Experimental human muscle pain and muscular hyperalgesia induced by combinations of serotonin and bradykinin

In the present study, we assessed the muscle pain and possible development of muscular hyperalgesia to mechanical stimuli after two subsequent intramu scular infusions of serotonin (5-HT) and bradykinin (BKN). The pain intensi ty after the infusions was continuously scored on a visual analogue scale ( VAS). The subjects drew the distribution of the pain areas on a map. Pressu re pain thresholds (PPTs) and suprapressure pain thresholds (SPPTs) stimula tions as 150% of the pre-infusion PPTs were assessed with a pressure algome ter at the injection site (10 cm below the patella), at the ankle, and at t he contralateral leg and ankle. Skin sensibility was assessed with a Von Fr ey hair at the same sites. This was done before and after an infusion into the tibialis anterior (TA) muscle on the right leg in ten volunteers. The f irst infusion in each combination was either serotonin (20 nmol) or isotoni c saline (NaCl 0.98). The second infusion was bradykinin (5 or 10 nmol) or isotonic saline. The two infusions were given over 20 s and separated by 3 min. The isotonic saline followed by BKN did not induce muscle pain or musc ular hyperalgesia. However, the combination of 5-HT and BKN (10 nmol) produ ced: (1) significantly higher VAS scores (P < 0.05) compared with all other combinations; (2) significantly longer pain offset(P < 0.05) compared with the combinations of isotonic saline and BKN; (3) significantly lower PPTs at 5, 20, and 40 min post-infusion (P < 0.05) compared with baseline PPT an d PPTs after all other combinations. Cutaneous sensibility to mechanical st imuli and SPPTs were not affected by any of the combinations. The combinati ons of serotonin and bradykinin produce experimental muscle pain and muscul ar hyperalgesia to mechanical stimuli. Pre-treatment with serotonin may enh ance the effect of bradykinin in the generation of muscle pain and muscular hyperalgesia in humans. (C) 1999 International Association for the Study o f Pain. Published by Elsevier Science B.V.