6-(18)F-DOPA PET study in patients with schizophrenia. Positron emission tomography

Investigating dopaminergic abnormalities in schizophrenia and first-episode psychosis with normative modelling and multisite molecular neuroimaging

Molecular neuroimaging techniques, like PET and SPECT, offer invaluable insights into the brain's in-vivo biology and its dysfunction in neuropsychiatric patients. However, the transition of molecular neuroimaging into diagnostics and precision medicine has been limited to a few clinical applications, hindered by issues like practical feasibility, high costs, and high between-subject heterogeneity of neuroimaging measures. In this study, we explore the use of normative modelling (NM) to identify individual patient alterations by describing the physiological variability of molecular functions. NM potentially addresses challenges such as small sample sizes and diverse acquisition protocols typical of molecular neuroimaging studies. We applied NM to two PET radiotracers targeting the dopaminergic system ([11C]-(+)-PHNO and [18F]FDOPA) to create a reference-cohort model of healthy controls. The models were subsequently utilized on different independent cohorts of patients with psychosis in different disease stages and treatment outcomes. Our results showed that patients with psychosis exhibited a higher degree of extreme deviations (~3-fold increase) than controls, although this pattern was heterogeneous, with minimal overlap of extreme deviations topology (max 20%). We also confirmed that striatal [18F]FDOPA signal, when referenced to a normative distribution, can predict treatment response (striatal AUC ROC: 0.77-0.83). In conclusion, our results indicate that normative modelling can be effectively applied to molecular neuroimaging after proper harmonization, enabling insights into disease mechanisms and advancing precision medicine. In addition, the method is valuable in understanding the heterogeneity of patient populations and can contribute to maximising cost efficiency in studies aimed at comparing cases and controls.

Imaging the Vesicular Acetylcholine Transporter in Schizophrenia: A Positron Emission Tomography Study Using [18F]-VAT

BACKGROUND

Despite longstanding interest in the central cholinergic system in schizophrenia (SCZ), cholinergic imaging studies with patients have been limited to receptors. Here, we conducted a proof-of-concept positron emission tomography study using [18F]-VAT, a new radiotracer that targets the vesicular acetylcholine transporter as a proxy measure of acetylcholine transmission capacity, in patients with SCZ and explored relationships of vesicular acetylcholine transporter with clinical symptoms and cognition.

METHODS

A total of 18 adult patients with SCZ or schizoaffective disorder (the SCZ group) and 14 healthy control participants underwent a positron emission tomography scan with [18F]-VAT. Distribution volume (VT) for [18F]-VAT was derived for each region of interest, and group differences in VT were assessed with 2-sample t tests. Functional significance was explored through correlations between VT and scores on the Positive and Negative Syndrome Scale and a computerized neurocognitive battery (PennCNB).

RESULTS

No group differences in [18F]-VAT VT were observed. However, within the SCZ group, psychosis symptom severity was positively associated with VT in multiple regions of interest, with the strongest effects in the hippocampus, thalamus, midbrain, cerebellum, and cortex. In addition, in the SCZ group, working memory performance was negatively associated with VT in the substantia innominata and several cortical regions of interest including the dorsolateral prefrontal cortex.

CONCLUSIONS

In this initial study, the severity of 2 important features of SCZ-psychosis and working memory deficit-was strongly associated with [18F]-VAT VT in several cortical and subcortical regions. These correlations provide preliminary evidence of cholinergic activity involvement in SCZ and, if replicated in larger samples, could lead to a more complete mechanistic understanding of psychosis and cognitive deficits in SCZ and the development of therapeutic targets.

The substantia nigra in the pathology of schizophrenia: A review on post-mortem and molecular imaging findings

Dysregulation of striatal dopamine is considered to be an important driver of pathophysiological processes in schizophrenia. Despite being one of the main origins of dopaminergic input to the striatum, the (dys)functioning of the substantia nigra (SN) has been relatively understudied in schizophrenia. Hence, this paper aims to review different molecular aspects of nigral functioning in patients with schizophrenia compared to healthy controls by integrating post-mortem and molecular imaging studies. We found evidence for hyperdopaminergic functioning in the SN of patients with schizophrenia (i.e. increased AADC activity in antipsychotic-free/-naïve patients and elevated neuromelanin accumulation). Reduced GABAergic inhibition (i.e. decreased density of GABAergic synapses, lower VGAT mRNA levels and lower mRNA levels for GABA_A receptor subunits), excessive glutamatergic excitation (i.e. increased NR1 and Glur5 mRNA levels and a reduced number of astrocytes), and several other disturbances implicating the SN (i.e. immune functioning and copper concentrations) could potentially underlie this nigral hyperactivity and associated striatal hyperdopaminergic functioning in schizophrenia. These results highlight the importance of the SN in schizophrenia pathology and suggest that some aspects of molecular functioning in the SN could potentially be used as treatment targets or biomarkers.

Magnetic resonance imaging of the dopamine system in schizophrenia - A scoping review

For decades, aberrant dopamine transmission has been proposed to play a central role in schizophrenia pathophysiology. These theories are supported by human in vivo molecular imaging studies of dopamine transmission, particularly positron emission tomography. However, there are several downsides to such approaches, for example limited spatial resolution or restriction of the measurement to synaptic processes of dopaminergic neurons. To overcome these limitations and to measure complementary aspects of dopamine transmission, magnetic resonance imaging (MRI)-based approaches investigating the macrostructure, metabolism, and connectivity of dopaminergic nuclei, i.e., substantia nigra pars compacta and ventral tegmental area, can be employed. In this scoping review, we focus on four dopamine MRI methods that have been employed in patients with schizophrenia so far: neuromelanin MRI, which is thought to measure long-term dopamine function in dopaminergic nuclei; morphometric MRI, which is assumed to measure the volume of dopaminergic nuclei; diffusion MRI, which is assumed to measure fiber-based structural connectivity of dopaminergic nuclei; and resting-state blood-oxygenation-level-dependent functional MRI, which is thought to measure functional connectivity of dopaminergic nuclei based on correlated blood oxygenation fluctuations. For each method, we describe the underlying signal, outcome measures, and downsides. We present the current state of research in schizophrenia and compare it to other disorders with either similar (psychotic) symptoms, i.e., bipolar disorder and major depressive disorder, or dopaminergic abnormalities, i.e., substance use disorder and Parkinson's disease. Finally, we discuss overarching issues and outline future research questions.

Clinical correlation but no elevation of striatal dopamine synthesis capacity in two independent cohorts of medication-free individuals with schizophrenia

Dysregulation of dopamine systems has been considered a foundational driver of pathophysiological processes in schizophrenia, an illness characterized by diverse domains of symptomatology. Prior work observing elevated presynaptic dopamine synthesis capacity in some patient groups has not always identified consistent symptom correlates, and studies of affected individuals in medication-free states have been challenging to obtain. Here we report on two separate cohorts of individuals with schizophrenia spectrum illness who underwent blinded medication withdrawal and medication-free neuroimaging with [¹⁸F]-FDOPA PET to assess striatal dopamine synthesis capacity. Consistently in both cohorts, we found no significant differences between patient and matched, healthy comparison groups; however, we did identify and replicate robust inverse relationships between negative symptom severity and tracer-specific uptake widely throughout the striatum: [¹⁸F]-FDOPA specific uptake was lower in patients with a greater preponderance of negative symptoms. Complementary voxel-wise and region of interest analyses, both with and without partial volume correction, yielded consistent results. These data suggest that for some individuals, striatal hyperdopaminergia may not be a defining or enduring feature of primary psychotic illness. However, clinical differences across individuals may be significantly linked to variability in striatal dopaminergic tone. These findings call for further experimentation aimed at parsing the heterogeneity of dopaminergic systems function in schizophrenia.

Dopamine D1R Receptor Stimulation as a Mechanistic Pro-cognitive Target for Schizophrenia

Decades of research have highlighted the importance of optimal stimulation of cortical dopaminergic receptors, particularly the D1R receptor (D1R), for prefrontal-mediated cognition. This mechanism is particularly relevant to the cognitive deficits in schizophrenia, given the abnormalities in cortical dopamine (DA) neurotransmission and in the expression of D1R. Despite the critical need for D1R-based therapeutics, many factors have complicated their development and prevented this important therapeutic target from being adequately interrogated. Challenges include determination of the optimal level of D1R stimulation needed to improve cognitive performance, especially when D1R expression levels, affinity states, DA levels, and the resulting D1R occupancy by DA, are not clearly known in schizophrenia, and may display great interindividual and intraindividual variability related to cognitive states and other physiological variables. These directly affect the selection of the level of stimulation necessary to correct the underlying neurobiology. The optimal mechanism for stimulation is also unknown and could include partial or full agonism, biased agonism, or positive allosteric modulation. Furthermore, the development of D1R targeting drugs has been complicated by complexities in extrapolating from in vitro affinity determinations to in vivo use. Prior D1R-targeted drugs have been unsuccessful due to poor bioavailability, pharmacokinetics, and insufficient target engagement at tolerable doses. Newer drugs have recently become available, and these must be tested in the context of carefully designed paradigms that address methodological challenges. In this paper, we discuss how a better understanding of these challenges has shaped our proposed experimental design for testing a new D1R/D5R partial agonist, PF-06412562, renamed CVL-562.

Dopamine dysregulation in psychotic relapse after antipsychotic discontinuation: an [18F]DOPA and [11C]raclopride PET study in first-episode psychosis

Although antipsychotic drugs are effective for relieving the psychotic symptoms of first-episode psychosis (FEP), psychotic relapse is common during the course of the illness. While some FEPs remain remitted even without medication, antipsychotic discontinuation is regarded as the most common risk factor for the relapse. Considering the actions of antipsychotic drugs on presynaptic and postsynaptic dopamine dysregulation, this study evaluated possible mechanisms underlying relapse after antipsychotic discontinuation. Twenty five FEPs who were clinically stable and 14 matched healthy controls were enrolled. Striatal dopamine activity was assessed as K_i^cer value using [¹⁸F]DOPA PET before and 6 weeks after antipsychotic discontinuation. The D2/3 receptor availability was measured as BP_ND using [¹¹C]raclopride PET after antipsychotic discontinuation. Healthy controls also underwent PET scans according to the corresponding schedule of the patients. Patients were monitored for psychotic relapse during 12 weeks after antipsychotic discontinuation. 40% of the patients showed psychotic relapse after antipsychotic discontinuation. The change in K_i^cer value over time significantly differed between relapsed, non-relapsed patients and healthy controls (Week*Group: F = 4.827, df = 2,253.193, p = 0.009). In relapsed patients, a significant correlation was found between baseline striatal K_i^cer values and time to relapse after antipsychotic discontinuation (R² = 0.518, p = 0.018). BP_ND were not significantly different between relapsed, non-relapsed patients and healthy controls (F = 1.402, df = 2,32.000, p = 0.261). These results suggest that dysfunctional dopamine autoregulation might precipitate psychotic relapse after antipsychotic discontinuation in FEP. This finding could be used for developing a strategy for the prevention of psychotic relapse related to antipsychotic discontinuation.

GMP production of 6-[18F]Fluoro-L-DOPA for PET/CT imaging by different synthetic routes: a three center experience

BACKGROUND

The radiofluorinated levodopa analogue 6-[¹⁸F]F-L-DOPA (3,4-dihydroxy-6-¹⁸F-L-phenylalanine) is a commonly employed radiotracer for PET/CT imaging of multiple oncological and neurological indications. An unusually large number of different radiosyntheses have been published to the point where two different Ph. Eur. monographs exist depending on whether the chemistry relies on electrophilic or nucleophilic radiosubstitution of appropriate chemical precursors. For new PET imaging sites wishing to adopt [¹⁸F]FDOPA into clinical practice, selecting the appropriate production process may be difficult and dependent on the clinical needs of the site.

METHODS

Data from four years of [¹⁸F]FDOPA production at three different clinical sites are collected and compared. These three sites, Aarhus University Hospital (AUH), Odense University Hospital (OUH), and Herlev University Hospital (HUH), produce the radiotracer by different radiosynthetic routes with AUH adopting an electrophilic strategy, while OUH and HUH employ two different nucleophilic approaches. Production failure rates, radiochemical yields, and molar activities are compared across sites and time. Additionally, the clinical use of the radiotracer over the time period considered at the different sites are presented and discussed.

RESULTS

The electrophilic substitution route suffers from being demanding in terms of cyclotron operation and maintenance. This challenge, however, was found to be compensated by a production failure rate significantly below that of both nucleophilic approaches; a result of simpler chemistry. The five-step nucleophilic approach employed at HUH produces superior radiochemical yields compared to the three-step approach adopted at OUH but suffers from the need for more comprehensive synthesis equipment given the multi-step nature of the procedure, including HPLC purification. While the procedure at OUH furnishes the lowest radiochemical yield of the synthetic routes considered, it produces the highest molar activity. This is of importance across the clinical applications of the tracer discussed here, including dopamine synthesis in striatum of subjects with schizophrenia and congenital hyperinsulinism in infants.

CONCLUSION

For most sites either of the two nucleophilic substitution strategies should be favored. However, which of the two will depend on whether a given site wishes to optimize the radiochemical yield or the ease of the use.

Intrinsic Connectivity Patterns of Task-Defined Brain Networks Allow Individual Prediction of Cognitive Symptom Dimension of Schizophrenia and Are Linked to Molecular Architecture

BACKGROUND

Despite the marked interindividual variability in the clinical presentation of schizophrenia, the extent to which individual dimensions of psychopathology relate to the functional variability in brain networks among patients remains unclear. Here, we address this question using network-based predictive modeling of individual psychopathology along 4 data-driven symptom dimensions. Follow-up analyses assess the molecular underpinnings of predictive networks by relating them to neurotransmitter-receptor distribution patterns.

METHODS

We investigated resting-state functional magnetic resonance imaging data from 147 patients with schizophrenia recruited at 7 sites. Individual expression along negative, positive, affective, and cognitive symptom dimensions was predicted using a relevance vector machine based on functional connectivity within 17 meta-analytic task networks following repeated 10-fold cross-validation and leave-one-site-out analyses. Results were validated in an independent sample. Networks robustly predicting individual symptom dimensions were spatially correlated with density maps of 9 receptors/transporters from prior molecular imaging in healthy populations.

RESULTS

Tenfold and leave-one-site-out analyses revealed 5 predictive network-symptom associations. Connectivity within theory of mind, cognitive reappraisal, and mirror neuron networks predicted negative, positive, and affective symptom dimensions, respectively. Cognitive dimension was predicted by theory of mind and socioaffective default networks. Importantly, these predictions generalized to the independent sample. Intriguingly, these two networks were positively associated with D₁ receptor and serotonin reuptake transporter densities as well as dopamine synthesis capacity.

CONCLUSIONS

We revealed a robust association between intrinsic functional connectivity within networks for socioaffective processes and the cognitive dimension of psychopathology. By investigating the molecular architecture, this work links dopaminergic and serotonergic systems with the functional topography of brain networks underlying cognitive symptoms in schizophrenia.

Dysregulation of Midbrain Dopamine System and the Pathophysiology of Schizophrenia

Dysregulation of the dopamine system is central to many models of the pathophysiology of psychosis in schizophrenia. However, emerging evidence suggests that this dysregulation is driven by the disruption of upstream circuits that provide afferent control of midbrain dopamine neurons. Furthermore, stress can profoundly disrupt this regulatory circuit, particularly when it is presented at critical vulnerable prepubertal time points. This review will discuss the dopamine system and the circuits that regulate it, focusing on the hippocampus, medial prefrontal cortex, thalamic nuclei, and medial septum, and the impact of stress. A greater understanding of the regulation of the dopamine system and its disruption in schizophrenia may provide a more complete neurobiological framework to interpret clinical findings and develop novel treatments.

The Topography of Striatal Dopamine and Symptoms in Psychosis: An Integrative Positron Emission Tomography and Magnetic Resonance Imaging Study

Background

Striatal dopamine dysfunction is thought to underlie symptoms in psychosis, yet it remains unclear how a single neurotransmitter could cause the diverse presentations that are observed clinically. One hypothesis is that the consequences of aberrant dopamine signaling vary depending on where within the striatum the dysfunction occurs. Positron emission tomography allows for the quantification of dopamine function across the striatum. In the current study, we used a novel method to investigate the relationship between spatial variability in dopamine synthesis capacity and psychotic symptoms.

Methods

We used a multimodal imaging approach combining ¹⁸F-DOPA positron emission tomography and resting-state magnetic resonance imaging in 29 patients with first-episode psychosis and 21 healthy control subjects. In each participant, resting-state functional connectivity maps were used to quantify the functional connectivity of each striatal voxel to well-established cortical networks. Network-specific striatal dopamine synthesis capacity (Ki^cer) was then calculated for the resulting connectivity-defined parcellations.

Results

The connectivity-defined parcellations generated Ki^cer values with equivalent reliability, and significantly greater orthogonality compared with standard anatomical parcellation methods. As a result, dopamine-symptom associations were significantly different from one another for different subdivisions, whereas no unique subdivision relationships were found when using an anatomical parcellation. In particular, dopamine function within striatal areas connected to the default mode network was strongly associated with negative symptoms (p < .001).

Conclusions

These findings suggest that individual differences in the topography of dopamine dysfunction within the striatum contribute to shaping psychotic symptomatology. Further validation of the novel approach in future studies is necessary.

Impaired brain glucose metabolism and presynaptic dopaminergic functioning in a mouse model of schizophrenia

Background

Schizophrenia is a disease diagnosed by visible signs and symptoms from late adolescence to early adulthood. The etiology of this disease remains unknown. An objective diagnostic approach is required. Here, we used a mouse model that shows schizophrenia-like phenotypes to study brain glucose metabolism and presynaptic dopaminergic functioning by positron emission tomography (PET) and immunohistochemistry. PET scannings were performed on mice after the administration of [¹⁸F]-FDG or [¹⁸F]-F-DOPA. Glucose metabolism was evaluated in basal conditions and after the induction of a hyperdopaminergic state.

Results

Mutant animals show reduced glucose metabolism in prefrontal cortex, amygdala, and nucleus reuniens under the hyperdopaminergic state. They also show reduced [¹⁸F]-F-DOPA uptake in prefrontal cortex, substantia nigra reticulata, raphe nucleus, and ventral striatum but increased [¹⁸F]-F-DOPA uptake in dorsal striatum. Mutant animals also show reduced tyrosine hydroxylase expression on midbrain neurons.

Conclusions

Dopamine D2 mutant animals show reduced glucose metabolism and impaired presynaptic dopaminergic functioning, in line with reports from human studies. This mouse line may be a valuable model of schizophrenia, useful to test novel tracers for PET scanning diagnostic.

Reduced striatal dopamine synthesis capacity in patients with schizophrenia during remission of positive symptoms

While there is consistent evidence for increased presynaptic dopamine synthesis capacity in the striatum of patients with schizophrenia during psychosis, it is unclear whether this also holds for patients during psychotic remission. This study investigates whether striatal dopamine synthesis capacity is altered in patients with schizophrenia during symptomatic remission of positive symptoms, and whether potential alterations relate to symptoms other than positive, such as cognitive difficulties. Twenty-three patients with schizophrenia in symptomatic remission of positive symptoms according to Andreasen, and 24 healthy controls underwent 18F-DOPA-PET and behavioural-cognitive assessment. Imaging data were analysed with voxel-wise Patlak modelling with cerebellum as reference region, resulting in the influx constant kicer reflecting dopamine synthesis capacity. For the whole striatum and its subdivisions (i.e. limbic, associative, and sensorimotor), averaged regional kicer values were calculated, compared across groups, and correlated with behavioural-cognitive scores, including a mediation analysis. Patients had negative symptoms (Positive and Negative Syndrome Scale-negative 14.13 ± 5.91) and cognitive difficulties, i.e. they performed worse than controls in Trail-Making-Test-B (TMT-B; P = 0.01). Furthermore, kicer was reduced in patients for whole striatum (P = 0.004) and associative (P = 0.002) and sensorimotor subdivisions (P = 0.007). In patients, whole striatum kicer was negatively correlated with TMT-B (rho = -0.42, P = 0.04; i.e. the lower striatal kicer, the worse the cognitive performance). Mediation analysis showed that striatal kicer mediated the group difference in TMT-B. Results demonstrate that patients with schizophrenia in symptomatic remission of positive symptoms have decreased striatal dopamine synthesis capacity, which mediates the disorder's impact on cognitive difficulties. Data suggest that striatal dopamine dysfunction contributes to cognitive difficulties in schizophrenia.

Dopamine and glutamate in schizophrenia: biology, symptoms and treatment

Glutamate and dopamine systems play distinct roles in terms of neuronal signalling, yet both have been proposed to contribute significantly to the pathophysiology of schizophrenia. In this paper we assess research that has implicated both systems in the aetiology of this disorder. We examine evidence from post-mortem, preclinical, pharmacological and in vivo neuroimaging studies. Pharmacological and preclinical studies implicate both systems, and in vivo imaging of the dopamine system has consistently identified elevated striatal dopamine synthesis and release capacity in schizophrenia. Imaging of the glutamate system and other aspects of research on the dopamine system have produced less consistent findings, potentially due to methodological limitations and the heterogeneity of the disorder. Converging evidence indicates that genetic and environmental risk factors for schizophrenia underlie disruption of glutamatergic and dopaminergic function. However, while genetic influences may directly underlie glutamatergic dysfunction, few genetic risk variants directly implicate the dopamine system, indicating that aberrant dopamine signalling is likely to be predominantly due to other factors. We discuss the neural circuits through which the two systems interact, and how their disruption may cause psychotic symptoms. We also discuss mechanisms through which existing treatments operate, and how recent research has highlighted opportunities for the development of novel pharmacological therapies. Finally, we consider outstanding questions for the field, including what remains unknown regarding the nature of glutamate and dopamine function in schizophrenia, and what needs to be achieved to make progress in developing new treatments.

Heterogeneity of Striatal Dopamine Function in Schizophrenia: Meta-analysis of Variance

BACKGROUND

It has been hypothesized that dopamine function in schizophrenia exhibits heterogeneity in excess of that seen in the general population. However, no previous study has systematically tested this hypothesis.

METHODS

We employed meta-analysis of variance to investigate interindividual variability of striatal dopaminergic function in patients with schizophrenia and in healthy control subjects. We included 65 studies that reported molecular imaging measures of dopamine synthesis or release capacities, dopamine D_2/3 receptor (D2/3R) or dopamine transporter (DAT) availabilities, or synaptic dopamine levels in 983 patients and 968 control subjects. Variability differences were quantified using variability ratio (VR) and coefficient of variation ratio.

RESULTS

Interindividual variability of striatal D2/3R (VR = 1.26, p < .0001) and DAT (VR = 1.31, p = .01) availabilities and synaptic dopamine levels (VR = 1.38, p = .045) but not dopamine synthesis (VR = 1.12, p = .13) or release (VR = 1.08, p = .70) capacities were significantly greater in patients than in control subjects. Findings were robust to variability measure. Mean dopamine synthesis (g = 0.65, p = .004) and release (g = 0.66, p = .03) capacities, as well as synaptic levels (g = 0.78, p = .0006), were greater in patients overall, but mean synthesis capacity did not differ from that of control subjects in treatment-resistant patients (p > .3). Mean D2/3R (g = 0.17, p = .14) and DAT (g = -0.20, p = .28) availabilities did not differ between groups.

CONCLUSIONS

Our findings demonstrate significant heterogeneity of striatal dopamine function in schizophrenia. They suggest that while elevated dopamine synthesis and release capacities may be core features of the disorder, altered D2/3R and DAT availabilities and synaptic dopamine levels may occur only in a subgroup of patients. This heterogeneity may contribute to variation in treatment response and side effects.

Acute anxiety disorder, major depressive disorder, bipolar disorder and schizophrenia are related to different patterns of nigrostriatal and mesolimbic dopamine dysfunction

Dopamine (DA) receptor and transporter dysfunctions play a major role in the pathophysiology of neuropsychiatric diseases including anxiety disorder (AD), major depressive disorder (MDD), bipolar disorder (BD) in the manic (BDman) or depressive (BDdep) state and schizophrenia (SZ). We performed a PUBMED search, which provided a total of 239 in vivo imaging studies with either positron emission tomography (PET) or single-proton emission computed tomography (SPECT). In these studies, DA transporter binding, D1 receptor (R) binding, D2R binding, DA synthesis and/or DA release in patients with the primary diagnosis of acute AD (n=310), MDD (n=754), BDman (n=15), BDdep (n=49) or SZ (n=1532) were compared to healthy individuals. A retrospective analysis revealed that AD, MDD, BDman, BDdep and SZ differed as to affected brain region(s), affected synaptic constituent(s) and extent as well as direction of dysfunction in terms of either sensitization or desensitization of transporter and/or receptor binding sites. In contrast to AD and SZ, in MDD, BDman and BDdep, neostriatal DA function was normal, whereas MDD, BDman, and BDdep were characterized by the increased availability of prefrontal and frontal DA. In contrast to AD, MDD, BDman and BDdep, DA function in SZ was impaired throughout the nigrostriatal and mesolimbocortical system with an increased availability of DA in the striatothalamocortical and a decreased availability in the mesolimbocortical pathway.

Dysfunction of Magnocellular/dorsal Processing Stream in Schizophrenia

Background:

Patients with schizophrenia show not only cognitive, but also perceptual deficits. Perceptual deficits may affect different sensory modalities. Among these, the impairment of visual information processing is of particular relevance as demonstrated by the high incidence of visual disturbances. In recent years, the study of neurophysiological mechanisms that underlie visuo-perceptual, -spatial and -motor disorders in schizophrenia has increasingly attracted the interest of researchers.

Objective:

The study aims to review the existent literature on magnocellular/dorsal (occipitoparietal) visual processing stream impairment in schizophrenia. The impairment of relatively early stages of visual information processing was examined using experimental paradigms such as backward masking, contrast sensitivity, contour detection, and perceptual closure. The deficits of late processing stages were detected by examining visuo-spatial and -motor abilities.

Results:

Neurophysiological and behavioral studies support the existence of deficits in the processing of visual information along the magnocellular/dorsal pathway. These deficits appear to affect both early and late stages of visual information processing.

Conclusion:

The existence of disturbances in the early processing of visual information along the magnocellular/dorsal pathway is strongly supported by neurophysiological and behavioral observations. Early magnocellular dysfunction may provide a substrate for late dorsal processing impairment as well as higher-level cognition deficits.

Defining the Locus of Dopaminergic Dysfunction in Schizophrenia: A Meta-analysis and Test of the Mesolimbic Hypothesis

BACKGROUND

Studies using positron emission tomography to image striatal dopamine function, have demonstrated that individuals with schizophrenia display increases in presynaptic function. Mesolimbic dysfunction specifically, has previously been suggested to underlie psychotic symptoms. This has not been directly tested in vivo, and the precise anatomical locus of dopamine dysfunction within the striatum remains unclear. The current article investigates the magnitude of dopaminergic abnormalities in individuals with schizophrenia, and determines how the magnitude of abnormality varies across functional subdivisions of the striatum.

METHODS

EMBASE, PsychINFO, and MEDLINE were searched from January 1, 1960, to December 1, 2016. Inclusion criteria were molecular imaging studies that had measured presynaptic striatal dopamine functioning. Effects sizes for whole striatum and functional subdivisions were calculated separately. The magnitude of difference between functional subdivisions in patients and controls was meta-analyzed.

RESULTS

Twenty-one eligible studies were identified, including 269 patients and 313 controls. Individuals with schizophrenia (Hedges' g = 0.68, P < .001) demonstrated elevated presynaptic dopamine functioning compared to controls. Seven studies examined functional subdivisions. These demonstrated significant increases in patients compared to controls in associative (g = 0.73, P = .002) and sensorimotor (g = 0.54, P = .005) regions, but not limbic (g = 0.29, P = .09). The magnitude of the difference between associative and limbic subdivisions was significantly greater in patients compared to controls (g = 0.39, P = .003).

CONCLUSION

In individuals with schizophrenia dopaminergic dysfunction is greater in dorsal compared to limbic subdivisions of the striatum. This is inconsistent with the mesolimbic hypothesis and identifies the dorsal striatum as a target for novel treatment development.

Cognitive Function and Monoamine Neurotransmission in Schizophrenia: Evidence From Positron Emission Tomography Studies

Positron emission tomography (PET) is a non-invasive imaging technique used to assess various brain functions, including cerebral blood flow, glucose metabolism, and neurotransmission, in the living human brain. In particular, neurotransmission mediated by the monoamine neurotransmitters dopamine, serotonin, and norepinephrine, has been extensively examined using PET probes, which specifically bind to the monoamine receptors and transporters. This useful tool has revealed the pathophysiology of various psychiatric disorders, including schizophrenia, and the mechanisms of action of psychotropic drugs. Because monoamines are implicated in various cognitive processes such as memory and executive functions, some PET studies have directly investigated the associations between monoamine neurotransmission and cognitive functions in healthy individuals and patients with psychiatric disorders. In this mini review, I discuss the findings of PET studies that investigated monoamine neurotransmission under resting conditions, specifically focusing on cognitive functions in patients with schizophrenia. With regard to the dopaminergic system, some studies have examined the association of dopamine D1 and D2/D3 receptors, dopamine transporters, and dopamine synthesis capacity with various cognitive functions in schizophrenia. With regard to the serotonergic system, 5-HT1A and 5-HT2A receptors have been studied in the context of cognitive functions in schizophrenia. Although relatively few PET studies have examined cognitive functions in patients with psychiatric disorders, these approaches can provide useful information on enhancing cognitive functions by administering drugs that modulate monoamine transmission. Moreover, another paradigm of techniques such as those exploring the release of neurotransmitters and further development of radiotracers for novel targets are warranted.

A Test of the Transdiagnostic Dopamine Hypothesis of Psychosis Using Positron Emission Tomographic Imaging in Bipolar Affective Disorder and Schizophrenia

Importance

The dopamine hypothesis suggests that dopamine abnormalities underlie psychosis, irrespective of diagnosis, implicating dopamine dysregulation in bipolar affective disorder and schizophrenia, in line with the research domain criteria approach. However, this hypothesis has not been directly examined in individuals diagnosed with bipolar disorder with psychosis.

Objectives

To test whether dopamine synthesis capacity is elevated in bipolar disorder with psychosis and how this compares with schizophrenia and matched controls and to examine whether dopamine synthesis capacity is associated with psychotic symptom severity, irrespective of diagnostic class.

Design, Setting, and Participants

This cross-sectional case-control positron emission tomographic study was performed in the setting of first-episode psychosis services in an inner-city area (London, England). Sixty individuals participated in the study (22 with bipolar psychosis [18 antipsychotic naive or free], 16 with schizophrenia [14 antipsychotic naive or free], and 22 matched controls) and underwent fluorodihydroxyphenyl-l-alanine ([18F]-DOPA) positron emission tomography to examine dopamine synthesis capacity. Standardized clinical measures, including the Positive and Negative Syndrome Scale, Young Mania Rating Scale, and Global Assessment of Functioning, were administered. The study dates were March 2013 to November 2016.

Main Outcomes and Measures

Dopamine synthesis capacity (Kicer) and clinical measures (Positive and Negative Syndrome Scale, Young Mania Rating Scale, and Global Assessment of Functioning).

Results

The mean (SD) ages of participants were 23.6 (3.6) years in 22 individuals with bipolar psychosis (13 male), 26.3 (4.4) years in 16 individuals with schizophrenia (14 male), and 24.5 (4.5) years in controls (14 male). There was a significant group difference in striatal dopamine synthesis capacity (Kicer) (F2,57 = 6.80, P = .002). Kicer was significantly elevated in both the bipolar group (mean [SD], 13.18 [1.08] × 10-3 min-1; P = .002) and the schizophrenia group (mean [SD], 12.94 [0.79] × 10-3 min-1; P = .04) compared with controls (mean [SD], 12.16 [0.92] × 10-3 min-1). There was no significant difference in striatal Kicer between the bipolar and schizophrenia groups. Kicer was significantly positively correlated with positive psychotic symptom severity in the combined bipolar and schizophrenia sample experiencing a current psychotic episode, explaining 27% of the variance in symptom severity (n = 32, r = 0.52, P = .003). There was a significant positive association between Kicer and positive psychotic symptom severity in individuals with bipolar disorder experiencing a current psychotic episode (n = 16, r = 0.60, P = .01), which remained significant after adjusting for manic symptom severity.

Conclusions and Relevance

These findings are consistent with a transdiagnostic role for dopamine dysfunction in the pathoetiology of psychosis and suggest dopamine synthesis capacity as a potential novel drug target for bipolar disorder and schizophrenia.

Presynaptic Dopamine Capacity in Patients with Treatment-Resistant Schizophrenia Taking Clozapine: An [18F]DOPA PET Study

Some patients with schizophrenia show poor response to first-line antipsychotic treatments and this is termed treatment-resistant schizophrenia. The differential response to first-line antipsychotic drugs may reflect a different underlying neurobiology. Indeed, a previous study found dopamine synthesis capacity was significantly lower in patients with treatment-resistant schizophrenia. However, in this study, the treatment-resistant patients were highly symptomatic, whereas the responsive patients showed no or minimal symptoms. The study could not distinguish whether this was a trait effect or reflected the difference in symptom levels. Thus, we aimed to test whether dopaminergic function is altered in patients with a history of treatment resistance to first-line drugs relative to treatment responders when both groups are matched for symptom severity levels by recruiting treatment-resistant patients currently showed low symptom severity with the clozapine treatment. Healthy controls (n=12), patients treated with clozapine (n=12) who had not responded to first-line antipsychotics, and patients who had responded to first-line antipsychotics (n=12) were recruited. Participants were matched for age and sex and symptomatic severity level in patient groups. Participants' dopamine synthesis capacity was measured by using [¹⁸F]DOPA PET. We found that patients treated with clozapine show lower dopamine synthesis capacity than patients who have responded to first-line treatment (Cohen's d=0.9191 (whole striatum), 0.7781 (associative striatum), 1.0344 (limbic striatum), and 1.0189 (sensorimotor striatum) in line with the hypothesis that the dopaminergic function is linked to treatment response. This suggests that a different neurobiology may underlie treatment-resistant schizophrenia and that dopamine synthesis capacity may be a useful biomarker to predict treatment responsiveness.

Pathway-Specific Dopamine Abnormalities in Schizophrenia

In light of the clinical evidence implicating dopamine in schizophrenia and the prominent hypotheses put forth regarding alterations in dopaminergic transmission in this disease, molecular imaging has been used to examine multiple aspects of the dopaminergic system. We review the imaging methods used and compare the findings across the different molecular targets. Findings have converged to suggest early dysregulation in the striatum, especially in the rostral caudate, manifesting as excess synthesis and release. Recent data showed deficit extending to most cortical regions and even to other extrastriatal subcortical regions not previously considered to be "hypodopaminergic" in schizophrenia. These findings yield a new topography for the dopaminergic dysregulation in schizophrenia. We discuss the dopaminergic innervation within the individual projection fields to provide a topographical map of this dual dysregulation and explore potential cellular and circuit-based mechanisms for brain region-dependent alterations in dopaminergic parameters. This refined knowledge is essential to better guide translational studies and efforts in early drug development.

Recent Developments in Molecular Brain Imaging of Neuropsychiatric Disorders

Molecular imaging with PET or SPECT has been an important research tool in psychiatry for as long as these modalities have been available. Here, we discuss two areas of neuroimaging relevant to current psychiatry research. The first is the use of imaging to study neurotransmission. We discuss the use of pharmacologic probes to induce changes in levels of neurotransmitters that can be inferred through their effects on outcome measures of imaging experiments, from their historical origins focusing on dopamine transmission through recent developments involving serotonin, GABA, and glutamate. Next, we examine imaging of neuroinflammation in the context of psychiatry. Imaging markers of neuroinflammation have been studied extensively in other areas of brain research, but they have more recently attracted interest in psychiatry research, based on accumulating evidence that there may be an inflammatory component to some psychiatric conditions. Furthermore, new probes are under development that would allow unprecedented insights into cellular processes. In summary, molecular imaging would continue to offer great potential as a unique tool to further our understanding of brain function in health and disease.

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.

Role of (18) F-FDOPA PET/CT imaging in endocrinology

(18) F-FDOPA (6-[18F]-L-fluoro-L-3, 4-dihydroxyphenylalanine)-based PET/CT imaging can be a useful tool for the detection of different neuroendocrine tumours (NETs). (18) F-FDOPA is taken up into the cells via the neutral amino acid transporter (LAT1/4F2hc). This transporter is also coupled to the mammalian target of rapamycin (mTOR) signalling pathway. (18) F-FDOPA PET/CT may be performed for confirmation of diagnosis of pheochromocytoma/paraganglioma, staging at initial presentation, restaging and follow-up of patients. In SDHx-related syndromes, (18) F-FDG PET/CT should be performed in addition to (18) F-FDOPA PET/CT. (18) F-FDOPA PET/CT is also invaluable in the detection staging/restaging of carcinoid tumours and has greater sensitivity as compared to somatostatin receptor scintigraphy. (18) F-FDOPA PET/CT can also distinguish between focal vs diffuse CHI. It is not as useful in adult hyperinsulinism due to increased background uptake, but the problem may be overcome with the help of premedication with carbidopa. It has limited use in pancreatic NETs. (18) F-FDOPA PET/CT is a good modality for detection of persistent and residual medullary thyroid cancer (MTC), but (18) F-FDG PET/CT may be needed in aggressive tumours. In summary, F-DOPA PET/CT has widespread utility in the diagnosis of different neuroendocrine tumours.

Neurochemical dysfunction in treated and nontreated schizophrenia - a retrospective analysis of in vivo imaging studies

To evaluate the contribution of individual synaptic constituents, all available in vivo imaging studies on schizophrenic patients were subjected to a retrospective analysis. For the pool of drug-naïve, drug-free, and acutely medicated patients, major findings were increases in neostriatal dopamine (DA) synthesis and release and decreases in neostriatal DA transporters and D1 receptors, neostriatal, thalamic, frontal, and parietal D2 receptors, mesencephalic/pontine and temporal 5-HT1A receptors, frontal and temporal HT2A and μ-amino butyric acid (GABA)A receptors. Based on the findings on drug-naïve and drug-free patients, it may be hypothesized that schizophrenia initially is characterized by an impaired mechanism of D2 autoreceptor and heteroreceptor sensitization leading to sensitization instead of desensitization in response to increased levels of neostriatal DA. Neuroleptic medication blocks neostriatal D2 autoreceptor and heteroreceptors, reducing neostriatal DA and disinhibiting DA action mediated by D2 heteroreceptor binding sites. Ultimately, this may result in a restitution of GABA function, leading to a recovery of inhibitory input to the target regions of the descending corticothalamostriatal efferents. Furthermore, a blockade of inhibitory and excitatory neocortical 5-HT function may be inferred, which is likely to reduce (excitatory) DAergic input to the mesolimbic target regions of corticothalamostriatal projections.

Alterations in cortical and extrastriatal subcortical dopamine function in schizophrenia: systematic review and meta-analysis of imaging studies

BACKGROUND

The hypothesis that cortical dopaminergic alterations underlie aspects of schizophrenia has been highly influential.

AIMS

To bring together and evaluate the imaging evidence for dopaminergic alterations in cortical and other extrastriatal regions in schizophrenia.

METHOD

Electronic databases were searched for in vivo molecular studies of extrastriatal dopaminergic function in schizophrenia. Twenty-three studies (278 patients and 265 controls) were identified. Clinicodemographic and imaging variables were extracted and effect sizes determined for the dopaminergic measures. There were sufficient data to permit meta-analyses for the temporal cortex, thalamus and substantia nigra but not for other regions.

RESULTS

The meta-analysis of dopamine D2/D3 receptor availability found summary effect sizes of d = -0.32 (95% CI -0.68 to 0.03) for the thalamus, d = -0.23 (95% CI -0.54 to 0.07) for the temporal cortex and d = 0.04 (95% CI -0.92 to 0.99) for the substantia nigra. Confidence intervals were wide and all included no difference between groups. Evidence for other measures/regions is limited because of the small number of studies and in some instances inconsistent findings, although significant differences were reported for D2/D3 receptors in the cingulate and uncus, for D1 receptors in the prefrontal cortex and for dopamine transporter availability in the thalamus.

CONCLUSIONS

There is a relative paucity of direct evidence for cortical dopaminergic alterations in schizophrenia, and findings are inconclusive. This is surprising given the wide influence of the hypothesis. Large, well-controlled studies in drug-naive patients are warranted to definitively test this hypothesis.

Abnormal striatal dopamine transmission in schizophrenia

Despite numerous revisions and reformulations, dopamine (DA) hypothesis of schizophrenia remains a pivotal neurochemical hypothesis of this illness. The aim of this review is to expose and discuss findings from positron emission tomography (PET) or single-photon-emission computed tomography (SPECT) studies investigating DA function in the striatum of medicated, drug-naïve or drug-free patients with schizophrenia and in individuals at risk compared with healthy volunteers.

DA function was studied at several levels: i) at a presynaptic level where neuroimaging studies investigating DOPA uptake capacity clearly show an increase of DA synthesis in patients with schizophrenia; ii) at a synaptic level where neuroimaging studies investigating dopamine transporter availability (DAT) does not bring any evidence of dysfunction; iii) and finally, neuroimaging studies investigating DA receptor density show a mild increase of D2 receptor density in basic condition and, an hyperreactivity of DA system in dynamic condition.

These results are discussed regarding laterality, sub-regions of striatum and implications for the at-risk population. Striatal DA abnormalities are now clearly demonstrated in patients with schizophrenia and at risk population and could constitute an endophenotype of schizophrenia. Subtle sub-clinical striatal DA abnormalities in at risk population could be a biomarker of transition from a vulnerability state to the expression of frank psychosis.

Midbrain dopamine function in schizophrenia and depression: a post-mortem and positron emission tomographic imaging study

Elevated in vivo markers of presynaptic striatal dopamine activity have been a consistent finding in schizophrenia, and include a large effect size elevation in dopamine synthesis capacity. However, it is not known if the dopaminergic dysfunction is limited to the striatal terminals of dopamine neurons, or is also evident in the dopamine neuron cell bodies, which mostly originate in the substantia nigra. The aim of our studies was therefore to determine whether dopamine synthesis capacity is altered in the substantia nigra of people with schizophrenia, and how this relates to symptoms. In a post-mortem study, a semi-quantitative analysis of tyrosine hydroxylase staining was conducted in nigral dopaminergic cells from post-mortem tissue from patients with schizophrenia (n = 12), major depressive disorder (n = 13) and matched control subjects (n = 13). In an in vivo imaging study, nigral and striatal dopaminergic function was measured in patients with schizophrenia (n = 29) and matched healthy control subjects (n = 29) using (18)F-dihydroxyphenyl-L-alanine ((18)F-DOPA) positron emission tomography. In the post-mortem study we found that tyrosine hydroxylase staining was significantly increased in nigral dopaminergic neurons in schizophrenia compared with both control subjects (P < 0.001) and major depressive disorder (P < 0.001). There was no significant difference in tyrosine hydroxylase staining between control subjects and patients with major depressive disorder, indicating that the elevation in schizophrenia is not a non-specific indicator of psychiatric illness. In the in vivo imaging study we found that (18)F-dihydroxyphenyl-L-alanine uptake was elevated in both the substantia nigra and in the striatum of patients with schizophrenia (effect sizes = 0.85, P = 0.003 and 1.14, P < 0.0001, respectively) and, in the voxel-based analysis, was elevated in the right nigra (P < 0.05 corrected for family wise-error). Furthermore, nigral (18)F-dihydroxyphenyl-L-alanine uptake was positively related with the severity of symptoms (r = 0.39, P = 0.035) in patients. However, whereas nigral and striatal (18)F-dihydroxyphenyl-L-alanine uptake were positively related in control subjects (r = 0.63, P < 0.001), this was not the case in patients (r = 0.30, P = 0.11). These findings indicate that elevated dopamine synthesis capacity is seen in the nigral origin of dopamine neurons as well as their striatal terminals in schizophrenia, and is linked to symptom severity in patients.

A proposal for reframing schizophrenia research

The current schizophrenia concept is built on experts' agreement on the matter, and it is basically rooted in the epidemiological and clinical evidence. However, the numerous and intensive attempts to find the biological underpinnings of this syndrome face almost constantly a low degree of replication of the results. We have reviewed previously published work to contribute to identify some reasons underlying that failure. The difficulty in replicating biological findings in schizophrenia may relate to the intrinsic heterogeneity among patient samples, acquired through the current diagnostic criteria. As a result, the necessary replication for any finding to be accepted as characteristic data for schizophrenia would be impeded. Therefore, a new frame based on identification of correlates of the most replicated biological anomalies in schizophrenia to date may contribute to overcome those difficulties.

Novel nutritional treatment for manic and psychotic disorders: a review of tryptophan and tyrosine depletion studies and the potential of protein-based formulations using glycomacropeptide

RATIONALE

Current amino acid (AA) mixtures used in acute tryptophan (Trp) and tyrosine (Tyr) plus phenylalanine (Phe) depletion and loading tests are unpalatable and lack specificity. Specificity is improved by reducing content of branched-chain amino acids (BCAA) and palatability to a certain extent by dose reduction.

OBJECTIVES

This study aims to identify a palatable naturally occurring alternative(s) to amino acids with the desired BCAA content for use in the above tests.

METHODS

A palatable alternative lacking in Trp, Tyr and Phe has been identified in the whey protein fraction caseino-glycomacropeptide (c-GMP). The absence of these three aromatic amino acids renders GMP suitable as a template for seven formulations for separate and combined depletion or loading and a placebo control. The absence of Phe and Tyr enables GMP to provide a unique nutritional therapy of manic and psychotic disorders by inhibition of cerebral dopamine synthesis and release and possibly also by enhancing glutamatergic function, in general, and in patients resistant to anti-psychotic medication, in particular.

RESULTS

Seven GMP-based formulations for the above tests are proposed, two of which can be used in the above nutritional therapy and a third formulation as a placebo control in clinical trials.

CONCLUSIONS

Development of these formulations should advance the above research and diagnostic tests, open new avenues for neuroscience research on monoamine function, and improve the therapy of bipolar and psychotic disorders and enhance the quality of life of sufferers.

Presynaptic striatal dopamine dysfunction in people at ultra-high risk for psychosis: findings in a second cohort

BACKGROUND

Using positron emission tomography (PET), we previously observed increases in 3,4-dihydroxy-6-[(18)F]fluoro-L-phenylalanine ((18)F-DOPA) uptake in the striatum of subjects at ultra-high risk (UHR) for psychosis, indicating elevated presynaptic dopamine synthesis capacity. The purpose of this study was to test if this finding would be replicated in a second UHR cohort.

METHODS

(18)F-DOPA PET was used to estimate dopamine synthesis capacity in the striatum of an entirely new cohort of 26 individuals at UHR for psychosis (14 males, mean±SD age = 22.7±4.7 years) and 20 healthy volunteers matched for age and gender (11 males, mean±SD age = 24.5±4.5 years).

RESULTS

Dopamine synthesis capacity was elevated in the whole [t(44) = 2.6; p = .01, effect size = .81] and associative striatum [t(44) = 2.6; p = .01, effect size = .73] of UHR compared with control subjects. When the two samples were combined to give a final sample of 32 control and 50 UHR subjects, the higher levels of dopamine synthesis capacity in the UHR group reached significance across the whole [F(1,81) = 11.0; p = .001], associative [F(1,81) = 12.7; p = .001], and sensorimotor [F(1,81) = 4.7; p = .03], but not the limbic [F(1,81) = 2.1; p = .2], striatum.

CONCLUSIONS

The findings indicate that elevated dopamine synthesis capacity in the dorsal striatum is a robust feature of individuals at UHR for psychosis and provide further evidence that dopaminergic abnormalities precede the onset of psychosis.

Striatal presynaptic dopamine in schizophrenia, part II: meta-analysis of [(18)F/(11)C]-DOPA PET studies

BACKGROUND

Alterations in striatal dopamine neurotransmission are central to the emergence of psychotic symptoms and to the mechanism of action of antipsychotics. Although the functional integrity of the presynaptic system can be assessed by measuring striatal dopamine synthesis capacity (DSC), no quantitative meta-analysis is available.

METHODS

Eleven striatal (caudate and putamen) [(11)C/(18)F]-DOPA positron emission tomography studies comparing 113 patients with schizophrenia and 131 healthy controls were included in a quantitative meta-analysis of DSC. Demographic, clinical, and methodological variables were extracted from each study or obtained from the authors and tested as covariates. Hedges' g was used as a measure of effect size in Comprehensive Meta-Analysis. Publication bias was assessed with funnel plots and Egger's intercept. Heterogeneity was addressed with the Q statistic and I(2) index.

RESULTS

Patients and controls were well matched in sociodemographic variables (P > .05). Quantitative evaluation of publication bias was nonsignificant (P = .276). Heterogeneity across study was modest in magnitude and statistically nonsignificant (Q = 19.19; P = .078; I (2) = 39.17). Patients with schizophrenia showed increased striatal DSC as compared with controls (Hedges' g = 0.867, CI 95% from 0.594 to 1.140, Z = 6.222, P < .001). The DSC schizophrenia/control ratio showed a relatively homogenous elevation of around 14% in schizophrenic patients as compared with controls. DSC elevation was regionally confirmed in both caudate and putamen. Controlling for potential confounders such as age, illness duration, gender, psychotic symptoms, and exposure to antipsychotics had no impact on the results. Sensitivity analysis confirmed robustness of meta-analytic findings.

CONCLUSIONS

The present meta-analysis showed consistently increased striatal DSC in schizophrenia, with a 14% elevation in patients as compared with healthy controls.

The nature of dopamine dysfunction in schizophrenia and what this means for treatment

CONTEXT

Current drug treatments for schizophrenia are inadequate for many patients, and despite 5 decades of drug discovery, all of the treatments rely on the same mechanism: dopamine D(2) receptor blockade. Understanding the pathophysiology of the disorder is thus likely to be critical to the rational development of new treatments for schizophrenia.

OBJECTIVE

To investigate the nature of the dopaminergic dysfunction in schizophrenia using meta-analysis of in vivo studies.

DATA SOURCES

The MEDLINE, EMBASE, and PsycINFO databases were searched for studies from January 1, 1960, to July 1, 2011.

STUDY SELECTION

A total of 44 studies were identified that compared 618 patients with schizophrenia with 606 controls, using positron emission tomography or single-photon emission computed tomography to measure in vivo striatal dopaminergic function.

DATA EXTRACTION

Demographic, clinical, and imaging variables were extracted from each study, and effect sizes were determined for the measures of dopaminergic function. Studies were grouped into those of presynaptic function and those of dopamine transporter and receptor availability. Sensitivity analyses were conducted to explore the consistency of effects and the effect of clinical and imaging variables.

DATA SYNTHESIS

There was a highly significant elevation (P.<001) in presynaptic dopaminergic function in schizophrenia with a large effect size (Cohen d=0.79). There was no evidence of alterations in dopamine transporter availability. There was a small elevation in D(2/3) receptor availability (Cohen d=0.26), but this was not evident in drug-naive patients and was influenced by the imaging approach used.

CONCLUSIONS

The locus of the largest dopaminergic abnormality in schizophrenia is presynaptic, which affects dopamine synthesis capacity, baseline synaptic dopamine levels, and dopamine release. Current drug treatments, which primarily act at D(2/3) receptors, fail to target these abnormalities. Future drug development should focus on the control of presynaptic dopamine synthesis and release capacity.

Role of dopamine D(2) receptors for antipsychotic activity

This review summarizes the current state of knowledge regarding the proposed mechanisms by which antipsychotic agents reduce the symptoms of schizophrenia while giving rise to adverse side effects. The first part summarizes the contribution of neuroimaging studies to our understanding of the neurochemical substrates of schizophrenia, putting emphasis on direct evidence suggestive of a presynaptic rather than a postsynaptic dysregulation of dopaminergic neurotransmission in this disorder. The second part addresses the role of D(2) and non-D(2) receptor blockade in the treatment of schizophrenia and highlights a preponderant role of D(2) receptors in the mechanism of antipsychotic action. Neuroimaging studies have defined a narrow, but optimal, therapeutic window of 65-78 % D(2) receptor blockade within which most antipsychotics achieve optimal clinical efficacy with minimal side effects. Some antipsychotics though do not conform to that therapeutic window, notably clozapine. The reasons for its unexcelled clinical efficacy despite subthreshold levels of D(2) blockade are unclear and current theories on clozapine's mechanisms of action are discussed, including transiency of its D(2) receptor blocking effects or preferential blockade of limbic D(2) receptors. Evidence is also highlighted to consider the use of extended antipsychotic dosing to achieve transiency of D(2) blockade as a way to optimize functional outcomes in patients. We also present some critical clinical considerations regarding the mechanisms linking dopamine disturbance to the expression of psychosis and its blockade to the progressive resolution of psychosis, keeping in perspective the speed and onset of antipsychotic action. Finally, we discuss potential novel therapeutic strategies for schizophrenia.

The dopamine dysfunction in schizophrenia revisited: new insights into topography and course

Schizophrenia has long been associated with an imbalance in dopamine (DA) neurotransmission, and brain imaging has played an important role in advancing our knowledge and providing evidence for the dopaminergic abnormalities. This chapter reviews the evidence for DA dysfunction in different brain regions in schizophrenia, in particular striatal, extrastriatal, and prefrontal regions, with emphasis on recently published findings. As opposed to the traditional view that most striatal dopaminergic excess, associated with the positive symptoms of schizophrenia, involves the dopaminergic mesolimbic pathway, recent evidence points to the nigrostriatal pathway as the area of highest dysregulation. Furthermore, evidence from translational research suggests that dopaminergic excess may be present in the prodromal phase, and may by itself, as suggested by the phenotype observed in transgenic mice with developmental overexpression of dorso-striatal D(2) receptors, be an early pathogenic condition, leading to irreversible cortical dysfunction.

Imaging as tool to investigate psychoses and antipsychotics

The results of imaging studies have played an important role in the formulation of hypotheses regarding the etiology of psychosis and schizophrenia, as well as in our understanding of the mechanisms of action of antipsychotics. Since this volume is primarily directed to molecular aspects of psychosis and antipsychotics, only the results of molecular imaging techniques addressing these topics will be discussed here.One of the most consistent findings of molecular imaging studies in schizophrenia is an increased uptake of DOPA in the striatum, which may be interpreted as an increased synthesis of L-DOPA. Also, several studies reported an increased release of dopamine induced by amphetamine in schizophrenia patients. These findings played an important role in reformulating the dopamine hypothesis of schizophrenia. To study the roles of the neurotransmitters γ-aminobutyric acid (GABA) and glutamate in schizophrenia, SPECT as well as MR spectroscopy have been used. The results of preliminary SPECT studies are consistent with the hypothesis of NMDA receptor dysfunction in schizophrenia. Regarding the GABA deficit hypothesis of schizophrenia, imaging results are inconsistent. No changes in serotonin transporters were demonstrated in imaging studies in schizophrenia, but studies of several serotonin receptors showed conflicting results. The lack of selective radiotracers for muscarinic receptors may have hampered examination of this system in schizophrenia as well as its role in the induction of side effects of antipsychotics. Interestingly, preliminary molecular imaging studies on the cannabinoid-1 receptor and on neuroinflammatory processes in schizophrenia have recently been published. Finally, a substantial number of PET/SPECT studies have examined the occupancy of receptors by antipsychotics and an increasing number of studies is now focusing on the effects of these drugs using techniques like spectroscopy and pharmacological MRI.

Striatal dopamine synthesis capacity in twins discordant for schizophrenia

BACKGROUND

Elevated striatal dopamine synthesis capacity is thought to be fundamental to the pathophysiology of schizophrenia and has also been reported in people at risk of psychosis. It is therefore unclear if striatal hyperdopaminergia is a vulnerability marker for schizophrenia, or a state feature related to the psychosis itself. Relatives of patients with schizophrenia are themselves at increased risk of developing the condition. In this study we examined striatal dopamine synthesis capacity in both members of twin pairs discordant for schizophrenia.

METHOD

In vivo striatal dopamine synthesis capacity was examined using fluorine-18-l-dihydroxyphenylalanine (18F-DOPA) positron emission tomography (PET) scans in seven twin pairs discordant for schizophrenia and in a control sample of 10 healthy control twin pairs.

RESULTS

Striatal 18F-DOPA uptake was not elevated in the unaffected co-twins of patients with schizophrenia (p=0.65) or indeed in the twins with schizophrenia (p=0.89) compared to the control group. Levels of psychotic symptoms were low in the patients with schizophrenia who were in general stable [mean (s.d.) Positive and Negative Syndrome Scale (PANSS) total=56.8 (25.5)] whereas the unaffected co-twins were largely asymptomatic.

CONCLUSIONS

Striatal dopamine synthesis capacity is not elevated in symptom-free individuals at genetic risk of schizophrenia, or in well-treated stable patients with chronic schizophrenia. These findings suggest that striatal hyperdopaminergia is not a vulnerability marker for schizophrenia.

Presynaptic regulation of dopamine transmission in schizophrenia

A role for dopamine (DA) release in the hallucinations and other positive symptoms associated with schizophrenia has long been inferred from the antipsychotic response to D2 DA receptor antagonists and because the DA releaser amphetamine can be psychotogenic. Recent studies suggest that patients with schizophrenia, including those never exposed to antipsychotic drugs, maintain high presynaptic DA accumulation in the striatum. New laboratory approaches are elucidating mechanisms that control the level of presynaptic DA stores, thus contributing to fundamental understanding of the basic pathophysiologic mechanism in schizophrenia.

Dysfunctional brain networks and genetic risk for schizophrenia: specific neurotransmitter systems

Multiple neurotransmitter circuits are disturbed in schizophrenia, and the dopamine hypothesis of schizophrenia prevails as the hypothesis with most empirical support. On the other hand, schizophrenia is highly heritable with a pattern consistent with both common and rare allelic variants and gene × environment interaction. Advances in the field of neuroimaging have expanded our knowledge of intermediate phenotypes, the neurobiological processes that convey the risk from the genes to the complex phenotype. In this article, we review the recent and continuously accumulating evidence from in vivo imaging studies aiming at characterizing neurochemical intermediate phenotypes of schizophrenia. Dopaminergic alterations in schizophrenia are shared by individuals at genetic risk who do not express the illness, suggesting a "dopamine hypothesis of schizophrenia vulnerability." This hypothesis has the potential to help us better understand the dopaminergic dysfunction in the context of the complex pathophysiological process leading to schizophrenia. In the future, neurotransmitter imaging studies should investigate the gene × environment interaction in schizophrenia, and try to identify neurobiological correlates of heightened sensitivity to environmental stressors (e.g., cannabis, childhood trauma, and other psychosocial stress) in genetically vulnerable individuals.

Motor behavior abnormalities in drug-naïve patients with schizophrenia spectrum disorders

Prevalence and correlates of primary motor abnormalities in schizophrenia are presently ill defined. This study was aimed at examining the prevalence, syndromic structure, external correlates, and response to antipsychotic medication of a broad array of primary motor abnormalities. Two-hundred antipsychotic-naive patients with schizophrenia spectrum disorders were examined for motor abnormalities using the Modified Rogers Scale. Thirty-one motor signs were subjected to factor analysis, and the resulting factors examined for association with a number of risk factors, clinical and psychopathological variables. One-hundred and eighty-nine patients were reassessed for motor abnormalities after completing a 4-week trial with antipsychotic medication. Prevalence rates for at least one motor sign and syndrome at baseline were 66% and 40%, respectively. Motor signs clustered together into seven clinically interpretable factors: abnormal involuntary movements, hypokinesia, retarded catatonia, echo-phenomena, excited catatonia, catalepsy, and parkinsonism. All motor domains but parkinsonism were inter-related. Abnormal involuntary movements were associated with variables indicating both neurodevelopmental dysfunction and illness severity, and most motor domains were closely related to negative or disorganization symptoms. Change scores in motor domains after treatment with antipsychotic medication indicated improvement for abnormal involuntary movements, hypokinesia, retarded catatonia, excited catatonia and echophenomena, and worsening for parkinsonism. It is concluded that primary motor dysfunction is a prevalent and heterogeneous condition of schizophrenia. Motor abnormalities segregate into various syndromes, which have different clinical correlates and a differential response pattern to antipsychotic medication. It is hypothesized that the existence of a differential dopaminergic dysfunction in the nigroestriatal circuitry is responsible for the generation of those motor domains that improve and worsen with antipsychotic drugs.

Positron emission tomography in schizophrenia: a new perspective

PET is an important functional imaging technique that can be used to investigate neurotransmitter receptors and transporters directly by mapping human brain function. PET is increasingly being used greatly to advance our understanding of the neurobiology and pathophysiology of schizophrenia.

METHODS

This review focuses on the use of PET tracers and kinetic modeling in identifying regional brain abnormalities and regions associated with cognitive functioning in schizophrenia. A variety of PET tracers have been used to identify brain abnormalities, including (11)C, (15)O-water, (18)F-fallypride, and L-3,4-dihydroxy-6-(18)F-fluorophenylalanine ((18)F-FDOPA).

RESULTS

Some studies have used compartmental modeling to determine tracer binding kinetics. The most consistent findings show a difference in the dopamine content in the prefrontal cortex, anterior cingulate gyrus, and hippocampus between healthy controls and patients with schizophrenia. Studies also show a higher density of D(2) receptors in the striatum and neural brain dysconnectivity.

CONCLUSION

Future investigations integrating clinical, imaging, genetic, and cognitive aspects are warranted to gain a better understanding of the pathophysiology of this disorder.

Executive function, neural circuitry, and genetic mechanisms in schizophrenia

After decades of research aimed at elucidating the pathophysiology and etiology of schizophrenia, it has become increasingly apparent that it is an illness knowing few boundaries. Psychopathological manifestations extend across several domains, impacting multiple facets of real-world functioning for the affected individual. Even within one such domain, arguably the most enduring, difficult to treat, and devastating to long-term functioning-executive impairment-there are not only a host of disrupted component processes, but also a complex underlying dysfunctional neural architecture. Further, just as implicated brain structures (eg, dorsolateral prefrontal cortex) through postmortem and neuroimaging techniques continue to show alterations in multiple, interacting signaling pathways, so too does evolving understanding of genetic risk factors suggest multiple molecular entry points to illness liability. With this expansive network of interactions in mind, the present chapter takes a systems-level approach to executive dysfunction in schizophrenia, by identifying key regions both within and outside of the frontal lobes that show changes in schizophrenia and are important in cognitive control neural circuitry, summarizing current knowledge of their relevant functional interactions, and reviewing emerging links between schizophrenia risk genetics and characteristic executive circuit aberrancies observed with neuroimaging methods.

Neurochemical imaging in schizophrenia

Recent advances in the development and applications of neurochemical brain imaging methods have improved the ability to study the neurochemistry of the living brain in normal processes as well as psychiatric disorders. In particular, positron emission tomography (PET) and single photon emission computed tomography (SPECT) have been used to determine neurochemical substrates of schizophrenia and to uncover the mechanism of action of antipsychotic medications. The growing availability of radiotracers for monoaminergic neurotransmitter synthesis, transporters and receptors, has enabled the evaluation of hypotheses regarding neurotransmitter function in schizophrenia derived from preclinical and clinical observations. This chapter reviews the studies using neurochemical brain imaging methods for (1) detection of abnormalities in indices of dopamine and serotonin transmission in patients with schizophrenia compared to controls, (2) development of new tools to study other neurotransmitters systems, such as gamma-aminobutyric acid (GABA) and glutamate, and (3) characterization of target occupancy by antipsychotic drugs, as well as its relationship to efficacy and side effects. As more imaging tools become available, this knowledge will expand and will lead to better detection of disease, as well as better therapeutic approaches.

How have developments in molecular imaging techniques furthered schizophrenia research?

Molecular imaging techniques have led to significant advances in understanding the pathophysiology of schizophrenia and contributed to knowledge regarding potential mechanisms of action of the drugs used to treat this illness. The aim of this article is to provide a review of the major findings related to the application of molecular imaging techniques that have furthered schizophrenia research. This article focuses specifically on neuroreceptor imaging studies with PET and SPECT. After providing a brief overview of neuroreceptor imaging methodology, we consider relevant findings from studies of receptor availability, and dopamine synthesis and release. Results are discussed in the context of current hypotheses regarding neurochemical alterations in the illness. We then selectively review pharmacological occupancy studies and the role of neuroreceptor imaging in drug development for schizophrenia.