Dopamine D4 versus D2 receptor selectivity of dopamine receptor antagonists: possible therapeutic implications

Exhaustive in silico design and screening of novel antipsychotic compounds with improved pharmacodynamics and blood-brain barrier permeation properties

Antipsychotic drugs or neuroleptics are widely used in the treatment of psychosis as a manifestation of schizophrenia and bipolar disorder. However, their effectiveness largely depends on the blood-brain barrier (BBB) permeation (pharmacokinetics) and drug-receptor pharmacodynamics. Therefore, in this study, we developed and implemented the in silico pipeline to design novel compounds (n = 260) as leads using the standard drug scaffolds with improved PK/PD properties from the standard scaffolds. As a result, the best candidates (n = 3) were evaluated in molecular docking to interact with serotonin and dopamine receptors. Finally, haloperidol (HAL) derivative (1-(4-fluorophenyl)-4-(4-hydroxy-4-{4-[(2-phenyl-1,3-thiazol-4-yl)methyl]phenyl}piperidin-1-yl)butan-1-one) was identified as a "magic shotgun" lead compound with better affinity to the 5-HT2A, 5-HT1D, D2, D3, and 5-HT1B receptors than the control molecule. Additionally, this hit substance was predicted to possess similar BBB permeation properties and much lower toxicological profiles in comparison to HAL. Overall, the proposed rational drug design platform for novel antipsychotic drugs based on the BBB permeation and receptor binding might be an invaluable asset for a medicinal chemist or translational pharmacologist.Communicated by Ramaswamy H. Sarma.

Clozapine's multiple cellular mechanisms: What do we know after more than fifty years? A systematic review and critical assessment of translational mechanisms relevant for innovative strategies in treatment-resistant schizophrenia

Almost fifty years after its first introduction into clinical care, clozapine remains the only evidence-based pharmacological option for treatment-resistant schizophrenia (TRS), which affects approximately 30% of patients with schizophrenia. Despite the long-time experience with clozapine, the specific mechanism of action (MOA) responsible for its superior efficacy among antipsychotics is still elusive, both at the receptor and intracellular signaling level. This systematic review is aimed at critically assessing the role and specific relevance of clozapine's multimodal actions, dissecting those mechanisms that under a translational perspective could shed light on molecular targets worth to be considered for further innovative antipsychotic development. In vivo and in vitro preclinical findings, supported by innovative techniques and methods, together with pharmacogenomic and in vivo functional studies, point to multiple and possibly overlapping MOAs. To better explore this crucial issue, the specific affinity for 5-HT₂R, D1R, α_2c, and muscarinic receptors, the relatively low occupancy at dopamine D2R, the interaction with receptor dimers, as well as the potential confounder effects resulting in biased ligand action, and lastly, the role of the moiety responsible for lipophilic and alkaline features of clozapine are highlighted. Finally, the role of transcription and protein changes at the synaptic level, and the possibility that clozapine can directly impact synaptic architecture are addressed. Although clozapine's exact MOAs that contribute to its unique efficacy and some of its severe adverse effects have not been fully understood, relevant information can be gleaned from recent mechanistic understandings that may help design much needed additional therapeutic strategies for TRS.

Present and future antipsychotic drugs: a systematic review of the putative mechanisms of action for efficacy and a critical appraisal under a translational perspective

Antipsychotics represent the mainstay of schizophrenia pharmacological therapy, and their role has been expanded in the last years to mood disorders treatment. Although introduced in 1952, many years of research were required before an accurate picture of how antipsychotics work began to emerge. Despite the well-recognized characterization of antipsychotics in typical and atypical based on their liability to induce motor adverse events, their main action at dopamine D2R to elicit the "anti-psychotic" effect, as well as the multimodal action at other classes of receptors, their effects on intracellular mechanisms starting with receptor occupancy is still not completely understood. Significant lines of evidence converge on the impact of these compounds on multiple molecular signaling pathways implicated in the regulation of early genes and growth factors, dendritic spine shape, brain inflammation, and immune response, tuning overall the function and architecture of the synapse. Here we present, based on PRISMA approach, a comprehensive and systematic review of the above mechanisms under a translational perspective to disentangle those intracellular actions and signaling that may underline clinically relevant effects and represent potential targets for further innovative strategies in antipsychotic therapy.

Dopamine D4 receptor stimulation contributes to novel object recognition: Relevance to cognitive impairment in schizophrenia

Several atypical antipsychotic drugs (APDs) have high affinity for the dopamine (DA) D₄ receptor, but the relevance to the efficacy for the treatment of cognitive impairment associated with schizophrenia (CIAS) is poorly understood. The aim of this study was to investigate the effects of D₄ receptor stimulation or blockade on novel object recognition (NOR) in normal rats and on the sub-chronic phencyclidine (PCP)-induced novel object recognition deficit. The effect of the D₄ agonist, PD168077, and the D₄ antagonist, L-745,870, were studied alone, and in combination with clozapine and lurasidone. In normal rats, L-745,870 impaired novel object recognition, whereas PD168077 had no effect. PD168077 acutely reversed the sub-chronic phencyclidine-induced novel object recognition deficit. Co-administration of a sub-effective dose (SED) of PD168077 with a sub-effective dose of lurasidone also reversed this deficit, but a sub-effective dose of PD168077 with a sub-effective dose of clozapine, a more potent D₄ antagonist than lurasidone, did not reverse the sub-chronic phencyclidine-induced novel object recognition deficit. At a dose that did not induce a novel object recognition deficit, L-745,870 blocked the ability of clozapine, but not lurasidone, to reverse the novel object recognition deficit. D₄ receptor agonism has a beneficial effect on novel object recognition in sub-chronic PCP-treated rats and augments the cognitive enhancing efficacy of an atypical antipsychotic drug that lacks affinity for the D₄ receptor, lurasidone.

Methylone-induced hyperthermia and lethal toxicity: role of the dopamine and serotonin transporters

Methylone (2-methylamino-1-[3,4-methylenedioxy-phenyl]propan-1-one), an amphetamine analog, has emerged as a popular drug of abuse worldwide. Methylone induces hyperthermia, which is thought to contribute toward the lethal consequences of methylone overdose. Methylone has been assumed to induce hyperthermic effects through inhibition of serotonin and/or dopamine transporters (SERT and DAT, respectively). To examine the roles of each of these proteins in methylone-induced toxic effects, we used SERT and DAT knockout (KO) mice and assessed the hyperthermic and lethal effects caused by a single administration of methylone. Methylone produced higher rates of lethal toxicity compared with other amphetamine analogs in wild-type mice. Compared with wild-type mice, lethality was significantly lower in DAT KO mice, but not in SERT KO mice. By contrast, only a slight diminution in the hyperthermic effects of methylone was observed in DAT KO mice, whereas a slight enhancement of these effects was observed in SERT KO mice. Administration of the selective D1 receptor antagonist SCH 23390 and the D2 receptor antagonist raclopride reduced methylone-induced hyperthermia, but these drugs also had hypothermic effects in saline-treated mice, albeit to a smaller extent than the effects observed in methylone-treated mice. In contradistinction to 3,4-methylenedioxymethamphetamine, which induces its toxicity through SERT and DAT, these data indicate that DAT, but not SERT, is strongly associated with the lethal toxicity produced by methylone, which did not seem to be dependent on the hyperthermic effects of methylone. DAT is therefore a strong candidate molecule for interventions aimed at preventing acute neurotoxic and lethal effects of methylone.

Striatal dopamine receptor plasticity in neurotensin deficient mice

Schizophrenia is thought to be caused, at least in part, by dysfunction in striatal dopamine neurotransmission. Both clinical studies and animal research have implicated the dopamine neuromodulator neurotensin (NT) in the pathophysiology of schizophrenia. Utilizing male mice lacking the NT gene (NT(-/-)), these studies examined the consequences of NT deficiency on dopaminergic tone and function, investigating (1) dopamine concentrations and dopamine receptor and transporter expression and binding in dopaminergic terminal regions, and (2) the behavioral effects of selective dopamine receptor agonists on locomotion and sensorimotor gating in adult NT(-/-) mice compared to wildtype (NT(+/+)) mice. NT(-/-) mice did not differ from NT(+/+) mice in concentrations of dopamine or its metabolite DOPAC in any brain region examined. However, NT(-/-) mice showed significantly increased D1 receptor, D2 receptor, and dopamine transporter (DAT) mRNA in the caudate putamen compared to NT(+/+) controls. NT(-/-) mice also showed elevated D2 receptor binding densities in both the caudate putamen and nucleus accumbens shell compared to NT(+/+) mice. In addition, some of the behavioral effects of the D1-type receptor agonist SKF-82958 and the D2-type receptor agonist quinpirole on locomotion, startle amplitude, and prepulse inhibition were dose-dependently altered in NT(-/-) mice, showing altered D1-type and D2-type receptor sensitivity to stimulation by agonists in the absence of NT. The results indicate that NT deficiency alters striatal dopamine receptor expression, binding, and function. This suggests a critical role for the NT system in the maintenance of striatal DA system homeostasis and implicates NT deficiency in the etiology of dopamine-associated disorders such as schizophrenia.

Design, synthesis, lipophilic properties, and binding affinities of potential ligands in positron emission tomography (PET) for visualization of brain dopamine D4 receptors

We report the synthesis of compounds structurally related to the high-affinity dopamine D4 receptor ligand N-{2-[4-(3-cyanopyridin-2-yl)piperazin-1-yl]ethyl}-3-methoxybenzamide (1e). All compounds were specifically designed as potential PET radioligands for brain D4 receptor visualization, having lipophilicity within a range for brain uptake and weak non-specific binding (0.75<cLogP<3.15) and bearing a substituent for easy access to labeling with the positron emitter isotope (11) C or (18) F. The best compound of the series, N-{2-[4-(4-chlorophenyl)piperazin-1-yl]ethyl}-6-fluoropyridine-3-carboxamide (7a), displayed excellent selectivity over D2 and D3 receptors (>100-fold), but its D4 receptor affinity was suboptimal for imaging of brain D4 receptors (Ki =30 nM).

Aberrant neural synchrony in the maternal immune activation model: using translatable measures to explore targeted interventions

Maternal exposure to infection occurring mid-gestation produces a three-fold increase in the risk of schizophrenia in the offspring. The critical initiating factor appears to be the maternal immune activation (MIA) that follows infection. This process can be induced in rodents by exposure of pregnant dams to the viral mimic Poly I:C, which triggers an immune response that results in structural, functional, behavioral, and electrophysiological phenotypes in the adult offspring that model those seen in schizophrenia. We used this model to explore the role of synchronization in brain neural networks, a process thought to be dysfunctional in schizophrenia and previously associated with positive, negative, and cognitive symptoms of schizophrenia. Exposure of pregnant dams to Poly I:C on GD15 produced an impairment in long-range neural synchrony in adult offspring between two regions implicated in schizophrenia pathology; the hippocampus and the medial prefrontal cortex (mPFC). This reduction in synchrony was ameliorated by acute doses of the antipsychotic clozapine. MIA animals have previously been shown to have impaired pre-pulse inhibition (PPI), a gold-standard measure of schizophrenia-like deficits in animal models. Our data showed that deficits in synchrony were positively correlated with the impairments in PPI. Subsequent analysis of LFP activity during the PPI response also showed that reduced coupling between the mPFC and the hippocampus following processing of the pre-pulse was associated with reduced PPI. The ability of the MIA intervention to model neurodevelopmental aspects of schizophrenia pathology provides a useful platform from which to investigate the ontogeny of aberrant synchronous processes. Further, the way in which the model expresses translatable deficits such as aberrant synchrony and reduced PPI will allow researchers to explore novel intervention strategies targeted to these changes.

The Concise Guide to PHARMACOLOGY 2013/14: G protein-coupled receptors

The Concise Guide to PHARMACOLOGY 2013/14 provides concise overviews of the key properties of over 2000 human drug targets with their pharmacology, plus links to an open access knowledgebase of drug targets and their ligands (www.guidetopharmacology.org), which provides more detailed views of target and ligand properties. The full contents can be found at http://onlinelibrary.wiley.com/doi/10.1111/bph.12444/full. G protein-coupled receptors are one of the seven major pharmacological targets into which the Guide is divided, with the others being G protein-coupled receptors, ligand-gated ion channels, ion channels, catalytic receptors, nuclear hormone receptors, transporters and enzymes. These are presented with nomenclature guidance and summary information on the best available pharmacological tools, alongside key references and suggestions for further reading. A new landscape format has easy to use tables comparing related targets. It is a condensed version of material contemporary to late 2013, which is presented in greater detail and constantly updated on the website www.guidetopharmacology.org, superseding data presented in previous Guides to Receptors and Channels. It is produced in conjunction with NC-IUPHAR and provides the official IUPHAR classification and nomenclature for human drug targets, where appropriate. It consolidates information previously curated and displayed separately in IUPHAR-DB and the Guide to Receptors and Channels, providing a permanent, citable, point-in-time record that will survive database updates.

Evaluation of 18F-labeled benzodioxine piperazine-based dopamine D4 receptor ligands: lipophilicity as a determinate of nonspecific binding

Derivatization of the putative neuroleptic 1-(2,3-dihydrobenzo[1,4]dioxin-6-yl)-4-(4-fluorobenzyl)piperazine (3a) led to a series of new dopamine receptor D4 ligands displaying high affinity (Ki=1.1-15 nM) and D2/D4 subtype selectivities of about 800-6700. These ligands were labeled with the short-lived positron emitter fluorine-18 and analyzed for their potential application for imaging studies by positron emission tomography (PET). In vitro autoradiography was used to determine their nonspecific binding behavior as a result of their structural and thus physicochemical properties. The biodistribution, in vivo stability, and brain uptake of the most promising D4 radioligand candidate were determined. This proved to be 1-(2,3-dihydrobenzo[b][1,4]dioxin-6-yl)-4-((6-fluoropyridin-3-yl)methyl)piperazine ([18F]3d), which revealed an excellent binding pattern with a high selectivity and limited nonspecific binding in vitro. This analogue also exhibited a high stability and an extremely high brain uptake in vivo with specific binding in hippocampus, cortex, colliculus, and cerebellum as determined by ex vivo autoradiography. Thus, [18F]3d appears as a suitable D4 radioligand for in vivo imaging, encouraging continued evaluation by PET studies.

Melperone in treatment-refractory schizophrenia: a case series

OBJECTIVE

Clozapine is the treatment of choice in refractory schizophrenia, but a substantial proportion of patients experience inadequate response or tolerate the drug poorly. Melperone has been suggested as a possible alternative in such patients. This case series examines the efficacy of melperone in refractory schizophrenia.

METHOD

All patients prescribed melperone at the South London and Maudsley NHS Foundation Trust were identified using a pharmacy database. The main outcome was to determine the proportion of patients discharged on melperone.

RESULTS

Three of 21 patients were discharged on melperone. The primary reason for discontinuation was lack of efficacy.

CONCLUSION

Melperone may be an option in a very few patients with refractory schizophrenia, but it should not be considered as an alternative to clozapine.

Design, synthesis, radiolabeling, and in vivo evaluation of carbon-11 labeled N-[2-[4-(3-cyanopyridin-2-yl)piperazin-1-yl]ethyl]-3-methoxybenzamide, a potential positron emission tomography tracer for the dopamine D(4) receptors

Here we describe the design, synthesis, and evaluation of physicochemical and pharmacological properties of D(4) dopamine receptor ligands related to N-[2-[4-(4-chlorophenyl)piperazin-1-yl]ethyl]-3-methoxybenzamide (2). Structural features were incorporated to increase affinity for the target receptor, to improve selectivity over D(2) and σ(1) receptors, to enable labeling with carbon-11 or fluorine-18, and to adjust lipophilicity within the range considered optimal for brain penetration and low nonspecific binding. Compounds 7 and 13 showed the overall best characteristics: nanomolar affinity for the D(4) receptor, >100-fold selectivity over D(2) and D(3) dopamine receptors, 5-HT(1A), 5-HT(2A), and 5-HT(2C) serotonin receptors and σ(1) receptors, and log P = 2.37-2.55. Following intraperitoneal administration in mice, both compounds rapidly entered the central nervous system. The methoxy of N-[2-[4-(3-cyanopyridin-2-yl)piperazin-1-yl]ethyl]-3-methoxybenzamide (7) was radiolabeled with carbon-11 and subjected to PET analysis in non-human primate. [(11)C]7 time-dependently accumulated to saturation in the posterior eye in the region of the retina, a tissue containing a high density of D(4) receptors.

Allosteric effects of the antipsychotic drug trifluoperazine on the energetics of calcium binding by calmodulin

Trifluoperazine (TFP; Stelazine) is an antagonist of calmodulin (CaM), an essential regulator of calcium-dependent signal transduction. Reports differ regarding whether, or where, TFP binds to apo CaM. Three crystallographic structures (1CTR, 1A29, and 1LIN) show TFP bound to (Ca(2+))(4)-CaM in ratios of 1, 2, or 4 TFP per CaM. In all of these, CaM domains adopt the "open" conformation seen in CaM-kinase complexes having increased calcium affinity. Most reports suggest TFP also increases calcium affinity of CaM. To compare TFP binding to apo CaM and (Ca(2+))(4)-CaM and explore differential effects on the N- and C-domains of CaM, stoichiometric TFP titrations of CaM were monitored by (15)N-HSQC NMR. Two TFP bound to apo CaM, whereas four bound to (Ca(2+))(4)-CaM. In both cases, the preferred site was in the C-domain. During the titrations, biphasic responses for some resonances suggested intersite interactions. TFP-binding sites in apo CaM appeared distinct from those in (Ca(2+))(4)-CaM. In equilibrium calcium titrations at defined ratios of TFP:CaM, TFP reduced calcium affinity at most levels tested; this is similar to the effect of many IQ-motifs on CaM. However, at the highest level tested, TFP raised the calcium affinity of the N-domain of CaM. A model of conformational switching is proposed to explain how TFP can exert opposing allosteric effects on calcium affinity by binding to different sites in the "closed," "semi-open," and "open" domains of CaM. In physiological processes, apo CaM, as well as (Ca(2+))(4)-CaM, needs to be considered a potential target of drug action.

Differential effects of antipsychotic and glutamatergic agents on the phMRI response to phencyclidine

Acute administration of NMDA receptor (NMDAR) antagonists such as phencyclidine (PCP) or ketamine induces symptoms that closely resemble those of schizophrenia in humans, a finding that has led to the hypothesis that a decreased NMDAR function may be a predisposing or even causative factor in schizophrenia. However, the precise neuropharmacological mechanisms underlying these effects remain to be fully elucidated. Here, we applied pharmacological MRI (phMRI) to examine the brain circuitry underlying the psychotomimetic action of PCP in the anesthetized rat, and investigated how these functional changes are modulated by drugs that possess distinct pharmacological mechanisms. Acute administration of PCP (0.5 mg/kg i.v.) produced robust and sustained positive relative cerebral blood volume (rCBV) changes in discrete cortico-limbo-thalamic regions. Pretreatment with the selective D2 dopamine antagonist raclopride (0.3 mg/kg i.p.) did not significantly affect the rCBV response to PCP, while the atypical antipsychotic clozapine (5 mg/kg i.p.) produced region-dependent effects, with complete suppression of the rCBV response in the thalamus, and weaker attenuation of the response in cortical and hippocampal structures. The response to PCP was strongly suppressed in all regions by pretreatment with two drugs that can inhibit aberrant glutamatergic activity: the anticonvulsant lamotrigine (10 mg/kg i.p.) and the mGluR2/3 agonist LY354740 (10 mg/kg i.p.). Taken together, our findings corroborate the pivotal role of dysfunctional glutamatergic neurotransmission in the functional response elicited by PCP, while the lack of effect of raclopride argues against a primary role of dopamine D2 receptor activation in this process. Finally, the thalamic effect of clozapine could be key to elucidating the functional basis of its pharmacological action.

Synthesis and biodistribution of 8-iodo-11-(4-methylpiperazino)-5H-dibenzo[b,e][1,4]-diazepine: Iozapine

8-Iodo-11-(4-methylpiperazino)-5H-dibenzo[b,e][1,4]-diazepine: Iozapine, a potential D(4)-receptor ligand was synthesized using oxidative iodo-destannylation reaction. The preliminary biodistribution studies of radioiodinated iozapine have shown that the compound is taken up in the brains of mice and rabbits.

Suppression of P-glycoprotein expression by antipsychotics trifluoperazine in adriamycin-resistant L1210 mouse leukemia cells

Multidrug resistance (MDR) to unrelated chemotherapeutic drugs can be mediated by overexpression of P-glycoprotein (P-gp), the mdr gene product. Trifluoperazine (TFP), a phenothiazine derivative antipsychotics, is known to reverse MDR of tumor cell lines by blocking P-gp efflux function. In the present study, we evaluated the effect of TFP on the expression of P-gp in multidrug-resistant L1210/Adr mouse leukemic cell lines, which are characterized by overexpession of P-gp. We found that TFP induced the downregulation of P-gp protein and mdr1b mRNA in a dose- and time-dependent manner in L1210/Adr cells. TFP reduction of mdr1b mRNA was paralleled by transcriptional suppression of the mdr1b promoter. Moreover, TFP restored the adriamycin-induced apoptosis in L1210/Adr cells. These results suggest that TFP may have utility as an adjuvant in the therapy of leukemia for the reversal of P-gp-dependent MDR as well as for the management of psychological symptoms in the cancer patients.

Antagonistic Activity of Ascorbic Acid (Vitamin C) on Dopaminergic Modulation: Apomorphine-Induced Stereotypic Behavior in Mice

Among the various neurotransmitter systems implicated in the mechanism of action of ascorbic acid (vitamin C), the relationship between the dopaminergic system and ascorbic acid is not particularly clear. Ascorbic acid is speculated to have an antagonistic effect on dopaminergic modulation. With this background in mind, in the present study we have seen the effect of ascorbic acid per se and in combination with typical and atypical antipsychotic agents against apomorphine-induced stereotypic behavior in mice. Male Laka mice weighing 20-25 g were used in the present study. Apomorphine-induced stereotypic behavior was used as an animal model. Various dopaminergic modulators were used. Ascorbic acid dose-dependently inhibited stereotypic behavior produced by apomorphine in mice. It potentiated the antipsychotic activity of haloperidol (0.1 mg/kg i.p.), a typical antipsychotic agent. When administered along with atypical antipsychotics, clozapine (1-2 mg/kg i.p.), sulpiride (10-20 mg/kg i.p.) and risperidone (0.0025 mg/kg i.p.), ascorbic acid also potentiated their activity. Also when given along with SCH-23390, a selective D(1) antagonist, an additive effect was observed. Ascorbic acid also inhibited the supersensitization response of apomorphine on reserpinization (2 mg/kg i.p.). Interestingly, at a lower dose (100 mg/kg i.p.), ascorbic acid potentiated the dopaminergic activity of apomorphine (0.5 mg/kg) and BHT-920 (0.25 mg/kg i.p.). However, when given concomitantly with SKF-38393, it failed to alter the response of SKF-38393. The data substantiate the hypothesis that ascorbic acid potentiated the activity of typical as well as atypical antipsychotics and that the effect of ascorbic acid on the dopaminergic system is markedly dose dependent; a low dose (100 mg/kg i.p.) potentiated the dopaminergic action while higher doses (400-1,600 mg/kg i.p.) blocked it.

A theoretical study to investigate D2DAR/D4DAR selectivity: receptor modeling and molecular docking of dopaminergic ligands

Molecular modeling methods have been applied to construct three-dimensional models for dopaminergic ligand complexes with D2 and D4 receptor subtypes (D2DAR and D4DAR), using the bovine rhodopsin crystal structure as a template for the modeling study. Different dopaminergic ligands, in particular the N-n-propyl-substituted 3-aryl- and 3-cyclohexylpiperidines, were docked into the D2DAR and the D4DAR, to evaluate the agreement between theoretical and experimental results as regards their D2/D4 selectivity. The different position of an aromatic region in the two receptors might explain the structural basis of this biological property.

Classification of dopamine antagonists using functional feature hypothesis and topological descriptors

The designing of selective dopamine antagonists for their own subreceptors can be useful in individual therapy of various neuropsychiatric disorders. Three-dimensional pharmacophore hypothesis and two-dimensional topological descriptors were used to investigate and compare different classes of dopamine antagonists. The structurally diverse D(3) and D(4) antagonists above preclinical trials were selected to map common structural features of highly selective and efficacious antagonists. The generated pharmacophore hypotheses were successfully employed as discriminative probe for database screening. To filter out the false positive from screening hits, the classification models by two-dimensional topological descriptors were built. Molconn-Z and BCUT topological descriptors were employed to develop a classification model for 1328 dopamine antagonists from MDDR database. The soft independent modeling of class analogy and artificial neural network, two supervised classification techniques, successfully classified D(1), D(3), and D(4) antagonists at the average of 80% rates into their own active classes. The mean classification rates for D(2) antagonists were obtained to 60% due to insufficient selective D(2) antagonists. In this paper, we report the validity of our models generated using functional feature hypotheses and topological descriptors. The combining both of classification using functional feature hypotheses and topological descriptors would be a useful tool to predict selective antagonists.

Invited Review: Fluphenazine Augments Retinal Oxidative Stress

Recently, fluphenazine, a phenothiazine neuroleptic, has been associated with idiosyncratic retinopathy. Neuroleptic-induced retinopathy appears to be isolated to only a few structurally related phenothiazines, suggesting that the causality is not the result of dopamine antagonism. The chemical structure of fluphenazine is very similar to that of chlorpromazine and thioridazine, agents known to produce retinopathy. Like chlorpromazine and thioridazine, fluphenazine may be oxidized by retinal cytochrome P450 and/or myeloperoxidase to an electrophile, producing injury in susceptible patients.

Identification and Characterization of the Acidic pH Binding Sites for Growth Regulatory Ligands of Low Density Lipoprotein Receptor-related Protein-1

The type V TGF-beta receptor (TbetaR-V) plays an important role in growth inhibition by IGFBP-3 and TGF-beta in responsive cells. Unexpectedly, TbetaR-V was recently found to be identical to the LRP-1/alpha(2)M receptor; this has disclosed previously unreported growth regulatory functions of LRP-1. Here we demonstrate that, in addition to expressing LRP-1, all cells examined exhibit low affinity but high density acidic pH binding sites for LRP-1 growth regulatory ligands (TGF-beta(1), IGFBP-3, and alpha(2)M(*)). These sites, like LRP-1, are sensitive to receptor-associated protein and calcium depletion but, unlike LRP-1, are also sensitive to chondroitin sulfate and heparin and capable of directly binding ligands, which do not bind to LRP-1. Annexin VI has been identified as a major membrane-associated protein capable of directly binding alpha(2)M(*) at acidic pH. This is evidenced by: 1) structural and Western blot analyses of the protein purified from bovine liver plasma membranes by alpha(2)M(*) affinity column chromatography at acidic pH, and 2) dot blot analysis of the interaction of annexin VI and (125)I-alpha(2)M(*). Cell surface annexin VI is involved in (125)I-TGF-beta(1) and (125)I-alpha(2)M(*) binding to the acidic pH binding sites and (125)I-alpha(2)M(*) binding to LRP-1 at neutral pH as demonstrated by the sensitivity of cells to pretreatment with anti-annexin VI IgG. Cell surface annexin VI is also capable of mediating internalization and degradation of cell surface-bound (125)I-TGF-beta(1) and (125)I-alpha(2)M(*) at pH 6 and of forming ternary complexes with (125)I-alpha(2)M(*) and LRP-1 at neutral pH as demonstrated by co-immunoprecipitation. Trifluoperazine and fluphenazine, which inhibit ligand binding to the acidic pH binding sites, block degradation after internalization of cell surface-bound (125)I-TGF-beta(1) or (125)I-alpha(2)M(*). These results suggest that cell surface annexin VI may function as an acidic pH binding site or receptor and may also function as a co-receptor with LRP-1 at neutral pH.

Weight gain associated with atypical antipsychotic use in children and adolescents: prevalence, clinical relevance, and management

Atypical antipsychotics are increasingly prescribed to children and adolescents with neuropsychiatric disorders. Although their profile of potent antagonism at specific serotonin and dopamine receptors offers certain advantages compared with typical antipsychotics, their use has been associated with various adverse effects, including significant weight gain. This adverse effect is of particular concern in children and adolescents, secondary to the immediate and long-term health risks associated with weight gain, including obesity, diabetes mellitus, and hyperlipidemia. Indeed, from 1963 to 1991, the prevalence of obesity has approximately doubled in youth. Prior to selecting an atypical antipsychotic, a detailed review of the predictors of weight gain is necessary for every child and adolescent. Published data suggest that clozapine and olanzapine are associated with considerable weight gain, whereas risperidone and quetiapine have a moderate risk. Alternatively, ziprasidone and aripiprazole may exhibit a low risk for this adverse effect. Whereas behavioral and pharmacologic measures are available to manage weight gain associated with atypical antipsychotics, research is needed to establish more effective and safe interventions for this adverse effect in children and adolescents.

Development of Novel 1,2,3,4-Tetrahydroisoquinoline Derivatives and Closely Related Compounds as Potent and Selective Dopamine D3 Receptor Ligands

Based on N-alkylated 1,2,3,4-tetrahydroisoquinoline derivatives, which are structurally related to the partial agonist BP 897, a series of novel, selective dopamine D3 receptor antagonists has been synthesised. Derivatisation included changes in the arylamide moiety and the tetrahydroisoquinoline substructure leading to compounds with markedly improved selectivities and affinities in the low nanomolar concentration range. From the 55 structures presented here, (E)-3-(4-iodophenyl)-N-(4-(1,2,3,4-tetrahydroisoquinolin-2-yl)butyl)acrylamide (51) has high affinity (Ki(hD3)=12 nM) and a 123-fold preference for the D3 receptor relative to the D2 receptor subtype. Its pharmacological profile offers the prospect of a novel radioligand as a tool for various dopamine D3-receptor-related in vitro and in vivo investigations.

In vitro and in vivo pharmacological profile of 4-(4-fluorobenzylidene)-1-[2-[5-(4-fluorophenyl)-1H-pyrazol-4-yl] ethyl] piperidine (NRA0161)

Atypical antipsychotic properties of 4-(4-fluorobenzylidene)-1-[2-[5-(4-fluorophenyl)-1H-pyrazol-4-yl]ethyl] piperidine (NRA0161) were investigated by in vitro receptor affinities, in vivo receptor occupancies and findings were compared with those of risperidone and haloperidol in rodent behavioral studies. In in vitro receptor binding studies, NRA0161 has a high affinity for human cloned dopamine D(4) and 5-HT(2A) receptor with Ki values of 1.00 and 2.52 nM, respectively. NRA0161 had a relatively high affinity for the alpha(1) adrenoceptor (Ki; 10.44 nM) and a low affinity for the dopamine D(2) receptor (Ki; 95.80 nM). In in vivo receptor binding studies, NRA0161 highly occupied the 5-HT(2A) receptor in rat frontal cortex. In contrast, NRA0161 did not occupy the striatal D(2) receptor. In behavioral studies, NRA0161, risperidone and haloperidol antagonized the locomotor hyperactivity in mice, as induced by methamphetamine (MAP). At a higher dosage, NRA0161, risperidone and haloperidol dose-dependently antagonized the MAP-induced stereotyped behavior in mice and NRA0161 dose-dependently and significantly induced catalepsy in rats. The ED(50) value in inhibiting the MAP-induced locomotor hyperactivity was 30 times lower than that inhibiting the MAP-induced stereotyped behavior and 50 times lower than that which induced catalepsy. These findings suggest that NRA0161 may have atypical antipsychotic activities yet without producing extrapyramidal side effects.

Dopamine D3 receptor antagonist effects on the motivational effects of ethanol

Dopaminergic systems are thought to play important roles in the motivational effects of ethanol. In the present experiments, we examined the effects of U99194A, a putative dopamine D(3) receptor antagonist, on ethanol-induced conditioned place preference, locomotor stimulation, taste aversion, and self-administration. In two separate studies with the use of a place conditioning procedure, adult male Swiss-Webster mice received six pairings of a tactile stimulus with ethanol (1 or 3 g/kg, i.p.), U99194A (20 mg/kg, i.p.), or ethanol + U99194A. For determination of ethanol-stimulated activity, subjects received U99194A at a dose of 0, 10, 20, or 30 mg/kg 15 min before ethanol at 0, 1, or 2 g/kg immediately before a 30-min locomotor activity test. In a taste conditioning procedure, subjects received five 1-h access periods to 0.2 M NaCl. After the first four access periods, subjects received ethanol at 0, 2, or 4 g/kg and U99194A at 0, 10, or 20 mg/kg. In an oral self-administration procedure, male C57BL/6J mice received U99194A at 0, 10, or 20 mg/kg, followed by 30-min access to 10% (wt./vol.) sucrose or 10% (vol./vol.) ethanol in 10% sucrose. The acquisition of ethanol-induced conditioned place preference was enhanced by U99194A. However, U99194A did not produce significant preference alone. U99194A did not alter locomotor stimulation produced by an injection of ethanol at 2 g/kg. U99194A also did not alter the acquisition of ethanol-induced conditioned taste aversion and did not change oral ethanol self-administration. These results support the suggestion that dopamine D(3) receptors have specific involvement in ethanol reward, as measured by place conditioning, but are not important for ethanol-stimulated activity, ethanol taste aversion, or ethanol intake.

Tolerability profile of atypical antipsychotics in children and adolescents

Antipsychotics are frequently used in the treatment of a variety of neuropsychiatric conditions in children and adolescents. Atypical antipsychotics have come to the forefront in child psychiatry due largely to their tolerability profiles as well as their efficacy. Potential treatment options include clozapine, risperidone, olanzapine, quetiapine and ziprasidone. A number of studies investigating the use of clozapine have been published in children; however, owing to the frequent monitoring required for agranulocytosis, the use of clozapine may be restricted to patients with treatment-refractory disease. With accumulating data on the development of glucose intolerance in adults receiving clozapine, closer monitoring of bodyweight and fasting blood glucose is imperative. Clozapine also has an increased seizure risk, therefore a baseline electroencephalogram should be performed, as well as continued vigilance for this adverse effect. Risperidone is an atypical antipsychotic that is generally well tolerated and numerous studies have been published investigating this drug in children. Unlike clozapine, its receptor interaction profile lends itself toward increased risk of extrapyramidal symptoms (EPS) and hyperprolactinaemia. Bodyweight gain is a common adverse effect, although somewhat less than that reported with olanzapine. Baseline liver function studies prior to initiation of this medication are recommended. Risperidone-induced mania has been reported in adults and, therefore, increased caution should be used when deciding to treat children and adolescents with risperidone, particularly in those with a predisposition toward mania. Olanzapine, like risperidone, has also been associated with onset of mania in adults. Olanzapine has a receptor profile that results in significant risk for bodyweight gain and sedation. Furthermore, this drug has been linked to the development of glucose intolerance; thus, it is important to monitor bodyweight and fasting blood glucose on a frequent basis. Less information is known about quetiapine in children and adolescents. Reports about its efficacy and tolerability vary. Quetiapine appears to have increased risk for sedation and bodyweight gain, albeit less than that of olanzapine. The compound appears to be less likely to induce EPS. Finally, ziprasidone has recently been approved for use in the adult population. This compound, in terms of its receptor profile, has more in common with risperidone. This suggests a potential for increased risk of EPS and hyperprolactinaemia. It also has an increased risk of QTc prolongation; thus, a baseline electrocardiogram is suggested, particularly in those patients with a history of cardiovascular illness. Lack of evidence for bodyweight gain with ziprasidone is a considerable advantage.

Genetic variations in human G protein-coupled receptors: implications for drug therapy

Numerous genes encode G protein-coupled receptors (GPCRs)-a main molecular target for drug therapy. Estimates indicate that the human genome contains approximately 600 GPCR genes. This article addresses therapeutic implications of sequence variations in GPCR genes. A number of inactivating and activating receptor mutations have been shown to cause a variety of (mostly rare) genetic disorders. However, pharmacogenetic and pharmacogenomic studies on GPCRs are scarce, and therapeutic relevance of variant receptor alleles often remains unclear. Confounding factors in assessing the therapeutic relevance of variant GPCR alleles include 1) interaction of a single drug with multiple closely related receptors, 2) poorly defined binding pockets that can accommodate drug ligands in different orientations or at alternative receptor domains, 3) possibility of multiple receptor conformations with distinct functions, and 4) multiple signaling pathways engaged by a single receptor. For example, antischizophrenic drugs bind to numerous receptors, several of which might be relevant to therapeutic outcome. Without knowing accurately what role a given receptor subtype plays in clinical outcome and how a sequence variation affects drug-induced signal transduction, we cannot predict the therapeutic relevance of a receptor variant. Genome-wide association studies with single nucleotide polymorphisms could identify critical target receptors for disease susceptibility and drug efficacy or toxicity.

Synthesis of new 2-(aminomethyl)-4-phenylpyrrolo[1,2-a]-quinoxalines and their preliminary in-vivo central dopamine antagonist activity evaluation in mice

In the search for antipsychotic agents that are not associated with extrapyramidal side effects, efforts have been focused on finding selective D4-receptor antagonists and investigating their pharmacology. Our laboratory has developed a synthesis program for new pyrroloquinoxalines with therapeutic potential. We have described the synthesis of some new pyrroloquinoxalines with substituted arylpiperazino or aryltetrahydropyrido chain at position 3 of the quinoxaline ring (2-(4-phenylpiperazin-1-ylmethyl)-4-phenylpyrrolo[1,2-a]quinoxalinium oxalate (3a), 2-[4-(2-methoxyphenyl)piperazin-1-ylmethyl]-4-phenylpyrrolo[1,2-a]quinoxalinium oxalate (3b), 2-[4-(3-trifluoromethylphenyl)piperazin-1-ylmethyl]-4-phenylpyrrolo[1,2-a]quinoxalinium oxalate (3c), 2-[4-(4-chlorophenyl)piperazin-1-ylmethyl]-4-phenylpyrrolo[1,2-a]quinoxalinium oxalate (3d), 2-(4-pyridin-2-ylpiperazin-1-ylmethyl)-4-phenylpyrrolo[1,2-a]quinoxalinium oxalate (3e), and 2-(4-phenyl1,2,3,6-tetrahydropyridin-1-ylmethyl)-4-phenylpyrrolo[1,2-a]quinoxalinium oxalate (3f)). A preliminary pharmacological study of these products was conducted using climbing behaviour induced by apomorphine (2.5 mg kg(-1), s.c.) in mice. The derivatives were administered intraperitoneally 30 min before apomorphine. Haloperidol, chlorpromazine and clozapine were used as references. Among this series, 3b, 3c and 3f revealed a central dopamine antagonist activity. The most active derivative was 3b, which exhibited a profile relatively close to clozapine.

Antidyskinetic effect of JL-18, a clozapine analog, in parkinsonian monkeys

Clozapine reduces L-3,4-dihydroxyphenylalanine (L-Dopa)-induced dyskinesias in parkinsonian patients. To test if the antidyskinetic effect of clozapine is related to antagonism at the dopamine D(4) receptor, we investigated the effect of 8-methyl-6-(4-methyl-1-piperazinyl)-11H-pyrido[2,3-b][1, 4]benzodiazepine (JL-18), a structural analog of clozapine which is more selective for this receptor. Four 1-methyl-4-phenyl-1,2,3, 6-tetrahydropyridine (MPTP)-treated cynomolgus monkeys with a stable parkinsonian syndrome and reproducible dyskinesias to L-Dopa were used in this study. They were injected subcutaneously (s.c.) with L-Dopa methyl ester (125 mg per animal) plus benserazide (50 mg per animal; L-Dopa/benserazide) alone or in combination with JL-18 (at the doses of 0.1, 0.3, or 0.9 mg/kg, s.c.). Subcutaneous injection of sterile saline was used as control. L-Dopa/benserazide increased locomotion and improved parkinsonism but also induced dyskinesias. Co-administration of JL-18, at low doses (0.1, 0.3 mg/kg) with L-Dopa/benserazide, produced a dose-dependent reduction in L-Dopa-induced dyskinesias without a parallel return to parkinsonism. The present results suggest that novel selective dopamine D(4) receptor antagonists may represent a useful tool to reduce L-Dopa-induced dyskinesias.

Effect of calmodulin antagonists on the compound action potential of the cochlea

This study aimed to evaluate the effect of calmodulin antagonists on the threshold of the compound action potential (CAP) and the functional recovery of the cochlea after transient ischemia. When trifluoperazine and W-7 were administered to albino guinea pigs with perilymphatic perfusion, these drugs did not significantly affect the CAP thresholds. Transient cochlear ischemia of 30-min duration was obtained via a skull base approach. Although trifluoperazine significantly ameliorated the post-ischemic CAP threshold shifts 4 h after the onset of reperfusion, 1 to 50 microM W-7 did not affect the CAP threshold shifts. These results suggest that the action antagonizing calmodulin has no effect on the CAP threshold, while the role that calmodulin plays in cochlear ischemia-reperfusion injury still remains unclear.

A selective dopamine D4 receptor antagonist, NRA0160: a preclinical neuropharmacological profile

NRA0160, 5 - [2- ( 4- ( 3 - fluorobenzylidene) piperidin-1-yl) ethyl] - 4 -(4-fluorophenyl) thiazole-2-carboxamide, has a high affinity for human cloned dopamine D4.2, D4.4 and D4.7 receptors, with Ki values of 0.5, 0.9 and 2.7 nM, respectively. NRA0160 is over 20,000fold more potent at the dopamine D4.2 receptor compared with the human cloned dopamine D2L receptor. NRA0160 has negligible affinity for the human cloned dopamine D3 receptor (Ki=39 nM), rat serotonin (5-HT)2A receptors (Ki=180 nM) and rat alpha1 adrenoceptor (Ki=237 nM). NRA0160 and clozapine antagonized locomotor hyperactivity induced by methamphetamine (MAP) in mice. NRA0160 and clozapine antagonized MAP-induced stereotyped behavior in mice, although their effects did not exceed 50% inhibition, even at the highest dose given. NRA0160 and clozapine significantly induced catalepsy in rats, although their effects did not exceed 50% induction even at the highest dose given. NRA0160 and clozapine significantly reversed the disruption of prepulse inhibition (PPI) in rats produced by apomorphine. NRA0160 and clozapine significantly shortened the phencyclidine (PCP)-induced prolonged swimming latency in rats in a water maze task. These findings suggest that NRA0160 may have unique antipsychotic activities without the liability of motor side effects typical of classical antipsychotics.

Paradoxical effect of neuroleptic drugs on prolactin secretion by rat pituitary cell cultures

Several antipsychotic drugs reverse the dopamine-induced inhibition of prolactin release by rat pituitary cell cultures. Paradoxically, at high doses and without dopamine, antipsychotic drugs can also inhibit prolactin secretion. The mechanism underlying this phenomenon is unclear. Some evidence suggests that these drugs have an agonistic action. We sought to verify whether clozapine and fluphenazine, at doses higher than those reversing dopamine-induced inhibition of prolactin secretion in vitro, show this paradoxical effect and eventually a partial agonistic action. Both antipsychotics inhibited prolactin secretion, clozapine at doses starting from 10(-6) M and fluphenazine from 10(-7) M. Haloperidol reversed clozapine-induced prolactin inhibition but left fluphenazine-induced inhibition unchanged. These in vitro findings suggest that clozapine has a partial agonistic action on dopaminergic receptors but fluphenazine does not.

Receptor density as a factor governing the efficacy of the dopamine D4 receptor ligands, L-745,870 and U-101958 at human recombinant D4.4 receptors expressed in CHO cells

1. The relationships between the density of dopamine D4.4 receptors and the agonist efficacies of L-745,870 (3-(4-[4-chlorophhenyl]piperazin-1-yl)-methyl-1H-pyrrolo [2, 3-b]pyridine) and U-101958 ((1-benzyl-piperidin-4-yl)-(3-isopropoxy-pyridin-2-yl)-methyl-a min e) were investigated in Chinese hamster ovary (CHO) cells, after treatment with the gene expression enhancer, sodium butyrate. 2. In CHO cells expressing D4.4 receptors (CHO/D4 cells), dopamine inhibited forskolin-stimulated cyclic AMP accumulation (Emax 56+/-1% inhibition, pEC50 7.4+/-0.1, n=10). U-101958 behaved as a partial agonist (39+/-7% the efficacy of dopamine, pEC50 8.1+/-0.3, n=4), whereas L-745,870 had no detectable agonist effect. 3. Receptor density, as estimated by [3H]-spiperone saturation binding was 240+/-30 fmol mg-1 protein (n=8) in CHO/D4 cell homogenates. It reached 560+/-150 (n=6), 1000+/-190 (n=4) and 840+/-120 (n=4) fmol mg-1 protein after treatment with sodium butyrate (5 mM) for 6, 18 and 48 h, respectively. 4. The increase in receptor density was associated with a gradual enhancement of the agonist effects (increased Emax and pEC50 values) of dopamine. The efficacy of U-101958 (relative to dopamine) doubled and L-745,870 was turned into a partial agonist (efficacy 49% relative to dopamine, pEC50 8. 6+/-0.2, n=6, after 48 h treatment with sodium butyrate). These agonist effects of U-101958 and L-745,870 could be antagonized by spiperone (0.1 microM) but not by raclopride (10 microM). 5. The results show that U-101958 and L-745,870 are partial agonists at human dopamine D4.4 receptors expressed in CHO cells. Their efficacy is governed by receptor density. Agonist effects of these two compounds in vivo cannot be excluded under circumstances of increased receptor levels.

Effect of amphetamine, alpha-methyl-p-tyrosine (alpha-MPT) and antipsychotic agents on dopamine D2-type receptor occupancy in rats

1. EEDQ inactivates unoccupied receptors in vivo in brain tissue and is useful in determining which receptors are occupied by a drug treatment. 2. alpha-MPT, inhibits the synthesis of dopamine, reducing D2-type receptor occupancy by dopamine and enhances the amount of receptor inactivation by EEDQ. 3. Amphetamine releases dopamine resulting in increased occupancy of dopamine D2-type receptors and we have shown that it protects those receptors from EEDQ. 4. Clozapine and remoxipride, two antipsychotic agents, occupied the dopamine receptors in both the caudate and cortex. 5. These findings are important because they substantiate other results obtained with amphetamine and SPECT, which demonstrated an exaggerated dopamine neurotransmission in schizophrenic patients versus normal controls.

The agonist activities of the putative antipsychotic agents, L-745,870 and U-101958 in HEK293 cells expressing the human dopamine D4.4 receptor

1. Dopamine D4 receptor antagonists are being developed by several pharmaceutical companies as putative novel antipsychotics, possibly with low propensity to side-effects. Two such compounds, L-745,870 and U-101958 have been recently introduced. 2. The radioligand binding and functional activities of L-745,870 and U-101958 were investigated in human embryonic kidney (HEK)293 cells expressing the human recombinant dopamine D4.4 receptor (HEK293/D4 cells). [3H]-spiperone binding experiments were performed and inhibition of forskolin-stimulated cyclic AMP accumulation was used as the functional response. 3. [3H]-spiperone was found to label a homogeneous and saturable population of specific binding sites in HEK293/D4 cell homogenates (Bmax 505+/-90 fmol mg(-1) protein, pK(D) 9.5+/-0.1, n=3). Inhibition of specific [3H]-spiperone binding was observed with spiperone (pKi 9.6+/-0.1, n=3), clozapine (pKi 7.4+/-0.1, n=4), L-745,870 (pKi 8.5+/-0.1, n=3) and U-101958 (pKi 8.9+/-0.1, n=3). By contrast, raclopride was very weak (pKi < 5, n=3). 4. Dopamine inhibited forskolin-stimulated cyclic AMP accumulation in HEK293/D4 cells in a concentration-dependent fashion (Emax 71+/-2% inhibition of forskolin-stimulated levels, pEC50 8.7+/-0.1, n=10). This effect was mimicked by the dopamine D2-like receptor agonists, quinpirole and 7-hydroxy-2-dipropylaminotetralin (7-OH-DPAT). 5. L-745,870 and U-101958 also inhibited forskolin-stimulated cyclic AMP accumulation in HEK293/D4 cells in a concentration-dependent way. L-745,870 was less efficacious than dopamine (71% the efficacy of dopamine), whereas U-101958 behaved as a full agonist compared to dopamine. Potencies (pEC50) values of L-745,870 and U-101958 were 9.0+/-0.2 (n=4) and 8.7+/-0.3 (n=3), consistent with pKi values determined in radioligand binding studies. 6. Dopamine, L-745,870 and U-101958 (up to 1 microM) were devoid of effect on forskolin-stimulated cyclic AMP accumulation in control, non-transfected HEK293 cells. 7. The agonist effects of dopamine, L-745,870 and U-101958 in HEK293/D4 cells could be antagonized by spiperone (pK(B) 8.2-8.8) and clozapine (pK(B) 7.1), but not by raclopride (pK(B) < 5). None of these antagonists had any significant agonist activity at concentrations up to 10 microM. 8. These results show that the putative dopamine D4 receptor antagonists, L-745,870 and U-101958 are not devoid of intrinsic activity at human recombinant dopamine D4.4 receptors. Therefore, they may not represent the most appropriate drugs for testing the benefit of D4 receptor antagonism in schizophrenic patients, if agonism should translate in vivo.

The preferential dopamine D3 receptor agonist cis-8-OH-PBZI induces limbic Fos expression in rat brain

The affinity, selectivity and agonistic properties of a constrained dopaminergic compound, the benz[e]indole cis-8-hydroxy-3-(n-propyl)1,2,3a.4,5,9b-hexahydro-1H-benz[e]indole (cis-8-OH-PBZI), for the dopamine D3 receptor were evaluated in competition binding experiments with cloned human dopamine receptor subtypes and, to further extend its profile, in in vitro radioligand binding assays. The Ki value measured for competition binding of this compound to the dopamine D3 receptor was 27.4+/-3.1 nM; this was 775-fold, 550-fold, 90-fold and 10-fold higher affinity than that measured at dopamine D1A, D5, D2s and D4 receptors, respectively. Evidence of dopamine receptor activation by cis-8-OH-PBZI was obtained by measuring dose-dependent increases in extracellular acidification rates and decreases in cAMP synthesis. In vivo, cis-8-OH-PBZI potently induced Fos protein immunoreactivity in the rat medial prefrontal cortex and shell region of the nucleus accumbens, but only marginally in the motor dorsolateral striatum, indicating a selective limbic site of action. In conclusion, the present data identify cis-8-OH-PBZI as having preference for the dopamine D3 receptor in vitro, and as having dopamine agonist activity and limbic sites of action in vivo.

Distribution and postnatal ontogeny of adenosine A2A receptors in rat brain: comparison with dopamine receptors

In adult rat brain, adenosine A2A receptors and dopamine D2 receptors are known to be located on the same cells where they interact in an antagonistic manner. In the present study we wanted to examine when this situation develops and compared the postnatal ontogeny of the binding of the adenosine A2A receptor agonist [3H]CGS 21680, the binding of the dopamine D1 receptor antagonist [3H]SCH 23390 and the dopamine D2 receptor antagonist [3H]raclopride. All three radioligands bound to the striatum at birth and this binding increased several-fold during the postnatal period. [3H]SCH 23390 binding developed first (mostly during the first week), followed by [3H]raclopride binding (first to third week) and [3H]CGS 21680 binding (only during second and third week). For all three radioligands the binding tended to decrease between 21 days and adulthood. This occurred earlier and was more pronounced in the globus pallidus than in the other examined structures. The increase in [3H]CGS 21680 binding from newborn to adult was mainly due to four-fold increase in the number of binding sites. The pharmacology of [3H]CGS 21680 binding to caudate-putamen was similar in newborn, one-week-old and adult animals, and was indicative of A2A receptors. The binding was inhibited by guanylyl imidodiphosphate at all ages, indicating that A2A receptors are G-protein-coupled already at birth. In contrast to the large increase in [3H]CGS 21680 binding, there was a decrease in the levels of A2A messenger RNA during the postnatal period in the caudate-putamen. In cerebral cortex [3H]CGS 21680 bound to a different site than the A2A receptor. From birth to adulthood cortical binding of [3H]CGS 21680 increased four-fold and that of the adenosine A1 agonist [3H]cyclohexyladenosine 19-fold. During early postnatal development [3H]SCH 23390 binding was higher in deep than in superficial cortical layers, but this difference disappeared in adult animals. There was binding of both [3H]CGS 21680 and [3H]cyclohexyladenosine to the olfactory bulb, suggesting a role of the two adenosine receptors in processing of olfactory information. [3H]CGS 21680 binding was present in the external plexiform layer and glomerular layer, and increased during development, but the density of binding sites was about one tenth of that seen in caudate putamen. [3H]cyclohexyladenosine showed a very different labelling pattern, resembling that observed with [3H]SCH 23390. Postnatal changes in adenosine receptors may explain age-dependent differences in stimulatory caffeine effects and endogenous protection against seizures. Since A2A receptors show a co-distribution with D2 receptors throughout development, caffeine may partly exert such actions by regulating the activity of D2 receptor-containing striatopallidal neurons.

Molecular approaches to receptors as targets for drug discovery

The cloning of a great number of receptors and channels has revealed that many of these targets for drug discovery can be grouped into superfamilies based on sequence and structural similarities. This review presents an overview of how molecular biological approaches have revealed a plethora of receptor subtypes, led to new definitions of subtypes and isoforms, and played a role in the development of high selective drugs. Moreover, the diversity of subtypes has molded current views of the structure and function of receptor families. Practical difficulties and limitations inherent in the characterization of the ligand binding and signaling properties of expressed recombinant receptors are discussed. The importance of evaluating drug-receptor interactions that differ with temporally transient and distinct receptor conformational states is emphasized. Structural motifs and signal transduction features are presented for the following major receptor superfamilies: ligand-gated ion channel, voltage-dependent ion channel, G-protein coupled, receptor tyrosine-kinase, receptor protein tyrosine-phosphatase, cytokine and nuclear hormone. In addition, a prototypic receptor is analyzed to illustrate functional properties of a given family. The review concludes with a discussion of future directions in receptor research that will impact drug discovery, with a specific focus on orphan receptors as targets for drug discovery. Methods for classifying orphan receptors based upon homologies with members of existing superfamilies are presented together with molecular approaches to the greater challenge of defining their physiological roles. Besides revealing new orphan receptors, the human genome sequencing project will result in the identification of an abundance of novel receptors that will be molecular targets for the development of highly selective drugs. These findings will spur the discovery and development of an exciting new generation of receptor-subtype specific drugs with enhanced therapeutic specificity.

Neuroleptic binding to sigma receptors: possible involvement in neuroleptic-induced acute dystonia

Several antipsychotic drugs, belonging to various chemical classes, were compared for their affinity for the sigma, dopamine-D2, and muscarinic receptors. Many neuroleptic drugs were found to bind with high affinity to sigma 2 receptors, and the binding affinity was clearly different from that observed for dopamine-D2 receptors. The dopaminergic and muscarinic theories for the physiopathology of acute dystonia are not completely satisfactory. Since the sigma receptors were reported to play a role in the control of movement, the high affinity of some neuroleptics for these sites suggests their possible involvement in some side effects, such as drug-induced dystonia. There was a correlation between the clinical incidence of neuroleptic-induced acute dystonia and binding affinity of drugs for the sigma receptor, except for some drugs, with a lower incidence, displaying significant affinity for the cholinergic muscarinic receptor. Therefore, we conclude that the affinity for the sigma receptor might be involved in neuroleptic-induced acute dystonia, but this might be partially corrected by the intrinsic anticholinergic properties of the drug.

The atypical antipsychotic profile of NRA0045, a novel dopamine D4 and 5-hydroxytryptamine2A receptor antagonist, in rats

1. The atypical antipsychotic profile of (R)-(+)-2-amino-4-(4-fluorophenyl)-5-[1-[4-(4-fluorophenyl)-4-oxobutyl] pyrrolidin-3-yl] thiazole (NRA0045), a potent dopamine D4 and 5-hydroxytryptamine (5-HT)2A receptor antagonist, was examined in rats. 2. Spontaneous locomotor activity was decreased dose-dependently with i.p. administration of clozapine (ED50 3.7 mg kg-1), haloperidol (ED50 0.1 mg kg-1) and chlorpromazine (ED50 0.9 mg kg-1), whereas inhibition of this type of behaviour induced by i.p. administration of NRA0045, at doses up to 10 mg kg-1, did not exceed 50%. 3. Locomotor hyperactivity induced by methamphetamine (MAP, 2 mg kg-1, i.p.) in rats (a model of antipsychotic activity) was dose-dependently antagonized by NRA0045 (ED50 0.4 mg kg-1, i.p., and 0.3 mg kg-1, p.o., respectively), clozapine (ED50 0.3 mg kg-1, i.p. and 0.8 mg kg-1, p.o., respectively), haloperidol (ED50 0.02 mg kg-1, i.p. and 0.1 mg kg-1, p.o., respectively), chlorpromazine (ED50 0.3 mg kg-1, i.p. and 3.3 mg kg-1, p.o., respectively). In contrast, the MAP (3 mg kg-1, i.v.)-induced stereotyped behaviour in rats (a model of extrapyramidal symptoms) was not affected by NRA0045 or clozapine, at the highest dose given (30 mg kg-1, i.p.). Haloperidol (ED50 0.3 mg kg-1, i.p.) and chlorpromazine (ED50 4.8 mg kg-1, i.p.) strongly blocked the MAP-induced stereotyped behaviour. NRA0045 and clozapine selectively blocked behaviour associated with activation of the mesolimbic/mesocortical dopamine neurones rather than nigrostriatal dopamine neurones. 4. Extracellular single-unit recording studies demonstrated that MAP (1 mg kg-1, i.v.) decreased the firing rate in the substantia nigra (A9) and ventral tegmental area (A10) dopamine neurones in anaesthetized rats. NRA0045 completely reversed the inhibitory effects of MAP on A10 dopamine neurones (ED50 0.1 mg kg-1, i.v.), whereas the inhibitory effects of MAP on A9 dopamine neurones were not affected by NRA0045, in doses up to 1 mg kg-1 (i.v.). Clozapine completely reversed the inhibitory effects of MAP on A10 dopamine neurones (ED50 1.9 mg kg-1, i.v.) and on A9 dopamine neurones (ED50 2.5 mg kg-1, i.v.). Haloperidol completely reversed the inhibitory effects of MAP on A10 (ED50 0.03 mg kg-1, i.v.) and on A9 dopamine neurones (0.02 mg kg-1, i.v.). NRA0045, like clozapine, was more potent in reversing the effects of MAP on A10 than A9 dopamine neurones. 5. Prepulse inhibition (PPI) is impaired markedly in humans with schizophrenia. The disruption of PPI in rats by apomorphine (0.5 mg kg-1, s.c.) was reversed significantly by NRA0045 (3 mg kg-1, i.p.), clozapine (3 mg kg-1, i.p.) and haloperidol (0.3 mg kg-1, i.p.). 6. Phencyclidine (PCP) elicits predominantly psychotic symptoms in normal humans and in schizophrenics. NRA0045 (0.03-0.3 mg kg-1, i.p.) and clozapine (0.1-1 mg kg-1, i.p.) significantly and dose-dependently shortened the PCP(1.25 mg kg-1, i.p.)-induced prolonged swimming latency in rats in a water maze task, whereas haloperidol (0.01-0.1 mg kg-1, i.p.) did not significantly alter swimming latency. 7. These findings suggest that NRA0045 may have unique antipsychotic activities without the liability of motor side effects typical of classical antipsychotics.

Autoradiographic localization of the putative D4 dopamine receptor in rat brain

The putative dopamine D4 receptor protein in rat brain was labelled and quantified autoradiographically using two selective benzamides: [3H]YM-09151-2 which labels D2, D3 and D4 dopamine receptors and [3H]Raclopride which labels D2 and D3. The difference in densities of both ligands at saturable concentrations, show a regional distribution for the putative D4 receptor in the following rank order: hippocampus > caudate putamen > olfactory tubercle = substancia nigra > nucleus accumbens core > cerebral cortex > cerebellum. A calculated value of 0.34 pmol/mg protein was attributable to D4 receptor maximum capacity in caudate putamen and was obtained after subtracting the Bmax of the ligands. Our results show that the distribution of D4 receptor only partially overlaps with the D4 mRNA localization reported earlier and is not only associated to limbic structures but to motor areas as well.

Distribution of D4 dopamine receptor in rat brain with sequence-specific antibodies

The distribution of the dopaminergic D4 receptor in rat brain was studied employing site directed polyclonal antibodies. Antisera were raised in rabbits to two oligopeptides corresponding to amino acids 160-172 of the second extracellular loop (P1) and amino acids 260-273 of the third intracellular loop (P2) of the D4 receptor sequence. Affinity-purified antibodies (anti-P1 and anti-P2) specifically recognized two major bands of 42-45 and 95 kDa in Western blots of denatured preparations of various rat brain areas. Immunocyto-chemistry studies showed that D4 receptor is widely distributed in rat central nervous system (CNS) showing higher labelling in the hippocampus (CA1, CA2, CA3 and dentate gyrus) frontal cortex, entorhinal cortex, caudate putamen, nucleus accumbens, olfactory tubercle, cerebellum, supraoptic nucleus and sustancia nigra pars compacta. In addition, anti-P1 decreased the binding of the antagonist [3H]YM-09151-2 selective for D2, D3 and D4 receptors but did not modify the binding of [3H]raclopride an antagonist selective for D2 and D3, in striatal synaptosomes. Anti-P2 did not modify the binding of these ligands. These results confirm the selectivity of the antibodies towards the D4 receptor and suggest that the binding site for the antagonists might be located at or close to the second extracellular loop of the protein sequence. D4 receptor protein is mainly expressed in plasma membranes and in the peripheral cytoplasm of neurons and is more widely distributed than was originally proposed based on mRNA localization, since it is present both in limbic, diencephalic and motor areas of rat brain.

Substituted 4-aminopiperidines having high in vitro affinity and selectivity for the cloned human dopamine D4 receptor

We have discovered two substituted 4-aminopiperidine compounds having high in vitro affinity and selectivity for the human dopamine D1 receptor. Both compounds, 3-ethoxy-N-methyl-N-[1-(phenylmethyl)-4-piperidinyl]-2-pyridinylamine (U-99363E), and its 3-isopropoxy analog (U-101958), were found through a routine receptor binding screen. The determined affinities (Ki) of these compounds for the cloned human dopamine D4 receptor were 2.2 and 1.4 nM, respectively. They exhibited at least 100-fold lower affinities for dopamine D2 and for other dopaminergic, serotonergic and adrenergic receptors. Both compounds were found to antagonize quinpirole-induced mitogenesis in Chinese hamster ovary cells expressing the human dopamine D4 receptor. In spite of their poor metabolic stability and low bioavailability. U-99363E and U-101958 appear to be among the first high-affinity, highly selective dopamine D4 receptor antagonists reported, and may have utility in in vitro investigations requiring selective tagging or blockade of dopamine D4 sites.

Quantification of neuroreceptors in the living human brain: IV. Effect of aging and elevations of D2-like receptors in schizophrenia and bipolar illness

In a previous study of 10 drug-naive schizophrenic patients, the density of D2 dopamine receptors was found to be elevated in the caudate nucleus. The study raised questions about the influence of the age of the patients, the relationship of receptor density to psychosis, and the accuracy of the method used to obtain this evidence. Using positron emission tomography and constrained analysis of the brain uptake of the radioligand N-[11C]methyl-spiperone ([11C]NMSP), we tested four questions: Were the assumptions underlying the quantitation valid? Is there an age decline of the density of D2-like dopamine receptors in drug-naive schizophrenia and bipolar illness? If so, is it different from that observed in normal aging? Are D2-like dopamine receptors elevated at any age in either drug-naive schizophrenic or psychotic bipolar illness patients? NMSP and haloperidol partition volumes and plasma protein fractions were not significantly different among patient groups and normal volunteers. The model-derived assay of radioligand metabolites in plasma was confirmed by high-performance liquid chromatography in the patient groups. D2-like dopamine receptors declined with age, and the slope did not differ significantly between the schizophrenic patients, bipolar affective illness patients, and normal controls. Taking the effect of age into account, increases in D2 dopamine receptor density were found in seven psychotic patients with bipolar affective illness compared with seven nonpsychotic patients and 24 control subjects as well as in 22 drug-naive schizophrenic patients compared with the 24 control subjects.

Mechanism of action of clozapine-induced modification of motor behavior in an animal model of the "super-off" phenomenon

We tested the effects of clozapine, and "atypical" neuroleptic with high affinity for the D4 (dopaminergic), and the 5-HT1c and 5-HT2 (serotonergic) receptor subtypes on locomotor activity in an animal model of Parkinson's disease showing a bimodal response curve to increasing doses of a D2 agonist. Sulpiride (D2 antagonist) and ritanserin (5-HT1c and 5-HT2 antagonist) were used for comparison. The D1 agonist SKF 38393 at a dose of 8 mg/kg significantly reversed the akinesia induced by chronic reserpine treatment (1 mg/kg for 5 days) and alpha-methyl-p-tyrosine pretreatment (300 mg/kg). In this model, the addition of a low dose of a D2 agonist, LY 171555 (quinpirole, 1 microgram/kg), inhibited the effects of SKF 38393, whereas the same drug at higher doses (5-50 microgram/kg) restored and potentiated the stimulatory response to D1 stimulation. Clozapine inhibited the inhibitory phase and potentiated the stimulatory phase of the curve. Sulpiride inhibited both phases of the dose-response curve (inhibitory/stimulatory), whereas ritanserin had no effect. We believe these results may reflect a disinhibition phenomenon possible mediated by the blockade by clozapine of a subpopulation of inhibitory, dopamine (DA) receptors belonging to the D2 "family."