Evidence for a dopaminergic innervation of the pedunculopontine nucleus in monkeys, and its drastic reduction after MPTP intoxication

Olfactory Projections to Locomotor Control Centers in the Sea Lamprey

Although olfaction is well known to guide animal behavior, the neural circuits underlying the motor responses elicited by olfactory inputs are poorly understood. In the sea lamprey, anatomical evidence shows that olfactory inputs project to the posterior tuberculum (PT), a structure containing dopaminergic (DA) neurons homologous to the mammalian ventral tegmental area and the substantia nigra pars compacta. Olfactory inputs travel directly from the medial olfactory bulb (medOB) or indirectly through the main olfactory bulb and the lateral pallium (LPal). Here, we characterized the transmission of olfactory inputs to the PT in the sea lamprey, Petromyzon marinus. Abundant projections from the medOB were observed close to DA neurons of the PT. Moreover, electrophysiological experiments revealed that PT neurons are activated by both the medOB and LPal, and calcium imaging indicated that the olfactory signal is then relayed to the mesencephalic locomotor region to initiate locomotion. In semi-intact preparations, stimulation of the medOB and LPal induced locomotion that was tightly associated with neural activity in the PT. Moreover, PT neurons were active throughout spontaneously occurring locomotor bouts. Altogether, our observations suggest that the medOB and LPal convey olfactory inputs to DA neurons of the PT, which in turn activate the brainstem motor command system to elicit locomotion.

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.

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.

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.

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.

Cerebellar Microstructural Abnormalities in Parkinson's Disease: a Systematic Review of Diffusion Tensor Imaging Studies

Diffusion tensor imaging (DTI) is now having a strong momentum in research to evaluate the neural fibers of the CNS. This technique can study white matter (WM) microstructure in neurodegenerative disorders, including Parkinson's disease (PD). Previous neuroimaging studies have suggested cerebellar involvement in the pathogenesis of PD, and these cerebellum alterations can correlate with PD symptoms and stages. Using the PRISMA 2020 framework, PubMed and EMBASE were searched to retrieve relevant articles. Our search revealed 472 articles. After screening titles and abstracts, and full-text review, and implementing the inclusion criteria, 68 papers were selected for synthesis. Reviewing the selected studies revealed that the patterns of reduction in cerebellum WM integrity, assessed by fractional anisotropy, mean diffusivity, radial diffusivity, and axial diffusivity measures can differ symptoms and stages of PD. Cerebellar diffusion tensor imaging (DTI) changes in PD patients with "postural instability and gait difficulty" are significantly different from "tremor dominant" PD patients. Freezing of the gate is strongly related to cerebellar involvement depicted by DTI. The "reduced cognition," "visual disturbances," "sleep disorders," "depression," and "olfactory dysfunction" are not related to cerebellum microstructural changes on DTI, while "impulsive-compulsive behavior" can be linked to cerebellar WM alteration. Finally, higher PD stages and longer disease duration are associated with cerebellum white matter alteration depicted by DTI. Depiction of cerebellar white matter involvement in PD is feasible by DTI. There is an association with disease duration and severity and several clinical presentations with DTI findings. This clinical-imaging association may eventually improve disease management.

Brainstem Circuits for Locomotion

Locomotion is a universal motor behavior that is expressed as the output of many integrated brain functions. Locomotion is organized at several levels of the nervous system, with brainstem circuits acting as the gate between brain areas regulating innate, emotional, or motivational locomotion and executive spinal circuits. Here we review recent advances on brainstem circuits involved in controlling locomotion. We describe how delineated command circuits govern the start, speed, stop, and steering of locomotion. We also discuss how these pathways interface between executive circuits in the spinal cord and diverse brain areas important for context-specific selection of locomotion. A recurrent theme is the need to establish a functional connectome to and from brainstem command circuits. Finally, we point to unresolved issues concerning the integrated function of locomotor control.

Expected final online publication date for the Annual Review of Neuroscience, Volume 45 is July 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Optogenetic stimulation of glutamatergic neurons in the cuneiform nucleus controls locomotion in a mouse model of Parkinson's disease

In Parkinson's disease (PD), the loss of midbrain dopaminergic cells results in severe locomotor deficits, such as gait freezing and akinesia. Growing evidence indicates that these deficits can be attributed to the decreased activity in the mesencephalic locomotor region (MLR), a brainstem region controlling locomotion. Clinicians are exploring the deep brain stimulation of the MLR as a treatment option to improve locomotor function. The results are variable, from modest to promising. However, within the MLR, clinicians have targeted the pedunculopontine nucleus exclusively, while leaving the cuneiform nucleus unexplored. To our knowledge, the effects of cuneiform nucleus stimulation have never been determined in parkinsonian conditions in any animal model. Here, we addressed this issue in a mouse model of PD, based on the bilateral striatal injection of 6-hydroxydopamine, which damaged the nigrostriatal pathway and decreased locomotor activity. We show that selective optogenetic stimulation of glutamatergic neurons in the cuneiform nucleus in mice expressing channelrhodopsin in a Cre-dependent manner in Vglut2-positive neurons (Vglut2-ChR2-EYFP mice) increased the number of locomotor initiations, increased the time spent in locomotion, and controlled locomotor speed. Using deep learning-based movement analysis, we found that the limb kinematics of optogenetic-evoked locomotion in pathological conditions were largely similar to those recorded in intact animals. Our work identifies the glutamatergic neurons of the cuneiform nucleus as a potentially clinically relevant target to improve locomotor activity in parkinsonian conditions. Our study should open avenues to develop the targeted stimulation of these neurons using deep brain stimulation, pharmacotherapy, or optogenetics.

Descending Dopaminergic Inputs to Reticulospinal Neurons Promote Locomotor Movements

Meso-diencephalic dopaminergic neurons are known to modulate locomotor behaviors through their ascending projections to the basal ganglia, which in turn project to the mesencephalic locomotor region, known to control locomotion in vertebrates. In addition to their ascending projections, dopaminergic neurons were found to increase locomotor movements through direct descending projections to the mesencephalic locomotor region and spinal cord. Intriguingly, fibers expressing tyrosine hydroxylase (TH), the rate-limiting enzyme of dopamine synthesis, were also observed around reticulospinal neurons of lampreys. We now examined the origin and the role of this innervation. Using immunofluorescence and tracing experiments, we found that fibers positive for dopamine innervate reticulospinal neurons in the four reticular nuclei of lampreys. We identified the dopaminergic source using tracer injections in reticular nuclei, which retrogradely labeled dopaminergic neurons in a caudal diencephalic nucleus (posterior tuberculum [PT]). Using voltammetry in brain preparations isolated in vitro, we found that PT stimulation evoked dopamine release in all four reticular nuclei, but not in the spinal cord. In semi-intact preparations where the brain is accessible and the body moves, PT stimulation evoked swimming, and injection of a D1 receptor antagonist within the middle rhombencephalic reticular nucleus was sufficient to decrease reticulospinal activity and PT-evoked swimming. Our study reveals that dopaminergic neurons have access to command neurons that integrate sensory and descending inputs to activate spinal locomotor neurons. As such, our findings strengthen the idea that dopamine can modulate locomotor behavior both via ascending projections to the basal ganglia and through descending projections to brainstem motor circuits.SIGNIFICANCE STATEMENT Meso-diencephalic dopaminergic neurons play a key role in modulating locomotion by releasing dopamine in the basal ganglia, spinal networks, and the mesencephalic locomotor region, a brainstem region that controls locomotion in a graded fashion. Here, we report in lampreys that dopaminergic neurons release dopamine in the four reticular nuclei where reticulospinal neurons are located. Reticulospinal neurons integrate sensory and descending suprareticular inputs to control spinal interneurons and motoneurons. By directly modulating the activity of reticulospinal neurons, meso-diencephalic dopaminergic neurons control the very last instructions sent by the brain to spinal locomotor circuits. Our study reports on a new direct descending dopaminergic projection to reticulospinal neurons that modulates locomotor behavior.

Nurr1, Pitx3, and α7 nAChRs mRNA Expression in Nigral Tissue of Rats with Pedunculopontine Neurotoxic Lesion

Background and Objectives: The knowledge that the cholinergic neurons from pedunculopontine nucleus (PPN) are vulnerable to the degeneration in early stages of the Parkinson disease progression has opened new perspectives to the development of experimental model focused in pontine lesions that could increase the risk of nigral degeneration. In this context it is known that PPN lesioned rats exhibit early changes in the gene expression of proteins responsible for dopaminergic homeostasis. At the same time, it is known that nicotinic cholinergic receptors (nAChRs) mediate the excitatory influence of pontine-nigral projection. However, the effect of PPN injury on the expression of transcription factors that modulate dopaminergic neurotransmission in the adult brain as well as the α7 nAChRs gene expression has not been studied. The main objective of the present work was the study of the effects of the unilateral neurotoxic lesion of PPN in nuclear receptor-related factor 1 (Nurr1), paired-like homeodomain transcription factor 3 (Pitx3), and α7 nAChRs mRNA expression in nigral tissue. Materials and Methods: The molecular biology studies were performed by means of RT-PCR. The following experimental groups were organized: Non-treated rats, N-methyl-D-aspartate (NMDA)-lesioned rats, and Sham operated rats. Experimental subjects were sacrificed 24 h, 48 h and seven days after PPN lesion. Results: Nurr1 mRNA expression, showed a significant increase both 24 h (p < 0.001) and 48 h (p < 0.01) after PPN injury. Pitx3 mRNA expression evidenced a significant increase 24 h (p < 0.001) followed by a significant decrease 48 h and seven days after PPN lesion (p < 0.01). Finally, the α7 nAChRs nigral mRNA expression remained significantly diminished 24 h, 48 h (p < 0.001), and 7 days (p < 0.01) after PPN neurotoxic injury. Conclusion: Taking together these modifications could represent early warning signals and could be the preamble to nigral neurodegeneration events.

Relationship between Lower Urinary Tract Dysfunction and Clinical Features in Chinese Parkinson's Disease Patients

Background

Lower urinary tract (LUT) dysfunction is very common in Parkinson's disease (PD) patients. However, the number of studies conducted on LUT dysfunction and its related factors in Chinese PD patients is very limited, and there is no international consensus concerning the results.

Methods

This cross-sectional study enrolled 100 Chinese PD patients. The patients were classified based on their overactive bladder symptom score (OABSS) and then assigned to either a PD with overactive bladder (PD-OAB) group or a PD with no overactive bladder (PD-NOAB) group. A binary logistic regression analysis was performed to identify the accompanying factors for overactive bladder (OAB). Next, correlations between the OABSS and patient sex, age, age of onset, disease duration, MDS-UPDRS-III, H-Y stage, PD subtype, treatment, education, and nonmotor symptoms were analyzed to identify factors correlated with LUT dysfunction.

Results

Eighty nine (89%) of the PD patients suffered from LUT dysfunction, and OAB was diagnosed in 45 (45%) of those PD patients. The most common lower urinary tract (LUT) symptom in the PD patients was nighttime frequency (86%), followed by urgency (50%), urge incontinence (34%), and daytime frequency (17%). Patients in the PD-OAB group had an older age and age of onset, were at a more advanced Hoehn–Yahr stage, and had more severe motor symptoms and nonmotor symptoms, including worse cognition, and a greater incidence of REM sleep behavior disorder (RBD). A binary logistic regression analysis showed that a lower Frontal Assessment Battery (FAB) score, higher H-Y stage, and RBD accompanied with a higher prevalence of OAB in PD patients. A multiple linear regression analysis showed that the OABSS was significantly influenced by the FAB score, H-Y stage, RBD, and age.

Conclusions

The FAB score, H-Y stage, and RBD are accompanying factors for OAB. A higher OABSS in PD patients was related to a lower FAB score for frontal lobe executive dysfunction, a higher H-Y stage for severity of motor disorders, RBD, and an older age.

Reciprocal interaction between monoaminergic systems and the pedunculopontine nucleus: Implication in the mechanism of L-DOPA

The pedunculopontine nucleus (PPN) is part of the mesencephalic locomotor region (MLR) and has been involved in the control of gait, posture, locomotion, sleep, and arousal. It likely participates in some motor and non-motor symptoms of Parkinson's disease and is regularly proposed as a surgical target to ameliorate gait, posture and sleep disorders in Parkinsonian patients. The PPN overlaps with the monoaminergic systems including dopamine, serotonin and noradrenaline in the modulation of the above-mentioned functions. All these systems are involved in Parkinson's disease and the mechanism of the anti-Parkinsonian agents, mostly L-DOPA. This suggests that PPN interacts with monoaminergic neurons and vice versa. Some evidence indicates that the PPN sends cholinergic, glutamatergic and even gabaergic inputs to mesencephalic dopaminergic cells, with the data regarding serotonergic or noradrenergic cells being less well known. Similarly, the control exerted by the PPN on dopaminergic neurons, is multiple and complex, and more extensively explored than the other monoaminergic systems. The data on the influence of monoaminergic systems on PPN neuron activity are rather scarce. While there is evidence that the PPN influences the therapeutic response of L-DOPA, it is still difficult to discerne the reciprocal action of the PPN and monoaminergic systems in this action. Additional data are required to better understand the functional organization of monoaminergic inputs to the MLR including the PPN to get a clearer picture of their interaction.

Spike discharge characteristic of the caudal mesencephalic reticular formation and pedunculopontine nucleus in MPTP-induced primate model of Parkinson disease

The pedunculopontine nucleus (PPN) included in the caudal mesencephalic reticular formation (cMRF) plays a key role in the control of locomotion and wake state. Regarding its involvement in the neurodegenerative process observed in Parkinson disease (PD), deep brain stimulation of the PPN was proposed to treat levodopa-resistant gait disorders. However, the precise role of the cMRF in the pathophysiology of PD, particularly in freezing of gait and other non-motor symptoms is still not clear. Here, using micro electrode recording (MER) in 2 primates, we show that dopamine depletion did not alter the mean firing rate of the overall cMRF neurons, particularly the putative non-cholinergic ones, but only a decreased activity of the regular neurons sub-group (though to be the cholinergic PPN neurons). Interestingly, a significant increase in the relative proportion of cMRF neurons with a burst pattern discharge was observed after MPTP intoxication. The present results question the hypothesis of an over-inhibition of the CMRF by the basal ganglia output structures in PD. The decreased activity observed in the regular neurons could explain some non-motor symptoms in PD regarding the strong involvement of the cholinergic neurons on the modulation of the thalamo-cortical system. The increased burst activity under dopamine depletion confirms that this specific spike discharge pattern activity also observed in other basal ganglia nuclei and in different pathologies could play a mojor role in the pathophysiology of the disease and could explain several symptoms of PD including the freezing of gait. The present data will have to be replicated in a larger number of animals and will have to investigate more in details how the modification of the spike discharge of the cMRF neurons in the parkinsonian state could alter functions such as locomotion and attentional state. This will ultimely allow a better comprehension of the pathophysiology of freezing of gait.

Parallel descending dopaminergic connectivity of A13 cells to the brainstem locomotor centers

The mesencephalic locomotor region (MLR) is an important integrative area for the initiation and modulation of locomotion. Recently it has been realized that dopamine (DA) projections from the substantia nigra pars compacta project to the MLR. Here we explore DA projections from an area of the medial zona incerta (ZI) known for its role in motor control onto the MLR. We provide evidence that dopaminergic (DAergic) A13 neurons have connectivity to the cuneiform nucleus (CnF) and pedunculopontine tegmental nucleus (PPTg) of the MLR. No ascending connectivity to the dorsolateral striatum was observed. On the other hand, DAergic A13 projections to the medullary reticular formation (MRF) and the lumbar spinal cord were sparse. A small number of non-DAergic neurons within the medial ZI projected to the lumbar spinal cord. We then characterized the DA A13 cells and report that these cells differ from canonical DA neurons since they lack the Dopamine Transporter (DAT). The lack of DAT expression, and possibly the lack of a dopamine reuptake mechanism, points to a longer time of action compared to typical dopamine neurons. Collectively our data suggest a parallel descending DAergic pathway from the A13 neurons of the medial ZI to the MLR, which we expect is important for modulating movement.

A Review of the Pedunculopontine Nucleus in Parkinson's Disease

The pedunculopontine nucleus (PPN) is situated in the upper pons in the dorsolateral portion of the ponto-mesencephalic tegmentum. Its main mass is positioned at the trochlear nucleus level, and is part of the mesenphalic locomotor region (MLR) in the upper brainstem. The human PPN is divided into two subnuclei, the pars compacta (PPNc) and pars dissipatus (PPNd), and constitutes both cholinergic and non-cholinergic neurons with afferent and efferent projections to the cerebral cortex, thalamus, basal ganglia (BG), cerebellum, and spinal cord. The BG controls locomotion and posture via GABAergic output of the substantia nigra pars reticulate (SNr). In PD patients, GABAergic BG output levels are abnormally increased, and gait disturbances are produced via abnormal increases in SNr-induced inhibition of the MLR. Since the PPN is vastly connected with the BG and the brainstem, dysfunction within these systems lead to advanced symptomatic progression in Parkinson's disease (PD), including sleep and cognitive issues. To date, the best treatment is to perform deep brain stimulation (DBS) on PD patients as outcomes have shown positive effects in ameliorating the debilitating symptoms of this disease by treating pathological circuitries within the parkinsonian brain. It is therefore important to address the challenges and develop this procedure to improve the quality of life of PD patients.

Tyrosine Hydroxylase, Vesicular Monoamine Transporter and Dopamine Transporter mRNA Expression in Nigrostriatal Tissue of Rats with Pedunculopontine Neurotoxic Lesion

BACKGROUND

The degeneration of the pedunculopontine nucleus (PPN) precedes the degeneration of the nigral cells in the pre-symptomatic stages of Parkinson's disease (PD). Although the literature recognizes that a lesion of the PPN increases the vulnerability of dopaminergic cells, it is unknown if this risk is associated with the loss of capability of handling the dopaminergic function.

METHODS

In this paper, the effects of a unilateral neurotoxic lesion of the PPN in tyrosine hydroxylase (TH), vesicular monoamine transporter 2 (VMAT2) and dopamine transporter (DAT) mRNA expression in nigrostriatal tissue were evaluated. Three experimental groups were organized: non-treated rats, NMDA-lesioned rats and Sham-operated rats.

RESULTS

Seven days after the PPN lesion, in nigral tissue, TH mRNA expression was higher in comparison with control groups (p < 0.05); in contrast, VMAT2 mRNA expression showed a significant decrease (p < 0.01). DAT mRNA expression showed a significant decrease (p < 0.001) in the striatal tissue. Comparing nigral neuronal density of injured and control rats revealed no significant difference seven days post-PPN injury.

CONCLUSIONS

Findings suggest that the PPN lesion modifies the mRNA expression of the proteins associated with dopaminergic homeostasis at nigrostriatal level. It could represent vulnerability signals for nigral dopaminergic cells and further increase the risk of degeneration of these cells.

Integration of Descending Command Systems for the Generation of Context-Specific Locomotor Behaviors

Over the past decade there has been a renaissance in our understanding of spinal cord circuits; new technologies are beginning to provide key insights into descending circuits which project onto spinal cord central pattern generators. By integrating work from both the locomotor and animal behavioral fields, we can now examine context-specific control of locomotion, with an emphasis on descending modulation arising from various regions of the brainstem. Here we examine approach and avoidance behaviors and the circuits that lead to the production and arrest of locomotion.

On the Role of the Pedunculopontine Nucleus and Mesencephalic Reticular Formation in Locomotion in Nonhuman Primates

The mesencephalic reticular formation (MRF) is formed by the pedunculopontine and cuneiform nuclei, two neuronal structures thought to be key elements in the supraspinal control of locomotion, muscle tone, waking, and REM sleep. The role of MRF has also been advocated in modulation of state of arousal leading to transition from wakefulness to sleep and it is further considered to be a main player in the pathophysiology of gait disorders seen in Parkinson's disease. However, the existence of a mesencephalic locomotor region and of an arousal center has not yet been demonstrated in primates. Here, we provide the first extensive electrophysiological mapping of the MRF using extracellular recordings at rest and during locomotion in a nonhuman primate (NHP) (Macaca fascicularis) model of bipedal locomotion. We found different neuronal populations that discharged according to a phasic or a tonic mode in response to locomotion, supporting the existence of a locomotor neuronal circuit within these MRF in behaving primates. Altogether, these data constitute the first electrophysiological characterization of a locomotor neuronal system present within the MRF in behaving NHPs under normal conditions, in accordance with several studies done in different experimental animal models.

SIGNIFICANCE STATEMENT

We provide the first extensive electrophysiological mapping of the two major components of the mesencephalic reticular formation (MRF), namely the pedunculopontine and cuneiform nuclei. We exploited a nonhuman primate (NHP) model of bipedal locomotion with extracellular recordings in behaving NHPs at rest and during locomotion. Different MRF neuronal groups were found to respond to locomotion, with phasic or tonic patterns of response. These data constitute the first electrophysiological evidences of a locomotor neuronal system within the MRF in behaving NHPs.

Dopamine and the Brainstem Locomotor Networks: From Lamprey to Human

In vertebrates, dopamine neurons are classically known to modulate locomotion via their ascending projections to the basal ganglia that project to brainstem locomotor networks. An increased dopaminergic tone is associated with increase in locomotor activity. In pathological conditions where dopamine cells are lost, such as in Parkinson's disease, locomotor deficits are traditionally associated with the reduced ascending dopaminergic input to the basal ganglia. However, a descending dopaminergic pathway originating from the substantia nigra pars compacta was recently discovered. It innervates the mesencephalic locomotor region (MLR) from basal vertebrates to mammals. This pathway was shown to increase locomotor output in lampreys, and could very well play an important role in mammals. Here, we provide a detailed account on the newly found dopaminergic pathway in lamprey, salamander, rat, monkey, and human. In lampreys and salamanders, dopamine release in the MLR is associated with the activation of reticulospinal neurons that carry the locomotor command to the spinal cord. Dopamine release in the MLR potentiates locomotor movements through a D1-receptor mechanism in lampreys. In rats, stimulation of the substantia nigra pars compacta elicited dopamine release in the pedunculopontine nucleus, a known part of the MLR. In a monkey model of Parkinson's disease, a reduced dopaminergic innervation of the brainstem locomotor networks was reported. Dopaminergic fibers are also present in human pedunculopontine nucleus. We discuss the conserved locomotor role of this pathway from lamprey to mammals, and the hypothesis that this pathway could play a role in the locomotor deficits reported in Parkinson's disease.

Role of the pedunculopontine nucleus in controlling gait and sleep in normal and parkinsonian monkeys

Patients with Parkinson's disease (PD) develop cardinal motor symptoms, including akinesia, rigidity, and tremor, that are alleviated by dopaminergic medication and/or subthalamic deep brain stimulation. Over the time course of the disease, gait and balance disorders worsen and become resistant to pharmacological and surgical treatments. These disorders generate debilitating motor symptoms leading to increased dependency, morbidity, and mortality. PD patients also experience sleep disturbance that raise the question of a common physiological basis. An extensive experimental and clinical body of work has highlighted the crucial role of the pedunculopontine nucleus (PPN) in the control of gait and sleep, and its potential major role in PD. Here, we summarise our investigations in the monkey PPN in the normal and parkinsonian states. We first examined the anatomy and connectivity of the PPN and the cuneiform nucleus which both belong to the mesencephalic locomotor region. Second, we conducted experiments to demonstrate the specific effects of PPN cholinergic lesions on locomotion in the normal and parkinsonian monkey. Third, we aimed to understand how PPN cholinergic lesions impair sleep in parkinsonian monkeys. Our final goal was to develop a novel model of advanced PD with gait and sleep disorders. We believe that this monkey model, even if it does not attempt to reproduce the exact human disease with all its complexities, represents a good biomedical model to characterise locomotion and sleep in the context of PD.

Increased Motor Activity During REM Sleep Is Linked with Dopamine Function in Idiopathic REM Sleep Behavior Disorder and Parkinson Disease

STUDY OBJECTIVES

Rapid eye movement (REM) sleep behavior disorder (RBD) is a parasomnia characterized by impaired motor inhibition during REM sleep, and dream-enacting behavior. RBD is especially associated with α-synucleinopathies, such as Parkinson disease (PD). Follow-up studies have shown that patients with idiopathic RBD (iRBD) have an increased risk of developing an α-synucleinopathy in later life. Although abundant studies have shown that degeneration of the nigrostriatal dopaminergic system is associated with daytime motor function in Parkinson disease, only few studies have investigated the relation between this system and electromyographic (EMG) activity during sleep. The objective of this study was to investigate the relationship between the nigrostriatal dopamine system and muscle activity during sleep in iRBD and PD.

METHODS

10 iRBD patients, 10 PD patients with PD, 10 PD patients without RBD, and 10 healthy controls were included and assessed with (123)I-N-omega-fluoropropyl-2-beta-carboxymethoxy-3beta-(4-iodophenyl) nortropane ((123)I-FP-CIT) Single-photon emission computed tomography (SPECT) scanning ((123)I-FP-CIT SPECT), neurological examination, and polysomnography.

RESULTS

iRBD patients and PD patients with RBD had increased EMG-activity compared to healthy controls. (123)I-FP-CIT uptake in the putamen-region was highest in controls, followed by iRBD patients, and lowest in PD patients. In iRBD patients, EMG-activity in the mentalis muscle was correlated to (123)I-FP-CIT uptake in the putamen. In PD patients, EMG-activity was correlated to anti-Parkinson medication.

CONCLUSIONS

Our results support the hypothesis that increased EMG-activity during REM sleep is at least partly linked to the nigrostriatal dopamine system in iRBD, and with dopamine function in PD.

A descending dopamine pathway conserved from basal vertebrates to mammals

Dopamine neurons are classically known to modulate locomotion indirectly through ascending projections to the basal ganglia that project down to brainstem locomotor networks. Their loss in Parkinson's disease is devastating. In lampreys, we recently showed that brainstem networks also receive direct descending dopaminergic inputs that potentiate locomotor output. Here, we provide evidence that this descending dopaminergic pathway is conserved to higher vertebrates, including mammals. In salamanders, dopamine neurons projecting to the striatum or brainstem locomotor networks were partly intermingled. Stimulation of the dopaminergic region evoked dopamine release in brainstem locomotor networks and concurrent reticulospinal activity. In rats, some dopamine neurons projecting to the striatum also innervated the pedunculopontine nucleus, a known locomotor center, and stimulation of the dopaminergic region evoked pedunculopontine dopamine release in vivo. Finally, we found dopaminergic fibers in the human pedunculopontine nucleus. The conservation of a descending dopaminergic pathway across vertebrates warrants re-evaluating dopamine's role in locomotion.

Resting-state functional magnetic resonance imaging of the subthalamic microlesion and stimulation effects in Parkinson's disease: Indications of a principal role of the brainstem

During implantation of deep-brain stimulation (DBS) electrodes in the target structure, neurosurgeons and neurologists commonly observe a “microlesion effect” (MLE), which occurs well before initiating subthalamic DBS. This phenomenon typically leads to a transitory improvement of motor symptoms of patients suffering from Parkinson's disease (PD). Mechanisms behind MLE remain poorly understood. In this work, we exploited the notion of ranking to assess spontaneous brain activity in PD patients examined by resting-state functional magnetic resonance imaging in response to penetration of DBS electrodes in the subthalamic nucleus. In particular, we employed a hypothesis-free method, eigenvector centrality (EC), to reveal motor-communication-hubs of the highest rank and their reorganization following the surgery; providing a unique opportunity to evaluate the direct impact of disrupting the PD motor circuitry in vivo without prior assumptions. Penetration of electrodes was associated with increased EC of functional connectivity in the brainstem. Changes in connectivity were quantitatively related to motor improvement, which further emphasizes the clinical importance of the functional integrity of the brainstem. Surprisingly, MLE and DBS were associated with anatomically different EC maps despite their similar clinical benefit on motor functions. The DBS solely caused an increase in connectivity of the left premotor region suggesting separate pathophysiological mechanisms of both interventions. While the DBS acts at the cortical level suggesting compensatory activation of less affected motor regions, the MLE affects more fundamental circuitry as the dysfunctional brainstem predominates in the beginning of PD. These findings invigorate the overlooked brainstem perspective in the understanding of PD and support the current trend towards its early diagnosis.

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DBS surgery in Parkinson's patients is often associated with a “microlesion effect” (MLE).

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Mechanisms behind MLE remain poorly understood.

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Using resting-state fMRI, we identified the brainstem as the principal hub responding to MLE.

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This invigorates the overlooked brainstem perspective in the understanding of Parkinson's disease.

Pathophysiology of L-dopa-induced motor and non-motor complications in Parkinson's disease

Involuntary movements, or dyskinesia, represent a debilitating complication of levodopa (L-dopa) therapy for Parkinson's disease (PD). L-dopa-induced dyskinesia (LID) are ultimately experienced by the vast majority of patients. In addition, psychiatric conditions often manifested as compulsive behaviours, are emerging as a serious problem in the management of L-dopa therapy. The present review attempts to provide an overview of our current understanding of dyskinesia and other L-dopa-induced dysfunctions, a field that dramatically evolved in the past twenty years. In view of the extensive literature on LID, there appeared a critical need to re-frame the concepts, to highlight the most suitable models, to review the central nervous system (CNS) circuitry that may be involved, and to propose a pathophysiological framework was timely and necessary. An updated review to clarify our understanding of LID and other L-dopa-related side effects was therefore timely and necessary. This review should help in the development of novel therapeutic strategies aimed at preventing the generation of dyskinetic symptoms.

Pedunculopontine tegmental nucleus neurons provide reward, sensorimotor, and alerting signals to midbrain dopamine neurons

Dopamine (DA) neurons in the midbrain are crucial for motivational control of behavior. However, recent studies suggest that signals transmitted by DA neurons are heterogeneous. This may reflect a wide range of inputs to DA neurons, but which signals are provided by which brain areas is still unclear. Here we focused on the pedunculopontine tegmental nucleus (PPTg) in macaque monkeys and characterized its inputs to DA neurons. Since the PPTg projects to many brain areas, it is crucial to identify PPTg neurons that project to DA neuron areas. For this purpose we used antidromic activation technique by electrically stimulating three locations (medial, central, lateral) in the substantia nigra pars compacta (SNc). We found SNc-projecting neurons mainly in the PPTg, and some in the cuneiform nucleus. Electrical stimulation in the SNc-projecting PPTg regions induced a burst of spikes in presumed DA neurons, suggesting that the PPTg-DA (SNc) connection is excitatory. Behavioral tasks and clinical tests showed that the SNc-projecting PPTg neurons encoded reward, sensorimotor and arousal/alerting signals. Importantly, reward-related PPTg neurons tended to project to the medial and central SNc, whereas sensorimotor/arousal/alerting-related PPTg neurons tended to project to the lateral SNc. Most reward-related signals were positively biased: excitation and inhibition when a better and worse reward was expected, respectively. These PPTg neurons tended to retain the reward value signal until after a reward outcome, representing 'value state'; this was different from DA neurons which show phasic signals representing 'value change'. Our data, together with previous studies, suggest that PPTg neurons send positive reward-related signals mainly to the medial-central SNc where DA neurons encode motivational values, and sensorimotor/arousal signals to the lateral SNc where DA neurons encode motivational salience.

Neuromuscular complexity during gait is not responsive to medication in persons with Parkinson's disease

The purpose of this study was to investigate the effects of dopaminergic therapy on neuromuscular complexity during gait and on the relationship between neuromuscular complexity and gait speed in persons with Parkinson's disease (PD). Nine persons with PD walked at self-selected speed for 5 min after having withdrawn from dopaminergic medication for at least 12 h and while optimally-medicated. Electromyographic recordings were taken from eight leg muscles bilaterally. Non-negative matrix factorization was applied to reduce the dimensionality of the electromyographic signals into motor modules. We assessed neuromuscular complexity by investigating the number, structure, and timing of the modules. We also investigated the influence of dopaminergic medication on the relationships between neuromuscular complexity and gait speed. Though gait speed increased significantly after medication intake, medication did not affect neuromuscular complexity. Neuromuscular complexity was significantly associated with gait speed only while the participants were medicated. Thus, the supraspinal structures that govern neuromuscular complexity during gait do not appear to be solely dopaminergically-influenced in PD. The lack of dopaminergic influence on neuromuscular complexity may explain why persons with PD exhibit gait slowness even while medicated, and an intervention that restores neuromuscular complexity may result in gait speed improvement in PD.

Immediate-early gene expression in structures outside the basal ganglia is associated to l-DOPA-induced dyskinesia

Long-term l-3,4-dihydroxyphenylalanine (l-DOPA) treatment in Parkinson's disease (PD) leads to l-DOPA-induced dyskinesia (LID), a condition thought to primarily involve the dopamine D1 receptor-expressing striatal medium spiny neurons. Activation of the D1 receptor results in increased expression of several molecular markers, in particular the members of the immediate-early gene (IEG) family, a class of genes rapidly transcribed in response to an external stimulus. However, several dopaminoceptive structures in the brain that are likely to be affected by the exogenously produced DA have received little attention although they might play a key role in mediating those l-DOPA-induced abnormal behaviours. ΔFosB, ARC, FRA2 and Zif268 IEGs expression patterns were thus characterised, using unbiased stereological methods, in the whole brain of dyskinetic and non-dyskinetic rats to identify brain nuclei displaying a transcriptional response specifically related to LID. Within the basal ganglia, the striatum and the substantia nigra pars reticulata showed an increased expression of all four IEGs in dyskinetic compared to non-dyskinetic rats. Outside the basal ganglia, there was a striking increased expression of the four IEGs in the motor cortex, the bed nucleus of the stria terminalis, the dorsal hippocampus, the pontine nuclei, the cuneiform nucleus and the pedunculopontine nuclei. Moreover, the zona incerta and the lateral habenula displayed an overexpression of ΔFosB, ARC and Zif268. Among these structures, the IEG expression in the striatum, the bed nucleus of the stria terminalis, the lateral habenula, the pontine nuclei and the cuneiform nucleus correlate with LID severity. These results illustrate a global transcriptional response to a dyskinetic state in the whole brain suggesting the possible involvement of these structures in LID.

Forebrain dopamine neurons project down to a brainstem region controlling locomotion

The contribution of dopamine (DA) to locomotor control is traditionally attributed to ascending dopaminergic projections from the substantia nigra pars compacta and the ventral tegmental area to the basal ganglia, which in turn project down to the mesencephalic locomotor region (MLR), a brainstem region controlling locomotion in vertebrates. However, a dopaminergic innervation of the pedunculopontine nucleus, considered part of the MLR, was recently identified in the monkey. The origin and role of this dopaminergic input are unknown. We addressed these questions in a basal vertebrate, the lamprey. Here we report a functional descending dopaminergic pathway from the posterior tuberculum (PT; homologous to the substantia nigra pars compacta and/or ventral tegmental area of mammals) to the MLR. By using triple labeling, we found that dopaminergic cells from the PT not only project an ascending pathway to the striatum, but send a descending projection to the MLR. In an isolated brain preparation, PT stimulation elicited excitatory synaptic inputs into patch-clamped MLR cells, accompanied by activity in reticulospinal cells. By using voltammetry coupled with electrophysiological recordings, we demonstrate that PT stimulation evoked DA release in the MLR, together with the activation of reticulospinal cells. In a semi-intact preparation, stimulation of the PT elicited reticulospinal activity together with locomotor movements. Microinjections of a D1 antagonist in the MLR decreased the locomotor output elicited by PT stimulation, whereas injection of DA had an opposite effect. It appears that this descending dopaminergic pathway has a modulatory role on MLR cells that are known to receive glutamatergic projections and promotes locomotor output.

Internal pallidum and substantia nigra control different parts of the mesopontine reticular formation in primate

The locomotor area has recently emerged as a target for deep brain stimulation to lessen gait disturbances in advanced parkinsonian patients. An important step in choosing this target is to define anatomical limits of its 2 components, the pedunculopontine nucleus and the cuneiform nucleus, their connections with the basal ganglia, and their output descending pathway. Based on the hypothesis that pedunculopontine nucleus controls locomotion whereas cuneiform nucleus controls axial posture, we analyzed whether both nuclei receive inputs from the internal pallidum and substantia nigra using anterograde and retrograde tract tracing in monkeys. We also examined whether these nuclei convey descending projections to the reticulospinal pathway. Pallidal terminals were densely distributed and restricted to the pedunculopontine nucleus, whereas nigral terminals were diffusely observed in the whole extent of both the pedunculopontine nucleus and the cuneiform nucleus. Moreover, nigral terminals formed symmetric synapses with pedunculopontine nucleus and cuneiform nucleus dendrites. Retrograde tracing experiments confirmed these results because labeled cell bodies were observed in both the internal pallidum and substantia nigra after pedunculopontine nucleus injection, but only in the substantia nigra after cuneiform nucleus injection. Furthermore, anterograde tracing experiments revealed that the pedunculopontine nucleus and cuneiform nucleus project to large portions of the pontomedullary reticular formation. This is the first anatomical evidence that the internal pallidum and the substantia nigra control different parts of the brain stem and can modulate the descending reticulospinal pathway in primates. These findings support the functional hypothesis that the nigro-cuneiform nucleus pathway could control axial posture whereas the pallido-pedunculopontine nucleus pathway could modulate locomotion.

Topographical organization of the pedunculopontine nucleus

Neurons in the pedunculopontine nucleus (PPN) exhibit a wide heterogeneity in terms of their neurochemical nature, their discharge properties, and their connectivity. Such characteristics are reflected in their functional properties and the behaviors in which they are involved, ranging from motor to cognitive functions, and the regulation of brain states. A clue to understand this functional versatility arises from the internal organization of the PPN. Thus, two main areas of the PPN have been described, the rostral and the caudal, which display remarkable differences in terms of the distribution of neurons with similar phenotype and the projections that originate from them. Here we review these differences with the premise that in order to understand the function of the PPN it is necessary to understand its intricate connectivity. We support the case that the PPN should not be considered as a homogeneous structure and conclude that the differences between rostral and caudal PPN, along with their intrinsic connectivity, may underlie the basis of its complexity.

Sing the mind electric - principles of deep brain stimulation

The remarkable efficacy of deep brain stimulation (DBS) for a range of treatment-resistant disorders is still not matched by a comparable understanding of the underlying neural mechanisms. Some progress has been made using translational research with a range of neuroscientific techniques, and here we review the most promising emerging principles. On balance, DBS appears to work by restoring normal oscillatory activity between a network of key brain regions. Further research using this causal neuromodulatory tool may provide vital insights into fundamental brain function, as well as guide targets for future treatments. In particular, DBS could have an important role in restoring the balance of the brain's default network and thus repairing the malignant brain states associated with affective disorders, which give rise to serious disabling problems such as anhedonia, the lack of pleasure. At the same time, it is important to proceed with caution and not repeat the errors from the era of psychosurgery.

Cholinergic mesencephalic neurons are involved in gait and postural disorders in Parkinson disease

Gait disorders and postural instability, which are commonly observed in elderly patients with Parkinson disease (PD), respond poorly to dopaminergic agents used to treat other parkinsonian symptoms. The brain structures underlying gait disorders and falls in PD and aging remain to be characterized. Using functional MRI in healthy human subjects, we have shown here that activity of the mesencephalic locomotor region (MLR), which is composed of the pedunculopontine nucleus (PPN) and the adjacent cuneiform nucleus, was modulated by the speed of imagined gait, with faster imagined gait activating a discrete cluster within the MLR. Furthermore, the presence of gait disorders in patients with PD and in aged monkeys rendered parkinsonian by MPTP intoxication correlated with loss of PPN cholinergic neurons. Bilateral lesioning of the cholinergic part of the PPN induced gait and postural deficits in nondopaminergic lesioned monkeys. Our data therefore reveal that the cholinergic neurons of the PPN play a central role in controlling gait and posture and represent a possible target for pharmacological treatment of gait disorders in PD.