Dopamine D4 receptors - Nature

Molecular Psychiatry (1999) 4, 529–538  1999 Stockton Press All rights reserved 1359–4184/99 $15.00

MILLENNIUM ARTICLE

Dopamine D4 receptors: significance for molecular psychiatry at the millennium FI Tarazi1,2 and RJ Baldessarini1,2 1

Mailman Research Center, McLean Division of Massachusetts General Hospital, Belmont, MA 02478; 2Consolidated Department of Psychiatry & Neuroscience Program, Harvard Medical School, Boston, MA 02115, USA Extraordinary progress has been made in the molecular, genetic, anatomical, and pharmacological characterization of dopamine D4 receptors in animal and human brain. Clarification of the neurochemical and physiological roles of these cerebral receptors is emerging. Postmortem neuropathological studies have inconsistently linked D4 receptors to psychotic disorders, and genetic studies have failed to sustain conclusive associations between D4 receptors and schizophrenia. However, associations are emerging between D4 receptors and other neuropsychiatric disorders, including attention deficit hyperactivity disorder, mood disorders, and Parkinson’s disease, as well as specific personality traits such as novelty-seeking. Selective D4 agonists and antagonists have been developed as useful experimental probes. D4 antagonists, so far, have proved ineffective in treatment of schizophrenia, but testing in a broader range of disorders may yield clinically useful drugs. D4 receptors appear to have broad implications for the pathophysiology of neuropsychiatric illnesses and their improved treatment. Keywords: antipsychotics; attention deficit hyperactivity disorder; D4; dopamine; mRNA; Parkinson’s disease; psychotic disorders; receptors; schizophrenia

Dopamine (DA) D4 receptors are members of D2-like DA receptor family, which also includes D2 and D3 receptors. Other well characterized DA receptors (D1, D5) constitute a D1-like family. Members of each family share similar molecular structures and pharmacological profiles, but differ greatly in their cerebral abundance and anatomical distribution.1,2 In the past decade, there have been remarkable advances in understanding the molecular, genetic, anatomical, and pharmacological characteristics of one D2-like DA receptor type—the D4 receptors—following their initial description by Van Tol and his colleagues in 1991.3

Cloning of dopamine D4 receptor genes The human D4 receptor gene (D4DR) was initially cloned from human neuroblastoma cells using probes complementary to mRNA sequences required to synthesize D2 receptor proteins, and was found to encode a protein product typically containing 387 amino acids.3 The D4DR gene was soon localized to region 15.5 of the long (q) arm of human chromosome 11.4 D4 membrane proteins, like other monoaminergic neurotransmitter receptors, are organized topographically into seven relatively hydrophobic, and putatively transmembrane-spanning segments (TM) joined by three extracellular and three intracellular peptide segments (IL), Correspondence: Dr FI Tarazi, Mailman Research Center, McLean Hospital, Harvard Medical School, 115 Mill Street, Belmont, MA 02478, USA. E-mail: ftarazi얀hms.harvard.edu Received 17 August 1999; accepted 24 August 1999

with an initial external amino terminus and final intracellular segment ending with a carboxy moiety. The third intracellular cytoplasmic loop (IL3) of the receptor peptide chain between the proposed fifth and sixth transmembrane segments (TMV, TMVI) and the intracytoplasmic carboxyl terminal segment are believed to couple to G-proteins and interact with other molecular elements involved in DA-mediated synaptic neurotransmission.3 Indeed, the molecular differentiation of the five main members of the DA receptor family is based largely on the length and amino acid sequences of IL3 and the intracellular carboxy terminal segment.1 The D4 receptor protein contains several post-translationally modifiable elements common to D2-like DA receptors, including at least one potential site for Nlinked glycosylation in the extracellular amino terminus, several likely phosphorylation sites in IL3 that may act as targets for protein kinases A and C, a cysteine residue at the end of the carboxyl terminal segment, two serine residues in the third cytoplasmic domain that may provide positive charge involving in ‘docking’ of the electronegative catechol groups of DA, and two acidic aspartate residues within TMII and TMIII that may account for docking of the amino group of DA.1,3 Additional features include disulfide bridging of TMIII and TMIV that may contribute to the stability of a proposed cup-like arrangement of the D4 receptor protein in the lipid membrane environment.5 The D4 receptor peptide sequence also contains several putative domains (the Src homology 3 [SH3] binding regions that strongly interact with small adapter peptides, including Grb2 and Nck, required for full func-

Molecular pharmacology of dopamine D4 receptors FI Tarazi and RJ Baldessarini

530

tional activity of G-protein coupled receptors.6 Finally, a methionine residue located in TMVI and adjacent to phospholipid headgroup may be involved in facilitating the methylation of membrane phospholipids.7 The rat D4 gene was cloned after the initial work with the human DRD4 gene,8 and found to encode a slightly shorter protein of 368 amino acids (19 fewer) than the human D4 receptor, and without a 48-amino acid repeat sequence found in the human IL3 region. The rat D4 receptor also has one fewer exons (n = 4) and introns (n = 3) than the human D4DR. However, it shares with the human D4 receptor 73% overall sequence homology and 89% homology within the seven putative TM domains.8

Polymorphisms of dopamine D4 receptor genes During its synthesis, the human D4 receptor protein can be transcribed into different polymorphic variants that differ in the number of repeats of a small, 48-amino acid sequence within the functionally critical IL3. There can be 2–11 repeats, but 2, 4 or 7 are relatively common (D4.2, D4.4, D4.7).9,10 The presence of these polymorphic variants in the human D4DR has not been found in D4 genes in other species.9,11 Pharmacological consequences of these structural variants are not well defined, although patients carrying the common shorter variants of D4 receptors (D4.2, D4.4) may be more sensitive and responsive to the therapeutic effects of typical neuroleptics than those carrying the D4.7 allele.12 Several genetic linkage studies have tried to associate D4DR variants with specific neuropsychiatric disorders. Most have failed to find conclusive evidence of linkage between D4DR and schizophrenia, Tourette’s syndrome, bipolar manic-depressive disorder, alcoholism or drug abuse.13–19 Other studies have suggested associations between the 7-repeat allele of D4DR and disorders including attention deficit hyperactivity disorder (ADHD),20–22 major depressive disorder,23 and Parkinson’s disease,24 or personality traits including ‘novelty seeking’.25,26 All of these potentially interesting proposals concerning associations of the D4DR gene and its structural variants to clinical conditions remain inconclusive, in part, perhaps, owing to variations in the statistical analyses and other methods used.19

Expression of brain D4 receptor mRNA Considerable variation has been found in D4 receptor mRNA expression between brain regions and species. In rat brain, the highest concentrations of D4 mRNA arise in frontal cortex, thalamus and hypothalamus.3,8 Mouse brain has highest levels in cerebellum, followed by frontal cortex, caudate-putamen (CPu) and thalamus.27 In monkey brain, relatively high levels of D4 mRNA expression occur in frontal cortex, amygdala and medulla oblongata. In human brain, localization of D4 mRNA is more controversial, as several studies attempting to define its regional distribution in undis-

eased human brain tissue have yielded inconsistent results.28–32 Overall, these studies suggest that greater expression of D4 mRNA occurs in cortical and limbic regions of human forebrain, with variable levels in cerebellum and thalamus, and substantially lower amounts in the basal ganglia. Inconsistencies in estimating D4 mRNA levels have also arisen in studies comparing postmortem brain tissue from patients diagnosed with schizophrenia or normal controls. One study found significant elevation of D4 mRNA expression in frontal cortex of schizophrenic patients,33 two others found no differences,34,35 and a fourth found substantially reduced levels in the orbitofrontal cortex of schizophrenic patients.31 Such inconsistencies may arise from multiple sources, including clinical heterogeneity in both patients and controls, and variance in postmortem delay, tissue handling, and assay procedures.

Detection of brain D4 receptor protein Radioligand assays D4 receptor proteins initially were quantified in human postmortem brain tissue by subtracting the number of binding sites defined with [3H]raclopride, a selective D2/D3 receptor antagonist, from the total defined with nonselective D2/D3/D4 inhibitors, [3H]nemonapride or [3H]spiperone. These methods initially detected substantial levels of D4 receptor binding in normal postmortem human CPu,36–38 but were disconfirmed by others.39,40 A recent autoradiographic study with a proposed, experimental D4-selective agent [3H]NGD 94–1 found relatively high levels of radioligand binding in normal postmortem human hippocampus and entorhinal cortex, and low, but detectable, levels in CPu and nucleus accumbens (NAc).41 In diseased postmortem human tissue, several studies using such radioligand binding methods reported significantly higher D4 receptor-density in CPu tissue from subjects diagnosed with schizophrenia than in normal controls.36–38 Again, others failed to replicate these findings.40,42,43 A recent analysis with the D4selective radioligand [3H]NGD 94–1, detected no changes in radioligand binding in postmortem CPu tissue from brains of normal controls and patients with schizophrenia.41 These findings require further verification with other highly D4-selective radioligands as they become available, and testing for specificity in comparisons with other subjects diagnosed with major mental illnesses other than schizophrenia. In rat brain tissue, we estimated concentrations of D4 receptor proteins with [3H]nemonapride or [3H]spiperone in the presence of masking concentrations of unlabeled raclopride to occlude D2/D3 receptors, and addition of agents to prevent binding to other nonspecific sites.44–47 This method identified a subset of raclopride-insensitive binding sites (‘D4-like’ receptors) in rat brain tissue that were largely displaced by the D4selective agent L-745,870, indicating that most sites labeled under such conditions represent D4 receptors.44 Such binding was found mainly in rat hippocampus


and entorhinal and frontal cortex, with less in CPu and NAc. Assays with D4-selective [3H]NGD 94–1 also found D4 receptors mainly in rat cerebral cortex and hippocampus, with less in the basal ganglia.48 D4 receptors in rat CPu seem to have a particularly intriguing anatomical localization. We found that a subset of them co-localizes with glutamate receptors of the N-methyl-d-aspartic acid (NMDA) type on terminals of corticostriatal projections innervating the basal ganglia, or perhaps on parallel fibers projecting from cortical layer V where the majority of glutamatergic corticostriatal projections originate.46,49,50 This proposed co-localization of NMDA and D4 receptors seems to be limited to terminals of corticostriatal projections, but not the projections from hippocampus to CPu and NAc.51 Thus, hippocampal lesioning localized NMDA and kainate, but neither D4 nor other DA receptors on rat hippocampostriatal projection terminals.52,53 Degeneration of DA afferents projecting from substantia nigra to CPu further indicated that D4 autoreceptors are unlikely to be found with presynaptic D2/D3 autoreceptors or ionotropic glutamate NMDA, AMPA and kainate heteroceptors at nigrostriatal projection terminals.46,49,50,54 Immunological assays Development of D4-selective antibodies provides another approach to selective localization of D4 receptor proteins in brain tissue. A specific monoclonal antibody raised against the extracellular amino-terminal of the human D4 receptor labeled cerebral cortex and substantia nigra.55 In nonhuman primate brain, another D4 receptor-antibody raised against the amino terminal of the human D4 receptor labeled GABA-containing interneurons, and pyramidal glutamatergic neurons in cerebral cortex, hippocampus, thalamic reticular nucleus, and globus pallidus.56 In rat brain, D4-selective polyclonal antibodies generated against a specific fragment in the amino terminal of rat D4 receptor indicated significant densities of D4 receptors in frontal cortex, followed by thalamus, globus pallidus, and hippocampus, with lesser amounts in CPu.57 Another immunocytochemical study using different D4 antibodies generated against a selective segment from the third intracellular loop of rat D4 receptor indicated a wider distribution of D4 receptors in rat brain, including substantial levels in CPu and NAc.58 Such differences in D4 receptor localization may stem from differences in the specific epitopic peptide sequences used to generate antibodies against D4 receptor protein. Positron-emission tomography (PET) PET imaging was recently introduced for labeling D4 receptors in vivo. In primate brain, specific binding signals for D4 receptors were detected by PET in frontal, temporal, parietal and occipital lobes of cerebral cortex, as well as in CPu.59 A complex, indirect approach visualized D4 receptors by combining a non-selective DA receptor antagonist [11C]SDZ-GLC 756 with saturating concentrations of SCH-23390 and raclopride to occlude D1 and D2/D3 receptors, respectively. Clearly,

there is still a need for selective D4-radioligands that can be applied in neuroradiologic studies to visualize and quantify D4 receptors in living human brain, as has been done with the much more abundant cerebral D1 and D2 receptors.17

Effector responses to D4 receptors DA has high affinity for the three main human recombinant D4 receptors: D4.2, D4.4 and D4.7 (KD = 0.9 nM)60 as well as for rat D4 receptors (KD = 3 nM).11 However, stimulation of D4 receptors is not entirely specific to DA or DA-selective agonists. Both (−)-epinephrine (EPI) and (−)-norepinephrine (NE) also bind with high affinity to human D4 receptors (Ki = 14 and 33 nM, respectively), although DA remained more potent.60 In addition, submicromolar concentrations of DA, EPI and NE stimulated GTP-binding in human CHO-D4 receptor-containing membranes, and decreased forskolin-stimulated cyclic-AMP (cAMP) levels in CHO-D4.4 cells.60 These findings implicate D4 receptors in a wider range of signal transduction pathways than has been proposed for other DA receptor subtypes, all of which are more selective for DA. The common human D4 receptor variants D4.2, D4.4 and D4.7 have similar pharmacological binding profiles except for the atypical antipsychotic clozapine which was twice as potent at the shorter D4.2 and D4.4 variants compared to the longer D4.7 variant.61 When expressed by transfection of cultured carrier cells, the three variants were converted to a low-affinity binding state in the presence of the stable, nonhydrolyzable GTP analog Gpp(NH)p.11 This observation indicates that they, like D1 and D2 receptors, probably function through a Gprotein/receptor coupling mechanism.1 Furthermore, stimulation of D4 receptors by DA and DA agonists has produced concentration-dependent inhibition of synthesis of cAMP by adenylyl cyclase in transfected cells expressing D4 receptors.61,62 Moreover, such responses can vary with the number of repeat sequences in human D4 receptors: DA was only half as potent in inhibiting adenylyl cyclase in cells expressing the human D4.7 receptors compared to the shorter and more sensitive D4.2 and D4.4 variants.61 Effects of D4 receptors are not limited to inhibiting the adenylyl cyclase cascade. Other effects of activating D4 receptors include: (1) stimulating release of arachidonic acid in D4-transfected cells;62 (2) blocking the Ltype calcium current in cultured granule cells from neonatal rat cerebellum;63 (3) inhibiting potassium currents in nerve terminals of the neurohypophysis;64 and (4) facilitating membrane phospholipid methylation in cultured neuroblastoma cell lines.7 In contrast to their ability to reduce cAMP production in mammotrophic cell lines derived from pituitary lactotrophs, stimulation of the three main human D4 receptor variants in transfected mammotrophs neither altered the activity of the prolactin promoter region necessary for regulation of prolactin gene transcription nor increased prolactin secretion.65 Studies of genetically altered, D4-knockout mice also

531


532

support a role for D4 receptors in modulating normal and psychostimulant-induced motor behaviors. Notably, such D4-deficient mice were found to be more responsive to locomotor-stimulating effects of ethanol, cocaine, and methamphetamine.66 This genetic approach, however, has potentially serious limitations, including genetic dysequilibrium as well as complex neurodevelopmental adaptations tending to compensate for the loss of the targeted protein. Such pervasive genetic alterations may yield models that are physiologically nonequivalent to pharmacological or chemical modification or blockade of a receptor in a normally developed animal. Pharmacological manipulations of D4 systems now possible include use of a growing series of D4-selective antagonists described below. In addition, we found recently that D4 receptors can be selectively and dose-dependently alkylated in rat brain tissue and transfected cells by both the nonspecific agent EEDQ (1-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline) and the D2-like receptor-selective agent NIPS (N-[p-isothiocyanatophenethyl]-spiperone).67 Such agents may help in further molecular, cellular, and other functional characterizations of cerebral D4 receptors.

D4 receptors and antipsychotic drugs Several laboratories have confirmed the initial observation by Van Tol and his colleagues3 that the unusual antipsychotic agent clozapine has some selectivity for D4 over D2 and other DA receptors.45,68,69 These findings stimulated the hypothesis that actions by clozapine at D4 receptors may contribute to its status as an atypical and unusually effective antipsychotic agent. Clozapine virtually lacks risk of adverse extrapyramidal motor effects (dystonia, parkinsonism, tardive dyskinesias) characteristic of most typical neuroleptics, and remains unique in having substantial evidence of superior effectiveness to standard antipsychotics, particularly in patients with treatmentresistant schizophrenia and perhaps severe schizoaffective and bipolar disorders.70–72 Olanzapine, a recently introduced structural analog of clozapine, shares with clozapine some D4-over-D2 selectivity.68,73 However, other clozapine-like analogs including quetiapine and loxapine do not share the D4over-D2 selectivity.68,73 In addition, other innovative antipsychotic drugs including melperone, remoxipride, risperidone, sertindole and ziprasidone are not D4- selective, indicating that preferential selectivity for D4 receptors does not uniquely distinguish atypical antipsychotics from typical neuroleptics.68,69,74 Many of these other atypical agents are, notably, also strongly antiserotonergic.75 In addition, some S(+) enantiomers of the classic DAagonist R(−)-aporphines, and particularly S(+)-N-n-propylnorapomorphine (S[+]-NPA), also show moderate selectivity for D4 over D2 receptors.45,76,77 These compounds have several behavioral and neurochemical properties that support their candidacy as potential atypical antipsychotic agents with a low risk of extra-

pyramidal side effects.77–80 D4 affinity and selectivity (over D2) of representative antipsychotic agents are summarized in Table 1. Repeated treatment with the typical neuroleptics fluphenazine, haloperidol and raclopride, the atypical antipsychotic clozapine, or the experimental atypical antipsychotic S(+)-NPA all produced D4 receptor-upregulation in rat CPu and NAc.45,47,81–83 Increasing D4 binding by both typical and atypical antipsychotic agents suggests that D4 receptors represent a common site of action of antipsychotic drugs that may contribute selectively to their antipsychotic effects.45,47,82,83 Inconsistently reported elevation of D4 receptors in postmortem brain tissue (usually basal ganglia) from patients with schizophrenia36–38 also may reflect adaptations to variable long-term antipsychotic drug exposure, rather than representing a component of the neuropathology of the disease. Upregulation of D4 receptors in rat brain seems to be limited to the CPu and NAc, since identical antipsychotic drug treatment failed to upregulate D4 receptors significantly in rat frontal cortex.45,47,81–84 These anatomical differences may reflect regional differences in adaptive responses of D4 mRNA to antipsychotic drugs, as evidenced by the ability of haloperidol to increase expression of D4 mRNA selectively in rat striatum but not in frontal cerebral cortex.81 Further studies are required to examine long-term effects of other antipsychotic drugs, including clozapine, on D4 mRNA expression in specific brain regions and different species. In contrast to their regionally selective effects on D4 receptors, typical and atypical antipsychotics have all upregulated D2 receptors in cerebral cortex of both rats and primates.45,47,81,85 Therefore, cortical D2 receptors, in addition to the striatolimbic D4 receptors, are Table 1 Affinity of representative antipsychotic agents at D4 and D2 receptors Agents

S(+)-NPA Clozapine Olanzapine Melperone Loxapine Quetiapine Risperidone Sertindole Ziprasidone Chlorpromazine Fluphenazine Haloperidol

Receptor potencya D4

D2

56 40 28 230 12 1600 16 21 39 12 9 10

774 190 31 199 8 700 5.9 7 4.6 1.2 0.5 0.5

D4 vs D2 selectivity

13.82b 4.75c 1.11c 0.87d 0.67d 0.44c 0.37c 0.33c 0.12c 0.10d 0.06d 0.05d

a Receptor potency = inhibitory concentration constants (Ki, in nM units) in competitive radioreceptor assays with rat or human brain tissue or transfected cell membranes, corrected for radioligand affinity, and ranked by D4 selectivity. NPA is N-n-propylnorapomorphine. b Ref. 45; c Ref. 73; d Ref. 68.


533

Figure 1

Antagonists with selectivity for D4 receptors.

candidates as common sites of antipsychotic drug action, that might guide development of novel psychotropic agents.45,47,86

D4-selective agents A growing number of novel compounds with much greater D4 selectivity over other DA receptors than that of clozapine have been developed recently.18,87,88 Among the earliest D4-selective antagonists reported were L-745,870,89 and its iodinated congener L750,667.90 Other chemically novel D4-selective agents usually considered D4 antagonists (Figure 1), in their approximate order of introduction, include: YM-50001 and YM-43611;91,92 PNU-101387G and PNU-101958;93 as well as RBI-257, the p-iodinated derivative of PNU101958;94,95 NRA-0045;96,97 NGD 94–1;98 PD-172,938;99 S 18126;100 and CP-293,019.101 A potent serotonin 5HT2A antagonist, fananserin, is also relatively D4-selective.102 In addition, at least two novel D4 receptor agonists have been developed (Figure 2): PD-168,077103 and CP-226,269104 although the balance of antagonist/ agonist activity of some agents usually considered antagonists has been challenged recently,105,106 and whereas selectivity for D4 over other D2-like receptors

Figure 2

Agonists with selectivity for D4 receptors.

has held up, interactions at other sites have emerged.18 These include high affinity of L-745,870, L-750,667 and PNU-101958 for so-called sigma sites (␴1 ‘receptors’), whose functional status remains unclear.90,95,107 All of the new D4-selective compounds exhibit high D4 receptor-affinity (0.3–8.7 nM Ki values), as well as high selectivity for D4 sites over other DA receptor types (Table 2). One of these compounds has been tritiated ([3H]NGD 94–1) and used for autoradiographic visualization of D4 receptors in rat48 and human postmortem brain tissue.41 However, this radioligand has limitations in terms of its relatively low specific radio activity, long periods of exposure to tritium-sensitive films (3 months in rats),48 and it is not available commercially. Three other D4-selective radioligands


534

Table 2

Affinity (Ki, nM) of novel compounds at dopamine D4 receptors

Compounds Agonists CP-226,269 PD-168,077 Antagonists RBI-257 L-745,870 L-750,667 YM-43611 S 18126 NRA-0045 PNU-101958 CP-293,019 NGD 94-1 PNU-101387G YM-50001 PD-172,938

D1

D2

D3

D4

ND ND

1760 3740

ND 2810

6 8.7

2800 ND ⱖ4500 ⬎10 000 ⬎3000 22 ⬎10 000 ⬎1000 ⬎10 000 8300 ⬎10 000 ND

568 ⬎1700 ⬎1700 220 738 231 2000 ⬎3310 2230 5100 793 ⬎5800

145 ⬎4500 ⬎4500 21 2840 107 552 ⬎2000 ⬎10 000 2700 915 ⬎5000

0.30 0.43 0.51 2.1 2.4 2.5 2.7 3.4 3.4 3.6 5.8 7.8

D5

Reference

ND ND

104

⬎10 000 ND ⱖ4500 ⬎10 000 ⬎3000 2217 ⬎10 000 ND ⬎10 000 270 ND ND

94

103

89 90 92 100 96 94 101 98 93 91 99

Compounds are listed in rank-order of D4 affinity; ND: not determined.

([3H]PNU-101958, [125I]RBI-257, and [125I]L-745,870) failed to provide selective and well-localized binding signals that were clearly differentiated from nonspecific background labeling.94,95,108 [3H]PNU-101958 was also found to bind with high affinity at ␴1 sites in cultured neuroblastoma cells.95,107 The ␴1 sites probably can be masked with agents such as ditolyguanidine to avoid inaccurate interpretation of radioligand binding.95 Important challenges in developing radioligands for D4 receptors include: (1) sufficient D4-over-D2 selectivity to overcome the high prevalence of D2-over-D4 sites in most brain regions; and (2) ability to provide signals from low-abundance D4 receptors that are distinguishable from nonspecific background interactions. Further development of better D4-selective radioligands, or selective means of occluding D2 receptors, would be of great interest, and represent an important advance over current indirect methods for visualizing D4 receptors in brain tissues. Behavioral effects of several D4-selective compounds were evaluated in animal models believed to be predictive of antipsychotic activity, including blockade of motor hyperactivity induced by DA agonists (eg, R[−]apomorphine) and releasers (eg, [+]-amphetamine), inhibition of conditioned avoidance responses, inhibition of climbing elicited by apomorphine, or reversal of deficits in prepulse inhibition (PPI) of acoustic startle responses.109 Behavioral tests indicative of extrapyramidal side effects have also been used, including induction of catalepsy, inhibition of gnawing elicited by methylphenidate, and development of ‘vacuous-chewing’ jaw movements after long-term administration of a test agent.110,111 Both L-745,870 and S 18126 neither induced catalepsy nor inhibited methylphenidate-induced gnawing.100,112 However, they failed to induce significant changes in behavioral tests predictive of antipsychotic efficacy, suggesting that these compounds may neither display antipsy-

chotic-like activity nor induce adverse extrapyramidal motor effects.100,112 Moreover, pretreatment with L745,870 neither altered the discriminative stimulus property of cocaine nor modified the behavioral arousal induced by this stimulant.113 Other D4-selective agents showed various behavioral effects. For example, NGD 94–1 reversed deficits in an object retrieval-detour behavioral paradigm in monkeys produced by repeated pretreatment with the psychotomimetic drug phencyclidine.114 PNU-101387G blocked amphetamine-induced behavioral sensitization and apomorphine-induced PPI deficits, but did not affect acute behavioral activation induced by apomorphine or amphetamine.93,115 CP-293,019 and NRA0045 did not induce catalepsy or block apomorphineinduced stereotypy. In addition, both compounds attenuated hyperlocomotion induced by DA agonists and significantly reversed apomorphine-induced disruption of PPI.96,97,116,117 The findings suggest that these compounds may have potential for clinical psychotropic activity with a low risk of extrapyramidal motor effects associated with effective D2 receptor blockade that seems to be characteristic of all typical neuroleptics.1,17 Surprisingly, preliminary clinical trials with at least two agents with D4-over-D2 selectivity have failed to indicate any evidence of antipsychotic benefits. L745,870 showed lack of improvement, or even slight worsening, in psychotic symptoms of 26 hospitalized, acutely psychotic schizophrenic patients compared to 12 others given only a placebo.118 The results are hard to interpret since effects of carry-over or withdrawal of previous medication were not necessarily precluded by the brief (3- to 5-day) placebo run-in period involved. Fananserin (RP-62203; 2-[3-(4-[p-fluorophenyl]-1piperazinyl)-propyl]-2H-naphth[1,8-cd]isothiazole-1, 1-dioxide) is a potent antagonist at D4 and serotonin 5HT2A receptors;102 in a recent trial in 98 patients with


schizophrenia in which 63 patients were given fananserin vs 34 patients who received placebo, fananserin also showed no evidence of antipsychotic efficacy.119 Additional complications to interpreting the preceding preliminary clinical observations include evidence that L-745,870 is not entirely D4-selective. It is also a potent sigma-ligand, as well as exerting only subtle behavioral effects conventionally expected to predict antipsychotic activity.100,112 Moreover, the status of this and some other D4-over-D2-selective ligands as putative D4-antagonists has been questioned. L-745,870 blocked DA-stimulated GTP-binding in human D4.4-expressing CHO cells, suggesting that it may act as a D4 antagonist.100,120 However, another study found that L-745,870, similar to DA, inhibited forskolin-stimulated cyclicAMP accumulation in HEK-293 cells uniquely expressing human D4.4 receptors—a biochemical property expected of a D4 agonist.105 Two other putative D4antagonists (PNU-101958 and NGD 94–1) may also have agonist-like activities in the same cell line expressing human D4.4 receptors.105,106 Agonist activity may have contributed to the failure of clinical trials with L-745,870, but the failure of fananserin is not readily explained.

ation of the results open to further consideration. Moreover, relationships of the physiology of D4 systems and animal behavioral—let alone clinical—activity remains tentative and incomplete. The suggested genetic associations of the D4DR genes with various neuropsychiatric disorders, including attention deficit hyperactivity disorder, mood disorders, and Parkinson’s disease, suggest that D4 receptors may have broader implications for human illnesses than has been implied by early focus on clozapine as a D4-selective lead agent, and on schizophrenia as a clinical target. The exciting leads reviewed here should open new avenues of research on the basic and clinical aspects of D4 receptors in the 21st century, and emerging D4-selective compounds may yet prove useful for the treatment of neuropsychiatric diseases other than schizophrenia. Acknowledgements Supported by NIMH grants MH-19905, MH-34006, and MH-47370, a grant from the Bruce J Anderson Foundation, and by the Mailman Research Center Private Donors Neuropharmacology Research Fund. Based, in part, on previous reviews.17,18

Conclusions Cloning of the human D4 receptor gene D4DR and its different polymorphic alleles, clarification of the sites of expression of D4 mRNA and protein in mammalian brain, partial characterization of their second-messenger and other molecular actions, and identification of D4-selective drug molecules have rapidly advanced understanding of these novel DA receptors, even though their neurophysiological and behavioral roles in the brain remain unclear. Preferential regional localization of D4 receptors in cortical and limbic regions of the brain, and the lack of behavioral or clinical effects of proposed D4 antagonists suggestive of adverse extrapyramidal effects make them especially attractive targets for development of novel psychotropic drugs. Moreover, their special localization on the terminals of corticostriatal glutamatergic projections suggests that D4 receptors may normally control release of glutamate into the basal ganglia, including its limbic components, with the probability of a direct influence on potentially behaviorally important interactions of the dopamine and glutamate systems. The localization of D4 receptors to limbic and cortical systems of forebrain, as well as the selectivity of clozapine for D4 over other DA receptors, strongly encouraged interest in D4 receptors as a target for improved agents, and led to remarkably rapid development and even preliminary clinical testing of D4-selective drugs. Animal studies have also indicated that D4 receptor levels are modified by repeated treatment with clozapine and other antipsychotic agents. At least two clinical trials involving lead agents with D4-selectivity failed to improve the psychotic symptoms in patients with schizophrenia. However, both drugs have other complex pharmacological properties, leaving interpret-

References 1 Baldessarini RJ, Tarazi FI. Brain dopamine receptors: a primer on their current status, basic and clinical. Harvard Rev Psychiatry 1996; 3: 301–325. 2 Neve KA, Neve RL. The Dopamine Receptors. Humana Press: Totowa, New Jersey, 1997. 3 Van Tol HHM, Bunzow JR, Guan H-C, Sunahara RK, Seeman P, Niznik HB et al. Cloning of a human dopamine D4 receptor gene with high affinity for the antipsychotic clozapine. Nature 1991; 350: 614–619. 4 Gerlenter J, Kennedy JL, Van Tol HHM, Civelli O, Kidd KK. The D4 dopamine receptor (DRD4) maps to distal 11p close to HRAS. Genomics 1992; 13: 208–210. 5 Dahl S, Edvardsen O, Sylte I. Molecular dynamics of dopamine at the D2 receptor. Proc Natl Acad Sci USA 1991; 88: 8111–8115. 6 Oldenhof J, Vickery R, Anafi M, Oak J, Ray A, Schoots O et al. SH3 binding domains in the dopamine D4 receptor. Biochemistry 1998; 37: 15726–15736. 7 Sharma A, Kramer ML, Wick PF, Liu D, Chari S, Shim S et al. D4 Dopamine receptor-mediated phospholipid methylation and its implications for mental illnesses such as schizophrenia. Mol Psychiatry 1999; 4: 235–246. 8 O’Malley KL, Harmon S, Tang L, Todd RD. The rat dopamine D4 receptor: sequence, gene structure and demonstration of expression in the cardiovascular system. New Biologist 1992; 4: 137–146. 9 Van Tol HHM, Wu CM, Guan H-C, Ohara K, Bunzow JR, Civelli O et al. Multiple dopamine D4 receptor variants in the human population. Nature 1992; 358: 149–152. 10 Lichter JB, Barr CL, Kennedy JL, Van Tol HHM, Kidd KK, Livak KJ. A hypervariable segment in the human dopamine receptor D4 (DRD4) gene. Hum Mol Genet 1993; 2: 767–773. 11 Asghari V, Schoots O, Van Kats S, Ohara K, Jobanovic V, Guan HC et al. Dopamine D4 receptor repeat: analysis of different native and mutant forms of the human and rat genes. J Pharmacol Exp Ther 1994; 46: 364–373. 12 Cohen BM, Ennulat DJ, Centorrino F, Matthysse S, Konieczna H, Chu H-M et al. Polymorphisms of the dopamine D4 receptor and response to antipsychotic drugs. Psychopharmacology 1999; 141: 6–10. 13 Adamson MD, Kennedy J, Petronis A, Dean M, Virkkunen M, Linnoila M et al. DRD4 dopamine receptor genotype and CSF

535


536 14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32 33

34

monoamine metabolites in Finnish alcoholics and controls. Am J Med Genet 1995; 60: 199–205. Barr CL, Wigg KG, Zovko E, Sandor P, Tsui LC. No evidence for a major gene effect of the dopamine D4 receptor gene in the susceptibility to Gilles de La Tourette syndrome in Five Canadian families. Am J Med Genet 1996; 67: 301–305. De Bruyn A, Mandelbaum K, Sandkuijl L, Delvenne V, Hirsch D, Staner L et al. Nonlinkage of bipolar illness to tyrosine hydroxylase, tyrosinase, and D2 and D4 dopamine receptor genes on chromosome 11. Am J Psychiatry 1994; 151: 1202–1206. Macciardi F, Verga M, Kennedy J, Petronis A, Bersani G, Pancheri P. An association study between schizophrenia and the dopamine receptor genes DRD3 and DRD4 using haplotype relative risk. Hum Heredity 1994; 44: 328–336. Baldessarini RJ. Dopamine receptors and clinical medicine. In: Neve KA, Neve RL (eds). The Dopamine Receptors. Humana Press: Totowa, New Jersey, 1997, pp 457–498. Tarazi FI, Baldessarini RJ. Brain dopamine D4 receptors: current basic and clinical status. Int J Neuropsychopharmacol 1999; 2: 41–58. Paterson AD, Sunohara GA, Kennedy JL. Dopamine D4 receptor gene: novelty or nonsense? Neuropsychopharmacology 1999; 21: 3–16. Lahoste GJ, Swanson JM, Wigal SB, Glabe C, Wigal T, King N et al. Dopamine D4 receptor gene polymorphism is associated with attention deficit hyperactivity disorder. Mol Psychiatry 1996; 1: 121–124. Swanson JM, Sunohara GA, Kennedy JL, Regino R, Fineberg E, Wigal T et al. Association of the dopamine receptor D4 (DRD4) gene with a refined phenotype of attention deficit hyperactivity disorder (ADHD), a family-based approach. Mol Psychiatry 1998; 3: 38–41. Faraone SV, Biederman J, Weiffenbach B, Keith T, Chu MP, Weaver A et al. Dopamine D4 gene 7-repeat allele and attention deficit hyperactivity disorder. Am J Psychiatry 1999; 156: 768–770. Manki H, Kanba S, Muramatsu T, Higuchi S, Suzuki E, Matsushita S et al. Dopamine D2, D3 and D4 receptor and transporter gene polymorphisms and mood disorders. J Affect Disord 1996; 40: 7–13. Ricketts MH, Hamer RM, Manowitz P, Feng F, Sage JI, Dipaola R et al. Association of long variants of the dopamine D4 receptor exon 3 repeat polymorphism with Parkinson’s disease. Clin Genet 1998; 54: 33–38. Benjamin J, Li L, Patterson C, Greenberg BD, Murphy DL, Hamer DH. Population and familial association between the D4 dopamine receptor gene and measures of novelty seeking. Nature Genet 1996; 12: 81–84. Ebstein RP, Novick O, Umansky R, Priel B, Osher Y, Blaine D et al. Dopamine D4 receptor (D4DR) exon III polymorphism associated with the human personality trait of novelty seeking. Nature Genet 1996; 12: 78–80. Suzuki T, Kobayashi K, Nagatsu T. Genomic structure and tissue distribution of the mouse dopamine D4 receptor. Neurosci Lett 1995; 199: 69–72. Matsumoto M, Hidaka K, Tada S, Tasaki Y, Yamaguchi T. Fulllength cDNA and distribution of human dopamine D4 receptor. Mol Brain Res 1995; 29: 157–162. Matsumoto M, Hidaka K, Tada S, Tasaki Y, Yamaguchi T. Low levels of mRNA for dopamine D4 receptor in human cerebral cortex and striatum. J Neurochem 1996; 66: 915–919. Meador-Woodruff JH, Damask SP, Wang J, Haroutunian V, Davis KL, Watson SJ. Dopamine receptor mRNA expression in human striatum and neocortex. Neuropsychopharmacology 1996; 15: 17– 29. Meador-Woodruff JH, Haroutunian V, Powchik P, Davidson M, Davis KL, Watson SJ. Dopamine receptor transcript expression in striatum and prefrontal and occipital cortex. Focal abnormalities in orbitofrontal cortex in schizophrenia. Arch Gen Psychiatry 1997; 54: 1089–1095. Mulcrone J, Kerwin RW. The regional pattern of D4 gene expression in human brain. Neurosci Lett 1997; 234: 147–150. Stefanis NC, Bresnick JN, Kerwin RW. Schofield WN, McAllister G. Elevation of D4 dopamine receptor mRNA in postmortem schizophrenic brain. Mol Brain Res 1998; 53: 112–119. Roberts DA. Balderson D, Pickering-Brown SM, Deakin JFW. Owen F. The relative abundance of dopamine D4 receptor mRNA in post

35

36 37

38

39

40

41

42

43

44

45

46

47

48

49

50

51

52

53

54

mortem brains of schizophrenics and controls. Schizophrenia Res 1996; 20: 171–174. Mulcrone J, Kerwin RW. No difference in the expression of the D4 gene in post-mortem frontal cortex from controls and schizophrenics. Neurosci Lett 1996; 219: 163–166. Seeman P, Guan H-C, Van Tol HHM. Dopamine D4 receptors elevated in schizophrenia. Nature 1993; 365: 441–445. Murray AM, Hyde TM, Knable MB, Herman MM, Bigelow LB, Carter JM et al. Distribution of putative D4 dopamine receptors in postmortem striatum from patients with schizophrenia. J Neurosci 1995; 15: 2186–2191. Sumiyoshi T, Stockmeier CA, Overholser JC, Thompson PA, Meltzer HY, Dopamine D4 receptors and effects of guanine nucleotides on [3H] raclopride binding in postmortem caudate nucleus of subjects with schizophrenia or major depression. Brain Res 1995; 681: 109–116. Lahti RA, Roberts RC, Tamminga CA. D2-Family receptor distribution in human postmortem tissue: an autoradiographic study. NeuroReport 1995; 6: 2505–2512. Reynolds GP, Mason SL. Absence of detectable striatal dopamine D4 receptors in drug-treated schizophrenia. Eur J Pharmacol 1995; 281: R5–R6. Lahti RA, Roberts RC, Cochrane EV, Primus RJ, Gallager DW, Conley RR et al. Direct determination of dopamine D4 receptors in normal and schizophrenic postmortem brain tissue—A [3H]NGD 94–1 study. Mol Psychiatry 1998; 3: 528–533. Lahti RA, Roberts RC, Conley RR, Cochrane EV, Mutin A, Tamminga CA. D2-type dopamine receptors in postmortem human brain sections from normal and schizophrenic subjects. NeuroReport 1996; 7: 1945–1948. Helmeste DM, Tang SW, Bunney WE Jr, Potkin SG, Jones EG. Decrease in ␴ but no increase in striatal dopamine D4 sites in schizophrenic brains. Eur J Pharmacol 1996; 314: R3–R5. Tarazi FI, Kula NS, Baldessarini RJ. Regional distribution of dopamine D4 receptors in rat forebrain. NeuroReport 1997; 8: 3423– 3426. Tarazi FI, Yeghiayan Sk, Baldessarini RJ, Kula NS, Neumeyer JL. Long-term effects of S(+)N-n- propylnorapomorphine compared with typical and atypical antipsychotics: differential increases of cerebrocortical D2-like and striatolimbic D4-like dopamine receptors. Neuropsychopharmacology 1997; 17: 186–196. Tarazi FI, Campbell A, Yeghiayan SK, Baldessarini RJ. Localization of dopamine receptor subtypes in caudate-putamen and nucleus accumbens septi of rat brain: comparison of D1-, D2-, and D4-like receptors. Neuroscience 1998; 83: 169–176. Tarazi FI, Yeghiayan SK, Neumeyer JL, Baldessarini RJ. Medial prefrontal cortical D2-like and striatolimbic D4-like dopamine receptors: common targets for typical, atypical and experimental antipsychotics. Prog Neuro-Psychopharmacol Biol Psychiatry 1998; 22: 693–707. Primus RJ, Thurkauf A, Xu J, Yevich E, Mcinerney S, Shaw K et al. Localization and characterization of dopamine D4 binding sites in rat and human brain by use of the novel, D4 receptor-selective ligand [3H]-NGD-94–1. J Pharmacol Exp Ther 1997; 282: 1020– 1027. Tarazi FI, Baldessarini RJ. Regional localization of dopamine and ionotropic glutamate receptor subtypes in striatolimbic brain regions. J Neurosci Res 1999; 55: 401–410. Tarazi FI, Campbell A, Yeghiayan SK, Baldessarini RJ. Localization of glutamate receptor subtypes in caudate-putamen and nucleus accumbens septi: comparison of NMDA, AMPA and kainate receptors. Synapse 1998; 30: 227–235. Kelley AE, Domesick VB. The distribution of the projection from the hippocampal formation to the nucleus accumbens in the rat: an anterograde and retrograde horseradish peroxidase study. Neuroscience 1982; 7: 2321–2335. Tarazi FI, Campbell A, Baldessarini RJ. Effects of hippocampal kainic acid lesion on striatolimbic dopamine D1-, D2-, and D4-like receptors. Neuroscience 1998; 87: 1–4. Tarazi FI, Campbell A, Baldessarini RJ. Effects of hippocampal kainic acid lesion on striatolimbic ionotropic glutamate NMDA, AMPA and KA receptors. Neurosci Lett 1998; 250: 13–16. Tepper JM, Sun B-C, Martin LP, Creese I. Functional roles of dopa-


55

56

57

58

59

60

61

62

63

64

65

66

67

68

69

70 71

72

73

74

75

mine D2 and D3 autoreceptors on nigrostriatal neurons analyzed by antisense knockdown in vivo. J Neurosci 1997; 17: 2519–2530. Lanau F, Brockhuas M, Pink JRL, Franchet C, Wildt-Perinic D, Goepfert C et al. Development and characterization of antibodies against the N-terminus of the human D4 receptor. J Neurochem 1997; 69: 2169–2178. Mrzljak L, Bergson C, Pappy M, Huff R, Levenson R, GoldmanRakic P. Localization of dopamine D4 receptors in GABAergic neurons of the primate brain. Nature 1996; 381: 245–248. Ariano MA, Wang J, Noblett KL, Larson ER, Sibley DR. Cellular distribution of the rat D4 dopamine receptor protein in the CNS using anti-receptor antisera. Brain Res 1997; 752: 26–34. Defagot MC, Malchiodi EL, Villar MJ, Antonelli MC. Distribution of D4 dopamine receptor in rat brain with sequence-specific antibodies. Mol Brain Res 1997; 45: 1–12. Boy C, Klimke A, Holschbach M, Herzog H, Muhlensiepen H, Kops ER et al. Imaging dopamine D4 receptors in the living primate brain: a positron emission tomography study using the novel D1/D4 antagonist [11C]SDZ GLC 756. Synapse 1998; 30: 341–350. Lanau F, Zenner M-T, Civelli O, Hartman DS. Epinephrine and norepinephrine act as potent agonists at the recombinant human D4 receptor. J Neurochem 1997; 68: 804–812. Asghari V, Sanyal S, Buchwaldt S, Paterson A, Jovanovic V, Van Tol HHM. Modulation of intracellular cyclic AMP levels by different human dopamine D4 receptor variants. J Neurochem 1995; 65: 1157–1165. Chio CL, Drong RF, Riley DT, Gill GS, Slightom JL, Huff RM. D4 dopamine receptor-mediated signaling events determined in transfected chinese hamster ovary cells. J Biol Chem 1994; 269: 11813–11819. Mei YA, Griffon N, Buquet C, Martres MP, Vaudry H, Schwartz JC et al. Activation of dopamine D4 receptor inhibits an L-type calcium current in cerebellar granule cells. Neuroscience 1995; 68: 107–116. Wilke RA, Hsu S-F, Jackson MB. Dopamine D4 receptor mediated inhibition of potassium current in neurohypophysial nerve terminals. J Pharmacol Exp Ther 1998; 284: 542–548. Sanyal S, Van Tol HHM. Dopamine D4 receptor-mediated inhibition of cyclic adenosine 3⬘, 5⬘- monophosphate production does not affect prolactin regulation. Endocrinology 1997; 138: 1871– 1878. Rubinstein M, Phillips TJ, Bunzow JR, Falzone TL, Dziewczapolski G, Zhang G et al. Mice lacking dopamine D4 receptors are supersensitive to ethanol, cocaine, and methamphetamine. Cell 1997; 90: 991–1001. Zhang K, Tarazi FI, Kula NS, Baldessarini RJ, Neumeyer JL. Selective alkylation of dopamine D2 and D4 receptors in rat brain by NIPS. Neurosci Lett 1999; (in press). Roth BL, Tandra S, Burgess LH, Sibley DR, Meltzer HY. D4 dopamine receptor affinity does not distinguish between typical and atypical antipsychotic drugs. Psychopharmacology 1995; 120: 365–368. Seeman P, Corbett R, Van Tol HHM. Atypical neuroleptics have low affinity for dopamine D2 receptors or are selective for D4 receptors. Neuropsychopharmacology 1997; 16: 93–110. Baldessarini RJ, Frankenburg FR. Clozapine—a novel antipsychotic agent. N Engl J Med 1991; 324: 746–754. Brunello N, Masotto C, Steardo L, Markstein R, Racagni G. New insights into the biology of schizophrenia through the mechanism of action of clozapine. Neuropsychopharmacology 1995; 13: 177– 213. Baldessarini RJ. Drugs and the treatment of psychiatric disorders. In: Hardman JG, Limbird LE, Molinoff PB, Ruddon RW, Gilman AG (eds). Goodman and Gilman’s The Pharmacologic Basis of Therapeutics. McGraw-Hill: New York, 1996, pp 399–459. Schotte A, Janssen PFM, Gommeren W, Luyten WHML, Vangompel P, Lesage AS et al. Risperidone compared with new and reference antipsychotic drugs—in vitro and in vivo receptor binding. Psychopharmacology 1996; 124: 57–73. Wilson JM, Sanyal S, Van Tol HHM. Dopamine D2 and D4 receptor ligands: relation to antipsychotic action. Eur J Pharmacol 1998; 351: 273–286. Meltzer HY, Nash JF. Effects of antipsychotic drugs on serotonin receptors. Pharmacol Rev 1991; 43: 587–604.

76 Seeman P, Van Tol HHM. Dopamine D4 receptors bind inactive (+)aporphines, suggesting neuroleptic role; sulpiride not stereoselective. Eur J Pharmacol 1993; 233: 173–174. 77 Baldessarini RJ, Kula NS, Zong R, Neumeyer JL. Receptor affinities of aporphine enantiomers in rat brain tissue. Eur J Pharmacol 1994; 254: 199–203. 78 Campbell A, Baldessarini RJ, Teicher MH, Neumeyer JL. Behavioral effects of apomorphine isomers in the rat: selective locomotorinhibitory effects of S(+)N-n-propylnorapomorphine. Psychopharmacology 1986; 88: 158–164. 79 Campbell A, Yeghiayan S, Baldessarini RJ, Neumeyer JL. Selective antidopaminergic effects of S(+)-N-propylnoraporphines in limbic vs extrapyramidal sites in rat brain: comparisons with typical and atypical antipsychotic agents. Psychopharmacology 1991; 103: 323–329. 80 Nemeroff CB, Kilts CD, Levant B, Bissette G, Campbell A, Baldessarini RJ. Effects of aporphine isomers and haloperidol on regional concentrations of neurotensin in rat brain. Neuropsychopharmacology 1991; 4: 27–33. 81 Schoots O, Seeman P, Guan HC, Paterson AD, Van Tol HHM. Longterm haloperidol elevates dopamine D4 receptors by 2-fold in rats. Eur J Pharmacol 1995; 289: 67–72. 82 Florijn WJ, Tarazi FI, Creese I. Dopamine receptor subtypes: differential regulation following 8 months treatment with antipsychotic drugs. J Pharmacol Exp Ther 1997; 280: 561–569. 83 Tarazi FI, Florijn WJ, Creese I. Differential regulation of dopamine receptors following chronic typical and atypical antipsychotic drug treatment. Neuroscience 1997; 78: 985–996. 84 Kusumi I, Ishikane T, Matsubara S, Koyama T. Long-term treatment with haloperidol or clozapine does not affect dopamine D4 receptors in rat frontal cortex. J Neural Transm 1995; 101: 231–235. 85 Lidow MS, Goldman-Rakic PS. A common action of clozapine, haloperidol, and remoxipride on D1- and D2-dopaminergic receptors in the primate cerebral cortex. Proc Natl Acad Sci USA 1994; 91: 4353–4356. 86 Lidow MS, William GV, Goldman-Rakic PS. The cerebral cortex: a case for a common site of action of antipsychotics. Trends Pharmacol Sci 1998; 19: 136–140. 87 Kebabian JW, Tarazi FI, Kula NS. Baldessarini RJ. Compounds selective for dopamine receptor subtypes. Drug Discovery Today 1997; 2: 333–340. 88 Sanner MA. Selective dopamine D4 receptor antagonists. Expert Opin Ther Patents 1998; 8: 383–393. 89 Kulagowski JJ, Broughton HB, Curtis NR, Mawer IM, Ridgill MP, Baker R et al. 3-([4-(4-Chlorophenyl)-piperazin-1-yl]-methyl)-1 Hpyrrolo[2,3-b]pyridine: an antagonist with high affinity and selectivity for the human dopamine D4 receptor. J Med Chem 1996; 39: 1941–1942. 90 Patel S, Patel S, Marwood R, Emms F, Marston D, Leeson PD et al. Identification and pharmacological characterization of [125I]L750,667, a novel radioligand for the dopamine D4 receptor. Mol Pharmacol 1996; 50: 1658–1664. 91 Hidaka K, Tada S, Matsumoto M, Ohmori J, Maeno K, Yamaguchi T. YM-50001: a novel, potent and selective dopamine D4 receptor antagonist. NeuroReport 1996; 7: 2543–2546. 92 Ohmori J, Maeno K, Hidaka K, Nakato K, Matsumoto M, Tada S et al. Dopamine D3 and D4 receptor antagonists: synthesis and structure-activity relationships of (S)(+)-N-(1-benzyl-3-pyrrolidinyl)-5chloro-4[(cyclopropylcarbonyl)-amino]-2-methoxybenzamide (YM-43611) and related compounds. J Med Chem 1996; 39: 2764–2772. 93 Merchant KM, Gill GS, Harris DW, Huff RM, Eaton MJ, Lookingland K et al. Pharmacological characterization of U-101387, a dopamine D4 receptor selective antagonist. J Pharmacol Exp Ther 1996; 279: 1392–1403. 94 Kula NS, Baldessarini RJ, Kebabian JW, Bakthavachalam V, Xu L. RBI-257: a highly potent D4-selective dopamine receptor ligand. Eur J Pharmacol 1997; 331: 333–336. 95 Kula NS, Tarazi FI, Baldessarini RJ, Xu L, Bakthavachalam V. Neuropharmacological assessment of potential dopamine D4-selective radioligands. Eur J Pharmacol 1999; 367: 139–142. 96 Okuyama S, Chaki S, Yoshikawa R, Suzuki Y, Ogawa SI, Imagawa Y et al. In vitro and in vivo characterization of the dopamine D4, serotonin 5-HT2A receptor and alpha-1 adrenoceptor antagonist (R)-

537


538

(+)-2-amino- 4-(4-fluorophenyl)-5-[1-[4-(4-fluorophenyl)-4-oxobutyl]pyrrolidin-3-yl]thiazole (NRA-0045). J Pharmacol Exp Ther 1997; 282: 56–63. 97 Okuyama S, Chaki S, Yoshikawa R, Suzuki Y, Ogawa SI, Kumagai T et al. The atypical profile of NRA-0045, a novel dopamine D4 and 5-hydroxytryptamine-2A receptor antagonist, in rats. Br J Pharmacol 1997; 121: 515–525. 98 Tallman JF, Primus RJ, Brodbeck R, Cornfield L, Meade R, Woodruff K et al. NGD-94-1: identification of a novel, high-affinity antagonist at the human dopamine D4 receptor. J Pharmacol Exp Ther 1997; 282: 1011–1019. 99 Wright JL, Gregory TF, Heffner TG, Mackenzie RG, Pugsley TA, Meulen SV et al. Discovery of selective dopamine D4 receptor antagonists: 1-aryloxy-3-(4-aryloxypiperidinyl)-2-propanols. Bioorg Med Chem Lett 1997; 7: 1377–1380. 100 Millan MJ, Newman-Tancredi A, Brocco M. Gobert A, Lejeune F, Audinot V et al. S 18126 ({2-[4-(2,3-dihydrobenzo [1,4]dioxin6yl)piperazin-1-ylmethyl]indan-2-yl}), a potent, selective and competitive antagonist at dopamine D4 receptors: an in vitro and in vivo comparison with L 745,870 (3-(4-[4-chlorophenyl]piperazin-1-yl)methyl-1H-pyrrolo[2,3b]pyridine) and raclopride. J Pharmacol Exp Ther 1998; 287: 176–186. 101 Sanner MA, Chappie TA, Dunaiskis AR, Fliri AF, Desai KA, Zorn SH et al. Synthesis, SAR, and pharmacology of CP-293,019—a potent, selective dopamine D4 receptor antagonist. Bioorganic Med Chem Lett 1998; 8: 725–730. 102 Heuillet E, Petitet F, Mignani S, Malleron J-L, Lavayre J, Neliat G et al. The naphtosultam derivative RP 62203 (fanaserin) has high affinity for the dopamine D4 receptor. Eur J Pharmacol 1996; 314: 229–233. 103 Glase SA, Akunne HC, Georgic LM, Heffner TG, MacKenzie RG, Manley PJ et al. Substituted [(4-phenylpiperazinyl)-methyl]benzamides: selective dopamine D4 agonists. J Med Chem 1997; 40: 1771–1772. 104 Zorn SH, Jackson ER, Johnson CG, Lewis JM, Fliri A. CP-226,269 is a selective dopamine D4 receptor agonist (Abstract 271.11). Soc Neurosci Abst 1997; 23: 685. 105 Gazi L, Bobirnac I, Danzeisen M, Schupbach E, Bruinvels AT, Geisse S et al. Agonist activities of the putative antipsychotic agents, L-745,870 and U-101958 in HEK-293 cells expressing the human dopamine D4.4 receptor. Br J Pharmacol 1998; 124: 889– 896. 106 Gazi L, Sommer B, Nozulak J, Schoeffter P. Agonist activities of the putative antipsychotic agents, L-745,870 and U-101958 in HEK-293 cells expressing the human dopamine D4.4 receptor. Eur J Pharmacol 1999; 372: R9–R10. 107 Helmeste DM, Shioiri T, Mitsuhashi M, Tang SW. Binding of [3H]U-101958 to ␴1 receptor-like sites in human cerebellum and neuroblastoma cells. Eur J Pharmacol 1999; 370: 205–209. 108 Kung MP, Stevenson DA, Zhuang ZP, Vessotskie JM, Chumpradit

109

110

111

112

113

114

115

116

117

118

119

120

S, Sun X-M et al. Characterization of a novel iodinated ligand, IPMPP, for human dopamine D4 receptors expressed in CHO cells. Life Sci 1997; 60: 1–100. Swerdlow NR, Braff DL, Taaid N, Geyer MA. Assessing the validity of an animal model of deficient sensorimotor gating in schizophrenic patients. Arch Gen Psychiatry 1994; 51: 139–154. Jackson DM, Westlind-Danielsson A. Dopamine receptors: molecular biology, biochemistry and behavioral aspects. Pharmacol Ther 1994; 64: 291–369. Arnt J, Skarsfeldt T. Do novel antipsychotics have similar pharmacological characteristics? A review of the evidence. Neuropsychopharmacology 1998; 18: 63–101. Bristow LJ, Collinson N, Cook GP, Curtis N, Freedman SB, Kulagowski JJ et al. L745,870, a subtype selective dopamine D4 receptor antagonist does not exhibit a neuroleptic-like profile in rodent behavioral tests. J Pharmacol Exp Ther 1997; 283: 1256–1263. Costanza RM, Terry P. The dopamine D4 receptor antagonist L745,870: effects in rats discriminating cocaine from saline. Eur J Pharmacol 1998; 345: 129–132. Jentsch JD, Taylor JR, Redmond DE Jr, Elsworth JD, Youngren KD, Roth RH. Dopamine D4 receptor antagonist reversal of subchronic phencyclidine-induced object retrieval/detour deficits in monkeys. Psychopharmacology 1999; 142: 78–84. Feldpausch DL, Needham LM, Stone MP, Althaus JS, Yamamoto BK, Svensson KA et al. KM. Role of dopamine D4 receptor in the induction of behavioral sensitization to amphetamine and accompanying biochemical and molecular adaptations: treatment with antipsychotic drugs. J Pharmacol Exp Ther 1998; 286: 497–508. Zorn SH, Johnson CG, Jackson ER, Seymour P, Majchrzak M, Mansbach R et al. CP-293,029: a D4 dopamine antagonist with an in vitro and in vivo profile suggestive of an atypical antipsychotic (abstract 697.1). Soc Neurosci Abst 1996: 22: 1771. Mansbach RS, Brooks EW, Sanner MA, Zorn SH. Selective dopamine D4 receptor antagonists reverse apomorphine-induced blockade of prepulse inhibition. Psychopharmacology 1998; 135: 194–200. Kramer MS, Last B, Getson A, Reines SA, and the D4 Dopamine Antagonist Group. The effects of a selective D4 dopamine receptor antagonist (L-745, 870) in acutely psychotic inpatients with schizophrenia. Arch Gen Psychiatry 1997; 54: 567–572. Truffinet P, Tamminga CA, Fabre LF, Meltzer HY, Riviere M-E, Papillon-Downey C. Placebo controlled study of the D4/5-HT2A antagonist fanaserin in the treatment of schizophrenia. Am J Psychiatry 1999; 156: 419–425. Newman-Tancredi A, Audinot V, Chaput C, Verriele L. [35S] Guanosine-5⬘-O-(3-thio) triphosphate binding as a measure of efficacy at human recombinant dopamine D4,4 receptors: actions of antiparkinsonina and antipsychotic agents. J Pharmacol Exp Ther 1997; 282: 181–191.