Direct effects of metabotropic glutamate receptor compounds on native and recombinant N -methyl- d -aspartate receptors

Abstract

The actions of glutamate in the central nervous system are mediated through interaction with fast activating ionotropic receptors and G protein-coupled metabotropic glutamate receptors (mGluRs). Studies of these receptors have relied on the availability of agonists and antagonists selective for each receptor class. Compounds that were thought to be selective for mGluRs have been extensively used to study the role of these receptors in the brain. Their use has implicated mGluRs in a wide range of physiological and pathological processes including the modulation of N-methyl-D-aspartate (NMDA) receptors and NMDA receptordependent processes. We report that some of the most commonly used mGluR compounds act as antagonists on NMDA receptors at concentrations commonly used to activate or block mGluRs. In addition, several of the drugs also act as agonists at higher concentrations due at least in part to high levels of contaminant amino acids. Our results indicate that caution should be used when using these drugs to study the roles of mGluRs in various NMDA-dependent processes. The antagonist effects were dependent on the concentration of the NMDA receptor coagonists, preventing reappraisal of previously published work.

Expression of Functional Metabotropic Glutamate Receptors in Primary Cultured Rat Osteoblasts

“…3 We are therefore confident that 100 mM NMDA is a saturating or nearly saturating dose. (ii) At the doses used in our experiments (100 M NMDA, 10 M glycine), 1S,3R-ACPD may have a minor inhibitory effect (up to15%) on the amplitude of NMDA currents (39). We observed 75-80% inhibition of the response to NMDA.…”

“…We observed 75-80% inhibition of the response to NMDA. (iii) At the NMDA receptor agonist dosages that we employed, (Ϯ)MCPG, like 1S,3R-ACPD, may act as a weak antagonist of the NMDA receptor (39). However, (Ϯ)MCPG enhanced the response of the osteoblastic NMDA receptor, consistent with a reduction of mGluR-mediated inhibition of NMDARs.…”

“…It has been reported that compounds that interact with mGluRs may also act at NMDARs, primarily as antagonists in the presence of subsaturating doses of agonist (39). The modulation of NMDA currents and [Ca 2ϩ ] i responses, by mGluR ligands, reported here might therefore reflect a direct action on the NMDAR.…”

Co-stimulation of mGluR5 and N-Methyl-D-aspartate Receptors Is Required for Potentiation of Excitatory Synaptic Transmission in Hippocampal Neurons

“…This transient depression was not blocked by MPEP, and it was also seen in cells taken from mGluR5 knockout mice providing evidence that it is due to a direct interaction with NMDARs rather than through mGluR5 signaling. Such a direct effect of mGluR reagents has been previously reported (36 -38) and reflects in part an interaction of these agents with the glycine binding site of the NMDA receptor (38). Indeed, at the concentrations of NMDA and glycine used by us, CHPG inhibited recombinant NR1 Stop838 /NR2a currents in HEK293 cells by 60.9 Ϯ 2.0% at 100 M and 98.5 Ϯ 0.8% at 1 mM (n ϭ 4, Fig.…”

Positive Allosteric Modulation of Metabotropic Glutamate 5 (mGlu5) Receptors Reverses N-Methyl-D-Aspartate Antagonist-Induced Alteration of Neuronal Firing in Prefrontal Cortex

“…The significant effect of CDPPB was long lasting in that the firing rate remained elevated above baseline for over an hour after injection, suggesting that this drug does not lead to rapid desensitization of mGlu5 receptors. This is interesting given that agonist stimulation of mGlu1/5 receptors leads to rapid desensitization (Contractor et al 1998;Gereau and Heinemann 1998). Another reason for the sustained effects of CDPPB may be that functional mGlu5 receptors have been localized on the nuclear membrane of cortical and midbrain neurons, where their activation increases intranuclear calcium release (Jong et al 2005;O'Malley et al 2003).…”