Simulation of GABA function in the basal ganglia: computational models of GABAergic mechanisms in basal ganglia function

Pathological cell assembly dynamics in a striatal MSN network model

Under normal conditions the principal cells of the striatum, medium spiny neurons (MSNs), show structured cell assembly activity patterns which alternate sequentially over exceedingly long timescales of many minutes. It is important to understand this activity since it is characteristically disrupted in multiple pathologies, such as Parkinson's disease and dyskinesia, and thought to be caused by alterations in the MSN to MSN lateral inhibitory connections and in the strength and distribution of cortical excitation to MSNs. To understand how these long timescales arise we extended a previous network model of MSN cells to include synapses with short-term plasticity, with parameters taken from a recent detailed striatal connectome study. We first confirmed the presence of sequentially switching cell clusters using the non-linear dimensionality reduction technique, Uniform Manifold Approximation and Projection (UMAP). We found that the network could generate non-stationary activity patterns varying extremely slowly on the order of minutes under biologically realistic conditions. Next we used Simulation Based Inference (SBI) to train a deep net to map features of the MSN network generated cell assembly activity to MSN network parameters. We used the trained SBI model to estimate MSN network parameters from ex-vivo brain slice calcium imaging data. We found that best fit network parameters were very close to their physiologically observed values. On the other hand network parameters estimated from Parkinsonian, decorticated and dyskinetic ex-vivo slice preparations were different. Our work may provide a pipeline for diagnosis of basal ganglia pathology from spiking data as well as for the design pharmacological treatments.

Anti-Hebbian plasticity drives sequence learning in striatum

Spatio-temporal activity patterns have been observed in a variety of brain areas in spontaneous activity, prior to or during action, or in response to stimuli. Biological mechanisms endowing neurons with the ability to distinguish between different sequences remain largely unknown. Learning sequences of spikes raises multiple challenges, such as maintaining in memory spike history and discriminating partially overlapping sequences. Here, we show that anti-Hebbian spike-timing dependent plasticity (STDP), as observed at cortico-striatal synapses, can naturally lead to learning spike sequences. We design a spiking model of the striatal output neuron receiving spike patterns defined as sequential input from a fixed set of cortical neurons. We use a simple synaptic plasticity rule that combines anti-Hebbian STDP and non-associative potentiation for a subset of the presented patterns called rewarded patterns. We study the ability of striatal output neurons to discriminate rewarded from non-rewarded patterns by firing only after the presentation of a rewarded pattern. In particular, we show that two biological properties of striatal networks, spiking latency and collateral inhibition, contribute to an increase in accuracy, by allowing a better discrimination of partially overlapping sequences. These results suggest that anti-Hebbian STDP may serve as a biological substrate for learning sequences of spikes.

Impaired striatal glutamate/GABA regulation in violent offenders with antisocial personality disorder and psychopathy

Men with antisocial personality disorder (ASPD) with or without psychopathy (+/-P) are responsible for most violent crime in society. Development of effective treatments is hindered by poor understanding of the neurochemical underpinnings of the condition. Men with ASPD with and without psychopathy demonstrate impulsive decision-making, associated with striatal abnormalities in functional neuroimaging studies. However, to date, no study has directly examined the potential neurochemical underpinnings of such abnormalities. We therefore investigated striatal glutamate: GABA ratio using Magnetic Resonance Spectroscopy in 30 violent offenders (16 ASPD-P, 14 ASPD + P) and 21 healthy non-offenders. Men with ASPD +/- P had a significant reduction in striatal glutamate : GABA ratio compared to non-offenders. We report, for the first time, striatal Glutamate/GABA dysregulation in ASPD +/- P, and discuss how this may be related to core behavioral abnormalities in the disorders.

Computational insights on asymmetrical D1 and D2 receptor-mediated chunking: implications for OCD and Schizophrenia

Repetitive thoughts and motor programs including perseveration are bridge symptoms characteristic of obsessive compulsive disorder (OCD), schizophrenia and in the co-morbid overlap of these conditions. The above pathologies are sensitive to altered activation and kinetics of dopamine D 1 and D 2 receptors that differently influence sequence learning and recall. Recognizing start and stop elements of motor and cognitive behaviors has crucial importance. During chunking, frequent components of temporal strings are concatenated into single units. We extended a published computational model (Asabuki et al. 2018), where two populations of neurons are connected and simulated in a reservoir computing framework. These neural pools were adopted to represent D1 and D2 striatal neuronal populations. We investigated how specific neural and striatal circuit parameters can influence start/stop signaling and found that asymmetric intra-network connection probabilities, synaptic weights and differential time constants may contribute to signaling of start/stop elements within learned sequences. Asymmetric coupling between the striatal D 1 and D 2 neural populations was also demonstrated to be beneficial. Our modeling results predict that dynamical differences between the two dopaminergic striatal populations and the interaction between them may play complementary roles in chunk boundary signaling. Start and stop dichotomies can arise from the larger circuit dynamics as well, since neural and intra-striatal connections only partially support a clear division of labor.

A realistic morpho-anatomical connection strategy for modelling full-scale point-neuron microcircuits

The modeling of extended microcircuits is emerging as an effective tool to simulate the neurophysiological correlates of brain activity and to investigate brain dysfunctions. However, for specific networks, a realistic modeling approach based on the combination of available physiological, morphological and anatomical data is still an open issue. One of the main problems in the generation of realistic networks lies in the strategy adopted to build network connectivity. Here we propose a method to implement a neuronal network at single cell resolution by using the geometrical probability volumes associated with pre- and postsynaptic neurites. This allows us to build a network with plausible connectivity properties without the explicit use of computationally intensive touch detection algorithms using full 3D neuron reconstructions. The method has been benchmarked for the mouse hippocampus CA1 area, and the results show that this approach is able to generate full-scale brain networks at single cell resolution that are in good agreement with experimental findings. This geometric reconstruction of axonal and dendritic occupancy, by effectively reflecting morphological and anatomical constraints, could be integrated into structured simulators generating entire circuits of different brain areas facilitating the simulation of different brain regions with realistic models.

Early-life adversity selectively interrupts the dendritic differentiation of dorsolateral striatal neurons in male mice

The effects of early-life adversity (ELA) on dendritic differentiation of striatal neurons were investigated in the dorsal striatum including the dorsomedial striatum and dorsolateral striatum (DMS and DLS, respectively). An animal model of ELA was created by changing the growth environment of newborn mouse pups by giving limited bedding and nesting materials from postnatal day 2 to day 9 (P2-P9). One week after the stress paradigm (P16), the dendritic branches and spines of striatal spiny neurons as well as the synapses represented by postsynaptic density protein-95 (PSD-95) in DMS and DLS were stereologically analyzed. Adverse experience in early life selectively affected the spiny neurons in DLS, leading to abundant proximal dendritic branches and an increased number of filopodia-like protrusions, but a reduced number of dendritic spines. The selective effects of stress on neurons in DLS were further identified by reduced expression of PSD-95, including a reduced optical density of PSD-95 immunoreactivity and fewer individual PSD-95 immunoreactive synapses in this region. Notably, stress in early life affected either D1 or D2 dopamine receptor-expressing DLS neurons. These findings suggest that adverse early-life experience delayed the maturation of dendritic spines on neurons in the dorsolateral striatum. Altered dendritic differentiation provoked by stress in early life may contribute critically to the formation of proper neuronal circuits in the dorsal striatum and, therefore, affect striatum-dependent habitual behavior and emotional function later in life.

Striatal network modeling in Huntington's Disease

Medium spiny neurons (MSNs) comprise over 90% of cells in the striatum. In vivo MSNs display coherent burst firing cell assembly activity patterns, even though isolated MSNs do not burst fire intrinsically. This activity is important for the learning and execution of action sequences and is characteristically dysregulated in Huntington's Disease (HD). However, how dysregulation is caused by the various neural pathologies affecting MSNs in HD is unknown. Previous modeling work using simple cell models has shown that cell assembly activity patterns can emerge as a result of MSN inhibitory network interactions. Here, by directly estimating MSN network model parameters from single unit spiking data, we show that a network composed of much more physiologically detailed MSNs provides an excellent quantitative fit to wild type (WT) mouse spiking data, but only when network parameters are appropriate for the striatum. We find the WT MSN network is situated in a regime close to a transition from stable to strongly fluctuating network dynamics. This regime facilitates the generation of low-dimensional slowly varying coherent activity patterns and confers high sensitivity to variations in cortical driving. By re-estimating the model on HD spiking data we discover network parameter modifications are consistent across three very different types of HD mutant mouse models (YAC128, Q175, R6/2). In striking agreement with the known pathophysiology we find feedforward excitatory drive is reduced in HD compared to WT mice, while recurrent inhibition also shows phenotype dependency. We show that these modifications shift the HD MSN network to a sub-optimal regime where higher dimensional incoherent rapidly fluctuating activity predominates. Our results provide insight into a diverse range of experimental findings in HD, including cognitive and motor symptoms, and may suggest new avenues for treatment.

Thalamic degeneration in MPTP-treated Parkinsonian monkeys: impact upon glutamatergic innervation of striatal cholinergic interneurons

In both Parkinson's disease (PD) patients and MPTP-treated non-human primates, there is a profound neuronal degeneration of the intralaminar centromedian/parafascicular (CM/Pf) thalamic complex. Although this thalamic pathology has long been established in PD (and other neurodegenerative disorders), the impact of CM/Pf cell loss on the integrity of the thalamo-striatal glutamatergic system and its regulatory functions upon striatal neurons remain unknown. In the striatum, cholinergic interneurons (ChIs) are important constituents of the striatal microcircuitry and represent one of the main targets of CM/Pf-striatal projections. Using light and electron microscopy approaches, we have analyzed the potential impact of CM/Pf neuronal loss on the anatomy of the synaptic connections between thalamic terminals (vGluT2-positive) and ChIs neurons in the striatum of parkinsonian monkeys treated chronically with MPTP. The following conclusions can be drawn from our observations: (1) as reported in PD patients, and in our previous monkey study, CM/Pf neurons undergo profound degeneration in monkeys chronically treated with low doses of MPTP. (2) In the caudate (head and body) nucleus of parkinsonian monkeys, there is an increased density of ChIs. (3) Despite the robust loss of CM/Pf neurons, no significant change was found in the density of thalamostriatal (vGluT2-positive) terminals, and in the prevalence of vGluT2-positive terminals in contact with ChIs in parkinsonian monkeys. These findings provide new information about the state of thalamic innervation of the striatum in parkinsonian monkeys with CM/Pf degeneration, and bring up an additional level of intricacy to the consequences of thalamic pathology upon the functional microcircuitry of the thalamostriatal system in parkinsonism. Future studies are needed to assess the importance of CM/Pf neuronal loss, and its potential consequences on the neuroplastic changes induced in the synaptic organization of the thalamostriatal system, in the development of early cognitive impairments in PD.

Dopaminergic modulation of striatal function and Parkinson's disease

The striatum is richly innervated by mesencephalic dopaminergic neurons that modulate a diverse array of cellular and synaptic functions that control goal-directed actions and habits. The loss of this innervation has long been thought to be the principal cause of the cardinal motor symptoms of Parkinson's disease (PD). Moreover, chronic, pharmacological overstimulation of striatal dopamine (DA) receptors is generally viewed as the trigger for levodopa-induced dyskinesia (LID) in late-stage PD patients. Here, we discuss recent advances in our understanding of the relationship between the striatum and DA, particularly as it relates to PD and LID. First, it has become clear that chronic perturbations of DA levels in PD and LID bring about cell type-specific, homeostatic changes in spiny projection neurons (SPNs) that tend to normalize striatal activity. Second, perturbations in DA signaling also bring about non-homeostatic aberrations in synaptic plasticity that contribute to disease symptoms. Third, it has become evident that striatal interneurons are major determinants of network activity and behavior in PD and LID. Finally, recent work examining the activity of SPNs in freely moving animals has revealed that the pathophysiology induced by altered DA signaling is not limited to imbalance in the average spiking in direct and indirect pathways, but involves more nuanced disruptions of neuronal ensemble activity.

Treadmill Exercise Improves Motor Dysfunction and Hyperactivity of the Corticostriatal Glutamatergic Pathway in Rats with 6-OHDA-Induced Parkinson's Disease

Hyperactivity in the corticostriatal glutamatergic pathway (CGP) induces basal ganglia dysfunction, contributing to parkinsonian syndrome (PS). Physical exercise can improve PS. However, the effect of exercise on the CGP, and whether this pathway is involved in the improvement of PS, remains unclear. Parkinson's disease (PD) was induced in rats by 6-hydroxydopamine injection into the right medial forebrain bundle. Motor function was assessed using the cylinder test. Striatal neuron (SN) spontaneous and evoked firing activity was recorded, and the expression levels of Cav1.3 and CaMKII in the striatum were measured after 4 weeks of treadmill exercise. The motor function in PD rats was improved by treadmill exercise. SN showed significantly enhanced excitability, and treadmill exercise reduced SN excitability in PD rats. In addition, firing activity was evoked in SNs by stimulation of the primary motor cortex, and SNs exhibited significantly decreased stimulus threshold, increased firing rates, and reduced latency. The expression of Cav1.3 and p-CaMKII (Thr286) in the striatum were enhanced in PD rats. However, these effects were reversed by treadmill exercise. These findings suggest that treadmill exercise inhibits CGP hyperactivity in PD rats, which may be related to improvement of PS.

Loss and remodeling of striatal dendritic spines in Parkinson's disease: from homeostasis to maladaptive plasticity?

In Parkinson's disease (PD) patients and animal models of PD, the progressive degeneration of the nigrostriatal dopamine (DA) projection leads to two major changes in the morphology of striatal projection neurons (SPNs), i.e., a profound loss of dendritic spines and the remodeling of axospinous glutamatergic synapses. Striatal spine loss is an early event tightly associated with the extent of striatal DA denervation, but not the severity of parkinsonian motor symptoms, suggesting that striatal spine pruning might be a form of homeostatic plasticity that compensates for the loss of striatal DA innervation and the resulting dysregulation of corticostriatal glutamatergic transmission. On the other hand, the remodeling of axospinous corticostriatal and thalamostriatal glutamatergic synapses might represent a form of late maladaptive plasticity that underlies changes in the strength and plastic properties of these afferents and the resulting increased firing and bursting activity of striatal SPNs in the parkinsonian state. There is also evidence that these abnormal synaptic connections might contribute to the pathophysiology of L-DOPA-induced dyskinesia. Despite the significant advances made in this field over the last thirty years, many controversial issues remain about the striatal SPN subtypes affected, the role of spine changes in the altered activity of SPNs in the parkinsonisn state, and the importance of striatal spine plasticity in the pathophysiology of L-DOPA-induced dyskinesia. In this review, we will examine the current state of knowledge of these issues, discuss the limitations of the animal models used to address some of these questions, and assess the relevance of data from animal models to the human-diseased condition.

Untangling Basal Ganglia Network Dynamics and Function: Role of Dopamine Depletion and Inhibition Investigated in a Spiking Network Model

The basal ganglia are a crucial brain system for behavioral selection, and their function is disturbed in Parkinson's disease (PD), where neurons exhibit inappropriate synchronization and oscillations. We present a spiking neural model of basal ganglia including plausible details on synaptic dynamics, connectivity patterns, neuron behavior, and dopamine effects. Recordings of neuronal activity in the subthalamic nucleus and Type A (TA; arkypallidal) and Type I (TI; prototypical) neurons in globus pallidus externa were used to validate the model. Simulation experiments predict that both local inhibition in striatum and the existence of an indirect pathway are important for basal ganglia to function properly over a large range of cortical drives. The dopamine depletion-induced increase of AMPA efficacy in corticostriatal synapses to medium spiny neurons (MSNs) with dopamine receptor D2 synapses (CTX-MSN D2) and the reduction of MSN lateral connectivity (MSN-MSN) were found to contribute significantly to the enhanced synchrony and oscillations seen in PD. Additionally, reversing the dopamine depletion-induced changes to CTX-MSN D1, CTX-MSN D2, TA-MSN, and MSN-MSN couplings could improve or restore basal ganglia action selection ability. In summary, we found multiple changes of parameters for synaptic efficacy and neural excitability that could improve action selection ability and at the same time reduce oscillations. Identification of such targets could potentially generate ideas for treatments of PD and increase our understanding of the relation between network dynamics and network function.

Dopaminergic treatment weakens medium spiny neuron collateral inhibition in the parkinsonian striatum

The striatal medium spiny neurons (MSNs) are critical to both motor and cognitive functions. A potential regulator of MSN activity is the GABAergic collateral axonal input from neighboring MSNs. These collateral axon terminals are further under the regulation of presynaptic dopamine (DA) receptors that may become dysfunctional when the intense striatal DA innervation is lost in Parkinson's disease (PD). We show that DA D1 receptor-expressing MSNs (D1-MSNs) and D2 receptor-expressing MSNs (D2-MSNs) each formed high-rate, one-way collateral connections with a homotypic preference in both normal and DA-denervated mouse striatum. Furthermore, whereas the homotypic preference, one-way directionality and the basal inhibitory strength were preserved, DA inhibited GABA release at the D2-MSN→D2-MSN collateral synapse in a supersensitive manner in the DA-denervated striatum. In contrast, for D1-MSN-originated collateral connections, whereas D1 agonism facilitated D1-MSN→D1-MSN collateral inhibition in the normal striatum, this presynaptic D1R facilitation of GABA release was lost in the parkinsonian striatum. These results indicate that in the parkinsonian striatum, dopaminergic treatment can presynaptically weaken the D2-MSN→D2-MSN collateral inhibition and disinhibit the surrounding D2-MSNs, whereas the D1-MSN→D1-MSN collateral inhibition is weakened by the loss of the presynaptic D1 receptor facilitation, disinhibiting the surrounding D1-MSNs. Together, these newly discovered effects can disrupt the MSN circuits in the parkinsonian striatum and may contribute to dopaminergic treatment-induced aberrant motor and nonmotor behaviors in PD.NEW & NOTEWORTHY With the use of a large database, this study establishes that neighboring homotypic striatal spiny projection neurons have a 50% chance to form one-way collateral inhibitory connection, a substantially higher rate than previous estimates. This study also shows that dopamine denervation may alter presynaptic dopamine receptor function such that dopaminergic treatment of Parkinson's disease can weaken the surround inhibition and may reduce the contrast of the striatal outputs, potentially contributing to dopamine's profound motor and nonmotor behavioral effects.

A Low-Correlation Resting State of the Striatum during Cortical Avalanches and Its Role in Movement Suppression

During quiet resting behavior, involuntary movements are suppressed. Such movement control is attributed to cortico-basal ganglia loops, yet population dynamics within these loops during resting and their relation to involuntary movements are not well characterized. Here, we show by recording cortical and striatal ongoing population activity in awake rats during quiet resting that intrastriatal inhibition maintains a low-correlation striatal resting state in the presence of cortical neuronal avalanches. Involuntary movements arise from disturbed striatal resting activity through two different population dynamics. Nonselectively reducing intrastriatal γ-aminobutyric acid (GABA) receptor-A inhibition synchronizes striatal dynamics, leading to involuntary movements at low rate. In contrast, reducing striatal interneuron (IN)-mediated inhibition maintains decorrelation and induces intermittent involuntary movements at high rate. This latter scenario was highly effective in modulating cortical dynamics at a subsecond timescale. To distinguish intrastriatal processing from loop dynamics, cortex-striatum-midbrain cultures, which lack feedback to cortex, were used. Cortical avalanches in vitro were accompanied by low-correlated resting activity in the striatum and nonselective reduction in striatal inhibition synchronized striatal neurons similar to in vivo. Importantly, reduction of inhibition from striatal INs maintained low correlations in the striatum while reorganizing functional connectivities among striatal neurons. Our results demonstrate the importance of two major striatal microcircuits in distinctly regulating striatal and cortical resting state dynamics. These findings suggest that specific functional connectivities of the striatum that are maintained by local inhibition are important in movement control.

Why don’t neuronal “avalanches” in resting-state cortex cause involuntary movements? This study shows that a low-correlation striatal resting state suppresses such movements and explores mechanisms that disrupt this inhibition.

Even in the absence of apparent motor output, the brain produces a rich repertoire of neuronal activity patterns known as “resting state” activity. In the outer layer of the cortex, resting state patterns emerge as neuronal avalanches, precisely scale-invariant spatiotemporal bursts that often engage large populations of neurons. Little is known about how the brain suppresses involuntary movements during such activity. Here, we show that the striatum, which is part of the cortex-basal ganglia loop, maintains a low-correlation state during resting activity. By using a combination of in vivo and in vitro approaches with pharmacological manipulations, we demonstrate that the precise configuration of this low-correlation state effectively contributes to involuntary movements. Nonselective blockade of intra-striatal inhibition abolished the low-correlation striatal resting state, barely affected cortical avalanches, and led to involuntary movements at low rate. In contrast, selectively reducing striatal interneuron inhibition strongly affected cortical avalanches and triggered involuntary movements at high rate while maintaining a relatively decorrelated striatal resting state. Our results demonstrate the importance of different inhibitory striatal circuits in the suppression of involuntary movements and suggest that the precise spatiotemporal configuration of striatal activity plays an active role in the regulation of cortical resting state activity and motor control.

Presynaptic Inhibition in the Striatum of the Basal Ganglia Improves Pattern Classification and Thus Promotes Superior Goal Selection

This review article takes a multidisciplinary approach to understand how presynaptic inhibition in the striatum of the basal ganglia (BG) contributes to pattern classification and the selection of goals that control behavior. It is a difficult problem both because it is multidimensional and because it is has complex system dynamics. We focus on the striatum because, as the main site for input to the BG, it gets to decide what goals are important to consider.

Morphological changes of glutamatergic synapses in animal models of Parkinson's disease

The striatum and the subthalamic nucleus (STN) are the main entry doors for extrinsic inputs to reach the basal ganglia (BG) circuitry. The cerebral cortex, thalamus and brainstem are the key sources of glutamatergic inputs to these nuclei. There is anatomical, functional and neurochemical evidence that glutamatergic neurotransmission is altered in the striatum and STN of animal models of Parkinson’s disease (PD) and that these changes may contribute to aberrant network neuronal activity in the BG-thalamocortical circuitry. Postmortem studies of animal models and PD patients have revealed significant pathology of glutamatergic synapses, dendritic spines and microcircuits in the striatum of parkinsonians. More recent findings have also demonstrated a significant breakdown of the glutamatergic corticosubthalamic system in parkinsonian monkeys. In this review, we will discuss evidence for synaptic glutamatergic dysfunction and pathology of cortical and thalamic inputs to the striatum and STN in models of PD. The potential functional implication of these alterations on synaptic integration, processing and transmission of extrinsic information through the BG circuits will be considered. Finally, the significance of these pathological changes in the pathophysiology of motor and non-motor symptoms in PD will be examined.

Melatonin reduces projection neuronal injury induced by 3-nitropropionic acid in the rat striatum

BACKGROUND

Melatonin has shown a protective effect against various oxidative damages in the nervous system. Our previous studies have also confirmed its effect on behavioral dysfunction of experimental rats and injury of striatal interneurons induced by 3-nitropropionic acid. The present study aimed to further determine the effect of melatonin on the injury of striatal projection neurons induced by 3-nitropropionic acid.

METHODS

Classic histology, immunohistochemistry, Western blotting and immunoelectron microscopy were applied in this study.

RESULTS

The results were as follows: (1) in the striatum, 3-nitropropionic acid induced a clear lesion area with a transition zone around it, in which both D1+ and D2+ fibers were decreased significantly. However, in the group with melatonin treatment, the striatal lesion area was smaller than in the 3-nitropropionic acid group and the loss of D1+ and D2+ fibers was less pronounced than in the 3-nitropropionic acid group. (2) Histochemical results showed that the dendritic spine density of striatal projection neurons was decreased more seriously after 3-nitropropionic acid treatment, whereas the loss of dendritic spines was less marked in the melatonin-treated group than in the 3- nitropropionic acid group. Immunoelectron microscopy showed that the density of D1+ and D2+ dendrites and spines was significantly decreased in the 3-nitropropionic acid group, and the loss of D1+ and D2+ spines as well as D2+ dendrites was significantly reversed by melatonin administration. (3) Western blotting showed that the expression level of projection neuron protein markers decreased more significantly in the 3-nitropropionic acid group than in the control group and increased significantly in the melatonin-treated group.

CONCLUSIONS

The present results suggest that 3-nitropropionic acid induces serious injury of striatal projection neurons and that melatonin effectively protects against this pathological damage.

Differential cortical activation of the striatal direct and indirect pathway cells: reconciling the anatomical and optogenetic results by using a computational method

The corticostriatal system is considered to be crucially involved in learning and action selection. Anatomical studies have shown that two types of corticostriatal neurons, intratelencephalic (IT) and pyramidal tract (PT) cells, preferentially project to dopamine D1 or D2 receptor-expressing striatal projection neurons, respectively. In contrast, an optogenetic study has shown that stimulation of IT axons evokes comparable responses in D1 and D2 cells and that stimulation of PT axons evokes larger responses in D1 cells. Since the optogenetic study applied brief stimulation only, however, the overall impacts of repetitive inputs remain unclear. Moreover, the apparent contradiction between the anatomical and optogenetic results remains to be resolved. I addressed these issues by using a computational approach. Specifically, I constructed a model of striatal response to cortical inputs, with parameters regarding short-term synaptic plasticity and anatomical connection strength for each connection type. Under the constraint of the optogenetic results, I then explored the parameters that best explain the previously reported paired-pulse ratio of response in D1 and D2 cells to cortical and intrastriatal stimulations, which presumably recruit different compositions of IT and PT fibers. The results indicate that 1) IT→D1 and PT→D2 connections are anatomically stronger than IT→D2 and PT→D1 connections, respectively, consistent with the previous findings, and that 2) IT→D1 and PT→D2 synapses entail short-term facilitation, whereas IT→D2 and PT→D1 synapses would basically show depression, and thereby 3) repetitive IT or PT inputs have larger overall impacts on D1 or D2 cells, respectively, supporting a recently proposed hypothesis on the roles of corticostriatal circuits in reinforcement learning.

Investigating habits: strategies, technologies and models

Understanding habits at a biological level requires a combination of behavioral observations and measures of ongoing neural activity. Theoretical frameworks as well as definitions of habitual behaviors emerging from classic behavioral research have been enriched by new approaches taking account of the identification of brain regions and circuits related to habitual behavior. Together, this combination of experimental and theoretical work has provided key insights into how brain circuits underlying action-learning and action-selection are organized, and how a balance between behavioral flexibility and fixity is achieved. New methods to monitor and manipulate neural activity in real time are allowing us to have a first look “under the hood” of a habit as it is formed and expressed. Here we discuss ideas emerging from such approaches. We pay special attention to the unexpected findings that have arisen from our own experiments suggesting that habitual behaviors likely require the simultaneous activity of multiple distinct components, or operators, seen as responsible for the contrasting dynamics of neural activity in both cortico-limbic and sensorimotor circuits recorded concurrently during different stages of habit learning. The neural dynamics identified thus far do not fully meet expectations derived from traditional models of the structure of habits, and the behavioral measures of habits that we have made also are not fully aligned with these models. We explore these new clues as opportunities to refine an understanding of habits.

Differential striatal spine pathology in Parkinson's disease and cocaine addiction: a key role of dopamine?

In the striatum, the dendritic tree of the two main populations of projection neurons, called "medium spiny neurons (MSNs)", are covered with spines that receive glutamatergic inputs from the cerebral cortex and thalamus. In Parkinson's disease (PD), striatal MSNs undergo an important loss of dendritic spines, whereas aberrant overgrowth of striatal spines occurs following chronic cocaine exposure. This review examines the possibility that opposite dopamine dysregulation is one of the key factors that underlies these structural changes. In PD, nigrostriatal dopamine degeneration results in a significant loss of dendritic spines in the dorsal striatum, while rodents chronically exposed to cocaine and other psychostimulants, display an increase in the density of "thin and immature" spines in the nucleus accumbens (NAc). In rodent models of PD, there is evidence that D2 dopamine receptor-containing MSNs are preferentially affected, while D1-positive cells are the main targets of increased spine density in models of addiction. However, such specificity remains to be established in primates. Although the link between the extent of striatal spine changes and the behavioral deficits associated with these disorders remains controversial, there is unequivocal evidence that glutamatergic synaptic transmission is significantly altered in both diseased conditions. Recent studies have suggested that opposite calcium-mediated regulation of the transcription factor myocyte enhancer factor 2 (MEF2) function induces these structural defects. In conclusion, there is strong evidence that dopamine is a major, but not the sole, regulator of striatal spine pathology in PD and addiction to psychostimulants. Further studies of the role of glutamate and other genes associated with spine plasticity in mediating these effects are warranted.

Optimal balance of the striatal medium spiny neuron network

Slowly varying activity in the striatum, the main Basal Ganglia input structure, is important for the learning and execution of movement sequences. Striatal medium spiny neurons (MSNs) form cell assemblies whose population firing rates vary coherently on slow behaviourally relevant timescales. It has been shown that such activity emerges in a model of a local MSN network but only at realistic connectivities of 10 ~ 20% and only when MSN generated inhibitory post-synaptic potentials (IPSPs) are realistically sized. Here we suggest a reason for this. We investigate how MSN network generated population activity interacts with temporally varying cortical driving activity, as would occur in a behavioural task. We find that at unrealistically high connectivity a stable winners-take-all type regime is found where network activity separates into fixed stimulus dependent regularly firing and quiescent components. In this regime only a small number of population firing rate components interact with cortical stimulus variations. Around 15% connectivity a transition to a more dynamically active regime occurs where all cells constantly switch between activity and quiescence. In this low connectivity regime, MSN population components wander randomly and here too are independent of variations in cortical driving. Only in the transition regime do weak changes in cortical driving interact with many population components so that sequential cell assemblies are reproducibly activated for many hundreds of milliseconds after stimulus onset and peri-stimulus time histograms display strong stimulus and temporal specificity. We show that, remarkably, this activity is maximized at striatally realistic connectivities and IPSP sizes. Thus, we suggest the local MSN network has optimal characteristics - it is neither too stable to respond in a dynamically complex temporally extended way to cortical variations, nor is it too unstable to respond in a consistent repeatable way. Rather, it is optimized to generate stimulus dependent activity patterns for long periods after variations in cortical excitation.

Input dependent cell assembly dynamics in a model of the striatal medium spiny neuron network

The striatal medium spiny neuron (MSN) network is sparsely connected with fairly weak GABAergic collaterals receiving an excitatory glutamatergic cortical projection. Peri-stimulus time histograms (PSTH) of MSN population response investigated in various experimental studies display strong firing rate modulations distributed throughout behavioral task epochs. In previous work we have shown by numerical simulation that sparse random networks of inhibitory spiking neurons with characteristics appropriate for UP state MSNs form cell assemblies which fire together coherently in sequences on long behaviorally relevant timescales when the network receives a fixed pattern of constant input excitation. Here we first extend that model to the case where cortical excitation is composed of many independent noisy Poisson processes and demonstrate that cell assembly dynamics is still observed when the input is sufficiently weak. However if cortical excitation strength is increased more regularly firing and completely quiescent cells are found, which depend on the cortical stimulation. Subsequently we further extend previous work to consider what happens when the excitatory input varies as it would when the animal is engaged in behavior. We investigate how sudden switches in excitation interact with network generated patterned activity. We show that sequences of cell assembly activations can be locked to the excitatory input sequence and outline the range of parameters where this behavior is shown. Model cell population PSTH display both stimulus and temporal specificity, with large population firing rate modulations locked to elapsed time from task events. Thus the random network can generate a large diversity of temporally evolving stimulus dependent responses even though the input is fixed between switches. We suggest the MSN network is well suited to the generation of such slow coherent task dependent response which could be utilized by the animal in behavior.

Opioidergic interactions between striatal projection neurons

Medium spiny striatal projection neurons (MSNs) release opioid neuropeptides, but the role of these neurotransmitters is still poorly understood. While presynaptic inhibition of corticostriatal axons by opioid receptors has been demonstrated using exogenous ligands, the action of synaptically released opioids in the striatum has not been investigated. We performed single and paired whole-cell recordings from rat MSNs while corticostriatal fibers were electrically activated. In single recording experiments, we also activated antidromically the axons of a population of MSNs. Corticostriatal fibers were stimulated once every 10 s and every other stimulation was preceded by 5 antidromic spikes (at 100 Hz). This burst of antidromic spikes produced robust inhibition of evoked corticostriatal responses. This inhibition was not affected by the δ-opioid receptor antagonist SDM25N, but was completely abolished by the μ-opioid receptor antagonist CTOP. Inhibitory effects were maximal (on average 29.6 ± 11.4%) when the burst preceded the corticostriatal stimulation by 500 ms and became undetectable for intervals >2 s. Paired recordings from MSNs located <100 μm apart revealed that, in 30 of 56 (54%) pairs, a burst of five action potentials in one of the MSNs caused significant inhibition (17.1 ± 5.7%) of evoked glutamatergic responses in the other MSN. In 5 of these pairs, reciprocal inhibition of corticostriatal inputs was present. These effects were maximal 500 ms after the burst and were completely blocked by CTOP. Thus, these results reveal a novel, strong opioid-mediated communication between MSNs and provide a new cellular substrate for competitive dynamics in the striatum.

Significance of input correlations in striatal function

The striatum is the main input station of the basal ganglia and is strongly associated with motor and cognitive functions. Anatomical evidence suggests that individual striatal neurons are unlikely to share their inputs from the cortex. Using a biologically realistic large-scale network model of striatum and cortico-striatal projections, we provide a functional interpretation of the special anatomical structure of these projections. Specifically, we show that weak pairwise correlation within the pool of inputs to individual striatal neurons enhances the saliency of signal representation in the striatum. By contrast, correlations among the input pools of different striatal neurons render the signal representation less distinct from background activity. We suggest that for the network architecture of the striatum, there is a preferred cortico-striatal input configuration for optimal signal representation. It is further enhanced by the low-rate asynchronous background activity in striatum, supported by the balance between feedforward and feedback inhibitions in the striatal network. Thus, an appropriate combination of rates and correlations in the striatal input sets the stage for action selection presumably implemented in the basal ganglia.

Ethanol affects striatal interneurons directly and projection neurons through a reduction in cholinergic tone

The acute effects of ethanol on the neurons of the striatum, a basal ganglia nucleus crucially involved in motor control and action selection, were investigated using whole-cell recordings. An intoxicating concentration of ethanol (50 mM) produced inhibitory effects on striatal large aspiny cholinergic interneurons (LAIs) and low-threshold spike interneurons (LTSIs). These effects persisted in the presence of tetrodotoxin and were because of an increase in potassium currents, including those responsible for medium and slow afterhyperpolarizations. In contrast, fast-spiking interneurons (FSIs) were directly excited by ethanol, which depolarized these neurons through the suppression of potassium currents. Medium spiny neurons (MSNs) became hyperpolarized in the presence of ethanol, but this effect did not persist in the presence of tetrodotoxin and was mimicked and occluded by application of the M1 muscarinic receptor antagonist telenzepine. Ethanol effects on MSNs were also abolished by 100 μM barium. This showed that the hyperpolarizations observed in MSNs were because of decreased tonic activation of M1 muscarinic receptors, resulting in an increase in Kir2 conductances. Evoked GABAergic responses of MSNs were reversibly decreased by ethanol with no change in paired-pulse ratio. Furthermore, ethanol impaired the ability of thalamostriatal inputs to inhibit a subsequent corticostriatal glutamatergic response in MSNs. These results offer the first comprehensive description of the highly cell type-specific effects of ethanol on striatal neurons and provide a cellular basis for the interpretation of ethanol influence on a brain area crucially involved in the motor and decisional impairment caused by this drug.

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'.

Genetic dissection of an amygdala microcircuit that gates conditioned fear

The role of different amygdala nuclei (neuroanatomical subdivisions) in processing Pavlovian conditioned fear has been studied extensively, but the function of the heterogeneous neuronal subtypes within these nuclei remains poorly understood. We used molecular genetic approaches to map the functional connectivity of a subpopulation of GABAergic neurons, located in the lateral subdivision of the central amygdala (CEl), which express protein kinase C-delta (PKCδ). Channelrhodopsin-2 assisted circuit mapping in amygdala slices and cell-specific viral tracing indicate that PKCδ⁺ neurons inhibit output neurons in the medial CE (CEm), and also make reciprocal inhibitory synapses with PKCδ⁻ neurons in CEl. Electrical silencing of PKCδ⁺ neurons in vivo suggests that they correspond to physiologically identified units that are inhibited by the conditioned stimulus (CS), called CEl_off units (Ciocchi et al, this issue). This correspondence, together with behavioral data, defines an inhibitory microcircuit in CEl that gates CEm output to control the level of conditioned freezing.

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.

Electrophysiological and morphological characteristics and synaptic connectivity of tyrosine hydroxylase-expressing neurons in adult mouse striatum

Whole-cell recordings were obtained from tyrosine hydroxylase-expressing (TH(+)) neurons in striatal slices from bacterial artificial chromosome transgenic mice that synthesize enhanced green fluorescent protein (EGFP) selectively in neurons expressing TH transcriptional regulatory sequences. Stereological cell counting indicated that there were approximately 2700 EGFP-TH(+) neurons/striatum. Whole-cell recordings in striatal slices demonstrated that EGFP-TH(+) neurons comprise four electrophysiologically distinct neuron types whose electrophysiological properties have not been reported previously in striatum. EGFP-TH(+) neurons were identified in retrograde tracing studies as interneurons. Recordings from synaptically connected pairs of EGFP-TH(+) interneurons and spiny neurons showed that the interneurons elicited GABAergic IPSPs/IPSCs in spiny neurons powerful enough to significantly delay evoked spiking. EGFP-TH(+) interneurons responded to local or cortical stimulation with glutamatergic EPSPs. Local stimulation also elicited GABA(A) IPSPs, at least some of which arose from identified spiny neurons. Single-cell reverse transcription-PCR showed expression of VMAT1 in EGFP-TH(+) interneurons, consistent with previous suggestions that these interneurons may be dopaminergic as well as GABAergic. All four classes of interneurons were medium sized with modestly branching, varicose dendrites, and dense, highly varicose axon collateral fields. These data show for the first time that there exists in the normal rodent striatum a substantial population of TH(+)/GABAergic interneurons comprising four electrophysiologically distinct subtypes whose electrophysiological properties differ significantly from those of previously described striatal GABAergic interneurons. These interneurons are likely to play an important role in striatal function through fast GABAergic synaptic transmission in addition to, and independent of, their potential role in compensation for dopamine loss in experimental or idiopathic Parkinson's disease.

Sequentially switching cell assemblies in random inhibitory networks of spiking neurons in the striatum

The striatum is composed of GABAergic medium spiny neurons with inhibitory collaterals forming a sparse random asymmetric network and receiving an excitatory glutamatergic cortical projection. Because the inhibitory collaterals are sparse and weak, their role in striatal network dynamics is puzzling. However, here we show by simulation of a striatal inhibitory network model composed of spiking neurons that cells form assemblies that fire in sequential coherent episodes and display complex identity-temporal spiking patterns even when cortical excitation is simply constant or fluctuating noisily. Strongly correlated large-scale firing rate fluctuations on slow behaviorally relevant timescales of hundreds of milliseconds are shown by members of the same assembly whereas members of different assemblies show strong negative correlation, and we show how randomly connected spiking networks can generate this activity. Cells display highly irregular spiking with high coefficients of variation, broadly distributed low firing rates, and interspike interval distributions that are consistent with exponentially tailed power laws. Although firing rates vary coherently on slow timescales, precise spiking synchronization is absent in general. Our model only requires the minimal but striatally realistic assumptions of sparse to intermediate random connectivity, weak inhibitory synapses, and sufficient cortical excitation so that some cells are depolarized above the firing threshold during up states. Our results are in good qualitative agreement with experimental studies, consistent with recently determined striatal anatomy and physiology, and support a new view of endogenously generated metastable state switching dynamics of the striatal network underlying its information processing operations.

Amygdala inhibitory circuits and the control of fear memory

Classical fear conditioning is a powerful behavioral paradigm that is widely used to study the neuronal substrates of learning and memory. Previous studies have clearly identified the amygdala as a key brain structure for acquisition and storage of fear memory traces. Whereas the majority of this work has focused on principal cells and glutamatergic transmission and its plasticity, recent studies have started to shed light on the intricate roles of local inhibitory circuits. Here, we review current understanding and emerging concepts of how local inhibitory circuits in the amygdala control the acquisition, expression, and extinction of conditioned fear at different levels.

From real-world events to psychosis: the emerging neuropharmacology of delusions

The earliest stages of delusion are characterized by an overabundance of meaningful coincidences impinging on the sufferer's existing worldview. Successive events are seen by him as pointing to, and then confirming, a fundamentally new reality that takes him over and engulfs his everyday life. Research over the last 4 decades has revealed the importance of dopamine (DA), D2 receptors, and the basal ganglia in psychotic thinking. Recent work has implicated the aberrant reward learning initiated by the excess release of striatal DA in the attribution of excessive importance or "salience" to insignificant stimuli and events. But our knowledge of what is happening beyond D2 receptors has remained scant. The gap is especially apparent at the cellular and microcircuit levels, encompassing the plastic changes, which are believed to be essential for new learning, and whose processes may go awry in major mental illness. Now new pharmacological findings are advancing our understanding of information processing and learning within the striatum. DA has an important role in setting the strength of individual striatal connections, but it does not act in isolation. Two other modulator systems are critical, the endocannabinoids and adenosine. Thus, at medium spiny neurons belonging to the indirect pathway, D2 stimulation evokes endocannabinoid-mediated depression of cortical inputs. Adenosine acting at A2A receptors elicits the opposite effect. Remarkably, drugs that target the endocannabinoid and purinergic systems also have pro- or antipsychotic properties. Here, we discuss how the 3 modulators regulate learning within the striatum and how their dysfunction may lead to delusional thinking.

Gap junctions between striatal fast-spiking interneurons regulate spiking activity and synchronization as a function of cortical activity

Striatal fast-spiking (FS) interneurons are interconnected by gap junctions into sparsely connected networks. As demonstrated for cortical FS interneurons, these gap junctions in the striatum may cause synchronized spiking, which would increase the influence that FS neurons have on spiking by the striatal medium spiny (MS) neurons. Dysfunction of the basal ganglia is characterized by changes in synchrony or periodicity, thus gap junctions between FS interneurons may modulate synchrony and thereby influence behavior such as reward learning and motor control. To explore the roles of gap junctions on activity and spike synchronization in a striatal FS population, we built a network model of FS interneurons. Each FS connects to 30-40% of its neighbors, as found experimentally, and each FS interneuron in the network is activated by simulated corticostriatal synaptic inputs. Our simulations show that the proportion of synchronous spikes in FS networks with gap junctions increases with increased conductance of the electrical synapse; however, the synchronization effects are moderate for experimentally estimated conductances. Instead, the main tendency is that the presence of gap junctions reduces the total number of spikes generated in response to synaptic inputs in the network. The reduction in spike firing is due to shunting through the gap junctions; which is minimized or absent when the neurons receive coincident inputs. Together these findings suggest that a population of electrically coupled FS interneurons may function collectively as input detectors that are especially sensitive to synchronized synaptic inputs received from the cortex.

Substance P mediates excitatory interactions between striatal projection neurons

The striatum is the largest nucleus of the basal ganglia, and is crucially involved in motor control. Striatal projection cells are medium-size spiny neurons (MSNs) and form functional GABAergic synapses with other MSNs through their axon collaterals. A subpopulation of MSNs also release substance P (SP), but its role in MSN-MSN communication is unknown. We studied this issue in rat brain slices, in the presence of antagonists for GABA, acetylcholine, dopamine, and opioid receptors; under these conditions, whole-cell paired recordings from MSNs (located <100 microm apart) revealed that, in 31/137 (23%) pairs, a burst of five spikes in a MSN caused significant facilitation (14.2 +/- 8.9%) of evoked glutamatergic responses in the other MSN. Reciprocal facilitation of glutamatergic responses was present in 4 of these pairs. These facilitatory effects were maximal when spikes preceded glutamatergic responses by 100 ms, and were completely blocked by the NK1 receptor antagonist L-732,138. Furthermore, in 31/57 (54%) MSNs, a burst of 5 antidromic stimuli delivered to MSN axons in the globus pallidus significantly potentiated glutamatergic responses evoked 250 or 500 ms later by stimulation of the corpus callosum. These effects were larger at 250 than 500 ms intervals, were completely blocked by L-732,138, and facilitated spike generation. These data demonstrate that MSNs facilitate glutamatergic inputs to neighboring MSNs through spike-released SP acting on NK1 receptors. The current view that MSNs form inhibitory networks characterized by competitive dynamics will have to be updated to incorporate the fact that groups of MSNs interact in an excitatory manner.

Actions of adenosine A 2A receptors on synaptic connections of spiny projection neurons in the neostriatal inhibitory network

There is growing evidence that adenosine plays a crucial role in basal ganglia function, particularly in the modulation of voluntary movement. An adenosine-based treatment for Parkinson's disease shows promise in recent clinical studies. Adenosine A(2A) receptors, the receptors involved in this treatment, are highly expressed in the neostriatum. Previous studies have suggested opposing actions of these receptors on synaptic transmission at striatal and pallidal terminals of the same spiny projection neurons, but the cells of origin of the intrastriatal terminals mediating these actions have not been identified. We used dual whole cell recordings to record simultaneously from pairs of striatal cells; this enabled definitive identification of the presynaptic and postsynaptic cells mediating the effects of A(2A) receptors. We found that A(2A) receptors facilitate GABAergic synaptic transmission by intrastriatal collaterals of the spiny projection neurons, consistent with their previously reported actions on synaptic transmission at pallidal terminals. This neuromodulatory action on lateral inhibition in the striatum may underlie, in part, the therapeutic efficacy of adenosine-based treatments for Parkinson's disease.

Feedforward and feedback inhibition in neostriatal GABAergic spiny neurons

There are two distinct inhibitory GABAergic circuits in the neostriatum. The feedforward circuit consists of a relatively small population of GABAergic interneurons that receives excitatory input from the neocortex and exerts monosynaptic inhibition onto striatal spiny projection neurons. The feedback circuit comprises the numerous spiny projection neurons and their interconnections via local axon collaterals. This network has long been assumed to provide the majority of striatal GABAergic inhibition and to sharpen and shape striatal output through lateral inhibition, producing increased activity in the most strongly excited spiny cells at the expense of their less strongly excited neighbors. Recent results, mostly from recording experiments of synaptically connected pairs of neurons, have revealed that the two GABAergic circuits differ markedly in terms of the total number of synapses made by each, the strength of the postsynaptic response detected at the soma, the extent of presynaptic convergence and divergence and the net effect of the activation of each circuit on the postsynaptic activity of the spiny neuron. These data have revealed that the feedforward inhibition is powerful and widespread, with spiking in a single interneuron being capable of significantly delaying or even blocking the generation of spikes in a large number of postsynaptic spiny neurons. In contrast, the postsynaptic effects of spiking in a single presynaptic spiny neuron on postsynaptic spiny neurons are weak when measured at the soma, and unable to significantly affect spike timing or generation. Further, reciprocity of synaptic connections between spiny neurons is only rarely observed. These results suggest that the bulk of the fast inhibition that has the strongest effects on spiny neuron spike timing comes from the feedforward interneuronal system whereas the axon collateral feedback system acts principally at the dendrites to control local excitability as well as the overall level of activity of the spiny neuron.