Dopamine receptor parameters detected by [3H]spiperone depend on tissue concentration: analysis and examples

The Problems of Applying Classical Pharmacology Analysis to Modern In Vitro Drug Discovery Assays: Slow Binding Kinetics and High Target Concentration

The analysis framework used to quantify drug potency in vitro (e.g., K_d or K_i) was initially developed for classical pharmacology bioassays, for example, organ bath experiments testing moderate-affinity natural products. Modern drug discovery can infringe the assumptions of the classical pharmacology analysis equations, owing to the reduction of assay volume in miniaturization, target overexpression, and the increase of compound-target affinity in medicinal chemistry. These assumptions are that (1) the compound concentration greatly exceeds the target concentration (i.e., minimal ligand depletion), and (2) the compound is at equilibrium with the receptor (i.e., rapid ligand binding kinetics). Unappreciated infringement of these assumptions can lead to substantial underestimation of compound affinity, which negatively impacts the drug discovery process, from early-stage lead optimization to prediction of human dosing. This study evaluates the real-world impact of these factors on the target interaction assays used in drug discovery using literature examples, database searches, and simulations. The ranges of compound affinity and the assay types that are prone to depletion and equilibration artifacts are identified. Importantly, the highest-affinity compounds, usually the highest value chemical matter in drug discovery, are the most affected. Methods and simulation tools are provided to enable investigators to evaluate, manage, and minimize depletion or equilibration artifacts. This study enables the correct application of pharmacological data analysis to accurately quantify affinity using modern drug discovery assay technology.

Tactics for preclinical validation of receptor-binding radiotracers

INTRODUCTION

Aspects of radiopharmaceutical development are illustrated through preclinical studies of [125I]-(E)-1-(2-(2,3-dihydrobenzofuran-5-yl)ethyl)-4-(iodoallyl)piperazine ([125I]-E-IA-BF-PE-PIPZE), a radioligand for sigma-1 (σ1) receptors, coupled with examples from the recent literature. Findings are compared to those previously observed for [125I]-(E)-1-(2-(2,3-dimethoxy-5-yl)ethyl)-4-(iodoallyl)piperazine ([125I]-E-IA-DM-PE-PIPZE).

METHODS

Syntheses of E-IA-BF-PE-PIPZE and [125I]-E-IA-BF-PE-PIPZE were accomplished by standard methods. In vitro receptor binding studies and autoradiography were performed, and binding potential was predicted. Measurements of lipophilicity and protein binding were obtained. In vivo studies were conducted in mice to evaluate radioligand stability, as well as specific binding to σ1 sites in brain, brain regions and peripheral organs in the presence and absence of potential blockers.

RESULTS

E-IA-BF-PE-PIPZE exhibited high affinity and selectivity for σ1 receptors (Ki = 0.43 ± 0.03 nM, σ2/σ1 = 173). [125I]-E-IA-BF-PE-PIPZE was prepared in good yield and purity, with high specific activity. Radioligand binding provided dissociation (koff) and association (kon) rate constants, along with a measured Kd of 0.24 ± 0.01 nM and Bmax of 472 ± 13 fmol/mg protein. The radioligand proved suitable for quantitative autoradiography in vitro using brain sections. Moderate lipophilicity, Log D7.4 2.69 ± 0.28, was determined, and protein binding was 71 ± 0.3%. In vivo, high initial whole brain uptake, >6% injected dose/g, cleared slowly over 24 h. Specific binding represented 75% to 93% of total binding from 15 min to 24 h. Findings were confirmed and extended by regional brain biodistribution. Radiometabolites were not observed in brain (1%).

CONCLUSIONS

Substitution of dihydrobenzofuranylethyl for dimethoxyphenethyl increased radioligand affinity for σ1 receptors by 16-fold. While high specific binding to σ1 receptors was observed for both radioligands in vivo, [125I]-E-IA-BF-PE-PIPZE displayed much slower clearance kinetics than [125I]-E-IA-DM-PE-PIPZE. Thus, minor structural modifications of σ1 receptor radioligands lead to major differences in binding properties in vitro and in vivo.

Cannabidiol is a partial agonist at dopamine D2High receptors, predicting its antipsychotic clinical dose

Although all current antipsychotics act by interfering with the action of dopamine at dopamine D2 receptors, two recent reports showed that 800 to 1000 mg of cannabidiol per day alleviated the signs and symptoms of schizophrenia, although cannabidiol is not known to act on dopamine receptors. Because these recent clinical findings may indicate an important exception to the general rule that all antipsychotics interfere with dopamine at dopamine D2 receptors, the present study examined whether cannabidiol acted directly on D2 receptors, using tritiated domperidone to label rat brain striatal D2 receptors. It was found that cannabidiol inhibited the binding of radio-domperidone with dissociation constants of 11 nm at dopamine D2High receptors and 2800 nm at dopamine D2Low receptors, in the same biphasic manner as a dopamine partial agonist antipsychotic drug such as aripiprazole. The clinical doses of cannabidiol are sufficient to occupy the functional D2High sites. it is concluded that the dopamine partial agonist action of cannabidiol may account for its clinical antipsychotic effects.

Binding Interactions of Dopamine and Apomorphine in D2High and D2Low States of Human Dopamine D2 Receptor Using Computational and Experimental Techniques

We have recently reported G-protein coupled receptor (GPCR) model structures for the active and inactive states of the human dopamine D2 receptor (D2R) using adrenergic crystal structures as templates. Since the therapeutic concentrations of dopamine agonists that suppress the release of prolactin are the same as those that act at the high-affinity state of the D2 receptor (D2High), D2High in the anterior pituitary gland is considered to be the functional state of the receptor. In addition, the therapeutic concentrations of anti-Parkinson drugs are also related to the dissociation constants in the D2High form of the receptor. The discrimination between the high- and low-affinity (D2Low) components of the D2R is not obvious and requires advanced computer-assisted structural biology investigations. Therefore, in this work, the derived D2High and D2Low receptor models (GPCR monomer and dimer three-dimensional structures) are used as drug-binding targets to investigate binding interactions of dopamine and apomorphine. The study reveals a match between the experimental dissociation constants of dopamine and apomorphine at their high- and low-affinity sites of the D2 receptor in monomer and dimer and their calculated dissociation constants. The allosteric receptor-receptor interaction for dopamine D2R dimer is associated with the accessibility of adjacent residues of transmembrane region 4. The measured negative cooperativity between agonist ligand at dopamine D2 receptor is also correctly predicted using the D2R homodimerization model.

Acute social defeat does not alter cerebral 5-HT2A receptor binding in male Wistar rats

It has been hypothesized that effects of uncontrollable stress on serotonin receptor expression contribute to the etiology of stress-related disorders like depression. While the serotonin-2A receptors (5-HT2A R) are thought to be important in this context, only few studies examined effects of stress on this receptor subtype. In this study, we therefore assessed acute and long-term changes in 5HT2A R binding after social defeat stress in rats. Male Wistar rats were subjected to social defeat by placing them in the home cage of an aggressive, dominant Long Evans rat. Acute social defeat suppressed growth, but did not affect anxiety-like behavior in an open field test. A positron emission tomography scan with the 5-HT2A R tracer [11C]MDL 100907 1 day and 3 weeks after defeat did not show significant changes in receptor binding. To verify these results, [3H]MDL 100907 binding assays were performed in homogenates of prefrontal cortex and hippocampus, which also did not indicate any changes in Bmax or Kd . These findings do not support the hypothesis that changes in 5-HT2A R function are a vital mechanism through which uncontrollable stress contributes to stress-related pathologies such as depression. It remains to be determined whether effects of stress on 5HT2A R binding depend on the nature of the stressor or on the characteristics of the rat strain.

Clozapine, a fast-off-D2 antipsychotic

Ever since clozapine was first synthesized and tested, it showed the unique property of having antipsychotic action but no Parkinson-like motor side effects. The antipsychotic basis of clozapine is to transiently occupy dopamine D2 receptors in the human striatum, in contrast to haloperidol and chlorpromazine, which have a prolonged occupation of D2 receptors. The chemical structure of clozapine facilitates a relatively rapid dissociation from D2 receptors. After short-term occupation of D2 receptors, peak neural activity raises synaptic dopamine, which then displaces clozapine. While clozapine also occupies other types of receptors, they may not have a significant role in preventing parkinsonism. Clozapine's transient occupation of D2 receptors permits patients to move easily and comfortably.

History of the discovery of the antipsychotic dopamine D2 receptor: a basis for the dopamine hypothesis of schizophrenia

The 1975 publication of Seeman et al. (Proc Nat Acad Sci, USA), reporting the discovery of the antipsychotic receptor in the brain, is a classic example of translational medicine research. In searching for a pathophysiological mechanism of psychosis, the team sought to identify sites that bound the antipsychotic drug haloperidol. Their criterion was that haloperidol bound to the site at one to two nanomoles per liter, corresponding to haloperidol concentrations found in spinal fluid or plasma water in treated patients. They requested de novo synthesis of tritiated haloperidol, and it readily detected specific haloperidol binding sites in brain striatum. With dopamine binding the haloperidol-labeled sites with higher potency than other neurotransmitters, the sites were named antipsychotic/dopamine receptors (now designated dopamine D2 receptors). Most significantly, they found that all antipsychotics bound these sites at concentrations and with a rank order of potencies that were directly related to the mean daily antipsychotic dose taken by patients with schizophrenia. Their findings enabled screening for new antipsychotics, initiated D2 receptor measurements in brain of living patients, and determination of minimum occupancy (65%) of D2 receptors for antipsychotic benefit. The collective work is generally viewed as providing a fundamental basis for the dopamine hypothesis of schizophrenia.

All roads to schizophrenia lead to dopamine supersensitivity and elevated dopamine D2(high) receptors

BACKGROUND

The dopamine D2 receptor is the common target for antipsychotics, and the antipsychotic clinical doses correlate with their affinities for this receptor. Antipsychotics quickly enter the brain to occupy 60-80% of brain D2 receptors in patients (the agonist aripiprazole occupies up to 90%), with most clinical improvement occurring within a few days. The D2 receptor can exist in a state of high-affinity (D2(High) ) or in a state of low-affinity for dopamine (D2Low).

AIM

The present aim is to review why individuals with schizophrenia are generally supersensitive to dopamine-like drugs such as amphetamine or methyphenidate, and whether the D2(High) state is a common basis for dopamine supersensitivity in the animal models of schizophrenia.

RESULTS

All animal models of schizophrenia reveal elevations in D2(High) receptors. These models include brain lesions, sensitization by drugs (amphetamine, phencyclidine, cocaine, corticosterone), birth injury, social isolation, and gene deletions in pathways for NMDA, dopamine, GABA, acetylcholine, and norepinephrine.

CONCLUSIONS

These multiple abnormal pathways converge to a final common pathway of dopamine supersensitivity and elevated D2(High) receptors, presumably responsible for psychotic symptoms. Although antipsychotics alleviate psychosis and reverse the elevation of D2(High) receptors, long-term antipsychotics can further enhance dopamine supersensitivity in patients. Therefore, switching from a traditional antipsychotic to an agonist antipsychotic (aripiprazole) can result in psychotic signs and symptoms. Clozapine and quetiapine do not elicit parkinsonism or tardive dyskinesia because they are released from D2 within 12 to 24 h. Traditional antipsychotics remain attached to D2 receptors for days, preventing relapse, but allowing accumulation that can lead to tardive dyskinesia. Future goals include imaging D2(High) receptors and desensitizing them in early-stage psychosis.

Hyperactive mice show elevated D2(High) receptors, a model for schizophrenia: Calcium/calmodulin-dependent kinase II alpha knockouts

The cerebral frontal cortex of patients who had schizophrenia shows elevated levels of RNA for calcium/calmodulin-dependent protein kinase II beta (CaMKIIbeta). In addition, recent research shows that animal models for schizophrenia, such as amphetamine-sensitized rats, consistently show elevated levels of D2 receptors in their high-affinity state (D2(High)), the major target for antipsychotic medication. The present study was done, therefore, to examine whether an alteration in the levels of CaMKIIbeta could lead to altered levels of D2(High) receptors. We found that the CaMKII inhibitor, KN-93, markedly reduced D2(High) states in rat striatum. In addition, we studied heterozygous CaMKIIalpha knock-out mice that show features analogous to schizophrenia. The striata of these mice revealed a 2.8-fold increase in D2(High) receptors. In frontal cortex of the heterozygous CaMKIIalpha knock-out mice, CaMKIIalpha mRNA levels were reduced by 50%, while CaMKIIbeta mRNA levels were unaltered. In striatum, CaMKIIbeta mRNA levels were increased by 29%, suggesting the presence of a new CaMKIIbeta regulatory pathway not previously described. The elevated levels of CaMKIIbeta mRNA in the striatum suggest that this enzyme may increase D2(High) in animals and possibly in schizophrenia itself.

Schizophrenia, amphetamine-induced sensitized state and acute amphetamine exposure all show a common alteration: increased dopamine D2 receptor dimerization

Background

All antipsychotics work via dopamine D2 receptors (D2Rs), suggesting a critical role for D2Rs in psychosis; however, there is little evidence for a change in receptor number or pharmacological nature of D2Rs. Recent data suggest that D2Rs form dimers in-vitro and in-vivo, and we hypothesized that schizophrenia, as well as preclinical models of schizophrenia, would demonstrate altered dimerization of D2Rs, even though the overall number of D2Rs was unaltered.

Methods

We measured the expression of D2Rs dimers and monomers in patients with schizophrenia using Western blots, and then in striatal tissue from rats exhibiting the amphetamine-induced sensitized state (AISS). We further examined the interaction between D2Rs and the dopamine transporter (DAT) by co-immunoprecipitation, and measured the expression of dopamine D2^High receptors with ligand binding assays in rat striatum slices with or without acute amphetamine pre-treatment.

Results

We observed significantly enhanced expression of D2Rs dimers (277.7 ± 33.6%) and decreased expression of D2Rs monomers in post-mortem striatal tissue of schizophrenia patients. We found that amphetamine facilitated D2Rs dimerization in both the striatum of AISS rats and in rat striatal neurons. Furthermore, amphetamine-induced D2Rs dimerization may be associated with the D2R-DAT protein-protein interaction as an interfering peptide that disrupts the D2R-DAT coupling, blocked amphetamine-induced up-regulation of D2Rs dimerization.

Conclusions

Given the fact that amphetamine induces psychosis and that the AISS rat is a widely accepted animal model of psychosis, our data suggest that D2R dimerization may be important in the pathophysiology of schizophrenia and may be a promising new target for novel antipsychotic drugs.

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.

The dopaminergic stabilizer ASP2314/ACR16 selectively interacts with D2(High) receptors

Dopaminergic stabilizers are recognized as compounds that can either enhance or antagonize dopamine (DA)-dependent behaviors depending on the prevailing dopaminergic tone. The dopaminergic stabilizer ASP2314 is being tested clinically and has been reported to have antipsychotic effects in a clinical trial as an add on medication. To elucidate the mechanisms of action of this dopaminergic stabilizer, its potency on the functional dopamine D2(High) receptors was examined. In competition with D2 receptors selectively labeled by [3H]domperidone, ASP2314 had a dissociation constant, Ki(High), of 1.62 microM for D2(High) in human cloned D2Long receptors and 0.83 muM for rat homogenized striata. Using the D2 agonist ligand [3H](+)-4-propyl-3,4,4a,5,6,10b-hexahydro-2H-naphtho[1,2-b][1,4]oxazin-9-ol ((+)PHNO), ASP2314 had a high-affinity Ki of 32 nM for D2(High) for rat homogenized striata. ASP2314 stimulated the incorporation of [35S]GTP-gamma-S into rat striata by 50% at 43 nM, and into the cloned D2Long membranes by 50% at 3.2 microM (compared to 100% stimulation by 10 microM dopamine). With similar concentrations of ASP2314 inhibiting the binding of ligands at D2(High) and stimulating [35S]GTP-gamma-S incorporation, the data indicate that the dopaminergic stabilizing action of ASP2314 may be related to the selectivity for the D2(high) state of the D2 receptor.

Dopamine D2High receptors stimulated by phencyclidines, lysergic acid diethylamide, salvinorin A, and modafinil

Although it is commonly stated that phencyclidine is an antagonist at ionotropic glutamate receptors, there has been little measure of its potency on other receptors in brain tissue. Although we previously reported that phencyclidine stimulated cloned-dopamine D2Long and D2Short receptors, others reported that phencyclidine did not stimulate D2 receptors in homogenates of rat brain striatum. This study, therefore, examined whether phencyclidine and other hallucinogens and psychostimulants could stimulate the incorporation of [(35)S]GTP-gamma-S into D2 receptors in homogenates of rat brain striatum, using the same conditions as previously used to study the cloned D2 receptors. Using 10 microM dopamine to define 100% stimulation, phencyclidine elicited a maximum incorporation of 46% in rat striata, with a half-maximum concentration of 70 nM for phencyclidine, when compared with 80 nM for dopamine, 89 nM for salvinorin A (48 nM for D2Long), 105 nM for lysergic acid diethylamide (LSD), 120 nM for R-modafinil, 710 nM for dizocilpine, 1030 nM for ketamine, and >10,000 nM for S-modafinil. These compounds also inhibited the binding of the D2-selective ligand [(3)H]domperidone. The incorporation was inhibited by the presence of 200 microM guanylylimidodiphosphate and also by D2 blockade, using 10 microM S-sulpiride, but not by D1 blockade with 10 microM SCH23390. Hypertonic buffer containing 150 mM NaCl inhibited the stimulation by phencyclidine, which may explain negative results by others. It is concluded that phencyclidine and other psychostimulants and hallucinogens can stimulate dopamine D2 receptors at concentrations related to their behavioral actions.

Increased dopamine D2High receptors in rats reared in social isolation

Postweaning social isolation in the rat induces lasting alterations that parallel several of the core symptoms seen in human schizophrenics, including hyperreactivity to novel environments, cognitive impairment, and deficits in sensorimotor gating. The current study determined whether these changes are accompanied by any elevation in the proportion of striatal dopamine receptors in the functional high affinity state (D(2) (High)), as observed in other preclinical models of psychosis. Male Lister hooded rats (20-24 days) were housed in groups of three or alone. On Day 36 postweaning locomotor activity was monitored for 60 min in a novel arena, and on Day 37 novel object discrimination was assessed using a 2 h intertrial interval. Three days later striata were collected, homogenized, washed three times to remove endogenous dopamine, and the proportion of D(2) (High) determined by competition between dopamine and 2.27 nM [(3)H]domperidone. Isolates were significantly more active than group housed controls for both ambulation and rears. Although both groups exhibited comparable levels of familiarization trial object exploration, group housed animals were able to discriminate between novel and familiar objects during the choice trial while isolates were not. Social isolation was associated with a highly significant elevation in the proportion of striatal D(2) (High), equivalent to a 3.3-fold increase (group 15.2% +/- 1.4%, isolate 49.8% +/- 4.8%; P < 0.0001, Student's unpaired t-test). These findings support both the hypothesis that elevated D(2) (High) is a common feature of multiple animal models of psychosis, and the validity of isolation rearing as a neurodevelopmental model of a "schizophrenia-like" state.

Phencyclidine and glutamate agonist LY379268 stimulate dopamine D2High receptors: D2 basis for schizophrenia

It has previously been reported that the glutamate ionotropic antagonist phencyclidine directly inhibits the release of prolactin in anterior pituitary cells in culture, suggesting that phencyclidine has a dopamine (DA)-like action on prolactin-releasing cells. It has also been reported that the glutamate metabotropic agonist LY379268 can stimulate the incorporation of [35S]GTP-gamma-S into DA D2Long receptors. The present study was done to examine whether such glutamatergic drugs had similar actions on the DA D2Short receptor. The present results show that phencyclidine, ketamine, and LY379268 also stimulated the incorporation of [35S]GTP-gamma-S into D2Short receptors. The proportion of D2Long and D2Short receptors existing in the high-affinity state were both markedly reduced by NaCl. While phencyclidine and LY379268 each stimulated the incorporation of GTP-gamma-S into D2Long and D2Short receptors, this stimulation was reduced by NaCl, with D2Short being much more sensitive than D2Long to the inhibition by NaCl. The binding of phencyclidine and LY379268 to D2High receptors in vivo was directly confirmed by the i.v. injection of phencyclidine and LY379268 in which 50% inhibited the binding of [3H]PHNO to the striatum ex vivo at 0.25 and 1.5 mg/kg, respectively. The results confirm that glutamate agonists and antagonists have a significant affinity for DA D2High receptors. The psychotogenic action of phencyclidine may stem from a combination or synergistic action of glutamate receptor antagonism and DA D2 agonism. In addition, the antipsychotic clinical action of LY379268 congeners such as LY404039 may be related to a combined or synergistic action of glutamate receptor stimulation together with a partial DA agonist action that reduces endogenous DA neurotransmission.

Glutamate receptor mGlu2 and mGlu3 knockout striata are dopamine supersensitive, with elevated D2(High) receptors and marked supersensitivity to the dopamine agonist (+)PHNO

The finding that the mGlu2/3 metabotropic glutamate receptor agonist, LY404039, improves clinical symptoms in schizophrenia warrants a search for a possible interaction between mGlu2/3 receptors and dopamine D2 receptors. Here, this topic is examined in striatal tissue of mice lacking either mGlu2 or mGlu3 receptor. Such mice are known to be behaviorally supersensitive to dopamine receptor agonists. Therefore, to determine the basis of this dopamine supersensitivity, the proportion of dopamine D2(High) receptors was measured in the striata of mGlu2 and mGlu3 receptor knockout mice. The proportion of D2(High) receptors was found to be elevated by 220% in the striata of both knockouts. To measure the functional dopamine supersensitivity, the D2 agonist (+)PHNO was used to stimulate the incorporation of GTP-gamma-S in the striatal homogenates in the presence of drugs that blocked the dopamine D1, D3, and D5 receptors. Compared with control striata, the mGlu2 receptor knockout tissues were 67-fold more sensitive to (+)PHNO, while the mGlu3 receptor knockout tissues were 17-fold more sensitive. These data suggest that group II mGlu receptors-mGlu2 receptors in particular-may normally regulate D2 receptors by reducing the proportion of high-affinity D2 receptors in membranes. Such regulation may contribute to the antipsychotic action of mGlu2/3 receptor agonists.

Dopamine receptors in human lymphocytes: radioligand binding and quantitative RT-PCR assays

Analysis of dopamine receptors (DR) in lymphocytes of the human peripheral blood mononuclear cell (PBMC) fraction is an attractive tool for evaluation of functional properties of dopaminergic function underlying variation in complex psychological/psychopathological traits. Receptor binding assays (RBAs) with selective radioligands, which are widely used in CNS studies, have not produced consistent results when applied to isolated PBMC. We tested the assay conditions that could be essential for detection of DR in human PBMC and their membrane preparations. Using [(3)H]SCH23390, a dopamine D1-like receptor antagonist, we demonstrated the presence of two binding sites in PBMC-derived membrane fraction. One of them is characterized by the K(d) value consistent with that reported for D5 dopamine receptors in human lymphocytes, whereas the other K(d) value possibly corresponds to serotonin receptor(s). Although D5 receptor binding sites in PBMC membranes could be characterized by binding assays, the low protein expression and the large volume of blood needed for membrane preparation render the binding method impracticable for individual phenotyping. In contrast, real-time RT-PCR may be used for this purpose, contingent on the relationship between DR expression in the brain and in lymphocytes. The expression of the DRD2-DRD5 genes, as detected by this method, varied widely among samples, whereas the DRD1 expression was not detected. The expression levels were comparable with those in the brain for DRD3 and DRD4, and were significantly lower for DRD2 and DRD5.

Cocaine self-administration produces a persistent increase in dopamine D2 High receptors

Cocaine addicts are reported to have decreased numbers of striatal dopamine D2 receptors. However, in rodents, repeated cocaine administration consistently produces hypersensitivity to the psychomotor activating effects of both indirect dopamine agonists, such as cocaine itself, and importantly, to direct-acting D2 receptor agonists. The current study reports a possible resolution to this long-standing paradox. The dopamine D2 receptor exists in both a low and a high-affinity state, and dopamine exerts its effects via the more functionally relevant high-affinity D2 receptor (D2 High). We report here that cocaine self-administration experience produces a large (approximately 150%) increase in the proportion of D2 High receptors in the striatum with no change in the total number of D2 receptors, and this effect is evident both 3 and 30 days after the discontinuation of cocaine self-administration. Changes in D2 High receptors would not be evident with the probes used in human (and non-human primate) imaging studies. We suggest, therefore, that cocaine addicts and animals previously treated with cocaine may be hyper-responsive to dopaminergic drugs in part because an increase in D2 High receptors results in dopamine supersensitivity. This may also help explain why stimuli that increase dopamine neurotransmission, including drugs themselves, are so effective in producing relapse in individuals with a history of exposure to cocaine.

Dopamine D2(High) receptors on intact cells

In humans, behavioral dopamine supersensitivity occurs in schizophrenia and in Parkinson's disease. In animals, behavioral dopamine supersensitivity is consistently associated with increased dopamine D2(High) receptors in homogenized striata in vitro. Because D2(High) receptors have not yet been detected in intact cells, we used [(3)H]domperidone to detect D2(High) sites in intact rat anterior pituitary adenoma culture cells. Although [(3)H]raclopride and [(3)H]spiperone did not detect D2(High) receptors in intact cells or in rat fresh striatal slices, [(3)H]domperidone readily detected D2(High) receptors, warranting an in vivo search for D2(High) variations in human diseases.

Dopamine D2(High) receptors moderately elevated by sertindole

Because long-term administration of antipsychotics can cause behavioral dopamine supersensitivity, this study examined whether the antipsychotic sertindole could elicit biochemical changes indicative of dopamine supersensitivity. In rats, behavioral dopamine supersensitivity is consistently associated with an increased proportion of dopamine receptors that have high affinity for dopamine, namely D2(High), in homogenized striata. Nine days of subcutaneously injected sertindole (1.25 mg/kg/day) increased the proportion of D2(High) receptors between 186% and 215%, although the total population of D2 receptors did not change. Although the findings suggest that rats or patients treated with sertindole might exhibit behavioral dopamine supersensitivity, the drug-induced increase in D2(High) receptors was less than that previously found with haloperidol.

Repeated administration of a dopamine D1 receptor agonist reverses the increased proportions of striatal dopamine D1High and D2High receptors in methamphetamine-sensitized rats

Repeated administration of psychostimulants produces a behavioural sensitization. Amphetamine-sensitized animals are known to have a higher proportion of high-affinity states of dopamine D2 receptors (D2(High) receptors) in the striatum. We recently reported that repeated administration of a dopamine D1 receptor agonist, R-(+)-SKF38393, reverses the established behavioural sensitization to methamphetamine (MAP). To investigate the mechanisms for reversal of behavioural sensitization, we examined the effect of repeated administration of the dopamine D1 receptor agonist on the proportions of D2(High) receptors and the high-affinity states of dopamine D1 receptors (D1(High) receptors) in the striatum. In the striatum from the MAP-sensitized rats, the proportions of D1(High) and D2(High) receptors (28.5 +/- 1.96 and 57.5 +/- 3.58%) were higher than those in the saline-control rats (12.0 +/- 1.01 and 21.9 +/- 1.60%, respectively). Repeated administration of R-(+)-SKF38393 to the MAP-sensitized rats reduced the increased proportions of D1(High) and D2(High) receptors to 12.4 +/- 1.57 and 31.0 +/- 2.14%, respectively, which were similar to the proportions in the saline-control rats. The total densities of dopamine D1 and D2 receptors were not altered in each treatment condition. The results demonstrate that the proportions of D1(High) and D2(High) receptors in the striatum are elevated in MAP-sensitized rats, and that repeated administration of the dopamine D1 receptor agonist to the MAP-sensitized rats reverses the increased proportions of D1(High) and D2(High) receptors. The findings reveal postsynaptic mechanisms for the development of behavioural sensitization to MAP and the reversal of established sensitization by repeated administration of the dopamine D1 receptor agonist.

Dopamine partial agonist actions of the glutamate receptor agonists LY 354,740 and LY 379,268

Because glutamate compounds alter the release of dopamine and prolactin, the present study examined whether group II metabotropic receptor agonists, LY 354,740 and LY 379,268, had any direct in vitro action on dopamine D2 receptors on rat striatal tissue, cloned D2Long receptors, and prolactin release from anterior pituitary cells. In competition versus the D2-specific ligand [(3)H]domperidone, LY 354,740 had a dissociation constant of 24 nM at D2(High) (the functional high-affinity state of dopamine D2 receptors), while the value for LY 379,268 was 21 nM. LY 354,740 also stimulated by 50% the incorporation of [(35)S]-GTP-gamma-S at a concentration of 120 nM, but its maximal stimulation was only 22% of the maximum elicited by dopamine. LY 379,268 stimulated by 50% the incorporation of [(35)S]-GTP-gamma-S at 280 nM, but its maximal stimulation was also only 22% of the maximum elicited by dopamine. However, both LY 354,740 and LY 379,268 potently inhibited the dopamine-induced incorporation of [(35)S]-GTP-gamma-S with inhibitory Ki values of 43 nM and 30 nM, respectively. The release of prolactin from rat isolated anterior pituitary cells in culture was 50% inhibited by 20 nM LY 379,268 and by 100 nM LY 354,740. These Ki values are similar to those known for the mGluR II receptor, suggesting that these compounds may have both glutamate and dopamine actions in vivo. The dopamine agonist and antagonist actions of these compounds indicate that these drugs have properties of a dopamine partial agonist, and may, therefore, have antipsychotic action.

Antiparkinson therapeutic potencies correlate with their affinities at dopamine D2(High) receptors

To determine whether antiparkinson dopamine agonists preferentially act on the high-affinity or the low-affinity states of dopamine D1 and D2 receptors, the agonist potencies were obtained by competition against [(3)H]SCH23390 for D1(High) and D1(Low), and against [(3)H]domperidone for D2(High) and D2(Low). N-propylnorapomorphine and cabergoline were the most potent at D2(High), with dissociation constants of 0.18 and 0.36 nM, respectively. Other agonists had D2(High)K(i) values of 0.52 nM for quinagolide, 0.6 nM for (+)PHNO, 0.9 for bromocriptine, 1.8 nM for apomorphine, 2.4 nM for pergolide, 3 nM for quinpirole, and 6.2 nM for lergotrile. There was a clear correlation between the K(i) values at D2(High) and their therapeutic concentrations in the plasma water, as derived from the known concentrations after correction for the fraction bound to the human plasma proteins. The data suggest that D2(High) is the primary and common target for the antiparkinson action of dopamine agonists. Bromocriptine, cabergoline, lergotrile, pergolide, and pramipexole had no affinity for D1(High), consistent with the clinical observations that the D2-selective bromocriptine and pramipexole elicit low levels of dyskinesia.

Elevated dopamine D2High receptors in alpha-1b-adrenoceptor knockout supersensitive mice

Although amphetamine induces hyperactivity by releasing dopamine (DA), mice that lack alpha1b-adrenoceptors do not release DA in response to amphetamine and do not, therefore, exhibit locomotor supersensitivity to amphetamine. However, such mice reveal hyperlocomotion to p-chloroamphetamine (PCA). Because these alpha1b-adrenoceptor knockout mice have no alterations in the striatal densities of DA D1 or D2 receptors, the basis for any possible dopaminergic contribution to the PCA-induced hyperlocomotion to PCA is unclear. Therefore, because supersensitive animals are generally known to have a higher proportion of DA D2 receptors in the high-affinity state for DA D2(High), we investigated whether there was any change in the alpha1b-adrenoceptor knockout striata in the proportion of DA D2(High) receptors to determine whether there could be a DA-based contribution to the PCA-induced hyperlocomotion. We found that the proportion of D2(High) in the wild type striata was 23 +/- 3.3%, whereas that in the alpha1b-adrenoceptor knockout striata was 52 +/- 2.9%, an increase of 2.3-fold. This elevation agrees with other types of DA-supersensitive animal striata and could assist in eliciting a supersensitive response in these alpha1b-adrenoceptor knockout mice.

"Breakthrough" dopamine supersensitivity during ongoing antipsychotic treatment leads to treatment failure over time

Antipsychotics often lose efficacy in patients despite chronic continuous treatment. Why this occurs is not known. It is known, however, that withdrawal from chronic antipsychotic treatment induces behavioral dopaminergic supersensitivity in animals. How this emerging supersensitivity might interact with ongoing treatment has never been assessed. Therefore, we asked whether dopamine supersensitivity could overcome the behavioral and neurochemical effects of antipsychotics while they are still in use. Using two models of antipsychotic-like effects in rats, we show that during ongoing treatment with clinically relevant doses, haloperidol and olanzapine progressively lose their efficacy in suppressing amphetamine-induced locomotion and conditioned avoidance responding. Treatment failure occurred despite high levels of dopamine D2 receptor occupancy by the antipsychotic and was at least temporarily reversible by an additional increase in antipsychotic dose. To explore potential mechanisms, we studied presynaptic and postsynaptic elements of the dopamine system and observed that antipsychotic failure was accompanied by opposing changes across the synapse: tolerance to the ability of haloperidol to increase basal dopamine and dopamine turnover on one side, and 20-40% increases in D2 receptor number and 100-160% increases in the proportion of D2 receptors in the high-affinity state for dopamine (D2(High)) on the other. Thus, the loss of antipsychotic efficacy is linked to an increase in D2 receptor number and sensitivity. These results are the first to demonstrate that "breakthrough" supersensitivity during ongoing antipsychotic treatment undermines treatment efficacy. These findings provide a model and a mechanism for antipsychotic treatment failure and suggest new directions for the development of more effective antipsychotics.

Syntheses and in vitro evaluation of fluorinated naphthoxazines as dopamine D2/D3 receptor agonists: radiosynthesis, ex vivo biodistribution and autoradiography of [18F]F-PHNO

INTRODUCTION

Carbon-11-labeled (+)-4-propyl-3,4,4a,5,6,10b-hexahydro-2H-naphtho[1,2-b][1,4]oxazin-9-ol ([(11)C]-(+)-PHNO) is a dopamine D2/D3 agonist radioligand that is currently used to image the high-affinity state of dopamine receptors in humans with positron emission tomography (PET). The present study reports the preparation and evaluation of fluorinated (+)-PHNO derivatives.

METHODS

Five fluorinated (+)-PHNO derivatives were synthesized and tested in vitro for inhibition of binding of [(3)H]domperidone in homogenates of rat striatum and inhibition of binding to [(3)H]-(+)-PHNO in homogenates of human-cloned D2Long receptors in Chinese hamster ovary cells and rat striatum. Radiolabeling with fluorine-18 was carried out for the most promising candidate, N-fluoropropyl-(+)-HNO (F-PHNO), and ex vivo biodistribution and autoradiography studies with this radiopharmaceutical were performed in rodents.

RESULTS

(+)-PHNO and the fluorinated analogs inhibited binding of [(3)H]domperidone and [(3)H]-(+)-PHNO to the high- and low-affinity states of dopamine D2 receptors, consistent with D2 agonist behavior. The average dissociation constant at the high-affinity state of D2, K(i)(High), was 0.4 nM for F-PHNO and proved to be equipotent with (+)-PHNO (0.7 nM). All other fluorinated derivatives were significantly less potent (K(i)(High)=2-102 nM). The most promising candidate, F-PHNO, was labeled with fluorine-18 in 5% uncorrected radiochemical yield, with respect to starting fluoride. Ex vivo biodistribution and autoradiography studies in rodents revealed that [(18)F]F-PHNO rapidly enters the rodent brain. However, this radiotracer does not reveal specific binding in the brain and is rapidly cleared.

CONCLUSIONS

Five novel dopamine D2/D3 agonists based on (+)-PHNO were synthesized and evaluated in vitro. F-PHNO was shown to behave as a potent D2 agonist in vitro and was therefore radiolabeled with fluorine-18. Despite the promising in vitro pharmacological profile, [(18)F]F-PHNO did not display in vivo behavior suitable to image dopaminergic receptor expression using PET.

Increased dopamine D2High receptors in amphetamine-sensitized rats, measured by the agonist [3H](+)PHNO

Repeated injections of amphetamine causes animals to become sensitized and supersensitive to DA. Previous work showed that the striata from such sensitized rats revealed a 3.5-fold increase in the density of D2(High) DA receptors, as measured by the guanine-nucleotide-sensitive component of [(3)H]raclopride binding. The present study was done to confirm these earlier findings by different methods and different ligands. The striata from amphetamine-sensitized rats showed an increase of 2.2-fold in the density of guanine-nucleotide-sensitive D2 receptors labeled by saturation experiments with [(3)H](+)PHNO. The proportion of D2(High) receptors was also found to increase 2.5-fold using the method of competition between DA and [(3)H]domperidone. The overall 2.2-3.5-fold increase of DA D2(High) receptors may explain why amphetamine-sensitized animals are much more sensitive to DA agonists, even though the total density of D2 receptors may apparently be unchanged or even decreased.

The Trace Amine 1 receptor knockout mouse: an animal model with relevance to schizophrenia

Trace amines have been implicated in a number of neuropsychiatric disorders including depression and schizophrenia. Although long known to modulate neurotransmission indirectly through the release of catecholamines, the identification of the Trace Amine 1 receptor (TA1) offers a mechanism by which trace amines can influence synaptic activity directly. TA1 binds and is activated by trace amines such as beta-phenylethylamine and tyramine. Our pharmacological characterization of mouse TA1 showed that, as in rat and primate, amphetamine is an agonist at this receptor but with surprisingly high potency. Without selective ligands for TA1 that do not also possess catecholamine-releasing properties, however, it has not been possible to study its physiological role in the central nervous system. To that end, a line of mice lacking the TA1 receptor was generated to characterize its contribution to the regulation of behavior. Compared with wild-type littermates, TA1 knockout (KO) mice displayed a deficit in prepulse inhibition. Knockout animals, in which the TA1-agonist influence of amphetamine was absent, showed enhanced sensitivity to the psychomotor-stimulating effect of this drug, which was temporally correlated with significantly larger increases in the release of both dopamine and norepinephrine in the dorsal striatum and associated with a 262% increase in the proportion of striatal high-affinity D2 receptors. TA1 therefore appears to play a modulatory role in catecholaminergic function and represents a potentially novel mechanism for the treatment of neuropsychiatric disorders. Furthermore, the TA1 KO mouse may provide a useful model for the development of treatments for some positive symptoms of schizophrenia.

Psychosis pathways converge via D2high dopamine receptors

The objective of this review is to identify a target or biomarker of altered neurochemical sensitivity that is common to the many animal models of human psychoses associated with street drugs, brain injury, steroid use, birth injury, and gene alterations. Psychosis in humans can be caused by amphetamine, phencyclidine, steroids, ethanol, and brain lesions such as hippocampal, cortical, and entorhinal lesions. Strikingly, all of these drugs and lesions in rats lead to dopamine supersensitivity and increase the high-affinity states of dopamine D2 receptors, or D2High, by 200-400% in striata. Similar supersensitivity and D2High elevations occur in rats born by Caesarian section and in rats treated with corticosterone or antipsychotics such as reserpine, risperidone, haloperidol, olanzapine, quetiapine, and clozapine, with the latter two inducing elevated D2High states less than that caused by haloperidol or olanzapine. Mice born with gene knockouts of some possible schizophrenia susceptibility genes are dopamine supersensitive, and their striata reveal markedly elevated D2High states; suchgenes include dopamine-beta-hydroxylase, dopamine D4 receptors, G protein receptor kinase 6, tyrosine hydroxylase, catechol-O-methyltransferase, the trace amine-1 receptor, regulator of G protein signaling RGS9, and the RIIbeta form of cAMP-dependent protein kinase (PKA). Striata from mice that are not dopamine supersensitive did not reveal elevated D2High states; these include mice with knockouts of adenosine A2A receptors, glycogen synthase kinase GSK3beta, metabotropic glutamate receptor 5, dopamine D1 or D3 receptors, histamine H1, H2, or H3 receptors, and rats treated with ketanserin or aD1 antagonist. The evidence suggests that there are multiple pathways that convergetoelevate the D2High state in brain regions and that this elevation may elicit psychosis. This proposition is supported by the dopamine supersensitivity that is a common feature of schizophrenia and that also occurs in many types of genetically altered, drug-altered, and lesion-altered animals. Dopamine supersensitivity, in turn, correlates with D2High states. The finding that all antipsychotics, traditional and recent ones, act on D2High dopamine receptors further supports the proposition.

Dopamine D2 and D3 receptors in human putamen, caudate nucleus, and globus pallidus

Because radioactive raclopride and radioactive (+)-4-propyl-9-hydroxynaphthoxazine ((+)-PHNO) are used to image dopamine (DA) D2 and D3 receptors in the striatum and globus pallidus in humans, the present study examined the proportions of D2 and D3 receptors in postmortem tissues from these regions. Conflicting results were obtained when using a single concentration of remoxipride to occlude D2 receptors or using a single concentration of U99194A or FAUC 365 to occlude D3 receptors. However, using a range of concentrations of FAUC 365, a D3-selective antagonist, to inhibit the binding [(3)H]raclopride or [(3)H]-(+)-PHNO to D3 receptors at low concentrations (1-10 nM) and to inhibit ligand binding to D2 receptors at higher concentrations (100-2000 nM), it was possible to measure the proportion of D2 and D3 receptors in the tissues. This method revealed that these two radioligands detected only D2 receptors in the dorsal putamen and the dorsal caudate nucleus, but detected a mixed population of two-thirds D2 and one-third D3 DA receptors in the ventral putamen, the ventral caudate, and the globus pallidus. The present findings are in good agreement with the known gene expression data for D2 and D3 receptors in these human brain regions.

Antiparkinson concentrations of pramipexole and PHNO occupy dopamine D2(high) and D3(high) receptors

Because the high-affinity state of dopamine D2 receptors, D2(High), is the functional state of D2, and because the proportion of D2 receptors in the high-affinity state correlates with dopamine behavioral supersensitivity, the present study was designed to determine the affinities of antiparkinson dopamine agonists at the D2(High) site by means of competition with [3H]domperidone. In contrast to [125I]iodosulpride or [3H]spiperone, which are not sensitive to low concentrations of dopamine agonists, [3H]domperidone readily reveals dissociation constants (K(i)) for antiparkinson agonists at D2(High) and D3(High) receptors. The K(i) values for the human cloned D2(High) and D3(High) receptors, respectively, were 19 and 9 nM for pramipexole, 0.24 and 0.6 nM for +PHNO, 0.7 and 1.3 nM for bromocriptine, 0.5 and 2.6 nM for apomorphine, and 0.09 and 0.25 nM for (-)N-propylnorapomorphine. After correcting for the fraction of drug bound to plasma proteins, the known clinical concentrations in plasma or plasma water of these drugs, including pramipexole and +PHNO, are sufficient to occupy and activate the high-affinity state of D2, D2(High), in treating Parkinson's disease. The D3(High) receptors are less selectively occupied by +PHNO, bromocriptine, apomorphine, and -NPA.

Dopamine receptor contribution to the action of PCP, LSD and ketamine psychotomimetics

Although phencyclidine and ketamine are used to model a hypoglutamate theory of schizophrenia, their selectivity for NMDA receptors has been questioned. To determine the affinities of phencyclidine, ketamine, dizocilpine and LSD for the functional high-affinity state of the dopamine D2 receptor, D2High, their dissociation constants (Ki) were obtained on [3H]domperidone binding to human cloned dopamine D2 receptors. Phencyclidine had a high affinity for D2High with a Ki of 2.7 nM, in contrast to its low affinity for the NMDA receptor, with a Ki of 313 nM, as labeled by [3H]dizocilpine on rat striatal tissue. Ketamine also had a high affinity for D2High with a Ki of 55 nM, an affinity higher than its 3100 nM Ki for the NMDA sites. Dizocilpine had a Ki of 0.3 nM at D2High, but a Kd of 1.8 nM at the NMDA receptor. LSD had a Ki of 2 nM at D2High. Because the psychotomimetics had higher potency at D2High than at the NMDA site, the psychotomimetic action of these drugs must have a major contribution from D2 agonism. Because these drugs have a combined action on both dopamine receptors and NMDA receptors, these drugs, when given in vivo, test a combined hyperdopamine and hypoglutamate theory of psychosis.

Effect of dopamine D3 receptor blockade on renal function and glomerular size in diabetic rats

Dopamine D2-like receptors, including D2, D3, and D4 receptors, are involved in the regulation of glomerular hyperfiltration due to diabetes mellitus. These hemodynamic alterations represent a risk factor for the later development of diabetic nephropathy. The aim of the present study was to determine whether the D3 receptor subtype modulates the diabetes-induced increase in glomerular filtration rate (GFR) in rats. Renal function was studied in Sprague-Dawley rats 14 days after induction of a moderate diabetes mellitus (DM) by streptozotocin and in non-diabetic controls (CON). Rats were orally treated either with the peripherally acting, selective dopamine D3 receptor antagonist BSF 135170 (BSF, 10 mg/kg per day for 2 weeks) or with vehicle (VHC). Perfusion-fixed kidneys were used for estimation of glomerular volume. In conscious rats, which were treated with BSF, the DM-induced increase in fluid intake, urinary output, and renal sodium excretion was significantly less pronounced than in the vehicle group (DM-VHC). In the clearance experiments, GFR in CON was about 0.84+/-0.04 ml/min per 100 g body weight. The DM-VHC group presented a significant glomerular hyperfiltration (1.09+/-0.04 ml/min per 100 g body weight). Treatment with BSF significantly lowered GFR towards levels of CON. The estimated glomerular volume was 0.73+/-0.03 x 10(6) microm3 in the CON-VHC group and 0.86+/-0.04 x 10(6) microm3 in the DM-VHC animals. Interestingly, treatment with BSF decreased the glomerular volume in both groups. Irrespective of BSF treatment, kidney wet weight related to body weight was about 36% higher in DM animals compared with CON animals. We conclude that dopamine D3 receptors represent a target for the modulation of diabetes-induced glomerular hyperfiltration. Therefore, the results encourage the testing of the possible beneficial effects of long-term D3 receptor blockade on the development of diabetic nephropathy.

Alcohol-withdrawn animals have a prolonged increase in dopamine D2high receptors, reversed by general anesthesia: relation to relapse?

The biochemical basis for alcohol addiction and relapse is not known. Although ethanol promotes the release of dopamine like other drugs of abuse, many unknown factors remain to be investigated concerning the biochemical abnormalities which persist after ethanol drinking and which contribute to alcohol relapse. Although ethanol withdrawal is associated with enhanced sensitivity to dopamine in animals and humans, only minor changes in the striatal density of dopamine D2 receptors have been found in humans, and animals show a small reduction in striatal D2 receptors. But how can dopamine-related functions be increased in ethanol withdrawal in the face of an unchanged or reduced density of dopamine D2 receptors? Considering that ethanol sensitizes rats to amphetamine, and that the high-affinity state of D2, or D2High, is markedly increased in striata from amphetamine-sensitized rats, we measured the density of D2High in striata from rats withdrawn from ethanol. These sites were elevated by 360% (7.2 pmol/g) for at least 8 days after stopping ethanol and returned to normal levels of 2 pmol/g after 2 weeks of ethanol withdrawal. In addition, 1 h of deep general anesthesia given 5 days into withdrawal resulted in a normal level of D2High within 24 h. Because the D2High states are the functional form of D2, their elevated density in ethanol withdrawal may be related to ethanol relapse in humans. General anesthesia may alleviate aspects of alcohol or amphetamine abuse or psychosis associated with elevated D2High.

Dopamine displaces [3H]domperidone from high-affinity sites of the dopamine D2 receptor, but not [3H]raclopride or [3H]spiperone in isotonic medium: Implications for human positron emission tomography

Because the high-affinity state of the dopamine D2 receptor, D2High, is the functional state of the receptor, has a role in demarcating typical from atypical antipsychotics, and is markedly elevated in amphetamine-sensitized rats, it is important to have a method for the convenient detection of this state by a ligand. The present data show that, in contrast to [(3)H]spiperone or [(3)H]raclopride, [(3)H]domperidone labels D2High sites in the presence of isotonic NaCl in either striatum or cloned D2Long receptors, yielding a dopamine dissociation constant (1.75 nM) in agreement with that found with [(3)H]dopamine. Increased labeling of D2High sites occurred with [(3)H]domperidone after severe disruption of the cells, suggesting that [(3)H]domperidone has better access to the D2 receptor from the cytoplasmic aspect of the cell membrane. The density of the [(3)H]domperidone-labeled D2 receptors was the same as that of the [(3)H]raclopride-labeled D2 receptors, but twice the density of [(3)H]spiperone sites for human cloned D2Long receptors, compatible with the monomer-dimer concept of the D2 receptor. [(3)H]domperidone readily labels the D2High sites in postmortem human brain homogenates. Although [(3)H]spiperone or [(3)H]raclopride can occupy D2High sites, the inability of 1-10 nM dopamine to displace these ligands under isotonic conditions suggests that these ligands may not be suitable for monitoring the physiological high-affinity state of the dopamine D2 receptor by means of [(11)C]methylspiperone or [(11)C]raclopride in humans.

Link between dopamine D1 and D2 receptors in rat and human striatal tissues

Many psychomotor behaviors depend on an interaction between dopamine D(1) and D(2) receptors. This study tested the hypothesis that agonist stimulation of dopamine D(1) receptors leads to the conversion of D(2) receptors from a state of high affinity for dopamine into a state of low affinity for dopamine. To test this hypothesis, dopamine was competed against [(3)H]raclopride for binding to rat and human striatal homogenates. Although the detection of high-affinity states of the dopamine D(2) receptor in rat or postmortem human striatum is normally difficult because the proportions of such sites are very low in the presence of physiological concentrations of sodium ions, the present work found that in the presence of 100 nM SCH 23390 to block D(1) receptors, a significant proportion of D(2) high-affinity sites were unmasked and readily revealed to be 10-35% of the D(2) population, illustrating the presence of a strong D(1)-D(2) link in both rat and human striata.

NMDA receptor antagonists ketamine and PCP have direct effects on the dopamine D(2) and serotonin 5-HT(2)receptors-implications for models of schizophrenia

Ketamine and PCP are commonly used as selective NMDA receptor antagonists to model the putative hypoglutamate state of schizophrenia and to test new antipsychotics. Recent findings question the NMDA receptor selectivity of these agents. To examine this further, we measured the affinity of ketamine and PCP for the high-affinity states of the dopamine D(2) and serotonin 5-HT(2) receptor and found that ketamine shows very similar affinity at the NMDA receptor and D(2) sites with a slightly lower affinity for 5-HT(2) (0.5 microM, 0.5 microM and 15 microM respectively), while PCP shows similar affinity for the NMDA and 5-HT(2) sites, with a slightly lower affinity for the D(2) site (2 microM, 5 microM and 37 microM respectively). Further, ketamine and PCP in clinically relevant doses caused a significant increase in the incorporation of [(35)S]GTP-gamma-S binding in CHO-cells expressing D(2) receptors, which was prevented by raclopride, suggesting a partial agonist effect at the D(2) receptor. Thus, ketamine and PCP may not produce a selective hypoglutamate state, but more likely produce a non-selective multi-system neurochemical perturbation via direct and indirect effects. These findings confound the inferences one can draw from the ketamine/PCP models of schizophrenia.

Amphetamine-sensitized animals show a marked increase in dopamine D2 high receptors occupied by endogenous dopamine, even in the absence of acute challenges

While a range of dopamine D(2)-related behaviors are exaggerated in amphetamine-sensitized animals, studies of the dopamine D(2) receptor have reported either no change or a decrease in dopamine D(2) receptor density--especially when measured using radioraclopride. We hypothesized that a decrease in D(2) receptors may actually be "apparent" and that these receptors may still be present, but are noncompetitively "occupied" by endogenous dopamine. Animals sensitized to amphetamine (and their saline controls) were examined 4 weeks after their last injection. We first measured the [(3)H]raclopride binding in vivo, and observed that sensitized animals showed a 29% lower level of raclopride binding in vivo, suggesting an apparently lower level of dopamine D(2) receptors. To assess the reason for this we examined the density of receptors (using Scatchard analysis in vitro) measured by [(3)H]raclopride in the presence and absence of guanilylimidodiphosphate. This guanine nucleotide converts the dopamine-bound high-affinity state of D(2) receptors into low-affinity states, thereby making measurable the absolute density of the sites. As previously reported, the amphetamine-sensitized animals showed a 31% lower number of D(2) receptors in conventional binding (B(max) 15.6 vs. 22.7 pmol/g). However, with the addition of guanilylimidodiphosphate there was an equalization of both groups (B(max) 25.9 vs. 25.6 pmol/g), revealing an additional 10.3 pmol/g in the sensitized animals, but only 2.9 pmol/g in saline controls. There were no changes in the dissociation constant of [(3)H]raclopride for the receptors. The nearly four-fold increase of dopamine D(2) receptors in the high-affinity state occupied by dopamine may explain why amphetamine-sensitized animals show almost an order of magnitude greater response to dopamine-releasing challenges or dopamine agonists, even though the absolute receptor number is unchanged and the "apparent" receptor number is decreased.

In vivo receptor assay with multiple ligand concentrations: an equilibrium approach

The ligand-receptor binding potential determined by PET studies at high ligand-specific radioactivity reflects both the receptor density and ligand-receptor affinity. This ambiguity has been resolved by various methods based on the administration of multiple unlabeled ligand concentrations. The authors aimed to implement and refine an approach to multiple ligand concentration receptor assay that combined maximum simplicity and a minimum of assumptions and model dependence that would nonetheless reliably distinguish density from affinity effects. The approach uses administration by bolus followed by infusion to obtain a true equilibrium between bound ligand and the other components of the ligand concentration, and does not require measurements of ligand in blood plasma. Four approaches to the optimization of the desired density and affinity parameters from the measured equilibrium data were implemented and compared in the analysis of raclopride studies performed in both normal control and MPTP-lesioned nonhuman primates. The authors conclude that the method is simple enough for routine use and yet reliable enough to apply in ongoing studies of both chronic and acute drug effects in the dopamine system.

High-resolution phosphor imaging: validation for use with human brain tissue sections to determine the affinity and density of radioligand binding

This study investigated the suitability of high-resolution storage phosphor imaging for the quantitative analysis of radioligand binding to human brain tissue. Hence, the binding of [(3)H]mazindol to the dopamine transporter in caudate-putamen tissue homogenates or frozen tissue sections apposed to either autoradiographic film or phosphor imaging plates was measured. Estimates of binding affinity were similar for homogenate studies and phosphor imaging plates (Kd=6.44+/-0.14 and 6.91+/-0.47 nM, respectively), but higher values were obtained with film autoradiography (Kd=11.31+/-0.82 nM). The density of binding was similar for both autoradiographic techniques (Bmax=371.9+/-30.8 fmol/mg estimated tissue equivalent, ETE (imaging plate) and 425+/-13.77 fmol/mg ETE (film)), although lower values were obtained from tissue homogenates (Bmax=64.27+/-6.74 fmol/mg wet weight). These results suggest that high resolution phosphor imaging can be used to analyse radioligand binding parameters in human brain tissue. Moreover, the reduced exposure time of phosphor imaging plates (e.g. 7 days vs 5 weeks) allows results to be obtained more rapidly than with conventional film autoradiography.

Cooperativity and oligomeric status of cardiac muscarinic cholinergic receptors

Muscarinic cholinergic receptors can appear to be more numerous when labeled by [(3)H]quinuclidinylbenzilate (QNB) than by N-[(3)H]methylscopolamine (NMS). The nature of the implied heterogeneity has been studied with M(2) receptors in detergent-solubilized extracts of porcine atria. The relative capacity for [(3)H]NMS and [(3)H]QNB was about 1 in digitonin-cholate, 0.56 in cholate-NaCl, and 0.44 in Lubrol-PX. Adding digitonin to extracts in cholate-NaCl increased the absolute capacity for both radioligands, and the relative capacity increased to near 1. The latency cannot be attributed to a chemically impure radioligand, instability of the receptor, an irreversible effect of NMS, or a failure to reach equilibrium. Binding at near-saturating concentrations of [(3)H]QNB in cholate-NaCl or Lubrol-PX was blocked fully by unlabeled NMS, which therefore appeared to inhibit noncompetitively at sites inaccessible to radiolabeled NMS. Such an effect is inconsistent with the notion of functionally distinct, noninterconverting, and mutually independent sites. Both the noncompetitive effect of NMS on [(3)H]QNB and the shortfall in capacity for [(3)H]NMS can be described quantitatively in terms of cooperative interactions within a receptor that is at least tetravalent; no comparable agreement is possible with a receptor that is only di- or trivalent. The M(2) muscarinic receptor therefore appears to comprise at least four interacting sites, presumably within a tetramer or larger array, and ligands appear to bind in a cooperative manner under at least some conditions.

Apparent capacity of cardiac muscarinic receptors for different radiolabeled antagonists

Muscarinic receptors in sarcolemmal membranes, digitonin-solubilized extracts, and purified preparations from porcine atria have revealed a shortfall in the apparent capacity for N-[3H]methylscopolamine, which was only about 75% of that for [3H]quinuclidinylbenzilate. Since binding at near-saturating concentrations of [3H]quinuclidinylbenzilate was inhibited fully at comparatively low concentrations of unlabeled N-methylscopolamine, the data are inconsistent with the notion that [3H]quinuclidinylbenzilate binds selectively to a subclass of distinct, non-interconverting, and mutually independent sites. The discrepancy is resolved by adjusting the specific activity of N-[3H]methylscopolamine to account for unlabeled scopolamine that was identified in some batches of the radioligand. Also, there was no shortfall in capacity when N-[3H]methylscopolamine was devoid of scopolamine, and the predicted effect was obtained when pure N-[3H]methylscopolamine was supplemented with known amounts of scopolamine. A small discrepancy in the levels of scopolamine estimated pharmacologically and by mass spectrometry can be attributed largely to a difference in the efficiency of ionization between scopolamine and N-methylscopolamine. Different capacities for different radioligands are not uncommon with muscarinic and other G protein-coupled receptors, and in some cases the effect may have been due wholly or in part to an unlabeled impurity. Binding data can be mechanistically ambiguous, particularly when acquired only at graded concentrations of the radioligand. The predicted effects of an unlabeled impurity mimic or resemble those of alternative scenarios such as sequestration behind a hydrophobic barrier, a nucleotide-regulated interconversion from one state of affinity to another, and cooperativity between interacting sites.

Dopamine D2 receptor dimer formation: evidence from ligand binding

We have examined the binding of two radioligands ([(3)H]spiperone and [(3)H]raclopride) to D(2) dopamine receptors expressed in Chinese hamster ovary cells. In saturation binding experiments in the presence of sodium ions, both radioligands labeled a similar number of sites, whereas in the absence of sodium ions [(3)H]raclopride labeled about half the number of sites labeled by [(3)H]spiperone. In competition experiments in the absence of sodium ions, however, raclopride was able to inhibit [(3)H]spiperone binding fully. In saturation analyses with [(3)H]spiperone in the absence of sodium ions raclopride exerted noncompetitive effects, decreasing the number of sites labeled by the radioligand. These data are interpreted in terms of a model where the receptor exists as a dimer, and in the absence of sodium ions, raclopride exerts negative cooperativity across the dimer both for its own binding and the binding of spiperone. A model of the receptor has been produced that provides a good description of the experimental phenomena described here.

An Analytical Approach to the Measurement of Equilibrium Binding Constants: Application to EGF Binding to EGF Receptors in Intact Cells Measured by Flow Cytometry

In ligand binding studies, ligand depletion often limits the accuracy of the results obtained. This problem is approached by employing the simple observation that as the concentration of receptor in the assay is reduced, ligand depletion is also reduced. Measuring apparent K(D)'s of a ligand at multiple concentrations of receptor with extrapolation to infinitely low receptor concentration takes ligand depletion into account and, depending on the binding model employed, yields a K(D) within the defined limits of accuracy. We apply this analysis to the binding of epidermal growth factor (EGF) to the EGF receptor expressed in intact 32D cells, using a homogeneous fluorescein-labeled preparation of EGF and measuring binding by flow cytometry. Binding isotherms were carried out at varying cell densities with each isotherm fit to the generally applied model with two independent binding sites. Examination of the variation in the K(D)'s versus cell density yields a high-affinity site that accounts for 18% of the sites and a lower affinity site that accounts for the remainder. However, further examination of these data suggests that while consistent with each individual isotherm, the simple model of two independent binding sites that is generally applied to EGF binding to the EGF receptor is inconsistent with the changes in the apparent K(D)'s seen across varying cell densities.