No release of interstitial glutamate in experimental human model of muscle pain

Glutamate may be released from muscle nociceptors and thereby contribute to mechanisms underlying acute and chronic muscle pain. In vivo concentration of glutamate during muscle pain has not previously been studied in either animals or humans. In the present study, we aimed to study the in vivo concentration of glutamate before, during and after acute pain of trapezius muscle in humans using the microdialysis technique. In addition, we examined the nutritive skeletal muscle blood flow and the interstitial concentrations of lactate, glucose, glycerol, pyruvate and urea. Experimental pain and tenderness were induced by intramuscular infusion of a chemical mixture consisting of bradykinin, prostaglandin E(2), histamine and serotonin. One EMG-needle and one microdialysis catheter were inserted into non-dominant and dominant trapezius muscles on a standard anatomical point in 19 healthy subjects. Dialysates were collected at rest, during infusion and 60 and 120 min after stop of infusion. Local tenderness was recorded at baseline and at the end of experiment. Local pain was recorded during infusion. The infusion of chemical mixture was more painful than infusion of placebo (p < 0.05) and resulted in significantly higher local tenderness score than placebo (p = 0.007). There was no difference in change in interstitial concentrations of glutamate, lactate, glucose, glycerol, pyruvate and urea from baseline to infusion and post-infusion periods between chemical mixture and placebo (p > 0.05). Muscle blood flow increased significantly over time in response to infusion of chemical mixture and placebo (p = 0.001). However, we found no difference in changes in muscle blood flow between chemical mixture and placebo (p > 0.05). In conclusion, the present study demonstrates no signs of increased release of glutamate from myofascial nociceptors during and after acute experimentally induced muscle pain and tenderness.