Effects of endogenous neurotrophins on sympathetic sprouting in the dorsal root ganglia and allodynia following spinal nerve injury

Peripheral nerve injury is often complicated by a chronic pain syndrome that is difficult to treat. In animal models of peripheral nerve injury, sympathetic nerve terminals in the dorsal root ganglia (DRG) sprout to form baskets around large diameter neurons, an anatomical change that has been implicated in the induction of neuropathic pain. In the present study, we have investigated whether neurotrophins derived from peripheral sources play any roles in sympathetic sprouting and neuropathic pain in a rat model of peripheral nerve injury. After transection of the left lumbar (L) 5 spinal nerve, antisera specific to neurotrophins were injected intraperitoneally twice a week for 2 weeks. The foot withdrawal response to von Frey hairs was examined on days 1, 3, 7, 10, and 14 postlesion. After completion of behavioral tests, sympathetic sprouting in DRG was examined by tyrosine hydroxylase (TH) immunohistochemistry. The number of TH-immunoreactive (ir) fibers and baskets around large neurons within the lesioned DRG was dramatically increased in the rats treated with control normal sheep serum. Antisera specific to nerve growth factor (NGF), neurotrophin-3 (NT3), and brain-derived neurotrophic factor (BDNF) significantly reduced the sympathetic sprouting and the formation of baskets. L5 spinal nerve lesion induced a significant increase in foot withdrawal responses to von Frey hair stimuli, which was attenuated by treatment of antisera to neurotrophins with a different time sequential. The effect of BDNF antiserum occurred earlier and lasted longer than those of NGF and NT3 antisera. These results implicate that peripherally derived neurotrophins are involved in the induction of sympathetic sprouting and neuropathic pain following peripheral nerve injury.