Metabotropic glutamate receptor agonists for schizophrenia

A drug acting at metabotropic glutamate receptors has recently been reported to be an effective antipsychotic, breaking the rule that only dopamine receptor-blocking drugs have this property. The finding complements accumulating evidence that glutamatergic abnormalities are important in the pathophysiology of schizophrenia.     

Metabotropic glutamate receptor agonists reduce glutamate release from cultured astrocytes

Astrocytes are thought to control extracellular glutamate concentrations ([Glu]_o) in the brain, thereby protecting neurons from excitotoxic injury. We investigated the effects of metabotropic glutamate receptor (mGluR) agonists on glutamate transport and [Glu]_o in primary hippocampal astrocytic cultures. Acute or chronic exposure of astrocytes to the mGluR agonist trans‐1‐aminocyclopentane‐1,3‐dicarboxylic acid (trans‐ACPD) or its active isomer 1S,3R‐ACPD reduced [Glu]_o in a time‐ and dose‐dependent manner (44.5 ± 3.6% reductions of [Glu]_o in astrocytes from P0–P10 rats and 65.9 ± 4.1 % from rats P20 by 100 μM 1S,3R‐ACPD, EC₅₀ ∼ 5 μM). 1S,3R‐ACPD effects developed slowly (median effective at ∼60 min) and persisted for several hours after agonist removal. ACPD‐pretreated astrocytes established lower steady‐state [Glu]_o levels. ACPD effects persisted in the presence of the glutamate uptake inhibitors D ,L ‐threo‐β‐hydroxyaspartate (THA) and L ‐trans‐pyrrolidine‐2,4‐dicarboxylate (PDC) but were impaired by disruption of the transmembrane Na⁺, K⁺, or H⁺ gradients. In addition, 1S,3R‐ACPD had no effects on intracellular glutamate content and did not directly block glutamate transport. Furthermore, ACPD effects could be mimicked by glutamate per se and several other compounds presumed to be mGluR agonists, although (S)‐3,5‐dihydroxyphenylglycine (DHPG), (2S,2R,3R)‐2‐(2,3‐dicarboxycyclopropyl)glycine (DCG‐IV), and L ‐(+)‐2‐amino‐4‐phosphonobutyric acid (L ‐AP4) were without effect. These data suggest that glutamate and certain mGluR agonists may regulate [Glu]_o by modulating the transmembrane equilibrium of glutamate transport, especially by attenuating glutamate release. GLIA 25:270–281, 1999. © 1999 Wiley‐Liss, Inc.     

Metabotropic glutamate receptor agonists reduce epileptiform activity in the rat cortex.

Slices of rat cingulate cortex, when incubated in magnesium-free medium, produce spontaneous epileptiform spikes. Here it is demonstrated that the metabotropic glutamate receptor (mGluR) agonists +/- trans-1-amino-cyclopentane-1,3-dicarboxylic acid (+/- Trans-ACPD), IS3R-ACPD and quisqualic acid (quis) can reduce the frequency of these bursts in a concentration-dependent manner. The IC50 values were 16, 12 and 26 microM, respectively. The low concentrations of +/- trans- and IS3R-ACPD used, plus the lack of NMDA antagonism shown by these compounds, suggest that the effect may be via a presynaptic reduction in glutamate release. The relative potency of the agonists IS3R-ACPD > Trans-ACPD > quis would seem to suggest that the mGluR1 receptor is not involved.     

Metabotropic glutamate receptor agonists potentiate a slow afterdepolarization in CNS neurons.

We have previously reported that, in the rat dorsolateral septal nucleus (DLSN), metabotropic glutamate receptor (met-GluR) agonists evoked a slow depolarization accompanied by an increase in membrane conductance and burst firing. We have speculated that the burst firing elicited by met-GluR agonists may be due to activation or enhancement of a non-specific cation current, which exists in some DLSN neurons. Now we report that a slow afterdepolarization (sADP) mediated by a non-specific cation current was potentiated by both 1S,3R-ACPD and quisqualate. In addition, met-GluR agonists unmask a sADP in DLSN neurons which did not show a sADP under control conditions. Our data suggest that a non-specific cation current can be potentiated by activation of the met-GluR.     

Metabotropic glutamate receptor antagonists and agonists: Potential neuroprotectors in diffuse brain injury

Our previous study has suggested that metabotropic glutamate receptors (mGluRs) were significantly involved in the secondary processes after diffuse brain injury (DBI) and that mGluRs antagonists or agonists may be used for the treatment of DBI. In the present study, the neuroprotective effects of antagonists or agonists of mGluRs on DBI were further investigated. Sprague-Dawly rats were randomized into the following six groups: (i) normal control; (ii) sham-operated control; (iii) DBI; (iv) DBI treated with normal saline (NS); (v) DBI treated with alpha-methyl-4-carboxy-phenylglycine (MCPG); and (vi) DBI treated with (2S,1'R,2'R,3'R)-2-(2,3-dicarboxycyclopropyl)glycine (DCG-IV). Animals were injected intracerebroventricularly (icv) with 10 microL MCPG (100mmol/L), DCG-IV (10nmol/L) or the equivalent volume of normal saline 1 h after injury. The neurological severity score (NSS), brain water content and the number of damaged neurons were determined 6, 12, 24, 72 and 168 h after injury. In rats with DBI, it was found that the NSS was improved and the water content in the frontal cortex and the number of damaged neurons in the parietal cortex were significantly reduced following icv injection of either MCPG or DCG-IV. This suggests that icv injection of the mGluR group I antagonist MCPG or the mGluR group II agonist DCG-IV may exert neuroprotective effects in the early stage after DBI.     

Metabotropic glutamate receptor regulation of neuronal cell death

The metabotropic glutamate receptors (mGluRs) are a family of glutamate-sensitive receptors that regulate neuronal function separately from the ionotropic glutamate receptors. By coupling to guanosine nucleotide-binding proteins (G proteins), mGluRs are able to regulate neuronal injury and survival, likely through a series of downstream protein kinase and cysteine protease signaling pathways that affect mitochondrial regulated programmed cell death (PCD). The physiological relevance of this system is supported by evidence that mGluRs are associated with cell survival in several central nervous system neurodegenerative diseases. Evidence is presented that mGluRs are also able to prevent PCD in the peripheral nervous system, and that this may provide a novel mechanism for treatment of diabetic neuropathy. In dorsal root ganglion (DRG) neurons, a high glucose load increases generation of reactive oxygen species (ROS), destabilizes the inner mitochondrial membrane potential (Deltapsi(M)), induces cytochrome c release from the mitochondrial intermembrane space, and induces downstream activation of caspases. In high-glucose conditions, the group II metabotropic glutamate agonist N-acetylaspartylglutamate (NAAG) blocks caspase activation and is completely reversed by the mGluR3 antagonist (S)-alpha-ethylglutamic acid (EGLU). Furthermore, the direct mGluR3 agonist (2R,4R)-4-aminopyrrolidine-2, 4-dicarboxylate (APDC) prevents induction of ROS. Together these findings are consistent with an emerging concept that mGluRs may protect against cellular injury by regulating oxidative stress in the neuron. More complete understanding of the complex PCD regulatory pathways mediated by mGluRs will provide new therapeutic approaches for the treatment of a wide variety of neurodegenerative diseases.     

Metabotropic receptors and 'slow' excitatory actions of glutamate agonists in the hippocampus.

The actions of glutamate in the CNS can be divided into ionotropic and metabotropic effects. The ionotropic receptors participate in synaptic transmission by directly opening nonselective cation channels. Recently, a so-called 'metabotropic effect' of glutamate has been described and is attributed to a novel metabotropic glutamate receptor. This effect consists of increased hydrolysis of membrane phosphoinositides, production of the second messengers diacylglycerol and inositol 1,4,5-trisphosphate, and mobilization of intracellular Ca2+. Activation of metabotropic glutamate receptors blocks the slow Ca(2+)-dependent K+ conductance and increases the membrane excitability of neurones. In addition, metabotropic agonists block the excitatory synaptic transmission supported by the ionotropic glutamate receptor, and may therefore play a critical role in synaptic plasticity. However, intracellular mechanisms linking metabotropic glutamate receptors with ionic channels remain unclear. This article discusses recent findings concerning metabotropic agonist effects on membrane currents and synaptic transmission, the pharmacology of the agonists and the roles played by G proteins and second messengers in mediating their effects.     

Interaction of dopamine and cardiac sac modulatory inputs on the pyloric network in the lobster stomatogastric ganglion

Dopamine and cardiac sac network activity each have strong, and different, modulatory actions on the pyloric rhythm in the stomatogastric ganglion of the spiny lobster. When combined, the two modulatory inputs have a complex effect. Dopamine and cardiac sac activity cancel one another's effects to restore normal pyloric activity to 4 of the 6 classes of pyloric neurons. In the remaining two pyloric neurons, dopamine's strong modulatory effects are completely overruled during cardiac sac network related activity. Possible cellular mechanisms underlying these interactions are discussed.     

Metabotropic glutamate receptors: phosphorylation and receptor signaling

Metabotropic glutamate receptors (mGluRs) play important roles in neurotransmission, neuronal development, synaptic plasticity, and neurological disorders. Recent studies have revealed a sophisticated interplay between mGluRs and protein kinases: activation of mGluRs regulates the activity of a number of kinases, and direct phosphorylation of mGluRs affects receptor signaling, trafficking, and desensitization. Here we review the emerging literature on mGluR phosphorylation, signaling, and synaptic function.     

Metabotropic glutamate receptor subtypes independently modulate neuronal intracellular calcium

Metabotropic glutamate receptors (mGluRs) modulate several G-protein-related signal transduction pathways including intracellular calcium (iCa(2+)) that control both neuronal development and demise. As an initial investigation, we characterized the ability of specific mGluR subtypes to modulate iCa(2+) by using Fura-2 microfluorometry in primary hippocampal neurons. Activation rather than inhibition of the metabotropic system with the group I and group II mGluR agonist 1S, 3R-1-aminocyclopentane-1,3-dicarboxylic acid (ACPD), the specific group I agonist (S)-3,5-dihydroxyphenylglycine (DHPG), and the specific group II agonist (2S,1'S,2'S)-2-(carboxycyclopropyl)glycine (LCCG-I) increased iCa(2+) with increasing concentrations. In contrast, the group III mGluR agonist, L(+)-2-amino-4-phosphonobutyric acid (L-AP4) produced no significant increase in iCa(2+). Through the pharmacological modulation of individual mGluR subtypes, we further examined the role of iCa(2+) release by the mGluR system. Release of iCa(2+) by both 1S,3R-ACPD and LCCG-I was prevented only through the administration of the antagonists (2S)-alpha-ethylglutamic acid (EGlu; mGluR2 and mGluR3) and (2S,1'S,2'S,3'R)-2-(2'-carboxy-3'-phenylcyclopropyl)glycine (PCCG-IV; mGluR2), suggesting that the mGluR2 subtype was responsible for the release of iCa(2+). As a control, the group I antagonists, L(+)-2-amino-3-phosphonopropionic acid (L-AP3) and (RS)-1-aminoindan-1,5-dicarboxylic acid (AIDA), prevented DHPG release of iCa(2+) but were ineffective against iCa(2+) release by 1S,3R-ACPD. Although extracellular calcium influx did not significantly contribute to the release of iCa(2+) by the mGluR system, pharmacological inhibition of calcium-induced calcium-release-sensitive calcium pools played a critical role in the release of iCa(2+). Further characterization of the cellular calcium pools modulated by the mGluR subtypes may provide greater insight into the mechanisms that mediate neuronal function.     

Biophysical properties and responses to glutamate receptor agonists of identified subpopulations of rat geniculate ganglion neurons

The goal of the current study was to evaluate the electrophysiological properties and responses to glutamate receptor agonists of rat geniculate ganglion (GG) neurons innervating the tongue. Subpopulations of GG neurons were labeled by injecting Fluoro-Gold (FG) or True Blue chloride into the anterior tongue and soft palate (AT and SP neurons) and applying FG crystals to the posterior auricular branch of the facial nerve (PA neurons). Three to 12 days later, the GG neurons were acutely isolated and patch clamped. Although many biophysical properties of the AT, SP and PA neurons were similar, significant differences were found among these groups in properties related to cell excitability. For example, the average amount of current necessary to elicit an action potential was 61 pA in AT neurons (n=55), 90 pA in SP neurons (n=41) and 189 pA in PA neurons (n=35, P<0.001). In addition, AT neurons tended to fire significantly more action potentials during depolarization as well as following hyperpolarizing pulses than SP or PA neuron types. Most GG neurons responded to application of glutamate receptor agonists. The neurons responded with a depolarization accompanied by a reduction in input resistance. These results suggest that subpopulations of neurons in the geniculate ganglion have distinct biophysical properties and express functional glutamate receptors. The differing biophysical properties of GG neurons is possibly related to their functional heterogeneity and glutaminergic neurotransmission may function in the processing of gustatory, and other sensory information, within the geniculate ganglion and its projections.     

Metabotropic glutamate receptor subtypes as targets for neuroprotective drugs.

Metabotropic glutamate (mGlu) receptors have been considered as potential targets for neuroprotective drugs, but the lack of specific drugs has limited the development of neuroprotective strategies in experimental models of acute or chronic central nervous system (CNS) disorders. The advent of potent and centrally available subtype-selective ligands has overcome this limitation, leading to an extensive investigation of the role of mGlu receptor subtypes in neurodegeneration during the last 2 years. Examples of these drugs are the noncompetitive mGlu1 receptor antagonists, CPCCOEt and BAY-36-7620; the noncompetitive mGlu5 receptor antagonists, 2-methyl-6-(phenylethynyl)pyridine, SIB-1893, and SIB-1757; and the potent mGlu2/3 receptor agonists, LY354740 and LY379268. Pharmacologic blockade of mGlu1 or mGlu5 receptors or pharmacologic activation of mGlu2/3 or mGlu4/7/8 receptors produces neuroprotection in a variety of in vitro or in vivo models. MGlu1 receptor antagonists are promising drugs for the treatment of brain ischemia or for the prophylaxis of neuronal damage induced by synaptic hyperactivity. MGlu5 receptor antagonists may limit neuronal damage induced by a hyperactivity of N-methyl-d-aspartate (NMDA) receptors, because mGlu5 and NMDA receptors are physically and functionally connected in neuronal membranes. A series of observations suggest a potential application of mGlu5 receptor antagonists in chronic neurodegenerative disorders, such as amyotrophic lateral sclerosis and Alzheimer disease. MGlu2/3 receptor agonists inhibit glutamate release, but also promote the synthesis and release of neurotrophic factors in astrocytes. These drugs may therefore have a broad application as neuroprotective agents in a variety of CNS disorders. Finally, mGlu4/7/8 receptor agonists potently inhibit glutamate release and have a potential application in seizure disorders. The advantage of all these drugs with respect to NMDA or AMPA receptor agonists derives from the evidence that mGlu receptors do not "mediate," but rather "modulate" excitatory synaptic transmission. Therefore, it can be expected that mGlu receptor ligands are devoid of the undesirable effects resulting from the inhibition of excitatory synaptic transmission, such as sedation or an impairment of learning and memory.     

Tetanic stimulation and metabotropic glutamate receptor agonists modify synaptic responses and protein kinase activity in rat auditory cortex

We investigated whether tetanic-stimulation and activation of metabotropic glutamate receptors (mGluRs) can modify field-synaptic-potentials and protein kinase activity in rat auditory cortex, specifically protein kinase A (PKA) and protein kinase C (PKC). Tetanic stimulation (50 Hz, 1 s) increases PKA and PKC activity only if the CNQX-sensitive field-EPSP (f-EPSP) is also potentiated. If the f-EPSP is unchanged, then PKA and PKC activity remains unchanged. Tetanic stimulation decreases a bicuculline-sensitive field-IPSP (f-IPSP), and this occurs whether the f-EPSP is potentiated or not. Potentiation of the f-EPSP is blocked by antagonists of mGluRs (MCPG) and PKC (calphostin-C, tamoxifen), suggesting that the potentiation of the f-EPSP is dependent on mGluRs and PKC. PKC antagonists block the rise in PKC and PKA activity, which suggests that these may be coupled. In contrast, ACPD (agonist at mGluRs) decreases both the f-EPSP and the f-IPSP, but increases PKC and PKA activity. Quisqualate (group I mGluR agonist), decreases the f-IPSP, and increases PKA activity, suggesting that the increase in PKA activity is a result of activation of group I mGluRs. Additionally, the increase in PKC and PKA activity appears to be independent of the decrease of the f-EPSP and f-IPSP, because PKC antagonists block the increase in PKC and PKA activity levels but do not block ACPD's effect on the f-EPSP or f-IPSP. These data suggest that group I mGluRs are involved in potentiating the f-EPSP by a PKC and possibly PKA dependent mechanism which is separate from the mechanism that decreases the f-EPSP and f-IPSP.     

Oxidative stress induced by glutamate receptor agonists

The effect of various selective glutamate agonists upon the rate of generation of reactive oxygen species (ROS), was examined in an isolated synaptoneurosomal (microsac) fraction derived from rat cerebral cortex. The rates of ROS generation were determined by a fluorescent probe. Agonists specific for each of the three major glutamate inotropic receptor sites (NMDA, kainic acid, -amino-3-hydroxy-5-methyl-4-isoxalolpropionic acid, AMPA), were able to enhance rates or ROS generation. The metabotropic glutamate agonisttrans-1-aminocyclopentane-1,3-dicarboxylic acid, (ACPD), was inactive in this regard. Stimulation of ROS was most pronounced in the case of kinate. Such results could not be replicated by use of ion-channel active agents, veratridine and A23817. Pretreatment with the kainate antagonist, 6-cyano-7-quinoxaline-2,3-dione (CNQX), was not able to block the kainate-induced elvation of ROS. Domoic acid, a kainate agonist, also enhanced microsac ROS generation. Neurological damage may result from generation of excess free radicals, and this may be effected by glutamate agonists acting by means independent of their ionotropic properties.     

Metabotropic glutamate receptor activation selectively limits excitotoxic damage in the intact neostriatum

The present study was designed to compare the protective consequences of activation of metabotropic glutamate receptors (mGluRs) onN-methyl-d-aspartate (NMDA)- and kainic acid (KA)-induced excitotoxicity in vivo. Pretreatment with the mGluR agonist 1SR,3RS-1-aminocyclo-pentane-1,3-dicarboxylic acid (tACPD) limited the anatomical and behavioral consequences of the intrastriatal administration of the NMDA agonist quinolinic acid (QA). In contrast, pretreatment with tACPD did not alter the effects of intrastriatal injection of KA.     

Metabotropic glutamate receptor 5 activation inhibits microglial associated inflammation and neurotoxicity

The Group I metabotropic glutamate receptor 5 (mGluR5) can modulate addiction, pain, and neuronal cell death. Expression of some mGluRs, such as Group II and III mGluRs, has been reported in microglia and may affect their activation. However, the expression and role of mGluR5 in microglia is unclear. Using immunocytochemistry and Western blot, we demonstrate that mGluR5 protein is expressed in primary microglial cultures. Activation of mGluR5 using the selective agonist (RS)-2-chloro-5-hydroxyphenylglycine (CHPG) significantly reduces microglial activation in response to lipopolysaccharide, as indicated by a reduction in nitric oxide, reactive oxygen species, and TNFalpha production. Microglial induced neurotoxicity is also markedly reduced by CHPG treatment. The anti-inflammatory effects of CHPG are not observed in microglial cultures from mGluR5 knockout mice and are blocked by selective mGluR5 antagonists, suggesting that these actions are mediated by the mGluR5 receptor. Anti-inflammatory actions of mGluR5 activation are attenuated by phospholipase C and protein kinase C inhibitors, as well as by calcium chelators, suggesting that the mGluR5 activation in microglia involves the G(alphaq)-protein signal transduction pathway. These data indicate that microglial mGluR5 may represent a novel target for modulating neuroinflammation, an important component of both acute and chronic neurodegenerative disorders.     

Metabotropic glutamate receptor mGluR1 distribution and ultrastructural localization in hypothalamus

The metabotropic glutamate receptor mGluR1 is a G-protein-coupled glutamate receptor whose activation induces phosphotidylinositol hydrolysis and increases diacylglycerol and cytoplasmic calcium. By using affinity-purified antisera against a partial amino acid sequence of mGluR1α, deduced from the nucleotide sequence of the cloned gene, the heterogeneous expression of this glutamate receptor was studied immunocytochemically with light and electron microscopy in the rat hypothalamus. Immunoreactivity was restricted to cell bodies and dendrites throughout many regions of the adult hypothalamus, including the preoptic area, anterior hypothalamus, suprachiasmatic nucleus, dorsomedial hypothalamus, and periventricular region. Strong immunolabeling was found in the lateral hypothalamus where immunoreactivity could be detected as early as embryonic day 18. Intense immunoreactivity was also found in the medial mammillary nuclei. In contrast to the strong labeling in many other regions, the neuroendocrine neurons of the arcuate, supraoptic, and paraventricular nuclei showed relatively little staining in adults. With light microscopy, immunoperoxidase labeling was found distributed in patches on the cytoplasmic side of the plasma membrane of immunoreactive neurons. When the same tissue was examined ultrastructurally, the patches were not restricted to synaptic specializations but were also found distributed on perikaryal and dendritic membranes sometimes associated with synapses and sometimes not. Some immunoreactive membranes showed no immunolabeling at the synaptic junction. When the tissue was strongly stained, labeling could be found in the cytoplasm of immunoreactive cells. No immunostaining was found on axons or presynaptic boutons. Together with other evidence showing a widespread expression of many different subtypes of both ionotropic and metabotropic receptors, these data support the hypothesis that glutamate may regulate hypothalamic cellular activity with a number of physiologycally different mechanisms, and these mechanisms include second-messenger systems activated by G proteins. © 1994 Wiley-Liss, Inc.