Agonist and antagonist properties of antipsychotics at human dopamine D4.4 receptors: G-protein activation and K+ channel modulation in transfected cells

Interaction at dopamine D4 receptors may improve cognitive function, which is highly impaired in individuals with schizophrenia, but comparative studies of recent antipsychotics in cellular models of D4 receptor activation are lacking. Here, we report the in-vitro profile of over 30 ligands at recombinant hD4.4 receptors. In [35S]GTPgammaS binding experiments using membranes of CHO-hD4.4 cells, apomorphine, preclamol and the selective D4 agonists, ABT724, CP226269, Ro-10-5824 and PD168077, behaved as partial agonists (Emax 20-60% vs. dopamine), whereas L745870 and RBI257, displayed antagonist properties. The 'conventional' antipsychotic, haloperidol and the 'atypicals', clozapine and risperidone, exhibited antagonist properties, while 'third generation' compounds bifeprunox, SLV313 and F15063, acted as partial agonists (10-30%). Aripiprazole and SSR181507 slightly stimulated [35S]GTPgammaS binding at micromolar concentrations. In Xenopus laevis oocytes co-expressing hD4.4 receptors with G-protein-coupled inwardly rectifying potassium (GIRK) channels, apomorphine, preclamol, ABT724, CP226269, and PD168077 stimulated GIRK currents (Emax 70-80%). The 5-HT1A receptor ligands, WAY100635 and flibanserin, also exhibited partial agonist activity (30% and 15%, respectively). Haloperidol, clozapine, olanzapine and nemonapride did not stimulate GIRK currents, whereas aripiprazole, bifeprunox, SLV313 and F15063, but not SSR181507, exhibited partial agonism (Emax 20-35%). In-vitro responses depended on experimental conditions: increasing NaCl concentration (30 mm to 100 mm) reduced agonist efficacy in [35S]GTPgammaS binding, whereas decreasing the amount of hD4.4 cRNA injected into oocytes (from 2.0 to 0.5 ng/oocyte) reduced agonist efficacy of several compounds. These data indicate that, unlike conventional or 'atypical' antipsychotics, several 'third generation' agents display D4 receptor partial agonism that may be sufficient to influence physiological D4 receptor activity in vivo.     

Differential profile of typical, atypical and third generation antipsychotics at human 5-HT7a receptors coupled to adenylyl cyclase: detection of agonist and inverse agonist properties

5-HT₇ receptors are present in thalamus and limbic structures, and a possible role of these receptors in the pathology of schizophrenia has been evoked. In this study, we examined binding affinity and agonist/antagonist/inverse agonist properties at these receptors of a large series of antipsychotics, i.e., typical, atypical, and third generation compounds preferentially targeting D₂ and 5-HT_1A sites. Adenylyl cyclase (AC) activity was measured in HEK293 cells stably expressing the human (h) 5-HT_7a receptor isoform. 5-HT and 5-CT increased cyclic adenosine monophosphate level by about 20-fold whereas (+)-8-OH-DPAT, the antidyskinetic agent sarizotan, and the novel antipsychotic compound bifeprunox exhibited partial agonist properties at h5-HT_7a receptors stimulating AC. Other compounds antagonized 5-HT-induced AC activity with pK _B values which correlated with their pK _i as determined by competition binding vs [³H]5-CT. The selective 5-HT₇ receptor ligand, SB269970, was the most potent antagonist. For antipsychotic compounds, the following rank order of antagonism potency (pK _B) was ziprasidone > tiospirone > SSR181507 ≥ clozapine ≥ olanzapine > SLV-314 > SLV-313 ≥ aripiprazole ≥ chlorpromazine > nemonapride > haloperidol. Interestingly, pretreatment of HEK293-h5-HT_7a cells with forskolin enhanced basal AC activity and revealed inverse agonist properties for both typical and atypical antipsychotics as well as for aripiprazole. In contrast, other novel antipsychotics exhibited diverse 5-HT_7a properties; SLV-313 and SLV-314 behaved as quasi-neutral antagonists, SSR181507 acted as an inverse agonist, and bifeprunox as a partial agonist, as mentioned above. In conclusion, the differential properties of third generation antipsychotics at 5-HT₇ receptors may influence their antipsychotic profile.     

Inverse agonist properties of atypical antipsychotic drugs

Mechanisms of action of several atypical antipsychotic drugs have been examined at the D(2) dopamine receptor expressed in CHO cells. The drugs tested were found to exhibit inverse agonist activity at the D(2) dopamine receptor based on their effects to potentiate forskolin-stimulated cyclic AMP (cAMP) accumulation. Each of the antipsychotic drugs tested (clozapine, olanzapine, quetiapine and risperidone) increased cAMP accumulation to the same extent. The increase in cAMP was also similar to that seen with typical antipsychotic drugs. Inverse agonism at the D(2) dopamine receptor seems, therefore, to be a property common to all classes of antipsychotic drugs. The effect of sodium ions on the binding of the drugs to the receptor was also assessed. Each of the atypical antipsychotic drugs tested here bound with higher affinity in the absence of sodium ions. Previous studies have shown that some antipsychotic drugs are insensitive to sodium ions and some bind with higher affinity in the presence of sodium ions. Given that all of these antipsychotic drugs are inverse agonists, it may be concluded that this sodium ion sensitivity is unrelated to mechanisms of inverse agonism.     

Putative antipsychotics with pronounced agonism at serotonin 5-HT1A and partial agonist activity at dopamine D2 receptors disrupt basal PPI of the startle reflex in rats

INTRODUCTION: Prepulse inhibition (PPI) of the startle reflex has been extensively studied because it is disrupted in several psychiatric diseases, most notably schizophrenia. In rats, and to a lesser extent, in humans, PPI can be diminished by dopamine (DA) D(2)/D(3) and serotonin 5-HT(1A) receptor agonists. A novel class of potential antipsychotics (SSR181507, bifeprunox, and SLV313) possess partial agonist/antagonist properties at D(2) receptors and various levels of 5-HT(1A) activation. MATERIALS AND METHODS: It thus appeared warranted to assess, in Sprague-Dawley rats, the effects of these antipsychotics on basal PPI. RESULTS: SSR181507, sarizotan, and bifeprunox decreased PPI, with a near-complete abolition at 2.5-10 mg/kg; SLV313 had a significant effect at 0.16 mg/kg only. Co-treatment with the 5-HT(1A) receptor antagonist WAY100,635 (0.63 mg/kg) showed that the 5-HT(1A) agonist activity of SSR181507 was responsible for its effect. By contrast, antipsychotics with low affinity and/or efficacy at 5-HT(1A) receptors, such as aripiprazole (another DA D(2)/D(3) and 5-HT(1A) ligand), and established typical and atypical antipsychotics (haloperidol, clozapine, risperidone, olanzapine, quetiapine, and ziprasidone) had no effect on basal PPI (0.01-2.5 to 2.5-40 mg/kg). DISCUSSION: The present data demonstrate that some putative antipsychotics with pronounced 5-HT(1A) agonist activity, coupled with partial agonist activity at DA D(2) receptors, markedly diminish PPI of the startle reflex in rats. CONCLUSIONS: These data raise the issue of the influence of such compounds on sensorimotor gating in humans.     

Inverse agonist properties of antipsychotic agents at cloned, human (h) serotonin (5-HT)(1B) and h5-HT(1D) receptors.

The actions of diverse antipsychotics at cloned h5-HT(1B) and h5-HT(1D) receptors were examined employing [3H]-GR125,743 and [35S]-GTPgammaS for determination of affinities and efficacies, respectively. Compared with hD(2) receptors, haloperidol, chlorpromazine and olanzapine showed markedly (>100-fold) lower affinity for h5-HT(1D) and h5-HT(1B) receptors at which they expressed inverse agonist properties. Clozapine, risperidone and ocaperidone likewise behaved as inverse agonists at h5-HT(1B) and h5-HT(1D) receptors but their affinities were only approximately 10-fold lower than at hD(2) receptors. Moreover, ziprasidone, S16924 and ORG5222 interacted at h5-HT(1B) and h5-HT(1D) receptors with affinities similar to hD(2) sites. While S16924 and ORG5222 were inverse agonists at h5-HT(1B) and h5-HT(1D) sites, ziprasidone was an inverse agonist at h5-HT(1D) receptors yet a partial agonist at h5-HT(1B) receptors. These actions of antipsychotics were abolished by the selective, neutral antagonist, S18127. In conclusion, with the exception of ziprasidone, all antipsychotics were inverse agonists at h5-HT(1B) and h5-HT(1D) receptors, although they differed markedly in their potency at these sites as compared to hD(2) receptors.     

Action of novel antipsychotics at human dopamine D3 receptors coupled to G protein and ERK1/2 activation

The effects of new generation antipsychotic drugs (APDs) targeting dopamine D(2) and serotonin 5-HT(1A) receptors were compared with typical and atypical APDs on phosphorylation of extracellular signal-regulated kinase 1/2 (ERK 1/2) and measures of G protein activation in CHO cell lines stably expressing the human dopamine D(3) receptor. The preferential dopamine D(3) agonists (+)-7-OH-DPAT and PD128907, like dopamine and quinelorane, efficaciously stimulated ERK 1/2 phosphorylation at dopamine D(3) receptors. In contrast, in [(35)S]GTPgammaS binding experiments, (+)-7-OH-DPAT exhibited partial agonist properties, while PD128907 and quinelorane maintained full agonist properties. The preferential dopamine D(3) ligand BP 897 and the antidyskinetic sarizotan partially activated ERK 1/2 phosphorylation while exerting no agonist activity on GTPgammaS binding, suggesting signal amplification at the MAP kinase level. Antipsychotics differed in their ability to inhibit both agonist-stimulated GTPgammaS binding and ERK 1/2 phosphorylation, but all typical and atypical compounds tested acted as dopamine D(3) receptor antagonists with the exception of n-desmethylclozapine, the active metabolite of clozapine, which partially activated dopamine D(3) receptor-mediated ERK 1/2 phosphorylation. Among the new generation dopamine D(2)/serotonin 5-HT(1A) antipsychotics, only F 15063 and SLV313 acted as pure dopamine D(3) receptor antagonists, bifeprunox was highly efficacious whereas SSR181507 and aripiprazole showed marked partial agonist properties for ERK 1/2 phosphorylation. In contrast, in the GTPgammaS binding study, aripiprazole was devoid of agonist properties and bifeprunox, and to an even lesser extent SSR181507, only weakly stimulated GTPgammaS binding. In summary, these findings underline the differences of dopamine D(3) properties of new generation antipsychotics which may need to be considered in understanding their diverse therapeutic actions.     

Functional potencies of dopamine agonists and antagonists at human dopamine D₂ and D₃ receptors

We measured the functional agonist potencies of dopamine agonists including antiparkinson drugs, and functional antagonist potencies of antipsychotics at human dopamine D(2) and D(3) receptors. In vitro pharmacological assessment included inhibition of forskolin-stimulated cAMP accumulation and the reversal of dopamine-induced inhibition in clonal Chinese hamster ovary cells expressing low and high densities of human dopamine D(2L) and D(2S) receptors (hD(2L)-Low, hD(2L)-High, hD(2S)-Low and hD(2S)-High, respectively) and human dopamine D(3) Ser-9 and D(3) Gly-9 receptors (hD(3)-Ser-9 and hD(3)-Gly-9, respectively). Cabergoline, bromocriptine, pergolide, (±)-7-hydroxy-N,N-di-n-propyl-2-aminotetralin (7-OH-DPAT), talipexole, pramipexole, R-(+)-trans-3,4,4a,10b-tetrahydro-4-propyl-2H,5H-[1]benzopyrano[4,3-b]-1,4-oxazin-9-olhydrochloride (PD128907) and ropinirole behaved as dopamine D(2) and D(3) receptor full agonists and showed higher potencies in hD(2L)-High and hD(2S)-High compared to hD(2L)-Low and hD(2S)-Low. In hD(3)-Ser-9 and hD(3)-Gly-9 compared to hD(2L)-Low and hD(2S)-Low, dopamine, ropinirole, PD128907, and pramipexole potencies were clearly higher; talipexole and 7-OH-DPAT showed slightly higher potencies; pergolide showed slightly lower potency; and, cabergoline and bromocriptine potencies were lower. Aripiprazole acted as an antagonist in hD(2L)-Low; a low intrinsic activity partial agonist in hD(2S)-Low; a moderate partial agonist in hD(3)-Ser-9 and hD(3)-Gly-9; a robust partial agonist in hD(2L)-High; and a full agonist in hD(2S)-High. Amisulpride, sulpiride and perphenazine behaved as preferential antagonists; and chlorpromazine and asenapine behaved as modest preferential antagonists; whereas fluphenazine, haloperidol, and blonanserin behaved as non-preferential antagonists in hD(2S)-Low and hD(2S)-High compared to hD(3)-Ser-9 and hD(3)-Gly-9. These findings may help to elucidate the basis of therapeutic benefit observed with these drugs, with varying mechanisms of action, in the treatment of Parkinson's disease, depression and schizophrenia.     

Identification of selective agonists and antagonists to g protein-activated inwardly rectifying potassium channels: candidate medicines for drug dependence and pain

G protein-activated inwardly rectifying K(+) (GIRK) channels have been known to play a key role in the rewarding and analgesic effects of opioids. To identify potent agonists and antagonists to GIRK channels, we examined various compounds for their ability to activate or inhibit GIRK channels. A total of 503 possible compounds with low molecular weight were selected from a list of fluoxetine derivatives at Pfizer Japan Inc. We screened these compounds by a Xenopus oocyte expression system. GIRK1/2 and GIRK1/4 heteromeric channels were expressed on Xenopus laevis oocytes at Stage V or VI. A mouse IRK2 channel, which is another member of inwardly rectifying potassium channels with similarity to GIRK channels, was expressed on the oocytes to examine the selectivity of the identified compounds to GIRK channels. For electrophysiological analyses, a two-electrode voltage clamp method was used. Among the 503 compounds tested, one compound and three compounds were identified as the most effective agonist and antagonists, respectively. All of these compounds induced only negligible current responses in the oocytes expressing the IRK2 channel, suggesting that these compounds were selective to GIRK channels. These effective and GIRK-selective compounds may be useful possible therapeutics for drug dependence and pain.     

Effects of recent and reference antipsychotic agents at human dopamine D2 and D3 receptor signaling in Chinese hamster ovary cells

RATIONALE: Central dopamine D2 receptor blockade is an essential property of antipsychotic agents in the treatment of schizophrenia. However, for certain of the newer antipsychotics (e.g., sertindole), the in vitro D2 receptor binding affinity does not correlate with in vivo central dopamine antagonism. OBJECTIVE: This study aimed to investigate the effect and potency of haloperidol, pipamperone, clozapine, risperidone, sertindole, zotepine, olanzapine, and quetiapine on signaling pathways of human dopamine D2S and D3 receptors expressed in Chinese hamster ovary cells and to relate this to their dopamine antagonist potency in vivo. METHODS: Chinese hamster ovary cells, stably expressing high levels of hD2S and hD3 receptors were cultured: dopamine-stimulated [35S]-GTPgammaS binding was investigated in cell membrane preparations, and forskolin-induced cAMP formation was measured in intact cells. RESULTS: The antipsychotic agents inhibited dopamine-stimulated [35S]-GTPgammaS binding mediated by hD2S and hD3 receptors with potencies equal to their receptor binding affinities. The antipsychotics reversed dopamine inhibition of cAMP formation (equally well detectable with both hD2S and hD3 receptors) dose dependently at both receptors. Partial agonist effects were not observed with any of the antipsychotics. Antagonistic potencies of haloperidol, risperidone, and pipamperone in the cAMP test were equal to their receptor binding affinities. Sertindole and olanzapine were more than ten times less potent dopamine antagonists in the intact cell assay than in the assay using cell membranes; the other compounds showed less marked potency differences. CONCLUSIONS: Olanzapine and sertindole were less efficacious dopamine antagonists in intact cell assays, possibly due to avid uptake in cells. For sertindole, the weak hD2S receptor antagonism in intact cells corresponded to a weak in vivo central dopamine antagonism assessed in rats. However, for olanzapine, hD2S receptor binding affinity correlated better with its in vivo dopamine antagonist potency. Such discrepancies may be further explained by relative differences of the compounds in penetrating into the brain.     

Typical antipsychotics exhibit inverse agonist activity at rat dopamine D1-like receptors expressed in Sf9 cells

The baculovirus system has been used to express the rat dopamine D1 receptors in Spodoptera frugiperda (Sf9) cells. A panel of typical antipsychotics including, -flupenthixol, fluphenazine and thioridizine were found to inhibit dopamine-dependent stimulation of adenylyl cyclase. However, these compounds were also found to inhibit adenylyl cyclase activity in the absence of agonist in Sf9 cells expressing dopamine D1-like receptors. Therefore, these nonselective dopamine receptor compounds displayed negative intrinsic or inverse agonist activity. None of the compounds tested were neutral antagonists.     

Pharmacological actions of a novel and selective dopamine D3 receptor antagonist, KCH-1110

1-(2-ethoxy-phenyl)-4-[3-(3-thiophen-2-yl-isoxazolin-5-yl)-propyl]-piperazine (KCH-1110), has a high affinity for human dopamine D3 (hD3) receptor (Ki=1.28 nM) with about 90-fold selectivity over the human dopamine D2L (hD2L) receptor. Antipsychotic or antidopaminergic activity of KCH-1110 was investigated in the models for the positive symptoms of schizophrenia, apomorphine-induced climbing and cocaine-induced hyperlocomotion, in mice. Intraperitoneal (i.p.) or oral (p.o.) administration of KCH-1110 potently inhibited the apomorphine-induced cage climbing without any rotarod ataxia in mice. Cocaine-induced hyperactivity was also antagonised by KCH-1110. In addition, KCH-1110 attenuated the hypothermia induced by a selective dopamine D3 agonist, 7-OH-DPAT in mice. KCH-1110 did not induce catalepsy in mice, but at much higher doses only a slight catalepsy response was shown. Although high doses of KCH-1110 significantly enhanced serum prolactin secretion in rats, low dose of KCH-1110 did not increase prolactin levels in rats. The present studies, therefore, suggest that KCH-1110 is a potent and relatively selective dopamine D3 receptor antagonist with antipsychotic actions.     

5-HT1A receptor agonist properties of antipsychotics determined by [35S]GTPgammaS binding in rat hippocampal membranes

1. 5-Hydroxytryptamine 1A (5-HT1A) receptors have attracted increasing attention as a promising target for antipsychotic therapy. Although many atypical antipsychotic drugs, including the prototype clozapine, have been reported to be partial agonists at 5-HT1A receptors, these results are often fragmental and derived mainly from experiments that used cultured cells. 2. In the present study, [35S]guanosine 5'-O-(3-thiotriphosphate) ([35S]GTPgammaS) binding assay in rat hippocampal membranes was applied to a series of antipsychotic drugs, especially atypical antipsychotics. 3. Most, but not all, of atypical antipsychotic drugs and the classical antipsychotic drug nemonapride behaved as partial agonists at 5-HT1A receptors with varied potencies and relative efficacies. The most potent compound was perospirone with a mean EC50 of 27 nmol/L, followed by aripiprazole (45 nmol/L) > ziprasidone (480 nmol/L) > nemonapride (790 nmol/L) > clozapine (3900 nmol/L) > quetiapine (26,000 nmol/L). The maximal percentage increases over the basal binding (%Emax) for these antipsychotic drugs were 30-50%, with the exception of perospirone (approximately 15%), whereas 5-HT stimulated the binding to a mean %Emax of 105%. 4. Increasing concentrations of the selective and neutral 5-HT1A antagonist WAY100635 shifted the concentration-response curve of nemonapride-stimulated [35S]GTPgammaS binding to the right and in parallel. 5. The relative efficacy or intrinsic activity of a compound was affected differently by the differing concentrations of guanosine diphosphate (GDP) in the assay buffer, which should be taken into consideration when determining the relative efficacies of these antipsychotics as 5-HT1A receptor agonists. 6. These results provide important information concerning the relevance of 5-HT1A receptor partial agonist properties in the treatment for schizophrenic patients with most, if not all, of atypical antipsychotic drugs.     

Affinity of cyamemazine,an anxiolytic antipsychotic drug,for human recombinant dopamine vs. serotonin receptor subtypes

Animal studies indicate that the anxiolytic properties of the antipsychotic agent cyamemazine may result from blockade of serotonin 5-HT(2C) receptors and to a lesser extent from blockade of serotonin 5-HT(3) receptors. Here, we used human recombinant receptors to determine the relative affinity of cyamemazine for serotonin and dopamine receptor subtypes. In addition, cyamemazine was tested in other brain receptor types and subtypes which are considered to mediate central nervous systems effects of drugs. Hence, cyamemazine affinity was determined in human recombinant receptors expressed in CHO cells (hD(2), hD(3), and hD(4.4) receptors, h5-HT(1A), h5-HT(2A), h5-HT(2C), and h5-HT(7), and hM(1), hM(2), hM(3), hM(4), and hM(5) receptors), L-cells (hD(1) receptor), and HEK-293 cells (h5-HT(3) receptors) or natively present in N1E-115 cells (5-HT(3) receptors) or in rat cerebral cortex (non-specific alpha(1)- and alpha(2)-adrenoceptors, GABA(A) and GABA(B) receptors, H(3) histamine receptors), and guinea-pig cerebellum (H(1) central and H(2) histamine receptors) membranes. Similarly to atypical antipsychotics, cyamemazine exhibited high affinity for: (i) h5-HT(2A) receptors (K(i)=1.5+/-0.7 nM, mean+/-SEM, N=3) and this was four times higher than for hD(2) receptors (K(i)=5.8+/-0.8 nM), (ii) h5-HT(2C) receptors (K(i)=11.8+/-2.2nM), and (iii) 5-HT(7) receptors (K(i)=22 nM). Conversely, cyamemazine exhibited very low affinity for h5-HT(3) receptors (K(i)=2.9+/-0.4 microM). In conclusion, similarly to atypical antipsychotic agents, cyamemazine, possesses high affinity for h5-HT(2A), h5-HT(2C), and h5-HT(7) receptors, a feature which can explain its low propensity to cause extrapyramidal adverse reactions in clinical practice. The high affinity for h5-HT(2C) receptors, but not for h5-HT(3) receptors, can account for the anxiolytic activity of cyamemazine in human subjects.     

PD 158771, a potential antipsychotic agent with D(2)/D(3) partial agonist and 5-HT(1A) agonist actions. I. Neurochemical effects

The neurochemical effects of a novel dopamine (DA) D(2)-like and serotonin (5-HT) 5-HT(1A) agonist, PD 158771, are described. PD 158771 exhibited affinities for human D(2L), D(3) and D(4.2) receptors expressed in Chinese hamster ovary (CHO)-K1 cells with K(i) (nM) values of 5.2, 13.7 and 34.8 respectively. PD 158771 showed high affinity for cloned human 5-HT(1A) (K(i) = 2.6 nM) and rat hippocampal 5-HT(1A) receptors (K(i) = 3.5 nM). Weaker affinities were observed at alpha 1-adrenergic (K(i) = 43 nM), histamine H(1) (IC(50) = 30 nM), 5-HT(2A) (K(i) = 24.5 nM) and sigma (sigma) -1 binding sites (K(i) = 24.5 nM). In measures of in vitro functional activity, PD 158771 stimulated [(3)H]thymidine uptake in CHO p-5 cells transfected with hD(3) receptors with a maximal effect of 23% relative to quinpirole. In hD(2)L, the corresponding value was 60% with an EC(50) of 29 nM, again indicating partial DA agonist action of PD 158771. In vivo, PD 158771 produced a dose-related decrease in DA synthesis in the striatum and mesolimbic regions of rat brain treated with gamma-butyrolactone (GBL), indicating a DA autoreceptor agonist action. In animals not treated with GBL, PD 158771 produced a dose-related decrease in DA synthesis and extracellular DA. A decrease in 5-HT synthesis in several brain areas was observed consistent with an agonist response. Further support for DA autoreceptor agonist action is that PD 158771 produced a partial inhibition of the firing of substantia nigra zona compacta DA neurons, an effect reversed by haloperidol. In conclusion, PD 158771 exhibited affinities for DA and 5-HT receptors, appears to possess DA and 5-HT agonist actions; and it could provide improved antipsychotic profile with minimal side effects.     

Aripiprazole and its human metabolite are partial agonists at the human dopamine D2 receptor, but the rodent metabolite displays antagonist properties

Aripiprazole is a novel antipsychotic drug, which displays partial agonist activity at the dopamine D(2) receptor. Aripiprazole has been extensively studied pre-clinically, both in vitro and in vivo, and these results have been correlated with clinical findings. However, aripiprazole is metabolised differently in rats and man and these metabolites may contribute to the profile of aripiprazole observed in vivo. We have therefore studied the interaction of aripiprazole and its principal rat and human metabolites in both in vitro models of dopamine hD(2) receptor function and affinity, and of in vivo models of dopamine rat D(2) receptor function. The human metabolite displayed similar levels of partial agonist activity to aripiprazole at the dopamine hD(2) receptor and displayed similar behavioural profile to aripiprazole in vivo, suggesting that in man the metabolite may maintain the effects of aripiprazole. In contrast, the rat metabolite displayed antagonist activity both in vitro and in vivo. Thus care must be taken in ascribing effects seen in vivo with aripiprazole in rats to dopamine D(2) receptor partial agonist activity in man, and that care must also be taken in extrapolating effects seen in rats to man, particularly from long-term studies.     

The effects of the dopamine D2 agonist sumanirole on prepulse inhibition in rats

Dopamine agonists reduce prepulse inhibition (PPI) of startle in rats. While it is used to predict antipsychotic efficacy, the specific receptor subtypes mediating this effect of dopamine agonists remain unclear. We characterized the effects of sumanirole, a highly selective D2 agonist, on PPI in rats. Sumanirole decreased PPI at 60–120 ms prepulse intervals, and increased PPI at 10–20 ms intervals. PPI deficits were antagonized by low doses of the preferential D2 antagonist L741626, supporting a D2 mechanism of action. Sumanirole is a valuable tool for parsing the role of dopamine receptor subtypes in the regulation of PPI.     

WAY-163909, a 5-HT2C agonist, enhances the preclinical potency of current antipsychotics

Introduction  5-HT_2C agonists, by decreasing mesolimbic dopamine without affecting nigrostriatal dopamine, are predicted to have antipsychotic efficacy with low extrapyramidal side effects (EPS). Combining 5-HT_2C agonists with low doses of existing antipsychotics could increase treatment efficacy while reducing treatment liabilities such as EPS (typical antipsychotics), and the propensity for weight gain (atypical antipsychotics). Objectives  The objectives of these studies were to combine WAY-163909, a selective 5-HT_2C agonist, with either the typical antipsychotic haloperidol, or the atypical antipsychotic clozapine, at doses that were ineffective on their own, with the expectation that a shift in potency in several rodent behavior models predictive of antipsychotic activity would occur. Results and discussion  In mice, co-administration of either haloperidol, or clozapine, produced a significant leftward shift in the ability of WAY-163909 to block apomorphine-induced climbing behavior, without any affect on apomorphine-induced stereotypy or an increased propensity for catalepsy. In the rat-conditioned avoidance model, WAY-163909 was combined with either haloperidol or clozapine at doses that individually produced reductions in avoidance response on the order of 10%, while the combination of WAY-163909 and either of the antipsychotics resulted in a greater than 70% reduction in avoidance, with no evidence of response failures, or pharmacokinetic interaction. Conclusion  Doses of either haloperidol or clozapine, that failed to antagonize an MK-801 induced deficit in prepulse inhibition, significantly attenuated the sensory gating deficit when combined with WAY-163909. Data support the notion that 5-HT_2C receptor agonists, co-administered with other marketed antipsychotics, allow for dose sparing with a more favorable side-effect profile.     

Bupivacaine induces apoptosis via mitochondria and p38 MAPK dependent pathways

N-desmethylclozapine (NDMC or norclozapine) is the major active metabolite of the antipsychotic clozapine in humans. The activity of NDMC differs from clozapine at a number of neurotransmitter receptors, probably influencing the pharmacological effects of clozapine treatment. Here, we tested the properties of NDMC in comparison with clozapine at recombinant human dopamine D(2) and serotonin 5-HT(1A) receptors, using a panel of functional assays implicating diverse signalling pathways. At dopamine D(2) receptors, NDMC as well as clozapine did not display agonist activity in measures of G protein activation by [(35)S]GTPγS binding and in the sensitive Extracellular Signal-Regulated Kinase 1/2 (ERK1/2) phosphorylation assay. In contrast, there were weak partial agonist actions of NDMC (but not of clozapine) for dopamine D(2)-dependent activation of Ca(2+) liberation via coexpressed chimeric Gα(q/o) proteins and for G protein-coupled inward rectifier potassium channel (GIRK) current induction in Xenopus oocytes. Intriguingly, GIRK currents induced by NDMC via dopamine D(2) receptors showed a rapid and transient time course, strikingly different from currents recorded with other receptor agonists. At serotonin 5-HT(1A) receptors, NDMC was a more efficacious partial agonist than clozapine for [(35)S]GTPγS binding, ERK1/2 phosphorylation and GIRK activation. Respective low and moderate partial agonist properties of NDMC at dopamine D(2) and serotonin 5-HT(1A) receptors thus differentiate the metabolite from its parent drug and may contribute to the overall effects of clozapine pharmacotherapy.     

The human histamine H3 receptor couples to GIRK channels in Xenopus oocytes

The histamine H(3) receptor mediates inhibitory responses in the nervous system. Here, we demonstrate the coupling of the human histamine H(3) receptor to G protein-coupled inward rectifier potassium (GIRK) channels in Xenopus oocytes, using voltage-clamp. The histamine H(3) receptor agonist (R)-alpha-methylhistamine increased GIRK currents with an EC(50) of 2.5 nM. The response to (R)-alpha-methylhistamine was inhibited by the specific antagonist/inverse agonist clobenpropit. GIRK channels represent a novel effector pathway for the histamine H(3) receptor, also suggesting the use of electrophysiology assays in histamine H(3) receptor drug screening, allowing for the resolution of G protein activation kinetics.     

Aripiprazole: a partial dopamine D2 receptor agonist antipsychotic

This paper reviews the clinical pharmacology, efficacy and safety of the novel antipsychotic drug aripiprazole. All published citations regarding aripiprazole were reviewed using a Medline^® search (completed for citations through mid-year, 2002). In addition, abstracts from recent scientific meetings presenting data not yet published (nor peer-reviewed) were reviewed. Aripiprazole has a unique mechanism of action as a dopamine D2 partial agonist, serotonin 5-HT_1A partial agonist and serotonin 5-HT_2A antagonist. Like other new antipsychotics, aripiprazole has the profile of an atypical agent, with efficacy in the treatment of positive and negative symptoms of psychosis as well as mood symptoms, a low rate of neurological side effects and no significant adverse effect on serum prolactin concentrations. In addition, aripiprazole was not associated with significant weight gain or QTc prolongation in both acute and long-term treatment trials.