Inflammation-induced GluA1 trafficking and membrane insertion of Ca2+ permeable AMPA receptors in dorsal horn neurons is dependent on spinal tumor necrosis factor, PI3 kinase and protein kinase A.

Peripheral inflammation induces sensitization of nociceptive spinal cord neurons. Both spinal tumor necrosis factor (TNF) and neuronal membrane insertion of Ca2+ permeable AMPA receptor (AMPAr) contribute to spinal sensitization and resultant pain behavior, molecular mechanisms connecting these two events have not been studied in detail. Intrathecal (i.t.) injection of TNF-blockers attenuated paw carrageenan-induced mechanical and thermal hypersensitivity. Levels of GluA1 and GluA4 from dorsal spinal membrane fractions increased in carrageenan-injected rats compared to controls. In the same tissue, GluA2 levels were not altered. Inflammation-induced increases in membrane GluA1 were prevented by i.t. pre-treatment with antagonists to TNF, PI3K, PKA and NMDA. Interestingly, administration of TNF or PI3K inhibitors followed by carrageenan caused a marked reduction in plasma membrane GluA2 levels, despite the fact that membrane GluA2 levels were stable following inhibitor administration in the absence of carrageenan. TNF pre-incubation induced increased numbers of Co2+ labeled dorsal horn neurons, indicating more neurons with Ca2+ permeable AMPAr. In parallel to Western blot results, this increase was blocked by antagonism of PI3K and PKA. In addition, spinal slices from GluA1 transgenic mice, which had a single alanine replacement at GluA1 ser 845 or ser 831 that prevented phosphorylation, were resistant to TNF-induced increases in Co2+ labeling. However, behavioral responses following intraplantar carrageenan and formalin in the mutant mice were no different from littermate controls, suggesting a more complex regulation of nociception. Co-localization of GluA1, GluA2 and GluA4 with synaptophysin on identified spinoparabrachial neurons and their relative ratios were used to assess inflammation-induced trafficking of AMPAr to synapses. Inflammation induced an increase in synaptic GluA1, but not GluA2. Although total GluA4 also increased with inflammation, co-localization of GluA4 with synaptophysin, fell short of significance. Taken together these data suggest that peripheral inflammation induces a PI3K and PKA dependent TNFR1 activated pathway that culminates with trafficking of calcium permeable AMPAr into synapses of nociceptive dorsal horn projection neurons.

Increased miR-132-3p expression is associated with chronic neuropathic pain.

Alterations in the neuro-immune balance play a major role in the pathophysiology of chronic neuropathic pain. MicroRNAs (miRNA) can regulate both immune and neuronal processes and may function as master switches in chronic pain development and maintenance. We set out to analyze the role of miR-132-3p, first in patients with peripheral neuropathies and second in an animal model of neuropathic pain. We initially determined miR-132-3p expression by measuring its levels in white blood cells (WBC) of 30 patients and 30 healthy controls and next in sural nerve biopsies of 81 patients with painful or painless inflammatory or non-inflammatory neuropathies based on clinical diagnosis. We found a 2.6 fold increase in miR-132-3p expression in WBC of neuropathy patients compared to healthy controls (p<0.001). MiR-132-3p expression was also slightly up-regulated in sural nerve biopsies from neuropathy patients suffering from neuropathic pain compared to those without pain (1.2 fold; p<0.001). These promising findings were investigated further in an animal model of neuropathic pain, the spared nerve injury model (SNI). For this purpose miR-132-3p expression levels were measured in dorsal root ganglia and spinal cord of rats. Subsequently, miR-132-3p expression was pharmacologically modulated with miRNA antagonists or mimetics, and evoked pain and pain aversion were assessed. Spinal miR-132-3p levels were highest 10days after SNI, a time when persistent allodynia was established (p<0.05). Spinal administration of miR-132-3p antagonists via intrathecal (i.t.) catheters dose dependently reversed mechanical allodyina (p<0.001) and eliminated pain behavior in the place escape avoidance paradigm (p<0.001). Intrathecal administration of miR-132-3p mimetic dose-dependently induced pain behavior in naïve rats (p<0.001). Taken together these results indicate a pro-nociceptive effect of miR-132-3p in chronic neuropathic pain.

MKK3, an upstream activator of p38, contributes to formalin phase 2 and late allodynia in mice.

Spinal p38 mitogen activated (MAP) kinase plays a key role in chronic pain behavior. However, clinical development of p38 inhibitors has been hindered by significant toxicity. To evaluate alternative strategies of p38 regulation, we determined if known upstream activators of p38 (mitogen activated kinase kinase [MKK] 3 and MKK6), are involved in development and maintenance of pain and spinal p38 phosphorylation. Acute pain behaviors were not altered in MKK3 or MKK6 deficient mice. The phase 2 formalin response was delayed in MKK3-/- mice, but unchanged in magnitude, while the response remained normal in MKK6-/- mice. More striking, late formalin allodynia (3-18 days post-injection) was prominent in wild type and MKK6-/- mice, but was delayed for several days in MKK3-/- mice. In wild type, but not MKK3-/- mice, intraplantar formalin elicited increases in ipsilateral spinal MKK3/6 phosphorylation acutely and again at 9 days postinjection. Phosphorylation of MKK3/6 correlated with phase 2 formalin behavior. Wild type (WT) and MKK3-/- mice both expressed increases in spinal phosphorylated p38, however in WT mice this response began several days earlier, and was of higher magnitude and duration than in MKK3-/- mice. This phosphorylation correlated with the late allodynia. Phosphorylated MKK3/6 was detected only in astrocytes, given that phosphorylated p38 (P-p38) is usually not seen in astrocytes this argues for astrocytic release of soluble mediators that affect p38 phosphorylation in microglia. Taking these data together, MKK3, but not MKK6, is necessary for normal development of chronic pain behavior and phosphorylation of spinal p38.

Selective stimulation of either tumor necrosis factor receptor differentially induces pain behavior in vivo and ectopic activity in sensory neurons in vitro.

Recent studies suggest that tumor necrosis factor-alpha (TNF) sensitizes primary afferent neurons, and thus facilitates neuropathic pain. Here, we separately examined the roles of tumor necrosis factor receptor (TNFR) 1 and 2 by parallel in vivo and in vitro paradigms using proteins that selectively activate TNFR1 or TNFR2 (R1 and R2). In vivo, intrathecally injected R1, but not R2 slightly reduced mechanical and thermal withdrawal thresholds in rats, whereas co-injection resulted in robust, at least additive pain-associated behavior. In vitro, the electrophysiological responses of dorsal root ganglia (DRG) from rats with spinal nerve ligation were measured utilizing single-fiber recordings of teased dorsal root filaments. In naïve DRG, only R1 (10-1000 pg/ml) induced firing in Ass- and Adelta-fibers, whereas R2 had no effect. In injured DRG, both R1 and R2 at significantly lower concentrations (1 pg/ml) increased discharge rates of Adelta-fibers. Most interesting, in adjacent uninjured DRG, R2 and not R1, increased ectopic activity in both Ass- and Adelta-fibers. We conclude that TNFR1 may be predominantly involved in the excitation of sensory neurons and induction of pain behavior in the absence of nerve injury, TNFR2 may contribute in the presence of TNFR1 activation. Importantly, the effects of individually applied R1 and R2 on injured and adjacent uninjured fibers imply that the role of TNFR2 in the excitation of sensory neurons increases after injury.

[Spinal glutamate receptor antagonists differentiate primary and secondary mechanical hyperalgesia caused by incision]. / Spinale Glutamatrezeptorantagonisten. Differenzierung von primärer und sekundärer mechanischer Hyperalgesie nach operativer Schnittinzision im Tierexperiment.

Secondary mechanical hyperalgesia has been demonstrated in postoperative patients indicating that central sensitization occurs after surgery. However, the underlying mechanisms are unknown. Here, we studied the role of spinal AMPA/kainate receptors for pain behaviors indicating secondary hyperalgesia caused by gastrocnemius incision in the rat. These were reduced by NBQX, a selective antagonist of AMPA/kainate receptors. However, administration of NMDA receptor antagonists caused no or only a modest decrease in behaviors for secondary hyperalgesia but produced associated motor deficits and supraspinal side effects. We further determined that only secondary mechanical hyperalgesia was reversed by JSTX, a selective antagonist of calcium-permeable AMPA receptor; primary mechanical hyperalgesia and guarding behavior were unchanged. These findings indicate that JSTX influenced a spinal amplification process that leads to secondary hyperalgesia but does not contribute to primary hyperalgesia and guarding after incision. This amplification process likely requires Ca(2) influx through spinal AMPA/KA (but not NMDA) receptors. Behaviors for secondary mechanical hyperalgesia after incision can be inhibited without affecting primary mechanical hyperalgesia and guarding. Mechanisms for central sensitization causing secondary hyperalgesia in postoperative patients may therefore be separated from spontaneous pain and hyperalgesia that arises adjacent to the area of the incision.

Spinal nerve ligation induces transient upregulation of tumor necrosis factor receptors 1 and 2 in injured and adjacent uninjured dorsal root ganglia in the rat.

Evidence indicates a role for tumor necrosis factor-alpha (TNF) in neuropathic pain. We correlated pain behavior in response to mechanical stimulation with immunoreactivity for TNF receptor (TNFR) 1 and 2 at 6, 24, 76 and 120 h following L5 and L6 spinal nerve ligation (SNL). Allodynia began in both L4 and L5 dermatomes within 6 h following SNL, peaking by 24 h. In L5 (injured) dorsal root ganglia (DRG), TNFR1 and TNFR2 levels displayed a bimodal increase, peaking at 6 and 120 h after SNL. In L4 (uninjured) DRG, TNFR1 and TNFR2 immunoreactivity peaked at 24 h returning to basal levels by 120 h. TNFR upregulation in injured and adjacent uninjured DRG neurons may be essential for mediating enhanced TNF effects and thus contribute to the development of pain-related behavior.

Antibody directed against GD(2) produces mechanical allodynia, but not thermal hyperalgesia when administered systemically or intrathecally despite its dependence on capsaicin sensitive afferents.

Anti-GD(2) antibodies have been shown to be effective for immunotherapy of neuroblastoma and other GD(2) enriched malignancies. Infusion of anti-GD(2) antibodies frequently causes spontaneous pain and allodynia for the duration of the immunotherapy and occasionally longer lasting neuropathic pain. Bolus intravenous injection of anti-GD(2) in rats initiates mechanical allodynia as measured by withdrawal threshold of the hindpaws. In this study, thermal thresholds were measured prior to and for up to 6 h following systemic anti-GD(2) administration in adult rats. In addition, both thermal and mechanical thresholds were tested following intrathecal administration of anti-GD(2) and IgG(2a). Murine anti-GD(2) elicited mechanical allodynia when administered into either the vasculature or the intrathecal space. Effective systemic doses were 1--3 mg/kg as previously shown. Intrathecally, optimal doses ranged from 0.01 to 0.1 ng; a higher dose was ineffective. Thermal hyperalgesia was not observed via either route of administration. Intrathecal pretreatment 48--72 h prior to the experiment with capsaicin at doses sufficient to cause a 50% depletion of dorsal horn CGRP, caused a total blockade of the mechanical allodynia indicating an involvement of peptidergic fine afferent fibers. It is likely that the antibody reacts with an antigen on peripheral nerve and/or myelin to initiate its effect. The lack of observed thermal hyperalgesia is surprising especially in light of the capsaicin-associated blockade, however, it is consistent with several other immune system related models of pain.

Pain models display differential sensitivity to Ca2+-permeable non-NMDA glutamate receptor antagonists.

BACKGROUND: Ca2+-permeable non-N-methyl-D-aspartate receptors are found in the spinal dorsal horn and represent a presumptive target for glutamatergic transmission in nociceptive processing. This study characterized the analgesic profile associated with the blockade of these spinal receptors by intrathecally delivered agents known to act at these receptors, the spider venom Joro toxin (JST) and philanthotoxin. METHODS: Philanthotoxin (0.5, 2.5, or 5 microg) or JST (5 microg) was given spinally before thermal injury to the paw. JST (5 microg) was also given 10 min before subcutaneous formalin injection, after intraplantar administration of carrageenan, and to rats that were allodynic due to tight ligation of spinal nerves. Lower doses of JST (0.25 and 1.0 microg) were given before formalin injection and testing of thermal latencies. Thermal latencies were measured using a Hargreaves box, mechanical thresholds using von Frey hairs, and formalin response by means of counting flinches. RESULTS: Both agents blocked thermal injury-induced mechanical allodynia. JST (5 microg) given 1 h after carrageenan blocked induction of thermal hyperalgesia and mechanical allodynia. JST (5 microg) had no effect in the formalin test, on allodynia after spinal nerve ligation, or when given 3 h after carrageenan. The lowest dose (0.25 microg JST) at pretreatment intervals of 60-120 min resulted in modest hypoalgesia during phase 1 formalin and thermal testing. CONCLUSIONS: The behavioral effect of intrathecal Ca2+-permeable non-N-methyl-D-aspartate antagonists indicates an important role for this spinal receptor in regulating hyperalgesic states induced by tissue injury and inflammation and reveals an action that is distinct from those observed with other glutamate receptor antagonists.

Upregulation of dorsal root ganglion (alpha)2(delta) calcium channel subunit and its correlation with allodynia in spinal nerve-injured rats.

Peripheral nerve injury can lead to a persistent neuropathic pain state in which innocuous tactile stimulation elicits pain behavior (tactile allodynia). Spinal administration of the anticonvulsant gabapentin suppresses allodynia by an unknown mechanism. In vitro studies indicate that gabapentin binds to the alpha(2)delta-1 (hereafter referred to as alpha(2)delta) subunit of voltage-gated calcium channels. We hypothesized that nerve injury may result in altered alpha(2)delta subunit expression in spinal cord and dorsal root ganglia (DRGs) and that this change may play a role in neuropathic pain processing. Using a rat neuropathic pain model in which gabapentin-sensitive tactile allodynia develops after tight ligation of the left fifth and sixth lumbar spinal nerves, we found a >17-fold, time-dependent increase in alpha(2)delta subunit expression in DRGs ipsilateral to the nerve injury. Marked alpha(2)delta subunit upregulation was also evident in rats with unilateral sciatic nerve crush, but not dorsal rhizotomy, indicating a peripheral origin of the expression regulation. The increased alpha(2)delta subunit expression preceded the allodynia onset and diminished in rats recovering from tactile allodynia. RNase protection experiments indicated that the DRG alpha(2)delta regulation was at the mRNA level. In contrast, calcium channel alpha(1B) and beta(3) subunit expression was not co-upregulated with the alpha(2)delta subunit after nerve injury. These data suggest that DRG alpha(2)delta regulation may play an unique role in neuroplasticity after peripheral nerve injury that may contribute to allodynia development.

Isoprostanes induce plasma extravasation in rat skin.

Isoprostane E2 (8-iso PGE) and isoprostane F2 alpha (8-iso PGF) contribute to numerous vascular, proinflammatory, and nociceptive functions. The underlying mechanisms for many of their actions are still under investigation. We examined the ability of isoprostanes to promote cutaneous inflammation using the Evan's blue dye method. Our data show that 4 micrograms subcutaneously (s.c.) injected 8-iso PGE or 8-iso PGF induced plasma extravasation in glabrous rat skin. Dye extravasation was also elicited in hairy skin after injections of 8-iso PGE, but not after 8-iso PGF. Isoprostane-evoked dye extravasation can be reduced by pretreatment with both the S+ and R- isomers of the cyclooxygenase (COX)-inhibitor ibuprofen (30 mg/kg intraperitoneally), indicating perhaps a nonspecific inhibition; pretreatment with ketorolac (1 and 10 mg/kg i.v.) was without effect. Unlike isoprostane-induced cutaneous nociceptor sensitization, which is blocked in a stereospecific and dose-dependent manner by COX-inhibitors, the effect of these drugs on isoprostane-induced cutaneous plasma extravasation is less consistent. We conclude that at least a large component of the isoprostane effect on cutaneous plasma extravasation is COX-independent.

Epineurial application of TNF elicits an acute mechanical hyperalgesia in the awake rat.

Tumor necrosis factor alpha (TNF) injected into the sciatic nerve and neutralizing antibodies to its receptor injected around the nerve are respectively associated with inducing and blocking pain behavior beginning 1 to 3 days post-injection. This study examined the acute effects of TNF applied around the nerve trunk on the mechanical threshold (determined with von Frey hairs) and withdrawal latency to radiant heat. TNF (0.9 and 7.7 ng in 90 microL) injected onto the nerve via an indwelling catheter elicited a decrease in mechanical threshold. Following the low dose of TNF, no change in thermal latency was observed; after the 7.7 ng dose, thermal thresholds decreased and returned to baseline multiple times within the 3-hour observation period. Identical doses of TNF injected near, but not on the nerve, 90 ng of TNF injected on the nerve, and vehicle were without effect on either modality. These data indicate that effects of acutely administered TNF to the nerve trunk are capable of producing modality specific pain behavior. These changes may represent a first step in TNF-induced neuropathic pain.

C-nociceptor sensitization by isoprostanes is cyclooxygenase dependent.

Isoprostane E(2) (8-iso-PGE) and F(2alpha) (8-iso-PGF) sensitize nociceptors and capsaicin-sensitive DRG neurons. In this study we investigated the cyclooxygenase-dependence of isoprostane-induced C-nociceptor sensitization. Systemic pretreatment of rats with ketorolac (1 and 10 mg/kg) abolished 8-iso-PGF sensitization and reduced the effects of 8-iso-PGE. Ibuprofen (30 mg/kg) blocked all sensitizing effects. These data suggest that some algesic properties of isoprostanes are mediated via prostanoid synthesis.

Isoprostanes, novel eicosanoids that produce nociception and sensitize rat sensory neurons.

Isoprostanes are a novel class of eicosanoids primarily formed by peroxidation of arachidonic acid. Because of their potential as inflammatory and/or hyperalgesic agents whose formation is largely independent of cyclooxygenases, we examined whether 8-iso prostaglandin E(2) (8-iso PGE(2)) or 8-iso prostaglandin F(2alpha) (8-iso PGF(2alpha)) reduces mechanical and thermal withdrawal threshold in rats, and whether they sensitize rat sensory neurons. Injection of 1 microg of 8-iso PGE(2) (in 2.5 microl) into the hindpaw of rats significantly reduced mechanical and thermal withdrawal thresholds, whereas 1 microg of 8-iso PGF(2alpha) elicited a transient decrease in only the mechanical withdrawal threshold. Both isoprostanes enhanced the firing of C-nociceptors in a concentration-dependent manner when injected into peripheral receptive fields. Exposing sensory neurons grown in culture to 1 microM 8-iso PGE(2) or 8-iso PGF(2alpha) augmented the number of action potentials elicited by a ramp of depolarizing current. In contrast, 8-iso PGE(2) but not 8-iso PGF(2alpha) enhanced the release of substance P- and calcitonin gene-related peptide-like immunoreactivity from isolated sensory neurons. Ten micromolar 8-iso PGE(2) stimulated peptide release directly, whereas treatment with 1 microM 8-iso PGE(2) augmented the release evoked by either bradykinin or capsaicin. Pretreating neuronal cultures with the nonsteroidal anti-inflammatory drug ketorolac did not alter the sensitizing action of 8-iso PGE(2) on peptide release, suggesting that this action of the isoprostane was not secondary to the production of prostaglandins via the cyclooxygenase pathway. These data support the notion that isoprostanes are an important class of inflammatory mediators that augment nociception.

Nociceptive and inflammatory effects of subcutaneous TNFalpha.

Tumor necrosis factor alpha (TNF) is a potent pro-inflammatory cytokine that produces pain and hyperalgesia following injection. Its algesic effects are due to sensitizing actions on nociceptive primary afferents and to the upregulation of other pro-inflammatory and algesic proteins. In anesthetized rats, we investigated the effect of subcutaneously injected TNF on background activity and mechanical sensitivity of C nociceptors of the sural nerve, as well as its effects on cutaneous plasma extravasation. TNF sensitized C nociceptors dose-dependently; the optimal dose (5 ng) lowered threshold in 66.7% of the tested fibers. This sensitization occurred within 30 min and could last for 2 or more hours. Injected TNF had no effect on Abeta mechanoreceptive fibers. In addition, TNF evoked ongoing activity in 14% of C nociceptors and caused significant and dose-related increases in vascular permeability in glabrous skin. Our data suggest that TNF released during disease or after tissue injury participates in the generation of hyperalgesia and inflammation.

Antibody activation and immune reactions: potential linkage to pain and neuropathy.

Immune responses are an input source of modulation/modification for the peripheral nervous system that can result in pain and/or peripheral neuropathy. The resulting pain can be a significant debilitating component of many diseases as well as an untoward side effect of treatment. This paper briefly describes three sources of peripheral neuropathy generated in the presence of, or associated with, an immune response. Two are classified as autoimmune diseases. The body, in an attempt to rid itself of a tumor or an invading bacterial infection or virus, attacks its nervous system due to molecular mimicry; this results in, respectively, paraneoplastic neuropathy or inflammatory polyneuropathy. The third neuropathic pain syndrome is iatrogenic and occurs after administration of an antibody to GD2 ganglioside as an immunotherapy for neuroblastoma. This paper will attempt to point out some common elements in their neuropathologies and mechanisms.

Acute pain mechanisms.

The systems activated by tissue-injuring stimuli are complex. The nociceptive primary afferents have little spontaneous activity under normal conditions; however, after tissue injury, they display longlasting, ongoing activity. This results, in part, because the injury elicits the release of active factors that sensitize or excite the peripheral nerve terminal. A threshold that is lowered to the extent that body temperature and the pressure of edema are adequate stimuli results in spontaneous pain. This phenomenon is mediated by a variety of blood-borne active factors released during plasma extravasation, by agents released from local inflammatory cells, and by neurotransmitters released from the peripheral terminals of the primary afferent fibers themselves. Well-defined projections into the dorsal horn convey the "pain message" to at least two well-defined populations of neurons: those that are nociceptive specific and those that display an intensity-linked discharge over a range of stimuli from innocuous to noxious. Convergence from various fiber types, modalities, and end organs permits the encoding of afferent traffic with respect to intensity and location. The convergence of axons from somatic and visceral structures reflects the mechanism for the so-called "referred pain state." Most importantly, these dorsal horn systems have a dynamic component in addition to the hard-wiring; their output can be regulated both up and down. The up-regulation provides the basis for much of the facilitated processing that is believed to account for a significant percentage of the postinjury pain state. The facilitated state has a unique pharmacology, with the underlying mechanisms reflecting a cascade of actions that starts with the NMDA receptor and proceeds through the spinal release of intermediaries, such as prostaglandins and nitric oxide. Conversely, the ability to down-regulate the dorsal horn stimulus response function accounts for the powerful control exerted by a wide variety of diverse factors, including the spinal delivery of opioid and nonopioid analgesics and the "endogenous analgesia system." These linkages reflect the complexity of the encoding mechanisms that transduce the tissue injury into the behavioral sequela known as pain. This article also emphasizes that, although considerable progress has been made in the past decade, the current pace of research promises greater insights.

Mechanical allodynia in rats is blocked by a Ca2+ permeable AMPA receptor antagonist.

Mild thermal injury to the hindpaw induces tactile allodynia distal to the injury. The allodynia is blocked by non-NMDA, but not NMDA, antagonists. The calcium permeable subtype of non-NMDA receptors is blocked by Joro spider toxin (JSTX). We injected JSTX or saline intrathecally followed after 5 min, 6 or 24 h by thermal injury. Rats receiving saline had decreased mechanical thresholds. Rats receiving 3 microg JSTX 5 min or 6 h prior to burn showed no allodynia. JSTX had no prominent side effects at doses between 1 and 5 microg. JSTX (5 microg) had no effect on thermal threshold. These results are consistent with the hypothesis that spinal mechanisms leading to tactile allodynia in this injury model act via a calcium permeable AMPA linkage.

Systemic gabapentin and S(+)-3-isobutyl-gamma-aminobutyric acid block secondary hyperalgesia.

Gabapentin (GBP) and S(+)-3-isobutyl-gamma-aminobutyric acid (IBG) are anticonvulsant agents which are effective against many clinical and experimental neuropathic pain states. We examined the efficacy of these agents in a new rat model of secondary mechanical hyperalgesia generated by a mild thermal injury. Under brief halothane anesthesia, an injury was induced by applying one heel to a hot surface (52.5 degreesC) for 45 s. GBP, IBG or saline was injected i.p. just prior to the injury. Mean mechanical withdrawal threshold (MWT) was determined using von Frey hairs before and at 30 min intervals for 3 h following the injury. MWT outside the injury area decreased post-injury (secondary hyperalgesia, allodynia), but primary (site of injury) mechanical hyperalgesia was not observed. Secondary hyperalgesia exhibited a tendency toward recovery over time. Time to onset of the anti-allodynic effect of GBP was 30-60 min. The minimum effective GBP dose was 100 mg/kg; 300 mg/kg GBP totally inhibited the drop in MWT, but was accompanied by pronounced sedation. Anti-allodynic effects of IBG were apparent at the first post-injury measure of MWT (30 min). Thirty milligrams per kilogram was the minimum effective dose; 100 mg/kg IBG totally blocked the allodynia with minimal side effects. Our findings demonstrate a dose-dependent blockade of the mechanical sensitivity caused by a mild thermal injury by both GBP and IBG. Results indicate that IBG is more effective than GBP in this model at doses which do not cause sedation. These observations support the suggested use of these or related gamma-amino acid analogues as an effective treatment for post-operative pain.

Gabapentin reverses the allodynia produced by the administration of anti-GD2 ganglioside, an immunotherapeutic drug.

UNLABELLED: Systemically administered, the anti-GD2 antibody produces allodynia demonstrated by decreased mechanical withdrawal threshold. Electrophysiologic recordings indicate a probable neuropathic origin, as small-diameter sensory fibers develop continuous high-frequency discharge after antibody administration. Gabapentin (GBP) is a gamma-aminobutyric acid analog originally synthesized for its anticonvulsant actions. Several open-label clinical studies, as well as a wealth of anecdotal evidence, suggest that GBP may be beneficial for the treatment of neuropathic pain. This study examined the effects of GBP given as a posttreatment after induction of an anti-GD2-associated allodynia. Anti-GD2 (1 mg/kg intravenously [i.v.]) administered to Sprague-Dawley rats reduced the mean withdrawal threshold from 14.71 to 4.95 g (P < 0.001), as measured by using von Frey hairs. This was reversed by GBP in a dose-dependent fashion; the minimal effective dose was between 3 and 30 mg/kg i.v. The maximal percent analgesic effect of GBP was 76% and 93% at doses of 30 and 100 mg/kg, respectively (P < 0.001). With these doses, side effects were minimal and were manifested as slightly decreased spontaneous movement and startle response. No changes were seen in reflex responses to corneal or pinna stimulation, and no motor deficits were observed. These data support the use of GBP as an effective therapy for neuropathic pain. IMPLICATIONS: After the administration of anti-GD2 antibody, rats display an escape reaction to light touch, increased blood pressure, and aberrant firing in nerve fibers associated with pain transmission. Systemic gabapentin reduced or eliminated the escape response and reversed the hypertension with minimal side effects. This suggests that gabapentin blocked the antibody-associated (neuropathic) pain.

Tumour necrosis factor-alpha induces ectopic activity in nociceptive primary afferent fibres.

Tumour necrosis factor-alpha, a pro-inflammatory cytokine, is expressed endoneurially following a variety of local and systemic pathophysiological insults which give rise to pain. We administered tumour necrosis factor-alpha to pentobarbital-anaesthetized rats, either topically along a restricted portion of the sciatic nerve or injected subcutaneously within the distribution of the sural nerve. Single nociceptive primary afferent fibres were assessed for ectopic discharge and receptor sensitization. Low concentrations (0.001-0.01 ng/ml) of tumour necrosis factor-alpha applied along the nerve elicited a dose-dependent, rapid onset (1-3 min) increase in discharge; higher concentrations led to reduced firing rates. C-fibres developed higher mean firing frequencies than A delta-fibres. Bursting frequency in both fibre types reached several (6) Hz. No change in mechanical threshold was observed. Intradermal injection (50 pg in 50 microliters) led to ectopic discharge and a decrease in mechanical threshold; these effects developed at different rates, suggesting multiple actions of the cytokine. Our data suggest that acute application of tumour necrosis factor-alpha to the axon can lead to aberrant electrophysiologic activity independent of peripheral receptor involvement. This low level of ectopic firing of nociceptive axons may produce wind-up in dorsal horn neurons or may, by itself, be interpreted as pain.