An experimental arthritis in rats: Dorsal horn aspartate and glutamate increases

Spinal pain processing in arthritis: Neuron and glia (inter)actions

Diseases of joints are among the most frequent causes of chronic pain. In the course of joint diseases, the peripheral and the central nociceptive system develop persistent hyperexcitability (peripheral and central sensitization). This review addresses the mechanisms of spinal sensitization evoked by arthritis. Electrophysiological recordings in anesthetized rats from spinal cord neurons with knee input in a model of acute arthritis showed that acute spinal sensitization is dependent on spinal glutamate receptors (AMPA, NMDA, and metabotropic glutamate receptors) and supported by spinal actions of neuropeptides such as neurokinins and CGRP, by prostaglandins, and by proinflammatory cytokines. In several chronic arthritis models (including immune-mediated arthritis and osteoarthritis) spinal glia activation was observed to be coincident with behavioral mechanical hyperalgesia which was attenuated or prevented by intrathecal application of minocycline, fluorocitrate, and pentoxyfylline. Some studies identified specific pathways of micro- and astroglia activation such as the purinoceptor- (P2X7-) cathepsin S/CX3CR1 pathway, the mobility group box-1 protein (HMGB1), and toll-like receptor 4 (TLR4) activation, spinal NFκB/p65 activation and others. The spinal cytokines TNF, interleukin-6, interleukin-1β, and others form a functional spinal network characterized by an interaction between neurons and glia cells which is required for spinal sensitization. Neutralization of spinal cytokines by intrathecal interventions attenuates mechanical hyperalgesia. This effect may in part result from local suppression of spinal sensitization and in part from efferent effects which attenuate the inflammatory process in the joint. In summary, arthritis evokes significant spinal hyperexcitability which is likely to contribute to the phenotype of arthritis pain in patients.

The cornucopia of central disinhibition pain - An evaluation of past and novel concepts

Central disinhibition (CD), as applied to pain, decreases thresholds of endogenous systems. This provokes onset of spontaneous or evoked pain in an individual beyond the ability of the nervous system to inhibit pain resulting from a disease or tissue damage. The original CD concept as proposed by Craig entails a shift from the lateral pain pathway (i.e. discriminative pain processing) towards the medial pain pathway (i.e. emotional pain processing), within an otherwise neurophysiological intact environment. In this review, the original CD concept as proposed by Craig is extended by the primary "nociceptive pathway damage - CD" concept and the secondary "central pathway set point - CD". Thereby, the original concept may be transferred into anatomical and psychological non-functional conditions. We provide examples for either primary or secondary CD concepts within different clinical etiologies as well as present surrogate models, which directly mimic the underlying pathophysiology (A-fiber block) or modulate the CD pathway excitability (thermal grill). The thermal grill has especially shown promising advancements, which may be useful to examine CD pathway activation in the future. Therefore, within this topical review, a systematic review on the thermal grill illusion is intended to stimulate future research. Finally, the authors review different mechanism-based treatment approaches to combat CD pain.

Tyrosine Kinase Inhibitors Reduce NMDA NR1 Subunit Expression, Nuclear Translocation, and Behavioral Pain Measures in Experimental Arthritis

In the lumbar spinal cord dorsal horn, release of afferent nerve glutamate activates the neurons that relay information about injury pain. Here, we examined the effects of protein tyrosine kinase (PTK) inhibition on NMDA receptor NR1 subunit protein expression and subcellular localization in an acute experimental arthritis model. PTK inhibitors genistein and lavendustin A reduced cellular histological translocation of NMDA NR1 in the spinal cord occurring after the inflammatory insult and the nociceptive behavioral responses to heat. The PTK inhibitors were administered into lumbar spinal cord by microdialysis, and secondary heat hyperalgesia was determined using the Hargreaves test. NMDA NR1 cellular protein expression and nuclear translocation were determined by immunocytochemical localization with light and electron microscopy, as well as with Western blot analysis utilizing both C- and N-terminal antibodies. Genistein and lavendustin A (but not inactive lavendustin B or diadzein) effectively reduced (i) pain related behavior, (ii) NMDA NR1 subunit expression increases in spinal cord, and (iii) the shift of NR1 from a cell membrane to a nuclear localization. Genistein pre-treatment reduced these events that occur in vivo within 4 h after inflammatory insult to the knee joint with kaolin and carrageenan (k/c). Cycloheximide reduced glutamate activated upregulation of NR1 content confirming synthesis of new protein in response to the inflammatory insult. In addition to this in vivo data, genistein or staurosporin inhibited upregulation of NMDA NR1 protein and nuclear translocation in vitro after treatment of human neuroblastoma clonal cell cultures (SH-SY5Y) with glutamate or NMDA (4 h). These studies provide evidence that inflammatory activation of peripheral nerves initiates increase in NMDA NR1 in the spinal cord coincident with development of pain related behaviors through glutamate non-receptor, PTK dependent cascades.

Emerging Trends in Pain Modulation by Metabotropic Glutamate Receptors

Pain is an essential protective mechanism meant to prevent tissue damages in organisms. On the other hand, chronic or persistent pain caused, for example, by inflammation or nerve injury is long lasting and responsible for long-term disability in patients. Therefore, chronic pain and its management represents a major public health problem. Hence, it is critical to better understand chronic pain molecular mechanisms to develop innovative and efficient drugs. Over the past decades, accumulating evidence has demonstrated a pivotal role of glutamate in pain sensation and transmission, supporting glutamate receptors as promising potential targets for pain relieving drug development. Glutamate is the most abundant excitatory neurotransmitter in the brain. Once released into the synapse, glutamate acts through ionotropic glutamate receptors (iGluRs), which are ligand-gated ion channels triggering fast excitatory neurotransmission, and metabotropic glutamate receptors (mGluRs), which are G protein-coupled receptors modulating synaptic transmission. Eight mGluRs subtypes have been identified and are divided into three classes based on their sequence similarities and their pharmacological and biochemical properties. Of note, all mGluR subtypes (except mGlu6 receptor) are expressed within the nociceptive pathways where they modulate pain transmission. This review will address the role of mGluRs in acute and persistent pain processing and emerging pharmacotherapies for pain management.

Alleviating pain hypersensitivity through activation of type 4 metabotropic glutamate receptor

Hyperactivity of the glutamatergic system is involved in the development of central sensitization in the pain neuraxis, associated with allodynia and hyperalgesia observed in patients with chronic pain. Herein we study the ability of type 4 metabotropic glutamate receptors (mGlu4) to regulate spinal glutamate signaling and alleviate chronic pain. We show that mGlu4 are located both on unmyelinated C-fibers and spinal neurons terminals in the inner lamina II of the spinal cord where they inhibit glutamatergic transmission through coupling to Cav2.2 channels. Genetic deletion of mGlu4 in mice alters sensitivity to strong noxious mechanical compression and accelerates the onset of the nociceptive behavior in the inflammatory phase of the formalin test. However, responses to punctate mechanical stimulation and nocifensive responses to thermal noxious stimuli are not modified. Accordingly, pharmacological activation of mGlu4 inhibits mechanical hypersensitivity in animal models of inflammatory or neuropathic pain while leaving acute mechanical perception unchanged in naive animals. Together, these results reveal that mGlu4 is a promising new target for the treatment of chronic pain.

Nitric oxide metabolites in the lumbosacral spinal cord interstice and cerebrospinal fluid in female rats with acute cyclophosphamide-induced cystitis. An in vivo microdialysis study

Objective:

To determine the concentration of nitrate/nitrite in the cerebrospinal fluid and in the dorsal horn interstice of the L6-S1 spinal cord boundary in rats with or without cystitis induced by cyclophosphamide.

Methods:

All experiments were conducted using Wistar female rats. A microdialysis probe was implanted in the subarachnoid space or in the spinal cord tissue at the L6-S1 segments (confirmed histologically). Two days later, the microdialysis probe was perfused with artificial cerebrospinal fluid, containing or not NGmonomethyl-L-arginine. Samples were collected every 15 minutes and kept at −20°C. Nitrite/nitrate concentrations were determined by chemiluminescence.

Results:

In normal animals, the mean values of nitrite/nitrate concentrations in the first microdialysate sample of the cerebrospinal fluid and of the spinal cord interstice were similar (482.5±90.2pmol/75μL, n=20, and 505.7±11.5pmol/75μL, n=6, respectively), whereas, in the samples from rats with cystitis, these values were significantly greater (955.5±66.3pmol/75μL, n=8, and 926.5±131.7pmol/75μL, n=11, respectively). In both groups, NGmonomethyl-L- arginine caused a significant reduction in the nitrite/nitrate concentration. Interestingly, the maximal reduction of nitrite/nitrate concentration caused by NG-monomethyl-L- arginine was no greater than 30% of the initial values.

Conclusions:

These results constitute the first demonstration that nitrite/nitrate concentrations in the cerebrospinal fluid and spinal cord interstice are elevated between 20- and 22 hours after cyclophosphamide-induced cystitis, and indicate that cystitis is associated with changes in the production of nitric oxide in the spinal cord segments, where most primary bladder afferents end.

Investigation of serum amino acid and serum amyloid A concentrations in chickens with amyloid arthropathy

BACKGROUND

Increased proteolytic cleavage of serum amyloid A (SAA) may potentially contribute to the development of AA amyloid deposition.

OBJECTIVE

To study the possible relationship between amyloid artropathy and expression of SAA and some serum amino acids.

ANIMALS AND METHODS

Values of 15 serum amino acids and SAA were investigated in chickens with experimentally induced amyloid arthropathy. Thirty-four, 5-week-old chicks were allocated into two groups: one group was injected intra-articularly with 0.25 mL complete Freund's adjuvant at the left tibio-metatarsal joint to induce amyloid arthropathy, whereas the other group served as control. All pullets were necropsied 13 weeks after injection. Collected tissue samples were examined histopathologically. Blood samples were collected and SAA concentrations were measured with enzyme-linked immunosorbent assay. High-performance liquid chromatography was used to assess the amino acid concentrations in serum.

RESULTS

Amyloid accumulation in joints occurred only in the experimental group (89%). SAA concentrations of 166 ± 17 and 423 ± 39 (SD) ng/mL were found in the control and experimental groups, respectively (p < 0.001). In the experimental group, an increase was observed in all examined amino acid concentrations except for citrulline. The most significant (p < 0.001) increases were noticed in serine (from 159 ± 15 to 360 ± 29 µmol/L), glycine (from 151 ± 20 to 279 ± 16 µmol/L), isoleucine (from 48 ± 2 to 80 ± 6 µmol/L), and phenylalanine (from 49 ± 2 to 90 ± 3 µmol/L).

CONCLUSION

The results of this study suggest that there is a positive correlation between some serum amino acid values, especially serine, glycine, isoleucine, and phenylalanine, and the high concentrations of SAA in chickens with amyloid arthropathy.

Anti-inflammatory effect of triterpene 3β, 6β, 16β-trihydroxylup-20(29)-ene obtained from Combretum leprosum Mart & Eich in mice

ETHNOPHARMACOLOGICAL RELEVANCE

The 3β, 6β, 16β-trihydroxylup-20(29)-ene (TTHL) is a pentacyclic triterpene obtained from a medicinal plant named Combretum leprosum. In folk medicine, this plant is used to treat several diseases associated with inflammation and pain. We previously demonstrated that TTHL presents a significant antinociceptive effect, suggesting the involvement of the glutamatergic system.

AIM OF THE STUDY

This study was designed to investigate the effect of TTHL on nociception and vascular permeability induced by acetic acid. We also evaluated the effect of TTHL on carrageenan-induced peritonitis and the levels of cytokines (interleukin 1-β [IL-1β], tumor necrosis factor α [TNF-α] and interleukin 10 [IL-10]) on peritoneal fluid.

MATERIALS AND METHODS

TTHL was administered orally by intra-gastric gavage (i.g.) 60 min prior to experimentation. Abdominal contractions and vascular permeability were induced by an intraperitoneal (i.p.) injection of acetic acid (0.6%). We also investigated whether TTHL decreases carrageenan-induced peritonitis (750 μg/cavity) by measuring leukocyte migration and vascular permeability. In addition, we evaluated the effects of TTHL on TNF-α, IL-1β and IL-10 release induced by carrageenan on peritoneal fluid. The levels of these cytokines were measured by ELISA.

RESULTS

TTHL (0.01-10 mg/kg) administered by intra-gastric (i.g.) gavage inhibited (69±3%) acetic acid-induced abdominal constrictions, with an ID₅₀ of 0.15 (0.03-0.8) mg/kg. TTHL (10mg/kg) also reduced the leukocyte infiltration induced by acetic acid, with an inhibition of 59±9 but had no effect on abdominal vascular permeability. In addition, indomethacin (10 mg/kg, i.p.) reduced the nociceptive behavior (92±1%), total leukocyte migration (29±3%) and capillary permeability (71±3%) induced by acetic acid. While the glucocorticoid dexamethasone (2 mg/kg, s.c.) reduced partially but significantly the nociception (31±1%), besides to promote a marked reduction on total leukocyte migration (60±2%) to the peritoneal cavity caused by acetic acid. In a model of peritonitis induced by carrageenan, TTHL also reduced total leukocyte migration, mainly neutrophils (inhibition of 84±3% and 85±2% at 30 mg/kg and 100 mg/kg, respectively). Likewise, dexamethasone (0.5 mg/kg, i.p.) resulted in an inhibition of 93±3%. Nevertheless, carrageenan-induced abdominal vascular permeability was reduced by dexamethasone but was not altered by TTHL. Furthermore, dexamethasone and TTHL significantly reduced the TNF-α and IL-1β levels in peritoneal fluid, whereas the IL-10 levels were unchanged.

CONCLUSIONS

Altogether, our data confirm the antinociceptive effect of TTHL and demonstrate its effect in inflammatory animal models, providing novel data about this compound, which could be useful as an anti-inflammatory drug.

Differential release of neurotransmitters from superficial and deep layers of the dorsal horn in response to acute noxious stimulation and inflammation of the rat paw

Experimental evidence suggests that release of neurotransmitters in response to acute noxious stimulation and inflammation can differ in superficial and deeper dorsal horn (DH) laminae. Using two different microdialysis probes, we studied changes in levels of glutamate, aspartate, arginine and GABA in dialysates collected from the surface of the spinal cord and within the DH induced by pinching the paw or paw inflammation. In penthotal anaesthetized rats, a flexible microdialysis probe was placed on the dorsal surface of the L4-L5 or L6-S2 spinal segments. In other rats, a rigid microdialysis probe was implanted within the DH of the same segments. Samples were collected every minute before, during and after pinching the hind paw (acute pain), and every half an hour after injecting either carrageenan or saline into the same paw (inflammation-induced pain). Amino acids were measured by capillary zone electrophoresis with laser-induced fluorescence detection (CZE-LIFD). Pinching the paw induced a significant but short lasting increase in extracellular glutamate and aspartate in dialysates from the surface of the DH. Carrageenan, but not saline, injected into the paw significantly increased concentrations of glutamate, aspartate and arginine both on the surface and within the DH of L4-L5 and also within the DH of the L6-S2 segments. The GABA level was significantly increased following carrageenan only within the DH. The maximum increase on the surface was detected 60-120 min after the onset of inflammation whereas the response within the DH reached a maximum between 150 and 180 min after carrageenan. These results indicate that unlike acute mechanical noxious stimulation which enhances amino acid neurotransmitters in surface dialysate, inflammation induced neurotransmitter release in all layers of the DH suggesting sensitization of the DH.

Regulation of peripheral inflammation by the central nervous system

In inflammatory disorders such as rheumatoid arthritis, cytokines and danger signals are sensed by the central nervous system, which adapts behavior and physiologic responses during systemic stress. The central nervous system can also signal the periphery to modulate inflammation through efferent hormonal and neuronal pathways. The brain and spinal cord are involved in this bidirectional interaction. A variety of neuronal pathways that modulate synovial inflammation have been implicated, including the sympathetic and the parasympathetic branches of the autonomic system. Another mechanism, the dorsal root reflex, involves antidromic signaling along somatic afferent fibers that influences joint inflammation by releasing neuropeptides and other neuromediators in the periphery. Some of the neurotransmitters and neuroreceptors involved have been identified in preclinical models and represent novel targets for the treatment of rheumatic diseases.

Metabotropic glutamate receptors (mGluRs) regulate noxious stimulus-induced glutamate release in the spinal cord dorsal horn of rats with neuropathic and inflammatory pain

In rats with persistent pain, spinal group I metabotropic glutamate receptor (mGluR) activity has been shown to be pronociceptive, whereas spinal group II/III activity is anti-nociceptive. In brain, group I mGluR activity produces positive feedback effects on glutamate release, whereas group II/III activity produces negative feedback effects. It is unknown whether the nociceptive versus anti-nociceptive effects of spinal group I versus group II/III mGluR activity depend on differential regulation of spinal glutamate release. Here, we used behavioral nociceptive testing and in vivo microdialysis to assess the effect of intrathecal treatment with group I mGluR antagonists [cyclopropan[b] chromen-1a-carboxylate, (CPCCOEt), 2-methyl-6-(phenylethynyl) pyridine (MPEP)] or groups II [aminopyrrolidine-2R,4R-dicarboxylate (APDC)] and III [l-2-amino-4-phosphonobutyrate (l-AP4)] mGluR agonists or vehicle, on nociception and noxious stimulus-induced increases in glutamate release in the spinal cord dorsal horn of rats with a chronic constriction injury (CCI) of the sciatic nerve or hind paw injection of complete Freund's adjuvant (CFA). None of the treatments significantly influenced basal spinal glutamate concentrations in either CCI or CFA rats. In CCI rats, formalin-induced nociception and increases in spinal glutamate concentrations were significantly attenuated by pre-treatment with CPCCOEt, MPEP, APDC, or l-AP4. In CFA rats, capsaicin-induced increases in nociception and spinal glutamate concentrations were significantly attenuated by pre-treatment with CPCCOEt, MPEP, or APDC, but not l-AP4. This study demonstrates that group I antagonists and group II/III mGluR agonists attenuated the enhanced nociception and noxious stimulus-induced glutamate release in spinal cord dorsal horn of CCI and/or CFA rats in vivo, and suggests a possible mechanism for their anti-hyperalgesic effects.

A peripheral neuroimmune link: glutamate agonists upregulate NMDA NR1 receptor mRNA and protein, vimentin, TNF-alpha, and RANTES in cultured human synoviocytes

Human primary and clonal synovial cells were incubated with glutamate receptor agonists to assess their modulating influence on glutamate receptors N-methyl-d-aspartate (NMDA) NR1 and NR2 and inflammatory cytokines to determine potential for paracrine or autocrine (neurocrine) upregulation of glutamate receptors, as has been shown for bone and chondrocytes. Clonal SW982 synoviocytes constitutively express vimentin, smooth muscle actin (SMA), and NMDA NR1 and NR2. Coincubation (6 h) with glutamate agonists NMDA (5 microM), and the NMDA NR1 glycine site activator (+/-)1-aminocyclopentane-cis-1,3-dicarboxylic acid (5 muM), significantly increases cellular mRNA and protein levels of glutamate receptors, as well as increasing vimentin, SMA, tumor necrosis factor-alpha, and RANTES (regulated on activation, normal T-cell expressed and secreted), assessed qualitatively and quantitatively with nucleotide amplification, image analysis of immunocytochemical staining, fluorescein-activated cell sorting, Western blotting, and immunoassays. Human primary synovial cells harvested from patients with arthritic conditions also constitutively expressed NMDA NR1 with increases after agonist treatment. Glutamate receptor agonist-induced increases were blocked by the noncompetitive glutamate antagonist MK-801 (8 microg/ml) and NR1 blocking antibody. Coincubation with glutamate agonists and phorbol 12-myristate 13-acetate, a protein kinase C activator, significantly enhanced mean levels of TNF-alpha and RANTES in SW982 cell supernatants compared with incubation with either agent alone. Increases were diminished with protein kinase inhibitor and NR1 blocking antibody. The functional activation of glutamate receptors on human synoviocytes establishes a neurogenic cell signaling link between neurotransmitter glutamate released from nerve terminals and target cells in the joint capsule. The influence of glutamate on subsequent release of cellular proinflammatory mediators in non-neural tissue for activation of downstream immune events supports a peripheral neuroimmune link in arthritis.

Evidence that pregabalin reduces neuropathic pain by inhibiting the spinal release of glutamate

Pregabalin is an anti-convulsant that successfully treats many neuropathic pain syndromes, although the mechanism of its anti-hyperalgesic action remains elusive. This study aims to help delineate pregabalin's anti-hyperalgesic mechanisms. We assessed the effectiveness of pregabalin at decreasing mechanical and cold hypersensitivity induced in a rat model of neuropathic pain. Thus, we compared the effectiveness of pre- or post-treatment with systemic or intrathecal (i.t.) pregabalin at reducing the development and maintenance of the neuropathic pain symptoms. Pregabalin successfully decreased mechanical and cold hypersensitivity, as a pre-treatment, but was less effective at suppressing cold hypersensitivity when administered as a post-treatment. Furthermore, both i.t. and systemic administration of pregabalin were effective in reducing the behavioral hypersensitivity, with the exception of systemic post-treatment on cold hypersensitivity. We also examined pregabalin's effects at inhibiting hind paw formalin-induced nociception in naïve rats and formalin-induced release of excitatory amino acids in the spinal cord dorsal horn (SCDH) both in naïve rats and in rats with neuropathic pain. Pregabalin dose-dependently reduced nociceptive scores in the formalin test. We also present the first evidence that pregabalin reduces the formalin-induced release of glutamate in SCDH. Furthermore, i.t. pregabalin reduces the enhanced noxious stimulus-induced spinal release of glutamate seen in neuropathic rats. These data suggest that pregabalin reduces neuropathic pain symptoms by inhibiting the release of glutamate in the SCDH.

Imbalance between excitatory and inhibitory amino acids at spinal level is associated with maintenance of persistent pain-related behaviors

Although the postsynaptic events responsible for development of pathological pain have been intensively studied, the relative contribution of presynaptic neurotransmitters to the whole process remains less elucidated. In the present investigation, we sought to measure temporal changes in spinal release of both excitatory amino acids (EAAs, glutamate and aspartate) and inhibitory amino acids (IAAs, glycine, ?-aminobutyric acid and taurine) in response to peripheral inflammatory pain state. The results showed that following peripheral chemical insult induced by subcutaneous bee venom (BV) injection, there was an initial, parallel increase in spinal release of both EAAs and IAAs, however, the balance between them was gradually disrupted when pain persisted longer, with EAAs remaining at higher level but IAAs at a level below the baseline. Moreover, the EAAs-IAAs imbalance at the spinal level was dependent upon the ongoing activity from the peripheral injury site. Intrathecal blockade of ionotropic (NMDA and non-NMDA) and metabotropic (mGluRI, II, III) glutamate receptors, respectively, resulted in a differential inhibition of BV-induced different types of pain (persistent nociception vs. hyperalgesia, or thermal vs. mechanical hyperalgesia), implicating that spinal antagonism of any specific glutamate receptor subtype fails to block all types of pain-related behaviors. This result provides a new line of evidence emphasizing an importance of restoration of EAAs-IAAs balance at the spinal level to prevent persistence or chronicity of pain.

Gene transfer of GLT-1, a glial glutamate transporter, into the spinal cord by recombinant adenovirus attenuates inflammatory and neuropathic pain in rats

Background

The glial glutamate transporter GLT-1 is abundantly expressed in astrocytes and is crucial for glutamate removal from the synaptic cleft. Decreases in glutamate uptake activity and expression of spinal glutamate transporters are reported in animal models of pathological pain. However, the lack of available specific inhibitors and/or activators for GLT-1 makes it difficult to determine the roles of spinal GLT-1 in inflammatory and neuropathic pain. In this study, we examined the effect of gene transfer of GLT-1 into the spinal cord with recombinant adenoviruses on the inflammatory and neuropathic pain in rats.

Results

Intraspinal infusion of adenoviral vectors expressing the GLT-1 gene increased GLT-1 expression in the spinal cord 2–21 days after the infusion. Transgene expression was primarily localized to astrocytes. The spinal GLT-1 gene transfer had no effect on acute mechanical and thermal nociceptive responses in naive rats, whereas it significantly reduced the inflammatory mechanical hyperalgesia induced by hindlimb intraplantar injection of carrageenan/kaolin. Spinal GLT-1 gene transfer 7 days before partial sciatic nerve ligation recovered the extent of the spinal GLT-1 expression in the membrane fraction that was decreased following the nerve ligation, and prevented the induction of tactile allodynia. However, the partial sciatic nerve ligation-induced allodynia was not reversed when the adenoviruses were infused 7 or 14 days after the nerve ligation.

Conclusion

These results suggest that overexpression of GLT-1 on astrocytes in the spinal cord by recombinant adenoviruses attenuates the induction, but not maintenance, of inflammatory and neuropathic pain, probably by preventing the induction of central sensitization, without affecting acute pain sensation. Upregulation or functional enhancement of spinal GLT-1 could be a novel strategy for the prevention of pathological pain.

Reversal of chronic inflammatory pain by acute inhibition of Ca2+/calmodulin-dependent protein kinase II

Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) is a major protein kinase that is capable of regulating the activities of many ion channels and receptors. In the present study, the role of CaMKII in the complete Freund's adjuvant (CFA)-induced inflammatory pain was investigated. Intraplantarly injected CFA was found to induce spinal activity of CaMKII (phosphorylated CaMKII), which was blocked by KN93 [[2-[N-(2-hydroxyethyl)]-N-(4-methoxybenzenesulfonyl)]amino-N-(4-chlorocinnamyl)-N-methylbenzylamine)], a CaMKII inhibitor. Pretreatment with KN93 (i.t.) dose-dependently prevented the development of CFA-induced thermal hyperalgesia and mechanical allodynia. Acute treatment with KN93 (i.t.) also dose-dependently reversed CFA-induced thermal hyperalgesia and mechanical allodynia. The action of KN93 started in 30 min and lasted for at least 2 to 4 h. KN92 (45 nmol i.t.) [2-[N-(4-methoxybenzenesulfonyl)]amino-N-(4-chlorocinnamyl)-N-methylbenzylamine], an inactive analog of KN93, showed no effect on CFA-induced CaMKII activation, allodynia, or hyperalgesia. Furthermore, our previous studies identified trifluoperazine, a clinically used antipsychotic drug, to be a potent CaMKII inhibitor. Inhibition of CaMKII activity by trifluoperazine was confirmed in the study. In addition, trifluoperazine (i.p.) dose-dependently reversed CFA-induced mechanical allodynia and thermal hyperalgesia. The drug was also effectively when given orally. In conclusion, our findings support a critical role of CaMKII in inflammatory pain. Blocking CaMKII or CaMKII-mediated signaling may offer a novel therapeutic target for the treatment of chronic pain.

The peripheral role of group I metabotropic glutamate receptors on nociceptive behaviors in rats with knee joint inflammation

We examined whether the mGluR1 and mGluR5 were involved in development and maintenance of behavioral signs of non-evoked pain and secondary mechanical hyperalgesia induced by knee joint inflammation. Selective mGluR1 antagonist, (RS)-1-aminoindan-1,5-dicarboxylic acid (AIDA: 50, 100, 200 microM/25 microl, n=10 per group) and selective mGluR5 antagonist, 2-methyl-6-(phenylethynyl)-pyridine (MPEP: 50, 100, 200 nM/25 microl, n=10 per group) was intra-articularly (i.a.) injected 30 min before and 4h after carrageenan injection and behavioral tests were conducted. In the pre-treatment, only a higher dose (200 nM) of MPEP significantly prevented the magnitude of weight load reduction, whereas AIDA (200 microM) and MEPE (50, 100 and 200 nM) significantly reduced the development of mechanical hyperalgesia compared to saline treated group. In the post-treatment, AIDA (200 microM) and MPEP at 100 and 200 nM partially reversed the reduction of weight load induced by carrageenan. MPEP significantly increased the withdrawal threshold to mechanical stimulation in a dose-dependent manner, whereas AIDA had significantly reversed the decreased the paw withdrawal threshold only at 200 microM. The present study demonstrated that i.a. MPEP, selective mGluR5 antagonist is more effective than selective mGluR1 antagonist, AIDA on non-evoked pain as well as mechanical hyperalgesia in both induction and maintenance phase in knee joint inflammation. It is suggested that peripheral mGlu5 receptors play a more prominent role in inflammatory pain including evoked and spontaneous pain. Thus, selective mGluR5 antagonist could be effective therapeutic tools in clinical setting.

Release of GABA and activation of GABA(A) in the spinal cord mediates the effects of TENS in rats

Transcutaneous electrical nerve stimulation (TENS) is a commonly utilized non-pharmacological, non-invasive treatment for pain. GABA is a neurotransmitter in the dorsal horn of the spinal cord that mediates analgesia locally, and also through activation of supraspinal sites. TENS reduces hyperalgesia through activation of receptor-mediated pathways at the level of the spinal cord, and supraspinally. The current study tested the hypothesis that either high or low frequency TENS applied to the inflamed knee joint increases GABA in the spinal cord dorsal horn and activates GABA receptors spinally. We utilized microdialysis to sample the extracellular fluid before, during and after TENS and analyzed GABA in dialysates with high performance liquid chromatography. We analyzed the extracellular GABA concentrations in animals with and without knee joint inflammation induced by intra-articular injection of kaolin and carrageenan. We further tested if spinal blockade of GABA receptors prevents the antihyperalgesia produced by TENS in rats with joint inflammation. We show that high frequency TENS increases extracellular GABA concentrations in the spinal cord in animals with and without joint inflammation. The increases in GABA do not occur in response to low frequency TENS, and there are no increases in glycine in response to low or high frequency TENS. However, the reduction in primary hyperalgesia by both high and low frequency TENS is prevented by spinal blockade of GABA(A) receptors with bicuculline. Thus, high frequency TENS increases release of GABA in the deep dorsal horn of the spinal cord, and both high and low frequency TENS reduce primary hyperalgesia by activation of GABA(A) receptors spinally.

Role of neurogenic inflammation in pancreatitis and pancreatic pain

Pain arising from pancreatic diseases can become chronic and difficult to treat. There is a paucity of knowledge regarding the mechanisms that sensitize neural pathways that transmit noxious information from visceral organs. In this review, neurogenic inflammation is presented as a possible amplifier of the noxious signal from peripheral organs including the pancreas. The nerve pathways that transmit pancreatic pain are also reviewed as a conduit of the amplified signals. It is likely that components of these visceral pain pathways can also be sensitized after neurogenic inflammation.

Excitatory amino acid concentrations increase in the spinal cord dorsal horn after repeated intramuscular injection of acidic saline

Chronic muscle pain is common and often difficult to treat. In this study, we further characterize a model of chronic muscle pain induced by repeated intramuscular injection of acidic saline. Two injections of acid into muscle separated by 5 days result in secondary mechanical hyperalgesia that lasts for up to 4 weeks. Blockade of spinal NMDA receptors prior to the second injection intramuscular acid injection delays the onset of hyperalgesia, where as the maintenance phase of hyperalgesia, evaluated 1 week after the second intramuscular injection, is dependent on activation of spinal AMPA/kainate and NMDA receptors. In order to determine if behavioral hyperalgesia and glutamate receptor involvement are associated with increased concentrations of excitatory amino acids (EAA), we utilized microdialysis to evaluate extracellular glutamate and aspartate concentrations in the spinal dorsal horn during the first and second intramuscular acid injections, and 1 week after the development of mechanical hyperalgesia. The second intramuscular injection evoked a calcium-dependent increase in both spinal glutamate and aspartate concentrations. Glutamate concentrations within the dorsal horn were also increased 1 week after the second acid injection. Our data suggest increased release of spinal EAAs in the dorsal horn contributes to the development and maintenance of hyperalgesia.

High-frequency, but not low-frequency, transcutaneous electrical nerve stimulation reduces aspartate and glutamate release in the spinal cord dorsal horn

Transcutaneous electrical nerve stimulation (TENS) is a commonly utilized non-pharmacological treatment for pain. Studies show that low- and high-frequency TENS utilize opioid, serotonin and/or muscarinic receptors in the spinal cord to reduce hyperalgesia induced by joint inflammation in rats. As there is an increase in glutamate and aspartate levels in the spinal cord after joint inflammation, and opioids reduce glutamate and aspartate release, we hypothesized that TENS reduces release of glutamate and aspartate in animals with joint inflammation by activation of opioid receptors. Using microdialysis and HPLC with fluorescence detection, we examined the release pattern of glutamate and aspartate in the dorsal horn in response to either low-frequency (4 Hz) or high-frequency (100 Hz) TENS. We examined the effects of TENS on glutamate and aspartate release in animals with and without joint inflammation. High-frequency, but not low-frequency, TENS significantly reduced spinal glutamate and aspartate in animals with joint inflammation compared with levels in those without joint inflammation. The reduced release of glutamate and aspartate by high-frequency TENS was prevented by spinal blockade of delta-opioid receptors with naltrindole. Thus, we conclude that high-frequency TENS activates delta-opioid receptors consequently reducing the increased release of glutamate and aspartate in the spinal cord.

Evidence that gabapentin reduces neuropathic pain by inhibiting the spinal release of glutamate

Gabapentin is an anticonvulsant that successfully treats many neuropathic pain syndromes, although the mechanism of its antihyperalgesic action remains elusive. This study aims to help delineate gabapentin's antihyperalgesic mechanisms. We assessed the effectiveness of gabapentin at decreasing mechanical and cold hypersensitivity induced in a rat model of neuropathic pain. Thus, we compared the effectiveness of pre- or post-treatment with systemic or intrathecal (i.t.) gabapentin at reducing the development and maintenance of the neuropathic pain symptoms. Gabapentin successfully decreased mechanical and cold hypersensitivity, both as a pretreatment and post-treatment. Furthermore, both i.t. and systemic administration of gabapentin were effective in reducing the behavioral hypersensitivity; however, the i.t. administration was superior to the systemic. We also examined gabapentin's effects at inhibiting hindpaw formalin-induced release of excitatory amino acids (EAAs) in the spinal cord dorsal horn (SCDH) both in naïve rats and in rats with neuropathic pain. We present the first evidence that gabapentin reduces the formalin-induced release of both glutamate and aspartate in SCDH. Furthermore, i.t. gabapentin reduces the enhanced noxious stimulus-induced spinal release of glutamate seen in neuropathic rats. These data suggest that gabapentin reduces neuropathic pain symptoms by inhibiting the release of glutamate in the SCDH.

Excitatory amino acids, TNF-alpha, and chemokine levels in synovial fluids of patients with active arthropathies

The aim of this study was to assess the synovial fluid (SF) neurotransmitter excitatory amino acid (EAA) levels, including glutamate (Glu) and aspartate (Asp), in the context of SF levels of other amino acids, TNF-alpha and chemokines from patients with active arthropathies. The SF was collected from patients with active rheumatoid arthritis (RA), gout, or osteoarthritis (OA). The SF samples were analysed for levels of neurotransmitters glutamate and aspartate, tumour necrosis factor-alpha (TNF-alpha), Regulated upon Activation Normally T-cell Expressed and Secreted (RANTES), macrophage inhibitory factor-1 alpha (MIP-1alpha) and interleukin 8 (IL-8). SF WBC counts were also determined. Correlations between SF EAA, TNF-alpha and chemokines were determined by the Pearson product-moment correlation. Primary cultures derived from SF from active RA and gout patients were incubated with added l-glutamate, to assess if exposure to Glu could increase TNF-alpha levels. There were significant elevations in SF EAA, SF TNF-alpha and SF RANTES in RA patients compared to gout or OA patients. Significant correlations between SF EAA and SF RANTES, MIP-1alpha and IL-8 levels were seen, and SF EAA and SF TNF-alpha or SF WBC levels approached significance. Addition of exogenous neurotransmitter glutamate significantly increased TNF-alpha levels in primary cell cultures derived from RA and gout patients. The SF neurotransmitter EAA levels significantly correlated to selected SF chemokine levels, in clinically active RA, gout and OA patients, independent of disease. Added Glu resulted in significantly increased TNF-alpha levels in primary synovial cell cultures. These data expand the relationship of SF neurotransmitter EAA levels to SF cytokines and chemokines in patients with clinically active arthritis, and suggest that neurotransmitters Glu and Asp contribute to peripheral inflammatory processes.

Involvement of NMDA receptors in Zif/268 expression in the trigeminal nucleus caudalis following formalin injection into the rat whisker pad

We investigated the involvement of N-methyl-D-aspartate (NMDA) glutamate receptor in the expression of the proteins Zif/268 and c-Fos elicited by painful stimuli. To this purpose, the effect of the administration of MK-801, an NMDA receptor antagonist, on Zif/268 and c-Fos expression following a noxious stimulus, represented by formalin injection into the whisker pad of rats, was examined in neurons of the trigeminal nucleus caudalis. Furthermore, the co-localization of formalin injection-evoked Zif/268 and c-Fos expression and subunit 1 of the NMDA receptor (NR1) was studied in this nucleus. Zif/268 or c-Fos immunoreactivity elicited by formalin injection was significantly reduced by pretreatment with MK-801 in the superficial layer of the trigeminal nucleus caudalis; more than 40% of the neurons expressing Zif/268 and c-Fos in this layer were also immunolabeled by NR1. On the other hand, there was little effect of MK-801 administration on Zif/268 and c-Fos immunoreactivity in the nucleus proprius and deep lamina V of the trigeminal nucleus caudalis, while most neurons expressing Zif/268 or c-Fos in these two regions were labeled by NR1. These results point out differences between the superficial and deeper layers of the trigeminal nucleus caudalis in the involvement of NMDA receptor in the mechanisms underlying the expression of protein products of immediate early genes induced by painful stimuli.

Effects of GCP-II inhibition on responses of dorsal horn neurones after inflammation and neuropathy: an electrophysiological study in the rat

N-Acetylaspartylglutamate (NAAG) is a peptide neurotransmitter present in the brain and spinal cord. It is hydrolysed by glutamate carboxypeptidase II (GCPII); thus, the GCP-II inhibitor 2-[phosphono-methyl]-pentanedioic acid (2-PMPA) protects endogenous NAAG from degradation, allowing its effects to be studied in vivo. We recorded the effect of spinal 2-PMPA (50-1000 microg) on the electrical-evoked activity of dorsal horn neurones in normal and carrageenan-inflamed animals, and in the spinal nerve ligation (SNL) model of neuropathy and sham-operated animals. In normal animals, 1000 microg 2-PMPA selectively inhibited noxious-evoked activity (input, post-discharge and C- and Adelta-fibre-evoked responses), and not low threshold Abeta-fibre-evoked responses. After carrageenan inflammation, the lower dose of 100 microg 2-PMPA inhibited input, post-discharge, C- and Adelta-fibre-evoked responses by a significantly greater amount than the same dose in normal animals. 2-PMPA inhibited neuronal responses less consistently in sham-operated and SNL animals, and effects were not significantly different from those seen in normal animals. NAAG is an agonist at the inhibitory metabotropic glutamate receptor mGluR3, and 2-PMPA may inhibit nociceptive transmission in normal animals by elevating synaptic NAAG levels, allowing it to activate mGluR3 and thus reducing transmitter release from afferent nerve terminals. mGluR3 expression in the superficial dorsal horn is upregulated after peripheral inflammation, perhaps explaining the greater inhibition of neuronal responses we observed after carrageenan inflammation. These results support an important role of endogenous NAAG in the spinal processing of noxious information.

Unilateral carrageenan injection into muscle or joint induces chronic bilateral hyperalgesia in rats

Chronic musculoskeletal pain is a major clinical problem and there is a general lack of animal models to study this condition. Carrageenan is commonly used to produce short-lasting acute inflammation and hyperalgesia in animal models. However, the potential of carrageenan to produce chronic, long-lasting hyperalgesia has not been evaluated. In the present study, we investigated the long-term effects of carrageenan injected into joint or muscle in rats. Rats were injected with 0.3, 1 or 3% carrageenan in one knee joint or gastrocnemius muscle and hyperalgesia to mechanical (measured as decreased withdrawal threshold) and heat (measured as decreased withdrawal latency) stimuli of both paws assessed before and at varying times after injection, through 8 weeks. Histological changes were examined only after injection of 3% carrageenan. Three percent carrageenan injected in the muscle or knee produced hyperalgesia to mechanical and heat stimuli ipsilaterally, which lasted 7-8 weeks and spread to the contralateral side 1-2 weeks after injection. One percent carrageenan injected to the knee joint or gastrocnemius muscle, produced hyperalgesia that was shorter-lasting and remained ipsilateral; 0.3% carrageenan injected into the knee joint or gastrocnemius muscle had no effect. Three percent carrageenan injected into the skin surrounding the knee joint did not produce hyperalgesia. A similar pattern of inflammatory changes was observed histologically for both the joint and muscle tissues. Acute inflammation was observed for the first 24 h with edema and neutrophilic infiltration evident as early as 4 h. At 1 week, the inflammation converted to primarily a macrophage response with scattered mast cells. The data suggest that animals injected with 1 or 3% carrageenan in the knee joint or gastrocnemius muscle could be used as models of acute inflammation through 24 h and chronic inflammation after 1 week. Furthermore, 3% carrageenan injected into deep tissues produces hyperalgesia that spreads to the contralateral side, at the same time period as the inflammation transforms from acute to chronic.

Comparison of microdialysis and push-pull perfusion for retrieval of serotonin and norepinephrine in the spinal cord dorsal horn

Both push-pull and microdialysis methods are utilized to measure norepinephrine and serotonin in the dorsal horn of the spinal cord. This experiment was designed to determine which technique is better for measurement of norepinephrine and serotonin in the spinal cord and also to determine if the samples are best collected with or without perchloric acid. Sample stability and an assay validation for precision, limit of quantification, and limit of detection were also performed. Push-pull or microdialysis catheters were placed transversely through the dorsal horn and the catheter was perfused with artificial cerebrospinal fluid. Noxious pinch (20 s/min for 10 min) was used to evoke a change in the concentration of catecholamines. Samples were collected before, during and after pinch. No basal concentrations of epinephrine and serotonin were found with microdialysis. Although basal concentrations of norepinephrine were measured by microdialysis, there was no change in response to noxious pinch. The push-pull technique coupled with collection of samples without perchloric acid showed that significant increases in serotonin and norepinephrine are measurable in response to noxious pinch. In contrast, when samples were collected with perchloric acid present there was no change in serotonin or norepinephrine in response to pinch. The stability of catecholamines is greatly affected by perchloric acid such that there is a near complete loss of ability to detect serotonin and norepinephrine by 24 h in samples collected by push-pull. In contrast, samples collected without perchloric acid showed only a 20% reduction in concentration by 24 h. Even without perchloric acid, by 1 wk there was a 50% or greater loss in the concentrations of norepinephrine in push-pull samples. Thus, to measure changes in catecholamines in the dorsal horn, push-pull collected without perchloric provides measurable, reliable and valid results if analyzed by high performance liquid chromatography within 24 h.

Differential efficacy of intrathecal NMDA receptor antagonists on inflammatory mechanical and thermal hyperalgesia in rats

Spinal cord dorsal horn N-methyl-D-aspartate (NMDA) receptors have been implicated in central sensitization, enhanced responsiveness to peripheral stimuli following peripheral injury. Since hyperalgesia is a behavioral consequence of central sensitization, it should be attenuated at the level of the dorsal horn with NMDA receptor antagonists. However, responsiveness to thermal and mechanical hyperalgesia may be distinct, and have thus far not been directly compared in chronic inflammatory pain models. In the present study, inflammation was induced with complete Freund's adjuvant (CFA) injected into the rat hind paw and NMDA receptor antagonists dizocilpine (MK-801) or 2-amino-5-phosphonovaleric acid (AP-5) were intrathecally injected in rats to determine the effects on both mechanical and thermal hyperalgesia. Locomotor tests and reflexes were also conducted to evaluate potential motor side effects. The NMDA receptor antagonists dose-dependently ameliorated mechanical hyperalgesia, but had marginal effects on thermal hyperalgesia. In ranges near antihyperalgesic doses, significant disruption of motor coordination was observed for both antagonists. These results suggest that, depending on the stimulus, NMDA receptors may have variable significance for central sensitization-mediated hyperalgesia, and that NMDA receptor antagonists may have therapeutic potential for some, but not all components in the clinical manifestation of inflammatory pain.

Spinal adenosine receptor activation inhibits inflammation and joint destruction in rat adjuvant-induced arthritis

OBJECTIVE

To examine the effect of spinal cord adenosine (Ado) receptor stimulation on rat adjuvant-induced arthritis (AIA).

METHODS

Long-term intrathecal (IT) catheters were implanted into rats to provide spinal access for drug delivery. Animals were immunized with complete Freund's adjuvant at the tail base. Eight days later and every other day thereafter until day 20, rats were treated IT with the selective Ado A1 receptor agonist cyclohexyladenosine (CHA) or vehicle. In some experiments, animals received an additional daily intraperitoneal injection of the nonselective Ado antagonist theophylline. Paw swelling was measured by water displacement plethysmometry. The effect of IT CHA on the activation of activator protein 1 (AP-1) was determined by electromobility shift assay. Spinal cord c-Fos expression was determined by immunohistochemistry.

RESULTS

Spinal CHA significantly inhibited inflammation in AIA, with a mean +/- SEM 20.9 +/- 16.9% increase in paw swelling in the IT CHA group compared with 81.3 +/- 10.6% in the saline group. The antiinflammatory effect of CHA was mediated through Ado receptors since the effect was reversed by coadministration of systemic theophylline. In addition, radiographs showed significantly less bone and cartilage destruction in the CHA-treated animals. Synovial expression of AP-1, which is a key regulator of metalloproteinase expression, was lower in IT CHA-treated animals. C-Fos expression was localized to spinal laminae I-VI, with a modest decrease observed in the superficial laminae in IT CHA-treated rats.

CONCLUSION

These data demonstrate that the spinal cord can regulate peripheral inflammation. Therapeutic strategies that target the central nervous system might be useful in arthritis.

Excitatory amino acid release in the spinal cord caused by plantar incision in the rat

There is extensive evidence that spinal excitatory amino acids (EAAs) like glutamate (Glu) and aspartate (Asp) are important in the processing of nociceptive behaviors caused by incisions. To better understand EAA-induced dorsal horn sensitization caused by surgery, we examined the time course and extent of spinal amino acid (AA) release during and after a plantar incision utilizing in vivo microdialysis. We also examined the role of primary afferent input and axonal conduction by measuring spinal EAAs in rats after hindpaw denervation and in rats treated with spinal tetrodotoxin (TTX). In halothane-anesthetized rats, a microdialysis filament (200 microm diameter, 45,000 MW cut off, Hospal AN69) was passed transversely through the deep dorsal horn of the spinal cord (L4-L6). After 18-24h, the dialysis filament was perfused with artificial cerebrospinal fluid (ACSF), dialysate samples collected and analyzed for AAs (Glu, Asp, asparagine, glutamine, serine, and glycine). Rats underwent anesthetic induction with halothane followed by a plantar incision (n=8) or sham operation (n=8). AAs were also measured in incised rats that underwent hindpaw denervation (n=4), in rats that had the filament placed outside the L4-L6 spinal segments (n=7), in rats with a microdialysis catheter placed in the ventral horn at L4-L6 (n=7) and in rats treated with spinally administered TTX (n=5). AAs were measured during recovery from anesthesia and for the next 8h. In the sham-operated group, Asp and Glu did not change throughout the experiment. In rats undergoing plantar incision, Asp and Glu increased from 10 to 30 min after incision to 200+/-30 and 138+/-12 percentage of control, respectively. The EAAs returned to baseline by 1h after incision. For the other AAs, only serine and asparagine increased after incision. No increase in AA release by incision was observed after hindpaw denervation, TTX treatment or placement of the filament outside the L4-L6 segments. In rats with a filament implanted in the ventral horn (L4-L6), EAAs increased during halothane induction and sham preparation. Thus, the EAA release required an intact afferent nerve barrage and segmental excitatory nerve transmission. The incision-induced nociceptive afferent barrage increased the release of Glu and Asp in the lumbar dorsal horn for 45 min. The concentrations of AAs returned to baseline by 1h. The percentage increase is in some cases less and for a shorter period of time compared to other models of persistent pain, perhaps because the incision injury is less severe compared to others models. This profile of EAA release further explains why models of inflammation and chemical irritation do not translate well to human postoperative pain.

Differential effects of NMDA and AMPA/KA receptor antagonists on c-Fos or Zif/268 expression in the rat spinal dorsal horn induced by noxious thermal or mechanical stimulation, or formalin injection

The involvement of N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA)/kainate (KA) receptors in the induction of c-Fos and Zif/268 expression in spinal dorsal horn neurons following noxious thermal or mechanical stimulation, or formalin injection into the rat hind paw was examined by intrathecal administration of a competitive NMDA receptor antagonist, 2-amino-5-phosphonopentanoic acid (APV) or an AMPA/KA receptor antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), or both, 30 min prior to noxious stimulation. APV caused a significant reduction in the level of c-Fos expression in the superficial layer induced by each of these three noxious stimuli. The effects of APV on Zif/268 expression or of CNQX on c-Fos or Zif/268 expression in the superficial layer induced by these three noxious stimuli were dependent on the type of stimulus applied to the rat hind paw. The noxious thermal stimulus-evoked c-Fos expression level was reduced by APV and/or CNQX, while Zif/268 expression was hardly changed. Both c-Fos and Zif/268 expressions following formalin injection were reduced by APV alone and APV+CNQX, but not by CNQX alone. Zif/268 expression following noxious mechanical stimulation was significantly reduced only by APV+CNQX although APV or CNQX alone did not affect the expression, while c-Fos expression was reduced by APV and APV+CNQX but not by CNQX alone. These findings suggest that NMDA and AMPA/KA receptors are differentially involved in c-Fos and Zif/268 expression in the spinal dorsal horn following noxious thermal, formalin and mechanical stimulation.

Effects of NMDA and non-NMDA ionotropic glutamate receptor antagonists on the development and maintenance of hyperalgesia induced by repeated intramuscular injection of acidic saline

Two unilateral injections of pH 4.0 saline into the gastrocnemius muscle result in a bilateral decrease in mechanical withdrawal threshold after the second injection. This decrease is significant by 4h and lasts through 1 week. The purpose of this study was to characterize the involvement of both N-methyl-D-aspartate (NMDA) and non-NMDA glutamate receptors in the spinal cord dorsal horn in the development and maintenance of mechanical hyperalgesia from repeated intramuscular injections of acidic saline. 2-amino-5-phosphonovaleric acid (AP5) (2-20 nmol, 10 microl, pH 7) or 1,2,3,4-tetrahydro-6-nitro-2,3-dioxo[f]quinoxaline-7-sulfonamide (NBQX) (1-10 nmol, 10 microl, pH 8-9) was administered intrathecally to the lumbar spinal cord to block NMDA and non-NMDA ionotropic glutamate receptors in the dorsal horn, respectively. Drugs were administered at one of three different time points: (1) prior to the first intramuscular injection of pH 4.0 saline on Day 0, (2) prior to the second intramuscular injection of pH 4.0 saline on Day 5, and (3) 1 week after the second injection. Mechanical withdrawal thresholds were measured with von Frey filaments before, 4h, and 24h after injection 1 and before, 4h, 24h, and 1 week after injection 2. AP5 had no effect on mechanical withdrawal thresholds when administered prior to the first intramuscular injection of pH 4.0 saline. When AP5 was administered before the second intramuscular injection, the bilateral decrease in mechanical withdrawal thresholds was delayed for up to 24h. Intrathecal administration of AP5 1 week after the second intramuscular injection of pH 4.0 saline produced a bilateral increase in mechanical withdrawal thresholds. Blockade of non-NMDA glutamate receptors in the spinal cord dorsal horn prior to either the first or second intramuscular injection of pH 4.0 saline had no effect on the development of mechanical hyperalgesia. However, spinal injection of NBQX 1 week after the second intramuscular injection of pH 4.0 saline resulted in an increase in mechanical withdrawal thresholds when compared to vehicle controls. These data suggest that both NMDA and non-NMDA glutamate receptors are involved in the maintenance of chronic, muscle-induced hyperalgesia.

Systemic pre-treatment with a group II mGlu agonist, LY379268, reduces hyperalgesia in vivo

1. Previous studies investigating the role of metabotropic glutamate (mGlu) receptors in nociceptive processing have been hampered by the lack of systemically active, selective, ligands. This study investigates the possible analgesic and/or anti-hyperalgesic properties of the most potent compound to date that has systemic agonist activity at group II mGlu receptors, LY379268. 2. In testing the drug in rats as an analgesic to acute noxious stimuli, LY379268 (in doses up to 3 mg kg(-1) i.p.) did not affect withdrawal latencies to either mechanical or thermal stimulation. 3. However, when a 3 mg kg(-1) dose was given prior to an intraplantar injection of carrageenan, the inflammatory hyperalgesia that developed was significantly delayed compared to saline pre-treated controls, without affecting the inflammation of the paw. A similar dose of the mGlu-inactive enantiomer, LY379267, was not anti-hyperalgesic. 4. In a model of mouse tail withdrawal to warm water, LY379268 (12 mg kg(-1) i.p.), given before a subcutaneous tail injection of capsaicin, reduced the subsequent neurogenic hyperalgesia. 5. Rota-rod testing showed that the drug did not produce a motor impairment in rats at antihyperalgesic doses. 6. The results indicate that systemic activation of this group of mGlu receptors reduces both inflammatory and neurogenic thermal hyperalgesia.

Mechanisms underlying the nociception and paw oedema caused by injection of glutamate into the mouse paw

This study characterizes the receptor subtypes and investigates some of the mechanisms by which glutamate, injected intraplantarly (i.pl.) into the mouse paw, produces nociception and paw oedema. I.pl. injection of glutamate induced a rapid-onset, dose-related pain response associated with oedema formation, with mean ED(50) values of 2.6 (1.6-4.3) and 0.5 (0.4-0.7) micromol/kg, respectively. Pretreatment with Chicago sky blue 6B (100 microg/kg), an inhibitor of glutamate uptake, caused a significant (about sixfold) reduction of the mean ED(50) value for glutamate-induced nociception, but not paw oedema. NMDA receptor antagonist MK 801, given by systemic (i.p.), intracerebroventricular (i.c.v.), i.pl. or intrathecal (i.t.) routes, produced graded inhibition of glutamate-induced nociception. Non-NMDA receptor antagonists NBQX or GAMS, metabotropic antagonist E4CPG, and also the antagonist that acts at the NMDA receptor-associated glycine binding site felbamate, significantly inhibited the nociception induced by glutamate. L(omega)-N-nitro-arginine (given i.p., i.t., i.pl. or i.c.v.) prevented the nociception and paw oedema caused by glutamate, an effect that was reversed by L-arginine but not by D-arginine. S-nitroso-N-acetyl-D,L-penicillamine (SNAP), given i.pl., greatly potentiated glutamate-induced nociception and oedema formation. Finally, the i.pl. injection of glutamate was accompanied by a graded increase in the nitrite levels of the hindpaw exudate. It is concluded that the nociception caused by i.pl. injection of glutamate probably involves the activation of NMDA and non-NMDA receptors by a mechanism which largely depends on the activation of L-arginine-nitric oxide pathway. Glutamate-induced paw oedema seems to be primarily mediated by non-NMDA ionotropic glutamate receptors and release of nitric oxide.

Group I metabotropic glutamate receptor antagonists block secondary thermal hyperalgesia in rats with knee joint inflammation

Activation of ionotropic glutamate receptors has been shown previously to be essential for the development of secondary thermal hyperalgesia. The present study assessed involvement of group I metabotropic glutamate receptors (mGlu) in both the induction and maintenance phases of secondary thermal hyperalgesia initiated by knee joint inflammation in rats. The dose dependence of each drug in antagonism of thermal hypersensitivity was demonstrated in pre- and post-treatment paradigms. Knee joint inflammation was induced by injection of kaolin and carrageenan. Four hours later the paw withdrawal latencies were significantly shorter than baseline values. Rats were pretreated by spinal microdialysis infusion of group I mGlu receptor antagonists, LY393053 [(+/-)-2-amino-2-(3-cis and trans-carboxycyclobutyl-3-(9-thioxanthyl)propionic acid], LY367385 [(S)-(+)-alpha-amino-4-carboxy-2-methylbenzeneacetic acid], or AIDA [(R,S)-1-aminoindan-1,5-dicarboxylic acid/UPF 523] before knee joint injection. The paw withdrawal latencies measured 4 h after the injection were significantly longer in the presence of group I mGlu receptor antagonists than those of the artificial cerebrospinal fluid-treated arthritic control group. Post-treatment with the group I mGlu receptor antagonists LY367385 and AIDA allowed significant recovery of the paw withdrawal latencies after the onset of the knee joint inflammation. The knee joint inflammation itself was not affected by either treatment. The results of the present study indicate that secondary thermal hyperalgesia can be effectively attenuated during both the development and maintenance phases of acute knee joint inflammation by spinal application of specific group I mGlu receptor antagonists.

Glutamate uptake inhibition modulates dorsal horn neurotransmission: a comparison between normal and arthritic rats

This study determined the effects of glutamate uptake inhibition on primary-afferent excitatory postsynaptic potentials (DR-EPSPs) in spinal dorsal horn neurones in vitro from naive rats and rats with localised arthritis. The glutamate uptake inhibitor L-PDC (1 mM) significantly reduced DR-EPSP amplitude and duration with a greater reduction in arthritic than in naive rats. The group II/III selective metabotropic glutamate receptor antagonist CPPG (100 microM) reversed L-PDC-induced DR-EPSP inhibition in naive but not arthritic rats. L-AP4 (30 microM), a group III metabotropic agonist, inhibited DR-EPSPs with no difference between naive and arthritic rats. These data suggest the existence of an autoregulatory feedback mechanism that limits spinal postsynaptic excitation especially during inflammation. The putative contribution of metabotropic glutamate receptors to this phenomenon is discussed.

Glutamate release from adult primary sensory neurons in culture is modulated by growth factors

The aim of this study was to examine possible modulatory effects of some trophic molecules, i.e. nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF) and basic fibroblast growth factor (bFGF), on potassium (K(+))-, bradykinin (BK)- or capsaicin (CAPS)-evoked release of glutamate (GLU) from dorsal root ganglion (DRG) neurons in vitro. BK (0.5 and 1 microM) induced a dramatic and significant increase in glutamate release. Neither CAPS nor K(+) (60 mM) produced any significant increase of GLU release vs. basal levels during a 5-min stimulation. The BK-evoked release of GLU was almost completely blocked by HOE 140, a selective BK2-receptor antagonist at high doses. Basal release of GLU was significantly reduced in cultures grown in the presence of bFGF, whereas BDNF and NGF had no significant effect. Incubation with growth factors generally decreased the BK-stimulated GLU release, an effect most pronounced for bFGF, which completely blocked BK-stimulated release. The rise in intracellular [Ca(2+)] following stimulation with BK (100 nM-1 microM), potassium (60 mM) or ATP (10 microM) was also studied using a Ca(2+)-sensitive indicator, Fura-2, in cultures grown in basal medium with or without bFGF. None of the bFGF-treated cells exhibited strong Ca(2+) responses to BK or ATP stimulation, while 10-20% of the responding cells grown in basal medium exhibited strong responses. The K(+)-induced increase of [Ca(2+)] did not vary between the different groups. The present findings suggest that sensory neurotransmission involving glutamate may be modulated by growth factors and that regulation of intracellular Ca(2+) homeostasis may be a contributing factor.

GV196771A, an NMDA receptor/glycine site antagonist, attenuates mechanical allodynia in neuropathic rats and reduces tolerance induced by morphine in mice

The effects of the N-methyl-D-aspartate (NMDA) receptor/glycine site antagonist, GV196771A (E-4,6-dichloro-3-(2-oxo-1-phenyl-pyrrolidin-3-ylidenemethyl)-1H-indole-2-carboxylic acid sodium salt), on mechanical allodynia and on tolerance to the antinociceptive effects induced by morphine were evaluated. Its antiallodynic properties were studied in a model of chronic constriction injury applied to rat sciatic nerve. GV196771A (0.3-10 mg/kg, p.o.) dose-dependently inhibited established mechanical allodynia when tested 14 or 21 days after nerve ligation. In the formalin test in mice, GV196771A (10 or 20 mg/kg, p.o.), administered for 8 days together with morphine 10 mg/kg, i.p. inhibited morphine tolerance development in both early and late phases of the test. This finding reinforces the key role of the NMDA receptors in the plastic event, such as allodynia, which develops in some conditions of painful neuropathy. Moreover, the capability to strongly reduce morphine-induced tolerance suggests that GV196771A could be an alternative agent for the treatment of difficult pain states not only when given alone, but also in combination, in order to prolong the analgesic effects of the opiates.

Amino acids are still as exciting as ever

Glutamate is probably the most important excitatory transmitter in the vertebrate central nervous system. Its multiple functional roles in the brain and spinal cord make therapeutic manipulation of these systems fraught with difficulties. There has, however, been recent progress in pharmacological manipulations of NMDA receptor subtypes and non-NMDA receptors, and understanding of the roles of NAAG, that promise rapid advances in pain control.