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 g iven into the tibialis anterior (TA) muscle over 20 s with an inter-infusio ns interval of 3 min. Infusions of isotonic saline (NaCl, 0.9%) were given as control. Pain intensity was continuously scored on a visual analogue sca le (VAS), and subjects drew the distribution of the pain areas on an anatom ical 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 PP Ts were done at the contralateral TA and ankle. Skin sensibility was assess ed with a Von Frey hair at the same sites. All measurements were done befor e and 5, 20, 40, and 60 min after infusions. The VAS-peak after BKN was sig nificantly 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 significantl y longer (P < 0.05) compared with the infusions of NaCl. The local pain are a 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 signifi cantly decreased(P < 0.05) after 5-HT + BKN at 5 cm (5 and 20 min) and 10 c m (5 min). Serotonin may enhance the effect of bradykinin in producing expe rimental muscle pain and muscle hyperalgesia to mechanical stimuli. The com bination of serotonin and bradykinin can produce muscle hyperalgesia, laste d for up to 40 min and located within the muscle. No widespread hyperalgesi a to the ankle and other leg (tested at 10 cm below the patella and ankle) was observed suggesting a predominant peripheral origin of the experimental ly induced hyperalgesic stage. (C) 2000 Elsevier Science B.V. All rights re served.