Distribution of the mRNA for a metabotropic glutamate receptor (mGluR3) in the rat brain: an in situ hybridization study

Regulation of synaptic transmission in the mossy fibre-granule cell pathway of rat cerebellum by metabotropic glutamate receptors

The role of metabotropic glutamate receptors (mGluRs) in the mossy fibre-granule cell pathway in rat cerebellum was studied using slice preparations and electrophysiological techniques. Application of the group I selective agonist (S)-3,5-dihydroxyphenylglycine (DHPG) evoked, in a concentration-dependent manner (EC50 = 33 microM), a depolarising/hyperpolarising complex response from granule cells which was preferentially inhibited by the group I selective antagonist (S)-4-carboxyphenylglycine (4CPG). The group III selective agonist L-amino-4-phosphonobutyrate (AP4) evoked a hyperpolarising response (EC50 = 10 microM) which was inhibited by the group II/III selective antagonist (S)-alpha-methyl-4-phosphonophenylglycine (MPPG). The group II agonist (2S,2'R,3'R)-2-(2',3'-dicarboxylcyclopropyl)glycine (DCG-IV) elicited no measurable voltage change. The amplitude of the synaptically-mediated mossy fibre response in granule cells was unaffected during application of AP4, was reduced by DHPG and was enhanced by DCG-IV (EC50 = 80 nM). These effects were inhibited by the group selective antagonists 4CPG and (2S,1'S,2'S,3'R)-2-(2'-carboxy-3'-phenylcyclopropyl)glycine (PCCG-4), respectively. Further investigation using patch-clamp recording revealed that DCG-IV potently inhibited spontaneous GABAergic currents. We conclude that group I and III (but not group II) mGluRs are functionally expressed by granule cells, whereas unexpectedly group II or III mGluRs do not appear to be present presynaptically on mossy fibre terminals. Group II mGluRs are located on Golgi cell terminals; when activated these receptors cause disinhibition, a function which may be important for gating information transfer from the mossy fibres to the granule cells.

Dopamine D1 receptor modulation of glutamate receptor messenger RNA levels in the neocortex and neostriatum of unilaterally 6-hydroxydopamine-lesioned rats

The effect of treatment with the D1 dopamine receptor agonist SKF 38393 on the expression of metabotropic glutamate receptor 1, 3, 4 and 5 receptor subtypes and of the glutamate N-methyl-D-aspartate ionotropic receptor subunits NRI, NR2A and NR2B was analysed using in situ hybridization. We studied the neocortex and neostriatum of normal rats and of rats unilaterally treated with 6-hydroxydopamine, a neurotoxin that, after intracerebral injection into the ventral tegmental area, causes selective degeneration of the ascending dopamine pathway. In the 6-hydroxydopamine-lesioned rats, metabotropic glutamate receptor subtype 3 messenger RNA levels were ipsilaterally increased in the neocortex and neostriatum, while the levels of metabotropic glutamate receptor subtype 4 messenger RNA were bilaterally increased in both regions. When administered to the 6-hydroxydopamine-lesioned rats, the D1 receptor agonist SKF 38393 (3 x 20 mg/kg, s.c.) produced a bilateral decrease in the expression of the metabotropic glutamate receptor subtype 1 and 5 receptor messenger RNA levels in the neocortex and neostriatum. In the neostriatum, SKF 38393 attenuated the ipsilateral increase in the expression of striatal metabotropic glutamate receptor subtype 3 messenger RNA produced by the 6-hydroxydopamine lesion. Furthermore, SKF 38393 produced a bilateral decrease in the levels of NRI receptor subunit messenger RNA and, in contrast, an increase in the striatal NR2B messenger RNA levels. All of these effects were abolished by the D1 receptor antagonist SCH 23360. These results indicate a differential D1 receptor-mediated modulation of the expression of some glutamate receptor subtypes in the neostriatum and neocortex, in agreement with the idea of a functional coupling between dopamine and excitatory amino acid systems in both regions. Thus, pharmacological targeting of excitatory amino acid systems could provide alternative or complementary treatment strategies for diseases involving dopaminergic systems in the striatum (e.g., Parkinson's disease) and cortex (e.g., schizophrenia).

Distribution of metabotropic glutamate receptors in the parabrachial and Kölliker-Fuse nuclei of the rat

In the present study, we analysed the distribution and cellular localization of metabotropic glutamate receptors (1alpha, 2/3, 5) in parabrachial and Kölliker-Fuse nuclei using subtype-specific antisera. Immunolabelling revealed that different nuclei express different sets of metabotropic glutamate receptors. Metabotropic glutamate receptor la immunoreactivity was found in the Kölliker-Fuse nucleus and in several parabrachial nuclei, including the waist area, lateral crescent, medial, external medial and ventral lateral nuclei. The external lateral and internal lateral parabrachial nuclei were devoid of metabotropic glutamate receptor 1alpha immunoreactivity. Metabotropic glutamate receptor 5 immunoreactivity was observed in the Kölliker-Fuse and in the medial parabrachial nuclei, while in the remaining nuclei the staining was very weak. Again, the external lateral nucleus was devoid of metabotropic glutamate receptor 5 immunoreactivity. The metabotropic glutamate receptor 2/3 antisera stained all lateral parabrachial nuclei as well as the Kölliker-Fuse nucleus, while staining in the medial parabrachial nucleus was weak. Metabotropic glutamate receptor 1alpha immunoreactivity was observed on presumed dendritic profiles, while metabotropic glutamate receptor 5 immunoreactivity was found predominantly on neuronal cell bodies. Metabotropic glutamate receptor 2/3 immunoreactivity was present as a fine, punctate immunostaining in the neuropil. Our data suggest that glutamate release in the parabrachial and Kölliker-Fuse nuclei might induce a variety of second messenger cascades, as indicated by the presence or absence of certain types of metabotropic glutamate receptors.

Metabotropic glutamate receptors and the generation of locomotor activity: interactions with midbrain dopamine

Interactions between excitatory amino acid (EAA) and dopamine (DA) pathways in the basal ganglia have been known for some time to contribute importantly to the generation of motor behaviors. In particular, the role played by ionotropic glutamate receptors (iGluRs) in such interactions and in the production of locomotion has received considerable attention, particularly in brain areas such as the ventral tegmental area (VTA) where EAA afferants are known to modulate the activity of DA neurons and the nucleus accumbens (NAcc) where descending EAA projections and ascending DA mesencephalic projections come in close apposition to each other and co-innervate intrinsic neurons projecting to motor output regions. Recently, the growing importance of the metabotropic glutamate receptor (mGluR) in the generation of motor behaviors and various forms of plasticity has begun to emerge. The known coupling of the mGluR to second messenger systems and its demonstrated role in the long-term modulation of synaptic transmission make it a logical candidate not only for the generation of locomotion involving EAA-DA interactions, but also for the induction and expression of locomotor plasticity involving these neurotransmitters. In this review, we examine the evidence supporting a role for mGluRs in the generation of DA-dependent locomotion as well as in one form of locomotor plasticity: the sensitization of locomotor activity by psychomotor stimulant drugs.

The selective mGlu2/3 receptor agonist LY354740 attenuates morphine-withdrawal-induced activation of locus coeruleus neurons and behavioral signs of morphine withdrawal

Naltrexone-precipitated morphine withdrawal induces hyperactivity of locus coeruleus (LC) neurons, as well as a plethora of behavioral withdrawal signs. Previous research has demonstrated that an increased release of glutamate and activation of AMPA receptors, particularly in the LC, play an important role in opiate withdrawal. LY354740 is a novel Group II metabotropic glutamate mGlu2/3 receptor agonist that decreases the release of glutamate. Therefore, we investigated the effect of LY354740 on naltrexone-precipitated morphine-withdrawal-induced activation of LC neurons and behavioral signs of morphine withdrawal. In in vivo recordings from anesthetized rats, pretreatment with LY354740 (3-30 mg/kg, s.c.) dose-dependently attenuated the morphine-withdrawal-induced activation of LC neurons. In unanesthetized, morphine-dependent animals, pretreatment with LY354740 (3-30 mg/kg, s.c.) dose-dependently suppressed the severity and occurrence of many naltrexone-precipitated morphine-withdrawal signs. These results indicate mGlu2/3 receptor agonists: (1) can attenuate the morphine-withdrawal-induced activation of LC neurons and many behavioral signs of morphine withdrawal; and (2) may have therapeutic effects in man for the treatment of opiate withdrawal.

Distribution and developmental changes in metabotropic glutamate receptor messenger RNA expression in the rat lumbar spinal cord

Using in situ hybridisation, the regional distribution of primary transcripts and splice variants of all metabotropic glutamate receptor subtypes (mGluR) currently known to be expressed in the spinal cord have been studied in the lumbar enlargement of the rat spinal cord. In adult animals, the messenger RNA of the mGluR subtypes 1, 5, 3, 4 and 7 were differentially expressed. The transcripts of mGluR1 and 5 were most abundant with mGluR5 messenger RNA being concentrated in the superficial dorsal horn. In contrast, the mGluR2 transcript was not detectable with the sensitivity of the method. Secondly, age related changes (postnatal days 1, 7, 12, 21) in the postnatal expression of mGluR1-5 and 7 transcripts have been investigated. mGluR1 and 7 messenger RNA showed a general decrease in spinal expression from postnatal day 1 to day 21. Quantitative densitometry showed high mGluR3 and 5 messenger RNA levels especially in the superficial dorsal horn at birth, however these levels decreased with age. In addition to changes in density, the regional distribution of mGluR3 messenger RNA was altered with postnatal development. Up to postnatal day 12, mGluR3 messenger RNA expression was almost exclusively restricted to the spinal grey matter, but with postnatal day 21 a strong additional expression in the white matter occurred. Distribution of mGluR4 messenger RNA showed little change in the dorsal horn, however motoneuronal expression emerged during development. These changes may suggest different roles for mGluRs in the maturation of spinal transmission of the rat nervous system.

Antisense ablation of type I metabotropic glutamate receptor mGluR1 inhibits spinal nociceptive transmission

Electrophysiological and behavioral studies point to a role of group I metabotropic glutamate receptors (mGluR1 and mGluR5) in mediating spinal nociceptive responses in rats. However, antagonists with a high degree of specificity for each of these sites are not yet available. We, therefore, examined the effects of antisense deletion of spinal mGluR1 expression in assays of behavioral analgesia and of electrophysiological responses of dorsal horn neurons. Rats treated with an mGluR1 antisense oligonucleotide reagent, delivered continuously to the intrathecal space of the lumbar spinal cord, developed marked analgesia as measured by an increase in the latency to tail-flick (55 degreesC) over a period of 4-7 d. This correlated with a selective reduction in mGluR1, but not mGluR5, immunoreactivity in the superficial dorsal horn compared with untreated control rats, in parallel with a significant reduction in the proportion of neurons activated by the mGluR group I agonist 3, 5-dihydroxyphenylglycine (DHPG), whereas the proportion of cells excited by the mGluR5 agonist, trans-azetidine-2,4-dicarboxylic acid (t-ADA) remained unaffected. In contrast, rats treated with mGluR1 sense or mismatch probes showed none of these changes compared with untreated, control rats. Furthermore, multireceptive dorsal horn neurons in mGluR1 antisense-treated rats were strongly excited by innocuous stimuli to their peripheral receptive fields, but showed severe reductions in their sustained excitatory responses to the selective C-fiber activator mustard oil and in responses to DHPG.

The role of nigrostriatal dopamine in metabotropic glutamate agonist-induced rotation

Metabotropic glutamate receptors are a major class of excitatory amino acid receptors. Eight metabotropic glutamate receptors subtypes have been cloned and have been classified into three groups based on their amino acid sequence homology, effector systems, and pharmacological profile. Previous results have shown that striatal group I metabotropic glutamate receptor stimulation produces vigorous contralateral rotation in intact rats, thought to be due to increased striatal dopamine release. Examination of FOS-like immunoreactivity and local cerebral glucose metabolism suggests that this occurs secondary to activation of the subthalamic nucleus. We sought to determine the contribution of dopamine by examining metabotropic glutamate receptor agonist-induced rotation in rats following acute dopamine depletion by reserpine/alpha-methyl-para-tyrosine treatment, or chronic dopamine depletion by 6-hydroxydopamine treatment. In unilateral 6-hydroxydopamine lesioned rats, the group I metabotropic glutamate receptor agonist (RS)-3,5-dihydroxyphenylglycine induced contralateral rotation with a coincident increase in striatal 3,4-dihydroxyphenylacetic acid. The rotation was attenuated by the group I antagonist 1-aminoindan-1,5-dicarboxylate. Examination of FOS-like immunoreactivity and [14C]2-deoxyglucose uptake in chronically dopamine depleted rats also revealed similar patterns to those seen previously in intact rats. However, acutely dopamine depleted rats do not exhibit metabotropic glutamate receptor agonist-induced rotation and show a different pattern of [14C]2-deoxyglucose uptake, with no increase in glucose utilization in the intermediate and deep layers of the superior colliculus. These results suggest that there are compensatory changes under conditions of chronic dopamine denervation which permit metabotropic glutamate receptor agonist-induced rotation to occur, which may include dopamine receptor supersensitivity, increased dopamine turnover, and/or changes in sensitivity of striatal group I metabotropic glutamate receptors. The group III metabotropic glutamate receptor agonist L-(+)-2-amino-4-phosphonobutyrate induced contralateral rotation in 6-hydroxydopamine lesioned rats, while it had no effect in intact rats. Additionally, examination of FOS-like immunoreactivity revealed a distinct pattern following L-(+)-2-amino-4-phosphonobutyrate administration in 6-hydroxydopamine lesioned versus intact rats. These results suggest that there is a change in the effect of striatal group III stimulation under conditions of dopamine depletion.

Glycine receptors in adult guinea pig brain stem auditory nuclei: regulation after unilateral cochlear ablation

In young adult guinea pigs, the effects of unilateral cochlear ablation were determined on the specific binding of [3H]strychnine measured in subdivisions of the cochlear nucleus (CN), the superior olivary complex, and the auditory midbrain, after 2, 7, 31, 60, and 147 postlesion days. Changes in binding relative to that in age-matched controls were interpreted as altered activity and/or expression of synaptic glycine receptors. Postlesion binding declined ipsilaterally in most of the ventral CN and in the lateral superior olive (LSO). Binding was modestly deficient in the ipsilateral dorsal CN and in the anterior part of the contralateral anteroventral CN. Binding was elevated in the contralateral LSO. Transient changes also occurred. Binding was elevated transiently, between 2 and 31 days, contralaterally in parts of the anteroventral CN, bilaterally in the medial superior olive (MSO), and bilaterally in most of the midbrain nuclei. Binding was deficient transiently, at 60 days, in most of the contralateral CN and bilaterally in the midbrain nuclei. The present findings, together with previously reported postlesion changes in glycine release, were consistent with persistently weakened glycinergic inhibitory transmission ipsilaterally in the ventral CN and the LSO and bilaterally in the dorsal CN. Glycinergic inhibitory transmission was strengthened in the contralateral LSO and transiently strengthened in the MSO bilaterally. A hypothetical model of the findings suggested that glycine receptor regulation may depend on excitatory and glycinergic input to auditory neurons. The present changes in glycine receptor activity may contribute to altered auditory functions, which often accompany hearing loss.

N-acetylaspartylglutamate activates cyclic AMP-coupled metabotropic glutamate receptors in cerebellar astrocytes

N-Acetylaspartylglutamate (NAAG) is the most prevalent peptide in the mammalian nervous system. NAAG meets the traditional criteria of a neurotransmitter, including localization in synaptic vesicles, depolarization-induced release, low potency activation of some N-methyl-D-aspartate receptors, and highly selective activation of a cAMP-coupled metabotropic glutamate receptor (mGluR) with potency approaching that of glutamate. The peptide is present in cultured cortical glia in high concentration and is hydrolyzed by cell surface peptidase activity. In the present work, we tested the hypothesis that NAAG selectively activates a subclass of metabotropic receptors on cultured rat cerebellar glia, primarily astrocytes. These glial cells express mRNA for mGluR subtypes 1, 3, 4, 5, 6, 7, and 8. We were not able to detect message for mGluR2 in these cells using the reverse transcriptase-polymerase chain reaction. Cerebellar glia responded to NAAG, glutamate, and trans-ACPD with a decrease in forskolin-stimulated cAMP formation. AP4, an agonist of the group III receptors mGluR4, mGluR6, mGluR7, and mGluR8, had little or no effect on stimulated cAMP levels. Treatment with low micromolar NAAG significantly increased uptake of radioactive thymidine by cultured astrocytes through activation of metabotropic glutamate receptors. Antagonists of group II mGluRs prevented the decrease in cAMP and the increase in uptake of thymidine by NAAG. Cultured cerebellar astrocytes expressed 20 pmol NAAG per mg protein, a value that is at least 30-fold lower than that expressed by mixed glial cultures prepared from mouse cortex. We conclude that cerebellar astrocytes respond to NAAG via the mGluR3 receptor and that the peptide may selectively activate this receptor subtype in astrocytes following release from neurons or glia.

Developmental regulation of group III metabotropic glutamate receptors modulating adenylate cyclase activity in the avian retina

The regulation of adenylate cyclase by neurotransmitters is observed in early development of the chick retina. In the present work we show that L-2-amine-4-phosphonobutyric acid (L-AP4), the major agonist of group III metabotropic glutamate receptors (mGluRs), inhibits the accumulation of cyclic AMP induced by forskolin in the chick retina. This effect is observed after 8 days of development (E8), is maximal from E12-E17 and decreases at the post-hatching period (PH). The inhibition is also observed in cultures of retinal cells incubated for 2-8 days. We have also investigated the interaction between group III mGluRs and other receptors coupled to adenylate cyclase in the developing retina. The inhibition by L-AP4 is partially additive with that induced by the A1 adenosine agonist Cyclohexyladenosine and is not observed when cyclic AMP levels are increased with 2-chloroadenosine or dopamine. The group II mGluR agonist trans-(1S,3R)-1-amino-cyclopentanedicarboxylic acid has an inhibitory effect only on PH retinas, indicating that group II and group III mGluRs have a differential ontogenesis in this tissue. The results show that Group III mGluRs are expressed early during chick retina development and do not interact with other receptors known to be coupled to adenylate cyclase in the developing retina.

Immunohistochemical localisation of mGluR7 protein in the rodent and human cerebellar cortex using subtype specific antibodies

Metabotropic glutamate receptors (mGluRs) are a heterogeneous family of G protein coupled receptors that are linked to multiple second messenger systems to regulate neuronal excitability and synaptic transmission. To characterise the protein expression of the two mGluR7 receptor splice variants in human and rat cerebellar cortex, antibodies specific to mGluR7 were generated. Antibodies were raised against a glutathione-S-transferase fusion protein containing amino acid residues located in the extracellular domain common to both the human and rat mGluR7 splice variants. These antibodies specifically detected human mGluR7a in mammalian cells transfected with this receptor. In agreement with mGluR7 in situ hybridisation studies, immunohistochemistry performed at the light microscope level revealed that mGluR7 protein expression occurred most prominently in a particular population of nerve cells common to both the human and rat, located within the cerebellar cortex of gray matter contained within transverse folia. Moreover, strong mGluR7-like immunoreactivity was seen in Purkinje cell body cytoplasm of the Purkinje cell layer. In the most superficial cerebellar cortical layer, the molecular layer, immunostaining was observed in Purkinje cell associated proximal and distal dendritic trees. No detectable labelling was evident in intrinsic deep cerebellar nuclei known to contain GABAergic terminals of projecting Purkinje cell axons. These data are suggestive of a post-synaptic location of mGluR7 in this central nervous system structure. In the rodent, additional non-Purkinje cells thought to represent inhibitory interneurones were labelled at all levels in the molecular layer. mGluR7-like immunoreactivity was not associated with glial cells.

Effect of metabotropic glutamate receptor activation on receptor-mediated cyclic AMP responses in primary cultures of rat striatal neurones

Co-activation of group I metabotropic glutamate (mGlu) receptors and adenosine receptors resulted in an augmented cyclic AMP response in primary cultures of rat striatal neurones. L-glutamate and the selective group I agonist, (S)-dihydroxyphenylglycine (S-DHPG) evoked concentration-dependent potentiations of cyclic AMP accumulation stimulated by the adenosine receptor agonist, 5'-N-ethylcarboxamidoadenosine (NECA), with EC50 values of 3.41+/-0. 39 and 5.69+/-1.64 microM, respectively, and maximal augmentations of approximately 350% at concentrations of 100 microM. The S-DHPG potentiation was inhibited by group I mGlu receptor antagonists and a protein kinase C inhibitor, Ro 31-8220, implicating products of PI hydrolysis in this effect. Furthermore, L-glutamate and S-DHPG stimulated PI hydrolysis in striatal neuronal cultures with similar EC50 values to those observed for the augmentation of NECA cyclic AMP responses (5.19+/-1.18 and 3.78+/-1.42 microM, respectively). In situ hybridization and immunofluorescence techniques indicate that group I mGlu receptor-evoked potentiations are likely to be mediated via mGlu5 receptors, which are expressed at high levels in these cultures. In contrast to cross-chopped slices of neonatal rat striatum, of equivalent age, the group II mGlu receptor agonist, (2S, 2'R,3'R)-2-(2',3'-dicarboxycyclopropyl)glycine (DCG-IV) was without effect on NECA- or forskolin-stimulated cyclic AMP responses in primary striatal neuronal cultures. This lack of effect might be due to a low level of expression of group II mGlu receptors in cultured striatal neurones.

Inflammation alters the effects of mGlu receptor agonists on spinal nociceptive neurones

Several types of metabotropic glutamate receptor are known to be located in the spinal cord. This study examined the effects of the metabotropic glutamate receptor agonists (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid ((1S,3R)-ACPD), (S)-3,5-dihydroxyphenylglycine ((S)-3,5-DHPG) and (1S,3S)-1-aminocyclopentane-1,3-dicarboxylic acid ((1S,3S)-ACPD) on the electrically evoked responses of dorsal horn neurones recorded in normal animals and in animals 3 h after the induction of carrageenan inflammation. The group I and II agonist (1S,3R)-ACPD produced facilitations of the noxious evoked neuronal responses in normal animals, but inhibited these responses following carrageenan inflammation. The group II agonist (1S,3S)-ACPD also produced inhibitions in the carrageenan animals, in contrast to the mixed effects seen in normal animals. The group I agonist (S)-3,5-DHPG produced mixed effects (inhibitions and facilitations) in both normal and carrageenan animals. This in vivo study shows that the effects of metabotropic glutamate receptor agonists are more complex than in vitro studies have suggested to date.

The involvement of metabotropic glutamate receptors in sensory transmission in dorsal horn of the rat spinal cord

The role of metabotropic glutamate receptors in the processing of somatosensory information was studied in dorsal horn neurons of the rat spinal cord. Activation of metabotropic glutamate receptors by local iontophoresis of (1S,3R)-1-amino-cyclopentane-1,3-dicarboxylic acid resulted in an increased response of dorsal horn neurons to ionotropic glutamate receptor agonists (N-methyl-D-aspartate and kainic acid) applied by iontophoresis. Greater amounts of 1S,3R-1-amino-cyclopentane-1,3-dicarboxylic acid, ejected at high iontophoresis currents, directly excited dorsal horn neurons. Application of (1S,3R)-1-amino-cyclopentane-1,3-dicarboxylic acid also led to a significant increase in responses to innocuous (brush, pressure) but not in responses to noxious (pinch, squeeze) mechanical stimulation. The excitatory effects of (1S,3R)-1-amino-cyclopentane-1,3-dicarboxylic acid were selectively blocked by (S)-4-carboxy-3-hydroxyphenyl-glycine, an antinociceptive phenylglycine derivative which is a selective group 1 metabotropic glutamate receptor antagonist, confirming the involvement of these receptors. In wide dynamic range neurons, wind-up, the progressive potentiation of C-fibre-evoked responses during a train of stimuli, was increased by iontophoretic application of (1S,3R)-1-amino-cyclopentane-1,3-dicarboxylic acid or decreased by iontophoresis of (S)-4-carboxy-3-hydroxyphenyl-glycine without significant change in the C-fibre input. The results suggest an interaction between metabotropic and ionotropic glutamate receptors in spinal dorsal horn neurons. Metabotropic glutamate receptors proved to be involved in the frequency-dependent potentiation of C-fibre responses possibly via modulation of ionotropic glutamate receptors. The long-lasting effects of (1S,3R)-1-amino-cyclopentane-1,3-dicarboxylic acid on wind-up and on responses to peripheral mechanical stimuli strongly support the view that metabotropic glutamate receptors in these neurons may play a significant role in spinal synaptic plasticity, and therefore, may contribute to the central sensitization during mechanical hyperalgesia.

Localization of mGluR1a-like immunoreactivity and mGluR5-like immunoreactivity in identified populations of striatal neurons

Metabotropic glutamate receptors are important mediators of excitatory amino acid neurotransmission in the striatum. Two-color immunofluorescence histochemistry and immunohistochemistry in combination with retrograde tract-tracing techniques were used to examine the distribution of metabotropic glutamate receptor subtypes 1a and 5 (mGluR1a and mGluR5) among identified subpopulations of striatal projection neurons and interneurons. The majority of striatopallidal and striatonigral neurons were double-labeled for both mGluR1a or mGluR5. Approximately 60% to 70% of either striatonigral or striatopallidal neurons expressed mGluR1a- or mGluR5-like immunoreactivity. The percentage of double-labeled striatopallidal or striatonigral projection neurons did not differ among striatal quadrants. Striatal interneurons expressing parvalbumin or somatostatin or choline acetyltransferase exhibited varying degrees of expression of mGluR1a or mGluR5. Virtually all (94%) parvalbumin-immunoreactive striatal neurons expressed mGluR1a-like immunoreactivity with a majority (79%) of these neurons expressing mGluR5-like immunoreactivity. A high percentage (89%) of striatal choline acetyltransferase-immunoreactive neurons were double-labeled for mGluR1a-like immunoreactivity. Approximately 65% of striatal choline acetyltransferase-immunoreactive neurons expressed mGluR5-like immunoreactivity. A majority (65%) of somatostatin-immunoreactive striatal interneurons expressed mGluR1a-like immunoreactivity with a slightly lower percentage (55%) expressing mGluR5-like immunoreactivity. These findings indicate considerable heterogeneity among striatal projection and interneurons with respect to mGluR1a and mGluR5 expression. There may be subpopulations of striatonigral and striatopallidal projection neurons. These results are consistent as well with prior data indicating subpopulations of the different classes of striatal interneurons.

Distribution of a metabotropic glutamate receptor, mGluR2, in the central nervous system of the rat and mouse: an immunohistochemical study with a monoclonal antibody

The distribution of a metabotropic glutamate receptor mGluR2 in the central nervous system was immunohistochemically examined in the rat and mouse with a monoclonal antibody raised against an N-terminal sequence of rat mGluR2 (amino acid residues 87-134). Neuronal cell bodies with mGluR2-like immunoreactivity (mGluR2-LI) were clearly shown in the horizontal cells of Cajal in the cerebral cortex, neurons in the triangular septal nucleus and medial mammillary nucleus, Golgi cells and the unipolar brush cells in the cerebellar cortex, and Golgi-like and unipolar brush-like cells in the cochlear nucleus. Neuropil was intensely immunostained in the accessory olfactory bulb, bed nucleus of the accessory olfactory tract, neocortex, cingulate cortex, retrosplenial cortex, subicular and entorhinal cortices, stratum lacunosum-moleculare of CA1 and CA3, molecular layer of the dentate gyrus, periamygdaloid cortex, basolateral amygdaloid nucleus, bed nucleus of the anterior commissure, caudate-putamen, accumbens nucleus, thalamic reticular nucleus, anteroventral and paraventricular thalamic nuclei, granular layer of the cerebellar cortex, anterior and ventral tegmental nuclei, granular layer of the cochlear nucleus, and parvicellular part of the lateral reticular nucleus. Many axons in the white matter and fiber bundles were also immunostained. No glial cells with mGluR2-LI were found. No particular species differences were found in the distribution pattern of mGluR2-LI between the rat and mouse. The results indicate that mGluR2 is expressed not only in somato-dendritic domain, but also in axonal domain of excitatory and inhibitory neurons.

Enhanced expression of metabotropic glutamate receptor 3 messenger RNA in the rat spinal cord during ultraviolet irradiation induced peripheral inflammation

Metabotropic glutamate receptors are thought to play a role in the development and maintenance of spinal hyperexcitability resulting in hyperalgesia and pain. In this study we have used in situ hybridization to investigate the distribution of metabotropic glutamate receptors mGluR1-7 messenger RNA in the rat spinal cord in a model of inflammatory hyperalgesia. Hyperalgesia was induced in nine-day-old rats by exposure of the left hindpaw to an ultraviolet light source. Lumbar portions of spinal cords were removed from control and ultraviolet-treated animals. In situ hybridization with specific oligonucleotide probes was used to localize metabotropic glutamate receptor messenger RNAs. mGluR1, 3-5 and 7 subtype messenger RNA was detected in the gray matter of the spinal cord with distribution being specific for the different subtypes. A significant increase in the expression of mGluR3 messenger RNA was seen in cells of the dorsal laminae in both sides of the lumbar spinal cord. This increase was most pronounced in laminae II, III and IV but gradually decreased and disappeared by the third day of inflammation. In parallel with this, behavioural experiments revealed mechanical hyperalgesia in both hindlimbs after ultraviolet irradiation. There was no change in mGluR3 messenger RNA expression in the thoracic segments. No changes have been detected in the levels of expression of mGluR 1,2,4,5,7 subtype messenger RNA in spinal cords taken from hyperalgesic animals. These observations show that during ultraviolet irradiation induced inflammation, the synthesis of mGluR3 messenger RNA is altered suggesting that regulation of metabotropic glutamate receptor expression may be instrumental in plastic changes within the spinal cord during the development of hyperalgesia and pain.

Glutamate receptor-mediated calcium responses in acutely isolated hippocampal astrocytes

GFAP(+) cells were acutely isolated from the hippocampi of 1-10 day old rats, and the intracellular calcium responses to L-glutamate, ATP, and 5-HT were studied in single cells. Eighty-two percent of such cells responded to glutamate, 20% of them responded to ATP, and none responded to 5-HT. The same cells that failed to respond to ATP and 5-HT often responded to glutamate. These proportions of cells responding to glutamate and ATP are very similar to those reported for GFAP(+) astrocytes in hippocampal slices (Porter and McCarthy, 1995a,b). After culturing for 1-2 days in serum-containing medium, 60% of such acutely isolated cells responded to either glutamate or ATP, and 5% to 5-HT. After 1 week in culture, the percentage of cells responding to glutamate remained essentially the same (62%) but the percentages of cells responding to ATP and 5-HT increased to 92% and 62%, respectively. These percentages were very close to the results obtained from primary hippocampal astrocyte cultures prepared from 1 day old rats and cultured for 1-2 weeks in vitro. Pharmacological characterization showed that the Ca2+ responses of acutely isolated hippocampal astrocytes from P1-10 rats was due to activation of a group I metabotropic glutamate receptor. The calcium responses to ATP and 5-HT in acutely isolated cells that were then cultured were mediated by P2y and 5-HT2A receptors, respectively. These data show that, like cortical astrocytes (Kimelberg et al., 1997), GFAP(+) astrocytes cultured from the hippocampi of young rats showed marked differences in receptor expression compared to their acutely isolated counterparts. Also, since the astrocytes acutely isolated from these 2 different brain regions showed qualitatively the same responses for the 3 receptors selected, it indicates a degree of homogeneity of receptor expression for astrocytes from these 2 brain regions.

Differential presynaptic localization of metabotropic glutamate receptor subtypes in the rat hippocampus

Neurotransmission in the hippocampus is modulated variously through presynaptic metabotropic glutamate receptors (mGluRs). To establish the precise localization of presynaptic mGluRs in the rat hippocampus, we used subtype-specific antibodies for eight mGluRs (mGluR1-mGluR8) for immunohistochemistry combined with lesioning of the three major hippocampal pathways: the perforant path, mossy fiber, and Schaffer collateral. Immunoreactivity for group II (mGluR2) and group III (mGluR4a, mGluR7a, mGluR7b, and mGluR8) mGluRs was predominantly localized to presynaptic elements, whereas that for group I mGluRs (mGluR1 and mGluR5) was localized to postsynaptic elements. The medial perforant path was strongly immunoreactive for mGluR2 and mGluR7a throughout the hippocampus, and the lateral perforant path was prominently immunoreactive for mGluR8 in the dentate gyrus and CA3 area. The mossy fiber was labeled for mGluR2, mGluR7a, and mGluR7b, whereas the Schaffer collateral was labeled only for mGluR7a. Electron microscopy further revealed the spatial segregation of group II and group III mGluRs within presynaptic elements. Immunolabeling for the group III receptors was predominantly observed in presynaptic active zones of asymmetrical and symmetrical synapses, whereas that for the group II receptor (mGluR2) was found in preterminal rather than terminal portions of axons. Target cell-specific segregation of receptors, first reported for mGluR7a (Shigemoto et al,., 1996), was also apparent for the other group III mGluRs, suggesting that transmitter release is differentially regulated by 2-amino-4-phosphonobutyrate-sensitive mGluRs in individual synapses on single axons according to the identity of postsynaptic neurons.

Differential plasma membrane distribution of metabotropic glutamate receptors mGluR1 alpha, mGluR2 and mGluR5, relative to neurotransmitter release sites

Two group I metabotropic glutamate receptor subtypes, mGluR1 and mGluR5, have been reported to occur in highest concentration in an annulus surrounding the edge of the postsynaptic membrane specialisation. In order to determine whether such a distribution is uniform amongst postsynaptic mGluRs, their distribution was compared quantitatively by a pre-embedding silver-intensified immunogold technique at electron microscopic level in hippocampal pyramidal cells (mGluR5), cerebellar Purkinje cells (mGluR1 alpha) and Golgi cells (mGluR2). The results show that mGluR1 alpha, mGluR5 and mGluR2 each have a distinct distribution in relation to the glutamatergic synaptic junctions. On dendritic spines, mGluR1 alpha and mGluR5 showed the highest receptor density in a perisynaptic annulus (defined as within 60 nm of the edge of the synapse) followed by a decreasing extrasynaptic (60-900 nm) receptor level, but the gradient of decrease and the proportion of the perisynaptic pool (mGluR1 alpha, approximately 50%; vs mGluR5, approximately 25%) were different for the two receptors. The distributions of mGluR1 alpha and mGluR5 also differed significantly from simulated random distributions. In contrast, mGluR2 was not closely associated with glutamatergic synapses in the dendritic plasma membrane of cerebellar Golgi cells and its distribution relative to synapses is not different from simulated random distribution in the membrane. The somatic membrane, the axon and the synaptic boutons of the GABAergic Golgi cells also contained immunoreactive mGluR2 that is not associated with synaptic specialisations. In the hippocampal CA1 area the distribution of immunoparticles for mGluR5 on individual spines was established using serial sections. The results indicate that dendritic spines of pyramidal cells are heterogeneous with respect to the ratio of perisynaptic to extrasynaptic mGluR5 pools and about half of the immunopositive spines lack the perisynaptic pool. The quantitative comparison of receptor distributions demonstrates that mGluR1 alpha and mGluR5, but not mGluR2, are highly compartmentalised in different plasma membrane domains. The unique distribution of each mGluR subtype may reflect requirements for different transduction and effector mechanisms between cell types and different domains of the same cell, and suggests that the precise placement of receptors is a crucial factor contributing to neuronal communication.

Metabotropic glutamate receptor expression in cultured rat astrocytes and human gliomas

In order to confirm the existence of metabotropic glutamate receptors in astroglial cultures and to provide information on different receptor subtypes, the expression of different mGluRs was analysed in cultures highly enriched in rat astroglial cells. mRNA levels for mGluR1, 2, 3, 4, 7 were undetectable by Northern blot analysis in primary type-1 astroglial cultures derived from total cerebral hemispheres, cerebral cortex and striatum. Interestingly, these cultures expressed a low, but detectable, level of mGluR5 mRNA. The more sensitive technique Reverse Transcription-Polymerase Chain Reaction (RT-PCR) confirmed the presence of mGluR5 transcript in cultured astrocytes and, in addition, revealed the presence of mGluR3 mRNA. The lack of expression of mGluR5 in CG-4 cells, a rat cell line able to differentiate in type-2 astrocytes or oligodendrocytes depending on the culture conditions, suggested that the presence of mGluR5 was not a general feature of cells of glial origin. Moreover, all the examined mGluR transcript were undetectable by RT-PCR in CG4 cells. In order to confirm the possible expression of mGluR5 in cell of glial origin we examined the mRNA levels for this receptor in tissue samples from human gliomas obtained after surgical resection of the tumors: only 1 sample (grade II astrocytoma), out of 8 examined, showed the presence of mGluR5 mRNA. In conclusion our data show that the only cloned metabotropic receptor linked to phosphoinositide hydrolysis, whose expression is detectable in cultured type-1 astrocytes, in mGluR5. It remains to be established if the low level of expression of mGluR3 could be responsible for the group II metabotropic glutamate receptor activity previously observed in cultured astroglial cells.

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.

Developmentally regulated gene expression of all eight metabotropic glutamate receptors in hypothalamic suprachiasmatic and arcuate nuclei--a PCR analysis

Previous studies have demonstrated the critical role glutamate plays in the hypothalamus, both in the developing and adult brain. The expression of metabotropic glutamate receptor (mGluR) mRNA (mGluR1-8) was studied in the suprachiasmatic (SCN) and arcuate (ARC) nuclei. Using reverse Northern blots and cDNA-PCR, we found that all eight cloned mGluRs were expressed in these brain regions. Most had not previously been detected here. Surprisingly, this included mGluRs that had previously been thought to be restricted to the retina, such as mGluR6. We also detected, cloned, and sequenced a splice variant of mGluR7 (mGluR7b). Developmentally, the age of maximal expression of mGluRs was dependent on the region. For instance, mGluR5 was more strongly expressed in neonatal ARC than in adult, whereas the opposite was true in the SCN. Compared with P10 neonates, mGluR1, R3, R6, R7a, R7b, and R8 showed a greater expression in adult SCN and ARC.

Cloning and stable expression of the mGluR1b subtype of human metabotropic receptors and pharmacological comparison with the mGluR5a subtype

We isolated and characterized a cDNA encoding the human metabotropic glutamate receptor subtype 1b (hmGluR1b). In situ hybridization studies in human brain regions revealed a higher distribution of mGluR1 mRNA in the dentate gyrus of the hippocampus, the substantia nigra pars compacta and the Purkinje cell layer of the cerebellum compared to other regions studied. We established stable expression of recombinant hmGluR1b in L(tk-) mouse fibroblast and Chinese hamster ovary (CHO-dhfr-) cells. In both expression systems, agonist activation of hmGluR1b stimulated inositol phosphate (InsP) formation and elevation of the cytosolic free calcium ([Ca2+]i), and both responses were blocked by (S)-MCPG. The rank order of potency for agonists was quisqualate > glutamate > (1S,3R)-ACPD in both expression systems. Comparison of the agonist profiles of hmGluR1b and hmGluR5a, both stably expressed in L(tk-) cells, indicated the same rank order of potency (quisqualate > glutamate > or = (RS)-3,5-DHPG > or = (1S,3R)-ACPD), but each of the four agonists were more potent on hmGluR5a than on hmGluR1b. In antagonist studies, (S)-MCPG inhibited the agonist-induced InsP formation and elevation of [Ca2+]i in both hmGluR1b- and hmGluR5a-expressing cells. (S)-4CPG and (S)-4C3HPG both inhibited agonist responses only in hmGluR1b-expressing cells. However, in hmGluR5a-expressing cells the antagonist activity of (S)-4CPG and (S)-4C3HPG was dependent on the agonist used in the study, since they inhibited responses to glutamate but not to quisqualate. Stable cell lines expressing specific subtypes of human mGluRs represent valuable tools for the study of the mechanism of action of mGluRs at the molecular and cellular level and as screening targets for identification of subtype-selective agonists or antagonists.

Pharmacological characterization of metabotropic glutamate receptors linked to the inhibition of adenylate cyclase activity in rat striatal slices

The pharmacological profile of mGlu receptors negatively linked to adenylyl cyclase was characterized in adult rat striatal slices. Among the mGlu agonists tested, (+)-2-aminobicyclo-[3.1.0]-hexane-2,6-di carboxylate (LY354740), was the most potent inhibitor of forskolin-stimulated cAMP formation (EC50 = 11 +/- 2 nM). Inhibition of forskolin stimulation by the group III agonist L-2-amino-4-phosphono-butanoate (L-AP4) was biphasic, the two parts of the concentration curve having EC50 values of 6 +/- 1 microM and 260 +/- 4 microM, suggesting a sequential recruitment of mGlu4/8 and mGlu7. The effects of several new phenylglycine derivative antagonists were tested on the inhibition of forskolin cAMP response by (2S,1'S,2'S)-2-(carboxy-cyclopropyl)-glycine (L-CCG I) and L-AP4. At 500 microM, (RS)-alpha-methyl-3-carboxy-methyl-pheny lglycine was unable to antagonize the effect of L-CCG I or L-AP4 but (S)-alpha-methyl-3-carboxy-phenylalanine inhibited the effect of L-AP4 with a low potency. Finally, (RS)-alpha-methyl-4-tetrazolylphenylglyc ine and particularly (RS)-alpha-methyl-4-phosphonophenylglyci ne, appeared to be the most potent and selective antagonists of L-AP4 induced inhibition of forskolin-stimulated cAMP production in adult rat striatal slices.

Metabotropic glutamate receptors in the rat nucleus accumbens

The effects of glutamate metabotropic receptors (mGluRs) on excitatory transmission in the nucleus accumbens were investigated using electrophysiological techniques in rat nucleus accumbens slices. The broad-spectrum mGluR agonist (1S,3R)-1-aminocyclopentyl-1,3-dicarboxylate, the mGluR group 2 selective agonists (S)-4-carboxy-3-hydroxyphenylglycine, (1S,3S)-ACPD) and (2S,1'S,2'S)-2-(2'-carboxycyclopropyl)glycine (L-CCG1), and the mGluR group 3 specific agonist L-2-amino-4-phosphonobutyrate (L-AP4) all reversibly inhibited evoked excitatory synaptic responses. The specific group 1 mGluR agonist (R,S)-3,5-dihydroxyphenylglycine [(R,S)-DHPG] did not depress transmission. Dose-response curves showed that the rank order of agonist potencies was: L-CCG1 > L-AP4 > (1S,3S)-ACPD. Group 2 and 3 mGluRs inhibited transmission via a presynaptic mechanism, as they increased paired-pulse facilitation, decreased the frequency of miniature excitatory postsynaptic currents and had no effect on their amplitude. The mGluRs did not inhibit transmitter release by reducing voltage-dependent Ca2+ currents through N- or P-type Ca2+ channels, as inhibition persisted in the presence of omega-conotoxin-GVIA or omega-Aga-IVA. The depression induced by mGluRs was not affected by specific antagonists of dopamine D1, GABA-B or adenosine A1 receptors, indicating direct effects. Finally, (R,S)-DHPG specifically blocked the postsynaptic afterhyperpolarization current (I(AHP)). Our results represent the first direct demonstration of functional mGluRs in the nucleus accumbens of the rat.

Metabotropic glutamate agonist-induced rotation: a pharmacological, FOS immunohistochemical, and [14C]-2-deoxyglucose autoradiographic study

Metabotropic glutamate receptors (mGluRs) are a major class of excitatory amino acid receptors. Eight mGluR subtypes, coupled to a variety of effector systems, have been cloned. These receptors have been classified into three groups based on amino acid sequence homology, effector systems, and pharmacological profile. Group I mGluRs increase phosphoinositide turnover, whereas groups II and III mGluRs are negatively coupled to adenylyl cyclase. The striatum possesses a high density of mGluR binding sites, and several mGluR mRNAs and proteins are expressed by striatal neurons. In rats, unilateral striatal injection of the nonsubtype selective mGluR agonist 1-aminocyclopentane-1S,3R-dicarboxylic acid (1S,3R-ACPD) results in contralateral rotation with delayed onset, thought to be secondary to an increase in dopamine release. We sought to determine the mGluR subtype(s) involved, the modulation of the rotation by other basal ganglia neurotransmitter systems, and the functional anatomy underlying the rotational behavior. The group I mGluR agonist 3,5-dihydroxyphenylglycine (DHPG) induced contralateral rotation in a dose-dependent manner, whereas group II and group III agonists were ineffective. Rotation induced by DHPG or 1S,3R-ACPD was attenuated by group I antagonists, but not by group II or group III antagonists. This suggests that the rotation is mediated by group I mGluRs. Rotation induced by DHPG or 1S,3R-ACPD was attenuated by pretreatment with antagonists at muscarinic cholinergic, adenosine A2, dopamine D2, or dopamine D1 receptors. Examination of FOS-like immunoreactivity after group I and group II mGluR agonist administration suggests increased activity in the striatopallidal pathway. However, [14C]-2-deoxyglucose uptake studies indicate increased activity in nuclei of the striatopallidal (indirect) pathway, particularly in the subthalamic nucleus, only after group I mGluR activation.

Metabotropic glutamate receptor mediated long-term depression in developing hippocampus

The effects of bath application of the metabotropic glutamate receptor (mGluR) agonist 1S,3R-1-aminocyclopentane-1,3-dicarboxylic acid (ACPD, 10 microM) were studied at the Schaffer collateral-CA1 synapse in hippocampal slices from rats of 8-33 days postnatal age. In immature animals (8-12 days) ACPD induced a biphasic response characterized by an acute decrease in field EPSP slope (approximately 50-60% of baseline) in the presence of the agonist, followed by long-term depression (LTD, approximately 75-80% of baseline) after washout. In animals older than 20 days, ACPD induced a slow onset potentiation or minimal change. Both the acute depression and LTD were blocked by the mGluR antagonist alpha-methyl-4-carboxyphenyl glycine (MCPG). ACPD-induced LTD was blocked by the N-methyl-D-aspartate receptor (NMDAR) antagonists D(-)-2-amino-5 phosphopentanoic acid (AP5) and dizocilpine maleate (MK-801), and by ethanol. Glutamic pyruvic transaminase, an enzyme that selectively metabolizes endogenous extracellular glutamate, also blocked LTD suggesting that the requisite NMDA currents were tonically activated by extracellular rather than synaptically released glutamate. ACPD-induced LTD was blocked by staurosporine, indicating a requirement for serinethreonine kinase activation, and was unaffected by the L-type voltage sensitive calcium channel blocker nitrendipine and the A1 adenosine receptor antagonist 8-cyclopentyl-1,3-dimethylxanthine (CPT). Because mGluR-mediated LTD was observed only in immature CA1, mGluRs may play a role in hippocampal development, perhaps by contributing to synapse pruning in a temporally restricted fashion.

The pharmacology of amino-acid responses in septal neurons

Septal cholinergic neurons are known to play an important role in cognitive processes including learning and memory through afferent innervation of the hippocampal formation and cerebral cortex. The septum contains not only cholinergic neurons but also various types of neurons including GABA (gamma-aminobutyric acid)-ergic neurons. Although synaptic transmission in the septum is mediated primarily by the activation of excitatory and inhibitory amino-acid receptors, it is possible that a distinct phenotype of neuron is endowed with a different type for each of the amino-acid receptors and thus they play different roles from each other, since it has been demonstrated within the septum that there is a regional distribution of various types of amino-acid receptor subunits, their expression as different combinations within a specific cell may produce receptor channels with disparate functional properties. As a first step towards knowing the various functions of septal cholinergic neurons, we characterized the functional properties of glutamate, GABA (type A; GABAA) and glycine receptor channels on cultured rat septal neurons which were histologically identified to be cholinergic. These were similar to those of receptor channels on other types of neurons, except for the actions of some neuromodulators. The septal N-methyl-D-aspartate receptor channel was distinct in being less sensitive to Mg2+ and in a voltage-dependent action of Zn2+. The septal GABAA receptor channel exhibited a lanthanide site whose activation resulted in a positive allosteric interaction with a binding site of pentobarbital. The septal glycine receptor channel was only positively modulated by Zn2+; this action of Zn2+ was not accompanied by an inhibitory effect. Our data suggest that the amino-acid receptors on septal cholinergic neurons may play a distinct role compared to other types of neurons; this difference depends on the actions of neuromodulators and metal cations. It would be interesting to compare these effects recorded in tissue culture to those observed with septal cholinergic neurons in slice preparations.

Cerebellar Grafts Partially Reverse Amino Acid Receptor Changes Observed in the Cerebellum of Mice with Hereditary Ataxia: Quantitative Autoradiographic Studies

We used quantitative autoradiography of [3H]CNQX (200 nM), [3H]muscimol (13 nM), and [3H]flunitrazepam (10 nM) binding to study the distribution of non-NMDA and GABAA receptors in the cerebellum of pcd mutant mice with unilateral cerebellar grafts. Nonspecific binding was determined by incubation with 1 mM Glu, 200 μM GABA, or 1 μM clonazepam, respectively. Saturation parameters were defined in wild-type and mutant cerebella. In mutants, non-NMDA receptors were reduced by 38% in the molecular layer and by 47% in the granule cell layer. The reduction of non-NMDA receptors in the pcd cerebellar cortex supports their localization on Purkinje cells. [3H] CNQX binding sites were visualized at higher density in grafts that had migrated to the cerebellar cortex of the hosts (4.1 and 11.0 pmol/mg protein, respectively, at 23 and 37 days after grafting) than in grafts arrested intraparen-chymally (2.6 and 6.2 pmol/mg protein, respectively, at 23 and 37 days after grafting). The pattern of expression of non-NMDA receptors in cortical vs. parenchymal grafts suggests a possible regulation of their levels by transacting elements from host parallel fibers. GABAA binding levels in the grafts for both ligands used were similar to normal molecular layer. Binding was increased in the deep cerebellar nuclei of pcd mutants: the increase in [3H]muscimol binding over normal was 215% and the increase in [3H]flunitrazepam binding was 89%. Such increases in the pcd deep cerebellar nuclei may reflect a denervation-induced supersensitivity subsequent to the loss of Purkinje axon terminal innervation. In the deep nuclei of pcd mutants with unilateral cerebellar grafts, [3H]muscimol binding was 31% lower in the grafted side than in the contralateral nongrafted side at 37 days after transplantation; [3H]fluni-trazepam binding was also lower in the grafted side by 15% compared to the nongrafted side. Such changes in GABAA receptors suggest a significant, albeit partial, normalizing trend of cerebellar grafts on the state of postsynaptic supersensitive receptors in the host cerebellar nuclei.

Activation of group-II metabotropic glutamate receptors blocks induction of long-term potentiation and depotentiation in area CA1 of the rat in vivo

The metabotropic glutamate receptor group-II agonist (1S,3S)-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3S-ACPD; 5 microliters/10 mM, i.c.v.) prevented the induction of long-term potentiation and depotentiation in the CA1 area of the hippocampus in urethane-anaesthetised rats. These effects were prevented by the group-II metabotropic glutamate receptor antagonists alpha-methyl-(2S,3S,4S)-alpha-(carboxycyclopropyl)glycine (MCCG; 5 microliters/100 mM) and (RS)-alpha-methyl-4-tetrazolylphenylglycine (MTPG; 5 microliters/500 mM). The group-I antagonist (RS)-1-aminoindan-1,5-dicarboxylic acid (AIDA; 5 microliters/200 mM) or the group-III antagonist alpha-methyl-L-2-amino-4-phosphonobutyrate (MAP4; 5 microliters/100 mM) did not affect the block of the induction of long-term potentiation by 1S,3S-ACPD. It is concluded that activation of group-II metabotropic glutamate receptors can block both high-frequency stimulation-induced long term potentiation and low-frequency stimulation-induced depotentiation in the CA1 area in vivo.

Control of the phosphorylation of the astrocyte marker glial fibrillary acidic protein (GFAP) in the immature rat hippocampus by glutamate and calcium ions: possible key factor in astrocytic plasticity

The present review describes recent research on the regulation by glutamate and Ca2+ of the phosphorylation state of the intermediate filament protein of the astrocytic cytoskeleton, glial fibrillary acidic protein (GFAP), in immature hippocampal slices. The results of this research are discussed against a background of modern knowledge of the functional importance of astrocytes in the brain and of the structure and dynamic properties of intermediate filament proteins. Astrocytes are now recognized as partners with neurons in many aspects of brain function with important roles in neural plasticity. Site-specific phosphorylation of intermediate filament proteins, including GFAP, has been shown to regulate the dynamic equilibrium between the polymerized and depolymerized state of the filaments and to play a fundamental role in mitosis. Glutamate was found to increase the phosphorylation state of GFAP in hippocampal slices from rats in the post-natal age range of 12-16 days in a reaction that was dependent on external Ca2+. The lack of external Ca2+ in the absence of glutamate also increased GFAP phosphorylation to the same extent. These effects of glutamate and Ca2+ were absent in adult hippocampal slices, where the phosphorylation of GFAP was completely Ca(2+)-dependent. Studies using specific agonists of glutamate receptors showed that the glutamate response was mediated by a G protein-linked group II metabotropic glutamate receptor (mGluR). Since group II mGluRs do not act by liberating Ca2+ from internal stores, it is proposed that activation of the receptor by glutamate inhibits Ca2+ entry into the astrocytes and consequently down-regulates a Ca(2+)-dependent dephosphorylation cascade regulating the phosphorylation state of GFAP. The functional significance of these results may be related to the narrow developmental window when the glutamate response is present. In the rat brain this window corresponds to the period of massive synaptogenesis during which astrocytes are known to proliferate. Possibly, glutamate liberated from developing synapses during this period may signal an increase in the phosphorylation state of GFAP and a consequent increase in the number of mitotic astrocytes.

Astrocytic neurotransmitter receptors in situ and in vivo

In the brain, astrocytes are associated intimately with neurons and surround synapses. Due to their close proximity to synaptic clefts, astrocytes are in a prime location for receiving synaptic information from released neurotransmitters. Cultured astrocytes express a wide range of neurotransmitter receptors, but do astrocytes in vivo also express neurotransmitter receptors and, if so, are the receptors activated by synaptically released neurotransmitters? In recent years, considerable efforts has gone into addressing these issues. The experimental results of this effort have been compiled and are presented in this review. Although there are many different receptors which have not been identified on astrocytes in situ, it is clear that astrocytes in situ express a number of different receptors. There is evidence of glutamatergic, GABAergic, adrenergic, purinergic, serotonergic, muscarinic, and peptidergic receptors on protoplasmic, fibrous, or specialized (Bergmann glia, pituicytes, Müller glia) astrocytes in situ and in vivo. These receptors are functionally coupled to changes in membrane potential or to intracellular signaling pathways such as activation of phospholipase C or adenylate cyclase. The expression of neurotransmitter receptors by astrocytes in situ exhibits regional and intraregional heterogeneity and changes during development and in response to injury. There is also evidence that receptors on astrocytes in situ can be activated by neurotransmitter(s) released from synaptic terminals. Given the evidence of extra-synaptic signaling and the expression of neurotransmitter receptors by astrocytes in situ, direct communication between neurons and astrocytes via neurotransmitters could be a widespread form of communication in the brain which may affect many different aspects of brain function, such as glutamate uptake and the modulation of extracellular space.

Characterization of metabotropic glutamate receptor-mediated nitric oxide production in vivo

We tested the hypothesis that stimulation of metabotropic glutamate receptors (mGluRs) increases nitric oxide (NO) production in the hippocampus in vivo. Microdialysis probes were placed bilaterally into the CA3 region of the hippocampus of adult Sprague-Dawley rats under pentobarbital anesthesia. Probes were perfused for 5 h with artificial cerebrospinal fluid (CSF) containing 3 microM [14C]-L-arginine. Recovery of [14C]-L-citrulline in the effluent was used as a marker of NO production. In nine groups of rats, increases in [14C]-L-citrulline recovery were compared between right- and left-sided probes perfused with various combinations of the selective mGluR agonist, trans-(1S,3R)-1-amino-1,3-cyclopentanedicarboxylic acid (ACPD); the mGluR antagonist, (+/-)-alpha-methyl-4-carboxyphenylglycine (MCPG); the NO synthase inhibitor, N-nitro-L-arginine (LNNA); the ryanodine sensitive calcium-release channel inhibitor dantrolene, the non-N-methyl-D-aspartate (NMDA); receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX); the NMDA receptor antagonist (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d] cyclohepten-5,10-imine (MK-801); and the Na+ channel blocker, tetrodotoxin. Recovery of [14C]-L-citrulline during perfusion with artificial CSF progressively increased to 90 +/- 21 fmol/min (+/-SD) over 5 h. Perfusion in the contralateral hippocampus with 1 mM ACPD augmented [14C]-L-citrulline recovery to 250 +/- 81 fmol/min. Perfusion of 1 mM nitroarginine + ACPD inhibited [14C]-L-citrulline recovery compared to that with ACPD alone. Perfusion with 1 mM MCPG + ACPD attenuated ACPD enhanced [14C]-L-citrulline recovery. Perfusion of 1 mM dantrolene + ACPD inhibited the ACPD-evoked increase in [14C]-L-citrulline recovery. Perfusion of 1 mM MCPG or dantrolene without ACPD did not decrease [14C]-L-citrulline recovery as compared to CSF alone. ACPD-enhanced [14C]-L-citrulline recovery was not attenuated by CNQX, MK-801, or tetrodotoxin (TTX). Using an indirect method of assessing NO production in vivo, these data demonstrate that mGluR stimulation enhances NO production in rat hippocampus. Inhibition with dantrolene suggests that calcium-induced calcium release amplifies the inositol triphosphate-mediated calcium signal associated with mGluR stimulation, thereby resulting in augmented calcium-dependent NO production.

Metabotropic glutamate receptors mGluR2 and mGluR5 are expressed in two non-overlapping populations of Golgi cells in the rat cerebellum

The metabotropic glutamate receptor subtypes mGluR2 and mGluR5, which are thought to be coupled respectively to the inhibitory cyclic adenosine monophosphate (cAMP) cascade and the phosphatidylinositol hydrolysis/Ca2+ cascade, are known to be expressed on Golgi cells in the granular layer of the rat cerebellar cortex. In the present immunohistochemical study with a monoclonal antibody against mGluR2 and a polyclonal antibody for mGluR5, we examined whether or not mGluR2- and mGluR5-like immunoreactivities were both present in single Golgi cells in the rat cerebellar cortex. In double immunofluorescence histochemistry, no Golgi cells showed mGluR2- and mGluR5-like immunoreactivities simultaneously. Of the total number of Golgi cells immunoreactive for mGluR2 or mGluR5, about 90% were mGluR2-like immunoreactive, and about 10% were mGluR5-like immunoreactive. Golgi cells with mGluR2-like immunoreactivity were distributed evenly in the granular layer of all the cerebellar regions, while those with mGluR5-like immunoreactivity were distributed more frequently in the I, II, VII-X lobules of the vermis and the copula pyramidis of the hemisphere than in other cerebellar regions. The results indicate that Golgi cells containing mGluR2 are segregated from those possessing mGluR5. These two populations of Golgi cells, each equipped with a different metabolic glutamate receptor coupled to a different intracellular signal transduction system, may play different roles in the glutamatergic neuronal circuits in the cerebellar cortex.

Glutamate as a candidate retrograde messenger at interneurone-Purkinje cell synapses of rat cerebellum

1. Depolarization-induced suppression of inhibition (DSI) is a form of synaptic plasticity which involves a retrograde messenger. We have performed experiments in Purkinje cells of rat cerebellar slices to determine the nature of this messenger. 2. DSI is mimicked by 2-(2,3-dicarboxycyclopropyl)-glycine (DCG-IV), a specific agonist of group II metabotropic glutamate receptors (mGluRs). 3. DSI is reduced if transmitter release is inhibited by saturating doses of DCG-IV. 4. Both DSI and DCG-IV-induced inhibition are inhibited by L-2-amino-3-phosphonopropionic acid (L-AP3), a drug which interferes with several subtypes of mGluRs. 5. DSI is reduced if synaptic activity is enhanced by application of forskolin. 6. We propose that glutamate or a glutamate-like substance is the retrograde messenger implicated in DSI, and that the inhibition resulting from presynaptic glutamate binding is mediated by a decrease in the presynaptic concentration of cAMP.

The effects of (RS)-alpha-cyclopropyl-4-phosphonophenylglycine ((RS)-CPPG), a potent and selective metabotropic glutamate receptor antagonist

1. In this study we describe the potent antagonist activity of a novel metabotropic glutamate (mGlu) receptor antagonist (RS)-alpha-cyclopropyl-4-phosphonophenylglycine ((RS)-CPPG) which exhibits selectivity for mGlu receptors (group II and III) negatively coupled to adenylyl cyclase in the adult rat cortex. 2. Both the L-2-amino-4-phosphonobutyrate (L-AP4) and (2S, 1'S, 2'S)-2-(carboxycyclopropyl)glycine (L-CCG-1) inhibition of forskolin-stimulated cyclic AMP accumulation were potently reversed by (RS)-CPPG (IC50 values: 2.2 +/- 0.6 nM and 46.2 +/- 18.2 nM, respectively). 3. In contrast, (RS)-CPPG acted as a weak antagonist against group I mGlu receptors. In neonatal rat cortical slices, (RS)-CPPG antagonized (KB = 0.65 +/- 0.07 mM) (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid ((1S,3R)-ACPD)-stimulated phosphoinositide hydrolysis. (RS)-CPPG (100 microM) failed to influence L-quisqualate-stimulated phosphoinositide hydrolysis in cultured cerebellar granule cells. 4. In the rat cerebral cortex, (RS)-CPPG is the most potent antagonist of group II/III mGlu receptors yet described (with 20 fold selectivity for group III mGlu receptors), having negligible activity at group I mGlu receptors.

Immunohistochemical distribution of metabotropic glutamate receptor subtypes mGluR1b, mGluR2/3, mGluR4a and mGluR5 in human hippocampus

The metabotropic glutamate receptors (mGluRs) can be classified into three families based on amino acid sequence homology, signal transduction mechanisms and pharmacological properties. Generally, class I mGluRs mediate an excitation of neurons while activation of class II and III mGluRs results in a depression of synaptic transmission. In this study we have analyzed the expression pattern of mGluRs in human hippocampus using a panel of polyclonal antibodies specific for mGluR1b, mGluR2/3, mGluR4a, and mGluR5. Immunoreactivity for mGluR1b and mGluR5, i.e., the subtypes representing class I mGluRs, was found in all hippocampal neurons. The mGluR1b antiserum stained perikarya and proximal dendrites, whereas immunoreactivity for mGluR5 was also detectable in the distal dendritic compartments. Immunoreactivity for mGluR2/3, members of class II mGluRs, was present in all principle neurons in the dentate gyrus as well as in the CA4, CA3 and CA2 regions. Pyramidal cells of the CA1 region exhibited only weak labeling for mGluR2/3. Glial cells were also mGluR2/3-immunoreactive. The reaction obtained with an antiserum directed against mGluR4a, a member of class III mGluRs, was confined to the mossy fiber projection field in CA3 stratum lucidum. These data demonstrate differential expression of mGluR variants in the human hippocampus and may provide an important basis for future studies of mGluRs under various neuropathological conditions such as temporal lobe epilepsy, ischemia and neurodegenerative disorders.

Metabotropic glutamate receptor activation contributes to nociceptive reflex activity in the rat spinal cord in vitro

The contribution of metabotropic glutamate receptor activation to the spinal segmental reflex response evoked at high-intensity electrical stimulation suggesting a role in nociception, has been examined in an in vitro preparation of neonatal rat spinal cord. Segmental reflex responses were recorded as a ventral root depolarization evoked following drug perfusion to the spinal cord or by electrical activation of high-threshold nociceptive afferent fibres. Superfusion of the selective metabotropic glutamate receptor agonist, (1S, 3R)-1-aminocyclopentane-1,3-dicarboxylic acid [(1S,3R)-ACPD], to the spinal cord produced a dose-dependent, reversible ventral root depolarization (EC50 = 58 +/- 7 microM; n = 4), which was antagonized by the selective metabotropic glutamate receptor antagonist, (+)-alpha-methyl-4-carboxyphenylglycine (MCPG; IC50 = 243 +/- 61 microM; n = 4). MCPG, over the same concentration range (10 microM-5.0 mM) did not affect N-methyl-D-aspartate-induced ventral root depolarizations. In contrast, the specific N-methyl-D-aspartate receptor antagonist D(-)-2-amino-5-phosphonopentanoic acid (D-AP5) reduced N-methyl-D-aspartate-evoked ventral root depolarization but did not affect the depolarization evoked by (1S,3R)-ACPD, thus indicating the specificity of the antagonists for these aggregate responses. MCPG significantly reduced the prolonged phase of the single shock C-fibre-evoked ventral root depolarization (IC50 = 2.9 +/- 0.2 mM; n = 3-5). Low frequency high intensity stimulation of the dorsal root evoked a wind-up response, the amplitude of which was attenuated by both D-AP5 and MCPG in a dose-dependent manner. The ventral root depolarization evoked by capsaicin application (1.0 microM, 30 s) was blocked by both MCPG (IC50 = 809 +/- 35 microM; n = 4) and D-AP5 (IC50 = 143 +/- 43 microM; n = 4). These data suggest that both D-AP5 and MCPG reduced C-fibre-induced ventral root responses. In addition to N-methyl-D-aspartate receptor, metabotropic glutamate receptor activation appears to be involved in the generation of the segmental spinal reflex evoked by high-intensity stimulation in the neonatal rat spinal cord in vitro.

Ionotropic and metabotropic glutamate receptors show unique postsynaptic, presynaptic, and glial localizations in the dorsal cochlear nucleus

The dorsal cochlear nucleus (DCN) is a major brain center for integration of auditory information, and excitatory amino acid neurotransmission plays a central role in the processing of this information. In this study, the distribution of glutamate receptors was examined with preembedding immunocytochemistry, using 14 antibodies to ionotropic (GluR1, GluR2/3, GluR4, GluR5-7, GluR6/7, KA2, NR1, NR2A/B, delta 1/2) and metabotropic (mGluR1 alpha, mGluR2/3, mGluR5) glutamate receptor subtypes. Each of these antibodies produced a specific immunolabeling pattern, including a variety of postsynaptic, presynaptic, and glial localizations. Some antibodies showed widespread distribution patterns, notably the antibodies to the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptor subunits, GluR2 and GluR3, and the N-methyl-D-aspartate (NMDA) receptor subunit, NR1. In contrast, antibodies to other glutamate receptor subunits produced more restricted distribution patterns, especially that to GluR1, which stained the outer neuropil of the DCN, cartwheel cells, and a small population of presumptive interneurons associated with the dorsal acoustic stria, but produced little or no staining in fusiform cells or deep DCN neurons. Staining of the postsynaptic density and membrane of the granule cell-parallel fiber/cartwheel cell spins synapse was most prevalent with delta 1/2 and mGluR1 alpha antibodies. A unique pattern of staining was found with mGluR2/3 antibody--with staining concentrated in Golgi cells and unipolar brush cells of the middle to deep DCN. Distribution of some glutamate receptors in the DCN shows similarities to that of the cerebellum, where delta 2 and mGluR1 alpha may modulate neurotransmission at parallel fiber synapses, while mGluR2 and/or mGluR3 may modulate mossy terminal function.

Molecular characterization and localization of human metabotropic glutamate receptor type 3

A human brain cDNA library was screened using amplified human metabotropic glutamate receptor (mGluR) cDNA sequences as probes. The resulting clones included one containing the complete coding sequence of mGluR3. This sequence has 90% DNA sequence identity with rat mGluR3 and the predicted protein sequence has 97% identity. The mGluR3 cDNA was transfected in Chinese hamster ovary (CHO) cells. Stimulation of the expressed receptor by (2S,3S,4S)-alpha-(carboxycyclopropyl) glycine (L-CCG-I) resulted in a reduction of forskolin-stimulated cyclic AMP (cAMP) with EC50 values of 0.15-0.3 microM. A specific probe from the human mGluR3 clone was used to hybridise to Northern blots of mRNA from various human tissues and different brain regions. The mGluR3 mRNA is brain-specific, and is expressed in all the brain regions represented on the blot. In-situ hybridization studies on human brain sections confirmed this widespread distribution with expression in neurones in the cerebral cortex, caudate-putamen, thalamus and cerebellum.

News on glutamate receptors in glial cells

Glutamate (Glu) receptors convey most of the excitatory synaptic transmission in the mammalian CNS. Distinct Glu-receptor genes and different subtypes of glutamate-activated channels are expressed ubiquitously throughout the developing and mature brain in the two major macroglial cell types, astrocytes and oligodendrocytes. These glial receptors are found in acutely isolated cells and in brain slices, and are therefore functional in vivo. Glutamate receptors in glial cells are activated during neuronal activity, and their activation modulates gene expression in astrocytes and oligodendrocytes. The proliferation and differentiation of glial precursor cells are also regulated by activation of Glu receptors, suggesting that the excitatory transmitter might be one of the environmental signals that regulate glial-cell development.

Molecular cloning, functional expression, pharmacological characterization and chromosomal localization of the human metabotropic glutamate receptor type 3

Glutamic acid is the major excitatory amino acid of the central nervous system which interacts with two receptor families, the ionotropic and metabotropic glutamate receptors. The metabotropic glutamate receptors (mGluRs) are coupled to G proteins and can be divided into three subgroups based on their sequence homology, signal transduction pathway and pharmacology. In this study, we describe the cloning of the cDNA encoding the human metabotropic glutamate receptor type 3 (HmGluR3). It was obtained by reverse transcription-polymerase chain reaction (RT-PCR) with degenerate oligonucleotides corresponding to highly conserved sequences between rat mGluRs. The receptor shows 879 amino acids with 96% amino acid sequence identity with rat mGluR3. It is strongly expressed in fetal and adult whole brain, especially in caudate nucleus and corpus callosum. The gene was identified by fluorescence in situ hybridization on chromosome 7 band q22. Activation of the human mGluR3, permanently expressed in Baby Hamster Kidney (BHK) cells, by excitatory amino acid inhibits the forskolin-stimulated accumulation of intracellular cAMP. The rank order of potency is L-glutamic acid > or = (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R)-ACPD) >> ibotenic acid > quisqualic acid. (RS)-alpha-methyl-4-carboxyphenylglycine [(RS)-MCPG, 1 mM] is without effect on inhibition of forskolin-induced cAMP accumulation by L-glutamic acid.