A PET study of D1-like dopamine receptor ligand binding

Rationale: Several positron emission tomography (PET) studies have shown that radioligand binding to D₂-like dopamine receptors competes with endogenous dopamine. Objective: The purpose of this PET study was to examine the effect of amphetamine and reserpine on D₁-like dopamine receptor binding. Methods: Three Cynomolgus monkeys were examined with the radioligands [¹¹C]SCH 23390 or [¹¹C]NNC 112 at baseline condition and after pretreatment with amphetamine (2 mg/kg IV). The B/F values (binding potential) in the striatum and the neocortex were calculated at transient equilibrium. In two monkeys, the effect of the long-lasting dopamine depletion after reserpine (1 mg/kg IV) was followed by a repeated Scatchard procedure in up to 77 days after drug administration. The Scatchard analysis was based on two PET measurements with high and low specific radioactivity and allowed the calculation of D₁-like dopamine receptor density (B_max) and apparent affinity (K_D ^app). Results: The effect of amphetamine on the B/F values was between –14 and 6%. These changes can be considered as within the range of the test-retest reliability. Thus, there was no evident effect of amphetamine-induced dopamine release on D₁-like dopamine receptor binding. Five hours after reserpine administration, there was no change in B_max or K_D ^app. At 3, 23, and 28 days after reserpine administration, the Scatchard analyses indicated a 13–20% reduction in B_max without any evident change in K_D ^app in both the striatum and the neocortex. Conclusions: The lack of evident effects of amphetamine and reserpine on D₁-like dopamine receptor binding is markedly different from the 20% amphetamine-induced decrease and 50% reserpine-induced increase that has been consistently reported for D₂-like dopamine receptor binding. The data indicated that D₁-like dopamine receptor occupancy of endogenous dopamine is low at physiological condition. It is thus unlikely that D₁-like dopamine receptor radioligands can be used to measure changes in the concentration of endogenous dopamine. Received: 8 March 1999 / Final version: 12 May 1999     

In vivo occupancy of dopamine D2 receptors by antipsychotic drugs and novel compounds in the mouse striatum and olfactory tubercles

Interaction with dopamine D₂-like receptors plays a major role in the therapeutic effects of antipsychotic drugs. We examined in vivo dopamine D₂ receptor occupancy of various established and potential antipsychotics in mouse striatum and olfactory tubercles 1 h after administration of the compound, using [³H]nemonapride as a ligand. All the compounds reduced in vivo binding of [³H]nemonapride in the striatum. When administered systemically, conventional antipsychotics, D₂ antagonists, nemonapride (ID₅₀: 0.034 mg/kg), eticlopride (0.047), haloperidol (0.11) and raclopride (0.11) potently inhibited [³H]nemonapride binding. The ‘atypical’ antipsychotics, risperidone (0.18), ziprasidone (0.38), aripiprazole (1.6), olanzapine (0.99), and clozapine (11.1) were less potent for occupying D₂-like receptors. New compounds, displaying marked agonism at 5-HT_1A receptors in addition to D₂ receptor affinity, exhibited varying D₂ receptor occupancy: bifeprunox (0.25), SLV313 (0.78), SSR181507 (1.6) and sarizotan (6.7). ID₅₀ values for inhibition of [³H]nemonapride binding in the striatum correlated with those in the olfactory tubercles (r=0.95, P<0.0001). These values also correlated with previously-reported in vitro affinity of the compounds at rat D₂ receptors (r=0.85, P=0.0001) and with inhibition of apomorphine-induced climbing in mice (r=0.79 P=0.0005). In contrast, there was no significant correlation between ID₅₀ values herein and previously-reported ED₅₀ values for catalepsy in mice. These data indicate that: (1) there is no difference in D₂ receptor occupancy in limbic versus striatal regions between most classical and atypical or potential antipsychotics; and (2) high occupancy of D₂ receptors can be dissociated from catalepsy, if the drugs also activate 5-HT_1A receptors. Taken together, these data support the strategy of simultaneously targeting D₂ receptor blockade and 5-HT_1A receptor activation for new antipsychotics.     

Prolactin plasma levels and D2-dopamine receptor occupancy measured with IBZM-SPECT

By the application of¹²³([¹²³I]IBZM), an iodine-labelled dopamine D₂-receptor antagonist, brain D₂ receptors in humans can be visualized with single photon emission computed tomography (SPECT). The ratio of IBZM binding to striatal regions versus binding to frontal cortex (ST/FC ratio) provided a semiquantitative measurement of D₂ receptor binding in the striatum. This study investigated the relationship between receptor occupancy and plasma prolactin levels in 12 male patients treated with haloperidol, benperidol or clozapine. Prolactin levels were positively correlated with D₂ receptor occupancy, reflecting at least in part a comparable dopamine receptor antagonism in different dopaminergic pathways.     

Antipsychotics possessing antidepressive efficacy increase G<Subscript>olf</Subscript> protein in rat striatum

Rationale Paliperidone ER is a novel antipsychotic drug in an extended-release (ER) formulation. As with all antipsychotics, careful dose setting is necessary to avoid side effects. Objectives In this study, we measured striatal and extrastriatal dopamine D₂ receptor occupancy during paliperidone ER treatment in patients with schizophrenia using positron emission tomography (PET) to compare regional occupancy and to estimate the optimal dose. Materials and methods Thirteen male patients with schizophrenia participated in this 6-week multiple-dose study. Six of them took 3 mg of paliperidone ER per day, four took 9 mg, and three took 15 mg. Two to 6 weeks after first drug intake, two PET scans, one with [¹¹C]raclopride and one with [¹¹C]FLB 457, were performed in each patient on the same day. The relationship between the dose or plasma concentration of paliperidone and dopamine D₂ receptor occupancy was calculated. Results The dopamine D₂ receptor occupancies in the striatum measured with [¹¹C]raclopride and the temporal cortex measured with [¹¹C]FLB 457 were 54.2–85.5% and 34.5–87.3%, respectively. ED₅₀ values of the striatum and temporal cortex were 2.38 and 2.84 mg/day, respectively. There was no significant difference in dopamine D₂ receptor occupancy between the striatum and the temporal cortex. Conclusions The data from this study suggest that paliperidone ER at 6–9 mg provides an estimated level of dopamine D₂ receptor occupancy between 70–80% and that the magnitude of dopamine D₂ receptor occupancy is similar between the striatum and temporal cortex.     

PET examination of [11C]NNC 687 and [11C]NNC 756 as new radioligands for the D1-dopamine receptor

The benzazepines NNC 687 and NNC 756 have in animal studies been described as selective D₁-dopamine receptor antagonists. Both compounds have been labeled with¹¹C for examination by positron emission tomography (PET). In the present study central receptor binding was studied in monkeys and healthy men. After IV injection of both radioligands in Cynomolgus monkeys radioactivity accumulated markedly in the striatum, a region with a high density of D₁-dopamine receptors. This striatal uptake was displaced by high doses of the selective D₁-antagonist SCH 23390 (2 mg/kg) but not by the 5HT₂-antagonist ketanserin (1.5 mg/kg) or the selective D₂-antagonist raclopride (3 mg/kg). The cortical uptake after injection of [¹¹C]NNC 687 was not reduced in displacement experiments with ketanserin. The cortical uptake of [¹¹C]NNC 756 was reduced in displacement and protection experiments with ketanserin by 24–28% (1.5 mg/kg), whereas no reduction could be demonstrated on striatal uptake. In healthy males both compounds accumulated markedly in the striatum. For [¹¹C]NNC 687 the ratio of radioactivity in the putamen to cerebellum was about 1.5. For [¹¹C]NNC 756 the ratio was about 5. This ratio of 5 for [¹¹C]NNC 756 is the highest obtained so far for PET radioligands for the D₁-dopamine receptor.     

Evidence for the sensitivity of operant timing behaviour to stimulation of D1 dopamine receptors

Rationale Temporal differentiation of operant behaviour is sensitive to dopaminergic manipulations. Previous studies using the fixed-interval peak procedure implicated D₂-like dopamine receptors in these effects. However, recent findings suggest that d-amphetamine alters timing performance on the free-operant psychophysical procedure via D₁-like receptors. It is not known whether this effect of d-amphetamine is mimicked by direct D₁-like receptor stimulation. Objective The effects of a D₁-like receptor agonist 6-chloro-2,3,4,5-tetrahydro-1-phenyl-1H-3-benzazepine (SKF-81297) on performance on the free-operant psychophysical procedure and the interaction between SKF-81297 and a D₁-like receptor antagonist 8-bromo-2,3,4,5-tetrahydro-3-methyl-5-phenyl-1H-3-benzazepin-7-ol (SKF-83566) and a D₂-like receptor antagonist haloperidol, were examined. Materials and methods Rats were trained to respond on two levers (A and B) under a free-operant psychophysical schedule, in which sucrose reinforcement was provided intermittently for responding on A during the first half and on B during the second half of 50-s trials. Logistic psychometric functions were fitted to the relative response rate data (percent responding on B [%B] vs time from trial onset [t]) under each treatment condition, and quantitative indices of timing (T₅₀ [value of t corresponding to %B = 50] and the Weber fraction [(T₇₅-T₂₅)/2T₅₀; T₂₅ and T₇₅ are values of t corresponding to %B = 25 and %B = 75] were compared among treatments. Results SKF-81297 (0.8 mg kg⁻¹) reduced T₅₀; this effect was antagonized by SKF-83566 (0.03 mg kg⁻¹) but not by haloperidol (0.05, 0.1 mg kg⁻¹). Conclusions Stimulation of D₁-like dopamine receptors affects performance in the free-operant psychophysical procedure.     

Imaging apomorphine stimulation of brain arachidonic acid signaling <Emphasis Type="Italic">via</Emphasis> D<Subscript>2</Subscript>-like receptors in unanesthetized rats

Rationale and objective Because of the important role of dopamine in neurotransmission, it would be useful to be able to image brain dopamine receptor-mediated signal transduction in animals and humans. Administering the D₁–D₂ receptor agonist apomorphine may allow us to do this, as the D₂-like receptor is reported to be coupled to cytosolic phospholipase A₂ activation and arachidonic acid (AA) release from membrane phospholipid. Methods Unanesthetized adult rats were given intraperitoneally apomorphine (0.5 mg/kg) or saline, with or without pretreatment with 6 mg/kg intravenous raclopride, a D₂/D₃ receptor antagonist. [1–¹⁴C]AA was injected intravenously, then AA incorporation coefficients k*—brain radioactivity divided by integrated plasma radioactivity—markers of AA signaling, were measured using quantitative autoradiography in 62 brain regions. Results Apomorphine significantly elevated k* in 26 brain regions, including the frontal cortex, motor and somatosensory cortex, caudate-putamen, thalamic nuclei, and nucleus accumbens. Raclopride alone did not change baseline values of k*, but raclopride pretreatment prevented the apomorphine-induced increments in k*. Conclusions A mixed D₁–D₂ receptor agonist, apomorphine, increased the AA signal by activating only D₂-like receptors in brain circuits containing regions with high D₂-like receptor densities. Thus, apomorphine might be used with positron emission tomography to image brain D₂-like receptor-mediated AA signaling in humans in health and disease.     

F15063, a potential antipsychotic with dopamine D(2)/D(3) antagonist, 5-HT(1A) agonist and D(4) partial agonist properties: (IV) duration of brain D2-like receptor occupancy and antipsychotic-like activity versus plasma concentration in mice

F15063 (N-[(2,2-dimethyl-2,3-dihydro-benzofuran-7-yloxy)ethyl]-3-(cyclopent-1-enyl)-benzylamine fumarate salt) is a novel potential antipsychotic with dopamine D₂/D₃ blocking properties and agonist activity at 5-HT_1A and D₄ receptors. The pertinent parameter for pharmacological activity of antipsychotics appears to be central D2-like receptor occupancy. However, its duration is not necessarily correlated with drug plasma levels, on which clinical dosing regimens are often based. Thus, we compared in mice the duration of actions of F15063 and haloperidol to (1) inhibit apomorphine-induced climbing and sniffing (behavioural measures of D2-like receptor antagonism) and (2) occupy D2-like receptors in vivo in the striatum and olfactory tubercles (inhibition of [³H]nemonapride binding). Finally, we measured plasma levels of F15063. D2-like receptor occupancy in the striatum remained elevated at 1, 4 and 8 h postadministration, with both F15063 (ID₅₀: 7.1, 3.6 and 16.5 mg/kg p.o., respectively) and the typical antipsychotic, haloperidol (ID₅₀: 1.4, 0.52 and 0.53 mg/kg p.o., respectively). This was paralleled by a protracted inhibition of apomorphine-induced climbing (ED₅₀: 0.9, 2.8 and 3.6 mg/kg p.o., and 0.21, 0.37 and 0.87 mg/kg p.o., respectively, for F15063 and haloperidol). In contrast, after administration of 10 mg/kg p.o. of F15063, its plasma levels decreased rapidly: 15.2, 2.1 and 0.6 ng/ml, 1, 4 and 8 h after administration, respectively. A similar pattern of results was observed when F15063 and haloperidol were administered i.p. and s.c., respectively. To summarise, the time-course of D2-like receptor occupancy and inhibition of apomorphine-climbing (and sniffing) behaviours was similarly long lasting with F15063 and haloperidol. In addition, the durations of action of F15063 and haloperidol in a behavioural model of antipsychotic-like activity were closely correlated to their occupancy of central D2-like receptors, and much longer than their presence in plasma.     

High 5-HT2 receptor occupancy in clozapine treated patients demonstrated by PET

The clinical benefit of the atypical antipsychotic drug clozapine may be related to a combined effect on D₂ and 5-HT₂ receptors. To examine the basis for this hypothesis, positron emission tomography (PET) and the radioligand [¹¹C]N-methylspiperone were used to determine cortical 5-HT₂ receptor occupancy in three psychotic patients treated with 125 mg, 175 mg and 200 mg clozapine daily. The uptake of [¹¹C]N-methylspiperone in the frontal cortex was very low compared to that in neuroleptic naive schizophrenic patients. 5-HT₂ receptor occupancy calculated in the clozapine treated patients was 84%, 87% and 90%. The results show that clinical treatment with clozapine induces a high 5-HT₂ receptor occupancy in psychotic patients at a low dose level.     

Oral administration of NNC 756 — a placebo controlled PET study of D1-dopamine receptor occupancy and pharmacodynamics in man

NNC 756 is a new benzazepine with high affinity and selectivity for D₁-dopamine receptors. In a double-blind, placebo controlled, cross-over study, positron emission tomography and the radioligand [¹¹C]SCH 23390 were used to determine central D₁-do-pamine receptor occupancy after a single oral dose of 80 mg NNC 756 in three healthy men. NNC 756 induced 75, 66 and 47% occupancy of D₁-dopamine receptors in the putamen of at 1.5 h after drug administration and 46, 36 and 24% after 7.5 h. There was a hyperbolic relationship between the occupancy values and the serum concentrations. The K_i value for the hyperbola was 6.4 ng/ml (±SD 1.4). The occupancy at 1.5 h is on the same level as that shown to induce effects in animal models for prediction of antipsychotic effect. Restlessness (akathisia) appeared in two subjects and nausea in one subject at time of peak drug concentration in serum. The oral dose level of 80 mg should be appropriate to investigate the potential antipsychotic effect of NNC 756.     

Regulation of ionotropic glutamate receptors following subchronic and chronic treatment with typical and atypical antipsychotics

Quantitative in vitro receptor autoradiography was used to examine changes in three ionotropic glutamate receptor subtypes using ³H-MK801 (NMDA-R antagonist), ³H-CNQX (AMPA-R antagonist) and ³H-kainic acid (kainate-R agonist) following subchronic (28 days) and chronic (8 months) treatment of rats with a typical antipsychotic, haloperidol (1.5 mg/kg per day), atypical antipsychotic, clozapine (25 mg/kg per day), the dopamine D₂/D₃ receptor antagonist, raclopride (10 mg/kg per day), and the dopamine D1 (D₁/D₅) receptor antagonist SCH23390 (0.5 mg/kg per day). Subchronic and chronic drug treatments did not significantly alter ³H-CNQX or ³H-kainate binding in any of brain regions examined. Subchronic SCH23390 treatment elevated ³H-MK801 binding in the hippocampal formation with significant increases in the CA₁ and dentate gyrus, suggesting a specific role for dopamine D1 receptors in the regulation of hippocampal NMDA receptor function. Subchronic, but not chronic, haloperidol and clozapine treatment significantly reduced ³H-MK801 binding in the medial prefrontal cortex. This suggests that typical and atypical antipsychotics may exert some of their clinical effects by affecting NMDA receptors in the medial prefrontal cortex. Both subchronic and chronic clozapine treatment decreased ³H-MK801 binding in the caudate putamen. The minimal extrapyramidal side effects produced by clozapine may result, in part, from the reduction in NMDA receptor binding in the caudate putamen. Received: 29 February 1996 /Final version: 15 July 1996     

Time course of central nervous dopamine-D2 and 5-HT2 receptor blockade and plasma drug concentrations after discontinuation of quetiapine (Seroquel®) in patients with schizophrenia

Quetiapine (Seroquel) is a novel antipsychotic with an atypical profile in animal models and a relatively short plasma half-life of 2.5–5 h. In the present study, we used PET to compare the time course of blockade of dopamine D₂ and serotonin 5HT₂ receptors of quetiapine using C¹¹-raclopride and C¹¹-N-methyl-spiperone as ligands, parallel to monitoring plasma drug concentrations. It was an open study in 11 schizophrenic men using a fixed dose of 450 mg quetiapine. Eight men completed the 29 days treatment, followed by four PET scans performed over a 26-h period after withdrawal of the compound. Quetiapine was shown to bind to dopamine D₂ receptors in striatum and 2 h (t_max) after the last dose, 44% receptor occupancy was calculated. After 26 h it had dropped to the same level as was found in untreated healthy volunteers. Serotonin 5HT₂ receptor blockade in the frontal cortex was 72% after 2 h, which declined to 50% after 26 h. The terminal plasma half-life of quetiapine was 5.3 h. Clinically, our eight patients had good antipsychotic effect without any extrapyramidal side-effects. Our data shows that quetiapine has a relatively low affinity for dopamine D₂ receptors, with an occupancy half-life (10 h), which was about twice as long as that for plasma. A more prolonged blockade of the serotonin 5HT₂ receptors was found in the frontal cortex, with receptor occupancy half-life of 27 h. Compared to clozapine, as demonstrated in other studies, quetiapine has much the same ratio of D₂/5HT₂ occupancy. This could suggest that the combination of D₂/5HT₂ receptor blockade contributes to the antipsychotic effect and a low incidence of EPS seen with quetiapine in comparative phase three trials. Our results also confirm the clinical data that quetiapine can be administered twice daily. Received: 13 December 1996/Final version: 13 June 1997     

Decreased prepulse inhibition and increased sensitivity to muscarinic,but not dopaminergic drugs in M<Subscript>5</Subscript> muscarinic acetylcholine receptor knockout mice

Rationale Schizophrenic patients show decreased measures of sensorimotor gating, such as prepulse inhibition of startle (PPI). In preclinical models, these measures may be used to predict antipsychotic activity. While current antipsychotic drugs act largely at dopamine receptors, the muscarinic acetylcholine receptors offer promising novel pharmacotherapy targets. Of these, the M₅ receptor gene was recently implicated in susceptibility to schizophrenia. Due to the lack of selective ligands, muscarinic receptor knockout mice have been generated to elucidate the roles of the five receptor subtypes (M₁–M₅). Objectives Here, we used M₅ receptor knockout (M₅−/−) mice to investigate the involvement of M₅ receptors in behavioral measures pertinent to schizophrenia. We tested the hypothesis that disruption of M₅ receptors affected PPI or the effects of muscarinic or dopaminergic agents in PPI or psychomotor stimulation. Materials and methods We measured PPI in M₅−/−, heterozygous and wild-type mice without drugs, and with clozapine (0.56–3.2 mg/kg) or haloperidol (0.32–3.2 mg/kg) alone, and as pretreatment to d-amphetamine. In addition, we evaluated locomotor stimulation by the muscarinic antagonist trihexyphenidyl (0.56–56 mg/kg) and by cocaine (3.2–56 mg/kg). Results The M₅−/− mice showed decreased PPI relative to wild-type mice, and clozapine appeared to reduce this difference, while haloperidol increased PPI regardless of genotype. The M₅−/− mice also showed more locomotor stimulation by trihexyphenidyl than wild-type mice, while cocaine had similar effects between genotypes. Conclusions These data suggest that disruption of the M₅ receptor gene affected sensorimotor gating mechanisms, increased sensitivity to clozapine and to the psychostimulant effects of muscarinic antagonists without modifying the effect of dopaminergic drugs.     

Antipsychotic regulation of dopamine D1, D2 and D3 receptor mRNA

A range of antipsychotic drugs, both “typical” and “atypical”, was administered to rats over a time course and at several different dosages. The mRNA levels of dopamine D₁ , D₂ and D₃ receptor were measured in either whole brain or dissected brain regions. D₃ receptor mRNA was up-regulated in whole brain by clozapine (10 and 30 but not 3 mg/kg/day), sulphide (50 and 100 but not 20 mg/kg/day), haloperidol (3 but not 1 or 0.3 mg/kg/day), flupenthixol (3 but not 1 or 0.3 mg/kg/day), pimozide (4.5 but not 1.5 or 0.5 mg/kg/day) and loxapine (1.2 and 4 mg/kg/day but not 0.4 mg/kg/day). Sulphide (100 mg/kg/day), clozapine (30 mg/kg/ day) and haloperidol (3 mg/kg/day) all up-regulated the D₃ receptor mRNA in nucleus accumbens and olfactory tubercles but not striatum. D₁ and D₂ receptor mRNA was up-regulated in whole brain by haloperidol and loxapine only, and in the case of haloperidol this was localized to striatum and prefrontal cortex. Haloperidol, clozapine and sulphide all down-regulated D₁ mRNA in hippocampus and additionally haloperidol and sulpiride down-regulated it in the cerebellum. This work shows that all the drugs tested upregulated D₃ receptor, but effects on D₁ and D₂ receptors were less general.     

Neonatal exposure to epidermal growth factor induces dopamine D<Subscript>2</Subscript>-like receptor supersensitivity in adult sensorimotor gating

Rational Abnormality in the neurotrophic factor for dopamine neurons, epidermal growth factor (EGF), is associated with schizophrenia. Thus, rats treated with EGF as neonates are used as a putative animal model for schizophrenia showing impaired prepulse inhibition (PPI) and other cognitive deficits in the adult stage. Objectives To elucidate the abnormal behavioral traits of this animal model, the EGF effects on the dopaminergic system were analyzed pharmacologically and biochemically at the adult stage. Results We examined the effects of subthreshold doses of dopamine agonists on PPI in this model. A non-selective dopamine agonist, apomorphine (0.1 mg/kg), decreased PPI in EGF-treated rats, but not in controls. Further, a D₂-like receptor agonist, quinpirole (0.01 and 0.03 mg/kg), similarly decreased PPI in EGF-treated rats but had no effect in the control animals. In contrast, a D₁-like receptor agonist, SKF38393 (3 and 10 mg/kg), had no effect on PPI in both groups. To explore the molecular mechanism underlying the change in sensorimotor gating, we assessed D₁ and D₂ receptors expression in the prefrontal cortex, striatum and hippocampus and their downstream signaling. Although there were no significant differences in basal receptor levels, quinpirole administration significantly enhanced phosphorylation of extracellular signal-regulated kinase (ERK) and cAMP response element binding protein (CREB) in the striatum of EGF-treated rats. Conclusion These results suggest that circulating EGF in the early development substantially influences D₂ receptor-dependent regulation of sensorimotor gating.     

Selective dopamine D4 receptor antagonists reverse apomorphine-induced blockade of prepulse inhibition

 Recent evidence suggests that the dopamine D₄ receptor may play a role in schizophrenia, and that the atypical properties of the antipsychotic clozapine may be attributable in part to its antagonistic actions at this receptor. In the present study, clozapine and three other compounds having D₄ dopamine receptor antagonist properties were examined for their effectiveness in reducing losses in prepulse inhibition (PPI) induced in rats by the dopamine receptor agonist apomorphine. Previously, activity in the PPI model has been shown to correlate highly with the antipsychotic potency of a number of neuroleptics. As previously reported, clozapine (1–5.6 mg/kg) significantly reduced apomorphine-induced PPI deficits. The three D₄-selective compounds, CP-293,019 (5.6–17.8 mg/kg), U-101,387 (3–30 mg/kg) and L-745,870 (1–10 mg/kg), also significantly blocked the losses in PPI produced by apomorphine. Taken together, these results suggest that dopamine receptor antagonists with selectivity for the D₄ dopamine receptor subtype may be effective in the treatment of schizophrenia, while being less likely to produce dyskinesias associated with D₂ receptor antagonists. Received: 13 May 1997/Final version: 15 July 1997     

Partial agonistic action of clozapine at dopamine D2 receptors in dopamine depleted animals

 In the present experiments, the locomotor and stereotypic stimulant effects of the atypical antipsychotic agent, clozapine were studied in mice depleted of their brain dopamine by reserpine and alpha-methyl-p-tyrosine pretreatment. Clozapine at a dose of 1mg/kg produced slight stimulation of locomotor activity, but failed to produce any stereotypy. However, clozapine at 0.5 or 2 mg/kg failed to show increase in locomotor activity or stereotypy. Clozapine potentiated the locomotor activity and stereotypy induced by selective dopamine D₁ receptor agonist, SKF 38393 but blocked locomotor activity and stereotypy induced by dopamine D₂ agonist, B-HT 920 (1 mg/kg) and stereotypy induced by higher dose of B-HT 920 (2 mg/kg). The synergistic action of clozapine and SKF 38393 was partially blocked by the dopamine D₁ selective antagonist SCH 23390 and the dopamine D₂ selective antagonist, sulpiride, respectively. Clozapine potentiated the stereotypy induced by apomorphine (0.5 and 1 mg/kg) but did not affect locomotor stimulating activity of apomorphine. These behavioral effects of clozapine suggested that it has partial agonistic action at dopamine D₂ receptors. Received: 22 July 1997/Final version: 17 August 1997     

Non-uniform blockade of intrastriatal D<Subscript>2</Subscript>/D<Subscript>3</Subscript> receptors by risperidone and amisulpride

Rationale Atypical antipsychotic drugs have been shown to preferentially affect extrastriatal (mesolimbic) D₂/D₃ receptors over those within the striatum (nigrostriatal). The striatum does not contain exclusively nigrostriatal dopamine tracts, however. The caudate nucleus and ventral parts of the striatum primarily contain limbic and associative dopamine pathways more relevant to psychosis. Objectives We tested the hypothesis that two pharmacologically distinct atypical antipsychotic drugs, amisulpride and risperidone, would preferentially occupy of D₂/D₃ dopamine receptors in limbic and associative regions of the striatum. Methods Eight amisulpride-treated patients, six risperidone-treated patients and six age- and sex-matched healthy controls were recruited. Dynamic SPET studies were performed after bolus injection of [¹²³I]epidepride. Binding potential (BP) images were generated using a modified Logan method and aligned between subjects. Regions of interest (ROIs) were placed around head of caudate and putamen bilaterally on an average BP map derived from aligned control images. These ROIs were then applied user-independently to the BP maps for each subject to calculate BP for head of caudate and putamen. Mean occupancy of D₂/D₃ receptors in each ROI was determined by reference to the drug-free healthy volunteer group. Occupancy values for head of caudate and putamen were compared using paired Student’s t test. Results D₂/D₃ receptor occupancy was 42% in caudate and 31% in putamen for risperidone (t=5.9, df=11, p=0.0001) and 51% in caudate and 37% in putamen for amisulpride (t=11.1, df=15, p<0.0001). Conclusions Amisulpride and risperidone both show selective occupancy for limbic and associative D₂/D₃ receptors within the striatum.     

Striatal dopamine D<Subscript>2</Subscript> receptors in medication-naive patients with major depressive disorder as assessed with [<Superscript>11</Superscript>C]raclopride PET

Rationale Among other monoamine neurotransmitters, dopamine is implicated in the pathophysiology of major depression. Experimental studies suggest the involvement of the mesolimbic dopamine system in the mechanism of action of antidepressant drugs. Previous in vivo imaging studies have studied striatal dopamine D₂ receptor availability in depression but the results are equivocal thus far. Objective To study the striatal and thalamic dopamine D₂ receptor availability in drug-naive patients with major depression was the aim of this study. Materials and methods Caudate, putamen, and thalamic dopamine D₂ receptor availability was estimated using positron emission tomography and [¹¹C]raclopride in 25 treatment-seeking drug-free patients (of whom 24 were drug-naive) with major depression (primary care patients) as well as in 19 demographically similar healthy control subjects. Receptor availability was expressed as the binding potential (BP_ND), and analyses were carried out based on both regional and voxel-level BP_ND estimates. Results No statistically significant differences in [¹¹C]raclopride BP_ND were observed between the groups either in the caudate nucleus (+1.7%, CI −4.8% to +8.3%), putamen (−1.0%, CI −7.2% to 5.1%), thalamus (−2.4%, CI −8.7% to 4.0%), or ventral striatum (−3.8%, CI −9.3% to +1.6%). In the patients, depressive symptoms were not associated with [¹¹C]raclopride BP_ND in any region. Conclusions The findings in this sample of treatment-seeking, drug-naive and predominantly first-episode patients with major depression do not support the involvement of striatal dopamine D₂ receptors in the pathophysiology of the illness, but do not exclude the potential importance of dopaminergic mechanisms in antidepressant drug action.     

5-HT2 and D2 dopamine receptor occupancy in the living human brain

It has been suggested that a combined blockade of 5-HT₂ and D₂ dopamine receptors may be superior to D₂ dopamine antagonists alone in the treatment of schizophrenia. Risperidone, which has a high affinity for 5-HT₂ and D₂ dopamine receptors in vitro, is a new antipsychotic drug that has been developed according to this hypothesis. The aim of this study was to examine if risperidone indeed induces 5-HT₂ and D₂ dopamine receptor occupancy in vivo in humans. Central receptor occupancy was examined by positron emission tomography (PET) in three healthy men after oral administration of 1 mg risperidone. [¹¹C]N-methylspiperone ([¹¹C]NMSP) was used as a radioligand for determination of 5-HT₂ receptor occupancy in the neocortex. Both an equilibrium ratio analysis and a kinetic three-compartmental analysis indicated a 5-HT₂ receptor occupancy about 60%. [¹¹C]raclopride was used as a radioligand for determination of D₂ dopamine receptor occupancy in the striatum and the calculated occupancy was about 50%. This is the first quantitative determination of 5-HT₂ receptor occupancy induced by an antipsychotic drug in the living human brain. The results indicate that 5-HT₂ receptor occupancy should be very high at the dose level of 4–10 mg risperidone daily, as suggested for clinical use. Risperidone is thus an appropriate compound for clinical evaluation of the benefit of combined 5-HT₂ and D₂ dopamine receptor blockade in the treatment of schizophrenia.This paper was presented in part at the XVIIIth C.I.N.P. Congress, Nice, France, 28th June-2nd July 1992.