Changes in nerve growth factor levels in dorsal root ganglia and spinal nerves in a rat neuropathic pain model

The purpose of this study was to measure the changes in levels of nerve growth factor (NGF) in dorsal root ganglia (DRG) and spinal nerves with the aim of investigating the role of NGF in a rat neuropathic pain model. Nerve injuries were made by tight ligation of the left L5 and L6 spinal nerves using 6-0 silk thread in male Sprague-Dawley rats. Before surgery and 1, 3, 5, 7, and 14 days after surgery, tissue samples collected included the L3-6 DRGs bilaterally, segments of the ipsilateral L5-6 spinal nerves proximal and distal to ligation sites, and corresponding sites of the contralateral L3-6 and the ipsilateral L3-4 spinal nerves. NGF levels in the DRGs of the injured spinal nerves (the left L5 and L6) did not change significantly from control values. The spinal nerve segments distal to ligation sites had higher levels of NGF than the control values. Unlesioned sites did not show any significant changes in NGF levels. The increase of NGF in distal segments of injured spinal nerves may be due to an accumulation of retrogradely transported NGF. The maintenance of NGF levels in the DRGs that had lost peripheral connections may reflect local synthesis after nerve injury.