Comparison of (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD)- and 1R,3S-ACPD-stimulated brain phosphoinositide hydrolysis.

(1S,3R)-1-Aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD) and (1R,3S)-1-aminocyclopentane-1,3-dicarboxylic acid (1R,3S-ACPD) were characterized for potency, efficacy, and selectivity at metabotropic excitatory amino acid receptors. 1S,3R-ACPD stimulated [3H]phosphoinositide hydrolysis in slices of the neonatal and adult rat hippocampus with full efficacy and twice the potency relative to what has been shown for (+/-)-trans-ACPD. 1S,3R-ACPD was up to 30 times more potent in activating metabotropic excitatory amino acid receptors, compared to its affinity for [3H]CGS19755 binding to NMDA receptors. In contrast, 1R,3S-ACPD was much less potent, efficacious, and selective than 1S,3R-ACPD. Although 1S,3R-ACPD is not specific, it is a most selective and efficacious agonist at metabotropic excitatory amino acid receptors.     

Effects of the mGluR8 agonist (S)-3,4-DCPG in the lateral amygdala on acquisition/expression of fear-potentiated startle, synaptic transmission, and plasticity

The lateral amygdala plays an important role in emotional learning. Previous studies found that amygdaloid plasticity processes involve the activation of metabotropic glutamate receptors. In the present study we examined the effect of the highly specific mGluR8 agonist (S)-3,4-DCPG on conditioned fear in vivo measuring fear-potentiated startle. Both, acquisition and expression of conditioned fear were dose-dependently inhibited by (S)-3,4-DCPG injections into the amygdala. Since synaptic long-term potentiation in the lateral amygdala has been correlated with the acquisition of conditioned fear in rats, the effect of (S)-3,4-DCPG in vitro on synaptic transmission, short- and long-term plasticity in the lateral amygdala was evaluated in parallel. Patch clamp recordings in rat brain slices revealed that (S)-3,4-DCPG strongly attenuated synaptic transmission from sensory afferents. The lack of detectable effects on postsynaptic neurons and altered short-term plasticity indicate that (S)-3,4-DCPG acts at presynaptic sites. Long-term potentiation of thalamic afferent fiber synapses induced by a pairing protocol was slightly attenuated in the presence of (S)-3,4-DCPG, but long-term potentiation by tetanic afferent stimulation was inhibited. We conclude that mGluR8 activation is not specifically involved in long-term plasticity processes but that it rather provides a powerful inhibitory control of synaptic transmission within the lateral amygdala, with the ability to reduce activity in such a way that the expression and the acquisition of learned fear become strongly impaired in vivo.     

Fractalkine/CX3CL1 depresses central synaptic transmission in mouse hippocampal slices

This work reports the effect of chemokine fractalkine/CX3CL1, an endogenous small peptide highly expressed in the central nervous system, on evoked synaptic responses investigated in mouse CA1 stratum radiatum using an electrophysiological approach. We report that in acute mouse hippocampal slices, superfusion of CX3CL1 resulted in a reversible depression of the field excitatory postsynaptic potential (fEPSP) which developed within few seconds, increased for up to 10 min of application and disappeared within 30 min after the end of CX3CL1 treatment. We also show that CX3CL1-induced synaptic depression is (i) dose-dependent with IC50 and nH values of 0.7 nM and 1, respectively, (ii) not associated with a change in paired-pulse facilitation, (iii) mediated through CX3CL1 receptor (CX3CR1), being absent in CX3CR1-/- mice and inhibited in wild-type mice by a specific blocking antibody, and (iv) occluded by the induction of homosynaptic long-term depression (LTD). We conclude that CX3CL1 is a potent neuromodulator of the evoked excitatory synaptic transmission, sharing common mechanisms with LTD.     

Acute effects of antidepressant drugs on long-term potentiation (LTP) in rat hippocampal slices

Summary The actions of three clinically effective antidepressant drugs with different pharmacological profiles were investigated in the CAI area of rat hippocampal slices. Imipramine and (+) or (–)-oxaprotiline had negligible effects on population spikes evoked by stratum radiatum stimulation, but reduced postsynaptic excitability in low Ca high Mg medium after an exposure of more than 15 min. Imipramine and (+)-oxaprotiline at 10 mol/l enhanced long-term potentiation (LTP) when a lower stimulation strength was applied while (+)-oxaprotiline reduced UP when a higher stimulus amplitude was used to evoke population spikes. (–)-oxaprotiline (levoprotiline) had a similar effect which was, however, not significant in either stimulation paradigm at the P<0.05 level. Imipramine actions were also studied on epileptiform discharges in Mg²⁺-free medium: a facilitation-inhibition sequence with a slow time course was seen with 50 mol/l but no effect with 10 mol/l. An involvement of N-methyl-D-aspartate (NMDA)-receptors in acute actions of antidepressants is unlikely but long-term potentiation in the hippocampus is modulated by these drugs. Send offprint requests to H.L. Haas, Düsseldorf     

Effects of dimethylarsinic and dimethylarsinous acid on evoked synaptic potentials in hippocampal slices of young and adult rats

In this study, the effects of pentavalent dimethylarsinic acid ((CH(3))(2)AsO(OH); DMA(V)) and trivalent dimethylarsinous acid ((CH(3))(2)As(OH); DMA(III)) on synaptic transmission generated by the excitatory Schaffer collateral-CA1 synapse were tested in hippocampal slices of young (14-21 day-old) and adult (2-4 month-old) rats. Both compounds were applied in concentrations of 1 to 100 micromol/l. DMA(V) had no effect on the amplitudes of evoked fEPSPs or the induction of LTP recorded from the CA1 dendritic region either in adult or in young rats. However, application of DMA(III) significantly reduced the amplitudes of evoked fEPSPs in a concentration-dependent manner with a total depression following application of 100 micromol/l DMA(III) in adult and 10 micromol/l DMA(III) in young rats. Moreover, DMA(III) significantly affected the LTP-induction. Application of 10 micromol/l DMA(III) resulted in a complete failure of the postsynaptic potentiation of the fEPSP amplitudes in slices taken both from adult and young rats. The depressant effect was not reversible after a 30-min washout of the DMA(III). In slices of young rats, the depressant effects of DMA(III) were more pronounced than in those taken from adult ones. Compared to the (absent) effect of DMA(V) on synaptic transmission, the trivalent compound possesses a considerably higher neurotoxic potential.     

Effects of monomethylarsonic and monomethylarsonous acid on evoked synaptic potentials in hippocampal slices of adult and young rats

Arsenite and its metabolites, dimethylarsinic or dimethylarsinous acid, have previously been shown to disturb synaptic transmission in hippocampal slices of rats (Krüger, K., Gruner, J., Madeja, M., Hartmann, L.M., Hirner, A.V., Binding, N., Muβhoff, U., 2006a. Blockade and enhancement of glutamate receptor responses in Xenopus oocytes by methylated arsenicals. Arch. Toxicol. 80, 492-501, Krüger, K., Straub, H., Binding, N., Muβhoff, U., 2006b. Effects of arsenite on long-term potentiation in hippocampal slices from adult and young rats. Toxicol. Lett. 165, 167-173, Krüger, K., Repges, H., Hippler, J., Hartmann, L.M., Hirner, A.V., Straub, H., Binding, N., Muβhoff, U., 2007. Effects of dimethylarsinic and dimethylarsinous acid on evoked synaptic potentials in hippocampal slices of young and adult rats. Toxicol. Appl. Pharmacol. 225, 40-46). The present experiments investigate, whether the important arsenic metabolites monomethylarsonic acid (MMA^V) and monomethylarsonous acid (MMA^III) also influence the synaptic functions of the hippocampus.In hippocampal slices of young (14-21 days-old) and adult (2-4 months-old) rats, evoked synaptic field potentials from the Schaffer collateral-CA1 synapse were measured under control conditions and during and after 30 and 60 min of application of the arsenic compounds.MMA^V had no effect on the synapse functions neither in slices of adult nor in those from young rats. However, MMA^III strongly influenced the synaptic transmission: it totally depressed the amplitudes of fEPSPs at concentrations of 50 μmol/l (adult rats) and 25 μmol/l (young rats) and LTP amplitudes at concentrations of 25 μmol/l (adult rats) and 10 μmol/l (young rats), respectively. In contrast, application of 1 μmol/l MMA^III led to an enhancement of the LTP amplitude in young rats, which is interpretable by an enhancing effect on NMDA receptors and a lack of the blocking effect on AMPA receptors at this concentration (Krüger, K., Gruner, J., Madeja, M., Hartmann, L.M., Hirner, A.V., Binding, N., Muβhoff, U., 2006a. Blockade and enhancement of glutamate receptor responses in Xenopus oocytes by methylated arsenicals. Arch. Toxicol. 80, 492-501).These effects are probably not mediated by changes in cell excitability or in presynaptic glutamate release rates, since antidromically induced population spikes and paired-pulse facilitation failed to show any MMA^III effect. The impairment of the excitatory CA1 synapse is more likely caused by the action of MMA^III on postsynaptic glutamatergic receptors and may be jointly responsible for dysfunctions of cognitive effects in arsenic toxicity.     

Anticonvulsant activity of a mGlu(4alpha) receptor selective agonist, (1S,3R,4S)-1-aminocyclopentane-1,2,4-tricarboxylic acid

The metabotropic Group III agonist, (1S,3R,4S)-1-aminocyclopentane-1,2,4-tricarboxylic acid (ACPT-1), selective for the mGlu(4alpha) receptor, suppresses sound-induced seizures in DBA/2 mice following its intracerebroventricular (i.c.v.) administration (ED(50) 5.6 [2.9-10.7], nmol i.c.v., 15 min, clonic phase) and in genetically epilepsy-prone (GEP) rats following focal administration into the inferior colliculus (ED(50) 0.08 [0.01-0.50], nmol, 60 min, clonic phase). ACPT-1 also protects against clonic seizures induced in DBA/2 mice by the Group I agonist, (RS)-3,5-dihydroxyphenylglycine (3,5-DHPG) (ED(50) 0.60 [0.29-1.2], nmol i.c.v.) and by the Group III antagonist, (RS)-alpha-methylserine-O-phosphate (MSOP) (ED(50) 49.3 [37.9-64.1], nmol i.c.v.). Another Group III agonist, (RS)-4-phosphonophenyl-glycine (PPG), preferentially activating the mGlu(8) receptor, previously shown to protect against sound-induced seizures in DBA/2 mice and GEP rats, also protects against seizures induced in DBA/2 by 3,5-DHPG (ED(50) 3.7 [2.4-5.7], nmol i.c.v.) and by the Group III antagonist, MSOP (ED(50) 40.2 [21.0-77.0], nmol i.c.v.). At very high doses (500 nmol i.c.v. and above), Group III antagonists have pro-convulsant and convulsant activity. The anticonvulsant protection against sound-induced seizures in DBA/2 mice provided by a fully protective dose (20 nmol, i.c.v.) of the mGlu(4) receptor agonist ACPT-1, is partially reversed by the co-administration of the Group III antagonists, MSOP, (RS)-alpha-methyl-4-phosphonophenylglycine (MPPG) or (S)-2-amino-2-methyl-4-phosphonobutanoic acid (MAP4), in the 20-50 nmol dose range. At doses of 50-200 nmol, MPPG and MAP4 cause further reversal of the ACPT-1 anticonvulsant protection, while the MSOP effect on ACPT-1 protection is abolished at higher doses. In contrast, the anticonvulsant protection against sound-induced seizures in DBA/2 mice provided by a fully protective dose (20 nmol, i.c.v.) of the mGlu(8) receptor agonist PPG, is not significantly affected by the co-administration of the same Group III antagonists, MSOP, MPPG or MAP4. We conclude that activation of either mGlu(4alpha) or mGlu(8) receptors confer anticonvulsant protection in DBA/2 mice. Furthermore, the metabotropic Group III receptor antagonists, MSOP, MPPG, and MAP4 appear to be functionally selective for the mGlu(4) receptor in this system.     

Anxiolytic- and antidepressant-like effects of group III metabotropic glutamate agonist (1S,3R,4S)-1-aminocyclopentane-1,3,4-tricarboxylic acid (ACPT-I) in rats

We examined the anxiolytic-like activity of (1S,3R,4S)-1-aminocyclopentane-1,3,4-tricarboxylic acid (ACPT-I) using the Vogel conflict drinking test, while antidepressant-like effects of this compound were evaluated using Porsolt's test. ACPT-I, a selective group III mGlu receptor agonist, produced a dose-dependent anticonflict effect after intrahippocampal injections and antidepressant-like effect in rats after intraventricular injections. These data suggest that selective group III mGlu receptor agonists may become a new class of anxiolytics and/or antidepressants.     

Differential regulation of synaptic transmission by mGlu2 and mGlu3 at the perforant path inputs to the dentate gyrus and CA1 revealed in mGlu2 -/- mice

Group II metabotropic glutamate (mGlu) receptors can act as presynaptic autoinhibitory receptors at perforant path inputs to the hippocampus under conditions of high frequency synaptic activation. We have used mGlu2 -/- mice to examine the relative roles of mGlu2 and mGlu3 in the regulation of perforant path synaptic transmission mediated by both the selective group II receptor agonist, DCG-IV, and by synaptically released glutamate. Field excitatory postsynaptic potentials evoked by stimulation of either the perforant path inputs to the dentate gyrus mid-moleculare or the CA1 stratum lacunosum moleculare were inhibited by DCG-IV with IC(50) values and maximum percentage inhibition of: 169 nM (60%) and 41 nM (72%) in wild-type mice and 273 nM (19%) and 116 nM (49%) in mGlu2 -/- mice, respectively. Activation of presynaptic group II mGlu autoreceptors by synaptically released glutamate, as revealed by a LY341495-mediated increase in the relative amplitude of a test fEPSP evoked after a conditioning burst, was observed in both the dentate gyrus and the stratum lacunosum of wild-type, but not mGlu2 -/- mice. These observations demonstrate that activation of mGlu3 receptors can regulate synaptic transmission at perforant path synapses but suggest that mGlu2 is the major presynaptic group II autoreceptor activated by synaptically released glutamate.     

Plasticity-specific phosphorylation of CaMKII, MAP-kinases and CREB during late-LTP in rat hippocampal slices in vitro

Processes of learning, memory formation and functional plasticity, such as long-term potentiation (LTP) are associated with activity changes of alphaCaMKII, MAPKs and CREB proteins. Little is known on the temporal regulation of the phosphorylation states of these proteins during late-LTP, a period which requires the synthesis of new macromolecules and has been postulated to correlate with the consolidation of a memory trace at the cellular level. We now present data from hippocampal slices in vitro obtained from adult rats that describe such specific phosphorylation changes of the above mentioned three molecules during early- and late-LTP. We detail that LTP induction and its maintenance reveals a delayed onset of continuous CREB phosphorylation with two separate peaks at 45 min and 6 h before it decays back to baseline phosphorylation levels; whereas alphaCaMKII and MAPK2 remain in a specific, but enhanced phosphorylation state throughout the induction, early- and late-LTP. These LTP-regulated phosphorylation events were NMDAR-dependent and upon the activity of a translated serine-threonine kinase. Interestingly, only the late enhancement of pCREB was clearly dependent on protein synthesis. Our data extend results describing the regulation of alphaCaMKII, MAPKs and CREB phosphorylation during early stages of LTP, suggesting a specific role of these enzymes also during phases of LTP consolidation in adult animals.     

石杉碱甲增强大鼠海马脑片CA1锥体神经元的兴奋性突触传递

目的:观察石杉碱甲(Hup-A)对海马CA1锥体神经元兴奋性突触传递的影响,以探讨其增强学习记忆功能的神经细胞电生理机制。方法:应用大鼠海马脑片CA1锥体神经元细胞内记录技术,观察Hup-A对大鼠海马CA1锥体神经元膜电性质和刺激Schaffer侧支诱发的兴奋性突触后电位(EPSP)的影响。结果:(1)Hup-A(1μmol/L)灌流15min对CA1锥体神经元的膜电性质没有显著性影响。(2)Hup-A(0.3～3.0μmol/L)浓度依赖性使EPSP幅度升高、时程延长、曲线下面积增大,该作用可被阿托品(10μmol/L)预处理取消。(3)Hup-A对外源性谷氨酸诱导的去极化反应无明显影响。结论:Hup-A可增强CA1锥体神经元的兴奋性突触传递,其增强突触传递作用与M型乙酰胆碱受体激动有关。     

Chronic administration of dehydroepiandrosterone sulfate (DHEAS) primes for facilitated induction of long-term potentiation via sigma 1 (sigma1) receptor: optical imaging study in rat hippocampal slices

Dehydroepiandrosterone sulfate (DHEAS), one of the most abundant neurosteroids synthesized de novo in the nervous system, has well characterized effects on memory and cognitive performances. However, little is known about the underlying synaptic mechanisms. In this study, we investigated the effects of chronic administration of DHEAS (20 mg/kg for 7 days) on the plasticity of Schaffer collateral-CA1 synapses by applying an optical recording technique on the hippocampal slices stained with voltage-sensitive dyes. We report here that chronically administered DHEAS significantly facilitated the induction of frequency-dependent LTP, termed DHEAS-facilitated LTP. While tetanus of at least 50 pulses (at 100 Hz) were required to induce LTP in control rats, only 20 pulses were needed in DHEAS-treated animals. In contrast DHEA, the non-sulfated form of DHEAS, had no facilitating effect on the induction of LTP. We found that chronically administered DHEAS did not alter the presynaptic glutamate release in response to both single pulse and tetanic stimulation, suggesting that certain alterations happened in postsynaptic neurons. Co-administration of the sigma 1 (sigma1) receptor antagonists, haloperidol or NE100, with DHEAS completely inhibited the DHEAS-facilitated LTP. However, acute administration of sigma1 receptor antagonists to the slices did not affect the induction of DHEAS-facilitated LTP, suggesting that sigma1 receptor is a key target of chronic actions of DHEAS but is not involved in the induction of DHEAS-facilitated LTP. Our findings provide evidence that chronically administered DHEAS plays a priming role in inducing a facilitated synaptic plasticity probably via a chronic activation of sigma1 receptor in rat hippocampal CA1 pyramidal cells.     

Inhibition of long-term potentiation in rat hippocampal slices by anandamide and WIN55212-2: reversal by SR141716 A,a selective antagonist of CB1 cannabinoid receptors

It has been reported previously that ⁹-tetrahydrocannabinol and the synthetic cannabinoid agonist HU-210 [(–)-11-OH-⁸-dimethylheptyl tetrahydrocannabinol] prevent long-term potentiation (LTP) induction in rat hippocampal slices. In this study we confirm that both WIN55212-2 {R-(+)-(2,3-dihydro-5-methyl-3-[{4-morpholinyl} methyl] pyrol [1,2,3-de]-1,4-benzoxazin-6-yl) (1-naphtalenyl) methanone monomethanesulphonate} (3 and 10 M), another synthetic cannabinoid agonist, and anandamide (10 M), considered to be the endogenous ligand of cannabinoid receptors, inhibit LTP formation in the Schaffer collateral-CA1 field complex. In addition, we show that SRt417l6A [N-(piperidin-1-yl)-5-(4-chlorophenyl)-1-(2, 4-dichlorophenyl)-4-methyl-lH-pyrazole-3-carboxamide hydrochloride] at 0.1–10 M, a potent and selective antagonist of CB1 cannabinoid receptors, concentration-dependently reversed the inhibition of LTP induced by both WIN55212-2 and anandamide. These data indicate that cannabinoid receptor agonists inhibit hippocampal LTP formation through CB1 receptor activation and that anandamide could be a candidate for an endogenous neuromessenger involved in memory processes.     

Effects of Cd on AMPA receptor-mediated synaptic transmission in rat hippocampal CA1 area

Cadmium (Cd²⁺) is a common pollutant that causes a wide variety of toxic effects on the central nervous system. However, the mechanism of Cd²⁺ neurotoxicity remains to be elucidated. In the present study, we examined the effects of Cd²⁺ on AMPA receptor-mediated synaptic transmission and short-term synaptic plasticity in hippocampal CA1 area, using whole-cell patch clamp technique. Cd²⁺ significantly inhibited the peak amplitude of evoked EPSCs (eEPSCs) in a concentration-dependent manner and enhanced the short-term synaptic plasticity including paired-pulse facilitation and frequency facilitation. Cd²⁺ also decreased the frequency and amplitude of spontaneous EPSCs (sEPSCs) but had no effect on those of miniature EPSCs (mEPSCs). These effects of Cd²⁺ may involve a presynaptic mechanism of blockade of action potential-sensitive, calcium-dependent release of glutamate. In addition, Cd²⁺ prolonged the decay time of both sEPSCs and mEPSCs, which suggested a postsynaptic action site of Cd²⁺. This study demonstrates that Cd²⁺ impairs the Schaffer collateral-commissural-CA1 glutamatergic synaptic transmission and short-term plasticity in rat hippocampal slices, which may be a possible contributing mechanism for the Cd²⁺-induced neurotoxic effects.     

Glucocorticoid receptors and beta-adrenoceptors in basolateral amygdala modulate synaptic plasticity in hippocampal dentate gyrus, but not in area CA1

The basolateral amygdala (BLA) is a key structure in a memory-modulatory system that regulates stress and stress hormones (glucocorticoid and noradrenaline) effects on hippocampal functioning. We have shown previously that priming the amygdala differentially affects plasticity in the hippocampal dentate gyrus (DG) and CA1, and mimicked acute stress effect on plasticity in these two subregions. In the present study, we investigated the mechanisms that mobilize the BLA to differentially alter plasticity in DG and CA1. Glucocorticoid receptors antagonist RU 38486 or beta-adrenoceptor antagonist propranolol were microinfused in the BLA, 10 min prior to BLA activation-induced modulation of long-term potentiation (LTP) in DG and CA1. The results showed that neither glucocorticoid nor noradrenergic transmissions in the BLA are necessary for LTP induction and for the impairing effect of amygdala activation on CA1 LTP. In contrast, blockade of glucocorticoid or noradrenergic transmission in BLA, increased baseline synaptic transmission in the DG, but suppressed the enhancing effect of BLA activation on DG LTP. These findings provide further evidence for a differential amygdala control of hippocampal subregions as well as for differential memory processes involving CA1 and DG. They also provide insight into how stress hormones exert their actions on the circuits involved in these processes.     

Conformational analysis and structural comparisons of (1R,3S)-(+)- and (1S,3R)-(-)-tefludazine, (S)-(+)- and (R)-(-)-octoclothepin, and (+)-dexclamol in relation to dopamine receptor antagonism and amine-uptake inhibition

Conformational analysis with molecular mechanics (MM2(85] and molecular superimposition studies of (1R,3S)-(+)- and (1S,3R)-(-)-4-[3-(4-fluorophenyl)-6-(trifluoromethyl)indan-1-yl]-1- piperazineethanol (tefludazine) and (S)-(+)- and (R)-(-)-octoclothepin have been employed to identify biologically active conformations of these compounds with respect to dopamine receptor antagonism and amine-uptake inhibition. In contrast to what is commonly assumed, these studies indicate that the conformation of (S)-(+)-octoclothepin responsible for the dopamine receptor antagonism is different from the one observed in the crystal. From least-squares molecular superimpositions with the potent and stereoselective dopamine receptor antagonist (1R,3S)-tefludazine, biologically active conformations for the two compounds on the dopamine receptor have been deduced. This analysis also rationalizes the enantioselectivity of octoclothepin on the dopamine receptor. The X-ray structure of (S)-(+)-octoclothepin is shown to correspond structurally to the 1S,3R enantiomer of tefludazine, which is an amine-uptake inhibitor. This correspondence provides a structural basis for the norepinephrine (NE) uptake blocking properties of octoclothepin. It is predicted that the enantioselectivity of the NE-uptake inhibition of octoclothepin should be low with the S-(+) enantiomer as the more active optical isomer. A comparison of the deduced biologically active conformation of (S)-(+)-octoclothepin with (+)-dexclamol is also discussed on the basis of earlier derived superimposition studies with (+)-dexclamol.     

Rational design and enantioselective synthesis of (1R,4S,5R,6S)-3-azabicyclo[3.3.0]octane-4,6-dicarboxylic acid - a novel inhibitor at human glutamate transporter subtypes 1, 2, and 3

The natural product kainic acid is used as template for the rational design of a novel conformationally restricted (S)-glutamic acid (Glu) analogue, (1R,4S,5R,6S)-3-azabicyclo[3.3.0]octane-4,6-dicarboxylic acid (1a). The target structure 1a was synthesized from commercially available (S)-pyroglutaminol, in an enantioselective fashion, in 14 steps. Pharmacological characterization of 1a at human glutamate transporter subtypes 1, 2, and 3 yielded K(i) values of 127, 52, and 46 microM, respectively. Furthermore, binding studies at native ionotropic Glu (iGlu) receptors revealed low affinity for alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA)-preferring iGlu receptors (IC(50) > 100 microM), whereas affinities for the KAIN-preferring iGlu receptors and the N-methyl-d-aspartate (NMDA)-preferring group of iGlu receptors were in the low micromolar range (IC(50) = 14 and 2.9 microM, respectively). At metabotropic Glu receptors (mGluR), EC(50) values for 1a were >1000 microM for mGluR1 and 4, representing group I and III, respectively, and approximately 1000 microM (agonist) for mGluR2, representing group II.     

Selective inhibition of gamma-aminobutyric acid aminotransferase by (3R,4R),(3S,4S)- and (3R,4S),(3S,4R)-4-amino-5-fluoro-3-phenylpentanoic acids

(3R,4R),(3S,4S)- and (3R,4S),(3S,4R)-4-amino-5-fluoro-3-phenylpentanoic acid (1a and 1b) were synthesized and studied as selective inactivators of gamma-aminobutyric acid (GABA) aminotransferase. Neither compound caused time-dependent inactivation of the enzyme. Neither compound underwent enzyme-catalyzed transamination nor was fluoride ion eliminated from either compound by the enzyme. No 3-phenyllevulinic acid, the product of elimination of HF followed by enamine hydrolysis, was detected. However, both 1a and 1b were competitive reversible inhibitors of GABA aminotransferase; the Ki for 1a was smaller than the Km for GABA. These results suggest that 1a and 1b bind to the active site of GABA aminotransferase, but gamma-proton removal does not occur. Whereas (S)-4-amino-5-fluoropentanoic acid (AFPA) is a potent inhibitor of L-glutamic acid decarboxylase (GAD), neither 1a nor 1b at concentrations 40 times the Ki of AFPA caused any detectable competitive inhibition of GAD. Therefore, the incorporation of a phenyl substituent at the 3-position of AFPA confirms selective inhibition of GABA aminotransferase over GAD.     

Solid-phase synthesis of combinatorial libraries based on enatiomerically pure (1S,2S,4R,5S)-4,5-dihydroxycyclohexan-1,2-dicarboxylic acid scaffolds

The conditions for solid-phase functionalization of the new enantio-pure scaffold 1 to give libraries of general formula 4 have been derived.     

3-Methyl pyrrole-2,4-dicarboxylic acid 2-propyl ester 4-(1,2,2-trimethyl-propyl) ester: an exploration of the C-2 position. Part I

Following the recent disclosure of 3-methyl pyrrole-2,4-dicarboxylic acid 2-propyl ester 4-(1,2,2-trimethyl-propyl) ester, a potent and selective mGluR1 non-competitive antagonist, we report here a detailed exploration of the C-2 position of this scaffold with the preparation of differently substituted amides. Great improvement of the pharmacokinetic properties has been achieved through this exercise.