A brief opportunity to run does not function as a reinforcer for mice selected for high daily wheel-running rates

A meta-analysis of whole-body and heart mass effect sizes from a long-term artificial selection experiment for high voluntary exercise

Selection experiments play an increasingly important role in comparative and evolutionary physiology. However, selection experiments can be limited by relatively low statistical power, in part because replicate line is the experimental unit for analyses of direct or correlated responses (rather than number of individuals measured). One way to increase the ability to detect correlated responses is through a meta-analysis of studies for a given trait across multiple generations. To demonstrate this, we applied meta-analytic techniques to two traits (body mass and heart ventricle mass, with body mass as a covariate) from a long-term artificial selection experiment for high voluntary wheel-running behavior. In this experiment, all four replicate High Runner (HR) lines reached apparent selection limits around generations 17-27, running approximately 2.5- to 3-fold more revolutions per day than the four non-selected Control (C) lines. Although both traits would also be expected to change in HR lines (relative heart size expected to increase, expected direction for body mass is less clear), their statistical significance has varied, despite repeated measurements. We compiled information from 33 unique studies and calculated a measure of effect size (Pearson's R). Our results indicate that, despite a lack of statistical significance in most generations, HR mice have evolved larger hearts and smaller bodies relative to controls. Moreover, plateaus in effect sizes for both traits coincide with the generational range during which the selection limit for wheel-running behavior was reached. Finally, since reaching the selection limit, absolute effect sizes for body mass and heart ventricle mass have become smaller (i.e. closer to 0).

Effects of food restriction on voluntary wheel-running behavior and body mass in selectively bred High Runner lines of mice

Food restriction can have profound effects on various aspects of behavior, physiology, and morphology. Such effects might be amplified in animals that are highly active, given that physical activity can represent a substantial fraction of the total daily energy budget. More specifically, some effects of food restriction could be associated with intrinsic, genetically based differences in the propensity or ability to perform physical activity. To address this possibility, we studied the effects of food restriction in four replicate lines of High Runner (HR) mice that have been selectively bred for high levels of voluntary wheel running. We hypothesized that HR mice would respond differently than mice from four non-selected Control (C) lines. Healthy adult females from generation 65 were housed individually with wheels and provided access to food and water ad libitum for experimental days 1-19 (Phase 1), which allowed mice to attain a plateau in daily running distances. Ad libitum food intake of each mouse was measured on days 20-22 (Phase 2). After this, each mouse experienced a 20 % food restriction for 7 days (days 24-30; Phase 3), and then a 40 % food restriction for 7 additional days (days 31-37; Phase 4). Mice were weighed on experimental days 1, 8, 9, 15, 20, and 23-37 and wheel-running activity was recorded continuously, in 1-minute bins, during the entire experiment. Repeated-measures ANOVA of daily wheel-running distance during Phases 2-4 indicated that HR mice always ran much more than C, with values being 3.29-fold higher during the ad libitum feeding trial, 3.58-fold higher with -20 % food, and 3.06-fold higher with -40 % food. Seven days of food restriction at -20 % did not significantly reduce wheel-running distance of either HR (-5.8 %, P = 0.0773) or C mice (-13.3 %, P = 0.2122). With 40 % restriction, HR mice showed a further decrease in daily wheel-running distance (P = 0.0797 vs. values at 20 % restriction), whereas C mice did not (P = 0.4068 vs. values at 20 % restriction) and recovered to levels similar to those on ad libitum food (P = 0.3634). For HR mice, daily running distances averaged 11.4 % lower at -40 % food versus baseline values (P = 0.0086), whereas for C mice no statistical difference existed (-4.8 %, P = 0.7004). Repeated-measures ANOVA of body mass during Phases 2-4 indicated a highly significant effect of food restriction (P = 0.0001), but no significant effect of linetype (P = 0.1764) and no interaction (P = 0.8524). Both HR and C mice had a significant reduction in body mass only when food rations were reduced by 40 % relative to ad libitum feeding, and even then the reductions averaged only -0.60 g for HR mice (-2.6 %) and -0.49 g (-2.0 %) for C mice. Overall, our results indicate a surprising insensitivity of body mass to food restriction in both high-activity (HR) and ordinary (C) mice, and also insensitivity of wheel running in the C lines of mice, thus calling for studies of compensatory mechanisms that allow this insensitivity.

Does Behavior Evolve First? Correlated Responses to Selection for Voluntary Wheel-Running Behavior in House Mice

AbstractHow traits at multiple levels of biological organization evolve in a correlated fashion in response to directional selection is poorly understood, but two popular models are the very general "behavior evolves first" (BEF) hypothesis and the more specific "morphology-performance-behavior-fitness" (MPBF) paradigm. Both acknowledge that selection often acts relatively directly on behavior and that when behavior evolves, other traits will as well but most with some lag. However, this proposition is exceedingly difficult to test in nature. Therefore, we studied correlated responses in the high-runner (HR) mouse selection experiment, in which four replicate lines have been bred for voluntary wheel-running behavior and compared with four nonselected control (C) lines. We analyzed a wide range of traits measured at generations 20-24 (with a focus on new data from generation 22), coinciding with the point at which all HR lines were reaching selection limits (plateaus). Significance levels (226 P values) were compared across trait types by ANOVA, and we used the positive false discovery rate to control for multiple comparisons. This meta-analysis showed that, surprisingly, the measures of performance (including maximal oxygen consumption during forced exercise) showed no evidence of having diverged between the HR and C lines, nor did any of the life history traits (e.g., litter size), whereas body mass had responded (decreased) at least as strongly as wheel running. Overall, results suggest that the HR lines of mice had evolved primarily by changes in motivation rather than performance ability at the time they were reaching selection limits. In addition, neither the BEF model nor the MPBF model of hierarchical evolution provides a particularly good fit to the HR mouse selection experiment.

Development and implementation of a Dependable, Simple, and Cost-effective (DSC), open-source running wheel in High Drinking in the Dark and Heterogeneous Stock/Northport mice

Maintaining healthy and consistent levels of physical activity (PA) is a clinically proven and low-cost means of reducing the onset of several chronic diseases and may provide an excellent strategy for managing mental health and related outcomes. Wheel-running (WR) is a well-characterized rodent model of voluntary PA; however, its use in biomedical research is limited by economical and methodical constraints. Here, we showcase the DSC (Dependable, Simple, Cost-effective), open-source running wheel by characterizing 24-h running patterns in two genetically unique mouse lines: inbred High Drinking in the Dark line 1 [iHDID-1; selectively bred to drink alcohol to intoxication (and then inbred to maintain phenotype)] and Heterogeneous Stock/Northport (HS/Npt; the genetically heterogeneous founders of iHDID mice). Running distance (km/day), duration (active minutes/day) and speed (km/hour) at 13-days (acute WR; Experiment 1) and 28-days (chronic WR; Experiment 2) were comparable to other mouse strains, suggesting the DSC-wheel reliably captures murine WR behavior. Analysis of 24-h running distance supports previous findings, wherein iHDID-1 mice tend to run less than HS/Npt mice in the early hours of the dark phase and more than HS/Npt in the late hours of dark phase/early light phase. Moreover, circadian actograms were generated to highlight the broad application of our wheel design across disciplines. Overall, the present findings demonstrate the ability of the DSC-wheel to function as a high-throughput and precise tool to comprehensively measure WR behaviors in mice.

Hippocampal, Whole Midbrain, Red Nucleus, and Ventral Tegmental Area Volumes Are Increased by Selective Breeding for High Voluntary Wheel-Running Behavior

Uncovering relationships between neuroanatomy, behavior, and evolution are important for understanding the factors that control brain function. Voluntary exercise is one key behavior that both affects, and may be affected by, neuroanatomical variation. Moreover, recent studies suggest an important role for physical activity in brain evolution. We used a unique and ongoing artificial selection model in which mice are bred for high voluntary wheel-running behavior, yielding four replicate lines of high runner (HR) mice that run ∼3-fold more revolutions per day than four replicate nonselected control (C) lines. Previous studies reported that, with body mass as a covariate, HR mice had heavier whole brains, non-cerebellar brains, and larger midbrains than C mice. We sampled mice from generation 66 and used high-resolution microscopy to test the hypothesis that HR mice have greater volumes and/or cell densities in nine key regions from either the midbrain or limbic system. In addition, half of the mice were given 10 weeks of wheel access from weaning, and we predicted that chronic exercise would increase the volumes of the examined brain regions via phenotypic plasticity. We replicated findings that both selective breeding and wheel access increased total brain mass, with no significant interaction between the two factors. In HR compared to C mice, adjusting for body mass, both the red nucleus (RN) of the midbrain and the hippocampus (HPC) were significantly larger, and the whole midbrain tended to be larger, with no effect of wheel access nor any interactions. Linetype and wheel access had an interactive effect on the volume of the periaqueductal gray (PAG), such that wheel access increased PAG volume in C mice but decreased volume in HR mice. Neither linetype nor wheel access affected volumes of the substantia nigra, ventral tegmental area, nucleus accumbens, ventral pallidum (VP), or basolateral amygdala. We found no main effect of either linetype or wheel access on neuronal densities (numbers of cells per unit area) for any of the regions examined. Taken together, our results suggest that the increased exercise phenotype of HR mice is related to increased RN and hippocampal volumes, but that chronic exercise alone does not produce such phenotypes.

The Inverse Krogh Principle: All Organisms Are Worthy of Study

AbstractKrogh's principle states, "For such a large number of problems there will be some animal of choice, or a few such animals, on which it can be most conveniently studied." The downside of picking a question first and then finding an ideal organism on which to study it is that it will inevitably leave many organisms neglected. Here, we promote the inverse Krogh principle: all organisms are worthy of study. The inverse Krogh principle and the Krogh principle are not opposites. Rather, the inverse Krogh principle emphasizes a different starting point for research: start with a biological unit, such as an organism, clade, or specific organism trait, then seek or create tractable research questions. Even the hardest-to-study species have research questions that can be asked of them: Where does it fall within the tree of life? What resources does it need to survive and reproduce? How does it differ from close relatives? Does it have unique adaptations? The Krogh and inverse Krogh approaches are complementary, and many research programs naturally include both. Other considerations for picking a study species include extreme species, species informative for phylogenetic analyses, and the creation of models when a suitable species does not exist. The inverse Krogh principle also has pitfalls. A scientist that picks the organism first might choose a research question not really suited to the organism, and funding agencies rarely fund organism-centered grant proposals. The inverse Krogh principle does not call for all organisms to receive the same amount of research attention. As knowledge continues to accumulate, some organisms-models-will inevitably have more known about them than others. Rather, it urges a broader search across organismal diversity to find sources of inspiration for research questions and the motivation needed to pursue them.

Effect of Voluntary Wheel-Running Exercise on the Endocrine and Inflammatory Response to Social Stress: Conditioned Rewarding Effects of Cocaine

The present paper evaluates the effect of physical activity on the increase of the conditioned rewarding effects of cocaine induced by intermittent social stress and on the neuroinflammatory response that contributes to the enhancement of drug response. For that purpose, three studies were designed in which social stress was induced in different samples of mice through a social-defeat protocol; the mice underwent an increase of physical activity by different modalities of voluntary wheel running (continuous and intermittent access). The results showed that continuous access to running wheels prior to stress enhanced the establishment of cocaine place preference, whereas an intermittent access exerted a protective effect. Wheel running contingent to cocaine administration prevented the development of conditioned preference, and if applied during the extinction of drug memories, it exerted a dual effect depending on the stress background of the animal. Our biological analysis revealed that increased sensitivity to cocaine may be related to the fact that wheel running promotes inflammation though the increase of IL-6 and BDNF levels. Together, these results highlight that physical exercise deeply impacts the organism’s response to stress and cocaine, and these effects should be taken into consideration in the design of a physical intervention.

Trade-Offs (and Constraints) in Organismal Biology

AbstractTrade-offs and constraints are inherent to life, and studies of these phenomena play a central role in both organismal and evolutionary biology. Trade-offs can be defined, categorized, and studied in at least six, not mutually exclusive, ways. (1) Allocation constraints are caused by a limited resource (e.g., energy, time, space, essential nutrients), such that increasing allocation to one component necessarily requires a decrease in another (if only two components are involved, this is referred to as the Y-model, e.g., energy devoted to size versus number of offspring). (2) Functional conflicts occur when features that enhance performance of one task decrease performance of another (e.g., relative lengths of in-levers and out-levers, force-velocity trade-offs related to muscle fiber type composition). (3) Shared biochemical pathways, often involving integrator molecules (e.g., hormones, neurotransmitters, transcription factors), can simultaneously affect multiple traits, with some effects being beneficial for one or more components of Darwinian fitness (e.g., survival, age at first reproduction, fecundity) and others detrimental. (4) Antagonistic pleiotropy describes genetic variants that increase one component of fitness (or a lower-level trait) while simultaneously decreasing another. (5) Ecological circumstances (or selective regime) may impose trade-offs, such as when foraging behavior increases energy availability yet also decreases survival. (6) Sexual selection may lead to the elaboration of (usually male) secondary sexual characters that improve mating success but handicap survival and/or impose energetic costs that reduce other fitness components. Empirical studies of trade-offs often search for negative correlations between two traits that are the expected outcomes of the trade-offs, but this will generally be inadequate if more than two traits are involved and especially for complex physiological networks of interacting traits. Moreover, trade-offs often occur only in populations that are experiencing harsh environmental conditions or energetic challenges at the extremes of phenotypic distributions, such as among individuals or species that have exceptional athletic abilities. Trade-offs may be (partially) circumvented through various compensatory mechanisms, depending on the timescale involved, ranging from acute to evolutionary. Going forward, a pluralistic view of trade-offs and constraints, combined with integrative analyses that cross levels of biological organization and traditional boundaries among disciplines, will enhance the study of evolutionary organismal biology.

Food avoidance learning based on entirely voluntary wheel running in laboratory mice (Mus musculus)

Previous studies (Nakajima, 2019a,b) demonstrated food avoidance learning based on wheel running in laboratory mice: Consumption of a target snack becomes suppressed if it is repeatedly paired with an opportunity to run in an activity wheel. This is a kind of Pavlovian conditioning, because the avoidance is specific to the paired snack. For example, in an experiment, mice were initially trained to run in closed wheels. Then, access to one of the two kinds of snacks (cheese or raisins, counterbalanced) was followed by confinement in a large pet cage with an open wheel, while access to the other snack was not. After several repetitions of these two types of trials, differentiation in consumption between the two snacks emerged: The intake of the unpaired snack increased gradually over days, while the increase was attenuated for the running-paired snack. The present study replicated this food avoidance learning without the pretraining of running in a closed wheel, emphasizing the intrinsic capacity of running to establish food avoidance. The results somewhat suggest that pretraining in open wheels facilitates running-based food avoidance, but this effect was too weak in the present study to draw a clear conclusion.

Exercise craving potentiates excitatory inputs to ventral tegmental area dopaminergic neurons

Physical exercise, which can be addictogenic on its own, is considered a therapeutic alternative for drug craving. Exercise might thus share with drugs the ability to strengthen excitatory synapses onto ventral tegmental area (VTA) dopaminergic neurones, as assessed by the ratio of AMPA receptor (AMPAR)-mediated excitatory postsynaptic currents (EPSCs) to NMDA receptor (NMDAR)-mediated EPSCs. As did acute cocaine, amphetamine, or Δ⁹ -tetrahydrocannabinol (THC) pretreatments, an acute 1-h wheel-running session increased the AMPAR/NMDAR ratio in VTA dopaminergic neurones. To dissect the respective influences of wheel-running seeking and performance, mice went through an operant protocol wherein wheel-running was conditioned by nose poking under fixed ratio schedules of reinforcement. Conditioned wheel-running increased the AMPAR/NMDAR ratio to a higher extent than free wheel-running, doing so although running performance was lower in the former paradigm than in the latter. Thus, the cue-reward association, rather than reward consumption, played a major role in this increase. The AMPAR/NMDAR ratio returned to baseline levels in mice that had extinguished the cued-running motivated task, but it increased after a cue-induced reinstatement session. The amplitude of this increase correlated with the intensity of exercise craving, as assessed by individual nose poke scores. Finally, cue-induced reinstatement of running seeking proved insensitive to acute cocaine or THC pretreatments. Our study reveals for the first time that the drive for exercise bears synaptic influences on VTA dopaminergic neurones which are reminiscent of drug actions. Whether these influences play a role in the therapeutic effects of exercise in human drug craving remains to be established.

Cannabis and exercise: Effects of Δ9-tetrahydrocannabinol on preference and motivation for wheel-running in mice

Recent surveys have revealed close links between cannabis and exercise. Specifically, cannabis usage before and/or after exercise is an increasingly common habit primarily aimed at boosting exercise pleasure, motivation, and performance whilst facilitating post-exercise recovery. However, whether these beliefs reflect the true impact of cannabis on these aspects of exercise is unknown. This study has thus examined the effects of cannabis' main psychoactive ingredient, namely Δ⁹-tetrahydrocannabinol (THC), on (i) mouse wheel-running preference and performance and (ii) running motivation and seeking behaviour. Wheel-running preference and performance were investigated using a T-maze with free and locked wheels located at the extremity of either arm. Running motivation and seeking were assessed by a cued-running operant task wherein wheel-running was conditioned by nose poking. Moreover, because THC targets cannabinoid type 1 (CB₁) receptors, i.e. receptors previously documented to control running motivation, this study also assessed the role of these receptors in running preference, performance, and craving-like behaviour. Whilst acute blockade or genetic deletion of CB₁ receptors decreased running preference and performance in the T-maze, THC proved ineffective on either variable. The failure of THC to affect running variables in the T-maze extended to running motivation, as assessed by cued-running under a progressive ratio (PR) reinforcement schedule. This ineffectiveness of THC was not related to the treatment protocol because it successfully increased motivation for palatable food. Although craving-like behaviour, as indexed by a cue-induced reinstatement of running seeking, was found to depend on CB₁ receptors, THC again proved ineffective. Neither running motivation nor running seeking were affected when CB₁ receptors were further stimulated by increasing the levels of the endocannabinoid 2-arachidonoylglycerol. These results, which suggest that the drive for running is insensitive to the acute stimulation of CB₁ receptors, raise the hypothesis that cannabis is devoid of effect on exercise motivation. Future investigation using chronic administration of THC, with and without other cannabis ingredients (e.g. cannabidiol), is however required before conclusions can be drawn.

Genetic Basis of Aerobically Supported Voluntary Exercise: Results from a Selection Experiment with House Mice

The biological basis of exercise behavior is increasingly relevant for maintaining healthy lifestyles. Various quantitative genetic studies and selection experiments have conclusively demonstrated substantial heritability for exercise behavior in both humans and laboratory rodents. In the "High Runner" selection experiment, four replicate lines of Mus domesticus were bred for high voluntary wheel running (HR), along with four nonselected control (C) lines. After 61 generations, the genomes of 79 mice (9-10 from each line) were fully sequenced and single nucleotide polymorphisms (SNPs) were identified. We used nested ANOVA with MIVQUE estimation and other approaches to compare allele frequencies between the HR and C lines for both SNPs and haplotypes. Approximately 61 genomic regions, across all somatic chromosomes, showed evidence of differentiation; 12 of these regions were differentiated by all methods of analysis. Gene function was inferred largely using Panther gene ontology terms and KO phenotypes associated with genes of interest. Some of the differentiated genes are known to be associated with behavior/motivational systems and/or athletic ability, including Sorl1, Dach1, and Cdh10 Sorl1 is a sorting protein associated with cholinergic neuron morphology, vascular wound healing, and metabolism. Dach1 is associated with limb bud development and neural differentiation. Cdh10 is a calcium ion binding protein associated with phrenic neurons. Overall, these results indicate that selective breeding for high voluntary exercise has resulted in changes in allele frequencies for multiple genes associated with both motivation and ability for endurance exercise, providing candidate genes that may explain phenotypic changes observed in previous studies.

Conditioned place preference for cocaine and methylphenidate in female mice from lines selectively bred for high voluntary wheel-running behavior

Behavioral addictions can come in many forms, including overeating, gambling and overexercising. All addictions share a common mechanism involving activation of the natural reward circuit and reinforcement learning, but the extent to which motivation for natural and drug rewards share similar neurogenetic mechanisms remains unknown. A unique mouse genetic model in which four replicate lines of female mice were selectively bred (>76 generations) for high voluntary wheel running (High Runner or HR lines) alongside four non-selected control (C) lines were used to test the hypothesis that high motivation for exercise is associated with greater reward for cocaine (20 mg/kg) and methylphenidate (10 mg/kg) using the conditioned place preference (CPP) test. HR mice run ~three times as many revolutions/day as C mice, but the extent to which they have increased motivation for other rewards is unknown. Both HR and C mice displayed significant CPP for cocaine and methylphenidate, but with no statistical difference between linetypes for either drug. Taken together, results suggest that selective breeding for increased voluntary running has modified the reward circuit in the brain in a way that increases motivation for running without affecting cocaine or methylphenidate reward.

The ergogenic impact of the glucocorticoid prednisolone does not translate into increased running motivation in mice

Glucocorticoids, such as prednisolone, are considered sport doping agents owing to their ergogenic properties. These are accounted for by peripheral mechanisms associated with energetic and anti-inflammatory processes. However, because glucocorticoids target brain tissues, it is likely that these ergogenic impacts are associated with central effects. One of these might be reward motivation, which relies on glucocorticoid receptor-expressing mesocorticolimbic dopaminergic neurons. In keeping with this possibility, this study has explored in mice whether repeated prednisolone administration (5 or 15 μg/ml of drinking water for 10 days) affected intrinsic motivation for running, a strong reinforcer in rodents. Running motivation was assessed by means of a cued-reward motivated instrumental task wherein wheel-running was conditioned by prior nose poke responses under fixed (FR), and then progressive (PR), ratio reinforcement schedules. Sub-chronic ingestion of prednisolone decreased the running distance covered during each rewarded sequence under FR schedules. This finding did not extend to wheel-running performances in mice provided free (i.e. unconditioned) wheel-running opportunities. Running motivation, as estimated under a PR reinforcement schedule, was found to be decreased (lowest concentration) or to remain unaffected (highest concentration) by prednisolone concentration. Lastly, an inter-individual analysis of the respective effects of prednisolone on muscular endurance (as assessed in the wire grid-hanging test) and on running motivation indicated that the former was not predictive of the latter. This observation suggests that prednisolone ergogenic impacts might occur without any concomitant increase in intrinsic exercise motivation.

Further demonstration of running-based food avoidance learning in laboratory mice (Mus musculus)

Voluntary wheel running has hedonically bivalent properties in laboratory rats and mice. While it works as a reward for instrumental performance such as bar pressing, it also functions as an aversive stimulus to establish Pavlovian conditioned avoidance of the paired stimulus. The present study focused on the latter case. Running in closed wheels hampered habituation of a reluctance to eat a target snack in rats (Experiment 1A) and mice (Experiment 1B) trained by pairing access to a target snack with confinement to a wheel attached to the cage. Experiment 2 successfully confirmed and extended this finding with mice running in both open and closed wheels. A differential conditioning procedure employed in Experiment 3 ensured that this phenomenon is specific to the snack paired with running, implying that it reflects Pavlovian conditioned flavor avoidance (CFA). Free exploration in cages without wheels, however, did not results in a CFA.

Beyond the Activity-Based Anorexia Model: Reinforcing Values of Exercise and Feeding Examined in Stressed Adolescent Male and Female Mice

Anorexia nervosa (AN), mostly observed in female adolescents, is the most fatal mental illness. Its core is a motivational imbalance between exercise and feeding in favor of the former. The most privileged animal model of AN is the "activity-based anorexia" (ABA) model wherein partly starved rodents housed with running wheels exercise at the expense of feeding. However, the ABA model bears face and construct validity limits, including its inability to specifically assess running motivation and feeding motivation. As infant/adolescent trauma is a precipitating factor in AN, this study first analyzed post-weaning isolation rearing (PWIR) impacts on body weights and wheel-running performances in female mice exposed to an ABA protocol. Next, we studied through operant conditioning protocols i) whether food restriction affects in a sex-dependent manner running motivation before ii) investigating how PWIR and sex affect running and feeding drives under ad libitum fed conditions and food restriction. Besides amplifying ABA-elicited body weight reductions, PWIR stimulated wheel-running activities in anticipation of feeding in female mice, suggesting increased running motivation. To confirm this hypothesis, we used a cued-reward motivated instrumental task wherein wheel-running was conditioned by prior nose poke responses. It was first observed that food restriction increased running motivation in male, but not female, mice. When fed grouped and PWIR mice were tested for their running and palatable feeding drives, all mice, excepted PWIR males, displayed increased nose poke responses for running over feeding. This was true when rewards were proposed alone or within a concurrent test. The increased preference for running over feeding in fed females did not extend to running performances (time, distance) during each rewarded sequence, confirming that motivation for, and performance during, running are independent entities. With food restriction, mice displayed a sex-independent increase in their preference for feeding over running in both group-housed and PWIR conditions. This study shows that the ABA model does not specifically capture running and feeding drives, i.e. components known to be affected in AN.

The motivation for exercise over palatable food is dictated by cannabinoid type-1 receptors

The lack of intrinsic motivation to engage in, and adhere to, physical exercise has major health consequences. However, the neurobiological bases of exercise motivation are still unknown. This study aimed at examining whether the endocannabinoid system (ECS) is involved in this process. To do so, we developed an operant conditioning paradigm wherein mice unlocked a running wheel with nose pokes. Using pharmacological tools and conditional mutants for cannabinoid type-1 (CB1) receptors, we provide evidence that CB1 receptors located on GABAergic neurons are both necessary and sufficient to positively control running motivation. Conversely, this receptor population proved dispensable for the modulation of running duration per rewarded sequence. Although the ECS mediated the motivation for another reward, namely palatable food, such a regulation was independent from CB1 receptors on GABAergic neurons. In addition, we report that the lack of CB1 receptors on GABAergic neurons decreases the preference for running over palatable food when mice were proposed an exclusive choice between the two rewards. Beyond providing a paradigm that enables motivation processes for exercise to be dissected either singly or in concurrence, this study is the first to our knowledge to identify a neurobiological mechanism that might contribute to sedentary behavior.

Food avoidance learning based on voluntary wheel running in laboratory mice (Mus musculus)

Mice show a reluctance to eat unfamiliar food, when they first encounter it. This neophobic reaction is conventionally habituated by repeated trials: the mice gradually increase their consumption of the novel food. The new finding reported here is that the consumption remains low in mice that voluntarily run in activity wheels after the novel food access. This effect implies that running yields Pavlovian conditioned flavor aversion, which suppresses, otherwise increasing, consumption of the novel food. In the present research, the effect was demonstrated with a between-group design by pitting experimental mice receiving cheese-running paired treatment against cheese/running unpaired control mice (Experiment 1). The running-based food avoidance in mice was also shown in a differential conditioning paradigm, where one of two novel snacks (chocolate and marshmallow) was paired with running while the other was not, in non-deprived animals (Experiment 2 A) and food-deprived animals (Experiment 2B). These results concord with those previously reported in rats, indicating the generality of the phenomenon.

Brain region-dependent gene networks associated with selective breeding for increased voluntary wheel-running behavior

Mouse lines selectively bred for high voluntary wheel-running behavior are helpful models for uncovering gene networks associated with increased motivation for physical activity and other reward-dependent behaviors. The fact that multiple brain regions are hypothesized to contribute to distinct behavior components necessitates the simultaneous study of these regions. The goals of this study were to identify brain-region dependent and independent gene expression patterns, regulators, and networks associated with increased voluntary wheel-running behavior. The cerebellum and striatum from a high voluntary running line and a non-selected control line were compared. Neuropeptide genes annotated to reward-dependent processes including neuropeptide S receptor 1 (Npsr1), neuropeptide Y (Npy), and proprotein convertase subtilisin/kexin type 9 (Pcsk9), and genes implicated in motor coordination including vitamin D receptor (Vdr) and keratin, type I cytoskeletal 25 (Krt25) were among the genes exhibiting activity line-by-region interaction effects. Genes annotated to the Parkinson pathway presented consistent line patterns, albeit at different orders of magnitude between brain regions, suggesting some parallel events in response to selection for high voluntary activity. The comparison of gene networks between brain regions highlighted genes including transcription factor AP-2-delta (Tfap2d), distal-less homeobox 5 gene (Dlx5) and sine oculis homeobox homolog 3 (Six3) that exhibited line differential expression in one brain region and are associated with reward-dependent behaviors. Transcription factors including En2, Stat6 and Eomes predominated among regulators of genes that differed in expression between lines. Results from the simultaneous study of striatum and cerebellum confirm the necessity to study molecular mechanisms associated with voluntary activity and reward-dependent behaviors in consideration of brain region dependencies.

High-runner mice have reduced incentive salience for a sweet-taste reward when housed with wheel access

To explore reward substitution in the context of voluntary exercise, female mice from four replicate high-runner (HR) lines (bred for wheel running) and four non-selected control (C) lines were given simultaneous access to wheels and palatable solutions as competing rewards (two doses of saccharin [0.1, 0.2% w/v]; two doses of common artificial sweetener blends containing saccharin [Sweet 'N Low^®: 0.1, 0.2% w/v], aspartame [Equal^®: 0.04, 0.08% w/v], or sucralose [Splenda^®: 0.08, 0.16% w/v]; or two doses of sucrose [3.5, 10.5% w/v]). Wheel running and fluid consumption were measured daily, with each dose (including plain water) lasting two days and two "washout" days between solutions. In a separate set of mice, the experiment was repeated without wheel access. The artificial sweeteners had no statistical effect on wheel running. However, based on proportional responses, both doses of sucrose significantly elevated wheel running in C but not HR mice. In contrast, the high dose of sucrose suppressed home-cage activity for both linetypes. Both sucrose and the artificial blends generally increased fluid consumption in a dose-dependent manner. When they had access to wheels, HR had a significantly smaller increase in consumption of artificial sweetener blends when compared with C mice, but not when housed without wheels. Overall, these results provide further evidence that the reward system of HR mice has evolved, and specifically suggest that HR mice have a reduced incentive salience for some artificial sweetener blends, likely attributable to the stronger competing reward of wheel running that has evolved in these lines.

Physical Activity Modulates Common Neuroplasticity Substrates in Major Depressive and Bipolar Disorder

Mood disorders (MDs) are chronic, recurrent mental diseases that affect millions of individuals worldwide. Although the biogenic amine model has provided some clinical utility, a need remains to better understand the interrelated mechanisms that contribute to neuroplasticity deficits in MDs and the means by which various therapeutics mitigate them. Of those therapeutics being investigated, physical activity (PA) has shown clear and consistent promise. Accordingly, the aims of this review are to (1) explicate key modulators, processes, and interactions that impinge upon multiple susceptibility points to effectuate neuroplasticity deficits in MDs; (2) explore the putative mechanisms by which PA mitigates these features; (3) review protocols used to induce the positive effects of PA in MDs; and (4) highlight implications for clinicians and researchers.

Caffeine stimulates voluntary wheel running in mice without increasing aerobic capacity

The "energy drink" Red Bull and the "sports drink" Gatorade are often marketed to athletes, with claims that they cause performance gains. However, both are high in sugars, and also consumed by non-athletes. Few studies have addressed the effects of these drinks or their biologically active components in rodent exercise models. We used three experiments to test effects on both voluntary exercise behavior and maximal aerobic capacity in lines of mice known to differ in "athletic" traits. Mice from four replicate High Runner (HR) lines have been selectively bred for voluntary running on wheels, and run approximately three times as many revolutions per day as do mice from four non-selected Control (C) lines. HR mice also have higher endurance and maximal oxygen consumption (VO₂max) during forced treadmill exercise. In Experiment 1, we tested the hypothesis that Gatorade or Red Bull might cause or allow mice to increase their voluntary wheel running. On days 5 and 6 of 6days of wheel access, as is used to select breeders, HR mice ran 3.3-fold more than C, and females ran 1.2-fold more than males, with no linetype by sex interaction. On day 7, mice were administered Gatorade, Red Bull or tap water. During the subsequent 19-hour period, Gatorade had no statistical effect on running, but Red Bull significantly increased distance run by both sexes and in both HR and C lines. The increase in distance run caused by Red Bull was attributable to time spent running, not an increase in mean (or maximum) speed. As previous studies have found that sucrose alone does not generally increase wheel running, we tested two other active ingredients in Red Bull, caffeine and taurine, in Experiment 2. With a similar testing protocol, caffeine alone and caffeine+taurine increased running by about half the magnitude of Red Bull. In Experiment 3, we tested the hypothesis that Red Bull or caffeine alone can increase physiological performance ability during aerobic exercise, measured as VO₂max. In a repeated-measures design spanning 6days, females were housed with water bottles containing Red Bull, caffeine or water in a randomized order, and tested for VO₂max twice while receiving each fluid (6 total trials). Neither Red Bull nor caffeine significantly affected either VO₂max or a measure of trial cooperativity (rated on a scale of 1-5), but both treatments significantly reduced tiredness (rated on a scale of 1-3) scored at the end of trials for both HR and C lines. Taken together, our results suggest that caffeine increases voluntary exercise levels of mice by delaying fatigue, rather than increasing aerobic capacity.

Preference for Western diet coadapts in High Runner mice and affects voluntary exercise and spontaneous physical activity in a genotype-dependent manner

Do animals evolve (coadapt) to choose diets that positively affect their performance abilities? We addressed this question from a microevolutionary perspective by examining preference for Western diet (WD: high in fat and sugar, but lower in protein) versus standard rodent chow in adults of both sexes from 4 lines of mice selectively bred for high levels of voluntary wheel running (High Runner or HR lines) and 4 non-selected control (C) lines. We also assessed whether food preference or substitution affects physical activity (wheel running and/or spontaneous physical activity [SPA] in the attached home cages). In experiment 1 (generation 56), mice were given 6days of wheel acclimation (as is used routinely to pick breeders in the selection experiment) prior to a 2-day food choice trial. In experiment 2 (generation 56), 17days of wheel acclimation allowed mice to reach a stable level of daily running, followed by a 7-day food-choice trial. In experiment 3 (generation 58), mice had 6days of wheel acclimation with standard chow, after which half were switched to WD for two days. In experiment 1, WD was highly preferred by all mice, with somewhat greater preference in male C mice. In experiment 2, wheel running increased and SPA decreased continuously for the first 14days of adult wheel testing, followed by 3-day plateaus in both. During the subsequent 7-day food choice trial, HR mice of both sexes preferred WD significantly more than did C mice; moreover, wheel running increased in all groups except males from C lines, with the increase being significantly greater in HR than C, while SPA declined further in all groups. In experiment 3, the effect of being switched to WD depended on both linetype and sex. On standard chow, only HR females showed a significant change in wheel running during nights 7+8, increasing by 10%. In contrast, when switched to WD, C females (+28%), HR females (+33%), and HR males (+10%) all significantly increased their daily wheel-running distances. Our results show for the first time that dietary preferences can coadapt in response to selection on activity levels.

Cerebellum Transcriptome of Mice Bred for High Voluntary Activity Offers Insights into Locomotor Control and Reward-Dependent Behaviors

The role of the cerebellum in motivation and addictive behaviors is less understood than that in control and coordination of movements. High running can be a self-rewarding behavior exhibiting addictive properties. Changes in the cerebellum transcriptional networks of mice from a line selectively bred for High voluntary running (H) were profiled relative to an unselected Control (C) line. The environmental modulation of these changes was assessed both in activity environments corresponding to 7 days of Free (F) access to running wheel and to Blocked (B) access on day 7. Overall, 457 genes exhibited a significant (FDR-adjusted P-value < 0.05) genotype-by-environment interaction effect, indicating that activity genotype differences in gene expression depend on environmental access to running. Among these genes, network analysis highlighted 6 genes (Nrgn, Drd2, Rxrg, Gda, Adora2a, and Rab40b) connected by their products that displayed opposite expression patterns in the activity genotype contrast within the B and F environments. The comparison of network expression topologies suggests that selection for high voluntary running is linked to a predominant dysregulation of hub genes in the F environment that enables running whereas a dysregulation of ancillary genes is favored in the B environment that blocks running. Genes associated with locomotor regulation, signaling pathways, reward-processing, goal-focused, and reward-dependent behaviors exhibited significant genotype-by-environment interaction (e.g. Pak6, Adora2a, Drd2, and Arhgap8). Neuropeptide genes including Adcyap1, Cck, Sst, Vgf, Npy, Nts, Penk, and Tac2 and related receptor genes also exhibited significant genotype-by-environment interaction. The majority of the 183 differentially expressed genes between activity genotypes (e.g. Drd1) were under-expressed in C relative to H genotypes and were also under-expressed in B relative to F environments. Our findings indicate that the high voluntary running mouse line studied is a helpful model for understanding the molecular mechanisms in the cerebellum that influence locomotor control and reward-dependent behaviors.

Hormones and the Evolution of Complex Traits: Insights from Artificial Selection on Behavior

Although behavior may often be a fairly direct target of natural or sexual selection, it cannot evolve without changes in subordinate traits that cause or permit its expression. In principle, changes in endocrine function could be a common mechanism underlying behavioral evolution because they are well positioned to mediate integrated responses to behavioral selection. More specifically, hormones can influence both motivational (e.g., brain) and performance (e.g., muscles) components of behavior simultaneously and in a coordinated fashion. If the endocrine system is often "used" as a general mechanism to effect responses to selection, then correlated responses in other aspects of behavior, life history, and organismal performance (e.g., locomotor abilities) should commonly occur because any cell with appropriate receptors could be affected. Ways in which behavior coadapts with other aspects of the phenotype can be studied directly through artificial selection and experimental evolution. Several studies have targeted rodent behavior for selective breeding and reported changes in other aspects of behavior, life history, and lower-level effectors of these organismal traits, including endocrine function. One example involves selection for high levels of voluntary wheel running, one aspect of physical activity, in four replicate High Runner (HR) lines of mice. Circulating levels of several hormones (including insulin, testosterone, thyroxine, triiodothyronine) have been characterized, three of which-corticosterone, leptin, and adiponectin-differ between HR and control lines, depending on sex, age, and generation. Potential changes in circulating levels of other behaviorally and metabolically relevant hormones, as well as in other components of the endocrine system (e.g., receptors), have yet to be examined. Overall, results to date identify promising avenues for further studies on the endocrine basis of activity levels.

Repeatability of locomotor performance and of morphology – locomotor performance relationships

There is good evidence that natural selection drives the evolution of locomotor performance, but the processes that generate among individual variation in locomotion, the substrate upon which selection acts, are relatively poorly understood. We measured prolonged swimming performance, Ucrit, and morphology in a large cohort (n=461) of wildtype zebrafish, Danio rerio, at ∼6 months and again at ∼9 months. Using mixed model analyses to estimate repeatability as the intraclass correlation coefficient, we determined that Ucrit was significantly repeatable (r = 0.55; 95% CI: 0.45 -0.64). Performance differences between the sexes (males 12% faster than females) and changes with age (decreasing 0.07% per day) both contributed to variation in Ucrit and, therefore, the repeatability estimate. Accounting for mean differences between sexes within the model decreased the estimate of Ucrit repeatability to 21% below the naïve estimate, while fitting age in the models increased the estimate to 14% above the naïve estimate. Greater consideration of factors such as age and sex is therefore necessary for the interpretation of performance repeatability in wild populations. Body shape significantly predicted Ucrit in both sexes in both assays, with the morphology – performance relationship significantly repeatable at the population level. However, morphology was more strongly predicative of performance in older fish, suggesting a change in the contribution of morphology relative to other factors such as physiology and behaviour. The morphology – performance relationship changed with age to a greater extent in males than females.

Voluntary Exercise Improves Estrous Cyclicity in Prenatally Androgenized Female Mice Despite Programming Decreased Voluntary Exercise: Implications for Polycystic Ovary Syndrome (PCOS)

Prenatal androgen (PNA) exposure in mice produces a phenotype resembling lean polycystic ovary syndrome. We studied effects of voluntary exercise on metabolic and reproductive parameters in PNA vs vehicle (VEH)-treated mice. Mice (8 wk of age) were housed individually and estrous cycles monitored. At 10 weeks of age, mice were divided into groups (PNA, PNA-run, VEH, VEH-run, n = 8-9/group); those in the running groups received wheels allowing voluntary running. Unexpectedly, PNA mice ran less distance than VEH mice; ovariectomy eliminated this difference. In ovary-intact mice, there was no difference in glucose tolerance, lower limb muscle fiber types, weight, or body composition among groups after 16 weeks of running, although some mitochondrial proteins were mildly up-regulated by exercise in PNA mice. Before running, estrous cycles in PNA mice were disrupted with most days in diestrus. There was no change in cycles during weeks 1-6 of running (10-15 wk of age). In contrast, from weeks 11 to 16 of running, cycles in PNA mice improved with more days in proestrus and estrus and fewer in diestrus. PNA programs reduced voluntary exercise, perhaps mediated in part by ovarian secretions. Exercise without weight loss improved estrous cycles, which if translated could be important for fertility in and counseling of lean women with polycystic ovary syndrome.

Genetic approaches in comparative and evolutionary physiology

Whole animal physiological performance is highly polygenic and highly plastic, and the same is generally true for the many subordinate traits that underlie performance capacities. Quantitative genetics, therefore, provides an appropriate framework for the analysis of physiological phenotypes and can be used to infer the microevolutionary processes that have shaped patterns of trait variation within and among species. In cases where specific genes are known to contribute to variation in physiological traits, analyses of intraspecific polymorphism and interspecific divergence can reveal molecular mechanisms of functional evolution and can provide insights into the possible adaptive significance of observed sequence changes. In this review, we explain how the tools and theory of quantitative genetics, population genetics, and molecular evolution can inform our understanding of mechanism and process in physiological evolution. For example, lab-based studies of polygenic inheritance can be integrated with field-based studies of trait variation and survivorship to measure selection in the wild, thereby providing direct insights into the adaptive significance of physiological variation. Analyses of quantitative genetic variation in selection experiments can be used to probe interrelationships among traits and the genetic basis of physiological trade-offs and constraints. We review approaches for characterizing the genetic architecture of physiological traits, including linkage mapping and association mapping, and systems approaches for dissecting intermediary steps in the chain of causation between genotype and phenotype. We also discuss the promise and limitations of population genomic approaches for inferring adaptation at specific loci. We end by highlighting the role of organismal physiology in the functional synthesis of evolutionary biology.

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.

Both food restriction and high-fat diet during gestation induce low birth weight and altered physical activity in adult rat offspring: the "Similarities in the Inequalities" model

We have previously described a theoretical model in humans, called "Similarities in the Inequalities", in which extremely unequal social backgrounds coexist in a complex scenario promoting similar health outcomes in adulthood. Based on the potential applicability of and to further explore the "similarities in the inequalities" phenomenon, this study used a rat model to investigate the effect of different nutritional backgrounds during gestation on the willingness of offspring to engage in physical activity in adulthood. Sprague-Dawley rats were time mated and randomly allocated to one of three dietary groups: Control (Adlib), receiving standard laboratory chow ad libitum; 50% food restricted (FR), receiving 50% of the ad libitum-fed dam's habitual intake; or high-fat diet (HF), receiving a diet containing 23% fat. The diets were provided from day 10 of pregnancy until weaning. Within 24 hours of birth, pups were cross-fostered to other dams, forming the following groups: Adlib_Adlib, FR_Adlib, and HF_Adlib. Maternal chow consumption and weight gain, and offspring birth weight, growth, physical activity (one week of free exercise in running wheels), abdominal adiposity and biochemical data were evaluated. Western blot was performed to assess D2 receptors in the dorsal striatum. The "similarities in the inequalities" effect was observed on birth weight (both FR and HF groups were smaller than the Adlib group at birth) and physical activity (both FR_Adlib and HF_Adlib groups were different from the Adlib_Adlib group, with less active males and more active females). Our findings contribute to the view that health inequalities in fetal life may program the health outcomes manifested in offspring adult life (such as altered physical activity and metabolic parameters), probably through different biological mechanisms.

Mice selectively bred for high voluntary wheel running have larger midbrains: support for the mosaic model of brain evolution

Increased brain size, relative to body mass, is a primary characteristic distinguishing the mammalian lineage. This greater encephalization has come with increased behavioral complexity and, accordingly, it has been suggested that selection on behavioral traits has been a significant factor leading to the evolution of larger whole-brain mass. In addition, brains may evolve in a mosaic fashion, with functional components having some freedom to evolve independently from other components, irrespective of, or in addition to, changes in size of the whole brain. We tested whether long-term selective breeding for high voluntary wheel running in laboratory house mice results in changes in brain size, and whether those changes have occurred in a concerted or mosaic fashion. We measured wet and dry brain mass via dissections and brain volume with ex vivo magnetic resonance imaging of brains that distinguished the caudate-putamen, hippocampus, midbrain, cerebellum and forebrain. Adjusting for body mass as a covariate, mice from the four replicate high-runner (HR) lines had statistically larger non-cerebellar wet and dry brain masses than those from four non-selected control lines, with no differences in cerebellum wet or dry mass or volume. Moreover, the midbrain volume in HR mice was ~13% larger (P<0.05), while volumes of the caudate-putamen, hippocampus, cerebellum and forebrain did not differ statistically between HR and control lines. We hypothesize that the enlarged midbrain of HR mice is related to altered neurophysiological function in their dopaminergic system. To our knowledge, this is the first example in which selection for a particular mammalian behavior has been shown to result in a change in size of a specific brain region.

Mice from lines selectively bred for high voluntary wheel running exhibit lower blood pressure during withdrawal from wheel access

Exercise is known to be rewarding and have positive effects on mental and physical health. Excessive exercise, however, can be the result of an underlying behavioral/physiological addiction. Both humans who exercise regularly and rodent models of exercise addiction sometimes display behavioral withdrawal symptoms, including depression and anxiety, when exercise is denied. However, few studies have examined the physiological state that occurs during this withdrawal period. Alterations in blood pressure (BP) are common physiological indicators of withdrawal in a variety of addictions. In this study, we examined exercise withdrawal in four replicate lines of mice selectively bred for high voluntary wheel running (HR lines). Mice from the HR lines run almost 3-fold greater distances on wheels than those from non-selected control lines, and have altered brain activity as well as increased behavioral despair when wheel access is removed. We tested the hypothesis that male HR mice have an altered cardiovascular response (heart rate, systolic, diastolic, and mean arterial pressure [MAP]) during exercise withdrawal. Measurements using an occlusion tail-cuff system were taken during 8 days of baseline, 6 days of wheel access, and 2 days of withdrawal (wheel access blocked). During withdrawal, HR mice had significantly lower systolic BP, diastolic BP, and MAP than controls, potentially indicating a differential dependence on voluntary wheel running in HR mice. This is the first characterization of a cardiovascular withdrawal response in an animal model of high voluntary exercise.

Exercise offers anxiolytic potential: a role for stress and brain noradrenergic-galaninergic mechanisms

Although physical activity reduces anxiety in humans, the neural basis for this response is unclear. Rodent models are essential to understand the mechanisms that underlie the benefits of exercise. However, it is controversial whether exercise exerts anxiolytic-like potential in rodents. Evidence is reviewed to evaluate the effects of wheel running, an experimental mode of exercise in rodents, on behavior in tests of anxiety and on norepinephrine and galanin systems in neural circuits that regulate stress. Stress is proposed to account for mixed behavioral findings in this literature. Indeed, running promotes an adaptive response to stress and alters anxiety-like behaviors in a manner dependent on stress. Running amplifies galanin expression in noradrenergic locus coeruleus (LC) and suppresses stress-induced activity of the LC and norepinephrine output in LC-target regions. Thus, enhanced galanin-mediated suppression of brain norepinephrine in runners is supported by current literature as a mechanism that may contribute to the stress-protective effects of exercise. These data support the use of rodents to study the emotional and neurobiological consequences of exercise.

Functional genomic architecture of predisposition to voluntary exercise in mice: expression QTL in the brain

The biological basis of voluntary exercise is complex and simultaneously controlled by peripheral (ability) and central (motivation) mechanisms. The accompanying natural reward, potential addiction, and the motivation associated with exercise are hypothesized to be regulated by multiple brain regions, neurotransmitters, peptides, and hormones. We generated a large (n = 815) advanced intercross line of mice (G(4)) derived from a line selectively bred for increased wheel running (high runner) and the C57BL/6J inbred strain. We previously mapped multiple quantitative trait loci (QTL) that contribute to the biological control of voluntary exercise levels, body weight, and composition, as well as changes in body weight and composition in response to short-term exercise. Currently, using a subset of the G(4) population (n = 244), we examined the transcriptional landscape relevant to neurobiological aspects of voluntary exercise by means of global mRNA expression profiles from brain tissue. We identified genome-wide expression quantitative trait loci (eQTL) regulating variation in mRNA abundance and determined the mode of gene action and the cis- and/or trans-acting nature of each eQTL. Subsets of cis-acting eQTL, colocalizing with QTL for exercise or body composition traits, were used to identify candidate genes based on both positional and functional evidence, which were further filtered by correlational and exclusion mapping analyses. Specifically, we discuss six plausible candidate genes (Insig2, Socs2, DBY, Arrdc4, Prcp, IL15) and their potential role in the regulation of voluntary activity, body composition, and their interactions. These results develop a potential initial model of the underlying functional genomic architecture of predisposition to voluntary exercise and its effects on body weight and composition within a neurophysiological framework.

Sex differences in cannabinoid receptor-1 (CB1) pharmacology in mice selectively bred for high voluntary wheel-running behavior

The endocannabinoid system (ECS) is involved in regulation of various physiological functions, including locomotion, antinociception, emotional states, and motivated behaviors. The ECS has been implicated in regulation of voluntary wheel running in mice via actions at the cannabinoid receptor-1 (CB1). Previously, we showed that four replicate lines of mice bred for high levels of voluntary wheel running (high-runner or HR lines) sex-specifically (females only) decreased running in response to antagonism of the CB1 receptor, as compared with four unselected Control lines. Here, we administered a CB1 receptor agonist, WIN 55,212-2 (WIN). We predicted that if CB1 activation is involved in the regulation of voluntary wheel running, then HR mice would show a greater response to CB1 agonism. Following our previous protocols, mice from generation 53 were acclimated to running wheels for 24 days, then received, in random order, either an intra-peritoneal injection of vehicle or a low (0.5 mg/kg), medium (1 mg/kg) or high dosage (3 mg/kg) of WIN. Each mouse received an injection and then experienced two nights without injections, for a total period of 12 days. Response to WIN was quantified as wheel revolutions, time spent running, and average running speed in the 10-120 min immediately following injection. Injection decreased wheel revolutions in all mice, but male HR mice decreased their running to a greater degree relative to Controls in response to the high dose of WIN over the entire period analyzed, whereas HR females showed a differential response relative to Controls only in the latter 70-120 min post-injection. These results, in conjunction with our previous study, show that (a) aspects of endocannabinoid signaling have diverged in four lines of mice bred for high levels of voluntary exercise and (b) male and female HR mice differ from one another in CB1 signaling as it relates to wheel running.

Sex-specific heterosis in line crosses of mice selectively bred for high locomotor activity

When populations with similar histories of directional selection are crossed, their offspring may differ in mean phenotype as compared with the average for the parental populations, often exhibiting enhancement of the mean phenotype (termed heterosis or hybrid vigor). We tested for heterosis in a cross of two replicate lines of mice selectively bred for high voluntary wheel running for 53 generations. Mice were paired to produce four sets of F1 offspring: two purebred High Runner (HR) lines and the hybrid reciprocal crosses. The purebred HR showed statistically significant, sex-dependent differences in body mass, wheel revolutions, running duration, mean running speed, and (controlling for body mass) organ masses (heart ventricles, liver, spleen, triceps surae muscle). Hybrid males ran significantly more revolutions than the purebred males, mainly via increased running speeds, but hybrid females ran intermediate distances, durations, and speeds, as compared with the purebred females. In both sexes, ventricles were relatively smaller in hybrids as compared with purebred HR. Overall, our results demonstrate differential and sex-specific responses to selection in the two HR lines tested, implying divergent genetic architectures underlying high voluntary exercise.

How to run far: multiple solutions and sex-specific responses to selective breeding for high voluntary activity levels

The response to uniform selection may occur in alternate ways that result in similar performance. We tested for multiple adaptive solutions during artificial selection for high voluntary wheel running in laboratory mice. At generation 43, the four replicate high runner (HR) lines averaged 2.85-fold more revolutions per day as compared with four non-selected control (C) lines, and females ran 1.11-fold more than males, with no sex-by-linetype interaction. Analysis of variance indicated significant differences among C lines but not among HR for revolutions per day. By contrast, average speed varied significantly among HR lines, but not among C, and showed a sex-by-linetype interaction, with the HR/C ratio being 2.02 for males and 2.45 for females. Time spent running varied among both HR and C lines, and showed a sex-by-linetype interaction, with the HR/C ratio being 1.52 for males but only 1.17 for females. Thus, females (speed) and males (speed, but also time) evolved differently, as did the replicate selected lines. Speed and time showed a trade-off among HR but not among C lines. These results demonstrate that uniform selection on a complex trait can cause consistent responses in the trait under direct selection while promoting divergence in the lower-level components of that trait.

The biological control of voluntary exercise, spontaneous physical activity and daily energy expenditure in relation to obesity: human and rodent perspectives

Mammals expend energy in many ways, including basic cellular maintenance and repair, digestion, thermoregulation, locomotion, growth and reproduction. These processes can vary tremendously among species and individuals, potentially leading to large variation in daily energy expenditure (DEE). Locomotor energy costs can be substantial for large-bodied species and those with high-activity lifestyles. For humans in industrialized societies, locomotion necessary for daily activities is often relatively low, so it has been presumed that activity energy expenditure and DEE are lower than in our ancestors. Whether this is true and has contributed to a rise in obesity is controversial. In humans, much attention has centered on spontaneous physical activity (SPA) or non-exercise activity thermogenesis (NEAT), the latter sometimes defined so broadly as to include all energy expended due to activity, exclusive of volitional exercise. Given that most people in Western societies engage in little voluntary exercise, increasing NEAT may be an effective way to maintain DEE and combat overweight and obesity. One way to promote NEAT is to decrease the amount of time spent on sedentary behaviours (e.g. watching television). The effects of voluntary exercise on other components of physical activity are highly variable in humans, partly as a function of age, and have rarely been studied in rodents. However, most rodent studies indicate that food consumption increases in the presence of wheels; therefore, other aspects of physical activity are not reduced enough to compensate for the energetic cost of wheel running. Most rodent studies also show negative effects of wheel access on body fat, especially in males. Sedentary behaviours per se have not been studied in rodents in relation to obesity. Several lines of evidence demonstrate the important role of dopamine, in addition to other neural signaling networks (e.g. the endocannabinoid system), in the control of voluntary exercise. A largely separate literature points to a key role for orexins in SPA and NEAT. Brain reward centers are involved in both types of physical activities and eating behaviours, likely leading to complex interactions. Moreover, voluntary exercise and, possibly, eating can be addictive. A growing body of research considers the relationships between personality traits and physical activity, appetite, obesity and other aspects of physical and mental health. Future studies should explore the neurobiology, endocrinology and genetics of physical activity and sedentary behaviour by examining key brain areas, neurotransmitters and hormones involved in motivation, reward and/or the regulation of energy balance.

Locomotion in response to shifting climate zones: not so fast

Although a species' locomotor capacity is suggestive of its ability to escape global climate change, such a suggestion is not necessarily straightforward. Species vary substantially in locomotor capacity, both ontogenetically and within/among populations, and much of this variation has a genetic basis. Accordingly, locomotor capacity can and does evolve rapidly, as selection experiments demonstrate. Importantly, even though this evolution of locomotor capacity may be rapid enough to escape changing climate, genetic correlations among traits (often due to pleiotropy) are such that successful or rapid dispersers are often limited in colonization or reproductive ability, which may be viewed as a trade-off. The nuanced assessment of this variation and evolution is reviewed for well-studied models: salmon, flying versus flightless insects, rodents undergoing experimental evolution, and metapopulations of butterflies. This work reveals how integration of physiology with population biology and functional genomics can be especially informative.

Erythropoietin elevates VO2,max but not voluntary wheel running in mice

Voluntary activity is a complex trait, comprising both behavioral (motivation, reward) and anatomical/physiological (ability) elements. In the present study, oxygen transport was investigated as a possible limitation to further increases in running by four replicate lines of mice that have been selectively bred for high voluntary wheel running and have reached an apparent selection limit. To increase oxygen transport capacity, erythrocyte density was elevated by the administration of an erythropoietin (EPO) analogue. Mice were given two EPO injections, two days apart, at one of two dose levels (100 or 300 microg kg(-1)). Hemoglobin concentration ([Hb]), maximal aerobic capacity during forced treadmill exercise (VO2,max) and voluntary wheel running were measured. [Hb] did not differ between high runner (HR) and non-selected control (C) lines without EPO treatment. Both doses of EPO significantly (P<0.0001) increased [Hb] as compared with sham-injected animals, with no difference in [Hb] between the 100 microg kg(-1) and 300 microg kg(-1) dose levels (overall mean of 4.5 g dl(-1) increase). EPO treatment significantly increased VO2,max by approximately 5% in both the HR and C lines, with no dosexline type interaction. However, wheel running (revolutions per day) did not increase with EPO treatment in either the HR or C lines, and in fact significantly decreased at the higher dose in both line types. These results suggest that neither [Hb] per se nor VO2,max is limiting voluntary wheel running in the HR lines. Moreover, we hypothesize that the decrease in wheel running at the higher dose of EPO may reflect direct action on the reward pathway of the brain.

Running behavior and its energy cost in mice selectively bred for high voluntary locomotor activity

Locomotion is central to behavior and intrinsic to many fitness-critical activities (e.g., migration, foraging), and it competes with other life-history components for energy. However, detailed analyses of how changes in locomotor activity and running behavior affect energy budgets are scarce. We quantified these effects in four replicate lines of house mice that have been selectively bred for high voluntary wheel running (S lines) and in their four nonselected control lines (C lines). We monitored wheel speeds and oxygen consumption for 24-48 h to determine daily energy expenditure (DEE), resting metabolic rate (RMR), locomotor costs, and running behavior (bout characteristics). Daily running distances increased roughly 50%-90% in S lines in response to selection. After we controlled for body mass effects, selection resulted in a 23% increase in DEE in males and a 6% increase in females. Total activity costs (DEE - RMR) accounted for 50%-60% of DEE in both S and C lines and were 29% higher in S males and 5% higher in S females compared with their C counterparts. Energetic costs of increased daily running distances differed between sexes because S females evolved higher running distances by running faster with little change in time spent running, while S males also spent 40% more time running than C males. This increase in time spent running impinged on high energy costs because the majority of running costs stemmed from "postural costs" (the difference between RMR and the zero-speed intercept of the speed vs. metabolic rate relationship). No statistical differences in these traits were detected between S and C females, suggesting that large changes in locomotor behavior do not necessarily effect overall energy budgets. Running behavior also differed between sexes: within S lines, males ran with more but shorter bouts than females. Our results indicate that selection effects on energy budgets can differ dramatically between sexes and that energetic constraints in S males might partly explain the apparent selection limit for wheel running observed for over 15 generations.

Western diet increases wheel running in mice selectively bred for high voluntary wheel running

OBJECTIVE

Mice from a long-term selective breeding experiment for high voluntary wheel running offer a unique model to examine the contributions of genetic and environmental factors in determining the aspects of behavior and metabolism relevant to body-weight regulation and obesity. Starting with generation 16 and continuing through to generation 52, mice from the four replicate high runner (HR) lines have run 2.5-3-fold more revolutions per day as compared with four non-selected control (C) lines, but the nature of this apparent selection limit is not understood. We hypothesized that it might involve the availability of dietary lipids.

METHODS

Wheel running, food consumption (Teklad Rodent Diet (W) 8604, 14% kJ from fat; or Harlan Teklad TD.88137 Western Diet (WD), 42% kJ from fat) and body mass were measured over 1-2-week intervals in 100 males for 2 months starting 3 days after weaning.

RESULTS

WD was obesogenic for both HR and C, significantly increasing both body mass and retroperitoneal fat pad mass, the latter even when controlling statistically for wheel-running distance and caloric intake. The HR mice had significantly less fat than C mice, explainable statistically by their greater running distance. On adjusting for body mass, HR mice showed higher caloric intake than C mice, also explainable by their higher running. Accounting for body mass and running, WD initially caused increased caloric intake in both HR and C, but this effect was reversed during the last four weeks of the study. Western diet had little or no effect on wheel running in C mice, but increased revolutions per day by as much as 75% in HR mice, mainly through increased time spent running.

CONCLUSION

The remarkable stimulation of wheel running by WD in HR mice may involve fuel usage during prolonged endurance exercise and/or direct behavioral effects on motivation. Their unique behavioral responses to WD may render HR mice an important model for understanding the control of voluntary activity levels.

Dopaminergic dysregulation in mice selectively bred for excessive exercise or obesity

Dysregulation of the dopamine system is linked to various aberrant behaviors, including addiction, compulsive exercise, and hyperphagia leading to obesity. The goal of the present experiments was to determine how dopamine contributes to the expression of opposing phenotypes, excessive exercise and obesity. We hypothesized that similar alterations in dopamine and dopamine-related gene expression may underly obesity and excessive exercise, as competing traits for central reward pathways. Moreover, we hypothesized that selective breeding for high levels of exercise or obesity may have influenced genetic variation controlling these pathways, manifesting as opposing complex traits. Dopamine, dopamine-related peptide concentrations, and gene expression were evaluated in dorsal striatum (DS) and nucleus accumbens (NA) of mice from lines selectively bred for high rates of wheel running (HR) or obesity (M16), and the non-selected ICR strain from which these lines were derived. HPLC analysis showed significantly greater neurotransmitter concentrations in DS and NA of HR mice compared to M16 and ICR. Microarray analysis showed significant gene expression differences between HR and M16 compared to ICR in both brain areas, with changes revealed throughout the dopamine pathway including D1 and D2 receptors, associated G-proteins (e.g., Golf), and adenylate cyclase (e.g., Adcy5). The results suggest that similar modifications within the dopamine system may contribute to the expression of opposite phenotypes in mice, demonstrating that alterations within central reward pathways can contribute to both obesity and excessive exercise.

Stimulus-food pairings produce stimulus-directed touch-screen responding in cynomolgus monkeys (macaca fascicularis) with or without a positive response contingency

Acquisition and maintenance of touch-screen responding was examined in naïve cynomolgus monkeys (Macaca fascicularis) under automaintenance and classical conditioning arrangements. In the first condition of Experiment 1, we compared acquisition of screen touching to a randomly positioned stimulus (a gray square) that was either stationary or moving under automaintenance (i.e., banana pellet delivery followed an 8-s stimulus presentation or immediately upon a stimulus touch). For all subjects stimulus touching occurred within the first session and increased to at least 50% of trials by the end of four sessions (320 trials). In the subsequent condition, stimulus touching further increased under a similar procedure in which pellets were only delivered if a stimulus touch occurred (fixed ratio 1 with 8-s limited hold). In Experiment 2, 6 naive subjects were initially exposed to a classical conditioning procedure (8-s stimulus preceded pellet delivery). Despite the absence of a programmed response contingency, all subjects touched the stimulus within the first session and responded on about 50% or more of trials by the second session. Responding was also sensitive to negative, neutral, and positive response contingencies introduced in subsequent conditions. Similar to other species, monkeys engaged in stimulus-directed behavior when stimulus presentations were paired with food delivery. However, stimulus-directed behavior quickly conformed to response contingencies upon subsequent introduction. Video recordings of sessions showed topographies of stimulus-directed behavior that resembled food acquisition and consumption.