The Roles And Mechanisms Of Spinal 5-hydroxytryptomine System In Exercise-induced Fatigue

Exercise-induced fatigue may arise not only because of peripheral changes at the level of muscle,but also because of the central nervous system failing to drive the motoneurons adequately(central fatigue).As central fatigue appears so commonly in human performance,researchers of the sports field have always been focusing on the CNS mechanisms of fatigue.The spinal motoneurons(MNs)as the final pathway not only send their signals directly to the muscle but receive nearly all signals related to exercise or posture,then regulate the firing rate of motor unit and the contraction capability of muscles by integrating all these information derived from central or peripheral system.Recently,more and more scientists have been concerning about the mechanism of central fatigue at spinal level.Studies by electrophysiological methods confirme that the excitability and discharge frequency of spinal MNs declined during sustained fatiguing contraction of muscles.The inhibition of the MNs may be an important contributor to the development of fatigue during sustained exercise,but the cause is unsolved.It's an urgent problem why spinal MNs are inhibited during or after fatiguing exercise.At spinal level the presynaptic modulation by neuromodulatory or neurotransmitter acting on MNs are likely to be an important influences on the behavior of the MNs.The present study were first designed to determine the relationship between exercise-induced fatigue and neurontransmitter within spinal cord.After establishment of the animal models,the changes of main monoamine and amino acid neurotransmitter were analysed by HPLC in the cervical spinal cord in different period after fatigue.The results showed that monoamine neurotransmitter serotonin(5-hydroxytriotamine,5-HT) decreased significantly and didn't recover even after 24h of the last bout fatiguing exercise.These data regarded the 5-HT system in spinal cord as an important contributor to exercise-induced fatigue.So the changes of electromyography(EMG)and the excitability of MNs were recorded by microinjection of 5-HT in the local ventral horn of spinal cord after fatigue.Further study about the changes of the synthesis, metabolism,transportation and related receptors of 5-HT after exercise-induced fatigue were carried out.These work had great importance to interpretation of the central fatigue and might open up new research strategies for reducing fatigue or delaying the onset of the fatigue.Here are the results:1.Establishment of the exercise-induced fatigue model.Rats were divided into 2 groups:the control group(C)and the exercise-induced fatigue group(E).After 10 days of loading-increasing treadmill running,the rats in E group showed the condition of fatigue:the body weight increased slowly,the exercise ability had obvious tendency to decline;the Hb decreased dramatically(p＜0.05); whereas blood BUN,lactic acid and CK increased significantly(p＜0.05).Both the behavior and chemical indexes showed that the exercise-induce fatigue model was established.2.Investigation of the changes of neurotransmitter concentration in spinal cord after exercise-induced fatigue.The results showed:amino acid neurotransmitters in spinal cord increased immediately after the last time of training(0E),Glu,GABA increased significantly(p＜0.05),Gly also increased but without statistic difference;while 3h after the last time of training(3E),amino acid neurotransmitters got back to normal level.Monoamine neurotransmitters:NE,5-HT tended to decrease in 0E group but in 3E group 5-HT decreased dramatically(p＜0.05);even at low level 24h after the last time of training. The results suggested that neurotransmitter 5-HT in spinal cord might have been involved in exercise-induced fatigue. 3.Effect of 5-HT on EMG/excitability of motoneurons after exercise-induced fatigue.The ratio of EMG Hmax/Mmax decreased significantly after exercise(p＜0.05). Whereas after microinjection of serotonin the decreased ratio of Hmax/Mmax recovered almost to normal level.So the results confirmed our hypothesis that 5-HT in spinal cord had an important effect on facilitating the excitability of motoneuron after exercise-induced fatigue.4.Detection of the changes of 5-HT-nergic system after exercise-induced fatigue.The most important finding of our work was:There was TPH1mRNA expression in spinal cord without detectable TPH2mRNA expression;furthermore,fluorescent quantitative real-time PCR revealed that the expression of TPH1mRNA after fatigue decreased significantly(p＜0.05)to 5.48%of the control value.In situ hybridization histochemistry(ISHH)results showed that TPH1 hybridization signals in spinal cord and NRO/NRP region of the rats in exercise group decreased dramatically the(p＜0.05).All the findings might implicate that:(1)Independent serotonergic neurons might be located in spinal cord.(2)TPH1 as a causal factor in regulating the synthesis of 5-HT in spinal cord after exercise-induced fatigue would be worthy of further research.However 5-HT transporter and related 5-HT receptors had no statistic changes after exercise induced fatigue.Overall,the above resuls supported the hypothesis that the decreased concentration of 5-HT especially in the ventral horn of spinal cord after exercise-induced fatigue might contribute to disfacilitating the excitability of spinal motoneurons and reducing the motor output which eventrally lead to increase in exertion during exercise and thereby hasten the onset of fatigue.TPH1 might be a likely target in the modulatory pathway for serotonergic functions in spinal cord after fatigue.Moreover,it is worthy of research for the modulation of TPH1 expression after fatigue.