Partial sciatic nerve ligation induced more dramatic increase of neuropeptide Y immunoreactive axonal fibers in the gracile nucleus of middle-aged rats than in young adult rats.

Neuropeptide changes in primary sensory neurons caused by partial nerve injury are likely involved in the development of neuropathic pain. In this study, using immunocytochemistry, we examined neuropeptide Y (NPY) expression in lumbar dorsal root ganglion (DRG) cells of young adult (2-3 months old) and middle-aged (8-10 months old) rats 4 weeks after partial sciatic nerve ligation (PSNL). Significantly higher NPY immunoreactivity was induced in the injured side DRG neurons, the dorsal horn and the gracile nuclei in middle-aged rats than in young rats. Using combined fluorescent dye tracing and NPY immunostaining, we found in middle-aged rats that 46% injured DRG neurons projected to the gracile nucleus and 45% of injured neurons were also NPY-IR, whereas 42% spared DRG neurons projected to the gracile nucleus and 18% of spared neurons were also NPY-IR. Thus PSNL induces NPY up-regulation in spared as well as injured DRG neurons, both contribute to the increased NPY immunoreactivity in the gracile nucleus in the middle-aged rats. The more dramatic increase of NPY in DRG neurons of middle-aged rats after PSNL shows that the responses to partial nerve injury are age-dependent, that suggests a possible relevance to the higher incidence of neuropathic pain in human middle age.