Learning new sequential stepping patterns requires striatal plasticity during the earliest phase of acquisition

Representation of rhythmic chunking in striatum of mice executing complex continuous movement sequences

We used a step-wheel system to examine the activity of striatal projection neurons as mice practiced stepping on complexly arranged foothold pegs in this Ferris-wheel-like device to receive reward. Sets of dorsolateral striatal projection neurons were sensitive to specific parameters of repetitive motor coordination during the runs. They responded to combinations of the parameters of continuous movements (interval, phase, and repetition), forming "chunking responses"-some for combinations of these parameters across multiple body parts. Recordings in sensorimotor cortical areas exhibited notably fewer such responses but were documented for smaller neuron sets whose heterogeneity was significant. Striatal movement encoding via chunking responsivity could provide insight into neural strategies governing effective motor control by the striatum. It is possible that the striking need for external rhythmic cuing to allow movement sequences by Parkinson's patients could, at least in part, reflect dysfunction in such striatal coding.

Role of the basal ganglia in innate and learned behavioural sequences

Integrating individual actions into coherent, organised behavioural units, a process called chunking, is a fundamental, evolutionarily conserved process that renders actions automatic. In vertebrates, evidence points to the basal ganglia - a complex network believed to be involved in action selection - as a key component of action sequence encoding, although the underlying mechanisms are only just beginning to be understood. Central pattern generators control many innate automatic behavioural sequences that form some of the most basic behaviours in an animal's repertoire, and in vertebrates, brainstem and spinal pattern generators are under the control of higher order structures such as the basal ganglia. Evidence suggests that the basal ganglia play a crucial role in the concatenation of simpler behaviours into more complex chunks, in the context of innate behavioural sequences such as chain grooming in rats, as well as sequences in which innate capabilities and learning interact such as birdsong, and sequences that are learned from scratch, such as lever press sequences in operant behaviour. It has been proposed that the role of the striatum, the largest input structure of the basal ganglia, might lie in selecting and allowing the relevant central pattern generators to gain access to the motor system in the correct order, while inhibiting other behaviours. As behaviours become more complex and flexible, the pattern generators seem to become more dependent on descending signals. Indeed, during learning, the striatum itself may adopt the functional characteristics of a higher order pattern generator, facilitated at the microcircuit level by striatal neuropeptides.

Rhythm Receptive Fields in Striatum of Mice Executing Complex Continuous Movement Sequences

By the use of a novel experimental system, the step-wheel, we investigated the neural underpinnings of complex and continuous movements. We recorded neural activities from the dorsolateral striatum and found neurons sensitive to movement rhythm parameters. These neurons responded to specific combinations of interval, phase, and repetition of movement, effectively forming what we term "rhythm receptive fields." Some neurons even responsive to the combination of movement phases of multiple body parts. In parallel, cortical recordings in sensorimotor areas highlighted a paucity of neurons responsive to multiple parameter combinations, relative to those in the striatum. These findings have implications for comprehending motor coordination deficits seen in brain disorders including Parkinson's disease. Movement encoding by rhythm receptive fields should streamline the brain's capacity to encode temporal patterns, help to resolve the degrees of freedom problem. Such rhythm fields hint at the neural mechanisms governing effective motor control and processing of rhythmic information.

Emergence of rhythmic chunking in complex stepping of mice

Motor chunking is important for motor execution, allowing atomization and efficiency of movement sequences. However, it remains unclear why and how chunks contribute to motor execution. To analyze the structure of naturally occurring chunks, we trained mice to run in a complex series of steps and identified the formation of chunks. We found that intervals (cycle) and the positional relationship between the left and right limbs (phase) of steps inside the chunks, unlike those outside the chunks, were consistent across occurrences. Further, licking by the mice was also more periodic and linked to the specific phases of limb movements within the chunk. Based on these findings, we propose the rhythm chunking hypothesis, whereby within chunks, the repetitive movements of many body parts are linked by the rhythm parameters: cycle and phase. The computational complexity of movement may thereby be reduced by adjusting movements as the combination of rhythms.

Striatal synaptic adaptations in Parkinson's disease

The striatum is densely innervated by mesencephalic dopaminergic neurons that modulate acquisition and vigor of goal-directed actions and habits. This innervation is progressively lost in Parkinson's disease (PD), contributing to the defining movement deficits of the disease. Although boosting dopaminergic signaling with levodopa early in the course of the disease alleviates these deficits, later this strategy leads to the emergence of debilitating dyskinesia. Here, recent advances in our understanding of how striatal cells and circuits adapt to this progressive de-innervation and to levodopa therapy are discussed. First, we discuss how dopamine (DA) depletion triggers cell type-specific, homeostatic changes in spiny projection neurons (SPNs) that tend to normalize striatal activity but also lead to disruption of the synaptic architecture sculpted by experience. Second, we discuss the roles played by cholinergic and nitric oxide-releasing interneurons in these adaptations. Third, we examine recent work in freely moving mice suggesting that alterations in the spatiotemporal dynamics of striatal ensembles contributes to PD movement deficits. Lastly, we discuss recently published evidence from a progressive model of PD suggesting that contrary to the classical model, striatal pathway imbalance is necessary but not sufficient to produce frank parkinsonism.

CalDAG-GEFI mediates striatal cholinergic modulation of dendritic excitability, synaptic plasticity and psychomotor behaviors

CalDAG-GEFI (CDGI) is a protein highly enriched in the striatum, particularly in the principal spiny projection neurons (SPNs). CDGI is strongly down-regulated in two hyperkinetic conditions related to striatal dysfunction: Huntington’s disease and levodopa-induced dyskinesia in Parkinson’s disease. We demonstrate that genetic deletion of CDGI in mice disrupts dendritic, but not somatic, M1 muscarinic receptors (M1Rs) signaling in indirect pathway SPNs. Loss of CDGI reduced temporal integration of excitatory postsynaptic potentials at dendritic glutamatergic synapses and impaired the induction of activity-dependent long-term potentiation. CDGI deletion selectively increased psychostimulant-induced repetitive behaviors, disrupted sequence learning, and eliminated M1R blockade of cocaine self-administration. These findings place CDGI as a major, but previously unrecognized, mediator of cholinergic signaling in the striatum. The effects of CDGI deletion on the self-administration of drugs of abuse and its marked alterations in hyperkinetic extrapyramidal disorders highlight CDGI’s therapeutic potential.

Blocking NK1 receptors disrupts the sequential and temporal organization of chain grooming in rats

The basal ganglia are a group of sub-cortical structures believed to play a critical role in action selection and sequencing. The striatum is the largest input structure of the basal ganglia and contains the neuropeptide substance P in abundance. Recent computational work has suggested that substance P could play a critical role in action sequence performance and acquisition, but this has not been tested experimentally before. The aim of the present study was to test how blocking substance P's main NK1-type receptors affected the sequential and temporal organization of spontaneous behavioral patterns. We did this in rats by focusing on the grooming chain, an innate and highly stereotyped ordered sequence. We performed an open field experiment in which the NK1 receptor antagonist L-733,060 was injected intraperitoneally in rats at two doses (2 and 4 mg/kg/ml), in a within-subject counterbalanced design. We used first order transition probabilities, Variable Length Markov Models, entropy metrics and T-pattern analysis to evaluate the effects of L-733,060 on sequential and temporal aspects of spontaneously ordered behavioral sequences. Our results suggest that blocking NK1 receptors made the transitions between the grooming chain elements significantly more variable, the transition structure of the grooming bouts simpler, and it increased the probability of transitioning from active to inactive states. Overall, this suggest that blocking substance P receptors led to a general break down in the fluency of spontaneous behavioral sequences, suggesting that substance P could be playing a key role in the implementation of sequential patterns.

Synaptic Zinc: An Emerging Player in Parkinson's Disease

Alterations of zinc homeostasis have long been implicated in Parkinson's disease (PD). Zinc plays a complex role as both deficiency and excess of intracellular zinc levels have been incriminated in the pathophysiology of the disease. Besides its role in multiple cellular functions, Zn²⁺ also acts as a synaptic transmitter in the brain. In the forebrain, subset of glutamatergic neurons, namely cortical neurons projecting to the striatum, use Zn²⁺ as a messenger alongside glutamate. Overactivation of the cortico-striatal glutamatergic system is a key feature contributing to the development of PD symptoms and dopaminergic neurotoxicity. Here, we will cover recent evidence implicating synaptic Zn²⁺ in the pathophysiology of PD and discuss its potential mechanisms of actions. Emphasis will be placed on the functional interaction between Zn²⁺ and glutamatergic NMDA receptors, the most extensively studied synaptic target of Zn²⁺.

Pyramidal cell subtype-dependent cortical oscillatory activity regulates motor learning

The cortex processes information through intricate circuitry and outputs to multiple brain areas by different sets of pyramidal cells (PCs). PCs form intra- and inter-laminar subnetworks, depending on PC projection subtypes. However, it remains unknown how individual PC subtypes are involved in cortical network activity and, thereby, in distinct brain functions. Here, we examined the effects of optogenetic manipulations of specific PC subtypes on network activity in the motor cortex. In layer V, the beta/gamma frequency band of oscillation was evoked by photostimulation, depending on PC subtypes. Our experimental and simulation results suggest that oscillatory activity is generated in reciprocal connections between pyramidal tract (PT) and fast-spiking cells. A similar frequency band was also observed in local field potentials during a pattern learning task. Manipulation of PT cell activity affected beta/gamma band power and learning. Our results suggest that PT cell-dependent oscillations play important roles in motor learning.

Otsuka and Kawaguchi investigate how manipulation of pyramidal cell subtypes in the motor cortex affects cortical network activity. Their findings suggest that pyramidal cell type cell-dependent oscillatory activity play an important role in motor learning.

Spontaneous grouping of saccade timing in the presence of task-irrelevant objects

Sequential movements are often grouped into several chunks, as evidenced by the modulation of the timing of each elemental movement. Even during synchronized tapping with a metronome, we sometimes feel subjective accent for every few taps. To examine whether motor segmentation emerges during synchronized movements, we trained monkeys to generate a series of predictive saccades synchronized with visual stimuli which sequentially appeared for a fixed interval (400 or 600 ms) at six circularly arranged landmark locations. We found two types of motor segmentations that featured periodic modulation of saccade timing. First, the intersaccadic interval (ISI) depended on the target location and saccade direction, indicating that particular combinations of saccades were integrated into motor chunks. Second, when a task-irrelevant rectangular contour surrounding three landmarks ("inducer") was presented, the ISI significantly modulated depending on the relative target location to the inducer. All patterns of individual differences seen in monkeys were also observed in humans. Importantly, the effects of the inducer greatly decreased or disappeared when the animals were trained to generate only reactive saccades (latency >100 ms), indicating that the motor segmentation may depend on the internal rhythms. Thus, our results demonstrate two types of motor segmentation during synchronized movements: one is related to the hierarchical organization of sequential movements and the other is related to the spontaneous grouping of rhythmic events. This experimental paradigm can be used to investigate the underlying neural mechanism of temporal grouping during rhythm production.

Drumming Motor Sequence Training Induces Apparent Myelin Remodelling in Huntington's Disease: A Longitudinal Diffusion MRI and Quantitative Magnetization Transfer Study

Background:

Impaired myelination may contribute to Huntington’s disease (HD) pathogenesis.

Objective:

This study assessed differences in white matter (WM) microstructure between HD patients and controls, and tested whether drumming training stimulates WM remodelling in HD. Furthermore, it examined whether training-induced microstructural changes are related to improvements in motor and cognitive function.

Methods:

Participants undertook two months of drumming exercises. Working memory and executive function were assessed before and post-training. Changes in WM microstructure were investigated with diffusion tensor magnetic resonance imaging (DT-MRI)-based metrics, the restricted diffusion signal fraction (Fr) from the composite hindered and restricted model of diffusion (CHARMED) and the macromolecular proton fraction (MPF) from quantitative magnetization transfer (qMT) imaging. WM pathways linking putamen and supplementary motor areas (SMA-Putamen), and three segments of the corpus callosum (CCI, CCII, CCIII) were studied using deterministic tractography. Baseline MPF differences between patients and controls were assessed with tract-based spatial statistics.

Results:

MPF was reduced in the mid-section of the CC in HD subjects at baseline, while a significantly greater change in MPF was detected in HD patients relative to controls in the CCII, CCIII, and the right SMA-putamen post-training. Further, although patients improved their drumming and executive function performance, such improvements did not correlate with microstructural changes. Increased MPF suggests training-induced myelin changes in HD.

Conclusion:

Though only preliminary and based on a small sample size, these results suggest that tailored behavioural stimulation may lead to neural benefits in early HD, that could be exploited for delaying disease progression.

Endogenous opiates and behavior: 2017

This paper is the fortieth consecutive installment of the annual anthological review of research concerning the endogenous opioid system, summarizing articles published during 2017 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides and receptors as well as effects of opioid/opiate agonists and antagonists. The review is subdivided into the following specific topics: molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors (1), the roles of these opioid peptides and receptors in pain and analgesia in animals (2) and humans (3), opioid-sensitive and opioid-insensitive effects of nonopioid analgesics (4), opioid peptide and receptor involvement in tolerance and dependence (5), stress and social status (6), learning and memory (7), eating and drinking (8), drug abuse and alcohol (9), sexual activity and hormones, pregnancy, development and endocrinology (10), mental illness and mood (11), seizures and neurologic disorders (12), electrical-related activity and neurophysiology (13), general activity and locomotion (14), gastrointestinal, renal and hepatic functions (15), cardiovascular responses (16), respiration and thermoregulation (17), and immunological responses (18).

Dopamine D1 and muscarinic acetylcholine receptors in dorsal striatum are required for high speed running

Dopamine (DA) signaling in the basal ganglia plays important roles in motor control. Motor deficiencies were previously reported in dopamine receptor D1 (D1R) and D2 (D2R) knockout mice. While these results indicate the involvement of DA receptors in motor execution, the null knockout (KO) mouse lacks the specificity necessary to determine when and where in the brain D1R and D2R function in motor execution. To address these questions, we restricted the loss of function temporally and spatially by using D1R conditional knockdown (cKD) mice and mice injected with antagonists against DA receptors directly into the dorsal striatum. In addition, we address the DA and acetylcholine (ACh) balance hypothesis by using antagonists against ACh receptors. We tested the motor ability of the mice with a newly devised task named the accelerating step-wheel. In this task, the maximum running speed was measured in a situation where the wheel rotation speed was gradually accelerated in one trial. We found significant decreases in the maximum running speed of D1R cKD mice and the mice injected with the antagonist against D1R or muscarinic ACh receptor. These results indicated that D1R and muscarinic ACh receptor in the dorsal striatum play pivotal roles in the execution of walking/running.

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.

Selective Role of RGS9-2 in Regulating Retrograde Synaptic Signaling of Indirect Pathway Medium Spiny Neurons in Dorsal Striatum

In the striatum, medium spiny neurons (MSNs) are heavily involved in controlling movement and reward. MSNs form two distinct populations expressing either dopamine receptor 1 (D1-MSN) or dopamine receptor 2 (D2-MSN), which differ in their projection targets and neurochemical composition. The activity of both types of MSNs is shaped by multiple neuromodulatory inputs processed by GPCRs that fundamentally impact their synaptic properties biasing behavioral outcomes. How these GPCR signaling cascades are regulated and what downstream targets they recruit in D1-MSN and D2-MSN populations are incompletely understood. In this study, we examined the cellular and molecular mechanisms underlying the action of RGS9-2, a key GPCR regulator in MSNs implicated in both movement control and actions of addictive drugs. Imaging cultured striatal neurons, we found that ablation of RGS9-2 significantly reduced calcium influx through NMDARs. Electrophysiological recordings in slices confirmed inhibition of NMDAR function in MSNs, resulting in enhanced AMPAR/NMDAR ratio. Accordingly, male mice lacking RGS9-2 displayed behavioral hypersensitivity to NMDAR blockade by MK-801 or ketamine. Recordings from genetically identified populations of striatal neurons revealed that these changes were selective to D2-MSNs. Surprisingly, we found that these postsynaptic effects resulted in remodeling of presynaptic inputs to D2-MSNs increasing the frequency of mEPSC and inhibiting paired-pulse ratio. Pharmacological dissection revealed that these adaptations were mediated by the NMDAR-dependent inhibition of retrograde endocannabinoid signaling from D2-MSNs to CB1 receptor on presynaptic terminals. Together, these data demonstrate a novel mechanism for pathway selective regulation of synaptic plasticity in MSNs controlled by GPCR signaling.SIGNIFICANCE STATEMENT This study identifies a role for a major G-protein regulator in controlling synaptic properties of striatal neurons in a pathway selective fashion.

Kainate Receptors Inhibit Glutamate Release Via Mobilization of Endocannabinoids in Striatal Direct Pathway Spiny Projection Neurons

Kainate receptors are members of the glutamate receptor family that function by both generating ionotropic currents through an integral ion channel pore and coupling to downstream metabotropic signaling pathways. They are highly expressed in the striatum, yet their roles in regulating striatal synapses are not known. Using mice of both sexes, we demonstrate that GluK2-containing kainate receptors expressed in direct pathway spiny projection neurons (dSPNs) inhibit glutamate release at corticostriatal synapses in the dorsolateral striatum. This inhibition requires postsynaptic kainate-receptor-mediated mobilization of a retrograde endocannabinoid (eCB) signal and activation of presynaptic CB1 receptors. This pathway can be activated during repetitive 25 Hz trains of synaptic stimulation, causing short-term depression of corticostriatal synapses. This is the first study to demonstrate a role for kainate receptors in regulating eCB-mediated plasticity at the corticostriatal synapse and demonstrates an important role for these receptors in regulating basal ganglia circuits.SIGNIFICANCE STATEMENT The GRIK2 gene, encoding the GluK2 subunit of the kainate receptor, has been linked to several neuropsychiatric and neurodevelopmental disorders including obsessive compulsive disorder (OCD). Perseverative behaviors associated with OCD are known to result from pathophysiological changes in the striatum and kainate receptor knock-out mice have striatal-dependent phenotypes. However, the role of kainate receptors in striatal synapses is not known. We demonstrate that GluK2-containing kainate receptors regulate corticostriatal synapses by mobilizing endocannabinoids from direct pathway spiny projection neurons. Synaptic activation of GluK2 receptors during trains of synaptic input causes short-term synaptic depression, demonstrating a novel role for these receptors in regulating striatal circuits.