Inhibition of AMPA receptor-stimulated 57Co2+ influx by D- and L-2-amino-4-phosphonobutanoic acid (D- and L-AP4) and L-serine-O-phosphate (L-SOP) in cultured cerebellar granule cells

Group III metabotropic glutamate receptor-mediated, chemically induced long-term depression differentially affects cell viability in the hippocampus

In vivo, activation of group III metabotropic glutamate (mGlu) receptors leads to a reduction of basal synaptic transmission in the hippocampus, and depending on the experimental conditions in vitro, leads to neuroprotection or neurotoxicity. Here, the cellular response to cerebral application of L(+)-2-amino-4-phosphonobutanoic acid (AP4) was investigated in the CA1 region and dentate gyrus of freely moving rats. Drugs were applied via the lateral ventricle, and electrophysiological measurements were obtained via chronically implanted electrodes. AP4 produced a slowly developing depression of evoked responses in both hippocampal regions which lasted for over 4 h. Effects could be reversed by application of high frequency tetanus. Histological evaluation, 4 h or 7 d, following a single, acute AP4 injection into the lateral cerebral ventricle showed that AP4-mediated synaptic depression either amplified (CA1) or attenuated (dentate gyrus) excitotoxic neuronal death, strongly depending on the sub-region investigated. Effects were long-lasting, being still evident 7 days after AP4 application. In both hippocampal areas, the effects obtained were subtle, however, with the CA1 region being more potently affected. Interestingly, effects in the dentate gyrus comprised a slight enhancement of live cell number coupled with deterioration in cell area, suggesting that cell proliferation triggered by group III mGlu receptor activation may have masked neurotoxic effects mediated by activation of this receptor. These results show that although AP4 induces a slow-onset synaptic depression in both sub-regions, cell viability is differentially influenced by activation of group III mGlu receptors in the CA1 region and dentate gyrus.

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.

Distinct influence of the group III metabotropic glutamate receptor agonist (R,S)-4-phosphonophenylglycine [(R,S)-PPG] on different forms of neuronal damage

With this study we evaluated the influence of (R, S)-4-phosphonophenylglycine [(R,S)-PPG], a selective group III metabotropic glutamate receptor agonist, on excitotoxic, hypoxic/hypoglycaemic and ischaemic cerebral damage in rodents. Consistent with previous data showing neuroprotective and anticonvulsive effects (Gasparini, F., Bruno, V., Battaglia, G., Lukic, S., Leonhardt, T., Inderbitzin, W., et al., 1999. (R, S)-4-Phosphonophenylglycine, a potent and selective group III metabotropic glutamate receptor agonist, is anticonvulsive and neuroprotective in vivo. Journal of Pharmacology and Experimental Therapeutics 290, 1678-1687), we found pronounced neuroprotective effects with (R,S)-PPG (300 nmol) in a model of excitotoxicity, i.e. quinolinic acid-induced striatal lesions in rats. However, neither in focal cerebral ischaemia in mice nor in global cerebral ischaemia in gerbils or rats did (R,S)-PPG have any significant influence on the extent of neuronal damage. In a model of hypoxia/hypoglycaemia in acutely isolated hippocampal slices, however, (R,S)-PPG led to an improved recovery of population spike amplitude. As acutely isolated hippocampal slices are only viable for a few hours, these electrophysiological recordings can only be performed in a limited time window after the challenge-when most probably excitotoxicity is still the predominant influence in hypoxic pathophysiology. From this we conclude that group III mGluR agonists might be promising drugs against damage mediated mainly by excitotoxicity, but less likely against development of neuronal death due to ischaemia.

mGluR7-like metabotropic glutamate receptors inhibit NMDA-mediated excitotoxicity in cultured mouse cerebellar granule neurons

Glutamate-induced glutamate release may be involved in the delayed neuronal death induced by N-methyl-D-aspartate (NMDA). In order to examine a possible modulatory effect of the presynaptic group III mGluRs on glutamate excitotoxicity, the effect of L-2-amino-4-phosphonobutyrate (L-AP4) was examined on NMDA-induced delayed death of mouse cerebellar granule neurons in culture. We found that L-AP4, at high concentration (in the millimolar range), inhibited in a non-competitive manner the NMDA-induced toxicity. This effect was mimicked by high concentration of L-serine-o-phosphate (L-SOP), and was inhibited by pertussis toxin (PTX) indicating the involvement of a Gi/o protein. This suggests the involvement of mGluR7 in the L-AP4 effect, and this was consistent with the detection of both mGluR7 protein and mRNA in these cultured neurons. To examine the mechanism of the L-AP4-induced protection from excitotoxic damage, the effect of L-AP4 on glutamate release was examined. L-AP4 (> or = 1 mM) noncompetitively inhibited by more than 60% the glutamate release induced by NMDA during the insult. We also observed that the 10-min NMDA receptor stimulation resulted in a dramatic increase in the extracellular glutamate concentration reaching 6000% of the control value 24 h after the insult. This large increase was also inhibited when NMDA was applied in the presence of > or = 1 mM L-AP4. Part of the L-AP4-induced protection from excitotoxic damage of granule neurons may therefore result from the inhibition of the vicious cycle: dying cells release glutamate, glutamate induced cell death. The present results add to the hypothesis that presynaptic mGluRs, probably mGluR7, may be the targets of drugs decreasing glutamate release and then neuronal death observed in some pathological situations.