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

Temporomandibular disorders (TMD) comprise an assortment of clinical conditions characterized by pain in the temporomandibular joint (TMJ). TMD patients have a variety of symptoms, including jaw movement disorder and TMJ pain. Metabotropic glutamate receptor subtype 5 (mGluR5) was reported to be involved in pain processing in several animal models of neuropathic and inflammatory pain. In this study, the head withdrawal threshold and mGluR5 expression were investigated in rats with complete Freund's adjuvant (CFA)-induced TMJ inflammatory pain. CFA injection into the TMJ significantly decreased the mechanical head withdrawal thresholds relative to vehicle injection, and the effects were blocked by pre-injection of 2-methyl-6-(phenylethynyl)-pyridine (MPEP). mGluR5 expression in the trigeminal ganglion was predominantly increased in the CFA-injected group compared with the normal control group. Pretreatment with MPEP, a selective mGluR5 antagonist, reduced mGluR5 expression in the trigeminal ganglion compared with the CFA group, as measured by immunohistochemistry, western blotting, and RT-PCR. Significant differences in the proportion or intensity of mGluR5 expression were found in animals with inflammation versus control animals at the examined time point. These findings indicate a role for peripheral mGluR5 in CFA-induced nociceptive behavior and TMJ inflammation. Peripheral application of mGluR5 antagonists could provide therapeutic benefits for inflammatory TMJ pain.

Topics: Animals; Freund's Adjuvant; Inflammation; Joint Diseases; Male; Pyridines; Rats; Rats, Sprague-Dawley; Receptor, Metabotropic Glutamate 5; Temporomandibular Joint; Trigeminal Ganglion

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

The loss of nigrostriatal dopaminergic neurons that characterizes Parkinson's disease (PD) causes complex functional alterations in the basal ganglia circuit. Increased glutamatergic activity at crucial points of the circuit may be central to these alterations, thereby contributing to the onset of PD motor symptoms. Signs of neuroinflammation accompanying the neuronal loss have also been observed; also in this case, glutamate-mediated mechanisms may be involved. Glutamate may therefore intervene at multiple levels in PD pathophysiology, possibly through the modulation of metabotropic receptors. To address this issue, we evaluated the effects of systemic treatment with MPEP (2-methyl-6-(phenylethynyl)-pyridine), an antagonist of metabotropic receptor mGluR5, in a rodent model of progressive nigrostriatal degeneration based on the intrastriatal injection of 6-hydroxydopamine (6-OHDA). Following 6-OHDA injection, Sprague-Dawley rats underwent a 4-week, daily treatment with MPEP (1.5mg/kg, i.p.). To investigate whether the effects varied with the progression of the lesion, subgroups of lesioned animals started the treatment at different time-points: (1) immediately, (2) 1 week, or (3) 4 weeks after the neurotoxin injection. Akinesia, dopaminergic nigrostriatal damage and neuroinflammatory response (microglial and astroglial activation) were investigated. MPEP prompted immediate amelioration of 6-OHDA-induced akinesia, as measured by the Adjusting step test, in all subgroups, regardless of the degree of nigrostriatal damage. Conversely, MPEP did not modify neuronal survival or neuroinflammatory response in the nigrostriatal pathway. In conclusion, chronic treatment with MPEP exerted a pure symptomatic effect, further supporting that mGluR5 modulation may be a viable strategy to counteract the basal ganglia functional modifications underlying PD motor symptoms.

Topics: Animals; Behavior, Animal; Corpus Striatum; Disease Models, Animal; Dopamine; Excitatory Amino Acid Antagonists; Humans; Inflammation; Male; Motor Activity; Neurons; Oxidopamine; Parkinsonian Disorders; Pyridines; Random Allocation; Rats; Rats, Sprague-Dawley; Receptor, Metabotropic Glutamate 5; Receptors, Metabotropic Glutamate; Substantia Nigra

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

The present study investigated the role of peripheral group I metabotropic glutamate receptors (mGluRs) in MO-induced nociceptive behaviour and inflammation in the masseter muscles of lightly anesthetized rats. Experiments were carried out on male Sprague-Dawley rats weighing 300-400 g. After initial anesthesia with sodium pentobarbital (40 mg/kg, i.p.), one femoral vein was cannulated and connected to an infusion pump for intravenous infusion of sodium pentobarbital. The rate of infusion was adjusted to provide a constant level of anesthesia. Mustard oil (MO, 30 microl) was injected into the mid-region of the left masseter muscle via a 30-gauge needle over 10s. After 30 microl injection of 5, 10, 15, or 20% MO into the masseter muscle, the total number of hindpaw shaking behaviour and extravasated Evans' blue dye concentration in the masseter muscle were significantly higher in the MO-treated group in a dose-dependent manner compared with the vehicle (mineral oil)-treated group. Intramuscular pretreatment with 3 or 5% lidocaine reduced MO-induced hindpaw shaking behaviour and increases in extravasated Evans' blue dye concentration. Intramuscular pretreatment with 5 mM MCPG, non-selective group I/II mGluR antagonist, or MPEP, a selective group I mGluR5 antagonist, produced a significant attenuation of MO-induced hindpaw shaking behaviour and increases in extravasated Evans' blue dye concentration in the masseter muscle while LY367385, a selective group I mGluR1 antagonist, did not affect MO-induced nociceptive behaviour and inflammation in the masseter muscle. These results indicate that peripheral mGluR5 plays important role in mediating MO-induced nociceptive behaviour and inflammation in the craniofacial muscle.

Topics: Anesthetics, Local; Animals; Behavior, Animal; Benzoates; Dose-Response Relationship, Drug; Drug Interactions; Excitatory Amino Acid Antagonists; Facial Muscles; Facial Pain; Functional Laterality; Glycine; Inflammation; Lidocaine; Male; Mustard Plant; Pain Measurement; Plant Oils; Pyridines; Rats; Rats, Sprague-Dawley

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

Previous studies have demonstrated that the metabotropic glutamate receptor subtype 5 (mGlu5 receptor) is expressed in the cell bodies of rat primary afferent neurones. We have further investigated the function and expression of mGlu5 receptors in primary afferent neurones, and their role in inflammatory nociception. Freund's complete adjuvant-induced inflammatory hyperalgesia of the rat hind paw was significantly reduced by intraplantar, but not by intracerebroventricular or intrathecal microinjection of the selective mGlu5 receptor antagonist, 2-methyl-6-(phenylethynyl)-pyridine (MPEP). Pharmacological comparison in vivo of the nociceptive effects of glutamate, and ionotropic and metabotropic glutamate (mGlu) receptor agonists applied to the rat hind paw, indicated that group I mGlu receptor agonists induce a dose-dependent decrease in paw withdrawal threshold (mechanical hyperalgesia). Group I mGlu agonist-induced hyperalgesia was inhibited by co-microinjection of MPEP, but not by the mGlu1 receptor antagonist (S)-4-carboxy-phenylglycine (4-CPG). Carrageenan-induced inflammatory hyperalgesia was inhibited by pre-treatment of the inflamed hind paw with MPEP, but not following MPEP injection into the contralateral hind paw. Dorsal horn neurones receiving peripheral nociceptive and non-nociceptive afferent input were recorded in anaesthetized rats following microinjection of CHPG into their peripheral receptive fields. CHPG significantly increased the frequency and duration of firing of dorsal horn wide dynamic range (WDR) neurones and this activity was prevented by co-administration of CHPG and MPEP into their receptive fields. Immunohistochemical experiments revealed the co-expression of mGlu5 receptor protein and betaIII tubulin in skin from naive rats, indicating the constitutive expression of mGlu5 receptors on peripheral neurones. Double-labelling of adult rat DRG cells with mGlu5 receptor and vanilloid receptor subtype 1 antisera also supports the expression of mGlu5 receptors on peripheral nociceptive afferents. These results suggest that mGlu5 receptors expressed on the peripheral terminals of sensory neurones are involved in nociceptive processes and contribute to the hyperalgesia associated with inflammation.

Topics: Analgesics, Opioid; Animals; Carrageenan; Excitatory Amino Acid Agonists; Hyperalgesia; Immunohistochemistry; Inflammation; Male; Morphine; Neurons, Afferent; Pain Measurement; Peripheral Nervous System; Pyridines; Rats; Rats, Sprague-Dawley; Receptor, Metabotropic Glutamate 5; Receptors, Metabotropic Glutamate; Skin

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