Cell-Type-Specific Control of Brainstem Locomotor Circuits by Basal Ganglia

Inhibitory basal ganglia nuclei differentially innervate pedunculopontine nucleus subpopulations and evoke opposite motor and valence behaviors

The canonical basal ganglia model predicts that the substantia nigra pars reticulata (SNr) and the globus pallidus externa (GPe) will have specific effects on locomotion: the SNr inhibiting locomotion and the GPe enhancing it. In this manuscript, we use in vivo optogenetics to show that a projection-defined neural subpopulation within each structure exerts non-canonical effects on locomotion. These non-canonical subpopulations are defined by their projection to the pedunculopontine nucleus (PPN) and mediate opposing effects on reward. To understand how these structures differentially modulate the PPN, we use ex vivo whole-cell recording with optogenetics to comprehensively dissect the SNr and GPe connections to regionally- and molecularly-defined populations of PPN neurons. The SNr inhibits all PPN subtypes, but most strongly inhibits caudal glutamatergic neurons. The GPe selectively inhibits caudal glutamatergic and GABAergic neurons, avoiding both cholinergic and rostral cells. This circuit characterization reveals non-canonical basal ganglia pathways for locomotion and valence.

Why so slow? Models of parkinsonian bradykinesia

Bradykinesia, or slowness of movement, is a defining feature of Parkinson disease (PD) and a major contributor to the negative effects on quality of life associated with this disorder and related conditions. A dominant pathophysiological model of bradykinesia in PD has existed for approximately 30 years and has been the basis for the development of several therapeutic interventions, but accumulating evidence has made this model increasingly untenable. Although more recent models have been proposed, they also appear to be flawed. In this Perspective, I consider the leading prior models of bradykinesia in PD and argue that a more functionally related model is required, one that considers changes that disrupt the fundamental process of accurate information transmission. In doing so, I review emerging evidence of network level functional connectivity changes, information transfer dysfunction and potential motor code transmission error and present a novel model of bradykinesia in PD that incorporates this evidence. I hope that this model may reconcile inconsistencies in its predecessors and encourage further development of therapeutic interventions.

Muscarinic receptor activation preferentially inhibits rebound in vulnerable dopaminergic neurons

Dopaminergic subpopulations of the substantia nigra pars compacta (SNc) differentially degenerate in Parkinson's disease and are characterized by unique electrophysiological properties. The vulnerable population expresses a T-type calcium channel-mediated afterdepolarization (ADP) and shows rebound activity upon release from inhibition, whereas the resilient population does not have an ADP and is slower to fire after hyperpolarization. This rebound activity can trigger dopamine release in the striatum, an important component of basal ganglia function. Using whole-cell patch clamp electrophysiology on ex vivo slices from adult mice of both sexes, we find that muscarinic activation with the non-selective muscarinic agonist Oxotremorine inhibits rebound activity more strongly in vulnerable vs resilient SNc neurons. Here, we show that this effect depends on the direct activation of muscarinic receptors on the SNc dopaminergic neurons. Through a series of pharmacological and transgenic knock-out experiments, we tested whether the muscarinic inhibition of rebound was mediated through the canonical rebound-related ion channels: T-type calcium channels, hyperpolarization-activated cation channels (HCN), and A-type potassium channels. We find that muscarinic receptor activation inhibits HCN-mediated current (Ih) in vulnerable SNc neurons, but that Ih activity is not necessary for the muscarinic inhibition of rebound activity. Similarly, we find that Oxotremorine inhibits rebound activity independently of T-type calcium channels and A-type potassium channels. Together these findings reveal new principles governing acetylcholine and dopamine interactions, showing that muscarinic receptors directly affect SNc rebound activity in the midbrain at the somatodendritic level and differentially modify information processing in distinct SNc subpopulations.

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.

Three paradoxes related to the mode of action of pramipexole: The path from D2/D3 dopamine receptor stimulation to modification of dopamine-modulated functions

Pramipexole, a D2/D3 dopamine receptor agonist, is used to treat the motor symptoms of Parkinson's disease, caused by degeneration of the dopaminergic nigrostriatal pathway. There are three paradoxes associated with its mode of action. Firstly, stimulation of D2/D3 receptors leads to neuronal inhibition, although pramipexole does not inhibit but promotes some dopamine-modulated functions, such as locomotion and reinforcement. Secondly, another dopamine-modulated function, arousal, is not promoted but inhibited by pramipexole, leading to sedation. Thirdly, pramipexole-evoked sedation is associated with an increase in pupil diameter, although sedation is expected to cause pupil constriction. To resolve these paradoxes, the path from stimulation of D2/D3 receptors to the modification of dopamine-modulated functions has been tracked. The functions considered are modulated by midbrain dopaminergic nuclei: locomotion - substantia nigra pars compacta (SNc), reinforcement/motivation - ventral tegmental area (VTA), sympathetic activity (as reflected in pupil function) - VTA; arousal - ventral periaqueductal grey (vPAG), with contributions from VTA and SNc. The application of genetics-based molecular techniques (optogenetics and chemogenetics) has enabled tracing the chains of neurones from the dopaminergic nuclei to their final targets executing the functions. The functional neuronal circuits linked to the D2/D3 receptors in the dorsal and ventral striata, stimulated by inputs from SNc and VTA, respectively, may explain how neuronal inhibition induced by pramipexole is translated into the promotion of locomotion, reinforcement/motivation and sympathetic activity. As the vPAG may increase arousal mainly by stimulating cortical D1 dopamine receptors, pramipexole would stimulate only presynaptic D2/D3 receptors on vPAG neurones, curtailing their activity and leading to sedation.

The mesopontine tegmentum in reward and aversion: From cellular heterogeneity to behaviour

The mesopontine tegmentum, comprising the pedunculopontine tegmentum (PPN) and the laterodorsal tegmentum (LDT), is intricately connected to various regions of the basal ganglia, motor systems, and limbic systems. The PPN and LDT can regulate the activity of different brain regions of these target systems, and in this way are in a privileged position to modulate motivated behaviours. Despite recent findings, the PPN and LDT have been largely overlooked in discussions about the neural circuits associated with reward and aversion. This review aims to provide a timely and comprehensive resource on past and current research, highlighting the PPN and LDT's connectivity and influence on basal ganglia and limbic, and motor systems. Seminal studies, including lesion, pharmacological, and optogenetic/chemogenetic approaches, demonstrate their critical roles in modulating reward/aversive behaviours. The review emphasizes the need for further investigation into the associated cellular mechanisms, in order to clarify their role in behaviour and contribution for different neuropsychiatric disorders.

Multimodal convergence in the pedunculopontine tegmental nucleus: Motor, sensory and theta-frequency inputs influence activity of single neurons

The pedunculopontine tegmental nucleus of the brainstem (PPTg) has extensive interconnections and neuronal-behavioural correlates. It is implicated in movement control and sensorimotor integration. We investigated whether single neuron activity in freely moving rats is correlated with components of skilled forelimb movement, and whether individual neurons respond to both motor and sensory events. We found that individual PPTg neurons showed changes in firing rate at different times during the reach. This type of temporally specific modulation is like activity seen elsewhere in voluntary movement control circuits, such as the motor cortex, and suggests that PPTg neural activity is related to different specific events occurring during the reach. In particular, many neuronal modulations were time-locked to the end of the extension phase of the reach, when fine distal movements related to food grasping occur, indicating strong engagement of PPTg in this phase of skilled individual forelimb movements. In addition, some neurons showed brief periods of apparent oscillatory firing in the theta range at specific phases of the reach-to-grasp movement. When movement-related neurons were tested with tone stimuli, many also responded to this auditory input, allowing for sensorimotor integration at the cellular level. Together, these data extend the concept of the PPTg as an integrative structure in generation of complex movements, by showing that this function extends to the highly coordinated control of the forelimb during skilled reach to grasp movement, and that sensory and motor-related information converges on single neurons, allowing for direct integration at the cellular level.

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.

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

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

The Mesencephalic Locomotor Region: Multiple Cell Types, Multiple Behavioral Roles, and Multiple Implications for Disease

The mesencephalic locomotor region (MLR) controls locomotion in vertebrates. In humans with Parkinson disease, locomotor deficits are increasingly associated with decreased activity in the MLR. This brainstem region, commonly considered to include the cuneiform and pedunculopontine nuclei, has been explored as a target for deep brain stimulation to improve locomotor function, but the results are variable, from modest to promising. However, the MLR is a heterogeneous structure, and identification of the best cell type to target is only beginning. Here, I review the studies that uncovered the role of genetically defined MLR cell types, and I highlight the cells whose activation improves locomotor function in animal models of Parkinson disease. The promising cell types to activate comprise some glutamatergic neurons in the cuneiform and caudal pedunculopontine nuclei, as well as some cholinergic neurons of the pedunculopontine nucleus. Activation of MLR GABAergic neurons should be avoided, since they stop locomotion or evoke bouts flanked with numerous stops. MLR is also considered a potential target in spinal cord injury, supranuclear palsy, primary progressive freezing of gait, or stroke. Better targeting of the MLR cell types should be achieved through optimized deep brain stimulation protocols, pharmacotherapy, or the development of optogenetics for human use.

Brain Mechanisms Underlying Panic Attack and Panic Disorder

Panic disorder is a psychiatric disorder characterized by recurrent panic attacks, with a prevalence of ~ 4% in the general population, causing heavy personal and socioeconomic burdens. The similarities of animal defense responses to clinical panic attack symptoms in humans make it possible to translate neuroanatomical pathways identified in animal studies to panic disorder in humans. Therefore, in this review we first present a basic overview of panic disorder in humans including the main subtypes, models commonly used to trigger panic attacks, related hypotheses, the neurotransmitter systems that may be involved, and the current clinical treatments to give the reader a comprehensive understanding of panic disorder. The animal section introduces the models that trigger panic-like behavior in animals and the brain regions that may be involved, providing insights for future elucidation of the neural circuit mechanisms behind panic attacks.

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 ontogenetic excitation typically has opposing effects. Subthalamic and motor activity are also inversely correlated in movement disorders. However, most STN neurons 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 treadmill locomotion. The majority of STN neurons (type 1) exhibited locomotion-dependent increases in activity, with half encoding the locomotor cycle. A minority of neurons exhibited dips in activity or were uncorrelated with movement. Brief optogenetic inhibition of the dorsolateral STN (where type 1 neurons are concentrated) slowed and prematurely terminated locomotion. In Q175 Huntington's disease mice abnormally brief, low-velocity locomotion was specifically associated with type 1 hyperactivity. Together these data argue that movement-related increases in STN activity contribute to optimal locomotor performance.

Dopamine-sensitive neurons in the mesencephalic locomotor region control locomotion initiation, stop, and turns

The locomotor role of dopaminergic neurons is traditionally attributed to their ascending projections to the basal ganglia, which project to the mesencephalic locomotor region (MLR). In addition, descending dopaminergic projections to the MLR are present from basal vertebrates to mammals. However, the neurons targeted in the MLR and their behavioral role are unknown in mammals. Here, we identify genetically defined MLR cells that express D1 or D2 receptors and control different motor behaviors in mice. In the cuneiform nucleus, D1-expressing neurons promote locomotion, while D2-expressing neurons stop locomotion. In the pedunculopontine nucleus, D1-expressing neurons promote locomotion, while D2-expressing neurons evoke ipsilateral turns. Using RNAscope, we show that MLR dopamine-sensitive neurons comprise a combination of glutamatergic, GABAergic, and cholinergic neurons, suggesting that different neurotransmitter-based cell types work together to control distinct behavioral modules. Altogether, our study uncovers behaviorally relevant cell types in the mammalian MLR based on the expression of dopaminergic receptors.

Influence of maternal high-fat diet on offspring's locomotor activity during anxiety-related behavioral tests: A systematic review

The aim of this review was to summarize and discuss the impact of a maternal high-fat diet on the locomotor activity of offspring during anxiety-related behavioral tests. A search was performed in the LILACS, Web of Science, SCOPUS and PUMBED databases, using the following inclusion criteria: studies in which rodent dams were submitted to a high-fat diet during gestation and/or lactation and in which the locomotor activity parameters of offspring were evaluated during an anxiety-related test. Twenty-three articles met these criteria and were included. Most studies, 14 out of 23, found that a maternal high-fat diet did not alter offspring locomotor activity. Six articles found that a maternal high-fat diet increased the locomotor activity of offspring, while three found decreased locomotion. This effect may be associated with the initial response to the test and the fact that it was the first day of exposure to the apparatus.

Electrical stimulation of the cuneiform nucleus enhances the effects of rehabilitative training on locomotor recovery after incomplete spinal cord injury

Most human spinal cord injuries are anatomically incomplete, leaving some fibers still connecting the brain with the sublesional spinal cord. Spared descending fibers of the brainstem motor control system can be activated by deep brain stimulation (DBS) of the cuneiform nucleus (CnF), a subnucleus of the mesencephalic locomotor region (MLR). The MLR is an evolutionarily highly conserved structure which initiates and controls locomotion in all vertebrates. Acute electrical stimulation experiments in female adult rats with incomplete spinal cord injury conducted in our lab showed that CnF-DBS was able to re-establish a high degree of locomotion five weeks after injury, even in animals with initially very severe functional deficits and white matter lesions up to 80-95%. Here, we analyzed whether CnF-DBS can be used to support medium-intensity locomotor training and long-term recovery in rats with large but incomplete spinal cord injuries. Rats underwent rehabilitative training sessions three times per week in an enriched environment, either with or without CnF-DBS supported hindlimb stepping. After 4 weeks, animals that trained under CnF-DBS showed a higher level of locomotor performance than rats that trained comparable distances under non-stimulated conditions. The MLR does not project to the spinal cord directly; one of its main output targets is the gigantocellular reticular nucleus in the medulla oblongata. Long-term electrical stimulation of spared reticulospinal fibers after incomplete spinal cord injury via the CnF could enhance reticulospinal anatomical rearrangement and in this way lead to persistent improvement of motor function. By analyzing the spared, BDA-labeled giganto-spinal fibers we found that their gray matter arborization density after discontinuation of CnF-DBS enhanced training was lower in the lumbar L2 and L5 spinal cord in stimulated as compared to unstimulated animals, suggesting improved pruning with stimulation-enhanced training. An on-going clinical study in chronic paraplegic patients investigates the effects of CnF-DBS on locomotor capacity.

Dopamine lesions alter the striatal encoding of single-limb gait

The striatum serves an important role in motor control, and neurons in this area encode the body's initiation, cessation, and speed of locomotion. However, it remains unclear whether the same neurons also encode the step-by-step rhythmic motor patterns of individual limbs that characterize gait. By combining high-speed video tracking, electrophysiology, and optogenetic tagging, we found that a sizable population of both D1 and D2 receptor expressing medium spiny projection neurons (MSNs) were phase-locked to the gait cycle of individual limbs in mice. Healthy animals showed balanced limb phase-locking between D1 and D2 MSNs, while dopamine depletion led to stronger phase-locking in D2 MSNs. These findings indicate that striatal neurons represent gait on a single-limb and step basis, and suggest that elevated limb phase-locking of D2 MSNs may underlie some of the gait impairments associated with dopamine loss.

Grid cells: the missing link in understanding Parkinson's disease?

The mechanisms underlying Parkinson's disease (PD) are complex and not fully understood, and the box-and-arrow model among other current models present significant challenges. This paper explores the potential role of the allocentric brain and especially its grid cells in several PD motor symptoms, including bradykinesia, kinesia paradoxa, freezing of gait, the bottleneck phenomenon, and their dependency on cueing. It is argued that central hubs, like the locus coeruleus and the pedunculopontine nucleus, often narrowly interpreted in the context of PD, play an equally important role in governing the allocentric brain as the basal ganglia. Consequently, the motor and secondary motor (e.g., spatially related) symptoms of PD linked with dopamine depletion may be more closely tied to erroneous computation by grid cells than to the basal ganglia alone. Because grid cells and their associated central hubs introduce both spatial and temporal information to the brain influencing velocity perception they may cause bradykinesia or hyperkinesia as well. In summary, PD motor symptoms may primarily be an allocentric disturbance resulting from virtual faulty computation by grid cells revealed by dopamine depletion in PD.

Inhibitory medial zona incerta pathway drives exploratory behavior by inhibiting glutamatergic cuneiform neurons

The cuneiform nucleus (CnF) regulates locomotor activity, which is canonically viewed as being primarily involved in initiating locomotion and regulating speed. Recent research shows greater context dependency in the locomotor functions of this nucleus. Glutamatergic neurons, which contain vesicular glutamate transporter 2 (vGLUT2), regulate context-dependent locomotor speed in the CnF and play a role in defensive behavior. Here, we identify projections from the medial zona incerta (mZI) to CnF vGLUT2 neurons that promote exploratory behavior. Using fiber photometry recordings in male mice, we find that mZI gamma-aminobutyric acid (GABA) neurons increase activity during periods of exploration. Activation of mZI GABAergic neurons is associated with reduced spiking of CnF neurons. Additionally, activating both retrogradely labeled mZI-CnF GABAergic projection neurons and their terminals in the CnF increase exploratory behavior. Inhibiting CnF vGLUT2 neuronal activity also increases exploratory behavior. These findings provide evidence for the context-dependent dynamic regulation of CnF vGLUT2 neurons, with the mZI-CnF circuit shaping exploratory behavior.

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.

Dopamine lesions alter the striatal encoding of single-limb gait

The striatum serves an important role in motor control, and neurons in this area encode the body's initiation, cessation, and speed of locomotion. However, it remains unclear whether the same neurons also encode the step-by-step rhythmic motor patterns of individual limbs that characterize gait. By combining high-speed video tracking, electrophysiology, and optogenetic tagging, we found that a sizable population of both D1 and D2 receptor expressing medium spiny projection neurons (MSNs) were phase-locked to the gait cycle of individual limbs in mice. Healthy animals showed balanced limb phase-locking between D1 and D2 MSNs, while dopamine depletion led to stronger phase-locking in D2 MSNs. These findings indicate that striatal neurons represent gait on a single-limb and step basis, and suggest that elevated limb phase-locking of D2 MSNs may underlie some of the gait impairments associated with dopamine loss.

The Basal Ganglia and Mesencephalic Locomotor Region Connectivity Matrix

Although classically considered a relay station for basal ganglia (BG) output, the anatomy, connectivity, and function of the mesencephalic locomotor region (MLR) were redefined during the last two decades. In striking opposition to what was initially thought, MLR and BG are actually reciprocally and intimately interconnected. New viral-based, optogenetic, and mapping technologies revealed that cholinergic, glutamatergic, and GABAergic neurons coexist in this structure, which, in addition to extending descending projections, send long-range ascending fibers to the BG. These MLR projections to the BG convey motor and non-motor information to specific synaptic targets throughout different nuclei. Moreover, MLR efferent fibers originate from precise neuronal subpopulations located in particular MLR subregions, defining independent anatomo-functional subcircuits involved in particular aspects of animal behavior such as fast locomotion, explorative locomotion, posture, forelimb- related movements, speed, reinforcement, among others. In this review, we revised the literature produced during the last decade linking MLR and BG. We conclude that the classic framework considering the MLR as a homogeneous output structure passively receiving input from the BG needs to be revisited. We propose instead that the multiple subcircuits embedded in this region should be taken as independent entities that convey relevant and specific ascending information to the BG and, thus, actively participate in the execution and tuning of behavior.

The regulation of the pedunculopontine tegmental nucleus in sleep-wake states

The pedunculopontine tegmental nucleus (PPTg) plays a vital role in sleep/wake states. There are three main kinds of heterogeneous neurons involved: cholinergic, glutamatergic, and gamma-aminobutyric acidergic (GABAergic) neurons. However, the precise roles of cholinergic, glutamatergic and GABAergic PPTg cell groups in regulating sleep-wake are unknown. Recent work suggests that the cholinergic, glutamatergic, and GABAergic neurons of the PPTg may activate the main arousal-promoting nucleus, thus exerting their wakefulness effects. We review the related projection pathways and functions of various neurons of the PPTg, especially the mechanisms of the PPTg in sleep-wake, thus providing new perspectives for research of sleep-wake mechanisms.

Dopamine control of downstream motor centers

The role of dopamine in the control of movement is traditionally associated with ascending projections to the basal ganglia. However, more recently descending dopaminergic pathways projecting to downstream brainstem motor circuits were discovered. In lampreys, salamanders, and rodents, these include projections to the downstream Mesencephalic Locomotor Region (MLR), a brainstem region controlling locomotion. Such descending dopaminergic projections could prime brainstem networks controlling movement. Other descending dopaminergic projections have been shown to reach reticulospinal cells involved in the control of locomotion. In addition, dopamine directly modulates the activity of interneurons and motoneurons. Beyond locomotion, dopaminergic inputs modulate visuomotor transformations within the optic tectum (mammalian superior colliculus). Loss of descending dopaminergic inputs will likely contribute to pathological conditions such as in Parkinson's disease.

How deep is the brain? The shallow brain hypothesis

Deep learning and predictive coding architectures commonly assume that inference in neural networks is hierarchical. However, largely neglected in deep learning and predictive coding architectures is the neurobiological evidence that all hierarchical cortical areas, higher or lower, project to and receive signals directly from subcortical areas. Given these neuroanatomical facts, today's dominance of cortico-centric, hierarchical architectures in deep learning and predictive coding networks is highly questionable; such architectures are likely to be missing essential computational principles the brain uses. In this Perspective, we present the shallow brain hypothesis: hierarchical cortical processing is integrated with a massively parallel process to which subcortical areas substantially contribute. This shallow architecture exploits the computational capacity of cortical microcircuits and thalamo-cortical loops that are not included in typical hierarchical deep learning and predictive coding networks. We argue that the shallow brain architecture provides several critical benefits over deep hierarchical structures and a more complete depiction of how mammalian brains achieve fast and flexible computational capabilities.

Emotion in action: When emotions meet motor circuits

The brain is a remarkably complex organ responsible for a wide range of functions, including the modulation of emotional states and movement. Neuronal circuits are believed to play a crucial role in integrating sensory, cognitive, and emotional information to ultimately guide motor behavior. Over the years, numerous studies employing diverse techniques such as electrophysiology, imaging, and optogenetics have revealed a complex network of neural circuits involved in the regulation of emotional or motor processes. Emotions can exert a substantial influence on motor performance, encompassing both everyday activities and pathological conditions. The aim of this review is to explore how emotional states can shape movements by connecting the neural circuits for emotional processing to motor neural circuits. We first provide a comprehensive overview of the impact of different emotional states on motor control in humans and rodents. In line with behavioral studies, we set out to identify emotion-related structures capable of modulating motor output, behaviorally and anatomically. Neuronal circuits involved in emotional processing are extensively connected to the motor system. These circuits can drive emotional behavior, essential for survival, but can also continuously shape ongoing movement. In summary, the investigation of the intricate relationship between emotion and movement offers valuable insights into human behavior, including opportunities to enhance performance, and holds promise for improving mental and physical health. This review integrates findings from multiple scientific approaches, including anatomical tracing, circuit-based dissection, and behavioral studies, conducted in both animal and human subjects. By incorporating these different methodologies, we aim to present a comprehensive overview of the current understanding of the emotional modulation of movement in both physiological and pathological conditions.

Updates on brain regions and neuronal circuits of movement disorders in Parkinson's disease

Parkinson's disease (PD) is a progressive neurodegenerative disease with a global burden that affects more often in the elderly. The basal ganglia (BG) is believed to account for movement disorders in PD. More recently, new findings in the original regions in BG involved in motor control, as well as the new circuits or new nucleuses previously not specifically considered were explored. In the present review, we provide up-to-date information related to movement disorders and modulations in PD, especially from the perspectives of brain regions and neuronal circuits. Meanwhile, there are updates in deep brain stimulation (DBS) and other factors for the motor improvement in PD. Comprehensive understandings of brain regions and neuronal circuits involved in motor control could benefit the development of novel therapeutical strategies in PD.

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.

Paradoxical kinesia may no longer be a paradox waiting for 100 years to be unraveled

Parkinson's disease (PD) is a progressive neurodegenerative disorder mainly characterized by bradykinesia and akinesia. Interestingly, these motor disabilities can depend on the patient emotional state. Disabled PD patients remain able to produce normal motor responses in the context of urgent or externally driven situations or even when exposed to appetitive cues such as music. To describe this phenomenon Souques coined the term "paradoxical kinesia" a century ago. Since then, the mechanisms underlying paradoxical kinesia are still unknown due to a paucity of valid animal models that replicate this phenomenon. To overcome this limitation, we established two animal models of paradoxical kinesia. Using these models, we investigated the neural mechanisms of paradoxical kinesia, with the results pointing to the inferior colliculus (IC) as a key structure. Intracollicular electrical deep brain stimulation, glutamatergic and GABAergic mechanisms may be involved in the elaboration of paradoxical kinesia. Since paradoxical kinesia might work by activation of some alternative pathway bypassing basal ganglia, we suggest the IC as a candidate to be part of this pathway.

Layer 5 Intratelencephalic Neurons in the Motor Cortex Stably Encode Skilled Movement

The primary motor cortex (M1) and the dorsal striatum play a critical role in motor learning and the retention of learned behaviors. Motor representations of corticostriatal ensembles emerge during motor learning. In the coordinated reorganization of M1 and the dorsal striatum for motor learning, layer 5a (L5a) which connects M1 to the ipsilateral and contralateral dorsal striatum, should be a key layer. Although M1 L5a neurons represent movement-related activity in the late stage of learning, it is unclear whether the activity is retained as a memory engram. Here, using Tlx3-Cre male transgenic mice, we conducted two-photon calcium imaging of striatum-projecting L5a intratelencephalic (IT) neurons in forelimb M1 during late sessions of a self-initiated lever-pull task and in sessions after 6 d of nontraining following the late sessions. We found that trained male animals exhibited stable motor performance before and after the nontraining days. At the same time, we found that M1 L5a IT neurons strongly represented the well-learned forelimb movement but not uninstructed orofacial movements. A subset of M1 L5a IT neurons consistently coded the well-learned forelimb movement before and after the nontraining days. Inactivation of M1 IT neurons after learning impaired task performance when the lever was made heavier or when the target range of the pull distance was narrowed. These results suggest that a subset of M1 L5a IT neurons continuously represent skilled movement after learning and serve to fine-tune the kinematics of well-learned movement.SIGNIFICANCE STATEMENT Motor memory persists even when it is not used for a while. IT neurons in L5a of the M1 gradually come to represent skilled forelimb movements during motor learning. However, it remains to be determined whether these changes persist over a long period and how these neurons contribute to skilled movements. Here, we show that a subset of M1 L5a IT neurons retain information for skilled forelimb movements even after nontraining days. Furthermore, suppressing the activity of these neurons during skilled forelimb movements impaired behavioral stability and adaptability. Our results suggest the importance of M1 L5a IT neurons for tuning skilled forelimb movements over a long period.

Substantia Nigra Pars Reticulata Projections to the Pedunculopontine Nucleus Modulate Dyskinesia

BACKGROUND

Long-term use of levodopa for Parkinson's disease (PD) treatment is often hindered by development of motor complications, including levodopa-induced dyskinesia (LID). The substantia nigra pars reticulata (SNr) and globus pallidus internal segment (GPi) are the output nuclei of the basal ganglia. Dysregulation of SNr and GPi activity contributes to PD pathophysiology and LID.

OBJECTIVE

The objective of this study was to determine whether direct modulation of SNr GABAergic neurons and SNr projections to the pedunculopontine nucleus (PPN) regulates PD symptoms and LID in a mouse model.

METHODS

We expressed Cre-recombinase activated channelrhodopsin-2 (ChR2) or halorhodopsin adeno-associated virus-2 (AAV2) vectors selectively in SNr GABAergic neurons of Vgat-IRES-Cre mice in a 6-hydroxydopamine model of PD to investigate whether direct optogenetic modulation of SNr neurons or their projections to the PPN regulates PD symptoms and LID expression. The forepaw stepping task, mouse LID rating scale, and open-field locomotion were used to assess akinesia and LID to test the effect of SNr modulation.

RESULTS

Akinesia was improved by suppressing SNr neuron activity with halorhodopsin. LID was significantly reduced by increasing SNr neuronal activity with ChR2, which did not interfere with the antiakinetic effect of levodopa. Optical stimulation of ChR2 in SNr projections to the PPN recapitulated direct SNr stimulation.

CONCLUSIONS

Modulation of SNr GABAergic neurons alters akinesia and LID expression in a manner consistent with the rate model of basal ganglia circuitry. Moreover, the projections from SNr to PPN likely mediate the antidyskinetic effect of increasing SNr neuronal activity, identifying a potential novel role for the PPN in LID. © 2023 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

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

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

Inhibitory Pedunculopontine Neurons Gate Dopamine-Mediated Motor Actions of Unsigned Valence

BACKGROUND

The pedunculopontine nucleus (PPN) maintains a bidirectional connectivity with the basal ganglia that supports their shared roles in the selection and execution of motor actions. Previous studies identified a role for PPN neurons in goal-directed behavior, but the cellular substrates underlying this function have not been elucidated. We recently revealed the existence of a monosynaptic GABAergic input from the PPN that inhibits dopamine neurons of the substantia nigra. Activation of this pathway interferes with the execution of learned motor sequences when the actions are rewarded, even though the inhibition of dopamine neurons did not shift the value of the action, hence suggesting executive control over the gating of behavior.

OBJECTIVE

To test the attributes of the inhibition of dopamine neurons by the PPN in the context of goal-directed behavior regardless of whether the outcome is positive or negative.

METHODS

We delivered optogenetic stimulation to PPN GABAergic axon terminals in the substantia nigra during a battery of behavioral tasks with positive and negative valence.

RESULTS

Inhibition of dopamine neurons by PPN optogenetic activation during an appetitive task impaired the initiation and overall execution of the behavioral sequence without affecting the consumption of reward. During an active avoidance task, the same activation impaired the ability of mice to avoid a foot shock, but their escape response was unaffected. In addition, responses to potential threats were significantly attenuated.

CONCLUSION

Our results show that PPN GABAergic neurons modulate learned, goal-directed behavior of unsigned valence without affecting overall motor behavior.

Pedunculopontine Chx10+ neurons control global motor arrest in mice

Arrest of ongoing movements is an integral part of executing motor programs. Behavioral arrest may happen upon termination of a variety of goal-directed movements or as a global motor arrest either in the context of fear or in response to salient environmental cues. The neuronal circuits that bridge with the executive motor circuits to implement a global motor arrest are poorly understood. We report the discovery that the activation of glutamatergic Chx10-derived neurons in the pedunculopontine nucleus (PPN) in mice arrests all ongoing movements while simultaneously causing apnea and bradycardia. This global motor arrest has a pause-and-play pattern with an instantaneous interruption of movement followed by a short-latency continuation from where it was paused. Mice naturally perform arrest bouts with the same combination of motor and autonomic features. The Chx10-PPN-evoked arrest is different to ventrolateral periaqueductal gray-induced freezing. Our study defines a motor command that induces a global motor arrest, which may be recruited in response to salient environmental cues to allow for a preparatory or arousal state, and identifies a locomotor-opposing role for rostrally biased glutamatergic neurons in the PPN.

Reduced thalamic excitation to motor cortical pyramidal tract neurons in parkinsonism

Degeneration of midbrain dopaminergic (DA) neurons alters the connectivity and functionality of the basal ganglia-thalamocortical circuits in Parkinson's disease (PD). Particularly, the aberrant outputs of the primary motor cortex (M1) contribute to parkinsonian motor deficits. However, cortical adaptations at cellular and synaptic levels in parkinsonism remain poorly understood. Using multidisciplinary approaches, we found that DA degeneration induces cell subtype- and input-specific reduction of thalamic excitation to M1 pyramidal tract (PT) neurons. At molecular level, we identified that N-methyl-d-aspartate (NMDA) receptors play a key role in mediating the reduced thalamocortical excitation to PT neurons. At circuit level, we showed that the reduced thalamocortical transmission in parkinsonian mice can be rescued by chemogenetically suppressing basal ganglia outputs. Together, our data suggest that cell subtype- and synapse-specific adaptations in M1 contribute to altered cortical outputs in parkinsonism and are important aspects of PD pathophysiology.

Locomotor pattern generation and descending control: a historical perspective

The ability to generate and control locomotor movements depends on complex interactions between many areas of the nervous system, the musculoskeletal system, and the environment. How the nervous system manages to accomplish this task has been the subject of investigation for more than a century. In vertebrates, locomotion is generated by neural networks located in the spinal cord referred to as central pattern generators. Descending inputs from the brain stem initiate, maintain, and stop locomotion as well as control speed and direction. Sensory inputs adapt locomotor programs to the environmental conditions. This review presents a comparative and historical overview of some of the neural mechanisms underlying the control of locomotion in vertebrates. We have put an emphasis on spinal mechanisms and descending control.

The indirect pathway of the basal ganglia promotes transient punishment but not motor suppression

Optogenetic stimulation of Adora2a receptor-expressing spiny projection neurons (A2A-SPNs) in the striatum drives locomotor suppression and transient punishment, results attributed to activation of the indirect pathway. The sole long-range projection target of A2A-SPNs is the external globus pallidus (GPe). Unexpectedly, we found that inhibition of the GPe drove transient punishment but not suppression of movement. Within the striatum, A2A-SPNs inhibit other SPNs through a short-range inhibitory collateral network, and we found that optogenetic stimuli that drove motor suppression shared a common mechanism of recruiting this inhibitory collateral network. Our results suggest that the indirect pathway plays a more prominent role in transient punishment than in motor control and challenges the assumption that activity of A2A-SPNs is synonymous with indirect pathway activity.

Relationship between temporal dynamics of intrinsic brain activity and motor function remodeling in patients with acute BGIS

Background

patients with acute basal ganglia ischemic stroke (BGIS) show changes in local brain activity represented by the amplitude of low-frequency fluctuation (ALFF), but the time-varying characteristics of this local nerve activity are still unclear. This study aimed to investigate the abnormal time-varying local brain activity of patients with acute BGIS by using the ALFF method combined with the sliding-window approach.

Methods

In this study, 34 patients with acute BGIS with motor dysfunction and 44 healthy controls (HCs) were recruited. The dynamic amplitude of low-frequency fluctuation (dALFF) was employed to detect the alterations in brain activity induced by acute BGIS patients. A two-sample t-test comparison was performed to compare the dALFF value between the two groups and a Spearman correlation analysis was conducted to assess the relationship between the local brain activity abnormalities and clinical characteristics.

Results

Compared with HCs, the activity of neurons in the left temporal pole (TP), parahippocampal gyrus (paraHIP), middle occipital gyrus (MOG), dorsolateral superior frontal gyrus (SFGdl), medial cingulate cortex (MCC), right rectus, precuneus (PCu) and right cerebellum crus1 were significantly increased in patients with BGIS. In addition, we found that there was a negative correlation (r = -0.458, p = 0.007) between the dALFF value of the right rectus and the scores of the National Institutes of Health Stroke Scale (NIHSS), and a positive correlation (r = 0.488, 0.499, p < 0.05) with the scores of the Barthel Index scale (BI) and the Fugl Meyer motor function assessment (FMA). ROC analysis results demonstrated that the area under the curves (AUC) of the right rectus was 0.880, p<0.001.

Conclusion

The pattern of intrinsic brain activity variability was altered in patients with acute BGIS compared with HCs. The abnormal dALFF variability might be a potential tool to assess motor function in patients with acute BGIS and potentially inform the diagnosis of this disease.

Mechanism of Bisphenol F Affecting Motor System and Motor Activity in Zebrafish

Bisphenol F (BPF; 4,4'-dihydroxydiphenylmethane) is one of the most frequently used compounds in the manufacture of plastics and epoxy resins. Previous studies have demonstrated that BPF affects locomotor behavior, oxidative stress, and neurodevelopment in zebrafish. However, its neurotoxic effects are controversial, and the underlying mechanisms are unclear. In order to determine whether BPF affects the motor system, we exposed zebrafish embryos to BPF and assessed behavioral, histological, and neurochemical changes. Spontaneous locomotor behavior and startle response were significantly decreased in BPF-treated zebrafish larvae compared with control larvae. BPF induced motor degeneration and myelination defects in zebrafish larvae. In addition, embryonic exposure to BPF resulted in altered metabolic profiles of neurochemicals, including neurotransmitters and neurosteroids, which may impact locomotion and motor function. In conclusion, exposure to BPF has the potential to affect survival, motor axon length, locomotor activity, myelination, and neurochemical levels of zebrafish larvae.

Upregulation of breathing rate during running exercise by central locomotor circuits in mice

While respiratory adaptation to exercise is compulsory to cope with the increased metabolic demand, the neural signals at stake remain poorly identified. Using neural circuit tracing and activity interference strategies in mice, we uncover here two systems by which the central locomotor network can enable respiratory augmentation in relation to running activity. One originates in the mesencephalic locomotor region (MLR), a conserved locomotor controller. Through direct projections onto the neurons of the preBötzinger complex that generate the inspiratory rhythm, the MLR can trigger a moderate increase of respiratory frequency, prior to, or even in the absence of, locomotion. The other is the lumbar enlargement of the spinal cord containing the hindlimb motor circuits. When activated, and through projections onto the retrotrapezoid nucleus (RTN), it also potently upregulates breathing rate. On top of identifying critical underpinnings for respiratory hyperpnea, these data also expand the functional implication of cell types and pathways that are typically regarded as "locomotor" or "respiratory" related.

A proprioceptive feedback circuit drives Caenorhabditis elegans locomotor adaptation through dopamine signaling

An animal adapts its motor behavior to navigate the external environment. This adaptation depends on proprioception, which provides feedback on an animal's body postures. How proprioception mechanisms interact with motor circuits and contribute to locomotor adaptation remains unclear. Here, we describe and characterize proprioception-mediated homeostatic control of undulatory movement in the roundworm Caenorhabditis elegans. We found that the worm responds to optogenetically or mechanically induced decreases in midbody bending amplitude by increasing its anterior amplitude. Conversely, it responds to increased midbody amplitude by decreasing the anterior amplitude. Using genetics, microfluidic and optogenetic perturbation response analyses, and optical neurophysiology, we elucidated the neural circuit underlying this compensatory postural response. The dopaminergic PDE neurons proprioceptively sense midbody bending and signal to AVK interneurons via the D2-like dopamine receptor DOP-3. The FMRFamide-like neuropeptide FLP-1, released by AVK, regulates SMB head motor neurons to modulate anterior bending. We propose that this homeostatic behavioral control optimizes locomotor efficiency. Our findings demonstrate a mechanism in which proprioception works with dopamine and neuropeptide signaling to mediate motor control, a motif that may be conserved in other animals.

Glutamatergic and GABAergic neurons in pontine central gray mediate opposing valence-specific behaviors through a global network

Extracting the valence of environmental cues is critical for animals' survival. How valence in sensory signals is encoded and transformed to produce distinct behavioral responses remains not well understood. Here, we report that the mouse pontine central gray (PCG) contributes to encoding both negative and positive valences. PCG glutamatergic neurons were activated selectively by aversive, but not reward, stimuli, whereas its GABAergic neurons were preferentially activated by reward signals. The optogenetic activation of these two populations resulted in avoidance and preference behavior, respectively, and was sufficient to induce conditioned place aversion/preference. Suppression of them reduced sensory-induced aversive and appetitive behaviors, respectively. These two functionally opponent populations, receiving a broad range of inputs from overlapping yet distinct sources, broadcast valence-specific information to a distributed brain network with distinguishable downstream effectors. Thus, PCG serves as a critical hub to process positive and negative valences of incoming sensory signals and drive valence-specific behaviors with distinct circuits.

Neurocircuitry of Predatory Hunting

Predatory hunting is an important type of innate behavior evolutionarily conserved across the animal kingdom. It is typically composed of a set of sequential actions, including prey search, pursuit, attack, and consumption. This behavior is subject to control by the nervous system. Early studies used toads as a model to probe the neuroethology of hunting, which led to the proposal of a sensory-triggered release mechanism for hunting actions. More recent studies have used genetically-trackable zebrafish and rodents and have made breakthrough discoveries in the neuroethology and neurocircuits underlying this behavior. Here, we review the sophisticated neurocircuitry involved in hunting and summarize the detailed mechanism for the circuitry to encode various aspects of hunting neuroethology, including sensory processing, sensorimotor transformation, motivation, and sequential encoding of hunting actions. We also discuss the overlapping brain circuits for hunting and feeding and point out the limitations of current studies. We propose that hunting is an ideal behavioral paradigm in which to study the neuroethology of motivated behaviors, which may shed new light on epidemic disorders, including binge-eating, obesity, and obsessive-compulsive disorders.

Contrasting walking styles map to discrete neural substrates in the mouse brainstem

Walking is a slow gait which is particularly adaptable to meet internal or external needs and is prone to maladaptive alterations that lead to gait disorders. Alterations can affect speed, but also style (the way one walks). While slowed speed may signify the presence of a problem, style represents the hallmark essential for clinical classification of gait disorders. However, it has been challenging to objectively capture key stylistic features while uncovering neural substrates driving these features. Here we revealed brainstem hotspots that drive strikingly different walking styles by employing an unbiased mapping assay that combines quantitative walking signatures with focal, cell type specific activation. We found that activation of inhibitory neurons that mapped to the ventromedial caudal pons induced slow motion-like style. Activation of excitatory neurons that mapped to the ventromedial upper medulla induced shuffle-like style. Contrasting shifts in walking signatures distinguished these styles. Activation of inhibitory and excitatory neurons outside these territories or of serotonergic neurons modulated walking speed, but without walking signature shifts. Consistent with their contrasting modulatory actions, hotspots for slow-motion and shuffle-like gaits preferentially innervated different substrates. These findings lay the basis for new avenues to study mechanisms underlying (mal)adaptive walking styles and gait disorders.

Graphical abstract

Structural-Functional Correlates of Response to Pedunculopontine Stimulation in a Randomized Clinical Trial for Axial Symptoms of Parkinson's Disease

BACKGROUND

Axial symptoms of Parkinson's disease (PD) can be debilitating and are often refractory to conventional therapies such as dopamine replacement therapy and deep brain stimulation (DBS) of the subthalamic nuclei (STN).

OBJECTIVE

Evaluate the efficacy of bilateral DBS of the pedunculopontine nucleus area (PPNa) and investigate structural and physiological correlates of clinical response.

METHODS

A randomized, double-blind, cross-over clinical trial was employed to evaluate the efficacy of bilateral PPNa-DBS on axial symptoms. Lead positions and neuronal activity were evaluated with respect to clinical response. Connectomic cortical activation profiles were generated based on the volumes of tissue activated.

RESULTS

PPNa-DBS modestly improved (p = 0.057) axial symptoms in the medication-off condition, with greatest positive effects on gait symptoms (p = 0.027). Electrode placements towards the anterior commissure (ρ= 0.912; p = 0.011) or foramen caecum (ρ= 0.853; p = 0.031), near the 50% mark of the ponto-mesencephalic junction, yielded better therapeutic responses. Recording trajectories of patients with better therapeutic responses (i.e., more anterior electrode placements) had neurons with lower firing-rates (p = 0.003) and higher burst indexes (p = 0.007). Structural connectomic profiles implicated activation of fibers of the posterior parietal lobule which is involved in orienting behavior and locomotion.

CONCLUSION

Bilateral PPNa-DBS influenced gait symptoms in patients with PD. Anatomical and physiological information may aid in localization of a favorable stimulation target.

Brainstem neural mechanisms controlling locomotion with special reference to basal vertebrates

Over the last 60 years, the basic neural circuitry responsible for the supraspinal control of locomotion has progressively been uncovered. Initially, significant progress was made in identifying the different supraspinal structures controlling locomotion in mammals as well as some of the underlying mechanisms. It became clear, however, that the complexity of the mammalian central nervous system (CNS) prevented researchers from characterizing the detailed cellular mechanisms involved and that animal models with a simpler nervous system were needed. Basal vertebrate species such as lampreys, xenopus embryos, and zebrafish became models of choice. More recently, optogenetic approaches have considerably revived interest in mammalian models. The mesencephalic locomotor region (MLR) is an important brainstem region known to control locomotion in all vertebrate species examined to date. It controls locomotion through intermediary cells in the hindbrain, the reticulospinal neurons (RSNs). The MLR comprises populations of cholinergic and glutamatergic neurons and their specific contribution to the control of locomotion is not fully resolved yet. Moreover, the downward projections from the MLR to RSNs is still not fully understood. Reporting on discoveries made in different animal models, this review article focuses on the MLR, its projections to RSNs, and the contribution of these neural elements to the control of locomotion. Excellent and detailed reviews on the brainstem control of locomotion have been recently published with emphasis on mammalian species. The present review article focuses on findings made in basal vertebrates such as the lamprey, to help direct new research in mammals, including humans.

The integrated brain network that controls respiration

Respiration is a brain function on which our lives essentially depend. Control of respiration ensures that the frequency and depth of breathing adapt continuously to metabolic needs. In addition, the respiratory control network of the brain has to organize muscular synergies that integrate ventilation with posture and body movement. Finally, respiration is coupled to cardiovascular function and emotion. Here, we argue that the brain can handle this all by integrating a brainstem central pattern generator circuit in a larger network that also comprises the cerebellum. Although currently not generally recognized as a respiratory control center, the cerebellum is well known for its coordinating and modulating role in motor behavior, as well as for its role in the autonomic nervous system. In this review, we discuss the role of brain regions involved in the control of respiration, and their anatomical and functional interactions. We discuss how sensory feedback can result in adaptation of respiration, and how these mechanisms can be compromised by various neurological and psychological disorders. Finally, we demonstrate how the respiratory pattern generators are part of a larger and integrated network of respiratory brain regions.

Functional contribution of mesencephalic locomotor region nuclei to locomotor recovery after spinal cord injury

Spinal cord injury (SCI) results in a disruption of information between the brain and the spinal circuit. Electrical stimulation of the mesencephalic locomotor region (MLR) can promote locomotor recovery in acute and chronic SCI rodent models. Although clinical trials are currently under way, there is still debate about the organization of this supraspinal center and which anatomic correlate of the MLR should be targeted to promote recovery. Combining kinematics, electromyographic recordings, anatomic analysis, and mouse genetics, our study reveals that glutamatergic neurons of the cuneiform nucleus contribute to locomotor recovery by enhancing motor efficacy in hindlimb muscles, and by increasing locomotor rhythm and speed on a treadmill, over ground, and during swimming in chronic SCI mice. In contrast, glutamatergic neurons of the pedunculopontine nucleus slow down locomotion. Therefore, our study identifies the cuneiform nucleus and its glutamatergic neurons as a therapeutical target to improve locomotor recovery in patients living with SCI.

Mesencephalic locomotor region stimulation-cuneiform or pedunculopontine?

Roussel et al.¹ provide new insight into mecencephalic locomotor region (MLR) stimulation to treat spinal cord injury in mice. Previously, it was unclear which part of the MLR to target. Now, evidence converges on cuneiform nucleus activation.

Deconstructing the modular organization and real-time dynamics of mammalian spinal locomotor networks

Locomotion empowers animals to move. Locomotor-initiating signals from the brain are funneled through descending neurons in the brainstem that act directly on spinal locomotor circuits. Little is known in mammals about which spinal circuits are targeted by the command and how this command is transformed into rhythmicity in the cord. Here we address these questions leveraging a mouse brainstem-spinal cord preparation from either sex that allows locating the locomotor command neurons with simultaneous Ca²⁺ imaging of spinal neurons. We show that a restricted brainstem area - encompassing the lateral paragigantocellular nucleus (LPGi) and caudal ventrolateral reticular nucleus (CVL) - contains glutamatergic neurons which directly initiate locomotion. Ca²⁺ imaging captures the direct LPGi/CVL locomotor initiating command in the spinal cord and visualizes spinal glutamatergic modules that execute the descending command and its transformation into rhythmic locomotor activity. Inhibitory spinal networks are recruited in a distinctly different pattern. Our study uncovers the principal logic of how spinal circuits implement the locomotor command using a distinct modular organization.

Brainstem circuits encoding start, speed, and duration of swimming in adult zebrafish

The flexibility of locomotor movements requires an accurate control of their start, duration, and speed. How brainstem circuits encode and convey these locomotor parameters remains unclear. Here, we have combined in vivo calcium imaging, electrophysiology, anatomy, and behavior in adult zebrafish to address these questions. We reveal that the detailed parameters of locomotor movements are encoded by two molecularly, topographically, and functionally segregated glutamatergic neuron subpopulations within the nucleus of the medial longitudinal fasciculus. The start, duration, and changes of locomotion speed are encoded by vGlut2⁺ neurons, whereas vGlut1⁺ neurons encode sudden changes to high speed/high amplitude movements. Ablation of vGlut2⁺ neurons compromised slow-explorative swimming, whereas vGlut1⁺ neuron ablation impaired fast swimming. Our results provide mechanistic insights into how separate brainstem subpopulations implement flexible locomotor commands. These two brainstem command subpopulations are suitably organized to integrate environmental cues and hence generate flexible swimming movements to match the animal's behavioral needs.