Prefrontal dopamine D1 receptors and working memory in schizophrenia

Improper activation of D1 and D2 receptors leads to excess noise in prefrontal cortex

The dopaminergic system has been shown to control the amount of noise in the prefrontal cortex (PFC) and likely plays an important role in working memory and the pathophysiology of schizophrenia. We developed a model that takes into account the known receptor distributions of D1 and D2 receptors, the changes these receptors have on neuron response properties, as well as identified circuitry involved in working memory. Our model suggests that D1 receptor under-stimulation in supragranular layers gates internal noise into the PFC leading to cognitive symptoms as has been proposed in attention disorders, while D2 over-stimulation gates noise into the PFC by over-activation of cortico-striatal projecting neurons in infragranular layers. We apply this model in the context of a memory-guided saccade paradigm and show deficits similar to those observed in schizophrenic patients. We also show set-shifting impairments similar to those observed in rodents with D1 and D2 receptor manipulations. We discuss how the introduction of noise through changes in D1 and D2 receptor activation may account for many of the symptoms of schizophrenia depending on where this dysfunction occurs in the PFC.

The relationship between dopamine receptor D1 and cognitive performance

BACKGROUND

Cognitive impairment cuts across traditional diagnostic boundaries and is one of the most typical symptoms in various psychiatric and neurobiological disorders.

AIMS

The objective of this study was to examine the genetic association between 94 candidate genes, including receptors and enzymes that participate in neurotransmission, with measures of cognition.

METHODS

The Clinical Dementia Rating (CDR), a global measure of cognition, and genotypes derived from a custom array of 1,536 single-nucleotide polymorphisms (SNPs) in 94 genes were available for a large postmortem cohort of Caucasian cases with Alzheimer's disease (AD), schizophrenia and controls (n=727). A cohort of healthy young males (n=1,493) originating from the LOGOS project (Learning On Genetics Of Schizophrenia Spectrum) profiled across multiple cognitive domains was available for targeted SNP genotyping. Gene expression was quantified in the superior temporal gyrus of control samples (n=109). The regulatory effect on transcriptional activity was assessed using the luciferase reporter system.

RESULTS

The rs5326-A allele at the promoter region of dopamine receptor D1 (DRD1) locus was associated with: (i) poorer cognition (higher CDR) in the postmortem cohort (P=9.325×10(-4)); (ii) worse cognitive performance relevant to strategic planning in the LOGOS cohort (P=0.008); (iii) lower DRD1 gene expression in the superior temporal gyrus of controls (P=0.038); and (iv) decreased transcriptional activity in human neuroblastoma (SH-SY5Y) cells (P=0.026).

CONCLUSIONS

An interdisciplinary approach combining genetics with cognitive and molecular neuroscience provided a possible mechanistic link among DRD1 and alterations in cognitive performance.

(-)-Stepholidine is a potent pan-dopamine receptor antagonist of both G protein- and β-arrestin-mediated signaling

RATIONALE

(-)-Stepholidine is a tetrahydroberberine alkaloid that is known to interact with dopamine receptors and has also been proposed as a novel antipsychotic agent. Its suggested novelty lies in the fact that it has been proposed to have D1-like receptor agonist and D2-like receptor antagonist properties. Thus, it might be effective in treating both positive and negative (cognition) symptoms of schizophrenia. However, its activity on specific dopamine receptor subtypes has not been clarified, especially with respect to its ability to activate D1-like receptors.

OBJECTIVES

We wished to examine the affinity and functional activity of (-)-stepholidine at each of the human dopamine receptor subtypes expressed in a defined cellular environment.

METHODS

D1-D5 dopamine receptors were stably expressed in cell lines and their interactions with (-)-stepholidine were examined using radioligand binding and various functional signaling assays. Radioligand binding assays were also performed using bovine striatal membranes.

RESULTS

(-)-Stepholidine exhibited high (nM) affinity for D1 and D5 receptors, somewhat lower (two- to four-fold) affinity for D2 and D3 receptors, and low micromolar affinity for D4 receptors. Functionally, (-)-stepholidine was ineffective in activating G protein-mediated signaling of D1-like and D2 receptors and was also ineffective in stimulating β-arrestin recruitment to any dopamine receptor subtype. It did, however, antagonize all of these responses. It also antagonized D1-D2 heteromer-mediated Ca(2+) mobilization. Radioligand binding assays of D1-like receptors in brain membranes also indicated that (-)-stepholidine binds to the D1 receptor with antagonist-like properties.

CONCLUSIONS

(-)-Stepholidine is a pan-dopamine receptor antagonist and its in vivo effects are largely mediated through dopamine receptor blockade with potential cross-talk to other receptors or signaling proteins.

Motor and behavioral phenotype in conditional mutants with targeted ablation of cortical D1 dopamine receptor-expressing cells

D1-dopamine receptors (Drd1a) are highly expressed in the deep layers of the cerebral cortex and the striatum. A number of human diseases such as Huntington disease and schizophrenia are known to have cortical pathology involving dopamine receptor expressing neurons. To illuminate their functional role, we exploited a Cre/Lox molecular paradigm to generate Emx-1(tox) MUT mice, a transgenic line in which cortical Drd1a-expressing pyramidal neurons were selectively ablated. Emx-1(tox) MUT mice displayed prominent forelimb dystonia, hyperkinesia, ataxia on rotarod testing, heightened anxiety-like behavior, and age-dependent abnormalities in a test of social interaction. The latter occurred in the context of normal working memory on testing in the Y-maze and for novel object recognition. Some motor and behavioral abnormalities in Emx-1(tox) MUT mice overlapped with those in CamKIIα(tox) MUT transgenic mice, a line in which both striatal and cortical Drd1a-expressing cells were ablated. Although Emx-1(tox) MUT mice had normal striatal anatomy, both Emx-1(tox) MUT and CamKIIα(tox) MUT mice displayed selective neuronal loss in cortical layers V and VI. This study shows that loss of cortical Drd1a-expressing cells is sufficient to produce deficits in multiple motor and behavioral domains, independent of striatal mechanisms. Primary cortical changes in the D1 dopamine receptor compartment are therefore likely to model a number of core clinical features in disorders such as Huntington disease and schizophrenia.

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

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

The neurobiology and treatment of first-episode schizophrenia

It is evident that once psychosis is present in patients with schizophrenia, the underlying biological process of the illness has already been ongoing for many years. At the time of diagnosis, patients with schizophrenia show decreased mean intracranial volume (ICV) as compared with healthy subjects. Since ICV is driven by brain growth, which reaches its maximum size at approximately 13 years of age, this finding suggests that brain development in patients with schizophrenia is stunted before that age. The smaller brain volume is expressed as decrements in both grey and white matter. After diagnosis, it is mainly the grey matter loss that progresses over time whereas white matter deficits are stable or may even improve over the course of the illness. To understand the possible causes of the brain changes in the first phase of schizophrenia, evidence from treatment studies, postmortem and neuroimaging investigations together with animal experiments needs to be incorporated. These data suggest that the pathophysiology of schizophrenia is multifactorial. Increased striatal dopamine synthesis is already evident before the time of diagnosis, starting during the at-risk mental state, and increases during the onset of frank psychosis. Cognitive impairment and negative symptoms may, in turn, result from other abnormalities, such as NMDA receptor hypofunction and low-grade inflammation of the brain. The latter two dysfunctions probably antedate increased dopamine synthesis by many years, reflecting the much earlier presence of cognitive and social dysfunction. Although correction of the hyperdopaminergic state with antipsychotic agents is generally effective in patients with a first-episode psychosis, the effects of treatments to correct NMDA receptor hypofunction or low-grade inflammation are (so far) rather modest at best. Improved efficacy of these interventions can be expected when they are applied at the onset of cognitive and social dysfunction, rather than at the onset of psychosis.

Effects of the D1 dopamine receptor agonist dihydrexidine (DAR-0100A) on working memory in schizotypal personality disorder

Pharmacological enhancement of prefrontal D1 dopamine receptor function remains a promising therapeutic approach to ameliorate schizophrenia-spectrum working memory deficits, but has yet to be rigorously evaluated clinically. This proof-of-principle study sought to determine whether the active enantiomer of the selective and full D1 receptor agonist dihydrexidine (DAR-0100A) could attenuate working memory impairments in unmedicated patients with schizotypal personality disorder (SPD). We performed a randomized, double-blind, placebo-controlled trial of DAR-0100A (15 mg/150 ml of normal saline administered intravenously over 30 min) in medication-free patients with SPD (n=16) who met the criteria for cognitive impairment (ie, scoring below the 25th percentile on tests of working memory). We employed two measures of verbal working memory that are salient to schizophrenia-spectrum cognitive deficits, and that clinical data implicate as being associated with prefrontal D1 availability: (1) the Paced Auditory Serial Addition Test (PASAT); and (2) the N-back test (ratio of 2-back:0-back scores). Study procedures occurred over four consecutive days, with working memory testing on Days 1 and 4, and DAR-0100A/placebo administration on Days 2-4. Treatment with DAR-0100A was associated with significantly improved PASAT performance relative to placebo, with a very large effect size (Cohen's d=1.14). Performance on the N-back ratio was also significantly improved; however, this effect rested on both a non-significant enhancement and diminution of 2-back and 0-back performance, respectively; therefore interpretation of this finding is more complicated. DAR-0100A was generally well tolerated, with no serious medical or psychiatric adverse events; common side effects were mild to moderate and transient, consisting mainly of sedation, lightheadedness, tachycardia, and hypotension; however, we were able to minimize these effects, without altering the dose, with supportive measures, eg, co-administered normal saline. Although preliminary, these findings lend further clinical support to the potential of D1 receptor agonists to treat schizophrenia-spectrum working memory impairments. These data suggest a need for further studies with larger group sizes, serum DAR-0100A levels, and a more comprehensive neuropsychological battery.

Phospholipase C-β1 Hypofunction in the Pathogenesis of Schizophrenia

Schizophrenia is a mental disorder that is characterized by various abnormal symptoms. Previous studies indicate decreased expression of phospholipase C-β1 (PLC-β1) in the brains of patients with schizophrenia. PLC-β1-null (PLC-β1(-/-)) mice exhibit multiple endophenotypes of schizophrenia. Furthermore, a study of PLC-β1 knockdown in the medial prefrontal cortex of mice has shown a specific behavioral deficit, impaired working memory. These results support the notion that disruption of PLC-β1-linked signaling in the brain is strongly involved in the pathogenesis of schizophrenia. In this review, we broadly investigate recent studies regarding schizophrenia-related behaviors as well as their various clinical and biological correlates in PLC-β1(-/-) and knockdown mouse models. This will provide a better understanding of the pathological relevance of the altered expression of PLC-β1 in the brains of patients with schizophrenia. Evidence accumulated will shed light on future in-depth studies, possibly in human subjects.

Social-Cognitive Deficits in Schizophrenia

Patients with schizophrenia not only suffer from prototypical psychotic symptoms such as delusions and hallucinations and from cognitive deficits, but also from tremendous deficits in social functioning. However, little is known about the interplay between the cognitive and the social-cognitive deficits in schizophrenia. Our chapter gives an overview on behavioral, as well as functional imaging studies on social cognition in schizophrenia. Main findings on cognitive and motivational deficits in schizophrenia are reviewed and introduced within the context of the dopamine hypothesis of schizophrenia. The reviewed findings suggest that disturbed "social brain" functioning in schizophrenia, depending on the specific context, can either lead to a neglect of the emotions and intentions of others or to the false attribution of these emotions and intentions in an emotionally neutral social content. We integrate these findings with the current knowledge about aberrant dopaminergic firing in schizophrenia by presenting a comprehensive model explaining core symptoms of the disorder. The main implication of the presented model is that neither cognitive-motivational, nor social-cognitive deficits alone cause schizophrenia symptoms, but that symptoms only emerge by the interplay of disturbed social brain functioning with aberrant dopaminergic firing.

Changes in the adolescent brain and the pathophysiology of psychotic disorders

Adolescence is a time of extensive neuroanatomical, functional, and chemical reorganisation of the brain which parallels substantial maturational changes in cognition and affect regulation. This period is characterised by stabilisation of synapses to diminish redundancy and increase efficiency of neural function, fine-tuning of excitatory and inhibitory neurotransmitter systems, beginning of integration between late maturing and early maturing brain structures, and development of effective connections. In effect, these so-called moving parts create a state of dynamic change that might underlie adolescent behaviours. Imbalances or changes in timing of these developmental processes clearly increase the risk for psychiatric disorders. Genetic, environmental, and epigenetic factors that shape brain development and hormonal changes that affect stress reactivity could be reasons why some, but not all, adolescents are at a heightened risk of developing a psychopathological disorder. In this Series paper, we assess the neurobiology of the changing adolescent brain, implications of this knowledge, and future research in major psychiatric disorders, particularly for psychotic disorders.

MicroRNAs in Schizophrenia: Implications for Synaptic Plasticity and Dopamine-Glutamate Interaction at the Postsynaptic Density. New Avenues for Antipsychotic Treatment Under a Theranostic Perspective

Despite dopamine-glutamate aberrant interaction that has long been considered a relevant landmark of psychosis pathophysiology, several aspects of these two neurotransmitters reciprocal interaction remain to be defined. The emerging role of postsynaptic density (PSD) proteins at glutamate synapse as a molecular "lego" making a functional hub where different signals converge may add a new piece of information to understand how dopamine-glutamate interaction may work with regard to schizophrenia pathophysiology and treatment. More recently, compelling evidence suggests a relevant role for microRNA (miRNA) as a new class of dopamine and glutamate modulators with regulatory functions in the reciprocal interaction of these two neurotransmitters. Here, we aimed at addressing the following issues: (i) Do miRNAs have a role in schizophrenia pathophysiology in the context of dopamine-glutamate aberrant interaction? (ii) If miRNAs are relevant for dopamine-glutamate interaction, at what level this modulation takes place? (iii) Finally, will this knowledge open the door to innovative diagnostic and therapeutic tools? The biogenesis of miRNAs and their role in synaptic plasticity with relevance to schizophrenia will be considered in the context of dopamine-glutamate interaction, with special focus on miRNA interaction with PSD elements. From this framework, implications both for biomarkers identification and potential innovative interventions will be considered.

Serotonin transporter availability in impulsive aggressive personality disordered patients: a PET study with [11C]DASB

Serotonin (5-HT) has consistently been implicated in the pathophysiology of impulsive aggression. In the current study, we tested the hypothesis that 5-HT transporter (5-HTT) binding is reduced in the anterior cingulate cortex (ACC) in impulsive aggressive patients. Additionally, we characterized pathological personality dimensions, with a specific focus on callousness (i.e. emotional indifference, a facet of psychopathy). Callousness is putatively positively correlated with presynaptic 5-HT, and thus could potentially confound the hypothesized negative relation between 5-HTT levels and trait aggression. We determined 5-HTT binding with positron emission tomography and [(11)C]DASB in 29 patients with intermittent explosive disorder (IED-IR) and 30 controls. We assessed group differences in 5-HTT binding in the pregenual ACC, amygdala and subcortical regions and examined correlations between 5-HTT binding and clinical measures. There were no significant differences in 5-HTT binding between IED-IR patients and controls. Trait callousness exhibited a significant, positive correlation with ACC 5-HTT availability. Among IED-IR patients, a trend-level negative partial correlation was observed between trait aggression and ACC 5-HTT availability, while covarying for callousness and age. Exploratory analyses revealed a significant negative correlation between state aggression levels and 5-HTT availability in subcortical regions, namely striatum and thalamus. We did not confirm our hypothesis of lower ACC 5-HTT availability in impulsive aggressive patients, however, the positive correlation between callousness and ACC 5-HTT availability likely played a confounding role. Subtypes of aggression (e.g., reactive vs. proactive aggression), which are differentially associated with pathological personality dimensions such as callousness, may contribute to variability between 5-HT functioning and aggression.

The 4th Schizophrenia International Research Society Conference, 5-9 April 2014, Florence, Italy: a summary of topics and trends

The 4th Schizophrenia International Research Society Conference was held in Florence, Italy, April 5-9, 2014 and this year had as its emphasis, "Fostering Collaboration in Schizophrenia Research". Student travel awardees served as rapporteurs for each oral session, summarized the important contributions of each session and then each report was integrated into a final summary of data discussed at the entire conference by topic. It is hoped that by combining data from different presentations, patterns of interest will emerge and thus lead to new progress for the future. In addition, the following report provides an overview of the conference for those who were present, but could not participate in all sessions, and those who did not have the opportunity to attend, but who would be interested in an update on current investigations ongoing in the field of schizophrenia research.

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.

The therapeutic potential of the cerebellum in schizophrenia

The cognitive role of the cerebellum is critically tied to its distributed connections throughout the brain. Accumulating evidence from anatomical, structural and functional imaging, and lesion studies advocate a cognitive network involving indirect connections between the cerebellum and non-motor areas in the prefrontal cortex. Cerebellar stimulation dynamically influences activity in several regions of the frontal cortex and effectively improves cognition in schizophrenia. In this manuscript, we summarize current literature on the cingulocerebellar circuit and we introduce a method to interrogate this circuit combining opotogenetics, neuropharmacology, and electrophysiology in awake-behaving animals while minimizing incidental stimulation of neighboring cerebellar nuclei. We propose the novel hypothesis that optogenetic cerebellar stimulation can restore aberrant frontal activity and rescue impaired cognition in schizophrenia. We focus on how a known cognitive region in the frontal cortex, the anterior cingulate, is influenced by the cerebellum. This circuit is of particular interest because it has been confirmed using tracing studies, neuroimaging reveals its role in cognitive tasks, it is conserved from rodents to humans, and diseases such as schizophrenia and autism appear in its aberrancy. Novel tract tracing results presented here provide support for how these two areas communicate. The primary pathway involves a disynaptic connection between the cerebellar dentate nuclei (DN) and the anterior cingulate cortex. Secondarily, the pathway from cerebellar fastigial nuclei (FN) to the ventral tegmental area, which supplies dopamine to the prefrontal cortex, may play a role as schizophrenia characteristically involves dopamine deficiencies. We hope that the hypothesis described here will inspire new therapeutic strategies targeting currently untreatable cognitive impairments in schizophrenia.

Potential enhancing effects of histamine H₁ agonism/H₃ antagonism on working memory assessed by performance and bold response in healthy volunteers

BACKGROUND AND PURPOSE

Schizophrenia is a highly debilitating disorder characterized by hallucinations and delusions, but also impaired cognition such as memory. While hallucinations and delusions are the main target for pharmacological treatment, cognitive impairments are rarely treated. Evidence exists that histamine has a role in the cognitive deficits in schizophrenia, which could be the basis of the development of a histamine-type treatment. Histamine H₃ antagonists have been shown to improve memory performance in experimental animals, but these effects have been little investigated in humans within the context of impaired cognition in schizophrenia and using sensitive measures of brain activity. In the present study, the effects of betahistine (H₃ antagonist/H₁ agonist) on learning and memory, and associated brain activity were assessed.

EXPERIMENTAL APPROACH

Sixteen healthy volunteers (eight female) aged between 18 and 50 years received two p.o. doses of betahistine (48 mg) or placebo separated by 30 min, on separate days according to a two-way, double-blind, crossover design. Volunteers performed an N-back working memory task and a spatial paired associates learning task while being scanned using a MRI scanner.

KEY RESULTS

Task-related activity changes in well-defined networks and performance were observed. No betahistine-induced changes in brain activity were found in these networks. Alternatively, liberal whole-brain analyses showed activity changes in areas outside task networks, like the lateral geniculate nucleus.

CONCLUSIONS AND IMPLICATIONS

Clear effects of betahistine on working memory could not be established. Future studies should use higher doses and explore the role of histamine in visual information processing.

Working memory training impacts the mean diffusivity in the dopaminergic system

Dopaminergic transmission plays a critical role in working memory (WM). Mean diffusivity (MD) is a sensitive and unique neuroimaging tool for detecting microstructural differences particularly in the areas of the dopaminergic system. Despite previous investigation of the effects of WM training (WMT) on dopamine receptor binding potentials, the effects of WMT on MD remain unknown. In this study, we investigated these effects in young adult subjects who either underwent WMT or received no intervention for 4 weeks. Before and after the intervention or no-intervention periods, subjects underwent scanning sessions in diffusion-weighted imaging to measure MD. Compared with no intervention, WMT resulted in an increase in MD in the bilateral caudate, right putamen, left dorsolateral prefrontal cortex (DLPFC), right anterior cingulate cortex (ACC), right substantia nigra, and ventral tegmental area. Furthermore, the increase in performance on WMT tasks was significantly positively correlated with the mean increase in MD in the clusters of the left DLPFC and of the right ACC. These results suggest that WMT caused microstructural changes in the regions of the dopaminergic system in a way that is usually interpreted as a reduction in neural components.

Schizotypal personality disorder: a current review

The study of schizotypal personality disorder (SPD) is important clinically, as it is understudied, challenging to treat, often under-recognized or misdiagnosed, and associated with significant functional impairment. SPD also represents an intermediate schizophrenia-spectrum phenotype, and therefore, can provide a better understanding of the genetics, pathogenesis, and treatment of related psychotic illnesses. In this review we discuss recent findings of SPD related to epidemiology and functional impairment, heritability and genetics, working memory and cognitive impairments, social-affective disturbances, and neurobiology. Additionally, we examine the challenges associated with treating patients with SPD, as well as clinical recommendations. Finally, we address future directions and areas in need of further exploration.

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.

Brain stimulation and functional imaging with fMRI and PET

The use of functional brain imaging techniques, including positron emission tomography (PET), single-photon emission computed tomography (SPECT), and functional magnetic resonance imaging (fMRI), has allowed for monitoring neuronal and neurochemical activities in the living human brain and identifying abnormal changes in various neurological and psychiatric diseases. Combining these methods with techniques such as deep brain stimulation (DBS) and transcranial magnetic stimulation (TMS) has greatly advanced our understanding of the effects of such treatment on brain activity at targeted regions as well as specific disease-related networks. Indeed, recent network-level analysis focusing on inter-regional covarying activities in data interpretation has unveiled several key mechanisms underlying the therapeutic effects of brain stimulation. However, non-negligible discrepancies have been reported in the literature, attributable in part to the heterogeneity of both imaging and brain stimulation techniques. This chapter summarizes recent studies that combine brain imaging and brain stimulation, and includes discussion of future direction in these lines of research.

The role of prefrontal catecholamines in attention and working memory

While much progress has been made in identifying the brain regions and neurochemical systems involved in the cognitive processes disrupted in mental illnesses, to date, the level of detail at which neurobiologists can describe the chain of events giving rise to cognitive functions is very rudimentary. Much of the intense interest in understanding cognitive functions is motivated by the hope that it might be possible to understand these complex functions at the level of neurons and neural circuits. Here, we review the current state of the literature regarding how modulations in catecholamine levels within the prefrontal cortex (PFC) alter the neuronal and behavioral correlates of cognitive functions, particularly attention and working memory.

Reviewing the ketamine model for schizophrenia

The observation that antagonists of the N-methyl-D-aspartate receptor (NMDAR), such as phencyclidine (PCP) and ketamine, transiently induce symptoms of acute schizophrenia had led to a paradigm shift from dopaminergic to glutamatergic dysfunction in pharmacological models of schizophrenia. The glutamate hypothesis can explain negative and cognitive symptoms of schizophrenia better than the dopamine hypothesis, and has the potential to explain dopamine dysfunction itself. The pharmacological and psychomimetic effects of ketamine, which is safer for human subjects than phencyclidine, are herein reviewed. Ketamine binds to a variety of receptors, but principally acts at the NMDAR, and convergent genetic and molecular evidence point to NMDAR hypofunction in schizophrenia. Furthermore, NMDAR hypofunction can explain connectional and oscillatory abnormalities in schizophrenia in terms of both weakened excitation of inhibitory γ-aminobutyric acidergic (GABAergic) interneurons that synchronize cortical networks and disinhibition of principal cells. Individuals with prenatal NMDAR aberrations might experience the onset of schizophrenia towards the completion of synaptic pruning in adolescence, when network connectivity drops below a critical value. We conclude that ketamine challenge is useful for studying the positive, negative, and cognitive symptoms, dopaminergic and GABAergic dysfunction, age of onset, functional dysconnectivity, and abnormal cortical oscillations observed in acute schizophrenia.

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.

Treating working memory deficits in schizophrenia: a review of the neurobiology

Cognitive deficits are a core feature of schizophrenia. Among these deficits, working memory impairment is considered a central cognitive impairment in schizophrenia. The prefrontal cortex, a region critical for working memory performance, has been demonstrated as a critical liability region in schizophrenia. As yet, there are no standardized treatment options for working memory deficits in schizophrenia. In this review, we summarize the neuronal basis for working memory impairment in schizophrenia, including dysfunction in prefrontal signaling pathways (e.g., γ-aminobutyric acid transmission) and neural network synchrony (e.g., gamma/theta oscillations). We discuss therapeutic strategies for working memory dysfunction such as pharmacological agents, cognitive remediation therapy, and repetitive transcranial magnetic stimulation. Despite the drawbacks of current approaches, the advances in neurobiological and translational treatment strategies suggest that clinical application of these methods will occur in the near future.

Schizophrenia: from dopaminergic to glutamatergic interventions

Schizophrenia might be considered a neurodevelopmental disease. However, the fundamental process(es) associated with this disease remain(s) uncertain. Many lines of evidence suggest that schizophrenia is associated with excessive stimulation of dopamine D2 receptors in the associative striatum, with a lack of stimulation of dopamine D1 receptors in prefrontal cortex, and with modifications in prefrontal neuronal connectivity involving glutamate transmission at N-methyl aspartate (NMDA) receptors. This article, whilst briefly discussing the current knowledge of the disease, mainly concentrates on the NMDA hypofunction hypothesis. However, there are also potential consequences for a Dopamine imbalance on NMDA function. Thus, it is proposed that schizophrenia has a complex aetiology associated with strongly interconnected aberrations of dopamine and glutamate transmission.

Common polymorphisms in dopamine-related genes combine to produce a 'schizophrenia-like' prefrontal hypoactivity

Individual changes in dopamine-related genes influence prefrontal activity during cognitive-affective processes; however, the extent to which common genetic variations combine to influence prefrontal activity is unknown. We assessed catechol-O-methyltransferase (COMT) Val108/158Met (rs4680) and dopamine D2 receptor (DRD2) G-T (rs2283265) single nucleotide polymorphisms and functional magnetic resonance imaging during an emotional response inhibition test in 43 healthy adults and 27 people with schizophrenia to determine the extent to which COMT Val108/158Met and DRD2 G-T polymorphisms combine to influence prefrontal response to cognitive-affective challenges. We found an increased number of cognitive-deficit risk alleles in these two dopamine-regulating genes predict reduced prefrontal activation during response inhibition in healthy adults, mimicking schizophrenia-like prefrontal hypoactivity. Our study provides evidence that functionally related genes can combine to produce a disease-like endophenotype.

Impaired working memory by repeated neonatal MK-801 treatment is ameliorated by galantamine in adult rats

Early life blockade of the NMDA receptor by using MK-801, a non-competitive NMDA receptor antagonist, induces behavioral changes that mimic several features of schizophrenia. In the current study, we first examined the effects of neonatal MK-801 treatment in male Sprague-Dawley rats on locomotor activity, prepulse inhibition and spatial working memory in adolescence (postnatal day 35, PND35) and adulthood (PND63). Next, we investigated the effects of an acetylcholinesterase inhibitor, galantamine, on working memory deficits induced by MK-801 treatment. Rats were treated with either saline or MK-801 (0.25mg/kg twice daily) at PND 5-14, and the long-term behavioral effects were investigated. MK-801 treated rats showed moderate working memory impairments in adolescence but a pronounced deficit in adulthood. However, locomotion and prepulse inhibition at two life stages were not affected by this treatment. Systemic administration of galantamine (1mg/kg) 30 min before each training session significantly improved neonatal MK-801-induced working memory deficits in adulthood. In conclusion, these results suggest that the neonatal MK-801 treatment-induced selective working memory deficit is related to a change in brain cholinergic systems.

Memory deficits in aging and neurological diseases

Memory is central to our ability to perform daily life activities and correctly function in society. Improvements in public health and medical treatment for a variety of diseases have resulted in longer life spans; however, age-related memory impairments have been significant sources of morbidity. Loss in memory function is not only associated with aging population but is also a feature of neurodegenerative diseases such as Alzheimer's disease and other psychiatric and neurological disorders. Here, we focus on current understanding of the impact of normal aging on memory and what is known about its mechanisms, and further review pathological mechanisms behind the cause of dementia in Alzheimer's disease. Finally, we discuss schizophrenia and look into abnormalities in circuit function and neurotransmitter systems that contribute to memory impairment in this illness.

Problems and solutions to filling the drying drug pipeline for psychiatric disorders: a report from the inaugural 2012 CINP Think Tank

The inaugural Collegium Internationale Neuro-Psychopharmacologicum (CINP) Think Tank, a small open meeting sponsored by the CINP, discussed impediments to developing new drugs for psychiatric disorders and approaches to overcome these impediments. Whilst neuropsycharmacology has a rich pharmacopeia (current treatments benefiting many individuals), issues of treatment resistance, sub-optimal response and unwanted side effects remain problematic. Many scientific, economic and social issues are impeding the development of drugs (e.g. higher risk of failure, placebo effects, problematic regulatory environments, pressures imposed by patent protection, downward pressure on reimbursements and financial, legal and social risk aversion). A consensus of the meeting was that efforts to understanding the core pathophysiology of psychiatric disorders are fundamental to increasing the chance of developing new drugs. However, findings from disorders such as Huntington's chorea, have shown that knowing the cause of a disorder may not reveal new drug targets. By contrast, clinically useful biomarkers that define target populations for new drugs and models that allow findings to be accurately translated from animals to humans will increase the likelihood of developing new drugs. In addition, a greater accent on experimental medicine, creative clinical investigations and improved communication between preclinical neuropsychopharmacologists, clinicians committed to neuropsychopharmacological research, industry and the regulators would also be a driver to the development of new treatments. Finally, it was agreed that the CINP must continue its role as a conduit facilitating vibrant interactions between industry and academia as such communications are a central component in identifying new drug targets, developing new drugs and transitioning new drugs into the clinic.

Imaging glutamate in schizophrenia: review of findings and implications for drug discovery

Currently, all treatments for schizophrenia (SCZ) function primarily by blocking D(2)-type dopamine receptors. Given the limitations of these medications, substantial efforts have been made to identify alternative neurochemical targets for treatment development in SCZ. One such target is brain glutamate. The objective of this article is to review and synthesize the proton magnetic resonance spectroscopy ((1)H MRS) and positron emission tomography (PET)/single-photon emission computed tomography (SPECT) investigations that have examined glutamatergic indices in SCZ, including those of modulatory compounds such as glutathione (GSH) and glycine, as well as data from ketamine challenge studies. The reviewed (1)H MRS and PET/SPECT studies support the theory of hypofunction of the N-methyl-D-aspartate receptor (NMDAR) in SCZ, as well as the convergence between the dopamine and glutamate models of SCZ. We also review several advances in MRS and PET technologies that have opened the door for new opportunities to investigate the glutamate system in SCZ and discuss some ways in which these imaging tools can be used to facilitate a greater understanding of the glutamate system in SCZ and the successful and efficient development of new glutamate-based treatments for SCZ.

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.

Genetic association between the dopamine D1-receptor gene and paranoid schizophrenia in a northern Han Chinese population

OBJECTIVE

Dysregulation of dopaminergic neurotransmission at the D1 receptor in the prefrontal cortex has been implicated in the pathogenesis of schizophrenia. Genetic polymorphisms of the dopamine D1-receptor gene have a plausible role in modulating the risk of schizophrenia. To determine the role of DRD1 genetic polymorphisms as a risk factor for schizophrenia, we undertook a case-control study to look for an association between the DRD1 gene and schizophrenia.

MATERIALS AND METHODS

We genotyped eleven single-nucleotide polymorphisms within the DRD1 gene by deoxyribonucleic acid sequencing involving 173 paranoid schizophrenia patients and 213 unrelated healthy individuals. Statistical analysis was performed to identify the difference of genotype, allele, or haplotype distribution between cases and controls.

RESULTS

A significantly lower risk of paranoid schizophrenia was associated with the AG + GG genotype of rs5326 and the AG + GG genotype of rs4532 compared to the AA genotype and the AA genotype, respectively. Distribution of haplotypes was no different between controls and paranoid schizophrenia patients. In the males, the genotype distribution of rs5326 was statistically different between cases and controls. In the females, the genotype distribution of rs4532 was statistically different between cases and controls. However, the aforementioned statistical significances were lost after Bonferroni correction.

CONCLUSION

It is unlikely that DRD1 accounts for a substantial proportion of the genetic risk for schizophrenia. As an important dopaminergic gene, DRD1 may contribute to schizophrenia by interacting with other genes, and further relevant studies are warranted.

Computational neuropsychiatry - schizophrenia as a cognitive brain network disorder

Computational modeling of functional brain networks in fMRI data has advanced the understanding of higher cognitive function. It is hypothesized that functional networks mediating higher cognitive processes are disrupted in people with schizophrenia. In this article, we review studies that applied measures of functional and effective connectivity to fMRI data during cognitive tasks, in particular working memory fMRI studies. We provide a conceptual summary of the main findings in fMRI data and their relationship with neurotransmitter systems, which are known to be altered in individuals with schizophrenia. We consider possible developments in computational neuropsychiatry, which are likely to further our understanding of how key functional networks are altered in schizophrenia.

Expression of 5-HT2A receptors in prefrontal cortex pyramidal neurons projecting to nucleus accumbens. Potential relevance for atypical antipsychotic action

The prefrontal cortex (PFC) is involved in higher brain functions altered in schizophrenia. Classical antipsychotic drugs modulate information processing in cortico-limbic circuits via dopamine D2 receptor blockade in nucleus accumbens (NAc) whereas atypical antipsychotic drugs preferentially target cortical serotonin (5-HT) receptors. The brain networks involved in the therapeutic action of atypical drugs are not fully understood. Previous work indicated that medial PFC (mPFC) pyramidal neurons projecting to ventral tegmental area express 5-HT2A receptors suggesting that atypical antipsychotic drugs modulate dopaminergic activity distally, via 5-HT2A receptor (5-HT2A-R) blockade in PFC. Since the mPFC also projects heavily to NAc, we examined whether NAc-projecting pyramidal neurons also express 5-HT2A-R. Using a combination of retrograde tracing experiments and in situ hybridization we report that a substantial proportion of mPFC-NAc pyramidal neurons in rat brain express 5-HT2A-R mRNA in a layer- and area-specific manner (up to 68% in layer V of contralateral cingulate). The functional relevance of 5-HT2A-R to modulate mPFC-NAc projections was examined in dual-probe microdialysis experiments. The application of the preferential 5-HT2A-R agonist DOI into mPFC enhanced glutamate release locally (+66 ± 18%) and in NAc (+74 ± 12%) indicating that cortical 5-HT2A-R activation augments glutamatergic transmission in NAc. Since NAc integrates glutamatergic and dopaminergic inputs, blockade of 5-HT2A-R by atypical drugs may reduce cortical excitatory inputs onto GABAergic neurons of NAc, adding to dopamine D2 receptor blockade. Together with previous observations, the present results suggest that atypical antipsychotic drugs may control the activity of the mesolimbic pathway at cell body and terminal level.

Schizotypal personality disorder

Early phenomenological descriptions of schizophrenia have acknowledged the existence of milder schizophrenia spectrum disorders characterized by the presence of attenuated symptoms typically present in chronic schizophrenia. The investigation of the schizophrenia spectrum disorders offers an opportunity to elucidate the pathophysiological mechanisms giving rise to schizophrenia. Differences and similarities between subjects with schizotypal personality disorder (SPD), the prototypical schizophrenia personality disorder, and chronic schizophrenia have been investigated with genetic, neurochemical, imaging, and pharmacological techniques. Patients with SPD and the more severely ill patients with chronic schizophrenia share cognitive, social, and attentional deficits hypothesized to result from common neurodevelopmentally based cortical temporal and prefrontal pathology. However, these deficits are milder in SPD patients due to their capacity to recruit other related brain regions to compensate for dysfunctional areas. Individuals with SPD are also less vulnerable to psychosis due to the presence of protective factors mitigating subcortical DA hyperactivity. Given the documented close relationship to other schizophrenic disorders, SPD will be included in the psychosis section of DSM-5 as a schizophrenia spectrum disorder as well as in the personality disorder section.

Localized infusions of the partial alpha 7 nicotinic receptor agonist SSR180711 evoke rapid and transient increases in prefrontal glutamate release

The ability of local infusions of the alpha 7 nicotinic acetycholine receptor (α7 nAChR) partial agonist SSR180711 to evoke glutamate release in prefrontal cortex was determined in awake rats using a microelectrode array. Infusions of SSR180711 produced dose-dependent increases in glutamate levels. The lower dose (1.0μg in 0.4μL) evoked a rapid rise (∼1.0s) in glutamate (1.41±0.30μM above baseline). The higher dose (5.0μg) produced a similarly rapid, yet larger increase (3.51±0.36μM above baseline). After each dose, the glutamate signal was cleared to basal levels within 7-18s. SSR180711-evoked glutamate was mediated by the α7 nAChR as co-infusion of the selective α7 nAChR antagonist α-bungarotoxin (10.0μM)+SSR1808711 (5.0μg) reduced the effect of 5.0μg alone by 87% (2.62 vs. 0.35μM). Finally, the clearance of the SSR180711 (5.0μg)-evoked glutamate was bidirectionally affected by drugs that inhibited (threo-beta-benzyl-oxy-aspartate (TβOA), 100.0μM) or facilitated (ceftriaxalone, 200mg/kg, i.p.) excitatory amino acid transporters. TβOA slowed both the clearance (s) and rate of clearance (μM/s) by 10-fold, particularly at the mid-late stages of the return to baseline. Ceftriaxone reduced the magnitude of the SSR180711-evoked increase by 65%. These results demonstrate that pharmacological stimulation of α7 nAChRs within the prefrontal cortex is sufficient to evoke rapid yet transient increases in glutamate levels. Such increases may underlie the cognition-enhancing effects of the drug in animals; further justifying studies on the use of α7 nAChR-positive modulators in treating cognition-impairing disorders in humans.

Impaired prefrontal-basal ganglia functional connectivity and substantia nigra hyperactivity in schizophrenia

BACKGROUND

The theory that prefrontal cortex (PFC) dysfunction in schizophrenia leads to excess subcortical dopamine has generated widespread interest because it provides a parsimonious account for two core features of schizophrenia, cognitive deficits and psychosis, respectively. However, there has been limited empirical validation of this model. Moreover, the identity of the specific subcortical brain regions and circuits that may be impaired as a result of PFC dysfunction and mediate its link to psychosis in schizophrenia remains unclear. We undertook this event-related functional magnetic resonance imaging study to test the hypothesis that PFC dysfunction is associated with altered function of and connectivity with dopamine regulating regions of the basal ganglia.

METHODS

Eighteen individuals with schizophrenia or schizoaffective disorder and 19 healthy control participants completed event-related functional magnetic resonance imaging during working memory. We conducted between-group contrasts of task-evoked, univariate activation maps to identify regions of altered function in schizophrenia. We also compared the groups on the level of functional connectivity between a priori identified PFC and basal ganglia regions to determine if prefrontal disconnectivity in patients was present.

RESULTS

We observed task-evoked hyperactivity of the substantia nigra that occurred in association with prefrontal and striatal hypoactivity in the schizophrenia group. The magnitude of prefrontal functional connectivity with these dysfunctional basal ganglia regions was decreased in the schizophrenia group. Additionally, the level of nigrostriatal functional connectivity predicted the level of psychosis.

CONCLUSIONS

These results suggest that functional impairments of the prefrontal striatonigral circuit may be a common pathway linking the pathogenesis of cognitive deficits and psychosis in schizophrenia.

PET neuroimaging of extrastriatal dopamine receptors and prefrontal cortex functions

The role of prefrontal dopamine D1 receptors in prefrontal cortex (PFC) functions, including working memory, is widely investigated. However, human (healthy volunteers and schizophrenia patients) positron emission tomography (PET) studies about the relationship between prefrontal D1 receptors and PFC functions are somewhat inconsistent. We argued that several factors including an inverted U-shaped relationship between prefrontal D1 receptors and PFC functions might be responsible for these inconsistencies. In contrast to D1 receptors, relatively less attention has been paid to the role of D2 receptors in PFC functions. Several animal and human pharmacological studies have reported that the systemic administration of D2 receptor agonist/antagonist modulates PFC functions, although those studies do not tell us which region(s) is responsible for the effect. Furthermore, while prefrontal D1 receptors are primarily involved in working memory, other PFC functions such as set-shifting seem to be differentially modulated by dopamine. PET studies of extrastriatal D2 receptors including ours suggested that orchestration of prefrontal dopamine transmission and hippocampal dopamine transmission might be necessary for a broad range of normal PFC functions. In order to understand the complex effects of dopamine signaling on PFC functions, measuring a single index related to basic dopamine tone is not sufficient. For a better understanding of the meanings of PET indices related to neurotransmitters, comprehensive information (presynaptic, postsynaptic, and beyond receptor signaling) will be required. Still, an interdisciplinary approach combining molecular imaging techniques with cognitive neuroscience and clinical psychiatry will provide new perspectives for understanding the neurobiology of neuropsychiatric disorders and their innovative drug developments.

Role of concomitant inhibition of the norepinephrine transporter for the antipsychotic effect of quetiapine

Quetiapine alleviates both positive and negative symptoms as well as certain cognitive impairments in schizophrenia despite a low D2 receptor occupancy and may also be used as monotherapy in bipolar and major depressive disorder. The mechanisms underlying the broad clinical utility of quetiapine remain to be clarified, but may be related to the potent inhibition of the norepinephrine transporter (NET) by norquetiapine, the major metabolite of quetiapine in humans. Since norquetiapine is not formed in rodents we here investigated in rats whether NET-inhibition may, in principle, contribute to the clinical effectiveness of quetiapine and allow for its low D2 receptor occupancy, by combining quetiapine with the selective NET-inhibitor reboxetine. Antipsychotic-like activity was assessed using the conditioned avoidance response (CAR) test, dopamine output in the medial prefrontal cortex (mPFC) and the nucleus accumbens was measured using in vivo microdialysis, and NMDA receptor-mediated transmission was measured using intracellular electrophysiological recordings in pyramidal cells of the mPFC in vitro. Adjunct reboxetine potentiated the suppression of CAR by quetiapine. Moreover, concomitant administration of quetiapine and reboxetine resulted in a synergistic increase in cortical, but not accumbal, dopamine output. The combination of low, clinically relevant concentrations of quetiapine (60 nM) and reboxetine (20 nM) markedly facilitated cortical NMDA receptor-mediated transmission in contrast to either drug alone, an effect that could be inhibited by the D₁ receptor antagonist SCH23390. We conclude that concomitant NET-inhibition by norquetiapine may contribute to the overall antipsychotic effectiveness of quetiapine in spite of its relatively low level of D₂ occupancy.

In vivo binding of the dopamine-1 receptor PET tracers [¹¹C]NNC112 and [¹¹C]SCH23390: a comparison study in individuals with schizophrenia

RATIONALE

A deficit in dopamine-1 (D1) receptor function in the prefrontal cortex is suggested to play a role in the cognitive dysfunction observed in patients with schizophrenia. However, the results from positron emission tomography imaging studies of D1 receptor levels in individuals with schizophrenia are mixed.

OBJECTIVES

The aim of this investigation was to determine whether the in vivo characteristics of the different D1 receptor tracers used in previous reports, [(11)C]SCH23390 and [(11)C]NNC112, may have contributed to these discrepancies reported in the literature.

METHODS

Eight patients with schizophrenia and 12 healthy control subjects were scanned with both [(11)C]SCH23390 and [(11)C]NNC112.

RESULTS

[(11)C]SCH23390 and [(11)C]NNC112 binding potentials in both patients and control subjects were compared and no tracer by diagnosis interactions were observed.

CONCLUSIONS

The results of this study suggest that differences in the binding of [(11)C]SCH23390 and [(11)C]NNC112 observed in previous studies are not due to differences in the in vivo behavior of these tracers.

Memory deficits in schizophrenia: a selective review of functional magnetic resonance imaging (FMRI) studies

Schizophrenia is a complex chronic mental illness that is characterized by positive, negative and cognitive symptoms. Cognitive deficits are most predictive of long-term outcomes, with abnormalities in memory being the most robust finding. The advent of functional magnetic resonance imaging (fMRI) has allowed exploring neural correlates of memory deficits in vivo. In this article, we will give a selective review of fMRI studies probing brain regions and functional networks that are thought to be related to abnormal memory performance in two memory systems prominently affected in schizophrenia; working memory and episodic memory. We revisit the classic "hypofrontality" hypothesis of working memory deficits and explore evidence for frontotemporal dysconnectivity underlying episodic memory abnormalities. We conclude that fMRI studies of memory deficits in schizophrenia are far from universal. However, the current literature does suggest that alterations are not isolated to a few brain regions, but are characterized by abnormalities within large-scale brain networks.

Detection of cell surface dopamine receptors

Dopamine receptors are a class of metabotropic G protein-coupled receptors. Plasma membrane expression is a key determinant of receptor signaling, and one that is regulated both by extra and intracellular cues. Abnormal dopamine receptor signaling is implicated in several neuropsychiatric disorders, including schizophrenia and attention deficit hyperactivity disorder, as well as drug abuse. Here, we describe in detail the application of two complementary applications of protein biotinylation and enzyme-linked immunoabsorbent assay (ELISA) for detecting and quantifying levels of dopamine receptors expressed on the cell surface. In the biotinylation method, cell surface receptors are labeled with Sulfo-NHS-biotin. The charge on the sulfonyl facilitates water solubility of the reactive biotin compound and prevents its diffusion across the plasma membrane. In the ELISA method, surface labeling is achieved with antibodies specific to extracellular epitopes on the receptors, and by fixing the cells without detergent such that the plasma membrane remains intact.