GABA agonists. Resolution, absolute stereochemistry, and enantioselectivity of (S)-(+)- and (R)-(-)-dihydromuscimol

Visualizing GABA transporters in vivo: an overview of reported radioligands and future directions

By clearing GABA from the synaptic cleft, GABA transporters (GATs) play an essential role in inhibitory neurotransmission. Consequently, in vivo visualization of GATs can be a valuable diagnostic tool and biomarker for various psychiatric and neurological disorders. Not surprisingly, in recent years several research attempts to develop a radioligand have been conducted, but so far none have led to suitable radioligands that allow imaging of GATs. Here, we provide an overview of the radioligands that were developed with a focus on GAT1, since this is the most abundant transporter and most of the research concerns this GAT subtype. Initially, we focus on the field of GAT1 inhibitors, after which we discuss the development of GAT1 radioligands based on these inhibitors. We hypothesize that the radioligands developed so far have been unsuccessful due to the zwitterionic nature of their nipecotic acid moiety. To overcome this problem, the use of non-classical GAT inhibitors as basis for GAT1 radioligands or the use of carboxylic acid bioisosteres may be considered. As the latter structural modification has already been used in the field of GAT1 inhibitors, this option seems particularly viable and could lead to the development of more successful GAT1 radioligands in the future.

A radical-mediated 1,3,4-trifunctionalization cascade of 1,3-enynes with sulfinates and tert-butyl nitrite: facile access to sulfonyl isoxazoles

An unprecedented indium-promoted three-component 1,3,4-trifunctionalization cascade of 1,3-enynes with sulfinates and tert-butyl nitrite for producing 5-sulfonylisoxazoles via [3+2] annulation is reported.

Poly(ε-caprolactone)-block-polysarcosine by Ring-Opening Polymerization of Sarcosine N-Thiocarboxyanhydride: Synthesis and Thermoresponsive Self-Assembly

Biocompatible amphiphilic block copolymers composed of polysarcosine (PSar) and poly(ε-caprolactone) (PCL) were synthesized using ring-opening polymerization of sarcosine N-thiocarboxyanhydride initiated by oxyamine-ended PCL and characterized by NMR, SEC, and DSC. Self-assembling of two triblock copolymers PSar8-b-PCL28-b-PSar8 (CS7) and PSar16-b-PCL40-b-PSar16 (CS10) in dilute solution was studied in detail toward polymersome formation using thin-film hydration and nanoprecipitation techniques. A few giant vesicles were obtained by thin-film hydration from both copolymers and visualized by confocal laser scanning microscope. Unilamellar sheets and nanofibers (with 8-10 nm thickness or diameter) were obtained by nanoprecipitation at room temperature and observed by Cryo-TEM. These lamellae and fibrous structures were transformed into worm-like cylinders and spheres (D∼30-100 nm) after heating to 65 °C (>Tm,PCL). Heating CS10 suspensions to 90 °C led eventually to multilamellar polymersomes (D∼100-500 nm). Mechanism II, where micelles expand to vesicles through water diffusion and hydrophilic core forming, was proposed for polymersome formation. A cell viability test confirmed the self-assemblies were not cytotoxic.

Probing the orthosteric binding site of GABAA receptors with heterocyclic GABA carboxylic acid bioisosteres

The ionotropic GABAA receptors (GABAARs) are widely distributed in the central nervous system where they play essential roles in numerous physiological and pathological processes. A high degree of structural heterogeneity of the GABAAR has been revealed and extensive effort has been made to develop selective and potent GABAAR agonists. This review investigates the use of heterocyclic carboxylic acid bioisosteres within the GABAAR area. Several heterocycles including 3-hydroxyisoxazole, 3-hydroxyisoxazoline, 3-hydroxyisothiazole, and the 1- and 3-hydroxypyrazole rings have been employed in order to map the orthosteric binding site. The physicochemical properties of the heterocyclic moieties making them suitable for bioisosteric replacement of the carboxylic acid in the molecule of GABA are discussed. A variety of synthetic strategies for synthesis of the heterocyclic scaffolds are available. Likewise, methods for introduction of substituents into specific positions of the heterocyclic scaffolds facilitate the investigation of different regions in the orthosteric binding pocket in close vicinity of the core scaffolds of muscimol/GABA. The development of structural models, from pharmacophore models to receptor homology models, has provided more insight into the molecular basis for binding. Similar binding modes are proposed for the heterocyclic GABA analogues covered in this review by use of ligand-receptor docking into the receptor homology model and the presented structure-activity relationships. A network of interactions between the analogues and the binding pocket is leaving no room for substituents and underline the limited space in the GABAAR orthosteric binding site when in the agonist conformation.

Differentiating enantioselective actions of GABOB: a possible role for threonine 244 in the binding site of GABA(C) ρ(1) receptors

Designing potent and subtype-selective ligands with therapeutic value requires knowledge about how endogenous ligands interact with their binding site. 4-Amino-3-hydroxybutanoic acid (GABOB) is an endogenous ligand found in the central nervous system in mammals. It is a metabolic product of GABA, the major inhibitory neurotransmitter. Homology modeling of the GABA(C) ρ(1) receptor revealed a potential H-bond interaction between the hydroxyl group of GABOB and threonine 244 (T244) located on loop C of the ligand binding site of the ρ(1) subunit. Using site-directed mutagenesis, we examined the effect of mutating T244 on the efficacy and pharmacology of GABOB and various ligands. It was found that mutating T244 to amino acids that lacked a hydroxyl group in their side chains produced GABA insensitive receptors. Only by mutating ρ(1)T244 to serine (ρ(1)T244S) produced a GABA responsive receptor, albeit 39-fold less sensitive to GABA than ρ(1)wild-type. We also observed changes in the activities of the GABA(C) receptor partial agonists, muscimol and imidazole-4-acetic acid (I4AA). At the concentrations we tested, the partial agonists antagonized GABA-induced currents at ρ(1)T244S mutant receptors (Muscimol: ρ(1)wild-type, EC(50) = 1.4 μM; ρ(1)T244S, IC(50) = 32.8 μM. I4AA: ρ(1)wild-type, EC(50) = 8.6 μM; ρ(1)T244S, IC(50) = 21.4 μM). This indicates that T244 is predominantly involved in channel gating. R-(-)-GABOB and S-(+)-GABOB are full agonists at ρ(1)wild-type receptors. In contrast, R-(-)-GABOB was a weak partial agonist at ρ(1)T244S (1 mM activates 26% of the current produced by GABA EC(50) versus ρ(1)wild-type, EC(50) = 19 μM; I(max) 100%), and S-(+)-GABOB was a competitive antagonist at ρ(1)T244S receptors (ρ(1)wild-type, EC(50) = 45 μM versus ρ(1)T244S, IC(50) = 417.4 μM, K(B) = 204 μM). This highlights that the interaction of GABOB with T244 is enantioselective. In contrast, the potencies of a range of antagonists tested, 3-aminopropyl(methyl)phosphinic acid (3-APMPA), 3-aminopropylphosphonic acid (3-APA), S- and R-(3-amino-2-hydroxypropyl)methylphosphinic acid (S-(-)-CGP44532 and R-(+)-CGP44533), were not altered. This suggests that T244 is not critical for antagonist binding. Receptor gating is dynamic, and this study highlights the role of loop C in agonist-evoked receptor activation, coupling agonist binding to channel gating.

First Example of Direct RuO4-Catalyzed Oxidation of Isoxazolidines to 3-Isoxazolidones

RuO2/NaIO4 oxidation of 3-unsubstituted isoxazolidines, under ethyl acetate/water biphasic conditions, affords 3-isoxazolidones in good yields. The methodology can be used on both racemic and optically active isoxazolidines.

GABA(A) agonists and partial agonists: THIP (Gaboxadol) as a non-opioid analgesic and a novel type of hypnotic

The GABA(A) receptor system is implicated in a number of central nervous system (CNS) disorders, making GABA(A) receptor ligands interesting as potential therapeutic agents. Only a few different classes of structures are currently known as ligands for the GABA recognition site on the hetero-pentameric GABA(A) receptor complex, reflecting the very strict structural requirements for GABA(A) receptor recognition and activation. A large number of the compounds showing agonist activity at the GABA(A) receptor site are structurally derived from the GABA(A) agonists muscimol, THIP (Gaboxadol), or isoguvacine, which we developed at the initial stage of the project. Using recombinant GABA(A) receptors, functional selectivity has been shown for a number of compounds, including THIP, showing subunit-dependent potency and maximal response. The pharmacological and clinical activities of THIP probably reflect its potent effects at extrasynaptic GABA(A) receptors insensitive to benzodiazepines and containing alpha(4)beta(3)delta subunits. The results of ongoing clinical studies on the effect of the partial GABA(A) agonist THIP on human sleep pattern show that the functional consequences of a directly acting agonist are distinctly different from those seen after administration of GABA(A) receptor modulators, such as benzodiazepines. In the light of the interest in partial GABA(A) receptor agonists as potential therapeutics, structure-activity studies of a number of analogues of 4-PIOL, a low-efficacy partial GABA(A) agonist derived from THIP, have been performed. In this connection, a series of GABA(A) ligands has been developed showing pharmacological profiles ranging from low-efficacy partial GABA(A) agonist activity to selective antagonist effect.

Selective inhibitors of GABA uptake: synthesis and molecular pharmacology of 4-N-methylamino-4,5,6,7-tetrahydrobenzo[d]isoxazol-3-ol analogues

A series of lipophilic diaromatic derivatives of the glia-selective GABA uptake inhibitor (R)-4-amino-4,5,6,7-tetrahydrobenzo[d]isoxazol-3-ol [(R)-exo-THPO, 4] were synthesized via reductive amination of 3-ethoxy-4,5,6,7-tetrahydrobenzo[d]isoxazol-4-one (9) or via N-alkylation of O-alkylatedracemic 4. The effects of the target compounds on GABA uptake mechanisms in vitro were measured using a rat brain synaptosomal preparation or primary cultures of mouse cortical neurons and glia cells (astrocytes), as well as HEK cells transfected with cloned mouse GABA transporter subtypes (GAT1-4). The activity against isoniazid-induced convulsions in mice after subcutaneous administration of the compounds was determined. All of the compounds were potent inhibitors of synaptosomal uptake the most potent compound being (RS)-4-[N-(1,1-diphenylbut-1-en-4-yl)amino]-4,5,6,7-tetrahydrobenzo[d]isoxazol-3-ol (17a, IC50 = 0.14 microM). The majority of the compounds showed a weak preference for glial, as compared to neuronal, GABA uptake. The highest degree of selectivity was 10-fold corresponding to the glia selectivity of (R)-N-methyl-exo-THPO (5). All derivatives showed a preference for the GAT1 transporter, as compared with GAT2-4, with the exception of (RS)-4-[N-[1,1-bis(3-methyl-2-thienyl)but-1-en-4-yl]-N-methylamino]-4,5,6,7-tetrahydrobenzo[d]isoxazol-3-ol (28d), which quite surprisingly turned out to be more potent than GABA at both GAT1 and GAT2 subtypes. The GAT1 activity was shown to reside in (R)-28d whereas (R)-28d and (S)-28d contributed equally to GAT2 activity. This makes (S)-28d a GAT2 selective compound, and (R)-28d equally effective in inhibition of GAT1 and GAT2 mediated GABA transport. All compounds tested were effective as anticonvulsant reflecting that these compounds have blood-brain barrier permeating ability.

Specific GABA(A) agonists and partial agonists

The GABA(A) receptor system is implicated in a number of neurological and psychiatric diseases, making GABA(A) receptor ligands interesting as potential therapeutic agents. Only a few different classes of structures are currently known as ligands for the GABA recognition site on the hetero-pentameric GABA(A) receptor complex, reflecting the very strict structural requirements for GABA(A) receptor recognition and activation. Within the series of compounds showing agonist activity at the GABA(A) receptor site that have been developed, most of the ligands are structurally derived from the GABA(A) agonists muscimol, THIP, or isoguvacine, which we developed in the initial stages of the project. Using recombinant GABA(A) receptors, functional selectivity was demonstrated for a number of compounds, including THIP, showing highly subunit-dependent potency and maximal response. In light of the interest in partial GABA(A) receptor agonists as potential therapeutics, structure-activity studies of a number of analogs of 4-PIOL, a low-efficacy partial GABA(A) agonist derived from THIP, have been performed. In this connection, a series of GABA(A) ligands has been developed that exhibit pharmacological profiles from moderately potent low-efficacy partial GABA(A) agonist activity to potent and selective antagonist effects. Very little information is available on direct-acting GABA(A) receptor agonists in clinical studies. However, the results of clinical studies on the effect of the partial GABA(A) agonist THIP on human sleep patterns show that the functional consequences of a direct-acting agonist are different from those seen after the administration of GABA(A) receptor modulators, such as benzodiazepines and barbiturates.

Superimposition-based protocol as a tool for determining bioactive conformations. II. Application to the GABA(A) receptor

The natural templates (NT) superimposition method is used to determine the pharmacophoric requirements of the A subtype of the gamma-aminobutyric acid (GABA) receptor. Bioactive conformations for antagonists and agonists are found by superimposing them on a relatively rigid alkaloid bicuculline, which itself is a competitive antagonist at this ligand-gated ion channel receptor. As has been usual in the application of this modeling method, consideration of available experimental data is the cornerstone for obtaining realistic models. The identification of two substructural fragments of bicuculline permitted classification of the ligands. Analysis of the antagonists and agonists with respect to the two substructural fragments revealed two bioactive conformations of the highly flexible GABA molecule, one of which is extended with the nonhydrogenic atoms roughly coplanar torsional angles of -37 and -179 degrees at N-C-C-C and C-C-C-C (carboxyl), respectively. The second bioactive compound is clearly non planar (torsional angles of -81 and -109 degrees at N-C-C-C and C-C-C-C (carboxyl), respectively).

Preparation of (S)-N-substituted 4-hydroxy-pyrrolidin-2-ones by regio- and stereoselective hydroxylation with Sphingomonas sp. HXN-200

[reaction: see text] Enantiopure (S)-N-substituted 4-hydroxy-pyrrolidin-2-ones have been prepared for the first time by regio- and stereoselective hydroxylation of the corresponding pyrrolidin-2-ones by use of a biocatalyst. Hydroxylation of 6 and 8 with Sphingomonas sp. HXN-200 afforded 68% of (S)-7 in >99.9% ee and 46% of (S)-9 in 92% ee, respectively. Simple crystallization increased the ee of (S)-9 to 99. 9% in 82% yield.

Synthesis and pharmacology of seleninic acid analogues of the inhibitory neurotransmitter gamma-aminobutyric acid

[structure: see text] The first seleninic acid analogues of gamma-aminobutyric acid (GABA) and of the specific GABA(A) agonist piperidine-4-carboxylic acid (isonipecotic acid), 1 and 2, respectively, have been synthesized and shown to be potent agonists at the GABA receptors.

GABAA receptor pharmacology

gamma-Aminobutyric acid (GABA)A receptors for the inhibitory neurotransmitter GABA are likely to be found on most, if not all, neurons in the brain and spinal cord. They appear to be the most complicated of the superfamily of ligand-gated ion channels in terms of the large number of receptor subtypes and also the variety of ligands that interact with specific sites on the receptors. There appear to be at least 11 distinct sites on GABAA receptors for these ligands.

Identifying agonistic and antagonistic mechanisms operative at the GABA receptor

Based on our molecular modeling investigations of the glycinergic receptor, we expanded our studies to similarly investigate the GABAergic receptor. New data suggest there may exist a slightly different agonistic mechanism for the molecules described herein as compared to glycine. The origin of this is undoubtedly the fact that, while glycine has a positive and two negative binding sites, it is significantly shorter than GABA and the other GABA agonists. Clearly, discovery of more glycine agonists is needed to further clarify this point. Moreover, we find a remarkedly different antagonistic mechanism exists for this phylogenetically newer inhibitory system in the central nervous system (CNS) than recently reported for strychnine and eight weaker glycine antagonists. We used GABA and six agonists (muscimol, dihydromuscimol, THIP, isoguvacine, trans-3-aminocyclopentane-1-carboxylic acid, piperidine-4-sulfonic acid) and five antagonists (bicuculline-N15-methobromide, R5135, pitrazepin, iso-THAZ and securinine) to derive our conclusions. We found that each of the agonists have three clearly defined atoms that can serve as attachment points at the GABAA receptor site. One of the three attachment atoms includes a carbonyl or carboxylate oxygen. The role of the carbonyl or carboxylate atom is very important. First, we theorize that a rapid two-point attachment occurs (one from the positive end and one from one of the other two negative atoms on the ligand) at the recognition site in the receptor where GABA or a GABAergic agonist binds. The positive end of the agonist perhaps associates through hydrogen bonding to a beta-carboxyl group in one of the aspartate molecules in the polypeptide. The negative attachment points perhaps bind through hydrogen bonding to arginine molecules in this polypeptide. The second negative site in the agonist immediately triggers a conformational change by pulling together the aforementioned groups by electrostatic attraction, and hence opening the chloride channel. We propose the carbonyl oxygen is partly responsible for triggering the opening by formation of a double hydrogen bond to arginine. We postulate that this attraction is the first step inducing the conformational change. In the case of the GABA antagonists investigated, a fourth attachment site was not found. In fact only two sites have been identified similar to the group II glycine antagonists. Our data support a hypothesis for GABAergic antagonist activity which suggests that the antagonist simply binds to the recognition site and blocks the neurotransmitter, GABA, from entering this site thereby preventing the opening of the chloride channel; it just stays closed. This mechanism is different from the mechanism proposed for the large number of Group I glycine antagonists (Aprison et al.: J Neurosci Res 41: 259-269, 1995).

Synthesis of (S)-3,4-diaminobutanenitriles as precursors for 3-amino-GABA derivatives

Starting from natural asparagine (1) a synthesis of the protected (S)-3,4-diaminobutanenitriles 5 and 8a-c via the beta-homoserine derivative 2 is described. The amino function in position 4 was introduced by Mitsunobu-coupling or by reductive amination when a strange deformylation of the amino aldehyde 7 was observed as a side reaction. The Mitsunobu-product 5 was converted into the dibenzylamine substituted GABA 6b which was investigated for its affinity at the GABA-A receptor.

Hydroxylated analogues of 5-aminovaleric acid as 4-aminobutyric acidB receptor antagonists: stereostructure-activity relationships

The (R) and (S) forms of 5-amino-2-hydroxyvaleric acid (2-OH-DAVA) and 5-amino-4-hydroxyvaleric acid (4-OH-DAVA) were designed as structural hybrids of the 4-aminobutyric acidB (GABAB) agonist (R)-(-)-4-amino-3-hydroxybutyric acid [(R)-(-)-3-OH-GABA] and the GABAB antagonist 5-aminovaleric acid (DAVA). (S)-(-)-2-OH-DAVA and (R)-(-)-4-OH-DAVA showed a moderately potent affinity for GABAB receptor sites in rat brain and showed GABAB antagonist effects in a guinea pig ileum preparation. The respective enantiomers, (R)-(+)-2-OH-DAVA and (S)-(+)-4-OH-DAVA, were markedly weaker in both test systems. All four compounds were weak inhibitors of GABAA receptor binding in rat brain, and none of them significantly affected synaptosomal GABA uptake. Based on molecular modeling studies it has been demonstrated that low-energy conformations of (R)-(-)-3-OH-GABA, (S)-(-)-2-OH-DAVA, and (R)-(-)-4-OH-DAVA can be superimposed. These conformations may reflect the shapes adopted by these conformationally flexible compounds during their interaction with GABAB receptors. The present studies emphasize the similar, but distinct, constraints imposed on agonists and antagonists for GABAB receptors.

Ligand structural specificity of GABAA receptors in guinea pig ileum

The ligand structural specificity of ileal GABAA receptors was examined using the strength and half-life of contractions in guinea-pig myenteric plexus-longitudinal muscle preparations. The agonists used differ by more than a factor of 1000 in affinity to central GABAA receptors and include both conformationally flexible and restricted molecules as well as pairs of enantiomers. The overall correlation between ileal contractile activity and rat brain receptor affinity was poor (r = 0.75), but within groups of conformationally flexible or conformationally restricted molecules a high correlation was found (r greater than 0.9999). When comparing data for ileal contractile activity with available data for agonist activity in the CNS no difference between ligand specificity of ileal and central GABAA receptors was apparent with the present range of ligands. The half-lives of ileal contractile responses were found to decrease with increasing GABAB agonist activity.

Enantioselectivity at the physiologically active GABAA receptor

A subfraction of cortical tissue from rat brain, containing membrane vesicles was prepared freshly with added protease inhibitors and antioxidant. The preparation was used to measure stimulation of transmembrane 36C1- flux and inhibition of bicuculline-sensitive [3H] muscimol binding by (+)-(S) and (-)-(R) enantiomers of dihydromuscimol at 30 degrees in physiological salt solution. Displacement of bound [3H]muscimol and stimulation of 36Cl- flux appeared in the 0.1-10 microM concentration range of the enantiomers, channel gating, however, required rather high concentrations. Degrees of enantioselectivity for channel gating, desensitization of and binding to GABAA receptors were estimated by the concentration ratios of dihydromuscimol enantiomers, [(-)-(R)]/[(+)-(S)], at the same level of response or displacement. Different enantioselectives were observed for channel gating (6 +/- 3), receptor binding (3 +/- 2) and desensitization (no selectivity). The low and concentration-dependent enantioselectives found for channel gating and receptor binding can be explained by desensitization and heterogeneity of GABAA receptors.

Modeling of agonist binding to the ligand-gated ion channel superfamily of receptors

A generalized model is presented of agonist binding to ligand-gated ion channels (LGICs). Broad similarity in the structure of agonists suggests that the binding sites of LGICs may have evolved from a protobinding site. Aligned sequence data identified as a candidate for such a site a highly conserved 15 residue stretch of primary structure in the N-terminal extracellular region of all known LGIC subunits. We modeled this subregion, termed the cys-loop, as a rigid, amphiphilic beta-hairpin and propose that it may form a major determinant of a conserved structural binding cleft. In the model of the binding complex (1) an invariant aspartate residue at position 11 of the cys-loop is the anionic site interacting with the positively charged amine group of agonists, (2) a local dipole within the pi-electron system of agonists is favorably oriented in the electrostatic field of the invariant aspartate, (3) the epsilon ring-proton of a conserved aromatic residue at the turn of the cys-loop interacts orthogonally with the agonist pi-electron density at its electronegative center, and (4) selective recognition is partly a result of the type of amino acid residue at position 6 of the cys-loop. Additionally, formation of a hydrogen bond between the electronegative atom of the pi-electron system of agonist and a complementary group in the receptor may be important in the high-affinity binding of agonists.

GABA receptors on the somatic muscle cells of the parasitic nematode, Ascaris suum: stereoselectivity indicates similarity to a GABAA-type agonist recognition site

1. The gamma-aminobutyric acid (GABA) receptors on the somatic muscle cells of Ascaris, which mediate muscle cell hyperpolarization and relaxation, have been characterized by use of intracellular recording techniques. 2. These receptors are like mammalian GABAA-receptors in that the response is mediated by an increase conductance to chloride ions. The GABAA-mimetic, muscimol, has a relative potency of 0.40 +/- 0.02 (n = 3) compared to GABA. 3. The stereoselectivity of the GABA receptor on Ascaris is identical to that for the mammalian GABAA-receptor, as determined from the relative potency of three pairs of enantiomers of structural analogues of GABA. 4. The most potent agonist is (S)-(+)-dihydromuscimol which is 7.53 +/- 0.98 (n = 5) times more potent than GABA. 5. The Ascaris GABA receptor is not significantly blocked, at concentrations below 100 microM by the potent, competitive GABAA-receptor antagonist, SR95531. 6. The Ascaris GABA receptor does not recognise agents that are known to block the GABA gated chloride channel in mammalian preparations such as t-butylbicyclophosphorothionate (TBPS, 10 microM, n = 2) or the insecticide dieldrin (100 microM, n = 3). 7. GABAergic responses in Ascaris are not potentiated by pentobarbitone (100 microM, n = 3) or flurazepam (100 microM, n = 3). 8. The potencies of various GABA-mimetics in the Ascaris preparation have been compared with their potency at displacing GABAA-receptor binding in mammalian brain. Excluding the sulphonic acid derivatives of GABA, the correlation coefficient (r) between the potencies of compounds in the two systems is 0.74 (P less than 0.01). The significance of this correlation is discussed. 9. The pharmacology of the Ascaris GABA receptor is discussed in relation to other invertebrate systems and the mammalian subclassification of GABA receptors.

Comparative stereostructure-activity studies on GABAA and GABAB receptor sites and GABA uptake using rat brain membrane preparations

The affinities of a number of analogues of gamma-aminobutyric acid (GABA) for GABAA and GABAB receptor sites and GABA uptake were studied using rat brain membrane preparations. Studies on the (S)-(+)- and (R)-(-)-isomers of baclofen, 3-hydroxy-4-aminobutyric acid (3-OH-GABA), and 4,5-dihydromuscimol (DHM) revealed different stereoselectivities of these synaptic mechanisms in vitro. Although (S)-3-OH-GABA and, in particular, (S)-DHM were more potent than the corresponding (R)-isomers as inhibitors of GABAA binding, the opposite stereoselectivity was demonstrated for the GABAB binding sites. Thus, (R)-3-OH-GABA and (R)-baclofen were more potent than the (S)-isomers as inhibitors of GABAB binding, (R)-baclofen being some five times more potent than (R)-3-OH-GABA. These two (R)-isomers actually have opposite orientation of the substituents on the GABA backbones, suggesting that the lipophilic substituent of (R)-baclofen interacts with a structural element of the GABAB receptor site different from that that binds the very polar hydroxy group of (R)-3-OH-GABA. The O-methylated analogue of 3-OH-GABA, 3-methoxy-4-aminobutyric acid (3-OCH3-GABA), did not interact significantly with GABAB sites. The homologues of GABA, trans-4-aminocrotonic acid (trans-ACA), muscimol, and 3-OH-GABA, that is, 5-aminovaleric acid (DAVA), trans-5-aminopent-2-enoic acid, homomuscimol, and 3-hydroxy-5-aminovaleric acid (3-OH-DAVA), respectively, were generally much weaker than the parent compounds, whereas 2-hydroxy-5-aminovaleric acid (2-OH-DAVA) showed a significantly higher affinity for GABAB sites than the corresponding GABA analogue.(ABSTRACT TRUNCATED AT 250 WORDS)