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

Abstract

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.

Peripheral role of glutamate in orofacial pain

“…A clinical study showed that there were differences in the masseter muscle levels of glutamate during acute nociception in patients with myofascial TMD compared with healthy subjects ( Bajramaj et al, 2019 ). Injecting glutamate into the human masseter muscle or temporomandibular joint causes acute pain and/or mechanical allodynia ( Svensson et al, 2005 ; Castrillon et al, 2012 ; Suzuki et al, 2017 ; Exposto et al, 2018 ; Udagawa et al, 2018 ; Shimada et al, 2020 ; Alhilou et al, 2021a ), which is confirmed in animals ( Cairns et al, 2002 ; Lam et al, 2009 ; Jie et al, 2018 ). When NMDA is injected into the rat masseter muscle, an increase occurs in muscle afferent discharge in a dose-related manner ( Dong et al, 2007 ).…”

“…The glutamatergic system plays a key role in the pathogenesis of orofacial pain. Glutamate was injected into the masseter muscle in healthy participants and elicited pain and muscle-referred sensations ( Suzuki et al, 2017 ; Exposto et al, 2018 ; Udagawa et al, 2018 ; Shimada et al, 2020 ). Masseteric injection of NGF and glutamate increased expression of peripheral NMDA receptors and could be associated with masseter muscle pain sensitivity in healthy humans ( Alhilou et al, 2021a , b ).…”