The role of group I metabotropic glutamate receptors in schizophrenia

Evaluation of [18F]FMTEB in Sprague Dawley rats as a PET tracer for metabotropic glutamate receptor 5

INTRODUCTION

[¹⁸F]FMTEB, along with other tracers, was developed as a promising PET radioligand for imaging metabotropic glutamate receptor subtype 5 (mGluR5). Despite favorable preliminary results, it has not been used further for studies of mGluR5. This paper presents an in-depth preclinical evaluation of [¹⁸F]FMTEB in healthy Sprague Dawley rats.

METHODS

[¹⁸F]FMTEB was synthesized from a boronic ester precursor using copper-mediated fluorination. In vivo PET imaging was performed on six rats, of which three were pre-treated with a high affinity mGluR5 receptor antagonist. An additional 18 rats were used for ex vivo experiments for metabolite analyses in plasma, brain and urine, and for biodistribution and ex vivo brain autoradiography at different time points.

RESULTS

[¹⁸F]FMTEB was synthesized in adequate radiochemical yield and a molar activity of 154 ± 64 GBq/μmol. Both in vivo imaging and ex vivo brain autoradiography showed high specificity for mGluR5, and the blocking experiments showed a clear decrease in radioactivity in mGluR5-rich brain areas. Metabolite analyses confirmed fast metabolism of the tracer in plasma. The percentage of parent compound in brain tissue exceeded 90 % up to 90 min after injection.

CONCLUSION

[¹⁸F]FMTEB produced via copper-mediated ¹⁸F-fluorination fulfilled the requirements for preclinical evaluation in rats. The absence of specific uptake in cerebellum and absence of defluorination of the tracer allowed cerebellum to be used as a reference tissue. Due to the fast kinetics in rats, the region-to-cerebellum ratios equilibrated within 30 min. These results prove [¹⁸F]FMTEB to be a good candidate for mapping mGluR5 in rat brain and a suitable alternative to [¹⁸F]FPEB.

Excitatory amino acid transporter (EAAT)1 and EAAT2 mRNA levels are altered in the prefrontal cortex of subjects with schizophrenia

Excitatory amino acid transporter (EAAT)1 and EAAT2 mediate glutamatergic neurotransmission and prevent excitotoxicity through binding and transportation of glutamate into glia. These EAATs may be regulated by metabotropic glutamate receptor 5 (mGluR5), which is also expressed by glia. Whilst we have data from an Affymetrix™ Human Exon 1.0 ST Array showing higher levels of EAAT1 mRNA (+36%) in Brodmann's are (BA)9 of subjects with schizophrenia, there is evidence that EAAT1 and EAAT2, as well as mGluR5 levels, are altered in the cortex of subjects with the disorder. Hence, we measured mRNA levels of these genes in other cortical regions in subjects with that disorder. EAAT1, EAAT2 and mGluR5 mRNA were measured, in triplicate, using Quantitative PCR in BA10 and BA46 from subjects with schizophrenia (n = 20) and age and sex matched controls (n = 18). Levels of mRNA were normalised to the geometric mean of two reference genes, transcription factor B1, mitochondrial (TFB1M) and S-phase kinase-associated protein 1A (SKP1A), for which mRNA did not vary between diagnostic groups in either region. Normalised levels of EAAT1 and EAAT2 mRNA were significantly higher in BA10 (EAAT1: U = 58, p = 0.0002; EAAT2 U = 70, p = 0.0009), but not BA46 (EAAT1: U = 122, p = 0.09; EAAT2: U = 136, p = 0.21), from subjects with schizophrenia compared to controls. mGluR5 levels in BA10 (U = 173, p=0.85) and BA46 (U = 178, p = 0.96) did not vary by cohort. Our data suggests that region-specific increases in cortical EAAT1 and EAAT2 mRNA are involved in schizophrenia pathophysiology and that disrupted glutamate uptake in schizophrenia may be of particular significance in BA10.

Activation of the mGlu1 metabotropic glutamate receptor has antipsychotic-like effects and is required for efficacy of M4 muscarinic receptor allosteric modulators

Recent clinical and preclinical studies suggest that selective activators of the M₄ muscarinic acetylcholine receptor have potential as a novel treatment for schizophrenia. M₄ activation inhibits striatal dopamine release by mobilizing endocannabinoids, providing a mechanism for local effects on dopamine signaling in the striatum but not in extrastriatal areas. G protein-coupled receptors (GPCRs) typically induce endocannabinoid release through activation of Gα_q/11-type G proteins whereas M₄ transduction occurs through Gα_i/o-type G proteins. We now report that the ability of M₄ to inhibit dopamine release and induce antipsychotic-like effects in animal models is dependent on co-activation of the Gα_q/11-coupled mGlu₁ subtype of metabotropic glutamate (mGlu) receptor. This is especially interesting in light of recent findings that multiple loss of function single nucleotide polymorphisms (SNPs) in the human gene encoding mGlu₁ (GRM1) are associated with schizophrenia, and points to GRM1/mGlu₁ as a gene within the "druggable genome" that could be targeted for the treatment of schizophrenia. Herein, we report that potentiation of mGlu₁ signaling following thalamo-striatal stimulation is sufficient to inhibit striatal dopamine release, and that a novel mGlu₁ positive allosteric modulator (PAM) exerts robust antipsychotic-like effects through an endocannabinoid-dependent mechanism. However, unlike M₄, mGlu₁ does not directly inhibit dopamine D₁ receptor signaling and does not reduce motivational responding. Taken together, these findings highlight a novel mechanism of cross talk between mGlu₁ and M₄ and demonstrate that highly selective mGlu₁ PAMs may provide a novel strategy for the treatment of positive symptoms associated with schizophrenia.

Effects of P-gp and Bcrp as brain efflux transporters on the uptake of [18 F]FPEB in the murine brain

The purpose of this study was to determine whether the brain uptake of [¹⁸ F]FPEB is influenced by P-glycoprotein (P-gp) and breast cancer resistance protein (Bcrp) as efflux transporters in rodents. To assess this possible modulation, positron emission tomography studies were performed in animal models of pharmacological or genetic ablation of these transporters. Compared with the control conditions, when P-gp was blocked with tariquidar, there was an 8%-12% increase in the brain uptake of [¹⁸ F]FPEB. In P-gp knockout mice, such as Mdr1a/b^(-/-) and Mdr1a/b^(-/-) Bcrp1^(-/-) , genetic ablation models, there was an increment of 8%-53% in [¹⁸ F]FPEB uptake compared with that in the wild-type mice. In contrast, Bcrp knockout mice showed a decrement of 5%-12% uptake and P-gp/Bcrp knockout group displayed an increment of 5%-17% compared with wild type. These results indicate that [¹⁸ F]FPEB is possibly a weak substrate for P-gp.

Cholinergic Neurons of the Medial Septum Are Crucial for Sensorimotor Gating

Hypofunction of NMDA receptors has been considered a possible cause for the pathophysiology of schizophrenia. More recently, indirect ways to regulate NMDA that would be less disruptive have been proposed and metabotropic glutamate receptor subtype 5 (mGluR5) represents one such candidate. To characterize the cell populations involved, we demonstrated here that knock-out (KO) of mGluR5 in cholinergic, but not glutamatergic or parvalbumin (PV)-positive GABAergic, neurons reduced prepulse inhibition of the startle response (PPI) and enhanced sensitivity to MK801-induced locomotor activity. Inhibition of cholinergic neurons in the medial septum by DREADD (designer receptors exclusively activated by designer drugs) resulted in reduced PPI further demonstrating the importance of these neurons in sensorimotor gating. Volume imaging and quantification were used to compare PV and cholinergic cell distribution, density, and total cell counts in the different cell-type-specific KO lines. Electrophysiological studies showed reduced NMDA receptor-mediated currents in cholinergic neurons of the medial septum in mGluR5 KO mice. These results obtained from male and female mice indicate that cholinergic neurons in the medial septum represent a key cell type involved in sensorimotor gating and are relevant to pathologies associated with disrupted sensorimotor gating such as schizophrenia.SIGNIFICANCE STATEMENT The mechanistic complexity underlying psychiatric disorders remains a major challenge that is hindering the drug discovery process. Here, we generated genetically modified mouse lines to better characterize the involvement of the receptor mGluR5 in the fine-tuning of NMDA receptors, specifically in the context of sensorimotor gating. We evaluated the importance of knocking-out mGluR5 in three different cell types in two brain regions and performed different sets of experiments including behavioral testing and electrophysiological recordings. We demonstrated that cholinergic neurons in the medial septum represent a key cell-type involved in sensorimotor gating. We are proposing that pathologies associated with disrupted sensorimotor gating, such as with schizophrenia, could benefit from further evaluating strategies to modulate specifically cholinergic neurons in the medial septum.

Altered expression of schizophrenia-related genes in mice lacking mGlu5 receptors

The evidence underlying the so-called glutamatergic hypothesis ranges from NMDA receptor hypofunction to an imbalance between excitatory and inhibitory circuits in specific brain structures. Among all glutamatergic system components, metabotropic receptors play a main role in regulating neuronal excitability and synaptic plasticity. Here, we investigated, using qRT-PCR and western blot, consequences in the hippocampus and prefrontal/frontal cortex (PFC/FC) of mice with a genetic deletion of the metabotropic glutamate receptor 5 (mGlu5), addressing key components of the GABAergic and glutamatergic systems. We found that mGlu5 knockout (KO) mice showed a significant reduction of reelin, GAD65, GAD67 and parvalbumin mRNA levels, which is specific for the PFC/FC, and that is paralleled by a significant reduction of protein levels in male KO mice. We next analyzed the main NMDA and AMPA receptor subunits, namely GluN1, GluN2A, GluN2B and GluA1, and we found that mGlu5 deletion determined a significant reduction of their mRNA levels, also within the hippocampus, with differences between the two genders. Our data suggest that neurochemical abnormalities impinging the glutamatergic and GABAergic systems may be responsible for the behavioral phenotype associated with mGlu5 KO animals and point to the close interaction of these molecular players for the development of neuropsychiatric disorders such as schizophrenia. These data could contribute to a better understanding of the involvement of mGlu5 alterations in the molecular imbalance between excitation and inhibition underlying the emergence of a schizophrenic-like phenotype and to understand the potential of mGlu5 modulators in reversing the deficits characterizing the schizophrenic pathology.

Phospholipase C-β1 and schizophrenia-related behaviors

Abnormal expression patterns of phospholipase C-β1(PLC-β1) in specific brain areas of patients with schizophrenia, and its high genetic linkage to the disorder implicated a pathogenetical involvement of PLC-β1 signaling system. The schizophrenia-related behavioral phenotypes displayed in the mutant mice lacking PLC-β1 (PLC-β1 KO) suggested that PLCβ1-linked signaling pathways may be involved in the neural system whose function is disrupted in the pathogenesis of schizophrenia. In the brain, PLC-β1 is known to be linked to muscarinic acetylcholine receptors, metabotropic glutamatergic, serotonergic, and oxytocinergic systems. The objective of this review is to provide an overview of the current knowledge regarding these schizophrenia-related behaviors and discuss the probable ways in which PLC-β1signalling can be involved in the neural mechanisms for each behavior, which may help suggest future directions for research in this area.

Synthesis and characterization in monkey of [11C]SP203 as a radioligand for imaging brain metabotropic glutamate 5 receptors

PURPOSE

[(18)F]SP203 (3-fluoro-5-(2-(2-([(18)F]fluoromethyl)-thiazol-4-yl)ethynyl)benzonitrile) is an effective high-affinity and selective radioligand for imaging metabotropic 5 receptors (mGluR5) in human brain with PET. To provide a radioligand that may be used for more than one scanning session in the same subject in a single day, we set out to label SP203 with shorter-lived (11)C (t (1/2) = 20.4 min) and to characterize its behavior as a radioligand with PET in the monkey.

METHODS

Iodo and bromo precursors were obtained by cross-coupling 2-fluoromethyl-4-((trimethylsilyl)ethynyl)-1,3-thiazole with 3,5-diiodofluorobenzene and 3,5-dibromofluorobenzene, respectively. Treatment of either precursor with [(11)C]cyanide ion rapidly gave [(11)C]SP203, which was purified with high-performance liquid chromatography. PET was used to measure the uptake of radioactivity in brain regions after injecting [(11)C]SP203 intravenously into rhesus monkeys at baseline and under conditions in which mGluR5 were blocked with 3-[(2-methyl-1,3-thiazol-4-yl)ethynyl]pyridine (MTEP). The emergence of radiometabolites in monkey blood in vitro and in vivo was assessed with radio-HPLC. The stability of [(11)C]SP203 in human blood in vitro was also measured.

RESULTS

The iodo precursor gave [(11)C]SP203 in higher radiochemical yield (>98 %) than the bromo precursor (20-52 %). After intravenous administration of [(11)C]SP203 into three rhesus monkeys, radioactivity peaked early in brain (average 12.5 min) with a regional distribution in rank order of expected mGluR5 density. Peak uptake was followed by a steady decline. No radioactivity accumulated in the skull. In monkeys pretreated with MTEP before [(11)C]SP203 administration, radioactivity uptake in brain was again high but then declined more rapidly than in the baseline scan to a common low level. [(11)C]SP203 was unstable in monkey blood in vitro and in vivo, and gave predominantly less lipophilic radiometabolites. By contrast, [(11)C]SP203 was stable in human blood in vitro.

CONCLUSION

[(11)C]SP203 emulates [(18)F]SP203 with regard to providing a sizeable mGluR5-specific signal in monkey brain, and advantageously avoids troublesome accumulation of radioactivity in bone. Although [(11)C]SP203 is unsuitable for mGluR5 quantification in monkey brain, its evaluation as a PET radioligand for studying human brain mGluR5 is nevertheless warranted.

Phencyclidine treatment increases NR2A and NR2B N-methyl-D-aspartate receptor subunit expression in rats

Administration of noncompetitive N-methyl-D-aspartate receptor (NMDAR) antagonist phencyclidine to rats on postnatal days 7, 9, and 11 induces apoptosis in prefrontal cortex and hippocampus. In adulthood, these animals display cognitive impairment of working memory, reversal learning and attention that are similar to clinical observations in schizophrenia. In this study, expression of different NMDAR subunits, the postsynaptic mGlu5 receptor and the connecting NMDAR-mGluR5 intracellular postsynaptic density proteins have been measured in adult rats after treatment with phencyclidine on postnatal days 7, 9, and 11. We found that these animals exhibited elevated expression in medial prefrontal cortex of the NR2A and NR2B NMDA receptor subunits in adulthood. These results indicate how behavioral changes in a developmental model for cognitive dysfunction involve changes to specific molecular subsets of the cortical glutamate system.

Syntheses of 2-amino and 2-halothiazole derivatives as high-affinity metabotropic glutamate receptor subtype 5 ligands and potential radioligands for in vivo imaging

The structure of the potent selective mGlu(5) ligand, SP203 (1, 3-fluoro-5-[[2-(fluoromethyl)thiazol-4-yl]ethynyl]benzonitrile), was modified by replacing the 2-fluoromethyl substituent with an amino or halo substituent and by variation of substituents in the distal aromatic ring to provide a series of new high-affinity mGlu(5) ligands. In this series, among the most potent ligands obtained, the 2-chloro-thiazoles 7a and 7b and the 2-fluorothiazole 10b showed subnanomolar mGlu(5) affinity. 10b also displayed >10000-fold selectivity over all other metabotropic receptor subtypes plus a wide range of other receptors and binding sites. The 2-fluorothiazoles 10a and 10b were labeled using [(18)F]fluoride ion (t(1/2) = 109.7 min) in moderately high radiochemical yield to provide potential radioligands that may resist troublesome radiodefluorination during the imaging of brain mGlu(5) with position emission tomography. The iodo compound 9b has nanomolar affinity for mGlu(5) and may also serve as a lead to a potential (123)I-labeled ligand for imaging brain mGlu(5) with single photon emission computed tomography.

Interactive effects of mGlu5 and 5-HT2A receptors on locomotor activity in mice

RATIONALE

Metabotropic glutamate (mGlu) receptors have been suggested to play a role in neuropsychiatric disorders including schizophrenia, drug abuse, and depression. Because serotonergic hallucinogens increase glutamate release and mGlu receptors modulate the response to serotonin (5-HT)(2A) activation, the interactions between serotonin 5-HT(2A) receptors and mGlu receptors may prove to be important for our understanding of these diseases.

OBJECTIVE

We tested the effects of the serotonergic hallucinogen and 5-HT(2A) agonist, 2,5-dimethoxy-4-methylamphetamine (DOM), and the selective 5-HT(2A) antagonist, M100907, on locomotor activity in the mouse behavioral pattern monitor (BPM) in mGlu5 wild-type (WT) and knockout (KO) mice on a C57 background.

RESULTS

Both male and female mGlu5 KO mice showed locomotor hyperactivity and diminished locomotor habituation compared with their WT counterparts. Similarly, the mGlu5-negative allosteric modulator 2-methyl-6-(phenylethynyl)pyridine (MPEP) also increased locomotor hyperactivity, which was absent in mGlu5 KO mice. The locomotor hyperactivity in mGlu5 receptor KO mice was potentiated by DOM (0.5 mg/kg, subcutaneously (SC)) and attenuated by M100907 (1.0 mg/kg, SC). M100907 (0.1 mg/kg, SC) also blocked the hyperactivity induced by MPEP.

CONCLUSIONS

These studies demonstrated that loss of mGlu5 receptor activity either pharmacologically or through gene deletion leads to locomotor hyperactivity in mice. Additionally, the gene deletion of mGlu5 receptors increased the behavioral response to the 5-HT(2A) agonist DOM, suggesting that mGlu5 receptors either mitigate the behavioral effects of 5-HT(2A) hallucinogens or that mGlu5 KO mice show an increased sensitivity to 5-HT(2A) agonists. Taken together, these studies indicate a functional interaction between mGlu5 and 5-HT(2A) receptors.

Design and synthesis of substituted N-(1,3-diphenyl-1H-pyrazol-5-yl)benzamides as positive allosteric modulators of the metabotropic glutamate receptor subtype 5

Based on SAR in the alkyne class of mGlu5 receptor negative allosteric modulators and a set of amide-based positive allosteric modulators, optimized substitution of the aryl 'b' ring was used to create substituted N-(1,3-diphenyl-1H-pyrazol-5-yl)benzamides. Results from an mGlu5 receptor functional assay, using calcium fluorescence, revealed varying efficacies and potencies that provide evidence that subtle changes in compounds within a close structural class can have marked effects on functional activity including switches in modes of efficacy (i.e., negative to positive allosteric modulation).

Potential psychiatric applications of metabotropic glutamate receptor agonists and antagonists

Drugs acting at metabotropic glutamate receptors (mGluRs) are among the most promising agents under development for the treatment of psychiatric disorders. The research in this area is at a relatively early stage, as there are no drugs acting at mGluRs that have been approved for the treatment of any psychiatric disorder. However, in the areas of schizophrenia, anxiety disorders and mood disorders, research conducted in animal models appears to translate well into efficacy in human laboratory-based models of psychopathology and in preliminary clinical trials. Further, the genes coding for mGluRs are implicated in the risk for a growing number of psychiatric disorders. This review highlights the best studied mGluR strategies for psychiatry, based on human molecular genetics, studies in animal models and preliminary clinical trials. It describes the potential value of mGluR2 and mGluR5 agonists and positive allosteric modulators for the treatment of schizophrenia. It also reviews evidence that group II mGluR agonists and positive allosteric modulators as well as group I mGluR antagonists might also treat anxiety disorders and some forms of depression, while mGluR2 and group I mGluR antagonists (particularly mGluR5 antagonists) might have antidepressant properties. This review also links growing insights into the role of glutamate in the pathophysiology of these disorders to hypothesized mGluR-related treatment mechanisms.

Modeling the positive symptoms of schizophrenia in genetically modified mice: pharmacology and methodology aspects

In recent years, there have been huge advances in the use of genetically modified mice to study pathophysiological mechanisms involved in schizophrenia. This has allowed rapid progress in our understanding of the role of several proposed gene mechanisms in schizophrenia, and yet this research has also revealed how much still remains unresolved. Behavioral studies in genetically modified mice are reviewed with special emphasis on modeling psychotic-like behavior. I will particularly focus on observations on locomotor hyperactivity and disruptions of prepulse inhibition (PPI). Recommendations are included to address pharmacological and methodological aspects in future studies. Mouse models of dopaminergic and glutamatergic dysfunction are then discussed, reflecting the most important and widely studied neurotransmitter systems in schizophrenia. Subsequently, psychosis-like behavior in mice with modifications in the most widely studied schizophrenia susceptibility genes is reviewed. Taken together, the available studies reveal a wealth of available data which have already provided crucial new insight and mechanistic clues which could lead to new treatments or even prevention strategies for schizophrenia.

Proteomic analysis reveals novel binding partners of metabotropic glutamate receptor 1

Regulated trafficking of neurotransmitter receptors is critical to normal neurodevelopment and neuronal signaling. Group I mGluRs (mGluR1/5 and their splice variants) are G protein-coupled receptors enriched at excitatory synapses, where they serve to modulate glutamatergic transmission. The mGluR1 splice variants mGluR1a and mGluR1b are broadly expressed in the central nervous system and differ in their signaling and trafficking properties. Several proteins have been identified that selectively interact with mGluR1a and participate in receptor trafficking but no proteins interacting with mGluR1b have thus far been reported. We have used a proteomic strategy to isolate and identify proteins that co-purify with mGluR1b in Madin-Darby Canine Kidney (MDCK) cells, an established model system for trafficking studies. Here, we report the identification of 10 novel candidate mGluR1b-interacting proteins. Several of the identified proteins are structural components of the cell cytoskeleton, while others serve as cytoskeleton-associated adaptors and motors or endoplasmic reticulum-associated chaperones. Findings from this work will help unravel the complex cellular mechanisms underlying mGluR trafficking under physiological and pathological conditions.

Normal electrocortical facilitation but abnormal target identification during visual sustained attention in schizophrenia

Attentional deficits in schizophrenia have been investigated using target identification tasks which conflate the abilities to successfully (1) attend to possible target locations and (2) detect target events. Whether compromised attentional selectivity or abnormal target detection causes schizophrenia subjects' poor performance on visual attention tasks, therefore, is unknown. To address this issue, we measured the neural activity (using electroencephalography) of 17 schizophrenia and 17 healthy subjects during a target identification task. Participants viewed superimposed images (horizontal and vertical bars differing in color) and attended to one image to identify bar width changes in specific locations. Bars were frequency tagged so attention directed to unique parts of the images could be tracked. Steady-state visual evoked potentials (ssVEPs) were used to quantify attention-related neural activity to specific parts of the visual images. Behavioral performance and event-related potentials (ERPs) in response to the target events were used to quantify target detection abilities. For both schizophrenia and healthy subjects, attending to specific parts of the attended image enhanced brain activity related to attended bars and reduced activity evoked by unattended bars. Activity in relation to the spatially overlapping unattended image was unaffected. Schizophrenia patients, however, were impaired on target detection abilities on both behavioral and brain activity measures. Target-related behavioral and brain activity measures were highly correlated in both groups. These findings indicate that deficient target detection rather than compromised attentional selectivity accounts for previously reported visual attention deficits in schizophrenia.

Radiodefluorination of 3-fluoro-5-(2-(2-[18F](fluoromethyl)-thiazol-4-yl)ethynyl)benzonitrile ([18F]SP203), a radioligand for imaging brain metabotropic glutamate subtype-5 receptors with positron emission tomography, occurs by glutathionylation in rat brain

Metabotropic glutamate subtype-5 receptors (mGluR5) are implicated in several neuropsychiatric disorders. Positron emission tomography (PET) with a suitable radioligand may enable monitoring of regional brain mGluR5 density before and during treatments. We have developed a new radioligand, 3-fluoro-5-(2-(2-[(18)F](fluoromethyl)thiazol-4-yl)ethynyl)benzonitrile ([(18)F]SP203), for imaging brain mGluR5 in monkey and human. In monkey, radioactivity was observed in bone, showing release of [(18)F]-fluoride ion from [(18)F]SP203. This defluorination was not inhibited by disulfiram, a potent inhibitor of CYP2E1. PET confirmed bone uptake of radioactivity and therefore defluorination of [(18)F]SP203 in rats. To understand the biochemical basis for defluorination, we administered [(18)F]SP203 plus SP203 in rats for ex vivo analysis of metabolites. Radio-high-performance liquid chromatography detected [(18)F]fluoride ion as a major radiometabolite in both brain extract and urine. Incubation of [(18)F]SP203 with brain homogenate also generated this radiometabolite, whereas no metabolism was detected in whole blood in vitro. Liquid chromatography-mass spectrometry analysis of the brain extract detected m/z 548 and 404 ions, assignable to the [M + H](+) of S-glutathione (SP203Glu) and N-acetyl-S-l-cysteine (SP203Nac) conjugates of SP203, respectively. In urine, only the [M + H](+) of SP203Nac was detected. Mass spectrometry/mass spectrometry and multi-stage mass spectrometry analyses of each metabolite yielded product ions consistent with its proposed structure, including the former fluoromethyl group as the site of conjugation. Metabolite structures were confirmed by similar analyses of SP203Glu and SP203Nac, prepared by glutathione S-transferase reaction and chemical synthesis, respectively. Thus, glutathionylation at the 2-fluoromethyl group is responsible for the radiodefluorination of [(18)F]SP203 in rat. This study provides the first demonstration of glutathione-promoted radiodefluorination of a PET radioligand.

[Development of antipsychotic drug: new directions]

The treatment of psychotic disorders has been dramatically changed by the discovery, in 1952, of antipsychotic activity of chlorpromazine and the definition of criteria for a new therapeutic class. Since then, although major improvements were done in terms of neurological tolerance and easiness to handle, today's antipsychotics are highly similar to former ones. Those treatments do not answer to all expectations of patients or practitioners, in particular regarding their metabolic or cardiologic side effects and potency to reduce cognitive deficit. Moreover, despite increasing understanding of the pathophysiology of schizophrenic disorders, this new knowledge is not or very little used in the field of therapeutics, justifying to resume research efforts to improve therapeutics in psychotic disorders. The development of a new medication follows several steps, based on precise strategies and regulations. The main streams of research concern compounds that act on dopaminergic, serotoninergic or peptidergic systems.

The possible involvement of metabotropic glutamate receptors in schizophrenia

Glutamate disruption is thought to have a major role in schizophrenia brain processes, possibly involving NMDA hypofunction. The metabotropic glutamate receptors are distributed in brain regions related to schizophrenia and seem to affect glutamate release in a moderate way. Compounds modulating these receptors are being investigated in animal models of schizophrenia, in an attempt to discover new antipsychotics. This article reviews the current research data regarding the role of these receptors in schizophrenia animal models. It was found that more research was done on Group I and II metabotropic receptors while investigation of group III receptors is still trailing behind. Accumulating evidence shows that mGluR5 antagonists by themselves do not necessarily disrupt pre-pulse inhibition (PPI), but can exacerbate disruption of PPI caused by MK-801 and PCP, while positive modulation of this receptor has beneficial effects on these models of psychosis. Group II agonists are also showing beneficial effects in animal models. It seems that metabotropic glutamate receptor modulators could be developed into a novel treatment of schizophrenia by altering glutamate release, thus overcoming the putative NMDA hypofunction. Although the implications from these pre-clinical studies to human schizophrenia patients are premature, the data obtained with some compounds point to promising results for drug development. More studies, with agents active at other mGluRs in animal models and schizophrenia patients as well as with human subjects are needed in order to clarify the role of the metabotropic glutamate receptors in the pathophysiology and pharmacotherapy of schizophrenia.