Modulation of sensory neurone excitatory and inhibitory responses in the ventrobasal thalamus by activation of metabotropic excitatory amino acid receptors

The role of thalamic group II mGlu receptors in health and disease

The thalamus plays a pivotal role in the integration and processing of sensory, motor, and cognitive information. It is therefore important to understand how the thalamus operates in states of both health and disease. In the present review, we discuss the function of the Group II metabotropic glutamate (mGlu) receptors within thalamic circuitry, and how they may represent therapeutic targets in treating disease states associated with thalamic dysfunction.

Group II metabotropic glutamate receptor (mGlu2 and mGlu3 ) roles in thalamic processing

BACKGROUND AND PURPOSE

As the thalamus underpins almost all aspects of behaviour, it is important to understand how the thalamus operates. Group II metabotropic glutamate (mGlu₂ /mGlu₃ ) receptor activation reduces inhibition in thalamic nuclei originating from the surrounding thalamic reticular nucleus (TRN). Whilst an mGlu₂ component to this effect has been reported, in this study, we demonstrate that it is likely, largely mediated via mGlu₃ .

EXPERIMENTAL APPROACH

The somatosensory ventrobasal thalamus (VB) is an established model for probing fundamental principles of thalamic function. In vitro slices conserving VB-TRN circuitry from wild-type and mGlu₃ knockout mouse brains were used to record IPSPs and mIPSCs. In vivo extracellular recordings were made from VB neurons in anaesthetised rats. A range of selective pharmacological agents were used to probe Group II mGlu receptor function (agonist, LY354740; antagonist, LY341495; mGlu₂ positive allosteric modulator, LY487379 and mixed mGlu₂ agonist/mGlu₃ antagonist LY395756).

KEY RESULTS

The in vitro and in vivo data are complementary and suggest that mGlu₃ receptor activation is largely responsible for potentiating responses to somatosensory stimulation by reducing inhibition from the TRN.

CONCLUSIONS AND IMPLICATIONS

mGlu₃ receptor activation in the VB likely enables important somatosensory information to be discerned from background activity. These mGlu₃ receptors are likely to be endogenously activated via 'glutamate spillover'. In cognitive thalamic nuclei, this mechanism may be of importance in governing attentional processes. Positive allosteric modulation of endogenous mGlu₃ receptor activation may therefore enhance cognitive function in pathophysiological disease states, such as schizophrenia, thus representing a highly specific therapeutic target.

Glutamate and Its Receptors as Therapeutic Targets for Migraine

There is substantial evidence indicating a role for glutamate in migraine. Levels of glutamate are higher in the brain and possibly also in the peripheral circulation in migraine patients, particularly during attacks. Altered blood levels of kynurenines, endogenous modulators of glutamate receptors, have been reported in migraine patients. Population genetic studies implicate genes that are involved with glutamate signaling in migraine, and gene mutations responsible for familial hemiplegic migraine and other familial migraine syndromes may influence glutamate signaling. Animal studies indicate that glutamate plays a key role in pain transmission, central sensitization, and cortical spreading depression. Multiple therapies that target glutamate receptors including magnesium, topiramate, memantine, and ketamine have been reported to have efficacy in the treatment of migraine, although with the exception of topiramate, the evidence for the efficacy of these therapies is not strong. Also, because all of these therapies have other mechanisms of action, it is not possible to conclude that the efficacy of these drugs is entirely due to their effects on glutamate receptors. Further studies are needed to more clearly delineate the possible roles of glutamate and its specific receptor subtypes in migraine and to identify new ways of targeting glutamate for migraine therapy.

Neuronal activity patterns in the mediodorsal thalamus and related cognitive circuits are modulated by metabotropic glutamate receptors

The mediodorsal thalamus (MD) likely plays an important role in cognition as it receives abundant afferent connections from the amygdala and prefrontal cortex (PFC). Indeed, disturbed activity within the MD is thought to precipitate cognitive deficits associated with schizophrenia. As compounds acting at the Group II metabotropic glutamate (mGlu) receptors (subtypes mGlu2/mGlu3) have efficacy in animal models of schizophrenia, we investigated whether a Group II agonist and an mGlu2 positive allosteric modulator (PAM) could modulate MD activity. Extracellular single-unit recordings were made in vivo from MD neurones in anaesthetised rats. Responses were elicited by electrical stimulation of the PFC and/or amygdala, with Group II compounds locally applied as required. The Group II agonist reduced inhibition evoked in the MD: an effect manifested as an increase in short-latency responses, and a decrease in long-latency burst-firing. This disinhibitory action of the Group II receptors in the MD represents a mechanism of potential therapeutic importance as increased inhibition in the MD has been associated with cognitive deficit-onset. Furthermore, as co-application of the mGlu2 PAM did not potentiate the Group II agonist effects in the MD, we suggest that the Group II disinhibitory effect is majority-mediated via mGlu3. This heterogeneity in Group II receptor thalamic physiology bears consequence, as compounds active exclusively at the mGlu2 subtype are unlikely to perturb maladapted MD firing patterns associated with cognitive deficits, with activity at mGlu3 receptors possibly more appropriate. Indeed, polymorphisms in the mGlu3, but not the mGlu2, gene have been detected in patients with schizophrenia.

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There is heterogeneity in Group II receptor physiology across thalamic nuclei.

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This differential distribution may facilitate multimodal thalamic nuclei functions.

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Group II receptor activation reduced burst firing via reducing thalamic inhibition.

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Increased thalamic inhibition precipitates impairments in cognitive function.

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Activating the Group II receptors may therefore enhance cognitive function.

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.

The function of metabotropic glutamate receptors in thalamus and cortex

Metabotropic glutamate receptors (mGluRs) are found throughout thalamus and cortex and are clearly important to circuit behavior in both structures, and so considering only participation of ionotropic glutamate receptors (e.g., [R,S]-α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid [AMPA] and N-methyl-d-aspartate receptors [NMDA] receptors) in glutamatergic processing would be an unfortunate oversimplification. These mGluRs are found both postsynaptically, on target cells of glutamatergic afferents, and presynaptically, on various synaptic terminals themselves, and when activated, they produce prolonged effects lasting at least hundreds of msec to several sec and perhaps longer. Two main types exist: activation of group I mGluRs causes postsynaptic depolarization, and group II, hyperpolarization. Both types are implicated in synaptic plasticity, both short term and long term. Their evident importance in functioning of thalamus and cortex makes it critical to develop a better understanding of how these receptors are normally activated, especially because they also seem implicated in a wide range of neurological and cognitive pathologies.

Regulators of synaptic transmission: roles in the pathogenesis and treatment of epilepsy

Synaptic transmission is the communication between a presynaptic and a postsynaptic neuron, and the subsequent processing of the signal. These processes are complex and highly regulated, reflecting their importance in normal brain functioning and homeostasis. Sustaining synaptic transmission depends on the continuing cycle of synaptic vesicle formation, release, and endocytosis, which requires proteins such as dynamin, syndapin, synapsin, and synaptic vesicle protein 2A. Synaptic transmission is regulated by diverse mechanisms, including presynaptic modulators of synaptic vesicle formation and release, postsynaptic receptors and signaling, and modulators of neurotransmission. Neurotransmitters released presynaptically can bind to their postsynaptic receptors, the inhibitory γ-aminobutyric acid (GABA)ergic receptors or the excitatory glutamate receptors. Once released, glutamate activates a variety of postsynaptic receptors including α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), N-methyl-D-aspartate (NMDA), kainate, and metabotropic receptors. The activation of the receptors triggers downstream signaling cascades generating a vast array of effects, which can be modulated by a numerous auxiliary regulatory subunits. Moreover, different neuropeptides such as neuropeptide Y, brain-derived neurotrophic factor (BDNF), somatostatin, ghrelin, and galanin, act as regulators of diverse synaptic functions and along with the classic neurotransmitters. Abnormalities in the regulation of synaptic transmission play a critical role in the pathogenesis of numerous brain diseases, including epilepsy. This review focuses on the different mechanisms involved in the regulation of synaptic transmission, which may play a role in the pathogenesis of epilepsy: the presynaptic modulators of synaptic vesicle formation and release, postsynaptic receptors, and modulators of neurotransmission, including the mechanism by which drugs can modulate the frequency and severity of epileptic seizures.

Actions of Xanthurenic acid, a putative endogenous Group II metabotropic glutamate receptor agonist, on sensory transmission in the thalamus

Xanthurenic acid (XA), a molecule arising from tryptophan metabolism by transamination of 3-hydroxykynurenine, has recently been identified as an endogenous Group II (mGlu2 and mGlu3) metabotropic glutamate (mGlu) receptor ligand in vitro. Impairments in Group II mGlu receptor expression and function have been implicated in the pathophysiology of schizophrenia, as have multiple steps in the kynurenine metabolism pathway. Therefore, we examined XA in vivo to further investigate its potential as a Group II mGlu receptor ligand using a preparation that has been previously demonstrated to efficiently reveal the action of other Group II mGlu receptor ligands in vivo. Extracellular single-neurone recordings were made in the rat ventrobasal thalamus (VB) in conjunction with iontophoresis of agonists, an antagonist and a positive allosteric modulator and/or intravenous (i.v.) injection of XA. We found the XA effect on sensory inhibition, when applied iontophoretically and i.v., was similar to that of other Group II mGlu receptor agonists in reducing inhibition evoked in the VB from the thalamic reticular nucleus upon physiological sensory stimulation. Furthermore, we postulate that XA may be the first potential endogenous allosteric agonist (termed 'endocoid') for the mGlu receptors. As the Group II receptors and kynurenine metabolism pathway have both been heavily implicated in the pathophysiology of schizophrenia, XA could play a pivotal role in antipsychotic research as this potential endocoid represents both a convergence within these two biological parameters and a novel class of Group II mGlu receptor ligand. This article is part of a Special Issue entitled 'Metabotropic Glutamate Receptors'.

Positive allosteric modulation reveals a specific role for mGlu2 receptors in sensory processing in the thalamus

Group II metabotropic glutamate receptor (mGlu) modulation of sensory processing in the rat ventrobasal thalamic nucleus (VB) has been extensively studied in vivo. However, it is not yet known what the relative contributions are of the Group II mGlu receptor subtypes (mGlu2 and mGlu3) to this modulation, nor to what extent these receptors may be activated under physiological conditions during this process. Using single-neurone recording in the rat VB in vivo with local application of the selective Group II agonist LY354740 and the subtype selective mGlu2 positive allosteric modulator (PAM) LY487379, our findings were twofold. Firstly, we found that there is an mGlu2 component to the effects of LY354740 on sensory responses in the VB. Secondly, we have demonstrated that application of the PAM alone can modulate sensory responses of single neurones in vivo. This indicates that mGlu2 receptors can be activated by endogenous agonist following physiological sensory stimulation. We speculate that the mGlu2 subtype could be activated under physiological stimulus-evoked conditions by 'glutamate spillover' from synapses between excitatory sensory afferents and VB neurones that can lead to a reduction in sensory-evoked inhibition arising from the thalamic reticular nucleus (TRN). We propose that this potential mGlu2 receptor modulation of inhibition could play an important role in discerning relevant information from background activity upon physiological sensory stimulation. Furthermore, this could be a site of action for mGlu2 PAMs to modulate cognitive processes.

Temporal summation of heat pain in humans: Evidence supporting thalamocortical modulation

Noxious cutaneous contact heat stimuli (48 degrees C) are perceived as increasingly painful when the stimulus duration is extended from 5 to 10s, reflecting the temporal summation of central neuronal activity mediating heat pain. However, the sensation of increasing heat pain disappears, reaching a plateau as stimulus duration increases from 10 to 20s. We used functional magnetic resonance imaging (fMRI) in 10 healthy subjects to determine if active central mechanisms could contribute to this psychophysical plateau. During heat pain durations ranging from 5 to 20s, activation intensities in the bilateral orbitofrontal cortices and the activation volume in the left primary (S1) somatosensory cortex correlated only with perceived stimulus intensity and not with stimulus duration. Activation volumes increased with both stimulus duration and perceived intensity in the left lateral thalamus, posterior insula, inferior parietal cortex, and hippocampus. In contrast, during the psychophysical plateau, both the intensity and volume of thalamic and cortical activations in the right medial thalamus, right posterior insula, and left secondary (S2) somatosensory cortex continued to increase with stimulus duration but not with perceived stimulus intensity. Activation volumes in the left medial and right lateral thalamus, and the bilateral mid-anterior cingulate, left orbitofrontal, and right S2 cortices also increased only with stimulus duration. The increased activity of specific thalamic and cortical structures as stimulus duration, but not perceived intensity, increases is consistent with the recruitment of a thalamocortical mechanism that participates in the modulation of pain-related cortical responses and the temporal summation of heat pain.

Group II metabotropic glutamate receptors reduce excitatory but not inhibitory neurotransmission in rat barrel cortex in vivo

Group II metabotropic (mGlu) receptors are known to play an important role in regulating the release of excitatory transmitter in a number of brain areas. Previous experiments demonstrated that (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD) depressed excitatory transmission in the adult rat barrel cortex. Here we show, using in vivo extracellular single unit recordings and iontophoretic application of drugs, that selective activation of Group II mGlu receptors depresses excitatory but not inhibitory transmission. The selective Group II receptor agonist (2R,4R)-4-aminopyrrolidine-2,4-dicarboxylate (2R,4R-APDC) had similar depressant effects to 1S,3R-ACPD on tactile evoked responses of rapidly adapting neurons. The depressant effects were seen on shorter latency (<12 ms) responses, were most pronounced in layers 3-4 (and 5b for 2R,4R-APDC only), and were reversibly antagonized by the Group II receptor antagonist (2S)-alpha-ethylglutamic acid (EGLU) relative to depressions produced by iontophoretic GABA. Where 1S,3R-ACPD and 2R,4R-APDC depressed excitatory transmission, there was little or no effect on postsynaptic excitations produced by iontophoretic AMPA--a result that supports a presynaptic location of Group II receptors on excitatory terminals. To assess the possible involvement of Group II mGlu receptors in the modulation of inhibition, we studied the effect of iontophoretic 1S,3R-ACPD in a condition-test protocol. The results contrasted markedly from those previously observed using the Group III agonist L(+)-2-amino-4-phosphonobutyric acid in that activation of Group II receptors using 1S,3R-ACPD did not modulate inhibition. Therefore our results show that Group II mGlu receptors play an important role in modulating excitatory, but not inhibitory, transmission. We propose that the Group II mGlu receptors are located on excitatory terminals, and act as autoreceptors. Their role appears to be important in the early stages of cortical processing, by keeping excitatory inputs within specified physiological limits, and possibly by mediating depression evidenced during synaptic plasticity.

Metabotropic glutamate receptors as a strategic target for the treatment of epilepsy

Epilepsy is a chronic neurological disorder that has many known types, including generalized epilepsies that involve cortical and subcortical structures. A proportion of patients have seizures that are resistant to traditional anti-epilepsy drugs, which mainly target ion channels or postsynaptic receptors. This resistance to conventional therapies makes it important to identify novel targets for the treatment of epilepsy. Given the involvement of the neurotransmitter glutamate in the etiology of epilepsy, targets that control glutamatergic neurotransmission are of special interest. The metabotropic glutamate receptors (mGluRs) are of a family of eight G-protein-coupled receptors that serve unique regulatory functions at synapses that use the neurotransmitter glutamate. Their distribution within the central nervous system provides a platform for both presynaptic control of glutamate release, as well as postsynaptic control of neuronal responses to glutamate. In recent years, substantial efforts have been made towards developing selective agonists and antagonists which may be useful for targeting specific receptor subtypes in an attempt to harness the therapeutic potential of these receptors. We examine the possibility of intervening at these receptors by considering the specific example of absence seizures, a form of generalized, non-convulsive seizure that involves the thalamus. Views of the etiology of absence seizures have evolved over time from the "centrencephalic" concept of a diffuse subcortical pacemaker toward the "cortical focus" theory in which cortical hyperexcitability leads the thalamus into the 3-4 Hz rhythms that are characteristic of absence seizures. Since the cortex communicates with the thalamus via a massive glutamatergic projection, ionotropic glutamate receptor (iGluR) blockade has held promise, but the global nature of iGluR intervention has precluded the clinical effectiveness of drugs that block iGluRs. In contrast, mGluRs, because they modulate iGluRs at glutamatergic synapses only under certain conditions, may quell seizure activity by selectively reducing hyperactive glutamatergic synaptic communication within the cortex and thalamus without significantly affecting normal response rates. In this article, we review the circuitry and events leading to absence seizure generation within the corticothalamic network, we present a comprehensive review of the synaptic location and function of mGluRs within the thalamus and cerebral cortex, and review the current knowledge of mGluR modulation and seizure generation. We conclude by reviewing the potential advantages of Group II mGluRs, specifically mGluR2, in the treatment of both convulsive and non-convulsive seizures.

Unique presynaptic and postsynaptic roles of Group II metabotropic glutamate receptors in the modulation of thalamic network activity

The thalamic reticular nucleus (TRN) is a sheet of GABAergic neurons that project to other TRN neurons and to associated thalamocortical relay nuclei. The TRN receives glutamatergic synaptic inputs from cortex as well as reciprocal inputs from the collaterals of thalamocortical neurons. In addition to ionotropic glutamate receptors, metabotropic glutamate receptors (mGluRs) are present in the TRN circuitry. Using whole cell voltage clamp recordings, we pharmacologically characterized unique pre- and postsynaptic functions for Group II mGluRs (mGluR 2 and mGluR 3) within the TRN circuitry in ferrets. mGluR 2 was found on presynaptic cortical axon terminals in the TRN, where it reduced glutamate release, while mGluR 3 acted postsynaptically on TRN cells to increase membrane conductance. Using miniature inhibitory postsynaptic current analysis, we also found that picrotoxin-sensitive intra-TRN GABA-mediated neurotransmission was not affected by administration of a Group II mGluR agonist, indicating that neither mGluR 2 nor 3 acts on presynaptic GABA-containing terminals within the TRN. Because strong corticothalamic activation is implicated in abnormal thalamic rhythms, we used extracellular recordings in the lateral geniculate nucleus to study the effect of Group II mGluR agonists upon these slow oscillations. We induced approximately 3 Hz spike-and-wave discharge activity through corticothalamic stimulation, and found that such activity was reduced in the presence of the Group II mGluR agonist, (-)-2-oxa-4-aminobicyclo[3.1.0]hexane-4,6-dicarboxylate (LY379268). These data indicate that Group II mGluR reduce the impact of corticothalamic excitation, and that they may be a useful target in the reduction of absence-like rhythms.

Analgesic effects of the selective group II (mGlu2/3) metabotropic glutamate receptor agonists LY379268 and LY389795 in persistent and inflammatory pain models after acute and repeated dosing

Group II (mGluR2/3) metabotropic glutamate receptors have been implicated in the mechanisms of persistent pain states. In the present study, the effects of the selective group II metabotropic glutamate receptor agonists LY379268 and LY389795 were evaluated in the formalin test, carrageenan-induced thermal hyperalgesia and mechanical allodynia, and capsaicin-induced mechanical allodynia in rats. The agonists LY379268 and LY389795 produced dose-dependent decreases in formalin-induced behaviors that were antagonized by the mGlu2/3 receptor antagonist LY341495. The group II antagonist LY341495 produced parallel shifts in the LY379268 dose-response curve, consistent with a competitive antagonism. LY379268 decreased formalin-induced behaviors after intracisternal but not intrathecal administration, suggesting primarily a supraspinal site of action. Both LY379268 and LY389795 produced a dose-related reversal of carrageenan-induced thermal hyperalgesia and capsaicin-induced mechanical allodynia, but had no effect on carrageenan-induced mechanical allodynia. Both agonists also increased response latencies in the hot plate test, but were without effect in the tail-flick test. However, both agonists produced motor impairment on the inverted screen at doses that were analgesic. Moreover, tolerance to the analgesic effects of LY379268 developed after 4 days of once-daily repeated administration in the formalin, carrageenan, capsaicin and hot plate tests. The present findings indicate that group II (mGluR2/3) metabotropic glutamate receptors may be involved in the mechanisms of hyperalgesia and allodynia, however tolerance rapidly develops to these effects.

Presynaptic inhibition of corticothalamic feedback by metabotropic glutamate receptors

The thalamus relays sensory information to cortex, but this information may be influenced by excitatory feedback from cortical layer VI. The full importance of this feedback has only recently been explored, but among its possible functions are influences on the processing of sensory features, synchronization of thalamic firing, and transitions in response mode of thalamic relay cells. Uncontrolled, corticothalamic feedback has also been implicated in pathological thalamic rhythms associated with certain neurological disorders. We have found a form of presynaptic inhibition of corticothalamic synaptic transmission that is mediated by a Group II metabotropic glutamate receptor (mGluR) and activated by high-frequency corticothalamic activity. We tested putative retinogeniculate and corticogeniculate synapses for Group II mGluR modulation within the dorsal lateral geniculate nucleus of the ferret thalamus. Stimulation of optic-tract fibers elicited paired-pulse depression of excitatory postsynaptic currents (EPSCs), whereas stimulation of the optic radiations elicited paired-pulse facilitation. Paired-pulse responses were subsequently used to characterize the pathway of origin of stimulated synapses. Group II mGluR agonists (LY379268 and DCG-IV) applied to thalamic neurons under voltage-clamp conditions reduced the amplitude of corticogeniculate EPSCs. Stimulation with high-frequency trains produced a facilitating response that was reduced by Group II mGluR agonists, but was enhanced by the selective antagonist LY341495, revealing a presynaptic, mGluR-mediated reduction of high-frequency corticogeniculate feedback. Agonist treatment did not affect EPSCs from stimulation of the optic tract. NAAG (reported to be selective for mGluR3) was ineffective at the corticogeniculate synapse, implicating mGluR2 in the observed effects. Our data are the first to show a synaptically elicited form of presynaptic inhibition of corticothalamic synaptic transmission that is mediated by presynaptic action of mGluR2. This presynaptic inhibition may partially mute sensory feedback and prevent reentrant excitation from initiating abnormal thalamic rhythms.

The roles of different types of glutamate receptors involved in the mediation of nucleus submedius (Sm) glutamate-evoked antinociception in the rat

Based on our previous findings that glutamate microinjected into the thalamic nucleus submedius (Sm) inhibits dose-dependently the rat tail-flick (TF) reflex, this study investigated which glutamate receptor subtype is involved in mediating this effect. The effects of an NMDA (N-methyl-D-aspartate), non-NMDA or metabotropic glutamate receptor (mGluR) antagonist microinjected into Sm on the TF reflex were examined in untreated or in Sm glutamate treated (microinjection into the Sm) rats. The TF latencies were measured in each of these groups of rats every 5 min. Injection of DNQX [6,7-dinitroquinoxaline-2,3(1H,4H)-dione], a non-NMDA receptor antagonist, or (+/-)-MCPG [(+/-)-alpha-methyl-4-carboxyphenylglycine], a mGluR antagonist, into the Sm blocked the inhibitory effects induced by a subsequent microinjection of glutamate into the same Sm site. The TF latency increased only by 6.6+/-1.6 or 9.0+/-1.1%, respectively, of the baseline value, which was markedly less than that (51.3+/-8.4 or 50.7+/-5.3%) obtained from injection of glutamate only (P<0.001, n=8). However, pre-microinjection of MK-801 [(+)-5-methyl-10,11-dibenzo[a,d]cyclohepten-5,10-imine], an NMDA receptor antagonist, into the Sm had no effect on the Sm glutamate-evoked inhibition of the TF reflex. The TF latency change (40.0+/-11.1%) was not significantly different (P>0.05, n=8) compared with that obtained from glutamate injection alone. These observations suggest that non-NMDA and metabotropic glutamate receptors, but not NMDA receptors, are involved in mediating Sm glutamate-evoked antinociception.

Increased seizure susceptibility in mice lacking metabotropic glutamate receptor 7

To study the role of mGlu7 receptors (mGluR7), we used homologous recombination to generate mice lacking this metabotropic receptor subtype (mGluR7(-/-)). After the serendipitous discovery of a sensory stimulus-evoked epileptic phenotype, we tested two convulsant drugs, pentylenetetrazole (PTZ) and bicuculline. In animals aged 12 weeks and older, subthreshold doses of these drugs induced seizures in mGluR7(-/-), but not in mGluR7(+/-), mice. PTZ-induced seizures were inhibited by three standard anticonvulsant drugs, but not by the group III selective mGluR agonist (R,S)-4-phosphonophenylglycine (PPG). Consistent with the lack of signs of epileptic activity in the absence of specific stimuli, mGluR7(-/-) mice showed no major changes in synaptic properties in two slice preparations. However, slightly increased excitability was evident in hippocampal slices. In addition, there was slower recovery from frequency facilitation in cortical slices, suggesting a role for mGluR7 as a frequency-dependent regulator in presynaptic terminals. Our findings suggest that mGluR7 receptors have a unique role in regulating neuronal excitability and that these receptors may be a novel target for the development of anticonvulsant drugs.

Elevated levels of group-III metabotropic glutamate receptors in the inferior colliculus of genetically epilepsy-prone rats following intracollicular administration of L-serine-O-phosphate

The selective group-III metabotropic glutamate receptor agonist, L-serine-O-phosphate (L-SOP), when injected bilaterally into the inferior colliculus of the sound sensitive genetically epilepsy-prone (GEP) rats produces a short proconvulsant excitation followed by a long phase of protection against sound-induced seizures lasting for 2-4 days. We have studied this prolonged suppression of audiogenic seizures using pharmacological and molecular biological approaches including semiquantitative RT-PCR and western blotting. The intracerebroventricular injection of the protein synthesis inhibitor cycloheximide (120 microg) 30 min beforehand significantly reduces the proconvulsant seizure activity and the prolonged anticonvulsant effect of intracollicular L-SOP (500 nmol/side). The sensitive semiquantitative RT-PCR revealed a significant up-regulation in mGlu(4) and mGlu(7) mRNA levels in the inferior colliculus at 2 days (maximum suppression of audiogenic seizures) after intracollicular L-SOP injection compared with the non-injected, 2-day post-vehicle treated and 7-day (return to expressing audiogenic seizures) post-drug or vehicle-treated groups. No significant changes were observed in mGlu(6) or mGlu(8) mRNA expression levels in drug-treated compared with control groups. Examination of mGlu(4a) and mGlu(7a) protein levels using western blotting showed a significant increase in mGlu(7a) but no significant change in mGlu(4a) protein levels 2 days after L-SOP treatment compared with the control groups (non-injected and 2-day vehicle-injected group). These results suggest that up-regulation of mGlu(7) receptors is involved in the prolonged anticonvulsant effect of L-SOP against sound-induced seizures in GEP rats. The potential use of mGlu(7) agonists as novel anti-epileptic agents merits investigation.

Metabotropic and NMDA glutamate receptors participate in the cannabinoid-induced antinociception

The purpose of this study was to evaluate the possible contribution of metabotropic glutamate receptors (mGluRs) to cannabinoid-induced antinociception in the periaqueductal grey (PAG) matter of rats. Intra-PAG microinjection of WIN 55,212-2, a cannabinoid receptor agonist, increased the latency of the nociceptive reaction (NR) in a dose-dependent fashion in the plantar test. This effect was prevented by pretreatment with SR141716A, a selective antagonist of CB1 receptors. When injected alone, SR141716A produced, with the highest dosage used, a significant reduction in the latency of the NR. CPCCOEt, a selective mGlu1 receptor antagonist, was unable to prevent the analgesia produced by WIN 55,212-2. On the contrary, MPEP, a selective mGlu5 receptor antagonist, completely antagonized the effect of WIN 55,212-2. However, the analgesia induced by CHPG, a selective mGlu5 receptor agonist, was blocked by MPEP but not by SR141716A. When injected alone, CPCOOEt produced no effect, whereas MPEP produced, with the highest dosage used, a significant reduction in the latency of the NR. These data emphasize that mGlu5 receptors, but not mGluR1, may modulate nociception in the PAG. Similarly, a pretreatment with either 2-(S)-alpha-EGlu or (RS)-alpha-MSOP, selective antagonists for group II and III mGluRs, respectively, prevented the WIN 55,212-2-induced analgesia. When the higher dosage of (RS)-alpha-MSOP was used a decrease in the latency of the NR was observed. This was not the case for 2-(S)-alpha-EGlu. Pretreatment with DL-AP5, a selective antagonist of N-methyl-D-aspartate (NMDA) receptors, blocked the effect of WIN 55,212-2, and by increasing the dosage strongly reduced per se the latency of the NR. This study suggests that endogenous glutamate could tonically modulate nociception through mGlu and NMDA receptors in the PAG matter. In particular, the physiological stimulation of these receptors seems to be required for the cannabinoid-induced analgesia in this midbrain area.

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

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

The metabotropic glutamate receptor subtype mGluR 2/3 is located at extrasynaptic loci in rat spinal dorsal horn synapses

The position of neurotransmitter receptors relative to active neurotransmitter release sites may be a major factor influencing neuronal responses. The location of the metabotropic glutamate receptor subtype mGluR2/3 was investigated in synaptic structures in the rat superficial spinal dorsal horn laminae by using a pre-embedding immunogold technique. Immunostaining for mGluR2/3 occurred in laminae I through III. Gold particles were encountered both in the cytosol and along the plasma membrane. Distinctive plasmalemmal immunodeposits were detected in vesicle-containing profiles, where they were located to membrane compartments distant from active release sites rather than in the close vicinity of synaptic specialisations. No distinct immunolabelling was observed in profiles meeting characteristics of primary afferent terminals. The extrasynaptic occurrence of mGluR2/3 suggests a presynaptic heteroreceptor role for these receptor subtypes in the spinal dorsal horn.

Periaqueductal gray matter metabotropic glutamate receptors modulate formalin-induced nociception

The role played by periaqueductal gray (PAG) matter metabotropic glutamate receptors (mGluRs) in the modulation of persistent noxious stimulation was investigated in mice. The formalin test was used as a model of persistent pain. Intra-PAG microinjections of (S)-3, 5-DHPG (25 and 50 nmol/mouse) and L-CCG-I (30 and 60 nmol/mouse), agonists of group I and group II mGluRs, respectively, decreased the nociceptive response (-92+/-6% and -89+/-8%, respectively) during the late phase. No change of the early nociceptive phase was observed after (S)-3,5-DHPG or L-CCG-I treatments. These effects were antagonized by a pretreatment with CPCCOEt (40 nmol/mouse) and (2S)-alpha-EGlu (30 nmol/mouse). CPCCOEt is a selective antagonist of group I mGlu receptors, while (2S)-alpha-EGlu is an antagonist of group II. Intra-PAG microinjections of L-SOP (60 and 120 nmol/mouse), a selective agonist of group III mGluRs, induced an increase of the nociceptive response (+95+/-7%) during the late hyperalgesic phase. (R,S)-alpha-M-SOP (70 nmol/mouse), a selective antagonist of group III mGluRs, completely antagonized the L-SOP-induced effect. These results show that PAG mGluRs participate in modulating the late hyperalgesic behaviours induced by formalin. It seems, therefore, possible that group I and group II mGluRs positively modulate PAG antinociceptive descending pathway following a persistent noxious stimulation, while group III mGluRs modulate it negatively.

Simultaneous multi-site recordings and iontophoretic drug and dye applications along the trigeminal system of anesthetized rats

A multi-electrode system that permits simultaneous recordings from multiple neurons and iontophoretic applications at two or three different brain sites during acute experiments is described. This system consists of two or three microdrive terminals, each of which includes four electrodes that can be moved independently and used for both extracellular recordings and microiontophoretic drug administration. Drug applications were performed during standard extracellular recordings of multiple single-units via specialized combined electrodes (CEs), which enable ejection of neuroactive substances and recording of neuronal activity from the same electrode. With this system, we were able to successfully record simultaneously from different levels (brainstem, thalamus, and cortex) of the vibrissal ascending pathway of the anesthetized rat. Herein, examples of simultaneous recordings from the brainstem and thalamus and from the thalamus and cortex are presented. An effect of iontophoretic applications of agonists and antagonists of metabotropic glutamate receptors (mGluRs) in the thalamus is demonstrated, and the extent of drug diffusion in the barrel cortex is demonstrated with biocytin. This new multi-electrode system will facilitate the study of transformations of sensory information acquired by the whiskers into cortical representations.

Group I metabotropic glutamate receptors: implications for brain diseases

Glutamate is the major excitatory neurotransmitter in the brain and plays a unique role in a variety of central nervous system (CNS) functions. The discovery of the metabotropic receptors (mGluRs), a family of G-protein coupled receptors than can be activated by glutamate, has led to an impressive number of studies in recent years aimed at understanding their biochemical, physiological and pharmacological characteristics. The eight mGluRs now known are divided into three groups according to their sequence homology, signal transduction mechanisms, and agonist selectivity. Group I mGluRs include mGluR1 and mGluR5, which are linked to the activation of phospholipase C; Groups II and III include all others and are negatively coupled to adenylyl cyclases. The availability in recent years of agents selective for Group I mGluRs has made possible the study of the physiological roles of these receptors in the CNS. In addition to mediating glutamatergic neurotransmission, Group I mGluRs can modulate other neurotransmitter receptors, including GABA and the ionotropic glutamate receptors. Group I mGluRs are involved in many CNS functions and may participate in a variety of disorders such as pain, epilepsy, ischemia, and chronic neurodegenerative diseases. This class of receptor may provide important pharmacological therapeutic targets and elucidating its functions will be relevant to develop new treatments for neurological and psychiatric disorders in which glutamatergic neurotransmission is abnormally regulated. In this review anatomical, physiological and pharmacological results are presented with a special emphasis on the role of Group I mGluRs in functional and pathological processes.

Metabotropic glutamate receptors 2 and 3 expressed by astrocytes in rat ventrobasal thalamus

It has been reported that pharmacologic activation of metabotropic receptors mGluR2/3 may produce inhibition of GABAergic transmission in rat ventrobasal thalamus. We use double immunolabeling to show that: (i) mGluR2/3 is expressed in glial processes surrounding GABAergic terminals, rather than in the terminals themselves, and (ii) mR2/3 positive glial lamellae more frequently surround GABAergic terminals and ascending terminals, than cortical terminals. This suggests involvement of glia in glutamate/GABA interactions in the sensory thalamus.

The induction of pain: an integrative review

The highly disagreeable sensation of pain results from an extraordinarily complex and interactive series of mechanisms integrated at all levels of the neuroaxis, from the periphery, via the dorsal horn to higher cerebral structures. Pain is usually elicited by the activation of specific nociceptors ('nociceptive pain'). However, it may also result from injury to sensory fibres, or from damage to the CNS itself ('neuropathic pain'). Although acute and subchronic, nociceptive pain fulfils a warning role, chronic and/or severe nociceptive and neuropathic pain is maladaptive. Recent years have seen a progressive unravelling of the neuroanatomical circuits and cellular mechanisms underlying the induction of pain. In addition to familiar inflammatory mediators, such as prostaglandins and bradykinin, potentially-important, pronociceptive roles have been proposed for a variety of 'exotic' species, including protons, ATP, cytokines, neurotrophins (growth factors) and nitric oxide. Further, both in the periphery and in the CNS, non-neuronal glial and immunecompetent cells have been shown to play a modulatory role in the response to inflammation and injury, and in processes modifying nociception. In the dorsal horn of the spinal cord, wherein the primary processing of nociceptive information occurs, N-methyl-D-aspartate receptors are activated by glutamate released from nocisponsive afferent fibres. Their activation plays a key role in the induction of neuronal sensitization, a process underlying prolonged painful states. In addition, upon peripheral nerve injury, a reduction of inhibitory interneurone tone in the dorsal horn exacerbates sensitized states and further enhance nociception. As concerns the transfer of nociceptive information to the brain, several pathways other than the classical spinothalamic tract are of importance: for example, the postsynaptic dorsal column pathway. In discussing the roles of supraspinal structures in pain sensation, differences between its 'discriminative-sensory' and 'affective-cognitive' dimensions should be emphasized. The purpose of the present article is to provide a global account of mechanisms involved in the induction of pain. Particular attention is focused on cellular aspects and on the consequences of peripheral nerve injury. In the first part of the review, neuronal pathways for the transmission of nociceptive information from peripheral nerve terminals to the dorsal horn, and therefrom to higher centres, are outlined. This neuronal framework is then exploited for a consideration of peripheral, spinal and supraspinal mechanisms involved in the induction of pain by stimulation of peripheral nociceptors, by peripheral nerve injury and by damage to the CNS itself. Finally, a hypothesis is forwarded that neurotrophins may play an important role in central, adaptive mechanisms modulating nociception. An improved understanding of the origins of pain should facilitate the development of novel strategies for its more effective treatment.

Characterisation of mGluRs which modulate nociception in the PAG of the mouse

The contribution of metabotropic glutamate receptors (mGluRs) to the modulation of nociception by the periaqueductal gray (PAG) matter was investigated in mice. Intra-PAG microinjection of (IS,3R)-ACPD, an agonist of groups I and II mGluRs, as well as (S)-3,5-DHPG, a selective agonist of group I mGluRs, increased the latency of the nociceptive reaction (NR) in the hot plate test. (RS)-AIDA, an antagonist of group I mGluRs, antagonized the effect of (S)-3,5-DHPG, but changed the effect induced by (1S,3R)-ACPD in that a decrease in the latency for the NR could now be observed. L-CCG-I and L-SOP, which are agonists of groups II and III mGluRs respectively, decreased the latency of the NR. (2S)-alpha-EGlu and (RS)-alpha-MSOP, which are antagonists of groups II and III mGluRs, respectively, antagonized the effect of L-CCG-I and L-SOP. (RS)-AIDA and (RS)-alpha-MSOP alone decreased and increased, respectively, the latency of the NR with the highest doses used. (2S)-alpha-EGlu alone did not change significantly the latency of the NR. Intra-PAG microinjection of LH, an agonist of ionotropic glutamate receptors, induced a dose-dependent analgesia which was blocked by pretreatment with DL-AP5, a selective antagonist of NMDA receptors. No mGluRs antagonists were able to prevent LH-induced analgesia. These results emphasize the possible involvement of mGluRs in the modulation of nociception. It seems that activation of group I mGluRs potentiates, while groups II and III mGluRs decrease, the activity of the PAG for the modulation of nociception.

Differential expression of rat and human type I metabotropic glutamate receptor splice variant messenger RNAs

The type I metabotropic glutamate receptor (mGlu1) messenger RNA and protein are known to be widely expressed in rat brain, but knowledge of the regional expression of splice variants other than mGlu1a is limited. Probes were designed for in situ hybridization that specifically recognize each of the carboxy-terminal splice variants mGlu1a, -1b, -1c and -1d. The novel rat mGlu1d sequence was obtained by polymerase chain reaction and the predicted protein is highly homologous to the human sequence but contains both conservative and radical substitutions and is slightly longer (912 vs 908 amino acids). Each rat mGlu1 splice variant messenger RNA was found in a unique expression pattern. The messenger RNA encoding mGlu1a was abundant in cerebellar Purkinje cells and in mitral and tufted cells of the olfactory bulb. Strong expression was also detected in hippocampal interneurons, and neurons of the thalamus and substantia nigra, while moderate expression was found in colliculi and cerebellar granule cells. The mGlu1b messenger RNA was strongly expressed in Purkinje cells, hippocampal pyramidal neurons, dentate gyrus granule cells and lateral septum, and moderately expressed in striatal, superficial cortical and cerebellar granule neurons. The mGlu1d messenger RNA was expressed in all regions where mGlu1a and -1b were detected; abundant in Purkinje cells, mitral and tufted cells, and hippocampal principal neurons and interneurons, strong in thalamus and substantia nigra, and moderate in lateral septum, cortex, striatum and colliculi. Human mGlu1 splice variant expression in the cerebellum matched that found for the rat. No specific signal was found with a probe capable of hybridizing to the rat mGlu1c splice junction, although another probe designed against a more 3' sequence of mGlu1c gave strong signals in the cerebellum and hippocampus, and moderate signals in thalamus and colliculi. It is concluded that mGlu1d messenger RNA is widely expressed, that mGlu1a and -1b messenger RNAs are expressed in almost complementary patterns and that formation of the mGlu1c splice junction is a rare event.

Reduction of sensory and metabotropic glutamate receptor responses in the thalamus by the novel metabotropic glutamate receptor-1-selective antagonist S-2-methyl-4-carboxy-phenylglycine

Previous work has shown that responses of thalamic neurons in vivo to the metabotropic glutamate receptor agonists 1S,3R-aminocyclopentane-1,3-dicarboxylate and S-3,5-dihydroxyphenylglycine can be reduced by a variety of phenylglycine antagonists. Responses of thalamic neurons to noxious thermal somatosensory stimuli were reduced in parallel by these antagonists, indicating that these responses are mediated by Group I metabotropic glutamate receptors (i.e. metabotropic glutamate receptor-1 and/or metabotropic glutamate receptor-5), which are known to be linked to phosphoinositol phosphate hydrolysis. The recent development of S-2-methyl-4-carboxyphenylglycine as an antagonist which is highly selective for metabotropic glutamate receptor-1 compared to metabotropic glutamate receptor-5 on human receptors expressed in AV-12 cells, now offers the possibility of discriminating between these two receptor subtypes in order to distinguish which is involved in thalamic responses. We have made recordings from single somatosensory neurons in the thalamus of the rat, and find that S-2-methyl-4-carboxy-phenylglycine is able to reduce responses of neurons to 1S,3R-aminocyclopentane-1,3-dicarboxylate, S-3,5-dihydroxyphenylglycine, and noxious stimuli without significant effect on responses to either N-methyl-D-aspartate or (+/-)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate. These results suggest that excitatory responses of thalamic neurons to 1S,3R-aminocyclopentane-1,3-dicarboxylate and S-3,5-dihydroxyphenylglycine may be mediated by metabotropic glutamate receptor-1. Furthermore, the reduction of nociceptive responses by S-2-methyl-4-carboxy-phenylglycine indicates that metabotropic glutamate receptor-1 is involved in thalamic nociceptive processing and that such antagonists may have analgesic properties.

Glutamate-like immunoreactivity in ascending spinofugal afferents to the rat periaqueductal grey

The midbrain periaqueductal gray is a key structure for the mediation of an integrated defence behaviour. Although a prominent role for glutamate in PAG mechanisms is supported by both behavioural and morphological studies, whether PAG afferents conveying somatosensory information constitute a source of glutamatergic input to the PAG remains unknown. Here, we have compared the projection pattern of orthogradely-labelled spinoannular fibres with the distribution of glutamate-like immunoreactivity in the PAG at the light microscopic level. Transaxonal labelling was observed throughout the whole rostrocaudal axis of the PAG except for the dorsolateral regions. Cell-processes and terminal-reminiscent puncta were strongly immunoreactive in all PAG regions, including the dorsolateral areas. To ascertain whether glutamate-immunoreactive puncta observed at light microscopy indeed constituted axon terminals of the spinoannular system, glutamate-like immunoreactivity was assessed in orthogradely-labelled synaptic terminals using a post-embedding immunogold procedure for electron microscopy. Quantitative analysis of gold particle densities revealed over twice as strong an immunoreactivity in anatomically-identified spinoannular axon terminals as in dendrites postsynaptic to them, perikarya and inhibitory Gray II synapses, as well as an over 5-fold heavier immunolabelling than in glial profiles. These findings reveal that glutamate is accumulated in synaptic terminals of the spinoannular system, supporting a neurotransmitter role for this acidic amino acid in spinofugal afferents to the PAG.

Multiple pathways for regulation of the KCl-induced [3H]-GABA release by metabotropic glutamate receptors, in primary rat cortical cultures

In rat cortical primary cultures, group II- and III-metabotropic glutamate receptor-selective agonists concentration-dependently reduced KCl-induced [3H]GABA release, with IC50 values of 11 nM for LY354740, 80 nM for L(+)-2-amino-4-phosphonobutyric acid (L-AP4), 180 nM for DCG-IV, and 330 nM for L-SOP. The group II antagonists, LY341495 and EGLU, reversed the effect of LY354740, and the group III antagonist MTPG reversed the effect of L-AP4. In the presence of omega-conotoxin GVIA, LY354740 inhibited the remaining [3H]GABA release, whereas L-AP4 was inactive. In contrast, in the presence of nifedipine, L-AP4 inhibited the remaining [3H]GABA release, but LY354740 was no longer active. The PKA inhibitor, H89, blocked the effects of both L-AP4 and LY354740, whereas the PKC inhibitor Ro 31-8220 blocked only the effect of LY354740. Both Ro 31-8220 and H89 reduced the [3H]GABA release to 60% of control. In whole-cell, voltage-clamp experiments, LY354740 and L-AP4 inhibited voltage-gated calcium channel currents with IC50 values of 28 nM and 22 microM, respectively. The results suggest that, in these cells, KCl-induced [3H]GABA release is modulated by two different mechanisms, one involving group II receptors and a direct control of the Ca2+ channel activity, and the other mediated by group III receptors and possibly involving a regulation located downstream of the Ca2+ channel activation.

Modulation of excitatory synaptic transmission in locus coeruleus by multiple presynaptic metabotropic glutamate receptors

Metabotropic glutamate receptors have been implicated in modulation of synaptic transmission in many different systems. This study reports the effects of selective activation of metabotropic glutamate receptors on synaptic transmission in intracellularly recorded locus coeruleus neurons in brain slice preparations. Perfusion of either L-2-amino-4-phosphonobutyric acid (L-AP4; 0.1-500 microM) or (+/-)-1-aminocyclopentane-trans-1,3,dicarboxylic acid (t-ACPD; 0.1-500 microM) caused a depression of excitatory postsynaptic potentials in a dose-dependent fashion to about 70% inhibition. Both agonists exerted their effects at relatively low concentrations with estimated EC50s of 2.6 microM and 11.5 microM for L-AP4 and t-ACPD, respectively. This inhibition was not observed with the potent group I metabotropic glutamate receptor agonist (RS)-3,5-dihydroxyphenylglycine (DHPG; 100 microM). Conversely, (R)-4-carboxy-3-hydroxyphenyl-glycine (4C-3H-PG), a group I antagonist/group II agonist, and 2R,4R-4-aminopyrrolidine-2,4-dicarboxylate (APDC), a novel and specific group II agonist, also caused an inhibition of excitatory postsynaptic potentials. Both t-ACPD and L-AP4 produced an increase in paired-pulse facilitation, and failed to change the locus coeruleus response to focally applied glutamate, indicating a presynaptic locus of action. The L-AP4 inhibition was antagonized by (S)-amino-2-methyl-4-phosphonobutanoic acid (MAP4: group III antagonist) but not by (RS)-alpha-methyl-4-carboxyphenylglycine [(RS)-MCPG; mixed antagonist], suggesting that this agonist acts through a type 4 metabotropic glutamate receptor. Conversely, t-ACPD was antagonized by MCPG and by ethyl glutamate (group II antagonist), but not by aminoindan dicarboxylic acid (AIDA; group I antagonist) or MAP4, suggesting that this agonist acts on a type 2 or 3 metabotropic glutamate receptor. Taken together, these results suggest that two pharmacologically distinct presynaptic metabotropic glutamate receptors function in an additive fashion to inhibit excitatory synaptic transmission in locus coeruleus neurons. These receptors may be involved in a feedback mechanism and as such may function as autoreceptors for excitatory amino acids.

The L-AP4 receptor

1. The L-2-amino-4-phosphonobutyric acid (L-AP4) receptor was originally discovered by the ability of L-AP4 to depress synaptic transmission in hippocampal glutamatergic pathways and in the retina. 2. The molecular identity of the L-AP4 receptor is not yet resolved; however, with the molecular cloning of subtypes of metabotropic glutamate receptors (mGluRs), high affinity targets for L-AP4 have been identified. 3. As the information on the pharmacology of the mGluRs and the electrophysiological and biochemical studies on L-AP4 receptor physiology becomes elaborated it seems evident that the L-AP4 receptor is not a single molecular target but may involve multiple receptor subtypes.

Epileptogenesis in vivo enhances the sensitivity of inhibitory presynaptic metabotropic glutamate receptors in basolateral amygdala neurons in vitro

Modulation of excitatory synaptic transmission by presynaptic metabotropic glutamate receptors (mGluRs) was examined in brain slices from control rats and rats with amygdala-kindled seizures. Using whole-cell voltage-clamp and current-clamp recordings, this study shows for the first time that in control and kindled basolateral amygdala neurons, two pharmacologically distinct presynaptic mGluRs mediate depression of synaptic transmission. Moreover, in kindled neurons, agonists at either group II- or group III-like mGluRs exhibit a 28- to 30-fold increase in potency and suppress synaptically evoked bursting. The group II mGluR agonist (2S,3S,4S)-2-(carboxycyclopropyl)glycine (L-CCG) dose-dependently depressed monosynaptic EPSCs evoked by stimulation in the lateral amygdala with EC50 values of 36 nM (control) and 1.2 nM (kindled neurons). The group III mGluR agonist L-2-amino-4-phosphonobutyrate (L-AP4) was less potent, with EC50 values of 297 nM (control) and 10.8 nM (kindled neurons). The effects of L-CCG and L-AP4 were fully reversible. Neither L-CCG (0.0001-10 microM) nor L-AP4 (0.001-50 microM) caused membrane currents or changes in the current-voltage relationship. The novel mGluR antagonists (2S,3S,4S)-2-methyl-2-(carboxycyclopropyl)-glycine (MCCG; 100 microM) and (S)-2-methyl-2-amino-4-phosphonobutyrate (MAP4; 100 microM) selectively reversed the inhibition by L-CCG and L-AP4 to 81.3 +/- 12% and 65.3 +/- 6.6% of predrug, respectively. MCCG and MAP4 (100-300 microM) themselves did not significantly affect synaptic transmission. The exquisite sensitivity of agonists in the kindling model of epilepsy and the lack of evidence for endogenous receptor activation suggest that presynaptic group II- and group III-like mGluRs might be useful targets for suppression of excessive synaptic activation in neurological disorders such as epilepsy.