Avermectin binding in Caenorhabditis elegans. A two-state model for the avermectin binding site

Specific binding sites for ivermectin (IVM; 22,23-dihydroavermectin-B1) were identified and characterized in a crude membrane fraction prepared from the nematode, Caenorhabditis elegans (C. elegans). Specific [3H]IVM binding was saturable with an apparent dissociation constant, Kd, of 0.26 nM and a receptor concentration of 3.53 pmol/mg protein. [3H]IVM binding in C. elegans was linear with tissue protein concentration, and optimal binding occurred within a pH range of 7.3 to 7.6. Kinetic analysis of the binding showed that the reaction proceeded by a two-step mechanism. Initially, a rapidly reversible complex was formed and, after additional incubation, this complex was transformed to a much more slowly reversible complex. Stereospecificity of [3H]IVM binding to C. elegans membranes was demonstrated by competition with a series of avermectin derivatives. The in vivo effects of IVM and its derivatives on C. elegans motility were concentration dependent and correlated well with their relative binding affinities. Several putative neurotransmitters including gamma-aminobutyric acid (GABA), carbamyl choline, taurine, glutamate and dopamine were tested and found to have no effect on IVM binding. Specific IVM binding sites were also identified in rat brain; however, the affinity was approximately 100-fold lower than that observed in C. elegans and stereospecificity studies demonstrated structural differences in the two binding sites. These results are the first direct demonstration of a specific IVM binding site in nematodes and thus are important in furthering our understanding of its mode of action.     

Evidence for association of a high affinity avermectin binding site with the benzodiazepine receptor

Avermectin B1a concentrations corresponding to the KD value of a specific high affinity binding site for [3H]avermectin B1a are sufficient to stimulate the specific high affinity binding of [3H]flunitrazepam to its receptors. This stimulation of [3H]flunitrazepam binding by low concentrations of avermectin B1a was enhanced by chloride ions and picrotoxinin and reduced by the GABA agonists THIP and PSA. The similar modulation by chloride ions, GABA agonists and picrotoxinin and the resistance against washing of membranes of avermectin B1a bound to its specific high affinity binding site or to the site modulating [3H]flunitrazepam binding indicate a close association of the specific high affinity binding site for [3H]avermectin B1a with the GABA-benzodiazepine receptor complex.     

High- and low-affinity binding sites for [3H]-alpha, beta-methylene ATP in rat urinary bladder membranes.

1. The characteristics of [3H]-alpha, beta-methylene adenosine 5'triphosphate ([3H]-alpha, beta-MeATP) binding to membrane preparations of rat urinary bladder detrusor were studied. 2. The rat bladder membrane preparation was obtained by multiple centrifugation. [3H]-quinuclidinyl benzilate [( 3H]-QNB) binding to this preparation demonstrated that the muscarinic receptor density was 4.32 times higher than that in the homogenate. [3H]-alpha, beta-MeATP binding was increased 3.88 times. 3. Saturation analysis revealed that the rat bladder membrane contained a high density of [3H]-alpha, beta-MeATP binding sites, which could be divided into a high-affinity component (Kd = 8.1-8.9 nM) and a low-affinity component (Kd = 67.0-119.8 nM). 4. Magnesium ions inhibited the maximum binding in a concentration-dependent manner. The maximum high-affinity binding was reduced from 10.32 pmol mg-1 protein in magnesium-free buffer to 4.62 pmol mg-1 protein with 25 mM MgCl2, while the maximum low-affinity binding was reduced from 58.84 pmol mg-1 protein to 14.24 pmol mg-1 protein. Kd values were not greatly affected. 5. The binding was a rapid reversible process. The association rate constants were 7.64 x 10(7) M-1 min-1 for high-affinity binding, and 7.31 x 10(6) M-1 min-1 for low-affinity binding. The dissociation rate constants were 0.2896 min-1 for high-affinity binding, and 0.6348 min-1 for the low-affinity binding. 6. Displacement experiments with unlabelled purinoceptor ligands confirmed that [3H]-alpha, beta-MeATP mainly binds to P2X-purinoceptors.(ABSTRACT TRUNCATED AT 250 WORDS)     

Characterization of angiotensin converting enzyme by [3H]captopril binding

We demonstrate that [3H]captopril selectively labels angiotensin converting enzyme (EC 3.14.15.1) (ACE) and employ this technique to probe enzyme-inhibitor interactions. [3H]Captopril binding sites copurify with ACE activity from rat lung or rat brain. At each stage of the purification the Vmax/Bmax ratio, or kcat is 17,000 min-1 with hippuryl-L-histidyl-L-leucine as substrate. The specificity of [3H]captopril binding is apparent in the similar pharmacologic profile of inhibition in crude and pure enzyme preparations. Furthermore, binding sites and enzyme activity comigrate in gel filtration and sucrose gradient sedimentation experiments. Equilibrium analysis of [3H]captopril binding to purified ACE reveals a Bmax of 6 nmol/mg of protein (KD = 2 nM), demonstrating the presence of one inhibitor binding site per polypeptide chain. The kinetics of [3H]captopril binding are characterized by monophasic association and dissociation rate constants of 0.026 nM-1 min-1 and 0.034 min-1, respectively. The affinity of ACE for both [3H] captopril and enalaprilat is greater at 37 degrees than at 0 degree, demonstrating that these interactions are entropically driven, perhaps by an isomerization of the enzyme molecule. The ionic requirements for [3H]captopril binding and substrate catalysis differ. Chloride and bromide ion, but not fluoride, are about 100-fold more potent stimulators of binding than catalysis. When the active site Zn2+ ion is replaced by Co2+, catalysis was stimulated 2-fold, whereas binding activity was decreased by 70%.     

Electrophysiological actions of phenytoin on N-methyl-D-aspartate receptor-mediated responses in rat hippocampus in vitro.

1. [3H]-bradykinin was used to characterize the bradykinin receptors associated with canine cultured tracheal smooth muscle cells (TSMCs). Receptor binding assay showed that TSMCs had specific, saturable, high-affinity binding sites for [3H]-bradykinin. 2. The specific [3H]-bradykinin binding increased linearly with increasing cell concentrations. The equilibrium for association of [3H]-bradykinin with the bradykinin receptors was attained within 2 h at 4 degrees C and 1 h at room temperature, respectively. 3. Analysis of binding isotherms yielded an apparent equilibrium dissociation constant (KD) of 2.5 +/- 0.3 nM and a maximum receptor density (Bmax) of 25.1 +/- 0.3 fmol mg-1 protein. The Hill coefficient for [3H]-bradykinin binding was 1.00 +/- 0.02. The association (K1) and dissociation (K-1) rate constants were (8.67 +/- 2.60) x 10(6) M-1 min-1 and 0.024 +/- 0.005 min-1, respectively. KD, calculated from the ratio of K-1 and K1 was 2.8 +/- 0.5 nM, a value close to that of KD calculated from Scatchard plots of binding isotherms. 4. The B1 receptor selective agonist, (des-Arg9-bradykinin, 0.1 nM-10 microM) and antagonist ([Leu8, des-Arg9]-bradykinin, 0.1 nM-10 microM) did not did not inhibit the [3H]-bradykinin binding to TSMCs, which excludes the presence of B1 receptors in canine TSMCs.(ABSTRACT TRUNCATED AT 250 WORDS)     

Binding of [3H]-muscimol, a potent gamma-aminobutyric acid receptor agonist, to membranes of the bovine retina.

1 The binding of [3H]-muscimol, a potent gamma-aminobutyric acid (GABA) receptor agonist, to crude membrane preparations of bovine retina was studied, using a filtration method to isolate membrane-bound ligand. 2 Specific binding was found to be saturable and occurred at two binding sites with affinity constants of 4.3 nM and 38.2 nM. 3 Binding was sodium-independent, enhanced by both freezing and Triton X-100 treatment but abolished with sodium laurylsulphate. 4 The binding sites demonstrated a high degree of pharmacological specificity, GABA being a potent displacer of [3H]-muscimol. 5 A higher degree of [3H]-muscimol binding was associated with subcellular fractions enriched with photoreceptor synaptosomes rather than with fractions enriched with conventional synaptosomes.     

Properties of a high affinity binding site for [3H]avermectin B1a

The specific high affinity binding of [3H]avermectin B1a was investigated in membranes from several rat brain regions. Binding occurred rapidly, was reversible and partially dependent on the presence of chloride ions in the incubation medium. Specific high affinity binding of [3H]avermectin B1a was partially inhibited by GABA receptor agonists and this effect was blocked by GABA receptor antagonists. Pentobarbital and etazolate inhibited, and picrotoxin, picrotoxinin and IPTBO stimulated high affinity binding of [3H]avermectin B1a. All these effects were influenced by the presence of chloride ions in the incubation medium. The results indicate that the high affinity binding site of [3H]avermectin B1a is associated with the GABA-benzodiazepine receptor-chloride ion channel complex.     

Interaction of pitrazepin with the GABA/benzodiazepine receptor complex and with glycine receptors

Pitrazepin (3-(piperazinyl-1)-9H-dibenz(c,f)triazolo(4,5-a)azepin) is a new GABAA receptor antagonist reported to antagonize electrophysiological effects of GABA. We have investigated in some detail the interaction of pitrazepin with the GABA/benzodiazepine receptor chloride channel complex. Pitrazepin was found to be a competitive inhibitor of the GABAA receptor which is coupled to [3H]diazepam and [35S]TBPS binding sites; the KI value obtained by Schild analyses was 80 nM. Although pitrazepin interacted weakly with BZ receptors the compound did not affect the chloride gating mechanism (labelled with [35S]TBPS or [3H]avermectin B1a). Further, pitrazepin was a non-selective GABA antagonist since glycine receptors, labelled with [3H]strychnine, were affected at low concentrations (the KI values in rat brain-stem were 71-110 nM).     

The binding of 1,3-[3H]-dipropyl-8-cyclopentylxanthine to adenosine A1 receptors in rat smooth muscle preparations.

1. The binding of 1,3-[3H]-dipropyl-8-cyclopentylxanthine ([3H]-DPCPX), an antagonist radioligand selective for adenosine A1 receptors, was studied in rat duodenum, colon muscularis mucosae and longitudinal muscle, urinary bladder and vasa deferentia. 2. [3H]-DPCPX bound with high affinity to a single site in all membrane preparations studied with the exception of the rat urinary bladder in which no specific binding was detected. The affinity (Kd) of the binding site for [3H]-DPCPX was similar in all membrane preparations, the colon longitudinal muscle (1.18 +/- 0.47 nM), colon muscularis mucosae (0.84 +/- 0.15 nM), duodenum (1.59 +/- 0.18 nM) and vasa deferentia (0.93 +/- 0.17 nM). The density of [3H]-DPCPX binding sites was similar in the duodenum (38.8 +/- 4 fmol mg-1 protein), muscularis mucosae (43 +/- 3.5 fmol mg-1 protein) and vasa deferentia (43.3 +/- 12.2 fmol mg-1 protein), but in the longitudinal muscle 6-7 fold more binding sites (295 +/- 70 fmol mg-1 protein) were identified. 3. Inhibition studies using DPCPX (0.1-100 nM), N6-cyclopentyladenosine (CPA) (0.1-100 nM), 5'-N-ethylcarboxamidoadenosine (NECA) (2 nM-10 microM) and (R)-N6-phenylisopropyladenosine (R-PIA) (1 nM-1 microM) to displace the binding of [3H]-DPCPX at a concentration around the Kd value (1 nM), demonstrated an order of potency of displacement in all tissues of DPCPX > or = CPA > R-PIA > NECA. This potency order is characteristic of an A1 receptor, indicating that [3H]-DPCPX binds to adenosine A1 receptors in the rat duodenum, colon and vasa deferentia.(ABSTRACT TRUNCATED AT 250 WORDS)     

AT-1015, a newly synthesized 5-HT2 receptor antagonist,dissociates slowly from the 5-HT2 receptor sites in rabbit cerebral cortex membrane

The purpose of this study was to investigate the association and dissociation kinetics of [3H]AT-1015 from 5-HT2 receptors in rabbit cerebral cortex membranes using a radioligand binding assay method and to make a comparison with those of [3H]ketanserin binding. Scatchard analysis of [3H]AT-1015 binding in rabbit cerebral cortex membranes indicated the existence of a single class of binding sites (dissociation constant, Kd = 2.18 nM). The specific binding of [3H]AT-1015 increased slowly with time and the association rate constant of [3H]AT-1015 binding (k1 = 0.1229 min-1 nM-1) was two times slower than that of [3H]ketanserin binding (k1 = 0.2451 min-1 nM-1). The dissociation rate constant of [3H]AT-1015 binding (t 1/2 = 37.03 min) was six times slower than that of [3H]ketanserin binding (t 1/2 = 6.29 min), when the addition of excess unlabelled ligands were AT-1015 and ketanserin, respectively. The dissociation rate constant of [3H]AT-1015 was slowed to a greater degree (t 1/2 = 163.40 min and t 1/2 = 198.12 min) by the addition of ketanserin and sarpogrelate as excess unlabelled ligands than was that of [3H]ketanserin (t 1/2 = 17.76 min and t 1/2 = 18.45 min) by the addition of AT-1015 and sarpogrelate as an excess unlabelled ligand, respectively. These findings on the dissociation kinetics of [3H]AT-1015 have confirmed and supported previously reported evidence of the slower dissociation of AT-1015 from 5-HT2 receptors.     

Pharmacological and functional characterization of bradykinin receptors in canine cultured tracheal epithelial cells.

1. A direct [3H]-bradykinin ([3H]-BK) binding assay has been used to characterize the BK receptors in canine cultured tracheal epithelial cells (TECs). Based on receptor binding assay, TECs have specific, saturable, high-affinity binding sites for [3H]-BK. 2. The specific [3H]-BK binding was time- and temperature-dependent. Equilibrium of association of [3H]-BK with the BK receptors was attained within 30 min at room temperature and 1 h at 4 degrees C, respectively. 3. Analysis of binding isotherms yielded an apparent equilibrium dissociation constant (KD) of 1.5 +/- 0.2 nM and a maximum receptor density (Bmax) of 53.2 +/- 5.2 fmol mg-1 protein. The Hill coefficient for [3H]-BK binding was 1.00 +/- 0.02. The association (K1) and dissociation (K-1) rate constants were (7.6 +/- 1.1) x 10(6) M-1 min-1 and (9.2 +/- 1.5) x 10 M-3 min-1, respectively. KD, calculated from the ratio of K-1 and K1, was 1.2 +/- 0.3 nM, a value close to that calculated from Scatchard plots of binding isotherms. 4. Neither a B1 receptor selective agonist (des-Arg9-BK, 0.1 nM - 10 microM) nor antagonist ([Leu8, des-Arg9]-BK, 0.1 nM - 10 microM) significantly inhibited [3H]-BK binding to TECs, which excludes the presence of B1 receptors in canine TECs. 5. The specific binding of [3H]-BK to canine TECs was inhibited by the B2 receptor selective antagonists ([D-Arg0, Hyp3, Thi5, D-Tic7, Oic8]-BK (Hoe 140, 0.1 nM-10 microM) and [D-Arg0, Hyp3, Thi5.8, D-Phe7]-BK, 0.1 nM - 10 microM) and agonists (BK and kallidin, 0.1 nM-10 microM) with a best fit by a one-binding site model. The order of potency for the inhibition of [3H]-BK binding was kallidin = BK = Hoe 140 > [D-Arg0, Hyp3, Thi5,8, D-Phe7]-BK. 6. BK and kallidin significantly induced concentration-dependent accumulation of IPs with a half-maximal response (EC50) at 17.6 +/- 3.5 and 26.6 +/- 5.3 nM, respectively, while the B1-selective agonist, des-Arg9-BK did not stimulate IPs accumulation and the B1-selective antagonist [Leu8, des-Arg9]-BK did not inhibit BK-induced IPs accumulation. Two B2-selective antagonists, Hoe 140 and [D-Arg0, Hyp3, Thi5,8, D-Phe7]-BK, inhibited BK-stimulated IPs accumulation with apparent pKB values of 8.8 +/- 0.3 and 7.0 +/- 0.3, respectively. 7. It is concluded that the pharmacological characteristics of the BK receptors in canine cultured TECs are primarily of the B2 receptor subtype which might regulate the function of tracheal epithelium through the activation of this receptor subtype coupling to PI hydrolysis.     

Binding of the cage convulsant, [3H]TBOB, to sites linked to the GABAA receptor complex

[3H]t-Butylbicycloorthobenzoate ([3H]TBOB) binds to specific sites on crude synaptic rat brain membranes. The dissociation constant, Kd, determined from saturation experiments is near 8 nM and the receptor density Bmax is about 20 pmol/g wet tissue. Non-specific binding constitutes about 35% of the total binding at 4 nM [3H]TBOB. The association of [3H]TBOB is monophasic but its dissociation is biphasic. Kd values of 8 nM (70% of the binding sites) and 20 nM (30% of the binding sites) were estimated from the kinetic data. These values differ from those previously reported. Specifically bound [3H]TBOB is displaced by picrotoxin and by t-butylbicyclophosphorothionate (TBPS). No simple competitive interaction of picrotoxin with [3H]TBOB binding was found. Micromolar quantities of the GABAergic facilitating compounds, GABA, muscimol and diazepam inhibited [3H]TBOB binding in an allosteric manner.     

Characterization of guinea pig myocardial leukotriene C4 binding sites. Regulation by cations and sulfhydryl-directed reagents

Using [3H]leukotriene C4 (LTC4) and radioligand-binding techniques, specific leukotriene C4 binding sites have been identified in membranes derived from guinea pig ventricular myocardium. High performance liquid chromatography analyses indicated that, in the presence of the gamma-glutamyl transpeptidase inhibitor L-serine-borate (80 mM), less than 2% of membrane-bound [3H]LTC4 was converted at 20 degrees to [3H]leukotriene D4 or [3H]leukotriene E4. The specific binding of 4 nM [3H]LTC4, in the presence of 80 mM L-serine-borate, reached a stable steady state within 15 min at 20 degrees (pH 7.5). A monophasic Scatchard plot of saturation binding data yielded a dissociation constant (Kd) of 27.5 +/- 6.0 nM and a maximum number of binding sites (Bmax) of 19.9 +/- 5.2 pmol/mg of membrane protein. Competition binding studies of [3H]LTC4 with synthetic leukotriene C4, leukotriene D4, and leukotriene E4 and the putative peptidoleukotriene antagonists FPL 55712, SKF 88046, and 4R-hydroxy-5S-1-cysteinylglycine-6Z-nonadecanoic acid revealed an order of potency of leukotriene C4 much greater than 4R-hydroxy-5S-1-cysteinylglycine-6Z-nonadecanoic acid greater than SKF 88046 greater than LTE4 greater than LTD4 greater than FPL 55712. The specific [3H]LTC4 binding was stimulated by the divalent cations Ca2+, Mg2+, and Mn2+ and to a lesser degree by the monovalent cations Na+, K+, Li+, and NH4+. CaCl2 (3 mM) and NaCl (150 mM) stimulated the LTC4 binding by increasing the Bmax to 42.6 +/- 5.9 and 35.0 +/- 2.0 pmol/mg, respectively, but had minimal effects on Kd. Pretreatment of the heart membranes with the sulfhydryl reagent N-ethylmaleimide decreased the specific [3H]LTC4 binding in a concentration-dependent manner. The N-ethylmaleimide-induced inactivation of [3H]LTC4 binding sites was protected by occupation of the binding site with the agonist leukotriene C4, but no protection was observed with the antagonist SKF 88046. Scatchard analyses of saturation isotherms indicated that 30 microM N-ethylmaleimide pretreatment reduced the Bmax of the [3H]LTC4 binding to 8.2 +/- 3.1 pmol/mg with minimal effects on Kd. The data provide direct biochemical evidence for specific [3H]LTC4 binding sites in the guinea pig heart membranes. The [3H]LTC4 binding sites appear to be modulated by divalent and monovalent cations and free sulfhydryl group(s) may be associated with the agonist-binding site. The results suggest that the physiological effects of the leukotrienes on the guinea pig heart may be mediated through membrane-bound receptors.     

Syntheses and biological activities of 13-substituted avermectin aglycons

The reactions of sulfonate esters of the allylic/homoallylic 13-alcohol of 5-O-(tert-butyldimethylsilyl)-22,23-dihydroavermectin B1a aglycon (1a) were investigated. Nucleophilic substitution gave 13 beta-chloro and 13 beta-iodo derivatives, while solvolytic reaction conditions yielded 13 alpha-methoxy, 13 alpha-fluoro, and 13 alpha-chloro products. A mixture of 13 alpha- and 13 beta-fluorides was obtained upon reaction with DAST. The 13 beta-iodide gave, upon elimination with lutidine, the 8(9),10(11),12(13),14(15)-tetraene. The 13 beta-alcohol and the rearranged 15-ol 13(14)-ene and 15-amino 13(14)-ene derivatives were obtained by substitution via the allylic carbonium ion. MEM ethers 11 and 12 of the two epimeric 13-ols were prepared by alkylation with MEM chloride. In contrast, methylation of 1a with MeI and Ag2O in CH2Cl2 occurred exclusively at the tertiary 7-hydroxy group and not at the secondary 13 alpha-ol. Oxidation of the allylic alcohol 1a proceeded under Swern conditions but not with MnO2 to the 13-oxo aglycon, which was reduced by NaBH4 exclusively to the natural 13 alpha-ol, while reductive amination with NaCNBH3-NH4OAc gave the 13 alpha-amine. The methoxime derivative was obtained in the form of the two geometric isomers. Anthelmintic activities against the sheep nematode Trichostrongylus colubriformis, miticidal activities against the two-spotted spider mite (Tetranychus urticae), and insecticidal activities against the southern armyworm (Spodoptera eridania) as well as the binding constants to a free living nematode (Caenorhabditis elegans) derived receptor assay were obtained and compared to avermectin B1a, 22,23-dihydroavermectin B1a, and the 13-deoxy-22,23-dihydroavermectin B1 aglycon related to the milbemycins. None of the newly prepared derivatives exceeded the potency of the three reference compounds. Lipophilic 13-substituents such as halogen, alkoxy, and methoxime retained high biological activities in all assays, while the more polar substituents hydroxy and amino had weaker activities. Rearranged 15-substituted 13(14)-ene derivatives were completely inactive. The 13-oxo and the 12,13-dehydro analogues were only weakly active in vivo despite having good binding affinity to the receptor, possibly due to instability or poor absorption.     

Adenosine receptors in cortical-derived vesicles of the rat: studies on binding sites and accumulation of cyclic AMP

A vesicular preparation derived from the cerebral cortex of the rat was used to obtain, under the same experimental condition, binding parameters and stimulation data for cyclic AMP. Two analogues of adenosine were employed in the binding studies: [3H]NECA, a mixed A1/A2 agonist and [3H]CHA, a more selective A1 agonist. The [3H]CHA seemed to bind to a single high affinity site (Kd = 1.31 nM, Bmax = 0.327 pmol bound); saturation data for [3H]NECA were resolved for the presence of a high and a low affinity binding site (Kd1 = 3.08 nM, Bmax1 0.115 pmol bound; Kd2 = 204 nM, Bmax2 1.59 pmol bound), but only when calcium ions were omitted from the incubation medium. At 0 degree C, [3H]NECA bound to a single, low affinity site; the presence of calcium ions (1 mM) significantly reduced the affinity of [3H]NECA (Kd 419 nM), with respect to the absence of calcium (Kd 208 nM), without affecting the Bmax value. The influence of calcium ions was also investigated on the binding of [3H]CHA and a reduction of the Bmax value (36%) was found. Regardless of the presence or the absence of calcium ions, NECA stimulated accumulation of cyclic AMP in a dose-dependent way with an EC50 of 2.79 microM; this value did not correlate with the Kd of the low affinity binding site for [3H]NECA. Thus, the purpose of establishing a correlation between binding sites for analogues of adenosine and the site in the cerebral cortex through which the accumulation of cyclic AMP is induced, was not achieved. It is concluded that the stimulatory effect of analogues of adenosine on adenylate cyclase might not be a receptor-mediated effect. The complex influence of calcium ions on affinity and binding capacity of analogues of adenosine is discussed.     

Gamma-hydroxybutyric acid binding sites in rat and human brain synaptosomal membranes

The binding of gamma-hydroxy[2,3-3H]butyric acid (GHB) was characterized in rat and human brain synaptosomal membranes. Binding was shown to be saturable, pH dependent, and linear with protein concentration. There was a distinct regional distribution of binding sites in both rat and human brain, with the hippocampus being the richest and the cerebellum the poorest, in density of [3H]GHB binding sites. Competition and saturation experiments revealed two different population of binding sites, a high-affinity site with a KD1 of 580 nM and a B max1 of 1.8 pmoles/mg protein and a low-affinity site with a KD2 of 2.3 microM and a B max2 of 11.3 pmoles/mg protein. [3H]GHB binding was not inhibited by gamma-aminobutyric acid (GABA), GABA receptor agonists, opiate antagonists or anticonvulsant drugs. These data suggest that GHB may play a role as a neurotransmitter or neuromodulator in brain independent of GABA.     

Characterization of [3H]thiocolchicoside binding sites in rat spinal cord and cerebral cortex.

Thiocolchicoside, a semi-synthetic derivative of the naturally occurring compound colchicoside with a relaxant effect on skeletal muscle, has been found to displace both [3H]gamma-aminobutyric acid ([3H]GABA) and [3H]strychnine binding, suggesting an interaction with both GABA and strychnine-sensitive glycine receptors. In order to gain further insight into the interaction of thiocolchicoside with these receptors, the binding of [3H]thiocolchicoside in rat spinal cord-brainstem and cortical synaptic membranes was characterized. [3H]Thiocolchicoside binding was saturable in both tissues examined. In spinal cord-brainstem membranes, we found a K(D) of 254 +/- 47 nM and a Bmax of 2.39 +/- 0.36 pmol/mg protein, whereas in cortical membranes, a K(D) of 176 nM and a Bmax of 4.20 pmol/mg protein was observed. A similar K(D) value was found in kinetic experiments performed in spinal cord-brainstem membranes. Heterologous displacement experiments showed that GABA and strychnine displaced the binding in a dose-dependent manner, whereas glycine was ineffective. [3H]Thiocolchicoside binding was also displaced by several GABA(A) receptor agonists and antagonists, but not by baclofen, flunitrazepam, guvacine, picrotoxin or by other drugs unrelated to GABA transmission. In spinal cord-brainstem, and to a lower extent, in cortical membranes, GABA and its analogs were not able to completely displace [3H]thiocolchicoside specific binding indicating that, besides GABA(A) receptors, thiocolchicoside can bind to another unidentified site. Unlabelled thiocolchicoside, however, completely displaced [3H]muscimol binding both in cortical and in spinal cord-brainstem synaptic membranes with an IC50 in the low microM range. Neurosteroids were found to modulate the binding in cortical but not in spinal cord-brainstem synaptic membranes. We conclude that [3H]thiocolchicoside binding shows a pharmacological profile indicating an interaction with the GABA(A) receptor. The different affinities for the GABA(A) receptor agonists and antagonists and sensitivity to neurosteroids obtained in the cerebral cortex and in the spinal cord may indicate a preferential interaction of the compound with a subtype of the GABA(A) receptor. The data also indicate that [3H]thiocolchicoside binds to another site(s), whose nature remains to be elucidated.     

Kinetic and pharmacological profiles of the in vitro binding of the potent dopamine agonist [3H]N,N-dipropyl-5,6-dihydroxy-2-aminotetralin to rat striatal membranes

The in vitro binding of the new tritiated dopaminergic ligand [3H]N,N-dipropyl-5,6-dihydroxy-2-aminotetralin to rat striatal membranes is described. Binding assays were performed in 50 mM Tris-HCl pH 7.5 containing 1 mM EDTA and 0.01% ascorbic acid at 25 degrees C for 30 min. Specific binding at 0.5 nM [3H]DP-5,6-ADTN and 5 mg/ml membrane suspension was very high (87-93%) and was found to be linearly related with homogenate concentration over the range of 0.5-10 mg/ml (r = 0.9968). (+)Butaclamol 1 microM was used to define specific binding. Specific binding disappeared completely within 20 min when the tissue was incubated at 55 degrees C. Association and dissociation curves (obtained after addition of 1 microM (-)-DP-5,6-ADTN) were converted to linear logarithmic plots and the kinetic constants were computed (k1 = 0.054 min-1 and k-1 = 0.0188 min-1) as were the t1/2 for association (3.52 min) and dissociation (20.8 min). This resulted in an apparent KD of 0.34 nM. Increasing concentrations of [3H]DP-5,6-ADTN (0.05-3.0 nM) were found to saturate the receptor site. Linear transformation of the saturation curves and presentation of the curves as Eadie-Hofstee plots showed that only one set of receptors was labeled (nHill was 0.995 +/- 0.07) with a high affinity constant KD of 0.57 +/- 0.10 nM and a maximum number of binding sites Bmax of 18.6 +/- 2.4 pmol/g tissue. Various compounds were tested for their potency to displace the specific binding of [3H]DP-5,6-ADTN to striatal membranes (2.5 mg/ml).(ABSTRACT TRUNCATED AT 250 WORDS)     

Regional modification of [(3)H]Ethynylbicycloorthobenzoate binding in mouse brain GABA(A) receptor by endosulfan, fipronil, and avermectin B(1a)

[(3)H]Ethynylbicycloorthobenzoate ([(3)H]EBOB), a high affinity radioligand for the noncompetitive blocker site of the GABA(A) receptor, is used here for quantitative autoradiography to determine regional binding in mouse brain and the effects on this binding of administering toxic doses of endosulfan, fipronil, and avermectin B(1a) (AVM). Animals were euthanized 4-8 min after 1 LD50 or 2 LD50 doses of the two channel blockers and 32 min after 1 LD50 or 4 LD50 doses of the channel activator AVM. Specific binding of [(3)H]EBOB was determined for 20-microm brain sections as the difference in labeling on incubation with 2 nM [(3)H]EBOB either alone (total binding) or with 5 microM alpha-endosulfan (nonspecific binding). The highest specific labeling was observed for layers I and IV of the cerebral cortex, the globus pallidus, and the medial septal nucleus/nucleus of the vertical limb of the diagonal band. Dose-dependent inhibition by endosulfan was highest in the nucleus accumbens and least in the cerebellum and periaqueductal gray matter. Fipronil had much less effect on binding even at severely toxic doses. AVM increased [(3)H]EBOB binding in most regions and was the only one of the three agents inhibiting in vitro [(3)H]strychnine binding to the glycine receptor. In summary, the noncompetitive blocker site was strongly inhibited with dose dependence and regional selectively by alpha-endosulfan but was generally poorly inhibited or activated by fipronil and was activated by avermectin.     

3H](-)Baclofen: an improved ligand for GABAB sites

[3H](-)Baclofen, the radiolabelled form of the active isomer of baclofen, has been used as a ligand for GABAB (gamma-aminobutyric acid) receptors on synaptic membranes from whole brain of rat. The pharmacological profile for displacement of this ligand was observed to be identical with that for the racemic ligand [3H](+/-)baclofen and [3H]GABA under conditions where GABAB, but not GABAA sites, were labelled. The displaceable (specific) portion of membrane-bound [3H](-)baclofen was 47.5 +/- 2.3% of the total which was twice that obtained previously with [3H](+/-)baclofen. Two binding components were observed with affinities of 19 and 304 nM and binding capacities of 0.37 and 1.58 pmol/mg protein respectively. It is suggested that [3H](-)baclofen is an improvement over the labelled racemic form and binds to the same sites. It should provide a more reliable tool for studying GABAB receptors.