Atsuro Nakazato

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.

NE-100, a novel sigma receptor ligand: In vivo tests

It has been suggested that sigma receptor antagonists may be useful as antipsychotic drugs. N, N-dipropyl-2-[4-methoxy-3-(2-phenylethoxy)phenyl]-ethylamine monohydrochloride (NE-100) is a novel compound with high affinity for the sigma receptor (IC50 = 4.16 nM), but low affinity (IC50 > 10,000 nM) for D1, D2, 5-HT1A, 5-HT2 and phencyclidine (PCP) receptors. The head-weaving behavior induced by either (+)SKF10047 or PCP was dose-dependently antagonized by NE-100 with oral ED50 at 0.27 and 0.12 mg/kg, respectively. NE-100 did not affect dopamine agonists-induced stereotyped behavior and/or hyperactivity. NE-100 failed to induce catalepsy in rats. These findings indicate that NE-100 may have antipsychotic activity without the liability of motor side effects typical of neuroleptics.

Neuropharmacological profile of peripheral benzodiazepine receptor agonists, DAA1097 and DAA1106.

Receptor binding and behavioral profiles of N-(4-chloro-2-phenoxyphenyl)-N-(2-isopropoxybenzyl)acetamide (DAA1097) and N-(2,5-dimethoxybenzyl)-N-(5-fluoro-2-phenoxyphenyl)acetamide (DAA1106), novel, selective agonists for the peripheral benzodiazepine receptor (PBR) were examined. DAA1097 and DAA1106 inhibited [3H]PK 11195 binding to crude mitochondrial preparations of rat whole brain, with IC50 values of 0.92 and 0.28 nM. Likewise, DAA1097 and DAA1106 inhibited [3H]Ro 5-4864 binding to the same mitochondrial preparation, with IC50 values of 0.64 and 0.21 nM. In contrast, DAA1097 and DAA1106 did not inhibit [3H]-flunitrazepam, the central benzodiazepine receptor (CBR) ligand, binding to membranes of rat whole brain (IC50>10,000nM). Oral administration of DAA1097 and DAA1106 had anxiolytic effects in the mouse light/dark exploration test and in the rat elevated plus- maze test. Oral administration of DAA1106, diazepam and buspirone but not DAA1097 significantly increased sleeping time in hexobarbital-induced anesthesia in mice. The order of potency of potentiation of hexobarbital anesthesia was diazepam> buspirone> DAA1106> DAA1097. Oral administration of DAA1097 and DAA1106 but not diazepam and buspirone did not affect spontaneous locomotor activity in mice. These findings indicate that DAA1097 and DAA1106 are PBR selective ligands with potent anxiolytic-like properties, in laboratory animals.

Binding characteristics of [3H]DAA1106, a novel and selective ligand for peripheral benzodiazepine receptors

Here, we investigated the binding characteristics of [3H]N-(2,5-dimethoxybenzyl)-N-(5-fluoro-2-phenoxyphenyl)acetamide ([3H]DAA1106), a potent and selective ligand for peripheral benzodiazepine receptors, in mitochondrial fractions of the rat brain. [3H]DAA1106 bound to the mitochondrial fraction of the rat brain in a saturable manner. The dissociation constant (Kd) and maximal number of binding sites (Bmax) obtained from Scatchard plot analysis of the saturation curve of [3H]DAA1106 binding were 0.12 +/- 0.03 nM and 161.03 +/- 5.80 fmol/mg protein, respectively. [3H]DAA1106 binding to mitochondrial preparations of the rat cerebral cortex was inhibited by several peripheral benzodiapine receptor ligands, and DAA1106 was the most potent inhibitor in inhibiting [3H]DAA1106 binding among the peripheral benzodiazepine receptor ligands we tested. The binding of [3H]DAA1106 was not affected by several neurotransmitter-related compounds, including adrenoceptor, gamma-aminobutyric acid (GABA), dopamine, 5-hydroxytryptamine (5-HT), acetylcholine, histamine, glutamate and central benzodiazepine receptor ligands even at a concentration of 10 microM. In the cerebral cortex of rhesus monkeys, DAA1106 and 1-(2-chlorophenyl)-N-methyl-(1-methylpropyl)-3-isoquinoline carboxamide (PK11195) potently inhibited [3H]DAA1106 binding, while 7-chloro-5-(4-chlorophenyl)-1-methyl-1,3-dihydrobenzo[e][1,4]diazepin -2-one (Ro5-4864) did not. The highest [3H]DAA1106 binding was observed in the olfactory bulb, followed by the cerebellum. In autoradiographic studies, practically the same results were obtained, in that the highest binding of [3H]DAA1106 was in the olfactory bulb. Potent labeling was also noted in ventricular structures such as the choroid plexus. Thus, [3H]DAA1106 is a potent and selective ligand for peripheral benzodiazepine receptors and should prove useful for elucidating the physiological relevance of events mediated through peripheral benzodiazepine receptors.

Recent advances in feeding suppressing agents: potential therapeutic strategy for the treatment of obesity

Obesity is an increasingly prevalent health problem for which no ideal treatments are available, especially for long-term maintenance of body loss. The only approved drugs are centrally acting appetite suppressants that modulate monoamine neurotransmitters in the brain. Since the discovery of leptin, and the fact it modulates neuropeptide systems in the CNS, attention has shifted from amines to peptides. Many researchers have focused on these neuropeptide systems in the CNS and their involvement in energy balance. With an effort to identify molecules involved in feeding behaviour and energy expenditure in the brain, especially in the hypothalamic nuclei, a series of novel potential targets for the development of anti-obesity agents, as well as potential drug candidates, are being given attention. Many pharmaceutical companies have large programmes directed at the development of new modulators of neuropeptide receptors, antagonists of appetite enhancing peptides and agonists of appetite suppressing peptides, including neuropeptide Y receptors and melanocortin receptors. This review covers recent targets for drug discovery of anti-obesity agents and patents focusing on the receptors.

Neuropharmacological profiles of antagonists of group II metabotropic glutamate receptors

Glutamatergic abnormalities play roles in several psychiatric disorders. Glutamate acts at two classes of receptors, ionotropic and metabotropic glutamate receptors (mGluR), the latter is classified into three group, based on receptor homology and signaling mechanisms. Among them, recent pharmacological and histochemical studies suggest that the group II mGluR (mGluR2 and mGluR3) plays crucial roles in the control of emotional states. We previously reported that MGS0039, a selective group II mGluR antagonist, exhibited dose-dependent antidepressant-like effects in some animal models. However, the mechanism by which group II mGluR antagonists exhibit such effects is still unclear. In the present two studies, we examined neuropharmacological effects of group II mGluR antagonists on monoaminergic neurons. In an electrophysiological study, MGS0039 dose-dependently and significantly increased the firing rate of dorsal raphe nucleus (DRN) serotonergic neurons. LY341495, another group II mGluR antagonist, also increased DRN serotonergic neural activity significantly. Consistent with the findings of this electrophysiological study, MGS0039 significantly increased extracellular level of serotonin in rat medial prefrontal cortex in a microdialysis study. In contrast, MGS0039 had no effect on the activity of locus coeruleus noradrenergic neurons. These findings suggest that modulation of serotonergic neuron might be, at least in part, responsible for the antidepressant-like effects of group II mGluR antagonists.

Amino acid residues in the transmembrane domain of the type 1 sigma receptor critical for ligand binding

The type 1 sigma receptor expressed in Xenopus oocytes showed binding abilities for the sigma‐1 ligands, [3H](+)pentazocine and [3H]NE‐100, with similar kinetic properties as observed in native tissue membranes. Amino acid substitutions (Ser99Ala, Tyr103Phe and di‐Leu105,106di‐Ala) in the transmembrane domain did not alter the expression levels of the type 1 sigma receptor as determined by immunoblot analysis using an anti‐type 1 sigma receptor antiserum. By contrast, ligand binding was significantly suppressed by the substitutions. These findings provide evidence that the transmembrane domain of the type 1 sigma receptor plays a critical role in ligand binding of this receptor.

Metabotropic glutamate 2/3 receptor antagonists improve behavioral and prefrontal dopaminergic alterations in the chronic corticosterone-induced depression model in mice.

Metabotropic glutamate 2/3 (mGlu2/3) receptor antagonists have an antidepressant-like effect, but the exact mechanism still remains unclear. This study examined the effects of mGlu2/3 receptor antagonists in chronic corticosterone-treated mice which could be used as an animal model of depression. In the forced swim test, the mGlu2/3 receptor antagonists MGS0039 (1.0 mg/kg, i.p.) and LY341495 (0.3 mg/kg, i.p) significantly reduced the increased immobility time of mice pretreated with corticosterone (20 mg/kg, s.c.) for 21 days, while desipramine (30 mg/kg, i.p.) and fluoxetine (30 mg/kg, i.p.) did not. The antidepressant-like effect of LY341495 was not blocked by the α-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) receptor antagonist NBQX (10 mg/kg, i.p.). Systemic administration of LY341495 did not affect basal release of glutamate, dopamine or serotonin in the prefrontal cortex of the control or chronic corticosterone-treated mice. Chronic corticosterone markedly enhanced high K(+)-induced release of dopamine, but not serotonin or glutamate, in the prefrontal cortex. This neurochemical change was blocked by systemic administration of MGS0039 and LY341495, but not desipramine or fluoxetine. These results suggest that chronic corticosterone-treated mice could be used as an animal model of treatment-resistant depression. This study also suggests that the prefrontal dopaminergic system is involved in the antidepressant-like effect of mGlu2/3 receptor antagonists in the chronic corticosterone-induced depression model.

Effect of NE-100, a novel sigma receptor ligand, on phencyclidine-induced delayed cognitive dysfunction in rats

Phencyclidine (PCP), in a dose of 15 mg/kg, produced delayed cognitive dysfunction (at 24 h) in rats subjected to water maze tasks. At 24 h after PCP administration, ataxia, hyperlocomotion and stereotyped behavior were not induced. NE-100, N,N-dipropyl-2-[4-methoxy-3-(2-phenylethoxy)phenyl]-enthylamine monohydrochloride, a selective and potent sigma receptor ligand, was administered orally 10 min after PCP administration or 15 min before the first trial (24 h after PCP administration). In both cases, NE-100 dose-dependently attenuated the delayed cognitive dysfunction induced by PCP. As these findings show that ingestion of PCP led to delayed cognitive dysfunction similar to the cognitive signs of psychosis seen in humans, NE-100 is being further studied for possible treatment of subjects with schizophrenia.

The atypical antipsychotic profile of NRA0045, a novel dopamine D4 and 5-hydroxytryptamine2A receptor antagonist, in rats

The atypical antipsychotic profile of (R)‐(+)‐2‐amino‐4‐(4‐fluorophenyl)‐5‐[1‐[4‐(4‐fluorophenyl)‐4‐oxobutyl] pyrrolidin‐3‐yl] thiazole (NRA0045), a potent dopamine D4 and 5‐hydroxytryptamine (5‐HT)2A receptor antagonist, was examined in rats. Spontaneous locomotor activity was decreased dose‐dependently with i.p. administration of clozapine (ED50 3.7 mg kg−1), haloperidol (ED50 0.1 mg kg−1) and chlorpromazine (ED50 0.9 mg kg−1), whereas inhibition of this type of behaviour induced by i.p. administration of NRA0045, at doses up to 10 mg kg−1, did not exceed 50%. Locomotor hyperactivity induced by methamphetamine (MAP, 2 mg kg−1, i.p.) in rats (a model of antipsychotic activity) was dose‐dependently antagonized by NRA0045 (ED50 0.4 mg kg−1, i.p., and 0.3 mg kg−1, p.o., respectively), clozapine (ED50 0.3 mg kg−1, i.p. and 0.8 mg kg−1, p.o., respectively), haloperidol (ED50 0.02 mg kg−1, i.p. and 0.1 mg kg−1, p.o., respectively), chlorpromazine (ED50 0.3 mg kg−1, i.p. and 3.3 mg kg−1, p.o., respectively). In contrast, the MAP (3 mg kg−1, i.v.)‐induced stereotyped behaviour in rats (a model of extrapyramidal symptoms) was not affected by NRA0045 or clozapine, at the highest dose given (30 mg kg−1, i.p.). Haloperidol (ED50 0.3 mg kg−1, i.p.) and chlorpromazine (ED50 4.8 mg kg−1, i.p.) strongly blocked the MAP‐induced stereotyped behaviour. NRA0045 and clozapine selectively blocked behaviour associated with activation of the mesolimbic/mesocortical dopamine neurones rather than nigrostriatal dopamine neurones. Extracellular single‐unit recording studies demonstrated that MAP (1 mg kg−1, i.v.) decreased the firing rate in the substantia nigra (A9) and ventral tegmental area (A10) dopamine neurones in anaesthetized rats. NRA0045 completely reversed the inhibitory effects of MAP on A10 dopamine neurones (ED50 0.1 mg kg−1, i.v.), whereas the inhibitory effects of MAP on A9 dopamine neurones were not affected by NRA0045, in doses up to 1 mg kg−1 (i.v.). Clozapine completely reversed the inhibitory effects of MAP on A10 dopamine neurones (ED50 1.9 mg kg−1, i.v.) and on A9 dopamine neurones (ED50 2.5 mg kg−1, i.v.). Haloperidol completely reversed the inhibitory effects of MAP on A10 (ED50 0.03 mg kg−1, i.v.) and on A9 dopamine neurones (0.02 mg kg−1, i.v.). NRA0045, like clozapine, was more potent in reversing the effects of MAP on A10 than A9 dopamine neurones. Prepulse inhibition (PPI) is impaired markedly in humans with schizophrenia. The disruption of PPI in rats by apomorphine (0.5 mg kg−1, s.c.) was reversed significantly by NRA0045 (3 mg kg−1, i.p.), clozapine (3 mg kg−1, i.p.) and haloperidol (0.3 mg kg−1, i.p.). Phencyclidine (PCP) elicits predominantly psychotic symptoms in normal humans and in schizophrenics. NRA0045 (0.03–0.3 mg kg−1, i.p.) and clozapine (0.1–1 mg kg−1, i.p.) significantly and dose‐dependently shortened the PCP(1.25 mg kg−1, i.p.)‐induced prolonged swimming latency in rats in a water maze task, whereas haloperidol (0.01–0.1 mg kg−1, i.p.) did not significantly alter swimming latency. These findings suggest that NRA0045 may have unique antipsychotic activities without the liability of motor side effects typical of classical antipsychotics.