6-cyano-7-nitroquinoxaline-2-3-dione and Pain

This study was designed to investigate a possible interaction between 17β‑estradiol and glutamate receptors of the paragigantocellularis lateralis (LPGi) nucleus on pain coping behavior using the formalin test in ovariectomized (OVX) rats. The results showed that intra‑LPGi injection of 17β‑estradiol declined flexing behavior in both phases of the formalin test. Still, it only diminished the late phase of licking behavior in the OVX rats. NMDA receptor antagonist, AP5, reversed the analgesic effect of 17β‑estradiol on flexing behavior in both phases of the formalin test in the OVX rats. The 17β‑estradiol‑induced anti‑nociceptive effect on the flexing duration was prevented by CNQX (AMPA receptor antagonist) only in the early phase of the formalin test in the OVX rats. AP5 and CNQX reduced the anti‑nociceptive effect of 17β‑estradiol in the late phase, but not the early phase of licking response in the OVX rats. These results suggested: (i) The intra‑LPGi injection of 17β‑estradiol is satisfactory in producing modest analgesia on the formalin‑induced inflammatory pain in the OVX rats; (ii) Co‑treatment of glutamate receptors (NMDA and AMPA) antagonists and 17β‑estradiol in the LPGi nucleus decrease the analgesic effect of 17β‑estradiol in the OVX rats; (iii) There is a possible association between 17β‑estradiol and glutamate receptors of the LPGi nucleus on pain coping behavior in the OVX rats.

Topics: 6-Cyano-7-nitroquinoxaline-2,3-dione; Analgesics; Animals; Estradiol; Excitatory Amino Acid Antagonists; Microinjections; Pain; Rats; Receptors, Glutamate; Receptors, N-Methyl-D-Aspartate

Locus coeruleus (LC) neurones extend noradrenergic projections throughout the neuroaxis and are involved in homeostatic functions such as pain modulation, arousal and cardio-respiratory control. To address the cellular mechanisms underlying pain modulation we have developed a patch-clamp recording technique from LC neurones in anaesthetized rats. These recordings showed LC discharge in vivo to be driven by both spontaneous membrane potential oscillations and CNQX-sensitive EPSCs opposed by bicuculine-sensitive IPSCs. Hindlimb pinch evoked a biphasic action potential response underpinned by a slow monophasic excitatory current. This approach allows detailed characterisation of the synaptic and integrative mechanisms of LC responses to naturalistic stimulation.

Topics: 6-Cyano-7-nitroquinoxaline-2,3-dione; Animals; Bicuculline; Excitatory Amino Acid Antagonists; Excitatory Postsynaptic Potentials; GABA-A Receptor Antagonists; Inhibitory Postsynaptic Potentials; Locus Coeruleus; Male; Neurons; Pain; Patch-Clamp Techniques; Rats; Rats, Sprague-Dawley; Receptors, AMPA

To elucidate the mechanisms of antinociception mediated by the dopaminergic descending pathway in the spinal cord, we investigated the actions of dopamine (DA) on substantia gelatinosa (SG) neurons by in vivo whole-cell patch-clamp methods. In the voltage-clamp mode (V(H)=-70mV), the application of DA induced outward currents in about 70% of SG neurons tested. DA-induced outward current was observed in the presence of either Na(+) channel blocker, tetrodotoxin (TTX) or a non-NMDA receptor antagonist, CNQX, and was inhibited by either GDP-β-S in the pipette solution or by perfusion of a non-selective K(+) channel blocker, Ba(2+). The DA-induced outward currents were mimicked by a selective D2-like receptor agonist, quinpirole and attenuated by a selective D2-like receptor antagonist, sulpiride, indicating that the DA-induced outward current is mediated by G-protein-activated K(+) channels through D2-like receptors. DA significantly suppressed the frequency and amplitude of glutamatergic spontaneous excitatory postsynaptic currents (EPSCs). DA also significantly decreased the frequency of miniature EPSCs in the presence of TTX. These results suggest that DA has both presynaptic and postsynaptic inhibitory actions on synaptic transmission in SG neurons. We showed that DA produced direct inhibitory effects in SG neurons to both noxious and innocuous stimuli to the skin. Furthermore, electrical stimulation of dopaminergic diencephalic spinal neurons (A11), which project to the spinal cord, induced outward current and suppressed the frequency and amplitude of EPSCs. We conclude that the dopaminergic descending pathway has an antinociceptive effect via D2-like receptors on SG neurons in the spinal cord.

Topics: 2,3,4,5-Tetrahydro-7,8-dihydroxy-1-phenyl-1H-3-benzazepine; 6-Cyano-7-nitroquinoxaline-2,3-dione; Action Potentials; Afferent Pathways; Animals; Barium Compounds; Chlorides; Dopamine; Dopamine Agents; Drug Interactions; Excitatory Amino Acid Antagonists; Excitatory Postsynaptic Potentials; Guanosine Diphosphate; Male; Nociceptors; Pain; Patch-Clamp Techniques; Physical Stimulation; Quinpirole; Rats; Rats, Sprague-Dawley; Skin; Sodium Channel Blockers; Spinal Cord; Substantia Gelatinosa; Tetrodotoxin; Thionucleotides

Mechanisms of inflammatory pain are not fully understood. We investigated the role of TRPV1 (transient receptor potential subtype V1) and TNF-α, two critical mediators for inflammatory pain, in regulating spinal cord synaptic transmission. We found in mice lacking Trpv1 the frequency but not the amplitude of spontaneous EPSCs (sEPSCs) in lamina II neurons of spinal cord slices is reduced. Further, C-fiber-induced spinal long-term potentiation (LTP) in vivo is abolished in Trpv1 knock-out mice. TNF-α also increases sEPSC frequency but not amplitude in spinal outer lamina II (lamina IIo) neurons, and this increase is abolished in Trpv1 knock-out mice. Single-cell PCR analysis revealed that TNF-α-responding neurons in lamina IIo are exclusively excitatory (vGluT2(+)) neurons. Notably, neuroprotectin-1 (NPD1), an anti-inflammatory lipid mediator derived from ω-3 polyunsaturated fatty acid (docosahexaenoic acid), blocks TNF-α- and capsaicin-evoked sEPSC frequency increases but has no effect on basal synaptic transmission. Strikingly, NPD1 potently inhibits capsaicin-induced TRPV1 current (IC(50) = 0.4 nm) in dissociated dorsal root ganglion neurons, and this IC(50) is ≈ 500 times lower than that of AMG9810, a commonly used TRPV1 antagonist. NPD1 inhibition of TRPV1 is mediated by GPCRs, since the effects were blocked by pertussis toxin. In contrast, NPD1 had no effect on mustard oil-induced TRPA1 currents. Spinal injection of NPD1, at very low doses (0.1-10 ng), blocks spinal LTP and reduces TRPV1-dependent inflammatory pain, without affecting baseline pain. NPD1 also reduces TRPV1-independent but TNF-α-dependent pain hypersensitivity. Our findings demonstrate a novel role of NPD1 in regulating TRPV1/TNF-α-mediated spinal synaptic plasticity and identify NPD1 as a novel analgesic for treating inflammatory pain.

Topics: 6-Cyano-7-nitroquinoxaline-2,3-dione; Acrylamides; Analysis of Variance; Animals; Bridged Bicyclo Compounds, Heterocyclic; Cells, Cultured; Disease Models, Animal; Docosahexaenoic Acids; Enzyme Inhibitors; Excitatory Amino Acid Antagonists; Excitatory Postsynaptic Potentials; Freund's Adjuvant; Ganglia, Spinal; In Vitro Techniques; Inflammation; Long-Term Potentiation; Mice; Mice, Inbred C57BL; Mice, Knockout; Neurons; Pain; Pain Measurement; Patch-Clamp Techniques; Receptors, Tumor Necrosis Factor; Receptors, Tumor Necrosis Factor, Type I; Spinal Cord; TRPV Cation Channels; Tumor Necrosis Factor-alpha

Estradiol is a neuroactive steroid found in several brain areas such as locus coeruleus (LC). It modulates nociception by binding to its receptors and also by allosteric interaction with other membrane-bound receptors like glutamate and GABA(A) receptors. LC is involved in noradrenergic descending pain modulation. In order to study the effect of 17beta-estradiol on both acute and persistent pain modulation and its mechanisms, formalin was injected into the male rat's hind paw. Formalin-induced responses including licking, flexing duration and paw jerking frequency were recorded for 60 min after injection of 50 microl of 2% formalin. The results of the current study showed that intra-locus coeruleus injection of 17beta-estradiol attenuated the second phase, but not the acute phase of formalin-induced pain (P<0.05). AMPA receptor antagonists CNQX had no effect on pain-modulatory effect of 17beta-estradiol. Estrogen and GABA(A) receptor antagonists (ICI 182,780 and bicuculline, respectively) could not reverse the antinociceptive effect of 17beta-estradiol. However, NMDA receptor antagonist APV significantly antagonized the analgesic effect of 17beta-estradiol on flexing behaviour (P<0.05). It may be concluded that the analgesic effect of 17beta-estradiol in formalin-induced inflammatory pain is mediated through interaction with membrane-bound receptors, probably the NMDA receptors.

Topics: 6-Cyano-7-nitroquinoxaline-2,3-dione; Animals; Bicuculline; Disease Models, Animal; Estradiol; Estrogen Antagonists; Estrogens; Formaldehyde; Fulvestrant; Inflammation; Locus Coeruleus; Male; Microinjections; Pain; Rats; Rats, Sprague-Dawley; Valine

The anterior cingulate cortex (ACC) has been demonstrated to play an important role in the affective dimension of pain. Although much evidence has pointed to an increased excitatory synaptic transmission in the ACC in some of the pathological pain state, the inhibitory synaptic transmission in this process has not been well studied. Also, the overall changes of excitatory and inhibitory synaptic transmission have not been comparatively studied in an animal model displaying both long-term persistent nociception and hyperalgesia. Here we used patch clamp recordings in ACC brain slices to observe the changes in synaptic transmission in a pain model induced by peripheral bee venom injection. First, we show that, comparing with those of naive and saline controlled rats, there was a significant increase in spike frequency in ACC neurons harvested from rats after 2 h period of peripheral persistent painful stimuli. Second, it is further shown that the frequency, amplitude and half-width were all increased in spontaneous excitatory post-synaptic currents (sEPSCs), while the amplitude of spontaneous inhibitory post-synaptic currents (sIPSCs) was decreased. The recordings of miniature post-synaptic currents demonstrate an increase in frequency of miniature excitatory post-synaptic currents (mEPSCs) and a decrease in both frequency and amplitude of miniature inhibitory post-synaptic currents (mIPSCs) in rats' ACC slice of bee venom treatment. Taken together, the present results demonstrate an unparalleled change between excitatory and inhibitory synaptic transmission in the ACC under a state of peripheral persistent nociception that might be underlying mechanisms of the excessive excitability of the ACC neurons. We propose that the painful stimuli when lasts or becomes persistent may cause a disruption of the balance between excitatory and inhibitory synaptic transmission that can contribute to the functional change in the ACC.

Topics: 6-Cyano-7-nitroquinoxaline-2,3-dione; Action Potentials; Analysis of Variance; Animals; Bee Venoms; Bicuculline; Biophysics; Disease Models, Animal; Electric Stimulation; Excitatory Amino Acid Antagonists; GABA-A Receptor Antagonists; Gyrus Cinguli; Male; Neural Inhibition; Neurons; Pain; Patch-Clamp Techniques; Rats; Rats, Sprague-Dawley; Sodium Channel Blockers; Synaptic Transmission; Tetrodotoxin

Pain is known to be processed by a complex neural network (neuromatrix) in the brain. It is hypothesized that under pathological state, persistent or chronic pain can affect various higher brain functions through ascending pathways, leading to co-morbidities or mental disability of pain. However, so far the influences of pathological pain on the higher brain functions are less clear and this may hinder the advances in pain therapy. In the current study, we studied spatiotemporal plasticity of synaptic connection and function in the hippocampal formation (HF) in response to persistent nociception.. On the hippocampal slices of rats which had suffered from persistent nociception for 2 h by receiving subcutaneous bee venom (BV) or formalin injection into one hand paw, multisite recordings were performed by an 8 x 8 multi-electrode array probe. The waveform of the field excitatory postsynaptic potential (fEPSP), induced by perforant path electrical stimulation and pharmacologically identified as being activity-dependent and mediated by ionotropic glutamate receptors, was consistently positive-going in the dentate gyrus (DG), while that in the CA1 was negative-going in shape in naïve and saline control groups. For the spatial characteristics of synaptic plasticity, BV- or formalin-induced persistent pain significantly increased the number of detectable fEPSP in both DG and CA1 area, implicating enlargement of the synaptic connection size by the injury or acute inflammation. Moreover, the input-output function of synaptic efficacy was shown to be distinctly enhanced by the injury with the stimulus-response curve being moved leftward compared to the control. For the temporal plasticity, long-term potentiation produced by theta burst stimulation (TBS) conditioning was also remarkably enhanced by pain. Moreover, it is strikingly noted that the shape of fEPSP waveform was drastically deformed or split by a TBS conditioning under the condition of persistent nociception, while that in naïve or saline control state was not affected. All these changes in synaptic connection and function, confirmed by the 2-dimentional current source density imaging, were found to be highly correlated with peripheral persistent nociception since pre-blockade of nociceptive impulses could eliminate all of them. Finally, the initial pharmacological investigation showed that AMPA/KA glutamate receptors might play more important roles in mediation of pain-associated spatiotemporal plasticity than NMDA receptors.. Peripheral persistent nociception produces great impact upon the higher brain structures that lead to not only temporal plasticity, but also spatial plasticity of synaptic connection and function in the HF. The spatial plasticity of synaptic activities is more complex than the temporal plasticity, comprising of enlargement of synaptic connection size at network level, deformed fEPSP at local circuit level and, increased synaptic efficacy at cellular level. In addition, the multi-synaptic model established in the present investigation may open a new avenue for future studies of pain-related brain dysfunctions at the higher level of the neuromatrix.

Topics: 6-Cyano-7-nitroquinoxaline-2,3-dione; Animals; Electrodes; Electrophysiology; Excitatory Postsynaptic Potentials; Hippocampus; In Vitro Techniques; Long-Term Potentiation; Male; Neuronal Plasticity; Nociceptors; Pain; Perforant Pathway; Rats; Rats, Sprague-Dawley; Receptors, AMPA; Receptors, N-Methyl-D-Aspartate; Synaptic Transmission; Time Factors

The present study investigated the involvement of spinal glutamate receptors in the induction and maintenance of the pain-related behaviors induced by the venom of scorpion Buthus martensi Karsch (BmK). (5R,10S)-(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]-cyclohepten-5-10-imine hydrogen maleate (MK-801; 40nmol; a non-competitive NMDA receptor antagonist), 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 40nmol; a non-NMDA receptor antagonist), dl-amino-3-phosphonopropionic acid (dl-AP3; 100nmol; a group I metabotropic glutamate receptor antagonist) and 4-aminopyrrolidine-2,4-dicarboxylate (APDC; 100nmol; a group II metabotropic glutamate receptor agonist) were employed. On intrathecal injection of glutamate receptor antagonists/agonist before BmK venom administration by 10min, BmK venom-induced spontaneous nociceptive responses could be suppressed by all tested agents. Primary thermal hyperalgesia could be inhibited by MK-801 and dl-AP3, while bilateral mechanical hyperalgesia could be inhibited by CNQX and dl-AP3 and contralateral mechanical hyperalgesia could be inhibited by APDC. On intrathecal injection of glutamate receptor antagonists/agonist after BmK venom injection by 4.5h, primary thermal hyperalgesia could be partially reversed by all tested agents, while bilateral mechanical hyperalgesia could only be inhibited by APDC. The results suggest that the role of spinal glutamate receptors may be different on the various manifestations of BmK venom-induced pain-related behaviors.

Topics: 6-Cyano-7-nitroquinoxaline-2,3-dione; Alanine; Animals; Dizocilpine Maleate; Excitatory Amino Acid Antagonists; Injections, Spinal; Male; Pain; Proline; Rats; Rats, Sprague-Dawley; Receptors, Glutamate; Receptors, N-Methyl-D-Aspartate; Scorpion Venoms

Considerable evidence has demonstrated that alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor blockade has an antinociceptive effect on inflammatory pain. Stargazin (STG) is the first transmembrane protein known to associate with AMPA receptors and regulate their synaptic targeting. However, it is not known whether STG is involved in inflammatory pain processing by regulating AMPA receptor function. In the present study, we investigated the effect of knockdown of spinal STG on AMPA receptor-mediated pain sensitization after inflammation. Antisense technology was employed to knock down STG expression in the spinal cord. We show that STG was expressed and interacted with AMPA receptor subunit GluR2 in the spinal cord. Intrathecally injected STG antisense oligodeoxyribonucleotide (ODN) specifically decreased STG expression in the lumbar spinal cord and dose dependently inhibited formalin-induced inflammatory pain in the second phase. More important was our finding for the first time that this specific STG antisense ODN diminished AMPA (0.1 mug)-enhanced formalin pain and lost its effect if pretreated with AMPA receptor antagonist CNQX. Our results demonstrate a new role for STG in central sensitization of inflammatory pain by interacting with AMPA receptors in the spinal cord.

Topics: 6-Cyano-7-nitroquinoxaline-2,3-dione; alpha-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic Acid; Analysis of Variance; Animals; Calcium Channels; Disease Models, Animal; Disks Large Homolog 4 Protein; Dose-Response Relationship, Drug; Excitatory Amino Acid Agonists; Excitatory Amino Acid Antagonists; Fluorescent Antibody Technique; Gene Expression; Gene Expression Regulation; Immunoprecipitation; Inflammation; Intracellular Signaling Peptides and Proteins; Male; Membrane Proteins; Motor Activity; Oligonucleotides, Antisense; Pain; Pain Measurement; Rats; Rats, Sprague-Dawley; Receptors, AMPA; Spinal Cord

Recent work has demonstrated that a brief high-frequency conditioning stimulation to the primary afferent nerve fibers can induce a long-term potentiation (LTP) of synaptic transmission in neurons in the superficial layer of the trigeminal caudal nucleus; however, the cellular and molecular mechanisms underlying this synaptic potentiation remain unclear. Using both extracellular field potential and whole-cell patch-clamp recordings in brainstem parasagital slices of juvenile rat with the mandibular nerve attached, we show here that the induction of trigeminal primary afferent LTP: (1) does not require the activation of ionotropic glutamate receptors; (2) is dependent on extracellular Ca(2+) and the release of Ca(2+) from intracellular stores; (3) is specifically prevented by the metabotropic glutamate receptor subtype 5 (mGluR5) antagonist 2-methyl-6-(phenylethynyl)pyridine but not the mGluR1 antagonist LY367385, group II mGluR antagonist LY341495 or group III mGluR antagonist MAP4; (4) is mimicked by the bath-applied group I mGluR agonist (S)-3,5-dihydroxyphenylglycine and mGluR5 agonist (RS)-2-chloro-5-hydroxyphenylglycine; (5) requires the activation of phospholipase C (PLC) and protein kinase C (PKC); and (6) is concomitantly with a decrease in paired-pulse depression. These results demonstrate that the activation of mGluR5 and in turn triggering a PLC/PKC-dependent signaling cascade may contribute to the induction of LTP of primary afferent synaptic transmission in the superficial layer of trigeminal caudal nucleus of juvenile rats. This may be relevant to the processing of nociceptive information.

Topics: 2-Amino-5-phosphonovalerate; 6-Cyano-7-nitroquinoxaline-2,3-dione; Afferent Pathways; Animals; Calcium; Excitatory Amino Acid Antagonists; Excitatory Postsynaptic Potentials; Long-Term Potentiation; Male; Pain; Presynaptic Terminals; Protein Kinase C; Rats; Rats, Sprague-Dawley; Receptor, Metabotropic Glutamate 5; Receptors, Metabotropic Glutamate; Synapses; Trigeminal Caudal Nucleus; Type C Phospholipases

Chronic opioid-induced drug dependence and withdrawal syndrome after opioid cessation remain a severe obstacle in clinical treatment of chronic pain and opioid drug addiction. One of the key symptoms during opioid withdrawal is a state of sensitized pain. The most significant molecular adaptation induced by chronic opioids in the brain is upregulation of the cAMP-signaling pathway. Although the cAMP system is known to have multiple effects on central neuron functions, how its upregulation mediates behavioral opioid dependence and withdrawal-induced pain in vivo remains unclear. In this study, we demonstrate that withdrawal from chronic morphine significantly upregulates the mRNA level of adenylyl cyclase (AC) VI and VIII isoforms and immunoreactivity of ACV/VI in the nucleus raphe magnus (NRM), a brainstem site critically involved in opioid modulation of pain. In cellular studies of NRM neurons containing mu-opioid receptors, we show that morphine withdrawal significantly increases glutamate synaptic transmission via a presynaptic mechanism mediated by an upregulated cAMP pathway. Morphine withdrawal also enhances the hyperpolarization-activated current in these neurons by increased intracellular cAMP. Both of the withdrawal-induced cAMP actions increase the excitability of these mu-receptor-containing neurons, which are thought to facilitate spinal pain transmission. Furthermore, in morphine-dependent rats in vivo, blocking the cAMP pathway significantly reduces withdrawal-induced pain sensitization. These results illustrate neurobiological mechanisms for the cAMP-mediated withdrawal pain and provide potential therapeutic targets for the treatment of opioid dependence and withdrawal-related problems.

Topics: 2-Amino-5-phosphonovalerate; 6-Cyano-7-nitroquinoxaline-2,3-dione; 8-Bromo Cyclic Adenosine Monophosphate; Adenylyl Cyclases; Animals; Animals, Newborn; Behavior, Animal; Blotting, Western; Colforsin; Cyclic AMP; Dose-Response Relationship, Radiation; Drug Administration Schedule; Drug Interactions; Electric Stimulation; Enzyme Inhibitors; Excitatory Amino Acid Antagonists; Excitatory Postsynaptic Potentials; Gene Expression Regulation; Imines; In Vitro Techniques; Isoquinolines; Male; Membrane Potentials; Microinjections; Morphine; Naloxone; Narcotic Antagonists; Narcotics; Opioid-Related Disorders; Oxidoreductases; Pain; Pain Measurement; Patch-Clamp Techniques; Pyrimidines; Raphe Nuclei; Rats; Rats, Wistar; Reverse Transcriptase Polymerase Chain Reaction; RNA, Messenger; Sulfonamides

Intrathecal pretreatment with N-methyl-D-aspartate (NMDA) receptor antagonists blocks development of spinal sensitization in a number of pain models. In contrast, secondary mechanical allodynia evoked by thermal injury (52.5 degrees C for 45 s) applied to the hind paw of the rat is not blocked by intrathecal pretreatment with NMDA receptor antagonists. It is, however, blocked by antagonists to the non-NMDA, alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid/kainate (AMPA/KA) and calcium-permeable AMPA/KA receptors. These findings suggest a role for these receptors in the development of spinal sensitization. The present study used the same thermal injury model to assess the effects of the AMPA/KA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and specific calcium-permeable AMPA/KA receptor antagonists philanthotoxin (PHTx) and joro spider toxin (JST) when given as postinjury treatments. Intrathecal saline injection at 5 and 30 min postinjury had no effect on thermal injury-evoked allodynia as measured by calibrated von Frey filaments. In contrast, 36 nmol of CNQX given at either time point reversed allodynia. Intrathecal 13 nmol of PHTx or 9 nmol of JST (higher doses than that required for pretreatment) reversed allodynia at the 5-min time point, but neither drug was antiallodynic at the 30-min time point. Thus, secondary mechanical allodynia in this model is not maintained by calcium-permeable AMPA/KA receptors, but instead requires activation of calcium-impermeable AMPA/KA receptors. This finding supports a role for AMPA/KA receptor function in responses occurring during spinal sensitization.

Topics: 6-Cyano-7-nitroquinoxaline-2,3-dione; alpha-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic Acid; Animals; Burns; Calcium; Disease Models, Animal; Excitatory Amino Acid Antagonists; Injections, Spinal; Kainic Acid; Motor Cortex; Pain; Pain Threshold; Rats; Receptors, AMPA; Receptors, Kainic Acid; Spider Venoms

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.

Topics: 2-Amino-5-phosphonovalerate; 6-Cyano-7-nitroquinoxaline-2,3-dione; Animals; DNA-Binding Proteins; Drug Combinations; Early Growth Response Protein 1; Excitatory Amino Acid Antagonists; Formaldehyde; Hot Temperature; Immediate-Early Proteins; Injections; Male; Pain; Physical Stimulation; Proto-Oncogene Proteins c-fos; Rats; Rats, Sprague-Dawley; Receptors, AMPA; Receptors, Kainic Acid; Receptors, N-Methyl-D-Aspartate; Spinal Cord; Transcription Factors

The present study examined the effect of a spinally administered excitatory amino acid antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 1, 2.5, 5 microg) on responses of spinal dorsal horn neurons to graded intensities (20, 40, 60, 80 mmHg) of colorectal distention (CRD). Extracellular single unit recordings were made from 28 dorsal horn neurons in the L6-S2 spinal cord. Neurons excited by CRD were subclassified as short latency abrupt (SLA) neurons and short latency sustained (SLS) neurons. The response to graded intensities of CRD was dose-dependently attenuated in 9/17 SLA neurons (53%). The response to CRD was also dose-dependently attenuated in 8/11 SLS neurons (73%). The response to CRD in the remaining eight SLA neurons and three SLS neurons was not attenuated by CNQX. Comparing only neurons that were significantly attenuated by the CNQX, it was found that the magnitude of attenuation of the response to noxious CRD (80 mmHg) produced by 5 microg CNQX was significantly greater in SLA (63 +/-6%) vs. SLS (40 +/- 6%) neurons. While CNQX produced a significant attenuation of the response to innocuous CRD (20 mmHg), there was no difference between the SLA and SLS neurons. The effects of CNQX on the response to somatic stimulation (touch, pinch) of the cutaneous receptive field of these 28 neurons were qualitatively examined in all neurons and quantitatively examined in nine neurons (five SLA and four SLS neurons). CNQX generally decreased the response to pinch or touch, even if CNQX did not attenuate the response to CRD. These results suggest that subpopulations of SLA and SLS neurons are differentially modulated by non-NMDA ionotropic excitatory amino acid receptors and that these neuronal subtypes contribute differently to visceral sensory processing. Furthermore, the lack of correlation between the effects of CNQX on visceral and somatic sensory processing in the same neuron underscores potential differences in processing of visceral and somatic pain.

Topics: 6-Cyano-7-nitroquinoxaline-2,3-dione; Animals; Colon; Dilatation; Excitatory Amino Acid Antagonists; Male; Pain; Posterior Horn Cells; Rats; Rats, Sprague-Dawley; Receptors, AMPA; Receptors, Kainic Acid; Rectum; Visceral Afferents

We investigated the effect of topical application of capsaicin cream on withdrawal latency in the hind foot of rat in response to radiant heat in an experimental model of neuropathic pain. A neuropathic state was induced by loose ligation of the sciatic nerve with chromic gut suture. A marked thermal hyperalgesia was observed in response to heat stimulus applied to the operated side from 3 days through 2 weeks, followed by a gradual return to the control level by 35 days after surgery. Capsaicin cream applied to both the bilateral hind instep and sole once a day for a continuous period of 2 weeks or 4 weeks alleviated thermal hyperalgesia in a dose-dependent manner. A remarkable effect was observed 2 weeks after the start of the application and this effect proved to be reversible. On the other hand, in sham-operated animals when capsaicin cream was applied once daily from day 7 after the sham operation, from 1 day through 3 weeks following capsaicin application, withdrawal latency of the sham-operated paws of the capsaicin-treated group was significantly increased as compared to that of the vehicle cream-treated group. The effects of antagonists of glutamate receptor and tachykinin receptors were investigated 7 days post surgery. Pretreatment with MK-801 (0.5 mg kg(-1), i.p.), but not with CNQX (0.5 mg kg(-1), i.p.), reversed the thermal hyperalgesia following nerve injury. Neither of RP67580 (1--10 mg kg(-1), i.p.) nor SR48968 (1--10 mg kg(-1), i.p.) had any effect on the withdrawal latency in the injured and non-injured hind paw. These results suggest that although the manifestation of effectiveness may be delayed by changes in networks of neurotransmitters related to the nociceptive pathways following nerve injury, longer-term repetitive application of capsaicin cream has a significant therapeutic effect on subjects with painful peripheral neuropathy.

Topics: 6-Cyano-7-nitroquinoxaline-2,3-dione; Analgesics; Animals; Benzamides; Capsaicin; Carrageenan; Disease Models, Animal; Dizocilpine Maleate; Excitatory Amino Acid Antagonists; Hyperalgesia; Indoles; Isoindoles; Male; Neuroprotective Agents; Pain; Peripheral Nervous System Diseases; Piperidines; Postoperative Period; Rats; Rats, Sprague-Dawley; Sciatic Nerve; Substance Withdrawal Syndrome; Time Factors

Spinal nociceptive transmission is mediated by glutamate and neuropeptides such as substance P (SP) and neurokinin A (NKA). The neuropeptide-mediated excitatory postsynaptic potentials (EPSPs) had a slow onset and long duration. Here, we demonstrate SP- and NKA-mediated excitatory postsynaptic currents (EPSCs) in dorsal horn neurons of young rats using whole-cell patch-clamp recording techniques. After complete blockade of glutamate receptor-mediated currents, we observed a small residual EPSC. The residual EPSCs exhibited temporal summation in response to a train of stimulation (six pulses delivered at 10-50 Hz). High intensity stimulation (the same or greater than the stimulation threshold for nociceptive fibers in vivo) was required for evoking these summated EPSCs. Summated EPSCs were attenuated or abolished by capsaicin pretreatment, which depletes SP and NKA from presynaptic terminals; SP and NKA pretreatment; NK(1) or NK(2) receptor antagonists; and inhibition of postsynaptic G proteins. EPSCs were neither blocked by a metabotropic glutamate receptor antagonist nor a gamma-aminobutyric acid(B) receptor antagonist. The summated EPSCs were also sensitive to voltage-gated calcium channel antagonists or mu-opioid receptor activation by DAMGO. The present study provides electrophysiological evidence that suggests the possible contribution of SP and NKA to sensory synaptic transmission between primary afferent fibers and dorsal horn neurons.

Topics: 2-Amino-5-phosphonovalerate; 6-Cyano-7-nitroquinoxaline-2,3-dione; Age Factors; Analgesics, Opioid; Animals; Calcium Channel Blockers; Calcium Channels, P-Type; Enkephalin, Ala(2)-MePhe(4)-Gly(5)-; Excitatory Amino Acid Antagonists; Excitatory Postsynaptic Potentials; GABA Antagonists; Guanosine Diphosphate; In Vitro Techniques; Neurokinin A; omega-Conotoxin GVIA; Pain; Patch-Clamp Techniques; Phosphinic Acids; Posterior Horn Cells; Propanolamines; Rats; Rats, Sprague-Dawley; Receptors, Glutamate; Receptors, Neurokinin-1; Receptors, Neurokinin-2; Substance P; Synaptic Transmission; Thionucleotides

GABAergic neurons play an important role in the generation of primary afferent depolarization, which results in presynaptic inhibition and, if large enough, triggers dorsal root reflexes. Recent electrophysiological studies by our group have suggested that increased excitation of spinal GABAergic neurons by activation of N-methyl-D-aspartate (NMDA) and non-NMDA receptors following intradermal injection of capsaicin results in the generation of DRRs that contribute to neurogenic inflammation. The present study was to determine if changes in the expression of Fos protein occur in GABAergic neurons in the lumbosacral spinal cord following injection of capsaicin into the glabrous skin of one hind paw of anesthetized rats and if pretreatment with an NMDA receptor antagonist, D-(-)-2-amino-7-phosphonoheptanoic acid (AP7) or a non-NMDA receptor antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) blocks Fos expression in these neurons. The experiments used western blots and immunofluorescence double labeling staining following capsaicin or vehicle injection. Western blots showed that Fos protein was increased on the ipsilateral side in spinal cord tissue 0.5 h after capsaicin injection. Pretreatment with AP7 or CNQX caused a decrease in capsaicin-induced Fos expression. Immunofluorescence double labeling showed that the proportion of Fos-positive GABAergic neuronal profiles was significantly increased following capsaicin injection (48.8+/-4.8%) compared to the vehicle injection (23.8+/-5.1%) in superficial laminae on the ipsilateral side in lumbosacral spinal cord (P<0.05). However, when the spinal cord was pretreated with AP7 (5 microg) or CNQX (0.2 microg), only 9.1+/-0.6% or 7.1+/-0.8% of GABA-immunoreactive neuronal profiles were stained for Fos following capsaicin injection. The blockade of the capsaicin-evoked Fos staining was dose-dependent. These findings suggest that GABAergic neurons take part in dorsal horn circuits that modulate nociceptive information and that the function of GABAergic neurons following capsaicin injection is partially mediated by NMDA and non-NMDA receptors.

Topics: 2-Amino-5-phosphonovalerate; 6-Cyano-7-nitroquinoxaline-2,3-dione; Animals; Blotting, Western; Capsaicin; Dose-Response Relationship, Drug; Excitatory Amino Acid Antagonists; Fluorescent Antibody Technique; Functional Laterality; gamma-Aminobutyric Acid; Inflammation; Injections, Intradermal; Interneurons; Male; Membrane Potentials; Neural Conduction; Neural Inhibition; Nociceptors; Pain; Posterior Horn Cells; Proto-Oncogene Proteins c-fos; Rats; Rats, Sprague-Dawley; Receptors, N-Methyl-D-Aspartate; Up-Regulation

Glutamate and non-N-methyl-D-aspartate (NMDA) receptors have been implicated in the development of neuroplasticity in the spinal cord in neuropathic pain. The spinal cord has been identified as one of the sites of the analgesic action of gabapentin. In the current study, the authors determined the antiallodynic effect of intrathecal 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) in a rat model of neuropathic pain. Also tested was a hypothesis that intrathecal injection of CNQX and gabapentin produces a synergistic effect on allodynia in neuropathic rats.. Allodynia was produced in rats by ligation of the left L5 and L6 spinal nerves. Allodynia was determined by application of von Frey filaments to the left hind paw. Through an implanted intrathecal catheter, 10-100 microg gabapentin or 0.5-8 microg CNQX disodium (a water-soluble formulation of CNQX) was injected in conscious rats. Isobolographic analysis was performed comparing the interaction of intrathecal gabapentin and CNQX using the ED50 dose ratio of 15:1.. Intrathecal treatment with gabapentin or CNQX produced a dose-dependent increase in the withdrawal threshold to mechanical stimulation. The ED50 for gabapentin and CNQX was 45.9+/-4.65 and 3.4+/-0.22 microg, respectively. Intrathecal injection of a combination of CNQX and gabapentin produced a strong synergistic antiallodynic effect in neuropathic rats.. This study shows that intrathecal administration of CNQX exhibits an antiallodynic effect in this rat model of neuropathic pain. Furthermore, CNQX and gabapentin, when combined intrathecally, produce a potent synergistic antiallodynic effect on neuropathic pain in spinal nerve-ligated rats.

Topics: 6-Cyano-7-nitroquinoxaline-2,3-dione; Acetates; Amines; Animals; Anticonvulsants; Cyclohexanecarboxylic Acids; Dose-Response Relationship, Drug; Drug Synergism; Excitatory Amino Acid Antagonists; Gabapentin; gamma-Aminobutyric Acid; Injections, Spinal; Ligation; Male; Pain; Pain Threshold; Rats; Rats, Sprague-Dawley; Spinal Nerves

The effect of (+/-)-5-methyl-10,11-dihydro-5H-dibenzo(a,d) cyclohepten-5, 10-imine maleate (MK-801) or 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) injected intrathecally (i.t.) on the inhibition of the tail-flick response induced by morphine, D-Ala(2)-NmePhe(4)-Gly-ol-enkephalin (DAMGO), beta-endorphin, D-Pen(2,5)-enkephalin (DPDPE), or ¿(trans-3, 4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl) cyclohexyl] benzeocetamide)¿ (U50, 488H) administered i.t. was studied in ICR mice. The i.t. injection of MK-801 (2 microg) or CNQX (1 microg) alone did not affect the basal tail-flick response. Morphine (0.2 microg), DAMGO (0.8 ng), beta-endorphin (0.1 microg), DPDPE (0.5 microg) or U50, 488H (6 microg) caused only slight inhibition of the tail-flick response. CNQX injected i.t., but not MK-801, enhanced the inhibition of the tail-flick response induced by i.t. administered morphine, DAMGO, DPDPE or U50, 488H. However, CNQX or MK-801 injected i.t. was not effective in enhancing the inhibition of the tail-flick response induced by beta-endorphin administered i.t. The potentiating effect of CNQX on tail-flick inhibition induced by morphine, DAMGO, DPDPE or U50, 488H was blocked by naloxone (from 1 to 20 microg), yohimbine (from 1 to 20 microg) or methysergide (from 1 to 20 microg) injected i.t. in a dose-dependent manner. Our results suggest that the blockade of AMPA/kainate receptors located in the spinal cord appears to be involved in enhancing the inhibition of the tail-flick response induced by stimulation of spinal mu-, delta-, and kappa-opioid receptors. Furthermore, this potentiating action may be mediated by spinal noradrenergic and serotonergic receptors. However, N-methyl-D-aspartate receptors may not be involved in modulating the inhibition of the tail-flick response induced by various opioids administered spinally.

Topics: 3,4-Dichloro-N-methyl-N-(2-(1-pyrrolidinyl)-cyclohexyl)-benzeneacetamide, (trans)-Isomer; 6-Cyano-7-nitroquinoxaline-2,3-dione; Analgesics, Opioid; Animals; beta-Endorphin; Dizocilpine Maleate; Enkephalin, Ala(2)-MePhe(4)-Gly(5)-; Enkephalin, D-Penicillamine (2,5)-; Excitatory Amino Acid Antagonists; Injections, Spinal; Male; Mice; Mice, Inbred ICR; Pain; Spinal Cord; Time Factors

We found that spinorphin, a novel neuropeptide showed analgesia in a different manner compared with morphine. By measuring flexor responses induced by the intraplanter injection of substances, the presence of three different types of sensory neurons were demonstrated. Although spinorphin completely blocked 2-metylthioadenosine (2-MeS ATP, a P2X(3) agonist)-induced responses, morphine did not. On the other hand, morphine-induced blockade of bradykinin (BK, a B(2)-receptor agonist)-responses was attenuated by pertussis toxin (PTX) treatment, whereas that of spinorphin was not. Thus it is suggested that spinorphin has a spectrum of analgesia which covers the blockade of nociception insensitive to morphine.

Topics: 6-Cyano-7-nitroquinoxaline-2,3-dione; Adenosine; Analgesics, Opioid; Animals; Animals, Newborn; Bradykinin; Capsaicin; Dizocilpine Maleate; Dose-Response Relationship, Drug; Excitatory Amino Acid Antagonists; Male; Mice; Morphine; Neurons, Afferent; Neuroprotective Agents; Oligopeptides; Pain; Pertussis Toxin; Piperidines; Protease Inhibitors; Purinergic Agonists; Thionucleosides; Time Factors; Virulence Factors, Bordetella

Glutamate, the major excitatory neurotransmitter in the central nervous system, activates three different receptors that directly gate ion channels, namely receptors for AMPA (alpha-amino-3-hydroxy-5-methyl isoxozole propionic acid), NMDA (N-methyl-D-aspartate), and kainate, a structural analogue of glutamate. The contribution of AMPA and NMDA receptors to synaptic transmission and plasticity is well established. Recent work on the physiological function of kainate receptors has focused on the hippocampus, where repetitive activation of the mossy-fibre pathway generates a slow, kainate-receptor-mediated excitatory postsynaptic current (EPSC). Here we show that high-intensity single-shock stimulation (of duration 200 microseconds) of primary afferent sensory fibres produces a fast, kainate-receptor-mediated EPSC in the superficial dorsal horn of the spinal cord. Activation of low-threshold afferent fibres generates typical AMPA-receptor-mediated EPSCs only, indicating that kainate receptors may be restricted to synapses formed by high-threshold nociceptive (pain-sensing) and thermoreceptive primary afferent fibres. Consistent with this possibility, kainate-receptor-mediated EPSCs are blocked by the analgesic mu-opiate-receptor agonist Damgo and spinal blockade of both kainate and AMPA receptors produces antinociception. Thus, spinal kainate receptors contribute to transmission of somatosensory inputs from the periphery to the brain.

Topics: 6-Cyano-7-nitroquinoxaline-2,3-dione; alpha-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic Acid; Animals; Cells, Cultured; Excitatory Postsynaptic Potentials; Hippocampus; In Vitro Techniques; Neurons, Afferent; Pain; Rats; Receptors, Kainic Acid; Spinal Cord; Synaptic Transmission

Non-NMDA glutamate receptor antagonists produce antinociceptive effects, but the antinociceptive interaction between non-NMDA glutamate receptor antagonists and local anesthetics has not been demonstrated. We designed this study to evaluate the antinociceptive effects of a non-NMDA glutamate receptor antagonist and its interaction with lidocaine in rats. Intrathecal catheters were implanted at the L4-5 level in rats. The tail flick (TF) and colorectal distension (CD) tests were used to assess somatic and visceral antinociceptive effects, respectively. The TF latency and CD threshold were measured before and for 180 min after the intrathecal administration of lidocaine (20-100 micrograms), 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) (0.4-4.0 micrograms), a combination of CNQX (0.2-0.6 microgram) and lidocaine (10-30 micrograms), or isotonic sodium chloride solution. The TF latency and CD threshold were converted to the percent maximal possible effect (%MPE). To determine synergistic interaction, isobolographic analysis was used. Lidocaine or CNQX increased %MPEs in both the TF and CD tests. The coadministration of CNQX 0.4 microgram and lidocaine 20 micrograms, which had no effect by alone, significantly increased %MPEs in the TF and CD tests for 30 min and 10 min, respectively. Isobolographic analysis revealed the synergistic antinociception of CNQX and lidocaine in the TF test. Motor impairment was not observed after that combination. We conclude that CNQX and lidocaine produce synergistic analgesia on somatic and visceral pain at the spinal level.. We investigated the antinociceptive effects of 6-cyano-7-nitroquinoxaline-2,3-dione and its interaction with lidocaine at the spinal level in rats. Intrathecal 6-cyano-7-nitroquinoxaline-2,3-dione produced both somatic and visceral antinociception, and its coadministration with lidocaine showed synergistic antinociceptive effects.

Topics: 6-Cyano-7-nitroquinoxaline-2,3-dione; Anesthetics, Local; Animals; Colon; Drug Synergism; Excitatory Amino Acid Antagonists; Injections, Spinal; Lidocaine; Male; Pain; Pain Threshold; Rats; Rats, Sprague-Dawley; Sensory Thresholds

Intravenous administration of N-methyl-D-aspartate (NMDA) receptor antagonists and alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptor antagonists reportedly reduce the minimum alveolar anaesthetic concentration (MAC) for inhalation anaesthetics. If pain perception can be prevented by the intrathecal administration of antinociceptive receptor antagonists, these agents may reduce the requirements for inhalation anaesthetics. We studied the effect of intrathecal administration of an AMPA/kainate receptor antagonist, a metabotropic glutamate (mGlu) receptor antagonist and co-administration of NMDA and a neurokinin-1(NK-1) receptor antagonist drugs at low doses on the MAC.. After Wistar rats (n=36) were fitted with indwelling intrathecal catheters, the MAC of isoflurane was determined following intrathecal administration of a non-NMDA receptor antagonist (CNQX) at 10 microg, a mGlu receptor antagonist (AP3) at 10 microg, or a combination of NMDA receptor antagonist (APV) at 0.01 microg to 1 microg with NK-1 receptor antagonist (CP96345, CP) at 0.1 microg to 10 microg. Subsequently, a reversal dose of intrathecal NMDA with substance P (SP) was administered, and the MAC of isoflurane was redetermined. Conscious rats (n=15) were also examined for the presence of locomotor dysfunction following the intrathecal co-administration of APV and CP.. Neither CNQX nor AP3 reduced the MAC of isoflurane. APV at 0.01 microg plus CP at 1 microg, as well as APV at 0.1 microg plus CP at 10 microg, reduced the MAC of isoflurane, with respective reductions of 7.6% and 14%; (P<0.05). Co-administration of NMDA plus SP reversed the decrease in the MAC of isoflurane. Locomotive activity was not changed.. The NMDA receptor and the NK-1 receptor are important determinants of the MAC of isoflurane, exerting this influence by inhibition of pain transmission in the spinal cord, while mGlu and AMPA receptors have no effect on the MAC of isoflurane.

Topics: 6-Cyano-7-nitroquinoxaline-2,3-dione; Alanine; Anesthetics, Inhalation; Animals; Biphenyl Compounds; Catheters, Indwelling; Drug Combinations; Excitatory Amino Acid Antagonists; Injections, Spinal; Isoflurane; Locomotion; Male; Neurokinin-1 Receptor Antagonists; Nociceptors; Pain; Rats; Rats, Wistar; Receptors, AMPA; Receptors, Kainic Acid; Receptors, N-Methyl-D-Aspartate; Spinal Cord; Substance P; Synaptic Transmission; Valine

To study the roles of peripheral excitatory amino acids receptor subtypes N-methyl-D-aspartate (NMDA) and non-NMDA receptors in persistent nociception, extracellular single unit recording technique was used to assess the effects of a single dose NMDA and non-NMDA receptor antagonists, AP(5) (5-aminophosphonovaleric acid) and CNQX (6-cyano-7-nitroquinoxaline-2,3-dione) or DNQX (6,7-dinitroquinoxaline-2,3-dione), on s.c. bee venom-induced increase in firing of wide-dynamic-range (WDR) neurons in the spinal dorsal horn of the urethane-chloralose anesthetized cats. Subcutaneous bee venom injection into the cutaneous receptive field resulted in a single phase of increased firing of WDR neurons over the background activity for more than 1 h. Local pre-administration of AP(5) (200 microg/100 microl) or CNQX (8.3 microg/100 microl) into the bee venom injection site produced 94% (1.01+/-0.96 spikes/s, n=5) or 76% (2.97+/-0.58 spikes/s, n=4) suppression of the increased neuronal firing when compared with local saline (16.32+/-4.55 spikes/s, n=10) or dimethyl sulfoxide (DMSO) (12.37+/-6.36 spikes/s, n=4) pre-treated group, respectively. Local post-administration of the same dose of AP(5) produced a similar result to the pre-treatment group with a 67% inhibition of the mean firing rate, however, the same treatment with CNQX and even a higher dose of DNQX (100 microg/100 microl) did not produce any inhibition of the neuronal firing induced by s.c. bee venom injection (DNQX vs. DMSO: 23.91+/-0. 25 vs. 22.14+/-0.04 spikes/s, P=0.0298, n=5). In the control experiments, local pre-administration of the same dose of AP(5) or CNQX into a region on the contralateral hindpaw symmetrical to the bee venom injection site produced no significant influence on the increased firing of the WDR neurons [contralateral AP(5) vs. saline: 14.17+/-6.27 spikes/s (n=5) vs. 16.32+/-4.55 spikes/s (n=10), P0.05; contralateral CNQX vs. DMSO: 12.85+/-6.38 spikes/s (n=4) vs. 12. 37+/-6.36 spikes/s (n=4), P0.05], implicating that the suppressive action of local AP(5) or CNQX was not the result of systemic effects. The present results suggest that activation of the peripheral NMDA receptors is involved in both induction and maintenance, while activation of non-NMDA receptors is only involved in induction, but not in the maintenance of persistent firing of the dorsal horn WDR neurons induced by s.c. bee venom injection.

Topics: 2-Amino-5-phosphonovalerate; 6-Cyano-7-nitroquinoxaline-2,3-dione; Action Potentials; Animals; Bee Venoms; Cats; Disease Models, Animal; Excitatory Amino Acid Agonists; Excitatory Amino Acid Antagonists; Female; Injections, Subcutaneous; Male; N-Methylaspartate; Nociceptors; Pain; Posterior Horn Cells; Quinoxalines; Receptors, AMPA; Receptors, N-Methyl-D-Aspartate

It is known that Substance P (SP) enhances glutamate- and N-methyl-D-aspartate (NMDA)-induced activity in spinal cord dorsal horn neurons and that this enhancement is important in the generation of wind-up and central sensitization. It is now known that SP and glutamate receptors are present on sensory axons in rat glabrous skin. This raises the issue as to whether SP and glutamate interact in the periphery. Using the tail skin in rats, the present study demonstrates 1) that unmyelinated axons at the dermal-epidermal junction immunostain for antibodies directed against NMDA, non-NMDA or SP (NK1) receptors; 2) that glutamate injected into the tail skin results in dose-dependent nociceptive behaviors interpreted as mechanical hyperalgesia, mechanical allodynia and thermal hyperalgesia, which are blocked following co-injection with glutamate antagonists; 3) that peripheral injection of SP potentiates glutamate-induced nociceptive behaviors in that the co-injection of SP+glutamate results in a significantly longer duration of behavioral responses compared to the responses seen following injection of either substance alone. These data provide support for the hypothesis that primary afferent neurons might well be subject to similar mechanisms that result in wind-up or central sensitization of spinal cord neurons.

Topics: 6-Cyano-7-nitroquinoxaline-2,3-dione; Animals; Axons; Behavior, Animal; Dizocilpine Maleate; Dose-Response Relationship, Drug; Excitatory Amino Acid Antagonists; Glutamic Acid; Hot Temperature; Hyperalgesia; Male; Microscopy, Immunoelectron; Neurons, Afferent; Nociceptors; Pain; Physical Stimulation; Rats; Rats, Sprague-Dawley; Receptors, AMPA; Receptors, N-Methyl-D-Aspartate; Receptors, Neurokinin-1; Skin; Substance P; Tail

Tetanic stimulation of high-threshold primary afferent fibers in the dorsal root was found to elicit intrinsic optical signals (IOSs) in transverse slices of 11- to 20-day-old rat spinal cords. The IOS, lasting for 30 s or longer, was most prominent in the lamina II of the dorsal horn. Treatment with a Na+-K+-2Cl- co-transport blocker, furosemide, abolished the IOS, suggesting that the origin of the IOS is the cellular swelling due to an activity-dependent rise in extracellular K+. Substance P antagonist spantide, glutamate antagonists 2-amino-5-phosphonovaleric acid and 6-cyano-7-nitroquinoxaline-2,3-dione, and the mu-opioid agonist [d-Ala2,N-Me-Phe4,Gly5-ol]-enkephalin suppressed IOSs. Thus, IOSs represent at least in part the slow excitatory response that is known to be generated in dorsal horn neurons after tetanic activation of unmyelinated afferent fibers.

Topics: 2-Amino-5-phosphonovalerate; 6-Cyano-7-nitroquinoxaline-2,3-dione; Action Potentials; Analgesics; Analgesics, Opioid; Animals; Coloring Agents; Diuretics; Electric Stimulation; Enkephalin, Ala(2)-MePhe(4)-Gly(5)-; Enkephalins; Excitatory Amino Acid Antagonists; Excitatory Postsynaptic Potentials; Furosemide; Ganglia, Spinal; Neurons, Afferent; Pain; Rats; Substance P; Substantia Gelatinosa

Severe or prolonged tissue or nerve injury can induce hyperexcitability of dorsal horn neurons of the spinal cord, resulting in persistent pain, an exacerbated response to noxious stimuli (hyperalgesia), and a lowered pain threshold (allodynia). These changes are mediated by NMDA (N-methyl-D-aspartate)-type glutamate receptors in the spinal cord. Here we report that activation of the NMDA receptor causes release of substance P, a peptide neurotransmitter made by small-diameter, primary, sensory 'pain' fibres. Injection of NMDA in the cerebrospinal fluid of the rat spinal cord mimicked the changes that occur with persistent injury, and produced not only pain, but also a large-scale internalization of the substance P receptor into dorsal horn neurons, as well as structural changes in their dendrites. Both the pain and the morphological changes produced by NMDA were significantly reduced by substance P-receptor antagonists or by elimination of substance P-containing primary afferent fibres with the neurotoxin capsaicin. We suggest that presynaptic NMDA receptors located on the terminals of small-diameter pain fibres facilitate and prolong the transmission of nociceptive messages, through the release of substance P and glutamate. Therapies directed at the presynaptic NMDA receptor could therefore ameliorate injury-evoked persistent pain states.

Topics: 2-Amino-5-phosphonovalerate; 6-Cyano-7-nitroquinoxaline-2,3-dione; Animals; Injections, Spinal; N-Methylaspartate; Neurons, Afferent; Nociceptors; Pain; Rats; Receptors, N-Methyl-D-Aspartate; Receptors, Neurokinin-1; Receptors, Presynaptic; Spinal Cord; Substance P

The present study demonstrates that local cutaneous administration of either the N-methyl-D-aspartate (NMDA) glutamate receptor antagonist MK-801 or the non-NMDA glutamate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) significantly attenuates formalin-induced nociceptive behaviors. Specifically, pretreatment with either drug reduced the magnitude and time course of lifting and licking behavior in the late phase of formalin pain; however, flinching behavior was not affected. In contrast, post-treatment of formalin pain with either antagonist did not affect lifting and licking behavior, although flinching behavior was mildly attenuated. We hypothesize that these actions result from blocking of peripheral glutamate receptors located on unmyelinated axons at the dermal-epidermal junction. These data suggest that peripheral glutamate receptors on cutaneous axons can be manipulated to reduce certain aspects of pain of peripheral origin. This route of administration offers the advantage of avoiding the side effects of systemic administration.

Topics: 6-Cyano-7-nitroquinoxaline-2,3-dione; Animals; Axons; Dizocilpine Maleate; Excitatory Amino Acid Antagonists; Formaldehyde; Pain; Rats; Rats, Sprague-Dawley; Receptors, Glutamate; Receptors, N-Methyl-D-Aspartate; Skin; Stereotyped Behavior; Time Factors

The involvement of excitatory amino acid receptors in peripheral nociceptive processing was assessed in two separate experiments. In the first, one knee joint cavity of rats was injected with 0.1 ml of L-glutamate (0.001 mM; 0.1 mM; 1.0 mM), L-aspartate (0.001 mM; 0.1 mM: 1.0 mM), L-arginine (0.1 mM) or different combinations of these amino acids. The animals tested for paw withdrawal latency to radiant heat and withdrawal threshold to von Frey filaments at different time points. Combinations of glutamate/aspartate, aspartate/arginine or glutamate/aspartate/arginine when injected into the joint, in the absence of any other treatment, reduced the paw withdrawal latency and withdrawal threshold immediately after the injection and persisting up to 5 h indicating the development of hyperalgesia and allodynia. Subsequent intra-articular injection of either an NMDA or a non-NMDA glutamate receptor antagonist ((+/-)-2-amino-7-phosphonoheptanoic acid (AP7), 0.2 mM) or 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), 0.1 mM) attenuated the thermal hyperalgesia and the mechanical allodynia produced by glutamate/aspartate/arginine. On the other hand, in a second experiment intra-articular injection of AP7, ketamine or CNQX reversed the hyperalgesia and allodynia produced by injection of a mixture of kaolin and carrageenan into the joint. These receptor antagonists, however, did not have an effect on the joint edema. These findings provide evidence for a potential role of peripheral NMDA and non-NMDA receptors in nociceptive transmission.

Topics: 2-Amino-5-phosphonovalerate; 6-Cyano-7-nitroquinoxaline-2,3-dione; Animals; Excitatory Amino Acid Antagonists; Excitatory Amino Acids; Hyperalgesia; Injections, Intra-Articular; Knee Joint; Male; Pain; Rats; Rats, Sprague-Dawley; Reaction Time; Synaptic Transmission

We examined the antinociceptive effect of smilaxin B administered intracerebroventricularly (i.c.v.) in ICR mice. The tail-flick test was used as an analgesic assay. Smilaxin B showed a strong antinociceptive effect in a dose-dependent manner. Sulfated cholecystokinin (CCK-8s, 0.5 ng), muscimol (50ng), or MK-801 [(+/-)-5-methyl-10, 11-dihydro-5H-dibenzo [a,d]cyclohepten-5, 10-imine maleate, 1 microgram] injected i.c.v. significantly reduced inhibition of the tail-flick response induced by smilaxin B administered i.c.v. However, naloxone (2 microgram), baclofen (10 ng), or CNQX (6-cyano-7- nitroquinoxaline-2,3-dione, 0.5 microgram) injected i.c.v. did not affect inhibition of the tail-flick response induced by similaxin B administered i.c.v. The intrathecal (i.t.) injection of yohimbine (20 micrograms), but not methysergide (20 micrograms) and naloxone (2 microgram), significantly attenuated inhibition of the tail-flick response. induced by smilaxin B administered i.c.v. Our results suggest that GABAA or NMDA receptors but not opioid, GABAB, and non-NMDA receptors located at the supraspinal level may play important roles in the production of antinociception induced by smilaxin B administered supraspinally. Furthermore, smilaxin B administered supraspinally. may produce its antinociception by activating descending noradrenergic- but not opioidergic- and serotonergic-neurons.

Topics: 6-Cyano-7-nitroquinoxaline-2,3-dione; Animals; Baclofen; Cerebral Ventricles; Dizocilpine Maleate; Glycosides; Injections, Intraventricular; Injections, Spinal; Male; Methysergide; Mice; Mice, Inbred ICR; Muscimol; Naloxone; Pain; Sincalide; Spinal Cord; Spirostans; Yohimbine

1. The purpose of this study was to investigate a proposed role for the postsynaptic dorsal column (PSDC) pathway in mediating visceral nociceptive input into the dorsal column (DC) nuclei. 2. In one group of animals, the hypogastric nerves were sectioned, thereby restricting colorectal input into the cord to pelvic afferent pathways known to coverage on lower lumbar and sacral segments. Extracellular recording were made from 41 nucleus gracilis (NG) cells that responded to colorectal distension (CRD). Results reported are from 15 NG cells that were tested before and after the administration of morphine into the sacral cord by microdialysis. 3. The responses of 11 NG cells to CRD were dramatically reduced by morphine infused into the sacral cord through a microdialysis fiber. This reduction was reversed by an intravenous injection of naloxone. Microdialysis administration of 6-cyano-7-nitro-quinoxaline-2,3-dione (CNQX) or a lesion of the DC also abolished the responses of the NG cells to CRD. 4. Four NG cells that responded to CRD showed an increase in their background activity approximately 25 min after an injection of mustard oil (MO). This increase in activity was counteracted by morphine or by a lesion of the DC. 5. In a second group of animals, recordings were made from 28 PSDC cells in the L0-S1 segments of the cord. These units were antidromically activated by stimulation of the upper cervical fasciculus gracilis. The projections of five PSDC neurons into the NG were traced with the use of antidromic mapping. Results are reported for the responses of 12 PSDC cells to CRD and to cutaneous stimuli before and after morphine administration into the sacral cord by microdialysis. 6. Morphine given spinally reduced the responses of 12 PSDC cells to CRD. This reduction was reversed by an intravenous injection of naloxone. CNQX administered spinally also abolished the responses to CRD of the PSDC cells tested. 7. Four other PSDC cells were studied before and after an injection of MO into the colon. Their background activity started to increase within 25 min after the injection. Morphine suppressed this increase in background activity and this effect of morphine was reversed by naloxone. 8. The responses of NG cells to cutaneous stimuli were not significantly affected by morphine in the dose used. On the other hand, morphine significantly reduced the responses of PSDC cells to noxious cutaneous stimuli although this effect was not as dramatic as that on resp

Topics: 6-Cyano-7-nitroquinoxaline-2,3-dione; Analgesics, Opioid; Animals; Electric Stimulation; Excitatory Amino Acid Antagonists; Male; Medulla Oblongata; Morphine; Neural Pathways; Neurons, Afferent; Pain; Pelvis; Rats; Rats, Sprague-Dawley; Skin; Spinal Cord; Stress, Mechanical; Viscera

The responses of convergent dorsal horn neurones to peripheral injection of formalin (5% formaldehyde, 50 microliters volume) were recorded extracellularly in the halothane anaesthetized rat. The control response of dorsal horn neurones to formalin was biphasic, with a first phase from 0-10 min and the second inflammatory phase from 10-60 min. Pre-administered intrathecal CNQX (5, 50 and 500 micrograms), 5 min before formalin injection, significantly reduced both the first phase (40 +/- 22, 52 +/- 20 and 40 +/- 28% inhibition, respectively, P < 0.05) and the second phase of the formalin response (40 +/- 20% inhibition, P < 0.05, 93 +/- 4% inhibition, P < 0.0001 and 65 +/- 17% inhibition, P < 0.05, respectively). Post-administered CNQX, administered 5 min after the peripheral injection of formalin, was less efficacious, as compared to pre-administered CNQX, at reducing the second phase of the formalin response. The lowest dose of post-administered CNQX (5 micrograms) facilitated the second phase of the response (47 +/- 19% facilitation, P < 0.05), and the higher dose (50 micrograms) produced smaller inhibitions of the response (42 +/- 10% inhibition, P < 0.05) than those observed with pre-administration of the same dose. However, the highest dose of CNQX (500 micrograms) studied produced similar inhibitions of the second phase of the formalin response, irrespective of the timing of administration. Intrathecal administration of 7-chlorokynurenate (7CK, 0.25-2.5 micrograms), a functional antagonist at the glycine site of the NMDA receptor, did not alter the first phase of the formalin response. The second phase of the formalin response was significantly inhibited, and to a similar extent, by both pre- and post-administration of 2.5 micrograms of 7CK (67 +/- 10% and 56 +/- 7% inhibition respectively, P < 0.05 for both). Overall, our results clearly demonstrate differential time-related roles of different transmitter systems in the induction and maintenance of inflammatory evoked persistent pain responses, and such events may become increasingly relevant to the control of pain in the clinic.

Topics: 6-Cyano-7-nitroquinoxaline-2,3-dione; Animals; Evoked Potentials; Excitatory Amino Acid Antagonists; Formaldehyde; Injections, Spinal; Kynurenic Acid; Male; Neurons; Pain; Rats; Rats, Sprague-Dawley; Receptors, AMPA; Receptors, Glycine; Receptors, N-Methyl-D-Aspartate; Spinal Cord; Time Factors

In vivo microdialysis was used to assess the hypothesis that the stress-induced increase in dopamine release in the prefrontal cortex is mediated by stress-activated glutamate neurotransmission in this region. Local perfusion of an alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA)/kainate receptor antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione, blocked the stress-induced increase in dopamine levels, whereas an NMDA receptor antagonist, 2-amino-5-phosphonopentanoic acid, at the dose tested, was not able to alter this response significantly. These data indicate that the effect of stress on dopamine release in the prefrontal cortex is mediated locally by activation of AMPA/kainate receptors, which modulate the release of dopamine in this region.

Topics: 2-Amino-5-phosphonovalerate; 6-Cyano-7-nitroquinoxaline-2,3-dione; alpha-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic Acid; Analysis of Variance; Animals; Dopamine; Excitatory Amino Acid Antagonists; Extracellular Space; Male; Microdialysis; Pain; Prefrontal Cortex; Quinoxalines; Rats; Rats, Sprague-Dawley; Receptors, AMPA; Receptors, Glutamate; Receptors, Kainic Acid; Receptors, N-Methyl-D-Aspartate; Stress, Psychological

In a rat model of morphine tolerance, we examined the hypotheses that thermal hyperalgesia to radiant heat develops in association with the development of morphine tolerance and that both the development and expression of thermal hyperalgesia in morphine-tolerant rats are mediated by central NMDA and non-NMDA receptors and subsequent protein kinase C (PKC) activation. Tolerance to the analgesic effect of morphine was developed in rats utilizing an intrathecal repeated treatment regimen. The development of morphine tolerance and thermal hyperalgesia was examined by employing the tail-flick test and paw-withdrawal test, respectively. Intrathecal MK 801 (an NMDA receptor antagonist), 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; a non-NMDA receptor antagonist), or GM1 ganglioside (an intracellular PKC inhibitor) treatment was given to examine the effects of these agents on the development and expression of thermal hyperalgesia in morphine-tolerant rats. Tolerance to the analgesic effect of morphine was reliably developed in rats following once daily intrathecal (onto the lumbosacral spinal cord) injection of 10 micrograms of morphine sulfate for 8 consecutive days as demonstrated by the decreased analgesia following morphine administration on day 8 as compared to that on day 1. In association with the development of morphine tolerance, thermal hyperalgesia to radiant heat developed in these same rats. Paw-withdrawal latencies were reliably decreased in morphine-tolerant rats as compared to nontolerant (saline) controls when tested on day 8 before the last morphine treatment and on day 10 (i.e., 48 hr after the last morphine treatment). The coincident development of morphine tolerance and thermal hyperalgesia was potently prevented by intrathecal coadministration of morphine with MK 801 (10 nmol) or GM1 (160 nmol), and partially by CNQX (80 nmol). MK 801 (5, 10 nmol, not 2.5 nmol) and CNQX (80, 160 nmol, not 40 nmol), but not GM1 (160 nmol), also reliably reversed thermal hyperalgesia in rats rendered tolerant to morphine when tested 30 min after each drug treatment on day 10 (48 hr after the last morphine treatment). The data indicate that thermal hyperalgesia develops in association with the development of morphine tolerance and that the coactivation of central NMDA and non-NMDA receptors is crucial for both the development and expression of thermal hyperalgesia in morphine-tolerant rats. Furthermore, intracellular PKC activation plays a critical role in the

Topics: 6-Cyano-7-nitroquinoxaline-2,3-dione; Animals; Dizocilpine Maleate; Drug Tolerance; Excitatory Amino Acid Antagonists; G(M1) Ganglioside; Hot Temperature; Hyperalgesia; Injections, Spinal; Male; Morphine; Pain; Protein Kinase C; Quinoxalines; Rats; Rats, Sprague-Dawley; Receptors, Glutamate; Receptors, N-Methyl-D-Aspartate

Unilateral adjuvant inflammation was induced at the rat ankle 2 or 20 days before an evaluation of the contribution of N-methyl-D-aspartate (NMDA) and non-NMDA receptors to the processing of nociceptive information by wide dynamic range neurons in the spinal cord. Microionophoretic application of either the NMDA receptor antagonists ketamine and DL-2-amino-5-phosphonovalerate (AP5) or the non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) reduced the responses to innocuous and noxious mechanical stimulation of the inflamed ankle. The pattern of these effects was comparable to that in rats with acute inflammation suggesting that non-NMDA and NMDA receptors are similarly involved in acute, prolonged acute and chronic inflammation-evoked activity.

Topics: 2-Amino-5-phosphonovalerate; 6-Cyano-7-nitroquinoxaline-2,3-dione; Afferent Pathways; Animals; Ankle Joint; Arthritis, Experimental; Chronic Disease; Electrophysiology; Ketamine; Male; Neurons; Pain; Pressure; Quinoxalines; Rats; Rats, Wistar; Receptors, Amino Acid; Receptors, N-Methyl-D-Aspartate; Spinal Cord

In anaesthetized rats the involvement of N-methyl-D-aspartate (NMDA) and non-NMDA receptors in the processing of innocuous and noxious sensory inflow from joints was assessed in 27 spinal dorsal horn neurons. Microionophoretic application of either the NMDA antagonists ketamine and D,L-2-amino-5-phosphonovalerate (AP5) or the non-NMDA antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) reduced the responses to noxious compression of the knee and ankle joint. By contrast, the responses to innocuous pressure were consistently reduced by CNQX but only exceptionally by NMDA antagonists. Therefore non-NMDA receptors are involved in the processing of innocuous and noxious mechanical stimuli applied to the normal joint whereas NMDA receptors are activated mainly by nociceptive input.

Topics: 2-Amino-5-phosphonovalerate; 6-Cyano-7-nitroquinoxaline-2,3-dione; alpha-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic Acid; Animals; Ibotenic Acid; Ketamine; Male; Neurons; Pain; Physical Stimulation; Quinoxalines; Rats; Rats, Wistar; Receptors, Amino Acid; Receptors, N-Methyl-D-Aspartate; Spinal Cord

Central activation of excitatory amino acid receptors has been implicated in neuropathic pain following nerve injury. In a rat model of painful peripheral mononeuropathy, we compared the effects of non-competitive NMDA receptor antagonists (MK 801 and HA966) and a non-NMDA receptor antagonist (CNQX) on induction and maintenance of thermal hyperalgesia induced by chronic constrictive injury (CCI) of the rat common sciatic nerve. Thermal hyperalgesia to radiant heat was assessed by using a foot-withdrawal test and NMDA/non-NMDA receptor antagonists were administered intrathecally onto the lumbar spinal cord before and after nerve injury. Four daily single treatments with 20 nmol HA966 or CNQX beginning 15 min prior to nerve ligation (pre-injury treatment), reliably reduced thermal hyperalgesia in CCI rats on days 3, 5, 7 and 10 after nerve ligation. Thermal hyperalgesia was also reduced in CCI rats receiving a single post-injury treatment with HA966 (20 or 80 nmol) or MK 801 (5 or 20 nmol) on day 3 after nerve ligation when thermal hyperalgesia was well developed. In contrast, a single post-injury CNQX (20 or 80 nmol) treatment failed to reduce thermal hyperalgesia or to potentiate effects of HA966 or MK 801 (5 or 20 nmol) on thermal hyperalgesia in CCI rats. Moreover, multiple post-injury CNQX treatments utilizing the same dose regime as employed for the pre-injury treatment attenuated thermal hyperalgesia but only when the treatment began 1 or 24 h (but not 72 h) after nerve ligation.(ABSTRACT TRUNCATED AT 250 WORDS)

Topics: 6-Cyano-7-nitroquinoxaline-2,3-dione; Animals; Dizocilpine Maleate; Hot Temperature; Hyperalgesia; Injections, Spinal; Male; Pain; Peripheral Nervous System Diseases; Pyrrolidinones; Quinoxalines; Rats; Rats, Sprague-Dawley; Receptors, N-Methyl-D-Aspartate; Sciatic Nerve

The contribution of C-fiber neuropeptides and excitatory amino acids (EAAs) as central mediators of secondary hyperalgesia was assessed by examining the effects of intrathecal (i.t.) administration of receptor-selective agonists and antagonists on foot-withdrawal latencies (from 48 degrees C water), both before and after heat injury of the contralateral hindpaw. The hyperalgesia which develops in the hindpaw contralateral to a heat injury, could be reproduced in uninjured rats following i.t. injection of substance P, neurokinin A and N-methyl-D-aspartate (NMDA) but not following calcitonin gene related peptide (CGRP), neurokinin B, kainate or (R,S)-alpha-amino-3-hydroxy-5-methylisozazole-4-propionic acid hydrobromide (AMPA). Contralateral hyperalgesia was reversed by the substance P antagonist Arg1,D-Pro2,D-Phe2-D-His9-substance P, and the NMDA receptor antagonist D-2-amino-5-phosphonovalerate (APV), but not by the non-NMDA EAA antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). When the limb of the injured hindpaw was pretreated with the C-fiber neurotoxin capsaicin, hyperalgesia in the contralateral hindpaw was unaffected. Furthermore, prior to injury, the capsaicin pretreatment itself produced hyperalgesia in the contralateral hindpaw. The results give support for a contribution of both C-fiber neuropeptides and EAAs to central nervous system plasticity and secondary hyperalgesia following heat injury.

Topics: 2-Amino-5-phosphonovalerate; 6-Cyano-7-nitroquinoxaline-2,3-dione; alpha-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic Acid; Amino Acids; Animals; Capsaicin; Functional Laterality; Hot Temperature; Ibotenic Acid; Kainic Acid; N-Methylaspartate; Neuronal Plasticity; Neuropeptides; Pain; Quinoxalines; Rats; Reflex; Time Factors

The responses of single thalamic neurones to noxious thermal stimulation were recorded in anaesthetized rats. The selective N-methyl-D-aspartate (NMDA) receptor antagonist, 3-((+-)-2-carboxypiperazin-4-yl)propyl-l-phosphonate (CPP), antagonised nociceptive responses when ejected iontophoretically with currents which produced selective antagonism at NMDA receptors. In contrast, the non-NMDA excitatory amino acid receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNOX) had little or no effect on nociceptive responses, although it was able to reduce responses to non-nociceptive mechanoreceptor input. These results show that there is substantial NMDA receptor involvement in thalamic nociceptive responses, and that the contribution of CNQX-sensitive non-NMDA receptors to these responses is not extensive. Furthermore, it appears that nociceptive and non-nociceptive input to the thalamus have distinct synaptic pharmacologies.

Topics: 6-Cyano-7-nitroquinoxaline-2,3-dione; Action Potentials; Animals; Hot Temperature; Male; Nociceptors; Pain; Piperazines; Quinoxalines; Rats; Rats, Inbred Strains; Receptors, N-Methyl-D-Aspartate; Receptors, Neurotransmitter; Thalamus