Fig. 1

The experimental design of this study is shown. At baseline (BL), 15 min after the systemic administration, 5 and 10 min after each injection, and after again 90 min after systemic administration of monosodium glutamate (MSG) or NaCl, numeric rating scale (NRS) of pain intensity was reported by each participant. The participants were asked to draw painful area at the same time points as NRS. Pressure pain threshold (PPT) was also measured in the left and right Masseter (MAL, MAR), left and right Temporalis, the right Trapezius (TRP), the left Thenar (TH), together with systolic and diastolic blood pressure (sBP, dBP) and heart rate (HR). The participants were asked to report adverse effects 15 min after the systemic intervention. Saliva samples were collected at baseline, just before each injection was applied, and 90 min post oral ingestion of MSG or NaCl