Cortical signatures of sleep are altered following effective deep brain stimulation for depression

Deep brain stimulation (DBS) of the subcallosal cingulate cortex (SCC) is an experimental therapy for treatment-resistant depression (TRD). Chronic SCC DBS leads to long-term changes in the electrophysiological dynamics measured from local field potential (LFP) during wakefulness, but it is unclear how it impacts sleep-related brain activity. This is a crucial gap in knowledge, given the link between depression and sleep disturbances, and an emerging interest in the interaction between DBS, sleep, and circadian rhythms. We therefore sought to characterize changes in electrophysiological markers of sleep associated with DBS treatment for depression. We analyzed key electrophysiological signatures of sleep-slow-wave activity (SWA, 0.5-4.5 Hz) and sleep spindles-in LFPs recorded from the SCC of 9 patients who responded to DBS for TRD. This allowed us to compare the electrophysiological changes before and after 24 weeks of therapeutically effective SCC DBS. SWA power was highly correlated between hemispheres, consistent with a global sleep state. Furthermore, SWA occurred earlier in the night after chronic DBS and had a more prominent peak. While we found no evidence for changes to slow-wave power or stability, we found an increase in the density of sleep spindles. Our results represent a first-of-its-kind report on long-term electrophysiological markers of sleep recorded from the SCC in patients with TRD, and provides evidence of earlier NREM sleep and increased sleep spindle activity following clinically effective DBS treatment. Future work is needed to establish the causal relationship between long-term DBS and the neural mechanisms underlying sleep.

Coordinating brain-distributed network activities in memory resistant to extinction

Certain memories resist extinction to continue invigorating maladaptive actions. The robustness of these memories could depend on their widely distributed implementation across populations of neurons in multiple brain regions. However, how dispersed neuronal activities are collectively organized to underpin a persistent memory-guided behavior remains unknown. To investigate this, we simultaneously monitored the prefrontal cortex, nucleus accumbens, amygdala, hippocampus, and ventral tegmental area (VTA) of the mouse brain from initial recall to post-extinction renewal of a memory involving cocaine experience. We uncover a higher-order pattern of short-lived beta-frequency (15-25 Hz) activities that are transiently coordinated across these networks during memory retrieval. The output of a divergent pathway from upstream VTA glutamatergic neurons, paced by a slower (4-Hz) oscillation, actuates this multi-network beta-band coactivation; its closed-loop phase-informed suppression prevents renewal of cocaine-biased behavior. Binding brain-distributed neural activities in this temporally structured manner may constitute an organizational principle of robust memory expression.

The aperiodic exponent of subthalamic field potentials reflects excitation/inhibition balance in Parkinsonism

Periodic features of neural time-series data, such as local field potentials (LFPs), are often quantified using power spectra. While the aperiodic exponent of spectra is typically disregarded, it is nevertheless modulated in a physiologically relevant manner and was recently hypothesised to reflect excitation/inhibition (E/I) balance in neuronal populations. Here, we used a cross-species in vivo electrophysiological approach to test the E/I hypothesis in the context of experimental and idiopathic Parkinsonism. We demonstrate in dopamine-depleted rats that aperiodic exponents and power at 30-100 Hz in subthalamic nucleus (STN) LFPs reflect defined changes in basal ganglia network activity; higher aperiodic exponents tally with lower levels of STN neuron firing and a balance tipped towards inhibition. Using STN-LFPs recorded from awake Parkinson's patients, we show that higher exponents accompany dopaminergic medication and deep brain stimulation (DBS) of STN, consistent with untreated Parkinson's manifesting as reduced inhibition and hyperactivity of STN. These results suggest that the aperiodic exponent of STN-LFPs in Parkinsonism reflects E/I balance and might be a candidate biomarker for adaptive DBS.

Subthalamic and nigral neurons are differentially modulated during parkinsonian gait

The parkinsonian gait disorder and freezing of gait are therapeutically demanding symptoms with considerable impact on quality of life. The aim of this study was to assess the role of subthalamic and nigral neurons in the parkinsonian gait control using intraoperative microelectrode recordings of basal ganglia neurons during a supine stepping task. 12 male patients (56±7 years) suffering from moderate idiopathic Parkinson's disease (disease duration 10±3 years, Hoehn & Yahr stage 2) participated in the study. After 10 seconds resting period, stepping at self-paced speed for 35 seconds was followed by short intervals of stepping in response to random "start" and "stop" cues. Single- and multi-unit activity was analysed offline in relation to different aspects of the stepping task (attentional "start" and "stop" cues, heel strikes, stepping irregularities) in terms of firing frequency, firing pattern, and oscillatory activity. Subthalamic nucleus and the substantia nigra neurons responded to different aspects of the stepping task. 24% of the subthalamic nucleus neurons exhibited movement-related activity modulation as an increase of the firing rate, suggesting a predominant role of the subthalamic nucleus in motor aspects of the task, while 8% of subthalamic nucleus neurons showed a modulation in response to the attentional cues. In contrast, responsive substantia nigra neurons showed activity changes exclusively associated with attentional aspects of the stepping task (15%). The firing pattern of subthalamic nucleus neurons revealed gait-related firing regulation and a drop of beta oscillations during regular stepping performance. During freezing episodes instead, there was a rise of beta oscillatory activity. This study shows for the first time specific, task-related, subthalamic nucleus and substantia nigra single unit activity during gait-like movements in humans with differential roles in motor and attentional control of gait. The emergence of perturbed firing patterns in the subthalamic nucleus indicates a disturbed information transfer within the gait network, resulting in freezing of gait.

Diurnal modulation of subthalamic beta oscillatory power in Parkinson’s disease patients during deep brain stimulation

Beta-band activity in the subthalamic local field potential (LFP) is correlated with Parkinson’s disease (PD) symptom severity and is the therapeutic target of deep brain stimulation (DBS). While beta fluctuations in PD patients are well characterized on shorter timescales, it is not known how beta activity evolves around the diurnal cycle, outside a clinical setting. Here, we obtained chronic recordings (34 ± 13 days) of subthalamic beta power in PD patients implanted with the Percept DBS device during high-frequency DBS and analysed their diurnal properties as well as sensitivity to artifacts. Time of day explained 41 ± 9% of the variance in beta power (p < 0.001 in all patients), with increased beta during the day and reduced beta at night. Certain movements affected LFP quality, which may have contributed to diurnal patterns in some patients. Future DBS algorithms may benefit from taking such diurnal and artifactual fluctuations in beta power into account.

Stable, interactive modulation of neuronal oscillations produced through brain-machine equilibrium

Closed-loop interaction has the potential to regulate ongoing brain activity by continuously binding an external stimulation to specific dynamics of a neural circuit. Achieving interactive modulation requires a stable brain-machine feedback loop. Here, we demonstrate that it is possible to maintain oscillatory brain activity in a desired state by delivering stimulation accurately aligned with the timing of each cycle. We develop a fast algorithm that responds on a cycle-by-cycle basis to stimulate basal ganglia nuclei at predetermined phases of successive cortical beta cycles in parkinsonian rats. Using this approach, an equilibrium emerges between the modified brain signal and feedback-dependent stimulation pattern, leading to sustained amplification or suppression of the oscillation depending on the phase targeted. Beta amplification slows movement speed by biasing the animal's mode of locomotion. Together, these findings show that highly responsive, phase-dependent stimulation can achieve a stable brain-machine interaction that leads to robust modulation of ongoing behavior.

Computationally efficient neural network classifiers for next generation closed loop neuromodulation therapy - a case study in epilepsy

This work explores the potential utility of neural network classifiers for real-time classification of field-potential based biomarkers in next-generation responsive neuromodulation systems. Compared to classical filter-based classifiers, neural networks offer an ease of patient-specific parameter tuning, promising to reduce the burden of programming on clinicians. The paper explores a compact, feed-forward neural network architecture of only dozens of units for seizure-state classification in refractory epilepsy. The proposed classifier offers comparable accuracy to filterclassifiers on clinician-labeled data, while reducing detection latency. As a trade-off to classical methods, the paper focuses on keeping the complexity of the architecture minimal, to accommodate the on-board computational constraints of implantable pulse generator systems.

Clinical relevance—A neural network-based classifier is presented for responsive neurostimulation, with comparable accuracy to classical methods at reduced latency.

Current approaches to characterize micro- and macroscale circuit mechanisms of Parkinson's disease in rodent models

Accelerating technological progress in experimental neuroscience is increasing the scale as well as specificity of both observational and perturbational approaches to study circuit physiology. While these techniques have also been used to study disease mechanisms, a wider adoption of these approaches in the field of experimental neurology would greatly facilitate our understanding of neurological dysfunctions and their potential treatments at cellular and circuit level. In this review, we will introduce classic and novel methods ranging from single-cell electrophysiological recordings to state-of-the-art calcium imaging and cell-type specific optogenetic or chemogenetic stimulation. We will focus on their application in rodent models of Parkinson’s disease while also presenting their use in the context of motor control and basal ganglia function. By highlighting the scope and limitations of each method, we will discuss how they can be used to study pathophysiological mechanisms at local and global circuit levels and how novel frameworks can help to bridge these scales.

Input Zone-Selective Dysrhythmia in Motor Thalamus after Dopamine Depletion

The cerebral cortex, basal ganglia and motor thalamus form circuits important for purposeful movement. In Parkinsonism, basal ganglia neurons often exhibit dysrhythmic activity during, and with respect to, the slow (∼1 Hz) and beta-band (15-30 Hz) oscillations that emerge in cortex in a brain state-dependent manner. There remains, however, a pressing need to elucidate the extent to which motor thalamus activity becomes similarly dysrhythmic after dopamine depletion relevant to Parkinsonism. To address this, we recorded single-neuron and ensemble outputs in the basal ganglia-recipient zone (BZ) and cerebellar-recipient zone (CZ) of motor thalamus in anesthetized male dopamine-intact rats and 6-OHDA-lesioned rats during two brain states, respectively defined by cortical slow-wave activity and activation. Two forms of thalamic input zone-selective dysrhythmia manifested after dopamine depletion: (1) BZ neurons, but not CZ neurons, exhibited abnormal phase-shifted firing with respect to cortical slow oscillations prevalent during slow-wave activity; and (2) BZ neurons, but not CZ neurons, inappropriately synchronized their firing and engaged with the exaggerated cortical beta oscillations arising in activated states. These dysrhythmias were not accompanied by the thalamic hypoactivity predicted by canonical firing rate-based models of circuit organization in Parkinsonism. Complementary recordings of neurons in substantia nigra pars reticulata suggested that their altered activity dynamics could underpin the BZ dysrhythmias. Finally, pharmacological perturbations demonstrated that ongoing activity in the motor thalamus bolsters exaggerated beta oscillations in motor cortex. We conclude that BZ neurons are selectively primed to mediate the detrimental influences of abnormal slow and beta-band rhythms on circuit information processing in Parkinsonism.SIGNIFICANCE STATEMENT Motor thalamus neurons mediate the influences of basal ganglia and cerebellum on the cerebral cortex to govern movement. Chronic depletion of dopamine from the basal ganglia causes some symptoms of Parkinson's disease. Here, we elucidate how dopamine depletion alters the ways motor thalamus neurons engage with two distinct oscillations emerging in cortico-basal ganglia circuits in vivo We discovered that, after dopamine depletion, neurons in the thalamic zone receiving basal ganglia inputs are particularly prone to becoming dysrhythmic, changing the phases and/or synchronization (but not rate) of their action potential firing. This bolsters cortical dysrhythmia. Our results provide important new insights into how aberrant rhythmicity in select parts of motor thalamus could detrimentally affect neural circuit dynamics and behavior in Parkinsonism.

Diversity in striatal synaptic circuits arises from distinct embryonic progenitor pools in the ventral telencephalon

Synaptic circuits in the brain are precisely organized, but the processes that govern this precision are poorly understood. Here, we explore how distinct embryonic neural progenitor pools in the lateral ganglionic eminence contribute to neuronal diversity and synaptic circuit connectivity in the mouse striatum. In utero labeling of Tα1-expressing apical intermediate progenitors (aIP), as well as other progenitors (OP), reveals that both progenitors generate direct and indirect pathway spiny projection neurons (SPNs) with similar electrophysiological and anatomical properties and are intermingled in medial striatum. Subsequent optogenetic circuit-mapping experiments demonstrate that progenitor origin significantly impacts long-range excitatory input strength, with medial prefrontal cortex preferentially driving aIP-derived SPNs and visual cortex preferentially driving OP-derived SPNs. In contrast, the strength of local inhibitory inputs among SPNs is controlled by birthdate rather than progenitor origin. Combined, these results demonstrate distinct roles for embryonic progenitor origin in shaping neuronal and circuit properties of the postnatal striatum.

•

The Tα1 promoter distinguishes two embryonic progenitor pools in the LGE

•

Both pools generate intermixed spiny projection neurons in dorsomedial striatum

•

Excitatory cortical inputs are biased toward SPNs of different embryonic origin

•

Neurogenic stage rather impacts local inhibitory connections among SPNs

DyNeuMo Mk-2: An Investigational Circadian-Locked Neuromodulator with Responsive Stimulation for Applied Chronobiology

Deep brain stimulation (DBS) for Parkinson's disease, essential tremor and epilepsy is an established palliative treatment. DBS uses electrical neuromodulation to suppress symptoms. Most current systems provide a continuous pattern of fixed stimulation, with clinical follow-ups to refine settings constrained to normal office hours. An issue with this management strategy is that the impact of stimulation on circadian, i.e. sleep-wake, rhythms is not fully considered; either in the device design or in the clinical follow-up. Since devices can be implanted in brain targets that couple into the reticular activating network, impact on wakefulness and sleep can be significant. This issue will likely grow as new targets are explored, with the potential to create entraining signals that are uncoupled from environmental influences. To address this issue, we have designed a new brain-machine-interface for DBS that combines a slow-adaptive circadian-based stimulation pattern with a fast-acting pathway for responsive stimulation, demonstrated here for seizure management. In preparation for first-in-human research trials to explore the utility of multi-timescale automated adaptive algorithms, design and prototyping was carried out in line with ISO risk management standards, ensuring patient safety. The ultimate aim is to account for chronobiology within the algorithms embedded in brain-machine-interfaces and in neuromodulation technology more broadly.

Parkinson's disease uncovers an underlying sensitivity of subthalamic nucleus neurons to beta-frequency cortical input in vivo

Abnormally sustained beta-frequency synchronisation between the motor cortex and subthalamic nucleus (STN) is associated with motor symptoms in Parkinson's disease (PD). It is currently unclear whether STN neurons have a preference for beta-frequency input (12-35 Hz), rather than cortical input at other frequencies, and how such a preference would arise following dopamine depletion. To address this question, we combined analysis of cortical and STN recordings from awake human PD patients undergoing deep brain stimulation surgery with recordings of identified STN neurons in anaesthetised rats. In these patients, we demonstrate that a subset of putative STN neurons is strongly and selectively sensitive to magnitude fluctuations of cortical beta oscillations over time, linearly increasing their phase-locking strength with respect to the full range of instantaneous amplitude in the beta-frequency range. In rats, we probed the frequency response of STN neurons in the cortico-basal-ganglia-network more precisely, by recording spikes evoked by short bursts of cortical stimulation with variable frequency (4-40 Hz) and constant amplitude. In both healthy and dopamine-depleted rats, only beta-frequency stimulation led to a progressive reduction in the variability of spike timing through the stimulation train. This suggests, that the interval of beta-frequency input provides an optimal window for eliciting the next spike with high fidelity. We hypothesize, that abnormal activation of the indirect pathway, via dopamine depletion and/or cortical stimulation, could trigger an underlying sensitivity of the STN microcircuit to beta-frequency input.

•

STN-neurons are selectively entrained to cortical beta oscillations in PD patients.

•

Phase-locking of STN-neurons is linearly dependent on oscillation magnitude.

•

Beta bursts in LFP/EEG are accompanied by transient synchronisation of STN spiking.

•

STN neurons are selectively entrained to cortical beta stimulation in rats.

•

Beta-selectivity of STN neurons is present in control and dopamine-depleted rats.

Frequency and Phase Synchronization in Distributed (Implantable-Transcutaneous) Neural Interfaces

Synchronized oscillations are a ubiquitous feature of neuronal circuits and can modulate online information transfer and plasticity between brain areas. The disruption of these oscillatory processes is associated with the symptoms of several brain disorders. While conventional therapeutic high-frequency deep brain stimulation can perturb neuronal oscillations, manipulating the timing of oscillatory activity between areas more precisely could provide a more efficient and effective method of modulating these activities. Here we describe a prototype circuit for synchronizing the clocks between an active implantable and an external sensing and stimulation system that could be used to achieve this goal. Our specific focus is on synchronizing the systems for paired-associative stimulation. The ability to repetitively drive two brain regions with a fixed latency has specific implications for neural plasticity. Furthermore, the general concept can be applied for many potential applications involving distributed neural interfaces.

Thalamocortical dynamics underlying spontaneous transitions in beta power in Parkinsonism

Parkinson's disease (PD) is a neurodegenerative condition in which aberrant oscillatory synchronization of neuronal activity at beta frequencies (15–35 Hz) across the cortico-basal ganglia-thalamocortical circuit is associated with debilitating motor symptoms, such as bradykinesia and rigidity. Mounting evidence suggests that the magnitude of beta synchrony in the parkinsonian state fluctuates over time, but the mechanisms by which thalamocortical circuitry regulates the dynamic properties of cortical beta in PD are poorly understood. Using the recently developed generic Dynamic Causal Modelling (DCM) framework, we recursively optimized a set of plausible models of the thalamocortical circuit (n = 144) to infer the neural mechanisms that best explain the transitions between low and high beta power states observed in recordings of field potentials made in the motor cortex of anesthetized Parkinsonian rats. Bayesian model comparison suggests that upregulation of cortical rhythmic activity in the beta-frequency band results from changes in the coupling strength both between and within the thalamus and motor cortex. Specifically, our model indicates that high levels of cortical beta synchrony are mainly achieved by a delayed (extrinsic) input from thalamic relay cells to deep pyramidal cells and a fast (intrinsic) input from middle pyramidal cells to superficial pyramidal cells. From a clinical perspective, our study provides insights into potential therapeutic strategies that could be utilized to modulate the network mechanisms responsible for the enhancement of cortical beta in PD. Specifically, we speculate that cortical stimulation aimed to reduce the enhanced excitatory inputs to either the superficial or deep pyramidal cells could be a potential non-invasive therapeutic strategy for PD.

•

Coupling changes within and between circuit nodes lead to cortical beta enhancement.

•

Input propagation delays play a crucial role in the up-regulation of cortical beta.

•

Beta power could be modulated by altering lamina specific inputs.

A Hippocampus-Accumbens Tripartite Neuronal Motif Guides Appetitive Memory in Space

Retrieving and acting on memories of food-predicting environments are fundamental processes for animal survival. Hippocampal pyramidal cells (PYRs) of the mammalian brain provide mnemonic representations of space. Yet the substrates by which these hippocampal representations support memory-guided behavior remain unknown. Here, we uncover a direct connection from dorsal CA1 (dCA1) hippocampus to nucleus accumbens (NAc) that enables the behavioral manifestation of place-reward memories. By monitoring neuronal ensembles in mouse dCA1→NAc pathway, combined with cell-type selective optogenetic manipulations of input-defined postsynaptic neurons, we show that dCA1 PYRs drive NAc medium spiny neurons and orchestrate their spiking activity using feedforward inhibition mediated by dCA1-connected parvalbumin-expressing fast-spiking interneurons. This tripartite cross-circuit motif supports spatial appetitive memory and associated NAc assemblies, being independent of dorsal subiculum and dispensable for both spatial novelty detection and reward seeking. Our findings demonstrate that the dCA1→NAc pathway instantiates a limbic-motor interface for neuronal representations of space to promote effective appetitive behavior.

Synchronised spiking activity underlies phase amplitude coupling in the subthalamic nucleus of Parkinson's disease patients

Both phase-amplitude coupling (PAC) and beta-bursts in the subthalamic nucleus have been significantly linked to symptom severity in Parkinson's disease (PD) in humans and emerged independently as competing biomarkers for closed-loop deep brain stimulation (DBS). However, the underlying nature of subthalamic PAC is poorly understood and its relationship with transient beta burst-events has not been investigated. To address this, we studied macro- and micro electrode recordings of local field potentials (LFPs) and single unit activity from 15 hemispheres in 10 PD patients undergoing DBS surgery. PAC between beta phase and high frequency oscillation (HFO) amplitude was compared to single unit firing rates, spike triggered averages, power spectral densities, inter spike intervals and phase-spike locking, and was studied in periods of beta-bursting. We found a significant synchronisation of spiking to HFOs and correlation of mean firing rates with HFO-amplitude when the latter was coupled to beta phase (i.e. in the presence of PAC). In the presence of PAC, single unit power spectra displayed peaks in the beta and HFO frequency range and the HFO frequency was correlated with that in the LFP. Furthermore, inter spike interval frequencies peaked in the same frequencies for which PAC was observed. Finally, PAC significantly increased with beta burst-duration. Our findings offer new insight in the pathology of Parkinson's disease by providing evidence that subthalamic PAC reflects the locking of spiking activity to network beta oscillations and that this coupling progressively increases with beta-burst duration. These findings suggest that beta-bursts capture periods of increased subthalamic input/output synchronisation in the beta frequency range and have important implications for therapeutic closed-loop DBS.

Propagation of beta/gamma rhythms in the cortico-basal ganglia circuits of the parkinsonian rat

Much of the motor impairment associated with Parkinson’s disease is thought to arise from pathological activity in the networks formed by the basal ganglia (BG) and motor cortex. To evaluate several hypotheses proposed to explain the emergence of pathological oscillations in parkinsonism, we investigated changes to the directed connectivity in BG networks following dopamine depletion. We recorded local field potentials (LFPs) in the cortex and basal ganglia of rats rendered parkinsonian by injection of 6-hydroxydopamine (6-OHDA) and in dopamine-intact controls. We performed systematic analyses of the networks using a novel tool for estimation of directed interactions (nonparametric directionality, NPD). We used a “conditioned” version of the NPD analysis that reveals the dependence of the correlation between two signals on a third reference signal. We find evidence of the dopamine dependency of both low-beta (14–20 Hz) and high-beta/low-gamma (20–40 Hz) directed network interactions. Notably, 6-OHDA lesions were associated with enhancement of the cortical “hyperdirect” connection to the subthalamic nucleus (STN) and its feedback to the cortex and striatum. We find that pathological beta synchronization resulting from 6-OHDA lesioning is widely distributed across the network and cannot be located to any individual structure. Furthermore, we provide evidence that high-beta/gamma oscillations propagate through the striatum in a pathway that is independent of STN. Rhythms at high beta/gamma show susceptibility to conditioning that indicates a hierarchical organization compared with those at low beta. These results further inform our understanding of the substrates for pathological rhythms in salient brain networks in parkinsonism.

NEW & NOTEWORTHY We present a novel analysis of electrophysiological recordings in the cortico-basal ganglia network with the aim of evaluating several hypotheses concerning the origins of abnormal brain rhythms associated with Parkinson’s disease. We present evidence for changes in the directed connections within the network following chronic dopamine depletion in rodents. These findings speak to the plausibility of a “short-circuiting” of the network that gives rise to the conditions from which pathological synchronization may arise.

Structural and molecular heterogeneity of calretinin-expressing interneurons in the rodent and primate striatum

Calretinin‐expressing (CR+) interneurons are the most common type of striatal interneuron in primates. However, because CR+ interneurons are relatively scarce in rodent striatum, little is known about their molecular and other properties, and they are typically excluded from models of striatal circuitry. Moreover, CR+ interneurons are often treated in models as a single homogenous population, despite previous descriptions of their heterogeneous structures and spatial distributions in rodents and primates. Here, we demonstrate that, in rodents, the combinatorial expression of secretagogin (Scgn), specificity protein 8 (SP8) and/or LIM homeobox protein 7 (Lhx7) separates striatal CR+ interneurons into three structurally and topographically distinct cell populations. The CR+/Scgn+/SP8+/Lhx7− interneurons are small‐sized (typically 7–11 µm in somatic diameter), possess tortuous, partially spiny dendrites, and are rostrally biased in their positioning within striatum. The CR+/Scgn−/SP8−/Lhx7− interneurons are medium‐sized (typically 12–15 µm), have bipolar dendrites, and are homogenously distributed throughout striatum. The CR+/Scgn−/SP8−/Lhx7+ interneurons are relatively large‐sized (typically 12–20 µm), and have thick, infrequently branching dendrites. Furthermore, we provide the first in vivo electrophysiological recordings of identified CR+ interneurons, all of which were the CR+/Scgn−/SP8−/Lhx7− cell type. In the primate striatum, Scgn co‐expression also identified a topographically distinct CR+ interneuron population with a rostral bias similar to that seen in both rats and mice. Taken together, these results suggest that striatal CR+ interneurons comprise at least three molecularly, structurally, and topographically distinct cell populations in rodents. These properties are partially conserved in primates, in which the relative abundance of CR+ interneurons suggests that they play a critical role in striatal microcircuits.

A Population of Indirect Pathway Striatal Projection Neurons Is Selectively Entrained to Parkinsonian Beta Oscillations

Classical schemes of basal ganglia organization posit that parkinsonian movement difficulties presenting after striatal dopamine depletion stem from the disproportionate firing rates of spiny projection neurons (SPNs) therein. There remains, however, a pressing need to elucidate striatal SPN firing in the context of the synchronized network oscillations that are abnormally exaggerated in cortical-basal ganglia circuits in parkinsonism. To address this, we recorded unit activities in the dorsal striatum of dopamine-intact and dopamine-depleted rats during two brain states, respectively defined by cortical slow-wave activity (SWA) and activation. Dopamine depletion escalated striatal net output but had contrasting effects on "direct pathway" SPNs (dSPNs) and "indirect pathway" SPNs (iSPNs); their firing rates became imbalanced, and they disparately engaged in network oscillations. Disturbed striatal activity dynamics relating to the slow (∼1 Hz) oscillations prevalent during SWA partly generalized to the exaggerated beta-frequency (15-30 Hz) oscillations arising during cortical activation. In both cases, SPNs exhibited higher incidences of phase-locked firing to ongoing cortical oscillations, and SPN ensembles showed higher levels of rhythmic correlated firing, after dopamine depletion. Importantly, in dopamine-depleted striatum, a widespread population of iSPNs, which often displayed excessive firing rates and aberrant phase-locked firing to cortical beta oscillations, preferentially and excessively synchronized their firing at beta frequencies. Conversely, dSPNs were neither hyperactive nor synchronized to a large extent during cortical activation. These data collectively demonstrate a cell type-selective entrainment of SPN firing to parkinsonian beta oscillations. We conclude that a population of overactive, excessively synchronized iSPNs could orchestrate these pathological rhythms in basal ganglia circuits.SIGNIFICANCE STATEMENT Chronic depletion of dopamine from the striatum, a part of the basal ganglia, causes some symptoms of Parkinson's disease. Here, we elucidate how dopamine depletion alters striatal neuron firing in vivo, with an emphasis on defining whether and how spiny projection neurons (SPNs) engage in the synchronized beta-frequency (15-30 Hz) oscillations that become pathologically exaggerated throughout basal ganglia circuits in parkinsonism. We discovered that a select population of so-called "indirect pathway" SPNs not only fire at abnormally high rates, but are also particularly prone to being recruited to exaggerated beta oscillations. Our results provide an important link between two complementary theories that explain the presentation of disease symptoms on the basis of changes in firing rate or firing synchronization/rhythmicity.

Secretagogin expression delineates functionally-specialized populations of striatal parvalbumin-containing interneurons

Corticostriatal afferents can engage parvalbumin-expressing (PV+) interneurons to rapidly curtail the activity of striatal projection neurons (SPNs), thus shaping striatal output. Schemes of basal ganglia circuit dynamics generally consider striatal PV+ interneurons to be homogenous, despite considerable heterogeneity in both form and function. We demonstrate that the selective co-expression of another calcium-binding protein, secretagogin (Scgn), separates PV+ interneurons in rat and primate striatum into two topographically-, physiologically- and structurally-distinct cell populations. In rats, these two interneuron populations differed in their firing rates, patterns and relationships with cortical oscillations in vivo. Moreover, the axons of identified PV+/Scgn+ interneurons preferentially targeted the somata of SPNs of the so-called 'direct pathway', whereas PV+/Scgn- interneurons preferentially targeted 'indirect pathway' SPNs. These two populations of interneurons could therefore provide a substrate through which either of the striatal output pathways can be rapidly and selectively inhibited to subsequently mediate the expression of behavioral routines.

The highs and lows of beta activity in cortico-basal ganglia loops

Oscillatory activity in the beta (13-30 Hz) frequency band is widespread in cortico-basal ganglia circuits, and becomes prominent in Parkinson's disease (PD). Here we develop the hypothesis that the degree of synchronization in this frequency band is a critical factor in gating computation across a population of neurons, with increases in beta band synchrony entailing a loss of information-coding space and hence computational capacity. Task and context drive this dynamic gating, so that for each state there will be an optimal level of network synchrony, and levels lower or higher than this will impair behavioural performance. Thus, both the pathological exaggeration of synchrony, as observed in PD, and the ability of interventions like deep brain stimulation (DBS) to excessively suppress synchrony can potentially lead to impairments in behavioural performance. Indeed, under physiological conditions, the manipulation of computational capacity by beta activity may itself present a mechanism of action selection and maintenance.

Cortical and thalamic excitation mediate the multiphasic responses of striatal cholinergic interneurons to motivationally salient stimuli

Cholinergic interneurons are key components of striatal microcircuits. In primates, tonically active neurons (putative cholinergic interneurons) exhibit multiphasic responses to motivationally salient stimuli that mirror those of midbrain dopamine neurons and together these two systems mediate reward-related learning in basal ganglia circuits. Here, we addressed the potential contribution of cortical and thalamic excitatory inputs to the characteristic multiphasic responses of cholinergic interneurons in vivo. We first recorded and labeled individual cholinergic interneurons in anesthetized rats. Electron microscopic analyses of these labeled neurons demonstrated that an individual interneuron could form synapses with cortical and, more commonly, thalamic afferents. Single-pulse electrical stimulation of ipsilateral frontal cortex led to robust short-latency (<20 ms) interneuron spiking, indicating monosynaptic connectivity, but firing probability progressively decreased during high-frequency pulse trains. In contrast, single-pulse thalamic stimulation led to weak short-latency spiking, but firing probability increased during pulse trains. After initial excitation from cortex or thalamus, interneurons displayed a "pause" in firing, followed by a "rebound" increase in firing rate. Across all stimulation protocols, the number of spikes in the initial excitation correlated positively with pause duration and negatively with rebound magnitude. The magnitude of the initial excitation, therefore, partly determined the profile of later components of multiphasic responses. Upon examining the responses of tonically active neurons in behaving primates, we found that these correlations held true for unit responses to a reward-predicting stimulus, but not to the reward alone, delivered outside of any task. We conclude that excitatory inputs determine, at least in part, the multiphasic responses of cholinergic interneurons under specific behavioral conditions.

Asymmetric pallidal neuronal activity in patients with cervical dystonia

The origin of asymmetric clinical manifestation of symptoms in patients suffering from cervical dystonia (CD) is hitherto poorly understood. Dysregulated neuronal activity in the basal ganglia has been suggested to have a role in the pathophysiology of CD. Here, we re-assessed the question to what extent relative changes occur in the direct vs. indirect basal ganglia pathway in CD, whether these circuit changes are lateralized, and how these alterations relate to CD symptoms. To this end, we recorded ongoing single cell and local field potential (LFP) activity from the external (GPe) and internal pallidal segment (GPi) of 13 CD patients undergoing microelectrode-guided stereotactic surgery for deep brain stimulation in the GPi. We compared pallidal recordings from CD patients operated under local anaesthesia (LA) with those obtained in CD patients operated under general anaesthesia (GA). In awake patients, mean GPe discharge rate (52 Hz) was lower than that of GPi (72 Hz). Mean GPi discharge ipsilateral to the side of head turning was higher than contralateral and correlated with torticollis symptom severity. Lateralized differences were absent at the level of the GPe and in recordings from patients operated under GA. Furthermore, in the GPi of CD patients there was a subpopulation of theta-oscillatory cells with unique bursting characteristics. Power and coherence of GPe– and GPi–LFPs were dominated by a theta peak and also exhibited band-specific interhemispheric differences. Strong cross-frequency coupling of low-gamma amplitude to theta phase was a feature of pallidal LFPs recorded under LA, but not GA. These results indicate that CD is associated with an asymmetric pallidal outflow. Based on the finding of symmetric neuronal discharges in the GPe, we propose that an imbalanced interhemispheric direct pathway gain may be involved in CD pathophysiology.

Decomposition of abnormal free locomotor behavior in a rat model of Parkinson's disease

Poverty of spontaneous movement, slowed execution and reduced amplitudes of movement (akinesia, brady- and hypokinesia) are cardinal motor manifestations of Parkinson's disease that can be modeled in experimental animals by brain lesions affecting midbrain dopaminergic neurons. Most behavioral investigations in experimental parkinsonism have employed short-term observation windows to assess motor impairments. We postulated that an analysis of longer-term free exploratory behavior could provide further insights into the complex fine structure of altered locomotor activity in parkinsonian animals. To this end, we video-monitored 23 h of free locomotor behavior and extracted several behavioral measures before and after the expression of a severe parkinsonian phenotype following bilateral 6-hydroxydopamine (6-OHDA) lesions of the rat dopaminergic substantia nigra. Unbiased stereological cell counting verified the degree of midbrain tyrosine hydroxylase positive cell loss in the substantia nigra and ventral tegmental area. In line with previous reports, overall covered distance and maximal motion speed of lesioned animals were found to be significantly reduced compared to controls. Before lesion surgery, exploratory rat behavior exhibited a bimodal distribution of maximal speed values obtained for single movement episodes, corresponding to a "first" and "second gear" of motion. 6-OHDA injections significantly reduced the incidence of second gear motion episodes and also resulted in an abnormal prolongation of these fast motion events. Likewise, the spatial spread of such episodes was increased in 6-OHDA rats. The increase in curvature of motion tracks was increased in both lesioned and control animals. We conclude that the discrimination of distinct modes of motion by statistical decomposition of longer-term spontaneous locomotion provides useful insights into the fine structure of fluctuating motor functions in a rat analog of Parkinson's disease.

Temporal coupling with cortex distinguishes spontaneous neuronal activities in identified basal ganglia-recipient and cerebellar-recipient zones of the motor thalamus

Neurons of the motor thalamus mediate basal ganglia and cerebellar influences on cortical activity. To elucidate the net result of γ-aminobutyric acid-releasing or glutamatergic bombardment of the motor thalamus by basal ganglia or cerebellar afferents, respectively, we recorded the spontaneous activities of thalamocortical neurons in distinct identified “input zones” in anesthetized rats during defined cortical activity states. Unexpectedly, the mean rates and brain state dependencies of the firing of neurons in basal ganglia-recipient zone (BZ) and cerebellar-recipient zone (CZ) were matched during slow-wave activity (SWA) and cortical activation. However, neurons were distinguished during SWA by their firing regularities, low-threshold spike bursts and, more strikingly, by the temporal coupling of their activities to ongoing cortical oscillations. The firing of neurons across the BZ was stronger and more precisely phase-locked to cortical slow (∼1 Hz) oscillations, although both neuron groups preferentially fired at the same phase. In contrast, neurons in BZ and CZ fired at different phases of cortical spindles (7–12 Hz), but with similar strengths of coupled firing. Thus, firing rates do not reflect the predicted inhibitory–excitatory imbalance across the motor thalamus, and input zone-specific temporal coding through oscillatory synchronization with the cortex could partly mediate the different roles of basal ganglia and cerebellum in behavior.

Effect of sensory stimulation in rat barrel cortex, dorsolateral striatum and on corticostriatal functional connectivity

The striatum integrates sensory information to enable action selection and behavioural reinforcement. In the rat, a large topographical projection from the rat barrel cortex to widely distributed areas of the striatum is assumed to be an important structural component supporting these processes. The striatal sensory response is, however, not comprehensively understood at a network level. We used a 10-Hz, 100-ms air puff, allowing undamped movement of multiple whiskers, to look at functional connectivity in contralateral cortex and striatum in response to sensory stimulation. Simultaneous recordings of cortical and striatal local field potentials (LFPs) were made under isoflurane anaesthesia in 15 male Brown Norway rats. Four electrodes were placed in the barrel cortex while the dorsolateral striatum was mapped with a 500-μm resolution, resulting in a maximum of 315 recording positions per animal. Significant event-related responses were unevenly distributed throughout the striatum in 34.8% of positions recorded within this area. Only 10.3% of recorded positions displayed significant total power increases in the LFPs during the stimulation period at the stimulus frequency. This suggests that the responses seen in the LFPs are due to phase rearrangement rather than an amplitude increase in the signal. Analysis of corticostriatal imaginary coherence revealed stimulus-induced changes in the functional connectivity of 12% of corticostriatal pairs, the sensory response of sparsely distributed neuronal ensembles within the dorsolateral striatum is reflected in the phase relationship between the cortical and striatal local fields.

Functional connectivity between motor cortex and globus pallidus in human non-REM sleep

Recent evidence suggests that the motor system undergoes very specific modulation in its functional state during the different sleep stages. Here we test the hypothesis that changes in the functional organization of the motor system involve both cortical and subcortical levels and that these distributed changes are interrelated in defined frequency bands. To this end we evaluated functional connectivity between motor and non-motor cortical sites (fronto-central, parieto-occipital) and the globus pallidus (GP) in human non-REM sleep in seven patients undergoing deep brain stimulation (DBS) for dystonia using a variety of spectral measures (power, coherence, partial coherence and directed transfer function (DTF)). We found significant coherence between GP and fronto-central cortex as well as between GP and parieto-occipital cortex in circumscribed frequency bands that correlated with sleep specific oscillations in 'light sleep' (N2) and 'slow-wave sleep' (N3). These sleep specific oscillations were also reflected in significant coherence between the two cortical sites corroborating previous studies. Importantly, we found two different physiological activities represented within the broad band of significant coherence between 9.5 and 17 Hz. One component occurred in the frequency range of sleep spindles (12.5-17 Hz) and was maximal in the coherence between fronto-central and parieto-occipital cortex as well as between GP and both cortical sites during N2. This component was still present between fronto-central and parieto-occipital cortex in N3. Functional connectivity in this frequency band may be due to a common input to both GP and cortex. The second component consisted of a spectral peak over 9.5-12.5 Hz. Coherence was elevated in this band for all topographical constellations in both N2 and N3, but especially between GP and fronto-central cortex. The DTF suggested that the 9.5-12.5 Hz activity consisted of a preferential drive from GP to the fronto-central cortex in N2, whereas in N3 the DTF between GP and fronto-central cortex was symmetrical. Partial coherence supported distinctive patterns for the 9.5-12.5 and 12.5 and 17 Hz component, so that only coherence in the 9.5-12.5 Hz band was reduced when the effects of GP were removed from the coherence between the two cortical sites. The data suggest that activities in the GP and fronto-central cortex are functionally connected over 9.5-12.5 Hz, possibly as a specific signature of the motor system in human non-REM sleep. This finding is pertinent to the longstanding debate about the nature of alpha-delta sleep as a physiological or pathological feature of non-REM sleep.

Waking up the brain: a case study of stimulation-induced wakeful unawareness during anaesthesia

Hitherto, little is known about the specific functional contributions of extrathalamic arousal systems to the regulation of wakefulness in humans. Here, we describe a 42-year-old woman with treatment resistant tremulous cervical dystonia who underwent microelectrode-guided stereotactic implantation of deep brain stimulation (DBS) electrodes in the internal segment of the globus pallidus internus (GPi) under general anaesthesia. Acute unilateral DBS of circumscribed sites within the subpallidal fibre-field with 130 Hz caused a transient state of wakefulness with an increased responsiveness to external stimuli but without detectable signs of conscious awareness. The extent of behavioural arousal could be titrated as a function of stimulus intensity. At lower stimulation intensities, bilateral eye opening occurred in response to verbal commands or tactile stimulation. At suprathreshold intensities, the patient's eyes remained open and conjugated throughout the stimulation period. The arousal effect ceased abruptly when DBS was discontinued. Behavioural arousal was accompanied by global cortical EEG activation in the gamma-frequency range (40-120 Hz) and by autonomic activation as evidenced by increased heart rate. The observed effect was reproducible in both hemispheres and topographically restricted to 6 out of 15 tested sites in the fibre-field between the GPi and the posterior aspect of the basal nucleus of Meynert. We conclude that the stimulated neural substrate in the subpallidal basal forebrain is involved in the premotor control of lid and eye position and the control of the activation state of the human neocortex. It may thus be important for the induction and maintenance of anaesthesia-induced unconsciousness in humans. It is suggested that subpallidal DBS released a downstream arousal circuit from anaesthesia-related inhibitory modulation either by direct excitation of an arousal nucleus or by inhibition of a sleep-promoting centre in the basal forebrain.

Changes in functional connectivity within the rat striatopallidal axis during global brain activation in vivo

The functional organization of the basal ganglia (BG) is often defined according to one of two opposing schemes. The first proposes multiple, essentially independent channels of information processing. The second posits convergence and lateral integration of striatal channels at the level of the globus pallidus (GP). We tested the hypothesis that these proposed aspects of functional connectivity within the striatopallidal axis are dynamic and related to brain state. Local field potentials (LFPs) were simultaneously recorded from multiple sites in striatum and GP in anesthetized rats during slow-wave activity (SWA) and during global activation evoked by sensory stimulation. Functional connectivity was inferred from comparative analyses of the internuclear and intranuclear coherence between bipolar derivations of LFPs. During prominent SWA, as shown in the electrocorticogram and local field potentials in the basal ganglia, intranuclear coherence, and, thus, lateral functional connectivity within striatum or globus pallidus was relatively weak. Furthermore, the temporal coupling of LFPs recorded across these two nuclei involved functional convergence at the level of GP. Global activation, indicated by a loss of SWA, was accompanied by a rapid functional reorganization of the striatopallidal axis. Prominent lateral functional connectivity developed within GP and, to a significantly more constrained spatial extent, striatum. Additionally, functional convergence on GP was no longer apparent, despite increased internuclear coherence. These data demonstrate that functional connectivity within the BG is highly dynamic and suggest that the relative expression of organizational principles, such as parallel, independent processing channels, striatopallidal convergence, and lateral integration within BG nuclei, is dependent on brain state.

Delayed synchronization of activity in cortex and subthalamic nucleus following cortical stimulation in the rat

Oscillations may play a role in the functional organization of cortico-basal ganglia-thalamocortical circuits, and it is important to understand their underlying mechanisms. The cortex often drives basal ganglia (BG) activity, and particularly, oscillatory activity in the subthalamic nucleus (STN). However, the STN may also indirectly influence cortex. The aim of this study was to characterize the delayed (>200 ms) responses of STN neurons to synchronized cortical inputs, focusing on their relationship with oscillatory cortical activity. We recorded the short-latency and delayed responses of STN units and frontal electrocorticogram (ECoG) to cortical stimulation in anaesthetized rats. Similar to previous studies, stimulation of ipsilateral frontal cortex, but not temporal cortex, evoked a short-latency triphasic response, followed by a sustained reduction or pause in firing, in rostral STN units. Caudal STN units did not show the short-latency triphasic response but often displayed a prolonged firing reduction. Oscillations in STN unit activity and ECoG were common after this sustained firing reduction, particularly between 200 and 600 ms after frontal cortical stimulation. These delayed oscillations were significantly coherent in a broad frequency band of 5-30 Hz. Coherence with ECoG at 5-15 Hz was observed throughout STN, though coherence at 15-30 Hz was largely restricted to rostral STN. Furthermore, oscillatory responses at 5-30 Hz in rostral STN predominantly led those in cortex (mean latency of 29 ms) after frontal cortical stimulation. These findings suggest that STN neurons responding to corticosubthalamic inputs may provide a delayed input to cortex, via BG output nuclei, and thence, thalamocortical pathways.

Dopamine depletion increases the power and coherence of β-oscillations in the cerebral cortex and subthalamic nucleus of the awake rat

Local field potentials (LFPs) recorded from the subthalamic nucleus (STN) of untreated patients implanted with stimulation electrodes for the treatment of Parkinson's disease (PD) demonstrate strong coherence with the cortical electroencephalogram over the beta-frequency range (15-30 Hz). However, studies in animal models of PD emphasize increased temporal coupling in cortico-basal ganglia circuits at substantially lower frequencies, undermining the potential usefulness of these models. Here we show that 6-hydroxydopamine (6-OHDA) lesions of midbrain dopamine neurons are associated with significant increases in the power and coherence of beta-frequency oscillatory activity present in LFPs recorded from frontal cortex and STN of awake rats, as compared with the healthy animal. Thus, the pattern of synchronization between population activity in the STN and cortex in the 6-OHDA-lesioned rodent model of PD closely parallels that seen in the parkinsonian human. The peak frequency of coherent activity in the beta-frequency range was increased in lesioned animals during periods of spontaneous and sustained movement. Furthermore, administration of the dopamine receptor agonist apomorphine to lesioned animals suppressed beta-frequency oscillations, and increased coherent activity at higher frequencies in the cortex and STN, before producing the rotational behaviour indicative of successful lesion. Taken together, these results support a crucial role for dopamine in the modulation of population activity in cortico-basal ganglia circuits, whereby dopaminergic mechanisms effectively filter out synchronized, rhythmic activity at beta-frequencies at the systems level, and shift temporal couplings in these circuits to higher frequencies. These changes may be important in regulating movement.

Coherent spike-wave oscillations in the cortex and subthalamic nucleus of the freely moving rat

The basal ganglia play a critical role in controlling seizures in animal models of idiopathic non-convulsive (absence) epilepsy. Inappropriate output from the substantia nigra pars reticulata (SNr) is known to exacerbate seizures, but the precise neuronal mechanisms underlying abnormal activity in SNr remain unclear. To test the hypothesis that cortical spike-wave oscillations, often considered indicative of absence seizures, propagate to the subthalamic nucleus, an important afferent of SNr, we simultaneously recorded local field potentials from the frontal cortex and subthalamic nucleus of freely moving rats. Spontaneous spike-wave oscillations in cortex (mean dominant frequency of 7.4 Hz) were associated with similar oscillations in the subthalamic nucleus (mean of 7.9 Hz). The power of oscillations at 5-9 Hz was significantly higher during spike-wave activity as compared with rest periods without this activity. Importantly, spike-wave oscillations in cortex and subthalamic nucleus were significantly coherent across a range of frequencies (3-40 Hz), and the dominant (7-8 Hz) oscillatory activity in the subthalamic nucleus typically followed that in cortex with a small time lag (mean of 2.7 ms). In conclusion, these data suggest that ensembles of subthalamic nucleus neurons are rapidly recruited into oscillations during cortical spike-wave activity, thus adding further weight to the importance of the subthalamic nucleus in absence epilepsy. An increase in synchronous oscillatory input from the subthalamic nucleus could thus partly underlie the expression of pathological activity in SNr that could, in turn, aggravate seizures. Finally, these findings also reiterate the importance of oscillations in these circuits in normal behaviour.

Directional analysis of coherent oscillatory field potentials in the cerebral cortex and basal ganglia of the rat

Population activity in cortico-basal ganglia circuits is synchronized at different frequencies according to brain state. However, the structures that are likely to drive the synchronization of activity in these circuits remain unclear. Furthermore, it is not known whether the direction of transmission of activity is fixed or dependent on brain state. We have used the directed transfer function (DTF) to investigate the direction in which coherent activity is effectively driven in cortico-basal ganglia circuits. Local field potentials (LFPs) were simultaneously recorded in the subthalamic nucleus (STN), globus pallidus (GP) and substantia nigra pars reticulata (SNr), together with the ipsilateral frontal electrocorticogram (ECoG) of anaesthetized rats. Directional analysis was performed on recordings made during robust cortical slow-wave activity (SWA) and "global activation". During SWA, there was coherence at approximately 1 Hz between ECoG and basal ganglia LFPs, with much of the coherent activity directed from cortex to basal ganglia. There were similar coherent activities at approximately 1 Hz within the basal ganglia, with more activity directed from SNr to GP and STN, and from STN to GP rather than vice versa. During global activation, peaks in coherent activity were seen at higher frequencies (15-60 Hz), with most coherence also directed from cortex to basal ganglia. Within the basal ganglia, however, coherence was predominantly directed from GP to STN and SNr. Together, these results highlight a lead role for the cortex in activity relationships with the basal ganglia, and further suggest that the effective direction of coupling between basal ganglia nuclei is dynamically organized according to brain state, with activity relationships involving the GP displaying the greatest capacity to change.

Brain State–Dependency of Coherent Oscillatory Activity in the Cerebral Cortex and Basal Ganglia of the Rat

The nature of the coupling between neuronal assemblies in the cerebral cortex and basal ganglia (BG) is poorly understood. We tested the hypothesis that coherent population activity is dependent on brain state, frequency range, and/or BG nucleus using data from simultaneous recordings of electrocorticogram (ECoG) and BG local field potentials (LFPs) in anesthetized rats. The coherence between ECoG and LFPs simultaneously recorded from subthalamic nucleus (STN), globus pallidus (GP), and substantia nigra pars reticulata (SNr) was largely confined to slow- (∼1 Hz) and spindle- (7–12 Hz) frequency oscillations during slow-wave activity (SWA). In contrast, during cortical activation, coherence was mostly restricted to high-frequency oscillations (15–60 Hz). The coherence between ECoG and LFPs also depended on BG recording site. Partial coherence analyses showed that, during SWA, STN and SNr shared the same temporal coupling with cortex, thereby forming a single functional axis. Cortex was also tightly, but independently, correlated with GP in a separate functional axis. During activation, STN, GP, and, to a lesser extent, SNr shared the same coherence with cortex as part of one functional axis. In addition, GP formed a second, independently coherent loop with cortex. These data suggest that coherent oscillatory activity is present at the level of LFPs recorded in cortico-basal ganglia circuits, and that synchronized population activity is dynamically organized according to brain state, frequency, and nucleus. These attributes further suggest that synchronized activity should be considered as one of a number of candidate mechanisms underlying the functional organization of these brain circuits.

Synchronous Unit Activity and Local Field Potentials Evoked in the Subthalamic Nucleus by Cortical Stimulation

The responses of single subthalamic nucleus (STN) neurons to cortical activation are complex and depend on the relative activation of several neuronal circuits, making theoretical extrapolation of single neuron responses to the population level difficult. To understand better the degree of synchrony imposed on STN neurons and associated neuronal networks by cortical activation, we recorded the responses of single units, pairs of neighboring neurons, and local field potentials (LFPs) in STN to discrete electrical stimulation of the cortex in anesthetized rats. Stimulation of ipsilateral frontal cortex, but not temporal cortex, generated synchronized “multiphasic” responses in neighboring units in rostral STN, usually consisting of a brief, short-latency excitation, a brief inhibition, a second excitation, and a long-duration inhibition. Evoked LFPs in STN consistently mirrored unit responses; brief, negative deflections in the LFP coincided with excitations and brief, positive deflections with inhibitions. This characteristic LFP was dissimilar to potentials evoked in cortex and structures surrounding STN and was resistant to fluctuations in forebrain activity. The short-latency excitation and associated LFP deflection exhibited the highest fidelity to low-intensity cortical stimuli. Unit response failures, which mostly occurred in caudal STN, were not associated with LFPs typical of rostral STN. These data suggest that local populations of STN neurons can be synchronized by both direct and indirect cortical inputs. Synchronized ensemble activity is dependent on topography and input intensity. Finally, the stereotypical, multiphasic profile of the evoked LFP indicates that it might be useful for locating the STN in clinical as well as nonclinical settings.

Effects of combined cortical and acoustic stimuli on muscle activity

Hitherto, it has proven difficult to investigate interactions between cerebral and brainstem motor systems in the human. We hypothesised that transcranial magnetic stimulation (TMS) centred over the dorsal premotor and primary motor cortices might elicit net facilitatory cortico-reticular effects that could interact at the level of the brainstem with a habituated startle to give a reticulospinal discharge and electromyographic (EMG) response with a longer latency than the direct corticospinal response. Conversely, any reticulo-cortical activity evoked by a habituated startle should influence the size of the direct response to cortical TMS. EMG was recorded from active left deltoid muscle in nine healthy volunteers. Acoustic stimulation was delivered binaurally through headphones and repeated until the startle response was habituated. When TMS was centred over the right dorsal premotor or primary motor cortices and delivered 50 ms after the habituated acoustic stimulus, the contralateral direct motor evoked potential was inhibited, compared with the response elicited by TMS alone. The contralateral silent period was shortened and associated with less of a decrease in EMG levels relative to TMS alone. Indeed, an actual increase in EMG over baseline levels occurred in the later half of the silent period in all subjects. We conclude that both cortico-reticular and reticular-cortical effects could be elicited in deltoid through the combination of acoustic stimulation and TMS at short interstimulus intervals. Effects were similar with TMS over premotor and primary motor cortex.

Motor cortex inhibition induced by acoustic stimulation

The influence of the brainstem motor system on cerebral motor areas may play an important role in motor control in health and disease. A new approach to investigate this interaction in man is combining acoustic stimulation activating the startle system with transcranial magnetic stimulation (TMS) over the motor cortex. However, it is unclear whether the inhibition of TMS responses following acoustic stimulation occurs at the level of the motor cortex through reticulo-cortical projections or subcortically, perhaps through reticulo-spinal projections. We compared the influence of acoustic stimulation on motor effects elicited by TMS over motor cortical areas to those evoked with subcortical electrical stimulation (SES) through depth electrodes in five patients treated with deep brain stimulation for Parkinson's disease. SES bypasses the motor cortex, demonstrating any interaction with acoustic stimuli at the subcortical level. EMG was recorded from the contralateral biceps brachii muscle. Acoustic stimulation was delivered binaurally through headphones and used as a conditioning stimulus at an interstimulus interval of 50 ms. When TMS was used as the test stimulus, the area and amplitude of the conditioned motor response was significantly inhibited (area: 57.5+/-12.9%, amplitude: 47.9+/-7.4%, as percentage of unconditioned response) whereas facilitation occurred with SES (area: 110.1+/-4.3%, amplitude: 116.9+/-6.9%). We conclude that a startle-evoked activation of reticulo-cortical projections transiently inhibits the motor cortex.

Primary orthostatic tremor is an exaggeration of a physiological response to instability

Primary orthostatic tremor (POT) is a rare disorder characterised by an intense sense of unsteadiness upon standing and a 16-Hz tremor in which the timing between tremor bursts in different muscles (unilateral and bilateral) remains constant. Hitherto, similar EMG activity has not been described in healthy subjects and it has been postulated that the oscillations seen in POT are primarily pathological. In this study, EMG was recorded from tibialis anterior in healthy subjects who were made unsteady through vestibular galvanic stimulation or leaning backwards. Under these conditions, a peak at approximately 16 Hz was seen in the coherence between the left and right tibialis anterior. This bilateral coherence was absent when the subjects activated the same muscles when not unsteady. These data indicate the existence of a physiological system involved in organising postural responses under circumstances of imbalance and characterised by a highly synchronised output at approximately 16 Hz. In addition, the results suggest that the core abnormality in POT may be an exaggerated sense of unsteadiness when standing still, which then elicits activity from a 16-Hz oscillator normally engaged in postural responses.

Role of muscarinic receptors in the activation of the ventral subiculum and the consequences for dopamine release in the nucleus accumbens

The nucleus accumbens receives limbic inputs from a number of brain regions, including the ventral subiculum. In rats, activation of the ventral subiculum following microinjection of N-methyl-D-aspartate (NMDA) or carbachol increases locomotor activity, whilst ventral subiculum application of NMDA also increases dopamine efflux in the ipsilateral nucleus accumbens. Microdialysis experiments were therefore conducted to ascertain the consequences for dopamine release in the nucleus accumbens following ventral subiculum administration of carbachol, and to explore the acetylcholine receptor subtype(s) that might be involved. We report that, in anaesthetised rats, ventral subiculum administration of carbachol increased dopamine levels in the nucleus accumbens. The response was attenuated by co-administration with atropine, whilst administration of nicotine and the alpha-7 nicotinic acetylcholine receptor agonist AR-R17779 (spiro[1-azabicyclo[2,2,2]octane-3,5'-oxazolidine]-2'-one monohydrochloride) failed to evoke a response. Oxotremorine-M produced a dose-dependent increase in dopamine efflux confirming sensitivity to muscarinic receptor stimulation. However, the ventral subiculum was insensitive to xanomeline and pilocarpine, muscarinic M(1) receptor-preferring agonists, but sensitive to BuTAC ([5R-[exo]-6-[butylthio]-1,2,5-thiadiazol-3-yl]-1-azabicyclo[3.2.1])octane), a muscarinic M(2)/M(4) receptor agonist. The dopamine response to oxotremorine-M was significantly attenuated, although not abolished by co-administration with the M(2)/M(4) receptor antagonist methoctramine, and studies combining oxotremorine-M with (-)-bicuculline, indicated a dual action in the ventral subiculum that was dependent and independent of reduced GABA neurotransmission. The data presented indicates that activation of the ventral subiculum by carbachol increases dopamine efflux in the nucleus accumbens by stimulation of muscarinic receptors, and that the ventral subiculum-nucleus accumbens projection system is sensitive to muscarinic M(2)/M(4) receptor stimulation.

Oscillatory local field potentials recorded from the subthalamic nucleus of the alert rat

Hitherto, high-frequency local field potential oscillations in the upper gamma frequency band (40-80 Hz) have been recorded only from the region of subthalamic nucleus (STN) in parkinsonian patients treated with levodopa. Here we show that local field potentials recorded from the STN in the healthy alert rat also have a spectral peak in the upper gamma band (mean 53 Hz, range 46-70 Hz). The power of this high-frequency oscillatory activity was increased by 30 +/- 4% (+/-SEM) during motor activity compared to periods of alert immobility. It was also increased by 86 +/- 36% by systemic injection of the D2 dopamine receptor agonist quinpirole. The similarities between the high-frequency activities in the STN of the healthy rat and in the levodopa-treated parkinsonian human argue that this oscillatory activity may be physiological in nature and not a consequence of the parkinsonian state.