Peripheral nerve injury can lead to a persistent neuropathic pain state in which innocuous tactile stimulation elicits pain behavior (tactile allodynia). Spinal administration of the anticonvulsant gabapentin suppresses allodynia by an unknown mechanism. In vitro studies indicate that gabapentin binds to the ␣ 2 ␦-1 (hereafter referred to as ␣ 2 ␦) subunit of voltage-gated calcium channels. We hypothesized that nerve injury may result in altered ␣ 2 ␦ subunit expression in spinal cord and dorsal root ganglia (DRGs) and that this change may play a role in neuropathic pain processing. Using a rat neuropathic pain model in which gabapentin-sensitive tactile allodynia develops after tight ligation of the left fifth and sixth lumbar spinal nerves, we found a Ͼ17-fold, time-dependent increase in ␣ 2 ␦ subunit expression in DRGs ipsilateral to the nerve injury. Marked ␣ 2 ␦ subunit upregulation was also evident in rats with unilateral sciatic nerve crush, but not dorsal rhizotomy, indicating a peripheral origin of the expression regulation. The increased ␣ 2 ␦ subunit expression preceded the allodynia onset and diminished in rats recovering from tactile allodynia. RNase protection experiments indicated that the DRG ␣ 2 ␦ regulation was at the mRNA level. In contrast, calcium channel ␣ 1B and ␤ 3 subunit expression was not co-upregulated with the ␣ 2 ␦ subunit after nerve injury. These data suggest that DRG ␣ 2 ␦ regulation may play an unique role in neuroplasticity after peripheral nerve injury that may contribute to allodynia development. Key words: ␣ 2 ␦ calcium channel subunit; peripheral nerve injury; rhizotomy; allodynia; dorsal root ganglia; spinal cord; sensory neuronsPeripheral nerve injury may lead to neuropathic syndromes characterized by both spontaneous and evoked painful sensations. Allodynia, or an exaggerated response to otherwise innocuous tactile stimuli, is considered both a hallmark and the most troublesome of these syndromes (Price et al., 1989;Wahren and Torebjork, 1992;Koltzenburg et al., 1994). The molecular mechanisms of neuropathic pain states are not clear. It has been hypothesized that disordered sensory processing arises from long-term changes in the function of sensory afferents and the organization of sensory input into the dorsal horn (Coderre et al., 1993;Woolf and Doubell, 1994).Pharmacological evidence suggests that spinal N-type voltagegated calcium channels (N-VGCCs) play a role in neuropathic pain transduction. Intrathecal delivery of N-type ( -conopeptides), but not L-or P-type, VGCC antagonists suppresses allodynia in neuropathic rat models (Chaplan et al., 1994b;Calcutt and Chaplan, 1997). Autoradiographic studies showed the highest density of conopeptide-binding sites in the spinal dorsal horn (lamina I and II) where primary afferents terminate (Kerr et al., 1988;Takemura et al., 1989). N-VGCCs are also found in dorsal root ganglion (DRG) neurons where they are differentially modulated after sciatic nerve damage (Abdulla and Smith, 1997).High-threshold neuronal VGCCs contain thr...

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.

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The aims of this study were two-fold: first, to simplify the method for creating a recently described neuropathic pain model in the rat, and second, to evaluate the effects of a number of drugs with analgesic or antihyperalgesic properties, in this model. Continuous intravenous vincristine infusion (1-100 microg kg(-1) day (-1)) for 14 days resulted in a dose dependent tactile allodynia (as measured by von Frey filaments) by 7 days at doses between 30 - 100 microg kg(-1) day (-1), with a hindlimb motor deficit observed at doses greater than 50 microg kg(-1) day (-1). No thermal hyperalgesia was observed. Systemic morphine, lidocaine, mexiletine and pregabalin (given intraperitoneally) produced significant reduction of the allodynia, while tetrodotoxin was without effect. Continuous intravenous infusion of vincristine in rats thus provides a reliable model of chemotherapy induced neuropathy which may be used in defining the mechanism and pharmacology of this clinically relevant condition.

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Vincristine belongs to the family of vinca alkaloids used for treatment of malignant tumors. Clinical application of these agents is often associated with dose-dependent painful neuropathy due to damages to the peripheral axons. A rat model of vincristine-induced hyperalgesia was developed through intravenous injection of vincristine by Aley et al. (1996) and was later modified by Nozaki-Taguchi et al. (2001) using continuous intravenous infusion of vincristine. This model provides consistent and long-lasting neuropathic pain states mimicking vincristine-induced pain conditions in human patients. Therefore, this model is a valuable means of studying the mechanisms and pharmacology of vincristine-induced neuropathic pain. In this chapter we describe in detail steps the generation of vincristine-induced neuropathy in rats through continuous intravenous infusion of vincristine.