Antipsychotic drugs,dopamine receptors,and schizophrenia

The clinical potencies of antipsychotic drugs are directly related to their affinities for the dopamine D2 receptor. In addition, the concentrations of antipsychotic drugs (given at therapeutic maintenance doses) in the plasma water or in the spinal fluid are almost identical to the antipsychotic dissociation constants at the dopamine D2 receptor. A consistent 70–75% of brain D2 receptors are occupied by antipsychotic drugs, as calculated from the therapeutic concentration and the antipsychotic dissociation constant. The D3 and D4 dopamine receptors, however, are not consistently occupied by antipsychotic drugs, the occupancies being 0–85% for D3, and 0–95% for D4. Human brain imaging also reveals that therapeutic doses of antipsychotic drugs occupy ∼70% of D2 receptors. Between 2 and 4 h after the daily oral dose, clozapine and quetiapine occupy high levels (∼70%) of the dopamine D2 receptors in schizophrenia patients, with lower occupancies at 6 and 12 h. Although clozapine and quetiapine occupy low levels of D2 receptors many hours after the oral dose, the observed fraction of D2 receptors occupied by these drugs, however, depends on the radioligand used, with high occupancy seen when using [¹¹C]raclopride, and low occupancy seen with [¹¹C]methylspiperone (which is tightly bound to D2). This dependence on the radioligand occurs because clozapine and quetiapine are loosely bound to D2. The loose binding of clozapine and quetiapine to D2 permits endogenous dopamine to displace these antipsychotic drugs much more quickly than haloperidol. In addition, the small dose of radioactive raclopride injected (in brain imaging) can displace a little of the D2-bound clozapine. Hence, the observed low level of D2 occupancy by clozapine in patients may arise from a combination of the above three factors – the ligand dependency, the endogenous dopamine, and the displacement by the imaging dose. Parkinsonism and extrapyramidal effects occur with antipsychotics which have a high affinity for D2 and which are, therefore, tightly bound to D2. Clozapine and quetiapine have a low affinity for D2, and, being readily displaced by endogenous dopamine, do not give rise to extrapyramidal effects. Because the loosely bound antipsychotics dissociate from D2 more rapidly, clinical relapse may occur earlier than that found with the tightly bound traditional antipsychotics. The dopamine hypothesis of schizophrenia is supported by the fact that D2 is the main target of antipsychotic action, that monomers of D2 appear elevated in schizophrenia, and that the synaptic levels of dopamine in schizophrenia are at least two-fold higher than in control subjects.     

Equivalent occupancy of dopamine D1 and D2 receptors with clozapine: differentiation from other atypical antipsychotics

OBJECTIVE: Clozapine, the prototype of atypical antipsychotics, remains unique in its efficacy in the treatment of refractory schizophrenia. Its affinity for dopamine D(4) receptors, serotonin 5-HT(2A) receptor antagonism, effects on the noradrenergic system, and its relatively moderate occupancy of D(2) receptors are unlikely to be the critical mechanism underlying its efficacy. In an attempt to elucidate the molecular/synaptic mechanism underlying clozapine's distinctiveness in refractory schizophrenia, the authors studied the in vivo D(1) and D(2) receptor profile of clozapine compared with other atypical antipsychotics. METHOD: Positron emission tomography with the radioligands [(11)C]SCH23390 and [(11)C]raclopride was used to investigate D(1) and D(2) receptor occupancy in vivo in 25 schizophrenia patients receiving atypical antipsychotic treatment with clozapine, olanzapine, quetiapine, or risperidone. RESULTS: Mean striatal D(1) occupancies ranged from 55% with clozapine to 12% with quetiapine (rank order: clozapine > olanzapine > risperidone > quetiapine). The striatal D(2) occupancy ranged from 81% with risperidone to 30% with quetiapine (rank order: risperidone > olanzapine > clozapine > quetiapine). The ratio of striatal D(1)/D(2) occupancy was significantly higher for clozapine (0.88) relative to olanzapine (0.54), quetiapine (0.41), or risperidone (0.31). CONCLUSIONS: Among the atypical antipsychotics, clozapine appears to have a simultaneous and equivalent occupancy of dopamine D(1) and D(2) receptors. Whether its effect on D(1) receptors represents agonism or antagonism is not yet clear, as this issue is still unresolved in the preclinical arena. This distinctive effect on D(1)/D(2) receptors may be responsible for clozapine's unique effectiveness in patients with schizophrenia refractory to other typical and atypical antipsychotics.     

Atypical antipsychotics: mechanism of action.

BACKGROUND: Although the principal brain target that all antipsychotic drugs attach to is the dopamine D2 receptor, traditional or typical antipsychotics, by attaching to it, induce extrapyramidal signs and symptoms (EPS). They also, by binding to the D2 receptor, elevate serum prolactin. Atypical antipsychotics given in dosages within the clinically effective range do not bring about these adverse clinical effects. To understand how these drugs work, it is important to examine the atypical antipsychotics' mechanism of action and how it differs from that of the more typical drugs. METHOD: This review analyzes the affinities, the occupancies, and the dissociation time-course of various antipsychotics at dopamine D2 receptors and at serotonin (5-HT) receptors, both in the test tube and in live patients. RESULTS: Of the 31 antipsychotics examined, the older traditional antipsychotics such as trifluperazine, pimozide, chlorpromazine, fluphenazine, haloperidol, and flupenthixol bind more tightly than dopamine itself to the dopamine D2 receptor, with dissociation constants that are lower than that for dopamine. The newer, atypical antipsychotics such as quetiapine, remoxipride, clozapine, olanzapine, sertindole, ziprasidone, and amisulpride all bind more loosely than dopamine to the dopamine D2 receptor and have dissociation constants higher than that for dopamine. These tight and loose binding data agree with the rates of antipsychotic dissociation from the human-cloned D2 receptor. For instance, radioactive haloperidol, chlorpromazine, and raclopride all dissociate very slowly over a 30-minute time span, while radioactive quetiapine, clozapine, remoxipride, and amisulpride dissociate rapidly, in less than 60 seconds. These data also match clinical brain-imaging findings that show haloperidol remaining constantly bound to D2 in humans undergoing 2 positron emission tomography (PET) scans 24 hours apart. Conversely, the occupation of D2 by clozapine or quetiapine has mostly disappeared after 24 hours. CONCLUSION: Atypicals clinically help patients by transiently occupying D2 receptors and then rapidly dissociating to allow normal dopamine neurotransmission. This keeps prolactin levels normal, spares cognition, and obviates EPS. One theory of atypicality is that the newer drugs block 5-HT2A receptors at the same time as they block dopamine receptors and that, somehow, this serotonin-dopamine balance confers atypicality. This, however, is not borne out by the results. While 5-HT2A receptors are readily blocked at low dosages of most atypical antipsychotic drugs (with the important exceptions of remoxipride and amisulpride, neither of which is available for use in Canada) the dosages at which this happens are below those needed to alleviate psychosis. In fact, the antipsychotic threshold occupancy of D2 for antipsychotic action remains at about 65% for both typical and atypical antipsychotic drugs, regardless of whether 5-HT2A receptors are blocked or not. At the same time, the antipsychotic threshold occupancy of D2 for eliciting EPS remains at about 80% for both typical and atypical antipsychotics, regardless of the occupancy of 5-HT2A receptors. RELEVANCE: The "fast-off-D2" theory, on the other hand, predicts which antipsychotic compounds will or will not produce EPS and hyperprolactinemia and which compounds present a relatively low risk for tardive dyskinesia. This theory also explains why L-dopa psychosis responds to low atypical antipsychotic dosages, and it suggests various individualized treatment strategies.     

Time course of dopamine D2 receptor occupancy by clozapine with medium and high plasma concentrations

Most antipsychotics were thought to induce antipsychotic action at an excess of 70% striatal dopamine D2 receptor occupancy, while the clinical dose of clozapine was reported to show less than 60% occupancy. High-dose clozapine could occupy as high as 80% of striatal dopamine D2 receptor in monkey PET studies. Although the time course of dopamine D2 receptor occupancy is an important property of antipsychotics, that by clozapine has not been investigated in a clinical setting. We measured the time course of extrastriatal dopamine D2 receptor occupancy with different doses of clozapine and evaluated whether the measured occupancies fitted the binding theory. Three consecutive PET scans with [11C]FLB 457 were performed for two patients with schizophrenia, chronically taking 600 mg/day and 200 mg/day of clozapine, respectively. Series of occupancies were also measured in combination with fluvoxamine or paroxetine in one patient. Dopamine D2 receptor occupancies were also simulated using individual clozapine plasma data and previously determined in vivo ED50 value. The occupancy of one patient with high plasma concentration (1207 ng/ml at peak time) was around 75% at peak and around 60% after 26 h. Another patient with medium plasma concentration (649 ng/ml at peak time) showed less than 50% occupancy at peak, decreasing to 15% after 25 h. The measured occupancy values fitted well with the simulated occupancy values. At high plasma concentration, clozapine can induce high extrastriatal dopamine D2 receptor occupancy in the human brain, and this finding fitted well with the theoretical estimation.     

Specific binding of [(11)C]raclopride and N-[(3)H]propyl-norapomorphine to dopamine receptors in living mouse striatum: occupancy by endogenous dopamine and guanosine triphosphate-free G protein.

According to the ternary complex model of G-protein linkage to receptors, agonists increase the affinity of the receptors for the G protein. The model predicts that an endogenous agonist's constant of inhibition toward an agonist radioligand is lower than that toward an antagonistic radioligand. The authors hypothesized that competition from endogenous dopamine in striatum of living mice should have a greater effect on the binding of the D2,3 partial agonist N-[3H]propylnorapomorphine than on the binding of the D2,3 antagonist [(11)C]raclopride. The baseline binding potential (pB(0)), defined as the ratio of bound-to-unbound ligand in the absence of competition from endogenous dopamine, was simultaneously measured in mouse striatum for [(11)C]raclopride (pB(0) = 8.5) and N-[(3)H]propylnorapomorphine (p'B(0) = 5.3). The baseline was established by treatment with alpha-methyl-p-tyrosine and reserpine. Relative to these baseline values in saline-treated mice, the pB of N-[(3)H]propylnorapomorphine decreased 52% whereas the pB of [(11)C]raclopride decreased only 30%, indicating greater sensitivity of the former compound to inhibition by synaptic dopamine. Furthermore, amphetamine decreased the pB of N-[(3)H]propylnorapomorphine to a greater extent (73%) than that of [(11)C]raclopride (43%) relative to the reserpine condition. For both radioligands, the occupancy of the dopamine receptors by endogenous agonist obeyed Michaelis-Menten kinetics over a wide range of agonist concentrations established by the pharmacologic treatments. The apparent inhibition constant of endogenous dopamine depended on the dopamine occupancy and decreased to a value 1.66 times greater for N-[(3)H]propylnorapomorphine than for [(11)C]raclopride at its highest occupancies. The results are consistent with the hypothesis that agonist binding is more sensitive than antagonist binding to competition from endogenous dopamine. Therefore, dopamine agonist ligands may be superior to benzamide antagonist ligands for the estimation of dopamine receptor occupancy by endogenous synaptic dopamine. The analysis of the effect of dopamine occupancy on the inhibition of N-[(3)H]propylnorapomorphine binding indicated a limited supply of G protein with a maximum ternary complex fraction of 40% of maximum agonist binding capacity.     

Treatment response to olanzapine and haloperidol and its association with dopamine D receptor occupancy in first-episode psychosis.

OBJECTIVE: Response to typical antipsychotic medication has been associated with achieving a level of striatal dopamine D2 receptor occupancy in the range of 65% to 70%. We undertook this study to determine whether response to the atypical antipsychotic olanzapine occurs at lower levels of D2 receptor occupancy. METHOD: Eighteen patients who presented with a first episode of psychosis were randomized to receive olanzapine 5 mg daily or haloperidol 2 mg daily in a double-blind design. We acquired positron emission tomography (PET) scans using the D2 ligand [11C]raclopride within the first 15 days of treatment to determine the percentage of D2 receptors occupied by the medication. According to response, dosage was then adjusted to a maximum dosage of 20 mg daily of either drug. PET scans were repeated after 10 to 12 weeks of treatment. RESULTS: At the first PET scan, the 8 olanzapine-treated patients had significantly lower D2 receptor occupancies (mean 63.4%, SD 7.3) than those observed in the 10 patients treated with haloperidol (mean 73.0%, SD 6.1). When patients were rescanned following dosage adjustment, mean D2 receptor occupancies were greater than 70% in both groups. D2 receptor occupancies did not differ significantly between the olanzapine-treated group (mean 72.0%, SD 5.7) and the haloperidol-treated group (mean 78.7%, SD 7.6). CONCLUSIONS: These results suggest that, in patients being treated for a first episode of psychosis, olanzapine has its antipsychotic effect at approximately the same levels of D2 receptor occupancy as are achieved with low dosages of haloperidol.     

Autoradiographic analysis of regional alterations in brain receptors following chronic administration and withdrawal of typical and atypical neuroleptics in rats

Summary Rats were administered haloperidol, clozapine, raclopride, or no drug for 28 days or 8 months. Following a 3 week withdrawal period, in vitro autoradiography was utilized to examine receptor binding for dopamine D2 ([³H]spiperone and [³H]raclopride), dopamine D1 ([³H]SCH23390), GABA_A ([³H]muscimol), benzodiazepine ([³H]RO15-1788), and muscarinic ACh receptors ([³H]QNB). [³H]spiperone was elevated in striatal subregions only in haloperidol-treated rats, with the largest increases seen in the 8 month duration animals. Striatal [³H]raclopride binding was increased after both short- and long-term treatment in both haloperidol and raclopride, but not clozapinetreated animals. Clozapine-treated rats showed significant increases in [³H]SCH23390 in the nucleus accumbens after 28-day administration; otherwise no changes were seen for this ligand in any other groups. Increases in [³H]muscimol binding in the substantia nigra reticulata were seen in haloperidol-treated rats after 8 month treatment. Binding of [³H]QNB and [³H]RO15-1788 were not significantly different from control for any of the drug-treated groups. These data suggest that persisting alterations in receptor binding are primarily seen in dopamine D2 and GABA receptors after withdrawal from chronic administration of haloperidol but not the atypical neuroleptics, clozapine and raclopride.     

Receptor occupancy and antipsychotic drug action measured by PET and SPECT]

Positron emission tomography (PET) and single photon emission computed tomography (SPECT) studies have demonstrated consistent findings of high dopamine D2 receptor occupancy (> 65-70%) in patients treated with antipsychotic drugs. Further, the risk of extrapyramidal side effects has been shown high in patients with occupancy above 80%. On the basis of these findings, an optimal interval for D2 receptor occupancy between 70% and 80% has been suggested. It has also been shown that several atypical antipsychotics induce marked occupancy of central 5-HT2 and D2 receptors in vivo. However, a low D2 occupancy has been observed in patients with clinical dose of clozapine or quetiapine. The antipsychotic effect of these atypical drugs with a low D2 receptor occupancy has been widely discussed with respect to actions on other receptor systems, limbic selectivity of antipsychotic action and episodic transient occupancy. The recent advances in PET/SPECT and developments of new radioligands have made it possible to evaluate antipsychotic drug actions directly in humans. The empirical data from occupancy measurements will enable us to open future directions of investigation of antipsychotic action and improvement of antipsychotic treatment.     

Dopamine D2 receptor partial agonists display differential or contrasting characteristics in membrane and cell-based assays of dopamine D2 receptor signaling

Clinical evidence suggests that dopamine D(2) receptor partial agonists must have a sufficiently low intrinsic activity to be effective as antipsychotics. Here, we used dopamine D(2) receptor signaling assays to compare the in vitro functional characteristics of the antipsychotic aripiprazole with other dopamine D(2) receptor partial agonists (7-{3-[4-(2,3-dimethylphenyl)-piperazinyl]propoxy}-2(1H)-quinolinone [OPC-4392], (-)-3-(3-hydroxy-phenyl)-N-n-propylpiperidine [(-)3-PPP] and (+)terguride) and dopamine D(2) receptor antagonists. Aripiprazole and OPC-4392 were inactive in a guanosine-5'-O-(3-[(35)S]thio)-triphosphate ([(35)S]GTPgammaS) binding assay using Chinese Hamster Ovary (CHO) cell membranes expressing cloned human dopamine D(2Long) (hD(2L)) receptors, whereas (-)3-PPP and (+)terguride displayed low intrinsic activity. Aripiprazole also had no effect on [(35)S]GTPgammaS binding to CHO-hD(2L) cells, while OPC-4392, (-)3-PPP and (+)terguride were partial agonists. In contrast, aripiprazole, OPC-4392, (-)3-PPP, and (+)terguride were inactive in a [(35)S]GTPgammaS binding assay using rat striatal membranes. However, at a more downstream level of CHO-hD(2L) cell signalling, these drugs all behaved as dopamine hD(2L) receptor partial agonists, with aripiprazole displaying an intrinsic activity 2 to 3-fold lower (inhibition of forskolin-induced adenosine 3',5'-cyclic monophosphate accumulation) and almost half as high (enhancement of adenosine triphosphate-stimulated [(3)H]arachidonic acid release) as OPC-4392, (-)3-PPP and (+)terguride. Dopamine activity was blocked in each case by (-)raclopride, which was inactive on its own in every assay, as were the antipsychotics haloperidol, olanzapine, ziprasidone and clozapine. Together, these data, whilst preclinical in nature, are consistent with clinical evidence suggesting the favorable antipsychotic profile of aripiprazole, compared with the other clinically ineffective partial agonists, is dependent on its low intrinsic activity at dopamine D(2) receptors. This study also highlights the limitations of using [(35)S]GTPgammaS binding assays to identify dopamine D(2) receptor partial agonists.     

An animal model of extrapyramidal side effects induced by antipsychotic drugs: relationship with D2 dopamine receptor occupancy

1. Muscle rigidity was assessed quantitatively and objectively as increases in electromyographic (EMG) activity (muscle rigidity) in the hindlimb muscles of the rat following subcutaneous administration of haloperidol, fluphenazine and thioridazine. 2. Behavioural changes were assessed as increases in the catalepsy score, defined as the time taken for an animal to move off an inclined grid. 3. Increased tonic EMG activity, or the presence of catalepsy was related to the level of occupancy of dopamine D2 receptors in the striatum and substantia nigra of the brain, measured using ex vivo quantitative autoradiography. 4. Increases in tonic EMG activity and the induction of catalepsy were associated with >80% occupancy of striatal and nigral D2 receptors by fluphenazine, while haloperidol increased tonic EMG activity at D2 occupancies of >57%. 5. Thioridazine at doses ranging from 1-15 mg/kg failed to increase EMG activity and occupied <61% of striatal D2 receptors. 6. Overall the findings support the hypothesis that muscle rigidity is observed when a threshold level of D2 receptors in the striatum and substantia nigra are occupied by antipsychotic drugs. 7. This conclusion is consistent with the results of positron emission tomography (PET) studies in humans, and those from our past studies in rats using raclopride, chlorpromazine and clozapine, in which a threshold of approximately 70% striatal and nigral D2 receptor occupancy has been demonstrated.     

Alcohol dependence is associated with blunted dopamine transmission in the ventral striatum.

BACKGROUND: A decrease in dopamine type 2 receptors (D2) and mesolimbic dopamine transmission predisposes animals to consume alcohol. This study measured D2 receptors and dopamine transmission in human alcohol-dependent (AD) subjects using positron emission tomography (PET) and [11C]raclopride. METHODS: Fifteen AD and 15 healthy control (HC) subjects were scanned before and after a psychostimulant challenge (amphetamine .3 mg/kg intravenous). The outcome measures for baseline D2 receptor availability were binding potential (BP) and the equilibrium partition coefficient (V3'). Amphetamine-induced [11C]raclopride displacement was measured as the difference in V3' between the two scans. RESULTS: [11C]raclopride BP was significantly reduced by 16.6% in the limbic striatum, 19.2% in the associative striatum, and 21.3% in the sensorimotor striatum in AD subjects compared with HC. The alcohol-dependent subjects showed a blunting of amphetamine-induced dopamine release in the limbic striatum: [11C]raclopride displacement was -5.2% +/- 3.6% in AD subjects compared with -13.0% +/- 8.8% in HC. However, no significant difference in [11C]raclopride displacement was seen in the associative (-4.6% +/- 5.8% in AD subjects vs. -6.7 +/- 5.4% in HC) and sensorimotor (-12.3% +/- 7.3% in AD subjects vs. -13.7 +/- 7.5% in HC) subdivisions of the striatum between the two groups. CONCLUSIONS: Alcohol dependence was associated with a decrease in D2 receptors in each striatal subdivision, whereas amphetamine-induced dopamine release was reduced in the limbic striatum only.     

Distribution of dopamine D1 and D2 receptors in rabbit cortical areas, hippocampus, and neostriatum in relation to dopamine contents

The densities of dopamine D1 and D2 receptors were measured by using [3H]SCH23390 and [3H]raclopride, respectively, in the rabbit cingulate, visual, sensorimotor, and entorhinal-piriform cortical areas; the dorsal and ventral hippocampus; and the putamen as well as the medial and lateral caudate. Endogenous dopamine (DA) and its metabolites 3,4-dihydroxyphenylacetic acid (DOPAC), 4-hydroxy-3-methoxyphenylacetic acid (HVA), and 3-methoxytyramine (3-MT) were assayed by HPLC with electrochemical detection. The distributions of [3H]SCH23390 and [3H]raclopride binding were heterogenous with the greatest densities in the neostriatum. The concentrations of DA and its metabolites were also highest in this structure. Regions with low DA content, i.e., cortex and hippocampus, had lower densities of [3H]SCH23390 and [3H]raclopride binding. Furthermore, these sites were differentially localized within the various regions and there were substantially more D1 than D2 receptors. The functional significance and heterogeneities in the distribution of D1 and D2 receptors are discussed in relation to the dopaminergic innervation and the turnover estimated by the ratios between endogenous DA and its metabolites.     

Time Course of Dopamine-D2 and Serotonin-5-HT2 Receptor Occupancy Rates by Haloperidol and Clozapine In Vivo

Abstract: In vivo occupancy of dopamine-D₁, D₂ and serotonin-5-HT₂ receptors by haloperidol 10 mg/kg and clozapine 20 mg/kg were studied. Rats were injected intravenously with [³H]-YM-09151-2, [³H]-SCH23390, or [³H]-ketanserin 10 min after the administration of the tested drugs. Fifteen to 240 min after the ligand injection, the receptor occupancy rates of the drugs in the striatum and frontal cortex were calculated. Clozapine demonstrated the higher 5-HT₂ and lower D₂ occupancies in the respective regions. A dose-response analysis of D₂ and 5-HT₂, receptor occupancy by the drugs consolidated the higher 5-HT₂ binding affinity of clozapine in comparison with haloperidol. The present methodology may serve as an accurate tool to evaluate the peculiarity of various antipsychotics.     

A PET study of dopamine D2 and serotonin 5-HT2 receptor occupancy in patients with schizophrenia treated with therapeutic doses of ziprasidone

OBJECTIVE: Ziprasidone is an atypical antipsychotic drug that shows a higher affinity for serotonin 5-HT(2) receptors compared with dopamine D(2) receptors in vitro. The affinity of ziprasidone for these receptors in vivo in patients was examined in a positron emission tomography (PET) study. METHOD: The authors conducted a PET study to evaluate D(2) occupancy (using [(11)C]raclopride) and 5-HT(2) occupancy (using [(18)F]setoperone) in brain regions of interest in 16 patients with schizophrenia or schizoaffective disorder randomly assigned to receive 40, 80, 120, or 160 mg/day of ziprasidone, which reflected the recommended dose range. PET scanning was done after 3 weeks of administration and at trough plasma levels, i.e., 12-16 hours after the last dose. RESULTS: The mean 5-HT(2) receptor occupancy was significantly higher than the mean D(2) receptor occupancy (mean=76%, SD=15%, and mean=56%, SD=18%, respectively). The estimated plasma ziprasidone concentration associated with 50% maximal 5-HT(2) receptor occupancy was almost four times lower than that for D(2) receptor occupancy. CONCLUSIONS: These data affirm that ziprasidone is similar to other novel antipsychotics in having greater 5-HT(2) than D(2) receptor occupancy at therapeutic doses and suggest that the optimal effective dose of ziprasidone is closer to 120 mg/day than to the lower doses suggested by previous PET studies. The relatively high D(2) receptor occupancy, even at trough plasma levels, suggests that ziprasidone is more similar to risperidone and olanzapine in receptor occupancy profile than to clozapine and quetiapine. Since ziprasidone plasma levels show significant (more than twofold) variation within a single dose cycle, studies that are aimed at peak plasma levels (6 hours after the last dose) and that examine extrastriatal regions are required to fully characterize the in vivo occupancy profile of ziprasidone.     

Comparable changes in synaptic dopamine induced by methylphenidate and by cocaine in the baboon brain

Though the blockade of dopamine transporters (DAT) is associated with cocaine's and methylphenidate's reinforcing effects, it is the stimulation of dopamine (DA) receptors, achieved by increases in synaptic DA, that enables these effects to occur. Positron emission tomography (PET) and [11C]raclopride were used to assess the levels of occupancy of DA D2 receptors by dopamine achieved by doses of cocaine or methylphenidate previously documented to block over 70% of DAT. Studies were performed in five baboons using a paired scan protocol designed to measure DA D2 receptor availability (Bmax/Kd) at baseline conditions and after intravenous administration of either cocaine or methylphenidate. Cocaine (1-2 mg/kg) or methylphenidate (0.5 mg/kg) administered 5 min prior to [11C]raclopride decreased Bmax/Kd by 29+/-3% and 32 + 4%, respectively. Smaller reductions in Bmax/Kd (13% for cocaine given 30 min before [11C]raclopride and 25+/-10% for methylphenidate given 40 min before [11C]raclopride) were seen with longer periods between drug and radioligand. These observations are consistent with the slower striatal clearance kinetics of [11C]methylphenidate than [1C]cocaine observed in previous PET experiments and with the approximately twofold higher potency of methylphenidate than cocaine in in vitro experiments. Though the elevation of synaptic DA induced by >70% occupancy of DAT by these drugs lead to a modest increase in occupancy of D2 receptors (25-30%), further studies are required to assess if this is an underestimation because of differences in D2 receptor binding kinetics between raclopride and DA.     

Effect of amphetamine, alpha-methyl-p-tyrosine (alpha-MPT) and antipsychotic agents on dopamine D2-type receptor occupancy in rats

1. EEDQ inactivates unoccupied receptors in vivo in brain tissue and is useful in determining which receptors are occupied by a drug treatment. 2. alpha-MPT, inhibits the synthesis of dopamine, reducing D2-type receptor occupancy by dopamine and enhances the amount of receptor inactivation by EEDQ. 3. Amphetamine releases dopamine resulting in increased occupancy of dopamine D2-type receptors and we have shown that it protects those receptors from EEDQ. 4. Clozapine and remoxipride, two antipsychotic agents, occupied the dopamine receptors in both the caudate and cortex. 5. These findings are important because they substantiate other results obtained with amphetamine and SPECT, which demonstrated an exaggerated dopamine neurotransmission in schizophrenic patients versus normal controls.     

Increased dopamine d(2) receptor occupancy and elevated prolactin level associated with addition of haloperidol to clozapine

OBJECTIVE: The authors added haloperidol, a potent D(2) blocker, to ongoing treatment with clozapine in patients with schizophrenia to determine the effects of this combination on dopamine D(2) receptor blockade, prolactin level, and extrapyramidal side effects. METHOD: At baseline and 4-8 weeks after the addition of haloperidol (4 mg/day) to ongoing clozapine treatment, five patients were examined for prolactin elevation, extrapyramidal side effects, drug plasma levels, and D(2) receptor occupancy measured with [(11)C]raclopride and positron emission tomography imaging. RESULTS: Adding haloperidol significantly increased D(2) receptor occupancy, from a mean of 55% to 79%, and significantly increased the prolactin level. One patient developed akathisia, and another manifested mild extrapyramidal side effects. CONCLUSIONS: Adding a modest dose of haloperidol to clozapine results in the high D(2) receptor occupancy and sustained prolactin elevation usually associated with typical antipsychotics. These findings suggest that the lack of prolactin elevation associated with clozapine derives mainly from low D(2) receptor occupancy and not from the medication's effects on other receptors.     

Dopamine displaces [3H]domperidone from high-affinity sites of the dopamine D2 receptor,but not [3H]raclopride or [3H]spiperone in isotonic medium: Implications for human positron emission tomography

Because the high-affinity state of the dopamine D2 receptor, D2High, is the functional state of the receptor, has a role in demarcating typical from atypical antipsychotics, and is markedly elevated in amphetamine-sensitized rats, it is important to have a method for the convenient detection of this state by a ligand. The present data show that, in contrast to [(3)H]spiperone or [(3)H]raclopride, [(3)H]domperidone labels D2High sites in the presence of isotonic NaCl in either striatum or cloned D2Long receptors, yielding a dopamine dissociation constant (1.75 nM) in agreement with that found with [(3)H]dopamine. Increased labeling of D2High sites occurred with [(3)H]domperidone after severe disruption of the cells, suggesting that [(3)H]domperidone has better access to the D2 receptor from the cytoplasmic aspect of the cell membrane. The density of the [(3)H]domperidone-labeled D2 receptors was the same as that of the [(3)H]raclopride-labeled D2 receptors, but twice the density of [(3)H]spiperone sites for human cloned D2Long receptors, compatible with the monomer-dimer concept of the D2 receptor. [(3)H]domperidone readily labels the D2High sites in postmortem human brain homogenates. Although [(3)H]spiperone or [(3)H]raclopride can occupy D2High sites, the inability of 1-10 nM dopamine to displace these ligands under isotonic conditions suggests that these ligands may not be suitable for monitoring the physiological high-affinity state of the dopamine D2 receptor by means of [(11)C]methylspiperone or [(11)C]raclopride in humans.     

Receptor-receptor link in membranes revealed by ligand competition: Example for dopamine D1 and D2 receptors

An interaction or link between dopamine D1 receptors and dopamine D2 receptors was found by a ligand competition method, using [³H]raclopride to label dopamine D2 receptors and SCH 23390 to block dopamine D1 receptors. In the presence of endogenous or exogenous dopamine, SCH 23390 increased the binding of [³H]raclopride in post-mortem human striata homogenates or in tissue culture cells containing human dopamine D1 and D2 receptors. In order to reveal such intramembrane receptor-receptor interactions in general, therefore, it appears essential to add two agonists, one for each receptor, and then to block one of the receptors when measuring the binding of a ligand to the second receptor. © 1994 Wiley-Liss, Inc.     

Alpha-adrenoceptor modulation hypothesis of antipsychotic atypicality

Although all currently used antipsychotic drugs act as dopamine (DA) D2 receptor antagonists, clozapine, the prototype for atypical antipsychotics, shows superior efficacy, especially regarding negative and cognitive symptoms, in spite of a significantly reduced central D2 receptor occupancy compared with typical (conventional) antipsychotic drugs. Clozapine, as well as several other atypicals, displays significant affinities also for several other neurotransmitter receptors, including other dopaminergic receptors, alpha-adrenergic receptors and different serotonergic and cholinergic receptors, which in several ways may contribute to the clinical effectiveness of the drugs. Preclinical and clinical results suggest a dysregulated mesocorticolimbic DA system in schizophrenia, with an impaired prefrontal DA projection, which may relate to negative and cognitive symptoms, concomitant with an overactive or overreactive striatal DA projection, with bearing on psychotic (positive) symptomatology. Available data suggest that blockage of alpha1-adrenoceptors by antipsychotics may contribute to suppress positive symptoms, especially in acute schizophrenia, whereas alpha2-adrenoceptor blockage, a prominent effect of clozapine and, to some extent, risperidone but not other antipsychotics, may rather be involved in relief of negative and cognitive symptoms. Whereas alpha1-adrenoceptor blockage may act by suppressing, at the presynaptic level, striatal hyperdopaminergia, alpha2-adrenoceptor blockage may act by augmenting and improving prefrontal dopaminergic functioning. Thus, the prominent alpha1- and alpha2-adrenoceptor blocking effects of clozapine may generally serve to stabilize dysregulated central dopaminergic systems in schizophrenia, allowing for improved efficacy in spite of a reduced central D2 receptor occupancy compared with typical antipsychotic drugs.