Dopamine D-1/D-5 receptor activation is required for long-term potentiation in the rat neostriatum in vitro

Distinguishing whether dopamine regulates liking, wanting, and/or learning about rewards

To determine whether dopamine regulates liking, wanting, and/or learning about rewards during goal-directed behavior, the authors tested genetically engineered dopamine-deficient (DD) mice for acquisition of an appetitive T-maze task with and without endogenous dopamine signaling. Experiment 1 established that DD mice treated with L-dihydroxyphenylalanine (L-dopa [LD]) perform similarly to controls on a T-maze task designed to measure liking, wanting, and learning about rewards. Experiment 2, which tested saline-, caffeine-, and LD-treated DD mice on the T maze, separated performance factors from cognitive processes and revealed that dopamine is not necessary for mice to like or learn about rewards but is necessary for mice to seek (want) rewards during goal-directed behavior.

Coincident stimulation of convergent cortical inputs enhances immediate early gene induction in the striatum

The effect of coincident stimulation of convergent corticostriatal inputs was analyzed by the induction of immediate early genes in striatal neurons. Cortical motor areas were stimulated through implanted electrodes in awake, behaving rats, and the induction of the mRNAs encoding the immediate early genes (IEGs) c-fos and arc was analyzed in the striatum with in situ hybridization histochemistry. In the first experiment, unilateral stimulation of the medial agranular cortex, orofacial region of the lateral agranular cortex or the forelimb region of the lateral agranular cortex resulted in IEG induction in the striatum, which was restricted to the topographically related area receiving input from the stimulated cortical area. In a second experiment, stimulation parameters were altered, including frequency, number of pulses/train, and number of trains/s. These parameters did not have a significant effect on IEG induction. Notably, in some cases, in which there was IEG induction not only in the stimulated cortical region, but also in the homologous area in the contralateral hemisphere, very robust IEG induction was observed in the striatum. In a third experiment, the orofacial regions of the lateral agranular cortex of both hemispheres were stimulated coincidently. All of these animals showed robust striatal IEG induction. This IEG induction was attenuated by pretreatment with an NMDA antagonist MK-801. In a fourth experiment, we tested whether the coincidence of bilateral cortical stimulation contributed to the efficacy of striatal IEG induction. Either alternating stimulation or coincident stimulation of non-homologous cortical regions produced significantly lower striatal IEG induction than obtained with coincident stimulation of homologous cortical areas. Enhanced striatal IEG induction occurred in indirect striatal neurons, labeled with enkephalin, but was also present in a large number of enkephalin-negative neurons, most of which are likely direct pathway neurons. These results suggest that regional and temporal convergence of cortical inputs enhances striatal IEG induction.

Dynamic Dopamine Modulation in the Basal Ganglia: A Neurocomputational Account of Cognitive Deficits in Medicated and Nonmedicated Parkinsonism

Dopamine (DA) depletion in the basal ganglia (BG) of Parkinson's patients gives rise to both frontal-like and implicit learning impairments. Dopaminergic medication alleviates some cognitive deficits but impairs those that depend on intact areas of the BG, apparently due to DA “overdose.” These findings are difficult to accommodate with verbal theories of BG/DA function, owing to complexity of system dynamics: DA dynamically modulates function in the BG, which is itself a modulatory system. This article presents a neural network model that instantiates key biological properties and provides insight into the underlying role of DA in the BG during learning and execution of cognitive tasks. Specifically, the BG modulates the execution of “actions” (e.g., motor responses and working memory updating) being considered in different parts of the frontal cortex. Phasic changes in DA, which occur during error feedback, dynamically modulate the BG threshold for facilitating/suppressing a cortical command in response to particular stimuli. Reduced dynamic range of DA explains Parkinson and DA overdose deficits with a single underlying dysfunction, despite overall differences in raw DA levels. Simulated Parkinsonism and medication effects provide a theoretical basis for behavioral data in probabilistic classification and reversal tasks. The model also provides novel testable predictions for neuropsychological and pharmacological studies, and motivates further investigation of BG/DA interactions with the prefrontal cortex in working memory.

Dual synaptic sites of D1-dopaminergic regulation of ethanol sensitivity of NMDA receptors in nucleus accumbens

Regulation of NMDAreceptor-mediated synaptic transmission onto accumbal medium spiny neurons (MSN) may constitute an important site in drug reward and reinforcement in mesolimbic structures. Previously, we reported that D(1)-like dopamine receptors activate a postsynaptic cAMP/PKA/DARPP-32 signaling cascade culminating in phosphorylation of SER897-NR1 subunits and a reduction in the sensitivity to ethanol of NMDA receptor-mediated synaptic transmission. Here, we use a detailed electrophysiological analysis of D(1)-like receptor regulation of the ethanol sensitivity of accumbal NMDA receptors (NMDARs) through recordings of quantal Sr(2+)-supported NMDA miniature synaptic currents (mEPSCs) in reduced Mg(2+) (0.6 mM) and report dual presynaptic and postsynaptic components of D(1)-like regulation of ethanol sensitivity of NMDARs. Ethanol inhibited NMDA mEPSC amplitude and frequency in a dose-dependent manner (25-75 mM), indicating inhibitory effects on presynaptic and postsynaptic components NMDA receptor-mediated synaptic transmission. The presynaptic inhibitory effect was corroborated by analysing the ratio of paired-pulse facilitation (PPF) of Ca(2+)-supported NMDA EPSCs. Activation of D(1) receptors with the agonist, SKF 38393 (25 microM), reversed ethanol suppression of NMDA mEPSC frequency and amplitude. Furthermore, the Mg(2+)-dependent decay off-rate of NMDA mEPSCs was substantially reduced by ethanol in a manner strongly reversed by the D(1) agonist. D(1) receptor-mediated attenuation of both the presynaptic and postsynaptic actions of ethanol was completely blocked by a D(1) selective antagonist (SCH 23390). These data suggest that D(1)-like receptors modulate both the presynaptic and postsynaptic effects of ethanol on NMDA receptor-mediated synaptic transmission in nucleus accumbens (NAc) and that these interactions may contribute to ethanol-induced neuroadaptation of the reward pathway.

Memory and addiction: shared neural circuitry and molecular mechanisms

An important conceptual advance in the past decade has been the understanding that the process of drug addiction shares striking commonalities with neural plasticity associated with natural reward learning and memory. Basic mechanisms involving dopamine, glutamate, and their intracellular and genomic targets have been the focus of attention in this research area. These two neurotransmitter systems, widely distributed in many regions of cortex, limbic system, and basal ganglia, appear to play a key integrative role in motivation, learning, and memory, thus modulating adaptive behavior. However, many drugs of abuse exert their primary effects precisely on these pathways and are able to induce enduring cellular alterations in motivational networks, thus leading to maladaptive behaviors. Current theories and research on this topic are reviewed from an integrative systems perspective, with special emphasis on cellular, molecular, and behavioral aspects of dopamine D-1 and glutamate NMDA signaling, instrumental learning, and drug cue conditioning.

Corticostriatal plasticity: life after the depression

Experience-dependent changes in corticostriatal transmission efficacy are likely to support the role of the striatum in reinforcement-based motor learning. Whereas long-term depression at glutamatergic corticostriatal synapses has long been regarded as the normal form of striatal plasticity, recent work provides evidence that use-dependent potentiation can naturally occur at these connections through an increase in both synaptic efficacy and postsynaptic intrinsic excitability. By decreasing the weight of cortical inputs required to fire striatal output neurons, short-term and long-term potentiation at corticostriatal connections can jointly participate in the formation of sensorimotor links by which specific context-dependent patterns of cortical activity can engage selected motor programs.

Induction of striatal long-term synaptic depression by moderate frequency activation of cortical afferents in rat

The striatum regulates motor output, and it is thought that changes in the synaptic efficacy of inputs to the striatum contribute to motor learning and habit formation. Previously, several laboratories have observed that brief high frequency stimulation (HFS) of cortical afferents innervating the dorsolateral striatum induces a long-term decrease in synaptic efficacy called long-term depression (LTD). We recently showed that HFS-induced striatal LTD requires retrograde signalling involving postsynaptic release of endocannabinoids and activation of presynaptic CB1 cannabinoid receptors. In the present study we have employed whole-cell recording in brain slices to examine a new form of LTD at corticostriatal synapses that can be induced by a 10 Hz, 5 min train. The decrease in synaptic efficacy is associated with a decrease in presynaptic release probability, as demonstrated by a decrease in frequency but not amplitude of spontaneous excitatory postsynaptic currents (sEPSCs) and an increase in the paired pulse ratio (PPR). This form of LTD is blocked by antagonists for CB1 and D2 dopamine receptors and impaired by blockers of L-type calcium channels. However, 10 Hz-induced LTD does not depend on postsynaptic depolarization, unlike HFS-induced LTD. Furthermore, this new form of LTD is not prevented by treatments known to block HFS-induced LTD, including antagonism of metabotropic glutamate receptors (mGluRs), chelation of postsynaptic calcium, or intracellular application of an anandamide membrane transport inhibitor (VDM11). From these findings it is not clear that the endocannabinoid responsible for this form of LTD acts in a retrograde fashion, and the cellular source of endocannabinoid necessary for 10 Hz-induced LTD is as yet unknown. Our results demonstrate that a prolonged moderate frequency train induces cannabinoid-dependent LTD, further supporting the idea that endocannabinoids play a prominent role in the regulation of long-lasting changes in striatal output.

Place preference induced by nucleus accumbens amphetamine is impaired by antagonists of ERK or p38 MAP kinases in rats

The nucleus accumbens (NAc) plays a role in conditioned place preference (CPP). The authors tested the hypothesis that inhibition of mitogen-activated protein kinases (MAPKs) would inhibit NAc-amphetamine-produced CPP. Results confirmed that NAc amphetamine increased levels of the MAPK extracellular signal-regulated kinase (ERK). In CPP studies, NAc injections (0.5 microl per side) of the ERK inhibitor PD98059 (1.0-2.5 microg) or the p38 kinase inhibitor SB203580 (15-500 ng) dose dependently impaired CPP. The c-Jun-N-terminal kinase (JNK) inhibitor SP600125 (1.0-2.5 microg) failed to block the CPP effect. The drugs did not block amphetamine-induced motor activity. Results suggest that ERK and p38, but not JNK, MAPKs may be necessary for the establishment of NAc amphetamine-produced CPP and may also mediate other forms of reward-related learning dependent on NAc.

Anatomical funneling, sparse connectivity and redundancy reduction in the neural networks of the basal ganglia

The major anatomical characteristics of the main axis of the basal ganglia are: (1) Numerical reduction in the number of neurons across layers of the feed-forward network, (2) lateral inhibitory connections within the layers, and (3) neuro-modulatory effects of dopamine and acetylcholine, both on the basal ganglia neurons and on the efficacy of information transmission along the basal ganglia axis. We recorded the simultaneous activity of neurons in the output stages of the basal ganglia as well as the activity of dopaminergic and cholinergic neurons during the performance of a probability decision-making task. We found that the functional messages of the cholinergic and dopaminergic neurons differ, and that the cholinergic message is less specific than that of the dopaminergic neurons. The output stage of the basal ganglia showed uncorrelated neuronal activity. We conclude that despite the huge numerical reduction from the cortex to the output nuclei of the basal ganglia, the activity of these nuclei represents an optimally compressed (uncorrelated) version of distinctive features of cortical information.

Firing properties of dopamine neurons in freely moving dopamine-deficient mice: effects of dopamine receptor activation and anesthesia

To examine the regulation of midbrain dopamine neurons, recordings were obtained from single neurons of freely moving, genetically engineered dopamine-deficient (DD) mice. DD mice were tested without dopamine signaling (basal state) and with endogenous dopamine signaling (after L-dopa administration). In the basal state, when dopamine concentration in DD mice is <1% of that in control animals, the firing properties of midbrain dopamine neurons were remarkably similar among genotypes. However, L-dopa treatment, which restores dopamine and feeding and locomotor behavior in DD mice, profoundly inhibited the firing rate and bursting of dopamine neurons in DD mice. In addition, dopamine neurons in DD mice were hypersensitive to the dopamine receptor agonists quinpirole and SKF 81297. Anesthesia markedly reduced the firing rate of dopamine neurons in DD mice but did not significantly decrease the firing rate in control dopamine neurons. These data suggest that restoration of endogenous dopamine signaling activates hypersensitive long-loop feedback pathways that serve to limit dopamine release and underscore the importance of recording from awake animals.

Contrast enhancement: a physiological effect of striatal dopamine?

Dopamine functions as an important neuromodulator in the dorsal striatum and ventral striatum/nucleus accumbens. Evidence is accumulating for the idea that striatal neurons compete with each other for control over the animal's motor resources, and that dopamine plays an important modulatory role that allows a particular subset of neurons, encoding a specific behavior, to predominate in this competition. One means by which dopamine could facilitate selection among competing neurons is to enhance the contrast between stronger and weaker excitations (or to increase the "signal to noise ratio" among neurons, where the firing of the most excited neurons is assumed to transmit signal and the firing of the least excited to transmit noise). Here, we review the electrophysiological evidence for this hypothesis and discuss potential cellular mechanisms by which dopamine-mediated contrast enhancement could occur.

Coincident but Distinct Messages of Midbrain Dopamine and Striatal Tonically Active Neurons

Midbrain dopamine and striatal tonically active neurons (TANs, presumed acetylcholine interneurons) signal behavioral significance of environmental events. Since striatal dopamine and acetylcholine affect plasticity of cortico-striatal transmission and are both crucial to learning, they may serve as teachers in the basal ganglia circuits. We recorded from both neuronal populations in monkeys performing a probabilistic instrumental conditioning task. Both neuronal types respond robustly to reward-related events. Although different events yielded responses with different latencies, the responses of the two populations coincided, indicating integration at the target level. Yet, while the dopamine neurons' response reflects mismatch between expectation and outcome in the positive domain, the TANs are invariant to reward predictability. Finally, TAN pairs are synchronized, compared to a minority of dopamine neuron pairs. We conclude that the striatal cholinergic and dopaminergic systems carry distinct messages by different means, which can be integrated differently to shape the basal ganglia responses to reward-related events.

Association of the dopamine D5 receptor with attention deficit hyperactivity disorder (ADHD) and scores on a continuous performance test (TOVA)

Towards further clarifying the role of dopamine D5 receptor (DRD5) microsatellite polymorphism in the etiology of ADHD, we used a robust family based strategy to test for association between DRD5 and this disorder. Additionally, a neuropsychological mechanism by which this allele may confer risk was explored by examining the relationship between DRD5 genotype and scores on a continuous performance test. DNA was obtained from 164 probands and their parents. Additionally, the majority of these probands were administered a computerized continuous performance test, the Test Of Variables of Attention (TOVA). We first confirmed preferential transmission (TDT chi(2) = 7.02, P = 0.008) of the 148 base pair allele in 155 informative transmissions (94 transmitted and 61 non-transmitted 148 bp allele). Additionally, we used a family-based association test (FBAT) and observed significant multivariate association using FBAT between TOVA scores before methylphenidate administration and the DRD5 microsatellite polymorphism across all four TOVA variables: multi-allelic, multivariate test chi(2) = 16.32, P = 0.037 when the 148 bp allele was compared to all others (collapsed genotype) that was also significant (chi(2) = 59.20, P = 0.025) when all 14 alleles (full genotype) were analyzed. Following methylphenidate, no significant association was observed (chi(2) = 12.08, P = 0.147 for 148 bp versus all others) and, similarly, for all 14 alleles (chi(2) = 47.18, P = 0.343). In summary, the main finding of this report is that the DRD5 repeat polymorphism confers a small but significant risk for ADHD consistent with previous reports. Provisional results in this single investigation suggest that the DRD5 microsatellite also affects performance scores on the TOVA.

Action potential timing determines dendritic calcium during striatal up-states

Up-states represent a key feature of synaptic integration in cortex and striatum that involves activation of many synaptic inputs. In the striatum, the sparse firing and tight control of action potential timing is in contrast to the large intracellular membrane potential depolarizations observed during the up-state. One hallmark of striatal spiny projection neurons is the delay to action potential generation in both up-states and suprathreshold depolarization by somatic current injection. By studying somatic and dendritic intracellular calcium ([Ca2+]i) transients during spontaneous up-states in cortex-striatum-substantia nigra organotypic cultures, we show that the delay between up-state onset and action potential generation determines dendritic peak [Ca2+]i. Peak [Ca2+]i from single action potentials reached maximum values when action potentials were close to up-state onset and sharply decayed to near subthreshold up-state [Ca2+] levels as a function of time (tau = 47 +/- 26 msec for tertiary dendrite). Similarly, a precisely timed action potential elicited during subthreshold up-states through somatic current injection established that the delay between up-state onset and action potential generation is the critical variable that controls peak [Ca2+]i. Blocking NMDA channels internally with high intracellular Mg2+ ([Mg2+]i) (10 mm) abolished the dependency of peak [Ca2+]i on action potential timing during spontaneous up-states. Finally, high [Mg2+]i specifically blocked [Ca2+]i transients that resulted from local NMDA application in conjunction with backpropagating action potentials. We conclude that precisely timed, single action potentials during striatal up-states control peak dendritic calcium levels. We suggest that this mechanism might play an important role in synaptic plasticity of the corticostriatal pathway.

Long-term potentiation in an avian basal ganglia nucleus essential for vocal learning

Vocal learning in songbirds provides an excellent model for sensorimotor learning in vertebrates, with an accessible, well-defined behavior and discrete neural substrate. The rich behavioral plasticity exhibited by songbirds, however, contrasts starkly with the scarcity of candidate cellular mechanisms. Here, we report for the first time on an activity-dependent form of synaptic plasticity in area X, a component of the song system required for song learning and song maintenance. In slice preparations of zebra finch area X, pairing of high-frequency presynaptic stimulation with postsynaptic depolarization induces Hebbian long-term potentiation (LTP) of the glutamatergic inputs to spiny neurons. This form of LTP requires activation of NMDA receptors and D1-like dopamine receptors. In addition, LTP is observed in birds as young as 47 d after hatching and also in adult birds but not in younger birds, providing evidence of developmental regulation of the onset of synaptic plasticity. These properties make this form of LTP the best known candidate mechanism for reinforcement-based vocal learning in juveniles and song maintenance in adult birds.

Neural mechanisms of reward-related motor learning

The analysis of the neural mechanisms responsible for reward-related learning has benefited from recent studies of the effects of dopamine on synaptic plasticity. Dopamine-dependent synaptic plasticity may lead to strengthening of selected inputs on the basis of an activity-dependent conjunction of sensory afferent activity, motor output activity, and temporally related firing of dopamine cells. Such plasticity may provide a link between the reward-related firing of dopamine cells and the acquisition of changes in striatal cell activity during learning. This learning mechanism may play a special role in the translation of reward signals into context-dependent response probability or directional bias in movement responses.

Differential contribution of dopamine D2S and D2L receptors in the modulation of glutamate and GABA transmission in the striatum

Compelling evidence indicates that the long (D2L) and the short (D2S) isoform of dopamine (DA) D2 receptors serve distinct physiological functions in vivo. To address the involvement of these isoforms in the control of synaptic transmission in the striatum, we measured the sensitivity to D2 receptor stimulation of glutamate- and GABA-mediated currents recorded from striatal neurons of three mutant mice, in which the expression of D2L and D2S receptors was either ablated or variably altered. Our data indicate that both isoforms participate in the presynaptic inhibition of GABA transmission in the striatum, while the D2-receptor-dependent modulation of glutamate release preferentially involves the D2S receptor. Accordingly, the inhibitory effects of the DA D2 receptor agonist quinpirole (10 microM) on GABA(A)-mediated spontaneous inhibitory postsynaptic currents (IPSCs)correlate with the total number of D2 receptor sites in the striatum, irrespective of the specific receptor isoform expressed. In contrast, glutamate-mediated spontaneous excitatory postsynaptic currents (EPSCs) were significantly inhibited by quinpirole only when the total number of D2 receptor sites, normally composed by both D2L and D2S receptors in a ratio favoring the D2L isoform, was modified to express only the D2S isoform at higher than normal levels. Understanding the physiological roles of DA D2 receptors in the striatum is essential for the treatment of several neuropsychiatric conditions, such as Parkinson's disease, Tourette's syndrome, schizophrenia, and drug addiction.

Information processing, dimensionality reduction and reinforcement learning in the basal ganglia

Modeling of the basal ganglia has played a major role in our understanding of this elusive group of nuclei. Models of the basal ganglia have undergone evolutionary and revolutionary changes over the last 20 years, as new research in the fields of anatomy, physiology and biochemistry of these nuclei has yielded new information. Early models dealt with a single pathway through the nuclei and focused on the nature of the processing performed within it, convergence of information versus parallel processing of information. Later, the Albin-DeLong "box-and-arrow" model characterized the inter-nuclei interaction as multiple pathways while maintaining a simplistic scalar representation of the nuclei themselves. This model made a breakthrough by providing key insights into the behavior of these nuclei in hypo- and hyper-kinetic movement disorders. The next generation of models elaborated the intra-nuclei interactions and focused on the role of the basal ganglia in action selection and sequence generation which form the most current consensus regarding basal ganglia function in both normal and pathological conditions. However, new findings challenge these models and point to a different neural network approach to information processing in the basal ganglia. Here, we take an in-depth look at the reinforcement driven dimensionality reduction (RDDR) model which postulates that the basal ganglia compress cortical information according to a reinforcement signal using optimal extraction methods. The model provides new insights and experimental predictions on the computational capacity of the basal ganglia and their role in health and disease.

Plastic Control of Striatal Glutamatergic Transmission by Ensemble Actions of Several Neurotransmitters and Targets for Drugs of Abuse

Long-lasting alterations in the efficacy of glutamatergic synapses, such as long-term potentiation (LTP) and long-term depression (LTD), are prominent models for mechanisms of information storage in the brain. It has been suggested that exposure to drugs of abuse produces synaptic plasticity at glutamatergic synapses that shares many features with LTP and LTD, and that these synaptic changes may play roles in addiction. We have examined the involvement of particular neurotransmitters in synaptic plasticity at glutamatergic synapses within the striatum, a brain region with prominent roles in initiation and sequencing of actions, as well as habit formation. Our studies indicate that multiple neurotransmitters interact to produce striatal synaptic plasticity, and that the relative strength and patterning of the afferent inputs that release the various neurotransmitters determines whether LTP or LTD is activated. Drugs of abuse interact with glutamatergic synaptic plasticity in multiple ways, including alterations in dopamine release and more direct effects on glutamate release and glutamate receptors. We hypothesize that these effects contribute to addiction by facilitating the formation of new, drug-centered habits, and by disruption of more adaptive behaviors.

Distinct roles of D1 and D5 dopamine receptors in motor activity and striatal synaptic plasticity

Stimulation of dopamine (DA) receptors in the striatum is essential for voluntary motor activity and for the generation of plasticity at corticostriatal synapses. In the present study, mice lacking DA D1 receptors have been used to investigate the involvement of the D1-like class (D1 and D5) of DA receptors in locomotion and corticostriatal long-term depression (LTD) and long-term potentiation (LTP). Our results suggest that D1 and D5 receptors exert distinct actions on both activity-dependent synaptic plasticity and spontaneous motor activity. Accordingly, the ablation of D1 receptors disrupted corticostriatal LTP, whereas pharmacological blockade of D5 receptors prevented LTD. On the other side, genetic ablation of D1 receptors increased locomotor activity, whereas the D1/D5 receptor antagonist SCH 23390 decreased motor activity in both control mice and mice lacking D1 receptors. Endogenous DA stimulated D1 and D5 receptors in distinct subtypes of striatal neurons to induce, respectively, LTP and LTD. In control mice, in fact, LTP was blocked by inhibiting the D1-protein kinase A pathway in the recorded spiny neuron, whereas the striatal nitric oxide-producing interneuron was presumably the neuronal subtype stimulated by D5 receptors during the induction phase of LTD. Understanding the role of DA receptors in striatal function is essential to gain insights into the neural bases of critical brain functions and of dramatic pathological conditions such as Parkinson's disease, schizophrenia, and drug addiction.

Receptor subtypes involved in the presynaptic and postsynaptic actions of dopamine on striatal interneurons

By stimulating distinct receptor subtypes, dopamine (DA) exerts presynaptic and postsynaptic actions on both large aspiny (LA) cholinergic and fast-spiking (FS) parvalbumin-positive interneurons of the striatum. Lack of receptor- and isoform-specific pharmacological agents, however, has hampered the progress toward a detailed identification of the specific DA receptors involved in these actions. To overcome this issue, in the present study we used four different mutant mice in which the expression of specific DA receptors was ablated. In D1 receptor null mice, D1R-/-, DA dose-dependently depolarized both LA and FS interneurons. Interestingly, SCH 233390 (10 microm), a D1-like (D1 and D5) receptor antagonist, but not l-sulpiride (3-10 microm), a D2-like (D2, D3, D4) receptor blocker, prevented this effect, implying D5 receptors in this action. Accordingly, immunohistochemical analyses in both wild-type and D1R-/- mice confirmed the expression of D5 receptors in both cholinergic and parvalbumin-positive interneurons of the striatum. In mice lacking D2 receptors, D2R-/-, the DA-dependent inhibition of GABA transmission was lost in both interneuron populations. Both isoforms of D2 receptor, D2L and D2S, were very likely involved in this inhibitory action, as revealed by the electrophysiological analysis of the effect of the DA D2-like receptor agonist quinpirole in two distinct mutants lacking D2L receptors and expressing variable contents of D2S receptors. The identification of the receptor subtypes involved in the actions of DA on different populations of striatal cells is essential to understand the circuitry of the basal ganglia and to develop pharmacological strategies able to interfere selectively with specific neuronal functions.

Modulation of striatal single units by expected reward: a spiny neuron model displaying dopamine-induced bistability

Single-unit activity in the neostriatum of awake monkeys shows a marked dependence on expected reward. Responses to visual cues differ when animals expect primary reinforcements, such as juice rewards, in comparison to secondary reinforcements, such as tones. The mechanism of this reward-dependent modulation has not been established experimentally. To assess the hypothesis that direct neuromodulatory effects of dopamine on spiny neurons can account for this modulation, we develop a computational model based on simplified representations of key ionic currents and their modulation by D1 dopamine receptor activation. This minimal model can be analyzed in detail. We find that D1-mediated increases of inward rectifying potassium and L-type calcium currents cause a bifurcation: the native up/down state behavior of the spiny neuron model becomes truly bistable, which modulates the peak firing rate and the duration of the up state and introduces a dependence of the response on the past state history. These generic consequences of dopamine neuromodulation through bistability can account for both reward-dependent enhancement and suppression of spiny neuron single-unit responses to visual cues. We validate the model by simulating responses to visual targets in a memory-guided saccade task; our results are in close agreement with the main features of the experimental data. Our model provides a conceptual framework for understanding the functional significance of the short-term neuromodulatory actions of dopamine on signal processing in the striatum.

When inhibition goes incognito: feedback interaction between spiny projection neurons in striatal function

Theories about basal ganglia function have always been driven by our knowledge about the spiny projection neurons of the striatum. At the core of these theories lies the question of how, precisely, spiny projection neurons process cortical inputs. Most recently, studies demonstrating the role of spiny projection neurons in local synaptic GABA transmission have provided several new avenues for exploring striatal dynamics. They have also suggested new experimental directives for examining the specific ways in which spiny projection neurons both compete and cooperate through their local axon collaterals during cortical input processing.

Regulation of dopamine D1 receptor trafficking and desensitization by oligomerization with glutamate N-methyl-D-aspartate receptors

Activation of dopamine D1 receptors is critical for the generation of glutamate-induced long-term potentiation at corticostriatal synapses. In this study, we report that, in striatal neurons, D1 receptors are co-localized with N-methyl-d-aspartate (NMDA) receptors in the postsynaptic density and that they co-immunoprecipitate with NMDA receptor subunits from postsynaptic density preparations. Using modified bioluminescence resonance energy transfer, we demonstrate that D1 and NMDA receptor clustering reflects the existence of direct interactions. The tagged D1 receptor and NR1 subunit cotransfected in COS-7 cells generated a significant bioluminescence resonance energy transfer signal that was insensitive to agonist stimulation and that did not change in the presence of the NR2B subunit, suggesting that the D1 receptor constitutively and selectively interacts with the NR1 subunit of the NMDA channel. Oligomerization with the NR1 subunit substantially modified D1 receptor trafficking. In individually transfected HEK293 cells, NR1 was localized in the endoplasmic reticulum, whereas the D1 receptor was targeted to the plasma membrane. In cotransfected cells, both the D1 receptor and NR1 subunit were retained in cytoplasmic compartments. In the presence of the NR2B subunit, the NR1-D1 receptor complex was translocated to the plasma membrane. These data suggest that D1 and NMDA receptors are assembled within intracellular compartments as constitutive heteromeric complexes that are delivered to functional sites. Coexpression with NR1 and NR2B subunits also abolished agonist-induced D1 receptor cytoplasmic sequestration, indicating that oligomerization with the NMDA receptor could represent a novel regulatory mechanism modulating D1 receptor desensitization and cellular trafficking.

Loss of bidirectional striatal synaptic plasticity in L-DOPA-induced dyskinesia

Long-term treatment with the dopamine precursor levodopa (L-DOPA) induces dyskinesia in Parkinson's disease (PD) patients. We divided hemiparkinsonian rats treated chronically with L-DOPA into two groups: one showed motor improvement without dyskinesia, and the other developed debilitating dyskinesias in response to the treatment. We then compared the plasticity of corticostriatal synapses between the two groups. High-frequency stimulation of cortical afferents induced long-term potentiation (LTP) of corticostriatal synapses in both groups of animals. Control and non-dyskinetic rats showed synaptic depotentiation in response to subsequent low-frequency synaptic stimulation, but dyskinetic rats did not. The depotentiation seen in both L-DOPA-treated non-dyskinetic rats and intact controls was prevented by activation of the D1 subclass of dopamine receptors or inhibition of protein phosphatases. The striata of dyskinetic rats contained abnormally high levels of phospho[Thr34]-DARPP-32, an inhibitor of protein phosphatase 1. These results indicate that abnormal information storage in corticostriatal synapses is linked with the development of L-DOPA-induced dyskinesia.

Dopamine: a potential substrate for synaptic plasticity and memory mechanisms

It is only recently that a number of studies on synaptic plasticity in the hippocampus and other brain areas have considered that a heterosynaptic modulatory input could be recruited as well as the coincident firing of pre- and post-synaptic neurons. So far, the strongest evidence for such a regulation has been attributed to dopaminergic (DA) systems but other modulatory pathways have also been considered to influence synaptic plasticity. This review will focus on dopamine contribution to synaptic plasticity in different brain areas (hippocampus, striatum and prefrontal cortex) with, for each region, a few lines on the distribution of DA projections and receptors. New insights into the possible mechanisms underlying these plastic changes will be considered. The contribution of various DA systems in certain forms of learning and memory will be reviewed with recent advances supporting the hypothesis of similar cellular mechanisms underlying DA regulation of synaptic plasticity and memory processes in which the cyclic adenosine monophosphate/protein kinase A (cAMP/PKA) pathway has a potential role. To summarize, endogenous DA, which depends on the activity patterns of DA midbrain neurons in freely moving animals, appears as a key regulator in specific synaptic changes observed at certain stages of learning and memory and of synaptic plasticity.

It could be habit forming: drugs of abuse and striatal synaptic plasticity

Drug addiction can take control of the brain and behavior, activating behavioral patterns that are directed excessively and compulsively toward drug usage. Such patterns often involve the development of repetitive and nearly automatic behaviors that we call habits. The striatum, a subcortical brain region important for proper motor function as well as for the formation of behavioral habits, is a major target for drugs of abuse. Here, we review recent studies of long-term synaptic plasticity in the striatum, emphasizing that drugs of abuse can exert pronounced influences on these processes, both in the striatum and in the dopaminergic midbrain. Synaptic plasticity in the ventral striatum appears to play a prominent role in early stages of drug use, whereas dopamine- and endocannabinoid-dependent synaptic plasticity in the dorsal striatum could contribute to the formation of persistent drug-related habits when casual drug use progresses towards compulsive drug use and addiction.

Opposing roles of D1 and D2 receptors in appetitive conditioning

Previous studies have shown that D(1) receptor blockade disrupts and D(2) receptor blockade enhances long-term potentiation. These data lead to the prediction that D(1) antagonists will attenuate and D(2) antagonists will potentiate at least some types of learning. The prediction is difficult to test, however, because disruptions in either D(1) or D(2) transmission lead to reduced locomotion, exploration, and response execution and are therefore likely to impair learning that requires behavioral responding (including exploration of an environment) during the learning episode. Under a paradigm that minimizes motor requirements, rats were trained to enter a food compartment during pellet presentation. Animals then received tone-food pairings under the influence of D(1) antagonist SCH23390 (0, 0.4, 0.8, and 0.16 mg/kg) or D(2) antagonist raclopride (0, 0.2, 0.4, and 0.8 mg/kg). An additional group received unpaired presentations of tone and food. On a drug-free test day 24 hr later, animals that had been under the influence of SCH23390 (like animals that had received unpaired presentations of tone and food) showed reduced head entries in response to the tone, whereas animals that had been under the influence of raclopride showed increased head entries in response to the tone compared with vehicle controls. These data demonstrate that, under a conditioned approach paradigm, D(1) and D(2) family receptor antagonists disrupt and promote learning, respectively, as predicted by the effects of D(1) and D(2) receptor blockade on neuronal plasticity.

D5 (not D1) dopamine receptors potentiate burst-firing in neurons of the subthalamic nucleus by modulating an L-type calcium conductance

Dopamine is a crucial factor in basal ganglia functioning. In current models of basal ganglia, dopamine is postulated to act on striatal neurons. However, it may also act on the subthalamic nucleus (STN), a key nucleus in the basal ganglia circuit. The data presented here were obtained in brain slices using whole-cell patch clamp. They reveal that D5 dopamine receptors strengthen electrical activity in the subset of subthalamic neurons endowed with burst-firing capacity, resulting in longer discharges of spontaneous or evoked bursts. To distinguish between D1 and D5 subtypes, the action of agonists in the D1/D5 receptor family was first investigated on rat subthalamic neurons. Single-cell reverse transcription-PCR profiling showed that burst-competent neurons only expressed D5 receptors. Accordingly, receptors localized in postsynaptic membranes within the STN were labeled by a D5-specific antibody. Second, agonists in the D1/D5 family were tested in mouse brain slices. It was found that these agonists were active in D1 receptor knock-out mice in a similar way to wild-type mice or rats. This proved that D5 rather than D1 receptors were involved. Pharmacological tools (dihydropyridines, omega-conotoxins, and calciseptine) were used to identify the target of D5 receptors as an L-type channel. This was reached via G-protein and protein kinase A. The action of dopamine on D5 receptors therefore shapes neuronal activity. It contributes to normal information processing in basal ganglia outside striatum. This finding may be useful in drug therapy for various disorders involving changes in STN activity, such as Parkinson's disease and related disorders.

Dopamine enhances EPSCs in layer II-III pyramidal neurons in rat prefrontal cortex

Dopaminergic inputs to the prefrontal cortex (PFC) are important for the integration of neuronal signals, the formation of working memory, and the establishment of memory fields. A detailed characterization of cellular mechanisms underlying the effects of dopamine on PFC is still emerging. We have examined how dopamine affects excitatory synaptic transmission in the PFC using whole-cell patch-clamp recording from visually identified layer II-III pyramidal cells in vitro. Bath application of dopamine significantly enhanced EPSC amplitudes. Pharmacologically isolated AMPA and NMDA receptor-mediated EPSCs were increased to a similar extent. Application of the specific D1-like receptor agonist SKF38393 [(+/-)-1-phenyl-2,3,4,5-tetrahydro-(1H)-3-benzazepine-7,8-diol hydrobromide] significantly increased EPSC amplitude, whereas the D2-like receptor agonist quinpirole had no effect. Responses to pressure-applied glutamate were also enhanced by dopamine, indicating a postsynaptic mechanism. Inclusion of the Ca(2+) chelator BAPTA in the recording pipette blocked the dopamine enhancement. When the PKA inhibitory peptide PKI [5-24] was included in the recording pipette, dopamine did not affect EPSCs. Similarly, when the Ca(2+)/calmodulin-kinase II (CaMKII) inhibitory peptide was present in the pipette, dopamine enhancement of EPSCs was not observed in any of the cells tested. These results indicate that EPSC enhancement may be attributable to a postsynaptic signaling cascade involving Ca(2+), PKA, and CaMKII.

Dopamine gating of forebrain neural ensembles

Dopamine may exert different actions depending on a number of factors. A common view is that D1 receptors may be responsible for excitatory actions whereas D2 receptors are involved in inhibitory actions. However, this position cannot be reconciled with several findings indicating otherwise. The role of dopamine on forebrain neural ensembles may be better understood in the light of functional states of the system. Pyramidal cortical neurons and striatal medium spiny neurons alternate between two membrane potential states ('up' and 'down') that could shape dopamine actions. It is proposed that D1 receptors can act as state-stabilizers by sustaining up states and thereby facilitating plasticity mechanisms by providing postsynaptic depolarization and increasing NMDA function. In this way, dopamine can sustain activity in depolarized units. This action is accompanied by a decrease in cell firing (perhaps mediated by D2 receptors), which renders the cells responsive only to strong stimuli. The result would be a net increase in signal-to-noise ratio in a selected assembly of neurons.

Calcyon transcript expression in macaque brain

The anatomical distribution of the transcript encoding calcyon, a 24 kDa membrane protein associated with coupling D1-like dopamine receptor activation to potentiated intracellular calcium release, was examined using in situ hybridization in the macaque brain. Calcyon mRNA is found to be abundantly distributed throughout the primate brain. In neocortex, moderately dense, diffuse signal is found in all areas, with increased intensity present in a superficial isodense band corresponding generally to cortical layers II and III. Increased intensity of signal is also seen in the pyramidal cell layers of medial prefrontal and anterior cingulate cortex. Calcyon mRNA is present abundantly in subcortical limbic areas such as the nucleus accumbens, septum, hypothalamus, amygdala, and hippocampus. Moderate calcyon transcript expression is seen in caudate and putamen, with lower levels in globus pallidus. Thalamic nuclei, including the reticular nucleus, express low to moderate levels. Very dense expression is noted in the substantia nigra pars compacta. Numerous brainstem regions express this transcript, notably monoaminergic nuclei including the locus coeruleus and dorsal raphe. The cerebellum has detectable levels of expression in both cortex and deep nuclei. Although calcyon is hypothesized as a means for D1-like receptors to modulate "cross-talk" with other neurotransmitter receptor systems, it is notable that abundant calcyon transcript is detected in brain regions not associated with D1-like neurotransmission, particularly the substantia nigra pars compacta and other dopamine-synthesizing cell groups. A substantial proportion of this may relate to the reported association of calcyon with the D5 receptor, or in addition, may suggest that calcyon has a wider role as a regulator of intracellular signal transduction.

Dopamine gating of glutamatergic sensorimotor and incentive motivational input signals to the striatum

Dopamine (DA) neurons of the substantia nigra (SN) and ventral tegmental area (VTA) respond to a wide category of salient stimuli. Activation of SN and VTA DA neurons, and consequent release of nigrostriatal and mesolimbic DA, modulates the processing of concurrent glutamate inputs to dorsal and ventral striatal target regions. According to the view described here, this occurs under conditions of unexpected environmental change regardless of whether that change is rewarding or aversive. Nigrostriatal and mesolimbic DA activity gates the input of sensory, motor, and incentive motivational (e.g. reward) signals to the striatum. In light of recent single-unit and brain imaging data, it is suggested that the striatal reward signals originate in the orbitofrontal cortex and basolateral amygdala (BLA), regions that project strongly to the striatum. A DA signal of salient unexpected event occurrence, from this framework, gates the throughput of the orbitofrontal glutamate reward input to the striatum just as it gates the throughput of corticostriatal sensory and motor signals needed for normal response execution. Processing of these incoming signals is enhanced when synaptic DA levels are high, because DA enhances the synaptic efficacy of strong concurrent glutamate inputs while reducing the efficacy of weak glutamate inputs. The impairments in motor performance and incentive motivational processes that follow from nigrostriatal and mesolimbic DA loss can be understood in terms of a single mechanism: abnormal processing of sensorimotor and incentive motivation-related glutamate input signals to the striatum.

Dopamine enhancement of NMDA currents in dissociated medium-sized striatal neurons: role of D1 receptors and DARPP-32

Dopamine (DA), via activation of D1 receptors, enhances N-methyl-D-aspartate (NMDA)-evoked responses in striatal neurons. The present investigation examined further the properties of this enhancement and the potential mechanisms by which this enhancement might be effected. Dissociated medium-sized striatal neurons were obtained from intact rats and mice or mutant mice lacking the DA and cyclic adenosine 3',5' monophosphate (cAMP)-regulated phosphoprotein of M(R) 32,000 (DARPP-32). NMDA (10-1,000 microM) induced inward currents in all neurons. In acutely dissociated neurons from intact rats or mice, activation of D1 receptors with the selective agonist, SKF 81297, produced a dose-dependent enhancement of NMDA currents. This enhancement was reduced by the selective D1 receptor antagonist SKF 83566. Quinpirole, a D2 receptor agonist alone, produced small reductions of NMDA currents. However, it consistently and significantly reduced the enhancement of NMDA currents by D1 agonists. In dissociated striatal neurons, in conditions that minimized the contributions of voltage-gated Ca(2+) conductances, the D1-induced potentiation was not altered by blockade of L-type voltage-gated Ca(2+) conductances in contrast to results in slices. The DARPP-32 signaling pathway has an important role in D1 modulation of NMDA currents. In mice lacking DARPP-32, the enhancement was significantly reduced. Furthermore, okadaic acid, a protein phosphatase 1 (PP-1) inhibitor, increased D1-induced potentiation, suggesting that constitutively active PP-1 attenuates D1-induced potentiation. Finally, activation of D1 receptors produced differential effects on NMDA and gamma aminobutyric acid (GABA)-induced currents in the same cells, enhancing NMDA currents and inhibiting GABA currents. Thus simultaneous activation of D1, NMDA, and GABA receptors could predispose medium-sized spiny neurons toward excitation. Taken together, the present findings indicate that the unique potentiation of NMDA receptor function by activation of the D1 receptor signaling cascade can be controlled by multiple mechanisms and has major influences on neuronal function.

Tissue plasminogen activator is required for corticostriatal long-term potentiation

Several experimental data indicate that tissue plasminogen activator (tPA) is involved in memory formation and synaptic plasticity in different brain areas. In the attempt to highlight the role of this serine protease in striatal neuron activity, mice lacking tPA have been used for electrophysiological, immunohistochemical and Western blot experiments. Disruption of tPA gene prevented corticostriatal long-term potentiation, an NMDA-dependent form of synaptic plasticity requiring the stimulation of both dopamine and acetylcholine receptors. Spontaneous and evoked glutamatergic transmission was intact in the striatum of tPA-deficient mice, as was the nigrostriatal dopamine innervation and the expression of dopamine D1 receptors. Conversely, the sensitivity of striatal cholinergic interneurons to dopamine D1 receptor stimulation was lost in these mutants, suggesting that tPA facilitates long-term potentiation (LTP) induction in the striatum by favouring the D1 receptor-mediated excitation of acetylcholine-producing interneurons. The demonstration that tPA ablation interferes with the induction of corticostriatal LTP and with the dopamine receptor-mediated control of cholinergic interneurons might help to explain the altered striatum-dependent learning deficits observed in tPA-deficient mice and provides new insights into the molecular mechanisms underlying synaptic plasticity in the striatum.

Learning and memory functions of the Basal Ganglia

Although the mammalian basal ganglia have long been implicated in motor behavior, it is generally recognized that the behavioral functions of this subcortical group of structures are not exclusively motoric in nature. Extensive evidence now indicates a role for the basal ganglia, in particular the dorsal striatum, in learning and memory. One prominent hypothesis is that this brain region mediates a form of learning in which stimulus-response (S-R) associations or habits are incrementally acquired. Support for this hypothesis is provided by numerous neurobehavioral studies in different mammalian species, including rats, monkeys, and humans. In rats and monkeys, localized brain lesion and pharmacological approaches have been used to examine the role of the basal ganglia in S-R learning. In humans, study of patients with neurodegenerative diseases that compromise the basal ganglia, as well as research using brain neuroimaging techniques, also provide evidence of a role for the basal ganglia in habit learning. Several of these studies have dissociated the role of the basal ganglia in S-R learning from those of a cognitive or declarative medial temporal lobe memory system that includes the hippocampus as a primary component. Evidence suggests that during learning, basal ganglia and medial temporal lobe memory systems are activated simultaneously and that in some learning situations competitive interference exists between these two systems.

Getting formal with dopamine and reward

Recent neurophysiological studies reveal that neurons in certain brain structures carry specific signals about past and future rewards. Dopamine neurons display a short-latency, phasic reward signal indicating the difference between actual and predicted rewards. The signal is useful for enhancing neuronal processing and learning behavioral reactions. It is distinctly different from dopamine's tonic enabling of numerous behavioral processes. Neurons in the striatum, frontal cortex, and amygdala also process reward information but provide more differentiated information for identifying and anticipating rewards and organizing goal-directed behavior. The different reward signals have complementary functions, and the optimal use of rewards in voluntary behavior would benefit from interactions between the signals. Addictive psychostimulant drugs may exert their action by amplifying the dopamine reward signal.

Change in bi-directional plasticity at CA1 synapses in hippocampal slices taken from 6-hydroxydopamine-treated rats: the role of endogenous norepinephrine

The object of the present study is to investigate the role of endogenous adrenergic innervation in regulating bi-directional synaptic plasticity in rat hippocampal CA1 synapses. The endogenous adrenergic system was eliminated by giving subcutaneous injection of 6-hydroxydopamine (6-OHDA) to rats immediately after birth, and the animals were killed for experiments at postnatal ages of 25-35 days. In hippocampal slices taken from 6-OHDA-treated animals, theta-burst stimulation at 100 Hz failed to induce long-term potentiation (LTP) at CA1 synapses. However, the induction of long-term depression (LTD) by prolonged low frequency stimulation at 1 Hz was unaffected in slices from 6-OHDA-treated animals. Bath application of norepinephrine (NE) restored LTP to control levels and blocked LTD. This effect was mimicked by beta- but not alpha-adrenergic receptor agonists, i.e. by isoproterenol but not phenylephrine. The activators of adenylyl cyclase and protein kinase A (PKA), i.e. forskolin and 8-bromoadenosine-3', 5'-cyclic monophosphate, respectively, restored LTP in slices from 6-OHDA-treated animals. In addition, application of the D1/D5 receptor agonist, dihydrexidine, also restored LTP in slices from 6-OHDA-treated animals. These results suggest that physiologically the recruitment of catecholamine innervation may be important for induction of LTP at hippocampal CA1 synapses during tetanic stimulation, while it may not be essential for LTD induction by prolonged 1 Hz stimulation. The released NE and dopamine exert their role in modulating synaptic plasticity via activation of beta-adrenergic and D1/D5 receptors, respectively, which in turn increase the levels of cytoplasm adenosine-3',5'-cyclic monophosphate and PKA.

Dopamine-dependent plasticity of corticostriatal synapses

Knowledge of the effect of dopamine on corticostriatal synaptic plasticity has advanced rapidly over the last 5 years. We consider this new knowledge in relation to three factors proposed earlier to describe the rules for synaptic plasticity in the corticostriatal pathway. These factors are a phasic increase in dopamine release, presynaptic activity and postsynaptic depolarisation. A function is proposed which relates the amount of dopamine release in the striatum to the modulation of corticostriatal synaptic efficacy. It is argued that this function, and the experimental data from which it arises, are compatible with existing models which associate the reward-related firing of dopamine neurons with changes in corticostriatal synaptic efficacy.

Dendritic calcium encodes striatal neuron output during up-states

Striatal spiny projection neurons control basal ganglia outputs via action potential bursts conveyed to the globus pallidus and substantia nigra. Accordingly, burst activity in these neurons contributes importantly to basal ganglia function and dysfunction. These bursts are driven by multiple corticostriatal inputs that depolarize spiny projection neurons from their resting potential of approximately -85 mV, which is the down-state, to a subthreshold up-state of -55 mV. To understand dendritic processing of bursts during up-states, changes in intracellular calcium concentration ([Ca2+]i) were measured in striatal spiny projection neurons from cortex-striatum-substantia nigra organotypic cultures grown for 5-6 weeks using somatic whole-cell patch recording and Fura-2. During up-states, [Ca2+]i transients at soma and primary, secondary, and tertiary dendrites were highly correlated with burst strength (i.e., the number of spontaneous action potentials). During down-states, the action potentials evoked by somatic current pulses elicited [Ca2+]i transients in higher-order dendrites that were also correlated with burst strength. Evoked bursts during up-states increased dendritic [Ca2+]i transients supralinearly by >200% compared with the down-state. In the presence of tetrodotoxin, burst-like voltage commands failed to elicit [Ca2+]i transients at higher-order dendrites. Thus, dendritic [Ca2+]i transients in spiny projection neurons encode somatic bursts supralinearly during up-states through active propagation of action potentials along dendrites. We suggest that this conveys information about the contribution of a spiny projection neuron to a basal ganglia output specifically back to the corticostriatal synapses involved in generating these outputs.

Inactivation of adenosine A2A receptor impairs long term potentiation in the accumbens nucleus without altering basal synaptic transmission

The nucleus accumbens is considered to be critically involved in the control of complex motivated behaviors. By modulating its glutamatergic excitatory input, mesolimbic dopaminergic afferents have been implicated in the reinforcing properties of drugs of abuse. However, they might not represent the only path for influencing the accumbens output. The aim of this study was to investigate possible modulation of synaptic transmission at this glutamatergic synapse by adenosine receptors. The standard field potential recording technique was used on brain slices from wild-type and A2A receptor-deficient mice. Neither the stimulus-response relationship nor paired-pulse facilitation was altered in the mutant mice. In both genotypes, the activation of A1 receptors by 2-chloro-N6-cyclopentyladenosine reduced the field excitatory postsynaptic potential (fEPSP) slope to a similar extent. In wild-type slices, activation or blockade of A2A receptors by 2-[4-(carboxyethyl)phenylethylamino]-5'-N-ethylcarboxamidoadenosine and 4-(2-[7-amino-2-(2-furyl)[1,2,4]-triazolo-[2,3-a][1,3,5]triazin-5-ylamino]ethyl)phenol, respectively, did not modify the synaptic transmission. Moreover, a long lasting pre-activation of these A2A receptors did not influence the A1 receptor-mediated reduction in fEPSP slope. Long term potentiation (LTP) of the alpha-amino-3-hydroxy-5-methyl-4-isoxazole proprionate (AMPA) receptor-mediated synaptic transmission could be elicited in both wild-type and A2A receptor-deficient mice. However, LTP appeared to be quantitatively modulated by the A2A receptor pathway since the level of potentiation was reduced in A2A receptor-deficient mice as well as in slices of wild-type mice in which the A2A receptor pathway was blocked. The involvement of the cAMP-dependent protein kinase was supported by the reduction in potentiation level in slices of wild-type mice treated with adenosine 3',5'-cyclic monophosphorothiotate, 8-(4-chlorophenylthio)-Rp isomer, an inhibitor of this enzyme. These data provide evidence that the adenosine acting at the A2A receptor is implicated in events directly or indirectly related to LTP induction in the accumbens whereas it is not involved in the regulation of the basal AMPA receptor-mediated excitatory synaptic transmission.

Appetitive instrumental learning requires coincident activation of NMDA and dopamine D1 receptors within the medial prefrontal cortex

Through its complex role in cognition, memory, and emotion, the mammalian prefrontal cortex is thought to contribute to the organization of adaptive behavioral actions. In the present studies we examined the role of dopaminergic D1 and glutamatergic NMDA receptors within the prefrontal cortex of the rat during the development of adaptive instrumental learning. Hungry rats with bilateral indwelling cannulas aimed at the medial prefrontal cortex were trained to lever-press for food. Infusion of the selective D1 antagonist SCH-23390 (0.15, 0.3, 3.0 nmol) dose-dependently impaired acquisition of this behavior. Higher doses also impaired expression of this task. Co-infusion of the lowest dose of SCH 23390 with a low dose of the NMDA antagonist AP-5 (0.5 nmol), each of which had no effect on learning when infused alone, potently reduced the ability to acquire the response. Inhibition of intracellular protein kinase A with the selective PKA inhibitor Rp-cAMPS also disrupted acquisition, suggesting that PKA is an intracellular substrate for a D1-NMDA receptor interaction. In control experiments, drug infusions that impaired learning did not affect food intake or locomotion, suggesting a specific effect on learning. We hypothesize that coincident detection of D1-NMDA receptor activation and its transcriptional consequences, within multiple sites of a distributed corticostriatal network, may represent a conserved molecular mechanism for instrumental learning.

Stepping out of the box: information processing in the neural networks of the basal ganglia

The Albin-DeLong 'box and arrow' model has long been the accepted standard model for the basal ganglia network. However, advances in physiological and anatomical research have enabled a more detailed neural network approach. Recent computational models hold that the basal ganglia use reinforcement signals and local competitive learning rules to reduce the dimensionality of sparse cortical information. These models predict a steady-state situation with diminished efficacy of lateral inhibition and low synchronization. In this framework, Parkinson's disease can be characterized as a persistent state of negative reinforcement, inefficient dimensionality reduction, and abnormally synchronized basal ganglia activity.

Reward signaling by dopamine neurons

Dopamine projections from the midbrain to the striatum and frontal cortex are involved in behavioral reactions controlled by rewards, as inferred from deficits in parkinsonism, schizophrenia, and drug addiction. Recent experiments have shown that dopamine neurons are not directly modulated in relation to movements. Rather, they appear to code the rewarding aspects of environmental stimuli. They show short, phasic increases of activity following primary food and liquid rewards ("unconditioned stimuli") and conditioned, reward-predicting stimuli of visual, auditory, and somatosensory modalities. They also display smaller activation-depression sequences after stimuli resembling rewards and after novel or particularly intense stimuli. Rewards are only reported as far as they occur differently than predicted. According to learning theories, a "prediction error" message may constitute a powerful teaching signal for behavior and learning. The phasic reward message is different from the more tonic enabling function of dopamine that is deficient in Parkinson's disease, indicating that dopamine neurons subserve different functions at different time scales. Neurons in other brain structures, such as the striatum, orbitofrontal cortex, and amygdala, code the quality, quantity, and preference of rewards. The dopamine reward prediction error signal may cooperate with these reward perception signals during the learning and performance of behavioral reactions to motivating environmental stimuli.

Dopaminergic control of synaptic plasticity in the dorsal striatum

Cortical glutamatergic and nigral dopaminergic afferents impinge on projection spiny neurons of the striatum, providing the most significant inputs to this structure. Isolated activation of glutamate or dopamine (DA) receptors produces short-term effects on striatal neurons, whereas the combined stimulation of both glutamate and DA receptors is able to induce long-lasting modifications of synaptic excitability. Repetitive stimulation of corticostriatal fibres causes a massive release of both glutamate and DA in the striatum and, depending on the glutamate receptor subtype preferentially activated, produces either long-term depression (LTD) or long-term potentiation (LTP) of excitatory synaptic transmission. D1-like and D2-like DA receptors interact synergistically to allow LTD formation, while they operate in opposition during the induction phase of LTP. Corticostriatal synaptic plasticity is severely impaired after chronic DA denervation and requires the stimulation of DARPP-32, a small protein expressed in dopaminoceptive spiny neurons which acts as a potent inhibitor of protein phosphatase-1. In addition, the formation of LTD and LTP requires the activation of PKG and PKA, respectively, in striatal projection neurons. These kinases appear to be stimulated by the activation of D1-like receptors in distinct neuronal populations.