The spiking activity of basal ganglia neurons can be characterized by summary statistics such as the average firing rate, or by measures of firing patterns, such as burst discharges, or oscillatory fluctuations of firing rates. Many of these features are altered by the presence of parkinsonism. This study examined another distinct attribute of firing activity, i.e., the occurrence of repeating sequences of interspike intervals (ISIs). We studied this feature in extracellular electrophysiological recordings that were made in the basal ganglia of rhesus monkeys, before and after they had been rendered parkinsonian by treatment with the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine. Neurons in both pallidal segments and in the subthalamic nucleus tended to fire in repeating sequences, typically two ISIs long (i.e., involving three spikes). In recordings that were 5,000 interspike intervals long, 20%-40% of spikes participated in one of many sequences with each ISI replicating the sequence pattern with a timing error of ≤1%. Compared with similar analyses in shuffled representations of the same data, sequences were more common in the original representation of ISIs in all of the tested structures. Induction of parkinsonism reduced the proportion of sequence spikes in the external pallidum but increased it in the subthalamic nucleus. We found no relation between the sequence generation and the firing rate of neurons, and, at most, a weak correlation between sequence generation and the incidence of bursts. We conclude that basal ganglia neurons fire in recognizable sequences of ISIs, whose incidence is influenced by the induction of parkinsonism.NEW & NOTEWORTHY Previous work has shown that the timing of the electrical activity of basal ganglia neurons has nonstochastic properties, resulting in oscillatory firing patterns, or bursting. This article describes another such property in the monkey brain; a surprisingly large proportion of action potentials generated by cells in the extrastriatal basal ganglia are part of precisely timed recurring sequences of spiking events. We also found that the generation of these sequences changes substantially in the parkinsonian state.

Parkinson’s disease (PD) is a progressive neurodegenerative disorder that primarily affects elderly people and constitutes a major source of disability worldwide. Notably, the neuropathological hallmarks of PD include nigrostriatal loss and the formation of intracellular inclusion bodies containing misfolded α-synuclein protein aggregates. Cardinal motor symptoms, which include tremor, rigidity and bradykinesia, can effectively be managed with dopaminergic therapy for years following symptom onset. Nonetheless, patients ultimately develop symptoms that no longer fully respond to dopaminergic treatment. Attempts to discover disease-modifying agents have increasingly been supported by translational molecular imaging concepts, targeting the most prominent pathological hallmark of PD, α-synuclein accumulation, as well as other molecular pathways that contribute to the pathophysiology of PD. Indeed, molecular imaging modalities such as positron emission tomography (PET) and single-photon emission computed tomography (SPECT) can be leveraged to study parkinsonism not only in animal models but also in living patients. For instance, mitochondrial dysfunction can be assessed with probes that target the mitochondrial complex I (MC-I), while nigrostriatal degeneration is typically evaluated with probes designed to non-invasively quantify dopaminergic nerve loss. In addition to dopaminergic imaging, serotonin transporter and N-methyl-D-aspartate (NMDA) receptor probes are increasingly used as research tools to better understand the complexity of neurotransmitter dysregulation in PD. Non-invasive quantification of neuroinflammatory processes is mainly conducted by targeting the translocator protein 18 kDa (TSPO) on activated microglia using established imaging agents. Despite the overwhelming involvement of the brain and brainstem, the pathophysiology of PD is not restricted to the central nervous system (CNS). In fact, PD also affects various peripheral organs such as the heart and gastrointestinal tract – primarily via autonomic dysfunction. As such, research into peripheral biomarkers has taken advantage of cardiac autonomic denervation in PD, allowing the differential diagnosis between PD and multiple system atrophy with probes that visualize sympathetic nerve terminals in the myocardium. Further, α-synuclein has recently gained attention as a potential peripheral biomarker in PD. This review discusses breakthrough discoveries that have led to the contemporary molecular concepts of PD pathophysiology and how they can be harnessed to develop effective imaging probes and therapeutic agents. Further, we will shed light on potential future trends, thereby focusing on potential novel diagnostic tracers and disease-modifying therapeutic interventions.

In the primate thalamus, the parvocellular ventral anterior nucleus (VApc) and the centromedian nucleus (CM) receive GABAergic projections from the internal globus pallidus (GPi) and glutamatergic inputs from motor cortices. In this study, we used electron microscopy to assess potential structural changes in GABAergic and glutamatergic microcircuits in the VApc and CM of MPTP-treated parkinsonian monkeys. The intensity of immunostaining for GABAergic markers in VApc and CM did not differ between control and parkinsonian monkeys. In the electron microscope, three major types of terminals were identified in both nuclei: (a) vesicular glutamate transporter 1 (vGluT1)-positive terminals forming asymmetric synapses (type As), which originate from the cerebral cortex, (b) GABAergic terminals forming single symmetric synapses (type S1), which likely arise from the reticular nucleus and GABAergic interneurons, and (c) GABAergic terminals forming multiple symmetric synapses (type S2), which originate from GPi. The density of As terminals outnumbered that of S1 and S2 terminals in VApc and CM of control and parkinsonian animals. No significant change was found in the abundance and synaptic connectivity of S1 and S2 terminals in VApc or CM of MPTP-treated monkeys, while the prevalence of “As” terminals in VApc of parkinsonian monkeys was 51.4% lower than in controls. The cross-sectional area of vGluT1-positive boutons in both VApc and CM of parkinsonian monkeys was significantly larger than in controls, but their pattern of innervation of thalamic cells was not altered. Our findings suggest that the corticothalamic system undergoes significant synaptic remodeling in the parkinsonian state.

(1) Background: Non-invasive neuromodulation is a promising alternative to medication or deep-brain stimulation treatment for Parkinson’s Disease or essential tremor. In previous work, we developed and tested a wearable system that modulates tremor via the non-invasive, electrical stimulation of peripheral nerves. In this article, we examine the proper range and the effects of various stimulation parameters for phase-locked stimulation. (2) Methods: We recruited nine participants with essential tremor. The subjects performed a bean-transfer task that mimics an eating activity to elicit kinetic tremor while using the wearable stimulation system. We examined the effects of stimulation with a fixed duty cycle, at different stimulation amplitudes and frequencies. The epochs of stimulation were locked to one of four phase positions of ongoing tremor, as measured with an accelerometer. We analyzed stimulation-evoked changes of the frequency and amplitude of tremor. (3) Results: We found that the higher tremor amplitude group experienced a higher rate of tremor power reduction (up to 65%) with a higher amplitude of stimulation when the stimulation was applied at the ±peak of tremor phase. (4) Conclusions: The stimulation parameter can be adjusted to optimize tremor reduction, and this study lays the foundation for future large-scale parameter optimization experiments for personalized peripheral nerve stimulation.

Optogenetics has revolutionized neuroscience in small laboratory animals, but its effect on animal models more closely related to humans, such as non-human primates (NHPs), has been mixed. To make evidence-based decisions in primate optogenetics, the scientific community would benefit from a centralized database listing all attempts, successful and unsuccessful, of using optogenetics in the primate brain. We contacted members of the community to ask for their contributions to an open science initiative. As of this writing, 45 laboratories around the world contributed more than 1,000 injection experiments, including precise details regarding their methods and outcomes. Of those entries, more than half had not been published. The resource is free for everyone to consult and contribute to on the Open Science Framework website. Here we review some of the insights from this initial release of the database and discuss methodological considerations to improve the success of optogenetic experiments in NHPs.

In order to evaluate the specific interactions between cortical oscillations and basal ganglia–spiking activity under normal and parkinsonian conditions, we examined the relationship between frontal cortex electroencephalographic (EEG) signals and simultaneously recorded neuronal activity in the internal and external segments of the pallidum or the subthalamic nucleus (STN) in 3 rhesus monkeys. After we made recordings in the normal state, hemiparkinsonism was induced with intracarotid injections of the dopaminergic neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) in one animal, followed by additional recordings. Spiking activity in the pallidum and STN was associated with significant shifts in the level of EEG synchronization. We also found that the spectral power of beta- and gamma-band EEG rhythms covaried positively before the basal ganglia spikes but did not covary or covaried negatively thereafter. In parkinsonism, changes in cortical synchronization and phase coherence were reduced in EEG segments aligned to STN spikes, whereas both were increased in data segments aligned to pallidal spikes. Spiking-related changes in beta/gamma-band covariance were reduced. The findings indicate that basal ganglia and cortex interact in the processing of cortical rhythms that contain oscillations across a broad range of frequencies and that this interaction is severely disrupted in parkinsonism.

The availability of suitable animal models and the opportunity to record electrophysiologic data in movement disorder patients undergoing neurosurgical procedures has allowed researchers to investigate parkinsonism-related changes in neuronal firing patterns in the basal ganglia and associated areas of the thalamus and cortex. These studies have shown that parkinsonism is associated with increased activity in the basal ganglia output nuclei, along with increases in burst discharges, oscillatory firing and synchronous firing patterns throughout the basal ganglia. Computational approaches have the potential to play an important role in the interpretation of these data. Such efforts can provide a formalized view of neuronal interactions in the network of connections between the basal ganglia, thalamus, and cortex, allow for the exploration of possible contributions of particular network components to parkinsonism, and potentially result in new conceptual frameworks and hypotheses that can be subjected to biological testing. It has proven very difficult, however, to integrate the wealth of the experimental findings into coherent models of the disease. In this review, we provide an overview of the abnormalities in neuronal activity that have been associated with parkinsonism. Subsequently, we discuss some particular efforts to model the pathophysiologic mechanisms that may link abnormal basal ganglia activity to the cardinal parkinsonian motor signs and may help to explain the mechanisms underlying the therapeutic efficacy of deep brain stimulation for Parkinson's disease. We emphasize the logical structure of these computational studies, making clear the assumptions from which they proceed and the consequences and predictions that follow from these assumptions. Parkinsonism has been linked with changes in neuronal firing patterns in the basal ganglia (BG) and associated areas of the thalamus and cortex. We provide an overview of these findings and discuss some efforts to use computational models to understand these relationships as well as the therapeutic effects of deep brain stimulation (DBS). In particular, several modeling studies that we consider focus on the idea that DBS works by regularizing BG outputs. For example, models show how parkinsonian basal ganglia outputs may compromise thalamocortical relay of excitatory inputs (curly brackets), while DBS- induced regularization may restore relay fidelity, and these ideas lead to predictions about the importance of particular BG outputs in the emergence of parkinsonian signs and of particular DBS properties in alleviating these signs.

The role of the corticothalamic projection in the ventral motor thalamus remains poorly understood. Therefore, we studied the electrophysiological responses of neurons in the basal ganglia and cerebellar receiving-territories of the motor thalamus (BGMT and CbMT, respectively) using optogenetic activation of corticothalamic projections in awake rhesus macaques. After injections of viral vectors carrying the excitatory opsins ChR2 or C1V1 into the primary motor and premotor cortices of two monkeys, we used optrodes to light activate opsin-expressing neurons in cortex or their terminals in the thalamus while simultaneously recording the extracellular activity of neuronsin the vicinity of the stimulationsites. Asexpected, light activation of opsinsin the cerebral cortex evoked robust,short-latency increases in firing of cortical neurons. In contrast, light stimulation of corticothalamic terminals induced small-amplitude, long-latency increases and/or decreases of activity in thalamic neurons. In postmortemmaterial, opsins were found to be expressed in cell bodies and dendrites of cortical neurons and along their corticothalamic projections. At the electron microscopic level, opsin labeling was confined to unmyelinated preterminal axons and small terminals that formed asymmetric synapses with dendrites of projection neurons or GABAergic interneurons in BGMT and CbMT and with neurons in the reticular thalamic nucleus. The morphological features of the transfected terminals, along with the long latency and complex physiological responses of thalamic neurons to their activation, suggest a modulatory role of corticothalamic afferents upon the primate ventral motor thalamus.

Analysis of the coupling between the phases and amplitudes of oscillations within the same continuously sampled signal has provided interesting insights into the physiology of memory and other brain process, and, more recently, the pathophysiology of parkinsonism and other movement disorders. Technical aspects of the analysis have a significant impact on the results. We present an empirical exploration of a variety of analysis design choices that need to be considered when measuring phase-amplitude coupling (PAC). We studied three alternative filtering approaches to the commonly used Kullback–Leibler distance-based method of PAC analysis, including one method that uses wavelets, one that uses constant filter settings, and one in which filtering of the data is optimized for individual frequency bands. Additionally, we introduce a time-dependent PAC analysis technique that takes advantage of the inherent temporality of wavelets. We examined how the duration of the sampled data, the stability of oscillations, or the presence of artifacts affect the value of the “modulation index”, a commonly used parameter describing the degree of PAC. We also studied the computational costs associated with calculating modulation indices by the three techniques. We found that wavelet-based PAC performs better with similar or less computational cost than the two other methods while also allowing to examine temporal changes of PAC. We also show that the reliability of PAC measurements strongly depends on the duration and stability of PAC, and the presence (or absence) of artifacts. The best parameters to be used for PAC analyses of long samples of data may differ, depending on data characteristics and analysis objectives. Prior to settling on a specific PAC analysis approach for a given set of data, it may be useful to conduct an initial analysis of the time-dependence of PAC using our time-resolved PAC analysis.

Parkinsonism is associated with changes in oscillatory activity patterns and increased synchronization of neurons in the basal ganglia and cortex in patients and animal models of Parkinson's disease, but the relationship between these changes and the severity of parkinsonian signs remains unclear. We examined this relationship by studying changes in local field potentials (LFPs) in the internal pallidal segment (GPi) and the subthalamic nucleus (STN), and in encephalographic signals (EEG) from the primary motor cortex (M1) in Rhesus monkeys which were rendered progressively parkinsonian by repeated systemic injections of small doses of the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). Observations during wakefulness and sleep (defined by EEG and video records) were analyzed separately. The severity of parkinsonism correlated with increases in spectral power at frequencies below 15.5. Hz in M1 and GPi and reductions in spectral power at frequencies above 15.6. Hz with little change in STN. The severity of parkinsonism also correlated with increases in the coherence between M1 EEG and basal ganglia LFPs in the low frequency band. Levodopa treatment reduced low-frequency activity and increased high-frequency activity in all three areas, but did not affect coherence. The state of arousal also affected LFP and EEG signals in all three structures, particularly in the STN. These results suggest that parkinsonism-associated changes in alpha and low-beta band oscillatory activity can be detected early in the parkinsonian state in M1 and GPi. Interestingly, oscillations detectable in STN LFP signals (including oscillations in the beta-band) do not appear to correlate strongly with the severity of mild-to-moderate parkinsonism in these animals. Levodopa-induced changes in oscillatory M1 EEG and basal ganglia LFP patterns do not necessarily represent a normalization of abnormalities caused by dopamine depletion.