Solubilization of rat kidney benzodiazepine binding sites

3-Iodothyroacetic acid (TA1), a by-product of thyroid hormone metabolism, reduces the hypnotic effect of ethanol without interacting at GABA-A receptors

3-iodothyroacetic acid (TA₁) is among the by-products of thyroid hormone metabolism suspected to mediate the non-genomic effects of the hormone (T3). We aim to investigate whether TA₁ systemically administered to mice stimulated mice wakefulness, an effect already described for T3 and for another T3 metabolite (i.e. 3-iodothryonamine; T1AM), and whether TA₁ interacted at GABA-A receptors (GABA-AR). Mice were pre-treated with either saline (vehicle) or TA₁ (1.32, 4 and 11 μg/kg) and, after 10 min, they received ethanol (3.5 g/kg, i.p.). In another set of experiments, TA₁ was administered 5 min after ethanol. The latency of sleep onset and the time of sleep duration were recorded. Voltage-clamp experiments to evaluate the effect of 1 μM TA₁ on bicuculline-sensitive currents in acute rat hippocampal slice neurons and binding experiments evaluating the capacity of 1, 10, 100 μM TA₁ to displace [³H]flumazenil from mice brain membranes were also performed. 4 μg/kg TA₁ increases the latency of onset and at 1.32 and 4 μg/kg it reduces the duration of ethanol-induced sleep only if administered before ethanol. TA₁ does not functionally interact at GABA-AR. Overall these results indicate a further similarity between the pharmacological profile of TA₁ and that of T₁AM.

Effects of newly-developed benzodiazepine ligands on noise-induced mitochondrial damage in the rat

In this study we measured the ability of three newly-synthesized N-arylalkylindol-3-ylglyoxylylamide derivatives, which have recently been characterized as partial agonists at central benzodiazepine binding sites, to prevent the rat cardiac mitochondrial alterations resulting from acute loud noise exposure. In particular, we evaluated the effects of these new compounds on the ultrastructural damage induced by noise stress on the right atrium and ventricle after 6 and 12 hr of loud noise exposure. In parallel experiments, we measured the affinity of these compounds for peripheral benzodiazepine binding sites. Following a single injection of the test products, we observed a cardioprotective effect which was more marked after 6 hr compared with 12 hr of noise exposure. Confirming our recent data showing that full agonists at benzodiazepine receptors produce cardioprotection, we demonstrate in this study that partial agonists, like indolylglyoxylylamides, can also produce a cardioprotective effect. Based on their greater affinity in binding studies, the protective activity seems to be related more to their action at central than at peripheral benzodiazepine receptors.

Characterization of peripheral-type benzodiazepine binding sites from rat and pig pancreas

The binding of [3H]1-(2-chlorophenyl-N-methyl-1-methyl-propyl)-3-isoquinolinecarboxa mide ([3H]PK-11195) and [3H]7-chloro-1,3-dihydro-1-methyl-5-(p-chlorophenyl)-2H-1,4-benzodiaz epin-2 - on ([3H]Ro5-4864) to membrane preparations of pancreas was studied in the rat and pig. [3H]PK-11195 bound with high affinity to rat and pig membrane preparations yielding maximal numbers of binding sites (Bmax) of 2393 +/- 160 and 777 +/- 65 fmol/mg of protein, respectively, and equilibrium dissociation constant (Kd) values of 3.01 +/- 0.25 and 3.9 +/- 0.23 nM, respectively. [3H]Ro5-4864 successfully labelled rat but not pig pancreatic membranes, yielding a Kd value of 6.45 +/- 0.5 nM and a Bmax value of 551 +/- 43 fmol/mg of protein. Displacement studies showed a similar rank order of potency of various unlabelled ligands against both [3H]Ro5-4864 and [3H]PK-11195 binding to rat and pig membrane preparations (PK-11195 > or = Ro5-4864 > diazepam > flunitrazepam >> flumazenil). These results suggest that [3H]PK-11195 binds with high affinity and specificity to rat and pig pancreas and [3H]Ro5-4864 binds with high affinity and specificity to rat but not pig pancreas.

The peripheral-type benzodiazepine receptor ligands [3H]Ro 5-4864 and [3H]PK 11195 bind to the retina of rabbit, but not of turtle

The binding of [3H]flunitrazepam, [3H]Ro 5-4864, and [3H]PK 11195 to membrane preparations of the retina was studied in the turtle and rabbit. Only a single population of [3H]flunitrazepam binding sites was detected in the turtle, whereas two populations appeared to be present in the rabbit. No specific binding for [3H]Ro 5-4864 and [3H]PK 11195 could be detected in the turtle. In rabbit, both ligands bound with high affinity, revealing a significant population of binding sites (KD values of 24 +/- 2.3 and 2.2 +/- 0.8 nM, and Bmax values of 440 +/- 35 and 1,482 +/- 110 fmol/mg of protein, respectively). The binding was temperature- and protein-dependent. Displacement studies showed a similar rank order of potency of various unlabeled ligands against both [3H]Ro 5-4864 and [3H]PK 11195 (PK 11195 > Ro 5-4864 > flunitrazepam > flumazenil). These results suggest that peripheral-type benzodiazepine receptors are present in the retina of the rabbit, but not of the turtle.

Peripheral-type benzodiazepine receptors

Since their first description as anomalous high affinity diazepam binding sites in rat peripheral tissues, the peripheral-type benzodiazepine receptor (PBR) has been increasingly studied to better understand nonneural effects of the benzodiazepines. The mammalian PBR is ubiquitously distributed with high concentrations in the outer mitochondrial membrane of secretory tissues. In regions of the brain, the density of PBR can equal or exceed the density of central-type benzodiazepine receptors. High affinity PK 11195 binding is diagnostic for the receptor while the affinity for benzodiazepines is species dependent. Recent cDNA cloning of a PBR component, the isoquinoline binding protein (IBP), shows no apparent sequence homology with any GABAA receptor subunits known to comprise central benzodiazepine receptor subtypes. The PBR seems at best only distantly related to CBRs. Recent advances in the pharmacology, biochemistry and molecular biology of the PBR are reviewed.

Stabilization of the peripheral-type benzodiazepine acceptor by specific phospholipids

The peripheral-type benzodiazepine acceptor from rat adrenal gland was solubilized with Triton X-100. The soluble acceptor exhibited isoquinoline carboxamide and benzodiazepine binding activity only when supplemented with lipid. Phosphatidylserine and phosphatidylinositol were especially efficacious. Such selectivity may reflect a specific structural requirement for acceptor activity. The requirement for lipid for acceptor activity in fractions subsequent to solubilization was demonstrated. This stabilization of acceptor activity by the phospholipids promises to facilitate purification of the functional acceptor.

Biochemical, physiological, and pathological aspects of the peripheral benzodiazepine receptor

The PBR is a mitochondrial protein composed of at least two subunits, an approximately 30-kDa subunit that contains the site for BZs and an approximately 18-kDa subunit that binds isoquinoline carboxamide derivatives. Porphyrins and diazepam binding inhibitor are putative endogenous ligands for these receptors, which are under neural and hormonal control. Alterations in the density of PBR seem to be a sensitive indicator of stress: up-regulation after acute stress and down-regulation induced by repeated stress. PBR-specific ligands are involved in the control of cell proliferation and differentiation, and their binding is increased in some cancer tumors. Numerous studies in various endocrine organs have revealed that PBR are located in specific regions or tissues in the organs. Furthermore, PBR densities in various organs subject to hormonal control are regulated by organotropic hormones. At least in some cases, BZ ligands do not exert a specific effect in an organ, but rather modulate the well-documented effects of that particular hormone. To the best of our knowledge, BZ ligand action in peripheral tissues is dependent on recognition of PBR, which may suggest a receptor-mediated action.

Cofractionation of the 17-kD PK 14105 binding site protein with solubilized peripheral-type benzodiazepine binding sites

To examine the relationship between PKBS, a 17-kD protein covalently photolabeled by [3H]PK 14105, and its association with peripheral-type benzodiazepine binding sites, rat adrenal mitochondrial fractions were photolabeled with [3H]PK 14105, solubilized in digitonin, and subjected to anion-exchange chromatography over Q-Sepharose. The chromatographic behavior of PKBS was evident as principally two major fractions, signified as Q-I and Q-II. Specific binding sites for [3H]Ro5-4864 and [3H]PK 11195 were also assayed and found to cofractionate with each other and in a manner which coincided with the photolabeled PKBS profile. The Q-I and Q-II fractions were further distinguished based on their different molecular sizes observed by gel filtration, yet both fractions were characterized as containing peripheral-type benzodiazepine recognition sites according to the following criteria. Scatchard analysis of both subpopulations revealed a single class of binding sites for [3H]Ro5-4864 with an apparent KD of 14 nM for Q-I and 22 nM for Q-II; these affinities were slightly lower than those found in mitochondrial membrane preparations used as the starting material for solubilization. The rank order of potency to inhibit [3H]Ro5-4864 binding in both subpopulations was PK 11195 greater than Ro5-4864 greater than diazepam greater than clonazepam, in connection with the pharmacological specificity of membrane-associated peripheral-type benzodiazepine binding sites. These studies provide direct biochemical evidence that the recognition sites for benzodiazepines and isoquinoline carboxamides cofractionate in unison with the 17-kD PKBS protein, demonstrating an intimate relationship between this protein and the binding domains for peripheral-type benzodiazepine ligands.

Species differences and heterogeneity of solubilized peripheral-type benzodiazepine binding sites

The pharmacological characteristics of digitonin-solubilized peripheral-type benzodiazepine binding sites (PBS) from kidney membranes of various species were investigated to determine whether the species differences and heterogeneity observed in membrane-bound binding sites would be maintained after solubilization. [3H]PK 11195 (0.05 to 10 nM) bound with high affinity to rat, guinea pig, calf, and cat kidney solubilized preparations yielding maximal numbers of binding sites (Bmax) of 3,593 +/- 381, 25,645 +/- 1,795, 1,327 +/- 141, and 2,446 +/- 148 fmol/mg protein, respectively, and equilibrium dissociation constant (KD) values of 1.74 +/- 0.18, 2.15 +/- 0.15, 0.85 +/- 0.09, and 1.02 +/- 0.06 nM, respectively. On the other hand, the respective Bmax and KD values for [3H]Ro 5-4864 (1.25 to 40 nM) were 2,688 +/- 275, 14,182 +/- 1,134, 144 +/- 23 and 205 +/- 17 fmol/mg protein (about 75, 55, 11, and 8%, respectively, of that of [3H]PK 11195) and 13.8 +/- 1.5, 14.6 +/- 1.1, 10.6 +/- 1.7, and 19.9 +/- 1.2 nM. Unlabeled Ro 5-4864 was two orders of magnitude more potent in displacing [3H]PK 11195 binding from rat kidney solubilized preparations than from calf kidney solubilized preparations, whereas the potency of unlabeled PK 11195 in displacing [3H]PK 11195 binding from both rat and calf kidney solubilized preparations was almost identical. Analysis of these displacement data revealed that PK 11195 bound to a single population of binding sites (nH approximately equal to 1.0), whereas Ro 5-4864 bound to two populations of binding sites (nH less than 1.0) in both rat and calf kidney solubilized preparations. These results indicate that PBS species differences and heterogeneity observed in membrane-bound binding sites are retained in the soluble state and are probably attributable to variations in the molecular structure of PBS rather than to differences in membrane environment.

Synthesis of 1,5-diaryl-3-methyl-1H-pyrazolo[4,5-c]isoquinolines and studies of binding to specific peripheral benzodiazepine binding sites

Some 1,5-diaryl-3-methyl-1H-pyrazolo[4,5-c]isoquinolines were synthesized and tested for their ability to displace [3H]clonazepam or [3H]Ro 5-4864 from their specific binding on the central and peripheral benzodiazepine receptors. None of the tested compounds showed any activity as central binding inhibitors, while most of them were specific as peripheral binding inhibitors, although they were not very potent.

Solubilization of peripheral-type benzodiazepine binding sites from cat cerebral cortex

The present study demonstrates for the first time the solubilization of peripheral-type benzodiazepine binding sites (PBS) from cat cerebral cortex. Of all detergents tested [digitonin, 3-[(3-cholamidopropyl)dimethylammonio]-1-propane sulfonate (CHAPS), Tween 20, deoxycholate, and Triton X-100] in the presence of NaCl, the best solubilization (15% of initial activity) was obtained using 0.5% of the zwitterionic detergent CHAPS plus 2 M NaCl. Specific binding of [3H]PK 11195 to membrane-bound and solubilized PBS was saturable, yielding equilibrium dissociation constants (KD) of 1.3 +/- 0.2 and 1.9 +/- 0.3 nM, respectively, and maximal numbers of binding sites of 1,435 +/- 150 and 980 +/- 126 fmol/mg protein, respectively. The KD value of PK 11195 binding to solubilized PBS obtained from experimental kinetic analysis was 0.95 +/- 0.09 nM. The relative potencies of various compounds (PK 11195, Ro 5-4864, diazepam, flunitrazepam, clonazepam, methyl-beta-carboline-3-carboxylate, and Ro 15-1788) in displacing [3H]PK 11195 specific binding from membrane-bound and solubilized PBS were similar. Most of the solubilized binding activity was destroyed by heating at 60 degrees C for 30 min or by treatment with 2 M guanidinium chloride, which indicates the presence of a protein-binding site in the solubilized preparation. Over 85% of the solubilized binding activity was retained after 1 week at 4 degrees C, which will enable future application of purification procedures without major concern for stability of the material.

[3H]AHN 086 acylates peripheral benzodiazepine receptors in the rat pineal gland

AHN 086, an isothiocyanato derivative of Ro 5-4864 (4'-chlorodiazepam), inhibits radioligand binding to peripheral benzodiazepine receptors with characteristics of an irreversible (acylating) ligand. We now report that [3H]AHN 086 labels a approximately 30 kDa protein in the rat pineal gland determined by both SDS-polyacrylamide gel electrophoresis and gel filtration high-performance liquid chromatography of digitonin-solubilized membranes. Specific incorporation of [3H]AHN 086 into this protein was inhibited by preincubating membranes with excess AHN 086. Moreover, significant specific binding of [3H]AHN 086 was not observed in either bovine pineal gland (which does not possess high-affinity binding sites for Ro 5-4864) or ovalbumin. These findings suggest that the approximately 30 kDa protein labeled by [3H]AHN 086 in rat pineal gland is associated with peripheral benzodiazepine receptors in this tissue.

Heterogeneity between rat and calf peripheral-type benzodiazepine binding sites: differential sensitivity to Triton X-100

The effect of various detergents treatment on the specific binding of [3H]PK 11195 (2nM) to peripheral-type benzodiazepine binding sites (PBS) in calf and rat kidney, adrenal gland, and cerebral cortex membranes was studied. At a concentration of 0.025%, Triton X-100 increased [3H]PK 11195 specific binding to calf kidney, adrenal gland, and cerebral cortex membranes by 20-40%. At the same concentration, Triton X-100 scarcely affected specific binding of [3H]PK 11195 to rat cerebral cortex but decreased binding to rat kidney and adrenal gland membranes by 20-30%. At a concentration of 0.05% of Triton X-100, [3H]PK 11195 specific binding to calf kidney, adrenal gland, and cerebral cortex membranes was increased by 10-20%; whereas [3H]PK 11195 specific binding to rat kidney, adrenal gland, and cerebral cortex membranes was decreased by more than 40%. The increase in [3H]PK 11195 specific binding to calf kidney membranes following Triton X-100 (0.05%) treatment was apparently due to an increase in the binding affinity of PBS, since the density remained unaltered; whereas, the decrease in [3H]PK 11195 specific binding to rat kidney membranes was due to a decrease in both binding affinity and density of PBS. On the other hand, the detergents 3- [(3- cholamidopropyl)- dimethylammonio] - 1 - propane sulfonate (CHAPS), Tween 20, deoxycholic acid, and digitonin have a similar effect on [3H]PK 11195 specific binding to PBS in both calf and rat kidney membranes.

Specific inhibition of benzodiazepine receptor binding by some 1,2,3-triazole derivatives

Certain 1,2,3-triazole derivatives were prepared and tested for their ability to displace [3H]diazepam from bovine brain membranes. From these compounds, the quinolytriazole derivatives (14, 15, 16, 17) were clearly the most potent, while the naphthyl- and the naphthyridyl-triazoles were considerably less active. The p-nitrophenyl derivative (15) was the compound that bound with the highest affinity within the quinolyltriazole compounds class. The replacement of the p-nitrophenyl group with other substituents greatly decreased the binding activity. From a Lineweaver-Burk analysis of 11, it appears that the inhibition is competitive.

Differential effect of detergents on [3H]Ro 5-4864 and [3H]PK 11195 binding to peripheral-type benzodiazepine-binding sites

The present study demonstrates a differential effect of various detergent treatments on [3H]Ro 5-4864 and [3H]PK 11195 binding to peripheral benzodiazepine binding sites (PBS). Triton X-100 (0.0125%) caused a decrease of about 70% in [3H]Ro 5-4864 binding to membranes from various peripheral tissues of rat, but had only a negligible effect on [3H]PK 11195 binding. A similar effect of Triton X-100 was observed on guinea pig and rabbit kidney membranes. The decrease in [3H]Ro 5-4864 binding after treatment with Triton X-100 was apparently due to a decrease in the density of PBS, since the affinity remained unaltered. The detergents 3-[(3-cholamidopropyl)-dimethylammonio]-1-propane sulfonate (CHAPS), Tween 20, deoxycholic acid, or digitonin (0.0125%) caused only a minor change in [3H]Ro 5-4864 and [3H]PK 11195 binding to rat kidney membranes; but when concentrations were substantially increased (0.1%), all detergents caused a decrease of at least 50% in [3H]Ro 5-4864 binding, while [3H]PK 11195 binding to rat kidney membranes remained unaffected by the first three detergents, with only a minor decrease (15%) after treatment with digitonin. These results may further support the assumption that Ro 5-4864 and PK 11195 are agonist and antagonist, respectively, of PBS and interact with two different conformations or domains in the peripheral-type benzodiazepine binding site molecule.

Partial purification and pharmacology of peripheral-type benzodiazepine receptors

This report describes the results obtained with a new photoaffinity ligand for the "peripheral-type" benzodiazepine binding site (PBS), using a digitonin solubilized preparation from rat heart or adrenals. The specific binding activity of the solubilized adrenal preparation is higher than 50 pmol/mg protein, with binding properties and pharmacological specificity identical to the membrane bound PBS. The apparent molecular weight of the solubilized PBS, determined by gel filtration is 215 KDa. The photoaffinity ligand (PK 14105) is a nitrophenyl derivative of PK 11195, which attaches covalently and specifically to all the PBS when cardiac membranes are irradiated with this compound under ultraviolet light. After photolabelling with [3H]PK 14105 and solubilization in SDS of heart or adrenal membranes, gel electrophoresis indicates the existence of a single protein band whose molecular weight (18 KDa) is unaltered by incubation with sulphydryl-reducing or protein cross-linking agents. This molecule seems to be a low molecular weight, acidic protein. Diethylpyrocarbonate decreases partially (60%) the binding of [3H]PK 11195 without affecting [3H] RO5-4864 binding, which implies a vital histidine residue in the binding domain of [3H]-PK 11195. Treatment with phospholipase A2 or mellitin, a stimulant of endogenous PLA2, led to a selective loss of [3H] RO5-4864 binding with no change in the binding of [3H]PK 11195. Such differences between a benzodiazepine ligand and an isoquinoline ligand suggest that these compounds may induce, on binding, different conformational changes in the PBS, which is compatible with the hypothesis that RO5-4864 and PK 11195 may be an agonist and an antagonist respectively at the PBS.

Pyrazolo[4,5-c]quinolines. 3. Synthesis, receptor binding, and 13C NMR study

Some 1-aryl-3-methylpyrazolo[4,5-c]quinolin-4-ones, were prepared and tested for their ability to displace specific [3H]flunitrazepam binding from bovine brain membranes. The 1-meta-aryl derivatives were the compounds that bound with the highest affinity within this class. Our 13C NMR study suggested a correlation between the binding affinity and the chemical shift value of a carbon atom of the tricyclic system.

Dihydropyridine and peripheral type benzodiazepine binding sites: subcellular distribution and molecular size determination

Electrophysiological and pharmacological studies have shown that peripheral-type benzodiazepine receptors modulate voltage-sensitive calcium channels in the heart. We have compared these binding sites with binding sites for [3H]dihydropyridines, which are believed to label such channels. Although no direct or allosteric interaction could be demonstrated between the two sites, their subcellular distribution--sarcolemma and ryanodine-sensitive sarcoplasmic reticulum--was parallel. Size determination of the two sites suggests that the receptors for these two classes of compounds are separate molecules packaged in the same membrane compartment.

Heterogeneity of benzodiazepine binding sites: a review of recent research

This paper reviews selected aspects of benzodiazepine binding site heterogeneity. These include receptor heterogeneity revealed by biochemical determinations of receptor numbers, autoradiographic localization in histological sections of brain, lesion studies, solubilization of receptors, and photoaffinity labelling. The data summarized support the concept of benzodiazepine receptor multiplicity. In addition, we have reviewed recent work on peripheral-type benzodiazepine binding sites and suggest that further study of these sites may increase our understanding of both the central and peripheral actions of benzodiazepines and other ligands.

Characterization of solubilized "peripheral type" benzodiazepine binding sites from rat adrenals by using [3H]PK 11195, an isoquinoline carboxamide derivative

"Peripheral type" benzodiazepine binding sites have been solubilized with digitonin. Binding site density for the solubilized material is increased 1.7 times compared to membranes. A decrease in the affinity for [3H]-PK 11195 (a new ligand for the peripheral type benzodiazepine binding sites) was also observed. Pharmacological specificity of displacing agents was conserved during solubilization. The apparent molecular weight determined by gel filtration was 215,000 +/- 20,000. The high Bmax value of the solubilized preparation (greater than 50 pmole/mg protein) makes it advantageous as the starting point for a purification procedure.

Benzodiazepine receptors along the nephron: [3H]PK 11195 binding in rat tubules

Binding of [3H]PK 11195, an isoquinoline carboxamide derivative, was measured in microdissected tubule segments of rat nephron. High specific binding capacities (1.1-1.8 fmol X mm-1) were found in the thick ascending limb of the Henle's loop and in the collecting tubule, whereas specific binding could not be detected in the proximal tubule. In the medullary collecting tubule, the association and dissociation rate constants at 4 degrees C were k1 = 3.0 X 10(6) M-1 X min-1 and k-1 = 0.021 min -1; the ratio k-1/k1 = 7.0 nM was in agreement with the estimated equilibrium dissociation constant (Kd = 2.4 nM). [3H]PK 11195 binding sites from medullary ascending limb and medullary collecting tubule revealed the following sequence of specificity: PK 11195 = Ro 5-4864 much greater than clonazepam, indicating that tubule binding sites might be the peripheral benzodiazepine receptors of the rat kidney.

Pharmacological and physiological properties of benzodiazepine binding sites in rodent brown adipose tissue

[3H]Diazepam and [3H] Ro5 -4864 were used as ligands to identify and characterize peripheral-type benzodiazepine binding sites in mouse and rat brown adipose tissue (BAT) membranes. [3H]Diazepam and [3H] Ro5 -4864 binding sites in BAT are pharmacologically similar to peripheral-type benzodiazepine binding sites in other tissues. Stimulators of central-type benzodiazepine receptors had no effect on or inhibited ligand binding to BAT membranes. Brown adipose tissue benzodiazepine binding sites are highly localized to mitochondria-containing subcellular fractions. These binding sites decrease with age in BAT from Fischer 344 rats. Stimulation of BAT thermogenesis in mice with 1-norepinephrine led to a decrease in [3H] Ro5 -4864 binding in the tissue.