Increased prefrontal cortical D₁ receptors in drug naive patients with schizophrenia: a PET study with [¹¹C]NNC112

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.

An NIMH Workshop on Non-Affective Psychosis in Midlife and Beyond: Research Agenda on Phenomenology, Clinical Trajectories, Underlying Mechanisms, and Intervention Targets

We present a review of the state of the research in the phenomenology, clinical trajectories, biological mechanisms, aging biomarkers, and treatments for middle-aged and older people with schizophrenia (PwS) discussed at the NIMH sponsored workshop "Non-affective Psychosis in Midlife and Beyond." The growing population of PwS has specific clinical needs that require tailored and mechanistically derived interventions. Differentiating between the effects of aging and disease progression is a key challenge of studying older PwS. This review of the workshop highlights the recent findings in this understudied clinical population and the critical gaps in knowledge and consensus for research priorities. This review showcases the major challenges and opportunities for research to advance clinical care for this growing and understudied population.

Gradients of striatal function in antipsychotic-free first-episode psychosis and schizotypy

Both psychotic illness and subclinical psychosis-like experiences (PLEs) have been associated with cortico-striatal dysfunction. This work has largely relied on a discrete parcellation of the striatum into distinct functional areas, but recent evidence suggests that the striatum comprises multiple overlapping and smoothly varying gradients (i.e., modes) of functional organization. Here, we investigated two of these functional connectivity modes, previously associated with variations in the topographic patterning of cortico-striatal connectivity (first-order gradient), and dopaminergic innervation of the striatum (second-order gradient), and assessed continuities in striatal function from subclinical to clinical domains. We applied connectopic mapping to resting-state fMRI data to obtain the first-order and second-order striatal connectivity modes in two distinct samples: (1) 56 antipsychotic-free patients (26 females) with first-episode psychosis (FEP) and 27 healthy controls (17 females); and (2) a community-based cohort of 377 healthy individuals (213 females) comprehensively assessed for subclinical PLEs and schizotypy. The first-order "cortico-striatal" and second-order "dopaminergic" connectivity gradients were significantly different in FEP patients compared to controls bilaterally. In the independent sample of healthy individuals, variations in the left first-order "cortico-striatal" connectivity gradient were associated with inter-individual differences in a factor capturing general schizotypy and PLE severity. The presumed cortico-striatal connectivity gradient was implicated in both subclinical and clinical cohorts, suggesting that variations in its organization may represent a neurobiological trait marker across the psychosis continuum. Disruption of the presumed dopaminergic gradient was only noticeable in patients, suggesting that neurotransmitter dysfunction may be more apparent to clinical illness.

Polymorphism in the ZNF804A Gene and Variation in D1 and D2/D3 Dopamine Receptor Availability in the Healthy Human Brain: A Dual Positron Emission Tomography Study

BACKGROUND

The rs1344706 single nucleotide polymorphism in the ZNF804A gene has been associated with risk for psychosis in multiple genome-wide association studies, yet mechanisms underlying this association are not known. Given preclinical work suggesting an impact of ZNF804A on dopamine receptor gene transcription and clinical studies establishing dopaminergic dysfunction in patients with schizophrenia, we hypothesized that the ZNF804A risk single nucleotide polymorphism would be associated with variation in dopamine receptor availability in the human brain.

METHODS

In this study, 72 healthy individuals genotyped for rs1344706 completed both [18F]fallypride and [11C]NNC-112 positron emission tomography scans to measure D2/D3 and D1 receptor availability, respectively. Genetic effects on estimates of binding potential for each ligand were tested first with canonical subject-specific striatal regions of interest analyses, followed by exploratory whole-brain voxelwise analyses to test for more localized striatal signals and for extrastriatal effects.

RESULTS

Region of interest analyses revealed significantly less D2/D3 receptor availability in risk-allele homozygotes (TT) compared with non-risk allele carriers (G-allele carrier group: TG and GG) in the associative striatum and sensorimotor striatum, but no significant differences in striatal D1 receptor availability.

CONCLUSIONS

These data suggest that ZNF804A genotype may be meaningfully linked to dopaminergic function in the human brain. The results also may provide information to guide future studies of ZNF804A-related mechanisms of schizophrenia risk.

Unusual Molecular Regulation of Dorsolateral Prefrontal Cortex Layer III Synapses Increases Vulnerability to Genetic and Environmental Insults in Schizophrenia

Schizophrenia is associated with reduced numbers of spines and dendrites from layer III of the dorsolateral prefrontal cortex (dlPFC), the layer that houses the recurrent excitatory microcircuits that subserve working memory and abstract thought. Why are these synapses so vulnerable, while synapses in deeper or more superficial layers are little affected? This review describes the special molecular properties that govern layer III neurotransmission and neuromodulation in the primate dlPFC and how they may render these circuits particularly vulnerable to genetic and environmental insults. These properties include a reliance on NMDA receptor rather than AMPA receptor neurotransmission; cAMP (cyclic adenosine monophosphate) magnification of calcium signaling near the glutamatergic synapse of dendritic spines; and potassium channels opened by cAMP/PKA (protein kinase A) signaling that dynamically alter network strength, with built-in mechanisms to take dlPFC "offline" during stress. A variety of genetic and/or environmental insults can lead to the same phenotype of weakened layer III connectivity, in which mechanisms that normally strengthen connectivity are impaired and those that normally weaken connectivity are intensified. Inflammatory mechanisms, such as increased kynurenic acid and glutamate carboxypeptidase II expression, are especially detrimental to layer III dlPFC neurotransmission and modulation, mimicking genetic insults. The combination of genetic and inflammatory insults may cross the threshold into pathology.

Present and future antipsychotic drugs: a systematic review of the putative mechanisms of action for efficacy and a critical appraisal under a translational perspective

Antipsychotics represent the mainstay of schizophrenia pharmacological therapy, and their role has been expanded in the last years to mood disorders treatment. Although introduced in 1952, many years of research were required before an accurate picture of how antipsychotics work began to emerge. Despite the well-recognized characterization of antipsychotics in typical and atypical based on their liability to induce motor adverse events, their main action at dopamine D2R to elicit the "anti-psychotic" effect, as well as the multimodal action at other classes of receptors, their effects on intracellular mechanisms starting with receptor occupancy is still not completely understood. Significant lines of evidence converge on the impact of these compounds on multiple molecular signaling pathways implicated in the regulation of early genes and growth factors, dendritic spine shape, brain inflammation, and immune response, tuning overall the function and architecture of the synapse. Here we present, based on PRISMA approach, a comprehensive and systematic review of the above mechanisms under a translational perspective to disentangle those intracellular actions and signaling that may underline clinically relevant effects and represent potential targets for further innovative strategies in antipsychotic therapy.

Neuromodulation of prefrontal cortex cognitive function in primates: the powerful roles of monoamines and acetylcholine

The primate prefrontal cortex (PFC) subserves our highest order cognitive operations, and yet is tremendously dependent on a precise neurochemical environment for proper functioning. Depletion of noradrenaline and dopamine, or of acetylcholine from the dorsolateral PFC (dlPFC), is as devastating as removing the cortex itself, and serotonergic influences are also critical to proper functioning of the orbital and medial PFC. Most neuromodulators have a narrow inverted U dose response, which coordinates arousal state with cognitive state, and contributes to cognitive deficits with fatigue or uncontrollable stress. Studies in monkeys have revealed the molecular signaling mechanisms that govern the generation and modulation of mental representations by the dlPFC, allowing dynamic regulation of network strength, a process that requires tight regulation to prevent toxic actions, e.g., as occurs with advanced age. Brain imaging studies in humans have observed drug and genotype influences on a range of cognitive tasks and on PFC circuit functional connectivity, e.g., showing that catecholamines stabilize representations in a baseline-dependent manner. Research in monkeys has already led to new treatments for cognitive disorders in humans, encouraging future research in this important field.

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.

Multitask Learning Based Three-Dimensional Striatal Segmentation of MRI: fMRI and PET Objective Assessments

BACKGROUND

Recent studies have established a clear topographical and functional organization of projections to and from complex subdivisions of the striatum. Manual segmentation of these functional subdivisions is labor-intensive and time-consuming, and automated methods are not as reliable as manual segmentation.

PURPOSE

To utilize multitask learning (MTL) as a method to segment subregions of the striatum consisting of pre-commissural putamen (prePU), pre-commissural caudate (preCA), post-commissural putamen (postPU), post-commissural caudate (postCA), and ventral striatum (VST).

STUDY TYPE

Retrospective.

POPULATION

Eighty-seven total data sets from patients with schizophrenia and matched controls.

FIELD STRENGTH/SEQUENCE

1.5 T and 3.0 T, T₁ -weighted (SPGR SENSE, 3D BRAVO).

ASSESSMENT

MTL-generated segmentations were compared to the Imperial College London Clinical Imaging Center (CIC) atlas. Dice similarity coefficient (DSC) was used to compare the automated methods to manual segmentations. Positron emission tomography (PET) imaging: 60 minutes of emission data were acquired using [¹¹ C]raclopride. Data were reconstructed by filtered back projection (FBP) with computed tomography (CT) used for attenuation correction. Binding potential values, BP_ND , and region of interest (ROI) time series and whole-brain connectivity using functional magnetic resonance imaging (fMRI) images were compared between manual and both automated segmentations.

STATISTICAL TESTS

Pearson correlation and paired t-test.

RESULTS

MTL-generated segmentations showed excellent spatial agreement with manual (DSC ≥0.72 across all striatal subregions). BP_ND values from MTL-generated segmentations were shown to correlate well with manual segmentations with R²  ≥ 0.91 in all caudate and putamen subregions, and R²  = 0.69 in VST. Mean Pearson correlation coefficients of the fMRI data between MTL-generated and manual segmentations were also high in time series (≥0.86) and whole-brain connectivity (≥0.89) across all subregions.

DATA CONCLUSION

Across both PET and fMRI task-based assessments, results from MTL-generated segmentations more closely corresponded to results from manually drawn ROIs than CIC-generated segmentations did. Therefore, the proposed MTL approach is a fast and reliable method for three-dimensional striatal subregion segmentation with results comparable to manually segmented ROIs.

LEVEL OF EVIDENCE

2 TECHNICAL EFFICACY STAGE: 1.

Phase 1 randomized study on the safety, tolerability, and pharmacodynamic cognitive and electrophysiological effects of a dopamine D1 receptor positive allosteric modulator in patients with schizophrenia

ASP4345, a novel dopamine D₁ receptor positive allosteric modulator, is being evaluated for the treatment of cognitive impairment associated with schizophrenia (CIAS). This phase 1 multiple ascending-dose study (NCT02720263) assessed the safety, tolerability, and pharmacodynamics of ASP4345 in patients with schizophrenia/schizoaffective disorder. Pharmacodynamic assessments were Cogstate cognitive tests and electrophysiological biomarkers, including gamma-band power and phase synchronization in response to 40-Hz auditory steady-state stimulation, as well as mismatch negativity (MMN) and P3a event-related potentials. The sample size determination was based on standard practice in assessing safety and tolerability of a new chemical entity. Data were summarized by conversion of this data into effect sizes using descriptive and inferential statistics. A total of 36 randomized patients received ASP4345 (3, 15, 50, and 150 mg; n = 9 each dose) and 12 patients received placebo. Patients in the ASP4345 group experienced 73 treatment-emergent adverse events (TEAEs) and 34 TEAEs were reported for the placebo group. The most common TEAEs were headache and somnolence and nearly all TEAEs were mild in severity. No changes in mood or self-reports of suicidal ideation/behavior were observed. Improvements in performance on cognitive tests were noted, which suggests a potential improvement in psychomotor function and visual attention. Furthermore, positive changes in neurophysiological biomarkers (auditory steady-state response [ASSR] and MMN) suggest improvement in information processing. The findings need to be confirmed in studies with a larger patient population. Nonetheless, the trends in safety and pharmacodynamic data support further clinical development of ASP4345 for the treatment of CIAS.

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.

Carbon-11: Radiochemistry and Target-Based PET Molecular Imaging Applications in Oncology, Cardiology, and Neurology

The positron emission tomography (PET) molecular imaging technique has gained its universal value as a remarkable tool for medical diagnosis and biomedical research. Carbon-11 is one of the promising radiotracers that can report target-specific information related to its pharmacology and physiology to understand the disease status. Currently, many of the available carbon-11 (t_1/2 = 20.4 min) PET radiotracers are heterocyclic derivatives that have been synthesized using carbon-11 inserted different functional groups obtained from primary and secondary carbon-11 precursors. A spectrum of carbon-11 PET radiotracers has been developed against many of the upregulated and emerging targets for the diagnosis, prognosis, prediction, and therapy in the fields of oncology, cardiology, and neurology. This review focuses on the carbon-11 radiochemistry and various target-specific PET molecular imaging agents used in tumor, heart, brain, and neuroinflammatory disease imaging along with its associated pathology.

Molecular imaging of schizophrenia: Neurochemical findings in a heterogeneous and evolving disorder

The past four decades have seen enormous efforts placed on a search for molecular markers of schizophrenia using positron emission tomography (PET) and single photon emission computed tomography (SPECT). In this narrative review, we cast a broad net to define and summarize what researchers have learned about schizophrenia from molecular imaging studies. Some PET studies of brain energy metabolism with the glucose analogue FDGhave have shown a hypofrontality defect in patients with schizophrenia, but more generally indicate a loss of metabolic coherence between different brain regions. An early finding of significantly increased striatal trapping of the dopamine synthesis tracer FDOPA has survived a meta-analysis of many replications, but the increase is not pathognomonic of the disorder, since one half of patients have entirely normal dopamine synthesis capacity. Similarly, competition SPECT studies show greater basal and amphetamine-evoked dopamine occupancy at post-synaptic dopamine D_2/3 receptors in patients with schizophrenia, but the difference is likewise not pathognomonic. We thus propose that molecular imaging studies of brain dopamine indicate neurochemical heterogeneity within the diagnostic entity of schizophrenia. Occupancy studies have established the relevant target engagement by antipsychotic medications at dopamine D_2/3 receptors in living brain. There is evidence for elevated frontal cortical dopamine D₁ receptors, especially in relation to cognitive deficits in schizophrenia. There is a general lack of consistent findings of abnormalities in serotonin markers, but some evidence for decreased levels of nicotinic receptors in patients. There are sparse and somewhat inconsistent findings of reduced binding of muscarinic, glutamate, and opioid receptors ligands, inconsistent findings of microglial activation, and very recently, evidence of globally reduced levels of synaptic proteins in brain of patients. One study reports a decline in histone acetylase binding that is confined to the dorsolateral prefrontal cortex. In most contexts, the phase of the disease and effects of past or present medication can obscure or confound PET and SPECT findings in schizophrenia.

Dopamine, Cognitive Impairments and Second-Generation Antipsychotics: From Mechanistic Advances to More Personalized Treatments

The pharmacological treatment of cognitive impairments associated with schizophrenia is still a major unmet clinical need. Indeed, treatments with available antipsychotics generate highly variable cognitive responses among patients with schizophrenia. This has led to the general assumption that antipsychotics are ineffective on cognitive impairment, although personalized medicine and drug repurposing approaches might scale down this clinical issue. In this scenario, evidence suggests that cognitive improvement exerted by old and new atypical antipsychotics depends on dopaminergic mechanisms. Moreover, the newer antipsychotics brexpiprazole and cariprazine, which might have superior clinical efficacy on cognitive deficits over older antipsychotics, mainly target dopamine receptors. It is thus reasonable to assume that despite more than 50 years of elusive efforts to develop novel non-dopaminergic antipsychotics, dopamine receptors remain the most attractive and promising pharmacological targets in this field. In the present review, we discuss preclinical and clinical findings showing dopaminergic mechanisms as key players in the cognitive improvement induced by both atypical antipsychotics and potential antipsychotics. We also emphasize the concept that these mechanistic advances, which help to understand the heterogeneity of cognitive responses to antipsychotics, may properly guide treatment decisions and address the unmet medical need for the management of cognitive impairment associated with schizophrenia.

Links Between Human and Animal Models of Trauma and Psychosis: A Narrative Review

Traumatic experiences during development are associated with an increased risk of developing psychosis. Individuals with psychosis also report a higher rate of past trauma than healthy control subjects and worse outcomes than those who do not have these experiences. It is thought that traumatic experiences negatively impact specific neurobiological processes to confer this increased risk, and that systems affected by trauma are similarly changed in individuals with psychosis. Examining animal models of psychosis and the shared neurobiological changes in response to stressors can offer valuable insight into biological mechanisms that mediate symptoms and targets for intervention. This targeted review highlights a subset of models of psychosis across humans and animals, examines the similarities with the brain's response to stress and traumatic events, and discusses how these models may interact. Suggestions for future research are described.

Dopamine D1 receptor availability is not associated with delusional ideation measures of psychosis proneness

The dopamine D1 receptor (D1R) is thought to play a role in psychosis and schizophrenia, however positron emission tomography studies comparing patients and controls have been inconsistent. To circumvent some of the limitations of clinical studies, such as antipsychotic exposure, an alternative approach is to examine subclinical psychotic symptoms within the general population, i.e. psychosis proneness traits. In this study, we investigated whether D1R availability is associated with delusional ideation in healthy controls, in four experiments, using [¹¹C]SCH23390 PET (n = 76) and psychometric questionnaires (n = 217). We performed exploratory analyses, direct self-replication, and confirmatory analyses using Bayesian statistical modelling. Collectively, we found strong evidence that there is little to no linear association between delusional ideation and D1R. If hypothesised changes in D1R in drug-naive psychosis patients can be confirmed, our results suggest that they may either occur at disease onset, or that they are associated with specific aspects of psychosis other than delusional ideation.

An integrative framework for perceptual disturbances in psychosis

Perceptual disturbances in psychosis, such as auditory verbal hallucinations, are associated with increased baseline activity in the associative auditory cortex and increased dopamine transmission in the associative striatum. Perceptual disturbances are also associated with perceptual biases that suggest increased reliance on prior expectations. We review theoretical models of perceptual inference and key supporting physiological evidence, as well as the anatomy of associative cortico-striatal loops that may be relevant to auditory perceptual inference. Integrating recent findings, we outline a working framework that bridges neurobiology and the phenomenology of perceptual disturbances via theoretical models of perceptual inference.

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.

A novel approach to evaluate the pharmacodynamics of a selective dopamine D1/D5 receptor partial agonist (PF-06412562) in patients with stable schizophrenia

BACKGROUND

PF-06412562 is an orally bioavailable, selective dopamine D1/D5 receptor partial agonist with a non-catechol structure under evaluation for treatment of cognitive impairment in schizophrenia.

AIMS

This randomized, double-blind, placebo-controlled, parallel-group, Phase 1b study examined the pharmacokinetics and pharmacodynamics of three doses of PF-06412562 (3 mg, 9 mg, and 45 mg twice daily) over 15 days in patients with schizophrenia receiving antipsychotics.

METHODS

Primary endpoints included adjunctive safety/tolerability and effects on MATRICS Consensus Cognitive Battery Working Memory domain and reward processing (Monetary Incentive Delay) tasks. Exploratory endpoints included other behavioral/neurophysiological tasks, including the N-back task.

RESULTS

Among 95 subjects (78% male; mean age 34.8 years), baseline characteristics were similar across groups. The MATRICS Consensus Cognitive Battery Working Memory composite change from baseline on Day 13 improved in all groups, the smallest improvement was observed in the 45 mg group and was significantly smaller than that in the placebo group (two-sided p=0.038). For the Monetary Incentive Delay task (change from baseline in blood-oxygen-level-dependent functional magnetic resonance imaging activation in anterior ventral striatum for the contrast of cue gain>cue no gain on Day 15), no PF-06412562 dose was significantly different from placebo. No doses of PF-06412562 showed a significant difference on two-back task accuracy versus placebo.

CONCLUSIONS

Adjunctive treatment with PF-06412562 was safe and well tolerated in patients with schizophrenia. PF-06412562 failed to show clinical benefit relative to placebo on assessments of cognition or reward processing in symptomatically stable patients over a 15-day treatment period. Numerous limitations due to the safety study design warrant further efficacy evaluation for this drug mechanism.

Prefrontal cortical dopamine release in clinical high risk for psychosis during a cognitive task: a [11C]FLB457 positron emission tomography study

Research suggests decreased cortical dopamine is a neural correlate of cognitive deficits in schizophrenia. Evidence of impaired cognitive task-induced cortical dopamine release was demonstrated in patients with psychosis. However, whether cortical dopamine release in response to a cognitive task in clinical high risk for psychosis (CHR) is also impaired, is currently unknown. We aimed to test dopamine release in the dorsolateral prefrontal cortex (DLPFC) and the anterior cingulate cortex (ACC) in antipsychotic-free CHR participants and healthy controls (HC) performing the Wisconsin Card Sorting Task (WCST). Two [¹¹C]FLB457 PET scans were conducted for 13 CHR and 15 HC while performing the WCST and the sensorimotor control task (SMCT), respectively. A magnetic resonance image was acquired for anatomical delineation. Percentage change in binding potential (ΔBP_ND) in ACC and DLPFC in WCST were compared with the SMCT between CHR and HC. Mixed model analysis revealed no statistically significant differences in the cognitive task induced ΔBP_ND in any ROIs. There were no main effect of group (F(1, 26) = 0.348; p = 0.560) or ROI (F(1, 26) = 1.080; p = 0.308) and no significant Group x ROI interaction (F(1, 26) = 0.049; p = 0.826). Our findings suggest no statistically significant difference between CHR and HC in cognitive task-induced cortical dopamine release. This is the first in vivo study to illustrate that the cortical hypodopaminergic state observed in schizophrenia may not be present in its putative high-risk state.

PET Measures of D1, D2, and DAT Binding Are Associated With Heightened Tactile Responsivity in Rhesus Macaques: Implications for Sensory Processing Disorder

Sensory processing disorder (SPD), a developmental regulatory condition characterized by marked under- or over-responsivity to non-noxious sensory stimulation, is a common but poorly understood disorder that can profoundly affect mood, cognition, social behavior and adaptive life skills. Little is known about the etiology and neural underpinnings. Clinical research indicates that children with SPD show greater prevalence of difficulties in complex cognitive behavior including working memory, behavioral flexibility, and regulation of sensory and affective functions, which are related to prefrontal cortex (PFC), striatal, and midbrain regions. Neuroimaging may provide insight into mechanisms underlying SPD, and animal experiments provide important evidence that is not available in human studies. Rhesus monkeys (N = 73) were followed over a 20-year period from birth into old age. We focused on a single sensory modality, the tactile system, measured at 5-7 years, because of its critical importance for nourishment, attachment, and social reward in development. Positron emission tomography imaging was conducted at ages 12-18 years to quantify the availability of the D1 and D2 subtypes of the DA receptor (D1R and D2R), and the DA transporter (DAT). Heightened tactile responsivity was related to (a) elevated D1R in PFC overall, including lateral, ventrolateral, medial, anterior cingulate (aCg), frontopolar, and orbitofrontal (OFC) subregions, as well as nucleus accumbens (Acb), (b) reduced D2R in aCg, OFC, and substantia nigra/ventral tegmental area, and (c) elevated DAT in putamen. These findings suggest a mechanism by which DA pathways may be altered in SPD. These pathways are associated with reward processing and pain regulation, providing top-down regulation of sensory and affective processes. The balance between top-down cognitive control in the PFC-Acb pathway and bottom-up motivational function of the VTA-Acb-PFC pathway is critical for successful adaptive function. An imbalance in these two systems might explain DA-related symptoms in children with SPD, including reduced top-down regulatory function and exaggerated responsivity to stimuli. These results provide more direct evidence that SPD may involve altered DA receptor and transporter function in PFC, striatal, and midbrain regions. More work is needed to extend these results to humans.

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.

Antipsychotics: Mechanisms underlying clinical response and side-effects and novel treatment approaches based on pathophysiology

Antipsychotic drugs are central to the treatment of schizophrenia and other psychotic disorders but are ineffective for some patients and associated with side-effects and nonadherence in others. We review the in vitro, pre-clinical, clinical and molecular imaging evidence on the mode of action of antipsychotics and their side-effects. This identifies the key role of striatal dopamine D2 receptor blockade for clinical response, but also for endocrine and motor side-effects, indicating a therapeutic window for D2 blockade. We consider how partial D2/3 receptor agonists fit within this framework, and the role of off-target effects of antipsychotics, particularly at serotonergic, histaminergic, cholinergic, and adrenergic receptors for efficacy and side-effects such as weight gain, sedation and dysphoria. We review the neurobiology of schizophrenia relevant to the mode of action of antipsychotics, and for the identification of new treatment targets. This shows elevated striatal dopamine synthesis and release capacity in dorsal regions of the striatum underlies the positive symptoms of psychosis and suggests reduced dopamine release in cortical regions contributes to cognitive and negative symptoms. Current drugs act downstream of the major dopamine abnormalities in schizophrenia, and potentially worsen cortical dopamine function. We consider new approaches including targeting dopamine synthesis and storage, autoreceptors, and trace amine receptors, and the cannabinoid, muscarinic, GABAergic and glutamatergic regulation of dopamine neurons, as well as post-synaptic modulation through phosphodiesterase inhibitors. Finally, we consider treatments for cognitive and negative symptoms such dopamine agonists, nicotinic agents and AMPA modulators before discussing immunological approaches which may be disease modifying. This article is part of the issue entitled 'Special Issue on Antipsychotics'.

D1-Dopamine Receptor Availability in First-Episode Neuroleptic Naive Psychosis Patients

BACKGROUND

Positron emission tomography studies examining differences in D1-dopamine receptor binding between control subjects and patients with schizophrenia have been inconsistent, reporting higher, lower, and no difference in the frontal cortex. Exposure to antipsychotic medication has been suggested to be a likely source of this heterogeneity, and thus there is a need for studies of patients at early stages of the disorder who have not been exposed to such drugs.

METHODS

Here, we compared 17 healthy control subjects and 18 first-episode neuroleptic naive patients with schizophrenia or schizophreniform psychosis using positron emission tomography and the D1-dopamine receptor radioligand [11C]SCH23390.

RESULTS

We observed a statistically significant difference in the dorsolateral prefrontal cortex. Contrary to our expectations, patients had less D1-dopamine receptor availability with a moderate effect size. In a Bayesian analysis, we show that the data are over 50 times more likely to have occurred under the decrease as opposed to the increase hypothesis. This effect was not global, as our analysis showed that the null hypothesis was preferred over either hypothesis in the striatum.

CONCLUSIONS

This investigation represents the largest single sample of neuroleptic-naive patients examined for D1-dopamine receptor availability using PET and suggests a reduction of prefrontal D1-dopamine receptor density in the pathophysiology of schizophrenia. However, further work will be required to reach a consensus.

Impaired Prefrontal Cortical Dopamine Release in Schizophrenia During a Cognitive Task: A [11C]FLB 457 Positron Emission Tomography Study

Evidence from several lines of research suggests decreased dopamine release in the prefrontal cortex as the neurochemical correlates of cognitive deficits in schizophrenia (SCZ). However, in vivo examination of cortical hypodopaminergia using positron emission tomography (PET) during cognitive task performance in SCZ remains to be investigated. We examined dopamine release in anterior cingulate cortex (ACC) and dorsolateral prefrontal cortex (DLPFC), using PET while participants were performing a cognitive task. Thirteen drug-free patients with SCZ and 13 healthy volunteers (HV) matched for age and sex participated in the study. Data were acquired between 2011 and 2015. Two PET scans with [11C]FLB 457 were acquired while the participants were performing the Wisconsin Card Sorting Test (WCST) and a sensorimotor control task (SMCT). A magnetic resonance image was acquired for anatomical delineation. Differences in cortical dopamine release between SCZ and HV, indexed as percentage change in binding potential between WCST and SMCT (ΔBPND), were calculated in ACC and DLPFC. We observed significant differences in the ΔBPND in ACC (HV = 4.40 ± 6.00; SCZ = -11.48 ± 15.08; t = 3.52; P = .003) and a trend-level difference in ΔBPND in DLPFC (HV = -0.58 ± 8.45; SCZ = -7.79 ± 11.28; t = 1.84; P = .079), suggesting dopamine depletion in cortical brain regions in patients with SCZ while performing a cognitive task. These results provide the first in vivo evidence for reduced dopamine release or even dopamine depletion while performing cognitive task in ACC and DLPFC in patients with SCZ. The present results provide support for the frontal hypodopaminergia hypothesis of cognitive symptoms in SCZ.

A novel dopamine D1 receptor agonist excites delay-dependent working memory-related neuronal firing in primate dorsolateral prefrontal cortex

Decades of research have emphasized the importance of dopamine (DA) D1 receptor (D1R) mechanisms to dorsolateral prefrontal cortex (dlPFC) working memory function, and the hope that D1R agonists could be used to treat cognitive disorders. However, existing D1R agonists all have had high affinity for D1R, and engage β-arrestin signaling, and these agonists have suppressed task-related neuronal firing. The current study provides the first physiological characterization of a novel D1R agonist, PF-3628, with low affinity for D1R -more similar to endogenous DA actions- as well as little engagement of β-arrestin signaling. PF-3628 was applied by iontophoresis directly onto dlPFC neurons in aged rhesus monkeys performing a delay-dependent working memory task. Aged monkeys have naturally-occurring loss of DA, and naturally-occurring reductions in dlPFC neuronal firing and working memory performance. We found the first evidence of excitatory actions of a D1R agonist on dlPFC task-related firing, and this PF-3628 beneficial response was blocked by co-application of a D1R antagonist. These D1R actions likely occur on pyramidal cells, based on previous immunoelectron microscopic studies showing expression of D1R on layer III spines, and current microarray experiments showing that D1R are four times more prevalent in pyramidal cells than in parvalbumin-containing interneurons laser-captured from layer III of the human dlPFC. These results encourage the translation of D1R mechanisms from monkey to human, with the hope PF-3628 and related, novel D1R agonists will be more appropriate for enhancing dlPFC cognitive functions in patients with mental disorders.

Computational psychopharmacology: a translational and pragmatic approach

RATIONALE

Psychopharmacology needs novel quantitative measures and theoretical approaches based on computational modelling that can be used to help translate behavioural findings from experimental animals to humans, including patients with neuropsychiatric disorders.

OBJECTIVES

This brief review exemplifies this approach when applied to recent published studies of the effects of manipulating central dopaminergic and serotoninergic systems in rodents and marmoset monkeys, and possible comparisons with healthy human volunteers receiving systemic agents or patients with depression and schizophrenia.

METHODS

Behavioural effects of central depletions of dopamine or serotonin in monkeys in probabilistic learning paradigms are characterised further by computational modelling methods and related to rodent and human data.

RESULTS

Several examples are provided of the power of computational modelling to derive new measures and reappraise conventional explanations of regional neurotransmitter depletion and other drug effects, whilst enhancing construct validation in patient groups. Specifically, effects are shown on such parameters as 'stimulus stickiness' and 'side stickiness', which occur over and above effects on standard parameters of reinforcement learning, reminiscent of some early innovations in data analysis in psychopharmacology.

CONCLUSIONS

Computational modelling provides a useful methodology for further detailed analysis of behavioural mechanisms that are affected by pharmacological manipulations across species and will aid the translation of experimental findings to understand the therapeutic effects of medications in neuropsychiatric disorders, as well as facilitating future drug discovery.

Altered hippocampal gene expression and structure in transgenic mice overexpressing neuregulin 1 (Nrg1) type I

Transgenic mice overexpressing the type I isoform of neuregulin 1 (Nrg1; NRG1) have alterations in hippocampal gamma oscillations and an age-emergent deficit in hippocampus-dependent spatial working memory. Here, we examined the molecular and morphological correlates of these findings. Microarrays showed over 100 hippocampal transcripts differentially expressed in Nrg1^{tg-type I} mice, with enrichment of genes related to neuromodulation and, in older mice, of genes involved in inflammation and immunity. Nrg1^{tg-type I} mice had an enlarged hippocampus with a widened dentate gyrus. The results show that Nrg1 type I impacts on hippocampal gene expression and structure in a multifaceted and partly age-related way, complementing the evidence implicating Nrg1 signaling in aspects of hippocampal function. The findings are also relevant to the possible role of NRG1 signaling in the pathophysiology of schizophrenia or other disorders affecting this brain region.

[11C]SCH23390 binding to the D1-dopamine receptor in the human brain-a comparison of manual and automated methods for image analysis

Background

The D₁-dopamine receptor radioligand [¹¹C]SCH23390 has been frequently used in PET studies. In drug-naïve patients with schizophrenia, the findings have been inconsistent, with decreases, increases, and no change in the frontal cortex D₁-dopamine receptors. While these discrepancies are likely primarily due to a lack of statistical power in these studies, we speculated that an additional explanation may be the differences due to methods of image analysis between studies, affecting reliability as well as bias between groups.

Methods

Fifteen healthy subjects underwent two PET measurements with [¹¹C]SCH23390 on the same day. The binding potential (BP_ND) was compared using a 95% confidence interval following manual and automated delineation of a region of interest (ROI) as well as with and without frame-by-frame realignment.

Results

Automated target region delineation produced lower BP_ND values, while automated delineation of the reference region yielded higher BP_ND values. However, no significant differences were observed for repeatability using automated and manual delineation methods. Frame-by-frame realignment generated higher BP_ND values and improved repeatability.

Conclusions

The results suggest that the choice of ROI delineation method is not an important factor for reliability, whereas the improved results following movement correction confirm its importance in PET image analysis. Realignment is therefore especially important for measurements in patient populations such as schizophrenia or Parkinson’s disease, where motion artifacts may be more prevalent.

Electronic supplementary material

The online version of this article (10.1186/s13550-018-0416-2) contains supplementary material, which is available to authorized users.

Prefronto-cortical dopamine D1 receptor sensitivity can critically influence working memory maintenance during delayed response tasks

The dopamine (DA) hypothesis of cognitive deficits suggests that too low or too high extracellular DA concentration in the prefrontal cortex (PFC) can severely impair the working memory (WM) maintenance during delay period. Thus, there exists only an optimal range of DA where the sustained-firing activity, the neural correlate of WM maintenance, in the cortex possesses optimal firing frequency as well as robustness against noisy distractions. Empirical evidences demonstrate changes even in the D1 receptor (D1R)-sensitivity to extracellular DA, collectively manifested through D1R density and DA-binding affinity, in the PFC under neuropsychiatric conditions such as ageing and schizophrenia. However, the impact of alterations in the cortical D1R-sensitivity on WM maintenance has yet remained poorly addressed. Using a quantitative neural mass model of the prefronto-mesoprefrontal system, the present study reveals that higher D1R-sensitivity may not only effectuate shrunk optimal DA range but also shift of the range to lower concentrations. Moreover, higher sensitivity may significantly reduce the WM-robustness even within the optimal DA range and exacerbates the decline at abnormal DA levels. These findings project important clinical implications, such as dosage precision and variability of DA-correcting drugs across patients, and failure in acquiring healthy WM maintenance even under drug-controlled normal cortical DA levels.

Effects of cariprazine on extracellular levels of glutamate, GABA, dopamine, noradrenaline and serotonin in the medial prefrontal cortex in the rat phencyclidine model of schizophrenia studied by microdialysis and simultaneous recordings of locomotor activity

RATIONALE

Aberrant glutamatergic, dopaminergic, and GABAergic neurotransmission has been implicated in schizophrenia. Cariprazine reverses the behavioral effects observed in the rat phencyclidine (PCP)-induced model of schizophrenia; however, little is known about its in vivo neurochemistry.

OBJECTIVES

The study aims to compare the effects of cariprazine and aripiprazole on PCP-induced changes in the extracellular levels of glutamate, dopamine, serotonin, noradrenaline, and GABA in the rat medial prefrontal cortex (mPFC), and on locomotor activation.

METHODS

Microdialysis was performed in awake rats with probes placed into the mPFC. Rats (n = 7/group) received vehicle (saline), cariprazine (0.05, 0.2, or 0.8 mg/kg), or aripiprazole (3 or 20 mg/kg) via gavage. After 60 min, 5 mg/kg PCP was administered intraperitoneally (i.p.). Samples were taken before drug administration, during pretreatment, and after PCP injection. Locomotor activity recording and microdialysis sampling occurred simultaneously.

RESULTS

PCP treatment increased extracellular levels of all the neurotransmitters tested except GABA, for which there were no significant changes. Cariprazine and aripiprazole dose-dependently inhibited the PCP-induced increases of tested neurotransmitters. Overall effects were significant for higher cariprazine doses and both aripiprazole doses for glutamate and noradrenaline, for higher cariprazine doses and 20 mg/kg aripiprazole for dopamine, and for 0.8 mg/kg cariprazine and 20 mg/kg aripiprazole for serotonin and locomotor activity.

CONCLUSION

Both cariprazine and aripiprazole dose-dependently attenuated PCP-induced hyperlocomotion and acute increases in glutamate, dopamine, noradrenaline, and serotonin levels in the mPFC; cariprazine was approximately 5-fold more potent than aripiprazole.

PET radioligands for the dopamine D1-receptor: Application in psychiatric disorders

The dopamine (DA) system is considered to be centrally involved in the pathophysiology of several major psychiatric disorders. Using positron emission tomography (PET), aberrations in dopamine D2/D3-receptors (D2-R) levels and uptake of the DA precursor FDOPA have been shown for schizophrenia, substance abuse and depression. Radioligands for the dopamine D1-receptor (D1-R) have been available for more than three decades, however this receptor subtype has received much less attention in psychiatry research. Here, studies investigating D1-R in psychiatric patients in comparison to healthy control subjects are summarized. Although small sample sizes, medication effects and heterogeneous methods of quantification limit the conclusions that can be drawn, the data is suggestive of higher levels of cortical D1-R in drug naïve patients with psychosis, and lower D1-R in patients with affective disorders. Data sharing and reanalysis using harmonized methodology are important next steps towards clarifying the role of D1-R in these disorders.

The direct basal ganglia pathway is hyperfunctional in focal dystonia

See Fujita and Eidelberg (doi:10.1093/brain/awx305) for a scientific commentary on this article. Focal dystonias are the most common type of isolated dystonia. Although their causative pathophysiology remains unclear, it is thought to involve abnormal functioning of the basal ganglia-thalamo-cortical circuitry. We used high-resolution research tomography with the radioligand 11C-NNC-112 to examine striatal dopamine D1 receptor function in two independent groups of patients, writer’s cramp and laryngeal dystonia, compared to healthy controls. We found that availability of dopamine D1 receptors was significantly increased in bilateral putamen by 19.6–22.5% in writer’s cramp and in right putamen and caudate nucleus by 24.6–26.8% in laryngeal dystonia (all P ≤ 0.009). This suggests hyperactivity of the direct basal ganglia pathway in focal dystonia. Our findings paralleled abnormally decreased dopaminergic function via the indirect basal ganglia pathway and decreased symptom-induced phasic striatal dopamine release in writer’s cramp and laryngeal dystonia. When examining topological distribution of dopamine D1 and D2 receptor abnormalities in these forms of dystonia, we found abnormal separation of direct and indirect pathways within the striatum, with negligible, if any, overlap between the two pathways and with the regions of phasic dopamine release. However, despite topological disorganization of dopaminergic function, alterations of dopamine D1 and D2 receptors were somatotopically localized within the striatal hand and larynx representations in writer’s cramp and laryngeal dystonia, respectively. This finding points to their direct relevance to disorder-characteristic clinical features. Increased D1 receptor availability showed significant negative correlations with dystonia duration but not its severity, likely representing a developmental endophenotype of this disorder. In conclusion, a comprehensive pathophysiological mechanism of abnormal basal ganglia function in focal dystonia is built upon upregulated dopamine D1 receptors that abnormally increase excitation of the direct pathway, downregulated dopamine D2 receptors that abnormally decrease inhibition within the indirect pathway, and weakened nigro-striatal phasic dopamine release during symptomatic task performance. Collectively, these aberrations of striatal dopaminergic function underlie imbalance between direct and indirect basal ganglia pathways and lead to abnormal thalamo-motor-cortical hyperexcitability in dystonia.

Small Molecule Receptor Ligands for PET Studies of the Central Nervous System-Focus on G Protein Coupled Receptors

G protein-coupled receptors (GPRCs) are a class of proteins that are expressed in high abundance and are responsible for numerous signal transduction pathways in the central nervous system. Consequently, alterations in GPRC function have been associated with a wide variety of neurologic and neuropsychiatric disorders. The development of PET probes for imaging GPRCs has served as a major emphasis of PET radiotracer development and PET imaging studies over the past 30 years. In this review, a basic description of the biology of G proteins and GPRCs is provided. This includes recent evidence of the existence of dimeric and multimeric species of GPRCs that have been termed "receptor mosaics," with an emphasis on the different GPRCs that form complexes with the dopamine D2 receptor. An overview of the different PET radiotracers for imaging the component GPRC within these different multimeric complexes of the D2 receptor is also provided.

Test-retest measurements of dopamine D1-type receptors using simultaneous PET/MRI imaging

PURPOSE

The role of dopamine D₁-type receptor (D₁R)-expressing neurons in the regulation of motivated behavior and reward prediction has not yet been fully established. As a prerequisite for future research assessing D₁-mediated neuronal network regulation using simultaneous PET/MRI and D₁R-selective [¹¹C]SCH23390, this study investigated the stability of central D₁R measurements between two independent PET/MRI sessions under baseline conditions.

METHODS

Thirteen healthy volunteers (7 female, age 33 ± 13 yrs) underwent 90-min emission scans, each after 90-s bolus injection of 486 ± 16 MBq [¹¹C]SCH23390, on two separate days within 2-4 weeks using a PET/MRI system. Parametric images of D₁R distribution volume ratio (DVR) and binding potential (BP_ND) were generated by a multi-linear reference tissue model with two parameters and the cerebellar cortex as receptor-free reference region. Volume-of-interest (VOI) analysis was performed with manual VOIs drawn on consecutive transverse MRI slices for brain regions with high and low D₁R density.

RESULTS

The DVR varied from 2.5 ± 0.3 to 2.9 ± 0.5 in regions with high D₁R density (e.g. the head of the caudate) and from 1.2 ± 0.1 to 1.6 ± 0.2 in regions with low D₁R density (e.g. the prefrontal cortex). The absolute variability of the DVR ranged from 2.4% ± 1.3% to 5.1% ± 5.3%, while Bland-Altman analyses revealed very low differences in mean DVR (e.g. 0.013 ± 0.17 for the nucleus accumbens). Intraclass correlation (one-way, random) indicated very high agreement (0.93 in average) for both DVR and BP_ND values. Accordingly, the absolute variability of BP_ND ranged from 7.0% ± 4.7% to 12.5% ± 10.6%; however, there were regions with very low D₁R content, such as the occipital cortex, with higher mean variability.

CONCLUSION

The test-retest reliability of D₁R measurements in this study was very high. This was the case not only for D₁R-rich brain areas, but also for regions with low D₁R density. These results will provide a solid base for future joint PET/MRI data analyses in stimulation-dependent mapping of D₁R-containing neurons and their effects on projections in neuronal circuits that determine behavior.

Current Concepts on the Physiopathological Relevance of Dopaminergic Receptors

Dopamine (DA) is a key neurotransmitter modulating essential functions of the central nervous system (CNS), like voluntary movement, reward, several cognitive functions and goal-oriented behaviors. The factual relevance of DAergic transmission can be well appreciated by considering that its dysfunction is recognized as a core alteration in several devastating neurological and psychiatric disorders, including Parkinson’s disease (PD) and associated movement disorders, as well as, schizophrenia, bipolar disorder, attention deficit hyperactivity disorder (ADHD) and addiction. Here we present an overview of the current knowledge on the involvement of DAergic receptors in the regulation of key physiological brain activities, and the consequences of their dysfunctions in brain disorders such as PD, schizophrenia and addiction.

Novel Dopamine Therapeutics for Cognitive Deficits in Schizophrenia

Schizophrenia is characterized by profound cognitive deficits that are not alleviated by currently available medications. Many of these cognitive deficits involve dysfunction of the newly evolved, dorsolateral prefrontal cortex (dlPFC). The brains of patients with schizophrenia show evidence of dlPFC pyramidal cell dendritic atrophy, likely reductions in cortical dopamine, and possible changes in dopamine D1 receptors (D1R). It has been appreciated for decades that optimal levels of dopamine are essential for dlPFC working memory function, with many beneficial actions arising from D1R stimulation. D1R are concentrated on dendritic spines in the primate dlPFC, where their stimulation produces an inverted-U dose response on dlPFC neuronal firing and cognitive performance during working memory tasks. Research in both academia and the pharmaceutical industry has led to the development of selective D1 agonists, e.g., the first full D1 agonist, dihydrexidine, which at low doses improved working memory in monkeys. Dihydrexidine has begun to be tested in patients with schizophrenia or schizotypal disorder. Initial results are encouraging, but studies are limited by the pharmacokinetics of the drug. These data, however, have spurred efforts toward the discovery and development of improved or novel new compounds, including D1 agonists with better pharmacokinetics, functionally selective D1 ligands, and D1R positive allosteric modulators. One or several of these approaches should allow optimization of the beneficial effects of D1R stimulation in the dlPFC that can be translated into clinical practice.

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.

The Roles of Dopamine D1 Receptor on the Social Hierarchy of Rodents and Nonhuman Primates

BACKGROUND

Although dopamine has been suggested to play a role in mediating social behaviors of individual animals, it is not clear whether such dopamine signaling contributes to attributes of social groups such as social hierarchy.

METHODS

In this study, the effects of the pharmacological manipulation of dopamine D1 receptor function on the social hierarchy and behavior of group-housed mice and macaques were investigated using a battery of behavioral tests.

RESULTS

D1 receptor blockade facilitated social dominance in mice at the middle, but not high or low, social rank in the groups without altering social preference among mates. In contrast, the administration of a D1 receptor antagonist in a macaque did not affect social dominance of the drug-treated animal; however, relative social dominance relationships between the drug-treated and nontreated subjects were altered indirectly through alterations of social affiliative relationships within the social group.

CONCLUSIONS

These results suggest that dopamine D1 receptor signaling may be involved in social hierarchy and social relationships within a group, which may differ between rodents and primates.

Dynamic Connectivity between Brain Networks Supports Working Memory: Relationships to Dopamine Release and Schizophrenia

Connectivity between brain networks may adapt flexibly to cognitive demand, a process that could underlie adaptive behaviors and cognitive deficits, such as those observed in neuropsychiatric conditions like schizophrenia. Dopamine signaling is critical for working memory but its influence on internetwork connectivity is relatively unknown. We addressed these questions in healthy humans using functional magnetic resonance imaging (during ann-back working-memory task) and positron emission tomography using the radiotracer [(11)C]FLB457 before and after amphetamine to measure the capacity for dopamine release in extrastriatal brain regions. Brain networks were defined by spatial independent component analysis (ICA) and working-memory-load-dependent connectivity between task-relevant pairs of networks was determined via a modified psychophysiological interaction analysis. For most pairs of task-relevant networks, connectivity significantly changed as a function of working-memory load. Moreover, load-dependent changes in connectivity between left and right frontoparietal networks (Δ connectivity lFPN-rFPN) predicted interindividual differences in task performance more accurately than other fMRI and PET imaging measures. Δ Connectivity lFPN-rFPN was not related to cortical dopamine release capacity. A second study in unmedicated patients with schizophrenia showed no abnormalities in load-dependent connectivity but showed a weaker relationship between Δ connectivity lFPN-rFPN and working memory performance in patients compared with matched healthy individuals. Poor working memory performance in patients was, in contrast, related to deficient cortical dopamine release. Our findings indicate that interactions between brain networks dynamically adapt to fluctuating environmental demands. These dynamic adaptations underlie successful working memory performance in healthy individuals and are not well predicted by amphetamine-induced dopamine release capacity.

SIGNIFICANCE STATEMENT

It is unclear how communication between brain networks responds to changing environmental demands during complex cognitive processes. Also, unknown in regard to these network dynamics is the role of neuromodulators, such as dopamine, and whether their dysregulation could underlie cognitive deficits in neuropsychiatric illness. We found that connectivity between brain networks changes with working-memory load and greater increases predict better working memory performance; however, it was not related to capacity for dopamine release in the cortex. Patients with schizophrenia did show dynamic internetwork connectivity; however, this was more weakly associated with successful performance in patients compared with healthy individuals. Our findings indicate that dynamic interactions between brain networks may support the type of flexible adaptations essential to goal-directed behavior.

Dopamine-Related Disruption of Functional Topography of Striatal Connections in Unmedicated Patients With Schizophrenia

IMPORTANCE

Despite the well-established role of striatal dopamine in psychosis, current views generally agree that cortical dysfunction is likely necessary for the emergence of psychotic symptoms. The topographic organization of striatal-cortical connections is central to gating and integration of higher-order information, so a disruption of such topography via dysregulated dopamine could lead to cortical dysfunction in schizophrenia. However, this hypothesis remains to be tested using multivariate methods ascertaining the global pattern of striatal connectivity and without the confounding effects of antidopaminergic medication.

OBJECTIVES

To examine whether the pattern of brain connectivity across striatal subregions is abnormal in unmedicated patients with schizophrenia and whether this abnormality relates to psychotic symptoms and extrastriatal dopaminergic transmission.

DESIGN, SETTING, AND PARTICIPANTS

In this multimodal, case-control study, we obtained resting-state functional magnetic resonance imaging data from 18 unmedicated patients with schizophrenia and 24 matched healthy controls from the New York State Psychiatric Institute. A subset of these (12 and 17, respectively) underwent positron emission tomography with the dopamine D2 receptor radiotracer carbon 11-labeled FLB457 before and after amphetamine administration. Data were acquired between June 16, 2011, and February 25, 2014. Data analysis was performed from September 1, 2014, to January 11, 2016.

MAIN OUTCOMES AND MEASURES

Group differences in the striatal connectivity pattern (assessed via multivariable logistic regression) across striatal subregions, the association between the multivariate striatal connectivity pattern and extrastriatal baseline D2 receptor binding potential and its change after amphetamine administration, and the association between the multivariate connectivity pattern and the severity of positive symptoms evaluated with the Positive and Negative Syndrome Scale.

RESULTS

Of the patients with schizophrenia (mean [SEM] age, 35.6 [11.8] years), 9 (50%) were male and 9 (50%) were female. Of the controls (mean [SEM] age, 33.7 [8.8] years), 10 (42%) were male and 14 (58%) were female. Patients had an abnormal pattern of striatal connectivity, which included abnormal caudate connections with a distributed set of associative cortex regions (χ229 = 53.55, P = .004). In patients, more deviation from the multivariate pattern of striatal connectivity found in controls correlated specifically with more severe positive symptoms (ρ = -0.77, P = .002). Striatal connectivity also correlated with baseline binding potential across cortical and extrastriatal subcortical regions (t25 = 3.01, P = .01, Bonferroni corrected) but not with its change after amphetamine administration.

CONCLUSIONS AND RELEVANCE

Using a multimodal, circuit-level interrogation of striatal-cortical connections, it was demonstrated that the functional topography of these connections is globally disrupted in unmedicated patients with schizophrenia. These findings suggest that striatal-cortical dysconnectivity may underlie the effects of dopamine dysregulation on the pathophysiologic mechanism of psychotic symptoms.

Dopamine D1 signaling organizes network dynamics underlying working memory

Local prefrontal dopamine signaling supports working memory by tuning pyramidal neurons to task-relevant stimuli. Enabled by simultaneous positron emission tomography-magnetic resonance imaging (PET-MRI), we determined whether neuromodulatory effects of dopamine scale to the level of cortical networks and coordinate their interplay during working memory. Among network territories, mean cortical D1 receptor densities differed substantially but were strongly interrelated, suggesting cross-network regulation. Indeed, mean cortical D1 density predicted working memory-emergent decoupling of the frontoparietal and default networks, which respectively manage task-related and internal stimuli. In contrast, striatal D1 predicted opposing effects within these two networks but no between-network effects. These findings specifically link cortical dopamine signaling to network crosstalk that redirects cognitive resources to working memory, echoing neuromodulatory effects of D1 signaling on the level of cortical microcircuits.

Biological mechanisms underlying evolutionary origins of psychotic and mood disorders

Psychotic and mood disorders are brain dysfunctions that are caused by gene environment interactions. Although these disorders are disadvantageous and involve behavioral phenotypes that decrease the reproductive success of afflicted individuals in the modern human society, the prevalence of these disorders have remained constant in the population. Here, we propose several biological mechanisms by which the genes associated with psychotic and mood disorders could be selected for in specific environmental conditions that provide evolutionary bases for explanations of when, why, and where these disorders emerged and have been maintained in humans. We discuss the evolutionary origins of psychotic and mood disorders with specific focuses on the roles of dopamine and serotonin in the conditions of social competitiveness/hierarchy and maternal care and other potential mechanisms, such as social network homophily and symbiosis.

A proof-of-concept, randomized controlled trial of DAR-0100A, a dopamine-1 receptor agonist, for cognitive enhancement in schizophrenia

BACKGROUND

Evidence from preclinical and human studies indicates the presence of reduced dopamine-1 receptor (D1R) signaling in the cortex, where D1Rs predominate, in patients with schizophrenia (SCZ), which may contribute to their cognitive deficits. Furthermore, studies in nonhuman primates (NHP) have suggested that intermittent administration of low doses of D1R agonists produce long-lasting reversals in cognitive deficits. The purpose of this trial was to test whether a similar design, involving subacute intermittent administration of low doses of a full, selective agonist at D1Rs, DAR-0100A, would improve cognitive deficits in SCZ.

METHODS

We randomized 49 clinically stable individuals with SCZ to three weeks of intermittent treatment with 0.5 mg or 15 mg of DAR-0100A, or placebo (normal saline). Functional magnetic resonance imaging (fMRI) BOLD was used to evaluate the effects of drug administration on brain activity during a working memory (WM) task. Effects on cognition were also assessed using the MATRICS and the N-back task as primary endpoints. The CogState battery was used as a secondary endpoint.

RESULTS

There were no observed treatment effects on either the BOLD fMRI signal during WM tasks or the WM domains of the MATRICS. Moderate improvement was detected on the CogState battery and on the attention domain of the MATRICS.

CONCLUSION

These results suggest that low doses of D1 agonists that do not result in measureable occupancy of the D1R do not reliably improve cognition in SCZ, unlike the observations in NHP. As this drug is limited by its pharmacokinetic profile, better D1R agonists that can achieve adequate levels of D1R occupancy are needed to test the efficacy of this mechanism for cognitive enhancement in SCZ.

Associations between a locus downstream DRD1 gene and cerebrospinal fluid dopamine metabolite concentrations in psychosis

Dopamine activity, mediated by the catecholaminergic neurotransmitter dopamine, is prominent in the human brain and has been implicated in schizophrenia. Dopamine targets five different receptors and is then degraded to its major metabolite homovanillic acid (HVA). We hypothesized that genes encoding dopamine receptors may be associated with cerebrospinal fluid (CSF) HVA concentrations in patients with psychotic disorder. We searched for association between 67 single nucleotide polymorphisms (SNPs) in the five dopamine receptor genes i.e., DRD1, DRD2, DRD3, DRD4 and DRD5, and the CSF HVA concentrations in 74 patients with psychotic disorder. Nominally associated SNPs were also tested in 111 healthy controls. We identified a locus, located downstream DRD1 gene, where four SNPs, rs11747728, rs11742274, rs265974 and rs11747886, showed association with CSF HVA concentrations in psychotic patients. The associations between rs11747728, which is a regulatory region variant, and rs11742274 with HVA remained significant after correction for multiple testing. These associations were restricted to psychotic patients and were absent in healthy controls. The results suggest that the DRD1 gene is implicated in the pathophysiology of psychosis and support the dopamine hypothesis of schizophrenia.

Dopamine's Actions in Primate Prefrontal Cortex: Challenges for Treating Cognitive Disorders

The prefrontal cortex (PFC) elaborates and differentiates in primates, and there is a corresponding elaboration in cortical dopamine (DA). DA cells that fire to both aversive and rewarding stimuli likely project to the dorsolateral PFC (dlPFC), signaling a salient event. Since 1979, we have known that DA has an essential influence on dlPFC working memory functions. DA has differing effects via D1 (D1R) versus D2 receptor (D2R) families. D1R are concentrated on dendritic spines, and D1/5R stimulation produces an inverted U-shaped dose response on visuospatial working memory performance and Delay cell firing, the neurons that generate representations of visual space. Optimal levels of D1R stimulation gate out "noise," whereas higher levels, e.g., during stress, suppress Delay cell firing. These effects likely involve hyperpolarization-activated cyclic nucleotide-gated channel opening, activation of GABA interneurons, and reduced glutamate release. Dysregulation of D1R has been related to cognitive deficits in schizophrenia, and there is a need for new, lower-affinity D1R agonists that may better mimic endogenous DA to enhance mental representations and improve cognition. In contrast to D1R, D2R are primarily localized on layer V pyramidal cell dendrites, and D2/3R stimulation speeds and magnifies the firing of Response cells, including Response Feedback cells. Altered firing of Feedback neurons may relate to positive symptoms in schizophrenia. Emerging research suggests that DA may have similar effects in the ventrolateral PFC and frontal eye fields. Research on the orbital PFC in monkeys is just beginning and could be a key area for future discoveries.