Striatal and extrastriatal dopamine D2/D3 receptors in schizophrenia evaluated with [18F]fallypride positron emission tomography

Inhibition of striatal indirect pathway during second postnatal week leads to long-lasting deficits in motivated behavior

Schizophrenia is a neuropsychiatric disorder with postulated neurodevelopmental etiology. Genetic and imaging studies have shown enhanced dopamine and D2 receptor occupancy in the striatum of patients with schizophrenia. However, whether alterations in postnatal striatal dopamine can lead to long-lasting changes in brain function and behavior is still unclear. Here, we approximated striatal D2R hyperfunction in mice via designer receptor-mediated activation of inhibitory Gi-protein signaling during a defined postnatal time window. We found that Gi-mediated inhibition of the indirect pathway (IP) during postnatal days 8-15 led to long-lasting decreases in locomotor activity and motivated behavior measured in the adult animal. In vivo photometry further showed that the motivational deficit was associated with an attenuated adaptation of outcome-evoked dopamine levels to changes in effort requirements. These data establish a sensitive time window of D2R-regulated striatal development with long-lasting impacts on neuronal function and behavior.

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.

A neural modeling approach to study mechanisms underlying the heterogeneity of visual spatial frequency sensitivity in schizophrenia

Patients with schizophrenia exhibit abnormalities in spatial frequency sensitivity, and it is believed that these abnormalities indicate more widespread dysfunction and dysregulation of bottom-up processing. The early visual system, including the first-order Lateral Geniculate Nucleus of the thalamus (LGN) and the primary visual cortex (V1), are key contributors to spatial frequency sensitivity. Medicated and unmedicated patients with schizophrenia exhibit contrasting changes in spatial frequency sensitivity, thus making it a useful probe for examining potential effects of the disorder and antipsychotic medications in neural processing. We constructed a parameterized, rate-based neural model of on-center/off-surround neurons in the early visual system to investigate the impacts of changes to the excitatory and inhibitory receptive field subfields. By incorporating changes in both the excitatory and inhibitory subfields that are associated with pathophysiological findings in schizophrenia, the model successfully replicated perceptual data from behavioral/functional studies involving medicated and unmedicated patients. Among several plausible mechanisms, our results highlight the dampening of excitation and/or increase in the spread and strength of the inhibitory subfield in medicated patients and the contrasting decreased spread and strength of inhibition in unmedicated patients. Given that the model was successful at replicating results from perceptual data under a variety of conditions, these elements of the receptive field may be useful markers for the imbalances seen in patients with schizophrenia.

Medial Prefrontal Cortex Dysfunction Mediates Working Memory Deficits in Patients With Schizophrenia

Background

Schizophrenia (SCZ) is marked by working memory (WM) deficits, which predict poor functional outcome. While most functional magnetic resonance imaging studies of WM in SCZ have focused on the dorsolateral prefrontal cortex (PFC), some recent work suggests that the medial PFC (mPFC) may play a role. We investigated whether task-evoked mPFC deactivation is associated with WM performance and whether it mediates deficits in SCZ. In addition, we investigated associations between mPFC deactivation and cortical dopamine release.

Methods

Patients with SCZ (n = 41) and healthy control participants (HCs) (n = 40) performed a visual object n-back task during functional magnetic resonance imaging. Dopamine release capacity in mPFC was quantified with [11C]FLB457 in a subset of participants (9 SCZ, 14 HCs) using an amphetamine challenge. Correlations between task-evoked deactivation and performance were assessed in mPFC and dorsolateral PFC masks and were further examined for relationships with diagnosis and dopamine release.

Results

mPFC deactivation was associated with WM task performance, but dorsolateral PFC activation was not. Deactivation in the mPFC was reduced in patients with SCZ relative to HCs and mediated the relationship between diagnosis and WM performance. In addition, mPFC deactivation was significantly and inversely associated with dopamine release capacity across groups and in HCs alone, but not in patients.

Conclusions

Reduced WM task-evoked mPFC deactivation is a mediator of, and potential substrate for, WM impairment in SCZ, although our study design does not rule out the possibility that these findings could relate to cognition in general rather than WM specifically. We further present preliminary evidence of an inverse association between deactivation during WM tasks and dopamine release capacity in the mPFC.

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.

[18F]F13640: a selective agonist PET radiopharmaceutical for imaging functional 5-HT1A receptors in humans

PURPOSE

F13640 (a.k.a. befiradol, NLX-112) is a highly selective 5-HT_1A receptor ligand that was selected as a PET radiopharmaceutical-candidate based on animal studies. Due to its high efficacy agonist properties, [¹⁸F]F13640 binds preferentially to functional 5-HT_1A receptors, which are coupled to intracellular G-proteins. Here, we characterize brain labeling of 5-HT_1A receptors by [¹⁸F]F13640 in humans and describe a simplified model for its quantification.

METHODS

PET/CT and PET-MRI scans were conducted in a total of 13 healthy male volunteers (29 ± 9 years old), with arterial input functions (AIF) (n = 9) and test-retest protocol (n = 8). Several kinetic models were compared (one tissue compartment model, two-tissue compartment model, and Logan); two models with reference region were also evaluated: simplified reference tissue model (SRTM) and the logan reference model (LREF).

RESULTS

[¹⁸F]F13640 showed high uptake values in raphe nuclei and cortical regions. SRTM and LREF models showed a very high correlation with kinetic models using AIF. As concerns test-retest parameters and the prolonged binding kinetics of [¹⁸F]F13640, better reproducibility, and reliability were found with the LREF method. Cerebellum white matter and frontal lobe white matter stand out as suitable reference regions.

CONCLUSION

The favorable brain labeling and kinetic profile of [¹⁸F]F13640, its high receptor specificity and its high efficacy agonist properties open new perspectives for studying functionally active 5-HT_1A receptors, unlike previous radiopharmaceuticals that act as antagonists. [¹⁸F]F13640's kinetic properties allow injection outside of the PET scanner with delayed acquisitions, facilitating the design of innovative longitudinal protocols in neurology and psychiatry.

TRIAL REGISTRATION

Trial Registration EudraCT 2017-002,722-21.

Thalamic dopamine D2-receptor availability in schizophrenia: a study on antipsychotic-naive patients with first-episode psychosis and a meta-analysis

Pharmacological and genetic evidence support a role for an involvement of the dopamine D2-receptor (D2-R) in the pathophysiology of schizophrenia. Previous molecular imaging studies have suggested lower levels of D2-R in thalamus, but results are inconclusive. The objective of the present study was to use improved methodology to compare D2-R density in whole thalamus and thalamic subregions between first-episode psychosis patients and healthy controls. Differences in thalamocortical connectivity was explored based on the D2-R results. 19 antipsychotic-naive first-episode psychosis patients and 19 age- and sex-matched healthy controls were examined using high-resolution Positron Emission Tomography (PET) and the high-affinity D2-R radioligand [¹¹C]FLB457. The main outcome was D2-R binding potential (BP_ND) in thalamus, and it was predicted that patients would have lower binding. Diffusion tensor imaging (DTI) was performed in a subgroup of 11 patients and 15 controls. D2-R binding in whole thalamus was lower in patients compared with controls (Cohen's dz = -0.479, p = 0.026, Bayes Factor (BF) > 4). Among subregions, lower BP_ND was observed in the ROI representing thalamic connectivity to the frontal cortex (Cohen's dz = -0.527, p = 0.017, BF > 6). A meta-analysis, including the sample of this study, confirmed significantly lower thalamic D2-R availability in patients. Exploratory analyses suggested that patients had lower fractional anisotropy values compared with controls (Cohen's d = -0.692, p = 0.036) in the inferior thalamic radiation. The findings support the hypothesis of a dysregulation of thalamic dopaminergic neurotransmission in schizophrenia, and it is hypothesized that this could underlie a disturbance of thalamocortical connectivity.

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.

How changes in dopamine D2 receptor levels alter striatal circuit function and motivation

It was first posited, more than five decades ago, that the etiology of schizophrenia involves overstimulation of dopamine receptors. Since then, advanced clinical research methods, including brain imaging, have refined our understanding of the relationship between striatal dopamine and clinical phenotypes as well as disease trajectory. These studies point to striatal dopamine D2 receptors, the main target for all current antipsychotic medications, as being involved in both positive and negative symptoms. Simultaneously, animal models have been central to investigating causal relationships between striatal dopamine D2 receptors and behavioral phenotypes relevant to schizophrenia. We begin this article by reviewing the circuit, cell-type and subcellular locations of dopamine D2 receptors and their downstream signaling pathways. We then summarize results from several mouse models in which D2 receptor levels were altered in various brain regions, cell-types and developmental periods. Behavioral, electrophysiological and anatomical consequences of these D2 receptor perturbations are reviewed with a selective focus on striatal circuit function and alterations in motivated behavior, a core negative symptom of schizophrenia. These studies show that D2 receptors serve distinct physiological roles in different cell types and at different developmental time points, regulating motivated behaviors in sometimes opposing ways. We conclude by considering the clinical implications of this complex regulation of striatal circuit function by D2 receptors.

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 receptor density and white mater integrity: 18F-fallypride positron emission tomography and diffusion tensor imaging study in healthy and schizophrenia subjects

Dopaminergic dysfunction and changes in white matter integrity are among the most replicated findings in schizophrenia. A modulating role of dopamine in myelin formation has been proposed in animal models and healthy human brain, but has not yet been systematically explored in schizophrenia. We used diffusion tensor imaging and ¹⁸F-fallypride positron emission tomography in 19 healthy and 25 schizophrenia subjects to assess the relationship between gray matter dopamine D₂/D₃ receptor density and white matter fractional anisotropy in each diagnostic group. AFNI regions of interest were acquired for 42 cortical Brodmann areas and subcortical gray matter structures as well as stereotaxically placed in representative white matter areas implicated in schizophrenia neuroimaging literature. Welch's t-test with permutation-based p value adjustment was used to compare means of z-transformed correlations between fractional anisotropy and ¹⁸F-fallypride binding potentials in hypothesis-driven regions of interest in the diagnostic groups. Healthy subjects displayed an extensive pattern of predominantly negative correlations between ¹⁸F-fallypride binding across a range of cortical and subcortical gray matter regions and fractional anisotropy in rostral white matter regions (internal capsule, frontal lobe, anterior corpus callosum). These patterns were disrupted in subjects with schizophrenia, who displayed significantly weaker overall correlations as well as comparatively scant numbers of significant correlations with the internal capsule and frontal (but not temporal) white matter, especially for dopamine receptor density in thalamic nuclei. Dopamine D₂/D₃ receptor density and white matter integrity appear to be interrelated, and their decreases in schizophrenia may stem from hyperdopaminergia with dysregulation of dopaminergic impact on axonal myelination.

Dopamine and Neuroinflammation in Schizophrenia - Interpreting the Findings from Translocator Protein (18kDa) PET Imaging

Schizophrenia is a complex disease whose pathophysiology is not yet fully understood. In addition to the long prevailing dopaminergic hypothesis, the evidence suggests that neuroinflammation plays a role in the pathophysiology of the disease. Recent studies using positron emission tomography (PET) that target a 18kDa translocator protein (TSPO) in activated microglial cells in an attempt to measure neuroinflammation in patients have shown a decrease or a lack of an increase in TSPO binding. Many biological and methodological considerations have been formulated to explain these findings. Although dopamine has been described as an immunomodulatory molecule, its potential role in neuroinflammation has not been explored in the aforementioned studies. In this review, we discuss the interactions between dopamine and neuroinflammation in psychotic states. Dopamine may inhibit neuroinflammation in activated microglia. Proinflammatory molecules released from microglia may decrease dopaminergic transmission. This could potentially explain why the levels of neuroinflammation in the brain of patients with schizophrenia seem to be unchanged or decreased compared to those in healthy subjects. However, most data are indirect and are derived from animal studies or from studies performed outside the field of schizophrenia. Further studies are needed to combine TSPO and dopamine imaging to study the association between microglial activation and dopamine system function.

Dopamine transporter neuroimaging accurately assesses the maturation of dopamine neurons in a preclinical model of Parkinson's disease

BACKGROUND

Significant developments in stem cell therapy for Parkinson's disease (PD) have already been achieved; however, methods for reliable assessment of dopamine neuron maturation in vivo are lacking. Establishing the efficacy of new cellular therapies using non-invasive methodologies will be critical for future regulatory approval and application. The current study examines the utility of neuroimaging to characterise the in vivo maturation, innervation and functional dopamine release of transplanted human embryonic stem cell-derived midbrain dopaminergic neurons (hESC-mDAs) in a preclinical model of PD.

METHODS

Female NIH RNu rats received a unilateral stereotaxic injection of 6-OHDA into the left medial forebrain bundle to create the PD lesion. hESC-mDA cell and sham transplantations were carried out 1 month post-lesion, with treated animals receiving approximately 4 × 10⁵ cells per transplantation. Behavioural analysis, [¹⁸F]FBCTT and [¹⁸F]fallypride microPET/CT, was conducted at 1, 3 and 6 months post-transplantation and compared with histological characterisation at 6 months.

RESULTS

PET imaging revealed transplant survival and maturation into functional dopaminergic neurons. [¹⁸F]FBCTT-PET/CT dopamine transporter (DAT) imaging demonstrated pre-synaptic restoration and [¹⁸F]fallypride-PET/CT indicated functional dopamine release, whilst amphetamine-induced rotation showed significant behavioural recovery. Moreover, histology revealed that the grafted cells matured differently in vivo producing high- and low-tyrosine hydroxylase (TH) expressing cohorts, and only [¹⁸F]FBCTT uptake was well correlated with differentiation.

CONCLUSIONS

This study provides further evidence for the value of in vivo functional imaging for the assessment of cell therapies and highlights the utility of DAT imaging for the determination of early post-transplant cell maturation and differentiation of hESC-mDAs.

Positive association between cerebral grey matter metabolism and dopamine D2/D3 receptor availability in healthy and schizophrenia subjects: An 18F-fluorodeoxyglucose and 18F-fallypride positron emission tomography study

Objectives: Overlapping decreases in extrastriatal dopamine D₂/D₃-receptor availability and glucose metabolism have been reported in subjects with schizophrenia. It remains unknown whether these findings are physiologically related or coincidental.Methods: To ascertain this, we used two consecutive ¹⁸F-fluorodeoxyglucose and ¹⁸F-fallypride positron emission tomography scans in 19 healthy and 25 unmedicated schizophrenia subjects. Matrices of correlations between ¹⁸F-fluorodeoxyglucose uptake and ¹⁸F-fallypride binding in voxels at the same xyz location and AFNI-generated regions of interest were evaluated in both diagnostic groups.Results:¹⁸F-fluorodeoxyglucose uptake and ¹⁸F-fallypride binding potential were predominantly positively correlated across the striatal and extrastriatal grey matter in both healthy and schizophrenia subjects. In comparison to healthy subjects, significantly weaker correlations in subjects with schizophrenia were confirmed in the right cingulate gyrus and thalamus, including the mediodorsal, lateral dorsal, anterior, and midline nuclei. Schizophrenia subjects showed decreased D₂/D₃-receptor availability in the hypothalamus, mamillary bodies, thalamus and several thalamic nuclei, and increased glucose uptake in three lobules of the cerebellar vermis.Conclusions: Dopaminergic system may be involved in modulation of grey matter metabolism and neurometabolic coupling in both healthy human brain and psychopathology. Hyperdopaminergic state in untreated schizophrenia may at least partly account for the corresponding decreases in grey matter metabolism.

Plasma monoamines change under dopamine supersensitivity psychosis in patients with schizophrenia: A comparison with first-episode psychosis

BACKGROUND

Patients with first-episode psychosis respond well to initial antipsychotic treatment, but among patients experiencing a relapse of psychosis, the response rate falls to approximately 30%. The mechanism of this discrepancy has not been clarified, but the development of dopamine supersensitivity psychosis with the underlying up-regulation of post-synaptic dopamine D2 receptors could be involved in this lesser response. It is uncertain whether elevated dopamine synthesis and release occurs in patients with dopamine supersensitivity psychosis, in contrast to those with first-episode psychosis.

PATIENTS AND METHODS

We examined a first-episode psychosis group (n=6) and a chronic schizophrenia group, i.e. patients experiencing relapse (n=23) including those who relapsed due to dopamine supersensitivity psychosis (n=18). Following the initiation of treatment, we measured the patients' blood concentrations of homovanillic acid and 3-methoxy-4-hydroxyphenylglycol at two weeks and four weeks after the baseline measurements.

RESULTS

The first-episode psychosis group tended to show decreased homovanillic acid, accompanied by an improvement of symptoms. The chronic schizophrenia group showed no alteration of homovanillic acid or 3-methoxy-4-hydroxyphenylglycol over the treatment period. These results were the same in the dopamine supersensitivity psychosis patients alone.

CONCLUSIONS

Our findings suggest that unlike first-episode psychosis, the release of dopamine from presynaptic neurons did not increase in relapse episodes in the patients with dopamine supersensitivity psychosis. This indirectly indicates that the development of supersensitivity of post-synapse dopamine D2 receptor is involved in relapse in dopamine supersensitivity psychosis patients.

The relationship between excitement symptom severity and extrastriatal dopamine D2/3 receptor availability in patients with schizophrenia: a high-resolution PET study with [18F]fallypride

The purpose of this study was to investigate the relationship between specific symptom severity and D_2/3 receptor availability in extrastriatal regions in outpatients with schizophrenia to shed light on the role of extrastriatal dopaminergic neurotransmission in the pathophysiology of symptoms of schizophrenia. Sixteen schizophrenia patients receiving relatively low-dose maintenance atypical antipsychotics and seventeen healthy controls underwent 3-Tesla magnetic resonance imaging and high-resolution positron emission tomography with [¹⁸F]fallypride. For D_2/3 receptor availability, the binding potential with respect to non-displaceable compartment (BP_ND) was derived using the simplified reference tissue model. The BP_ND values were lower in patients on antipsychotic treatment than in controls across all regions with large effect sizes (1.03-1.42). The regions with the largest effect size were the substantia nigra, amygdala, and insula. Symptoms of schizophrenia were assessed using a five-factor model of the Positive and Negative Syndrome Scale (PANSS). The region of interest-based analysis showed that PANSS excitement factor score had a significant positive correlation with the [¹⁸F]fallypride BP_ND in the insula. The equivalent dose of antipsychotics was not significantly correlated with PANSS factor scores or regional BP_ND values. The voxel-based analysis also revealed a significant positive association between the PANSS excitement factor and the [¹⁸F]fallypride BP_ND in the insula. The present study revealed a significant association between excitement symptom severity and D_2/3 receptor availability in the insula in schizophrenia, suggesting a possible important role of D_2/3 receptor-mediated neurotransmission in the insula and related limbic system in the pathophysiology of this specific symptom cluster.

α7 Nicotinic receptor-modulating agents reverse the hyperdopaminergic tone in the MAM model of schizophrenia

Recent evidence has emerged supporting a role for the cholinergic system in schizophrenia, including the potential of α7 modulators as a treatment strategy. However, preclinical studies to date have relied on studies in normal systems rather than on a validated developmental model of schizophrenia. Furthermore, there have been only few studies on whether orthosteric and allosteric modulators have differential impacts in such models. Thus, we investigated the effects of α7 agonists and positive allosteric modulators (PAMs) on dopamine (DA) neuron activity in the ventral tegmental area (VTA) in the methylazoxymethanol acetate (MAM) developmental disruption model of schizophrenia. Four different drugs were evaluated: PNU282987 (full agonist), SSR180711 (partial agonist) NS1738 (PAM type I) and PNU120596 (PAM type II). PNU120596 increased the number of spontaneously active VTA DA neurons in normal rats. In contrast, PNU282987 and SSR180711 reduced the hyperdopaminergic tone in MAM rats. This appeared to be due to effects on DA afferent regulation, in that PNU282987 or SSR180711 infusion into the ventral hippocampus of MAM rats replicated the decrease in the number of spontaneously active VTA DA neurons. In contrast, infusion of the same drugs into the basolateral amygdala increased the number of spontaneously active VTA DA neurons in normal rats without impacting MAM rats. These data suggest that α7 receptors may represent a promising target in the development of new pharmacological therapies for schizophrenia.

Selectivity of probes for PET imaging of dopamine D3 receptors

Dopamine D3 receptors have key roles in behavioral reward, addiction, Parkinson's disease, and schizophrenia, and there is interest in studying their role in these disorders using PET. However, current PET radiotracers for studying D3 receptors in humans all bind to both D2 and D3 due to similarities between the two receptors. Selective D2 and D3 radioligands would aid investigation of the differences between D2 and D3 circuitry in the central nervous system. While there are currently in vitro measures of ligand D3/D2 selectivity, there is a need for an in vivo PET measure of D3/D2 selectivity. This review discusses current PET imaging of dopamine D2/D3 receptors and proposes methodology for quantitating in vivo selectivity of probes for PET imaging of dopamine D3 receptors.

Dopamine, psychosis and schizophrenia: the widening gap between basic and clinical neuroscience

The stagnation in drug development for schizophrenia highlights the need for better translation between basic and clinical research. Understanding the neurobiology of schizophrenia presents substantial challenges but a key feature continues to be the involvement of subcortical dopaminergic dysfunction in those with psychotic symptoms. Our contemporary knowledge regarding dopamine dysfunction has clarified where and when dopaminergic alterations may present in schizophrenia. For example, clinical studies have shown patients with schizophrenia show increased presynaptic dopamine function in the associative striatum, rather than the limbic striatum as previously presumed. Furthermore, subjects deemed at high risk of developing schizophrenia show similar presynaptic dopamine abnormalities in the associative striatum. Thus, our view of subcortical dopamine function in schizophrenia continues to evolve as we accommodate this newly acquired information. However, basic research in animal models has been slow to incorporate these clinical findings. For example, psychostimulant-induced locomotion, the commonly utilised phenotype for positive symptoms in rodents, is heavily associated with dopaminergic activation in the limbic striatum. This anatomical misalignment has brought into question how we assess positive symptoms in animal models and represents an opportunity for improved translation between basic and clinical research. The current review focuses on the role of subcortical dopamine dysfunction in psychosis and schizophrenia. We present and discuss alternative phenotypes that may provide a more translational approach to assess the neurobiology of positive symptoms in schizophrenia. Incorporation of recent clinical findings is essential if we are to develop meaningful translational animal models.

Abdominal Vagal Afferents Modulate the Brain Transcriptome and Behaviors Relevant to Schizophrenia

Reduced activity of vagal efferents has long been implicated in schizophrenia and appears to be responsible for diminished parasympathetic activity and associated peripheral symptoms such as low heart rate variability and cardiovascular complications in affected individuals. In contrast, only little attention has been paid to the possibility that impaired afferent vagal signaling may be relevant for the disorder's pathophysiology as well. The present study explored this hypothesis using a model of subdiaphragmatic vagal deafferentation (SDA) in male rats. SDA represents the most complete and selective vagal deafferentation method existing to date as it leads to complete disconnection of all abdominal vagal afferents while sparing half of the abdominal vagal efferents. Using next-generation mRNA sequencing, we show that SDA leads to brain transcriptional changes in functional networks annotating with schizophrenia. We further demonstrate that SDA induces a hyperdopaminergic state, which manifests itself as increased sensitivity to acute amphetamine treatment and elevated accumbal levels of dopamine and its major metabolite, 3,4-dihydroxyphenylacetic acid. Our study also shows that SDA impairs sensorimotor gating and the attentional control of associative learning, which were assessed using the paradigms of prepulse inhibition and latent inhibition, respectively. These data provide converging evidence suggesting that the brain transcriptome, dopamine neurochemistry, and behavioral functions implicated in schizophrenia are subject to visceral modulation through abdominal vagal afferents. Our findings may encourage the further establishment and use of therapies for schizophrenia that are based on vagal interventions.SIGNIFICANCE STATEMENT The present work provides a better understanding of how disrupted vagal afferent signaling can contribute to schizophrenia-related brain and behavioral abnormalities. More specifically, it shows that subdiaphragmatic vagal deafferentation (SDA) in rats leads to (1) brain transcriptional changes in functional networks related to schizophrenia, (2) increased sensitivity to dopamine-stimulating drugs and elevated dopamine levels in the nucleus accumbens, and (3) impairments in sensorimotor gating and the attentional control of associative learning. These findings may encourage the further establishment of novel therapies for schizophrenia that are based on vagal interventions.

Cognitive function in schizophrenia: conflicting findings and future directions

INTRODUCTION

Schizophrenia is a severe mental disorder with multiple psychopathological domains being affected. Several lines of evidence indicate that cognitive impairment serves as the key component of schizophrenia psychopathology. Although there have been a multitude of cognitive studies in schizophrenia, there are many conflicting results. We reasoned that this could be due to individual differences among the patients (i.e. variation in the severity of positive vs. negative symptoms), different task designs, and/or the administration of different antipsychotics.

METHODS

We thus review existing data concentrating on these dimensions, specifically in relation to dopamine function. We focus on most commonly used cognitive domains: learning, working memory, and attention.

RESULTS

We found that the type of cognitive domain under investigation, medication state and type, and severity of positive and negative symptoms can explain the conflicting results in the literature.

CONCLUSIONS

This review points to future studies investigating individual differences among schizophrenia patients in order to reveal the exact relationship between cognitive function, clinical features, and antipsychotic treatment.

Dopamine D2 Receptors in the Paraventricular Thalamus Attenuate Cocaine Locomotor Sensitization

Alterations in thalamic dopamine (DA) or DA D2 receptors (D2Rs) have been measured in drug addiction and schizophrenia, but the relevance of thalamic D2Rs for behavior is largely unknown. Using in situ hybridization and mice expressing green fluorescent protein (GFP) under the Drd2 promoter, we found that D2R expression within the thalamus is enriched in the paraventricular nucleus (PVT) as well as in more ventral midline thalamic nuclei. Within the PVT, D2Rs are inhibitory as their activation inhibits neuronal action potentials in brain slices. Using Cre-dependent anterograde and retrograde viral tracers, we further determined that PVT neurons are reciprocally interconnected with multiple areas of the limbic system including the amygdala and the nucleus accumbens (NAc). Based on these anatomical findings, we analyzed the role of D2Rs in the PVT in behaviors that are supported by these areas and that also have relevance for schizophrenia and drug addiction. Male and female mice with selective overexpression of D2Rs in the PVT showed attenuated cocaine locomotor sensitization, whereas anxiety levels, fear conditioning, sensorimotor gating, and food-motivated behaviors were not affected. These findings suggest the importance of PVT inhibition by D2Rs in modulating the sensitivity to cocaine, a finding that may have novel implications for human drug use.

The impact of Disrupted-in-Schizophrenia 1 (DISC1) on the dopaminergic system: a systematic review

Disrupted-in-Schizophrenia 1 (DISC1) is a gene known as a risk factor for mental illnesses possibly associated with dopamine impairments. DISC1 is a scaffold protein interacting with proteins involved in the dopamine system. Here we summarise the impact of DISC1 disruption on the dopamine system in animal models, considering its effects on presynaptic dopaminergic function (tyrosine hydroxylase levels, dopamine transporter levels, dopamine levels at baseline and after amphetamine administration) and postsynaptic dopaminergic function (dopamine D1 and D2 receptor levels, dopamine receptor-binding potential and locomotor activity after amphetamine administration). Our findings show that many but not all DISC1 models display (1) increased locomotion after amphetamine administration, (2) increased dopamine levels after amphetamine administration in the nucleus accumbens, and (3) inconsistent basal dopamine levels, dopamine receptor levels and binding potentials. There is also limited evidence for decreased tyrosine hydroxylase levels in the frontal cortex and increased dopamine transporter levels in the striatum but not nucleus accumbens, but these conclusions warrant further replication. The main dopaminergic findings are seen across different DISC1 models, providing convergent evidence that DISC1 has a role in regulating dopaminergic function. These results implicate dopaminergic dysregulation as a mechanism underlying the increased rate of schizophrenia seen in DISC1 variant carriers, and provide insights into how DISC1, and potentially DISC1-interacting proteins such as AKT and GSK-3, could be used as novel therapeutic targets for schizophrenia.

Reduced effects of age on dopamine D2 receptor levels in physically active adults

Physical activity has been shown to ameliorate dopaminergic degeneration in non-human animal models. However, the effects of regular physical activity on normal age-related changes in dopamine function in humans are unknown. Here we present cross-sectional data from forty-four healthy human subjects between 23 and 80 years old, showing that typical age-related dopamine D2 receptor loss, assessed with PET [18F]fallypride, was significantly reduced in physically active adults compared to less active adults.

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.

Consensus paper of the WFSBP Task Force on Biological Markers: Criteria for biomarkers and endophenotypes of schizophrenia part II: Cognition, neuroimaging and genetics

OBJECTIVES

Schizophrenia is a group of severe psychiatric disorders with high heritability but only low odds ratios of risk genes. Despite progress in the identification of pathophysiological processes, valid biomarkers of the disease are still lacking.

METHODS

This comprehensive review summarises recent efforts to identify genetic underpinnings, clinical and cognitive endophenotypes and symptom dimensions of schizophrenia and presents findings from neuroimaging studies with structural, functional and spectroscopy magnetic resonance imaging and positron emission tomography. The potential of findings to be biomarkers of schizophrenia is discussed.

RESULTS

Recent findings have not resulted in clear biomarkers for schizophrenia. However, we identified several biomarkers that are potential candidates for future research. Among them, copy number variations and links between genetic polymorphisms derived from genome-wide analysis studies, clinical or cognitive phenotypes, multimodal neuroimaging findings including positron emission tomography and magnetic resonance imaging, and the application of multivariate pattern analyses are promising.

CONCLUSIONS

Future studies should address the effects of treatment and stage of the disease more precisely and apply combinations of biomarker candidates. Although biomarkers for schizophrenia await validation, knowledge on candidate genomic and neuroimaging biomarkers is growing rapidly and research on this topic has the potential to identify psychiatric endophenotypes and in the future increase insight on individual treatment response in schizophrenia.

Using human brain imaging studies as a guide toward animal models of schizophrenia

Schizophrenia is a heterogeneous and poorly understood mental disorder that is presently defined solely by its behavioral symptoms. Advances in genetic, epidemiological and brain imaging techniques in the past half century, however, have significantly advanced our understanding of the underlying biology of the disorder. In spite of these advances clinical research remains limited in its power to establish the causal relationships that link etiology with pathophysiology and symptoms. In this context, animal models provide an important tool for causally testing hypotheses about biological processes postulated to be disrupted in the disorder. While animal models can exploit a variety of entry points toward the study of schizophrenia, here we describe an approach that seeks to closely approximate functional alterations observed with brain imaging techniques in patients. By modeling these intermediate pathophysiological alterations in animals, this approach offers an opportunity to (1) tightly link a single functional brain abnormality with its behavioral consequences, and (2) to determine whether a single pathophysiology can causally produce alterations in other brain areas that have been described in patients. In this review we first summarize a selection of well-replicated biological abnormalities described in the schizophrenia literature. We then provide examples of animal models that were studied in the context of patient imaging findings describing enhanced striatal dopamine D2 receptor function, alterations in thalamo-prefrontal circuit function, and metabolic hyperfunction of the hippocampus. Lastly, we discuss the implications of findings from these animal models for our present understanding of schizophrenia, and consider key unanswered questions for future research in animal models and human patients.

Age-related changes in binding of the D2/3 receptor radioligand [(11)C](+)PHNO in healthy volunteers

OBJECTIVE

Previous imaging studies with positron emission tomography (PET) have reliably demonstrated an age-associated decline in the dopamine system. Most of these studies have focused on the densities of dopamine receptor subtypes D2/3R (D2R family) in the striatum using antagonist radiotracers that are largely nonselective for D2R vs. D3R subtypes. Therefore, less is known about any possible age effects in D3-rich extrastriatal areas such as the substantia nigra/ventral tegmental area (SN/VTA) and hypothalamus. This study sought to investigate whether the receptor availability measured with [(11)C](+)PHNO, a D3R-preferring agonist radiotracer, also declines with age.

METHODS

Forty-two healthy control subjects (9 females, 33 males; age range 19-55 years) were scanned with [(11)C](+)PHNO using a High Resolution Research Tomograph (HRRT). Parametric images were computed using the simplified reference tissue model (SRTM2) with cerebellum as the reference region. Binding potentials (BPND) were calculated for the amygdala, caudate, hypothalamus, pallidum, putamen, SN/VTA, thalamus, and ventral striatum and then confirmed at the voxel level with whole-brain parametric images.

RESULTS

Regional [(11)C](+)PHNO BPND displayed a negative correlation between receptor availability and age in the caudate (r=-0.56, corrected p=0.0008) and putamen (r=-0.45, corrected p=0.02) in healthy subjects (respectively 8% and 5% lower per decade). No significant correlations with age were found between age and other regions (including the hypothalamus and SN/VTA). Secondary whole-brain voxel-wise analysis confirmed these ROI findings of negative associations and further identified a positive correlation in midbrain (SN/VTA) regions.

CONCLUSION

In accordance with previous studies, the striatum (an area rich in D2R) is associated with age-related declines of the dopamine system. We did not initially find evidence of changes with age in the SN/VTA and hypothalamus, areas previously found to have a predominantly D3R signal as measured with [(11)C](+)PHNO. A secondary analysis did find a significant positive correlation in midbrain (SN/VTA) regions, indicating that there may be differential effects of aging, whereby D2R receptor availability decreases with age while D3R availability stays unchanged or is increased.

Evaluation of the early response and mechanism of treatment of Parkinson's disease with L-dopa using 18F-fallypride micro-positron emission tomography scanning

The aim of the present study was to investigate the use of ¹⁸F-fallypride micro-positron emission tomography (micro-PET) imaging in the evaluation of the early therapeutic efficacy of L-dopa in the treatment of Parkinson's disease (PD) and the underlying mechanism. ¹⁸F-fallypride was synthesized and its specific binding with dopamine (DA) receptors in normal mouse brain was studied. Following the establishment of a mouse model of PD, the animals were divided into normal control, PD model and L-dopa treatment groups. General behavior, swimming test, locomotor activity counts, transmission electron microscopy, immunohistochemical analysis, high performance liquid chromatography-electrochemical detection and ¹⁸F-fallypride micro-PET imaging were used to study intergroup differences and the correlation between the changes of striatal uptake of ¹⁸F-fallypride and the therapeutic efficacy. The general behavioral features of PD model mice were similar to the clinical symptoms of PD patients and were alleviated after treatment. The swimming time, locomotor activity and frequency of standing posture of PD model mice were lower than those of the control mice, but had no difference from those of the control mice after L-dopa treatment. The number of tyrosine hydroxylase-positive neurons and the striatal contents of glutathione peroxidase, superoxide dismutase, DA and its metabolites 3,5-dihydroxyphenylacetic acid and homovanillic acid in the PD group were lower than those in the control group, but were significantly improved following the treatment; the significant reduction in DOPAC/DA and HVA/DA ratios post treatment suggested that the rate of DA metabolism decreased significantly. The striatal malondialdehyde content in the PD group increased compared with that in the control group, but was reduced after L-dopa treatment. Micro-PET imaging indicated that the uptake of ¹⁸F-fallypride in the mouse striatum of the PD group was lower than that of the control group and was significantly increased after the treatment. The mechanism of treatment of PD with L-dopa in mice may involve increasing the number of TH-positive cells and DA receptor levels, as well as reducing the rate of DA metabolism; such changes can be noninvasively observed in vitro by ¹⁸F-fallypride imaging.

Deficits in prefrontal cortical and extrastriatal dopamine release in schizophrenia: a positron emission tomographic functional magnetic resonance imaging study

IMPORTANCE

Multiple lines of evidence suggest a deficit in dopamine release in the prefrontal cortex (PFC) in schizophrenia. Despite the prevalence of the concept of prefrontal cortical hypodopaminergia in schizophrenia, in vivo imaging of dopamine release in the PFC has not been possible until now, when the validity of using the positron emission tomographic D2/3 radiotracer carbon 11-labeled FLB457 in combination with the amphetamine paradigm was clearly established.

OBJECTIVES

To (1) test amphetamine-induced dopamine release in the dorsolateral PFC (DLPFC) in drug-free or drug-naive patients with schizophrenia (SCZ) and healthy control (HC) individuals matched for age, sex, race/ethnicity, and familial socioeconomic status;(2) test blood oxygenation level-dependent (BOLD) functional magnetic resonance imaging activation during a working memory task in the same participants; and (3) examine the relationship between positron emission tomographic and functional magnetic resonance imaging outcome measures.

DESIGN, SETTING AND PARTICIPANTS

Positron emission tomographic imaging with carbon 11-labeled FLB457 before and following 0.5 mg/kg of amphetamine by mouth. Blood oxygenation level-dependent functional magnetic resonance imaging during the self-ordered working memory task. Twenty patients with schizophrenia recruited from the inpatient and outpatient research facilities at New York State Psychiatric Institute and 21 healthy control individuals participated, and data were acquired between June 16, 2011, and February 25, 2014.

MAIN OUTCOMES AND MEASURE

The percentage change in binding potential (∆BPND) in the DLPFC following amphetamine, BOLD activation during the self-ordered working memory task compared with the control task, and the correlation between these 2 outcome measures.

RESULTS

We observed significant differences in the effect of amphetamine on DLPFC BPND (mean [SD], ∆BPND in HC: -7.5% [11%]; SCZ: +1.8% [11%]; P = .01); a generalized blunting in dopamine release in SCZ involving most extrastriatal regions and the midbrain; and a significant association between ∆BPND and BOLD activation in the DLPFC in the overall sample including patients with SCZ and HC individuals.

CONCLUSIONS AND RELEVANCE

To our knowledge, these results provide the first in vivo evidence for a deficit in the capacity for dopamine release in the DLPFC in SCZ and suggest a more widespread deficit extending to many cortical and extrastriatal regions including the midbrain. This contrasts with the well-replicated excess in dopamine release in the associative striatum in SCZ and suggests a differential regulation of striatal dopamine release in associative striatum vs extrastriatal regions. Furthermore, dopamine release in the DLPFC relates to working memory-related activation of this region, suggesting that blunted release may affect frontal cortical function.

Imaging-based neurochemistry in schizophrenia: a systematic review and implications for dysfunctional long-term potentiation

Cognitive deficits are commonly observed in patients with schizophrenia. Converging lines of evidence suggest that these deficits are associated with impaired long-term potentiation (LTP). In our systematic review, this hypothesis is evaluated using neuroimaging literature focused on proton magnetic resonance spectroscopy, positron emission tomography, and single-photon emission computed tomography. The review provides evidence for abnormal dopaminergic, GABAergic, and glutamatergic neurotransmission in antipsychotic-naive/free patients with schizophrenia compared with healthy controls. The review concludes with a model illustrating how these abnormalities could lead to impaired LTP in patients with schizophrenia and consequently cognitive deficits.

Prefrontal dopamine D1 receptors and working memory in schizotypal personality disorder: a PET study with [¹¹C]NNC112

RATIONALE

Schizotypal personality disorder (SPD) is associated with working memory (WM) impairments that are similar to those observed in schizophrenia. Imaging studies have suggested that schizophrenia is associated with alterations in dopamine D1 receptor availability in the prefrontal cortex (PFC) that may be related to the WM impairments that characterize this disorder.

OBJECTIVES

The aim of this study was to characterize prefrontal D1 receptor availability and its relation to WM performance in SPD.

METHODS

We used positron emission tomography (PET) and the radiotracer [(11)C]NNC112 with 18 unmedicated SPD and 21 healthy control participants; as an index of D1 receptor availability, binding potential (BP) measures (BPF, BPND, and BPP) were calculated for prefrontal and striatal subregions. To assess WM, SPD participants completed the 2-back and Paced Auditory Serial Addition Test (PASAT).

RESULTS

There were no significant group differences in PFC BP. BPF and BPP in the medial PFC were significantly negatively related to PASAT performance (r s  = -0.551, p = .022 and r s  = -0.488, p = .047, respectively), but BP was not related to 2-back performance.

CONCLUSIONS

In contrast to what has been found in schizophrenia, SPD was not associated with significant alterations in prefrontal D1 receptor availability. Similar to previous schizophrenia findings, however, higher prefrontal D1 receptor availability was associated with poorer WM performance (as measured by the PASAT) in SPD. These findings suggest that schizophrenia and SPD may share a common pathophysiological feature related to prefrontal dopamine functioning that contributes to WM dysfunction, but that in SPD, alterations in D1 may occur only in a subset of individuals and/or to an extent that is minor relative to what occurs in schizophrenia.

Basic visual dysfunction allows classification of patients with schizophrenia with exceptional accuracy

Basic visual dysfunctions are commonly reported in schizophrenia; however their value as diagnostic tools remains uncertain. This study reports a novel electrophysiological approach using checkerboard visual evoked potentials (VEP). Sources of spectral resolution VEP-components C1, P1 and N1 were estimated by LORETA, and the band-effects (BSE) on these estimated sources were explored in each subject. BSEs were Z-transformed for each component and relationships with clinical variables were assessed. Clinical effects were evaluated by ROC-curves and predictive values. Forty-eight patients with schizophrenia (SZ) and 55 healthy controls participated in the study. For each of the 48 patients, the three VEP components were localized to both dorsal and ventral brain areas and also deviated from a normal distribution. P1 and N1 deviations were independent of treatment, illness chronicity or gender. Results from LORETA also suggest that deficits in thalamus, posterior cingulum, precuneus, superior parietal and medial occipitotemporal areas were associated with symptom severity. While positive symptoms were more strongly related to sensory processing deficits (P1), negative symptoms were more strongly related to perceptual processing dysfunction (N1). Clinical validation revealed positive and negative predictive values for correctly classifying SZ of 100% and 77%, respectively. Classification in an additional independent sample of 30 SZ corroborated these results. In summary, this novel approach revealed basic visual dysfunctions in all patients with schizophrenia, suggesting these visual dysfunctions represent a promising candidate as a biomarker for schizophrenia.

The simplified reference tissue model with 18F-fallypride positron emission tomography: choice of reference region

The development of high-affinity radiotracers for positron emission tomography (PET) has allowed for quantification of dopamine receptors in extrastriatal and striatal regions of the brain. As these new radiotracers have distinctly different kinetic properties than their predecessors, it is important to examine the suitability of kinetic models to represent their uptake, distribution, and in vivo washout. Using the simplified reference tissue model, we investigated the influence of reference region choice on the striatal binding potential of 18F-fallypride, a high-affinity dopamine D2/D3 receptor ligand. We compared the use of the visual cortex and a white matter region (superior longitudinal fasciculus) to the cerebellum, a commonly used reference tissue, in a PET-fallypride study of healthy and methamphetamine-dependent subjects. Compared to the cerebellum, use of the visual cortex produced significantly greater sample variance in binding potential relative to nondisplaceable uptake (BP(ND)). Use of the white matter region was associated with BP(ND) values and sample variance similar to those obtained with the cerebellum and a larger effect size for the group differences in striatal BP(ND) between healthy and methamphetamine-dependent subjects. Our results do not support the use of the visual cortex as a reference region in 18F-fallypride studies and suggest that white matter may be a reasonable alternative to the cerebellum as it displays similar statistical and kinetic properties.

Effects of smoking on D₂/D₃ striatal receptor availability in alcoholics and social drinkers

Studies have reported lower striatal D₂/D₃ receptor availability in both alcoholics and cigarette smokers relative to healthy controls. These substances are commonly co-abused, yet the relationship between comorbid alcohol/tobacco abuse and striatal D₂/D₃ receptor availability has not been examined. We sought to determine the degree to which dual abuse of alcohol and tobacco is associated with lower D₂/D₃ receptor availability. Eighty-one subjects (34 nontreatment-seeking alcoholic smokers [NTS-S], 21 social-drinking smokers [SD-S], and 26 social-drinking non-smokers [SD-NS]) received baseline [(11)C]raclopride scans. D₂/D₃ binding potential (BPND ≡ Bavail/KD) was estimated for ten anatomically defined striatal regions of interest (ROIs). Significant group effects were detected in bilateral pre-commissural dorsal putamen, bilateral pre-commissural dorsal caudate; and bilateral post-commissural dorsal putamen. Post-hoc testing revealed that, regardless of drinking status, smokers had lower D₂/D₃ receptor availability than non-smoking controls. Chronic tobacco smokers have lower striatal D₂/D₃ receptor availability than non-smokers, independent of alcohol use. Additional studies are needed to identify the mechanisms by which chronic tobacco smoking is associated with striatal dopamine receptor availability.

Alterations of GABAergic and dopaminergic systems in mutant mice with disruption of exons 2 and 3 of the Disc1 gene

Disrupted-in-schizophrenia-1 (DISC1) has been widely associated with several psychiatric disorders, including schizophrenia, mood disorders and autism. We previously reported that a deficiency of DISC1 may induce low anxiety and/or high impulsivity in mice with disruption of exons 2 and 3 of the Disc1 gene (Disc1(Δ2-3/Δ2-3)). It remains unclear, however, if deficiency of DISC1 leads to specific alterations in distinct neuronal systems. In the present study, to understand the role of DISC1 in γ-aminobutyric acid (GABA) interneurons and mesocorticolimbic dopaminergic (DAergic) neurons, we investigated the number of parvalbumin (PV)-positive interneurons, methamphetamine (METH)-induced DA release and the expression levels of GABAA, DA transporter (DAT) and DA receptors in wild-type (Disc1(+/+)) and Disc1(Δ2-3/Δ2-3) mice. Female Disc1(Δ2-3/Δ2-3) mice showed a significant reduction of PV-positive interneurons in the hippocampus, while no apparent changes were observed in mRNA expression levels of GABAA receptor subunits. METH-induced DA release was significantly potentiated in the nucleus accumbens (NAc) of female Disc1(Δ2-3/Δ2-3) mice, although there were no significant differences in the expression levels of DAT. Furthermore, the expression levels of DA receptor mRNA were upregulated in the NAc of female Disc1(Δ2-3/Δ2-3) mice. Male Disc1(Δ2-3/Δ2-3) mice showed no apparent differences in all experiments. DISC1 may play a critical role in gender-specific developmental alteration in GABAergic inhibitory interneurons and DAergic neurons.

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.

Shuttle-cargo fusion molecules of transport peptides and the hD2/3 receptor antagonist fallypride: a feasible approach to preserve ligand-receptor binding?

To determine if the conjugation of a small receptor ligand to a peptidic carrier to potentially facilitate transport across the blood-brain barrier (BBB) by "molecular Trojan horse" transcytosis is feasible, we synthesized several transport peptide-fallypride fusion molecules as model systems and determined their binding affinities to the hD2 receptor. Although they were affected by conjugation, the binding affinities were found to be still in the nanomolar range (between 1.5 and 64.2 nM). In addition, homology modeling of the receptor and docking studies for the most potent compounds were performed, elucidating the binding modes of the fusion molecules and the structure elements contributing to the observed high receptor binding. Furthermore, no interaction between the hybrid compounds and P-gp, the main excretory transporter of the BBB, was found. From these results, it can be inferred that the approach to deliver small neuroreceptor ligands across the BBB by transport peptide carriers is feasible.

α2-Adrenoceptors are targets for antipsychotic drugs

RATIONALE

Almost all antipsychotic drugs (APDs), irrespective of whether they belong to the first-generation (e.g. haloperidol) or second-generation (e.g. clozapine), are dopamine D2 receptor antagonists. Second-generation APDs, which differ from first-generation APDs in possessing a lower propensity to induce extrapyramidal side effects, target a variety of monoamine receptors such as serotonin (5-hydroxytryptamine) receptors (e.g. 5-HT1A, 5-HT2A, 5-HT2C, 5-HT6, 5-HT7) and α1- and α2-adrenoceptors in addition to their antagonist effects at D2 receptors.

OBJECTIVE

This short review is focussed on the potential role of α2-adrenoceptors in the antipsychotic therapy.

RESULTS

Schizophrenia is characterised by three categories of symptoms: positive symptoms, negative symptoms and cognitive deficits. α2-Adrenoceptors are classified into three distinct subtypes in mammals, α2A, α2B and α2C. Whereas the α2B-adrenoceptor seems to play only a minor role in the brain, activation of postsynaptic α2A-adrenoceptors in the prefrontal cortex improves cognitive functions. Preclinical models such as D-amphetamine-induced locomotion, the conditioned avoidance response and the pharmacological N-methyl-D-aspartate receptor hypofunction model have shown that α2C-adrenoceptor blockade or the combination of D2 receptor antagonists with idazoxan (α2A/2C-adrenoceptor antagonist) could be useful in schizophrenia. A potential benefit of a treatment combination of first-generation APDs with the α2A/2C-adrenoceptor antagonists idazoxan or mirtazapine was also demonstrated in patients with schizophrenia.

CONCLUSIONS

It is concluded that α2-adrenoceptors may be promising targets in the antipsychotic therapy.

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.

Is schizophrenia a dopamine supersensitivity psychotic reaction?

Adolf Meyer (1866-1950) did not see schizophrenia as a discrete disorder with a specific etiology but, rather, as a reaction to a wide variety of biopsychosocial factors. He may have been right. Today, we have evidence that gene mutations, brain injury, drug use (cocaine, amphetamine, marijuana, phencyclidine, and steroids), prenatal infection and malnutrition, social isolation and marginalization, can all result in the signs and symptoms of schizophrenia. This clinical picture is generally associated with supersensitivity to dopamine, and activates dopamine neurotransmission that is usually alleviated or blocked by drugs that block dopamine D2 receptors. While the dopamine neural pathway may be a final common route to many of the clinical symptoms, the components of this pathway, such as dopamine release and number of D2 receptors, are approximately normal in schizophrenia patients who are in remission. Postmortem findings, however, reveal more dimers of D1D2 and D2D2 receptors in both human schizophrenia brains and in animal models of schizophrenia. Another finding in animal models is an elevation of high-affinity state D2High receptors, but no radioactive ligand is yet available to selectively label D2High receptors in humans. It is suggested that synaptic dopamine supersensitivity in schizophrenia is an attempt at compensation for the original damage by heightening dopamine neurotransmission pathways (preparing the organism for fight or flight). The dopamine overactivity is experienced subjectively as overstimulation, which accounts for some of the clinical symptoms, with attempts at dampening down the stimulation leading to still other symptoms. Reaction and counter-reaction may explain the symptoms of schizophrenia.

The role of dopamine in schizophrenia from a neurobiological and evolutionary perspective: old fashioned, but still in vogue

Dopamine is an inhibitory neurotransmitter involved in the pathology of schizophrenia. The revised dopamine hypothesis states that dopamine abnormalities in the mesolimbic and prefrontal brain regions exist in schizophrenia. However, recent research has indicated that glutamate, GABA, acetylcholine, and serotonin alterations are also involved in the pathology of schizophrenia. This review provides an in-depth analysis of dopamine in animal models of schizophrenia and also focuses on dopamine and cognition. Furthermore, this review provides not only an overview of dopamine receptors and the antipsychotic effects of treatments targeting them but also an outline of dopamine and its interaction with other neurochemical models of schizophrenia. The roles of dopamine in the evolution of the human brain and human mental abilities, which are affected in schizophrenia patients, are also discussed.

Schizophrenia thalamus imaging: low benzamide binding to dopamine D2 receptors suggests fewer D2Short receptors and fewer presynaptic terminals

The dopamine D2 receptor continues to be the major target for the treatment of schizophrenia and is one of many genes genetically associated with this disease. Recent data show that fewer short forms of the D2 receptor protein are synthesized if there is a genetic variant in the D2 receptor (with a T in rs 1076560 in intron 6). At the same time, at least six publications report that the binding of radioactive benzamides is reduced in the schizophrenia thalamus. A review of the benzamide pharmacology of the short and long forms of the D2 receptor shows that benzamides have a 2.4-fold higher affinity for the D2Short receptor relative to the D2Long form. Hence, the reduced amount of benzamide binding to the D2 receptors in the schizophrenia thalamus suggests that there is a reduced amount of D2Short receptors in this diseased region, and may possibly also mean fewer presynaptic terminals because that is where D2Short receptors mostly reside. If so, fewer presynaptic dopamine terminals in various brain regions may be the basis of the known behavioural dopamine supersensitivity in schizophrenia.

Are dopamine D2 receptors out of control in psychosis?

It is known that schizophrenia patients are behaviorally supersensitive to dopamine-like drugs (amphetamine, methylphenidate). There is evidence for an increased release of dopamine, a slight increase of dopamine D2 receptors and an increase of dopamine D2High receptors in schizophrenia, all possibly explaining the clinical supersensitivity to dopamine. The elevation in apparent D2High receptors in vivo in schizophrenia matches the elevation in D2High receptors in many animal models of psychosis. The increased amounts of D2High receptors in psychotic-like behavior in animals may result from a loss of control of D2 by various factors. These factors include the rate of phosphorylation and desensitization of D2 receptors by kinases, the attachment of arrestin to D2 receptors, internalization of D2 receptors, the rate of receptor de-phosphorylation, formation of D2 receptor dimers, and GTP regulation by various GTPases. While at present there are no statistically significant associations of any of these controlling factors and their genes with schizophrenia, investigation of D2High receptors in schizophrenia will require a new radioligand in order to selectively label D2High receptors in vivo in patients. Finally, haloperidol reduces the number of D2High receptors that are elevated by amphetamine, indicating that this therapeutic effect may occur clinically.

Schizophrenia and dopamine receptors

Schizophrenia patients are behaviorally supersensitive to dopamine-like drugs such as amphetamine or methylphenidate, meaning that patients respond to such drugs with increased psychotic symptoms, as compared to control subjects. A basis of such supersensitivity may be an increased pre-synaptic release of dopamine or a post-synaptic elevation of D2 receptors or of D2High receptors in active stages of schizophrenia. While the pre-synaptic release of dopamine is normal in stable patients with schizophrenia, brain imaging studies find that D2 receptors are increased by an average of 5.8% in antipsychotic-free schizophrenia patients. It is possible that the behavioral supersensitivity may stem from more D2High receptors in schizophrenia. Although the antipsychotic/dopamine D2 receptor can exist in vitro in a state of high affinity for dopamine (as D2High), or in a state of low affinity for dopamine (as D2Low), there is no clear evidence that D2High states can be selectively labeled or stably exist in vivo. Nevertheless, two studies revealed an 80% increase in apparent D2High receptors in schizophrenia patients after reducing endogenous dopamine. The elevation in apparent D2High receptors in vivo in schizophrenia matches the elevation in D2High receptors in vitro in animal models of psychosis, including dopamine-supersensitive animals pretreated with amphetamine, marijuana, or phencyclidine, or animals with gene knockouts in various neurotransmitter pathways, including those for glutamate receptors. The elevation of D2High receptors in vitro and the increased apparent D2High receptors in vivo is consistent with behavioral dopamine supersensitivity in schizophrenia patients.

[(18)F]Fallypride: metabolism studies and quantification of the radiotracer and its radiometabolites in plasma using a simple and rapid solid-phase extraction method

INTRODUCTION

[(18)F]Fallypride, a fluorinated and substituted benzamide with high affinity for D2/D3 receptors, is a useful PET radioligand for the study of striatal/extrastriatal areas. Since [(18)F]fallypride is extensively metabolized in vivo and since PET examinations are long lasting in humans, the rapid measurement of the unchanged radiotracer in plasma is essential for the quantification of images. The present study aims: i) to evaluate if the radiometabolites of [(18)F]fallypride cross the blood-brain barrier in rodents, ii) to identify these radiometabolites in baboon plasma and iii) to develop a rapid solid phase extraction method (SPE) suitable for human applications to quantify both [(18)F]fallypride and its radiometabolites in plasma.

METHODS

The metabolites P450-dependant in rat and human liver microsomes were characterized by LC-MS-MS and compared to those detected in vivo. Sequential solvent elution on Oasis®-MCX-SPE cartridges was used to quantify [(18)F]fallypride and its radiometabolites.

RESULT

In rat microsomal incubations, five metabolites generated upon N/O-dealkylation or hydroxylation at the pyrrolidine and/or at the benzamide moiety were identified. No radiometabolite was detected in the rat brain. N-dealkylated and hydroxylated derivatives were detected in human microsomal incubations as well as in baboon plasma. The use of SPE (total recovery 100.2%± 2.8%, extraction yield 95.5%± 0.3%) allowed a complete separation of [(18)F]fallypride from its radiometabolites in plasma and evaluate [(18)F]fallypride at 150 min pi to be 22%± 5% of plasma radioactivity.

CONCLUSIONS

The major in vivo radiometabolites of [(18)F]fallypride were produced by N-dealkylation and hydroxylation. Allowing the rapid analysis of multiple plasma samples, SPE is a method of choice for the determination of [(18)F]fallypride until late images required for quantitative PET imaging in humans.

Vulnerability to psychotogenic effects of ketamine is associated with elevated D2/3-receptor availability

Previous positron emission tomography (PET) studies employing competition paradigms have shown either no change or substantial declines in striatal [(11)C]-raclopride binding after challenge with psychotogenic doses of the N-methyl-D-aspartate antagonist ketamine. We sought to probe the relationship between the severity of ketamine-induced psychotic symptoms and altered dopamine D(2/3) receptor availability throughout brain using the high affinity ligand [(18)F]-fallypride (FP). PET recordings were obtained in a group of 10 healthy, young male volunteers, in a placebo condition, and in the course of an infusion with ketamine at a psychotomimetic dose. Administration of the Positive and Negative Syndrome Scale and the Thought and Language Index in both conditions revealed a substantial emergence of mainly negative symptoms of schizophrenia, persisting until the end of the 3 h PET recordings. The baseline FP binding in cortex, caudate nucleus and other brain regions was highly predictive of the individual severity of psychotic symptoms in the ketamine condition. However, there was no evidence of ketamine-evoked reductions in FP binding. In the context of earlier findings, we speculate that high baseline D(2/3)-receptor availability may impart benefits with regard to cognitive flexibility, but increases the risk of maladaptive information processing in the face of environmental stresses and challenges.

Dopamine D2 and D3 binding in people at clinical high risk for schizophrenia, antipsychotic-naive patients and healthy controls while performing a cognitive task

BACKGROUND

The dopamine (DA) D2 receptors exist in 2 states: a high-affinity state (D2 high) that is linked to second messenger systems, responsible for functional effects, exhibits high affinity for agonists (e.g., DA), and a low-affinity state that is functionally inert exhibits lower affinity for agonists. The DA D3 receptor subtype exhibits high agonist affinity, whereas the existence of the multiple affinity states is controversial. Preclinical studies in animal models of psychosis have shown a selective increase of D2 high as the common factor in psychosis, and the D3 receptor has been suggested to be involved in the pathophysiology of schizophrenia.

METHODS

We studied D2 high and D3 in people at clinical high risk (CHR) for schizophrenia and in antipsychotic-naive patients with schizophrenia using the novel positron emission tomography radiotracer, [11C]-(+)-PHNO. The binding potential nondisplaceable (BP(ND)) was examined in the regions of interest (ROI; caudate, putamen, ventral striatum, globus pallidus, substantia nigra and thalamus) using an ROI and a voxel-wise approach while participants performed a cognitive task.

RESULTS

We recruited 12 CHR individuals and 13 antipsychotic-naive patients with schizophrenia-spectrum disorder, whom we compared with 12 age- and sex-matched healthy controls. The BP(ND) between patients and controls did not differ in any of the ROIs, consistent with the voxel-wise analysis. Correlations between the BP(ND) in D3-rich regions and psychopathology warrant further investigation.

LIMITATIONS

In the absence of resting-state (baseline) BP(ND) data, or following a depletion paradigm (i.e., α-methyl partyrosine), it is not possible to ascertain whether the lack of difference among the groups is owing to different levels of baseline DA or to release during the cognitive task.

CONCLUSION

To our knowledge, the present study represents the first effort to measure the D2 and D3 receptors under a cognitive challenge in individuals putative/prodromal for schizophrenia using [11C]-(+)-PHNO.

Synapse-specific contributions in the cortical pathology of schizophrenia

Schizophrenia (SZ) is often described as a disease of neuronal connectivity. Cognitive processes such as working memory, which are particularly dependent on the proper functioning of complex cortical circuitry, are disturbed in the disease. Reciprocal connections between pyramidal neurons and interneurons, as well as dopaminergic innervations, form the basis for higher cognition in the cortex. Nonetheless, only a few review articles are available which address how each synapse operates, and is possibly disturbed in SZ, at least in part by the mechanisms involving genetic susceptibility factors for SZ. In this review, we provide an overview of cortical glutamatergic, GABAergic, and dopaminergic circuitry, review SZ-associated deficits at each of these synapses, and discuss how genetic factors for SZ may contribute to SZ-related phenotype deficits in a synapse-specific manner. Pinpointing the spatially and temporally distinct sites of action of putative SZ susceptibility factors may help us better understand the pathological mechanisms of SZ, especially those associated with synaptic functioning and neuronal connectivity.