5-Substituted imidazole-4-acetic acid analogues: synthesis, modeling, and pharmacological characterization of a series of novel gamma-aminobutyric acid(C) receptor agonists

Convergent Syntheses of Isomeric Imidazolospiroketones as Templates for Acetyl-CoA Carboxylase (ACC) Inhibitors

The synthesis of imidazole fused spirocyclic ketones as templates for acetyl-CoA carboxylase (ACC) inhibitors is reported. By completing the spirocyclic ring closure via divergent pathways, the synthesis of these regioisomers from common intermediates was developed. Through an aldehyde homologation/transmetalation strategy, one isomer was formed selectively. The second desired isomer was obtained via an intramolecular aromatic homolytic substitution reaction. Preparation of these isomeric spiroketones provided templates which, upon elaboration, led to key structure-activity relationship (SAR) points for delivery of potent ACC inhibitors.

Pharmacological Effect of GABA Analogues on GABA-ϱ2 Receptors and Their Subtype Selectivity

GABAρ receptors are distinctive GABAergic receptors from other ionotropic GABA_A and metabotropic GABA_B receptors in their pharmacological, biochemical, and electrophysiological properties. Although GABA-ρ1 receptors are the most studied in this subfamily, GABA-ρ2 receptors are widely distributed in the brain and are considered a potential target for treating neurological disorders such as stroke. The structure of GABA-ρ2 receptors and their pharmacological features are poorly studied. We generated the first homology model of GABA-ρ2 channel, which predicts similar major interactions of GABA with the binding-site residues in GABA-ρ1 and GABA-ρ2 channels. We also investigated the pharmacological properties of several GABA analogues on the activity of GABA-ρ2 receptors. In comparison to their pharmacological effect on GABA-ρ1 receptors, the activation effect of these ligands and their potentiation/inhibition impact on GABA response have interestingly shown inter-selectivity between the two GABA-ρ receptors. Our results suggest that several GABA analogues can be used as research tools to study the distinctive physiology of GABA-ρ1 and GABA-ρ2 receptors. Furthermore, their partial agonist effect may hold promise for the future discovery of selective modulatory agents on GABA_A receptors.

Roles of hydrophilic residues in GABA binding site of GABA-ρ1 receptor explain the addition/inhibition effects of competitive ligands

The orthosteric binding site of GABA-gated ion channels has been widely explored. Many residues in the binding site of GABA were studied. The interactions due to the binding of GABA into the binding site drive channel activation and determine the potency and efficacy of GABA response. The combined effect of a competitive ligand and GABA on GABA-ρ1 receptors has been poorly studied. Here, we used point mutations, molecular modeling, and electrophysiological studies to explore the role of two hydrophilic residues (Serine 168 and Serine 243) of the GABA-ρ1 receptors in response to the binding of GABA and other studied ligands. Our results suggested that Ser168 residue stabilizes either closed state or open conformation depending on the other determinant interactions of each state. On the other hand, Ser243 residue is predicted to form different inter-subunit interactions with residues in the adjacent subunit at different states of the channel. Our current findings enlighten us to reasonably explain the additive/inhibitive effects of applying a competitive ligand with GABA simultaneously. Understanding the mixed effect of potentiation and inhibition would facilitate the discovery of new drugs to work as a direct GABA's activity modulators with more selectivity at various subunits forming GABA-gated ion channels.

Synthesis and Evaluation of α-Asaronol Esters with LDH and GABAA Receptor Modulation as Anticonvulsant Agents

Background:

Our previous studies showed that α-asaronol was a potential antiepileptic candidate. Here, twelve O-terminus modified ester derivatives of α-asaronol were designed, synthesized and evaluated their anticonvulsant activity.

Methods:

All synthetic compounds were subjected to three animal models of seizure (MES, scPTZ and sc3-MP models) combined with neurotoxicity test, as well as the LDH inhibitory test. Furthermore, GABAA Receptor modulation and pharmacokinetic evaluation of compound 4k were also performed.

Results:

Five compounds (4a, 4b, 4d, 4e and 4k) showed significant anticonvulsant properties at the dose of 30-300 mg/kg in MES and scPTZ test, but weak activity in sc3-MP model. Meanwhile, 4a, 4b, 4d and 4k showed good LDH inhibitory activity in vitro. Specifically, 4k was the best compound in above evaluation, and better than that of α-asaronol and reference compound (stiripentol). In addition, 4k could increase chloride ion influx by modulating GABAA receptor α1β2γ2 subtype with EC50 of 48.65 ± 10.31 μM and showed good PK profiles in rats with moderate oral bioavailability (51.5%).

Conclusion:

These results suggested 4k possesses potential effectiveness in treatment of therapyresistant seizures and is expected to be developed as a novel molecule for safer and efficient anticonvulsants having neuroprotective effects as well as low toxicity.

Functional characterization of 5-HT1A and 5-HT1B serotonin receptor signaling through G-protein-activated inwardly rectifying K+ channels in a fluorescence-based membrane potential assay

The 5-HT_1A and 5-HT_1B serotonin receptors are abundantly expressed in the CNS and constitute validated as well as putative drug targets in a variety of psychiatric and cognitive disorders, alcoholism/addiction, pain and migraine. In the present study we have characterized the functional properties of human 5-HT_1A and 5-HT_1B stably co-expressed with the human G-protein-activated inwardly rectifying K⁺ channel 2 (GIRK2) in HEK293 cells in the fluorescence-based FLIPR® Membrane Potential Blue (FMP) assay. Serotonin and other agonists induced robust decreases in fluorescence levels in the 5-HT_1A/GIRK2- and 5-HT_1B/GIRK2-HEK293 cells in a concentration-dependent manner in the assay, and these responses could be inhibited by selective 5-HT_1A/5-HT_1B antagonists and by the Gα_i/o-protein inhibitor pertussis toxin (PTX). Five additional stable HEK293 cell lines co-expressing 5-HT_1A or 5-HT_1B with GIRK2 and one of the PTX-insensitive Gα_i/o-subunit mutants Gα_i1^C351I, Gα_i2^C352I and Gα_o1^C351I were constructed, and 5-HT_1A/5-HT_1B-mediated responses through these specific Gα_i/o-subunits were measured in these cells pretreated with PTX in the FMP assay. The functional properties of 16 reference 5-HT₁ agonists were characterized at the seven cell lines, which constitutes the most detailed pharmacological profiling and comparison of 5-HT_1A and 5-HT_1B receptor signaling in the same assay published to date. We propose that this approach to assay 5-HT₁-mediated signaling through endogenous Gα_i/o-proteins in HEK293 cells or through specific Gα_i/o-subunits in a fairly high-throughput manner holds some advantages to other functional assays for Gα_i/o-coupled receptors. The assay will facilitate detailed profiling of the Gα_i/o- and Gβγ-mediated signaling of 5-HT_1A and 5-HT_1B at the molecular level, and it could also be used to identify novel modulators for the receptors.

Synthesis of 2-bromo- and 2-phenyl-neo-confused porphyrins

New examples of neo-confused porphyrins are reported. These retain global diatropic characteristics but are relatively unstable in solution.

Fragment screening for a protein-protein interaction inhibitor to WDR5

The WD40-repeat protein WDR5 scaffolds various epigenetic writers and is a critical component of the mammalian SET/MLL histone methyltransferase complex. Dysregulation of the MLL1 catalytic function is associated with mixed-lineage leukemia, and antagonism of the WDR5-MLL1 interaction by small molecules has been proposed as a therapeutic strategy for MLL-rearranged cancers. Small molecule binders of the "WIN" site of WDR5 that cause displacement from chromatin have been additionally implicated to be of broader use in cancer treatment. In this study, a fragment screen with Surface Plasmon Resonance (SPR) was used to identify a highly ligand-efficient imidazole-containing compound that is bound in the WIN site. The subsequent medicinal chemistry campaign-guided by a suite of high-resolution cocrystal structures with WDR5-progressed the initial hit to a low micromolar binder. One outcome from this study is a moiety that substitutes well for the side chain of arginine; a tripeptide containing one such substitution was resolved in a high resolution structure (1.5 Å) with a binding mode analogous to the native tripeptide. SPR furthermore indicates a similar residence time (k _d = ∼0.06 s^-1) for these two analogs. This novel scaffold therefore represents a possible means to overcome the potential permeability issues of WDR5 ligands that possess highly basic groups like guanidine. The series reported here furthers the understanding of the WDR5 WIN site and functions as a starting point for the development of more potent WDR5 inhibitors that may serve as cancer therapeutics.

Identification and Structure-Activity Relationship Study of Imidazo[1,2-a]pyridine-3-amines as First Selective Inhibitors of Excitatory Amino Acid Transporter Subtype 3 (EAAT3)

In the present study, screening of a library of 49,087 compounds at the excitatory amino acid transporter subtype 3 (EAAT3) led to the identification of 2-(furan-2-yl)-8-methyl-N-(o-tolyl)imidazo[1,2-a]pyridin-3-amine (3a) which showed a >20-fold preference for inhibition of EAAT3 (IC₅₀ = 13 μM) over EAAT1,2,4 (EAAT1: IC₅₀ ∼ 250 μM; EAAT2,4: IC₅₀ > 250 μM). It was shown that a small lipophilic substituent (methyl or bromine) at the 7- and/or 8-position was essential for activity. Furthermore, the substitution pattern of the o-tolyl group (compound 5b) and the chemical nature of the substituent in the 2-position (compound 7b) were shown to be essential for the selectivity toward EAAT3 over EAAT1,2. The most prominent analogues to come out of this study are 3a and 3e that display ∼35-fold selectivity for EAAT3 (IC₅₀ = 7.2 μM) over EAAT1,2,4 (IC₅₀ ∼ 250 μM).

Polygala tenuifolia-Acori tatarinowii herbal pair as an inspiration for substituted cinnamic α-asaronol esters: Design, synthesis, anticonvulsant activity, and inhibition of lactate dehydrogenase study

Inspired by the traditional Chinese herbal pair of Polygala tenuifolia-Acori Tatarinowii for treating epilepsy, 33 novel substituted cinnamic α-asaronol esters and analogues were designed by Combination of Traditional Chinese Medicine Molecular Chemistry (CTCMMC) strategy, synthesized and tested systematically not only for anticonvulsant activity in three mouse models but also for LDH inhibitory activity. Thereinto, 68-70 and 75 displayed excellent and broad spectra of anticonvulsant activities with modest ability in preventing neuropathic pain, as well as low neurotoxicity. The protective indices of these four compounds compared favorably with stiripentol, lacosamide, carbamazepine and valproic acid. 68-70 exhibited good LDH1 and LDH5 inhibitory activities with noncompetitive inhibition type, and were more potent than stiripentol. Notably, 70, as a representative agent, was also shown as a moderately positive allosteric modulator at human α1β2γ2 GABA_A receptors (EC₅₀ 46.3 ± 7.3 μM). Thus, 68-70 were promising candidates for developing into anti-epileptic drugs, especially for treatment of refractory epilepsies such as Dravet syndrome.

A Direct Approach to Orthogonally Protected α-Amino Aldehydes

O-Neopentyl-xanthate 19 bearing a masked α-amino aldehyde, with the two functional groups orthogonally protected, reacts cleanly with many functional alkenes. The radical addition-transfer furnishes densely functionalized adducts that can be further transformed into an array of amino-substituted carbocycles and heteroaromatics. They are also easily converted into imidazolones.

Anticonvulsive evaluation of THIP in the murine pentylenetetrazole kindling model: lack of anticonvulsive effect of THIP despite functional δ-subunit-containing GABAA receptors in dentate gyrus granule cells

THIP (4,5,6,7‐tetrahydroisoxazolo[5,4‐c]pyridin‐3‐ol) is a GABA_A receptor agonist with varying potencies and efficacies at γ‐subunit‐containing receptors. More importantly, THIP acts as a selective superagonist at δ‐subunit‐containing receptors (δ‐GABA_ARs) at clinically relevant concentrations. Evaluation of THIP as a potential anticonvulsant has given contradictory results in different animal models and for this reason, we reevaluated the anticonvulsive properties of THIP in the murine pentylenetetrazole (PTZ) kindling model. As loss of δ‐GABA_AR in the dentate gyrus has been associated with several animal models of epilepsy, we first investigated the presence of functional δ‐GABA_A receptors. Both immunohistochemistry and Western blot data demonstrated that δ‐GABA_AR expression is not only present in the dentate gyrus, but also the expression level was enhanced in the early phase after PTZ kindling. Whole‐cell patch‐clamp studies in acute hippocampal brain slices revealed that THIP was indeed able to induce a tonic inhibition in dentate gyrus granule cells. However, THIP induced a tonic current of similar magnitude in the PTZ‐kindled mice compared to saline‐treated animals despite the observed upregulation of δ‐GABA_ARs. Even in the demonstrated presence of functional δ‐GABA_ARs, THIP (0.5–4 mg/kg) showed no anticonvulsive effect in the PTZ kindling model using a comprehensive in vivo evaluation of the anticonvulsive properties.

GABA-ρ receptors: distinctive functions and molecular pharmacology

The homomeric GABA-ρ ligand-gated ion channels (also known as GABA_C or GABA_A -ρ receptors) are similar to heteromeric GABA_A receptors in structure, function and mechanism of action. However, their distinctive pharmacological properties and distribution make them of special interest. This review focuses on GABA-ρ ion channel structure, ligand selectivity toward ρ receptors over heteromeric GABA_A receptor sub-types and selectivity between different homomeric ρ sub-type receptors. Several GABA analogues show selectivity at homomeric GABA-ρ receptors over heteromeric GABA_A receptors. More recently, some synthetic ligands have been found to show selectivity at receptors formed from one ρ subtype over others. The unique pharmacological profiles of these agents are discussed in this review. The classical binding site of GABA within the orthosteric site of GABA-ρ homomeric receptors is discussed in detail regarding the loops and residues that constitute the binding site. The ligand-residue interactions in this classical binding and those of mutant receptors are discussed. The structure and conformations of GABA are discussed in regard to its flexibility and molecular properties. Although the binding mode of GABA is difficult to predict, several interactions between GABA and the receptor assist in predicting its potential conformation and mode of action. The structure-activity relationships of GABA and structurally key ligands at ρ receptors are described and discussed.

Analysis of Differential Efficacy and Affinity of GABAA (α1/α2) Selective Modulators

Selective modulators of the γ-amino butyric acid (GABA_A) family of receptors have the potential to treat a range of disease states related to cognition, pain, and anxiety. While the development of various α subunit-selective modulators is currently underway for the treatment of anxiety disorders, a mechanistic understanding of the correlation between their bioactivity and efficacy, based on ligand-target interactions, is currently still lacking. In order to alleviate this situation, in the current study we have analyzed, using ligand- and structure-based methods, a data set of 5440 GABA_A modulators. The Spearman correlation (ρ) between binding activity and efficacy of compounds was calculated to be 0.008 and 0.31 against the α1 and α2 subunits of GABA receptor, respectively; in other words, the compounds had little diversity in structure and bioactivity, but they differed significantly in efficacy. Two compounds were selected as a case study for detailed interaction analysis due to the small difference in their structures and affinities (ΔpK_{i(comp1_α1 - comp2_α1)} = 0.45 log units, ΔpK_{i(comp1_α2 - comp2_α2)} = 0 log units) as compared to larger relative efficacies (ΔRE_{(comp1_α1 - comp2_α1)} = 1.03, ΔRE_{(comp1_α2 - comp2_α2)} = 0.21). Docking analysis suggested that His-101 is involved in a characteristic interaction of the α1 receptor with both compounds 1 and 2. Residues such as Phe-77, Thr-142, Asn-60, and Arg-144 of the γ chain of the α1γ2 complex also showed interactions with heterocyclic rings of both compounds 1 and 2, but these interactions were disturbed in the case of α2γ2 complex docking results. Binding pocket stability analysis based on molecular dynamics identified three substitutions in the loop C region of the α2 subunit, namely, G200E, I201T, and V202I, causing a reduction in the flexibility of α2 compared to α1. These amino acids in α2, as compared to α1, were also observed to decrease the vibrational and dihedral entropy and to increase the hydrogen bond content in α2 in the apo state. However, freezing of both α1 and α2 was observed in the ligand-bound state, with an increased number of internal hydrogen bonds and increased entropy. Therefore, we hypothesize that the amino acid differences in the loop C region of α2 are responsible for conformational changes in the protein structure compared to α1, as well as for the binding modes of compounds and hence their functional signaling.

Current advances in the synthesis and biological potencies of tri- and tetra-substituted 1H-imidazoles

In this report, we review the current chemistry progress and in particular the synthesis approaches of tri- and tetra-substituted imidazoles.

The orthosteric GABAA receptor ligand Thio-4-PIOL displays distinctly different functional properties at synaptic and extrasynaptic receptors

BACKGROUND AND PURPOSE

Explorations into the heterogeneous population of native GABA type A receptors (GABAA Rs) and the physiological functions governed by the multiple GABAA R subtypes have for decades been hampered by the lack of subtype-selective ligands.

EXPERIMENTAL APPROACH

The functional properties of the orthosteric GABAA receptor ligand 5-(4-piperidyl)-3-isothiazolol (Thio-4-PIOL) have been investigated in vitro, ex vivo and in vivo.

KEY RESULTS

Thio-4-PIOL displayed substantial partial agonist activity at the human extrasynaptic GABAA R subtypes expressed in Xenopus oocytes, eliciting maximal responses of up to ∼30% of that of GABA at α5 β3 γ2S , α4 β3 δ and α6 β3 δ and somewhat lower efficacies at the corresponding α5 β2 γ2S , α4 β2 δ and α6 β2 δ subtypes (maximal responses of 4-12%). In contrast, it was an extremely low efficacious agonist at the α1 β3 γ2S , α1 β2 γ2S , α2 β2 γ2S , α2 β3 γ2S , α3 β2 γ2S and α3 β3 γ2S GABAA Rs (maximal responses of 0-4%). In concordance with its agonism at extrasynaptic GABAA Rs and its de facto antagonism at the synaptic receptors, Thio-4-PIOL elicited robust tonic currents in electrophysiological recordings on slices from rat CA1 hippocampus and ventrobasal thalamus and antagonized phasic currents in hippocampal neurons. Finally, the observed effects of Thio-4-PIOL in rat tests of anxiety, locomotion, nociception and spatial memory were overall in good agreement with its in vitro and ex vivo properties.

CONCLUSION AND IMPLICATIONS

The diverse signalling characteristics of Thio-4-PIOL at GABAA Rs represent one of the few examples of a functionally subtype-selective orthosteric GABAA R ligand reported to date. We propose that Thio-4-PIOL could be a useful pharmacological tool in future studies exploring the physiological roles of native synaptic and extrasynaptic GABAA Rs.

Synthesis and biological evaluation of 4-(aminomethyl)-1-hydroxypyrazole analogues of muscimol as γ-aminobutyric acid(a) receptor agonists

A series of bioisosteric 4-(aminomethyl)-1-hydroxypyrazole (4-AHP) analogues of muscimol, a GABA(A) receptor agonist, has been synthesized and pharmacologically characterized at native and selected recombinant GABA(A) receptors. The unsubstituted 4-AHP analogue (2a) (EC(50) 19 μM, R(max) 69%) was a moderately potent agonist at human α(1)β(2)γ(2) GABA(A) receptors, and in SAR studies substitutions in the 3- and/or 5-position were found to be detrimental to binding affinities. Ligand-receptor docking in an α(1)β(2)γ(2) GABA(A) receptor homology model along with the obtained SAR indicate that 2a and muscimol share a common binding mode, which deviates from the binding mode of the structurally related antagonist series based on 4-(piperidin-4-yl)-1-hydroxypyrazole (4-PHP, 1). Selectivity for α(1)β(2)γ(2) over ρ(1) GABA(A) receptors was observed for the 5-chloro, 5-bromo, and 5-methyl substituted analogues of 2a illustrating that even small differences in structure can give rise to subtype selectivity.

A unified model of the GABA(A) receptor comprising agonist and benzodiazepine binding sites

We present a full-length α(1)β(2)γ(2) GABA receptor model optimized for agonists and benzodiazepine (BZD) allosteric modulators. We propose binding hypotheses for the agonists GABA, muscimol and THIP and for the allosteric modulator diazepam (DZP). The receptor model is primarily based on the glutamate-gated chloride channel (GluCl) from C. elegans and includes additional structural information from the prokaryotic ligand-gated ion channel ELIC in a few regions. Available mutational data of the binding sites are well explained by the model and the proposed ligand binding poses. We suggest a GABA binding mode similar to the binding mode of glutamate in the GluCl X-ray structure. Key interactions are predicted with residues α(1)R66, β(2)T202, α(1)T129, β(2)E155, β(2)Y205 and the backbone of β(2)S156. Muscimol is predicted to bind similarly, however, with minor differences rationalized with quantum mechanical energy calculations. Muscimol key interactions are predicted to be α(1)R66, β(2)T202, α(1)T129, β(2)E155, β(2)Y205 and β(2)F200. Furthermore, we argue that a water molecule could mediate further interactions between muscimol and the backbone of β(2)S156 and β(2)Y157. DZP is predicted to bind with interactions comparable to those of the agonists in the orthosteric site. The carbonyl group of DZP is predicted to interact with two threonines α(1)T206 and γ(2)T142, similar to the acidic moiety of GABA. The chlorine atom of DZP is placed near the important α(1)H101 and the N-methyl group near α(1)Y159, α(1)T206, and α(1)Y209. We present a binding mode of DZP in which the pending phenyl moiety of DZP is buried in the binding pocket and thus shielded from solvent exposure. Our full length GABA(A) receptor is made available as Model S1.

Discovery of a novel allosteric modulator of 5-HT3 receptors: inhibition and potentiation of Cys-loop receptor signaling through a conserved transmembrane intersubunit site

The ligand-gated ion channels in the Cys-loop receptor superfamily mediate the effects of neurotransmitters acetylcholine, serotonin, GABA, and glycine. Cys-loop receptor signaling is susceptible to modulation by ligands acting through numerous allosteric sites. Here we report the discovery of a novel class of negative allosteric modulators of the 5-HT(3) receptors (5-HT(3)Rs). PU02 (6-[(1-naphthylmethyl)thio]-9H-purine) is a potent and selective antagonist displaying IC(50) values of ~1 μM at 5-HT(3)Rs and substantially lower activities at other Cys-loop receptors. In an elaborate mutagenesis study of the 5-HT(3)A receptor guided by a homology model, PU02 is demonstrated to act through a transmembrane intersubunit site situated in the upper three helical turns of TM2 and TM3 in the (+)-subunit and TM1 and TM2 in the (-)-subunit. The Ser(248), Leu(288), Ile(290), Thr(294), and Gly(306) residues are identified as important molecular determinants of PU02 activity with minor contributions from Ser(292) and Val(310), and we propose that the naphthalene group of PU02 docks into the hydrophobic cavity formed by these. Interestingly, specific mutations of Ser(248), Thr(294), and Gly(306) convert PU02 into a complex modulator, potentiating and inhibiting 5-HT-evoked signaling through these mutants at low and high concentrations, respectively. The PU02 binding site in the 5-HT(3)R corresponds to allosteric sites in anionic Cys-loop receptors, which emphasizes the uniform nature of the molecular events underlying signaling through the receptors. Moreover, the dramatic changes in the functional properties of PU02 induced by subtle changes in its binding site bear witness to the delicate structural discrimination between allosteric inhibition and potentiation of Cys-loop receptors.

Potentiating action of propofol at GABAA receptors of retinal bipolar cells

PURPOSE

Propofol (2,6-diisopropyl phenol), a widely used systemic anesthetic, is known to potentiate GABA(A) receptor activity in a number of CNS neurons and to produce changes in electroretinographically recorded responses of the retina. However, little is known about propofol's effects on specific retinal neurons. The authors investigated the action of propofol on GABA-elicited membrane current responses of retinal bipolar cells, which have both GABA(A) and GABA(C) receptors.

METHODS

Single, enzymatically dissociated bipolar cells obtained from rat retina were treated with propofol delivered by brief application in combination with GABA or other pharmacologic agents or as a component of the superfusing medium.

RESULTS

When applied with GABA at subsaturating concentrations and with TPMPA (a known GABA(C) antagonist), propofol markedly increased the peak amplitude and altered the kinetics of the response. Propofol increased the response elicited by THIP (a GABA(A)-selective agonist), and the response was reduced by bicuculline (a GABA(A) antagonist). The response to 5-methyl I4AA, a GABA(C)-selective agonist, was not enhanced by propofol. Serial brief applications of (GABA + TPMPA + propofol) led to a progressive increase in peak response amplitude and, at higher propofol concentrations, additional changes that included a prolonged time course of response recovery. Pre-exposure of the cell to perfusing propofol typically enhanced the rate of development of potentiation produced by (GABA + TPMPA + propofol) applications.

CONCLUSIONS

Propofol exerts a marked and selective potentiation on GABA(A) receptors of retinal bipolar cells. The data encourage the use of propofol in future studies of bipolar cell function.

Neurovascular interaction and the pathophysiology of diabetic retinopathy

Diabetic retinopathy (DR) is the most severe of the several ocular complications of diabetes, and in the United States it is the leading cause of blindness among adults 20 to 74 years of age. Despite recent advances in our understanding of the pathogenesis of DR, there is a pressing need to develop novel therapeutic treatments that are both safe and efficacious. In the present paper, we identify a key mechanism involved in the development of the disease, namely, the interaction between neuronal and vascular activities. Numerous pathological conditions in the CNS have been linked to abnormalities in the relationship between these systems. We suggest that a similar situation arises in the diabetic retina, and we propose a logical strategy aimed at therapeutic intervention.

Development of imidazole alkanoic acids as mGAT3 selective GABA uptake inhibitors

A new series of potential GABA uptake inhibitors starting from of 1H-imidazol-4-ylacetic acid with the carboxylic acid side chain originating from different positions and varying in length have been synthesized and tested for the inhibitory potency at the four GABA uptake transporters mGAT1-4 stably expressed in HEK cells. Further two bicyclic compounds with a rigidified carboxylic acid side chain were included in this study. The results of the biological tests indicated that most ω-imidazole alkanoic and alkenoic acid derivatives exhibit the highest potencies as GABA uptake inhibitors at mGAT3.

Medicinal chemistry of ρ GABAC receptors

The inhibitory neurotransmitter, GABA, is a low-molecular-weight molecule that can achieve many low-energy conformations, which are recognized by GABA receptors and transporters. In this article, we assess the structure–activity relationship profiles of GABA analogs at the ionotropic ρ GABAC receptor. Such studies have significantly contributed to the design and development of potent and selective agonists and antagonists for this subclass of GABA receptors. With these tools in hand, the role of ρ GABAC receptors is slowly being realized. Of particular interest is the development of selective phosphinic acid analogs of GABA and their potential use in sleep disorders, inhibiting the development of myopia, and in improving learning and memory.

Neurochemicals for the investigation of GABA(C) receptors

GABA(C) receptors are being investigated for their role in many aspects of nervous system function including memory, myopia, pain and sleep. There is evidence for functional GABA(C) receptors in many tissues such as retina, hippocampus, spinal cord, superior colliculus, pituitary and the gut. This review describes a variety of neurochemicals that have been shown to be useful in distinguishing GABA(C) receptors from other receptors for the major inhibitory neurotransmitter GABA. Some selective agonists (including (+)-CAMP and 5-methyl-IAA), competitive antagonists (such as TPMPA, (±)-cis-3-ACPBPA and aza-THIP), positive (allopregnanolone) and negative modulators (epipregnanolone, loreclezole) are described. Neurochemicals that may assist in distinguishing between homomeric ρ1 and ρ2 GABA(C) receptors (2-methyl-TACA and cyclothiazide) are also covered. Given their less widespread distribution, lower abundance and relative structural simplicity compared to GABA(A) and GABA(B) receptors, GABA(C) receptors are attractive drug targets.

Population Patch-Clamp Electrophysiology Analysis of Recombinant GABAA α1β3γ2 Channels Expressed in HEK-293 Cells

γ-Amino butyric acid (GABA)—activated Cl— channels are critical mediators of inhibitory postsynaptic potentials in the CNS. To date, rational design efforts to identify potent and selective GABAA subtype ligands have been hampered by the absence of suitable high-throughput screening approaches. The authors describe 384-well population patch-clamp (PPC) planar array electrophysiology methods for the study of GABAA receptor pharmacology. In HEK293 cells stably expressing human α1β3γ2 GABAA channels, GABA evoked outward currents at 0 mV of 1.05 ± 0.08 nA, measured 8 s post GABA addition. The IGABA was linear and reversed close to the theoretical ECl (—56 mV). Concentration-response curve analysis yielded a mean pEC50 value of 5.4 and Hill slope of 1.5, and for a series of agonists, the rank order of potency was muscimol > GABA > isoguvacine. A range of known positive modulators, including diazepam and pentobarbital, produced concentration-dependent augmentation of the GABA EC 20 response (1 µM). The competitive antagonists bicuculline and gabazine produced concentration-dependent, parallel, rightward displacement of GABA curves with pA2 and slope values of 5.7 and 1.0 and 6.7 and 1.0, respectively. In contrast, picrotoxin (0.2-150 µM) depressed the maximal GABA response, implying a non-competitive antagonism. Overall, the pharmacology of human α1β3γ2 GABAA determined by PPC was highly similar to that obtained by conventional patch-clamp methods. In small-scale single-shot screens, Z′ values of >0.5 were obtained in agonist, modulator, and antagonist formats with hit rates of 0% to 3%. The authors conclude that despite the inability of the method to resolve the peak agonist responses, PPC can rapidly and usefully quantify pharmacology for the α1β3γ2 GABAA isoform. These data suggest that PPC may be a valuable approach for a focused set and secondary screening of GABAA receptors and other slow ligand-gated ion channels. ( Journal of Biomolecular Screening 2009:769-780)

A molecular basis for agonist and antagonist actions at GABA(C) receptors

We modelled the N-terminal ligand-binding domain of the rho1 GABA(C) receptor based on the Lymnaea stagnalis acetylcholine-binding protein (L-AChBP) crystal structure using comparative modelling and validated using flexible docking guided by known mutagenesis studies. A range of known rho1 GABA(C) receptor ligands comprising seven full agonists, 10 partial agonists, 43 antagonists and 12 inactive molecules were used to evaluate and validate the models. Of the 50 models identified, six models that allowed flexible ligand docking in accordance with the experimental data were selected and used to study detailed receptor-ligand interactions. The most refined model to accommodate all known active ligands featured a cavity comprising of a volume of 488 A(3). A detailed analysis of the interaction between the rho1 GABA(C) receptor model and the docked ligands revealed possible H-bonds and cation-pi interactions between the different ligands and binding site residues. Based on quantum mechanical/molecular mechanical (QM/MM) calculations, the model showed distinctive conformations of loop C that provided a molecular basis for agonist and antagonist actions. Agonists elicit loop C closure, while a more open loop C was observed upon antagonist binding. The model differentiates the role for key residues known to be involved in either binding and/or gating.