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

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

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

Performance in the Wisconsin Card Sorting Test and the C957T polymorphism of the DRD2 gene in healthy volunteers

INTRODUCTION

Previous studies have associated a decreased striatal D2 dopamine receptor (DRD2) binding with impaired performance in cognitive tasks. In vivo studies have found a lower DRD2 binding associated with the CC genotype of the C957T single nucleotide polymorphism (SNP) of the DRD2 gene.

OBJECTIVE

The aim of this study was to investigate the relationship between executive functions and the C957T DRD2 SNP. We hypothesized that the CC genotype would be associated with a poorer executive functioning.

METHODS

Our sample consisted of 83 healthy volunteers (28 males and 55 females; mean age 25.2, SD 1.7 years). To assess executive functions, the Wisconsin Card Sorting Test was used, considering the variables perseverative errors, perseverative responses, and number of categories achieved. The genotype distribution was 13 CC, 41 CT, and 29 TT, satisfying Hardy-Weinberg equilibrium.

RESULTS

Carriers of the CC genotype, compared with carriers of the CT/TT genotypes, achieved significantly fewer categories (5.00 vs. 5.81; p = 0.004), made a greater number of perseverative errors (13.46 vs. 8.39; p = 0.018), and had a greater number of perseverative responses (14.92 vs. 8.94; p = 0.014).

CONCLUSIONS

Our results support the hypothesis that the C957T DRD2 SNP may influence cognitive performance through its repercussions on central dopaminergic function.

Mother-infant bonding and the evolution of mammalian social relationships

A wide variety of maternal, social and sexual bonding strategies have been described across mammalian species, including humans. Many of the neural and hormonal mechanisms that underpin the formation and maintenance of these bonds demonstrate a considerable degree of evolutionary conservation across a representative range of these species. However, there is also a considerable degree of diversity in both the way these mechanisms are activated and in the behavioural responses that result. In the majority of small-brained mammals (including rodents), the formation of a maternal or partner preference bond requires individual recognition by olfactory cues, activation of neural mechanisms concerned with social reward by these cues and gender-specific hormonal priming for behavioural output. With the evolutionary increase of neocortex seen in monkeys and apes, there has been a corresponding increase in the complexity of social relationships and bonding strategies together with a significant redundancy in hormonal priming for motivated behaviour. Olfactory recognition and olfactory inputs to areas of the brain concerned with social reward are downregulated and recognition is based on integration of multimodal sensory cues requiring an expanded neocortex, particularly the association cortex. This emancipation from olfactory and hormonal determinants of bonding has been succeeded by the increased importance of social learning that is necessitated by living in a complex social world and, especially in humans, a world that is dominated by cultural inheritance.

Postnatal alterations in dopaminergic markers in the human prefrontal cortex

Dopamine in the prefrontal cortex plays a critical role in normal cognition throughout the lifespan and has been implicated in the pathophysiology of neuropsychiatric disorders such as schizophrenia and attention deficit disorder. Little is known, however, about the postnatal development of the dopaminergic system in the human prefrontal cortex. In this study, we examined pre- and post-synaptic markers of the dopaminergic system in postmortem tissue specimens from 37 individuals ranging in age from 2 months to 86 years. We measured the levels of tyrosine hydroxylase, the rate limiting enzyme in dopamine biosynthesis, using Western immunoblotting. We also examined the gene expression of the three most abundant dopamine receptors (DARs) in the human prefrontal cortex: DAR1, DAR2 and DAR4, by in situ hybridization. We found that tyrosine hydroxylase concentrations and DAR2 mRNA levels were highest in the cortex of neonates. In contrast, the gene expression of DAR1 was highest in adolescents and young adults. No significant changes across age groups were detected in mRNA levels of DAR4. Both DAR1 and DAR2 mRNA were significantly lower in the aged cortex. Taken together, our data suggest dynamic changes in markers of the dopamine system in the human frontal cortex during postnatal development at both pre-and post-synaptic sites. The peak in DAR1 mRNA levels around adolescence/early adulthood may be of particular relevance to neuropsychiatric disorders such as schizophrenia in which symptoms manifest during the same developmental period.

On feeling in control: A biological theory for individual differences in control perception

This review aims to create a cross-disciplinary framework for understanding the perception of control. Although, the personality trait locus of control, the most common measure of control perception, has traditionally been regarded as a product of social learning, it may have biological antecedents as well. It is suggested that control perception follows from the brain's capacity for self regulation, leading to flexible and goal directed behaviours. To this account, a model is presented which spans several levels of analyses. On a behavioural level, control perception may be a corollary of emotion regulation, executive functions, and social cognition. On a neural level, these self-regulatory functions are substantiated in part by the dorsolateral and ventral prefrontal cortex and the anterior cingulate cortex. In addition, a possible role of subcortical-cortical dopamine pathways underlying control perception is discussed.

Subcellular localization of the dopamine D2 receptor and coexistence with the calcium-binding protein neuronal calcium sensor-1 in the primate prefrontal cortex

Structures of the cerebral cortex expressing the D2 dopamine receptor subtype (D2) are important sites of action of antipsychotic drugs. It has also been repeatedly suggested that the prefrontal cortex plays a significant role in neuropsychiatric disorders, including schizophrenia. Here, by using single and double immunohistochemical techniques with electron microscopy, we investigated in the primate prefrontal cortex the ultrastructural localization of D2 and we compared it with that of the neuronal calcium sensor-1 (NCS-1), a neuron-specific calcium-binding and D2-interacting protein. D2 immunoreactivity, revealed with preembedding immunoperoxidase in single labeling and with preembedding immunogold for double labeling, was localized in cell bodies with ultrastructural characteristics of both neurons and astroglia. D2 was localized in pre- and postsynaptic structures, including spines and dendrites, and in both excitatory- and inhibitory-like axon terminals. Immunogold labeling revealed peri- and extrasynaptic localization of D2 in postsynaptic structures, whereas extrasynaptic labeling was typically found in boutons. NSC-1 immunoreactivity was abundant in pre- and postsynaptic structures, in which it was also colocalized with D2. With the present strategy (that has high resolution but relatively limited sensitivity), NSC-1 was observed in about 10% of the D2-immunopositive spines and in a lower proportion of D2-immunopositive dendrites and boutons. The data demonstrate the localization of D2 in pre- and postsynaptic as well as extra- and perisynaptic structures of the primate prefrontal cortex. The data also show the coexistence of NCS-1 and D2 at the ultrastructural level. The latter finding suggests a role for NCS-1 in desensitization of D2 in the prefrontal cortex.

Acquired neuronal channelopathies in HIV-associated dementia

A gene expression profile of the human brain cortex was performed in people with HIV-1-associated dementia (HAD) using Affymetrix HG-U133 chips. Messenger RNA transcripts in middle frontal gyrus from subjects with HAD or milder neurocognitive dysfunction were compared to HIV-negative people. The analysis focused on ionic conductance carriers that control membrane excitation. Overexpressed ionic channel genes in brain cortex of subjects with dementia included (1) a calcium-driven K+ channel that prolongs afterhyperpolarization (AHP) current, (2) a leak type of K+ channel that prolongs the AHP, (3) an adenosine receptor that modulates cationic current via G proteins, (4) a G protein-coupled serotonin receptor that modulates cyclic AMP-linked current transduction, (5) a G protein-coupled dopamine receptor, (6) a GABA receptor subunit that conducts chloride current. Underexpressed current generators in the demented subjects included (1) two voltage-gated K+ channels that influence refractory periods and the onset of AHP, (2) a Na+ channel subunit that modifies current inactivation and the onset of the AHP, (3) a neuronal type of voltage-sensitive Ca+ channel that controls postsynaptic membrane excitability, (4) a metabotropic glutamate receptor that regulates cationic gating via G protein coupling, (5) A specific Galpha protein that transduces metabotropic cationic current, (6) an NMDA receptor subunit, (7) a glycine receptor subunit that modulates chloride current. These gene expression shifts probably occurred in neurons because they were not present in gyral white matter. Acquired neuronal channelopathies were not associated with a generalized shift of neuronal or glial cell markers, which suggest that they were not an artifact produced by neurodegeneration and/or glial cell proliferation. Channelopathies were not correlated with a generalized increase of inflammatory cell transcripts and were present in demented people without, and with HIV encephalitis (HIVE). We surveyed experimentally induced perturbations of these channels to determine the implications for brain function. Eleven experimental channelopathies produced decreased neuronal firing frequencies and pacemaker rates in model neurons; seven channelopathies increase neuronal firing rates experimentally. The implied disruption of neuronal excitability is consistent with some features of HAD, including its potential reversibility after HIV-1 replication is suppressed, the abnormal electroencephalographic recordings, the lack of clear-cut correlation with neurodegeneration and the lack of strict correlation with brain inflammation. The channelopathy concept may have wide relevance to the subcortical dementias.

The principal features and mechanisms of dopamine modulation in the prefrontal cortex

Mesocortical [corrected] dopamine (DA) inputs to the prefrontal cortex (PFC) play a critical role in normal cognitive process and neuropsychiatic pathologies. This DA input regulates aspects of working memory function, planning and attention, and its dysfunctions may underlie positive and negative symptoms and cognitive deficits associated with schizophrenia. Despite intense research, there is still a lack of clear understanding of the basic principles of actions of DA in the PFC. In recent years, there has been considerable efforts by many groups to understand the cellular mechanisms of DA modulation of PFC neurons. However, the results of these efforts often lead to contradictions and controversies. One principal feature of DA that is agreed by most researchers is that DA is a neuromodulator and is clearly not an excitatory or inhibitory neurotransmitter. The present article aims to identify certain principles of DA mechanisms by drawing on published, as well as unpublished data from PFC and other CNS sites to shed light on aspects of DA neuromodulation and address some of the existing controversies. Eighteen key features about DA modulation have been identified. These points directly impact on the end result of DA neuromodulation, and in some cases explain why DA does not yield identical effects under all experimental conditions. It will become apparent that DA's actions in PFC are subtle and depend on a variety of factors that can no longer be ignored. Some of these key factors include distinct bell-shaped dose-response profiles of postsynaptic DA effects, different postsynaptic responses that are contingent on the duration of DA receptor stimulation, prolonged duration effects, bidirectional effects following activation of D1 and D2 classes of receptors and membrane potential state and history dependence of subsequent DA actions. It is hoped that these factors will be borne in mind in future research and as a result a more consistent picture of DA neuromodulation in the PFC will emerge. Based on these factors, a theory is proposed for DA's action in PFC. This theory suggests that DA acts to expand or contract the breadth of information held in working memory buffers in PFC networks.

D1 but not D2 dopamine receptors or adrenoceptors mediate dopamine-induced potentiation of N-methyl-d-aspartate currents in the rat prefrontal cortex

Dopamine-glutamate interactions in the prefrontal cortex (PFC) are associated with higher order cognitive functions, and are involved in the pathophysiology of schizophrenia and addiction. Recordings with intracellular sharp microelectrodes and patch-clamp pipettes were used to investigate these interactions in layer V pyramidal cells of brain slices obtained from the rat PFC. Dopamine (100 microM) potentiated N-methyl-d-aspartate (NMDA; 10mM)-evoked depolarizations, but did not change those elicited by alpha-amino-3-hydroxy-5-methyl-4-isoxazole-4-propionic acid (AMPA; 1mM). Dopamine (100 microM) increased the amplitude of the NMDA (30 microM)-induced currents as well, and 1-phenyl-2,3,4,5-tetrahydro-(1H)-3-benzazepine-7,8-diol (SKF 38393; 1, 10 microM), a D(1) receptor agonist, concentration-dependently reproduced this effect. Furthermore, 7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-(1H)-3-benzapine hydrochloride (SCH 23390; 10 microM), a D(1) receptor antagonist, reversed both the dopamine- and the SKF 38393-evoked potentiation. The D(2) receptor agonists lisuride and quinpirole (10 microM both), as well as noradrenaline (100 microM) failed to mimic the stimulatory effect of dopamine. Isoproterenol (1, 10 microM) concentration-dependently facilitated NMDA responses. However, neither this effect at 10 microM nor that of dopamine at 100 microM could be antagonized by propranolol (10 microM), a non-selective beta adrenoceptor blocker. The isoproterenol-induced facilitation of NMDA currents was abolished by SCH 23390 (10 microM). The results indicate that dopamine potentiates NMDA responses in layer V pyramidal cells of the PFC solely by activating D(1) receptors. D(2) receptors and alpha or beta adrenoceptors are not involved in the dopamine-NMDA interaction.

8,9-dihydroxy-1,2,3,11b-tetrahydrochromeno[4,3,2,-de]isoquinoline (dinoxyline), a high affinity and potent agonist at all dopamine receptor isoforms

The synthesis and preliminary pharmacological evaluation of 8,9-dihydroxy-1,2,3,11b-tetrahydrochromeno[4,3,2,-de]isoquinoline (5, now named dinoxyline) is described. This molecule was designed as a potential bioisostere that would conserve the essential elements of our beta-phenyldopamine D(1) pharmacophore (i.e., position and orientation of the nitrogen, hydroxyls, and phenyl rings). Previously, we have rigidified these elements using alkyl bridges, as exemplified in the dopamine D(1) full agonist molecules dihydrexidine (1) and dinapsoline (2). This approach has been modified and we now show that it is possible to tether these elements using an ether linkage. Preliminary pharmacology has revealed that 5 is a potent full D(1) agonist (K(0.5) <10 nM; EC(50)=30 nM), but also has high affinity for brain D(2)-like and cloned D(2) and D(3) receptors. Interestingly, whereas 1 and 2 and their analogues have only moderate affinity for the human D(4) receptor, 5 also has high affinity for this isoform. Moreover, although N-alkylation of 1 and 2 increases D(2) affinity, the N-allyl (15) and N-n-propyl (17) derivatives of 5 had decreased D(2) affinity. Therefore, 5 may be engaging different amino acid residues than do 1 and 2 when they bind to the D(2) receptor. This is the first example of a ligand with high affinity at all dopamine receptors, yet with functional characteristics similar to dopamine. These rigid ligands also will be useful tools to determine specific residues of the receptor transmembrane domains that are critical for agonist ligand selectivity for the D(4) receptor.

Selective D2 receptor actions on the functional circuitry of working memory

Prefrontal neurons engaged by working memory tasks express a sequence of phasic and tonic activations linked to a train of sensory, mnemonic, and response-related events. Here, we report that the dopamine D2 receptor selectively modulates the neural activities associated with memory-guided saccades in oculomotor delayed-response tasks yet has little or no effect on the persistent mnemonic-related activity, which is instead modulated by D1 receptors. This associates the D2 receptor with a specific component of working memory circuitry and fractionates the modulatory effects of D1 and D2 receptors on the neural machinery of a cognitive process.

Effectiveness of the selective D4 antagonist sonepiprazole in schizophrenia: a placebo-controlled trial

BACKGROUND

Selective localization of dopamine D(4) receptors in the prefrontal cortex and preferential affinity of clozapine for the dopamine D(4) receptor over the D(2) receptor led to the hypothesis that the superior efficacy of clozapine may be mediated via blockade of the D(4) receptor. This hypothesis was tested by evaluating sonepiprazole, a selective D(4) dopamine antagonist, in schizophrenia patients.

METHODS

We treated 467 hospitalized schizophrenia patients with scores of > or = 60 on the Positive and Negative Syndrome Scale (PANSS) with sonepiprazole, olanzapine, or placebo once daily for 6 weeks. The primary efficacy end point was the mean change from baseline in the PANSS total score at 6 weeks. Secondary efficacy end points were the mean change from baseline in the PANSS factor scores, the Brief Psychiatric Rating Scale score, the Clinical Global Impressions Severity of Illness score, and the Calgary Depression Scale score.

RESULTS

No statistically significant differences were observed between placebo and any sonepiprazole dose on the primary or any secondary end point after 6 weeks of treatment. Statistically significant differences, favoring olanzapine over placebo, were observed on all efficacy end points but the Calgary Depression Scale.

CONCLUSIONS

Sonepiprazole was ineffective for the treatment of patients with schizophrenia.

Structural and functional maturation of the developing primate brain

Descriptive studies have established that the developmental events responsible for the assembly of neural systems and circuitry are conserved across mammalian species. However, primates are unique regarding the time during which histogenesis occurs and the extended postnatal period during which myelination of pathways and circuitry formation occur and are then subsequently modified, particularly in the cerebral cortex. As in lower mammals, the framework for subcortical-cortical connectivity in primates is established before midgestation and already begins to remodel before birth. Association systems, responsible for modulating intracortical circuits that integrate information across functional domains, also form before birth, but their growth and reorganization extend into puberty. There are substantial differences across species in the patterns of development of specific neurochemical systems. The complexity is even greater when considering that the development of any particular cellular component may differ among cortical areas in the same primate species. Developmental and behavioral neurobiologists, psychologists, and pediatricians are challenged with understanding how functional maturation relates to the evolving anatomical organization of the human brain during childhood, and moreover, how genetic and environmental perturbations affect the adaptive changes exhibited by neural circuits in response to developmental disruption.

D1 dopamine receptors in the mouse prefrontal cortex: Immunocytochemical and cognitive neuropharmacological analyses

Dopamine D1 receptors have critical neuromodulatory influences on the working memory functions of the prefrontal cortex, a brain region affected in many neuropsychiatric disorders. When D1 receptor agents are administered to rats or monkeys performing working memory tasks, an "inverted U" dose/response function is typically observed, whereby either too little or too much D1 receptor stimulation impairs working memory. There are two subtypes of D1 receptors, the D1A and the D1B (also known as the D1 and D5, respectively), but the relative contributions of these subtypes to prefrontal cortical function are not known, as there are no pharmacological agents that can distinguish between these receptors. Thus, genetically altered mice are needed to address this question. However, it is not known whether the mouse prefrontal cortex contains both D1A and D1B receptor subtypes, nor is it known whether mice will exhibit responses to D1 receptor agonists similar to those seen in rats and monkeys. The current study examined these issues by immunostaining the mouse brain with specific antibodies directed at the D1A and D1B receptor subtypes and by assessing the effects of increasing doses of a D1 receptor agonist, SKF81297, on spatial working memory performance in mice. Results indicate that mice are generally similar to monkeys and rats, expressing both D1A and D1B receptors in the prefrontal cortex and exhibiting an inverted "U" dose/response curve when administered SKF81297.

Involvement of the medial prefrontal cortex in mediating behavioural responses to odour cues rather than olfactory recognition memory

Sheep form an olfactory recognition memory for their lambs within 2 h of parturition and will subsequently reject the approaches of any strange lamb and protest vocally. In this study we report that following olfactory memory formation, ewes exposed to either their own or a strange lamb show c-fos mRNA expression in the medial frontal cortex, although levels of expression in the pyramidal output cell layer V were significantly higher in ewes that rejected strange lambs. Reversibly inactivating this region by the retrodialysis of the anaesthetic tetracaine before birth reduced aggressive motor responses towards lambs but not protest vocalisations. Similar treatment during the critical period for olfactory memory formation and lamb recognition (0-4 h post-partum) had no effect on ewes maternal behaviour towards their own lambs. It did, however, prevent the normal selective expression of aggressive rejection, and reduced protest vocalisation behaviours directed towards strange lambs. These rejection behaviours did appear 1 h after the termination of tetracaine infusions despite the ewes not being given the opportunity to interact with their own lambs during this time. Therefore, tetracaine blockade of the medial frontal cortex prevents animals from responding with motor aggression, but not vocal aggression, to odour cues from strange lambs, but has no effect on the formation of an olfactory recognition memory for their own lambs. Both pre- and post-partum aggressive rejection of strange lambs was associated with increased concentrations of dopamine, serotonin, glutamate and GABA. When these behaviours were inhibited by the tetracaine infusions, extracellular concentrations of these neurotransmitters were all increased by the anaesthetic but did not change in response to lambs. These findings suggest that a functional medial frontal cortex is not required for the formation of an olfactory recognition memory or for mediating pro-active maternal behaviours. It is however required for the mediation of motor but not vocal aspects of aggressive rejection responses directed towards aversive odour cues from strange lambs.

Layer-specific reductions in GFAP-reactive astroglia in the dorsolateral prefrontal cortex in schizophrenia

Neuroimaging studies have implicated the prefronto-striatal loop as a substrate for the cognitive deficits in schizophrenia (SCHZ). Postmortem morphometric studies reveal that layers III and V of the dorsolateral prefrontal cortex (dlPFC), which gave rise to glutamatergic projections to neostriatum, demonstrate the most structural pathology in this region of the SCHZ. These neuronal alterations in SCHZ are not accompanied by marked glial changes as revealed by Nissl staining. We examined the glial-type specific pathology in SCHZ by analyzing the glial fibrillary acidic protein- (GFAP) immunoreactive astroglia in contrast to the Nissl-stained general pool of glial cells in dlPFC (area 9) from 9 subjects with SCHZ and 15 psychiatrically normal control subjects. In layer V of the dlPFC in SCHZ, there was a significant 32% reduction in the GFAP-area fraction, 81% increase in the density of the GFAP-positive cell bodies and a 14% decrease in the width of the cortical layer V, as compared to the control subjects. None of these parameters were affected in layers III and IV in the SCHZ. Therefore, only subtle, type- and layer-specific glial pathology is present in the dlPFC in SCHZ. Astroglial pathology in dlPFC may reflect disturbances of the neuron-glia interactions in layer V and may be related to the dysfunctional prefronto-striatal circuits, dopaminergic alterations and cognitive pathology in SCHZ.

Neuromodulatory transmitter systems in the cortex and their role in cortical plasticity

Cortical neuromodulatory transmitter systems refer to those classical neurotransmitters such as acetylcholine and monoamines, which share a number of common features. For instance, their centers are located in subcortical regions and send long projection axons to innervate the cortex. The same transmitter can either excite or inhibit cortical neurons depending on the composition of postsynaptic transmitter receptor subtypes. The overall functions of these transmitters are believed to serve as chemical bases of arousal, attention and motivation. The anatomy and physiology of neuromodulatory transmitter systems and their innervations in the cerebral cortex have been well characterized. In addition, ample evidence is available indicating that neuromodulatory transmitters also play roles in development and plasticity of the cortex. In this article, the anatomical organization and physiological function of each of the following neuromodulatory transmitters, acetylcholine, noradrenaline, serotonin, dopamine, and histamine, in the cortex will be described. The involvement of these transmitters in cortical plasticity will then be discussed. Available data suggest that neuromodulatory transmitters can modulate the excitability of cortical neurons, enhance the signal-to-noise ratio of cortical responses, and modify the threshold for activity-dependent synaptic modifications. Synaptic transmissions of these neuromodulatory transmitters are mediated via numerous subtype receptors, which are linked to multiple signal transduction mechanisms. Among the neuromodulatory transmitter receptor subtypes, cholinergic M(1), noradrenergic beta(1) and serotonergic 5-HT(2C) receptors appear to be more important than other receptor subtypes for cortical plasticity. In general, the contribution of neuromodulatory transmitter systems to cortical plasticity may be made through a facilitation of NMDA receptor-gated processes.

Interaction between P2Y and NMDA receptors in layer V pyramidal neurons of the rat prefrontal cortex

In the first part of this study, monosynaptic excitatory postsynaptic potentials (EPSPs) in layer V of the rat prefrontal cortex were evoked by electrical stimulation of layer I. Recordings by intracellular sharp microelectrodes showed that EPSPs were concentration-dependently facilitated by the P2 receptor antagonistic ATP analogue 2-methylthio ATP (2-MeSATP), while ATP itself depressed the synaptic potentials. The inhibitory effect of ATP turned into facilitation in the presence of the adenosine A(1) receptor antagonist DPCPX. The 2-MeSATP-induced potentiation of EPSP amplitudes were prevented by the P2 receptor antagonists PPADS and Suramin. The EPSP was almost abolished by coapplication of the NMDA receptor antagonist AP-5 and the AMPA/kainate receptor antagonist CNQX. After blockade of the NMDA receptor-mediated part of the EPSP by AP-5, the stimulatory effect of 2-MeSATP disappeared. When NMDA or AMPA were pressure-applied onto pyramidal cells, only the NMDA-induced depolarization was potentiated by 2-MeSATP. In the second part of the study, NMDA-induced currents were measured by whole-cell patch-clamp pipettes. ATP, 2-MeSATP, UDP and UTP potentiated the response to NMDA, while ADP-beta-S was inactive. PPADS antagonized the effect of ATP. Synaptic isolation of pyramidal neurons by a Ca(2+)-free medium or tetrodotoxin did not alter the effect of ATP which, however, was markedly depressed when GTP in the micropipette was replaced by GDP-beta-S. These observations suggest that in layer V pyramidal neurons of the prefrontal cortex postsynaptically localized P2Y receptors interact with NMDA receptor-channels.

Dopamine D2 receptors are present in prefrontal cortical afferents and their targets in patches of the rat caudate-putamen nucleus

Glutamatergic neurons within the deep layers of the prefrontal cortex and dopaminergic neurons of the substantia nigra pars compacta preferentially terminate in patch-like regions within the caudate putamen nucleus (CPN). Activation of dopamine D2 receptors is known to potently modulate striatal glutamatergic transmission and may play a role in reward-based motor learning. To determine the cellular substrate for D2-mediated regulation of prefrontal corticostriatal transmission in striatal patches, we combined anterograde transport of biotinylated dextran amine (BDA) with immunogold-silver labeling of a D2 receptor antipeptide antiserum in rat brain. Injections centered in deep layers of the dorsal part of the anterior cingulate cortex, one of the prefrontal cortical regions, produced varicose axonal BDA labeling in a patch-like distribution in the dorsomedial CPN. Electron microscopy showed that in these patch compartments, BDA labeling was present exclusively in axons and terminals (total number = 581), 9% of which contained detectable D2-like immunoreactivity. Thirty percent of the BDA-labeled terminals formed asymmetric excitatory synapses with dendritic spine heads, and the remainder were without recognizable junctions. The recipient spines were unlabeled or contained immunogold-silver particles for D2 receptors. A few of the D2-labeled spines also received convergent, often nonsynaptic contact from D2-labeled terminals resembling dopaminergic afferents. In addition, the corticostriatal terminals often apposed spiny and nonspiny neuronal profiles that contained D2 labeling. These results suggest that dopamine D2 receptors are strategically positioned for presynaptic and postsynaptic modulation of prefrontal corticostriatal excitation of spiny neurons in striatal patches. The findings have direct implications for D2-mediated control of reward-related motor learning.

D1 and D2 dopamine receptor mRNA expression in whole hemisphere sections of the human brain

Understanding dopamine signaling in human behavior requires knowledge of the distribution of all molecular components involved in dopamine pathways throughout the human brain. In the present study, the relative distributions of D1 and D2 dopamine receptor mRNAs were determined by in situ hybridization histochemistry in whole hemisphere sections from normal human post mortem brains. The findings confirmed information documented from single structure examination that the highest expression of both the D1 and D2 mRNAs were localized to the striatum. The cerebral cortex expressed moderate D1 mRNA in all regions with the highest signal in the medial orbital frontal area (Brodmann areas 11, 14), the paraterminal gyrus (Brodmann area 32) and the insular cortex (Brodmann areas 13-16), whereas the D2 mRNA expression had very low cortical expression. The bed nucleus of the stria terminalis and islands of Calleja had high expression of the D1 mRNA and moderate D2 mRNA levels. Moderate to high expression of the D2 mRNA was evident in the hippocampal formation, parafascicular and paraventricular thalamic nuclei, geniculate bodies, subthalamic nucleus, and pineal gland, all of which were devoid of, or showed only faint, D1 mRNA expression. Brainstem regions, e.g. substantia nigra, red nucleus, inferior colliculus, medial lemniscus, and pontine nuclei expressed D2, but not D1, mRNA. These results emphasize the differential anatomical localization of D1 and D2 dopamine receptor mRNA neuronal populations in the human brain. The restricted expression of the D1 mRNA to the cortical mantle and to a few forebrain structures indicates a strong involvement of the D1 system in cognitive function.

Pharmacology and behavioral pharmacology of the mesocortical dopamine system

The prefrontal cortex (PFC) has long been known to be involved in the mediation of complex behavioral responses. Considerable research efforts are directed towards refining the knowledge about the function of this brain area and the role it plays in cognitive performance and behavioral output. In the first part, this review provides, from a pharmacological perspective, an overview of anatomical, electrophysiological and neurochemical aspects of the function of the PFC, with an emphasis on the mesocortical dopamine system. Anatomy of the mesocortical system, basic physiological and pharmacological properties of neurotransmission within the PFC, and interactions between dopamine and glutamate as well as other transmitters within the mesocorticolimbic circuit are included. The coverage of these data is largely restricted to what is relevant for the second part of the review which focuses on behavioral studies that have examined the role of the PFC in a variety of phenomena, behaviors and paradigms. These include reward and addiction, locomotor activity and sensitization, learning, cognition, and schizophrenia. Although the focus of this review is on the mesocortical dopamine system, given the intricate interactions of dopamine with other transmitter systems within the PFC and the importance of the PFC as a source of glutamate in subcortical areas, these aspects are also covered in some detail where appropriate. Naturally, a topic as complex as this cannot be covered comprehensively in its entirety. Therefore this review is largely limited to data derived from studies using rats, and it is also specifically restricted to data concerning the medial PFC (mPFC). Since in several fields of research the findings concerning the function or role of the mPFC are relatively inconsistent, the question is addressed whether these inconsistencies might, at least in part, be related to the anatomical and functional heterogeneity of this brain area.

Dopamine increases excitability of pyramidal neurons in primate prefrontal cortex

Dopaminergic modulation of neuronal networks in the dorsolateral prefrontal cortex (PFC) is believed to play an important role in information processing during working memory tasks in both humans and nonhuman primates. To understand the basic cellular mechanisms that underlie these actions of dopamine (DA), we have investigated the influence of DA on the cellular properties of layer 3 pyramidal cells in area 46 of the macaque monkey PFC. Intracellular voltage recordings were obtained with sharp and whole cell patch-clamp electrodes in a PFC brain-slice preparation. All of the recorded neurons in layer 3 (n = 86) exhibited regular spiking firing properties consistent with those of pyramidal neurons. We found that DA had no significant effects on resting membrane potential or input resistance of these cells. However DA, at concentrations as low as 0.5 microM, increased the excitability of PFC cells in response to depolarizing current steps injected at the soma. Enhanced excitability was associated with a hyperpolarizing shift in action potential threshold and a decreased first interspike interval. These effects required activation of D1-like but not D2-like receptors since they were inhibited by the D1 receptor antagonist SCH23390 (3 microM) but not significantly altered by the D2 antagonist sulpiride (2.5 microM). These results show, for the first time, that DA modulates the activity of layer 3 pyramidal neurons in area 46 of monkey dorsolateral PFC in vitro. Furthermore the results suggest that, by means of these effects alone, DA modulation would generally enhance the response of PFC pyramidal neurons to excitatory currents that reach the action potential initiation site.

The dopamine D(4) receptor: one decade of research

Dopamine is an important neurotransmitter involved in motor control, endocrine function, reward, cognition and emotion. Dopamine receptors belong to the superfamily of G protein-coupled receptors and play a crucial role in mediating the diverse effects of dopamine in the central nervous system (CNS). The dopaminergic system is implicated in disorders such as Parkinson's disease and addiction, and is the major target for antipsychotic medication in the treatment of schizophrenia. Molecular cloning studies a decade ago revealed the existence of five different dopamine receptor subtypes in mammalian species. While the presence of the abundantly expressed dopamine D(1) and D(2) receptors was predicted from biochemical and pharmacological work, the cloning of the less abundant dopamine D(3), D(4) and D(5) receptors was not anticipated. The identification of these novel dopamine receptor family members posed a challenge with respect to determining their precise physiological roles and identifying their potential as therapeutic targets for dopamine-related disorders. This review is focused on the accomplishments of one decade of research on the dopamine D(4) receptor. New insights into the biochemistry of the dopamine D(4) receptor include the discovery that this G protein-coupled receptor can directly interact with SH3 domains. At the physiological level, converging evidence from transgenic mouse work and human genetic studies suggests that this receptor has a role in exploratory behavior and as a genetic susceptibility factor for attention deficit hyperactivity disorder.

In vivo autoradiography of [3H]SCH 39166 in rat brain: selective displacement by D1/D5 antagonists

The purpose of this study was to examine the receptor occupancy of D1/D5 antagonists for D1-like dopamine receptors in rat brain using [3H]SCH 39166, a highly selective D1/D5 antagonist with low affinity for 5HT2 receptors. A single concentration of triated SCH 39166 was administered to rats, with or without competing doses of the Dl/D5 antagonist SCH 23390 and unlabeled SCH 39166. the D2-like antagonists haloperidol or the 5-HT, antagonist ketanserin. The bound radioactivity in the cortex, striatum, nucleus accumbens and olfactory tubercle was then quantified using an in vivo autoradiographic procedure. The results indicated that [3H]SCH 39166 was dose dependently displaced by the Dl/D5 antagonists in regions associated with both the nigro-striatal pathway and the mesolimbic dopamine pathway, particularly the nucleus accumbens. Neither haloperidol nor ketanserin displaced [3H]SCH 39166 in any of the regions examined. The data were compared with previously published data examining the in vivo binding of [3H]SCH 39166 in rat brain homogenates. The relative values obtained were comparable to values detected in rat brain homogenates after in vivo binding of [3H]SCH 39166.

Dopamine-mediated stabilization of delay-period activity in a network model of prefrontal cortex

The prefrontal cortex (PFC) is critically involved in working memory, which underlies memory-guided, goal-directed behavior. During working-memory tasks, PFC neurons exhibit sustained elevated activity, which may reflect the active holding of goal-related information or the preparation of forthcoming actions. Dopamine via the D1 receptor strongly modulates both this sustained (delay-period) activity and behavioral performance in working-memory tasks. However, the function of dopamine during delay-period activity and the underlying neural mechanisms are only poorly understood. Recently we proposed that dopamine might stabilize active neural representations in PFC circuits during tasks involving working memory and render them robust against interfering stimuli and noise. To further test this idea and to examine the dopamine-modulated ionic currents that could give rise to increased stability of neural representations, we developed a network model of the PFC consisting of multicompartment neurons equipped with Hodgkin-Huxley-like channel kinetics that could reproduce in vitro whole cell and in vivo recordings from PFC neurons. Dopaminergic effects on intrinsic ionic and synaptic conductances were implemented in the model based on in vitro data. Simulated dopamine strongly enhanced high, delay-type activity but not low, spontaneous activity in the model network. Furthermore the strength of an afferent stimulation needed to disrupt delay-type activity increased with the magnitude of the dopamine-induced shifts in network parameters, making the currently active representation much more stable. Stability could be increased by dopamine-induced enhancements of the persistent Na(+) and N-methyl-D-aspartate (NMDA) conductances. Stability also was enhanced by a reduction in AMPA conductances. The increase in GABA(A) conductances that occurs after stimulation of dopaminergic D1 receptors was necessary in this context to prevent uncontrolled, spontaneous switches into high-activity states (i.e., spontaneous activation of task-irrelevant representations). In conclusion, the dopamine-induced changes in the biophysical properties of intrinsic ionic and synaptic conductances conjointly acted to highly increase stability of activated representations in PFC networks and at the same time retain control over network behavior and thus preserve its ability to adequately respond to task-related stimuli. Predictions of the model can be tested in vivo by locally applying specific D1 receptor, NMDA, or GABA(A) antagonists while recording from PFC neurons in delayed reaction-type tasks with interfering stimuli.

Enhancement of N-methyl-D-aspartate (NMDA) immunoreactivity in residual dendritic spines in the caudate-putamen nucleus after chronic haloperidol administration

Glutamate receptors of the N-methyl-D-aspartate (NMDA) subtype in the caudate-putamen nucleus (CPN) have been implicated in the adverse motor effects produced by chronic administration of the typical antipsychotic drug haloperidol. To determine the functionally relevant sites, we examined the electron microscopic immunocytochemical localization of the R1 receptor subunit (NMDAR1) in the dorsolateral CPN of rats receiving 4 months of biweekly depot intramuscular injections of either haloperidol or vehicle. In all animals, NMDAR1 immunoreactivity was seen mainly in dendritic spines, but was also present in a few somata and dendrites of spiny neurons, axon terminals, and glia. In comparison with controls, the dissector stereological analysis showed a significant reduction in the numerical density of total NMDAR1-labeled and unlabeled dendritic spines in the dorsolateral CPN after haloperidol administration. When labeled spines were identified separately based exclusively on the presence of immunoreactivity within a single plane of section, there was, however, a significant increase in the numerical density of NMDAR1-containing spines in haloperidol vs. control animals. This increase was not seen using a classic dissector, suggesting that the enhancement was mainly attributed to more frequent detection of spines having higher levels of NMDA immunoreactivity. Our results are the first to identify dendritic spines in the dorsolateral CPN as preferential sites for the regulated expression of NMDA receptors following chronic administration of haloperidol.

Opposite modulation of cortical N-methyl-D-aspartate receptor-mediated responses by low and high concentrations of dopamine

To examine whether dopamine modulates cortical N-methyl-D-aspartate receptor-mediated glutamate transmission, whole-cell recordings were made from identified pyramidal cells located in layers V and VI of the medial prefrontal cortex of the rat using a slice preparation. In the presence of tetrodotoxin and the absence of Mg2+, a brief local application of N-methyl-D-aspartate evoked an inward current which was blocked by the N-methyl-D-aspartate antagonist dizocilpine maleate but not affected by the non-N-methyl-D-aspartate antagonist 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo(f)quinoxaline, suggesting that the observed current is mediated by N-methyl-D-aspartate receptors located on recorded cells. Bath application of dopamine produced opposite effects on the N-methyl-D-aspartate current depending on the concentrations of dopamine applied. At low concentrations (<50 microM), dopamine enhanced the N-methyl-D-aspartate current, whereas at higher concentrations, dopamine suppressed the current. The same concentrations of dopamine did not significantly affect the inward current induced by the non-N-methyl-D-aspartate agonist alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid. The enhancing effect of dopamine on the N-methyl-D-aspartate response was mimicked by the D1 agonist SKF38393 and blocked by the D1 antagonist SCH31966, whereas the suppressing effect was mimicked by the D2 agonist quinpirole and blocked by the D2 antagonist eticlopride. The above results suggest that dopamine at low concentrations acts preferentially on D1-like receptors to promote N-methyl-D-aspartate receptor-mediated transmission, while at high concentrations dopamine also activates D2-like receptors, leading to a suppression of the N-methyl-D-aspartate function. This differential modulation of N-methyl-D-aspartate function may have significant implications for understanding behaviors and disorders involving both cortical dopamine- and glutamate-mediated neurotransmission.