Striatal dopamine D2-receptor blockade by typical and atypical neuroleptics

IBZM-SPECT imaging of dopamine D2 receptors with typical and atypical antipsychotics

Considerable progress has been achieved over the past 15 years in uncovering the biological basis of major psychiatric disorders. Since psychopharmacological treatment is thought tc act on the underlying biological basis of the disease, brain imaging techniques enable us to understand the mechanism of action of such compounds. One important tool used to determine patterns of brain dysfunction and how psychopharmacological agents such as antipsychotic compounds work is single-photon emission computerised tomography (SPECT). This technique allows determination of striatal D2 receptor occupancy rates, which are associated with the extrapyramidal side effects (EPS) of antipsychotic drugs. Studies have confirmed that atypical antipsychotic agents have lower occupancy rates than typical agents. No association has been found between D2 receptor occupancy rates n the striatum and antipsychotic efficacy, and it therefore appears that striatal D2 receptor occupancy rates are not necessary for the antipsychotic effect of such agents in schizophrenia. The availability of more refined radioligands will help us not only to understand the action of antipsychotics but also the pathophysiology of schizophrenia.

The Role of Dopamine in Temporal Uncertainty

The temporal preparation of motor responses to external events (temporal preparation) relies on internal representations of the accumulated elapsed time (temporal representations) before an event occurs and on estimates about its most likely time of occurrence (temporal expectations). The precision (inverse of uncertainty) of temporal preparation, however, is limited by two sources of uncertainty. One is intrinsic to the nervous system and scales with the length of elapsed time such that temporal representations are least precise for longest time durations. The other is external and arises from temporal variability of events in the outside world. The precision of temporal expectations thus decreases if events become more variable in time. It has long been recognized that the processing of time durations within the range of hundreds of milliseconds (interval timing) strongly depends on dopaminergic (DA) transmission. The role of DA for the precision of temporal preparation in humans, however, remains unclear. This study therefore directly assesses the role of DA in the precision of temporal preparation of motor responses in healthy humans. In a placebo-controlled double-blind design using a selective D2-receptor antagonist (sulpiride) and D1/D2 receptor antagonist (haloperidol), participants performed a variable foreperiod reaching task, under different conditions of internal and external temporal uncertainty. DA blockade produced a striking impairment in the ability of extracting temporal expectations across trials and on the precision of temporal representations within a trial. Large Weber fractions for interval timing, estimated by fitting subjective hazard functions, confirmed that this effect was driven by an increased uncertainty in the way participants were experiencing time. This provides novel evidence that DA regulates the precision with which we process time when preparing for an action.

Glutamate and dopamine in schizophrenia: an update for the 21st century

The glutamate and dopamine hypotheses are leading theories of the pathoaetiology of schizophrenia. Both were initially based on indirect evidence from pharmacological studies supported by post-mortem findings, but have since been substantially advanced by new lines of evidence from in vivo imaging studies. This review provides an update on the latest findings on dopamine and glutamate abnormalities in schizophrenia, focusing on in vivo neuroimaging studies in patients and clinical high-risk groups, and considers their implications for understanding the biology and treatment of schizophrenia. These findings have refined both the dopamine and glutamate hypotheses, enabling greater anatomical and functional specificity, and have been complemented by preclinical evidence showing how the risk factors for schizophrenia impact on the dopamine and glutamate systems. The implications of this new evidence for understanding the development and treatment of schizophrenia are considered, and the gaps in current knowledge highlighted. Finally, the evidence for an integrated model of the interactions between the glutamate and dopamine systems is reviewed, and future directions discussed.

Mechanisms underlying psychosis and antipsychotic treatment response in schizophrenia: insights from PET and SPECT imaging

Molecular imaging studies have generated important in vivo insights into the etiology of schizophrenia and treatment response. This article first reviews the PET and SPECT evidence implicating dopaminergic dysfunction, especially presynaptic dysregulation, as a mechanism for psychosis. Second, it summarises the neurochemical imaging studies of antipsychotic action, focussing on D2/3 receptors. These studies show that all currently licensed antipsychotic drugs block striatal D2/3 receptors in vivo- a site downstream of the likely principal dopaminergic pathophysiology in schizophrenia- and that D2/3 occupancy above a threshold is required for antipsychotic treatment response. However, adverse events, such as extra-pyramidal side-effects or hyperprolactinemia, become much more likely at higher occupancy levels, which indicates there is an optimal 'therapeutic window' for D2/3 occupancy, and questions the use of high doses of antipsychotic treatment in clinical practice and trials. Adequate D2/3 blockade by antipsychotic drugs is necessary but not always sufficient for antipsychotic response. Molecular imaging studies of clozapine, the one antipsychotic licensed for treatment resistant schizophrenia, have provided insights into the mechanisms underlying its unique efficacy. To link this pharmacology to the phenomenology of the illness, we discuss the role of dopamine in motivational salience and show how i) psychosis could be viewed as a process of aberrant salience, and ii) antipsychotics might provide symptomatic relief by blocking this aberrant salience. Finally, we discuss the implications of these PET and SPECT findings for new avenues of drug development.

Dopamine D2 receptor occupancy by risperidone: implications for the timing and magnitude of clinical response

The objective of the study is to investigate whether dopamine D2 receptor occupancy by risperidone and plasma levels over time can account for therapeutic efficacy and the latency period to response. Thirty-eight examinations with (123)I-IBZM single photon emission computed tomography were performed on 22 patients with schizophrenia, at diagnosis, 48 h after starting risperidone treatment and at a stable dose. Risperidone plasma levels were determined and psychopathologic evaluations (Brief Psychiatric Rating Scale, Positive and Negative Syndrome Scale) were carried out. No differences in the striatal/occipital (S/O) ratio or plasma levels were found between examinations at the 48-h time point and when a stable dose level had been established, so these parameters could not account for the latency period required for clinical response. D2 receptor occupancy at 48 h correlated positively with clinical improvement after 2 weeks of treatment. Therefore, if these results are confirmed, D2 receptor occupancy at the beginning of treatment with risperidone may be a predictor of subsequent clinical response.

Developments in the pharmacological treatment of schizophrenia

Schizophrenia is, at once, a biological disease, a neuropsychological disorder and a dysfunction of social interactions. This presents clinicians with a series of problems with regards to therapy. In the first section of this article, some of the clinical challenges that face those attempting to develop new drugs, are summarised. Several potential pharmacological therapeutic targets that have been, and are continuing to be used, in the development of new antipsychotic drugs, are then considered. This is followed by an outline of the pharmacological and clinical profiles of some of the newer generation antipsychotics, as well as investigational drugs in the pipeline for schizophrenia. Finally, the implications of the introduction of these new drugs for the management of schizophrenia, are discussed.

Receptor and transporter imaging studies in schizophrenia, depression, bulimia and Tourette's disorder--implications for psychopharmacology

Considerable progress has been achieved over the past 15 years in uncovering the biological basis of major psychiatric disorders. To determine patterns of brain dysfunction and to uncover the mechanism of action of centrally active compounds we used single photon emission computerized tomography (SPECT) as well as positron emission tomography (PET) in patients diagnosed with schizophrenia, depression, bulimia and Tourette's disorder. Striatal D2 and 5-HT1A receptors were studied in schizophrenia and 5-HT transporters (5-HTT) in depression and bulimia. Patients were either drug-naïve or drug free, or we studied the influence of specifically acting compounds on receptor/transporter occupancy. We could demonstrate that atypical antipsychotics have a dose-dependent (with the exception of clozapine and quetiapine) lower striatal D2 receptor occupancy rate compared with typical neuroleptics, paralleling the more favourable extrapyramidal side effects of atypical antipsychotics. However, no association between striatal D2 receptor occupancy rates and antipsychotic efficacy has been found. The measurement of 5-HT1A receptors in drug-naïve schizophrenic patients using the in vivo PET methodology revealed an increase of cortical 5-HT1A receptor binding potential in schizophrenia. beta-CIT as a ligand for measurement of 5-HT transporter densities (5-HTT) revealed lower rates in depression compared to age- and sex-matching healthy controls, a measurement that has also been obtained for bulimia. We also documented seasonal variations in brain serotonergic function by our finding of reduced brain 5-HTT availability in winter (compared to summer) in healthy controls. Furthermore, displaceable [123I] beta-CIT binding in the area corresponding to the left striatum (representing predominantly the density of dopamine transporters) was significantly reduced in SAD patients compared to healthy controls. In depression as well as in bulimia, selective serotonin reuptake inhibitors significantly decreased the beta-CIT binding potential, however, no significant dose relationship has been obtained in depression. Genotyping depressed patients for the serotonin transporter promoter gene region (5-HTTLPR) did not provide evidence for in vivo functional regulation of 5-HTT availability by 5-HTTLPR in the thalamus-hypothalamus and mesencephalon-pons of healthy subjects. In patients suffering from Tourette's disorder (TD) we were unable to detect differences of dopamine transporter densities between psychotropic drug-naïve TD patients and controls. Furthermore, no difference could be found between currently treated (with antipsychotics) and psychotropic drug-naïve TD patients. Our data provide insight into the pathophysiology of neuropsychiatric disorders and may guide future psychopharmacological drug developments.

Contributions of PET and SPECT to the understanding of the pathophysiology of Parkinson's disease

Positron emission tomography (PET) and single photon emission computed tomography (SPECT) provide the means to studying in vivo the neurochemical, hemodynamic or metabolic consequences of the degeneration of the nigrostriatal dopaminergic system in Parkinson's disease (PD). The extent of striatal dopaminergic denervation can be quantified with radiotracers as [18F]FDopa for PET and [123I]tropanes for SPECT. There are other radiotracers such as [11C]Dopa and meta-tyrosines as well as PET tracers for uptake sites. Striatal uptake of [18F]FDopa and [123I]tropanes is markedly decreased in PD, more in the putamen than in the caudate nucleus, and inversely correlates with the severity of motor signs and with duration of disease. PET and SPECT make possible the assessment by noninvasive means of the changes in dopamine receptor density, the effect of neuronal transplants or neuroprotective treatments in PD patients, or the nigrostriatal dopaminergic function in at-risk subjects. Activation studies using cerebral blood flow and metabolism measurements during a motor task reveal an impaired ability to activate the supplementary motor area and dorsolateral prefrontal cortex in PD. This functional disability is reversed by the use of dopaminergic medication or by surgical treatment by pallidotomy or deep brain stimulation. The differential diagnosis between PD and multiple system atrophy, progressive supranuclear palsy or corticobasal degeneration is not yet clearly established by PET and SPECT, even though these syndromes have some particular neurochemical and metabolic profiles. On the other hand, PET and SPECT are useful for distinguishing PD from Dopa-responsive dystonia, or for assessing the integrity of the nigrostriatal dopaminergic pathway in atypical cases of postural tremor or iatrogenic parkinsonian syndromes.

D2 receptor occupancy under recommended and high doses of olanzapine: an iodine-123-iodobenzamide SPECT study

The aim of this study was to compare the degree of striatal dopamine D2 receptor availability in patients treated with recommended (5-20 mg, mean dose 11.9 +/- 6.3 mg daily) and higher doses (25-40 mg, mean 32.1 +/- 5.6 mg daily) of the novel antipsychotic drug olanzapine by means of [123I] IBZM Single photon emission computed tomography (SPECT). The results were compared to those of a group of 10 untreated, healthy, age- and sex-matched controls. The degree of dopamine D2 receptor occupancy in the patient group was correlated with the presence of extrapyramidal symptoms (EPS). A total of 20 patients who met the DSM III R criteria for schizophrenia or schizoaffective disorder received a clinically effective antipsychotic monotherapy with olanzapine. The mean daily dose of olanzapine ranged from 0.05-0.6 mg/kg body weight. The dopamine D2 receptor binding was reduced in all patients treated with olanzapine. Specific IBZM binding expressed as the [STR-BKG]/BKG ratio ranged from 0.13-0.61 (healthy controls 0.95). The D2 receptor availability revealed an exponential dose-response relationship (r = - 0.85, p < 0.001). The frequency of EPS induced by olanzapine was considerably lower. Only one patient, treated with 40 mg olanzapine, suffered from severe EPS symptoms and had to be given biperiden. There were no significant differences in the presence of EPS symptoms between patients with recommended doses and those with higher doses of olanzapine.

In vivo occupancy of striatal and temporal cortical D2/D3 dopamine receptors by typical antipsychotic drugs. [123I]epidepride single photon emission tomography (SPET) study

BACKGROUND

The dopamine hypothesis proposes that antipsychotic drugs act primarily through limbic cortical D2/D2-like dopamine receptor blockade.

AIM

To evaluate this hypothesis with the D2/D3-selective SPET probe [123I]-epidepride.

METHOD

[123I]-epidepride SPET scans were performed on 12 patients with schizophrenia treated with antipsychotics and II age-matched healthy controls. [123I]-epidepride 'specific binding' to D2/D3 dopamine receptors was estimated, and relative percentage D2/D3 receptor occupancy by typical antipsychotic drugs determined.

RESULTS

Mean (s.d.) daily dose was 669.12 (516.8) mg chlorpromazine equivalents. Mean percentage D2/D3 receptor occupancy was 81.6 (8.1) and 73.2 (13.9) in the temporal cortex and striatum respectively.

CONCLUSIONS

Typical antipsychotic drug treatment is associated with substantial temporal cortical D2/D3 receptor occupancy. The relationship between this and efficacy is poor in patients with treatment-resistant schizophrenia.

Extrapyramidal side effects with risperidone and haloperidol at comparable D2 receptor occupancy levels

Risperidone is an antipsychotic drug with high affinity at dopamine D2 and serotonin 5-HT2 receptors. Previous clinical studies have proposed that risperidone's pharmacologic profile may produce improved efficacy for negative psychotic symptoms and decreased propensity for extrapyramidal side effects; features shared by so-called 'atypical' neuroleptics. To determine if routine risperidone treatment is associated with a unique degree of D2 receptor occupancy and pattern of clinical effects, we used [123I]IBZM SPECT to determine D2 occupancy in subjects treated with routine clinical doses of risperidone (n = 12) or haloperidol (n = 7). Both risperidone and haloperidol produced D2 occupancy levels between approximately 60 and 90% at standard clinical doses. There was no significant difference between occupancy levels obtained with haloperidol or risperidone. Drug-induced parkinsonism was observed in subjects treated with risperidone (42%) and haloperidol (29%) and was observed at occupancy levels above 60%. Based on these observations, it is concluded that 5-HT2 blockade obtained with risperidone at D2 occupancy rates of 60% and above does not appear to protect against the risk for extrapyramidal side effects.

[123I]IBZM SPECT in patients treated with typical and atypical neuroleptics: relationship to drug plasma levels and extrapyramidal side effects

[123I]Iodobenzamide (IBZM) is an iodine-labeled dopamine receptor ligand and can be used to visualize brain D2 receptors in humans with single photon emission computerized tomography (SPECT). The ratio of striatal IBZM uptake to uptake in frontal cortex (ST/FC ratio) represents a semiquantitative measure of D2 receptor binding in the striatum. Our study sample included six patients treated with haloperidol (3.0-8.0 mg/day orally; one patient with an average of 0.9 mg/day intramuscularly), five patients with benperidol (9.0-15.0 mg/day orally) and nine patients treated with clozapine (200.0-600.0 mg/day orally). Typical neuroleptics (TNs) and atypical neuroleptics (ANs) were significantly different in their ST/FC ratios. The ST/FC ratios indicated that patients treated with benperidol exhibited the lowest ST/FC ratios, with increasingly higher ratios in patients on haloperidol or clozapine. We found a curvilinear relationship between the ST/FC ratios and the dose/kg body wt. of TNs and ANs on the basis of a dose-normalization according to Ki-values of the neuroleptic at D2 receptors and a weaker, but also curvilinear relationship between ST/FC ratios and normalized dosages according to clinically defined chlorpromazine equivalents. The specific uptake of IBZM did not correlate with the plasma levels of the TN haloperidol at the present dose range (0-12.4 ng/ml). For clozapine, a meaningful negative correlation between plasma levels and ST/FC ratio could be established. There was a negative continuous correlation between uptake of IBZM and extrapyramidal side effects, which is different from the threshold-based relationship between extrapyramidal side effects and IBZM uptake reported previously.

D2-dopamine-receptor occupancy during treatment with haloperidol decanoate

We investigated in an open, explanatory study a total of 24 patients meeting DSM-III-R criteria for schizophrenia. Eighteen patients were treated for at least 4 weeks with a fixed dose of orally administered haloperidol for at least 4 weeks (mean daily dosage ranging from 0.07 to 0.35 mg/kg b.w.), and 6 patients received haloperidol decanoate with a fixed dose for at least 4 months (dosage range 50-150 mg/4 weeks; calculated mean daily dosage ranging from 0.02 to 0.09 mg/kg b.w.). One week after injection of haloperidol decanoate, the single photon emission computed tomography examination was performed. Our data suggest that D2-dopamine-receptor occupancy of 50 mg/4 weeks haloperidol decanoate 1 week after injection corresponds to an oral dose of 4.5 mg/day haloperidol.

Striatal dopamine-2 receptor occupancy in psychotic patients treated with risperidone

Seventeen psychiatric patients (11 with schizophrenia, 5 with other psychotic disorders, and 1 with obsessive-compulsive disorder) were examined by single photon emission computed tomography with 123I-iodobenzamide (IBZM) as tracer. Patients were treated with risperidone in two different dosage groups (3 mg and 8 mg) and haloperidol (10-20 mg) and compared with eight healthy control subjects. There was a statistically significant difference in basal ganglia/frontal cortex ratios of IBZM binding between controls and all treatment groups. A statistically significant difference was also found concerning these ratios and percentage of dopamine D2 receptor occupancy rates between the treatment groups with lowest ratios and highest percentage of D2 receptor occupancy in the group of patients treated with haloperidol, followed by the group treated with 8 mg of risperidone and the group treated with 3 mg of risperidone.

Interventional brain SPECT--a review

Brain SPECT with HMPAO or ECD has-due to its short accumulation period--a rather high time resolution of approx 60 sec. Compared to isopropyl amphetamine (I-123) and FDG-PET, shortlasting interventions may be evaluated by SPECT. Usually, a two-step approach is used, injecting one third of the dose under baseline conditions and two thirds during intervention. The first study is then subtracted from the second study, resulting in a "difference" image which allows to calculate the effect of the intervention. These interventional procedures may include drug, mechanical, and mental intervention as well as ictal, blood pressure and receptor intervention. Moreover, the difference of pCO2 after hyperventilation or hypoventilation may also be used as a stimulus. The above mentioned procedures are described in detail.

Dopamine D2 receptor occupancy in vivo by the novel atypical antipsychotic olanzapine--a 123I IBZM single photon emission tomography (SPET) study

We have studied striatal D2 dopamine binding in schizophrenic patients treated with the novel atypical antipsychotic drug, olanzapine. 123I iodobenzamide (IBZM) single photon emission tomography (SPET) was used to estimate striatal dopamine D2 receptor binding in vivo. Patients were recruited from a prospective, double blind controlled trial of olanzapine versus haloperidol treatment. In vivo striatal D2 binding data from olanzapine treated patients (n = 6) were compared with previously reported data from typical antipsychotic responsive (n = 10); clozapine (n = 10); and risperidone (n = 6) treated patient groups. Mean % Brief Psychiatric Rating Scale score (BPRS) improvement following olanzapine treatment was 49% (SD 44). The hypothesis that clinical improvement in olanzapine treated patients would be associated with higher mean striatal D2 binding of 123I IBZM (reflecting lower levels of D2 occupancy) than typical antipsychotic (1.25 +/- 0.05) or risperidone (1.24 +/- 0.04) treatment was confirmed. Olanzapine treated patients had similar levels of striatal D2 binding in vivo (1.41 +/- 0.06) as those treated with clozapine (1.49 +/- 0.04). This preliminary evidence suggests olanzapine is another atypical antipsychotic drug in which therapeutic response is not associated with a high degree of striatal D2 receptor occupancy in vivo.

Traditional receptor theory and its application to neuroreceptor measurements in functional imaging

The mathematical, biological discipline of receptor pharmacology and the graphical methods of describing receptor behaviour evolved over a century of painstaking observation and model testing. Laws regarding in vitro theories are classically considered inoperative in vivo. Nevertheless, functional imaging techniques have rapidly evolved to allow receptor measurement and rules of thumb have been developed which clearly prove valid receptor parameters can be derived from functional imaging studies. The field is evolving so rapidly now that nuclear medicine researchers are in danger in applying these techniques without recourse to an understanding of the orthodox discipline of receptor pharmacology. This review attempts to document the basis of receptor pharmacology and to give an account of the theoretical and practical basis on which this can be applied in vivo. The review is targeted towards single-photon emission tomography because of the rapid growth in the area, but many parts draw on the literature relating to positron emission tompgraphy since the first translation of in vitro to in vivo measurement was performed with this technique.

D2 receptor blockade by flunarizine and cinnarizine explains extrapyramidal side effects. A SPECT study

Twenty-six patients under treatment with the calcium channel blockers flunarizine (Fz) or cinnarizine (Cz) were examined-with single-photon emission computed tomography using [123I]iodobenzamide as a ligand. The striatal dopamine D2 receptor-binding potential was determined and found to be reduced by 14 to 63% (39.5 +/- 15.0%; p < 0.0001) in patients compared with age-matched control values. This reduction was larger in 12 patients with extrapyramidal symptoms and was only slowly reversible after discontinuation of treatment. Patients treated for > 6 months had significantly larger reductions than patients treated for a shorter period. Parkinsonian symptoms were only seen in patients older than 50 years. Our findings prove a neuroleptic-like action of Fz and Cz, which seems to be the major reason for their extrapyramidal side effects. Older age and long-term treatment are predisposing factors for these effects.

D2-receptor imaging with [123I]IBZM and single photon emission tomography in psychiatry: a survey of current status

D2-dopamine receptors can be visualized in the human brain in vivo by Single Photon Emission Tomography (SPECT) and the radiolabeled benzamide [123I]IBZM. The present paper reviews the current status of this type of functional brain imaging with respect to basic methodological aspects, data analysis and quantification. The results from published clinical studies in different psychiatric patient populations and normal controls with [123I]IBZM are reviewed. [123I]IBZM-SPECT is a powerful tool for the investigation of D2-dopamine receptor status in psychiatric disorders, different types of drug treatment as well as therapeutic and side effects of pharmacologic agents. However, there still is a need for standardized imaging times and image-processing procedures. Advantages and disadvantages of SPECT with special regard to Positron Emission Tomography (PET) are also discussed.

D2-dopamine receptor occupancy differs between patients with and without extrapyramidal side effects

To investigate whether the occurrence of extrapyramidal side effects was related to D2 dopamine receptor occupancy, iodobenzamide single positron emission computed tomography was carried out in 27 schizophrenic patients and 10 controls. Eighteen patients were treated with haloperidol; 9 patients were treated with clozapine. Our data suggest a relationship between D2 receptor occupancy and extrapyramidal side effects as well as the existence of a neuroleptic threshold of a striatal:frontal cortex ratio of 1.2, below which drug-induced exptrapyramidal side effects can be expected.

Dopamine receptor pharmacology

Dopamine receptors are the primary targets in the treatment of schizophrenia, Parkinson's disease, and Huntington's chorea, and are discussed in this review by Philip Seeman and Hubert Van Tol. Improved therapy may be obtained by drugs that selectively target a particular subtype of dopamine receptor. Most antipsychotic drugs block D2 receptors in direct correlation to clinical potency, except clozapine, which prefers D4 receptors. D1 and D2 receptors can enhance each other's actions, possibly through subunits of the G proteins. In schizophrenia, the D2 and D3 receptor density is elevated by 10%, while the D4 receptor density is elevated by 600%. Therefore, D4 receptors may be a target for future antipsychotic drugs. While antipsychotics originally helped to discover dopamine receptors, the five cloned dopamine receptors are now facilitating the discovery of selective antipsychotic and antiparkinson drugs.

Clozapine and haloperidol produce a differential pattern of immediate early gene expression in rat caudate-putamen, nucleus accumbens, lateral septum and islands of Calleja

Acute administration of the typical neuroleptic haloperidol (HAL, 2 mg/kg) induced the immediate-early gene proteins (IEGPs) c-Fos, Fos-related antigens (FRAs), FosB, JunB, JunD and Krox24 in the striatum and nucleus accumbens of the rat brain. In contrast, acute administration of the atypical antipsychotic drug clozapine (CLOZ, 30 mg/kg) induced only FRAs, JunB and Krox24 IEGPs in the striatum, and c-Fos, FRAs, and Krox24 IEGPs in the nucleus accumbens. c-Jun was not induced by acute administration of HAL or CLOZ in the rat brain. Differential induction of IEGs by HAL and CLOZ was also observed in the lateral septal nucleus and the islands of Calleja complex of the rat brain. These differences in IEG induction by HAL and CLOZ may be related to the different clinical profiles of the two drugs. Specifically, CLOZ induces FRAs in the islands of Calleja and lateral septum and this action may be involved in its therapeutic effects on the negative symptoms of schizophrenia, whereas HAL produces a coordinate induction of Fos and JunB in striatal neurons and this dimer combination may be involved in producing the extrapyramidal side-effects of typical neuroleptics.

D2 dopamine receptor binding in the basal ganglia of antipsychotic-free schizophrenic patients. An 123I-IBZM single photon emission computerised tomography study

We used SPECT to examine striatal D2 receptor binding in 20 antipsychotic-free DSM-III-R schizophrenic patients and 20 age- and sex-matched normal controls. Dynamic single-slice SPECT, at a slice chosen to include the basal ganglia, began immediately following intravenous injection of 185 MBq of 123I-IBZM. A semiquantitative approach was used to generate indices of specific D2 receptor binding in the basal ganglia. There was no overall elevation of D2 receptor binding between patients and controls. A male sex-specific left lateralised asymmetry of striatal D2 receptor binding was found in the patient group. Age-dependent decline of striatal D2 receptors was confirmed in controls, but not in patients. These results suggest that alterations in striatal D2 receptor distribution and density do occur in schizophrenia, and possibly reflect wider disruptions in prefrontal-striatal-limbic circuits.

Antipsychotic medication, D2 dopamine receptor blockade and clinical response: a 123I IBZM SPET (single photon emission tomography) study

The hypothesis that poor response to antipsychotic medication is due to inadequate occupancy of central D2 receptors was tested in vivo. We assessed striatal D2 dopamine receptor availability for binding with the specific ligand 123I IBZM by single photon emission tomography (SPET) in two groups of DSM-III-R diagnosed schizophrenic patients on typical antipsychotic medication, and a group of healthy controls (N = 20). Patients were characterized by clinical ratings as antipsychotic responders (N = 10) or non-responders (N = 8). Dynamic single slice SPET, at a slice chosen to include the basal ganglia, began immediately following intravenous injection of 185 MBq 123I IBZM. Semiquantitative analysis generated indices of D2 receptor availability for binding. There was no difference in striatal D2 receptor availability between the patient groups, both showing a similar degree of occupancy by antipsychotic medication compared to the control group. Thus, poor clinical response does not appear to be accounted for by differential blockade, or inadequate occupancy of striatal dopamine D2 receptors by antipsychotic medication.

Low density of dopamine D4 receptors in Parkinson's, schizophrenia, and control brain striata

The purpose of this study was to determine whether dopamine D4 receptors could be detected in the human brain striatum by means of an indirect ligand-binding method, because no dopamine D4 receptor-selective ligand presently exists. The antipsychotic clozapine is more selective for the dopamine D4 receptor than for other dopamine receptors. Although most antipsychotic drugs act in the striatum to elicit Parkinson-like side-effects, clozapine is atypical in that it does not produce Parkinsonism. To understand this atypical action of clozapine, it would be helpful to know whether the presumed target for clozapine, the dopamine D4 receptor, is or is not present in the human striatum. We measured dopamine D4 receptors indirectly, using [3H]emonapride and [3H]raclopride. Emonapride has a high affinity (K = 90 pM) for the dopamine D4 receptor, while raclopride has a very low affinity for this receptor (K = 240 nM); thus, any difference in the densities of these two [3H]ligands (in the absence of dopamine) could be attributed to the presence of dopamine D4 receptors. Since the binding of [3H]raclopride is sensitive to endogenous dopamine, we used Parkinson-diseased tissue which has little dopamine. We found that the densities of the two ligands were identical in Parkinson striata, indicating a low density (< 1 pmol/g) for dopamine D4 receptors in the human striatum. This low or undetectable density of dopamine D4 receptors in the striatum is consistent with other data indicating that clozapine does not have its major action in the human striatum.(ABSTRACT TRUNCATED AT 250 WORDS)

Glutamate receptor antagonists for neuroleptic malignant syndrome and akinetic hyperthermic parkinsonian crisis

Schizophrenia and Parkinson's disease have been considered inversely related neuropsychiatric disorders since the former has been attributed to increased dopaminergic transmission while the latter is thought to result from loss of dopaminergic neurons. It is in line with this concept that the classical neuroleptic (anti-schizophrenic) drugs cause as a side effect a drug-induced type of Parkinsonism. Most etiopathogenetic models hold that the "neuroleptic malignant syndrome" may result from "over-therapy" of schizophrenia, causing too widespread a block of dopaminergic transmission. The same clinical condition can be triggered by rapid discontinuation of dopaminergic medication in Parkinson's disease. Further, neuroleptic malignant syndrome shares key clinical features such as extrapyramidal motor disturbances and hyperthermia with a severe form of clinical deterioration in Parkinson's disease patients, the akinetic Parkinsonian crisis. Both conditions, neuroleptic malignant syndrome and Parkinsonian crisis, are resistant to anticholinergic treatment but may well respond to drugs with N-methyl-D-aspartate (NMDA) antagonistic properties such as amantadine and memantine. We advocate the use of NMDA receptor antagonists in these medical emergencies and link their clinical efficacy to the common pathophysiological pathway of increased excitatory amino acid neurotransmitter activity in neuroleptic malignant syndrome, Parkinsonian crisis, and dopamine agonist withdrawal states.

Clozapine, single photon emission tomography, and the D2 dopamine receptor blockade hypothesis of schizophrenia

According to the dopamine hypothesis of schizophrenia, D2 receptor blockade is essential for a drug to have antipsychotic potency, and antipsychotic potency and D2 blockade are linearly related in vitro. To test this assumption in vivo, we have compared clinical response with central D2 dopamine receptor availability measured by 123I-iodobenzamide single photon emission tomography in two groups of schizophrenic patients. 6 patients were on typical antipsychotic drugs and 10 were on the atypical antipsychotic clozapine, including 2 patients from the first group. The patients on typical antipsychotics showed poor therapeutic response despite D2 receptor blockade. Significant clinical improvement occurred in all patients on clozapine, but at a lower level of D2 blockade by the drug. These findings suggest a more complex relation between D2 blockade and clinical efficacy than was previously thought.