Selective Activation of Subthalamic Nucleus Output Quantitatively Scales Movements.

The subthalamic nucleus (STN) is a common target for deep brain stimulation (DBS) treatments of Parkinsonian motor symptoms. According to the dominant model, the STN output can suppress movement by enhancing inhibitory basal ganglia (BG) output via the indirect pathway, and disrupting STN output using DBS can restore movement in Parkinson's patients. But the mechanisms underlying STN DBS remain poorly understood, as previous studies usually relied on electrical stimulation, which cannot selectively target STN output neurons. Here, we selectively stimulated STN projection neurons using optogenetics and quantified behavior in male and female mice using 3D motion capture. STN stimulation resulted in movements with short latencies (10-15 ms). A single pulse of light was sufficient to generate movement, and there was a highly linear relationship between stimulation frequency and kinematic measures. Unilateral stimulation caused movement in the ipsiversive direction (toward the side of stimulation) and quantitatively determined head yaw and head roll, while stimulation of either STN raises the head (pitch). Bilateral stimulation does not cause turning but raised the head twice as high as unilateral stimulation of either STN. Optogenetic stimulation increased the firing rate of STN neurons in a frequency-dependent manner, and the increased firing is responsible for stimulation-induced movements. Finally, stimulation of the STN's projection to the brainstem mesencephalic locomotor region was sufficient to reproduce the behavioral effects of STN stimulation. These results question the common assumption that the STN suppresses movement, and instead suggest that STN output can precisely specify action parameters via direct projections to the brainstem.SIGNIFICANCE STATEMENT Our results question the common assumption that the subthalamic nucleus (STN) suppresses movement, and instead suggest that STN output can precisely specify action parameters via direct projections to the brainstem.

Training-induced circuit-specific excitatory synaptogenesis in mice is required for effort control.

Synaptogenesis is essential for circuit development; however, it is unknown whether it is critical for the establishment and performance of goal-directed voluntary behaviors. Here, we show that operant conditioning via lever-press for food reward training in mice induces excitatory synapse formation onto a subset of anterior cingulate cortex neurons projecting to the dorsomedial striatum (ACC→DMS). Training-induced synaptogenesis is controlled by the Gabapentin/Thrombospondin receptor α2δ-1, which is an essential neuronal protein for proper intracortical excitatory synaptogenesis. Using germline and conditional knockout mice, we found that deletion of α2δ-1 in the adult ACC→DMS circuit diminishes training-induced excitatory synaptogenesis. Surprisingly, this manipulation does not impact learning but results in a significant increase in effort exertion without affecting sensitivity to reward value or changing contingencies. Bidirectional optogenetic manipulation of ACC→DMS neurons rescues or phenocopies the behaviors of the α2δ-1 cKO mice, highlighting the importance of synaptogenesis within this cortico-striatal circuit in regulating effort exertion.

Cortical regulation of neurogenesis and cell proliferation in the ventral subventricular zone.

Neurogenesis and differentiation of neural stem cells (NSCs) are controlled by cell-intrinsic molecular pathways that interact with extrinsic signaling cues. In this study, we identify a circuit that regulates neurogenesis and cell proliferation in the lateral ventricle-subventricular zone (LV-SVZ). Our results demonstrate that direct glutamatergic projections from the anterior cingulate cortex (ACC), as well as inhibitory projections from calretinin+ local interneurons, modulate the activity of cholinergic neurons in the subependymal zone (subep-ChAT+). Furthermore, in vivo optogenetic stimulation and inhibition of the ACC-subep-ChAT+ circuit are sufficient to control neurogenesis in the ventral SVZ. Both subep-ChAT+ and local calretinin+ neurons play critical roles in regulating ventral SVZ neurogenesis and LV-SVZ cell proliferation.

The role of the parafascicular thalamic nucleus in action initiation and steering.

The parafascicular (Pf) nucleus of the thalamus has been implicated in arousal and attention, but its contributions to behavior remain poorly characterized. Here, using in vivo and in vitro electrophysiology, optogenetics, and 3D motion capture, we studied the role of the Pf nucleus in behavior using a continuous reward-tracking task in freely moving mice. We found that many Pf neurons precisely represent vector components of velocity, with a strong preference for ipsiversive movements. Their activity usually leads velocity, suggesting that Pf output is critical for self-initiated orienting behavior. To test this hypothesis, we expressed excitatory or inhibitory opsins in VGlut2+ Pf neurons to manipulate neural activity bidirectionally. We found that selective optogenetic stimulation of these neurons consistently produced ipsiversive head turning, whereas inhibition stopped turning and produced downward movements. Taken together, our results suggest that the Pf nucleus can send continuous top-down commands that specify detailed action parameters (e.g., direction and speed of the head), thus providing guidance for orienting and steering during behavior.

Elucidating a locus coeruleus-dentate gyrus dopamine pathway for operant reinforcement.

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 dentate gyrus in supporting operant reinforcement.

A one-photon endoscope for simultaneous patterned optogenetic stimulation and calcium imaging in freely behaving mice.

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.

Striatal mechanisms of turning behaviour following unilateral dopamine depletion in mice.

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.

Neural circuit pathology driven by Shank3 mutation disrupts social behaviors.

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.

Achieving natural behavior in a robot using neurally inspired hierarchical perceptual control.

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.

Thalamic projections to the subthalamic nucleus contribute to movement initiation and rescue of parkinsonian symptoms.

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.

Hypothalamic-Extended Amygdala Circuit Regulates Temporal Discounting.

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 STATEMENT Temporal 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.

Protocol for Recording from Ventral Tegmental Area Dopamine Neurons in Mice while Measuring Force during Head-Fixation.

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).

Dysregulation of the Synaptic Cytoskeleton in the PFC Drives Neural Circuit Pathology, Leading to Social Dysfunction.

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.

Mediodorsal Thalamus Contributes to the Timing of Instrumental Actions.

The perception of time is critical to adaptive behavior. While prefrontal cortex and basal ganglia have been implicated in interval timing in the seconds to minutes range, little is known about the role of the mediodorsal thalamus (MD), which is a key component of the limbic cortico-basal ganglia-thalamocortical loop. In this study, we tested the role of the MD in timing, using an operant temporal production task in male mice. In this task, that the expected timing of available rewards is indicated by lever pressing. Inactivation of the MD with muscimol produced rightward shifts in peak pressing on probe trials as well as increases in peak spread, thus significantly altering both temporal accuracy and precision. Optogenetic inhibition of glutamatergic projection neurons in the MD also resulted in similar changes in timing. The observed effects were found to be independent of significant changes in movement. Our findings suggest that the MD is a critical component of the neural circuit for interval timing, without playing a direct role in regulating ongoing performance.SIGNIFICANCE STATEMENT The mediodorsal nucleus (MD) of the thalamus is strongly connected with the prefrontal cortex and basal ganglia, areas which have been implicated in interval timing. Previous work has shown that the MD contributes to working memory and learning of action-outcome contingencies, but its role in behavioral timing is poorly understood. Using an operant temporal production task, we showed that inactivation of the MD significantly impaired timing behavior.

Ventral Tegmental Dopamine Neurons Control the Impulse Vector during Motivated Behavior.

The ventral tegmental area (VTA) is a major source of dopamine, especially to the limbic brain regions. Despite decades of research, the function of VTA dopamine neurons remains controversial. Here, using a novel head-fixed behavioral system with five orthogonal force sensors, we show for the first time that the activity of dopamine neurons precisely represents the impulse vector (force exerted over time) generated by the animal. Distinct populations of VTA dopamine neurons contribute to components of the impulse vector in different directions. Optogenetic excitation of these neurons shows a linear relationship between signal injected and impulse generated. Optogenetic inhibition paused force generation or produced force in the backward direction. At the same time, these neurons also regulate the initiation and execution of anticipatory licking. Our results indicate that VTA dopamine controls the magnitude, direction, and duration of force used to move toward or away from any motivationally relevant stimuli.

Opponent regulation of action performance and timing by striatonigral and striatopallidal pathways.

The basal ganglia have been implicated in action selection and timing, but the relative contributions of the striatonigral (direct) and striatopallidal (indirect) pathways to these functions remain unclear. We 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, whereas indirect pathway stimulation suppresses licking and results in rebound licking after stimulation. Moreover, direct and indirect pathways also play distinct roles in timing. Direct pathway stimulation produced a resetting of the internal timing process, whereas indirect pathway stimulation transiently paused timing, 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.

A Head-Fixation System for Continuous Monitoring of Force Generated During Behavior.

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 a new head-fixed setup 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 presentation of stimuli and rewards, but also during periods in between these events, when there is no apparent overt behavior. 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, 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.

Striatal Projection Neurons Require Huntingtin for Synaptic Connectivity and Survival.

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.