The effects of L-AP4 and L-serine-O-phosphate on inhibition in primary somatosensory cortex of the adult rat in vivo

Prepulse inhibition of the acoustic startle reflex impairment by 5-HT2A receptor activation in the inferior colliculusis prevented by GABAA receptor blockade in the pedunculopontine tegmental nucleus

The relationship between serotonin dysfunction and schizophrenia commenced with the discovery of the effects of lysergic acid diethylamide (LSD) that has high affinity for 5-HT_2A receptors. Activation of these receptors produces perceptual and behavioural changes such as illusions, visual hallucinations and locomotor hyperactivity. Using prepulse inhibition (PPI) of the acoustic startle, which is impaired in schizophrenia,we aimed to investigate:i) the existence of a direct and potentially inhibitory neural pathway between the inferior colliculus (IC) and the pedunculopontine tegmental nucleus (PPTg) involved in the mediation of PPI responses by a neural tract tracing procedure;ii) if the microinjection of the 5-HT_2A receptors agonist DOI in IC would activate neurons in this structure and in the PPTg by a c-Fos protein immunohistochemistry study;iii) whether the deficits in PPI responses, observed after the administration of DOI in the IC, could be prevented by the concomitant microinjection of the GABA_A receptor antagonist bicuculline in the PPTg.Male Wistar rats were used in this study. An IC-PPTg reciprocated neuronal pathway was identified by neurotracing. The number of c-Fos labelled cells was lower in the DOI group in IC and PPTg, suggesting that this decrease could be due to the high levels of GABA in both structures. The concomitant microinjections of bicuculline in PPTg and DOI in IC prevented the PPI deficit observed after the IC microinjection of DOI. Ourfindings suggest that IC 5-HT_2A receptors may be at least partially involved in the regulation of inhibitory pathways mediating PPI response in IC and PPTg structures.

Metabotropic glutamate receptor 4 (mGlu4)-positive allosteric modulators for the treatment of Parkinson's disease: historical perspective and review of the patent literature

INTRODUCTION

Metabotropic glutamate receptor 4 (mGlu(4)) is a group III GPCR and has been demonstrated to play a major role in a number of therapeutic areas within the CNS. As the orthosteric site of all glutamate receptors is highly conserved, modulating mGlu(4) via allosteric modulation has emerged as a very attractive mode-of-action and has been validated preclinically in a number of animal models for Parkinson's disease, anxiety, pain, and neuroinflammation.

AREAS COVERED

In this review, the patent literature for mGlu(4)-positive allosteric modulators over the past 4 years will be provided. Patents from all companies are discussed and an overview of the chemical matter and relevant biological properties will be given.

EXPERT OPINION

Although there has yet to be an mGlu(4)-positive allosteric modulator progressed into clinical trials, there is a wealth of preclinical data from the primary literature that shows the promise of this emerging target. A number of academic and industry laboratories have recently published exciting patent data covering a multitude of chemical matter. Positive allosteric modulation of mGlu(4) remains one of the more attractive non-dopaminergic therapies for Parkinson's disease, as well as emerging data for other indications such as pain, neuroinflammation, schizophrenia and diabetes, which could potentially make mGlu(4) a significant therapeutic target going forward.

Recent progress on the identification of metabotropic glutamate 4 receptor ligands and their potential utility as CNS therapeutics

This Review describes recent activity in the advancement of ligands for the metabotropic glutamate 4 receptor subtype and their potential utility as central nervous system (CNS) therapeutics. Until recently, there was a paucity of compounds with suitable selectivity and druglike properties to elucidate the value of this target. The search for selective entities has led several groups to the investigation of allosteric modulators as a path to optimization of potential ligands. Recent efforts, discussed here, have afforded a variety of derivatives with improvements in potency, solubility, and pharmacokinetic properties that garner support for continued investigation and optimization.

mGluR4-positive allosteric modulation as potential treatment for Parkinson's disease

Although Parkinson's disease was first diagnosed nearly 200 years ago, its effective treatment still remains elusive for most of those diagnosed. The gold standard of treatment for most patients is 3,4-dihydroxy-L-phenylalanine. This drug works for most individuals early in the disease; however, resistant symptoms start to emerge after several years of treatment. There has been increased interest in finding novel therapies to help Parkinson's disease patients. Such strategies may have the benefit of not only treating the symptomatic issues of the disorder, but might also offer promise in protecting dopaminergic neurons from further degeneration. One such target that is now receiving much attention from the scientific community is the metabotropic glutamate receptor mGluR4. In this article, we briefly review Parkinson's disease and then recent work in the mGluR area, with a focus on the efforts being made toward finding and optimizing novel mGluR4 positive allosteric modulators (PAMs). Preclinically in rodent models, mGluR4 activation has offered much promise as a novel treatment of Parkinson's disease. Additionally, the specific use of PAMs, rather than direct-acting agonists at the orthosteric glutamate site, continues to be validated as a viable treatment option for this target. It is anticipated that continued progress in this area will further our understanding of the potential of mGluR4 modulation as a novel symptomatic and potentially disease-modifying treatment for Parkinson's disease.

PICK1 uncoupling from mGluR7a causes absence-like seizures

Absence epilepsy is a neurological disorder that causes a recurrent loss of consciousness and generalized spike-and-wave discharges on an electroencephalogram (EEG). The role of metabotropic glutamate receptors (mGluRs) and associated scaffolding proteins in absence epilepsy has been unclear to date. We investigated a possible role for these proteins in absence epilepsy, focusing on the mGluR7a receptor and its PDZ-interacting protein, protein interacting with C kinase 1 (PICK1), in rats and mice. Injection of a cell-permeant dominant-negative peptide or targeted mutation of the mGluR7a C terminus, both of which disrupt the interaction between the receptor and PDZ proteins, caused behavioral symptoms and EEG discharges that are characteristic of absence epilepsy. Inactivation of the Pick1 gene also facilitated pharmacological induction of the absence epilepsy phenotype. The cortex and thalamus, which are known to participate in absence epilepsy, were involved, but the hippocampus was not. Our results indicate that disruption of the mGluR7a-PICK1 complex is sufficient to induce absence epilepsy-like seizures in rats and mice, thus providing, to the best of our knowledge, the first animal model of metabotropic glutamate receptor-PDZ protein interaction in absence epilepsy.

Group II metabotropic glutamate receptors reduce excitatory but not inhibitory neurotransmission in rat barrel cortex in vivo

Group II metabotropic (mGlu) receptors are known to play an important role in regulating the release of excitatory transmitter in a number of brain areas. Previous experiments demonstrated that (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD) depressed excitatory transmission in the adult rat barrel cortex. Here we show, using in vivo extracellular single unit recordings and iontophoretic application of drugs, that selective activation of Group II mGlu receptors depresses excitatory but not inhibitory transmission. The selective Group II receptor agonist (2R,4R)-4-aminopyrrolidine-2,4-dicarboxylate (2R,4R-APDC) had similar depressant effects to 1S,3R-ACPD on tactile evoked responses of rapidly adapting neurons. The depressant effects were seen on shorter latency (<12 ms) responses, were most pronounced in layers 3-4 (and 5b for 2R,4R-APDC only), and were reversibly antagonized by the Group II receptor antagonist (2S)-alpha-ethylglutamic acid (EGLU) relative to depressions produced by iontophoretic GABA. Where 1S,3R-ACPD and 2R,4R-APDC depressed excitatory transmission, there was little or no effect on postsynaptic excitations produced by iontophoretic AMPA--a result that supports a presynaptic location of Group II receptors on excitatory terminals. To assess the possible involvement of Group II mGlu receptors in the modulation of inhibition, we studied the effect of iontophoretic 1S,3R-ACPD in a condition-test protocol. The results contrasted markedly from those previously observed using the Group III agonist L(+)-2-amino-4-phosphonobutyric acid in that activation of Group II receptors using 1S,3R-ACPD did not modulate inhibition. Therefore our results show that Group II mGlu receptors play an important role in modulating excitatory, but not inhibitory, transmission. We propose that the Group II mGlu receptors are located on excitatory terminals, and act as autoreceptors. Their role appears to be important in the early stages of cortical processing, by keeping excitatory inputs within specified physiological limits, and possibly by mediating depression evidenced during synaptic plasticity.

Increased seizure susceptibility in mice lacking metabotropic glutamate receptor 7

To study the role of mGlu7 receptors (mGluR7), we used homologous recombination to generate mice lacking this metabotropic receptor subtype (mGluR7(-/-)). After the serendipitous discovery of a sensory stimulus-evoked epileptic phenotype, we tested two convulsant drugs, pentylenetetrazole (PTZ) and bicuculline. In animals aged 12 weeks and older, subthreshold doses of these drugs induced seizures in mGluR7(-/-), but not in mGluR7(+/-), mice. PTZ-induced seizures were inhibited by three standard anticonvulsant drugs, but not by the group III selective mGluR agonist (R,S)-4-phosphonophenylglycine (PPG). Consistent with the lack of signs of epileptic activity in the absence of specific stimuli, mGluR7(-/-) mice showed no major changes in synaptic properties in two slice preparations. However, slightly increased excitability was evident in hippocampal slices. In addition, there was slower recovery from frequency facilitation in cortical slices, suggesting a role for mGluR7 as a frequency-dependent regulator in presynaptic terminals. Our findings suggest that mGluR7 receptors have a unique role in regulating neuronal excitability and that these receptors may be a novel target for the development of anticonvulsant drugs.

Modulation of GABAergic signaling among interneurons by metabotropic glutamate receptors

Synapses between hippocampal interneurons are an important potential target for modulatory influences that could affect overall network behavior. We report that the selective group III metabotropic receptor agonist L(+)-2-amino-4-phosphonobutyric acid (L-AP4) depresses GABAergic transmission to interneurons more than to pyramidal neurons. The L-AP4-induced depression is accompanied by changes in trial-to-trial variability and paired-pulse depression that imply a presynaptic site of action. Brief trains of stimuli in Schaffer collaterals also depress GABAergic transmission to interneurons. This depression persists when GABA(B) receptors are blocked, is enhanced by blocking glutamate uptake, and is abolished by the group III metabotropic receptor antagonist (alpha-methylserine-O-phosphate (MSOP). The results imply that GABAergic transmission among interneurons is modulated by glutamate spillover from excitatory afferent terminals.

Effect of the group II metabotropic glutamate agonist, 2R,4R-APDC, varies with age, layer, and visual experience in the visual cortex

Group II metabotropic glutamate receptors (mGluR 2/3) are distributed differentially across the layers of cat visual cortex, and this distribution varies with age. At 3-4 wk, mGluR 2/3 receptor immunoreactivity is present in all layers. By 6-8 wk of age, it is still present in extragranular layers (2, 3, 5, and 6) but has disappeared from layer 4, and dark-rearing postpones the disappearance of Group II receptors from layer 4. We examined the physiological effects of Group II activation, to see if these effects varied similarly. The responses of single neurons in cat primary visual cortex were recorded to visual stimulation, then the effect of iontophoresis of 2R,4R-4 aminopyrrolidine-2, 4-decarboxylate (2R,4R-APDC), a Group II specific agonist, was observed in animals between 3 wk and adulthood. The effect of 2R, 4R-APDC was generally suppressive, reducing both the visual response and spontaneous activity of single neurons. The developmental changes were in agreement with the immunohistochemical results: 2R, 4R-APDC had effects on cells in all layers in animals of 3-4 wk but not in layer 4 of animals >6 wk old. Moreover, the effect of 2R, 4R-APDC was reduced in the cortex of older animals (>22 wk). Dark-rearing animals to 47-54 days maintained the effects of 2R, 4R-APDC in layer 4. The disappearance of Group II mGluRs from layer 4 between 3 and 6 wk of age is correlated with the segregation of ocular dominance columns in that layer, raising the possibility that mGluRs 2/3 are involved in this process.

The group III metabotropic glutamate receptor agonist, l-AP4, reduces EPSPs in some layers of rat visual cortex

The action of the specific Group III metabotropic glutamate receptor, l-2-amino-4-phosphonobutanoic acid (l-AP4) was tested in slices of rat visual cortex. When the predominant input to the cell was stimulated, l-AP4 generally reduced the EPSP that was produced. This result was specific to the layer: it was found when recording cells in layers II/III, V and VI, but not when recording cells in layer IV. The effect was the same when G-proteins in the cell recorded were inactivated. Also, l-AP4 had little effect on membrane potential and input impedance of the cell recorded, and little effect on the response to NMDA in that cell. Thus, Group III metabotropic glutamate receptors act presynaptically to reduce the release of glutamate onto cells in layers II/III, V and VI in visual cortex, but not cells in layer IV.

Prolonged anticonvulsant action of glutamate metabotropic receptor agonists in inferior colliculus of genetically epilepsy-prone rats

The anticonvulsant activity of (S)-4-carboxy-3-hydroxyphenylglycine ((S)-4C3HPG) (an antagonist of Group I and an agonist of Group II metabotropic glutamate (mGlu) receptors), of (1S,3S)-1-aminocyclopentane-1,3-dicarboxylic acid ((1S,3S)-ACPD) (an agonist of Group II mGlu receptors), and of L-serine-O-phosphate (an agonist of Group III mGlu receptors) was studied against sound-induced seizures in genetically epilepsy-prone (GEP) rats following bilateral microinjection into the inferior colliculus. All 3 drugs produce dose-dependent suppression of all phases of sound-induced seizures (wild running, clonic and tonic). (S)-4C3HPG produces an immediate and short-lasting (< 2 h) protection against sound-induced seizures with an ED50 value of 4.3 (3.2-5.7) nmol, at 5 min. The preferential agonists of Group II and Group III mGlu receptors produce an immediate, transient (< 10 min) proconvulsant effect followed by a prolonged (> 1 day) anticonvulsant effect against sound-induced seizures. The anticonvulsant ED50 value for (1S,3S)-ACPD is 9 (5-18) nmol at 2 h, and for L-serine-O-phosphate is 36 (6.5-199) nmol at 2 days. It is concluded that mGlu receptor activation potently modifies seizure threshold.

Metabotropic glutamate receptor activation modulates kainate and serotonin calcium response in astrocytes

Although metabotropic glutamate receptor (mGluR) modulation has been studied extensively in neurons, it has not been investigated in astrocytes. We studied modulation of glutamate-evoked calcium rises in primary astrocyte cultures using fura-2 ratiometric digital calcium imaging. Calcium plays a key role as a second messenger system in astrocytes, both in regulation of many subcellular processes and in long distance intercellular signaling. Suprachiasmatic nucleus (SCN) and cortical astrocytes showed striking differences in sensitivity to glutamate and to mGluR agonists, even after several weeks in culture. Kainate-evoked intracellular calcium rises were inhibited by concurrent application of the type I and II mGluR agonists quisqualate (10 micro;M), trans-(+/-)-1-amino-1,3-cyclopentanedicarboxylate (100-500 micro;M), and (2S-1'S-2'S)-2-(carboxycyclopropyl)glycine (L-CCG-I) (10 micro;M). Inhibition mediated by L-CCG-I had long-lasting effects (>45 min) in approximately 30% of the SCN astrocytes tested. The inhibition could be mimicked by the L-type calcium channel blocker nimodipine (1 micro;M) as well as by protein kinase C (PKC) activators phorbol 12,13-dibutyrate (10 micro;M) and phorbol 12-myristate 13-acetate (500 nM), and blocked by the PKC inactivator (+/-)-1-(5-isoquinolinesulfonyl)-2-methylpiperazine (200 micro;M), suggesting a mechanism involving PKC modulation of L-type calcium channels. In contrast, mGluRs modulated serotonin (5HT)-evoked calcium rises through a different mechanism. The type III mGluR agonist L-2-amino-4-phosphonobutyrate consistently inhibited 5HT-evoked calcium rises, whereas in a smaller number of cells quisqualate and L-CCG-I showed both inhibitory and additive effects. Unlike the mGluR-kainate interaction, which required a pretreatment with an mGluR agonist and was insensitive to pertussis toxin (PTx), the mGluR modulation of 5HT actions was rapid and was blocked by PTx. These data suggest that glutamate, acting at several metabotropic receptors expressed by astrocytes, could modulate glial activity evoked by neurotransmitters and thereby influence the ongoing modulation of neurons by astrocytes.