P2X7 receptor-mediated release of microglial prostanoids and miRNAs correlates with reversal of neuropathic hypersensitivity in rats.

BACKGROUND: P2X7 receptor antagonists have potential for treating various central nervous system (CNS) diseases, including neuropathic pain, although none have been approved for clinical use. Reasons may include insufficient understanding of P2X7 receptor signalling in pain, and the lack of a corresponding preclinical mechanistic biomarker. METHODS: Lu AF27139 is a highly selective and potent small molecule antagonist at rat, mouse and human forms of the P2X7 receptor, with excellent pharmacokinetic and CNS permeability properties. In the current experiments, we probed the utility of previously characterized and novel signalling cascades exposed to Lu AF27139 using cultured microglia combined with release assays. Subsequently, we assessed the biomarker potential of identified candidate molecules in the rat chronic constriction injury (CCI) model of neuropathic pain; study design limitations precluded their assessment in spared nerve injury (SNI) rats. RESULTS: Lu AF27139 blocked several pain-relevant pathways downstream of P2X7 receptors in vitro. At brain and spinal cord receptor occupancy levels capable of functionally blocking P2X7 receptors, it diminished neuropathic hypersensitivity in SNI rats, and less potently in CCI rats. Although tissue levels of numerous molecules previously linked to neuropathic pain and P2X7 receptor function (e.g. IL-6, IL-1ß, cathepsin-S, 2-AG) were unaffected by CCI, Lu AF27139-mediated regulation of spinal PGE2 and miRNA (e.g. rno-miR-93-5p) levels increased by CCI aligned with its ability to diminish neuropathic hypersensitivity. CONCLUSIONS: We have identified a pain-relevant P2X7 receptor-regulated mechanism in neuropathic rats, which could hold promise as a translatable biomarker and by association enhance the clinical progression of P2X7 receptor antagonists in neuropathic pain. SIGNIFICANCE: Sub-optimal translation of preclinical molecules has hindered the clinical development of novel mechanism of action analgesics. We have undertaken a comprehensive in vitro analysis of migroglial signalling mechanisms recruited upon P2X7 receptor activation, a number of which were shown to be modulated by a selective P2X7 receptor antagonist in a well characterized animal model of neuropathic pain. Subject to further confirmation in other neuropathic models, this opens up the possibility to investigate their clinical utility as potential pain biomarkers in patients.

Sodium channel cleavage is associated with aberrant neuronal activity and cognitive deficits in a mouse model of Alzheimer's disease.

BACE1 is the rate-limiting enzyme that cleaves amyloid precursor protein (APP) to produce the amyloid ß peptides that accumulate in Alzheimer's disease (AD). BACE1, which is elevated in AD patients and APP transgenic mice, also cleaves the ß2-subunit of voltage-gated sodium channels (Navß2). Although increased BACE1 levels are associated with Navß2 cleavage in AD patients, whether Navß2 cleavage occurs in APP mice had not yet been examined. Such a finding would be of interest because of its potential impact on neuronal activity: previous studies demonstrated that BACE1-overexpressing mice exhibit excessive cleavage of Navß2 and reduced sodium current density, but the phenotype associated with loss of function mutations in either Navß-subunits or pore-forming α-subunits is epilepsy. Because mounting evidence suggests that epileptiform activity may play an important role in the development of AD-related cognitive deficits, we examined whether enhanced cleavage of Navß2 occurs in APP transgenic mice, and whether it is associated with aberrant neuronal activity and cognitive deficits. We found increased levels of BACE1 expression and Navß2 cleavage fragments in cortical lysates from APP transgenic mice, as well as associated alterations in Nav1.1α expression and localization. Both pyramidal neurons and inhibitory interneurons exhibited evidence of increased Navß2 cleavage. Moreover, the magnitude of alterations in sodium channel subunits was associated with aberrant EEG activity and impairments in the Morris water maze. Together, these results suggest that altered processing of voltage-gated sodium channels may contribute to aberrant neuronal activity and cognitive deficits in AD.

4-aryl piperazine and piperidine amides as novel mGluR5 positive allosteric modulators.

Positive allosteric modulation of metabotropic glutamate receptor 5 (mGluR5) is regarded as a potential novel treatment for schizophrenic patients. Herein we report the synthesis and SAR of 4-aryl piperazine and piperidine amides as potent mGluR5 positive allosteric modulators (PAMs). Several analogs have excellent activity and desired drug-like properties. Compound 2b was further characterized as a PAM using several in vitro experiments, and produced robust activity in several preclinical animal models.

Comparative study of five antiepileptic drugs on a translational cognitive measure in the rat: relationship to antiepileptic property.

RATIONALE: Antiepileptic drugs (AEDs) have been available for many years; yet, new members of this class continue to be identified and developed due to the limitations of existing drugs, which include a propensity for cognitive impairment. However, there is little preclinical information about the cognitive effects they produce, which clinically include deficits in attention and slowing of reaction time. OBJECTIVES: The purpose of this study was to profile two first-generation AEDs, phenytoin and valproate, and three second-generation AEDs, levetiracetam, pregabalin and lacosamide. Initially, each drug was examined across a range of well characterised preclinical seizure tests, and then each drug was evaluated in the five-choice serial reaction time test (5-CSRTT) based on efficacious doses from the seizure tests. MATERIALS AND METHODS: Each AED was tested for anti-seizure efficacy in either (1) the maximal electroshock seizure test, (2) s.c. PTZ seizure test, (3) amygdala-kindled seizures and (4) the genetic absence epilepsy rat of Strasbourg model of absence seizures. On completion of these studies, each drug was tested in rats trained to asymptotic performance in the 5-CSRTT (0.5 s SD, 5 s ITI, 100 trials). Male rats were used in all studies. RESULTS: Each AED was active in at least one of the seizure tests, although only valproate was active in each test. In the 5-CSRT test, all drugs with the exception of levetiracetam, significantly slowed reaction time and increased omissions. Variable effects were seen on accuracy. The effect on omissions was reversed by increasing stimulus duration from 0.5 to 5 s, supporting a drug-induced attention deficit. Levetiracetam had no negative effect on performance; indeed, reaction time was slightly increased (i.e. faster). CONCLUSIONS: These results highlight somewhat similar effects of phenytoin, valproate, pregabalin and lacosamide on attention and reaction time, and comparison to efficacious doses from the seizure tests support the view that there may be a better separation with the newer AEDs. Levetiracetam had no detrimental effect in the 5-CSRTT, which may be consistent with clinical experience where the drug is considered to be well tolerated amongst the AED class.

The anti-epileptic drug lacosamide (Vimpat) has anxiolytic property in rodents.

Lacosamide ((R)-2-acetamido-N-benzyl-3-methoxypropionamide; formerly harkoseride, SPM 927; Vimpat), has been recently approved by US and European regulatory authorities for use as add-on therapy for partial-onset seizures in adults. Because a number of anti-epileptic drugs are used to treat conditions beyond epilepsy, including anxiety, in the present study we investigated the anxiolytic potential of lacosamide in a conditioned emotional response (CER) model in rat, and the mouse marble burying assay. In each test lacosamide produced a significant effect consistent with anxiolysis, i.e. lacosamide increased suppression ratio in the CER test, and reduced the number of marbles buried in the marble burying assay. The doses necessary for an anxiolytic effect were higher than those necessary for efficacy in seizure tests conducted in the same species. For example in the mouse, the lacosamide oral ED(50) in the maximal electroshock seizure (MES) and 6 Hz tests was 5.3 and 9.6 mg/kg respectively, and the minimal effective dose in the marble burying assay was 30 mg/kg. In both seizure and anxiety tests, the (S)-enantiomer of lacosamide was inactive suggesting a similar mechanism of action, possibly use-dependent inhibition of sodium channel function (Errington et al., 2008). Efficacy in the CER model was equivalent to diazepam and pregabalin (Lyrica). In tests of side-effects, lacosamide had no effect on choice accuracy in the delayed match to position task of working memory, although at the 30 mg/kg dose, response rates and response latencies were significantly affected. In sum, the present results identify for the first time, an anxiolytic potential of lacosamide.

Optimization of continuous in vivo DOPA production and studies on ectopic DA synthesis using rAAV5 vectors in Parkinsonian rats.

Viral vector-mediated gene transfer is emerging as a novel therapeutic approach with clinical utility in treatment of Parkinson's disease. Recombinant adeno-associated viral (rAAV) vector in particular has been utilized for continuous l-3,4 dihydroxyphenylalanine (DOPA) delivery by expressing the tyrosine hydroxylase (TH) and GTP cyclohydrolase 1 (GCH1) genes which are necessary and sufficient for efficient synthesis of DOPA from dietary tyrosine. The present study was designed to determine the optimal stoichiometric relationship between TH and GCH1 genes for ectopic DOPA production and the cellular machinery involved in its synthesis, storage, and metabolism. For this purpose, we injected a fixed amount of rAAV5-TH vector and increasing amounts of rAAV5-GCH1 into the striatum of rats with complete unilateral dopamine lesion. After 7 weeks the animals were killed for either biochemical or histological analysis. We show that increasing the availability of 5,6,7,8-tetrahydro-l-biopterin (BH4) in the same cellular compartment as the TH enzyme resulted in better efficiency in DOPA synthesis, most likely by hindering inactivation of the enzyme and increasing its stability. Importantly, the BH4 synthesis from ectopic GCH1 expression was saturable, yielding optimal TH enzyme functionality between GCH1 : TH ratios of 1 : 3 and 1 : 7.

Positron emission tomography imaging demonstrates correlation between behavioral recovery and correction of dopamine neurotransmission after gene therapy.

In vivo gene transfer using viral vectors is an emerging therapy for neurodegenerative diseases with a clinical impact recently demonstrated in Parkinson's disease patients. Recombinant adeno-associated viral (rAAV) vectors, in particular, provide an excellent tool for long-term expression of therapeutic genes in the brain. Here we used the [(11)C]raclopride [(S)-(-)-3,5-dichloro-N-((1-ethyl-2-pyrrolidinyl)methyl)-2-hydroxy-6-methoxybenzamide] micro-positron emission tomography (PET) technique to demonstrate that delivery of the tyrosine hydroxylase (TH) and GTP cyclohydrolase 1 (GCH1) enzymes using an rAAV5 vector normalizes the increased [(11)C]raclopride binding in hemiparkinsonian rats. Importantly, we show in vivo by microPET imaging and postmortem by classical binding assays performed in the very same animals that the changes in [(11)C]raclopride after viral vector-based enzyme replacement therapy is attributable to a decrease in the affinity of the tracer binding to the D(2) receptors, providing evidence for reconstitution of a functional pool of endogenous dopamine in the striatum. Moreover, the extent of the normalization in this non-invasive imaging measure was highly correlated with the functional recovery in motor behavior. The PET imaging protocol used in this study is fully adaptable to humans and thus can serve as an in vivo imaging technique to follow TH + GCH1 gene therapy in PD patients and provide an additional objective measure to a potential clinical trial using rAAV vectors to deliver l-3,4-dihydroxyphenylanaline in the brain.

Rodent model of attention: the 5-choice serial reaction time task.

The 5-choice serial reaction time task (5-CSRTT) is the most widely used test to measure attentional performance in rodents. The basic test design involves training animals to respond to a brief visual stimulus presented unpredictably in one of five locations. Once trained to stable performance levels, the effects of experimental manipulations on response speed and choice accuracy are measured and each are related to attentional performance. Increasingly, the test is also used to examine aspects of response control. Having been adapted from a human task, the test has also been successfully extended to the mouse and primate, thus highlighting its translational value. Increasingly this test is being applied in drug discovery efforts, primarily to identify novel drug treatments for conditions associated with attention deficits.

The functional impact of the intrastriatal dopamine neuron grafts in parkinsonian rats is reduced with advancing disease.

Clinical trials involving intrastriatal transplants of human embryonic mesencephalic tissue have provided proof-of-principle that nigral dopamine (DA) neurons can survive and functionally integrate into the host neural circuitry. However, the degree of graft-induced symptomatic relief differs significantly between the patients. This variability has led to investigations aimed at identifying factors that could affect the clinical outcome. The extent and pattern of dopaminergic denervation in the brain may be one of the major determinants of the functional outcome after intrastriatal DA cell grafts. Here, we report that in animals subjected to an intrastriatal 6-hydroxydopamine lesion of the striatal dopaminergic afferent, the integrity of the host dopaminergic innervation outside the areas innervated by the graft is critical for optimal function of DA neurons placed in the striatum. Established graft-induced functional recovery, as assessed in the stepping and cylinder tests, was compromised in animals in which the dopaminergic lesion was extended to include also the medial and ventral striatum as well as the cortical and limbic DA projections. Poor clinical outcome after transplantation may, thus, at least in part, be caused by dopaminergic denervation in areas outside the graft-innervated territories, and similarly beneficial effects initially observed in patients may regress if the degeneration of the host extrastriatal DA projection systems proceeds with advancing disease. This would have two implications: first, patients with advanced disease involving the ventral striatum and/or nonstriatal DA projections would be unlikely to respond well to intrastriatal DA grafts and, second, to retain the full benefit of the grafts, progression of the disease should be avoided by, for example, combining cell therapy with a neuroprotective approach.

Dysfunction of the cortico-basal ganglia-cortical loop in a rat model of early parkinsonism is reversed by metabotropic glutamate receptor 5 antagonism.

This study examined the cellular correlates of the akinetic deficits produced in Wistar rats by discrete bilateral 6-hydroxydopamine (6-OHDA) striatal infusions in the dorsolateral striatum, mimicking the preferential denervation of the motor striatal territory in early symptomatic stage of Parkinson's disease (PD). Intraneuronal gene expression of cytochrome oxidase subunit I (COI), a metabolic index of neuronal activity, was increased in the subthalamic nucleus, substantia nigra pars reticulata and decreased in frontal cortical areas, but paradoxically unchanged in the striatum, globus pallidus, entopeduncular nucleus and ventrolateral thalamic nucleus. Neither preproenkephalin A nor preprotachykinin mRNA expression, markers of striatal projection neurons, were modified in the denervated striatal area despite 90% loss of dopamine (DA) terminals. Preproenkephalin A mRNA expression was however, decreased in the nondepleted striatal region, suggesting compensatory increase of dopamine tone from those spared areas. A chronic treatment with the metabotropic glutamate receptor 5 (mGluR5) antagonist 2-methyl-6-(phenylethylnyl)-pyridine (MPEP), which alleviated the akinetic disorders produced by the lesion, reversed the lesion-induced variations of COI gene expression, moderately increased this marker in the structures unaffected by the lesion and did not modify the striatal neuropeptides gene expression. These data suggest that the expression of akinetic deficits in early parkinsonism is associated with focused metabolic changes in the cortico-basal ganglia-cortical loop downstream of the striatum and pallidal complex.

Imaging in cell-based therapy for neurodegenerative diseases.

Fetal cell transplantation for the treatment of Parkinson's and Huntington's diseases has been developed over the past two decades and is now in early clinical testing phase. Direct assessment of the graft's survival, integration into the host brain and impact on neuronal functions requires advanced in vivo neuroimaging techniques. Owing to its high sensitivity, positron emission tomography is today the most widely used tool to evaluate the viability and function of the transplanted tissue in the brain. Nuclear magnetic resonance techniques are opening new possibilities for imaging neurochemical events in the brain. The ultimate goal will be to use the combination of multiple imaging modalities for complete functional monitoring of the repair processes in the central nervous system.

Functional interaction between mGlu 5 and NMDA receptors in a rat model of Parkinson's disease.

RATIONALE: Electrophysiological evidence suggests a synergistic relationship between metabotropic (mGlu) and ionotropic (iGlu) glutamate receptors. The functional consequences of these interactions have not been investigated in neurodegenerative diseases such as in Parkinson's disease. OBJECTIVE: The goals of this study are as follows: (1) to investigate the effects of 2-methyl-6-(phenylethynyl)-pyridine (MPEP) and dizocilpine (MK-801), antagonists at metabotropic glutamate 5 (mGlu5) and NMDA receptors, respectively, on the akinetic syndrome observed in bilateral 6-OHDA-lesioned rats; (2) to investigate if the effects of MPEP were potentiated by co-treatment with a behaviorally inactive dose of MK-801; and (3) to investigate the effects of L-DOPA alone and in combination with MPEP on the akinetic syndrome observed in 6-OHDA-lesioned rats. METHODS: The effects of the different treatments (single and co-treatment) administered for 3 weeks were measured in 6-OHDA-lesioned rats trained to release a lever rapidly after a visual stimulus onset in a simple reaction time task. RESULTS: MPEP 0.75 mg/kg reversed the akinetic deficits produced by striatal dopamine depletion, while MPEP 0.375 mg/kg had no effect. Co-administration with MK-801 0.02 mg/kg, ineffective alone, failed to speed the recovery process of MPEP 0.75 mg/kg but revealed the anti-akinetic action of MPEP 0.375 mg/kg. L-DOPA 3 mg/kg alone had a potent anti-akinetic effect in 6-OHDA lesioned rats, and this effect was not potentiated by a subthreshold MPEP treatment. CONCLUSION: These results support a critical role for mGlu5 receptor blockade in improving parkinsonian symptomatology either as a single treatment or in combination with low concentrations of L-DOPA and demonstrate an interaction between NMDA and mGluR5 in regulating these effects.

Simultaneous blockade of adenosine A2A and metabotropic glutamate mGlu5 receptors increase their efficacy in reversing Parkinsonian deficits in rats.

Recent evidence suggest that antagonism of adenosine A2A receptors represent an alternative therapeutic approach to Parkinson's disease (PD). Coactivation of A2A and the glutamate subtype 5 metabotropic receptors (mGlu5) synergistically stimulates DARPP-32 phosphorylation and c-fos expression in the striatum. This study therefore tested the effects of a joint blockade of these receptors to alleviate the motor dysfunction in a rat model of PD. 6-Hydroxydopamine infusions in the striatum produced akinetic deficits in rats trained to release a lever after a stimulus in a reaction time (RT) task. At 2 weeks after the lesion, A2A and mGlu5 receptors selective antagonists 8-(3-chlorostyryl)caffeine (CSC) and 2-methyl-6-(phenylethynyl)-pyridine (MPEP) were administered daily for 3 weeks either as a single or joint treatment. Injections of CSC (1.25 mg/kg) and MPEP (1.5 mg/kg) separately or in combination reduced the increase of delayed responses and RTs induced by 6-OHDA lesions, while the same treatment had no effect in controls. Furthermore, coadministration of lower doses of 0.625 mg/kg CSC and 0.375 mg/kg MPEP noneffective as a single treatment promoted a full and immediate recovery of akinesia, which was found to be more efficient than the separate blockade of these receptors. These results demonstrate that the combined inactivation of A2A and mGlu5 receptor potentiate their beneficial effects supporting this pharmacological strategy as a promising anti-Parkinsonian therapy.

Metabotropic glutamate 5 receptor blockade alleviates akinesia by normalizing activity of selective basal-ganglia structures in parkinsonian rats.

Glutamate overactivity within the basal ganglia has been shown to be central to the expression of motor symptoms in advanced stages of Parkinson's disease, and metabotropic glutamate receptors (mGluRs) represent promising targets for new therapeutic strategies in this pathology. Little is known, however, about the cellular and behavioral changes occurring in the early stages of the disease when dopamine depletion is moderate. Here, we report that rats with partial bilateral dopamine lesions exhibit akinetic deficits associated with dramatically increased neuronal metabolic activity in selective structures of the basal ganglia such as the subthalamic nucleus and the substantia nigra pars reticulata, but not in the entopeduncular nucleus. Furthermore, chronic treatment with the mGluR5 antagonist 2-methyl-6-(phenylethylnyl)-pyridine alleviated the akinesia and was associated with a normalization of the activity of these two overactive structures. These data stress the therapeutic potential of mGluR5 antagonists in the treatment of parkinsonian patients in the early stages of the disease.

D1 and D2 dopamine receptors contribute to the locomotor response induced by Group II mGluRs activation in the rat nucleus accumbens.

Whereas the involvement of ionotropic glutamate receptors (iGluRs) in the functional interaction between glutamate and dopamine (DA) systems in the nucleus accumbens (N. Acc.) is well established, the role of metabotropic glutamate receptors (mGluRs) is less clear. This study was thus aimed to investigate the mechanisms involving DA and glutamate systems via mGluRs in the generation of motor activity in rats. Intra-accumbens infusion of the Group II agonist (2S,3S,4S)-2-(carboxycyclopropyl)glycine (L-CCG-I; 25, 50, 100 nmol) increased locomotor activity, whereas the Group I agonist (S)-3,5-dihydroxyphenylglycine (S-3,5-DHPG) at the same doses had no effect. The effects of L-CCG-I were blocked by a selective Group II mGluRs antagonist (2S,3S,4S)-2-methyl-2-(carboxypropyl)glycine (MCCG; 50 nmol). The locomotor stimulant effect induced by L-CCG-I might be partly DA mediated, as it is abolished by a pretreatment with the DA receptor antagonist haloperidol (0.1 mg/kg ip) and potentiated by D-amphetamine systemic injection (0.5 mg/kg sc). Furthermore, selective D1 (SCH 23390; 0.005, 0.01 and 0.02 mg/kg) or D2 (raclopride; 0.05, 0.1 and 0.2 mg/kg) antagonists injected systemically were also effective in decreasing L-CCG-I induced hyperactivity. Taken together, these results demonstrate that stimulation of Group II but not Group I mGluRs contributes to the regulation of motor behavior in the N. Acc. and that this increased activity requires the activation of both D1 and D2 DA receptors.

Chronic but not acute treatment with a metabotropic glutamate 5 receptor antagonist reverses the akinetic deficits in a rat model of parkinsonism.

Metabotropic glutamate receptors (mGluRs) have recently been considered as potential pharmacological targets in the treatment of neurodegenerative disorders and particularly in parkinsonism. Within the basal ganglia, receptors of group I (mGluR1 and mGluR5) are widely expressed; the present study was thus aimed at blocking these receptors in a 6-hydroxydopamine (6-OHDA) model of Parkinson's disease in the rat. Considering the prominent expression of mGluR5, we have used the selective mGluR5 antagonist 2-methyl-6-(phenylethynyl)-pyridine (MPEP) to target these receptors. In rats trained to quickly depress a lever after a visual cue, bilateral lesions of the dopaminergic nerve terminals in the striatum produced severe akinetic deficits, which were expressed by increases in delayed responses and reaction times. Acute MPEP injection (1.5, 3, and 6 mg/kg, i.p.) had no effect, whereas chronic administration, ineffective in a control group, significantly reversed the akinetic deficits. Alleviation of these deficits was seen after 1 week of treatment, and the preoperative performance was fully recovered after a 3 week treatment of MPEP at all doses. Chronic MPEP also induced ipsilateral rotation in the unilateral 6-OHDA circling model. However, no effect was seen of MPEP (1.5, 3, or 6 mg/kg, i.p.) on haloperidol-induced catalepsy (1 mg/kg, i.p.). Altogether, these results suggest a specific role of mGluRs in the regulation of extrapyramidal motor functions and a potential therapeutic value for mGluR5 antagonists in the treatment of Parkinson's disease.