Use of cholate/sodium chloride for solubilisation of brain D2 dopamine receptors

Coupling of D2 dopamine receptors to G-proteins in solubilized preparations of bovine caudate nucleus

1. The coupling of D2 dopamine receptors and G-proteins has been examined in cholate-solubilized preparations of bovine caudate nucleus. 2. No receptor-G-protein coupling could be detected in solubilized preparations obtained in 0.3% cholate, but if this preparation is diluted 5-fold, coupling is re-established. 3. The dilution process was examined, and it was shown that the change in ionic strength was an important factor in modulating the observed receptor-G-protein interaction. 4. Ionic strength was shown, however, not to be the primary determinant of receptor-G-protein coupling. This is likely to be the formation, upon dilution of the preparation, of vesicles in which receptor and G-protein reassociate. 5. The formation of vesicles upon dilution was examined by a variety of techniques, including thermal-stability studies, gel filtration, centrifugation and electron microscopy.

Investigation of rat striatal dopamine D-1 receptors solubilized by digitonin with a precipitation method

[3H]SCH 23390 binding sites solubilized from rat striatal membranes by the detergent digitonin were investigated by using a polyethylene glycol precipitation method to separate the bound [3H]SCH 23390 from the free [3H]SCH 23390. The binding of [3H]SCH 23390 to the solubilized preparations was specific and saturable with a KD of 4.99 +/- 0.03 nM and a Bmax of 619 +/- 13 fmol/mg protein. The rank order of potency of dopamine agonists and antagonists for competing with [3H]SCH 23390 binding for the solubilized preparations was appropriate for dopamine D-1 receptors. The competition of SCH 23390 and S(-)-SCH 23388 with [3H]SCH 23390 binding for the solubilized preparations was stereoselective. However, the sensitivity of the dopamine agonist high-affinity binding to guanine nucleotide GTP was almost lost upon digitonin solubilization. Preincubating the membranes with dopamine preserved the guanine nucleotide sensitivity of agonist binding for membranes in solubilized preparations. These results proved that the polyethylene glycol precipitation method can be used for assay of digitonin-solubilized dopamine D-1 receptors in rat striatum.

Solubilization and reconstitution of the D-1 dopamine receptor: potentiation of the agonist high-affinity state of the receptor

The D-1 dopamine receptor was extracted from rat striatal membranes with sodium cholate and NaCl in the presence of a specific agonist and phospholipids. The soluble receptor then was reconstituted into phospholipid vesicles by further addition of phospholipids prior to detergent removal. Of the total membrane receptors, up to 48% were extracted and 36% were reconstituted into phospholipid vesicles. Yields were greatly reduced if the agonist was omitted or replaced with an antagonist. The solubilized and reconstituted D-1 receptors retained the pharmacological properties of the membrane-bound receptors, including the ability to discriminate between active and inactive enantiomers of specific agonists and antagonists. In this regard, the affinity of the reconstituted receptors for the D-1 specific antagonist 125I SCH 23982 was similar to that of the membrane-bound receptors with a Kd of 1.5 nM. Both the soluble and reconstituted forms of the D-1 receptor exhibited two affinity states for the D-1 specific agonist SK&F R-38393. In contrast to the low proportion of the receptors that had a high affinity for the agonists in striatal membranes (less than 6%), there was a dramatic increase following solubilization (22%) and reconstitution (40%). Similar results were obtained by using dopamine; the proportion of high-affinity sites increased from 4% (membrane-bound) to 48% (reconstituted) of the total receptor population. These high-affinity sites were coupled to G proteins, as guanyl nucleotides completely abolished them. Addition of guanyl nucleotides prior to solubilization or to reconstitution, however, had no effect on the subsequent yield of the reconstituted receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

The glycosylation properties of D2 dopamine receptors from striatal and limbic areas of bovine brain

D2 dopamine receptors from bovine brain (caudate nucleus and olfactory tubercle) have been solubilized using sodium cholate/NaCl and their glycoprotein properties studied in terms of their interaction with wheat-germ agglutinin-agarose (WGA-agarose). Under optimal conditions about 65% of the applied D2 dopamine receptors bound to WGA-agarose and could be eluted with N-acetylglucosamine. The ability of receptors to adsorb to the affinity column was shown to be dependent on the cholate and salt concentrations used. Digestion of the membrane bound D2 dopamine receptors with neuraminidase prior to solubilisation reduced the ability of the receptors to bind to WGA-agarose (50% of applied receptors bound) whereas digestion with N-acetylglucosaminidase did not significantly affect binding to WGA-agarose. Digestion with the two enzymes together resulted in a larger decrease in binding to WGA-agarose than was seen with the two enzymes alone (40% of applied receptors bound). Stepwise elution of bound receptors from the WGA-agarose columns using 2.5 mM- and 100-mM-N-acetylglucosamine showed that about 40% of the bound receptors interacted with WGA-agarose in a low-affinity manner, the remainder showing a high-affinity interaction. Neuraminidase treatment reduced the low-affinity population suggesting that the interaction of oligosaccharides bearing sialic acid with WGA-agarose is of lower affinity and that higher-affinity binding is via N-acetylglucosamine. These data are discussed in terms of the heterogeneity of carbohydrate moieties on the D2 dopamine receptors within a brain region. In all the tests applied here, however, receptors from caudate nucleus and olfactory tubercle behaved identically so their glycosylation patterns must be very similar.

Biochemical properties of D1 and D2 dopamine receptors

The physiological action of dopamine are mediated by two distinct subtypes of receptors, D1 and D2 dopamine receptors. D1-receptors are linked to stimulation of adenylate cyclase whereas D2-receptors inhibit the enzyme and may also couple to other signal transduction systems such as ion channels. In order to characterize these receptors at the biochemical level we have developed specific probes for the identification and purification of these proteins. The ligand binding sites of the two receptors have been identified by photoaffinity labeling and reside on distinct polypeptides. In rat striatum, the D1 receptor binding site can be identified as a peptide of Mr = 72,000. In contrast, the D2 receptors appears to reside on an Mr = 94,000 peptide in most tissues. A larger peptide of Mr = 120,000 identified in the intermediate lobe of pituitary may represent the unproteolyzed form of this receptor. An affinity chromatography purification procedure has been developed for the D2 dopamine receptor. This procedure affords a substantial purification (greater than 1000 fold) of the receptor solubilized from bovine anterior pituitary glands with complete retention of its binding properties. These biochemical tools should eventually lead to the complete characterization of these two receptor subtypes.

Dopamine receptors: molecular structure and function

Two distinct categories of dopamine receptors, termed D1 and D2, have been identified on the basis of pharmacological and biochemical criteria. Some of the progress made in our understanding of the subunit structure, function and signal transduction properties of these important membrane proteins are reviewed.

Characterization of D2 dopamine receptors in dopamine-resistant prolactin-secreting rat pituitary tumors 7315a and MtTW15

We have investigated the structure of D2 receptors present in two prolactin-secreting, dopamine-resistant, transplantable rat pituitary tumors, 7315a and MtTW15. These receptors specifically bind with high affinity the dopamine antagonist [3H]spiroperidol when membrane bound or solubilized by [3-(3-cholamidopropyl)-dimethyl-ammonio]-1-propane sulfonate 10 mM and are pharmacologically characterized as D2 type. Target-size analysis by radiation inactivation indicated a molecular mass of approximately 100,000 and 200,000 daltons for receptors present respectively in 7315a and MtTW15 tumors either membrane bound or solubilized. The minimal size of the D2 binding site was evaluated at 94,000 daltons by photoaffinity labeling with [125I]azido-N-(p-aminophenethyl)-spiperone followed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. A guanine nucleotide had no effect on the displacing potency of the agonist N-propylnorapomorphine evaluated with membrane-bound or solubilized receptors obtained from either tumor. These results suggest the absence or inactivation of a guanine nucleotide binding protein in the receptorial complex of these tumors. Thus, our data indicate that a structural anomaly is present in the D2 receptorial complex of these prolactin-secreting rat pituitary tumors, which may be responsible for their resistance to the inhibitory effects of 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 dopamine D2 receptors with a zwitterionic detergent DCHAPS

Dopamine D2 receptors were solubilized from synaptosomal membranes of the bovine caudate nucleus using a novel zwitterionic detergent 3-[(3-deoxycholamidopropyl)-dimethylammonio]-1-propane sulfonate (DCHAPS) supplemented with 1,2-propylene glycol. Optimal conditions for solubilization were: 0.12% DCHAPS, 5% 1,2-propylene glycol, 8 mg/ml membrane protein, 30 min, 4 degrees and the yield of the D2 receptors was 36.1%. The soluble extract retained the ability to bind [3H]spiperone. This binding was of high affinity (Kd = 2.28 +/- 0.16 nM), reversible and saturable (Bmax = 1.68 +/- 0.06 pmol/mg protein). The order of potencies of dopamine agonists and antagonists for inhibition of binding, paralleled that observed on membrane-bound D2 receptors (correlation factor r = 0.96). The stereo-specificity of solubilized receptors toward the pairs (+)-[(-)butaclamol, cis(Z)-]trans(E)flupenthixol and dihydroergosine/dihydroergosinine was pronounced.

Characterization of high-affinity dopamine D2 receptors and modulation of affinity states by guanine nucleotides in cholate-solubilized bovine striatal preparations

3,4-Dihydroxyphenylethylamine (dopamine) D2 receptors, solubilized from bovine striatal membranes using a cholic acid-NaCl combination, exhibited the typical pharmacological characteristics of both agonist and antagonist binding. The rank order potency of the agonists and antagonists to displace [3H]spiroperidol binding was the same as that observed with membrane-bound receptors. Computer-assisted analysis of the [3H]spiroperidol/agonist competition curves revealed the retention of high- and low-affinity states of the D2 receptor in the solubilized preparations and the proportions of receptor subpopulations in the two affinity states were similar to those reported in membrane. Guanine nucleotide almost completely converted the high-affinity sites to low-affinity sites for the agonists. The binding of the high-affinity agonist [3H]N-n-propylnorapomorphine ([3H]NPA) was clearly demonstrated in the solubilized preparations for the first time. Addition of guanylyl-imidodiphosphate completely abolished the [3H]NPA binding. When the solubilized receptors were subjected to diethylaminoethyl-Sephacel chromatography, the dopaminergic binding sites eluted in two distinct peaks, showing six- to sevenfold purification of the receptors in the major peak. Binding studies performed on both peaks indicated that the receptor subpopulation present in the first peak may have a larger proportion of high-affinity binding sites than the second peak. The solubilized preparation also showed high-affinity binding of [35S]guanosine-5'-(gamma-thio)triphosphate, a result suggesting the presence of guanine nucleotide binding sites, which may interact with the solubilized D2 receptors. These data are consistent with the retention of the D2 receptor-guanine nucleotide regulatory protein complex in the solubilized preparations and should provide a suitable model system to study the receptor-effector interactions.

Solubilization and characterization of D2-dopamine receptors in an estrone-induced, prolactin-secreting rat pituitary adenoma

D2-dopamine (3,4-dihydroxyphenylethylamine) receptors were successfully solubilized with 3-[(3-cholamidopropyl)-dimethylammonio]-1-propane sulfonate from an estrone-induced rat pituitary adenoma. Forty-five percent of initial protein and 48% of initial [3H]spiroperidol binding sites were solubilized. The high affinity as well as the stereoselectivity of the sites was preserved. The order of potency of dopaminergic agonists was found to be typical of D2 receptors. Target size analysis by radiation inactivation indicated a molecular weight of 143,000 +/- 3,000 and of 106,000 +/- 4,000 daltons for membrane-bound and solubilized receptors, respectively. This suggests the loss of a 37,000-dalton subunit during solubilization without significant modification of binding characteristics. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of receptor protein preparation photolabeled with N-(p-azido-m[125I]iodophenethyl)spiroperidol confirmed the existence of a 94,000-dalton peptide which probably constitutes the ligand binding site of the receptor. Thus, our data indicate that chronic estrogen treatment of rats, although inducing a pituitary adenoma, does not modify the pharmacological characteristics of D2 receptors. These data suggest therefore that these adenoma may represent an ideal source of material for further biochemical characterization of D2 receptors.

Optimisation of conditions for solubilisation of the bovine dopamine D2 receptor

Solubilisation of the dopamine D2 receptor from a membrane preparation of bovine corpus striatum using cholate and NaCl was independently optimised with regard to cholate (0.2%, wt/vol), NaCl (1.5 M), and membrane protein (4 mg/ml) concentrations. A maximum solubilisation yield of 58% was obtained and receptors were measured using a [3H]spiperone binding assay incorporating a polyethylene glycol precipitation step. Solubilisation was confirmed by ultracentrifugation studies, passage of the receptor through fine-pore filters, increased thermolability, and by retention of the prelabelled receptor on gel filtration. The soluble receptor showed saturability and reversibility of binding. Displacement of [3H]spiperone from the soluble receptor by competing compounds correlated closely with displacement from the membrane-bound receptors. [3H]Spiperone binding was found to be pH-dependent, with maximum binding occurring at pH 7.8. A comparison of solubilisation was made with six other agents both with and without added NaCl and it was concluded that the cholate/NaCl solubilisation system provides an efficient, inexpensive, and reliable method for the preparation of functional bovine dopamine D2 receptors.

Attempts to obtain anti(D2 dopamine receptor) antibodies via the anti-idiotypic route

Five stable hybridomas have been obtained that secrete monoclonal antibodies against the D2-dopamine receptor-selective drug spiperone. Each monoclonal antibody has been characterized in terms of its ability to bind a range of dopamine-receptor-selective ligands. One monoclonal antibody has been purified by Protein A affinity chromatography and used to immunize mice. Anti-idiotypic antisera and one hybridoma secreting an anti-idiotypic monoclonal antibody were obtained and shown to inhibit [3H]spiperone binding to the anti-spiperone antibody used for immunization. Neither the antisera nor the anti-idiotypic monoclonal antibody, however, inhibited binding of [3H]spiperone to D2-dopamine receptors.

Solubilization of the D-1 dopamine receptor from rat striatum

The D-1 dopamine receptor was extracted from rat striatal membranes with 0.7% sodium cholate and 1 M NaCl. Pretreatment of the membranes with a D-1 specific agonist, inclusion of crude phospholipids in the solubilization buffer, and subsequent removal of the detergent led to a maximal extraction of 48% of the receptor binding sites. The D-1 antagonist, [125I]SCH 23982, bound to single class of sites with a Kd of 1.8 nM and a Bmax of 1.65 pmol/mg protein. The solubilized receptors retained the ability to discriminate between active and inactive enantiomers of agonists and antagonists selective for the D-1 receptor.

[125I]Spiperone is not a useful ligand for studying the CHAPS solubilized dopamine D-2 receptor from rat striatum

The interaction of [125I]spiperone and [3H]spiperone with CHAPS solubilized preparations of rat striatal tissue has been compared. Only about 15% of [125I]spiperone binding was displaced by sulpiride compared with about 67% of [3H]spiperone binding. In the presence of (+)-butaclamol the displacement of [125I]spiperone was twice that found with sulpiride suggesting an interaction with sites other than D-2 receptors. The specific binding of [125I]spiperone was not saturable within the maximum concentration range that could be employed and its affinity for soluble preparations was far lower than that of [3H]spiperone. Despite its very high specific activity [125I]spiperone offers no advantage over [3H]spiperone in the identification of dopamine receptors in soluble tissue preparations.

Specific [3H]piflutixol binding to CHAPS-solubilised rat striatal preparations involves dopamine D-2 but not D-1 binding sites

[3H]Piflutixol binding to rat striatal membrane preparations identifies both D-1 and D-2 sites. We used [3H]piflutixol to characterise those binding sites present in 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate (CHAPS)-solubilised rat striatal preparations. The specific binding of [3H]piflutixol, as defined using cis-flupenthixol, to CHAPS-solubilised rat striatal tissue was saturable and of high affinity. Specific [3H]piflutixol binding to the solubilised preparations was displaced stereoselectively by the isomers of butaclamol and to an equal extent by both cis-flupenthixol and (+/-)-sulpiride. A positive correlation was found between the capacity of a range of drugs to displace [3H]piflutixol binding and the displacement of [3H]spiperone to the same preparations. The Bmax of [3H]piflutixol binding was not different from that of [3H]spiperone binding to the same preparation. These studies suggest that, in contrast to specific binding of membrane preparations, the specific binding of [3H]piflutixol to CHAPS-solubilised preparations involves mainly D-2 sites. Specific [3H]piflutixol binding, in contrast to [3H]spiperone binding, showed only slow dissociation from soluble preparations. The binding of [3H]piflutixol to CHAPS-solubilised preparations was retained during passage through a gel filtration column. This prelabelling of solubilised striatal preparations using [3H]piflutixol may aid in the purification of CHAPS-solubilised rat striatal D-2 sites.

Properties of rat striatal D-2 dopamine receptors solubilized with the zwitterionic detergent CHAPS

The specific binding of [3H]spiperone was determined in membrane preparations of rat striatum and following solubilization treatment with the zwitterionic detergent CHAPS. Membrane protein solubilization was confirmed by ultrafiltration, gel filtration and increased heat sensitivity. Specific binding of [3H]spiperone to the solubilized preparation was saturable and of high affinity, although solubilization led to an approximate 10 fold decrease in receptor affinity for [3H]spiperone. The drug displacement profile of binding to the CHAPS solubilized preparation corresponded to that of the dopamine D-2 receptor; binding was stereoselectively displaced by the isomers of butaclamol. The sodium dependence of sulpiride displacement of specific [3H]spiperone binding was retained in the CHAPS solubilized preparation. GTP (100 microM) only altered the ability of dopamine to displace [3H]spiperone binding to the solubilized preparation in the presence of 120 mM sodium chloride. The GTP effect was small compared with that observed in the membranes. Specific [3H]spiperone binding sites in the solubilized preparation were preferentially retained by a wheat germ agglutinin affinity column and subsequently eluted with N-acetyl-D-glucosamine. Gel filtration of the solubilized preparation using a Sepharose column resulted in two peaks of specific [3H]spiperone binding, the larger component had a Stokes radius of 7.7 nm. CHAPS treatment of rat striatal membranes results in solubilization of the D-2 receptor in an active form. The D-2 site appears to be a glycoprotein of high molecular weight.

Partial purification of dopamine D2 receptors using lectin affinity columns

Dopamine D2 receptors, detected by [3H]spiperone binding, were solubilized from bovine caudate nucleus by cholate/sodium chloride and were found to bind to wheat germ agglutinin immobilized on agarose. Specific elution could be achieved with N-acetylglucosamine whereas other sugars tested were inactive in this regard. The eluted preparation was enriched in solubilized receptors about sevenfold. The pharmacological properties of the preparation were essentially unchanged by the lectin affinity purification procedure. The D2 dopamine receptor is therefore a glycoprotein.

Characterization of cholate-solubilized dopamine receptors from human, dog and rat brain

[3H]Spiperone binding sites were solubilized in high yield from human, dog and rat brain with a mixture of sodium cholate (0.3% w/v) and sodium chloride (1.4 M). The binding sites were not sedimented after one hour at 100,000 g, they passed freely through 0.20 micron filters, migrated as a single peak in gradient sedimentation and were retarded upon gel filtration, proving that they were truly solubilized. The solubilized binding sites were definitely of dopaminergic nature. They showed saturable, reversible, high affinity binding of [3H]spiperone; displacement of [3H]spiperone binding by nanomolar concentrations of dopamine antagonists and micromolar concentrations of serotonin antagonists; stereo-specificity and a good correlation with drug affinities for membrane preparations. The non-displaceable, non-specific [3H]spiperone binding was very low. Gradient sedimentation analysis revealed a sedimentation coefficient of 12 S for dog solubilized preparations, 9 S for rat solubilized preparations and only 2.5 S for human solubilized preparations (values, uncorrected for detergent binding). Gel filtration experiments seem to confirm these molecular characteristics. Therefore the present results show that the dopamine receptor reveals the same pharmacological properties when solubilized with cholate-salt from rat, dog or human brain, while physico-chemical properties seem to indicate some differences.

Photoaffinity labelling of dopamine receptors. Synthesis and binding characteristics of azapride

A new compound, azapride, i.e. 4-azido-5-chloro-2-methoxy-N-[1-(phenylmethyl)-4-piperidinyl]benza mide, the azide derivative of the dopamine antagonist clebopride, was synthesized and tested for its usefulness as a photoaffinity probe for dopamine receptors. Without photolysis the azide behaved as a very selective dopamine antagonist. When a microsomal preparation of dog striatum was pretreated with the azide and subsequently irradiated with ultraviolet light, dopamine receptors became irreversibly blocked. This reaction was dependent on both azide concentration and time of irradiation. The irradiation-induced inhibition was quite selective for the dopamine receptor; various other receptor systems were not affected. Moreover the irreversible inhibition of dopamine receptors from dog striatum could be prevented by dopamine antagonists as well as by a dopamine agonist, but not by a serotonin antagonist. The irradiation-induced inactivation by azapride remained unchanged in the presence of the scavenger p-aminobenzoic acid. These findings provide evidence that azapride labels dopamine receptors specifically and irreversibly and by a true photoaffinity mechanism. It should therefore be of great value for further molecular characterization and purification of dopamine receptors.

Solubilization of dopamine-D2 receptors from synaptosomal membranes of the bovine caudate nucleus

Dopamine D2-receptors were solubilized from synaptosomal membranes of the bovine caudate nucleus using different detergents. They were labelled with [3H]-spiperone and assayed by polyethylene glycol precipitation. CHAPS was found to be the best solubilizing agent among all detergents used. Optimal conditions for solubilization were: 0.25% CHAPS, 3.5 mg ml-1 protein, 25 min, 4 degrees C and the yield of D2-receptors was 18.6%. Addition of some sulphobetain detergents increased the extent of solubilization, 125 mM NaCl and 0.25 M sucrose decreased it, while SH-group protecting agents (2 mM dithiothreitol and 6 mM beta-mercaptoethanol), as well as MEGA-9 and MEGA-12 were almost ineffective. -log IC50 values for solubilized dopamine D2-receptors are in linear correlation with the corresponding values for membrane-bound receptors (r = 0.962, slope factor 0.96) and Kd value of solubilized receptors was 3.61 +/- 0.94 nM, while that of membrane-bound receptors was 1.25 +/- 0.10 nM. Specific binding of [3H]-spiperone to the solubilized receptors resolved by linear sucrose density gradient centrifugation shows two maxima, one in the first several fractions from the bottom and the other with an apparent S value of 7.3.

Improvement in conditions for solubilisation and characterisation of brain D2 dopamine receptors using various detergents

A series of detergents of varying chemical properties has been tested for solubilisation of bovine caudate nucleus D2 dopamine receptors using [3H]spiperone binding to assay the solubilised sites. The properties of the lysophosphatidylcholine (LPC)- and 3-[(3-cholamidopropyl)dimethylammonio]-1-propane-sulphonate (CHAPS)-solubilised preparations are described in detail. The preparations are truly solubilised, and sucrose density gradient and gel filtration data are reported. Specific [3H]spiperone binding in the LPC-solubilised preparation assayed at 4 degrees C is solely to D2 dopamine receptors. If the assay temperature is raised to 25 degrees C, the amount of specific [3H]spiperone binding is largely unchanged, but it forms a greater proportion of the total [3H]spiperone binding owing to a reduction in nonstereospecific (spirodecanone) [3H]spiperone binding at the higher temperature. The effect of raising the assay temperature is important as it enables more precise determinations of specific [3H]spiperone binding to be made. Part of the specific [3H]spiperone binding at 25 degrees C is to solubilised S2 serotonin receptors in addition to D2 dopamine receptors. Good correlations are observed between the affinities for binding of ligands to the solubilised D2 receptors and corresponding data obtained on membrane-bound receptors. Agonist binding in LPC-solubilised preparations is insensitive to guanine nucleotides. It is speculated that the spirodecanone sites represent, in part, proteolysed or damaged D2 dopamine, or S2 serotonin, receptors. In the CHAPS-solubilised preparation the pharmacological profile of [3H]spiperone binding is unclear when assayed at 4 degrees C, but in assays at 25 degrees C a clear serotonin S2 receptor component of specific [3H]spiperone binding can be discerned.(ABSTRACT TRUNCATED AT 250 WORDS)

Reconstitution of solubilised brain D2 dopamine receptors into phospholipid vesicles

D2 dopamine receptors have been solubilised from bovine caudate nucleus using cholate/sodium chloride in the presence of soyabean phospholipid. Reconstitution of the receptors into soyabean phospholipid vesicles has been achieved by dialysis to remove detergent and salt. The receptors are truly reconstituted as judged by sedimentation, electron microscopy, heat stability and analysis on sucrose density gradients. The ligand-binding properties of the reconstituted receptors resemble those of the solubilised preparation.