The connections of the dopaminergic system with the striatum in rats and primates: an analysis with respect to the functional and compartmental organization of the striatum

Model-based fMRI and its application to reward learning and decision making

In model-based functional magnetic resonance imaging (fMRI), signals derived from a computational model for a specific cognitive process are correlated against fMRI data from subjects performing a relevant task to determine brain regions showing a response profile consistent with that model. A key advantage of this technique over more conventional neuroimaging approaches is that model-based fMRI can provide insights into how a particular cognitive process is implemented in a specific brain area as opposed to merely identifying where a particular process is located. This review will briefly summarize the approach of model-based fMRI, with reference to the field of reward learning and decision making, where computational models have been used to probe the neural mechanisms underlying learning of reward associations, modifying action choice to obtain reward, as well as in encoding expected value signals that reflect the abstract structure of a decision problem. Finally, some of the limitations of this approach will be discussed.

How to make a mesodiencephalic dopaminergic neuron

Dopaminergic neurons located in the ventral mesodiencephalon are essential for the control of voluntary movement and the regulation of emotion, and are severely affected in neurodegenerative diseases such as Parkinson's disease. Recent advances in molecular biology and mouse genetics have helped to unravel the mechanisms involved in the development of mesodiencephalic dopaminergic (mdDA) neurons, including their specification, migration and differentiation, as well as the processes that govern axonal pathfinding and their specific patterns of connectivity and maintenance. Here, we follow the developmental path of these neurons with the goal of generating a molecular code that could be exploited in cell-replacement strategies to treat diseases such as Parkinson's disease.

Understanding the Placebo Effect: Contributions from Neuroimaging

Neuroimaging studies have provided a major contribution to our understanding of the mechanisms of the placebo effect in neurological and psychiatric disorders. Expectation of symptom improvement has long been believed to play a critical role in the placebo effect, and is associated with increased endogenous striatal dopamine release in Parkinson's disease and increased endogenous opioid transmission in placebo analgesia. Evidence from positron emission tomography and functional magnetic resonance imaging studies suggests that expectations of symptom improvement are driven by frontal cortical areas, particularly the dorsolateral prefrontal, orbitofrontal, and anterior cingulate cortices. The ventral striatum is involved in the expectation of rewarding stimuli and, together with the prefrontal cortex, has also been shown to play an important role in the placebo-induced expectation of therapeutic benefit. Understanding the mechanisms of the placebo effect has important implications for treatment of several medical conditions, including depression, pain, and Parkinson's disease.

Aging of the rat mesostriatal system: Differences between the nigrostriatal and the mesolimbic compartments

The impairment of the mesostriatal dopaminergic system has been considered responsible for motor and affective disturbances associated with aging and a risk factor for Parkinson's disease. However, the basic mechanisms underlying this phenomenon are still unknown. Here we used biochemical, molecular and morphological techniques directed at detecting flaws in the dopamine synthesis route and signs of dopaminergic degeneration in the rat mesostriatal system during normal aging. We found two different age-related processes. One is characterized by a dopa decarboxylase decrease, and involves both the nigrostriatal and mesolimbic compartments, and is responsible for a moderate dopamine loss in the dorsal striatum, where other parameters of dopamine synthesis are not affected. The other is characterized by axonal degeneration with aggregation of phosphorylated forms of tyrosine hydroxylase (TH) and amyloid precursor protein in degenerate terminals, and alpha-synuclein in their original somata. This process is restricted to mesolimbic regions and is responsible for the decline of TH activity and l-dopa levels and the greater decrease in dopamine levels in this compartment. These findings suggest that both the nigrostriatal and the mesolimbic systems are vulnerable to aging, but in contrast to what occurs in Parkinson's disease, the mesolimbic system is more vulnerable to aging than the nigrostriatal one.

Caudate nucleus volume and its clinical and cognitive correlations in first episode schizophrenia

OBJECTIVE

Striatal dysfunction has been traditionally implicated in the pathophysiology of schizophrenia. The purpose of this study is to examine the relationship between caudate nucleus volumes and clinical and cognitive features of schizophrenic patients in an early phase of their illness.

METHODS

Caudate nucleus volumes in previously untreated first episode patients with non-affective psychosis (N=76) and healthy comparison subjects (N=45) were measured. Caudate nucleus volume in the right and left hemispheres were automatically segmented and analyzed using BRAINS2. Analysis of covariance was used to control for intracranial volume. Severity of clinical symptoms was assessed using SAPS and SANS total scores. The relationship between cognitive dimensions, and caudate nucleus volume was evaluated. Finally, we examined the correlation between caudate volumes and the duration of untreated illness (DUI), duration of untreated psychosis (DUP) and duration of prodrome period (DPP).

RESULTS

Right, left, and total caudate nucleus volumes did not differ significantly between patients and controls. Those patients with a longer DUP have smaller caudate nucleus. In addition, caudate nucleus volume was positively correlated with the severity of psychotic symptomatology. No significant associations were found between caudate nucleus volume and cognitive functioning.

CONCLUSION

This group of first episode schizophrenia patients did not exhibit significant volumetric anomalies of the caudate nucleus. Despite this lack of volumetric abnormalities, a delay in receiving antipsychotic treatment and the severity of initial positive symptomatology were significantly associated with reduced caudate volume.

Reinforcement learning signals predict future decisions

Optimal behavior in a competitive world requires the flexibility to adapt decision strategies based on recent outcomes. In the present study, we tested the hypothesis that this flexibility emerges through a reinforcement learning process, in which reward prediction errors are used dynamically to adjust representations of decision options. We recorded event-related brain potentials (ERPs) while subjects played a strategic economic game against a computer opponent to evaluate how neural responses to outcomes related to subsequent decision-making. Analyses of ERP data focused on the feedback-related negativity (FRN), an outcome-locked potential thought to reflect a neural prediction error signal. Consistent with predictions of a computational reinforcement learning model, we found that the magnitude of ERPs after losing to the computer opponent predicted whether subjects would change decision behavior on the subsequent trial. Furthermore, FRNs to decision outcomes were disproportionately larger over the motor cortex contralateral to the response hand that was used to make the decision. These findings provide novel evidence that humans engage a reinforcement learning process to adjust representations of competing decision options.

Selective enrichment of DJ-1 protein in primate striatal neuronal processes: implications for Parkinson's disease

Mutations in DJ-1 cause autosomal recessive, early-onset Parkinson's disease (PD). The precise function and distribution of DJ-1 in the central nervous system remain unclear. In this study, we performed a comprehensive analysis of DJ-1 expression in human, monkey, and rat brains with antibodies that recognize distinct, evolutionarily conserved epitopes of DJ-1. We found that DJ-1 displays region-specific neuronal and glial labeling in human and nonhuman primate brain, sharply contrasting with the primarily neuronal expression pattern observed throughout rat brain. Further immunohistochemical analysis of DJ-1 expression in human and nonhuman primate brains showed that DJ-1 protein is expressed in neurons within the substantia nigra pars compacta and striatum, two regions critically involved in PD pathogenesis. Moreover, immunoelectron microscopic analysis revealed a selective enrichment of DJ-1 within primate striatal axons, presynaptic terminals, and dendritic spines with respect to the DJ-1 expression in prefrontal cortex. Together, these findings indicate neuronal and synaptic expression of DJ-1 in primate subcortical brain regions and suggest a physiological role for DJ-1 in the survival and/or function of nigral-striatal neurons.

A physiologically plausible model of action selection and oscillatory activity in the basal ganglia

The basal ganglia (BG) have long been implicated in both motor function and dysfunction. It has been proposed that the BG form a centralized action selection circuit, resolving conflict between multiple neural systems competing for access to the final common motor pathway. We present a new spiking neuron model of the BG circuitry to test this proposal, incorporating all major features and many physiologically plausible details. We include the following: effects of dopamine in the subthalamic nucleus (STN) and globus pallidus (GP), transmission delays between neurons, and specific distributions of synaptic inputs over dendrites. All main parameters were derived from experimental studies. We find that the BG circuitry supports motor program selection and switching, which deteriorates under dopamine-depleted and dopamine-excessive conditions in a manner consistent with some pathologies associated with those dopamine states. We also validated the model against data describing oscillatory properties of BG. We find that the same model displayed detailed features of both gamma-band (30-80 Hz) and slow (approximately 1 Hz) oscillatory phenomena reported by Brown et al. (2002) and Magill et al. (2001), respectively. Only the parameters required to mimic experimental conditions (e.g., anesthetic) or manipulations (e.g., lesions) were changed. From the results, we derive the following novel predictions about the STN-GP feedback loop: (1) the loop is functionally decoupled by tonic dopamine under normal conditions and recoupled by dopamine depletion; (2) the loop does not show pacemaking activity under normal conditions in vivo (but does after combined dopamine depletion and cortical lesion); (3) the loop has a resonant frequency in the gamma-band.

Coexpression of prodynorphin and corticotrophin-releasing hormone in the rat central amygdala: Evidence of two distinct endogenous opioid systems in the lateral division

The lateral subdivision of the central nucleus of the amygdala (CeA) comprises two groups of gamma-aminobutyric acid (GABA) neurons that express corticotrophin-releasing hormone (CRH) and enkephalin. Regulation of the expression and release of these neuropeptides by glucocorticoids and other factors has been suggested to have a regulatory function on the diverse somatic, autonomic, and neuroendocrine responses that are coordinated by the CeA. Because another opioid peptide, dynorphin, has been reported to be also expressed by neurons in the lateral CeA, this study examined the neuronal expression of this kappa-opioid (KOP) receptor-preferring ligand by using immunohistochemistry for the precursor peptide prodynorphin. Prodynorphin neurons in the extended amygdala were observed mostly in the medial and central regions of the lateral CeA and the oval of the bed nucleus of the stria terminalis (BST). About one-third of the prodynorphin neurons in the CeA coexpressed CRH, whereas no coexpression with CRH was detected in the BST. Prodynorphin was not expressed by calbindin neurons in the medial part of the lateral CeA, and indirect evidence suggested that it was not expressed by enkephalin neurons. Coexpression of prodynorphin in extrahypothalamic CRH neurons in the CeA could provide an anatomical basis for regulation of the stress responses and other CRH-related functions by the brain dynorphin/KOP receptor system.

The role of protein synthesis in striatal long-term depression

Long-term depression (LTD) at the corticostriatal synapse is postsynaptically induced but presynaptically expressed, the depression being a result of retrograde endocannabinoid signaling that activates presynaptic cannabinoid CB1 receptors and reduces the probability of glutamate release. To study the role of protein synthesis in striatal LTD, we used a striatum-only preparation in which the presynaptic cell body is cut off, leaving intact only its axons, whose terminals synapse on medium spiny neurons. LTD (duration >150 min) was induced in this preparation, thus providing evidence that transcription in the presynaptic cell nucleus is not necessary for this form of plasticity. The maintenance of striatal LTD, however, was blocked by bath application of protein translation inhibitors but not by the same inhibitors loaded into the postsynaptic cell. These results suggest that local translation is critical for the expression of striatal LTD, distinguishing this form of mammalian synaptic plasticity from other forms that require postsynaptic protein synthesis. Possible roles of axonal or glial translation in striatal LTD are considered.

Separate neural substrates for skill learning and performance in the ventral and dorsal striatum

It is widely accepted that the striatum of the basal ganglia is a primary substrate for the learning and performance of skills. We provide evidence that two regions of the rat striatum, ventral and dorsal, play distinct roles in instrumental conditioning (skill learning), with the ventral striatum being critical for learning and the dorsal striatum being important for performance but, notably, not for learning. This implies an actor (dorsal) versus director (ventral) division of labor, which is a new variant of the widely discussed actor-critic architecture. Our results also imply that the successful performance of a skill can ultimately result in its establishment as a habit outside the basal ganglia.

Dopamine reduction of GABA currents in striatal medium-sized spiny neurons is mediated principally by the D(1) receptor subtype

Dopamine modulates voltage- and ligand-gated currents in striatal medium-sized neurons (MSNs) through the activation of D1- and D2-like family receptors. GABA(A) receptor-mediated currents are reduced by D1 receptor agonists, but the relative contribution of D(1) or D(5 )receptors in this attenuation has been elusive due to the lack of selective pharmacological agents. Here we examined GABA(A) receptor-mediated currents and the effects of D1 agonists on MSNs from wildtype and D(1) or D(5 )receptor knockout (KO) mice. Immunohistochemical and single-cell RT-PCR studies demonstrated a lack of compensatory effects after genetic deletion of D(1) or D(5) receptors. However, the expression of GABA(A )receptor alpha1 subunits was reduced in D(5) KO mice. At the functional level, whole-cell patch clamp recordings in dissociated MSNs showed that GABA peak current amplitudes were smaller in cells from D(5) KO mice indicating that lack of this receptor subtype directly affected GABA(A)-mediated currents. In striatal slices, addition of a D1 agonist reduced GABA currents significantly more in D(5) KO compared to D(1) KO mice. We conclude that D(1) receptors are the main D1-like receptor subtype involved in the modulation of GABA currents and that D(5) receptors contribute to the normal expression of these currents in the striatum.

Comparing nucleus accumbens and dorsal striatal dopamine responses to self-administered cocaine in naïve rats

Dopamine (DA) responses in the nucleus accumbens (NAcc) and dorsal striatum (DS) are commonly associated with different aspects of cocaine effects. Enhanced NAcc DA has been most convincingly linked with the positive reinforcing effects of cocaine, while DS DA is thought to mediate cocaine-induced motoric effects. Though several studies have shown NAcc DA enhancement following cocaine self-administration, very little work has examined the effects of cocaine self-administration on DS DA. In this study, DA levels in the NAcc and DS, and locomotor responses to a single self-administered cocaine injection (1.5mg/kg) were assessed in operant-trained, drug-naïve Sprague-Dawley rats. Locomotor activity, NAcc and DS DA levels increased significantly over baseline activity immediately after cocaine injection. However, while basal and cocaine-stimulated NAcc DA concentrations (nM) were significantly greater than DS DA levels, the magnitude of response was statistically comparable between brain regions. These findings indicate that, though both the NAcc and DS are importantly involved in the dopaminergic response to self-administered cocaine in drug-naïve rats, basal DA differences in dialysis data are obscured by statistical conversions to baseline percentages.

The role of the basal ganglia in habit formation

Many organisms, especially humans, are characterized by their capacity for intentional, goal-directed actions. However, similar behaviours often proceed automatically, as habitual responses to antecedent stimuli. How are goal-directed actions transformed into habitual responses? Recent work combining modern behavioural assays and neurobiological analysis of the basal ganglia has begun to yield insights into the neural basis of habit formation.

Heterogeneous Dopamine Neurochemistry in the Striatum: The Fountain-Drain Matrix

In contrast to the relatively high attention paid to the structural heterogeneity of striatal dopamine (DA) innervation, little attention has been focused on the possible striatal heterogeneity for release and uptake of DA. By using amperometric methods, we found striatal regions showing a DA decrease during the medial forebrain bundle stimulation (drain areas) near to other zones that showed an increase in DA concentration (fountain areas). Both areas were intermixed to form a tridimensional matrix to regulate DA concentration throughout the striatum (fountain-drain matrix). The response to electrical stimuli of different amplitudes and durations and to different drugs (alpha-methyl-l-tyrosine, cocaine, gamma-butyrolactone, and haloperidol) suggests that regional differences for both DA release/DA uptake and DA cell firing autoregulation are behind the striatal fountain-drain matrix. The high diversity of DA activity observed in the striatum is a new framework for analyzing experimental and clinical phenomena.

Reduction of caudate nucleus volumes in neuroleptic-naïve female subjects with schizotypal personality disorder

BACKGROUND

The caudate nucleus might contribute to the psychopathological and cognitive deficits observed in schizotypal personality disorder (SPD), a schizophrenia spectrum disorder. Here we focused on female patients, because this group is underrepresented in studies of SPD and schizophrenia, and we might learn more about the caudate and clinical and cognitive impairments that are unique to female patients diagnosed with SPD.

METHODS

Magnetic resonance imaging scans, obtained on a 1.5-T magnet with 1.5-mm contiguous slices, were used to measure the caudate in 32 neuroleptic-naïve women with SPD and in 29 female normal comparison subjects. Subjects were group-matched for age, parental socioeconomic status, and intelligence quotient.

RESULTS

We found significantly reduced left and right caudate relative volume (8.3%, 7.7%) in female SPD subjects compared with normal comparison subjects. In female SPD subjects, we found significant correlations between smaller total caudate relative volume and worse performance on the Wisconsin Card Sorting test (nonperseverative errors) and on the California Verbal Learning Test (verbal memory and learning), and significant correlations between smaller total caudate relative volume and both positive and negative symptoms on the Structured Interview for Schizotypy.

CONCLUSIONS

These findings demonstrate that, for female SPD subjects, smaller caudate volume is associated with poorer cognitive performance and more schizotypal symptomatology.

Relation between rotation in the 6-OHDA lesioned rat and dopamine loss in striatal and substantia nigra subregions

The relation between the rotation response to drug-induced activation of the dopamine (DA) receptor in the rat unilaterally lesioned with 6-hydroxydopamine (6-OHDA) in the substantia nigra (SN) and the loss of DA in subregions of the SN and caudate-putamen (C/PUT) is not clear. Here this relation was examined in 23 rats classified as rotators to amphetamine (5 mg/kg). After their response was characterized in terms of ipsilateral rotation, contralateral rotation, and oral stereotypy in one place, they were divided into high, medium, low, and very low rotators. The loss of DA in each group was visualized on brain sections immunoreacted to tyrosine hydroxylase (TH). The density of the TH label on the side of the lesion was compared to that on the intact side. In the ventral midbrain, the density was determined in the SN subdivided into far lateral, lateral, central, and medial subregions and also in the ventral tegmental area (VTA). In the forebrain, it was determined in the C/PUT subdivided into lateral, central, and medial subregions and also in the nucleus accumbens (ACC). These measurements led to three principal findings. The first was a positive overall correlation between rotation and loss of TH label. The second was a correlation between rotation and penetration of the loss from the lateral subregions into more medial areas. The third was a larger loss in SN and VTA (midbrain) than in C/PUT and ACC (forebrain). These findings show that rotation depended not only on the overall loss of DA but also on its distribution across subregions. The loss in the lateral subregion, always the largest regardless of the rate of rotation, may have been the first step in inducing the motor abnormality, and the loss in the central and medial subregions may have served to enhance the abnormality due to the loss in the lateral subregion.

Properties and opioid inhibition of mesolimbic dopamine neurons vary according to target location

The mesolimbic dopamine system, which mediates the rewarding properties of nearly all drugs of abuse, originates in the ventral tegmental area (VTA) and sends major projections to both the nucleus accumbens (NAc) and the basolateral amygdala (BLA). To address whether differences occur between neurons that project to these separate areas, retrograde microspheres were injected to either the BLA or the NAc of DBA/2J mice. Whole-cell recordings were made from labeled VTA dopamine neurons. We found that identified neurons that projected to the BLA and NAc originated within different quadrants of the VTA with neither group exhibiting large-amplitude h-currents. Neurons that projected to the NAc exhibited a greater outward current in response to the kappa-opioid agonist (5alpha,7alpha,8alpha)-(+)-N-methyl-N-[7-(pyrrolidinyl)-1-oxaspiro [4,5]dec-8-yl]-benzeneacetamide (U69593; 200 nM), whereas neurons that projected to the BLA exhibited greater inhibition to the mu/delta opioid agonist [Met5] enkephalin (ME; 3 microM). In addition, we found that the presynaptic inhibition of GABAergic transmission at both GABAA and GABAB receptors was differentially regulated by U69593 between the two groups. When dopamine IPSCs were examined, U69593 caused a greater inhibition in NAc- than BLA-projecting neurons. ME had no effect on either. Finally, the regulation of extracellular dopamine by dopamine uptake transporters was equal across the VTA. These results suggest that opioids differentially inhibit mesolimbic neurons depending on their target projections. Identifying the properties of projecting mesolimbic VTA dopamine neurons is crucial to understanding the action of drugs of abuse.

Making working memory work: a computational model of learning in the prefrontal cortex and basal ganglia

The prefrontal cortex has long been thought to subserve both working memory (the holding of information online for processing) and executive functions (deciding how to manipulate working memory and perform processing). Although many computational models of working memory have been developed, the mechanistic basis of executive function remains elusive, often amounting to a homunculus. This article presents an attempt to deconstruct this homunculus through powerful learning mechanisms that allow a computational model of the prefrontal cortex to control both itself and other brain areas in a strategic, task-appropriate manner. These learning mechanisms are based on subcortical structures in the midbrain, basal ganglia, and amygdala, which together form an actor-critic architecture. The critic system learns which prefrontal representations are task relevant and trains the actor, which in turn provides a dynamic gating mechanism for controlling working memory updating. Computationally, the learning mechanism is designed to simultaneously solve the temporal and structural credit assignment problems. The model's performance compares favorably with standard backpropagation-based temporal learning mechanisms on the challenging 1-2-AX working memory task and other benchmark working memory tasks.

Glutamate release in the nucleus accumbens during competitive presentation of aversive and appetitive stimuli

In vivo intracerebral microdialysis/HPLC studies on Sprague-Dawley rats showed that simultaneous presentation of food and a tone previously combined with a pain stimulus was accompanied by transient elevation of the glutamate level in the extracellular space of the nucleus accumbens, the extent of this increase being proportional to the latent period of initiation of the act of eating. Conversely, sequential presentation of the conditioned aversive signal followed by food and isolated presentation of the conditioned aversive signal alone were not accompanied by changes in the level of extracellular glutamate. It is suggested that glutamate release in the nucleus accumbens in conditions of competition between presentation of food and a tone previously combined with pain stimulation is not related to the process of selecting between the aversive and appetitive strategies of behavior, but may be associated with inhibition of preparation to perform or performance of the food-related behavior.

Enhanced sensitivity of dopaminergic neurons to rotenone-induced toxicity with aging

Rotenone has been reported to induce various degrees of Parkinsonism in rats. We tested whether advancing age alters the sensitivity of dopaminergic neurons to rotenone. A low, systemic dose of rotenone had no effect on young rats, but led to a 20-30% reduction of tyrosine hydroxylase-positive neurons in the substantia nigra of older rats. The effect was specific to nigral dopaminergic neurons and may be associated with the increase of glial cell activation in older rats. These data suggest that age enhances the sensitivity of dopaminergic neurons to rotenone and should be considered when assessing models of Parkinson's disease.

Molecular mapping of striatal subdivisions in juvenile Macaca Mulata

The striatum of the primate brain can be subdivided into three distinct anatomical subregions: caudate (CAU), putamen (PUT), and ventral striatum (VS). Although these subregions share several anatomical connections, cell morphological, and histochemical features, they differ considerably in their vulnerability to different neurological and psychiatric diseases, and these brain regions have significantly different functions in health and disease. In order to better understand the molecular underpinnings of the different disease and functional vulnerabilities, transcriptional profiles were generated from the CAU, PUT, and VS of five juvenile rhesus macaques (Macaca mulatta) using human cDNA neuromicroarrays containing triplicate spots of 1227 cDNAs. Differences in microarray gene expression were assessed using z score analysis and 1.5-fold change between paired subregions. Clustering of genes based on dissimilarity of expression patterns between regions revealed subregion specific expression profiles encoding G-protein-coupled receptor signaling transcripts, transcription factors, kinases and phosphatases, and cell signaling and signal transduction transcripts. Twelve transcripts were examined using quantitative real-time PCR (qPCR), and 81% demonstrated alterations similar to those seen with microarray analysis, some of which were statistically significant. Subregion specific transcription profiles support the anatomical differentiation and potential disease vulnerabilities of the respective subregions.

Brain activation during craving for alcohol measured by positron emission tomography

OBJECTIVE

Craving for alcohol is probably involved in acquisition and maintenance of alcohol dependence to a substantial degree. However, the brain substrates and mechanisms that underlie alcohol craving await more detailed elucidation.

METHOD

Positron emission tomography was used to map regional cerebral blood flow (CBF) in 21 detoxified patients with alcohol dependence during exposure to alcoholic and non-alcoholic beverages.

RESULTS

During the alcohol condition compared with the control condition, significantly increased CBF was found in the ventral putamen. Additionally, activated areas included insula, dorsolateral prefrontal cortex and cerebellum. Cerebral blood flow increase in these regions was related to self-reports of craving assessed in the alcoholic patients.

CONCLUSIONS

In this investigation, cue-induced alcohol craving was associated with activation of brain regions particularly involved in brain reward mechanisms, memory and attentional processes. These results are consistent with studies on craving for other addictive substances and may offer strategies for more elaborate studies on the neurobiology of addiction.

Possible Mechanisms of the Involvement of Dopaminergic Cells and Cholinergic Interneurons in the Striatum in the Conditioned-Reflex Selection of Motor Activity

A possible mechanism for the involvement of cholinergic interneurons in the striatum and dopaminergic cells in the substantia nigra in the selection from among several types of motor activity during learning is proposed. Selection is triggered by simultaneous increases in the activity of dopaminergic neurons and a pause in the activity of cholinergic interneurons in response to the conditioned signal. The appearance of the pause may facilitate activation of GABAergic interneurons in the striatum and the action of dopamine on D2 receptors on cholinergic interneurons. Differently directed changes in dopamine and acetylcholine levels synergistically modulate the efficiency of corticostriatal inputs, such that the rules for modulation of the "strong" and "weak" inputs are opposite in sign. The subsequent reorganization of neuron activity in the cortex-basal ganglia-thalamus-cortex circuit leads to increased activity in those cortical neurons providing "strong" innervation to the striatum with simultaneous decreases in the activity of neurons providing "weak" innervation to the striatum, which may underlie the selection of the movement reaction, in which the neocortex is involved. It follows from this model that if the delay between the conditioned and unconditioned stimuli is not longer than the latent period of the reactions of dopaminergic and cholinergic cells (about 100 msec), selection of movement activity in response to the conditioned signal and learning is hindered.

A mechanistic account of striatal dopamine function in human cognition: Psychopharmacological studies with cabergoline and haloperidol

The authors test a neurocomputational model of dopamine function in cognition by administering to healthy participants low doses of D2 agents cabergoline and haloperidol. The model suggests that DA dynamically modulates the balance of Go and No-Go basal ganglia pathways during cognitive learning and performance. Cabergoline impaired, while haloperidol enhanced, Go learning from positive reinforcement, consistent with presynaptic drug effects. Cabergoline also caused an overall bias toward Go responding, consistent with postsynaptic action. These same effects extended to working memory and attentional domains, supporting the idea that the basal ganglia/dopamine system modulates the updating of prefrontal representations. Drug effects interacted with baseline working memory span in all tasks. Taken together, the results support a unified account of the role of dopamine in modulating cognitive processes that depend on the basal ganglia.

Influence of oestrogenic compounds on monoamine transporters in rat striatum

Oestrogens have been reported to modulate rat membrane (DAT) and vesicular (VMAT(2)) dopamine transporters. A recent pilot study of postmenopausal women showed that chronic oestrogen replacement therapy increases striatal DAT. In the present study, we first investigated whether the oestrogen receptors alpha and beta mediate the effects of oestradiol on DAT and VMAT(2). Two days after ovariectomy, Sprague-Dawley rats were treated for 2 weeks with oestradiol or specific ligands for oestrogen receptor alpha, 4,4',4''-(4-propyl-[1H]-pyrazole-1,3,5-triyl)trisphenol (PPT) or oestrogen receptor beta, 2,3-bis(4-hydroxyphenyl)-propionitrile (DPN). Ovariectomy caused a decrease in [(125)I]-3beta-(4-iodophenyl)-tropane-2beta-carboxylic acid isopropyl ester ([(125)I] RTI-121) specific binding to DAT transporters in the middle striatum compared to values for intact rats, and this was reversed by oestradiol replacement therapy. DPN, but not PPT, mimicked the effect of oestradiol. [(125)I] RTI-121 specific binding in the anterior and posterior striatum was not affected by ovariectomy or any of the drug treatments. Second, we investigated whether oestradiol increased DAT specific binding after a longer period of hormonal withdrawal (a model of hormonal withdrawal at menopause) and whether the selective oestrogen receptor modulators (SERMs), tamoxifen and raloxifene, could reproduce the oestradiol-induced increase of [(125)I] RTI-121 specific binding in long-term ovariectomised rats. Four months after ovariectomy, Sprague-Dawley rats were treated for 2 weeks with oestradiol, tamoxifen or raloxifene, and then killed. Ovariectomy decreased [(3)H] RTI-121 specific binding to DAT transporters in the middle striatum compared to values for intact rats. Treatment with oestradiol, tamoxifen and raloxifene reversed this effect. [(125)I] RTI-121 specific binding in anterior and posterior striatum was not affected by ovariectomy or treatment with oestrogen receptor ligands. In both experiments, neither ovariectomy nor the oestrogenic treatments modulated striatal [(3)H] tetrahydrobenazine specific binding to VMAT(2). Overall, these results suggest that oestrogen receptor beta mediates the oestradiol-induced increase of striatal DAT and that oestradiol can increase DAT density even after long-term steroid withdrawal. The results also support the premise that the SERMs tamoxifen and raloxifene exert oestrogenic agonist effects in the brain.

Neurons in dopamine-rich areas of the rat medial midbrain predominantly encode the outcome-related rather than behavioural switching properties of conditioned stimuli

Midbrain dopamine neurons are phasically activated by a variety of sensory stimuli. It has been hypothesized that these activations contribute to reward prediction or behavioural switching. To test the latter hypothesis we recorded from 131 single neurons in the ventral tegmental area and retrorubral field of thirsty rats responding during a modified go/no-go task. One-quarter (n = 33) of these neurons responded to conditioned stimuli in the task, which varied according to the outcome with which they were associated (saccharin or quinine solution) and according to whether they triggered a switch in the ongoing sequence of the animal's behaviour ('behavioural switching'). Almost half the neurons (45%) responded differentially to saccharin- vs. quinine-conditioned stimuli; the activity of a minority (15%) correlated with an aspect of behavioural switching (mostly exhibiting changes from baseline activity in the absence of a behavioural switch) and one-third (33%) encoded various outcome-switch combinations. The strongest response was excitation to the saccharin-conditioned stimulus. Additionally, a proportion (38%) of neurons responded during outcome delivery, typically exhibiting inhibition during saccharin consumption. The neurons sampled did not fall into distinct clusters on the basis of their electrophysiological characteristics. However, most neurons that responded to the outcome-related properties of conditioned stimuli had long action potentials (> 1.2 ms), a reported characteristic of dopamine neurons. Moreover, responses to saccharin-conditioned stimuli were functionally akin to dopamine responses found in the macaque and rat nucleus accumbens responses observed within the same task. In conclusion, our data are more consistent with the reward-prediction than the behavioural switching hypothesis.

Dopamine-glutamate reciprocal modulation of release and motor responses in the rat caudate-putamen and nucleus accumbens of "intact" animals

Functional interactions between dopaminergic neurotransmission and glutamatergic neurotransmission are well known to play a crucial integrative role in the striatum, the major input structure of the basal ganglia now widely recognized to contribute to the control of motor activity and movements but also to the processing of cognitive and limbic functions. However, the nature of these interactions is still a matter of debate and controversy. This review (1) summarizes anatomical data on the distribution of dopaminergic and glutamatergic receptors in the striatum-accumbens complex, (2) focuses on the dopamine-glutamate interactions in the modulation of each other's release in the striatum-accumbens complex, and (3) examines the dopamine-glutamate interactions in the entire striatum involved in the control of locomotor activity. The effects of dopaminergic and glutamatergic receptor selective agonists and antagonists on dopamine and glutamate release as well on motor responses are analyzed in the entire striatum, by reviewing both in vitro and in vivo data. Regarding in vivo data, only findings from focal injections studies in the nucleus accumbens or the caudate-putamen of "intact" animals are reviewed. Altogether, the available data demonstrate that dopamine and glutamate do not uniformly interact to modulate each others' release and postsynaptic modulation of striatal output neurons. Depending on the receptor subtypes involved, interactions between dopaminergic and glutamatergic transmission vary as a multiple and complex combination of tonic, phasic, facilitatory, and inhibitory properties.

Error-related negativity predicts reinforcement learning and conflict biases

The error-related negativity (ERN) is an electrophysiological marker thought to reflect changes in dopamine when participants make errors in cognitive tasks. Our computational model further predicts that larger ERNs should be associated with better learning to avoid maladaptive responses. Here we show that participants who avoided negative events had larger ERNs than those who were biased to learn more from positive outcomes. We also tested for effects of response conflict on ERN magnitude. While there was no overall effect of conflict, positive learners had larger ERNs when having to choose among two good options (win/win decisions) compared with two bad options (lose/lose decisions), whereas negative learners exhibited the opposite pattern. These results demonstrate that the ERN predicts the degree to which participants are biased to learn more from their mistakes than their correct choices and clarify the extent to which it indexes decision conflict.

Direct receptor cross-talk can mediate the modulation of excitatory and inhibitory neurotransmission by dopamine

Dopamine is a neurotransmitter that can regulate both excitatory and inhibitory fast synaptic transmission. The overlapping dopaminergic, glutamatergic, and GABAergic systems provide a basis for the interaction between these three neurotransmitters. Although there is considerable evidence for the involvement of second-messenger systems to mediate receptor cross-talk between these receptor systems, there is emerging evidence that receptors can interact through direct protein-protein interactions. The functional implications and overall significance of the dopamine/glutamate/GABA interactions will be examined.

A study on the role of the dorsal striatum and the nucleus accumbens in allocentric and egocentric spatial memory consolidation

There is now accumulating evidence that the striatal complex in its two major components, the dorsal striatum and the nucleus accumbens, contributes to spatial memory. However, the possibility that different striatal subregions might modulate specific aspects of spatial navigation has not been completely elucidated. Therefore, in this study, two different learning procedures were used to determine whether the two striatal components could be distinguished on the basis of their involvement in spatial learning using different frames of reference: allocentric and egocentric. The task used involved the detection of a spatial change in the configuration of four objects placed in an arena, after the mice had had the opportunity to experience the objects in a constant position for three previous sessions. In the first part of the study we investigated whether changes in the place where the animals were introduced into the arena during habituation and testing could induce a preferential use of an egocentric or an allocentric frame of reference. In the second part of the study we performed focal injections of the N-methyl-d-aspartate (NMDA) receptors' antagonist, AP-5, within the two subregions immediately after training. The results indicate that using the two behavioral procedures, the animals rely on an egocentric and an allocentric spatial frame of reference. Furthermore, they demonstrate that AP-5 (37.5, 75, and 150 ng/side) injections into the dorsal striatum selectively impaired consolidation of spatial information in the egocentric but not in the allocentric procedure. Intra-accumbens AP-5 administration, instead, impaired animals trained using both procedures.

Molecular and cellular alterations in the Pitx3-deficient midbrain dopaminergic system

Parkinson's disease (PD) is a progressive neurodegenerative disorder characterized by loss of midbrain dopaminergic (mDA) neurons in the substantia nigra compacta (SNc). In order to provide insights into adaptive mechanisms of the mDA system in pathology, specific molecular and cellular parameters of the mDA system were studied in Pitx3-deficient Aphakia (ak) mice, which suffer from severe developmental failure of SNc mDA neurons. Here, we demonstrate differential changes in striatal gene expression, reflecting the specific neuronal loss in these mice. In addition, the neuronal activity of remaining mDA neurons in the ventral tegmental area (VTA) was significantly increased in ak mice. In conclusion, ak mice display specific molecular and cellular alterations in the mDA system that provide new insights in compensatory mechanisms present in mDA-associated disorders such as PD.

No sequence dependent modulation of the Simon effect in Parkinson's disease

This study sought to evaluate impaired response tendencies and modulation of automatic processes in Parkinson's disease (PD), utilising a saccadic Simon task with stimulus-response (S-R) compatibility determined on the basis of cue shape. The appearance of either a circle or a square in one of two boxes presented peripherally required the generation of a leftward or rightward horizontal saccade, respectively. These goal-directed responses were considered behaviourally relevant to an examination of visuospatial performance. Although response times are typically faster when stimulus and response are spatially compatible than when they are not, sequence-dependent modulation of this effect results in large differences between S-R compatible and S-R incompatible trials when stimulus and response are spatially compatible in the preceding trial, and reduced or absent differences when stimulus and response are spatially incompatible in the preceding trial. Unlike control subjects, PD patients demonstrated significantly shorter saccadic latencies overall, compared to a baseline condition involving endogenously-driven saccades. Patients also responded erroneously to cue stimuli with greater frequency. Analyses of both saccadic latency and errors to cue demonstrated a Simon effect (relatively faster response for S-R compatible trials), irrespective of the preceding trial. This suggests impaired modulation of the Simon effect in PD, consistent with predictions of inhibitory dysfunction, or impaired episodic memory. These results demonstrate the pivotal role of the basal ganglia in the regulation of context-dependent neural activity.

Striatal dopamine release in the rat during a cued lever-press task for food reward and the development of changes over time measured using high-speed voltammetry

Substantia nigra dopamine neuronal activity in the primate is thought to be related to the error in predicting reward delivery. Dopamine release in rat nucleus accumbens has been shown to increase in relation to drug/food-seeking behaviour. It is not known how the release of dopamine in the striatum corresponds to the many distinct steps of a rewarded, cued task (e.g. recognizing the cue, executing the behaviour, anticipating the reward, receiving the reward) and how dopamine release then changes over time as task performance improves. To investigate dopamine release during a rewarded, cued task and the development of changes in dopamine release over time, changes in extracellular striatal dopamine concentration during a rewarded, cued lever-press task were measured a few days every week for 5 months using high-speed in vivo voltammetry. Rats were trained to press a lever after a tone to obtain a food reward. The reaction time for the lever press decreased gradually as training continued. Changes in dopamine concentration were measured in the anterior striatum (ventral portion) during the task performance after an initial 6-day familiarization period, in which the animals learned that a lever press yielded food, and a 5-week period for surgery, recovery, and electrode preparation. During the task performance, dopamine concentration started to increase just after the cue, peaked near the time of the lever press, and returned to basal levels 1-2 s after the lever press. This pattern of changes in dopamine concentration was observed over the 5 months of testing, the peak dopamine concentration increasing steadily until peaking at week 7, at which time the task performance had not yet improved significantly from week 2. By week 13, task performance had significantly improved and peak dopamine concentration had begun to subside. Thus, the increase in dopamine concentration after the cue was highest while the task was not yet perfected and subsided toward the end of the learning process. It was concluded that striatal dopamine release during a cued lever-press task is relevant to the novelty of the conditions.

The striatum in the hedgehog tenrec: histochemical organization and cortical afferents

In order to get insight into the striopallidal organization in mammals with little differentiated brain the striatum of the lesser hedgehog tenrec (Afrotheria) was characterized histochemically and analysed with regard to its cortical afferents using axonal tracer substances. The majority of neocortical cells projecting to the striatum were found bilaterally in the layers 2 and 3 of the frontal hemisphere; caudalwards the relative number of cells increased somewhat in the upper layer 5. There was a topographical organization as far as the allocortical projections appeared confined to the ventral striatum, and the efferents from hippocampal, posterior paleocortical, somatosensory and audiovisual areas were distributed in largely different striatal territories. Projections from the anterior frontal cortex, on the other hand, terminated extensively upon the caudate-putamen and also involved the nucleus accumbens and the olfactory tubercle. In the latter region the molecular layer was especially involved. The entorhinal cortex also projected heavily to the olfactory tubercle but unlike other species it scarcely involved the nucleus accumbens. The cortical fibers were distributed in a relatively homogenous fashion within their striatal territory and there was little evidence for patches of high density terminations. Islands of low density labeling, however, were noted occasionally in the caudate-putamen. These islands were partly similar in size as the patches of neuropil staining obtained with anti-calretinin and anti-substance P. There were also hints for the presence of a shell-like region in the nucleus accumbens stained with anti-dopamine transporter and NADPh-diaphorase. The classical striosome-matrix markers such as calbindin, acetylcholinesterase and enkephalin, however, failed to reveal any compartmental organization.

The primate working memory networks

Working memory has long been associated with the prefrontal cortex, since damage to this brain area can critically impair the ability to maintain and update mnemonic information. Anatomical and physiological evidence suggests, however, that the prefrontal cortex is part of a broader network of interconnected brain areas involved in working memory. These include the parietal and temporal association areas of the cerebral cortex, cingulate and limbic areas, and subcortical structures such as the mediodorsal thalamus and the basal ganglia. Neurophysiological studies in primates confirm the involvement of areas beyond the frontal lobe and illustrate that working memory involves parallel, distributed neuronal networks. In this article, we review the current understanding of the anatomical organization of networks mediating working memory and the neural correlates of memory manifested in each of their nodes. The neural mechanisms of memory maintenance and the integrative role of the prefrontal cortex are also discussed.

Striatal expression of GDNF and differential vulnerability of midbrain dopaminergic cells

Glial cell line-derived neurotrophic factor (GDNF) is a member of the transforming growth factor-beta superfamily that when exogenously administrated exerts a potent trophic action on dopaminergic (DA) cells. Although we know a lot about its signalling mechanisms and pharmacological effects, physiological actions of GDNF on the adult brain remain unclear. Here, we have used morphological and molecular techniques, and an experimental model of Parkinson's disease in rats, to investigate whether GDNF constitutively expressed in the adult mesostriatal system plays a neuroprotective role on midbrain DA cells. We found that although all midbrain DA cells express both receptor components of GDNF (GFRalpha1 and Ret), those in the ventral tegmental area (VTA) and rostromedial substantia nigra (SNrm) also contain GDNF but not GDNFmRNA. The levels of GDNFmRNA are significantly higher in the ventral striatum (vSt), the target region of VTA and SNrm cells, than in the dorsal striatum (dSt), the target region of DA cells in the caudoventral substantia nigra (SNcv). After fluoro-gold injection in striatum, VTA and SNrm DA cells show triple labelling for tyrosine hydroxylase, GDNF and fluoro-gold, and after colchicine injection in the lateral ventricle, they become GDNF-immunonegative, suggesting that GDNF in DA somata comes from their striatal target. As DA cells in VTA and SNrm are more resistant than those in SNcv to intracerebroventricular injection of 6-OHDA, as occurs in Parkinson's disease, we can suggest that the fact that they project to vSt, where GDNF expression is significantly higher than in the dSt, is a neuroprotective factor involved in the differential vulnerability of midbrain DA neurons.

Behavioral and anatomical effects of quinolinic acid in the striatum of the hemiparkinsonian rat

Parkinson's disease (PD), a hypokinetic disorder, and Huntington's disease (HD), a hyperkinetic disorder, share the fact that in the motor pathways the dysfunction starts in the striatum. In PD the projection neurons are overactive due to decreased inhibitory regulation by lost dopamine afferents, while in HD the output from the striatum is insufficient due to loss of projection neurons. This study aimed to determine whether the introduction of a mild HD condition in the PD striatum can counter the hypokinetic condition. The experiment was carried out in the 6-OHDA rat model for PD in which amphetamine, 5 mg/kg, evokes an asymmetric rotation response toward the side of the 6-OHDA lesion (ipsilateral rotation). The response to amphetamine in this study was fractionated into multiple components and measured automatically. After baseline measurements, the subjects were divided into four groups. Group I was unilaterally sham-lesioned in medial, central, and lateral striatum. Group II was injected quinolinic acid (QA) 20 nM in medial, central, and lateral striatum. Group III was injected QA 60 nM in central striatum. Group IV was injected QA 120 nM in central striatum. The effects of QA were measured weekly. The sham lesions in Group I had no effects. In Group II, ipsilateral rotation was reduced and replaced by oral stereotypy, a competitive behavior. In Group III, ipsilateral rotation decreased, but to a lesser degree than in Group II. In Group IV, QA had no effects. Histological findings show a unilateral loss of tyrosine immunoreactive (TH) neurons in substantia nigra and of fibers in striatum in all subjects. In addition, in Group II the striatum was atrophied. These findings suggest that the shift in Group II from ipsilateral rotation to oral stereotypy after QA was due to reduced striatal output caused by a loss of projection neurons, a loss insufficient to induce HD symptoms, but sufficient to counter the PD condition.

Systems-level integration of interval timing and reaction time

Reaction time (RT) procedures are a prominent tool for the study of information processing by humans and other animals. The interpretation of how RT changes after manipulating the appropriate experimental variables has contributed to the contemporary understanding of a variety of cognitive constructs, including attention and memory. With the use of properly designed tasks, evaluating how RT is modified in response to various neural perturbations has become common within the realms of behavioral and cognitive neuroscience. One interesting observation made during both human and animal RT experiments is that the RT to a signal often speeds-up as more time is allotted to prepare for the signal's onset-referred to as the preparatory interval (PI) effect. In the human RT literature, the PI effect has been used as evidence for time estimation playing a fundamental role in the determination of RT. On the other hand, our theoretical understanding of time estimation remains largely divorced from the RT findings in the animal cognition literature. In order to bridge these different perspectives, we provide here a review of the behavioral parallels between RT and interval-timing experiments. Moreover, both the PI effect and interval timing are shown to be jointly influenced by neuropathologies such as Parkinson's disease in humans or dopamine-depleting brain lesions in experimental animals. The primary goal of this review is to consider human and animal RT experiments within the broader context of interval timing. This is accomplished by first integrating human RT theory with scalar timing theory-the leading model of interval timing. Following this, both RT and interval timing are discussed at a brain systems level insofar as these two processes share common neural substrates. Our conclusion is that interval timing and RT processes are in fact two sides of the same coin.

Localization of immunoreactivity for Deleted in Colorectal Cancer (DCC), the receptor for the guidance factor netrin-1, in ventral tier dopamine projection pathways in adult rodents

DCC (deleted in colorectal cancer)-the receptor of the netrin-1 neuronal guidance factor-is expressed and is active in the central nervous system (CNS) during development, but is down-regulated during maturation. The substantia nigra contains the highest level of netrin-1 mRNA in the adult rodent brain, and corresponding mRNA for DCC has also been detected in this region but has not been localized to any particular neuron type. In this study, an antibody raised against DCC was used to determine if the protein was expressed by adult dopamine neurons, and identify their distribution and projections. Significant DCC-immunoreactivity was detected in midbrain, where it was localized to ventrally displaced A9 dopamine neurons in the substantia nigra, and ventromedial A10 dopamine neurons predominantly situated in and around the interfascicular nucleus. Strong immunoreactivity was not detected in dopamine neurons found elsewhere, or in non-dopamine-containing neurons in the midbrain. Terminal fields selectively labeled with DCC antibody corresponded to known nigrostriatal projections to the dorsolateral striatal patches and dorsomedial shell of the accumbens, and were also detected in prefrontal cortex, septum, lateral habenular and ventral pallidum. The unique distribution of DCC-immunoreactivity in adult ventral midbrain dopamine neurons suggests that netrin-1/DCC signaling could function in plasticity and remodeling previously identified in dopamine projection pathways. In particular, a recent report that DCC is regulated through the ubiquitin-proteosome system via Siah/Sina proteins, is consistent with a potential involvement in genetic and sporadic forms of Parkinson's disease.

Expression of dopamine and vesicular monoamine transporters and differential vulnerability of mesostriatal dopaminergic neurons

Numerous studies suggest that the dopamine transporter (DAT), responsible for dopamine reuptake, may act as a vulnerability factor in the pathogenesis of Parkinson's disease (PD) and the vesicular monoamine transporter (VMAT2), responsible for its vesicular storage, as a neuroprotective factor. However, the relevance of each on the differential vulnerability of midbrain DA cells remains unknown. Here we studied the relationship between the expression pattern (mRNA and protein) of both transporters and the differential vulnerability of midbrain DA cells in a model of PD (intracerebroventricular injection of 6-OHDA in rats) and in monkey and human midbrain. Our results revealed that the expression patterns for VMAT2 mRNA and protein and DAT mRNA are similar, with the highest levels in the rostromedial region of substantia nigra (SNrm), followed by the caudoventral region of SN (SNcv), the ventral tegmental area and pigmented parabrabraquial nucleus (VTA/PBP), and finally the linear and interfascicular nuclei (Li/IF). In contrast, the expression of DAT protein in rats, monkeys, and humans followed a caudoventrolateral-to-rostrodorsomedial decreasing gradient (SNcv > SNrm > VTA/PBP > Li/IF), matching the degeneration profile observed after intracerebroventricular injection of 6-OHDA and in PD. In addition, DAT blockade made all midbrain DA cells equally resistant to 6-OHDA. These data indicate that DAT protein levels, but not DAT mRNA levels, are closely related to the differential vulnerability of midbrain DA cells and that this relationship is unaffected by the relative levels of VMAT2. Furthermore, the difference between DAT mRNA and protein profiles suggests internuclear differences in its posttransductional regulation.

How do memory systems interact? Evidence from human classification learning

Studies of human classification learning using functional neuroimaging have suggested that basal ganglia and medial temporal lobe memory systems may interact during learning. We review these results and outline a set of possible mechanisms for such interactions. Effective connectivity analyses suggest that interaction between basal ganglia and medial temporal lobe are mediated by prefrontal cortex rather than by direct connectivity between regions. A review of possible neurobiological mechanisms suggests that interactions may be driven by neuromodulatory systems in addition to mediation by interaction of inputs to prefrontal cortical neurons. These results suggest that memory system interactions may reflect multiple mechanisms that combine to optimize behavior based on experience.

Contrasting effects of dopamine and glutamate receptor antagonist injection in the nucleus accumbens suggest a neural mechanism underlying cue-evoked goal-directed behavior

Discriminative stimuli (DSs) inform animals that reward can be obtained contingent on the performance of a specific behavior. Such stimuli reinstate drug-seeking behavior, evoke dopamine release in the nucleus accumbens (NAc) and excite and inhibit specific subpopulations of NAc neurons. Here we show in rats that DSs can reinstate food-seeking behavior. In addition, we compare the effects of injecting dopamine receptor antagonists into the NAc with those of general NAc inactivation on the performance of a DS task. Selective antagonism of D1 receptors reduced responding to the DS and increased the latency to respond, whereas general inactivation of NAc neuronal activity increased the latency to respond to the DS and increased behaviors extraneous to the task, such as responding in the absence of cues and responding on the inactive lever. Based on these results and our previous findings that NAc neuronal responses to DSs are dependent on the ventral tegmental area, we propose a model for the functional role of NAc neurons in controlling behavioral responses to reward-predictive stimuli.

Organization of the thalamic projections of the striopallidum of the dog brain

Experiments based on double luminescent labeling were performed to study the distribution of labeled neurons in the thalamic nuclei depending on the injection sites of luminescent markers into functionally similar or functionally different areas of the striopallidum of 16 dogs. The organizational characteristics of the thalamo-striopallidal projection system in dogs provide evidence for its high level of specificity, as not only the motor and limbic areas of the striopallidum, but also its functionally related areas, receive separate inputs mainly from diverse cellular groups. The centromedian nucleus contained groups of diffusely mixed cells, labeled with different markers and innervating functionally diverse segments of the caudate nucleus. In the centromedian, parafascicular, central medial, and medial dorsal nuclei of the thalamus, projection neurons form analogous cell populations innervating different segments of the striopallidal structures belonging to the same functional system. These striopallidal areas receive projections from small numbers of neurons via axon collaterals.

Circadian-dependent effect of melatonin on dopaminergic D2 antagonist-induced hypokinesia and agonist-induced stereotypies in rats

Although a melatonin/dopamine relationship has been well established in nonmotor systems wherein dopamine and melatonin share an antagonist relationship, less clear is the role melatonin may play in extrapyramidal dopaminergic function. Therefore, the purpose of the present experiments was to examine the relationship between melatonin and the dopaminergic D2 receptor system and behavior. Hypokinesia was induced in male Sprague-Dawley rats with fluphenazine (D2 antagonist, 0.4 mg/kg ip) and stereotypies with apomorphine (D2 agonist, 0.6 mg/kg sc) during the light (1200 h) and dark (2200 h) phases. As expected, fluphenazine induced severe hypokinesia during the light phase (482 +/- 176 s); however, unexpectedly, fluphenazine-induced hypokinesia during the dark was almost nonexistent (25 +/- 6 s). Furthermore, melatonin treatment (30 mg/kg ip) produced a strong interaction with fluphenazine in that it reduced fluphenazine-induced hypokinesia by nearly 80% in the light (112 +/- 45 s) but paradoxically increased the minimal fluphenazine-induced hypokinesia in the dark by more than 60% (70 +/- 17 s). Melatonin also reduced apomorphine-induced stereotypies by nearly 40% in the light but had no effect in the dark. Taken together, these data show (1) a strong and unexpected nocturnal effect of fluphenazine on hypokinesia and (2) provide support for an antagonistic melatonin/dopaminergic interaction in the context of motor behavior and D2 receptor function which appears to be critically dependent on the light/dark status of the dopaminergic system.

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.