Metabotropic glutamate receptors--important modulators of nociception and pain behavior

Ecdysone-Based System for Controlled Inducible Expression of Metabotropic Glutamate Receptor Subtypes 2, 5, and 8

Stable and inducible expression of human metabotropic glutamate receptor types 2, 5, and 8 was achieved in HEK293 cells using the ecdysone inducible system. Treatment of the respective cell lines with ponasterone A resulted in time and concentration-dependent induction of receptor expression. In all cases, the functional activation of receptorswas determined bymeasuring increases in intracellular calcium. The physiologically G• i-coupled receptors mGluR2 andm GluR8 were successfully coupled to phospholipase Cactivation using the chimeric Gprotein G• q/o. The pharmacological properties of recombinant receptors were characterized and proved to be similar to native receptors. Our data suggest that the ecdysone system has a number of characteristics thatmake itwell suited for expressing mGluRs and that the combined use of this systemand chimeric G proteins allows receptors to be characterized using a rapid and straightforward Ca 2+assay.

Excitatory amino acid neurotransmission

In recent years great progress has been made in understanding the function of ionotropic and metabotropic glutamate receptors; their pharmacology and potential therapeutic applications. It should be stressed that there are already N-methyl-D-aspartate (NMDA) antagonists in clinical use, such as memantine, which proves the feasibility of their therapeutic potential. It seems unlikely that competitive NMDA receptor antagonists and high-affinity channel blockers will find therapeutic use due to limiting side-effects, whereas agents acting at the glycineB site, NMDA receptor subtype-selective agents and moderate-affinity channel blockers are far more promising. This is supported by the fact that there are several glycineB antagonists, NMDA moderate-affinity channel blockers and NR2B-selective agents under development. Positive and negative modulators of AMPA receptors such as the AMPAkines and 2,3-benzodiazepines also show more promise than e.g. competitive antagonists. Great progress has also been made in the field of metabotropic glutamate receptors since the discovery of novel, allosteric modulatory sites for these receptors. Selective agents acting at these transmembrane sites have been developed that are more drug-like and have a much better access to the central nervous system than their competitive counterparts. The chapter will critically review preclinical and scarce clinical experience in the development of new ionotropic and metabotropic glutamate receptor modulators according to the following scheme: rational, preclinical findings in animal models and finally clinical experience, where available.

Signal transduction pathways of group I metabotropic glutamate receptor-induced long-term depression at sensory spinal synapses

Activation of spinal group I metabotropic glutamate receptors (mGluRs) may have antinociceptive or pro-nociceptive effects in different pain models. Pharmacological activation of group I mGluRs leads to long-term depression (LTD) of synaptic strength between Adelta-fibers and neurons in lamina II of spinal dorsal horn of the rat. Here, we studied the signal transduction pathways involved. Synaptic strength between Adelta-fibers and lamina II neurons was assessed by perforated whole-cell patch-clamp recordings in a spinal cord-dorsal root slice preparation of young rats. Bath application of the specific group I mGluR agonist (S)-3,5-dihydroxyphenylglycine [(S)-3,5-DHPG] produced an LTD of Adelta-fiber-evoked responses. LTD induction by (S)-3,5-DHPG was prevented, when intracellular Ca(2+) stores were depleted by thapsigargin or cyclopiazonic acid (CPA). Preincubation with ryanodine to inhibit Ca(2+)-induced Ca(2+) release had no effect on LTD-induction by (S)-3,5-DHPG. In contrast, pretreatment with 2-aminoethoxydiphenyl borate (2-APB), an inhibitor of inositol-1,4,5-trisphosphate (IP(3))-sensitive Ca(2+) stores prevented LTD induction. Preincubation with the specific protein kinase C (PKC) inhibitors bisindolylmaleimide I (BIM) or chelerythrine, respectively, had no effect. Inhibition of L-type VDCCs by verapamil or nifedipine prevented LTD-induction by (S)-3,5-DHPG. The presently identified signal transduction cascade may be relevant to the long-term depression of sensory information in the spinal cord, including nociception.

Evidence for the involvement of glutamatergic system in the antinociceptive effect of ascorbic acid

This study examined the role of glutamatergic system in the ascorbic acid (AA)-induced antinociception in chemical behavioural models of nociception in mice. AA (0.3-10 mg/kg, i.p.) produced significant inhibition of both phases of formalin-induced licking, with mean ID50 values of 4.0 and 3.2 mg/kg and inhibitions of 56+/-4 and 60+/-7% for the early and second phase of the nociception caused by formalin, respectively. AA (1-5 mg/kg, i.p.) also produced significant inhibition of glutamate-induced nociception with mean ID50 value of 2.1 mg/kg and inhibition of 66+/-5%. Furthermore, AA (3 mg/kg, i.p.) caused marked inhibition of nociceptive response induced by intrathecal injection of glutamate, NMDA, AMPA, kainate and substance P, with inhibitions of 49+/-9, 42+/-7, 34+/-8, 38+/-5 and 65+/-8%, respectively. In contrast, AA at the same dose did not affect the biting response induced by the metabotropic agonist trans-ACPD. Taken together, present results indicate that AA, at low systemic doses, produces a rapid onset and consistent antinociception in mice when assessed in several models of chemical nociception, an action that is likely mediated by an interaction with ionotropic, but not metabotropic, glutamate receptors.

The sperm, a neuron with a tail: 'neuronal' receptors in mammalian sperm

A number of plasma membrane receptor types originally thought to be specific to neurons have been found in other somatic cells. More surprisingly, the mammalian sperm and neuron appear to share many of these 'neuronal' receptors. The morphology, chromosome number, genomic activity, and functions of those two cell types are as unlike as any two cells in the body, but they both achieve their highly disparate goals with the aid of a number of the same receptors. Exocytosis in neurons and sperm is essential to the functions of these cells and is strongly influenced by similar receptors. 'Neuronal' receptor types in sperm may also play a role in the control of sperm motility (a function of course not shared by neurons). This review will consider the evidence for the presence of sperm plasma membrane 'neuronal' receptors and for their significance to mammalian sperm function. The persuasiveness of the evidence varies depending on the receptor being considered, but there is strong experimental support for the presence and importance of a number of 'neuronal' receptors in sperm.

Assessing the role of metabotropic glutamate receptor 5 in multiple nociceptive modalities

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

Synthesis, structure-activity relationship, and receptor pharmacology of a new series of quinoline derivatives acting as selective, noncompetitive mGlu1 antagonists

We describe the discovery and the structure-activity relationship of a new series of quinoline derivatives acting as selective and highly potent noncompetitive mGlu1 antagonists. We first identified cis-10 as a fairly potent mGlu1 antagonist (IC(50) = 20 nM) in a cell-based signal transduction assay on the rat mGlu1 receptor expressed in CHO-K1 cells, and then we were able to design and synthesize highly potent compounds on both rat and human mGlu1 receptors as exemplified by compound cis-64a, which has an antagonist potency of 0.5 nM for the human mGlu1 receptor. We briefly present and discuss the in vitro metabolic stability of the compounds in human liver microsomes. We finally report the pharmacokinetic properties of our lead compound cis-64a.

mGluR1 and mGluR5 antagonists in the amygdala inhibit different components of audible and ultrasonic vocalizations in a model of arthritic pain

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

Understanding neuropathic pain

Neuropathic pain is defined as a chronic pain condition that occurs or persists after a primary lesion or dysfunction of the peripheral or central nervous system. Traumatic injury of peripheral nerves also increases the excitability of nociceptors in and around nerve trunks and involves components released from nerve terminals (neurogenic inflammation) and immunological and vascular components from cells resident within or recruited into the affected area. Action potentials generated in nociceptors and injured nerve fibers release excitatory neurotransmitters at their synaptic terminals such as L-glutamate and substance P and trigger cellular events in the central nervous system that extend over different time frames. Short-term alterations of neuronal excitability, reflected for example in rapid changes of neuronal discharge activity, are sensitive to conventional analgesics, and do not commonly involve alterations in activity-dependent gene expression. Novel compounds and new regimens for drug treatment to influence activity-dependent long-term changes in pain transducing and suppressive systems (pain matrix) are emerging.

Virtual Screening Workflow Development Guided by the “Receiver Operating Characteristic” Curve Approach. Application to High-Throughput Docking on Metabotropic Glutamate Receptor Subtype 4

The "receiver operating characteristic" (ROC) curve method is a well-recognized metric used as an objective way to evaluate the ability of a given test to discriminate between two populations. This facilitates decision-making in a plethora of fields in which a wrong judgment may have serious consequences including clinical diagnosis, public safety, travel security, and economic strategies. When virtual screening is used to speed-up the drug discovery process in pharmaceutical research, taking the right decision upon selecting or discarding a molecule prior to in vitro evaluation is of paramount importance. Characterizing both the ability of a virtual screening workflow to select active molecules and the ability to discard inactive ones, the ROC curve approach is well suited for this critical decision gate. As a case study, the first virtual screening workflow focused on metabotropic glutamate receptor subtype 4 (mGlu4R) agonists is reported here. Six compounds out of 38 selected and tested in vitro were shown to have agonist activity on this target of therapeutic interest.

Protein kinase A-dependent enhanced NMDA receptor function in pain-related synaptic plasticity in rat amygdala neurones

Mechanisms of pain-related plasticity in the amygdala, a key player in emotionality, were studied at the cellular and molecular levels in a model of arthritic pain. The influence of the arthritis pain state induced in vivo on synaptic transmission and N-methyl-d-aspartate (NMDA) receptor function was examined in vitro using whole-cell voltage-clamp recordings of neurones in the latero-capsular part of the central nucleus of the amygdala (CeA), which is now defined as the 'nociceptive amygdala'. Synaptic transmission was evoked by electrical stimulation of afferents from the pontine parabrachial area (part of the spino-parabrachio-amygdaloid pain pathway) in brain slices from control rats and from arthritic rats. This study shows that pain-related synaptic plasticity is accompanied by protein kinase A (PKA)-mediated enhanced NMDA-receptor function and increased phosphorylation of NMDA-receptor 1 (NR1) subunits. Synaptic plasticity in the arthritis pain model, but not normal synaptic transmission in control neurones, was inhibited by a selective NMDA receptor antagonist. Accordingly, an NMDA receptor-mediated synaptic component was recorded in neurones from arthritic animals, but not in control neurones, and was blocked by inhibition of PKA but not protein kinase C (PKC). Exogenous NMDA evoked a larger inward current in neurones from arthritic animals than in control neurones, indicating a postsynaptic effect. Paired-pulse facilitation, a measure of presynaptic mechanisms, was not affected by an NMDA-receptor antagonist. Increased levels of phosphorylated NR1 protein, but not of total NR1, were measured in the CeA of arthritic rats compared to controls. Our results suggest that pain-related synaptic plasticity in the amygdala involves a critical switch of postsynaptic NMDA receptor function through PKA-dependent NR1 phosphorylation.

Group I metabotropic glutamate receptor NMDA receptor coupling and signaling cascade mediate spinal dorsal horn NMDA receptor 2B tyrosine phosphorylation associated with inflammatory hyperalgesia

Hindpaw inflammation induces tyrosine phosphorylation (tyr-P) of the NMDA receptor (NMDAR) 2B (NR2B) subunit in the rat spinal dorsal horn that is closely related to the initiation and development of hyperalgesia. Here, we show that in rats with Freund's adjuvant-induced inflammation, the increased dorsal horn NR2B tyr-P is blocked by group I metabotropic glutamate receptor (mGluR) antagonists [7-(hydroxyimino)cyclopropa[b] chromen-1a-carboxylate ethyl ester (CPCCOEt) and 2-methyl-6-(phenylethynyl)-pyridine (MPEP), by the Src inhibitor CGP 77675, but not by the MAP kinase inhibitor 2'-amino-3'-methoxyflavone. Analysis of the calcium pathways shows that the in vivo NR2B tyr-P is blocked by an IP3 receptor antagonist 2-aminoethoxydiphenylborate (2APB) but not by antagonists of ionotropic glutamate receptors and voltage-dependent calcium channels, suggesting that the NR2B tyr-P is dependent on intracellular calcium release. In a dorsal horn slice preparation, the group I (dihydroxyphenylglycine), but not group II [(2R,4R)-4-aminopyrrolidine-2,3-dicarboxylate] and III [L-AP 4 (L-(+)-2-amino-4-phosphonobutyric acid)], mGluR agonists, an IP3 receptor (D-IP3) agonist, and a PKC (PMA) activator, induces NR2B tyr-P similar to that seen in vivo after inflammation. Coimmunoprecipitation indicates that Shank, a postsynaptic density protein associated with mGluRs, formed a complex involving PSD-95 (postsynaptic density-95), NR2B, and Src in the spinal dorsal horn. Double immunofluorescence studies indicated that NR1 is colocalized with mGluR5 in dorsal horn neurons. mGluR5 also coimmunoprecipitates with NR2B. Finally, intrathecal pretreatment of CPCCOEt, MPEP, and 2APB attenuates inflammatory hyperalgesia. Thus, inflammation and mGluR-induced NR2B tyr-P share similar mechanisms. The group ImGluR-NMDAR coupling cascade leads to phosphorylation of the NMDAR and appears necessary for the initiation of spinal dorsal horn sensitization and behavioral hyperalgesia after inflammation.

Glutamate and tachykinin receptors in central sensitization of withdrawal reflexes in the decerebrated rabbit

This study assessed the involvement of NMDA and group I metabotropic glutamate receptors, and tachykinin NK1 and NK3 receptors, in central sensitization of withdrawal reflexes in the decerebrated rabbit. Reflexes evoked in the ankle flexor tibialis anterior and the knee flexor semitendinosus by electrical stimulation at the base of the toes were enhanced for 29-63 min after application of 20% mustard oil to the tips of the toes. Selective antagonists of mGlu1, mGlu5, NMDA and NR2B-subunit-containing NMDA glutamate receptors, as well as NK1, and NK3 receptors, and a non-selective blocker of all tachykinin receptors, were assessed for their effects on the magnitude and duration of the increase in reflexes induced by mustard oil. Dizocilpine, an antagonist of all NMDA receptors (1 mg intrathecal) abolished facilitation of tibialis anterior reflexes and significantly reduced the magnitude and duration of increase of the semitendinosus response. The NR2B-subtype selective antagonist CP-101,606 decreased the magnitude of facilitation of both reflexes but had no effect on duration of enhancement. Selective antagonists for the mGlu1 (CPCCOEt, 1-3 mg intrathecal), mGlu5 (MPEP, 0.2-1 mg intrathecal), NK1 (L-733,060, 0.3 mg intrathecal) or NK3 (SR 142,801, 1 mg kg(-1) i.v.) receptors had no effect on the amplitude or duration of sensitization. However, the non-selective tachykinin receptor blocker ZD-6021 (0.3 mg intrathecal) reduced the amplitude but not the duration of sensitization in the flexor reflexes. Combination of ZD-6021 with CP-101,606 (doses as above) decreased both aspects of the sensitization response. Dizocilpine reduced reflexes evoked from the heel per se, and dizocilpine, CP-101,606 and ZD-6021 reduced arterial blood pressure. Otherwise the drugs used had no effects on baseline variables. The present data confirm the importance of NMDA receptors as a critical part of the process of central sensitization, provide no evidence for a role of metabotropic glutamate receptors, and show that simultaneous blockade of all tachykinin receptors is required to reveal their role in hyperalgesia. The data further indicate that a combined pharmacological approach offers a potential way forward for the development of new antihyperalgesic agents.

Inflammation persistently enhances nocifensive behaviors mediated by spinal group I mGluRs through sustained ERK activation

Group I metabotropic glutamate receptors (mGluRs) and their downstream signaling pathways, which involve the extracellular signal-regulated kinases (ERKs), have been implicated as mediators of plasticity in several pain models. In this study, we report that inflammation leads to a long-lasting enhancement of behavioral responses induced by activation of spinal group I mGluRs. Thus, the nocifensive response to intrathecal injection of the group I mGluR agonist (RS)-3,5-Dihydroxyphenylglycine (DHPG) is significantly potentiated seven days following Complete Freund's Adjuvant (CFA)-induced inflammation of the hind paw. This potentiation is not associated with increased mGlu1 or mGlu5 receptor expression but is associated with increased levels of phosphorylated ERK in dorsal horn neurons. We also tested whether the increased behavioral response to DHPG following inflammation may be explained by enhanced coupling of the group I mGluRs to ERK activation. DHPG-induced ERK phosphorylation in the dorsal horn is not potentiated following inflammation. However, inhibiting ERK activation using a MEK inhibitor, U0126, following inflammation attenuates the intrathecal DHPG-induced behavioral responses to a greater extent than in control animals. The results from this study indicate that persistent ERK activation is required for the enhanced behavioral responses to spinal group I mGluR activation following inflammation and suggest that tonic modulation of ERK activity may underlie a component of central sensitization in dorsal horn neurons.

Antinociceptive effects of N-acetylaspartylglutamate (NAAG) peptidase inhibitors ZJ-11, ZJ-17 and ZJ-43 in the rat formalin test and in the rat neuropathic pain model

The peptide neurotransmitter N-acetylaspartylglutamate (NAAG) acts as an agonist at group II metabotropic glutamate receptors (mGluRs). NAAG is inactivated by extracellular peptidase activity yielding glutamate and N-acetylaspartate. We recently developed a series of potent NAAG peptidase inhibitors, including ZJ-11, ZJ-17 and ZJ-43. In the present study, we examined the effects of intrathecally administered ZJ-11 and ZJ-17 and intravenously administered ZJ-11 and ZJ-43 in the rat formalin test (an inflammatory pain model) and in the rat partial sciatic nerve ligation model (a neuropathic pain model). Intrathecal injection of ZJ-11 or ZJ-17 or intravenous injection of ZJ-11 or ZJ-43 suppressed both phases of the agitation behaviour induced by paw formalin injection. Intrathecal and intravenous injection of ZJ-11 suppressed the expression of Fos-like immunoreactivity, induced by paw formalin injection, in laminae I-II in segments L4-L5 of the spinal cord, suggesting an action on sensory spinal transmission. Partial sciatic nerve ligation induced significant mechanical allodynia 7 days after the nerve injury. Intrathecal injection of ZJ-11 or ZJ-17 or intravenous administration of ZJ-11 or ZJ-43 attenuated the level of mechanical allodynia induced by this nerve ligation. These effects of intrathecally or intravenously administered ZJ compounds in both the formalin test and the partial sciatic nerve ligation model were completely antagonized by pretreatment with LY-341495, a highly selective group II mGluR antagonist. Thus, elevation of extracellular NAAG, induced by the inhibition of NAAG peptidase, activates group II mGluRs and produces an analgesic effect in neuropathic and inflammatory and pain models. In contrast, peptidase inhibition did not affect the threshold for withdrawal from a noxious mechanical stimulus or from an acute thermal stimulus in the hotplate test.

Specificity of membrane specializations in mechanoreceptors of birds--A freeze-etching study

The detailed knowledge of the molecular process of mechanotransduction is still an unsolved question. The investigation of the intramembranous structure of the cutaneous mechanoreceptors may play an important role in elucidating this problem. In this relation, Herbst sensory corpuscles in ducks were studied for the first time using the freeze-etching and thin sectioning techniques. Herbst corpuscles have the basic structural components valid for most of the encapsulated mechanoreceptors in mammals: a capsule made of perineural cells, a lamellar complex of modified Schwann cells, surrounding the non-myelinated part of the receptor nerve fiber and its ending. Freeze-etching replicas reveal that the plasmalemmae of the capsule cells, modified Schwann cells and axolemmae of parts of the nerve fiber differ in both density and pattern of distribution of intramembranous particles (IMPs) as well as IMP size. On all the plasmalemmae the IMP density is higher on the P-face (2000-3300 microm(-2)) than the respective E-face (800-1500 microm(-2)). The axolemma of the ending of the receptor nerve fiber expresses higher density of IMPs than its shaft. The mean IMP size for all the plasmalemmae varies between 5.5 and 7.5 nm. Many tight junctions occur between the capsule cells. These results indicate that the non-myelinated axolemma as well as the plasmalemmae of other components of Herbst corpuscles are specialized in terms of content and distribution of IMPs. The IMPs may represent various kinds of mechanosensitive channel proteins or related membrane-bound proteins participating in the process of mechanotransduction.

The amygdala and persistent pain

A reciprocal relationship exists between persistent pain and negative affective states such as fear, anxiety, and depression. Accumulating evidence points to the amygdala as an important site of such interaction. Whereas a key role of the amygdala in the neuronal mechanisms of emotionality and affective disorders has been well established, the concept of the amygdala as an important contributor to pain and its emotional component is still emerging. This article will review and discuss evidence from anatomical, neuroimaging, behavioral, electrophysiological, pharmacological, and biochemical data that implicate the amygdala in pain modulation and emotional responses to pain. The latero-capsular division of the central nucleus of the amygdala is now defined as the "nociceptive amygdala" and integrates nociceptive information with poly-modal information about the internal and external bodily environment. Dependent on environmental conditions and affective states, the amygdala appears to play a dual facilitatory and inhibitory role in the modulation of pain behavior and nociceptive processing at different levels of the pain neuraxis. Only recently, electrophysiological, pharmacological, and biochemical neuroplastic changes were shown in the nociceptive amygdala in persistent pain. It is conceivable, however, that amygdala plasticity plays an important role in emotional pain behavior and its modulation by affective state.

Differential expression of central metabotropic glutamate receptor (mGluR) subtypes in a clinical model of post-surgical pain

Tissue damage during surgery can induce 'central sensitization' and the development of pain and hyperalgesia post-operatively. Metabotropic glutamate receptors (mGluRs) contribute to nociception, inflammatory pain and hyperalgesia. This study characterized the temporal expression of group I (mGluR(1), mGluR(5)) and II (mGluR(2), mGluR(3)) mGluRs in spinal cord following abdominal surgery. Lumbar spinal cord was recovered from adult sheep euthanased 5 h, 1, 2, 3 and 6 days after undergoing a midline laparotomy, and processed for mGluR mRNA (real-time PCR, in situ hybridization) and protein (Western blotting). mGluR(5) mRNA was up-regulated 5 h and 1 day post-surgery in laminae I-II of the spinal cord dorsal horn. mGluR(5) protein was increased 1 day post-surgery. A delayed induction of mGluR(2) and mGluR(3) mRNAs and mGluR(2/3) protein occurred in spinal cord 3 days after surgery. By 6 days, mGluR(2) mRNA levels had returned to normal, however, mGluR(3) mRNA and mGluR(2/3) protein remained elevated. No change was detected in mGluR(1). These results demonstrate that mGluRs are differentially regulated following surgery and support a link between mGluR-mediated activity and post-surgical pain.

Enhanced group III mGluR-mediated inhibition of pain-related synaptic plasticity in the amygdala

Pain has a strong emotional component. A key player in emotionality, the amygdala is also involved in pain processing. Our previous studies showed synaptic plasticity in the central nucleus of the amygdala (CeA) in a model of arthritic pain. Here, we address the role of group III metabotropic glutamate receptors (mGluRs) in the regulation of synaptic transmission in CeA neurons. Whole-cell current- and voltage-clamp recordings were made from neurons in the latero-capsular part of the CeA in brain slices from control rats and arthritic rats (>6 h postinduction). The latero-capsular part of the CeA is the target of the spino-parabrachio-amygdaloid pain pathway and is now designated as the "nociceptive amygdala". Monosynaptic excitatory postsynaptic currents (EPSCs) were evoked by electrical stimulation of afferents from the pontine parabrachial (PB) area. LAP4 decreased the amplitude of EPSCs more potently in CeA neurons from arthritic rats (EC(50)=1.2 nM) than in control animals (EC(50)=11.5 nM). The inhibitory effect of LAP4 was reversed by a selective group III mGluR antagonist (UBP1112). During the application of LAP4, paired-pulse facilitation was increased, while no significant changes in slope conductance and action potential firing rate of CeA neurons were observed. These data suggest that presynaptic group III mGluRs are involved in the regulation of synaptic plasticity in the amygdala in an arthritis pain model.

Metabotropic glutamate 1 receptor distribution and occupancy in the rat brain: a quantitative autoradiographic study using [3H]R214127

We used the selective metabotropic glutamate (mGlu) 1 receptor antagonist [3H]1-(3,4-dihydro-2H-pyrano[2,3-b]quinolin-7-yl)-2-phenyl-1-ethanone ([3H]R214127) to investigate the distribution of mGlu1 receptor binding sites in rat brain. We found high mGlu1 receptor binding in the cerebellum, thalamus, dentate gyrus and medial central gray, moderate binding within the CA3 of the hippocampus and hypothalamus, and low mGlu1 receptor binding in the basal ganglia and cortex. The mGlu1 receptor is also present in variable degree in the dorsal lateral septal nucleus, amygdala, interpeduncular nucleus and median raphe nucleus. Additionally, we employed [3H]R214127 autoradiography as a means of investigating the occupancy of central mGlu1 receptors following in vivo administration of mGlu1 receptor antagonists that prevent binding of this radioligand. We found that the mGlu1 receptor antagonist (3aS,6aS)-6a-naphtalan-2-ylmethyl-5-methyliden-hexahydro-cyclopenta[c]furan-1-on (BAY 36-7620), administered subcutaneously (s.c.) at 10 mg/kg, only occupied about 30% of cerebellar and thalamic mGlu1 receptors. The mGlu1/5 receptor antagonist 2-quinoxaline-carboxamide-N-adamantan-1-yl (NPS 2390) exhibited a relatively high potency in occupying mGlu1 receptors in rat cerebellum (ED50 = 0.75 mg/kg, s.c.) and thalamus (ED50 = 0.63 mg/kg, s.c). In the future, this method can be employed to gain more insight into the in vivo profile and central activity of potential therapeutic agents that act upon the mGlu1 receptor.

Multiple metabotropic glutamate receptor subtypes modulate GABAergic neurotransmission in rat periaqueductal grey neurons in vitro

The effect of metabotropic glutamate receptor (mGluR) activation on GABAergic synaptic transmission in rat periaqueductal grey (PAG) neurons was examined using whole-cell patch-clamp recordings in brain slices. The selective groups I, II and III mGluR agonists DHPG (10-30 microM), DCG-IV (1-3 microM) and L-AP4 (10-30 microM) inhibited electrically evoked GABA(A) mediated inhibitory postsynaptic currents (IPSCs) in all PAG neurons tested. DCG-IV and L-AP4 also reduced the frequency of spontaneous IPSCs, while DHPG produced both increases and decreases in spontaneous IPSC frequency in a dose dependent manner. In the presence of TTX, DHPG, DCG-IV and L-AP4 all reduced the frequency of spontaneous miniature IPSCs, but had no effect on their amplitudes. The DHPG, DCG-IV and L-AP4 effects on miniature IPSCs were dose dependent (EC(50)s=1.4, 0.055 and 0.52 microM, respectively) and were reduced by the selective mGluR antagonists MCPG, EGLU and MSOP, respectively. These results indicate that GABAergic synaptic transmission within the PAG is reduced by groups I, II and III mGluR activation via a presynaptic mechanism and is increased by group I mGluR activation via an action potential dependent mechanism. The finding of convergent groups I, II and III mGluR-mediated inhibition of synaptic transmission is novel and indicates that all groups of mGluRs have the potential to modulate the constellation of analgesic, behavioural and autonomic functions within the PAG.

Synthesis of Urea-Based Inhibitors as Active Site Probes of Glutamate Carboxypeptidase II: Efficacy as Analgesic Agents

The neuropeptidase glutamate carboxypeptidase II (GCPII) hydrolyzes N-acetyl-L-aspartyl-L-glutamate (NAAG) to liberate N-acetylaspartate and glutamate. GCPII was originally cloned as PSMA, an M(r) 100,000 type II transmembrane glycoprotein highly expressed in prostate tissues. PSMA/GCPII is located on the short arm of chromosome 11 and functions as both a folate hydrolase and a neuropeptidase. Inhibition of brain GCPII may have therapeutic potential in the treatment of certain disease states arising from pathologically overactivated glutamate receptors. Recently, we reported that certain urea-based structures act as potent inhibitors of GCPII (J. Med. Chem. 2001, 44, 298). However, many of the potent GCPII inhibitors prepared to date are highly polar compounds and therefore do not readily penetrate the blood-brain barrier. Herein, we elaborate on the synthesis of a series of potent, urea-based GCPII inhibitors from the lead compound 3 and provide assay data for these ligands against human GCPII. Moreover, we provide data revealing the ability of one of these compounds, namely, 8d, to reduce the perception of inflammatory pain. Within the present series, the gamma-tetrazole bearing glutamate isostere 7d is the most potent inhibitor with a K(i) of 0.9 nM. The biological evaluation of these compounds revealed that the active site of GCPII likely comprises two regions, namely, the pharmacophore subpocket and the nonpharmacophore subpocket. The pharmacophore subpocket is very sensitive to structural changes, and thus, it appears important to keep one of the glutamic acid moieties intact to maintain the potency of the GCPII inhibitors. The site encompassing the nonpharmacophore subpocket that binds to glutamate's alpha-carboxyl group is sensitive to structural change, as shown by compounds 6b and 7b. However, the other region of the nonpharmacophore subpocket can accommodate both hydrophobic and hydrophilic groups. Thus, an aromatic ring can be introduced to the inhibitor, as in 8b and 8d, thereby increasing its hydrophobicity and thus potentially its ability to cross the blood-brain barrier. Intrathecally administered 8d significantly reduced pain perception in the formalin model of rat sensory nerve injury. A maximal dose of morphine (10 mg) applied in the same experimental paradigm provided no significant increase in analgesia in comparison to 8d during phase 1 of this pain study and modestly greater analgesia than 8d in phase 2. These urea-based inhibitors of GCPII thus offer a novel approach to pain management.

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.

Up-regulation of metabotropic glutamate receptor subtypes 3 and 5 in spinal cord in a clinical model of persistent inflammation and hyperalgesia

Evidence from experimental pain research has revealed that metabotropic glutamate receptors (mGluRs) play a pivotal role in nociceptive processing, inflammatory pain and hyperalgesia. The aim of this study was to characterise expression of group I and II mGluRs in spinal cord in a model of naturally occurring persistent inflammation (sheep with unilateral lameness due to inflammation of the digital tissues of the feet, estimated to have been affected by the condition for >2 weeks) and an experimental model of acute inflammation (injection of intradermal carrageenan into lower forelimb in sheep). Animals with unilateral clinical inflammation displayed significant mechanical hyperalgesia on the affected limb. Carrageenan treatment produced significant bilateral limb mechanical hyperalgesia 3 h post-injection. Up-regulation of mGluR(3) and mGluR(5) mRNA was observed in ipsilateral spinal cord recovered from clinically lame animals, restricted to laminae II-V and I-II, respectively. Western blot analyses of protein extracts revealed a bilateral increase in mGluR(2/3) and mGluR(5). No change was detected in spinal cord mGluR(1) or mGluR(2) mRNA. There was no change in mGluR(1,2,3,5) subtype mRNA or proteins in spinal cord recovered from animals 3 h post-carrageenan. These results demonstrate for the first time that mGluR subtypes are differentially expressed in spinal cord dorsal horn in response to persistent inflammation, and suggest that mGluR activity may be involved in mediating altered behaviours associated with clinical inflammatory pain.

Differential roles of mGluR1 and mGluR5 in brief and prolonged nociceptive processing in central amygdala neurons

The laterocapsular division of the central nucleus of the amygdala (CeA) is now defined as the "nociceptive amygdala" because of its high content of neurons that respond to painful stimuli. The majority of these neurons become sensitized in a model of arthritis pain. Here we address the role of G protein-coupled group I metabotropic glutamate receptor subtypes mGluR1 and mGluR5 in nociceptive processing under normal conditions and in pain-related sensitization. Extracellular single-unit recordings were made from 65 CeA neurons in anesthetized rats. Each neuron's responses to brief mechanical stimuli, background activity, receptive field size, and threshold were measured before and after induction of the kaolin/carrageenan mono-arthritis in one knee and before and during applications of agonists and antagonists into the CeA by microdialysis. All neurons received excitatory input from the knee(s) and responded most strongly to noxious stimuli. Before arthritis, a group I mGluR1 and mGluR5 agonist (DHPG, n = 10) potentiated the responses to innocuous and noxious stimuli. This effect was mimicked by an mGluR5 agonist (CHPG, n = 15). In the arthritis pain state (>6 h after induction), the facilitatory effects of DHPG (n = 9), but not CHPG (n = 7), increased. An mGluR1 antagonist (CPCCOEt) had no effect before arthritis (n = 12) but inhibited the responses of sensitized neurons in the arthritis pain state (n = 8). An mGluR5 antagonist (MPEP) inhibited brief nociceptive responses under normal conditions (n = 19) and prolonged nociception in arthritis (n = 8). These data suggest a change of mGluR1 function and activation in the amygdala in pain-related sensitization, whereas mGluR5 is involved in brief as well as prolonged nociception.

Cognitive impairment in patients with chronic pain: the significance of stress

This review article examines the role of emotional distress and other aspects of suffering in the cognitive impairment that often is apparent in patients with chronic pain. Research suggests that pain-related negative emotions and stress potentially impact cognitive functioning independent of the effects of pain intensity. The anterior cingulate cortex is likely an integral component of the neural system that mediates the impact of pain-related distress on cognitive functions, such as the allocation of attentional resources. A maladaptive physiologic stress response is another plausible cause of cognitive impairment in patients with chronic pain, but a direct role for dysregulation of the hypothalamic-pituitary-adrenocortical axis has not been systematically investigated.

Dynamic balance of metabotropic inputs causes dorsal horn neurons to switch functional states

Sensory relay structures in the spinal cord dorsal horn are now thought to be active processing structures that function before supraspinal sensory integration. Dorsal horn neurons directly receive nociceptive (pain) signals from the periphery, express a high degree of functional plasticity and are involved in long-term sensitization and chronic pain. We show here that deep dorsal horn neurons (DHNs) in Wistar rats can switch their intrinsic firing properties from tonic to plateau or endogenous bursting patterns, depending upon the balance of control by metabotropic glutamate (mGlu) and GABA(B) receptors. We further show that this modulation acts on at least one common target, the inwardly rectifying potassium channel (Kir3). Finally, we found that these firing modes correspond to specific functional states of information transfer in which dorsal horn neurons can faithfully transmit, greatly enhance or block the transfer of nociceptive information.

Synaptic plasticity in the amygdala in a model of arthritic pain: differential roles of metabotropic glutamate receptors 1 and 5

Pain has a strong emotional-affective dimension, and the amygdala plays a key role in emotionality. Mechanisms of pain-related changes in the amygdala were studied at the cellular and molecular levels in a model of arthritis pain. The influence of the arthritic condition induced in vivo on synaptic transmission and group I metabotropic glutamate receptor (mGluR1 and mGluR5) function was examined in vitro using whole-cell voltage-clamp recordings of neurons in the central nucleus of the amygdala (CeA). G-protein-coupled mGluRs are implicated in various forms of neuroplasticity as well as in neurological and psychiatric disorders. Synaptic transmission was evoked by electrical stimulation of afferents from the basolateral amygdala (BLA) and the pontine parabrachial (PB) area in brain slices from control (untreated or saline-injected) rats and from arthritic rats. This study shows enhanced synaptic transmission of nociceptive-specific inputs (PB-->CeA synapse) and polymodal sensory inputs (BLA-->CeA synapse) in the arthritis model. CeA neurons from arthritic rats also developed increased excitability compared with control CeA neurons. Synaptic plasticity in the CeA was accompanied by increased presynaptic mGluR1 function and upregulation of mGluR1 and mGluR5. A selective mGluR1 antagonist reduced transmission in CeA neurons from arthritic animals but not in control neurons, and increased levels of mGluR1 and mGluR5 protein were measured in the CeA of arthritic rats compared with controls. Our results show that plastic changes in the amygdala in an arthritis model that produces prolonged pain involve a critical switch of presynaptic mGluR1 expression and function.