Dopamine D1 receptors characterized with [3H]SCH 23390. Solubilization of a guanine nucleotide-sensitive form of the receptor

Mutations in GNAL cause primary torsion dystonia

Dystonia is a movement disorder characterized by repetitive twisting muscle contractions and postures^1,2. Its molecular pathophysiology is poorly understood, in part due to limited knowledge of the genetic basis of the disorder. Only three genes for primary torsion dystonia (PTD), TOR1A (DYT1)³, THAP1 (DYT6)⁴, and CIZ1⁵ have been identified. Using exome sequencing in two PTD families we identified a novel causative gene, GNAL, with a nonsense p.S293X mutation resulting in premature stop codon in one family and a missense p.V137M mutation in the other. Screening of GNAL in 39 PTD families, revealed six additional novel mutations in this gene. Impaired function of several of the mutations was shown by bioluminescence resonance energy transfer (BRET) assays.

Postsynaptic nigrostriatal dopamine receptors and their role in movement regulation

The article presents the hypothesis that nigrostriatal dopamine may regulate movement by modulation of tone and contraction in skeletal muscles through a concentration-dependent influence on the postsynaptic D1 and D2 receptors on the follow manner: nigrostriatal axons innervate both receptor types within the striatal locus somatotopically responsible for motor control in agonist/antagonist muscle pair around a given joint. D1 receptors interact with lower and D2 receptors with higher dopamine concentrations. Synaptic dopamine concentration increases immediately before movement starts. We hypothesize that increasing dopamine concentrations stimulate first the D1 receptors and reduce muscle tone in the antagonist muscle and than stimulate D2 receptors and induce contraction in the agonist muscle. The preceded muscle tone reduction in the antagonist muscle eases the efficient contraction of the agonist. Our hypothesis is applicable for an explanation of physiological movement regulation, different forms of movement pathology and therapeutic drug effects. Further, this hypothesis provides a theoretical basis for experimental investigation of dopaminergic motor control and development of new strategies for treatment of movement disorders.

Dopamine D2 receptors as treatment targets in schizophrenia

The antipsychotic effectiveness of chlorpromazine and haloperidol started a search for their therapeutic targets. The antipsychotic receptor target turned out to be a dopamine receptor, now cloned as the dopamine D2 receptor. The D2 receptor is the common target for antipsychotics. Antipsychotic clinical doses correlate with their affinities for this receptor. Therapeutic doses of antipsychotics occupy 60 to 80% of brain D2 receptors in patients, but aripiprazole occupies up to 90%. While antipsychotics may take up to six hours to occupy D2 receptors, much clinical improvement occurs within a few days. The receptor has high- and low-affinity states. The D2High state is functional for dopamine-like agonists such as aripiprazole. Most individuals with schizophrenia are supersensitive to dopamine. Animal models of psychosis show that a variety of risk factors, genetic and nongenetic, are associated with behavioral supersensitivity to dopamine, reflected in elevated levels of dopamine D2High receptors. Although antipsychotics such as haloperidol alleviate psychosis and reverse the elevation of D2High receptors, long-term use of traditional antipsychotics can further enhance dopamine supersensitivity in patients. Therefore, switching from a traditional antipsychotic to an agonist antipsychotic such as aripiprazole can result in the emergence of psychotic signs and symptoms. Clozapine and quetiapine do not elicit parkinsonism and rarely result in tardive dyskinesia because they are released from D2 within 12 to 24 hours. Traditional antipsychotics remain attached to D2 receptors for days, preventing relapse, but allowing accumulation that can lead to tardive dyskinesia. Future goals include imaging D2High receptors and desensitizing them in early-stage psychosis.

Rhesus monkey trace amine-associated receptor 1 signaling: enhancement by monoamine transporters and attenuation by the D2 autoreceptor in vitro

Trace amine-associated receptor 1 (TAAR1) is a G protein-coupled receptor that directly responds to endogenous monoamines as well as amphetamine-related psychostimulants, including methamphetamine. In the present study, we demonstrate TAAR1 mRNA and protein expression in rhesus monkey brain regions associated with monoaminergic systems, variable cellular distribution of TAAR1 in rhesus monkey brain, and TAAR1 coexpression with the dopamine transporter (DAT) in a subset of dopamine neurons in both rhesus monkey and mouse substantia nigra. On this basis, we evaluated rhesus monkey TAAR1 activation by different compounds and its functional relation with monoamine transporters and the dopamine D2 receptor (D2) short isoform (D2s) autoreceptor in vitro using a cAMP response element-luciferase assay. TAAR1 activation by monoamines and amphetamine-related compounds was greatly enhanced by coexpression of dopamine, norepinephrine, or serotonin transporters, and the activation enhancement was blocked by monoamine transporter inhibitors. This enhancement did not occur in control experiments in which the dopamine D1 receptor (D1) was substituted for TAAR1. Furthermore, activation of TAAR1 by dopamine was completely inhibited by D2s when coexpressed with TAAR1, and this inhibition was blocked by the D2 antagonist raclopride. Last, dopamine activation of TAAR1 could induce c-FOS-luciferase expression but only in the presence of DAT, whereas dopamine activation of D1 resulted in equivalent c-FOS expression in the presence or absence of DAT. Together, these data reveal a broad agonist spectrum for TAAR1, a functional relation of TAAR1 with monoamine transporters and D2s, and a mechanism by which D2 receptor drugs can influence brain monoaminergic function and have efficacy through affecting TAAR1 signaling.

Discovery and characterization of orthosteric and allosteric muscarinic M2 acetylcholine receptor ligands by affinity selection-mass spectrometry

Screening assays using target-based affinity selection coupled with high-sensitivity detection technologies to identify small-molecule hits from chemical libraries can provide a useful discovery approach that complements traditional assay systems. Affinity selection-mass spectrometry (AS-MS) is one such methodology that holds promise for providing selective and sensitive high-throughput screening platforms. Although AS-MS screening platforms have been used to discover small-molecule ligands of proteins from many target families, they have not yet been used routinely to screen integral membrane proteins. The authors present a proof-of-concept study using size exclusion chromatography coupled to AS-MS to perform a primary screen for small-molecule ligands of the purified muscarinic M2 acetylcholine receptor, a G-protein-coupled receptor. AS-MS is used to characterize the binding mechanisms of 2 newly discovered ligands. NGD-3350 is a novel M2-specific orthosteric antagonist of M2 function. NGD-3366 is an allosteric ligand with binding properties similar to the allosteric antagonist W-84, which decreases the dissociation rate of N-methyl-scopolamine from the M2 receptor. Binding properties of the ligands discerned from AS-MS assays agree with those from in vitro biochemical assays. The authors conclude that when used with appropriate small-molecule libraries, AS-MS may provide a useful high-throughput assay system for the discovery and characterization of all classes of integral membrane protein ligands, including allosteric modulators.

Dopamine D(5) receptor immunolocalization in rat and monkey brain

Dopamine D(5) receptor localization has been difficult because even the most specific ligands cannot distinguish between molecular subtypes of the D(1)-like receptor subfamily. Antifusion protein rabbit polyclonal antibodies directed against the C-terminus of human D(5) receptor were therefore developed for immunolocalization of the D(5) receptor protein in brain. The antibodies were characterized by immunoblot analysis and immunoprecipitation and used for light microscopic immunocytochemistry in rat and monkey brain. Affinity purified D(5) antibodies were specific for D(5) fusion protein as well as cloned and native D(5) receptor on Western blots, and D(5) antisera specifically immunoprecipitated solubilized, cloned D(5) receptor. Regional distribution of D(5) receptor immunoreactivity was consistent across species and correlated well with D(5) mRNA distribution previously reported in monkey brain. Immunoreactivity was widespread and tended to label perikarya and proximal dendrites of neurons in cerebral cortex, basal ganglia, basal forebrain, hippocampus, diencephalon, brainstem, and cerebellum. Neuropil was immunoreactive in olfactory bulb, islands of Calleja, cerebral cortex, superior colliculus, and molecular layer of cerebellum. The distribution of D(5) in brain was clearly different from that of other dopamine receptor subtypes, including D(1), the other member of the D(1)-like receptor subfamily. This unique distribution corroborates the idea that the D(5) receptor subtype has a distinct role in dopamine neurotransmission.

Solubilization and pharmacological characterization of a glucocorticoid membrane receptor from an amphibian brain

Physiological functions of steroid hormones involve activation of intracellular receptors as well as poorly understood membrane receptors. We report the pharmacological characterization of a solubilized corticosterone receptor from neuronal membranes. This receptor previously was shown to localize with plasma membrane subcellular fractions and to be involved in the modulation of courtship behaviors in the roughskin newt (Taricha granulosa). We describe procedures with non-ionic detergents that solubilize the receptor and maintain high affinity [3H]corticosterone binding. The pharmacology of the solubilized corticosterone receptor resembles that of the membrane receptor with high affinity for [3H]corticosterone and an identical rank-order potency for other steroid ligands (corticosterone>cortisol>aldosterone>dexamethasone). Unlike binding in membrane preparations, [3H]corticosterone binding to the solubilized receptor is insensitive to negative modulation by guanyl nucleotides and only modestly sensitive to the presence of Mg2+. We also identified two ligands that exhibit high affinity binding to the solubilized receptor and have the potential to be used in an affinity purification scheme. They are corticosterone-3-carboxymethyloxime (CORT-3-CMO), which may be covalently attached to a Sepharose resin, and a derivitized azide form of CORT-3-CMO which can be covalently coupled to the solubilized receptor itself. The stability of the solubilized [3H]corticosterone receptor in the detergent system will facilitate further purification and molecular characterization.

Solubilization and biochemical characterization of the human myometrial corticotrophin-releasing hormone receptor

We have solubilized an active form of the myometrial corticotrophin-releasing hormone (CRH) receptor using 1% w/v digitonin. The solubilized receptor retains its capacity for high-affinity binding as demonstrated by Scatchard analysis, although there was a shift in dissociation constant (Kd) from 83.6 +/- 15-195 +/- 35 pM for the membrane-bound and soluble receptor respectively. There was no difference in the maximum binding site concentrations (Bmax) of 13 +/- 5 and 21.5 +/- 6 fmol/mg protein for the membrane-bound and soluble receptor respectively. Sauvagine unlike CRH had no effect on radiolabeled CRH binding which suggests that the CRH-R2 receptor is not present in the myometrium. The solubilized receptor did not retain guanine-nucleotide sensitivity. The isoelectric focusing (IEF) profile of the human myometrial CRH receptors was significantly different from that of the rat cerebral cortex. Furthermore, solubilization of human myometrial membrane proteins followed by gel filtration and SDS-PAGE revealed a specifically labeled protein with an apparent molecular weight of 42000-47000 kDa. Our results suggest that during solubilization the human myometrial CRH receptor is dissociated from the guanine nucleotide-binding protein (Gs) and that high affinity binding for soluble CRH receptors is not dependent on the coupling of a guanine nucleotide-binding protein.

In vivo binding of [11C]SKF 75670 and [11C]SKF 82957 in rat brain: two dopamine D-1 receptor agonist ligands

The high affinity benzazepine D1 agonists SKF 75670 and SKF 82957 labeled with C-11 were evaluated in vivo in rats as potential radioligands for imaging dopamine D1 receptors with positron emission tomography (PET). Their in vivo pharmacological profile revealed selective binding for both tracers in rat brain regions rich in D1 receptors such as the caudate-putamen. The more lipophilic [11C]SKF 82957 (6-chloro-[11C]SKF 75670) showed a higher brain uptake (more than 2-fold up to 30 min), higher specific uptake in the striatum and higher signal-to-noise ratio (striatum-to-cerebellum = 3.2 +/- 0.4 for [11C]SKF 75670 and 9.7 +/- 2.5 for [11C]SKF 82957 at 60 min post-injection) as compared to [11C]SKF 75670. Both radiotracers exhibited high specificity and selectivity for D1 receptors, since only D1 competitors but not the D2 antagonist sulpiride or the 5-HT2 antagonist ritanserin reduced significantly their binding the striatum with [11C]SKF 75670 or the striatum and olfactory tubercles with [11C]SKF 82957. Previous reports have shown that only D1 agonists can recognize the functional high-affinity state from the low-affinity state of D1 receptors. [11C]SKF 75670 and especially [11C]SKF 82957 are D1 agonist radioligands that can potentially be used to study in vivo the functional high-affinity state of D1 receptors using PET.

Dopamine-induced reduction in the density of guanine nucleotide-sensitive D1 receptors in human postmortem brain in the absence of apparent D1: D2 interactions

The effects of dopamine and guanine nucleotides on the binding of the D1 dopamine receptor antagonist ligand [3H]SCH 23390 were examined in membranes prepared from putamen, caudate and nucleus accumbens of human postmortem brain. Dopamine induced a concentration-dependent decrease in the apparent maximum number of binding sites (Bmax) in each brain region studied, and displaced binding in a biphasic manner consistent with the presence of both high and low affinity states of the D1 receptor; the GTP analogue Gpp(NH)p transformed this biphasic displacement to a monophasic pattern consistent with a shift of high affinity sites to a low affinity state. However, the selective D2 antagonist eticlopride did not reverse the action of dopamine to decrease Bmax. These data suggest that dopamine decreases Bmax for D1 receptors through a high affinity, guanine nucleotide-sensitive agonist binding site, but fail to reveal D1:D2 interactions at this synaptic level.

Presence of dopamine D1 receptors and absence of dopamine D2 receptors in human cerebral meningioma tissue

Preliminary studies have shown that the dopamine D1 receptor is expressed in cerebral meningioma tissue. The current study presents evidence that the iodinated dopamine D1 antagonist [125I]SCH-23982 bound to dopamine binding sites in 33 of the 45 human cerebral meningiomas examined for this. Saturation curves and the linearity of the Scatchard analysis indicate that [125]SCH-23982 binds to a homogeneous population of binding sites. Competition curves reveal the presence of a dopamine D1 receptor by rank order of various dopaminergic and nondopaminergic antagonists ((+)-SCH-23390 greater than (+/-)-SKF-83566 greater than (cis)-flupentixol greater than (+)-butaclamol greater than chlorpromazine greater than 1-sulpiride greater than mianserin greater than (-)-butaclamol). Stereoselectivity was evaluated by (+)- and (-)-butaclamol. The mean (+/- standard deviation) dissociation rate constant was 369 +/- 196 pM with a density of 31.9 +/- 12.5 fmol/mg membrane protein among 33 meningiomas. The dopamine D2 receptor was not present in the 30 meningiomas examined for this. These findings indicate that the dopamine D1 receptor identified is expressed alone and is therefore regulated independent of a D2 receptor in cerebral meningioma tissue. Although the function of the dopamine D1 receptor in cerebral meningiomas has not so far been defined, previous studies have suggested that the D1 receptor might be involved in the control of proliferative growth of meningiomatous tissue.

Solubilization of dopamine D-1 receptors with a zwitterionic detergent DCHAPS and their reconstitution

1. Dopamine D-1 receptors of the bovine caudate nucleus were solubilized with different detergents. They were labelled with [3H]SCH 23390 and assayed by filtration through PEI-coated glass fibre filters and Sephadex G-50 columns. 2. DCHAPS was the best solubilizer among all detergents used and at 0.075% DCHAPS, 10 mg/ml protein, 30 min, 4 degrees C, gave the yield of 48.7%. 3. Reconstitution of solubilized receptors was performed using SM-2 Bio-Beads. Phosphatidylcholine did not improve reconstitution suggesting that DCHAPS solubilized sufficient amounts of the membrane phospholipids. 4. Loss of affinity of solubilized receptors to [3H]SCH 23390 binding was reversible. Apparent Kd values of 0.36 +/- 0.02, 21.3 +/- 3.2 and 0.77 +/- 0.05 nM were obtained for membrane-bound, solubilized and reconstituted receptors, respectively.

Activation of the 5-HT1C receptor expressed in Xenopus oocytes by the benzazepines SCH 23390 and SKF 38393

1. A cloned 5-HT1C receptor expressed in Xenopus laevis oocytes was used to characterize the action of four dopamine D1-selective benzazepines at the 5-HT1C receptor. Additionally, the apparent binding of the D1-selective benzazepines to 5-HT1C receptors was measured in the choroid plexus of the pig. 2. In voltage-clamped oocytes expressing the cloned 5-HT1C receptor, 5-hydroxytryptamine (5-HT) elicited a characteristic inward current response with an EC50 of 13 nM. SCH 23390 acted as a stereoselective agonist (or partial agonist) with an EC50 of about 550 nM. SKF 38393 (1 microM-1 mM), SKF 77434 (100 microM), and SKF 82958 (100 microM) also acted as agonists (or partial agonists) at the cloned 5-HT1C receptor. SKF 38393 was not stereoselective at the 5-HT1C receptor. 3. The response to SCH 23390 activated slowly and, although the response contained many oscillations characteristic of the activation of the phosphatidylinositol signal transduction system, SCH 23390 rarely elicited the rapid spike-like response seen routinely in response to 5-HT. However, the responses to SKF 38393, SKF 77434, and SKF 82958 were identical in appearance to the response to 5-HT, except that the responses to the benzazepines were smaller. These comparisons were made by applying both a benzazepine and 5-HT to each individual oocyte expressing the cloned 5-HT1C receptor. 4. Consistent with the responses measured in oocytes, SCH 23390 bound stereoselectively to 5-HT1C receptors in the choroid plexus of the pig (Ki = 6.3 nM), and SKF 38393 bound non-stereoselectively with lower affinity (Ki = 2.0-2.2 microM).5. It is concluded that while these benzazepines demonstrate selectivity for the dopamine D1 receptor, they also can act as agonists or partial agonists at the 5-HT1c receptor in situ and as expressed in Xenopus oocytes. The oocyte expression system is useful for studies of the functional pharmacology of these 5-HTic receptors. Information about the pharmacological actions and variations in stereoselectivity among dopamine and 5-HT receptors should be of interest in modelling the interactions of ligands with these G-protein coupled receptors, and in the testing of such models through receptor mutagenesis.

On the mechanism of mergocryptine-induced suppression of dopamine turnover in the rat striatum

The effect of mergocryptine, a new ergot alkaloid, on the cerebral dopaminergic systems was examined using Wistar rats. The administration of mergocryptine (1 and 10 mg/kg i.p.) induced a significant suppression of striatal dopamine (DA) turnover. In vitro addition of mergocryptine (0.01-100 microM) induced a dose-dependent suppression of the release of [3H]DA from striatal slices. Mergocryptine inhibited [3H]apomorphine binding to a striatal synaptosomal fraction, and its IC50 value was found to be 0.23 microM. Pretreatment with apomorphine (100 micrograms/kg s.c.) showed an additive effect on the mergocryptine (10 mg/kg)-induced suppression of DA turnover. These results suggest that mergocryptine may induce the suppression of striatal DA turnover by reducing DA release via the stimulation of presynaptic dopaminergic autoreceptors.

Characterization of the dopamine receptor mediating the hyperpolarization of cockroach salivary gland acinar cells in vitro

1. Intracellular recordings have been made of the hyperpolarization of cockroach salivary gland cells induced by nerve stimulation and dopamine. 2. The relative potency of a number of dopamine antagonists in inhibiting the dopamine- and nerve-mediated hyperpolarization was studied. SCH23390 (10-50 microM), chlorpromazine (0.1-5 microM), haloperidol (10-100 microM) and metoclopramide (1 mM) inhibited the hyperpolarization. 3. In contrast, domperidone and (+/-)-sulpiride potentiated the hyperpolarization induced by both nerve stimulation and dopamine. 4. Apparent dissociation constants (KDapp) were obtained for the blockade of the dopamine-induced hyperpolarization. The rank order of potency (KDapp in parentheses) was as follows: chlorpromazine (0.2 microM); haloperidol (3.3 microM); SCH23390 (4.1 microM); metoclopramide (265 microM); domperidone and (+/-)-sulpiride (inactive). 5. It is concluded that the receptor subserving the dopamine-induced hyperpolarization of the salivary gland acinar cells is the same as that mediating the secretory response to dopamine. In addition these data support our findings, which suggested that this receptor is similar to the D1 dopamine receptor, but distinct from the D2 receptor found in mammalian systems.

Solubilization of A1 adenosine receptor from pig brain: characterization and evidence of the role of the cell membrane on the coexistence of high- and low-affinity states

The present solubilization strategy recognizes the important role of detergent cocktails in the solubilization and subsequent stability of adenosine A1, receptors from pig brain cortical membranes. The 3-[3-(cholamidopropyl)dimethylammonio]-1-propane-sulfonate-digitonin mixture produced the extraction of up to 52% of the receptor with an enrichment of 1.2-fold with respect to crude membranes. The binding activity of the soluble extract was very stable even in the absence of glycerol. In crude membranes the existence of high- and low-affinity states was detected, but in the soluble extract and in the detergent-treated membranes only the high-affinity state was detected. Association-dissociation curves showed that in crude membranes no interconversion between high- and low-affinity sites is produced by the association of the ligand [3H]R-N6-phenylisopropyladenosine. These results suggest that the high- and low-affinity states are different conformations induced by the structure of the membrane. The modulation of the binding activity by (Gpp(NH)p) 5'-guanylylimidodiphosphate and Mg2+ was studied. In crude membranes Gpp(NH)p shifted the high-affinity state to the low-affinity state, whereas the contrary occurred when Mg2+ was used. The effect of both Mg2+ and Gpp(NH)p was also assayed with the soluble extract and with the detergent-treated membranes. In addition to a decrease of the overall binding capacity, Gpp(NH)p promoted a conversion to all low-affinity states in the detergent-treated membranes or to all very-low-affinity sites in the soluble extract. Mg2+ and Gpp(NH)p counteracted their effects in intact membranes, whereas Mg2+ could not reverse the uncoupling effect of Gpp(NH)p with solubilized or detergent-treated membranes. Thus, it is suggested that Mg2+ acts at sites other than guanine-nucleotide-sensitive sites. If high-affinity states correspond to receptor/G protein complexes and low-affinity states correspond to the uncoupled receptor, we should conclude that Mg2+, as well as the loss of membrane integrity, favours the interaction of A1 receptor molecule with G protein.

Apparent precoupling of kappa- but not mu-opioid receptors with a G protein in the absence of agonist

Rabbit and guinea-pig cerebellum membranes contain a very high (greater than 80%) proportion of mu- and kappa-opioid receptors, respectively. Rabbit (mu) and guinea-pig (kappa) cerebellum membranes were (i) labeled either with the opiate agonist, [3H]etorphine (Kd = 0.1-0.2 nM), or with the opiate antagonist, [3H]diprenorphine (Kd = 0.1 nM), in the absence or presence of Na+ and/or 5'-guanylylimidodiphosphate (GppNHp), (ii) solubilized with digitonin (1%, w:v) and (iii) the radioactivity in the soluble extracts analyzed by ultracentrifugation in sucrose gradients. In the soluble extracts from rabbit cerebellum (mu) membranes, bound [3H]etorphine sedimented faster (S20,w congruent to 12S) than bound [3H]diprenorphine (10S), while in those from guinea-pig cerebellum (kappa) membranes, bound [3H]etorphine and bound [3H]diprenorphine sedimented at the same position (12S). Na+ selectively decreased recovery of the bound tritiated agonist in the two soluble preparations. When they had been generated in the presence of GppNHp but in the absence of Na+, the [3H]etorphine complexes of the mu- and kappa-opioid receptors as well as the [3H]diprenorphine complex of the kappa-opioid receptor were all recovered at position 10S, indicating that GppNHp had induced a decrease of the apparent molecular size of the two types of opioid receptors. These data are interpreted in terms of mu- and kappa-opioid receptors being capable of physically interacting with a G protein (GTP binding regulatory protein) yet, unlike the mu-opioid receptor which does so only in the presence of an agonist, the kappa-opioid receptor appears to be precoupled with a G protein.

Solubilization and reconstitution of dopamine D1 receptor from bovine striatal membranes: effects of agonist and antagonist pretreatment

The bovine striatal dopamine D1 receptor was solubilized with a combination of sodium cholate and NaCl in the presence of phospholipids, following treatment of membranes with a dopaminergic agonist (SKF-82526-J) or antagonist (SCH-23390). The solubilized receptors were subsequently reconstituted into lipid vesicles by gel-filtration. A comparison of ligand-binding properties shows that the solubilized and reconstituted receptors bound [3H]SCH-23390 to a homogeneous site in a saturable, stereospecific and reversible manner with a Kd of 0.95 and 1.1 nM and a Bmax of 918 and 885 fmol/mg protein respectively for agonist- and antagonist-pretreated preparations. These values are very similar to those obtained for membrane-bound receptors. The competition of antagonists for [3H]SCH-23390 binding exhibited a clear D1 dopaminergic order in the reconstituted preparation obtained from either agonist or antagonist-pretreated membranes, except that (+)butaclamol was about four-fold more potent than cis-flupentixol in displacing [3H]SCH-23390 binding in preparation obtained from agonist-pretreated membranes compared to antagonist-pretreated membranes. The agonist/[3H]SCH-23390 competition studies revealed the presence of a high-affinity component of agonist binding in both the reconstituted receptor preparations. The number of high-affinity agonist binding sites, however, is 40-80% higher in reconstituted preparation obtained from antagonist-treated membrane compared to that obtained from the agonist-treated membrane. In both the preparations, 100 microM guanylylimidodiphosphate (Gpp(NH)p) completely abolished the high-affinity component of agonist binding compared to partial abolition in the native membranes, indicating a close association of a G-protein with the solubilized receptors. Whether the receptor was solubilized following agonist or antagonist preincubation of the membranes, the receptor-detergent complex eluted from a steric-exclusion HPLC column with an apparent molecular size of 360,000. Preincubation of the solubilized preparations with Gpp(NH)p had virtually no effect on the elution profile suggesting a lack of guanine nucleotide-dependent dissociation of G-protein receptor complex.

The dopamine transporter and cytochrome P45OIID1 (debrisoquine 4-hydroxylase) in brain: resolution and identification of two distinct [3H]GBR-12935 binding proteins

Two [3H]GBR-12935 binding proteins, identified as the dopamine transporter and cytochrome P45OIID1, were solubilized in digitonin from canine striatal membranes, and were resolved following wheat germ agglutinin (WGA)-lectin column chromatography. Protein adsorbed to and specifically eluted from WGA-lectin with N-acetylglucosamine displayed saturable, high affinity (KD approximately 3 nM), and sodium-dependent binding of [3H]GBR-12935, which was inhibited in a concentration-dependent and stereoselective manner by dopamine uptake blockers and substrates with a pharmacological profile indicative of the dopamine uptake site. Protein not adsorbed to WGA-lectin also bound [3H]-GBR-12935 with high affinity (approximately 7 nM), in a sodium-independent manner, and was insensitive to classical dopamine uptake blockers and substrates such as mazindol or dopamine, corresponding to the so-called "piperazine acceptor" site seen in native membranes. [3H]GBR-12935 binding to this latter protein was, however, inhibited by various compounds with a pharmacological profile indicative of a form of cytochrome P450 designated P45OIID1 (debrisoquine/sparteine monooxygenase) with the following rank order of inhibitory potency: GBR-12909 greater than budipine greater than alpha-lobeline greater than quinidine greater than alpha flupenthixol greater than SKF-525A greater than sparteine greater than quinine. Ki values obtained for inhibition of [3H]-GBR-12935 binding to neuronal WGA passthrough fractions by these drugs correlate well with their respective Ki values for liver P45OIID1 activity. Western blotting and immunoprecipitation analysis with rabbit anti-rat P45OIID1 antibody also supported the identity of the mazindol-insensitive [3H]GBR-12935 binding site (or piperazine acceptor site) as P45OIID1. Furthermore, a [3H]GBR-12935 binding protein with pharmacological and immunological characteristics similar to those of P45OIID1 was solubilized from both bovine and human liver membranes, and GBR-12909 was found to be a potent competitive inhibitor (Ki approximately 100 nM) of sparteine monooxygenase activity in human liver microsomes. These data clearly indicate that [3H]GBR-12935 and its analogs display similar affinities for both the dopamine transporter and neuronal P45OIID1, and that this radioligand may be a useful probe of P45OIID1 activity in brain and liver. The exact molecular and functional association (if any) between these two distinct binding protein populations remains to be established; however, it is tempting to speculate that P45OIID1 is involved in the catabolism and processing of neurotransmitters subsequent to their reuptake into target cells.

The action of dopamine receptor antagonists on the secretory response of the cockroach salivary gland in vitro

1. A study has been made of the potency of a number of dopamine antagonists to inhibit dopamine-induced secretion from the cockroach salivary gland in vitro. 2. Chlorpromazine (0.5-5 microM), SCH23390 (10-100 microM), haloperidol (10-100 microM) and metoclopramide (2 mM) competitively inhibited the secretory response to dopamine. In contrast (+/-)sulpiride (1-100 microM) and domperidone (1-100 microM) had no effect on either basal or dopamine-induced secretion. 3. Apparent dissociation constants (KDapp) were obtained using a 'three point assay'. The rank order of potency (KDapp in parentheses) was as follows: chlorpromazine (0.2 microM) greater than SCH23390 (2.2 microM) greater than haloperidol (17.5 microM) much greater than metoclopramide (1.2 mM). 4. It is concluded that the receptor mediating dopamine-induced secretion in the cockroach salivary gland is similar to the D1/DA1 dopamine receptor and distinct from the D2/DA2 receptor found in mammalian systems.

Stabilization of soluble active rat liver glucagon receptor

Active glucagon receptor was solubilized with 3-(3-cholamidopropyl)dimethylammonio-1-propanesulfonate (Chaps) from rat liver plasma membranes but rapidly (less than 8 h) lost activity. Either inclusion of 1X Hanks' balanced salt solution in the 3 mM Chaps solubilization buffer or its addition after solubilization increased the percentage of total binding attributable to specific glucagon binding from approximately 10 to greater than 80%; of great importance, it increased the stability from near zero binding at 8 h to 50% binding at 48 h (4 degrees C). Of the Hanks' solution components, either NaCl (137 mM) or CaCl2 (1.26 mM) was effective in increasing specific binding to approximately 70 and 60% respectively: Mg salts were ineffective. Soluble receptor binding activity was assayed by dextran-coated charcoal adsorption of free hormone. The assay is rapid, simple, and reproducible. It is suitable for monitoring receptor activity during purification and molecular characterization. Competition binding studies gave an IC50 value of 10-20 nM (slope factor approximately 1), with or without GTP. Dissociation assays revealed GTP sensitivity when receptors were solubilized either as glucagon-receptor complexes or free receptor. Active glucagon-receptor complexes could be eluted from wheat germ lectin-agarose: neither concanavalin A-agarose nor soybean agglutinin-agarose bind receptor. A glucagon degrading activity which co-solubilized with the receptor but did not require detergent for extraction was distinguishable from the soluble receptor not only by solubility but also by its heat stability (30 degrees C), its inhibition by bacitracin, its affinity for glucagon, its retention of activity for at least 1 week at 4 degrees C, and its size.

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.

Mono- and divalent cations modulate the affinities of brain D1 and D2 receptors for dopamine by a mechanism independent of receptor coupling to guanyl nucleotide binding proteins

In order to clarify the question of whether the modulatory effects of cations on dopamine receptor affinities are brought about by shifts in the equilibrium of receptor - G protein - coupling, it was investigated whether mono- and divalent cations were still able to modulate rat striatal D1 and D2 receptor affinities after selective inactivation of the G-proteins linked to the two receptors. The Gs-protein coupled to the D1 receptor was eliminated by mild thermal inactivation, and the Gi- (or Go-) protein associated with the D2 receptor by alkylation with a low concentration of N-ethyl-maleimide. Incubation of striatal membranes at 60 degrees C completely abolished the specific binding of 3H-GTP. Both treatments resulted in an increase of the IC50-values for dopamine as a displacer of 3H-SCH 23390 from D1- and of 3H-spiperone from D2 receptors. Concomitantly, the formerly shallow D1 displacement curves became steeper, with their Hill coefficients increasing. This effect was less evident at D2 receptors. Guanosine triphosphate (GTP), which increased the IC50's of dopamine for both receptors approximately two-fold in control membranes, was without effect in pretreated samples, indicating an effective inactivation of the G-proteins. Na+ ions were still able to lower, and Ca2+ ions to increase the affinities of D1 and D2 receptors for dopamine after such inactivation of the respective G-proteins. It is concluded that the mechanism underlying the regulation of dopamine receptor affinities by mono- and divalent cations is independent of and superimposed upon the coupling of these receptors to guanyl nucleotide binding proteins.

Effects of long-term administration of antidepressants and neuroleptics on receptors in the central nervous system

1. A review of the effects of long-term administration of antidepressants and neuroleptics on receptors in the central nervous system is presented. 2. The effects of antidepressants on adenylate cyclase activity and on receptor binding in brain tissue are discussed. Effects on a variety of receptor types are considered. 3. The utilization of electrophysiological, behavioral, and neurochemical studies to assess receptor function after chronic antidepressant administration is discussed, as is the use of peripheral receptor estimations in clinical studies. 4. Animal studies on the actions of chronic administration of neuroleptics on pre- and postsynaptic dopamine receptors are reviewed. Effects of these drugs on dopamine receptors in humans are considered from the following perspectives: postmortem and in vivo binding studies in schizophrenia, tardive dyskinesia, and central versus peripheral receptor estimation.

Dopamine D1 receptor development in the rat striatum: early localization in striosomes

The development of dopamine D1-receptors in rat striatum during the early postnatal period is examined, using autoradiography and [3H]SCH 23390 as ligand. Dopamine D1-receptors are present in striatum at birth and are more dense than in any adjacent region. The receptors are preferentially localized in striosomes and to some extent in a subcallosal streak. The density of D1-receptors in the matrix increases with age so that by two weeks postnatally the striosomal pattern is no longer evident, and the overall dense labelling is the same as seen in adults. Dopamine D1-receptor development seems to take place earlier than that of dopamine terminals but at the same time as or somewhat later than that of acetylcholinesterase. The D1-defined striosomes move from ventrolateral towards dorso-medial striatum with increasing age and from anterior to posterior striatum. This direction is nearly perpendicular to the direction of development of several other markers, including dopamine terminals and D2-receptors. The present studies indicate that for markers appearing in the striosomal compartment there are different patterns of development with respect to time and spatial pattern. Regulation of striatal development by interaction of neuronal systems with one another and with other factors is complex and will require extensive study to clarify the mechanisms involved.

Solubilization of a guanine nucleotide-sensitive form of vasopressin V2 receptors from porcine kidney

Vasopressin (V2) receptors were solubilized from porcine kidney membranes with the detergent egg lysolecithin. Binding of [3H]vasopressin to the solubilized fraction was rapid, specific, and saturable. The agonist dissociation constants observed in membranes and solubilized fractions were 1.7 +/- 0.3 and 2.3 +/- 0.2 nM, respectively. In competition binding experiments, the solubilized fraction exhibited the same pharmacological profile as the membranes. Chemical crosslinking of [125I]vasopressin to the solubilized fraction followed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis demonstrated a 62-kDa band which was specifically labeled with [125I]vasopressin. Vasopressin binding sites from the solubilized fractions were resolved by gel filtration and ultracentrifugation on a sucrose gradient. In addition, agonist high affinity binding to V2 receptors and its sensitivity to guanine nucleotides were preserved even after solubilization in the absence of prebound agonist prior to solubilization. Addition of guanine nucleotides such as GTP gamma S decreased the specific binding of [3H]arginine vasopressin to these solubilized fractions in a dose-dependent manner, suggesting the solubilization of a V2 receptor-G protein complex. [32P]ADP ribosylation of the solubilized fraction by cholera and pertussis toxins revealed specifically labeled proteins with molecular weights of 42,000-43,000 and 39,000-41,000, respectively, on sodium dodecyl sulfate polyacrylamide gels. Furthermore [35S]GTP gamma S binding to these solubilized fractions was enhanced by vasopressin, confirming that a significant proportion of the vasopressin receptors must be closely coupled to G proteins even when these receptors are solubilized in the absence of agonist. These results are in contrast with those reported for beta, alpha 2 adrenergic and D2 dopaminergic receptor systems, but in agreement with D1 dopaminergic and A1 adenosine receptors. The molecular mechanism responsible for this difference remains to be determined.

Anatomical and affinity state comparisons between dopamine D1 and D2 receptors in the rat central nervous system

The anatomical distributions and affinity states of dopamine D1 and D2 receptors were compared in the rat central nervous system using quantitative autoradiography. [3H]SCH23390 and [3H]spiperone (in the presence of 100 nM mianserin) were used to label the D1 and D2 receptors, respectively. The densities of D1 and D2 receptors displayed a positive correlation among 21 brain regions (Pearson correlation coefficient, r = 0.80, P less than 0.001). The affinity states for the D1 and D2 receptors were found to be quite different from each other, and different from the results obtained by others using homogenate preparations. Both the D1 and D2 receptors were best modeled using a two-state model. In the absence of exogenous guanine nucleotides and using the nonselective agonist dopamine as the competitor, the D1 receptor was primarily in a low affinity agonist state (RH = 21 +/- 6%), whereas the D2 receptor was primarily in the high affinity agonist state (RH = 77 +/- 3%). In the presence of 10 microM guanylyl-imidodiphosphate or guanosine-5'-O-(2-thiophosphate) both the D1 and the D2 receptor were completely in a low affinity agonist state (RL = 100%). These affinity states were found both in the nucleus accumbens and olfactory tubercle using dopamine as the competitor and in the striatum using selective D1 or D2 agonists as competitors. Receptor occupancy of the D2 receptor with either an agonist or antagonist did not alter the affinity states of the D1 receptor, and conversely, receptor occupancy of the D1 receptor did not alter the affinity states of the D2 receptor. The correlation between densities of D1 and D2 receptors provides an anatomical framework for evaluating behavioral and electrophysiological evidence of an interaction between the two dopamine receptor subtypes. This interaction does not appear to be due to a sharing or coupling of G-proteins in such a way that binding to one dopamine receptor subtype alters the affinity state of the other receptor subtype. The differences between dopamine receptor distributions described by labeled agonists and antagonists may be due in part to differences in their affinity states. The low proportion of high affinity state D1 receptors may explain some of the difficulties in assigning specific behavioral roles to the D1 receptor.

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)

D1 and D2 dopamine receptors in caudate-putamen of nonhuman primates (Macaca fascicularis)

D1 and D2 dopamine receptors were characterized in the caudate-putamen region of nonhuman primate brains (Macaca fascicularis). D1 dopamine receptors were identified with [3H]SCH 23390 and D2 receptors with [3H]-spiperone. Scatchard analysis of [3H]SCH 23390 saturation data using washed membranes revealed a single high-affinity binding site (KD, 0.352 +/- 0.027 nM) with a density (Bmax) of 35.7 +/- 2.68 pmol/g original wet tissue weight (n = 10). The affinity of [3H]spiperone for the D2 site was 0.039 +/- 0.007 nM and the density was 25.7 +/- 1.97 pmol/g original wet tissue weight (n = 10). D1 and D2 receptors in nonhuman primates may be differentiated on the basis of drug affinities and stereoselectivity. In competition experiments, RS-SKF 38393 was the most selective D1 agonist, whereas (+)-4-propyl-9-hydroxynaphthoxazine [(+)-PHNO] was the most selective D2 agonist. Apomorphine was essentially nonselective for D1 or D2 binding sites. Of the antagonists, R-SKF 83566 and SCH 23390 were the most selective for the D1 site, whereas YM-09151-2 was the most selective for the D2 site. cis-Flupentixol and (S)-butaclamol were the least selective dopamine antagonists. D1 receptors bound benzazepine antagonists (SCH 23390/SCH 23388, R-SKF 83692/RS-SKF 83692) stereoselectively whereas D2 receptors did not. Conversely D2 receptors bound (S)-sulpiride and (+)-PHNO more potently than their enantiomers whereas D1 receptors showed little stereoselectively for each of these isomeric pairs. These binding characteristics may be utilized for evaluation of individual receptor function in vivo.

Dopamine D1 receptors with enhanced agonist affinity and reduced antagonist affinity revealed by chemical modification

In order to investigate the possibility that there may be two conformationally distinct dopamine D1 binding sites, the effect of lysine-modifying agents on striatal dopamine D1 receptors was investigated. Treatment with the distilbene derivative, 4,4'-diisothiocyanostilbene-2,2'-disulfonate, (DIDS), resulted in an irreversible D1 receptor inactivation that was associated with a 70% loss of binding sites. The remaining DIDS-insensitive sites displayed both a decreased affinity (approximately 5 fold) for the D1 antagonist SCH-23390 and an enhanced affinity of dopaminergic agonists (approximately 10 fold) for the agonist high-affinity form of the receptor. Pretreatment with Gpp(NH)p, a non-hydrolysable guanine nucleotide, prevented the formation of the agonist high-affinity form, indicating that these sites are G-protein-linked. Prior occupancy of D1 receptors with dopaminergic agonists and antagonists afforded no protection against DIDS inactivation, suggesting that a site outside the ligand binding subunit of the D1 receptor was modified. Taken together, these data suggest that [3H]SCH-23390 labels two conformationally distinct populations of dopamine D1 receptors.

Glycoprotein nature of D2 dopamine receptors

The glycoprotein nature of the ligand binding subunit of photoaffinity-labeled striatal D2 receptors was investigated. Upon photolysis, [125I]N-azidophenethylspiperone covalently incorporated into a major band of Mr 94000 with an appropriate pharmacological profile for D2 receptors as assessed by autoradiography following SDS-polyacrylamide gel electrophoresis. The exoglycosidase, neuraminidase, altered the electrophoretic mobility of the 94 kDa labeled band to 54 kDa with a slight modification in the binding affinity of [3H]spiperone. Endoglycosidase treatment (glycopeptidase-F) produced a further increase in the mobility of the 94 kDa peptide to approximately 43 kDa. A smaller specifically photolabeled D2 receptor peptide of 34 kDa does not contain terminal sialic acid but is an N-linked glycoprotein as assessed by lectin affinity chromatography and susceptibility to digestion by glycopeptidase-F to a peptide of approximately 23 kDa.

Biochemical and functional characterization of central dopamine receptors

In order to characterize the D2 dopamine receptors at the molecular level we have developed procedures for purification and characterization of the receptor binding protein. The ligand binding sites of the D2 dopamine receptor have been identified by photoaffinity labeling with 3H-7-azidofluphenazine. The D2 dopamine receptor protein was partially purified by Fast Performance Liquid Chromatography on a Mono Q column and on a wheat-germ agglutinin agarose column. Some behavioral expressions which are probably mediated by D1 dopamine receptors were described. Evidence was obtained that catalepsy is associated with the blockade of D1 dopamine receptors and that D1 and D2 dopamine receptor systems interact either directly or indirectly in mediating this behavior. The administration of a dopamine agonist to monkeys with unilateral ventromedial tegmental lesions of the brainstem produced a biting behavior which, in some aspects, resembles the behavior in Lesch-Nyhan patients. The prevention of the dopamine agonist-induced biting behavior by the D1 dopamine antagonists indicates that D1 dopamine receptors are involved in the control of the expression of this behavior. The biting behavior, like other motor functions, might be controlled by the mesolimbic dopamine systems, and supersensitive mesolimbic D1 dopamine receptors might be associated with the pathology of this behavior.

Dopamine receptors in the developing sheep kidney

These studies were designed to characterize dopamine receptor density and affinity in kidneys removed from sheep of varying ages (fetal, newborn, and adult) using radioligand binding methods. Three different radioligands were used: the specific dopamine-1 antagonist 3H-SCH 23390, the dopamine-1/dopamine-2 antagonist 3H-haloperidol, and the dopamine-2 antagonist 3H-spiroperidol. The specific binding of 3H-haloperidol and 3H-spiroperidol was saturable with time and ligand concentration, being indicative of dopamine receptors. The specific binding of the dopamine-1 selective radioligand 3H-SCH 23390 was also saturable with time but displayed several points of saturation with increasing ligand concentration. The specific binding of 3H-haloperidol, which had a low affinity and is indicative of dopamine-1 receptors, showed no age-related changes in maximum receptor density or affinity. On the other hand, the maximum receptor density of dopamine-2 receptors measured by 3H-spiroperidol decreased with age. The observations that renal dopamine-1 receptor density or affinity do not change with maturation are in agreement with our previous studies that showed no age-related changes in dopamine-receptor-mediated renal vasodilatation in sheep. The significance of the decrease in renal dopamine-2 receptor density with age remains to be determined.

The D-1 dopamine receptor

The D-1 receptor will provide a fruitful ground for many scientists in the coming years. Pure biochemists will attempt to isolate, purify and sequence the molecule itself. Functional biochemists will study the mechanisms whereby the receptor regulates adenylate cyclase activity. Physiologists will attempt to study the consequences of stimulating the receptor in either the brain or in peripheral tissues. Animal behavioralists will attempt to understand how the receptor participates in the generation of animals response to dopaminergic drugs (both agonists and antagonists). Finally, it remains to be determined if any novel therapeutic agents targeted towards the D-1 receptor will become commercially viable compounds.

Pharmacological characterisation of the dopamine-sensitive adenylate cyclase in cockroach brain: evidence for a distinct dopamine receptor

Dopamine increases cyclic AMP production in crude membrane preparations of cockroach brain with plateaus in cyclic AMP production occurring between 1-10 microM and at 10 mM. Maximal production of cyclic AMP is 2.25 fold greater than that of control values. Octopamine also increases cyclic AMP production with a Ka of 1.4 microM and maximal production 3.5 fold greater than that of control. 5-Hydroxytryptamine does not increase cyclic AMP production. The effects of octopamine and dopamine are fully additive. The vertebrate dopamine agonists ADTN and epinine stimulate the dopamine-sensitive adenylate cyclase (AC) with Ka values of 4.5 and 0.6 microM respectively and with maximal effectiveness 1.7 fold greater than that of control. The selective D2-dopamine agonist LY-171555 stimulates cyclic AMP production to a similar extent with a Ka of 50 microM. Other dopamine agonists (apomorphine, SKF-82526, SKF-38393) have no stimulatory effects. The octopamine-sensitive AC is inhibited by a variety of antagonists known to affect octopamine and dopamine receptors, with the following order of potency: mianserin greater than phentolamine greater than cyproheptadine greater than piflutixol greater than cis-flupentixol greater than SCH-23390 greater than (+)-butaclamol greater than SKF-83566 greater than SCH-23388 greater than sulpiride greater than spiperone greater than haloperidol. The dopamine-sensitive AC is inhibited by the same compounds with the following order of potency: piflutixol greater than cis-flupentixol greater than (+)-butaclamol greater than spiperone greater than or equal to SCH-23390 greater than cyproheptadine greater than SKF-83566 greater than SCH 23388 greater than mianserin greater than phentolamine greater than sulpiride greater than haloperidol. With the exception of mianserin, 3H-piflutixol is displaced from brain membranes by dopamine antagonists with an order of potency similar to that observed for the inhibition of dopamine-sensitive AC. The results indicate that the octopamine- and dopamine-sensitive AC in cockroach brain can be distinguished pharmacologically and the dopamine receptors coupled to AC have pharmacological characteristics distinct from vertebrate D1- and D2-dopamine receptors.

Elevation of plasma prolactin concentrations by intravenous SCH 23390 and SKF 38393 in conscious rats

The effect of the selective dopamine-1 (D-1) antagonist, SCH 23390 and the selective D-1 agonist, SKF 38393 on plasma prolactin levels was investigated in conscious, freely moving male rats. Plasma prolactin was markedly increased by the intravenous injection of either SCH 23390 or SKF 38393 in a dose-related fashion. The maximal prolactin response was observed 15 min after drug injection. The d-isomer of SKF 38393 was significantly more potent than the l-isomer. In urethane-anesthetized rats, the two drugs produced similar effects on prolactin as those in the conscious animal preparation. Both SCH 23390 and SKF 38393 potentiated the sulpiride-induced prolactin elevation. These observations suggest that pharmacologic stimulation, or blockade of D-1 receptors in the living animal, can mimic the action of a D2 antagonist at the anterior pituitary lactotroph. We have speculated on the possible mechanisms in pituitary and higher in the central nervous system which might be responsible for the prolactin changes caused by either SCH 23390 or SKF 38393.

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.

Comparison of dopamine binding sites in the rat superior cervical ganglion and caudate nucleus

A comparison of morphological and biochemical characteristics of the D1-type dopamine receptor has been assessed in two experimental tissues, the superior cervical ganglion (SCG) and caudate nucleus of the rat. Correlation of the distribution of this dopaminergic binding site using in vitro autoradiographic localization of [3H]SCH 23390, a selective D1-binding antagonist, demonstrated no specific association of this receptor subtype with cyclic AMP immunoreactive structures in the superior cervical ganglion. In contrast, the caudate nucleus demonstrated specific D1-binding sites associated with cyclic AMP immunoreactive elements. Biochemical analyses of the D1-dopamine binding sites showed only 20% of the amount of radioligand bound in SCG homogenates as compared to the quantity bound in homogenates of the caudate nucleus. The non-cyclase linked dopaminergic receptor, assessed using D2-type radioligand binding, was much less prevalent than the binding of the D1-subtype in either experimental tissue. Only a small amount of [3H]sulpiride binding, indicative of the D2-receptor subtype, could be measured in the SCG as compared to the caudate nucleus. This work has demonstrated differences in the amount, and the cellular association of dopamine binding sites in peripheral versus central nervous system areas with dopamine sensitive adenylate cyclase mechanisms.

Cholecystokinin induces the interaction of its receptor with a guanine nucleotide binding protein

To evaluate the relation between the pancreatic cholecystokinin (CCK) receptor and guanine nucleotide-binding protein(s) we studied the effects of nucleotides on 125I-CCK binding to pancreatic acinar plasma membranes, 125I-CCK binding to solubilized 125I-CCK receptors, and the stability of the solubilized 125I-CCK-receptor complex. In plasma membranes, guanine nucleotides both inhibited CCK binding and increased the dissociation of CCK from its receptor. The potency of the nucleotides studied was GTP gamma S = GMP-PNP greater than GTP much greater than ATP. When membranes were solubilized with digitonin, subsequent binding of CCK was insensitive to guanine nucleotides including GTP, GMP-PNP and GTP gamma S. However, if CCK binding occurred before solubilization of the membranes, guanine nucleotides increased dissociation at concentrations and with a specificity similar to that observed for effects on intact pancreatic membranes. It is concluded that guanine nucleotides act via a protein which is separable from the receptor to induce dissociation of bound CCK. Moreover, CCK binding induces an association in the plasma membrane of the CCK receptor with this guanine nucleotide binding protein.

Dopamine agonist high-affinity state in solubilized D2 receptors in striatum, but not in anterior pituitary

To determine whether brain and pituitary dopamine D2 receptors have similar characteristics on solubilization, canine striatal and bovine anterior pituitary tissues were solubilized with digitonin, and their D2 dopamine receptors were characterized by competition studies and by high pressure liquid chromatography. Solubilized striatal membranes retained the ability to bind agonists with high affinity and retained sensitivity to guanine nucleotides. In contrast, however, solubilized pituitary membranes no longer revealed high-affinity binding of agonists and sensitivity to guanine nucleotide unless they had been solubilized in the presence of agonist. Agonist-pre-labelled pituitary receptors were of larger apparent molecular weight than antagonist-pre-labelled receptors. However, striatal receptors pre-labelled by agonist or antagonist were of similar apparent molecular weight. Thus, unlike the pituitary, striatal receptors probably remain associated with a guanine nucleotide binding protein (N) upon solubilization.

Dopamine receptors and the dopamine hypothesis of schizophrenia

The discovery of neuroleptic drugs in 1952 provided a new strategy for seeking a biological basis of schizophrenia. This entailed a search for a primary site of neuroleptic action. The Parkinsonian effects caused by neuroleptics suggested that dopamine transmission may be disrupted by these drugs. In 1963 it was proposed that neuroleptics blocked "monoamine receptors" or impeded the release of monoamine metabolites. The neuroleptic concentration in plasma water or cerebrospinal fluid was of the order of 2 nM for haloperidol in clinical therapy. A systematic research was made between 1963 and 1974 for a primary site of neuroleptic action which would be sensitive to 2 nM haloperidol and stereoselective for (+)-butaclamol. Direct evidence that neuroleptics selectively blocked dopamine receptors occurred in 1974 with the finding that nanomolar concentrations of these drugs stereoselectively inhibited the binding of [3H]-dopamine or [3H]-haloperidol. These binding sites, now termed D2 dopamine receptors (which inhibit adenylate cyclase), are blocked by neuroleptics in direct relation to the antipsychotic potencies of the neuroleptics. No such correlation exists for D1 receptors (which stimulate adenylate cyclase). Based on the fact that dopamine-mimetic drugs elicited hallucinations, and that neuroleptics caused rigidity, Van Rossum in 1966 had suggested a hypothesis that dopamine pathways may be overactive in schizophrenia. The D2-selective blockade by all neuroleptics (except the monoamine-depleting reserpine) provided strong support for the dopamine hypothesis. Further support now comes from postmortem data and in vivo positron tomographic data, both of which indicate that the density of D2 receptors are elevated in the schizophrenic brain. The postmortem data indicate a bimodal pattern with half the schizophrenics having striatal D2 densities of 14 pmol/g (control is 13 pmol/g) and the other half having 26 pmol/g. Current positron tomographic data indicate D2 densities of 14 pmol/g in control subjects, but values of 34 pmol/g in drug-naive schizophrenics. Future tests of the dopamine hypothesis of schizophrenia may entail an examination of the amino acid composition and genes for D2 receptors in schizophrenic tissue, an examination of the ability of the D2 receptor to become phosphorylated and to desensitize into the low-affinity state, and an examination of the interaction of D2 receptors with D1 receptors or other neurotransmitters.