اثر متقابل استرس بر وابسته بودن مدولاسیون درد به شدت پاسخ استرس فردی Opposite Effects of Stress on Pain Modulation Depend on the Magnitude of Individual Stress Response

Introduction Anecdotes on the ability of individuals to perform under stressful conditions, despite injuries causing excruciating pain, suggest that acute stress may induces analgesia or hypoalgesia. Animal models of acute stress, such as inescapable foot shock or exposure to predators indeed show that acute stress can produce stress-induced analgesia/ hypoalgesia (SIA) [e.g., 6,76]. Similarly, acute stress manipulations applied to human subjects, such as public speaking and erythematic tasks produce SIA, manifested as an increase in pain threshold [20,78] and intolerance levels [24,78] and reduced pain ratings [20,78]. However, similar stress manipulations were also reported to produce the opposite effect, namely of stress-induced hyperalgesia (SIH). SIH was observed both in animals [37,38] and human subjects, manifested in decreased pain and intolerance [7,8,15,61] thresholds and increased pain ratings [45]. These contradictory results are not easy to explain, especially as both SIA and SIH were reported in different studies using the same stress manipulation [e.g. 13,29]. One factor that may underlie the contradictory results is the variability in individuals’ stress response. Stress is regarded as a cognitive perception of uncontrollability and unpredictability that is expressed in a physiological and behavioral responses [39]. The focus on appraisal stem from the understanding of the significant influence of cognitive factors (e.g. thoughts, attitudes and beliefs) on the individual’s response to stressors along with the demonstration that both appetitive and aversive stimuli can cause comparable physiological stress responses even though individuals are motivated to obtain or avoid them, respectively [37,49,69]. Individual differences can thus determine if, and to what extent a particular stimulus (stressor) is perceived as positive or negative. Stress perception may in turn, influence individual pain responses under stressful conditions and can possibly underlie the inconsistent effects of stress on pain threshold and tolerance reported in the literature. However, since perceived stress was seldom evaluated in these studies, whether or not the effect of acute stress on pain depends on individual stress responsiveness is an open question.