Duration and distribution of experimental muscle hyperalgesia in humans following combined infusions of serotonin and bradykinin

The present study examined distribution and duration of muscle hyperalgesia to pressure stimuli after intramuscular bolus-infusions of serotonin (5-HT, 20 nmol) and bradykinin (BKN, 10 nmol) in 10 volunteers. Infusions were given into the tibialis anterior (TA) muscle over 20 s with an inter-infusions interval of 3 min. Infusions of isotonic saline (NaCl, 0.9%) were given as control. Pain intensity was continuously scored on a visual analogue scale (VAS), and subjects drew the distribution of the pain areas on an anatomical map. Pressure pain thresholds (PPTs) were assessed with an electronic algometer at the injection site (10 cm below the patella), 2, 5, and 10 cm discal from the injection site, and at the ankle. Control assessments of PPTs were done at the contralateral TA and ankle. Skin sensibility was assessed with a Von Frey hair at the same sites. All measurements were done before and 5, 20, 40, and 60 min after infusions. The VAS-peak after BKN was significantly higher (P < 0.05) compared with 5-HT and the second infusion of NaCl. The duration of the increase in VAS after 5-HT + BKN was significantly longer (P < 0.05) compared with the infusions of NaCl. The local pain area after infusion of BKN was significantly larger (P < 0.05) compared with 5-HT and control infusions. Cutaneous sensibility to tactile stimuli was not affected by any of the combinations. PPTs at the injection site and 2 cm (5, 20, and 40 min) were significantly decreased (P < 0.05) after 5-HT + BKN compared with baseline and isotonic saline. In addition, PPTs were significantly decreased(P < 0.05) after 5-HT + BKN at 5 cm (5 and 20 min) and 10 cm (5 min). Serotonin may enhance the effect of bradykinin in producing experimental muscle pain and muscle hyperalgesia to mechanical stimuli. The combination of serotonin and bradykinin can produce muscle hyperalgesia, lasted for up to 40 min and located within the muscle. No widespread hyperalgesia to the ankle and other leg (tested at 10 cm below the patella and ankle) was observed suggesting a predominant peripheral origin of the experimentally induced hyperalgesic stage. (C) 2000 Elsevier Science B.V. All rights reserved.