Actions of pentobarbitone and derivatives with modified 5-butyl substituents on GABA and diazepam binding to rat brain synaptosomal membranes

The effects of a variety of factors known to influence the enhancement of GABA binding by diazepam, were studied upon pentobarbitone stimulation of GABA binding to washed synaptosomal membranes prepared from whole rat brains. The differential kinetics of, and effects of temperature, chloride ions, a benzodiazepine receptor antagonist (Ro15-1788) and picrotoxinin upon pentobarbitone and diazepam enhancement of GABA binding, suggest that these drugs exert their actions upon GABA binding at different loci. The degree of enhancement of diazepam binding and of high affinity GABA binding in chloride-containing media at 25 degrees C by members of a series of twelve side chain methyl substituted and/or unsaturated derivatives of 5-butyl-5-ethyl-barbituric acid (pentobarbitone analogs) correlated significantly. For the sedative members of the series, enhancement of high affinity GABA binding correlated with their anaesthetic but not their anticonvulsant activities. It appears likely that the anaesthetic and anticonvulsant activities of barbiturates arise from different molecular actions.     

Phospholipid Methylation Increases [3H]Diazepam and [3H]GABA Binding in Membrane Preparations of Rat Cerebellum

The effect of phospholipid methylation on both [3H]diazepam and [3H]GABA ( [3H]gamma-aminobutyric acid) binding to crude synaptic plasma membrane from rat cerebellum has been studied. S-Adenosylmethionine (SAM) stimulates [3H]methyl group incorporation into membrane phospholipids and enhances [3H]diazepam binding by increasing the apparent Bmax. Conversely, inhibition of [3H]methyl group transfer from [3H]SAM to phospholipids by preincubation with SAM at 0 degrees C or with SAH abolishes the increase of binding. After preincubation with SAM, analysis of the GABA binding reveals the presence of binding sites with high affinity, a property absent in control membranes preincubated without SAM. Among the neurotransmitter bindings tested, only those of GABA and benzodiazepine in the cerebellum and beta-adrenergic ligands in the cerebral cortex are enhanced upon stimulation of phospholipid methyltransferase activity. [3H]Dihydromorphine, [3H]dihydro-alpha-ergokryptine and [3H]spiroperidol bindings are not affected by SAM. The present data suggest an involvement of phospholipid methylation in regulation of both [3H]GABA and [3H]-diazepam binding.     

GABA-benzodiazepine interactions: physiological, pharmacological and developmental aspects

Many of the pharmacological actions of the benzodiazepines can be attributed to their actions on gamma-aminobutyric acid (GABA) systms in the brain. Electrophysiological studies on dorsal raphe neurons indicate that the benzodiazepines act postsynaptically to potentiate GABAergic inhibition in this midbrain nucleus. Direct binding studies have shown that both in vitro and in vivo binding of [3H]diazepam to a specific high affinity benzodiazepine binding site in cerebral cortical tissue are enhanced by the direct in vitro addition of GABA and GABA agonists or by pretreatment of animals with GABA analogs and agents that elevate GABA levels in brain. Ontogenic development of [3H]diazepam binding in brain parallels the development of the sodium-independent [3H]GABA binding. The ability of GABA to enhance benzodiazepine binding is present throughout development and inversely related to age. These data suggest that there is a functionally significant interaction between the benzodiazepines and GABA throughout development and at maturity. A model is proposed to relate these interactions to conformational changes in a benzodiazepine/GABA/Cl- ionophore complex.     

Characteristics of Flunitrazepam Binding to Intact Primary Cultured Spinal Cord Neurons and Its Modulation by GABAergic Drugs

The interaction of [3H]flunitrazepam and its modulation by various drugs was studied in intact primary cultured spinal cord neurons. In the intact cells, the [3H]-flunitrazepam binding was rapid and saturable. The benzodiazepine binding sites exhibited high affinity and saturability, with an apparent KD of 6.1 +/- 1.6 nM and Bmax of 822 +/- 194 fmol/mg protein. The association and dissociation of [3H]flunitrazepam binding exhibited monoexponential kinetics. Specifically bound [3H]flunitrazepam was displaced in a concentration-dependent manner by benzodiazepines like flunitrazepam, clonazepam, diazepam, Ro 15-1788, and beta-carbolines like methyl-6,7-dimethoxy-4-ethyl-beta-carboline-3'-carboxylate. Specific [3H]flunitrazepam binding to intact cells was enhanced in a concentration-dependent manner by gamma-aminobutyric acid (GABA) agonists and drugs which facilitate GABAergic transmission like etazolate, (+)-etomidate, and pentobarbital. The enhancing effect of GABA agonists was antagonized by bicuculline and picrotoxinin. These results suggest that the intact cultured spinal cord neurons exhibit the properties of benzodiazepine GABA receptor-ionophore complex. Since these cells can also be studied in parallel for characterizing GABA-induced 36Cl-influx, they provide an ideal in vitro assay preparation to study GABA synaptic pharmacology.     

Chronic exposure of cells expressing recombinant GABAA receptors to benzodiazepine antagonist flumazenil enhances the maximum number of benzodiazepine binding sites

The aim of this study was to better understand the mechanisms that underlie adaptive changes in GABAA receptors following their prolonged exposure to drugs. Exposure (48 h) of human embryonic kidney (HEK) 293 cells stably expressing recombinant alpha1beta2gamma2S GABAA receptors to flumazenil (1 or 5 microM) in the presence of GABA (1 microM) enhanced the maximum number (Bmax) of [3H]flunitrazepam binding sites without affecting their affinity (Kd). The flumazenil-induced enhancement in Bmax was not counteracted by diazepam (1 microM). GABA (1 nM-1 mM) enhanced [3H]flunitrazepam binding to membranes obtained from control and flumazenil-pretreated cells in a concentration-dependent manner. No significant differences were observed in either the potency (EC50) or efficacy (Emax) of GABA to potentiate [3H]flunitrazepam binding. However, in flumazenil pretreated cells the basal [3H]flunitrazepam and [3H]TBOB binding were markedly enhanced. GABA produced almost complete inhibition of [3H]TBOB binding to membranes obtained from control and flumazenil treated cells. The potencies of GABA to inhibit this binding, as shown by a lack of significant changes in the IC50 values, were not different between vehicle and drug treated cells. The results suggest that chronic exposure of HEK 293 cells stably expressing recombinant alpha1beta2gamma2S GABAA receptors to flumazenil (in the presence of GABA) up-regulates benzodiazepine and convulsant binding sites, but it does not affect the allosteric interactions between these sites and the GABA binding site. Further studies are needed to elucidate these phenomena.     

Enhancement of diazepam and gamma-aminobutyric acid binding by (+)etomidate and pentobarbital

Abstract: (+) Etomidate and pentobarbital enhance [³H]diazepam and [³H]γ-aminobutyric acid ([³H]GABA) binding to cerebral cortex membranes. Both (+)etomidate and pentobarbital increase the affinity of [³H]diazepam for its binding sites. In contrast, they increase the B _max of both the high- and low-affinity GABA receptor sites. The enhancement of [³H]diazepam and [³H]GABA by (+)etomidate and pentobarbital is blocked by GABA antagonists. These results indicate that hypnotic drugs such as (+)etomidate and pentobarbital, which are not structurally related, modulate diazepam and GABA binding sites via similar mechanisms.     

γ-Aminobutyric Acid Receptor Ionophore Complexes: Differential Effects of Deltamethrin, Dichlorodiphenyltrichloroethane, and Some Novel Insecticides in a Rat Brain Membrane Preparation

The binding of [35S]t-butylbicyclophosphorothionate ([35S]TBPS), a gamma-aminobutyric acid (GABA)-activated chloride ionophore ligand; [3H]diazepam, a benzodiazepine agonist; and [3H]muscimol, a GABA receptor probe, were used to assess the effects at 100 microM of deltamethrin, dichlorodiphenyltrichloroethane (DDT), and three experimental insecticides--a DDT-pyrethroid hybrid, GH414 (cycloprothrin), and two DDT-analogues, GH266 and GH149 (EDO), on GABA receptor ionophore complexes in a rat brain membrane preparation. GH266 and GH149 were found to inhibit a greater percentage of [35S]TBPS binding than the same concentration of deltamethrin or DDT, although GH414 had little effect. GH266 and GH149 enhanced [3H]diazepam binding by nearly 200%, in contrast to the inhibitory effects of deltamethrin, DDT, and GH414. GH266 and GH149 also caused a dramatic enhancement of [3H]muscimol binding, 367 and 236% of control, respectively, whereas DDT and deltamethrin caused only a moderate enhancement. The effects of the insecticides on binding affinity and density were examined for each of the ligands. The results show a differential interaction of the insecticides on the various ligand binding sites.     

Kinetic Modulation by GABAergic Agents of High- and Low-Affinity Binding of [3H]Methyl β-Carboline-3-Carboxylate

The kinetics of dissociation of [3H]methyl beta-carboline-3-carboxylate (beta-CCM) binding was studied in a synaptosomal membrane preparation of rat cerebral cortex. Dissociation was biphasic: a faster phase (10-30% contribution) was followed by a slower phase. Picrotoxin pretreatment at 22 degrees C enhanced the equilibrium binding of [3H]beta-CCM. The half-life of the slower phase of beta-CCM dissociation (t1/2II) was increased by 60 muM picrotoxin from 1.7 min to 3.3 min. The dissociation of [3H]beta-CCM was identical when initiated by an excess of either diazepam or beta-CCM. Quasi-equilibrium Scatchard analysis of [3H]beta-CCM binding was performed by a kinetic separation of the rapid and slow phases of dissociation. The slow and rapid phases represented beta-CCM binding sites of high and low affinity, respectively. The dissociation of [3H]beta-CCM (control t1/2II = 2.0 min) was decelerated by the gamma-aminobutyric acid (GABA) antagonist 3-alpha-hydroxy-16-imino-5 beta-17-aza-androstan-11-one (R 5135) (t1/2II = 2.5 min) and accelerated by GABA (t1/2II = 1.6 min). GABA inhibited both high- and low-affinity beta-CCM bindings.     

Enhancement of γ-Aminobutyric Acid Binding by Quazepam, a Benzodiazepine Derivative with Preferential Affinity for Type I Benzodiazepine Receptors

We evaluated the effect of the two N-trifluoroethyl benzodiazepines, quazepam and its 2-oxo metabolite SCH 15725, which possess preferential affinity for type I benzodiazepine recognition sites, on the binding of [3H] gamma-aminobutyric acid ([3H]GABA) to rat brain membrane preparations. The study also included compounds such as diazepam and N-desalkyl-2-oxoquazepam (SCH 17514), which have equal affinity for the type I and type II receptor subtypes. Binding of [3H]GABA was studied in frozen-thawed and repeatedly washed cortical membranes incubated in 20 mM KH2PO4 plus 50 mM KCl, pH 7.4, at 4 degrees C in the absence and presence of quazepam or its metabolites. Addition of 10(-6) M quazepam increased by 30% specific [3H]GABA binding; as revealed by Scatchard plot analysis, the effect was due to an increase in the total number of GABA receptors. The effect of quazepam was concentration dependent, and it was shared by its active metabolite SCH 15725. The potency of quazepam and SCH 15725 in enhancing [3H]GABA binding was similar to that of diazepam, whereas CL 218872 and SCH 17514 were less active. Moreover, the [3H]GABA binding-enhancing effect of quazepam was mediated by an occupancy of benzodiazepine receptors, because it was specifically antagonized by 5 X 10(-6) M Ro15-1788.     

Effects of Guanyl Nucleotides on [3H]Flunitrazepam Binding to Rat Hippocampal Synaptic Membranes: Equilibrium Binding and Dissociation Kinetics

The effects of guanyl nucleotides on the binding of [3H]flunitrazepam to rat hippocampal synaptic membranes were studied. In equilibrium binding studies, gamma-amino-n-butyric acid (GABA) increased and GTP decreased the binding affinity of [3H]flunitrazepam; GTP also caused a decrease in binding capacity. The effect, however, is variable. In studies of the dissociation kinetics of [3H]flunitrazepam using diazepam and the antagonist Ro 15-1788 as the displacers, there was evidence of two dissociation rate constants. GTP increased both the fast- and slow-dissociation rate constants and increased the ratio of the slow-dissociation binding state. The effect of GTP was mimicked by its nonhydrolyzable analogue 5'-guanylylimidodiphosphate but not by ATP and occurred when diazepam, but not when Ro 15-1788, was used as the displacer. GABA antagonized the effect of GTP on the dissociation of [3H]flunitrazepam. The nature of the benzodiazepine receptor, its actions, and the possible role of cyclic AMP as a second messenger are discussed.     

Effect of Diethyl Pyrocarbonate Modification of Benzodiazepine Receptors on [3H]Ro 15-4513 Binding

The effects of treatment of brain membranes with diethyl pyrocarbonate (DEP), a histidine-modifying reagent, on the binding of 3H-labeled Ro 15-4513 (ethyl-8-azido-5,6-dihydro-5-methyl-6-oxo-4H-imidazo[1,5-a]- [1,4]benzodiazepine-3-carboxylate) and [3H]diazepam were compared. DEP pretreatment produced a dose-dependent decrease in [3H]diazepam binding, whereas low DEP concentrations enhanced the binding of [3H]Ro 15-4513. These effects were reversed by incubation with hydroxylamine after the treatment. The enhancement of [3H]Ro 15-4513 binding was due to an increase in the affinity of the binding sites (KD), without any effect on binding capacity (Bmax). The enhancement was perceived in cerebral cortical, cerebellar, and hippocampal membranes. DEP treatment decreased the displacement of [3H]Ro 15-4513 binding by diazepam and FG 7142 (N-methyl-beta-carboline-3-carboxamide) but not by Ro 15-4513 and Ro 19-4603 (tert-butyl-5,6-dihydro-5-methyl-6-oxo-4H-imidazol[1,5- a]thieno[2,3-f][1,4]diazepine-3-carboxylate). Although the stimulating effect of gamma-aminobutyric acid (GABA) on [3H]-diazepam binding was not affected by DEP treatment, such treatment reduced the inhibitory effect of GABA on [3H]Ro 15-4513 binding. The enhancement of [3H]Ro 15-4513 binding was observed in membranes pretreated with DEP in the presence of flunitrazepam, whereas such pretreatment reduced significantly the inhibitory effect of DEP on [3H]-diazepam binding.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of taurine on a benzodiazepine-GABA-chloride ionophore receptor complex in rat brain membranes

Taurine at 10 mM had no effect on basal binding of [³H]diazepam to the membranes, while it significantly inhibited a GABA-stimulated binding of [³H]diazepam in cerebral cortex, hippocampus, but not in cerebellum. The inhibition by taurine in the presence of GABA (1M to 1 mM) was not competitive. At low concentrations (0.04 to 0.2 nM) the binding of [³H]propyl--carboline-3-carboxylate, a ligand exhibiting higher affinity for type I than type II benzodiazepine receptors, was not enhanced by GABA, while the binding of higher concentrations (0.5 nM) was. This GABA enhancement of [³H]propyl--carboline-3-carboxylate binding was also selectively blocked by taurine. Pentobarbital increased the binding of [³H]diazepam in a medium containing chloride and this effect was potentiated by taurine at 1–10 mM. These findings may be relevant to the modulatory role of taurine in the central nervous system.     

[3H]Propyl β-Carboline-3-Carboxylate Binds Specifically to Brain Benzodiazepine Receptors

Ethyl beta-carboline-3-carboxylate has recently been isolated from human urine and it was proposed that derivatives of this compound might be related to an endogenous ligand for benzodiazepine receptors. In the present study we investigated high-affinity binding of [3H]propyl beta-carboline-3-carboxylate ([3H]PrCC) to rat brain membranes. [3H]PrCC binds specifically and with high affinity (half-maximal binding at ca. 1nM) to rat brain membranes. The regional and subcellular distributions of specific [3H]PrCC binding are similar, but not identical, to the distributions of [3H]flunitrazepam or [3H]-diazepam binding. The total numbers of binding sites labelled by [3H]PrCC and [3H]flunitrazepam in rat cerebellum are closely similar, and both ligands bind to cerebellar membranes in a mutually exclusive way. The pharmacological selectivity of [3H]PrCC and [3H]diazepam binding is almost identical. Binding of [3H]PrCC like binding of [3H]diazepam, can be increased in vitro by muscimol, GABA and SQ 20.009. Although subtle differences in binding characteristics were observed, these results indicate that [3H]PrCC and benzodiazepines bind to a common recognition site on benzodiazepine receptors.     

Properties of [3H] ß-carboline-3-carboxylate ethyl ester binding to the benzodiazepine receptor

β-Carbolines are inhibitors of [³H] diazepam binding with the most potent inhibitor being β-carboline-3-carboxylate ethyl ester (β-CCE). In this report the binding of [³H] β-CCE to extensively washed rat forebrain membranes is characterized. [³H] ß-CCE binds with high affinity (K_D = 1.4 nM) to an apparently homogenous population of benzodiazepine receptor. The rank order of potency for inhibition of [³H] ß-CCE binding by different benzodiazepines is clonazepam > diazepam > chlordiazepoxide, which is similar to that observed for inhibition of [³H] diazepam binding. In marked contrast to [³H] diazepam, the binding of [³H] ß-CCE is not modulated by GABA since concentrations of GABA as high as 10^?3 M had no effect. [³H] ß-CCE is also less potent than [³H] diazepam in its interaction with the peripheral type kidney benzodiazepine receptor indicating that this ligand has a higher degree of specificity for the central brain type benzodiazepine receptor.     

Characterization of the Influence of Unsaturated Free Fatty Acids on Brain GABA/Benzodiazepine Receptor Binding In Vitro

Abstract: We have investigated the effect of unsaturated free fatty acids (FFAs) on the brain GABA/benzodiazepine receptor chloride channel complex from mammalian, avian, amphibian, and fish species in vitro. Unsaturated FFAs with a carbon chain length between 16 and 22 carbon atoms enhanced [³H]diazepam binding in rat brain membrane preparations, whereas the saturated analogues had no effect. The enhancement of [³H]diazepam binding by oleic acid was independent of the incubation temperature (0-30°C) of the binding assay and not additive to the enhancement by high concentrations of C1. In rat brain preparations, the stimulation of [³H]diazepam binding by oleic acid (10^?4M) was independent of the ontogenetic development. Phylogenetically, large differences were found in the effect of unsaturated FFAs on [³H]diazepam and [³H]muscimol binding: In mammals and amphibians, unsaturated FFAs enhanced both [³H]-muscimol and [³H]diazepam binding to 150-250% of control binding. In 17 fish species studied, oleic acid (10^?4M) stimulation of [³H]diazepam binding was weak (11 species), absent (four species), or reversed to inhibition (two species), whereas stimulation of [³H]muscimol binding was of the same magnitude as in mammals and amphibians. In 10 bird species studied, only weak enhancement of [³H]muscimol binding (110–130% of control) by oleic acid (10^?4M) was found, whereas [³H]diazepam binding enhancement was similar to values in mammal species. Radiation inactivation of the receptor complex in situ from frozen rat cortex showed that the functional target size for oleic acid to stimulate [³H]flunitrazepam binding has a molecular mass of ～200,000 daltons. Our data show that unsaturated FFAs have distinct effects on membranebound GABA/benzodiazepine receptors in vitro.     

Ethylenediamine and GABA Potentiation of [3H]Diazepam Binding to Benzodiazepine Receptors in Rat Cerebral Cortex

Specific binding of [3H]diazepam at a free concentration of 2 nM was found to be maximally potentiated by 117% in Tris-HCl buffer and 160% in Tris-citrate buffer by ethylenediamine (EDA), but only at relatively high concentrations of EDA (ED50 = 5 X 10(-5) M), although this potentiation was susceptible to a low dose (6 microM) of bicuculline. Dose-response curves show that EDA differs from GABA with respect to both potency and efficacy. In additivity experiments no evidence was found that EDA could act as a partial agonist at GABA receptors, and it was concluded that EDA and GABA apparently do not potentiate [3H]diazepam binding by acting on the same receptor. Scatchard analysis lends support to this hypothesis, indicating that the potentiation of [3H]diazepam binding by 3.16 X 10(-3) M EDA is due to an increase in receptor number (from 930 to 1170 fmol/mg protein) and not receptor affinity (remaining constant about 20 nM). Subsequent studies showed the potentiation to be reversible. It is concluded that EDA can act on the GABA-benzodiazepine receptor ionophore complex but that this is probably not a direct action on the GABA receptor. It is suggested that EDA can be used to differentiate GABA receptors linked to benzodiazepine receptors from those not so linked.     

Anticonvulsant effects of the wasp Polybia ignobilis venom on chemically induced seizures and action on GABA and glutamate receptors

Venoms of spiders and wasps are well recognized to present high affinity to the central nervous tissue of many mammalian species. Here we describe the effects of direct exposure of rat (Rattus norvegicus) brains to the crude and denatured venom of the Brazilian social wasp Polybia ignobilis. Lower doses of crude venom injected via intracerebroventricular (i.c.v.) inhibited the exploratory activity of animals, while higher doses provoked severe generalized tonic-clonic seizures, with hind limb extension. The status epilepticus lasted for few minutes leading the animals to respiratory depression and death. In contrast, the denatured venom was anticonvulsant against acute seizures induced by the i.c.v. injection of bicuculline, picrotoxin and kainic acid, but it was ineffective against seizures caused by systemic pentylenetetrazole. Moreover, the [3H]-glutamate binding in membranes from rat brain cortex was inhibited by the denatured venom in lower concentrations than the [3H]-GABA binding. The denatured venom contains free GABA and glutamate (34 and 802 pg/microg of venom, respectively), but they are not the major binding inhibitors. These interactions of venom components with GABA and glutamate receptors could be responsible for the anticonvulsant effects introducing the venom from P. ignobilis as a potential pharmacological source of anticonvulsant drugs.     

Action of analogues on γ-aminobutyric acid recognition sites in rat brain

With a view to finding potential GABA-mimetics, the effects of a number of structural analogues of GABA were studied on three parameters associated with GABA neural transmission of rat brain. These were (1) the binding of [³H]GABA to its receptor, (2) the binding of [³H]GABA to its transporter (sodium-dependent binding), and (3) the activity of GABA aminotransferase. Thirteen of the 21 compounds tested competitively inhibited both the low and the high affinity GABA receptor binding components. The most potent inhibitors were morpholinopropane sulphonic acid (MOPS) and aminoethylthiosulphonic acid (AETS). All of the compounds were markedly less effective in inhibiting the high affinity GABA receptor binding system than the low affinity system. The effect of each of the inhibitors was measured on [³H]diazepam receptor binding. Only 6-(morpholinomethyl)kojic acid, kojic amine, 1-piperidinepropane sulphonic acid and 4(4′-azido-benzoimidylamino)butanoic acid (ABBA) were able to induce a stimulation of binding. Four of the inhibitors of [³H]GABA binding were able to appreciably reduce GABA-induced enhancement of diazepam binding. These were N-(2-nitro,4-azidophenyl)aminopropane sulphonic acid, 8-amino-1-napthalene sulphonic acid, narcotine-N-oxide and 5-phenyl-2-pyrrolepropionic acid. These results demonstrate that MOPS and AETS are good inhibitors of GABA receptor binding although there is no other evidence that they might be agonists since they have no effect on diazepam receptor binding. Based on their ability to block GABA-induced stimulation of diazepam binding ABBA, 8-amino-1-naphthalene sulphonic acid and 5-phenyl-2-pyrrolepropionic acid may possess antagonistic properties. ABBA was the only compound to inhibit sodium-dependent [³H]GABA binding. None of the compounds had an effect on the activity of GABA aminotransferase. From this study at least two analogues, MOPS and AETS, have emerged that hold potential as GABA-mimetics. Also, the three GABA recognition sites of rat brain have been shown to possess marked pharmacological differences.     

The gamma-aminobutyric acid receptor in catfish brain

When the binding of [3H]gamma-aminobutyric acid (GABA) to its receptor in catfish synaptic membranes was studied, a high affinity (Kd = 8.4 nM) and a low affinity (Kd = 65 nM) binding component was observed. Muscimol, thiomuscimol, tetrahydroisoxazole-5,4-c-pyridin-3-ol, imidazole acetic acid and bicuculline each competitively inhibited both high affinity and low affinity [3H]GABA binding. The potency of these inhibitors was similar to that reported for the GABA receptor from mammalian brain. It is concluded that the GABA receptor from catfish brain has very similar properties to the receptor from mammalian central nervous system and consequently has not undergone any obvious evolutionary changes.     

Modulation of [3H]Diazepam Binding in Rat Cortical Membranes by GABAA Agonists

GABAA receptor agonists modulate [3H]diazepam binding in rat cortical membranes with different efficacies. At 23 degrees C, the relative potencies for enhancement of [3H]diazepam binding by agonists parallel their potencies in inhibiting [3H]gamma-aminobutyric acid [( 3H]GABA) binding. The agonist concentrations needed for enhancement of [3H]diazepam binding are up to 35 times higher than for [3H]GABA binding and correspond closely to the concentrations required for displacement of [3H]bicuculline methochloride (BMC) binding. The maximum enhancement of [3H]diazepam varied among agonists: muscimol = GABA greater than isoguvacine greater than 3-aminopropane sulphonic acid (3APS) = imidazoleacetic acid (IAA) greater than 4,5,6,7-tetrahydroisoxazolo (4,5,6)-pyridin-3-ol (THIP) = taurine greater than piperidine 4-sulphonic acid (P4S). At 37 degrees C, the potencies of agonists remained unchanged, but isoguvacine, 3 APS, and THIP acquired efficacies similar to GABA, whereas IAA, taurine, and P4S maintained their partial agonist profiles. At both temperatures the agonist-induced enhancement of [3H]diazepam binding was reversible by bicuculline methobromide and by the steroid GABA antagonist RU 5135. These results stress the importance of studying receptor-receptor interaction under near-physiological conditions and offer an in vitro assay that may predict the agonist status of putative GABA receptor ligands.