Experimental human muscle pain induced by intramuscular injections of bradykinin, serotonin, and substance P

After intramuscular (m. tibialis anterior) injection of three different algogenic substances, the pain intensity was continuously scored on a visual analogue scale (VAS) in eight volunteers. The subject drew the distribution of the local and referred pain areas on a map. Four times within the first hour after injection, the pressure pain-thresholds (PPTs) and supra pressure-pain thresholds were assessed at the injection point, 2 cm distal from the injection site, at the arm, and at the contralateral leg. Measurements were done before and after injection of 0.5 ml of the algogenic substance [bradykinin (BKN), serotonin (5-HT), substance P (SP)], and isotonic saline as control. Cutaneous sensitivity to mechanical stimuli was assessed with a Von Frey hair at the same location as PPT determinations. The pain intensity (VAS-peak) after BKN (2, 4, and 10 nmol) and 5-HT (2, 4, and 20 nmol) was significantly higher (p < 0.05) than after SP (0.2, 0.4, and 0.8 nmol) and isotonic saline. The VAS-peak after infusions of hypertonic saline was significantly higher (p < 0.05) compared with VAS-peaks after all other substances. A significantly larger (p < 0.05) local pain area was found after BKN compared with isotonic saline. After injections of hypertonic saline, the offsets of evoked pain were significantly longer (p < 0.05) and the local and referred pain areas were significantly larger (p < 0.05) compared with all other substances. There was no dose-response relation between the pain intensity and the different doses of BKN, 5-HT: and SP. PPTs and skin sensitivity were not affected by any of the injections. We conclude that under the present experimental conditions, BKN and 5-HT can produce low levels of muscle pain after intramuscular injection. In the used concentrations, however, BKN, 5-HT, and SP did not generate cutaneous or muscular hyperalgesia.