The regulation of subcortical dopamine systems by the prefrontal cortex: interactions of central dopamine systems and the pathogenesis of schizophrenia

Augmentation therapy with serotonin1A receptor partial agonists on neurocognitive function in schizophrenia: A systematic review and meta-analysis

Background

In a previous meta-analysis, the use of serotonin1A(5-HT1A) receptor partial agonists of the azapirone class as an add-on therapy was associated with beneficial effects on positive symptoms and attention/processing speed in schizophrenia patients. This meta-analysis builds on that study by examining the effects of adjunctive treatment with 5-HT1A partial agonists in improving other domains of neurocognitive function in schizophrenia patients.

Methods

A literature search was performed from 1987 to May 2023 to identify randomized controlled trials. The standardized mean difference (SMD) with 95 % confidence intervals (CI) was calculated when there were two or more studies. Four studies, involving 313 patients, met the inclusion criteria and were used in the analysis.

Results

5-HT1A partial agonists (buspirone or tandospirone) did not have a significant effect on verbal learning (SMD = 0.08, 95 % CI = -0.31 to 0.47) or working memory (SMD = 0.15, 95 % CI = -0.09 to 0.39). Regarding executive functions (Wisconsin Card Sorting Test), positive but non-significant results were seen with the category number (SMD = 0.26, 95 % CI = -0.81 to 1.32), while non-significant effects were noted for percent preservation errors (SMD = -0.10, 95 % CI = -0.53 to 0.33).

Conclusions

The absence of any significant benefits in the cognitive domains studied here may have been due to the variance in the concomitant medication (typical vs atypical antipsychotic drugs), the level of cognition at baseline, or other factors. Further studies with various types of 5-HT1A agonists are warranted to examine the potential cognitive efficacy of stimulating these receptors.

The Relationship Between Frontostriatal Connectivity and Striatal Dopamine Function in Schizophrenia: An 18F-DOPA PET and Diffusion Tensor Imaging Study in Treatment Responsive and Resistant Patients

OBJECTIVE

Striatal dopamine dysfunction caused by cortical abnormalities is a leading hypothesis of schizophrenia. Although prefrontal cortical pathology is negatively correlated with striatal dopamine synthesis, the relationship between structural frontostriatal connectivity and striatal dopamine synthesis has not been proved in patients with schizophrenia with different treatment response. We therefore investigated the relationship between frontostriatal connectivity and striatal dopamine synthesis in treatment-responsive schizophrenia (non-TRS) and compared them to treatment-resistant schizophrenia (TRS) and healthy controls (HC).

METHODS

Twenty-four patients with schizophrenia and twelve HC underwent [18F] DOPA PET scans to measure dopamine synthesis capacity (the influx rate constant Kicer) and diffusion 3T MRI to measure structural connectivity (fractional anisotropy, FA). Connectivity was assessed in 2 major frontostriatal tracts. Associations between Kicer and FA in each group were evaluated using Spearman's rho correlation coefficients.

RESULTS

Non-TRS showed a negative correlation (r=-0.629, p=0.028) between connectivity of dorsolateral prefrontal cortex-associative striatum (DLPFC-AST) and dopamine synthesis capacity of associative striatum but this was not evident in TRS (r=-0.07, p=0.829) and HC (r=-0.277, p=0.384).

CONCLUSION

Our findings are consistent with the hypothesis of dysregulation of the striatal dopaminergic system being related to prefrontal cortex pathology localized to connectivity of DLPFC-AST in non-TRS, and also extend the hypothesis to suggest that different mechanisms underlie the pathophysiology of non-TRS and TRS.

The relationship between grey matter volume and striatal dopamine function in psychosis: a multimodal 18F-DOPA PET and voxel-based morphometry study

A leading hypothesis for schizophrenia and related psychotic disorders proposes that cortical brain disruption leads to subcortical dopaminergic dysfunction, which underlies psychosis in the majority of patients who respond to treatment. Although supported by preclinical findings that prefrontal cortical lesions lead to striatal dopamine dysregulation, the relationship between prefrontal structural volume and striatal dopamine function has not been tested in people with psychosis. We therefore investigated the in vivo relationship between striatal dopamine synthesis capacity and prefrontal grey matter volume in treatment-responsive patients with psychosis, and compared them to treatment non-responsive patients, where dopaminergic mechanisms are not thought to be central. Forty patients with psychosis across two independent cohorts underwent ¹⁸F-DOPA PET scans to measure dopamine synthesis capacity (indexed as the influx rate constant K_i^cer) and structural 3T MRI. The PET, but not MR, data have been reported previously. Structural images were processed using DARTEL-VBM. GLM analyses were performed in SPM12 to test the relationship between prefrontal grey matter volume and striatal K_i^cer. Treatment responders showed a negative correlation between prefrontal grey matter and striatal dopamine synthesis capacity, but this was not evident in treatment non-responders. Specifically, we found an interaction between treatment response, whole striatal dopamine synthesis capacity and grey matter volume in left (pFWE corr. = 0.017) and right (pFWE corr. = 0.042) prefrontal cortex. We replicated the finding in right prefrontal cortex in the independent sample (pFWE corr. = 0.031). The summary effect size was 0.82. Our findings are consistent with the long-standing hypothesis of dysregulation of the striatal dopaminergic system being related to prefrontal cortex pathology in schizophrenia, but critically also extend the hypothesis to indicate it can be applied to treatment-responsive schizophrenia only. This suggests that different mechanisms underlie the pathophysiology of treatment-responsive and treatment-resistant schizophrenia.

The Pharmacology of Visual Hallucinations in Synucleinopathies

Visual hallucinations (VH) are commonly found in the course of synucleinopathies like Parkinson's disease and dementia with Lewy bodies. The incidence of VH in these conditions is so high that the absence of VH in the course of the disease should raise questions about the diagnosis. VH may take the form of early and simple phenomena or appear with late and complex presentations that include hallucinatory production and delusions. VH are an unmet treatment need. The review analyzes the past and recent hypotheses that are related to the underlying mechanisms of VH and then discusses their pharmacological modulation. Recent models for VH have been centered on the role played by the decoupling of the default mode network (DMN) when is released from the control of the fronto-parietal and salience networks. According to the proposed model, the process results in the perception of priors that are stored in the unconscious memory and the uncontrolled emergence of intrinsic narrative produced by the DMN. This DMN activity is triggered by the altered functioning of the thalamus and involves the dysregulated activity of the brain neurotransmitters. Historically, dopamine has been indicated as a major driver for the production of VH in synucleinopathies. In that context, nigrostriatal dysfunctions have been associated with the VH onset. The efficacy of antipsychotic compounds in VH treatment has further supported the notion of major involvement of dopamine in the production of the hallucinatory phenomena. However, more recent studies and growing evidence are also pointing toward an important role played by serotonergic and cholinergic dysfunctions. In that respect, in vivo and post-mortem studies have now proved that serotonergic impairment is often an early event in synucleinopathies. The prominent cholinergic impairment in DLB is also well established. Finally, glutamatergic and gamma aminobutyric acid (GABA)ergic modulations and changes in the overall balance between excitatory and inhibitory signaling are also contributing factors. The review provides an extensive overview of the pharmacology of VH and offers an up to date analysis of treatment options.

Organization of afferents to the orbitofrontal cortex in the rat

The prefrontal cortex (PFC) is usually defined as the frontal cortical area receiving a mediodorsal thalamic (MD) innervation. Certain areas in the medial wall of the rat frontal area receive a MD innervation. A second frontal area that is the target of MD projections is located dorsal to the rhinal sulcus and often referred to as the orbitofrontal cortex (OFC). Both the medial PFC and OFC are comprised of a large number of cytoarchitectonic regions. We assessed the afferent innervation of the different areas of the OFC, with a focus on projections arising from the mediodorsal thalamic nucleus, the basolateral nucleus of the amygdala, and the midbrain dopamine neurons. Although there are specific inputs to various OFC areas, a simplified organizational scheme could be defined, with the medial areas of the OFC receiving thalamic inputs, the lateral areas of the OFC being the recipient of amygdala afferents, and a central zone that was the target of midbrain dopamine neurons. Anterograde tracer data were consistent with this organization of afferents, and revealed that the OFC inputs from these three subcortical sites were largely spatially segregated. This spatial segregation suggests that the central portion of the OFC (pregenual agranular insular cortex) is the only OFC region that is a prefrontal cortical area, analogous to the prelimbic cortex in the medial prefrontal cortex. These findings highlight the heterogeneity of the OFC, and suggest possible functional attributes of the three different OFC areas.

Imaging Genetics and Genomics in Psychiatry: A Critical Review of Progress and Potential

Imaging genetics and genomics research has begun to provide insight into the molecular and genetic architecture of neural phenotypes and the neural mechanisms through which genetic risk for psychopathology may emerge. As it approaches its third decade, imaging genetics is confronted by many challenges, including the proliferation of studies using small sample sizes and diverse designs, limited replication, problems with harmonization of neural phenotypes for meta-analysis, unclear mechanisms, and evidence that effect sizes may be more modest than originally posited, with increasing evidence of polygenicity. These concerns have encouraged the field to grow in many new directions, including the development of consortia and large-scale data collection projects and the use of novel methods (e.g., polygenic approaches, machine learning) that enhance the quality of imaging genetic studies but also introduce new challenges. We critically review progress in imaging genetics and offer suggestions and highlight potential pitfalls of novel approaches. Ultimately, the strength of imaging genetics and genomics lies in their translational and integrative potential with other research approaches (e.g., nonhuman animal models, psychiatric genetics, pharmacologic challenge) to elucidate brain-based pathways that give rise to the vast individual differences in behavior as well as risk for psychopathology.

Multimodal connectivity-based parcellation reveals a shell-core dichotomy of the human nucleus accumbens

The subdifferentiation of the nucleus accumbens (NAc) has been extensively studied using neuroanatomy and histochemistry, yielding a well-accepted dichotomic shell/core architecture that reflects dissociable roles, such as in reward and aversion, respectively. However, in vivo parcellation of these structures in humans has been rare, potentially impairing future research into the structural and functional characteristics and alterations of putative NAc subregions. Here, we used three complementary parcellation schemes based on tractography, task-independent functional connectivity, and task-dependent co-activation to investigate the regional differentiation within the NAc. We found that a 2-cluster solution with shell-like and core-like subdivisions provided the best description of the data and was consistent with the earlier anatomical shell/core architecture. The consensus clusters from this optimal solution, which was based on the three schemes, were used as the final parcels for the subsequent connection analyses. The resulting connectivity patterns presented inter-hemispheric symmetry, convergence and divergence across the modalities, and, most importantly, clearly distinct patterns between the two subregions. This convergent connectivity patterns also confirmed the connections in animal models, supporting views that the two subregions could have antagonistic roles in some circumstances. Finally, the identified parcels should be helpful in further neuroimaging studies of the NAc. Hum Brain Mapp 38:3878-3898, 2017. © 2017 Wiley Periodicals, Inc.

Maternal Malnutrition in the Etiopathogenesis of Psychiatric Diseases: Role of Polyunsaturated Fatty Acids

Evidence from human studies indicates that maternal metabolic state and malnutrition dramatically influence the risk for developing psychiatric complications in later adulthood. In this regard, the central role of polyunsaturated fatty acids (PUFAs), and particularly n-3 PUFAs, is emerging considering that epidemiological evidences have established a negative correlation between n-3 PUFA consumption and development of mood disorders. These findings were supported by clinical studies indicating that low content of n-3 PUFAs in diet is linked to an increased susceptibility to psychiatric disorders. PUFAs regulate membrane fluidity and exert their central action by modulating synaptogenesis and neurotrophic factor expression, neurogenesis, and neurotransmission. Moreover, they are precursors of molecules implicated in modulating immune and inflammatory processes in the brain. Importantly, their tissue concentrations are closely related to diet intake, especially to maternal consumption during embryonal life, considering that their synthesis from essential precursors has been shown to be inefficient in mammals. The scope of this review is to highlight the possible mechanisms of PUFA functions in the brain during pre- and post-natal period and to evaluate their role in the pathogenesis of psychiatric diseases.

Sex Differences Distinguish Intracortical Glutamate Receptor-Mediated Regulation of Extracellular Dopamine Levels in the Prefrontal Cortex of Adult Rats

Executive functions of the prefrontal cortex (PFC) are sensitive to local dopamine (DA) levels. Although sex differences distinguish these functions and their dysfunction in disease, the basis for this is unknown. We asked whether sex differences might result from dimorphisms in the glutamatergic mechanisms that regulate PFC DA levels. Using antagonists selective for α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and N-methyl-d-aspartate (NMDA) receptors, we compared drug effects on in vivo microdialysis DA measurements in the PFC of adult male and female rats. We found that baseline DA levels were similar across sex, AMPA antagonism decreased PFC DA in both sexes, and NMDA antagonism increased DA in males but decreased DA in females. We also found that, at subseizure-producing drug levels, γ-aminobutyric acid (GABA)-A antagonism did not affect DA in either sex but that GABA-B antagonism transiently increased PFC DA in both sexes, albeit more so in females. Finally, when NMDA antagonism was coincident with GABA-B antagonism, PFC DA levels in males responded as if to GABA-B antagonism alone, whereas in females, DA effects mirrored those induced by NMDA antagonism. Taken together, these data suggest commonalities and fundamental differences in the intracortical amino acid transmitter mechanisms that regulate DA homeostasis in the male and female rat PFCs.

Cytokine pathway disruption in a mouse model of schizophrenia induced by Munc18-1a overexpression in the brain

Background

An accumulating body of evidence points to the significance of neuroinflammation and immunogenetics in schizophrenia, and an imbalance of cytokines in the central nervous system (CNS) has been suggested to be associated with the disorder. Munc18-overexpressing mice (Munc18-OE) have provided a model for the study of the alterations that may underlie the symptoms of subjects with schizophrenia. The aim of the present study was to elucidate the involvement of neuroinflammation and cytokine imbalance in this model.

Methods

Cytokines were evaluated in the cortex and the striatum of Munc18-OE and wild-type (WT) mice by enzyme-linked immunosorbent assay (ELISA). Protein levels of specific microglia and macrophage, astrocytic and neuroinflammation markers were quantified by western blot in the cortex and the striatum of Munc18-OE and WT mice.

Results

Each cytokine evaluated (Interferon-gamma (IFN-γ), Tumor Necrosis Factor-alpha (TNF-α), Interleukin-2 (IL-2) and CCL2 chemokine) was present at higher levels in the striatum of Munc18-OE mice than WT. Cortical TNF-α and IL-2 levels were significantly lower in Munc18-OE mice than WT mice. The microglia and macrophage marker CD11b was lower in the cortexes of Munc18-OE mice than WT, but no differences were observed in the striatum. Glial Fibrillary Acidic Protein (GFAP) and Nuclear Factor-kappaB (NF-κB)p65 levels were not different between the groups. Interleukin-1beta (IL-1β) and IL-6 levels were beneath detection limits.

Conclusions

The disrupted levels of cytokines detected in the brain of Munc18-OE mice was found to be similar to clinical reports and endorses study of this type for analysis of this aspect of the disorder. The lower CD11b expression in the cortex but not in the striatum of the Munc18-OE mice may reflect differences in physiological activity. The cytokine expression pattern observed in Munc18-OE mice is similar to a previously published model of schizophrenia caused by maternal immune activation. Together, these data suggest a possible role for an immune imbalance in this disorder.

Neurogenetics of depression: a focus on reward processing and stress sensitivity

Major depressive disorder (MDD) is etiologically complex and has a heterogeneous presentation. This heterogeneity hinders the ability of molecular genetic research to reliably detect the small effects conferred by common genetic variation. As a result, significant research efforts have been directed at investigating more homogenous intermediate phenotypes believed to be more proximal to gene function and lie between genes and/or environmental effects and disease processes. In the current review we survey and integrate research on two promising intermediate phenotypes linked to depression: reward processing and stress sensitivity. A synthesis of this burgeoning literature indicates that a molecular genetic approach focused on intermediate phenotypes holds significant promise to fundamentally improve our understanding of the pathophysiology and etiology of depression, which will be required for improved diagnostic definitions and the development of novel and more efficacious treatment and prevention strategies. We conclude by highlighting challenges facing intermediate phenotype research and future development that will be required to propel this pivotal research into new directions.

Disulfiram stimulates dopamine release from noradrenergic terminals and potentiates cocaine-induced dopamine release in the prefrontal cortex

RATIONALE

Disulfiram efficacy in treatment of cocaine addiction is attributed to the inhibition of dopamine-β-hydroxylase and reduction in brain noradrenaline (NA)/dopamine (DA) ratio.

OBJECTIVES

Using microdialysis, we investigated if disulfiram causes DA release from noradrenergic terminals and modifies cocaine-induced DA release.

RESULTS

Disulfiram reduced extracellular NA in the medial prefrontal (mPF) cortex, occipital cortex, accumbens and caudate nuclei, while it markedly increased DA not only in mPF but also in the occipital cortex, despite its scanty dopaminergic afferences, and modestly increased DA in the accumbens and caudate nuclei, despite their dense dopaminergic innervation. Disulfiram-induced DA accumulation was reversed in both cortices by tetrodotoxin infusion and by systemic administration of the α(2)-adrenoceptor agonist clonidine, but was not modified by the α(2)-adrenoceptor antagonist RS 79948 or the D(2)-like agonist quinpirole. Disulfiram prevented cocaine-induced NA release in the mPF cortex and nucleus accumbens, potentiated cocaine-induced DA release in the mPF cortex but failed to modify cocaine effect in the nucleus accumbens. DA release induced by disulfiram-cocaine combination in the mPF cortex was prevented by clonidine but not by quinpirole.

CONCLUSIONS

We suggested that disulfiram, by removing NA-mediated inhibitory control on noradrenergic terminals, causes an unrestrained cocaine-induced DA release from those terminals in the mPF cortex. In the accumbens and caudate nuclei, "allogenic" DA concentration might be clouded by DA originated from dopaminergic terminals. The possible role of "allogenic" DA in disulfiram ability to prevent stress-induced reinstatement of cocaine seeking is discussed.

Natural rewards, neuroplasticity, and non-drug addictions

There is a high degree of overlap between brain regions involved in processing natural rewards and drugs of abuse. "Non-drug" or "behavioral" addictions have become increasingly documented in the clinic, and pathologies include compulsive activities such as shopping, eating, exercising, sexual behavior, and gambling. Like drug addiction, non-drug addictions manifest in symptoms including craving, impaired control over the behavior, tolerance, withdrawal, and high rates of relapse. These alterations in behavior suggest that plasticity may be occurring in brain regions associated with drug addiction. In this review, I summarize data demonstrating that exposure to non-drug rewards can alter neural plasticity in regions of the brain that are affected by drugs of abuse. Research suggests that there are several similarities between neuroplasticity induced by natural and drug rewards and that, depending on the reward, repeated exposure to natural rewards might induce neuroplasticity that either promotes or counteracts addictive behavior.

Correlation of individual differences in schizotypal personality traits with amphetamine-induced dopamine release in striatal and extrastriatal brain regions

OBJECTIVE

Schizotypal personality traits are associated with schizophrenia spectrum disorders, and individuals with schizophrenia spectrum disorders demonstrate increased dopamine transmission in the striatum. The authors sought to determine whether individual differences in normal variation in schizotypal traits are correlated with dopamine transmission in the striatum and in extrastriatal brain regions.

METHOD

Sixty-three healthy volunteers with no history of psychiatric illness completed the Schizotypal Personality Questionnaire and underwent positron emission tomography imaging with [(18)F]fallypride at baseline and after administration of oral d-amphetamine (0.43 mg/kg). Dopamine release, quantified by subtracting each participant's d-amphetamine scan from his or her baseline scan, was correlated with Schizotypal Personality Questionnaire total and factor scores using region-of-interest and voxel-wise analyses.

RESULTS

Dopamine release in the striatum was positively correlated with overall schizotypal traits. The association was especially robust in the associative subdivision of the striatum. Voxel-wise analyses identified additional correlations between dopamine release and schizotypal traits in the left middle frontal gyrus and left supramarginal gyrus. Exploratory analyses of Schizotypal Personality Questionnaire factor scores revealed correlations between dopamine release and disorganized schizotypal traits in the striatum, thalamus, medial prefrontal cortex, temporal lobe, insula, and inferior frontal cortex.

CONCLUSIONS

The association between dopamine signaling and psychosis phenotypes extends to individual differences in normal variation in schizotypal traits and involves dopamine transmission in both striatal and extrastriatal brain regions. Amphetamine-induced dopamine release may be a useful endophenotype for investigating the genetic basis of schizophrenia spectrum disorders.

Opponency revisited: competition and cooperation between dopamine and serotonin

Affective valence lies on a spectrum ranging from punishment to reward. The coding of such spectra in the brain almost always involves opponency between pairs of systems or structures. There is ample evidence for the role of dopamine in the appetitive half of this spectrum, but little agreement about the existence, nature, or role of putative aversive opponents such as serotonin. In this review, we consider the structure of opponency in terms of previous biases about the nature of the decision problems that animals face, the conflicts that may thus arise between Pavlovian and instrumental responses, and an additional spectrum joining invigoration to inhibition. We use this analysis to shed light on aspects of the role of serotonin and its interactions with dopamine.

Similarities in the behavior and molecular deficits in the frontal cortex between the neurotensin receptor subtype 1 knockout mice and chronic phencyclidine-treated mice: relevance to schizophrenia

Much evidence suggests that targeting the neurotensin (NT) system may provide a novel and promising treatment for schizophrenia. Our recent work shows that: NTS1 knockout (NTS1(-/-)) mice may provide a potential animal model for studying schizophrenia by investigating the effect of deletion NTS1 receptor on amphetamine-induced hyperactivity and neurochemical changes. The data indicate a hyper-dopaminergic state similar to the excessive striatal DA activity reported in schizophrenia. The present study was done to determine if NTS1(-/-) mice also have similar changes in behavior, in prefrontal neurotransmitters, and in protein expression, as observed in wild type (WT) mice treated with the psychotomimetic phencylclidine (PCP), an animal model for schizophrenia. Our results showed many similarities between untreated NTS1(-/-) mice and WT mice chronically treated with PCP (as compared with untreated WT mice): 1) lower PCP-induced locomotor activity; 2) similar avolition-like behavior in forced-swim test and tail suspension test; 3) lower prefrontal glutamate levels; 4) less PCP-induced dopamine release in medial prefrontal cortex (mPFC); and 5) down-regulation of mRNA and protein for DA D(1), DA D(2), and NMDAR2A in mPFC. Therefore, these data strengthen the hypothesis that the NTS1(-/-) mouse is an animal model of schizophrenia, particularly for the dysfunction of the prefrontal cortex. In addition, after chronic PCP administration, the DA D(1) receptor was up-regulated in NTS1(-/-) mice, results which suggest a possible interaction of NTS1/DA D(1) in mPFC contributing to chronic PCP-induced schizophrenia-like signs.

The interrelationship of dopamine D2-like receptor availability in striatal and extrastriatal brain regions in healthy humans: a principal component analysis of [18F]fallypride binding

Individual differences in dopamine D2-like receptor availability arise across all brain regions expressing D2-like receptors. However, the interrelationships in receptor availability across brain regions are poorly understood. To address this issue, we examined the relationship between D2-like binding potential (BPND) across striatal and extrastriatal regions in a sample of healthy participants. PET imaging was performed with the high affinity D2/D3 ligand [18F]fallypride in 45 participants. BPND images were submitted to voxel-wise principal component analysis to determine the pattern of associations across brain regions. Individual differences in D2-like BPND were explained by three distinguishable components. A single component explained almost all of the variance within the striatum, indicating that individual differences in receptor availability vary in a homogenous manner across the caudate, putamen, and ventral striatum. Cortical BPND was only modestly related to striatal BPND and mostly loaded on a distinct component. After controlling for the general level of cortical D2-like BPND, an inverse relationship emerged between receptor availability in the striatum and the ventral temporal and ventromedial frontal cortices, suggesting possible cross-regulation of D2-like receptors in these regions. The analysis additionally revealed evidence of: (1) a distinct component involving the midbrain and limbic areas; (2) a dissociation between BPND in the medial and lateral temporal regions; and (3) a dissociation between BPND in the medial/midline and lateral thalamus. In summary, individual differences in D2-like receptor availability reflect several distinct patterns. This conclusion has significant implications for neuropsychiatric models that posit global or regionally specific relationships between dopaminergic tone and behavior.

Prenatal exposure to infection: a primary mechanism for abnormal dopaminergic development in schizophrenia

RATIONALE

Prenatal exposure to infection is a notable environmental risk factor in the development of schizophrenia. One prevalent hypothesis suggests that infection-induced disruption of early prenatal brain development predisposes the organism to long-lasting structural and functional brain abnormalities. Many of the prenatal infection-induced functional brain abnormalities appear to be closely associated with imbalances in the mesocorticolimbic dopamine system in adult life, suggesting that disruption of functional and structural dopaminergic development may be at the core of the developmental neuropathology associated with psychosis-related abnormalities induced by prenatal exposure to infection.

OBJECTIVES

In this review, we integrate recent findings derived from experimental models in animals with parallel research in humans which supports this hypothesis. We thereby highlight the developmental perspective of abnormal DA functions following in-utero exposure to infection in relation to the developmental and maturational mechanisms potentially involved in schizophrenia.

RESULTS

Experimental investigations show that early prenatal immune challenge can lead to the emergence of early structural and functional alterations in the mesocorticolimbic DA system, long before the onset of the full spectrum of psychosis-associated behavioral and cognitive abnormalities in adulthood.

CONCLUSIONS

Dopaminergic mal-development in general, and following prenatal immune activation in particular, may represent a primary etiopathological mechanism in the development of schizophrenia and related disorders. This hypothesis differs from the view that dopaminergic abnormalities in schizophrenia may be secondary to abnormalities in other brain structures and/or neurotransmitter systems. The existence of primary dopaminergic mechanisms may have important implications for the identification and early treatment of individuals prodromally symptomatic for schizophrenia.

Dissociable effects of noradrenaline, dopamine, and serotonin uptake blockade on stop task performance in rats

RATIONALE

The stop-signal paradigm measures the ability to stop a motor response after its execution has been initiated. Impairments in inhibiting inappropriate behavior and prolonged stop-signal reaction times (SSRTs) are characteristic of several psychiatric disorders, most notably attention deficit/hyperactivity disorder. While there is relative consensus regarding the anatomical substrates of behavioral inhibition, the neurochemical imbalance responsible for the deficits in stopping displayed by impulsive individuals is still a matter of debate.

OBJECTIVE

The aim of this study was to investigate the effects of manipulating brain monoamine levels on stop task parameters.

METHODS

Lister-hooded rats were trained on the rodent version of the stop-signal task and administered different monoamine transporter inhibitors: citalopram, which selectively blocks the serotonin transporter; atomoxetine, which selectively blocks the noradrenaline transporter; and GBR-12909, which selectively blocks the dopamine transporter (DAT), and the alpha-2 adrenergic agonist guanfacine.

RESULTS

Atomoxetine speeded SSRT and increased accuracy for go-trials. Citalopram slowed go reaction time and decreased go accuracy at the highest dose (1 mg/kg). GBR-12909 speeded go reaction time and impaired both go and stop accuracy. Guanfacine negatively modulated all principal stop and go measures at the highest dose used (0.3 mg/kg).

CONCLUSIONS

The results suggest that atomoxetine exerts its beneficial effects on SSRT via its action on noradrenaline re-uptake, as the specific DAT blocker GBR-12909 and serotonin reuptake blockade had only minor effects on SSRT. The speeding of the go reaction time by dopamine reuptake blockade is consistent with the hypothesis that the hypothetical stop and go processes are modulated by distinct monoaminergic systems.

Dopamine D2 receptor levels in striatum, thalamus, substantia nigra, limbic regions, and cortex in schizophrenic subjects

BACKGROUND

Studies in schizophrenic patients have reported dopaminergic abnormalities in striatum, substantia nigra, thalamus, anterior cingulate, hippocampus, and cortex that have been related to positive symptoms and cognitive impairments.

METHODS

[(18)F]fallypride positron emission tomography studies were performed in off-medication or never-medicated schizophrenic subjects (n = 11, 6 men, 5 women; mean age of 30.5 +/- 8.0 [SD] years; 4 drug-naive) and age-matched healthy subjects (n = 11, 5 men, 6 women, mean age of 31.6 +/- 9.2 [SD]) to examine dopamine D(2) receptor (DA D(2)r) levels in the caudate, putamen, ventral striatum, medial thalamus, posterior thalamus, substantia nigra, amygdala, temporal cortex, anterior cingulate, and hippocampus.

RESULTS

In schizophrenic subjects, increased DA D(2)r levels were seen in the substantia nigra bilaterally; decreased levels were seen in the left medial thalamus. Correlations of symptoms with ROI data demonstrated a significant correlation of disorganized thinking/nonparanoid delusions with the right temporal cortex ROI (r = .94, p = .0001), which remained significant after correction for multiple comparisons (p < .03). Correlations of symptoms with parametric images of DA D(2)r levels revealed no significant clusters of correlations with negative symptoms but significant clusters of positive correlations of total positive symptoms, delusions and bizarre behavior with the lateral and anterior temporal cortex, and hallucinations with the left ventral striatum.

CONCLUSIONS

The results of this study demonstrate abnormal DA D(2)r-mediated neurotransmission in the substantia nigra consistent with nigral dysfunction in schizophrenia and suggest that both temporal cortical and ventral striatal DA D(2)r mediate positive symptoms.

Interregional cerebral metabolic associativity during a continuous performance task (Part I): healthy adults

One emerging hypothesis regarding psychiatric illnesses is that they arise from the dysregulation of normal circuits or neuroanatomical patterns. In order to study mood disorders within this framework, we explored normal metabolic associativity patterns in healthy volunteers as a prelude to examining the same relationships in affectively ill patients (Part II). We applied correlational analyses to regional brain activity as measured with FDG-PET during an auditory continuous performance task (CPT) in 66 healthy volunteers. This simple attention task controlled for brain activity that otherwise might vary amongst affective and cognitive states. There were highly significant positive correlations between homologous regions in the two hemispheres in thalamic, extrapyramidal, orbital frontal, medial temporal and cerebellar areas. Dorsal frontal, lateral temporal, cingulate, and especially insula, and inferior parietal areas showed less significant homologous associativity, suggesting more specific lateralized function. The medulla and bilateral thalami exhibited the most diverse interregional associations. A general pattern emerged of cortical regions covarying inversely with subcortical structures, particularly the frontal cortex with cerebellum, amygdala and thalamus. These analytical data may help to confirm known functional and neuroanatomical relationships, elucidate others as yet unreported, and serve as a basis for comparison to patients with psychiatric illness.

Subchronic and chronic PCP treatment produces temporally distinct deficits in attentional set shifting and prepulse inhibition in rats

RATIONALE

We have previously demonstrated that subchronic (five daily administrations of 2.6 mg/kg PCP) and chronic intermittent administration of 2.6 mg/kg PCP to rats produces hypofrontality and other neurochemical changes akin to schizophrenia pathology (Cochran et al., Neuropsychopharmacology, 28:265-275, 2003).

OBJECTIVES

We sought to determine whether behavioral alterations related to discrete aspects of schizophrenia are also induced by these PCP treatment regimes.

MATERIALS AND METHODS

Following administration of vehicle or PCP according to the protocols described above, rats were assessed for attentional set shifting ability, prepulse inhibition (PPI), or social interaction and the locomotor response to a challenge dose of amphetamine.

RESULTS

Ability to shift attentional set was impaired 72 h after the last PCP administration following the subchronic and chronic intermittent treatment regimes. PPI was disrupted after each acute administration of PCP in animals under the subchronic treatment regime. However, PPI deficits were not sustained 72 h after the last of five daily administrations. In subchronic and chronic PCP treated animals, no change was found in social interaction behavior, and there was little change in baseline or amphetamine-stimulated locomotor activity, employed as an indicator of dopaminergic hyperfunction.

CONCLUSIONS

The temporally distinct behavioral effects of these PCP treatment regimes suggest that PPI deficits relate directly to acute NMDA receptor antagonism, whereas the more enduring set shifting deficits relate to the longer term consequences of NMDA receptor blockade. Therefore, these subchronic and chronic PCP treatment regimes produce hypofrontality (Cochran et al., Neuropsychopharmacology, 28:265-275, 2003) and associated prefrontal cortex-dependent deficits in behavioral flexibility which mirror core deficits in schizophrenia.

Dopamine depletion of the prefrontal cortex induces dendritic spine loss: reversal by atypical antipsychotic drug treatment

Dystrophic changes in dendrites of cortical neurons are present in several neuro-psychiatric disorders, including schizophrenia. The mechanisms that account for dendritic changes in the prefrontal cortex (PFC) in schizophrenia are unclear. Cognitive deficits in schizophrenia have been linked to compromised cortical dopamine function, and the density of the PFC dopamine innervation is decreased in schizophrenia. We determined if 6-hydroxydopamine lesions of the ventral tegmental area that disrupt the PFC dopamine innervation cause dystrophic changes in cortical neurons. Three weeks post-operatively we observed a marked decrease in basal dendritic length and spine density of layer V pyramidal cells in the prelimbic cortex; no change was seen in neurons of the motor cortex. We then examined rats in which the PFC dopamine innervation was lesioned and 3 weeks later were started on chronic treatment with an atypical (olanzapine) or typical (haloperidol) antipsychotic drug. Olanzapine but not haloperidol reversed lesion-induced changes in PFC pyramidal cell dendrites. These data suggest that dopamine regulates dendritic structure in PFC neurons. Moreover, the findings are consistent with a decrease in cortical dopaminergic tone contributing to the pathological changes in the cortex of schizophrenia, and suggest that the progressive cortical loss in schizophrenia may be slowed or reversed by treatment with atypical antipsychotic drugs.

D2 receptor regulation of synaptic burst firing in prefrontal cortical pyramidal neurons

The efficacy of antipsychotics in the treatment of schizophrenia depends on their ability to block dopamine (DA) D2 receptors. D2 receptor excitatory mediation of glutamatergic receptors has been implicated in in vivo studies. However, D2 receptor enhancement of glutamatergic transmission has rarely been reported in slice recordings. Instead, D2 receptor depression of both alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and N-methyl-D-aspartate (NMDA) action was obtained in previous slice studies. To obtain insight into this paradox, we examined DA's actions on synaptic responses of layer V pyramidal cells to minimal extracellular stimulation in layer III of ferret prefrontal cortical slices under NMDA and gamma-aminobutyric acid type A blockade. This experimental design models the proposed hypofunction of NMDA receptor and gamma-aminobutyric acid type A deficiency in schizophrenia. We found that DA and D2 receptor agonists promoted burst firing in a subset of pyramidal cells, which was reversed by haloperidol, a D2 antagonist and a D3 agonist, compounds having antipsychotic efficacy. In contrast, a D4 antagonist, which has not proven clinically effective, was not effective in blocking DA-promoted bursts. These results revealed excitatory effects of DA mediated mainly via D2 receptors, potentially providing a cellular mechanism for the D2 antagonism in treating schizophrenia.

Perinatal distress leads to lateralized medial prefrontal cortical dopamine hypofunction in adult rats

Obstetric complications involving anoxia or prolonged hypoxia are suspected to increase the risk for such mental disorders as schizophrenia and attention deficit-hyperactivity disorder. In previous studies, we reported evidence of enhanced nucleus accumbens (NAcc) dopamine (DA) function in adult rats subjected to intrauterine anoxia during cesarean (C) section birth. In the present study, we used voltammetry and monoamine-sensitive electrodes to investigate the possibility that this functional hyperactivity of the meso-NAcc system is attributable to a loss of inhibitory control from the medial prefrontal cortex (PFC). We monitored the DA responses to repeated once-daily stress in the right or left PFC of adult male rats born vaginally (VAG) or by C-section, either with (C + 15) or without (C + 0) an additional 15 min of intrauterine anoxia. In C + 15 animals, we observed a pronounced and persistent blunting of stress-induced DA release in the right PFC but not in the left; with repeated testing, a similar pattern of dampened right PFC DA stress responses emerged in C + 0 animals. In addition, C + 15 animals were spontaneously more active than VAG and C + 0 animals and displayed an increase in PFC DA transporter density that was also lateralized to the right hemisphere. There was no evidence, however, that PFC D(1) and D(2) receptor levels differed between birth groups or hemisphere. These findings suggest a mechanism by which perinatal complications involving anoxia might contribute to the etiology of mental disorders that have been linked to disturbances in central DA transmission and lateralized PFC dysfunction.

Dopamine terminals in the rat prefrontal cortex synapse on pyramidal cells that project to the nucleus accumbens

Afferents to the prefrontal cortex (PFC) from dopamine neurons in the ventral tegmental area have been implicated in working memory processes and in the pathogenesis of schizophrenia. Previous anatomical investigations have demonstrated that dopamine terminals synapse on dendritic spines and shafts of pyramidal cells in the PFC. Moreover, neurochemical and physiological studies suggest that dopamine modulates the activity of PFC neurons that project to the nucleus accumbens. However, whether this modulation involves direct synaptic input to cortico-accumbens projection neurons has not been determined. To address this question, retrograde transport of an attenuated strain of pseudorabies virus (PRV) from the nucleus accumbens was combined with immunoperoxidase labeling of tyrosine hydroxylase (TH) to identify dopamine terminals in the PFC. At survival times <48 hr, extensive dendritic distribution of immunogold labeling for PRV was observed in cortico-accumbens neurons. However, evidence consistent with trans-synaptic passage of PRV within this timeframe was observed only rarely. When examined at the electron microscopic level, immunogold labeling for PRV was localized to neuronal somata, proximal and distal dendrites, and dendritic spines. Some of these dendritic processes received symmetric synaptic input from TH-immunoreactive terminals. These data represent the first demonstration of dopamine synaptic contacts onto an identified population of pyramidal cells in the PFC. The findings have important implications for understanding how dopamine modulates cortical outflow to limbic regions in normal brain and pathological states such as schizophrenia.

Modulation of basolateral amygdala neuronal firing and afferent drive by dopamine receptor activation in vivo

The basolateral amygdala (BLA) is implicated in responding to affective stimuli. Dopamine (DA) is released in the BLA during numerous conditions; however, the neurophysiological effects of DA in the BLA have not been examined in depth. In this study, the effects of DA receptor manipulation on spontaneous and afferent-driven neuronal firing were examined using in vivo extracellular single-unit recordings in parallel with systemic and iontophoretic drug application, and stimulation of the substantia nigra/ventral tegmental area in the rat. The effects of DA receptor activation in the BLA were found to depend on the characteristics of the BLA neuron examined, causing an increase in the firing rate of putative interneurons and a decrease in the firing of identified projection neurons. Additionally, DA receptor activation attenuated short-latency spikes evoked by electrical stimulation of prefrontal cortical and mediodorsal thalamic inputs to the BLA while potentiating the responses evoked by electrical stimulation of sensory association cortex. DA receptor activation can thus attenuate BLA projection neuron firing via two mechanisms: (1) by a direct inhibition, and (2) by indirect actions mediated via activation of BLA interneurons. This is hypothesized to lead to a global filtration of weaker inputs. Moreover, DA potentiates sensory inputs and attenuates medial prefrontal cortex inputs to the BLA. Conditions in which DA is released in the BLA, such as during the presentation of an affective stimulus, will lead to a potentiation of the strongest sensory input and a dampening of cortical inhibition over the BLA, thus augmenting the response to affective sensory stimuli.

Phencyclidine increases forebrain monoamine metabolism in rats and monkeys: modulation by the isomers of HA966

The noncompetitive NMDA receptor antagonist phencyclidine (PCP) has psychotomimetic properties in humans and activates the frontal cortical dopamine innervation in rats, findings that have contributed to a hyperdopaminergic hypothesis of schizophrenia. In the present studies, the effects of the enantiomers of 3-amino-1-hydroxypyrrolid-2-one (HA966) on PCP-induced changes in monoamine metabolism in the forebrain of rats and monkeys were examined, because HA966 has been shown previously to attenuate stress- or drug-induced activation of dopamine systems. In rats, PCP (10 mg/kg, i.p.) potently activated dopamine (DA) turnover in the medial prefrontal cortex (PFC) and nucleus accumbens. Serotonin utilization was also increased in PFC. Pretreatment with either R-(+)HA966 (15 mg/kg, i.p.) or S-(-)HA966 (3 mg/kg, i.p.) partially blocked PCP-induced increases in PFC DA turnover, whereas neither enantiomer altered the effect of PCP on DA turnover in the nucleus accumbens or the PCP-induced increases in serotonin turnover in PFC. PCP (0.3 mg/kg, i.m.) exerted regionally selective effects on the dopaminergic and serotonergic innervation of the monkey frontal cortex, effects blocked by pretreatment with S-(-)HA966 (3 mg/kg, i. m.). Importantly, these data demonstrate that in the primate, PCP has potent effects on dopamine transmission in the frontal cortex, a brain region thought to be dysfunctional in schizophrenia. In addition, a role for S-(-)HA966 as a modulator of cortical monoamine transmission in primates is posited.

Behavior-relevant changes in nucleus accumbens dopamine transmission elicited by food reinforcement: an electrochemical study in rat

Voltammetry was used to monitor dopamine (DA) transmission in nucleus accumbens (NAcc) of rats lever-pressing for food. Under standard conditions, animals responded on a fixed ratio 1 (FR1) schedule for 0.2 ml of milk delivered over 30 sec; milk delivery was paired with a 30 sec cue light. Consumption of the initial few milk rewards of the session caused DA signal increases. These initial signal increases were typical of the first and, at times, the second test days. On subsequent days, the most pronounced initial signal increases coincided with presentation of conditioned stimuli that marked the start of the session. Biphasic changes in DA signal that were time-locked to each reinforced lever-press were also observed; responses were preceded by increases and were followed, during milk consumption, by decreases in DA signal. At the end of milk delivery, the signal increased again in apparent anticipation of the next lever-press. Delaying milk delivery caused a corresponding delay in DA signal decreases, and the amount of time signals remained depressed was bound by the duration of milk consumption. Greater decreases in DA signal were observed when the rate of milk delivery was doubled or tripled, and such increases in reward value were associated with more pronounced signal increases during the period that preceded each lever-press. In contrast, DA signal increases were seen when milk was delivered at half the usual rate or was withheld altogether or when animals were denied access to the lever. Under partial reinforcement conditions, reinforced lever-presses were preceded by more pronounced signal increases and decreases of comparable magnitude accompanied milk consumption. These results suggest that meso-NAcc DA neurons are activated primarily in response to the incentive rather than to the reinforcing properties of rewards.

Dopamine and spatial working memory in rats and monkeys: pharmacological reversal of stress-induced impairment

The anxiogenic benzodiazepine inverse agonist FG7142 increases dopamine turnover in rodent prefrontal cortex but not in other dopamine terminal field areas. FG7142-induced increases in prefrontal cortical dopamine receptor stimulation impair prefrontal-dependent, but not nonprefrontal-dependent, cognitive tasks in rats and monkeys. The degree of impairment correlates with levels of prefrontal cortical dopamine turnover in rats and can be blocked in rats and monkeys with dopamine receptor antagonists, suggesting that increased dopamine turnover is directly related to the cognitive deficits. The current study examined nondopaminergic drug effects on FG7142-perturbed biochemistry and cognition. Both the noradrenergic alpha-2 agonist clonidine and the glycine/NMDA antagonist (+)HA966 prevented the FG7142-induced increase in dopamine turnover in rodent prefrontal cortex. Infusion of (+)HA966 into the ventral tegmental area (VTA) also blocked this increase in dopamine turnover, indicating that critical modulatory effects of (+)HA966 on FG7142-induced changes in dopamine turnover are occurring at the level of mesoprefrontal dopamine neuron cell bodies. Systemic (+)HA966 and clonidine, but not propranolol or D-cycloserine, prevented FG7142-associated spatial working memory deficits in rats and monkeys. These results support the idea of a critical range of dopamine turnover for optimal prefrontal cortical cognitive functioning, with excessive dopamine turnover leading to cognitive impairment. These studies also provide evidence for the regulation of prefrontal cortical dopamine turnover and cognition by multiple neurotransmitter systems and suggest that the VTA is an important regulatory site for these effects.

Enhanced amphetamine sensitivity and increased expression of dopamine D2 receptors in postpubertal rats after neonatal excitotoxic lesions of the medial prefrontal cortex

Functional and structural abnormalities in the medial prefrontal cortex (MPFC) and overactive dopamine (DA) neurotransmission are thought to be the key pathologies in schizophrenia. To understand the role of MPFC in the pre- and postpubertal development of the subcortical DA system, the effects of neonatal [postnatal day 7 (PD7)] MPFC excitotoxic lesions on locomotor behaviors and the expression of DA receptor subtypes and DA transporter were investigated in Sprague Dawley rats at PD35 and PD56, respectively. No significant differences in the novelty of d-amphetamine-induced locomotion were observed between sham-operated and ibotenic acid-lesioned rats at PD35. Postpubertally (at PD56), however, the locomotor activity of lesioned rats in the novel environment and after d-amphetamine administration was enhanced significantly compared with controls. The expressions of DA D1, D2, D3, and D4 receptors and DA transporter were then estimated in MPFC-lesioned and sham-operated rats at PD59 and PD60. The levels of DA D2 receptors, measured using [3H]-YM-09151-2 binding, and its mRNA by in situ hybridization, were observed to be significantly increased at PD60 in striatal and limbic areas of lesioned rats. Levels of other DA receptor subtypes were not significantly affected at any time points. Lesioned rats at PD39 show a small increase in DA transporter level in the shell of nucleus accumbens; however, this effect seems to wear off at PD60. The data suggest that neonatal MPFC lesions may alter the functional development and maturation of mesolimbic/nigrostriatal DA systems in that neonatally lesioned rats grow into a behavioral/neurochemical deficit.