Injury type-specific calcium channel alpha 2 delta-1 subunit up-regulation in rat neuropathic pain models correlates with antiallodynic effects of gabapentin

Current concepts of nociception: nociceptive molecular sensors in sensory neurons

PURPOSE OF REVIEW

A large number of channels that are in some way linked to sensory transduction including nociception have been discovered in recent years. This review summarizes newly discovered channels that are implicated in nociception. Furthermore, details are discussed with emphasis on their possible application to clinical use as analgesics.

RECENT FINDINGS

Studies with null mutant animals deficient in these channel genes reveal that the channels are indeed implicated in physiological as well as pathological nociception.

SUMMARY

Many transient receptor potential channels are thermosensors that detect cold, warm and hot temperatures. These channels are activated not only by natural chemicals such as capsaicin, menthol, and camphor, but by various inflammatory signaling pathways. The acid-sensing ion channel and P2X channel that detect extracellular acidosis and ATP are also implicated in some types of pain. Voltage-gated sodium or calcium channels draw attention because of their involvement in neuropathic pain.

Gabapentin evoked changes in functional activity in nociceptive regions in the brain of the anaesthetized rat: an fMRI study

BACKGROUND AND PURPOSE

Gabapentin (GBP; 1-(aminomethyl)cyclohexane acetic acid) is used clinically in the treatment of pain. Nevertheless, the sites and mechanisms of action of GBP are poorly defined. Herein, the effects of GBP on brain activation have been studied.

EXPERIMENTAL APPROACH

Changes in blood oxygen level dependent (BOLD) haemodynamic signal following intravenous infusion of GBP (equivalent to 30 mg kg(-1) p.o., followed by 100 mg kg(-1) p.o.), compared to saline control, were studied in isofluorane anaesthetized rats (n=8 per group). Effects of GBP on mean arterial blood pressure (MAP) were also recorded.

RESULTS

Random effect analysis revealed that the lower dose of GBP produced significant (P<0.001) increases in BOLD signal intensity in several brain regions, including the thalamus and periaqueductal grey (PAG), compared to basal. This dose of GBP also produced significant (P<0.001) decreases in BOLD signal intensity in the amygdala and the entorhinal cortex. Increasing the dose of GBP (100 mg kg(-1)) produced significantly greater changes in BOLD signal intensity in several brain regions including the thalamus and PAG. MAP was not significantly altered by GBP, compared to saline.

CONCLUSIONS AND IMPLICATIONS

GBP had marked positive and negative effects on BOLD signal intensity in a number of brain regions in naïve rats. The activation of key areas involved in nociceptive processing indicate a supraspinal site of action of GBP and this may contribute to its well-described analgesic effects in animal models of pain and clinical studies.

Pregabalin in the management of central neuropathic pain

Central neuropathic pain is a painful condition, often severe, that occurs in a person who is already affected by an injury or disease of the brain or spinal cord. This dual insult is especially threatening to the quality of life of a person and their ability to perform even the most basic of tasks. Despite this high level of suffering there are relatively few trials investigating the management of central neuropathic pain. However, two randomised placebo-controlled studies have recently emerged demonstrating efficacy of pregabalin in reducing central neuropathic pain due to spinal cord injury and central poststroke pain. Pregabalin, an anticonvulsant, has been shown to be efficacious in the management of peripheral neuropathic pain of various causes and now may have a role to play in central neuropathic pain.

Efficacy of gabapentin in the management of chemotherapy-induced peripheral neuropathy: a phase 3 randomized, double-blind, placebo-controlled, crossover trial (N00C3)

BACKGROUND

The antiepileptic agent, gabapentin, has been demonstrated to relieve symptoms of peripheral neuropathy due to various etiologies. On the basis of these data, a multicenter, double-blind, placebo-controlled, crossover, randomized trial was conducted to evaluate the effect of gabapentin on symptoms of chemotherapy-induced peripheral neuropathy (CIPN).

METHODS

Patients with symptomatic CIPN who complained of 'average' daily pain scores of either 1) >/=4 on a 0-10 numerical rating scale (NRS); or 2) >/=1 on the 0-3 Eastern Cooperative Oncology Group neuropathy scale (ENS) were eligible (higher numbers indicate greater severity of symptoms in both scales). Patients were randomized to receive gabapentin (target dose, 2700 mg) or placebo for 6 weeks. Crossover occurred after a 2-week washout period. CIPN-related symptoms were evaluated weekly by questionnaires. Statistical methods followed established methods for crossover designs, including Student t tests to compare average intrapatient differences between treatments and linear models to adjust for potential concomitant covariates.

RESULTS

There were 115 patients who were randomly assigned to the treatment or control arm. Both groups were well matched by symptoms at study entry. Changes in symptom severity were statistically similar between the 2 groups during the study. Adverse events were mild and similar in both groups.

CONCLUSIONS

This trial failed to demonstrate any benefit to using gabapentin to treat symptoms caused by CIPN.

Immunohistochemical demonstration of the calcium channel α2 subunit in the chicken dorsal root ganglion and spinal cord: A special reference to colocalization with calbindin-D28k in dorsal root ganglion neurons

We used immunohistochemical methods to examine the distribution of the calcium channel alpha2 (CCalpha2) subunit in the chicken spinal cord and dorsal root ganglion (DRG) neurons and determine its relationship with calbindin-D28k (CB) in the DRG neurons. In the spinal cord, CCalpha2 subunit was detected in nerve terminals, which were observed as dot-like structures, and in laminae I, II, III and Lissauer's tract in the dorsal horn. In the DRG neurons, approximately 65% of the total neurons were CCalpha2 subunit positive, and most (86%) of these neurons were small to medium sized, suggesting that the CCalpha2 subunit and/or a complex of the CCalpha2 and delta subunits is possibly localized in a number of nociceptive neurons. A majority (77%) of the positive neurons showed CB immunoreactivity and most (88%) of these neurons were small to medium sized. This may indicate a close correlation between the CCalpha2 subunit and CB in the nociceptive neurons. Thus, it is postulated that the mode of nociceptive transmission may involve a cellular Ca(2+)-regulating system that consists of both Ca(2+) entry via calcium channels with the alpha2delta subunit and intracellular Ca(2+)-binding activity of CB in the nociceptive neurons of the DRG.

Gabapentin and pregabalin for chronic neuropathic and early postsurgical pain: current evidence and future directions

PURPOSE OF REVIEW

Gabapentin and pregabalin bind to the alpha-2-delta calcium channel subunit and represent a novel analgesic drug class. The evidence base supporting their use for chronic neuropathic and early postsurgical pain is reviewed.

RECENT FINDINGS

Multiple, large, high-quality trials have demonstrated the safety and efficacy of gabapentin and pregabalin in neuropathic pain. Treatment-related improvement of pain and sleep positively impact upon quality of life. Sedation, dizziness and ataxia are important and relatively common adverse effects, however. Accumulating evidence indicates that gabapentin, and possibly pregabalin, also exert important effects following surgery. Multiple high-quality trials have demonstrated analgesic and opioid-sparing efficacy with gabapentin following various surgical procedures. Gabapentin and pregabalin reduce movement-evoked pain and this can lead to enhanced functional postoperative recovery. Postoperative opioid sparing is of questionable relevance since few trials have shown reduced opioid-related adverse effects. Sedation, dizziness and ataxia have been reported in only a few trials. Future larger-scale perioperative trials focused on safety assessment are needed, however.

SUMMARY

Gabapentin and pregabalin are efficacious treatments for neuropathic and postsurgical pain. Future research addressing several specific questions would serve to better delineate their optimal roles in treating these and other pain conditions.

The Cav2.3 calcium channel antagonist SNX-482 reduces dorsal horn neuronal responses in a rat model of chronic neuropathic pain

Neuropathic pain is a difficult state to treat, characterized by alterations in sensory processing that can include allodynia (touch-evoked pain). Evidence exists for nerve damage-induced plasticity in both transmission and modulatory systems, including changes in voltage-dependent calcium channel (VDCC) expression and function; however, the role of Ca(v)2.3 calcium channels has not clearly been defined. Here, the effects of SNX-482, a selective Ca(v)2.3 antagonist, on sensory transmission at the spinal cord level have been investigated in the rat. The spinal nerve ligation (SNL) model of chronic neuropathic pain [Kim & Chung, (1992)Pain, 50, 355-363] was used to induce mechanical allodynia, as tested on the ipsilateral hindpaw. In vivo electrophysiological measurements of dorsal horn neuronal responses to innocuous and noxious electrical and natural stimuli were made after SNL and compared to sham-operated animals. Spinal SNX-482 (0.5-4 microg/50 microL) exerted dose-related inhibitions of noxious C-fibre- and Adelta-fibre-mediated neuronal responses in conditions of neuropathy, but not in sham-operated animals. Measures of spinal cord hyperexcitability and nociception were most susceptible to SNX-482. In contrast, non-noxious Abeta-mediated responses were not affected by SNX-482. Moreover, responses to innocuous mechanical and also thermal stimuli were more sensitive to SNX-482 in SNL than control animals. This study is the first to demonstrate an antinociceptive role for SNX-482-sensitive channels in dorsal horn neurons during neuropathy. These data are consistent with plasticity in Ca(V)2.3 calcium channel expression and suggest a potential selective target to reduce nociceptive transmission during conditions of nerve damage.

Pregabalin, S-(+)-3-isobutylgaba, activates the descending noradrenergic system to alleviate neuropathic pain in the mouse partial sciatic nerve ligation model

We have previously demonstrated that gabapentin supraspinally activates the descending noradrenergic system to alleviate neuropathic pain. In this study, we investigated whether pregabalin, an antiepileptic and analgesic drug that is also designed as a structural analogue of gamma-aminobutyric acid (GABA), exhibits supraspinal analgesic effects similar to those of gabapentin involving the descending noradrenergic system. Both systemically (intraperitoneally; i.p.) and locally (intracerebroventricularly or intrathecally; i.c.v. or i.t.) injected pregabalin reduced thermal and mechanical hypersensitivity in a murine chronic pain model that was prepared by partial ligation of the sciatic nerve (the Seltzer model), suggesting that pregabalin acts at both supraspinal and spinal loci. The supraspinal analgesic action of pregabalin was observed only after peripheral nerve injury, and pregabalin (i.p. and i.c.v.) did not affect acute thermal and mechanical nociception. Depletion of spinal noradrenaline (NA) or pharmacological blockade of spinal alpha(2)-adrenoceptors with yohimbine (i.p. or i.t.), but not alpha(1)-adrenoceptors with prazosin (i.p.), reduced the analgesic effects of pregabalin (i.p. or i.c.v.) on thermal and mechanical hypersensitivity. Moreover, i.c.v.-administered pregabalin dose-dependently increased the spinal 4-hydroxy-3-methoxyphenylglycol (MHPG) content and the MHPG/NA ratio only in mice with neuropathic pain, whereas the concentrations of NA, serotonin, 5-hydroxyindoleacetic acid and dopamine were unchanged, demonstrating that supraspinal pregabalin accelerated the spinal turnover of NA. Together, these results indicate that pregabalin supraspinally activates the descending noradrenergic pain inhibitory system coupled with spinal alpha(2)-adrenoceptors to ameliorate neuropathic pain.

Computer modeling of siRNA knockdown effects indicates an essential role of the Ca2+ channel alpha2delta-1 subunit in cardiac excitation-contraction coupling

L-type Ca(2+) currents determine the shape of cardiac action potentials (AP) and the magnitude of the myoplasmic Ca(2+) signal, which regulates the contraction force. The auxiliary Ca(2+) channel subunits alpha(2)delta-1 and beta(2) are important regulators of membrane expression and current properties of the cardiac Ca(2+) channel (Ca(V)1.2). However, their role in cardiac excitation-contraction coupling is still elusive. Here we addressed this question by combining siRNA knockdown of the alpha(2)delta-1 subunit in a muscle expression system with simulation of APs and Ca(2+) transients by using a quantitative computer model of ventricular myocytes. Reconstitution of dysgenic muscle cells with Ca(V)1.2 (GFP-alpha(1C)) recapitulates key properties of cardiac excitation-contraction coupling. Concomitant depletion of the alpha(2)delta-1 subunit did not perturb membrane expression or targeting of the pore-forming GFP-alpha(1C) subunit into junctions between the outer membrane and the sarcoplasmic reticulum. However, alpha(2)delta-1 depletion shifted the voltage dependence of Ca(2+) current activation by 9 mV to more positive potentials, and it slowed down activation and inactivation kinetics approximately 2-fold. Computer modeling revealed that the altered voltage dependence and current kinetics exert opposing effects on the function of ventricular myocytes that in total cause a 60% prolongation of the AP and a 2-fold increase of the myoplasmic Ca(2+) concentration during each contraction. Thus, the Ca(2+) channel alpha(2)delta-1 subunit is not essential for normal Ca(2+) channel targeting in muscle but is a key determinant of normal excitation and contraction of cardiac muscle cells, and a reduction of alpha(2)delta-1 function is predicted to severely perturb normal heart function.

The importance of genetic background on pain behaviours and pharmacological sensitivity in the rat spared serve injury model of peripheral neuropathic pain

Neuropathic pain conditions can encompass a diverse constellation of signs and symptoms consisting of sensory deficits, allodynia and hyperalgesia. Animal models of neuropathic pain have enabled the identification of key pathophysiological changes occurring within nociceptive pathways as a result of injury, and serve an invaluable role for preclinical screening of novel analgesic candidates. We have produced the first systematic description of the development and maintenance, and the pharmacological sensitivity of nociceptive behaviours in four rat strains with different genetic background (outbred Sprague-Dawley and inbred Brown Norway, Lewis and Fischer 344 rats), using the spared nerve injury model of peripheral neuropathic pain. Hindpaw mechanical hypersensitivity was evident from 7 to 30 days post-injury in all four strains, developing most quickly and severely in Fischer 344 rats; Lewis rats were least affected. Morphine (6 but not 3 mg/kg, s.c.) and gabapentin (100 but not 50 mg/kg, s.c.) had significant antiallodynic and antihyperalgesic actions in all four strains after spared nerve injury, although marked differences in potency across strains were observed. Two strains (Fischer 344 rats and Lewis) were insensitive to the antihyperalgesic properties of gaboxadol (15 mg/kg) whereas gaboxadol (6 mg/kg) was equipotent to morphine (6 mg/kg) in two other strains (Sprague-Dawley and Brown Norway). The observed pharmacogenetic variations have important implications for the preclinical testing of drugs, and provide a basis for development of pharmacogenomics in neuropathic pain.

Alteration of the cell adhesion molecule L1 expression in a specific subset of primary afferent neurons contributes to neuropathic pain

The cell adhesion molecule L1 (L1-CAM) plays important functional roles in the developing and adult nervous systems. Here we show that peripheral nerve injury induced dynamic post-transcriptional alteration of L1-CAM in the rat dorsal root ganglia (DRGs) and spinal cord. Sciatic nerve transection (SCNT) changed the expression of L1-CAM protein but not L1-CAM mRNA. In DRGs, SCNT induced accumulation of the L1-CAM into the surface of somata, which resulted in the formation of immunoreactive ring structures in a number of unmyelinated C-fiber neurons. These neurons with L1-CAM-immunoreactive ring structures were heavily colocalized with phosphorylated p38 MAPK. Western blot analysis revealed the increase of full-length L1-CAM and decrease of fragments of L1-CAM after SCNT in DRGs. Following SCNT, L1-CAM-immunoreactive profiles in the dorsal horn showed an increase mainly in pre-synaptic areas of laminae I–II with a delayed onset and colocalized with growth-associated protein 43. In contrast to DRGs, SCNT increased the proteolytic 80-kDa fragment of L1-CAM and decreased full-length L1-CAM in the spinal cord. The intrathecal injection of L1-CAM antibody for the extracellular domain of L1-CAM inhibited activation of p38 MAPK and emergence of ring structures of L1-CAM immunoreactivity in injured DRG neurons. Moreover, inhibition of extracellular L1-CAM binding by intrathecal administration of antibody suppressed the mechanical allodynia and thermal hyperalgesia induced by partial SCNT. Collectively, these data suggest that the modification of L1-CAM in nociceptive pathways might be an important pathomechanism of neuropathic pain.

[3H] pregabalin binding is increased in ipsilateral dorsal horn following chronic constriction injury

Pregabalin, a 3-substituted analogue of gamma-amino butyric acid has recently been approved for treatment of neuropathic pain. We have investigated the anatomical binding profile of [(3)H] pregabalin following chronic constriction injury (CCI) and compared this with alpha 2 delta 1 subunit expression using in situ hybridisation. We report here that the intensity and distribution pattern of [(3)H] pregabalin binding is altered in the ipsilateral dorsal horn following CCI and this is associated with a corresponding increase in alpha 2 delta 1 mRNA in the ipsilateral dorsal root ganglion (DRG). It is likely that increased DRG mRNA production leads to increased alpha 2 delta 1 protein production and subsequent transport by primary afferents to the dorsal horn. The increased expression of calcium channel subunits and protein in central terminals is interesting, given that abnormal activity within sensory nerves is likely to significantly contribute to the symptomatology of neuropathic pain. The upregulation of pregabalin binding sites in sensory nerve terminals may occur as part of the response to nerve damage in neuropathic pain patients, and therefore, preferential actions of pregabalin at these sites may contribute to its mechanism of action in man.

Role of spinal voltage-dependent calcium channel α2δ-1 subunit in the expression of a neuropathic pain-like state in mice

The present study was undertaken to investigate the role of spinal voltage-dependent calcium channel alpha(2)delta-1 subunit in the expression of a neuropathic pain-like state induced by partial sciatic nerve ligation in mice. In cultured spinal neurons, gabapentin (GBP), which displays the inhibitory effect of alpha(2)delta-1 subunit, suppressed the extracellular Ca(2+) influx induced by KCl, whereas it failed to inhibit the intracellular Ca(2+) release induced by inositol-1,4,5-triphosphate. Seven days after sciatic nerve ligation, the protein level of alpha(2)delta-1 subunit in the ipsilateral spinal cord was clearly increased compared to that observed in sham-operated mice. In addition, the mRNA level of alpha(2)delta-1 subunit was significantly increased in the dorsal root ganglion, but not in the spinal cord, of nerve-ligated mice. Under these conditions, a marked decrease in the latency of paw-withdrawal against a thermal stimulation and tactile stimulation, induced by sciatic nerve ligation was abolished by repeated intrathecal (i.t.) treatment with GBP. Additionally, the persistent reduction in the nociceptive threshold by i.t. treatment with GBP at the early stage of the neuropathic pain-like state was maintained for 7 days even after GBP withdrawal. It is of interest to note that a single i.t. post-injection of GBP showed a marked and transient inhibitory effect on the developed neuropathic pain-like state, whereas repeated i.t. post-treatment with GBP produced a persistent inhibitory effect during the treatment. In conclusion, we propose here that the neuropathic pain-like state with sciatic nerve ligation is associated with the increased level of the alpha(2)delta-1 subunit of Ca(2+) channels at the sensory nerve terminal in the spinal dorsal horn of mice. Furthermore, the present data provide evidence that the neuropathic pain may be effectively controlled by repeated treatment with GBP at the early stage.

Postoperative pain management: new, convenient analgesic therapies

Despite the potential benefits to patient health that can result from effective pain management, survey results continue to indicate that acute postoperative pain remains inadequately managed worldwide. The development of novel analgesics and advanced analgesic delivery techniques has the potential to improve current strategies for postoperative pain management. This article outlines the pharmacological principles and clinical utility of recently developed agents and a novel drug delivery device indicated for the management of moderate-to-severe acute postoperative pain.

Antiallodynic effect of pregabalin in rat models of sympathetically maintained and sympathetic independent neuropathic pain

Pregabalin binds to the voltage-dependent calcium channel alpha2delta subunit and modulates the release of neurotransmitters, resulting in analgesic effects on neuropathic pain. Neuropathic pain has both sympathetically maintained pain (SMP) and sympathetic independent pain (SIP) components. We studied the antiallodynic effects of pregabalin on tactile allodynia (TA) and cold allodynia (CA) in SMP-and SIP-dominant neuropathic pain models. Allodynia was induced by ligation of the L5 and L6 spinal nerves (SMP model) or by transection of the tibial and sural nerves (SIP model) in rats. For intrathecal drug administration, a PE-10 catheter was implanted through the atlantooccipital membrane to the lumbar enlargement. Pregabalin was administered either intraperitoneally (IP) or intrathecally (IT) and dosed up incrementally until an antiallodynic effect without sedation or motor impairment was apparent. TA was assessed using von Frey filaments, and CA was assessed using acetone drops. IP-administered pregabalin dose-dependently attenuated TA in both models and CA in the SMP model, but not CA in the SIP model. IT-administered pregabalin dose-dependently attenuated both TA and CA in both models. However, the dose response curve of IT-administered pregabalin in SMP was shifted to left from that of SIP and the ED50 of IT-administered pregabalin for CA in SMP was about 900 times less than that in SIP. These findings suggest that pregabalin exerts its antiallodynic effect mainly by acting at the spinal cord, and that IT-administered pregabalin has more potent antiallodynic effects in SMP. The alpha2delta subunit might be less involved in the CA in SIP.

Neurological manifestations in Fabry's disease

Fabry's disease is an X-linked lysosomal storage disorder caused by a defect in the gene that encodes the lysosomal enzyme alpha-galactosidase A. Symptoms arise because of accumulation of globotriaosylceramide in multiple organs, resulting in severely reduced quality of life and premature death. Neurological symptoms, such as burning sensations (occasionally accompanied by acroparesthesia) and stroke, are among the first to appear, and occur in both male and female patients. A delay in establishing the diagnosis of Fabry's disease can cause unnecessary problems, especially now that enzyme replacement treatment is available to prevent irreversible organ damage. Females with Fabry's disease who present with pain have often been ignored and misdiagnosed because of the disorder's X-linked inheritance. This Review will stress the importance of recognizing neurological symptoms for the diagnosis of Fabry's disease. The possible pathophysiological background will also be discussed.

Anti-convulsants and anti-depressants

Damage to a nerve should only lead to sensory loss. While this is common, the incidence of spontaneous pain, allodynia and hyperalgesia indicate marked changes in the nervous system that are possible compensations for the loss of normal function that arises from the sensory loss. Neuropathic pain arises from changes in the damaged nerve which then alter function in the spinal cord and the brain and lead to plasticity in areas adjacent to those directly influenced by the neuropathy. The peripheral changes drive central compensations so that the mechanisms involved are multiple and located at a number of sites. Nerve damage increases the excitability of both the damaged and undamaged nerve fibres, neuromas and the cell bodies in the dorsal root ganglion. These peripheral changes are substrates for the ongoing pain and the efficacy of excitability blockers such as carbamazepine, lamotrigine and mexiletine, all anti-convulsants. A better understanding of ion channels at the sites of injury has shown important roles of particular sodium, potassium and calcium channels in the genesis of neuropathic pain. Within the spinal cord, increases in the activity of calcium channels and the receptors for glutamate, especially the N-methyl-D-aspartate (NMDA) receptor, trigger wind-up and central hyperexcitability. Increases in transmitter release, neuronal excitability and receptive field size result from the damage to the peripheral nerves. Ketamine and gabapentin/pregabalin, again with anti-convulsant activity, may interact with these mechanisms. Ketamine acts on central spinal mechanisms of excitability whereas gabapentin acts on a subunit of calcium channels that is responsible for the release of pain transmitters into the spinal cord. In addition to these spinal mechanisms of hyperexcitability, spinal cells participate in a spinal-supraspinal loop that involves parts of the brain involved in affective responses to pain but also engages descending excitatory and inhibitory systems that use the monoamines. These pathways become more active after nerve injury and are the site of action of anti-depressants. This chapter reviews the evidence and mechanisms of drugs, both anti-depressants and anti-convulsants, that are believed to be effective in pain control, with a major emphasis on the neuropathic state.

Chemotherapy-evoked painful peripheral neuropathy: analgesic effects of gabapentin and effects on expression of the alpha-2-delta type-1 calcium channel subunit

Chemotherapeutics in the taxane and vinca-alkaloid classes sometimes produce a painful peripheral neuropathy for which there is no validated treatment. Experiments with rat models of paclitaxel- and vincristine-evoked pain suggest that these conditions may not respond to all of the analgesics that have efficacy in other models of painful peripheral neuropathy. We tested gabapentin as a potential analgesic for paclitaxel- and vincristine-evoked pain. We used a repeated dosing paradigm because there are precedents showing that repeated drug exposure may be necessary to demonstrate analgesia in neuropathic pain models. Gabapentin is believed to work via binding to voltage-gated calcium channels that contain the alpha-2-delta type-1 (alpha(2)delta-1) subunit, and the expression of this subunit is known to be increased in some painful peripheral neuropathy models. Thus we also examined whether the paclitaxel-evoked pain syndrome was accompanied by an alpha(2)delta-1 increase, and whether gabapentin had any effect on subunit expression. We found that the paclitaxel- and vincristine-evoked mechano-allodynia and mechano-hyperalgesia were significantly reduced by gabapentin, but only with repeated dosing. Paclitaxel-evoked painful peripheral neuropathy was associated with an increased expression of the alpha(2)delta-1 subunit in the spinal dorsal horn, but not in the dorsal root ganglia. The spinal cord increase was normalized by repeated gabapentin injections. Together, these findings suggest that repeated dosing with gabapentin may be beneficial in patients with chemotherapy-evoked painful peripheral neuropathy and that gabapentin's mechanisms of action may include normalization of the nerve injury-evoked increase in calcium channel alpha(2)delta-1 subunit expression.

Anti-nociceptive and anti-allodynic effects of a high affinity NOP hexapeptide [Ac-RY(3-Cl)YRWR-NH2] (Syn 1020) in rodents

There has been a flurry of activity to develop agonists and antagonists for the member of the opioid receptor family, NOP receptor (also known as ORL1), in part to understand its role in pain. Modifications of a hexapeptide originally identified from a combinatorial library have led to the discovery of a high affinity hexapeptide agonist Ac-RY(3-Cl)YRWR-NH2 (Syn 1020). In the following experiments we characterized the anti-nociceptive effects of Syn 1020 in the tail-flick model of acute pain and the diabetic neuropathy model of chronic pain in mice and rats, respectively. Acute antinociception was assessed using the tail-flick assay in mice in which animals received intracerebroventricular (i.c.v.) or subcutaneous (s.c.) injections of Syn 1020 alone or with morphine and were tested for tail-flick latencies. In the chronic pain model, diabetic neuropathy was induced by injections of streptozotocin in rats. Tactile allodynia was measured, with von Frey hair filaments, following intraperitoneal (i.p.) injections of Syn 1020 or gabapentin (positive control). In mice, i.c.v. injections of Syn 1020 did not have any pro- or anti-nociceptive effects, however, Syn 1020 reversed morphine antinociception with a similar potency as N/OFQ (the natural ligand to NOP). S.c. injections of Syn 1020 in mice also produced analgesic effects. In rats, i.p, injections of Syn 1020 produced anti-allodynic effects. Thus, Syn 1020, a NOP receptor directed peptide, administered systemically has anti-nociceptive activity in both acute and chronic pain models in mice and rats respectively.

Arthritis and pain. Future targets to control osteoarthritis pain

Clinical presentation of osteoarthritis (OA) is dominated by pain during joint use and at rest. OA pain is caused by aberrant functioning of a pathologically altered nervous system with key mechanistic drivers from peripheral nerves and central pain pathways. This review focuses on symptomatic pain therapy exemplified by molecular targets that alter sensitization and hyperexcitability of the nervous system, for example, opioids and cannabinoids. We highlight opportunities for targeting inflammatory mediators and their key receptors (for example, prostanoids, kinins, cytokines and chemokines), ion channels (for example, NaV1.8, NaV1.7 and CaV2.2) and neurotrophins (for example, nerve growth factor), noting evidence that relates to their participation in OA etiology and treatment. Future neurological treatments of pain appear optimistic but will require the systematic evaluation of emerging opportunities.

Ion channels in analgesia research

The distribution of ion channels in neurons associated with pain pathways is becoming better understood. In particular, we now have insights into the molecular nature of the channels that are activated by tissue-damaging stimuli, as well as the mechanisms by which voltage-gated channels alter the sensitivity of peripheral neurons to change pain thresholds. This chapter details the evidence that individual channels may be associated with particular pain states, and describes genetic approaches to test the possible utility of targeting individual channels to treat pain.

Spinal alpha(2)-adrenergic and muscarinic receptors and the NO release cascade mediate supraspinally produced effectiveness of gabapentin at decreasing mechanical hypersensitivity in mice after partial nerve injury

After partial nerve injury, the central analgesic effect of systemically administered gabapentin is mediated by both supraspinal and spinal actions. We further evaluate the mechanisms related to the supraspinally mediated analgesic actions of gabapentin involving the descending noradrenergic system. Intracerebroventricularly (i.c.v.) administered gabapentin (100 microg) decreased thermal and mechanical hypersensitivity in a murine chronic pain model that was prepared by partial ligation of the sciatic nerve. These effects were abolished by intrathecal (i.t.) injection of either yohimbine (3 microg) or idazoxan (3 microg), alpha(2)-adrenergic receptor antagonists. Pretreatment with atropine (0.3 mg kg(-1), i.p. or 0.1 microg, i.t.), a muscarinic receptor antagonist, completely suppressed the effect of i.c.v.-injected gabapentin on mechanical hypersensitivity, whereas its effect on thermal hypersensitivity remained unchanged. Similar effects were obtained with pirenzepine (0.1 microg, i.t.), a selective M(1)-muscarinic receptor antagonist, but not with methoctramine (0.1 and 0.3 microg, i.t.), a selective M(2)-muscarinic receptor antagonist. The cholinesterase inhibitor neostigmine (0.3 ng, i.t.) potentiated only the analgesic effect of i.c.v. gabapentin on mechanical hypersensitivity, confirming spinal acetylcholine release downstream of the supraspinal action of gabapentin. Moreover, the effect of i.c.v. gabapentin on mechanical but not thermal hypersensitivity was reduced by i.t. injection of L-NAME (3 microg) or L-NMMA (10 microg), both of which are nitric oxide (NO) synthase inhibitors. Systemically administered naloxone (10 mg kg(-1), i.p.), an opioid receptor antagonist, failed to suppress the analgesic actions of i.c.v. gabapentin, indicating that opioid receptors are not involved in activation of the descending noradrenergic system by gabapentin. Thus, the supraspinally mediated effect of gabapentin on mechanical hypersensitivity involves activation of spinal alpha(2)-adrenergic receptors followed by muscarinic receptors (most likely M(1)) and the NO cascade. In contrast, the effect of supraspinal gabapentin on thermal hypersensitivity is independent of the spinal cholinergic-NO system.

Identification of the alpha2-delta-1 subunit of voltage-dependent calcium channels as a molecular target for pain mediating the analgesic actions of pregabalin

Neuropathic pain is a debilitating condition affecting millions of people around the world and is defined as pain that follows a lesion or dysfunction of the nervous system. This type of pain is difficult to treat, but the novel compounds pregabalin (Lyrica) and gabapentin (Neurontin) have proven clinical efficacy. Unlike traditional analgesics such as nonsteroidal antiinflammatory drugs or narcotics, these agents have no frank antiinflammatory actions and no effect on physiological pain. Although extensive preclinical studies have led to a number of suggestions, until recently their mechanism of action has not been clearly defined. Here, we describe studies on the analgesic effects of pregabalin in a mutant mouse containing a single-point mutation within the gene encoding a specific auxiliary subunit protein (alpha2-delta-1) of voltage-dependent calcium channels. The mice demonstrate normal pain phenotypes and typical responses to other analgesic drugs. We show that the mutation leads to a significant reduction in the binding affinity of pregabalin in the brain and spinal cord and the loss of its analgesic efficacy. These studies show conclusively that the analgesic actions of pregabalin are mediated through the alpha2-delta-1 subunit of voltage-gated calcium channels and establish this subunit as a therapeutic target for pain control.

Neurochemical evidence that supraspinally administered gabapentin activates the descending noradrenergic system after peripheral nerve injury

We have previously demonstrated that gabapentin supraspinally activates the descending noradrenergic system to produce analgesic effects after peripheral nerve injury. To further establish the neurochemical basis for its supraspinally mediated analgesic action, concentrations of spinal noradrenaline, 4-hydroxy-3-methoxyphenylglycol (MHPG), serotonin (5-HT), 5-hydroxyindoleacetic acid (5-HIAA) and dopamine were measured using high-performance liquid chromatography in a murine neuropathic pain model that was prepared by partial ligation of the sciatic nerve (the Seltzer model). Intracerebroventricularly (i.c.v.) administered gabapentin (100 and 300 microg) increased the spinal MHPG concentration and the MHPG/noradrenaline ratio and alleviated mechanical hypersensitivity, whereas the concentrations of noradrenaline, 5-HT, 5-HIAA and dopamine were unchanged. By contrast, i.c.v. gabapentin neither affected the spinal MHPG concentration and MHPG/noradrenaline ratio nor exhibited analgesic effects in animals subjected to a sham operation. In addition, spinal monoamine levels in ligated animals were not changed after intrathecal administration of gabapentin which however generated analgesic effects. Thus, the supraspinally mediated analgesic effects of gabapentin are correlated with an increase in spinal noradrenaline turnover.

Calcium channel alpha2delta1 subunit mediates spinal hyperexcitability in pain modulation

Mechanisms of chronic pain, including neuropathic pain, are poorly understood. Upregulation of voltage-gated calcium channel (VGCC) alpha2delta1 subunit (Ca(v)alpha2delta1) in sensory neurons and dorsal spinal cord by peripheral nerve injury has been suggested to contribute to neuropathic pain. To investigate the mechanisms without the influence of other injury factors, we have created transgenic mice that constitutively overexpress Ca(v)alpha2delta1 in neuronal tissues. Ca(v)alpha2delta1 overexpression resulted in enhanced currents, altered kinetics and voltage-dependence of VGCC activation in sensory neurons; exaggerated and prolonged dorsal horn neuronal responses to mechanical and thermal stimulations at the periphery; and pain behaviors. However, the transgenic mice showed normal dorsal horn neuronal responses to windup stimulation, and behavioral responses to tissue-injury/inflammatory stimuli. The pain behaviors in the transgenic mice had a pharmacological profile suggesting a selective contribution of elevated Ca(v)alpha2delta1 to the abnormal sensations, at least at the spinal cord level. In addition, gabapentin blocked VGCC currents concentration-dependently in transgenic, but not wild-type, sensory neurons. Thus, elevated neuronal Ca(v)alpha2delta1 contributes to specific pain states through a mechanism mediated at least partially by enhanced VGCC activity in sensory neurons and hyperexcitability in dorsal horn neurons in response to peripheral stimulation. Modulation of enhanced VGCC activity by gabapentin may underlie at least partially its antihyperalgesic actions.

Multiple dose gabapentin attenuates cutaneous pain and central sensitisation but not muscle pain in healthy volunteers

Various muscle pains constitute a large clinical problem, both for patients and clinicians. Gabapentin is an established therapy in neuropathic pain and reduces cutaneous pain in healthy volunteers. Gabapentin in combination with other analgesics reduces post-operative pain. No data exist on the effect of gabapentin on muscle pain. This study investigates the effect of gabapentin on muscle and cutaneous pain in healthy volunteers. Sixteen healthy volunteers, 8 male/8 female, were included in this double-blind three-session crossover study comparing the effects of 0, 1200, 1800 and 2600 mg (pre-treatment, titrated over 4 doses) gabapentin and placebo. Muscle pain was induced by infusing 0.5 ml of hypertonic saline into the anterior tibial muscle. Simultaneously, subjects graded pain on a computerized visual analog scale (VAS, 0-10). Total (AUC, VAS*duration in s) and maximal pain (VAS(max)) were assessed. Areas of local and referred pain were measured. Further, continuous intracutaneous electrical stimulation was applied to the forearm. Current was increased until pain intensity 5/10 or until subjects reached a cut-off of 70 mA. Spontaneous pain (VAS 0-10), areas of secondary hyperalgesia to pinprick (cm2) and mechanical pain threshold (g) within this area were assessed. Gabapentin pre-treatment reduced sensitivity to electrical induction of skin pain by 14%, p=0.016. Secondary hyperalgesia was induced, but areas were reduced after pre-treatment, p<0.05. Mechanical pain thresholds were unaffected. Pain induced by intramuscular infusion of hypertonic saline was not affected by gabapentin. In conclusion, single or repeated dosing of gabapentin reduced cutaneous but not muscle pain in healthy volunteers.

Analgesic action of gabapentin on chronic pain in the masticatory muscles: a randomized controlled trial

Chronic masticatory myalgia (CMM) can be defined as constant pain in the masticatory muscles for more than 6 months and is influenced by the central nervous system. The antiepileptic agent gabapentin acts centrally and is used for managing different types of chronic pain conditions. The objective of this study was to evaluate the analgesic action of gabapentin on CMM. In this 12-week randomized controlled clinical trial 50 patients were randomly allocated into two study groups: 25 received gabapentin and 25 received placebo. The outcome measures utilized were pain reported on a VAS (VAS-pain), Palpation Index (PI) and impact of CMM on daily functioning reported on a VAS (VAS-function). Thirty-six patients completed the study. Gabapentin showed to be clinically and statistically superior to placebo in reducing pain reported by patients (gabapentin=51.04%; placebo=24.30%; P=0.037), masticatory muscle hyperalgesia (gabapentin=67.03%; placebo=14.37%; P=0.001) and impact of CMM on daily functioning (gabapentin=57.70%; placebo=16.92%; P=0.022). It can be concluded from this study that gabapentin is effective for the management of CMM.

α2δ and the mechanism of action of gabapentin in the treatment of pain

Gabapentin is a drug that has been widely used in the treatment of chronic pain states. Despite its widespread usage, it is only recently that light has been shed on the mechanism of action of this agent. In the current review, the authors document the pharmacological, biochemical and molecular information that has led to the identification of the alpha2delta1 auxilliary subunit of voltage gated calcium channels as the target for this drug's actions.

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.

(L)-Phenylglycine, but not necessarily other alpha2delta subunit voltage-gated calcium channel ligands, attenuates neuropathic pain in rats

Gabapentin and pregabalin have been demonstrated, both in animal pain models and clinically, to be effective analgesics particularly for the treatment of neuropathic pain. The precise mechanism of action for these two drugs is unknown, but they are generally believed to function via initially binding to the alpha2delta subunit of voltage-gated Ca2+ channels. In this study, we used a pharmacological approach to test the hypothesis whether high affinity interactions with the alpha2delta subunit alone could lead to attenuation of neuropathic pain in rats. The anti-allodynic effects of gabapentin and pregabalin, along with three other compounds--(L)-phenylglycine, m-chlorophenylglycine and 3-exo-aminobicyclo[2.2.1]heptane-2-exo-carboxylic acid (ABHCA)--discovered to be potent alpha2delta ligands, were tested in the rat spinal nerve ligation model of neuropathic pain. Gabapentin (Ki = 120 nM), pregabalin (180 nM) and (L)-phenylglycine (180 nM) were shown to be anti-allodynic, with respective ED50 values of 230, 90 and 80 micromol/kg (p.o.). (L)-Phenylglycine was as potent as pregabalin and equi-efficacious in reversing mechanical allodynia. In contrast, two ligands with comparable or superior alpha2delta binding affinities, m-chlorophenylglycine (Ki = 54 nM) and ABHCA (150 nM), exhibited no anti-allodynic effects at doses of 30-300 micromol/kg (p.o.), although these compounds achieved substantial brain levels. The data demonstrate that, at least in the rat spinal nerve ligation model of neuropathic pain, (L)-phenylglycine has an anti-allodynic effect, but two equally potent alpha2delta subunit ligands do not. These results suggest that additional mechanisms, besides alpha2delta interactions, may contribute to the effects of compounds like gabapentin, pregabalin and (L)-phenylglycine in neuropathic pain.

Inhibition of Paclitaxel-Induced A-Fiber Hypersensitization by Gabapentin

Paclitaxel (Taxol) is a widely used chemotherapeutic agent in the treatment of several tumors. However, its use is often associated with the generation of peripheral neuropathic pain expressed as mechanical allodynia and thermal hyperalgesia. The molecular mechanism behind this debilitating side effect is obscure, and efficient drugs for its prevention are required. We sought to clarify the cellular changes in the involved nociceptor types underlying paclitaxel-induced neuropathic pain and to test for an alleviating effect of gabapentin treatment in a murine model of paclitaxel-induced neuropathic pain. We found that a single treatment with paclitaxel (4 mg/kg i.p.) led to a decrease in both thermal and mechanical nociceptive thresholds as well as a reduction in the thresholds for 250-Hz (A delta-fiber) and 2000 Hz (A beta-fiber) but not 5-Hz (C-fiber) sine wave electrical stimuli-induced paw withdrawal. The paclitaxel-induced neuropathic pain was completely abrogated by gabapentin (30 mg/kg i.p.) treatment. Furthermore, we found that mRNA and protein levels of the voltage-gated calcium channel (alpha 2)delta-1 subunit (Ca(alpha 2)delta-1), one of the putative targets for gabapentin, was up-regulated in dorsal root ganglions (DRGs), as well as increased expression of Ca(alpha 2)delta-1 protein in medium/large-sized DRG neurons by immunohistochemistry, following paclitaxel treatment. This suggests that paclitaxel induces A-fiber-specific hypersensitization, which may contribute to the functional mechanical allodynia and hyperalgesia, and that gabapentin could be a potential therapeutic agent for paclitaxel-induced neuropathic pain.

Central Hypersensitivity in Chronic Pain: Mechanisms and Clinical Implications

The available literature consistently shows increased pain sensitivity after sensory stimulation of healthy tissues in patients who have various chronic pain conditions. This indicates a state of hypersensitivity of the CNS that amplifies the nociceptive input arising from damaged tissues. Experimental data indicate that central hypersensitivity is probably induced primarily by nociceptive input arising from a diseased tissue. In patients, imbalance of descending modulatory systems connected with psychologic distress may play a role. There is experimental support in animal studies for the persistence of central hypersensitivity after complete resolution of tissue damage. This is particularly true for neuropathic pain conditions, whereby potentially irreversible plasticity changes of the CNS have been documented in animal studies. Whether such changes are present in musculoskeletal pain states is at present uncertain. Despite the likely importance of central hypersensitivity in the pathophysiology of chronic pain, this mechanism should not be used to justify the lack of understanding on the anatomic origin of the pain complaints in several pain syndromes, which is mostly due to limitations of the available diagnostic tools. Treatment strategies for central hypersensitivity in patients have been investigated mostly in neuropathic pain states. Possible therapy modalities for central hypersensitivity in chronic pain of musculoskeletal origin are largely unexplored. The limited evidence available and everyday practice show, at best, modest efficacy of the available treatment modalities for central hypersensitivity. The gap between basic knowledge and clinical benefits remains large and should stimulate further intensive research.

Pharmacological characterisation of place escape/avoidance behaviour in the rat chronic constriction injury model of neuropathic pain

RATIONALE

Classical pain tests performed in animals routinely measure evoked nociceptive behaviours. These almost exclusively reflect sensory processing of nociceptive transmission, although a recently described place escape/avoidance paradigm may be used to selectively assess affective pain processing.

OBJECTIVE

To establish if drugs with proven analgesic efficacy selectively attenuate sensory-discriminative or affective-motivational aspects of nociceptive processing.

METHODS

The mu-opioid receptor agonist morphine, the anti-epileptic gabapentin, the anti-depressant duloxetine, the 5HT1A receptor agonist 8-OH-DPAT, the GABA(A) receptor agonist gaboxadol and the mixed cannabinoid receptor agonist WIN55,212-2 were tested after systemic administration in the chronic constriction injury (CCI) model of neuropathic pain. For the place escape/avoidance paradigm, CCI rats had free access between the 'non-aversive' dark and 'aversive' light side of an enclosed chamber. Either the injured or non-injured hindpaw was routinely stimulated if the rat was in the dark or light area, respectively. Escape/avoidance behaviour was defined as a shift from the dark to the light area. Mechanical allodynia and hyperalgesia were determined prior to and following escape/avoidance testing.

RESULTS

Morphine (3 and 6 mg/kg), gabapentin (50 and 100 mg/kg), duloxetine (10 and 30 mg/kg) and 8-OH-DPAT (0.1 and 0.5 mg/kg) attenuated the time spent by CCI rats in the light area; gaboxadol (1 and 3 mg/kg) and WIN55,212-2 (0.3 and 1 mg/kg) were ineffective. Only gabapentin and 8-OH-DPAT attenuated mechanical nociceptive behaviours at non-sedative doses.

CONCLUSIONS

The place escape/avoidance paradigm may enable discrimination between selected drug classes on distinct components of sensory and affective pain processing in rats with neuropathic pain.

Therapeutic efficacy of prosaposin-derived peptide on different models of allodynia

We have previously demonstrated that the prosaposin-derived 14-mer peptide TX14(A) prevents structural and functional abnormalities associated with peripheral neuropathy in diabetic rats. Unusually, this neuroprotective peptide also exhibited acute anti-hyperalgesic properties in the same model, suggesting a dual action of TX14(A) that could allow therapeutic targeting of both degenerative neuropathy and neuropathic pain. In the present study, we have extended investigation of the anti-allodynic properties of TX14(A) to a range of models in which allodynia is induced using metabolic, physical, neurotoxic or chemical/inflammatory damage to the peripheral nerve. Single systemic doses of TX14(A) rapidly alleviated tactile allodynia in rats in which nerve injury was induced by diabetes, sciatic nerve hemiligation, systemic paclitaxel treatment or paw formalin injection. Further, TX14(A) pre-treatment prevented onset of allodynia in the paclitaxel and formalin injection models. These results indicate that TX14(A) has anti-allodynic properties in diverse models of neuropathic pain and support further exploration of its potential as a therapeutic agent for a wide range of peripheral neuropathies and neuropathic pain states.

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.

The L-type calcium channel in the heart: the beat goes on

Sydney Ringer would be overwhelmed today by the implications of his simple experiment performed over 120 years ago showing that the heart would not beat in the absence of Ca2+. Fascination with the role of Ca2+ has proliferated into all aspects of our understanding of normal cardiac function and the progression of heart disease, including induction of cardiac hypertrophy, heart failure, and sudden death. This review examines the role of Ca2+ and the L-type voltage-dependent Ca2+ channels in cardiac disease.

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.

The nitric oxide-cyclic GMP-protein kinase G-K+ channel pathway participates in the antiallodynic effect of spinal gabapentin

The possible participation of the nitric oxide (NO)-cyclic GMP-protein kinase G (PKG) pathway on gabapentin-induced spinal antiallodynic activity was assessed in spinal nerve injured rats. Intrathecal gabapentin, diazoxide or pinacidil reduced tactile allodynia in a dose-dependent manner. Pretreatment with NG-L-nitro-arginine methyl ester (L-NAME, non-specific inhibitor of NO synthase NOS), 7-nitroindazole (neuronal NO synthase inhibitor), 1H-[1,2,4] -oxadiazolo [4,3-a] quinoxalin-1-one (ODQ, guanylyl cyclase inhibitor) or (9S, 10R, 12R)-2,3,9,10,11,12-hexahydro-10-methoxy-2,9-dimethyl-1-oxo-9,12-epoxy-1H-diindolo-[1,2,3-fg:3',2',1'-kl]pyrrolo[3,4-i][1,6]benzodiazocine-10-carboxylic acid methyl ester (KT-5823, specific PKG inhibitor), but not NG-D-nitro-arginine methyl ester (D-NAME) or okadaic acid (protein phosphatase 1 and 2 inhibitor) prevented gabapentin-induced antiallodynia. Pinacidil activity was not blocked by L-NAME, D-NAME, 7-nitroindazole, ODQ, KT-5823 or okadaic acid. Moreover, KT-5823, glibenclamide (ATP-sensitive K+ channel blocker), apamin and charybdotoxin (small- and large-conductance Ca2+-activated K+ channel blockers, respectively), but not margatoxin (voltage-gated K+ channel blocker), L-NAME, 7-nitroindazole, ODQ or okadaic acid, reduced diazoxide-induced antiallodynia. Data suggest that gabapentin-induced spinal antiallodynia could be due to activation of the NO-cyclic GMP-PKG-K+ channel pathway.

Spinal nerve ligation does not alter the expression or function of GABA(B) receptors in spinal cord and dorsal root ganglia of the rat

Loss of GABA-mediated inhibition in the spinal cord is thought to mediate allodynia and spontaneous pain after nerve injury. Despite extensive investigation of GABA itself, relatively little is known about how nerve injury alters the receptors at which GABA acts. This study examined levels of GABA(B) receptor protein in the spinal cord dorsal horn, and in the L4 and L5 (lumbar designations) dorsal root ganglia one to 18 weeks after L5 spinal nerve ligation. Mechanical allodynia was maximal by 1 week and persisted at blunted levels for at least 18 weeks after injury. Spontaneous pain behaviors were evident for 6 weeks. Western blotting of dorsal horn detected two isoforms of the GABA(B(1)) subunit and a single GABA(B(2)) subunit. High levels of GABA(B(1a)) and low levels of GABA(B(1b)) protein were present in the dorsal root ganglia. However, GABA(B(2)) protein was not detected in the dorsal root ganglia, consistent with the proposed existence of an atypical receptor composed of GABA(B(1)) homodimers. The levels of GABA(B(1a)), GABA(B(1b)), and GABA(B(2)) protein in the ipsilateral dorsal horn were unchanged at any time after injury. Immunohistochemical staining also did not detect a change in GABA(B(1)) or GABA(B(2)) subunits in dorsal horn segments having a robust loss of isolectin B4 staining. The levels of GABA(B(1a)) protein were also unchanged in the L4 or L5 dorsal root ganglia at any time after spinal nerve ligation. Levels of GABA(B(2)) remained undetectable. Finally, baclofen-stimulated binding of guanosine-5'-(gamma-O-thio)triphosphate in dorsal horn did not differ between sham and ligated rats. Collectively, these results argue that a loss of GABA(B) receptor-mediated inhibition, particularly of central terminals of primary afferents, is unlikely to mediate the development or maintenance of allodynia or spontaneous pain behaviors after spinal nerve injury.

Differential distribution of voltage-gated calcium channel alpha-2 delta (alpha2delta) subunit mRNA-containing cells in the rat central nervous system and the dorsal root ganglia

Voltage-gated calcium channels (VGCCs) play an essential role in controlling neurotransmitter release, neuronal excitability, and gene expression in the nervous system. The distribution of cells that contain mRNAs encoding the auxiliary alpha2delta-1, alpha2delta-2, and alpha2delta-3 subunits of the VGCCs in the central nervous system (CNS) and the dorsal root ganglia (DRG) was examined in rats by using in situ hybridization. Specific labeling of alpha2delta-1, alpha2delta-2, and alpha2delta-3 mRNAs appeared to be largely confined to neurons and was widely, although differentially, distributed in the brain, the spinal cord, and the DRG. Importantly, alpha2delta-2 mRNA was found to be expressed in interneurons in the cortex, the hippocampus, the striatum, and in regions that contain dense cholinergic neurons. Our results suggest that different alpha2delta subunits may exert distinctive functions in the CNS. The alpha2delta-1 subunit mRNA is localized in brain regions known to be involved in cortical processing, learning and memory, defensive behavior, neuroendocrine secretion, autonomic activation, primary sensory transmission, and general arousal. The alpha2delta-2 subunit mRNA is present in brain regions known to modulate the overall activities of the cortex, the hippocampus, and the thalamus. The alpha2delta-2 subunit is also found in brain regions known to be involved in olfaction, somatic motor control, fluid homeostasis, ingestive and defensive behaviors, neuroendocrine functions, and circadian rhythm. In addition to being localized in brain regions that express alpha2delta-1 and alpha2delta-2 subunit mRNAs, alpha2delta-3 subunit mRNA is highly expressed in regions involved in auditory information processing and somatic movement.