Targeting prefrontal cortical dopamine D1 and N-methyl-D-aspartate receptor interactions in schizophrenia treatment

A dopamine gradient controls access to distributed working memory in the large-scale monkey cortex

Dopamine is required for working memory, but how it modulates the large-scale cortex is unknown. Here, we report that dopamine receptor density per neuron, measured by autoradiography, displays a macroscopic gradient along the macaque cortical hierarchy. This gradient is incorporated in a connectome-based large-scale cortex model endowed with multiple neuron types. The model captures an inverted U-shaped dependence of working memory on dopamine and spatial patterns of persistent activity observed in over 90 experimental studies. Moreover, we show that dopamine is crucial for filtering out irrelevant stimuli by enhancing inhibition from dendrite-targeting interneurons. Our model revealed that an activity-silent memory trace can be realized by facilitation of inter-areal connections and that adjusting cortical dopamine induces a switch from this internal memory state to distributed persistent activity. Our work represents a cross-level understanding from molecules and cell types to recurrent circuit dynamics underlying a core cognitive function distributed across the primate cortex.

Deletion of Glycogen Synthase Kinase-3β in D2 Receptor-Positive Neurons Ameliorates Cognitive Impairment via NMDA Receptor-Dependent Synaptic Plasticity

BACKGROUND

Cortical dopaminergic systems are critically involved in prefrontal cortex (PFC) functions, especially in working memory and neurodevelopmental disorders such as schizophrenia. GSK-3β (glycogen synthase kinase-3β) is highly associated with cAMP (cyclic adenosine monophosphate)-independent dopamine D₂ receptor (D₂R)-mediated signaling to affect dopamine-dependent behaviors. However, the mechanisms underlying the GSK-3β modulation of cognitive function via D₂Rs remains unclear.

METHODS

This study explored how conditional cell-type-specific ablation of GSK-3β in D₂R+ neurons (D₂R-GSK-3β^-/-) in the brain affects synaptic function in the medial PFC (mPFC). Both male and female (postnatal days 60-90) mice, including 140 D₂R, 24 D₁R, and 38 DISC1 mice, were used.

RESULTS

This study found that NMDA receptor (NMDAR) function was significantly increased in layer V pyramidal neurons in mPFC of D₂R-GSK-3β^-/- mice, along with increased dopamine modulation of NMDAR-mediated current. Consistently, NR2A and NR2B protein levels were elevated in mPFC of D₂R-GSK-3β^-/- mice. This change was accompanied by a significant increase in enrichment of activator histone mark H3K27ac at the promoters of both Grin2a and Grin2b genes. In addition, altered short- and long-term synaptic plasticity, along with an increased spine density in layer V pyramidal neurons, were detected in D₂R-GSK-3β^-/- mice. Indeed, D₂R-GSK-3β^-/- mice also exhibited a resistance of working memory impairment induced by injection of NMDAR antagonist MK-801. Notably, either inhibiting GSK-3β or disrupting the D₂R-DISC1 complex was able to reverse the mutant DISC1-induced decrease of NMDAR-mediated currents in the mPFC.

CONCLUSIONS

This study demonstrates that GSK-3β modulates cognition via D₂R-DISC1 interaction and epigenetic regulation of NMDAR expression and function.

Tetrahydroprotoberberines: A Novel Source of Pharmacotherapies for Substance Use Disorders?

Tetrahydroprotoberberines (THPBs) are a class of compounds that target both dopamine D1 and D2 families of receptors, making them attractive candidates for treating substance use disorder (SUD). The binding of some THPBs to serotonin and adrenergic receptors, in addition to dopamine receptors, gives rise to complex pharmacological profiles. Significant progress has been made over the last decade in examining these compounds for their therapeutic potential. Here, we evaluate recent discoveries relating to the neural mechanism and therapeutic effects of THPBs, focusing on compounds that have shown promise in animal models of SUD and preliminary clinical studies. Advancements in structure-activity relationship studies and in silico modeling of THPB binding to dopamine receptors have facilitated the synthesis of novel THPBs with enhanced therapeutic properties and provide insights regarding use of the THPB scaffold to serve as a template for innovative drug designs.

Clozapine augmentation strategies

Clozapine is established as the gold standard for antipsychotic treatment of patients suffering from treatment-resistant schizophrenia. Over virtually 3 decades, the level of inadequate response to clozapine was found to range from 40% to 60%. A heightened interest developed in the augmentation of clozapine to try to achieve response or maximize partial response. A large variety of drug groups have been investigated. This article focuses on the meta-analyses of these trials to discover reasonable evidence-based approaches to the management of patients not responding to clozapine.

Effect of Damaging Rare Mutations in Synapse-Related Gene Sets on Response to Short-term Antipsychotic Medication in Chinese Patients With Schizophrenia: A Randomized Clinical Trial

IMPORTANCE

The underlying mechanism for individual differences in patient response to antipsychotic medication remains unknown.

OBJECTIVE

To discover genes and gene sets harboring rare variants associated with short-term antipsychotic medication efficacy.

DESIGN, SETTING, AND PARTICIPANTS

In this multicenter, open-label, randomized clinical trial conducted between July 6, 2010, and December 31, 2011, 3023 patients recruited in China of Chinese Han descent with schizophrenia with total Positive and Negative Syndrome Scale (PANSS) score ≥ 60 received a 6-week treatment of antipsychotic medications randomly chosen from 5 atypical and 2 typical antipsychotic medications. Whole-exome sequencing (WES) was performed in 316 participants (grouped into those with the best response [n=156] and those who had no response [n=160] to the antipsychotic medication prescribed), according to the total PANSS score reduction rate after 6 weeks of treatment. Validation was performed using targeted sequencing in an independent sample of 1920 patients. Data analyses was performed between March 15, 2016, and March 1, 2017.

MAIN OUTCOMES AND MEASURES

Drug efficacy at week 6 was assessed according to the change in PANSS scores from baseline. Extremely good and extremely poor responders were selected for an initial WES association study, from which a subset of genes showing putative association was selected for independent replication with a targeted sequencing approach.

RESULTS

Of the 3023 patients (1549 [51.24%] female and 1474 [48.8%] male; mean [SD] age, 31.2 [7.9] years), 2336 (77.3%) were eligible for genetic analysis. After quality-control exclusions, 316 patients (10.5%) were included for WES and 1920 (63.5%) were included for replication. In the WES discovery stage, 2 gene sets (reduced NMDA [N-methyl-D-aspartate]-mediated synaptic currents and reduced AMPA [α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid]-mediated synaptic currents) were found to be enriched with rare damaging variants in the nonresponder group, suggesting the involvement of these gene sets in antipsychotic medication efficacy. Reduced NMDA-mediated synaptic currents gene set was further replicated in an independent sample using targeting sequencing. No statistically significant differences in antipsychotic drug response were found among the patients who received different antipsychotic drugs.

CONCLUSIONS AND RELEVANCE

Genetic variation in glutamatergic or NMDA neurotransmission is implicated in short-term antipsychotic medication efficacy; WES may have utility in the study of rare genetic variation in pharmacogenetics.

TRIAL REGISTRATION

Chinese Clinical Trials Registry Identifier: ChiCTR-TRC-10000934.

An Allosteric Potentiator of the Dopamine D1 Receptor Increases Locomotor Activity in Human D1 Knock-In Mice without Causing Stereotypy or Tachyphylaxis

Allosteric potentiators amplify the sensitivity of physiologic control circuits, a mode of action that could provide therapeutic advantages. This hypothesis was tested with the dopamine D1 receptor potentiator DETQ [2-(2,6-dichlorophenyl)-1-((1S,3R)-3-(hydroxymethyl)-5-(2-hydroxypropan-2-yl)-1-methyl-3,4-dihydroisoquinolin-2(1H)-yl)ethan-1-one]. In human embryonic kidney 293 (HEK293) cells expressing the human D1 receptor, DETQ induced a 21-fold leftward shift in the cAMP response to dopamine, with a K_b of 26 nM. The maximum response to DETQ alone was ∼12% of the maximum response to dopamine, suggesting weak allosteric agonist activity. DETQ was ∼30-fold less potent at rat and mouse D1 receptors and was inactive at the human D5 receptor. To enable studies in rodents, an hD1 knock-in mouse was generated. DETQ (3–20 mg/kg orally) caused a robust (∼10-fold) increase in locomotor activity (LMA) in habituated hD1 mice but was inactive in wild-type mice. The LMA response to DETQ was blocked by the D1 antagonist SCH39166 and was dependent on endogenous dopamine. LMA reached a plateau at higher doses (30–240 mg/kg) even though free brain levels of DETQ continued to increase over the entire dose range. In contrast, the D1 agonists SKF 82958, A-77636, and dihydrexidine showed bell-shaped dose-response curves with a profound reduction in LMA at higher doses; video-tracking confirmed that the reduction in LMA caused by SKF 82958 was due to competing stereotyped behaviors. When dosed daily for 4 days, DETQ continued to elicit an increase in LMA, whereas the D1 agonist A-77636 showed complete tachyphylaxis by day 2. These results confirm that allosteric potentiators may have advantages compared with direct-acting agonists.

Psychostimulants and atomoxetine alter the electrophysiological activity of prefrontal cortex neurons, interaction with catecholamine and glutamate NMDA receptors

RATIONALE

Attention-deficit hyperactivity disorder (ADHD) is the most frequently diagnosed neuropsychiatric disorder in childhood. Currently available ADHD drugs include the psychostimulants methylphenidate (MPH) and D-amphetamine (D-AMP), acting on norepinephrine and dopamine transporters/release, and atomoxetine (ATX), a selective norepinephrine uptake inhibitor. Recent evidence suggests an involvement of glutamate neurotransmission in the pathology and treatment of ADHD, via mechanisms to be clarified.

OBJECTIVE

We have investigated how ADHD drugs could modulate, through interaction with catecholamine receptors, basal and glutamate-induced excitability of pyramidal neurons in the prefrontal cortex (PFC), a region which plays a major role in control of attention and impulsivity.

METHODS

We have used the technique of extracellular single-unit recording in anaesthetised rats coupled with microiontophoresis.

RESULTS

Both MPH (1-3 mg/kg) and D-AMP (1-9 mg/kg) increased the firing activity of PFC neurons in a dopamine D1 receptor-dependent manner. ATX administration (1-6 mg/kg) also increased the firing of neurons, but this effect is not significantly reversed by D1 (SCH 23390) or alpha1 (prazosin) receptor antagonists but potentiated by alpha2 antagonist (yohimbine). All drugs induced a clear potentiation of the excitatory response of PFC neurons to the microiontophoretic application of the glutamate agonist N-methyl-D-aspartate (NMDA), but not to the glutamate agonist α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA). The potentiating effect of D-AMP on NMDA-induced activation of PFC neurons was partially reversed or prevented by dopamine D1 receptor blockade.

CONCLUSION

Our data shows that increase in excitability of PFC neurons in basal conditions and via NMDA receptor activation may be involved in the therapeutic response to ADHD drugs.

Strain-dependent differences in corticolimbic processing of aversive or rewarding stimuli

Aberrations in the elaboration of both aversive and rewarding stimuli characterize several psychopathologies including anxiety, depression and addiction. Several studies suggest that different neurotrasmitters, within the corticolimbic system, are critically involved in the processing of positive and negative stimuli. Individual differences in this system, depending on genotype, have been shown to act as a liability factor for different psychopathologies. Inbred mouse strains are commonly used in preclinical studies of normal and pathological behaviors. In particular, C57BL/6J (C57) and DBA/2J (DBA) strains have permitted to disclose the impact of different genetic backgrounds over the corticolimbic system functions. Here, we summarize the main findings collected over the years in our laboratory, showing how the genetic background plays a critical role in modulating amminergic and GABAergic neurotransmission in prefrontal-accumbal-amygdala system response to different rewarding and aversive experiences, as well as to stress response. Finally, we propose a top-down model for the response to rewarding and aversive stimuli in which amminergic transmission in prefrontal cortex (PFC) controls accumbal and amygdala neurotransmitter response.

Potential role of the combination of galantamine and memantine to improve cognition in schizophrenia

The Measurement and Treatment Research to Improve Cognition in Schizophrenia (MATRICS) and Treatment Units for Research on Neurocognition and Schizophrenia projects were designed to facilitate the development of new drugs for the treatment of cognitive impairments in people with schizophrenia. The MATRICS project identified three drug mechanisms of particular interest: dopaminergic, cholinergic, and glutamatergic. As a group, while people with schizophrenia have moderate cognitive impairment, it is the best predictor of long-term outcome. Unfortunately, there are no approved medications for cognitive impairment in this population. Hence, the development of new pharmacological approaches is critical for reducing illness-related disability. The combination of an acetylcholinesterase inhibitor (AChEI) and memantine is more effective than either medication alone to improve cognition in Alzheimer's dementia. Galantamine is not only an AChEI, but also a positive allosteric modulator of the α4β2 and α7 nicotinic receptors. Hypofunction of N-methyl-d-aspartate (NMDA) receptors has been implicated in the pathophysiology of cognitive symptoms in schizophrenia and hence memantine may positively impact cognition. Memantine decreases the tonic NMDA current and galantamine enhances the action potential mediated by a postsynaptic NMDA current. This results in an increased signal transmission; therefore, a greater signal-to-noise ratio occurs with the combination than memantine alone. Galantamine improves the α-amino-3-hydroxy-5-methyl-4-isoxazol-propionate (AMPA)-mediated signaling which could be neuroprotective and may improve memory coding. The combination of galantamine and memantine may be particularly effective in schizophrenia in order to increase the selective cognition enhancement produced by either medication alone. In the future, multitarget-directed ligands may play a role in the treatment of complex diseases like schizophrenia.

Interaction between effects of genes coding for dopamine and glutamate transmission on striatal and parahippocampal function

The genes for the dopamine transporter (DAT) and the D-Amino acid oxidase activator (DAOA or G72) have been independently implicated in the risk for schizophrenia and in bipolar disorder and/or their related intermediate phenotypes. DAT and G72 respectively modulate central dopamine and glutamate transmission, the two systems most robustly implicated in these disorders. Contemporary studies have demonstrated that elevated dopamine function is associated with glutamatergic dysfunction in psychotic disorders. Using functional magnetic resonance imaging we examined whether there was an interaction between the effects of genes that influence dopamine and glutamate transmission (DAT and G72) on regional brain activation during verbal fluency, which is known to be abnormal in psychosis, in 80 healthy volunteers. Significant interactions between the effects of G72 and DAT polymorphisms on activation were evident in the striatum, parahippocampal gyrus, and supramarginal/angular gyri bilaterally, the right insula, in the right pre-/postcentral and the left posterior cingulate/retrosplenial gyri (P < 0.05, FDR-corrected across the whole brain). This provides evidence that interactions between the dopamine and the glutamate system, thought to be altered in psychosis, have an impact in executive processing which can be modulated by common genetic variation.

How Nox2-containing NADPH oxidase affects cortical circuits in the NMDA receptor antagonist model of schizophrenia

SIGNIFICANCE

Schizophrenia is a complex neuropsychiatric disorder affecting around 1% of the population worldwide. Its mode of inheritance suggests a multigenic neurodevelopmental disorder with symptoms appearing during late adolescence/early adulthood, with its onset strongly influenced by environmental stimuli. Many neurotransmitter systems, including dopamine, glutamate, and gamma-aminobutyric acid, show alterations in affected individuals, and the behavioral and physiological characteristics of the disease can be mimicked by drugs that produce blockade of N-methyl-d-aspartate glutamate receptors (NMDARs).

RECENT ADVANCES

Mounting evidence suggests that drugs that block NMDARs specifically impair the inhibitory capacity of parvalbumin-expressing (PV+) fast-spiking neurons in adult and developing rodents, and alterations in these inhibitory neurons is one of the most consistent findings in the schizophrenic postmortem brain. Disruption of the inhibitory capacity of PV+ inhibitory neurons will alter the functional balance between excitation and inhibition in prefrontal cortical circuits producing impairment of working memory processes such as those observed in schizophrenia.

CRITICAL ISSUES

Mechanistically, the effect of NMDAR antagonists can be attributed to the activation of the Nox2-dependent reduced form of nicotinamide adenine dinucleotide phosphate oxidase pathway in cortical neurons, which is consistent with the emerging role of oxidative stress in the pathogenesis of mental disorders, specifically schizophrenia. Here we review the mechanisms by which NMDAR antagonists produce lasting impairment of the cortical PV+ neuronal system and the roles played by Nox2-dependent oxidative stress mechanisms.

FUTURE DIRECTIONS

The discovery of the pathways by which oxidative stress leads to unbalanced excitation and inhibition in cortical neural circuits opens a new perspective toward understanding the biological underpinnings of schizophrenia.

Treatment with a clinically-relevant dose of methylphenidate alters NMDA receptor composition and synaptic plasticity in the juvenile rat prefrontal cortex

Methylphenidate (Ritalin, MPH) is the most commonly prescribed psychoactive drug for children. Used to treat attention-deficit/hyperactivity disorder (ADHD) and for cognitive enhancement in healthy individuals, its cellular mechanisms of action and potential long-term effects are poorly understood. We recently reported that a clinically relevant (1 mg/kg i.p., single injection) dose of MPH significantly decreased neuronal excitability in the juvenile rat prefrontal cortical neurons. Here we further explore the actions of acute treatment with MPH on the level of NMDA receptor subunits and NMDA receptor-mediated short- and long-term synaptic plasticity in the juvenile rat prefrontal cortical neurons. We found that a single dose of MPH treatment (1 mg/kg, intraperitoneal) significantly decreased the surface and total protein levels of NMDA receptor subunits NR1 and NR2B, but not NR2A, in the juvenile prefrontal cortex. In addition, the amplitude, decay time and charge transfer of NMDA receptor-mediated EPSCs were significantly decreased whereas the amplitude and short-term depression of AMPA receptor-mediated EPSCs were significantly increased in the prefrontal neurons. Furthermore, MPH treatment also significantly increased the probability and magnitude of LTP induction, but had only a small effect on LTD induction in juvenile rat prefrontal cortical neurons. Our data thus present a novel mechanism of action of MPH, i.e., changes in glutamatergic receptor-mediated synaptic plasticity following early-life treatment. Furthermore, since a single dosage resulted in significant changes in NMDA receptors, off-label usage by healthy individuals, especially children and adolescents, may result in altered potential for plastic learning.

Head-twitch response in rodents induced by the hallucinogen 2,5-dimethoxy-4-iodoamphetamine: a comprehensive history, a re-evaluation of mechanisms, and its utility as a model

Two primary animal models persist for assessing hallucinogenic potential of novel compounds and for examining the pharmacological and neurobiological substrates underlying the actions of classical hallucinogens, the two-lever drug discrimination procedure and the drug-induced head-twitch response (HTR) in rodents. The substituted amphetamine hallucinogen, serotonin 2 (5-HT(2) ) receptor agonist, 2,5-dimethoxy-4-iodoamphetamine (DOI) has emerged as the most popular pharmacological tool used in HTR studies of hallucinogens. Synthesizing classic, recent, and relatively overlooked findings, addressing ostensibly conflicting observations, and considering contemporary theories in receptor and behavioural pharmacology, this review provides an up-to-date and comprehensive synopsis of DOI and the HTR model, from neural mechanisms to utility for understanding psychiatric diseases. Also presented is support for the argument that, although both the two-lever drug discrimination and the HTR models in rodents are useful for uncovering receptors, interacting proteins, intracellular signalling pathways, and neurochemical processes affected by DOI and related classical hallucinogens, results from both models suggest they are not reporting hallucinogenic experiences in animals.

Normalization of cortical gray matter deficits in nonpsychotic siblings of patients with childhood-onset schizophrenia

OBJECTIVE

Cortical gray matter (GM) abnormalities in patients with childhood-onset schizophrenia (COS) progress during adolescence ultimately localizing to prefrontal and temporal cortices by early adult age. A previous study of 52 nonpsychotic siblings of COS probands had significant prefrontal and temporal GM deficits that appeared to "normalize" by age 17 years. Here we present a replication with nonoverlapping groups of healthy full siblings and healthy controls.

METHOD

Using an automated measure and prospectively acquired anatomical brain magnetic resonance images, we mapped cortical GM thickness in nonpsychotic full siblings (n = 43, 68 scans; ages 5 through 26 years) of patients with COS, contrasting them with age-, gender-, and scan interval-matched healthy controls (n = 86, 136 scans). The false-discovery rate procedure was used to control for type I errors due to multiple comparisons.

RESULTS

As in our previous study, young nonpsychotic siblings (<17 years) showed significant GM deficits in bilateral prefrontal and left temporal cortices and, in addition, smaller deficits in the parietal and right inferior temporal cortices. These deficits in nonpsychotic siblings normalized with age with minimal abnormalities remaining by age 17.

CONCLUSIONS

Our results support previous findings showing nonpsychotic siblings of COS probands to have early GM deficits that ameliorate with time. At early ages, prefrontal and/or temporal loss may serve as a familial/trait marker for COS. Late adolescence appears to be a critical period for greatest localization of deficits in probands or normalization in nonpsychotic siblings.

Simvastatin prevents dopaminergic neurodegeneration in experimental parkinsonian models: the association with anti-inflammatory responses

BACKGROUND

In addition to their original applications to lowering cholesterol, statins display multiple neuroprotective effects. N-methyl-D-aspartate (NMDA) receptors interact closely with the dopaminergic system and are strongly implicated in therapeutic paradigms of Parkinson's disease (PD). This study aims to investigate how simvastatin impacts on experimental parkinsonian models via regulating NMDA receptors.

METHODOLOGY/PRINCIPAL FINDINGS

Regional changes in NMDA receptors in the rat brain and anxiolytic-like activity were examined after unilateral medial forebrain bundle lesion by 6-hydroxydopamine via a 3-week administration of simvastatin. NMDA receptor alterations in the post-mortem rat brain were detected by [³H]MK-801(Dizocilpine) binding autoradiography. 6-hydroxydopamine treated PC12 was applied to investigate the neuroprotection of simvastatin, the association with NMDA receptors, and the anti-inflammation. 6-hydroxydopamine induced anxiety and the downregulation of NMDA receptors in the hippocampus, CA1(Cornu Ammonis 1 Area), amygdala and caudate putamen was observed in 6-OHDA(6-hydroxydopamine) lesioned rats whereas simvastatin significantly ameliorated the anxiety-like activity and restored the expression of NMDA receptors in examined brain regions. Significant positive correlations were identified between anxiolytic-like activity and the restoration of expression of NMDA receptors in the hippocampus, amygdala and CA1 following simvastatin administration. Simvastatin exerted neuroprotection in 6-hydroxydopamine-lesioned rat brain and 6-hydroxydopamine treated PC12, partially by regulating NMDA receptors, MMP9 (matrix metalloproteinase-9), and TNF-a (tumour necrosis factor-alpha).

CONCLUSIONS/SIGNIFICANCE

Our results provide strong evidence that NMDA receptor modulation after simvastatin treatment could partially explain its anxiolytic-like activity and anti-inflammatory mechanisms in experimental parkinsonian models. These findings contribute to a better understanding of the critical roles of simvastatin in treating PD via NMDA receptors.

Prefrontal stimulation of GABAA receptors counteracts the corticolimbic hyperactivity produced by NMDA antagonists in the prefrontal cortex of the rat

RATIONALE

The hypofunction of NMDA receptors in the prefrontal cortex (PFC) has been suggested to produce corticolimbic hyperactivity through the reduction of cortical GABA transmission.

OBJECTIVES

The present study investigates the effects of injections of the NMDA antagonist 3-[(R)-2-carboxypiperazin-4-yl]-propyl-1-phosphonic acid (CPP) into the PFC on (1) the release of dopamine and/or acetylcholine in the amygdala and hippocampus, (2) the levels of corticosterone in the hippocampus and (3) spontaneous motor activity. Also, the stimulation of GABA(A) receptors, by prefrontal injections of muscimol, on the effects produced by NMDA antagonists on these same neurochemical, hormonal and behavioural parameters was evaluated.

METHODS

Male Wistar rats were implanted with guide cannulae to perform bilateral microinjections into the PFC and microdialysis experiments in the amygdala and/or ventral hippocampus, simultaneously. Spontaneous motor activity was monitored in the open field.

RESULTS

Injections of CPP (1 μg/0.5 μl) into the PFC increased dialysate concentrations of dopamine and acetylcholine in the amygdala, acetylcholine and free corticosterone in the hippocampus and also motor activity. Simultaneous injections of muscimol (0.5 μg/0.5 μl) into the PFC counteracted the increases of dopamine and acetylcholine in the amygdala and hippocampus and also significantly reduced the peak increase of corticosterone in the hippocampus. Injections of muscimol (0.05 and 0.5 μg/0.5 μl) reduced the increases of motor activity produced by prefrontal NMDA antagonists.

CONCLUSIONS

These results suggest that the hypofunction of NMDA receptors in the PFC produces corticolimbic hyperactivity through the activation of prefrontal efferent projections to subcortical/limbic areas.

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.

Functional and dysfunctional synaptic plasticity in prefrontal cortex: roles in psychiatric disorders

Prefrontal cortex (PFC) mediates an assortment of cognitive functions including working memory, behavioral flexibility, attention, and future planning. Unlike the hippocampus, where induction of synaptic plasticity in the network is well-documented in relation to long-term memory, cognitive functions mediated by the PFC have been thought to be independent of long-lasting neuronal adaptation of the network. Nonetheless, accumulating evidence suggests that prefrontal cortical neurons possess the cellular machinery of synaptic plasticity and exhibit lasting changes of neural activity associated with various cognitive processes. Moreover, deficits in the mechanisms of synaptic plasticity induction in the PFC might be involved in the pathophysiology of psychiatric and neurological disorders such as schizophrenia, drug addiction, mood disorders, and Alzheimer's disease.

Activation of glycogen synthase kinase-3 beta is required for hyperdopamine and D2 receptor-mediated inhibition of synaptic NMDA receptor function in the rat prefrontal cortex

The interactions between dopamine and glutamate systems play an essential role in normal brain functions and neuropsychiatric disorders. The mechanism of NMDA receptor regulation through high concentrations of dopamine, however, remains unclear. Here, we show the signaling pathways involved in hyperdopaminergic regulation of NMDA receptor functions in the prefrontal cortex by incubating cortical slices with high concentration of dopamine or administering dopamine reuptake inhibitor 1-(2-[bis-(4-fluorophenyl)methoxy]ethyl)- 4-(3-phenylpropyl)piperazine (GBR12909) in vivo. We found that, under both conditions, the synaptic NMDA receptor-mediated currents were significantly attenuated by excessive dopamine stimulation through activation of D(2) receptors. Furthermore, high dose of dopamine failed to affect NMDA receptor-mediated currents after blockade of NR2B subunits but triggered a dynamin-dependent endocytosis of NMDA receptors. The high-dose dopamine/D(2) receptor-mediated suppression of NMDA receptors was involved in the increase of glycogen synthase kinase-3beta (GSK-3beta) activity, which in turn phosphorylates beta-catenin and disrupts beta-catenin-NR2B interaction, but was dependent on neither Gq11 nor PLC (phospholipase C). Moreover, the hyperdopamine induced by GBR12909 significantly decreased the expression of both surface and intracellular NR2B proteins, as well as NR2B mRNA levels, suggesting an inhibition of protein synthesis. These effects were, however, completely reversed by administration of either GSK-3beta inhibitor or D(2) receptor antagonist. These results therefore suggest that GSK-3beta is required for the hyperdopamine/D(2) receptor-mediated inhibition of NMDA receptors in the prefrontal neurons and these actions may underlie D(2) receptor-mediated psychostimulant effects and hyperdopamine-dependent behaviors in the brain.

Evaluation of the antipsychotic effect of bi-acetylated l-stepholidine (l-SPD-A), a novel dopamine and serotonin receptor dual ligand

Bi-acetylated l-stepholidine (l-SPD-A), a novel derivate of l-stepholidine (l-SPD), possesses a pharmacological profile of D(1)/5-HT(1A) agonism and D(2) antagonism. In the present study, we examined the potential antipsychotic effect of l-SPD-A in a phencyclidine (PCP)-induced rat model of schizophrenia. Pretreatment with l-SPD-A blocked acute PCP-induced hyperlocomotion and reversed prepulse inhibition (PPI) deficits. Chronic l-SPD-A administration (i.p., 10mg/kg/day for 14 days) improved social interaction and novel object recognition impairments in rats that were pretreated with PCP (i.p., 5mg/kg/day for 14 days). Moreover, in a conditioned avoidance response (CAR) test, l-SPD-A, with either i.p. or oral administration, significantly decreased active avoidance without affecting the escape response of rats. Importantly, compared to that of the parent compound l-SPD, l-SPD-A showed stronger suppression of CARs. Lastly, using a [(35)S]GTPgammaS binding assay, we demonstrated that l-SPD-A improved impaired dopamine D(1) receptor function in the prefrontal cortex (PFC) in chronic PCP-treated rats. Taken together, these results indicate that l-SPD-A was not only effective against the hyperactivity, but also improved the sensorimotor gating deficit, social withdrawal and cognitive impairment in an animal model of schizophrenia. The present data suggest that l-SPD-A, a potential neurotransmitter stabilizer, is a promising novel candidate drug for the treatment of schizophrenia.

Oral bioavailability and brain penetration of (-)-stepholidine, a tetrahydroprotoberberine agonist at dopamine D(1) and antagonist at D(2) receptors, in rats

BACKGROUND AND PURPOSE

(-)-Stepholidine has high affinity for dopamine D(1) and D(2) receptors. The aims of the present study were to examine the oral bioavailability and brain penetration of (-)-stepholidine and to gain understanding of mechanisms governing its transport across the enterohepatic barrier and the blood-brain barrier.

EXPERIMENTAL APPROACH

The pharmacokinetics of (-)-stepholidine was studied in rats and microdialysis was used to measure delivery to the brain. These studies were supported by biological measurement of unbound (-)-stepholidine. Membrane permeability was assessed using Caco-2 cell monolayers. Metabolite profiling of (-)-stepholidine in rat bile and plasma was performed. Finally, in vitro metabolic stability and metabolite profile of (-)-stepholidine were examined to compare species similarities and differences between rats and humans.

KEY RESULTS

Orally administered (-)-stepholidine was rapidly absorbed from the gastrointestinal tract; two plasma concentration peaks were seen, and the second peak might result from enterohepatic circulation. Due to extensive pre-systemic metabolism, the oral bioavailability of (-)-stepholidine was poor (<2%). However, the compound was extensively transported across the blood-brain barrier, demonstrating an AUC (area under concentration-time curve) ratio of brain : plasma of approximately 0.7. (-)-Stepholidine showed good membrane permeability that was unaffected by P-glycoprotein and multidrug resistance-associated protein 2. In vitro (-)-stepholidine was metabolized predominantly by glucuronidation and sulphation in rats and humans, but oxidation of this substrate was very low.

CONCLUSIONS AND IMPLICATIONS

Although (-)-stepholidine exhibits good brain penetration, future development efforts should aim at improving its oral bioavailability by protecting against pre-systemic glucuronidation or sulphation. In this regard, prodrug approaches may be useful.

Cognitive effects of second-generation antipsychotics: current insights into neurochemical mechanisms

Historically, pharmacotherapy for schizophrenia was mainly focused on finding drugs to treat psychotic symptoms only, without addressing other crucial domains of the disorder such as cognitive impairments. As a result, these domains have remained undertreated. In this review, we discuss recent preclinical research efforts, including investigation of synaptic mechanisms as well as intracellular signalling pathways and mechanisms involved in neuroplasticity and cell resilience, that may represent new mechanisms participating in the pathogenesis of schizophrenia, particularly at the level of the prefrontal cortex and hippocampus, and that might lead to the development of drugs that can counteract, at least partially, the cognitive impairments typical of schizophrenia.

Astrocyte-derived kynurenic acid modulates basal and evoked cortical acetylcholine release

We tested the hypothesis that fluctuations in the levels of kynurenic acid (KYNA), an endogenous antagonist of the alpha7 nicotinic acetylcholine (ACh) receptor, modulate extracellular ACh levels in the medial prefrontal cortex in rats. Decreases in cortical KYNA levels were achieved by local perfusion of S-ESBA, a selective inhibitor of the astrocytic enzyme kynurenine aminotransferase II (KAT II), which catalyses the formation of KYNA from its precursor L-kynurenine. At 5 mm, S-ESBA caused a 30% reduction in extracellular KYNA levels, which was accompanied by a two-threefold increase in basal cortical ACh levels. Co-perfusion of KYNA in the endogenous range (100 nm), which by itself tended to reduce basal ACh levels, blocked the ability of S-ESBA to raise extracellular ACh levels. KYNA perfusion (100 nm) also prevented the evoked ACh release caused by d-amphetamine (2.0 mg/kg). This effect was duplicated by the systemic administration of kynurenine (50 mg/kg), which resulted in a significant increase in cortical KYNA formation. Jointly, these data indicate that astrocytes, by producing and releasing KYNA, have the ability to modulate cortical cholinergic neurotransmission under both basal and stimulated conditions. As cortical KYNA levels are elevated in individuals with schizophrenia, and in light of the established role of cortical ACh in executive functions, our findings suggest that drugs capable of attenuating the production of KYNA may be of benefit in the treatment of cognitive deficits in schizophrenia.

Dysconnection in schizophrenia: from abnormal synaptic plasticity to failures of self-monitoring

Over the last 2 decades, a large number of neurophysiological and neuroimaging studies of patients with schizophrenia have furnished in vivo evidence for dysconnectivity, ie, abnormal functional integration of brain processes. While the evidence for dysconnectivity in schizophrenia is strong, its etiology, pathophysiological mechanisms, and significance for clinical symptoms are unclear. First, dysconnectivity could result from aberrant wiring of connections during development, from aberrant synaptic plasticity, or from both. Second, it is not clear how schizophrenic symptoms can be understood mechanistically as a consequence of dysconnectivity. Third, if dysconnectivity is the primary pathophysiology, and not just an epiphenomenon, then it should provide a mechanistic explanation for known empirical facts about schizophrenia. This article addresses these 3 issues in the framework of the dysconnection hypothesis. This theory postulates that the core pathology in schizophrenia resides in aberrant N-methyl-D-aspartate receptor (NMDAR)-mediated synaptic plasticity due to abnormal regulation of NMDARs by neuromodulatory transmitters like dopamine, serotonin, or acetylcholine. We argue that this neurobiological mechanism can explain failures of self-monitoring, leading to a mechanistic explanation for first-rank symptoms as pathognomonic features of schizophrenia, and may provide a basis for future diagnostic classifications with physiologically defined patient subgroups. Finally, we test the explanatory power of our theory against a list of empirical facts about schizophrenia.

A randomized, placebo-controlled study of memantine as adjunctive treatment in patients with schizophrenia

Memantine, an uncompetitive antagonist of glutamate receptors of the N-methyl-D-aspartate type is approved for the treatment of moderate to severe Alzheimer's disease. A growing body of evidence supports a link between the glutamatergic neurotransmission and schizophrenia. The purpose of this study (MEM-MD-29) was to examine the efficacy and safety of memantine as an adjunctive treatment to atypical antipsychotics in patients with persistent residual psychopathology of schizophrenia. In this double-blind, placebo-controlled study, participants were assigned to receive 20 mg/day memantine (n=70) or placebo (n=68), in addition to continuing treatment with atypical antipsychotics, for 8 weeks. The primary efficacy measure was the total score on the Positive and Negative Symptom Scale (PANSS). Secondary measures were positive and negative PANSS scores, PANSS responders, Calgary Depression Scale for Schizophrenia (CDSS), Clinical Global Impression of Severity (CGI-S), Clinical Global Impression of Improvement (CGI-I), and Brief Assessment of Cognition in Schizophrenia (BACS). Missing data were imputed using the last observation carried forward (LOCF) approach. Safety was assessed by means of physical examination, clinical laboratory evaluation, recording of adverse events (AEs), and measures of extrapyramidal symptoms. At end point, total PANSS scores did not differ between the memantine and the placebo group (p=0.570, LOCF). A similar outcome was observed for all secondary measures. The frequency of serious AEs in the memantine vs placebo group was 8.7 vs 6.0%; treatment discontinuations because of AEs occurred in 11.6 and 3.0% of patients in these groups, respectively. Memantine showed no efficacy as an adjunctive therapy in schizophrenia patients with residual psychopathology and was associated with a higher incidence of AEs than placebo.

Dopaminergic suppression of synaptic transmission in the lateral entorhinal cortex

Dopaminergic projections to the superficial layers of the lateral entorhinal cortex can modulate the strength of olfactory inputs to the region. We have found that low concentrations of dopamine facilitate field EPSPs in the entorhinal cortex, and that higher concentrations of dopamine suppress synaptic responses. Here, we have used whole-cell current clamp recordings from layer II neurons to determine the mechanisms of the suppression. Dopamine (10 to 50 μM) hyperpolarized membrane potential and reversibly suppressed the amplitude of EPSPs evoked by layer I stimulation. Both AMPA- and NMDA-mediated components were suppressed, and paired-pulse facilitation was also enhanced indicating that the suppression is mediated largely by reduced glutamate release. Blockade of D₂-like receptors greatly reduced the suppression of EPSPs. Dopamine also lowered input resistance, and reduced the number of action potentials evoked by depolarizing current steps. The drop in input resistance was mediated by activation of D₁-like receptors, and was prevented by blocking K⁺ channels with TEA. The dopaminergic suppression of synaptic transmission is therefore mediated by a D₂ receptor-dependent reduction in transmitter release, and a D₁ receptor-dependent increase in a K⁺ conductance. This suppression of EPSPs may dampen the strength of sensory inputs during periods of elevated mesocortical dopamine activity.

Activation of dopamine D1 receptors blocks phencyclidine-induced neurotoxicity by enhancing N-methyl-D-aspartate receptor-mediated synaptic strength

Early postnatal blockade of NMDA receptors by phencyclidine (PCP) causes cortical apoptosis in animals. This is associated with the development of schizophrenia-like behaviors in rats later in life. Recent studies show that the mechanism involves a loss of neurotrophic support from the phosphoinositol-3 kinase/Akt pathway, which is normally maintained by synaptic NMDA receptor activation. Here we report that activation of dopamine D1 receptors (D1R) with dihydrexidine (DHX) prevents PCP-induced neurotoxicity in cortical neurons by enhancing the efficacy of NMDAergic synapses. DHX increases serine phosphorylation of the NR1 subunit through protein kinase A activation and tyrosine phosphorylation of the NR2B subunit via Src kinase. DHX enhances recruitment of NR1 and NR2B, but not NR2A, into synapses. DHX also facilitated the synaptic response in cortical slices and this was blocked by an NR2B antagonist. DHX pre-treatment of rat pups prior to PCP on postnatal days 7, 9 and 11 inhibited PCP-induced caspase-3 activation on PN11 and deficits in pre-pulse inhibition of acoustic startle measured on PN 26-28. In summary, these data demonstrate that PCP-induced deficits in NMDA receptor function, neurotoxicity and subsequent behavioral deficits may be prevented by D1R activation in the cortex and further, it is suggested that D1R activation may be beneficial in treating schizophrenia.

High dose of simvastatin induces hyperlocomotive and anxiolytic-like activities: The association with the up-regulation of NMDA receptor binding in the rat brain

Statins are widely being used for the treatment of a variety of conditions beyond their original indication for lowering cholesterol. We have previously reported that simvastatin affected the dopaminergic system in the rat brain. This study aims to investigate locomotor and anxiety effects along with the regional changes of N-methyl-d-aspartate (NMDA) receptors in the rat brain after 4-week administration of simvastatin. Hyperlocomotive and anxiolytic-like activities in the rat were observed after chronic administration of high dose simvastatin (10 mg/kg/day). Distributions and alterations of NMDA receptors in the post-mortem rat brain were detected by [(3)H] MK-801 binding autoradiography. Simvastatin increased [(3)H] MK-801 binding, predominantly in the prefrontal cortex (20%, p=0.003), primary motor cortex (20%, p<0.001), cingulate cortex (28%, p<0.001), hippocampus (41%, p<0.001), caudate putamen (30%, p=0.029), nucleus accumbens (27%, p=0.035) and amygdala (45%, p<0.001) compared to controls. Significant positive correlations were identified between hyperlocomotive as well as anxiolytic-like activities and the upregulation of NMDA receptors in different brain regions. Our results also provide strong evidence that chronic high dose simvastatin administration is to exhibit NMDA antagonist-like effects, which would partially explain the anxiolytic and hyperlocomotor activities. These findings contribute to a better understanding of the critical roles of simvastatin in modulating psycho-neurodegenerative disorders, via NMDA receptors.

Blockade of NMDA receptors in the prefrontal cortex increases dopamine and acetylcholine release in the nucleus accumbens and motor activity

OBJECTIVES

The present study investigates the effects of injections of a specific N-methyl-D-aspartic acid (NMDA) antagonist 3-[(R)-2-carboxypiperazin-4-yl]-propyl-1-phophonic acid (CPP) into the prefrontal cortex (PFC) on the extracellular concentrations of dopamine and acetylcholine in the nucleus accumbens (NAc) and on motor activity in the freely moving rat.

MATERIALS AND METHODS

Sprague-Dawley male rats were implanted with guide cannulas into the medial PFC and NAc to perform bilateral microinjections and microdialysis experiments. Spontaneous motor activity was monitored in the open field.

RESULTS

Injections of CPP (1 microg/0.5 microL) into the PFC produced a significant increase of the baseline extracellular concentrations of dopamine (up to 130%), dihydroxyphenylacetic acid (DOPAC; up to 120%), homovanillic acid (HVA; up to 130%), and acetylcholine (up to 190%) in the NAc as well as motor hyperactivity. In the NAc, perfusion of the NMDA and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) glutamate antagonists CPP (50 microM)+6,7-dinitroquinoxaline-2,3-dione (DNQX; 50 microM) through the microdialysis probe blocked acetylcholine release, but not DOPAC and HVA increases produced by CPP injections into the PFC. Also, increases in motor activity produced by prefrontal injections of CPP were significantly reduced by bilateral injections into the NAc of a mixed D1/D2 antagonist, flupenthixol (5 and 25 microg/0.5 microL). Injections into the NAc of the muscarinic antagonist scopolamine (1 and 10 microg/0.5 microL) further increased, and of the nicotinic antagonist mecamylamine (1 and 10 microg/0.5 microL) did not change, the increases in motor activity produced by prefrontal CPP injections.

CONCLUSIONS

These results suggest that the dysfunction of NMDA receptors in the PFC could be a key factor in the neurochemical and motor effects associated with corticolimbic hyperactivity.

Activation of phosphatidylinositol-linked novel D1 dopamine receptor contributes to the calcium mobilization in cultured rat prefrontal cortical astrocytes

Recent evidences indicate the existence of an atypical D(1) dopamine receptor other than traditional D(1) dopamine receptor in the brain that mediates PI hydrolysis via activation of phospholipase C(beta) (PLC(beta)). To further understand the basic physiological function of this receptor in brain, the effects of a selective phosphoinositide (PI)-linked D(1) dopamine receptor agonist SKF83959 on cytosolic free calcium concentration ([Ca(2+)](i)) in cultured rat prefrontal cortical astrocytes were investigated by calcium imaging. The results indicated that SKF83959 caused a transient dose-dependent increase in [Ca(2+)](i). Application of D(1) receptor, but not D(2), alpha(1) adrenergic, 5-HT receptor, or cholinergic antagonist prevented SKF83959-induced [Ca(2+)](i) rise, indicating that activation of the D(1) dopamine receptor was essential for this response. Increase in [Ca(2+)](i) was a two-step process characterized by an initial increase in [Ca(2+)](i) mediated by release from intracellular stores, supplemented by influx through voltage-gated calcium channels, receptor-operated calcium channels, and capacitative Ca(2+) entry. Furthermore, SKF83959-stimulated increase in [Ca(2+)](i) was abolished following treatment with a PLC inhibitor. Overall, these results suggested that activation of D(1) receptor by SKF83959 mediates a dose-dependent mobilization of [Ca(2+)](i) via the PLC signaling pathway in cultured rat prefrontal cortical astrocytes.

The antipsychotic potential of l-stepholidine--a naturally occurring dopamine receptor D1 agonist and D2 antagonist

RATIONALE

l-Stepholidine, a dopamine D(2) antagonist with D(1) agonist activity, should in theory control psychosis and treat cognitive symptoms by enhancing cortical dopamine transmission. Though several articles describe its impact on the dopamine system, it has not been systematically evaluated and compared to available antipsychotics.

MATERIALS AND METHODS

We examined its in vitro interaction with dopamine D(2) and D(1) receptors and compared its in vivo pharmacokinetic profile to haloperidol (typical) and clozapine (atypical) in animal models predictive of antipsychotic activity.

RESULTS

In vitro, l-stepholidine showed significant activity on dopamine receptors, and in vivo, l-stepholidine demonstrated a dose-dependent striatal receptor occupancy (RO) at D(1) and D(2) receptors (D(1) 9-77%, 0.3-30 mg/kg; D(2) 44-94%, 1-30 mg/kg), though it showed a rather rapid decline of D(2) occupancy related to its quick elimination. In tests of antipsychotic efficacy, it was effective in reducing amphetamine- and phencyclidine-induced locomotion as well as conditioned avoidance response, whereas catalepsy and prolactin elevation, the main side effects, appeared only at high D(2)RO (>80%). This preferential therapeutic profile was supported by a preferential immediate early gene (Fos) induction in the nucleus accumbens over dorsolateral striatum. We confirmed its D(1) agonism in vitro, and then using D(2) receptor, knockout mice showed that l-stepholidine shows D(1) agonism in the therapeutic dose range.

CONCLUSIONS

Thus, l-stepholidine shows efficacy like an "atypical" antipsychotic in traditional animal models predictive of antipsychotic activity and shows in vitro and in vivo D(1) agonism, and, if its rapid elimination does not limit its actions, it could provide a unique therapeutic approach to schizophrenia.

Prefrontal cortex-nucleus accumbens interaction: in vivo modulation by dopamine and glutamate in the prefrontal cortex

Previous experimental studies have shown that the prefrontal cortex (PFC) regulates the activity of the nucleus accumbens (NAc), and in particular the release of dopamine in this area of the brain. In the present report we review recent microinjections/microdialysis studies from our laboratory on the effects of stimulation/blockade of dopamine and glutamate receptors in the PFC that modulate dopamine, and also acetylcholine release in the NAc. Stimulation of prefrontal D2 dopamine receptors, but not group I mGlu glutamate receptors, reduces the release of dopamine and acetylcholine in the NAc and spontaneous motor activity. This inhibitory role of prefrontal D2 receptors is not changed by acute systemic injections of the NMDA antagonist phencyclidine. On the other hand, the blockade of NMDA receptors in the PFC increases the release of dopamine and acetylcholine in the NAc as well as motor activity which suggests that the hypofunction of prefrontal NMDA receptors is able to produce the neurochemical and behavioural changes associated with a dysfunction of the corticolimbic circuit. We suggest here that dopamine and glutamate receptors are, in part, segregated in specific cellular circuits in the PFC. Thus, the stimulation/blockade of these receptors would have a different net impact on PFC output projections to regulate dopamine and acetylcholine release in the NAc and in guided behaviour. Finally, it is speculated that environmental enrichment might produce plastic changes that modify the functional interaction between the PFC and the NAc in both physiological and pathological conditions.

The cognitive effects of modulating the glycine site of the NMDA receptor with high-dose glycine in healthy controls

N-methyl-D-aspartate (NMDA) receptors play an important role in learning and memory. Targeting the glycine modulatory site of the NMDA receptor has been suggested as a therapeutic strategy to improve cognition, although findings have not been convincing. We used the Cognitive Drug Research computerised assessment system to examine the effects of high-dose glycine on a number of cognitive processes in healthy young subjects. The study was a randomised placebo controlled repeated measures design in which each subject received acute placebo or glycine (0.8 g/kg) orally, with treatment conditions separated by a 5-day washout period. No significant effects of glycine were found on measures of working memory, declarative memory, attention or perceptual processing. These findings, together with those of previous studies, cast doubt over the ability of acute high-dose glycine to improve cognitive function in healthy subjects and suggest that the optimum dose of glycine for improving cognition may vary between different populations, possibly due to differences in endogenous glycine levels and the functional status of NMDA receptors.

Comparison between intraperitoneal and subcutaneous phencyclidine administration in Sprague-Dawley rats: a locomotor activity and gene induction study

In a putative model of acute phencyclidine (PCP)-induced psychosis we evaluated effects of the drug on locomotor activity (LMA) and immediate early gene (IEG) induction in the rat using two routes of drug administration, intraperitoneal (i.p.) and subcutaneous (s.c.). Adult male rats received saline or PCP (1.0-5.0 mg/kg) either i.p or s.c. and were assessed for LMA for 60 min. At the end of the LMA testing animals were culled and blood and brain samples were collected for PCP concentration analysis. Separate cohorts of animals received 5.0 mg/kg PCP (i.p. or s.c.) and were used to investigate (1) the pharmacokinetics of PCP or (2) induction of IEG (Arc, c-fos, BDNF, junB, Krox-20, sgk-1, NURR1, fra-2, Krox-24, and egr-3) mRNA expression in the prefrontal cortex (PFC). Administration of PCP resulted in locomotor hyperactivity which was more robust and longer-lasting in animals dosed s.c. compared to i.p.-treated-animals. Differences in hyperlocomotion were paralleled by higher concentrations of PCP in the blood and in the brain of s.c.-treated animals compared to i.p.-treated animals. The differences in the concentration of PCP between the two routes of administration were detected 30 min after dosing and persisted for up to 4 h. Administration of PCP via the s.c. route resulted in induction of more IEGs and consistently larger magnitudes of induction than that via the i.p. route. Therefore, we have outlined the dosing conditions to induce rapid and robust effect of acute PCP on behaviour, gene induction, and pharmacokinetic profile, to allow investigation of this as a potential animal model of acute psychosis.

Behavioral and neurochemical alterations in mice deficient in anaplastic lymphoma kinase suggest therapeutic potential for psychiatric indications

The receptor tyrosine kinase product of the anaplastic lymphoma kinase (ALK) gene has been implicated in oncogenesis as a product of several chromosomal translocations, although its endogeneous role in the hematopoietic and neural systems has remained poorly understood. We describe that the generation of animals homozygous for a deletion of the ALK tyrosine kinase domain leads to alterations in adult brain function. Evaluation of adult ALK homozygotes (HOs) revealed an age-dependent increase in basal hippocampal progenitor proliferation and alterations in behavioral tests consistent with a role for this receptor in the adult brain. ALK HO animals displayed an increased struggle time in the tail suspension test and the Porsolt swim test and enhanced performance in a novel object-recognition test. Neurochemical analysis demonstrates an increase in basal dopaminergic signalling selectively within the frontal cortex. Altogether, these results suggest that ALK functions in the adult brain to regulate the function of the frontal cortex and hippocampus and identifies ALK as a new target for psychiatric indications, such as schizophrenia and depression, with an underlying deregulated monoaminergic signalling.

The Yin and Yang of dopamine release: a new perspective

Dopamine has undergone extensive investigation due to its known involvement in a number of neurological and psychiatric disorders. In particular, studies into pathological conditions have focused on the roles of high amplitude, phasically evoked dopamine release in regions such as the prefrontal cortex and striatum. However, research has shown that dopamine release can be more complex than just phasic release; thus, there is also a tonic, background dopamine release, with alterations in tonic dopamine release likely having unique and important functional roles. Unfortunately, however, tonic dopamine release has received relatively little attention. In this review, we summarize our recent studies and discuss how modulation of the dopamine system, both in terms of phasic activation and attenuation of tonic dopamine are important for the functions of brain regions receiving this dopamine innervation, and that imbalances in these dopamine release mechanisms may play a significant role in psychiatric disorders such as schizophrenia.

Effects of (-)-stepholidine on NMDA receptors: comparison with haloperidol and clozapine

AIM

To examine whether (-)-stepholidine (SPD) has a direct effect on the N-methyl- D-aspartic acid receptors (NMDAR) containing the NMDA receptor subunits NR2A or NR2B and to compare its effect with those of haloperidol (Hal) and clozapine (Cloz).

METHODS

NMDAR was transiently expressed in human embryonic kidney 293 (HEK293) cells. Changes in intracellular calcium concentration ([Ca2+]i) induced by NMDAR activation were monitored with Fura-2 ratio imaging techniques.

RESULTS

SPD had no significant effects on either subunit of NMDAR at a concentration of less than 100 micromol/L. Hal selectively inhibited NMDAR containing the NR2B subunit, whereas Cloz inhibited both subunits of NMDAR. Although both Hal and Cloz inhibited NR1a/NR2B receptor-mediated Ca2+ influx, their effects were different. Hal was more potent and had a faster peak effect than Cloz.

CONCLUSION

Both Hal and Cloz inhibit NMDAR-mediated function, whereas SPD produced only a little inhibition at a high concentration. Based on our other studies, the modulation of SPD on NMDAR function may be via D1 receptor action underlying an indirect mechanism.

l-Stepholidine increases the frequency of sEPSC via the activation of D1 dopamine signaling pathway in rat prelimbic cortical neurons

AIM

To investigate the effect of l-stepholidine (SPD) on the frequency of spontaneous excitatory postsynaptic currents (sEPSC) in the pyramidal cells between layers V and VI in the prelimbic cortex (PL).

METHODS

A whole-cell patch clamp in rat brain slices was used.

RESULTS

SPD significantly increased the frequency of sEPSC in a concentration-dependent manner. A selective D1 dopamine receptor antagonist SCH23390 blocked SPD-mediated effects, whereas the D1 agonist SKF38393, but not the D2/3 antagonist sulpiride, mimicked SPD-mediated increase in the frequency of sEPSC. Moreover, both protein kinase A (PKA) inhibitor N-(2- [p-bromocinnamylamino]-ethyl)-5-isoquinolinesulfonamide hydrochloride and protein kinase C (PKC) inhibitor chelerythrine attenuated the effect of SPD on sEPSC.

CONCLUSION

SPD elicits its effect on the frequency of sEPSC on the PL pyramidal cells via presynaptic D1 receptors, and is dependent on PKA and PKC signaling pathways.

The ability of the mesocortical dopamine system to operate in distinct temporal modes

BACKGROUND

This review discusses evidence that cells in the mesocortical dopamine (DA) system influence information processing in target areas across three distinct temporal domains.

DISCUSSIONS

Phasic bursting of midbrain DA neurons may provide temporally precise information about the mismatch between expected and actual rewards (prediction errors) that has been hypothesized to serve as a learning signal in efferent regions. However, because DA acts as a relatively slow modulator of cortical neurotransmission, it is unclear whether DA can indeed act to precisely transmit prediction errors to prefrontal cortex (PFC). In light of recent physiological and anatomical evidence, we propose that corelease of glutamate from DA and/or non-DA neurons in the VTA could serve to transmit this temporally precise signal. In contrast, DA acts in a protracted manner to provide spatially and temporally diffuse modulation of PFC pyramidal neurons and interneurons. This modulation occurs first via a relatively rapid depolarization of fast-spiking interneurons that acts on the order of seconds. This is followed by a more protracted modulation of a variety of other ionic currents on timescales of minutes to hours, which may bias the manner in which cortical networks process information. However, the prolonged actions of DA may be curtailed by counteracting influences, which likely include opposing actions at D1 and D2-like receptors that have been shown to be time- and concentration-dependent. In this way, the mesocortical DA system optimizes the characteristics of glutamate, GABA, and DA neurotransmission both within the midbrain and cortex to communicate temporally precise information and to modulate network activity patterns on prolonged timescales.

Dopamine D1/5 receptor-mediated long-term potentiation of intrinsic excitability in rat prefrontal cortical neurons: Ca2+-dependent intracellular signaling

Prefrontal cortex (PFC) dopamine D1/5 receptors modulate long- and short-term neuronal plasticity that may contribute to cognitive functions. Synergistic to synaptic strength modulation, direct postsynaptic D1/5 receptor activation also modulates voltage-dependent ionic currents that regulate spike firing, thus altering the neuronal input-output relationships in a process called long-term potentiation of intrinsic excitability (LTP-IE). Here, the intracellular signals that mediate this D1/5 receptor-dependent LTP-IE were determined using whole cell current-clamp recordings in layer V/VI rat pyramidal neurons from PFC slices. After blockade of all major amino acid receptors (V(hold) = -65 mV) brief tetanic stimulation (20 Hz) of local afferents or application of the D1 agonist SKF81297 (0.2-50 microM) induced LTP-IE, as shown by a prolonged (>40 min) increase in depolarizing pulse-evoked spike firing. Pretreatment with the D1/5 antagonist SCH23390 (1 microM) blocked both the tetani- and D1/5 agonist-induced LTP-IE, suggesting a D1/5 receptor-mediated mechanism. The SKF81297-induced LTP-IE was significantly attenuated by Cd(2+), [Ca(2+)](i) chelation, by inhibition of phospholipase C, protein kinase-C, and Ca(2+)/calmodulin kinase-II, but not by inhibition of adenylate cyclase, protein kinase-A, MAP kinase, or L-type Ca(2+) channels. Thus this form of D1/5 receptor-mediated LTP-IE relied on Ca(2+) influx via non-L-type Ca(2+) channels, activation of PLC, intracellular Ca(2+) elevation, activation of Ca(2+)-dependent CaMKII, and PKC to mediate modulation of voltage-dependent ion channel(s). This D1/5 receptor-mediated modulation by PKC coexists with the previously described PKA-dependent modulation of K(+) and Ca(2+) currents to dynamically regulate overall excitability of PFC neurons.

Acute clozapine suppresses synchronized pyramidal synaptic network activity by increasing inhibition in the ferret prefrontal cortex

Recent studies have indicated that impaired neural circuitry in the prefrontal cortex is a prominent feature of the neuropathology of schizophrenia. Clozapine is one of the most effective antipsychotic drugs used for this debilitating disease. Despite its effectiveness, the mechanism by which clozapine acts on prefrontal cortical circuitry remains poorly understood. In this study, in vitro multiple whole cell recordings were performed in slices of the ferret prefrontal cortex. Clozapine, which effectively inhibited the spontaneous synchronized network activities in the prefrontal neurons, achieved the suppressive effect by decreasing the recurrent excitation among pyramidal neurons and by enhancing the inhibitory inputs onto pyramidal cells through a likely network mechanism. Indeed, under the condition of disinhibition, the depressing effects were reversed and clozapine enhanced the recurrent excitation. These results suggest that the therapeutic actions of clozapine in alleviating the positive symptoms of schizophrenia are achieved, at least partially, through the readjustment of synaptic balance between the excitation and inhibition in the prefrontal cortical circuitry.

Schizophrenia susceptibility genes converge on interlinked pathways related to glutamatergic transmission and long-term potentiation, oxidative stress and oligodendrocyte viability

Over 130 genes have been associated with schizophrenia in genetic studies. None of these has reached a sufficient level of confidence to be accepted as a universal susceptibility gene and problems of replicability suggest that many may be false positives. Nevertheless, these genes can be grouped into distinct families related to glutamate transmission (in particular related to NMDA receptor function), the control of synaptic plasticity, dopaminergic transmission, oxidative stress, glutathione and quinone metabolism and oligodendrocyte viability. These families mirror the processes disrupted in the schizophrenic brain and certain gene families can be linked together to form a clearly defined signalling cascade involved in the phenomenon of NMDA receptor-dependent long-term potentiation and synaptic plasticity, that may be interconnected with oligodendrocyte and oxidative stress-related pathways. Many of the protein products of these genes interact with each other, forming complex integrated networks. Certain high-interest genes (for example DISC1, NRG1, COMT) may exert multiple effects on different areas of these pathways, while others exert more specific effects on certain branches. The convergence of a large number of genes on a definable signaling network raises the possibility of numerous interactions between gene candidates, and suggests that a targeted multigenic pathway approach would be useful in gene association studies.

Where do you think you are going? The NMDA-D1 receptor trap

The number and outcomes of reciprocal interactions between dopamine (DA) D1 receptors and N-methyl-D-aspartate (NMDA)-type glutamate receptors continue to increase. Recent studies have demonstrated close physical interactions in which activation of one receptor affects the function of the other. In one physical interaction, the activation of NMDA receptors alters the topography and movement of D1 receptors by trapping them in dendritic spines and thus altering their distribution. In a second physical interaction, D1 and subunits of NMDA receptors form heterodimers, which are translocated from the cell interior to the surface. Finally, a third physical interaction posits that the C terminus of D1 receptors makes contact with subunits of the NMDA receptor. These physical interactions can attenuate or potentiate receptor function. In contrast, the more traditional interactions mediated by second messengers generally cause NMDA receptor function to be potentiated through the activation of D1 receptors and the cAMP-PKA-DARPP-32 [adenosine 3',5'-monophosphate (cAMP)-protein kinase A-cAMP-regulated phosphoprotein of 32 kD] or PKC (protein kinase C) cascades. Together, these mechanisms provide a basis for understanding the increasing complexity of D1-NMDA receptor interactions and their importance in physiological and pathological processes.