Melanin-concentrating hormone MCH1 receptor antagonists: a potential new approach to the treatment of depression and anxiety disorders.

Melanin-concentrating hormone (MCH) is a cyclic 19-amino-acid neuropeptide that has been considered to play a key role in the regulation of feeding and energy homeostasis. To date, two receptor subtypes for MCH (designated MCH(1) and MCH(2)) have been identified; the MCH(1) receptor has been proposed to mediate the physiological functions of MCH in rodents. In addition to the crucial roles of MCH in feeding behaviour, anatomical and neurochemical studies suggest that the MCH/MCH(1) system is involved in the regulation of emotion and stress responses. This assumption has been supported by a recent series of neurochemical and behavioural studies. Indeed, several lines of evidence show that MCH activates stress responses and induces depressive- and anxiety-like behaviours, while the blockade of MCH(1) receptors results in antidepressant and anxiolytic effects in various rodent models. Moreover, MCH may decrease reward activity while increasing hypothalamus-pituitary adrenal axis activity, both of which may underlie the neurochemical mechanisms of the depression and anxiety-like effects induced by MCH. The effects of MCH(1) receptor antagonists in animal models, together with their rapid onset of effect and lack of adverse CNS effects, suggest that they deserve further investigation as potential new treatments for depression and anxiety disorders.

An mGluR2/3 antagonist, MGS0039, exerts antidepressant and anxiolytic effects in behavioral models in rats.

RATIONALE: Abnormalities of glutamatergic neurotransmission have been reportedly observed in psychiatric disorders. Previously, we demonstrated that (1R, 2R, 3R, 5R, 6R)-2-Amino-3-(3,4-dichlorobenzyloxy)-6-fluorobicyclo[3.1.0]hexane-2,6-dicarboxylic acid (MGS0039) is a selective antagonist for group II metabotropic glutamate receptors (mGluR2/3), and that it exerted antidepressant effects in some animal behavioral tests. OBJECTIVES: In the present study, we provide additional evidence that MGS0039 exhibits antidepressant and anxiolytic effects in experimental rodent models, which are predictive of clinical efficacy. METHODS: The learned helplessness (LH) paradigm, which is a common model used to examine the depressive state, was used to assess antidepressant effects of MGS0039. Moreover, anxiolytic effects of MGS0039 were investigated in the conditioned fear stress (CFS) model, which represents emotional abnormality, including anxiety. RESULTS: Intraperitoneal administration of MGS0039 (10 mg/kg) to rats for 7 days elicited a significant reduction in escape failures in the LH paradigm. In addition, rats treated with MGS0039 (2 mg/kg) showed significantly attenuated freezing behavior in a CFS model, indicating the anxiolytic-like potential of MGS0039. CONCLUSIONS: These results suggest that the blockade of mGluR2/3 with MGS0039 may be effective in the treatment of depressive and anxiety disorders.

A metabotropic glutamate 2/3 receptor antagonist, MGS0039, increases extracellular dopamine levels in the nucleus accumbens shell.

(1R,2R,3R,5R,6R)-2-Amino-3-(3,4-dichlorobenzyloxy)-6-fluorobicyclo[3.1.0]hexane-2,6-dicarboxylic acid (MGS0039), a potent and selective metabotropic glutamate 2/3 (mGlu 2/3) receptor antagonist, exhibits antidepressant-like activities in some animal models. In the present study, we examined the effect of MGS0039 on extracellular dopamine levels in the rat nucleus accumbens (NAc) shell using in vivo microdialysis evaluation because accumbal dopamine has been implicated in depression. Local application of MGS0039 into the NAc shell at 10 microM significantly increased extracellular dopamine levels in the NAc shell in freely moving rats. In contrast, local application of 10 microM of LY354740, an mGlu 2/3 receptor agonist, significantly decreased extracellular dopamine levels in the same brain region. These findings suggest that dopamine release in the NAc shell is regulated by mGlu 2/3 receptors, and that the effect on dopamine levels in the NAc shell may partially explain the antidepressant-like properties of mGlu 2/3 receptor antagonists.

The role of the DRY motif of human MC4R for receptor activation.

We constructed several mutant human MC4R cDNAs by site directed mutagenesis and expressed these receptors in COS-1 cells. The conserved DRY motif among GPCRs was mutated to generate eight mutants. While no MC4R ligand binding was detected in any of the mutants, one mutant, D146A, resulted in higher cAMP production in cells than the wild-type receptor without ligand stimulation.

MGS0039: a potent and selective group II metabotropic glutamate receptor antagonist with antidepressant-like activity.

The present study describes the pharmacological profile of (1R,2R,3R,5R,6R)-2-Amino-3-(3,4-dichlorobenzyloxy)-6-fluorobicyclo[3.1.0]hexane-2,6-dicarboxylic acid (MGS0039), a novel group II mGluR antagonist. MGS0039 showed high affinity for both mGluR2 (Ki = 2.2 nM) and mGluR3 (Ki = 4.5 nM), which are comparable to LY341495, another group II mGluR antagonist. MGS0039 attenuated both glutamate-induced inhibition of forskolin-evoked cyclic AMP formation in CHO cells expressing mGluR2 (IC50 = 20 nM) or mGluR3 (IC50 = 24 nM) and glutamate-increased [35S]GTPgammaS binding to mGluR2 (pA2 = 8.2), which means that MGS0039 acts as an antagonist. MGS0039 shifted the dose-response curve of glutamate-increased [35S]GTPgammaS binding rightward without altering the maximal response, and thereby indicating competitive antagonism. MGS0039 showed no significant effects on other mGluRs as well as the other receptors and transporters we studied. MGS0039 (0.3-3 mg/kg, i.p.) as well as LY341495 (0.1-3 mg/kg, i.p.) had dose-dependent antidepressant-like effects in the rat forced swim test and in the mouse tail suspension test. In contrast, MGS0039 (0.3-3 mg/kg, i.p.) had no apparent effect in the rat social interaction test and in the rat elevated plus-maze. These results indicate that MGS0039 is a potent and selective antagonist of group II mGluR, and that group II mGluR antagonists, like MGS0039, have an antidepressant-like potential in experimental animal models.

Increased cell proliferation in the adult mouse hippocampus following chronic administration of group II metabotropic glutamate receptor antagonist, MGS0039.

We have previously reported that MGS0039, a novel antagonist of group II metabotropic glutamate receptors (mGluRs), exerts antidepressant-like effects in experimental animal models. Recent studies suggest that the behavioral effects of chronic antidepressant treatment are mediated by the stimulation of neurogenesis in the hippocampus. In the present study, we examined the effects of MGS0039 on cell proliferation in the adult mouse hippocampus. MGS0039 (5 or 10mg/kg) or fluvoxamine was administered chronically to male ICR mice over a period of 14 days. Multiple bromodeoxyuridine (BrdU) administrations were performed after the last drug injection to label dividing cells. Immunohistochemical analyses after BrdU injections revealed that chronic MGS0039 treatment enhanced BrdU-positive cells in the dentate gyrus ( approximately 62% increase) in the same manner as chronic fluvoxamine treatment. This is the first in vivo study to demonstrate an increase in cell proliferation following a blockade of group II mGluRs. These findings raise the possibility that MGS0039 may exert antidepressant-like effects by modulating cell proliferation in the hippocampus.

Diversity of mouse proton-translocating ATPase: presence of multiple isoforms of the C, d and G subunits.

Vacuolar-type proton-translocating ATPases (V-ATPases), multimeric proton pumps, are involved in a wide variety of physiological processes. For their diverse functions, V-ATPases utilize a specific subunit isoform(s). Here, we reported the molecular cloning and characterization of three novel subunit isoforms, C2, d2 and G3, of mouse V-ATPase. These isoforms were expressed in a tissue-specific manner, in contrast to the ubiquitously expressed C1, d1 and G1 isoforms. C2 was expressed predominantly in lung and kidney, and d2 and G3 specifically in kidney. We introduced these isoforms into yeasts lacking the corresponding genes. Although the G3 and d2 did not rescue the vmaDelta phenotype, d1 and the two C isoforms functionally complemented the Deltavma6 and Deltavma5, respectively, indicating that they are bona fide subunits of V-ATPase.

Differential localization of the vacuolar H+ pump with G subunit isoforms (G1 and G2) in mouse neurons.

Vacuolar H(+)-ATPases (V-ATPases), a family of multimeric proton pumps, are involved in a wide variety of physiological processes. We have identified two mouse genes, Atp6g1 and Atp6g2, encoding the G1 and G2 isoforms of the V-ATPase G subunit, respectively. G1 was distributed ubiquitously in the tissues examined, whereas G2 was specifically distributed in central nervous system neurons. G1 was expressed at an early embryonic stage, whereas G2 transcription was significantly induced at 10.5 days postcoitus (embryonic day 10.5, i.e. 2 days before axon outgrowth). Both G1 and G2 were strongly expressed in cortical and hippocampal neurons, cerebellar granule cells, and Purkinje cells. Immunohistochemistry with isoform-specific antibodies revealed that G2 was localized in cell bodies, dendrites, and axons. In addition, electron microscopy and subcellular fractionation indicated that G2 was localized in synaptic vesicles, whereas G1 was not detectable. G1 and G2 exhibit 62% identity, and both isoforms were immunoprecipitated with the c and A subunits of V-ATPase. G2 could complement the yeast deletion mutant Deltavma10, which lacks the G subunit. The V-ATPases containing the G1 and G2 isoforms, respectively, showed similar K(m)((ATP)) values and maximal velocity. These results indicate that G1 and G2 are bona fide subunits of V-ATPases and that the enzyme with the G2 isoform is involved in synaptic vesicle acidification.