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

Effect of dopamine D4 receptor agonists on sleep architecture in rats

Dopamine plays a key role in the regulation of sleep-wake states, as revealed by the observation that dopamine-releasing agents such as methylphenidate have wake-promoting effects. However, the precise mechanisms for the wake-promoting effect produced by the enhancement of dopamine transmission are not fully understood. Although dopamine D1, D2, and D3 receptors are known to have differential effects on sleep architecture, the role of D4 receptors (D4Rs), and particularly the influence of D4R activation on the sleep-wake state, has not been studied so far. In this study, we investigated for the first time the effects of two structurally different D4R agonists, Ro 10-5824 and A-412997, on the sleep-wake states in rats. We found that both D4R agonists generally increased waking duration, and conversely, reduced non-rapid eye movement (NREM) sleep duration in rats. The onset of NREM sleep was also generally delayed. However, only the A-412997 agonist (but not the Ro 10-5824) influenced rapid eye movement sleep onset and duration. Furthermore, these effects were accompanied with an enhancement of EEG spectral power in the theta and the gamma bands. Our results suggest the involvement of dopamine D4R in the regulation of sleep-wake states. The activation of the D4R could enhance the arousal states as revealed by the behavioral and electrophysiological patterns in this study. Dopamine D4R may contribute to the arousal effects of dopamine-releasing agents such as methylphenidate.

A selective role for dopamine D₄ receptors in modulating reward expectancy in a rodent slot machine task

BACKGROUND

Cognitive distortions regarding gambling outcomes confer vulnerability to pathological gambling. Using a rat slot machine task (rSMT), we previously demonstrated that the nonspecific D₂ agonist quinpirole enhances erroneous expectations of reward on near-miss trials, suggesting a pivotal role for the D₂ receptor family in mediating the near-miss effect. Identifying which receptor subtype is involved could facilitate treatment development for compulsive slot machine play.

METHODS

Thirty-two male Long Evans rats learned the rSMT. Three flashing lights could be set to on or off. A win was signaled if all three lights were set to on, whereas any other light pattern indicated a loss. Rats then chose between responding on the collect lever, which delivered 10 sugar pellets on win trials but a 10-second time penalty on loss trials, or to start a new trial instead. Performance was assessed following systemic administration of selective D₂, D₃, and D₄ receptor ligands.

RESULTS

The selective D₂ antagonist L-741,626, the D₃ antagonist SB-277011-A, and the D₃ agonist PD128,907 had no effect. In contrast, the selective D₄ agonist PD168077 partially mimicked quinpirole's effects, increasing erroneous collect responses on nonwin trials, whereas the D₄ antagonist L-745,870 improved the error rate. L-745,870 was also the only antagonist that could attenuate the deleterious effects of quinpirole.

CONCLUSIONS

The dopamine D₄ receptor is critically involved in signaling reward expectancy in the rSMT. The ability of L-745,870 to reduce the classification of losses as wins suggests that D₄ antagonists could be effective in treating problematic slot machine play.

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.

Three amino acids in the D2 dopamine receptor regulate selective ligand function and affinity

The D(2) dopamine receptor is an important therapeutic target for the treatment of psychotic, agitated, and abnormal behavioral states. To better understand the specific interactions of subtype-selective ligands with dopamine receptor subtypes, seven ligands with high selectivity (>120-fold) for the D(4) subtype of dopamine receptor were tested on wild-type and mutant D(2) receptors. Five of the selective ligands were observed to have 21-fold to 293-fold increases in D(2) receptor affinity when three non-conserved amino acids in TM2 and TM3 were mutated to the corresponding D(4) amino acids. The two ligands with the greatest improvement in affinity for the D(2) mutant receptor [i.e., 3-{[4-(4-iodophenyl) piperazin-1-yl]methyl}-1H-pyrrolo[2,3-b]pyridine (L-750,667) and 1-[4-iodobenzyl]-4-[N-(3-isopropoxy-2-pyridinyl)-N-methyl]-aminopiperidine (RBI-257)] were investigated in functional assays. Consistent with their higher affinity for the mutant than for the wild-type receptor, concentrations of L-750,667 or RBI-257 that produced large reductions in the potency of quinpirole's functional response in the mutant did not significantly reduce quinpirole's functional response in the wild-type D(2) receptor. In contrast to RBI-257 which is an antagonist at all receptors, L-750,667 is a partial agonist at the wild-type D(2) but an antagonist at both the mutant D(2) and wild-type D(4) receptors. Our study demonstrates for the first time that the TM2/3 microdomain of the D(2) dopamine receptor not only regulates the selective affinity of ligands, but in selected cases can also regulate their function. Utilizing a new docking technique that incorporates receptor backbone flexibility, the three non-conserved amino acids that encompass the TM2/3 microdomain were found to account in large part for the differences in intermolecular steric contacts between the ligands and receptors. Consistent with the experimental data, this model illustrates the interactions between a variety of subtype-selective ligands and the wild-type D(2), mutant D(2), or wild-type D(4) receptors.

Synthesis and in vitro binding studies of substituted piperidine naphthamides. Part II: Influence of the substitution on the benzyl moiety on the affinity for D2L, D4.2, and 5-HT2A receptors

In continuation of our work on N-(piperidin-4-yl)-naphthamides, the effect of substituted benzyl groups on D(2L), D(4.2), and 5-HT(2A) receptor affinity was evaluated. In the 1-naphthamide series most compounds were highly selective for D(4.2) over D(2L) and 5-HT(2A) receptors. Halogen and methyl substitution in position 3 or 4 of the benzyl group increased D(4.2) affinity. In the 2-naphthamide series a similar high D(4.2) over D(2L) selectivity was retained while 5-HT(2A) affinity was increased. 3-Methoxy, 3-methyl, and 4-methyl substituents were favorable for D(4.2) affinity while halogens reduced affinity. 2-Naphthamides with a 3-bromo- or a 3-methyl group were mixed D(4.2)/5-HT(2A) ligands similar to their unsubstituted parent compound. All compounds from both series with significant affinity for D(4.2) and 5-HT(2A) receptors were antagonists.

PIP3EA and PD-168077, two selective dopamine D4 receptor agonists, induce penile erection in male rats: site and mechanism of action in the brain

PIP3EA (2-[4-(2-methoxyphenyl)piperazin-1-yl-methyl]imidazo[1,2-a]pyridine) and PD-168077 (N-[4-2-cyanophenylpiperazin-1-ylmethyl]-3-methylbenzamide maleate), two selective dopamine D4 agonists, administered systemically, intracerebroventricularly or into the paraventricular nucleus of the hypothalamus induce penile erection in male Sprague-Dawley rats. A U-inverted dose-response curve was found with either compound when given subcutaneously (1-100 microg/kg) or intracerebroventricularly (0.1-20 microg/rat), but not into the paraventricular nucleus (10-200 ng/rat). The pro-erectile effect of PIP3EA and of PD-168077 occurs concomitantly with an increased nitric oxide (NO) production in the paraventricular nucleus, as measured by the increased concentration of nitrites and nitrates found in the dialysate obtained from the paraventricular nucleus by intracerebral microdialysis. These effects of PIP3EA and PD-168077 were reduced by L-745,870 (3-[4-(4-chlorophenyl)piperazin-1-ylmethyl]-1H-pyrrolo[2,3-b]pyridine trihydrochloride), a selective dopamine D4 receptors antagonist, by omega-conotoxin, a blocker of voltage-dependent Ca2+ channels of the N-type, by S-methyl-thiocitrulline, a neuronal nitric oxide synthase inhibitor, and by d(CH2)5Tyr(Me)2-Orn8-vasotocin, an oxytocin receptor antagonist, given into the lateral ventricles, but not into the paraventricular nucleus. Comparison of the dose-response curves of PIP3EA and PD-168077 revealed that PIP3EA is as potent as PD-168077 when given into the paraventricular nucleus, but more potent when given systemically. However, both compounds are less efficacious (e.g. induce a lower number of penile erection episodes) than apomorphine, a classical mixed dopamine receptor agonist, irrespective of the route of administration. These results confirm previous findings showing that central D4 receptors mediate penile erection and show that dopamine D4 receptor agonists act in the paraventricular nucleus to facilitate penile erection by increasing central oxytocinergic neurotransmission.

Synthesis and evaluation of 3-aryl piperidine analogs as potent and efficacious dopamine D4 receptor agonists

A series of 3-aryl piperidine analogs with 2-piperidinoalkylamino or 2-piperidinoalkyloxy fused bicyclic rings were prepared and found to be potent and efficacious human dopamine D4 agonists. The synthesis and structure-activity relationship (SAR) studies that led to the identification of these compounds are discussed.

2-[4-(3,4-Dimethylphenyl)piperazin-1-ylmethyl]-1H benzoimidazole (A-381393), a selective dopamine D4 receptor antagonist

2-[4-(3,4-Dimethylphenlyl)piperazin-1-ylmethyl]-1H benzoimidazole (A-381393) was identified as a potent dopamine D4 receptor antagonist with excellent receptor selectivity. [3H]-spiperone competition binding assays showed that A-381393 potently bound to membrane from cells expressing recombinant human dopamine D4.4 receptor (Ki=1.5 nM), which was 20-fold higher than that of clozapine (Ki=30.4 nM). A-381393 exhibited highly selective binding for the dopamine D4.4 receptor (>2700-fold) when compared to D1, D2, D3 and D5 dopamine receptors. Furthermore, in comparison to clozapine and L-745870, A-381393 exhibits better receptor selectivity, showing no affinity up to 10 microM for a panel of more than 70 receptors and channels, with the exception of moderate affinity for 5-HT2A (Ki=370 nM). A-381393 potently inhibited the functional activity of agonist-induced GTP-gamma-S binding assay and 1 microM dopamine induced-Ca2+ flux in human dopamine D4.4 receptor expressing cells, but not in human dopamine D2L or D3 receptor cells. In contrast to L-745870, A-381393 did not exhibit any significant intrinsic activity in a D4.4 receptor. In vivo, A-381393 has good brain penetration after subcutaneous administration. A-381393 inhibited penile erection induced by the selective D4 agonist PD168077 in conscious rats. Thus, A-381393 is a novel selective D4 antagonist that will enhance the ability to study dopamine D4 receptors both in vitro and in vivo.

Certain 1,4-Disubstituted Aromatic Piperidines and Piperazines with Extreme Selectivity for the Dopamine D4 Receptor Interact with a Common Receptor Microdomain

We previously demonstrated that, in the D4 dopamine receptor, the aromatic microdomain that spans the interface of the second and third transmembrane segments influences the high-affinity interactions with the D4-selective ligand L750,667 [3-[[4-(4-iodophenyl) piperazin-1-yl]methyl]-1H-pyrrolo[2,3-b]pyridine] and the D2-selective ligands methylspiperone, aripiprazole, and its congener OPC4392 [7-[3-(4-(2,3-dimethylphenyl) piperazinyl) propoxy] 2-(1H)-quinolinone] (Schetz et al., 2000). Here we tested a variety of 1,4-disubstituted aromatic piperidines/piperazines (1,4-DAPs) with different subtype selectivities and functional properties against a panel of D4 receptor mutations in the aromatic microdomain to ascertain whether these ligands recognize this common site. Mutant D4 receptors were constructed by substituting the nonconserved amino acid(s) from the corresponding locations in the D2 receptor. The D4-L2.60W, D4-F2.61V, and D4-LM3.28-3.29FV substitutions result in alterations of the relative position of members of the aromatic microdomain. From these results and molecular models of the ligand-receptor complexes, we conclude that 9 of the 11 D4-selective 1,4-DAPs, including L750,667, have a common pattern of ligand-receptor recognition that depends upon favorable interactions with the phenylalanine at position 2.61 (D4-F2.61V, 20-96-fold decrease). Like methylspiperone, aripiprazole, and OPC4392, the two D4-selective 1,4-DAPs that are insensitive to the D4-F2.61V mutation are sensitive to aromatics at position 2.60 (D4-L2.60W, 7-20-fold increase), and they all have longer spacer arms that permit their tethered aromatics to adopt alternative orientations in the binding-site crevice. All 11 of the D4-selective 1,4-DAPs were sensitive to the D4-LM3.28-3.29FV mutation (13-494-fold decrease) but not the moderately D2-selective methylspiperone. The inferences suggest that subtype selectivity involves two different modes of interaction with the microdomain for the D4-selective 1,4-DAPs and a third mode for D2-selective 1,4-DAPs.

FAUC 213, a highly selective dopamine D4 receptor full antagonist, exhibits atypical antipsychotic properties in behavioural and neurochemical models of schizophrenia

RATIONALE

2-[4-(4-Chlorophenyl)piperazin-1-ylmethyl]pyrazolo[1,5-a]pyridine (FAUC 213) is a highly selective antagonist at the dopamine D(4) receptor subtype. It was designed as a derivative of two partial antagonists and has been proven to be a complete antagonist in mitogenesis assay.

OBJECTIVES

In the present study, FAUC 213 was examined for antipsychotic properties in animal models of behavioural neurobiology and neurochemistry.

METHODS

Different concentrations of FAUC 213 were screened for effects on spontaneous, as well as amphetamine-induced, locomotor activity and apomorphine-induced prepulse disruption. The liability of causing extrapyramidal side effects was investigated in models of catalepsy and by high-performance liquid chromatography (HPLC) detection of dopamine turnover in several brain regions. The application schedule was validated, and the bioavailability of the compound determined, by means of a HPLC-pharmacokinetic study.

RESULTS

A significant effect in both the reduction of amphetamine-induced locomotor hyperactivity and the restoration of apomorphine-disrupted prepulse inhibition was found at 30 mg/kg. This dose proved not to be high enough to induce catalepsy or to increase dopamine turnover in the dorsal striatum, nucleus accumbens and medial prefrontal cortex. The selective D(4) antagonist FAUC 213, therefore, is not believed to mediate the above-mentioned effects via D(2) receptor antagonism, but a partial involvement of 5-HT(2)- and alpha(1)-receptors cannot be ruled out at present.

CONCLUSIONS

We have gathered evidence that FAUC 213 exhibits atypical antipsychotic characteristics.

Dopamine D4 Ligands and Models of Receptor Activation: 2-(4-Pyridin-2-ylpiperazin-1-ylmethyl)-1H-benzimidazole and Related Heteroarylmethylarylpiperazines Exhibit a Substituent Effect Responsible for Additional Efficacy Tuning

A series of subtype selective dopamine D(4) receptor ligands from the hetroarylmethylphenylpiperazine class have been discovered that exhibit a remarkable structure-activity relationship (SAR), revealing a substituent effect in which regiosubstitution on the terminal arylpiperazine ring can modulate functional or intrinsic activity. Other structure-dependent efficacy studies in the dopamine D(4) field have suggested a critical interaction of the heteroarylmethyl moiety with specific protein microdomains in controlling intrinsic activity. Our studies indicate that for some binding orientations, the phenylpiperazine moiety also plays a key role in determining efficacy. These data also implicate a kinetic or efficiency term, contained within measured functional affinities for agonists, which support a sequential binding and conformational stabilization model for receptor activation. The structural similarity between partial agonist and antagonist, within this subset of ligands, and lack of bioisosterism for this substituent effect are key phenomena for these hypotheses.

The dopamine D4 receptors and mechanisms of antipsychotic atypicality

The dopamine D4 receptor (D4) is a target for most common neuroleptic medications. After its initial discovery, it was found to possess the highest affinity of all dopamine receptor subtypes for the archetypical, atypical, antipsychotic clozapine. Nevertheless, initial clinical trials have not provided evidence that this receptor is a primary target for antipsychotic drugs. Considering the accumulated in vivo evidence that at least a subgroup of psychotic patients have altered dopamine signaling, all dopamine receptor subtypes likely contribute to the phenotypic expression of schizophrenia. New insights into the function of this receptor and its role in the modulation of excitatory signaling support the view that this dopamine receptor may affect attention and cognition. In this review, the authors outline some recent developments that provide insight into D4 receptor physiology, function and its possible relationship to schizophrenia treatment.

Facilitatory effect of the dopamine D4 receptor agonist PD168,077 on memory consolidation of an inhibitory avoidance learned response in C57BL/6J mice

The still unknown contribution of the D4 receptors to memory consolidation was studied examining the memory effects of the dopamine D4 agonist PD168,077, the putative dopamine D4 antagonist L745,870, their mutual combination, and the combination of the D4 agonist with representative compounds acting as agonist or antagonist on the D1, D2 and the D3 receptors. Memory consolidation was assessed in C57BL/6J mice using the one-trial step-through inhibitory avoidance task, the compounds being injected immediately after training (foot-shock) and performance measured 24h later. PD168,077 (0.5-10mg/kg) dose-dependently improved memory performance and L745,870 (0.05-5mg/kg) at doses lower than 1mg/kg increased and at doses higher than 1mg/kg impaired memory performance. PD168,077 did not affect the paradoxical promnesic effect of low doses (0.1-0.5mg/kg) of L745,870, but antagonised the memory-impairing effect induced by 5mg/kg L745,870. The D1 antagonist SCH23390 (0.025-0.05 mg/kg) and the D2 antagonist eticlopride (0.01-0.05 mg/kg) antagonised the promnesic effects of PD168,077, which attenuated the decreasing effect on memory consolidation of both D1 and D2 antagonists. Accordingly, the D1 agonist SKF38393 (5-20mg/kg) and the D2 agonist quinelorane (0.1-1 mg/kg) both synergistically magnified the memory-improving effects of the D4 agonist. The dopamine D3 antagonist U99194A (2.5-10mg/kg) did not affect the promnesic effects induced by the D4 agonist, which nevertheless abolished the U99194A-induced promnesic effects. Additionally, the amnesic effects produced by the D3 agonist 7-OH-DPAT (0.01-1 microg/kg) was attenuated by PD168,077. These results suggest a potential role of dopamine D4 receptors in memory consolidation, which would be similar to that of the D1 and D2 receptors and probably opposite to that of the D3 receptors.

Pharmacological and functional characterisation of dopamine D4 receptors in the rat retina

In the retina, activation of dopamine receptors, particularly the D2-like family (D2, D3, D4 receptor subtypes), with quinpirole suppresses the light sensitive cAMP pool and inhibits melatonin synthesis in photoreceptor cells. We have characterised rat retinal D4 receptors using the D4 selective radioligand [(125)I] L-750667 which bound specifically and saturably to rat retinal membranes with high affinity (K(d) 0.06+/-0.02 nM) and exhibited a D4 receptor pharmacology. Comparison of the binding kinetics of [(125)I] L-750667 and [(3)H] spiperone revealed B(max) values of 134+/-27 fmol/mg and 219+/-47 fmol/mg respectively, indicating that the dopamine D4 receptor is a major component of D2-like dopamine receptors in the rat retina. Modulation of retinal cAMP levels by quinpirole was used to evaluate the functional relevance of rat retinal dopamine D4 receptors. Quinpirole (0.03-3 micro ) produced a dose-related decrease of the light sensitive cAMP pool which was reversed by haloperidol, clozapine and the D4 selective antagonist, L-745870 with a rank order of potency suggesting that the quinpirole effect is due to activation of the dopamine D4 receptors. The D2 selective ligand L-741626 had no effect on the quinpirole response confirming that the D4 receptor is the major receptor subtype mediating dopamine induced suppression of adenylate cyclase in the retina.

JL 13, an atypical antipsychotic: a preclinical review

The extensive pharmacological evaluation of JL 13 as an atypical antipsychotic drug has revealed a close similarity to clozapine, however with some major advantages. JL 13 was characterized as a weak D(2) antagonist, both in vitro and in vivo, with a strong affinity for the D(4) and the 5-HT(2A) receptors. It has no affinity for the 5-HT(2C) receptor. In vivo microdialysis experiments in rat showed that JL 13, like clozapine, preferentially increased extracellular dopamine concentrations in the prefrontal cortex compared to nucleus accumbens or striatum. Behavioral studies showed that JL 13, like clozapine, has the profile of an atypical antipsychotic. Thus, JL 13 did not antagonize apomorphine-induced stereotypy nor did it produce catalepsy, but it antagonized apomorphine-induced climbing in rodents. It was inactive against d-amphetamine-induced stereotypy but antagonized d-amphetamine-induced hyperactivity in the mouse. Likewise, in the paw test, it was more effective in prolonging hindlimb retraction time than prolonging forelimb retraction time. Like other antipsychotic drugs, JL 13 reversed the apomorphine- and amphetamine-induced disruption of prepulse inhibition. In a complex temporal regulation schedule in the dog, JL 13 showed a high resemblance with clozapine without inducing sialorrhea, palpebral ptosis or any significant motor side effects. In rats and squirrel monkeys JL 13 induced a high degree of generalization (70%) to clozapine. Regarding behavioral toxicology, JL 13 did not produce dystonia or Parkinsonian symptoms in haloperidol-sensitized monkeys. After acute administration, again like clozapine, JL 13 induced only a transient increase in circulating prolactin. Last but not the least, regarding a possible hematological toxicity, unlike clozapine, JL 13 did not present sensitivity to peroxidase-induced oxidation. Moreover, its electrooxidation potential was close to that of loxapine and far from that of clozapine. Taking all these preclinical data into account, it appears that JL 13 is a promising atypical antipsychotic drug.

The discovery of PD 89211 and related compounds: selective dopamine D4 receptor antagonists

The dopamine (DA) D2 family of receptors consists of the D2, D3, and D4 receptors. The DA D4 receptor is of interest as a target for drugs to treat schizophrenia based upon its high affinity for the atypical antipsychotic clozapine and its localization to the limbic and cortical regions of the brain. As part of a program to identify novel DA D4 receptor antagonists, a high-volume screen using the Parke-Davis compound library was initiated. This led to the discovery of PD 89211 (benzenemethanol, 2-chloro-4-[4-[(1H-benzimidazol-2-yl)methyl]-1-piperzinyl]) that displaced [3H]spiperone binding to hD4.2 with an affinity (Ki) of 3.7 nM. PD 89211 exhibited high selectivity for the DA D4.2 receptor (> 800-fold) as compared to other hDA receptor subtypes, rat brain serotonin, and adrenergic receptors. In vitro, PD 89211 had D4 receptor antagonist activity reversing quinpirole-induced [3H]thymidine uptake in CHOpro5 cells (IC50 = 2.1 nM). Limited structure-activity relationship (SAR) studies indicated that compounds with a 4-chloro-, 4-methyl-, and 3-chloro- substituents on the phenyl ring retained high affinity for D4 receptors, while those with a 4-methoxy- and no substituent had less affinity. While all clinically effective antipsychotics increase DA synthesis (DOPA accumulation) in rodents, PD 89211 did not increase DA synthesis in the DA-enriched striatum, indicating no effect on DA turnover and low propensity for exhibiting motor side effects. However, it did increase catecholamine synthesis in rat hippocampus, as did clozapine. Moreover, PD 89211 selectivity increased catecholamine synthesis in the hippocampus of wild type but not in mice lacking D4 receptors, suggesting that one function of D4 receptors may be to modulate DA/norepinephrine (NE) turnover in this brain area known to possess D4 receptors. The discovery of compounds like PD 89211 provides a tool to help in understanding the function of DA D4 receptors in the CNS.

Cyanoindole derivatives as highly selective dopamine D(4) receptor partial agonists: solid-phase synthesis, binding assays, and functional experiments

Traceless linking of diethoxymethyl (DEM)-protected 5- and 6-cyanoindoles and subsequent incorporation of phenylpiperazine derivatives led to the 2- and 3-piperazinylmethyl-substituted cyanoindoles 3a-m. Dopamine receptor binding studies on the final products 3a-m clearly indicated strong and selective recognition of the D(4) subtype which is known as a promising target for the treatment of neuropsychiatric disorders. The most interesting binding properties were observed for the 2-aminomethyl-5-cyanoindoles FAUC 299 (3f) and FAUC 316 (3j) (K(i) = 0.52 and 1.0 nM, respectively) when the fluoro derivative 3j proved extraordinary selectivity over D(1), D(2long), D(2short), and D(3) (>8600). To determine ligand efficacy, mitogenesis experiments were performed indicating partial agonist effects for the test compounds 3f,j (35% and 30%, when compared to the full agonist quinpirole).

Dopamine D4 receptors and development of newer antipsychotic drugs

The last ten years have witnessed the generation of a large amount of information on the neurobiology of dopamine receptors. Molecular biology and pharmacology studies have revealed existence of at least five dopamine receptor subtypes, namely D1, D2, D3, D4 and D5. The discovery of D4 receptors and the putative affinity of clozapine for D4 receptors have kindled development of selective D4 receptor antagonists for the treatment of schizophrenia. Studies on expression of D4 receptor proteins have shown selective localisation of D4 receptors in mesolimbic/mesocortical areas which could probably explain the lack of motor side effects with atypical antipsychotics like clozapine and olanzapine. However, neuropathological and genetic studies on the role of D4 receptors in the pathophysiology of schizophrenia and preliminary clinical studies with selective D4 receptor antagonists have been disappointing. There have been, however, complimentary findings between selective D4 receptor antagonism and genetic approaches such as antisense treatment or gene targeting. The therapeutic potential of D4 receptors as a target for developing antipsychotics will be known only when selective D4 receptor-antagonists with varying D2/D4 and D4/5-HT2A ratios are developed and tested in psychiatric patients.

Inverse agonism and constitutive activity as functional correlates of serotonin h5-HT(1B) receptor/G-protein stoichiometry

This study evaluated the influence of receptor/G-protein (R:G) stoichiometry on constitutive activity and the efficacy of agonists, partial agonists, and inverse agonists at human (h) 5-hydroxytryphamine 1B (5-HT(1B)) receptors. Two Chinese hamster ovary cell lines were used; they expressed 8.5 versus 0.4 pmol h5-HT(1B) receptors/mg (determined by [(3)H]GR125,743 saturation analysis) and 3.0 versus 1.5 pmol receptor-activated G-proteins/mg [determined by guanosine-5'-O-(3-[(35)S]thio)-triphosphate ([(35)S]GTPgammaS) isotopic dilution], respectively. Thus, they displayed R:G ratios of approximately 3.0 (RGhigh) and approximately 0.3 (RGlow), respectively. In competition-binding experiments, the agonists, 5-HT and sumatriptan, displayed fewer high-affinity (HA)-binding sites and the partial agonists, BMS181, 101 and L775,606, displayed decreased affinity in RGhigh versus RGlow membranes. In contrast, the inverse agonists, SB224,289 and, to a lesser extent, methiothepin, showed increased affinity. In G-protein activation experiments, both basal and 5-HT-activated [(35)S]GTPgammaS binding were higher in RGhigh than in RGlow membranes. Constitutive activity (determined by inhibition of basal [(35)S]GTPgammaS binding with GTPgammaS in the absence of receptor ligands) was more pronounced in RGhigh versus RGlow membranes, as revealed by the >5-fold greater proportion of HA sites. Correspondingly, the negative efficacy of inverse agonists was strikingly augmented, inasmuch as they suppressed approximately two-thirds of HA [(35)S]GTPgammaS binding in RGhigh membranes, but only approximately one-third in RGlow membranes. Furthermore, the efficacy of partial agonists was greater at RGhigh versus RGlow membranes, as estimated by their ability to enhance [(35)S]GTPgammaS binding. In conclusion, an increase in R:G ratios at h5-HT(1B) receptors was associated with an increase in relative efficacy of partial agonists and, most notably, an increase in both constitutive G-protein activation and negative efficacy of inverse agonists.

The dopamine D(4) receptor: one decade of research

Dopamine is an important neurotransmitter involved in motor control, endocrine function, reward, cognition and emotion. Dopamine receptors belong to the superfamily of G protein-coupled receptors and play a crucial role in mediating the diverse effects of dopamine in the central nervous system (CNS). The dopaminergic system is implicated in disorders such as Parkinson's disease and addiction, and is the major target for antipsychotic medication in the treatment of schizophrenia. Molecular cloning studies a decade ago revealed the existence of five different dopamine receptor subtypes in mammalian species. While the presence of the abundantly expressed dopamine D(1) and D(2) receptors was predicted from biochemical and pharmacological work, the cloning of the less abundant dopamine D(3), D(4) and D(5) receptors was not anticipated. The identification of these novel dopamine receptor family members posed a challenge with respect to determining their precise physiological roles and identifying their potential as therapeutic targets for dopamine-related disorders. This review is focused on the accomplishments of one decade of research on the dopamine D(4) receptor. New insights into the biochemistry of the dopamine D(4) receptor include the discovery that this G protein-coupled receptor can directly interact with SH3 domains. At the physiological level, converging evidence from transgenic mouse work and human genetic studies suggests that this receptor has a role in exploratory behavior and as a genetic susceptibility factor for attention deficit hyperactivity disorder.

[(3)H]PNU-101958, a D(4) dopamine receptor probe, accumulates in prefrontal cortex and hippocampus of non-human primate brain

The D(4) dopamine receptor has been investigated for its potential role in neuropsychiatric disorders, "novelty-seeking" behaviors, and effects produced by some psychostimulants. An accurate map of D(4) distribution and density in brain is essential to clarify the role of this receptor subtype in normal brain function and in neuropsychiatric disorders. We investigated the autoradiographic distribution of D(4) receptors in non-human primate (Macaca mulatta) brain (N = 3) with the novel D(4) receptor probe [(3)H]PNU-101958. Quantification of [(3)H]PNU-101958 binding sites in 77 brain regions revealed dense levels of D(4) receptors in several cortical areas, especially in prefrontal cortex, uncus, hypothalamic median eminence, hippocampal formation, and distinct thalamic nuclei, but were significantly lower in striatum. The results correspond well with previous reports of brain distribution of D(4) receptors using other radiolabeled probes, and of D(4) mRNA localization (with some exceptions). Overall, this study reveals that [(3)H]PNU-101958 binding sites in non-human primate brain appear to reflect D(4) dopamine receptor distribution. The significance of a dense localization of D(4) receptors in prefrontal cortex and hippocampus, and broad distribution in other brain areas, allows for investigation of the relationship of these receptors to specific neuropsychiatric disorders and effects produced by psychostimulants.

Dopamine D4 receptors

Initial investigations on dopamine D4 receptors generated much interest in the role of this receptor in schizophrenia and other aspects of human behavior, as well as new opportunities for novel therapeutics. However, attempts to treat patients suffering from schizophrenia with dopamine D4 agents have failed to yield satisfactory results so far. An examination of the dopamine D4 literature shows that contrasting and conflicting data seemed to be the norm in this field of research. This paper reviews the literature on the dopamine D4 receptor and discusses many of the associated methodological problems that might have contributed to the paradoxical findings.

A series of 6- and 7-piperazinyl- and -piperidinylmethylbenzoxazinones with dopamine D4 antagonist activity: discovery of a potential atypical antipsychotic agent

As part of a program to develop dopamine D4 antagonists for the treatment of schizophrenia, we discovered a series of 6- and 7-(phenylpiperazinyl)- and -(phenylpiperidinyl)methylbenzoxazinones through mass screening of our compound library. A structure-activity relationship SAR study was carried out involving substituents on the phenyl ring, and several selective D4 antagonists were identified. The 7-substituted benzoxazinones showed more activity in neurochemical and behavioral tests than the 6-substituted series. One of the most potent and selective compounds (26) was found to have potent activity in animal tests predictive of antipsychotic activity in humans after oral administration. This paper describes the SAR of the benzoxazinone series and the preclinical characterization of 26.

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.

Native human neocortex release-regulating dopamine D2 type autoreceptors are dopamine D2 subtype

Dopamine (DA) autoreceptors expressed at DA nerve terminals regulate DA release. Considerable evidence has indicated that, in rodents, these autoreceptors belong to the D2 type of the DA receptor family, which, in turn, comprises the D2, D3 and D4 subtypes. We investigated here, for the first time, the subclassification of native human DA autoreceptors by studying the release of [3H]DA evoked by electrical stimulation in fresh human neocortical slices. The results have been compared with those obtained in three animal systems: rat neocortical and striatal slices and rat mesencephalic neuronal cultures. In human neocortical slices, the D2/D3 receptor agonist quinpirole (1 nM-10 microM) inhibited tritium release with a calculated EC50 of 17 nM and a maximal inhibition of approximately 75% reached at 1 microM. In the presence of the D2/D3 receptor antagonist (-)-sulpiride (0.1 and 1 microM), the concentration-response curve of quinpirole was shifted to the right, and the apparent pA2 mean value was 8.5 (8.14-8.77); on the other hand, the inhibitory effects of quinpirole were not affected by the D3 receptor-selective antagonist [7-N,N-dipropylamino-5,6,7, 8-tetrahydro-naphtho(2,3b) dihydro,2,3-furane] (S 14297) and the D4 receptor-selective antagonist 3-(4-[4-chlorophenyl]piperazin-1-yl)-methyl-1H-pyrrolo [2,3-b]pyridine (L-745,870) (0.01-1 microM in each case). Superimposable results have been obtained when the release was elicited from rat striatal slices or dopamine mesencephalic neurons in culture, whereas quantitative differences emerged in the case of rat cortical slices. It is concluded that in human brain, as well as in rat brain, the release of DA in the terminal region of midbrain dopaminergic neurons is regulated through autoreceptors of the D2 subtype.

NGD 94-1 as an agonist at human recombinant dopamine D4.4 receptors expressed in HEK293 cells

The atypical antipsychotic, clozapine has some selectivity for dopamine D4 receptors and is a silent antagonist at these receptors. NGD 94-1 (2-phenyl-4(5)-[4-(2-pyrimidinyl)-piperazin-1-yl-methyl]-imidazole ) is a highly selective dopamine D4 receptor ligand recently introduced as a putative antipsychotic mimicking the dopamine D4 receptor antagonist effects of clozapine. We show that NGD 94-1 is not silent. It behaved as an agonist in human embryonic kidney 293 cells expressing human recombinant dopamine D4.4 receptors. This questions the clinical use of NGD 94-1.

Binding of [3H]U-101958 to sigma1 receptor-like sites in human cerebellum and neuroblastoma cells

1-Benzyl-4-[N-(3-isopropoxy-2-pyridinyl)-N-methyl]-amino-piperidine ([3H]U-101958), a dopamine D4 receptor ligand, was found to bind to a large sigma1 receptor-like component in human cerebellum and SK-N-MC neuroblastoma cells with high affinity (2-4 nM Kd). By contrast, binding to dopamine D4 receptors represented 10% or less of the sigma1 receptor-like site. Considering that U-101958 has been characterized as either a dopamine D4 receptor agonist or antagonist, depending on the system under study, the observation that U-101958 also binds to sigma1 receptor-like sites is important for accurate interpretation of the pharmacological actions of this compound. [3H]U-101958 may be a useful radioligand for sigma1 rather than dopamine D4 receptor sites.

Neuropharmacological assessment of potential dopamine D4 receptor-selective radioligands

Radiolabeled dopamine D4 receptor-selective agents ([3H]1-benzyl-4-[ N-(3-isopropoxy-2-pyridinyl)-N-methyl]-aminopiperidine maleate; [3 H]PNU-101958. and [125I]1-[4-iodobenzyl]-4-[ N-(3-isopropoxy-2-pyridinyl)-N-methyl]-aminopiperidine; [125I]RBI-257) were prepared and characterized. With D4.2- and D2L receptor-transfected cell membranes, [3H]PNU-101958 showed high dopamine D4 receptor affinity and selectivity, and potent inhibition by dopamine D4 receptor-selective compounds. However, its binding with rat brain homogenates showed little regional selectivity, and pharmacology inconsistent with selective dopamine D4 receptor labeling. Autoradiography indicated partial displacement of [3H]PNU-101958 by unlabeled dopamine D4 receptor ligands without regional selectivity, and lack of selective labeling with [125I]RBI-257. The results encourage further efforts to develop better dopamine D4 receptor-selective radioligands.