Up and down states in striatal medium spiny neurons simultaneously recorded with spontaneous activity in fast-spiking interneurons studied in cortex-striatum-substantia nigra organotypic cultures

Computing reward-prediction error: an integrated account of cortical timing and basal-ganglia pathways for appetitive and aversive learning

There are two prevailing notions regarding the involvement of the corticobasal ganglia system in value‐based learning: (i) the direct and indirect pathways of the basal ganglia are crucial for appetitive and aversive learning, respectively, and (ii) the activity of midbrain dopamine neurons represents reward‐prediction error. Although (ii) constitutes a critical assumption of (i), it remains elusive how (ii) holds given (i), with the basal‐ganglia influence on the dopamine neurons. Here we present a computational neural‐circuit model that potentially resolves this issue. Based on the latest analyses of the heterogeneous corticostriatal neurons and connections, our model posits that the direct and indirect pathways, respectively, represent the values of upcoming and previous actions, and up‐regulate and down‐regulate the dopamine neurons via the basal‐ganglia output nuclei. This explains how the difference between the upcoming and previous values, which constitutes the core of reward‐prediction error, is calculated. Simultaneously, it predicts that blockade of the direct/indirect pathway causes a negative/positive shift of reward‐prediction error and thereby impairs learning from positive/negative error, i.e. appetitive/aversive learning. Through simulation of reward‐reversal learning and punishment‐avoidance learning, we show that our model could indeed account for the experimentally observed features that are suggested to support notion (i) and could also provide predictions on neural activity. We also present a behavioral prediction of our model, through simulation of inter‐temporal choice, on how the balance between the two pathways relates to the subject's time preference. These results indicate that our model, incorporating the heterogeneity of the cortical influence on the basal ganglia, is expected to provide a closed‐circuit mechanistic understanding of appetitive/aversive learning.

From the neuron doctrine to neural networks

For over a century, the neuron doctrine--which states that the neuron is the structural and functional unit of the nervous system--has provided a conceptual foundation for neuroscience. This viewpoint reflects its origins in a time when the use of single-neuron anatomical and physiological techniques was prominent. However, newer multineuronal recording methods have revealed that ensembles of neurons, rather than individual cells, can form physiological units and generate emergent functional properties and states. As a new paradigm for neuroscience, neural network models have the potential to incorporate knowledge acquired with single-neuron approaches to help us understand how emergent functional states generate behaviour, cognition and mental disease.

A spiking Basal Ganglia model of synchrony, exploration and decision making

To make an optimal decision we need to weigh all the available options, compare them with the current goal, and choose the most rewarding one. Depending on the situation an optimal decision could be to either “explore” or “exploit” or “not to take any action” for which the Basal Ganglia (BG) is considered to be a key neural substrate. In an attempt to expand this classical picture of BG function, we had earlier hypothesized that the Indirect Pathway (IP) of the BG could be the subcortical substrate for exploration. In this study we build a spiking network model to relate exploration to synchrony levels in the BG (which are a neural marker for tremor in Parkinson's disease). Key BG nuclei such as the Sub Thalamic Nucleus (STN), Globus Pallidus externus (GPe) and Globus Pallidus internus (GPi) were modeled as Izhikevich spiking neurons whereas the Striatal output was modeled as Poisson spikes. The model is cast in reinforcement learning framework with the dopamine signal representing reward prediction error. We apply the model to two decision making tasks: a binary action selection task (similar to one used by Humphries et al., 2006) and an n-armed bandit task (Bourdaud et al., 2008). The model shows that exploration levels could be controlled by STN's lateral connection strength which also influenced the synchrony levels in the STN-GPe circuit. An increase in STN's lateral strength led to a decrease in exploration which can be thought as the possible explanation for reduced exploratory levels in Parkinson's patients. Our simulations also show that on complete removal of IP, the model exhibits only Go and No-Go behaviors, thereby demonstrating the crucial role of IP in exploration. Our model provides a unified account for synchronization, action section, and explorative behavior.

Integrative hippocampal and decision-making neurocircuitry during goal-relevant predictions and encoding

It has become clear that the hippocampus plays a critical role in the identification of new contexts and for the detection of changes in familiar contexts. The hippocampus accomplishes these goals through a continual process of comparing predicted features of a context or situation to those actually experienced. A mismatch between expected and experienced context expectations is thought to lead to the generation of a context prediction error (Mizumori, 2013) that functionally alerts connected brain areas to alter subsequent decision making and response selection. Little is understood about how hippocampal context analyses impact downstream decision processes. This issue is evaluated here first by comparing the nature of the information represented in hippocampus and decision-related midbrain-striatal structures, while rats perform a hippocampal-dependent spatial memory task in which rewards of different value are found at different locations. In contrast to place-specific and egocentric neural representations, neural representations of goal information are broadly distributed in hippocampal and decision neural circuitry, but they appear in different forms for different brain structures. It is suggested that further researching on how goal information processing occurs in hippocampus and decision neural circuitry may reveal insights into the nature of the interaction between memory and decision systems. The second part of this review describes neural pathways by which hippocampal context information might arrive within the decision circuit. The third section presents a hypothesis that the nature of the interactions between hippocampal and midbrain-striatal circuitry is regulated by the prefrontal cortex.

Lag threads organize the brain's intrinsic activity

It has been widely reported that intrinsic brain activity, in a variety of animals including humans, is spatiotemporally structured. Specifically, propagated slow activity has been repeatedly demonstrated in animals. In human resting-state fMRI, spontaneous activity has been understood predominantly in terms of zero-lag temporal synchrony within widely distributed functional systems (resting-state networks). Here, we use resting-state fMRI from 1,376 normal, young adults to demonstrate that multiple, highly reproducible, temporal sequences of propagated activity, which we term "lag threads," are present in the brain. Moreover, this propagated activity is largely unidirectional within conventionally understood resting-state networks. Modeling experiments show that resting-state networks naturally emerge as a consequence of shared patterns of propagation. An implication of these results is that common physiologic mechanisms may underlie spontaneous activity as imaged with fMRI in humans and slowly propagated activity as studied in animals.

A perspective on neural and cognitive mechanisms of error commission

Behavioral adaptation and cognitive control are crucial for goal-reaching behaviors. Every creature is ubiquitously faced with choices between behavioral alternatives. Common sense suggests that errors are an important source of information in the regulation of such processes. Several theories exist regarding cognitive control and the processing of undesired outcomes. However, most of these models focus on the consequences of an error, and less attention has been paid to the mechanisms that underlie the commissioning of an error. In this article, we present an integrative review of neuro-cognitive models that detail the determinants of the occurrence of response errors. The factors that may determine the likelihood of committing errors are likely related to the stability of task-representations in prefrontal networks, attentional selection mechanisms and mechanisms of action selection in basal ganglia circuits. An important conclusion is that the likelihood of committing an error is not stable over time but rather changes depending on the interplay of different functional neuro-anatomical and neuro-biological systems. We describe factors that might determine the time-course of cognitive control and the need to adapt behavior following response errors. Finally, we outline the mechanisms that may proof useful for predicting the outcomes of cognitive control and the emergence of response errors in future research.

A biologically constrained model of the whole basal ganglia addressing the paradoxes of connections and selection

The basal ganglia nuclei form a complex network of nuclei often assumed to perform selection, yet their individual roles and how they influence each other is still largely unclear. In particular, the ties between the external and internal parts of the globus pallidus are paradoxical, as anatomical data suggest a potent inhibitory projection between them while electrophysiological recordings indicate that they have similar activities. Here we introduce a theoretical study that reconciles both views on the intra-pallidal projection, by providing a plausible characterization of the relationship between the external and internal globus pallidus. Specifically, we developed a mean-field model of the whole basal ganglia, whose parameterization is optimized to respect best a collection of numerous anatomical and electrophysiological data. We first obtained models respecting all our constraints, hence anatomical and electrophysiological data on the intrapallidal projection are globally consistent. This model furthermore predicts that both aforementioned views about the intra-pallidal projection may be reconciled when this projection is weakly inhibitory, thus making it possible to support similar neural activity in both nuclei and for the entire basal ganglia to select between actions. Second, we predicts that afferent projections are substantially unbalanced towards the external segment, as it receives the strongest excitation from STN and the weakest inhibition from the striatum. Finally, our study strongly suggests that the intrapallidal connection pattern is not focused but diffuse, as this latter pattern is more efficient for the overall selection performed in the basal ganglia.

Dopamine regulates distinctively the activity patterns of striatal output neurons in advanced parkinsonian primates

Nigrostriatal dopamine denervation plays a major role in basal ganglia circuitry disarray and motor abnormalities of Parkinson's disease (PD). Studies in rodent and primate models have revealed that striatal projection neurons, namely, medium spiny neurons (MSNs), increase the firing frequency. However, their activity pattern changes and the effects of dopaminergic stimulation in such conditions are unknown. Using single-cell recordings in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated primates with advanced parkinsonism, we studied MSN activity patterns in the transition to different motor states following levodopa administration. In the "off" state (baseline parkinsonian disability), a burst-firing pattern accompanied by prolonged silences (pauses) was found in 34% of MSNs, and 80% of these exhibited a levodopa response compatible with dopamine D1 receptor activation (direct pathway MSNs). This pattern was highly responsive to levodopa given that bursting/pausing almost disappeared in the "on" state (reversal of parkinsonism after levodopa injection), although this led to higher firing rates. Nonbursty MSNs fired irregularly with marked pausing that increased in the on state in the MSN subset with a levodopa response compatible with dopamine D2 receptor activation (indirect pathway MSNs), although the pause increase was not sustained in some units during the appearance of dyskinesias. Data indicate that the MSN firing pattern in the advanced parkinsonian monkey is altered by bursting and pausing changes and that dopamine differentially and inefficiently regulates these behaviorally correlated patterns in MSN subpopulations. These findings may contribute to understand the impact of striatal dysfunction in the basal ganglia network and its role in motor symptoms of PD.

A novel neuroferritinopathy mouse model (FTL 498InsTC) shows progressive brain iron dysregulation, morphological signs of early neurodegeneration and motor coordination deficits

Neuroferritinopathy is a rare genetic disease with a dominant autosomal transmission caused by mutations of the ferritin light chain gene (FTL). It belongs to Neurodegeneration with Brain Iron Accumulation, a group of disorders where iron dysregulation is tightly associated with neurodegeneration. We studied the 498–499InsTC mutation which causes the substitution of the last 9 amino acids and an elongation of extra 16 amino acids at the C-terminus of L-ferritin peptide. An analysis with cyclic voltammetry on the purified protein showed that this structural modification severely reduces the ability of the protein to store iron. In order to analyze the impact of the mutation in vivo, we generated mouse models for the some pathogenic human FTL gene in FVB and C57BL/6J strains.

Transgenic mice in the FVB background showed high accumulation of the mutated ferritin in brain where it correlated with increased iron deposition with age, as scored by magnetic resonance imaging. Notably, the accumulation of iron–ferritin bodies was accompanied by signs of oxidative damage. In the C57BL/6 background, both the expression of the mutant ferritin and the iron levels were lower than in the FVB strain. Nevertheless, also these mice showed oxidative alterations in the brain. Furthermore, post-natal hippocampal neurons obtained from these mice experienced a marked increased cell death in response to chronic iron overload and/or acute oxidative stress, in comparison to wild-type neurons. Ultrastructural analyses revealed an accumulation of lipofuscin granules associated with iron deposits, particularly enriched in the cerebellum and striatum of our transgenic mice. Finally, experimental subjects were tested throughout development and aging at 2-, 8- and 18-months for behavioral phenotype. Rotarod test revealed a progressive impaired motor coordination building up with age, FTL mutant old mice showing a shorter latency to fall from the apparatus, according to higher accumulation of iron aggregates in the striatum. Our data show that our 498–499InsTC mouse models recapitulate early pathological and clinical traits of the human neuroferritinopathy, thus providing a valuable model for the study of the disease. Finally, we propose a mechanistic model of lipofuscine formation that can account for the etiopathogenesis of human neuroferritinopathy.

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We developed two new neuroferritinopathy mice models (NF).

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NF brains are characterized by iron/ferritin accumulation and oxidative damage.

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NF brains show granules of lipofuscine associated with iron.

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A mechanism of lipofuscine formation is proposed.

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NF mice show impaired motor coordination increasing with age.

Homeostatic regulation of memory systems and adaptive decisions

While it is clear that many brain areas process mnemonic information, understanding how their interactions result in continuously adaptive behaviors has been a challenge. A homeostatic-regulated prediction model of memory is presented that considers the existence of a single memory system that is based on a multilevel coordinated and integrated network (from cells to neural systems) that determines the extent to which events and outcomes occur as predicted. The “multiple memory systems of the brain” have in common output that signals errors in the prediction of events and/or their outcomes, although these signals differ in terms of what the error signal represents (e.g., hippocampus: context prediction errors vs. midbrain/striatum: reward prediction errors). The prefrontal cortex likely plays a pivotal role in the coordination of prediction analysis within and across prediction brain areas. By virtue of its widespread control and influence, and intrinsic working memory mechanisms. Thus, the prefrontal cortex supports the flexible processing needed to generate adaptive behaviors and predict future outcomes. It is proposed that prefrontal cortex continually and automatically produces adaptive responses according to homeostatic regulatory principles: prefrontal cortex may serve as a controller that is intrinsically driven to maintain in prediction areas an experience-dependent firing rate set point that ensures adaptive temporally and spatially resolved neural responses to future prediction errors. This same drive by prefrontal cortex may also restore set point firing rates after deviations (i.e. prediction errors) are detected. In this way, prefrontal cortex contributes to reducing uncertainty in prediction systems. An emergent outcome of this homeostatic view may be the flexible and adaptive control that prefrontal cortex is known to implement (i.e. working memory) in the most challenging of situations. Compromise to any of the prediction circuits should result in rigid and suboptimal decision making and memory as seen in addiction and neurological disease. © 2013 The Authors. Hippocampus Published by Wiley Periodicals, Inc.

Voltage drives diverse endocannabinoid signals to mediate striatal microcircuit-specific plasticity

The dorsolateral striatum and cannabinoid type 1 receptor (CB1) signaling mediate habitual action learning, which is thought to require a balance of activity in the direct and indirect striatal output pathways. However, very little is known about how the high CB1-expressing striatal inhibitory microcircuitry might contribute to long-term plasticity capable of sculpting direct/indirect pathway output. Using optogenetic and molecular interrogation of striatal GABAergic microcircuits, we describe novel mechanisms of voltage-dependent long-term depression of inhibitory synapses (iLTD) onto mouse and rat medium spiny projection neurons (MSNs). This iLTD involves recruitment of different endocannabinoid types and shows both presynaptic and postsynaptic selectivity for MSN subtypes, ultimately resulting in a powerful disinhibition of direct pathway MSNs. These results indicate a new role for voltage states in gating circuit-specific forms of synaptic plasticity and illuminate possible circuit dynamics underlying action control.

Parvalbumin tunes spike-timing and efferent short-term plasticity in striatal fast spiking interneurons

  Striatal fast spiking interneurons (FSIs) modulate output of the striatum by synchronizing medium-sized spiny neurons (MSNs). Recent studies have broadened our understanding of FSIs, showing that they are implicated in severe motor disorders such as parkinsonism, dystonia and Tourette syndrome. FSIs are the only striatal neurons to express the calcium-binding protein parvalbumin (PV). This selective expression of PV raises questions about the functional role of this Ca(2+) buffer in controlling FSI Ca(2+) dynamics and, consequently, FSI spiking mode and neurotransmission. To study the functional involvement of FSIs in striatal microcircuit activity and the role of PV in FSI function, we performed perforated patch recordings on enhanced green fluorescent protein-expressing FSIs in brain slices from control and PV-/- mice. Our results revealed that PV-/- FSIs fired more regularly and were more excitable than control FSIs by a mechanism in which Ca(2+) buffering is linked to spiking activity as a result of the activation of small conductance Ca(2+)-dependent K(+) channels. A modelling approach of striatal FSIs supports our experimental results. Furthermore, PV deletion modified frequency-specific short-term plasticity at inhibitory FSI to MSN synapses. Our results therefore reinforce the hypothesis that in FSIs, PV is crucial for fine-tuning of the temporal responses of the FSI network and for the orchestration of MSN populations. This, in turn, may play a direct role in the generation and pathology-related worsening of motor rhythms.

Chronic electrical stimulation homeostatically decreases spontaneous activity, but paradoxically increases evoked network activity

Neural dynamics generated within cortical networks play a fundamental role in brain function. However, the learning rules that allow recurrent networks to generate functional dynamic regimes, and the degree to which these regimes are themselves plastic, are not known. In this study we examined plasticity of network dynamics in cortical organotypic slices in response to chronic changes in activity. Studies have typically manipulated network activity pharmacologically; we used chronic electrical stimulation to increase activity in in vitro cortical circuits in a more physiological manner. Slices were stimulated with "implanted" electrodes for 4 days. Chronic electrical stimulation or treatment with bicuculline decreased spontaneous activity as predicted by homeostatic learning rules. Paradoxically, however, whereas bicuculline decreased evoked network activity, chronic stimulation actually increased the likelihood that evoked stimulation elicited polysynaptic activity, despite a decrease in evoked monosynaptic strength. Furthermore, there was an inverse correlation between spontaneous and evoked activity, suggesting a homeostatic tradeoff between spontaneous and evoked activity. Within-slice experiments revealed that cells close to the stimulated electrode exhibited more evoked polysynaptic activity and less spontaneous activity than cells close to a control electrode. Collectively, our results establish that chronic stimulation changes the dynamic regimes of networks. In vitro studies of homeostatic plasticity typically lack any external input, and thus neurons must rely on "spontaneous" activity to reach homeostatic "set points." However, in the presence of external input we propose that homeostatic learning rules seem to shift networks from spontaneous to evoked regimes.

Functional roles of neurotransmitters and neuromodulators in the dorsal striatum

The dorsal striatum, with its functional microcircuits galore, serves as the primary gateway of the basal ganglia and is known to play a key role in implicit learning. Initially, excitatory inputs from the cortex and thalamus arrive on the direct and indirect pathways, where the precise flow of information is then regulated by local GABAergic interneurons. The balance of excitatory and inhibitory transmission in the dorsal striatum is modulated by neuromodulators such as dopamine and acetylcholine. Under pathophysiological states in the dorsal striatum, an alteration in excitatory and inhibitory transmission may underlie dysfunctional motor control. Here, we review the cellular connections and modulation of striatal microcircuits and propose that modulating the excitatory and inhibitory balance in synaptic transmission of the dorsal striatum is important for regulating locomotion.

Balance between excitation and inhibition controls the temporal organization of neuronal avalanches

Neuronal avalanches, measured in vitro and in vivo, exhibit a robust critical behavior. Their temporal organization hides the presence of correlations. Here we present experimental measurements of the waiting time distribution between successive avalanches in the rat cortex in vitro. This exhibits a nonmonotonic behavior not usually found in other natural processes. Numerical simulations provide evidence that this behavior is a consequence of the alternation between states of high and low activity, named up and down states, leading to a balance between excitation and inhibition controlled by a single parameter. During these periods, both the single neuron state and the network excitability level, keeping memory of past activity, are tuned by homeostatic mechanisms.

Cannabinoid receptors mediate methamphetamine induction of high frequency gamma oscillations in the nucleus accumbens

Patients suffering from amphetamine-induced psychosis display repetitive behaviors, partially alleviated by antipsychotics, which are reminiscent of rodent stereotypies. Due to recent evidence implicating endocannabinoid involvement in brain disorders, including psychosis, we studied the effects of endocannabinoid signaling on neuronal oscillations of rats exhibiting methamphetamine stereotypy. Neuronal network oscillations were recorded with multiple single electrode arrays aimed at the nucleus accumbens of freely-moving rats. During the experiments, animals were dosed intravenously with the CB1 receptor antagonist rimonabant (0.3 mg/kg) or vehicle followed by an ascending dose regimen of methamphetamine (0.01, 0.1, 1, and 3 mg/kg; cumulative dosing). The effects of drug administration on stereotypy and local gamma oscillations were evaluated. Methamphetamine treatment significantly increased high frequency gamma oscillations (∼80 Hz). Entrainment of a subpopulation of nucleus accumbens neurons to high frequency gamma was associated with stereotypy encoding in putative fast-spiking interneurons, but not in putative medium spiny neurons. The observed ability of methamphetamine to induce both stereotypy and high frequency gamma power was potently disrupted following CB1 receptor blockade. The present data suggest that CB1 receptor-dependent mechanisms are recruited by methamphetamine to modify striatal interneuron oscillations that accompany changes in psychomotor state, further supporting the link between endocannabinoids and schizophrenia spectrum disorders.

Nonlinear-dynamics theory of up-down transitions in neocortical neural networks

The neurons of the neocortex show ~1-Hz synchronized transitions between an active up state and a quiescent down state. The up-down state transitions are highly coherent over large sections of the cortex, yet they are accompanied by pronounced, incoherent noise. We propose a simple model for the up-down state oscillations that allows analysis by straightforward dynamical systems theory. An essential feature is a nonuniform network geometry composed of groups of excitatory and inhibitory neurons with strong coupling inside a group and weak coupling between groups. The enhanced deterministic noise of the up state appears as the natural result of the proximity of a partial synchronization transition. The synchronization transition takes place as a function of the long-range synaptic strength linking different groups of neurons.

Origin and properties of striatal local field potential responses to cortical stimulation: temporal regulation by fast inhibitory connections

Evoked striatal field potentials are seldom used to study corticostriatal communication in vivo because little is known about their origin and significance. Here we show that striatal field responses evoked by stimulating the prelimbic cortex in mice are reduced by more than 90% after infusing the AMPA receptor antagonist CNQX close to the recording electrode. Moreover, the amplitude of local field responses and dPSPs recorded in striatal medium spiny neurons increase in parallel with increasing stimulating current intensity. Finally, the evoked striatal fields show several of the basic known properties of corticostriatal transmission, including paired pulse facilitation and topographical organization. As a case study, we characterized the effect of local GABA_A receptor blockade on striatal field and multiunitary action potential responses to prelimbic cortex stimulation. Striatal activity was recorded through a 24 channel silicon probe at about 600 µm from a microdialysis probe. Intrastriatal administration of the GABA_A receptor antagonist bicuculline increased by 65±7% the duration of the evoked field responses. Moreover, the associated action potential responses were markedly enhanced during bicuculline infusion. Bicuculline enhancement took place at all the striatal sites that showed a response to cortical stimulation before drug infusion, but sites showing no field response before bicuculline remained unresponsive during GABA_A receptor blockade. Thus, the data demonstrate that fast inhibitory connections exert a marked temporal regulation of input-output transformations within spatially delimited striatal networks responding to a cortical input. Overall, we propose that evoked striatal fields may be a useful tool to study corticostriatal synaptic connectivity in relation to behavior.

The ionic mechanism of gamma resonance in rat striatal fast-spiking neurons

Striatal fast-spiking (FS) cells in slices fire in the gamma frequency range and in vivo are often phase-locked to gamma oscillations in the field potential. We studied the firing patterns of these cells in slices from rats ages 16-23 days to determine the mechanism of their gamma resonance. The resonance of striatal FS cells was manifested as a minimum frequency for repetitive firing. At rheobase, cells fired a doublet of action potentials or doublets separated by pauses, with an instantaneous firing rate averaging 44 spikes/s. The minimum rate for sustained firing was also responsible for the stuttering firing pattern. Firing rate adapted during each episode of firing, and bursts were terminated when firing was reduced to the minimum sustainable rate. Resonance and stuttering continued after blockade of Kv3 current using tetraethylammonium (0.1-1 mM). Both gamma resonance and stuttering were strongly dependent on Kv1 current. Blockade of Kv1 channels with dendrotoxin-I (100 nM) completely abolished the stuttering firing pattern, greatly lowered the minimum firing rate, abolished gamma-band subthreshold oscillations, and slowed spike frequency adaptation. The loss of resonance could be accounted for by a reduction in potassium current near spike threshold and the emergence of a fixed spike threshold. Inactivation of the Kv1 channel combined with the minimum firing rate could account for the stuttering firing pattern. The resonant properties conferred by this channel were shown to be adequate to account for their phase-locking to gamma-frequency inputs as seen in vivo.

Striatal gating through up states and oscillations in the basal ganglia: Implications for Parkinson's disease

Up states are a hallmark of striatal physiology. Spontaneous activity in the thalamo-cortical network drives robust plateau depolarizations in the medium spiny projection neurons of the striatum. Medium spiny neuron firing is only possible during up states and is very tightly regulated by dopamine and NMDA receptors. In a rat model of Parkinson's disease the medium spiny neurons projecting to the globus pallidus (indirect pathway) show more depolarized up states and increased firing. This is translated into abnormal patterns of synchronization between the globus pallidus and frontal cortex, which are believed to underlie the symptoms of Parkinson's disease. Here we review our work in the field and propose a mechanism through which the lack of D2 receptor stimulation in the striatum allows the establishment of fixed routes of information flow in the cortico-striato-pallidal network.

Enhanced high-frequency membrane potential fluctuations control spike output in striatal fast-spiking interneurones in vivo

Fast-spiking interneurones (FSIs) constitute a prominent part of the inhibitory microcircuitry of the striatum; however, little is known about their recruitment by synaptic inputs in vivo. Here, we report that, in contrast to cholinergic interneurones (CINs), FSIs (n = 9) recorded in urethane-anaesthetized rats exhibit Down-to-Up state transitions very similar to spiny projection neurones (SPNs). Compared to SPNs, the FSI Up state membrane potential was noisier and power spectra exhibited significantly larger power at frequencies in the gamma range (55-95 Hz). The membrane potential exhibited short and steep trajectories preceding spontaneous spike discharge, suggesting that fast input components controlled spike output in FSIs. Spontaneous spike data contained a high proportion (43.6 ± 32.8%) of small inter-spike intervals (ISIs) of <30 ms, setting FSIs clearly apart from SPNs and CINs. Cortical-evoked inputs had slower dynamics in SPNs than FSIs, and repetitive stimulation entrained SPN spike output only if the stimulation was delivered at an intermediate frequency (20 Hz), but not at a high frequency (100 Hz). Pharmacological induction of an activated ECoG state, known to promote rapid FSI spiking, mildly increased the power (by 43 ± 55%, n = 13) at gamma frequencies in the membrane potential of SPNs, but resulted in few small ISIs (<30 ms; 4.3 ± 6.4%, n = 8). The gamma frequency content did not change in CINs (n = 8). These results indicate that FSIs are uniquely responsive to high-frequency input sequences. By controlling the spike output of SPNs, FSIs could serve gating of top-down signals and long-range synchronisation of gamma-oscillations during behaviour.

Synaptically driven state transitions in distal dendrites of striatal spiny neurons

Striatal spiny neurons (SPNs) associate a diverse array of cortically processed information to regulate action selection. But how this is done by SPNs is poorly understood. A key step in this process is the transition of SPNs from a hyperpolarized ‘down-state’ to a sustained, depolarized ‘up-state’. These transitions are thought to reflect a sustained synaptic barrage, involving the coordination of hundreds of pyramidal neurons. Indeed, in mice simulation of cortical input by glutamate uncaging on proximal dendritic spines produced potential changes in SPNs that tracked input time course. However, brief glutamate uncaging at spines on distal dendrites evoked somatic up-states lasting hundreds of milliseconds. These regenerative events depended upon both NMDA receptors and voltage-dependent Ca²⁺ channels. Moreover, they were bidirectionally regulated by dopamine receptor signaling. This capacity not only changes our model of how up-state are generated in SPNs, it has fundamental implications for the associative process underlying action selection.

Spike-timing dependent plasticity in striatal interneurons

Basal ganglia, an ensemble of interconnected subcortical nuclei, are involved in adaptive motor planning and procedural learning. Striatum, the primary input nucleus of basal ganglia, extracts the pertinent cortical and thalamic information from background noise in relation with the environmental stimuli and motivation. The striatum comprises different neuronal populations: the GABAergic striatal output neurons, three classes of GABAergic interneurons and the cholinergic cells. Striatal interneurons exert a powerful control of striatal output neuron excitability and therefore shape the cortico-basal ganglia information processing. Besides output neurons, striatal interneurons also receive directly cortical information and are able to adapt their behavior depending on the level of cortical and striatal activation. In this review, we focus on the corticostriatal long-term synaptic efficacy changes occurring in interneurons, and especially the spike-timing dependent plasticity (STDP), as a Hebbian synaptic learning rule. Combined with the striatal local interactions between interneurons and output neurons, we will consider the functional consequences of the interneuron plasticity on the striatal output. This article is part of a Special Issue entitled 'Synaptic Plasticity & Interneurons'.

Amantadine for dyskinesias in Parkinson's disease: a randomized controlled trial

Background

Dyskinesias are some of the major motor complications that impair quality of life for patients with Parkinson's disease. The purpose of the present study was to investigate the efficacy of amantadine in Parkinson's disease patients suffering from dyskinesias.

Methods

In this multi-center, double-blind, randomized, placebo-controlled, cross-over trial, 36 patients with Parkinson's disease and dyskinesias were randomized, and 62 interventions, which included amantadine (300 mg /day) or placebo treatment for 27 days, were analyzed. At 15 days after washout, the treatments were crossed over. The primary outcome measure was the changes in the Rush Dyskinesia Rating Scale (RDRS) during each treatment period. The secondary outcome measures were changes in the Unified Parkinson's Disease Rating Scale part IVa (UPDRS-IVa, dyskinesias), part IVb (motor fluctuations), and part III (motor function).

Results

RDRS improved in 64% and 16% of patients treated with amantadine or placebo, respectively, with significant differences between treatments. The adjusted odds-ratio for improvement by amantadine was 6.7 (95% confidence interval, 1.4 to 31.5). UPDRS-IVa was improved to a significantly greater degree in amantadine-treated patients [mean (SD) of 1.83 (1.56)] compared with placebo-treated patients [0.03 (1.51)]. However, there were no significant effects on UPDRS-IVb or III scores.

Conclusions

Results from the present study demonstrated that amantadine exhibited efficacious effects against dyskinesias in 60–70% of patients.

Trial Registration

UMIN Clinical Trial Registry UMIN000000780

Striatal structure and function in mood disorders: a comprehensive review

OBJECTIVES

  A large and diverse literature has implicated abnormalities of striatal structure and function in both unipolar and bipolar disorder. Recent functional imaging studies have greatly expanded this body of research. The aim of this review is to provide a comprehensive and critical appraisal of the relevant literature.

METHODS

  A total of 331 relevant articles were reviewed to develop an integrated overview of striatal function in mood disorders.

RESULTS

  There is compelling evidence from multiple studies that functional abnormalities of the striatum and greater corticostriatal circuitry exist in at least some forms of affective illness. The literature does not yet provide data to determine whether these aberrations represent primary pathology or they contribute directly to symptom expression. Finally, there is considerable evidence that bipolar disorder may be associated with striatal hyperactivity and some suggestion that unipolar illness may be associated with hypoactivation.

CONCLUSIONS

  Additional research investigating striatal function in affective disorders will be critical to the development of comprehensive models of the neurobiology of these conditions.

Fast-spiking interneurons of the rat ventral striatum: temporal coordination of activity with principal cells and responsiveness to reward

Although previous in vitro studies revealed inhibitory synaptic connections of fast-spiking interneurons to principal cells in the striatum, uncertainty remains about the nature of the behavioural events that correlate with changes in interneuron activity and about the temporal coordination of interneuron firing with spiking of principal cells under natural conditions. Using in vivo tetrode recordings from the ventral striatum in freely moving rats, fast-spiking neurons were distinguished from putative medium-sized spiny neurons on the basis of their spike waveforms and rates. Cross-correlograms of fast-spiking and putative medium-sized spiny neuron firing patterns revealed a variety of temporal relationships, including peaks of concurrent firing and transient decrements in medium-sized spiny neuron spiking around fast-spiking unit activity. Notably, the onset of these decrements was mostly in advance of the fast-spiking unit firing. Many of these temporal relationships were dependent on the sleep-wake state. Coordinated activity was also found amongst pairs of the same phenotype, both fast-spiking units and putative medium-sized spiny neurons, which was often marked by a broad peak of concurrent firing. When studying fast-spiking neurons in a reward-searching task, they generally showed a pre-reward ramping increment in firing rate but a decrement specifically when the rat received reward. In conclusion, our data indicate that various forms of temporally coordinated activity exist amongst ventral striatal interneurons and principal cells, which cannot be explained by feed-forward inhibitory circuits alone. Furthermore, firing patterns of ventral striatal fast-spiking interneurons do not merely correlate with the general arousal state of the animal but display distinct reward-related changes in firing rate.

Cortico-basal ganglia circuitry: a review of key research and implications for functional connectivity studies of mood and anxiety disorders

There is considerable evidence that dysfunction of the cortico-basal ganglia circuits may be associated with several mood and anxiety disorders. However, it is unclear whether circuit abnormalities contribute directly either to the neurobiology of these conditions or to the manifestation of symptoms. Understanding the role of these pathways in psychiatric illness has been limited by an incomplete characterization of normal function. In recent years, studies using animal models and human functional imaging have greatly expanded the literature describing normal cortico-basal ganglia circuit function. In this paper, recent key studies of circuit function using human and animal models are reviewed and integrated with findings from other studies conducted over the previous decades. The literature suggests several hypotheses of cortico-basal ganglia circuitry function in mood and anxiety disorders that warrant further exploration. Hypotheses are proposed herein based upon the cortico-basal ganglia mechanisms of: (1) feedforward and feedback control, (2) circuit integration and (3) emotional control. These are presented as models of circuit function, which may be particularly relevant to future investigations using neuroimaging and functional connectivity analyses.

Self-organized criticality occurs in non-conservative neuronal networks during Up states

During sleep, under anesthesia and in vitro, cortical neurons in sensory, motor, association and executive areas fluctuate between Up and Down states (UDS) characterized by distinct membrane potentials and spike rates [1, 2, 3, 4, 5]. Another phenomenon observed in preparations similar to those that exhibit UDS, such as anesthetized rats [6], brain slices and cultures devoid of sensory input [7], as well as awake monkey cortex [8] is self-organized criticality (SOC). This is characterized by activity "avalanches" whose size distributions obey a power law with critical exponent of about [Formula: see text] and branching parameter near unity. Recent work has demonstrated SOC in conservative neuronal network models [9, 10], however critical behavior breaks down when biologically realistic non-conservatism is introduced [9]. We here report robust SOC behavior in networks of non-conservative leaky integrate-and-fire neurons with short-term synaptic depression. We show analytically and numerically that these networks typically have 2 stable activity levels corresponding to Up and Down states, that the networks switch spontaneously between them, and that Up states are critical and Down states are subcritical.

Selective activation of striatal fast-spiking interneurons during choice execution

Basal ganglia circuits are essential for the organization and execution of voluntary actions. Within the striatum, fast-spiking interneurons (FSIs) are thought to tightly regulate the activity of medium-spiny projection neurons (MSNs) through feed-forward inhibition, yet few studies have investigated the functional contributions of FSIs in behaving animals. We recorded presumed MSNs and FSIs together with motor cortex and globus pallidus (GP) neurons, in rats performing a simple choice task. MSN activity was widely distributed across the task sequence, especially near reward receipt. By contrast, FSIs showed a coordinated pulse of increased activity as chosen actions were initiated, in conjunction with a sharp decrease in GP activity. Both MSNs and FSIs were direction selective, but neighboring MSNs and FSIs showed opposite selectivity. Our findings suggest that individual FSIs participate in local striatal information processing, but more global disinhibition of FSIs by GP is important for initiating chosen actions while suppressing unwanted alternatives.

Cocaine-induced homeostatic regulation and dysregulation of nucleus accumbens neurons

Homeostatic response is an endowed self-correcting/maintaining property for living units, ranging from subcellular domains, single cells, and organs to the whole organism. Homeostatic responses maintain stable function through the ever-changing internal and external environments. In central neurons, several forms of homeostatic regulation have been identified, all of which tend to stabilize the functional output of neurons toward their prior "set-point." Medium spiny neurons (MSNs) within the forebrain region the nucleus accumbens (NAc) play a central role in gating/regulating emotional and motivational behaviors including craving and seeking drugs of abuse. Exposure to highly salient stimuli such as cocaine administration not only acutely activates a certain population of NAc MSNs, but also induces long-lasting changes in these neurons. It is these long-lasting cellular alterations that are speculated to mediate the increasingly strong cocaine-craving and cocaine-seeking behaviors. Why do the potentially powerful homeostatic mechanisms fail to correct or compensate for these drug-induced maladaptations in neurons? Based on recent experimental results, this review proposes a hypothesis of homeostatic dysregulation induced by exposure to cocaine. Specifically, we hypothesize that exposure to cocaine generates false molecular signals which misleads the homeostatic regulation process, resulting in maladaptive changes in NAc MSNs. Thus, many molecular and cellular alterations observed in the addicted brain may indeed result from homeostatic dysregulation. This review is among the first to introduce the concept of homeostatic neuroplasticity to understanding the molecular and cellular maladaptations following exposure to drugs of abuse.

Neural representations and mechanisms for the performance of simple speech sequences

Speakers plan the phonological content of their utterances before their release as speech motor acts. Using a finite alphabet of learned phonemes and a relatively small number of syllable structures, speakers are able to rapidly plan and produce arbitrary syllable sequences that fall within the rules of their language. The class of computational models of sequence planning and performance termed competitive queuing models have followed K. S. Lashley [The problem of serial order in behavior. In L. A. Jeffress (Ed.), Cerebral mechanisms in behavior (pp. 112-136). New York: Wiley, 1951] in assuming that inherently parallel neural representations underlie serial action, and this idea is increasingly supported by experimental evidence. In this article, we developed a neural model that extends the existing DIVA model of speech production in two complementary ways. The new model includes paired structure and content subsystems [cf. MacNeilage, P. F. The frame/content theory of evolution of speech production. Behavioral and Brain Sciences, 21, 499-511, 1998 ] that provide parallel representations of a forthcoming speech plan as well as mechanisms for interfacing these phonological planning representations with learned sensorimotor programs to enable stepping through multisyllabic speech plans. On the basis of previous reports, the model's components are hypothesized to be localized to specific cortical and subcortical structures, including the left inferior frontal sulcus, the medial premotor cortex, the basal ganglia, and the thalamus. The new model, called gradient order DIVA, thus fills a void in current speech research by providing formal mechanistic hypotheses about both phonological and phonetic processes that are grounded by neuroanatomy and physiology. This framework also generates predictions that can be tested in future neuroimaging and clinical case studies.

Hierarchical interaction structure of neural activities in cortical slice cultures

Recent advances in the analysis of neuronal activities suggest that the instantaneous activity patterns can be mostly explained by considering only first-order and pairwise interactions between recorded elements, i.e., action potentials or local field potentials (LFP), and do not require higher-than-pairwise-order interactions. If generally applicable, this pairwise approach greatly simplifies the description of network interactions. However, an important question remains: are the recorded elements the units of interaction that best describe neuronal activity patterns? To explore this, we recorded spontaneous LFP peak activities in cortical organotypic cultures using planar, integrated 60-microelectrode arrays. We compared predictions obtained using a pairwise approach with those using a hierarchical approach that uses two different spatial units for describing the activity interactions: single electrodes and electrode clusters. In this hierarchical model, short-range interactions within each cluster were modeled by pairwise interactions of electrode activities and long-range interactions were modeled by pairwise interactions of cluster activities. Despite the relatively low number of parameters used, the hierarchical model provided a more accurate description of the activity patterns than the pairwise model when applied to ensembles of 10 electrodes. Furthermore, the hierarchical model was successfully applied to a larger-scale data of approximately 60 electrodes. Electrode activities within clusters were highly correlated and spatially contiguous. In contrast, long-range interactions were diffuse, suggesting the presence of higher-than-pairwise-order interactions involved in the LFP peak activities. Thus, the identification of appropriate units of interaction may allow for the successful characterization of neuronal activities in large-scale networks.

Embryonic MGE precursor cells grafted into adult rat striatum integrate and ameliorate motor symptoms in 6-OHDA-lesioned rats

We investigated a strategy to ameliorate the motor symptoms of rats that received 6-hydroxydopamine (6-OHDA) lesions, a rodent model of Parkinson's disease, through transplantation of embryonic medial ganglionic eminence (MGE) cells into the striatum. During brain development, embryonic MGE cells migrate into the striatum and neocortex where they mature into GABAergic interneurons and play a key role in establishing the balance between excitation and inhibition. Unlike most other embryonic neurons, MGE cells retain the capacity for migration and integration when transplanted into the postnatal and adult brain. We performed MGE cell transplantation into the basal ganglia of control and 6-OHDA-lesioned rats. Transplanted MGE cells survived, differentiated into GABA(+) neurons, integrated into host circuitry, and modified motor behavior in both lesioned and control rats. Our data suggest that MGE cell transplantation into the striatum is a promising approach to investigate the potential benefits of remodeling basal ganglia circuitry in neurodegenerative diseases.

Cortical regulation of striatal medium spiny neuron dendritic remodeling in parkinsonism: modulation of glutamate release reverses dopamine depletion-induced dendritic spine loss

Striatal medium spiny neurons (MSNs) receive glutamatergic afferents from the cerebral cortex and dopaminergic inputs from the substantia nigra (SN). Striatal dopamine loss decreases the number of MSN dendritic spines. This loss of spines has been suggested to reflect the removal of tonic dopamine inhibitory control over corticostriatal glutamatergic drive, with increased glutamate release culminating in MSN spine loss. We tested this hypothesis in two ways. We first determined in vivo if decortication reverses or prevents dopamine depletion-induced spine loss by placing motor cortex lesions 4 weeks after, or at the time of, 6-hydroxydopamine lesions of the SN. Animals were sacrificed 4 weeks after cortical lesions. Motor cortex lesions significantly reversed the loss of MSN spines elicited by dopamine denervation; a similar effect was observed in the prevention experiment. We then determined if modulating glutamate release in organotypic cocultures prevented spine loss. Treatment of the cultures with the mGluR2/3 agonist LY379268 to suppress corticostriatal glutamate release completely blocked spine loss in dopamine-denervated cultures. These studies provide the first evidence to show that MSN spine loss associated with parkinsonism can be reversed and point to suppression of corticostriatal glutamate release as a means of slowing progression in Parkinson's disease.

Singing-related neural activity distinguishes four classes of putative striatal neurons in the songbird basal ganglia

The striatum-the primary input nucleus of the basal ganglia-plays a major role in motor control and learning. Four main classes of striatal neuron are thought to be essential for normal striatal function: medium spiny neurons, fast-spiking interneurons, cholinergic tonically active neurons, and low-threshold spiking interneurons. However, the nature of the interaction of these neurons during behavior is poorly understood. The songbird area X is a specialized striato-pallidal basal ganglia nucleus that contains two pallidal cell types as well as the same four cell types found in the mammalian striatum. We recorded 185 single units in Area X of singing juvenile birds and, based on singing-related firing patterns and spike waveforms, find six distinct cell classes--two classes of putative pallidal neuron that exhibited a high spontaneous firing rate (> 60 Hz), and four cell classes that exhibited low spontaneous firing rates characteristic of striatal neurons. In this study, we examine in detail the four putative striatal cell classes. Type-1 neurons were the most frequently encountered and exhibited sparse temporally precise singing-related activity. Type-2 neurons were distinguished by their narrow spike waveforms and exhibited brief, high-frequency bursts during singing. Type-3 neurons were tonically active and did not burst, whereas type-4 neurons were inactive outside of singing and during singing generated long high-frequency bursts that could reach firing rates over 1 kHz. Based on comparison to the mammalian literature, we suggest that these four putative striatal cell classes correspond, respectively, to the medium spiny neurons, fast-spiking interneurons, tonically active neurons, and low-threshold spiking interneurons that are known to reside in area X.

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

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

Nrf2 activators provide neuroprotection against 6-hydroxydopamine toxicity in rat organotypic nigrostriatal cocultures

Oxidative stress and inflammation appear to play a critical role in the progression of Parkinson's disease. As a result, there has been growing interest in antioxidant pathways and how these pathways might be exploited to slow the progressive loss of dopamine neurons. One such pathway that has garnered attention recently is mediated by the transcription factor Nrf2 and is integral in orchestrating cells' antiinflammatory defense. Nrf2 controls the inducible expression of numerous antioxidant and phase 2 detoxification genes, such as glutathione S-transferase, heme oxygenase-1, and NAD(P)H:quinone oxidoreductase 1 (NQO1). Once activated, these genes work synergistically to maintain intracellular redox homeostasis. In this study, we test the hypothesis that Nrf2 activation can protect dopaminergic neurons against 6-hydroxydopamine (6-OHDA)-induced toxicity. Treatment of organotypic nigrostriatal cocultures with either tert-butylhydroquinone (tBHQ) or sulforaphane, known activators of Nrf2, mitigated dopaminergic cell loss. The observed protection appeared to be mediated, at least in part, by an increase in antioxidant activity. Simultaneous treatment of cultures with tBHQ and 6-OHDA increased NQO1 expression 17-fold compared with controls. Overall, these results suggest that Nrf2 may play an important role in cellular protection in neurodegenerative diseases and may be a viable therapeutic target in the future.

Good vibrations: cross-frequency coupling in the human nucleus accumbens during reward processing

The nucleus accumbens is critical for reward-guided learning and decision-making. It is thought to "gate" the flow of a diverse range of information (e.g., rewarding, aversive, and novel events) from limbic afferents to basal ganglia outputs. Gating and information encoding may be achieved via cross-frequency coupling, in which bursts of high-frequency activity occur preferentially during specific phases of slower oscillations. We examined whether the human nucleus accumbens engages such a mechanism by recording electrophysiological activity directly from the accumbens of human patients undergoing deep brain stimulation surgery. Oscillatory activity in the gamma (40-80 Hz) frequency range was synchronized with the phase of simultaneous alpha (8-12 Hz) waves. Further, losing and winning small amounts of money elicited relatively increased gamma oscillation power prior to and following alpha troughs, respectively. Gamma-alpha synchronization may reflect an electrophysiological gating mechanism in the human nucleus accumbens, and the phase differences in gamma-alpha coupling may reflect a reward information coding scheme similar to phase coding.

Activation of mGluR5 induces spike afterdepolarization and enhanced excitability in medium spiny neurons of the nucleus accumbens by modulating persistent Na+ currents

The involvement of metabotropic glutamate receptors type 5 (mGluR5) in drug-induced behaviours is well-established but limited information is available on their functional roles in addiction-relevant brain areas like the nucleus accumbens (NAc). This study demonstrates that pharmacological and synaptic activation of mGluR5 increases the spike discharge of medium spiny neurons (MSNs) in the NAc. This effect was associated with the appearance of a slow afterdepolarization (ADP) which, in voltage-clamp experiments, was recorded as a slowly inactivating inward current. Pharmacological studies showed that ADP was elicited by mGluR5 stimulation via G-protein-dependent activation of phospholipase C and elevation of intracellular Ca(2+) levels. Both ADP and spike aftercurrents were significantly inhibited by the Na(+) channel-blocker, tetrodotoxin (TTX). Moreover, the selective blockade of persistent Na(+) currents (I(NaP)), achieved by NAc slice pre-incubation with 20 nm TTX or 10 \#956;m riluzole, significantly reduced the ADP amplitude, indicating that this type of Na(+) current is responsible for the mGluR5-dependent ADP. mGluR5 activation also produced significant increases in I(NaP), and the pharmacological blockade of this current prevented the mGluR5-induced enhancement of spike discharge. Collectively, these data suggest that mGluR5 activation upregulates I(NaP) in MSNs of the NAc, thereby inducing an ADP that results in enhanced MSN excitability. Activation of mGluR5 will significantly alter spike firing in MSNs in vivo, and this effect could be an important mechanism by which these receptors mediate certain aspects of drug-induced behaviours.

Changes to interneuron-driven striatal microcircuits in a rat model of Parkinson's disease

Striatal interneurons play key roles in basal ganglia function and related disorders by modulating the activity of striatal projection neurons. Here we have injected rabies virus (RV) into either the rat substantia nigra pars reticulata or the globus pallidus and took advantage of the trans-synaptic spread of RV to unequivocally identify the interneurons connected to striatonigral- or striatopallidal-projecting neurons, respectively. Large numbers of RV-infected parvalbumin (PV+/RV+) and cholinergic (ChAT+/RV+) interneurons were detected in control conditions, and they showed marked changes following intranigral 6-hydroxydopamine injection. The number of ChAT+/RV+ interneurons innervating striatopallidal neurons increased concomitant with a reduction in the number of PV+/RV+ interneurons, while the two interneuron populations connected to striatonigral neurons were clearly reduced. These data provide the first evidence of synaptic reorganization between striatal interneurons and projection neurons, notably a switch of cholinergic innervation onto striatopallidal neurons, which could contribute to imbalanced striatal outflow in parkinsonian state.

Dysregulated information processing by medium spiny neurons in striatum of freely behaving mouse models of Huntington's disease

Huntington's disease (HD) is an autosomal dominant condition that compromises behavioral output. Dysfunction of medium spiny neurons (MSNs), which are the sole output system of the striatum, is thought to underlie HD pathophysiology. What is not known is how HD alters MSN information processing during behavior, which likely drives the HD behavioral phenotype. We recorded from populations of MSNs in two freely behaving and symptomatic HD mouse models: R6/2 transgenics are based on a C57BL/6J*CBA/J background and show robust behavioral symptoms, whereas knock-in (KI) mice have a 129sv background and express relatively mild behavioral signs. At the single-unit level, we found that the MSN firing rate was elevated in R6/2 but not in KI mice compared with their respective wild-type (WT) controls. In contrast, burst activity, which corresponds to periods of high-frequency firing, was altered in both HD models compared with WT. At the population level, we found that correlated firing between pairs of MSNs was a prominent feature in WT that was reduced in both HD models. Similarly, coincident bursts, which are bursts between pairs of neurons that overlap in time and occur more often in pairs of MSNs that exhibit correlated firing, were decreased in HD mice. Our results indicate an important role in both bursting and correlated burst firing for information processing in MSNs. Dysregulation of this processing scheme, moreover, is a key component of HD pathophysiology regardless of the severity of HD symptoms, genetic construct, and background strain of the mouse models.

Striatal synaptic changes in experimental parkinsonism: role of NMDA receptor trafficking in PSD

Parkinson's disease (PD) is a neurodegenerative disorder characterized by the progressive degeneration of dopaminergic terminals from substantia nigra pars compacts, which leads to the motor symptoms observed in this disorder. L-Dopa administration represents the most effective therapeutic treatment of PD, but the development of disabling dyskinetic movements is a dramatic consequence of the treatment. The organization and functional interactions of glutamate receptors within the striatum appear to be crucial both in the pathogenesis of PD and in the development of dyskinesia. At the molecular level, it has become increasingly evident that the glutamatergic NMDA receptor complex is a dynamic structure that is involved in the regulation of corticostriatal long-term synaptic changes, which is altered in experimental PD and in dyskinesia.

NR2A-containing NMDA receptors depress glutamatergic synaptic transmission and evoked-dopamine release in the mouse striatum

NMDA receptors play essential roles in the physiology and pathophysiology of the striatum, a brain nucleus involved in motor control and reward-motivated behaviors. NMDA receptors are composed of NR1 and NR2A-D subunits. Functional properties of NMDA receptors are determined by the type of NR2 subunit they contain. In this study, we have examined the involvement of NR2B and NR2A in the modulatory effect of NMDA on glutamatergic and dopaminergic synaptic transmission in the striatum. We found that bath application of NMDA decreased the amplitude of the field excitatory post-synaptic potential/population spike (fEPSP/PS) measured in corticostriatal mouse brain slices. This depression was not affected by the NR2B-selective antagonists Ifenprodil and Ro 25-6981, but was abolished by the NR2A antagonist NVP-AAM077. Activation of corticostriatal neurons by NMDA did not contribute to synaptic depression because similar results were obtained in decorticated striatal slices. Synaptic depression was not dependent on GABA release because the GABA(A) receptor antagonist bicuculline did not affect NMDA-induced decrease of the fEPSP/PS. NMDA also depressed evoked-dopamine release through NR2A- but not NR2B-containing NMDA receptors. Our results identify an important role for NR2A-containing NMDA receptors intrinsic to the striatum in regulating glutamatergic synaptic transmission and evoked-dopamine release.

Abnormal striatal GABA transmission in the mouse model for the fragile X syndrome

BACKGROUND

Structural and functional neuroimaging studies suggest abnormal activity in the striatum of patients with the fragile X syndrome (FXS), the most common form of inherited mental retardation.

METHODS

Neurophysiological and immunofluorescence experiments in striatal brain slices. We studied the synaptic transmission in a mouse model for FXS, as well as the subcellular localization of fragile X mental retardation protein (FMRP) and brain cytoplasmic (BC1) RNA in striatal axons.

RESULTS

Our results show that absence of FMRP is associated with apparently normal striatal glutamate-mediated transmission, but abnormal gamma-aminobutyric acid (GABA) transmission. This effect is likely secondary to increased transmitter release from GABAergic nerve terminals. We detected the presence of FMRP in axons of striatal neurons and observed a selective increase in the frequency of spontaneous and miniature inhibitory postsynaptic currents (sIPSCs, mIPSCs) in fmr1-knockout mice. We also observed reduced paired-pulse ratio of evoked IPSCs, a finding that is consistent with the idea that transmitter release probability from striatal GABAergic nerve terminals is higher than normal in these mutants. Finally, we have identified the small noncoding BC1 RNA as a critical coplayer of FMRP in the regulation of striatal synaptic transmission.

CONCLUSIONS

Understanding the physiologic action of FMRP and the synaptic defects associated with GABA transmission might be useful to design appropriate pharmacologic interventions for FXS.

Feed-forward excitation of striatal neuron activity by frontal cortical activation of nitric oxide signaling in vivo

The gaseous neurotransmitter nitric oxide plays an important role in the modulation of corticostriatal synaptic transmission. This study examined the impact of frontal cortex stimulation on striatal nitric oxide efflux and neuron activity in urethane-anesthetized rats using amperometric microsensor and single-unit extracellular recordings, respectively. Systemic administration of the neuronal nitric oxide synthase inhibitor 7-nitroindazole decreased spontaneous spike activity without affecting activity evoked by single-pulse stimulation of the ipsilateral cortex. Train (30 Hz) stimulation of the contralateral frontal cortex transiently increased nitric oxide efflux in a robust and reproducible manner. Evoked nitric oxide efflux was attenuated by systemic administration of 7-nitroindazole and the non-selective nitric oxide synthase inhibitor N(G)-nitro-L-arginine methyl ester. Train stimulation of the contralateral cortex, in a manner identical to that used to evoke nitric oxide efflux, had variable effects on spike activity assessed during the train stimulation trial, but induced a short-term depression of cortically evoked activity in the first post-train stimulation trial. Interestingly, 7-nitroindazole potently decreased cortically evoked activity recorded during the train stimulation trial. Moreover, the short-term depression of spike activity induced by train stimulation was enhanced following pretreatment with 7-nitroindazole and attenuated after systemic administration of the dopamine D2 receptor antagonist eticlopride. These results demonstrate that robust activation of frontal cortical afferents in the intact animal activates a powerful nitric oxide-mediated feed-forward excitation which partially offsets concurrent D2 receptor-mediated short-term inhibitory influences on striatal neuron activity. Thus, nitric oxide signaling is likely to play an important role in the integration of corticostriatal sensorimotor information in striatal networks.

Effects of Striatal GABAA-Receptor Blockade on Striatal and Cortical Activity in Monkeys

To elucidate the role of ambient striatal γ-aminobutyric acid (GABA) in the regulation of neuronal activity in the basal ganglia–thalamocortical circuits, we studied the effects of blocking striatal GABAA receptors on the electrical activities of single striatal neurons, on local field potentials (LFPs) in the striatum, and on motor cortical electroencephalograms (EEGs) in two monkeys. Striatal LFPs were recorded with a device that allowed us to simultaneously record field potentials and apply drugs by reverse microdialysis at the same site. Administration of the GABAA-receptor antagonist gabazine (SR95531, 10 and 500 μM) induced large-amplitude LFP fluctuations at the infusion site, occurring every 2–5 s for about 2 h after the start of the 20-min drug administration. These events were prevented by cotreatment with a GABAA-receptor agonist (muscimol, 100 μM) or a combination of ionotropic glutamate receptor antagonists (CNQX and MK-801, each given at 100 μM). Gabazine (10 μM) also increased the firing of single neurons recorded close to the injection site, but in most cases there was no correlation between single-neuron activity and the concomitantly recorded LFP signals from the same striatal region. In contrast, intrastriatal application of gabazine increased the correlation between striatal LFPs and EEG, and resulted in the appearance of recurrent EEG events that were temporally related to the striatal LFP events. These data provide evidence that a GABAergic “tone” in the monkey striatum controls the spontaneous activity of striatal neurons, as well as the level of striatal and cortical synchrony.