In vitro characterization of [3H]MethoxyPyEP, an mGluR5 selective radioligand

A Review of Molecular Imaging of Glutamate Receptors

Molecular imaging with positron emission tomography (PET) and single photon emission computed tomography (SPECT) is a well-established and important in vivo technique to evaluate fundamental biological processes and unravel the role of neurotransmitter receptors in various neuropsychiatric disorders. Specific ligands are available for PET/SPECT studies of dopamine, serotonin, and opiate receptors, but corresponding development of radiotracers for receptors of glutamate, the main excitatory neurotransmitter in mammalian brain, has lagged behind. This state of affairs has persisted despite the central importance of glutamate neurotransmission in brain physiology and in disorders such as stroke, epilepsy, schizophrenia, and neurodegenerative diseases. Recent years have seen extensive efforts to develop useful ligands for molecular imaging of subtypes of the ionotropic (N-methyl-D-aspartate (NMDA), kainate, and AMPA/quisqualate receptors) and metabotropic glutamate receptors (types I, II, and III mGluRs). We now review the state of development of radioligands for glutamate receptor imaging, placing main emphasis on the suitability of available ligands for reliable in vivo applications. We give a brief account of the radiosynthetic approach for selected molecules. In general, with the exception of ligands for the GluN2B subunit of NMDA receptors, there has been little success in developing radiotracers for imaging ionotropic glutamate receptors; failure of ligands for the PCP/MK801 binding site in vivo doubtless relates their dependence on the open, unblocked state of the ion channel. Many AMPA and kainite receptor ligands with good binding properties in vitro have failed to give measurable specific binding in the living brain. This may reflect the challenge of developing brain-penetrating ligands for amino acid receptors, compounded by conformational differences in vivo. The situation is better with respect to mGluR imaging, particularly for the mGluR5 subtype. Several successful PET ligands serve for investigations of mGluRs in conditions such as schizophrenia, depression, substance abuse and aging. Considering the centrality and diversity of glutamatergic signaling in brain function, we have relatively few selective and sensitive tools for molecular imaging of ionotropic and metabotropic glutamate receptors. Further radiopharmaceutical research targeting specific subtypes and subunits of the glutamate receptors may yet open up new investigational vistas with broad applications in basic and clinical research.

Palladium mediated ¹¹C-cyanation and characterization in the non-human primate brain of the novel mGluR5 radioligand [¹¹C]AZD9272

INTRODUCTION

The aims of the present positron emission tomography (PET) study were to set up a system for (11)C-cyanation labeling of the selective mGluR5-antagonist [(11)C]AZD9272 and to perform the first in vivo characterization of [(11)C]AZD9272 binding in cynomolgus monkeys.

METHODS

[(11)C]AZD9272 was labeled using palladium mediated (11)C-cyanation. Altogether seven PET measurements were performed in three cynomolgus monkeys including baseline and co-injection experiments with unlabelled AZD9272 (0.04 and 0.4 mg/kg). Radiometabolites in plasma were measured using HPLC.

RESULTS

[(11)C]AZD9272 was prepared in over 50% incorporation yield from hydrogen [(11)C]cyanide in a total synthesis time of 45-50 min. The radiochemical purity of the radioligand in its final formulation was high (>99%) and the mean specific radioactivity was 47 GBq/ μmol (1278 Ci/mmol, n=7) calculated at end of bombardment (EOB). In the baseline measurements 10% of the total injected radioactivity was present in monkey brain at five minutes after i.v. injection. The radioactivity concentration was high in the caudate, cingulate gyrus and thalamus whereas it was moderate in the temporal cortex and lower for the cerebellum. After co-injection with cold AZD9272 the binding of [(11)C]AZD9272 was reduced in a dose-dependent fashion. Analysis of radiometabolites showed relatively slow metabolism and resulted only in hydrophilic radiometabolites.

CONCLUSION

A fast and efficient method was developed to label AZD9272 with (11)C. PET-examination in Cynomolgus monkeys showed that [(11)C]AZD9272 entered the brain to a high extent, that binding was saturable and that the regional radioactivity pattern was in accordance with the known distribution of mGluR5. The results support further examination of [(11)C]AZD9272 binding in human subjects.

Potent mGluR5 antagonists: pyridyl and thiazolyl-ethynyl-3,5-disubstituted-phenyl series

We report the synthesis of four series of 3,5-disubstituted-phenyl ligands targeting the metabotropic glutamate receptor subtype 5: (2-methylthiazol-4-yl)ethynyl (1a-j,), (6-methylpyridin-2-yl)ethynyl (2a-j), (5-methylpyridin-2-yl)ethynyl (3a-j,), and (pyridin-2-yl)ethynyl (4a-j,). The compounds were evaluated for antagonism of glutamate-mediated mobilization of internal calcium in an mGluR5 in vitro assay. All compounds were found to be full antagonists and exhibited low nanomolar to subnanomolar activity.

Antagonists at metabotropic glutamate receptor subtype 5: structure activity relationships and therapeutic potential for addiction

As a result of intensive investigation, particularly in the pharmaceutical industry, a number of potent and selective metabotropic glutamate receptor subtype 5 (mGluR5) antagonists have been discovered. The structure activity relationship studies that led to the discovery of these mGluR5 antagonists are presented in this review. Results from studies on selected mGluR5 antagonists in animal models that simulate drug reward, reinforcement, and relapse appear promising. The comorbidity between drug abuse and anxiety and depression make drugs active in these disorders of great interest. Clinical studies showed that the mGluR5 antagonist fenobam was an active anxiolytic drug. Several new mGluR5 antagonists produced anxiolytic and antidepressant-like effects in animal models of these disorders. The results from the clinical and animal studies provide information for new approaches to finding mechanistically distinct pharmacotherapies to help patients achieve and maintain abstinence from cocaine, methamphetamine, opiates, ethanol, and nicotine (smoking).

Combined antagonism of glutamate mGlu5 and adenosine A2A receptors interact to regulate alcohol-seeking in rats

Adenosine and glutamate have been implicated as mediators involved in the self-administration of alcohol. In the present study we sought to determine whether adenosine receptors could interact with metabotropic glutamate receptors to regulate operant responding for alcohol and also the integration of the salience of alcohol-paired cues. Alcohol-preferring (iP) rats were trained to self-administer alcohol under operant conditions. The availability of alcohol was paired with an olfactory cue plus a stimulus light. Rats were examined under fixed ratio responding and also following extinction under a cue-induced reinstatement paradigm. Administration of the selective adenosine A2A receptor antagonist, SCH 58261, reduced fixed ratio responding for alcohol in iP rats in a dose-related manner. Furthermore, the combination of a subthreshold dose of SCH 58261 with a subthreshold dose of the mGlu5 receptor antagonist MTEP also reduced alcohol self-administration and increased the latency to the first reinforced response, suggesting a pre-ingestive effect. Moreover, this combination of SCH 58261 and MTEP also prevented the conditioned reinstatement of alcohol-seeking elicited by the re-presentation of cues previously paired with alcohol availability. In contrast, combinations of the selective adenosine A1 receptor antagonist, DPCPX, with either SCH 58261 or MTEP had no effect on alcohol responding. Collectively, these data suggest a functional interaction between adenosine A2A and mGlu5 receptors in relation to alcohol-seeking and the integration of the drug-related cues.

Synthesis and Simple 18F-Labeling of 3-Fluoro-5-(2-(2-(fluoromethyl)thiazol-4-yl)ethynyl)benzonitrile as a High Affinity Radioligand for Imaging Monkey Brain Metabotropic Glutamate Subtype-5 Receptors with Positron Emission Tomography

2-fluoromethyl analogs of (3-[(2-methyl-1,3-thiazol-4yl)ethynyl]pyridine) were synthesized as potential ligands for metabotropic glutamate subtype-5 receptors (mGluR5s). One of these, namely, 3-fluoro-5-(2-(2-(fluoromethyl)thiazol-4-yl)ethynyl)benzonitrile (3), was found to have exceptionally high affinity (IC50 = 36 pM) and potency in a phosphoinositol hydrolysis assay (IC50 = 0.714 pM) for mGluR5. Compound 3 was labeled with fluorine-18 (t1/2 = 109.7 min) in high radiochemical yield (87%) by treatment of its synthesized bromomethyl analog (17) with [18F]fluoride ion and its radioligand behavior was assessed with positron emission tomography (PET). Following intravenous injection of [18F]3 into rhesus monkey, radioactivity was avidly taken up into brain with high uptake in mGluR5 receptor-rich regions such as striata. [18F]3 was stable in monkey plasma and human whole blood in vitro and in monkey and human brain homogenates. In monkey in vivo, a single polar radiometabolite of [18F]3 appeared rapidly in plasma. [18F]3 merits further evaluation as a PET radioligand for mGluR5 in human subjects.

Quantitative evaluation of 11C-ABP688 as PET ligand for the measurement of the metabotropic glutamate receptor subtype 5 using autoradiographic studies and a beta-scintillator

In this study we assessed the new glutamatergic ligand (11)C-ABP688 with regard to the following characteristics: (A) brain distribution, (B) first pass extraction fraction, (C) suitable model to describe tracer kinetics and (D) specificity for the mGlu5 receptor. These parameters were assessed using autoradiography and a beta-scintillator positioned in the striatum. The study included 13 male rats. In 2 animals cerebral blood flow was measured using H(2)(15)O. The (11)C-ABP688 data were analyzed using compartmental modeling. A two-tissue compartment model turned out to fit the data more adequately (parameters: K(1), k(2)('), k(3)('), k(4), total distribution volume DV(tot)=K(1)/k(2)(') (1+k(3)(')/k(4)) than a one-tissue compartment model. The autoradiographic studies revealed high uptake in hippocampus, striatum and cortex and low accumulation in thalamus and cerebellum. The uptake was markedly reduced following blockade with the mGlu5 antagonist M-MPEP. The first pass extraction fraction exceeded 85%. Baseline DV(tot) was 15.16+/-2.67 ml plasma/ml tissue and decreased by 56, 67 and 72% following blockade with 1, 2 and 6 mg/kg M-MPEP, respectively. These results show that (11)C-ABP688 is a promising PET ligand for the quantification of mGlu5 receptors in humans and animals. It readily crosses the blood-brain barrier and binds with high specificity to the mGlu5 receptor. The study furthermore demonstrates the usefulness of a beta-scintillator, if necessary in connection with autoradiography, to evaluate new receptor tracers.

mGluR5 metabotropic glutamate receptors and dyskinesias in MPTP monkeys

Modulation of excessive glutamatergic transmission within the basal ganglia is considered as an alternative approach to reduce l-Dopa-induced dyskinesias (LIDs) in Parkinson's disease (PD). In this study receptor binding autoradiography of [3H]MPEP, a metabotropic glutamate receptor 5 (mGluR5) selective radioligand, was used to investigate possible changes in mGluR5 in the basal ganglia of l-Dopa-treated 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) monkeys having developed LIDs compared to animals in which LIDs was prevented by adjunct treatments. LIDs were associated with an increase of mGluR5 specific binding in the posterior putamen and pallidum (+41% and +56%) compared to controls. By contrast, prevention of dyskinesias was associated with an important decrease of mGluR5 specific binding in these areas (-37% and -48%) compared with dyskinetic animals. Moreover, an upregulation (+34%) of mGluR5 receptor binding was seen in the anterior caudate nucleus of saline treated MPTP monkeys. This study is the first to provide evidence that enhanced mGluR5 specific binding in the posterior putamen and pallidum may contribute to the pathogenesis of LIDs in PD.

ABP688, a novel selective and high affinity ligand for the labeling of mGlu5 receptors: Identification, in vitro pharmacology, pharmacokinetic and biodistribution studies

[(11)C]ABP688 (2) has recently been demonstrated to be a useful PET tracer for in vivo imaging of the metabotropic glutamate receptors type 5 (mGluR5) in rodents. We describe here the identification and preclinical profiling of ABP688 and its tritiated version [(3)H]ABP688, and show that its high affinity (K(d)=2nM), selectivity, and pharmacokinetic properties fulfill all requirements for development as a PET tracer for clinical imaging of the mGlu5 receptor.

In vitro and in vivo evaluation of [11C]MPEPy as a potential PET ligand for mGlu5 receptors

Excessive activation via the metabotropic glutamate receptor subtype 5 (mGluR(5)) has been implicated in depression, neuropathic pain and other psychiatric, neurological and neurodegenerative diseases. A mGluR(5) radioligand for in vivo quantification by positron emission tomography (PET) would facilitate studies of the role of this receptor in disease and treatment. 3-Methoxy-5-pyridin-2-ylethynylpyridine (MPEPy), a selective and high-affinity antagonist at the mGluR(5) receptor was selected as a candidate ligand; a recent publication by Yu et al. [Nucl Med Biol 32 (2005) 631-640] presented initial micro-PET results for [(11)C]MPEPy with enthusiasm. Building on their efforts, we report as unique contributions (1) an improved chemical synthesis method, (2) the first data using human tissue, (3) phosphor images for rat brain preparations, (4) a novel comparison of anesthetic agents and (5) in vivo data in baboon. In vitro phosphor imaging studies of this ligand using human and rat brain tissue demonstrated high specific binding in the hippocampus, striatum and cortex with minimal specific binding in the cerebellum. In contrast, in vivo micro-PET studies in rats using urethane anesthesia, PET studies in baboons using isoflurane anesthesia and ex vivo micro-PET studies in unanesthetized rats each showed little specific binding in the brain. Despite the promising in vitro results, the low signal-to-noise ratio found in vivo does not justify the use of [(11)C]MPEPy as a PET radiotracer in humans.

Substituent Effects ofN-(1,3-Diphenyl-1H-pyrazol-5-yl)benzamides on Positive Allosteric Modulation of the Metabotropic Glutamate-5 Receptor in Rat Cortical Astrocytes

CDPPB [3-cyano-N-(1,3-diphenyl-1H-pyrazol-5-yl)benzamide] was recently described as the first centrally active, positive allosteric modulator of rat and human metabotropic glutamate receptor (mGluR) mGluR5 subtype. We explored the structural requirements for potentiation of glutamate-induced calcium release in naturally expressed mGluR5 in cultured rat astrocytes and increasing affinity for the allosteric antagonist binding site by evaluating 50 analogues of CDPPB. In the fluorometric calcium assay, CDPPB exhibited an EC50 value of 77 +/- 15 nM in potentiating mGluR5-mediated responses in cortical astrocytes and a Ki value of 3760 +/- 430 nM in displacing [3H]methoxyPEPy binding in membranes of cultured HEK-293 cells expressing rat mGluR5. The structure-activity relationships showed that electronegative aromatic substituents in the para-position of the benzamide moiety of CDPPB increase potency. Both binding and functional activities were further increased with a halogen atom in the ortho-position of the 1-phenyl ring. These effects of substitution do not match those of either aromatic ring of MPEP [2-methyl-6-(phenylethynyl)pyridine] for the antagonist allosteric binding site. Combination of the optimal substituents and aromatic positions resulted in 4-nitro-N-(1-(2-fluorophenyl)-3-phenyl-1H-pyrazol-5-yl)benzamide (VU-1545) showing Ki = 156 +/- 29 nM and EC50 = 9.6 +/- 1.9 nM in the binding and functional assays, respectively.

Screening cascade and development of potential Positron Emission Tomography radiotracers for mGluR5: in vitro and in vivo characterization

PURPOSE

Use of mGluR5 receptor radiotracers to determine whether an in vitro binding assay is able to predict how good a radiotracer is likely to be in imaging receptor in the central nervous system (CNS) via positron emission tomography (PET).

PROCEDURES

Saturation and equilibrium competition studies in rat and rhesus membranes were used to determine receptor concentrations and tracer affinities. In addition, specific binding of metabotropic receptor subtype 5 (mGluR5) radioligands in rhesus and rat brain sections was determined using a "no-wash protocol," and the in vivo binding signal in rats was determined using micro-PET.

RESULTS

Affinity values were determined for a series of mGluR5 antagonists (1-5) and ranged from 0.1 to 11 nM in rat. A previously reported "no-wash protocol" was then employed to determine specific binding in tissue sections following a 20-min incubation, and the regional distribution of these mGluR5 radiotracers determined in rat brain via autoradiography. The analogs 1b, 2b, 3b, and 4b, but not 5b, displayed good signal-to-noise ratios under these conditions with high density of binding in caudate, cortex, and hippocampus and lower density in cerebellum. With this information it was predicted that 1c, 2c, 3b, and 4b would display measurable signal-to-noise ratios in vivo, and that the larger in vitro signals for 3b and 4b would translate to 3b and 4b yielding the best in vivo signals. These predictions were investigated using micro-PET imaging in rat. Compound 1c showed a rapid wash-in and rapid wash-out profile in rat brain. Compound 2c showed similar signal-to-noise ratio as 1b, but slower washout. Compounds 3b and 4b showed the best signal-to-noise ratio in vivo, while 5b did not provide a significant signal, as predicted. In vivo occupancy estimates for 2-methyl-6-(phenylethynyl)-pyridine (MPEP) following intravenous administration were determined using radiolabeled compounds 1c, 2c, and 3b; they were essentially the same and were on the order of 1 mg kg(-1) (ID(50)).

CONCLUSIONS

An in vitro screen of several mGluR5 tracers was used to rapidly predict whether radiolabeled mGluR5 analogs would be useful as PET radiotracers. Results provided an extension to previously reported data. Two of the four radiotracers with the best in vitro "no-wash" results also showed the best potential as measured noninvasively using micro-PET.

Methoxyphenylethynyl, methoxypyridylethynyl and phenylethynyl derivatives of pyridine: synthesis, radiolabeling and evaluation of new PET ligands for metabotropic glutamate subtype 5 receptors

We have synthesized three different PET ligands to investigate the physiological function of metabotropic glutamate subtype 5 receptors (mGluR5) in vivo: 2-[(11)C]methyl-6-(2-phenylethynyl)pyridine ([(11)C]MPEP), 2-(2-(3-[(11)C]methoxyphenyl)ethynyl)pyridine ([(11)C]M-MPEP) and 2-(2-(5-[(11)C]methoxypyridin-3-yl)ethynyl)pyridine ([(11)C]M-PEPy). [(11)C]Methyl iodide was used to label the compounds under basic conditions, and a Pd(0) catalyst was applied to label [(11)C]MPEP in a Stille coupling reaction. In vivo microPET imaging studies of the functional accumulation of radiolabeled ligands were conducted in 35 rats (Sprague-Dawley, 8 weeks old male, weight of 300 g). Specific binding was tested using pre-administration of unlabeled mGluR5 antagonist 2-methyl-6-(2-phenylethynyl)pyridine (MPEP) (10 mg/kg iv 5 min before radioactivity injection). In the radiolabeling of [(11)C]MPEP, [(11)C]M-MPEP and [(11)C]M-PEPy, a specific radioactivity of 700-1200 mCi/micromol and over 97% radiochemical purity were obtained. The microPET studies showed these three radiolabeled mGluR5 antagonists having the highest binding in the olfactory bulb followed by striatum, hippocampus and cortex. Pre-administration of the mGluR5 antagonist MPEP induced a 45.1% decrease in [(11)C]MPEP binding, a 59.7% decrease in [(11)C]M-MPEP binding and an 84.6% decrease in [(11)C]M-PEPy binding in the olfactory bulb at 5 min. The feasibility of synthesizing high-affinity and high-selectivity ligands for mGluR5 receptors and their suitability as PET imaging ligands for mGluR5 receptors in vivo are demonstrated.

Role of central and peripheral mGluR5 receptors in post-operative pain in rats

Metabotropic glutamate receptors (mGluRs) have previously been shown to play a role in pain transmission during inflammatory or neuropathic pain states. However, the role of mGluR5 in post-operative pain remains to be fully investigated. The present study was conducted to characterize analgesic activity of 2-methyl-6-(phenylethynyl)-pyridine (MPEP) in the skin-incision-induced post-operative pain model in rats. MPEP is a potent and selective mGluR5 antagonist with high affinity (K(i)=6.3+/-0.9 nM) in rat cortex using [(3)H]-MPEP as a radioligand, while not competing with the mGluR1-selective radioligand [(3)H]-R214127 (K(i)>10,000 nM) in rat cerebellum. Post-operative pain was examined 2 h following surgery using weight-bearing (WB) difference between injured and uninjured paws as a measure of non-evoked pain. In this model, MPEP, as morphine, showed dose-dependent effects and full efficacy after systemic administration (ED(50)=15 mg/kg, i.p. for MPEP, ED(50)=1.3 mg/kg, s.c. for morphine). In addition, intrathecal (i.t.) and intracerebroventricular (i.c.v.) MPEP reduced WB difference (ED(50)=65 microg/rat i.t. and ED(50)=200 microg/rat i.c.v.). Interestingly, intraplantar (i.pl.) injection of MPEP either before or after surgery induced a similar reduction in WB difference (ED(50)=90 microg/rat, i.pl.) while contralateral i.pl. MPEP injection did not produce any effect. These results demonstrate that both peripheral and central mGluR5 receptors play a role in nociceptive transmission observed during post-operative pain. In addition, the data suggest that mGluR5 antagonists could offer a new therapeutic approach to the treatment of post-operative pain.

Synthesis, characterization, and first successful monkey imaging studies of metabotropic glutamate receptor subtype 5 (mGluR5) PET radiotracers

Three metabotropic glutamate receptor subtype 5 (mGluR5) PET tracers have been labeled with either carbon-11 or fluorine-18 and their in vitro and in vivo behavior in rhesus monkey has been characterized. Each of these tracers share the common features of high affinity for mGluR5 (0.08-0.23 nM vs. rat mGluR5) and moderate lipophilicity (log P 2.8-3.4). Compound 1b was synthesized using a Suzuki or Stille coupling reaction with [11C]MeI. Compounds 2b and 3b were synthesized by a SNAr reaction using a 3-chlorobenzonitrile precursor. Autoradiographic studies in rhesus monkey brain slices using 2b and 3b showed specific binding in cortex, caudate, putamen, amygdala, hippocampus, most thalamic nuclei, and lower binding in the cerebellum. PET imaging studies in monkey showed that all three tracers readily enter the brain and provide an mGluR5-specific signal in all gray matter regions, including the cerebellum. The specific signal observed in the cerebellum was confirmed by the autoradiographic studies and saturation binding experiments that showed tracer binding in the cerebellum of rhesus monkeys. In vitro metabolism studies using the unlabeled compounds showed that 1a, 2a, and 3a are metabolized slower by human liver microsomes than by monkey liver microsomes. In vivo metabolism studies showed 3b to be long-lived in rhesus plasma with only one other more polar metabolite observed.

Metabotropic glutamate receptor activation regulates fragile x mental retardation protein and FMR1 mRNA localization differentially in dendrites and at synapses

Fragile X syndrome is caused by the absence of the mRNA-binding protein Fragile X mental retardation protein (FMRP), which may play a role in activity-regulated localization and translation of mRNA in dendrites and at synapses. We investigated whether neuronal activity and glutamatergic signals regulate trafficking of FMRP and its encoding Fmr1 mRNA into dendrites or at synapses. Using high-resolution fluorescence and digital imaging microscopy in cultured hippocampal neurons, FMRP and Fmr1 mRNA were localized in granules throughout dendrites and within spines. KCl depolarization rapidly increased FMRP and Fmr1 mRNA levels in dendrites. Metabotropic glutamate receptor (mGluR) activation, in particular mGluR5 activation, was necessary for localization of FMRP into dendrites. Blockade of either PKC or internal calcium prevented mGluR-dependent localization of both FMRP and Fmr1 mRNA in dendrites. The activity-dependent localization of FMRP was not dependent on protein synthesis. Fluorescence recovery after photobleaching analysis of live neurons transfected with enhanced green fluorescent protein-FMRP revealed increased granule trafficking in response to KCl depolarization. In contrast to its dendritic localization, mGluR activation diminished FMRP, but not Fmr1 mRNA, localization at synapses. These results demonstrate regulation of FMRP and Fmr1 mRNA trafficking in dendrites and synapses in response to specific glutamatergic signals.

In vivo receptor occupancy of mGlu5 receptor antagonists using the novel radioligand [3H]3-methoxy-5-(pyridin-2-ylethynyl)pyridine)

In vivo receptor occupancy of mGlu5 receptor antagonists was quantified in rat and mouse brain using the mGlu5 receptor selective antagonist [3H]3-methoxy-5-(pyridin-2-ylethynyl)pyridine) ([3H]methoxy-PEPy). Administration of [3H]methoxy-PEPy (50 microCi/kg i.v.) to mGlu5 receptor-deficient mice revealed binding at background levels in forebrain, whereas wild-type mice exhibited 14-fold higher binding in forebrain relative to cerebellum. Systemic administration of the mGlu5 receptor antagonists 2-methyl-6-(phenylethynyl)pyridine (MPEP) and 3-[(2-methyl-1,3-thiazol-4-yl)ethynyl]pyridine (MTEP) reduced the binding of [3H]methoxy-PEPy in rats and mice, reflecting mGlu5 receptor occupancy by these compounds. MPEP (10 mg/kg i.p.) and MTEP (3 mg/kg i.p.) maintained >75% receptor occupancy for 2 h in rats, while in mice MPEP and MTEP achieved >75% occupancy for only 30 and 15 min, respectively. Compound levels in plasma were substantially lower in mice suggesting species differences in receptor occupancy result from differences in absorption or metabolism of the compounds. These findings demonstrate that [3H]methoxy-PEPy is useful for determining the occupancy of mGlu5 receptors in the brain.

The role of metabotropic glutamate receptors in regulation of striatal proenkephalin expression: implications for the therapy of Parkinson′s disease

Overactivity of the striatopallidal pathway, associated with an enhancement of enkephalin expression, has been suggested to contribute to the development of parkinsonian symptoms. The aim of the present study was to examine whether the blockade of group I metabotropic glutamate receptors: subtypes 1 and 5 (mGluR1/5), or stimulation of group II: subtypes 2 and 3 (mGluR2/3) may normalize enkephalin expression in the striatopallidal pathway in an animal model of parkinsonism. The proenkephalin mRNA level measured by in situ hybridization in the striatum was increased by pretreatments with haloperidol (1.5 mg/kg s.c., three times, 3 h apart). Triple (3 h apart), bilateral, intrastriatal administration of selective antagonists of mGluR1: (S)-(+)-alpha-amino-4-carboxy-2-methylbenzeneacetic acid (3 x 5 microg/0.5 microl) or 7-(hydroxyimino)cyclopropa[b]chromen-1a-carboxylate (3 x 2.5 microg/0.5 microl), reversed the haloperidol-induced increases in proenkephalin mRNA levels in the rostral and central regions of the striatum. Similarly, repeated (6 times, 1.5 h apart), systemic injections of an antagonist of mGluR5, 2-methyl-6-(phenylethynyl)pyridine (6 x 10 mg/kg i.p.) counteracted an increase in the striatal proenkephalin mRNA expression elicited by haloperidol. None of the abovementioned antagonists of mGluR1 and mGluR5 per se influenced the proenkephalin expression. Differential effects were induced by agonists of the group II mGluRs, viz. (2S,2'R,3'R)-2-(2',3'-dicarboxycyclopropyl)glycine administered intraventricularly (3 times at 0.1-0.2 microg/4 microl, 3 h apart) increased both the normal and haloperidol-increased proenkephalin mRNA level, whereas (2R,4R)-4-aminopyrrolidine-2,4-dicarboxylate injected intrastriatally (3 times at 15 microg/0.5 microl, 3 h apart) was ineffective. The present study indicates that the blockade of striatal glutamate receptors belonging to the group I (mGluR1 and mGluR5) but not stimulation of the group II mGluRs may normalize the function of the striatopallidal pathway in an animal model of parkinsonism, which may be important for future antiparkinsonian therapy in humans.