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

Red nucleus mGluR1 and mGluR5 facilitate the development of neuropathic pain through stimulating the expressions of TNF-α and IL-1β

Our previous study has identified that glutamate in the red nucleus (RN) facilitates the development of neuropathic pain through metabotropic glutamate receptors (mGluR). Here, we further explored the actions and possible molecular mechanisms of red nucleus mGluR Ⅰ (mGluR1 and mGluR5) in the development of neuropathic pain induced by spared nerve injury (SNI). Our data indicated that both mGluR1 and mGluR5 were constitutively expressed in the RN of normal rats. Two weeks after SNI, the expressions of mGluR1 and mGluR5 were significantly boosted in the RN contralateral to the nerve injury. Administration of mGluR1 antagonist LY367385 or mGluR5 antagonist MTEP to the RN contralateral to the nerve injury at 2 weeks post-SNI significantly ameliorated SNI-induced neuropathic pain. However, unilateral administration of mGluRⅠ agonist DHPG to the RN of normal rats provoked a significant mechanical allodynia, this effect could be blocked by LY367385 or MTEP. Further studies indicated that the expressions of TNF-α and IL-1β in the RN were also elevated at 2 weeks post-SNI. Administration of mGluR1 antagonist LY367385 or mGluR5 antagonist MTEP to the RN at 2 weeks post-SNI significantly inhibited the elevations of TNF-α and IL-1β. However, administration of mGluR Ⅰ agonist DHPG to the RN of normal rats significantly enhanced the expressions of TNF-α and IL-1β, these effects were blocked by LY367385 or MTEP. These results suggest that activation of red nucleus mGluR1 and mGluR5 facilitate the development of neuropathic pain by stimulating the expressions of TNF-α and IL-1β. mGluR Ⅰ maybe potential targets for drug development and clinical treatment of neuropathic pain.

Neural Circuitry Polarization in the Spinal Dorsal Horn (SDH): A Novel Form of Dysregulated Circuitry Plasticity during Pain Pathogenesis

Pathological pain emerges from nociceptive system dysfunction, resulting in heightened pain circuit activity. Various forms of circuitry plasticity, such as central sensitization, synaptic plasticity, homeostatic plasticity, and excitation/inhibition balance, contribute to the malfunction of neural circuits during pain pathogenesis. Recently, a new form of plasticity in the spinal dorsal horn (SDH), named neural circuit polarization (NCP), was discovered in pain models induced by HIV-1 gp120 and chronic morphine administration. NCP manifests as an increase in excitatory postsynaptic currents (EPSCs) in excitatory neurons and a decrease in EPSCs in inhibitory neurons, presumably facilitating hyperactivation of pain circuits. The expression of NCP is associated with astrogliosis. Ablation of reactive astrocytes or suppression of astrogliosis blocks NCP and, concomitantly, the development of gp120- or morphine-induced pain. In this review, we aim to compare and integrate NCP with other forms of plasticity in pain circuits to improve the understanding of the pathogenic contribution of NCP and its cooperation with other forms of circuitry plasticity during the development of pathological pain.

Amygdala Metabotropic Glutamate Receptor 1 Influences Synaptic Transmission to Participate in Fentanyl-Induced Hyperalgesia in Rats

The underlying mechanisms of opioid-induced hyperalgesia (OIH) remain unclear. Herein, we found that the protein expression of metabotropic glutamate receptor 1 (mGluR1) was significantly increased in the right but not in the left laterocapsular division of central nucleus of the amygdala (CeLC) in OIH rats. In CeLC neurons, the frequency and the amplitude of mini-excitatory postsynaptic currents (mEPSCs) were significantly increased in fentanyl group which were decreased by acute application of a mGluR1 antagonist, A841720. Finally, the behavioral hypersensitivity could be reversed by A841720 microinjection into the right CeLC. These results show that the right CeLC mGluR1 is an important factor associated with OIH that enhances synaptic transmission and could be a potential drug target to alleviate fentanyl-induced hyperalgesia.

Targeting Chemokines and Chemokine GPCRs to Enhance Strong Opioid Efficacy in Neuropathic Pain

Neuropathic pain (NP) originates from an injury or disease of the somatosensory nervous system. This heterogeneous origin and the possible association with other pathologies make the management of NP a real challenge. To date, there are no satisfactory treatments for this type of chronic pain. Even strong opioids, the gold-standard analgesics for nociceptive and cancer pain, display low efficacy and the paradoxical ability to exacerbate pain sensitivity in NP patients. Mounting evidence suggests that chemokine upregulation may be a common mechanism driving NP pathophysiology and chronic opioid use-related consequences (analgesic tolerance and hyperalgesia). Here, we first review preclinical studies on the role of chemokines and chemokine receptors in the development and maintenance of NP. Second, we examine the change in chemokine expression following chronic opioid use and the crosstalk between chemokine and opioid receptors. Then, we examine the effects of inhibiting specific chemokines or chemokine receptors as a strategy to increase opioid efficacy in NP. We conclude that strong opioids, along with drugs that block specific chemokine/chemokine receptor axis, might be the right compromise for a favorable risk/benefit ratio in NP management.

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.

Underpinning the Neurobiological Intricacies Associated with Opioid Tolerance

The opioid crisis is a major threat of the 21st century, with a remarkable juxtaposition of use and abuse. Opioids are the most potent and efficacious class of analgesics, but despite their proven therapeutic efficacy, they have recently been degraded to third-line therapy for the management of chronic pain in clinics. The reason behind this is the development of potential side effects and tolerance after repeated dosing. Opioid tolerance is the major limiting factor leading to the withdrawal of treatment, severe side effects due to dose escalation, and sometimes even death of the patients. Every day more than 90 people die due to opioids overdose in America, and a similar trend has been seen across the globe. Over the past two decades, researchers have been trying to dissect the neurobiological mechanism of opioid tolerance. Research on opioid tolerance shifted toward central nervous system-based adaptations because tolerance is much more than just a cellular phenomenon. Thus, neurobiological adaptations associated with opioid tolerance are important to understand in order to find newer pain therapeutics. These adaptations are associated with alterations in ascending and descending pain pathways, reward circuitry modulations, receptor desensitization and down-regulation, receptor internalization, heterodimerization, and altered epigenetic regulation. The present Review is focused on novel circuitries associated with opioid tolerance in different areas of the brain, such as periaqueductal gray, rostral ventromedial medulla, dorsal raphe nucleus, ventral tegmental area, and nucleus accumbens. Understanding the neurobiological modulations associated with chronic opioid exposure and tolerance will pave the way for the development of novel pharmacological tools for safer and better management of chronic pain in patients.

Isoflurane Preconditioning Protects the Myocardium Against Ischemia and Reperfusion Injury by Upregulating GRM1 Expression

BACKGROUND

Reduction in myocardial I/R injury has become the key to the therapy of ischemic cardiovascular disease. Isoflurane (ISO) preconditioning can mimic the major potent protective mechanisms and attenuate ischemia injury. Nevertheless, the mechanisms involved in the cardioprotective effects afforded by isoflurane preconditioning have never been evaluated systematically.

METHODS

Mice were randomly divided into an ISO preconditioning group and control group. The size of the infarcted region was measured, and comparisons between ISO preconditioning and control animals were made. The metabotropic glutamate receptor type 1(GRM1) expression levels in all groups were determined by quantitative PCR. GRM1 protein expression and DNA damage relative protein γ-H2AX were measured by western blot analysis. The oxidative stress was detected by immunofluorescence after staining with the Dihydroethidium (DHE).

RESULTS

ISO preconditioning significantly reduced the IR induced infarct volumes and reversed the GRM1 protein expression level in I/R induced myocardial injury. Moreover, ISO preconditioning has a protective effect in reducing the I/R induced DNA damage and oxidative stress.

CONCLUSION

The results of the present study have demonstrated that the expression of GRM1 provides a protective role in ISO preconditioning against I/R-induced myocardial infarction by reducing the oxidative stress and DNA damage.

RNA control in pain: Blame it on the messenger

mRNA function is meticulously controlled. We provide an overview of the integral role that posttranscriptional controls play in the perception of painful stimuli by sensory neurons. These specialized cells, termed nociceptors, precisely regulate mRNA polarity, translation, and stability. A growing body of evidence has revealed that targeted disruption of mRNAs and RNA-binding proteins robustly diminishes pain-associated behaviors. We propose that the use of multiple independent regulatory paradigms facilitates robust temporal and spatial precision of protein expression in response to a range of pain-promoting stimuli. This article is categorized under: RNA in Disease and Development > RNA in Disease Translation > Translation Regulation RNA Turnover and Surveillance > Regulation of RNA Stability.

Emerging Trends in Pain Modulation by Metabotropic Glutamate Receptors

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

Why mu-opioid agonists have less analgesic efficacy in neuropathic pain?

Injury to peripheral nerves often leads to abnormal pain states (hyperalgesia, allodynia and spontaneous pain), which can remain long after the injury heals. Although opioid agonists remain the gold standard for the treatment of moderate to severe pain, they show reduced efficacy against neuropathic pain. In addition to analgesia, opioid use is also associated with hyperalgesia and analgesia tolerance, whose underlying mechanisms share some commonalities with nerve injury-induced hypersensitivity. Here, we reviewed up-to-day research exploring the contribution of mu-opioid receptor (MOR) on the pathophysiology of neuropathic pain and on analgesic opioid actions under these conditions. We focused on the specific contributions of MOR populations at peripheral, spinal and supraspinal level. Moreover, evidences of neuroplastic changes that may underlie the low efficacy of MOR agonists under neuropathic pain conditions are reviewed and discussed. Sensitization processes leading to pain hypersensitivity, molecular changes in signalling pathways triggered by MOR and glial activation are some of these mechanisms elicited by both nerve injury and opioid exposure. Nerve injury-induced pain hypersensitivity might be masking the initial analgesic effects of opioid agonists, and alternatively, sustained opioid treatment to individuals already suffering from neuropathic pain could aggravate their pathophysiological state. Finally, some combined therapies that can increase opioid analgesic effectiveness in neuropathic pain treatment are highlighted. SIGNIFICANCE: This review provides evidence of the low benefit of opioid monotherapy in neuropathic pain and analyses the reasons of this reduced effectiveness. Opioid agonists along with drugs targeted to block the sensitization processes induced by MOR stimulation might result in a better management of neuropathic pain.

Pharmacological Modulation of Endogenous Opioid Activity to Attenuate Neuropathic Pain in Rats

We showed previously that spinal metabotropic glutamate receptor 1 (mGluR₁) signaling suppresses or facilitates (depending on the stage of estrous cycle) analgesic responsiveness to intrathecal endomorphin 2, a highly mu-opioid receptor-selective endogenous opioid. Spinal endomorphin 2 antinociception is suppressed during diestrus by mGluR₁ when it is activated by membrane estrogen receptor alpha (mERα) and is facilitated during proestrus when mGluR₁ is activated by glutamate. In the current study, we tested the hypothesis that in female rats subjected to spinal nerve ligation (SNL), the inhibition of spinal estrogen synthesis or blockade of spinal mERα/mGluR₁ would be antiallodynic during diestrus, whereas during proestrus, mGluR₁ blockade would worsen the mechanical allodynia. As postulated, following SNL, aromatase inhibition or mERα/mGluR₁ blockade during diestrus markedly lessened the mechanical allodynia. This was observed only on the paw ipsilateral to SNL and was eliminated by naloxone, implicating endogenous opioid mediation. In contrast, during proestrus, mGluR₁ blockade worsened the SNL-induced mechanical allodynia of the ipsilateral paw. Findings suggest menstrual cycle stage-specific drug targets for and the putative clinical utility of harnessing endogenous opioids for chronic pain management in women, as well as the value of, if not the necessity for, considering menstrual cycle stage in clinical trials thereof. PERSPECTIVE: Intrathecal treatments that enhance spinal endomorphin 2 analgesic responsiveness under basal conditions lessen mechanical allodynia in a chronic pain model. Findings provide a foundation for developing drugs that harness endogenous opioid antinociception for chronic pain relief, lessening the need for exogenous opioids and thus prescription opioid abuse.

Current and Future Issues in the Development of Spinal Agents for the Management of Pain

Targeting analgesic drugs for spinal delivery reflects the fact that while the conscious experience of pain is mediated supraspinally, input initiated by high intensity stimuli, tissue injury and/or nerve injury is encoded at the level of the spinal dorsal horn and this output informs the brain as to the peripheral environment. This encoding process is subject to strong upregulation resulting in hyperesthetic states and downregulation reducing the ongoing processing of nociceptive stimuli reversing the hyperesthesia and pain processing. The present review addresses the biology of spinal nociceptive processing as relevant to the effects of intrathecally-delivered drugs in altering pain processing following acute stimulation, tissue inflammation/injury and nerve injury. The review covers i) the major classes of spinal agents currently employed as intrathecal analgesics (opioid agonists, alpha 2 agonists; sodium channel blockers; calcium channel blockers; NMDA blockers; GABA A/B agonists; COX inhibitors; ii) ongoing developments in the pharmacology of spinal therapeutics focusing on less studied agents/targets (cholinesterase inhibition; Adenosine agonists; iii) novel intrathecal targeting methodologies including gene-based approaches (viral vectors, plasmids, interfering RNAs); antisense, and toxins (botulinum toxins; resniferatoxin, substance P Saporin); and iv) issues relevant to intrathecal drug delivery (neuraxial drug distribution), infusate delivery profile, drug dosing, formulation and principals involved in the preclinical evaluation of intrathecal drug safety.

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.

Effect of naltrexone on neuropathic pain in mice locally transfected with the mutant μ-opioid receptor gene in spinal cord

BACKGROUND AND PURPOSE

Opioid antagonists, such as naloxone and naltrexone, exhibit agonistic properties at the mutated μ receptor, MOR-S196ACSTA. In our previous study, systemic naloxone (10 mg·kg(-1) , s.c.) elicited antinociceptive effect without the induction of tolerance, dependence or rewarding effect in mice 2 weeks after intrathecal administration of double-stranded adeno-associated virus-MOR-S196ACSTA-eGFP. Here, we have investigated if this antinociceptive paradigm would be effective in a mouse model of neuropathic pain.

EXPERIMENTAL APPROACH

Spinal nerves were ligated in male C57BL/6 mice 3 or 4 weeks after intrathecal injection of the lentivirus encoding the construct of MOR-S196ACSTA-eGFP (LV-MOR-S196ACSTA). Anti-allodynic effects of daily s.c.injections of saline, naltrexone (10 mg·kg(-1) ) or morphine (10 mg·kg(-1) ) were assessed by the von Frey test. After 14 days of treatment with saline, naltrexone or morphine, signs of natural withdrawal were measured at 22 and 46 h after the last injection. To determine the rewarding effects induced by morphine or naltrexone, the conditioned place preference test was carried out.

KEY RESULTS

Anti-allodynic effects, as measured by von Frey test, increased after naltrexone or morphine treatment in mice transfected with LV-MOR-S196ACSTA in the spinal cord. Cessation of treatment with morphine, but not naltrexone, induced natural withdrawal and rewarding effects.

CONCLUSIONS AND IMPLICATIONS

Systemic injection of naltrexone after the expression of a mutant μ opioid receptor, MOR-S196ACSTA, in the spinal cord may have therapeutic potential for chronic neuropathic pain, without the development of dependence or addiction.

LINKED ARTICLES

This article is part of a themed section on Opioids: New Pathways to Functional Selectivity. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2015.172.issue-2.

The hippocampus and TNF: Common links between chronic pain and depression

Major depression and chronic pain are significant health problems that seriously impact the quality of life of affected individuals. These diseases that individually are difficult to treat often co-exist, thereby compounding the patient's disability and impairment as well as the challenge of successful treatment. The development of efficacious treatments for these comorbid disorders requires a more comprehensive understanding of their linked associations through common neuromodulators, such as tumor necrosis factor-α (TNFα), and various neurotransmitters, as well as common neuroanatomical pathways and structures, including the hippocampal brain region. This review discusses the interaction between depression and chronic pain, emphasizing the fundamental role of the hippocampus in the development and maintenance of both disorders. The focus of this review addresses the hypothesis that hippocampal expressed TNFα serves as a therapeutic target for management of chronic pain and major depressive disorder (MDD).

Synthesis of novel dihydropyridothienopyrimidin-4,9-dione derivatives

A novel molecular scaffold, dihydropyridothienopyrimidin-4,9-dione, was synthesized from benzylamine or p-methoxybenzylamine in six steps involving successive ring closure to form a fused ring system composed of dihydropyridone, thiophene and pyrimidone. The pharmacological versatility of the dihydropyridothenopyrimidin-4,9-dione scaffold was demonstrated by inhibitory activity against metabotropic glutamate receptor subtype 1 (mGluR1), which shows that the title compounds can serve as an interesting scaffold for the discovery of potential bioactive molecules for the treatment of human diseases.

Altered nociception in mice with genetically induced hypoglutamatergic tone

Extensive pharmacological evidence supports the idea that glutamate plays a key role in both acute and chronic pain. In the present study, we investigated the implication of the excitatory amino acid in physiological nociception by using mutant mice deficient in phosphate-activated glutaminase type 1 (GLS1), the enzyme that synthesizes glutamate in central glutamatergic neurons. Because homozygous GLS1-/- mutants die shortly after birth, assays for assessing mechanical, thermal and chemical (formalin) nociception were performed on heterozygous GLS1+/- mutants, which present a clear-cut decrease in glutamate synthesis in central neurons. As compared to paired wild-type mice, adult male GLS1+/- mutants showed decreased responsiveness to mechanical (von Frey filament and tail-pressure, but not tail-clip, tests) and thermal (Hargreaves' plantar, tail-immersion and hot-plate tests) nociceptive stimuli. Genotype-related differences were also found in the formalin test for which GLS1+/- mice exhibited marked decreases in the nociceptive responses (hindlimb lift, lick and flinch) during both phase 1 (0-5 min) and phase 2 (16-45 min) after formalin injection. On the other hand, acute treatment with memantine (1mg/kg i.p.), an uncompetitive antagonist at NMDA glutamate receptors, reduced nociception responses in wild-type but not GLS1+/- mice. Conversely, antinociceptive response to acute administration of a low dose (1mg/kg s.c.) of morphine was significantly larger in GLS1+/- mutants versus wild-type mice. Our findings indicate that genetically driven hypoactivity of central glutamatergic neurotransmission renders mice hyposensitive to nociceptive stimulations, and promotes morphine antinociception, further emphasizing the critical role of glutamate in physiological nociception and its opioid-mediated control.

Investigating the expression of metabotropic glutamate receptors in trigeminal ganglion neurons and satellite glial cells: implications for craniofacial pain

CONTEXT/OBJECTIVE

Previous studies have demonstrated that various subtypes of the metabotropic glutamate receptors (mGluRs) are expressed in the dorsal root ganglion (DRG) of the peripheral nervous system (PNS), implicating that glutamate potentially contributes to sensory transmission through these receptors. While mGluR expression has been investigated largely in the DRG, the present study focused on mGluR expression on neurons and satellite glial cells (SGCs) of the trigeminal ganglion (TG).

MATERIALS AND METHODS

To address the presence of mGluRs in rat TG neurons and their corresponding SGCs, the trigeminal ganglia from six adult male Wistar rats were isolated and immunohistochemistry and immunocytochemistry were performed. The expression of mGluR1α-, mGluR2/3- and mGluR8 on TG neurons and SGCs was investigated in tissue slices and isolated cells.

RESULTS

35.1 ± 6.0% of the TG neurons were positive for mGluR1α, whereas 39.9 ± 7.7% and 55.5 ± 6.3% were positive for mGluR2/3 and mGluR8, respectively. Immunoreactive neurons expressing mGluRs were mainly medium- to large sized, with a smaller population of small-sized neurons showing immunoreactivity. The SGCs showed immunoreactivity toward mGluR1α and mGluR8, but not mGluR2/3, both in the tissue and in isolated cells.

CONCLUSIONS

Findings from the present study showed that trigeminal neurons express mGluR1α, mGluR2/3 and mGluR8, while SGCs only express mGluR1α and mGluR8. This novel evidence may advance investigations on a possible role of mGluRs in relation to trigeminal pain transmission within the craniofacial region.

The sciatic nerve cuffing model of neuropathic pain in mice

Neuropathic pain arises as a consequence of a lesion or a disease affecting the somatosensory system. This syndrome results from maladaptive changes in injured sensory neurons and along the entire nociceptive pathway within the central nervous system. It is usually chronic and challenging to treat. In order to study neuropathic pain and its treatments, different models have been developed in rodents. These models derive from known etiologies, thus reproducing peripheral nerve injuries, central injuries, and metabolic-, infectious- or chemotherapy-related neuropathies. Murine models of peripheral nerve injury often target the sciatic nerve which is easy to access and allows nociceptive tests on the hind paw. These models rely on a compression and/or a section. Here, the detailed surgery procedure for the "cuff model" of neuropathic pain in mice is described. In this model, a cuff of PE-20 polyethylene tubing of standardized length (2 mm) is unilaterally implanted around the main branch of the sciatic nerve. It induces a long-lasting mechanical allodynia, i.e., a nociceptive response to a normally non-nociceptive stimulus that can be evaluated by using von Frey filaments. Besides the detailed surgery and testing procedures, the interest of this model for the study of neuropathic pain mechanism, for the study of neuropathic pain sensory and anxiodepressive aspects, and for the study of neuropathic pain treatments are also discussed.

Lipid rafts serve as signaling platforms for mGlu1 receptor-mediated calcium signaling in association with caveolin

Background

Group I metabotropic glutamate receptors (mGlu1/5 receptors) have important roles in synaptic activity in the central nervous system. They modulate neuronal excitability by mobilizing intracellular Ca²⁺ following receptor activation. Also, accumulating evidence has indicated the association of Ca²⁺ signaling with lipid rafts. Caveolin, an adaptor protein found in a specialized subset of lipid rafts, has been reported to promote the localization of membrane proteins to lipid rafts.

Results

In the present study, we investigated the role of lipid rafts on the mGlu1α receptor-mediated Ca²⁺ signaling in association with caveolin in hippocampal primary neurons and HEK293 cells. We show that the disruption of lipid rafts using methyl-β-cyclodextrin markedly decreased mGlu1α receptor-mediated Ca²⁺ transients and lipid rafts localization of the receptor. Furthermore, transfection of mGlu1α receptor with mutated caveolin-binding domain reduced localization of the receptor to lipid rafts. Also, application of a peptide blocker of mGlu1α receptor and caveolin binding reduced the Ca²⁺ signaling and the lipid rafts localization.

Conclusions

Taken together, these results suggest that the binding of mGlu1α receptor to caveolin is crucial for its lipid rafts localization and mGlu1α receptor-mediated Ca²⁺ transients.

Metabotropic glutamate antagonists alone and in combination with morphine: comparison across two models of acute pain and a model of persistent, inflammatory pain

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.

Time-dependent cross talk between spinal serotonin 5-HT2A receptor and mGluR1 subserves spinal hyperexcitability and neuropathic pain after nerve injury

Emerging evidence implicates serotonergic descending facilitatory pathways from the brainstem to the spinal cord in the maintenance of pathologic pain. Upregulation of the serotonin receptor 2A (5-HT(2A)R) in dorsal horn neurons promotes spinal hyperexcitation and impairs spinal μ-opioid mechanisms during neuropathic pain. We investigated the involvement of spinal glutamate receptors, including metabotropic receptors (mGluRs) and NMDA, in 5-HT(2A)R-induced hyperexcitability after spinal nerve ligation (SNL) in rat. High-affinity 5-HT(2A)R agonist (4-bromo-3,6-dimethoxybenzocyclobuten-1-yl)methylamine hydrobromide (TCB-2) enhanced C-fiber-evoked dorsal horn potentials after SNL, which was prevented by mGluR1 antagonist AIDA [(RS)-1-aminoindan-1,5-dicarboxylic acid] but not by group II mGluR antagonist LY 341495 [(2S)-2-amino-2-[(1S,2S)-2-carboxycycloprop-1-yl]-3-(xanth-9-yl)propanoic acid] or NMDA antagonist d-AP5 [D-(-)-2-amino-5-phosphonopentanoic acid]. 5-HT(2A)R and mGluR1 were found to be coexpressed in postsynaptic densities in dorsal horn neurons. In the absence of SNL, pharmacological stimulation of 5-HT(2A)R with TCB-2 both induced rapid bilateral upregulation of mGluR1 expression in cytoplasmic and synaptic fractions of spinal cord homogenates, which was attenuated by PKC inhibitor chelerythrine, and enhanced evoked potentials during costimulation of mGluR1 with 3,5-DHPG [(RS)-3,5-dihydroxyphenylglycine]. SNL was followed by bilateral upregulation of mGluR1 in 5-HT(2A)R-containing postsynaptic densities. Upregulation of mGluR1 in synaptic compartments was partially prevented by chronic administration of selective 5-HT(2A)R antagonist M100907 [(R)-(+)-α-(2,3-dimethoxyphenyl)-1-[2-(4-fluorophenyl)ethyl]-4-pipidinemethanol], confirming 5-HT(2A)R-mediated control of mGluR1 upregulation triggered by SNL. Changes in thermal and mechanical pain thresholds following SNL were increasingly reversed over the days after injury by chronic 5-HT(2A)R blockade. These results emphasize a role for 5-HT(2A)R in hyperexcitation and pain after nerve injury and support mGluR1 upregulation as a novel feedforward activation mechanism contributing to 5-HT(2A)R-mediated facilitation.

Synthesis and SAR development of novel mGluR1 antagonists for the treatment of chronic pain

High throughput screening identified the pyridothienopyrimidinone 1 as a ligand for the metabotropic glutamate receptor 1 (mGluR1=10 nM). Compound 1 has an excellent in vivo profile; however, it displays unfavorable pharmacokinetic issues and metabolic stability. Therefore, using 1 as a template, novel analogues (10i) were prepared. These analogues displayed improved oral exposure and activity in the Spinal Nerve Ligation (SNL) pain model.

The glutamatergic system as a target for neuropathic pain relief

Glutamate is the major excitatory neurotransmitter in the mammalian CNS. The understanding of glutamatergic transmission in the nervous system has been greatly expanded with the discovery and investigation of the family of ionotropic and metabotropic glutamate receptors (mGluRs). Metabotropic glutamate receptors are localized at nerve terminals, postsynaptic sites and glial cells and thus, they can influence and modulate the action of glutamate at different levels in the synapse. Moreover, there is substantial evidence of glial participation in glutamate nociceptive processes and neuropathic pain. Metabotropic glutamate receptors have been shown to play a role in neuropathic pain, which is one of the most troublesome illnesses because the therapy is still not satisfactory. Recently, the development of selective mGluR ligands has provided important tools for further investigation of the role of mGluRs in the modulation of chronic pain processing. This paper presents a review of the literature of glutamate receptors in neuropathic pain and the role of glia in these effects. Specifically, pharmacological interventions aimed at inhibiting group I mGluRs and/or potentiating group II and III mGluR-mediated signalling is discussed. Moreover, we introduce data about the role of glutamate transporters. They are responsible for the level of glutamate in the synaptic cleft and thus regulate the effects of all three groups of mGluRs and, in consequence, the activity of this system in nociceptive transmission. Additionally, the question of how the modulation of the glutamatergic system influences the effectiveness of analgesic drugs used in neuropathic pain therapy is addressed.

Therapeutic potential of metabotropic glutamate receptor modulators

Glutamate is the main excitatory neurotransmitter in the central nervous system (CNS) and is a major player in complex brain functions. Glutamatergic transmission is primarily mediated by ionotropic glutamate receptors, which include NMDA, AMPA and kainate receptors. However, glutamate exerts modulatory actions through a family of metabotropic G-protein-coupled glutamate receptors (mGluRs). Dysfunctions of glutamatergic neurotransmission have been implicated in the etiology of several diseases. Therefore, pharmacological modulation of ionotropic glutamate receptors has been widely investigated as a potential therapeutic strategy for the treatment of several disorders associated with glutamatergic dysfunction. However, blockade of ionotropic glutamate receptors might be accompanied by severe side effects due to their vital role in many important physiological functions. A different strategy aimed at pharmacologically interfering with mGluR function has recently gained interest. Many subtype selective agonists and antagonists have been identified and widely used in preclinical studies as an attempt to elucidate the role of specific mGluRs subtypes in glutamatergic transmission. These studies have allowed linkage between specific subtypes and various physiological functions and more importantly to pathological states. This article reviews the currently available knowledge regarding the therapeutic potential of targeting mGluRs in the treatment of several CNS disorders, including schizophrenia, addiction, major depressive disorder and anxiety, Fragile X Syndrome, Parkinson’s disease, Alzheimer’s disease and pain.

Expression profile of flotillin-2 and its pathophysiological role after spinal cord injury

Some receptors that block axonal regeneration or promote cell death after spinal cord injury (SCI) are localized in membrane rafts. Flotillin-2 (Flot-2) is an essential protein associated with the formation of these domains and the clustering of membranal proteins, which may have signaling activities. Our hypothesis is that trauma will change Flot-2 expression and interference of this lipid raft marker will promote functional locomotor recovery after SCI. Analyses were conducted to determine the spatiotemporal profile of Flot-2 expression in adult rats after SCI, using the MASCIS impactor device. Immunoblots showed that SCI produced a significant decrease in the level of Flot-2 at 2 days post-injury (DPI) that increased until 28 DPI. Confocal microscopy revealed Flot-2 expression in neurons, reactive astrocytes and oligodendrocytes specifically associated to myelin structures near or close to the axons of the cord. In the open field test and grid walking assays, to monitor locomotor recovery of injured rats infused intrathecally with Flot-2 antisense oligonucleotides for 28 days showed significant behavioral improvement at 14, 21 and 28 DPI. These findings suggest that Flot-2 has a role in the nonpermissive environment that blocks locomotor recovery after SCI by clustering unfavorable proteins in membrane rafts.

Therapeutic promise and principles: metabotropic glutamate receptors

For a number of disease entities, oxidative stress becomes a significant factor in the etiology and progression of cell dysfunction and injury. Therapeutic strategies that can identify novel signal transduction pathways to ameliorate the toxic effects of oxidative stress may lead to new avenues of treatment for a spectrum of disorders that include diabetes, Alzheimer's disease, Parkinson's disease and immune system dysfunction. In this respect, metabotropic glutamate receptors (mGluRs) may offer exciting prospects for several disorders since these receptors can limit or prevent apoptotic cell injury as well as impact upon cellular development and function. Yet the role of mGluRs is complex in nature and may require specific mGluR modulation for a particular disease entity to maximize clinical efficacy and limit potential disability. Here we discuss the potential clinical translation of mGluRs and highlight the role of novel signal transduction pathways in the metabotropic glutamate system that may be vital for the clinical utility of mGluRs.

Contemporary and emergent pharmacological therapies for chronic pain: nonopioid analgesia

The American Pain Society estimates that 50 million Americans are partially or totally disabled by pain. This striking statistic is certain to increase as our population continues to age. In order to combat this growing problem, healthcare professionals must arm themselves with information. By developing the appropriate pain assessment skills and by staying abreast of the rapidly-changing therapies used in pain management, clinicians can dramatically impact the quality of life of those living with pain.

Regulation of increased glutamatergic input to spinal dorsal horn neurons by mGluR5 in diabetic neuropathic pain

Diabetic neuropathic pain is associated with increased glutamatergic input in the spinal dorsal horn. Group I metabotropic glutamate receptors (mGluRs) are involved in the control of neuronal excitability, but their role in the regulation of synaptic transmission in diabetic neuropathy remains poorly understood. Here we studied the role of spinal mGluR5 and mGluR1 in controlling glutamatergic input in a rat model of painful diabetic neuropathy induced by streptozotocin. Whole-cell patch-clamp recordings of lamina II neurons were performed in spinal cord slices. The amplitude of excitatory post-synaptic currents (EPSCs) evoked from the dorsal root and the frequency of spontaneous EPSCs (sEPSCs) were significantly higher in diabetic than in control rats. The mGluR5 antagonist 2-methyl-6-(phenylethynyl)-pyridine (MPEP) inhibited evoked EPSCs and sEPSCs more in diabetic than in control rats. Also, the percentage of neurons in which sEPSCs and evoked EPSCs were affected by MPEP or the group I mGluR agonist was significantly higher in diabetic than in control rats. However, blocking mGluR1 had no significant effect on evoked EPSCs and sEPSCs in either groups. The mGluR5 protein level in the dorsal root ganglion, but not in the dorsal spinal cord, was significantly increased in diabetic rats compared with that in control rats. Furthermore, intrathecal administration of MPEP significantly increased the nociceptive pressure threshold only in diabetic rats. These findings suggest that increased mGluR5 expression on primary afferent neurons contributes to increased glutamatergic input to spinal dorsal horn neurons and nociceptive transmission in diabetic neuropathic pain.

Discovery and SAR of 6-substituted-4-anilinoquinazolines as non-competitive antagonists of mGlu5

A high-throughput cell-based screen identified a series of 6-substituted-4-anilinoquinazolines as non-competitive antagonists of metabotropic glutamate receptor 5 (mGlu(5)). This Letter describes the SAR of this series and the profile of selected compounds in selectivity and radioligand binding assays.

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.

Group I metabotropic glutamate receptors control metaplasticity of spinal cord learning through a protein kinase C-dependent mechanism

Neurons within the spinal cord can support several forms of plasticity, including response-outcome (instrumental) learning. After a complete spinal transection, experimental subjects are capable of learning to hold the hindlimb in a flexed position (response) if shock (outcome) is delivered to the tibialis anterior muscle when the limb is extended. This response-contingent shock produces a robust learning that is mediated by ionotropic glutamate receptors (iGluRs). Exposure to nociceptive stimuli that are independent of limb position (e.g., uncontrollable shock; peripheral inflammation) produces a long-term (>24 h) inhibition of spinal learning. This inhibition of plasticity in spinal learning is itself a form of plasticity that requires iGluR activation and protein synthesis. Plasticity of plasticity (metaplasticity) in the CNS has been linked to group I metabotropic glutamate receptors (subtypes mGluR1 and mGluR5) and activation of protein kinase C (PKC). The present study explores the role of mGluRs and PKC in the metaplastic inhibition of spinal cord learning using a combination of behavioral, pharmacological, and biochemical techniques. Activation of group I mGluRs was found to be both necessary and sufficient for metaplastic inhibition of spinal learning. PKC was activated by stimuli that inhibit spinal learning, and inhibiting PKC activity restored the capacity for spinal learning. Finally, a PKC inhibitor blocked the metaplastic inhibition of spinal learning produced by a group I mGluR agonist. The data strongly suggest that group I mGluRs control metaplasticity of spinal learning through a PKC-dependent mechanism, providing a potential therapeutic target for promoting use-dependent plasticity after spinal cord injury.

Attenuation of morphine tolerance by minocycline and pentoxifylline in naive and neuropathic mice

We have previously demonstrated that glial inhibitors reduce the development of allodynia and hyperalgesia, potentiating the effect of a single morphine dose in a neuropathic pain model. This study explores the effects of two glial activation inhibitors, minocycline and pentoxifylline, on the development of tolerance to morphine in naive and chronic constriction injury (CCI)-exposed mice. Administration of morphine to naive (20 mg/kg; i.p.) and CCI-exposed mice (40 mg/kg; i.p.) twice daily resulted in tolerance to its anti-nociceptive effect after 6 days. Injections of morphine were combined with minocycline (30 mg/kg, i.p.) or pentoxifylline (20 mg/kg, i.p.) administered as two preemptive doses before first morphine administration in naive or pre-injury in CCI-exposed mice, and repeated twice daily 30 min before each morphine administration. With treatment, development of morphine tolerance was delayed by 5 days (from 6 to 11 days), as measured by the tail-flick test in naive and by tail-flick, von Frey, and cold plate tests in CCI-exposed mice. Western blot analysis of CD11b/c and GFAP protein demonstrated that minocycline and pentoxifylline, at doses delaying development of tolerance to morphine analgesia, significantly diminished the morphine-induced increase in CD11b/c protein level. We found that repeated systemic administration of glial inhibitors significantly delays development of morphine tolerance by attenuating the level of this microglial marker under normal and neuropathic pain conditions. Our results support the idea that targeting microglial activation during morphine therapy/treatment is a novel and clinically promising method for enhancing morphine's analgesic effects, especially in neuropathic pain.

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.

Glutamate receptor ligands attenuate allodynia and hyperalgesia and potentiate morphine effects in a mouse model of neuropathic pain

Recent studies have indicated that metabotropic glutamate receptors mGluR5, mGluR2/3 and mGluR7 are present in the regions of central nervous system important for nociceptive transmission, but their involvement in neuropathic pain has not been well established. We demonstrated that acute and chronic administration of MPEP (mGluR5 antagonist), LY379268 (mGluR2/3 agonist), and AMN082 (mGluR7 agonist) attenuated allodynia (von Frey test) and hyperalgesia (cold plate test) as measured in Swiss albino mice on day seven after chronic constriction injury (CCI) to the sciatic nerve. Moreover, single administration of MPEP (30 mg/kg; i.p.) or LY379268 (10mg/kg; i.p.) injected 30 min before morphine potentiated morphine's effects (20mg/kg; i.p.) in the mouse CCI model, as measured by both the tests mentioned above. However, a single administration of AMN082 (3mg/kg; i.p.) potentiated the effects of a single morphine injection (20mg/kg; i.p.) in the von Frey test only. Chronic administration (7 days) of low doses of MPEP, LY379268 or AMN082 (all drugs at 3mg/kg; i.p.) potentiated the effects of single doses of morphine (3, 10, and 20mg/kg; i.p.) administered on day seven; however, AMN082 only potentiated the effect in the cold plate test. Additionally, the same doses of MPEP and LY379268 (but not AMN082) chronically co-administered with morphine (40 mg/kg; i.p.) attenuated the development of morphine tolerance in CCI-exposed mice. Our data suggest that mGluR5, mGluR2/3, and mGluR7 are involved in injury-induced plastic changes in nociceptive pathways and that the mGluR5 and mGluR2/3 ligands enhanced morphine's effectiveness in neuropathy, which could have therapeutic implications.

Involvement of subtype 1 metabotropic glutamate receptors in apoptosis and caspase-7 over-expression in spinal cord of neuropathic rats

The effect of the non-selective, 1-aminoindan-1,5-dicarboxylic acid (AIDA), and selective (3,4-dihydro-2H-pyrano[2,3-b]quinolin-7-yl)-(cis-4-methoxycyclohexyl) methanone (JNJ16259685), metabotropic glutamate subtype 1 (mGlu1) receptor antagonists, on rat sciatic nerve chronic constrictive injury (CCI)-induced hyperalgesia, allodynia, spinal dorsal horn apoptosis, and gliosis was examined at 3 and 7 days post-injury. RT-PCR analysis showed increased expression of bax, apoptotic protease-activating factor-1 (apaf-1), nestin, GFAP, and caspase-7 mRNA in the dorsal horn spinal cord by 3 days post-CCI. At 7 days post-CCI, only over-expression of bcl-2, nestin and GFAP mRNA was observed. Administration of AIDA reduced thermal hyperalgesia and mechanical allodynia at 3 and 7 days post-CCI; administration of JNJ16259685 reduced thermal hyperalgesia at 3 and 7 days post-CCI, but not mechanical allodynia. AIDA decreased the mRNA levels of bax, apaf-1, GFAP and caspase-7 genes. JNJ16259685 increased the mRNA levels of bcl-2 and GFAP gene, and decreased APAF-1 and caspases-7 genes. Inhibiting mGlu1 receptors also reduced TUNEL-positive profiles and immunohistochemical reactivity for caspase-7. We report here that despite inhibiting CCI-induced over-expression of pro-apoptotic genes in the spinal cord dorsal horn, the selective mGlu1 receptor antagonist JNJ16259685 exerted only a slight and transient allodynic effect. Moreover, JNJ16259685, but not the non-selective AIDA, increased astrogliosis which may account for its decreased analgesic efficacy. This study provides evidence that the contemporary and partial blockade of group I and likely ionotropic glutamate receptors may be a more suitable therapy than selective blockade of mGlu1 subtype receptors condition to decrease neuropathic pain symptoms.

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.

Morphine in combination with metabotropic glutamate receptor antagonists on schedule-controlled responding and thermal nociception

The present study examined the interactive effects of morphine in combination with metabotropic glutamate (mGlu) receptor antagonists on schedule-controlled responding and thermal nociception. Drug interaction data were examined with isobolographic and dose-addition analysis. Morphine, the mGlu1 receptor antagonist JNJ16259685 [(3,4-dihydro-2H-pyrano-[2,3-b]quinolin-7-yl)-(cis-4-methoxycyclohexyl)-methanone], the mGlu5 receptor antagonist MPEP [2-methyl-6-(phenylethynyl)pyridine hydrochloride], and the mGlu2/3 receptor antagonist LY341495 [(2S)-2-amino-2-[(1S,2S-2-carboxycycloprop-1-yl]-3-(xanth-9-yl) propanoic acid] all decreased rates of schedule-controlled responding. JNJ16259685/morphine, MPEP/morphine, and LY341495/morphine mixtures produced additive effects on this endpoint. Morphine also produced dose-dependent antinociception in the assay of thermal nociception, whereas JNJ16259685, MPEP, and LY341495 failed to produce an effect. In this assay, JNJ16259685 and LY341495 potentiated the antinociceptive effects of morphine, whereas MPEP/morphine mixtures produced additive effects. These results suggest that an mGlu1 and an mGlu2/3 receptor antagonist, but not an mGlu5 receptor antagonist, selectively enhance the antinociceptive effects of morphine. In addition, these data confirm that the behavioral effects of drug mixtures depend on the endpoint under study.

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.