Exercise-induced pain requires NMDA receptor activation in the medullary raphe nuclei

Abstract

Purpose: 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.

Methods: 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.

Results: 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.

Conclusions: 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.

FIGURE 1

Graphs show the mechanical sensitivity of the paw in response to an exercise task in combination with muscle insult. The values before muscle insult are shown with open symbols, and those after the muscle insult, with or without exercise, are shown with closed symbols. The ipsilateral side is shown with circles, and the contralateral side is shown with squares. The number of withdrawals to repeated application of von Frey filaments of increasing bending forces applied to the paw in the group that did not receive the exercise task (A). *P < 0.05. The no-exercise group was significantly different from the group that performed the 2-h exercise task and with the second acid injection given immediately after the exercise task (B), the group that performed the 30-min exercise task and with the second acid injection given immediately after the exercise task (C), and the group that performed the 2-h exercise task and with the second acid injection given 2 h after the exercise task (D).

FIGURE 2

Withdrawal threshold of the muscle for the ipsilateral (A) and the contralateral (B) sides is shown for each group. *Significantly different from uninjected controls (P < 0.05); +significantly different from pH 5.0 injections without the exercise task (P < 0.05).

FIGURE 3

Line graphs represent the number of withdrawal to repeated application of von Frey filaments for different bending forces in animals that were pretreated in the NRO/NRP with different doses of AP5 or vehicle before the exercise task. The withdrawal thresholds were significantly lower in the animals treated with 1 and 0.3 nmol of AP5. *Significantly different from vehicle, P < 0.05.

FIGURE 4

Immunohistochemical staining for pNR1 in the NRO and the NRP in each group: unexercised control, 2-h exercise immediately after task, 2-h exercise 2 h after task, and 30-min exercise immediately after task. Notice the increased number of positively labeled cells after the fatigue task compared with controls.

FIGURE 5

Bar graph showing the number of cells in five sections counted from the NRO and the NRP in each group: unexercised control, 2-h exercise immediately after task, 2-h exercise 2 h after task, and 30-min exercise immediately after task. Significant increases occurred for all groups with the greatest increase immediately after the 2-h exercise task. *Significantly less than P < 0.05.