Pharmacology and functions of metabotropic glutamate receptors

Multiple pathways for regulation of the KCl-induced [3H]-GABA release by metabotropic glutamate receptors, in primary rat cortical cultures

In rat cortical primary cultures, group II- and III-metabotropic glutamate receptor-selective agonists concentration-dependently reduced KCl-induced [3H]GABA release, with IC50 values of 11 nM for LY354740, 80 nM for L(+)-2-amino-4-phosphonobutyric acid (L-AP4), 180 nM for DCG-IV, and 330 nM for L-SOP. The group II antagonists, LY341495 and EGLU, reversed the effect of LY354740, and the group III antagonist MTPG reversed the effect of L-AP4. In the presence of omega-conotoxin GVIA, LY354740 inhibited the remaining [3H]GABA release, whereas L-AP4 was inactive. In contrast, in the presence of nifedipine, L-AP4 inhibited the remaining [3H]GABA release, but LY354740 was no longer active. The PKA inhibitor, H89, blocked the effects of both L-AP4 and LY354740, whereas the PKC inhibitor Ro 31-8220 blocked only the effect of LY354740. Both Ro 31-8220 and H89 reduced the [3H]GABA release to 60% of control. In whole-cell, voltage-clamp experiments, LY354740 and L-AP4 inhibited voltage-gated calcium channel currents with IC50 values of 28 nM and 22 microM, respectively. The results suggest that, in these cells, KCl-induced [3H]GABA release is modulated by two different mechanisms, one involving group II receptors and a direct control of the Ca2+ channel activity, and the other mediated by group III receptors and possibly involving a regulation located downstream of the Ca2+ channel activation.

A cluster of basic residues in the carboxyl-terminal tail of the short metabotropic glutamate receptor 1 variants impairs their coupling to phospholipase C

Among phospholipase C-coupled metabotropic glutamate receptors (mGluRs), some have a surprisingly long carboxyl-terminal intracellular domain (mGluR1a, -5a, and -5b), and others have a short one (mGluR1b, -1c, and -1d). All mGluR1 sequences are identical up to 46 residues following the 7th transmembrane domain, followed by 313, 20, 11, and 26 specific residues in mGluR1a, mGluR1b, mGluR1c, and mGluR1d, respectively. Several functional differences have been described between the long isoforms (mGluR1a, -5a, and -5b) and the short ones (mGluR1b, -1c, and -1d). Compared with the long receptors, the short ones induce slower increases in intracellular Ca2+, are activated by higher concentration of agonists, and do not exhibit constitutive, agonist-independent activity. To identify the residues responsible for these functional properties, a series of truncated, chimeric, and mutated receptors were constructed. We found that the deletion of the last 19 carboxyl-terminal residues in mGluR1c changed its properties into those of mGluR1a. Moreover, the exchange of the long carboxyl-terminal domain of mGluR5a with that of mGluR1c generated a chimeric receptor that possessed functional properties similar to those of mGluR1c. Mutagenesis of specific residues within the 19 carboxyl-terminal residues of mGluR1c revealed the importance of a cluster of 4 basic residues in defining the specific properties of this receptor. Since this cluster is part of the sequence common to all mGluR1 variants, we conclude that the long carboxyl-terminal domain of mGluR1a suppresses the inhibitory action of this sequence element.

Effects of ethanol and anesthetics on type 1 and 5 metabotropic glutamate receptors expressed in Xenopus laevis oocytes

Previous studies have demonstrated that ethanol and volatile anesthetics inhibit the function of some metabotropic (G protein-coupled) receptors, including the 5-hydroxytryptamine2 and muscarinic cholinergic receptors. The metabotropic glutamate receptors (mGluRs) show little sequence homology with most other metabotropic receptors and are important modulators of synaptic transmission in the mammalian central nervous system. It was of interest to determine drug actions on these receptors, and we investigated the effects of ethanol, halothane, the anesthetic compound F3 (1-chloro-1,2,2-trifluorocyclobutane), and the nonanesthetics F6 (1,2-dichlorohexafluorocyclobutane) and F8 (2,3-chlorooctafluorobutane) on the function of mGluR1 and mGluR5 expressed in Xenopus laevis oocytes. Halothane, F3, and ethanol inhibited mGluR5-induced Ca(2+)-dependent Cl- currents, yet pharmacologically relevant concentrations of these compounds had little effect on the glutamate-induced currents in the oocytes expressing mGluR1. F6 had inhibitory effects on both receptors, and F8 did not affect either mGluR1 or mGluR5 function. The protein kinase C (PKC) inhibitor GF109203X enhanced the glutamate-induced current, and the PKC activator phorbol-12-myristate-13-acetate inhibited this current in the oocytes expressing mGluR5, but these compounds had little effect on mGluR1 function. GF109203X abolished the inhibitory effects of halothane, F3, and ethanol on mGluR5s. Conversely, the phosphatase inhibitor calyculin A prolonged the action of halothane and ethanol. Furthermore, mutation of a PKC consensus site (Ser890) of mGluR5 abolished the inhibitory effects of halothane, F3, and ethanol. These results suggest that ethanol and volatile anesthetics inhibit mGluR5 because they promote PKC-mediated phosphorylation.

Rat group I metabotropic glutamate receptors inhibit neuronal Ca2+ channels via multiple signal transduction pathways in HEK 293 cells

We have shown previously that metabotropic glutamate receptors with group I-like pharmacology couple to N-type and P/Q-type calcium channels in acutely isolated cortical neurons using G proteins most likely belonging to the Gi/Go subclass. To better understand the potential mechanisms forming the basis for group I mGluR modulation of voltage-gated calcium channels in the CNS, we have examined the ability of specific mGluRs to couple to neuronal N-type (alpha1B-1/alpha2delta/beta1b) and P/Q-type (alpha1A-2/alpha2delta/beta1b) voltage-gated calcium channels in an HEK 293 heterologous expression system. Using the whole cell patch-clamp technique where intracellular calcium is buffered to low levels, we have shown that group I receptors inhibit both N-type and P/Q-type calcium channels in a voltage-dependent fashion. Similar to our observations in cortical neurons, this voltage-dependent inhibition is mediated almost entirely by N-ethylmaleimide (NEM)-sensitive heterotrimeric G proteins, strongly suggesting that these receptors can use Gi/Go-like G proteins to couple to N-type and P/Q-type calcium channels. However, inconsistent with the apparent NEM sensitivity of group I modulation of calcium channels, modulation of N-type channels in group I mGluR-expressing cells was only partially sensitive to pertussis toxin (PTX), indicating the potential involvement of both PTX-sensitive and -resistant G proteins. The PTX-resistant modulation was voltage dependent and entirely resistant to NEM and cholera toxin. A time course of treatment with PTX revealed that this toxin caused group I receptors to slowly shift from using a primarily NEM-sensitive G protein to using a NEM-resistant form. The PTX-induced switch from NEM-sensitive to -resistant modulation was also dependent on protein synthesis, indicating some reliance on active cellular processes. In addition to these voltage-dependent pathways, perforated patch recordings on group I mGluR-expressing cells indicate that another slowly developing, calcium-dependent form of modulation for N-type channels may be seen when intracellular calcium is not highly buffered. We conclude that group I mGluRs can modulate neuronal Ca2+ channels using a variety of signal transduction pathways and propose that the relative contributions of different pathways may exemplify the diversity of responses mediated by these receptors in the CNS.

Role of protein kinase C phosphorylation in rapid desensitization of metabotropic glutamate receptor 5

Metabotropic glutamate receptors (mGluRs) coupled to phosphoinositide hydrolysis desensitize in response to prolonged or repeated agonist exposure, and evidence suggests that this involves activation of protein kinase C (PKC). The present studies were undertaken to determine if cloned mGluR5 undergoes similar PKC-mediated desensitization and to investigate the molecular mechanism underlying PKC-induced desensitization. In Xenopus oocytes, both mGluR5a and mGluR5b showed pronounced desensitization in response to a brief activation by glutamate. Pharmacological studies clearly suggest that this desensitization requires PKC-mediated phosphorylation. Analysis of PKC consensus phosphorylation site mutants suggests that PKC phosphorylates mGluR5 at multiple sites to induce a relatively rapid form of desensitization. Because mGluRs play important roles in synaptic plasticity and in excitotoxicity, this desensitization may be involved in the dynamic regulation of these processes.

Contribution of metabotropic glutamate receptors to the depression of excitatory postsynaptic potentials during hypoxia

We tested the hypothesis that activation of metabotropic glutamate receptors (mGluR) might contribute to the depression of excitatory postsynaptic potentials during hypoxia. The experiments were performed on hippocampal slices taken from young (12-14 days old) Wistar rats. The depression induced by hypoxia (14 min) was not modified in the presence of either the non-selective mGluR antagonist (which blocks mainly group I and II mGluR), MCPG (500 microM) or the selective group III mGluR antagonist, MPPG (500 microM). However, in experiments performed in the presence of the selective adenosine A1 receptor antagonist, DPCPX (50 nM), part of the hypoxia-induced depression could be prevented by MPPG (500 microM). Activation of group III mGluR may contribute to the hypoxia-induced depression, but this contribution is only revealed when adenosine A1 receptors are blocked.

The rat mGlu1d receptor splice variant shares functional properties with the other short isoforms of mGlu1 receptor

Three splice variants of the rat metabotropic glutamate receptor 1 (mGlu1a, 1b and 1c receptors) have been characterized so far. All have the same sequence up to the 46th residue following the 7th transmembrane domain, followed by different carboxyl-terminal tails. Whereas mGlu1b and mGlu1c receptors possess a short intracellular carboxyl-terminal tail, the mGlu1a receptor has a very long one. Compared to cells expressing mGlu1b or mGlu1c receptors, a higher agonist potency and basal phospholipase C activity were detected in cells expressing mGlu1a receptors. Another variant with a short carboxyl-terminal tail, the HmGlu1d receptor, has been recently isolated from human brain. Here we show that the mGlu1d receptor variant also exists in the rat. Like all rat mGlu1 receptor variants, the mGlu1d receptor activates phospholipase C upon stimulation with mGlu1 receptor agonists. Although the rank order of agonist potency is the same on mGlu1a and mGlu1d receptors, agonists are less potent in stimulating phospholipase C in mGlu1d receptor-expressing cells than in cells expressing mGlu1a receptors. Moreover, like the other short variants it has no significant constitutive activity. These results indicate that the mGlu1d receptor shares similar functional properties with the other short mGlu1 receptor splice variants, and further suggests that the long carboxyl-terminal tail of the mGlu1a receptor increases phospholipase C coupling efficacy.

Synthesis and pharmacological characterization of aminocyclopentanetricarboxylic acids: new tools to discriminate between metabotropic glutamate receptor subtypes

The four stereoisomers of 1-aminocyclopentane-1,3,4-tricarboxylic acid {ACPT-I (18) and -II (19), (3R, 4R)-III [(-)-20], and (3S,4S)-III [(+)-20]} have been synthesized and evaluated for their effects at glutamate receptors subtypes. ACPTs are ACPD analogues in which a third carboxylic group has been added at position 4 in the cyclopentane ring. None of the ACPT isomers showed a significant effect on ionotropic NMDA, KA, and AMPA receptors. On the other hand, ACPT-II (19) was found to be a general competitive antagonist for metabotropic receptors (mGluRs) and exhibited a similar affinity for mGluR1a (KB = 115 +/- 2 microM), mGluR2 (KB = 88 +/- 21 microM), and mGluR4a (KB = 77 +/- 9 microM), the representative members of group I, II and III mGluRs, respectively. Two other isomers, ACPT-I (18) and (+)-(3S,4S)-ACPT-III [(+)-20], were potent agonists at the group III receptor mGluR4a (EC50 = 7.2 +/- 2.3 and 8.8 +/- 3.2 microM) and competitive antagonists with low affinity for mGluR1a and mGluR2 (KB > 300 microM). Finally, (-)-(3R,4R)-ACPT-III [(-)-20] was a competitive antagonist with poor but significant affinity for mGluR4a (KB = 220 microM). These results demonstrate that the addition of a third carboxylic group to ACPD can change its activity (from agonist to antagonist) and either increase or decrease its selectivity and/or affinity for the various mGluR subtypes.