Rats bred as low intrinsic aerobic capacity runners (LCR) recover more slowly from chronic pain compared to rats bred as high intrinsic aerobic capacity runners (HCR)

Abstract

Low physical fitness and obesity are associated with chronic pain (J Pain 8: 430, 2007) and the mechanisms underlying this link are difficult to explore in humans. This study is testing susceptibility to experimentally induced chronic pain in two rat strains developed by two‐way artificial selection on a treadmill to contrast for both intrinsic (untrained) fitness and obesity. Experiments were performed using generation 19 LCR (n=3) and HCR (n=3) males. Pain threshold was measured as paw withdrawal latency (PWL) to a nociceptive thermal stimulus using an IITC Hargreaves system. PWL was measured before and weekly for six weeks after chronic constriction injury (CCI) of the sciatic nerve, a validated model of neuropathic pain (Pain 33: 87, 1988). PWL did not differ between HCR and LCR rats before CCI. Two‐way ANOVA for repeated measures showed that PWL varied significantly as a function of aerobic capacity, time after CCI, and by interaction. At days 21, 28, 35, and 42 after CCI the LCR rats had a 23% shorter PWL than the HCR rats. The finding that CCI produced a shorter PWL in LCR compared to HCR rats is consistent with the hypothesis that a low aerobic capacity phenotype is associated with greater hyperalgesia. Thus, LCR and HCR rats may provide contrasting models of susceptibility to chronic pain relevant for mechanistic studies in humans.Support: NIH grants HL6427, RR‐17718, HL57120, HL40881, MH45361 and Dept Anesthesiology