with LCR. We find little support for the hypothesis that resting metabolic rate is correlated with maximal aerobic capacity if body size and composition are fully considered; rather, the critical factor appears to be activity thermogenesis.