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.