Antisense ablation of type I metabotropic glutamate receptor mGluR1 inhibits spinal nociceptive transmission

A compendium of validated pain genes

The development of novel pain therapeutics hinges on the identification and rigorous validation of potential targets. Model organisms provide a means to test the involvement of specific genes and regulatory elements in pain. Here we provide a list of genes linked to pain-associated behaviors. We capitalize on results spanning over three decades to identify a set of 242 genes. They support a remarkable diversity of functions spanning action potential propagation, immune response, GPCR signaling, enzymatic catalysis, nucleic acid regulation, and intercellular signaling. Making use of existing tissue and single-cell high-throughput RNA sequencing datasets, we examine their patterns of expression. For each gene class, we discuss archetypal members, with an emphasis on opportunities for additional experimentation. Finally, we discuss how powerful and increasingly ubiquitous forward genetic screening approaches could be used to improve our ability to identify pain genes. This article is categorized under: Neurological Diseases > Genetics/Genomics/Epigenetics Neurological Diseases > Molecular and Cellular Physiology.

Impaired sensitivity to pain stimuli in plasma membrane calcium ATPase 2 (PMCA2) heterozygous mice: a possible modality- and sex-specific role for PMCA2 in nociception

Plasma membrane calcium ATPase 2 (PMCA2) is a calcium pump that plays important roles in neuronal function. Although it is expressed in pain-associated regions of the CNS, including in the dorsal horn (DH), its contribution to pain remains undefined. The present study assessed the role of PMCA2 in pain responsiveness and the link between PMCA2 and glutamate receptors, GABA receptors (GABARs), and glutamate transporters that have been implicated in pain processing in the DH of adult female and male PMCA2^+/+ and PMCA2^+/- mice. Behavioral assays evaluated mechanical and thermal pain responsiveness. Mechanical sensitivity was significantly increased by 52% and heat sensitivity was reduced by 29% in female, but not male, PMCA2^+/- mice compared with PMCA2^+/+ controls. There were female-specific changes in metabotropic glutamate receptor 1, NMDA receptor 2A, α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor subunit GluR1, GABA_BR1, and GABA_BR2 levels, whereas metabotropic glutamate receptor 5, NMDA receptor 2B, GluR2, and GABA_ARα2 levels were not altered. Glutamate aspartate transporter levels were higher and glial glutamate transporter 1 levels were lower in the DH of female, but not male, PMCA2^+/- mice. These findings indicate a novel role for PMCA2 in modality- and sex-dependent pain responsiveness. Female-specific molecular changes potentially account for the altered pain responses.-Khariv, V., Ni, L., Ratnayake, A., Sampath, S., Lutz, B. M., Tao, X.-X., Heary, R. F., Elkabes, S. Impaired sensitivity to pain stimuli in plasma membrane calcium ATPase 2 (PMCA2) heterozygous mice: a possible modality- and sex-specific role for PMCA2 in nociception.

Local group I mGluR antagonists reduce TMJ-evoked activity of trigeminal subnucleus caudalis neurons in female rats

Group I metabotropic glutamate receptors (mGluR1 and mGluR5) are functionally linked to estrogen receptors and play a key role in the plasticity of central neurons. Estrogen status strongly influences sensory input from the temporomandibular joint (TMJ) to neurons at the spinomedullary (Vc/C1-2) region. This study tested the hypothesis that TMJ input to trigeminal subnucleus caudalis/upper cervical cord (Vc/C1-2) neurons involved group I mGluR activation and depended on estrogen status. TMJ-responsive neurons were recorded in superficial laminae at the Vc/C1-2 region in ovariectomized (OvX) female rats treated with low-dose estradiol (2 μg/day, LE) or high-dose estradiol (20 μg/day, HE) for 2 days. TMJ-responsive units were activated by adenosine triphosphate (ATP, 1mM) injected into the joint space. Receptor antagonists selective for mGluR1 (CPCCOEt) or mGluR5 (MPEP) were applied topically to the Vc/C1-2 surface at the site of recording 10 min prior to the intra-TMJ ATP stimulus. In HE rats, CPCCOEt (50 and 500 μM) markedly reduced ATP-evoked unit activity. By contrast, in LE rats, a small but significant increase in neural activity was seen after 50 μM CPCCOEt, while 500 μM caused a large reduction in activity that was similar in magnitude as that seen in HE rats. Local application of MPEP produced a significant inhibition of TMJ-evoked unit activity independent of estrogen status. Neither mGluR1 nor mGluR5 antagonism altered the spontaneous activity of TMJ units in HE or LE rats. High-dose MPEP caused a small reduction in the size of the convergent cutaneous receptive field in HE rats, while CPCCOEt had no effect. These data suggest that group I mGluRs play a key role in sensory integration of TMJ nociceptive input to the Vc/C1-2 region and are largely independent of estrogen status.

Metabotropic glutamate 1 receptor: current concepts and perspectives

Almost 25 years after the first report that glutamate can activate receptors coupled to heterotrimeric G-proteins, tremendous progress has been made in the field of metabotropic glutamate receptors. Now, eight members of this family of glutamate receptors, encoded by eight different genes that share distinctive structural features have been identified. The first cloned receptor, the metabotropic glutamate (mGlu) receptor mGlu1 has probably been the most extensively studied mGlu receptor, and in many respects it represents a prototypical subtype for this family of receptors. Its biochemical, anatomical, physiological, and pharmacological characteristics have been intensely investigated. Together with subtype 5, mGlu1 receptors constitute a subgroup of receptors that couple to phospholipase C and mobilize Ca(2+) from intracellular stores. Several alternatively spliced variants of mGlu1 receptors, which differ primarily in the length of their C-terminal domain and anatomical localization, have been reported. Use of a number of genetic approaches and the recent development of selective antagonists have provided a means for clarifying the role played by this receptor in a number of neuronal systems. In this article we discuss recent advancements in the pharmacology and concepts about the intracellular transduction and pathophysiological role of mGlu1 receptors and review earlier data in view of these novel findings. The impact that this new and better understanding of the specific role of these receptors may have on novel treatment strategies for a variety of neurological and psychiatric disorders is considered.

Spinal Metabotropic Glutamate Receptors (mGluRs) are Involved in the Melittin-induced Nociception in Rats

Intraplantar injection of melittin has been known to induce sustained decrease of mechanical threshold and increase of spontaneous flinchings. The present study was undertaken to investigate how the melittin-induced nociceptive responses were modulated by changes of metabotropic glutamate receptor (mGluR) activity. Changes in paw withdrawal threshold (PWT), number of flinchings and paw thickness were measured at a given time point after injection of melittin (10 microg/paw) into the mid-plantar area of rat hindpaw. To observe the effects of mGluRs on the melittin-induced nociceptions, group I mGluR (AIDA, 100 microg and 200 microg), mGluR(1) (LY367385, 50 microg and 100 microg) and mGluR(5) (MPEP, 200 microg and 300 microg) antagonists, group II (APDC, 100 microg and 200 microg) and III (L-SOP, 100 microg and 200 microg) agonists were intrathecally administered 20 min before melittin injection. Intraplantar injection of melittin induced a sustained decrease of mechanical threshold, spontaneous flinchings and edema. The effects of melittin to reduce mechanical threshold and to induce spontaneous flinchings were significantly suppressed following intrathecal pre-administration of group I mGluR, mGluR(1) and mGluR(5) antagonists, group II and III mGluR agonists. Group I mGluR antagonists and group II and III mGluR agonists had no significant effect on melittin-induced edema. These experimental findings indicate that multiple spinal mGluRs are involved in the modulation of melittin-induced nociceptive responses.

Analgesic activity of metabotropic glutamate receptor 1 antagonists on spontaneous post-operative pain in rats

Activation of metabotropic glutamate (mGlu) receptors has previously been shown to play a role in inflammatory or neuropathic pain states. However, the role of mGlu type 1 receptors in post-operative pain remains to be investigated. In the present study, effects of potent and selective mGlu1 receptor antagonists A-841720, A-794282, A-794278, and A-850002 were evaluated in a skin incision-induced post-operative pain model in rats. Post-operative pain was examined 2 h following surgery using weight-bearing difference between injured and uninjured paws as a measure of spontaneous pain. In this model, A-841720, A-794282, A-794278, and A-850002 induced significant attenuation of spontaneous post-operative pain behavior, with ED50s of 10, 50, 50, and 65 micromol/kg i.p., respectively. Depending on the compound, significant motor side effects were also observed at 3 to 10 fold higher doses. These results support the notion that mGlu1 receptor activation plays a significant role in nociceptive transmission in post-operative pain, though motor impairment may be a limiting factor in developing mGlu1 receptor antagonists as novel analgesics.

Antinociceptive profile of a selective metabotropic glutamate receptor 1 antagonist YM-230888 in chronic pain rodent models

Metabotropic glutamate receptor 1 (mGlu(1) receptor) has been suggested to play an important role in pain transmission. In this study, the effects of a newly-synthesized mGlu(1) receptor antagonist, (R)-N-cycloheptyl-6-({[(tetrahydro-2-furyl)methyl]amino}methyl)thieno[2,3-d]pyrimidin-4-ylamine (YM-230888), were examined in a variety of rodent chronic pain models in order to characterize the potential analgesic profile of mGlu(1) receptor blockade. YM-230888 bound an allosteric site of mGlu(1) receptor with a K(i) value of 13+/-2.5 nM and inhibited mGlu(1)-mediated inositol phosphate production in rat cerebellar granule cells with an IC(50) value of 13+/-2.4 nM. It showed selectivity for mGlu(1) versus mGlu(2)-mGlu(7) subtypes and ionotropic glutamate receptors. YM-230888 recovered mechanical allodynia with an ED(50) value of 8.4 mg/kg p.o. in L5/L6 spinal nerve ligation models. It also showed antinociceptive response at doses of 10 and 30 mg/kg p.o. in streptozotocin-induced hyperalgesia models. In addition, it significantly reduced pain parameters at a dose of 30 mg/kg p.o. in complete Freund's adjuvant-induced arthritic pain models. Although YM-230888 showed no significant effect on rotarod performance time at doses of 10 or 30 mg/kg p.o., it significantly decreased it at a dose of 100 mg/kg p.o. On the other hand, YM-230888 showed no significant sedative effect in locomotor activity measurement up to 100 mg/kg p.o. These results suggest that the blockade of mGlu(1) receptors is an attractive target for analgesics. YM-230888 has potential as a new analgesic agent for the treatment of various chronic pain conditions. In addition, YM-230888 may be a useful tool for the investigation of mGlu(1) receptors.

Structure–activity relationships of novel non-competitive mGluR1 antagonists: A potential treatment for chronic pain

A series of novel mGluR1 antagonists have been prepared. Incorporation of fragments derived from weak lead matter into a library led to enhanced potency in a new chemical series. A chemistry driven second library iteration, covering a greatly enhanced area of chemical space, maintained good potency and introduced metabolic stability.

Blockade of mGluR1 receptor results in analgesia and disruption of motor and cognitive performances: effects of A-841720, a novel non-competitive mGluR1 receptor antagonist

BACKGROUND AND PURPOSE

To further assess the clinical potential of the blockade of metabotropic glutamate receptors (mGluR1) for the treatment of pain.

EXPERIMENTAL APPROACH

We characterized the effects of A-841720, a novel, potent and non-competitive mGluR1 antagonist in models of pain and of motor and cognitive function.

KEY RESULTS

At recombinant human and native rat mGluR1 receptors, A-841720 inhibited agonist-induced calcium mobilization, with IC50 values of 10.7+/-3.9 and 1.0 +/- 0.2 nM, respectively, while showing selectivity over other mGluR receptors, in addition to other neurotransmitter receptors, ion channels, and transporters. Intraperitoneal injection of A-841720 potently reduced complete Freund's adjuvant-induced inflammatory pain (ED50 = 23 micromol kg(-1)) and monoiodoacetate-induced joint pain (ED50 = 43 micromol kg(-1)). A-841720 also decreased mechanical allodynia observed in both the sciatic nerve chronic constriction injury and L5-L6 spinal nerve ligation (SNL) models of neuropathic pain (ED50 = 28 and 27 micromol kg(-1), respectively). Electrophysiological studies demonstrated that systemic administration of A-841720 in SNL animals significantly reduced evoked firing in spinal wide dynamic range neurons. Significant motor side effects were observed at analgesic doses and A-841720 also impaired cognitive function in the Y-maze and the Water Maze tests.

CONCLUSIONS AND IMPLICATIONS

The analgesic effects of a selective mGluR1 receptor antagonist are associated with motor and cognitive side effects. The lack of separation between efficacy and side effects in pre-clinical models indicates that mGluR1 antagonism may not provide an adequate therapeutic window for the development of such antagonists as novel analgesic agents in humans.

NMDA receptor antagonist treatment at the time of nerve injury prevents injury-induced changes in spinal NR1 and NR2B subunit expression and increases the sensitivity of residual pain behaviours to subsequently administered NMDA receptor antagonists

Spinal NMDA receptors (NMDA R) are important in neuropathic sensitisation and acute administration of antagonists can provide temporary attenuation of sensitisation. If establishment of the chronic pain state could be prevented by brief administration of such agents at or around the time of nerve injury (pre-emptive analgesia) it might be possible to avoid many of the unacceptable side effects associated with repeated administration of these or other antagonists. Several reports describe aspects of effective pre-emptive analgesia from NMDA R antagonists in animal models of neuropathic pain. The first aim of the present study was to make a direct comparison of changes in mechanical allodynia, cold allodynia and thermal hyperalgesia following nerve injury, demonstrating their increasing degree of susceptibility to pre-emptive NMDA R antagonist treatment. Secondly, we used immunoblotting and immunohistochemistry to investigate the effects of nerve injury on NMDA receptor subunit expression, revealing increased expression of NR2B, but not NR2A and reduced NR1 in the superficial dorsal horn. These changes were attenuated following NMDA receptor antagonist pre-treatment. Thirdly, we investigated the pharmacological properties of residual mechanical allodynia and cold allodynia that remained after pre-emptive treatment and revealed a greater sensitivity to NMDA R antagonists. These findings indicate that in addition to a marked suppression of thermal hyperalgesia and cold allodynia, pre-emptive treatment with NMDA R antagonist causes a lasting change in spinal NMDA R complexes such that remaining mechanical allodynia should be more effectively targeted by NMDA R antagonists.

Disinhibition of spinal responses to primary afferent input by antagonism at GABA receptors in urethane-anaesthetised rats is dependent on NMDA and metabotropic glutamate receptors

Disruption of spinal GABAergic circuits, which regulate the conveyance of sensory information to spinal cord neurones from the primary afferent system, leads to miscoding of afferent input and often results in hyperresponsiveness states. In the present work, extracellular field potentials elicited by electrical peripheral nerve activation were recorded in the urethane-anaesthetised rat following spinal administration of GABA(A) or GABA(B) receptor-antagonists, and the involvement of glutamate receptors of the NMDA and metabotropic types in changes induced by altered GABAergic function was examined by pre-treating the spinal dorsal horn with appropriate antagonist drugs. Spinal administration of the GABA(A) receptor antagonist bicuculline (BIC) dose-dependently augmented poly- but not monosynaptic field potentials elicited by activation of A fibres or potentials elicited by activation of C fibres, whereas application of the GABA(B) receptor antagonist CGP35348 significantly increased the amplitudes of C- but not A fibre-evoked potentials. BIC-induced augmentation was blocked by pre-treatment with the NMDA receptor antagonist D-(-)-2-amino-5-phosphonopentanoic acid (D-AP5) or the group I or II metabotropic glutamate receptor (mGluR)-antagonists (RS)-1-aminoindan-1,5-dicarboxylic acid (AIDA) or (2S)-alpha-ethylglutamic acid (EGLU), respectively, but not by the group III mGluR-antagonist (RS)-alpha-methylserine-O-phosphate (MSOP). Augmentation of spinal field potentials induced by CGP35348 was prevented by pre-treatment with D-AP5 but not with mGluR-antagonists. The present findings provide novel evidence that disparate synaptic mechanisms subserved by metabotropic and NMDA glutamate receptors may be involved in spinal hyperresponsiveness states secondary to decreased GABA(A) or GABA(B) receptor activity.

Radioligand Binding Properties and Pharmacological Characterization of 6-Amino-N-cyclohexyl-N,3-dimethylthiazolo[3,2-a]benzimidazole-2-carboxamide (YM-298198), a High-Affinity, Selective, and Noncompetitive Antagonist of Metabotropic Glutamate Receptor Type 1

Metabotropic glutamate receptor type 1 (mGluR1) is thought to play important roles in the neurotransmission and pathogenesis of several neurological disorders. Here, we describe the radioligand binding properties and pharmacological effects of a newly synthesized, high-affinity, selective, and noncompetitive mGluR1 antagonist, 6-amino-N-cyclohexyl-N,3-dimethylthiazolo[3,2-a]benzimidazole-2-carboxamide (YM-298198). YM-298198 inhibited glutamate-induced inositol phosphate production in mGluR1-NIH3T3 cells with an IC50 of 16 +/- 5.8 nM in a noncompetitive manner. Its radiolabeled form, [3H]YM-298198, bound to mGluR1-NIH3T3 cell membranes with a KD of 32 +/- 8.5 nM and a Bmax of 2297 +/- 291 fmol/mg protein. In ligand displacement experiments using rat cerebellum membrane, an existing noncompetitive mGluR1 antagonist 7-(hydroxyimino)cyclo-propa[b]chromen-1a-carboxylate ethyl ester (CPCCOEt) competitively displaced [3H]YM-298198 binding, although glutamate and other mGluR1 ligands acting on a glutamate site failed to inhibit [3H]YM-298198 binding, suggesting that YM-298198 binds to CPCCOEt (allosteric) binding sites but not to glutamate (agonist) binding sites. Specificity was demonstrated for mGluR1 over mGluR subtypes 2 to 7, ionotropic glutamate receptors, and other receptor, transporter, and ion channel targets. In in vivo experiments, orally administered YM-298198 showed a significant analgesic effect in streptozotocin-induced hyperalgesic mice at doses (30 mg/kg) that did not cause Rotarod performance impairment, indicating that it is also useful even for in vivo experiments. In conclusion, YM-298198 is a newly synthesized, high-affinity, selective, and noncompetitive antagonist of mGluR1 that will be a useful pharmacological tool due to its highly active properties in vitro and in vivo. Its radiolabeled form [3H]YM-298198 will also be a valuable tool for future investigation of the mGluR1.

Spinal distribution and metabolism of 2'-O-(2-methoxyethyl)-modified oligonucleotides after intrathecal administration in rats

Intrathecal (IT) delivery of antisense oligodeoxynucleotides (ASO) has been used to study the function of specific gene products in spinal nociception. However, a lack of systematic studies on the spinal distribution and kinetics of IT ASO is a major hurdle to the utilization of this technique. In the present study, we injected rats IT with 2'-O-(2-methoxyethyl) modified phosphorothioate ASO (2'-O-MOE ASO) and examined anatomical and cellular location of the ASO in the spinal cord and dorsal root ganglia (DRG) by immunocytochemistry. At 0.5 h after a single IT injection, immunostaining for ISIS 13920 (a 2'-O-MOE ASO targeting h-ras) localized superficially in the lumbar spinal cord, while at 24 h the immunostaining was distributed throughout the spinal cord and was predominantly intracellular. Double staining with cell type specific antibodies indicated that the ASO was taken up by both glia and neurons. ASO immunoreactivity was also observed in DRG after IT ISIS 13920. Capillary gel electrophoresis analysis showed that ISIS 22703, a 2'-O-MOE ASO targeting the alpha isozyme of protein kinase C (PKC), remained intact in spinal cord tissue and cerebrospinal fluid up to 24 h after the injection and no metabolites were detected. In contrast, after IT ISIS 11300, an unmodified phosphorothioate ASO with the same sequence as ISIS 22703, no full-length compound was detectable at 24 h, and metabolites were seen as early as 0.5 h. IT treatment with ISIS 22703 at doses that effectively down-regulated PKCalpha mRNA in spinal cord did not affect the mRNA expression in DRG. In summary, 2'-O-MOE ASO displayed high stability in spinal tissue after IT delivery, efficiently distributed to spinal cord, and internalized into both neuronal and non-neuronal cells. ASO are able to reach DRG after IT delivery; however, higher doses may be required to reduce target gene in DRG as compared with spinal cord.

Effects of mGlu1 receptor blockade on anxiety-related behaviour in the rat lick suppression test

RATIONALE

Group I metabotropic glutamate receptor antagonists, which block both the mGlu1 and mGlu5 receptors, have been shown to have anxiolytic effects in the lick suppression test in rats.

OBJECTIVE

The anxiolytic potential of the selective mGlu1 antagonist 3,4-dihydro-2H-pyrano[2,3]beta-quinolin-7-yl)(cis-4-methoxycyclohexyl)methanone (JNJ16259685) was investigated and compared with the mGlu5 antagonist MPEP.

METHODS

Anxiety-related behaviour was assessed in lick suppression and in the elevated zero maze in rats. Non-specific effects on pain threshold, water intake and locomotor activity were also measured.

RESULTS

Acute administration of JNJ16259685 or MPEP increased the number of licks (lowest active dose 2.5 mg/kg IP for each compound). JNJ16259685 did not increase water intake or reduce acute pain threshold, suggesting that the anxiolytic-like properties are specific. However, acute administration decreased locomotor activity. The effects of chronic administration of JNJ16259685 over 14 days (5 mg/kg bid) on lick suppression were comparable to those seen after acute administration, arguing against development of behavioural tolerance or sensitisation. Yet, there was a tendency for an increase in locomotor activity after cessation of chronic treatment. Acute co-administration of both JNJ16259685 and MPEP had additive effects on the number of licks. No anxiolytic-like properties of JNJ16259685 were observed in the elevated zero maze.

CONCLUSION

Our data suggest that the anxiolytic-like effects induced by group I metabotropic glutamate receptor antagonists are mediated through both mGlu1 and mGlu5 receptors. Rather than producing a general anxiolytic-like effect, the effects seen following mGlu1 antagonism seem task-dependent, as prominent effects were seen in a conflict procedure, but not in a task based on spontaneous exploration.

The antinociceptive and anxiolytic-like effects of the metabotropic glutamate receptor 5 (mGluR5) antagonists, MPEP and MTEP, and the mGluR1 antagonist, LY456236, in rodents: a comparison of efficacy and side-effect profiles

RATIONALE

Modulation of metabotropic glutamate receptor (mGluR) subtypes represents a novel approach for the treatment of neurological and psychiatric disorders.

OBJECTIVES

This study was conducted to investigate the role of the mGluR5 and mGluR1 subtypes in the modulation of pain and anxiety.

METHODS

The mGluR5 antagonists, 2-methyl-6-(phenylethynyl)pyridine (MPEP) and 3-[(2-methyl-1,3-thiazol-4-yl)ethynyl]pyridine (MTEP), and the mGluR1 antagonist, (4-methoxy-phenyl)-(6-methoxy-quinazolin-4-yl)-amine HCl (LY456236), were tested in models of pain [mouse formalin test, rat spinal nerve ligation (SNL)] and anxiety [Vogel conflict, conditioned lick suppression (CLS)], and their efficacious effects were compared to any associated side effects.

RESULTS

The systemic administration of MPEP, MTEP, and LY456236 reduced hyperalgesia induced by formalin and mechanical allodynia following SNL. However, only LY456236 completely reversed the allodynia. In the anxiety models, MPEP (3--30 mg/kg), MTEP (3--10 mg/kg), and LY456236 (10--30 mg/kg) produced anxiolytic-like effects similar to the benzodiazepine, chlordiazepoxide (CDP, 6 mg/kg). However, only MPEP and MTEP were able to produce a level of anxiolysis comparable to CDP. In a series of tests examining potential side effects, MPEP and MTEP reduced body temperature and locomotor activity and impaired operant responding for food and rotarod performance at doses of 3--30 and 1--30 mg/kg, respectively. LY456236 reduced operant responding at 30 mg/kg.

CONCLUSION

Both mGluR5 and mGluR1 antagonists are effective in models of pain and anxiety. However, an mGluR1 antagonist was more efficacious than the two mGluR5 antagonists in the pain models, which, conversely, appeared more efficacious in the anxiety models. These findings support the potential utility of mGluR5 and mGluR1 antagonists for both the treatment of chronic pain and as novel anxiolytics.

Induction of long-term potentiation of C fibre-evoked spinal field potentials requires recruitment of group I, but not group II/III metabotropic glutamate receptors

In superficial layers of the lumbar spinal dorsal horn, N-methyl-D-aspartate-dependent long-term potentiation (LTP) of C fibre-evoked field potentials, a synaptic model of central sensitisation and hyperalgesia, ensues the application of electrical high-frequency, high-intensity conditioning stimulation to the sciatic nerve. In order to investigate the putative involvement of the G protein-coupled metabotropic glutamate receptors (mGluRs) in the induction of this form of LTP, we applied a series of mGluR antagonists exhibiting distinct group-specific activity profiles to the spinal lumbar enlargement, prior to conditioning stimulation. The group I (mGluR1/5) and group II (mGluR2/3) mGluR antagonist (S)-alpha-methyl-4-carboxyphenylglycine or the selective mGluR1/5 antagonist (S)-4-carboxyphenylglycine consistently impaired the development of spinal LTP. However, potentiation occurred in the presence of the inactive enantiomer (R)-alpha-methyl-4-carboxyphenylglycine. LTP proved insensitive to the selective mGluR2/3 antagonists (2S)-alpha-ethylglutamic acid and LY341495, either spinally or intravenously delivered. LTP could also be induced in the presence of the selective group III (mGluR4/mGluR6-mGluR8) mGluR antagonist (RS)-alpha-methylserine-O-phosphate. However, none of the mGluR-active compounds alone noticeably altered the amplitudes of C fibre-evoked field potentials in the absence of conditioning stimulation. These findings suggest that the induction of LTP of C fibre-evoked field potentials in the spinal dorsal horn by high-frequency, high-intensity stimulation of afferent C fibres requires a group-specific mGluR recruitment, activation of mGluR1/5 but not that of mGluR4/6-8 and mGluR2/3 being a requisite step.

Antisense knockdown of spinal-mGluR1 reduces the sustained phase of formalin-induced nociceptive responses

To examine the role of mGluR1 (a subunit of the group I metabotropic glutamate receptor) in the nociceptive responses of rats following a subcutaneous injection of formalin into the plantar surface of the hind paw, we delivered antisense oligonucleotides (ODNs) against mGluR1 into the rat lumbar spinal cord (L3-L5) intrathecally using an HVJ-liposome-mediated gene transfer method. Rats treated with a single injection of mGluR1 antisense ODNs into the intrathecal space of the lumbar spinal cord showed a marked reduction of the early-sustained phase of formalin-induced nociceptive responses, but not of their acute phase. The reduction of nociceptive behavioral responses became apparent at day 2 after the antisense treatment and lasted for 2 days. This corresponded to a long-lasting down-regulation (46%) of mGluR1 expression in the lumbar cord. This down-regulated mGluR1 was observed at day 2 and persisted until day 4 after the intrathecal infusion of mGluR1 antisense ODN. In contrast, rats treated with mGluR1 sense or mismatch ODNs showed none of these changes. These results suggest that mGluR1 may play a crucial role in the sustained nociception of formalin-induced behavioral responses.

The pain of antisense: in vivo application of antisense oligonucleotides for functional genomics in pain and analgesia

As the genomic revolution continues to evolve, there is an increasing demand for efficient and reliable tools for functional characterization of individual gene products. Antisense oligonucleotide-mediated knockdown has been used successfully as a functional genomics tool in animal models of pain and analgesia yet skepticism regarding the validity and utility of antisense technology remains. Contributing to this uncertainty are the lack of systematic studies exploring antisense oligonucleotide use in vivo and the many technical and methodological challenges intrinsic to the method. This article reviews the contributions of antisense oligonucleotide-based studies to the field of pain and analgesia and the general principles of antisense technology. A special emphasis is placed on technical issues surrounding the successful application of antisense oligonucleotides in vivo, including sequence selection, antisense oligonucleotide chemistry, DNA controls, route of administration, uptake, dose-dependence, time-course and adequate evaluation of knockdown.

mGlu5 receptor antagonist decreases Fos expression in spinal neurons after noxious visceral stimulation

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.

Metabotropic glutamate receptor expression in the rat superior cervical ganglion

To begin to determine what role metabotropic glutamate receptors (mGluRs) play in the peripheral nervous system, reverse transcriptase-polymerase chain reaction (RT-PCR) was used to probe RNA isolated from sympathetic neurons of the rat superior cervical ganglion (SCG). RT-PCR primers were designed to detect each of the eight rat mGluR transcripts. Only one, mGluR7, was detected in RNA from rat SCG, though each appeared to be present in RNA from whole rat brain. Although mGluR7 messenger RNA is apparently present in rat SCG, functional mGluR7 was not observed, as application of neither glutamate nor the group III mGluR agonist L-2-amino-4-phosphonobutyrate (L-AP4) produced calcium current modulation in isolated SCG neurons, as would be expected. However, following mGluR7 heterologous expression, application of L-AP4 did produce moderate calcium current modulation in isolated neurons, indicating that mGluR7 can express on the surface of SCG soma, and that its activation can modulate calcium currents.

Attenuation of morphine tolerance after antisense oligonucleotide knock-down of spinal mGluR1

1. Chronic systemic treatment of rats with morphine leads to the development of opioid tolerance. This study was designed to examine the effects of intrathecal (i.t.) infusion of a metabotropic glutamate receptor 1 (mGluR1) antisense oligonucleotide, concomitant with chronic morphine treatment, on the development of tolerance to morphine's antinociceptive effects. 2. All rats received chronic (6 day) s.c. administration of morphine to induce opioid tolerance. Additionally, rats were treated with either mGluR1 antisense (AS), missense (MIS) or artificial cerebrospinal fluid (ACSF) by i.t. infusion via chronically implanted i.t. catheters connected to osmotic mini-pumps. The effects of acute i.t. or s.c. morphine on tail-flick latencies were assessed prior to and following chronic s.c. morphine treatment for all chronic i.t. infusion groups. mGluR1 protein level in the spinal cord was determined by Western blot analysis for all treatments, assessing the efficiency of knock-down with AS treatment. 3. Acute i.t. morphine dose-dependently produced antinociception in the tail-flick test in naïve rats. Systemic morphine-treated rats administered i.t. ACSF or MIS developed tolerance to i.t. morphine. Chronic i.t. infusion with mGluR1 AS significantly reduced the development of tolerance to i.t. morphine. 4. In contrast to i.t. morphine, tolerance developed to the antinociceptive effects of s.c. morphine, in all i.t. infusion groups, including the mGluR1 AS group. 5. The spinal mGluR1 protein level was dramatically decreased after mGluR1 AS infusion when compared to control animals (naïve and ACSF-treated animals). 6. These findings suggest that the spinal mGluR1 is involved in the development of tolerance to the antinociceptive effects of morphine. Selective blockade of mGluR1 may be beneficial in preventing the development of opioid analgesic tolerance.

Peripheral metabotropic glutamate receptors as drug targets for pain relief

The relatively new family of G-protein-coupled metabotropic glutamate receptors (mGluRs) is comprised of eight cloned subtypes, which are classified into three groups based on their sequence homology, signal transduction mechanisms and receptor pharmacology. It is now well-established that mGluRs in the central nervous system are essential for neuroplasticity associated with normal brain functions but are also critically involved in various neurological and psychiatric disorders. Recent anatomical and behavioural evidence suggests an important role of mGluRs in peripheral tissues in animal models of inflammatory and neuropathic pain. Once the cellular effects of peripheral mGluR activation and inhibition are better understood, certain peripheral mGluR subtypes may become important novel therapeutic targets for the relief of pain associated with peripheral tissue injury. Peripherally acting drugs that modulate nociceptive processing through mGluRs should have the advantage of lacking the central side effects commonly observed with drugs interfering with glutamatergic transmission in the central nervous system.

L-Acetylcarnitine induces analgesia by selectively up-regulating mGlu2 metabotropic glutamate receptors

L-Acetylcarnitine (LAC, 100 mg/kg, s.c.), a drug commonly used for the treatment of painful neuropathies, substantially reduced mechanical allodynia in rats subjected to monolateral chronic constriction injury (CCI) of the sciatic nerve and also attenuated acute thermal pain in intact rats. In both cases, induction of analgesia required repeated injections of LAC, suggesting that the drug induces plastic changes within the nociceptive pathway. In both CCI- and sham-operated rats, a 24-day treatment with LAC increased the expression of metabotropic glutamate (mGlu) receptors 2 and 3 in the lumbar segment of the spinal cord, without changing the expression of mGlu1a or -5 receptors. A similar up-regulation of mGlu2/3 receptors was detected in the dorsal horns and dorsal root ganglia of intact rats treated with LAC for 5-7 days, a time sufficient for the induction of thermal analgesia. Immunohistochemical analysis showed that LAC treatment enhanced mGlu2/3 immunoreactivity in the inner part of lamina II and in laminae III and IV of the spinal cord. An increased mGlu2/3 receptor expression was also observed in the cerebral cortex but not in the hippocampus or cerebellum of LAC-treated animals. Reverse transcription-polymerase chain reaction combined with Northern blot analysis showed that repeated LAC injections selectively induced mGlu2 mRNA in the dorsal horns and cerebral cortex (but not in the hippocampus). mGlu3 mRNA levels did not change in any brain region of LAC-treated animals. To examine whether the selective up-regulation of mGlu2 receptors had any role in LAC-induced analgesia, we have used the novel compound LY 341495, which is a potent and systemically active mGlu2/3 receptor antagonist. LAC-induced analgesia was largely reduced 45 to 75 min after a single injection of LY 341495 (1 mg/kg, i.p.) in both CCI rats tested for mechanical allodynia and intact rats tested for thermal pain. We conclude that LAC produces analgesia against chronic pain produced not only by peripheral nerve injury but also by acute pain in intact animals and that LAC-induced analgesia is associated with and causally related to a selective up-regulation of mGlu2 receptors. This offers the first example of a selective induction of mGlu2 receptors and discloses a novel mechanism for drug-induced analgesia.

Descending control of pain

Upon receipt in the dorsal horn (DH) of the spinal cord, nociceptive (pain-signalling) information from the viscera, skin and other organs is subject to extensive processing by a diversity of mechanisms, certain of which enhance, and certain of which inhibit, its transfer to higher centres. In this regard, a network of descending pathways projecting from cerebral structures to the DH plays a complex and crucial role. Specific centrifugal pathways either suppress (descending inhibition) or potentiate (descending facilitation) passage of nociceptive messages to the brain. Engagement of descending inhibition by the opioid analgesic, morphine, fulfils an important role in its pain-relieving properties, while induction of analgesia by the adrenergic agonist, clonidine, reflects actions at alpha(2)-adrenoceptors (alpha(2)-ARs) in the DH normally recruited by descending pathways. However, opioids and adrenergic agents exploit but a tiny fraction of the vast panoply of mechanisms now known to be involved in the induction and/or expression of descending controls. For example, no drug interfering with descending facilitation is currently available for clinical use. The present review focuses on: (1) the organisation of descending pathways and their pathophysiological significance; (2) the role of individual transmitters and specific receptor types in the modulation and expression of mechanisms of descending inhibition and facilitation and (3) the advantages and limitations of established and innovative analgesic strategies which act by manipulation of descending controls. Knowledge of descending pathways has increased exponentially in recent years, so this is an opportune moment to survey their operation and therapeutic relevance to the improved management of pain.

Group I metabotropic glutamate receptors in spinal cord injury: roles in neuroprotection and the development of chronic central pain

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.

Role of group II and group III metabotropic glutamate receptors in spinal cord injury

Spinal cord injury (SCI) produces an increase in extracellular excitatory amino acid (EAA) concentrations that results in glutamate receptor-mediated excitotoxic events. An important class of these receptors is the metabotropic glutamate receptors (mGluRs). mGluRs can activate a number of intracellular pathways that increase neuronal excitability and modulate neurotransmission. Group I mGluRs are known to modulate EAA release and the development of chronic central pain (CCP) following SCI; however, the role of group II and III mGluRs remains unclear. To begin evaluating group II and III mGluRs in SCI, we administered the specific agonists for group II, APDC, or group III, L-AP4, by interspinal injection immediately following SCI. Contusion injury was produced at spinal segment T10 with a New York University impactor (12.5-mm drop, 10-g rod 2 mm in diameter) in 30 adult male Sprague-Dawley rats (175-200 g). Evoked and spontaneous behavioral measures of CCP, locomotor recovery, changes in mGluR expression, and amount of spared tissue were examined. Neither APDC nor L-AP4 affected locomotor recovery or the development of thermal hyperalgesia; however, L-AP4 and APDC attenuated changes in mechanical thresholds and changes in exploratory behavior indicative of CCP. APDC- and L-AP4-treated groups had higher expression levels of mGluR2/3 at the epicenter of injury on post contusion day 28; however, there was no difference in the amount of spared tissue between treatment groups. These results demonstrate that treatment with agonists to group II and III mGluRs following SCI affects mechanical responses, exploratory behavior, and mGluR2/3 expression without affecting the amount of tissue spared, suggesting that the level of mGluR expression after SCI may modulate nociceptive responses.

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

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

Metabotropic glutamate mGlu1 receptor mRNA expression in dorsal root ganglia of rats after peripheral nerve injury

Although cerebral and spinal metabotropic glutamate mGlu(1) receptors are thought to be involved in nociception and in the development/maintenance of chronic pain, it is still unclear to what extent mGlu(1) receptors are present in the dorsal root ganglia of peripheral sensory afferents, and whether their expression is affected during development of chronic pain. It was found in the present study that mGlu(1) receptor messenger RNA (mRNA) is present in rat L5 dorsal root ganglia and that it is strongly downregulated after unilateral axotomy of the tibial branch of the sciatic nerve, a model of chronic neuropathic pain. However, as sham-operated animals showed a similar downregulation, it is suggested that peripheral tissue damage is sufficient to result in a reduction of peripheral mGlu(1) receptor expression.

Peripheral metabotropic glutamate receptors: fight the pain where it hurts

Recent anatomical and behavioral data show the expression of G-protein coupled metabotropic glutamate receptors in the periphery on nociceptive primary afferent nerve terminals, and provide evidence for a functional role of peripheral metabotropic glutamate receptors in inflammatory pain. These findings have important implications for new therapeutic strategies that target peripheral metabotropic glutamate receptors for pain relief. They also alert us to the necessity of assessing drug effects at different levels of the nervous system: peripheral and central.

Metabotropic glutamate 1alpha receptors on peripheral primary afferent fibers: their role in nociception

Several lines of evidence indicate that Group I metabotropic glutamate (mGlu) 1alpha receptors are involved in the processing of nociceptive information in the spinal cord. The goals of the present study are to document the role of mGlu1alpha receptors in peripheral nociception. To accomplish this we investigate the presence of mGlu1alpha receptors on peripheral primary afferent fibers and determine the behavioral effects of (S)-3,5-dihydroxyphenylglycine (S-DHPG), which is an mGlu1/5 receptor agonist and (RS)-1-aminoindan-1, 5-dicarboxylic acid (AIDA), a selective mGluR1alpha antagonist, on mechanical and thermal sensitivity and formalin-induced nociceptive behaviors. The anatomical studies at the electron microscopic level demonstrate that 32.4+/-2.9% of the unmyelinated axons and 21.6+/-4.7% of the myelinated axons are positively immunostained for mGlu1alpha receptors. Intraplantar injection of 0.1 or 1 mM S-DHPG results in a significant increase in mechanical sensitivity that persists for more than 60 min and this effect is blocked by co-injection of S-DHPG with 1 mM AIDA. Intraplantar injection of 40 microM AIDA+2% formalin significantly attenuates phase 2 lifting/licking and flinching behavior and this AIDA-induced effect is blocked with co-injection of 1 microM S-DHPG. In behavioral tests, intraplantar S-DHPG (0.1, 1.0, 10 mM) does not change tail flick latencies or paw withdrawal latencies to heat stimulation. These data indicate that mGlu1alpha receptors are present on peripheral cutaneous axons and activation of peripheral mGlu1alpha receptors contributes to mechanical allodynia and inflammatory pain but not thermal hyperalgesia.

In vivo control of NMDA receptor transcript level in motoneurons by viral transduction of a short antisense gene

Glutamate receptors play critical roles in normal and pathological processes. We developed an antisense gene delivery strategy to modulate the NMDA type of glutamate receptor. Using transient transfection in vitro and viral mediated gene transfer in vitro and in vivo, the effect of expression of an antisense gene fragment (60 bp) of the NR1 subunit was tested. Immunoblot analysis showed an antisense-concentration-dependent reduction in the NR1 subunit upon transient co-transfection of a plasmid expressing a sense NR1 gene and a plasmid expressing the antisense fragment into COS-7 cells. After recombination into an adenoviral vector, this antisense fragment reduced the amount of endogenous NR1 protein in PC12 cells. Finally, direct intraparenchymal injection of the viral vector into rat spinal cord resulted in diminished NR1 in motor neurons. Our results demonstrate the efficacy of this approach, which combines antisense with viral gene delivery to control the expression of specific genes in vivo. This approach may also be useful in reducing excitatory neurotransmission in vivo, with implications for the treatment of spinal disorders such as amyotrophic lateral sclerosis or chronic pain.

Changes in metabotropic glutamate receptor expression following spinal cord injury

Spinal cord injury (SCI) initiates biochemical events that lead to an increase in extracellular excitatory amino acid concentrations, resulting in glutamate receptor-mediated excitotoxic events. These receptors include the three groups of metabotropic glutamate receptors (mGluRs). Group I mGluR activation can initiate a number of intracellular pathways that increase neuronal excitability. Group II and III mGluRs may function as autoreceptors to modulate neurotransmission. Thus, all three groups may contribute to the mechanisms of central sensitization and chronic central pain. To begin evaluating mGluRs in SCI, we quantified the changes in mGluR expression after SCI in control (naive), sham, and impact injured adult male Sprague-Dawley rats (200-250 g). SCI was produced at spinal segment T10 with a New York University impactor (12.5-mm drop, 10-g rod of 2-mm diameter). Expression levels were determined by Western blot and immunohistochemistry analyses at the epicenter of injury, as well as segments rostral and caudal. The group I subtype mGluR1 was increased over control levels in segments rostral and caudal by postsurgical day (PSD) 7 and remained elevated through PSD 60. The group I subtype mGluR5 was unchanged in all segments rostral and caudal to the injury at every time point measured. Group II mGluRs were decreased compared to control levels from PSD 7 through PSD 60 in all segments. These results suggest that different subtypes of mGluRs have different spatial and temporal expression patterns following SCI. The expression changes in mGluRs parallel the development of mechanical allodynia and thermal hyperalgesia following SCI; therefore, understanding the expression of mGluRs after SCI may give insight into mechanisms underlying the development of chronic central pain.

Metabotropic glutamate receptor subtypes 1 and 5 are activators of extracellular signal-regulated kinase signaling required for inflammatory pain in mice

Metabotropic glutamate receptors are expressed abundantly in the spinal cord and have been shown to play important roles in the modulation of nociceptive transmission and plasticity. Most previous studies have focused on the group I metabotropic glutamate receptors (mGluR1 and mGluR5) and activation of phospholipase C signaling by these receptors in modulating nociception. Recently, it was shown that the extracellular signal-regulated kinases (ERKs)/mitogen-activated protein kinases are activated in spinal cord dorsal horn neurons in response to stimulation of nociceptors and that ERK signaling is involved in nociceptive plasticity. In the present studies, we sought to test the hypothesis that group I mGluRs modulate nociceptive transmission or plasticity via modulation of ERK signaling in dorsal horn neurons. We show that activation of mGluR1 and mGluR5 leads to activation of ERK1 and ERK2 in the spinal cord. Furthermore, we find that inflammation-evoked ERK activation, which is required for nociceptive plasticity, is downstream of mGluR1 and mGluR5. Finally, we show colocalization of group I mGluRs with activated ERK in dorsal horn neurons. These results show that mGluR1 and mGluR5 are activated in dorsal horn neurons in response to peripheral inflammation and that activation of these group I mGluRs leads to activation of ERK1 and ERK2, resulting in enhanced pain sensitivity.

Actions of the systemically active metabotropic glutamate antagonist MPEP on sensory responses of thalamic neurones

It is known that metabotropic glutamate receptors of the subtypes mGlu1 and mGlu5 participate in nociceptive processing in the thalamus, an area of prime importance in supra-spinal sensory processing. Antagonists of these receptors thus have potential as centrally-acting analgesics. We have investigated whether intravenous administration of the novel mGlu5-receptor antagonist 6-methyl-2-(phenylethynyl)-pyridine (MPEP) is able to reduce nociceptive responses of thalamic neurones. Extracellular recordings were made from single thalamic neurones of adult male Wistar rats anaesthetised with urethane. MPEP (1 mg/kg) reduced neuronal responses to noxious thermal stimuli to a mean of 24+/-4% of control within 10 min, whereas saline injections had no significant effect. Partial recovery was seen within 30-45 min after injection. Responses of neurones to non-noxious stimuli were not significantly affected by MPEP administration. In addition, MPEP caused an increase in the power of the slow-wave component (<1 Hz) of the electroencephalogram (EEG), but had no significant effect on peak frequency of the EEG or on heart rate. These results confirm that nociceptive responses of thalamic neurones are mediated in part by mGlu5 receptors. Furthermore, the effectiveness of intravenous MPEP suggests that such antagonists may be useful as centrally-acting analgesics.

Characterization with [3H]quisqualate of group I metabotropic glutamate receptor subtype in rat central and peripheral excitable tissues

Radioligand binding studies were performed to label metabotropic glutamate receptor (mGluR) in rat brain synaptic membranes using [3H]quisqualic acid (QA) synthesized in our laboratory as a radioligand. In the presence of ionotropic glutamate receptor (iGluR) agonists, including N-methyl-D-aspartic (NMDA), DL-alpha-amino-3-hydroxy-5-methylisoxasole-4-propionic (AMPA) and kainic acids (KA), at concentrations maximally effective in displacing each receptor binding, the agonists for group I mGluR subtype (+/-)-1-aminocyclopentane-trans-1,3-dicarboxylic acid (trans-ACPD) and (S)-3,5-dihydroxyphenylglycine ((S)-3,5-DHPG) more potently displaced [3H]QA binding in a concentration-dependent manner than their absence. The addition of these three iGluR agonists did not significantly affect potencies of (2S,2'R,3'R)-2-(2',3'-dicarboxycyclopropyl)glycine (DCG-IV) and L-(+)-2-amino-4-phosphonobutyric acid (L-AP4) to displace [3H]QA binding. Scatchard analysis revealed that [3H]QA binding consisted of a single component with a maximal number of binding sites (B(max)) of 431.6 fmol/mg protein and a dissociation constant (K(d)) of 50.9 nM, in the presence of the three iGluR agonists. [3H]QA binding was markedly inhibited by GTP and its analogues; but not by GDP, GMP and ATP, under these conditions. Inhibition by GTP was seen in all central structures examined, but [3H]QA binding was not detectable in peripheral tissues, such as pituitary and adrenal glands. Neither reverses transcription polymerase chain reaction nor immunoblotting analysis demonstrated the expression of mGluR1 and mGluR5 subunits in the aforementioned two peripheral tissues. These results suggest that [3H]QA indeed labels group I mGluR subtype functionally coupled to GTP binding protein in rat brain synaptic membranes under the experimental conditions employed. Group I mGluR subtype seems to be selectively distributed in central structures but not in pituitary and adrenal glands.

Contributions of mGlu1 and mGlu5 receptors to interactions with N-methyl-D-aspartate receptor-mediated responses and nociceptive sensory responses of rat thalamic neurons

The nociceptive responses of rat ventrobasal thalamus neurons can be reduced by N-methyl-D-aspartate antagonists and by selective metabotropic glutamate receptor mGlu1 antagonists. The recent development of the mGlu5-selective antagonist 6-methyl-2-(phenylethynyl)-pyridine now allows the direct probing of the possible involvement of mGlu5 receptors in thalamic nociceptive responses. Extracellular recordings were made from single neurons in the ventrobasal thalamus and immediately overlying dorsal thalamic nuclei of adult urethane-anaesthetized rats using multi-barrel electrodes. Responses of neurons to iontophoretic applications of the mGlu5-selective agonist (R,S)-2-chloro-5-hydroxyphenylglycine were selectively reduced during continuous iontophoretic applications of 6-methyl-2-(phenylethynyl)-pyridine. Similar applications of 6-methyl-2-(phenylethynyl)-pyridine reduced neuronal responses to noxious thermal stimuli to 53+/-9.5% of control responses. Co- application by iontophoresis of N-methyl-D-aspartate and metabotropic glutamate receptor agonists resulted in a mutual potentiation of excitatory responses. This effect could be reduced by either 6-methyl-2-(phenylethynyl)-pyridine or the mGlu1 antagonist LY367385. These results, taken together with previous data, suggest that acute thalamic nociceptive responses are mediated by a combination of mGlu1, mGlu5 and N-methyl-D-aspartate receptor activation, and that co-activation of these receptors produces a synergistic excitatory effect. Thus blockade of any of these receptor types would have a profound effect on the overall nociceptive response.

Glutamate-mediated neuroprotection against N-methyl-D-aspartate toxicity: a role for metabotropic glutamate receptors

We studied N-methyl-D-aspartate-induced cell death in organotypic hippocampal slices from seven-day-old Wistar rat pups cultured for 12-14 days in a medium containing no added glutamate. Propidium iodide fluorescence intensity was used as an indicator of cell death measured with the help of confocal microscopy. Exposure of slices for 2h to L-glutamate (1-500 microM) prior to the N-methyl-D-aspartate challenge significantly reduced N-methyl-D-aspartate-induced cell death. Glutamate at 10 and 500 microM concentrations was highly protective against N-methyl-D-aspartate-induced cell death, but was less protective at the 1 microM concentration. The protection was not blocked by the Na(+) channel blocker tetrodotoxin (1 microM), the N-methyl-D-aspartate receptor antagonist D-2-amino-5-phosphonopentanoic acid (20 microM) or the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (20 microM). 1S, 3R-1-Aminocyclopentane-trans-1,3-dicarboxylic acid, an agonist at metabotropic glutamate receptor types 1, 2/3 and 5, was protective at 100 microM but not at 50 microM. In contrast, the ionotropic glutamate receptor agonist aspartate (250 microM) facilitated N-methyl-D-aspartate toxicity. Treatment of slices with the protein kinase C inhibitor staurosporine (0.2 microM) or antisense oligonucleotide (10nM, 72 h) that selectively inhibits metabotropic glutamate receptor type 5 synthesis significantly reduced glutamate protection. These results suggest that ambient glutamate may reduce nerve cell susceptibility to injury caused by excessive N-methyl-D-aspartate receptor activation by acting at metabotropic glutamate receptors linked to protein kinase C.

mGlu5 receptors and nociceptive function II. mGlu5 receptors functionally expressed on peripheral sensory neurones mediate inflammatory hyperalgesia

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.

Metabotropic glutamate receptor subtype 5 (mGlu5) and nociceptive function. I. Selective blockade of mGlu5 receptors in models of acute, persistent and chronic pain

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.

Immunoreactivity for the group III metabotropic glutamate receptor subtype mGluR4a in the superficial laminae of the rat spinal dorsal horn

Studies indicate that metabotropic glutamate receptors (mGluRs) may play a role in spinal sensory transmission. We examined the cellular and subcellular distribution of the mGluR subtype 4a in spinal tissue by means of a specific antiserum and immunocytochemical techniques for light and electron microscopy. A dense plexus of mGluR4a-immunoreactive elements was seen in the dorsal horn, with an apparent accumulation in lamina II. The immunostaining was composed of sparse immunoreactive fibres and punctate elements. No perikaryal staining was seen. Immunostaining for mGluR4a was detected in small to medium-sized cells but not in large cells in dorsal root ganglia. At the electron microscopic level, superficial dorsal horn laminae demonstrated numerous immunoreactive vesicle-containing profiles. Labelling was present in the cytoplasmic matrix, but accretion of immunoreaction product to presynaptic specialisations was commonly observed. Axolemmal labelling was confirmed by using a preembedding immunogold technique, which revealed distinctive deposits of gold immunoparticles along presynaptic thickenings with an average centre-to-centre distance of 41 nm (41.145 +/- 13.59). Immunoreactive terminals often formed synaptic contacts with dendritic profiles immunonegative for mGluR4a. Immunonegative dendritic profiles were observed in apposition to both mGluR4a-immunoreactive and immunonegative terminals. Diffuse immunoperoxidase reaction product was also detected in dendritic profiles, some of which were contacted by mGluR4a-immunoreactive endings, but only occasionally were they observed to accumulate immunoreaction product along the postsynaptic density. Terminals immunoreactive for mGluR4a also formed axosomatic contacts. The present results reveal that mGluR4a subserves a complex spinal circuitry to which the primary afferent system seems to be a major contributor.

Glutamate receptors and nociception: implications for the drug treatment of pain

Evidence from the last several decades indicates that the excitatory amino acid glutamate plays a significant role in nociceptive processing. Glutamate and glutamate receptors are located in areas of the brain, spinal cord and periphery that are involved in pain sensation and transmission. Glutamate acts at several types of receptors, including ionotropic (directly coupled to ion channels) and metabotropic (directly coupled to intracellular second messengers). Ionotropic receptors include those selectively activated by N-methyl-D-aspartate, alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid and kainate. Metabotropic glutamate receptors are classified into 3 groups based on sequence homology, signal transduction mechanisms and receptor pharmacology. Glutamate also interacts with the opioid system, and intrathecal or systemic coadministration of glutamate receptor antagonists with opioids may enhance analgesia while reducing the development of opioid tolerance and dependence. The actions of glutamate in the brain seem to be more complex. Activation of glutamate receptors in some brain areas seems to be pronociceptive (e.g. thalamus, trigeminal nucleus), although activation of glutamate receptors in other brain areas seems to be antinociceptive (e.g. periaqueductal grey, ventrolateral medulla). Application of glutamate, or agonists selective for one of the several types of glutamate receptor, to the spinal cord or periphery induces nociceptive behaviours. Inhibition of glutamate release, or of glutamate receptors, in the spinal cord or periphery attenuates both acute and chronic pain in animal models. Similar benefits have been seen in studies involving humans (both patients and volunteers); however, results have been inconsistent. More research is needed to clearly define the role of existing treatment options and explore the possibilities for future drug development.

Knockdown of spinal metabotropic glutamate receptor 1 (mGluR(1)) alleviates pain and restores opioid efficacy after nerve injury in rats

1. Nerve injury often produces long-lasting spontaneous pain, hyperalgesia and allodynia that are refractory to treatment, being only partially relieved by clinical analgesics, and often insensitive to morphine. With the aim of assessing its therapeutic potential, we examined the effect of antisense oligonucleotide knockdown of spinal metabotropic glutamate receptor 1 (mGluR(1)) in neuropathic rats. 2. We chronically infused rats intrathecally with either vehicle, or 50 microg day(-1) antisense or missense oligonucleotides beginning either 3 days prior to or 5 days after nerve injury. Cold, heat and mechanical sensitivity was assessed prior to any treatment and again every few days after nerve injury. 3. Here we show that knockdown of mGluR(1) significantly reduces cold hyperalgesia, heat hyperalgesia and mechanical allodynia in the ipsilateral (injured) hindpaw of neuropathic rats. 4. Moreover, we show that morphine analgesia is reduced in neuropathic rats, but not in sham-operated rats, and that knockdown of mGluR(1) restores the analgesic efficacy of morphine. 5. We also show that neuropathic rats are more sensitive to the excitatory effects of intrathecally injected N-methyl-D-aspartate (NMDA), and have elevated protein kinase C (PKC) activity in the spinal cord dorsal horn, two effects that are reversed by knockdown of mGluR(1). 6. These results suggest that activity at mGluR(1) contributes to neuropathic pain through interactions with spinal NMDA receptors and PKC, and that knockdown of mGluR(1) may be a useful therapy for neuropathic pain in humans, both to alleviate pain directly, and as an adjunct to opioid analgesic treatment.

Dual modulation of excitatory synaptic transmission by agonists at group I metabotropic glutamate receptors in the rat spinal dorsal horn

The effects of group I metabotropic glutamate (mGlu) receptors on excitatory transmission in the rat dorsal horn, but mostly substantia gelatinosa, neurons were investigated using conventional intracellular recording in slices. The broad spectrum mGlu receptor agonist (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (1S, 3R-ACPD), the group I mGlu receptor selective agonist (S)-3, 5-dihydroxyphenylglycine (DHPG), and the selective mGlu subtype 5 agonist (RS)-2-chloro-5-hydroxyphenylglycine (CHPG), all induce long-lasting depression of A primary afferent fibers-mediated monosynaptic excitatory postsynaptic potential (EPSP), and long-lasting potentiation of polysynaptic EPSP, and EPSP in cells receiving C-afferent fiber input. The DHPG potentiation of polysynaptic EPSP was partially or fully reversed by (S)-4-carboxyphenylglycine (S-4CPG), the mGlu subtype 1 preferring antagonist. 2-Methyl-6-(phenylethynyl)-pyridine, the potent and selective mGlu subtype 5 antagonist, partially reversed the CHPG potentiation of polysynaptic EPSP. The effects of DHPG on monosynaptic and polysynaptic EPSPs were reduced, or abolished, by the N-methyl-D-aspartate (NMDA) receptor antagonist D(-)-2-amino-5-phosphonopentanoic acid (AP5). A clear and pronounced facilitation of the expression of DHPG- and CHPG-induced enhancement of polysynaptic EPSP, and EPSP evoked at C-fiber strength, was seen in the absence of gamma-aminobutyric acid subtype A receptor- and glycine-mediated synaptic inhibition. Besides dual modulation of excitatory synaptic transmission, DHPG induces depression of inhibitory postsynaptic potentials evoked by primary afferent stimulation in dorsal horn neurons. In addition, group I mGlu receptor agonists produced a direct persistent excitatory postsynaptic effect consisting of a slow membrane depolarization, an increase in input resistance, and an intense neuronal discharge. Cyclothiazide and (S)-4-CPG, the mGlu receptor subtype 1 preferring antagonists, significantly attenuated the DHPG-induced depolarization. These results demonstrate that the pharmacological activation of group I metabotropic glutamate receptors induces long-term depression (LTD) and long-term potentiation (LTP) of synaptic transmission in the spinal dorsal horn. These types of long-term synaptic plasticity may play a functional role in the generation of post-injury hypersensitivity (LTP) or antinociception (LTD).