Changes in the neuronal activity in the pedunculopontine nucleus in chronic MPTP-treated primates: an in situ hybridization study of cytochrome oxidase subunit I, choline acetyl transferase and substance P mRNA expression

Pedunculopontine Nucleus Degeneration Contributes to Both Motor and Non-Motor Symptoms of Parkinson's Disease

Parkinson's disease (PD) is a neurodegenerative disorder characterized by hypokinetic motor features; however, patients also display non-motor symptoms like sleep disorders. The standard treatment for PD is dopamine replacement with L-DOPA; however, symptoms including gait deficits and sleep disorders are unresponsive to L-DOPA. Notably, these symptoms have been linked to aberrant activity in the pedunculopontine nucleus (PPN). Of late, clinical trials involving PPN deep brain stimulation (DBS) have been employed to alleviate gait deficits. Although preclinical evidence implicating PPN cholinergic neurons in gait dysfunction was initially promising, DBS trials fell short of expected outcomes. One reason for the failure of DBS may be that the PPN is a heterogenous nucleus that consists of GABAergic, cholinergic, and glutamatergic neurons that project to a diverse array of brain structures. Second, DBS trials may have been unsuccessful because PPN neurons are susceptible to mitochondrial dysfunction, Lewy body pathology, and degeneration in PD. Therefore, pharmaceutical or gene-therapy strategies targeting specific PPN neuronal populations or projections could better alleviate intractable PD symptoms. Unfortunately, how PPN neuronal populations and their respective projections influence PD motor and non-motor symptoms remains enigmatic. Herein, we discuss normal cellular and neuroanatomical features of the PPN, the differential susceptibility of PPN neurons to PD-related insults, and we give an overview of literature suggesting a role for PPN neurons in motor and sleep deficits in PD. Finally, we identify future approaches directed towards the PPN for the treatment of PD motor and sleep symptoms.

The pedunclopontine nucleus and Parkinson's disease

In the last decade, scientific and clinical interest in the pedunculopontine nucleus (PPN) has grown dramatically. This growth is largely a consequence of experimental work demonstrating its connection to the control of gait and of clinical work implicating PPN pathology in levodopa-insensitive gait symptoms of Parkinson's disease (PD). In addition, the development of optogenetic and chemogenetic approaches has made experimental analysis of PPN circuitry and function more tractable. In this brief review, recent findings in the field linking PPN to the basal ganglia and PD are summarized; in addition, an attempt is made to identify key gaps in our understanding and challenges this field faces in moving forward.

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.

Basal ganglia, movement disorders and deep brain stimulation: advances made through non-human primate research

Studies in non-human primates (NHPs) have led to major advances in our understanding of the function of the basal ganglia and of the pathophysiologic mechanisms of hypokinetic movement disorders such as Parkinson's disease and hyperkinetic disorders such as chorea and dystonia. Since the brains of NHPs are anatomically very close to those of humans, disease states and the effects of medical and surgical approaches, such as deep brain stimulation (DBS), can be more faithfully modeled in NHPs than in other species. According to the current model of the basal ganglia circuitry, which was strongly influenced by studies in NHPs, the basal ganglia are viewed as components of segregated networks that emanate from specific cortical areas, traverse the basal ganglia, and ventral thalamus, and return to the frontal cortex. Based on the presumed functional domains of the different cortical areas involved, these networks are designated as 'motor', 'oculomotor', 'associative' and 'limbic' circuits. The functions of these networks are strongly modulated by the release of dopamine in the striatum. Striatal dopamine release alters the activity of striatal projection neurons which, in turn, influences the (inhibitory) basal ganglia output. In parkinsonism, the loss of striatal dopamine results in the emergence of oscillatory burst patterns of firing of basal ganglia output neurons, increased synchrony of the discharge of neighboring basal ganglia neurons, and an overall increase in basal ganglia output. The relevance of these findings is supported by the demonstration, in NHP models of parkinsonism, of the antiparkinsonian effects of inactivation of the motor circuit at the level of the subthalamic nucleus, one of the major components of the basal ganglia. This finding also contributed strongly to the revival of the use of surgical interventions to treat patients with Parkinson's disease. While ablative procedures were first used for this purpose, they have now been largely replaced by DBS of the subthalamic nucleus or internal pallidal segment. These procedures are not only effective in the treatment of parkinsonism, but also in the treatment of hyperkinetic conditions (such as chorea or dystonia) which result from pathophysiologic changes different from those underlying Parkinson's disease. Thus, these interventions probably do not counteract specific aspects of the pathophysiology of movement disorders, but non-specifically remove the influence of the different types of disruptive basal ganglia output from the relatively intact portions of the motor circuitry downstream from the basal ganglia. Knowledge gained from studies in NHPs remains critical for our understanding of the pathophysiology of movement disorders, of the effects of DBS on brain network activity, and the development of better treatments for patients with movement disorders and other neurologic or psychiatric conditions.

Role of pedunculopontine cholinergic neurons in the vulnerability of nigral dopaminergic neurons in Parkinson's disease

Pedunculopontine nucleus (PPN) cholinergic neurons, which exert excitatory nicotinic control over substantia nigra dopaminergic neurons, degenerate in Parkinson's disease (PD). This finding and other studies showing that nicotine, the preferential agonist of nicotinic acetylcholine receptors, is neuroprotective in experimental models of PD suggest that a deficit in PPN excitatory cholinergic inputs might contribute to the death of nigral dopaminergic neurons in PD. To explore this possibility, we used lesion paradigms of dopaminergic and/or cholinergic systems in rats and monkeys. Consistent with our hypothesis, we observed that stereotaxic lesioning of PPN cholinergic neurons with diphtheria toxin coupled to urotensin II resulted in a significant loss of nigral dopaminergic neurons in rats and induced morphological changes in these neurons in macaques. Unexpectedly, a lesion of dopaminergic neurons induced by unilateral striatal injection of 6-hydroxydopamine (6-OHDA) in rats, or by repeated systemic injections of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) in macaques, led to a 29% and 7% loss of PPN cholinergic neurons, respectively. Lastly, when the PPN cholinergic lesion was performed in rats in which the dopaminergic lesion induced by 6-OHDA was in progress, loss of cholinergic neurons was more drastic than when each neurotransmitter system was lesioned separately. Thus, our results suggest that strong PPN cholinergic and dopaminergic interactions may be an important mechanism in the pathophysiology of PD.

Low-frequency stimulation of the pedunculopontine nucleus affects gait and the neurotransmitter level in the ventrolateral thalamic nucleus in 6-OHDA Parkinsonian rats

The pedunculopontine nucleus (PPN) is connected to spinal, cerebellar and cerebral motor control structures and can be activated with external electrodes. Intrinsic cholinergic neuronal degeneration in the PPN is associated with postural instabilities and gait disturbances (PIGD) in advanced Parkinson's disease (PD). Clinical studies have demonstrated that PPN stimulation may improve PIGD. We investigated this claim and the underlying mechanisms using the 6-hydroxydopamine (6-OHDA) hemilesion model of PD. In this study, gait-related parameters, including the base of support (BOS), stride length, and maximum contact area, were analyzed via CatWalk gait analysis following PPN-low frequency stimulation (LFS) of rats with unilateral 6-OHDA lesions. Additionally, neurotransmitter concentrations in the ventrolateral thalamic nucleus (VL) were measured by microdialysis and liquid chromatography-mass spectrometry (LC-MS). Our data revealed that unilateral 6-OHDA lesions of the medial forebrain bundle (MFB) induced significant gait deficits. PPN-LFS significantly improved the BOS (hindlimb) and maximum contact area (impaired forelimb) scores, whereas no other gait parameters were significantly affected. Unilateral 6-OHDA MFB lesions significantly decreased acetylcholine (ACh) and moderately decreased noradrenaline (NA) concentrations in the VL. PPN-LFS mildly reversed the ACh loss in the VL in the lesioned rats but did not alter the NA levels. Taken together, our data indicate that PPN-LFS is useful for treating gait deficits of PD and that these effects are probably mediated by a rebalancing of ACh levels in the PPN-VL pathway. Thus, our findings provide possible insight into the mechanisms underlying PIGD in PD.

Alteration of daily and circadian rhythms following dopamine depletion in MPTP treated non-human primates

Disturbances of the daily sleep/wake cycle are common non-motor symptoms of Parkinson's disease (PD). However, the impact of dopamine (DA) depletion on circadian rhythms in PD patients or non-human primate (NHP) models of the disorder have not been investigated. We evaluated alterations of circadian rhythms in NHP following MPTP lesion of the dopaminergic nigro-striatal system. DA degeneration was assessed by in vivo PET ([(11)C]-PE2I) and post-mortem TH and DAT quantification. In a light∶dark cycle, control and MPTP-treated NHP both exhibit rest-wake locomotor rhythms, although DA-depleted NHP show reduced amplitude, decreased stability and increased fragmentation. In all animals, 6-sulphatoxymelatonin peaks at night and cortisol in early morning. When the circadian system is challenged by exposure to constant light, controls retain locomotor rest-wake and hormonal rhythms that free-run with stable phase relationships whereas in the DA-depleted NHP, locomotor rhythms are severely disturbed or completely abolished. The amplitude and phase relations of hormonal rhythms nevertheless remain unaltered. Use of a light-dark masking paradigm shows that expression of daily rest-wake activity in MPTP monkeys requires the stimulatory and inhibitory effects of light and darkness. These results suggest that following DA lesion, the central clock in the SCN remains intact but, in the absence of environmental timing cues, is unable to drive downstream rhythmic processes of striatal clock gene and dopaminergic functions that control locomotor output. These findings suggest that the circadian component of the sleep-wake disturbances in PD is more profoundly affected than previously assumed.

Sleep disturbances in Parkinson's disease: the contribution of dopamine in REM sleep regulation

Nearly all patients with Parkinson's disease (PD) have sleep disturbances. While it has been suggested that these disturbances involve a dopaminergic component, the specific mechanisms that contribute to this behavior are far from being fully understood. In this article, we have reviewed the current understanding of the linkage between sleep and PD, focusing on the participation of the dopaminergic system in the regulation of rapid eye movement (REM) sleep. The presence of an REM sleep behavior disorder in patients with PD might reflect the early involvement of dopaminergic neurotransmission in REM sleep-related structures. Therefore, it has been suggested that these structures are affected by an imbalance of dopamine levels. Several studies have demonstrated that neurons in the substantia nigra pars compacta (SNpc) and in the ventral tegmental area (VTA) are active during REM sleep and that sleep-related disturbances may result when these neurons are targeted by neurotoxins. We discuss current evidence suggesting the presence of a putative reciprocal connectivity between the SNpc, VTA, the pedunculopontine tegmental nucleus and reticular formation, which may exert an important influence on the REM sleep mechanism. This review provides a comprehensive overview of the literature that addresses this challenging and unrecognized component of PD.

Gait disorders in parkinsonian monkeys with pedunculopontine nucleus lesions: a tale of two systems

Gait and balance disorders unresponsive to dopaminergic drugs in Parkinson's disease (PD) are secondary to lesions located outside the dopaminergic system. However, available animal models of PD fail to display l-3,4-dihydroxyphenylalanine (DOPA)-responsive parkinsonism and drug-resistant gait and balance disorders, and this lack of appropriate model could account for the deficit of efficient treatments. Because the pedunculopontine nucleus (PPN) plays an important role in locomotion control, we conducted the present study to investigate the consequences of combined dopaminergic and PPN lesions in a same animal. We used macaques that received first 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) intoxication to render them parkinsonian and then local stereotaxic lesion of the PPN. Adding bilateral PPN lesions in MPTP-lesioned macaques induced dopamine-resistant gait and balance disorders but unexpectedly improved hypokinesia. Additional MPTP injections resulted in the association of a severe DOPA-responsive parkinsonism together with DOPA-unresponsive gait disorders. Histological examination assessed a severe dopaminergic degeneration and a significant loss of PPN cholinergic neurons. We observed similar results in aged monkeys intoxicated with MPTP: they developed severe DOPA-responsive hypokinesia and tremor together with unresponsive gait and balance disorders and displayed dopaminergic lesion and a weak but significant cholinergic PPN lesion. Our results highlight the complex role of the cholinergic PPN neurons in the pathophysiology of PD because its lesion induces a dual effect with an improvement of hypokinesia contrasting with a worsening of DOPA-unresponsive gait and balance disorders. Thus, we obtained a primate model of PD that could be useful to test symptomatic treatments for these heavily disabling symptoms.

Reduced cholinergic olfactory centrifugal inputs in patients with neurodegenerative disorders and MPTP-treated monkeys

Olfactory impairment is a common feature of neurodegenerative diseases such as Parkinson's disease (PD), Alzheimer's disease (AD) and dementia with Lewy bodies (DLB). Olfactory bulb (OB) pathology in these diseases shows an increased number of olfactory dopaminergic cells, protein aggregates and dysfunction of neurotransmitter systems. Since cholinergic denervation might be a common underlying pathophysiological feature, the objective of this study was to determine cholinergic innervation of the OB in 27 patients with histological diagnosis of PD (n = 5), AD (n = 14), DLB (n = 8) and 8 healthy control subjects. Cholinergic centrifugal inputs to the OB were clearly reduced in all patients, the most significant decrease being in the DLB group. We also studied cholinergic innervation of the OB in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated monkeys (n = 7) and 7 intact animals. In MPTP-monkeys, we found that cholinergic innervation of the OB was reduced compared to control animals (n = 7). Interestingly, in MPTP-monkeys, we also detected a loss of cholinergic neurons and decreased dopaminergic innervation in the horizontal limb of the diagonal band, which is the origin of the centrifugal cholinergic input to the OB. All these data suggest that cholinergic damage in the OB might contribute, at least in part, to the olfactory dysfunction usually exhibited by these patients. Moreover, decreased cholinergic input to the OB found in MPTP-monkeys suggests that dopamine depletion in itself might reduce the cholinergic tone of basal forebrain cholinergic neurons.

Computational modeling of pedunculopontine nucleus deep brain stimulation

OBJECTIVE

Deep brain stimulation (DBS) near the pedunculopontine nucleus (PPN) has been posited to improve medication-intractable gait and balance problems in patients with Parkinson's disease. However, clinical studies evaluating this DBS target have not demonstrated consistent therapeutic effects, with several studies reporting the emergence of paresthesia and oculomotor side effects. The spatial and pathway-specific extent to which brainstem regions are modulated during PPN-DBS is not well understood.

APPROACH

Here, we describe two computational models that estimate the direct effects of DBS in the PPN region for human and translational non-human primate (NHP) studies. The three-dimensional models were constructed from segmented histological images from each species, multi-compartment neuron models and inhomogeneous finite element models of the voltage distribution in the brainstem during DBS.

MAIN RESULTS

The computational models predicted that: (1) the majority of PPN neurons are activated with -3 V monopolar cathodic stimulation; (2) surgical targeting errors of as little as 1 mm in both species decrement activation selectivity; (3) specifically, monopolar stimulation in caudal, medial, or anterior PPN activates a significant proportion of the superior cerebellar peduncle (up to 60% in the human model and 90% in the NHP model at -3 V); (4) monopolar stimulation in rostral, lateral or anterior PPN activates a large percentage of medial lemniscus fibers (up to 33% in the human model and 40% in the NHP model at -3 V) and (5) the current clinical cylindrical electrode design is suboptimal for isolating the modulatory effects to PPN neurons.

SIGNIFICANCE

We show that a DBS lead design with radially-segmented electrodes may yield improved functional outcome for PPN-DBS.

The pedunculopontine tegmental nucleus: implications for a role in modulating spinal cord motoneuron excitability

There is evidence that deep brain stimulation (DBS) of the pedunculopontine tegmental nucleus (PPTg) improves parkinsonian motor signs. The mechanisms that mediate these effects and the modifications that occur in the PPTg in Parkinson's disease (PD) are not fully known and are the object of current debate. The aim of this paper was to critically review available data with respect to (1) the presence of PPTg neurons linked to reticulospinal projections, (2) the involvement of these neurons in modulating spinal reflexes, and (3) the participation of fibers close to or within the PPTg region in such modulation. The PPTg neurons are distributed in a large pontotegmental region, stimulation of which can evoke activity in hindlimb, shoulder and neck muscles, and potentiate motor responses evoked by stimulation of dorsal roots. This influence seems to be carried out by fast-conducting descending fibers, which likely run in the medial reticulospinal pathway. It is yet unclear which neurotransmitters are involved and on which elements of the gray matter of the spinal cord PPTg fibers synapse. The modulation of spinal cord activity which can be achieved by stimulating the PPTg region seems to be mediated not only by PPTg neurons, but also by tecto-reticular fibers which run in the pontotegmental area, and which likely are activated during PPTg-DBS. The importance of these fibers is discussed taking into account the degeneration of PPTg neurons in PD and the benefits in gait and postural control that PPTg-DBS exerts in PD. The potential usefulness of PPTg-DBS in other neurodegenerative disorders characterized by neuronal loss in the brainstem is also considered.

The pedunculopontine nucleus area: critical evaluation of interspecies differences relevant for its use as a target for deep brain stimulation

Recently, the pedunculopontine nucleus has been highlighted as a target for deep brain stimulation for the treatment of freezing of postural instability and gait disorders in Parkinson's disease and progressive supranuclear palsy. There is great controversy, however, as to the exact location of the optimal site for stimulation. In this review, we give an overview of anatomy and connectivity of the pedunculopontine nucleus area in rats, cats, non-human primates and humans. Additionally, we report on the behavioural changes after chemical or electrical manipulation of the pedunculopontine nucleus. We discuss the relation to adjacent regions of the pedunculopontine nucleus, such as the cuneiform nucleus and the subcuneiform nucleus, which together with the pedunculopontine nucleus are the main areas of the mesencephalic locomotor region and play a major role in the initiation of gait. This information is discussed with respect to the experimental designs used for research purposes directed to a better understanding of the circuitry pathway of the pedunculopontine nucleus in association with basal ganglia pathology, and with respect to deep brain stimulation of the pedunculopontine nucleus area in humans.

Functional disconnection of the substantia nigra pars compacta from the pedunculopontine nucleus impairs learning of a conditioned avoidance task

The pedunculopontine tegmental nucleus (PPTg) targets nuclei in the basal ganglia, including the substantia nigra pars compacta (SNc), in which neuronal loss occurs in Parkinson's disease, a condition in which patients show cognitive as well as motor disturbances. Partial loss and functional abnormalities of neurons in the PPTg are also associated with Parkinson's disease. We hypothesized that the interaction of PPTg and SNc might be important for cognitive impairments and so investigated whether disrupting the connections between the PPTg and SNc impaired learning of a conditioned avoidance response (CAR) by male Wistar rats. The following groups were tested: PPTg unilateral; SNc unilateral; PPTg-SNc ipsilateral (ipsilateral lesions in PPTg and SNc); PPTg-SNc contralateral (contralateral lesions in PPTg and SNc); sham lesions (of each type). SNc lesions were made with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine HCl (MPTP, 0.6micromol); PPTg lesions with ibotenate (24nmol). After recovery, all rats underwent 50-trial sessions of 2-way active avoidance conditioning for 3 consecutive days. Rats with unilateral lesions in PPTg or SNc learnt this, however rats with contralateral (but not ipsilateral) combined lesions in both structures presented no sign of learning. This effect was not likely to be due to sensorimotor impairment because lesions did not affect reaction time to the tone or footshock during conditioning. However, an increased number of non-responses were observed in the rats with contralateral lesions. The results support the hypothesis that a functional interaction between PPTg and SNc is needed for CAR learning and performance.

Region-specific protein abundance changes in the brain of MPTP-induced Parkinson's disease mouse model

Parkinson's disease (PD) is characterized by dopaminergic neurodegeneration in the nigrostriatal region of the brain; however, the neurodegeneration extends well beyond dopaminergic neurons. To gain a better understanding of the molecular changes relevant to PD, we applied two-dimensional LC-MS/MS to comparatively analyze the proteome changes in four brain regions (striatum, cerebellum, cortex, and the rest of brain) using a MPTP-induced PD mouse model with the objective to identify potential nigrostriatal-specific and other region-specific protein abundance changes. The combined analyses resulted in the identification of 4,895 nonredundant proteins with at least two unique peptides per protein. The relative abundance changes in each analyzed brain region were estimated based on the spectral count information. A total of 518 proteins were observed with substantial MPTP-induced abundance changes across different brain regions. A total of 270 of these proteins were observed with specific changes occurring either only in the striatum and/or in the rest of the brain region that contains substantia nigra, suggesting that these proteins are associated with the underlying nigrostriatal pathways. Many of the proteins that exhibit changes were associated with dopamine signaling, mitochondrial dysfunction, the ubiquitin system, calcium signaling, the oxidative stress response, and apoptosis. A set of proteins with either consistent change across all brain regions or with changes specific to the cortex and cerebellum regions were also detected. Ubiquitin specific protease (USP9X), a deubiquination enzyme involved in the protection of proteins from degradation and promotion of the TGF-beta pathway, exhibited altered abundance in all brain regions. Western blot validation showed similar spatial changes, suggesting that USP9X is potentially associated with neurodegeneration. Together, this study for the first time presents an overall picture of proteome changes underlying both nigrostriatal pathways and other brain regions potentially involved in MPTP-induced neurodegeneration. The observed molecular changes provide a valuable reference resource for future hypothesis-driven functional studies of PD.

Altered neuronal activity relationships between the pedunculopontine nucleus and motor cortex in a rodent model of Parkinson's disease

The pedunculopontine nucleus (PPN) is a new deep brain stimulation (DBS) target for Parkinson's disease (PD), but little is known about PPN firing pattern alterations in PD. The anesthetized rat is a useful model for investigating the effects of dopamine loss on the transmission of oscillatory cortical activity through basal ganglia structures. After dopamine loss, synchronous oscillatory activity emerges in the subthalamic nucleus and substantia nigra pars reticulata in phase with cortical slow oscillations. To investigate the impact of dopamine cell lesion-induced changes in basal ganglia output on activity in the PPN, this study examines PPN spike timing with reference to motor cortex (MCx) local field potential (LFP) activity in urethane- or ketamine-anesthetized rats. Seven to ten days after unilateral 6-hydroxydopamine lesion of the medial forebrain bundle, spectral power in PPN spike trains and coherence between PPN spiking and PPN LFP activity increased in the approximately 1 Hz range in urethane-anesthetized rats. PPN spike timing also changed from firing predominantly in phase with MCx slow oscillations in the intact urethane-anesthetized rat to firing predominantly antiphase to MCx oscillations in the hemi-parkinsonian rat. These changes were not observed in the ketamine-anesthetized preparation. These observations suggest that dopamine loss alters PPN spike timing by increasing inhibitory oscillatory input to the PPN from basal ganglia output nuclei, a phenomenon that may be relevant to motor dysfunction and PPN DBS efficacy in PD patients.

Mechanisms of deep brain stimulation in movement disorders as revealed by changes in stimulus frequency

Deep brain stimulation (DBS) is an established treatment for symptoms in movement disorders and is under investigation for symptom management in persons with psychiatric disorders and epilepsy. Nevertheless, there remains disagreement regarding the physiological mechanisms responsible for the actions of DBS, and this lack of understanding impedes both the design of DBS systems for treating novel diseases and the effective tuning of current DBS systems. Currently available data indicate that effective DBS overrides pathological bursts, low frequency oscillations, synchronization, and disrupted firing patterns present in movement disorders, and replaces them with more regularized firing. Although it is likely that the specific mechanism(s) by which DBS exerts its effects varies between diseases and target nuclei, the overriding of pathological activity appears to be ubiquitous. This review provides an overview of changes in motor symptoms with changes in DBS frequency and highlights parallels between the changes in motor symptoms and the changes in cellular activity that appear to underlie the motor symptoms.