Gender differences in pain modulation evoked by repeated injections of glutamate into the human trapezius muscle.

Gender differences in pain habituation, temporal summation, and pressure hyperalgesia evoked by repeated injections of glutamate into the dominant trapezius muscle were investigated. The glutamate-evoked muscle pain intensity and pressure pain threshold (PPT) were assessed. The PPTs were measured bilaterally in the trapezius muscles (local pain area) and posterolateral neck muscles (referred pain area) after glutamate injection in healthy and age-matched males and females (each n=14). Two glutamate injections (0.4 ml, 2M each) were injected with an interval of 5 min. One injection of glutamate (0.4 ml, 2M) served as a control. Males, but not females, rated the second injection (maximal pain intensity) significantly less painful than the first injection. The area under the visual analogue scale pain curve of the second injection was significantly larger than the first injection in females. Repeated glutamate injections, but not one-glutamate injection, significantly decreased PPTs in the local pain area, with no significant gender differences. No PPTs changes were observed either in the contralateral trapezius muscle or bilaterally in the referred pain areas in either sex. These results suggest that a less efficient pain habituation and a greater susceptibility to the development of temporal summation of muscle pain in females, but not in males, might be one of the contributing factors to the higher incidence of neck shoulder pain in females. In addition, the reduction of PPTs in the local pain area evoked by intramuscular glutamate injection may represent an early process of peripheral pressure hyperalgesia, which is most likely gender independent.