Synergistic effect between intrathecal non-NMDA antagonist and gabapentin on allodynia induced by spinal nerve ligation in rats

Calcineurin Regulates Synaptic Plasticity and Nociceptive Transmission at the Spinal Cord Level

Calcineurin, the predominant Ca2+/calmodulin-dependent serine/threonine protein phosphatase (also known as protein phosphatase 2B), is highly expressed in immune T cells and the nervous system, including the dorsal root ganglion and spinal cord. It controls synaptic transmission and plasticity by maintaining the appropriate phosphorylation status of many ion channels present at presynaptic and postsynaptic sites. As such, normal calcineurin activity in neurons and synapses is mainly involved in negative feedback regulation in response to increased neuronal activity and intracellular Ca2+ levels. Calcineurin inhibitors (e.g., cyclosporine and tacrolimus) are widely used as immunosuppressants in tissue and organ transplantation recipients and for treating autoimmune diseases but can cause severe pain in some patients. Furthermore, diminished calcineurin activity at the spinal cord level may play a major role in the transition from acute to chronic neuropathic pain after nerve injury. Restoring calcineurin activity at the spinal cord level produces long-lasting pain relief in animal models of neuropathic pain. In this article, we provide an overview of recent studies on the critical roles of calcineurin in regulating glutamate NMDA and AMPA receptors, voltage-gated Ca2+ channels, potassium channels, and transient receptor potential channels expressed in the spinal dorsal horn and primary sensory neurons.

α2δ-1 switches the phenotype of synaptic AMPA receptors by physically disrupting heteromeric subunit assembly

Many neurological disorders show an increased prevalence of GluA2-lacking, Ca2+-permeable AMPA receptors (CP-AMPARs), which dramatically alters synaptic function. However, the molecular mechanism underlying this distinct synaptic plasticity remains enigmatic. Here, we show that nerve injury potentiates postsynaptic, but not presynaptic, CP-AMPARs in the spinal dorsal horn via α2δ-1. Overexpressing α2δ-1, previously regarded as a Ca2+ channel subunit, augments CP-AMPAR levels at the cell surface and synapse. Mechanistically, α2δ-1 physically interacts with both GluA1 and GluA2 via its C terminus, inhibits the GluA1/GluA2 heteromeric assembly, and increases GluA2 retention in the endoplasmic reticulum. Consequently, α2δ-1 diminishes the availability and synaptic expression of GluA1/GluA2 heterotetramers in the spinal cord in neuropathic pain. Inhibiting α2δ-1 with gabapentin or disrupting the α2δ-1-AMPAR complex fully restores the intracellular assembly and synaptic dominance of heteromeric GluA1/GluA2 receptors. Thus, α2δ-1 is a pivotal AMPAR-interacting protein that controls the subunit composition and Ca2+ permeability of postsynaptic AMPARs.

Inhibitory effect of intrathecally administered AM404, an endocannabinoid reuptake inhibitor, on neuropathic pain in a rat chronic constriction injury model

BACKGROUND

The endocannabinoid system modulates a wide variety of pain conditions. Systemically administered AM404, an endocannabinoid reuptake inhibitor, exerts antinociceptive effects via activation of the endocannabinoid system. However, the mechanism and site of AM404 action are not fully understood. Here, we explored the effect of AM404 on neuropathic pain at the site of the spinal cord.

METHODS

Male Sprague-Dawley rats were subjected to chronic constriction injury (CCI) of the sciatic nerve. The effects of intrathecal administration of AM404 on mechanical and cold hyperalgesia were examined using the electronic von Frey test and cold plate test, respectively. Motor coordination was assessed using the rotarod test. To understand the mechanisms underlying the action of AM404, we tested the effects of pretreatment with the cannabinoid type 1 (CB₁) receptor antagonist AM251, CB₂ receptor antagonist AM630, and transient receptor potential vanilloid type 1 (TRPV1) antagonist capsazepine.

RESULTS

AM404 attenuated mechanical and cold hyperalgesia with minimal effects on motor coordination. AM251 significantly inhibited the antihyperalgesic action of AM404, whereas capsazepine showed a potentiating effect.

CONCLUSIONS

These results indicate that AM404 exerts antihyperalgesic effects primarily via CB₁, but not CB₂, receptor activation at the site of the spinal cord. TRPV1 receptors appear to play a pronociceptive role in CCI rats. The endocannabinoid reuptake inhibitor may be a promising candidate treatment for neuropathic pain.

Streptozotocin-Induced Diabetic Neuropathic Pain Is Associated with Potentiated Calcium-Permeable AMPA Receptor Activity in the Spinal Cord

Neuronal hyperactivity in the spinal dorsal horn can amplify nociceptive input in diabetic neuropathic pain. The glutamate N-methyl-d-aspartate and α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptors (NMDA receptors and AMPA receptors, respectively) are involved in spinal nociceptive transmission. It is unclear, however, whether painful diabetic neuropathy is associated with changes in the activity of synaptic NMDA receptors and AMPA receptors in spinal dorsal horn neurons. AMPA receptors lacking GluA2 are Ca²⁺-permeable (CP-AMPA receptors), and their currents display characteristic inward rectification. In this study, we showed that evoked excitatory postsynaptic currents (EPSCs), induced by streptozotocin, exhibited inward rectification in spinal dorsal neurons in diabetic rats. Presynaptic and postsynaptic NMDA receptor activity in the spinal dorsal horn was similar in diabetic and control rats. In the dorsal spinal cord, the membrane GluA2 protein level was significantly lower in diabetic than in control rats, whereas the cytosolic GluA2 level was greater in diabetic than in control rats. In contrast, the GluA1 subunit levels in the plasma membrane and cytosol did not differ between the two groups. Blocking CP-AMPA receptors significantly reduced the amplitude of EPSCs of dorsal horn neurons in diabetic but not in control rats. Furthermore, blocking spinal CP-AMPA receptors reduced pain hypersensitivity in diabetic rats but had no effect on nociception in control rats. Our study suggests that diabetic neuropathy augments CP-AMPA receptor activity in the spinal dorsal horn by causing intracellular retention of GluA2 and impairing GluA2 membrane trafficking. Increased prevalence of spinal CP-AMPA receptors sustains diabetic neuropathic pain. SIGNIFICANCE STATEMENT: This study demonstrates that the prevalence of synaptic calcium-permeable AMPA receptors is increased in the spinal dorsal horn, which mediates pain hypersensitivity in diabetic neuropathy. Thus, calcium-permeable AMPA receptors play an important role in glutamatergic synaptic plasticity in the spinal cord in painful diabetic neuropathy. This new knowledge improves our understanding of the mechanisms involved in central sensitization associated with diabetic neuropathic pain and suggests that calcium-permeable AMPA receptors are an alternative therapeutic target for treating this chronic pain condition.

Efficacy of a topical gabapentin gel in a cisplatin paradigm of chemotherapy-induced peripheral neuropathy

BACKGROUND

Chemotherapy induced peripheral neuropathy (CIPN) has been attributed to chemotherapeutic agents such as cisplatin which adversely affect disease outcome leading to increased cancer related morbidity. The clinical efficacy of systemic gabapentin in neuropathic pain management is limited by central side-effects in addition to a scarceness of conclusive evidence of its efficacy in CIPN management. The topical route therefore may provide a relatively safe alternative for neuropathic pain treatment in general and CIPN in particular.

METHODS

Cisplatin induced neuropathic nociception was established in rats after a single weekly cisplatin injection (3.0 mg/kg, intraperitoneally) for 4 weeks. The evoked neuropathic sensation of allodynia was assessed by plantar application of von Frey monofilaments as the paw withdrawal threshold (PWT), whereas the expression of heat-hypoalgesia was determined on a hot-plate as paw withdrawal latency (PWL). Gabapentin gel (10% w/w) was applied three-times daily on the hind paws while in a concurrent systemic study, gabapentin was administered daily (75 mg/kg, intraperitoneally) for 4 weeks. To assess any evidence of neurological adverse symptoms of cisplatin and the central side-effect propensity of systemic or topical gabapentin, evaluation of motor coordination (rotarod test) and gait (footprint analysis) were performed.

RESULTS

Cisplatin invoked a progressive development of neuropathic hind paw allodynia (decreased PWT, days 7-28) and heat hypoalgesia (increased PWL, days 21-28). Topical gabapentin significantly delayed the expression of both allodynia on protocol days 21 and 28 and heat-hypoalgesia (day 28). Systemic gabapentin displayed a comparative anti-neuropathic predisposition through a sustained suppression of tactile allodynia on days 14 and 21-28 as well as thermal hypoalgesia (days 21 and 28). Systemic gabapentin also impaired motor coordination and gait thus affirming its clinically documented central side effects, but these outcomes were not evident after topical treatment.

CONCLUSIONS

Both topical and systemic gabapentin exhibit a propensity to attenuate CIPN in a cisplatin paradigm. Gabapentin applied topically may therefore provide an adjunctive or alternative route for CIPN management upon cessation of systemic medications due to intolerable side-effects.

Multiple sites and actions of gabapentin-induced relief of ongoing experimental neuropathic pain

Gabapentin (GBP) is a first-line therapy for neuropathic pain, but its mechanisms and sites of action remain uncertain. We investigated GBP-induced modulation of neuropathic pain following spinal nerve ligation (SNL) in rats. Intravenous or intrathecal GBP reversed evoked mechanical hypersensitivity and produced conditioned place preference (CPP) and dopamine (DA) release in the nucleus accumbens (NAc) selectively in SNL rats. Spinal GBP also significantly inhibited dorsal horn wide-dynamic-range neuronal responses to a range of evoked stimuli in SNL rats. By contrast, GBP microinjected bilaterally into the rostral anterior cingulate cortex (rACC), produced CPP, and elicited NAc DA release selectively in SNL rats but did not reverse tactile allodynia and had marginal effects on wide-dynamic-range neuronal activity. Moreover, blockade of endogenous opioid signaling in the rACC prevented intravenous GBP-induced CPP and NAc DA release but failed to block its inhibition of tactile allodynia. Gabapentin, therefore, can potentially act to produce its pain relieving effects by (a) inhibition of injury-induced spinal neuronal excitability, evoked hypersensitivity, and ongoing pain and (b) selective supraspinal modulation of affective qualities of pain, without alteration of reflexive behaviors. Consistent with previous findings of pain relief from nonopioid analgesics, GBP requires engagement of rACC endogenous opioid circuits and downstream activation of mesolimbic reward circuits reflected in learned pain-motivated behaviors. These findings support the partial separation of sensory and affective dimensions of pain in this experimental model and suggest that modulation of affective-motivational qualities of pain may be the preferential mechanism of GBP's analgesic effects in patients.

Nerve Injury-Induced Chronic Pain Is Associated with Persistent DNA Methylation Reprogramming in Dorsal Root Ganglion

Nerve injury-induced hyperactivity of primary sensory neurons in the dorsal root ganglion (DRG) contributes to chronic pain development, but the underlying epigenetic mechanisms remain poorly understood. Here we determined genome-wide changes in DNA methylation in the nervous system in neuropathic pain. Spinal nerve ligation (SNL), but not paclitaxel treatment, in male Sprague Dawley rats induced a consistent low-level hypomethylation in the CpG sites in the DRG during the acute and chronic phases of neuropathic pain. DNA methylation remodeling in the DRG occurred early after SNL and persisted for at least 3 weeks. SNL caused DNA methylation changes at 8% of CpG sites with prevailing hypomethylation outside of CpG islands, in introns, intergenic regions, and repetitive sequences. In contrast, SNL caused more gains of methylation in the spinal cord and prefrontal cortex. The DNA methylation changes in the injured DRGs recapitulated developmental reprogramming at the neonatal stage. Methylation reprogramming was correlated with increased gene expression variability. A diet deficient in methyl donors induced hypomethylation and pain hypersensitivity. Intrathecal administration of the DNA methyltransferase inhibitor RG108 caused long-lasting pain hypersensitivity. DNA methylation reprogramming in the DRG thus contributes to nerve injury-induced chronic pain. Restoring DNA methylation may represent a new therapeutic approach to treat neuropathic pain.SIGNIFICANCE STATEMENT Epigenetic mechanisms are critically involved in the transition from acute to chronic pain after nerve injury. However, genome-wide changes in DNA methylation in the nervous system and their roles in neuropathic pain development remain unclear. Here we used digital restriction enzyme analysis of methylation to quantitatively determine genome-wide DNA methylation changes caused by nerve injury. We showed that nerve injury caused DNA methylation changes at 8% of CpG sites with prevailing hypomethylation outside of CpG islands in the dorsal root ganglion. Reducing DNA methylation induced pain hypersensitivity, whereas increasing DNA methylation attenuated neuropathic pain. These findings extend our understanding of the epigenetic mechanism of chronic neuropathic pain and suggest new strategies to treat nerve injury-induced chronic pain.

Dose-related beneficial and harmful effects of gabapentin in postoperative pain management - post hoc analyses from a systematic review with meta-analyses and trial sequential analyses

Background

During the last 15 years, gabapentin has become an established component of postoperative pain treatment. Gabapentin has been employed in a wide range of doses, but little is known about the optimal dose, providing the best balance between benefit and harm. This systematic review with meta-analyses aimed to explore the beneficial and harmful effects of various doses of gabapentin administered to surgical patients.

Materials and methods

Data in this paper were derived from an original review, and the subgroup analyses were predefined in an International Prospective Register of Systematic Reviews published protocol: PROSPERO (ID: CRD42013006538). The methods followed Cochrane guidelines. The Cochrane Library’s CENTRAL, PubMed, EMBASE, Science Citation Index Expanded, Google Scholar, and FDA database were searched for relevant trials. Randomized clinical trials comparing gabapentin versus placebo were included. Four different dose intervals were investigated: 0–350, 351–700, 701–1050, and >1050 mg. Primary co-outcomes were 24-hour morphine consumption and serious adverse events (SAEs), with emphasis put on trials with low risk of bias.

Results

One hundred and twenty-two randomized clinical trials, with 8466 patients, were included. Sixteen were overall low risk of bias. No consistent increase in morphine-sparing effect was observed with increasing doses of gabapentin from the trials with low risk of bias. Analyzing all trials, the smallest and the highest dose subgroups demonstrated numerically the most prominent reduction in morphine consumption. Twenty-seven trials reported 72 SAEs, of which 83% were reported in the >1050 mg subgroup. No systematic increase in SAEs was observed with increasing doses of gabapentin.

Conclusion

Data were sparse, and the small number of trials with low risk of bias is a major limitation for firm conclusions. Taking these limitations into account, we were not able to demonstrate a clear relationship between the dosage of gabapentin and opioid-sparing or harmful effects. These subgroup analyses are exploratory and hypothesis-generating for future trialists.

Synergistic Interaction of a Gabapentin- Mangiferin Combination in Formalin-Induced Secondary Mechanical Allodynia and Hyperalgesia in Rats Is Mediated by Activation of NO-Cyclic GMP-ATP-Sensitive K+ Channel Pathway

Preclinical Research Gabapentin is an anticonvulsant used to treat neuropathic pain. Mangiferin is an antioxidant that has antinociceptive and antiallodynic effects in inflammatory and neuropathic pain models. The purpose of this study was to determine the interaction between mangiferin and gabapentin in the development and maintenance of formalin-induced secondary allodynia and hyperalgesia in rats. Gabapentin, mangiferin, or their fixed-dose ratio combination were administrated peripherally. Isobolographic analyses was used to define the nature of the interaction of antiallodynic and/or antihyperalgesic effects of the two compounds. Theoretical ED₅₀ values for the combination were 74.31 µg/paw and 95.20 µg/paw for pre- and post-treatment, respectively. These values were higher than the experimental ED₅₀ values, 29.45 µg/paw and 37.73 µg/paw respectively, indicating a synergistic interaction in formalin-induced secondary allodynia and hyperalgesia. The antiallodynic and antihyperalgesic effect induced by the gabapentin/mangiferin combination was blocked by administration of L-NAME, the soluble guanylyl cyclase inhibitor, ODQ and glibenclamide. These data suggest that the gabapentin- mangiferin combination produces a synergistic interaction at the peripheral level. Moreover, the antiallodynic and hyperalgesic effect induced by the combination is mediated via the activation of an NO-cyclic GMP-ATP-sensitive K⁺ channel pathway. Drug Dev Res 78 : 390-402, 2017. © 2017 Wiley Periodicals, Inc.

Efficacy and Safety of Gabapentin vs. Carbamazepine in the Treatment of Trigeminal Neuralgia: A Meta-Analysis

To evaluate the safety and efficacy of gabapentin in comparison with carbamazepine in the treatment of trigeminal neuralgia, a meta-analysis of randomized controlled trials was performed. Two reviewers independently selected studies, assessed study quality, and extracted data. Sixteen randomized controlled trials that included 1,331 patients were assessed. The meta-analysis showed that the total effective rate of gabapentin therapy group was similar with carbamazepine therapy group (OR = 1.600, 95% CI 1.185, 2.161, P = 0.002). While the effective rate of gabapentin therapy for 4 weeks was higher than that of carbamazepine therapy (OR = 1.495, 95% CI 1.061, 2.107, P = 0.022, heterogeneity: x² = 7.12, P = 0.625, I² = 0.0%), the life satisfaction improvement is also better in the gabapentin therapy group after a 4-week treatment (SMD = 0.966, 95% CI 0.583, 1.348, P < 0.001). Furthermore, our meta-analysis suggested that the adverse reaction rate of gabapentin therapy group was significantly lower than that of carbamazepine therapy group (OR = 0.312, 95% CI 0.240, 0.407, P < 0.001). In conclusion, present trials comparing gabapentin with carbamazepine are all poor in terms of methodological quality. Based on the available evidence, it is not possible to draw conclusions regarding the efficacy and side effects of gabapentin being superior to carbamazepine.

A streptozotocin-induced diabetic neuropathic pain model for static or dynamic mechanical allodynia and vulvodynia: validation using topical and systemic gabapentin

Neuropathic vulvodynia is a state of vulval discomfort characterized by a burning sensation, diffuse pain, pruritus or rawness with an acute or chronic onset. Diabetes mellitus may cause this type of vulvar pain in several ways, so this study was conducted to evaluate streptozotocin-induced diabetes as a neuropathic pain model for vulvodynia in female rats. The presence of streptozotocin (50 mg/kg i.p.)-induced diabetes was initially verified by disclosure of pancreatic tissue degeneration, blood glucose elevation and body weight loss 5–29 days after a single treatment. Dynamic (shortened paw withdrawal latency to light brushing) and static (diminished von Frey filament threshold pressure) mechanical allodynia was then confirmed on the plantar foot surface. Subsequently, both static and dynamic vulvodynia was detected by application of the paradigm to the vulval region. Systemic gabapentin (75 mg/kg, i.p.) and topical gabapentin (10 % gel) were finally tested against allodynia and vulvodynia. Topical gabapentin and the control gel vehicle significantly increased paw withdrawal threshold in the case of the static allodynia model and also paw withdrawal latency in the model for dynamic allodynia when compared with the streptozotocin-pretreated group. Likewise, in the case of static and dynamic vulvodynia, there was a significant antivulvodynia effect of systemic and topical gabapentin treatment. These outcomes substantiate the value of this model not only for allodynia but also for vulvodynia, and this was corroborated by the findings not only with systemic but also with topical gabapentin.

Casein kinase II inhibition reverses pain hypersensitivity and potentiated spinal N-methyl-D-aspartate receptor activity caused by calcineurin inhibitor

Clinically used calcineurin inhibitors, including tacrolimus (FK506) and cyclosporine A, can induce calcineurin inhibitor-induced pain syndrome (CIPS), which is characterized as severe pain and pain hypersensitivity. Increased synaptic N-methyl-D-aspartate receptor (NMDAR) activity in the spinal dorsal horn plays a critical role in the development of CIPS. Casein kinase II (CK2), a serine/threonine protein kinase, can regulate synaptic NMDAR activity in the brain. In this study, we determined whether spinal CK2 is involved in increased NMDAR activity and pain hypersensitivity caused by systemic administration of FK506 in rats. FK506 treatment caused a large increase in the amplitude of NMDAR-mediated excitatory postsynaptic currents (EPSCs) evoked by primary afferent stimulation and in the frequency of miniature EPSCs of spinal dorsal horn neurons. CK2 inhibition with either 5,6-dichloro-1-β-d-ribofuranosylbenzimidazole (DRB) or 4,5,6,7-tetrabromobenzotriazole (TBB) completely normalized the amplitude of evoked NMDAR-EPSCs of dorsal horn neurons in FK506-treated rats. In addition, DRB or TBB significantly attenuated the amplitude of NMDAR currents elicited by puff application of N-methyl-D-aspartate to dorsal horn neurons in FK506-treated rats. Furthermore, treatment with DRB or TBB significantly reduced the frequency of miniature EPSCs of spinal dorsal horn neurons increased by FK506 treatment. In addition, intrathecal injection of DRB or TBB dose-dependently reversed tactile allodynia and mechanical hyperalgesia in FK506-treated rats. Collectively, our findings indicate that CK2 inhibition abrogates pain hypersensitivity and increased pre- and postsynaptic NMDAR activity in the spinal cord caused by calcineurin inhibitors. CK2 inhibitors may represent a new therapeutic option for the treatment of CIPS.

Calcineurin inhibitor induces pain hypersensitivity by potentiating pre- and postsynaptic NMDA receptor activity in spinal cords

Calcineurin inhibitors, such as cyclosporin A and tacrolimus (FK506), have played a pivotal role in the preservation of allograft function. However, these drugs can cause unexplained severe pain in patients, often referred to as calcineurin inhibitor-induced pain syndrome (CIPS). Although calcineurin can regulate NMDA receptor (NMDAR) activity, the causal relationship between spinal synaptic plasticity and CIPS remains unknown. In this study, we showed that systemic administration of FK506 (1.5 mg kg(-1) day(-1)) for 7 days in rats led to long-lasting nociceptive and mechanical hypersensitivity. Whole-cell patch-clamp recordings in spinal cord slices revealed that FK506 treatment caused a large increase in the amplitude of NMDAR-mediated excitatory postsynaptic currents (EPSCs) of dorsal horn neurons evoked by dorsal root stimulation. The amplitude of NMDAR currents elicited by puff NMDA application to dorsal horn neurons was also significantly greater in FK506-treated than in vehicle-treated rats. The frequency of spontaneous and miniature EPSCs in most dorsal horn neurons was profoundly increased in FK506-treated rats and was reduced by blocking NMDARs. Furthermore, blocking GluN2A or GluN2B subunits similarly reduced the amplitude of evoked EPSCs and the frequency of miniature EPSCs in dorsal horn neurons of FK506-treated rats. In addition, intrathecal injection of an NMDAR antagonist or systemic administration of memantine effectively reversed nociceptive and mechanical hypersensitivity in FK506-treated rats. Our findings indicate that calcineurin inhibition increases glutamate-mediated nociceptive input by potentiating presynaptic and postsynaptic NMDAR activity in spinal cords. NMDAR antagonists may represent a new therapeutic option for the treatment of CIPS.

Nerve injury increases GluA2-lacking AMPA receptor prevalence in spinal cords: functional significance and signaling mechanisms

The glutamate α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptors (AMPARs) are critically involved in the excitatory synaptic transmission, and blocking AMPARs at the spinal level reverses neuropathic pain. However, little is known about changes in the composition of synaptic AMPARs in the spinal dorsal horn after peripheral nerve injury. AMPARs lacking the GluA2 subunit are permeable to Ca(2+), and their currents show unique inward rectification. We found that AMPAR-mediated excitatory postsynaptic currents (AMPAR-EPSCs) of spinal dorsal horn neurons exhibited a linear current-voltage relationship in control rats, whereas AMPAR-EPSCs of dorsal horn neurons displayed inward rectification in rats with spinal nerve injury. In nerve-injured rats, compared with control rats, the GluA2 protein level was significantly less in the plasma membrane but was greater in the cytosolic vesicle fraction in the dorsal spinal cord. However, the GluA1 protein levels in these fractions did not differ significantly between nerve-injured and control rats. Blocking N-methyl-d-aspartate receptors (NMDARs) abolished inward rectification of AMPAR-EPSCs of dorsal horn neurons in nerve-injured rats. Furthermore, inhibition of calpain or calcineurin, but not protein kinase C, completely blocked nerve injury-induced inward rectification of AMPAR-EPSCs of dorsal horn neurons. In addition, blocking GluA2-lacking AMPARs at the spinal cord level reduced nerve injury-induced pain hypersensitivity. Our study suggests that nerve injury increases GluA2 internalization and the prevalence of GluA2-lacking AMPARs in the spinal dorsal horn to maintain chronic neuropathic pain. Increased prevalence of spinal GluA2-lacking AMPARs in neuropathic pain is mediated by NMDARs and subsequent stimulation of calpain and calcineurin signaling.

Upregulation of nuclear factor of activated T-cells by nerve injury contributes to development of neuropathic pain

Nerve injury induces long-term changes in gene expression in the nociceptive circuitry and can lead to chronic neuropathic pain. However, the transcriptional mechanism involved in neuropathic pain is poorly understood. Nuclear factor of activated T-cells (NFATc) is a transcriptional factor regulated by the Ca(2+)-dependent protein phosphatase calcineurin. In this study, we determined nerve injury-induced changes in the expression of NFATc1-c4 in the dorsal root ganglia (DRG) and spinal cords and their role in the development of neuropathic pain. The mRNA of NFATc1-c4 was detected in the rat DRG and dorsal spinal cord. Nerve injury transiently elevated NFATc1-c3 mRNA levels and persistently increased NFATc4 and C-C chemokine receptor type 2 (CCR2) mRNA levels in the DRG. However, NFATc1-c4 mRNA levels in the spinal cord were not altered significantly by nerve injury. Nerve injury also significantly increased the protein level of dephosphorylated NFATc4 in the DRG. Intrathecal injection of the specific NFATc inhibitor 11R-VIVIT or the calcineurin inhibitor FK-506 (tacrolimus) early after nerve injury significantly attenuated the development of tactile allodynia. In addition, treatment with FK-506 or 11R-VIVIT significantly reduced the mRNA levels of NFATc4 and CCR2 but not large-conductance Ca(2+)-activated K(+) channels, in the DRG after nerve injury. Our findings suggest that peripheral nerve injury causes a time-dependent change in NFATc1-c4 expression in the DRG. Calcineurin-NFATc-mediated expression of pronociceptive cytokines contributes to the transition from acute to chronic pain after nerve injury.

Up-regulation of Cavβ3 subunit in primary sensory neurons increases voltage-activated Ca2+ channel activity and nociceptive input in neuropathic pain

High voltage-activated calcium channels (HVACCs) are essential for synaptic and nociceptive transmission. Although blocking HVACCs can effectively reduce pain, this treatment strategy is associated with intolerable adverse effects. Neuronal HVACCs are typically composed of α(1), β (Cavβ), and α(2)δ subunits. The Cavβ subunit plays a crucial role in the membrane expression and gating properties of the pore-forming α(1) subunit. However, little is known about how nerve injury affects the expression and function of Cavβ subunits in primary sensory neurons. In this study, we found that Cavβ(3) and Cavβ(4) are the most prominent subtypes expressed in the rat dorsal root ganglion (DRG) and dorsal spinal cord. Spinal nerve ligation (SNL) in rats significantly increased mRNA and protein levels of the Cavβ(3), but not Cavβ(4), subunit in the DRG. SNL also significantly increased HVACC currents in small DRG neurons and monosynaptic excitatory postsynaptic currents of spinal dorsal horn neurons evoked from the dorsal root. Intrathecal injection of Cavβ(3)-specific siRNA significantly reduced HVACC currents in small DRG neurons and the amplitude of monosynaptic excitatory postsynaptic currents of dorsal horn neurons in SNL rats. Furthermore, intrathecal treatment with Cavβ(3)-specific siRNA normalized mechanical hyperalgesia and tactile allodynia caused by SNL but had no significant effect on the normal nociceptive threshold. Our findings provide novel evidence that increased expression of the Cavβ(3) subunit augments HVACC activity in primary sensory neurons and nociceptive input to dorsal horn neurons in neuropathic pain. Targeting the Cavβ(3) subunit at the spinal level represents an effective strategy for treating neuropathic pain.

Functional plasticity of group II metabotropic glutamate receptors in regulating spinal excitatory and inhibitory synaptic input in neuropathic pain

Metabotropic glutamate receptors (mGluRs) are involved in the modulation of synaptic transmission and plasticity. Group II mGluRs in the spinal cord regulate glutamatergic input, but their functional changes in neuropathic pain are not clear. In this study, we determined the plasticity of spinal group II mGluRs in controlling excitatory and inhibitory synaptic transmission and nociception in neuropathic pain. Neuropathic pain was induced by spinal nerve ligation in rats, and whole-cell voltage-clamp recordings of glutamatergic excitatory postsynaptic currents (EPSCs) and spontaneous and miniature GABAergic and glycinergic inhibitory postsynaptic currents (sIPSCs and mIPSCs, respectively) were performed in spinal cord slices. The specific group II mGluR agonist (2S,2'R,3'R)-2-(2',3'-dicarboxycyclopropyl)glycine (DCG-IV) had a similar inhibitory effect on monosynaptic EPSCs evoked from the dorsal root in sham and nerve-injured rats. However, DCG-IV produced a greater inhibitory effect on evoked polysynaptic EPSCs and the frequency of spontaneous EPSCs in nerve-injured rats than in control rats. Although DCG-IV similarly reduced the frequency of GABAergic sIPSCs and mIPSCs in both groups, it distinctly inhibited the frequency of glycinergic sIPSCs and mIPSCs only in nerve-injured rats. The DCG-IV effect was blocked by the group II mGluR antagonist but not by the N-methyl-D-aspartate receptor antagonist. Strikingly, intrathecal injection of DCG-IV dose-dependently attenuated allodynia and hyperalgesia in nerve-injured rats but produced hyperalgesia in control rats. Our study provides new information that nerve injury up-regulates group II mGluRs present on glutamatergic and glycinergic interneurons in the spinal cord. Activation of group II mGluRs reduces neuropathic pain probably by attenuating glutamatergic and glycinergic input to spinal dorsal horn neurons.

Gabapentin enacarbil - clinical efficacy in restless legs syndrome

Restless legs syndrome (RLS) is a sleep-related movement disorder commonly involving an unpleasant urge to move the limbs, typically the legs. Dopaminergic agents represent the first-line therapy for RLS; however, long-term use of such drugs results in worsening symptoms due to "augmentation" or other adverse events. Gabapentin, an analog of the inhibitory neurotransmitter gamma-aminobutyric acid (GABA), is an anticonvulsant/analgesic agent. Gabapentin is only mildly effective in relieving RLS symptoms, perhaps a result of its poor absorption from the gastrointestinal (GI) tract. Gabapentin enacarbil is a prodrug of gabapentin specifically designed to enhance absorption via the GI tract, and hence provide improved circulating levels of gabapentin on metabolism. Clinical trials to date have demonstrated favorable safety and (compared to traditional gabapentin) improved pharmacokinetics and efficacy in treating RLS symptoms. Thus, gabapentin enacarbil may prove to be a useful drug in treating RLS. An application of gabapentin enacarbil for treatment of RLS is currently pending with FDA for approval.

Current approaches with the glutamatergic system as targets in the treatment of neuropathic pain

Glutamate is the most widely distributed and a major excitatory neurotransmitter in the CNS. It has been found to play a critical role in various physiological functions in which increased glutamate or its subsequent stimulation is thought to have a role in pathophysiological mechanism of various CNS diseases like epilepsy, stroke, depression and pain. Early attempts to develop glutamatergic antagonists failed in clinical studies due to nonselective or competitive antagonism and have a lot of safety issues like loss of cognitive functions, psychomimetic effect and sedation. Neuropathic pain can be described as pain associated with damage or permanent alteration of the peripheral or central nervous system. At present, there are very few effective therapies for neuropathic pain. The current approach includes targeting specific or alternate binding sites of glutamate receptors, resulting in reduced CNS liabilities. Targeting the glutamatergic system shows a better efficacy and fewer side effects, compared with classical drugs for the treatment of neuropathic pain. This review discusses the various targets on glutamatergic system, which includes the receptors, transporters and enzymes, for the treatment of neuropathic pain and their advantages over classical glutamatergic antagonists. The review also highlights the newer drugs in clinical trials for neuropathic pain.

Antinociceptive effects of chronic administration of uncompetitive NMDA receptor antagonists in a rat model of diabetic neuropathic pain

Diabetic neuropathic pain remains an unmet clinical problem and is poorly relieved by conventional analgesics. N-methyl-D-aspartate (NMDA) receptors play an important role in central sensitization in neuropathic pain. Although NMDA antagonists are highly effective in reducing neuropathic pain, these agents cause severe side effects at therapeutic doses, which limit their clinical uses. Neramexane and memantine are uncompetitive NMDA antagonists with minimal side effects at therapeutic doses. Here we determined the antinociceptive effect of chronic administration of neramexane and compared its effect with that of memantine and gabapentin in a rat model of diabetic neuropathic pain. Mechanical hyperalgesia was measured with a noxious pressure stimulus, and tactile allodynia was assessed with von Frey filaments in diabetic rats induced by streptozotocin. Compared with vehicle-treated rats, treatment with neramexane (12.3, 24.6, and 49.2 mg/kg/day) for 2 weeks via an osmotic minipump produced dose-dependent and sustained effects on mechanical hyperalgesia and allodynia. Administration of memantine (20 mg/kg/day) or gabapentin (50 mg/kg/day) for 2 weeks also produced significant and persistent antinociceptive effects on mechanical hyperalgesia and allodynia. The magnitude of the antinociceptive effect produced by the intermediate and high doses of neramexane was comparable to that of gabapentin and memantine. The plasma level achieved by neramexane at 12.3, 24.6, and 49.2 mg/kg/day was 0.26 +/- 0.04, 0.50 +/- 0.05, and 1.21 +/- 0.16 microM, respectively. These data suggest that neramexane at therapeutically relevant doses attenuates diabetic neuropathic pain. Our study provides valuable information about the therapeutic potential of chronic administration of neramexane and memantine for painful diabetic neuropathy.

Stimulation of alpha(1)-adrenoceptors reduces glutamatergic synaptic input from primary afferents through GABA(A) receptors and T-type Ca(2+) channels

Activation of the descending noradrenergic system inhibits nociceptive transmission in the spinal cord. Although both alpha(1)- and alpha(2)-adrenoceptors in the spinal cord are involved in the modulation of nociceptive transmission, it is not clear how alpha(1)-adrenoceptors regulate excitatory and inhibitory synaptic transmission at the spinal level. In this study, inhibitory and excitatory postsynaptic currents (IPSCs and EPSCs, respectively) were recorded from lamina II neurons in rat spinal cord slices. The specific alpha(1)-adrenoceptor agonist phenylephrine significantly increased the frequency of GABAergic spontaneous IPSCs in a concentration dependent manner, and this effect was abolished by the alpha(1)-adrenoceptor antagonist 2-(2,6-dimethoxyphenoxy)ethylaminomethyl-1,4-benzodioxane (WB4101). Phenylephrine also significantly reduced the amplitude of monosynaptic and polysynaptic EPSCs evoked from primary afferents. The inhibitory effect of phenylephrine on evoked monosynaptic glutamatergic EPSCs was largely blocked by the GABA(A) receptor antagonist picrotoxin and, to a lesser extent, by the GABA(B) receptor antagonist CGP55845. Furthermore, blocking T-type Ca(2+) channels with amiloride or mibefradil diminished the inhibitory effect produced by phenylephrine or the GABA(A) receptor agonist muscimol on monosynaptic EPSCs evoked from primary afferents. Collectively, these findings suggest that activation of alpha(1)-adrenoceptors in the spinal cord increases synaptic GABA release, which attenuates glutamatergic input from primary afferents mainly through GABA(A) receptors and T-type Ca(2+) channels. This mechanism of presynaptic inhibition in the spinal cord may be involved in the regulation of nociception by the descending noradrenergic system.

Venlafaxine compromises the antinociceptive actions of gabapentin in rat models of neuropathic and persistent pain

RATIONALE

Neuropathic pain is associated with a number of disease states of diverse aetiology that can share common pathophysiological mechanisms. Antiepileptic drugs modulate ion channel function and antidepressants increase extracellular monoamine levels, and both drug classes variously attenuate signs and symptoms of neuropathic pain. Thus, coadministration of the antiepileptic gabapentin and the antidepressant venlafaxine may provide superior pain relief to administration of either drug alone.

OBJECTIVES

To systematically establish the pain relieving efficacies of venlafaxine and gabapentin alone and in combination.

MATERIALS AND METHODS

Gabapentin (50 and 100 mg/kg, s.c.) and venlafaxine (10, 25, 50 mg/kg, s.c.) were tested alone or in combination in the rat spared nerve injury (SNI) model of neuropathic pain and the rat formalin test of persistent pain. Diuresis was measured in a separate experiment after administration of venlafaxine.

RESULTS

Hindpaw mechanical allodynia was dose-dependently reversed by gabapentin (50 and 100 mg/kg, s.c.), whereas venlafaxine was ineffective (10 and 50 mg/kg, s.c.). Both gabapentin and venlafaxine also attenuated hindpaw mechanical hyperalgesia. Surprisingly, coadministration of venlafaxine (50 mg/kg) significantly lowered the antiallodynic effect of both doses of gabapentin by up to 60% in spared-nerve-injury rats and a negative antinociceptive interaction between gabapentin and venlafaxine was also observed in the rat formalin test. We demonstrated that venlafaxine administration was associated with a dose-dependent increase in urine output over the time course of the nociceptive experiments.

CONCLUSION

Venlafaxine compromises the antiallodynic effects of coadministered gabapentin most probably as consequence-increased diuresis.

Mechanisms of the antinociceptive action of gabapentin

Gabapentin, a gamma-aminobutyric acid (GABA) analogue anticonvulsant, is also an effective analgesic agent in neuropathic and inflammatory, but not acute, pain systemically and intrathecally. Other clinical indications such as anxiety, bipolar disorder, and hot flashes have also been proposed. Since gabapentin was developed, several hypotheses had been proposed for its action mechanisms. They include selectively activating the heterodimeric GABA(B) receptors consisting of GABA(B1a) and GABA(B2) subunits, selectively enhancing the NMDA current at GABAergic interneurons, or blocking AMPA-receptor-mediated transmission in the spinal cord, binding to the L-alpha-amino acid transporter, activating ATP-sensitive K(+) channels, activating hyperpolarization-activated cation channels, and modulating Ca(2+) current by selectively binding to the specific binding site of [(3)H]gabapentin, the alpha(2)delta subunit of voltage-dependent Ca(2+) channels. Different mechanisms might be involved in different therapeutic actions of gabapentin. In this review, we summarized the recent progress in the findings proposed for the antinociceptive action mechanisms of gabapentin and suggest that the alpha(2)delta subunit of spinal N-type Ca(2+) channels is very likely the analgesic action target of gabapentin.

Spinal-supraspinal serotonergic circuits regulating neuropathic pain and its treatment with gabapentin

Not all neuropathic pain patients gain relief from current therapies that include the anticonvulsant, gabapentin, thought to modulate calcium channel function. We report a neural circuit that is permissive for the effectiveness of gabapentin. Substance P-saporin (SP-SAP) was used to selectively ablate superficial dorsal horn neurons expressing the neurokinin-1 receptor for substance P. These neurons project to the brain as shown by retrograde labelling and engage descending brainstem serotonergic influences that enhance spinal excitability via a facilitatory action on 5HT(3) receptors. We show the integrity of this pathway following nerve injury contributes to the behavioural allodynia, neuronal plasticity of deep dorsal horn neurons and the injury-specific actions of gabapentin. Thus SP-SAP attenuated the tactile and cold hypersensitivity and abnormal neuronal coding (including spontaneous activity, expansion of receptive field size) seen after spinal nerve ligation. Furthermore the powerful actions of gabapentin after neuropathy were blocked by either ablation of NK-1 expressing neurones or 5HT(3) receptor antagonism using ondansetron. Remarkably, 5HT(3) receptor activation provided a state-dependency (independent of that produced by neuropathy) allowing GBP to powerfully inhibit in normal uninjured animals. This circuit is therefore a crucial determinant of the abnormal neuronal and behavioural manifestations of neuropathy and importantly, the efficacy of gabapentin. As this spino-bulbo-spinal circuit contacts areas of the brain implicated in the affective components of pain, this loop may represent a route by which emotions can influence the degree of pain in a patient, as well as the effectiveness of the drug treatment. These hypotheses are testable in patients.

Calcium Channels As Therapeutic Targets in Neuropathic Pain

Injury to the nerve can produce changes in dorsal horn function and pain. This facilitated processing may be mediated in part by voltage-sensitive calcium channels. Activation of these channels increases intracellular calcium, thereby mediating transmitter release and activating cascades serving to alter membrane excitability and initiate protein transcription. Molecular techniques reveal the complexity and multiplicity of these channels. At the spinal level, blocking of several of these calcium channels, notably those of the N type, can prominently alter pain behavior. These effects are consistent with the high levels of expression on primary afferents and dorsal horn neurons of these channels. More recently, agents binding to auxiliary subunits such as the alpha2delta of these calcium channels diminish excitability of the membrane without completely blocking channel function. Drugs that bind to this site, highly expressed in the superficial dorsal horn, will diminish neuropathic pain states. Continuing developments in our understanding of these channel functions promises to advance the control of aberrant spinal functions initiated by nerve injury.

PERSPECTIVE

Pharmacologic studies showing the role of spinal voltage-sensitive calcium channels in neuropathic pain models provide evidence suggesting their applicability in human pain states.

Attenuation of morphine tolerance by intrathecal gabapentin is associated with suppression of morphine-evoked excitatory amino acid release in the rat spinal cord

This study was designed to investigate the effect of acute and chronic intrathecal (i.t.) injection of gabapentin (GBP) on the antinociceptive effect of morphine and tolerance development using a tail-flick latency test. Levels of excitatory amino acids (EAA) in i.t. CSF dialysates were also measured by high performance liquid chromatography. Male Wistar rats were implanted with either one or two i.t. catheters for drug injection or pump infusion and with a microdialysis probe for CSF dialysate collection. The effect of acute GBP (10 microg i.t.) injection on the morphine dose response was examined in both naïve rats and rats made tolerant by continuous infusion of morphine (15 microg/h i.t.) for 5 days. At such a low dose (10 microg i.t.), GBP did not enhance morphine's antinociception in naïve rats. In morphine-tolerant rats, however, acute GBP (10 microg i.t.) injection potentiated morphine's antinociception and yielded a 14.6-fold shift in morphine's dose-response curve. When GBP (10 microg/h i.t.) was co-infused with morphine (15 microg/h i.t.) to examine its effect on the development of morphine tolerance, GBP attenuated the development of morphine tolerance. The effect of GBP and morphine on CSF glutamate and aspartate levels was examined in naïve rats, and the effect of morphine challenge on CSF glutamate and aspartate levels was examined in rats previously infused for 5 days with morphine alone or morphine plus GBP. Acute injection of GBP (10 microg i.t.), morphine (50 microg i.t.), or GBP (10 microg i.t.) followed by morphine (50 microg i.t.) 30 min later had no significant effect on CSF EAA concentration in naïve rats; however, in tolerant rats, morphine challenge (50 microg i.t.) increased aspartate and glutamate levels to 221 +/- 22% and 296 +/- 43%, respectively, of those before morphine challenge, and this phenomenon was inhibited by GBP co-infusion. Our results show that GBP, at a dose without enhanced effect on morphine's antinociception in naïve rats, not only potentiates morphine's antinociceptive effect in morphine-tolerant rats but also attenuates the development of morphine tolerance. The mechanism of the effect of GBP on morphine tolerance might be via suppression of the EAA concentration in spinal CSF dialysate.

Pharmacokinetic-pharmacodynamic correlations and biomarkers in the development of COX-2 inhibitors

The mechanism by which COX inhibitors exert their analgesic effect is well established. However, data show no direct correlation between drug concentrations in plasma and the analgesic or adverse effects in chronic inflammatory conditions. This represents a major problem in the development of COX inhibitors, since it is difficult to predict the appropriate dosing regimen for the treatment of chronic inflammatory pain, based upon information from pre-clinical studies and eventually early clinical studies. The factors that determine response in inflammatory pain must be understood in order to make predictions about the time course of the analgesic effect. In this review the determinants of drug response and their variability will be discussed: physicochemical properties, pharmacokinetics (PK), pathophysiology and disease progression. From a mechanistic point of view, endogenous mediators of inflammation might be used as a biomarker for the analgesic effect and safety assessment. Such a biomarker can be an intermediate step between drug exposure and response. In addition, its concentration-effect relationship could be characterized by pharmacokinetic-pharmacodynamic (PK/PD) modelling. Indeed, recent investigations have shown that COX-2 inhibition, as determined by modelling of prostaglandin E2 (PGE2) levels in the whole blood assay in vitro can be used as a marker to predict drug effects (analgesia) in humans. A model-derived parameter, IC80, (total and unbound) was found to correlate directly with the analgesic plasma concentration of different COX inhibitors varying in enzyme selectivity. These findings indicate that PGE2 and thromboxane B2 inhibition can be used to predict and select efficacious doses in humans.

Antinociceptive interactions between intrathecal gabapentin and MK801 or NBQX in rat formalin test

Antagonists for spinal N-methyl-D-aspartate (NMDA) and amino-hydroxy-methtyl-isoxazolepropionate (AMPA) receptors are effective in attenuating acute nociception or injury-induced hyperalgesia. The antinociception of spinal gabapentin is developed in injury-induced hyperalgesia without affecting acute nociception. The authors evaluated the effects of intrathecal gabapentin, NMDA antagonist (MK801) and AMPA antagonist (NBQX) in the formalin test which shows injury-induced hyperalgesia as well as acute pain. We further assessed the interactions between gabapentin and either MK801 or NBQX. Male Sprague-Dawley rats were implanted with intrathecal catheters. To evoke pain, 50 microL of 5% formalin solution was injected into the hindpaw. The interaction was investigated by a fixed dose analysis or an isobolographic analysis. MK801 and NBQX suppressed flinching responses during phase 1 of the formalin test, while gabapentin had little effect on phase 1. All three agents decreased the phase 2 flinching response. A fixed dose analysis in phase 1 showed that gabapentin potentiated the antinociceptive effect of MK801 and NBQX. Isobolographic analysis in phase 2 revealed a synergistic interaction after coadministration of gabapentin-MK801 or gabapentin-NBQX. Correspondingly, spinal gabapentin with NMDA or AMPA antagonist may be useful in managing acute pain and injury-induced hyperalgesia.

Effect of systemic and intrathecal gabapentin on allodynia in a new rat model of postherpetic neuralgia

Patients with postherpetic neuralgia often have an increased sensitivity to a tactile stimulus but impaired thermal sensitivity in the same affected dermatomes. We recently found that depletion of capsaicin-sensitive afferents by systemic treatment with a potent TRPV1 agonist, resiniferotoxin, in adult rats produces long-lasting paradoxical changes in mechanical and thermal sensitivities, which resemble the unique clinical features of postherpetic neuralgia. The anticonvulsant gabapentin is effective in reducing the subjective pain score in patients with postherpetic neuralgia. In this study, we quantified the potential effect of systemic and intrathecal gabapentin on tactile allodynia induced by resiniferotoxin in rats. Intraperitoneal injection of 200 microg/kg resiniferotoxin produced a rapid and sustained increase in the paw withdrawal latency to a radiant heat stimulus. Profound tactile allodynia developed in all the resiniferotoxin-treated rats within 3 weeks. Intraperitoneal injection of 30-60 mg/kg of gabapentin in resiniferotoxin-treated rats significantly increased the withdrawal threshold in response to von Frey filaments. Furthermore, intrathecal administration of 10-30 microg of gabapentin also produced a significant effect on the mechanical withdrawal threshold in all resiniferotoxin-treated rats. These data provide complementary new information that gabapentin administered systemically and spinally can effectively relieve tactile allodynia in this animal model of postherpetic neuralgia.

Polyanalgesic Consensus Conference 2003: an update on the management of pain by intraspinal drug delivery-- report of an expert panel

Intraspinal drug infusion using fully implantable pump and catheter systems is a safe and effective therapy for selected patients with chronic pain. The options for this approach are increasing, as drugs that are commercially available for systemic administration are adapted to this use and other drugs that are in development specifically for intraspinal administration become available. In 2000 a Polyanalgesic Consensus Conference was organized to evaluate the existing literature and develop guidelines for drug selection. The major outcome of this effort, an algorithm for drug selection, was based on the best available evidence at the time. Rapid changes have occurred in the science and practice of intraspinal infusion and a Polyanalgesic Consensus Conference 2003 was organized to pursue the following goals: 1) to review the literature on intraspinal drug infusion since 1999, 2) to revise the 2000 drug-selection algorithm, 3) to develop guidelines for optimizing drug dosage and concentration, 4) to create a process for documenting minimum evidence supporting the use of a drug for intraspinal infusion, and 5) to clarify issues pertaining to compounding of drugs. Based on the best available evidence and expert opinion, consensus recommendations were developed in all these areas. The panel's conclusions may provide a foundation for clinical practice and a rational basis for new research.

Effect of gabapentin derivates on mechanical allodynia-like behaviour in a rat model of chronic sciatic constriction injury

A series of mutual prodrugs derived from gabapentin, pregabalin, memantine, venlafaxine were synthesized and their pharmacological properties to treat neuropathic pain were investigated in a rat model of chronic sciatic nerve constriction injury (CCI). In vivo evaluation demonstrated that the mutual prodrugs 2002413A, 2002823A composed of two gabapentins, 2002414 composed of gabapentin and pregabalin were effective in reversal tactile allodynia in CCI rats. The prodrugs 2002413A, 2002414 had no significant influence on the rotarod activity. The result suggest that the prodrugs may be possible candidates for further development.

Gabapentin in the treatment of neuropathic pain

This paper reviews the pharmacology and clinical effectiveness of gabapentin in the treatment of neuropathic pain. Gabapentin has antihyperalgesic and antiallodynic properties but does not have significant actions as an anti-nociceptive agent. Its mechanisms of action appear to be a complex synergy between increased GABA synthesis, non-NMDA receptor antagonism and binding to the alpha2delta subunit of voltage dependent calcium channels. The latter action inhibits the release of excitatory neurotransmitters. Clinically, several large randomized controlled trials have demonstrated its effectiveness in the treatment of a variety of neuropathic pain syndromes. Patients with neuropathic pain can expect a mean reduction in pain score of 2.05 points on an 11 point numerical rating scale compared with a reduction of 0.94 points if they had taken the placebo. Around 30% of patients can expect to achieve more than 50% pain relief and a similar number will also experience minor adverse events; the most common of which are somnolence and dizziness. In patients with neuropathic pain due to cancer, higher response rates might be observed with gabapentin when administered with opioids because of a synergistic interaction.

Gabapentin: in postherpetic neuralgia

Gabapentin is a structural analogue of the neurotransmitter gamma-aminobutyric acid (GABA) approved for use in adults with postherpetic neuralgia. Gabapentin does not bind to GABA(A) or GABA(B) receptors. Its mechanism of action in humans is unclear, but may involve binding to alpha2delta calcium channel subunits in animal models. Reductions in the mean daily pain score from baseline to week 7 or 8 of treatment (primary endpoint) were significantly greater with gabapentin 1800-3600 mg/day than placebo therapy in two well designed trials in patients with postherpetic neuralgia. The proportion of responders (patients showing a > or =50% reduction in mean daily pain score at endpoint versus baseline) was significantly greater with gabapentin than placebo. Daily sleep rating scores, the Short Form McGill Pain Questionnaire (total pain scores), Patient and Clinician Global Impression of Change and measures on the Short Form-36 Health Survey (including physical functioning, role-physical, bodily pain, vitality or mental health) improved to a significantly greater extent with gabapentin than placebo. Adverse events associated with gabapentin in patients with postherpetic neuralgia were usually mild to moderate in intensity, with dizziness, somnolence and peripheral oedema being commonly reported.

Up-regulation of spinal muscarinic receptors and increased antinociceptive effect of intrathecal muscarine in diabetic rats

Spinally administered muscarinic receptor agonists or acetylcholinesterase inhibitors produce effective pain relief. Intrathecal injection of a small dose of neostigmine produces a profound antiallodynic effect in rats with diabetic neuropathy. However, the mechanisms of increased antinociceptive effect of cholinergic agents on diabetic neuropathic pain are not clear. In the present study, we tested the hypothesis that spinal muscarinic receptors are up-regulated in diabetes. The withdrawal threshold of the hindpaw in response to noxious heat and pressure stimuli was determined in streptozotocin-induced diabetic and age-matched normal rats. Muscarine-stimulated guanosine 5'-O-(3-[35S]thio)triphosphate ([35S]GTPgammaS) binding was used to assess the change of functional muscarinic receptors in the spinal cord in diabetes. The [3H]AF-DX 384 membrane binding was performed to determine the number and affinity of spinal cord M2 muscarinic receptors in normal and diabetic rats. We found that the antinociceptive effect of intrathecal 2 to 12 mug muscarine in diabetic animals was potentiated significantly compared with that in normal animals. The maximal muscarine-stimulated [35S]GTPgammaS binding was 112.5 +/- 8.3% in normal rats and 168.8 +/- 12.1% (P < 0.05) in diabetic rats. Although the KD value (2.9 nM) was similar in both groups, the Bmax of [3H]AF-DX 384 membrane binding was significantly higher in diabetic than in normal rats (255.2 +/- 5.9 versus 165.9 +/- 3.5 fmol/mg protein, P < 0.05). Collectively, these data strongly suggest that the muscarinic receptor is up-regulated in the dorsal spinal cord in diabetic rats. This finding probably accounts for the increased efficacy of the antinociceptive effect of intrathecal muscarinic agonists in diabetic neuropathic pain.

Role of central NMDA versus non-NMDA receptor in spinal withdrawal reflex in spinal anesthetized rats under normal and hyperexcitable conditions

The present study aimed to investigate the role of central N-methyl-D-aspartate (NMDA) and non-NMDA receptors in the spinal withdrawal reflex assessed by recording single motor unit (SMU) electromyogram (EMG) response to peripheral mechanical (pressure, pinch) stimuli and repeated electrical stimuli at 3 and 20 Hz. During normal conditions, intrathecal administration of MK-801 and CNQX apparently depressed mechanically and electrically (3 Hz) evoked EMG responses in a dose-dependent manner (10, 20 and 40 nmol in 10 microl). In contrast, the after-discharges to 20 Hz electrical stimuli were suppressed only by CNQX treatment, not by MK-801 treatment. This indicates that the central mechanisms underlying the different frequencies of electrically evoked withdrawal reflex may be different. During peripheral bee venom (BV, 0.2 mg/50 microl) induced inflammation and central sensitization, the enhanced SMU EMG responses including after-discharges to pinch stimuli and 3 Hz electrical stimuli were depressed significantly by treatments with both MK-801 and CNQX. However, the enhanced SMU activities to innocuous pressure stimuli were depressed only by treatment with CNQX. Likewise, enhanced long lasting after-discharges elicited by 20 Hz electrical stimuli were also only depressed by CNQX, indicating that different central mechanisms are involved in the persistent hyperexcitability during BV-induced inflammation. The data suggest that both central NMDA and non-NMDA receptors play important roles in the transmission of nociceptive information under normal conditions. In BV-induced inflammation, however, central non-NMDA receptors, but not NMDA receptors, play a pivotal role in the generation of persistent hyperexcitability to mechanical and electrical stimuli at different frequencies (3 Hz, 20 Hz).

Antihyperalgesic effect of levetiracetam in neuropathic pain models in rats

The purpose of this study was to assess, in rats, the antinociceptive effects of levetiracetam (i.p.), a novel antiepileptic drug, in acute pain tests and in two models of human neuropathic pain. Levetiracetam and carbamazepine contrasted morphine by an absence of effect in the tail flick and hot plate tests. In normal rats, carbamazepine failed to modify the vocalisation thresholds to paw pressure whereas levetiracetam slightly increased this threshold only at the highest dose (540 mg/kg) for 30 min. In the sciatic nerve with chronic constriction injury model, the highest dose of levetiracetam (540 mg/kg) and carbamazepine (30 mg/kg) reversed the hyperalgesia. In streptozocin-induced diabetic rats, levetiracetam dose-dependently increased the vocalization threshold from 17 to 120 mg/kg reaching a similar effect as 10 mg/kg of carbamazepine. These results indicate that levetiracetam induces an antihyperalgesic effect in two models of human neuropathic pain, suggesting a therapeutic potential in neuropathic pain patients.

Spinal GABAB receptors mediate antinociceptive actions of cholinergic agents in normal and diabetic rats

Spinally administered muscarinic receptor agonists or acetylcholinesterase inhibitors can produce antinociception. However, the mechanisms of the action of cholinergic agents in the spinal cord are not fully understood. Activation of spinal muscarinic receptors evokes gamma-aminobutyric acid (GABA) release, which reduces the glutamatergic synaptic input to dorsal horn neurons through GABA(B) receptors. In this study, we determined the functional role of spinal GABA(B) receptors in the antinociceptive action of intrathecal cholinergic agents in normal rats and in a rat model of diabetic neuropathic pain. Diabetes was induced by intraperitoneal streptozotocin in rats. The intrathecal catheter was inserted with its tip positioned at the lumbar spinal level. Nociceptive threshold was measured by the paw withdrawal latency in response to a radiant heat stimulus in normal rats. Mechanical allodynia in diabetic rats was determined by von Frey filaments applied to the hindpaw. The effect of intrathecal muscarine or neostigmine was examined through pretreatment with the specific GABA(B) receptor antagonist, CGP55845, or its vehicle. Intrathecal injection of muscarine or neostigmine significantly increased the withdrawal latency in response to a heat stimulus in normal rats and the withdrawal threshold in response to application of von Frey filaments in diabetic rats. Intrathecal pretreatment with CGP55845 significantly attenuated the effect of both muscarine or neostigmine in normal rats. Furthermore, the antiallodynic effect of intrathecal neostigmine and muscarine was largely eliminated by CGP55845 in diabetic rats. These data suggest that the GABA(B) receptors in the spinal cord mediate both the antinociceptive and antiallodynic actions of intrathecal muscarine or neostigmine in normal rats and in a rat model of diabetic neuropathic pain. This study provides new functional evidence that activation of spinal GABA(B) receptors is one of the important mechanisms underlying the antinociceptive action of intrathecal cholinergic agents.

Transgene-mediated enkephalin release enhances the effect of morphine and evades tolerance to produce a sustained antiallodynic effect in neuropathic pain

We examined the pharmacologic characteristics of herpes simplex virus (HSV) vector-mediated expression of proenkephalin in the dorsal root ganglion in a rodent model of neuropathic pain. We found that: (i). vector-mediated enkephalin produced an antiallodynic effect that was reversed by naloxone; (ii). vector-mediated enkephalin production in animals with spinal nerve ligation prevented the induction of c-fos expression in second order sensory neurons in the dorsal horn of spinal cord; (iii). the effect of vector-mediated enkephalin enhanced the effect of morphine, reducing the ED(50) of morphine 10-fold; (iv). animals did not develop tolerance to the continued production of vector-mediated enkephalin over a period of several weeks; and, (v). vector transduction continued to provide an analgesic effect despite the induction of tolerance to morphine. This is the first demonstration of gene transfer to provide an analgesic effect in neuropathic pain. The pharmacologic analysis demonstrates that transgene-mediated expression and local release of opioid peptides produce some effects that are distinct from peptide analogues delivered pharmacologically.

Role of presynaptic muscarinic and GABA(B) receptors in spinal glutamate release and cholinergic analgesia in rats

Spinally administered muscarinic receptor agonists or acetylcholinesterase inhibitors can produce effective pain relief. However, the analgesic mechanisms and the site of actions of cholinergic agents in the spinal cord are not fully understood. In this study, we investigated the mechanisms underlying cholinergic presynaptic regulation of glutamate release onto spinal dorsal horn neurons. The role of spinal GABA(B) receptors in the antinociceptive action of muscarine was also determined. Whole-cell voltage-clamp recordings were performed on visualized dorsal horn neurons in the lamina II in the spinal cord slice preparation of rats. The miniature excitatory postsynaptic currents (mEPSCs) and miniature inhibitory postsynaptic currents (mIPSCs) were recorded in the presence of tetrodotoxin. The evoked EPSCs (eEPSCs) were obtained by electrical stimulation of the dorsal root entry zone or the attached dorsal root. Nociception in rats was measured using a radiant heat stimulus and the effect of intrathecal administration of drugs tested. Acetylcholine (10-100 microM) reduced the amplitude of monosynaptic eEPSCs in a concentration-dependent manner. Acetylcholine also significantly decreased the frequency of non-NMDA receptor-mediated mEPSCs, which was antagonized by atropine but not mecamylamine. The frequency of GABA(A) receptor-mediated mIPSCs was significantly increased by acetylcholine and this excitatory effect was abolished by atropine. Existence of presynaptic M(2) muscarinic receptors in the spinal dorsal horn was further demonstrated by immunocytochemistry staining and dorsal rhizotomy. CGP55845, a GABA(B) receptor antagonist, significantly attenuated the inhibitory effect of acetylcholine on the frequency of mEPSCs and the amplitude of monosynaptic eEPSCs in lamina II neurons. Furthermore, the antinociceptive action produced by intrathecal muscarine was significantly reduced by CGP55845 pretreatment in rats. Therefore, data from this integrated study provide new information that acetylcholine inhibits the glutamatergic synaptic input to lamina II neurons through presynaptic muscarinic receptors. Inhibition of glutamate release onto lamina II neurons by presynaptic muscarinic and GABA(B) heteroreceptors in the spinal cord probably contributes to the antinociceptive action of cholinergic agents.

Downregulation of P2X3 receptor-dependent sensory functions in A/J inbred mouse strain

There is large variability in the various pain responses including those to tissue injury among inbred mouse strains. However, the determinant factors for the strain-specific differences remain unknown. The P2X3 sensory-specific ATP-gated channel has been implicated as a damage-sensing molecule that evokes a pain sensation by receiving endogenous ATP from injured tissue. In this study, to clarify the contribution of the sensory P2X3 signalling to strain-specific differences in tissue injury pain, we examined whether the P2X3-mediated in vivo and in vitro responses in dorsal root ganglion (DRG) neurons are changed in the A/J inbred mouse strain, which is known to be resistant to tissue injury pain caused by formalin. Here we found that A/J mice exhibited a low magnitude of nocifensive behaviour induced by the P2X agonist alpha,beta-methylene ATP (alpha beta meATP) into the hindpaw compared with C57BL/6 J mice. This behaviour was blocked by P2X3 antisense oligodeoxynucleotides. The low magnitude of the in vivo pain sensation could be observed similarly in the in vitro response; the increase in the intracellular Ca(2+) increase by alpha beta meATP in capsaicin-sensitive DRG neurons from A/J mice was significantly lower than that from C57BL/6 J mice. In A/J DRG neurons the P2X3 protein level was significantly lower compared with C57BL/6 J DRG neurons. The change in P2X3 protein was selective because P2X2 protein was expressed equally in both strains. The present study suggests that the downregulation of sensory P2X3 could be one of the molecular predispositions to low sensitivity to tissue injury pain in the A/J inbred mouse strain.