6-methyl-2-(phenylethynyl)pyridine and Pain

Glutamate regulates the function of central nervous system (CNS), in part, through the cAMP and/or IP3/DAG second messenger-associated metabotropic glutamate receptors (mGluRs). The mGluR5 antagonist 2-methyl-6-(phenylethynyl)-pyridine (MPEP) has been extensively used to elucidate potential physiological and pathophysiological functions of mGluR5. Unfortunately, recent evidence indicates significant non-specific actions of MPEP, including inhibition of NMDA receptors. In contrast, in vivo and in vitro characterization of the newer mGluR5 antagonist 3-[(2-methyl-1,3-thiazol-4-yl)ethynyl]pyridine (MTEP) indicates that it is more highly selective for mGluR5 over mGluR1, has no effect on other mGluR subtypes, and has fewer off-target effects than MPEP. This article reviews literature on both of these mGluR5 antagonists, which suggests their possible utility in neurodegeneration, addiction, anxiety and pain management.

Topics: Animals; Humans; Neurodegenerative Diseases; Pain; Pyridines; Receptor, Metabotropic Glutamate 5; Receptors, Metabotropic Glutamate; Substance-Related Disorders; Thiazoles

There is a need to identify subtype-specific ligands for mGlu receptors to elucidate the potential of these receptors for the treatment of nervous system disorders. To date, most mGlu receptor antagonists are amino acid-like compounds acting as competitive antagonists at the glutamate binding site located in the large extracellular N-terminal domain. We have characterized novel subtype-selective mGlu(5) receptor antagonists which are structurally unrelated to competitive mGlu receptor ligands. Using a series of chimeric receptors and point mutations we demonstrate that these antagonists act as inverse agonists with a novel allosteric binding site in the seven-transmembrane domain. Recent studies in animal models implicate mGlu(5) receptors as a potentially important therapeutic target particularly for the treatment of pain and anxiety.

Topics: Animals; Anti-Anxiety Agents; Anxiety; Binding Sites; Brain; Excitatory Amino Acid Antagonists; Ligands; Pain; Pyridines; Receptor, Metabotropic Glutamate 5; Receptors, Metabotropic Glutamate

Although multiple metabotropic glutamate (mglu) receptor subtypes were cloned in the early 1990s, progress in the characterization of these receptors has been slow because of difficulties in obtaining subtype-selective ligands. However, in the past few years exciting progress has been made on the mglu(5) receptor subtype following the identification of selective non-amino-acid-like ligands that implicate the mglu(5) receptor as a potentially important therapeutic target, particularly for the treatment of pain and anxiety.

Topics: Allosteric Regulation; Animals; Anxiety; Brain; Central Nervous System Diseases; Depression; Excitatory Amino Acid Antagonists; Humans; Neurons, Afferent; Pain; Parkinson Disease; Pyridines; Rats; Receptor, Metabotropic Glutamate 5; Receptors, Metabotropic Glutamate

Plastic changes in the amygdala and limbic cortex networks have been widely shown in chronic pain. We have here investigated the role of group I metabotropic glutamate receptors (mGluRs) in the basolateral amygdala (BLA) pre-infra-limbic (PL-IL) divisions of the medial prefrontal cortex (mPFC) neuron connections after carrageenan-induced inflammatory pain in the rat. Intra-plantar injection of carrageenan decreased either spontaneous or mechanically/electrically evoked activity of PL cortex pyramidal neurons which responded with excitation in a way prevented by CPCOOEt, a selective mGluR1 antagonist, though not by MPEP, a selective mGluR5 antagonist. Accordingly, intra-BLA microinjection of DHPG, a group I mGluR agonist, caused PL cortex neuron activity depression, antagonized by CPCCOEt. CPCOOEt, but not MPEP, reduced also carrageenan-induced mechanical allodynia. The PL cortex cell deactivation in inflammatory pain condition was associated with increased GABA (conversely glutamate was decreased) in the PL/IL cortex. The local application of bicuculline, a GABA(A) receptor selective antagonist, reduced mechanical allodynia. An over-expression of mGluR1, but not mGluR5, have been observed in the PL-IL cortex after inflammatory pain suggesting an increased mGluR1-dependent cross-talk among BLA and IL-PL cortex neurons in inflammatory pain conditions. This article is part of a Special Issue entitled 'Metabotropic Glutamate Receptors'.

Topics: Action Potentials; Amygdala; Animals; Bicuculline; Carrageenan; Chromones; Disease Models, Animal; Electric Stimulation; Excitatory Amino Acid Agonists; Excitatory Amino Acid Antagonists; Formaldehyde; GABA Antagonists; Gene Expression; Glycine; Male; Neural Inhibition; Neural Pathways; Neuronal Plasticity; Pain; Physical Stimulation; Prefrontal Cortex; Pyramidal Cells; Pyridines; Rats; Rats, Wistar; Receptor, Metabotropic Glutamate 5; Receptors, Metabotropic Glutamate; Resorcinols

Acute stress reduces pain sensitivity by engaging an endocannabinoid signaling circuit in the midbrain. The neural mechanisms governing this process and molecular identity of the endocannabinoid substance(s) involved are unknown. We combined behavior, pharmacology, immunohistochemistry, RNA interference, quantitative RT-PCR, enzyme assays, and lipidomic analyses of endocannabinoid content to uncover the role of the endocannabinoid 2-arachidonoyl-sn-glycerol (2-AG) in controlling pain sensitivity in vivo. Here, we show that footshock stress produces antinociception in rats by activating type 5 metabotropic glutamate receptors (mGlu(5)) in the dorsolateral periaqueductal gray (dlPAG) and mobilizing 2-AG. Stimulation of mGlu(5) in the dlPAG with DHPG [(S)-3,5-dihydroxyphenylglycine] triggered 2-AG formation and enhanced stress-dependent antinociception through a mechanism dependent upon both postsynaptic diacylglycerol lipase (DGL) activity, which releases 2-AG, and presynaptic CB(1) cannabinoid receptors. Pharmacological blockade of DGL activity in the dlPAG with RHC80267 [1,6-bis(cyclohexyloximinocarbonylamino)hexane] and (-)-tetrahydrolipstatin (THL), which inhibit activity of DGL-α and DGL-β isoforms, suppressed stress-induced antinociception. Inhibition of DGL activity in the dlPAG with THL selectively decreased accumulation of 2-AG without altering levels of anandamide. The putative 2-AG-synthesizing enzyme DGL-α colocalized with mGlu(5) at postsynaptic sites of the dlPAG, whereas CB(1) was confined to presynaptic terminals, consistent with a role for 2-AG as a retrograde signaling messenger. Finally, virally mediated silencing of DGL-α, but not DGL-β, transcription in the dlPAG mimicked effects of DGL inhibition in suppressing both endocannabinoid-mediated stress antinociception and 2-AG formation. The results indicate that activation of the postsynaptic mGlu(5)-DGL-α cascade triggers retrograde 2-AG signaling in vivo. This pathway is required for endocannabinoid-mediated stress-induced analgesia.

Topics: Analgesia; Analysis of Variance; Animals; Arachidonic Acids; Cannabinoid Receptor Modulators; Cyclohexanones; Dose-Response Relationship, Drug; Electroconvulsive Therapy; Endocannabinoids; Excitatory Amino Acid Antagonists; Glycerides; Lipoprotein Lipase; Male; Methoxyhydroxyphenylglycol; Mice; Microscopy, Immunoelectron; Pain; Periaqueductal Gray; Piperidines; Protease Inhibitors; Pyrazoles; Pyridines; Rats; Rats, Sprague-Dawley; Receptor, Cannabinoid, CB1; Receptor, Metabotropic Glutamate 5; Receptors, Metabotropic Glutamate; Rimonabant; RNA, Messenger; RNA, Small Interfering; Synapses; Tandem Mass Spectrometry

The present study examined the effects of the mGluR1 antagonist JNJ16259685 (JNJ) and the mGluR5 antagonist 2-methyl-6-phenylethynylpyridine (MPEP) alone and in combination with morphine in two acute pain models (hotplate, warm water tail-withdrawal), and a persistent, inflammatory pain model (capsaicin). In the hotplate and warm water tail-withdrawal procedures, JNJ and MPEP were ineffective when administered alone. In both procedures, JNJ potentiated morphine antinociception. In the hotplate procedure, MPEP potentiated morphine antinociception at the highest dose examined, whereas in the warm water tail-withdrawal procedure MPEP attenuated morphine antinociception at a moderate dose and potentiated morphine antinociception at a high dose. For both JNJ and MPEP, the magnitude of this morphine potentiation was considerably greater in the hotplate procedure. In the capsaicin procedure, the highest dose of MPEP produced intermediate levels of antihyperalgesia and also attenuated the effects of a dose of morphine that produced intermediate levels of antihyperalgesia. In contrast, JNJ had no effect when administered alone in the capsaicin procedure and did not alter morphine-induced antihyperalgesia. The present findings suggest that the effects produced by mGluR1 and mGluR5 antagonists alone and in combination with morphine can be differentiated in models of both acute and persistent pain.

Topics: Animals; Behavior, Animal; Capsaicin; Disease Models, Animal; Drug Therapy, Combination; Excitatory Amino Acid Antagonists; Inflammation; Male; Morphine; Narcotics; Pain; Pain Measurement; Pyridines; Quinolines; Rats; Rats, Inbred F344; Receptors, Metabotropic Glutamate

Extracellular single unit recording experiments were performed to examine response characteristics of wide dynamic range neurons in the Vc that receive masseter afferent input in Sprague-Dawley rats. Capsaicin, or its vehicle, was directly administered into the masseter muscle and changes in resting discharge, responses to mechanical stimulation on the cutaneous receptive field and the electrical threshold for masseter nerve stimulation were assessed. Intramuscular capsaicin induced significant increase in the background discharge and mechanical hypersensitivity to the cutaneous stimulation and lowered the threshold for masseter nerve stimulation-evoked responses in the majority of neurons. The capsaicin-induced increase in evoked responses, but not the resting discharge, was partially attenuated when the muscle was pretreated with a mGluR antagonist. The present study suggests that injury or inflammation in the masseter muscle induce generalized hyperexcitability of central trigeminal neurons and that the blockade of peripherally localized mGluR5 can effectively attenuate muscular hypersensitivity.

Topics: Animals; Capsaicin; Evoked Potentials; Excitatory Amino Acid Antagonists; Injections, Intramuscular; Male; Masseter Muscle; Microelectrodes; Neurons; Pain; Physical Stimulation; Pyridines; Rats; Rats, Sprague-Dawley; Receptor, Metabotropic Glutamate 5; Receptors, Metabotropic Glutamate; Sensory System Agents; Trigeminal Caudal Nucleus

The central nucleus of the amygdala (CeA) has been identified as a site of nociceptive processing important for sensitization induced by peripheral injury. However, the cellular signaling components underlying this function remain unknown. Here, we identify metabotropic glutamate receptor 5 (mGluR5) as an integral component of nociceptive processing in the CeA. Pharmacological activation of mGluRs with (R,S)-3,5-dihydroxyphenylglycine (DHPG) in the CeA of mice is sufficient to induce peripheral hypersensitivity in the absence of injury. DHPG-induced peripheral hypersensitivity is reduced via pharmacological blockade of mGluR5 or genetic disruption of mGluR5. Furthermore, pharmacological blockade or conditional deletion of mGluR5 in the CeA abrogates inflammation-induced hypersensitivity, demonstrating the necessity of mGluR5 in CeA-mediated pain modulation. Moreover, we demonstrate that phosphorylation of extracellular-signal regulated kinase 1/2 (ERK1/2) is downstream of mGluR5 activation in the CeA and is necessary for the full expression of peripheral inflammation-induced behavioral sensitization. Finally, we present evidence of right hemispheric lateralization of mGluR5 modulation of amygdalar nociceptive processing. We demonstrate that unilateral pharmacological activation of mGluR5 in the CeA produces distinct behavioral responses depending on whether the right or left amygdala is injected. We also demonstrate significantly higher levels of mGluR5 expression in the right amygdala compared with the left under baseline conditions, suggesting a potential mechanism for right hemispheric lateralization of amygdala function in pain processing. Together, these results establish an integral role for mGluR5 and ERK1/2 in nociceptive processing in the CeA.

Topics: Amygdala; Analysis of Variance; Animals; Butadienes; Enzyme Inhibitors; Excitatory Amino Acid Antagonists; Formaldehyde; Functional Laterality; Gene Expression Regulation; Green Fluorescent Proteins; Hyperalgesia; Methoxyhydroxyphenylglycol; Mice; Mice, Inbred C57BL; Mice, Knockout; Mitogen-Activated Protein Kinase 3; Nitriles; Pain; Pain Measurement; Pyridines; Receptors, Glucocorticoid; Receptors, Kainic Acid

In micturition control, the roles of ionotropic glutamate (iGlu) receptors NMDA and AMPA are well established, whereas little is known about the function of metabotropic glutamate (mGlu) receptors. Since antagonists for mGlu5 receptors are efficacious in animal models of inflammatory and neuropathic pain, we examined whether mGlu5 receptors play a role in the voiding reflex and bladder nociception and, if so, via centrally or peripherally localized receptors. The mGlu5 receptor antagonist MPEP dose-dependently increased the micturition threshold (MT) volume in the volume-induced micturition reflex (VIMR) model in anesthetized rats. Following doses of 5.2, 15.5 and 51.7micromol/kg of MPEP (intraduodenal), the MT was increased by 24.7+/-5.0%, 97.2+/-12.5% (P<0.01) and 128.0+/-28.3% (P<0.01) from the baseline, respectively (n=4-5; compared with 0.8+/-9.1% in the vehicle group). Infusing MPEP (0.3, 1mM) directly into the bladder also raised MT. However, the efficacious plasma concentrations of MPEP following intravesical dosing were similar to that after intraduodenal dosing (EC(50) of 0.11 and 0.27microM, respectively, P>0.05). MPEP also dose-dependently attenuated the visceromotor responses (VMR, total number of abdominal EMG spikes during phasic bladder distension) in anesthetized rats. The VMR was decreased to 1332.4+/-353.9 from control of 2886.5+/-692.2 spikes/distension (n=6, P<0.01) following MPEP (10micromol/kg, iv). Utilizing the isolated mouse bladder/pelvic nerve preparation, we found that neither MPEP (up to 3microM) nor MTEP (up to 10microM) affected afferent discharge in response to bladder distension (n=4-6). In contrast, MPEP attenuated the responses of the mesenteric nerves to distension of the mouse jejunum in vitro. These data suggest that mGlu5 receptors play facilitatory roles in the processing of afferent input from the urinary bladder, and that central rather than peripheral mGlu5 receptors appear to be responsible.

Topics: Action Potentials; Analysis of Variance; Animals; Dose-Response Relationship, Drug; Female; In Vitro Techniques; Jejunum; Mice; Models, Biological; Pain; Pyridines; Rats; Rats, Sprague-Dawley; Receptor, Metabotropic Glutamate 5; Receptors, Metabotropic Glutamate; Reflex; Reflex, Stretch; Thiazoles; Urinary Bladder; Urination

Neurotensin modulates pain via its actions within descending analgesic pathways which include brain regions such as the midbrain periaqueductal grey (PAG). The aim of this study was to examine the cellular actions of neurotensin on PAG neurons. Whole cell patch clamp recordings were made from rat midbrain PAG slices in vitro to examine the postsynaptic effects of neurotensin and its effects on GABA(A) mediated inhibitory postsynaptic currents (IPSCs). Neurotensin (100-300 nM) produced an inward current in subpopulations of opioid sensitive and insensitive PAG neurons which did not reverse over membrane potentials between -50 and -130 mV. The neurotensin induced current was abolished by the NTS1 and NTS1/2 antagonists SR48692 (300 nM) and SR142948A (300 nM). Neurotensin also produced a reduction in the amplitude of evoked IPSCs, but had no effect on the rate and amplitude of TTX-resistant miniature IPSCs. The neurotensin induced inhibition of evoked IPSCs was reduced by the mGluR5 antagonist MPEP (5microM) and abolished by the cannabinoid CB(1) receptor antagonist AM251 (3 microM). These results suggest that neurotensin produces direct neuronal depolarisation via NTS1 receptors and inhibits GABAergic synaptic transmission within the PAG. The inhibition of synaptic transmission is mediated by neuronal excitation and action potential dependent release of glutamate, leading to mGluR5 mediated production of endocannabinoids which activate presynaptic CB(1) receptors. Thus, neurotensin has cellular actions within the PAG which are consistent with both algesic and analgesic activity, some of which are mediated via the endocannabinoid system.

Topics: Adamantane; Animals; Cannabinoid Receptor Modulators; Endocannabinoids; Excitatory Amino Acid Antagonists; Female; gamma-Aminobutyric Acid; Imidazoles; In Vitro Techniques; Inhibitory Postsynaptic Potentials; Male; Miniature Postsynaptic Potentials; Neural Inhibition; Neurons; Neurotensin; Pain; Periaqueductal Gray; Piperidines; Presynaptic Terminals; Pyrazoles; Pyridines; Quinolines; Rats; Rats, Sprague-Dawley; Receptor, Cannabinoid, CB1; Receptor, Metabotropic Glutamate 5; Receptors, Metabotropic Glutamate; Receptors, Neurotensin; Synaptic Transmission; Tetrodotoxin; Time Factors

Metabotropic glutamate receptor subtype 5 (mGlu5) has been demonstrated to play a role in the modulation of numerous nociceptive modalities. When administered via peripheral, intrathecal, or systemic routes, mGlu5 antagonists have analgesic properties in a variety of preclinical pain models. Despite a wealth of data supporting the use of mGlu5 antagonists to treat pain, studies have been limited to preclinical animal models due to a lack of mGlu5 antagonists that are approved for use in humans. It has been demonstrated previously that fenobam [N-(3-chlorophenyl)-N'-(4,5-dihydro-1-methyl-4-oxo-1H-imidazole-2-yl)urea], an anxiolytic shown to be safe and effective in human trials, is a selective and potent noncompetitive antagonist of mGlu5 (J Pharmacol Exp Ther 315:711-721, 2005). Here, we report a series of studies aimed at testing whether fenobam, similar to the prototypical mGlu5 antagonist 2-methyl-6-(phenylethynyl)-pyridine (MPEP), has analgesic properties in mice. We show that fenobam reduces formalin-induced pain behaviors and relieves established inflammation-induced thermal hypersensitivity in mice. Similar results were seen with MPEP. Administration of fenobam resulted in an increase in locomotor activity in the open-field task but did not impair performance on the accelerating Rotarod. Analysis of brain and plasma fenobam levels indicated that fenobam is rapidly concentrated in brain after intraperitoneal administration in mice but is essentially cleared from circulation within 1 h after injection. Fenobam had no analgesic effect in mGlu5 knockout mice, whereas the prototypical antagonist MPEP retained significant analgesic efficacy in mGlu5 knockouts. These results demonstrate that fenobam is analgesic in mice and has an improved in vivo selectivity for mGlu5 over MPEP.

Topics: Analgesics, Non-Narcotic; Animals; Behavior, Animal; Brain; Calibration; Chromatography, High Pressure Liquid; Formaldehyde; Freund's Adjuvant; Hot Temperature; Imidazoles; Indicators and Reagents; Inflammation; Male; Mass Spectrometry; Mice; Mice, Inbred C57BL; Motor Activity; Pain; Pain Measurement; Postural Balance; Pyridines; Quality Control; Receptors, Kainic Acid

Metabotropic glutamate receptors (mGluRs) are expressed abundantly in the spinal cord and have been shown to play important roles in the modulation of nociceptive transmission and plasticity. In this study, the involvement of metabotropic glutamate receptor 5 (mGluRs) in the nociceptive response induced by intrathecal injection (i.t.) of excitatory aminoacids, substance P (SP), bradykinin (BK) and cytokines in mice was demonstrated. The administration of 2-methyl-6-(phenylethynyl)-pyridine (MPEP; 10-50 nmol/site, i.t.) caused a significant inhibition in the nociceptive response induced by glutamate and trans-ACPD with maximal inhibitory effects of 36 +/- 7% and 56 +/- 5%, respectively. MPEP completely failed to affect the nociception induced by alpha-amino-3-hydroxy-5-mehtyl-4-isoxazolepropionic acid (AMPA; 135 pmol/site), kainate (110 pmol/site) and N-methyl-D-aspartate (NMDA; 450 pmol/site). MPEP also reduced the nociceptive response induced by SP (135 ng/site, i.t.), BK (0.1 microg/site), tumor necrosis factor-alpha (TNF-alpha; 0.1 pg/site) and interleukin-1beta (IL-1beta; 1 pg/site) with maximal inhibitions of 29 +/- 5%, 37 +/- 5%, 83 +/- 3% and 88 +/- 1%, respectively. Together, these results indicate the involvement of mGluRs, more specifically of subtype-5, in the nociceptive response induced by i.t. injection of excitatory aminoacids, SP, BK and cytokines in mice.

Topics: Animals; Bradykinin; Cytokinins; Excitatory Amino Acid Antagonists; Excitatory Amino Acids; Injections, Spinal; Interleukin-1beta; Male; Mice; Nociceptors; Pain; Pyridines; Receptor, Metabotropic Glutamate 5; Receptors, Metabotropic Glutamate; Substance P; Tumor Necrosis Factor-alpha

The present study comparatively evaluated the potency of a series of new phenylethyl[1,2,4]methyltriazines which are analogues of the classical metabotropic glutamate (mGlu) receptor subtype 5 (mGluR5) antagonist 2-methyl-6-(phenylethynyl)pyridine (MPEP) in blocking hyperalgesia induced by the group I mGlu receptor agonist (S)-3,5-DHPG as well as in reversing morphine antinociceptive tolerance in mice. Hyperalgesia was assessed in mice using the tail immersion test. Intrathecal (i.t.) pre-treatment with the test compounds 5-methyl-3-phenylethynyl-[1,2,4]triazine (RTI-4229-707), 5-methyl-3-(4-phenoxy-phenylethynyl-[1,2,4]triazine (RTI-4229-766), and 3-(3-methylphenylethynyl)-5-methyl-[1,2,4]triazine (RTI-4229-787) resulted in a dose-dependent blockade of (S)-3,5-DHPG-induced hyperalgesia. The inhibitory dose-50 (ID(50)) values were 0.49, 0.72 and 0.44 nmol/mouse, for RTI-4229-707, RTI-4229-766 and RTI-4229-787, respectively, compared to 18.63 nmol/mouse for MPEP. The other two compounds tested 3-(2,5-dimethylphenylethynyl)-5-methyl[1,2,4]triazine (RTI-4229-785) and 3-(2-methylphenylethynyl)-5-methyl[1,2,4]triazine (RTI-4229-828) were totally inactive. Morphine tolerance was induced in mice by implanting a 75 mg morphine pellet and assessing morphine-induced antinociception 72-h later. The morphine-pelleted mice showed a 5.5-fold tolerance to the antinociceptive effect of acute morphine compared to placebo-pelleted mice in the tail immersion test. Intracerebroventricular (i.c.v.) administration of the three active mGluR5 antagonists dose-dependently reversed morphine antinociceptive tolerance. The ID(50) values were 57.7, 25.8 and 64.3 nmol/mouse, for RTI-4229-707, RTI-4229-766 and RTI-4229-787, respectively, compared to 1050 nmol/mouse for MPEP. Similar to the hyperalgesia study, test compounds RTI-4229-785 and RTI-4229-828 were totally inactive in reversing morphine tolerance. These results are in agreement with our previous study in which we demonstrated that the same active mGluR5 antagonists blocked glutamate-mediated mobilization of internal calcium in a selective mGluR5 in vitro efficacy assay.

Topics: Analgesics, Opioid; Animals; Calcium Signaling; Central Nervous System; Dose-Response Relationship, Drug; Drug Tolerance; Excitatory Amino Acid Agonists; Excitatory Amino Acid Antagonists; Glutamic Acid; Glycine; Hyperalgesia; Male; Mice; Morphine; Nociceptors; Pain; Placebo Effect; Pyridines; Receptor, Metabotropic Glutamate 5; Receptors, Metabotropic Glutamate; Resorcinols; Synaptic Transmission

Prolonged intrathecal (i.t.) administration of morphine results in tolerance to morphine-induced antinociception. We found that co-administration of selective metabotropic glutamate receptor subtype 5 antagonist MPEP with morphine could suppress the loss of morphine-induced antinociception and inhibit the development of tolerance to morphine-induced antinociceptive effect. Whereas, the specific metabotropic glutamate receptor subtype 5 agonist CHPG does the opposite. As the activation of NMDA receptor after chronic morphine administration has been verified, we suppose there is an enhanced activation of mGluR5 during the development of tolerance to morphine-induced antinociception. Activation of mGluR5 may mobilize the release of intracellular Ca(2+) and activate PKC, leading to morphine-induced antinociception suppression. We conclude that mGluR5 contributes to the development of tolerance to morphine-induced antinociception after chronic morphine exposure.

Topics: Analgesics, Opioid; Animals; Calcium; Drug Administration Schedule; Drug Tolerance; Excitatory Amino Acid Agonists; Excitatory Amino Acid Antagonists; Glutamic Acid; Glycine; Injections, Spinal; Male; Morphine; Neurons, Afferent; Nociceptors; Pain; Pain Threshold; Phenylacetates; Protein Kinase C; Pyridines; Rats; Receptor, Metabotropic Glutamate 5; Receptors, Metabotropic Glutamate; Spinal Cord; Synaptic Transmission; Up-Regulation

mGlu1 and mGlu5 receptors have been implicated in pain associated with inflammation. In the present study, the formalin test was used to measure sustained pain with components of tissue injury. The aims of the present study were to assess: (i) the role of mGlu1 and mGlu5 receptors in inflammatory pain using selective antagonist EMQMCM, 1.25-5 mg/kg, as the mGlu1 receptor antagonist, and MPEP or MTEP, 2.5-10 mg/kg, as mGlu5 receptor antagonist; (ii) the possible interaction between mGlu1 and mGlu5 receptor antagonists and morphine; and (iii) whether tolerance develops to the analgesic effects of these antagonists after prolonged treatment. EMQMCM, MTEP and MPEP significantly reduced the manifestation of both phases of formalin response. However, all these mGlu receptor antagonists did not affect the withdrawal latencies in a model of acute pain (Hargreaves test), which has a different underlying mechanism. In the present study, the suppressive effect on formalin-induced pain behaviour was much stronger when mGlu1 and mGlu5 receptor antagonists were co-injected compared to administration of a single antagonist, but this effect was not seen when mGlu receptor antagonist was co-administered with morphine. This is in contrast to the pronounced inhibitory effects after co-treatment with morphine and the uncompetitive NMDA receptor antagonist memantine. The present study also provides the first direct in vivo evidence that prolonged administration of MTEP (5 mg/kg) over 7 days leads to the development of tolerance to its antinociceptive effects. Such tolerance was not observed when EMQMCM (5 mg/kg) was administered in the same manner. In conclusion, these results provide additional arguments for the role of group I mGlu receptors in pain with inflammatory conditions.

Topics: Analgesics; Analysis of Variance; Animals; Behavior, Animal; Dose-Response Relationship, Drug; Drug Interactions; Drug Tolerance; Formaldehyde; Male; Morphine; Pain; Pain Measurement; Pain Threshold; Pyridines; Quinolines; Rats; Rats, Sprague-Dawley; Receptor, Metabotropic Glutamate 5; Receptors, Metabotropic Glutamate; Thiazoles

Pain has a strong emotional component. The amygdala plays a key role in emotionality and is also involved in pain processing and pain modulation. Our previous studies showed an important role of group I metabotropic glutamate receptors (mGluRs) in pain-related synaptic plasticity and sensitization of neurons in the central nucleus of the amygdala (CeA). Here we address the roles of mGluR1 and mGluR5 subtypes in the CeA in the modulation of supraspinally organized behavioral responses in a model of arthritic pain. Audible and ultrasonic (25+/-4 kHz) vocalizations were measured in awake rats during and after innocuous and noxious stimulation (15 s) of the knee joint. Vocalizations were recorded in the same animals before arthritis, 6 h after arthritis induction and during administration of antagonists selective for mGluR1 (CPCCOEt) and mGluR5 (MPEP) into the CeA through stereotaxically implanted microdialysis probes. The duration of audible and ultrasonic vocalizations increased in the arthritic pain state. The duration of vocalizations during stimulation (VDS), which are organized at the brainstem level, was significantly reduced by CPCCOEt but not by MPEP. Vocalizations that continued after stimulation (VAS), which are organized in the limbic forebrain, particularly the amygdala, were inhibited by CPCCOEt and MPEP. These findings suggest differential roles of mGluR1 and mGluR5 in the CeA in pain-related vocalizations. Both mGluR1 and mGluR5 contribute to vocalizations generated in the amygdala whereas mGluR1, but not mGluR5, is involved in the amygdala-mediated modulation of vocalizations originating from activity in the brainstem.

Topics: Amygdala; Analysis of Variance; Animals; Arthritis; Chondrus; Chromones; Disease Models, Animal; Excitatory Amino Acid Antagonists; Male; Microdialysis; Pain; Physical Stimulation; Pyridines; Rats; Rats, Sprague-Dawley; Receptor, Metabotropic Glutamate 5; Receptors, Metabotropic Glutamate; Vocalization, Animal

We investigated whether the metabotropic glutamate receptor subtype 5 (mGluR5) selective antagonist, 2-methyl-6-(phenylethynyl)-pyridine (MPEP) has direct effects on primary afferent fiber responses to noxious mechanical stimulation following inflammation. Mechanosensory primary afferent fibers innervating the hind paw were recorded in naïve and complete Freunds adjuvant (CFA) inflamed rats. Following intraplantar injection of CFA, afferent fibers showed property changes including expanded receptive fields, burst firing with fast adaptive mechanical responses and a higher incidence of cold and/or heat sensitivities compared to naïve rats. In eight afferent fibers tested following i.v. administration of MPEP, seven fibers showed significantly reduced responses to noxious mechanical stimulation. At a cumulative dose of 10 mg/kg, MPEP inhibited afferent responses to 32.66+/-11.48% of control. The mean ID50 value of MPEP was 6.49+/-0.43 mg/kg. In contrast to its inhibitory action in the CFA model, i.v. administration of MPEP produced only a mild reduction of mechanical responses in 3 fibers out of 11 in naïve rats. These results provide direct functional evidence that blockade of peripheral mGluR5 receptors inhibits nociceptive transmission and support previous studies demonstrating a peripheral site of action associated with the antinociceptive effect of MPEP following inflammation.

Topics: Action Potentials; Afferent Pathways; Animals; Disease Models, Animal; Dose-Response Relationship, Drug; Excitatory Amino Acid Antagonists; Freund's Adjuvant; Hindlimb; Inflammation; Mechanoreceptors; Pain; Pain Measurement; Physical Stimulation; Pyridines; Rats

In this study, we evaluated the effects of 2-methyl-6-(phenylethynyl)-pyridine (MPEP), a selective antagonist of metabotropic glutamate subtype 5 receptors (mGlu(5)), delivered through different paths on dorsal raphe serotonin (5-HT) and on thermoceptive responses in rats with inflammatory pain. Intraplantar formalin and carrageenan increased 5-HT (137+/-11% and 212+/-6% of pre-injection baseline, respectively) and reduced nociceptive threshold (23+/-7% and 19+/-3% of pre-injection baseline, respectively). MPEP (2 mg/kg i.p.) further enhanced formalin and carrageenan-induced 5-HT increases (180+/-11% and 260+/-12% of pre-injection baseline, respectively) and reduced thermal hyperalgesia (71+/-8% and 80+/-10% of pre-injection baseline, respectively). MPEP (1 mM) through microdialytic probe into the dorsal raphe did not change formalin- or carrageenan-induced 5-HT increases (147+/-10% and 189+/-10% of pre-injection baseline, respectively) and thermal hyperalgesia (35+/-8% and 25+/-9% of pre-injection baseline, respectively). Finally, MPEP (30 nmol/rat) into the hind paw reduced the formalin- and carrageenan-induced 5-HT increase (108+/-3% and 126+/-7% of pre-injection baseline, respectively) and thermal hyperalgesia (77+/-6% and 117+/-7% of pre-injection baseline, respectively). Dorsal raphe serotonergic neurons activity increased following a peripherally induced inflammatory injury. In these conditions, peripheral but not dorsal raphe mGlu(5) receptors blockade prevented over activation of dorsal raphe serotonergic neurons and reversed thermal hyperalgesia.

Topics: Animals; Carrageenan; Formaldehyde; Hindlimb; Inflammation; Male; Microdialysis; Pain; Pain Measurement; Pyridines; Raphe Nuclei; Rats; Rats, Wistar; Receptor, Metabotropic Glutamate 5; Receptors, Metabotropic Glutamate; Serotonin

Preclinical data, performed in a limited number of pain models, suggest that functional blockade of metabotropic glutamate (mGlu) receptors may be beneficial for pain management. In the present study, effects of 2-methyl-6-(phenylethynyl)-pyridine (MPEP), a potent, selective mGlu5 receptor antagonist, were examined in a wide variety of rodent nociceptive and hypersensitivity models in order to fully characterize the potential analgesic profile of mGlu5 receptor blockade. Effects of 3-[(2-methyl-1,3-thiazol-4-yl)ethynyl]pyridine (MTEP), as potent and selective as MPEP at mGlu5/mGlu1 receptors but more selective than MPEP at N-methyl-aspartate (NMDA) receptors, were also evaluated in selected nociceptive and side effect models. MPEP (3-30 mg/kg, i.p.) produced a dose-dependent reversal of thermal and mechanical hyperalgesia following complete Freund's adjuvant (CFA)-induced inflammatory hypersensitivity. Additionally, MPEP (3-30 mg/kg, i.p.) decreased thermal hyperalgesia observed in carrageenan-induced inflammatory hypersensitivity without affecting paw edema, abolished acetic acid-induced writhing activity in mice, and was shown to reduce mechanical allodynia and thermal hyperalgesia observed in a model of post-operative hypersensitivity and formalin-induced spontaneous pain. Furthermore, at 30 mg/kg, i.p., MPEP significantly attenuated mechanical allodynia observed in three neuropathic pain models, i.e. spinal nerve ligation, sciatic nerve constriction and vincristine-induced neuropathic pain. MTEP (3-30 mg/kg, i.p.) also potently reduced CFA-induced thermal hyperalgesia. However, at 100 mg/kg, i.p., MPEP and MTEP produced central nerve system (CNS) side effects as measured by rotarod performance and exploratory locomotor activity. These results suggest a role for mGlu5 receptors in multiple nociceptive modalities, though CNS side effects may be a limiting factor in developing mGlu5 receptor analgesic compounds.

Topics: Acetic Acid; Animals; Carrageenan; Central Nervous System; Constriction, Pathologic; Edema; Formaldehyde; Hyperalgesia; Male; Mice; Mice, Inbred ICR; Motor Activity; Pain; Pain Measurement; Pain, Postoperative; Psychomotor Performance; Pyridines; Rats; Rats, Sprague-Dawley; Receptor, Metabotropic Glutamate 5; Receptors, Metabotropic Glutamate; Spinal Nerves; Thiazoles; Vincristine

In this study we examined the effects of the glutamate metabotropic subtype 5 (mGlu5) receptor antagonist 2-methyl-6-(phenylethynyl)-pyridine (MPEP) on Fos expression in the spinal cord in a model of visceral pain in the rat. We show that noxious stimulation increases the number of Fos-positive neurons in the dorsal horn of the thoracic and lumbar spinal cord, and that pretreatment with MPEP significantly reduces the number of Fos-positive neurons in these areas. These data indicate that mGlu5 is involved in the transmission of visceral pain in the spinal cord.

Topics: Acetic Acid; Animals; Excitatory Amino Acid Antagonists; Gene Expression Regulation; Genes, fos; Male; Neurons; Pain; Pain Measurement; Physical Stimulation; Proto-Oncogene Proteins c-fos; Pyridines; Rats; Rats, Sprague-Dawley; Receptor, Metabotropic Glutamate 5; Receptors, Metabotropic Glutamate; Spinal Cord

Activation of spinal cord dorsal horn ionotropic glutamate receptors leads to pain-related behaviors. However, the role of spinal metabotropic glutamate receptors (mGlu), particularly the mGlu5 receptor subtype, in nociception has not been well characterized. A recently described subtype selective and potent mGluR5 antagonist, 2-methyl-6-(phenylethynyl)pyridine (MPEP) was used to evaluate the role of the mGlu5 receptor in cold sensitivity. Intrathecal (i.t.) injection of group I (mGlu1 and mGlu5 receptors) mGlu receptor-selective agonist (RS)-3,5-dihydroxyphenylglycine (DHPG) increased the hind paw frequency and duration of lifting of rats placed on a cold (4 degrees C) surface, a behavior similarly observed in rats with a chronic constriction injury (CCI) of the sciatic nerve. In contrast, rats i.t. injected with DHPG did not display increased lifting when placed on a room temperature surface. I.t. injection of MPEP before i.t. injection of DHPG blocked DHPG-evoked cold hypersensitivity, suggesting that activation of spinal mGlu5 receptors induces this behavioral response. In contrast, i.t. injection of MPEP after i.t. injection of DHPG had no effect. In addition, i.t. injection of MPEP did not affect cold hypersensitivity in rats with a CCI. These data suggest that acute activation of spinal cord mGlu5 receptors results in increased sensitivity to cold, but ongoing cold hypersensitivity does not involve activation of the mGlu5 receptor.

Topics: Animals; Behavior, Animal; Cold Temperature; Constriction, Pathologic; Dizocilpine Maleate; Excitatory Amino Acid Antagonists; Injections, Spinal; Male; Methoxyhydroxyphenylglycol; Pain; Pyridines; Rats; Rats, Sprague-Dawley; Receptor, Metabotropic Glutamate 5; Receptors, Metabotropic Glutamate; Sciatic Nerve; Spinal Cord

In rats with chronic constriction of one sciatic nerve (CCI rats), showing behavioural signs of neuropathic pain, 6-methyl-2-(phenylethynyl)-pyridine (MPEP), a selective mGluR5 antagonist, was intraperitoneally administered at 0.75, 1.0 and 1.5 mg/kg or spinally microejected and the effects on the lumbar wide dynamic range neurons activity were investigated. In CCI rats MPEP at 1.0 and 1.5 (but not at 0.75) mg/kg, or spinally microejected induced a significant reduction of the spontaneous (SA) and noxious evoked activity (NEA), and a significant decrease of the suppression of the afterdischarge duration. In sham rats SA was unaffected and NEA was significantly reduced by 1.0 and 1.5 mg/kg MPEP dosages. These findings indicate that the metabotropic GluR5 receptor plays a role in the spinal cord processes underlying neuropathic pain and represents a potential target for new therapeutic approaches.

Topics: Action Potentials; Animals; Electrophysiology; Excitatory Amino Acid Antagonists; Lumbosacral Region; Neurons; Pain; Pyridines; Rats; Receptor, Metabotropic Glutamate 5; Receptors, Metabotropic Glutamate; Sciatic Nerve; Spinal Cord

The present series of experiments were designed to examine the contribution of metabotropic glutamate receptor subtype 5 (mGluR5) to neuropathic pain by determining the effects of the selective mGluR5 antagonist MPEP (2-methyl-6-(phenylethynyl)-pyridine) on neuropathy-induced cold hypersensitivity. Unilateral chronic constriction injury (CCI) to the sciatic nerve in rats produced an increase in the number of hind paw withdrawals from a cold surface (4 +/- 2 degrees C) which was dose-dependently inhibited by systemic (i.p.) injection of MPEP (ID(50) = 11.3 mg/kg). In vivo brain mGluR5 receptor occupancy following systemic (i.p.) MPEP revealed that >90% occupancy is required for behavioral efficacy. Intracerebroventricular (i.c.v.) injection of MPEP dose-dependently inhibited CCI-induced cold hypersensitivity (ID(50) = 123.5 nmol), while microinjection of MPEP directly into the rostral ventromedial medulla (RVM) potently inhibited this hypersensitivity (ID(50) = 1.3 pmol). A role for mGluR5 in the RVM was further supported by the observation that intra-RVM injection of the mGluR5 agonist CHPG (10 nmol; 2-chloro-5-hydroxyphenylglycine) produced cold hypersensitivity in naïve rats that was blocked by pretreatment with intra-RVM MPEP (3 nmol). Intrathecal (500 nmol; i.t.) or intraplantar (300 nmol; i.pl.) injection of MPEP was ineffective in reversing CCI-induced cold hypersensitivity. These results demonstrate that mGluR5 contributes to cold hypersensitivity following peripheral neuropathy exclusively at supraspinal sites in the CNS. Additionally, mGluR5 in the RVM significantly contributes to the maintenance of cold hypersensitivity, likely via activation of descending nociceptive facilitatory systems.

Topics: Animals; Cold Temperature; Constriction, Pathologic; Excitatory Amino Acid Antagonists; Injections, Intraventricular; Injections, Spinal; Male; Medulla Oblongata; Microinjections; Pain; Peripheral Nervous System Diseases; Pyridines; Radioligand Assay; Rats; Rats, Sprague-Dawley; Receptor, Metabotropic Glutamate 5; Receptors, Metabotropic Glutamate; Sciatic Nerve

Intrathecal (i.t.) administration of the group I metabotropic glutamate receptor (mGluR) agonist (RS)-3,5-dihydroxyphenylglycine ((RS)-3,5-DHPG) to rats produces an immediate display of spontaneous nociceptive behaviors (SNBs) persisting for up to 10 h after injection (NeuroReport 7 (1996) 2743). The mechanisms underlying these behavioral effects are not entirely understood but may include enhanced release of glutamate within the dorsal horn of the spinal cord. The current experiments used microdialysis in awake moving animals to test: (1), whether i.t. (S)-3,5-DHPG increases the local release of glutamate at doses that also induce SNBs; and (2), whether the effects on glutamate release (as well as SNBs) can be blocked by pretreatment with the mGluR5 selective antagonist 2-methyl-6-(phenylethynyl)-pyridine (MPEP). Male Sprague-Dawley rats were implanted with a microdialysis probe inserted into the i.t. space of the spinal cord (J. Neurosci. Methods 62 (1995) 43) and then tested under i.t. drug conditions (0.01, 0.1 and 1 mM (S)-3,5-DHPG) following a 2-3 day recovery period. As predicted, local application of (S)-3,5-DHPG via the microdialysis probe increased the release of glutamate in a dose-dependent manner. Significant SNBs were also noted in the 0.1 and 1 mM groups in a manner paralleling the onset and duration of the glutamate response. Pretreatment with MPEP (55 mg/kg, intraperitoneally) blocked glutamate release to the 0.1 mM dose of (S)-3,5-DHPG, and also decreased the proportion of animals displaying SNBs in this dose group. No effects of MPEP were seen against the higher dose of (S)-3,5-DHPG (1 mM). These results suggest that stimulation of spinal mGluR5 leads to glutamate release within the spinal cord, a response that may in part account for the nociceptive behaviors evoked by i.t. (S)-3,5-DHPG.

Topics: Animals; Behavior, Animal; Excitatory Amino Acid Agonists; Excitatory Amino Acid Antagonists; Extracellular Space; Glutamic Acid; Glycine; Male; Microdialysis; Pain; Pyridines; Rats; Rats, Sprague-Dawley; Receptors, Metabotropic Glutamate; Resorcinols; Spinal Cord; Time Factors

Spinal cord injury (SCI) initiates a cascade of biochemical events that leads to an increase in extracellular excitatory amino acid (EAA) concentrations, which results in glutamate receptor-mediated excitotoxic events. An important division of these glutamate receptors is the metabotropic glutamate receptor (mGluR) class, which is divided into three groups. Of these three groups, group I (mGluR1 and mGluR5) activation can initiate a number of intracellular pathways that lead to increased extracellular EAA concentrations. To evaluate subtypes of group I mGluRs in SCI, we administered AIDA (group I antagonist), LY 367385 (mGluR1 specific antagonist), or MPEP (mGluR5 specific antagonist) by interspinal injection to adult male Sprague-Dawley rats (175-200 g) immediately following injury at T10 with an NYU impactor (12.5-mm drop, 10-g rod, 2 mm in diameter). AIDA- and LY 367385-treated subjects had improved locomotor scores and demonstrated an attenuation in the development of mechanical allodynia as measured by von Frey stimulation of the forelimbs; however, LY 367385 potentiated the development of thermal hyperalgesia. MPEP had no effect on locomotor recovery or mechanical allodynia, but attenuated the development of thermal hyperalgesia. AIDA and LY 367385 treatment resulted in a significant increase in tissue sparing compared to the vehicle-treated group at 4 weeks following SCI. These results suggest that mGluRs play an important role in EAA toxicity and have different acute pathophysiological roles following spinal cord injury.

Topics: Animals; Benzoates; Chronic Disease; Excitatory Amino Acid Antagonists; Glycine; Hyperalgesia; Locomotion; Male; Neuroprotective Agents; Neurotoxins; Pain; Pain Threshold; Physical Stimulation; Pyridines; Rats; Rats, Sprague-Dawley; Receptors, Metabotropic Glutamate; Spinal Cord Injuries

This study investigates the effect of microinjections of capsaicin in the periaqueductal grey matter of rats on nociceptive behaviour and the possible interactions with NMDA and mGlu receptors. Intra-periaqueductal grey microinjection of capsaicin (1-3-6 nmol/rat) increased the latency of the nociceptive reaction in the plantar test. This effect was prevented by pretreatment with capsazepine (6 nmol/rat), which had no effect per se on the latency of the nociceptive reaction. 7-(Hydroxyimino)cyclopropa[b]chromen-1alpha-carboxylate ethyl ester (CPCCOEt, 50 nmol/rat) and 2-Methyl-6-(phenylethynyl)pyridine (MPEP, 50 nmol/rat), antagonists of mGlu(1) and mGlu(5) receptors, respectively, completely blocked the effect of capsaicin. Similarly, pretreatment with DL-2-Amino-5-phosphonovaleric acid (DL-AP5, 5 nmol/rat) and riluzole (4 nmol/rat), an NMDA receptor antagonist and a voltage-dependent Na(+) channels blocker which inhibits glutamate release, respectively, completely antagonized the effect of capsaicin. However, pretreatment with (2S)-alpha-Ethylglutamic acid (30 nmol/rat) and (RS)-alpha-Methylserine-O-phosphate (MSOP, 30 nmol/rat), antagonists of group II and group III mGlu receptors, respectively, had no effects on capsaicin-induced analgesia. Similarly, pretreatment with N-(piperidin-1-yl)-5-(4-chlophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide (SR 141716A, 5 pmol/rat), a selective cannabinoid CB(1) receptor antagonist, did not affect the capsaicin-induced antinociception. In conclusion, this study shows that capsaicin might produce antinociception at the periaqueductal grey level by increasing glutamate release, which activates postsynaptic group I mGlu and NMDA receptors.

Topics: 2-Amino-5-phosphonovalerate; Animals; Capsaicin; Chromones; Excitatory Amino Acid Antagonists; Male; Pain; Periaqueductal Gray; Phosphoserine; Piperidines; Protein Binding; Pyrazoles; Pyridines; Rats; Rats, Wistar; Receptors, Drug; Receptors, Glutamate; Riluzole; Rimonabant; Time Factors

The excitatory neurotransmitter, glutamate, is particularly important in the transmission of pain information in the nervous system through the activation of ionotropic and metabotropic glutamate receptors. A potent, subtype-selective antagonist of the metabotropic glutamate-5 (mGlu5) receptor, 2-methyl-6-(phenylethynyl)-pyridine (MPEP), has now been discovered that has effective anti-hyperalgesic effects in models of inflammatory pain. MPEP did not affect rotarod locomotor performance, or normal responses to noxious mechanical or thermal stimulation in naïve rats. However, in models of inflammatory pain, systemic administration of MPEP produced effective reversal of mechanical hyperalgesia without affecting inflammatory oedema. In contrast to the non-steroidal anti-inflammatory drugs, indomethacin and diclofenac, the maximal anti-hyperalgesic effects of orally administered MPEP were observed without acute erosion of the gastric mucosa. In contrast to its effects in models of inflammatory pain, MPEP did not produce significant reversal of mechanical hyperalgesia in a rat model of neuropathic pain.

Topics: Animals; Anti-Inflammatory Agents, Non-Steroidal; Chronic Disease; Excitatory Amino Acid Antagonists; Hyperalgesia; Male; Motor Activity; Nociceptors; Pain; Pain Measurement; Pyridines; Rats; Rats, Sprague-Dawley; Receptor, Metabotropic Glutamate 5; Receptors, Metabotropic Glutamate; Stomach Ulcer

The metabotropic glutamate receptors (mGluRs) are found throughout the central nervous system, where they modulate neuronal excitability and synaptic transmission. Here we report the presence of phospholipase C-coupled group I mGluRs (mGluR1 and mGluR5) outside the central nervous system on peripheral unmyelinated sensory afferents. Given their localization on predominantly nociceptive afferents, we investigated whether these receptors modulate nociceptive signaling, and found that agonist-induced activation of peripheral group I mGluRs leads to increased sensitivity to noxious heat, a phenomenon termed thermal hyperalgesia. Furthermore, group I mGluR antagonists not only prevent, but also attenuate established formalin-induced pain. Taken together, these results suggest that peripheral mGluRs mediate a component of hyperalgesia and may be therapeutically targeted to prevent and treat inflammatory pain.

Topics: Animals; Chromones; Excitatory Amino Acid Antagonists; Glycine; Hindlimb; Hot Temperature; Immunohistochemistry; Inflammation; Male; Mice; Mice, Inbred C57BL; Neurons, Afferent; Pain; Pain Measurement; Pyridines; Receptors, Metabotropic Glutamate; Valine

The effect of the mGluR(5) antagonist, MPEP (2-Methyl-6-(phenylethynyl)-pyridine), and of the mGluR(1) antagonist, AIDA((RS)-1-Aminoindan-1,5-dicarboxylic acid), were examined on nociceptive neurons in the ventroposterolateral (VPL) nucleus of the thalamus in response to pressure stimuli to the contralateral hindpaw of rats under urethane anesthesia. Intravenous (i.v.) injection of MPEP (0.1, 1, and 10 mg/kg) blocked responses to noxious stimulation in a dose-dependent and reversible manner. AIDA (3 and 15 mg/kg, i.v.), in contrast, had no effect on these cells. MPEP action was selective to noxious stimulation because even when tested at the highest dose (10 mg/kg, i.v.) it did not alter the responses of non-nociceptive neurons to brush stimulation. To investigate the site of action of MPEP, intra-thalamic injections were made during electrophysiological recordings. Using this method, the mGluR(5) antagonist did not affect nociceptive responses, suggesting that thalamic receptors were not involved in this action. On the other hand, the NMDA thalamic receptors seem to be involved because the NMDA receptor antagonist, MK801, successfully blocked responses to noxious pressure stimulation following intra-thalamic injections. In the spinal cord in vitro model, MPEP (30 microM, 60 min) was also able to attenuate ventral root potentials after single shock electrical stimulation of the dorsal root and inhibit wind-up response evoked by repetitive stimulation. Taken together, these findings suggest that blockade of the mGluR(5), but not mGluR(1) decreases nociceptive transmission in the thalamus and that these effects may be mediated by spinal cord receptors.

Topics: Animals; Electric Stimulation; Excitatory Amino Acid Antagonists; In Vitro Techniques; Indans; Injections, Intravenous; Male; Neurons; Nociceptors; Pain; Pyridines; Rats; Rats, Sprague-Dawley; Receptor, Metabotropic Glutamate 5; Receptors, Metabotropic Glutamate; Receptors, N-Methyl-D-Aspartate; Spinal Cord; Synaptic Transmission; Ventral Thalamic Nuclei