A potent and selective metabotropic glutamate receptor 4 positive allosteric modulator improves movement in rodent models of Parkinson's disease.

Positive allosteric modulators (PAMs) of metabotropic glutamate receptor 4 (mGluR4) have been proposed as a novel therapeutic approach for the treatment of Parkinson's disease. However, evaluation of this proposal has been limited by the availability of appropriate pharmacological tools to interrogate the target. In this study, we describe the properties of a novel mGluR4 PAM. 5-Methyl-N-(4-methylpyrimidin-2-yl)-4-(1H-pyrazol-4-yl)thiazol-2-amine (ADX88178) enhances glutamate-mediated activation of human and rat mGluR4 with EC(50) values of 4 and 9 nM, respectively. The compound is highly selective for mGluR4 with minimal activities at other mGluRs. Oral administration of ADX88178 in rats is associated with high bioavailability and results in cerebrospinal fluid exposure of >50-fold the in vitro EC(50) value. ADX88178 reverses haloperidol-induced catalepsy in rats at 3 and 10 mg/kg. It is noteworthy that this compound alone has no impact on forelimb akinesia resulting from a bilateral 6-hydroxydopamine lesion in rats. However, coadministration of a low dose of L-DOPA (6 mg/kg) enabled a robust, dose-dependent reversal of the forelimb akinesia deficit. ADX88178 also increased the effects of quinpirole in lesioned rats and enhanced the effects of L-DOPA in MitoPark mice. It is noteworthy that the enhancement of the actions of L-DOPA was not associated with an exacerbation of L-DOPA-induced dyskinesias in rats. ADX88178 is a novel, potent, and selective mGluR4 PAM that is a valuable tool for exploring the therapeutic potential of mGluR4 modulation. The use of this novel tool molecule supports the proposal that activation of mGluR4 may be therapeutically useful in Parkinson's disease.

Molecular profiling of a 6-hydroxydopamine model of Parkinson's disease.

Convection enhanced delivery of 6-hydroxydopamine (6-OHDA) to the rat striatum results in a model of Parkinson's disease. An important feature of this unilateral model is the progressive loss of dopaminergic (DA) neurons over the course of several weeks. To improve the understanding of this model, gene expression changes in the substantia nigra, which contains the DA neuron cell bodies, and the striatum, which contains the DA neuron synaptic terminals, were examined using DNA microarrays. Samples were collected and behavior was analyzed from vehicle and toxin treated animals at 3 days, 1 week, 2 weeks and 4 weeks following 6-OHDA treatment. Tissue DA content was determined and samples from animals which exhibited a substantial depletion of striatal DA were included in the subsequent gene expression analysis. The results of the gene expression analysis indicated that 6-OHDA elicits a vigorous inflammatory response, comprised of several distinct pathways, in the striatum at the earliest time point tested. In contrast, relatively few gene expression changes were observed in the SN at the 3-day time point. In both tissues examined there was evidence for a vigorous inflammatory response at the 1- and 2-week time points, which was substantially diminished by the 4-week time point. Inflammation plays a prominent role in the 6-OHDA model of Parkinson's disease.

Identification and characterization of 4-methylbenzyl 4-[(pyrimidin-2-ylamino)methyl]piperidine-1-carboxylate, an orally bioavailable, brain penetrant NR2B selective N-methyl-D-aspartate receptor antagonist.

The discovery of a novel series of NR2B subtype selective N-methyl-d-aspartate (NMDA) antagonists is reported. Initial optimization of a high-throughput screening lead afforded an aminopyridine derivative 13 with significant NR2B antagonist potency but limited selectivity over hERG-channel and other off-target activities. Further structure-activity studies on the aminoheterocycle moiety and optimization of the carbamate led to the highly potent 2-aminopyrimidine derivative 20j with a significantly improved off-target activity profile and oral bioavailability in multiple species coupled with good brain penetration. Compound 20j demonstrated efficacy in in vivo rodent models of antinociception, allodynia, and Parkinson's disease.

Group III metabotropic glutamate receptor-mediated modulation of the striatopallidal synapse.

The globus pallidus (GP) is a key GABAergic nucleus in the basal ganglia (BG). The predominant input to the GP is an inhibitory striatal projection that forms the first synapse in the indirect pathway. The GP GABAergic neurons project to the subthalamic nucleus, providing an inhibitory control of these glutamatergic cells. Given its place within the BG circuit, it is not surprising that alterations in GP firing pattern are postulated to play a role in both normal and pathological motor behavior. Because the inhibitory striatal input to the GP may play an important role in shaping these firing patterns, we set out to determine the role that the group III metabotropic glutamate receptors (GluRs) play in modulating transmission at the striatopallidal synapse. In rat midbrain slices, electrical stimulation of the striatum evoked GABA(A)-mediated IPSCs recorded in all three types of GP neurons. The group III mGluR-selective agonist L-(+)-2-amino-4-phosphonobutyric acid (L-AP4) inhibited these IPSCs through a presynaptic mechanism of action. L-AP4 exhibited high potency and a pharmacological profile consistent with mediation by mGluR4. Furthermore, the effect of L-AP4 on striatopallidal transmission was absent in mGluR4 knock-out mice, providing convincing evidence that mGluR4 mediates this effect. The finding that mGluR4 may selectively modulate striatopallidal transmission raises the interesting possibility that activation of mGluR4 could decrease the excessive inhibition of the GP that has been postulated to occur in Parkinson's disease. Consistent with this, we find that intracerebroventricular injections of L-AP4 produce therapeutic benefit in both acute and chronic rodent models of Parkinson's disease.

Discovery of 3-substituted aminocyclopentanes as potent and orally bioavailable NR2B subtype-selective NMDA antagonists.

A series of 3-substituted aminocyclopentanes has been identified as highly potent and selective NR2B receptor antagonists. Incorporation of a 1,2,4-oxadiazole linker and substitution of the pendant phenyl ring led to the discovery of orally bioavailable analogues that showed efficient NR2B receptor occupancy in rats. Unlike nonselective NMDA antagonists, the NR2B-selective antagonist 22 showed no adverse affects on motor coordination in the rotarod assay at high dose. Compound 22 was efficacious following oral administration in a spinal nerve ligation model of neuropathic pain and in an acute model of Parkinson's disease in a dose dependent manner.

Discovery of 1,4-substituted piperidines as potent and selective inhibitors of T-type calcium channels.

The discovery of a novel series of potent and selective T-type calcium channel antagonists is reported. Initial optimization of high-throughput screening leads afforded a 1,4-substituted piperidine amide 6 with good potency and limited selectivity over hERG and L-type channels and other off-target activities. Further SAR on reducing the basicity of the piperidine and introducing polarity led to the discovery of 3-axial fluoropiperidine 30 with a significantly improved selectivity profile. Compound 30 showed good oral bioavailability and brain penetration across species. In a rat genetic model of absence epilepsy, compound 30 demonstrated a robust reduction in the number and duration of seizures at 33 nM plasma concentration, with no cardiovascular effects at up to 5.6 microM. Compound 30 also showed good efficacy in rodent models of essential tremor and Parkinson's disease. Compound 30 thus demonstrates a wide margin between CNS and peripheral effects and is a useful tool for probing the effects of T-type calcium channel inhibition.

Allosteric modulation of group III metabotropic glutamate receptor 4: a potential approach to Parkinson's disease treatment.

Parkinson's disease (PD) is a debilitating movement disorder that afflicts >1 million people in North America. Current treatments focused on dopamine-replacement strategies ultimately fail in most patients because of loss of efficacy and severe adverse effects that worsen as the disease progresses. The recent success of surgical approaches suggests that a pharmacological intervention that bypasses the dopamine system and restores balance in the basal ganglia motor circuit may provide an effective treatment strategy. We previously identified the metabotropic glutamate receptor 4 (mGluR4) as a potential drug target and predicted that selective activation of mGluR4 could provide palliative benefit in PD. We now report that N-phenyl-7-(hydroxylimino)cyclopropa[b]chromen-1a-carboxamide (PHCCC) is a selective allosteric potentiator of mGluR4. This compound selectively potentiated agonist-induced mGluR4 activity in cultured cells expressing this receptor and did not itself act as an agonist. Furthermore, PHCCC potentiated the effect of l-(+)-2-amino-4-phosphonobutyric acid in inhibiting transmission at the striatopallidal synapse. Modulation of the striatopallidal synapse has been proposed as a potential therapeutic target for PD, in that it may restore balance in the basal ganglia motor circuit. Consistent with this, PHCCC produced a marked reversal of reserpine-induced akinesia in rats. The closely related analogue 7-(hydroxylimino)cyclopropachromen-1a-carboxamide ethyl ester, which does not potentiate mGluR4, had no effect in this model. These results are evidence for in vivo behavioral effects of an allosteric potentiator of mGluRs and suggest that potentiation of mGluR4 may be a useful therapeutic approach to the treatment of PD.

A role for the melanocortin 4 receptor in sexual function.

By using a combination of genetic, pharmacological, and anatomical approaches, we show that the melanocortin 4 receptor (MC4R), implicated in the control of food intake and energy expenditure, also modulates erectile function and sexual behavior. Evidence supporting this notion is based on several findings: (i) a highly selective non-peptide MC4R agonist augments erectile activity initiated by electrical stimulation of the cavernous nerve in wild-type but not Mc4r-null mice; (ii) copulatory behavior is enhanced by administration of a selective MC4R agonist and is diminished in mice lacking Mc4r; (iii) reverse transcription (RT)-PCR and non-PCR based methods demonstrate MC4R expression in rat and human penis, and rat spinal cord, hypothalamus, brainstem, pelvic ganglion (major autonomic relay center to the penis), but not in rat primary corpus smooth muscle cavernosum cells; and (iv) in situ hybridization of glans tissue from the human and rat penis reveal MC4R expression in nerve fibers and mechanoreceptors in the glans of the penis. Collectively, these data implicate the MC4R in the modulation of penile erectile function and provide evidence that MC4R-mediated proerectile responses may be activated through neuronal circuitry in spinal cord erectile centers and somatosensory afferent nerve terminals of the penis. Our results provide a basis for the existence of MC4R-controlled neuronal pathways that control sexual function.