7-OH-DPAT,a dopamine D3-selective receptor agonist,produces contralateral rotation in 6-hydroxydopamine-lesioned rats

The purpose of the present study was to characterize the rotational behavior in unilateral 6-OHDA-lesioned rats produced by the high affinity and selective dopamine D₃ receptor ligand 7-OH-DPAT. Qualitatively similar to the direct-acting DA agonist apomorphine, 7-OH-DPAT causes rats to rotate in a direction contralateral to the side of the nigrostriatal DA pathway lesion. This effect is dose-dependent and the minimum effective dose is 0.03 mg (0.12 m?mol)/kg. 7-OH-DPAT-induced rotation is blocked in a dose-dependent manner by oral pretreatment with the “D₂-like” receptor antagonists haloperidol, eticlopride, or clozapine, but not by the “D₁-like” antagonist SCH 23390. The rank order potency for inhibition of 7-OH-DPAT rotation for haloperidol [ID₅₀ = 0.067 mg (0.18 m?mol)/kg], eticlopride [ID₅₀ = 0.41 mg (1.2 m?mol)/kg], clozapine [ID₅₀ = 13 mg (40 m?mol)/kg], and SCH 23390 [ID₅₀ > 90 mg (313 m?mol)/kg] closely parallels their rank order affinity for binding to either the D₂ or the D₃ receptor. Pretreatment with the non-DA receptor antagonists ritanserin (serotonin 5HT₂), scopolamine (muscarinic cholinergic), propranolol (betaadrenergic), or naltrexone (opiate), each at relevant pharmacological doses, failed to reduce 7-OH-DPAT rotation. Taken together, these results are consistent with mediation of 7-OH-DPAT-induced rotational behavior via an agonist interaction with one or more DA receptors. ©1995 Wiley-Liss, Inc.     

F15063, a compound with D2/D3 antagonist, 5-HT 1A agonist and D4 partial agonist properties. II. Activity in models of positive symptoms of schizophrenia

BACKGROUND AND PURPOSE: F15063 is a high affinity D(2)/D(3) antagonist, D(4) partial agonist, and high efficacy 5-HT(1A) agonist, with little affinity (40-fold lower than for D(2) receptors) at other central targets. Here, the profile of F15063 was evaluated in models of positive symptoms of schizophrenia and motor side-effects. EXPERIMENTAL APPROACH: Rodent behavioural tests were based on reversal of hyperactivity induced by psychostimulants and on measures of induction of catalepsy and 'serotonin syndrome'. KEY RESULTS: F15063 potently (ED(50)s: 0.23 to 1.10 mg kg(-1) i.p.) reversed methylphenidate-induced stereotyped behaviors, blocked d-amphetamine and ketamine hyperlocomotion, attenuated apomorphine-induced prepulse inhibition (PPI) deficits, and was active in the conditioned avoidance test. In mice, it reversed apomorphine-induced climbing (ED(50)=0.30 mg kg(-1) i.p.). F15063, owing to its 5-HT(1A) agonism, did not produce (ED(50)>40 mg kg(-1) i.p.) catalepsy in rats and mice, a behavior predictive of occurrence of extra-pyramidal syndrome (EPS) in man. This absence of cataleptogenic activity was maintained upon sub-chronic treatment of rats for 5 days at 40 mg kg(-1) p.o. Furthermore, F15063 did not induce the 'serotonin syndrome' in rats (flat body posture and forepaw treading: ED(50) >32 mg kg(-1) i.p.). CONCLUSIONS AND IMPLICATIONS: F15063 conformed to the profile of an atypical antipsychotic, with potent actions in models of hyperdopaminergic activity but without inducing catalepsy. These data suggest that F15063 may display potent antipsychotic actions with low EPS liability. This profile is complemented by a favourable profile in rodent models of negative symptoms and cognitive deficits of schizophrenia (companion paper).     

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.     

F15063, a compound with D2/D3 antagonist, 5-HT 1A agonist and D4 partial agonist properties. III. Activity in models of cognition and negative symptoms

BACKGROUND AND PURPOSE: The D(2)/D(3) receptor antagonist, D(4) receptor partial agonist, and high efficacy 5-HT(1A) receptor agonist F15063 was shown to be highly efficacious and potent in rodent models of activity against positive symptoms of schizophrenia. However F15063 induced neither catalepsy nor the 'serotonin syndrome'. Here, we evaluated its profile in rat models predictive of efficacy against negative symptoms/cognitive deficits of schizophrenia. EXPERIMENTAL APPROACH: F15063, given i.p., was assessed in models of behavioural deficits induced by interference with the NMDA/glutamatergic (phencyclidine: PCP) or cholinergic (scopolamine) systems. KEY RESULTS: Through 5-HT(1A) activation, F15063 partially alleviated (MED: 0.04 mg kg(-1)) PCP-induced social interaction deficit between two adult rats, without effect by itself, underlining its potential to combat negative symptoms. At doses above 0.16 mg kg(-1), F15063 reduced interaction by itself. F15063 (0.16 mg kg(-1)) selectively re-established PCP-impaired 'cognitive flexibility' in a reversal learning task, suggesting potential against adaptability deficits. F15063 (0.04-0.63 mg kg(-1)) also reversed scopolamine-induced amnesia in a juvenile-adult rat social recognition test, indicative of a pro-cholinergic influence. Activity in this latter test is consistent with its D(4) partial agonism, as it was blocked by the D(4) antagonist L745,870. Finally, F15063 up to 40 mg kg(-1) did not disrupt basal prepulse inhibition of startle reflex in rats, a marker of sensorimotor gating. CONCLUSIONS AND IMPLICATIONS: The balance of D(2)/D(3), D(4) and 5-HT(1A) receptor interactions of F15063 yields a promising profile of activity in models of cognitive deficits and negative symptoms of schizophrenia.     

F15063, a potential antipsychotic with D2/D3 antagonist, 5-HT 1A agonist and D4 partial agonist properties. I. In vitro receptor affinity and efficacy profile

BACKGROUND AND PURPOSE: Combining 5-HT(1A) receptor activation with dopamine D(2)/D(3) receptor blockade should improve negative symptoms and cognitive deficits in schizophrenia. We describe the in vitro profile of F15063 (N-[(2,2-dimethyl-2,3-dihydro-benzofuran-7-yloxy)ethyl]-3-(cyclopent-1-enyl)-benzylamine). EXPERIMENTAL APPROACH: F15063 was characterised in tests of binding affinity and in cellular models of signal transduction at monoamine receptors. KEY RESULTS: Affinities (receptor and pK(i) values) of F15063 were: rD(2) 9.38; hD(2L) 9.44; hD(2S) 9.25; hD(3) 8.95; hD(4) 8.81; h5-HT(1A) 8.37. F15063 had little affinity (40-fold lower than D(2)) at other targets. F15063 antagonised dopamine-activated G-protein activation at hD(2), rD(2) and hD(3) receptors with potency (pK (b) values 9.19, 8.29 and 8.74 in [(35)S]GTP gamma S binding experiments) similar to haloperidol. F15063 did not exhibit any hD(2) receptor agonism, even in tests of ERK1/2 phosphorylation and G-protein activation in cells with high receptor expression. In contrast, like (+/-)8-OH-DPAT, F15063 efficaciously activated h5-HT(1A) (E(max) 70%, pEC(50) 7.57) and r5-HT(1A) receptors (52%, 7.95) in tests of [(35)S]GTP gamma S binding, cAMP accumulation (90%, 7.12) and ERK1/2 phosphorylation (93%, 7.13). F15063 acted as a partial agonist for [(35)S]GTP gamma S binding at hD(4) (29%, 8.15) and h5-HT(1D) receptors (35%, 7.68). In [(35)S]GTP gamma S autoradiography, F15063 activated G-proteins in hippocampus, cortex and septum (regions enriched in 5-HT(1A) receptors), but antagonised quinelorane-induced activation of D(2)/D(3) receptors in striatum. CONCLUSIONS and IMPLICATIONS: F15063 antagonised dopamine D(2)/D(3) receptors, a property underlying its antipsychotic-like activity, whereas activation of 5-HT(1A) and D(4) receptors mediated its actions in models of negative symptoms and cognitive deficits of schizophrenia (see companion papers).     

Functional reactivity of the dopaminergic system following acute and chronic ketamine treatments

This study examined the effects of acute (15 mg/kg, i.p.) and chronic subanesthetic (15 mg/kg, i.p., t.i.d, for 6 days) doses of ketamine [a noncompetitive N-methyl-d-aspartate (NMDA) receptor antagonist] on amphetamine (presynaptic dopamine releasing agent; 10 mg/kg, i.p.) and apomorphine (a D₂ receptor agonist; 1 mg/kg, i.p.)-induced stereotyped behaviors. The effect of acute and chronic ketamine on haloperidol (a D₂ receptor antagonist; 1.6 mg/kg, i.p.)-induced catalepsy was also examined. Acute ketamine and chronic ketamine pretreatment increased amphetamine-induced stereotyped sniffing and locomotion compared with control groups. Acute ketamine significantly increased apomorphine-induced stereotyped sniffing. However, chronic ketamine had no significant effect on apomorphine-induced stereotyped sniffing. Acute, but not chronic ketamine treatment abolished haloperidol-induced catalepsy. The increase in amphetamine-induced stereotyped behaviors and the reversal of haloperidol-induced catalepsy by acute ketamine suggest that blockade of NMDA receptors by ketamine facilitates dopaminergic transmission. The absence of significant effect of chronic ketamine on apomorphine-induced stereotyped sniffing and haloperidol-induced catalepsy suggests that chronic ketamine does not modulate postsynaptic dopaminergic D₂ receptors. It is suggested that chronic ketamine increased amphetamine-induced behaviors by causing hypersensitivity of presynaptic dopamine releasing mechanisms on dopaminergic terminals.     

Heterogeneity of behavioural profile between three new putative selective D3 dopamine receptor antagonists using an ethologically based approach

The effects on behaviour of the putative selective D₃ dopamine receptor antagonists GR 103691, nafadotride and U 99194A were compared with those of the generic D₂-like antagonist haloperidol, using an ethologically based approach. Neither GR 103691 (0.008–1.0 mg/kg) nor nafadotride (0.025–1.6 mg/kg) influenced any element of behaviour. Conversely, U99194A (1.67–45 mg/kg) effected a dose-dependent stimulation of episodes of non-stereotyped sniffing, locomotion, chewing and eating, with some stimulation of rearing, and reduced baseline levels of grooming; thereafter, as sniffing and locomotion declined, stimulation of episodes of grooming emerged. Haloperidol (0.0008–0.1 mg/kg) failed to promote any element of behaviour and reduced baseline levels of grooming; responsivity to U99194A was antagonised by pretreatment with haloperidol. The lack of effect of GR 103691 (>100-fold D₃/D₂ selectivity) and nafadotride (10-fold D₃/D₂ preference), in contrast to the characteristic ‘‘ethogram’’ for U99194A (25-fold D₃/D₂ selectivity), indicated a fundamental difference in their mechanisms of action. This topography of responsivity to U99194A overlapped somewhat with the profiles of both D₂-like and D₁-like agonists, and its sensitivity to antagonism by haloperidol also indicated a dopaminergic basis thereto. However, differences among GR 103691, nafadotride and U99194A bore no relation to their relative selectivities for the D₃ receptor, and the basis thereof remains unclear. Theorising as to the behavioural role of the D₃ receptor may need to be tempered pending the identification of a range of chemically distinct D₃ antagonists of higher selectivity. Received: 14 August 1997/Final version: 10 October 1997     

D1/D2 dopamine and N-methyl-d-aspartate (NMDA) receptor participation in experimental catalepsy in rats

Mixed D₁/D₂ dopamine (DA) antagonists, perphenazine (5 mg/kg) and haloperidol (2 mg/kg) induced catalepsy in rats. SCH 23390 (1 mg/kg), a D₁ DA antagonist, also produced catalepsy. Co-administration of perphenazine (0.5 mg/kg) and SCH 23390 (0.1 mg/kg), at low doses, produced a marked increase in cataleptic response. B-HT 920, a D₂ agonist, reversed the cataleptogenic effects of perphenazine, haloperidol and SCH 23390. SKF 38893 (5 mg/kg) reduced the cataleptogenic effect of SCH 23390 but failed to reverse haloperidol- or perphenazine-induced catalepsy. SKF 38393 (10 mg/kg), however, protected the animals against perphenazine- induced catalepsy. Combined administration of B-HT 920 (0.1 mg/kg) and SKF 38393 (5 mg/kg) enhanced the protective effect of B-HT 920 in SCH 23390-treated animals but not in animals treated with haloperidol or perphenazine. MK-801 (0.025–0.5 mg/kg), a non-competitive N-methyl-d-aspartate (NMDA) receptor antagonist, reduced the cataleptogenic effects of perphenazine, haloperidol as well as SCH 23390. The anticataleptic action of MK-801 was enhanced by scopolamine (0.1 mg/kg) but not by bromocriptine (1 mg/kg) or clonidine (0.05 mg/kg) in perphenazine-treated rats. Unlike B-HT 920 (0.1 mg/kg), SKF 38393 (5 mg/kg) potentiated the anticataleptic effect of MK-801 (0.01 mg/kg) against SCH 23390-induced catalepsy. The above data suggests D₁/D₂ interdependence in catalepsy and a modulatory role of D₁ and D₂ DA receptor stimulation on the anticataleptic effect of MK-801.     

Clozapine binds preferentially to cortical D1-like dopamine receptors in the primate brain: a PET study

Rationale The D₁-like dopamine receptors have been suggested to play a role in the pathophysiology and treatment of schizophrenia. Previous positron emission tomography studies have demonstrated that the atypical antipsychotic clozapine occupies D₁-like dopamine receptors in the striatum in clozapine-treated patients. Objectives The aim of the present study was to compare striatal and cortical D₁-like dopamine receptor occupancy by clozapine in the primate brain. Methods Three monkeys were each examined three times at the same day with the radioligand (+)−[¹¹C]NNC 112. The first measurement was at baseline conditions, the second after 1.5 mg/kg and the third after 6 mg/kg clozapine IV. To compare regional levels of nonspecific binding in brain regions, an additional monkey was examined using the inactive enantiomer (−)−[¹¹C]NNC 112. Receptor occupancy was calculated using both the equilibrium–ratio analysis and the simplified reference tissue model. Results After 1.5 mg/kg the D₁-like dopamine receptor occupancy ranged from 30 to 38% in the striatum, whereas the range was 51 to 57% in the frontal cortex. After 6.0 mg/kg the occupancy was 53 to 64% in the striatum and 63 to 83% in the frontal cortex. The differences between striatal and cortical D₁-like receptors occupancy were between 12 and 25%. The study with (−)−[¹¹C]NNC 112 did not show regional differences in nonspecific binding that might explain the regional differences in occupancy. Conclusions The higher D₁-like dopamine receptor occupancy in the frontal cortex may reflect a different distribution of the D₁ and D₅ dopamine receptor subtypes among brain regions and different affinity of clozapine for the two subtypes. The finding supports the suggestion that binding to D₁-like dopamine receptors may explain clozapine’s atypical drug actions.     

Haloperidol-induced catalepsy is absent in dopamine D(2), but maintained in dopamine D(3) receptor knock-out mice

We have previously found that mice homozygous for the deletion of the dopamine D(2) receptor gene (D(2)(-/-) mice) do not present spontaneous catalepsy when tested in a "bar test". In the present study, we sought to analyse the reactivity of D(2) receptor mutant mice to the cataleptogenic effects of dopamine D(2)-like or D(1)-like receptor antagonists. In parallel, we assessed the cataleptogenic effects of these antagonists in dopamine D(3) receptor mutant mice. D(2)(-/-) mice were totally unresponsive to the cataleptogenic effects of the dopamine D(2)-like receptor antagonist haloperidol (0.125-2 mg/kg i.p.), while D(2)(+/-) mice, at the highest haloperidol doses tested, showed a level of catalepsy about half that of wild-type controls. The degree of haloperidol-induced catalepsy was thus proportional to the level of striatal dopamine D(2) receptor expression (0.50, 0.30 and 0.08 pmol/mg protein as measured at 0.25 nM [3H]spiperone for D(2)(+/+), D(2)(+/-) and D(2)(-/-) mice, respectively). However, D(2)(-/-) and D(2)(+/-) mice were as sensitive as their wild-type counterparts to the cataleptogenic effects of the dopamine D(1)-like receptor antagonist R-(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4, 5-tetrahydro-1H-3-benzazepine hydrochloride (SCH 23390: 0.03-0.6 mg/kg s.c.). Striatal dopamine D(1) receptor expression (as measured using [3H]SCH 23390 binding) was not significantly affected by the genotype. The ability of SCH 23390 to induce catalepsy in D(2)(-/-) mice suggests that their resistance to haloperidol-induced catalepsy is due to the absence of dopamine D(2) receptors, and not to the abnormal striatal synaptic plasticity that has been shown by others to occur in these mice. In agreement with the observation that dopamine D(2) and dopamine D(1) receptor expression was essentially identical in D(3)(+/+), D(3)(+/-) and D(3)(-/-) mice, dopamine D(3) receptor homozygous and heterozygous mutant mice, on the whole, did not differ from their controls in the time spent in a cataleptic position following administration of either haloperidol (0.5-2 mg/kg i.p.) or SCH 23390 (0.03-0.6 mg/kg s.c.). Also, dopamine D(3) receptor mutant mice were no more responsive than wild-type controls when co-administered subthreshold doses of haloperidol (0.125 mg/kg) and SCH 23390 (0.03 mg/kg), suggesting that dopamine D(3) receptor knock-out mice are not more sensitive than wild-types to the synergistic effects of concurrent blockade of dopamine D(2) and dopamine D(1) receptors in this model. These results suggest that the dopamine D(2) receptor subtype is necessary for haloperidol to produce catalepsy, and that the dopamine D(3) receptor subtype appears to exert no observable control over the catalepsy produced by dopamine D(2)-like, D(1)-like and the combination of D(1)-like and D(2)-like receptor antagonists.     

Partial agonist properties of the antipsychotics SSR181507, aripiprazole and bifeprunox at dopamine D2 receptors: G protein activation and prolactin release

Dopamine D2 receptor antagonists induce hyperprolactinemia depending on the extent of D2 receptor blockade. We compared the effects of the new antipsychotic agents SSR181507 ((3-exo)-8-benzoyl-N-[[(2 s)7-chloro-2,3-dihydro-1,4-benzodioxin-1-yl]methyl]-8-azabicyclo[3.2.1]octane-3-methanamine monohydrochloride), bifeprunox (DU127090: 1-(2-Oxo-benzoxazolin-7-yl)-4-(3-biphenyl)methylpiperazinemesylate) and SLV313 (1-(2,3-dihydro-benzo[1,4]dioxin-5-yl)-4-[5-(4-fluorophenyl)-pyridin-3-ylmethyl]-piperazine) with those of aripiprazole (7-{4-[4-(2,3-dichlorophenyl)-1-piperazinyl]-butyloxy)-3,4-dihydro-2(1 H)-quinolinone), clozapine and haloperidol, on functional measures of dopamine D2 receptor activity in vitro and in vivo: [35S]-GTPgammaS binding to membranes from Sf9 insect cells expressing human dopamine D2 Long (hD2 L) receptors, and serum prolactin levels in the rat. All compounds antagonized apomorphine-induced G protein activation at dopamine hD2 L receptors. Antagonist potencies of aripiprazole, bifeprunox and SLV313 were similar to haloperidol (pK(b) = 9.12), whereas SSR181507 (8.16) and clozapine (7.35) were less potent. Haloperidol, SLV313 and clozapine were silent antagonists but SSR181507, bifeprunox and aripiprazole stimulated [35S]-GTPgammaS binding by 17.5%, 26.3% and 25.6%, respectively, relative to 100 microM apomorphine (Emax = 100%). pEC50s were: SSR181507, 8.08; bifeprunox, 8.97; aripiprazole, 8.56. These effects were antagonized by raclopride. Following oral administration in vivo, the drugs increased prolactin release to different extents. SLV313 and haloperidol potently (ED50 0.12 and 0.22 mg/kg p.o., respectively) stimulated prolactin release up to 86 and 83 ng/ml. Aripiprazole potently (ED50 0.66 mg/kg p.o.) but partially (32 ng/ml) induced prolactin release. SSR181507 (ED50 4.9 mg/kg p.o.) also partially (23 ng/ml) enhanced prolactin release. Bifeprunox only weakly increased prolactin at high doses (13 ng/ml at 40 mg/kg) and clozapine only affected prolactin at the highest dose tested (41 ng/ml at 40 mg/kg). Prolactin levels of the corresponding vehicle-treated animals were <4.3 ng/ml. These data show that (1) SSR181507, aripiprazole and bifeprunox, but not SLV313, are partial agonists at dopamine hD2 L receptors in vitro; (2) SSR181507, bifeprunox and aripiprazole exhibit reduced prolactin release in vivo compared with drugs that are neutral antagonists at dopamine D2 receptors.     

The in vivo effects of olanzapine and other antipsychotic agents on receptor occupancy and antagonism of dopamine D1, D2, D3, 5HT2A and muscarinic receptors

The atypical antipsychotic olanzapine was compared to other atypical as well as typical antipsychotic agents for in vivo occupancy of D₁, D₂, D₃, 5HT₂, and muscarinic receptors in rat brain. Blockade of D₂ receptors was determined by measuring the levels of the dopamine metabolite 3,4-dihydroxyphenylacetic acid (DOPAC). To assess the interaction with phosphoinositide (PI)-coupled 5HT_2A and muscarinic receptors in vivo, we used a novel radiometric technique to measure in vivo PI hydrolysis. The antagonism of olanzapine and other antipsychotic agents on 5HT_2A and muscarinic receptors was determined by in vivo blockade of PI hydrolysis, stimulated by the 5HT₂ agonist 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) or the muscarinic agonist pilocarpine. Olanzapine inhibited 5HT₂, D₂, and D₃ in vivo binding with high potency (ID₅₀ = 0.15, 0.6 and 1.2 mg/kg, IP, respectively), while inhibiting D₁ and muscarinic in vivo binding with much less potency (ID₅₀ > 10 mg/kg, IP). The binding of olanzapine to D₂ receptors in neostriatum was well correlated with the increase of DOPAC (ED₂₀₀ = 0.8 mg/kg, IP) in vivo, indicating dopamine D₂ antagonism. In vivo PI hydrolysis was increased by DOI in frontal cortex and by pilocarpine in hippocampus up to 2- and 7-fold above the basal level, respectively. The agonist-induced increases in PI hydrolysis were fully blocked by the 5HT_2A antagonist MDL100907 and the muscarinic antagonist scopolamine, indicating the mediation by 5HT_2A receptors in frontal cortex and PI-coupled muscarinic receptors (m1, m3, and m5) in hippocampus, respectively. Olanzapine was about 8-fold more potent in vivo in blocking DOI-induced stimulation of PI hydrolysis (ID₅₀ = 0.1 mg/kg, IP) than pilocarpine-induced stimulation of PI hydrolysis (ID₅₀ = 0.8 mg/kg, IP). In conclusion, olanzapine is more potent in blocking the 5HT_2A receptor than D₁, D₂, D₃ and muscarinic receptors in vivo, consistent with its favorable clinical profiles. In addition, the novel in vivo PI hydrolysis assay proved to be a useful and reliable in vivo method to assess the functional efficacy of compounds that interact with the 5HT₂ and muscarinic receptors. Received: 10 February 1998/Final version: 22 April 1998     

IBZM SPECT imaging of striatal dopamine-2 receptors in psychotic patients treated with the novel antipsychotic substance quetiapine in comparison to clozapine and haloperidol

We investigated the striatal dopamine-2 (D₂) receptor occupancy caused by different antipsychotic substances in 18 psychotic patients (16 with schizophrenic and two with schizoaffective disorder according to DSM-IV) with single photon emission computed tomography (SPECT) using ¹²³I-iodobenzamide (IBZM) as tracer substance. Four patients were treated with the novel antipsychotic compound quetiapine (300–700 mg/day), six with clozapine (300–600 mg/ day) and eight with haloperidol (10–20 mg/day). They were compared with eight healthy controls. Measurement of S/F ratios and consecutive calculation of D₂ receptor occupancy revealed a significantly lower striatal D₂ occupancy rate with quetiapine and clozapine in comparison to haloperidol. In correspondence with the low striatal D₂ receptor occupancy rates and again in contrast to the haloperidol treatment group, there were no extrapyramidal motor side-effects (EPS) in the quetiapine and clozapine treatment groups. Therefore, the reported data support the position that quetiapine can be considered to be an atypical antipsychotic substance due to its relatively weak striatal D₂ receptor blocking property and therefore its low propensity to induce EPS. Received: 12 August 1996/Final version: 11 May 1997     

Differential effects of olanzapine at dopamine D1 and D2 receptors in dopamine depleted animals

We investigated the degree of striatal dopamine-2 (D₂) receptor occupancy in six schizophrenic patients receiving clinically effective antipsychotic treatment with olanzapine 10–25 mg/day in comparison to patients treated with clozapine 300–600 mg/day (n = 6) or haloperidol 5–20 mg/day (n = 10). ¹²³I Iodobenzamide (IBZM) and single photon emission computerized tomography (SPECT) were used for the visualization of striatal D₂ receptors. For the quantification of striatal D₂ receptor occupancy, striatal IBZM binding in patients treated with antipsychotics was compared to that in untreated healthy controls (n = 8) reported earlier. Olanzapine led to a mean striatal D₂ receptor occupancy rate of 75% (range 63–85). Haloperidol-treated patients showed dose-dependently (Pearson r = 0.64; P < 0.05) a significantly higher (P < 0.05) mean occupancy rate of 84% (range 67–94). During clozapine treatment, the mean D₂ receptor occupancy of 33% (range < 20–49) was significantly lower than with olanzapine (P < 0.005). The higher striatal D₂ receptor occupancy of haloperidol was correlated with the incidence and severity of extrapyramidal motor side-effects (EPS). No clinical relevant EPS occurred during treatment with olanzapine or clozapine. There was no correlation between the degree of striatal D₂ receptor occupancy and clinical improvement. Received: 18 March 1998/Final version: 10 June 1998     

Evidence for the involvement of the noradrenergic system, dopaminergic and imidazoline receptors in the antidepressant-like effect of tramadol in mice

The involvement of the noradrenergic system, imidazoline, dopaminergic and adenosinergic receptors in the antidepressant-like action of tramadol in the mouse forced swimming test (FST) was evaluated in this study. The antidepressant-like effect of tramadol (40 mg/kg, per oral, p.o.) in the FST was blocked with yohimbine (1 mg/kg, i.p., an α₂-adrenoceptor antagonist), α-methyl-para-tyrosine methyl ester (AMPT, 100 mg/kg, i.p., an inhibitor of tyrosine hydroxylase), efaroxan (1 mg/kg, i.p., an imidazoline I₁/α₂-adrenoceptor antagonist), idazoxan (0.06 mg/kg, i.p., an imidazoline I₂/α₂-adrenoceptor antagonist), antazoline (5 mg/kg, i.p., a ligand with high affinity for the I₂ receptor), haloperidol (0.2 mg/kg, i.p., a non selective dopamine receptor antagonist), SCH23390 (0.05 mg/kg, subcutaneously, s.c., a dopamine D₁ receptor antagonist), sulpiride (50 mg/kg, i.p., a dopamine D₂ and D₃ receptor antagonist) but was not reversed by prazosin (1 mg/kg, intraperitoneally, i.p., an α₁-adrenoceptor antagonist) and caffeine (3 mg/kg, i.p., a nonselective adenosine receptor antagonist). Monoamine oxidase-A and -B (MAO-A and MAO-B) activities were neither inhibited in the whole brain nor in specific brain regions of mice treated with tramadol. These data demonstrated that the antidepressant-like effect caused by oral administration of tramadol in the mouse FST is mediated by the noradrenergic system, dopaminergic and imidazoline receptors.     

Apomorphine-induced emesis in dogs: differential sensitivity to established and novel dopamine D2/5-HT(1A) antipsychotic compounds

Small rodents (mice, rats) are the species of choice for evaluating the pharmacology of centrally acting compounds, such as antipsychotics, whereas toxicology data are routinely obtained from other species (rabbits, dogs, monkeys). Whilst there is a substantial number of "therapeutically relevant" pharmacological models for "antipsychotic-like" activity in small rodents, based on hyperdopaminergic or hypoglutamatergic/NMDA approaches, there is a remarkable paucity of such models in other species. Here, we compared the efficacy and potency of reference and new generation dopamine D2/5-HT(1A) putative antipsychotics, administered orally, against apomorphine-induced emesis in dogs, a model of central D2 receptor activation that can be implemented with relative ease. Risperidone potently and fully (10 microg/kg) prevented emesis/retching induced by 0.1 mg/kg s.c. apomorphine. SLV313 and F15063 (D2 receptor antagonists/5-HT(1A) receptor agonists) also abolished emesis/retching, albeit less potently than risperidone (minimal effective dose, MEDs: 10 and 40 microg/kg, respectively). The D2 receptor partial agonists/5-HT(1A) receptor agonists aripiprazole and bifeprunox, (up to 80 microg/kg) only partially attenuated emesis, as did the peripheral D2 receptor antagonist domperidone. Under the present experimental conditions, haloperidol was only efficacious at the highest dose tested (320 microg/kg). To summarize, dogs are very sensitive to the dopaminergic blocking effects of antipsychotics in this model of D2 receptor activation. This model can thus be advantageously used to investigate the pharmacological activity of novel D2 receptor antagonists/partial agonists in dogs.     

The relationship between dopamine D2 receptor occupancy and the vacuous chewing movement syndrome in rats

Abstract Rationale. A dose–response relationship between dopamine D₂ occupancy and acute extrapyramidal symptoms (EPS) has been well established. However, the link with the induction of tardive dyskinesia (TD) is less clear. Objectives. To ascertain the nature and extent of D₂ receptor occupancy effects on haloperidol-induced vacuous chewing movements (VCMs) in a rat model of TD. Methods. Groups of eight rats received haloperidol decanoate injections corresponding to daily doses of 0, 0.08, 0.17, 0.33, or 1 mg/kg for 10–12 weeks. VCMs were measured on a weekly basis and D₂ occupancy levels were measured in vivo using [³H]-raclopride at the end of the experiment. Results. Final VCM scores were significantly different between haloperidol doses (P=0.001). Moderate but significant correlations were found between dose and average VCM scores (r=0.69, P<0.001) and between D₂ occupancy and average VCM scores (r=0.65, P<0.001). The rats that developed the VCM syndrome (≥8 VCMs) had higher occupancies than rats that did not. Of the rats with an occupancy above 70%, 63% developed VCMs, compared with 37% of the rats with D₂ occupancy below that. Conclusions. These results indicate that chronic haloperidol induces VCMs in a dose-dependent manner, with doses leading to high D₂ occupancy increasing the likelihood of emergence of the VCM syndrome. While a certain level of D₂ occupancy may be necessary for inducing VCMs, it is not sufficient in and of itself to induce the VCM syndrome. Electronic Publication     

The relationship between D2 receptor occupancy and plasma levels on low dose oral haloperidol: a PET study

The purpose of this study was to determine the relationship between dopamine D₂ receptor occupancy and plasma haloperidol. Twelve patients treated with 1–5 mg/day of haloperidol had their D₂ occupancy measured using [¹¹C]-raclopride and positron emission tomography and haloperidol plasma levels measured using gas chromatograph mass spectrophotometer. The patients exhibited haloperidol plasma levels ranging from 0.5 to 5.8 ng/ml and D₂ occupancy from 53 to 88%. The D₂ occupancy was related to the plasma level as a saturating rectangular hyperbola relationship (r ² = 0.84) and it showed that, on average, 50% D₂ occupancy was achieved with 0.51 ng/ml and 80% D₂ occupancy with 2.0 ng/ml. Our findings demonstrate that 2–5 mg/day of haloperidol, which usually leads to plasma levels of 1–2 ng/ml, would be expected to induce 60–80% dopamine D₂ receptor occupancy. If, as has been claimed, 70% D₂ occupancy is adequate for typical neuroleptic response, then the conventional use of >10 mg/day may have been too high, since 70% occupancy can be achieved in most patients by 2–5 mg/day. On the other hand, if as others have suggested, 8–12 ng/ml of haloperidol is the correct therapeutic window for plasma levels, then the required therapeutic D₂ occupancy is closer to 90%, not 70%. The implications of the D₂ occupancy findings for the optimal dosing of neuroleptics are discussed. Received: 6 August 1996/Final version: 22 November 1996     

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.     

Dopamine D1D2 antagonist combinations as antagonists of the discriminative stimulus effects of cocaine

Although data suggest that the dopaminergic system mediates the discriminative stimulus effects of cocaine, neither selective D₁ or D₂ dopamine agonists nor selective D₁ or D₂ antagonists substitute reliably for or consistently block these effects. These findings suggest that concurrent activity at these receptor subtypes may underlie this discrimination. Accordingly, it would be expected that simultaneous blockade of these receptors may be necessary to block it fully. The ability of various combinations of the D₁ antagonist, SCH 23390, and the D₂ antagonist, haloperidol, were tested for their ability to block the cocaine stimulus in rats trained to discriminate cocaine (7.5, 10, or 13 mg/kg) from vehicle. Antagonist combinations decreased the percentage of cocaine-appropriate responses 10–95% below the cocaine baseline at doses of the antagonist that were inactive when given separately. These findings support the position that activity at D₁-like and D₂-like receptor subtypes may account for more of the pharmacological action of cocaine than activation of a single dopamine receptor subtype.