The lactate-GPR81-PKA axis regulates exercise-induced central fatigue in male mice

Exercise-induced fatigue is regulated by central nervous system (CNS)-derived factors, including neurotransmitters and metabolic signals; however, the underlying mechanisms remain incompletely understood. This study aimed to test whether lactate activates Gi-protein coupled receptor 81 (GPR81) in the motor cortex to inhibit protein kinase A (PKA) phosphorylation, thereby contributing to central fatigue during exercise. Using a murine weight-loaded swimming model, we found that PKA phosphorylation in the motor cortex increased significantly after 15 min of swimming, but decreased markedly after swimming to exhaustion. Notably, inhibition of PKA phosphorylation by local administration of H-89 in the motor cortex shortened the swimming time of mice (H-89: 20 ± 5 min vs. saline 38 ± 6 min, P < 0.05). In addition, we found that activation of the lactate receptor GPR81 by local administration of 3-chloro-5-hydroxybenzoic acid (CHBA) or l-lactate attenuated exercise-induced upregulation of PKA phosphorylation. Conversely, genetic ablation of GPR81 (GPR81^-/-) mitigated the inhibitory effect of lactate on PKA phosphorylation, resulting in a 33% increase in swimming endurance. Despite comparable peripheral fatigue markers (blood lactate, skeletal muscle glycogen, and gastrocnemius p-AMPK/AMPK ratio) after 30 min of swimming, GPR81^-/- mice exhibited elevated motor cortical glutamate/GABA ratios, indicating preserved neuronal excitability. Therefore, our study reveals a vital role of the lactate-GPR81 signaling axis in the motor cortex during exercise and provides a potential target for alleviating exercise-induced central fatigue.NEW & NOTEWORTHY Exercise-induced fatigue is regulated by factors derived from the central nervous system (CNS), including neurotransmitters and metabolic signals. However, the underlying mechanisms remain largely obscure. Here, we demonstrate that lactate activates GPR81 in the motor cortex to inhibit PKA phosphorylation, thereby contributing to central fatigue during exercise. These findings reveal a vital role of lactate-GPR81-PKA axis in the motor cortex during exercise and provide a potential target for alleviating exercise-induced central fatigue.