Peripheral nerve injury can lead to persistent neuropathic pain, characterised by mechanical allodynia and hyperalgesia, thermal hyperalgesia and spontaneous pain. Gabapentin is an effective analgesic for neuropathic pain, but its mechanism of action is unknown. However, it is known to bind specifically to the voltage-dependent Ca²⁺ channel α₂δ subunit and a recent electrophysiological study in dorsal root ganglion (DRG) cells suggested a role for this subunit in this action of gabapentin (Alden & Garcia, 2001). We have therefore examined the expression of the α₂δ subunit in DRG neurones following nerve injury in animal models of neuropathic pain.

L4 and L5 spinal nerve ligation was performed on female Wistar rats (150-170 g, n = 4) under sodium pentobarbitone anaesthesia (40 mg kg^-1, I.P.). A loose chromic gut (5-0) loop was placed around the left L4 spinal nerve. The left L5 spinal nerve was then tightly ligated and transected proximal to its junction with L4 spinal nerve (modified Chung spinal nerve injury model, D. Lee and D. Lodge, personal communication). In control (sham-operated, n = 4) rats both left L4 and L5 spinal nerves were exposed but not ligated. Behavioural testing (threshold to von Frey hairs) was performed on all animals before and after surgical procedures. Seven days post-surgery rats were deeply anaesthetised with sodium pentobarbitone (60 mg kg^-1, I.P.) and perfused with Zamboni’s fixative. Immunocytochemistry and immuno-blotting was carried out on DRG tissue from these animals. The monoclonal α₂δ antibody was used in concentrations of 2 mg ml^-1 and 1:100 for immuno-cytochemistry and immunoblotting, respectively. In situ hybridisation was carried out on DRG tissue collected from rats (n = 5) 14 days after partial sciatic nerve ligation (Seltzer et al. 1990). All experimental procedures were carried out under licence according to the UK Animals (Scientific Procedures) Act of 1986.

All peripheral nerve-injured animals displayed an increased sensitivity to innocuous mechanical stimuli on the hindpaw ipsilateral to the injury compared with the contralateral side and sham-operated rats. Immuno-cytochemistry and in situ hybridisation showed moderately labelled small DRG neurones from both the contralateral side and sham-operated rats. Nerve-injured rats displayed a clear increase in the ipsilateral expression of both α₂δ protein and mRNA across the size distribution range of DRG neurones, with the most intensely labelled neurones after injury again being those of small cross-sectional area, a size range rich in units with C- and Aδ-fibres, i.e. an increase both in proportions of neurones labelled and intensity of labelling, found for both protein and mRNA. Western blotting revealed a band of the expected size for ipsilateral L4 and L5 DRGs from nerve-injured animals, confirming the specificity of the antibody.

Our data provide support for the suggestion that increased DRG α2δ subunit expression may contribute to peripheral nerve injury-induced neuropathic pain.This work was supported by a Wellcome Trust grant to S.N.L.; S.K. is supported by a Bristol University scholarship. The α₂δ antibody was a generous gift from Dr S.G. Volsen (Eli Lilly, UK).

Alden, K.J. & Garcia, J. (2001). J. Pharmac. Exp. Therap. 297, 727-735.

Seltzer, Z., Dubner, R. & Shir, Y. (1990). Pain 43, 205-218.