Neuroplasticity of dopamine circuits after exercise: Implications for central fatigue

Habitual exercise increases plasticity in a variety of neurotransmitter systems. The current review focuses on the effects of habitual physical activity on monoamine dopamine (DA) neurotransmission and the potential implication of these changes to exercise-induced fatigue. Although it is clear that peripheral adaptations in muscle and energy substrate utilization contribute to this effect, more recently it has been suggested that central nervous system pathways "upstream" of the motor cortex, which initiate activation of skeletal muscles, are also important. The contribution of the brain to exercise-induced fatigue has been termed "central fatigue." Given the well-defined role of DA in the initiation of movement, it is likely that adaptations in DA systems influence exercise capacity. A reduction in DA neurotransmission in the substantia nigra pars compacta (SNpc), for example, could impair activation of the basal ganglia and reduce stimulation of the motor cortex leading to central fatigue. Here we present evidence that habitual wheel running produces changes in DA systems. Using in situ hybridization techniques, we report that 6 weeks of wheel running was sufficient to increase tyrosine hydroxylase mRNA expression and reduce D2 autoreceptor mRNA in the SNpc. Additionally, 6 weeks of wheel running increased D2 postsynaptic receptor mRNA in the caudate putamen, a major projection site of the SNpc. These results are consistent with prior data suggesting that habitually physically active animals may have an enhanced ability to increase DA synthesis and reduce D2 autoreceptor-mediated inhibition of DA neurons in the SNpc compared to sedentary animals. Furthermore, habitually physically active animals, compared to sedentary controls, may be better able to increase D2 receptor-mediated inhibition of the indirect pathway of the basal ganglia. Results from these studies are discussed in light of our understanding of the role of DA in the neurobiological mechanisms of central fatigue.