Peripheral nerve injury leads to complex pathophysiological changes and often results in intractable neuropathic pain. Neutrotrophic factors can have neuroprotective roles and recently it has been shown that intrathecal administration of glial derived neurotophic factor (GDNF) to the spinal cord is highly effective in reversing pathological changes that occur following nerve injury (Boucher et al, 2000). Targeted delivery of neurotrophic factors can be achieved using viral vectors which produce the desired protein in a defined anatomic location. We investigated the efficiency of a recombinant lentiviral vector overexpressing GDNF in a rat model of neuropathic pain. The viral vector was injected under isofluorane anaesthesia (5%) into the spinal cord of male Wistar rats at the L5 level. The rats received a 1.5ml injection of rLV vector encoding either GDNF (n=5) or the marker gene green fluorescent protein (GFP, n=5). 4 weeks later, the rats received a ligation of the fifth lumbar spinal nerve (5% isofluorane for induction, 2% for maintenance) (modified from Kim & Chung, 1992). 14 days post-surgery the rats were anaesthetised with pentobarbitone (60 mg/kg, i.p.) and perfused with 4% paraformaldehyde and the dorsal root ganglia (DRGs) and spinal cord were collected for immunocytochemical processing. DRGs and spinal cords were labelled for IB4 (10 μg/ml), a marker of GDNF sensitive neurones and DRGs were also stained for ATF3 (1:500), a marker of axotomy. All procedures were carried out under licence according to the UK Animals (Scientific Procedures) Act. All values are means ± SEM. We followed the time-course of GFP expression in the spinal cord following the rLV-GFP. GFP was expressed from 2 weeks onwards, peaking at 4-5 weeks. GFP expression was restricted to one spinal segment and was detected in both neurons and glial cells. Spinal nerve ligation induces ATF3 upregulation in axotomised primary sensory neurones and downregulation of IB4 binding in the small, nonpeptidergic DRG cell population. We observed that 89 ± 2% of ipsilateral L5 DRG cells in the GFP virus treated rats were ATF3 positive, compared to none in the contralateral side. rLV-GDNF delivery reduced the incidence of ATF3 labelling by 35%. rLV-GFP treated rats also showed a complete loss of IB4 binding in ipsilateral L5 DRGs and a marked reduction of IB4 binding in the spinal cord (to 15% ± 7 of control values). GDNF virus treatment has significantly reversed this downregulation in both the DRGs to 74% of control values and in the spinal cord to 140% of control values (one way ANOVA). These findings suggest that spinal injection of recombinant virus may be a effective method of administration of GDNF to treat changes associated with nerve injury.