Pain Mechanisms and Sensory NeuroscienceResearch PaperPerformance of a repetitive task by aged rats leads to median neuropathy and spinal cord inflammation with associated sensorimotor declines

Epidemiological studies have demonstrated a relationship between advancing age and susceptibility to risk factors for median neuropathies and musculoskeletal disorders. In this study, we determined if performance of a voluntary reaching task by aged rats induced sensorimotor declines, median nerve dysfunction and increased inflammatory cytokines in peripheral nerves, muscle and spinal cord neurons. Aged (14 mon) rats were trained for 15 min/day for 4 weeks to learn a high repetition, low force (HRLF) task (19 reaches/min; 15% maximum pulling force). Aged task rats performed the task for 2 h/day, 3 days/wk, for 12 weeks (until they were 18 mon of age). No behavioral changes were detected in normal controls (NC) or food-restricted controls (FR C) as they aged. However, grip strength declined in HRLF rats in weeks 6-12 (P<0.01 each) and 12-week trained-only rats (TR; P<0.05), compared to NC. Mechanical hypersensitivity was present in weeks 9 and 12 HRLF reach limb forepaws (P<0.01 and P<0.05, respectively), and 12-week HRLF support limb forepaws (P<0.01) and hindpaws (P=0.03), compared to NC. By week 12, median nerve conduction velocity declined 23%, bilaterally, in HRLF (P<0.001 each), and 13% in TR (P<0.05), compared to NC. Tumor necrosis factor alpha (TNFα) increased in 12-week HRLF muscle (P=0.005), median nerve (P<0.01), and neurons in superficial lamina of HRLF cervical spinal cords (P<0.01), compared to NC. interleukin 1 beta (IL1β) also increased in superficial lamina neurons (P<0.01). Loss of grip strength was correlated with median nerve conduction slowing (r=0.70) as well as increased nerve and muscle TNFα (r=−0.38 and r=−0.41, respectively); decrease in forepaw withdrawal thresholds was correlated with median nerve conduction slowing (r=0.81), increased nerve TNFα (r=−0.59), and increased TNFα and IL1β in neurons in spinal cord dorsal horns (r=−0.52 and r=−0.47, respectively). Thus, aged rats performing a repetitive task exhibited sensorimotor declines that were associated with decreased median nerve conduction, and increased pro-inflammatory cytokines in the median nerve and cervical spinal cord neurons.