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Optogenetics and calcium imaging can be combined to simultaneously stimulate and record neural activity in vivo. However, this usually requires two-photon microscopes, which are not portable nor affordable. Here we report the design and implementation of a miniaturized one-photon endoscope for performing simultaneous optogenetic stimulation and calcium imaging. By integrating digital micromirrors, the endoscope makes it possible to activate any neuron of choice within the field of view, and to apply arbitrary spatiotemporal patterns of photostimulation while imaging calcium activity. We used the endoscope to image striatal neurons from either the direct pathway or the indirect pathway in freely moving mice while activating any chosen neuron in the field of view. The endoscope also allows for the selection of neurons based on their relationship with specific animal behaviour, and to recreate the behaviour by mimicking the natural neural activity with photostimulation. The miniaturized endoscope may facilitate the study of how neural activity gives rise to behaviour in freely moving animals.

Unilateral dopamine (DA) depletion produces ipsiversive turning behaviour, and the injection of DA receptor agonists can produce contraversive turning, but the underlying mechanisms remain unclear. We conducted in vivo recording and pharmacological and optogenetic manipulations to study the role of DA and striatal output in turning behaviour. We used a video-based tracking programme while recording single unit activity in both putative medium spiny projection neurons (MSNs) and fast-spiking interneurons (FSIs) in the dorsal striatum bilaterally. Our results suggest that unilateral DA depletion reduced striatal output from the depleted side, resulting in asymmetric striatal output. Depletion systematically altered activity in both MSNs and FSIs, especially in neurons that increased firing during turning movements. Like D1 agonist SKF 38393, optogenetic stimulation in the depleted striatum increased striatal output and reversed biassed turning. These results suggest that relative striatal outputs from the two cerebral hemispheres determine the direction of turning: Mice turn away from the side of higher striatal output and towards the side of the lower striatal output.

Animals can learn to repeat behaviors to earn desired rewards, a process commonly known as reinforcement learning. While previous work has implicated the ascending dopaminergic projections to the basal ganglia in reinforcement learning, little is known about the role of the hippocampus. Here we report that a specific population of hippocampal neurons and their dopaminergic innervation contribute to operant self-stimulation. These neurons are located in the dentate gyrus, receive dopaminergic projections from the locus coeruleus, and express D1 dopamine receptors. Activation of D1+ dentate neurons is sufficient for self-stimulation: mice will press a lever to earn optogenetic activation of these neurons. A similar effect is also observed with selective activation of the locus coeruleus projections to the dentate gyrus, and blocked by D1 receptor antagonism. Calcium imaging of D1+ dentate neurons revealed significant activity at the time of action selection, but not during passive reward delivery. These results reveal the role of dopaminergic innervation of the hippocampus in supporting operant reinforcement.

Dysfunctional sociability is a core symptom in autism spectrum disorder (ASD) that may arise from neural-network dysconnectivity between multiple brain regions. However, pathogenic neural-network mechanisms underlying social dysfunction are largely unknown. Here, we demonstrate that circuit-selective mutation (ctMUT) of ASD-risk Shank3 gene within a unidirectional projection from the prefrontal cortex to the basolateral amygdala alters spine morphology and excitatory-inhibitory balance of the circuit. Shank3 ctMUT mice show reduced sociability as well as elevated neural activity and its amplitude variability, which is consistent with the neuroimaging results from human ASD patients. Moreover, the circuit hyper-activity disrupts the temporal correlation of socially tuned neurons to the events of social interactions. Finally, optogenetic circuit activation in wild-type mice partially recapitulates the reduced sociability of Shank3 ctMUT mice, while circuit inhibition in Shank3 ctMUT mice partially rescues social behavior. Collectively, these results highlight a circuit-level pathogenic mechanism of Shank3 mutation that drives social dysfunction. Accepted version Source info: CP-COMMUNITY-D-21-00183

The ability to accurately determine when to perform an action is a fundamental brain function and vital to adaptive behavior. The behavioral mechanism and neural circuit for action timing, however, remain largely unknown. Using a new, self-paced action timing task in mice, we found that deprivation of auditory, but not somatosensory or visual input, disrupts learned action timing. The hearing effect was dependent on the auditory feedback derived from the animal’s own actions, rather than passive environmental cues. Neuronal activity in the secondary auditory cortex was found to be both correlated with and necessary for the proper execution of learned action timing. Closed-loop, action-dependent optogenetic stimulation of the specific task-related neuronal population within the secondary auditory cortex rescued the key features of learned action timing under auditory deprivation. These results unveil a previously underappreciated sensorimotor mechanism in which the secondary auditory cortex transduces self-generated audiomotor feedback to control action timing.

The subthalamic nucleus (STN), a diencephalic nucleus strongly connected with the basal ganglia, is a common target for deep brain stimulation (DBS) treatments of Parkinsonian motor symptoms. The STN is thought to suppress movement by enhancing inhibitory basal ganglia output via the indirect pathway, and disrupting STN output using DBS can restore movement in Parkinson’s patients. However, previous studies on STN DBS usually relied on electrical stimulation, which cannot selectively target STN output neurons. Moreover, most studies did not measure behavior precisely during STN manipulations. Here we selectively stimulated STN projection neurons, by expressing channelrhodopsin in vesicular glutamate transporter 2 (VGlut2)-positive projection neurons in the STN. We also quantified behavior in freely moving male mice with high spatial and temporal resolution using 3D motion capture at 200 frames per second. We found STN stimulation resulted in movements with very short latencies (10-15 ms). Unilateral stimulation quantitatively determines orientation, head yaw, and head roll consistently in the ipsiversive direction, toward thee side of stimulation. On the other hand, head raising (increased head pitch) was observed after stimulation on either side. Strikingly, a single pulse of light was sufficient to generate movement, and there was a highly linear relationship between stimulation frequency and kinematics. These results question the common assumption that STN suppresses movement; instead they suggest that STN output can precisely specific action parameters via direct projections to brainstem regions.

SummaryNeurogenesis and differentiation of the neural stem cells (NSCs) in the subventricular zone (SVZ) are controlled by cell-intrinsic molecular pathways that interact with extrinsic signaling cues. Here we identified a novel circuit that regulates neurogenesis and cellular proliferation in the lateral ventricle SVZ (LV-SVZ). Our results demonstrate direct glutamatergic inputs from the frontal cortex as well as local inhibitory interneurons, control the activity of distinctive cholinergic neurons in the subependymal zone (subep-ChAT+). In vivo optogenetic stimulation and inhibition in this circuit were sufficient to control local SVZ neurogenesis, LV NSCs proliferation, and SVZ cellular divisions in ventral SVZ. These findings shed light on local and distal neural circuit activity-dependent regulation of postnatal and adult SVZ neurogenesis and LV-SVZ cellular proliferation.

While in vivo calcium imaging makes it possible to record activity in defined neuronal populations with cellular resolution, optogenetics allows selective manipulation of neural activity. Recently, these two tools have been combined to stimulate and record neural activity at the same time, but current approaches often rely on two-photon microscopes that are difficult to use in freely moving animals, or one-photon fiberscopes with benchtop-based digital micromirror devices that limit system portability. To address these limitations, we have developed a new integrated system combining a one-photon endoscope and a digital micromirror device for simultaneous calcium imaging and precise optogenetic photo-stimulation (Miniscope with All-optical Patterned Stimulation and Imaging, MAPSI). Using this system, we were able to successfully image striatal neurons from either the direct pathway or the indirect pathway while simultaneously activating any neuron of choice within the field of view or synthesizing arbitrary spatio-temporal patterns of photo-stimulation in freely moving mice. We could also select neurons based on their relationship with behavior and recreate the behavior by mimicking the natural neural activity with photo-stimulation. MAPSI thus provides a powerful tool for in vivo interrogation of neural circuit function.

Significance Autism spectrum disorders (ASD) are heritable neurodevelopmental disorders affecting communication and behavior, and can include impaired intellectual ability. ASD is believed to result from altered synapse function and gene regulation. Among high-risk ASD genes, ANK2 is of particular interest since ANK2 mutations with an ASD diagnosis can occur with normal IQ. Here, we report that ASD ANK2 mutation in mice results in increased axon branching and ectopic connectivity associated with penetrant impairment in selected communicative and social behaviors combined with superior executive function. Thus, gain of axon branching is a candidate cellular mechanism to explain aberrant structural connectivity and dominantly inherited behavior that are compatible with normal intelligence and potentially can contribute to ASD and other forms of neurodiversity., Giant ankyrin-B (ankB) is a neurospecific alternatively spliced variant of ANK2, a high-confidence autism spectrum disorder (ASD) gene. We report that a mouse model for human ASD mutation of giant ankB exhibits increased axonal branching in cultured neurons with ectopic CNS axon connectivity, as well as with a transient increase in excitatory synapses during postnatal development. We elucidate a mechanism normally limiting axon branching, whereby giant ankB localizes to periodic axonal plasma membrane domains through L1 cell-adhesion molecule protein, where it couples microtubules to the plasma membrane and prevents microtubule entry into nascent axon branches. Giant ankB mutation or deficiency results in a dominantly inherited impairment in selected communicative and social behaviors combined with superior executive function. Thus, gain of axon branching due to giant ankB-deficiency/mutation is a candidate cellular mechanism to explain aberrant structural connectivity and penetrant behavioral consequences in mice as well as humans bearing ASD-related ANK2 mutations.

Most adaptive behaviors require precise tracking of targets in space. In pursuit behavior with a moving target, mice use distance to target to guide their own movement continuously. Here, we show that in the sensorimotor striatum, parvalbumin-positive fast-spiking interneurons (FSIs) can represent the distance between self and target during pursuit behavior, while striatal projection neurons (SPNs), which receive FSI projections, can represent self-velocity. FSIs are shown to regulate velocity-related SPN activity during pursuit, so that movement velocity is continuously modulated by distance to target. Moreover, bidirectional manipulation of FSI activity can selectively disrupt performance by increasing or decreasing the self-target distance. Our results reveal a key role of the FSI-SPN interneuron circuit in pursuit behavior and elucidate how this circuit implements distance to velocity transformation required for the critical underlying computation., Many natural behaviours involve tracking of a target in space. Here, the authors describe a task to assess this behaviour in mice and use in vivo electrophysiology, calcium imaging, optogenetics, and chemogenetics to investigate the role of the striatum in target pursuit.

Choice behavior is characterized by temporal discounting, i.e., preference for immediate rewards given a choice between immediate and delayed rewards. Agouti-related peptide (AgRP)-expressing neurons located in the arcuate nucleus of the hypothalamus (ARC) regulate food intake and energy homeostasis, yet whether AgRP neurons influence choice behavior and temporal discounting is unknown. Here, we demonstrate that motivational state potently modulates temporal discounting. Hungry mice (both male and female) strongly preferred immediate food rewards, yet sated mice were largely indifferent to reward delay. More importantly, selective optogenetic activation of AgRP-expressing neurons or their axon terminals within the posterior bed nucleus of stria terminalis (BNST) produced temporal discounting in sated mice. Furthermore, activation of neuropeptide Y (NPY) type 1 receptors (Y1Rs) within the BNST is sufficient to produce temporal discounting. These results demonstrate a profound influence of hypothalamic signaling on temporal discounting for food rewards and reveal a novel circuit that determine choice behavior.SIGNIFICANCE STATEMENTTemporal discounting is a universal phenomenon found in many species, yet the underlying neurocircuit mechanisms are still poorly understood. Our results revealed a novel neural pathway from agouti-related peptide (AgRP) neurons in the hypothalamus to the bed nucleus of stria terminalis (BNST) that regulates temporal discounting in decision-making.

Selective activation of Pf-STN pathway initiates actions in normal mice and restores movement in a mouse model of PD., The parafascicular nucleus (Pf) of the thalamus provides major projections to the basal ganglia, a set of subcortical nuclei involved in action initiation. Here, we show that Pf projections to the subthalamic nucleus (STN), but not to the striatum, are responsible for movement initiation. Because the STN is a major target of deep brain stimulation treatments for Parkinson’s disease, we tested the effect of selective stimulation of Pf-STN projections in a mouse model of PD. Bilateral dopamine depletion with 6-OHDA created complete akinesia in mice, but Pf-STN stimulation immediately and markedly restored a variety of natural behaviors. Our results therefore revealed a functionally novel neural pathway for the initiation of movements that can be recruited to rescue movement deficits after dopamine depletion. They not only shed light on the clinical efficacy of conventional STN DBS but also suggest more selective and improved stimulation strategies for the treatment of parkinsonian symptoms.

Terrestrial locomotion presents tremendous computational challenges on account of the enormous degrees of freedom in legged animals, and the complex and unpredictable properties of the natural environment and the effectors. Yet the nervous system can achieve locomotion with ease. Here we introduce a quadrupedal robot capable of goal-directed posture control and locomotion over rough terrain. The underlying control architecture is a hierarchical network of simple negative feedback control systems inspired by the organization of the vertebrate nervous system. Without using an internal model or feedforward planning, and without any training, our robot shows robust posture control and locomotor behavior in novel environments with unpredictable disturbances.

Loss of sociability is a core symptom in autism spectrum disorder (ASD) that may arise from neural network dysconnectivity between multiple brain regions contributing to social behaviors. However, the pathogenic neural network mechanisms underlying social dysfunction of ASD are largely unknown. Here, we demonstrate that the circuit-selective mutation of ASD-risk Shank3 gene within a unidirectional neural circuit from the prefrontal cortex to the basolateral amygdala reduces sociability as well as elevates the neural activity and its amplitude-variability in the circuit, which is consistent with the neuroimaging results from human ASD patients. Moreover, the abnormal neural circuit activities reduce the temporal correlation of socially-tuned neurons to the events of social interactions and alter the intra-circuit functional network involved in social behaviors. Finally, optogenetic mimicry of the circuit hyperactivity recapitulates the reduced sociability of Shank3 ctMUT mice. Collectively these results highlight a circuit-level pathogenic mechanism of Shank3 mutation that drives social dysfunction

Summary Many studies in systems neuroscience use head-fixation preparations for in vivo experimentation. While head-fixation confers several advantages, one major limitation is the lack of behavioral measures that quantify whole-body movements. Here, we detail a step-by-step protocol for using a novel head-fixation device that measures the forces exerted by head-fixed mice in multiple dimensions. We further detail how this system can be used in conjunction with in vivo electrophysiology and optogenetics to study dopamine neurons in the ventral tegmental area. For complete details on the use and execution of this protocol, please refer to Hughes et al. (2020a, 2020b), Graphical Abstract, Highlights • Protocol for using a novel head-fixation device to measure head and body forces in mice • Protocol allows recording from optogenetically tagged dopamine neurons • Device can identify start and end of movements for correlation with neuron activity, Many studies in systems neuroscience use head-fixation preparations for in vivo experimentation. While head fixation confers several advantages, one major limitation is the lack of behavioral measures that quantify whole-body movements. Here, we detail a step-by-step protocol for using a novel head-fixation device that measures the forces exerted by head-fixed mice in multiple dimensions. We further detail how this system can be used in conjunction with in vivo electrophysiology and optogenetics to study dopamine neurons in the ventral tegmental area.

Summary: Psychiatric disorders are highly heritable pathologies of altered neural circuit functioning. How genetic mutations lead to specific neural circuit abnormalities underlying behavioral disruptions, however, remains unclear. Using circuit-selective transgenic tools and a mouse model of maladaptive social behavior (ArpC3 mutant), we identify a neural circuit mechanism driving dysfunctional social behavior. We demonstrate that circuit-selective knockout (ctKO) of the ArpC3 gene within prefrontal cortical neurons that project to the basolateral amygdala elevates the excitability of the circuit neurons, leading to disruption of socially evoked neural activity and resulting in abnormal social behavior. Optogenetic activation of this circuit in wild-type mice recapitulates the social dysfunction observed in ArpC3 mutant mice. Finally, the maladaptive sociability of ctKO mice is rescued by optogenetically silencing neurons within this circuit. These results highlight a mechanism of how a gene-to-neural circuit interaction drives altered social behavior, a common phenotype of several psychiatric disorders.

Many studies in neuroscience use head-fixed behavioral preparations, which confer a number of advantages, including the ability to limit the behavioral repertoire and use techniques for large-scale monitoring of neural activity. But traditional studies using this approach use extremely limited behavioral measures, in part because it is difficult to detect the subtle movements and postural adjustments that animals naturally exhibit during head fixation. Here we report the development of an apparatus that is equipped with analog load cells capable of precisely monitoring the continuous forces exerted by mice. The load cells reveal the dynamic nature of movements generated not only around the time of task-relevant events, such as the regular presentation of stimuli and rewards, but also during periods of no apparent overt behavior that occur outside of the experimenter-defined events. It generates a new and rich set of behavioral measures that have been neglected in previous experiments. We detail the construction of the system, which can be 3D-printed and assembled at low cost (~$140), show behavioral results collected from head-fixed mice, and demonstrate that neural activity can be highly correlated with the subtle, whole-body movements continuously produced during head restraint.

The ability to represent time in the brain relies on network interactions across multiple brain areas. While much attention has been paid to the role of prefrontal cortex and basal ganglia in timing, little work has been aimed towards understanding the role of the thalamus in timing. To this end, we utilized pharmacological and optogenetic techniques in mice to test the role of the mediodorsal nucleus of the thalamus (MD) in interval timing. The MD is strongly connected with the prefrontal cortex and receives major inputs from the basal ganglia output nuclei. We used an operant temporal production task, in which mice indicate their expected timing of available rewards by pressing a lever in anticipation. On probe trials, inactivation of the MD with muscimol produced rightward shifts in peak pressing as well as increases in peak spread, thus significantly altering both temporal accuracy and precision. Optogenetic inhibition of glutamatergic projection neurons in the MD resulted in similar changes in timing; the shift in peak time is proportional to the duration of inhibition. The observed effects were found to be independent of significant changes in movement. Taken together, these findings provide strong evidence for the importance of thalamo-cortical projections in maintaining proper timing behavior. Significance Statement While extensive research has been dedicated to the involvement of the prefrontal cortex in interval timing behavior, the degree to which thalamic inputs contribute to timing is unclear. Using pharmacological and optogenetic inhibition of the MD, a prefrontal projecting thalamic nucleus, we demonstrate a key role of this thalamic nucleus in timing behavior.

The dominant paradigm in neuroscience, the linear causation paradigm, has led to a number of conceptual confusions about how the brain generates behavior. Because biological organisms are collections of closed loop control systems, their behaviors cannot be explained by a sequence of causes and effects. On account of simultaneous actions of forward and feedback paths in the closed loop, a major emergent property of control systems is circular causation. Methods used to analyze input-output systems are not only inadequate for understanding circular causation; they can produce misleading results. I shall use specific examples from the history of neuroscience to illustrate the fallacy of the conventional input/output analysis of behavior. I shall also discuss why a seemingly simple mechanism like negative feedback control can have complex and counterintuitive properties, and why generations of students, influenced by modern engineering conventions and the cybernetic tradition, have misunderstood the central features of control theory.

The basal ganglia are widely implicated in action selection and timing, but the relative contributions of the striatonigral (direct) and striatopallidal (indirect) pathways to these functions are still a matter of intense debate. This study investigated the effects of optogenetic stimulation of D1+ (direct) and A2A+ (indirect) neurons in the ventrolateral striatum in head-fixed mice on a fixed time reinforcement schedule. Direct pathway stimulation initiates licking, which can be frequency locked with the laser pulses. Indirect pathway stimulation suppresses licking and results in rebound licking immediately after the end of stimulation. During peak probe trials that measure the timing of expected reward, direct pathway stimulation produced a resetting of the internal timing process, as measured by timing of the next bout of anticipatory licking relative to time of stimulation. By contrast, indirect pathway stimulation transiently paused the timing mechanism, and proportionally delayed the next bout of licking. Our results provide evidence for the continuous and opposing contributions of the direct and indirect pathways in the production and timing of reward-guided behavior.

Habits have been studied for decades, but it was not until recent years that experiments began to elucidate the underlying cellular and circuit mechanisms. The latest experiments have been enabled by advances in cell-type specific monitoring and manipulation of activity in large neuronal populations. Here we will review recent efforts to understand the neural substrates underlying habit formation, focusing on rodent studies on corticostriatal circuits.

Astrocytes promote synapse formation during development via secreted factors including thrombospondin family proteins, which act through the neuronal calcium channel subunit α2δ-1. Risher et al. demonstrate that this process requires signaling via the Rho GTPase Rac1 to facilitate the maturation of dendritic spine synapses in the cortex., Astrocytes control excitatory synaptogenesis by secreting thrombospondins (TSPs), which function via their neuronal receptor, the calcium channel subunit α2δ-1. α2δ-1 is a drug target for epilepsy and neuropathic pain; thus the TSP–α2δ-1 interaction is implicated in both synaptic development and disease pathogenesis. However, the mechanism by which this interaction promotes synaptogenesis and the requirement for α2δ-1 for connectivity of the developing mammalian brain are unknown. In this study, we show that global or cell-specific loss of α2δ-1 yields profound deficits in excitatory synapse numbers, ultrastructure, and activity and severely stunts spinogenesis in the mouse cortex. Postsynaptic but not presynaptic α2δ-1 is required and sufficient for TSP-induced synaptogenesis in vitro and spine formation in vivo, but an α2δ-1 mutant linked to autism cannot rescue these synaptogenesis defects. Finally, we reveal that TSP–α2δ-1 interactions control synaptogenesis postsynaptically via Rac1, suggesting potential molecular mechanisms that underlie both synaptic development and pathology.

Summary Terrestrial locomotion presents tremendous computational challenges on account of the enormous degrees of freedom in legged animals, and complex, unpredictable properties of natural environments, including the body and its effectors, yet the nervous system can achieve locomotion with ease. Here we introduce a quadrupedal robot that is capable of posture control and goal-directed locomotion across uneven terrain. The control architecture is a hierarchical network of simple negative feedback control systems inspired by the organization of the vertebrate nervous system. This robot is capable of robust posture control and locomotion in novel environments with unpredictable disturbances. Unlike current robots, our robot does not use internal inverse and forward models, nor does it require any training in order to perform successfully in novel environments., Graphical abstract, Highlights • Inspired by a neural hierarchy with control of input at each level • Higher level specifies reference state for lower level • Successful posture control and locomotion despite unpredictable disturbances • No need for training or computation of inverse or forward kinematics, Biological sciences; Neuroscience; Sensory neuroscience; Robotics

Summary The ventral tegmental area (VTA) is a midbrain region implicated in a variety of motivated behaviors. However, the function of VTA GABAergic (Vgat+) neurons remains poorly understood. Here, using three-dimensional motion capture, in vivo electrophysiology, calcium imaging, and optogenetics, we demonstrate a novel function of VTAVgat+ neurons. We found three distinct populations of neurons, each representing head angle about a principal axis of rotation: yaw, roll, and pitch. For each axis, opponent cell groups were found that increase firing when the head moves in one direction and decrease firing in the opposite direction. Selective excitation and inhibition of VTAVgat+ neurons generate opposite rotational movements. Thus, VTAVgat+ neurons serve a critical role in the control of rotational kinematics while pursuing a moving target. This general-purpose steering function can guide animals toward desired spatial targets in any motivated behavior.

SummaryThe Ventral Tegmental Area (VTA) is a midbrain region implicated in a variety of motivated behaviors. However, the function of VTA GABAergic (Vgat+) neurons remains poorly understood. Here, using 3D motion capture, in vivo electrophysiology and calcium imaging, and optogenetics, we demonstrate a novel function of VTAVgat+ neurons. We found three distinct populations of neurons, each representing head angle about a principal axis of rotation: pitch, roll, and yaw. For each axis, opponent cell groups were found that increase firing when the head moves in one direction, and decrease firing in the opposite direction. Selective excitation and inhibition of VTAVgat+ neurons generate opposite rotational movements. The relationship between these neurons and head angle is degraded only at the time of reward consumption, at which point all head-angle related neuronal subpopulations show indistinguishable reward-related responses. Thus, VTAVgat+ neurons serve a critical role in the control of rotational kinematics while pursuing a moving target. This general-purpose steering function can guide animals toward desired spatial targets in any motivated behavior.

Choice behavior is characterized by temporal discounting, i.e., preference for immediate rewards over delayed rewards. Temporal discounting is often dysfunctional in psychiatric disorders, addiction, and eating disorders. However, the underlying neural mechanisms governing temporal discounting are still poorly understood. We found that food deprivation resulted in steep temporal discounting of food rewards, whereas satiation abolished discounting. In addition, optogenetic activation of AgRP-expressing neurons in the arcuate nucleus or their axon terminals in the posterior bed nucleus of stria terminalis (BNST) restored temporal discounting in sated mice. Activation of postsynaptic neuropeptide Y receptors (Y1Rs) within the BNST, which is influenced by neuropeptide released by AgRP neurons, was sufficient to restore temporal discounting. These results demonstrate for the first time a profound effect of motivational signals from hypothalamic feeding circuits on temporal discounting and reveal a novel neural circuit that regulates choice behavior.

Summary: Huntington’s disease (HD) is caused by an autosomal dominant polyglutamine expansion mutation of Huntingtin (HTT). HD patients suffer from progressive motor, cognitive, and psychiatric impairments, along with significant degeneration of the striatal projection neurons (SPNs) of the striatum. HD is widely accepted to be caused by a toxic gain-of-function of mutant HTT. However, whether loss of HTT function, because of dominant-negative effects of the mutant protein, plays a role in HD and whether HTT is required for SPN health and function are not known. Here, we delete Htt from specific subpopulations of SPNs using the Cre-Lox system and find that SPNs require HTT for motor regulation, synaptic development, cell health, and survival during aging. Our results suggest that loss of HTT function in SPNs could play a critical role in HD pathogenesis. : Burrus et al. show that striatal projection neurons require Huntingtin, the gene mutated in Huntington’s disease, for normal synaptic connectivity, regulated gene expression, and neuronal survival with aging. Loss of Huntingtin from striatal neurons recapitulates several features of Huntington’s disease pathology, an important consideration for therapies non-specifically targeting Huntingtin expression. Keywords: Huntington's Disease, basal ganglia, striatum, synaptic connectivity, neuronal survival

The basal ganglia (BG) are the major subcortical nuclei in the brain. Disorders implicating the BG are characterized by diverse symptoms, but it remains unclear what these symptoms have in common or how they can be explained by changes in the BG circuits. This review summarizes recent findings that not only question traditional assumptions about the role of the BG in movement but also elucidate general computations performed by these circuits. To explain these findings, a new conceptual framework is introduced for understanding the role of the BG in behavior. According to this framework, the cortico-BG networks implement transition control in an extended hierarchy of closed loop negative feedback control systems. The transition control model provides a solution to the posture/movement problem, by postulating that BG outputs send descending signals to alter the reference states of downstream position control systems for orientation and body configuration. It also explains major neurological symptoms associated with BG pathology as a result of changes in system parameters such as multiplicative gain and damping.

This review is an attempt to explain the role of basal ganglia (BG) outputs in generating movements. Recent work showed that opponent outputs from the BG represent instantaneous body position coordinates during behavior. On the other hand, projection neurons in the striatum, the major input nucleus, as well as dopaminergic neurons that form the nigrostriatal pathway, can represent movement velocity. To explain these findings, a new model is proposed, in which the BG implement the level of transition control in an extended control hierarchy. BG outputs represent descending reference signals that command diverse lower-level position controllers. This model not only explains major neurological symptoms but also makes quantitative and testable predictions.

The contribution of basal ganglia outputs to consummatory behavior remains poorly understood. We recorded from the substantia nigra pars reticulata (SNR), the major basal ganglia output nucleus, during self-initiated drinking in mice. The firing rates of many lateral SNR neurons were time-locked to individual licks. These neurons send GABAergic projections to the deep layers of the orofacial region of the lateral tectum (superior colliculus, SC). Many tectal neurons were also time-locked to licking, but their activity was usually in antiphase with that of SNR neurons, suggesting inhibitory nigrotectal projections. We used optogenetics to selectively activate the GABAergic nigrotectal afferents in the deep layers of the SC. Photo-stimulation of the nigrotectal projections transiently inhibited the activity of the lick-related tectal neurons, disrupted their licking-related oscillatory pattern and suppressed self-initiated drinking. These results demonstrate that GABAergic nigrotectal projections have a crucial role in coordinating drinking behavior.

The basal ganglia have long been implicated in action initiation. Using three-dimensional motion capture, we quantified the effects of optogenetic stimulation of the striatonigral (direct) pathway on movement kinematics. We generated transgenic mice with channelrhodopsin-2 expression in striatal neurons that express the D1-like dopamine receptor. With optic fibres placed in the sensorimotor striatum, an area known to contain movement velocity-related single units, photo-stimulation reliably produced movements that could be precisely quantified with our motion capture programme. A single light pulse was sufficient to elicit movements with short latencies (< 30 ms). Increasing stimulation frequency increased movement speed, with a highly linear relationship. These findings support the hypothesis that the sensorimotor striatum is part of a velocity controller that controls rate of change in body configurations.

The tight balance between synaptic excitation and inhibition (E/I) within neocortical circuits in the mammalian brain is important for complex behavior. Many loss-of-function studies have demonstrated that brain-derived neurotrophic factor (BDNF) and its cognate receptor tropomyosin receptor kinase B (TrkB) are essential for the development of inhibitory GABAergic neurons. However, behavioral consequences of impaired BDNF/TrkB signaling in GABAergic neurons remain unclear, largely due to confounding motor function deficits observed in previous animal models. In this study, we generated conditional knockout mice (TrkB cKO) in which TrkB was ablated from a majority of corticolimbic GABAergic interneurons postnatally. These mice showed intact motor coordination and movement, but exhibited enhanced dominance over other mice in a group-housed setting. In addition, immature fast-spiking GABAergic neurons of TrkB cKO mice resulted in an E/I imbalance in layer 5 microcircuits within the medial prefrontal cortex (mPFC), a key region regulating social dominance. Restoring the E/I imbalance via optogenetic modulation in the mPFC of TrkB cKO mice normalized their social dominance behavior. Taken together, our results provide strong evidence for a role of BDNF/TrkB signaling in inhibitory synaptic modulation and social dominance behavior in mice.

The parafascicular nucleus (Pf) of the thalamus sends major projections to the basal ganglia, a group of subcortical nuclei involved in action initiation and selection. Here, we used optogenetics, 3D motion capture, in vivo electrophysiology, and viral-based neuroanatomical tracing to examine the contribution of the Pf to volitional movements in mice. We discovered that Pf neurons are highly correlated with movement velocity. Stimulation of glutamatergic (Vglut2+) neurons in the Pf generates turning and orienting movements. This effect was not due to Pf projections to the striatum, but to its projections to the subthalamic nucleus (STN), a major target of deep brain stimulation (DBS) for Parkinson9s disease (PD) patients. Moreover, selective excitation of either Pf Vglut2+ terminals in the STN or Pf cell bodies that send glutamatergic projections to the STN can restore natural movements in a bilateral mouse model of PD with complete akinesia. Our results reveal a novel thalamo-subthalamic pathway regulating movement initiation, and demonstrate a circuit mechanism that could explain the clinical efficacy of DBS for relief of PD motor symptoms.

We previously reported a new line of Shank3 mutant mice which led to a complete loss of Shank3 by deleting exons 4−22 (Δe4−22) globally. Δe4−22 mice display robust ASD-like behaviors including impaired social interaction and communication, increased stereotypical behavior and excessive grooming, and a profound deficit in instrumental learning. However, the anatomical and neural circuitry underlying these behaviors are unknown. We generated mice with Shank3 selectively deleted in forebrain, striatum, and striatal D1 and D2 cells. These mice were used to interrogate the circuit/brain-region and cell-type specific role of Shank3 in the expression of autism-related behaviors. Whole-cell patch recording and biochemical analyses were used to study the synaptic function and molecular changes in specific brain regions. We found perseverative exploratory behaviors in mice with deletion of Shank3 in striatal inhibitory neurons. Conversely, self-grooming induced lesions were observed in mice with deletion of Shank3 in excitatory neurons of forebrain. However, social, communicative, and instrumental learning behaviors were largely unaffected in these mice, unlike what is seen in global Δe4−22 mice. We discovered unique patterns of change for the biochemical and electrophysiological findings in respective brain regions that reflect the complex nature of transcriptional regulation of Shank3. Reductions in Homer1b/c and membrane hyper-excitability were observed in striatal loss of Shank3. By comparison, Shank3 deletion in hippocampal neurons resulted in increased NMDAR-currents and GluN2B-containing NMDARs. These results together suggest that Shank3 may differentially regulate neural circuits that control behavior. Our study supports a dissociation of Shank3 functions in cortical and striatal neurons in ASD-related behaviors, and it illustrates the complexity of neural circuit mechanisms underlying these behaviors.