Topographic connectivity and cellular profiling reveal detailed input pathways and functionally distinct cell types in the subthalamic nucleus

Movement-related increases in subthalamic activity optimize locomotion

The subthalamic nucleus (STN) is traditionally thought to restrict movement. Lesion or prolonged STN inhibition increases movement vigor and propensity, while optogenetic excitation has opposing effects. However, STN neurons often exhibit movement-related increases in firing. To address this paradox, STN activity was recorded and manipulated in head-fixed mice at rest and during self-initiated and self-paced treadmill locomotion. We found that (1) most STN neurons (type 1) exhibit locomotion-dependent increases in activity, with half firing preferentially during the propulsive phase of the contralateral locomotor cycle; (2) a minority of STN neurons exhibit dips in activity or are uncorrelated with movement; (3) brief optogenetic inhibition of the lateral STN (where type 1 neurons are concentrated) slows and prematurely terminates locomotion; and (4) in Q175 Huntington's disease mice, abnormally brief, low-velocity locomotion is associated with type 1 hypoactivity. Together, these data argue that movement-related increases in STN activity contribute to optimal locomotor performance.

Connecto-informatics at the mesoscale: current advances in image processing and analysis for mapping the brain connectivity

Mapping neural connections within the brain has been a fundamental goal in neuroscience to understand better its functions and changes that follow aging and diseases. Developments in imaging technology, such as microscopy and labeling tools, have allowed researchers to visualize this connectivity through high-resolution brain-wide imaging. With this, image processing and analysis have become more crucial. However, despite the wealth of neural images generated, access to an integrated image processing and analysis pipeline to process these data is challenging due to scattered information on available tools and methods. To map the neural connections, registration to atlases and feature extraction through segmentation and signal detection are necessary. In this review, our goal is to provide an updated overview of recent advances in these image-processing methods, with a particular focus on fluorescent images of the mouse brain. Our goal is to outline a pathway toward an integrated image-processing pipeline tailored for connecto-informatics. An integrated workflow of these image processing will facilitate researchers' approach to mapping brain connectivity to better understand complex brain networks and their underlying brain functions. By highlighting the image-processing tools available for fluroscent imaging of the mouse brain, this review will contribute to a deeper grasp of connecto-informatics, paving the way for better comprehension of brain connectivity and its implications.

Arkypallidal neurons in the external globus pallidus can mediate inhibitory control by altering competition in the striatum

Reactive inhibitory control is crucial for survival. Traditionally, this control in mammals was attributed solely to the hyperdirect pathway, with cortical control signals flowing unidirectionally from the subthalamic nucleus (STN) to basal ganglia output regions. Yet recent findings have put this model into question, suggesting that the STN is assisted in stopping actions through ascending control signals to the striatum mediated by the external globus pallidus (GPe). Here we investigate this suggestion by harnessing a biologically-constrained spiking model of the corticobasal ganglia-thalamic (CBGT) circuit that includes pallidostriatal pathways originating from arkypallidal neurons. Through a series of experiments probing the interaction between three critical inhibitory nodes (the STN, arkypallidal cells, and indirect path-way spiny projection neurons), we find that the GPe acts as a critical mediator of both ascending and descending inhibitory signals in the CBGT circuit. In particular, pallidostriatal pathways regulate this process by weakening the direct pathway dominance of the evidence accumulation process driving decisions, which increases the relative suppressive influence of the indirect pathway on basal ganglia output. These findings delineate how pallidostriatal pathways can facilitate action cancellation by managing the bidirectional flow of information within CBGT circuits.

Rethinking the external globus pallidus and information flow in cortico-basal ganglia-thalamic circuits

For decades, the external globus pallidus (GPe) has been viewed as a passive way-station in the indirect pathway of the cortico-basal ganglia-thalamic (CBGT) circuit, sandwiched between striatal inputs and basal ganglia outputs. According to this model, one-way descending striatal signals in the indirect pathway amplify the suppression of downstream thalamic nuclei by inhibiting GPe activity. Here, we revisit this assumption, in light of new and emerging work on the cellular complexity, connectivity and functional role of the GPe in behaviour. We show how, according to this new circuit-level logic, the GPe is ideally positioned for relaying ascending and descending control signals within the basal ganglia. Focusing on the problem of inhibitory control, we illustrate how this bidirectional flow of information allows for the integration of reactive and proactive control mechanisms during action selection. Taken together, this new evidence points to the GPe as being a central hub in the CBGT circuit, participating in bidirectional information flow and linking multifaceted control signals to regulate behaviour.

The anterior cingulate cortex controls the hyperactivity in subthalamic neurons in male mice with comorbid chronic pain and depression

Neurons in the subthalamic nucleus (STN) become hyperactive following nerve injury and promote pain-related responses in mice. Considering that the anterior cingulate cortex (ACC) is involved in pain and emotion processing and projects to the STN, we hypothesize that ACC neurons may contribute to hyperactivity in STN neurons in chronic pain. In the present study, we showed that ACC neurons enhanced activity in response to noxious stimuli and to alterations in emotional states and became hyperactive in chronic pain state established by spared nerve injury of the sciatic nerve (SNI) in mice. In naïve mice, STN neurons were activated by noxious stimuli, but not by alterations in emotional states. Pain responses in STN neurons were attenuated in both naïve and SNI mice when ACC neurons were inhibited. Furthermore, optogenetic activation of the ACC-STN pathway induced bilateral hyperalgesia and depression-like behaviors in naive mice; conversely, inhibition of this pathway is sufficient to attenuate hyperalgesia and depression-like behaviors in SNI mice and naïve mice subjected to stimulation of STN neurons. Finally, mitigation of pain-like and depression-like behaviors in SNI mice by inhibition of the ACC-STN projection was eliminated by activation of STN neurons. Our results demonstrate that hyperactivity in the ACC-STN pathway may be an important pathophysiology in comorbid chronic pain and depression. Thus, the ACC-STN pathway may be an intervention target for the treatment of the comorbid chronic pain and depression.

Corticostriatal pathways for bilateral sensorimotor functions

Corticostriatal pathways are essential for a multitude of motor, sensory, cognitive, and affective functions. They are mediated by cortical pyramidal neurons, roughly divided into two projection classes: the pyramidal tract (PT) and the intratelencephalic tract (IT). These pathways have been the focus of numerous studies in recent years, revealing their distinct structural and functional properties. Notably, their synaptic connectivity within ipsi- and contralateral cortical and striatal microcircuits is characterized by a high degree of target selectivity, providing a means to regulate the local neuromodulatory landscape in the striatum. Here, we discuss recent findings regarding the functional organization of the PT and IT corticostriatal pathways and its implications for bilateral sensorimotor functions.

Neuronal and synaptic adaptations underlying the benefits of deep brain stimulation for Parkinson's disease

Deep brain stimulation (DBS) is a well-established and effective treatment for patients with advanced Parkinson's disease (PD), yet its underlying mechanisms remain enigmatic. Optogenetics, primarily conducted in animal models, provides a unique approach that allows cell type- and projection-specific modulation that mirrors the frequency-dependent stimulus effects of DBS. Opto-DBS research in animal models plays a pivotal role in unraveling the neuronal and synaptic adaptations that contribute to the efficacy of DBS in PD treatment. DBS-induced neuronal responses rely on a complex interplay between the distributions of presynaptic inputs, frequency-dependent synaptic depression, and the intrinsic excitability of postsynaptic neurons. This orchestration leads to conversion of firing patterns, enabling both antidromic and orthodromic modulation of neural circuits. Understanding these mechanisms is vital for decoding position- and programming-dependent effects of DBS. Furthermore, patterned stimulation is emerging as a promising strategy yielding long-lasting therapeutic benefits. Research on the neuronal and synaptic adaptations to DBS may pave the way for the development of more enduring and precise modulation patterns. Advanced technologies, such as adaptive DBS or directional electrodes, can also be integrated for circuit-specific neuromodulation. These insights hold the potential to greatly improve the effectiveness of DBS and advance PD treatment to new levels.

A role for the subthalamic nucleus in aversive learning

The subthalamic nucleus (STN) is critical for behavioral control; its dysregulation consequently correlated with neurological and neuropsychiatric disorders, including Parkinson's disease. Deep brain stimulation (DBS) targeting the STN successfully alleviates parkinsonian motor symptoms. However, low mood and depression are affective side effects. STN is adjoined with para-STN, associated with appetitive and aversive behavior. DBS aimed at STN might unintentionally modulate para-STN, causing aversion. Alternatively, the STN mediates aversion. To investigate causality between STN and aversion, affective behavior is addressed using optogenetics in mice. Selective promoters allow dissociation of STN (e.g., Pitx2) vs. para-STN (Tac1). Acute photostimulation results in aversion via both STN and para-STN. However, only STN stimulation-paired cues cause conditioned avoidance and only STN stimulation interrupts on-going sugar self-administration. Electrophysiological recordings identify post-synaptic responses in pallidal neurons, and selective photostimulation of STN terminals in the ventral pallidum replicates STN-induced aversion. Identifying STN as a source of aversive learning contributes neurobiological underpinnings to emotional affect.

The transcription regulator Lmo3 is required for the development of medial ganglionic eminence derived neurons in the external globus pallidus

The external globus pallidus (GPe) is an essential component of the basal ganglia, a group of subcortical nuclei that are involved in control of action. Changes in the firing of GPe neurons are associated with both passive and active body movements. Aberrant activity of GPe neurons has been linked to motor symptoms of a variety of movement disorders, such as Parkinson's Disease, Huntington's disease and dystonia. Recent studies have helped delineate functionally distinct subtypes of GABAergic GPe projection neurons. However, not much is known about specific molecular mechanisms underlying the development of GPe neuronal subtypes. We show that the transcriptional regulator Lmo3 is required for the development of medial ganglionic eminence derived Nkx2.1+ and PV+ GPe neurons, but not lateral ganglionic eminence derived FoxP2+ neurons. As a consequence of the reduction in PV+ neurons, Lmo3-null mice have a reduced GPe input to the subthalamic nucleus.

Dopaminergic Dependency of Cholinergic Pallidal Neurons

We employed the whole-cell patch-clamp method and ChAT-Cre mice to study the electrophysiological attributes of cholinergic neurons in the external globus pallidus. Most neurons were inactive, although approximately 20% displayed spontaneous firing, including burst firing. The resting membrane potential, the whole neuron input resistance, the membrane time constant and the total neuron membrane capacitance were also characterized. The current-voltage relationship showed time-independent inward rectification without a "sag". Firing induced by current injections had a brief initial fast adaptation followed by tonic firing with minimal accommodation. Intensity-frequency plots exhibited maximal average firing rates of about 10 Hz. These traits are similar to those of some cholinergic neurons in the basal forebrain. Also, we examined their dopamine sensitivity by acutely blocking dopamine receptors. This action demonstrated that the membrane potential, excitability, and firing pattern of pallidal cholinergic neurons rely on the constitutive activity of dopamine receptors, primarily D2-class receptors. The blockade of these receptors induced a resting membrane potential hyperpolarization, a decrease in firing for the same stimulus, the disappearance of fast adaptation, and the emergence of a depolarization block. This shift in physiological characteristics was evident even when the hyperpolarization was corrected with D.C. current. Neither the currents that generate the action potentials nor those from synaptic inputs were responsible. Instead, our findings suggest, that subthreshold slow ion currents, that require further investigation, are the target of this novel dopaminergic signaling.

Synaptic network structure shapes cortically evoked spatio-temporal responses of STN and GPe neurons in a computational model

Introduction

The basal ganglia (BG) are involved in motor control and play an essential role in movement disorders such as hemiballismus, dystonia, and Parkinson's disease. Neurons in the motor part of the BG respond to passive movement or stimulation of different body parts and to stimulation of corresponding cortical regions. Experimental evidence suggests that the BG are organized somatotopically, i.e., specific areas of the body are associated with specific regions in the BG nuclei. Signals related to the same body part that propagate along different pathways converge onto the same BG neurons, leading to characteristic shapes of cortically evoked responses. This suggests the existence of functional channels that allow for the processing of different motor commands or information related to different body parts in parallel. Neurological disorders such as Parkinson's disease are associated with pathological activity in the BG and impaired synaptic connectivity, together with reorganization of somatotopic maps. One hypothesis is that motor symptoms are, at least partly, caused by an impairment of network structure perturbing the organization of functional channels.

Methods

We developed a computational model of the STN-GPe circuit, a central part of the BG. By removing individual synaptic connections, we analyzed the contribution of signals propagating along different pathways to cortically evoked responses. We studied how evoked responses are affected by systematic changes in the network structure. To quantify the BG's organization in the form of functional channels, we suggested a two-site stimulation protocol.

Results

Our model reproduced the cortically evoked responses of STN and GPe neurons and the contributions of different pathways suggested by experimental studies. Cortical stimulation evokes spatio-temporal response patterns that are linked to the underlying synaptic network structure. Our two-site stimulation protocol yielded an approximate functional channel width.

Discussion/conclusion

The presented results provide insight into the organization of BG synaptic connectivity, which is important for the development of computational models. The synaptic network structure strongly affects the processing of cortical signals and may impact the generation of pathological rhythms. Our work may motivate further experiments to analyze the network structure of BG nuclei and their organization in functional channels.

Cell and circuit complexity of the external globus pallidus

The external globus pallidus (GPe) of the basal ganglia has been underappreciated owing to poor understanding of its cells and circuits. It was assumed that the GPe consisted of a homogeneous neuron population primarily serving as a 'relay station' for information flowing through the indirect basal ganglia pathway. However, the advent of advanced tools in rodent models has sparked a resurgence in interest in the GPe. Here, we review recent data that have unveiled the cell and circuit complexity of the GPe. These discoveries have revealed that the GPe does not conform to traditional views of the basal ganglia. In particular, recent evidence confirms that the afferent and efferent connections of the GPe span both the direct and the indirect pathways. Furthermore, the GPe displays broad interconnectivity beyond the basal ganglia, consistent with its emerging multifaceted roles in both motor and non-motor functions. In summary, recent data prompt new proposals for computational rules of the basal ganglia.

Single-neuron bursts encode pathological oscillations in subcortical nuclei of patients with Parkinson's disease and essential tremor

Deep brain stimulation procedures offer an invaluable opportunity to study disease through intracranial recordings from awake patients. Here, we address the relationship between single-neuron and aggregate-level (local field potential; LFP) activities in the subthalamic nucleus (STN) and thalamic ventral intermediate nucleus (Vim) of patients with Parkinson's disease (n = 19) and essential tremor (n = 16), respectively. Both disorders have been characterized by pathologically elevated LFP oscillations, as well as an increased tendency for neuronal bursting. Our findings suggest that periodic single-neuron bursts encode both pathophysiological beta (13 to 33 Hz; STN) and tremor (4 to 10 Hz; Vim) LFP oscillations, evidenced by strong time-frequency and phase-coupling relationships between the bursting and LFP signals. Spiking activity occurring outside of bursts had no relationship to the LFP. In STN, bursting activity most commonly preceded the LFP oscillation, suggesting that neuronal bursting generated within STN may give rise to an aggregate-level LFP oscillation. In Vim, LFP oscillations most commonly preceded bursting activity, suggesting that neuronal firing may be entrained by periodic afferent inputs. In both STN and Vim, the phase-coupling relationship between LFP and high-frequency oscillation (HFO) signals closely resembled the relationships between the LFP and single-neuron bursting. This suggests that periodic single-neuron bursting is likely representative of a higher spatial and temporal resolution readout of periodic increases in the amplitude of HFOs, which themselves may be a higher resolution readout of aggregate-level LFP oscillations. Overall, our results may reconcile "rate" and "oscillation" models of Parkinson's disease and shed light on the single-neuron basis and origin of pathophysiological oscillations in movement disorders.