Potentiation of NMDA receptor currents by dopamine D1 receptors in prefrontal cortex

Modulation of excitatory neurotransmission by neuronal/glial signalling molecules: interplay between purinergic and glutamatergic systems

Glutamate is the main excitatory neurotransmitter of the central nervous system (CNS), released both from neurons and glial cells. Acting via ionotropic (NMDA, AMPA, kainate) and metabotropic glutamate receptors, it is critically involved in essential regulatory functions. Disturbances of glutamatergic neurotransmission can be detected in cognitive and neurodegenerative disorders. This paper summarizes the present knowledge on the modulation of glutamate-mediated responses in the CNS. Emphasis will be put on NMDA receptor channels, which are essential executive and integrative elements of the glutamatergic system. This receptor is crucial for proper functioning of neuronal circuits; its hypofunction or overactivation can result in neuronal disturbances and neurotoxicity. Somewhat surprisingly, NMDA receptors are not widely targeted by pharmacotherapy in clinics; their robust activation or inhibition seems to be desirable only in exceptional cases. However, their fine-tuning might provide a promising manipulation to optimize the activity of the glutamatergic system and to restore proper CNS function. This orchestration utilizes several neuromodulators. Besides the classical ones such as dopamine, novel candidates emerged in the last two decades. The purinergic system is a promising possibility to optimize the activity of the glutamatergic system. It exerts not only direct and indirect influences on NMDA receptors but, by modulating glutamatergic transmission, also plays an important role in glia-neuron communication. These purinergic functions will be illustrated mostly by depicting the modulatory role of the purinergic system on glutamatergic transmission in the prefrontal cortex, a CNS area important for attention, memory and learning.

Age-Dependent D1-D2 Receptor Coactivation in the Lateral Orbitofrontal Cortex Potentiates NMDA Receptors and Facilitates Cognitive Flexibility

The orbitofrontal cortex (OFC) integrates information about the environment to guide decision-making. Glutamatergic synaptic transmission mediated through N-methyl-d-aspartate receptors is required for optimal functioning of the OFC. Additionally, abnormal dopamine signaling in this region has been implicated in impulsive behavior and poor cognitive flexibility. Yet, despite the high prevalence of psychostimulants prescribed for attention deficit/hyperactivity disorder, there is little information on how dopamine modulates synaptic transmission in the juvenile or the adult OFC. Using whole-cell patch-clamp recordings in OFC pyramidal neurons, we demonstrated that while dopamine or selective D2-like receptor (D2R) agonists suppress excitatory synaptic transmission of juvenile or adult lateral OFC neurons; in juvenile lateral OFC neurons, higher concentrations of dopamine can target dopamine receptors that couple to a phospholipase C (PLC) signaling pathway to enhance excitatory synaptic transmission. Interfering with the formation of a putative D1R-D2R interaction blocked the potentiation of excitatory synaptic transmission. Furthermore, targeting the putative D1R-D2R complex with a biased agonist, SKF83959, not only enhanced excitatory synaptic transmission in a PLC-dependent manner, but also improved the performance of juvenile rats on a reversal-learning task. Our results demonstrate that dopamine signaling in the lateral OFC differs between juveniles and adults, through potential crosstalk between dopamine receptor subtypes.

Novel Therapeutic GPCRs for Psychiatric Disorders

G protein-coupled receptors (GPCRs) are the most common targets of the neuropharmacological drugs in the central nervous system (CNS). GPCRs are activated by manifold neurotransmitters, and their activation in turn evokes slow synaptic transmission. They are deeply involved in multiple neurological and psychiatric disorders such as Parkinson’s disease and schizophrenia. In the brain, the striatum is strongly innervated by the ventral tegmental area (VTA) and plays a central role in manifestation of psychiatric disorders. Recently, anatomical and comprehensive transcriptome analysis of the non-odorant GPCR superfamily revealed that the orphan GPCRs GPR88, GPR6, and GPR52, as well as dopamine D1 and D2 receptors and the adenosine A2a receptor, are the most highly enriched in the rodent striatum. Genetically engineered animal models and molecular biological studies have suggested that these striatally enriched GPCRs have a potential to be therapeutic psychiatric receptors. This review summarizes the current understanding of the therapeutic GPCR candidates for psychiatric disorders.

Effect of dopaminergic D1 receptors on plasticity is dependent of serotoninergic 5-HT1A receptors in L5-pyramidal neurons of the prefrontal cortex

Major depression and schizophrenia are associated with dysfunctions of serotoninergic and dopaminergic systems mainly in the prefrontal cortex (PFC). Both serotonin and dopamine are known to modulate synaptic plasticity. 5-HT1A receptors (5-HT1ARs) and dopaminergic type D1 receptors are highly represented on dendritic spines of layer 5 pyramidal neurons (L5PyNs) in PFC. How these receptors interact to tune plasticity is poorly understood. Here we show that D1-like receptors (D1Rs) activation requires functional 5HT1ARs to facilitate LTP induction at the expense of LTD. Using 129/Sv and 5-HT1AR-KO mice, we recorded post-synaptic currents evoked by electrical stimulation in layer 2/3 after activation or inhibition of D1Rs. High frequency stimulation resulted in the induction of LTP, LTD or no plasticity. The D1 agonist markedly enhanced the NMDA current in 129/Sv mice and the percentage of L5PyNs displaying LTP was enhanced whereas LTD was reduced. In 5-HT1AR-KO mice, the D1 agonist failed to increase the NMDA current and orientated the plasticity towards L5PyNs displaying LTD, thus revealing a prominent role of 5-HT1ARs in dopamine-induced modulation of plasticity. Our data suggest that in pathological situation where 5-HT1ARs expression varies, dopaminergic treatment used for its ability to increase LTP could turn to be less and less effective.

Chronic cocaine disrupts mesocortical learning mechanisms

The addictive power of drugs of abuse such as cocaine comes from their ability to hijack natural reward and plasticity mechanisms mediated by dopamine signaling in the brain. Reward learning involves burst firing of midbrain dopamine neurons in response to rewards and cues predictive of reward. The resulting release of dopamine in terminal regions is thought to act as a teaching signaling to areas such as the prefrontal cortex and striatum. In this review, we posit that a pool of extrasynaptic dopaminergic D1-like receptors activated in response to dopamine neuron burst firing serve to enable synaptic plasticity in the prefrontal cortex in response to rewards and their cues. We propose that disruptions in these mechanisms following chronic cocaine use contribute to addiction pathology, in part due to the unique architecture of the mesocortical pathway. By blocking dopamine reuptake in the cortex, cocaine elevates dopamine signaling at these extrasynaptic receptors, prolonging D1-receptor activation and the subsequent activation of intracellular signaling cascades, and thus inducing long-lasting maladaptive plasticity. These cellular adaptations may account for many of the changes in cortical function observed in drug addicts, including an enduring vulnerability to relapse. Therefore, understanding and targeting these neuroadaptations may provide cognitive benefits and help prevent relapse in human drug addicts.

Methamphetamine-induced short-term increase and long-term decrease in spatial working memory affects protein Kinase M zeta (PKMζ), dopamine, and glutamate receptors

Methamphetamine (MA) is a toxic, addictive drug shown to modulate learning and memory, yet the neural mechanisms are not fully understood. We investigated the effects of 2 weekly injections of MA (30 mg/kg) on working memory using the radial 8-arm maze (RAM) across 5 weeks in adolescent-age mice. MA-treated mice show a significant improvement in working memory performance 1 week following the first MA injection compared to saline-injected controls. Following 5 weeks of MA abstinence mice were re-trained on a reference and working memory version of the RAM to assess cognitive flexibility. MA-treated mice show significantly more working memory errors without effects on reference memory performance. The hippocampus and dorsal striatum were assessed for expression of glutamate receptors subunits, GluA2 and GluN2B; dopamine markers, dopamine 1 receptor (D1), dopamine transporter (DAT) and tyrosine hydroxylase (TH); and memory markers, protein kinase M zeta (PKMζ) and protein kinase C zeta (PKCζ). Within the hippocampus, PKMζ and GluA2 are both significantly reduced after MA supporting the poor memory performance. Additionally, a significant increase in GluN2B and decrease in D1 identifies dysregulated synaptic function. In the striatum, MA treatment increased cytosolic DAT and TH levels associated with dopamine hyperfunction. MA treatment significantly reduced GluN2B while increasing both PKMζ and PKCζ within the striatum. We discuss the potential role of PKMζ/PKCζ in modulating dopamine and glutamate receptors after MA treatment. These results identify potential underlying mechanisms for working memory deficits induced by MA.

Canadian Association of Neurosciences Review: The Role of Dopamine Receptor Function in Neurodegenerative Diseases

Dopamine (DA) receptors, which are heavily expressed in the caudate/putamen of the brain, represent the molecular target of several drugs used in the treatment of various neurological disorders, such as Parkinson's disease. Although most of the drugs are very effective in alleviating the symptoms associated with these conditions, their long-term utilization could lead to the development of severe side-effects. In addition to uncovering novel mediators of physiological DA receptor functions, recent research advances are suggesting a role of these receptors in toxic effects on neurons. For instance, accumulating evidence indicates that DA receptors, particularly D1 receptors, are central in the neuronal toxicity induced by elevated synaptic levels of DA. In this review, we will discuss recent findings on DA receptors as regulators of long term neuronal dysfunction and neurodegenerative processes.

CREB modulates calcium signaling in cAMP-induced bone marrow stromal cells (BMSCs)

Calcium signaling has a versatile role in many important cellular functions. Despite its importance, regulation of calcium signaling in bone marrow stromal cells (BMSCs, also known as bone marrow-derived mesenchymal stem cells) has not been explored extensively. Our previous study revealed that cyclic adenosine monophosphate (cAMP) enabled BMSCs to generate calcium signal upon stimulation by dopamine, KCl and glutamate. Concurrently, cAMP transiently activated the transcription factor cAMP response element binding protein (CREB) in BMSCs. Activity of CREB can be modulated by the calcium/calmodulin-dependent kinase signaling pathway, however, whether the calcium signaling observed in cAMP-induced BMSCs requires CREB has not been investigated. In an effort to uncover the role of CREB in the generation of calcium signaling in response to modulators such as dopamine and KCl, we knocked down CREB activity in BMSCs. Our study indicated that BMSCs, but not its close relative fibroblasts, are responsive to dopamine and KCl after cAMP treatment. Calcium signal elicited by dopamine depends, in part, on calcium influx whereas that elicited by KCl depends completely on calcium influx. Knock-down of CREB activity significantly reduced or abolished the cAMP-induced calcium response, and reintroducing a constitutively active CREB partially restored the calcium response.

Dopamine modulation of avoidance behavior in Caenorhabditis elegans requires the NMDA receptor NMR-1

The nematode C. elegans utilizes a relatively simple neural circuit to mediate avoidance responses to noxious stimuli such as the volatile odorant octanol. This avoidance behavior is modulated by dopamine. cat-2 mutant animals that are deficient in dopamine biosynthesis have an increased response latency to octanol compared to wild type animals, and this defect can be fully restored with the application of exogenous dopamine. Because this avoidance behavior is mediated by glutamatergic signaling between sensory neurons and premotor interneurons, we investigated the genetic interactions between dopaminergic signaling and ionotropic glutamate receptors. cat-2 mutant animals lacking either the GLR-1 or GLR-2 AMPA/kainate receptors displayed an increased response latency to octanol, which could be restored via exogenous dopamine. However, whereas cat-2 mutant animals lacking the NMR-1 NMDA receptor had increased response latency to octanol they were insensitive to exogenous dopamine. Mutants that lacked both AMPA/kainate and NMDA receptors were also insensitive to exogenous dopamine. Our results indicate that dopamine modulation of octanol avoidance requires NMR-1, consistent with NMR-1 as a potential downstream signaling target for dopamine.

The effects of stress on glutamatergic transmission in the brain

Stress leads to detrimental effects on brain functions and results in various diseases. Recent studies highlight the involvement of glutamatergic transmission in pathogenesis of depressive behaviors and fears. Acute stress generates different impacts on the excitatory transmission compared to chronic stress. Different neuromodulators and epigenetic factors also participate in the alteration of synaptic transmission and the regulation of synaptic plasticity. Restoration of the glutamatergic transmission in stress-affected brain areas therefore provides novel directions of therapeutic interventions against stress.

The dopaminergic stabilizers pridopidine and ordopidine enhance cortico-striatal Arc gene expression

The dopaminergic stabilizers pridopidine [4-(3-(methylsulfonyl)phenyl)-1-propylpiperidine] and ordopidine [1-ethyl-4-(2-fluoro-3-(methylsulfonyl)phenyl)piperidine] inhibit psychostimulant-induced hyperactivity, and stimulate behaviour in states of hypoactivity. While both compounds act as dopamine D2 receptor antagonists in vitro, albeit with low affinity, their specific state-dependent behavioural effect profile is not shared by D2 receptor antagonists in general. To further understand the neuropharmacological effects of pridopidine and ordopidine, and how they differ from other dopaminergic compounds in vivo, we assessed the expression of activity-regulated cytoskeleton-associated protein/activity-regulated gene 3.1 (Arc), an immediate early gene marker associated with synaptic activation, in the frontal cortex and striatum. Furthermore, monoamine neurochemistry and locomotor activity were assessed. The effects of pridopidine and ordopidine were compared to reference dopamine D1 and D2 receptor agonists and antagonists, as well as the partial dopamine D2 agonist aripiprazole. Pridopidine and ordopidine induced significant increases in cortical Arc expression, reaching 2.2- and 1.7-fold levels relative to control, respectively. In contrast, none of the reference dopamine D1 and D2 compounds tested increased cortical Arc expression. In the striatum, significant increases in Arc expression were seen with both pridopidine and ordopidine as well as the dopamine D2 receptor antagonists, remoxipride and haloperidol. Interestingly, striatal Arc expression correlated strongly and positively with striatal 3,4-dihydroxyphenylacetic acid, suggesting that antagonism of dopamine D2 receptors increases Arc expression in the striatum. In conclusion, the concurrent increase in cortical and striatal Arc expression induced by pridopidine and ordopidine appears unique for the dopaminergic stabilizers, as it was not shared by the reference compounds tested. The increase in cortical Arc expression is hypothesized to reflect enhanced N-methyl-D-aspartic acid receptor-mediated signalling in the frontal cortex, which could contribute to the state-dependent locomotor effects of pridopidine and ordopidine.

Pharmacological rescue of Ras signaling, GluA1-dependent synaptic plasticity, and learning deficits in a fragile X model

Fragile X syndrome, caused by the loss of Fmr1 gene function, is the most common form of inherited mental retardation. Lim et al. find that compounds activating serotonin (5HT) subtype 2B receptors or dopamine (DA) subtype 1-like receptors and those inhibiting 5HT_2A-Rs or D₂-Rs enhance Ras signaling, GluA1-dependent synaptic plasticity, and learning in Fmr1 knockout mice. Combining 5HT and DA compounds at low doses synergistically restored normal learning. This suggests that properly dosed and combined FDA-approved psychoactive drugs may effectively treat the cognitive impairment associated with fragile X syndrome.

Fragile X syndrome, caused by the loss of Fmr1 gene function, is the most common form of inherited mental retardation, with no effective treatment. Using a tractable animal model, we investigated mechanisms of action of a few FDA-approved psychoactive drugs that modestly benefit the cognitive performance in fragile X patients. Here we report that compounds activating serotonin (5HT) subtype 2B receptors (5HT_2B-Rs) or dopamine (DA) subtype 1-like receptors (D₁-Rs) and/or those inhibiting 5HT_2A-Rs or D₂-Rs moderately enhance Ras–PI3K/PKB signaling input, GluA1-dependent synaptic plasticity, and learning in Fmr1 knockout mice. Unexpectedly, combinations of these 5HT and DA compounds at low doses synergistically stimulate Ras–PI3K/PKB signal transduction and GluA1-dependent synaptic plasticity and remarkably restore normal learning in Fmr1 knockout mice without causing anxiety-related side effects. These findings suggest that properly dosed and combined FDA-approved psychoactive drugs may effectively treat the cognitive impairment associated with fragile X syndrome.

Time course of functional connectivity in primate dorsolateral prefrontal and posterior parietal cortex during working memory

The dorsolateral prefrontal and posterior parietal cortex play critical roles in mediating attention, working memory, and executive function. Despite proposed dynamic modulation of connectivity strength within each area according to task demands, scant empirical data exist about the time course of the strength of effective connectivity, particularly in tasks requiring information to be sustained in working memory. We investigated this question by performing time-resolved cross-correlation analysis for pairs of neurons recorded simultaneously at distances of 0.2–1.5 mm apart of each other while monkeys were engaged in working memory tasks. The strength of effective connectivity determined in this manner was higher throughout the trial in the posterior parietal cortex than the dorsolateral prefrontal cortex. Significantly higher levels of parietal effective connectivity were observed specifically during the delay period of the task. These differences could not be accounted for by differences in firing rate, or electrode distance in the samples recorded in the posterior parietal and prefrontal cortex. Differences were present when we restricted our analysis to only neurons with significant delay period activity and overlapping receptive fields. Our results indicate that dynamic changes in connectivity strength are present but area-specific intrinsic organization is the predominant factor that determines the strength of connections between neurons in each of the two areas.

Dopamine-dependent long-term depression at subthalamo-nigral synapses is lost in experimental parkinsonism

Impairments of synaptic plasticity are a hallmark of several neurological disorders, including Parkinson's disease (PD) which results from the progressive loss of dopaminergic neurons of the substantia nigra pars compacta leading to abnormal activity within the basal ganglia (BG) network and pathological motor symptoms. Indeed, disrupted plasticity at corticostriatal glutamatergic synapses, the gateway of the BG, is correlated to the onset of PD-related movement disorders and thus has been proposed to be a key neural substrate regulating information flow and motor function in BG circuits. However, a critical question is whether similar plasticity impairments could occur at other glutamatergic connections within the BG that would also affect the inhibitory influence of the network on the motor thalamus. Here, we show that long-term plasticity at subthalamo-nigral glutamatergic synapses (STN-SNr) sculpting the activity patterns of nigral neurons, the main output of the network, is also affected in experimental parkinsonism. Using whole-cell patch-clamp in acute rat brain slices, we describe a molecular pathway supporting an activity-dependent long-term depression of STN-SNr synapses through an NMDAR-and D1/5 dopamine receptor-mediated endocytosis of synaptic AMPA glutamate receptors. We also show that this plastic property is lost in an experimental rat model of PD but can be restored through the recruitment of dopamine D1/5 receptors. Altogether, our findings suggest that pathological impairments of subthalamo-nigral plasticity may enhance BG outputs and thereby contribute to PD-related motor dysfunctions.

Dopaminergic enhancement of excitatory synaptic transmission in layer II entorhinal neurons is dependent on D₁-like receptor-mediated signaling

The modulatory neurotransmitter dopamine induces concentration-dependent changes in synaptic transmission in the entorhinal cortex, in which high concentrations of dopamine suppress evoked excitatory postsynaptic potentials (EPSPs) and lower concentrations induce an acute synaptic facilitation. Whole-cell current-clamp recordings were used to investigate the dopaminergic facilitation of synaptic responses in layer II neurons of the rat lateral entorhinal cortex. A constant bath application of 1 μM dopamine resulted in a consistent facilitation of EPSPs evoked in layer II fan cells by layer I stimulation; the size of the facilitation was more variable in pyramidal neurons, and synaptic responses in a small group of multiform neurons were not modulated by dopamine. Isolated inhibitory synaptic responses were not affected by dopamine, and the facilitation of EPSPs was not associated with a change in paired-pulse facilitation ratio. Voltage-clamp recordings of α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) glutamate receptor-mediated excitatory postsynaptic currents (EPSCs) were facilitated by dopamine, but N-methyl-D-aspartate receptor-mediated currents were not. Bath application of the dopamine D₁-like receptor blocker SCH23390 (50 μM), but not the D₂-like receptor blocker sulpiride (50 μM), prevented the facilitation, indicating that it is dependent upon D₁-like receptor activation. Dopamine D₁ receptors lead to activation of protein kinase A (PKA), and including the PKA inhibitor H-89 or KT 5720 in the recording pipette solution prevented the facilitation of EPSCs. PKA-dependent phosphorylation of inhibitor 1 or the dopamine- and cAMP-regulated protein phosphatase (DARPP-32) can lead to a facilitation of AMPA receptor responses by inhibiting the activity of protein phosphatase 1 (PP1) that reduces dephosphorylation of AMPA receptors, and we found here that inhibition of PP1 occluded the facilitatory effect of dopamine. The dopamine-induced facilitation of AMPA receptor-mediated synaptic responses in layer II neurons of the lateral entorhinal cortex is therefore likely mediated via a D₁ receptor-dependent increase in PKA activity and a resulting inhibition in PP1-dependent dephosphorylation of AMPA receptors.

Surface trafficking of NMDA receptors: gathering from a partner to another

Understanding the molecular and cellular pathways by which neurons integrate signals from different neurotransmitter systems has been among the major challenges of modern neuroscience. The ionotropic glutamate NMDA receptor plays a key role in the maturation and plasticity of glutamate synapses, both in physiology and pathology. It recently appeared that the surface distribution of NMDA receptors is dynamically regulated through lateral diffusion, providing for instance a powerful way to rapidly affect the content and composition of synaptic receptors. The ability of various neuromodulators to regulate NMDA receptor signaling revealed that this receptor can also serve as a molecular integrator of the ambient neuronal environment. Although still in its infancy, we here review our current understanding of the cellular regulation of NMDA receptor surface dynamics. We specifically discuss the roles of well-known modulators, such as dopamine, and membrane interactors in these regulatory processes, exemplifying the recent evidence that the direct interaction between NMDAR and dopamine receptors regulates their surface diffusion and distribution. In addition to the well-established modulation of NMDA receptor signaling by intracellular pathways, the surface dynamics of the receptor is now emerging as the first level of regulation, opening new pathophysiological perspectives for innovative therapeutical strategies.

Cytosolic sulfotransferase 1A3 is induced by dopamine and protects neuronal cells from dopamine toxicity: role of D1 receptor-N-methyl-D-aspartate receptor coupling

Dopamine neurotoxicity is associated with several neurodegenerative diseases, and neurons utilize several mechanisms, including uptake and metabolism, to protect them from injury. Metabolism of dopamine involves three enzymes: monoamine oxidase, catechol O-methyltransferase, and sulfotransferase. In primates but not lower order animals, a sulfotransferase (SULT1A3) is present that can rapidly metabolize dopamine to dopamine sulfate. Here, we show that SULT1A3 and a closely related protein SULT1A1 are highly inducible by dopamine. This involves activation of the D1 and NMDA receptors. Both ERK1/2 phosphorylation and calcineurin activation are required for induction. Pharmacological agents that inhibited induction or siRNA targeting SULT1A3 significantly increased the susceptibility of cells to dopamine toxicity. Taken together, these results show that dopamine can induce its own metabolism and protect neuron-like cells from damage, suggesting that SULT1A3 activity may be a risk factor for dopamine-dependent neurodegenerative diseases.

Trading speed and accuracy by coding time: a coupled-circuit cortical model

Our actions take place in space and time, but despite the role of time in decision theory and the growing acknowledgement that the encoding of time is crucial to behaviour, few studies have considered the interactions between neural codes for objects in space and for elapsed time during perceptual decisions. The speed-accuracy trade-off (SAT) provides a window into spatiotemporal interactions. Our hypothesis is that temporal coding determines the rate at which spatial evidence is integrated, controlling the SAT by gain modulation. Here, we propose that local cortical circuits are inherently suited to the relevant spatial and temporal coding. In simulations of an interval estimation task, we use a generic local-circuit model to encode time by ‘climbing’ activity, seen in cortex during tasks with a timing requirement. The model is a network of simulated pyramidal cells and inhibitory interneurons, connected by conductance synapses. A simple learning rule enables the network to quickly produce new interval estimates, which show signature characteristics of estimates by experimental subjects. Analysis of network dynamics formally characterizes this generic, local-circuit timing mechanism. In simulations of a perceptual decision task, we couple two such networks. Network function is determined only by spatial selectivity and NMDA receptor conductance strength; all other parameters are identical. To trade speed and accuracy, the timing network simply learns longer or shorter intervals, driving the rate of downstream decision processing by spatially non-selective input, an established form of gain modulation. Like the timing network's interval estimates, decision times show signature characteristics of those by experimental subjects. Overall, we propose, demonstrate and analyse a generic mechanism for timing, a generic mechanism for modulation of decision processing by temporal codes, and we make predictions for experimental verification.

Studies in neuroscience have characterized how the brain represents objects in space and how these objects are selected for detailed perceptual processing. The selection process entails a decision about which object is favoured by the available evidence over time. This period of time is typically in the range of hundreds of milliseconds and is widely believed to be crucial for decisions, allowing neurons to filter noise in the evidence. Despite the widespread belief that time plays this role in decisions and the growing recognition that the brain estimates elapsed time during perceptual tasks, few studies have considered how the encoding of time effects decision making. We propose that neurons encode time in this range by the same general mechanisms used to select objects for detailed processing, and that these temporal representations determine how long evidence is filtered. To this end, we simulate a perceptual decision by coupling two instances of a neural network widely used to simulate localized regions of the cerebral cortex. One network encodes the passage of time and the other makes decisions based on noisy evidence. The former influences the performance of the latter, reproducing signature characteristics of temporal estimates and perceptual decisions.

Interaction between the mGlu receptors 5 antagonist, MPEP, and amphetamine on memory and motor functions in mice

RATIONALE

Metabotropic glutamate mGlu receptors 5 (mGluR5) receptors are abundant in corticolimbic circuitry where they modulate glutamate and dopamine signal transduction.

OBJECTIVES

In this study, we explored the hypothesis that mGluR5 antagonist, (2-methyl-6-(phenylethynyl)pyridine hydrochloride) (MPEP), facilitates dopamine-dependent effects on memory and motor functions.

METHODS

To this aim, we examined the effects of different doses (from 0 to 24 mg/kg) of the mGluR5 antagonist, MPEP, on the modulation of amphetamine-dependent behaviors, namely passive avoidance, locomotor activity, and rotation behavior in intact and dopamine-depleted CD1 male mice.

RESULTS

We demonstrated that a low dose (3 mg/kg) of MPEP, which is void of behavioral effects on its own, facilitates amphetamine-induced effects independently on the behavior measured both in naïve and in dopamine-lesioned mice; this synergistic effect is lost when higher doses of MPEP are used.

CONCLUSION

The results are discussed in terms of possible balance between dopamine and glutamate activity in regulating the proper fine tuning of information processing.

Differences in intrinsic functional organization between dorsolateral prefrontal and posterior parietal cortex

The dorsolateral prefrontal and posterior parietal cortex are 2 components of the cortical network controlling attention, working memory, and executive function. Little is known about how the anatomical organization of the 2 areas accounts for their functional specialization. In order to address this question, we examined the strength of intrinsic functional connectivity between neurons sampled in each area by means of cross-correlation analyses of simultaneous recordings from monkeys trained to perform working memory tasks. In both areas, effective connectivity declined as a function of distance between neurons. However, the strength of effective connectivity was higher overall and more localized over short distances in the posterior parietal than the prefrontal cortex. The difference in connectivity strength between the 2 areas could not be explained by differences in firing rate or selectivity for the stimuli and task events, it was present when the fixation period alone was analyzed, and according to simulation results, was consistent with a systematic difference either in the strength or in the relative numbers of shared inputs between neurons. Our results indicate that the 2 areas are characterized by unique intrinsic functional organization, consistent with known differences in their response patterns during working memory.

Medial prefrontal cortex: multiple roles in fear and extinction

Anxiety disorders are among the most common mental health problems; deficits in extinction have been implicated as a possible risk factor for the development of these disorders. Fear extinction refers to the ability to adapt as situations change by learning to suppress a previously acquired fear. Attention is directed toward the medial prefrontal cortex (mPFC) and the interaction it has with the amygdala as this circuit has crucial roles in both the acquisition and the extinction of fear associations. Here, we review converging evidence from different laboratories pointing to multiple roles that the mPFC has in fear regulation. Research on rodents indicates opposing roles that the different subregions of the mPFC have in exciting and inhibiting fear. In addition, this review aims to survey the findings addressing the mechanisms by which the mPFC regulates fear. Data from our laboratory and others show that changes in plasticity in the mPFC could be one of the mechanisms mediating extinction of fear. Recent findings on rodents and nonhuman primates report that modifying plasticity in the mPFC alters fear and affects extinction, suggesting that targeting plasticity in the mPFC could constitute a therapeutic tool for the treatment of anxiety disorders.

D1 receptor modulation of action potential firing in a subpopulation of layer 5 pyramidal neurons in the prefrontal cortex

Dopamine modulation in the prefrontal cortex is important for cognitive processing and disrupted in diverse neuropsychiatric diseases. Activation of D1 receptors is thought to enable working memory by enhancing the firing properties of pyramidal neurons. However, these receptors are only sparsely expressed in the prefrontal cortex, and how they impact individual neurons remains unknown. Here we study D1 receptor modulation of layer 5 pyramidal neurons in acute slices of the mouse prefrontal cortex. Using whole-cell recordings and two-photon microscopy, we show that neurons expressing D1 receptors have unique morphological and physiological properties. We then demonstrate that activation of these receptors selectively enhances the firing of these neurons by signaling via the protein kinase A pathway. This finding of robust D1 receptor modulation in only a subpopulation of neurons has important implications for cognitive function and disease.

Dopaminergic modulation of synaptic transmission in cortex and striatum

Among the many neuromodulators used by the mammalian brain to regulate circuit function and plasticity, dopamine (DA) stands out as one of the most behaviorally powerful. Perturbations of DA signaling are implicated in the pathogenesis or exploited in the treatment of many neuropsychiatric diseases, including Parkinson's disease (PD), addiction, schizophrenia, obsessive compulsive disorder, and Tourette's syndrome. Although the precise mechanisms employed by DA to exert its control over behavior are not fully understood, DA is known to regulate many electrical and biochemical aspects of neuronal function including excitability, synaptic transmission, integration and plasticity, protein trafficking, and gene transcription. In this Review, we discuss the actions of DA on ionic and synaptic signaling in neurons of the prefrontal cortex and striatum, brain areas in which dopaminergic dysfunction is thought to be central to disease.

D(1) receptor agonists reverse the subchronic phencyclidine (PCP)-induced novel object recognition (NOR) deficit in female rats

Development of dopamine (DA) D(1) receptor agonists is a priority to improve cognitive impairment in schizophrenia (CIS). This study examined the dose-response relationship of the selective D(1) agonist, SKF38393 (0.5-40 mg/kg), to reverse the deficit in novel object recognition (NOR), an analog of declarative memory in man, produced by subchronic phencyclidine (PCP), an N-methyl-D-aspartate (NMDA) receptor non-competitive antagonist, and the ability of the D(1) antagonists, SCH23390 (0.05 mg/kg) and SKF83566 (0.15 mg/kg), to impair NOR in normal Long-Evans female rats. We also examined the ability of tandospirone, a serotonin (5-HT)(1A) receptor partial agonist, and LY341495, a mGluR2/3 receptor antagonist, to potentiate or block the effects of SKF38393 on NOR, respectively. SKF38393 reversed the persistent NOR deficit produced by subchronic PCP; the dose-response curve for SKF38393 was an inverted U-shape, with the peak effect at 6 mg/kg. SKF83566 and SCH23390 impaired NOR in normal rats. Co-administration of sub-effective doses of SKF38393 (0.25 mg/kg) and tandospirone (0.2 mg/kg) improved the PCP-induced NOR deficit, while LY341495 (1 mg/kg) blocked the ameliorating effect of SKF38393 (6 mg/kg), respectively. These data provide the first evidence that the reversal of the PCP-induced NOR deficit by D(1) agonism has an inverted U-shaped dose-response curve and that 5-HT(1A) and mGluR2/3 receptor signalling facilitates the efficacy of D(1) agonism to improve these deficits. These data suggest that although D(1) agonists may be useful to improve CIS, these agents, particularly higher doses, may also worsen cognitive function in some patients, because of an inverted U-shaped dose response curve.

CPB-K mice a mouse model of schizophrenia? Differences in dopaminergic, serotonergic and behavioral markers compared to BALB/cJ mice

Schizophrenia is characterized by disturbances in social behavior, sensorimotor gating and cognitive function, that are discussed to be caused by a termination of different transmitter systems. Beside morphological alterations in cortical and subcortical areas reduced AMPA- NMDA-, 5-HT2-receptor densities and increased 5-HT1-receptor densities are found in the hippocampus.The two inbred mouse strains CPB-K and BALB/cJ are known to display considerable differences in cognitive function and prepulse inhibition, a stable marker of sensorimotor gating. Furthermore, CPB-K mice exhibit lower NMDA-, AMPA- and increased 5-HT-receptor densities in the hippocampus as compared to BALB/cJ mice. We investigated both mouse strains in social interaction test for differences in social behavior and with immuncytochemical approaches for alterations of dopaminergic and serotonergic parameters. Our results can be summarized as follows: compared to BALB/cJ, CPB-K mice showed:(1) significantly reduced traveling distance and number of contacts in social interaction test, (2) differences in the number of serotonin transporter-immunoreactive neurons and volume of raphe nuclei and a lower serotonergic fiber density in the ventral and dorsal hippocampal subfields CA1 and CA3, (3) no alterations of dopaminergic markers like neuron number, neuron density and volume in subregions of substantia nigra and ventral tegmental area, but a significantly higher dopaminergic fiber density in the dorsal hippocampus, the ventral hippocampus of CA1 and gyrus dentatus, (4) no significant differences in serotonergic and dopaminergic fiber densities in the amygdala.Based on our results and previous studies, CPB-K mice compared to BALB/cJ may serve as an important model to understand the interaction of the serotonergic and dopaminergic system and their impact on sensorimotor gating and cognitive function as related to neuropsychiatric disorders like schizophrenia.

Group I mGluR antagonist rescues the deficit of D1-induced LTP in a mouse model of fragile X syndrome

BACKGROUND

Fragile X syndrome (FXS) is caused by the absence of the mRNA-binding protein Fragile X mental retardation protein (FMRP), encoded by the Fmr1 gene. Overactive signaling by group 1 metabotropic glutamate receptor (Grp1 mGluR) could contribute to slowed synaptic development and other symptoms of FXS. Our previous study has identified that facilitation of synaptic long-term potentiation (LTP) by D1 receptor is impaired in Fmr1 knockout (KO) mice. However, the contribution of Grp1 mGluR to the facilitation of synaptic plasticity by D1 receptor stimulation in the prefrontal cortex has been less extensively studied.

RESULTS

Here we demonstrated that DL-AP3, a Grp1 mGluR antagonist, rescued LTP facilitation by D1 receptor agonist SKF81297 in Fmr1KO mice. Grp1 mGluR inhibition restored the GluR1-subtype AMPA receptors surface insertion by D1 activation in the cultured Fmr1KO neurons. Simultaneous treatment of Grp1 mGluR antagonist with D1 agonist recovered the D1 receptor signaling by reversing the subcellular redistribution of G protein-coupled receptor kinase 2 (GRK2) in the Fmr1KO neurons. Treatment of SKF81297 alone failed to increase the phosphorylation of NR2B-containing N-methyl D-aspartate receptors (NMDARs) at Tyr-1472 (p-NR2B-Tyr1472) in the cultures from KO mice. However, simultaneous treatment of DL-AP3 could rescue the level of p-NR2B-Tyr1472 by SKF81297 in the cultures from KO mice. Furthermore, behavioral tests indicated that simultaneous treatment of Grp1 mGluR antagonist with D1 agonist inhibited hyperactivity and improved the learning ability in the Fmr1KO mice.

CONCLUSION

The findings demonstrate that mGluR1 inhibition is a useful strategy to recover D1 receptor signaling in the Fmr1KO mice, and combination of Grp1 mGluR antagonist and D1 agonist is a potential drug therapy for the FXS.

Unique and shared roles of the posterior parietal and dorsolateral prefrontal cortex in cognitive functions

The dorsolateral prefrontal cortex (PFC) and posterior parietal cortex (PPC) are two parts of a broader brain network involved in the control of cognitive functions such as working-memory, spatial attention, and decision-making. The two areas share many functional properties and exhibit similar patterns of activation during the execution of mental operations. However, neurophysiological experiments in non-human primates have also documented subtle differences, revealing functional specialization within the fronto-parietal network. These differences include the ability of the PFC to influence memory performance, attention allocation, and motor responses to a greater extent, and to resist interference by distracting stimuli. In recent years, distinct cellular and anatomical differences have been identified, offering insights into how functional specialization is achieved. This article reviews the common functions and functional differences between the PFC and PPC, and their underlying mechanisms.

Differential effects of AMPA receptor potentiators and glycine reuptake inhibitors on antipsychotic efficacy and prefrontal glutamatergic transmission

RATIONALE

The α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptor positive allosteric modulators (AMPA-PAMs), Org 24448 and Org 26576, and the glycine transporter-1 (GlyT-1) inhibitor Org 25935 are developed for treatment of schizophrenia.

OBJECTIVES

Here we examined experimentally the ability of co-administration of these AMPA-PAMs or the GlyT-1 inhibitor to augment the antipsychotic activity and effect on cortical N-methyl-D: -aspartate (NMDA) receptor-mediated transmission of risperidone, olanzapine, or haloperidol.

METHODS

We examined antipsychotic efficacy using the conditioned avoidance response (CAR) test, extrapyramidal side effect liability using a catalepsy test, and cortical NMDA receptor-mediated glutamatergic transmission using intracellular electrophysiological recording technique in vitro.

RESULTS

Both AMPA-PAMs enhanced the suppression of CAR induced by risperidone or olanzapine, and Org 24448 also enhanced the effect of haloperidol. In contrast, the GlyT-1 inhibitor did not cause any behaviorally significant effect in the CAR test. However, the GlyT-1 inhibitor, but not the AMPA-PAMs, produced a large facilitation of NMDA-induced currents. All three drugs potentiated the effect of risperidone but not haloperidol on these currents. The GlyT-1 inhibitor also facilitated the effect of olanzapine. All drugs potentiated the effect of risperidone on electrically stimulated excitatory postsynaptic potentials (EPSP) in cortical pyramidal cells, whereas only the GlyT inhibitor facilitated the effect of olanzapine.

CONCLUSIONS

Our results suggest that the AMPA-PAMs, when compared to the GlyT-1 inhibitor, show differential effects in terms of augmentation of antipsychotic efficacy, particularly when combined with risperidone or olanzapine. Both AMPA-PAMs and the GlyT-1 inhibitor may also improve negative symptoms and cognitive impairments in schizophrenia, in particular when combined with risperidone.

Neurons with inverted tuning during the delay periods of working memory tasks in the dorsal prefrontal and posterior parietal cortex

The dorsolateral prefrontal and posterior parietal cortices are two interconnected brain areas that are coactivated in tasks involving functions such as spatial attention and working memory. The response properties of neurons in the two areas are in many respects indistinguishable, yet only prefrontal neurons are able to resist interference by distracting stimuli when subjects are required to remember an initial stimulus. Several mechanisms have been proposed that could account for this functional difference, including the existence of specialized interneuron types, specific to the prefrontal cortex. Although such neurons with inverted tuning during the delay period of a working memory task have been described in the prefrontal cortex, no comparative data exist from other cortical areas that would establish a unique prefrontal role. To test this hypothesis, we analyzed a large database of recordings obtained in the dorsolateral prefrontal and posterior parietal cortex of the same monkeys as they performed working memory tasks. We found that in the prefrontal cortex, neurons with inverted tuning were more numerous and manifested unique properties. Our results give credence to the idea that a division of labor exists between separate neuron types in the prefrontal cortex and that this represents a functional specialization that is not present in its cortical afferents.

Circuits and brain rhythms in schizophrenia: a wealth of convergent targets

Few common neurological illnesses trace back to single molecular disturbances. Many disparate putative causes may co-associate with a single disease state. However, uncovering functional, hierarchical networks of underlying mechanisms can provide a framework in which many primary pathologies converge on more complex, single higher level correlates of disease. This article focuses on cognitive deficits associated with schizophrenia to illustrate: a) How non-invasive EEG biomarkers of cognitive function constitute such a 'higher level correlate' of underlying pathologies. b) How derangement of multiple, cell-specific, molecular processes can converge on such EEG-visible, correlates of disrupted cognitive function. This approach suggests that evidence-based design of multi-target therapies may take advantage of hierarchical patterns of convergence to improve both efficacy and selectivity of disease-intervention.

Exposure to novel environment is characterized by an interaction of D1/NMDA receptors underlined by phosphorylation of the NMDA and AMPA receptor subunits and activation of ERK1/2 signaling, leading to epigenetic changes and gene expression in rat hippocampus

Interactions between dopamine and glutamate receptors are essential for prefrontal cortical (PFC) and hippocampal cognitive functions. The hippocampus has been identified as a detector of a novel stimulus, where an association between incoming information and stored memories takes place. Further to our previous results which showed a strong synergistic interaction of dopamine D1 and glutamate NMDA receptors, the present study is going to investigate the functional status of that interaction in rats, following their exposure to a novel environment. Our results showed that the "spatial" novelty induced in rat hippocampus and PFC (a) a significant increase in phosphorylation of NMDA and AMPA receptor subunits, as well as a robust phosphorylation/activation of ERK1/2 signaling, which are both dependent on the concomitant stimulation of D1/NMDA receptors and are both abolished by habituation procedure, (b) chromatin remodeling events (phosphorylation-acetylation of histone H3) and (c) an increase in the immediate early genes (IEGs) c-Fos and zif-268 expression in the CA1 region of hippocampus, which is dependent on the co-activation of D1/NMDA and acetylcholine muscarinic receptors. In conclusion, our results clearly show that a strong synergistic interaction of D1/NMDA receptor is required for the novelty-induced phosphorylation of NMDA and AMPA receptor subunits and for the robust activation of ERK1/2 signaling, leading to chromatin remodeling events and the expression of the IEGs c-Fos and zif-268, which are involved in the regulation of synaptic plasticity and memory consolidation.

Dopamine D1 receptor antagonism impairs extinction of cocaine-cue memories

Increasingly, research suggests a role for dopamine D1 receptors in the consolidation of extinction of both appetitive and aversive memories. However, a role for D1 receptors in extinction of memories involving drug reward has yet to be established. Here we show that post-retrieval, but not delayed, systemic administration of the D1 receptor antagonist SCH23390 results in prolonged extinction of cocaine conditioned place preference (CPP), suggesting a critical role for D1 receptors in the consolidation of extinction of cocaine-cue memories.

Calcium-dependent phosphorylation regulates neuronal stability and plasticity in a highly precise pacemaker nucleus

Specific types of neurons show stable, predictable excitability properties, while other neurons show transient adaptive plasticity of their excitability. However, little attention has been paid to how the cellular pathways underlying adaptive plasticity interact with those that maintain neuronal stability. We addressed this question in the pacemaker neurons from a weakly electric fish because these neurons show a highly stable spontaneous firing rate as well as an N-methyl-D-aspartate (NMDA) receptor-dependent form of plasticity. We found that basal firing rates were regulated by a serial interaction of conventional and atypical PKC isoforms and that this interaction establishes individual differences within the species. We observed that NMDA receptor-dependent plasticity is achieved by further activation of these kinases. Importantly, the PKC pathway is maintained in an unsaturated baseline state to allow further Ca(2+)-dependent activation during plasticity. On the other hand, the Ca(2+)/calmodulin-dependent phosphatase calcineurin does not regulate baseline firing but is recruited to control the duration of the NMDA receptor-dependent plasticity and return the pacemaker firing rate back to baseline. This work illustrates how neuronal plasticity can be realized by biasing ongoing mechanisms of stability (e.g., PKC) and terminated by recruiting alternative mechanisms (e.g., calcineurin) that constrain excitability. We propose this as a general model for regulating activity-dependent change in neuronal excitability.

The physiology, signaling, and pharmacology of dopamine receptors

G protein-coupled dopamine receptors (D1, D2, D3, D4, and D5) mediate all of the physiological functions of the catecholaminergic neurotransmitter dopamine, ranging from voluntary movement and reward to hormonal regulation and hypertension. Pharmacological agents targeting dopaminergic neurotransmission have been clinically used in the management of several neurological and psychiatric disorders, including Parkinson's disease, schizophrenia, bipolar disorder, Huntington's disease, attention deficit hyperactivity disorder (ADHD(1)), and Tourette's syndrome. Numerous advances have occurred in understanding the general structural, biochemical, and functional properties of dopamine receptors that have led to the development of multiple pharmacologically active compounds that directly target dopamine receptors, such as antiparkinson drugs and antipsychotics. Recent progress in understanding the complex biology of dopamine receptor-related signal transduction mechanisms has revealed that, in addition to their primary action on cAMP-mediated signaling, dopamine receptors can act through diverse signaling mechanisms that involve alternative G protein coupling or through G protein-independent mechanisms via interactions with ion channels or proteins that are characteristically implicated in receptor desensitization, such as β-arrestins. One of the future directions in managing dopamine-related pathologic conditions may involve a transition from the approaches that directly affect receptor function to a precise targeting of postreceptor intracellular signaling modalities either directly or through ligand-biased signaling pharmacology. In this comprehensive review, we discuss dopamine receptor classification, their basic structural and genetic organization, their distribution and functions in the brain and the periphery, and their regulation and signal transduction mechanisms. In addition, we discuss the abnormalities of dopamine receptor expression, function, and signaling that are documented in human disorders and the current pharmacology and emerging trends in the development of novel therapeutic agents that act at dopamine receptors and/or on related signaling events.

Molecular mechanisms of working memory

Working memory is a process for temporary active maintenance of information and the role of prefrontal cortex in this memory has been known since the pioneering experiments of Fulton in the early 20th century. Sustained firing of prefrontal neurons during the delay period is considered the neural correlate of working memory. Evidence in literature suggests the involvement of areas beyond the frontal lobe and illustrate that working memory involves parallel, distributed neuronal networks. Prefrontal cortex is part of a complex neural circuit that includes both cortical and subcortical components and many of these regions play vital roles in working memory function. In this article, we review the current understanding of the neural mechanisms of memory maintenance in the brain.

cAMP initiates early phase neuron-like morphology changes and late phase neural differentiation in mesenchymal stem cells

The intracellular second messenger cAMP is frequently used in induction media to induce mesenchymal stem cells (MSCs) into neural lineage cells. To date, an understanding of the role cAMP exerts on MSCs and whether cAMP can induce MSCs into functional neurons is still lacking. We found cAMP initiated neuron-like morphology changes early and neural differentiation much later. The early phase changes in morphology were due to cell shrinkage, which subsequently rendered some cells apoptotic. While the morphology changes occurred prior to the expression of neural markers, it is not required for neural marker expression and the two processes are differentially regulated downstream of cAMP-activated protein kinase A. cAMP enabled MSCs to gain neural marker expressions with neuronal function, such as, calcium rise in response to neuronal activators, dopamine, glutamate, and potassium chloride. However, only some of the cells induced by cAMP responded to the three neuronal activators and further lack the neuronal morphology, suggesting that although cAMP is able to direct MSCs towards neural differentiation, they do not achieve terminal differentiation.

Dopamine D1 receptor-mediated NMDA receptor insertion depends on Fyn but not Src kinase pathway in prefrontal cortical neurons

BACKGROUND

Interactions between dopamine and glutamate in the prefrontal cortex are essential for cognitive functions such as working memory. Modulation of N-methyl-D-aspartic acid (NMDA) receptor functions by dopamine D1 receptor is believed to play a critical role in these functions. The aim of the work reported here is to explore the signaling pathway underlying D1 receptor-mediated trafficking of NMDA receptors in cultured rat prefrontal cortical neurons.

RESULTS

Activation of D1 receptor by selective agonist SKF-81297 significantly increased the expression of NR2B subunits. This effect was completely blocked by small interfering RNA knockdown of Fyn, but not Src. Under control conditions, neither Fyn nor Src knockdown exhibited significant effect on basal NR2B expression. D1 stimulation significantly enhanced NR2B insertion into plasma membrane in cultured PFC neurons, a process obstructed by Fyn, but not Src, knockdown.

CONCLUSIONS

Dopamine D1 receptor-mediated increase of NMDA receptors is thus Fyn kinase dependent. Targeting this signaling pathway may be useful in treating drug addiction and schizophrenia.

Seeking brain biomarkers for preventive therapy in Huntington disease

Huntington disease (HD) is a severe incurable nervous system disease that generally has an onset age of around 35-50, and is caused by a dominantly transmitted expansion mutation. A genetic test allows persons at risk, i.e., offspring or siblings of affected individuals, to discover their genetic status. Unaffected mutation-positive subjects will manifest HD sometime during life. Despite major advances in research on pathogenic mechanisms, no studies have yet fully validated preventive therapy or biomarkers for use before the symptoms become clinically manifest. Seeking brain and peripheral biomarkers is a requisite to develop a cure for HD. Changes in the brain can be observed in vivo using methods such as structural magnetic resonance imaging (MRI), diffusion tensor imaging (DTI), functional MRI (fMRI), and positron emission tomography (PET), detecting volumetric changes, microstructural and connectivity alterations, abnormalities in brain activity in response to specific tasks, and abnormalities in metabolism and receptor distribution. Although all these imaging techniques can detect early markers in asymptomatic HD gene carriers for premanifest screening and pharmacological responses to therapeutic interventions no single modality has yet provided and validated an optimal marker probably because this task requires an integrative multimodal imaging approach. In this article, we review the findings from imaging procedures in the attempt to identify potential brain markers, so-called dry biomarkers, for possible application to further, yet unavailable, neuroprotective preventive therapies for HD manifestations.

Amelioration of ketamine-induced working memory deficits by dopamine D1 receptor agonists

RATIONALE

Ketamine has been used in humans to model cardinal symptoms of schizophrenia, including working memory impairments and behavioral disorganization. Translational studies with ketamine in nonhuman primates promise to extend the neurobiological understanding of this model.

OBJECTIVES

By establishing the dose-dependent effects of ketamine on spatial working memory and behavior, we sought to test and compare the capacity of antipsychotic and procognitive agents to reverse these symptoms.

METHODS

Behavioral observations were taken following administration of placebo/ketamine (0.1-1.7 mg/kg, intramuscularly) and animals were tested on the spatial delayed response task 15 min post-injection. Pretreatments with risperidone as well as full and partial D1 receptor agonists were tested for their ability to reverse ketamine-induced impairments.

RESULTS

Ketamine (median 1.0 mg/kg) produced a profound cognitive impairment and behavioral sequelae reminiscent of positive and negative symptoms. Risperidone within the therapeutic dose range failed to antagonize behavioral or cognitive consequences of acute ketamine but A77636 (0.1 and 1 microg/kg) and SKF38393 (0.1 microg/kg-100 microg/kg) ameliorated the spatial working memory deficit. This effect of A77636 was blocked by the D1 receptor antagonist, SCH39166 (1 and 10 microg/kg).

CONCLUSIONS

These findings establish a valuable ketamine platform relevant to the treatment of cognitive dysfunction in schizophrenia. The reversal of ketamine-induced working memory deficits by a D1 receptor agonist, but not a commonly prescribed atypical antipsychotic, provides behavioral evidence for significant D1/N-methyl-D: -aspartate receptor interactions in prefrontal dysfunction and concurs with suggestions that D1 agonists may be useful in the treatment of cognitive impairments in schizophrenia.

Comparison of neural activity related to working memory in primate dorsolateral prefrontal and posterior parietal cortex

Neurons in a distributed network of cortical and subcortical areas continue to discharge after the presentation and disappearance of stimuli, providing a neural correlate for working memory. While it is thought that the prefrontal cortex plays a central role in this network, the relative contributions of other brain areas are not as well understood. In order to compare the contributions of the dorsolateral prefrontal and posterior parietal cortex, we recorded neurophysiological activity in monkeys trained to perform two different visuo-spatial working memory tasks: a Match/Nonmatch task, and a Spatial Delayed-Match-to-Sample Task. Neurons in both areas exhibited discharges in the delay periods of the tasks that could be classified in two forms. Sustained discharges persisted after the presentation of a stimulus in the receptive field with a constant or declining rate. Anticipatory responses increased in rate during the delay period, often appearing after presentation of a stimulus out of the receptive field. Despite similarities, we uncovered distinct differences between patterns of delay period in each brain area. Only in the prefrontal cortex sustained responses related to the original stimulus survived presentation of a second stimulus, in the context of the Match/Nonmatch task. Our results provide insights on the nature of processing in two areas active during working memory, and on the unique role of the prefrontal cortex in memory maintenance.

Roles of fragile X mental retardation protein in dopaminergic stimulation-induced synapse-associated protein synthesis and subsequent alpha-amino-3-hydroxyl-5-methyl-4-isoxazole-4-propionate (AMPA) receptor internalization

Fragile X syndrome, the most common form of inherited mental retardation, is caused by the absence of the RNA-binding protein fragile X mental retardation protein (FMRP). FMRP regulates local protein synthesis in dendritic spines. Dopamine (DA) is involved in the modulation of synaptic plasticity. Activation of DA receptors can regulate higher brain functions in a protein synthesis-dependent manner. Our recent study has shown that FMRP acts as a key messenger for DA modulation in forebrain neurons. Here, we demonstrate that FMRP is critical for DA D1 receptor-mediated synthesis of synapse-associated protein 90/PSD-95-associated protein 3 (SAPAP3) in the prefrontal cortex (PFC). DA D1 receptor stimulation induced dynamic changes of FMRP phosphorylation. The changes in FMRP phosphorylation temporally correspond with the expression of SAPAP3 after D1 receptor stimulation. Protein phosphatase 2A, ribosomal protein S6 kinase, and mammalian target of rapamycin are the key signaling molecules for FMRP linking DA D1 receptors to SAPAP3. Knockdown of SAPAP3 did not affect surface expression of alpha-amino-3-hydroxyl-5-methyl-4-isoxazole-4-propionate (AMPA) GluR1 receptors induced by D1 receptor activation but impaired their subsequent internalization in cultured PFC neurons; the subsequent internalization of GluR1 was also impaired in Fmr1 knock-out PFC neurons, suggesting that FMRP may be involved in subsequent internalization of GluR1 through regulating the abundance of SAPAP3 after DA D1 receptor stimulation. Our study thus provides further insights into FMRP involvement in DA modulation and may help to reveal the molecular mechanisms underlying impaired learning and memory in fragile X syndrome.