Striatonigrostriatal pathways in primates form an ascending spiral from the shell to the dorsolateral striatum

Obsessive-compulsive disorder as a disturbance of security motivation

The authors hypothesize that the symptoms of obsessive-compulsive disorder (OCD), despite their apparent nonrationality, have what might be termed an epistemic origin--that is, they stem from an inability to generate the normal "feeling of knowing" that would otherwise signal task completion and terminate the expression of a security motivational system. The authors compare their satiety-signal construct, which they term yedasentience, to various other senses of the feeling of knowing and indicate why OCD-like symptoms would stem from the abnormal absence of such a terminator emotion. In addition, they advance a tentative neuropsychological model to explain its underpinnings. The proposed model integrates many previous disparate observations and concepts about OCD and embeds it within the broader understanding of normal motivation.

The expanding effects of cocaine: studies in a nonhuman primate model of cocaine self-administration

Although neuroimaging investigations in human cocaine abusers have provided important insights into the brain changes that accompany drug use, the interpretation of reports in human abusers can be very difficult. Studies in nonhuman primates provide a way to systematically evaluate the structural and functional adaptations engendered by cocaine self-administration without the confounds of human research. Functional activity, measured with metabolic mapping methods, and markers of the dopamine system, assessed autoradiographically, were evaluated over the course of chronic cocaine self-administration (5 days, 3.3 months, and 15-22 months). Within the striatum the topography of these responses shifts dramatically over time. Changes in functional activity and alterations in the dopamine system occupy larger and larger portions of dorsal and ventral striatum with increasing durations of cocaine exposure. The growing impact of cocaine suggests that the elements of the behavioral repertoire outside of the influence of cocaine become smaller and smaller with increasing durations of exposure to drug use resulting in cocaine's dominance over all aspects of the addict's life.

The primate basal ganglia: parallel and integrative networks

The basal ganglia and frontal cortex operate together to execute goal directed behaviors. This requires not only the execution of motor plans, but also the behaviors that lead to this execution, including emotions and motivation that drive behaviors, cognition that organizes and plans the general strategy, motor planning, and finally, the execution of that plan. The components of the frontal cortex that mediate these behaviors, are reflected in the organization, physiology, and connections between areas of frontal cortex and in their projections through basal ganglia circuits. This comprises a series of parallel pathways. However, this model does not address how information flows between circuits thereby developing new learned behaviors (or actions) from a combination of inputs from emotional, cognitive, and motor cortical areas. Recent anatomical evidence from primates demonstrates that the neuro-networks within basal ganglia pathways are in a position to move information across functional circuits. Two networks are: the striato-nigral-striatal network and the thalamo-cortical-thalamic network. Within each of these sets of connected structures, there are both reciprocal connections linking up regions associated with similar functions and non-reciprocal connections linking up regions that are associated with different cortical basal ganglia circuits. Each component of information (from limbic to motor outcome) sends both feedback connection, and also a feedforward connection, allowing the transfer of information. Information is channeled from limbic, to cognitive, to motor circuits. Action decision-making processes are thus influenced by motivation and cognitive inputs, allowing the animal to respond appropriate to environmental cues.

The nucleus accumbens: a target for deep brain stimulation in obsessive-compulsive- and anxiety-disorders

We considered clinical observations in patients with obsessive-compulsive- and anxiety-disorders, who underwent bilateral anterior capsulotomy, as well as anatomical and pathophysiological findings. Based on these considerations, we choose the shell region of the right nucleus accumbens as target for deep brain stimulation (DBS) in a pilot-series of four patients with severe obsessive-compulsive- and anxiety-disorders. Significant reduction in severity of symptoms has been achieved in three of four patients treated. Clinical results as well as a 15-O-H(2)O-PET study, perfomed in one patient during stimulation, speak in favour of the following hypothesis. As a central relay-structure between amygdala, basal ganglia, mesolimbic dopaminergic areas, mediodorsal thalamus and prefrontal cortex, the accumbens nucleus seems to play a modulatory role in information flow from the amygdaloid complex to the latter areas. If disturbed, imbalanced information flow from the amygdaloid complex could yield obsessive-compulsive- and anxiety-disorders, which can be counteracted by blocking the information flow within the shell region of the accumbens nucleus by deep brain stimulation.

Phasic alterations in dopamine and serotonin release in striatum and prefrontal cortex in response to cocaine predictive cues in behaving rhesus macaques

The ability of environmental cues associated with cocaine availability to cause relapse may result from conditioned activation of dopamine (DA) release. We examined this hypothesis in macaque monkeys by conducting microdialysis studies in animals during exposure to a cocaine predictive compound cue. In addition to studying DA release in mesolimbic and sensorimotor striatum, both DA and serotonin levels were determined in the prefrontal cortex (medial orbitofrontal and anterior cingulate). The compound cue employed visual, auditory, and olfactory components, and was salient to the animals as demonstrated by anticipatory lever pressing in the absence of cocaine. During a 10-min period of exposure prior to cocaine availability, there was no significant increase in striatal or cortical DA. The addition of a DA uptake inhibitor to the striatal perfusate to reduce the potential interference of neuronal uptake did not alter the results. In contrast to the lack of any change in striatal DA, a significant decrease in extracellular serotonin in the prefrontal cortex during the 10 min of cue exposure was observed.

Differential control over cocaine-seeking behavior by nucleus accumbens core and shell

Nucleus accumbens (NAc) dopamine is widely implicated in mediating the reinforcing effects of drugs of abuse. However, the precise function of the NAc itself in drug self-administration has been difficult to establish. Here we show a neural double-dissociation of the behavioral processes that underlie cocaine self-administration in rats. Whereas selective excitotoxic lesions of the NAc core had only a minor effect on the acquisition of responding for cocaine under a standard schedule of continuous reinforcement, these lesions profoundly impaired the acquisition of drug-seeking behavior that was maintained by drug-associated conditioned reinforcers and assessed using a second-order schedule of cocaine reinforcement. In contrast, selective excitotoxic lesions of the NAc shell did not impair drug self-administration or the acquisition of cocaine-seeking, but they did attenuate the psychostimulant effects of cocaine. These results further our understanding of how the NAc controls drug-seeking and drug-taking behavior.

Endogenous dopamine release induced by repetitive transcranial magnetic stimulation over the primary motor cortex: an [11C]raclopride positron emission tomography study in anesthetized macaque monkeys

BACKGROUND

Repetitive transcranial magnetic stimulation (rTMS) has been used as a treatment for neuropsychiatric disorders such as depression and Parkinson's disease (PD). Despite the growing interest in therapeutic application of rTMS, precise mechanisms of its action remain unknown. With respect to PD, activation of the mesostriatal dopaminergic pathway is likely to be a candidate mechanism underlying the therapeutic effects; however, modulating effects of rTMS over the primary motor cortex (M1) on the dopaminergic system have not been studied.

METHODS

We used [11C]raclopride positron emission tomography to measure changes of extracellular dopamine concentration after 5Hz rTMS over the M1 in eight anesthetized monkeys.

RESULTS

rTMS over the right M1 induced a reduction of [11C]raclopride binding potential (BP) in the bilateral ventral striatum, including the nucleus accumbens, and a significant increase of BP in the right putamen; no significant BP reduction was found in the dorsal striatum. These data indicate that rTMS over the motor cortex induces a release of endogenous dopamine in the ventral striatum.

CONCLUSIONS

Our results suggest that therapeutic mechanisms of rTMS may be explained in part by an activation of the mesolimbic dopaminergic pathway, which plays critical roles in rewards, reinforcement, and incentive motivation.

Ovarian hormone influences on the density of immunoreactivity for tyrosine hydroxylase and serotonin in the primate corpus striatum

The serotonergic and dopaminergic inputs to the corpus striatum in human and non-human primates participate in diverse sensorimotor, cognitive, and affective functions, are implicated in dysfunction in diseases such as Parkinson's disease and schizophrenia, and are targets for many of the drugs used to treat these disorders. Sex differences in the incidence and/or clinical course of these disorders and in the effectiveness of related dopaminergic and serotonergic drug therapies suggest that primate striatal indolamines and catecholamines are also influenced by gonadal hormones. However, while well studied in rats, relatively little is known about precisely how gonadal steroids modulate stratial dopamine and serotonin systems in primates. To begin to address this issue, the present studies explored the effects of ovarian steroids on the serotonergic and dopaminergic innervation densities of the caudate, putamen, and the nucleus accumbens in young adult rhesus monkeys. Using densitometry to quantify immunoreactivity for serotonin and for the catecholamine-synthesizing enzyme tyrosine hydroxylase, innervation densities were compared in identified, functionally specialized striatal subdomains across animals that were either ovariectomized or ovariectomized and supplemented with estradiol and/or progesterone, i.e. in a primate model of surgical menopause, with and without hormone replacement therapy. These analyses revealed clear examples of structure-, hemisphere-, and replacement regimen-specific effects of changes in circulating steroids on the densities of each afferent system examined. Further, the predominantly stimulatory effects observed occurred in striatal areas analogous to those suspected as sites of localized dopamine and/or serotonin compromise in Parkinson's disease and schizophrenia. Thus, the hormone actions identified in this study could hold relevance for some of the sex differences identified in relation to these disorders, including the findings of decreased incidence and/or symptom severity in women that have led to hypotheses of protective effects for estrogen.

Cerebral metabolic correlates as potential predictors of response to anterior cingulotomy for treatment of major depression

OBJECT

Neurosurgical procedures are a viable intervention for severe, treatment-refractory major depression, although they have been associated with only modest rates of efficacy. The purpose of this study was to identify possible neuroimaging predictors of treatment response to anterior cingulotomy in patients with major depression.

METHODS

Thirteen patients underwent stereotactic anterior cingulotomy for treatment-refractory major depression. Symptom severity was measured using the Beck Depression Inventory (BDI) both before and approximately 12 months after surgery. The authors performed [18F]fluorodeoxyglucose-positron emission tomography (PET) studies in all patients preoperatively. Statistical parametric mapping methods were used to test for loci of significant correlation between preoperative regional cerebral metabolism and postoperative reduction in BDI scores. The mean (+/- standard deviation) change in the BDI score from the preoperative period (43.7 +/- 7.8) to the postoperative period (30.5 +/- 21.3) was 33.1 +/- 45.4%. Two loci--the left subgenual prefrontal cortex and left thalamus--were identified as sites at which preoperative metabolism was significantly correlated with subsequent improvement in depressive symptom severity following cingulotomy. Specifically, higher preoperative rates of metabolism at these loci were associated with better postoperative results.

CONCLUSIONS

Possible PET scanning predictors of treatment response were identified in patients with major depression who had undergone anterior cingulotomy. Further research will be necessary to determine the reproducibility of this finding. If confirmed, the availability of an index for noninvasively predicting a patient's response to cingulotomy for the treatment of major depression would be of great clinical value.

Inactivation of the infralimbic prefrontal cortex reinstates goal-directed responding in overtrained rats

Over the course of extended training, instrumental responding in rats shows a transition from goal-dependent performance to goal-independent performance, as assessed by sensitivity to reward-devaluation induced by taste aversions or specific satiety. It has been suggested that this reflects the gradual dominance of reflexive, habit-based responding over voluntary, goal-directed actions. Previous research suggests that lesions of the medial prefrontal cortex disrupt this interaction between goal-directed and habitual responding. More specifically, whereas lesions of the prelimbic prefrontal cortex appear to disrupt normal goal-directed responding, lesions of the infralimbic prefrontal cortex cause animals to remain goal-directed even after substantial overtraining. The current experiment explored further the nature of this interaction between actions and habits. Rats were given extended training of an instrumental lever press response before bilateral intracerebral cannulae giving access to the infralimbic cortex were implanted. Following further reminder training all animals were given a test of goal sensitivity by specific-satiety devaluation of the instrumental outcome, or a matched reward, prior to extinction tests. Before these tests, half of the animals received bilateral infusions of muscimol into the infralimbic cortex, and the remainder, control vehicle infusions. As expected after extended instrumental training, control-infused animals showed habitual performance that was not selectively influenced by devaluation of the instrumental outcome. In contrast, animals receiving temporary inactivation of the infralimbic cortex by muscimol showed selective sensitivity to devaluation of the instrumental outcome, indicating a reinstatement of goal-directed responding in these animals. This suggests that the development of habitual responding reflects the active inhibition of goal-directed responses that are mediated by action-outcome associations.

Dopamine hypofunction possibly results from a defect in glutamate-stimulated release of dopamine in the nucleus accumbens shell of a rat model for attention deficit hyperactivity disorder--the spontaneously hypertensive rat

RUSSELL, V.A. Dopamine hypofunction possibly results from a defect in glutamate-stimulated release of dopamine in the nucleus accumbens shell of a rat model for attention deficit hyperactivity disorder-the spontaneously hypertensive rat. NEUROSCI. BIOBEHAV. REV.27(2003). Disturbances in glutamate, dopamine and norepinephrine function in the brain of a genetic animal model for attention-deficit hyperactivity disorder (ADHD), the spontaneously hypertensive rat (SHR), and information obtained from patients with ADHD, suggest a defect in neuronal circuits that are required for reward-guided associative learning and memory formation. Evidence derived from (i). the neuropharmacology of drugs that are effective in treating ADHD symptoms, (ii). molecular genetic and neuroimaging studies of ADHD patients, as well as (iii). the behaviour and biochemistry of animal models, suggests dysfunction of dopamine neurons. SHR have decreased stimulation-evoked release of dopamine as well as disturbances in the regulation of norepinephrine release and impaired second messenger systems, cAMP and calcium. In addition, evidence supports a selective deficit in the nucleus accumbens shell of SHR which could contribute to impaired reinforcement of appropriate behaviour.

Effects of medial prefrontal cortex and dorsal striatum lesions on retrieval processes in rats

Exposure to training-related cues is known to reactivate associated memory and improves subsequent retention performance under various circumstances. The present studies investigated the neural basis of retrieval cue effects, by studying in two separate experiments, the involvement of the medial prefrontal cortex and of the dorsal striatum. Rats with lesions to the prelimbic-infralimbic cortex (PL-IL), to the anterior dorsal cingulate (ACd), and to the lateral and medial parts of the dorsal striatum (lDS and mDS) were first trained in a brightness discrimination avoidance task. One day later, rats were tested after being placed in the cueing box with either no training-related cue or with additional exposures to the light discriminative stimulus. None of the lesions affected the acquisition performance. During the retention test, control rats cued with the light in the box exhibited significantly better retention performance than those simply placed in the box, confirming our previous results. While mDS lesions did not modify effects of the retrieval cue, lDS as well as both PL-IL and ACd lesions blocked the facilitative effects of the discriminative stimulus. The present data indicate that ACd, PL-IL and lDS are involved in processes promoted by exposure to training cues, the nature of which are reviewed and discussed. This study in conjunction with previous ones suggests that retrieval cues activate several subcircuits mainly based on an amygdalo-prefrontal-striatum network. Activation of this network results in an improvement of the expression of the associated conditioned response, and may thus be viewed as increasing the efficacy of the retrieval processes.

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.

Does an imbalance between the dorsal and ventral striatopallidal systems play a role in Tourette's syndrome? A neuronal circuit approach

Tourette's syndrome is characterized by simple, involuntary muscle contractions and/or more complex movements or stereotyped behaviors, including vocalizations. There are strong indications that the basal ganglia play an important role in the pathophysiology of Tourette's syndrome. The present account reviews the functional anatomy of the basal ganglia, with an emphasis on the prefrontal cortex-ventral striatopallidal system. Different parts of the basal ganglia and thalamocortical system, with a focus on the premotor and prefrontal cortices, are connected with each other via parallel, functionally segregated basal ganglia-thalamocortical systems. These parallel circuits, representing sensorimotor, cognitive and emotional-motivational behavioral processes, are connected with each other through specific pathways that serve to integrate these various functions. In the context of the discussion on the pathophysiological mechanisms that lead to the expression of tics, emphasis is placed on the pathways that lead from the ventral striatum via the dopaminergic substantia nigra to the dorsal striatum. The dorsal striatum is crucial for habit formation. A conclusion of this overview of the anatomical organization of the basal ganglia is that via dopaminergic pathways limbic-relation information can influence the expression of (fragments of) motor and behavioral repertoires. Whether such mechanisms indeed play a role in the expression of tics in Tourette's syndrome remains to be established.

The nucleus accumbens: a target for deep brain stimulation in obsessive-compulsive- and anxiety-disorders

We considered clinical observations in patients with obsessive-compulsive- and anxiety-disorders, who underwent bilateral anterior capsulotomy, as well as anatomical and pathophysiological findings. Based on these considerations, we choose the shell region of the right nucleus accumbens as target for deep brain stimulation (DBS) in a pilot-series of four patients with severe obsessive-compulsive- and anxiety-disorders. Significant reduction in severity of symptoms has been achieved in three of four patients treated. Clinical results as well as a 15-O-H(2)O-PET study, perfomed in one patient during stimulation, speak in favour of the following hypothesis. As a central relay-structure between amygdala, basal ganglia, mesolimbic dopaminergic areas, mediodorsal thalamus and prefrontal cortex, the accumbens nucleus seems to play a modulatory role in information flow from the amygdaloid complex to the latter areas. If disturbed, imbalanced information flow from the amygdaloid complex could yield obsessive-compulsive- and anxiety-disorders, which can be counteracted by blocking the information flow within the shell region of the accumbens nucleus by deep brain stimulation.

Detecting subsecond dopamine release with fast-scan cyclic voltammetry in vivo

BACKGROUND

Dopamine is a potent neuromodulator in the brain, influencing a variety of motivated behaviors and involved in several neurologic diseases. Measurements of extracellular dopamine in the brains of experimental animals have traditionally focused on a tonic timescale (minutes to hours). However, dopamine concentrations are now known to fluctuate on a phasic timescale (subseconds to seconds).

APPROACH

Fast-scan cyclic voltammetry provides analytical chemical measurements of phasic dopamine signals in the rat brain.

CONTENT

Procedural aspects of the technique are discussed, with regard to appropriate use and in comparison with other methods. Finally, examples of data collected using fast-scan cyclic voltammetry are summarized, including naturally occurring dopamine transients and signals arising from electrical stimulation of dopamine neurons.

SUMMARY

Fast-scan cyclic voltammetry offers real-time measurements of changes in extracellular dopamine concentrations in vivo. With its subsecond time resolution, micrometer-dimension spatial resolution, and chemical selectivity, it is the most suitable technique currently available to measure transient concentration changes of dopamine.

Gene transcripts selectively down‐regulated in the shell of the nucleus accumbens long after heroin self‐administration are up‐regulated in the core independent of response contingency

Long-term drug-induced alterations in neurotransmission within the nucleus accumbens (NAc) shell and core may underlie relapse to drug-seeking behavior and drug-taking upon re-exposure to drugs and drug-associated stimuli (cues) during abstinence. Using an open screening strategy, we recently identified 25 gene transcripts, encoding for proteins involved in neuronal functioning and structure that are down-regulated in rat NAc shell after contingent (active), but not after non-contingent (passive), heroin administration. Studying the expression of the same transcripts in the NAc core by means of quantitative PCR, we now demonstrate that most of these transcripts are up-regulated in that NAc subregion long (3 weeks) after heroin self-administration in rats. A similar up-regulation in gene expression was also apparent in the NAc core of animals with a history of non-contingent heroin administration (yoked controls). These data indicate that heroin self-administration differentially regulates genes in the NAc core as compared with the shell. Moreover, whereas cognitive processes involved in active drug self-administration (e.g., instrumental learning) seems to direct gene expression in the NAc shell, neuroplasticity in the NAc core may be due to the pharmacological effects of heroin (including Pavlovian conditioning), as expressed in rats upon contingent as well as non-contingent administration of heroin.

Positron emission tomography imaging in depression: a neural systems perspective

PET measures of regional glucose metabolism, although chemically nonspecific, are sensitive indices of brain function in the untreated state and following disparate treatments. The continued development of imaging and multivariate statistical strategies is expected to provide an important perspective toward the full characterization of the depression phenotype at the neural systems level. An additional goal is the development of routine, brain-based clinical algorithms that optimize diagnosis and treatment of individual depressed patients.

Electrophysiological Interactions between Striatal Glutamatergic and Dopaminergic Systems

Glutamatergic and dopaminergic systems play a primary role in frontal-subcortical circuits involved in motor and cognitive functions. Considerable evidence has emerged indicating that the complex interaction between these neurotransmitter systems within the dorsal striatum and nucleus accumbens is critically involved in the gating of information flow in these highly integrative brain regions. As a result, disruptions of the interaction between glutamate and dopamine has been proposed as a pathological basis for a number of disorders, including the pathophysiology of schizophrenia. In this chapter, we discuss recent studies that have significantly advanced our understanding of the reciprocal interactions between glutamatergic and dopaminergic systems within the striatal complex in the normal brain and in pathological states.

Differential connections of the temporal pole with the orbital and medial prefrontal networks in macaque monkeys

Previous studies indicate that the orbital and medial prefrontal cortex (OMPFC) is organized into "orbital" and "medial" networks, which have distinct connections with cortical, limbic, and subcortical structures. In this study, retrograde and anterograde tracer experiments in monkeys demonstrated differential connections between the medial and orbital networks and the dorsal and ventral parts of the temporal pole. The dorsal part, including dysgranular and granular areas (TGdd and TGdg), is reciprocally connected with the medial network areas on the medial wall and gyrus rectus (areas 10m, 10o, 11m, 13a, 14c, 14r, 25, and 32) and on the lateral orbital surface (areas Iai and 12o). The strongest connections are with areas 10m (caudal part), 14c, 14r, 25, 32, and Iai. The agranular temporal pole (TGa) is connected with several areas, but most strongly with medial network area 25. The granular area around the superior temporal sulcus (TGsts) and the ventral dysgranular and granular areas (TGvd and TGvg) are reciprocally connected with the orbital network (especially areas 11l, 13b, 13l, 13m, Ial, Iam, and Iapm). TGsts is strongly connected with the entire orbital network, whereas areas TGvd and TGvg have lighter and more limited connections. Intrinsic connections within the temporal pole are also restricted to dorsal or ventral parts. Together with evidence that the dorsal and ventral temporal pole are differentially connected to auditory and visual areas of the superior and inferior temporal cortex, the results indicate separate connections between these systems and the medial and orbital prefrontal networks.

Alcohol promotes dopamine release in the human nucleus accumbens

Microdialysis experiments in rodents indicate that ethanol promotes dopamine release predominantly in the nucleus accumbens, a phenomenon that is implicated in the reinforcing effects of drugs of abuse. The aim of the present study was to test the hypothesis in humans that an oral dose of ethanol would lead to dopamine release in the ventral striatum, including the nucleus accumbens. Six healthy subjects underwent two [(11)C]raclopride PET scans following either alcohol (1 ml/kg) in orange juice or orange juice alone. Subjective mood changes, heart rate, and blood-alcohol levels were monitored throughout the procedure. Personality traits were evaluated using the tridimensional personality questionnaire. PET images were co-registered with MRI and transformed into stereotaxic space. Statistical parametric maps of [(11)C]raclopride binding potential change were generated. There was a significant reduction in [(11)C]raclopride binding potential bilaterally in the ventral striatum/nucleus accumbens in the alcohol condition compared to the orange juice condition, indicative of increased extracellular dopamine. Moreover, the magnitude of the change in [(11)C]raclopride binding correlated with the alcohol-induced increase in heart rate, which is thought to be a marker of the psychostimulant effects of the drug, and with the personality dimension of impulsiveness. The present study is the first report that, in humans, alcohol promotes dopamine release in the brain, with a preferential effect in the ventral striatum. These findings support the hypothesis that mesolimbic dopamine activation is a common property of abused substances, possibly mediating their reinforcing effects.

Behavioural sensitisation to repeated d-amphetamine: effects of excitotoxic lesions of the pedunculopontine tegmental nucleus

The pedunculopontine tegmental nucleus (PPTg) interacts with anatomical systems thought to be involved in mediating sensitisation of the locomotor response to repeated d-amphetamine. The PPTg has direct and indirect connections with the nucleus accumbens and prefrontal cortex, and also influences midbrain dopamine activity through direct projections to substantia nigra and ventral tegmental area. In this experiment, the development of behavioural sensitisation to the locomotor stimulant effects of repeated d-amphetamine was examined in rats bearing excitotoxic lesions of the PPTg, and sham-lesioned controls. Rats were given repeated d-amphetamine (1.5 mg/kg i.p.) treatment in an on-off procedure, with saline and d-amphetamine given on alternate days, such that rats received a total of seven d-amphetamine and seven saline treatments. Locomotor responses were measured in photocell cages. On the first day of d-amphetamine treatment, there was no difference between excitotoxin and sham-lesioned rats. Development of sensitisation to the locomotor stimulant effects of d-amphetamine was delayed in PPTg-lesioned rats, relative to the sham-lesioned control rats. However, there was no difference between lesion and control groups in the locomotion seen on saline-treatment days. These data suggest that the PPTg is involved in the development of behavioural sensitisation to the locomotor stimulant effects of repeated d-amphetamine, and indicate that traditional striatal circuitry models of the mechanisms underlying sensitisation should be extended to include the PPTg.

Three-dimensional organization of the recurrent axon collateral network of the substantia nigra pars reticulata neurons in the rat

The substantia nigra pars reticulata (SNR) constitutes a major output nucleus of the basal ganglia where the final stage of information processing within this system takes place. In this study, using juxtacellular labeling and three-dimensional reconstruction methods, we investigated the spatial organization of the intranigral innervation provided by single GABAergic projection neurons from the sensory-motor subdivision of the rat SNR. Confirming previous observations, most labeled SNR cells were found to possess a local axonal network innervating the pars reticulata and pars compacta (SNC). Within the SNR, axons of these cells were distributed along curved laminas enveloping a dorsolaterally located core, thus mostly respecting the onion-like compartmentalization of this nucleus. Although the axonal projection field mostly remained confined to the dendritic field of the parent neuron, it usually extended beyond its limits in caudal, lateral, and/or dorsal directions. Because SNR cells are GABAergic, this pattern of axonal projection suggests the existence of lateral inhibitory interactions between neurons belonging to the same as well as to adjacent functional subdivisions. Axonal projections of SNR cells to the SNC formed longitudinal bands. These bands partly occupied the SNC region projecting to the striatal sector from which parent SNR cells receive their afferents. These data indicate that SNR cells contribute to an indirect nigrostriatal loop circuit through which the striatum could upregulate its level of dopaminergic transmission via a disinhibition of nigrostriatal neurons. Spatial relationships between elements of this indirect nigrostriatonigral circuit indicate that this circuit operates in both a closed and open loop manner.

Variable dopamine release probability and short-term plasticity between functional domains of the primate striatum

Release of the neuromodulator dopamine (DA) is critical to the control of locomotion, motivation, and reward. However, the probability of DA release is not well understood. Current understanding of neurotransmitter release probability in the CNS is limited to the conventional synaptic amino acid transmitters (e.g., glutamate and GABA). These fast neurotransmitters are released with a repertoire of probabilities according to synapse type, and these probabilities show activity-dependent plasticity according to synapse use. Synapses for neuromodulators such as DA, however, are designed for signaling that diverges temporally and spatially from that for fast neurotransmitters: DA receptors are exclusively metabotropic and at sites that extend to extrasynaptic locations and neighboring synapses. In this study, the release probability of DA was explored in real time in limbicversus motor-associated functional domains of the striatum of a primate (marmoset; Callithrix jacchus) using fast-scan voltammetry at a carbon-fiber microelectrode. We show that the probability of axonal DA release varies with striatal domain. Furthermore, release probability exhibits a short-term, activity-dependent plasticity that ranges from depression to facilitation in motor-through limbic-associated regions, respectively. Rapid plasticity does not result from metabotropic D2-like DA receptor activation or ionotropic GABA(A) receptor effects but is dependent on Ca2+ availability. These data reveal that rapid dynamics in DA release probability will participate in the transmission of the patterns and frequencies encoded by DA neuron action potential discharge. Furthermore, the regional variation in these features indicates that limbic-versus motor-associated DA neurons are permitted to generate diverse DA signals in response to a given firing pattern.

Effects of expectations for different reward magnitudes on neuronal activity in primate striatum

In behavioral science, it is well known that humans and nonhuman animals are highly sensitive to differences in reward magnitude when choosing an outcome from a set of alternatives. We know that a realm of behavioral reactions is altered when animals begin to expect different levels of reward outcome. Our present aim was to investigate how the expectation for different magnitudes of reward influences behavior-related neurophysiology in the anterior striatum. In a spatial delayed response task, different instruction pictures are presented to the monkey. Each image represents a different magnitude of juice. By reaching to the spatial location where an instruction picture was presented, animals could receive the particular liquid amount designated by the stimulus. Reliable preferences in reward choice trials and differences in anticipatory licks, performance errors, and reaction times indicated that animals differentially expected the various reward amounts predicted by the instruction cues. A total of 374 of 2,000 neurons in the anterior parts of the caudate nucleus, putamen, and ventral striatum showed five forms of task-related activation during the preparation or execution of movement and activations preceding or following the liquid drop delivery. Approximately one-half of these striatal neurons showed differing response levels dependent on the magnitude of liquid to be received. Results of a linear regression analysis showed that reward magnitude and single cell discharge rate were related in a subset of neurons by a monotonic positive or negative relationship. Overall, these data support the idea that the striatum utilizes expectancies that contain precise information concerning the predicted, forthcoming level of reward in directing general behavioral reactions.

Neuroimaging and neurocircuitry models pertaining to the neurosurgical treatment of psychiatric disorders

Neurocircuitry models of obsessive-compulsive disorder (OCD) and major depression (MD) are described, focusing on relevant supporting neuroimaging data. Corticostriatothalamocortical circuitry is implicated in OCD. In MD, the relation between "dorsal" and "ventral" cortical compartments is emphasized; the amygdala, hippocampus, and pregenual anterior cingulate are implicated in the pathophysiology of MD and are potential targets for treatment. The neuroanatomy of psychiatric neurosurgical procedures and related neuroimaging findings are reviewed. Finally, anticipated future directions of research in this field are discussed.

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.

Imaging human mesolimbic dopamine transmission with positron emission tomography. Part II: amphetamine-induced dopamine release in the functional subdivisions of the striatum

The human striatum is functionally organized into limbic, associative, and sensorimotor subdivisions, which process information related to emotional, cognitive, and motor function. Dopamine projections ascending from the midbrain provide important modulatory input to these striatal subregions. The aim of this study was to compare activation of dopamine D2 receptors after amphetamine administration in the functional subdivisions of the human striatum. D2 receptor availability (V3") was measured with positron emission tomography and [11C]raclopride in 14 healthy volunteers under control conditions and after the intravenous administration of amphetamine (0.3 mg/kg). For each condition, [11C]raclopride was administered as a priming bolus followed by constant infusion, and measurements of D2 receptor availability were obtained under sustained binding equilibrium conditions. Amphetamine induced a significantly larger reduction in D2 receptor availability (DeltaV3") in limbic (ventral striatum, -15.3 +/- 11.8%) and sensorimotor (postcommissural putamen, -16.1 +/- 9.6%) regions compared with associative regions (caudate and precommissural putamen, -8.1 +/- 7.2%). Results of this region-of-interest analysis were confirmed by a voxel-based analysis. Correction for the partial volume effect showed even greater differences in DeltaV3" between limbic (-17.8 +/- 13.8%), sensorimotor (-16.6 +/- 9.9%), and associative regions (-7.5 +/- 7.5%). The increase in euphoria reported by subjects after amphetamine was associated with larger DeltaV3" in the limbic and sensorimotor regions, but not in the associative regions. These results show significant differences in the dopamine response to amphetamine between the functional subdivisions of the human striatum. The mechanisms potentially accounting for these regional differences in amphetamine-induced dopamine release within the striatum remain to be elucidated, but may be related to the asymmetrical feed-forward influences mediating the integration of limbic, cognitive, and sensorimotor striatal function via dopamine cell territories in the ventral midbrain.

Muscarinic and nicotinic cholinergic mechanisms in the mesostriatal dopamine systems

The striatum and its dense dopaminergic innervation originating in the midbrain, primarily from the substantia nigra pars compacta and the ventral tegmental area, compose the mesostriatal dopamine (DA) systems. The nigrostriatal system is involved mainly in motor coordination and in disorders such as Tourette's syndrome, Huntington's disease, and Parkinson's disease. The dopaminergic projections from the ventral tegmental area to the striatum participate more in the processes that shape behaviors leading to reward, and addictive drugs act upon this mesolimbic system. The midbrain DA areas receive cholinergic innervation from the pedunculopontine tegmentum and the laterodorsal pontine tegmentum, whereas the striatum receives dense cholinergic innervation from local interneurons. The various neurons of the mesostriatal systems express multiple types of muscarinic and nicotinic acetylcholine receptors as well as DA receptors. Especially in the striatum, the dense mingling of dopaminergic and cholinergic constituents enables potent interactions. Evidence indicates that cholinergic and dopaminergic systems work together to produce the coordinated functioning of the striatum. Loss of that cooperative activity contributes to the dysfunction underlying Parkinson's disease.

Modulating dysfunctional limbic-cortical circuits in depression: towards development of brain-based algorithms for diagnosis and optimised treatment

While characterization of pathogenetic mechanisms underlying major depression is a fundamental aim of neuroscience research, an equally critical clinical goal is to identify biomarkers that might improve diagnostic accuracy and guide treatment selection for individual patients. To this end, a synthesis of functional neuroimaging studies examining regional metabolic and blood flow changes in depression is presented in the context of a testable limbic-cortical network model. 'Network' dysfunction combined with active intrinsic compensatory processes is seen to explain the heterogeneity of depressive symptoms observed clinically, as well as variations in pretreatment scan patterns described experimentally. Furthermore, the synchronized modulation of these dysfunctional limbic-cortical pathways is considered critical for illness remission, regardless of treatment modality. Testing of response-specific functional relationships among regional 'nodes' within this network using multivariate approaches is discussed, with a perspective aimed at identifying biomarkers of treatment non-response, relapse risk and disease vulnerability. Characterization of adaptive and maladaptive functional interactions among these pathways is seen as a critical step towards future development of evidenced-based algorithms that will optimize the diagnosis and treatment of individual depressed patients.

Nucleus accumbens dopamine depletion impairs both acquisition and performance of appetitive Pavlovian approach behaviour: implications for mesoaccumbens dopamine function

The involvement of mesoaccumbens dopamine in adaptive learning and behaviour is unclear. For example, dopamine may act as a teaching signal to enable learning, or more generally modulate the behavioural expression, or selection, of an already-learned response. The present study investigated the involvement of the mesoaccumbens dopamine system in a fundamental form of learning: Pavlovian conditioning. In this case, the temporal association of a previously neutral visual stimulus and a biologically significant unconditioned stimulus (US), subsequently led to the production of the conditioned response (CR) of discriminated approach behaviour directed toward the conditioned stimulus (CS+), relative to a control (CS-) stimulus. 6-hydroxydopamine lesions of the nucleus accumbens (NAcc), leading to approximately 80% reductions in tissue dopamine, were made at varying time points in four experimental groups of rats, either before or subsequent to the acquisition of the CR. NAcc dopamine depletion produced long-term neuroadaptations in dopamine function 2 months after surgery, and profoundly impaired discriminated Pavlovian approach regardless of when the lesion was made. Thus, NAcc dopamine not only plays a role in conditioned behavioural activation, but also in making the appropriate discriminated response i.e. the direction of response. Further, acquisition lesions produced a far greater impact on discriminated approach than performance lesions. This difference in lesion-induced impairment implies that mesoaccumbens dopamine may play differential roles in the learning and performance of preparatory Pavlovian conditioning.

Cholinergic interneuron characteristics and nicotinic properties in the striatum

The neostriatum (dorsal striatum) is composed of the caudate and putamen. The ventral striatum is the ventral conjunction of the caudate and putamen that merges into and includes the nucleus accumbens and striatal portions of the olfactory tubercle. About 2% of the striatal neurons are cholinergic. Most cholinergic neurons in the central nervous system make diffuse projections that sparsely innervate relatively broad areas. In the striatum, however, the cholinergic neurons are interneurons that provide very dense local innervation. The cholinergic interneurons provide an ongoing acetylcholine (ACh) signal by firing action potentials tonically at about 5 Hz. A high concentration of acetylcholinesterase in the striatum rapidly terminates the ACh signal, and thereby minimizes desensitization of nicotinic acetylcholine receptors. Among the many muscarinic and nicotinic striatal mechanisms, the ongoing nicotinic activity potently enhances dopamine release. This process is among those in the striatum that link the two extensive and dense local arbors of the cholinergic interneurons and dopaminergic afferent fibers. During a conditioned motor task, cholinergic interneurons respond with a pause in their tonic firing. It is reasonable to hypothesize that this pause in the cholinergic activity alters action potential dependent dopamine release. The correlated response of these two broad and dense neurotransmitter systems helps to coordinate the output of the striatum, and is likely to be an important process in sensorimotor planning and learning.

Early consolidation of instrumental learning requires protein synthesis in the nucleus accumbens

It is widely held that long-term memories are established by consolidation of newly acquired information into stable neural representations, a process that requires protein synthesis and synaptic plasticity. Plasticity within the nucleus accumbens (NAc), a major component of the ventral striatum, is thought to mediate instrumental learning processes and many aspects of drug addiction. Here we show that the inhibition of protein synthesis within the NAc disrupts consolidation of an appetitive instrumental learning task (lever-pressing for food) in rats. Post-trial infusions of anisomycin immediately after the first several training sessions prevented consolidation, whereas infusions delayed by 2 or 4 hours had no effect. However, if the rats were allowed to learn the task, the behavior was not sensitive to disruption by intra-accumbens anisomycin. Control infusions into the medial NAc shell or the dorsolateral striatum did not impair learning; in fact, an enhancement was observed in the latter case. These results show that de novo protein synthesis within the NAc is necessary for the consolidation, but not reconsolidation, of appetitive instrumental memories.

Reward, motivation, and reinforcement learning

There is substantial evidence that dopamine is involved in reward learning and appetitive conditioning. However, the major reinforcement learning-based theoretical models of classical conditioning (crudely, prediction learning) are actually based on rules designed to explain instrumental conditioning (action learning). Extensive anatomical, pharmacological, and psychological data, particularly concerning the impact of motivational manipulations, show that these models are unreasonable. We review the data and consider the involvement of a rich collection of different neural systems in various aspects of these forms of conditioning. Dopamine plays a pivotal, but complicated, role.

Substantia nigra pars reticulata neurons code initiation of a serial pattern: implications for natural action sequences and sequential disorders

Sequences of movements are initiated abnormally in neurological disorders involving basal ganglia dysfunction, such as Parkinson's disease or Tourette's syndrome. The substantia nigra pars reticulata (SNpr) is one of the two primary output structures of the basal ganglia. However, little is known about how substantia nigra mediates the initiation of normal movement sequences. We studied its role in coding initiation of a sequentially stereotyped but natural movement sequence by recording neuronal activity in SNpr during behavioural performance of 'syntactic grooming chains'. These are rule-governed sequences of up to 25 grooming movements emitted in four predictable (syntactic) phases, which occur spontaneously during grooming behaviour by rats and other rodents. Our results show that neuronal activation in central SNpr codes the onset of this entire rule-governed sequential pattern of grooming actions, not elemental grooming movements. We conclude that the context of sequential pattern may be more important than the elemental motor parameters in determining SNpr neuronal activation.

Synaptic convergence of motor and somatosensory cortical afferents onto GABAergic interneurons in the rat striatum

Cortical afferents to the basal ganglia, and in particular the corticostriatal projections, are critical in the expression of basal ganglia function in health and disease. The corticostriatal projections are topographically organized but also partially overlap and interdigitate. To determine whether projections from distinct cortical areas converge at the level of single interneurons in the striatum, double anterograde labeling from the primary motor (M1) and primary somatosensory (S1) cortices in the rat, was combined with immunolabeling for parvalbumin (PV), to identify one population of striatal GABAergic interneurons. Cortical afferents from M1 and S1 gave rise to distinct, but partially overlapping, arbors of varicose axons in the striatum. PV-positive neurons were often apposed by cortical terminals and, in many instances, apposed by terminals from both cortical areas. Frequently, individual cortical axons formed multiple varicosities apposed to the same PV-positive neuron. Electron microscopy confirmed that the cortical terminals formed asymmetric synapses with the dendrites and perikarya of PV-positive neurons as well as unlabelled dendritic spines. Correlated light and electron microscopy revealed that individual PV-positive neurons received synaptic input from axon terminals derived from both motor and somatosensory cortices. These results demonstrate that, within areas of overlap of functionally distinct projections, there is synaptic convergence at the single cell level. Sensorimotor integration in the basal ganglia is thus likely to be mediated, at least in part, by striatal GABAergic interneurons. Furthermore, our findings suggest that the pattern of innervation of GABAergic interneurons by cortical afferents is different from the cortical innervation of spiny projection neurons.

Differential nicotinic receptor expression in monkey basal ganglia: effects of nigrostriatal damage

Our previous work showed that there were marked declines in (125)I-alpha-conotoxin MII labeled nicotinic receptors in monkey basal ganglia after nigrostriatal damage, findings that suggest alpha3/alpha6 containing nicotinic receptors sites may be of relevance to Parkinson's disease. We now investigate whether there are differential changes in the distribution pattern of nicotinic receptor subtypes in the basal ganglia in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-lesioned animals compared to controls to better understand the changes occurring with nigrostriatal damage. To approach this we used (125)I-alpha-conotoxin MII, a marker for alpha3/alpha6 nicotinic receptors, and (125)I-epibatidine, a ligand that labels multiple nicotinic subtypes. The results demonstrate that there were medial to lateral gradients in nicotinic receptor distribution in control striatum, as well as ventromedial to dorsolateral gradients in the substantia nigra, which resembled those of the dopamine transporter in these same brain regions. Treatment with MPTP, a neurotoxin that selectively destroys dopaminergic nigrostriatal neurons, led to a relatively uniform decrease in nicotinic receptor sites in the striatum, but a differential effect in the substantia nigra with significantly greater declines in the ventrolateral portion. Competition analysis in the striatum showed that alpha-conotoxin MII sensitive sites were primarily affected after lesioning, whereas multiple nicotinic receptor populations were decreased in the substantia nigra. From these data we suggest that in the striatum alpha3/alpha6 nicotinic receptors are primarily localized on dopaminergic nerve terminals, while multiple nicotinic receptor subtypes are present on dopaminergic cell bodies in the substantia nigra. Thus, if activation of striatal nicotinic receptors is key in the regulation of basal ganglia function, alpha3/alpha6-directed nicotinic receptor ligands may be more relevant for Parkinson's disease therapy. However, nicotinic receptor ligands with a broader specificity may be more important if receptors in the substantia nigra play a dominant role in controlling nigrostriatal activity.

Differential processing patterns of motor information via striatopallidal and striatonigral projections

The functional loop linking the frontal lobe and the basal ganglia plays an important role in the control of motor behaviors. To delineate the principal features of motor information processing in the cortico-basal ganglia loop, the present study aimed at investigating how corticostriatal inputs from the primary motor cortex (MI) and the supplementary motor area (SMA) are transposed onto the pallidal complex and the substantia nigra. In macaque monkeys, stimulating electrodes were chronically implanted into identified forelimb representations of the MI and SMA. Subsequently, the distribution of neurons exhibiting orthodromic responses was examined in the caudal putamen to demarcate striatal zones receiving inputs separately or confluently from the MI and SMA. Finally, anterograde double labeling was performed by paired injections of tracers into two of three identified zones: the MI-recipient zone, SMA-recipient zone, and the convergent zone. Data have revealed that inputs from the MI-recipient and SMA-recipient striatal zones were substantially segregated in the pallidal complex and that those from the convergent zone were distributed to fill in blanks made by terminal bands derived from the MI and SMA. On the other hand, striatonigral inputs from the SMA-recipient and convergent zones of the putamen largely overlapped, while the input from the MI-recipient zone was minimal. The present results clearly indicate that the mode to process corticostriatal motor information through the striatopallidal and striatonigral projections is target-dependent, such that the parallel versus convergent rules govern the arrangement of striatopallidal or striatonigral inputs, respectively.

Three-dimensional cartography of functional territories in the human striatopallidal complex by using calbindin immunoreactivity

This anatomic study presents an analysis of the distribution of calbindin immunohistochemistry in the human striatopallidal complex. Entire brains were sectioned perpendicularly to the mid-commissural line into 70-microm-thick sections. Every tenth section was immunostained for calbindin. Calbindin labeling exhibited a gradient on the basis of which three different regions were defined: poorly labeled, strongly labeled, and intermediate. Corresponding contours were traced in individual sections and reformatted as three-dimensional structures. The poorly labeled region corresponded to the dorsal part of the striatum and to the central part of the pallidum. The strongly labeled region included the ventral part of the striatum, the subcommissural part of the external pallidum but also the adjacent portion of its suscommissural part, and the anterior pole of the internal pallidum. The intermediate region was located between the poorly and strongly labeled regions. As axonal tracing and immunohistochemical studies in monkeys show a similar pattern, poorly, intermediate, and strongly labeled regions were considered as the sensorimotor, associative, and limbic territories of the human striatopallidal complex, respectively. However, the boundaries between these territories were not sharp but formed gradients of labeling, which suggests overlapping between adjacent territories. Similarly, the ventral boundary of the striatopallidal complex was blurred, suggesting a structural intermingling with the substantia innominata. This three-dimensional partitioning of the human striatopallidal complex could help to define functional targets for high-frequency stimulation with greater accuracy and help to identify new stimulation sites.

Amygdaloid projections to ventromedial striatal subterritories in the primate

The ventral striatum is the part of the striatum associated with reward and goal-directed behaviors, which are mediated in part by inputs from the amygdala. The ventral striatum is divided into 'shell' and 'core' subterritories which have different connectional, histochemical and pharmacological properties. Behavioral studies also indicate that subterritories of the ventral striatum are differentially involved in specific goal-directed behaviors. The amygdala is a heterogeneous structure which has multiple nuclei involved in processing emotional information. While the existence of an amygdalostriatal pathway has long been established, the relationship between amygdaloid afferents and specific subterritories of the ventral striatum is not known. In this study we operationally defined the ventromedial striatum as the region receiving cortical inputs primarily from the orbital and medial prefrontal cortex. We placed retrograde tracer injections into subregions of the ventromedial striatum of macaques monkeys to determine the relative contribution of specific amygdaloid inputs to each region. Calbindin-D28k immunostaining was used to further define the shell subterritory of the ventromedial striatum. Based on these definitions, the amygdala innervates the entire ventromedial striatum, and has few to no inputs to the central striatum. The basal and accessory basal nuclei are the major source of input to the ventromedial striatum, innervating both the shell and ventromedial striatum outside the shell. However, a restricted portion of the dorsomedial shell receives few basal nucleus inputs. Afferent inputs from the basal nucleus subdivisions are arranged such that the parvicellular subdivision projects mainly to the ventral shell and core, and the magnocellular subdivision targets the ventral shell and ventromedial putamen. In contrast, the intermediate subdivision of the basal nucleus projects broadly across the ventromedial striatum avoiding only the dorsomedial shell. The shell has a specific set of connections derived from the medial part of the central nucleus and periamygdaloid cortex. Within the shell, the dorsomedial region is distinguished by additional inputs from the medial nucleus. The ventromedial caudate nucleus forms a unique transition zone with the shell, based on inputs from the periamygdaloid cortex. Together, these results indicate that subterritories of the ventromedial striatum are differentially modulated by amygdaloid nuclei which play roles in processing olfactory, visual/gustatory, multimodal sensory, and 'drive'-related stimuli.

Actor-critic models of the basal ganglia: new anatomical and computational perspectives

A large number of computational models of information processing in the basal ganglia have been developed in recent years. Prominent in these are actor-critic models of basal ganglia functioning, which build on the strong resemblance between dopamine neuron activity and the temporal difference prediction error signal in the critic, and between dopamine-dependent long-term synaptic plasticity in the striatum and learning guided by a prediction error signal in the actor. We selectively review several actor-critic models of the basal ganglia with an emphasis on two important aspects: the way in which models of the critic reproduce the temporal dynamics of dopamine firing, and the extent to which models of the actor take into account known basal ganglia anatomy and physiology. To complement the efforts to relate basal ganglia mechanisms to reinforcement learning (RL), we introduce an alternative approach to modeling a critic network, which uses Evolutionary Computation techniques to 'evolve' an optimal RL mechanism, and relate the evolved mechanism to the basic model of the critic. We conclude our discussion of models of the critic by a critical discussion of the anatomical plausibility of implementations of a critic in basal ganglia circuitry, and conclude that such implementations build on assumptions that are inconsistent with the known anatomy of the basal ganglia. We return to the actor component of the actor-critic model, which is usually modeled at the striatal level with very little detail. We describe an alternative model of the basal ganglia which takes into account several important, and previously neglected, anatomical and physiological characteristics of basal ganglia-thalamocortical connectivity and suggests that the basal ganglia performs reinforcement-biased dimensionality reduction of cortical inputs. We further suggest that since such selective encoding may bias the representation at the level of the frontal cortex towards the selection of rewarded plans and actions, the reinforcement-driven dimensionality reduction framework may serve as a basis for basal ganglia actor models. We conclude with a short discussion of the dual role of the dopamine signal in RL and in behavioral switching.

Magnetic resonance imaging of neuronal connections in the macaque monkey

Recently, an MRI-detectable, neuronal tract-tracing method in living animals was introduced that exploits the anterograde transport of manganese (Mn2+). We present the results of experiments simultaneously tracing manganese chloride and wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) to evaluate the specificity of the former by tracing the neuronal connections of the basal ganglia of the monkey. Mn2+ and WGA-HRP yielded remarkably similar and highly specific projection patterns. By showing the sequential transport of Mn2+ from striatum to pallidum-substantia nigra and then to thalamus, we demonstrated MRI visualization of transport across at least one synapse in the CNS of the primate. Transsynaptic tract tracing in living primates will allow chronic studies of development and plasticity and provide valuable anatomical information for fMRI and electrophysiological experiments in primates.

Emotion and motivation: the role of the amygdala, ventral striatum, and prefrontal cortex

Emotions are multifaceted, but a key aspect of emotion involves the assessment of the value of environmental stimuli. This article reviews the many psychological representations, including representations of stimulus value, which are formed in the brain during Pavlovian and instrumental conditioning tasks. These representations may be related directly to the functions of cortical and subcortical neural structures. The basolateral amygdala (BLA) appears to be required for a Pavlovian conditioned stimulus (CS) to gain access to the current value of the specific unconditioned stimulus (US) that it predicts, while the central nucleus of the amygdala acts as a controller of brainstem arousal and response systems, and subserves some forms of stimulus-response Pavlovian conditioning. The nucleus accumbens, which appears not to be required for knowledge of the contingency between instrumental actions and their outcomes, nevertheless influences instrumental behaviour strongly by allowing Pavlovian CSs to affect the level of instrumental responding (Pavlovian-instrumental transfer), and is required for the normal ability of animals to choose rewards that are delayed. The prelimbic cortex is required for the detection of instrumental action-outcome contingencies, while insular cortex may allow rats to retrieve the values of specific foods via their sensory properties. The orbitofrontal cortex, like the BLA, may represent aspects of reinforcer value that govern instrumental choice behaviour. Finally, the anterior cingulate cortex, implicated in human disorders of emotion and attention, may have multiple roles in responding to the emotional significance of stimuli and to errors in performance, preventing responding to inappropriate stimuli.

I(h) channels contribute to the different functional properties of identified dopaminergic subpopulations in the midbrain

Dopaminergic (DA) midbrain neurons in the substantia nigra (SN) and ventral tegmental area (VTA) are involved in various brain functions such as voluntary movement and reward and are targets in disorders such as Parkinson's disease and schizophrenia. To study the functional properties of identified DA neurons in mouse midbrain slices, we combined patch-clamp recordings with either neurobiotin cell-filling and triple labeling confocal immunohistochemistry, or single-cell RT-PCR. We discriminated four DA subpopulations based on anatomical and neurochemical differences: two calbindin D28-k (CB)-expressing DA populations in the substantia nigra (SN/CB+) or ventral tegmental area (VTA/CB+), and respectively, two calbindin D28-k negative DA populations (SN/CB-, VTA/CB-). VTA/CB+ DA neurons displayed significantly faster pacemaker frequencies with smaller afterhyperpolarizations compared with other DA neurons. In contrast, all four DA populations possessed significant differences in I(h) channel densities and I(h) channel-mediated functional properties like sag amplitudes and rebound delays in the following order: SN/CB- --> VTA/CB- --> SN/CB+ --> VTA/CB+. Single-cell RT-multiplex PCR experiments demonstrated that differential calbindin but not calretinin expression is associated with differential I(h) channel densities. Only in SN/CB- DA neurons, however, I(h) channels were actively involved in pacemaker frequency control. In conclusion, diversity within the DA system is not restricted to distinct axonal projections and differences in synaptic connectivity, but also involves differences in postsynaptic conductances between neurochemically and topographically distinct DA neurons.

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.

Organization of inputs from cingulate motor areas to basal ganglia in macaque monkey

The cingulate motor areas reside within regions lining the cingulate sulcus and are divided into rostral and caudal parts. Recent studies suggest that the rostral and caudal cingulate motor areas participate in distinct aspects of motor function: the former plays a role in higher-order cognitive control of movements, whereas the latter is more directly involved in their execution. Here, we investigated the organization of cingulate motor areas inputs to the basal ganglia in the macaque monkey. Identified forelimb representations of the rostral and caudal cingulate motor areas were injected with different anterograde tracers and the distribution patterns of labelled terminals were analysed in the striatum and the subthalamic nucleus. Corticostriatal inputs from the rostral and caudal cingulate motor areas were located within the rostral striatum, with the highest density in the striatal cell bridges and the ventrolateral portions of the putamen, respectively. There was no substantial overlap between these input zones. Similarly, a certain segregation of input zones from the rostral and caudal cingulate motor areas occurred along the mediolateral axis of the subthalamic nucleus. It has also been revealed that corticostriatal and corticosubthalamic input zones from the rostral cingulate motor area considerably overlapped those from the presupplementary motor area, while the input zones from the caudal cingulate motor area displayed a large overlap with those from the primary motor cortex. The present results indicate that a parallel design underlies motor information processing in the cortico-basal ganglia loop derived from the rostral and caudal cingulate motor areas.