Inhibition of tryptophan hydroxylase abolishes fatigue induced by central tryptophan in exercising rats

Exercise-Induced Central Fatigue: Biomarkers, and Non-Medicinal Interventions

Fatigue, commonly experienced in daily life, is a feeling of extreme tiredness, shortage or lack of energy, exhaustion, and difficulty in performing voluntary tasks. Central fatigue, defined as a progressive failure to voluntarily activate the muscle, is typically linked to moderate- or light-intensity exercise. However, in some instances, high-intensity exercise can also trigger the onset of central fatigue. Exercise-induced central fatigue often precedes the decline in physical performance in well-trained athletes. This leads to a reduction in nerve impulses, decreased neuronal excitability, and an imbalance in brain homeostasis, all of which can adversely impact an athlete's performance and the longevity of their sports career. Therefore, implementing strategies to delay the onset of exercise-induced central fatigue is vital for enhancing athletic performance and safeguarding athletes from the debilitating effects of fatigue. In this review, we discuss the structural basis, measurement methods, and biomarkers of exercise-induced central fatigue. Furthermore, we propose non-pharmacological interventions to mitigate its effects, which can potentially foster improvements in athletes' performances in a healthful and sustainable manner.

Estrogen deficiency reduces maximal running capacity and affects serotonin levels differently in the hippocampus and nucleus accumbens in response to acute exercise

Introduction

Estrogen deficiency is associated with unfavorable changes in body composition and metabolic health. While physical activity ameliorates several of the negative effects, loss of ovarian function is associated with decreased physical activity levels. It has been proposed that the changes in brain neurochemical levels and /or impaired skeletal muscle function may underlie this phenomenon.

Methods

We studied the effect of estrogen deficiency induced via ovariectomy (OVX) in female Wistar rats (n = 64). Rats underwent either sham or OVX surgery and were allocated thereafter into four groups matched for body mass and maximal running capacity: sham/control, sham/max, OVX/control, and OVX/max, of which the max groups had maximal running test before euthanasia to induce acute response to exercise. Metabolism, spontaneous activity, and maximal running capacity were measured before (PRE) and after (POST) the surgeries. Three months following the surgery, rats were euthanized, and blood and tissue samples harvested. Proteins were analyzed from gastrocnemius muscle and retroperitoneal adipose tissue via Western blot. Brain neurochemical markers were measured from nucleus accumbens (NA) and hippocampus (HC) using ultra-high performance liquid chromatography.

Results

OVX had lower basal energy expenditure and higher body mass and retroperitoneal adipose tissue mass compared with sham group (p ≤ 0.005). OVX reduced maximal running capacity by 17% (p = 0.005) with no changes in muscle mass or phosphorylated form of regulatory light chain (pRLC) in gastrocnemius muscle. OVX was associated with lower serotonin metabolite 5-hydroxyindoleacetic acid (5-HIAA) level in the NA compared with sham (p = 0.007). In response to acute exercise, OVX was associated with low serotonin level in the HC and high level in the NA (p ≤ 0.024).

Discussion

Our results highlight that OVX reduces maximal running capacity and affects the response of brain neurochemical levels to acute exercise in a brain region-specific manner. These results may offer mechanistic insight into why OVX reduces willingness to exercise.

Tryptophan catabolites, inflammation, and insulin resistance as determinants of chronic fatigue syndrome and affective symptoms in long COVID

Critical COVID-19 disease is accompanied by depletion of plasma tryptophan (TRY) and increases in indoleamine-dioxygenase (IDO)-stimulated production of neuroactive tryptophan catabolites (TRYCATs), including kynurenine (KYN). The TRYCAT pathway has not been studied extensively in association with the physiosomatic and affective symptoms of Long COVID. In the present study, we measured serum TRY, TRYCATs, insulin resistance (using the Homeostatic Model Assessment Index 2-insulin resistance, HOMA2-IR), C-reactive protein (CRP), physiosomatic, depression, and anxiety symptoms in 90 Long COVID patients, 3-10 months after remission of acute infection. We were able to construct an endophenotypic class of severe Long COVID (22% of the patients) with very low TRY and oxygen saturation (SpO2, during acute infection), increased kynurenine, KYN/TRY ratio, CRP, and very high ratings on all symptom domains. One factor could be extracted from physiosomatic symptoms (including chronic fatigue-fibromyalgia), depression, and anxiety symptoms, indicating that all domains are manifestations of the common physio-affective phenome. Three Long COVID biomarkers (CRP, KYN/TRY, and IR) explained around 40% of the variance in the physio-affective phenome. The latter and the KYN/TRY ratio were significantly predicted by peak body temperature (PBT) and lowered SpO2 during acute infection. One validated latent vector could be extracted from the three symptom domains and a composite based on CRP, KYN/TRY, and IR (Long COVID), and PBT and SpO2 (acute COVID-19). In conclusion, the physio-affective phenome of Long COVID is a manifestation of inflammatory responses during acute and Long COVID, and lowered plasma tryptophan and increased kynurenine may contribute to these effects.

The persistence of stress-induced physical inactivity in rats: an investigation of central monoamine neurotransmitters and skeletal muscle oxidative stress

Introduction

Sedentary lifestyles have reached epidemic proportions world-wide. A growing body of literature suggests that exposures to adverse experiences (e.g., psychological traumas) are a significant risk factor for the development of physically inactive lifestyles. However, the biological mechanisms linking prior stress exposure and persistent deficits in physical activity engagement remains poorly understood.

Methods

The purpose of this study was twofold. First, to identify acute stress intensity thresholds that elicit long-term wheel running deficits in rats. To that end, young adult male rats were exposed to a single episode of 0, 50, or 100 uncontrollable tail shocks and then given free access to running wheels for 9 weeks. Second, to identify stress-induced changes to central monoamine neurotransmitters and peripheral muscle physiology that may be maladaptive to exercise output. For this study, rats were either exposed to a single episode of uncontrollable tail shocks (stress) or left undisturbed in home cages (unstressed). Eight days later, monoamine-related neurochemicals were quantified by ultra-high performance liquid chromatography (UHPLC) across brain reward, motor, and emotion structures immediately following a bout of graded treadmill exercise controlled for duration and intensity. Additionally, protein markers of oxidative stress, inflammation, and metabolic activity were assessed in the gastrocnemius muscle by Western blot.

Results

For experiment 1, stress exposure caused a shock number-dependent two to fourfold decrease in wheel running distance across the entire duration of the study. For experiment 2, stress exposure curbed an exercise-induced increase of dopamine (DA) turnover measures in the prefrontal cortex and hippocampus, and augmented serotonin (5HT) turnover in the hypothalamus and remaining cortical area. However, stress exposure also caused several monoaminergic changes independent of exercise that could underlie impaired motivation for physical activity, including a mild dopamine deficiency in the striatal area. Finally, stress potently increased HSP70 and lowered SOD2 protein concentrations in the gastrocnemius muscle, which may indicate prolonged oxidative stress.

Discussion

These data support some of the possible central and peripheral mechanisms by which exposure to adverse experiences may chronically impair physical activity engagement.

Determinants of body core temperatures at fatigue in rats subjected to incremental-speed exercise: The prominent roles of ambient temperature, distance traveled, initial core temperature, and measurement site

Understanding the factors that underlie the physical exercise-induced increase in body core temperature (T_CORE) is essential to developing strategies to counteract hyperthermic fatigue and reduce the risk of exertional heatstroke. This study analyzed the contribution of six factors to T_CORE attained at fatigue in Wistar rats (n = 218) subjected to incremental-speed treadmill running: ambient temperature (T_AMB), distance traveled, initial T_CORE, body mass, measurement site, and heat loss index (HLI). First, we ran hierarchical multiple linear regression analyses with data from different studies conducted in our laboratory (n = 353 recordings). We observed that T_AMB, distance traveled, initial T_CORE, and measurement site were the variables with predictive power. Next, regression analyses were conducted with data for each of the following T_CORE indices: abdominal (T_ABD), brain cortex (T_BRAIN), or colonic (T_COL) temperature. Our findings indicated that T_AMB, distance traveled (i.e., an exercise performance-related variable), initial T_CORE, and HLI predicted the three T_CORE indices at fatigue. Most intriguingly, HLI was inversely related to T_ABD and T_BRAIN but positively associated with T_COL. Lastly, we compared the temperature values at fatigue among these T_CORE indices, and the following descendent order was noticed - T_COL, T_ABD, and T_BRAIN - irrespective of T_AMB where experiments were conducted. In conclusion, T_CORE in rats exercised to fatigue depends primarily on environmental conditions, performance, pre-exercise T_CORE, and measurement site. Moreover, the influence of cutaneous heat loss on T_COL is qualitatively different from the influence on T_ABD and T_BRAIN, and the temperature values at fatigue are not homogenous within the body core.

Therapeutic Use and Chronic Abuse of CNS Stimulants and Anabolic Drugs

The available evidence suggests that affective disorders, such as depression and anxiety, increase risk for accelerated cognitive decline and late-life dementia in aging individuals. Behavioral neuropsychology studies also showed that cognitive decline is a central feature of aging impacting the quality of life. Motor deficits are common after traumatic brain injuries and stroke, affect subjective well-being, and are linked with reduced quality of life. Currently, restorative therapies that target the brain directly to restore cognitive and motor tasks in aging and disease are available. However, the very same drugs used for therapeutic purposes are employed by athletes as stimulants either to increase performance for fame and financial rewards or as recreational drugs. Unfortunately, most of these drugs have severe side effects and pose a serious threat to the health of athletes. The use of performance-enhancing drugs by children and teenagers has increased tremendously due to the decrease in the age of players in competitive sports and the availability of various stimulants in many forms and shapes. Thus, doping may cause serious health-threatening conditions including, infertility, subdural hematomas, liver and kidney dysfunction, peripheral edema, cardiac hypertrophy, myocardial ischemia, thrombosis, and cardiovascular disease. In this review, we focus on the impact of doping on psychopathological disorders, cognition, and depression. Occasionally, we also refer to chronic use of therapeutic drugs to increase physical performance and highlight the underlying mechanisms. We conclude that raising awareness on the health risks of doping in sport for all shall promote an increased awareness for healthy lifestyles across all generations.

Core body temperatures of rats subjected to treadmill exercise to fatigue or exhaustion: The journal Temperature toolbox

This study systematically reviewed the literature reporting the changes in rats' core body temperature (TCORE) induced by either incremental- or constant-speed running to fatigue or exhaustion. In addition, multiple linear regression analyses were used to determine the factors contributing to the TCORE values attained when exercise was interrupted. Four databases (EMBASE, PubMed, SPORTDiscus, and Web of Science) were searched in October 2021, and this search was updated in August 2022. Seventy-two studies (n = 1,538 rats) were included in the systematic review. These studies described heterogeneous experimental conditions; for example, the ambient temperature ranged from 5 to 40°C. The rats quit exercising with TCORE values varying more than 8°C among studies, with the lowest and highest values corresponding to 34.9°C and 43.4°C, respectively. Multiple linear regression analyses indicated that the ambient temperature (p < 0.001), initial TCORE (p < 0.001), distance traveled (p < 0.001; only incremental exercises), and running speed and duration (p < 0.001; only constant exercises) contributed significantly to explaining the variance in the TCORE at the end of the exercise. In conclusion, rats subjected to treadmill running exhibit heterogeneous TCORE when fatigued or exhausted. Moreover, it is not possible to determine a narrow range of TCORE associated with exercise cessation in hyperthermic rats. Ambient temperature, initial TCORE, and physical performance-related variables are the best predictors of TCORE at fatigue or exhaustion. From a broader perspective, this systematic review provides relevant information for selecting appropriate methods in future studies designed to investigate exercise thermoregulation in rats.

How training loads in the preparation and competitive period affect the biochemical indicators of training stress in youth soccer players?

Background

Physical fitness optimization and injury risk-reducing require extensive monitoring of training loads and athletes' fatigue status. This study aimed to investigate the effect of a 6-month training program on the training-related stress indicators (creatine kinase - CK; cortisol - COR; serotonin - SER; brain-derived neurotrophic factor - BDNF) in youth soccer players.

Methods

Eighteen players (17.8 ± 0.9 years old, body height 181.6 ± 6.9 cm, training experience 9.7 ± 1.7 years) were blood-tested four times: at the start of the preparation period (T0), immediately following the preparation period (T1), mid-competitive period (T2), and at the end of the competitive period (T3). CK activity as well as concentrations of serum COR, SER and BDNF were determined. Training loads were recorded using a session rating of perceived exertion (sRPE).

Results

Statistical analyzes revealed significant effects for all biochemical parameters in relation to their time measurements (T0, T1, T2, T3). The statistical analyzes of sRPE and differences of biochemical parameters in their subsequent measurements (T0-T1, T1-T2, T2-T3) also demonstrated significant effects observed for all variables: sRPE (H^KW = 13.189 (df = 2); p = 0.00), COR (H^KW = 9.261 (df = 2); p = 0.01), CK (H^KW = 12.492 (df = 2); p = 0.00), SER (H^KW = 7.781 (df = 2); p = 0.02) and BDNF (H^KW = 15.160 (df = 2); p < 0.001).

Discussion

In conclusion, it should be stated that the most demanding training loads applied in the preparation period (highest sRPE values) resulted in a significant increase in all analyzed biochemical training stress indicators. The reduction in the training loads during a competitive period and the addition of recovery training sessions resulted in a systematic decrease in the values of the measured biochemical indicators. The results of the study showed that both subjective and objective markers, including training loads, are useful in monitoring training stress in youth soccer players.

Physical exercise-induced thermoregulatory responses in trained rats: Effects of manipulating the duration and intensity of aerobic training sessions

This study investigated the effects of increasing the intensity and/or duration of aerobic training sessions on thermoregulatory responses in rats subjected to exercises in temperate and warm environments. Thirty-two adult male Wistar rats were divided into four groups: a control (CON) group and three groups that were subjected to an 8-week aerobic training, during which the physical overload was achieved by predominantly increasing the exercise intensity (INT), duration (DUR) or by increasing both in an alternate manner (ID). During the last week of training, the rats received an abdominal sensor implant to measure their core body temperature (TCORE) by telemetry. After the training protocol, the 32 rats were subjected to incremental speed-exercises in temperate (23 °C) and warm (32 °C) environments. The rats had their TCORE recorded while running on a treadmill, and the ratio between the increase in TCORE and distance traveled was calculated to estimate thermoregulatory efficiency. All training protocols increased the rats' thermoregulatory efficiency during the incremental-speed exercise at 23 °C; i.e., trained rats attained faster running speeds but unchanged TCORE at fatigue compared to CON rats. However, none of the load components of training sessions - intensity or duration - was more effective than the other in improving this efficiency. At 32 °C, the aerobic training protocols did not influence the exercise-induced thermoregulatory responses. Our data indicate that different progressions in aerobic training performed at temperate conditions improved thermoregulatory efficiency during incremental exercise in the same environment; this training-induced adaptation was not clearly observed when running in warmer conditions.

Analysis of the anti-fatigue activity of polysaccharides from Spirulina platensis: role of central 5-hydroxytryptamine mechanisms

This study evaluated the effects of polysaccharides from Spirulina platensis (PSP) on endurance during treadmill exercise; levels of some biochemical indicators and expressions of serotonin related genes in the caudate putamen of exercising rats.

Rats with higher intrinsic exercise capacities exhibit greater preoptic dopamine levels and greater mechanical and thermoregulatory efficiencies while running

The present study investigated whether intrinsic exercise capacity affects the changes in thermoregulation, metabolism and central dopamine (DA) induced by treadmill running. Male Wistar rats were subjected to three incremental exercises and ranked as low-performance (LP), standard-performance (SP), and high-performance (HP) rats. In the first experiment, abdominal (TABD) and tail (TTAIL) temperatures were registered in these rats during submaximal exercise (SE) at 60% of maximal speed. Immediately after SE, rats were decapitated and concentrations of DA and 3,4-dihydroxyphenylacetic acid (DOPAC) were determined in the preoptic area (POA). In the second experiment, oxygen consumption was measured and mechanical efficiency (ME) was calculated in these rats during an incremental exercise. HP rats ran for longer periods and were fatigued with higher TABD values, with no difference in TTAIL. Nevertheless, thermoregulatory efficiency was higher in HP rats, compared with other groups. DA and DOPAC concentrations in the POA were increased by SE, with higher levels in HP compared with LP and SP rats. V̇o2 also differed between groups, with HP rats displaying a lower consumption throughout the incremental exercise but a higher V̇o2 at fatigue. ME, in turn, was consistently higher in HP than in LP and SP rats. Thus, our results show that HP rats have greater TABD values at fatigue, which seem to be related to a higher dopaminergic activity in the POA. Moreover, HP rats exhibited a greater thermoregulatory efficiency during exercise, which can be attributed to a lower V̇o2, but not to changes in tail heat loss mechanisms.

NEW & NOTEWORTHY Our findings reveal that rats with higher intrinsic exercise capacities have greater thermoregulatory efficiencies and increased dopaminergic activity in the preoptic area, a key brain area in thermoregulatory control, while exercising. Moreover, higher intrinsic exercise capacities are associated with decreased oxygen consumption for a given exercise intensity, which indicates greater mechanical efficiencies. Collectively, these findings help to advance our knowledge of why some rats of a given strain can exercise for longer periods than others.

Characterizing the plasma metabolome during and following a maximal exercise cycling test

While complex in nature, a number of metabolites have been implicated in the onset of exercise-induced fatigue. The purpose of this study was to identify changes in the plasma-metabolome and specifically, identify candidate-metabolites associated with the onset of fatigue during prolonged cycling. Eighteen healthy and recreationally active men (mean{plus minus}SD age: 24.7{plus minus}4.8 years; mass 67.1{plus minus}6.1 kg; BMI: 22.8{plus minus}2.2; VO_2peak: 40.9{plus minus}6.1 ml.kg.min^-1) were recruited to this study. Participants performed a prolonged cycling Time-To-Exhaustion (TTE) test at an intensity corresponding to a fixed blood lactate concentration (3 mmol.L^-1). Plasma samples collected at 10 min of exercise, prior to fatigue (last sample prior to fatigue; &lt;10 min prior to fatigue), immediately post-fatigue (point of exhaustion) and 20 min post-fatigue were assessed using a liquid chromatography-mass spectrometry based metabolomic approach. Eighty metabolites were putatively identified, with 68 metabolites demonstrating a significant change during the cycling task (duration: ~80.9{plus minus}13.6 min). A clear multivariate structure in the data was revealed, with the first principal component (36% total variance) describing a continuous increase in metabolite concentration throughout the TTE trial and recovery; while the second principal component (14% total variance) showed an increase in metabolite concentration followed by a recovery trajectory, peaking at the point of fatigue. Six clusters of correlated metabolites demonstrating unique metabolite trajectories were identified, including significant separation in the metabolome between pre-fatigue and post-fatigue time-points. In accordance with our hypothesis, free-fatty acids and tryptophan contributed to differences in the plasma metabolome at fatigue.

Effects of Glutamine and Alanine Supplementation on Central Fatigue Markers in Rats Submitted to Resistance Training

Recent evidence suggests that increased brain serotonin synthesis impairs performance in high-intensity intermittent exercise and specific amino acids may modulate this condition, delaying fatigue. This study investigated the effects of glutamine and alanine supplementation on central fatigue markers in rats submitted to resistance training (RT). Wistar rats were distributed in: sedentary (SED), trained (CON), trained and supplemented with alanine (ALA), glutamine and alanine in their free form (G + A), or as dipeptide (DIP). Trained groups underwent a ladder-climbing exercise for eight weeks, with progressive loads. In the last 21 days, supplementations were offered in water with a 4% concentration. Albeit without statistically significance difference, RT decreased liver glycogen, and enhanced the concentrations of plasma glucose, free fatty acids (FFA), hypothalamic serotonin, and ammonia in muscle and the liver. Amino acids affected fatigue parameters depending on the supplementation form. G + A prevented the muscle ammonia increase by RT, whereas ALA and DIP augmented ammonia and glycogen concentrations in muscle. DIP also increased liver ammonia. ALA and G + A reduced plasma FFA, whereas DIP increased this parameter, free tryptophan/total tryptophan ratio, hypothalamic serotonin, and the serotonin/dopamine ratio. The supplementations did not affect physical performance. In conclusion, glutamine and alanine may improve or impair central fatigue markers depending on their supplementation form.

Pre-exercise exposure to the treadmill setup changes the cardiovascular and thermoregulatory responses induced by subsequent treadmill running in rats

Different methodological approaches have been used to conduct experiments with rats subjected to treadmill running. Some experimenters have exposed rats to the treadmill setup before initiating exercise to minimize the influences of handling and being placed in an anxiety-inducing environment on the physiological responses to subsequent running. Other experimenters have subjected rats to exercise immediately after placing them on the treadmill. Thus, the present study aimed to evaluate the effects of pre-exercise exposure to the treadmill on physical performance and cardiovascular and thermoregulatory responses during subsequent exercise. Male Wistar rats were subjected to fatiguing incremental-speed exercise at 24°C immediately after being placed on the treadmill or after being exposed to the treadmill for 70 min following removal from their home cages. Core body temperature (T_CORE), tail-skin temperature (T_SKIN), heart rate (HR) and mean arterial pressure (MAP) were recorded throughout the experiments. Rats exposed to the treadmill started exercise with higher T_CORE, lower HR and MAP, and unaltered T_SKIN. This exposure did not influence performance, but it markedly affected the exercise-induced increases in the four physiological parameters evaluated; for example, the T_SKIN increased earlier and at a higher T_CORE. Moreover, previous treadmill exposure notably allowed expected exercise-induced changes in cardiovascular parameters to be observed. Collectively, these data indicate that pre-exercise exposure to the treadmill induces important effects on physiological responses during subsequent treadmill running. The present data are particularly relevant for researchers planning experiments involving physical exercise and the recording of physiological parameters in rats.

Moderate intensity, exercise-induced catecholamine release in the preoptic area and anterior hypothalamus in rats is enhanced in a warm environment

Thermoeffector responses and core body temperature (T_core) homeostasis during exercise are affected by both ambient temperature and exercise intensity. We have previously reported that T_core, heat loss responses, and catecholamine release in the preoptic area and anterior hypothalamus (PO/AH) increased during incremental treadmill running. However, no previous study has examined whether changes in the thermoregulatory responses at warm ambient temperature are related to catecholamine responses during moderate intensity exercise in rats. Therefore, the aim of the present study was to investigate the responsiveness of neurotransmission in the PO/AH to moderate intensity exercise at different ambient temperatures, and to relate this to changes in thermoregulation. We measured the monoamine levels in the PO/AH and the thermoregulatory responses in exercising rats simultaneously using a combination of methods, including in vivo microdialysis, biotelemetry, and animal O²/CO² metabolism measuring system. On the day of experiments, rats ran for 60min at a speed of 18mmin^-1 on a treadmill at a 5% gradient, in an ambient temperature of 23°C or 30°C. T_core, tail skin temperature (T_tail; an index of heat loss), and oxygen consumption (V̇O₂: an index of heat production) were monitored. Dopamine (DA), noradrenaline (NA), and serotonin (5-HT) levels were measured by high performance liquid chromatography (HPLC) with electrochemical detection. Exercise significantly increased the T_core, T_tail, and V̇O₂ values, as well as DA and NA release in the PO/AH at both temperatures, and the increases were more pronounced at the warm ambient temperature. The results suggest that the increase in the T_core, heat production, and heat loss responses even during moderate intensity running in a warm environment are likely associated with an increase in DA and NA release in the PO/AH region.

Physical exercise-induced fatigue: the role of serotonergic and dopaminergic systems

Brain serotonin and dopamine are neurotransmitters related to fatigue, a feeling that leads to reduced intensity or interruption of physical exercises, thereby regulating performance. The present review aims to present advances on the understanding of fatigue, which has recently been proposed as a defense mechanism instead of a “physiological failure” in the context of prolonged (aerobic) exercises. We also present recent advances on the association between serotonin, dopamine and fatigue. Experiments with rodents, which allow direct manipulation of brain serotonin and dopamine during exercise, clearly indicate that increased serotoninergic activity reduces performance, while increased dopaminergic activity is associated with increased performance. Nevertheless, experiments with humans, particularly those involving nutritional supplementation or pharmacological manipulations, have yielded conflicting results on the relationship between serotonin, dopamine and fatigue. The only clear and reproducible effect observed in humans is increased performance in hot environments after treatment with inhibitors of dopamine reuptake. Because the serotonergic and dopaminergic systems interact with each other, the serotonin-to-dopamine ratio seems to be more relevant for determining fatigue than analyzing or manipulating only one of the two transmitters. Finally, physical training protocols induce neuroplasticity, thus modulating the action of these neurotransmitters in order to improve physical performance.

Intrinsic exercise capacity in rats influences dopamine neuroplasticity induced by physical training

The study evaluates whether the intrinsic capacity for physical exercise influences dopamine neuroplasticity induced by physical training. Male rats were submitted to three progressive tests until fatigue. Based on the maximal time of exercise (TE), rats were considered as low performance (LP), standard performance (SP) or high performance (HP) to exercise. Eight animals from each group (LP, SP, and HP) were randomly subdivided in sedentary (SED) or trained (TR). Physical training was performed for 6 wk. After that, concentrations of dopamine (DA), serotonin (5-HT), and their metabolites and mRNA levels of D1 receptor ( Drd1), D2 receptor ( Drd2), dopamine transporter ( Dat), tyrosine hydroxylase ( Th), glia cell line neurotrophic factor ( Gdnf), and brain-derived neurotrophic factor ( Bdnf) were determined in the caudate-putamen (CPu). TE was increased with training in all performance groups. However, the relative increase was markedly higher in LP rats, and this was associated with a training-induced increase in dopaminergic activity in the CPu, which was determined by the 3,4-dihydroxyphenylacetic acid (DOPAC)/DA ratio. An opposite monoamine response was found in HP-TR rats, in which physical training decreased the DOPAC/DA ratio in the CPu. Moreover, LP-SED rats displayed higher levels of Drd2 in the CPu compared with the other SED groups, and this higher expression was decreased by physical training. Physical training also decreased Dat and increased Gdnf in the CPu of LP rats. Physical training decreased Bdnf in the CPu only in HP rats. Thus, we provide evidence that the intrinsic capacity to exercise affects the neuroplasticity of the dopaminergic system in response to physical training. NEW & NOTEWORTHY The findings reported reveal that dopaminergic neuroplasticity in caudate-putamen induced by physical training is influenced by the intrinsic capacity to exercise in rats. To evaluate the dopaminergic neuroplasticity, we analyzed mRNA levels of D1 receptor, D2 receptor, dopamine transporter, tyrosine hydroxylase, glia cell line neurotrophic factor, and brain-derived neurotrophic factor as well as concentrations of dopamine, serotonin, and their metabolites. These results expand our knowledge about the interrelationship between genetic background, physical training, and dopaminergic neuroplasticity.

Serotonin regulation in a rat model of exercise-induced chronic fatigue

This study investigated the mechanisms underlying regulation of the serotonin system in the rat brain during exercise-induced chronic fatigue. High-performance liquid chromatography-mass spectrometry (HPLC-MS) was performed to measure serum tryptophan of the fatigued rat. HPLC was conducted to measure 5-hydroxytryptamine (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) in the frontal cortex and hippocampus. In addition, 5-HT_1A receptor and 5-HT transporter (5-HTT) mRNA expressions were measured at the same locations using real-time PCR. The results demonstrated a significant reduction in the serum tryptophan level in rats with exercise-induced chronic fatigue. Moreover, increased 5-HT and decreased 5-HIAA levels were detected in the frontal cortex and hippocampus, and these alterations were significant. Further, 5-HTT expression was significantly increased and 5-HT_1A receptor expression was significantly decreased. These results indicate that the 5-HT system plays an important role in the development of exercise-induced chronic fatigue. The 5-HT levels in different parts of the brain increased simultaneously, especially at synapses, and these alterations were associated with changes in 5-HTT and 5-HT_1A mRNA expressions.

Branched-chain amino acids, arginine, citrulline alleviate central fatigue after 3 simulated matches in taekwondo athletes: a randomized controlled trial

Background

The decline in cognitive performance has been shown after fatiguing exercise. Branched-chain amino acids (BCAA) have been suggested to alleviate exercise-induced central fatigue. Arginine and citrulline could remove the excess NH₃ accumulation accompanied with BCAA supplementation by increasing nitric oxide biosynthesis and/or urea cycle. The purpose of this study is to investigate the effect of the combined supplementation of BCAA, arginine, and citrulline on central fatigue after three simulated matches in well-trained taekwondo athletes.

Methods

In a double-blind randomized cross-over design, 12 male taekwondo athletes performed two trials containing three simulated matches each. Each match contained three 2-min rounds of high-intensity intermittent exercise. At the end of the second match, two different supplementations were consumed. In the AA trial, the subjects ingested 0.17 g/kg BCAA, 0.05 g/kg arginine and 0.05 g/kg citrulline, while placebo was consumed in the PL trial. A validated taekwondo-specific reaction test battery was used to measure the cognitive performance after each match.

Results

The premotor reaction time in the three single-task tests and the reaction time in the secondary task in the dual-task test were maintained in the AA trial after three matches, while they were impaired in the PL trial, resulting in significantly better performance in the AA trial. These improvements in the AA trial coincided with significantly lower plasma free tryptophan/BCAA ratio, increased NO_x concentrations, and similar NH₃ concentrations.

Conclusions

This study suggested that the combined supplementation could alleviate the exercise-induced central fatigue in elite athletes.

Increased brain L-arginine availability facilitates cutaneous heat loss induced by running exercise

The effects of increased brain availability of L-arginine (L-arg), a precursor for nitric oxide synthesis, on core body temperature (Tcore ) and cutaneous heat loss were evaluated in running rats. One week prior to the experiments, adult male Wistar rats received the following implants: a chronic guide cannula in the lateral cerebral ventricle and a temperature sensor in the abdominal cavity. On the day of the experiments, the rats were assigned to receive a 2-μL intracerebroventricular injection of either NaCl (0.15 mol/L) or L-arg solution (0.825, 1.65 or 3.30 mol/L); Tcore and tail skin temperature were measured while the rats ran at a speed of 18 m/min until they were fatigued. L-arginine induced a dose-dependent reduction in the threshold Tcore required for cutaneous heat loss (38.09 ± 0.20°C for 3.30-mol/L L-arg vs 38.61 ± 0.10°C for saline; P < 0.05), which attenuated the exercise-induced hyperthermia. Although the rats treated with L-arg presented a lower Tcore at the end of exercise (~0.7°C lower after treatment with the highest dose), no changes in the time to fatigue were observed relative to the control trial. These results suggest that brain L-arg controls heat loss during exercise, most likely by modulating the sympathetic vasoconstrictor tonus to skin vessels. Furthermore, despite facilitating cutaneous heat loss mechanisms, increased brain L-arg availability did not enhance physical performance.

Administration of caffeine inhibited adenosine receptor agonist-induced decreases in motor performance, thermoregulation, and brain neurotransmitter release in exercising rats

We examined the effects of an adenosine receptor agonist on caffeine-induced changes in thermoregulation, neurotransmitter release in the preoptic area and anterior hypothalamus, and endurance exercise performance in rats. One hour before the start of exercise, rats were intraperitoneally injected with either saline alone (SAL), 10 mg kg(-1) caffeine and saline (CAF), a non-selective adenosine receptor agonist (5'-N-ethylcarboxamidoadenosine [NECA]: 0.5 mg kg(-1)) and saline (NECA), or the combination of caffeine and NECA (CAF+NECA). Rats ran until fatigue on the treadmill with a 5% grade at a speed of 18 m min(-1) at 23 °C. Compared to the SAL group, the run time to fatigue (RTTF) was significantly increased by 52% following caffeine administration and significantly decreased by 65% following NECA injection (SAL: 91 ± 14.1 min; CAF: 137 ± 25.8 min; NECA: 31 ± 13.7 min; CAF+NECA: 85 ± 11.8 min; p<0.05). NECA decreased the core body temperature (Tcore), oxygen consumption, which is an index of heat production, tail skin temperature, which is an index of heat loss, and extracellular dopamine (DA) release at rest and during exercise. Furthermore, caffeine injection inhibited the NECA-induced decreases in the RTTF, Tcore, heat production, heat loss, and extracellular DA release. Neither caffeine nor NECA affected extracellular noradrenaline or serotonin release. These results support the findings of previous studies showing improved endurance performance and overrides in body limitations after caffeine administration, and imply that the ergogenic effects of caffeine may be associated with the adenosine receptor blockade-induced increases in brain DA release.

Exercise activates compensatory thermoregulatory reaction in rats: a modeling study

The importance of exercise is increasingly emphasized for maintaining health. However, exercise itself can pose threats to health such as the development of exertional heat shock in warm environments. Therefore, it is important to understand how the thermoregulation system adjusts during exercise and how alterations of this can contribute to heat stroke. To explore this we measured the core body temperature of rats (Tc) running for 15 min on a treadmill at various speeds in two ambient temperatures (Ta = 25°C and 32°C). We assimilated the experimental data into a mathematical model that describes temperature changes in two compartments of the body, representing the muscles and the core. In our model the core body generates heat to maintain normal body temperature, and dissipates it into the environment. The muscles produce additional heat during exercise. According to the estimation of model parameters, at Ta = 25°C, the heat generation in the core was progressively reduced with the increase of the treadmill speed to compensate for a progressive increase in heat production by the muscles. This compensation was ineffective at Ta = 32°C, which resulted in an increased rate of heat accumulation with increasing speed, as opposed to the Ta = 25°C case. Interestingly, placing an animal on a treadmill increased heat production in the muscles even when the treadmill speed was zero. Quantitatively, this "ready-to-run" phenomenon accounted for over half of the heat generation in the muscles observed at maximal treadmill speed. We speculate that this anticipatory response utilizes stress-related circuitry.

Branched-chain amino acids and arginine improve performance in two consecutive days of simulated handball games in male and female athletes: a randomized trial

The central nervous system plays a crucial role in the development of physical fatigue. The purpose of this study is to investigate the effect of combined supplementation of branched-chain amino acids (BCAA) and arginine on intermittent sprint performance in simulated handball games on 2 consecutive days.

METHODS

Fifteen male and seven female handball players consumed 0.17 g/kg BCAA and 0.04 g/kg arginine together (AA trial), or placebo (PB trial) before exercise. Each trial contained two 60-min simulated handball games on consecutive days. The game was consisted of 30 identical 2-min blocks and a 20 m all-out sprint was performed at the end of each block. The performance, measured by percentage changes of sprint time between day 1 and 2, was significantly better in the AA trial (first half: AA trial: -1.34 ± 0.60%, PB trial: -0.21 ± 0.69%; second half: AA trial: -1.68 ± 0.58%, PB trial: 0.49 ± 0.42%). The average ratings of perceive exertion throughout the 2-day trial was significantly lower in the AA trial (14.2 ± 0.3) than the PB trial (15.1 ± 0.4). Concurrently, post-exercise tryptophan/BCAA ratio on both days in the AA trial was significantly lower than the baseline. This study showed that BCAA and arginine supplementation could improve performance in intermittent sprints on the second consecutive day of simulated handball games in well-trained athletes by potentially alleviating central fatigue.

Serotonergic modulation of spinal motor control

Serotonin (5-HT) is a monoamine that powerfully modulates spinal motor control by acting on intrasynaptic and extrasynaptic receptors. Here we review the diversity of 5-HT actions on locomotor and motoneuronal activities. Two approaches have been used on in vitro spinal cord preparations: either applying 5-HT in the extracellular medium or inducing its synaptic release. They produced strikingly different results suggesting that the net effect of 5-HT depends on the identity of the activated receptors and their location. Recent findings suggest that moderate release of 5-HT facilitates locomotion and promotes the excitability of motoneurons, while stronger release inhibits rhythmic activity and motoneuron firing. This latter effect is responsible for central fatigue and secures rotation of motor units.

Fighting insomnia and battling lethargy: the yin and yang of palliative care

There is an interdependent relationship between insomnia and fatigue in the medical literature, but both remain distinct entities. Insomnia entails problematic sleep initiation, maintenance, or restoration with an accompanying decrease in perceived daytime function. Lethargy is a symptom that has a wide differential diagnosis that heavily overlaps with cancer-related fatigue; however, insomnia may contribute to worsened fatigue and lethargy in cancer patients. Insomnia is a major risk factor for mood disturbances such as depression, which may also contribute to lethargy in this at-risk population. The pathophysiology of fatigue and insomnia is discussed in this review, including their differential diagnoses as well as the emerging understanding of the roles of neurotransmitters, branched-chain amino acids, and inflammatory cytokines. Treatment approaches for insomnia and fatigue are also discussed and reviewed, including the role of hypnotics, psychotropics, hormonal agents, and alternative therapies.

Physical activity ameliorates cartilage degeneration in a rat model of aging: a study on lubricin expression

Osteoarthritis (OA) is a common musculoskeletal disorder characterized by slow progression and joint tissue degeneration. Aging is one of the most prominent risk factors for the development and progression of OA. OA is not, however, an inevitable consequence of aging and age-related changes in the joint can be distinguished from those that are the result of joint injury or inflammatory disease. The question that remains is whether OA can be prevented by undertaking regular physical activity. Would moderate physical activity in the elderly cartilage (and lubricin expression) comparable to a sedentary healthy adult? In this study we used physical exercise in healthy young, adult, and aged rats to evaluate the expression of lubricin as a novel biomarker of chondrocyte senescence. Immunohistochemistry and western blotting were used to evaluate the expression of lubricin in articular cartilage, while enzyme-linked immunosorbent assay was used to quantify lubricin in synovial fluid. Morphological evaluation was done by histology to monitor possible tissue alterations. Our data suggest that moderate physical activity and normal mechanical joint loading in elderly rats improve tribology and lubricative properties of articular cartilage, promoting lubricin synthesis and its elevation in synovial fluid, thus preventing cartilage degradation compared with unexercised adult rats.