Lack of sex differences in modulation of experimental intraoral pain by diffuse noxious inhibitory controls (DNIC)

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Abstract
The aims of this study were to investigate possible sex differences in (a) intraoral pain evoked by topical application of capsaicin to the gingiva, and (b) the modulation of this pain by diffuse noxious inhibitory controls (DNIC). Three groups with a total of fifty-four healthy volunteers (20 men, 20 women using oral contraceptives (W+OC), 14 women not using (W-OC)) completed the study. In two sessions, intraoral pain was evoked by topical application of 30microL 5% capsaicin to the gingiva. Conditioning stimuli were applied with three min hand immersion in ice water in one session and 30 degrees C water (control) in another session. The capsaicin-evoked pain and the water-evoked pain were evaluated by the participants on visual analogue scales (VAS). No main effects of group in capsaicin-evoked pain (P>0.062) or water-evoked pain (P>0.149) were found. There was a significant group x time interaction (P<0.001) with W+OC reporting lower capsaicin-evoked pain scores than W-OC in the early phase (2-3min) and lower pain scores than men in the later phase (5-11min). The degree of modulation by DNIC did not differ between groups (P=0.636). In conclusion, for a superficial type of intraoral pain, only minor sex differences were found in pain intensity and no differences in the degree of endogenous modulation by DNIC. Female sex and the use of OC may not consistently be associated with higher sensitivity to pain.