Cellular localization of a metabotropic glutamate receptor in rat brain

Regulation of main olfactory bulb mitral cell excitability by metabotropic glutamate receptor mGluR1

In the rodent main olfactory bulb (MOB), mitral cells (MCs) express high levels of the group I metabotropic glutamate receptor (mGluR) subtype, mGluR1. The significance of this receptor in modulating MC excitability is unknown. We investigated the physiological role of mGluR1 in regulating MC activity in rat and mouse MOB slices. The selective group I agonist (RS)-3,5-dihydroxyphenylglycine (DHPG), but not group II or III agonists, induced potent, dose-dependent, and reversible depolarization and increased firing of MCs. These effects persisted in the presence of blockers of fast synaptic transmission, indicating that they are due to direct activation of mGluRs on MCs. Voltage-clamp recordings showed that DHPG elicited a voltage-dependent inward current consisting of multiple components sensitive to potassium and calcium channel blockade and intracellular calcium chelation. MC excitatory responses to DHPG were absent in mGluR1 knockout mice but persisted in mGluR5 knockout mice. Broad-spectrum LY341495, MCPG, as well as preferential mGluR1 LY367385 antagonists blocked the excitatory effects of DHPG and also potently modulated MC spontaneous and olfactory nerve-evoked excitability. mGluR antagonists altered spontaneous membrane potential bistability, increasing the duration of the up and down states. mGluR antagonists also substantially attenuated MC responses to sensory input, decreasing the probability and increasing the latency of olfactory nerve-evoked spikes. These findings suggest that endogenous glutamate tonically modulates MC excitability and responsiveness to olfactory nerve input, and hence the operation of the MOB circuitry, via activation of mGluR1.

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

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

Somatosensory corticothalamic projections: distinguishing drivers from modulators

We used a juvenile mouse thalamocortical slice preparation with whole cell recording to investigate synaptic properties of corticothalamic inputs from somatosensory cortex to the ventral posterior medial and posterior medial nuclei (98 cells). We compared these data to those obtained from activating retinal and cortical inputs to the lateral geniculate nucleus (8 cells), the former representing a prototypical driver input and the latter, a typical modulator. Retinogeniculate activation evoked large, all-or-none excitatory postsynaptic potentials (EPSPs) that showed paired-pulse depression antagonized by N-methyl-d-aspartate (NMDA) and AMPA receptor blockers but with no sign of a metabotropic glutamate receptor (mGluR) component. Corticogeniculate activation evoked small, graded EPSPs showing paired-pulse facilitation, and the EPSPs showed both NMDA and AMPA receptor component plus an mGluR1 component. In the somatosensory thalamic relays, cortical stimulation elicited glutamatergic EPSPs in all thalamic cells, and these EPSPs fell into two groups. One, elicited from cortical layer 6 to cells of both nuclei, involved small, graded EPSPs with paired-pulse facilitation, and most also showed an mGluR1 component. The other, elicited from layer 5 to cells only of the posterior medial nucleus, involved large, all-or-none EPSPs with paired-pulse depression, and none showed an mGluR component. By analogy with results from the lateral geniculate nucleus, we conclude that the input from layer 6 to both nuclei acts as a modulator but that the layer 5 input to the posterior medial nucleus serves as a driver. Our data extend a common organizing principle from first-order nuclei to higher-order thalamic relays and further implicate the latter in corticocortical communication.

Metabotropic glutamate receptor 5 and dorsal raphe serotonin release in inflammatory pain in rat

In this study, we evaluated the effects of 2-methyl-6-(phenylethynyl)-pyridine (MPEP), a selective antagonist of metabotropic glutamate subtype 5 receptors (mGlu(5)), delivered through different paths on dorsal raphe serotonin (5-HT) and on thermoceptive responses in rats with inflammatory pain. Intraplantar formalin and carrageenan increased 5-HT (137+/-11% and 212+/-6% of pre-injection baseline, respectively) and reduced nociceptive threshold (23+/-7% and 19+/-3% of pre-injection baseline, respectively). MPEP (2 mg/kg i.p.) further enhanced formalin and carrageenan-induced 5-HT increases (180+/-11% and 260+/-12% of pre-injection baseline, respectively) and reduced thermal hyperalgesia (71+/-8% and 80+/-10% of pre-injection baseline, respectively). MPEP (1 mM) through microdialytic probe into the dorsal raphe did not change formalin- or carrageenan-induced 5-HT increases (147+/-10% and 189+/-10% of pre-injection baseline, respectively) and thermal hyperalgesia (35+/-8% and 25+/-9% of pre-injection baseline, respectively). Finally, MPEP (30 nmol/rat) into the hind paw reduced the formalin- and carrageenan-induced 5-HT increase (108+/-3% and 126+/-7% of pre-injection baseline, respectively) and thermal hyperalgesia (77+/-6% and 117+/-7% of pre-injection baseline, respectively). Dorsal raphe serotonergic neurons activity increased following a peripherally induced inflammatory injury. In these conditions, peripheral but not dorsal raphe mGlu(5) receptors blockade prevented over activation of dorsal raphe serotonergic neurons and reversed thermal hyperalgesia.

Group II and III metabotropic glutamate receptors and the control of the nucleus reticularis thalami input to rat thalamocortical neurones in vitro

Intracellular recordings were made from neurones in the thalamic reticular nucleus (TRN) and ventro-basal (VB) thalamus in slices of rat midbrain in vitro. Electrical stimulation of the medial lemniscus or TRN resulted in the generation of complex synaptic potentials containing disynaptic inhibitory post-synaptic potentials (IPSPs) in VB thalamocortical neurones. Analysis of the excitatory synaptic responses in TRN neurones indicates they can produce burst output response irrespective of the level of sub-threshold membrane potential. This suggests that network-evoked IPSPs in VB thalamocortical neurones occur following a burst of TRN action potentials. Using ionotropic glutamate receptor antagonists, the activation of these disynaptic events was blocked, and the monosynaptic IPSPs that resulted from the direct activation of the TRN could be isolated. The selective Group II agonists LY354740 (1-10 microM) and N-acetyl-aspartyl-glutamate (NAAG; 100-500 microM) both caused a reversible depression of these monosynaptic TRN IPSPs without any effect on membrane potential or input resistance. Likewise, the specific Group III agonist L-2-amino-4-phosphonobutanoate (10-500 microM), but not (RS)-4-phosphonophenylglycine (1 and 30 microM) also caused a reversible depression of these IPSPs, again without any effect on membrane potential or input resistance.Thus, the IPSPs recorded in VB thalamocortical neurones, evoked by TRN activation, can be depressed by the activation of either Group II or III metabotropic glutamate receptors. This is consistent with the location of these receptor types on the presynaptic terminals of TRN axons in the VB thalamus. This raises the possibility that, during periods of intense excitatory activity, glutamate release could influence the release of GABA from TRN axon terminals in the thalamus. In addition, as NAAG is located in the axons and terminals arising from the TRN, there is the possibility that this dipeptide is also released by these terminals to control the release of GABA during periods of high activity in the TRN.

In the amygdala anxiolytic action of mGlu5 receptors antagonist MPEP involves neuropeptide Y but not GABAA signaling

Several lines of evidence indicate that inhibition of the metabotropic glutamate (mGlu) receptor 5 produces anxiolytic-like effects in rodents. Peptide neurotransmitter neuropeptide Y (NPY) produces an anxiolytic effect in rats after intraventricular or intra-amygdalar administration. Many classes of anxiolytic drugs exert their effect through the GABA-benzodiazepine (BZD) receptor complex. Therefore, in the present study we have investigated whether the anxiolytic action of MPEP (2-methyl-6-(phenylethynyl)pyridyne), an mGlu5 receptor antagonist, is mediated by a mechanism involving either the GABA-BZD receptor complex or NPY receptor. In the behavioral studies, the anxiolytic activity of MPEP (10 mg/kg, i.p.) was examined using plus-maze test. The BZD antagonist flumazenil (10 mg/kg, i.p.) was given to one group of rats and Y1 receptor antagonist BIBO 3304 (((R)-N-[[4-(aminocarbonylaminomethyl) phenyl] methyl]-N2-(diphenylacetyl)-argininamide trifluoroacetate)3304) (200 pmol/site, intra-amygdala) to the other. It was found that anxiolytic effects of MPEP were not changed by flumazenil, but were abolished by BIBO 3304. Immunohistochemical studies showed a high density of mGlu5 receptor immunoreactivity (IR) in the amygdala. The effect of MPEP on NPY expression in the amygdala was studied using immunohistochemistry (IH) and radioimmunoassay (RIA). Both methods showed a diminution of NPY IR expression, to about 43% (IH) or 81% (RIA) of the control level after multiple administrations, but we observed an increase up to 148% of the control after single MPEP administration. These effects may suggest a release of NPY from nerve terminals after MPEP administration. Our results indicate that the anxiolytic action of MPEP is conveyed through NPY neurons with the involvement of Y1 receptors in the amygdala and that BZD receptors do not significantly contribute to these effects.

Synaptic Activation of Metabotropic Glutamate Receptors Regulates Dendritic Outputs of Thalamic Interneurons

Information gating through the thalamus is dependent on the output of thalamic relay neurons. These relay neurons receive convergent innervation from a number of sources, including GABA-containing interneurons that provide feed-forward inhibition. These interneurons are unique in that they have two distinct outputs: axonal and dendritic. In addition to conventional axonal outputs, these interneurons have presynaptic dendrites that may provide localized inhibitory influences. Our study indicates that synaptic activation of metabotropic glutamate receptors (mGluRs) increases inhibitory activity in relay neurons by increasing output of presynaptic dendrites of interneurons. Optic tract stimulation increases inhibitory activity in thalamic relay neurons in a frequency- and intensity-dependent manner and is attenuated by mGluR antagonists. Our data suggest that synaptic activation of mGluRs selectively alters dendritic output but not axonal output of thalamic interneurons. This mechanism could serve an important role in focal, feed-forward information processing in addition to dynamic information processing in thalamocortical circuits.

Immunolocalization of metabotropic glutamate receptor 1? (mGluR1?) in distinct classes of interneuron in the CA1 region of the rat hippocampus

In the hippocampal CA1 region, metabotropic glutamate subtype 1 (mGluR1) receptors have been implicated in a variety of physiological responses to glutamate, which include modulation of synaptic transmission and plasticity, as well as neuronal excitability and synchronization. The mGluR1alpha isoform is characteristically expressed only by nonprincipal cells, and it is particularly enriched in somatostatin (SS)-containing interneurons in stratum oriensalveus. Anatomical and physiological data have indicated the presence of mGluR1alpha in several distinct classes of interneurons with their somata located also in strata pyramidale, radiatum, and lacunosum moleculare. Each different interneuron subtype, as defined by functionally relevant criteria, including input/ output characteristics and expression of selective molecular markers, subserves distinct functions in local hippocampal circuits. We have investigated which of the different CA1 interneuron classes express mGluR1alpha by immunofluorescent labeling, combining antibodies to mGluR1alpha, calcium-binding proteins, and neuropeptides, and by intracellular labeling in vitro. Several types of interneuron that are immunopositive for mGluR1alpha each targeted different domains of pyramidal cells and included (1) O-LM inter-neurons, found to coexpress both SS and parvalbumin (PV); (2) interneurons with target selectivity for other interneurons, expressing vasoactive intestinal polypeptide (VIP) and/or the calcium-binding protein calretinin; (3) procholecystokinin-immunopositive interneurons probably non-basket and dendrite-targeting; and (4) an as-yet unidentified SS-immunoreactive but PV-immunonegative interneuron class, possibly corresponding to oriens-bistratified cells. Estimation of the relative proportion of mGluR1alpha-positive interneurons showed 43%, 46%, and 30% co-labeling with SS, VIP, or PV, respectively. The identification of the specific subclasses of CA1 interneurons expressing mGluR1alpha provides the network basis for assessing the contribution of this receptor to the excitability of the hippocampus.

Evidence that the metabotropic glutamate receptor 5 antagonist MPEP may act as an inhibitor of the norepinephrine transporter in vitro and in vivo

The mechanisms through which blockade of metabotropic glutamate receptors 5 (mGluR5) results in anxiolytic and antidepressant effects are currently unknown. In the present study, we therefore hypothesized that the anxiolytic- and antidepressant-like profile of the noncompetitive mGluR5 receptor antagonist 2-ethyl-6-(phenylethynyl)-pyridine (MPEP) may be mediated by inhibition of the norepinephrine transporter (NET). Accordingly, we first examined the potency of MPEP to bind to or inhibit uptake at the NET as well as the dopamine and serotonin transporters (DAT and SERT, respectively). We also examined the simultaneous in vivo effects of MPEP and desipramine (DMI) on both NE-like oxidation current in the amygdala (AMY) and cell firing in the locus coeruleus (LC) by means of differential pulse voltammetry (DPV) coupled with electrophysiology. MPEP completely displaced the binding of [3H]-nisoxetine on human NET with a pKi of 6.63 +/- 0.02. In addition, MPEP was able to inhibit [3H]-NE uptake in LLCPK cells expressing human NET, with a pIC50 of 5.55 +/- 0.09. In vivo DPV data revealed that both MPEP (30 mg/kg i.p.) and DMI (10 mg/kg i.p.) significantly increased NE-like voltammetric responses levels in the AMY, whereas both compounds also significantly decreased cell firing monitored concomitantly from the second microelectrode in the LC. Collectively, the results of the present study provide potential new mechanisms through which MPEP exerts its anxiolytic and antidepressant effects.

NCAM180 and glutamate receptor subtypes in potentiated spine synapses: an immunogold electron microscopic study

Activity-dependent changes in expression and localization of the largest major isoform of the neural cell adhesion molecule NCAM180 and three subtypes of glutamate receptors predominantly expressed in the outer part of the molecular layer of the dentate gyrus of adult rats-the NMDA receptor NR2A, the AMPA receptor GluR2/3, and the metabotropic glutamate receptor mGluR1 - were investigated using postembedding immunogold labeling, and electron microscopy. In synaptic membranes of nonstimulated spine synapses, NCAM180 and NR2A accumulated in the center of the postsynaptic density, whereas GluR2/3 and mGluR1 were distributed evenly. Twenty-four hours following induction of long-term potentiation in vivo, NCAM180 and NR2A accumulated at the edges of postsynaptic densities, whereas GluR2/3 was localized more centrally. Also, the distribution of gold particles per synapse significantly changed for NCAM180, NR2A, and mGluR1. Thus, changes in synaptic strength are associated with concomitant changes in the expression and distribution of NCAM180 and glutamate receptors, particularly of the NR2A subtype.

Reversibility of cisternal stack formation during hypoxic hypoxia and subsequent reoxygenation in cerebellar Purkinje cells

Cisternal stacks are induced during hypoxia, which may be associated with intracellular Ca2+ regulation. Although neurons are divided internally in different compartments, little is known about regional differences in cisternal stack formation. We investigated the effects of hypoxic hypoxia and later reoxygenation on cisternal stack formation and other ultrastructual changes in the proximal dendrite, dendritic spine, and cell body of cerebellar Purkinje cells in rats. After brief hypoxic events, cisternal stacks appeared predominantly in the proximal dendrites and after longer hypoxic events in dendritic spines and cell body. Following reoxygenation, cisternal stacks disappeared first in the cell body, followed by the dendritic spines, then the proximal dendrites. These results showed that stack formation occurred at different degrees and time courses among the three regions, and the effect was reversible, which suggests that these compartments are differentially sensitive to hypoxia.

Distribution of mGluR1? and mGluR5 immunolabeling in primate prefrontal cortex

Metabotropic glutamate receptors (mGluRs) mediate important modulatory glutamatergic influences throughout the brain. However, the specific localization and functions of group I mGluR subtypes (mGluR1alpha and mGluR5) in cortical neurotransmission are not well known, particularly in primates. To address this issue, we used immunoelectron microscopy to compare the subcellular localizations of mGluR1alpha and mGluR5 in the prefrontal cortex of macaque monkeys. Both receptor subtypes were found in a variety of subcellular compartments, including spines, dendrites, preterminal axons, axon terminals, and glia; however, quantitative differences were found in the relative abundance of labeled elements for each receptor. The mGluR1alpha-immunoreactive (-IR) elements were overwhelmingly the spines and dendrites, with labeled terminals, axons, and glia seen more rarely. The mGluR5-IR elements were also mostly spines and dendrites, but the proportion of labeled unmyelinated axons, terminals, and glia was higher than for mGluR1alpha-IR elements. Double labeling with SMI-32 and parvalbumin confirmed that both receptors were found in pyramidal cell and interneuron dendrites. The localization of mGluR1alpha to pyramidal cells in primate cortex contrasts with reports that mGluR1alpha is found almost exclusively in interneurons in rodent cortex. By using double labeling, we found no evidence for mGluR1alpha or mGluR5 in dopaminergic afferents to prefrontal cortex. The data presented here provide an anatomical substrate for a differential role of mGluR1alpha and mGluR5 in post-and presynaptic actions of glutamate in primate prefrontal cortex. They further suggest differences in the cortical distribution of group I mGluRs between primates and rodents.

Calbindin in cerebellar Purkinje cells is a critical determinant of the precision of motor coordination

Long-term depression (LTD) of Purkinje cell-parallel fiber synaptic transmission is a critical determinant of normal cerebellar function. Impairment of LTD through, for example, disruption of the metabotropic glutamate receptor-IP3-calcium signaling cascade in mutant mice results in severe deficits of both synaptic transmission and cerebellar motor control. Here, we demonstrate that selective genetic deletion of the calcium-binding protein calbindin D-28k (calbindin) from cerebellar Purkinje cells results in distinctly different cellular and behavioral alterations. These mutants display marked permanent deficits of motor coordination and sensory processing. This occurs in the absence of alterations in a form of LTD implicated in the control of behavior. Analysis of synaptically evoked calcium transients in spines and dendrites of Purkinje cells demonstrated an alteration of time course and amplitude of fast calcium transients after parallel or climbing fiber stimulation. By contrast, the delayed metabotropic glutamate receptor-mediated calcium transients were normal. Our results reveal a unique role of Purkinje cell calbindin in a specific form of motor control and suggest that rapid calcium buffering may directly control behaviorally relevant neuronal signal integration.

[3H]R214127: a novel high-affinity radioligand for the mGlu1 receptor reveals a common binding site shared by multiple allosteric antagonists

R214127 was shown to be a potent and noncompetitive metabotropic glutamate 1 (mGlu1) receptor-selective antagonist. The kinetics and pharmacology of [(3)H]1-(3,4-dihydro-2H-pyrano[2,3-b]quinolin-7-yl)-2-phenyl-1-ethanone (R214127) binding to rat mGlu1a receptor Chinese hamster ovary (CHO)-dhfr(-) membranes was investigated, as well as the distribution of [(3)H]R214127 binding in rat brain tissue and sections. Specific binding to rat mGlu1a receptor CHO-dhfr(-) membranes was approximately 92% of total and was optimal at 4 degrees C. Full association was reached within 5 min, and [(3)H]R214127 bound to a single binding site with an apparent K(D) of 0.90 +/- 0.14 nM and a B(max) of 6512 +/- 1501 fmol/mg of protein. Inhibition experiments showed that [(3)H]R214127 binding was completely blocked by 2-quinoxaline-carboxamide-N-adamantan-1-yl (NPS 2390), (3aS,6aS)-6a-naphtalan-2-ylmethyl-5-methyliden-hexahydro-cyclopenta[c]furan-1-on (BAY 36-7620), and 7-(hydroxyimino)cyclo-propa[b]chromen-1a-carboxylate ethyl ester (CPCCOEt), but was not displaced by competitive mGlu1 receptor ligands such as glutamate and quisqualate, suggesting that R214127, NPS 2390, BAY 36-7620, and CPCCOEt bind to the same site or mutually exclusive sites. Experiments using rat cortex, striatum, hippocampus and cerebellum revealed that [(3)H]R214127 labeled a single high-affinity binding site (K(D) approximately 1 nM). B(max) values were highest in the cerebellum (4302 +/- 2042 fmol/mg of protein) and were 741 +/- 48, 688 +/- 125, and 471 +/- 68 fmol/mg of protein in the striatum, hippocampus, and cortex, respectively. The distribution of [(3)H]R214127 binding in rat brain was investigated in more detail by radioligand autoradiography. A high density of binding sites was detected in the molecular layer of the cerebellum. Moderate labeling was seen in the CA3 and dentate gyrus of the hippocampus, thalamus, olfactory tubercle, amygdala, and substantia nigra reticulata. The cerebral cortex, caudate putamen, ventral pallidum, and nucleus accumbens showed lower labeling. The high affinity and selectivity of [(3)H]R214127 for mGlu1 receptors renders this compound the ligand of choice to study the native mGlu1 receptor in brain.

Neuroadaptive processes in GABAergic and glutamatergic systems in benzodiazepine dependence

Knowledge of the neural mechanisms underlying the development of benzodiazepine (BZ) dependence remains incomplete. The gamma-aminobutyric acid (GABA(A)) receptor, being the main locus of BZ action, has been the main focus to date in studies performed to elucidate the neuroadaptive processes underlying BZ tolerance and withdrawal in preclinical studies. Despite this intensive effort, however, no clear consensus has been reached on the exact contribution of neuroadaptive processes at the level of the GABA(A) receptor to the development of BZ tolerance and withdrawal. It is likely that changes at the level of this receptor are inadequate in themselves as an explanation of these neuroadaptive processes and that neuroadaptations in other receptor systems are important in the development of BZ dependence. In particular, it has been hypothesised that as part of compensatory mechanisms to diazepam-induced chronic enhancement of GABAergic inhibition, excitatory mechanisms (including the glutamatergic system) become more sensitive [Behav. Pharmacol. 6 (1995) 425], conceivably contributing to BZ tolerance development and/or expression of withdrawal symptoms on cessation of treatment, including increased anxiety and seizure activity. Glutamate is a key candidate for changes in excitatory transmission mechanisms and BZ dependence, (1) since there are defined neuroanatomical relationships between glutamatergic and GABAergic neurons in the CNS and (2) because of the pivotal role of glutamatergic neurotransmission in mediating many forms of synaptic plasticity in the CNS, such as long-term potentiation and kindling events. Thus, it is highly possible that glutamatergic processes are also involved in the neuroadaptive processes in drug dependence, which can conceivably be considered as a form of synaptic plasticity. This review provides an overview of studies investigating changes in the GABAergic and glutamatergic systems in the brain associated with BZ dependence, with particular attention to the possible differential involvement of N-methyl-D-aspartate and alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptors in these processes.

Neuron-to-glia signaling mediated by excitatory amino acid receptors regulates ErbB receptor function in astroglial cells of the neuroendocrine brain

Hypothalamic astroglial erbB tyrosine kinase receptors are required for the timely initiation of mammalian puberty. Ligand-dependent activation of these receptors sets in motion a glia-to-neuron signaling pathway that prompts the secretion of luteinizing hormone-releasing hormone (LHRH), the neuropeptide controlling sexual development, from hypothalamic neuroendocrine neurons. The neuronal systems that may regulate this growth factor-mediated back signaling to neuroendocrine neurons have not been identified. Here we demonstrate that hypothalamic astrocytes contain metabotropic receptors of the metabotropic glutamate receptor 5 subtype and the AMPA receptor subunits glutamate receptor 2 (GluR2) and GluR3. As in excitatory synapses, these receptors are in physical association with their respective interacting/clustering proteins Homer and PICK1. In addition, they are associated with erbB-1 and erbB-4 receptors. Concomitant activation of astroglial metabotropic and AMPA receptors results in the recruitment of erbB tyrosine kinase receptors and their respective ligands to the glial cell membrane, transactivation of erbB receptors via a mechanism requiring metalloproteinase activity, and increased erbB receptor gene expression. By facilitating erbB-dependent signaling and promoting erbB receptor gene expression in astrocytes, a neuron-to-glia glutamatergic pathway may represent a basic cell-cell communication mechanism used by the neuroendocrine brain to coordinate the facilitatory transsynaptic and astroglial input to LHRH neurons during sexual development.

Pedunculopontine tegmental stimulation evokes striatal dopamine efflux by activation of acetylcholine and glutamate receptors in the midbrain and pons of the rat

The pedunculopontine tegmental nucleus appears to influence striatal dopamine activity via cholinergic and glutamatergic afferents to dopaminergic cells of the substantia nigra pars compacta. We measured changes in striatal dopamine oxidation current (dopamine efflux) in response to electrical stimulation of the pedunculopontine tegmental nucleus using in vivo electrochemistry in urethane-anaesthetized rats. Pedunculopontine tegmental nucleus stimulation evoked a three-component change in striatal dopamine efflux, consisting of: (i) an initial rapid increase of 2 min duration; followed by (ii) a decrease below prestimulation levels of 9 min duration; then by (iii) a prolonged increase lasting 35 min. Intra-nigral infusions of the ionotropic glutamate receptor antagonist kynurenate (10 microg/ microL) or the nicotinic cholinergic receptor antagonist mecamylamine (5 microg/0.5 microL) selectively attenuated the rapid first component, while systemic injections of the muscarinic cholinergic antagonist scopolamine (5 mg/kg, i.p.) diminished the second and third components. In addition, intra-pedunculopontine tegmental nucleus infusions of the M2 muscarinic antagonist methoctramine (50 microg/ microL) selectively abolished the inhibitory second component, while intranigral infusions of scopolamine (200 microg/ microL) selectively abolished the prolonged third component. Intra-nigral infusions of the metabotropic glutamate receptor antagonist (+)-alpha-methyl-4-carboxyphenylglycine (2 microg/ microL) had no effect on pedunculopontine tegmental nucleus-elicited striatal dopamine efflux. These results suggest that the pedunculopontine tegmental nucleus utilizes nicotinic and ionotropic glutamate receptors in the substantia nigra to mediate rapid activation, M2-like muscarinic autoreceptors in the pedunculopontine tegmental nucleus to mediate decreased activation, and muscarinic receptors in the substantia nigra (probably of the M5 subtype) to mediate prolonged activation, of the nigrostriatal dopaminergic system.

Glutamate receptor functions in sensory relay in the thalamus

It is known that glutamate is a major excitatory transmitter of sensory and cortical afferents to the thalamus. These actions are mediated via several distinct receptors with postsynaptic excitatory effects predominantly mediated by ionotropic receptors of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) and N-methyl-D-aspartate varieties (NMDA). However, there are also other kinds of glutamate receptor present in the thalamus, notably the metabotropic and kainate types, and these may have more complex or subtle roles in sensory transmission. This paper describes recent electrophysiological experiments done in vitro and in vivo which aim to determine how the metabotropic and kainate receptor types can influence transmission through the sensory thalamic relay. A particular focus will be how such mechanisms might operate under physiological conditions.

Morphological study of organotypic cerebellar cultures

Organotypic cerebellar cultures from 8-days-old (P8) mouse pups were studied following 11 days of in vitro (I IDIV) culturing. The cerebellar cytoarchitectonic structure was maintained in most parasagittal cerebellar cortical slice cultures (also containing the deep cerebellar nuclei). The two main extrinsic excitatory inputs (the climbing and the mossy fibers) seem to be replaced by other axonal types: in the molecular layer mostly by parallel fibers (for climbing fibers) and in the granular layer by intrinsic mossy fiber collaterals of local excitatory interneurons, the unipolar brush cells. However, in a few organotypic cultures, which (although preserving the trilaminar cerebellar cortical structure) were "granuloprival" but also contained some of the deep cerebellar nuclei, the participation of extracortical axons from the deep cerebellar nuclei in the replacement of the missing afferents is suggested.

[3H]Methoxymethyl-3-[(2-methyl-1,3-thiazol-4-yl)ethynyl]pyridine binding to metabotropic glutamate receptor subtype 5 in rodent brain: in vitro and in vivo characterization

The binding of [3H]methoxymethyl-3-[(2-methyl-1,3-thiazol-4-yl)ethynyl]pyridine (methoxymethyl-MTEP), a potent and selective antagonist for metabotropic glutamate (mGlu)5 receptors, was characterized in rat brain both in vitro and in vivo. Nonspecific binding, as defined with 10 microM 2-methyl-6-(phenylethynyl)-pyridine (MPEP), was less than 10% of total binding in rat brain membranes. The binding of [3H]methoxymethyl-MTEP was of high affinity (K(d) = 20 +/- 2.7 nM), saturable (B(max) = 487 +/- 48 fmol/mg protein), and to a single site. The mGlu5 antagonists methoxymethyl-MTEP and MPEP displaced [3H]methoxymethyl-MTEP binding with IC50 values of 30 and 15 nM, respectively. In vivo administration of [3H]methoxymethyl-MTEP (50 microCi/kg i.v.) revealed 12-fold higher binding in hippocampus (an area enriched in mGlu5 receptors) relative to cerebellum (an area with few mGlu5 receptors) in rats. Similarly, administration of [3H]methoxymethyl-MTEP to mGlu5-deficient mice demonstrated binding at background levels in forebrain, whereas wild-type littermates exhibited 17-fold higher binding in forebrain relative to cerebellum. Systemic administration of unlabeled mGlu5 antagonists methoxymethyl-MTEP and MPEP to rats reduced the binding of [3H]methoxymethyl-MTEP with ID50 values of 0.8 and 2 mg/kg i.p., respectively, 1 h post-treatment. The mGlu5 agonist 2-chloro-5-hydroxyphenylglycine (CHPG) (0.3, 1, and 3 micromol) dose-dependently increased phosphoinositide (PI) hydrolysis in the hippocampus after i.c.v. administration in rats. CHPG-evoked increases in PI hydrolysis were blocked with MPEP at a dose (10 mg/kg i.p.) that markedly reduced [3H]methoxymethyl-MTEP binding in vivo. These results indicate that [3H]methoxymethyl-MTEP is a selective radioligand for labeling mGlu5 and is useful for studying the binding of mGlu5 receptors in rat brain in vitro and in vivo.

Metabotropic glutamate receptor involvement in phosphoinositide hydrolysis stimulation by an endogenous Na(+), K(+)-ATPase inhibitor and ouabain in neonatal rat brain

The mechanism of action of an endogenous Na(+), K(+)-ATPase inhibitor, termed endobain E, on phosphoinositide hydrolysis was studied in neonatal rat brain cortex and compared with that of ouabain. Lack of additivity for endobain E and glutamate paired stimulation on inositol phosphates accumulation suggested that they share at least a common step on inositol phosphate metabolism, as previously advanced for ouabain. In addition, Cd(2+) sensitivity of endobain E and ouabain effects strengthened the involvement of glutamate receptors. The participation of ionotropic glutamate receptors on endobain E- and ouabain-induced phosphoinositide hydrolysis seems untenable, since antagonists dizocilpine and CNQX proved unable to inhibit these effects. However, the endobain E effect was blocked by 2 x 10 (-4) M L-AP3 (an antagonist for group I mGluRs) when at least a 15-min preincubation protocol was employed. Maximal inhibition of endobain E effect (42%) occurred when L-AP3 preincubation was extended to 60 min, as already shown with glutamate, but only a trend to decrease was recorded with ouabain. At variance, the ouabain effect was reduced to 50% employing 5 x 10 (-4) M MCPG (a competitive antagonist for group I mGluRs), whereas no blockade was observed with endobain E or glutamate. In addition, MPEP (a selective mGluR5 antagonist) partially reduced ouabain, endobain E and glutamate responses and the selective mGluR1 antagonist LY367385 showed no activity at all. To sum up, the present findings support the involvement of mGluR5 in both endobain E and ouabain phosphoinositide hydrolysis stimulation in neonatal rat brain, in spite of dissimilar response to tested antagonists.

Different metabotropic glutamate receptors play opposite roles in synaptic plasticity of the rat medial vestibular nuclei

In the medial vestibular nuclei (MVN) of rat brainstem slices, the role of group II and III metabotropic glutamate receptors (mGluRs) and of the subtypes of group I mGluRs: mGluR1, mGluR5, was investigated in basal synaptic transmission and in the induction and maintenance of long-term potentiation (LTP). We used selective antagonists and agonists for mGluRs and we analysed the field potentials evoked by vestibular afferent stimulation before and after high-frequency stimulation (HFS) to induce LTP. The group II and III mGluR antagonist, (R,S)-alpha-2-methyl-4sulphonophenylglycine (MSPG), induced LTP per se and caused a reduction of the paired-pulse facilitation (PPF) ratio indicating an enhancement of glutamate release. This suggests that group II and III mGluRs are activated under basal conditions to limit glutamate release. Both the group II and III mGluR selective antagonists, 2S-2-amino-2-(1S,2S-2-carboxycycloprop-1-yl)-3-(xanth-9-yl)propanoate (LY341495) and (R,S)-alpha-methylserine-O-phosphate (MSOP), induced LTP, and the selective agonists, (2R,4R)-4-aminopyrrolidine-2,4-dicarboxylate (APDC) and L(+)-2-amino-4-phosphonobutyric acid (L-AP4) depressed the field potentials and prevented HFS-LTP, with a prevailing contribution of group II mGluRs over that of group III mGluRs. The mGluR1 antagonist, 7-(hydroxyimino)cyclopropa[b]chromen-1a-carboxylate ethyl ester (CPCCOEt) prevented the full development and maintenance of HFS-LTP. By contrast, the mGluR5 antagonist, 2-methyl-6-phenylethynylpyridine (MPEP) induced LTP per se, which was impeded by CPCCOEt, and it had no effect on LTP once induced by HFS. The PPF analysis showed an enhancement of glutamate release during MPEP potentiation. The group I mGluR agonist, (R,S)-3,5-dihydroxyphenylglycine (DHPG) induced LTP per se, which was blocked by CPCCOEt. By contrast the mGluR5 agonist, (R,S)-2-chloro-5-hydroxypheylglycine (CHPG) prevented LTP elicited by HFS and DHPG as well. In conclusion vestibular LTP is inhibited by group II and III mGluRs during the early induction phase while it is facilitated by mGluR1 for achieving its full expression and consolidation. An additional inhibitory control is exerted by mGluR5 at the level of this facilitatory phase.

Developmental regulation of metabotropic glutamate receptor 5b protein in rodent brain

The metabotropic glutamate receptor type 5 (mGlu5) is expressed in two splice variants, mGlu5a and mGlu5b, which differ in that mGlu5b has a 33-amino acid insert in the intracellular C-terminal domain. This receptor subtype is highly regulated, with higher levels found in developing animals, but the contributions of the individual splice variants to the receptor population at any time are unknown. An antibody that specifically reacts with the insert was developed and used to measure the regional and developmental distribution of mGlu5b in the mouse and rat brain. In contrast to total mGlu5 receptor protein, most brain regions exhibit a less than two-fold alteration between post-natal day 7 and adult levels of mGlu5b. In the adult cortex, there is a three-fold increase of mGlu5b protein relative to at post-natal day 7. Estimates of mGlu5a protein indicate that most of the developmental alteration in total mGlu5 is due to changes in expression of this variant. Comparison of mGlu5b protein and mRNA levels indicates that greatly different post-transcriptional regulation occurs across brain regions. These results indicate that mGlu5 expression is precisely and complexly controlled at the level of transcription and that different functions of mGlu5 during different developmental periods and in distinct regions are likely mediated by different splice variants.

Attenuation of morphine tolerance after antisense oligonucleotide knock-down of spinal mGluR1

1. Chronic systemic treatment of rats with morphine leads to the development of opioid tolerance. This study was designed to examine the effects of intrathecal (i.t.) infusion of a metabotropic glutamate receptor 1 (mGluR1) antisense oligonucleotide, concomitant with chronic morphine treatment, on the development of tolerance to morphine's antinociceptive effects. 2. All rats received chronic (6 day) s.c. administration of morphine to induce opioid tolerance. Additionally, rats were treated with either mGluR1 antisense (AS), missense (MIS) or artificial cerebrospinal fluid (ACSF) by i.t. infusion via chronically implanted i.t. catheters connected to osmotic mini-pumps. The effects of acute i.t. or s.c. morphine on tail-flick latencies were assessed prior to and following chronic s.c. morphine treatment for all chronic i.t. infusion groups. mGluR1 protein level in the spinal cord was determined by Western blot analysis for all treatments, assessing the efficiency of knock-down with AS treatment. 3. Acute i.t. morphine dose-dependently produced antinociception in the tail-flick test in naïve rats. Systemic morphine-treated rats administered i.t. ACSF or MIS developed tolerance to i.t. morphine. Chronic i.t. infusion with mGluR1 AS significantly reduced the development of tolerance to i.t. morphine. 4. In contrast to i.t. morphine, tolerance developed to the antinociceptive effects of s.c. morphine, in all i.t. infusion groups, including the mGluR1 AS group. 5. The spinal mGluR1 protein level was dramatically decreased after mGluR1 AS infusion when compared to control animals (naïve and ACSF-treated animals). 6. These findings suggest that the spinal mGluR1 is involved in the development of tolerance to the antinociceptive effects of morphine. Selective blockade of mGluR1 may be beneficial in preventing the development of opioid analgesic tolerance.

Group I metabotropic glutamate receptors (mGluRs) modulate visual responses in the superficial superior colliculus of the rat

Group I metabotropic glutamate receptors (mGluRs) are expressed in cells in the superficial layers of the rat superior colliculus (SSC) and SSC afferents. The purpose of this study was to investigate the physiological effect of Group I mGluR activation on visual responses of SSC neurones using both in vivo and in vitro techniques. In the in vivo preparation, agonists and antagonists were applied by iontophoresis and single neurone activity was recorded extracellularly in anaesthetised rats. Application of the Group I agonist (S)-3,5-dihydroxyphenylglycine (DHPG) resulted in a reversible inhibition of the visual response. The effect of DHPG could be blocked by concurrent application of the Group I (mGluR1/mGluR5) antagonist (S)-4-carboxyphenylglycine (4CPG) or mGluR1 antagonist (+)-2-methyl-4-carboxyphenylglycine (LY367385). Application of 4CPG alone resulted in a facilitation of the visual response and this effect was not changed when the visual stimulus contrast was varied. Response habituation was observed when visual stimuli were presented at 0.5 s intervals, but this was not affected by DHPG or 4CPG. In slices of the superior colliculus, stimulation of the optic tract resulted in a field EPSP recorded from the SSC whose duration was increased in the presence of the GABA antagonists picrotoxin and CGP55845. Application of DHPG (5-100 microM) reduced the field EPSP, and this effect could be reversed by the mGluR1 antagonist LY367385 (200 microM), but not by the mGluR5 antagonist MPEP (5 microM). These data show that activation of mGluR1, but probably not mGluR5, can modulate visual responses of SSC neurones in vivo, and that this could be via presynaptic inhibition of glutamate release from either retinal or, possibly, cortical afferents.

Effect of chronic imipramine or electroconvulsive shock on the expression of mGluR1a and mGluR5a immunoreactivity in rat brain hippocampus

Previous studies showed that chronic electroconvulsive shock (ECS) or imipramine treatment induced a subsensitivity of group I metabotropic glutamate receptors (mGluR) in hippocampus. In the present study effects of antidepressant treatment on the expression of mGluR1a and mGluR5a, belonging to the group I mGluR, were investigated in rat brain hippocampus using immunohistochemical and Western blot methods, respectively. Male Wistar rats were treated singly or chronically for 21 days with imipramine, 10 mg/kg, twice daily; with ECS (90 mA, 50 Hz, 0.5 s) every second day; or with haloperidol, 1.2 mg/kg, once daily. Appropriate controls were injected with saline. Rats were sacrificed 24 h after the last treatment and their hippocampi were taken out for analysis. It was found that the mGluR1a-immunoreactivity expression increased significantly in Ammon's horn (CA) regions after chronic ECS. The most pronounced effect was observed in the CA3. No significant effects were found after single treatment or after haloperidol. The expression of mGluR5a increased significantly after chronic imipramine in the CA1 and after chronic ECS in the CA3 region. The results obtained indicate an influence of antidepressant treatment on group I mGluR. This increase in the receptor protein level may be a compensatory mechanism developing after chronic treatment.

Reversal of neuronal polarity characterized by conversion of dendrites into axons in neonatal rat cortical neurons in vitro

The mechanisms for the establishment and maintenance of cell polarity in neurons are not well understood. Axon regeneration from dendrites has been reported after axotomy near the cell body in vivo. We report here in vitro a reversal of neuronal polarity characterized by the conversion of dendrites into axons. We isolated neurons from the neonatal rat cerebral cortex. Neurons that exhibited an apical dendrite with a length of >100 microm were monitored for 3 days in culture. In 66% of neurons examined, a new axon, as identified by reactivity with an antibody to dephosphorylated tau or by lack of reactivity with an antibody to the a and b isoforms of microtubule-associated protein 2, appeared to form from the tip of the original dendrite. Further analysis of such neurons revealed that the distal half of the original dendrite became positive for dephosphorylated tau or negative for microtubule-associated protein 2. Time-lapse video microscopy demonstrated the conversion of the original dendrite into an axon without dendritic retraction. Axon regeneration from dendritic tips required a significantly longer time than axon regeneration from minor processes. Our observations thus demonstrate in vitro a time-consuming reversal of neuronal polarity and the conversion of a dendritic cytoskeleton into an axonal one.

Distribution of metabotropic glutamate receptors in the superior colliculus of the adult rat, ferret and cat

The distribution of different metabotropic glutamate receptors (mGluRs 1a, 1b, 1c, 2/3, 4 and 5) has been compared in the superior colliculus of the rat, cat and ferret using immunohistochemical techniques and light microscopy. We found that although there are differences in labelling patterns between the species, there are also substantial similarities. In general, there was only light staining for the various mGluR1 splice variants, whereas labelling for the other Group I receptor, mGluR5, was heavier and with a pattern which suggested that at least some label arose from retinal afferents to the superficial superior colliculus. A further consistent feature in all species was labelling of astrocytes in the optic nerve/optic tract, superficial superior colliculus and brain at the collicular level with the antibody directed towards the Group II receptors, mGluR2 and mGluR3. Staining for the Group III receptor, mGluR4, was dense in the superficial superior colliculus in all species, with characteristics suggesting nerve fibre staining. mGluR4 staining was seen in the cat optic nerve/optic tract. One source of mGluR4 staining in the superior colliculus may thus be retinal axons, although other sources cannot be entirely excluded. These results demonstrate that distributions of mGluRs in these species have significant similarities but also some differences, suggesting that within the superior colliculus there may be some preservation of functional roles for some of the different receptor types. This is particularly so for the Group II and Group III receptors, which appear to have specific and distinct roles in the modulation of visual responses.

Distinct physiological roles of the Gq-coupled metabotropic glutamate receptors Co-expressed in the same neuronal populations

The group I metabotropic glutamate receptors, mGluR1 and mGluR5, exhibit a high degree of sequence homology, and are often found co-expressed in the same neuronal populations. These receptors couple to a broad array of effector systems, and are implicated in diverse physiological and pathophysiological functions. Due to the high degree of sequence homology, and the findings that these receptors couple identically in recombinant systems, it has been generally assumed that these two group I mGluR subtypes would exhibit redundant function when coexpressed in the same neurons. With the advent of subtype-selective pharmacological tools, it has become possible to tease apart the functions of mGluR1 and mGluR5 in the same neuron. The emerging picture is one of diverse function, which implies differential regulation. Interestingly, the group I mGluRs are modulated by a rich variety of regulatory systems, which may explain how these receptors can mediate divergent actions when present in the same cell.

Glutamate uptake controls expression of a slow postsynaptic current mediated by mGluRs in cerebellar Purkinje cells

At the cerebellar parallel fiber-Purkinje cell synapse, isolated presynaptic activity induces fast excitatory postsynaptic currents via ionotropic glutamate receptors while repetitive, high-frequency, presynaptic activity can also induce a slow excitatory postsynaptic current that is mediated by metabotropic glutamate receptors (mGluR1-EPSC). Here we investigated the involvement of glutamate uptake in the expression of the mGluR1-EPSC. Inhibitors of glutamate uptake led to a large increase of the mGluR1-EPSC. D-aspartate (0.4 mM) and L(-)-threo-3-hydroxyaspartate (0.4 mM) increased the mGluR1-EPSC approximately 4.5 and approximately 9-fold, respectively, while dihydrokainic acid (1 mM), had no significant effect on the mGluR1-EPSC. D-aspartate (0.4 mM) shifted the concentration-response curve of the depression of the mGluR1-EPSC by the low-affinity mGluR1 antagonist (S)-a-Methyl-4-carboxyphenylglycine [(S)-MCPG] to higher concentrations and decreased the stimulus intensity and the number of necessary stimuli to evoke an mGluR1-EPSC. Depression of the mGluR1-EPSC by rapid pressure application of (S)-MCPG at varying time intervals after tetanic stimulation of the parallel fibers indicated that the glutamate concentration in the peri- and extrasynaptic space decayed with time constants of 36 and 316 ms under control conditions and with inhibition of glutamate uptake, respectively. These results show that expression of the slow mGluR-mediated excitatory postsynaptic current is controlled by glutamate transporter activity. Thus in contrast to fast glutamatergic synaptic transmission, metabotropic glutamate receptor-mediated transmission is critically dependent on the activity and capacity of glutamate uptake.

Alternative splicing unmasks dendritic and axonal targeting signals in metabotropic glutamate receptor 1

Precise targeting of neurotransmitter receptors to different neuronal compartments is a fundamental step for the establishment and function of synaptic circuitry. Group I metabotropic glutamate receptors, mGluR1 and mGluR5, control glutamatergic neurotransmission by acting both postsynaptically and presynaptically. Four alternatively spliced variants of the mGluR1 gene exist, which differ in their signaling properties and subcellular localization. The present study was undertaken to identify the molecular signals responsible for trafficking of these receptors to different neuronal compartments. Here we report that targeting of mGluR1 to dendrites and axons of transfected retina neurons is controlled by alternative splicing. We have identified in the tail of the receptor a tripeptide motif, which is necessary and sufficient to exclude the splice variant mGluR1b from distal dendrites and to drive it to the axon. This motif, which is present in all the mGluR1 receptors, is masked in mGluR1a by a dominant dendritic signal sequence harbored by the extended C-terminal tail of this splice variant. Furthermore, we show that the identified axonal and dendritic targeting signals are also necessary and sufficient to localize mGluR1b and mGluR1a to the apical and basolateral compartment of Madin-Darby canine kidney cells, respectively, consistent with the existence of common trafficking components for polarized targeting in epithelial cells and neurons.

Role of excitatory amino acids in developmental epilepsies

Altered excitatory amino acid (EAA) neurotransmission, mediated primarily by glutamate, is a major cause of the imbalance of excitation and inhibition which characterizes both early development and epileptogenesis. Glutamate's actions are mediated by three classes of receptors: NMDA, non-NMDA (AMPA and kainate), and metabotropic. Several features of normal EAA development contribute to hyperexcitability in the immature brain, making it more prone to development of seizures. These features include increased density of NMDA receptors, differences in NMDA receptor subunit composition and activation kinetics, which result in reduced voltage-dependent Mg(2+) blockade and longer receptor openings in early development. Also, the unique subunit composition of AMPA receptors present at synapses during early development results in increased Ca(2+) influx. These and other differences in EAA signaling, in combination with developmental alterations in inhibitory neurotransmission, contribute to the increased seizure susceptibility seen in young animals and children. In turn, seizures themselves may alter EAA neurotransmission in an age-dependent manner. Age related changes in excitatory neurotransmission may, therefore, lead to differences in basic mechanisms of epileptogenesis between the immature and mature brain, and may also alter the activity and efficacy of antiepileptic drugs in the pediatric age group.

Tamalin, a PDZ domain-containing protein, links a protein complex formation of group 1 metabotropic glutamate receptors and the guanine nucleotide exchange factor cytohesins

In this investigation, we report identification and characterization of a 95 kDa postsynaptic density protein (PSD-95)/discs-large/ZO-1 (PDZ) domain-containing protein termed tamalin, also recently named GRP1-associated scaffold protein (GRASP), that interacts with group 1 metabotropic glutamate receptors (mGluRs). The yeast two-hybrid system and in vitro pull-down assays indicated that the PDZ domain-containing, amino-terminal half of tamalin directly binds to the class I PDZ-binding motif of group 1 mGluRs. The C-terminal half of tamalin also bound to cytohesins, the members of guanine nucleotide exchange factors (GEFs) specific for the ADP-ribosylation factor (ARF) family of small GTP-binding proteins. Tamalin mRNA is expressed predominantly in the telencephalic region and highly overlaps with the expression of group 1 mGluR mRNAs. Both tamalin and cytohesin-2 were enriched and codistributed with mGluR1a in postsynaptic membrane fractions. Importantly, recombinant and native mGluR1a/tamalin/cytohesin-2 complexes were coimmunoprecipitated from transfected COS-7 cells and rat brain tissue, respectively. Transfection of tamalin and mutant tamalin lacking a cytohesin-binding domain caused an increase and decrease in cell-surface expression of mGluR1a in COS-7 cells, respectively. Furthermore, adenovirus-mediated expression of tamalin and dominant-negative tamalin facilitated and reduced the neuritic distribution of endogenous mGluR5 in cultured hippocampal neurons, respectively. The results indicate that tamalin plays a key role in the association of group 1 mGluRs with the ARF-specific GEF proteins and contributes to intracellular trafficking and the macromolecular organization of group 1 mGluRs at synapses.

Role of calcium, glutamate neurotransmission, and nitric oxide in spreading acidification and depression in the cerebellar cortex

This study investigated the mechanisms underlying the recently reported fast spreading acidification and transient depression in the cerebellar cortex in vivo. Spreading acidification was evoked by surface stimulation in the rat and mouse cerebellar cortex stained with the pH-sensitive dye neutral red and monitored using epifluorescent imaging. The probability of evoking spreading acidification was dependent on stimulation parameters; greater frequency and/or greater amplitude were more effective. Although activation of the parallel fibers defined the geometry of the spread, their activation alone was not sufficient, because blocking synaptic transmission with low Ca(2+) prevented spreading acidification. Increased postsynaptic excitability was also a major factor. Application of either AMPA or metabotropic glutamate receptor antagonists reduced the likelihood of evoking spreading acidification, but stronger stimulation intensities were still effective. Conversely, superfusion with GABA receptor antagonists decreased the threshold for evoking spreading acidification. Blocking nitric oxide synthase (NOS) increased the threshold for spreading acidification, and nitric oxide donors lowered the threshold. However, spreading acidification could be evoked in neuronal NOS-deficient mice (B6;129S-Nos1(tm1plh)). The depression in cortical excitability that accompanies spreading acidification occurred in the presence of AMPA and metabotropic glutamate receptor antagonists and NOS inhibitors. These findings suggest that spreading acidification is dependent on extracellular Ca(2+) and glutamate neurotransmission with a contribution from both AMPA and metabotropic glutamate receptors and is modulated by nitric oxide. Therefore, spreading acidification involves both presynaptic and postsynaptic mechanisms. We hypothesize that a regenerative process, i.e., a nonpassive process, is operative that uses the cortical architecture to account for the high speed of propagation.

Activation of metabotropic glutamate receptor mediates upregulation of transcription factor mRNA expression in rat striatum induced by acute administration of amphetamine

Metabotropic glutamate receptors (mGluRs) are densely expressed on the medium spiny projection neurons of rat striatum. Intrastriatal injection of an mGluR agonist increases motor activity and dopamine release in the striatum. This study investigated the roles of mGluRs in the regulation of behavioral activity and transcription factor gene expression in striatal neurons in normal and amphetamine-treated rats. Acute injection of a behaviorally active dose of amphetamine (4.0 mg/kg, i.p.) elevated basal levels of the transcription factor c-fos and zif/268 mRNAs in the dorsal striatum as revealed by quantitative in situ hybridization. Pharmacological blockade of mGluRs with bilateral injections of a non-selective mGluR antagonist, (S)-alpha-methyl-4-carboxyphenylglycine (MCPG), into the dorsal striatum at 10 but not 0.4 nmol significantly attenuated amphetamine-stimulated c-fos mRNA expression in this area. In contrast to c-fos, striatal zif/268 induction stimulated by amphetamine was not altered by pretreatment with MCPG. MCPG by itself did not affect basal levels of either gene expression in the striatum. No significant effects of MCPG were found on spontaneous or amphetamine-stimulated behavioral activities. These data indicate that blockade of total mGluRs in the dorsal striatum has a selective effect on dopamine-stimulated gene expression. Activation of the MCPG-sensitive mGluRs is required for the upregulation of transcription factor c-fos, although not zif/268, mRNA expression in the striatum in response to acute injection of amphetamine.

Developmental changes in the localisation of the mGluR1alpha subtype of metabotropic glutamate receptors in Purkinje cells

The regulation of neurotransmitter receptors during synapse formation has been studied extensively at the neuromuscular junction, but little is known about the development of excitatory neurotransmitter receptors during synaptogenesis in central synapses. In this study we show qualitatively and quantitatively that a receptor undergoes changes in localisation on the surface of rat Purkinje cells during development in association with its excitatory synapses. The presence of mGluR1alpha at parallel and climbing fibre synapses on developing Purkinje cells was studied using high-resolution immunoelectron microscopy. Immunoreactivity for mGluR1alpha was detected from embryonic day 18 in Purkinje cells, and showed dramatic changes in its localisation with age. At early postnatal ages (P0 and P3), mGluR1alpha was found both in somata and stem dendrites but was not usually associated with synaptic contacts. At P7, mGluR1alpha became concentrated in somatic spines associated with climbing fibres and in the growing dendritic arborisation even before innervation by parallel fibres. During the second and third postnatal week, when spines and parallel fibre synapses were generated, mGluR1alpha became progressively concentrated in the molecular layer, particularly in the synaptic specialisations. As a result, during the fourth postnatal week, the pattern and level of mGluR1alpha expression became similar to the adult and mGluR1alpha appeared in high density in perisynaptic sites. Our results indicate that mGluR1alpha is present in the developing Purkinje cells prior to their innervation by climbing and parallel fibres and demonstrate that this receptor undergoes a dynamic and specific regulation during postnatal development in association with the establishment of synaptic inputs to Purkinje cell.

Retrograde modulation of transmitter release by postsynaptic subtype 1 metabotropic glutamate receptors in the rat cerebellum

1. The aim of the study was to elucidate the mechanisms underlying the depressant effect of the group I/II metabotropic glutamate receptor (mGluR) agonist 1S,3R-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD) on parallel fibre (PF) to Purkinje cell (PC) synaptic transmission. Experiments were performed in rat cerebellar slices using the whole-cell patch-clamp technique and fluorometric measurements of presynaptic calcium variation 2. Analysis of short-term plasticity, fluctuation of EPSC amplitude and responses of PCs to exogenous glutamate showed that depression caused by 1S,3R-ACPD is presynaptic. 3. The effects of 1S,3R-ACPD were blocked and reproduced by group I mGluR antagonists and agonists, respectively. 4. These effects remained unchanged in mGluR5 knock-out mice and disappeared in mGluR1 knock-out mice. 5. 1S,3R-ACPD increased calcium concentration in PFs. This effect was abolished by AMPA/kainate (but not NMDA) receptor antagonists and mimicked by focally applied agonists of these receptors. Thus, it is not directly due to mGluRs but to presynaptic AMPA/kainate receptors indirectly activated by 1S,3R-ACPD. 6. Frequencies of spontaneous and evoked unitary EPSCs recorded in PCs were respectively increased and decreased by mGluR1 agonists. Similar results were obtained when mGluR1s were activated by tetanic stimulation of PFs. 7. Injecting 30 mM BAPTA into PCs blocked the effects of 1S,3R-ACPD on unitary EPSCs. 8. In conclusion, 1S,3R-ACPD reduces evoked release of glutamate from PFs. This effect is triggered by postsynaptic mGluR1s and thus implies that a retrograde messenger, probably glutamate, opens presynaptic AMPA/kainate receptors and consequently increases spontaneous release of glutamate from PF terminals and decreases evoked synaptic transmission.

Activation of group I metabotropic glutamate receptors produces a direct excitation and disinhibition of GABAergic projection neurons in the substantia nigra pars reticulata

A pathological increase in excitatory glutamatergic input to substantia nigra pars reticulata (SNr) from the subthalamic nucleus (STN) is believed to play a key role in the pathophysiology of Parkinson's disease. We present an analysis of the physiological roles that group I metabotropic glutamate receptors (mGluRs) play in regulating SNr functions. Immunocytochemical analysis at the light and electron microscopic levels reveal that both mGuR1a and mGluR5 are localized postsynaptically in the SNr. Consistent with this, activation of group I mGluRs depolarizes SNr GABAergic neurons. Interestingly, although both group I mGluRs (mGluR1 and mGluR5) are expressed in these neurons, the effect is mediated solely by mGluR1. Light presynaptic staining for mGluR1a and mGluR5 was also observed in some terminals forming symmetric synapses and in small unmyelinated axons. Consistent with this, activation of presynaptic mGluR1a and mGluR5 decreases inhibitory transmission in the SNr. The combination of direct excitatory effects and disinhibition induced by activation of group I mGluRs could lead to a large excitation of SNr projection neurons. This suggests that group I mGluRs are likely to play an important role in the powerful excitatory control that the STN exerts on basal ganglia output neurons.

Expression of metabotropic glutamate receptor 1alpha in the hippocampus of rat pilocarpine model of status epilepticus

The expression of metabotropic glutamate receptor 1alpha was studied in the rat hippocampus after pilocarpine-induced status epilepticus by Western blot and immunocytochemistry at both light and electron microscopic levels. At 1 day after pilocarpine-induced status epilepticus, there was marked decrease in metabotropic glutamate receptor 1alpha immunoreactivity at the border between stratum oriens and alveus in CA1 and CA3, and in the hilus of dentate gyrus. Between 3 and 31 days after pilocarpine-induced status epilepticus, metabotropic glutamate receptor 1alpha-immunoreactive dendrites and cell bodies in the border between stratum oriens and alveus gradually reappeared. Upregulation of metabotropic glutamate receptor 1alpha, however, was observed in the stratum oriens of CA1 at day 1, but returned to baseline by day 7. By electron microscopy, the metabotropic glutamate receptor 1alpha-immunoreactive product was demonstrated only in the post-synaptic elements in the border between the stratum oriens and alveus of CA1 and the hilus of the dentate gyrus in both control and experimental rats. At 1 day after pilocarpine-induced status epilepticus, metabotropic glutamate receptor 1alpha-immunoreactive degenerating neurons were identified in the border between stratum oriens and alveus of CA1 and the hilus of the dentate gyrus. At 7 and 31 days, many degenerating axons were also found. Present results suggest that excitoneurotoxicity mediated through post-synaptic metabotropic glutamate receptor 1alpha may be involved in degeneration and death of interneurons in the hilus of dentate gyrus, and the border between stratum oriens and alveus of CA1 in the early stage after pilocarpine-induced status epilepticus.

Novel allosteric antagonists shed light on mglu(5) receptors and CNS disorders

Although multiple metabotropic glutamate (mglu) receptor subtypes were cloned in the early 1990s, progress in the characterization of these receptors has been slow because of difficulties in obtaining subtype-selective ligands. However, in the past few years exciting progress has been made on the mglu(5) receptor subtype following the identification of selective non-amino-acid-like ligands that implicate the mglu(5) receptor as a potentially important therapeutic target, particularly for the treatment of pain and anxiety.

Glutamate receptor subunit expression in primary neuronal and secondary glial cultures

We report on the expression of ionotropic glutamate receptor subunits in primary neuronal cultures from rat cortex, hippocampus and cerebellum and of metabotropic glutamate (mGlu) receptor subtypes in these neuronal cultures as well as in cortical astroglial cultures. We found that the NMDA receptor (NR) subunits NR1, NR2A and NR2B were expressed in all three cultures. Each of the three cultures showed also expression of the four AMPA receptor subunits. Although RT-PCR detected mRNA of all kainate (KA) subunits in the three cultures, western blot showed only expression of Glu6 and KA2 receptor subunits. The expression analysis of mGlu receptors indicated the presence of all mGlu receptor subtype mRNAs in the three neuronal cultures, except for mGlu2 receptor mRNA, which was not detected in the cortical and cerebellar culture. mGlu1a/alpha, -2/3 and -5 receptor proteins were present in all three cultures, whereas mGlu4a and mGlu8a receptor proteins were not detected. Astroglial cultures were grown in either serum-containing or chemically defined medium. Only mGlu5 receptor protein was found in astroglial cultures grown in serum-containing medium. When astrocytes were cultured in chemically defined medium, mGlu3, -5 and -8 receptor mRNAs were detected, but at the protein level, still only mGlu5 receptor was found.

Intensity-dependent, rapid activation of presynaptic metabotropic glutamate receptors at a central synapse

Synaptic signals from retinal bipolar cells were monitored by measuring EPSCs in ganglion cells voltage-clamped at -70 mV. Spontaneous EPSCs were strongly suppressed by l-2-amino-4-phosphonobutyrate (AP-4), an agonist at group III metabotropic glutamate receptors (mGluRs). Agonists of group I or II mGluRs were ineffective. AP-4 also suppressed ganglion cell EPSCs evoked by bipolar cell stimulation using potassium puffs, sucrose puffs, or zaps of current (0.5-1 microA). In addition, AP-4 suppressed Off EPSCs evoked by dim-light stimuli. This indicates that group III mGluRs mediate a direct suppression of bipolar cell transmitter release. An mGluR antagonist, (RS)-alpha-cyclopropyl-4-phosphonophenylyglycine (CPPG), blocked the action of AP-4. When bipolar cells were weakly stimulated, AP-4 produced a large suppression of the EPSC, but CPPG alone had little effect. Conversely, when bipolar cells were strongly stimulated, CPPG produced an enhancement of the EPSC, but AP-4 alone had little effect. This indicates that endogenous feedback regulates bipolar cell transmitter release and that the dynamic range of the presynaptic metabotropic autoreceptor is similar to that of the postsynaptic ionotropic receptor. Furthermore, the feedback is rapid and intensity-dependent. Hence, concomitant activation of presynaptic and postsynaptic glutamate receptors shapes the responses of ganglion cells.

D4 dopamine and metabotropic glutamate receptors in cerebral cortex and striatum in rat brain

The characterization of the functional interactions between the metabotropic glutamate receptors (mGluR) and the dopaminergic (DR) receptors in the corticostriatal projections may provide a possible interpretation of synaptic events in the basal ganglia. It has been suggested that presynaptic D2-type receptor located on glutamatergic corticostriatal neurons regulates the release of glutamate. In a first approach we have studied the cellular distribution of the D4R and the mGluRs in cerebral cortex and striatum employing immunocytochemistry. D4R positive neurons were particularly numerous in medial prefrontal cortex mainly occupying layers II and III. An even distribution was found on small round-shaped neurons in the striatum. Group I mGluR1alpha-like immunoreactivity (mGluR1alpha-LI) was found in medial and deep layers of the cerebral cortex while group III mGluR4a labeled more superficial layers; group II mGluR2/3 signal was intense on fine fibers with a punctate appearance. In the striatum, mGluR1alpha and mGluR2/3 stained mainly fibers while mGluR4a labeled round shaped cell bodies. After lateral ventricular injection of colchicine, an axonal transport and firing activity blocker, D4R labeling significantly increased in cerebral cortex and decreased in the striatum. mGluR1alpha and mGluR4a signal decreased in cerebral cortex and only mGluR4a signal decreased in the striatum. These results support previous reports indicating a presynaptic localization of D4R in the striatum. In contrast, striatal mGluR1alpha appears to be a postsynaptic receptor probably synthesized in situ. Our results do not support the hypothesis of a colocalization of D4 receptor and one or more of the metabotropic glutamatergic receptors studied here.

Group I metabotropic glutamate receptors are expressed in the chicken retina and by cultured retinal amacrine cells

Glutamate is well established as an excitatory neurotransmitter in the vertebrate retina. Its role as a modulator of retinal function, however, is poorly understood. We used immunocytochemistry and calcium imaging techniques to investigate whether metabotropic glutamate receptors are expressed in the chicken retina and by identified GABAergic amacrine cells in culture. Antibody labeling for both metabotropic glutamate receptors 1 and 5 in the retina was consistent with their expression by amacrine cells as well as by other retinal cell types. In double-labeling experiments, most metabotropic glutamate receptor 1-positive cell bodies in the inner nuclear layer also label with anti-GABA antibodies. GABAergic amacrine cells in culture were also labeled by metabotropic glutamate receptor 1 and 5 antibodies. Metabotropic glutamate receptor agonists elicited Ca(2+) elevations in cultured amacrine cells, indicating that these receptors were functionally expressed. Cytosolic Ca(2+) elevations were enhanced by metabotropic glutamate receptor 1-selective antagonists, suggesting that metabotropic glutamate receptor 1 activity might normally inhibit the Ca(2+) signaling activity of metabotropic glutamate receptor 5. These results demonstrate expression of group I metabotropic glutamate receptors in the avian retina and suggest that glutamate released from bipolar cells onto amacrine cells might act to modulate the function of these cells.

Differential synaptic distribution of AMPA receptor subunits in the ventral posterior and reticular thalamic nuclei of the rat

Although the mechanisms by which the cerebral cortex controls its ascending input are still poorly understood, it is known that cortical control at the thalamic level is via direct glutamatergic projections to relay nuclei and to the reticular nucleus. Here we confirm previous light microscopic reports of a high expression of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor subunit, GluR4, in reticular and ventral posterior thalamic nuclei of the rat, and moderate staining using an antibody recognizing both GluR2 and GluR3. In contrast only low levels of staining for GluR2, and barely detectable levels of GluR1 immunoreactivity were observed. After injections of biotinylated dextran, electron microscopy revealed that anterogradely-labeled cortical synapses in both thalamic nuclei were small with fewer mitochondria and more densely-packed vesicles than terminals likely to arise from intrinsic and ascending pathways. We performed post-embedding immunogold to provide quantitative data on the density of AMPA receptor subunits at morphologically-defined groups of synapses. We found that corticothalamic synapses in the reticular thalamic nucleus contain twice as much GluR2/3, and at least three times more GluR4 protein than do intrathalamic synapses. In the ventral posterior nucleus, corticothalamic synapses contain similar amounts of GluR2/3, but four times more GluR4 than do those from ascending afferents. Corticothalamic synapses in reticular nucleus contain slightly more GluR2/3, and three times more GluR4, than those in ventral posterior nucleus. We conclude that enrichment of GluR4 at morphologically-defined cortical synapses is a feature common to both thalamic nuclei, and those in the reticular nucleus express higher levels of AMPA receptors. The rapid kinetics of GluR4-rich AMPA receptors we suggest indicate that cortical descending control may be more temporally precise than previously recognized.

Development and molecular organization of dendritic spines and their synapses

Nearly all excitatory input in the hippocampus impinges on dendritic spines which serve as multifunctional compartments that can, at the very least, selectively isolate and amplify incoming signals. Their importance to normal brain function is highlighted by the severe mental impairment observed in most individuals having poorly developed spines (Purpura, Science 1974;186:1126-1128). Distinct groups of membrane proteins, cytoskeletal elements, scaffolding proteins, and second messenger-related proteins are concentrated particularly in dendritic spines, but their ability to generate, maintain, and coordinately regulate spine structure or function is poorly understood. Here we review the unique molecular composition of dendritic spines along with the factors known to influence dendritic spine development in order to construct a model of dendritic spine development in relation to synaptogenesis.

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.

Synaptic activation of the group I metabotropic glutamate receptor mGlu1 on the thalamocortical neurons of the rat dorsal lateral geniculate nucleus in vitro

Intracellular recordings were made from thalamocortical neurons in slices of rat dorsal lateral geniculate nucleus in vitro, where ionotropic glutamate receptors and ionotropic and metabotropic GABA receptors had been blocked. The activation of specific metabotropic glutamate receptors by exogenous agonists and by the electrical stimulation of the corticothalamic pathway was then assessed using selective antagonists. The specific group I agonist (S)-3, 5-dihydoxyphenylglycine and the non-selective agonist (1S, 3R)-1-aminocyclo-pentane-1,3-dicarboxylic acid both caused a concentration-dependent depolarization of membrane potential. These effects were associated with an increase in the apparent input resistance, and a more robust expression of both the depolarizing sag of the voltage response and the low-threshold Ca(2+) potential and an increase in thalamocortical neuron excitability. However, group I agonists selective for the mGlu5 receptor and agonists selective for group II and III receptors did not have these effects. Consequently, these data suggested that these actions were mediated specifically by the group I mGlu1 receptor. The activation of cortical fibres, with trains of 50 stimuli at 50Hz, resulted in a two-component depolarizing response. The first part of this synaptic response and the agonist-induced depolarization of membrane potential were depressed by the novel group I receptor antagonists LY367366 and LY367385, which are active at mGlu1 receptors. However, they were not blocked by 6-methyl-2-(phenylethyl)-pyridine, a highly selective mGlu5 receptor antagonist.Thus, the membrane potential depolarization of thalamocortical neurons caused either by exogenous agonists or by the stimulation of cortical fibres resulted from the specific activation of mGlu1 but not mGlu5 receptors. This result is consistent with the location of this receptor type on the distal dendrites of thalamocortical neurons in the dorsal lateral geniculate nucleus of the thalamus.

Contributions of mGlu1 and mGlu5 receptors to interactions with N-methyl-D-aspartate receptor-mediated responses and nociceptive sensory responses of rat thalamic neurons

The nociceptive responses of rat ventrobasal thalamus neurons can be reduced by N-methyl-D-aspartate antagonists and by selective metabotropic glutamate receptor mGlu1 antagonists. The recent development of the mGlu5-selective antagonist 6-methyl-2-(phenylethynyl)-pyridine now allows the direct probing of the possible involvement of mGlu5 receptors in thalamic nociceptive responses. Extracellular recordings were made from single neurons in the ventrobasal thalamus and immediately overlying dorsal thalamic nuclei of adult urethane-anaesthetized rats using multi-barrel electrodes. Responses of neurons to iontophoretic applications of the mGlu5-selective agonist (R,S)-2-chloro-5-hydroxyphenylglycine were selectively reduced during continuous iontophoretic applications of 6-methyl-2-(phenylethynyl)-pyridine. Similar applications of 6-methyl-2-(phenylethynyl)-pyridine reduced neuronal responses to noxious thermal stimuli to 53+/-9.5% of control responses. Co- application by iontophoresis of N-methyl-D-aspartate and metabotropic glutamate receptor agonists resulted in a mutual potentiation of excitatory responses. This effect could be reduced by either 6-methyl-2-(phenylethynyl)-pyridine or the mGlu1 antagonist LY367385. These results, taken together with previous data, suggest that acute thalamic nociceptive responses are mediated by a combination of mGlu1, mGlu5 and N-methyl-D-aspartate receptor activation, and that co-activation of these receptors produces a synergistic excitatory effect. Thus blockade of any of these receptor types would have a profound effect on the overall nociceptive response.