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

Publisher Website
Google Scholar
Abstract
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.