Introduction: Experimental animal studies provided evidence for a synergistic effect of immunological and psychological stressors on subsequent sickness behaviours. Up to now, little corroborating evidence for such synergy exists for humans, in whom it may provide a mechanism leading to the expression of functional somatic symptoms. The aim of the present study was to determine an interaction between stress(-vulnerability) and an immunological activation on experimental pain sensitivity, i.e., pressure pain threshold and tolerance in healthy humans.
Methods: In healthy female participants (n=25, mean age 22.3 years), negative affectivity (NA) and experienced stress were assessed by questionnaire before receiving a Salmonella typhi vaccine or saline control in a randomized blinded cross-over design. Pressure pain threshold was assessed at the lower back and calves and pain tolerance was assessed at the thumbnail, before and six hours after each injection.
Results: Vaccination induced leukocytosis (+100%) and increased serum IL-6 (+670%). NA predicted decreased pain tolerance after vaccination (β=-.57, p=.007), but not after placebo (β=.25, p=.26). Post-hoc analyses also demonstrated an association with administration order.
Discussion: NA moderated the effects of inflammation on pain tolerance. This finding is consistent with a synergistic model whereby inflammation may lower the threshold for pain reporting in individuals with increased vulnerability for somatic symptom reporting.
Keywords: Algometry; Cytokines; Experimental pain; Human; Inflammation; Inflammatory response; Interleukin-6; Life events; Negative affect; Negative affectivity; Pain sensitivity; Pain threshold; Pain tolerance; Placebo; Pressure pain; Randomized control; Stress; Vaccine.
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