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| SEX-RELATED DIFFERENCES IN THE RESPONSES TO EXPERIMENTALLY-INDUCED PAIN Roger B. Fillingim, Ph.D., Department of Psychology, School of Social & Behavioral Science, University of Alabama, Birmingham, Alabama and William Maixner, D.D.S., Ph.D., Department of Endodontics, School of Dentistry, University of North Carolina, Chapel Hill, North Carolina Considerable evidence indicates that females and males differ in their responses to experimental pain. For example, females routinely exhibit increased perceptual responses to mechanical and electrical stimuli, and somewhat less consistently females display greater thermal pain sensitivity. Data from our laboratory have replicated and extended previous findings regarding sex differences in experimental pain sensitivity. In one study, we reported greater thermal pain responses among females than males, and the psychological variables of control/efficacy and anxiety were related to thermal pain responses in a sex-dependent manner. Subsequently, we reported larger thermal and ischemic pain responses among females compared to males; however, after statistically correcting for resting blood pressure, the sex differences in pain perception were greatly diminished. Relatedly, we have found that family history of hypertension influences thermal pain threshold and tolerance in a sex-dependent manner. In a recent study, we demonstrated that while no sex differences emerged for sensory discrimination of thermal pain, females displayed greater temporal summation of painful heat than males, and preliminary data from another study also suggest greater spatial summation of thermal pain among females. Finally, females showed greater sensitivity to ischemic pain during the luteal (i.e. premenstrual) phase of the menstrual cycle compared to the follicular (i.e. post-menstrual) phase. In addition, higher plasma levels of estradiol were associated with greater thermal pain sensitivity regardless of menstrual cycle. These data suggest that sex differences in the responses to experimentally-induced pain result are influenced by multiple systems involved in pain modulation. Important moderating variables include (but are not limited to) psychosocial measures, cardiovascular factors, temporal and spatial characteristics of painful stimuli, and natural fluctuations of reproductive hormones. The clinical relevance of these findings has not been established. Given that some of the clinical pain conditions that are more prevalent among females are also characterized by enhanced laboratory pain sensitivity (e.g. fibromyalgia, TMD, and Tension-Type Headache), it is tempting to propose that sex related differences in pain modulation produce both increased experimental pain sensitivity and enhanced vulnerability to clinical pain among females. However, prospective studies are needed to determine whether experimental pain responses are associated with risk for development of clinical pain. Clearly, additional research is essential to further elucidate the mechanisms and the clinical relevance of sex differences in laboratory pain responses. Supported by NIH/NIDR grant # DE 12261
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