Diminished central fatigue by inhibition of the L-system transporter for the uptake of tryptophan

Metabolomic profiling of cancer-related fatigue involved in cachexia and chemotherapy

Patients with advanced cancer are frequently burdened with a severe sensation of fatigue called cancer-related fatigue (CRF). CRF is induced at various stages and treatments, such as cachexia and chemotherapy, and reduces the overall survival of patients. Objective and quantitative assessment of CRF could contribute to the diagnosis and prediction of treatment efficacy. However, such studies have not been intensively performed, particularly regarding metabolic profiles. Here, we conducted plasma metabolomics of 15 patients with urological cancer. The patients with and without fatigue, including those with cachexia or chemotherapy-induced fatigue, were compared. Significantly lower concentrations of valine and tryptophan were observed in fatigued patients than in non-fatigued patients. In addition, significantly higher concentrations of polyamine pathway metabolites were observed in patients with fatigue and cachexia than in those without cachexia. Patients with exacerbated fatigue due to chemotherapy showed significantly decreased cysteine and methionine metabolism before chemotherapy compared with those without fatigue exacerbation. These findings suggest that plasma metabolic profiles could help improve the diagnosis and monitoring of CRF.

The relationship between central fatigue and Attention Deficit/Hyperactivity Disorder of the inattentive type

Chronic fatigue and central fatigue with malaise significantly impair quality of life. Inattention caused by central fatigue is closely related to attention deficit/hyperactivity disorder (ADHD) symptoms, but the neurochemical mechanism of central fatigue remains hypothetical. The serotonin hypothesis of central fatigue was proposed first, serving as the central dogma for the molecular and neural mechanisms of central fatigue, and underpinning many studies. The tryptophan hypothesis was proposed because tryptophan released into the synaptic cleft of neurons in the brain coincides with and responds sensitively to development of fatigue. Tryptophan is highly bioactive, with brain concentrations of 50 to 200 times that of serotonin. The tryptophan-kynurenic acid-synergy hypothesis posits that central fatigue is not monocausal but a synergistic effect between tryptophan itself and its catabolite kynurenic acid. Central fatigue is associated with mental health problems and is a cause of inattention, thereby warranting scrutiny for its relationship with ADHD. Fatigability in ADHD is mediated by tryptophan, in which abnormal enhancement of the tryptophan-kynurenine-kynurenic acid pathway causes an imbalance in monoamine nervous system function. Notably, noradrenergic neuronal dysfunction is associated with the characteristic inattention of ADHD. Neutral amino acids such as branched-chain amino acids (BCAAs) can assist recovery from attentional and cognitive decline caused by central fatigue. Since they are transported by the same L-amino acid transporter as tryptophan, BCAAs compete with tryptophan to inhibit its brain uptake. Controlling central fatigue this way may improve attentional cognitive performance.

Potential Role of Neuroactive Tryptophan Metabolites in Central Fatigue: Establishment of the Fatigue Circuit

Central fatigue leads to reduced ability to perform mental tasks, disrupted social life, and impaired brain functions from childhood to old age. Regarding the neurochemical mechanism, neuroactive tryptophan metabolites are thought to play key roles in central fatigue. Previous studies have supported the “tryptophan-serotonin enhancement hypothesis” in which tryptophan uptake into extensive brain regions enhances serotonin production in the rat model of exercise-induced fatigue. However, serotonin was transiently released after 30 minutes of treadmill running to exhaustion, but this did not reflect the duration of fatigue. In addition, as the vast majority of tryptophan is metabolized along the kynurenine pathway, possible involvement of the tryptophan-kynurenine pathway in the mechanism of central fatigue induction has been pointed out. More recently, our study demonstrated that uptake of tryptophan and kynurenine derived from the peripheral circulation into the brain enhances kynurenic acid production in rat brain in sleep deprivation–induced central fatigue, but without change in serotonin activity. In particular, dynamic change in glial-neuronal interactive processes within the hypothalamus-hippocampal circuit causes central fatigue. Furthermore, increased tryptophan-kynurenine pathway activity in this circuit causes reduced memory function. This indicates a major potential role for the endogenous tryptophan-kynurenine pathway in central fatigue, which supports the “tryptophan-kynurenine enhancement hypothesis.” Here, we review research on the basic neuronal mechanism underlying central fatigue induced by neuroactive tryptophan metabolites. Notably, these basic findings could contribute to our understanding of latent mental problems associated with central fatigue.

Metformin Promotes Anxiolytic and Antidepressant-Like Responses in Insulin-Resistant Mice by Decreasing Circulating Branched-Chain Amino Acids

Epidemiological studies indicate that insulin resistance (IR), a hallmark of type 2 diabetes, is associated with an increased risk of major depression. Here, we demonstrated that male mice fed a high-fat diet (HFD) exhibited peripheral metabolic impairments reminiscent of IR accompanied by elevated circulating levels of branched-chain amino acids (BCAAs), whereas both parameters were normalized by chronic treatment with metformin (Met). Given the role of BCAAs in the regulation of tryptophan influx into the brain, we then explored the activity of the serotonin (5-HT) system. Our results indicated that HFD-fed mice displayed impairment in the electrical activity of dorsal raphe 5-HT neurons, attenuated hippocampal extracellular 5-HT concentrations and anxiety, one of the most visible and early symptoms of depression. On the contrary, Met stimulated 5-HT neurons excitability and 5-HT neurotransmission while hindering HFD-induced anxiety. Met also promoted antidepressant-like activities as observed with fluoxetine. In light of these data, we designed a modified HFD in which BCAA dietary supply was reduced by half. Deficiency in BCAAs failed to reverse HFD-induced metabolic impairments while producing antidepressant-like activity and enhancing the behavioral response to fluoxetine. Our results suggest that Met may act by decreasing circulating BCAAs levels to favor serotonergic neurotransmission in the hippocampus and promote antidepressant-like effects in mice fed an HFD. These findings also lead us to envision that a diet poor in BCAAs, provided either alone or as add-on therapy to conventional antidepressant drugs, could help to relieve depressive symptoms in patients with metabolic comorbidities.SIGNIFICANCE STATEMENT Insulin resistance in humans is associated with increased risk of anxiodepressive disorders. Such a relationship has been also found in rodents fed a high-fat diet (HFD). To determine whether insulin-sensitizing strategies induce anxiolytic- and/or antidepressant-like activities and to investigate the underlying mechanisms, we tested the effects of metformin, an oral antidiabetic drug, in mice fed an HFD. Metformin reduced levels of circulating branched-chain amino acids, which regulate tryptophan uptake within the brain. Moreover, metformin increased hippocampal serotonergic neurotransmission while promoting anxiolytic- and antidepressant-like effects. Moreover, a diet poor in these amino acids produced similar beneficial behavioral property. Collectively, these results suggest that metformin could be used as add-on therapy to a conventional antidepressant for the comorbidity between metabolic and mental disorders.

The effects of dexamethasone, light anesthesia, and tight glucose control on postoperative fatigue and quality of life after major noncardiac surgery: A randomized trial

STUDY OBJECTIVES

The postoperative period is associated with an inflammatory response that may contribute to a number of complications including postoperative fatigue (POF) that impair patients' quality of life (QoL). We studied the impact of three potentially anti-inflammatory interventions (steroid administration, tight intraoperative glucose control, and light anesthesia) on POF and QoL in patients having major noncardiac surgery.

DESIGN

A randomized Trial.

SETTING

Operating room and postoperative recovery area/ICU/hospital floors.

PATIENTS

Patients undergoing major noncardiac surgery.

INTERVENTIONS

Patients were randomized to perioperative IV dexamethasone (a total of 14 mg tapered over 3 days) versus placebo, intensive versus conventional glucose control (target 80-110 vs. 180-200 mg·dL^-1), and light versus deep anesthesia (Bispectral Index target of 55 vs. 35) in a 3-way factorial design.

MEASUREMENTS

In this planned sub-analysis, QoL was measured using SF-12 preoperatively and on postoperative day (POD) 30. POF was measured using Christensen VAS, pre-operatively, POD 1, and POD 3. We assessed the effect of each intervention on POF and on the physical and mental components of SF-12 summary scores with repeated-measures linear regression models.

MAIN RESULTS

326 patients with complete data were included in the SF-12 analysis and 306 were included in the QoL analysis. No difference was found between any of the intervention groups on fatigue or mean 30-day physical and mental components of SF-12 scores, after adjusting for preoperative score and imbalanced baseline variables (all P-value >0.07 for POF and >0.40 for QoL).

CONCLUSIONS

Steroid administration, tight intraoperative glucose control, and light anesthesia do not improve quality of life or postoperative fatigue after major surgery.

Thirteen-Week Oral Toxicity Study of Branched-Chain Amino Acids in Rats

Branched-chain amino acids (l-isoleucine, l-valine, and l-leucine) are being increasingly used in sport supplements. This study evaluated toxicological and behavioral effects of l-isoleucine (Ile), l-valine (Val), and l-leucine (Leu) during a dosing study with male and female Sprague-Dawley rats. The amino acids were incorporated into a standard diet at doses equal to 1.25%, 2.5%, and 5.0% ( w/ w). A control group of rats received a standard diet. All diets were administered ad libitum for 13 consecutive weeks. To examine stability of any potential effects, the administration period was followed by a 5-week recovery period, during which only the standard diet was provided to all animals. No significant, dose-related effects on body weight were found in rats fed a Leu-and Ile-supplemented diet. Val mixed into a diet at 5.0% ( w/ w) decreased slightly, but significantly body weight gain in females, but not males. Ile (5.0% w/ w) affected the urine electrolytes, protein, ketone bodies, urine glucose, and urobilinogen in both genders, yet the observed changes remained mostly within the range observed in controls. The random findings in hepatology and ophthalmology at the 13-week sacrifice were not considered toxicologically relevant to effects of the tested amino acids. No significant changes in organ weights were recorded. We estimate the no-observed-adverse-effect level (NOAEL) for Ile at 2.5% for both genders (male, 1.565 ± 0.060 g/kg/day; females, 1.646 ± 0.095 g/kg/day), Val at 5.0% for males (3.225 ± 0.135 g/kg/day) and 2.5% for females (1.853 ± 0.060 g/kg/day), and Leu at 5.0% for both genders (males, 3.333 ± 0.101 g/kg/day: females, 3.835 ± 0.257 g/kg/day).

The role of branched chain amino acid and tryptophan metabolism in rat's behavioral diversity: Intertwined peripheral and brain effects

Previously, we showed that a transient early-in-life interference with the expression of multiple genes by mithramycin (MTR) followed by later-in-life exposure to chronic stress, leads to a "daring" and novelty seeking behavior in rats. In this study we searched for molecular changes that contribute to this behavioral alteration. We applied a non-hypothesis driven strategy using whole genome cDNA array analysis (WGA) followed by Genome Scale Metabolic modeling analysis (GSMM). Gene expression validation was performed by qRT-PCR and immunoblotting. Brain and serum amino acids levels were measured by HPLC. WGA data directed us towards metabolic pathways and GSMM pointed at branched chain amino acids (BCAA) pathway. Out of 21 amino acids analyzed in the prefrontal cortex of MTR+Stress rats only tryptophan, whose brain levels depend on serum BCAA levels, showed a significant decrease. No change was observed in serotonin or kynurenine levels. However, a significant reduction in mRNA and protein levels of the large neutral amino acid transporter (LAT1), which transports BCAA and tryptophan into the brain, as well as in serum levels of tryptophan/BCAA ratio were observed. The latter may be attributed to the failure to increase serum insulin, following stress, in rats pre-exposed to mithramycin. Finally, significant correlations were observed between the anxiety index and tryptophan and between T-maze errors and LAT1. This study shows a specific behavioral pattern, which is linked to modulations in fluxes of amino acids both peripheral and central, which converge and reciprocally interact, and may thus be equally important targets for therapeutic intervention.

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.

Essential Role of Excessive Tryptophan and its Neurometabolites in Fatigue

Purpose:

Serotonin, a neurotransmitter synthesized from tryptophan, has been proposed to play a key role in central fatigue. In this study, we examined whether tryptophan itself and/or its two metabolites, kynurenic acid (KYNA) and quinolinic acid (QUIN), are involved in central fatigue.

Materials and Methods:

Experiments were conducted using Sprague-Dawley rats (SDR) and Nagase analbuminemic rats (NAR). Central fatigue was assessed by treadmill running and a Morris water maze test. Microdialysis was used to collect samples for measurement of extracellular concentration of tryptophan, serotonin and 5-hydroxyindoleacetic acid (5-HIAA) and to infuse test agents. To examine the kinetics of release, synaptosomes in the striatum were prepared in vitro to measure intra- and extrasynaptosomal concentration of tryptophan, serotonin and 5-HIAA.

Results:

The concentration of tryptophan secreted into the extracellular space of the striatum was higher during fatigue only, and quickly returned to basal levels with recovery from fatigue. Running time to exhaustion was reduced by activation of tryptophan receptors. Time to exhaustion was shorter in NAR, which maintain a higher extracellular level of striatum tryptophan than SDR. Impaired memory performance in a water maze task after tryptophan treatment was attributable to high levels of KYNA and QUIN in the hippocampus acting synergistically on N-methyl-D-aspartic acid receptors. When branched-chain amino acids were administered, tryptophan transport to the extracellular space of the striatum was drastically inhibited.

Conclusion:

Our findings demonstrate that the increase in fatigue which occurs because of excessively elevated brain tryptophan can be further amplified by the use of synthetic KYNA and QUIN.

International Society of Sports Nutrition position stand: energy drinks

Position Statement: The International Society of Sports Nutrition (ISSN) bases the following position stand on a critical analysis of the literature on the safety and efficacy of the use of energy drinks (ED) or energy shots (ES). The ISSN has concluded the following. 1. Although ED and ES contain a number of nutrients that are purported to affect mental and/or physical performance, the primary ergogenic nutrients in most ED and ES appear to be carbohydrate and/or caffeine. 2. The ergogenic value of caffeine on mental and physical performance has been well-established but the potential additive benefits of other nutrients contained in ED and ES remains to be determined. 3. Consuming ED 10-60 minutes before exercise can improve mental focus, alertness, anaerobic performance, and/or endurance performance. 4. Many ED and ES contain numerous ingredients; these products in particular merit further study to demonstrate their safety and potential effects on physical and mental performance. 5. There is some limited evidence that consumption of low-calorie ED during training and/or weight loss trials may provide ergogenic benefit and/or promote a small amount of additional fat loss. However, ingestion of higher calorie ED may promote weight gain if the energy intake from consumption of ED is not carefully considered as part of the total daily energy intake. 6. Athletes should consider the impact of ingesting high glycemic load carbohydrates on metabolic health, blood glucose and insulin levels, as well as the effects of caffeine and other stimulants on motor skill performance. 7. Children and adolescents should only consider use of ED or ES with parental approval after consideration of the amount of carbohydrate, caffeine, and other nutrients contained in the ED or ES and a thorough understanding of the potential side effects. 8. Indiscriminant use of ED or ES, especially if more than one serving per day is consumed, may lead to adverse events and harmful side effects. 9. Diabetics and individuals with pre-existing cardiovascular, metabolic, hepatorenal, and neurologic disease who are taking medications that may be affected by high glycemic load foods, caffeine, and/or other stimulants should avoid use of ED and/or ES unless approved by their physician.

Effects of high-dose large neutral amino acid supplementation on exercise, motor skill, and mental performance in Australian Rules Football players

This study investigated the effects of high-dose large neutral amino acid (LNAA) supplementation on attenuating fatigue-induced decrements in exercise and motor skill performance in Australian Rules Football (ARF) players. Fifteen subelite ARF players participated in 3 testing sessions separated by 7 days. Players completed an initial control trial involving a reactive motor skills test (RMST) and a reactive agility test (RAT) carried out before and after fatiguing exercise. In the subsequent experimental trials, players ingested a serotonin-depleting or protein control (PC) LNAA mixture 3 h before testing, allocated in a double-blind randomized cross-over design. Blood samples were taken at presupplementation and pre- and postexercise for analysis of plasma amino acid, insulin, and metabolite concentrations. The effect of the LNAA was established as the difference in the change in the mean RMST and RAT test scores among the depleting, PC, and baseline (BL) trials. Mean overall repetition time of the RAT was moderately improved by -5.2% ± 3.4% (mean ± 90% confidence limits; effect size -0.45 ± 0.28) after ingestion of the serotonin-depleting mixture compared with the BL trial. Serotonin-depleting and PC supplements had a divergent effect on mean repetition time after fatiguing exercise in RMST: depleting serotonin elicited a small improvement (-3.0% ± 2.7%) in motor skill performance in contrast to a small decrement (2.4% ± 2.7%) after ingestion of the PC mixture, when compared to the BL. High-dose serotonin-"depleting" LNAA supplementation given 3 h prior to intermittent high-intensity exercise improved reactive motor skill and agility performance in ARF players.

Lack of plasma albumin impairs intravascular lipolysis and explains the associated free fatty acids deficiency and hypertriglyceridemia

BACKGROUND

Abnormalities in lipid metabolism and transport are hallmarks in analbuminemic Nagase rats (NAR) and humans. Triglyceridemia is nearly 3- to 5-fold higher in female NAR than in control Sprague-Dawley rats (SDR). Also, NAR present with a severe plasma free fatty acid (FFA) deficit. There are conflicting results regarding the mechanisms underlying NAR hypertriglyceridemia.

OBJECTIVE

We aimed at investigating whether liver lipogenesis and triglyceride secretion rates into the plasma contribute to the hypertriglyceridemia in NAR. We also studied whether heparin or albumin administration would release the hypothesized lipolysis inhibition in NAR.

METHODS

The incorporation of tritiated water into lipids and the linear accumulation rate of plasma triglycerides after Triton WR1339 injection were the measures of liver lipogenesis and triglyceride secretion rates.

RESULTS

Lipogenesis (596 ± 40 vs. 929 ± 124 μmol 3H2O/g/h) and triglyceride (4.25 ± 1.00 vs. 7.04 ± 1.68 mg/dL/min) secretion rates were slower (P ≤ 0.05) in fasted NAR than in control SDR. The injection of either heparin or albumin elicited an increase in NAR plasma FFA levels over time. FFA levels reached control levels 90 min after the albumin administration, increasing from 0.36 ± 0.05 to 1.34 ± 0.16 mEq/L (P ≤ 0.05). These results indicate that the lack of plasma albumin inhibits intravascular lipolysis and causes the FFA deficit observed in NAR.

CONCLUSION

NAR hepatic triglyceride synthesis and output do not contribute to NAR hypertriglyceridemia. We propose that the lack of albumin diminishes intravascular lipolysis which reduces the plasma triglyceride removal rate and explain both NAR hypertriglyceridemia and FFA deficiency.

Brain serotonergic and dopaminergic modulators, perceptual responses and endurance exercise performance following caffeine co-ingested with a high fat meal in trained humans

BACKGROUND

The present study examined putative modulators and indices of brain serotonergic and dopaminergic function, perceptual responses, and endurance exercise performance following caffeine co-ingested with a high fat meal.

METHODS

Trained humans (n = 10) performed three constant-load cycling tests at 73% of maximal oxygen uptake (VO2max) until exhaustion at 10 degrees C remove space throughout. Prior to the first test, subjects consumed a 90% carbohydrate meal (Control trial) and for the remaining two tests, a 90% fat meal with (FC trial) and without (F trial) caffeine.

RESULTS

Time to exhaustion was not different between the F and FC trials (P > 0.05); [Control trial: 116(88-145) min; F trial: 122(96-144) min; FC trial: 127(107-176) min]. However, leg muscular discomfort during exercise was significantly lower on the FC relative to F trial (P < 0.01). There were no significant differences between F and FC trials in key modulators and indices of brain serotonergic (5-HT) and dopaminergic (DA) function [(i.e. plasma free and total tryptophan (Trp), tyrosine (Tyr), large neutral amino acids (LNAA), Trp:LNAA ratio, free-Trp:Tyr ratio, total Trp:Tyr ratio, and plasma prolactin] (P > 0.05) with the exception of plasma free-Trp:LNAA ratio which was higher at 90 min and at exhaustion during the FC trial (P < 0.05).

CONCLUSIONS

Neither brain 5-HT nor DA systems would appear to be implicated in the fatigue process when exercise is performed without significant thermoregulatory stress, thus indicating fatigue development during exercise in relatively cold temperatures to occur predominantly due to glycogen depletion.

Brain cytokines and the 5-HT system during poly I:C-induced fatigue

Fatigue is evoked not only by peripheral factors, such as muscle fatigue, but also by the central nervous system (CNS). For example, it is generally known that the feeling of fatigue is greatly influenced by psychological aspects, such as motivation. However, little is known about the central mechanisms of fatigue. The clinical symptoms of chronic fatigue syndrome (CFS) are shown to include disorders in neuroendocrine, autonomic, and immune systems. On the other hand, it has been demonstrated that cytokines produced in the brain play significant roles in neural-immune interactions through their various central actions, including hypothalamo-pituitary and sympathetic activation, as well as immunosuppression. In this article, using the immunologically induced fatigue model, which was achieved by intraperitoneal (i.p.) injection of synthetic double-stranded RNAs, polyriboinosinic: polyribocytidylic acid (poly I:C) in rats, we show an involvement of brain interferon-alpha (IFN-alpha) and serotonin (5-HT) transporter (5-HTT) in the central mechanisms of fatigue. In the poly I:C-induced fatigue rats, expression of IFN-alpha and 5-HTT increased, while extracellular concentration of 5-HT in the medial prefrontal cortex decreased, probably on account of the enhanced expression of 5-HTT. Since the poly I:C-induced reduction of the running wheel activity was attenuated by a 5-HT(1A) receptor agonist, but not by 5-HT(2), 5-HT(3), or dopamine D(3) receptor agonists, it is suggested that the decrease in 5-HT actions on 5-HT(1A) receptors may at least partly contribute to the poly I:C-induced fatigue.

Effects of elevated plasma tryptophan on brain activation associated with the Stroop task

RATIONALE

Central fatigue, such as that found in chronic fatigue syndrome, is a state in which cognition and action require increasing effort and performance is impaired without evidence for reduced peripheral motor responsiveness. Previous studies identified functional changes in subcortical regions in patients who experience central fatigue but did not address neural correlates of the subjective experience of fatigue.

OBJECTIVES

This study investigated responses to acute tryptophan feeding (after administration of 30 mg/kg body mass) using functional magnetic resonance imaging to investigate neural correlates of central fatigue during a cognitively demanding exercise, the counting Stroop task.

MATERIALS AND METHODS

In a double-blind, cross-over study, eight subjects ingested L: -tryptophan (Trp) or placebo (Plac) on two separate test days. Neutral (N) and interference (I) Stroop tasks were carried out.

RESULTS

Plasma-free tryptophan (p[FT]) increased tenfold after L: -Trp administration (P < 0.01). Although reaction times were longer after Trp (mean+/-SD, Plac-Neut 669 +/- 163 ms, I 715 +/- 174 ms, P < 0.01; Trp-Neut 712 +/- 193 ms, I 761 +/- 198 ms, P < 0.05), the Stroop effect was not significantly different between Plac and Trp. L: -Trp administration was associated with relatively decreased activation in regions, including the left postcentral, angular, inferior frontal, and the lateral orbital gyri and the inferior frontal sulcus relative to Plac. Relatively increased activation was found after Trp in the left precuneus and in the posterior cingulate gyrus.

CONCLUSIONS

Thus, Trp administration before the Stroop task caused distributed functional changes in primary sensory and in multimodal neocortex, including changes in a brain region, the activity of which has been shown previously to vary with conscious awareness (precuneus). Previous reports suggest that primary mechanisms of central fatigue may be predominantly subcortical. The present results demonstrate that neocortical activity changes are also found. Whether this activity contributes to the primary mechanisms underlying central fatigue or not, the neocortical activity changes may provide an index of the conscious experience.

Enhanced expression of brain interferon-alpha and serotonin transporter in immunologically induced fatigue in rats

Immunologically induced fatigue was induced in rats by intraperitoneal injection of a synthetic double-stranded RNA, polyriboinosinic : polyribocytidylic acid (poly I:C). An injection of poly I:C (3 mg/kg) decreased the daily amounts of spontaneous running wheel activity to approximately 60% of the preinjection level until day 8. Quantitative analysis of mRNA levels demonstrated that interferon-alpha (IFN-alpha) and p38 mitogen-activated protein kinase mRNAs increased in the medial preoptic, paraventricular and ventromedial hypothalamic nuclei and in cortex on both days 1 and 8, while interleukin-1beta and an inhibitor of nuclear factor kappaB (IkappaB)-beta mRNAs increased on day 1, but recovered within a week. Serotonin transporter (5-HTT) mRNA also increased on days 1 and 8 after poly I:C injection in the same brain regions where IFN-alpha mRNA increased. The increased 5-HTT had a functional significance, because in vivo brain microdialysis revealed that an i.p. injection of poly I:C induced a decrease in the extracellular concentration of 5-HT in the prefrontal cortex; the decrease was blocked by local perfusion with a nonselective 5-HT reuptake inhibitor, imipramine. Finally, the poly I:C-induced fatigue was attenuated by a 5-HT1A receptor agonist but not by 5-HT2, 5-HT3 or dopamine D3 agonists. These findings, taken together, suggest that disorders in brain IFN-alpha and 5-HTT expression may be involved in the neuronal mechanisms of the poly I:C-induced fatigue.

Exercise-dependent preference for a mixture of branched-chain amino acids and homeostatic control of brain serotonin in exercising rats

In this article we discuss studies showing that rats are able to regulate their intake of BCAA depending on the level of exercise, and that they will choose a solution of BCAA over water during times of intense exercise. We found that the voluntary intake of a solution made of BCAA, L-arginine and L-glutamine positively correlated with the timing and volume of exercise during the dark (active) period of the circadian rhythm. In the second behavioral protocol in which rats were fed BCAA fortified diet (2.0%, wt:wt), we observed voluntarily increased volume of physical activity beginning from d 4 of feeding on. In the second, neuro-behavioral, part of the study we measured the brain content of 5-hydroxytryptamine (5-HT) as well as plasma amino acid profiles in well-trained exercising rats to test a hypothesis that BCAA may alleviate central aspects of fatigue. A solution made of BCAA, L-arginine, and L-glutamine applied before running elevated the BCAA/tryptophan plasma ratio at the end of and after running, and decreased 5-HT release in the lateral hypothalamus and amygdala after running, when compared with the controls. The exercise-related shift in the fluid preference toward a BCAA-based solution suggests an ergogenic benefit. The forced-running study shows the lateral hypothalamus and possibly amygdala might be the critical brain regions implied in the central effects of a BCAA-based solution.

Structure of the blood-brain barrier and its role in the transport of amino acids

Brain capillary endothelial cells form the blood-brain barrier (BBB). They are connected by extensive tight junctions, and are polarized into luminal (blood-facing) and abluminal (brain-facing) plasma membrane domains. The polar distribution of transport proteins mediates amino acid (AA) homeostasis in the brain. The existence of two facilitative transporters for neutral amino acids (NAAs) on both membranes provides the brain access to essential AAs. Four Na(+)-dependent transporters of NAA exist in the abluminal membranes of the BBB. Together these systems have the capability to actively transfer every naturally occurring NAA from the extracellular fluid (ECF) to endothelial cells and from there into circulation. The presence of Na(+)-dependent carriers on the abluminal membrane provides a mechanism by which NAA concentrations in the ECF of brain are maintained at approximately 10% those of the plasma. Also present on the abluminal membrane are at least three Na(+)-dependent systems transporting acidic AAs (EAAT) and a Na(+)-dependent system transporting glutamine (N). Facilitative carriers for glutamine and glutamate are found only in the luminal membrane of the BBB. This organization promotes the net removal of acidic- and nitrogen-rich AAs from the brain and accounts for the low level of glutamate penetration into the central nervous system. The presence of a gamma-glutamyl cycle at the luminal membrane and Na(+)-dependent AA transporters at the abluminal membrane may serve to modulate movement of AAs from blood to the brain. The gamma-glutamyl cycle is expected to generate pyroglutamate (synonymous with oxyproline) within the endothelial cells. Pyroglutamate stimulates secondary active AA transporters at the abluminal membrane, thereby reducing the net influx of AAs to the brain. It is now clear that BBB participates in the active regulation of the AA content of the brain.

Prolonged oral treatment with an essential amino acid L-leucine does not affect female reproductive function and embryo-fetal development in rats

L-leucine, an essential amino acid, is one of the most popular ingredients in dietary supplements. To investigate a possibility of its embryo-fetal toxicity in rats, 11- to 12-week old dams were orally administered an aqueous solution of L-leucine at doses of 300 or 1000 mg/kg body weight on gestational days 7-17. Body weight and feed intake was evaluated throughout the whole course of pregnancy (days 0-20). L-Leucine did not influence body weight, but at a dose of 1000 mg/kg, slightly enhanced feed intake on days 14 and 18 of pregnancy. Caesarean section (day 20) revealed no influences on the litter size and weight of live-born fetuses, the number of corpora lutea, implantation index or the quality of placenta, and the minor increase in feed intake was considered irrelevant to the pregnancy outcomes. Fetuses were evaluated in a battery of external, visceral and skeletal examinations. No effects of L-leucine on gender ratio and external abnormalities, and no significant treatment-related variations in visceral and skeletal pathologies were observed. These results suggested that L-leucine, administered orally during organogenesis at doses up to 1000 mg/kg body weight, did not affect the outcome of pregnancy and did not cause fetotoxicity in rats.

Prolonged effects of polyriboinosinic:polyribocytidylic acid on spontaneous running wheel activity and brain interferon-alpha mRNA in rats: a model for immunologically induced fatigue

Following 2 weeks acclimation to the running wheel in the home cages, an i.p. injection of a synthetic double-stranded RNA, polyriboinosinic:polyribocytidylic acid (poly I:C, 3 mg/kg), was performed to produce the immunologically induced fatigue in rats. The daily amounts of spontaneous running wheel activity decreased to about 40-60% of the preinjection level until day 9 with normal circadian rhythm, then gradually returned to the baseline level by day 14. Rats given a heat exposure (36 degrees C for 1 h) for the consecutive 3 days showed an increase in activity except for the first day. In the open field test, the total moving distance and the number of rearing of the poly I:C-injected rats decreased on day 1, but they were not different from the saline-injected group on day 7, suggesting that the poly I:C-induced fatigue on day 7 was not due to the peripheral problems such as muscle/joint pain, but involved the CNS. Quantitative analysis of mRNA levels using a real-time capillary reverse transcriptase-polymerase chain reaction (RT-PCR) method revealed that interferon-alpha (IFN-alpha) mRNA contents in the cortex, hippocampus, hypothalamic medial preoptic, paraventricular, and ventromedial nuclei were higher in the poly I:C group than those in the saline and heat-exposed groups on day 7, although the amount of interleukin-1 beta mRNA showed no differences. Serum adrenocorticotropic hormone and catecholamine levels were not significantly different between groups. The present results indicate that the prolonged fatigue induced by poly I:C, which is evaluated by the spontaneous running wheel activity, can be used as an animal model for the immunologically induced fatigue associated with viral infection, and suggest that brain IFN-alpha may play a role in this model.

Evidence that tryptophan reduces mechanical efficiency and running performance in rats

It has been reported that exercise increases brain tryptophan (TRP), which is related to exhaustive fatigue. To study this further, the effect of increased TRP availability on the central nervous system (CNS) with regard to mechanical efficiency, oxygen consumption (VO(2)) and run-time to exhaustion was studied in normal untrained rats. Each rat was anesthetized with thiopental (30 mg/kg ip b. wt.) and fitted with a chronic guiding cannula attached to the right lateral cerebral ventricle 1 week prior to the experiments. Immediately before exercise, the rats were randomly injected through these cannulae with 2.0 microl of 0.15 M NaCl (n=6) or 20.3 microM L-TRP solution (n=6). Exercise consisted of running on a treadmill at 18 m min(-1) and 5% inclination until exhaustion. TRP-treated rats presented a decrease in their mechanical efficiency (21.25+/-0.84%, TRP group vs. 24.31+/-0.98%, saline-treated group; P< or =.05), and increased VO(2) at exhaustion (40.3+/-1.6 ml kg(-1) min(-1), TRP group vs. 36.0+/-0.8 ml kg(-1) min(-1), saline group; P< or =.05), indicating that the metabolic cost of exercise was higher in the former group. In addition, a highly significant reduction was also observed in run-time to exhaustion of TRP animals compared to those of the saline-treated group (15.2+/-1.52 min, TRP group vs. 50.6+/-5.4 min, saline group; P< or =.0001). It can be deduced from the data that intracerebroventricular TRP injection in rats increases O(2) consumption and reduces mechanical efficiency during exercise, diminishing running performance.

The Effect of Tryptophan Deficiency in the Brain on Rat Fatigue Levels: a Rat Model of Fatigue Reduction

The level of tryptophan and 5-hydroxyindoleacetic acid were measured in the brain (striatum) of rats on tryptophan-deficient diet and tryptophan-enriched diet. We measured concentrations of tryptophan and 5-hydroxyindoleacetic acid in striatum by using microdialysis and HPLC methods. The extracellular level of tryptophan and 5-hydroxyindoleacetic acid concentration in the striatum of a tryptophan-reduced rats was decreased by about 50% compared with a tryptophan-enriched diet. In the tryptophan-reduced condition, the rats an increased the running time of more than 100 min, compared with those on a tryptophan-enriched diet. These results suggest the tryptophan and 5-hydroxytryptamine in the central nervous system are involved in fatigue.

Preference for a solution of branched-chain amino acids plus glutamine and arginine correlates with free running activity in rats: involvement of serotonergic-dependent processes of lateral hypothalamus

Rats were given free access to a running wheel, food, water, and a solution composed of branched-chain amino acids plus glutamine and arginine (the "BCAA-based" solution). A positive relationship between dark-period running distance and preference for the BCAA-based solution was observed. Serotonin release in the lateral hypothalamus, the central nucleus of amygdala and the medial nucleus raphe in overnight fluid-deprived rats during their first subsequent free drinking was also measured. A lowered serotonin release in the lateral hypothalamus characterized the rats that consumed the BCAA-based solution. No drink-related changes were observed in the amygdala. A separate group of rats was trained on a treadmill. Following the training period, plasma amino acids and brain serotonin release were measured during running. The BCAA-based solution infused before running elevated the branched chain amino acids/tryptophan plasma ratio at the end of, and after, running. Additionally, a decreased lateral hypothalamus serotonin release was seen 80 min after running, when compared with water-infused rats. No fluid-related changes in the amygdala were observed. The exercise-related shift in the fluid preference towards a BCAA-based solution suggests an ergogenic benefit. The forced-running study shows the lateral hypothalamus as a critical region in the effects of a BCAA-based solution.