Pain & allodynia/hyperalgesia induced by intramuscular injection of serotonin in patients with fibromyalgia (FM) & healthy individuals

The aim of this study was to investigate the effect of injection of serotonin (5-HT) into the masseter muscle on pain and allodynia/hyperalgesia. Twelve female patients with fibromyalgia (FM) and 12 age-matched female healthy individuals (HI) participated in the study. The current pain intensity (CPI) and the pressure pain threshold (PPT) of the superficial masseter muscles were assessed bilaterally. 5-HT in one of three randomized concentrations (10(-3), 10(-5), 10(-7) M) or isotonic saline was then injected into either of the two masseter muscles in a double-blind manner. After the injections the CPI and PPT were recorded ten times during 30 min. The injections were repeated twice with the other concentrations of 5-HT after 1 and 2 weeks, respectively. In the FM-group there was a non-significant increase of CPI after injection that lasted during the entire 30-min period irrespective of whether 5-HT or saline was injected. Neither did the PPT change significantly. In the HI-group pain developed significantly after injection irrespective of whether 5-HT or saline was injected, but significantly more so after 5-HT at 10(-3) M than saline injection. CPI decreased quickly and then remained on a very low level for most of the experiment. 5-HT at both 10(-5) M and 10(-3) M caused a significantly greater decrease of PPT than saline. In conclusion, our results show that 5-HT injected into the masseter muscle of healthy female subjects elicits pain and allodynia/hyperalgesia, while no such responses occur in patients with fibromyalgia.

Ernberg M, Lundeberg T, Kopp S

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