Requirement of metabotropic glutamate receptors for the generation of inflammation-evoked hyperexcitability in rat spinal cord neurons

Spinal pain processing in arthritis: Neuron and glia (inter)actions

Diseases of joints are among the most frequent causes of chronic pain. In the course of joint diseases, the peripheral and the central nociceptive system develop persistent hyperexcitability (peripheral and central sensitization). This review addresses the mechanisms of spinal sensitization evoked by arthritis. Electrophysiological recordings in anesthetized rats from spinal cord neurons with knee input in a model of acute arthritis showed that acute spinal sensitization is dependent on spinal glutamate receptors (AMPA, NMDA, and metabotropic glutamate receptors) and supported by spinal actions of neuropeptides such as neurokinins and CGRP, by prostaglandins, and by proinflammatory cytokines. In several chronic arthritis models (including immune-mediated arthritis and osteoarthritis) spinal glia activation was observed to be coincident with behavioral mechanical hyperalgesia which was attenuated or prevented by intrathecal application of minocycline, fluorocitrate, and pentoxyfylline. Some studies identified specific pathways of micro- and astroglia activation such as the purinoceptor- (P2 X7 -) cathepsin S/CX3 CR1 pathway, the mobility group box-1 protein (HMGB1), and toll-like receptor 4 (TLR4) activation, spinal NFκB/p65 activation and others. The spinal cytokines TNF, interleukin-6, interleukin-1β, and others form a functional spinal network characterized by an interaction between neurons and glia cells which is required for spinal sensitization. Neutralization of spinal cytokines by intrathecal interventions attenuates mechanical hyperalgesia. This effect may in part result from local suppression of spinal sensitization and in part from efferent effects which attenuate the inflammatory process in the joint. In summary, arthritis evokes significant spinal hyperexcitability which is likely to contribute to the phenotype of arthritis pain in patients.

Differential Effects of Human P301L Tau Expression in Young versus Aged Mice

The greatest risk factor for developing Alzheimer’s disease (AD) is increasing age. Understanding the changes that occur in aging that make an aged brain more susceptible to developing AD could result in novel therapeutic targets. In order to better understand these changes, the current study utilized mice harboring a regulatable mutant P301L human tau transgene (rTg(TauP301L)4510), in which P301L tau expression can be turned off or on by the addition or removal of doxycycline in the drinking water. This regulatable expression allowed for assessment of aging independent of prolonged mutant tau expression. Our results suggest that P301L expression in aged mice enhances memory deficits in the Morris water maze task. These behavioral changes may be due to enhanced late-stage tau pathology, as evidenced by immunoblotting and exacerbated hippocampal dysregulation of glutamate release and uptake measured by the microelectrode array technique. We additionally observed changes in proteins important for the regulation of glutamate and tau phosphorylation that may mediate these age-related changes. Thus, age and P301L tau interact to exacerbate tau-induced detrimental alterations in aged animals.

The effects of estrogen on temporomandibular joint pain as influenced by trigeminal caudalis neurons

The signs and symptoms of persistent temporomandibular joint (TMJ)/muscle disorder (TMJD) pain suggest the existence of a central neural dysfunction or a problem of pain amplification. The etiology of chronic TMJD is not known; however, female sex hormones have been identified as significant risk factors. Converging lines of evidence indicate that the junctional region between the trigeminal subnucleus caudalis (Vc) and the upper cervical spinal cord, termed the Vc/C1-2 region, is the primary site for the synaptic integration of sensory input from TMJ nociceptors. In this paper, the mechanisms behind the estrogen effects on the processing of nociceptive inputs by neurons in the Vc/C1-2 region reported by human and animal studies are reviewed. The Vc/C1-2 region has direct connections to endogenous pain and autonomic control pathways, which are modified by estrogen status and are suggested to be critical for somatomotor and autonomic reflex responses of TMJ-related sensory signals.

Differential expression of mGluRs in rat spinal dorsal horns and their modulatory effects on nocifensive behaviors

Glutamate is a neurotransmitter present in most excitatory synapses in the nervous system. It also plays a key role in the spinal cord’s physiological excitatory circuit and is involved in pathological neurotransmissions such as those observed in inflammatory and neuropathic pain conditions. The actions of glutamate are mediated by different types of ionotropic glutamate receptors (iGluRs) and metabotropic glutamate receptors (mGluRs). Although expressions of iGluRs are well studied, those of mGluRs are not fully elucidated in the spinal cord. In this study, we examined the expressions of mGluRs (mGluR1-8) and investigated which mGluR subtypes can modulate pain transmission in the dorsal horn of the spinal cord using an inflammatory pain model. Reverse transcription-polymerase chain reaction revealed that mGluR mRNAs, except for mGluR2 and 6, were detected in the spinal cord. Double labeling analysis, in situ hybridization histochemistry with immunohistochemistry, was used to examine the distribution of each mGluR in neurons or glial cells in the lamina I–II of the spinal dorsal horn. mGluR1, 5, and 7 were generally, and 4 and 8 were frequently, expressed in neurons. mGluR3 was expressed not only in neurons but also in oligodendrocytes. We next examined the distribution of mGluR4 and 8 were expressed in excitatory or inhibitory neurons. Both mGluR4 and 8 were preferentially expressed in inhibitory neurons rather than in excitatory neurons. Furthermore, intrathecal delivery of CPPG((RS)-α-cyclopropyl-4-phosphonophenylglycine), an antagonist for mGluR 4 and 8, attenuated nocifensive behaviors and the increase in fos-positive-excitatory neurons of the dorsal horn induced by intraplantar injection of formalin. These findings suggest that mGluR4 and 8, which are preferentially expressed in inhibitory neurons, may play roles in the modulation of pain transmission in the spinal dorsal horn.

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

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

The role of the innate immune system in psychiatric disorders

There is by now substantial clinical evidence for an association between specific mood disorders and altered immune function. More recently, a number of hypotheses have been forwarded to explain how components of the innate immune system can regulate brain function at the cellular and systems levels and how these may underlie the pathology of disorders such as depression, PTSD and bipolar disorder. In this review we draw reference to biochemical, cellular and animal disease models, as well as clinical observations to elucidate the role of the innate immune system in psychiatric disorders. Proinflammatory cytokines, such as IL-1β IL-6 and TNFα, which feature prominently in the immune response to pathogen in the periphery, have unique and specific actions on neurons and circuits within the central nervous system. Effects of these signaling molecules on neurotransmission, memory, and glucocorticoid function, as well as animal behaviors such as social withdrawal and fear conditioning relevant to psychiatric disorders are elucidated. Finally, we highlight future directions for studies, including the use of peripheral biomarkers, relevant for developing new therapeutic approaches for treating psychiatric illnesses. This article is part of Special Issue entitled 'neuroinflammation in neurodegeneration and neurodysfunction'.

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.

Metabotropic glutamate receptors as targets for analgesia: antagonism, activation, and allosteric modulation

The metabotropic glutamate receptors (mGluRs) are expressed pre- and post-synaptically throughout the nervous system where they serve as modulators of synaptic transmission and neuronal excitability. Activation of mGluRs can be pro- or anti-nociceptive, depending on their anatomic location and the signaling cascades to which they couple. Antagonists of Group I mGluRs and agonists of Group II and III mGluRs have shown therapeutic promise in animal pain models. This article reviews the potential therapeutic utility of several agents that act predominantly via mGluRs, specifically focusing on their analgesic efficacy and discussing possible off-target effects. Glutamate, the primary excitatory neurotransmitter in the vertebrate nervous system, mediates its effects via activation of two main classes of receptors: ligand-gated ion channels known as ionotropic receptors and G-protein coupled metabotropic receptors. Antagonists of ionotropic glutamate receptors, such as ketamine, have robust analgesic properties; however, their analgesic utility is limited to monitored clinical settings due to the potential for psychomimetic effects.

Metabotropic glutamate subtype 7 and 8 receptors oppositely modulate cardiac nociception in the rat nucleus tractus solitarius

Recent study from our laboratory has indicated that microinjection of glutamate into the nucleus tractus solitarius (NTS) facilitates the cardiac-somatic reflex induced by pericardial capsaicin. Further, N-methyl-d-aspartate (NMDA) receptors and metabotropic glutamate receptors (mGluRs) mediate this function. However, the roles of the individual receptor subtypes or subunits in modulating cardiac nociception are unknown. Among the three groups of mGluRs, group III mGluRs are the primary mGluR subtype expressed in visceral afferent neurons in the NTS. The present study examined the roles of group III mGluRs and their subtype 7 and 8 receptors (mGluR7 and mGluR8) in modulating the cardiac-somatic reflex induced by pericardial capsaicin, which was monitored by recording electromyogram (EMG) activity from the spinotrapezius muscle in anesthetized rats. Intra-NTS microinjection of a group III mGluR agonist, l-(+)-2-Amino-4-phosphonobutyric acid (l-AP4, at 1, 10, and 20 nmol) or a selective mGluR7 agonist, N,N'-diphenylmethyl-1,2-ethanediamine dihydrochloride (AMN082, at 1, 2, and 4 nmol) both decreased the EMG response in a dose-dependent manner. This decrease was inhibited by the group III mGluR antagonist (RS)-α-Methylserine-O-phosphate (MSOP, at 20 nmol). In contrast, intra-NTS microinjection of a selective mGluR8 agonist, (S)-3, 4-dicarboxyphenylglycine (DCPG, at 6 and 8 nmol), significantly increased the EMG response above control levels. This effect was eliminated by intra-NTS MSOP and by vagal deafferentation. These data suggest that group III mGluRs and mGluR7 in the NTS display an inhibitory effect, while mGluR8 displays a facilitatory effect in modulating cardiac nociception, and this facilitatory effect is dependent on vagal afferents.

Chemical lesioning and glutamate administration reveal a major role for the nucleus tractus solitarius in the cardiac-somatic reflex in rats

Many patients suffer from secondary muscle hyperalgesia after experiencing angina pectoris. In this study, we examined the role of the nucleus tractus solitarius (NTS) and glutamate receptors in modulating cardiac-evoked muscle hyperalgesia induced by pericardial capsaicin, which was monitored by recording electromyogram (EMG) activity from the spinotrapezius muscle in the anesthetized rat. Unilateral chemical lesioning of the commissural NTS with the neurotoxin ibotenic acid significantly depressed the cardiac-somatic reflex; the EMG responses decreased to 56.4 ± 6.9% of that of the controls (5 of 5). Microinjection of the excitatory amino acid glutamate, at 10, 20, and 50 nmol, into the commissural NTS increased the EMG response, in a dose-dependent manner, to 116.9 ± 4.9%, 143.9 ± 10.2%, and 214.2 ± 15.8% (n=8), respectively, of that of the controls. In contrast, microinjection of the N-methyl-D-aspartate (NMDA) receptor antagonist (+)-5-methyl-10, 11-dihydro-5H-dibenzo [a, d]-cyclohepten-5,10-imine maleate (MK-801) at 4 and 6 nmol, decreased the EMG response to 45.2 ± 10.6% and 36.8 ± 14.3%, respectively, of that of the controls (n=8 for each dose). Similarly, the metabotropic glutamate receptor (mGluR) antagonist (RS)-a-methyl-4-carboxyphenylglycine (MCPG), at 2.5 and 5 nmol, decreased the EMG response to 65.2 ± 16.3% and 57.0 ± 4.2%, respectively, of that of the controls. When a combination of MK-801 and MCPG was administrated, the EMG response further decreased to 22.5 ± 13.2% (n=6) of that of the controls. However, administration of a non-NMDA receptor antagonist 6, 7-dinitroquinoxaline-2, 3-dione (DNQX), at 2 and 5 nmol, had no effect on the EMG response. These results suggest that the NTS is involved in the facilitation of the cardiac-somatic reflex, and that the NMDA receptor and mGluRs play an important role in mediating this effect.

Experiments on the nature of the signal that induces spinal neuroplastic changes following a peripheral lesion

This study aimed at identifying the signal(s) that elicit myositis-induced neuroplastic changes in background activity and responsiveness of spinal neurones. It is based on previous data suggesting that in dorsal horn neurones, responsiveness to peripheral input on one hand and background activity on the other are probably controlled by different mechanisms. In anaesthetized rats, myositis was induced in the gastrocnemius-soleus muscle and the activity of single dorsal horn neurones was recorded in segment L3. Impulse traffic and axoplasmatic transport in dorsal roots L4 and L5 were selectively blocked by lignocaine or vinblastine for various time periods relative to the induction of the myositis. The results show that the main triggering signal for the myositis-induced changes in both responsiveness and background activity is the altered impulse activity in primary afferent fibres. In contrast, 'no axonally transported chemical signal controlling the discharge behaviour of dorsal horn neurones was found. However, the time course of the electrical signals that cause the myositis-induced changes in background activity and responsiveness is different. For changes in responsiveness, a rather narrow time window of 2 h directly after induction of the myositis existed, during which the impulses from the inflamed muscle must reach the spinal cord. Accordingly, to prevent the increase in responsiveness, the electrical input had to be blocked during the first 2 h; a block of the same duration at another time had no effect. The change in background activity seems to be due to a continuous input from the inflamed muscle which adds up over the hours. Therefore, with regard to background activity, blocking the electrical signals is effective at any time, but only a block of long duration has a significant effect.

Impact of central and peripheral TRPV1 and ROS levels on proinflammatory mediators and nociceptive behavior

Background

Transient receptor potential vanilloid 1 (TRPV1) channels are important membrane sensors on peripheral nerve endings and on supportive non-neuronal synoviocytes in the knee joint. TRPV 1 ion channels respond with activation of calcium and sodium fluxes to pH, thermal, chemical, osmotic, mechanical and other stimuli abundant in inflamed joints. In the present study, the kaolin/carrageenan (k/c) induced knee joint arthritis model in rats, as well as primary and clonal human synoviocyte cultures were used to understand the reciprocal interactions between reactive nitroxidative species (ROS) and functional TRPV1 channels. ROS generation was monitored with ROS sensitive dyes using live cell imaging in vitro and in spinal tissue histology, as well as with measurement of ROS metabolites in culture media using HPLC.

Results

Functional responses in the experimental arthritis model, including increased nociceptive responses (thermal and mechanical hyperalgesia and allodynia), knee joint temperature reflecting local blood flow, and spinal cord ROS elevations were reduced by the ROS scavenger PBN after intraperitoneal pretreatment. Increases in TRPV1 and ROS, generated by synoviocytes in vitro, were reciprocally blocked by TRPV1 antagonists and the ROS scavenger. Further evidence is presented that synoviocyte responses to ROS and TRPV1 activation include increases in TNFα and COX-2, both measured as an indicator of the inflammation in vitro.

Conclusions

The results demonstrate that contributions of ROS to pronociceptive responses and neurogenic inflammation are mediated both centrally and peripherally. Responses are mediated by TRPV1 locally in the knee joint by synoviocytes, as well as by ROS-induced sensitization in the spinal cord. These findings and those of others reported in the literature indicate reciprocal interactions between TRPV1 and ROS play critical roles in the pathological and nociceptive responses active during arthritic inflammation.

Central sensitization: a generator of pain hypersensitivity by central neural plasticity

Central sensitization represents an enhancement in the function of neurons and circuits in nociceptive pathways caused by increases in membrane excitability and synaptic efficacy as well as to reduced inhibition and is a manifestation of the remarkable plasticity of the somatosensory nervous system in response to activity, inflammation, and neural injury. The net effect of central sensitization is to recruit previously subthreshold synaptic inputs to nociceptive neurons, generating an increased or augmented action potential output: a state of facilitation, potentiation, augmentation, or amplification. Central sensitization is responsible for many of the temporal, spatial, and threshold changes in pain sensibility in acute and chronic clinical pain settings and exemplifies the fundamental contribution of the central nervous system to the generation of pain hypersensitivity. Because central sensitization results from changes in the properties of neurons in the central nervous system, the pain is no longer coupled, as acute nociceptive pain is, to the presence, intensity, or duration of noxious peripheral stimuli. Instead, central sensitization produces pain hypersensitivity by changing the sensory response elicited by normal inputs, including those that usually evoke innocuous sensations.

PERSPECTIVE

In this article, we review the major triggers that initiate and maintain central sensitization in healthy individuals in response to nociceptor input and in patients with inflammatory and neuropathic pain, emphasizing the fundamental contribution and multiple mechanisms of synaptic plasticity caused by changes in the density, nature, and properties of ionotropic and metabotropic glutamate receptors.

Spinal antinociceptive effects of cyclooxygenase inhibition during inflammation: Involvement of prostaglandins and endocannabinoids

Both cyclooxygenase-1 and -2 are expressed in the spinal cord, and the spinal COX product prostaglandin E(2) (PGE(2)) contributes to the generation of central sensitization upon peripheral inflammation. Vice versa spinal COX inhibition is considered an important mechanism of antihyperalgesic pain treatment. Recently, however, COX-2 was shown to be also involved in the metabolism of endocannabinoids. Because endocannabinoids can have analgesic actions it is conceivable that inhibition of spinal COX produces analgesia not only by inhibition of PG synthesis but also by inhibition of endocannabinoid breakdown. In the present study, we recorded from spinal cord neurons with input from the inflamed knee joint and we measured the spinal release of PGE(2) and the endocannabinoid 2-arachidonoyl glycerol (2-AG) in vivo, using the same stimulation procedures. COX inhibitors were applied spinally. Selective COX-1, selective COX-2 and non-selective COX inhibitors attenuated the generation of spinal hyperexcitability when applied before and during development of inflammation but, when inflammation and spinal hyperexcitability were established, only selective COX-2 inhibitors reversed spinal hyperexcitability. During established inflammation all COX inhibitors reduced release of spinal PGE(2) almost equally but only the COX-2 inhibitor prevented breakdown of 2-AG. The reversal of spinal hyperexcitability by COX-2 inhibitors was prevented or partially reversed by AM-251, an antagonist at the cannabinoid-1 receptor. We conclude that inhibition of spinal COX-2 not only reduces PG production but also endocannabinoid breakdown and provide evidence that reversal of inflammation-evoked spinal hyperexcitability by COX-2 inhibitors is more related to endocannabinoidergic mechanisms than to inhibition of spinal PG synthesis.

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.

Metabotropic receptors for glutamate and GABA in pain

Glutamate and gamma-amino butyric acid (GABA) are respectively two major excitatory and inhibitory neurotransmitters of the adult mammalian central nervous system. These neurotransmitters exert their action through two types of receptors: ionotropic and metabotropic receptors. While ionotropic receptors are ligand gated ion channels involved in fast synaptic transmission, metabotropic receptors belong to the superfamily of G-protein coupled receptors (GPCRs) and are responsible for the neuromodulatory effect of glutamate and GABA. Metabotropic glutamate receptors (mGluRs) and metabotropic GABA receptors (GABA-B) are present at different levels of the pain neuraxis where they regulate nociceptive transmission and pain. The present review will focus on the role of these receptors in the modulation of pain perception.

Mechanisms of chronic central neuropathic pain after spinal cord injury

Not all spinal contusions result in mechanical allodynia, in which non-noxious stimuli become noxious. The studies presented use the NYU impactor at 12.5 mm drop or the Infinite Horizons Impactor (150 kdyn, 1 s dwell) devices to model spinal cord injury (SCI). Both of these devices and injury parameters, if done correctly, will result in animals with above level (forelimb), at level (trunk) and below level (hindlimb) mechanical allodynia that model the changes in evoked somatosensation experienced by the majority of people with SCI. The sections are as follows: 1) Mechanisms of remote microglial activation and pain signaling in "below-level" central pain 2) Intracellular signaling mechanisms in central sensitization in "at-level" pain 3) Peripheral sensitization contributes to "above level" injury pain following spinal cord injury and 4) Role of reactive oxygen species in central sensitization in regional neuropathic pain following SCI. To summarize, differential regional mechanisms contribute to the regional chronic pain states. We propose the importance of understanding the mechanisms in the differential regional pain syndromes after SCI in the chronic condition. Targeting regional mechanisms will be of enormous benefit to the SCI population that suffer chronic pain, and will contribute to better treatment strategies for other chronic pain syndromes.

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.

Spinal mechanisms of antinociceptive effect caused by oral administration of bis-selenide in mice

The present study was designed to investigate further the mechanisms involved in the antinociception caused by bis-selenide in behavioral model of pain in mice. Bis-selenide (5-50 mg/kg), given orally, produced significant inhibition of the antinociceptive behavior induced by intrathecal (i.t.) injection of glutamate (175 nmol/site), kainate (110 pmol/site) and (+/-)-1-aminocyclopentane-trans-1,3-dicarboxylic acid (trans-ACPD; 50 nmol/site) and the maximal inhibitions observed were 57+/-5, 46+/-7 and 73+/-3%, respectively. Bis-selenide failed to affect the nociception induced by alpha-amino-3-hydroxy-5-mehtyl-4-isoxazolepropionic acid (AMPA; 135 pmol/site) and N-methyl-d-aspartate (NMDA; 450 pmol/site). This compound also reduced the nociceptive response induced by tumor necrosis factor-alpha (TNF-alpha; 0.1 pg/site), interleukin-1beta (IL-1beta; 1 pg/site), substance P (SP) (135 ng/site, i.t.) and capsaicin (30 ng/site) and the inhibitions observed were 81+/-3%, 88+/-1%, 77+/-3 and 67+/-3, respectively. The oral administration of bis-selenide (25-50 mg/kg) in mice caused a significant increase in the reaction time to thermal stimuli in the hot plate test and the mean ID(50) value (and the 95% confidence limits) was 20.37 (15.00-25.74) mg/kg. The antinociceptive effect caused by bis-selenide (50 mg/kg, p.o.) on the hot plate test in mice was reversed by intrathecal (i.t.) injection of some K(+) channel blockers such as tetraethylammonium (TEA, non-selective voltage-dependent K(+) channel inhibitor) and glibenclamide (ATP-sensitive K(+) channel inhibitor), but not apamin and charybdotoxin (large- and small-conductance Ca(2+)-activated K(+) channel inhibitors, respectively). Together, these results indicate that bis-selenide produces antinociception at spinal sites through the activation of ATP-sensitive and voltage-gated K(+) channels and interaction with kainate and trans-ACDP receptors as well as vanilloid and neuropeptide receptors and pro-inflammatory cytokines.

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.

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.

Rapid hit to lead evaluation of pyrazolo[3,4-d]pyrimidin-4-one as selective and orally bioavailable mGluR1 antagonists

Our HTS effort yielded a preferential mGluR1 pyrimidinone antagonist 1 with lead-like characteristics. Rapid hit to lead (HTL) study identified compounds with improved functional activity and selectivity such as 1b with little improvements in ADME properties. Addition of an aminosulfonyl group on the N-1 aromatic ring led to 2f, a compound with similar in vitro biochemical profiles as those of 1b but drastically improved in vitro ADME properties. These improvements were paralleled by rat PK study characterized by low clearance and quantitative bioavailability. Compound 2f represented a true lead-like molecule that is amenable for further lead optimization (LO) evaluation.

Peripheral metabotropic glutamate receptor 5 mediates mechanical hypersensitivity in craniofacial muscle via protein kinase C dependent mechanisms

We previously demonstrated that peripherally located N-methyl-D-aspartic acid (NMDA) receptors contribute to acute muscle nociception and the development of chronic muscular hyperalgesia. In the present study, we investigated the potential role of peripheral group I metabotropic glutamate receptors (mGluRs 1/5) in the development of muscular hypersensitivity to mechanical stimulation, and attempted to elucidate intracellular signaling mechanisms associated with the mGluR activation in male Sprague-Dawley rats. First, our Western blot analyses revealed that mGluR 5 protein, but not mGluR 1 protein, is reliably detected in trigeminal ganglia and the masseter nerve. Subsequent behavioral studies demonstrated that the group I mGluR agonist, R,S-3,5-dihydroxyphenylglycol (DHPG), significantly decreased the mechanical threshold to noxious stimulation of the masseter, and that the DHPG-induced mechanical hypersensitivity can be effectively prevented by pretreatment of the masseter with 2-methyl-6-(phenylethynyl)pyridine hydrochloride (MPEP), a selective mGluR 5 antagonist, but not by 7-(hydroxyimino)cyclopropa[b]chromen-1a-carboxylate ethyl ester (CPCCOEt), a selective mGluR 1 antagonist. Moreover, the DHPG-induced mechanical hypersensitivity was significantly blocked by inhibiting either the alpha or epsilon isoform of protein kinase C (PKC). Collectively, these data provide evidence that peripherally located mGluR 5 may play an important role in the development of masseter hypersensitivity, and that PKC activation is required for the modulatory effect of peripheral mGluR 5 in the craniofacial muscle tissue. Thus, selective targeting of peripheral mGluR 5 and PKCalpha, as well as PKCepsilon, might serve as an effective therapeutic strategy in the management of chronic muscle pain conditions, such as temporomandibular disorders.

The peripheral role of group I metabotropic glutamate receptors on nociceptive behaviors in rats with knee joint inflammation

We examined whether the mGluR1 and mGluR5 were involved in development and maintenance of behavioral signs of non-evoked pain and secondary mechanical hyperalgesia induced by knee joint inflammation. Selective mGluR1 antagonist, (RS)-1-aminoindan-1,5-dicarboxylic acid (AIDA: 50, 100, 200 microM/25 microl, n=10 per group) and selective mGluR5 antagonist, 2-methyl-6-(phenylethynyl)-pyridine (MPEP: 50, 100, 200 nM/25 microl, n=10 per group) was intra-articularly (i.a.) injected 30 min before and 4h after carrageenan injection and behavioral tests were conducted. In the pre-treatment, only a higher dose (200 nM) of MPEP significantly prevented the magnitude of weight load reduction, whereas AIDA (200 microM) and MEPE (50, 100 and 200 nM) significantly reduced the development of mechanical hyperalgesia compared to saline treated group. In the post-treatment, AIDA (200 microM) and MPEP at 100 and 200 nM partially reversed the reduction of weight load induced by carrageenan. MPEP significantly increased the withdrawal threshold to mechanical stimulation in a dose-dependent manner, whereas AIDA had significantly reversed the decreased the paw withdrawal threshold only at 200 microM. The present study demonstrated that i.a. MPEP, selective mGluR5 antagonist is more effective than selective mGluR1 antagonist, AIDA on non-evoked pain as well as mechanical hyperalgesia in both induction and maintenance phase in knee joint inflammation. It is suggested that peripheral mGlu5 receptors play a more prominent role in inflammatory pain including evoked and spontaneous pain. Thus, selective mGluR5 antagonist could be effective therapeutic tools in clinical setting.

New role for spinal Stargazin in alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor-mediated pain sensitization after inflammation

Considerable evidence has demonstrated that alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor blockade has an antinociceptive effect on inflammatory pain. Stargazin (STG) is the first transmembrane protein known to associate with AMPA receptors and regulate their synaptic targeting. However, it is not known whether STG is involved in inflammatory pain processing by regulating AMPA receptor function. In the present study, we investigated the effect of knockdown of spinal STG on AMPA receptor-mediated pain sensitization after inflammation. Antisense technology was employed to knock down STG expression in the spinal cord. We show that STG was expressed and interacted with AMPA receptor subunit GluR2 in the spinal cord. Intrathecally injected STG antisense oligodeoxyribonucleotide (ODN) specifically decreased STG expression in the lumbar spinal cord and dose dependently inhibited formalin-induced inflammatory pain in the second phase. More important was our finding for the first time that this specific STG antisense ODN diminished AMPA (0.1 mug)-enhanced formalin pain and lost its effect if pretreated with AMPA receptor antagonist CNQX. Our results demonstrate a new role for STG in central sensitization of inflammatory pain by interacting with AMPA receptors in the spinal cord.

Metabotropic glutamate receptor subtype 5 antagonists MPEP and MTEP

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

Group I mGluR regulates the polarity of spike-timing dependent plasticity in substantia gelatinosa neurons

The spinal synaptic plasticity is associated with a central sensitization of nociceptive input, which accounts for the generation of hyperalgesia in chronic pain. However, how group I metabotropic glutamate receptors (mGluRs) may operate spinal plasticity remains essentially unexplored. Here, we have identified spike-timing dependent synaptic plasticity in substantia gelatinosa (SG) neurons, using perforated patch-clamp recordings of SG neuron in a spinal cord slice preparation. In the presence of bicuculline and strychnine, long-term potentiation (LTP) was blocked by AP-5 and Ca2+ chelator BAPTA-AM. The group I mGluR antagonist AIDA, PLC inhibitor U-73122, and IP3 receptor blocker 2-APB shifted LTP to long-term depression (LTD) without affecting acute synaptic transmission. These findings provide a link between postsynaptic group I mGluR/PLC/IP3-gated Ca2+ store regulating the polarity of synaptic plasticity and spinal central sensitization.

Molecular pharmacology and therapeutic prospects of metabotropic glutamate receptor allosteric modulators

The metabotropic glutamate receptors (mGluR) consist of a family of eight G-protein-coupled receptors that differ in their function, distribution and physiological roles within the central nervous system. In recent years substantial efforts have been made towards developing selective agonists and antagonists which have proven useful for elucidating their potential as novel targets for the treatment of psychiatric and neurological diseases. In the present review we will provide an update of the recent developments of functional allosteric modulators of the mGluR family and explore their therapeutic potential for anxiety/depression, schizophrenia, epilepsy/stroke, pain and Alzheimer's, Parkinson's and Huntington's diseases.

Upregulation of mRNAs coding for AMPA and NMDA receptor subunits and metabotropic glutamate receptors in the dorsal horn of the spinal cord in a rat model of diabetes mellitus

Recent studies suggest that glutamate plays a pivotal role in the processing of sensory information in the spinal cords of patients with diabetic neuropathy. However, the specific glutamate receptors that that are involved have yet to be determined. We therefore conducted a study to characterize the expression of messenger RNAs (mRNAs) coding for subunits of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors and N-methyl-d-aspartate (NMDA) receptors and for metabotropic glutamate receptors (mGluRs) in the dorsal horn of the lumbar segment of the spinal cord in a rat model (streptozotocin [STZ]-induced) of diabetic neuropathy. The levels of mRNAs coding for AMPA receptor subunits, GluR1, GluR2, and GluR3, were significantly increased in all layers (laminae I-V) of the dorsal horn in diabetic (STZ-injected) rats compared to control (vehicle-injected) rats. The hybridization signals for NR2A mRNA and NR2B mRNA were significantly elevated in the deep layer of the dorsal horn of diabetic rats. In diabetic (STZ-induced) rats, the levels of expression of mGluR1 mRNA and mGluR5 mRNA were significantly increased in all layers of the dorsal horn. These results suggest that abnormal expression of multiple glutamate receptors is involved in the development of diabetic neuropathy and that glutamate receptors are promising targets in the treatment of this disorder.

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.

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.

Role of calcitonin gene-related peptide in the sensitization of dorsal horn neurons to mechanical stimulation after intradermal injection of capsaicin

This study was designed to assess the role of calcitonin gene-related peptide (CGRP) and its receptor in the sensitization of dorsal horn neurons induced by intradermal injection of capsaicin in rats. Extracellular recordings were made from wide dynamic range (WDR) dorsal horn neurons with receptive fields on the hindpaw in the lumbar enlargement of anesthetized rats. The background activity and responses to brushing, pressing, and pinching the skin were assessed. A postsuperfusion or a presuperfusion of CGRP(8-37) paradigm was followed. When tested 30 min after capsaicin injection, there was an increase in background activity and responses to brush, press, and pinch applied to the receptive field. Superfusion of CGRP(8-37) into the spinal cord at 45 min after capsaicin injection significantly reversed the increased background activity and responses to brush, press, and pinch applied to the receptive field. On the other hand, spinal superfusion of CGRP(8-37) prior to capsaicin injection prevented the increased background activity and responses to brush, press, and pinch of WDR neurons that occurred following capsaicin injection in control experiments. A sensitization of spinal dorsal horn neurons could also be induced by superfusion of the spinal cord with CGRP. The effect could be blocked by CGRP(8-37) dose-dependently. Collectively, these results suggest that CGRP and its receptors are involved in the spinal cord central sensitization induced by intradermal injection of capsaicin.

Blockade of glutamate mGlu5 receptors in a rat model of neuropathic pain prevents early over-expression of pro-apoptotic genes and morphological changes in dorsal horn lamina II

We used rats with a sciatic nerve chronic constrictive injury (CCI) and combined behavioural, molecular and morphological approaches to assess the involvement of mGlu5 receptors in neuropathic pain-associated hyperalgesia and spinal cord neuron apoptosis. Mechanical and thermal hyperalgesia developed 2-3 days after surgery. Morphological changes in the ipsilateral L4-L5 lamina II consisted of: (i) cell loss (38 +/- 5%), (ii) increased TUNEL-positive profiles, (iii) decreased SP-immunoreactive primary afferents, and (iv) reactive gliosis. Molecular expression data suggested a bi-phasic response of bcl-2 family genes in CCI. An early (2-3 days post-CCI) E2F1- and p53-independent apoptosis appeared in the spinal cord as the pro-apoptotic bax gene increased (320 +/- 19%), followed by an increased expression of the anti-apoptotic bcl-2 and bcl-xL genes (60 +/- 11% and 110 +/- 15%, respectively) 7 days from CCI. The selective mGlu5 receptor antagonist, MPEP (2 mg/kg i.p. twice daily), prevented the development of thermal hyperalgesia and transiently reduced mechanical hyperalgesia. Despite the MPEP treatment, which normalised bax/bcl-2 and bcl-xL/bcl-xS ratios at all times post-CCI, mechanical hyperalgesia reappeared by 7 days after CCI. Similarly, MPEP was cytoprotective at 3, but not 7 days post-CCI. This study shows that: (a) spinal cord neuron loss may be triggered by a p53- and E2F1-independent apoptosis in lamina II with the participation of glutamate mGlu5 receptors, (b) these receptors seem to be involved transiently, as their blockade was no longer protective by 7 days CCI, and (c) this delayed cell death occurred in the absence of Bax activation, suggesting the involvement of an alternative death pathway.

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.

Group i metabotropic glutamate receptors in spiral ganglion neurons contribute to excitatory neurotransmissions in the cochlea

Evidence has accumulated over the years supporting glutamate as the primary neurotransmitter used by hair cells in afferent cochlear neurotransmission. Besides acting on ionotropic glutamate receptors, glutamate also activates second messenger systems via G-protein-coupled metabotropic glutamate receptors (mGluRs) to modulate neuronal excitability. However, it is unclear whether mGluRs participate in cochlear neurotransmission. We present evidence directly supporting a functional role for group I metabotropic glutamate receptors (mGluRIs) in spiral ganglion (SG) neurons. The presence of mGluRI and downstream G-protein subunits was demonstrated by molecular biology and immunolabeling methods. Direct activation of mGluRIs in cultured SG neurons resulted in transient increases of intracellular Ca(++) concentration and transient inward currents that gave rise to firings of multiple action potentials. These responses showed mGluRI pharmacological specificity and quickly desensitized. We next examined changes in cochlear function after noise exposure as a result of pharmacologically manipulating cochlear glutamate neurotransmission. These in vivo tests showed that blocking non-N-methyl-D-aspartic acid glutamate receptors was sufficient to eliminate compound action potentials of the auditory nerve, and pharmacologically inhibiting mGluRIs in the cochlea did not significantly affect the hearing threshold. In contrast, blocking mGluRIs lowered the amplitude of compound action potentials at louder sound levels and reduced the noise-induced temporary threshold shift. Our results suggest that although mGluRIs did not initiate fast excitatory cochlear neurotransmission, their activation contributed to the growth of excitatory responses of the cochlea. As a result, the cochlea was more resistant to noise-induced temporary hearing losses without the activation of mGluRIs in SG neurons.

Effects of glutamate receptor antagonists on spinal dorsal horn neurons during zymosan-induced inflammation in rats

These experiments examined the effects of spinal administration of the N-methyl-D-aspartate (NMDA) receptor antagonist DL-2-amino-5-phosphonovaleric acid (APV), the non-NMDA receptor antagonist 6,7-dinitroquinoxaline-2,3-dione (DNQX), or the metabotropic glutamate receptor antagonist DL-2-amino-3-phosphonoproprionic acid (AP3) on responses of spinal dorsal horn neurons evoked by thermal and mechanical stimuli applied to the rat hindpaw in either an inflamed or noninflamed state. Administration of APV, DNQX, or AP3 decreased heat-evoked neuronal discharges of wide dynamic range (WDR) neurons that were previously augmented by zymosan-induced inflammation. APV and DNQX also decreased heat-evoked discharges of WDR neurons that were previously unaffected by saline injection. Administration of either APV or DNQX, but not AP3, decreased heat-evoked neuronal discharges of nociceptive-specific (NS) neurons in both zymosan- and saline-injected rats. These data suggest that NMDA and non-NMDA receptors contribute to spinal processing of thermal stimuli in both the inflamed and noninflamed state, whereas metabotropic glutamate receptors might serve a role that is unique to WDR neurons in the inflamed state. Only DNQX consistently increased mechanical response thresholds and decreased slopes of the mechanical stimulus response functions (SRFs) of NS and WDR neurons, but this effect was observed in both inflamed and noninflamed states. These data suggest that spinal processing of mechanical stimuli is preferentially mediated by glutamate acting at non-NMDA receptors in either the inflamed or noninflamed state.

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 sensitization of amygdala neurons to afferent inputs in a model of arthritic pain

Pain is associated with negative affect such as anxiety and depression. The amygdala plays a key role in emotionality and has been shown to undergo neuroplastic changes in models of affective disorders. Many neurons in the central nucleus of the amygdala (CeA) are driven by nociceptive inputs, but the role of the amygdala in persistent pain states is not known. This study is the first to address nociceptive processing by CeA neurons in a model of prolonged pain. Extracellular single-unit recordings were made from 41 CeA neurons in anesthetized rats. Each neuron's responses to brief mechanical stimulation of joints, muscles, and skin and to cutaneous thermal stimuli were recorded. Background activity, receptive field size, and threshold were mapped, and stimulus-response functions were constructed. These parameters were measured repeatedly before and after induction of arthritis in one knee by intraarticular injections of kaolin and carrageenan. Multireceptive (MR) amygdala neurons (n = 20) with excitatory input from the knee joint responded more strongly to noxious than to innocuous mechanical stimuli of deep tissue (n = 20) and skin (n = 11). After induction of arthritis, 18 of 20 MR neurons developed enhanced responses to mechanical stimuli and expansion of receptive field size. These changes occurred with a biphasic time course (early peak: 1-1.5 h; persistent plateau phase: after 3-4 h). Responses to thermal stimuli did not change (7 of 7 neurons), but background activity (16 of 18 neurons) and electrically evoked orthodromic activity (11 of 12 neurons) increased in the arthritic state. Nociceptive-specific (NS) neurons (n = 13) showed no changes of their responses to mechanical, thermal, and electrical stimulation after induction of arthritis. A third group of neurons did not respond to somesthetic stimuli under control conditions (noSOM neurons; n = 8) but developed prolonged responses to mechanical, but not thermal, stimuli in arthritis (5 of 8 neurons). These data suggest that prolonged pain is accompanied by enhanced responsiveness of a subset of CeA neurons. Their sensitization to mechanical, but not thermal, stimuli argues against a nonspecific state of hyperexcitability. MR neurons could serve to integrate and evaluate information in the context of prolonged pain. Recruitment of noSOM neurons increases the gain of amygdala processing. NS neurons preserve the distinction between nociceptive and nonnociceptive inputs.

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.

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

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

Evidence for the involvement of glutamatergic receptors in the antinociception caused in mice by the sesquiterpene drimanial

Drimanial, a new sesquiterpene isolated from the barks of the plant Drimys winteri (Winteraceae), given systemically, intraplantarly, or by spinal or supraspinal routes, produced pronounced antinociception against both phases of formalin-induced licking. The systemic injection of drimanial also inhibited, in a graded manner, the pain-related behaviours induced by intraplantar or intrathecal (i.t.) administration of glutamate. Moreover, drimanial also caused marked inhibition of the nociception induced by i.t. administration of a metabotropic glutamate agonist (1S,3R)-ACPD, without affecting nociceptive responses induced by ionotropic agonists (NMDA, kainate, AMPA) or by substance P. The antinociception caused by drimanial was not influenced by naloxone, nor did it interfere with the motor coordination of animals in the rota-rod test. Furthermore, drimanial caused graded inhibition of [(3)H]glutamate binding in cerebral cortical membranes from mice, with an IC(50) value of 4.39 micro M. Together, these results provide strong evidence indicating that the sesquiterpene drimanial produces antinociception in mice at peripheral, spinal and supraspinal sites. An interaction with metabotropic glutamate receptors seems to contribute to the mechanisms underlying its antinociceptive action.

Peripheral metabotropic glutamate receptors as drug targets for pain relief

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