ABN99 Sigma-AldrichAnti-phospho NR1 (Ser897) Antibody

Chronic musculoskeletal pain is a significant health problem and is associated with increases in pain during acute physical activity. Regular physical activity is protective against many chronic diseases; however, it is unknown if it plays a role in development of chronic pain. The current study induced physical activity by placing running wheels in home cages of mice for 5 days or 8 wk and compared these to sedentary mice without running wheels in their home cages. Chronic muscle pain was induced by repeated intramuscular injection of pH 4.0 saline, exercise-enhanced pain was induced by combining a 2-h fatiguing exercise task with a low-dose muscle inflammation (0.03% carrageenan), and acute muscle inflammation was induced by 3% carrageenan. We tested the responses of the paw (response frequency) and muscle (withdrawal threshold) to nociceptive stimuli. Because the rostral ventromedial medulla (RVM) is involved in exercise-induced analgesia and chronic muscle pain, we tested for changes in phosphorylation of the NR1 subunit of the N-methyl-D-aspartate (NMDA) receptor in the RVM. We demonstrate that regular physical activity prevents the development of chronic muscle pain and exercise-induced muscle pain by reducing phosphorylation of the NR1 subunit of the NMDA receptor in the central nervous system. However, regular physical activity has no effect on development of acute pain. Thus physical inactivity is a risk factor for development of chronic pain and may set the nervous system to respond in an exaggerated way to low-intensity muscle insults.

Pain in response to physical activity is common in people with chronic musculoskeletal pain and is likely a barrier to regular exercise, which would lead to a sedentary lifestyle. We recently developed a model of exercise-induced pain that is associated with increased activation of neurons in the medullary raphe nuclei, i.e., the nucleus raphe obscurus (NRO) and nucleus raphe pallidus (NRP). Because the NRO and NRP not only modulate motor output but also respond to noxious stimuli, we hypothesized that the NRO and NRP were key nuclei in the interaction between pain and exercise. We tested whether exercise enhances hyperalgesia through activation of N-methyl D-aspartate (NMDA) receptors in the NRO/NRP.Muscle insult was induced by two injections of pH 5.0 saline 5 d apart into one gastrocnemius muscle. We initially tested whether hyperalgesia developed in mice injected with acidic saline (pH 5.0) into the gastrocnemius muscle immediately after a 30-min or 2-h exercise task or 2 h after a 2-h exercise task. Next, we tested whether blockade of NMDA receptors in the NRO/NRP during the exercise task prevented the development of exercise-induced hyperalgesia. Finally, we evaluated changes in phosphorylation of the NR1 subunit of the NMDA receptor (pNR1) after the exercise task at times in which muscle insult was given in behavioral experiments, i.e., immediately after a 30-min or 2-h exercise task or 2 h after the 2-h exercise task.All exercise conditions enhanced nociception (hyperalgesia) after combining with two injections of pH 5.0 saline. Microinjection of AP5 (1.0-0.1 nmol; 2-amino-5-phophonopenanoate) dose-dependently prevented the development of exercise-induced hyperalgesia. All exercise conditions increased pNR1 in the NRO and NRP.Thus, exercise-induced pain in sedentary mice is associated with increased phosphorylation and activation of NMDA receptors in the NRO/NRP, suggesting that changes in central excitability mediate an interaction between unaccustomed exercise and pain.