Solubilized serotonin receptors from rat and dog brain. Selective labelling with [3H]ketanserin

Specific binding of 3H-spiperone to peripheral blood cells: relevance for the interpretation of binding studies in psychiatric disorders

1. There is an ongoing discussion regarding the elevated binding of 3H-spiperone to lymphocytes of schizophrenic patients. Several authors described an atypical binding pattern with a saturable high-affinity and a nonsaturable binding site both displaceable by (+)-butaclamol. Recent findings are still controversial (Fartacek et al., 1997; Wodarz et al., 1996), possibly due to methodological differences. The authors investigated 3H-spiperone binding to different peripheral blood cells including B- and T-lymphoblastoids. 2. B-lymphocytes (KD = 0.081 nM; Bmax = 0.46 x 10(-15) mol/10(6) cells) and macrophages (KD = 0.1-1 nM, Bmax = 2.44 x 10(-15) mol/10(6) cells) are characterized by a minor but saturable binding of 3H-spiperone in a concentration range between 0.5 and 1 nM. Above 1 nM, only non-saturable binding was measurable. Interestingly, Epstein-Barr virus (EBV) transformed lymphoblastoids (KD = 0.13 nM) have nearly the same affinity for 3H-spiperone as native B-cells, but an increased number of binding sites (Bmax = 1.76 x 10(-15) mol/10(6) cells). 3. Membranes from B-lymphoblastoids displayed a saturable binding in a concentration between 0 and 1.8 nM of 3H-spiperone (KD = 0.5 nM and Bmax = 1.72 x 10(-15) mol/mg protein). Extraction with 1% digitonin resulted in a similar binding characteristic (KD = 0.17 nM and Bmax = 1.97 x 10(-15) mol/mg protein). 4. T-cells, granulocytes and MOLT-3-cells did not show is a saturable binding even not at high concentrations of 3H-spiperone. 5. The pharmacological profile of the high-affinity 3H-spiperone binding site is clearly different from the dopamine D2 and D4, serotonin 5-HT2 histamine H1 and noradrenergic alpha1 and alpha2 receptor, respectively. 6. In summary, the results suggest that spiperone binding studies to enriched lymphocytes of psychiatric patients should be interpreted cautiously. Variable amounts of leucocytes might result in a higher proportion of nonsaturable, butaclamol displaceable spiperone binding with relevance for the calculation of KD and Bmax of the saturable high-affinity site. Interestingly, homogenous B-lymphoblastoid cell lines have the same binding characteristics as native B-lymphocytes and therefore should be recommended for characterization of 3H-spiperone binding sites.

Development of polyclonal anti-D2 dopamine receptor antibodies to fusion proteins: inhibition of D2 receptor-G protein interaction

Portions of the cDNA encoding the third intracellular loop (i3 loop) of the long and short isoforms of the rat D2 dopamine receptor were subcloned into the vector pNMHUBpoly and expressed in Escherichia coli as fusion proteins. The fusion proteins were gel-purified and used to immunize rabbits for the production of polyclonal anti-receptor antisera. The anti-fusion protein antisera recognized synthetic peptides corresponding to segments of the i3 loops of D2 dopamine receptors in a solid-phase radioimmunoassay. Antisera were tested in an immunoprecipitation assay using the reversible D2 antagonist [125I]NCQ 298 and digitonin-solubilized extracts of canine and rat caudate. [125I]-NCQ 298 bound reversibly and with high affinity (KD = 0.14 nM) to receptors in solubilized extracts enriched by chromatography on heparin-agarose. The anti-UBI-D2i3L and anti-UBI-D2i3s antisera were able to immunoprecipitate quantitatively D2 dopamine receptors labeled with [125I]NCQ 298 from solubilized rat caudate. The antibodies were tested for their ability to affect the coupling of D2 dopamine receptors to GTP-binding proteins in digitonin-solubilized rat caudate. Both anti-UBI-D2i3L and anti-UBI-Di3s antisera were able to inhibit the high-affinity binding of the agonist N-propylnorapomorphine to digitonin-solubilized rat caudate. These findings indicate that the i3 loop of the D2 dopamine receptor is an important determinant for coupling of the G protein.

Antibodies with high affinity for spiroperidol--II. Cross reactivity with iodobenzamide and domperidone

Three radioligands, 3H-spiroperidol (3H-SPD), 3H-domperidone (3H-DOMP) and 125I-iodobenzamide (125I-IBZM), were used to investigate the antibody response to two haptens, aminospiroperidol (NH2SPD) and N-aminophenethylspiroperidol (NAPS). Although structurally different, these three radioligands each bind with high affinity to the D2 dopamine receptor. Antibodies with high affinity for 3H-SPD were elicited in rabbits following immunization with the hapten NH2SPD covalently linked to keyhole limpet hemocyanin (KLH). In addition, antibodies in the rabbit anti-NH2SPD antisera bound 125I-IBZM or 3H-DOMP. Rabbit anti-NH2SPD antibodies that bound 125I-IBZM or 3H-DOMP were found to have higher affinity for IBZM or DOMP, respectively, than for SPD. The binding properties of the anti-NH2SPD antibodies that bound 3H-SPD, 125I-IBZM and 3H-DOMP were characterized using a panel of competitive inhibitors and each radioligand appeared to bind to a distinct subpopulation of anti-NH2SPD antibodies. BALB/c mice were immunized with NH2SPD-KLH or NAPS-KLH. A population of antibodies that bound 3H-SPD and a population of antibodies that bound 3H-DOMP were detected. The population of antibodies that bound 3H-DOMP was found to be heteroclitic for DOMP, since DOMP was a more effective competitive inhibitor than SPD. Binding sites for 125I-IBZM were not detected in either the anti-NH2SPD or the anti-NAPS BALB/c antisera. However, two anti-NAPS monoclonal antibodies, N6-24 and N6-29, that bind 3H-SPD with high affinity (Kd = 10(-9) M), were also found to bind IBZM (Ki = 2 x 10(-7) M) and DOMP (Ki = 2 x 10(-6) M). Although anti-NH2SPD and anti-NAPS antibodies were identified that appeared to bind 3H-SPD, 3H-DOMP or 125I-IBZM with high affinity, none of the populations of polyclonal antibodies or monoclonal antibodies bound all three ligands with high affinity.

Monoclonal antibodies with high affinity for spiroperidol

A diverse panel of monoclonal antibodies was obtained from BALB/c mice immunized with two haptens structurally related to spiroperidol (SPD). Bromoacetyl derivatives of aminospiroperidol (NH2SPD) and N-amino-phenethylspiroperidol (NAPS) were synthesized to couple the haptens covalently to a protein carrier for immunization, thereby maintaining the butyrophenone portion of the immunogen. Hybridomas were selected based on their ability to secrete antibody that binds [3H]SPD with high affinity. Equilibrium dissociation constants for these antibodies ranged from 0.2 to greater than 100 nM. The antigen binding sites of the anti-NH2SPD and anti-NAPS antibodies were characterized in studies of the inhibition of the binding of [3H]-SPD by a series of ligands that are either (a) structurally related to SPD or (b) structurally unrelated to the butyrophenones but known to be selective antagonists of the D2 subtype of dopamine receptor. Based on the patterns of inhibition of the binding of [3H]SPD by these compounds, 12 classes of antibody combining sites were identified. Most of these antibodies bound butyrophenones with high affinity. One anti-NH2SPD and four anti-NAPS antibodies also bound domperidone, a nonbutyrophenone that has a high affinity for D2 receptors. None of the antibodies bound clebopride or sulpiride, D2-selective antagonists of the benzamide class, or the agonist dopamine.

Characterization of binding sites for [3H]spiroperidol

Experiments were designed to investigate the biochemical properties of binding sites for [3H]spiroperidol ([3H]SPD) solubilized from canine caudate and to define the effect of detergent on the binding of the radioligand. Extraction of canine caudate with 0.75-1.0% digitonin was found to generate the maximum yield of binding sites for [3H]SPD while minimizing extraction of membrane proteins. Although binding sites were solubilized with 1.0% digitonin, a 10-fold reduction in detergent concentration was necessary to achieve maximal binding of [3H]SPD. The rank order of affinity for agonists and antagonists was consistent with the pharmacologic properties of the D2 subtype of the dopamine receptor. However, the binding of antagonists was found to be complex. Studies with some preparations of pooled canine caudate resulted in competition curves for the D2-selective antagonists domperidone and sulpiride that best fit a single-site model. Other preparations exhibited biphasic inhibition curves with these antagonists. The class of binding sites for [3H]SPD with low affinity for D2-selective antagonists constituted as much as 30-40% of the binding sites. Enrichment of solubilized binding sites for [3H]SPD was achieved by size exclusion HPLC followed by adsorption to DEAE-Sephadex and elution with buffer of increasing ionic strength. Enrichment of binding sites was accompanied by a decrease in the affinity of solubilized sites for [3H]SPD.

Solubilization of serotonin1a and serotonin1b binding sites from bovine brain

Serotonin1 (5-hydroxytryptamine1, 5-HT1) binding sites have been solubilized from bovine brain cortex using a mixture of 0.1% Nonidet P-40 and 0.3% digitonin in a low-salt buffer containing 0.1% ascorbic acid. The affinity of [3H]5-HT for the soluble cortical binding sites (2.1 nM) is identical to its affinity at membrane-bound binding sites (2.1 nM). [3H]8-Hydroxy-2-(di-n-propylamino)tetralin ([3H]DPAT), a selective 5-HT1a radioligand, also binds to soluble cortical binding sites with high affinity (1.8 nM) comparable with its affinity in the crude membranes (1.7 nM). A significant correlation exists in the rank order potency of serotonergic agents for [3H]5-HT binding and for [3H]DPAT binding to crude and soluble membranes. The density of [3H]DPAT binding sites relative to the [3H]5-HT sites in the solubilized cortical membranes (35%) corresponds well with the proportion of 5-HT1a sites in the crude membranes determined by spiperone displacement (33%), suggesting that both the 5-HT1a and 5-HT1b binding sites have been cosolubilized. [3H]5-HT binding in the soluble preparations was inhibited by GTP, suggesting that a receptor complex may have been solubilized. [3H]Spiperone-specific binding was not detectable in this preparation, suggesting that 5-HT2 sites were not cosolubilized.

Photoaffinity labelling and solubilization on the central 5-HT1A receptor binding site

Two complementary approaches, covalent labelling and solubilization, have been used to study the biochemical properties of the central 5-HT1A receptor binding site. We have first designed a photoaffinity ligand containing the structure of 8-OH-DPAT, a potent and specific agonist of 5-HT1A sites. Thus, 8-methoxy-2[N-n-propyl,N-3-(2-nitro-4-azido-phenyl)- aminopropyl]aminotetralin or 8-methoxy-3'-NAP-amino-PAT, was found to displace, in the dark, [3H]8-OH-DPAT from 5-HT1A sites in rat hippocampal membranes with an IC50 of 6.6 nM. Under two cumulative UV irradiations (366 nm, for 20 min at 4 degrees C), 8-methoxy-3-'-NAP-amino-PAT (30 nM) blocked irreversibly 55-60% of 5-HT1A binding sites. This blockade was specific of 5-HT1A sites since the other serotoninergic sites, 5-HT1B, 5-HT2 and also the presynaptic 5-HT3 sites were not affected by the treatment. In addition, the binding of [3H]Spiperone and [3H]7-OH-DPAT to striatal dopamine sites remained unchanged under similar photolysis conditions. The tritiated derivative of the photoaffinity ligand (92 Ci/mmol) was then synthesized for the identification of the covalently bound protein(s). SDS-PAGE of solubilized membranes irradiated in the presence of 20 nM 3H-8-methoxy-3'-NAP-amino-PAT allowed the detection of a 63 kD protein whose labelling appeared specific. Thus, 3H-incorporation into the 63 kD band could be prevented by microM concentrations of 5-HT, 8-OH-DPAT and other selective 5-HT1A ligands such as isapirone. In contrast, the 5-HT2 antagonist ketanserin, norepinephrine and dopamine-related ligands (including 7-OH-DPAT) were ineffective. Direct solubilization of 5-HT1A receptor binding sites was also attempted from rat hippocampal membranes. The best results were obtained using CHAPS (10 mM) plus NaCl (0.2 M), which led to 50% recovery of 5-HT1A sites in the 100,000 g supernatant. The pharmacological properties and sensitivity to N-ethyl-maleimide and GppNHp of soluble sites appeared near identical to those of membrane-bound 5-HT1A sites.

Decreased serotonin S2 and increased dopamine D2 receptors in chronic schizophrenics

Serotonin S2 and dopamine D2 receptors in the prefrontal cortex and caudate nucleus of postmortem brains of chronic schizophrenics were studied using 3H-ketanserin and 3H-spiperone, respectively. In the prefrontal cortex of schizophrenics, we found a significant decrease in the maximum number of 3H-ketanserin binding sites (Bmax), with no change in the dissociation constant (Kd). Conversely, both Bmax and Kd of 3H-spiperone binding to the caudate nucleus were significantly increased in the schizophrenic patients. There were no differences in receptor indices between patients who were taking neuroleptics until their death and those who had taken none for 2 months or more prior to death. These findings suggest that alterations in S2 receptors in the prefrontal cortex may reflect the disease process, per se, and that the increase in the number of D2 receptors in the caudate nucleus of schizophrenics is not due solely to neuroleptic medication.

Pharmacological characterization of solubilized 5-HT1 serotonin binding sites from bovine brain

This report describes the pharmacologic characterization of [3H]serotonin binding activity solubilized from bovine frontal cortical membranes. The ability of a number of serotonin (5-HT) and lysergic acid diethylamide (LSD) analogs to compete with [3H]serotonin and D-[3H]LSD for binding to membrane and solubilized 5-HT1 sites has been investigated. The results indicate that the solubilized binding site is probably of the 5-HT1B type. Fifteen of the 21 compounds tested exhibit nearly identical affinity for membrane or solubilized 5-HT1 binding sites. However, some important differences were observed, and these may help elucidate the molecular structure of the binding site. In particular, some N-substituted tryptamine analogs show a markedly lower affinity for solubilized 5-HT1 sites compared to their binding to intact membranes. Further, the solubilized site does not distinguish stereoisomers of LSD: both D- and L-LSD bind to solubilized 5-HT1 sites with comparable high affinities, whereas D-LSD has a markedly higher affinity for the membrane 5-HT1 site. Methiothepin, which binds to the 5-HT1 site primarily through its amine groups, has virtually no affinity for the solubilized receptor, whereas it is quite potent at competing for [3H]serotonin binding to membrane sites. These observations lead to the conclusions that in bovine cortical membranes, the 5-HT1 site contains both indole and amine attachment sites. After solubilization, the indole attachment site retains its binding properties, but the amine attachment site has been significantly altered.(ABSTRACT TRUNCATED AT 250 WORDS)

Target size analysis of serotonin 5-HT1 and 5-HT2 receptors in bovine brain membranes

Freeze-dried crude synaptic membranes prepared from bovine cerebral cortex and striatum were exposed to high energy gamma ray from the source of 60Co. The size of serotonin 5-HT1 receptors labeled by [3H]serotonin and that of 5-HT2 receptors labeled by [3H]spiperone or [3H]ketanserin was determined by target size analyses. The values were 57,000 daltons, 145,000 daltons and 152,000 daltons for the cerebral cortex and 56,000 daltons, 141,000 daltons and 150,000 daltons for the striatum, respectively. The estimated sizes were deduced by reference to enzyme standards with known molecular masses and which were irradiated in parallel. Our results demonstrate that the molecular entities in situ for 5-HT1 receptors are distinct from those for 5-HT2 receptors, thus supporting data on the existence of two distinct populations of serotonin receptors, hitherto evidenced physiopharmacologically.

Solubilized ketanserin binding sites from pig platelet membranes

Plasma membranes of pig platelets were isolated and subjected to solubilization with digitonin. Binding of [3H]ketanserin to the solubilized membrane fraction, optimal at pH 7.5, was time and protein dependent and was greatly reduced by increasing ionic strength of the incubation buffer. Saturation analysis and inhibition of binding with various 5-HT2 antagonists showed the presence of two binding sites with high and low affinity, respectively. High affinity binding to solubilized membrane proteins correlated well with that found in native membranes with regard to the dissociation constant KD (1.6 and 1.2 nM, respectively) and the displacement by 5-HT2 antagonists. Binding to this site was not inhibited by the dopamine antagonist haloperidol which, however, displaced [3H]ketanserin bound to the low affinity site. At present the nature of the low affinity binding site (KD = 71 nM) is not well understood. Centrifugation on sucrose gradients revealed only one peak of [3H]ketanserin binding corresponding to the molecular weight of catalase (232,000 Da), thus probably reflecting a close association of the two binding sites.

Solubilization of rat brain serotonin-S2 receptors using CHAPS/salt

Serotonin-S2 receptors from rat frontal cortex were solubilized with a mixture of 6.8 mM CHAPS (3-[(3-cholamidopropyl)-dimethylammonio]-1-propane sulphonate) and 1.4 M sodium chloride. This new solubilization procedure solubilized about 40% of the membrane-bound serotonin-S2 receptors in an active form. The solubilized receptors were not sedimented after 1 h of centrifugation at 100 000 X g and they passed freely through 0.20 micron filters. The solubilized preparation showed high affinity binding of [3H]ketanserin and revealed a typical serotonin-S2 receptor profile: binding could be displaced by nanomolar concentrations of different serotonin antagonists and by micromolar concentrations of serotonin agonists. Compounds belonging to other pharmacological classes were poorly, or not active. Upon density gradient sedimentation, Svedberg coefficients of approximately 5 S were found on sucrose gradients made with H2O or D2O as the solvent. This was much lower than the value of 11.5 S previously reported from lysophosphatidylcholine-solubilized serotonin-S2 receptors.

Solubilization of serotonin S2-receptors from human brain

Serotonin S2-receptors were solubilized from human brain by means of the mild detergent, lysolecithin. Previous studies have shown that the serotonin S2-receptors in human brain are mainly enriched in the cortex. A total particulate fraction from human cortex was treated with 0.25% lysolecithin. [3H]Ketanserin binding sites from the soluble extract showed the binding characteristics of S2-receptors: potent 5HT antagonists like pirenperone, methysergide and pipamperone competed for [3H]ketanserin binding at nanomolar concentrations. The agonists bufotenin and serotonin themselves were more active than the potent dopamine agonist tetraline. Binding was saturable with a low KD (1.07 nM) and reversible. There was a good correlation between the drug potencies in both soluble and membrane preparations and also with the IC50 values previously obtained in membrane preparations and soluble extract from rat brain. Therefore, lysolecithin allows serotonin S2-receptors from human brain to be obtained in a molecularly dispersed form with the same high affinity properties as in the original membranes.

Characterization and regional distribution of serotonin S2-receptors in human brain

[3H]Ketanserin binding was characterized in vitro in human brain homogenates and the regional distribution of the sites was determined. In human brain, [3H]ketanserin was found to bind on serotonin (5-HT) S2-receptors; only 5-HT antagonists competed with the labelled ligand at nanomolar concentrations; other drugs were much less active or inactive. Special attention was paid to the choice of a displacer, here methysergide, to determine the blank value (non-displaceable binding). [3H]Ketanserin binding in human brain displayed similar binding characteristics to the S2-receptor in the rat frontal cortex, high affinity (Kd 0.69 nM) and relatively slow dissociation rate. The regional distribution of serotonin S2-receptors labelled with [3H]ketanserin was studied in 30 different regions of human brain. The highest number of receptors was measured in the cortex. However, within the cortex the distribution was also inhomogeneous, a much lower number of sites being found in the pre- and post-central gyri. In the dopaminergic areas and the cerebellum the number of sites was quite low, and only few binding sites were detected in the corpus callosum, the medulla and the hypophysis. The large number of serotonin S2-receptors in the human cortex suggests that serotonin has an important role in this brain region.

Solubilization and characterization of high-affinity [3H]serotonin binding sites from bovine cortical membranes

High-affinity [3H]serotonin binding activity has been solubilized from bovine cerebral cortical membranes by using Triton X-100, Tween-80, and octyl-beta-D-glucopyranoside. This mixture of detergents solubilizes the high-affinity [3H]serotonin binding activity present in crude membrane preparations with retention of 75-90% specific binding. The detergent mixture was chosen because it can easily be removed from the solubilized fraction by dialysis and polystyrene bead adsorption, thus permitting further purification and isolation of the binding sites. Saturation analysis reveals multiple components of high-affinity [3H]serotonin binding. In crude bovine cortical membranes, at least two binding components are present. A higher-affinity binding component, as defined from curvilinear Scatchard plots, has a Kd for [3H]serotonin of 1-3 nM, whereas a lower-affinity component has a Kd of 10-20 nM. In the solubilized preparation, only a single class of binding sites is apparent, with a Kd of 50-100 nM. Removal of detergents by dialysis and polystyrene bead adsorption results in restoration of the curvilinear Scatchard plot with apparent Kds similar to those observed in crude membrane preparations and with increased Bmax values for each component. [3H]Serotonin binding activity in the solubilized preparation is stable to Sephacryl S-300 column chromatography and to glycerol gradient sedimentation. Saturation analysis of the peak binding fractions from both these procedures once again yields curvilinear Scatchard plots, indicating that the multiple high-affinity binding components are preserved and migrate together. The molecular weight, Stokes radius, and frictional coefficient of the binding site(s) have been calculated. After detergent removal the solubilized material shows many of the characteristics usually attributed to S1 receptors, such as high affinity for [3H]serotonin and its analogs and low affinity for serotonin antagonists.