James A. Monn

Pharmacological agents acting at subtypes of metabotropic glutamate receptors

Metabotropic (G-protein-coupled) glutamate (mGlu) receptors have now emerged as a recognized, but still relatively new area of excitatory amino acid research. Current understanding of the roles and involvement of mGlu receptor subtypes in physiological/pathophysiological functions of the central nervous system has been recently propelled by the emergence of various structurally novel, potent, and mGlu receptor selective pharmacological agents. This article reviews the evolution of pharmacological agents that have been reported to target mGlu receptors, with a focus on the known receptor subtype selectivities of current agents.

Activation of mGlu2/3 receptors as a new approach to treat schizophrenia: a randomized Phase 2 clinical trial

Schizophrenia is a chronic, complex and heterogeneous mental disorder, with pathological features of disrupted neuronal excitability and plasticity within limbic structures of the brain. These pathological features manifest behaviorally as positive symptoms (including hallucinations, delusions and thought disorder), negative symptoms (such as social withdrawal, apathy and emotional blunting) and other psychopathological symptoms (such as psychomotor retardation, lack of insight, poor attention and impulse control). Altered glutamate neurotransmission has for decades been linked to schizophrenia, but all commonly prescribed antipsychotics act on dopamine receptors. LY404039 is a selective agonist for metabotropic glutamate 2/3 (mGlu2/3) receptors and has shown antipsychotic potential in animal studies. With data from rodents, we provide new evidence that mGlu2/3 receptor agonists work by a distinct mechanism different from that of olanzapine. To clinically test this mechanism, an oral prodrug of LY404039 (LY2140023) was evaluated in schizophrenic patients with olanzapine as an active control in a randomized, three-armed, double-blind, placebo-controlled study. Treatment with LY2140023, like treatment with olanzapine, was safe and well-tolerated; treated patients showed statistically significant improvements in both positive and negative symptoms of schizophrenia compared to placebo (P < 0.001 at week 4). Notably, patients treated with LY2140023 did not differ from placebo-treated patients with respect to prolactin elevation, extrapyramidal symptoms or weight gain. These data suggest that mGlu2/3 receptor agonists have antipsychotic properties and may provide a new alternative for the treatment of schizophrenia.

Metabotropic glutamate receptors as novel targets for anxiety and stress disorders

Anxiety and stress disorders are the most commonly occurring of all mental illnesses, and current treatments are less than satisfactory. So, the discovery of novel approaches to treat anxiety disorders remains an important area of neuroscience research. Glutamate is the major excitatory neurotransmitter in the mammalian central nervous system, and G-protein-coupled metabotropic glutamate (mGlu) receptors function to regulate excitability via pre- and postsynaptic mechanisms. Various mGlu receptor subtypes, including group I (mGlu1 and mGlu5), group II (mGlu2 and mGlu3), and group III (mGlu4, mGlu7 and mGlu8) receptors, specifically modulate excitability within crucial brain structures involved in anxiety states. In addition, agonists for group II (mGlu2/3) receptors and antagonists for group I (in particular mGlu5) receptors have shown activity in animal and/or human conditions of fear, anxiety or stress. These studies indicate that metabotropic glutamate receptors are interesting new targets to treat anxiety disorders in humans.

LY354740 is a Potent and Highly Selective Group II Metabotropic Glutamate Receptor Agonist in Cells Expressing Human Glutamate Receptors

The novel compound LY354740 is a conformationally constrained analog of glutamate, which was designed for interaction at metabotropic glutamate (mGlu) receptors. In this paper the selectivity of LY354740 for recombinant human mGlu receptor subtypes expressed in non-neuronal (RGT) cells is described. At human mGlu2 receptors, LY354740 produced > 90% suppression of forskolin-stimulated cAMP formation with an EC50 of 5.1 +/- 0.3 nM. LY354740 was six-fold less potent in activating human mGlu3 receptors (EC50 = 24.3 +/- 0.5 nM). LY354740 inhibition of forskolin-stimulated cAMP formation in human mGlu2 receptor-expressing cells was blocked by competitive mGlu receptor antagonists, including (+)-alpha-methyl-4-carboxyphenylglycine (MCPG) and LY307452 ((2S,4S)-2-amino-4-(4,4-diphenylbut-1-yl)-pentane-1,5-dioic acid). LY354740 had no agonist or antagonist activities at cells expressing human mGlu4 or mGlu7 (group III mGlu receptors) (EC50 > 100,000 nM). When tested at group I phosphoinositide-coupled human mGlu receptors (mGlu1a and mGlu5a), LY354740 did not activate or inhibit mGlu receptor agonist-evoked phosphoinositide hydrolysis at up to 100,000 nM. Electrophysiological experiments also demonstrated that LY354740 also had no appreciable activity in cells expressing human recombinant AMPA (GluR4) and kainate (GluR6) receptors. Thus, LY354740 is a highly potent, efficacious and selective group II (mGlu2/3) receptor agonist, useful to explore the functions of these receptors in situ.

In vivo inhibition of veratridine-evoked release of striatal excitatory amino acids by the group II metabotropic glutamate receptor agonist LY354740 in rats

In vivo microdialysis in freely moving rats was used to investigate the presynaptic mechanisms by which LY354740, a novel, potent, selective, and systemically active agonist for group II metabotropic glutamate receptors (mGluRs), alters glutamate neuronal transmission. Basal levels of glutamate and aspartate in striatal dialysates of LY354740 (10 mg/kg i.p.)-treated animals were not significantly different from the saline-treated control animals. In the saline treated controls, veratridine (100 microM) induced a 6-fold increase in glutamate and 9-fold increase in aspartate. However, following LY354740 administration the veratridine-evoked release of glutamate and aspartate was completely prevented. These data demonstrate that LY354740 blocks the evoked release of endogenous excitatory amino acids, and indicate a role for group II mGluRs in presynaptic modulation of glutamate neuronal transmission in vivo. Ireland Ltd.

Inhibition of cyclic AMP formation by a selective metabotropic glutamate receptor agonist.

Abstract: It is well documented that the effects of excitatory ammo acid (EAA) agonists on phosphoinositide hydrolysis involve a GTP‐binding protein‐linked or “metabotropic’ receptor mechanism. The mechanisms by which EAAs alter cyclic AMP levels in brain slices, however, are not yet clear. In this study, the selective metabotropic EAA agonist trans‐(±)‐l‐aminocyclopentane‐l,3‐dicarboxylic acid and its isomers were examined for effects on basal and forskolin‐stimulated cyclic AMP formation in slices of the rat hippocampus. Trans‐(±)‐l‐Aminocyclopentane‐l,3‐dicarboxylic acid had little effect on basal cyclic AMP but inhibited forskolin‐stimulated cyclic AMP formation in a biphasic manner. The 1S,3R isomer of 1‐aminocy‐clopentane‐l,3‐dicarboxylic acid produced potent but only partial (~50%) inhibition of forskolin‐stimulated cyclic AMP formation. 1R,3S–l‐Aminocyclopentane‐l,3‐dicarboxylic acid fully inhibited forskolin‐stimulated cyclic AMP but with lower potency than the IS,3R isomer. These results show that in addition to the formation of phosphoinositide‐derived second messengers, the cellular consequences of selectively activating hippocampal metabotropic EAA receptors include an alteration of cellular cyclic AMP levels.

Attenuation of specific PCP-evoked behaviors by the potent mGlu2/3 receptor agonist, LY379268 and comparison with the atypical antipsychotic, clozapine

Abstract Rationale: Recent studies using phencyclidine (PCP) as a model for psychosis have implicated metabotropic glutamate (mGlu) receptors in schizophrenia. We have shown, using an automated motor activity monitoring system, that selective group II mGlu receptor agonists attenuate PCP (5 mg/kg)-evoked increases in ambulations and fine motor movements with similar profiles to the atypical antipsychotic, clozapine. Objective and methods: Because the automated system does not discriminate between specific PCP-evoked behaviors, in this paper we examined the effects of the potent mGlu2/3 receptor agonist LY379268 on PCP-evoked behaviors as assessed by observational methods. Furthermore, we have compared the actions of LY379268 to the atypical antipsychotic clozapine. Results: LY379268 and clozapine reduced the expression of PCP-induced falling, turning and back pedaling in a dose-dependent manner. Thirty minutes post-PCP administration, 1 mg/kg LY379269 reduced falls and turns by 89% and 53%, respectively, and 1 mg/kg clozapine attenuated turning by 70%. Interestingly, low doses of clozapine increased PCP-elicited falls. Back-pedaling was particularly sensitive to LY379268 and clozapine, with 1 mg/kg of either agent completely abolishing back-pedaling 30 min after PCP administration. However, in contrast to LY379268, attenuation of these behaviors by clozapine only occurred at doses that augmented PCP-evoked ataxia. Furthermore, LY379268 did not affect PCP-evoked forepaw treading. Conclusions: These results indicate that mGlu2/3 receptors do not mediate a generalized reduction in motor activity, but instead selectively modulate specific PCP behaviors, further implicating group II mGlu receptors as viable drug targets in the treatment of schizophrenia.

LY354740 : a metabotropic glutamate receptor agonist which ameliorates symptoms of nicotine withdrawal in rats

LY354740 is a conformationally constrained analog of glutamate with high selectivity and nanomolar agonist activity at Group II metabotropic glutamate receptors (mGluRs). This orally active compound is a new drug candidate which is being developed for the treatment of anxiety. In this study, LY354740 was investigated in a model of nicotine withdrawal using the acoustic startle reflex (sensorimotor reactivity) in rats. Nicotine (6 mg/kg/day) was administered for 12 days subcutaneously by osmotic minipumps. After 12 days the pumps were removed and the animals were allowed to go through spontaneous withdrawal. Cessation of chronic nicotine exposure led to increased startle responding for 4 days following withdrawal. Treatment with LY354740 (0.0001-0.1 mg/kg, i.p.; 0.03-3 mg/kg, oral) produced a dose-dependent attenuation of the enhanced auditory startle responding following withdrawal of nicotine with intraperitoneal and oral ED50 values of 0.003 mg/kg and 0.7 mg/kg, respectively. These effects were stereoselective since the (-)-enantiomer of LY354740, LY366563, was without effect in this model. LY354740 produced no changes in the sensorimotor reactivity of rats not exposed to nicotine at oral doses up to 10 mg/kg. These data support the functional role of mGluR agonists in nicotine withdrawal and indicate that LY354740 may be efficacious in reducing the symptoms associated with nicotine withdrawal during smoking cessation in humans.

The mGlu2/3 receptor agonist LY379268 selectively blocks amphetamine ambulations and rearing

We have previously reported that the specific group II metabotropic glutamate receptor agonist LY379268 inhibited phencyclidine (PCP)-induced motor activations in rats, but had mixed effects on behaviors produced by amphetamine. Here, LY379268 (1 mg/kg subcutaneous (s.c.)) attenuated amphetamine-induced ambulations and rearing but did not alter amphetamine-evoked fine motor movements. Consistent with a mechanism involving mGlu(2/3) receptors, the inhibitory actions of LY379268 on ambulations and rearing were reversed by LY341495, a mGlu(2/3) receptor antagonist. These data further suggest antipsychotic actions of mGlu(2/3) receptor agonists with a low propensity for extra-pyramidal side effects.

Neuroprotection by metabotropic glutamate receptor agonists: LY354740, LY379268 and LY389795

In rat cortical neuronal cultures, metabotropic glutamate (mGlu) receptor agonists: LY354740 (+)-2-aminobicyclo[3.1.0]hexane-2,6-dicarboxylate); LY379268 (−)-2-oxa-4-aminobicyclo[3.1.0]hexane-4,6-dicarboxylate, and LY389795 (−)-2-thia-4-aminobicyclo[3.1.0]hexane-4,6-dicarboxylate, were neuroprotective against toxicity induced by N-methyl-d-aspartic acid (NMDA), kainic acid and staurosporine as measured by release of lactate dehydrogenase (LDH) activity into culture supernatants and DNA fragmentation by oligonucleosome formation. The potencies of the agonists were at least 100 times greater in reducing nucleosome formation than LDH release indicating a differential effect on neurons dying by apoptosis than by necrosis. In vivo studies showed that LY354740 was able to mediate a partial protection against apoptosis in CA1 hippocampal cells under ischaemic conditions where substantial CA1 cell loss occurred. The effects of the agonists in vitro were: (a) reversed by mGlu receptor antagonist LY341495, (b) enhanced by the presence of glial cells, (c) abrogated by RNA and protein synthesis inhibitors, and (d) unaltered by inhibition of endogenous adenosine activity. These results suggest that group II mGlu receptor agonists may represent a novel therapeutic strategy for the treatment of neurodegenerative diseases.