An open label trial of raclopride in acute schizophrenia. Confirmation of D2-dopamine receptor occupancy by PET

Beyond monoamines: II. Novel applications for PET imaging in psychiatric disorders

Early applications of positron emission tomography (PET) in psychiatry sought to identify derangements of cerebral blood flow and metabolism. The need for more specific neurochemical imaging probes was soon evident, and these probes initially targeted the sites of action of neuroleptic (dopamine D₂ receptors) and psychoactive (serotonin receptors) drugs. For nearly 30 years, the centrality of monoamine dysfunction in psychiatric disorders drove the development of an armamentarium of monoaminergic PET radiopharmaceuticals and imaging methodologies. However, continued investments in monoamine-enhancing drug development realized only modest gains in efficacy and tolerability. As patent protection for many widely prescribed and profitable psychiatric drugs lapsed, drug development pipelines shifted away from monoamines in search of novel targets with the promises of improved efficacy, or abandoned altogether. Over this period, PET radiopharmaceutical development activities closely parallelled drug development priorities, resulting in the development of new PET imaging agents for non-monoamine targets. In part two of this review, we survey clinical research studies using the novel targets and radiotracers described in part one across major psychiatric application areas such as substance use disorders, anxiety disorders, eating disorders, personality disorders, mood disorders, and schizophrenia. Important limitations of the studies described are discussed, as well as key methodologic issues, challenges to the field, and the status of clinical trials seeking to exploit these targets for novel therapeutics.

High long-term test-retest reliability for extrastriatal 11C-raclopride binding in healthy older adults

In vivo dopamine D2-receptor availability is frequently assessed with ¹¹C-raclopride and positron emission tomography. Due to low signal-to-noise ratios for ¹¹C-raclopride in areas with low D2 receptor densities, the ligand has been considered unreliable for measurements outside the dopamine-dense striatum. Intriguingly, recent studies show that extrastriatal ¹¹C-raclopride binding potential (BP_ND) values are (i) reliably higher than in the cerebellum (where D2-receptor levels are negligible), (ii) correlate with behavior in the expected direction, and (iii) showed good test-retest reliability in a sample of younger adults. The present work demonstrates high seven-month test-retest reliability of striatal and extrastriatal ¹¹C-raclopride BP_ND values in healthy, older adults (n = 27, age: 64-78 years). Mean ¹¹C-raclopride BP_ND values were stable between test sessions in subcortical nuclei, and in frontal and temporal cortices (p > 0.05). Across all structures analyzed, intraclass correlation coefficients were high (0.85-0.96), absolute variability was low (mean: 4-8%), and coefficients of variance ranged between 9 and 25%. Furthermore, regional ¹¹C-raclopride BP_ND values correlated with previously determined ¹⁸F-fallypride BP_ND values (ρ = 0.97 and 0.92 in correlations with and without striatal values, respectively, p < 0.01) and postmortem determined D2-receptor densities (including striatum: ρ = 0.92; p < 0.001; excluding striatum: ρ = 0.75; p = 0.067). These observations suggest that extrastriatal ¹¹C-raclopride measurements represent a true D2 signal.

Chronic antipsychotic treatment targets GIRK current suppression, loss of long-term synaptic depression and behavioural sensitization in a mouse model of amphetamine psychosis

BACKGROUND:

Antipsychotic drugs (APDs) are the mainstay of the pharmacological treatment of psychotic disorders like schizophrenia. While the clinical efficacy of APDs has long since been established, the neurobiological mechanisms underlying their therapeutic benefits are still not well understood.

METHODS:

Here, we used an escalating amphetamine regimen to induce a psychosis-like state in mice. To achieve clinically relevant drug concentrations in amphetamine-pretreated mice, the typical APD haloperidol or the atypical APD olanzapine were chronically administered via subcutaneously implanted osmotic mini-pumps.

RESULTS:

Demonstrating their therapeutic efficacy, both drugs dampened amphetamine-induced hyperlocomotion and restored normal behaviour in the light-induced activity test. Whole-cell recordings from dopaminergic neurons of the ventral tegmental area (VTA) in ex vivo brain slices revealed two pronounced aberrations associated with the psychosis-like state: Strongly enhanced spontaneous firing and a substantial loss of G protein-gated inwardly rectifying potassium (GIRK) current upon activation of GABAB receptors with baclofen. Chronic haloperidol and olanzapine restored normal firing and partially rescued the GIRK current response to baclofen. In ex vivo slices containing the nucleus accumbens, which receives a dopaminergic projection from the VTA, abrogation of long-term synaptic depression (LTD) and enhanced excitatory drive onto medium spiny neurons were identified as synaptic consequences of amphetamine-induced psychosis. Again, both alterations proved amenable to chronic APD treatment.

CONCLUSION:

Our data provide evidence for aberrant neuronal function and plasticity in the mesolimbic dopamine system during an induced psychotic state and identify these alterations as targets of chronic APD treatment.

Representativeness of clinical PET study participants with schizophrenia: A systematic review

While positron emission tomography (PET) studies have provided invaluable data on antipsychotic effects, selection bias remains a serious concern. A systematic review of PET studies that measured dopamine D₂ receptor blockade with antipsychotics was conducted to examine their inclusion/exclusion criteria, using PubMed, EMBASE, and ClinicalTrials.gov (last search, September 2016). PET studies were included if they measured D₂ receptor occupancy in patients with schizophrenia and included introduction of antipsychotic treatment or antipsychotic regimen change in a systematic manner. Twenty-six studies were identified. Age limit was included in 13 studies; one study solely included geriatric patients while others targeted younger adults. Eleven, 6, and 3 studies specifically targeted clinically stable patients, patients with severe psychopathology, and antipsychotic-free patients, respectively. Nineteen and 18 studies excluded patients with physical comorbidity and substance abuse, respectively. As a result, the mean age of subjects ranged from 23 to 42 years when one study that targeted geriatric patients was excluded. Mean Positive and Negative Syndrome Scale total scores ranged from 54 to 95. No comparison active-drug or placebo arm was employed in 24 studies. Blind assessment of symptomatology was performed in 5 studies. In general, subjects participating in clinical PET studies were relatively young, presented with mild symptomatology, and were free from substance abuse or physical comorbidities. These characteristics need to be taken into account when clinical PET data are interpreted. On the other hand, it should also be noted that this study was only qualitative and conservative interpretation is necessary for possibility of subjective bias.

Dysregulation of Striatal Dopamine Receptor Binding in Suicide

Inconsistent evidence implicates disruptions of striatal dopaminergic indices in suicide and major depression. To determine whether there are alterations in the striatal dopamine system in suicide, we conducted a quantitative autoradiographic survey of dopamine transporter (DAT; [³H]mazindol), D1 receptor ([³H]SCH23390), and D2 receptor ([³H]sulpiride) binding in the dorsal striatum postmortem from matched suicides and controls. Axis I and axis II psychiatric diagnosis, recent treatment history, and early life adversity (ELA) were determined by psychological autopsy. Mean DAT, D2, and D1 receptor binding did not differ in suicide. However, there was a positive correlation between D1 and D2 receptor binding in the dorsal striatum of control subjects (R²=0.31, p<0.05) that was not present in suicides (R²=0.00, p=0.97). In suicides and controls with reported ELA, there was no correlation between striatal DAT and D1 receptor binding (R²=0.07, p=0.33), although DAT and D1 receptor binding was positively correlated in subjects with no report of ELA (R²=0.32, p<0.05). After controlling for age, there were no significant ELA-related mean differences. Binding of D1 receptors and DAT throughout the striatum correlated negatively with age (D1 receptor: R²=0.12, p<0.05; DAT: R²=0.36, p<0.001). There appears to be an imbalance in dopaminergic receptor and transporter expression related to suicide that differs from that associated with ELA or age.

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.

Antipsychotics, dopamine D₂ receptor occupancy and clinical improvement in schizophrenia: a meta-analysis

OBJECTIVE

Treatment of schizophrenia (SCZ) was revolutionized with the development of the antipsychotic medications. Although imaging studies have linked antipsychotic D₂ receptor occupancy and clinical response in SCZ, heterogeneity between cohorts and methods has made it challenging to generalize findings across studies. The main objective of this meta-analysis was to analyze the relationship between in vivo estimation of typical and atypical antipsychotic D₂ receptor occupancy and treatment response in SCZ.

METHODS

Using the keywords "dopamine D₂ receptor occupancy," "schizophrenia," "PET/SPECT" and "antipsychotics," and further refining our search to journal articles with information on % striatal D₂ occupancy and % change in clinical symptoms as indexed by either the BPRS or the PANSS, our final analysis consisted of 16 imaging studies (20 cohorts; N=206).

RESULTS

The first step of the meta-analysis confirmed the positive relationship between antipsychotic medication and clinical improvement in SCZ (ES=1.36; 95% CI: 1.13-1.60). The second step of our analysis revealed that when D₂ occupancy was limited to less than 80% in order to control for the appearance of extrapyramidal symptoms, high D₂ occupancy was correlated with reduction in clinical scores (r=0.4, p<0.001) for medications other than clozapine or quetiapine.

CONCLUSIONS

Our results suggest that D₂ occupancy is a contributing factor for the mechanism of antipsychotic effect in SCZ for some but not all antipsychotic medications.

Dopamine D2 receptor occupancy and clinical effects: a systematic review and pooled analysis

Positron emission tomography (PET) studies proposed a therapeutic window of D2 receptor occupancy (65%-80%) of antipsychotics for the treatment of schizophrenia in young adults. However, this conclusion has been drawn from clinical PET studies using small sample sizes (<20). Prospective PET studies that measured D2 occupancy levels and assessed extrapyramidal side effects (EPS) and/or treatment response induced by antipsychotics (excluding partial agonists) were identified, using MEDLINE and EMBASE (last search: March 2010). Individual subjects were divided into 2 groups based on EPS status (ie, presence or lack of newly emergent EPS) and treatment response (ie, a ≥ 25% or ≥ 50% reduction in the Positive and Negative Syndrome Scale or Brief Psychiatric Rating Scale). To evaluate the performance of this binary classification, sensitivity, specificity, and accuracy of consecutive cutoff points in the D2 occupancy were calculated: Accuracy = (True Positive + True Negative) / Total N. Twelve studies, including a total of 82 subjects, were included in our analyses. The cutoff points associated with 0.5 or greater in both sensitivity and specificity with the greatest accuracy were 77% to 78% for EPS, 60% for a 25% or greater symptom reduction, and 72% for a 50% or greater symptom reduction. These findings support the presence of a therapeutic window of 60% to 78% D2 occupancy of antipsychotics in young adults with schizophrenia and may suggest the presence of a continuum of effectiveness with increasing occupancy within this therapeutic window.

Pharmacokinetic analysis of plasma curves obtained after i.v. injection of the PET radioligand [11C] raclopride provides a likely explanation for rapid radioligand metabolism

Positron emission tomography (PET) is an imaging technique that provides direct measurements of receptor binding in neurons. The present study was performed to find reasons for the common observation of rapid metabolism of receptor radioligands during time of a brain PET scan. To this aim, the 1-h phase during which imaging-data are acquired was evaluated by using a pharmacokinetic approach. The values of half-lives, volumes of distribution, and dilution calculated for a set of metabolite corrected plasma curves of D2-receptor radioligand [(11)C]raclopride (PETc) during 50 min after radioligand injection in tracer dose were compared with the reference values obtained from a set of plasma curves (REFc) during 30 h after i.v. infusion of unlabelled raclopride in pharmacological doses. We found that the half-life of PETc correspond to the distribution half-life of REFc. Accordingly, the distribution volume during the terminal phase of PETc (13.6 ± 10.8 L) was significantly lower than that during the terminal phase (82.2 ± 30.5 L) and at steady state (59.4 ± 20 L) for REFc, and the dilution of raclopride in body for PETc at 50 min was 38 L, whereas it was 1015 L for REFc at 30 h. The [(11)C]raclopride in plasma at 50 min was higher (10% of dose) than the value for unlabelled raclopride at 30 h (4%). We concluded that the kinetic behavior of the radiolabelled drug [(11)C]raclopride during the 1 h time of a PET corresponds to the distribution phase. The high percentage of [(11)C]raclopride in plasma during this phase is a likely reason for the observed rapid radioligand metabolism.

Antipsychotic drug binding in the substantia nigra: an examination of high metoclopramide binding in the brains of normal, Alzheimer's disease, Huntington's disease, and Multiple Sclerosis patients, and its relation to tardive dyskinesia

This project was done in order to determine why the annual incidence of metoclopramide-associated tardive dyskinesia is much higher than that for the commonly used antipsychotics. To test the hypothesis that metoclopramide tardive dyskinesia may be associated with high concentrations of metoclopramide in the substantia nigra under clinical conditions, the nonspecific binding of tritiated antipsychotics to the dissected melaninized regions of postmortem human substantia nigra was measured. The nonspecific binding at 1 nM [³H]ligand was 7.3, 4.2, 2.6, 0.91 and 0.66 fmoles/mg for [³H]haloperidol, [³H]clozapine, [³H]raclopride, [³H]metoclopramide, and [³H]olanzapine, respectively. After adjusting these values for the known free concentrations of these drugs in plasma or spinal fluid, the amounts that would be bound under clinical conditions would be 231, 113, 15, 11, and 3.4 fmoles/mg for metoclopramide, clozapine, raclopride, haloperidol, and olanzapine, respectively. Using rat striatum as baseline to define antipsychotic binding to nonnigral tissue, the excess amount of binding to the Alzheimer nigral tissue under clinical conditions would be 209, 19, 0, 3.4 and 0.8 fmole/mg for metoclopramide, clozapine, raclopride, haloperidol, and olanzapine, respectively, with a similar pattern for nigral tissues from Huntington and Multiple Sclerosis patients. The high accumulation of metoclopramide is sufficiently high to cause nigral nerve cell membrane damage by metoclopramide's detergent-like action, possibly explaining metoclopramide's toxic ability to elicit early tardive dyskinesia. In addition, the nonspecific binding of metoclopramide was much higher in Alzheimer-diseased substantia nigra, consistent with the fact that older individuals are relatively more vulnerable to metoclopramide tardive dyskinesia.

A highly sensitive LC-MS/MS method for the determination of S-raclopride in rat plasma: application to a pharmacokinetic study in rats

A highly sensitive and rapid assay method has been developed and validated for the estimation of S-(-)-raclopride (S-RCP) in rat plasma with liquid chromatography coupled to tandem mass spectrometry with electrospray ionization in the positive ion mode. The assay procedure involves a simple liquid-liquid extraction technique for extraction of S-RCP and phenacetin (internal standard, IS) from rat plasma. Chromatographic separation was achieved with 0.2% formic acid : acetonitrile (80:20, v/v) at a flow rate of 0.30 mL/min on a Phenomenex Prodigy C(18) column with a total run time of 4.5 min. The MS/MS ion transitions monitored were 347.2 → 112.1 for S-RCP and 180.1 → 110.1 for IS. Method validation and pre-clinical sample analysis were performed as per FDA guidelines and the results met the acceptance criteria. The lower limit of quantitation achieved was 0.05 ng/mL and the linearity range was extended from 0.05 to 152 ng/mL in rat plasma. The intra-day and inter-day precisions were 0.23-10.5 and 3.74-7.29%, respectively. This novel method was applied to a pharmacokinetic study of S-RCP in rats.

Antipsychotic dosing and drug delivery

This chapter addresses the current state of affairs regarding proposed mechanism of action for antipsychotic medications and how this mechanism relates to dosing and delivery strategies. The initial portion describes the history of antipsychotic medication, including key discoveries that contribute to the dopamine hypothesis of schizophrenia and provide evidence that dopamine D2 receptor antagonism remains the most copasetic explanation for both determination of dose and degree of efficacy for current antipsychotic medications. Early observations regarding the unique properties of clozapine and how those observations led to the misconception and misnomer of atypicality are also discussed. Subsequent sections relate the dosing of available medications using chlorpromazine equivalents, with a discussion of non-D2-related mechanisms to antipsychotic effects. The balance of the chapter explores the temporal pattern of receptor occupancy as a key determinant of antipsychotic effectiveness, noting that continuous infusion would present the optimal method of treatment. In addition to the pharmacodynamic benefits of continuous long-term delivery systems, the incidence, causes, and clinical consequences of poor adherence are addressed. These observations are then discussed in the context of clinical studies and meta-analyses, demonstrating superiority of long-term depot preparations over oral administration. However, despite overwhelming evidence in favor of long-term delivery systems, few options are available to provide such ideal medication delivery profiles. Barriers to creating traditional depot preparations for a large number of antipsychotic agents, as well as efforts to address these limitations with polymer-based microspheres are described. The potential extension of current formulations to very long-term delivery implants using biodegradable and nonbiodegradable platforms is then described. Benefits as well as limitations of such systems are discussed with respect to clinical and ethical issues as well as a brief description of potential regulatory and logistic barriers to developing better delivery options. In summary, this chapter describes the basis for relating the dose of all existing antipsychotic medications to dopamine D2 receptor affinity and the potential contribution of continuous occupancy to enhanced efficacy through superior biological effects and improved adherence.

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 D(2/3) receptor occupancy of apomorphine in the nonhuman primate brain--a comparative PET study with [11C]raclopride and [11C]MNPA

Binding studies in vitro have demonstrated that the dopamine D2 receptor may exist in two affinity states for agonists. The high affinity state is thought to represent the functional state of the receptor and proportions might alter during disease. In vitro studies further indicate that agonists induce measurable D(2) receptor occupancy at clinically relevant concentrations but only when measured at the high affinity state. Recently developed PET-radioligands, such as [11C]MNPA, have now made it possible to directly study agonist binding in vivo. The aim of this study was to compare the inhibition by apomorphine of agonist and antagonist radioligand binding to D(2/3) receptors in vivo. A total of 36 PET measurements were performed with the D(2/3) antagonist [11C]raclopride or the D(2/3) agonist [11C]MNPA in two cynomolgus monkeys. On each study day, a baseline measurement was followed by two consecutive pretreatment studies with rising doses of apomorphine (0.01, 0.05, 0.15, 0.5, 1.0, and 3.0 mg/kg). Binding potential (BP(ND)) values were calculated for the striatum with cerebellum as reference region. Apomorphine inhibited [11C]raclopride and [11C]MNPA binding in a dose-dependent manner and to a similar extent. ID(50) and K(i) values were 0.26 mg/kg and 29 ng/ml for [11C]raclopride and 0.50 mg/kg and 31 ng/ml for [11C]MNPA. The present observations do not support the existence of two affinity states in vivo. It might thus be speculated that all D(2/3) receptors are in the high affinity state at in vivo conditions.

Histone H3 phosphorylation is under the opposite tonic control of dopamine D2 and adenosine A2A receptors in striatopallidal neurons

The antipsychotic agent haloperidol regulates gene transcription in striatal medium spiny neurons (MSNs) by blocking dopamine D2 receptors (D2Rs). We examined the mechanisms by which haloperidol increases the phosphorylation of histone H3, a key step in the nucleosomal response. Using bacterial artificial chromosome (BAC)-transgenic mice that express EGFP under the control of the promoter of the dopamine D1 receptor (D1R) or the D2R, we found that haloperidol induced a rapid and sustained increase in the phosphorylation of histone H3 in the striatopallidal MSNs of the dorsal striatum, with no change in its acetylation. This effect was mimicked by raclopride, a selective D2R antagonist, and prevented by the blockade of adenosine A2A receptors (A2ARs), or genetic attenuation of the A2AR-associated G protein, Galpha(olf). Mutation of the cAMP-dependent phosphorylation site (Thr34) of the 32-kDa dopamine and cAMP-regulated phosphoprotein (DARPP-32) decreased the haloperidol-induced H3 phosphorylation, supporting the role of cAMP in H3 phosphorylation. Haloperidol also induced extracellular signal-regulated kinase (ERK) phosphorylation in striatopallidal MSNs, but this effect was not implicated in H3 phosphorylation. The levels of mitogen- and stress-activated kinase 1 (MSK1), which has been reported to mediate ERK-induced H3 phosphorylation, were lower in striatopallidal than in striatonigral MSNs. Moreover, haloperidol-induced H3 phosphorylation was unaltered in MSK1-knockout mice. These data indicate that, in striatopallidal MSNs, H3 phosphorylation is controlled by the opposing actions of D2Rs and A2ARs. Thus, blockade of D2Rs promotes histone H3 phosphorylation through the A2AR-mediated activation of Galpha(olf) and inhibition of protein phosphatase-1 (PP-1) through the PKA-dependent phosphorylation of DARPP-32.

Modulation of striatal dopamine release by 5-HT2A and 5-HT2C receptor antagonists: [11C]raclopride PET studies in the rat

RATIONALE

Antagonism at serotonin 5-HT2A and 5-HT2C receptors modulates cortical and striatal dopamine (DA) release and may underlie some aspects of the clinical efficacy of 'atypical' antipsychotic compounds. However, it is not known whether 5-HT2A/2C receptor-mediated modulation of DA release can be quantified with non-invasive neurochemical imaging, as would be required for investigation of these processes in man.

OBJECTIVE

The objective of the study was to perform a feasibility study in the rat in order to determine whether 5-HT2A/2C modulation of DA release can be observed using positron emission tomography (PET) imaging.

MATERIALS AND METHODS

Rats were administered with either vehicle, a combined 5-HT2A/2C antagonist (ketanserin, 3 mg/kg i.p.), or the more selective 5-HT2C antagonist SB 206,553 (10 mg/kg i.p.) 30 min before administration of the PET DA D2 receptor radiotracer [11C]raclopride ( approximately 11 MBq) and were then scanned for 60 min using a quad-high-density avalanche chamber small animal tomograph. Using the same technique, modulation of amphetamine (4 mg/kg)-induced decreases in [11C]raclopride binding by 5-HT2A antagonism (SR 46349B, 0.2 mg/kg i.v.) was also determined.

RESULTS

Consistent with the increase in DA release measured by others using microdialysis, 5-HT2C antagonism markedly reduced striatal [11C]raclopride binding (p < 0.003), while amphetamine-induced reductions in striatal [11C]raclopride binding (p < 0.001) were attenuated by 5-HT2A antagonist administration (p = 0.04).

CONCLUSIONS

These results inform the feasibility of monitoring 5-HT2A/2C receptor-mediated modulation of DA systems in man using PET and, more generally, demonstrate that D2 radiotracer PET imaging may be used to monitor the efficacy of new DA modulators in attenuating stimulated DA release.

A review of the discovery, pharmacological characterization, and behavioral effects of the dopamine D2-like receptor antagonist eticlopride

Eticlopride is a substituted benzamide analog with high affinity and selectivity for dopamine (DA) D2-like receptors that was initially developed as a potential antipsychotic agent. A great deal of research has utilized this drug to better understand central DA receptor function, the role of D2-like receptors in behavior, and the influence of blockade of these receptors on several preclinical animal models. This review highlights research utilizing this drug and compares it to typical and atypical antipsychotics used clinically. First, we describe structure-activity relationships as it relates to binding at DA receptors and the consequences on behavior. This is followed by a discussion of several imaging strategies including the use of eticlopride for in vivo, in vitro, and ex vivo examination of DA D2-like receptor densities and function. Finally, we discuss the use of eticlopride in several behavioral models predictive of antipsychotic activity, extrapyramidal side effects (EPS), and learning and memory. While eticlopride is not used clinically, it remains a viable research tool for understanding DA receptor function and behavior.

Striatal dopamine release in sequential learning

Sequential learning is an important aspect of cognitive processing. Neuropharmacological evidence acquired in laboratory animals suggests that striatal dopaminergic mechanisms may be important for processing of this form of learning. However, because experiments conducted on dopamine deficient patients have reported contradictory evidence, the role of dopamine and the striatum remains unclear in human sequential learning. We used a newly developed dynamic molecular imaging technique to determine whether striatal dopamine is released during performance of a sequential learning task. In this study we localized striatal regions where dopamine receptor ligand (11C-raclopride) was displaced from receptor sites, during performance of a motor sequence learning (serial reaction time) task. The results suggest that the task induces release of endogenous dopamine in the posterior two-third of dorsomedial aspect of left putamen and the anterior part of the body of caudate bilaterally. The activations of the left putamen and the right caudate coincided with the activations observed earlier during performance of a motor planning task. Since these activations are associated with the selection and execution of a response, the activation in the left caudate, which was not observed in motor planning, is probably associated with the detection of a change in the 'context', and in the formulation of a new 'rule'. Thus, the results suggest that sequential learning involves two striatal dopaminergic mechanisms, one for the detection of a change in context, and the other for selection and execution of the response.

Frontal dopamine D(2/3) receptor binding in drug-naive first-episode schizophrenic patients correlates with positive psychotic symptoms and gender

BACKGROUND

The aim of the study was to examine extrastriatal dopamine D(2/3) receptor binding and psychopathology in schizophrenic patients, and to relate binding potential (BP) values to psychopathology.

METHODS

Twenty-five drug-naive schizophrenic patients and 20 healthy controls were examined with single-photon emission computerized tomography (SPECT) using the D(2/3)-receptor ligand [123I]epidepride.

RESULTS

In the hitherto largest study on extrastriatal D(2/3) receptors we detected a significant correlation between frontal D(2/3) BP values and positive schizophrenic symptoms in the larger group of male schizophrenic patients, higher frontal BP values in male (n = 17) compared to female (n = 8) patients, and - in accordance with this - significantly fewer positive schizophrenic symptoms in the female patients. No significant differences in BP values were observed between patients and controls; the patients, however, had significantly higher BP in the right compared to the left thalamus, whereas no significant hemispheric imbalances were observed in the healthy subjects.

CONCLUSIONS

The present data are the first to confirm a significant correlation between frontal D(2/3) receptor BP values and positive symptoms in male schizophrenic patients. They are in agreement with the hypothesis that frontal D(2/3) receptor activity is significant for positive psychotic symptoms. Additionally, the data support a thalamic hemispheric imbalance in schizophrenia.

Clozapine markedly elevates pregnenolone in rat hippocampus, cerebral cortex, and serum: candidate mechanism for superior efficacy?

Clozapine demonstrates superior efficacy in patients with schizophrenia, but the precise mechanisms contributing to this clinical advantage are not clear. Clozapine and olanzapine increase the GABAergic neuroactive steroid (NS) allopregnanolone, and it has been hypothesized that NS induction may contribute to the therapeutic actions of these agents. Pregnenolone administration improves learning and memory in rodent models, and decreases in this NS have been associated with depressive symptoms in humans. These pregnenolone characteristics may be relevant to the actions of antipsychotics. We therefore investigated potential pregnenolone alterations in rat hippocampus and cerebral cortex following clozapine, olanzapine, and other second generation agents as a candidate NS mechanism contributing to antipsychotic efficacy. In the first set of experiments, intact, adrenalectomized, and sham-operated male rats received vehicle or clozapine (20 mg/kg) IP. In the second set, male rats received vehicle, olanzapine (5 mg/kg), quetiapine (20 mg/kg), ziprasidone (10 mg/kg) or aripiprazole (5 mg/kg) IP. Pregnenolone levels were determined by gas chromatography/mass spectrometry. Clozapine markedly elevates pregnenolone in rat hippocampus, cerebral cortex, and serum; hippocampal levels were strongly correlated with serum levels (r=0.987). Olanzapine also elevates pregnenolone levels, but to a lesser degree than clozapine. Pregnenolone induction may contribute to the clinical actions of clozapine and olanzapine.

Mechanism of action of atypical antipsychotic drugs and the neurobiology of schizophrenia

Atypical antipsychotics have greatly enhanced the treatment of schizophrenia. The mechanisms underlying the effectiveness and adverse effects of these drugs are, to date, not sufficiently explained. This article summarises the hypothetical mechanisms of action of atypical antipsychotics with respect to the neurobiology of schizophrenia.When considering treatment models for schizophrenia, the role of dopamine receptor blockade and modulation remains dominant. The optimal occupancy of dopamine D(2) receptors seems to be crucial to balancing efficacy and adverse effects - transient D(2) receptor antagonism (such as that attained with, for example, quetiapine and clozapine) is sufficient to obtain an antipsychotic effect, while permanent D(2) receptor antagonism (as is caused by conventional antipsychotics) increases the risk of adverse effects such as extrapyramidal symptoms. Partial D(2) receptor agonism (induced by aripiprazole) offers the possibility of maintaining optimal blockade and function of D(2) receptors. Balancing presynaptic and postsynaptic D(2) receptor antagonism (e.g. induced by amisulpride) is another mechanism that can, through increased release of endogenous dopamine in the striatum, protect against excessive blockade of D(2) receptors. Serotonergic modulation is associated with a beneficial increase in striatal dopamine release. Effects on the negative and cognitive symptoms of schizophrenia relate to dopamine release in the prefrontal cortex; this can be modulated by combined D(2) and serotonin 5-HT(2A) receptor antagonism (e.g. by olanzapine and risperidone), partial D(2) receptor antagonism or the preferential blockade of inhibitory dopamine autoreceptors. In the context of the neurodevelopmental disconnection hypothesis of schizophrenia, atypical antipsychotics (in contrast to conventional antipsychotics) induce neuronal plasticity and synaptic remodelling, not only in the striatum but also in other brain areas such as the prefrontal cortex and hippocampus. This mechanism may normalise glutamatergic dysfunction and structural abnormalities and affect the core pathophysiological substrates for schizophrenia.

Validation of the tremulous jaw movement model for assessment of the motor effects of typical and atypical antipychotics: effects of pimozide (Orap) in rats

Drug-induced tremulous jaw movements (TJMs) in rats have been used as a model of parkinsonian tremor. Previous studies demonstrated that the typical antipsychotic haloperidol induced TJMs after acute or subchronic administration, while atypical antipsychotics did not. Moreover, it has been suggested that the relative potency for suppression of tacrine-induced TJMs relative to the suppression of lever pressing can be used to discriminate between typical and atypical antipsychotics. In order to validate this model with additional drugs, the present studies assessed the effects of the typical antipsychotic pimozide. In the first series of experiments, the effects of acute pimozide on tacrine-induced TJMs and lever pressing were examined. As with haloperidol, pimozide failed to suppress tacrine-induced TJMs, even at doses considerably higher than those that suppressed lever pressing. In the second group of experiments, rats were given single daily injections of pimozide (0.125-1.0 mg/kg) or tartaric acid vehicle for 13 days, and were observed for TJMs on days 1, 7, and 13. Pimozide induced TJMs in a dose-related manner on all days. The jaw movements occurred largely in the 3-7 Hz frequency range characteristic of parkinsonian tremor. These data support the hypothesis that typical antipsychotics can induce TJMs in rats, and demonstrate that chronic administration of typical antipsychotics is not necessary for induction of TJMs. TJMs induced by acute or subchronic pimozide may be related to early-onset motor syndromes such as drug-induced parkinsonism.

Temporal characterisation of amphetamine-induced dopamine release assessed with [11C]raclopride in anaesthetised rodents

Competition between endogenous neurotransmitters and radiolabelled tracers, as measured by positron emission tomography (PET), may provide a measure of endogenous neurotransmitter flux in vivo. For example, carbon-11 labelled raclopride has been effectively used to monitor dopamine release following pharmacological and behavioural manipulations. The current study describes a rodent model of amphetamine-induced [11C]raclopride reduction, which allowed the characterisation of the dose-response and temporal dynamics of this reduction over a 24-h time course. Over the range studied, a monotonic dose-response relationship between amphetamine dose and [11C]raclopride reduction was observed. When compared with previously published microdialysis data, an approximate 16% reduction in [11C]raclopride binding potential was associated with a approximately 25-fold increase in extracellular dopamine. A reduction of 20-30% in raclopride binding was observed 30 min after amphetamine injection (4 mg/kg i.p.). This reduction in [11C]raclopride binding persisted for 4 h but returned to baseline by 8 h. The data suggest a persistent amphetamine-induced raclopride displacement in rodents and reinforce findings from nonhuman primates that a simple competitive occupancy model may not adequately explain the temporal characteristics of the amphetamine-induced decrease in radiotracer binding.

Dopamine and glutamate dysfunctions in schizophrenia: role of the dopamine D3 receptor

Symptoms of schizophrenia are improved by dopamine antagonists and exacerbated by dopamine-releasing agents, suggesting hyperactivity of dopamine. However, chronic blockade of glutamate neurotransmission by antagonists at the N-methyl-D-aspartate (NMDA) receptor subtype produces a pathophysiological state resembling schizophrenia. A link between cortical glutamate/NMDA deficiency and subcortical dopamine hyperactivity, particularly in the mesolimbic pathway, has been hypothesized in schizophrenia. Here we show that hyperactivity produced by NMDA receptor blockade is dependent upon stimulation of the dopamine D3 receptor subtype. Since D3 receptor antagonists and antipsychotics produced very similar effects, our results add to the growing evidence suggesting that D3 receptor blockade might produce antipsychotic effects.

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.

Plasma prolactin and central D2 receptor occupancy in antipsychotic drug-treated patients

Previous studies of the relationship between plasma prolactin and clinical effects in patients treated with antipsychotic drugs have yielded inconsistent results. A possible explanation may be that most studies have not included subtherapeutic or low doses of antipsychotics. In this exploratory, double-blind study, the relationship between plasma prolactin concentration and central D2 receptor occupancy was examined in 13 schizophrenic patients treated with the experimental antipsychotic drug raclopride (2, 6, or 12 mg daily). D2 receptor occupancy was determined by positron emission tomography and was related to antipsychotic effect as measured by the Brief Psychiatric Rating Scale. Plasma prolactin concentration was increased in eight of nine patients with a D2 receptor occupancy greater than 50%, whereas it was normal among patients with a D2 receptor occupancy less than 50% (p < 0.01). Plasma prolactin concentration measured 4 hours after the morning dose of raclopride correlated significantly with plasma raclopride concentration (r = 0.92, p < 0.01), the degree of D2 receptor occupancy (r = 0.81,p < 0.01), and the antipsychotic effect (r = 0.79, p < 0.01). Further controlled studies that include low doses of antipsychotic drugs may warrant a reconciliation of plasma prolactin as a useful tool in clinical monitoring of antipsychotic drug treatment.

Evolution of plasma homovanillic acid (HVA) in chronic schizophrenic patients treated with haloperidol

In a 4-week study of 14 drug-free schizophrenic patients (according to DSM-III-R), free and conjugated fractions of plasma homovanillic acid (pHVA) were repeatedly measured. Free HVA levels decreased during the first 2 h of haloperidol intake (P < 0.03). Conjugated HVA levels slowly decreased during the following weeks (P < 0.05), while free HVA levels remained stable. After 4 weeks, free HVA levels remained unchanged 2 h after morning haloperidol intake, but conjugated HVA levels tended to increase. In haloperidol responders, at baseline the free/total HVA ratio was significantly higher than that in non-responders (P < 0.01). Tolerant patients, i.e. those whose post-treatment free HVA levels decreased below pre-treatment levels, were not found to respond better to haloperidol than non-tolerant patients. The balance between free and conjugated pHVA may be a better reflection of the action of haloperidol than free pHVA levels and it may be of prognostic value in terms of drug response.

Emerging roles for novel antipsychotic medications in the treatment of schizophrenia

Antipsychotic medications are the mainstay of treatment for schizophrenia. The recent advent of atypical antipsychotics has provided new clinical options and set higher expectations for the treatment of schizophrenia. It is not yet clear how each different drug will fit within the therapeutic armamentarium and this lack is most evident with considering patients with treatment refractory schizophrenia. On the other hand, the expectation of superior efficacy, more benign side effect profile and potential to impact the longitudinal course of schizophrenia provide a rationale for the use of novel antipsychotics as a first-line treatment of schizophrenia.

In vivo kinetics of [18F](N-methyl)benperidol: a novel PET tracer for assessment of dopaminergic D2-like receptor binding

A novel D2-like receptor-binding radioligand, [18F](N-methyl)benperidol ([18F]NMB), was evaluated via positron emission tomographic (PET) imaging studies of baboons. [18F]NMB rapidly localized in vivo within dopaminergic receptor-rich cerebral tissues, and striatum-to-cerebellum ratios as high as 35 were achieved after 3 hours. Pretreatment of an animal with unlabeled receptor-specific antagonists before injection of [18F]NMB confirmed that the radioligand bound specifically to central D2-like receptors in vivo, and not to S2- or D1-like receptors. Unlabeled eticlopride displaced striatal [18F]NMB in vivo, showing that D2-like binding is reversible. Receptor-binding by the radioligand was resistant to competitive displacement by synaptic dopamine, as illustrated by the lack of effect of intravenous d-amphetamine on the in vivo localization of [18F]NMB. Studies involving sequential intravenous administration of [18F]NMB, d-amphetamine, and eticlopride show that the radioligand does not undergo agonist-mediated internalization with subsequent trapping. The feasibility of applying a three-compartment non-steady state model for quantification of [18F]NMB receptor binding was demonstrated. These in vivo characteristics give [18F]NMB distinct advantages over the PET radiopharmaceuticals currently used for clinical investigation of D2-like receptor binding.

Differential regulation of dopamine receptors after chronic typical and atypical antipsychotic drug treatment

Changes in dopamine receptor subtype binding in different brain regions were examined after 28 days treatment of rats with haloperidol, raclopride, clozapine or SCH23390 using in vitro receptor autoradiography. [3H]7-hydroxy-N,N-di-n-propyl-2-aminotetralin binding to dopamine D3 receptors was not changed in any brain region by any of the drug treatments. [3H]SCH23390 was only increased by chronic SCH23390 treatment. Haloperidol significantly increased [3H]nemonapride and [3H]spiperone binding to dopamine D2-like receptors in the caudate putamen. In contrast, haloperidol caused a small, significant increase in [3H]raclopride binding in the lateral caudate putamen only. Raclopride also elevated, but to a lesser extent [3H]nemonapride and [3H]spiperone binding in caudate putamen, whereas it did not affect [3H]raclopride binding. Clozapine did not significantly change D2-like striatal binding of [3H]nemonaipride, [3H]spiperone or [3H]raclopride. The differences in radioligand binding suggest that [3H]nemonapride and [3H]spiperone may be binding to additional subsets of dopamine D2-like receptors (including D4-like receptors) that are not recognized by [3H]raclopride, which has high affinity for D2 and D3 receptors only. Quantification of [3H]nemonapride or [3H]spiperone binding in the presence of 300 nM raclopride (to block D2 and D3 receptors) revealed that haloperidol, raclopride and clozapine up-regulated D4-like receptors in the caudate putamen using either radioligand. These results suggest that D4-like receptors may be a common site of action of both typical and atypical antipsychotics.

Effect of chronic antipsychotic drug treatment on preprosomatostatin and preprotachykinin A mRNA levels in the medial prefrontal cortex, the nucleus accumbens and the caudate putamen of the rat

In situ hybridization histochemistry was used to study the expression of preprosomatostatin (PPSOM) and preprotachykinin A (PPT-A) mRNA in the medial prefrontal cortex (mPFC), the nucleus accumbens (NAC) and the caudate putamen (CP) of the rat after chronic (21 days) treatment with the classical antipsychotic drug haloperidol (1 mg/kg i.p.), the atypical antipsychotic drugs clozapine (15 mg/kg i.p.) and amperozide (5 mg/kg i.p.), and the selective dopamine (DA)-D2/D3 receptor antagonist raclopride (2 mg/kg i.p.). Whereas amperozide markedly elevated the numerical density of PPSOM mRNA expressing neurons in the mPFC (52%), the other drugs did not significantly affect PPSOM mRNA levels in any of the brain regions studied. Amperozide also altered PPT-A mRNA expression in the mPFC, i.e. a decrease (22%) was found. Of the other drugs tested only haloperidol significantly decreased PPT-A mRNA levels in the NAC shell (14%), in the dorso-lateral CP (19%) and in the medial CP (15%). In view of the differences between amperozide and the other drugs studied, as regards both pre-clinical and clinical characteristics, we suggest that the specific effects of amperozide on PPSOM and PPT-A mRNA in the mPFC may be related to its 5-HT releasing action in the frontal cortex, an effect possibly caused by its alpha2-adrenoceptor blocking activity. This effect, in turn, may be related to an antidepressant-like action that this compound exhibits in animal studies. The decrease in PPT-A mRNA levels seen after the haloperidol treatment is probably due to its potent DA-D2 receptor antagonism and may be related to side-effects, rather than therapeutic effects of this drug.

Effects of intermittent and continuous subchronic administration of raclopride on motor activity, dopamine turnover and receptor occupancy in the rat

With the purpose of finding means to circumvent the marked pharmacokinetic differences of raclopride between rats and man, the effects of intermittent and continuous administration of raclopride were compared in rats. Intermittent administration of raclopride via subcutaneous injections resulted in a prompt increase of dopamine (DA) turnover and decrease of motor activity but these effects were of short duration, probably due to rapidly decreasing raclopride DA D2 receptor occupancy. In contrast, but similar to schizophrenic patients on raclopride treatment, stable plasma raclopride levels and a steady DA D2 receptor occupancy above 70% were produced in the caudate-putamen and nucleus accumbens/olfactory tubercle, when raclopride was administered continuously via minipumps at daily doses above 2 mg/kg. Tolerance to the acute effects of raclopride on DA turnover and locomotion was found with both routes of administration but it was more marked with continuous administration. At continuous raclopride administration, tolerance to the effects of raclopride on DA turnover and spontaneous motor activity as well as supersensitivity to amphetamine-induced motor activity occurred when 70% or more of DA D2 receptor sites were occupied, i.e. the same degree of receptor occupancy as found in patients given therapeutic doses of raclopride.

Correlation of vacuous chewing movements with morphological changes in rats following 1-year treatment with haloperidol

Long-term treatment with the typical antipsychotic drug, haloperidol, can lead to a sometimes irreversible motor disorder, tardive dyskinesia (TD). It has been hypothesized that increased release of glutamate due to prolonged neuroleptic drug treatment may result in an excitotoxic lesion in specific neuronal populations within the basal ganglia, leading to TD. We reported that treatment with haloperidol for 1 month results in an increase in the mean percentage of striatal asymmetric synapses containing a perforated postsynaptic density (PSD) and that these synapses are glutamatergic. Using quantitative immunocytochemistry, we found that depending on how long the animals had been off haloperidol following subchronic (30 d) treatment, there was either a decrease (1 day off) or increase (3-4 days off) in the density of glutamate immunolabeling within the presynaptic terminals of synapses with perforated PSDs. Using a rat model for TD, animals in the current study were treated for 1 year with haloperidol and spontaneous oral dyskinesias (i.e. vacuous chewing movements, VCMs) were recorded. In these long-term treated animals we wanted to determine if there was a correlation between glutamate function, as measured by changes in synapses with perforated PSDs and the density of nerve terminal glutamate immunoreactivity, and VCM behavior. In drug treated rats which demonstrated either a high or low rate of VCMs, there was a significant increase in the mean percentage of asymmetric synapses in the dorsolateral striatum with perforated PSDs in both haloperidol-treated groups compared to vehicle-treated rats. There was a small but significant increase in the density of glutamate immunolabeling within striatal nerve terminals of the high VCM group compared to the low VCM group. There was, however, no difference in the density of glutamate immunolabeling between the high VCM group compared to the vehicle-treated animals. One reason for this lack of difference was partially due to a significant increase in nerve terminal area within the high VCM group compared to either the low VCM- or vehicle-treated groups. The larger nerve terminal size in the high VCM group may be due to a small but sustained increase in glutamate neurotransmitter release with the ability of the terminal to maintain its supply of glutamate, while the terminals in the low VCM group showed evidence of glutamate depletion. This finding would be consistent with the hypothesis that increased glutamatergic activity may be associated with TD.

New drugs for the treatment of schizophrenic patients

Conventional neuroleptics are widely accepted as being effective against the positive symptoms of schizophrenia, but do not benefit all patients. Furthermore, they are relatively ineffective against negative symptoms and cognitive disorders, and most have unpleasant side effect profiles. New strategies for treating schizophrenia include the development of dopamine antagonists with high selectivity for different subtypes of dopamine receptors, dopamine partial agonists, antagonists at different serotonin (5-hydroxytryptamine; 5-HT) receptor subtypes, drugs with mixed pharmacological profiles and drugs which modify transmission via amino acids or peptides in the brain. The prospect is that some of these strategies will lead to the introduction of new drugs and that some of these will become the standards against which future drugs will be compared. The search for new drugs and their use in clinical practice will also lead to developments in our knowledge and understanding of schizophrenia.

GM1 ganglioside administration partially counteracts the morphological changes associated with haloperidol treatment within the dorsal striatum of the rat

Haloperidol, a typical antipsychotic drug, causes an increase in the mean percentage of synapses within the situation containing a discontinuous, or perforated, postsynaptic density (PSD) following 1 month of treatment (Meshul et al. 1994). This effect is not observed with the atypical antipsychotic drug, clozapine, following subchronic administration (Meshul et al. 1992a). This morphological change is also associated with an increase in the density of dopamine D2 receptors. The synapses containing the perforated PSD are asymmetrical and the nerve terminals contain the neurotransmitter, glutamate, as demonstrated by immunocytochemistry. We have also shown that subchronic treatment with haloperidol (0.5 mg/kg per day, 30 days) results in a decrease in the density of glutamate immunoreactivity within asymmetric nerve terminals associated with perforated and non-perforated PSDs (Meshul and Tan 1994). This could be due to an increase in glutamate release, perhaps due to activation of corticostriatal synapses. Agnati et el. (1983a) reported that administration of GM1 ganglioside blocks the increase in dopamine D2 receptors following haloperidol treatment. GM1 has also been shown to attenuate the release of glutamate (Nicoletti et al. 1989). In order to determine if similar treatment with ganglioside could block the haloperidol-induced ultrastructural changes notes above, rats were co-administered GM1 (10 mg/kg per day) and haloperidol (0.5 mg/kg per day) for 30 days. We report that GM1 blocked the haloperidol-induced increase in striatal asymmetric synapses containing a perforated PSD, but had no effect on the increase in dopamine D2 receptors or the decrease in nerve terminal glutamate immunoreactivity. GM1, either alone or co-administered with haloperidol, also caused a small, but significant, increase in the density of all asymmetric synapses within the striatum. It is possible that the effect of GM1 in attenuating the haloperidol-induced change in glutamate synapses with perforated PSDs is primarily postsynaptic, since GM1 did not block the change in density of glutamate immunoreactivity within asymmetric nerve terminals.

Lack of apparent antipsychotic effect of the D1-dopamine receptor antagonist SCH39166 in acutely ill schizophrenic patients

SCH 39166 is the first selective D1 dopamine receptor antagonist developed for the treatment of schizophrenic patients. To examine potential antipsychotic effect, tolerability and safety, SCH 39166 was given orally to 17 acutely ill drug free schizophrenic patients (DSMIIIR) in an open 4-week study. Doses were escalated from 10 to 100 mg b.i.d. according to a fixed schedule over 17 days and remained at 100 mg b.i.d. for another 11 days. The drug was withdrawn prematurely in ten patients because of deterioration or refusal to take SCH 39166. In the nine patients participating for more than 2 weeks, none had an apparent reduction of BPRS or CGI scores. Side effects were agitation, akathisia and emesis in single patients. After withdrawal of SCH 39166 of the patients improved when treated with classical neuroleptics or clozapine. The result of the study does not support the prediction that selective D1 dopamine receptor antagonism will produce antipsychotic effects in schizophrenia.

Drug-induced oral dyskinesias in rats after traditional and new neuroleptics

Tardive dyskinesia (TD) is a serious human side effect of neuroleptic treatment in psychotic disorders. Although the etiology is clear (i.e. chronic neuroleptic drugs), its pathophysiology has not yet been satisfactorily explained. This is important not only theoretically but also to inform drug development, allowing the introduction of antipsychotic compounds without TD liability. The development of an animal condition which putatively models these delayed onset dyskinesias, has provided a technique to differentiate between neuroleptic drug effect and dyskinesia correlates. We report here the development of oral dyskinesias in rats in response to a number of different neuroleptics, which have a range of neurochemical and clinical characteristics. Traditional neuroleptics (e.g. haloperidol) produced rat oral dyskinesias, in an open-cage environment. Clozapine, while it produced an increased rate of oral movements, showed a significantly decreased potency in this model. SCH23390 (D1 antagonist) neither produced the oral movements nor modified their onset by coadministration with raclopride. These data replicate and extend other similar studies in the literature. They suggest that clozapine differs from traditional neuroleptics with respect to motor side effects.

Effects of raclopride treatment on plasma and CSF HVA: relationships with clinical improvement in male schizophrenics

Thirty-two acutely psychotic, male schizophrenic patients received raclopride, at 2, 6, or 12 mg/day, or haloperidol, 15 mg/day for 4 weeks after randomized, double-blind assignment. Twenty-six patients, including 19 who had been assigned one of the three doses of raclopride, completed the study. Raclopride, particularly at 12 mg/day, increased CSF homovanillic acid (HVA) at 4 weeks, and plasma HVA at 2 days, of treatment. The clinical response to raclopride was significantly correlated with plasma raclopride concentrations and baseline plasma HVA concentrations. Although raclopride is a substituted benzamide with atypical properties in animals, these results suggest that the doses of raclopride required for clinical efficacy and elevation of clinical indices of brain dopamine turnover are similar.

Haloperidol-induced morphological changes in striatum are associated with glutamate synapses

Sub-chronic treatment with the typical neuroleptic, haloperidol (0.5 mg/kg/d, s.c.), but not the atypical neuroleptic, clozapine (35 mg/kg/day, s.c.), causes an increase in synapses containing a perforated postsynaptic density (referred to as 'perforated' synapses) and in dopamine (DA) D2 receptors within the caudate nucleus [46]. To determine if these perforated synapses are glutamatergic, we systemically co-administered MK-801 (0.3 mg/kg/day for 2 weeks), a non-competitive antagonist at the N-methyl-D-aspartate (NMDA) receptor-associated ion channel, and haloperidol. MK-801 blocked the haloperidol-induced increase in striatal perforated synapses, but not the haloperidol-induced increase in DA D2 receptors. Injection of MK-801 into the striatum also attenuated the haloperidol-induced increase in perforated synapses. Post-embedding immuno-gold electron microscopy using antibodies to glutamate indicated that the gold particles were localized within striatal presynaptic nerve terminals that make contact with perforated postsynaptic densities. These findings support the hypothesis that the haloperidol-induced increase in perforated synapses is regulated by the NMDA subtype of excitatory glutamate receptor. The increase in perforated synapses following administration of haloperidol, which is associated with a high incidence of extrapyramidal side effects (EPS), and the lack of a synaptic change following administration of clozapine, known to have a low frequency of EPS, suggests that glutamate synapses play a role in the motoric side effects that are observed with typical neuroleptic drug treatment.

Influence of rate of administration of raclopride on akathisia and prolactin response

The D2-dopamine receptor antagonist raclopride was administered to eight healthy male subjects, who had previously experienced akathisia following antipsychotic drugs. The influence of administration rate on onset, severity and duration of akathisia and on prolactin response was studied. Raclopride 3, 5 or 9 mg or placebo (P) was administered as single IV infusions during 10 min (R10 min/3 mg), 1 h (R1h/5 mg) or 4 h (R4h/9 mg) according to a randomized double-blind design. Despite a 24-fold difference in administration rate a similar peak raclopride concentration of about 350 nmol/l was obtained after all three infusions. Three of the eight subjects experienced akathisia following R10 min/3 mg and R1h/5 mg, respectively. After R4h/9 mg seven subjects experienced akathisia of longer duration but not more severe than after the short infusions. The incidence and duration of akathisia seem to be mainly related to the plasma raclopride concentrations over time, whereas the rate of administration might be more important for the severity. A maximal prolactin response was induced which was not markedly affected by the rate of administration.

Prazosin modulates the changes in firing pattern and transmitter release induced by raclopride in the mesolimbic, but not in the nigrostriatal dopaminergic system

Most antipsychotic drugs are, in addition to being dopamine (DA) D2 receptor antagonists, also relatively potent alpha 1 adrenoceptor antagonists. Here, we have studied the effects of the selective DA D2 receptor antagonist raclopride, alone and in combination with the selective alpha 1 adrenoceptor antagonist, prazosin, on midbrain DA neurons utilizing extracellular single cell recording techniques. As a reference compound, haloperidol (0.05-1.6 mg/kg, i.v.), a potent antagonist at both DA D2 receptors and alpha 1 adrenoceptors, was included in the electrophysiological part of the study. In addition, in vivo voltammetry was used to measure extracellular DA concentrations in the nucleus accumbens (NAC) and the dorsolateral striatum (STR) in anesthetized, pargyline pretreated rats treated with the above drugs. Raclopride (10-5120 micrograms/kg, i.v.) induced a dose dependent increase in firing rate of DA neurons in the ventral tegmental area (VTA), that was significant already at 10 micrograms/kg, and in the substantia nigra-zone compacta (SN-ZC), that reached significance at 2560 micrograms/kg. Burst firing of DA neurons was also increased in the VTA at 40 micrograms/kg, as well as in the SN-ZC at 640 micrograms/kg. A low dose of raclopride (80 micrograms/kg, cumulated dose) induced a significant increase in extracellular DA concentrations in NAC to 490% and in STR to 220%. A high dose of raclopride (2560 micrograms/kg, cumulated dose) induced a 930% increase in extracellular DA concentrations in NAC, but only a 280% increase in STR. These data demonstrate that raclopride exerts a relatively selective action on mesolimbic DA neurons. Prazosin (0.3 mg/kg, i.v.) decreased burst firing of VTA, but not SN-ZC DA neurons.(ABSTRACT TRUNCATED AT 250 WORDS)

An open clinical and biochemical study of ritanserin in acute patients with schizophrenia

The effect of the selective serotonin-2 antagonist ritanserin was investigated in an open study of patients with schizophrenia. The patients were in an acute psychotic state considered to require neuroleptic medication. No neuroleptic drug was allowed during the study or during the last month preceeding the study. Oxazepam or nitrazepam were allowed for sedation or sleep inducement. Safety, tolerability, potential antipsychotic effect, and drug effects on monoamine metabolites in serum and CSF and prolactin in serum were evaluated. Central D2-dopamine receptor occupancy was determined by positron emission tomography. Ten male patients (mean age 32.4) fulfilling DSM-III-R criteria for schizophrenia were included in the study. Nine of these patients completed 4 weeks' treatment with ritanserin 10 mg b.i.d. The clinical effect was evaluated by means of CPRS and SANS and significant improvement was seen after 4 weeks' treatment both in positive and negative symptoms. Ritanserin was well tolerated and no extrapyramidal symptoms or akathisia were seen. Concentrations of monoamine metabolites and prolactin did not change during treatment. Ritanserin did not occupy D2-dopamine receptors. Thus, no indications of any D2-dopamine-antagonistic activity were obtained. All patients had expected ritanserin levels in plasma during the whole study. This first study of a selective serotonin-2 antagonist in the treatment of acute schizophrenic patients demonstrated significant clinical effects. However, the open design of the study does not allow us to conclude with any certainty that the patients' improvement was due to a specific blockade of serotonin-2 receptors or unspecific factors, although a direct D2-dopamine blockade could be ruled out.

Comparison of the effect of substituted benzamides on midbrain dopamine neurones after treatment of rats for 21 days

We have tested five different substituted benzamides (nemonapride, D,L-sulpiride, remoxipride, raclopride and zacopride) for their potential to decrease the number of spontaneously active dopamine neurones in the rat midbrain after treatment for 21 days. Nemonapride, D,L-sulpiride, and remoxipride significantly reduced the number of spontaneously active dopamine neurones in the ventral tegmental area, indicating an antipsychotic potential, while raclopride and zacopride induced but minor effects. The number of active dopamine neurones in the substantia nigra pars compacta was reduced by nemonapride at higher doses which should indicate the propensity for developing extrapyramidal side-effects. In conclusion, several of the substituted benzamides showed an antipsychotic profile in this test model. In addition, some of the benzamides also showed a propensity for extrapyramidal side-effects and these results are in accordance with the profile reported from clinical trials. However, the results obtained with this model indicate that raclopride neither has antipsychotic potential nor induces extrapyramidal side-effects. The reason for this discrepancy is at present not known.

Synthesis and in vivo evaluation of [18F]-N-(p-nitrobenzyl)spiperone ([18F]PNBS) in rats

[18F]-N-(p-nitrobenzyl)spiperone, [18F]PNBS, was prepared and its in vivo uptake in rat brain was measured. A temporal increase in striatum:cerebellum ratio and frontal cortex:cerebellum (FC:Cb) ratio was observed. Blocking studies suggested the formation of a radiolabeled metabolite, presumably [18F]spiperone, that is capable of crossing the blood-brain barrier and labeling cortical 5-HT2 receptors. This was confirmed using the ISRP method of metabolite analysis indicating [18F]PNBS is not a suitable tracer for studying D2 receptors in vivo with PET.