The antipsychotic-like effects of positive allosteric modulators of metabotropic glutamate mGlu4 receptors in rodents

Background and Purpose

Because agonists at metabotropic glutamate receptors exert beneficial effects in schizophrenia, we have assessed the actions of Lu AF21934 and Lu AF32615, two chemically distinct, selective and brain-penetrant positive allosteric modulators (PAMs) of the mGlu₄ receptor, in several tests reflecting positive, negative and cognitive symptoms of schizophrenia in rodents.

Experimental Approach

Hyperactivity induced by MK-801 or amphetamine and head twitches induced by 2,5-dimethoxy-4-iodoamphetamine (DOI) in mice were used as models for positive symptoms. Disruption of social interaction and spatial delayed alternation tests induced by MK-801 in rats were used as models for negative and cognitive symptoms of schizophrenia, respectively.

Key Results

Lu AF21934 (0.1–5 mg·kg⁻¹) and Lu AF32615 (2–10 mg·kg⁻¹) dose-dependently inhibited hyperactivity induced by MK-801 or amphetamine. They also antagonized head twitches and increased frequency of spontaneous excitatory postsynaptic currents (EPSCs) in brain slices, induced by DOI. In mice lacking the mGlu₄ receptor (mGlu₄^−/−) mice, Lu AF21934 did not antagonize DOI-induced head twitches. MK-801-induced disruption in the social interaction test was decreased by Lu AF21934 at 0.5 mg·kg⁻¹ and by Lu AF32615 at 10 mg·kg⁻¹. In the delayed spatial alternation test, Lu AF21934 was active at 1 and 2 mg·kg⁻¹, while Lu AF32615 was active at 10 mg·kg⁻¹.

Conclusions and Implications

We propose that activation by PAMs of the mGlu₄ receptor is a promising approach to the discovery of novel antipsychotic drugs.     

Ⅱ、Ⅲ组亲代谢型谷氨酸受体激动剂逆转1-甲基-4-苯基吡啶离子抑制星形胶质细胞摄取谷氨酸

目的 :探讨Ⅱ、Ⅲ组亲代谢型谷氨酸受体(metabotropicglutamatereceptors ,mGluRs)激动剂对 1 甲基 4 苯基吡啶离子 (1 methyl 4 phenylpyridinium ,MPP )抑制星形胶质细胞摄取谷氨酸 (Glu)的影响。方法 :应用同位素标记法测定星形胶质细胞对培养液中 [3 H] D ,L 谷氨酸的摄取 ,应用四氮唑 (MTT)比色法检测星形胶质细胞的活性。结果 :MPP 15 0、2 0 0 μmol·L-1可以明显抑制星形胶质细胞摄取Glu ,抑制率达 5 8.3%和 70 .1% ,但并不影响细胞活性 ;Ⅱ组mGluRs激动剂 (2S ,2’R ,3’R) 2 (2’ ,3’ dicar boxycyclopropyl)glycine (DCG IV ) 0 .1、 1、 10 ,10 0 μmol·L-1和Ⅲ组mGluRs激动剂L( ) 2 amino 4 phosphonobutyricacid (L AP4 ) 1、10、10 0 μmol·L-1可以逆转MPP 对星形胶质细胞摄取Glu的抑制作用。结论 :MPP 抑制星形胶质细胞摄取Glu可能与直接影响谷氨酸转运体 (GluTs)的功能有关 ,激活星形胶质细胞上的Ⅱ、Ⅲ组mGluRs可以通过促进GluTs摄取Glu、进而降低细胞外液的Glu浓度而发挥神经保护作用。     

代谢型谷氨酸受体配体(s)-4C3HPG对红藻氨酸诱导性癫痫发作活动的调节研究

目的　探讨代谢型谷氨酸受体配体 (s) 4 羧基 3 羟苯基甘氨酸〔(s) 4C3HPG〕对红藻氨酸 (KA)诱导性癫痫发作活动的调节作用。方法　选用KA诱导大鼠复杂部分性发作模型 ,采用侧脑室注射 (s) 4C3HPG法 ,观察其对大鼠癫痫发作行为学、脑电图放电时间以及对大鼠海马的组织病理学影响。结果　 (s) 4C3HPG +KA组大鼠行为学评分与盐水 +KA组相比明显减少 ,在KA注射 40min之后差异有显著性 (P <0 0 1) ,湿狗样运动 (WDS)数目也减少 ,于KA注射后 40、6 0、80、10 0、12 0min差异有显著性 (P <0 0 1)。 (s) 4C3HPG +KA组大鼠EEG虽有癫痫样波发放 ,但放电持续时间较对照组缩短 ,在KA注射 30min之后差异有显著性 (P<0 0 1)。(s) 4C3HPG +KA组大鼠海马CA1、CA3区正常细胞数较对照组大鼠增多 (P <0 0 1) ,死亡或损伤细胞数要减少 (P <0 0 1)。结论　 (s) 4C3HPG可减轻KA诱导性癫痫发作的程度 ,具有神经保护性作用     

Activation of metabotropic glutamate (mGlu)2 receptors suppresses histamine release in limbic brain regions following acute ketamine challenge

In the present study we demonstrated that ketamine, an NMDA antagonist and possible psychotomimetic, increases extracellular histamine (HA) in the rat brain. We then examined the ability of the group II mGlu receptor agonist LY379268 to modulate the ketamine evoked increases in HA release in three limbic brain regions. Ketamine (25 mg/kg) increased HA in the medial prefrontal cortex (mPFC), ventral hippocampus (vHipp) and the nucleus accumbens (NAc) shell. LY379268 administered alone was without effect on basal HA efflux in the mPFC or vHipp but modestly decreased HA efflux in the NAc shell. Administration of LY379268 (3 and 10 mg/kg) prior to ketamine significantly attenuated the HA response in the mPFC, vHipp and the NAc shell. The inhibitory effects of LY379268 in the mPFC were mimicked by the systemic administration of the mGlu2 receptor positive allosteric modulator CBiPES (60 mg/kg). Finally, local perfusion experiments revealed that the effects of LY379268 on ketamine evoked HA efflux appear to be mediated by mGlu2 receptors outside the PFC as the intra-mPFC perfusion of LY379268 (100 μM or 300 μM) failed to attenuate ketamine evoked increases in HA efflux. Together, these novel observations reveal an effect of ketamine on histaminergic transmission in limbic brain areas and provide further insight into the possible antipsychotic mechanism of action of mGlu2/3 receptor agonists.     

Pharmacological characterization of stress-induced hyperthermia in DBA/2 mice using metabotropic and ionotropic glutamate receptor ligands

Rationale Accumulating evidence suggests that drugs acting on the glutamatergic system may represent promising novel therapeutic targets for the treatment of anxiety disorders. The stress-induced hyperthermia paradigm has been used widely to model some of the physiological symptoms associated with anxiety disorders and has produced results that are predictive of clinical efficacy. We have modified this paradigm to measure the autonomic consequences of stress induced by the fear of predation in mice.Objective To evaluate the efficacy of several classes of metabotropic and ionotropic glutamate receptor ligands, as well as known anxiolytics and psychotropic comparators, in attenuating predatory-stress-induced hyperthermia.Methods Male DBA/2 mice were implanted with radiotelemetric transmitters in the peritoneal cavity to measure stress-related increases in core body temperature, following placement in a novel cage containing soiled rat shavings.Results Clinically active compounds such as chlordiazepoxide (5–10 mg/kg), alprazolam (0.3–3 mg/kg), and buspirone (10–30 mg/kg) exhibited an anxiolytic profile. Assessment of glutamatergic agents indicated that the mGlu1 receptor antagonist LY456236 (10–30 mg/kg), mGlu5 receptor antagonist MPEP (10–30 mg/kg), mGlu2/3 receptor agonist LY354740 (3–10 mg/kg), mGlu2 receptor potentiator LY566332 (30 and 100 mg/kg), mGlu8 receptor agonist (S)-3,4-dicarboxyphenylglycine (30–60 mg/kg), competitive NMDA receptor antagonist LY235959 (1 mg/kg), AMPA receptor antagonist GYKI-52466 (10–20 mg/kg), and glycine transporter-1 (GlyT-1) inhibitor ALX-5407 (3–10 mg/kg) dose-dependently attenuated stress-induced hyperthermia. The AMPA receptor potentiator LY451646, iGlu5 kainate receptor antagonist LY382884, glycine_B receptor partial agonist d-cycloserine, and GlyT-1 inhibitor ORG-24461 were ineffective in this model.Conclusion Select metabotropic and ionotropic glutamate receptor ligands exhibited an anxiolytic profile, as measured by the attenuation of stress-induced hyperthermia, and may represent viable targets for the development of pharmacological treatments for anxiety-related disorders.     

Opportunities and challenges in drug discovery targeting metabotropic glutamate receptor 4

Introduction: Until recently, metabotropic glutamate receptor 4 (mGlu4) has not received adequate attention in terms of drug targeting when compared to other members of the same mGlu receptor family, possibly because of the difficulties encountered in developing highly selective, either orthosteric or allosteric, ligands for this receptor.

Areas covered: This review gives to discussion to the past and recent advances (between 2012–2017) in targeting the mGlu4 receptor for the treatment of disorders of the central nervous system (CNS) as well as immunological (neuroinflammation) and metabolic diseases (diabetes). Chemical structures, properties, and pharmacological properties discussed herein were retrieved from the scientific literature databases, PubMed and Google Scholar.

Expert opinion: The fertile field of mGlu receptor positive allosteric modulators (PAMs) has recently led to the discovery of foliglurax, a highly selective mGlu4 receptor PAM with optimal bioavailability after oral administration and excellent brain distribution. However, further elucidation of the biological properties of the mGlu4 receptor, including expression and its signalling profile in distinct tissues and cells are still awaited in order to establish the mGlu4 receptor as a definite drug target in several CNS and non-CNS diseases.     

AMPA and GABA(B) receptor antagonists and their interaction in rats with a genetic form of absence epilepsy.

The effects of combined and single administration of the -amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor antagonist, 7,8-methylenedioxy-1-(4-aminophenyl)-4-methyl-3-acetyl-4,5-dihydro-2,3-benzodiazepine (LY 300164), and of the GABA_B receptor antagonist γ-aminopropyl-n-butyl-phosphinic acid (CGP 36742), on spontaneously occurring spike-wave discharges were investigated in WAG/Rij rats. LY 300164 had minor effects; only the highest dose (16 mg/kg) reduced the number of spike-wave discharges in a short time window. CGP 36742 was more effective as it significantly reduced the number of spike-wave discharges and shortened their duration at the doses of 25 and 100 mg/kg. The ED₅₀ values for the inhibition of spike-wave discharges by LY 300164 and CGP 36742 in a time window 30–60 min after injection were 15.5 and 16.6 mg/kg, respectively. The ED₅₀ of CGP 36742 was reduced to 8.0 mg/kg when this antagonist was administered in combination with LY 300164 (6 mg/kg). The interaction between the two antagonists appeared to be additive according to isobolographic analysis. Importantly, CGP 36742 and LY 300164 administered either alone or in combination had no apparent effects on behavior. These results may provide information for a rational approach to polytherapy for the treatment of generalized absence epilepsy.     

The novel and systemically active metabotropic glutamate 1 (mGlu1) receptor antagonist 3-MATIDA reduces post-ischemic neuronal death

We examined the pharmacological properties of 3-methyl-aminothiophene dicarboxylic acid (3-MATIDA) by measuring second messenger responses in baby hamster kidney cells stably transfected with mGlu1a, mGlu2, mGlu4a or mGlu5a receptors and ionotropic glutamate receptor agonist-induced depolarizations in mouse cortical wedges. 3-MATIDA was a potent (IC(50)=6.3 microM, 95% confidence limits 3-15) and relatively selective mGlu1 receptor antagonist. When tested on mGlu2, mGlu4 or mGlu5 receptors its IC(50) was >300 microM. When tested in cortical wedges, however, 3-MATIDA was also able to antagonize AMPA or NMDA responses with an IC(50) of 250 microM. When present in the incubation medium of cultured murine cortical cells, 3-MATIDA (1-100 microM) significantly reduced the death of neurons induced by 60 min of oxygen and glucose deprivation (OGD), even when added up to 60 min after OGD. A similar neuroprotective activity was observed when 3-MATIDA was present at 10-100 microM in the medium of rat organotypic hippocampal slice cultures exposed to 30 min OGD. Systemic administration of 3-MATIDA (3-10 mg/kg, immediately and 1 h after the onset of ischemia) reduced the volume of brain infarcts following permanent middle cerebral artery occlusion in rats. Our results show that 3-MATIDA is a potent and possibly selective mGlu 1 receptor antagonist that may be considered as a novel prototype neuroprotective agent.     

Allosteric modulation of the group III mGlu(4) receptor provides functional neuroprotection in the 6-hydroxydopamine rat model of Parkinson's disease

BACKGROUND AND PURPOSE

We recently reported that broad spectrum agonist-induced activation of presynaptic group III metabotropic glutamate (mGlu) receptors within the substantia nigra pars compacta using L-2-amino-4-phosphonobutyrate provided functional neuroprotection in the 6-hydroxydopamine lesion rat model of Parkinson''s disease. The aim of this study was to establish whether selective activation of the mGlu₄ receptor alone could afford similar functional neuroprotection.

EXPERIMENTAL APPROACH

The neuroprotective effects of 8 days of supranigral treatment with a positive allosteric modulator of mGlu₄ receptors, (+/−)-cis-2-(3,5-dichlorphenylcarbamoyl)cyclohexanecarboxylic acid (VU0155041), were investigated in rats with unilateral 6-hydroxydopamine lesions. The effects of VU0155041 treatment on motor function were assessed using both habitual (cylinder test) and forced (adjusted stepping, amphetamine-induced rotations) behavioural tests. Nigrostriatal tract integrity was examined by analysis of tyrosine hydroxylase, dopa decarboxylase or dopamine levels in the striatum and tyrosine hydroxylase-positive cell counts in the substantia nigra pars compacta.

KEY RESULTS

VU0155041 provided around 40% histological protection against a unilateral 6-hydroxydopamine lesion as well as significant preservation of motor function. These effects were inhibited by pre-treatment with (RS)-α-cyclopropyl-4-phosphonophenylglycine, confirming a receptor-mediated response. Reduced levels of inflammatory markers were also evident in the brains of VU0155041-treated animals.

CONCLUSIONS AND IMPLICATIONS

Allosteric potentiation of mGlu₄ receptors in the substantia nigra pars compacta provided neuroprotective effects in the 6-hydroxydopamine rat model A reduced inflammatory response may contribute, in part, to this action. In addition to the reported symptomatic effects, activation of mGlu₄ receptors may also offer a novel approach for slowing the progressive degeneration observed in Parkinson''s disease.     

Activation of Group II and Group III metabotropic glutamate receptors by endogenous ligand(s) and the modulation of synaptic transmission in the superficial superior colliculus

Previous work from this laboratory indicates that Group II/III metabotropic glutamate (mGlu) receptors modulate responses of SC neurones to visual stimuli in vivo. It is thought that tonic levels of glutamate may be sufficient to activate some mGlu receptors. We wished to investigate if these receptors are activated under ambient conditions in SC. Field excitatory postsynaptic potentials (fEPSPs) evoked by optic tract stimulation were recorded from 300 microm slices of the adult pigmented rat superior colliculus at 34 degrees C. The Group II receptor selective agonist LY354740 (100-300 nM) had no significant effect on the peak amplitude of the fEPSP, although it did enhance the late phase of the fEPSP. In order to test for activation of Group II receptors by endogenous ligand, the selective antagonists LY341495 (50 nM) or EGLU (200 microM) were applied: these either enhanced or reduced the fEPSP amplitude. In similar experiments carried out at 22 degrees C, no effect was seen. The fEPSP enhancements, but not the fEPSP reductions, could be occluded by GABA antagonists. Application of higher concentrations of LY341495 (300, 600 nM-known to also affect Group III receptors, particularly mGlu8), or co-application of 50 nM LY341495 and the Group III-selective antagonist CPPG (100 microM) produced enhancements of responses, or counteracted response reductions over those seen with 50 nM LY341495 alone. The predominant Group II receptor in SC is mGlu3. It is known that this can be located presynaptically on GABAergic and glutamatergic terminals, postsynaptically, and on glia. Our results indicate that such receptors are tonically activated by endogenous transmitter, have distinct effects, and influence retino-collicular transmission. Furthermore, there is a segregation of effects where receptors exert some of their effects via modulation of GABAergic circuitry.     

Positive allosteric modulation of the human metabotropic glutamate receptor 4 (hmGluR4) by SIB-1893 and MPEP

We have identified 2-methyl-6-(2-phenylethenyl)pyridine (SIB-1893) and 2-methyl-6-phenylethynyl pyridine hydrochloride (MPEP) as positive allosteric modulators for the hmGluR4. SIB-1893 and MPEP enhanced the potency and efficacy of L-2-amino-4-phophonobutyrate (L-AP4) in guanosine 5'-O-(3-[(35)S]thiotriphosphate ([(35)S]GTPgammaS) binding and efficacy in cAMP studies. These effects were fully blocked by the mGluR4 competitive antagonist (RS)-alpha-cyclopropyl-4-phosphonophenylglycine (CPPG), indicating a dependency on receptor activation. Although SIB-1893 and MPEP had no effects alone in GTPgammaS binding, effects were observed in the cell-based cAMP assay due to media-derived activation as indicated by CPPG inhibition. Positive modulation of the mGluR4 was a receptor-specific effect since SIB-1893 and MPEP had neither effects on mGluR2-expressing cells nor on the parent BHK cell line. In [(3)H]L-AP4 binding, a two-fold decrease in K(D) but not in B(max) was observed with 100 micro M SIB-1893, whereas MPEP affected neither parameter. Finally, SIB-1893 and MPEP failed to displace [(3)H]L-AP4 binding. Taken together, these data identify positive allosteric modulators for the hmGluR4.     

Pharmacology of (S)-homoquisqualic acid and (S)-2-amino-5-phosphonopentanoic acid [(S)-AP5] at cloned metabotropic glutamate receptors

1. In this study we have determined the pharmacological profile of (S)-quisqualic acid, (S)-2-amino-4-phosphonobutyric acid ((S)-AP4) and their higher homologues (S)-homoquisqualic acid, (S)-2-amino-5-phosphonopentanoic acid ((S)-AP5), respectively, and (R)-AP5 at subtypes of metabotropic (S)-glutamic acid (mGlu) receptors expressed in Chinese hamster ovary cells.
2. (S)-Quisqualic acid was a potent mGlu₁/mGlu₅ agonist (EC₅₀ values of 1.1 μM and 0.055 μM, respectively) showing no activity at mGlu₂ and weak agonism at mGlu₄ (EC₅₀∼1000 μM).
3. (S)-Homoquisqualic acid displayed competitive antagonism at mGlu₁ (K_B=184 μM) and full agonism at mGlu₅ (EC₅₀=36 μM) and mGlu₂ (EC₅₀=23 μM), but was inactive at mGlu₄.
4. (S)-AP4 was a potent and selective mGlu₄ agonist (EC₅₀=0.91 μM) being inactive at mGlu₁, mGlu₂ and mGlu₅ both as agonist and antagonist.
5. (S)-AP5 displayed very weak agonist activity at mGlu₄. At the mGlu₂ receptor subtype (S)-AP5 acted as a competitive antagonist (K_B=205 μM), whereas the compound was inactive at mGlu₁ and mGlu₅. (R)-AP5 was inactive at all mGlu receptor subtypes tested both as agonist and antagonist.
6. These studies demonstrate that incorporation of an additional carbon atom into the backbone of (S)-glutamic acid and its analogues, to give the corresponding homologues, and replacement of the terminal carboxyl groups by isosteric acidic groups have profound effects on the pharmacological profiles at mGlu receptor subtypes. Furthermore, (S)-homoquisqualic acid has been shown to be a potentially useful tool for differentiating mGlu₁ and mGlu₅.

     

Antiabsence effects of carbenoxolone in two genetic animal models of absence epilepsy (WAG/Rij rats and lh/lh mice)

Carbenoxolone (CBX), the succinyl ester of glycyrrhetinic acid, is an inhibitor of gap junctional intercellular communication. We have tested its possible effects upon two genetic animal models of epilepsy (WAG/Rij rats and lethargic (lh/lh) mice). Systemic administration of CBX was unable to significantly affect the occurrence of absence seizures in WAG/Rij rats. In particular, intravenous (5-40 mg/kg) or intraperitoneal (i.p.; 10-80 mg/kg) administration of CBX was unable to significantly modify the number and duration of spike-wave discharges (SWDs) in WAG/Rij rats, whereas the bilateral microinjection (0.05, 0.1, 0.5 and 1 microg/0.5 microl) of CBX into nucleus reticularis thalami (NRT) and nucleus ventralis posterolateralis (VPL) thalami produced a decrease in the duration and the number of SWDs. Bilateral microinjection of CBX into nucleus ventroposteromedial (VPM) thalami did not produce any significant decrease in the number and duration of SWDs. On the contrary, i.p. (5-40 mg/kg) or intracerebroventricular (0.5, 1, 2 and 4 microg/2 microl) administration of CBX in lh/lh mice induced a marked decrease in the number and duration of SWDs in a dose-dependent manner. At the doses used no movement disorders, or other behavioural changes, were recorded in both WAG/Rij rats and lh/lh mice. No effects were observed in both animal models following systemic or focal administration of glycyrrhizin into the same brain areas where CBX was shown to be effective.     

Symptomatic and neuroprotective effects following activation of nigral group III metabotropic glutamate receptors in rodent models of Parkinson's disease

Background and purpose:

Increased glutamatergic innervation of the substantia nigra pars reticulata (SNpr) and pars compacta (SNpc) may contribute to the motor deficits and neurodegeneration, respectively, in Parkinson''s disease (PD). This study aimed to establish whether activation of pre-synaptic group III metabotropic glutamate (mGlu) receptors reduced glutamate release in the SN, and provided symptomatic or neuroprotective relief in animal models of PD.

Experimental approach:

Broad-spectrum group III mGlu receptor agonists, O-phospho-l-serine (l-SOP) and l-2-amino-4-phosphonobutyrate (l-AP4), were assessed for their ability to inhibit KCl-evoked [³H]-d-aspartate release in rat nigral prisms or inhibit KCl-evoked endogenous glutamate release in the SNpr in vivo using microdialysis. Reversal of akinesia in reserpine-treated rats was assessed following intranigral injection of l-SOP and l-AP4. Finally, the neuroprotective effect of 7 days'' supra-nigral treatment with l-AP4 was examined in 6-hydroxydopamine (6-OHDA)-lesioned rats.

Key results:

l-SOP and l-AP4 inhibited [³H]-d-aspartate release by 33 and 44% respectively. These effects were blocked by the selective group III mGlu antagonist (RS)-α-cyclopropyl-4-phosphonophenylglycine (CPPG). l-SOP also reduced glutamate release in the SNpr in vivo by 48%. Injection of l-SOP and l-AP4 into the SNpr reversed reserpine-induced akinesia. Following administration above the SNpc, l-AP4 provided neurochemical, histological and functional protection against 6-OHDA lesion of the nigrostriatal tract. Pretreatment with CPPG inhibited these effects.

Conclusions and implications:

These findings highlight group III mGlu receptors in the SN as potential targets for providing both symptomatic and neuroprotective relief in PD, and indicate that inhibition of glutamate release in the SN may underlie these effects.     

The selective metabotropic glutamate receptor 7 allosteric agonist AMN082 prevents reinstatement of extinguished ethanol-induced conditioned place preference in mice

Alcohol dependence is considered a major public health problem in modern societies. The role for glutamatergic neurotransmission in the reinforcing effects of ethanol is becoming increasingly evident. Our previous findings have shown that in rats, the mGluR7 positive allosteric agonist AMN082, but not its allosteric antagonist MMPIP, prevented ethanol consumption and preference in the two-bottle choice paradigm. This study was conducted to determine the effects of AMN082 and MMPIP on the extinction and reinstatement of ethanol-elicited place preference (CPP) in C57BL/6 mice. AMN082 and MMPIP were administered during extinction of ethanol CPP to determine whether mGluR7 signaling is required. Furthermore, the effects of AMN082 and MMPIP on reinstatement of CPP were also evaluated. Finally, spontaneous locomotor activity and ethanol pharmacokinetics were assessed following systemic administration of AMN082 and MMPIP. Our results indicate that mGluR7 pharmacological modulation had no effect on ethanol-elicited CPP extinction. In contrast, mGluR7 activation using AMN082 reduced ethanol-induced CPP reinstatement, an effect reversed by co-administration of MMPIP. Collectively, these results indicate, for the first time, that activation of the mGluR7 receptor is effective in reducing the reinstatement of conditioned rewarding effects of ethanol. Taken together, the efficacy of AMN082 on the various phases of alcohol-CPP could represent an interesting pharmacological approach and could open a new line of research for the development of therapies to reduce ethanol intake in patients.     

Mood disorders: regulation by metabotropic glutamate receptors

Medicinal therapies for mood disorders neither fully serve the efficacy needs of patients nor are they free of side-effect issues. Although monoamine-based therapies are the primary current treatment approaches, both preclinical and clinical findings have implicated the excitatory neurotransmitter glutamate in the pathogenesis of major depressive disorders. The present commentary focuses on the metabotropic glutamate receptors and their relationship to mood disorders. Metabotropic glutamate (mGlu) receptors regulate glutamate transmission by altering the release of neurotransmitter and/or modulating the post-synaptic responses to glutamate. Convergent biochemical, pharmacological, behavioral, and clinical data will be reviewed that establish glutamatergic neurotransmission via mGlu receptors as a biologically relevant process in the regulation of mood and that these receptors may serve as novel targets for the discovery of small molecule modulators with unique antidepressant properties. Specifically, compounds that antagonize mGlu2, mGlu3, and/or mGlu5 receptors (e.g. LY341495, MGS0039, MPEP, MTEP) exhibit biochemical effects indicative of antidepressant effects as well as in vivo activity in animal models predictive of antidepressant efficacy. Both preclinical and clinical data have previously been presented to define NMDA and AMPA receptors as important targets for the modulation of major depression. In the present review, we present a model suggesting how the interplay of glutamate at the mGlu and at the ionotropic AMPA and NMDA receptors might account for the antidepressant-like effects of glutamatergic- and monoaminergic-based drugs affecting mood in patients. The current data lead to the hypothesis that mGlu-based compounds and conventional antidepressants impact a network of interactive effects that converge upon a down regulation of NMDA receptor function and an enhancement in AMPA receptor signaling.     

T-current related effects of antiepileptic drugs and a Ca2+ channel antagonist on thalamic relay and local circuit interneurons in a rat model of absence epilepsy

Channel blocking, anti-oscillatory, and anti-epileptic effects of clinically used anti-absence substances (ethosuximide, valproate) and the T-type Ca2+ current (IT) blocker mibefradil were tested by analyzing membrane currents in acutely isolated local circuit interneurons and thalamocortical relay (TC) neurons, slow intrathalamic oscillations in brain slices, and spike and wave discharges (SWDs) occurring in vivo in Wistar Albino Glaxo rats from Rijswijk (WAG/Rij). Substance effects in vitro were compared between WAG/Rij and a non-epileptic control strain, the ACI rats. Ethosuximide (ETX) and valproate were found to block IT in acutely isolated thalamic neurons. Block of IT by therapeutically relevant ETX concentrations (0.25-0.75 mM) was stronger in WAG/Rij, although the maximal effect at saturating concentrations (>or=10 mM) was stronger in ACI. Ethosuximide delayed the onset of the low threshold Ca2+ spike (LTS) of neurons recorded in slice preparations. Mibefradil (>or=2 microM) completely blocked IT and the LTS, dampened evoked thalamic oscillations, and attenuated SWDs in vivo. Computational modeling demonstrated that the complete effect of ETX can be replicated by a sole reduction of IT. However, the necessary degree of IT reduction was not induced by therapeutically relevant ETX concentrations. A combined reduction of IT, the persistent sodium current, and the Ca2+ activated K+ current resulted in an LTS alteration resembling the experimental observations. In summary, these results support the hypothesis of IT reduction as part of the mechanism of action of anti-absence drugs and demonstrate the ability of a specific IT antagonist to attenuate rhythmic burst firing and SWDs.     

Antinociceptive activity of N-(4-hydroxyphenacetyl)-4-aminoclonidine, a novel analog of clonidine: role of opioid receptors and alpha-adrenoceptors

N-(4-hydroxyphenacetyl)-4-aminoclonidine, a derivative of the alpha-adrenoceptor agonist p-aminoclonidine, was found to exhibit dose-dependent antinociceptive activity in the mouse writhing assay. In this measure of antinociceptive activity it was less potent than clonidine or xylazine. Naloxone, an opioid receptor antagonist, at a dose sufficient to abolish the antinociceptive activity of morphine, did not affect the antinociceptive activity of N-(4-hydroxyphenacetyl)-4-aminoclonidine, clonidine or xylazine. In contrast, yohimbine, a alpha-adrenoceptor antagonist, reduced the antinociceptive activity of N-(4-hydroxyphenacetyl)-4-aminoclonidine, clonidine and xylazine, but not morphine. The affinity of N-(4-hydroxyphenacetyl)-4-aminoclonidine, clonidine and xylazine for alpha-adrenoceptors in rat aorta was correlated highly with the relative potency for writhing inhibition. These results suggest that the antinociceptive activity of N-(4-hydroxyphenacetyl)-4-aminoclonidine is mediated by alpha-adrenoceptors.     

Differential allosteric modulation by amiloride analogues of agonist and antagonist binding at A(1) and A(3) adenosine receptors

The diuretic drug amiloride and its analogues were found previously to be allosteric modulators of antagonist binding to A(2A) adenosine receptors. In this study, the possibility of the allosteric modulation by amiloride analogues of antagonist binding at A(1) and A(3) receptors, as well as agonist binding at A(1), A(2A), and A(3) receptors, was explored. Amiloride analogues increased the dissociation rates of two antagonist radioligands, [3H]8-cyclopentyl-1,3-dipropylxanthine ([3H]DPCPX) and [3H]8-ethyl-4-methyl-2-phenyl-(8R)-4,5,7,8-tetrahydro-1H-imidazo[2,1-i]purin-5-one ([3H]PSB-11), from A(1) and A(3) receptors, respectively. Amiloride and 5-(N,N-dimethyl)amiloride (DMA) were more potent at A(1) receptors than at A(3) receptors, while 5-(N,N-hexamethylene)amiloride (HMA) was more potent at A(3) receptors. Thus, amiloride analogues are allosteric inhibitors of antagonist binding at A(1), A(2A), and A(3) adenosine receptor subtypes. In contrast to their effects on antagonist-occupied receptors, amiloride analogues did not affect the dissociation rates of the A(1) agonist [3H]N(6)-[(R)-phenylisopropyl]adenosine ([3H]R-PIA) from A(1) receptors or the A(2A) agonist [3H]2-[p-(2-carboxyethyl)phenyl-ethylamino]-5'-N-ethylcarboxamidoadenosine ([3H]CGS21680) from A(2A) receptors. The dissociation rate of the A(3) agonist radioligand [125I]N(6)-(4-amino-3-iodobenzyl)adenosine-5'-N-methyluronamide ([125I]I-AB-MECA) from A(3) receptors was decreased significantly by amiloride analogues. The binding modes of amiloride analogues at agonist-occupied and antagonist-occupied receptors differed markedly, which was demonstrated in all three subtypes of adenosine receptors tested in this study. The effects of the amiloride analogues on the action of the A(3) receptor agonist were explored further using a cyclic AMP functional assay in intact CHO cells expressing the human A(3) receptor. Both binding and functional assays support the allosteric interactions of amiloride analogues with A(3) receptors.