Papers of the Week

Human epidemiological studies suggest that chronic pain can increase mortality risk. We investigated whether this was true in mice, so that underlying mechanisms might be identified. At 10 weeks of age, C57BL/6 mice of both sexes received sham or spared nerve injury (SNI) surgeries producing neuropathic pain. Mice were weighed monthly, tested behaviorally for mechanical and cold sensitivity and guarding behavior every three months post-surgery, and otherwise left undisturbed in their cages until death by natural causes. Evidence of pain over the lifespan displayed a strikingly sex-specific pattern. Male mice displayed largely stable mechanical and cold hypersensitivity and guarding at 6-30 months post-SNI. In contrast, female mice displayed a biphasic temporal pattern of mechanical hypersensitivity and guarding behavior, with a complete resolution of SNI-induced pain behavior at 6-9 months post-SNI followed by the return of pain thereafter. Mouse lifespan was not significantly altered by SNI in either sex, nor was frailty as assessed by cage inspection in the last six months of life. However, in male mice with SNI, we observe a significant correlation between average lifetime mechanical hypersensitivity and lifespan, such that death occurred sooner in male mice exhibiting more evidence of chronic pain. This relationship was not observed in female SNI mice nor in sham-operated mice of either sex. This experiment is the first to investigate pain behavior over an entire lifetime, and suggests that biology of relevance to human chronic pain is being ignored by the very short timespans of most extant preclinical pain research.