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The Brain and Fatigue : New Opportunities for Nutritional
Romain Meeusen1, Phil Watson2 , Jiri Dvorak3 1. Dept Human Physiology & Sportsmedicine - Faculty of Physical Education and Physiotherapy, Vrije Universiteit Brussel, Brussels , Belgium 2. School of Sport and Exercise Sciences, Loughborough University, Leicestershire, LE11 3TU, 3. Dept Neurology and F-MARC (FIFA Medical Assesment and Research Center) Schulthess Clinic Zurich, CH-8008 Zurich Corresponding author:
Romain Meeusen PhD Dept Human Physiology & Sportsmedicine Faculty of Physical Education and Physiotherapy, Vrije Universiteit Brussel, Pleinlaan 2 – B1050 Brussels Tel : +32-26292222 Email: rmeeusen@vub.ac.be Running Title: The role of central fatigue in football
Abstract
It is clear that the cause of fatigue is complex, influenced by events occurring in both the periphery and the central nervous system (CNS). Work conducted over the last 20 years has focused on the role of brain serotonin and catecholamines in the development of fatigue, and the possibility that manipulation of neurotransmitter precursors may delay the onset of fatigue. While there is some evidence that branched- chain amino acid and tyrosine ingestion can influence perceived exertion and some measures of mental performance, the results of several apparently well-controlled laboratory studies have not demonstrated a positive effect on exercise capacity or performance under temperate conditions. As football is highly reliant upon the successful execution of motor skills and tactics, the possibility that amino acid ingestion may attenuate a loss in cognitive function occurring during the later stages of a game would be desirable, even in the absence of no apparent benefit to physical performance. There are several reports of enhanced performance of high-intensity intermittent exercise with carbohydrate ingestion, but at present it is difficult to separate the peripheral effects from any potential impact on the CNS. The possibility that changes in central neurotransmission play a role in the aetiology of fatigue when exercise is performed in high ambient temperatures has recently been examined, although the significance of this in relation to the pattern of activity associated with football has yet to be determined.
Key words:
branched-chain amino acids, carbohydrate, dopamine, neurotransmission, serotonin, tyrosine
Introduction
Progressive fatigue occurring during high-intensity intermittent exercise, characteristic of many teams sports including football, has been typically ascribed to the depletion of muscle glycogen, reductions in circulating blood glucose, hyperthermia and the progressive loss of body fluids (Mohr et al., 2005). There is a reduction in distances covered, and the number and intensity of sprints undertaken by players towards the end of the second half of play (Mohr et al., 2003). While the idea that the central nervous system (CNS) is involved in feelings of tiredness, lethargy and mood disturbances is not new, evidence has accumulated over the past 20 years to support a significant role of the brain in the aetiology of fatigue during strenuous exercise. It is now acknowledged that the cause of fatigue is a complex phenomenon influenced by both events occurring in the periphery and the CNS (Meeusen and De Meirleir, 1995; Nybo and Secher, 2004). At the highest level, football players have been reported to cover distances in excess of 10km during competitive matches (Bangsbo et al., 1991). This activity typically consists of periods of walking and low-moderate intensity running, interspersed with explosive bursts of activity including sprinting, jumping, changes in speed and direction, and tackling. In many skill-based sports, participants have to simultaneously perform mechanical work, often with a great physical demand, coupled with the precise performance of decisional and/or perceptual tasks. Football matches show periods and situations of high intensity activity, and successful football performance depends upon numerous factors such as technical, tactical, physical, physiological and mental areas. Increased fatigue has commonly been observed after exercise and, although detrimental effects on mood or mental performance are typically small (Collardeau et al., 2001), in sports such as football even minor decrements in mental performance can significantly influence the outcome of a game. In this review we will discuss possible neurobiological mechanisms of fatigue and examine whether nutritional and pharmacological interventions to alter central neurotransmission are capable of influencing the development of fatigue during exercise.
The ‘Central fatigue Hypothesis'
The 'Central Fatigue Hypothesis' is based on the assumption that during prolonged exercise the synthesis and metabolism of central monoamines, in particular Serotonin (5-HT), Dopamine (DA) and Noradrenaline (NA) are influenced. It was first suggested by (Newsholme et al., 1987) that prolonged exercise resulted in an increase in brain serotonergic activity that may augment lethargy, cause an altered sensation of effort, perhaps a differing tolerance of pain/discomfort and a loss of drive and motivation, thus limiting physical and mental performance. The rate of 5-HT synthesis is largely dependent upon the peripheral availability of the essential amino acid tryptophan (TRP). An increase in the delivery of TRP to the CNS will increase serotonergic activity because the rate limiting enzyme, TRP hydroxylase, is not saturated under physiological conditions. Furthermore, free tryptophan (f-TRP) and the branched-chained amino acids (BCAA) share the same carrier in order to pass across the blood brain barrier (BBB), meaning that the plasma concentration ratio of f-TRP to BCAA is thought to be an important determinant of 5-HT synthesis. The underlying mechanism behind the central fatigue hypothesis as proposed by Newsholme et al. (1987) can be divided into two interrelated sections : 1. Under resting conditions, the majority of TRP, the precursor of 5-HT, circulates in the blood loosely bound to albumin, a transporter shared with free fatty acids (FFA). The shift in substrate mobilisation occurring as exercise progresses causes an increase in plasma FFA concentration. This displaces TRP from binding sites on albumin, leading to a marked increase in (f-TRP). Free- Tryptophan is then readily available for transport across the BBB. 2. Plasma BCAA concentrations either fall or are unchanged during prolonged exercise. Since f-TRP and BCAA share a common transporter across the BBB, a reduction in competing Large Neutral Amino Acids (LNAA) would increase the uptake of TRP into the CNS. The resulting elevation in TRP delivery results in an increased central synthesis of 5-HT.
Is there experimental evidence for central fatigue ?
Since neurotransmitters, including serotonin, dopamine and noradrenaline, have been implicated in the aetiology of a wide variety of psychiatric and mood disorders (e.g. depression, anxiety disorders, Parkinson's disease) a vast number of drugs have been developed to directly manipulate central neurotransmission. Through an understanding of the action of these pharmacological agents, it has been possible to examine the role of the CNS in the fatigue process. However, at present there appears to be no published reports of the effects of pharmacological manipulation of central neurotransmission on performance during high-intensity intermittent activity. Bailey and co-workers were amongst the first to examine the effects of pharmacological manipulation of brain 5-HT levels through the administration of specific 5-HT agonists and antagonists to rodents (Bailey et al., 1992; Bailey et al., 1993). This early work provided good evidence for a role of 5-HT in the development of fatigue, with a dose-dependent reduction in exercise capacity reported when central 5-HT activity was augmented by the acute administration of a general 5-HT agonist (Bailey et al., 1992). Brain 5-HT and DA content progressively increased during exercise, but at the point of exhaustion a marked fall in tissue DA content was apparent. Furthermore, exercise capacity was enhanced by a 5-HT antagonist (LY-53857), although this was apparent only when the highest dose was administered (Bailey et al., 1993). Selective Serotonin Reuptake inhibitors (SSRI) SSRIs are a class of drugs that selectively inhibit the reuptake of 5-HT into the presynaptic nerve terminal, thus increasing the extracellular concentration of 5-HT present at the postsynaptic receptors. These agents have been widely administered in the treatment of various psychiatric disorders, in particular depression, and were first to be employed in the study of central fatigue. To date, three studies have investigated the effects of an acute dose of paroxetine (Paxil, Seroxat), with two reporting a reduction in exercise time to exhaustion (Struder et al., 1998; Wilson and Maughan, 1992). A number of subsequent studies have examined the effects of pharmacological agents acting on central serotonergic neurotransmission during prolonged exercise, with largely negative results, making it difficult at this stage to make a firm decision regarding the importance of 5-HT in the fatigue process (Meeusen et al., 2001; Meeusen et al., 1997; Pannier et al., 1995; Strachan et al., 2004). The neuromuscular and performance effects of acute and long-term exposure to fluoxetine have also been examined (Parise et al., 2001). Serotonin has been demonstrated to alter an individuals' sensation of pain, and this study differs from many others in this area by investigating whether manipulation of serotonergic neurotransmission could alter the response to high-intensity and resistance exercise. Following periods of acute and chronic (2 weeks) administration it was concluded that SSRI do not influence measures of strength or high-intensity exercise performance, including maximum voluntary contractions, voluntary activation percentage, repeated Wingate and high-intensity exercise tests to volitional exhaustion in young adult men. Catecholaminergic drugs Because of the complexity of brain functioning, and the contradictory results from the studies that attempted to manipulate only serotonergic activity, it is unlikely that one single neurotransmitter is responsible for a centrally-mediated component of fatigue. In fact, alterations in catecholamines, as well as other excitatory and inhibitory neurotransmitters (glutamate, GABA and acetylcholine) have all been implicated as possible mediators of central fatigue during exercise (Meeusen and De Meirleir, 1995). These neurotransmitters are known to play a role in arousal, mood, motivation, vigilance, anxiety and reward mechanisms, and could therefore, if adversely affected, impair performance. It is therefore necessary to explore the different transmitter systems and their effect on the neuroendocrine response to endurance exercise. Dopamine (DA) and noradrenaline (NA) are neurotransmitters that have also been linked to the "central" component of fatigue, due to their well-known role in motivation and motor behaviour (Davis and Bailey, 1997; Meeusen and De Meirleir, 1995), and are therefore thought to have an enhancing effect on performance. DA and NA are synthesised through a shared metabolic pathway, with the amino acid tyrosine (TYR) acting as the precursor. Tyrosine is found in protein-rich dietary sources, including chicken and milk, but unlike TRP it is a non-essential LNAA that can also be synthesised from phenylalanine in the liver. Cerebral uptake of TYR is subject to competitive transport across the BBB by the LNAA-carrier system, which is shared with TRP and the other LNAA as discussed above. Early pharmacological manipulation of central neurotransmission to improve exercise performance focused largely on the effects of amphetamines, which have a long history of abuse in sport. Amphetamine is a close analogue of DA and NA, thought to act directly on catecholaminergic neurones to produce a marked elevation in extracellular DA concentrations. This response is believed to be mediated through the stimulation of DA release from storage vesicles, inhibition of DA reuptake and the inhibition of DA metabolism by monoamineoxidase (MAO). Amphetamines may also limit the synthesis of 5-HT through a reduction in TRP hydroxylase activity and a direct interaction between DA release and serotonergic neurotransmission. Studies have demonstrated a clear performance benefit following the administration of amphetamine to both rodents (Gerald, 1978) and humans (Borg et al., 1972; Chandler and Blair, 1980). The ergogenic action of amphetamine is thought to be mediated through the maintenance of DA release late in exercise. The importance of DA in the development of fatigue has been shown in animal studies (Heyes et al., 1985; Kalinski et al., 2001). It seems that at the point of fatigue extracellular DA concentrations are low, possibly due to the interaction with brain 5-HT (Bailey et al., 1992), or a depletion of central catecholamines (Davis and Bailey, 1997). In a series of studies we supplemented athletes with venlafaxine a combined 5-HT/NA reuptake inhibitor (SNRI; (Piacentini et al., 2002a), reboxetine a NA reuptake inhibitor (NARI; (Piacentini et al., 2002b) and Buproprion, a combined NA/DA reuptake inhibitor (Piacentini et al., 2004). Athletes performed two cycle-based time trials requiring the completion of a predetermined amount of work as quickly as possible ( 90 minutes), in a double- blind randomized crossover design. None of the above mentioned agents significantly influenced (either negatively or positively) exercise performance. Each drug clearly altered central neurotransmission since different neuroendocrine effects were observed depending on the type of reuptake inhibitor administered.
Central Fatigue and nutritional interventions
Much of the attraction of the hypothesis described by Newsholme and co-workers (1987) was the potential for nutritional manipulation of neurotransmitter precursors to delay the onset of central fatigue, potentially enhancing performance. In recent years a number of studies have attempted to attenuate the increase in central 5-HT levels and maintain/increase catecholaminergic neurotransmission through dietary supplementation with specific nutrients, including branched-chain amino acids, tyrosine and carbohydrate. Amino Acid Supplementation As f-TRP competes with BCAA for transport across the BBB into the CNS, reducing the plasma concentration ratio of f-TRP to BCAA through the provision of exogenous BCAA has been suggested as a practice to attenuate the development of central fatigue. The first investigation undertaken to test the efficacy of BCAA supplementation at attenuating 5-HT-mediated fatigue was a field study of the physical and mental performance of male volunteers competing in either a marathon or a 30 km cross- country race (Blomstrand et al., 1991a). The findings suggested that both physical (race time) and mental (colour and word tests) performance were enhanced in those receiving BCAA prior to exercise. However, enhanced performance was witnessed only in subjects completing the marathon in times slower than 3 hours 5 minutes, with the lack of a response in the faster runners attributed to an increased resistance to the feelings associated with central and peripheral fatigue. The reliability of these results has been subsequently questioned, due to a number of methodological problems largely relating to the field-based nature of the study (Davis and Bailey, 1997). While there is some additional evidence of BCAA ingestion influencing ratings of perceived exertion (RPE) (Blomstrand et al., 1997) and mental performance (Blomstrand et al., 1991b; Hassmen et al., 1994), the results of several apparently well-controlled laboratory studies have not demonstrated a positive effect on exercise capacity or performance. No ergogenic benefit has been reported during prolonged fixed intensity exercise to exhaustion (Blomstrand et al., 1995; Blomstrand et al., 1997; Galiano et al., 1991; Struder et al., 1998; van Hall et al., 1995), prolonged time trial (TT) performance (Hassmen et al., 1994; Madsen et al., 1996) or incremental exercice (Varnier et al., 1994). Work conducted by (Davis et al., 1999) investigated the effects of BCAA ingestion on the performance of a test specific to the intermittent, high-intensity activity involved in football. The effects of a sugar-free placebo, a carbohydrate solution (CHO) and a carbohydrate solution with added BCAA (CHO+BCAA) on exercise time to exhaustion were examined. Compared to the placebo trial, subjects were able to run significantly longer when the CHO and CHO+BCAA solutions were ingested, but the addition of BCAA resulted in no further benefit. The possible influence of CHO ingestion on the development of central fatigue is discussed below, but it is possible that the co-ingestion of BCAA along with CHO may have masked any performance effect. One possible explanation for a failure to observe an ergogenic effect in many BCAA studies, despite a good rationale for their use, is an increase in ammonia (NH3) production (Davis and Bailey, 1997). During prolonged intense exercise, the plasma concentration of NH3 increases, with this increase amplified by BCAA ingestion. Since NH3 can readily cross the BBB, it may enter the CNS where excessive accumulation may have a profound effect on cerebral function. Evidence suggests that hyperammonaemia has a marked effect of cerebral blood flow, energy metabolism, astrocyte function, synaptic transmission and the regulation of various neurotransmitter systems (Felipo and Butterworth, 2002). Therefore, it has been considered that exercise-induced hyperammonaemia could also be a mediator of CNS fatigue during prolonged exercise (Davis and Bailey, 1997). Recently Nybo et al (2005) reported that during prolonged exercise the cerebral uptake and accumulation of NH3 may provoke fatigue, through a disturbance to neurotransmitter metabolism. Marked increases in circulating ammonia concentrations have been reported during high level football matches (Mohr et al., 2005), thus an accumulation of serum ammonia may contribute to the development of fatigue through disruptions in peripheral and cerebral metabolism. The flip-side of the serotonin-fatigue hypothesis is the idea that increased catecholaminergic neurotransmission will favour feelings of arousal, motivation and reward, consequently enhancing exercise performance. In a similar manner to serotonin, central DA and NA synthesis is reliant on the delivery of the non-essential amino acid tyrosine, but the rate of synthesis appears to be also limited by the activity of the catecholaminergic neurons (Davis and Bailey, 1997). Despite a good rationale for its use, evidence of an ergogenic benefit of TYR supplementation during prolonged exercise is limited. Work by Struder and colleagues (1998) failed to observe any change in the capacity to perform prolonged exercise following the ingestion of TYR immediately before (10 g) and during exercise (10 g). It has been suggested that the high dose of TYR administered in this study may have resulted in an inhibition of dopamine synthesis, but a recent report administering half the dose employed by Struder et al. (1998) also produced no effect on time trial performance (Chinevere et al., 2002). Additionally, oral ingestion of TYR by humans had no measurable effect on endurance, muscle strength, or anaerobic power (Sutton et al., 2005). While evidence for an effect of TYR on physical performance is limited, stress-related decrements in mood and task performance are reported to be reduced by TYR supplementation during sustained military operations exceeding 12-hours, involving severe sleep deprivation and fatigue (Owasoyo et al., 1992). There are also several reports indicating that TYR ingestion improves stress-induced cognitive and behavioural deficits, in particular working memory, tracking, stress-sensitive attentional focus tasks (Banderet and Lieberman, 1989; Deijen et al., 1999; Dollins et al., 1995; Neri et al., 1995; Shurtleff et al., 1994; Sutton et al., 2005). As football is highly dependent on the successful execution of fine and gross motor skills, the possibility that TYR ingestion may attenuate a loss in cognitive function occurring during the later stages of a game would be desirable, despite no apparent benefit to physical performance. It is yet to be seen whether these results can be reproduced in a football-specific protocol. Carbohydrate (CHO) Supplementation Analysis of the activity patterns and muscle biopsy data taken from football players suggest that there is a large reliance on CHO utilisation, and ingestion of exogenous CHO before and during matches has been reported to enhance performance during the latter stages of a game (Kirkendall, 1993). The peripheral effects of CHO ingestion will be discussed elsewhere in this issue, but it is clear that the
provision of exogenous CHO during exercise can also have a profound effect on the CNS. Carbohydrate feeding suppresses lipolysis, consequently lowering the circulating concentration of plasma FFA. Recognising this, (Davis et al., 1992) suggested CHO ingestion as a means of reducing cerebral TRP uptake. A five- to sevenfold increase in the plasma concentration ratio of f-TRP to BCAA was reported under placebo conditions. Supplementation with a 6 or 12 % CHO solution attenuated the increase in plasma FFA and f-TRP, reducing the plasma concentration ratio of f-TRP to BCAA in a dose-dependent manner. Exercise capacity during CHO trials was increased over the placebo, suggesting CHO ingestion as an effective means of delaying the onset of central fatigue, but it is difficult to separate the contribution of central factors from the widely reported benefits of CHO at attenuating peripheral fatigue. Several studies have directly investigated the effect of CHO supplementation on the development of fatigue during team sport-based activity (Davis et al., 1999; Nicholas et al., 1995; Welsh et al., 2002; Winnick et al., 2005). It is clear that CHO ingestion can have a profound effect on measures of physical fatigue during this type of exercise, with marked improvements in time to volitional exhaustion, maintenance of sprint performance and vertical jump height. Recent work has also focused on the effects of CHO supplementation on measures of CNS fatigue, assessed largely through the performance of skills-based tasks and psychological inventories. Ingestion of CHO before and during exercise has been reported to attenuate losses in the performance of whole-body motor skills tasks (Welsh et al., 2002; Winnick et al., 2005) The beneficial effect of CHO supplementation during prolonged exercise could also relate to increased (or maintained) substrate delivery for the brain, with a number of studies indicating that hypoglycaemia affects brain function, and cognitive performance. Exercise-induced hypoglycaemia has been reported to reduce brain glucose uptake and overall cerebral metabolic rate (Nybo et al., 2003), and this is associated with a marked reduction in voluntary activation during sustained muscular contractions (Nybo, 2003). The reduction in CNS activation is abolished when euglycaemia was maintained. Ingestion of CHO has also been reported to minimise the negative effect of prolonged exercise on cognitive function, with an improvement in the performance of complex cognitive tasks observed following running (Collardeau et al., 2001). Data from animal work suggest that glucose plays an important role in the regulation central neurotransmission and alterations in extracellular glucose concentrations have been demonstrated to influence 5-HT release and reuptake significantly during exercise and recovery (Bequet et al., 2002). In addition to changes in circulating blood glucose, the possibility that the depletion of brain glycogen may be important to the development of fatigue during strenuous exercise has recently been explored (Dalsgaard et al., 2002).
What other factors might be responsible for ‘central fatigue'?
Fatigue and especially ‘Central Fatigue' is a complex and multifaceted phenomenon. There are several other possible cerebral factors that might limit exercise performance, all of them influencing signal transduction, since the brain cells communicate through chemical substances. Not all of these relationships have been explored in detail, and the complexity of brain neurochemical interactions make it difficult to construct a single or simple statement that covers the ‘Central Fatigue' phenomenon. Other neurotransmitters such as acetylcholine, GABA and glutamate have been suggested to a lesser extent to be involved with the development of central fatigue (Abdelmalki et al., 1997; Conlay et al., 1992) and as such will not be discussed in this review. Attention has also been given to the influence of ammonia (NH ) on the cerebral levels of glutamate, glutamine and GABA (Davis and Bailey, 1997; Nybo et al., 2005). In recent years, the role of central adenosine has been investigated, through its association with caffeine (Davis et al., 2003). The ergogenic effect of caffeine was originally thought to be mediated through an increase in fat oxidation rate, thus sparring muscle glycogen (Costill et al., 1978). Subsequent work has largely failed to provide convincing support for this mechanism, leading to the suggestion that the effects of caffeine supplementation are centrally mediated. Caffeine is a potent adenosine antagonist, that readily crosses the blood-brain barrier, producing a marked reduction in central adenosine neurotransmission. Adenosine inhibits the release of many excitory neurotransmitters, including dopamine and noradrenaline, consequently reducing arousal and spontaneous behavioural activity. The central effect of caffeine have recently been demonstrated by Davis and colleagues (2003), with a marked increase in exercise capacity observed following an infusion of caffeine into the brain of rodents. The influence of caffeine ingestion on both physical and mental performance will be discussed in the chapter by Peter Hespel. One area of the CNS that a received little attention in relation to exercise is the blood-brain barrier (BBB), and the possibility that changes in its integrity may be involved in the fatigue process. The relative impermeability of the BBB helps to maintain a stable environment for the brain by regulating exchange between the CNS and the extra-cerebral environment. While the BBB largely resistant to changes in permeability, there are situations where BBB function may be either acutely or chronically compromised, with changes potentially resulting in a disturbance of a wide range of homeostatic mechanisms. There is some evidence that prolonged exercise may lead to increased BBB permeability in both rodents (Sharma et al., 1996) and humans (Watson et al., 2005b). A recent human study reported an increase in circulating serum S100β, a proposed peripheral marker of BBB permeability, following prolonged exercise in a warm environment. This response was not apparent following exercise in temperate conditions (Watson et al., 2005b). A similar increase in serum S100β has been reported following football drills involving the repeated heading of a football (Mussack et al., 2003; Stalnacke et al., 2004), although the authors of these studies perhaps incorrectly interpret this change as an indication of neuronal damage. Serum S100β is now being employed as an index of brain trauma in individuals who suffer head injuries during sports. Changes in the permeability of the BBB to this protein may give misleading results in exercising individuals, particularly under conditions that lead to significant heat stress. At present the functional consequences of changes in BBB permeability during exercise and whether nutritional supplementation can alter this response are not clear. Other cerebral metabolic, thermodynamic, circulatory and humoral responses could all lead to a disturbance of cerebral homeostasis and eventually central fatigue. To date there is evidence that because of the extreme disturbance of homeostasis that occurs during prolonged exercise, peripheral and central regulatory mechanisms will be stressed. However, for the moment it is not possible to determine the exact regulation and the importance of each factor.

Hyperthermia, fatigue and central neurotransmission

Capacity to perform prolonged exercise is clearly impaired in high ambient temperatures (Galloway and Maughan, 1997; Parkin et al., 1999). While exercise capacity is thought to be primarily limited by thermoregulatory and fluid balance factors (Hargreaves and Febbraio, 1998), it has been suggested that the central nervous system (CNS) may become important in the development of fatigue when body temperature is significantly elevated (Nielsen, 1992). During prolonged exercise in the heat, exhaustion appears to coincide with the attainment of an internal body temperature of around 40.0oC (Gonzalez-Alonso et al., 1999; Nielsen et al., 1993). Hyperthermia has been proposed to accelerate the development of central fatigue during exercise, resulting in a reduction in maximal muscle activation (Nybo and Nielsen, 2001a), altered EEG brain activity (Nielsen et al., 2001) and increased perceived exertion (Nybo and Nielsen, 2001b). It is likely that this serves as a protective mechanism limiting further heat production when body temperature reaches levels that may be detrimental to the organism as a whole, but the neurobiological mechanisms for these responses are not clear at present. The suggestion that serotonin-mediated fatigue is important during exercise in the heat is partially supported by the work of Mittleman and colleagues (1998). A 14 % increase in time to exhaustion in warm ambient conditions (34.4oC) was reported following BCAA supplementation when compared to a polydextrose placebo, with no apparent difference in peripheral markers of fatigue between trials. The authors concluded that the supplementation regimen was successful in limiting the entry of TRP into the CNS, attenuating serotonin-mediated fatigue. It is perhaps important to note that from this study this conclusion seems premature as only TRP, BCAA, and other metabolic parameters (giving no insight on brain functioning) were measured. Furthermore, core temperature at fatigue was significantly below values suggested as limiting (< 38oC). Two subsequent studies have examined the effects of BCAA supplementation on human performance and thermoregulation in the heat (Cheuvront et al., 2004; Watson et al., 2004). Cheuvront et al. (2004) reported that BCAA, when combined with CHO, did not alter time-trial performance, cognitive performance, mood, RPE, thermal comfort and rectal temperature in the heat when subjects are hypohydrated. In this study hypohydration was used in order to increase plasma osmolality and increase thermoregulatory and cardiovascular strain. Additionally, ingestion of BCAA solution prior to, and during, prolonged exercise in glycogen- depleted subjects did not influence exercise capacity, rectal and skin temperature, heart rate, RPE and perceived thermal stress despite a four-fold reduction in the plasma concentration ratio of f-TRP to BCAA (Watson et al., 2004). To date there has been little investigation of the influence of pharmacological agents acting on the CNS on the response to prolonged exercise in a warm environment. A series of studies conducted by Strachan and colleagues have recently investigated the effects of acute 5-HT agonist (paroxetine) and 5-HT2C receptor antagonist (pizotifen) administration (Strachan et al., 2004; Strachan et al., 2005). Neither treatment influenced exercise performance, but pizotifen did produce a marked elevation in core temperature at rest and during exercise, suggesting a role for the 5-HT receptor in the regulation of core temperature. As DA and NA have been implicated in arousal, motivation, reinforcement and reward, the control of motor behaviour and mechanisms of addiction we recently explored the possible interaction between high ambient temperature, and possible underlying neurotransmitter drive during exercise, using a dual DA/NA reuptake inhibitor (Watson et al., 2005a). Subjects ingested either a placebo or bupropion (Zyban), prior to exercise in temperate (18oC) or warm (30oC) conditions. Two important findings arise from this study: 1) subjects completed a pre-loaded TT 9% faster when bupropion was taken before exercise in a warm environment compared to a placebo treatment. This ergogenic effect was not apparent 2) Seven (of 9) subjects in the heat attained core temperatures equal to, or greater than, 40°C in the bupropion trial, compared to only two during the placebo trial. It is possible that this drug may dampen or override inhibitory signals arising from the CNS to cease exercise due to hyperthermia, and enable an individual to continue to maintain a high power output. It is important to note, however, that this response appeared to occur with the same perception of effort and thermal stress reported during the placebo trial, and may potentially increase the risk of developing heat illness. As evidence for a role of 5-HT during exercise in the heat is limited (Cheuvront et al., 2004; Strachan et al., 2004; Watson et al., 2004) these data suggest that catecholaminergic neurotransmission may act as an important neurobiological mediator of fatigue under conditions of heat stress. It appears that when exercise is performed in high ambient temperatures, the development of central fatigue appears to be accelerated, leading to a loss of drive to continue. This may explain why individuals tend to cease exercise long before muscle glycogen stores reach levels thought to be limiting (Parkin et al., 1999). Until recently there have been few studies to focus directly on the relationship between brain neurotransmission, thermoregulation and exercise performance/exercise capacity in a warm environment. Therefore, further research, including both pharmacological and nutritional manipulation are necessary to elucidate the role of specific neurotransmitter functions during exercise in the heat. Additionally, the significance of this in relation to the pattern of activity associated with football has yet to be determined.
Conclusions
It is clear that the cause of fatigue is complex, influenced by both events occurring in the periphery and the CNS. The 'Central Fatigue Hypothesis' is based on the assumption that the synthesis and metabolism of central monoamines are influenced during prolonged exercise, consequently affecting subjective sensations of lethargy and tiredness, causing an altered sensation of effort, perhaps a differing tolerance of pain/discomfort and a loss of drive and motivation to continue exercise. Since its conception, Newsholme's original hypothesis has been developed to include the possibility that other neurotransmitters and neuromodulators, in particular the catecholamines, dopamine and noradrenaline, are also involved in the development of fatigue. Much of the attraction of neurotransmitter-mediated fatigue was the potential for nutritional manipulation of neurotransmitter precursors to delay the onset of central fatigue, potentially enhancing performance. When exercise is performed in temperate conditions it seems that manipulation of brain neurotransmission through amino acid supplementation or pharmacological means has little effect (either negatively or positively) on physical performance. While there is some evidence that BCAA and TYR ingestion can influence perceived exertion and various measures of mental performance (e.g. memory, tracking, cognitive function), the results of several apparently well-controlled laboratory studies have not demonstrated a positive effect on exercise capacity or performance. As football is highly dependent on the successful execution of motor skills and tactics, the possibility that amino acid ingestion may attenuate a loss in cognitive function occurring during the later stages of a game would be desirable, despite no apparent benefit to physical performance. It is clear that CHO ingestion can have a profound effect on measures of physical fatigue during high- intensity intermittent activity, with marked improvements in time to volitional exhaustion, maintenance of sprint performance and vertical jump height. The beneficial effect of CHO supplementation during prolonged exercise may also relate to increased (or maintained) substrate delivery for the brain. Several studies indicate that hypoglycaemia affects brain function, and cognitive performance. There are indications that CHO during exercise minimises the negative effect of central fatigue induced by prolonged exercise on cognitive function. These largely inconsistent findings, make it difficult to reach a firm decision regarding the role of central neurotransmission in the fatigue process at this stage, but it seems premature to discount its importance in the light of studies investigating amphetamines and other CNS stimulants. Additionally, evidence for a central component of fatigue during prolonged exercise in a warm environment appears to be convincing, with hyperthermia demonstrated to reduce maximal muscle activation, alter brain activity and increase perceived exertion. To date there has been little investigation of the influence of nutritional or pharmacological manipulation of central neurotransmission on the response to exercise in a warm environment, and further research in this area seems warranted. Fatigue and especially ‘Central Fatigue' is a complex and multifaceted phenomenon. There are several other possible cerebral factors that might limit exercise performance, all of them influencing signal transduction, since the brain cells communicate through chemical substances. Not all of these relationships have been explored in detail, and the complexity of brain neurochemical interactions will probably make it very difficult to construct a single or simple statement that covers the ‘Central Fatigue' phenomenon.
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