Ligands in Chronic Pain Syndromes - Journal of Pain, The

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*Palm Beach Neurological Center, Palm Beach Gardens, Florida. y Pfizer Global Medical, ... z Albert Einstein College of Medicine, Bronx, New York. x CP Taylor ...
The Journal of Pain, Vol 11, No 12 (December), 2010: pp 1241-1249 Available online at www.sciencedirect.com

Focus Article Central Sensitization and CaVa2d Ligands in Chronic Pain Syndromes: Pathologic Processes and Pharmacologic Effect Michael Tuchman,* Jeannette A. Barrett,y Sean Donevan,y Thomas G. Hedberg,z and Charles P. Taylorx * Palm Beach Neurological Center, Palm Beach Gardens, Florida. y Pfizer Global Medical, Pfizer Inc., New York, New York. z Albert Einstein College of Medicine, Bronx, New York. x CP Taylor Consulting, Chelsea, Michigan.

Abstract: Central sensitization is one form of long-term plasticity in the central nervous system. Sustained activation of primary sensory fibers supplying dorsal horn can induce long-lasting increases in the discharge amplitude of primary afferent synapses. This is similar to the long-term potentiation that occurs in many other CNS regions. Drugs that limit the short-duration wind-up component of central sensitization include sodium channel blockers, NMDA antagonists, fast-acting opioids and the calcium-channel ligands gabapentin and pregabalin (S-3-(aminomethyl)-5-methylhexanoic acid). Pregabalin, like gabapentin, binds selectively to the CaVa2d auxiliary subunit of presynaptic voltage-gated calcium channels. The conformational changes induced by this binding inhibit abnormally intense neuronal activity by reducing the synaptic release of glutamate and other neurotransmitters. Recent identification in animal models of increased CaVa2d protein expression in chronic pain, allodynia, and hyperalgesia have drawn additional interest to drugs that bind the CaVa2d site. Experimental studies with animal models and healthy human volunteers have shown that pregabalin reduces nociceptive responses, particularly in conditions involving central sensitization. Since these actions occur with relatively modest effects on physiological and cognitive functions, pregabalin may be an important consideration in the pharmacotherapy of otherwise difficult-to-treat pain syndromes. Perspective: This focus article discusses how the central nervous system plasticity phenomenon, central sensitization, is established in the induction and maintenance of chronic pain, allodynia, and hyperalgesia. In addition, it explores the neurophysiologic actions of the calcium-channel ligands gabapentin and pregabalin in limiting pathological manifestations of central sensitization. ª 2010 by the American Pain Society Key words: Central sensitization, pregabalin, chronic pain, plasticity, gabapentin.

C

entral sensitization was first described by Woolf 98 as an augmentation of peripheral processes in which an ‘‘injury-induced increase in excitability’’ arises from ‘‘changes in the activity of the spinal cord.’’ Subsequent to its initial definition, the term has evolved

Several of the studies discussed in this paper were funded by Pfizer Inc. Editorial support for the development of this manuscript was provided by UBC Scientific Solutions and funded by Pfizer Inc. Address reprint requests to Sean Donevan, Ph.D, Medical Director, Lyrica, Pfizer, 235 East 42nd Street, New York, NY. E-mail: Sean.Donevan@pfizer. com 1526-5900/$36.00 ª 2010 by the American Pain Society doi:10.1016/j.jpain.2010.02.024

to describe any change in physiology or anatomy within the central nervous system (CNS) that enhances the basal sensitivity to, or intensity of, pain perception. In central sensitization, the synaptic relay of nociception at the first synapse in the spinal cord dorsal horn becomes pathologically enhanced to the point of neural hyperactivity and hyperresponsiveness (Fig 1). Central sensitization can arise from nerve injury as well as various neuropathies, including diabetic peripheral neuropathy (DPN) and postherpetic neuralgia (PHN). Fibromyalgia47 may also have a central sensitization component.81 The clinical manifestations of central sensitization include hyperalgesia (increased painful response to mildly 1241

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Descending modulation

Local modulation

Normal pain response

Normal stimulus

Projections to brain Local modulation

Exaggerated pain response

Normal stimulus Neural injury Potentiated spontaneous discharge

Figure 1. Schematic view of the induction of plasticity in spinal cord dorsal horn via central sensitization. The sequence of plasticityrelated changes following injury is compared to a normal pattern of neurotransmission. Adapted from Tortora G, Grabowski SR: Principles of Anatomy and Physiology, 10th ed. Woolf and Mannion: Lancet 353:1959-1964, 1999

painful stimuli), allodynia (painful response to otherwise nonnoxious stimuli), expanded size of receptive fields to painful stimuli, and sensations of persistent pain following transient noxious stimuli. The term nociception as used here denotes neuronal processes in sensory pathways that trigger pain perception. In contrast, pain describes subjective experiential aspects such as suffering, which are not easily assessed in animal studies. Since most conventional drug trials measure only a single subjective pain score, inferences drawn from animal studies should be made with caution. When central sensitization of nociceptive input is chronically enhanced, as in certain neuropathic conditions,99,102 patient quality-of-life can decline substantially. This state of affairs is worsened in patients with conditions unresponsive or underresponsive to conventional analgesics (eg, nonsteroidal anti-inflammatory drugs and aspirin) and opiates. Given their probable association with neuropathic pain, the processes underlying central sensitization are a compelling target for pharmacologic intervention. These processes will be the focus of this paper relative to what is understood about pregabalin treatment of various pain conditions. Pregabalin (S-3-(aminomethyl)-5-methylhexanoic acid) is a substituted chemical analog of the CNS neurotransmitter g-aminobutyric acid (GABA) that is not active at any known GABA-binding site. Pregabalin binds selectively to a membrane-spanning protein in brain and spinal cord identified as CaVa2d, an auxiliary subunit of presynaptic, voltage-gated calcium channels. This selective binding reduces abnormally intense neuronal activity by reducing the stimulated synaptic release of several neurotransmitters, particularly glutamate.24,28,89 Several of these properties are similar to those of gabapentin. Preclinical and clinical findings suggest that pregabalin’s analgesic action can impact certain processes associated with central sensitization. In clinical trials with several painful disorders, pregabalin has shown marked efficacy in reducing pain symptoms. To date, pregabalin has been approved for the treatment of neuropathic pain in the European Union

and for the treatment of DPN, PHN, and fibromyalgia in the United States.

Central Sensitization, Nociception, and Pain Intense and abnormally sustained activation of primary sensory afferent fibers supplying the dorsal horn of the spinal cord can cause long-lasting increases in the synaptic activity of primary afferent pathways and their targets. This phenomenon is similar to the longterm potentiation (LTP) that occurs in many other CNS regions, and which was first described at glutamate synapses in the hippocampus.51 In cases of pathological pain arising either from peripheral or central lesion sites, or from established peripheral neuropathies, experimental results obtained over many years27,80,81,98 indicate that sustained input from sensory afferent fibers at the specific discharge frequencies, repetition intervals, and intensities characteristic of chronic nociceptive stimulation induce both short-term (eg, wind-up)19 and more sustained enhancements of both pre- and postsynaptic activation. Enhancements occurring in dorsal horn neurons that relay nociceptive signals from primary sensory afferent fibers to the thalamus and forebrain are particularly important to the evolution of pathologically enhanced nociception. As with long-term potentiation (LTP) at other glutamate synapses in the CNS, synaptic plasticity associated with central sensitization in the spinal dorsal horn results from a cascade of activitydependent events including sustained depolarization, increased calcium influx, nitric oxide synthesis, protein kinase C activation, and widespread receptor phosphorylation, accompanied by numerous downstream intracellular changes.44,97,99,101 In animal models, the neurotransmitter receptors that contribute to LTP in forebrain, and to enhanced synaptic strength in the dorsal horn, include G-protein-coupled metabotropic glutamate receptors as well as rapidly-gated glutamate ion channels (a-amino-3-hydroxy-5-methyl4-isoxazolepropionic acid [AMPA] receptors) and

Tuchman et al depolarization-dependent and calcium-permeable glutamate ion channels (N-methyl-D-aspartate [NMDA] receptors). Additionally, in the dorsal horn, the peptide neurotransmitter substance P and its binding at G-protein coupled neurokinin-1 tachykinin receptors also play an important role especially in inflammatory hyperalgesia.25,72,99 This role includes up-regulation of COX-2 expression, de novo COX-2 synthesis, auto-regulation of further substance-P release, and the activation of several second messenger systems critical to the maintenance of sustained nociception, including formation of 1,4,5-inositol trisphosphate [IP3] and cyclic AMP [cAMP] accumulation via adenylate cyclase.29,87 Finally, a number of studies with human volunteers and animal models indicate that synapses in the dorsal horn involved in nociception are modulated by activation of serotonin pathways descending from the raphe and noradrenergic fibers descending from locus coeruleus. These monoamine projections can have either analgesic or pain-enhancing effects in the dorsal horn. They are triggered and modulated by input from the periaqueductal gray of the rostroventral medulla, as well as by projections to the rostroventral medulla from forebrain areas such as the insular cortex and limbic system.10,103 Drugs that have been shown to limit the shortduration wind-up component of central sensitization in anesthetized lab animals include the sodium channel blockers lidocaine and carbamazepine; NMDA receptor antagonists; several fast-acting opioids; and the CaVa2d calcium-channel ligand gabapentin.39,79 Additionally, gabapentin has been shown to limit brain stem and insular neocortical activation following central sensitization in a capsaicin pain paradigm with healthy human volunteers.50 It is of interest in this context that nociceptive inputs to the insula and anterior cingulate cortex are thought to subsequently contribute to the activation of limbic structures (eg, amygdala, perirhinal cortex, and hippocampus) involved in the emotional and aversive features of pain perception.71 Some of these limbic and neocortical structures, along with other inputs, activate the rostral ventromedial medulla and the parabrachial area. It has been suggested50 that these structures modulate descending monoamine projections from the raphe nucleus and locus coeruleus. Thus, 5-HT3 and noradrenergic a2 receptors postsynaptic to the terminals of these projections in the spinal cord dorsal horn modulate both pain sensations and, to some degree, sensitivity to nociceptive stimuli. These descending spinal pathways have also been shown to contribute to central sensitization.11,70 Of interest here, these descending monoaminergic pathways appear to modulate the sensitivity of spinal nociceptive neurons to gabapentin and pregabalin and these pathways are modulated by pregabalin and gabapentin.43,84-86 Pregabalin, which is structurally similar to gabapentin, is a GABA derivative but is inactive at GABA binding sites. Although pregabalin and gabapentin have similar pharmacologic profiles, pregabalin shows a higher oral bioavailability and absorption; and, in contrast to gabapentin, its plasma concentration increases linearly with increasing dose.95 In light of recent findings on the

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involvement of the CaVa2d calcium-channel protein and its ligands in several pain models with associated allodynia and hyperalgesia,2,21,29,59 pregabalin has elicited interest in terms of its pharmacotherapeutic possibilities.

Pregabalin: Preclinical and Clinical Experience Pregabalin is an orally administered drug that binds to the CaVa2d protein of presynaptic voltage-gated calcium channels in the nervous system. While some evidence points to a direct and rapid effect of CaVa2d ligands on calcium channels at the cell surface,56,83 recent data with recombinant calcium channels in oocytes or mammalian cell lines suggest that with sustained application, pregabalin (or gabapentin) may bind to CaVa2d proteins inside the cell. This has been surmised to prevent translocation of calcium-channel complexes to the cell surface, thereby reducing functional calcium-channel density.45,64 Regardless of whether pregabalin acts at intracellular or extracellular sites, these findings suggest that it reduces calcium influx into nerve terminals in response to action potentialmediated excitation. This reduction in Ca11 influx presumably diminishes neurotransmitter release, as described in vitro.24,29,33 Neurotransmitters that pregabalin and gabapentin binding have been shown to inhibit in vitro following electrical stimulation include glutamate,23,61,65,92 norepinephrine,22 substance P, and CGRP.28 Interestingly, some evidence suggests that a reduction in stimulated norepinephrine release observed in the presence of CaVa2d drugs in vitro is secondary to decreased release of glutamate in the same tissues.34 Thus, in either prophylactic or acute applications, pregabalin may limit the types of high-frequency synaptic discharges that maintain central sensitization. Several recent findings suggest that increases in the number of type 1 CaVa2d proteins at synapses in the dorsal horn are important to the production of central sensitization in animal models. In particular, work by Luo et al8,58,59 has demonstrated upregulation of CaVa2d subunit expression in dorsal horn and dorsal root sensory ganglion in animal models of neuropathic pain and allodynia including spinal nerve ligation, chronic sciatic nerve constriction, and streptozocin-induced diabetes. Similarly, Melrose et al63 reported in situ hybridization studies showing increased CaVa2d-1 message in sensory ganglia during hyperalgesia from peripheral nerve injury. Finally, mice that are genetically engineered to overexpress CaVa2d-1 show that a behavioral phenotype, including allodynia and hyperalgesia, which can be reversed by gabapentin treatment.56 Both gabapentin45 and pregabalin reduce transmembrane expression of CaVa2d-1–linked calcium channels within sensory neurons. Bauer et al7 induced allodynia with spinal nerve ligation at L5/L6. This would be expected to increase production of CaVa2d protein and the amount of protein transported to nerve terminals at the dorsal horn.58 Indeed, spinal nerve ligation

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Table 1.

Effect of Pregabalin and Gabapentin in Animal Models With Behavioral Pain End Points EFFECT CODE: NP, NEUROPATHIC; CS, CENTRAL SENSITIZATION; PA, PREGABALIN ACTIVE; GA, GABAPENTIN ACTIVE

ANIMAL MODEL: R, RAT; M, MOUSE R-acute Randall-Selitto paw pressure test R/M-acute hot plate test R-acute footpad Hargreaves thermal test R-formalin test (early phase) R-formalin test (late phase) R-carrageenan inflammation footpad thermal hyperalgesia (Hargreaves) R/M-intrathecal substance P or NMDA tactile allodynia R-plantar incision delayed footpad Hargreaves thermal hyperalgesia R-plantar incision delayed footpad tactile allodynia R-footpad post-thermal injury tactile allodynia R-sciatic nerve chronic constriction (stroking) tactile allodynia R-sciatic nerve chronic constriction tactile allodynia R-sciatic nerve chronic constriction cold allodynia R-sciatic nerve partial ligation tactile allodynia R-spinal nerve ligation cold allodynia R-spinal nerve ligation tactile allodynia R-spared peripheral nerve injury tactile allodynia R-partial sciatic nerve ligation tactile allodynia R-brachial plexus evulsion cold and tactile allodynia R-infraorbital nerve ligation tactile allodynia R-tail nerve injury, cold, warm, and tactile allodynia R-spinal cord injury tactile allodynia R-streptozocin diabetes tactile allodynia R-vincristine (chemotherapy) tactile allodynia R-paclitaxel (chemotherapy) tactile allodynia R-GD2 ganglioside antibody (chemotherapy) tactile allodynia R-oxaliplatin (chemotherapy) cold allodynia R-intramuscular acid or capsaicin delayed tactile allodynia (similar to fibromyalgia) M-herpes simplex virus delayed tactile allodynia R-delayed TNBS colon pressure hyperalgesia R/M-bone cancer hyperalgesia M-plantar melanoma tactile allodynia R-plantar capsaicin delayed tactile allodynia R-post resiniferotoxin tactile allodynia R-surgical knee osteoarthritis weight bearing and tactile footpad allodynia R-leg ischemic nerve damage tactile allodynia

NP? No No No No No No No No No No Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes ?? ?? ?? ?? No

CS? No No No No Yes Yes Yes? Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes No Yes Yes Yes Yes Yes Yes Yes Yes Yes

PA? No* NT No32 No32 Yes32 Yes32,49 Yes6 Yes30 Yes30 Yes53 Yes31 Yes31 NT Yes14 Yes41 Yes41 Yes41 Yes14 NT NT NT NT Yes31 Yes67 NT NT NT Yes100

GA? No88 No52,55 No32 No32,55 Yes32,55 Yes32 Yes6,37 Yes30 Yes30 Yes53 Yes31 Yes31 Yes75 Yes35 Yes48 Yes1 Yes26 Yes14 Yes69 Yes16 Yes4 Yes42 Yes31 Yes96 Yes96 Yes38 Yes57 NT

Yes No Yes ?? No Yes ?? Yes

Yes Yes Yes Yes Yes Yes Yes Yes

NT Yes20 NT NT NT NT Yes* Yes66

Yes73 Yes20 Yes68 Yes54 Yes5 Yes13 Yes9 NT

Abbreviations: NMDA, N-methyl-D-aspartate; NT, not tested; ??, unknown. *Unpublished Pfizer data on file.

increased the CaVa2d-1 mRNA signal in sensory ganglion endoplasmic reticulum, increased CaVa2d-1 protein synthesis and transport, and ultimately increased immunogold and EM-verified CaVa2d-1 protein in synaptic terminals within the spinal dorsal horn. Animals treated with pregabalin for 8 days following spinal nerve ligation showed decreased CaVa2d-1 protein at the injured L5 and L6 nerve terminals in comparison to salinetreated controls.7 Since chronic treatment with pregabalin does not appear to alter CaVa2d-1 mRNA production, the observed decrease in CaVa2d-1 protein is likely caused by reduced traffic or axonal transport of CaVa2d-1 proteins to synaptic terminals. This suggests that, at least in animal models, either transient local anesthetic nerve block (ie, inhibition of discharges from spinal nerve ligation) or prophylactic treatment with pregabalin can reduce this particular enhancement of CaVa2d-1 protein expres-

sion in the dorsal horn that is a correlate of enhanced nociception and allodynia. These findings suggest that changes in CaVa2d-1 gene and protein expression are an important part of the entire process of central sensitization in commonly used laboratory models. In addition to data suggesting that central sensitization can accompany increases in CaVa2d-1 protein expression, there is extensive literature on the effects of gabapentin and pregabalin in various animal models of nociceptive behaviors. Importantly, behavioral responses to immediate noxious stimuli (painful situations that lack central sensitization) are either not altered or only slightly altered by pregabalin or gabapentin (Table 1). In contrast, most behavioral models of pain-related behaviors with evidence of neuropathy are likely to involve central sensitization, and in all cases studied to date, are sensitive to pregabalin and gabapentin (Table 1). In addition, several models involving inflammation, delayed

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Treatment Group

Survival Distribution Function

.95 .90 .85

Pregabalin Flexible Dose Pregabalin Fixed Dose Placebo

.80 .75 .70 .65 .60 .55 .50 .45 .40 .35 .30 .25 .20 .15 .10 .05 0.00 0 1 2 3 4 5 6 7 8 9 10

12

14

16

18

20

22

24

26

28

Time to First Occurrence (days)

Figure 2. Time to onset of pain relief in 269 patients with postherpetic neuralgia comparing flexibly dosed pregabalin (150– 600 mg/d), fixed-dose pregabalin (300 mg/d), and placebo.78

pain after peripheral tissue injury, or chronic painful stimulation likely to include central sensitization, are responsive to pregabalin and gabapentin. Although animal models are useful for measuring nociception, these studies give only limited information that may be relevant to the spontaneous pain that is a major component of pain scores in many clinical trials. Nevertheless, observed behaviors in animal studies are consistent with hyperalgesia or allodynia. Taken together, these results suggest that pain perception specifically associated with central sensitization from many different forms of injury or nerve stimulation may be sensitive to treatment with CaVa2d ligands. Studies with healthy human volunteers subjected to electrically induced central sensitization have shown that pregabalin treatment can reduce both punctate mechanical hyperalgesia and dynamic-touch allodynia.15 For example, Gottrup et al39 found that oral gabapentin given to healthy volunteers in a regimen approximating that used in clinical treatment of neuropathic pain significantly reduced the hyperalgesia evoked by intradermal capsaicin. Similarly, Segerdahl76 reported that pinprick hyperalgesia was significantly reduced by pretreatment with gabapentin in healthy volunteers. In another key study, Ianetti et al50 reported that tactile allodynia, subsequent to intradermal capsaicin and tactile stimulation of the ankle of healthy volunteers, caused brain activation (measured by functional magnetic resonance imaging) sensitive to acute gabapentin treatment. In that study, acute treatment with gabapentin reduced tactile allodynia and also reduced allodynia-associated activation of anterior cingulate cortex, brain stem, insular cortex, and thalamus without altering the activation of S-II secondary sensory cortex. In clinical trials in which central sensitization is likely to be a component of the disorder’s etiology, treatment

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with CaVa2d ligands has been associated with reductions in both allodynia and hyperalgesia. Further, a recent randomized study of pregabalin with a large group of PHN patients78 has shown that pregabalin substantially reduced PHN-associated allodynia. In that 4-week investigation, flexible-dose pregabalin (150–600 mg/day) was compared with fixed-dose pregabalin (300 mg/day) and placebo in assessment of the time to onset of meaningful analgesic effect. Thus, in a large (n = 269) patient population (95% of whom were positive for brush-evoked allodynia), pregabalin treatment yielded a greater improvement in visual analog-scale allodynia scores than did placebo. Treatment effect was significant beginning at week 1. As shown in Fig 2, median time to onset of pain relief was 1.5 days (fixed-dose), 3.5 days (flexibledose), and >4 weeks (placebo). A similar randomized, double-blind, crossover study by Werner et al94 found that pretreatment of 22 healthy volunteers with gabapentin, while not significantly attenuating pain from experimentally induced firstdegree burn, did reduce primary mechanical allodynia from thermal injury. Further, in a number of randomized clinical studies, pregabalin treatment at 150 to 600 mg/ day significantly decreased moderate-to-severe neuropathic pain from DPN and PHN.36,74,90,93 Moreover, pregabalin has been established as effective in decreasing pain scores in randomized trials of disorders likely to have a significant central sensitization component, ie, spinal cord injury, brain injury,77 and fibromyalgia.60 Preliminary data indicate that patients undergoing total knee replacement also might benefit from pregabalin.12 Collectively, these findings suggest that pregabalin treatment in humans may interrupt the progression of one or more components of central sensitization and ultimately reverse some of the symptoms associated with it. Another common and chronic condition in which patients often experience hyperalgesia and allodynia is fibromyalgia, estimated to have 2% to 3% prevalence in the US population.47,82 Although its underlying cause has not yet been established, recent data suggest that alterations in the CNS, including central sensitization, may contribute to the chronic, widespread pain which is its primary characteristic.40,46 Since 2005, there have been 4 randomized, controlled trials of pregabalin for fibromyalgia.3,17,18,62 As with other neuropathic pain trials, once significant relief was achieved, it was typically maintained for the duration of treatment. The analgesic efficacy of CaVa2d ligands, in treating the nociceptive conditions discussed herein, is not direct evidence that its antihyperalgesic and antiallodynic actions are caused by a specific interaction with one or more elements of the biochemical cascade (eg, LTP induction and/or upregulated expression of a2d subunit proteins) mediating central sensitization. However, it does suggest that CaVa2d ligand treatment may be a clinically appropriate intervention in several different conditions likely to involve central sensitization. The consistency of treatment effects in clinical trials for common neuropathic pain syndromes linked with elements of central sensitization across several different clinical populations appears

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to support this contention. Striking similarities in the characteristics of pregabalin analgesia as reported by these patients are particularly suggestive of 2 factors: 1) each of these pain syndromes appears to share mechanisms related to central sensitization; and 2) pregabalin acts either directly or indirectly on neuronal targets in the spinal cord and brain that are also involved with central sensitization.

Conclusions and Summary The findings reviewed suggest that CaVa2d ligands like pregabalin and gabapentin may be effective for the treatment of several syndromes in which central sensitization plays a major role. In addition, it is worth considering that CaVa2d drugs could ameliorate some of the symptoms of disorders involving central sensitization when used prophylactically. As described above, evidence with several different laboratory animal models suggests that the upregulation of the CaVa2d-1 subunit is an important step in the processes leading to the enhancement of nociceptive processes. Moreover, the finding that experimental overexpression of CaVa2d induces allodynia in mice supports a role for increased CaVa2d expression in central sensitization from multiple causes. By binding to CaVa2d, pregabalin may both downregulate calcium-channel function at presynaptic nerve terminals and reduce neurotransmitter release at hyperexcited synapses. Therefore, it is

reasonable to postulate that pregabalin modulates central sensitization through its actions at the CaVa2d subunit. While direct evidence for changes in CaVa2d expression in patients with neuropathic pain and other chronic pain conditions is lacking, pregabalin and gabapentin do reduce allodynia and hyperalgesia in humans, suggesting that similar processes are involved in both animal models and human trials. Given these experimental findings, early treatment with CaVa2d drugs to prevent incipient or worsening pain conditions may be one approach to a difficult problem. While directly applicable animal models of both spontaneous and chronic pain are difficult to achieve, further investigation into the pharmacotherapeutic inhibition of symptoms associated with central sensitization may contribute to the development of effective therapies for difficult-to-treat pain syndromes.

Acknowledgments Michael Tuchman and Charles Taylor did not receive financial support for the development of this manuscript. Charles Taylor was previously a full-time employee of Pfizer Inc. and is now employed part-time as a consultant to Pfizer. At the time this manuscript was prepared, Jeannette Barrett was a full-time employee of Pfizer Inc., and Thomas G. Hedberg was a full-time employee of UBC Scientific Solutions.

References

to presynaptic terminals in neuropathic pain is inhibited by the a2d ligand pregabalin. J Neurosci 29:4076-4088, 2009

1. Abdi S, Lee DH, Chung JM: The anti-allodynic effects of amitriptyline, gabapentin, and lidocaine in a rat model of neuropathic pain. Anesth Analg 87:1360-1366, 1998

8. Boroujerdi A, Kim HK, Lyu YS, Kim DS, Figueroa KW, Chung JM, Luo ZD: Injury discharges regulate calcium channel alpha-2-delta-1 subunit upregulation in the dorsal horn that contributes to initiation of neuropathic pain. Pain 139: 358-366, 2008

2. Arendt-Nielsen L, Frøkjaer JB, Staahl C, Graven-Nielsen T, Huggins JP, Smart TS, Drewes AM: Effects of gabapentin on experimental somatic pain and temporal summation. Reg Anesth Pain Med 32:382-388, 2007 3. Arnold LM, Russell IJ, Diri EW, Duan WR, Young JP Jr, Sharma U, Martin SA, Barrett JA, Haig G: A 14-week, randomized, double-blinded, placebo-controlled monotherapy trial of pregabalin in patients with fibromyalgia. J Pain 9: 792-805, 2008 4. Back SK, Won SY, Hong SK, Na HS: Gabapentin relieves mechanical, warm and cold allodynia in a rat model of peripheral neuropathy. Neurosci Lett 368:341-344, 2004 5. Bassani F, Bergamaschi M, Tonino Bolzoni P, Villetti G: CHF3381, a novel antinociceptive agent, attenuates capsaicin-induced pain in rats. Eur J Pharmacol 519: 231-236, 2005 6. Bauer C: Pregabalin-mediated anti-allodynia in neuropathic pain correlates with impaired alpha-2-delta 1 trafficking to presynaptic terminals. Presented at 12th World Congress on Pain, Glasgow, Scotland, August 12-17, 2008 7. Bauer CS, Nieto-Rostro M, Rahman W, Tran-Van-Minh A, Ferron L, Douglas L, Kadurin I, Sri Ranjan Y, FernandezAlacid L, Millar NS, Dickenson AH, Lujan R, Dolphin AC: The increased trafficking of the calcium channel subunit a2d-1

9. Bove SE, Laemont KD, Brooker RM, Osborn MN, Sanchez BM, Guzman RE, Hook KE, Juneau PL, Connor JR, Kilgore KS: Surgically induced osteoarthritis in the rat results in the development of both osteoarthritis-like joint pain and secondary hyperalgesia. Osteoarthritis Cartilage 14: 1041-1048, 2006 10. Brooks JC, Tracey I: The insula: a multidimensional integration site for pain. Pain 128:1-2, 2007 11. Burgess SE, Gardell LR, Ossipov MH, Malan TP Jr, Vanderah TW, Lai J, Porreca F: Time-dependent descending facilitation from the rostral ventromedial medulla maintains, but does not initiate, neuropathic pain. J Neurosci 22:5129-5136, 2002 12. Buvanedran A, Reuben SS, Kroin JS: Perioperative pregabalin reduces neuropathic pain at 6 months after total knee arthroplasty [presentation A595]. Presented at 2008 Annual Meeting of the American Society of Anesthesiologists, Orlando, FL, October 18-22, 2008 13. Chen SR, Pan HL: Effect of systemic and intrathecal gabapentin on allodynia in a new rat model of postherpetic neuralgia. Brain Res 1042:108-113, 2005 14. Chen SR, Xu Z, Pan HL: Stereospecific effect of pregabalin on ectopic afferent discharges and neuropathic pain

Tuchman et al induced by sciatic nerve ligation in rats. Anesthesiology 95: 1473-1479, 2001 15. Chizh BA, Go¨hring M, Tro¨ster A, Quartey GK, Schmelz M, Koppert W: Effects of oral pregabalin and aprepitant on pain and central sensitization in the electrical hyperalgesia model in human volunteers. Br J Anaesth 98:246-254, 2007 16. Christensen D, Gautron M, Guilbaud G, Kayser V: Effect of gabapentin and lamotrigine on mechanical allodynialike behaviour in a rat model of trigeminal neuropathic pain. Pain 93:147-153, 2001 17. Crofford LJ, Mease PJ, Simpson SL, Young JP Jr., Martin SA, Haig GM, Sharma U: Fibromyalgia relapse evaluation and efficacy for durability of meaningful relief (FREEDOM): A 6-month, double-blind, placebo-controlled trial with pregabalin. Pain 136:419-431, 2008 18. Crofford LJ, Rowbotham MC, Mease PJ, Russell IJ, Dworkin RH, Corbin AE, Young JP Jr., LaMoreaux LK, Martin SA, Sharma U: Pregabalin 1008-105 Study Group. Pregabalin for the treatment of fibromyalgia syndrome: Results of a randomized, double-blind, placebo-controlled trial. Arthritis Rheum 52:1264-1273, 2005 19. Dickenson AH, Sullivan AF: Evidence for a role of the NMDA receptor in the frequency dependent potentiation of deep rat dorsal horn nociceptive neurones following C fibre stimulation. Neuropharmacology 26:1235-1238, 1987 20. Diop L, Raymond F, Fargeau H, Petoux F, Chovet M, Doherty AM: Pregabalin (CI-1008) inhibits the trinitrobenzene sulfonic acid-induced chronic colonic allodynia in the rat. J Pharmacol Exp Ther 302:1013-1022, 2002 21. Dirks J, Petersen KL, Rowbotham MC, Dahl JB: Gabapentin suppresses cutaneous hyperalgesia following heatcapsaicin sensitization. Anesthesiology 97:102-107, 2002 22. Dooley DJ, Donovan CM, Meder WP, Whetzel SZ: Preferential action of gabapentin and pregabalin at P/Q-type voltage-sensitive calcium channels: Inhibition of K1-evoked [3H]-norepinephrine release from rat neocortical slices. Synapse 45:171-190, 2002 23. Dooley DJ, Mieske CA, Borosky SA: Inhibition of K1-evoked glutamate release from rat neocortical and hippocampal slices by gabapentin. Neurosci Lett 280:107-110, 2000 24. Dooley DJ, Taylor CP, Donevan S, Feltner D: Ca21 channel a2d ligands: Novel modulators of neurotransmission. Trends Pharmacol Sci 28:75-82, 2007 25. Dougherty PM, Palecek J, Zorn S, Willis WD: Combined application of excitatory amino acids and substance P produces long-lasting changes in responses of primate spinothalamic tract neurons. Brain Res Brain Res Rev 18:227-246, 1993 26. Erichsen HK, Blackburn-Munro G: Pharmacological characterisation of the spared nerve injury model of neuropathic pain. Pain 98:151-161, 2002 27. Farajidavar A, Saeb S, Behbehani K: Incorporating synaptic time-dependent plasticity and dynamic synapse into a computational model of wind-up. Neural Netw 21: 241-249, 2008 28. Fehrenbacher JC, Taylor CP, Vasko MR: Pregabalin and gabapentin reduce release of substance P and CGRP from rat spinal tissues only after inflammation or activation of protein kinase C. Pain 105:133-141, 2003

The Journal of Pain

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29. Field MJ, Cox PJ, Stott E, Melrose H, Offord J, Su TZ, Bramwell S, Corradini L, England S, Winks J, Kinloch RA, Hendrich J, Dolphin AC, Webb T, Williams D: Identification of the a2-d-1 subunit of voltage-dependent calcium channels as a molecular target for pain mediating the analgesic actions of pregabalin. Proc Natl Acad Sci U S A 103: 17537-17542, 2006 30. Field MJ, Holloman EF, McCleary S, Hughes J, Singh L: Evaluation of gabapentin and S-(1)-3-isobutylgaba in a rat model of postoperative pain. J Pharmacol Exp Ther 282: 1242-1246, 1997 31. Field MJ, McCleary S, Hughes J, Singh L: Gabapentin and pregabalin, but not morphine and amitriptyline, block both static and dynamic components of mechanical allodynia induced by streptozocin in the rat. Pain 80:391-398, 1999 32. Field MJ, Oles RJ, Lewis AS, McCleary S, Hughes J, Singh L: Gabapentin (neurontin) and S-(1)-3-isobutylgaba represent a novel class of selective antihyperalgesic agents. Br J Pharmacol 121:1513-1522, 1997 33. Fink K, Dooley DJ, Meder WP, Suman-Chauhan N, Duffy S, Clusmann H, Go¨thert M: Inhibition of neuronal Ca21 influx by gabapentin and pregabalin in the human neocortex. Neuropharmacology 42:229-236, 2002 34. Fink K, Meder W, Dooley DJ, Go¨thert M: Inhibition of neuronal Ca21 influx by gabapentin and subsequent reduction of neurotransmitter release from rat neocortical slices. Br J Pharmacol 130:900-906, 2000 35. Fox A, Gentry C, Patel S, Kesingland A, Bevan S: Comparative activity of the anti-convulsants oxcarbazepine, carbamazepine, lamotrigine and gabapentin in a model of neuropathic pain in the rat and guinea-pig. Pain 105: 355-362, 2003 36. Freynhagen R, Strojek K, Griesing T, Whalen E, Balkenohl M: Efficacy of pregabalin in neuropathic pain evaluated in a 12-week, randomised, double-blind, multicentre, placebo-controlled trial of flexible- and fixed-dose regimens. Pain 115:254-263, 2005 37. Gil DW, Cheevers CV, Donello JE: Transient allodynia pain models in mice for early assessment of analgesic activity. Br J Pharmacol 153:769-774, 2008 38. Gillin S, Sorkin LS: Gabapentin reverses the allodynia produced by the administration of anti-GD2 ganglioside, an immunotherapeutic drug. Anesth Analg 86:111-116, 1998 39. Gottrup H, Juhl G, Kristensen AD, Lai R, Chizh BA, Brown J, Bach FW, Jensen TS: Chronic oral gabapentin reduces elements of central sensitization in human experimental hyperalgesia. Anesthesiology 101:1400-1408, 2004 40. Gracely RH, Petzke F, Wolf JM, Clauw DJ: Functional magnetic resonance imaging evidence of augmented pain processing in fibromyalgia. Arthritis Rheum 46:1333-1343, 2002 41. Han DW, Kweon TD, Lee JS, Lee YW: Antiallodynic effect of pregabalin in rat models of sympathetically maintained and sympathetic independent neuropathic pain. Yonsei Med J 48:41-47, 2007 42. Hao JX, Xu XJ, Urban L, Wiesenfeld-Hallin Z: Repeated administration of systemic gabapentin alleviates allodynialike behaviors in spinally injured rats. Neurosci Lett 280: 211-214, 2000 43. Hayashida K, DeGoes S, Curry R, Eisenach JC: Gabapentin activates spinal noradrenergic activity in rats and humans

1248

The Journal of Pain

and reduces hypersensitivity after surgery. Anesthesiology 106:557-562, 2007 44. Hedberg TG, Stanton PK: Long-term potentiation and depression of synaptic transmission in rat posterior cingulate cortex. Brain Res 670:181-196, 1995 45. Hendrich J, Van Minh AT, Heblich F, Nieto-Rostro M, Watschinger K, Striessnig J, Wratten J, Davies A, Dolphin AC: Pharmacological disruption of calcium channel trafficking by the a2d ligand gabapentin. Proc Natl Acad Sci U S A 105:3628-3633, 2008 46. Henriksson KG: Fibromyalgia–from syndrome to disease. Overview of pathogenetic mechanisms. J Rehabil Med 41(Suppl):89-94, 2003 47. Hoffman DL, Dukes EM: The health status burden of people with fibromyalgia: A review of studies that assessed health status with the SF-36 or the SF-12. Int J Clin Pract 62: 115-126, 2008 48. Hunter JC, Gogas KR, Hedley LR, Jacobson LO, Kassotakis L, Thompson J, Fontana DJ: The effect of novel anti-epileptic drugs in rat experimental models of acute and chronic pain. Eur J Pharmacol 324:153-160, 1997 49. Hurley RW, Chatterjea D, Rose Feng M, Taylor CP, Hammond DL: Gabapentin and pregabalin can interact synergistically with naproxen to produce antihyperalgesia. Anesthesiology 97:1263-1273, 2002 50. Iannetti GD, Zambreanu L, Wise RG, Buchanan TJ, Huggins JP, Smart TS, Vennart W, Tracey I: Pharmacological modulation of pain-related brain activity during normal and central sensitization states in humans. Proc Natl Acad Sci U S A 102:18195-18200, 2005 51. Ji RR, Kohno T, Moore KA, Woolf CJ: Central sensitization and LTP: Do pain and memory share similar mechanisms? Trends Neurosci 26:696-705, 2003 52. Jones CK, Peters SC, Shannon HE: Efficacy of duloxetine, a potent and balanced serotonergic and noradrenergic reuptake inhibitor, in inflammatory and acute pain models in rodents. J Pharmacol Exp Ther 312:726-732, 2005

Pregabalin, Central Sensitization, Chronic Pain 59. Luo ZD, Chaplan SR, Higuera ES, Sorkin LS, Stauderman KA, Williams ME, Yaksh TL: Upregulation of dorsal root ganglion a2d calcium channel subunit and its correlation with allodynia in spinal nerve-injured rats. J Neurosci 21:1868-1875, 2001 60. Lyseng-Williamson KA, Siddiqui MA: Pregabalin: A review of its use in fibromyalgia. Drugs 68:2205-2223, 2008 61. Maneuf YP, Hughes J, McKnight AT: Gabapentin inhibits the substance P-facilitated K1-evoked release of [3H]glutamate from rat caudial trigeminal nucleus slices. Pain 93: 191-196, 2001 62. Mease PJ, Russell IJ, Arnold LM, Florian H, Young JP Jr., Martin SA, Sharma U: A randomized, double-blind, placebocontrolled, phase III trial of pregabalin in the treatment of patients with fibromyalgia. J Rheumatol 35:502-514, 2008 63. Melrose HL, Kinloch RA, Cox PJ, Field MJ, Collins D, Williams D: [3H] pregabalin binding is increased in ipsilateral dorsal horn following chronic constriction injury. Neurosci Lett 417:187-192, 2007 64. Mich PM, Horne WA: Alternative splicing of the Ca21 channel ß4 subunit confers specificity for gabapentin inhibition of Cav2.1 trafficking. Mol Pharmacol 74:904-912, 2008 65. Micheva KD, Taylor CP, Smith SJ: Pregabalin reduces the release of synaptic vesicles from cultured hippocampal neurons. Mol Pharmacol 70:467-476, 2006 66. Millecamps M, Coderre TJ: Rats with chronic postischemia pain exhibit an analgesic sensitivity profile similar to human patients with complex regional pain syndrome– type I. Eur J Pharmacol 583:97-102, 2008 67. Nozaki-Taguchi N, Chaplan SR, Higuera ES, Ajakwe RC, Yaksh TL: Vincristine-induced allodynia in the rat. Pain 93: 69-76, 2001 68. Partridge BJ, Chaplan SR, Sakamoto E, Yaksh TL: Characterization of the effects of gabapentin and 3-isobutylgamma-aminobutyric acid on substance P-induced thermal hyperalgesia. Anesthesiology 88:196-205, 1998

53. Jones DL, Sorkin LS: Systemic gabapentin and S(1)-3isobutyl-g-aminobutyric acid block secondary hyperalgesia. Brain Res 810:93-99, 1998

69. Peters CM, Ghilardi JR, Keyser CP, Kubota K, Lindsay TH, Luger NM, Mach DB, Schwei MJ, Sevcik MA, Mantyh PW: Tumor-induced injury of primary afferent sensory nerve fibers in bone cancer pain. Exp Neurol 193:85-100, 2005

54. Kuraishi Y, Iida Y, Zhang HW, Uehara S, Nojima H, Murata J, Saiki I, Takahata H, Ouchi H: Suppression by gabapentin of pain-related mechano-responses in mice given orthotopic tumor inoculation. Biol Pharm Bull 26:550-552, 2003

70. Porreca F, Ossipov MH, Gebhart GF: Chronic pain and medullary descending facilitation. Trends Neurosci 25: 319-325, 2002

55. Laughlin TM, Tram KV, Wilcox GL, Birnbaum AK: Comparison of antiepileptic drugs tiagabine, lamotrigine, and gabapentin in mouse models of acute, prolonged, and chronic nociception. J Pharmacol Exp Ther 302:1168-1175, 2002 56. Li CY, Zhang XL, Matthews EA, Li KW, Kurwa A, Boroujerdi A, Gross J, Gold MS, Dickenson AH, Feng G, Luo ZD: Calcium channel a2d1 subunit mediates spinal hyperexcitability in pain modulation. Pain 125:20-34, 2006 57. Ling B, Authier N, Balayssac D, Eschalier A, Coudore F: Behavioral and pharmacological description of oxaliplatininduced painful neuropathy in rat. Pain 128:225-234, 2007 58. Luo ZD, Calcutt NA, Higuera ES, Valder CR, Song YH, Svensson CI, Myers RR: Injury type-specific calcium channel a2d-1 subunit up-regulation in rat neuropathic pain models correlates with antiallodynic effects of gabapentin. J Pharmacol Exp Ther 303:1199-1205, 2002

71. Price DD: Psychological and neural mechanisms of the affective dimension of pain. Science 288:1769-1772, 2000 72. Ren K, Iadarola MJ, Dubner R: An isobolographic analysis of the effects of N-methyl-D-aspartate and NK1 tachykinin receptor antagonists on inflammatory hyperalgesia in the rat. Br J Pharmacol 117:196-202, 1996 73. Rodrigues-Filho R, Campos MM, Ferreira J, Santos AR, Bertelli JA, Calixto JB: Pharmacological characterisation of the rat brachial plexus avulsion model of neuropathic pain. Brain Res 1018:159-170, 2004 74. Rosenstock J, Tuchman M, LaMoreaux L, Sharma U: Pregabalin for the treatment of painful diabetic peripheral neuropathy: A double-blind, placebo-controlled trial. Pain 110:628-638, 2004 75. Sasaki A, Serizawa K, Andoh T, Shiraki K, Takahata H, Kuraishi Y: Pharmacological differences between static and

Tuchman et al dynamic allodynia in mice with herpetic or postherpetic pain. J Pharmacol Sci 108:266-273, 2008 76. Segerdahl M: Multiple dose gabapentin attenuates cutaneous pain and central sensitisation but not muscle pain in healthy volunteers. Pain 125:158-164, 2006 77. Siddall PJ, Cousins MJ, Otte A, Griesing T, Chambers R, Murphy TK: Pregabalin in central neuropathic pain associated with spinal cord injury: A placebo-controlled trial. Neurology 67:1792-1800, 2006

The Journal of Pain

1249

90. To¨lle T, Freynhagen R, Versavel M, Trostmann U, Young JP Jr: Pregabalin for relief of neuropathic pain associated with diabetic neuropathy: A randomized, double-blind study. Eur J Pain 12:203-213, 2008 91. Tuchman M, Durso-DeCruz E, Murphy TK, Barrett JA: Central sensitization, chronic pain syndromes, and pregabalin [poster 293]. Presented at American Pain Society 27th Annual Scientific Meeting, Tampa, FL, May 8-10, 2008. Available at: http://www.ampainsoc.org/db2/abstract/view? poster_id=3877#293. Accessed December 9, 2008

78. Stacey BR, Barrett JA, Whalen E, Phillips KF, Rowbotham MC: Pregabalin for postherpetic neuralgia: Placebo-controlled trial of fixed and flexible dosing regimens on allodynia and time to onset of pain relief. J Pain 9:1006-1017, 2008

92. van Hooft JA, Dougherty JJ, Endeman D, Nichols RA, Wadman WJ: Gabapentin inhibits presynaptic Ca21 influx and synaptic transmission in rat hippocampus and neocortex. Eur J Pharmacol 449:221-228, 2002

79. Stanfa LC, Singh L, Williams RG, Dickenson AH: Gabapentin, ineffective in normal rats, markedly reduces C-fibre evoked responses after inflammation. Neuroreport 8: 587-590, 1997

93. van Seventer R, Feister HA, Young JP Jr, Stoker M, Versavel M, Rigaudy L: Efficacy and tolerability of twicedaily pregabalin for treating pain and related sleep interference in postherpetic neuralgia: A 13-week, randomized trial. Curr Med Res Opin 22:375-384, 2006

80. Staud R, Bovee CE, Robinson ME, Price DD: Cutaneous C-fiber pain abnormalities of fibromyalgia patients are specifically related to temporal summation. Pain 139:315-323, 2008

94. Werner MU, Perkins FM, Holte K, Pedersen JL, Kehlet H: Effects of gabapentin in acute inflammatory pain in humans. Reg Anesth Pain Med 26:322-328, 2001

81. Staud R, Robinson ME, Price DD: Temporal summation of second pain and its maintenance are useful for characterizing widespread central sensitization of fibromyalgia patients. J Pain 8:893-901, 2007

95. Wesche D, Bockbrader H: A pharmacokinetic comparison of pregabalin and gabapentin [poster 684]. Presented at American Pain Society 24th Annual Scientific Meeting, San Diego, CA, March 30-April 2, 2005. Available at: http:// www.ampainsoc.org/db2/abstract/view?poster_id=2371#684. Accessed December 9, 2008

82. Staud R, Rodriguez ME: Mechanisms of disease: Pain in fibromyalgia syndrome. Nat Clin Pract Rheumatol 2:90-98, 2006 83. Stefani A, Spadoni F, Bernardi G: Gabapentin inhibits calcium currents in isolated rat brain neurons. Neuropharmacology 37:83-91, 1998 84. Suzuki R, Rahman W, Rygh LJ, Webber M, Hunt SP, Dickenson AH: Spinal-supraspinal serotonergic circuits regulating neuropathic pain and its treatment with gabapentin. Pain 117:292-303, 2005 85. Takeuchi Y, Takasu K, Honda M, Ono H, Tanabe M: Neurochemical evidence that supraspinally administered gabapentin activates the descending noradrenergic system after peripheral nerve injury. Eur J Pharmacol 556:69-74, 2007 86. Takeuchi Y, Takasu K, Ono H, Tanabe M: Pregabalin, S-(1)3-isobutylgaba, activates the descending noradrenergic system to alleviate neuropathic pain in the mouse partial sciatic nerve ligation model. Neuropharmacology 53:842-853, 2007 87. Tang HB, Li YS, Arihiro K, Nakata Y: Activation of the neurokinin-1 receptor by substance P triggers the release of substance P from cultured adult rat dorsal root ganglion neurons. Mol Pain 3:42, 2007 88. Tanimoto-Mori S, Nakazato-Imasato E, Toide K, Kita Y: Pharmacologic investigation of the mechanism underlying cold allodynia using a new cold plate procedure in rats with chronic constriction injuries. Behav Pharmacol 19: 85-90, 2008 89. Taylor CP, Angelotti T, Fauman E: Pharmacology and mechanism of action of pregabalin: The calcium channel a2d (alpha2-delta) subunit as a target for antiepileptic drug discovery. Epilepsy Res 73:137-150, 2007

96. Whiteside GT, Harrison J, Boulet J, Mark L, Pearson M, Gottshall S, Walker K: Pharmacological characterisation of a rat model of incisional pain. Br J Pharmacol 141:85-91, 2004 97. Willis WD: Long-term potentiation in spinothalamic neurons. Brain Res Brain Res Rev 40:202-214, 2002 98. Woolf CJ: Evidence for a central component of postinjury pain hypersensitivity. Nature 306:686-688, 1983 99. Woolf CJ, Salter MW: Neuronal plasticity: Increasing the gain in pain. Science 288:1765-1769, 2000 100. Xiao W, Boroujerdi A, Bennett GJ, Luo ZD: Chemotherapyevoked painful peripheral neuropathy: Analgesic effects of gabapentin and effects on expression of the alpha-2-delta type-1 calcium channel subunit. Neuroscience 144:714-720, 2007 101. Yashpal K, Pitcher GM, Parent A, Quirion R, Coderre TJ: Noxious thermal and chemical stimulation induce increases in 3H-phorbol 12,13-dibutyrate binding in spinal cord dorsal horn as well as persistent pain and hyperalgesia, which is reduced by inhibition of protein kinase C. J Neurosci 15: 3263-3272, 1995 102. Yunus MB: Fibromyalgia and overlapping disorders: The unifying concept of central sensitivity syndromes. Semin Arthritis Rheum 36:339-356, 2007 103. Zambreanu L, Wise RG, Brooks JC, Iannetti GD, Tracey I: A role for the brainstem in central sensitisation in humans. Evidence from functional magnetic resonance imaging. Pain 114:397-407, 2005