Layer V neurons bear the majority of mRNAs encoding the five distinct dopamine receptor subtypes in the primate prefrontal cortex

Computer-Aided Design and Biological Evaluation of Diazaspirocyclic D4R Antagonists

Parkinson's disease (PD) is a neurodegenerative disorder characterized by the progressive loss of dopaminergic neurons in the substantia nigra, resulting in motor dysfunction. Current treatments are primarily centered around enhancing dopamine signaling or providing dopamine replacement therapy and face limitations such as reduced efficacy over time and adverse side effects. To address these challenges, we identified selective dopamine receptor subtype 4 (D4R) antagonists not previously reported as potential adjuvants for PD management. In this study, a library screening and artificial neural network quantitative structure-activity relationship (QSAR) modeling with experimentally driven library design resulted in a class of spirocyclic compounds to identify candidate D4R antagonists. However, developing selective D4R antagonists suitable for clinical translation remains a challenge.

Adaptive control of synaptic plasticity integrates micro- and macroscopic network function

Synaptic plasticity configures interactions between neurons and is therefore likely to be a primary driver of behavioral learning and development. How this microscopic-macroscopic interaction occurs is poorly understood, as researchers frequently examine models within particular ranges of abstraction and scale. Computational neuroscience and machine learning models offer theoretically powerful analyses of plasticity in neural networks, but results are often siloed and only coarsely linked to biology. In this review, we examine connections between these areas, asking how network computations change as a function of diverse features of plasticity and vice versa. We review how plasticity can be controlled at synapses by calcium dynamics and neuromodulatory signals, the manifestation of these changes in networks, and their impacts in specialized circuits. We conclude that metaplasticity-defined broadly as the adaptive control of plasticity-forges connections across scales by governing what groups of synapses can and can't learn about, when, and to what ends. The metaplasticity we discuss acts by co-opting Hebbian mechanisms, shifting network properties, and routing activity within and across brain systems. Asking how these operations can go awry should also be useful for understanding pathology, which we address in the context of autism, schizophrenia and Parkinson's disease.

Flexible versus Fixed Spatial Self-Ordered Response Sequencing: Effects of Inactivation and Neurochemical Modulation of Ventrolateral Prefrontal Cortex

Previously, studies using human neuroimaging and excitotoxic lesions in non-human primate have demonstrated an important role of ventrolateral prefrontal cortex (vlPFC) in higher order cognitive functions such as cognitive flexibility and the planning of behavioral sequences. In the present experiments, we tested effects on performance of temporary inactivation (using GABA receptor agonists) and dopamine (DA) D2 and 5-HT2A-receptor (R) blockade of vlPFC via local intracerebral infusions in the marmoset. We trained common marmosets to perform spatial self-ordered sequencing tasks in which one cohort of animals performed two and three response sequences on a continuously varying spatial array of response options on a touch-sensitive screen. Inactivation of vlPFC produced a marked disruption of accuracy of sequencing which also exhibited significant error perseveration. There were somewhat contrasting effects of D2 and 5-HT2A-R blockade, with the former producing error perseveration on incorrect trials, though not significantly impairing accuracy overall, and the latter significantly impairing accuracy but not error perseveration. A second cohort of marmosets were directly compared on performance of fixed versus variable spatial arrays. Inactivation of vlPFC again impaired self-ordered sequencing, but only with varying, and not fixed spatial arrays, the latter leading to the consistent use of fewer, preferred sequences. These findings add to evidence that vlPFC is implicated in goal-directed behavior that requires higher-order response heuristics that can be applied flexibly over different (variable), as compared with fixed stimulus exemplars. They also show that dopaminergic and serotonergic chemomodulation has distinctive effects on such performance.SIGNIFICANCE STATEMENT This investigation employing local intracerebral infusions to inactivate the lateral prefrontal cortex (PFC) of the New World marmoset reveals the important role of this region in self-ordered response sequencing in variable but not fixed spatial arrays. These novel findings emphasize the higher order functions of this region, contributing to cognitive flexibility and planning of goal directed behavior. The investigation also reports for the first time somewhat contrasting neuromodulatory deficits produced by infusions of dopamine (DA) D2 and 5-HT2A receptor (R) antagonists into the same region, of possible significance for understanding cognitive deficits produced by anti-psychotic drugs.

The attention networks in benign epilepsy with centrotemporal spikes: A long-term follow-up study

PURPOSE

To evaluate the long-term prognosis of attention deficit in children with newly diagnosed benign childhood epilepsy with centrotemporal spikes (BECTS).

METHODS

Attention network test (ANT) was performed over a period of 7 years on 42 patients who were newly diagnosed with BECTS, in the Department of Neurology of Anhui Provincial Children's Hospital.

RESULTS

In the patients' group, the accuracy of ANT was lower (P = 0.000), the total response time was longer (P = 0.000), and the efficiency of orienting (P = 0.000) and alerting (P = 0.041) networks was lower than that of the control group. Accuracy was positively correlated with age of onset (b = 1.184) and negatively correlated with number of seizures (b = -1.321). After 7 years, there was no significant difference in the accuracy (P = 0.385); total response time (P = 0.661); and alerting (P = 0.797), orienting (P = 0.709), and executive control (P = 0.806) network efficiencies between the patients and controls. Accuracy was positively correlated with age of onset (b = 0.8583) and negatively correlated with number of seizures (b = -1.017) and duration of antiepileptic drugs therapy (b = -3.203).

CONCLUSIONS

In our study, the newly diagnosed BECTS patients had impaired attention network, mainly in the alerting and orienting domains. Age of onset, number of seizures, and time of antiepileptic treatment may affect the attention networks. With the remission of BECTS, the attention network dysfunction was reversed.

The epistatic interaction between the dopamine D3 receptor and dysbindin-1 modulates higher-order cognitive functions in mice and humans

The dopamine D2 and D3 receptors are implicated in schizophrenia and its pharmacological treatments. These receptors undergo intracellular trafficking processes that are modulated by dysbindin-1 (Dys). Indeed, Dys variants alter cognitive responses to antipsychotic drugs through D2-mediated mechanisms. However, the mechanism by which Dys might selectively interfere with the D3 receptor subtype is unknown. Here, we revealed an interaction between functional genetic variants altering Dys and D3. Specifically, both in patients with schizophrenia and in genetically modified mice, concomitant reduction in D3 and Dys functionality was associated with improved executive and working memory abilities. This D3/Dys interaction produced a D2/D3 imbalance favoring increased D2 signaling in the prefrontal cortex (PFC) but not in the striatum. No epistatic effects on the clinical positive and negative syndrome scale (PANSS) scores were evident, while only marginal effects on sensorimotor gating, locomotor functions, and social behavior were observed in mice. This genetic interaction between D3 and Dys suggests the D2/D3 imbalance in the PFC as a target for patient stratification and procognitive treatments in schizophrenia.

Neuronal Dopamine D3 Receptors: Translational Implications for Preclinical Research and CNS Disorders

Dopamine (DA), as one of the major neurotransmitters in the central nervous system (CNS) and periphery, exerts its actions through five types of receptors which belong to two major subfamilies such as D1-like (i.e., D1 and D5 receptors) and D2-like (i.e., D2, D3 and D4) receptors. Dopamine D3 receptor (D3R) was cloned 30 years ago, and its distribution in the CNS and in the periphery, molecular structure, cellular signaling mechanisms have been largely explored. Involvement of D3Rs has been recognized in several CNS functions such as movement control, cognition, learning, reward, emotional regulation and social behavior. D3Rs have become a promising target of drug research and great efforts have been made to obtain high affinity ligands (selective agonists, partial agonists and antagonists) in order to elucidate D3R functions. There has been a strong drive behind the efforts to find drug-like compounds with high affinity and selectivity and various functionality for D3Rs in the hope that they would have potential treatment options in CNS diseases such as schizophrenia, drug abuse, Parkinson’s disease, depression, and restless leg syndrome. In this review, we provide an overview and update of the major aspects of research related to D3Rs: distribution in the CNS and periphery, signaling and molecular properties, the status of ligands available for D3R research (agonists, antagonists and partial agonists), behavioral functions of D3Rs, the role in neural networks, and we provide a summary on how the D3R-related drug research has been translated to human therapy.

Methylphenidate Attenuates the Cognitive and Mood Alterations Observed in Mbnl2 Knockout Mice and Reduces Microglia Overexpression

Myotonic dystrophy type 1 (DM1) is a multisystem disorder affecting muscle and central nervous system (CNS) function. The cellular mechanisms underlying CNS alterations are poorly understood and no useful treatments exist for the neuropsychological deficits observed in DM1 patients. We investigated the progression of behavioral deficits present in male and female muscleblind-like 2 (Mbnl2) knockout (KO) mice, a rodent model of CNS alterations in DM1, and determined the biochemical and electrophysiological correlates in medial prefrontal cortex (mPFC), striatum and hippocampus (HPC). Male KO exhibited more cognitive impairment and depressive-like behavior than female KO mice. In the mPFC, KO mice showed an overexpression of proinflammatory microglia, increased transcriptional levels of Dat, Drd1, and Drd2, exacerbated dopamine levels, and abnormal neural spiking and oscillatory activities in the mPFC and HPC. Chronic treatment with methylphenidate (MPH) (1 and 3 mg/kg) reversed the behavioral deficits, reduced proinflammatory microglia in the mPFC, normalized prefrontal Dat and Drd2 gene expression, and increased Bdnf and Nrf2 mRNA levels. These findings unravel the mechanisms underlying the beneficial effects of MPH on cognitive deficits and depressive-like behaviors observed in Mbnl2 KO mice, and suggest that MPH could be a potential candidate to treat the CNS deficiencies in DM1 patients.

Underlying Susceptibility to Eating Disorders and Drug Abuse: Genetic and Pharmacological Aspects of Dopamine D4 Receptors

The dopamine D4 receptor (DRD4) has a predominant expression in the prefrontal cortex (PFC), brain area strictly involved in the modulation of reward processes related to both food and drug consumption. Additionally, the human DRD4 gene is characterized by a variable number of tandem repeats (VNTR) in the exon 3 and, among the polymorphic variants, the 7-repeat (7R) allele appears as a contributing factor in the neurobiological mechanisms underlying drug abuse, aberrant eating behaviors and related comorbidities. The 7R variant encodes for a receptor with a blunted intracellular response to dopamine, and carriers of this polymorphism might be more tempted to enhance dopamine levels in the brain, through the overconsumption of drugs of abuse or palatable food, considering their reinforcing properties. Moreover, the presence of this polymorphism seems to increase the susceptibility of individuals to engage maladaptive eating patterns in response to negative environmental stimuli. This review is focused on the role of DRD4 and DRD4 genetic polymorphism in these neuropsychiatric disorders in both clinical and preclinical studies. However, further research is needed to better clarify the complex DRD4 role, by using validated preclinical models and novel compounds more selective for DRD4.

Dopamine signals related to appetitive and aversive events in paradigms that manipulate reward and avoidability

Using environmental cues to acquire good and avoid harmful things is critical for survival. Rewarding and aversive outcomes both drive behavior through reinforcement learning and sometimes occur together in the environment, but it remains unclear how these signals are encoded within the brain and if signals for positive and negative reinforcement are encoded similarly. Recent studies demonstrate that the dopaminergic system and interconnected brain regions process both positive and negative reinforcement necessary for approach and avoidance behaviors, respectively. Here, we review these data with a special focus on behavioral paradigms that manipulate both expected reward and the avoidability of aversive events to reveal neural correlates related to value, prediction error encoding, motivation, and salience.

Dopamine and Cognitive Control in Prefrontal Cortex

Cognitive control, the ability to orchestrate behavior in accord with our goals, depends on the prefrontal cortex. These cognitive functions are heavily influenced by the neuromodulator dopamine. We review here recent insights exploring the influence of dopamine on neuronal response properties in prefrontal cortex (PFC) during ongoing behaviors in primates. This review suggests three major computational roles of dopamine in cognitive control: (i) gating sensory input, (ii) maintaining and manipulating working memory contents, and (iii) relaying motor commands. For each of these roles, we propose a neuronal microcircuit based on known mechanisms of action of dopamine in PFC, which are corroborated by computational network models. This conceptual approach accounts for the various roles of dopamine in prefrontal executive functioning.

Genetic labeling reveals temporal and spatial expression pattern of D2 dopamine receptor in rat forebrain

The D2 dopamine receptor (Drd2) is implicated in several brain disorders such as schizophrenia, Parkinson’s disease, and drug addiction. Drd2 is also the primary target of both antipsychotics and Parkinson’s disease medications. Although the expression pattern of Drd2 is relatively well known in mouse brain, the temporal and spatial distribution of Drd2 is lesser clear in rat brain due to the lack of Drd2 reporter rat lines. Here, we used CRISPR/Cas9 techniques to generate two knockin rat lines: Drd2::Cre and Rosa26::loxp-stop-loxp-tdTomato. By crossing these two lines, we produced Drd2 reporter rats expressing the fluorescence protein tdTomato under the control of the endogenous Drd2 promoter. Using fluorescence imaging and unbiased stereology, we revealed the cellular expression pattern of Drd2 in adult and postnatal rat forebrain. Strikingly, the Drd2 expression pattern differs between Drd2 reporter rats and Drd2 reporter mice generated by BAC transgene in prefrontal cortex and hippocampus. These results provide fundamental information needed for the study of Drd2 function in rat forebrain. The Drd2::Cre rats generated here may represent a useful tool to study the function of neuronal populations expressing Drd2.

Monoaminergic Neuromodulation of Sensory Processing

All neuronal circuits are subject to neuromodulation. Modulatory effects on neuronal processing and resulting behavioral changes are most commonly reported for higher order cognitive brain functions. Comparatively little is known about how neuromodulators shape processing in sensory brain areas that provide the signals for downstream regions to operate on. In this article, we review the current knowledge about how the monoamine neuromodulators serotonin, dopamine and noradrenaline influence the representation of sensory stimuli in the mammalian sensory system. We review the functional organization of the monoaminergic brainstem neuromodulatory systems in relation to their role for sensory processing and summarize recent neurophysiological evidence showing that monoamines have diverse effects on early sensory processing, including changes in gain and in the precision of neuronal responses to sensory inputs. We also highlight the substantial evidence for complementarity between these neuromodulatory systems with different patterns of innervation across brain areas and cortical layers as well as distinct neuromodulatory actions. Studying the effects of neuromodulators at various target sites is a crucial step in the development of a mechanistic understanding of neuronal information processing in the healthy brain and in the generation and maintenance of mental diseases.

Differential Expression of Dopamine D5 Receptors across Neuronal Subtypes in Macaque Frontal Eye Field

Dopamine signaling in the prefrontal cortex (PFC) is important for cognitive functions, yet very little is known about the expression of the D5 class of dopamine receptors (D5Rs) in this region. To address this, we co-stained for D5Rs, pyramidal neurons (neurogranin+), putative long-range projection pyramidal neurons (SMI-32+), and several classes of inhibitory interneuron (parvalbumin+, calbindin+, calretinin+, somatostatin+) within the frontal eye field (FEF): an area within the PFC involved in the control of visual spatial attention. We then quantified the co-expression of D5Rs with markers of different cell types across different layers of the FEF. We show that: (1) D5Rs are more prevalent on pyramidal neurons than on inhibitory interneurons. (2) D5Rs are disproportionately expressed on putative long-range projecting pyramidal neurons. The disproportionately high expression of D5Rs on long-range projecting pyramidals, compared to interneurons, was particularly pronounced in layers II-III. Together these results indicate that the engagement of D5R-dependent mechanisms in the FEF varies depending on cell type and cortical layer, and suggests that non-locally projecting neurons contribute disproportionately to functions involving the D5R subtype.

Neuromodulation of Prefrontal Cortex in Non-Human Primates by Dopaminergic Receptors during Rule-Guided Flexible Behavior and Cognitive Control

The prefrontal cortex (PFC) is indispensable for several higher-order cognitive and executive capacities of primates, including representation of salient stimuli in working memory (WM), maintenance of cognitive task set, inhibition of inappropriate responses and rule-guided flexible behavior. PFC networks are subject to robust neuromodulation from ascending catecholaminergic systems. Disruption of these systems in PFC has been implicated in cognitive deficits associated with several neuropsychiatric disorders. Over the past four decades, a considerable body of work has examined the influence of dopamine on macaque PFC activity representing spatial WM. There has also been burgeoning interest in neuromodulation of PFC circuits involved in other cognitive functions of PFC, including representation of rules to guide flexible behavior. Here, we review recent neuropharmacological investigations conducted in our laboratory and others of the role of PFC dopamine receptors in regulating rule-guided behavior in non-human primates. Employing iontophoresis, we examined the effects of local manipulation of dopaminergic subtypes on neuronal activity during performance of rule-guided pro- and antisaccades, an experimental paradigm sensitive to PFC integrity, wherein deficits in performance are reliably observed in many neuropsychiatric disorders. We found dissociable effects of dopamine receptors on neuronal activity for rule representation and oculomotor responses and discuss these findings in the context of prior studies that have examined the role of dopamine in spatial delayed response tasks, attention, target selection, abstract rules, visuomotor learning and reward. The findings we describe here highlight the common features, as well as heterogeneity and context dependence of dopaminergic neuromodulation in regulating the efficacy of cognitive functions of PFC in health and disease.

Buspirone Counteracts MK-801-Induced Schizophrenia-Like Phenotypes through Dopamine D3 Receptor Blockade

Background: Several efforts have been made to develop effective antipsychotic drugs. Currently, available antipsychotics are effective on positive symptoms, less on negative symptoms, but not on cognitive impairment, a clinically relevant dimension of schizophrenia. Drug repurposing offers great advantages over the long-lasting, risky and expensive, de novo drug discovery strategy. To our knowledge, the possible antipsychotic properties of buspirone, an azapirone anxiolytic drug marketed in 1986 as serotonin 5-HT_1A receptor (5-HT_1AR) partial agonist, have not been extensively investigated despite its intriguing pharmacodynamic profile, which includes dopamine D₃ (D₃R) and D₄ receptor (D₄R) antagonist activity. Multiple lines of evidence point to D₃R as a valid therapeutic target for the treatment of several neuropsychiatric disorders including schizophrenia. In the present study, we tested the hypothesis that buspirone, behaving as dopamine D₃R antagonist, may have antipsychotic-like activity.

Materials and Methods: Effects of acute administration of buspirone was assessed on a wide-range of schizophrenia-relevant abnormalities induced by a single administration of the non-competitive NMDAR antagonist MK-801, in both wild-type mice (WT) and D₃R-null mutant mice (D₃R^-/-).

Results: Buspirone (3 mg⋅kg^-1, i.p.) was devoid of cataleptogenic activity in itself, but resulted effective in counteracting disruption of prepulse inhibition (PPI), hyperlocomotion and deficit of temporal order recognition memory (TOR) induced by MK-801 (0.1 mg⋅kg^-1, i.p.) in WT mice. Conversely, in D₃R^-/- mice, buspirone was ineffective in preventing MK-801-induced TOR deficit and it was only partially effective in blocking MK-801-stimulated hyperlocomotion.

Conclusion: Taken together, these results indicate, for the first time, that buspirone, might be a potential therapeutic medication for the treatment of schizophrenia. In particular, buspirone, through its D₃R antagonist activity, may be a useful tool for improving the treatment of cognitive deficits in schizophrenia that still represents an unmet need of this disease.

D3 Receptors Regulate Excitability in a Unique Class of Prefrontal Pyramidal Cells

The D3 dopamine receptor, a member of the G_i-coupled D2 family of dopamine receptors, is expressed throughout limbic circuits affected in neuropsychiatric disorders, including prefrontal cortex (PFC). These receptors are important for prefrontal executive function because pharmacological and genetic manipulations that affect prefrontal D3 receptors alter anxiety, social interaction, and reversal learning. However, the mechanisms by which D3 receptors regulate prefrontal circuits and whether D3 receptors regulate specific prefrontal subnetworks remains unknown. Here, we combine dopamine receptor reporter lines, anatomical tracing techniques, and electrophysiology to show that D3 receptor expression defines a novel subclass of layer 5 glutamatergic pyramidal cell in mouse PFC (either sex). D3-receptor-expressing pyramidal neurons are electrophysiologically and anatomically separable from neighboring neurons expressing D1 or D2 receptors based on their dendritic morphology and subthreshold and suprathreshold intrinsic excitability. D3-receptor-expressing neurons send axonal projections to intratelencephalic (IT) targets, including contralateral cortex, nucleus accumbens, and basolateral amygdala. Within these neurons, D3 receptor activation was found to regulate low-voltage-activated Ca_V3.2 calcium channels localized to the axon initial segment, which suppressed action potential (AP) excitability, particularly when APs occurred at high frequency. Therefore, these data indicate that D3 receptors regulate the excitability of a unique, IT prefrontal cell population, thereby defining novel circuitry and cellular actions for D3 receptors in PFC.SIGNIFICANCE STATEMENT The D3 dopamine receptor, a member of the G_i-coupled D2 family of dopamine receptors, are expressed throughout limbic circuits, including prefrontal cortex (PFC). They are of broad interest as a site for therapeutic intervention in serious mental illness, yet we know very little about their distribution or function within PFC. Here, we show that D3 receptors define a unique population of glutamatergic principal cells in mouse PFC that largely lack expression of D1 or D2 receptors. Within these cells, we find that D3 receptors regulate the ability to generate high-frequency action potential bursts through mechanisms not supported by other dopamine receptors. These results define unique circuitry and cellular actions for D3 receptors in regulating PFC networks.

Inhibitory Effects of Dopamine Receptor D1 Agonist on Mammary Tumor and Bone Metastasis

Dopaminergic signaling plays a critical role in the nervous system, but little is known about its potential role in breast cancer and bone metabolism. A screening of ~1,000 biologically active compounds revealed that a selective agonist of dopamine receptor D1 (DRD1), A77636, inhibited proliferation of 4T1.2 mammary tumor cells as well as MDA-MB-231 breast cancer cells. Herein, we examined the effect of A77636 on bone quality using a mouse model of bone metastasis from mammary tumor. A77636 inhibited migration of cancer cells in a DRD1-dependent fashion and suppressed development of bone-resorbing osteoclasts by downregulating NFATc1 through the elevation of phosphorylation of eIF2α. In the mouse model of bone metastasis, A77636 reduced osteolytic lesions and prevented mechanical weakening of the femur and tibia. Collectively, we expect that dopaminergic signaling might provide a novel therapeutic target for breast cancer and bone metastasis.

Family-based association study of DRD4 gene in methylphenidate-responded Attention Deficit/Hyperactivity Disorder

The 48-basepair (48-bp) variable number tandem repeat (VNTR) polymorphism in exon 3 of the dopamine receptor D4 gene (DRD4) is implicated in the etiology of attention-deficit/ hyperactivity disorder (ADHD). In particular, ADHD in European-ancestry population is associated with an increased prevalence of the 7-repeat (7R) allele of the exon 3 VNTR. However, it is intriguing to note that the 7R allele has been found to be of very low prevalence in the Chinese general population. In a previous case-control study, our research team had found that the 7R allele was similarly absent in Chinese ADHD children in Hong Kong. Instead, there was an increased prevalence of the 2R allele in Chinese ADHD children. Interestingly, in Asian samples, the 2R allele had been found to be an evolutionary derivative of the 7R allele with equivalent biochemical functionality. So, the finding of an association between ADHD and 2R allele in Chinese population does not exactly contradict the original 7R allele finding in European-ancestry population. However, given the potential pitfall of population stratification in the previous case-control design, this current study tested the 2R allele and ADHD association using a methodologically more rigorous family-based approach on 33 Chinese ADHD probands who had favorable clinical responses to stimulant medication (methylphenidate). Haplotype Relative Risk (HRR) analysis and Transmission Disequilibrium Test (TDT) both showed a significant preferential transmission of the 2R allele from the biological parents to ADHD probands (p_one-tailed = 0.038, OR = 2.04; p_one-tailed = 0.048, OR = 2.29, respectively). A second hypothesis speculates that it is the deviation, including 7R and 2R alleles, from the conserved ancestral 4R allele which confers risk to ADHD. Thus, a preferential transmission of non-4R alleles, against the 4R allele, from biological parents to their ADHD probands is predicted. Both HRR analysis and TDT confirmed such prediction (p_one-tailed = 0.029, OR = 2.07; p_one-tailed = 0.032, OR = 2.43, respectively). This study re-confirmed the original finding of a previous study that in Chinese population, the 2R allele of the DRD4 exon 3 VNTR was related to ADHD. This endorses the general thesis that DRD4 exon 3 VNTR polymorphism is related to ADHD, despite that the exact length or number of repeats of the associated alleles varies across ethnicity. This in turn supports the dopamine dysregulation theory of ADHD.

Dopamine D1 and D2 Receptors Make Dissociable Contributions to Dorsolateral Prefrontal Cortical Regulation of Rule-Guided Oculomotor Behavior

Studies of neuromodulation of spatial short-term memory have shown that dopamine D1 receptor (D1R) stimulation in dorsolateral prefrontal cortex (DLPFC) dose-dependently modulates memory activity, whereas D2 receptors (D2Rs) selectively modulate activity related to eye movements hypothesized to encode movement feedback. We examined localized stimulation of D1Rs and D2Rs on DLPFC neurons engaged in a task involving rule representation in memory to guide appropriate eye movements toward or away from a visual stimulus. We found dissociable effects of D1R and D2R on DLPFC physiology. D1R stimulation degrades memory activity for the task rule and increases stimulus-related selectivity. In contrast, D2R stimulation affects motor activity tuning only when eye movements are made to the stimulus. Only D1R stimulation degrades task performance and increases impulsive responding. Our results suggest that D1Rs regulate rule representation and impulse control, whereas D2Rs selectively modulate eye-movement-related dynamics and not rule representation in the DLPFC.

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

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

Expression of a Novel D4 Dopamine Receptor in the Lamprey Brain. Evolutionary Considerations about Dopamine Receptors

Numerous data reported in lampreys, which belong to the phylogenetically oldest branch of vertebrates, show that the dopaminergic system was already well developed at the dawn of vertebrate evolution. The expression of dopamine in the lamprey brain is well conserved when compared to other vertebrates, and this is also true for the D2 receptor. Additionally, the key role of dopamine in the striatum, modulating the excitability in the direct and indirect pathways through the D1 and D2 receptors, has also been recently reported in these animals. The moment of divergence regarding the two whole genome duplications occurred in vertebrates suggests that additional receptors, apart from the D1 and D2 previously reported, could be present in lampreys. We used in situ hybridization to characterize the expression of a novel dopamine receptor, which we have identified as a D4 receptor according to the phylogenetic analysis. The D4 receptor shows in the sea lamprey a more restricted expression pattern than the D2 subtype, as reported in mammals. Its main expression areas are the striatum, lateral and ventral pallial sectors, several hypothalamic regions, habenula, and mesencephalic and rhombencephalic motoneurons. Some expression areas are well conserved through vertebrate evolution, as is the case of the striatum or the habenula, but the controversies regarding the D4 receptor expression in other vertebrates hampers for a complete comparison, especially in rhombencephalic regions. Our results further support that the dopaminergic system in vertebrates is well conserved and suggest that at least some functions of the D4 receptor were already present before the divergence of lampreys.

The attentional networks in benign epilepsy with centrotemporal spikes

Benign epilepsy affecting children with normal mental development often occurs at a particular age, responds well to medication, and could be resolved completely by puberty. Although several studies have shown neuropsychological disabilities of children with benign epilepsy with centrotemporal spikes (BECTS), there is no clear evidence about the impairment of attentional systems and the comorbidity of attentional problems. Our research was based on the attention network model and assessed the characteristics of three anatomically defined subnetworks (alerting, orienting, and executive control) of 90 children with BECTS and 90 healthy children. All the subjects enrolled in the study participated in the attention network test (ANT) with assessment of both the reaction time (RT) and accuracy of the test. The results indicated that the performance of healthy controls was significantly better in orienting of attentional system (P<0.001) and the accuracy of attention network test (P<0.001), compared with that of children affected by BECTS. The grand mean effect (higher score worse) was significantly higher (P<0.001) in the patient group than that in the control group. The multiple linear regression analysis revealed a positive correlation between the age of onset and the accuracy of attention network test results, and a negative correlation between the age of onset and the results of grand mean effect. A negative correlation was observed between spike index (SI) of the non-REM sleep stage and the accuracy of attention network test results. We found no relationship between the grand mean effect and clinical factors such as gender, duration of clinical course, duration of seizures, total number of seizures, severity of seizures (seizure frequency), hemispheric lateralization of electroencephalograph (EEG), and the awake SI. Furthermore, no relationship was observed between the clinical factors and the accuracy of the test results. The findings showed that BECTS is associated with impaired attentional networks, and impairments are greater at younger ages of onset.

Doping the Mind: Dopaminergic Modulation of Prefrontal Cortical Cognition

The prefrontal cortex is the center of cognitive control. Processing in prefrontal cortical circuits enables us to direct attention to behaviorally relevant events; to memorize, structure, and categorize information; and to learn new concepts. The prefrontal cortex receives strong projections from midbrain neurons that use dopamine as a transmitter. In this article, we review the crucial role dopamine plays as a modulator of prefrontal cognitive functions, in the primate brain in particular. Following a summary of the anatomy and physiology of the midbrain dopamine system, we focus on recent studies that investigated dopaminergic effects in prefrontal cortex at the cellular level. We then discuss how unregulated prefrontal dopamine signaling could contribute to major disorders of cognition. The studies highlighted in this review demonstrate the powerful influence dopamine exerts on the mind.

Disruption of columnar and laminar cognitive processing in primate prefrontal cortex following cocaine exposure

Prefrontal cortical activity in primate brain plays a critical role in cognitive processes involving working memory and the executive control of behavior. Groups of prefrontal cortical neurons within specified cortical layers along cortical minicolumns differentially generate inter- and intra-laminar firing to process relevant information for goal oriented behavior. However, it is not yet understood how cocaine modulates such differential firing in prefrontal cortical layers. Rhesus macaque nonhuman primates (NHPs) were trained in a visual delayed match-to-sample (DMS) task while the activity of prefrontal cortical neurons (areas 46, 8 and 6) was recorded simultaneously with a custom multielectrode array in cell layers 2/3 and 5. Animals were reinforced with juice for correct responses. The first half of the recording session (control) was conducted following saline injection and in the second half of the same session cocaine was administered. Prefrontal neuron activity with respect to inter- and intra-laminar firing in layers 2/3 and 5 was assessed in the DMS task before and after the injection of cocaine. Results showed that firing rates of both pyramidal cells and interneurons increased on Match phase presentation and the Match Response (MR) in both control and cocaine halves of the session. Differential firing under cocaine vs. control in the Match phase was increased for interneurons but decreased for pyramidal cells. In addition, functional' interactions between prefrontal pyramidal cells in layer 2/3 and 5 decreased while intra-laminar cross-correlations in both layers increased. These neural recordings demonstrate that prefrontal neurons differentially encode and process information within and between cortical cell layers via cortical columns which is disrupted in a differential manner by cocaine: administration.

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.

In vitro and in vivo evaluation of N-{2-[4-(3-Cyanopyridin-2-yl)piperazin-1-yl]ethyl}-3-[(11) C]methoxybenz-amide, a positron emission tomography (PET) radioligand for dopamine D4 receptors, in rodents

The D4 dopamine receptor belongs to the D2 -like family of dopamine receptors, and its exact regional distribution in the central nervous system is still a matter of considerable debate. The availability of a selective radioligand for the D4 receptor with suitable properties for positron emission tomography (PET) would help resolve issues of D4 receptor localization in the brain, and the presumed diurnal change of expressed protein in the eye and pineal gland. We report here on in vitro and in vivo characteristics of the high-affinity D4 receptor-selective ligand N-{2-[4-(3-cyanopyridin-2-yl)piperazin-1-yl]ethyl}-3-[(11) C]methoxybenzamide ([(11) C]2) in rat. The results provide new insights on the in vitro properties that a brain PET dopamine D4 radioligand should possess in order to have improved in vivo utility in rodents.

Neural Substrates of Dopamine D2 Receptor Modulated Executive Functions in the Monkey Prefrontal Cortex

Dopamine D2 receptors (D2R) play a major role in cognition, mood and motor movements. Their blockade by antipsychotic drugs reduces hallucinatory and delusional behaviors in schizophrenia, but often fails to alleviate affective and cognitive dysfunctions. The prefrontal cortex (PFC) expresses D2R and is altered in schizophrenia. We investigated how D2R modulate behavior and PFC function in monkeys. Two monkeys learned new and performed highly familiar visuomotor associations, where each cue was associated with a saccade to a right or left target. We recorded neural spikes and local field potentials from multiple electrodes while injecting the D2R antagonist eticlopride in the lateral PFC. Blocking prefrontal D2R impaired associative learning and cognitive flexibility, reduced motivation, but left the performance of familiar associations intact. Eticlopride reduced saccade-direction selectivity of prefrontal neurons, leading to a decrease in neural information about the associations, and an increase in alpha oscillations. These results, together with our recent study using a D1R antagonist, suggest that D1R and D2R in the primate lateral PFC cooperate to modulate several executive functions. Our findings help to gain insight into why antipsychotic drugs, with strong antagonistic actions on D2R, fail to ameliorate cognitive and emotional deficits in schizophrenia.

Design, synthesis, lipophilic properties, and binding affinities of potential ligands in positron emission tomography (PET) for visualization of brain dopamine D4 receptors

We report the synthesis of compounds structurally related to the high-affinity dopamine D4 receptor ligand N-{2-[4-(3-cyanopyridin-2-yl)piperazin-1-yl]ethyl}-3-methoxybenzamide (1e). All compounds were specifically designed as potential PET radioligands for brain D4 receptor visualization, having lipophilicity within a range for brain uptake and weak non-specific binding (0.75<cLogP<3.15) and bearing a substituent for easy access to labeling with the positron emitter isotope (11) C or (18) F. The best compound of the series, N-{2-[4-(4-chlorophenyl)piperazin-1-yl]ethyl}-6-fluoropyridine-3-carboxamide (7a), displayed excellent selectivity over D2 and D3 receptors (>100-fold), but its D4 receptor affinity was suboptimal for imaging of brain D4 receptors (Ki =30 nM).

The neurobiology of thought: the groundbreaking discoveries of Patricia Goldman-Rakic 1937-2003

Patricia S. Goldman-Rakic (1937–2003) transformed the study of the prefrontal cortex (PFC) and the neural basis of mental representation, the basic building block of abstract thought. Her pioneering research first identified the dorsolateral PFC (dlPFC) region essential for spatial working memory, and the extensive circuits of spatial cognition. She discovered the cellular basis of working memory, illuminating the dlPFC microcircuitry underlying spatially tuned, persistent firing, whereby precise information can be held “in mind”: persistent firing arises from recurrent excitation within glutamatergic pyramidal cell circuits in deep layer III, while tuning arises from GABAergic lateral inhibition. She was the first to discover that dopamine is essential for dlPFC function, particularly through D1 receptor actions. She applied a host of technical approaches, providing a new paradigm for scientific inquiry. Goldman-Rakic's work has allowed the perplexing complexities of mental illness to begun to be understood at the cellular level, including atrophy of the dlPFC microcircuits subserving mental representation. She correctly predicted that impairments in dlPFC working memory activity would contribute to thought disorder, a cardinal symptom of schizophrenia. Ten years following her death, we look back to see how she inspired an entire field, fundamentally changing our view of cognition and cognitive disorders.

NMDA receptors subserve persistent neuronal firing during working memory in dorsolateral prefrontal cortex

Neurons in the primate dorsolateral prefrontal cortex (dlPFC) generate persistent firing in the absence of sensory stimulation, the foundation of mental representation. Persistent firing arises from recurrent excitation within a network of pyramidal Delay cells. Here, we examined glutamate receptor influences underlying persistent firing in primate dlPFC during a spatial working memory task. Computational models predicted dependence on NMDA receptor (NMDAR) NR2B stimulation, and Delay cell persistent firing was abolished by local NR2B NMDAR blockade or by systemic ketamine administration. AMPA receptors (AMPARs) contributed background depolarization to sustain network firing. In contrast, many Response cells were sensitive to AMPAR blockade and increased firing after systemic ketamine, indicating that models of ketamine actions should be refined to reflect neuronal heterogeneity. The reliance of Delay cells on NMDAR may explain why insults to NMDARs in schizophrenia or Alzheimer's disease profoundly impair cognition.

Neuromodulation of thought: flexibilities and vulnerabilities in prefrontal cortical network synapses

This review describes unique neuromodulatory influences on working memory prefrontal cortical (PFC) circuits that coordinate cognitive strength with arousal state. Working memory arises from recurrent excitation within layer III PFC pyramidal cell NMDA circuits, which are afflicted in aging and schizophrenia. Neuromodulators rapidly and flexibly alter the efficacy of these synaptic connections, while leaving the synaptic architecture unchanged, a process called dynamic network connectivity (DNC). Increases in calcium-cAMP signaling open ion channels in long, thin spines, gating network connections. Inhibition of calcium-cAMP signaling by stimulating α2A-adrenoceptors on spines strengthens synaptic efficacy and increases network firing, whereas optimal stimulation of dopamine D1 receptors sculpts network inputs to refine mental representation. Generalized increases in calcium-cAMP signaling during fatigue or stress disengage dlPFC recurrent circuits, reduce firing and impair top-down cognition. Impaired DNC regulation contributes to age-related cognitive decline, while genetic insults to DNC proteins are commonly linked to schizophrenia.

Executive attention impairment in first-episode schizophrenia

BACKGROUND

We compared the attention abilities of a group of first-episode schizophrenia (FES) patients and a group of healthy participants using the Attention Network Test (ANT), a standard procedure that estimates the functional state of three neural networks controlling the efficiency of three different attentional behaviors, i.e., alerting (achieving and maintaining a state of high sensitivity to incoming stimuli), orienting (ability to select information from sensory input), and executive attention (mechanisms for resolving conflict among thoughts, feelings, and actions).

METHODS

We evaluated 22 FES patients from 17 to 29 years of age with a recent history of a single psychotic episode treated only with atypical neuroleptics, and 20 healthy persons matched with FES patients by sex, age, and educational level as the control group. Attention was estimated using the ANT in which participants indicate whether a central horizontal arrow is pointing to the left or the right. The central arrow may be preceded by spatial or temporal cues denoting where and when the arrow will appear, and may be flanked by other arrows (hereafter, flankers) pointing in the same or the opposite direction.

RESULTS

The efficiency of the alerting, orienting, and executive networks was estimated by measuring how reaction time was influenced by congruency between temporal, spatial, and flanker cues. We found that the control group only demonstrated significantly greater attention efficiency than FES patients in the executive attention network.

CONCLUSIONS

FES patients are impaired in executive attention but not in alerting or orienting attention, suggesting that executive attention deficit may be a primary impairment during the progression of the disease.

Chronic haloperidol-induced spatial memory deficits accompany the upregulation of D(1) and D(2) receptors in the caudate putamen of C57BL/6 mouse

AIMS

Haloperidol (HAL) is an antipsychotic drug that has high affinities to the dopamine D(2), but low affinities to D(1) receptors in the brain. Of brain regions, caudate putamen (CP) has the highest levels of the D(1) and D(2) receptors. In this study we evaluated the spatial memory of C57BL/6 mice following chronic administration of HAL and measured levels of D(1) and D(2) receptors in specific brain regions, with the hypothesis that the D(1) and D(2) receptors in CP are important players in spatial memory function of the brain.

MAIN METHODS

C57BL/6 mice received daily intraperitoneal injections of saline or HAL at 1.0 or 2.0mg/kg/day for 3 or 6 weeks. Two days after the last injection, spontaneous alternation of mice in a Y-maze was evaluated to measure their exploratory behavior and spatial working memory. The Morris water maze test was performed to measure their spatial learning and memory. D(1) and D(2) receptors in specific brain regions were measured by Western-blot analysis.

KEY FINDINGS

HAL treatment for 6 weeks decreased the spontaneous alternation of mice in Y-maze, altered the acquisition process and impaired spatial memory in Morris water maze. The same treatment increased levels of D(1) and D(2) receptors in CP and up-regulated D(2) receptors in the hippocampus, but did not change the receptors in the prefrontal cortex.

SIGNIFICANCE

These results suggest that the D(1) and D(2) receptors in CP are among the main targets of HAL and the receptors in CP play an important role in spatial learning and memory.

Synaptic activity unmasks dopamine D2 receptor modulation of a specific class of layer V pyramidal neurons in prefrontal cortex

Dopamine D2 receptors (D2Rs) play a major role in the function of the prefrontal cortex (PFC), and may contribute to prefrontal dysfunction in conditions such as schizophrenia. Here we report that in mouse PFC, D2Rs are selectively expressed by a subtype of layer V pyramidal neurons that have thick apical tufts, prominent h-current, and subcortical projections. Within this subpopulation, the D2R agonist quinpirole elicits a novel afterdepolarization that generates voltage fluctuations and spiking for hundreds of milliseconds. Surprisingly, this afterdepolarization is masked in quiescent brain slices, but is readily unmasked by physiologic levels of synaptic input which activate NMDA receptors, possibly explaining why this phenomenon has not been reported previously. Notably, we could still elicit this afterdepolarization for some time after the cessation of synaptic stimulation. In addition to NMDA receptors, the quinpirole-induced afterdepolarization also depended on L-type Ca(2+) channels and was blocked by the selective L-type antagonist nimodipine. To confirm that D2Rs can elicit this afterdepolarization by enhancing Ca(2+) (and Ca(2+)-dependent) currents, we measured whole-cell Ca(2+) potentials that occur after blocking Na(+) and K(+) channels, and found quinpirole enhanced these potentials, while the selective D2R antagonist sulpiride had the opposite effect. Thus, D2Rs can elicit a Ca(2+)-channel-dependent afterdepolarization that powerfully modulates activity in specific prefrontal neurons. Through this mechanism, D2Rs might enhance outputs to subcortical structures, contribute to reward-related persistent firing, or increase the level of noise in prefrontal circuits.

Prefrontal dopaminergic and enkephalinergic synaptic accommodation in HIV-associated neurocognitive disorders and encephalitis

Changes in synapse structure occur in frontal neocortex with HIV encephalitis (HIVE) and may contribute to HIV-associated neurocognitive disorders (HAND). A postmortem survey was conducted to determine if mRNAs involved in synaptic transmission are perturbed in dorsolateral prefrontal cortex (DLPFC) in subjects with HIVE or HAND. Expression of the opioid neurotransmitter preproenkephalin mRNA (PENK) was significantly decreased in a sampling of 446 brain specimens from HIV-1 infected people compared to 67 HIV negative subjects. Decreased DLPFC PENK was most evident in subjects with HIVE and/or increased expression of interferon regulatory factor 1 mRNA (IRF1). Type 2 dopamine receptor mRNA (DRD2L) was decreased significantly, but not in the same set of subjects with PENK dysregulation. DRD2L downregulation occurred primarily in the subjects without HIVE or neurocognitive impairment. Subjects with neurocognitive impairment often failed to significantly downregulate DRD2L and had abnormally high IRF1 expression. Conclusion: Dysregulation of synaptic preproenkephalin and DRD2L in frontal neocortex can occur with and without neurocognitive impairment in HIV-infected people. Downregulation of DRD2L in the prefrontal cortex was associated with more favorable neuropsychological and neuropathological outcomes; the failure to downregulate DRD2L was significantly less favorable. PENK downregulation was related neuropathologically to HIVE, but was not related to neuropsychological outcome independently. Emulating endogenous synaptic plasticity pharmacodynamically could enhance synaptic accommodation and improve neuropsychological and neuropathological outcomes in HIV/AIDS.

Evaluation of 18F-labeled benzodioxine piperazine-based dopamine D4 receptor ligands: lipophilicity as a determinate of nonspecific binding

Derivatization of the putative neuroleptic 1-(2,3-dihydrobenzo[1,4]dioxin-6-yl)-4-(4-fluorobenzyl)piperazine (3a) led to a series of new dopamine receptor D4 ligands displaying high affinity (Ki=1.1-15 nM) and D2/D4 subtype selectivities of about 800-6700. These ligands were labeled with the short-lived positron emitter fluorine-18 and analyzed for their potential application for imaging studies by positron emission tomography (PET). In vitro autoradiography was used to determine their nonspecific binding behavior as a result of their structural and thus physicochemical properties. The biodistribution, in vivo stability, and brain uptake of the most promising D4 radioligand candidate were determined. This proved to be 1-(2,3-dihydrobenzo[b][1,4]dioxin-6-yl)-4-((6-fluoropyridin-3-yl)methyl)piperazine ([18F]3d), which revealed an excellent binding pattern with a high selectivity and limited nonspecific binding in vitro. This analogue also exhibited a high stability and an extremely high brain uptake in vivo with specific binding in hippocampus, cortex, colliculus, and cerebellum as determined by ex vivo autoradiography. Thus, [18F]3d appears as a suitable D4 radioligand for in vivo imaging, encouraging continued evaluation by PET studies.

The role of neuromodulators in selective attention

Several classes of neurotransmitters exert modulatory effects on a broad and diverse population of neurons throughout the brain. Some of these neuromodulators, especially acetylcholine and dopamine, have long been implicated in the neural control of selective attention. We review recent evidence and evolving ideas about the importance of these neuromodulatory systems in attention, particularly visual selective attention. We conclude that, although our understanding of their role in the neural circuitry of selective attention remains rudimentary, recent research has begun to suggest unique contributions of neuromodulators to different forms of attention, such as bottom-up and top-down attention.

Catecholamine influences on dorsolateral prefrontal cortical networks

The symptoms of attention-deficit/hyperactivity disorder (ADHD) involve impairments in prefrontal cortical top-down regulation of attention and behavior. All current pharmacological treatments for ADHD facilitate catecholamine transmission, and basic research suggests that these compounds have prominent actions in the prefrontal cortex (PFC). The dorsolateral PFC is especially sensitive to levels of norepinephrine and dopamine, whereby either too little or too much markedly impairs PFC function. Recent physiological studies have shown that norepinephrine strengthens PFC network connectivity and maintains persistent firing during a working memory task through stimulation of postsynaptic α(2A)-adrenoceptors on PFC neurons. Conversely, dopamine acts at D1 receptors to narrow spatial tuning, sculpting network inputs to decrease noise (i.e., stabilization of the representation). The stimulant medications and atomoxetine appear to enhance PFC function by indirectly increasing these catecholamine actions through blockade of norepinephrine and/or dopamine transporters. In contrast, guanfacine mimics the enhancing effects of norepinephrine at postsynaptic α(2A)-receptors in the PFC, strengthening network connectivity. Stronger PFC regulation of attention, behavior, and emotion likely contributes to the therapeutic effects of these medications for the treatment of ADHD.

Control of visual cortical signals by prefrontal dopamine

The prefrontal cortex (PFC) is thought to modulate sensory signals in posterior cortices during top-down attention^1,2, yet little is known about the underlying neural circuitry. Experimental and clinical evidence suggest that prefrontal dopamine plays an important role in cognitive functions³, acting predominantly through D1 receptors (D1Rs). Here we show that dopamine D1Rs mediate prefrontal control of signals within visual cortex. We pharmacologically altered D1R-mediated activity within the frontal eye field (FEF) of the PFC and measured its effects on the responses of neurons within visual cortex. This manipulation was sufficient to enhance the response magnitude, orientation selectivity and response reliability of neurons in area V4 to an extent comparable with the known effects of top-down attention. The observed enhancement in V4 signals was restricted to neurons with response fields (RFs) overlapping the part of visual space affected by the D1R manipulation. Altering D1R or D2R-mediated FEF activity increased saccadic target selection, but the D2R manipulation did not enhance V4 signals. Our results identify a role of D1Rs in mediating the control of visual cortical signals by the PFC and demonstrate how processing within sensory areas can be altered in mental disorders involving prefrontal dopamine.

Infralimbic D2 receptors are necessary for fear extinction and extinction-related tone responses

BACKGROUND

Fear extinction is dependent on plasticity in the infralimbic prefrontal cortex, an area heavily innervated by midbrain dopaminergic inputs. Dopamine D2 receptors are concentrated in infralimbic output neurons that are involved in the suppression of conditioned fear after extinction. Here, we examined the specific role of infralimbic D2 receptors in mediating associative learning underlying fear extinction using the selective D2 antagonist raclopride.

METHODS

Raclopride was administered systemically or infused into the infralimbic prefrontal cortex before fear extinction, and extinction retention was tested the following day. Rats were also prepared for single-unit recording in the infralimbic prefrontal cortex to assess the effect of raclopride on firing properties.

RESULTS

We found that systemic injection of raclopride given before extinction impaired retrieval of extinction when rats were tested drug-free the next day but also induced catalepsy during extinction training. To determine whether impaired extinction was due to impaired motor function or disruption of extinction consolidation, we infused raclopride directly into the infralimbic prefrontal cortex. Raclopride infused immediately before extinction training did not produce motor deficits but impaired recall of extinction when tested drug-free. Furthermore, in animals that underwent extinction training, systemic raclopride reduced the tone responsiveness of infralimbic prefrontal cortex neurons in layers 5/6, with no changes in average firing rate.

CONCLUSIONS

We suggest that D2 receptors facilitate extinction by increasing the signal-to-noise of infralimbic prefrontal cortex neurons that consolidate extinction.

D2 and D4 dopamine receptor mRNA distribution in pyramidal neurons and GABAergic subpopulations in monkey prefrontal cortex: implications for schizophrenia treatment

D2 and D4 dopamine receptors play an important role in cognitive functions in the prefrontal cortex and they are involved in the pathophysiology of neuropsychiatric disorders such as schizophrenia. The eventual effect of dopamine upon pyramidal neurons in the prefrontal cortex depends on which receptors are expressed in the different neuronal populations. Parvalbumin and calbindin mark two subpopulations of cortical GABAergic interneurons that differently innervate pyramidal cells. Recent hypotheses about schizophrenia hold that the root of the illness is a dysfunction of parvalbumin chandelier cells that produces disinhibition of pyramidal cells. In the present work we report double in situ hybridization histochemistry experiments to determine the prevalence of D2 receptor mRNA and D4 receptor mRNA in glutamatergic neurons, GABAergic interneurons and both parvalbumin and calbindin GABAergic subpopulations in monkey prefrontal cortex layer V. We found that around 54% of glutamatergic neurons express D2 mRNA and 75% express D4 mRNA, while GAD-positive interneurons express around 34% and 47% respectively. Parvalbumin cells mainly expressed D4 mRNA (65%) and less D2 mRNA (15-20%). Finally, calbindin cells expressed both receptors in similar proportions (37%). We hypothesized that D4 receptor could be a complementary target in designing new antipsychotics, mainly because of its predominance in parvalbumin interneurons.

The dopamine D4 receptor: biochemical and signalling properties

Dopamine is an important neurotransmitter that regulates several key functions in the brain, such as motor output, motivation and reward, learning and memory, and endocrine regulation. Dopamine does not mediate fast synaptic transmission, but rather modulates it by triggering slow-acting effects through the activation of dopamine receptors, which belong to the G-protein-coupled receptor superfamily. Besides activating different effectors through G-protein coupling, dopamine receptors also signal through interaction with a variety of proteins, collectively termed dopamine receptor-interacting proteins. We focus on the dopamine D4 receptor, which contains an important polymorphism in its third intracellular loop. This polymorphism has been the subject of numerous studies investigating links with several brain disorders, such as attention-deficit hyperactivity disorder and schizophrenia. We provide an overview of the structure, signalling properties and regulation of dopamine D4 receptors, and briefly discuss their physiological and pathophysiological role in the brain.

A hybrid indoloquinolizidine peptide as allosteric modulator of dopamine D1 receptors

The indoloquinolizidine-peptide 28 [(3S,12bR)-N-((S)-1-((S)-1-((S)-2-carbamoylpyrrolidin-1-yl)-3-(4-fluorophenyl)-1-oxopropan-2-ylamino)-4-cyclohexyl-1-oxobutan-2-yl)-1,2,3,4,6,7,12, 12b-octahydroindolo[2,3-a]quinolizine-3-carboxamide], a trans-indoloquinolizidine-peptide hybrid obtained by a combinatorial approach, behaved as an orthosteric ligand of all dopamine D(2)-like receptors (D(2), D(3), and D(4)) and dopamine D(5) receptors, but as a negative allosteric modulator of agonist and antagonist binding to striatal dopamine D(1) receptors. Indoloquinolizidine-peptide 28 induced a concentration-dependent hyperbolic increase in the antagonist apparent equilibrium dissociation constant values and altered the dissociation kinetics of dopamine D(1) receptor antagonists. The negative allosteric modulation was also found when agonist binding to D(1) receptors was assayed. Indoloquinolizidine-peptide 28 was a weak ago-allosteric modulator but markedly led to a decreased potency without decreasing the maximum partial/full agonist-mediated effect on cAMP levels. Compounds able to decrease the potency while preserving the efficacy of D(1) receptor agonists are promising for exploration in psychotic pathologies.

Reward-seeking and discrimination deficits displayed by hypodopaminergic mice are prevented in mice lacking dopamine D4 receptors

The dopamine D4 receptor (D4R) is predominantly expressed in the prefrontal cortex, a brain area that integrates motor, rewarding, and cognitive information. Because participation of D4Rs in executive learning is largely unknown, we challenged D4R knockout mice (Drd4(-/-)) and their wild-type (WT) littermates, neonatally treated with 6-hydroxydopamine (6-OHDA; icv) or vehicle in two operant learning paradigms. A continuous reinforcement task, in which one food-pellet was delivered after every lever press, showed that 6-OHDA-treated mice (hypodopaminergic) WT mice pressed the reinforcing lever at much lower rates than normodopaminergic WT mice. In contrast, Drd4(-/-) mice displayed increased lever pressing rates, regardless of their dopamine content. In another study, mice were trained to solve an operant two-choice task in which a first showing lever was coupled to the delivery of one food pellet only after a second lever emerged. Interval between presentation of both levers was initially 12 s and progressively shortened to 6, 2, and finally 0.5 s. Normodopaminergic WT mice obtained a pellet reward in more than 75% of the trials at 12, 6, and 2 s, whereas hypodopaminergic WT mice were severely impaired to select the reward-paired lever. Absence of D4Rs was not detrimental in this task. Moreover, hypodopaminergic Drd4(-/-) mice were as efficient as their normodopaminergic Drd4(-/-) siblings in selecting the reward-paired lever. In summary, hypodopaminergic mice exhibit severe impairments to retrieve rewards in two operant positive reinforcement tasks, but these deleterious effects are totally prevented in the absence of functional D4Rs.