Differential modulation by dopamine of responses evoked by excitatory amino acids in human cortex

Pathways for Contextual Memory: The Primate Hippocampal Pathway to Anterior Cingulate Cortex

The anterior cingulate cortex (ACC) is one of the few prefrontal areas that receives robust direct hippocampal terminations. This pathway may enable current context and past experience to influence goal-directed actions and emotional regulation by prefrontal cortices. We investigated the still ill-understood organization of the pathway from anterior hippocampus to ACC (A24a, A25, A32) to identify laminar termination patterns and their postsynaptic excitatory and inhibitory targets from system to synapse in rhesus monkeys. The densest hippocampal terminations targeted posterior A25, a region that is involved in affective and autonomic regulation. Hippocampal terminations innervated mostly excitatory neurons (~90%), suggesting strong excitatory effects. Among the smaller fraction of inhibitory targets, hippocampal terminations in A25 preferentially innervated calretinin neurons, a pattern that differs markedly from rodents. Further, hippocampal terminations innervated spines with D1 receptors, particularly in the deep layers of A25, where D1 receptors are enriched in comparison with the upper layers. The proximity of hippocampal terminations to D1 receptors may enable dopamine to enhance information transfer from the hippocampus to A25 and contribute to dopaminergic influence downstream on goal-directed action and emotional control by prefrontal cortices, in processes that may be disrupted by excessive dopamine release during uncontrollable stress.

The Unexplored Territory of Neural Models: Potential Guides for Exploring the Function of Metabotropic Neuromodulation

The space of possible neural models is enormous and under-explored. Single cell computational neuroscience models account for a range of dynamical properties of membrane potential, but typically do not address network function. In contrast, most models focused on network function address the dimensions of excitatory weight matrices and firing thresholds without addressing the complexities of metabotropic receptor effects on intrinsic properties. There are many under-explored dimensions of neural parameter space, and the field needs a framework for representing what has been explored and what has not. Possible frameworks include maps of parameter spaces, or efforts to categorize the fundamental elements and molecules of neural circuit function. Here we review dimensions that are under-explored in network models that include the metabotropic modulation of synaptic plasticity and presynaptic inhibition, spike frequency adaptation due to calcium-dependent potassium currents, and afterdepolarization due to calcium-sensitive non-specific cation currents and hyperpolarization activated cation currents. Neuroscience research should more effectively explore possible functional models incorporating under-explored dimensions of neural function.

Translating striatal activity from brain slice to whole animal neurophysiology: A guide for neuroscience research integrating diverse levels of analysis

An important goal of this review is highlighting research in neuroscience as examples of multilevel functional and anatomical analyses addressing basic science issues and applying results to the understanding of diverse disorders. The research of Dr. Michael Levine, a leader in neuroscience, exemplifies this approach by uncovering fundamental properties of basal ganglia function and translating these findings to clinical applications. The review focuses on neurophysiological research connecting results from in vitro and in vivo recordings. A second goal is to utilize these research connections to produce novel, accurate descriptions for corticostriatal processing involved in varied, complex functions. Medium spiny neurons in striatum act as integrators combining input with baseline activity creating motivational "events." Basic research on corticostriatal synapses is described and links developed to issues with clinical relevance such as inhibitory gating, self-injurious behavior, and relative reward valuation. Work is highlighted on dopamine-glutamate interactions. Individual medium spiny neurons express both D₁ and D₂ receptors and encode information in a bivalent manner depending upon the mix of receptors involved. Current work on neurophysiology of reward processing has taken advantage of these basic approaches at the cellular and molecular levels. Future directions in studying physiology of reward processing and action sequencing could profit by incorporating the divergent ways dopamine modulates incoming neurochemical signals. Primary investigators leading research teams should mirror Mike Levine's efforts in "climbing the mountain" of scientific inquiry by performing analyses at different levels of inquiry, integrating the findings, and building comprehensive answers to problems unsolvable without this bold approach.

Dopaminergic Modulation of Synaptic Plasticity, Its Role in Neuropsychiatric Disorders, and Its Computational Modeling

Neuromodulators modify intrinsic characteristics of the nervous system in order to reconfigure the functional properties of neural circuits. This reconfiguration is crucial for the flexibility of the nervous system to respond on an input-modulated basis. Such a functional rearrangement is realized by modification of intrinsic properties of the neural circuits including synaptic interactions. Dopamine is an important neuromodulator involved in motivation and stimulus-reward learning process, and adjusts synaptic dynamics in multiple time scales through different pathways. The modification of synaptic plasticity by dopamine underlies the change in synaptic transmission and integration mechanisms, which affects intrinsic properties of the neural system including membrane excitability, probability of neurotransmitters release, receptors’ response to neurotransmitters, protein trafficking, and gene transcription. Dopamine also plays a central role in behavioral control, whereas its malfunction can cause cognitive disorders. Impaired dopamine signaling is implicated in several neuropsychiatric disorders such as Parkinson’s disease, drug addiction, schizophrenia, attention-deficit/hyperactivity disorder, obsessive-compulsive disorder and Tourette’s syndrome. Therefore, dopamine plays a crucial role in the nervous system, where its proper modulation of neural circuits may enhance plasticity-related procedures, but disturbances in dopamine signaling might be involved in numerous neuropsychiatric disorders. In recent years, several computational models are proposed to formulate the involvement of dopamine in synaptic plasticity or neuropsychiatric disorders and address their connection based on the experimental findings.

Presynaptic D1 heteroreceptors and mGlu autoreceptors act at individual cortical release sites to modify glutamate release

The aim of this work was to study release of glutamic acid (GLU) from one-axon terminal or bouton at-a-time using cortical neurons grown in vitro to study the effect of presynaptic auto- and heteroreceptor stimulation. Neurons were infected with release reporters SypHx2 or iGluSnFR at 7 or 3 days-in-vitro (DIV) respectively. At 13-15 DIV single synaptic boutons were identified from images obtained from a confocal scanning microscope before and after field electrical stimulation. We further stimulated release by raising intracellular levels of cAMP with forskolin (10µM). Forskolin-mediated effects were dependent on protein kinase A (PKA) and did not result from an increase in endocytosis, but rather from an increase in the size of the vesicle readily releasable pool. Once iGluSnFR was confirmed as more sensitive than SypHx2, it was used to study the participation of presynaptic auto- and heteroreceptors on GLU release. Although most receptor agonizts (carbamylcholine, nicotine, dopamine D2, BDNF) did not affect electrically stimulated GLU release, a significant increase was observed in the presence of metabotropic D1/D5 heteroreceptor agonist (SKF38393 10µM) that was reversed by PKA inhibitors. Interestingly, stimulation of group II metabotropic mGLU2/3 autoreceptors (LY379268 50nM) induced a decrease in GLU release that was reversed by the specific mGLU2/3 receptor antagonist (LY341495 1µM) and also by PKA inhibitors (KT5720 200nM and PKI14-22 400nM). These changes in release probability at individual release sites suggest another level of control of the distribution of transmitter substances in cortical tissue.

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

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

Differential effects of dopamine-directed treatments on cognition

Dopamine, a prominent neuromodulator, is implicated in many neuropsychiatric disorders. It has wide-ranging effects on both cortical and subcortical brain regions and on many types of cognitive tasks that rely on a variety of different learning and memory systems. As neuroscience and behavioral evidence for the existence of multiple memory systems and their corresponding neural networks accumulated, so did the notion that dopamine's role is markedly different depending on which memory system is engaged. As a result, dopamine-directed treatments will have different effects on different types of cognitive behaviors. To predict what these effects will be, it is critical to understand: which memory system is mediating the behavior; the neural basis of the mediating memory system; the nature of the dopamine projections into that system; and the time course of dopamine after its release into the relevant brain regions. Consideration of these questions leads to different predictions for how changes in brain dopamine levels will affect automatic behaviors and behaviors mediated by declarative, procedural, and perceptual representation memory systems.

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

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

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

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

Risk-taking behavior: dopamine D2/D3 receptors, feedback, and frontolimbic activity

Decision-making involves frontolimbic and dopaminergic brain regions, but how prior choice outcomes, dopamine neurotransmission, and frontostriatal activity are integrated to affect choices is unclear. We tested 60 healthy volunteers using the Balloon Analogue Risk Task (BART) during functional magnetic resonance imaging. In the BART, participants can pump virtual balloons to increase potential monetary reward or cash out to receive accumulated reward; each pump presents greater risk and potential reward (represented by the pump number). In a separate session, we measured striatal D2/D3 dopamine receptor binding potential (BPND) with positron emission tomography in 13 of the participants. Losses were followed by fewer risky choices than wins; and during risk-taking after loss, amygdala and hippocampal activation exhibited greater modulation by pump number than after a cash-out event. Striatal D2/D3 BPND was positively related to the modulation of ventral striatal activation when participants decided to cash out and negatively to the number of pumps in the subsequent trial; but negatively related to the modulation of prefrontal cortical activation by pump number when participants took risk, and to overall earnings. These findings provide in vivo evidence for a potential mechanism by which dopaminergic neurotransmission may modulate risk-taking behavior through an interactive system of frontal and striatal activity.

Decoding the dopamine signal in macaque prefrontal cortex: a simulation study using the Cx3Dp simulator

Dopamine transmission in the prefrontal cortex plays an important role in reward based learning, working memory and attention. Dopamine is thought to be released non-synaptically into the extracellular space and to reach distant receptors through diffusion. This simulation study examines how the dopamine signal might be decoded by the recipient neuron. The simulation was based on parameters from the literature and on our own quantified, structural data from macaque prefrontal area 10. The change in extracellular dopamine concentration was estimated at different distances from release sites and related to the affinity of the dopamine receptors. Due to the sparse and random distribution of release sites, a transient heterogeneous pattern of dopamine concentration emerges. Our simulation predicts, however, that at any point in the simulation volume there is sufficient dopamine to bind and activate high-affinity dopamine receptors. We propose that dopamine is broadcast to its distant receptors and any change from the local baseline concentration might be decoded by a transient change in the binding probability of dopamine receptors. Dopamine could thus provide a graduated ‘teaching’ signal to reinforce concurrently active synapses and cell assemblies. In conditions of highly reduced or highly elevated dopamine levels the simulations predict that relative changes in the dopamine signal can no longer be decoded, which might explain why cognitive deficits are observed in patients with Parkinson’s disease, or induced through drugs blocking dopamine reuptake.

The fine structure of the dopaminergic innervation of area 10 of macaque prefrontal cortex

In common with other areas of the prefrontal cortex, activity in frontopolar area 10 is probably modulated by dopamine. We studied the dopaminergic innervation of monkey prefrontal area 10 by immunostaining with tyrosine hydroxylase (TH) antibodies. TH-positive axons in layer 3 were examined by electron microscopy of series of ultrathin sections. TH-positive boutons containing vesicles were sparse (2 × 10(-4) per μm(3)) and the majority (94%, n = 52) had no identifiable synaptic specialization, which supports the hypothesis that dopamine is released non-synaptically and raises the question of whether the local microenvironment surrounding the boutons is special. Compared with unlabelled boutons TH-positive boutons had a higher proportion of their perimeter in contact with dendritic shafts and were more often in continuous contact with pairs of pre- and postsynaptic structures. However, this may result from exclusion from sites preferred by glutamatergic and GABAergic synapses as the density of all synapses in the closer vicinity was no different from any randomly selected site in the neuropil. This quantitative ultrastructural study presents basic features of the dopaminergic innervation in prefrontal area 10 and provides a more detailed understanding of the structural basis of dopamine signalling in the cortex.

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

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

Timing is not Everything: Neuromodulation Opens the STDP Gate

Spike timing dependent plasticity (STDP) is a temporally specific extension of Hebbian associative plasticity that has tied together the timing of presynaptic inputs relative to the postsynaptic single spike. However, it is difficult to translate this mechanism to in vivo conditions where there is an abundance of presynaptic activity constantly impinging upon the dendritic tree as well as ongoing postsynaptic spiking activity that backpropagates along the dendrite. Theoretical studies have proposed that, in addition to this pre- and postsynaptic activity, a “third factor” would enable the association of specific inputs to specific outputs. Experimentally, the picture that is beginning to emerge, is that in addition to the precise timing of pre- and postsynaptic spikes, this third factor involves neuromodulators that have a distinctive influence on STDP rules. Specifically, neuromodulatory systems can influence STDP rules by acting via dopaminergic, noradrenergic, muscarinic, and nicotinic receptors. Neuromodulator actions can enable STDP induction or – by increasing or decreasing the threshold – can change the conditions for plasticity induction. Because some of the neuromodulators are also involved in reward, a link between STDP and reward-mediated learning is emerging. However, many outstanding questions concerning the relationship between neuromodulatory systems and STDP rules remain, that once solved, will help make the crucial link from timing-based synaptic plasticity rules to behaviorally based learning.

Dopaminergic modulation of receptive fields in rat sensorimotor cortex

Dopaminergic projections to primary sensorimotor cortex (SMC) have been described anatomically, but their functional role is unknown. The objective here was to characterize how dopamine modulates the somatosensory evoked potential (SEP) and its receptive field in SMC. SEPs were evoked by median and tibial nerve stimulation and recorded using thin-film multielectrode arrays implanted epidurally over the caudal sensorimotor cortex of rats. SEP amplitudes and receptive fields were measured before and after intracortical injection of a D1- (SCH 23390) or a D2-receptor antagonist (raclopride). Both increased maximum SEP amplitudes by 107.5% and 82.1%, respectively (p<0.01), while vehicle application had no effect (5.9% change). SEP latencies and receptive fields remained unchanged. Dopamine antagonists increase the excitability of sensorimotor cortex to afferent signals. Dopamine, therefore, expectedly reduces SMC excitability thereby improving sensory signal-to-noise ratio. Dopaminergic modulation may render SMC circuitry more effective in processing sensory information from different sources.

Seeking brain biomarkers for preventive therapy in Huntington disease

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

Assemblies of glutamate receptor subunits with post-synaptic density proteins and their alterations in Parkinson's disease

N-methyl-D-aspartate (NMDA) receptors have been implicated as a mediator of neuronal injury associated with many neurological disorders including ischemia, epilepsy, brain trauma, dementia and neurodegenerative disorders such as Parkinson's disease (PD) and Alzheimer's disease. To this, non-selective NMDA receptor antagonists have been tried and have been shown to be effective in many experimental animal models of disease, and some of these compounds have moved into clinical trials. However, the initial enthusiasm for this approach has waned, because the therapeutic index for most NMDA antagonists is quite poor, with significant adverse effects at clinically effective doses, thus limiting their utility. More recently, the concept that the exact pathways downstream NMDA receptor activation could represent a key variable element among neurological disorders has been put forward. In particular, variations in NMDA receptor subunit composition could be important in different disorders, both in the pathophysiological mechanisms of cell death and in the application of specific symptomatic therapies. As to PD, NMDA receptor complex has been shown to be altered in experimental models of parkinsonism and in PD in humans. Further, it has become increasingly evident that the NMDA receptor complex is intimately involved in the regulation of corticostriatal long-term potentiation, which is altered in experimental parkinsonism. The following sections will examine the modifications of specific NMDA receptor subunits as well as post-synaptic associated signalling complex including kinases and scaffolding proteins in experimental parkinsonism. These findings may allow the identification of specific molecular targets whose pharmacological or genetic manipulation might lead to innovative therapies for PD.

PSD-95 uncouples dopamine-glutamate interaction in the D1/PSD-95/NMDA receptor complex

Classical dopaminergic signaling paradigms and emerging studies on direct physical interactions between the D(1) dopamine (DA) receptor and the NMDA glutamate receptor predict a reciprocally facilitating, positive feedback loop. This loop, if not controlled, may cause concomitant overactivation of both D(1) and NMDA receptors, triggering neurotoxicity. Endogenous protective mechanisms must exist. Here, we report that PSD-95, a prototypical structural and signaling scaffold in the postsynaptic density, inhibits D(1)-NMDA receptor subunit 1 (NR1) NMDA receptor association and uncouples NMDA receptor-dependent enhancement of D(1) signaling. This uncoupling is achieved, at least in part, via a disinhibition mechanism by which PSD-95 abolishes NMDA receptor-dependent inhibition of D(1) internalization. Knockdown of PSD-95 immobilizes D(1) receptors on the cell surface and escalates NMDA receptor-dependent D(1) cAMP signaling in neurons. Thus, in addition to its role in receptor stabilization and synaptic plasticity, PSD-95 acts as a brake on the D(1)-NMDA receptor complex and dampens the interaction between them.

Specific role of dopamine D1 receptors in spinal network activation and rhythmic movement induction in vertebrates

Dopamine (DA) is well-recognized for its determinant role in the modulation of various brain functions. DA was also found in in vitro isolated invertebrate preparations to activate per se the central pattern generator for locomotion. However, it is less clear whether such a role as an activator of central neural circuitries exists in vertebrate species. Here, we studied in vivo the effects induced by selective DA receptor agonists and antagonists on hindlimb movement generation in mice completely spinal cord-transected (Tx) at the low-thoracic level (Th9/10). Administration of D1/D5 receptor agonists (0.5-2.5 mg kg(-1), i.p.) was found to acutely elicit rhythmic locomotor-like movements (LMs) and non-locomotor movements (NLMs) in untrained and non-sensory stimulated animals. Comparable effects were found in mice lacking the D5 receptor (D5KO) whereas D1/D5 receptor antagonist-pretreated animals (wild-type or D5KO) failed to display D1/D5 agonist-induced LMs. In contrast, administration of broad spectrum or selective D2, D3 or D4 agonists consistently failed to elicit significant hindlimb movements. Overall, the results clearly show in mice the existence of a role for D1 receptors in spinal network activation and corresponding rhythmic movement generation.

Sustained activities and retrieval in a computational model of the perirhinal cortex

The perirhinal cortex is involved not only in object recognition and novelty detection but also in multimodal integration, reward association, and visual working memory. We propose a computational model that focuses on the role of the perirhinal cortex in working memory, particularly with respect to sustained activities and memory retrieval. This model describes how different partial informations are integrated into assemblies of neurons that represent the identity of an object. Through dopaminergic modulation, the resulting clusters can retrieve the global information with recurrent interactions between neurons. Dopamine leads to sustained activities after stimulus disappearance that form the basis of the involvement of the perirhinal cortex in visual working memory processes. The information carried by a cluster can also be retrieved by a partial thalamic or prefrontal stimulation. Thus, we suggest that areas involved in planning and memory coordination encode a pointer to access the detailed information encoded in the associative cortex such as the perirhinal cortex.

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.

Levodopa improves skilled hand functions in the elderly

The endogenous dopamine system is a potent modulator of motor function and learning. Previous studies have demonstrated that, in the elderly, age-related degeneration of the nigrostriatal dopamine system may contribute to deficits in execution of skilled motor functions. The present double-blind, randomized cross-over study examined whether pharmacologically replenishing dopamine improves the execution of complex motor tasks. Twenty healthy young and 20 healthy elderly subjects were studied in two different sessions: (i) after three doses of levodopa (each 100 mg levodopa plus 25 mg carbidopa) and (ii) after three doses of placebo. For each session, subjects completed a functional motor test that reflects hand activities of daily living (Jebsen-Taylor test). In the elderly, but not in the young, Jebsen-Taylor test performance improved significantly (4%) with levodopa compared with placebo, particularly for fine motor functions. Attention to the task, level of fatigue, and positive and negative feelings were similar between sessions. These results demonstrate that increasing the dopaminergic drive pharmacologically may be helpful when the motor system is challenged in the ageing process.

Dopamine and corticotropin-releasing factor synergistically alter basolateral amygdala-to-medial prefrontal cortex synaptic transmission: functional switch after chronic cocaine administration

Basolateral amygdala (BLA) neurons provide a major excitatory input to medial prefrontal cortex (mPFC)-layer V pyramidal neurons. Under stressful conditions, commonly associated with chronic cocaine abuse, altered BLA-to-mPFC synaptic transmission could lead to defective emotional information processing and decision making within the mPFC and result in misguided and inappropriate behaviors. We examined the effects of cocaine administered chronically in vivo on EPSCs recorded from a putative BLA-mPFC pathway in vitro and their modulation by dopamine (DA), corticotropin-releasing factor (CRF), and their combination (DA plus CRF). In saline-treated animals, activation of D(1/5) receptors depressed BLA-mPFC EPSCs, whereas CRF1 receptor activation alone had no effect on EPSCs. Activating D(1/5) and CRF1 receptors in combination, however, worked synergistically through presynaptic and postsynaptic mechanisms to depress EPSCs to levels greater than D(1/5) receptor activation alone. After chronic cocaine administration, the function of DA(1/5) and CRF receptors switched from inhibitory to excitatory. In slices from cocaine-treated animals, putative BLA-mPFC EPSCs were depressed through a presynaptic mechanism. Now, activation of either D(1/5) or CRF2 receptors increased the cocaine-induced, depressed EPSCs. Additionally, simultaneous activation of presynaptic D(1/5) and CRF2 receptors led to further enhancement of EPSCs. These data indicate that CRF acting synergistically with DA normally potentiates D(1/5)-induced synaptic depression. However, after chronic cocaine, the combined synergistic actions of DA and CRF switched polarity to enhance facilitation of BLA-mPFC glutamatergic transmission. Also unmasked after acute withdrawal from chronic cocaine are endogenous, tonic-inhibitory D2-like and tonic-facilitatory CRF2 receptor actions. These multiple functional and receptor changes may underlie the altered, possibly aberrant, decision-making process after chronic cocaine.

Motor intracortical inhibition in PD: L-DOPA modulation of high-frequency rTMS effects

Dopaminergic drugs and deep brain stimulation restore cortical inhibition in Parkinson disease (PD) patients. High-frequency rTMS was also found to increase cortical inhibition in PD but its therapeutic effect is still controversial. Here we hypothesize that, if dopaminergic drugs reverse to normal cortical excitability in M1, the effect of high-frequency (hf)-rTMS in PD patients could depend on whether they are in a medicated or unmedicated state. The present study aims to explore the lasting effects of sub-threshold hf rTMS trains over M1 on cortical inhibition in patients with "on" and without "off" L-DOPA treatment. Fourteen PD patients were examined twice while "on" and "off" medication. In both conditions, a paired-pulse paradigm was used to evaluate short intracortical inhibition (SICI) and long intracortical inhibition (LICI) that were evaluated before and after hf rTMS trains applied on the motor cortex. The results were compared with those obtained from normal controls. In baseline condition, SICI and LICI were significantly reduced in "off" compared to "on" patients and controls. hf-rTMS over the motor cortex significantly increased SICI and LICI in "off" medication PD patients. Magnetic stimulation proved to be ineffective when the same patients were in "on" state. The results showed that hf-rTMS affected intracortical inhibition (ICI) only in unmedicated patients. By restoring cortical inhibitory circuits dopaminergic drugs, normalize the excitability changes in M1 subsequent to motor rTMS. Whether patients are in a medicated or an unmedicated state would therefore appear to be critical for rTMS effects in PD patients. If a positive correlation exists between increased cortical inhibition and clinical improvement, hf-rTMS during the "off" state could be regarded as a potential add-on treatment to reduce the need of L-dopa and thus delay the adverse effects of its chronic use.

Distribution of neurons expressing tyrosine hydroxylase in the human cerebral cortex

Since the very first detailed description of the different types of cortical interneurons by Cajal, the tremendous variation in the morphology, physiology and neurochemical properties of these cells has become apparent. However, it still remains unclear whether all types of interneurons are present in all cortical areas and species. Here we have focused on tyrosine hydroxylase (TH)-immunoreactive cortical interneurons, which although only present in certain species, are particularly abundant in the human neocortex. We argue that this type of interneuron is more widespread in the human neocortex than in any other species examined so far and that, therefore, it is probably involved in a larger variety of cortical circuits. In addition, notable regional variation can be seen in relation to these interneurons. These differences further emphasize the variability in the design of microcircuits between cortical areas and species, and they probably reflect an evolutionary adaptation of cortical circuits to particular functions.

Levodopa increases memory encoding and dopamine release in the striatum in the elderly

Normal aging is associated with a decrease in dopaminergic function and a reduced ability to form new motor memories with training. This study examined the link between both phenomena. We hypothesized that levodopa would (a) ameliorate aging-dependent deficits in motor memory formation, and (b) increase dopamine availability at the dopamine type 2-like (D2) receptor during training in task-relevant brain structures. The effects of training plus levodopa (100mg, plus 25mg carbidopa) on motor memory formation and striatal dopamine availability were measured with [(11)C]raclopride (RAC) positron emission tomography (PET). We found that levodopa did not alter RAC-binding potential at rest but it enhanced training effects on motor memory formation as well as dopamine release in the dorsal caudate nucleus. Motor memory formation during training correlated with the increase of dopamine release in the caudate nucleus. These results demonstrate that levodopa may ameliorate dopamine deficiencies in the elderly by replenishing dopaminergic presynaptic stores, actively engaged in phasic dopamine release during motor training.

Enaminones and norepinephrine employ convergent mechanisms to depress excitatory synaptic transmission in the rat nucleus accumbensin vitro

We recently reported that anticonvulsant anilino enaminones depress excitatory postsynaptic currents (EPSCs) in the nucleus accumbens (NAc) indirectly via gamma-aminobutyric acid (GABA) acting on GABA(B) receptors [S.B. Kombian et al. (2005)Br. J. Pharmacol., 145, 945-953]. Norepinephrine (NE) and dopamine (DA), both known to be involved in seizure disorders, also depress EPSCs in this nucleus. The current study explored a possible interaction between enaminones and adrenergic and/or dopaminergic mechanisms that may contribute to their synaptic depression and anticonvulsant effect. Using whole-cell recording in rat forebrain slices containing the NAc, we show that NE-induced, but not DA-induced, EPSC depression occludes E139-induced EPSC depressant effect. UK14,304, a selective alpha(2) receptor agonist, mimicked the synaptic effect of NE and also occluded E139 effects. Phentolamine, a non-selective alpha-adrenergic antagonist that blocked NE-induced EPSC depression, also blocked the E139-induced EPSC depression. Furthermore, yohimbine, an alpha(2)-adrenoceptor antagonist, also blocked the E139-induced EPSC depression, while prazosin, a selective alpha(1)-adrenergic antagonist, and propranolol, a non-selective beta-adrenoceptor antagonist, did not block the E139 effect. Similar to the E139-induced EPSC depression, the NE-induced EPSC depression was also blocked by the GABA(B) receptor antagonist, CGP55845. By contrast, however, neither SCH23390 nor sulpiride, D1-like and D2-like DA receptor antagonists, respectively, blocked the E139-induced synaptic depression. These results suggest that NE and E139, but not DA, employ a similar mechanism to depress EPSCs in the NAc, and support the hypothesis that E139, like NE, may act on alpha(2)-adrenoceptors to cause the release of GABA, which then mediates synaptic depression via GABA(B) receptors.

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.

Under the curve: Critical issues for elucidating D1 receptor function in working memory

It has been postulated that spatial working memory operates optimally within a limited range of dopamine transmission and D1 dopamine receptor signaling in prefrontal cortex. Insufficiency in prefrontal dopamine, as in aging, and excessive transmission, as in acute stress, lead to impairments in working memory that can be ameliorated by D1 receptor agonist and antagonist treatment, respectively. Iontophoretic investigations of dopamine's influence on the cellular mechanisms of working memory have revealed that moderate D1 blockade can enhance memory fields in primate prefrontal pyramidal neurons while strong blockade abolishes them. The combined behavioral and physiological evidence indicates that there is a normal range of dopamine function in prefrontal cortex that can be described as an "inverted-U" relationship between dopamine transmission and the integrity of working memory. Both in vivo and in vitro studies have demonstrated a role for dopamine in promoting the excitability of prefrontal pyramidal cells and facilitating their N-methyl-d-aspartate inputs, while simultaneously restraining recurrent excitation and facilitating feedforward inhibition. This evidence indicates that there is a fine balance between the synergistic mechanisms of D1 modulation in working memory. Given the critical role of prefrontal function for cognition, it is not surprising that this balancing act is perturbed by both subtle genetic influences and environmental events. Further, there is evidence for an imbalance in these dopaminergic mechanisms in multiple neuropsychiatric disorders, particularly schizophrenia, and in related nonhuman primate models. Elucidating the orchestration of dopamine signaling in key nodes within prefrontal microcircuitry is therefore pivotal for understanding the influence of dopamine transmission on the dynamics of working memory. Here, we explore the hypothesis that the window of optimal dopamine signaling changes on a behavioral time-scale, dependent upon current cognitive demands and local neuronal activity as well as long-term alterations in signaling pathways and gene expression. If we look under the bell-shaped curve of prefrontal dopamine function, it is the relationship between neuromodulation and cognitive function that promises to bridge our knowledge between molecule and mind.

Depression of retinogeniculate synaptic transmission by presynaptic D2-like dopamine receptors in rat lateral geniculate nucleus

Extraretinal projections onto neurons in the dorsal lateral geniculate nucleus (dLGN) play an important role in modifying sensory information as it is relayed from the visual thalamus to neocortex. The dLGN receives dopaminergic innervation from the ventral tegmental area; however, the role of dopamine in synaptic transmission in dLGN has not been explored. In the present study, whole cell recordings were obtained to examine the actions of dopamine on glutamatergic synaptic transmission. Dopamine (2-100 microm) strongly suppressed excitatory synaptic transmission in dLGN relay neurons that was evoked by optic tract stimulation and mediated by both N-methyl-d-aspartate and non-N-methyl-d-aspartate glutamate receptors. In contrast, dopamine did not alter inhibitory synaptic transmission arising from either dLGN interneurons or thalamic reticular nucleus neurons. The suppressive action of dopamine on excitatory synaptic transmission was mimicked by the D(2)-like dopamine receptor agonist bromocriptine (2-25 microm) but not by the D(1)-like receptor agonist SKF38393 (10-25 microm). In addition, the dopamine-mediated suppression was antagonized by the D(2)-like receptor antagonist sulpiride (10-20 microm) but not by the D(1)-like receptor antagonist SCH23390 (5-25 microm). The dopamine-mediated decrease in evoked excitatory postsynaptic current amplitude was accompanied by an increase in the magnitude of paired-pulse depression. Furthermore, dopamine also reduced the frequency but not the amplitude of miniature excitatory postsynaptic currents. Taken together, these data suggest that dopamine may act presynaptically to regulate the release of glutamate at the retinogeniculate synapse and modify transmission of visual information in the dLGN.

Modification of Practice-dependent Plasticity in Human Motor Cortex by Neuromodulators

Practice-dependent plasticity underlies motor learning in everyday life and motor relearning after lesions of the nervous system. Previous studies showed that practice-dependent plasticity is modifiable by neuromodulating transmitters such as norepinephrine (NE), dopamine (DA) or acetylcholine (ACh). Here we explored, for the first time comprehensively and systematically, the modifying effects of an agonist versus antagonist in each of these neuromodulating transmitter systems on practice-dependent plasticity in healthy subjects in a placebo-controlled, randomized, double-blind crossover design. We found that the agonists in all three neuromodulating transmitter systems (NE: methylphenidate; DA: cabergoline; ACh: tacrine) enhanced practice-dependent plasticity, whereas the antagonists decreased it (NE: prazosin; DA: haloperidol; ACh: biperiden). Enhancement of plasticity under methylphenidate and tacrine was associated with an increase in corticomotoneuronal excitability of the prime mover of the practice, as measured by the motor evoked potential amplitude, but with a decrease under cabergoline. Our findings demonstrate that agonists and antagonists in various neuromodulating transmitter systems produce significant and oppositely directed modifications of practice-dependent plasticity in human motor cortex. Enhancement of plasticity occurred through different strategies that either favoured extrinsic (NE, ACh) or intrinsic (DA) modulating influence on the motor cortical output network.

Activation of D1, but not D2 receptors potentiates dizocilpine-mediated disruption of prepulse inhibition of the startle

Although substantial evidence has shown interactions between glutamatergic and dopaminergic systems play a cardinal role in the regulation of attentional processes, their involvement in informational filtering has been poorly investigated. Chiefly, little research has focused on functional correlations between the dopaminergic system and the mechanism of action of N-methyl-D-aspartate (NMDA) receptor antagonists on sensorimotor gating. The present study was targeted at evaluating whether the activation of D1 and D2 receptors is able to interact with the disruption of prepulse inhibition (PPI) of startle mediated by dizocilpine, a selective, noncompetitive NMDA receptor antagonist. We tested the effects of SKF 38393 ((+/-)-1-phenyl-2,3,4,5-tetrahydro-(1H)-3-benzazepine-7,8-diol) (10 mg/kg, s.c.), a selective D1 agonist, and quinpirole (0.3, 0.6 mg/kg, s.c.), a D2 agonist, in rats, per se and in cotreatment with different doses of dizocilpine, ranging from 0.0015 to 0.15 mg/kg (s.c.). Subsequently, the effect of the D1 antagonist SCH 23390 ((R)-(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine) (0.05, 0.1 mg/kg, s.c.) on PPI disruptions mediated by dizocilpine and by combination of dizocilpine and SKF 38393 was tested. Two further experiments were performed to verify whether the synergic effect of the D1 agonist with dizocilpine was counteracted by effective doses of haloperidol (0.1, 0.5 mg/kg, i.p.) and clozapine (5, 10 mg/kg, i.p.). All experiments were carried out using standard procedures for the assessment of PPI of the acoustic startle reflex. SKF 38393, while unable to impair sensorimotor gating alone, induced PPI disruption in cotreatment with 0.05 and 0.15 mg/kg of dizocilpine, both ineffective per se. Furthermore, this effect was reversed by SCH 23390, but not by haloperidol or clozapine. Conversely, no synergistic effect was exhibited between quinpirole and dizocilpine, at any given dose. These findings suggest that D1, but not D2 receptors, enhance the disruptive effect of dizocilpine on PPI.

Catecholaminergic innervation of pyramidal neurons in the human temporal cortex

In the human neocortex, catecholaminergic connections modulate the excitatory inputs of pyramidal neurons and are involved in higher cognitive functions. Catecholaminergic fibers form a dense network in which it is difficult to distinguish whether or not target specificity exists. In order to shed some light on this issue, we set out to quantify the catecholaminergic innervation of pyramidal cells in different layers of the human temporal cortex (II, IIIa, IIIb, V and VI). For this purpose, pyramidal cells were labeled in human cortical tissue by injecting them with Lucifer Yellow, and then performed immunocytochemistry for the rate limiting catecholamine synthesizing enzyme tyrosine hydroxylase (TH) to visualize catecholaminergic fibers in the same sections. Injected cells were reconstructed in three dimensions and appositions were quantified (n = 1503) in serial confocal microscopy images of each injected cell (n = 71). We found TH-immunoreactive appositions (TH-ir) in all the pyramidal cells analyzed, in both the apical and basal dendritic regions. In general, the density of TH-ir apposition was greater in layers II, V and VI than in layers IIIa and IIIb. Furthermore, TH-ir appositions showed a regular distribution in almost all dendritic compartments of the apical and basal dendritic arbors across all layers. Hence, it appears that all pyramidal neurons in the human neocortex receive catecholaminergic afferents in a rather regular pattern, independent of the layer in which they are located. Since pyramidal cells located in different layers are involved in different intrinsic and extrinsic circuits, these results suggest that catecholaminergic afferents may modify the function of a larger variety of circuits than previously thought. Thus, this aspect of human cortical organization is likely to have important implications in cortical function.

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.

Dopamine Receptor Signaling

The D1-like (D1, D5) and D2-like (D2, D3, D4) classes of dopamine receptors each has shared signaling properties that contribute to the definition of the receptor class, although some differences among subtypes within a class have been identified. D1-like receptor signaling is mediated chiefly by the heterotrimeric G proteins Galphas and Galphaolf, which cause sequential activation of adenylate cyclase, cylic AMP-dependent protein kinase, and the protein phosphatase-1 inhibitor DARPP-32. The increased phosphorylation that results from the combined effects of activating cyclic AMP-dependent protein kinase and inhibiting protein phosphatase 1 regulates the activity of many receptors, enzymes, ion channels, and transcription factors. D1 or a novel D1-like receptor also signals via phospholipase C-dependent and cyclic AMP-independent mobilization of intracellular calcium. D2-like receptor signaling is mediated by the heterotrimeric G proteins Galphai and Galphao. These pertussis toxin-sensitive G proteins regulate some effectors, such as adenylate cyclase, via their Galpha subunits, but regulate many more effectors such as ion channels, phospholipases, protein kinases, and receptor tyrosine kinases as a result of the receptor-induced liberation of Gbetagamma subunits. In addition to interactions between dopamine receptors and G proteins, other protein:protein interactions such as receptor oligomerization or receptor interactions with scaffolding and signal-switching proteins are critical for regulation of dopamine receptor signaling.

Dopamine-glutamate interactions controlling prefrontal cortical pyramidal cell excitability involve multiple signaling mechanisms

Although the importance of dopamine (DA) for prefrontal cortical (PFC) cognitive functions is widely recognized, the nature of DA actions in the PFC remains controversial. A critical component in DA actions is its modulation of glutamate transmission, which can be different when specific receptors are activated. To obtain a clear picture of cellular mechanisms involved in these interactions, we studied the effects of DA-glutamate coactivation on pyramidal cell excitability in brain slices obtained from developmentally mature rats using whole-cell patch-clamp recordings. Bath application of NMDA, AMPA, and the D1 agonist SKF38393 induced concentration-dependent excitability increases, whereas bath application of the D2 receptor agonist quinpirole induced a concentration-dependent excitability decrease. The NMDA-mediated response was potentiated by SKF38393. This NMDA-D1 synergism required postsynaptic intracellular Ca2+ and protein kinase A (PKA) and was independent of membrane depolarization. On the other hand, the excitatory effects of both NMDA and AMPA were attenuated by a D2 agonist. Surprisingly, the D2-NMDA interaction was also blocked by the GABA(A) antagonists bicuculline and picrotoxin, suggesting that the inhibitory action of D2 receptors on NMDA-induced responses in the PFC may be mediated by GABAergic interneurons. In contrast, the D2-AMPA interaction involves inhibition of PKA and activation of phospholipase lipase C-IP3 and intracellular Ca2+ at a postsynaptic level. Thus, the modulatory actions of D1 and D2 receptors on PFC pyramidal cell excitability are mediated by multiple intracellular mechanisms and by activation of GABA(A) receptors, depending on the glutamate receptor subtypes involved.

Targeting the dopamine D1 receptor in schizophrenia: insights for cognitive dysfunction

BACKGROUND AND RATIONALE

Reinstatement of the function of working memory, the cardinal cognitive process essential for human reasoning and judgment, is potentially the most intractable problem for the treatment of schizophrenia. Since deficits in working memory are associated with dopamine dysregulation and altered D(1) receptor signaling within prefrontal cortex, we present the case for targeting novel drug therapies towards enhancing prefrontal D(1) stimulation for the amelioration of the debilitating cognitive deficits in schizophrenia.

OBJECTIVES

This review examines the role of dopamine in regulating cellular and circuit function within prefrontal cortex in order to understand the significance of the dopamine dysregulation found in schizophrenia and related non-human primate models. By revealing the associations among prefrontal neuronal function, dopamine and D(1) signaling, and cognition, we seek to pinpoint the mechanisms by which dopamine modulates working memory processes and how these mechanisms may be exploited to improve cognitive function.

RESULTS AND CONCLUSIONS

Dopamine deficiency within dorsolateral prefrontal cortex leads to abnormal recruitment of this region by cognitive tasks. Both preclinical and clinical studies have demonstrated a direct relationship between prefrontal dopamine function and the integrity of working memory, suggesting that insufficient D(1) receptor signaling in this region results in cognitive deficits. Moreover, working memory deficits can be ameliorated by treatments that augment D(1) receptor stimulation, indicating that this target presents a unique opportunity for the restoration of cognitive function in schizophrenia.

Effect of amount and frequency of head-only stunning currents on the electroencephalogram and somatosensory evoked potentials in broilers

The effectiveness of head-only electrical stunning of broilers, with a root mean square (RMS) current of 100 or 150mA delivered using either 50, 400 or 1500Hz sine wave alternating current (AC), was investigated. The changes occurring in the spontaneous electroencephalogram (EEG) were evaluated using Fast Fourier Transformations (FFT) to determine the impact of the amount and frequency of stunning current on total (2-30 Hz) and relative (13-30 Hz) power contents in the EEG. Induction of epileptiform activity and reduction in the EEG power contents to less than 10% of pre-stun levels from the end of epileptiform activity were used as indicators of effective stunning. The duration of unconsciousness and insensibility was determined on the basis of the return of EEG power contents. In addition, the changes occurring in somatosensory evoked potentials (SEPs) were subjectively evaluated to determine the impact of stunning treatments. The results of ANOVA (repeated measures) showed statistically significant effects of interactions between the current frequencies, amount of current and repeated measures on changes in EEG power contents (P < 0.001). Stunning broilers with 150 mA delivered using 50 Hz resulted in EEG changes that were indicative of more pronounced neuronal inhibition following epileptiform activity and also lasted longer than was the case when broilers were stunned with 150 mA delivered using 400 Hz. Stunning broilers with 100 mA delivered using 50 Hz resulted in changes very similar to those observed after stunning with 150 mA of 50 Hz, but which lasted for a relatively shorter time. However, these changes were more pronounced and lasted longer than did stunning with 100 mA delivered using 400 Hz. The effects of stunning broilers with 150 mA of 400 Hz were similar to those found after stunning with 100 mA of 50 Hz. By contrast, stunning broilers with 100 mA of 1500 Hz failed to fulfil the criteria set out in this study. Stunning of broilers with 150 mA of 1500 Hz induced epileptiform activity but failed to reduce EEG power contents to less than 10% of pre-stun levels. Therefore, the stunning of broilers with 100 or 150 mA of 1500 Hz may not be adequate to avoid pain and suffering during slaughter. Thus, minimum currents of 100, 150 and 200 mA should be delivered whilst using 50, 400 and 1500 Hz, respectively, to achieve effective electrical stunning in broilers. Severing of the carotid arteries in the neck following head-only electrical stunning, and high frequency (> 125 Hz) electrical water bath stunning of broilers should also become a statutory requirement to prevent the return of consciousness during bleeding.

Regulation of dopamine D1 receptor function by physical interaction with the NMDA receptors

Functional interactions between dopamine D1-like receptors and NMDA subtype glutamate receptors have been implicated in the maintenance of normal brain activity and neurological dysfunction. Although modulation of NMDA receptor functions by D1 receptor activation has been the subject of extensive investigation, little is known as to how the activation of NMDA receptors alters D1 function. Here we report that NMDA receptors regulate D1 receptor function via a direct protein-protein interaction mediated by the carboxyl tail regions of both receptors. In both cotransfected cells and cultured hippocampal neurons the activation of NMDA receptors increases the number of D1 receptors on the plasma membrane surface and enhances D1 receptor-mediated cAMP accumulation via a SNARE-dependent mechanism. Furthermore, overexpression of mini-genes encoding either NR1 or D1 carboxyl tail fragments disrupts the D1-NR1 direct protein-protein interaction and abolishes NMDA-induced changes in both D1 cell surface expression and D1-mediated cAMP accumulation. Our results demonstrate that the D1-NR1 physical interaction enables NMDA receptors to increase plasma membrane insertion of D1 receptors and provides a novel mechanism by which the activation of NMDA receptors upregulates D1 receptor function. Understanding the molecular mechanisms by which D1 and NMDA receptors functionally interact may provide insight toward elucidating the molecular neurobiological mechanisms involved in many neuropsychiatric illnesses, such as schizophrenia.

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

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

A model of dopamine modulated cortical activation

Standard techniques from pharmacology and current theories on the biochemistry of dopamine (DA) are used to derive a new model of how DA modulates cortical activation. The model assumes that DA potentiates the glutamate response through the NMDA receptor and depresses the glutamate response through non-NMDA receptors. A static version of the model is used to examine the neurobiological plausibility of the Servan-Schreiber, Printz, and Cohen model [Science 249 (1990) 892]. A dynamic version can be used to model many behaviors that are outside the domain of the static [Science 249 (1990) 892] model, including single-cell recording data. As a preliminary test of the model, we show that it can account for some single-cell recording data that examined the effects of DA on the firing rate of glutamatergic cortical cells.