The calcium channel ␣ 2 ␦-1 subunit is a structural subunit important for functional calcium channel assembly. In vitro studies have shown that this subunit is the binding site for gabapentin, an anticonvulsant that exerts antihyperalgesic effects by unknown mechanisms. Increased expression of this subunit in the spinal cord and dorsal root ganglia (DRG) has been suggested to play a role in enhanced nociceptive responses of spinal nerve-injured rats to innocuous mechanical stimulation (allodynia). To investigate whether a common mechanism underlies allodynic states derived from different etiologies, and if so, whether similar ␣ 2 ␦-1 subunit up-regulation correlates with these allodynic states, we compared DRG and spinal cord ␣ 2 ␦-1 subunit levels and gabapentin sensitivity in allodynic rats with mechanical nerve injuries (sciatic nerve chronic constriction injury, spinal nerve transection, or ligation), a metabolic disorder (diabetes), or chemical neuropathy (vincristine neurotoxicity). Our data indicated that even though allodynia occurred in all types of nerve injury investigated, DRG and/or spinal cord ␣ 2 ␦-1 subunit up-regulation and gabapentin sensitivity only coexisted in the mechanical and diabetic neuropathies. Thus, induction of the ␣ 2 ␦-1 subunit in the DRG and spinal cord is likely regulated by factors that are specific for individual neuropathies and may contribute to gabapentin-sensitive allodynia. However, the calcium channel ␣ 2 ␦-1 subunit is not the sole molecular change that uniformly characterizes the neuropathic pain states.Peripheral nerve injury can lead to a neuropathic pain state, termed tactile allodynia, in which innocuous tactile stimulation elicits pain behavior. Spinal administration of gabapentin, a novel anticonvulsant that binds to the ␣ 2

Peripheral nerve injury induces upregulation of the calcium channel ␣ 2 ␦-1 structural subunit in dorsal root ganglia (DRG) and dorsal spinal cord of spinal nerve-ligated rats with neuropathic pain, suggesting a role of the calcium channel ␣ 2 ␦-1 subunit in central sensitization. To investigate whether spinal dorsal horn ␣ 2 ␦-1 subunit upregulation derives from increased DRG ␣ 2 ␦-1 subunit and plays a causal role in neuropathic pain development, we examined spinal dorsal horn ␣ 2 ␦-1 subunit expression with or without dorsal rhizotomy in spinal nerve-ligated rats and its correlation with tactile allodynia, a neuropathic pain state defined as reduced thresholds to nonnoxious tactile stimulation. We also examined the effects of intrathecal ␣ 2 ␦-1 antisense oligonucleotides on ␣ 2 ␦-1 subunit expression and neuropathic allodynia in the nerve-ligated rats. Our data indicated that spinal nerve injury resulted in time-dependent ␣ 2 ␦-1 subunit upregulation in the spinal dorsal horn that correlated temporally with neuropathic allodynia development and maintenance. Dorsal rhizotomy diminished basal level expression and blocked injury-induced expression of the spinal dorsal horn ␣ 2 ␦-1 subunit and reversed injury-induced tactile allodynia. In addition, intrathecal ␣ 2 ␦-1 antisense oligonucleotides blocked injury-induced dorsal horn ␣ 2 ␦-1 subunit upregulation and diminished tactile allodynia. These findings indicate that ␣ 2 ␦-1 subunit basal expression occurs presynaptically and postsynaptically in spinal dorsal horn. Nerve injury induces mainly presynaptic ␣ 2 ␦-1 subunit expression that derives from increased ␣ 2 ␦-1 subunit in injured DRG neurons. Thus, changes in presynaptic ␣ 2 ␦-1 subunit expression contribute to injury-induced spinal neuroplasticity and central sensitization that underlies neuropathic pain development and maintenance.

Genetic factors and nerve injury-induced changes of gene expression in sensory neurons are potential contributors to tactile allodynia, a neuropathic pain state manifested as hypersensitivity to innocuous mechanical stimulation. To uncover genes relevant to neuropathic allodynia, we analyzed gene expression profiles in dorsal root ganglia (DRG) of spinal nerve-ligated Harlan and Holtzman Sprague Dawley rats, strains with different susceptibilities to neuropathic allodynia. Using Affymetrix gene chips, we identified genes showing differential basal-level expression in these strains without injury-induced regulation. Of more than 8000 genes analyzed, less than 180 genes in each strain were regulated after injury, and 19-22% of that was regulated in a strain-specific manner.Importantly, we identified functionally related genes that were co-regulated post injury in one or both strains. In situ hybridization and real-time PCR analyses of a subset of identified genes confirmed the patterns of the microarray data, and the former also demonstrated that injury-induced changes occurred, not only in neurons, but also in non-neuronal cells. Together, our studies provide a global view of injury plasticity in DRG of these rat stains and support a plasticity-based mechanism mediating variations in allodynia susceptibility, thus providing a source for further characterization of neuropathic pain-relevant genes and potential pathways.