Interaction between metabotropic and NMDA glutamate receptors in the periaqueductal grey pain modulatory system

Cardiac-Gated Neuromodulation Increased Baroreflex Sensitivity and Reduced Pain Sensitivity in Female Fibromyalgia Patients

The study presents a novel approach of programing pain inhibition in chronic pain patients based on the hypothesis that pain perception is modulated by dysfunctional dorsal medial nucleus tractus solitarii (dmNTS) reflex arcs that produce diminished baroreflex sensitivity (BRS) resulting from a conditioned response. This study tested whether administration of noxious and non-noxious electrical stimuli synchronized with the cardiac cycle resets BRS, reestablishing pain inhibition. A total of 30 pain-free normotensives controls (NC) and 32 normotensives fibromyalgia (FM) patients received two, ≈8 min-epochs of cardiac-gated, peripheral electrical stimuli. Non-painful and painful electrical stimuli were synchronized to the cardiac cycle as the neuromodulation experimental protocol (EP) with two control conditions (CC1, CC2). BRS, heart-rate-variability (HRV), pain threshold and tolerance, and clinical pain intensity were assessed. Reduced BRS in FM at baseline increased by 41% during two, ≈8 min-epochs of stimulation. Thresholds in FM increased significantly during the experimental protocol (all Ps < 0.001) as did HRV. FM levels of clinical pain significantly decreased by 35.52% during the experimental protocol but not during control stimulations (p < 0.001). Baroreceptor training may reduce FM pain by BRS-mediated effects on intrinsic pain regulatory systems and autonomic responses.

Analgesic Activity of Cinnabarinic Acid in Models of Inflammatory and Neuropathic Pain

Cinnabarinic acid (CA) is a trace kynurenine metabolite, which activates both type-4 metabotropic glutamate (mGlu4) and arylic hydrocarbon (Ah) receptors. We examined the action of CA in models of inflammatory and neuropathic pain moving from the evidence that mGlu4 receptors are involved in the regulation of pain thresholds. Systemic administration of low doses of CA (0.125 and 0.25 mg/kg, i.p.) reduced nocifensive behaviour in the second phase of the formalin test. CA-induced analgesia was abrogated in mGlu4 receptor knockout mice, but was unaffected by treatment with the Ah receptor antagonist, CH223191 (1 mg/Kg, s.c.). Acute injection of low doses of CA (0.25 mg/kg, i.p.) also caused analgesia in mice subjected to Chronic Constriction Injury (CCI) of the sciatic nerve. Electrophysiological recording showed no effect of CA on spinal cord nociceptive neurons and a trend to a lowering effect on the frequency and duration of excitation of the rostral ventromedial medulla (RVM) ON cells in CCI mice. However, local application of CH223191 or the group-III mGlu receptor antagonist, MSOP disclosed a substantial lowering and enhancing effect of CA on both populations of neurons, respectively. When repeatedly administered to CCI mice, CA retained the analgesic activity only when combined with CH223191. Repeated administration of CA plus CH223191 restrained the activity of both spinal nociceptive neurons and RVM ON cells, in full agreement with the analgesic activity. These findings suggest that CA is involved in the regulation of pain transmission, and its overall effect depends on the recruitment of mGlu4 and Ah receptors.

Transcranial Direct Current Stimulation Alleviates the Chronic Pain of Osteoarthritis by Modulating NMDA Receptors in Midbrain Periaqueductal Gray in Rats

Purpose

Osteoarthritis (OA) is the most common cause to lead to chronic pain. Transcranial direct current stimulation (tDCS) has been widely used to treat nerve disorders and chronic pain. The benefits of tDCS for chronic pain are apparent, but its analgesic mechanism is still unclear. This study observed the analgesic effects of tDCS on OA-induced chronic pain and the changes of NMDA receptor levels in PAG after tDCS treatment in rats to explore the analgesic mechanism of tDCS.

Methods

After establishing chronic pain by injecting monosodium iodoacetate (MIA) into the rat ankle joint, the rats received tDCS for 14 consecutive days (20 min/day). Before tDCS treatment, Ifenprodil (the selective antagonist of NMDAR2B) was given to rats in different ways: intracerebroventricular (i.c.v.) injection or intraperitoneal (i.p.) injection. The Von Frey and hot plate tests were applied to assess the pain-related behaviors at different time points. The expression level of NMDAR2B was evaluated in midbrain periaqueductal gray (PAG) by Western blot. In addition, NMDAR2B and c-Fos were observed by the Immunohistochemistry staining after tDCS treatment.

Results

The mechanical allodynia and thermal hyperalgesia were produced after MIA injection. However, tDCS treatment reverted the mechanical allodynia and thermal hyperalgesia. Moreover, tDCS treatment significantly increased the expression of NMDAR2B and the proportion of positive stained cells of NMDAR2B. Besides that, the tDCS treatment also decreased the proportion of positive stained cells of c-Fos in PAG. However, these changes did not occur in the rats given the Ifenprodil (i.c.v.).

Conclusion

These results indicate that tDCS may increase the expression of NMDA receptors in PAG and strengthen the NMDA receptors-mediated antinociception to alleviate OA-induced chronic pain in rats.

Mu-opioid and CB1 cannabinoid receptors of the dorsal periaqueductal gray interplay in the regulation of fear response, but not antinociception

Evidence indicates that periaqueductal gray matter (PAG) plays an important role in defensive responses and pain control. The activation of cannabinoid type-1 (CB1) or mu-opioid (MOR) receptors in the dorsal region of this structure (dPAG) inhibits fear and facilitates antinociception induced by different aversive stimuli. However, it is still unknown whether these two receptors work cooperatively in order to achieve these inhibitory actions. This study investigated the involvement and a likely interplay between CB1 and MOR receptors localized into the dPAG on the regulation of fear-like defensive responses and antinociception (evaluated in tail-flick test) evoked by dPAG chemical stimulation with N-methyl-d-aspartate (NMDA). Before the administration of NMDA, animals were first intra-dPAG injected with the CB1 agonist ACEA (0.5 pmol), or with the MOR agonist DAMGO (0.5 pmol) in combination with the respective antagonists AM251 (CB1 antagonist, 100 pmol) or CTOP (MOR antagonist, 1 nmol). To investigate the interplay between these receptors, microinjection of CTOP was combined with ACEA, or microinjection of AM251 was combined with DAMGO. Our results showed that both the intra-PAG treatments with ACEA or DAMGO inhibited NMDA-induced freezing expression, whereas only the treatment with DAMGO increased antinociception induced with NMDA, which are completely blocked by its respective antagonists. Interestingly, the inhibitory effects of ACEA or DAMGO on freezing was blocked by CTOP and AM251, respectively, indicating a functional interaction between these two receptors in the mediation of defensive behaviors. However, this cooperative interaction was not observed during the NMDA-induced antinociception. Our findings indicate that there is a cooperative action between the MOR and CB1 receptors within the dPAG and it is involved in the mediation of NMDA-induced defensive responses. Additionally, the MORs into the dPAG are involved in the modulation of the antinociceptive effects that follow a fear-like defense-reaction induced by dPAG chemical stimulation with NMDA.

Supraspinal metabotropic glutamate receptors: a target for pain relief and beyond

Glutamate is the main excitatory neurotransmitter in the central nervous system, controlling the majority of synapses. Apart from neurodegenerative diseases, growing evidence suggests that glutamate is involved in psychiatric and neurological disorders, including pain. Glutamate signaling is mediated via ionotropic glutamate receptors (iGluRs) and metabotropic glutamate receptors (mGluRs). So far, drugs acting via modulation of glutamatergic system are few in number, and all are associated with iGluRs and important side effects. The glutamatergic system may be finely modulated by mGluRs. Signaling via these receptors is slower and longer-lasting, and permits fine-tuning of glutamate transmission. There have been eight mGluRs cloned to date (mGluR1-mGluR8), and these are further divided into three groups on the basis of sequence homology, pharmacological profile, and second messenger signaling. The pattern of expression of mGluRs along the pain neuraxis makes them suitable substrates for the design of novel analgesics. This review will focus on the supraspinal mGluRs, whose pharmacological manipulation generates a variety of effects, which depend on the synaptic location, the cell type on which they are located, and the expression in particular pain modulation areas, such as the periaqueductal gray, which plays a major role in the descending modulation of pain, and the central nucleus of the amygdala, which is an important center for the processing of emotional information associated with pain. A particular emphasis will also be given to the novel selective mGluR subtype ligands, as well as positive and negative allosteric modulators, which have permitted discrimination of the individual roles of the different mGluR subtypes, and subtle modulation of central nervous system functioning and related disorders.

Metabotropic glutamate receptor subtype 8 in the amygdala modulates thermal threshold, neurotransmitter release, and rostral ventromedial medulla cell activity in inflammatory pain

The amygdala is a crucial area in controlling the threshold of pain and its emotional component. The present study has evaluated the effect of a metabotropic glutamate 8 receptor (mGluR8) stimulation in the central nucleus of the amygdala (CeA) on the thermoceptive threshold and on CeA serotonin (5-HT), glutamate (Glu), and GABA release in normal and carrageenan-induced inflammatory pain conditions in rats. Furthermore, the activity of rostral ventromedial medulla (RVM) putative "pronociceptive" ON and "antinociceptive" OFF cells has been evaluated. (S)-3,4-Dicarboxyphenylglycine [(S)-3,4-DCPG], a selective mGluR8 agonist, administered into the CeA, did not change 5-HT, Glu, and GABA release, or the thermoceptive threshold, nor did it modify the activity of ON and OFF cells of the RVM in normal animals. In rats treated with carrageenan, intra-CeA (S)-3,4-DCPG perfusion produced antinociception, and increased 5-HT and Glu, whereas it decreased GABA release. Intra-CeA (S)-3,4-DCPG inhibited ON and increased OFF cell activities. Furthermore, an increase in mGluR8 gene, protein, and staining, the latter being associated with vesicular GABA transporter-positive profiles, has been found in the CeA after carrageenan-induced inflammatory pain. These results show that stimulation of mGluR8, which was overexpressed within the CeA in inflammatory pain conditions, inhibits nociceptive behavior. Such an effect is associated with an increase in 5-HT and Glu release, a decrease in GABA, and the inhibition of ON- and the stimulation of OFF-cell activities within RVM.

MK-801 changes the role of glutamic acid on modulation of algesia in nucleus accumbens

Dizocilpine maleate (MK-801) causes the blockage of the glutamic acid (Glu) receptors in the central nervous system that are involved in pain transmission. However, the mechanism of action of MK-801 in pain-related neurons is not clear, and it is still unknown whether Glu is involved in the modulation of this processing. This study examines the effect of MK-801, Glu on the pain-evoked response of pain-excitation neurons (PENs) and pain-inhibition neurons (PINs) in the nucleus accumbens (NAc) of rats. The trains of electric impulses applied to the sciatic nerve were used as noxious stimulation. The electrical activities of PENs or PINs in NAc were recorded by a glass microelectrode. Our results revealed that the lateral ventricle injection of Glu increased the discharged frequency and shortened the discharged latency of PEN, and decreased the discharged frequency and prolonged the discharged inhibitory duration (ID) of PIN in NAc of rats evoked by the noxious stimulation, while intra-NAc administration of MK-801 produced the opposite response. On the basis of above findings we can deduce that Glu, MK-801 and N-methyl-D-aspartate (NMDA) receptor are involved in the modulation of nociceptive information transmission in NAc.

A role of periaqueductal grey NR2B-containing NMDA receptor in mediating persistent inflammatory pain

The midbrain periaqueductal grey (PAG) is a structure known for its roles in pain transmission and modulation. Noxious stimuli potentiate the glutamate synaptic transmission and enhance glutamate NMDA receptor expression in the PAG. However, little is known about roles of NMDA receptor subunits in the PAG in processing the persistent inflammatory pain. The present study was undertaken to investigate NR2A- and NR2B-containing NMDA receptors in the PAG and their modulation to the peripheral painful inflammation. Noxious stimuli induced by hind-paw injection of complete Freund's adjuvant (CFA) caused up-regulation of NR2B-containing NMDA receptors in the PAG, while NR2A-containing NMDA receptors were not altered. Whole-cell patch-clamp recordings revealed that NMDA receptor mediated mEPSCs were increased significantly in the PAG synapse during the chronic phases of inflammatory pain in mice. PAG local infusion of Ro 25-6981, an NR2B antagonist, notably prolonged the paw withdrawal latency to thermal radian heat stimuli bilaterally in rats. Hyperoside (Hyp), one of the flavonoids compound isolated from Rhododendron ponticum L., significantly reversed up-regulation of NR2B-containing NMDA receptors in the PAG and exhibited analgesic activities against persistent inflammatory stimuli in mice. Our findings provide strong evidence that up-regulation of NR2B-containing NMDA receptors in the PAG involves in the modulation to the peripheral persistent inflammatory pain.

Alterations in extracellular levels of gamma-aminobutyric acid in the rat basolateral amygdala and periaqueductal gray during conditioned fear, persistent pain and fear-conditioned analgesia

Evidence suggests an important role for supraspinal gamma-aminobutyric acid (GABA) in conditioned fear and pain. Using dual probe microdialysis coupled to HPLC, we investigated alterations in extracellular levels of GABA simultaneously in the rat basolateral amygdala and dorsal periaqueductal gray during expression of conditioned fear, formalin-evoked nociception, and fear-conditioned analgesia. Re-exposure to a context previously paired with footshock significantly increased the duration of freezing and 22-kilohertz ultrasonic vocalization, and reduced formalin-evoked nociceptive behavior. Upon re-exposure to the context, GABA levels in the basolateral amygdala were significantly lower in fear-conditioned animals compared with non-fear-conditioned controls, irrespective of intraplantar formalin/saline injection. GABA levels in the dorsal periaqueductal gray were lower in rats receiving intraplantar injection of formalin, compared with saline-treated controls. GABA levels sampled were sensitive to nipecotic acid and calcium infusion. No specific fear-conditioned analgesia-related alterations in GABA efflux were observed in these regions despite the ability of rats undergoing dual probe microdialysis to express this important survival response. In conclusion, expression of contextually induced fear- and pain-related behavior are accompanied by suppression of GABA release in the basolateral amygdala and dorsal periaqueductal gray, respectively, compared with non-fear, non-pain controls.

PERSPECTIVE

This study investigates alterations in levels of the neurotransmitter GABA simultaneously in the rat amygdala and periaqueductal grey during expression of pain- and fear-related behavior and fear-induced analgesia. The results enhance our understanding of the role of this neurotransmitter in pain, memory of pain and control of pain during fear.

Glutamate modulation of antinociception, but not tolerance, produced by morphine microinjection into the periaqueductal gray of the rat

The periaqueductal gray (PAG) plays an important role in morphine antinociception and tolerance. Co-localization of mu-opioid and NMDA receptors on dendrites in the PAG suggests that glutamate may modulate morphine antinociception. Moreover, the involvement of glutamate in spinally mediated tolerance to morphine suggests that glutamate receptors may contribute to PAG mediated tolerance. These hypotheses were tested by microinjecting glutamate receptor antagonists and morphine into the ventrolateral PAG (vPAG) of the rat. Microinjection of the non-specific glutamate receptor antagonist kynurenic acid or the NMDA receptor antagonist MK-801 into the vPAG did not affect nociception. However, co-administration of these antagonists with morphine into the vPAG enhanced the acute antinociceptive effects of morphine as measured by a leftward shift in the morphine dose-response curves. Repeated microinjections of morphine into the vPAG caused a rightward shift in the dose-response curve for antinociception whether the glutamate receptor antagonists kynurenic acid or MK-801 were co-administered or not. The lack of effect of microinjecting glutamate receptor antagonists into the vPAG indicates that tonic glutamate release in the PAG does not contribute to nociceptive tone. That these antagonists enhance morphine antinociception indicates that endogenous glutamate counteracts the antinociceptive effect of morphine in the vPAG. However, this compensatory glutamate release does not contribute to tolerance to the antinociceptive effects of microinjecting morphine into the vPAG. Previous research showing that glutamate contributes to spinal mechanisms of tolerance indicate that different tolerance mechanisms are engaged in the vPAG and spinal cord.

Modulation of defensive responses and anxiety-like behaviors by group I metabotropic glutamate receptors located in the dorsolateral periaqueductal gray

Glutamatergic neurotransmission in the dorsolateral periaqueductal gray (dlPAG) is related to defensive responses. However, the role of group I glutamate metabotropic receptors (mGluR) in these responses has been poorly investigated. The objective of the present study, therefore, was to test the hypothesis that interference with group I mGluR-mediated neurotransmission in dlPAG could modulate defensive responses. Male Wistar rats with cannulae aimed at the dlPAG were submitted to the following experiments: 1. intra dlPAG injections of vehicle (veh, 0.2 microL) or (RS)1-aminoindan-1,5-dicarboxylic acid (AIDA, 30-100 nmol, an mGluR1 receptor competitive antagonist) followed, 5 min later, by veh or trans-(+)-1-amino-1,3-ciclopentanedicarboxylic acid (tACPD, a group I and II mGluR agonist, 30 nmol); 2. intra-dlPAG injections of veh, AIDA (30 nmol) or 2-methyl-6-(phenylethynyl)-pyridine (MPEP, an mGluR5 receptor non-competitive antagonist, 50 nmol) followed by trans-azetidine-2,4-dicarboxylic acid (tADA, a group I mGluR agonist, 10 nmol); 3. and 4. intra-dlPAG injections of vehicle, AIDA (10-30 nmol) or MPEP (10-50 nmol) before the elevated plus maze (EPM) test; 5. intra-dlPAG injections of vehicle, AIDA (30 nmol) or MPEP (50 nmol) before the Vogel punished licking test. tACPD induced defensive responses characterized by jumps and an increased number of crossings in the observation box. These responses were attenuated by AIDA (30 nmol). tADA produced similar responses, although of lower intensity. tADA effects were prevented by AIDA and MPEP. Both drugs also produced anxiolytic-like effects in the EPM and Vogel tests when injected alone. The results suggest that group I metabotropic glutamate receptors in the dlPAG facilitate defensive responses and may also be involved in regulating anxiety-like behavior.

Arginine vasopressin induces periaqueductal gray release of enkephalin and endorphin relating to pain modulation in the rat

Previous study has proven that microinjection of arginine vasopressin (AVP) into periaqueductal gray (PAG) raises the pain threshold, in which the antinociceptive effect of AVP can be reversed by PAG pretreatment with V2 rather than V1 or opiate receptor antagonist. The present work investigated the AVP effect on endogenous opiate peptides, oxytocin (OXT) and classical neurotransmitters in the rat PAG. The results showed that AVP elevated the concentrations of leucine-enkephalin (L-Ek), methionine-enkephalin (M-Ek) and beta-endorphin (beta-Ep), but did not change the concentrations of dynorphinA(1-13) (DynA(1-13)), OXT, classical neurotransmitters including achetylcholine (Ach), choline (Ch), serotonin (5-HT), gamma-aminobutyric acid (GABA), glutamate (Glu), dopamine (DA), norepinephrine (NE) and epinephrine (E), and their metabolic products in PAG perfusion liquid. Pain stimulation increased the concentrations of AVP, L-EK, M-Ek, beta-Ep, 5-HT and 5-HIAA (5-HT metabolic product), but did not influence the concentrations of DynA(1-13), OXT, the other classical neurotransmitters and their metabolic products. PAG pretreatment with naloxone - an opiate receptor antagonist completely attenuated the pain threshold increase induced by PAG administration of AVP, but local pretreatment of OXT or classical neurotransmitter receptor antagonist did not influence the pain threshold increase induced by PAG administration of AVP. The data suggested that AVP in PAG could induce the local release of enkephalin and endorphin rather than dynophin, OXT and classical neurotransmitters to participate in pain modulation.

Periaqueductal gray metabotropic glutamate receptor subtype 7 and 8 mediate opposite effects on amino acid release, rostral ventromedial medulla cell activities, and thermal nociception

The current study has investigated the involvement of periaqueductal gray (PAG) metabotropic glutamate subtype 7 and 8 receptors (mGluR(7) and mGluR(8)) in modulating rostral ventromedial medulla (RVM) ongoing and tail flick-related on and off cell activities. Our study has also investigated the role of PAG mGluR(7) on thermoceptive threshold and PAG glutamate and GABA release. Intra-ventrolateral PAG (S)-3,4-dicarboxyphenylglycine [(S)-3,4-DCPG (2 and 4 nmol/rat)] or N,N(I)-dibenzhydrylethane-1,2-diamin dihydrochloride (AMN082, (1 and 2 nmol/rat), selective mGluR(8) and mGluR(7) agonists, respectively, caused opposite effects on the ongoing RVM on and off cell activities. Tail flick latency was increased or decreased by (S)-3,4-DCPG or AMN082 (2 nmol/rat), respectively. (S)-3,4-DCPG reduced the pause and delayed the onset of the off cell pause. Conversely, AMN082 increased the pause and shortened the onset of off cell pause. (S)-3,4-DCPG or AMN082 did not change the tail flick-induced onset of on-cell peak firing. The tail flick latency and its related electrophysiological effects induced by (S)-3,4-DCPG or AMN082 were prevented by (RS)-alpha-methylserine-o-phosphate (100 nmol/rat), a group III mGluR antagonist. Intra-ventrolateral PAG perfusion with AMN082 (10 and 25 microM), decreased thermoceptive thresholds and glutamate extracellular levels. A decrease in GABA release was also observed. These results show that stimulation of PAG mGluR(8) or mGluR(7) could either relieve or worsen pain perception. The opposite effects on pain behavior correlate with the opposite roles played by mGluR(7) and mGluR(8) on glutamate and GABA release and the ongoing and tail flick-related activities of the RVM on and off cells.

Antiepileptic drugs in migraine: from clinical aspects to cellular mechanisms

Migraine and epilepsy share several clinical features, and epilepsy is a comorbid condition of migraine. Clinical studies have shown that some antiepileptic drugs are effective at preventing migraine attacks. A rationale for their use in migraine prophylaxis is the hypothesis that migraine and epilepsy share several common pathogenetic mechanisms. An imbalance between excitatory glutamate-mediated transmission and GABA-mediated inhibition in specific brain areas has been postulated in these two pathological conditions. Moreover, abnormal activation of voltage-operated ionic channels has been implicated in both migraine and epilepsy. Cortical spreading depression has been found to be involved in the pathophysiology of epilepsy, in addition to the generation of migraine aura.

Glutamate receptor ligands

Glutamate acts through a variety of receptors to modulate neurotransmission and neuronal excitability. Glutamate plays a critical role in neuroplasticity as well as in nervous system dysfunctions and disorders. Hyperfunction or dysfunction of glutamatergic neurotransmission also represents a key mechanism of pain-related plastic changes in the central and peripheral nervous system. This chapter will review the classification of glutamate receptors and their role in peripheral and central nociceptive processing. Evidence from preclinical pain models and clinical studies for the therapeutic value of certain glutamate receptor ligands will be discussed.

Effects of (S)-3,4-DCPG, an mGlu8 receptor agonist, on inflammatory and neuropathic pain in mice

In this study, the effect of (S)-3,4-dicarboxyphenylglycine (DCPG), a selective mGlu8 receptor agonist, has been investigated in inflammatory and neuropathic pain models in order to elucidate the role of mGlu8 receptor in modulating pain perception. Inflammatory pain was induced by the peripheral injection of formalin or carrageenan in awake mice. Systemic administration of (S)-3,4-DCPG, performed 15 min before formalin, decreased both early and delayed nociceptive responses of the formalin test. When this treatment was carried out 15 min after the peripheral injection of formalin it still reduced the late hyperalgesic phase. Similarly, systemic (S)-3,4-DCPG reduced carrageenan-induced thermal hyperalgesia and mechanical allodynia when administered 15 min before carrageenan, but no effect on pain behaviour was observed when (S)-3,4-DCPG was given after the development of carrageenan-induced inflammatory pain. When microinjected into the lateral PAG (RS)-alpha-methylserine-O-phoshate (MSOP), a group III receptor antagonist, antagonised the analgesic effect induced by systemic administration of (S)-3,4-DCPG in both of the inflammatory pain models. Intra-lateral PAG (S)-3,4-DCPG reduced pain behaviour when administered 10 min before formalin or carrageenan; both the effects were blocked by intra-lateral PAG MSOP. (S)-3,4-DCPG was ineffective in alleviating thermal hyperalgesia and mechanical allodynia 7 days after the chronic constriction injury of the sciatic nerve, whereas it proved effective 3 days after surgery. Taken together these results suggest that stimulation of mGlu8 receptors relieve formalin and carrageenan-induced hyperalgesia in inflammatory pain, whereas it would seem less effective in established inflammatory or neuropathic pain.

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.

Differential changes of group II and group III mGluR function in central amygdala neurons in a model of arthritic pain

Metabotropic glutamate receptors (mGluRs) play important roles in neuroplasticity and disorders such as persistent pain. Group I mGluRs contribute to pain-related sensitization and synaptic plasticity of neurons in the laterocapsular division of the central nucleus of the amygdala (CeLC), although the roles of groups II and III mGluRs are not known. Extracellular single-unit recordings were made from 60 CeLC neurons in anesthetized adult rats. Background activity and evoked responses were measured before and during the development of the kaolin/carrageenan-induced knee-joint arthritis. Drugs were administered into the CeLC by microdialysis before and/or after arthritis induction. A selective group III mGluR agonist (LAP4) inhibited CeLC neurons' responses to stimulation of the knee and ankle in arthritis (n = 7) more potently than under normal conditions (n = 14). A selective group II agonist (LY354740) inhibited responses under normal conditions (n = 12) and became more potent in inhibiting responses to noxious stimulation of the knee in arthritis (n = 10). The effect of LY354740 on innocuous stimulation of the knee and stimulation of the ankle did not change in arthritis. Antagonists for groups II (EGLU, n = 9) and III (UBP1112, n = 8) had no effects under normal conditions. In arthritis, UPB1112 (n = 5) facilitated the responses to stimulation of knee and ankle, whereas EGLU (n = 5) selectively increased the responses to stimulation of the knee. These data suggest that mGluRs of groups II and III can inhibit nociceptive processing in CeLC neurons. The increased function and endogenous activation of group II mGluRs in the arthritis pain model appear more input-selective than the general changes of group III mGluRs.

Assessment of neuroprotective effects of glutamate modulation on glaucoma-related retinal ganglion cell apoptosis in vivo

PURPOSE

To assess the neuroprotective effects of different glutamate modulation strategies, with a nonselective (MK801) and a selective (ifenprodil) NMDA receptor antagonist and a metabotropic glutamate receptor agonist (mGluR Group II, LY354740), in glaucoma-related in vivo rat models of retinal ganglion cell (RGC) apoptosis.

METHODS

RGC apoptosis was induced in Dark Agouti (DA) rats by staurosporine (SSP) treatment. Single agents MK801, ifenprodil, or LY354740, or MK801 and LY354740 combined, were administrated intravitreally at different doses. Eyes were imaged in vivo using a recently established technique and the results confirmed histologically. The most effective combined therapy regimen of MK801 and LY354740 was then assessed in a chronic ocular hypertension (OHT) rat model with application at 0, 1, and 2 weeks after OHT surgery and the effects assessed as described before.

RESULTS

All strategies of glutamate modulation reduced SSP-induced-RGC apoptosis compared with the control, in a dose-dependent manner: MK801 (R2= 0.8863), ifenprodil (R2= 0.4587), and LY354740 (R2= 0.9094), with EC50s of 0.074, 0.0138, and 19 nanomoles, respectively. The most effective combination dose of MK801 and LY354740 was 0.06 and 20 nanomoles (P < 0.05), respectively, and the optimal timing of the therapy was 0 weeks after OHT surgery (P < 0.05).

CONCLUSIONS

This novel SSP model was validated as a useful tool for screening neuroprotective strategies in vivo. Group II mGluR modulation may be a useful treatment for RGC death. Combination therapy optimized to limit neurotoxic effects of MK801 may be an effective neuroprotective approach in retinal degenerative disease. Furthermore, treatments that minimize secondary RGC degeneration may be most useful in glaucoma.

Defensive-like behaviors and antinociception induced by NMDA injection into the periaqueductal gray of mice depend on nitric oxide synthesis

Glutamate NMDA receptor activation within the periaqueductal gray (PAG) leads to antinociceptive, autonomic and behavioral responses characterized as the fear reaction. Considering that NMDA receptor triggers activation of neuronal nitric oxide synthase (nNOS), enzyme that produces nitric oxide (NO), this study investigated the effects of intra-PAG infusions of NPLA (Nomega-propyl-L-arginine), an nNOS inhibitor, on behavioral and antinociceptive responses induced by local injection of NMDA receptor agonist in mice. The behaviors measured were frequency of jumping and rearing as well as duration (in seconds) of running and freezing. Nociception was assessed during the second phase of the formalin test (injection of 50 microl of formalin 2.5% into the dorsal surface of the right hind paw). Five to seven days after stereotaxic surgery for intracerebral cannula implantation, mice were injected with formalin into the paw, and 10 min later, they received intra-dPAG injection of NPLA (0, 0.2, or 0.4 nmol/0.1 microl). Ten minutes later, they were injected with NMDA (N-methyl-D-aspartate: 0 or 0.04 nmol/0.1 microl) into the same midbrain site and were immediately placed in glass holding cage for recording the defensive behavior and the time spent on licking the injected paw with formalin during a period of 10 min. Microinjections of NMDA significantly decreased nociception response and produced jumping, running, and freezing reactions. Intra-dPAG injections of NPLA (0.4 nmol) completely blocked the NMDA effects without affecting either behavioral or nociceptive responses in intra-dPAG saline-injected animals, except for the rearing frequency that was increased by the nNOS inhibitor. These results strongly suggest the involvement of NO within the PAG in the antinociceptive and defensive reactions induced by local glutamate NMDA receptor activation in this midbrain structure.

Periaqueductal grey CB1 cannabinoid and metabotropic glutamate subtype 5 receptors modulate changes in rostral ventromedial medulla neuronal activities induced by subcutaneous formalin in the rat

This study was undertaken to analyze the involvement of periaqueductal gray (PAG) cannabinoid or group I metabotropic glutamate receptors in the formalin-induced changes on the rostral ventromedial medulla (RVM) ON- and OFF-cells activities. S.c. injection of formalin into the hind paw produced a transient decrease (4-6 min) followed by a longer increase (25-35 min) in tail flick latencies. Formalin also increased basal activity in RVM ON-cells (42+/-7%) and decreased it in OFF-cells (35+/-4%). Intra-PAG microinjection of (R)-(+)-[2,3-dihydro-5-methyl-3-(4-morpholinylmethyl) pyrrolo[1,2,3-de]-1,4-benzoxazin-6-yl]-1-naphthalenylmethanone mesylate (WIN 55,212-2) (2 nmol/rat), a cannabinoid receptor agonist, prevented the formalin-induced changes in RVM cell activities. Higher dosages of WIN 55,212-2 (4-8 nmol/rat) increased the tail flick latencies, delayed the tail flick-related onset to ON-cell burst, and decreased the duration of OFF-cell pause. Furthermore, WIN 55,212-2 at a dosage of 8 nmol/rat decreased RVM ON-cell (57+/-7%) and increased OFF-cell ongoing activities (26+/-4%). These effects were prevented by N-piperidino-5-(4-chlorophenyl)-1-(2,4dichlorophenyl)-4-methyl-3-pyrazolecarboxamide SR141716A, (1 pmol/rat), a CB1 cannabinoid receptor antagonist, or by 2-methyl-6-(phenylethynyl)pyridine (MPEP 20 nmol/rat), a selective mGlu5 glutamate receptor antagonist. T7-(hydroxyimino) cyclopropa[b]chromen-1alpha-carboxylate ethyl ester (CPCOOE/50 nmol/rat) and (S)-(+)-alpha-amino-4-carboxy-2-methylbenzeneacetic acid (LY367385, 20 nmol/rat), selective mGlu1 glutamate receptor antagonists, were ineffective in preventing the WIN-induced effects. This study suggests that s.c. injection of formalin modifies RVM neuronal activities and this effect is prevented by PAG cannabinoid receptor stimulation. Moreover, the physiological stimulation of PAG mGlu5, but not mGlu1 glutamate receptors, seems to be required for the cannabinoid-mediated effect.

Effect of various antagonists on the Channa striatus fillet extract antinociception in mice

The effects of an aqueous supernatant of haruan (ASH) (Channa striatus) fillet extract on various antinociception receptor system activities were examined using a mouse abdominal-constriction model. Mice that were pretreated with distilled water, s.c., followed 10 min later by administration of 25%, 50%, and 100% concentration ASH, s.c., produced a significant concentration-dependent antinociceptive activity (p < 0.001). Pretreatment with naloxone (0.3, 1.0, and 3.0 mg/kg body mass), 10 min before ASH administration, failed to block the extract antinociception. Pretreatment of the 100% concentration ASH with mecamylamine (5 mg/kg), pindolol (10 mg/kg), and haloperidol (1 mg/kg) also did not cause any significant change in its antinociception. However, pretreatment with atropine (5 mg/kg), bicuculline (10 mg/kg), phenoxybenzamine (10mg/kg), and methysergide (5 mg/kg) were found to reverse ASH antinociception. Based on the above findings, the ASH is suggested to contain different types of bioactive compounds that act synergistically on muscarinic, GABAA, α-adrenergic, and serotonergic receptor systems to produce the observed antinocicep tion. Key words: Haruan (Channa striatus), antinociceptive, writhing test, receptor systems

Differential roles of mGlu8 receptors in the regulation of glutamate and gamma-aminobutyric acid release at periaqueductal grey level

We investigated the role of group III metabotropic glutamate (mGlu) receptors on glutamate and GABA releases at the periaqueductal grey (PAG) level by using in vivo microdialysis in rats. Intra-PAG perfusion of either L-(+)-2-amino-4-phosphonobutyric acid (L-AP4, 100-300 microM), (RS)-4-phosphonophenylglycine ((RS)-PPG, 100-300 microM) selective agonists of group III mGlu receptors, or (S)-3,4-dicarboxyphenylglycine ((S)-3,4-DCPG, 50-100 microM), a selective agonist of mGlu8 receptor, increased glutamate and decreased GABA extracellular concentrations. (RS)-alpha-methylserine-O-phosphate (MSOP, 0.5 mM), a selective group III receptor antagonist, perfused in combination with (S)-3,4-DCPG, L-AP4 or (RS)-PPG, antagonised the effects induced by these agonists on both extracellular glutamate and GABA values. alpha-Methyl-3-methyl-4-phosphonophenylglycine (UBP1112, 300 microM), a group III mGlu receptor antagonist, perfused in combination with (RS)-PPG or (S)-3,4-DCPG, antagonised the effects induced by these agonists. Intra-PAG perfusion with forskolin (100 microM), an activator of adenylate cyclase, increased dialysate glutamate and GABA levels. Moreover, intra-PAG perfusion with N-[2-(p-bromocinnamyl-amino)ethyl]-5-isoquinolinesulfonamide dihydrochloride (H-89) (100 microM), a protein kinase (PKA) inhibitor, abolished the effect of (S)-3,4-DCPG on both glutamate and GABA releases. H-89, per se, did not modify glutamate release but reduced extracellular GABA value at the higher dosage used (200 microM). These data suggest that group III mGlu receptors in the PAG modulate the releases of glutamate and GABA conversely. In particular, both the facilitation of glutamate and the inhibition of GABA releases require the participation of coupling to adenylate cyclase and the subsequent activation of the PKA pathway.

Role of central and peripheral mGluR5 receptors in post-operative pain in rats

Metabotropic glutamate receptors (mGluRs) have previously been shown to play a role in pain transmission during inflammatory or neuropathic pain states. However, the role of mGluR5 in post-operative pain remains to be fully investigated. The present study was conducted to characterize analgesic activity of 2-methyl-6-(phenylethynyl)-pyridine (MPEP) in the skin-incision-induced post-operative pain model in rats. MPEP is a potent and selective mGluR5 antagonist with high affinity (K(i)=6.3+/-0.9 nM) in rat cortex using [(3)H]-MPEP as a radioligand, while not competing with the mGluR1-selective radioligand [(3)H]-R214127 (K(i)>10,000 nM) in rat cerebellum. Post-operative pain was examined 2 h following surgery using weight-bearing (WB) difference between injured and uninjured paws as a measure of non-evoked pain. In this model, MPEP, as morphine, showed dose-dependent effects and full efficacy after systemic administration (ED(50)=15 mg/kg, i.p. for MPEP, ED(50)=1.3 mg/kg, s.c. for morphine). In addition, intrathecal (i.t.) and intracerebroventricular (i.c.v.) MPEP reduced WB difference (ED(50)=65 microg/rat i.t. and ED(50)=200 microg/rat i.c.v.). Interestingly, intraplantar (i.pl.) injection of MPEP either before or after surgery induced a similar reduction in WB difference (ED(50)=90 microg/rat, i.pl.) while contralateral i.pl. MPEP injection did not produce any effect. These results demonstrate that both peripheral and central mGluR5 receptors play a role in nociceptive transmission observed during post-operative pain. In addition, the data suggest that mGluR5 antagonists could offer a new therapeutic approach to the treatment of post-operative pain.

Interactions between the cardiovascular and pain regulatory systems: an updated review of mechanisms and possible alterations in chronic pain

Endogenous pain regulatory system dysfunction appears to play a role in the maintenance of chronic pain. An important component of the pain regulatory process is the functional interaction between the cardiovascular and pain regulatory systems, which results in an association between elevated resting blood pressure (BP) and diminished acute pain sensitivity. This BP/pain sensitivity relationship is proposed to reflect a homeostatic feedback loop helping restore arousal levels in the presence of painful stimuli. Evidence is emerging that this normally adaptive BP/pain sensitivity relationship is significantly altered in chronic pain conditions, affecting responsiveness to both acute and chronic pain stimuli. Several mechanisms that may underlie this adaptive relationship in healthy individuals are overviewed, including endogenous opioid, noradrenergic, and baroreceptor-related mechanisms. Theoretical models are presented regarding how chronic pain-related alterations in the mechanisms above and increased pain facilatory system activity (central sensitization) may contribute to altered BP/pain sensitivity interactions in chronic pain. Clinical implications are discussed.

Prostaglandin E2 in the midbrain periaqueductal gray produces hyperalgesia and activates pain-modulating circuitry in the rostral ventromedial medulla

Recent years have seen significant advances in our understanding of the peripheral and spinal mechanisms through which prostaglandins contribute to nociceptive sensitization. By contrast, the possibility of a supraspinal contribution of these compounds to facilitated pain states has received relatively little attention. One possible mechanism through which prostaglandins could act supraspinally to facilitate nociception would be by recruitment of descending facilitation from brainstem pain-modulating systems. The rostral ventromedial medulla (RVM) is now known to contribute to enhanced responding in a variety of inflammatory and nerve injury models. Its major supraspinal input, the midbrain periaqueductal gray (PAG), expresses prostanoid receptors and synthetic enzymes. The aim of the present study was to determine whether direct application of prostaglandin E(2) (PGE(2)) within the ventrolateral PAG is sufficient to produce hyperalgesia, and whether any hyperalgesia could be mediated by recruiting nociceptive modulating neurons in the RVM. We determined the effects of focal application of PGE(2) in the PAG on paw withdrawal latency and activity of identified nociceptive modulating neurons in the RVM of lightly anesthetized rats. Microinjection of PGE(2) (50 fg in 200 nl) into the PAG produced a significant decrease in paw withdrawal latency. The PGE(2) microinjection activated on-cells, RVM neurons thought to facilitate nociception, and suppressed the firing of off-cells, RVM neurons believed to have an inhibitory effect on nociception. These data demonstrate a prostaglandin-sensitive descending facilitation from the PAG, and suggest that this is mediated by on- and off-cells in the RVM.

Effect of the synthetic polyamine N,N'-bis-(3-aminopropyl) cyclohexane-1,4-diamine (DCD) on rat spinal cord nociceptive transmission

In rats submitted to a C-fiber reflex response paradigm, intravenous (i.v.) administration of 2.5, 5 and 10 mg/kg of the synthetic polyamine N,N'-bis-(3-aminopropyl) cyclohexane-1,4-diamine (DCD) dose-dependently reduced both the integrated C reflex responses and wind-up activity. Inhibitory effects of the polyamine on spinal cord nociceptive transmission are likely to be consequence of blockade by extracellular DCD of NMDA receptor channels localized in dorsal horn neurons, although modulatory actions at supraspinal level and at other ion channels could also be possible.

Role of thalamic phospholipase C[beta]4 mediated by metabotropic glutamate receptor type 1 in inflammatory pain

Phospholipase C (PLC) beta4, one of the four isoforms of PLCbetas, is the sole isoform expressed in the mouse ventral posterolateral thalamic nucleus (VPL), a key station in pain processing. The mouse thalamus also has been shown to express a high level of metabotropic glutamate receptor type 1 (mGluR1), which stimulates PLCbetas through activation of Galphaq/11 protein. It is therefore expected that the thalamic mGluR1-PLCbeta4 cascade may play a functional role in nociceptive transmission. To test this hypothesis, we first studied behavioral responses to various nociceptive stimuli in PLCbeta4 knock-out mice. We performed the formalin test and found no difference in the pain behavior in the first phase of the formalin test, which is attributed to acute nociception, between PLCbeta4 knock-out and wild-type mice. Consistent with this result, acute pain responses in the hot plate and tail flick tests were also unaffected in the PLCbeta4 knock-out mice. However, the nociceptive behavior in the second phase of the formalin test, resulting from the tissue inflammation, was attenuated in PLCbeta4 knock-out mice. In the dorsal horn of the spinal cord where PLCbeta1 and PLCbeta4 mRNAs are expressed, no difference was found between the wild-type and knock-out mice in the number of Fos-like immunoreactive neurons, which represent neuronal activity in the second phase in the formalin test. Thus, it is unlikely that spinal PLCbeta4 is involved in the formalin-induced inflammatory pain. Next, we found that pretreatment with PLC inhibitors, mGluR1 antagonists, or both, by either intracerebroventricular or intrathalamic injection, attenuated the formalin-induced pain behavior in the second phase in wild-type mice. Furthermore, activation of mGluR1 at the VPL enhanced pain behavior in the second phase in the wild-type mice. In contrast, PLCbeta4 knock-out mice did not show such enhancement, indicating that mGluR1 is connected to PLCbeta4 in the VPL. Finally, in parallel with the behavioral results, we showed in an electrophysiological study that the time course of firing discharges in VPL corresponds well to that of pain behavior in the formalin test in both wild-type and PLCbeta4 knock-out mice. These findings indicate that the thalamic mGluR1-PLCbeta4 cascade is indispensable for the formalin-induced inflammatory pain by regulating the response of VPL neurons.

Capsaicin infused into the PAG affects rat tail flick responses to noxious heat and alters neuronal firing in the RVM

It is well established that the vanilloid receptor, VR1, is an important peripheral mediator of nociception. VR1 receptors are also located in several brain regions, yet it is uncertain whether these supraspinal VR1 receptors have any influence on the nociceptive system. To investigate a possible nociceptive role for supraspinal VR1 receptors, capsaicin (10 nmol in 0.4 microl) was microinjected into either the dorsal (dPAG) or ventral (vPAG) regions of the periaqueductal gray. Capsaicin-related effects on tail flick latency (immersion in 52 degrees C water) and on neuronal activity (on-, off-, and neutral cells) in the rostral ventromedial medulla (RVM) were measured in lightly anesthetized rats. Administration of capsaicin into the dPAG but not the vPAG caused an initial hyperalgesic response followed later by analgesia (125 +/- 20.96 min postinjection). The tail flick-related burst in on-cell activity was triggered earlier in the hyperalgesic phase and was delayed or absent during the analgesic phase. Spontaneous activity of on-cells increased at the onset of the hyperalgesic phase and decreased before and during the analgesic phase. The tail flick-related pause in off-cell activity as well as spontaneous firing for these cells was unchanged in the hyperalgesic phase. During the analgesic phase, off-cells no longer paused during noxious stimulation and had increased levels of spontaneous activity. Neutral cell firing was unaffected in either phase. Pretreatment with the VR1 receptor antagonist, capsazepine (10 nmol in 0.4 microl), into the dPAG blocked the capsaicin-induced hyperalgesia as well as the corresponding changes in on- and off-cell activity. VR1 receptor immunostaining was observed in the dPAG of untreated rats. Microinjection of capsaicin likely sensitized and then desensitized dPAG neurons affecting nocifensive reflexes and RVM neuronal activity. These results suggest that supraspinal VR1 receptors in the dPAG contribute to descending modulation of nociception.

Role of metabotropic glutamate receptor subtype 5 (mGluR5) in the maintenance of cold hypersensitivity following a peripheral mononeuropathy in the rat

The present series of experiments were designed to examine the contribution of metabotropic glutamate receptor subtype 5 (mGluR5) to neuropathic pain by determining the effects of the selective mGluR5 antagonist MPEP (2-methyl-6-(phenylethynyl)-pyridine) on neuropathy-induced cold hypersensitivity. Unilateral chronic constriction injury (CCI) to the sciatic nerve in rats produced an increase in the number of hind paw withdrawals from a cold surface (4 +/- 2 degrees C) which was dose-dependently inhibited by systemic (i.p.) injection of MPEP (ID(50) = 11.3 mg/kg). In vivo brain mGluR5 receptor occupancy following systemic (i.p.) MPEP revealed that >90% occupancy is required for behavioral efficacy. Intracerebroventricular (i.c.v.) injection of MPEP dose-dependently inhibited CCI-induced cold hypersensitivity (ID(50) = 123.5 nmol), while microinjection of MPEP directly into the rostral ventromedial medulla (RVM) potently inhibited this hypersensitivity (ID(50) = 1.3 pmol). A role for mGluR5 in the RVM was further supported by the observation that intra-RVM injection of the mGluR5 agonist CHPG (10 nmol; 2-chloro-5-hydroxyphenylglycine) produced cold hypersensitivity in naïve rats that was blocked by pretreatment with intra-RVM MPEP (3 nmol). Intrathecal (500 nmol; i.t.) or intraplantar (300 nmol; i.pl.) injection of MPEP was ineffective in reversing CCI-induced cold hypersensitivity. These results demonstrate that mGluR5 contributes to cold hypersensitivity following peripheral neuropathy exclusively at supraspinal sites in the CNS. Additionally, mGluR5 in the RVM significantly contributes to the maintenance of cold hypersensitivity, likely via activation of descending nociceptive facilitatory systems.

Descending control of pain

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

Interaction between vanilloid and glutamate receptors in the central modulation of nociception

This study investigates the effect of microinjections of capsaicin in the periaqueductal grey matter of rats on nociceptive behaviour and the possible interactions with NMDA and mGlu receptors. Intra-periaqueductal grey microinjection of capsaicin (1-3-6 nmol/rat) increased the latency of the nociceptive reaction in the plantar test. This effect was prevented by pretreatment with capsazepine (6 nmol/rat), which had no effect per se on the latency of the nociceptive reaction. 7-(Hydroxyimino)cyclopropa[b]chromen-1alpha-carboxylate ethyl ester (CPCCOEt, 50 nmol/rat) and 2-Methyl-6-(phenylethynyl)pyridine (MPEP, 50 nmol/rat), antagonists of mGlu(1) and mGlu(5) receptors, respectively, completely blocked the effect of capsaicin. Similarly, pretreatment with DL-2-Amino-5-phosphonovaleric acid (DL-AP5, 5 nmol/rat) and riluzole (4 nmol/rat), an NMDA receptor antagonist and a voltage-dependent Na(+) channels blocker which inhibits glutamate release, respectively, completely antagonized the effect of capsaicin. However, pretreatment with (2S)-alpha-Ethylglutamic acid (30 nmol/rat) and (RS)-alpha-Methylserine-O-phosphate (MSOP, 30 nmol/rat), antagonists of group II and group III mGlu receptors, respectively, had no effects on capsaicin-induced analgesia. Similarly, pretreatment with N-(piperidin-1-yl)-5-(4-chlophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide (SR 141716A, 5 pmol/rat), a selective cannabinoid CB(1) receptor antagonist, did not affect the capsaicin-induced antinociception. In conclusion, this study shows that capsaicin might produce antinociception at the periaqueductal grey level by increasing glutamate release, which activates postsynaptic group I mGlu and NMDA receptors.