Influence of glutamate-evoked pain and sustained elevated muscle activity on blood oxygenation in the human masseter muscle

This study aimed to investigate the effect of glutamate-evoked masseter muscle pain on intramuscular oxygenation during rest and sustained elevated muscle activity (SEMA). Seventeen healthy individuals participated in two sessions in which they were injected with glutamate and saline in random order. Each session was divided into three, 10-min periods. During the first (period 1) and the last (period 3) 10-min periods, participants performed five intercalated 1-min bouts of masseter SEMA with 1-min periods of 'rest'. At onset of the second 10-min period, glutamate (0.5 ml, 1 M; Ajinomoto, Tokyo, Japan) or isotonic saline (0.5 ml; 0.9%) was injected into the masseter muscle and the participants kept the muscle relaxed in a resting position for 10 min (period 2). The hemodynamic characteristics of the masseter muscle were recorded simultaneously during the experiment by a laser blood-oxygenation monitor. The results demonstrated that glutamate injections caused significant levels of self-reported pain in the masseter muscle; however, this nociceptive input did not have robust effects on intramuscular oxygenation during rest or SEMA tasks. Interestingly, these findings suggest an uncoupling between acute nociceptive activity and hemodynamic parameters in both resting and low-level active jaw muscles. Further studies are needed to explore the pathophysiological significance of blood-flow changes for persistent jaw-muscle pain conditions.