Changes induced by physical training were examined by in vivo microdialysis monitoring of extracellular dopamine and its metabolites in the rat nucleus accumbens, and by an immunohistochemical staining for fos protein as an index of nerve cell excitation.
The exercise training consisted of running on a treadmill 5 days/week for 4 weeks, with gradually increasing the running speed from 10 to 20 m/min for 20 min. The control group performed forced treadmill running for 3 days.
The sulpiride-induced increase in both dopamine release and dopamine metabolism was significantly attenuated in the nucleus accumbens of training rats in comparison with the controls, which indicated that physical training induced subsensitive DA D₂ receptors in the central dopaminergic neurons.
Fos protein expression following 1 h running was weaker in neurons of the raphe dorsalis, the locus ceruleus and the amygdala of trained rats than in those of controls. The present findings suggest that both neurochemical response to various stimuli, and adaptation induced by physical training differ from those in untrained animals.

To elucidate the phenomenon of central fatigue, we investigated the changes in the plasma amino acid level and false neurotransmitter (FNT) level during ultra-endurance exercise. Twenty-one male Japanese participants of the 1993 Ironman Japan Contest, held at Lake Biwa, were examined. Blood sampling and a questionnaire on the Profile of Mood States (POMS) test were collected two days before, immediately after and one-day following the race. The POMS questionnaire showed various changes especially due to fatigue. Indeed, the fatigue score significantly increased after the race. Although the confusion score significantly increased after the race, it quickly recovered to the pre race level the following day. The plasma concentration of the branched-chain amino acids (BCAA) decreased after the race, while the level of aromatic amino acids (AAA) increased, thus resulting in a significant decrease in the BCAA/AAA ratio. The plasma level of octopamine, one type of FNT, significantly increased after the race (p<0.01) . The blood ammonia level increased after the race, but recovered to the pre-race level one-day after the race. Although no change was observed in the total tryptophan (Trp) level, the level of free Trp significantly increased during the race. The significant increases in the plasma 5-hydroxyindoleacetic acid (5HIAA), homovanillic acid (HVA) and HVA/dihydroxyphenylacetic acid (DOPAC) ratios thus suggest the serotonin (5 HT) and dopamine (DA) neuron systems activated after the race. In conclusion, ultra-endurance exercise caused a disorder of both the amino acid and monoamine metabolisms. Furthermore these changes were also related with mood changes, especially fatigue, i. e. central fatigue. These results suggest that the measurement of both the plasma BCAA/AAA ratio and FNT may therefore be useful in quantifying exercise induced central fatigue.