Building action repertoires: memory and learning functions of the basal ganglia

Is there a brainstem substrate for action selection?

The search for the neural substrate of vertebrate action selection has focused on structures in the forebrain and midbrain, and particularly on the group of sub-cortical nuclei known as the basal ganglia. Yet, the behavioural repertoire of decerebrate and neonatal animals suggests the existence of a relatively self-contained neural substrate for action selection in the brainstem. We propose that the medial reticular formation (mRF) is the substrate's main component and review evidence showing that the mRF's inputs, outputs and intrinsic organization are consistent with the requirements of an action-selection system. The internal architecture of the mRF is composed of interconnected neuron clusters. We present an anatomical model which suggests that the mRF's intrinsic circuitry constitutes a small-world network and extend this result to show that it may have evolved to reduce axonal wiring. Potential configurations of action representation within the internal circuitry of the mRF are then assessed by computational modelling. We present new results demonstrating that each cluster's output is most likely to represent activation of a component action; thus, coactivation of a set of these clusters would lead to the coordinated behavioural response observed in the animal. Finally, we consider the potential integration of the basal ganglia and mRF substrates for selection and suggest that they may collectively form a layered/hierarchical control system.

Theories of autism

The purpose of the present paper was to review psychological theories of autism, and to integrate these theories with neurobiological findings. Cognitive, theory of mind, language and coherence theories were identified, and briefly reviewed. Psychological theories were found not to account for the rigid/repetitive behaviours universally described in autistic subjects, and underlying neurobiological systems were identified. When the developing brain encounters constrained connectivity, it evolves an abnormal organization, the features of which may be best explained by a developmental failure of neural connectivity, where high local connectivity develops in tandem with low long-range connectivity, resulting in constricted repetitive behaviours.

Slow Brain Potential and Oscillatory EEG Manifestations of Impaired Temporal Preparation in Parkinson's Disease

Performance in behavioral tasks is influenced by temporal expectations shaped by the temporal structure of the task. Such implicit temporal preparation is reflected in slow brain potentials and electroencephalographic oscillations and is attributed to interval timing mechanisms that probably depend on intact basal ganglia function. We investigated implicit timing in Parkinson's disease using a choice reaction task with two temporally regular stimulus presentation regimes, both including occasional deviant interstimulus intervals. Control subjects, but not patients, demonstrated temporal preparation in the form of an adjustment in time course of slow brain potentials to the duration of the interstimulus interval. However, in both groups, timing perturbations were accompanied by a slow brain potential amplitude drop at the time of expected stimulus occurrence, demonstrating intact representation of time in patients. In patients, oscillatory activity in beta and alpha bands showed attenuated preparatory desynchronization and reduced postmovement event-related synchronization, reflecting abnormal engagement and disengagement of sensorimotor and parietal areas. The results demonstrate profoundly deficient temporal preparation with preserved encoding of temporal information, a dissociation that may be explained by impaired dopamine-dependent motor learning. The results are discussed in the context of recent work on oscillatory activity in the basal ganglia.

GABAergic circuits in the basal ganglia and movement disorders

GABA is the major inhibitory neurotransmitter in the basal ganglia, and GABAergic pathways dominate information processing in most areas of these structures. It is therefore not surprising that abnormalities of GABAergic transmission are key elements in pathophysiologic models of movement disorders involving the basal ganglia. These include hypokinetic diseases such as Parkinson's disease, and hyperkinetic diseases, such as Huntington's disease or hemiballism. In this chapter, we will briefly review the major anatomic features of the GABAergic pathways in the basal ganglia, and then describe in greater detail the changes of GABAergic transmission, which are known to occur in movement disorders.

Bursting in substantia nigra pars reticulata neurons in vitro: possible relevance for Parkinson disease

Projection neurons of the substantia nigra reticulata (SNr) convey basal ganglia (BG) processing to thalamocortical and brain stem circuits responsible for movement. Two models try to explain pathological BG performance during Parkinson disease (PD): the rate model, which posits an overexcitation of SNr neurons due to hyperactivity in the indirect pathway and hypoactivity of the direct pathway, and the oscillatory model, which explains PD as the product of pathological pattern generators disclosed by dopamine reduction. These models are, apparently, incompatible. We tested the predictions of the rate model by increasing the excitatory drive and reducing the inhibition on SNr neurons in vitro. This was done pharmacologically with bath application of glutamate agonist N-methyl-d-aspartate and GABA(A) receptor blockers, respectively. Both maneuvers induced bursting behavior in SNr neurons. Therefore synaptic changes forecasted by the rate model induce the electrical behavior predicted by the oscillatory model. In addition, we found evidence that Ca(V)3.2 Ca(2+) channels are a critical step in generating the bursting firing pattern in SNr neurons. Other ion channels involved are: hyperpolarization-activated cation channels, high-voltage-activated Ca(2+) channels, and Ca(2+)-activated K(+) channels. However, although these channels shape the temporal structure of bursting, only Ca(V)3.2 Ca(2+) channels are indispensable for the initiation of the bursting pattern.

Differential involvement of M1-type and M4-type muscarinic cholinergic receptors in the dorsomedial striatum in task switching

Previous experiments have demonstrated that the rat dorsomedial striatum is one brain area that plays a crucial role in learning when conditions require a shift in strategies. Further evidence indicates that muscarinic cholinergic receptors in this brain area support adaptations in behavioral responses. Unknown is whether specific muscarinic receptor subtypes in the dorsomedial striatum contribute to a flexible shift in response patterns. The present experiments investigated whether blockade of M1-type and/or M4-type cholinergic receptors in the dorsomedial striatum underlie place reversal learning. Experiment 1 investigated the effects of the M1-type muscarinic cholinergic antagonist, muscarinic-toxin 7 (MT-7) infused into the dorsomedial striatum in place acquisition and reversal learning. Experiment 2 investigated the effects of the M4-type muscarinic cholinergic antagonist, muscarinic-toxin 3 (MT-3) injected into the dorsomedial striatum in place acquisition and reversal learning. All testing occurred in a modified cross-maze across two consecutive sessions. Bilateral injections of MT-7 into the dorsomedial striatum at 1 or 2 microg, but not 0.05 microg impaired place reversal learning. Analysis of the errors revealed that MT-7 at 1 and 2 microg significantly increased regressive errors, but not perseverative errors. An injection of MT-7 2 microg into the dorsomedial striatum prior to place acquisition did not affect learning. Experiment 2 revealed that dorsomedial striatal injections of MT-3 (0.05, 1 or 2 microg) did not affect place acquisition or reversal learning. The findings suggest that activation of M1-type muscarinic cholinergic receptors in the dorsomedial striatum, but not M4-type muscarinic cholinergic receptors facilitate the flexible shifting of response patterns by maintaining or learning a new choice pattern once selected.

Combined activation of L-type Ca2+ channels and synaptic transmission is sufficient to induce striatal long-term depression

Changes in synaptic strength at striatal synapses, such as long-term depression (LTD), may be involved in striatal-based learning and memory. Several molecular mechanisms have been implicated in striatal LTD, but it is not clear which mechanisms are crucial for LTD induction. We found that the activation of L-type calcium channels by 2,5-dimethyl-4-[2-(phenylmethyl)benzoyl]-1H-pyrrole-3-carboxylic acid methylester (FPL64176), combined with modest postsynaptic depolarization and synaptic activation, is sufficient to induce robust LTD (FPL-LTD). The L-channel activator 1,4-dihydro-2,6-dimethyl-5-nitro-4-[2(trifluoromethyl)phenyl]pyridine-3-carboxylic acid methyl ester (Bay K 8644) has a similar action. FPL-LTD occludes LTD induced by high-frequency stimulation (HFS-LTD) and requires elevated postsynaptic calcium and retrograde endocannabinoid signaling, properties similar to those of HFS-LTD. In contrast, FPL-LTD does not require the activation of metabotropic glutamate receptors (mGluRs), phospholipase C, or dopamine D2 receptors. FPL-LTD induction also requires afferent stimulation. These findings suggest a scenario in which L-type calcium channel activation is a crucial switch for LTD induction, and mGluRs and D2 receptors can be bypassed if this channel is activated.

Adenosine A2A receptors and basal ganglia physiology

Adenosine A2A receptors are highly enriched in the basal ganglia system. They are predominantly expressed in enkephalin-expressing GABAergic striatopallidal neurons and therefore are highly relevant to the function of the indirect efferent pathway of the basal ganglia system. In these GABAergic enkephalinergic neurons, the A2A receptor tightly interacts structurally and functionally with the dopamine D2 receptor. Both by forming receptor heteromers and by targeting common intracellular signaling cascades, A2A and D2 receptors exhibit reciprocal antagonistic interactions that are central to the function of the indirect pathway and hence to basal ganglia control of movement, motor learning, motivation and reward. Consequently, this A2A/D2 receptors antagonistic interaction is also central to basal ganglia dysfunction in Parkinson's disease. However, recent evidence demonstrates that, in addition to this post-synaptic site of action, striatal A2A receptors are also expressed and have physiological relevance on pre-synaptic glutamatergic terminals of the cortico-limbic-striatal and thalamo-striatal pathways, where they form heteromeric receptor complexes with adenosine A1 receptors. Therefore, A2A receptors play an important fine-tuning role, boosting the efficiency of glutamatergic information flow in the indirect pathway by exerting control, either pre- and/or post-synaptically, over other key modulators of glutamatergic synapses, including D2 receptors, group I metabotropic mGlu5 glutamate receptors and cannabinoid CB1 receptors, and by triggering the cAMP-protein kinase A signaling cascade.

Functional and ultrastructural analysis of group I mGluR in striatal fast-spiking interneurons

Striatal parvalbumin-containing fast-spiking (FS) interneurons provide a powerful feedforward GABAergic inhibition on spiny projection neurons, through a widespread arborization and electrical coupling. Modulation of FS interneuron activity might therefore strongly affect striatal output. Metabotropic glutamate receptors (mGluRs) exert a modulatory action at various levels in the striatum. We performed electrophysiological recordings from a rat striatal slice preparation to investigate the effects of group I mGluR activation on both the intrinsic and synaptic properties of FS interneurons. Bath-application of the group I mGluR agonist, (S)-3,5-dihydroxyphenylglycine (3,5-DHPG), caused a dose-dependent depolarizing response. Both (S)-(+)-alpha-amino-4-carboxy-2-methylbenzeneacetic acid (LY367385) and 7-(hydroxyimino)cyclopropa[b]chromen-1a-carboxylate ethyl ester (CPCCOEt), selective mGluR1 antagonists, significantly reduced the amplitude of the membrane depolarization caused by 3,5-DHPG application. Conversely, mGluR5 antagonists, 2-methyl-6-(phenylethylnyl)pyridine hydrochloride (MPEP) and 6-methyl-2-(phenylazo)-3-pyridinol (SIB1757), were unable to affect the response to 3,5-DHPG, suggesting that only mGluR1 contributes to the 3,5-DHPG-mediated excitatory action on FS interneurons. Furthermore, mGluR1 blockade significantly decreased the amplitude of the glutamatergic postsynaptic potentials, whereas the mGluR5 antagonist application produced a small nonsignificant inhibitory effect. Surprisingly, our electron microscopic data demonstrate that the immunoreactivity for both mGluR1a and mGluR5 is expressed extrasynaptically on the plasma membrane of parvalbumin-immunoreactive dendrites of FS interneurons. Together, these results suggest that despite a common pattern of distribution, mGluR1 and mGluR5 exert distinct functions in the modulation of FS interneuron activity.

Solving the credit assignment problem: explicit and implicit learning of action sequences with probabilistic outcomes

In most problem-solving activities, feedback is received at the end of an action sequence. This creates a credit-assignment problem where the learner must associate the feedback with earlier actions, and the interdependencies of actions require the learner to remember past choices of actions. In two studies, we investigated the nature of explicit and implicit learning processes in the credit-assignment problem using a probabilistic sequential choice task with and without a secondary memory task. We found that when explicit learning was dominant, learning was faster to select the better option in their first choices than in the last choices. When implicit reinforcement learning was dominant, learning was faster to select the better option in their last choices than in their first choices. Consistent with the probability-learning and sequence-learning literature, the results show that credit assignment involves two processes: an explicit memory encoding process that requires memory rehearsal and an implicit reinforcement-learning process that propagates credits backwards to previous choices.

Still at the Choice-Point: Action Selection and Initiation in Instrumental Conditioning

Contrary to classic stimulus-response (S-R) theory, recent evidence suggests that, in instrumental conditioning, rats encode the relationship between their actions and the specific consequences that these actions produce. It has remained unclear, however, how encoding this relationship acts to control instrumental performance. Although S-R theories were able to give a clear account of how learning translates into performance, the argument that instrumental learning constitutes the acquisition of information of the form "response R leads to outcome O" does not directly imply a particular performance rule or policy; this information can be used both to perform R and to avoid performing R. Recognition of this problem has forced the development of accounts that allow the O and stimuli that predict the O (i.e., S-O) to play a role in the initiation of specific Rs. In recent experiments, we have used a variety of behavioral procedures in an attempt to isolate the processes that contribute to instrumental performance, including outcome devaluation, reinstatement, and Pavlovian-instrumental transfer. Our results, particularly from experiments assessing outcome-selective reinstatement, suggest that both "feed-forward" (O-R) and "feed-back" (R-O) associations are critical and that although the former appear to be important to response selection, the latter-together with processes that determine outcome value-mediate response initiation. We discuss a conceptual model that integrates these processes and its neural implementation.

Plastic Corticostriatal Circuits for Action Learning: What's Dopamine Got to Do with It?

Reentrant corticobasal ganglia circuits are important for voluntary action and for action selection. In vivo and ex vivo studies show that these circuits can exhibit a plethora of short- and long-lasting plastic changes. Convergent evidence at the molecular, cellular, and circuit levels indicates that corticostriatal circuits are involved in the acquisition and automatization of novel actions. There is strong evidence that activity in corticostriatal circuits changes during the learning of novel actions, but the plastic changes observed during the early stages of learning a novel action are different than those observed after extensive training. A variety of studies indicate that the neural mechanisms and the corticostriatal subcircuits involved in the initial acquisition of actions and skills differ from those involved in their automatization or in the formation of habits. Dopamine, a critical modulator of short- and long-term plasticity in corticostriatal circuits, is differentially involved in early and late stages of action learning. Changes in dopaminergic transmission have several concomitant effects in corticostriatal function, which may be important for action selection and action learning. These diverse effects may subserve different roles for dopamine in reinforcement and action learning.

Leading tonically active neurons of the striatum from reward detection to context recognition

Tonically active neurons (TANs) in the primate striatum, which are presumed to be cholinergic interneurons, carry signals that are traditionally considered to be important for reward-related learning. Recent studies investigating the functional properties of TANs in behaving monkeys have shown that other factors beyond motivation can affect their responsiveness. There is now evidence that TAN responses reflect stimulus detection, movement control and recognition of a specific context, suggesting that these local circuit neurons contribute to different computations used in learning and action functions of the striatum. This is consistent with the view that TAN responses could represent an important component of the processes that are responsible for the ability to select the appropriate behavioral response to environmental events.

The phospholipid mediator platelet-activating factor mediates striatal synaptic facilitation

The phospholipid mediator platelet-activating factor (PAF), an endogenous modulator of glutamatergic neurotransmission, can also be secreted by brain mononuclear phagocytes during HIV-1 infection. Platelet-activating factor can induce neuronal apoptosis by NMDA receptor-dependent and independent mechanisms. We now demonstrate that acute administration of sublethal doses of PAF to striatal slices augments synaptic facilitation in striatal neurons following high-frequency stimulation, which can be blocked by PAF receptor antagonists, suggesting that striatal synaptic facilitation can be augmented by PAF receptor agonism. We also demonstrate that repeated sublethal doses of PAF during tetanic stimulation can greatly increase the magnitude of postsynaptic potentials and action potentials, but a lethal dose of PAF destroys the capacity of corticostriatal synapses to achieve this augmented synaptic facilitation. Thus, the relative concentration and temporal pattern of PAF expression at glutamatergic synapses may govern whether it acts in a physiologic or pathophysiologic manner during striatal neurotransmission.

Synchronous high-voltage spindles in the cortex-basal ganglia network of awake and unrestrained rats

Synchronous oscillations in various frequency ranges have been recorded in several nuclei of the basal ganglia (BG) and are thought to be an information processing mechanism. High-voltage spindles (HVSs) are 5-13 Hz spike-and-wave oscillations, which are commonly recorded in rats and which have been reported in some recent studies where their occurrence in the BG has been investigated. We recorded single neurons and local field potentials (LFPs) simultaneously in the motor cortex, striatum and substantia nigra pars reticulata (SNr) of the freely moving rat. We took advantage of the high level of synchronization observed during HVSs to study signal transmission in the cortex-BG network in the awake animals. The results show that LFPs are synchronized in the motor cortex, striatum and SNr during HVS episodes and that the latter propagate from the cortex to the SNr via the striatum. Moreover, > 50% of single neurons in each of these structures are triggered by the HVS. Following the discharge of cortical cells, SNr neurons are first inhibited after approximately 19 ms and then activated after approximately 45 ms. This response is probably driven by the direct and indirect pathways, respectively, without any involvement of the hyperdirect pathway. Here, it is shown that cortex-BG connectivity can be studied using physiological signals in the freely moving animal as opposed to artificial stimulation under anaesthetized conditions. This opens the door to further studies under various experimental conditions, such as animal models of basal ganglia disorders.

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.

Is there a tipping point in neuronal ensembles during learning?

Learning is important for humans and can be disrupted by disease. However, the essence of how learning may be represented within a neuronal network is still elusive. Spike trains generated by neurons have been demonstrated to carry information which is relevant for learning. The present study uses well-established mutual information (MI) analysis techniques to better understand learning within neuronal ensembles. Spike trains in tetrode recordings from the dorso-lateral striatum were used for computing MI as rats learnt a T-maze procedural task. We demonstrate that in in-vivo recordings the growth of MI is reflected in the behavioral response as learning proceeds. These changes in MI are seen to correspond to three phases, a low MI value, namely early learning, a rapid increase in MI value, task-acquisition and stabilization of MI, over-training. Over multiple training sessions, small changes in MI within the neuronal network suddenly produce a big change in ensemble MI during the task acquisition phase. This phase represents the "tipping point" in the neuronal network where the MI growth builds habits during motor learning in the striatum.

Frontal regions involved in learning of motor skill—A functional NIRS study

To investigate cerebral mechanisms underlying learning of motor skill, we assessed serial changes of cortical activation patterns during a pursuit rotor (PR) task in 18 right-handed, healthy subjects using a functional near-infrared spectroscopy (fNIRS) system. Subjects performed the task with the right hand for 30 s alternated with 30-s rest for 8 repetitions (cycle1 to 8). Gains in motor skill were evaluated by time for keeping the stylus on the target (max 30 s), surface EMG patterns and trajectories of the arm. Performance improved with repetitions of the task cycles (12.9/17.1/19.3/20.0/21.1/22.2/23.6/23.9 s on average) and reached plateau at the 7th cycle. Reciprocal EMG patterns and steady trajectories were associated with acquisition of the motor skill. Task-related increases of oxygenated hemoglobin (oxyHb) were observed in the channels covering the sensorimotor cortex (SMC), premotor and prefrontal regions. There were also task-related decreases of deoxygenated hemoglobin (deoxyHb) in these areas although the changes were smaller compared with those of oxyHb. The center of task-related increases of oxyHb was initially located in the presupplementary motor area (preSMA) and shifted caudally to the supplementary motor area (SMA) with cycle repetitions. The ratios of oxyHb changes in preSMA to SMA significantly decreased with task repetitions. DeoxyHb changes confirmed the activation patterns. These data suggest that preSMA plays an important role in the early phase of motor learning while the SMA might be more involved in the late learning phase of the motor skill.

Neural mechanisms of language comprehension: challenges to syntax

In 1980, the N400 event-related potential was described in association with semantic anomalies within sentences. When, in 1992, a second waveform, the P600, was reported in association with syntactic anomalies and ambiguities, the story appeared to be complete: the brain respected a distinction between semantic and syntactic representation and processes. Subsequent studies showed that the P600 to syntactic anomalies and ambiguities was modulated by lexical and discourse factors. Most surprisingly, more than a decade after the P600 was first described, a series of studies reported that semantic verb-argument violations, in the absence of any violations or ambiguities of syntax can evoke robust P600 effects and no N400 effects. These observations have raised fundamental questions about the relationship between semantic and syntactic processing in the brain. This paper provides a comprehensive review of the recent studies that have demonstrated P600s to semantic violations in light of several proposed triggers: semantic-thematic attraction, semantic associative relationships, animacy and semantic-thematic violations, plausibility, task, and context. I then discuss these findings in relation to a unifying theory that attempts to bring some of these factors together and to link the P600 produced by semantic verb-argument violations with the P600 evoked by unambiguous syntactic violations and syntactic ambiguities. I suggest that normal language comprehension proceeds along at least two competing neural processing streams: a semantic memory-based mechanism, and a combinatorial mechanism (or mechanisms) that assigns structure to a sentence primarily on the basis of morphosyntactic rules, but also on the basis of certain semantic-thematic constraints. I suggest that conflicts between the different representations that are output by these distinct but interactive streams lead to a continued combinatorial analysis that is reflected by the P600 effect. I discuss some of the implications of this non-syntactocentric, dynamic model of language processing for understanding individual differences, language processing disorders and the neuroanatomical circuitry engaged during language comprehension. Finally, I suggest that that these two processing streams may generalize beyond the language system to real-world visual event comprehension.

Subthalamic and striatal neurons concurrently process motor, limbic, and associative information in rats performing an operant task

Although the subthalamic nucleus (STN) is commonly assumed to be a relay for striatal (STR) output, anatomical evidence suggests the two structures are connected in parallel, raising the possibility that parallel STN and STR firing patterns mediate behavioral processes. The STR is known to play a role in associative and limbic processes, and although behavioral studies suggest that the STN may do so as well, evaluation of this hypothesis is complicated by a lack of pertinent STN physiological data. We recorded concurrent STN and STR firing patterns in rats learning an operant nose-poke task. Both structures responded in similar proportions to task events including instructive cues, discriminative nose-pokes, and sucrose reinforcement. Neuronal responses to reinforcement comprised phasic excitations preceding reinforcement and inhibitions afterward; the inhibition was attenuated when reinforcement was absent. Reinforcement responses occurred more frequently during later training sessions in which discriminative action was required, suggesting that responses were context-dependent. Nose-pokes were typically preceded by excitations; there also was a nonsignificant trend toward inhibition encoding correct nose-pokes. Sustained changes in firing rate coinciding with specific task events suggested that both nuclei were encoding behavioral sequences; this is the first report of such behavior in the STN. Our findings also reveal complex STN responses to reinforcement. Thus both STN and STR neurons show concurrent involvement in motor, limbic, and associative processes.

Dissociating language and word meaning in the bilingual brain

How do bilingual (or multilingual) persons keep different languages apart and switch between them as needs arise? Crinion et al. have used an ingenious method to dissociate brain regions sensitive to word meaning from those sensitive to the combination of meaning and language. This work should stimulate further research examining the role of subcortical areas in language processing and in context-appropriate language production.

Motor-skill learning-associated gene regulation in the striatum: effects of cocaine

Psychostimulant-induced molecular changes in cortico-basal ganglia-cortical circuits play a critical role in addiction and dependence. These changes include alterations in gene regulation particularly in projection neurons of the sensorimotor striatum. We previously showed that cocaine-induced gene regulation in such neurons is dependent on the behavior performed during drug action. Rats trained on a running wheel under the influence of cocaine for 4 days subsequently displayed greater c-fos induction by cocaine than untrained controls. This effect was selective for the sensorimotor striatum, which is known to mediate forms of motor learning. In the present study, we investigated whether this enhanced cellular responsiveness was associated with learning of wheel running or with prolonged running (exercising), by assessing c-fos inducibility after 1, 2, or 8 days of training. Wheel training was performed after injection of cocaine (25 mg/kg) or vehicle, and c-fos induction by a cocaine challenge was measured 24 h later. Rats that trained under cocaine (but not vehicle) showed a greater c-fos response in the striatum compared to locked-wheel controls. This effect was present after the 1-day training, peaked after 2 days, and dissipated by 8 days of training. Similar effects were found for substance P, but not enkephalin, expression. These changes in striatal gene regulation paralleled improvement in wheel running, which was facilitated by cocaine. Thus, these training-induced molecular changes do not appear to represent exercising effects, but may reflect motor learning-associated neuronal changes altered by cocaine. Such cocaine effects may contribute to aberrant motor learning implicated in psychostimulant addiction.

The effect of N-methyl-D-aspartate receptor blockade on acetylcholine efflux in the dorsomedial striatum during response reversal learning

Separate experiments found that activation of N-methyl-d-aspartate (NMDA) receptors or increased acetylcholine (ACh) efflux in the rat dorsomedial striatum is critical for learning when conditions require a shift in strategies. Increasing evidence indicates that NMDA receptor activity affects cholinergic efflux in the basal ganglia. The present studies determined whether NMDA receptor blockade in the dorsomedial striatum with dl-2-amino-5-phosphonopentanoic acid (AP-5) affects dorsomedial striatal ACh output in a resting condition, as well as during response reversal learning. Experiment 1 investigated the effects of AP-5 (12.5, 25 or 50 muM) infused into the dorsomedial striatum on ACh output in a resting condition. AP-5 infusion at 25 and 50 muM led to a 20% and 40% decrease in dorsomedial striatal ACh output, respectively. AP-5 (12.5 muM) infusion did not change dorsomedial striatal ACh output from basal levels. Experiment 2 determined whether dorsomedial striatal ACh efflux increases during response reversal learning and whether AP-5, at a dose that does not affect basal levels, modifies response reversal learning and ACh efflux. Following acquisition of a response discrimination, rats had microdialysis probes bilaterally inserted into the dorsomedial striatum prior to the reversal learning test. After baseline samples, rats received a response reversal learning test for 30 min. Control rats rapidly improved in the reversal learning session while simultaneously exhibiting an approximately 40% increase in ACh output compared with baseline levels. AP-5 (12.5 muM) treatment during testing significantly impaired response reversal learning while concomitantly blocking an increase in ACh output. These findings suggest that NMDA receptor activation in the dorsomedial striatum may facilitate a shift in response patterns, in part, by increasing ACh efflux.

Developmental neural networks in children performing a Categorical N-Back Task

The prefrontal and temporal networks subserving object working memory tasks in adults have been reported as immature in young children; yet children are adequately capable of performing such tasks. We investigated the basis of this apparent contradiction using a complex object working memory task, a Categorical n-back (CN-BT). We examined whether the neural networks engaged by the CN-BT in children consist of the same brain regions as those in adults, but with a different magnitude of activation, or whether the networks are qualitatively different. Event-related fMRI was used to study differences in brain activation between healthy children ages 6 and 10 years, and young adults (20-28 years). Performance accuracy and RTs in 10-year-olds and adults were comparable, but the performance in 6-year-olds was lower. In adults, the CN-BT was highly effective in engaging the bilateral (L>R) ventral prefrontal cortex, the bilateral fusiform gyrus, posterior cingulate and precuneus, thus suggesting an involvement of the ventral visual stream, with related feature extraction and semantic labeling strategies. In children, the brain networks were distinctly different. They involved the premotor and parietal cortex, anterior insula, caudate/putamen, and the cerebellum, thus suggesting a predominant involvement of the visual dorsal and sensory-motor pathways, with related visual-spatial and action cognitive strategies. The findings indicate engagement of developmental networks in children reflecting task-effective brain activation. The age-related pattern of fMRI activation suggests a working hypothesis of a developmental shift from reliance on the dorsal visual stream and premotor/striatal/cerebellar networks in young children to reliance on the ventral prefrontal and inferior temporal networks in adults.

Endocannabinoid-mediated synaptic plasticity in the CNS

Changes in synaptic efficacy are thought to be crucial to experience-dependent modifications of neural function. The diversity of mechanisms underlying these changes is far greater than previously expected. In the last five years, a new class of use-dependent synaptic plasticity that requires retrograde signaling by endocannabinoids (eCB) and presynaptic CB1 receptor activation has been identified in several brain structures. eCB-mediated plasticity encompasses many forms of transient and long-lasting synaptic depression and is found at both excitatory and inhibitory synapses. In addition, eCBs can modify the inducibility of non-eCB-mediated forms of plasticity. Thus, the eCB system is emerging as a major player in synaptic plasticity. Given the wide distribution of CB1 receptors in the CNS, the list of brain structures and synapses expressing eCB-mediated plasticity is likely to expand.

Post- versus presynaptic plasticity in L-DOPA-induced dyskinesia

L-3,4-dihydroxyphenylalanine (L-DOPA) remains the most efficacious drug for the treatment of Parkinson's disease (PD), but causes adverse effects that limit its utility. L-DOPA-induced dyskinesia (abnormal involuntary movements) is a significant clinical problem that attracts growing scientific interest. Current notions attribute the development of dyskinesia to two main factors, viz. the loss of nigrostriatal dopamine (DA) projections and the maladaptive changes produced by L-DOPA at sites postsynaptic to the nigrostriatal neuron. Basic research in the past 15 years has placed a lot of emphasis on the postsynaptic plasticity associated with dyskinesia, but recent experimental work shows that also some presynaptic factors, involving the regulation of L-DOPA/DA release and metabolism in the brain, may show plasticity during treatment. This review summarizes significant studies of L-DOPA-induced dyskinesia in patients and animal models, and outlines directions for future experiments addressing mechanisms of presynaptic plasticity. These investigations may uncover clues to the varying susceptibility to L-DOPA-induced dyskinesia among PD patients, paving the way for tailor-made treatments.

Delay period activity of monkey prefrontal neurones during duration-discrimination task

Evidence from brain imaging studies has indicated involvement of the prefrontal cortex (PFC) in time perception; however, the role of this area remains unclear. To address this issue, we recorded single neuronal activity from the PFC of two monkeys while they performed a duration-discrimination task. In the task, two visual cues (a blue or red square) were presented consecutively followed by delay periods and subjects then chose the cue presented for the longer duration. Durations of both cues, order of cue duration [long-short (LS) or short-long (SL)] and order of cue colour (blue-red or red-blue) were randomized on a trial-by-trial basis. We found that subjects responded differently between LS and SL trials and that most prefrontal neurones showed significantly different activity during either the first or the second delay period when comparing activity in LS and SL trials. The present result offers new insights into neural mechanisms of time perception. It appears that, during the delay periods, the PFC contributes to implement a strategic process in temporal processing associated with a trial type (LS or SL) such as representation of the trial type, retention of cue information and anticipation of the forthcoming cue.

Ethanol activates cAMP response element-mediated gene expression in select regions of the mouse brain

The specific brain regions that contribute to behavioral changes produced by ethanol are not clearly understood. We know that cAMP-PKA signaling has been strongly implicated in the CNS effects of ethanol. Ethanol promotes activation and translocation of the PKA catalytic subunit (Calpha) into the nucleus in cell lines and primary neuronal cultures. PKA Calpha translocation to the nucleus is followed by cAMP Response Element protein phosphorylation (pCREB) and cAMP Response Element (CRE)-mediated gene expression. Here, we use X-gal histochemistry to map CRE-mediated gene transcription in the brain of CRE-lacZ transgenic mice following ethanol injection.

RESULTS

3 h after i.p. ethanol injection (3.2 g/kg, 16% wt/vol.), the number of X-gal positive cells was increased in the nucleus accumbens (202 +/- 63 cells/field compared to 71 +/- 47 cells/field in saline injected controls, P < 0.05 by paired t-test, n = 10). Similar increases were found in other mesolimbic areas and brain regions associated with rewarding and addictive responses. These include: prefrontal cortex, lateral and medial septum, basolateral amygdala, paraventricular and anterior hypothalamus, centromedial thalamus, CA1 region of hippocampus and dentate gyrus, substantia nigra pars compacta, ventral tegmental area, geniculate nucleus and the superior colliculus.

CONCLUSION

these results confirm and extend current concepts that ethanol stimulates cAMP-PKA signaling in brain regions involved in CNS responses to ethanol.

Cortico–basal ganglia circuit mechanism for a decision threshold in reaction time tasks

Growing evidence from primate neurophysiology and modeling indicates that in reaction time tasks, a perceptual choice is made when the firing rate of a selective cortical neural population reaches a threshold. This raises two questions: what is the neural substrate of the threshold and how can it be adaptively tuned according to behavioral demands? Using a biophysically based network model of spiking neurons, we show that local dynamics in the superior colliculus gives rise to an all-or-none burst response that signals threshold crossing in upstream cortical neurons. Furthermore, the threshold level depends only weakly on the efficacy of the cortico-collicular pathway. In contrast, the threshold and the rate of reward harvest are sensitive to, and hence can be optimally tuned by, the strength of cortico-striatal synapses, which are known to be modifiable by dopamine-dependent plasticity. Our model provides a framework to describe the main computational steps in a reaction time task and suggests that separate brain pathways are critical to the detection and adjustment of a decision threshold.

Nigral injection of antisense oligonucleotides to synaptotagmin I using HVJ-liposome vectors causes disruption of dopamine release in the striatum and impaired skill learning

To produce an animal model of a dopa-responsive motor disorder with depletion of dopamine (DA) release in the striatum by dysfunction of the transmitter release machinery of the nigrostriatal DA system, we performed an intra-nigral injection of an HVJ-liposome gene transfer vector containing antisense oligodeoxynucleotides (ODNs) against synaptotagmin I (SytI), a key regulator of Ca(2+)-dependent exocytosis and endocytosis in adult rats. A unilateral intra-nigral injection of HVJ-liposome vectors containing antisense ODNs against SytI (syt-AS) caused a moderate disruption of methamphetamine-induced release of DA in the treated side of the striatum, while the syt-AS treatment did not affect physiological release of DA in the treated striatum. A bilateral intra-nigral injection of HVJ-liposome vectors containing syt-AS induced an impairment of the striatal DA-mediated acquisition of skilled behavior in a rotarod task without any deficits in general motor functions, such as spontaneous locomotor activity, motor adjusting steps, equilibrium function, or muscle strength. These findings suggest that an intra-nigral treatment with HVJ-liposome vectors containing syt-AS may cause a long-lasting nigral knockdown of SytI which, in turn, leads to a moderate dysfunction of the DA release machinery in the terminals of the nigrostriatal DA system and a subsequent mild depletion of DA release in the striatum.

Long-lasting spontaneous calcium transients in the striatal cells

The striatum plays an important role in linking cortical activity to basal ganglia output. We conducted the calcium (Ca2+) imaging to investigate the spontaneous activities of the striatum using acute slice preparations. Corticostriatal slices of rat brain were stained with Fura-PE3-AM. Long-lasting spontaneous intracellular Ca2+ ([Ca2+]i) transients, which lasted up to about 250 s, were observed. The amplitudes of the transients were variable even in a single cell. Most cells exhibited irregular frequencies, but some exhibited oscillatory features. These [Ca2+]i transients were not induced by action potentials because they were not inhibited by tetrodotoxin. Antagonists of the ionotropic glutamate receptors, 6-cyano-7-nitroquinoxaline-2,3-dione and D,L-2-amino-5-phosphonovaleric acid, did not block these transients. These results suggested that the action potentials and the excitatory synaptic inputs in these striatal network were not involved in the induction of the [Ca2+]i transients. In contrast, the number of the active cells, which exhibited the [Ca2+]i transients, was greatly reduced by the intracellular Ca2+ store depletor, thapsigargin. Therefore, the intracellular Ca2+ store is likely to contribute to the [Ca2+]i transients.

Effects of chronic nicotine administration and its withdrawal on striatal FosB/DeltaFosB and c-Fos expression in rats and mice

DeltaFosB, a member of Fos family of transcription factors, is implicated in behavioural responses and synaptic plasticity induced by abused drugs. We studied the expressions of FosB/DeltaFosB and c-Fos immunohistochemically in two dopaminergic brain areas, nucleus accumbens (NAcc) and caudate-putamen (CPu). In mice neither 2- nor 7-week oral nicotine treatment induced expression of long-lived DeltaFosB isoforms although during the treatment in the NAcc FosB/DeltaFosB expression was increased as was c-Fos in the CPu. In rats given nicotine subcutaneously once daily for 5days FosB/DeltaFosB expression was elevated in the NAcc still after 24-h withdrawal suggesting accumulation of DeltaFosB but in the CPu neither FosB/DeltaFosB nor c-Fos expression was altered. Thus, in rats repeated nicotine administration seems mainly affect the NAcc paralleling with the evidence that nicotine stimulates preferentially mesolimbic dopamine system. Also, repeated nicotine induced behavioural sensitization in rats agreeing with suggested role of DeltaFosB in the development of psychomotor sensitization. However, in mice given nicotine via drinking fluid although striatal fosB and c-fos were activated by nicotine even after 7-week treatment no evidence of accumulation of long-lived DeltaFosB was found suggesting perhaps a species difference or more likely a role for the manner of administration.

A model for integrating elementary neural functions into delayed-response behavior

It is well established that various cortical regions can implement a wide array of neural processes, yet the mechanisms which integrate these processes into behavior-producing, brain-scale activity remain elusive. We propose that an important role in this respect might be played by executive structures controlling the traffic of information between the cortical regions involved. To illustrate this hypothesis, we present a neural network model comprising a set of interconnected structures harboring stimulus-related activity (visual representation, working memory, and planning), and a group of executive units with task-related activity patterns that manage the information flowing between them. The resulting dynamics allows the network to perform the dual task of either retaining an image during a delay (delayed-matching to sample task), or recalling from this image another one that has been associated with it during training (delayed-pair association task). The model reproduces behavioral and electrophysiological data gathered on the inferior temporal and prefrontal cortices of primates performing these same tasks. It also makes predictions on how neural activity coding for the recall of the image associated with the sample emerges and becomes prospective during the training phase. The network dynamics proves to be very stable against perturbations, and it exhibits signs of scale-invariant organization and cooperativity. The present network represents a possible neural implementation for active, top-down, prospective memory retrieval in primates. The model suggests that brain activity leading to performance of cognitive tasks might be organized in modular fashion, simple neural functions becoming integrated into more complex behavior by executive structures harbored in prefrontal cortex and/or basal ganglia.

Before we do anything, our brain must construct neural representations of the operations required. Imaging and recording techniques are indeed providing ever more detailed insight into how different regions of the brain contribute to behavior. However, it has remained elusive exactly how these various regions then come to cooperate with each other, thus organizing the brain-scale activity patterns needed for even the simplest planned tasks. In the present work, the authors propose a neural network model built around the hypothesis of a modular organization of brain activity, where relatively autonomous basic neural functions useful at a given moment are recruited and integrated into actual behavior. At the heart of the model are regulating structures that restrain information from flowing freely between the different cortical areas involved, releasing it instead in a controlled fashion able to produce the appropriate response. The dynamics of the network, simulated on a computer, enables it to pass simple cognitive tests while reproducing data gathered on primates carrying out these same tasks. This suggests that the model might constitute an appropriate framework for studying the neural basis of more general behavior.

Translational Studies in Neurorehabilitation: From Bench to Bedside

Stroke is the leading cause of adult disability in the western world. Consensus has built over the last few years regarding the usefulness of training to improve motor disability resulting from stroke. Until recently, there were no accepted strategies to enhance the beneficial effects of training. However, the combination of basic and clinical science data over the last few years is changing this picture, and is highly relevant to the field of neurorehabilitation. Human studies in both healthy individuals and patients after brain damage demonstrate as a proof of principle that somatosensory input, cortical stimulation, interhemispheric interactions, and pharmacologic interventions can modulate cortical plasticity in neurorehabilitation after stroke. These findings strongly suggest directions in the development of novel strategies to enhance training effects on motor recovery. The intent of this review is to describe these strategies, the basic science principles on which they are based, and the clinical applications that have emerged so far.

The role of striatum in initiation and execution of learned action sequences in rats

To understand the role of striatum in motor sequence learning, we trained rats to perform a series of tasks measuring speed and accuracy of responding to luminance cues presented as discriminative stimuli for single nose pokes or for sequences of nose pokes in a serial reaction time task. Habit (stimulus-response) learning was measured by comparing performances when stimuli were repeated (predictable) with when they were selected randomly (unpredictable). Sequences had defined start and end points and were limited to five nose pokes to minimize chunking. When sequences were repeated, response time (RT) increased for nose pokes initiating the sequence and decreased for nose pokes completing it. These effects developed incrementally across sessions, consistent with the time course of habit learning. Medial (mCPu), lateral, and complete (CPu) caudate-putamen lesions affected speed and accuracy of single nose poke responses, confirming the role of these areas in guiding responses with external sensory stimuli. None of these lesions affected the short-term increase in accuracy observed when single nose poke responses were repeated. Both mCPu and CPu lesions increased RTs for initiating sequential responses, effects that were exacerbated across sessions in which specific sequences were repeated. None of the lesions affected the gradual decrease in RT for nose pokes completing repeated sequences. Correlational analyses confirmed the relationship between the extent of dorsal striatal damage and the ability to respond to brief luminance cues and to initiate learned sequences. These results provide evidence implicating dorsal striatum in higher-level organizational aspects of learning reflected in planning that precedes the execution of learned action sequences.

Activity of tonically active neurons in the monkey putamen during initiation and withholding of movement

Tonically active neurons (TANs) of the primate striatum are putative interneurons that respond to events of motivational significance, such as primary rewards, and to sensory stimuli that predict such events. Because TANs influence striatal projection neurons, TANs may play a role in the initiation and control of movement. To examine this issue, we recorded from putaminal TANs in macaque monkeys trained to make the same arm movement in two ways--in reaction to an external cue and also after a variable delay without an explicit instruction to move (self-timed movements). On other trials, the animals had to withhold movement following an external cue. The task design ensured that the three types of trials were effectively randomly interleaved, equally frequent, and similar in overall timing. Separately, we presented "playback" trials in which the same sequence of visual stimulation and reward was presented while the animals fixated without making the arm movement. We found that TAN responses were strongly affected by behavioral context. In particular, TAN responses were strikingly stronger when the animals actively withheld movements than on the corresponding playback trials, even though the stimulus sequence and reward timing were identical and no movement was made in either case. Many TANs also became active in the absence of a proximate sensory cue on self-timed movements, suggesting that TANs may reflect internal processes that are specific to self-timed movements. These results suggest that TANs may directly participate in, or monitor the motivational significance of, an animal's actions as well as external events.

Expression of protein kinase C-substrate mRNAs in the basal ganglia of adult and infant macaque monkeys

We performed in situ hybridization histochemistry on the monkey basal ganglia to investigate the mRNA localization of three protein kinase C substrates (GAP-43, MARCKS, and neurogranin), of which expression plays a role in structural changes in neurites and synapses. Weak hybridization signals for GAP-43 mRNA and intense signals for both MARCKS and neurogranin mRNAs were observed in the adult neostriatum. All three of the mRNAs were expressed in both substance P-positive direct pathway neurons and enkephalin-positive indirect pathway neurons. In the nucleus accumbens, the hybridization signals for the three mRNAs were weaker than those in the neostriatum. Double-label in situ hybridization histochemistry in the neostriatum revealed that GAP-43 and neurogranin mRNAs were expressed in a subset of MARCKS-positive neurons. While intense hybridization signals for MARCKS mRNA were observed in all of the other basal ganglia regions such as the globus pallidus, substantia innominata, subthalamic nucleus, and substantia nigra, intense signals for GAP-43 mRNA were restricted to the substantia innominata and substantia nigra pars compacta. No signal for neurogranin mRNA was observed in the basal ganglia regions outside the neostriatum and the nucleus accumbens. These results indicate that the protein kinase C substrates are abundant in some specific connections in cortico-basal ganglia circuits. Developmental analysis showed that the expression level in the putamen and nucleus accumbens, but not in the caudate nucleus, was higher in the infant than in the adult, suggesting that synaptic maturation in the caudate nucleus occurs earlier than that in the putamen and nucleus accumbens.

Premotor neuronal plasticity in monkeys adapting to a new dynamic environment

Recent evidence indicates that premotor cortex (PM) in addition to their well-established motor functions, also play a role in nonmotor processes such as spatial attention and working memory. In the present study, neuronal activities in dorsal PM (PMd) and ventral PM (PMv) were recorded in a force field adaptation task. This study found that PM neurons show learning-related plasticity and that a neuron demonstrates either one type or multiple types of properties (i.e. kinematic, dynamic, and memory). The current study reveals that memory properties could be displayed by one or a combination of the cell activity parameters [i.e. average firing rate (AFR), dynamic range (DR), and preferred direction (PD)]. A predominant percentage of cells displayed memory properties with AFR or AFR plus other parameters. This study investigated the memory properties vs. the time sequence of the task trial [i.e. delay time (DT), movement time (MT), and target holding time (THT)] and found that: (i) most neurons display memory properties only in one time window; (ii) few neurons display memory properties in three time windows, and (iii) there are significantly more cells showing memory properties during MT than during any other time windows. There are cells that show memory I (changing their tuning curves in the force field and retaining those changes after the force field was removed), memory II (changing their tuning curves after the force field was removed), or both properties. Significantly more cells display one type of memory property (memory I or memory II) rather than both types of memory properties (memory I and memory II).

A direct projection from superior colliculus to substantia nigra pars compacta in the cat

Dopaminergic neurons exhibit a short-latency, phasic response to unexpected, biologically salient stimuli. The midbrain superior colliculus also is sensitive to such stimuli, exhibits sensory responses with latencies reliably less than those of dopaminergic neurons, and, in rat, has been shown to send direct projections to regions of the substantia nigra and ventral tegmental area containing dopaminergic neurons (e.g. pars compacta). Recent electrophysiological and electrochemical evidence also suggests that tectonigral connections may be critical for relaying short-latency (<100 ms) visual information to midbrain dopaminergic neurons. By investigating the tectonigral projection in the cat, the present study sought to establish whether this pathway is a specialization of the rodent, or whether it may be a more general feature of mammalian neuroanatomy. Anterogradely and retrogradely transported anatomical tracers were injected into the superior colliculus and substantia nigra pars compacta, respectively, of adult cats. In the anterograde experiments, abundant fibers and terminals labeled with either biotinylated dextran amine or Phaseolus vulgaris leucoagglutinin were seen in close association with tyrosine hydroxylase-positive (dopaminergic) somata and processes in substantia nigra pars compacta and the ventral tegmental area. In the retrograde experiments, injections of biotinylated dextran amine into substantia nigra produced significant retrograde labeling of tectonigral neurons of origin in the intermediate and deep layers of the ipsilateral superior colliculus. Approximately half of these biotinylated dextran amine-labeled neurons were, in each case, shown to be immunopositive for the calcium binding proteins, parvalbumin or calbindin. Significantly, virtually no retrogradely labeled neurons were found either in the superficial layers of the superior colliculus or among the large tecto-reticulospinal output neurons. Taken in conjunction with recent data in the rat, the results of this study suggest that the tectonigral projection may be a common feature of mammalian midbrain architecture. As such, it may represent an additional route by which short-latency sensory information can influence basal ganglia function.

Steady-dose and escalating-dose "binge" administration of cocaine alter expression of behavioral stereotypy and striatal preprodynorphin mRNA levels in rats

This study examined the effects of chronic (14-day) steady-dose and escalating-dose "binge" pattern cocaine administration on striatal preprodynorphin (ppDyn) mRNA levels and behavioral stereotypies. Animals in the steady-state and escalating groups received cocaine in a "binge" pattern (three equal injections starting 30 min following the start of the daily light cycle, separated by 1 h). The dose of cocaine in the "steady-dose" group was 15 mg/kg/injection and remained constant throughout the study. The escalating group received 15 mg/kg/injection on days 1-3, 20 mg/kg/injection on days 4-6, 25 mg/kg/injection on days 7-9 and 30 mg/kg/injection thereafter, for a maximum daily dose of 90 mg/kg. Levels of ppDyn mRNA were determined by solution hybridization. Cocaine significantly affected body weight. Both steady-dose and escalating-dose "binge" cocaine administration resulted in expression of behavioral stereotypy and induced intense, rapid head movements which were dose- and time-dependent. Cocaine, independent of dose, increased ppDyn mRNA levels in the caudate putamen (CPu), but not in the nucleus accumbens (NAc). These data suggest that the ppDyn response to cocaine in the CPu is not dose-dependent or that it has reached a maximal level at the 45 mg/kg daily dose.

Psychostimulants, madness, memory... and RGS proteins?

The ingestion of psychostimulant drugs by humans imparts a profound sense of alertness and well-being. However, repeated use of these drugs in some individuals will induce a physiological state of dependence, characterized by compulsive behavior directed toward the acquisition and ingestion of the drug, at the expense of customary social obligations. Drugs of abuse and many other types of experiences share the ability to alter the morphology and density of neuronal dendrites and spines. Dopaminergic modulation of corticostriatal synaptic plasticity is necessary for these morphological changes. Changes in the density of dendritic spines on striatal neurons may underlie the development of this pathological pattern of drug-seeking behavior. Identifying proteins that regulate dopaminergic signaling are of value. A family of proteins, the regulators of G protein signaling (RGS) proteins, which regulate signaling from G protein-coupled receptors, such as dopamine and glutamate, may be important in this regard. By regulating corticostriatal synaptic plasticity, RGS proteins can influence presynaptic activity, neurotransmitter release, and postsynaptic depolarization and thereby play a key role in the development of this plasticity. Pharmacological agents that modify RGS activity in humans could be efficacious in ameliorating the dependence on psychostimulant drugs.

Bidirectional activity-dependent plasticity at corticostriatal synapses

Corticostriatal projections originate from the entire cerebral cortex and provide the major source of glutamatergic inputs to the basal ganglia. Despite the importance of corticostriatal connections in sensorimotor learning and cognitive functions, plasticity forms at these synapses remain strongly debated. Using a corticostriatal slice preserving the connections between the somatosensory cortex and the target striatal cells, we report the induction of both non-Hebbian and Hebbian forms of long-term potentiation (LTP) and long-term depression (LTD) on striatal output neurons (SONs). LTP and LTD can be induced selectively by different stimulation patterns (high-frequency trains vs low-frequency pulses) and were evoked with similar efficiency in non-Hebbian and Hebbian modes. Combination of LTP-LTD and LTD-LTP sequences revealed that bidirectional plasticity occurs at the same SONs and provides efficient homeostatic mechanisms leading to a resetting of corticostriatal synapses avoiding synaptic saturation. The effect of temporal relationship between cortical stimulation and SON activity was assessed using spike-timing-dependent plasticity (STDP) protocols. An LTP was observed when an action potential was triggered in the striatal neuron before the cortical stimulus, and, conversely, an LTD was induced when the striatal neuron discharge was triggered after the cortical stimulation. Such STDP was reversed when compared with those described so far in other mammalian brain structures. This mechanism may be essential for the role of the striatum in learning of motor sequences in which sensory and motor events are associated in a precise time sequence.

Neuron Activity in the Monkey Striatum of Identifies Integration Sequential Actions into Functional Blocks

Spike activity in monkey striatum (putamen) neurons was recorded during the performance of a complex multistep operant task. Tonic responses propagating beyond a single action were recorded, along with phasic responses seen within a given action. The tonic type of response was recorded in 132 of 148 cells. Only 11 of these neurons showed exclusively this type of activity. The beginnings and ends of tonic responses were generally associated with key moments in the behavior, corresponding to the triggering and completion of immediate aims during the performance of the behavioral program as a whole. These results provide evidence that the role of the striatum is not limited to controlling single sequentially performed actions, but spreads to the whole structure of a behavioral act.

Distinct basal ganglia territories are engaged in early and advanced motor sequence learning

In this study, we used functional MRI (fMRI) at high field (3T) to track the time course of activation in the entire basal ganglia circuitry, as well as other motor-related structures, during the explicit learning of a sequence of finger movements over a month of training. Fourteen right-handed healthy volunteers had to practice 15 min daily a sequence of eight moves using the left hand. MRI sessions were performed on days 1, 14 and 28. In both putamen, activation decreased with practice in rostrodorsal (associative) regions. In contrast, there was a significant signal increase in more caudoventral (sensorimotor) regions of the putamen. Subsequent correlation analyses between signal variations and behavioral variables showed that the error rate (movement accuracy) was positively correlated with signal changes in areas activated during early learning, whereas reaction time (movement speed) was negatively correlated with signal changes in areas activated during advanced learning stages, including the sensorimotor putamen and globus pallidus. These results suggest the possibility that motor representations shift from the associative to the sensorimotor territories of the striato-pallidal complex during the explicit learning of motor sequences, suggesting that motor skills are stored in the sensorimotor territory of the basal ganglia that supports a speedy performance.

Variable rate of singing and variable song duration are associated with high immediate early gene expression in two anterior forebrain song nuclei

The duration of songs and the intervals between these songs are more variable when wild, adult, free-ranging chipping sparrows sing at dawn than when they sing during the day. The more variable delivery is used to interact with males, and the stereotyped delivery is used to attract females. In captive birds, however, the variability observed at dawn persists during the day. We quantified the expression of an immediate early gene, ZENK, in wild and captive birds and found that the level of song-associated ZENK expression in two song nuclei, Area X and lMAN, was positively related to variability in song duration and intersong interval and could be dissociated from the social context in which the song occurred. Thus, a combination of field and laboratory approaches helped us identify nuclei, context, and behavioral features associated with a change in gene expression thought to be a marker of behavioral variability.

A positron emission tomography (PET) investigation of the role of striatal dopamine (D2) receptor availability in spatial cognition

The functional significance of age-related changes in regional brain dopamine (DA) function is poorly understood in health. Two recent studies have reported positive linear associations between measures of striatal DA (D2) receptor availability (binding potential) and specific aspects of motor and cognitive performance, after controlling for the effects of age [(Volkow, N.D., Gur, R.C., Wang, G.-J., Fowler, J.S., Moberg, P.J., Ding, Y.-S., Hitzemann, R., Smith, G., Logan, J., 1998. Association between decline in brain dopamine activity with age and cognitive and motor impairment in healthy individuals. Am. J. Psychiatry 155 (3), 344-349; Backman, L., Ginovart, N., Dixon, R.A., Wahlin, T.-B.R., Wahlin, A., Halldin, C., Farde, L., 2000. Age-related cognitive deficits mediated by changes in the striatal dopamine system. Am. J. Psychiatry 157 (4), 635-637)]. We investigated the relationship between measures of striatal DA (D2) receptor availability and visuo-spatial cognitive performance in thirty healthy post-menopausal women aged 58-90 years. [(11)C] Raclopride (RAC) positron emission tomography (PET) was used to assess dopamine (D2) receptor availability. The CANTAB neuropsychological test battery was used to assess spatial span, spatial working memory (SWM) and planning ability. Age showed significant linear correlations with several of the CANTAB performance measures. After controlling for age effects, DA (D2) receptor measures in left-sided striatal regions (caudate and putamen) were significantly and positively correlated with 'perfect solution' scores--the measure of performance accuracy--on the Tower of London (TOL) test of spatial planning. When this relationship was examined in relation to task difficulty, only perfect scores for the most 'difficult' (4-move) problems were significantly correlated with BP measures in all striatal regions, most notably the right and left caudate nuclei. Paradoxically, individuals with higher DA (D2) receptor measures in the right caudate performed less accurately on the SWM task, exhibiting a higher number of errors within each search sequence. The relative contribution of striatal DA (D2) receptor availability to specific aspects of cognitive performance needs to be evaluated in larger mixed-sex samples to facilitate the meaningful investigation of the potential therapeutic benefits of DA (D2) agonists in later life.

Specific language impairment is not specific to language: the procedural deficit hypothesis

Specific Language Impairment (SLI) has been explained by two broad classes of hypotheses, which posit either a deficit specific to grammar, or a non-linguistic processing impairment. Here we advance an alternative perspective. According to the Procedural Deficit Hypothesis (PDH), SLI can be largely explained by the abnormal development of brain structures that constitute the procedural memory system. This system, which is composed of a network of inter-connected structures rooted in frontal/basal-ganglia circuits, subserves the learning and execution of motor and cognitive skills. Crucially, recent evidence also implicates this system in important aspects of grammar. The PDH posits that a significant proportion of individuals with SLI suffer from abnormalities of this brain network, leading to impairments of the linguistic and non-linguistic functions that depend on it. In contrast, functions such as lexical and declarative memory, which depend on other brain structures, are expected to remain largely spared. Evidence from an in-depth retrospective examination of the literature is presented. It is argued that the data support the predictions of the PDH, and particularly implicate Broca's area within frontal cortex, and the caudate nucleus within the basal ganglia. Finally, broader implications are discussed, and predictions for future research are presented. It is argued that the PDH forms the basis of a novel and potentially productive perspective on SLI.

Thalamic regulation of striatal acetylcholine efflux is both direct and indirect and qualitatively altered in the dopamine-depleted striatum

Striatal cholinergic interneurons play a pivotal role in the integrative sensorimotor functions of the basal ganglia. The major excitatory input to these interneurons arises from glutamatergic neurons of the parafascicular nucleus of the thalamus (Pf). Thalamic regulation of cholinergic interneurons, however, may also include an indirect inhibitory component mediated by the axon collaterals of GABAergic medium spiny neurons that are also innervated by Pf. The present study examined thalamic regulation of striatal cholinergic interneurons by employing dual probe in vivo microdialysis in freely moving animals to determine the effect of pharmacological manipulation of Pf on acetylcholine (ACh) efflux in intact and dopamine-lesioned striata. In intact animals, reverse dialysis application of the GABA(A) antagonist bicuculline (50 microM) into Pf, likely disinhibiting Pf neurons, significantly decreased striatal ACh efflux. When striatal GABA(A) receptors were blocked by simultaneous reverse dialysis application of bicuculline (10 microM), however, the same manipulation significantly increased ACh efflux. Qualitatively similar results were obtained in experiments employing a higher concentration of bicuculline (200 microM). Application of the GABA agonist muscimol (500 microM) into Pf, likely inhibiting Pf neurons, decreased ACh efflux only when the experiment was conducted under blockade of striatal GABA(A) receptors. These data are consistent with the existence of an indirect, inhibitory, GABA(A) receptor-mediated component of ACh regulation that is most clearly manifested when Pf is disinhibited and with the existence of a direct excitatory component of ACh regulation, evident when Pf is inhibited. Manipulation of Pf using very high concentrations of drug (500 microM bicuculline, 2 mM muscimol), however, yielded data consistent only with direct excitatory thalamic regulation. In contrast to results obtained in intact animals, in animals with prior (3 weeks) unilateral lesion of the dopaminergic nigrostriatal pathway, bicuculline application (50 muM) in Pf significantly increased striatal ACh efflux, irrespective of simultaneous blockade of striatal GABA(A) receptors. The results of experiments in which muscimol (500 microM) was applied in Pf were similar to those obtained in intact animals, however. Baseline ACh efflux was not significantly elevated in dopamine-lesioned animals. These results indicate a qualitative alteration in the effectiveness of an inhibitory component of the thalamic regulation of ACh efflux in the dopamine depleted striatum, evident during increased thalamostriatal input. Such altered regulation of striatal ACh output is likely to have profound consequences for integrative function in the parkinsonian basal ganglia.

Feedforward inhibition of projection neurons by fast-spiking GABA interneurons in the rat striatum in vivo

Discharge activities and local field potentials were recorded in the orofacial motor cortex and in the corresponding rostrolateral striatum of urethane-anesthetized rats. Striatal projection neurons were identified by antidromic activation and fast-spiking GABAergic interneurons (FSIs) by their unique characteristics: briefer spike and burst responses. Juxtacellular injection of neurobiotin combined with parvalbumin immunohistochemistry validated this identification. Spontaneous activities and spike responses to cortical stimulation were recorded during both states of cortical activity: slow waves and desynchronization. Both FSI and projection neurons spontaneously discharged synchronously with slow waves at the maximum of cortical activity, but, on average, FSIs were much more active. Cortical desynchronization enhanced FSI activity and facilitated their spike responses to cortical stimulation, whereas opposite effects were observed regarding projection neurons. Experimental conditions favoring FSI discharge were always associated with a decrease in the firing activity of projection neurons. Spike responses to cortical stimulation occurred earlier (latency difference, 4.6 ms) and with a lower stimulation current for FSIs than for projection neurons. Moreover, blocking GABA(A) receptors by local picrotoxin injection enhanced the spike response of projection neurons, and this increase was larger in experimental conditions favoring FSI responses. Therefore, on average, FSIs exert in vivo a powerful feedforward inhibition on projection neurons. However, a few projection neurons were actually more sensitive to cortical stimulation than FSIs. Moreover, picrotoxin, which revealed FSI inhibition, preferentially affected projection neurons exhibiting the weakest sensitivity to cortical stimulation. Thus, feedforward inhibition by FSIs filters cortical information effectively transmitted by striatal projection neurons.