Input- and Output-Specific Regulation of Serial Order Performance by Corticostriatal Circuits

Complementary corticostriatal circuits orchestrate action repetition and switching

Action sequencing is fundamental to behavior. A critical decision for survival and reproduction is whether to repeat a current action or switch to a different one. However, the neural mechanisms governing action repetition and switching remain largely unknown. In mice trained to perform heterogeneous action sequences, we found that the M1-DLS circuit regulates action repetition, while the PrL-DMS pathway controls action switching. These distinct functions arise from preferential innervation of striatal D1-SPNs by M1 and D2-SPNs by PrL, respectively. In a Shank3 knockout mouse model of ASD, the D1/D2 innervation ratio in the PrL-DMS pathway was reversed, leading to impaired action switching and repetitive behaviors. Genetic restoration of Shank3 in the DMS rescued both physiological and behavioral deficits. These findings reveal how the brain orchestrates action sequencing in health and disease.

Role of posterior medial thalamus in the modulation of striatal circuitry and choice behavior

The posterior medial (POm) thalamus is heavily interconnected with sensory and motor circuitry and is likely involved in behavioral modulation and sensorimotor integration. POm provides axonal projections to the dorsal striatum, a hotspot of sensorimotor processing, yet the role of POm-striatal projections has remained undetermined. Using optogenetics with mouse brain slice electrophysiology, we found that POm provides robust synaptic input to direct and indirect pathway striatal spiny projection neurons (D1- and D2-SPNs, respectively) and parvalbumin-expressing fast spiking interneurons (PVs). During the performance of a whisker-based tactile discrimination task in head-restrained mice, POm-striatal projections displayed learning-related activation correlating with anticipatory, but not reward-related, pupil dilation. Inhibition of POm-striatal axons across learning caused slower reaction times and an increase in the number of training sessions for expert performance. Our data indicate that POm-striatal inputs provide a behaviorally relevant arousal-related signal, which may prime striatal circuitry for efficient integration of subsequent choice-related inputs.

Selective regulation of corticostriatal synapses by astrocytic phagocytosis

In the adult brain, neural circuit homeostasis depends on the constant turnover of synapses via astrocytic phagocytosis mechanisms. However, it remains unclear whether this process occurs in a circuit-specific manner. Here, we reveal that astrocytes target and eliminate specific type of excitatory synapses in the striatum. Using model mice lacking astrocytic phagocytosis receptors in the dorsal striatum, we found that astrocytes constantly remove corticostriatal synapses rather than thalamostriatal synapses. This preferential elimination suggests that astrocytes play a selective role in modulating corticostriatal plasticity and functions via phagocytosis mechanisms. Supporting this notion, corticostriatal long-term potentiation and the early phase of motor skill learning are dependent on astrocytic phagocytic receptors. Together, our findings demonstrate that astrocytes contribute to the connectivity and plasticity of the striatal circuit by preferentially engulfing a specific subset of excitatory synapses within brain regions innervated by multiple excitatory sources.

Dissociable roles of central striatum and anterior lateral motor area in initiating and sustaining naturalistic behavior

Understanding how corticostriatal circuits mediate behavioral selection and initiation in a naturalistic setting is critical to understanding behavior choice and execution in unconstrained situations. The central striatum (CS) is well poised to play an important role in these spontaneous processes. Using fiber photometry and optogenetics, we identify a role for CS in grooming initiation. However, CS-evoked movements resemble short grooming fragments, suggesting additional input is required to appropriately sustain behavior once initiated. Consistent with this idea, the anterior lateral motor area (ALM) demonstrates a slow ramp in activity that peaks at grooming termination, supporting a potential role for ALM in encoding grooming bout length. Furthermore, optogenetic stimulation of ALM-CS terminals generates sustained grooming responses. Finally, dual-region photometry indicates that CS activation precedes ALM during grooming. Taken together, these data support a model in which CS is involved in grooming initiation, while ALM may encode grooming bout length.

CBGTPy: An extensible cortico-basal ganglia-thalamic framework for modeling biological decision making

Here we introduce CBGTPy, a virtual environment for designing and testing goal-directed agents with internal dynamics that are modeled on the cortico-basal-ganglia-thalamic (CBGT) pathways in the mammalian brain. CBGTPy enables researchers to investigate the internal dynamics of the CBGT system during a variety of tasks, allowing for the formation of testable predictions about animal behavior and neural activity. The framework has been designed around the principle of flexibility, such that many experimental parameters in a decision making paradigm can be easily defined and modified. Here we demonstrate the capabilities of CBGTPy across a range of single and multi-choice tasks, highlighting the ease of set up and the biologically realistic behavior that it produces. We show that CBGTPy is extensible enough to apply to a range of experimental protocols and to allow for the implementation of model extensions with minimal developmental effort.

Neuronal encoding of behaviors and instrumental learning in the dorsal striatum

The dorsal striatum is instrumental in regulating motor control and goal-directed behaviors. The classical description of the two output pathways of the dorsal striatum highlights their antagonistic control over actions. However, recent experimental evidence implicates both pathways and their coordinated activities during actions. In this review, we examine the different models proposed for striatal encoding of actions during self-paced behaviors and how they can account for evidence harvested during goal-directed behaviors. We also discuss how the activation of striatal ensembles can be reshaped and reorganized to support the formation of instrumental learning and behavioral flexibility. Future work integrating these considerations may resolve controversies regarding the control of actions by striatal networks.

Role of Posterior Medial Thalamus in the Modulation of Striatal Circuitry and Choice Behavior

The posterior medial (POm) thalamus is heavily interconnected with sensory and motor circuitry and is likely involved in behavioral modulation and sensorimotor integration. POm provides axonal projections to the dorsal striatum, a hotspot of sensorimotor processing, yet the role of POm-striatal projections has remained undetermined. Using optogenetics with slice electrophysiology, we found that POm provides robust synaptic input to direct and indirect pathway striatal spiny projection neurons (D1- and D2-SPNs, respectively) and parvalbumin-expressing fast spiking interneurons (PVs). During the performance of a whisker-based tactile discrimination task, POm-striatal projections displayed learning-related activation correlating with anticipatory, but not reward-related, pupil dilation. Inhibition of POm-striatal axons across learning caused slower reaction times and an increase in the number of training sessions for expert performance. Our data indicate that POm-striatal inputs provide a behaviorally relevant arousal-related signal, which may prime striatal circuitry for efficient integration of subsequent choice-related inputs.

Striatal cell-type-specific molecular signatures reveal therapeutic targets in a model of dystonia

Striatal dysfunction is implicated in many forms of dystonia, including idiopathic, inherited and iatrogenic dystonias. The striatum is comprised largely of GABAergic spiny projection neurons (SPNs) that are defined by their long-range efferents. Direct SPNs (dSPNs) project to the internal globus pallidus/substantia nigra reticulata whereas indirect pathway SPNs (iSPNs) project to the external pallidum; the concerted activity of both SPN subtypes modulates movement. Convergent results from genetic, imaging and physiological studies in patients suggest that abnormalities of both dSPNs and iSPNs contribute to the expression of dystonia, but the molecular adaptations underlying these abnormalities are not known. Here we provide a comprehensive analysis of SPN cell-type-specific molecular signatures in a model of DOPA-responsive dystonia (DRD mice), which is caused by gene defects that reduce dopamine neurotransmission, resulting in dystonia that is specifically associated with striatal dysfunction. Individually profiling the translatome of dSPNs and iSPNs using translating ribosome affinity purification with RNA-seq revealed hundreds of differentially translating mRNAs in each SPN subtype in DRD mice, yet there was little overlap between the dysregulated genes in dSPNs and iSPNs. Despite the paucity of shared adaptations, a disruption in glutamatergic signaling was predicted for both dSPNs and iSPNs. Indeed, we found that both AMPA and NMDA receptor-mediated currents were enhanced in dSPNs but diminished in iSPNs in DRD mice. The pattern of mRNA dysregulation was specific to dystonia as the adaptations in DRD mice were distinct from those in parkinsonian mice where the dopamine deficit occurs in adults, suggesting that the phenotypic outcome is dependent on both the timing of the dopaminergic deficit and the SPN-specific adaptions. We leveraged the unique molecular signatures of dSPNs and iSPNs in DRD mice to identify biochemical mechanisms that may be targets for therapeutics, including LRRK2 inhibition. Administration of the LRRK2 inhibitor MLi-2 ameliorated the dystonia in DRD mice suggesting a novel target for therapeutics and demonstrating that the delineation of cell-type-specific molecular signatures provides a powerful approach to revealing both CNS dysfunction and therapeutic targets in dystonia.

Dopamine neurons drive spatiotemporally heterogeneous striatal dopamine signals during learning

Environmental cues, through Pavlovian learning, become conditioned stimuli that invigorate and guide animals toward rewards. Dopamine (DA) neurons in the ventral tegmental area (VTA) and substantia nigra (SNc) are crucial for this process, via engagement of a reciprocally connected network with their striatal targets. Critically, it remains unknown how dopamine neuron activity itself engages dopamine signals throughout the striatum, across learning. Here, we investigated how optogenetic Pavlovian cue conditioning of VTA or SNc dopamine neurons directs cue-evoked behavior and shapes subregion-specific striatal dopamine dynamics. We used a fluorescent biosensor to monitor dopamine in the nucleus accumbens (NAc) core and shell, dorsomedial striatum (DMS), and dorsolateral striatum (DLS). We demonstrate spatially heterogeneous, learning-dependent dopamine changes across striatal regions. Although VTA stimulation-evoked robust dopamine release in NAc core, shell, and DMS, predictive cues preferentially recruited dopamine release in NAc core, starting early in training, and DMS, late in training. Negative prediction error signals, reflecting a violation in the expectation of dopamine neuron activation, only emerged in the NAc core and DMS. Despite the development of vigorous movement late in training, conditioned dopamine signals did not emerge in the DLS, even during Pavlovian conditioning with SNc dopamine neuron activation, which elicited robust DLS dopamine release. Together, our studies show a broad dissociation in the fundamental prediction and reward-related information generated by VTA and SNc dopamine neuron populations and signaled by dopamine across the striatum. Further, they offer new insight into how larger-scale adaptations across the striatal network emerge during learning to coordinate behavior.

Rethinking dopamine-guided action sequence learning

As opposed to those requiring a single action for reward acquisition, tasks necessitating action sequences demand that animals learn action elements and their sequential order and sustain the behaviour until the sequence is completed. With repeated learning, animals not only exhibit precise execution of these sequences but also demonstrate enhanced smoothness and efficiency. Previous research has demonstrated that midbrain dopamine and its major projection target, the striatum, play crucial roles in these processes. Recent studies have shown that dopamine from the substantia nigra pars compacta (SNc) and the ventral tegmental area (VTA) serve distinct functions in action sequence learning. The distinct contributions of dopamine also depend on the striatal subregions, namely the ventral, dorsomedial and dorsolateral striatum. Here, we have reviewed recent findings on the role of striatal dopamine in action sequence learning, with a focus on recent rodent studies.

Hyperactivity of indirect pathway-projecting spiny projection neurons promotes compulsive behavior

Compulsive behaviors are a hallmark symptom of obsessive compulsive disorder (OCD). Striatal hyperactivity has been linked to compulsive behavior generation in correlative studies in humans and causal studies in rodents. However, the contribution of the two distinct striatal output populations to the generation and treatment of compulsive behavior is unknown. These populations of direct and indirect pathway-projecting spiny projection neurons (SPNs) have classically been thought to promote or suppress actions, respectively, leading to a long-held hypothesis that increased output of direct relative to indirect pathway promotes compulsive behavior. Contrary to this hypothesis, here we find that indirect pathway hyperactivity is associated with compulsive grooming in the Sapap3-knockout mouse model of OCD-relevant behavior. Furthermore, we show that suppression of indirect pathway activity using optogenetics or treatment with the first-line OCD pharmacotherapy fluoxetine is associated with reduced grooming in Sapap3-knockouts. Together, these findings highlight the striatal indirect pathway as a potential treatment target for compulsive behavior.

Information flow between motor cortex and striatum reverses during skill learning

The coordination of neural activity across brain areas during a specific behavior is often interpreted as neural communication involved in controlling the behavior. However, whether information relevant to the behavior is actually transferred between areas is often untested. Here, we used information-theoretic tools to quantify how motor cortex and striatum encode and exchange behaviorally relevant information about specific reach-to-grasp movement features during skill learning in rats. We found a temporal shift in the encoding of behaviorally relevant information during skill learning, as well as a reversal in the primary direction of behaviorally relevant information flow, from cortex-to-striatum during naive movements to striatum-to-cortex during skilled movements. Standard analytical methods that quantify the evolution of overall neural activity during learning-such as changes in neural signal amplitude or the overall exchange of information between areas-failed to capture these behaviorally relevant information dynamics. Using these standard methods, we instead found a consistent coactivation of overall neural signals during movement production and a bidirectional increase in overall information propagation between areas during learning. Our results show that skill learning is achieved through a transformation in how behaviorally relevant information is routed across cortical and subcortical brain areas and that isolating the components of neural activity relevant to and informative about behavior is critical to uncover directional interactions within a coactive and coordinated network.

Dopamine neurons drive spatiotemporally heterogeneous striatal dopamine signals during learning

Environmental cues, through Pavlovian learning, become conditioned stimuli that invigorate and guide animals toward acquisition of rewards. Dopamine neurons in the ventral tegmental area (VTA) and substantia nigra (SNC) are crucial for this process. Dopamine neurons are embedded in a reciprocally connected network with their striatal targets, the functional organization of which remains poorly understood. Here, we investigated how learning during optogenetic Pavlovian cue conditioning of VTA or SNC dopamine neurons directs cue-evoked behavior and shapes subregion-specific striatal dopamine dynamics. We used a fluorescent dopamine biosensor to monitor dopamine in the nucleus accumbens (NAc) core and shell, dorsomedial striatum (DMS), and dorsolateral striatum (DLS). We demonstrate spatially heterogeneous, learning-dependent dopamine changes across striatal regions. While VTA stimulation evoked robust dopamine release in NAc core, shell, and DMS, cues predictive of this activation preferentially recruited dopamine release in NAc core, starting early in training, and DMS, late in training. Corresponding negative prediction error signals, reflecting a violation in the expectation of dopamine neuron activation, only emerged in the NAc core and DMS, and not the shell. Despite development of vigorous movement late in training, conditioned dopamine signals did not similarly emerge in the DLS, even during Pavlovian conditioning with SNC dopamine neuron activation, which elicited robust DLS dopamine release. Together, our studies show broad dissociation in the fundamental prediction and reward-related information generated by different dopamine neuron populations and signaled by dopamine across the striatum. Further, they offer new insight into how larger-scale plasticity across the striatal network emerges during Pavlovian learning to coordinate behavior.

Comparative Roles of the Caudate and Putamen in the Serial Order of Behavior: Effects of Striatal Glutamate Receptor Blockade on Variable versus Fixed Spatial Self-Ordered Sequencing in Marmosets

Self-ordered sequencing is an important executive function involving planning and executing a series of steps to achieve goal-directed outcomes. The lateral frontal cortex is implicated in this behavior, but downstream striatal outputs remain relatively unexplored. We trained marmosets on a three-stimulus self-ordered spatial sequencing task using a touch-sensitive screen to explore the role of the caudate nucleus and putamen in random and fixed response arrays. By transiently blocking glutamatergic inputs to these regions, using intrastriatal CNQX microinfusions, we demonstrate that the caudate and putamen are both required for, but contribute differently to, flexible and fixed sequencing. CNQX into either the caudate or putamen impaired variable array accuracy, and infusions into both simultaneously elicited greater impairment. We demonstrated that continuous perseverative errors in variable array were caused by putamen infusions, likely due to interference with the putamen's established role in monitoring motor feedback. Caudate infusions, however, did not affect continuous errors, but did cause an upward trend in recurrent perseveration, possibly reflecting interference with the caudate's established role in spatial working memory and goal-directed planning. In contrast to variable array performance, while both caudate and putamen infusions impaired fixed array responding, the combined effects were not additive, suggesting possible competing roles. Infusions into either region individually, but not simultaneously, led to continuous perseveration. Recurrent perseveration in fixed arrays was caused by putamen, but not caudate, infusions. These results are consistent overall with a role of caudate in planning and flexible responding and the putamen in more rigid habitual or automatic responding.

Fear Learning: An Evolving Picture for Plasticity at Synaptic Afferents to the Amygdala

Unraveling the neuronal mechanisms of fear learning might allow neuroscientists to make links between a learned behavior and the underlying plasticity at specific synaptic connections. In fear learning, an innocuous sensory event such as a tone (called the conditioned stimulus, CS) acquires an emotional value when paired with an aversive outcome (unconditioned stimulus, US). Here, we review earlier studies that have shown that synaptic plasticity at thalamic and cortical afferents to the lateral amygdala (LA) is critical for the formation of auditory-cued fear memories. Despite the early progress, it has remained unclear whether there are separate synaptic inputs that carry US information to the LA to act as a teaching signal for plasticity at CS-coding synapses. Recent findings have begun to fill this gap by showing, first, that thalamic and cortical auditory afferents can also carry US information; second, that the release of neuromodulators contributes to US-driven teaching signals; and third, that synaptic plasticity additionally happens at connections up- and downstream of the LA. Together, a picture emerges in which coordinated synaptic plasticity in serial and parallel circuits enables the formation of a finely regulated fear memory.

Effective Regulation of Auditory Processing by Parvalbumin Interneurons in the Tail of the Striatum

Parvalbumin (PV) interneurons in the auditory cortex (AC) play a crucial role in shaping auditory processing, including receptive field formation, temporal precision enhancement, and gain regulation. PV interneurons are also the primary inhibitory neurons in the tail of the striatum (TS), which is one of the major descending brain regions in the auditory nervous system. However, the specific roles of TS-PV interneurons in auditory processing remain elusive. In this study, morphological and slice recording experiments in both male and female mice revealed that TS-PV interneurons, compared with AC-PV interneurons, were present in fewer numbers but exhibited longer projection distances, which enabled them to provide sufficient inhibitory inputs to spiny projection neurons (SPNs). Furthermore, TS-PV interneurons received dense auditory input from both the AC and medial geniculate body (MGB), particularly from the MGB, which rendered their auditory responses comparable to those of AC-PV interneurons. Optogenetic manipulation experiments demonstrated that TS-PV interneurons were capable of bidirectionally regulating the auditory responses of SPNs. Our findings suggest that PV interneurons can effectively modulate auditory processing in the TS and may play a critical role in auditory-related behaviors.

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.

Nucleus accumbens and dorsal medial striatal dopamine and neural activity are essential for action sequence performance

Separable striatal circuits have unique functions in Pavlovian and instrumental behaviors but how these roles relate to performance of sequences of actions with and without associated cues are less clear. Here, we tested whether dopamine transmission and neural activity more generally in three striatal subdomains are necessary for performance of an action chain leading to reward delivery. Male and female Long-Evans rats were trained to press a series of three spatially distinct levers to receive reward. We assessed the contribution of neural activity or dopamine transmission within each striatal subdomain when progression through the action sequence was explicitly cued and in the absence of cues. Behavior in both task variations was substantially impacted following microinfusion of the dopamine antagonist, flupenthixol, into nucleus accumbens core (NAc) or dorsomedial striatum (DMS), with impairments in sequence timing and numbers of rewards earned after NAc flupenthixol. In contrast, after pharmacological inactivation to suppress overall activity, there was minimal impact on total rewards earned. Instead, inactivation of both NAc and DMS impaired sequence timing and led to sequence errors in the uncued, but not cued task. There was no impact of dopamine antagonism or reversible inactivation of dorsolateral striatum on either cued or uncued action sequence completion. These results highlight an essential contribution of NAc and DMS dopamine systems in motivational and performance aspects of chains of actions, whether cued or internally generated, as well as the impact of intact NAc and DMS function for correct sequence performance.

Action Sequence Learning Is Impaired in Genetically Modified Mice with the Suppressed GABAergic Transmission from the Thalamic Reticular Nucleus to the Thalamus

The thalamic reticular nucleus (TRN) is a thin sheet of GABAergic neurons surrounding the thalamus, and it regulates the activity of thalamic relay neurons. The TRN has been reported to be involved in sensory gating, attentional regulation, and some other functions. However, little is known about the contribution of the TRN to sequence learning. In the present study, we examined whether the TRN is involved in reward-based learning of action sequence with no eliciting stimuli (operant conditioning), by analyzing the performance of male and female Avp-Vgat-/- mice (Vgatflox/flox mice crossed to an Avp-Cre driver line) on tasks conducted in an operant box having three levers. Our histological and electrophysiological data demonstrated that in adult Avp-Vgat-/- mice, vesicular GABA transporter (VGAT) was absent in most TRN neurons and the GABAergic transmission from the TRN to the thalamus was largely suppressed. The performance on a task in which mice needed to press an active lever for food reward showed that simple operant learning of lever pressing and learning of win-stay and lose-shift strategies are not affected in Avp-Vgat-/- mice. In contrast, the performance on a task in which mice needed to press three levers in a correct order for food reward showed that learning of the order of lever pressing (action sequence learning) was impaired in Avp-Vgat-/- mice. These results suggest that the TRN plays an important role in action sequence learning.

Mapping the neuroanatomical abnormalities in a phenotype of male compulsive rats

Compulsivity is considered a transdiagnostic dimension in obsessive-compulsive and related disorders, characterized by heterogeneous cognitive and behavioral phenotypes associated with abnormalities in cortico-striatal-thalamic-cortical circuitry. The present study investigated the structural morphology of white and gray matter in rats selected for low- (LD) and high- (HD) compulsive drinking behavior on a schedule-induced polydipsia (SIP) task. Regional brain morphology was assessed using ex-vivo high-resolution magnetic resonance imaging (MRI). Voxel-based morphometry of segmented MRI images revealed larger white matter volumes in anterior commissure and corpus callosum of HD rats compared with LD rats. HD rats also showed significantly larger regional volumes of dorsolateral orbitofrontal cortex, striatum, amygdala, hippocampus, midbrain, sub-thalamic nucleus, and cerebellum. By contrast, the medial prefrontal cortex was significantly smaller in HD rats compared with LD rats with no significant group differences in whole brain, ventricular, or cerebrospinal fluid volumes. These findings show that limbic cortico-basal ganglia structures implicated in impulse control disorders are distinct in rats that are vulnerable to develop compulsive behavior. Such abnormalities may be relevant to the etiology of compulsive disorders in humans.

The Secondary Motor Cortex-striatum Circuit Contributes to Suppressing Inappropriate Responses in Perceptual Decision Behavior

The secondary motor cortex (M2) encodes choice-related information and plays an important role in cue-guided actions. M2 neurons innervate the dorsal striatum (DS), which also contributes to decision-making behavior, yet how M2 modulates signals in the DS to influence perceptual decision-making is unclear. Using mice performing a visual Go/No-Go task, we showed that inactivating M2 projections to the DS impaired performance by increasing the false alarm (FA) rate to the reward-irrelevant No-Go stimulus. The choice signal of M2 neurons correlated with behavioral performance, and the inactivation of M2 neurons projecting to the DS reduced the choice signal in the DS. By measuring and manipulating the responses of direct or indirect pathway striatal neurons defined by M2 inputs, we found that the indirect pathway neurons exhibited a shorter response latency to the No-Go stimulus, and inactivating their early responses increased the FA rate. These results demonstrate that the M2-to-DS pathway is crucial for suppressing inappropriate responses in perceptual decision behavior.

Chronic alcohol exposure alters action control via hyperactive premotor corticostriatal activity

Alcohol use disorder (AUD) alters decision-making control over actions, but disruptions to the responsible neural circuit mechanisms are unclear. Premotor corticostriatal circuits are implicated in balancing goal-directed and habitual control over actions and show disruption in disorders with compulsive, inflexible behaviors, including AUD. However, whether there is a causal link between disrupted premotor activity and altered action control is unknown. Here, we find that mice chronically exposed to alcohol (chronic intermittent ethanol [CIE]) showed impaired ability to use recent action information to guide subsequent actions. Prior CIE exposure resulted in aberrant increases in the calcium activity of premotor cortex (M2) neurons that project to the dorsal medial striatum (M2-DMS) during action control. Chemogenetic reduction of this CIE-induced hyperactivity in M2-DMS neurons rescued goal-directed action control. This suggests a direct, causal relationship between chronic alcohol disruption to premotor circuits and decision-making strategy and provides mechanistic support for targeting activity of human premotor regions as a potential treatment in AUD.

Spinophilin Limits Metabotropic Glutamate Receptor 5 Scaffolding to the Postsynaptic Density and Cell Type Specifically Mediates Excessive Grooming

BACKGROUND

Grooming dysfunction is a hallmark of the obsessive-compulsive spectrum disorder trichotillomania. Numerous preclinical studies have utilized SAPAP3-deficient mice for understanding the neurobiology of repetitive grooming, suggesting that excessive grooming is caused by increased metabotropic glutamate receptor 5 (mGluR5) activity in striatal direct- and indirect-pathway medium spiny neurons (MSNs). However, the MSN subtype-specific signaling mechanisms that mediate mGluR5-dependent adaptations underlying excessive grooming are not fully understood. Here, we investigated the MSN subtype-specific roles of the striatal signaling hub protein spinophilin in mediating repetitive motor dysfunction associated with mGluR5 function.

METHODS

Quantitative proteomics and immunoblotting were utilized to identify how spinophilin impacts mGluR5 phosphorylation and protein interaction changes. Plasticity and repetitive motor dysfunction associated with mGluR5 action were measured using our novel conditional spinophilin mouse model in which spinophilin was knocked out from striatal direct-pathway MSNs and/or indirect-pathway MSNs.

RESULTS

Loss of spinophilin only in indirect-pathway MSNs decreased performance of a novel motor repertoire, but loss of spinophilin in either MSN subtype abrogated striatal plasticity associated with mGluR5 function and prevented excessive grooming caused by SAPAP3 knockout mice or treatment with the mGluR5-specific positive allosteric modulator VU0360172 without impacting locomotion-relevant behavior. Biochemically, we determined that the spinophilin-mGluR5 interaction correlates with grooming behavior and that loss of spinophilin shifts mGluR5 interactions from lipid raft-associated proteins toward postsynaptic density proteins implicated in psychiatric disorders.

CONCLUSIONS

These results identify spinophilin as a novel striatal signaling hub molecule in MSNs that cell subtype specifically mediates behavioral, functional, and molecular adaptations associated with repetitive motor dysfunction in psychiatric disorders.

Corticostriatal dynamics underlying components of binge-like consumption of palatable food in mice

Binge eating (BE) is a maladaptive repetitive feeding behavior present across nearly all eating disorder diagnoses. Despite the substantial negative impact of BE on psychological and physiological health, its underlying neural mechanisms are largely unknown. Other repetitive behavior disorders (e.g., obsessive compulsive disorder) show dysfunction within corticostriatal circuitry. However, to date, no work has investigated the in vivo neural dynamics underlying corticostriatal activity during BE episodes. The aim of the current study was to longitudinally examine in vivo neural activity within corticostriatal regions - the infralimbic cortex (IL) and dorsolateral striatum (DLS)- in a robust pre-clinical model for BE. Female C57BL6/J mice (N = 32) were randomized to receive: 1) intermittent (daily, 2-h) binge-like access to palatable food (sweetened condensed milk) (BE), or 2) continuous, non-intermittent (24-h) access to palatable food (control). In vivo calcium imaging was performed via fiber photometry at baseline and after chronic (4 weeks) engagement in the model for BE. Specific consummatory behaviors (feeding bout onset/offset) during recordings were captured using lickometers which generated TTL outputs for precise alignment of behavior to neural data. IL showed no specific changes in neural activity related to BE. However, BE animals showed decreased DLS activity at feeding onset and offset at the chronic timepoint when compared to activity at the baseline timepoint. Additionally, BE mice had significantly lower DLS activity at feeding onset and offset at the chronic timepoint compared to control mice. These results point to a role for DLS hypofunction in chronic BE, highlighting a potential target for future treatment intervention.

Human habit neural circuitry may be perturbed in eating disorders

Circuit-based mechanisms mediating the development and execution of habitual behaviors involve complex cortical-striatal interactions that have been investigated in animal models and more recently in humans. However, how human brain circuits implicated in habit formation may be perturbed in psychiatric disorders remains unclear. First, we identified the locations of the sensorimotor putamen and associative caudate in the human brain using probabilistic tractography from Human Connectome Project data. We found that multivariate connectivity of the sensorimotor putamen was altered in humans with binge eating disorder and bulimia nervosa and that the degree of alteration correlated with severity of disordered eating behavior. Furthermore, the extent of this circuit aberration correlated with mean diffusivity in the sensorimotor putamen and decreased basal dopamine D2/3 receptor binding potential in the striatum, consistent with previously reported microstructural changes and dopamine signaling mediating habit learning in animal models. Our findings suggest a neural circuit that links habit learning and binge eating behavior in humans, which could, in part, explain the treatment-resistant behavior common to eating disorders and other psychiatric conditions.

The deep cerebellar nuclei to striatum disynaptic connection contributes to skilled forelimb movement

Cerebellar-thalamo-striatal synaptic communication has been implicated in a wide range of behaviors, including goal-directed actions, and is altered in cerebellar dystonia. However, its detailed connectivity through the thalamus and its contribution to the execution of forelimb movements is unclear. Here, we use trans-synaptic and retrograde tracing, ex vivo slice recordings, and optogenetic inhibitions during the execution of unidirectional or sequential joystick displacements to demonstrate that the deep cerebellar nuclei (DCN) influence the dorsal striatum with a very high probability. We show that this mainly occurs through the centrolateral (CL), parafascicular (PF), and ventrolateral (VL) nuclei of the thalamus, observing that the DCN→VL and DCN→CL pathways contribute to the execution of unidirectional forelimb displacements while the DCN→PF and DCN→thalamo→striatal pathways contribute to the appropriate execution of forelimb reaching and sequential displacements. These findings highlight specific contributions of the different cerebellar-thalamo-striatal paths to the control of skilled forelimb movement.

The Sapap3-/- mouse reconsidered as a comorbid model expressing a spectrum of pathological repetitive behaviours

Symptom comorbidity is present amongst neuropsychiatric disorders with repetitive behaviours, complicating clinical diagnosis and impeding appropriate treatments. This is of particular importance for obsessive-compulsive disorder (OCD) and Tourette syndrome. Here, we meticulously analysed the behaviour of Sapap3 knockout mice, the recent rodent model predominantly used to study compulsive-like behaviours, and found that its behaviour is more complex than originally and persistently described. Indeed, we detected previously unreported elements of distinct pathologically repetitive behaviours, which do not form part of rodent syntactic cephalo-caudal self-grooming. These repetitive behaviours include sudden, rapid body and head/body twitches, resembling tic-like movements. We also observed that another type of repetitive behaviour, aberrant hindpaw scratching, might be responsible for the flagship-like skin lesions of this mouse model. In order to characterise the symptomatological nature of observed repetitive behaviours, we pharmacologically challenged these phenotypes by systemic aripiprazole administration, a first-line treatment for tic-like symptoms in Tourette syndrome and trichotillomania. A single treatment of aripiprazole significantly reduced the number of head/body twitches, scratching, and single-phase grooming, but not syntactic grooming events. These observations are in line with the high comorbidity of tic- and compulsive-like symptoms in Tourette, OCD and trichotillomania patients.

A striatal circuit balances learned fear in the presence and absence of sensory cues

During fear learning, defensive behaviors like freezing need to be finely balanced in the presence or absence of threat-predicting cues (conditioned stimulus, CS). Nevertheless, the circuits underlying such balancing are largely unknown. Here, we investigate the role of the ventral tail striatum (vTS) in auditory-cued fear learning of male mice. In vivo Ca²⁺ imaging showed that sizable sub-populations of direct (D1R+) and indirect pathway neurons (Adora+) in the vTS responded to footshocks, and to the initiation of movements after freezing; moreover, a sub-population of D1R+ neurons increased its responsiveness to an auditory CS during fear learning. In-vivo optogenetic silencing shows that footshock-driven activity of D1R+ neurons contributes to fear memory formation, whereas Adora+ neurons modulate freezing in the absence of a learned CS. Circuit tracing identified the posterior insular cortex (pInsCx) as an important cortical input to the vTS, and recording of optogenetically evoked EPSCs revealed long-term plasticity with opposite outcomes at the pInsCx synapses onto D1R+ - and Adora+ neurons. Thus, direct- and indirect pathways neurons of the vTS show differential signs of plasticity after fear learning, and balance defensive behaviors in the presence and absence of learned sensory cues.

Selective activation of striatal indirect pathway suppresses levodopa induced-dyskinesias

Levodopa (L-DOPA) administration remains the gold standard therapy for Parkinson's disease (PD). Despite several pharmacological advances in the use of L-DOPA, a high proportion of chronically treated patients continues to suffer disabling involuntary movements, namely, L-DOPA-induced dyskinesias (LIDs). As part of the effort to stop these unwanted side effects, the present study used a rodent model to identify and manipulate the striatal outflow circuitry responsible for LIDs. To do so, optogenetic technology was used to activate separately the striatal direct (D1R- expressing) and indirect (D2R- expressing) pathways in a mouse model of PD. Firstly, D1-cre or A2a-cre animals received unilateral injections of neurotoxin 6-hydroxydopamine (6-OHDA) to simulate the loss of dopamine observed in PD patients. The effects of independently stimulating each pathway were tested to see if experimental dyskinesias could be induced. Secondly, dopamine depleted A2a-cre animals received systemic L-DOPA to evoke dyskinetic movements. The ability of indirect pathway optogenetic stimulation to suppress pre-established LIDs was then tested. Selective manipulation of direct pathway evoked optodyskinesias both in dopamine depleted and intact animals, but optical inhibition of these neurons failed to suppress LIDs. On the other hand, selective activation of indirect striatal projection neurons produced an immediate and reliable suppression of LIDs. Thus, a functional dissociation has been found here whereby activation of D1R- and D2R-expressing projection neurons evokes and inhibits LIDs respectively, supporting the notion of tight interaction between the two striatal efferent systems in both normal and pathological conditions. This points to the importance of maintaining an equilibrium in the activity of both striatal pathways to produce normal movement. Finally, the ability of selective indirect pathway optogenetic activation to block the expression of LIDs in an animal model of PD sheds light on intrinsic mechanisms responsible for striatal-based dyskinesias and identifies a potential therapeutic target for suppressing LIDs in PD patients.

Cingulate-motor circuits update rule representations for sequential choice decisions

Anterior cingulate cortex mediates the flexible updating of an animal’s choice responses upon rule changes in the environment. However, how anterior cingulate cortex entrains motor cortex to reorganize rule representations and generate required motor outputs remains unclear. Here, we demonstrate that chemogenetic silencing of the terminal projections of cingulate cortical neurons in secondary motor cortex in the rat disrupts choice performance in trials immediately following rule switches, suggesting that these inputs are necessary to update rule representations for choice decisions stored in the motor cortex. Indeed, the silencing of cingulate cortex decreases rule selectivity of secondary motor cortical neurons. Furthermore, optogenetic silencing of cingulate cortical neurons that is temporally targeted to error trials immediately after rule switches exacerbates errors in the following trials. These results suggest that cingulate cortex monitors behavioral errors and updates rule representations in motor cortex, revealing a critical role for cingulate-motor circuits in adaptive choice behaviors.

The anterior cingulate cortex allows an animal to update its behaviour when the environment changes. In this work, the authors identify a pathway from cingulate to secondary motor cortex, critical for updating motor rules following behavioural errors.

GABAergic interneurons expressing the α2 nicotinic receptor subunit are functionally integrated in the striatal microcircuit

The interactions between the striatal cholinergic and GABAergic systems are crucial in shaping reward-related behavior and reinforcement learning; however, the synaptic pathways mediating them are largely unknown. Here, we use Chrna2-Cre mice to characterize striatal interneurons (INs) expressing the nicotinic α2 receptor subunit. Using triple patch-clamp recordings combined with optogenetic stimulations, we characterize the electrophysiological, morphological, and synaptic properties of striatal Chrna2-INs. Striatal Chrna2-INs have diverse electrophysiological properties, distinct from their counterparts in other brain regions, including the hippocampus and neocortex. Unlike in other regions, most striatal Chrna2-INs are fast-spiking INs expressing parvalbumin. Striatal Chrna2-INs are intricately integrated in the striatal microcircuit, forming inhibitory synaptic connections with striatal projection neurons and INs, including other Chrna2-INs. They receive excitatory inputs from primary motor cortex mediated by both AMPA and NMDA receptors. A subpopulation of Chrna2-INs responds to nicotinic input, suggesting reciprocal interactions between this GABAergic interneuron population and striatal cholinergic synapses.

Information normally considered task-irrelevant drives decision-making and affects premotor circuit recruitment

Decision-making is a continuous and dynamic process with prior experience reflected in and used by the brain to guide adaptive behavior. However, most neurobiological studies constrain behavior and/or analyses to task-related variables, not accounting for the continuous internal and temporal space in which they occur. We show mice rely on information learned through recent and longer-term experience beyond just prior actions and reward - including checking behavior and the passage of time - to guide self-initiated, self-paced, and self-generated actions. These experiences are represented in secondary motor cortex (M2) activity and its projections into dorsal medial striatum (DMS). M2 integrates this information to bias strategy-level decision-making, and DMS projections reflect specific aspects of this recent experience to guide actions. This suggests diverse aspects of experience drive decision-making and its neural representation, and shows premotor corticostriatal circuits are crucial for using selective aspects of experiential information to guide adaptive behavior.

Spatiotemporal reorganization of corticostriatal networks encodes motor skill learning

Motor skill learning requires the activity of the dorsal striatum, with a differential global implication of the dorsomedial and dorsolateral territories. We investigate here whether and how specific striatal neurons encode the acquisition and consolidation of a motor skill. Using ex vivo two-photon calcium imaging after rotarod training, we report that highly active (HA) striatal populations arise from distinct spatiotemporal reorganization in the dorsomedial (DMS) and dorsolateral (DLS) striatum networks and are correlated with learning performance. The DMS overall activity decreases in early training, with few and sparsely distributed HA cells, while the DLS shows a progressive and long-lasting formation of HA cell clusters. These reorganizations result from reinforcement of synaptic connections to the DMS and anatomical rearrangements to the DLS. Targeted silencing of DMS or DLS HA cells with the cFos-TRAP strategy strongly impairs individual performance. Our data reveal that discrete domains of striatal populations encode acquisition and long-lasting retention of a motor skill.

The prefrontal cortex and OCD

Obsessive Compulsive Disorder (OCD) is a highly prevalent and severe neuropsychiatric disorder, with an incidence of 1.5-3% worldwide. However, despite the clear public health burden of OCD and relatively well-defined symptom criteria, effective treatments are still limited, spotlighting the need for investigation of the neural substrates of the disorder. Human neuroimaging studies have consistently highlighted abnormal activity patterns in prefrontal cortex (PFC) regions and connected circuits in OCD during both symptom provocation and performance of neurocognitive tasks. Because of recent technical advances, these findings can now be leveraged to develop novel targeted interventions. Here we will highlight current theories regarding the role of the prefrontal cortex in the generation of OCD symptoms, discuss ways in which this knowledge can be used to improve treatments for this often disabling illness, and lay out challenges in the field for future study.

What Is the Role of Thalamostriatal Circuits in Learning Vocal Sequences?

Basal ganglia (BG) circuits integrate sensory and motor-related information from the cortex, thalamus, and midbrain to guide learning and production of motor sequences. Birdsong, like speech, is comprised of precisely sequenced vocal elements. Learning song sequences during development relies on Area X, a vocalization related region in the medial striatum of the songbird BG. Area X receives inputs from cortical-like pallial song circuits and midbrain dopaminergic circuits and sends projections to the thalamus. It has recently been shown that thalamic circuits also send substantial projections back to Area X. Here, we outline a gated-reinforcement learning model for how Area X may use signals conveyed by thalamostriatal inputs to direct song learning. Integrating conceptual advances from recent mammalian and songbird literature, we hypothesize that thalamostriatal pathways convey signals linked to song syllable onsets and offsets and influence striatal circuit plasticity via regulation of cholinergic interneurons (ChIs). We suggest that syllable sequence associated vocal-motor information from the thalamus drive precisely timed pauses in ChIs activity in Area X. When integrated with concurrent corticostriatal and dopaminergic input, this circuit helps regulate plasticity on medium spiny neurons (MSNs) and the learning of syllable sequences. We discuss new approaches that can be applied to test core ideas of this model and how associated insights may provide a framework for understanding the function of BG circuits in learning motor sequences.

Coupling between motor cortex and striatum increases during sleep over long-term skill learning

The strength of cortical connectivity to the striatum influences the balance between behavioral variability and stability. Learning to consistently produce a skilled action requires plasticity in corticostriatal connectivity associated with repeated training of the action. However, it remains unknown whether such corticostriatal plasticity occurs during training itself or 'offline' during time away from training, such as sleep. Here, we monitor the corticostriatal network throughout long-term skill learning in rats and find that non-rapid-eye-movement (NREM) sleep is a relevant period for corticostriatal plasticity. We first show that the offline activation of striatal NMDA receptors is required for skill learning. We then show that corticostriatal functional connectivity increases offline, coupled to emerging consistent skilled movements, and coupled cross-area neural dynamics. We then identify NREM sleep spindles as uniquely poised to mediate corticostriatal plasticity, through interactions with slow oscillations. Our results provide evidence that sleep shapes cross-area coupling required for skill learning.

The Contribution of Premotor Cortico-Striatal Projections to the Execution of Serial Order Sequences

Striatal activity is necessary to initiate and execute sequences of actions. The main excitatory input to the striatum comes from the cortex. While it is hypothesized that motor and premotor cortico-striatal projections are important to guide striatal activity during the execution of sequences of actions, technical limitations have made this challenging to address. Here, we implemented a task in mice that allows for the study of different moments to execute a serial order sequence consisting of two subsequences of actions. Using this task, we performed electrophysiological recordings in the premotor (M2) and primary motor (M1) cortices, and state-dependent optogenetic inhibitions of their cortico-striatal projections. We show that while both M2 and M1 contain activity modulations related to the execution of self-paced sequences, mainly, the premotor cortico-striatal projections contribute to the proper execution/structuring of these sequences.

Interhemispheric Cortico-Cortical Pathway for Sequential Bimanual Movements in Mice

Animals precisely coordinate their left and right limbs for various adaptive purposes. While the left and right limbs are clearly controlled by different cortical hemispheres, the neural mechanisms that determine the action sequence between them remains elusive. Here, we have established a novel head-fixed bimanual-press (biPress) sequence task in which mice sequentially press left and right pedals with their forelimbs in a predetermined order. Using this motor task, we found that the motor cortical neurons responsible for the first press (1P) also generate independent motor signals for the second press (2P) by the opposite forelimb during the movement transitions between forelimbs. Projection-specific calcium imaging and optogenetic manipulation revealed these motor signals are transferred from one motor cortical hemisphere to the other via corticocortical projections. Together, our results suggest the motor cortices coordinate sequential bimanual movements through corticocortical pathways.

α-Synuclein Selectively Impairs Motor Sequence Learning and Value Sensitivity: Reversal by the Adenosine A2A Receptor Antagonists

Parkinson's disease (PD) is characterized pathologically by alpha-synuclein (α-Syn) aggregates and clinically by the motor as well as cognitive deficits, including impairments in sequence learning and habit learning. Using intracerebral injection of WT and A53T mutant α-Syn fibrils, we investigate the behavioral mechanism of α-Syn for procedure-learning deficit in PD by critically determining the α-Syn-induced effects on model-based goal-directed behavior, model-free (probability-based) habit learning, and hierarchically organized sequence learning. 1) Contrary to the widely held view of habit-learning deficit in early PD, α-Syn aggregates in the dorsomedial striatum (DMS) and dorsolateral striatum (DLS) did not affect acquisition of habit learning, but selectively impaired goal-directed behavior with reduced value sensitivity. 2) α-Syn in the DLS (but not DMS) and SNc selectively impaired the sequence learning by affecting sequence initiation with the reduced first-step accuracy. 3) Adenosine A2A receptor (A2AR) antagonist KW6002 selectively improved sequence learning by preferentially improving sequence initiation and shift of sequence learning as well as behavioral reactivity. These findings established a casual role of α-Syn in the SN-DLS pathway in sequence-learning deficit and DMS α-Syn in goal-directed behavior deficit and suggest a novel therapeutic strategy to improve sequence-learning deficit in PD with enhanced sequence initiation by A2AR antagonists.

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.

Interpreting the role of the striatum during multiple phases of motor learning

The synaptic pathways in the striatum are central to basal ganglia functions including motor control, learning and organization, action selection, acquisition of motor skills, cognitive function, and emotion. Here, we review the role of the striatum and its connections in motor learning and performance. The development of new techniques to record neuronal activity and animal models of motor disorders using neurotoxin, pharmacological, and genetic manipulations are revealing pathways that underlie motor performance and motor learning, as well as how they are altered by pathophysiological mechanisms. We discuss approaches that can be used to analyze complex motor skills, particularly in rodents, and identify specific questions central to understanding how striatal circuits mediate motor learning.

Retrosplenial Cortical Connectivity with Frontal Basal Ganglia Networks

Previous studies of the retrosplenial cortex (RSC) have focused on its role in navigation and memory, consistent with its well-established medial temporal connections, but recent evidence also suggests a role for this region in reward and decision making. Because function is determined largely by anatomical connections, and to better understand the anatomy of RSC, we used tract-tracing methods to examine the anatomical connectivity between the rat RSC and frontostriatal networks (canonical reward and decision-making circuits). We find that, among frontal cortical regions, RSC bidirectionally connects most strongly with the anterior cingulate cortex, but also with an area of the central-medial orbito-frontal cortex. RSC projects to the dorsomedial striatum, and its terminal fields are virtually encompassed by the frontal-striatal projection zone, suggestive of functional convergence through the basal ganglia. This overlap is driven by anterior cingulate cortex, prelimbic cortex, and orbito-frontal cortex, all of which contribute to goal-directed decision making, suggesting that the RSC is involved in similar processes.

Spectral signatures of L-DOPA-induced dyskinesia depend on L-DOPA dose and are suppressed by ketamine

L-DOPA-induced dyskinesias (LID) are debilitating motor symptoms of dopamine-replacement therapy for Parkinson's disease (PD) that emerge after years of L-DOPA treatment. While there is an abundance of research into the cellular and synaptic origins of LID, less is known about how LID impacts systems-level circuits and neural synchrony, how synchrony is affected by the dose and duration of L-DOPA exposure, or how potential novel treatments for LID, such as sub-anesthetic ketamine, alter this activity. Sub-anesthetic ketamine treatments have recently been shown to reduce LID, and ketamine is known to affect neural synchrony. To investigate these questions, we measured movement and local-field potential (LFP) activity from the motor cortex (M1) and the striatum of preclinical rodent models of PD and LID. In the first experiment, we investigated the effect of the LID priming procedures and L-DOPA dose on neural signatures of LID. Two common priming procedures were compared: a high-dose procedure that exposed unilateral 6-hydroxydopamine-lesioned rats to 12 mg/kg L-DOPA for 7 days, and a low-dose procedure that exposed rats to 7 mg/kg L-DOPA for 21 days. Consistent with reports from other groups, 12 mg/kg L-DOPA triggered LID and 80-Hz oscillations; however, these 80-Hz oscillations were not observed after 7 mg/kg administration despite clear evidence of LID, indicating that 80-Hz oscillations are not an exclusive signature of LID. We also found that weeks-long low-dose priming resulted in the emergence of non-oscillatory broadband gamma activity (> 30 Hz) in the striatum and theta-to-high-gamma cross-frequency coupling (CFC) in M1. In a second set of experiments, we investigated how ketamine exposure affects spectral signatures of low-dose L-DOPA priming. During each neural recording session, ketamine was delivered through 5 injections (20 mg/kg, i.p.) administered every 2 h. We found that ketamine exposure suppressed striatal broadband gamma associated with LID but enhanced M1 broadband activity. We also found that M1 theta-to-high-gamma CFC associated with the LID on-state was suppressed by ketamine. These results suggest that ketamine's therapeutic effects are region specific. Our findings also have clinical implications, as we are the first to report novel oscillatory signatures of the common low-dose LID priming procedure that more closely models dopamine replacement therapy in individuals with PD. We also identify neural correlates of the anti-dyskinetic activity of sub-anesthetic ketamine treatment.

Striatal activity topographically reflects cortical activity

The cortex projects to the dorsal striatum topographically^1,2 to regulate behaviour^3-5, but spiking activity in the two structures has previously been reported to have markedly different relations to sensorimotor events^6-9. Here we show that the relationship between activity in the cortex and striatum is spatiotemporally precise, topographic, causal and invariant to behaviour. We simultaneously recorded activity across large regions of the cortex and across the width of the dorsal striatum in mice that performed a visually guided task. Striatal activity followed a mediolateral gradient in which behavioural correlates progressed from visual cue to response movement to reward licking. The summed activity in each part of the striatum closely and specifically mirrored activity in topographically associated cortical regions, regardless of task engagement. This relationship held for medium spiny neurons and fast-spiking interneurons, whereas the activity of tonically active neurons differed from cortical activity with stereotypical responses to sensory or reward events. Inactivation of the visual cortex abolished striatal responses to visual stimuli, supporting a causal role of cortical inputs in driving the striatum. Striatal visual responses were larger in trained mice than untrained mice, with no corresponding change in overall activity in the visual cortex. Striatal activity therefore reflects a consistent, causal and scalable topographical mapping of cortical activity.

Remembrance of things practiced with fast and slow learning in cortical and subcortical pathways

The learning of motor skills unfolds over multiple timescales, with rapid initial gains in performance followed by a longer period in which the behavior becomes more refined, habitual, and automatized. While recent lesion and inactivation experiments have provided hints about how various brain areas might contribute to such learning, their precise roles and the neural mechanisms underlying them are not well understood. In this work, we propose neural- and circuit-level mechanisms by which motor cortex, thalamus, and striatum support motor learning. In this model, the combination of fast cortical learning and slow subcortical learning gives rise to a covert learning process through which control of behavior is gradually transferred from cortical to subcortical circuits, while protecting learned behaviors that are practiced repeatedly against overwriting by future learning. Together, these results point to a new computational role for thalamus in motor learning and, more broadly, provide a framework for understanding the neural basis of habit formation and the automatization of behavior through practice.

The Functional Organization of Cortical and Thalamic Inputs onto Five Types of Striatal Neurons Is Determined by Source and Target Cell Identities

To understand striatal function, it is essential to know the functional organization of the numerous inputs targeting the diverse population of striatal neurons. Using optogenetics, we activated terminals from ipsi- or contralateral primary somatosensory cortex (S1) or primary motor cortex (M1), or thalamus while obtaining simultaneous whole-cell recordings from pairs or triplets of striatal medium spiny neurons (MSNs) and adjacent interneurons. Ipsilateral corticostriatal projections provided stronger excitation to fast-spiking interneurons (FSIs) than to MSNs and only sparse and weak excitation to low threshold-spiking interneurons (LTSIs) and cholinergic interneurons (ChINs). Projections from contralateral M1 evoked the strongest responses in LTSIs but none in ChINs, whereas thalamus provided the strongest excitation to ChINs but none to LTSIs. In addition, inputs varied in their glutamate receptor composition and their short-term plasticity. Our data revealed a highly selective organization of excitatory striatal afferents, which is determined by both pre- and postsynaptic neuronal identity.

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Whole-cell recordings are obtained from neighboring striatal neurons of different types

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FSIs receive the strongest inputs from S1, M1, and thalamic PF

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LTSIs are primarily excited by contralateral M1

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ChINs are primarily excited by PF and receive no input from contralateral M1 and S1

M2 cortex-dorsolateral striatum stimulation reverses motor symptoms and synaptic deficits in Huntington's disease

Huntington’s disease (HD) is a neurological disorder characterized by motor disturbances. HD pathology is most prominent in the striatum, the central hub of the basal ganglia. The cerebral cortex is the main striatal afferent, and progressive cortico-striatal disconnection characterizes HD. We mapped striatal network dysfunction in HD mice to ultimately modulate the activity of a specific cortico-striatal circuit to ameliorate motor symptoms and recover synaptic plasticity. Multimodal MRI in vivo indicates cortico-striatal and thalamo-striatal functional network deficits and reduced glutamate/glutamine ratio in the striatum of HD mice. Moreover, optogenetically-induced glutamate release from M2 cortex terminals in the dorsolateral striatum (DLS) was undetectable in HD mice and striatal neurons show blunted electrophysiological responses. Remarkably, repeated M2-DLS optogenetic stimulation normalized motor behavior in HD mice and evoked a sustained increase of synaptic plasticity. Overall, these results reveal that selective stimulation of the M2-DLS pathway can become an effective therapeutic strategy in HD.

Differential encoding of action selection by orbitofrontal and striatal population dynamics

Survival relies on the ability to flexibly choose between different actions according to varying environmental circumstances. Many lines of evidence indicate that action selection involves signaling in corticostriatal circuits, including the orbitofrontal cortex (OFC) and dorsomedial striatum (DMS). While choice-specific responses have been found in individual neurons from both areas, it is unclear whether populations of OFC or DMS neurons are better at encoding an animal's choice. To address this, we trained head-fixed mice to perform an auditory guided two-alternative choice task, which required moving a joystick forward or backward. We then used silicon microprobes to simultaneously measure the spiking activity of OFC and DMS ensembles, allowing us to directly compare population dynamics between these areas within the same animals. Consistent with previous literature, both areas contained neurons that were selective for specific stimulus-action associations. However, analysis of concurrently recorded ensemble activity revealed that the animal's trial-by-trial behavior could be decoded more accurately from DMS dynamics. These results reveal substantial regional differences in encoding action selection, suggesting that DMS neural dynamics are more specialized than OFC at representing an animal's choice of action.NEW & NOTEWORTHY While previous literature shows that both orbitofrontal cortex (OFC) and dorsomedial striatum (DMS) represent information relevant to selecting specific actions, few studies have directly compared neural signals between these areas. Here we compared OFC and DMS dynamics in mice performing a two-alternative choice task. We found that the animal's choice could be decoded more accurately from DMS population activity. This work provides among the first evidence that OFC and DMS differentially represent information about an animal's selected action.

Heterogeneity in striatal dopamine circuits: Form and function in dynamic reward seeking

The striatal dopamine system has long been studied in the context of reward learning, motivation, and movement. Given the prominent role dopamine plays in a variety of adaptive behavioral states, as well as diseases like addiction, it is essential to understand the full complexity of dopamine neurons and the striatal systems they target. A growing number of studies are uncovering details of the heterogeneity in dopamine neuron subpopulations. Here, we review that work to synthesize current understanding of dopamine system heterogeneity across three levels, anatomical organization, functions in behavior, and modes of action, wherein we focus on signaling profiles and local mechanisms for modulation of dopamine release. Together, these studies reveal new and emerging dimensions of the striatal dopamine system, informing its contribution to dynamic motivational and decision-making processes.