Abstract
BackgroundMonosodium glutamate (MSG) is often thought to be associated with headache and craniofacial pains like temporomandibular disorders. This randomized, double-blinded, placebo-controlled study was performed to investigate how ingestion of MSG affects muscle pain sensitivity before and after experimentally induced muscle pain.MethodsSixteen healthy adult subjects participated in 2 sessions with at least 1-week interval between sessions. In each session, two injections of glutamate (Glu, 0.5 M, 0.2 ml) and two injections of saline (0.9 %, 0.2 ml) into the masseter and temporalis muscles, respectively, were undertaken, with a 15 min interval between each injection. Injections of saline were made contralateral to Glu injections and done in a randomized order. Participants drank 400 mL of soda mixed with either MSG (150 mg/kg) or NaCl (24 mg/kg, placebo) 30 min before the intramuscular injections. Pressure pain thresholds (PPT), autonomic parameters and pain intensity were assessed prior to (baseline) and 30 min after ingestion of soda, as well as 5 min and 10 min after the intramuscular injections and at the end of the session. Whole saliva samples were collected prior to and 30, 45, 60, and 75 min after the ingestion of soda.ResultsMSG administration resulted in a significantly higher Glu level in saliva than administration of NaCl and was associated with a significant increase in systolic blood pressure. Injections of Glu were significantly more painful than injections of NaCl. However, ingestion of MSG did not change the intensity of Glu-evoked pain. Glu injections also significantly increased systolic and diastolic blood pressure, but without an additional effect of MSG ingestion. Glu injections into the masseter muscle significantly reduced the PPT. However, pre-injection MSG ingestion did not significantly alter this effect. Interestingly, PPT was significantly increased in the trapezius after MSG ingestion and intramuscular injection of Glu in the jaw muscles.ConclusionThe main finding in this study was that systemic intake of a substantial amount of MSG does not influence either pain intensity or pressure pain sensitivity in the masseter and temporalis muscles into which Glu injections were made.
Highlights
Monosodium glutamate (MSG) is often thought to be associated with headache and craniofacial pains like temporomandibular disorders
We have previously reported that injection of glutamate (Glu) into the masseter muscle of healthy subjects causes experimental muscle pain with features comparable to the pain suffered in myofascial temporomandibular disorder (TMD) [1]
The data are Experimental pain In both the MSG and placebo sessions, the mean pain intensity 5 min after Glu injection into the masseter or temporalis muscles was significantly higher than the isotonic saline (IS) injection into the same muscles (P < 0.001) (Fig. 2)
Summary
Monosodium glutamate (MSG) is often thought to be associated with headache and craniofacial pains like temporomandibular disorders. This randomized, double-blinded, placebo-controlled study was performed to investigate how ingestion of MSG affects muscle pain sensitivity before and after experimentally induced muscle pain. We have previously reported that injection of glutamate (Glu) into the masseter muscle of healthy subjects causes experimental muscle pain with features comparable to the pain suffered in myofascial temporomandibular disorder (TMD) [1]. Intramuscular injection of Glu produces pain in healthy subjects that is not different in intensity and quality from pain reported by patients with myofascial TMD [1]. Recent work indicates that repeated daily administration of MSG orally to healthy individuals leads to more frequent reports of adverse effect, such as headache and nausea and results in a transient but significant lowering of pressure pain threshold and tolerance in the masseter muscle [5]
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