Trans-ACPD inhibits cAMP formation via a pertussis toxin-sensitive G-protein

Phenotypic Trade-Offs: Deciphering the Impact of Neurodiversity on Drug Development in Fragile X Syndrome

Fragile X syndrome (FXS) is the most common single-gene cause of intellectual disability and autism spectrum disorder. Individuals with FXS present with a wide range of severity in multiple phenotypes including cognitive delay, behavioral challenges, sleep issues, epilepsy, and anxiety. These symptoms are also shared by many individuals with other neurodevelopmental disorders (NDDs). Since the discovery of the FXS gene, FMR1, FXS has been the focus of intense preclinical investigation and is placed at the forefront of clinical trials in the field of NDDs. So far, most studies have aimed to translate the rescue of specific phenotypes in animal models, for example, learning, or improving general cognitive or behavioral functioning in individuals with FXS. Trial design, selection of outcome measures, and interpretation of results of recent trials have shown limitations in this type of approach. We propose a new paradigm in which all phenotypes involved in individuals with FXS would be considered and, more importantly, the possible interactions between these phenotypes. This approach would be implemented both at the baseline, meaning when entering a trial or when studying a patient population, and also after the intervention when the study subjects have been exposed to the investigational product. This approach would allow us to further understand potential trade-offs underlying the varying effects of the treatment on different individuals in clinical trials, and to connect the results to individual genetic differences. To better understand the interplay between different phenotypes, we emphasize the need for preclinical studies to investigate various interrelated biological and behavioral outcomes when assessing a specific treatment. In this paper, we present how such a conceptual shift in preclinical design could shed new light on clinical trial results. Future clinical studies should take into account the rich neurodiversity of individuals with FXS specifically and NDDs in general, and incorporate the idea of trade-offs in their designs.

Elevated intracranial dopamine impairs the glutamate‑nitric oxide‑cyclic guanosine monophosphate pathway in cortical astrocytes in rats with minimal hepatic encephalopathy

In a previous study by our group memory impairment in rats with minimal hepatic encephalopathy (MHE) was associated with the inhibition of the glutamate-nitric oxide-cyclic guanosine monophosphate (Glu-NO-cGMP) pathway due to elevated dopamine (DA). However, the effects of DA on the Glu-NO-cGMP pathway localized in primary cortical astrocytes (PCAs) had not been elucidated in rats with MHE. In the present study, it was identified that when the levels of DA in the cerebral cortex of rats with MHE and high-dose DA (3 mg/kg)-treated rats were increased, the co-localization of N-methyl-d-aspartate receptors subunit 1 (NMDAR1), calmodulin (CaM), nitric oxide synthase (nNOS), soluble guanylyl cyclase (sGC) and cyclic guanine monophosphate (cGMP) with the glial fibrillary acidic protein (GFAP), a marker protein of astrocytes, all significantly decreased, in both the MHE and high-dose DA-treated rats (P<0.01). Furthermore, NMDA-induced augmentation of the expression of NMDAR1, CaM, nNOS, sGC and cGMP localized in PCAs was decreased in MHE and DA-treated rats, as compared with the controls. Chronic exposure of cultured cerebral cortex PCAs to DA treatment induced a dose-dependent decrease in the concentration of intracellular calcium, nitrites and nitrates, the formation of cGMP and the expression of NMDAR1, CaM, nNOS and sGC/cGMP. High doses of DA (50 μM) significantly reduced NMDA-induced augmentation of the formation of cGMP and the contents of NMDAR1, CaM, nNOS, sGC and cGMP (P<0.01). These results suggest that the suppression of DA on the Glu-NO-cGMP pathway localized in PCAs contributes to memory impairment in rats with MHE.

Inhibition of glutamate carboxypeptidase II (GCPII) activity as a treatment for cognitive impairment in multiple sclerosis

Half of all patients with multiple sclerosis (MS) experience cognitive impairment, for which there is no pharmacological treatment. Using magnetic resonance spectroscopy (MRS), we examined metabolic changes in the hippocampi of MS patients, compared the findings to performance on a neurocognitive test battery, and found that N-acetylaspartylglutamate (NAAG) concentration correlated with cognitive functioning. Specifically, MS patients with cognitive impairment had low hippocampal NAAG levels, whereas those with normal cognition demonstrated higher levels. We then evaluated glutamate carboxypeptidase II (GCPII) inhibitors, known to increase brain NAAG levels, on cognition in the experimental autoimmune encephalomyelitis (EAE) model of MS. Whereas GCPII inhibitor administration did not affect physical disabilities, it increased brain NAAG levels and dramatically improved learning and memory test performance compared with vehicle-treated EAE mice. These data suggest that NAAG is a unique biomarker for cognitive function in MS and that inhibition of GCPII might be a unique therapeutic strategy for recovery of cognitive function.

Differential negative coupling of type 3 metabotropic glutamate receptor to cyclic GMP levels in neurons and astrocytes

Metabotropic receptors may couple to different G proteins in different cells or perhaps even in different regions of the same cell. To date, direct studies of group II and group III metabotropic glutamate receptors' (mGluRs) relationships to second messenger cascades have reported negative coupling of these receptors to cyclic AMP (cAMP) levels in neurons, astrocytes and transfected cells. In the present study, we found that the peptide neurotransmitter N-acetylaspartylglutamate (NAAG), an mGluR3-selective agonist, decreased sodium nitroprusside (SNP)-stimulated cyclic GMP (cGMP) levels in cerebellar granule cells and cerebellar astrocytes. The mGluR3 and group II agonists FN6 and LY354740 had similar effects on cGMP levels. The mGluR3 and group II antagonists beta-NAAG and LY341495 blocked these actions. Treatment with pertussis toxin inhibited the effects of NAAG on SNP-stimulated cGMP levels in rat cerebellar astrocytes but not in cerebellar neurons. These data support the conclusion that mGluR3 is also coupled to cGMP levels and that this mGluR3-induced reduction of cGMP levels is mediated by different G proteins in cerebellar astrocytes and neurons. We previously reported that this receptor is coupled to a cAMP cascade via a pertussis toxin-sensitive G protein in cerebellar neurons, astrocytes and transfected cells. Taken together with the present data, we propose that mGluR3 is coupled to two different G proteins in granule cell neurons. These data greatly expand knowledge of the range of second messenger cascades induced by mGluR3, and have implications for clinical conditions affected by NAAG and other group II mGluR agonists.

Novel metabotropic glutamate receptor negatively coupled to adenylyl cyclase in cultured rat cerebellar astrocytes

Several excitatory amino acid ligands were found potently to inhibit forskolin-stimulated cAMP accumulation in rat cultured cerebellar astrocytes: L-cysteine sulfinic acid (L-CSA) = L-aspartate > L-glutamate >/= the glutamate uptake inhibitor, L-PDC. This property did not reflect activation of conventional glutamate receptors, since the selective ionotropic glutamate receptor agonists NMDA, AMPA, and kainate, as well as several mGlu receptor agonists [(1S,3R)-ACPD, (S)-DHPG, DCG-IV, L-AP4, L-quisqualate, and L-CCG-I], were without activity. In addition, the mGlu receptor antagonists, L-AP3, (S)-4CPG, Eglu, LY341495, (RS)-CPPG, and (S)-MCPG failed to reverse 30 microM glutamate-mediated inhibitory responses. L-PDC-mediated inhibition was abolished by the addition of the enzyme glutamate-pyruvate transaminase. This finding suggests that the effect of L-PDC is indirect and that it is mediated through endogenously released L-glutamate. Interestingly, L-glutamate-mediated inhibitory responses were resistant to pertussis toxin, suggesting that G(i)/G(o) type G proteins were not involved. However, inhibition of protein kinase C (PKC, either via the selective PKC inhibitor GF109203X or chronic PMA treatment) augmented glutamate-mediated inhibitory responses. Although mGlu3 receptors (which are negatively coupled to adenylyl cyclase) are expressed in astrocyte populations, in our study Western blot analysis indicated that this receptor type was not expressed in cerebellar astrocytes. We therefore suggest that cerebellar astrocytes express a novel mGlu receptor, which is negatively coupled to adenylyl cyclase, and possesses an atypical pharmacological profile.

Role of p/q-Ca2+ channels in metabotropic glutamate receptor 2/3-dependent presynaptic long-term depression at nucleus accumbens synapses

The nucleus accumbens (NAc) is an important cerebral area involved in reward and spatial memory (Pennartz et al., 1994), but little is known about synaptic plasticity in this region. Here, electron microscopy revealed that, in the NAc, metabotropic glutamate receptors 2/3 (mGlu2/3) immunostaining was essentially associated with axonal terminals and glial processes, whereas postsynaptic dendrites and neuronal cell bodies were unstained. Electrophysiological techniques in the NAc slice preparation demonstrated that activation of mGlu2/3 with synaptically released glutamate or specific exogenous agonist, such as LY354740 (200 nm, 10 min), induced long-term depression of excitatory synaptic transmission (mGlu2/3-LTD). Tetanic-LTD and pharmacological mGlu2/3-LTD occluded each other, suggesting common mechanisms. The mGlu2/3-LTD did not require synaptic activity but depended on the cAMP-protein kinase A cascade. Selective inhibition of P/Q-type Ca(2+) channels with omega-agatoxin-IVA occluded the expression of mGlu2/3-LTD, and, conversely, the inhibitory effects of omega-agatoxin-IVA were abolished during mGlu2/3-LTD. Thus, mGlu2/3 play an important role in the control of use-dependent synaptic plasticity at prelimbic cortex-NAc synapses: their activation causes a form of LTD mediated by the long-lasting reduction of P/Q-type Ca(2+)channels contribution to transmitter release.

Protein kinase C and A3 adenosine receptor activation inhibit presynaptic metabotropic glutamate receptor (mGluR) function and uncouple mGluRs from GTP-binding proteins

One of the most prominent roles of metabotropic glutamate receptors (mGluRs) in the CNS is to serve as presynaptic receptors that inhibit transmission at glutamatergic synapses. Previous reports suggest that the presynaptic effect of group II mGluRs at corticostriatal synapses can be inhibited by activators of protein kinase C (PKC). We now report that activation of PKC inhibits the ability of group II and group III mGluRs to regulate transmission at three major synapses in the hippocampal formation. Thus, this effect may be a widespread phenomenon that occurs at glutamatergic synapses throughout the CNS. We also report that this response is not limited to PKC-activating phorbol esters but that activation of A3 adenosine receptors induces a PKC-dependent inhibition of group III mGluR function at the Schaffer collateral-CA1 synapse. In addition to inhibiting mGluR modulation of excitatory synaptic transmission, we found that activation of PKC reduces inhibition of forskolin-stimulated cAMP accumulation by group II and group III mGluRs, suggesting that the effect of PKC on mGluR signaling is not specific to their effects on neurotransmitter release. This led us to test the hypothesis that PKC acts upstream from effector proteins regulated by mGluRs and acts at the level of the receptor or GTP-binding protein. Interestingly, we found that PKC inhibited mGluR-induced increases in [35S]-GTPgammaS binding in cortical synaptosomes. These data suggest that PKC-induced inhibition of mGluR signaling may be mediated by the inhibition of coupling of mGluRs to GTP-binding proteins.

Characterization of metabotropic glutamate receptors in rat C6 glioma cells

Metabotropic glutamate receptors in rat C6 glioma cells have been characterized by pharmacological and kinetic binding experiments, using both L-[3H]glutamate and [3H(+/-)-1-aminocyclopentane-trans-1,3-dicarboxylic acid ([3H](+/-)-trans-ACPD) radioligands. Saturation experiments revealed a single binding site with a Kd = 1250 +/- 101 nM and Bmax = 12.1 +/- 1.8 pmol/mg protein when the assays were performed with L-[3H]glutamate as radioligand in the presence of AMPA, kainate, NMDA and DL-threo-beta-hydroxyaspartic acid. When [3H](+/-)-trans-ACPD was used as radioligand, the kinetic parameters obtained were Kd = 2605 +/- 1042 nM and Bmax = 13.66 +/- 5.01 pmol/mg protein. Pharmacological characterization indicated that specific binding of L-[3H]glutamate was sensitive to different agonists of mGlu receptors, showing a rank order of affinity L-glutamate > L-quisqualic acid > (+/-)-1-aminocyclopentane-trans-1,3-dicarboxylic acid (trans-ACPD) > ibotenic acid >>> (2S, 'S,2'S)-2-(carboxycyclopropyl)glycine (L-CCG-I). Specific binding of L-[3H]glutamate to mGlu receptors is regulated by guanine nucleotides. Guanylyl imidodiphosphate (Gpp(NH)p) causes an affinity shift on the L-glutamate dose-response curve, increasing the IC50 value. These results support the evidence that metabotropic glutamate receptors are present in rat C6 glioma cells and they are coupled to a G-protein.

The enhancement and the inhibition of noradrenaline-induced cyclic AMP accumulation in rat brain by stimulation of metabotropic glutamate receptors

1. The actions of several metabotropic glutamate receptor and antagonists on noradrenaline (NA)-stimulated [3H]-cyclic AMP accumulation were investigated in rat cerebral cortical slices. 2. Quisqualate (QUIS), L-2-amino-3-phosphonopropionic acid (L-AP3) and glutamate (GLU) elicited concentration-dependent inhibition of (NA)-stimulated [3H]-cyclic AMP accumulation, with IC50 values of 105 +/- 29, 275 +/- 36 and 944 +/- 150 microM respectively. In contrast (Rs)-alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA) (0.5 mM) and N-methyl-D-aspartic acid (NMDA) (0.5 mM) had no effect. 3. (2S,3S,4S)-alpha-(Carboxycyclopropyl)glycine (L-CCGI), 1-Aminocyclo-pentane-1S,3R-dicarbo-xylate (1S,3R-ACPD), ibotenate (IBO) and (RS)-4-carboxy-3-hydroxy-phenylglycine (CHPG)elicited a concentration-dependent enhancement of NA-stimulated [3H]-cyclic AMP accumulation, with EC50 values of 2.5 +/- 0.11, 42 +/- 1.3, 97.8 +/- 2.1 and 157 +/- 13.4 microM, respectively. 4. (S)-3-carboxy-4-hydroxyphenylglycine (3C4HPG) and (S)-4-carboxy-3-hydroxyphenyl-glycine (4C3HPG) produced a biphasic effect, at concentrations up to 100 and 500 microM, respectively, they significantly enhanced the action of NA (100 microM), at 1mM concentration both compounds as well as alpha-methyl-4-carboxyphenylglycine (MCPG) produced a significant inhibition of NA-stimulated cyclic AMP accumulation. 5. A putative mGluR antagonist-L-AP3, inhibited the 1S,3R-ACPD (100 microM) induced enhancement of the action of NA (100 microM) on [3H]-cyclic AMP accumulation in a biphasic manner with an IC50 of 4.5 microM for the high affinity site, which represented 65% of the total and an IC50 of 283 microM for the low affinity site. 6. beta-adrenoceptor antagonist propranolol inhibited the interaction between 1S,3R-ACPD (100 microM) and NA (100 microM) on [3H]-cyclic AMP accumulation by about 80%, with an IC50 of 0.52 +/- 0.011 microM, to the level observed after 1S,3R-ACPD alone. Prazosin, an alpha 1-adrenoceptor antagonist was more potent (IC50 of 0.091 +/- 0.012 microM) but less efficacious (60% inhibition) as an inhibitor of the interaction either between NA and 1S,3R-ACPD while yohimbine, na alpha 2-adrenoceptor antagonist (up to 1 microM) had no effect. 7. Neither the protein kinase C inhibitor - staurosporine (10 microM) nor thapsigargin (1 microM), which depletes IP3 sensitive calcium stores, inhibited significantly the 1S,3R-ACPD (100 microM)-induced enhancement of the action of NA (100 microM) on [3H]-cyclic AMP accumulation. 8. Adenosine deaminase (0.5 U/ml) abolished both the 1S,3R-ACPD (100 microM)-induced [3H]-cyclic AMP accumulation and the synergistic interaction of this compound with NA (100 microM). 9. These results indicate the existence of different subtypes of metabotropic glutamate receptors in rat brain which either inhibit or enhance the NA-stimulated [3H]-cyclic AMP accumulation. The enhancement in cerebral cortical slices is mediated via receptors which are blocked with high affinity by L-AP3 and occurs via interactions with endogenous adenosine; the inhibition is mediated by receptors sensitive to quisqualate, L-AP3 and glutamate and may represent a predominant interaction between NA and excitatory amino acids (EAA), which in cerebral cortical slices is masked by excitatory effects.

1S,3R-ACPD-preferring inward current in rat dorsolateral septal neurons is mediated by a novel excitatory amino acid receptor

Metabotropic glutamate receptors (mGluRs) form a receptor family that consists of diverse receptor subtypes; now, numbering 8--exclusive of splice variants. (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD) has been suggested to be a selective agonist for the mGluRs. We have recently reported that, in rat dorsolateral septal nucleus (DLSN) neurones, a 1S,3R-ACPD-preferring inward current (ACPDi) persists in pertussis toxin-treated rats. We now report that this ACPDi-current: (1) persists in DLSN neurones dialyzed with a stable analog of GTP, namely, GTP gamma S; (2) exhibits a negative slope region with inward rectification in its I-V relationship; (3) persists in neurones superfused with tetrodotoxin or low calcium solutions; (4) is dependent upon both sodium and calcium ions; and (5) is independent of a reduction in temperature. Furthermore, pharmacological data suggest that this current may be activated by a unique type of excitatory amino acid (EAA) receptor, i.e. a receptor which prefers "metabotropic" EAA agonists and is insensitive to AP5 or CNQX. Activation by ACPD of inward currents associated with a conductance increase have also been reported at cultured mouse cerebellar Purkinje neurones; in slices of rat hippocampal CA1 neurones and slice cultures of hippocampal CA3 neurones. We suggest that this ACPDi current may play an important role within the CNS in the induction of long-term potentiation and other neurological processes; processes attributed previously to currents associated with NMDA receptor activation.

Metabotropic glutamate receptors modulate N-methyl-D-aspartate receptor function in neostriatal neurons

The functional roles played by metabotropic glutamate receptors in the neostriatum is just beginning to be examined. One possibility, raised by previous studies, is that metabotropic glutamate receptors act to modulate responses mediated by ionotropic glutamate receptors. In the present study, we examined this possibility in a neostriatal brain slice preparation using intracellular recording and iontophoretic techniques. We found that the iontophoretic application of the metabotropic glutamate receptor agonist 1-amino-cyclopentane-1,3-dicarboxylic acid markedly attenuated both the amplitude and duration of excitatory responses induced by the iontophoretic application of N-methyl-D-aspartate. These inhibitory effects were stereo-selective and relatively long-lasting. The metabotropic glutamate receptor antagonist 2-amino-3-phosphonopropionic acid applied either iontophoretically or in the bath prevented the inhibitory effects of 1-amino-cyclopentane-1,3-dicarboxylic acid. The inhibitory action of 1-amino-cyclopentane-1,3-dicarboxylic acid was specific to N-methyl-D-aspartate, as 1-amino-cyclopentane-1,3-dicarboxylic acid had no consistent action on the responses evoked by the iontophoretic application of glutamate, amino-3-hydroxy-5-methyl-4-isoazolepropionic acid or quisqualate. Bath application of 1-amino-cyclopentane-1,3-dicarboxylic acid inhibited the evoked depolarizing postsynaptic potentials recorded in neostriatal cells. Thus, activation of metabotropic glutamate receptors may play an important role in modulating N-methyl-D-aspartate receptor function in neostriatal neurons.

Requirement of metabotropic glutamate receptors for the generation of inflammation-evoked hyperexcitability in rat spinal cord neurons

In the central nervous system the transmitter L-glutamate activates both ionotropic receptors coupled to cation channels and metabotropic receptors coupled to G-proteins. The role of metabotropic receptors in the processing of mechanosensory and nociceptive information was studied in a subset of spinal cord neurons with afferent input from the knee joint in anaesthetized rats using electrophysiological methods. The ionophoretic administration of L-2-amino-3-phosphonopropionic acid (L-AP3), an antagonist at the metabotropic receptor, had no effect on the responses to innocuous and noxious pressure applied to the normal knee joint, although the antagonist prevented the potentiation of these responses evoked by the ionophoretic administration of a specific agonist at the metabotropic receptor, trans-(+/-)-1-amino-(1S,3R)-cyclopentane-dicarboxylic acid (t-ACPD). By contrast, in neurons that were rendered hyperexcitable by acute inflammation in the knee joint L-AP3 reduced the responses to pressure applied to the knee. When L-Ap3 was applied during induction of inflammation and throughout the subsequent 1.5 h the spinal neurons did not develop hyperexcitability over this time period. L-AP3 did not impair the activation of ionotropic N-methyl-D-aspartate (NMDA) and non-NMDA receptors by the specific agonists. We conclude that spinal metabotropic glutamate receptors are not involved in the mediation of responses to innocuous and noxious mechanical stimuli applied under normal conditions. They are required, however, for the generation of inflammation-evoked hyperexcitability of spinal cord neurons, a form of functional plasticity underlying the painfulness in pathophysiological conditions such as inflammation.

Trans-(+-)-1-amino-1,3-cyclopentanedicarboxylate (trans-ACPD) stimulates cAMP accumulation in rat cerebral cortical slices but not in glial or neuronal cultures

Recent cloning experiments indicate that multiple metabotropic receptors for excitatory amino acids (EAAs) exist, which are coupled to adenylate cyclase. Trans-(+-)-1-amino-1,3-cyclopentanedicarboxylate (trans-ACPD) is a selective agonist of metabotropic receptors for EAAs. One of the effects of trans-ACPD is stimulation of cAMP accumulation. In the present experiments, cAMP accumulation was measured using a [3H]adenine-prelabelling technique. It has been found that trans-AC-PD was able to induce significant stimulation of cAMP accumulation in rat cerebral cortical slices, with ED50 of 47.8 microM, which value is similar to that described earlier for hippocampal slices. However, trans-ACPD had no effect on cAMP accumulation either in primary neuronal or glial cell cultures. The reason for the lack of effects of trans-ACPD on cAMP accumulation in primary cultures from glial cells and neurons is discussed.

Pharmacological characterization of the metabotropic glutamate receptor inhibiting D-[3H]-aspartate output in rat striatum

1. The effects of several agonists of the metabotropic glutamate receptor (mGluR) were studied in adult rat striatal slices by measuring (i) KCl (30 mM)-induced output of previously taken up D-[3H]-aspartate (Asp), (ii) forskolin (30 microM)-induced adenosine 3':5'-cyclic monophosphate (cyclic AMP) accumulation and (iii) phophoinositide (PI) hydrolysis. 2. K(+)-induced efflux of D-[3H]-Asp was inhibited by the following mGluR agonists: (1S,3S,4S)-(carboxycyclopropyl)glycine (L-CCG-I), (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD) and quisqualic acid (Quis). 2-Amino-4-phosphonobutyrate (L-AP4) was inactive up to 300 microM. The maximal inhibition of D-[3H]-Asp output was 60 +/- 8%. The EC50s of mGluR agonists were: 0.5 microM for L-CCG-I, 100 microM for 1S,3R-ACPD and 100 microM for Quis. 3. Forskolin-induced cyclic AMP accumulation was also inhibited by mGluR agonists. The maximal inhibition was 50 +/- 4% and was obtained at a concentration of 10 microM for L-CCG-I and 100 microM for 1S,3R-ACPD. The EC50s for this inhibition were: 0.9 microM for L-CCG-I and 20 microM for 1S,3R-ACPD. Quis (300 microM) inhibited cyclic AMP accumulation by approximately 20%. L-AP4 slightly potentiated cyclic AMP accumulation. 4. PI hydrolysis was stimulated by mGluR agonists. The most potent compound was Quis (100 microM), which increased inositol phosphate formation up to 2.2 fold over control values. Its EC50 was 15 microM. L-CCG-I and 1S,3R-ACPD increased inositol phosphate formation by approximately 1.8 fold and their EC50 values were 30 and 25 microM, respectively. L-AP4 did not affect PI hydrolysis. 5. In conclusion, mGluR agonists that reduce D-[3H]-Asp output have a pharmacological profile similar to that of mGluR agonists inhibiting cyclic AMP accumulation. L-CCG-I appears to be a relatively selective agonist for the mGluR receptor which inhibits D-[3H]-Asp efflux and cyclic AMP accumulation,while Quis appears to act preferentially on the mGluR receptor linked to the metabolism of PIs.

Characterization of metabotropic glutamate receptors negatively linked to adenylyl cyclase in brain slices

We have characterized the pharmacological profile of activation of metabotropic glutamate receptors negatively linked to adenylyl cyclase (mGluR decreases cAMP) in brain slices. Among the putative mGluR agonists, (2S,1'R,2'R,3'R)-2-(2,3-dicarboxycyclopropyl)glycine (DCG-IV) and (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (ACPD), were the most potent inhibitors of forskolin-stimulated cAMP formation in hippocampal slices, followed by ibotenate, L-2-amino-3-phosphonopropionate (AP3), quisqualate, L-glutamate and beta-N-methylamino-L-alanine (BMAA). Inhibition of forskolin-stimulated cAMP formation by DL-2-amino-4-phosphonobutanoate (AP4) was biphasic, suggesting that the drug interacts with more than one mGluR decreases cAMP subtype. Both L-AP4 and L-serine-O-phosphate (a restricted analogue of AP4) were much more effective in inhibiting forskolin-stimulated cAMP formation than their D-isomers, indicating that interaction of these drugs with the mGluR decreases cAMP is stereoselective. Despite the fact that DCG-IV and ibotenate behave as NMDA receptor agonists, their effect was insensitive to MK-801. The regional pattern of expression of mGluR decreases cAMPS, as estimated by using 1S,3R-ACPD as an agonist, did not correlate with the steady-state levels of mGluR2 mRNA. Thus, 1S,3R-ACPD inhibited forskolin-stimulated cAMP in slices from hippocampus, cerebral cortex, corpus striatum, olfactory tubercle or hypothalamus, but not in slices from olfactory bulb or cerebellum; in contrast, mGluR2 mRNA levels were high in the olfactory bulb and very low in the corpus striatum. 1S,3R-ACPD also inhibited forskolin-stimulated cAMP formation in cortical membranes, excluding the involvement of trans-synaptic mechanisms in the activity of mGluR decreases cAMPS.(ABSTRACT TRUNCATED AT 250 WORDS)

A pharmacological characterization of the mGluR1 alpha subtype of the metabotropic glutamate receptor expressed in a cloned baby hamster kidney cell line

The pharmacological specificity of the mGluR1 alpha subtype of the metabotropic glutamate receptor (mGluR) was examined in a cloned baby hamster kidney cell line (BHK-ts13) measuring [3H]glutamate binding and inositol phosphate (PI) hydrolysis. PI-hydrolysis was maximally stimulated by quisqualate (1112 +/- 105% of basal), glutamate (1061 +/- 70% of basal), ibotenate (1097 +/- 115% of basal) and beta-N-methylamino-L-alanine (BMAA) (1010 +/- 104% of basal). In contrast, the maximal stimulation of PI-hydrolysis by (1S,3R)-1-amino-cyclopentane-1,3-dicarboxylic acid (t-ACPD) was only 673 +/- 78% of the basal level. The relative order of potency was quisqualate > glutamate > ibotenate > t-ACPD > BMAA. Agonist-stimulated PI-hydrolysis was attenuated (25 +/- 4% inhibition) by L-2-amino-3-phosphonopropionic acid and partially blocked (44 +/- 7%) by pertussis toxin treatment. Saturation binding studies with [3H]glutamate on membranes prepared from BHK-ts13 cells expressing the mGluR1 alpha subtype showed that glutamate binds to a single affinity state of this receptor with a limited capacity (Kd = 296 nM, Bmax = 0.8 pmol/mg protein). In competition experiments, [3H]glutamate was displaced by quisqualate, glutamate, ibotenate, t-ACPD and BMAA with a rank order of potency similar to that found for stimulation of PI-hydrolysis.

A postsynaptic G-protein in hippocampal long-term potentiation

The effects of guanosine triphosphate (GTP)-binding protein (G-protein) blockade on hippocampal LTP at stratum radiatum-CA1 synapses was studied. Bath application of 20 mM lithium chloride (LiCl) inhibited long-term potentiation (LTP) of extracellularly-recorded excitatory postsynaptic potentials (EPSPs). Inclusion of 100 mM LiCl in intracellular recording electrodes was shown to block postsynaptic G-proteins by bath-application of baclofen, an agonist at the G-protein linked gamma-aminobutyric acid (GABAB) receptor. Under normal conditions, GABAB receptor activation causes a hyperpolarization postsynaptically, and a decrease in neurotransmitter release presynaptically. With LiCl in the recording electrodes, the postsynaptically-mediated hyperpolarization was blocked, while the presynaptically-mediated depression of EPSPs was unaffected. With postsynaptic G-proteins blocked in this manner, LTP at these synapses was inhibited. These studies provide evidence for the involvement of a postsynaptic G-protein in LTP of stratum radiatum-CA1 synapses.

Subtypes of metabotropic excitatory amino acid receptor distinguished by stereoisomers of the rigid glutamate analogue, 1-aminocyclopentane-1,3-dicarboxylate

The 1R,3R- and 1R,3S-isomers of 1-aminocyclopentane-1,3-dicarboxylate (ACPD) failed to stimulate phosphoinositide turnover or modify A2b adenosine receptor-stimulated cyclic AMP accumulation in guinea-pig cerebral cortical slices. In contrast, both 1S,3R- and 1S,3S-ACPD elicited concentration-dependent stimulations of phosphoinositide turnover (EC50 values 35 and 97 microM, respectively) and potentiated A2b-stimulated cAMP formation (17 and 58 microM, respectively). When forskolin was used to elevate cyclic AMP levels, however, all four isomers elicited concentration-dependent inhibitions of cyclic AMP formation to the same extent (approximately 90% inhibition). For this response the rank order of potencies were (IC50 values): 1S,3S-(0.9 microM) > 1S,3R-(2.1 microM) > 1R,3R-(237 microM) > 1R,3S-ACPD (approximately 1 mM). These data suggest the presence in guinea-pig cerebral cortex of two distinct subtypes of ACPD receptor coupled to phosphoinositide hydrolysis (and the potentiation of A2b receptor-stimulated cAMP formation) and the inhibition of forskolin-stimulated cAMP accumulation. Furthermore, our results indicate the usefulness of 1S,3S-ACPD as a tool to selectively activate one of these subtypes.

Characterization of metabotropic glutamate receptors coupled to a pertussis toxin sensitive G-protein in bovine brain coated vesicles

Glutamate metabotropic receptors (mGluRs) in bovine brain coated vesicles have been characterized by pharmacological and kinetic binding experiments. Saturation experiments revealed a single binding site with a Kd = 607.9 +/- 78.5 nM and a Bmax = 6.45 +/- 0.88 pmol/mg protein. The specific binding of L-[3H]glutamate to mGluRs is regulated by guanine nucleotides. Guanosine-5'-triphosphate (GTP; 100 microM) shifts the agonist competition curves to the right, increasing the IC50 values. Pertussis toxin treatment produces a pharmacological binding profile for quisqualate similar to that obtained in the presence of 100 microM GTP. These results indicate the presence of metabotropic glutamate receptors in coated vesicles and its coupling to a pertussis toxin sensitive G-protein.

Inhibitory glutamate response on cyclic AMP formation in cultured astrocytes

We examined the effects of glutamate receptor agonists on cyclic AMP (cAMP) formation in cultured astrocytes. L-Glutamate reduced the cAMP formation induced by either isoproterenol (IC50 7 microM) or forskolin without affecting the basal level. Glutamate agonists reduced the cAMP formation in astrocytes with the following rank order of potency: L-glutamate > trans-(+/-)-1-aminocyclopentane-1,3-dicarboxylic acid (t-ACPD) = quisqualate. Pretreatment of astrocytes with pertussis toxin resulted in a partial reduction of the glutamate response and a complete attenuation of the t-ACPD response. These results suggest that astrocytes have another type of metabotropic glutamate receptor which inhibits adenylate cyclase through pertussis toxin-sensitive G-proteins.

Metabotropic glutamate receptors in brain function and pathology

Metabotropic glutamate receptors (mGluRs) are a novel family of recently cloned G protein-coupled receptors. These receptors are heterogeneous and coupled to multiple second messenger systems that include increases in phosphoinositide hydrolysis, activation of phospholipase D, decreases in cAMP formation, increases in cAMP formation, and changes in ion channel function. Using the selective mGluR agonist 1-aminocyclopentane-1,3-dicarboxylic acid (1s,3R-ACPD), considerable progress has been made towards understanding the role of this glutamate receptor class in the central nervous system. This article reviews the molecular aspects and pharmacology of mGluRs, and recent studies elucidating their role in brain function and pathology.

Metabotropic excitatory amino acid receptor activation stimulates phospholipase D in hippocampal slices

Metabotropic excitatory amino acid (EAA) receptors are coupled to effector systems through G proteins. Because various G protein-coupled receptors stimulate the hydrolysis of phosphatidylcholine by phospholipase D (PLD), we examined the possibility that metabotropic EAA receptors exist that are coupled to the activation of PLD. We found that the selective metabotropic glutamate receptor (mGluR) agonists 1S,3R-amino-1,3-cyclopentanedicarboxylic acid (ACPD) and 1S,3S-ACPD, but not the inactive isomer, 1R,3S-ACPD, induce a concentration-dependent increase in PLD activity in hippocampal slices. Selective ionotropic glutamate receptor (iGluR) antagonists did not block 1S,3R-ACPD-induced PLD stimulation. Furthermore, although selective iGluR agonists did not activate this response, the nonselective mGluR-iGluR agonists, ibotenate and quisqualate, caused significant increases in PLD activity (all in the presence of iGluR antagonists). L-2-Amino-3-phosphonopropionic acid, which blocks the mGluR that is coupled to phosphoinositide hydrolysis in various brain regions, activates PLD to the same extent as the active isomers of ACPD. These data suggest that metabotropic EAA receptors exist in hippocampus that are coupled to PLD activation and are pharmacologically distinct from phosphoinositide hydrolysis-coupled mGluRs.

Characterization of a metabotropic glutamate receptor: direct negative coupling to adenylyl cyclase and involvement of a pertussis toxin-sensitive G protein

We have characterized a G-protein-coupled glutamate receptor in primary cultures of striatal neurons. Glutamate, quisqualate, or trans-1-aminocyclopentane-1,3-dicarboxylate inhibited by 30-40% either forskolin-stimulated cAMP production in intact cells or forskolin plus vasoactive intestinal peptide-activated adenylyl cyclase assayed in neuronal membrane preparations. These inhibitory effects were suppressed after treatment of striatal neurons with Bordetella pertussis toxin, suggesting the involvement of a heterotrimeric guanine nucleotide-binding protein (G protein) of the G(i)/G(o) subtype. The pharmacological profile of this glutamate receptor negatively coupled to adenylyl cyclase was different from that of the metabotropic Qp glutamate receptor coupled to phospholipase C in striatal neurons and from that of the recently cloned "mGluR2" glutamate receptor, which is negatively coupled to adenylyl cyclase when expressed in non-neuronal cells.