The striopallidal pathway is involved in antiparkinsonian-like effects of the blockade of group I metabotropic glutamate receptors in rats

Haloperidol-induced catalepsy as an animal model for parkinsonism: A systematic review of experimental studies

Several useful animal models for parkinsonism have been developed so far. Haloperidol-induced catalepsy is often used as a rodent model for the study of motor impairments observed in Parkinson's disease and related disorders and for the screening of potential antiparkinsonian compounds. The objective of this systematic review is to identify publications that used the haloperidol-induced catalepsy model for parkinsonism and to explore the methodological characteristics and the main questions addressed in these studies. A careful systematic search of the literature was carried out by accessing articles in three different databases: Web of Science, PubMed and SCOPUS. The selection and inclusion of studies were performed based on the abstract and, subsequently, on full-text analysis. Data extraction included the objective of the study, study design and outcome of interest. Two hundred and fifty-five articles were included in the review. Publication years ranged from 1981 to 2020. Most studies used the model to explore the effects of potential treatments for parkinsonism. Although the methodological characteristics used are quite varied, most studies used Wistar rats as experimental subjects. The most frequent dose of haloperidol used was 1.0 mg/kg, and the horizontal bar test was the most used to assess catalepsy. The data presented here provide a framework for an evidence-based approach to the design of preclinical research on parkinsonism using the haloperidol-induced catalepsy model. This model has been used routinely and successfully and is likely to continue to play a critical role in the ongoing search for the next generation of therapeutic interventions for parkinsonism.

Advances in CNS PET: the state-of-the-art for new imaging targets for pathophysiology and drug development

PURPOSE

A limit on developing new treatments for a number of central nervous system (CNS) disorders has been the inadequate understanding of the in vivo pathophysiology underlying neurological and psychiatric disorders and the lack of in vivo tools to determine brain penetrance, target engagement, and relevant molecular activity of novel drugs. Molecular neuroimaging provides the tools to address this. This article aims to provide a state-of-the-art review of new PET tracers for CNS targets, focusing on developments in the last 5 years for targets recently available for in-human imaging.

METHODS

We provide an overview of the criteria used to evaluate PET tracers. We then used the National Institute of Mental Health Research Priorities list to identify the key CNS targets. We conducted a PubMed search (search period 1st of January 2013 to 31st of December 2018), which yielded 40 new PET tracers across 16 CNS targets which met our selectivity criteria. For each tracer, we summarised the evidence of its properties and potential for use in studies of CNS pathophysiology and drug evaluation, including its target selectivity and affinity, inter and intra-subject variability, and pharmacokinetic parameters. We also consider its potential limitations and missing characterisation data, but not specific applications in drug development. Where multiple tracers were present for a target, we provide a comparison of their properties.

RESULTS AND CONCLUSIONS

Our review shows that multiple new tracers have been developed for proteinopathy targets, particularly tau, as well as the purinoceptor P2X7, phosphodiesterase enzyme PDE10A, and synaptic vesicle glycoprotein 2A (SV2A), amongst others. Some of the most promising of these include 18F-MK-6240 for tau imaging, 11C-UCB-J for imaging SV2A, 11C-CURB and 11C-MK-3168 for characterisation of fatty acid amide hydrolase, 18F-FIMX for metabotropic glutamate receptor 1, and 18F-MNI-444 for imaging adenosine 2A. Our review also identifies recurrent issues within the field. Many of the tracers discussed lack in vivo blocking data, reducing confidence in selectivity. Additionally, late-stage identification of substantial off-target sites for multiple tracers highlights incomplete pre-clinical characterisation prior to translation, as well as human disease state studies carried out without confirmation of test-retest reproducibility.

Preclinical anaylses of [18F]cEFQ as a PET tracer for imaging metabotropic glutamate receptor type 1 (mGluR1)

Metabotropic glutamate receptor type 1 (mGluR1) is related with various neurological and psychiatric diseases, such as anxiety, depression, epilepsy, Parkinson's disease, and neuropathic pain. Hence, mGluR1 is an important target for drug development and imaging. We synthesized [¹⁸F]cEFQ (3-ethyl-2-[¹⁸F]fluoroquinolin-6-yl cis-(4-methoxycyclohexyl)methanone) as a PET tracer for selective mGluR1 imaging and evaluated its properties in rodents. A chloroquinoline precursor was labeled by a nucleophilic substitution reaction, and the resulting [¹⁸F]cEFQ was obtained with high radiochemical purity (>99%) and specific activity (63-246 GBq/µmol). The log D value was 3.24, and the initial brain uptake at 10 min was over 4% of injected dose per gram in BALB/c mice. According to PET/CT and autoradiography in SD rats, [¹⁸F]cEFQ showed wide distribution in the whole brain and the highest uptake in the cerebellum. Pre-treatment with unlabeled cEFQ or the mGluR1-specific antagonist JNJ16259685 blocked the uptake of [¹⁸F]cEFQ. However, the uptake was not blocked by pre-treatment with the mGluR5-specific antagonist ABP688. The trans isomer [¹⁸F]tEFQ did not show high uptake in the mGluR1-rich region. [¹⁸F]cEFQ was straightforwardly prepared using a chloro-derivative precursor. Its feasibility as a specific and selective PET agent for imaging mGluR1 was proved by in vitro and in vivo experiments using rodents.

Therapeutic potential of metabotropic glutamate receptor modulators

Glutamate is the main excitatory neurotransmitter in the central nervous system (CNS) and is a major player in complex brain functions. Glutamatergic transmission is primarily mediated by ionotropic glutamate receptors, which include NMDA, AMPA and kainate receptors. However, glutamate exerts modulatory actions through a family of metabotropic G-protein-coupled glutamate receptors (mGluRs). Dysfunctions of glutamatergic neurotransmission have been implicated in the etiology of several diseases. Therefore, pharmacological modulation of ionotropic glutamate receptors has been widely investigated as a potential therapeutic strategy for the treatment of several disorders associated with glutamatergic dysfunction. However, blockade of ionotropic glutamate receptors might be accompanied by severe side effects due to their vital role in many important physiological functions. A different strategy aimed at pharmacologically interfering with mGluR function has recently gained interest. Many subtype selective agonists and antagonists have been identified and widely used in preclinical studies as an attempt to elucidate the role of specific mGluRs subtypes in glutamatergic transmission. These studies have allowed linkage between specific subtypes and various physiological functions and more importantly to pathological states. This article reviews the currently available knowledge regarding the therapeutic potential of targeting mGluRs in the treatment of several CNS disorders, including schizophrenia, addiction, major depressive disorder and anxiety, Fragile X Syndrome, Parkinson’s disease, Alzheimer’s disease and pain.

Study of the neural basis of striatal modulation of the jaw-opening reflex

Previous experimental data from this laboratory demonstrated the participation of the striatum and dopaminergic pathways in central nociceptive processing. The objective of this study was to examine the possible pathways and neural structures associated with the analgesic action of the striatum. The experiments were carried out in rats anesthetized with urethane. The jaw-opening reflex (JOR) was evoked by electrical stimulation of the tooth pulp of lower incisors and recorded in the anterior belly of the digastric muscles. Intrastriatal microinjection of apomorphine, a nonspecific dopamine agonist, reduced or abolished the JOR amplitude. Electrolytic or kainic acid lesions, unilateral to the apomorphine-injected striatum, of the globus pallidus, substantia nigra pars reticulata, subthalamic nucleus and bilateral lesion the rostroventromedial medulla (RVM), blocked the inhibition of the JOR by striatal stimulation. These findings suggest that the main output nuclei of the striatum and the RVM may be critical elements in the neural pathways mediating the inhibition of the reflex response, evoked in jaw muscles by noxious stimulation of dental pulp.

Effect of the metabotropic glutamate antagonist MPEP on striatal expression of the Homer family proteins in levodopa-treated hemiparkinsonian rats

RATIONALE

Striatal glutamatergic hyperactivity through the metabotropic receptors and their intracellular signaling pathways is considered critical in the development of levodopa-induced dyskinesias in Parkinson's disease and in experimental parkinsonism.

OBJECTIVE

We investigated whether the administration of the metabotropic glutamate antagonist, MPEP, modifies striatal expression of Homer family proteins which are involved in the intracellular mechanisms mediated by these receptors.

MATERIALS AND METHODS

Sprague-Dawley rats were unilaterally lesioned in the nigrostriatal pathway with 6-hydroxydopamine (8 microg) and treated with: levodopa (12 mg/kg, i.p.) plus vehicle (n=10) divided in two daily injections; levodopa plus MPEP (1.5 and 3 mg/kg, i.p.; n=6-13) divided in two daily injections; or saline (n=7) for 10 consecutive days. Axial, limb, and orolingual dyskinesias were evaluated. Striatal expression of tyrosine hydroxylase (TH), Homer 1a, 1b/c, and deltaFosB were measured by Western Blot.

RESULTS

Animals treated with levodopa showed an increase of dyskinesia score (p<0.01) that was attenuated by the administration of MPEP (p<0.01). In the ipsilateral side of the lesion, striatal TH expression was decreased (p<0.01). No significant differences in striatal Homer 1a or b/c expression were observed between the groups of treatment. Striatal deltaFosB expression increased in the animals treated with levodopa (p<0.05) being attenuated after MPEP administration (p<0.05). MPEP effect was not paralleled by any modification of striatal Homer proteins expression.

CONCLUSIONS

These results suggest that Homer protein family is not causally involved in the development of dyskinetic movements induced by levodopa treatment in this animal model of parkinsonism.

Metabotropic glutamate 1 receptor: current concepts and perspectives

Almost 25 years after the first report that glutamate can activate receptors coupled to heterotrimeric G-proteins, tremendous progress has been made in the field of metabotropic glutamate receptors. Now, eight members of this family of glutamate receptors, encoded by eight different genes that share distinctive structural features have been identified. The first cloned receptor, the metabotropic glutamate (mGlu) receptor mGlu1 has probably been the most extensively studied mGlu receptor, and in many respects it represents a prototypical subtype for this family of receptors. Its biochemical, anatomical, physiological, and pharmacological characteristics have been intensely investigated. Together with subtype 5, mGlu1 receptors constitute a subgroup of receptors that couple to phospholipase C and mobilize Ca(2+) from intracellular stores. Several alternatively spliced variants of mGlu1 receptors, which differ primarily in the length of their C-terminal domain and anatomical localization, have been reported. Use of a number of genetic approaches and the recent development of selective antagonists have provided a means for clarifying the role played by this receptor in a number of neuronal systems. In this article we discuss recent advancements in the pharmacology and concepts about the intracellular transduction and pathophysiological role of mGlu1 receptors and review earlier data in view of these novel findings. The impact that this new and better understanding of the specific role of these receptors may have on novel treatment strategies for a variety of neurological and psychiatric disorders is considered.

An influence of ligands of metabotropic glutamate receptor subtypes on parkinsonian-like symptoms and the striatopallidal pathway in rats

Several data indicate that inhibition of glutamatergic transmission may be important to alleviate of parkinsonian symptoms. Therefore, the aim of the present paper is to review recent studies on the search for putative antiparkinsonian-like effects of mGluR ligands and their brain targets. In order to inhibit glutamatergic transmission, the group I mGluRs (mGluR1 and mGluR5) were blocked, and group II (mGluR2/3) or III (mGluR4/7/8) mGluRs were activated. Systemic or intrastriatal administration of group I mGluR antagonists (mGluR5 - MPEP, MTEP; mGluR1 - AIDA) was found to inhibit parkinsonian-like symptoms (catalepsy, muscle rigidity) in rats. MPEP administered systemically and mGluR1 antagonists (AIDA, CPCCOEt, LY367385) injected intrastriatally reversed also the haloperidol-increased proenkephalin (PENK) mRNA expression in the striatopallidal pathway. Similarly, ACPT-1, a group III mGluR agonist, administered into the striatum, globus pallidus or substantia nigra inhibited the catalepsy. Intrastriatal injection of this compound reduced the striatal PENK expression induced by haloperidol. In contrast, a group II mGluR agonist (2R,4R-APDC) administered intrastriatally reduced neither PENK expression nor the above-mentioned parkinsonian-like symptoms. Moreover, a mixed mGluR8 agonist/AMPA antagonist, (R,S)-3,4-DCPG, administered systemically evoked catalepsy and enhanced both the catalepsy and PENK expression induced by haloperidol. The results reviewed in this article seem to indicate that group I mGluR antagonists or some agonists of group III may possess antiparkinsonian properties, and point at the striatopallidal pathway as a potential target of therapeutic intervention.

Effects of group I metabotropic glutamate receptors blockade in experimental models of Parkinson's disease

The present study was devoted to investigate the effects of the metabotropic glutamate receptor(mGluR)5 antagonist [(2-methyl-1,3-thiazol-4-yl)ethynyl]pyridine (MTEP) and the mGluR1 antagonist, (3-ethyl-2-methyl-quinolin-6-yl)-(4-methoxy-cyclohexyl)-methanone methanesulfonate (EMQMCM), in animal studies indicative of antiparkinsonian-like activity such as haloperidol-induced catalepsy, hypoactivity in open field following haloperidol, and rotation in rats with unilateral 6-hydroxydopamine(OHDA)-induced lesions of the midbrain dopaminergic system (alone and in combination with L-DOPA). Moreover, antidyskinetic activity of different mGluR ligands was evaluated in the rat model of L-DOPA-induced dyskinesia. Both MTEP (5 mg/kg) and EMQMCM (4 mg/kg) slightly inhibited haloperidol (0.5 mg/kg)-induced catalepsy. However, neither substance reversed the hypoactivity produced by haloperidol (0.2 mg/kg). Although MTEP did not produce significant turning, it inhibited contralateral rotations after L-DOPA (at 5 mg/kg) and alleviated L-DOPA-induced dyskinesia (at 2.5 and 5 mg/kg) in 6-OHDA-lesioned rats. In contrast, mGluR1 antagonists EMQMCM and RS-1-aminoindan-1,5-dicarboxylic acid (AIDA) failed to modify L-DOPA-induced dyskinesia. The results of the present study suggest that either subtype of group I of mGluRs may be involved in the pathologically altered circuitry in the basal ganglia. However, the equivocal results do not strongly support the hypothesis that mGluR1 and mGluR5 antagonists may be beneficial in the symptomatic treatment of Parkinson's disease. However, mGluR5 antagonists may prove useful for the symptomatic treatment of L-DOPA-induced dyskinesia.

Endogenous opiates and behavior: 2003

This paper is the 26th consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning over a quarter-century of research. It summarizes papers published during 2003 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).

Abnormal metabotropic glutamate receptor expression and signaling in the cerebral cortex in diffuse Lewy body disease is associated with irregular alpha-synuclein/phospholipase C (PLCbeta1) interactions

Diffuse Lewy body disease (DLBD) is a degenerative disease of the nervous system, involving the brain stem, diencephalic nuclei and cerebral cortex, associated with abnormal a-synuclein aggregation and widespread formation of Lewy bodies and Lewy neurites. DLBD presents as pure forms (DLBDp) or in association with Alzheimer disease (AD) in the common forms (DLBDc). Several neurotransmitter abnormalities have been reported including those of the nigrostriatal and mesocorticolimbic dopaminergic system, and central noradrenergic, serotoninergic and cholinergic pathways. The present work examines metabotropic glutamate receptor (mGluR) expression and signaling in the frontal cortex of DLBDp and DLBDc cases in comparison with age-matched controls. Abnormal L-[3H]glutamate specific binding to group I and II mGluRs, and abnormal mGluR1 levels have been found in DLBD. This is associated with reduced expression levels of phospholipase C beta1 (PLCbeta1), the effector of group I mGluRs following protein G activation upon glutamate binding. Additional modification in the solubility of PLCbeta1 and reduced PLCbeta1 activity in pure and common DLBD further demonstrates for the first time abnormal mGluR signaling in the cerebral cortex in DLBD. In order to look for a possible link between abnormal mGluR signaling and a-synuclein accumulation in DLBD, immunoprecipitation studies have shown alpha-synuclein/PLCbeta1 binding in controls and decreased alpha-synuclein/PLCbeta1 binding in DLBD. This is accompanied by a shift in the distribution of a-synuclein, but not of PLCbeta1, in DLBD when compared with controls. Together, these results support the concept that abnormal a-synuclein in DLBD produces functional effects on cortical glutamatergic synapses, which are associated with reduced alpha-synuclein/PLCbeta1 interactions, and, therefore, that mGluRs are putative pharmacological targets in DLBD. Finally, these results emphasize the emergence of a functional neuropathology that has to be explored for a better understanding of the effects of abnormal protein interactions in degenerative diseases of the nervous system.

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.