Subdyskinetic apomorphine responses in globus pallidus and subthalamus of parkinsonian patients: lack of clear evidence for the 'indirect pathway'

Oscillatory vs. non-oscillatory subthalamic beta activity in Parkinson's disease

Parkinson's disease is characterized by exaggerated beta activity (13-35 Hz) in cortico-basal ganglia motor loops. Beta activity includes both periodic fluctuations (i.e. oscillatory activity) and aperiodic fluctuations reflecting spiking activity and excitation/inhibition balance (i.e. non-oscillatory activity). However, the relative contribution, dopamine dependency and clinical correlations of oscillatory vs. non-oscillatory beta activity remain unclear. We recorded, modelled and analysed subthalamic local field potentials in parkinsonian patients at rest while off or on medication. Autoregressive modelling with additive 1/f noise clarified the relationships between measures of beta activity in the time domain (i.e. amplitude and duration of beta bursts) or in the frequency domain (i.e. power and sharpness of the spectral peak) and oscillatory vs. non-oscillatory activity: burst duration and spectral sharpness are specifically sensitive to oscillatory activity, whereas burst amplitude and spectral power are ambiguously sensitive to both oscillatory and non-oscillatory activity. Our experimental data confirmed the model predictions and assumptions. We subsequently analysed the effect of levodopa, obtaining strong-to-extreme Bayesian evidence that oscillatory beta activity is reduced in patients on vs. off medication, with moderate evidence for absence of modulation of the non-oscillatory component. Finally, specifically the oscillatory component of beta activity correlated with the rate of motor progression of the disease. Methodologically, these results provide an integrative understanding of beta-based biomarkers relevant for adaptive deep brain stimulation. Biologically, they suggest that primarily the oscillatory component of subthalamic beta activity is dopamine dependent and may play a role not only in the pathophysiology but also in the progression of Parkinson's disease. KEY POINTS: Beta activity in Parkinson's disease includes both true periodic fluctuations (i.e. oscillatory activity) and aperiodic fluctuations reflecting spiking activity and synaptic balance (i.e. non-oscillatory activity). The relative contribution, dopamine dependency and clinical correlations of oscillatory vs. non-oscillatory beta activity remain unclear. Burst duration and spectral sharpness are specifically sensitive to oscillatory activity, while burst amplitude and spectral power are ambiguously sensitive to both oscillatory and non-oscillatory activity. Only the oscillatory component of subthalamic beta activity is dopamine-dependent. Stronger beta oscillatory activity correlates with faster motor progression of the disease.

Brain Networks Involved in Sensory Perception in Parkinson's Disease: A Scoping Review

Parkinson's Disease (PD) has historically been considered a disorder of motor dysfunction. However, a growing number of studies have demonstrated sensory abnormalities in PD across the modalities of proprioceptive, tactile, visual, auditory and temporal perception. A better understanding of these may inform future drug and neuromodulation therapy. We analysed these studies using a scoping review. In total, 101 studies comprising 2853 human participants (88 studies) and 125 animals (13 studies), published between 1982 and 2022, were included. These highlighted the importance of the basal ganglia in sensory perception across all modalities, with an additional role for the integration of multiple simultaneous sensation types. Numerous studies concluded that sensory abnormalities in PD result from increased noise in the basal ganglia and increased neuronal receptive field size. There is evidence that sensory changes in PD and impaired sensorimotor integration may contribute to motor abnormalities.

The subthalamic nucleus and the placebo effect in Parkinson's disease

The study of the placebo effect, or response, is related to the investigation of the psychologic component of different therapeutic rituals. The high rate of placebo responses in Parkinson's disease clinical trials provided the impetus for investigating the underlying mechanisms. Ruling out spontaneous remission and regression to the mean through the appropriate experimental designs, genuine psychologic placebo effects have been identified, in which both patients' expectations of therapeutic benefit and learning processes are involved. Specifically, placebo effects are associated with dopamine release in the striatum and changes in neuronal activity in the subthalamic nucleus, substantia nigra pars reticulata, and motor thalamus in Parkinson's disease, as assessed through positron emission tomography and single-neuron recording during deep brain stimulation, respectively. Conversely, verbal suggestions of clinical worsening or drug dose reduction induce nocebo responses in Parkinson's disease, which have been detected at both behavioral and electrophysiologic level. Important implications and applications emerge from this new knowledge. These include better clinical trial designs, whereby patients' expectations should always be assessed, as well as better drug dosage in order to reduce drug intake.

Teaching neurons to respond to placebos

KEY POINTS

We analysed the placebo response at the single-neuron level in the thalamus of Parkinson patients to see the differences between first-time administration of placebo and administration after pharmacological pre-conditioning. When the placebo was given for the first time, it induced neither clinical improvement, as assessed through muscle rigidity reduction at the wrist, nor neuronal changes in thalamic neurons. However, if placebo was given after two, three or four prior administrations of an anti-Parkinson drug, apomorphine, it produced both clinical and neuronal responses. Both the magnitude and the duration of these placebo responses depended on the number of prior exposures to apomorphine, according to the rule: the greater the number of previous apomorphine administrations, the larger the magnitude and the longer the duration of the clinical and neuronal placebo responses. These findings show that learning plays a crucial role in the placebo response and suggest that placebo non-responders can be turned into placebo responders, with important clinical implications.

ABSTRACT

Placebos have been found to affect the patient's brain in several conditions, such as pain and motor disorders. For example, in Parkinson's disease, a placebo treatment induces a release of dopamine in the striatum and changes the activity of neurons in both thalamic and subthalamic nuclei. The present study shows that placebo administration for the first time induces neither clinical nor neuronal improvement in Parkinson patients who undergo implantation of electrodes for deep brain stimulation. However, this lack of placebo responsiveness can be turned into substantial placebo responses following previous exposure to repeated administrations of the anti-Parkinson agent apomorphine. As the number of apomorphine administrations increased from one to four, both the clinical response and the neuronal activity in the ventral anterior and anterior ventrolateral thalamus increased. In fact, after four apomorphine exposures, placebo administration induced clinical responses that were as large as those to apomorphine, along with long-lasting neuronal changes. These clinical placebo responses following four apomorphine administrations were again elicited after a re-exposure to a placebo 24 h after surgery, but not after 48 h. These data indicate that learning plays a crucial role in placebo responsiveness and suggest that placebo non-responders can be turned into responders, with important implications in the clinical setting.

Placebo, nocebo, and learning mechanisms

Recent substantial laboratory and theoretical research hints for different learning mechanisms regulating the formation of placebo and nocebo responses. Moreover, psychological and biological variants may play a role as modulators of learning mechanisms underlying placebo and nocebo responses. In this chapter, we present pioneering and recent human and nonhuman research that has impressively increased our knowledge of learning mechanisms in the context of placebo and nocebo effects across different physiological processes and pathological conditions.

Non-linear dynamics in parkinsonism

Over the last 30 years, the functions (and dysfunctions) of the sensory-motor circuitry have been mostly conceptualized using linear modelizations which have resulted in two main models: the “rate hypothesis” and the “oscillatory hypothesis.” In these two models, the basal ganglia data stream is envisaged as a random temporal combination of independent simple patterns issued from its probability distribution of interval interspikes or its spectrum of frequencies respectively. More recently, non-linear analyses have been introduced in the modelization of motor circuitry activities, and they have provided evidences that complex temporal organizations exist in basal ganglia neuronal activities. Regarding movement disorders, these complex temporal organizations in the basal ganglia data stream differ between conditions (i.e., parkinsonism, dyskinesia, healthy control) and are responsive to treatments (i.e., l-DOPA, deep brain stimulation). A body of evidence has reported that basal ganglia neuronal entropy (a marker for complexity/irregularity in time series) is higher in hypokinetic state. In line with these findings, an entropy-based model has been recently formulated to introduce basal ganglia entropy as a marker for the alteration of motor processing and a factor of motor inhibition. Importantly, non-linear features have also been identified as a marker of condition and/or treatment effects in brain global signals (EEG), muscular activities (EMG), or kinetic of motor symptoms (tremor, gait) of patients with movement disorders. It is therefore warranted that the non-linear dynamics of motor circuitry will contribute to a better understanding of the neuronal dysfunctions underlying the spectrum of parkinsonian motor symptoms including tremor, rigidity, and hypokinesia.

Characterization of the thalamic-subthalamic circuit involved in the placebo response through single-neuron recording in Parkinson patients

The placebo effect, or response, is a complex phenomenon whereby an inert treatment can induce a therapeutic benefit if the subject is made to believe that it is effective. One of the main mechanisms involved is represented by expectations of clinical improvement which, in turn, have been found to either reduce anxiety or activate reward mechanisms. Therefore, the study of the placebo effect allows us to understand how emotions may affect both behavior and therapeutic outcome. The high rate of placebo responders in clinical trials of Parkinson's disease provided the motivation to investigate the biological underpinnings of the placebo response in Parkinsonian patients. The placebo effect in Parkinson's disease is induced through the administration of an inert substance which the patient believes to improve motor performance. By using this approach, different behavioral and neuroimaging studies have documented objective improvements in motor performance and an increase of endogenous dopamine release in both the dorsal and ventral striatum. Recently, single-neuron recording from the subthalamic and thalamic regions during the implantation of electrodes for deep brain stimulation has been used to investigate the firing pattern of different neurons before and after placebo administration. The results show that the subthalamic nucleus, the substantia nigra pars reticulata, and the ventral anterior thalamus are all involved in the placebo response in Parkinson patients, thus making intraoperative recording an excellent model to characterize the neuronal circuit that is involved in the placebo response in Parkinson's disease as well as in other disorders of movement.

An entropy-based model for basal ganglia dysfunctions in movement disorders

During this last decade, nonlinear analyses have been used to characterize the irregularity that exists in the neuronal data stream of the basal ganglia. In comparison to linear parameters for disparity (i.e., rate, standard deviation, and oscillatory activities), nonlinear analyses focus on complex patterns that are composed of groups of interspike intervals with matching lengths but not necessarily contiguous in the data stream. In light of recent animal and clinical studies, we present a review and commentary on the basal ganglia neuronal entropy in the context of movement disorders.

Does unilateral basal ganglia activity functionally influence the contralateral side? What we can learn from STN stimulation in patients with Parkinson's disease

In humans, the control of voluntary movement, in which the corticobasal ganglia (BG) circuitry participates, is mainly lateralized. However, several studies have suggested that both the contralateral and ipsilateral BG systems are implicated during unilateral movement. Bilateral improvement of motor signs in patients with Parkinson's disease (PD) has been reported with unilateral lesion or high-frequency stimulation (HFS) of the internal part of the globus pallidus or the subthalamic nucleus (STN-HFS). To decipher the mechanisms of production of ipsilateral movements induced by the modulation of unilateral BG circuitry activity, we recorded left STN neuronal activity during right STN-HFS in PD patients operated for bilateral deep brain stimulation. Left STN single cells were recorded in the operating room during right STN-HFS while patients experienced, or did not experience, right stimulation-induced dyskinesias. Most of the left-side STN neurons (64%) associated with the presence of right dyskinesias were inhibited, with a significant decrease in burst and intraburst frequencies. In contrast, left STN neurons not associated with right dyskinesias were mainly activated (48%), with a predominant increase 4–5 ms after the stimulation pulse and a decrease in oscillatory activity. This suggests that unilateral neuronal STN modulation is associated with changes in the activity of the contralateral STN. The fact that one side of the BG system can influence the functioning of the other could explain the occurrence of bilateral dyskinesias and motor improvement observed in PD patients during unilateral STN-HFS, as a result of a bilateral disruption of the pathological activity in the corticosubcortical circuitry.

The pharmacological properties and therapeutic use of apomorphine

Apomorphine (APO) is an aporphine derivative used in human and veterinary medicine. APO activates D₁, D_2S, D_2L, D₃, D₄, and D₅ receptors (and is thus classified as a non-selective dopamine agonist), serotonin receptors (5HT_1A, 5HT_2A, 5HT_2B, and 5HT_2C), and α-adrenergic receptors (α_1B, α_1D, α_2A, α_2B, and α_2C). In veterinary medicine, APO is used to induce vomiting in dogs, an important early treatment for some common orally ingested poisons (e.g., anti-freeze or insecticides). In human medicine, it has been used in a variety of treatments ranging from the treatment of addiction (i.e., to heroin, alcohol or cigarettes), for treatment of erectile dysfunction in males and hypoactive sexual desire disorder in females to the treatment of patients with Parkinson's disease (PD). Currently, APO is used in patients with advanced PD, for the treatment of persistent and disabling motor fluctuations which do not respond to levodopa or other dopamine agonists, either on its own or in combination with deep brain stimulation. Recently, a new and potentially important therapeutic role for APO in the treatment of Alzheimer’s disease has been suggested; APO seems to stimulate Aβ catabolism in an animal model and cell culture, thus reducing the rate of Aβ oligomerisation and consequent neural cell death.

Placebo-induced improvements: how therapeutic rituals affect the patient's brain

The placebo effect has evolved from being thought of as a nuisance in clinical research to a biological phenomenon worthy of scientific investigation. The study of the placebo effect and of its evil twin, the nocebo effect, is basically the study of the therapeutic ritual around the patient, and it plays a crucial role in the therapeutic outcome. In recent years, different types of placebo responses have been analyzed with sophisticated biological tools that have uncovered specific mechanisms at the neuroanatomical, neurophysiological, biochemical, and cellular levels. Most of our knowledge about the neurobiological mechanisms of the placebo response comes from pain and Parkinson's disease, whereby the neuronal networks involved in placebo responsiveness have been identified. In the first case, opioid, cannabinoid, and cholecystokinin circuits have been found to be involved. In the second case, dopaminergic activation in the striatum and neuronal changes in basal ganglia have been described. This recent research has revealed that these placebo-induced biochemical and cellular changes in a patient's brain are very similar to those induced by drugs. This new way of thinking may have profound implications in clinical trials and medical practice both for pharmacological interventions and for nonpharmacological treatments such as acupuncture.

Placebo mechanisms across different conditions: from the clinical setting to physical performance

Although the great increase in interest in the placebo phenomenon was spurred by the clinical implications of its use, the progressive elucidation of the neurobiological and pharmacological mechanisms underlying the placebo effect also helps cast new light on the relationship between mind (and brain) and body, a topic of foremost philosophical importance but also a major medical issue in light of the complex interactions between the brain on the one hand and body functions on the other. While the concept of placebo can be a general one, with a broad definition generally applicable to many different contexts, the description of the cerebral processes called into action in specific situations can vary widely. In this paper, examples will be given where physiological or pathological conditions are altered following the administration of an inert substance or verbal instructions tailored to induce expectation of a change, and explanations will be offered with details on neurotransmitter changes and neural pathways activated. As an instance of how placebo effects can extend beyond the clinical setting, data in the physical performance domain and implications for sport competitions will also be presented and discussed.

The placebo response: how words and rituals change the patient's brain

OBJECTIVE

The placebo effect, or response, has evolved from being thought of as a nuisance in clinical and pharmacological research to a biological phenomenon worthy of scientific investigation in its own right. The study of the placebo effect and of its negative counterpart, the nocebo effect, is basically the study of the psychosocial context around the treatment and the patient, and it plays a crucial role in the therapeutic outcome.

METHODS

In recent years, different types of placebo responses have been analyzed with sophisticated biological tools that have uncovered specific mechanisms at the anatomical, physiological, biochemical and cellular level.

RESULTS

Most of our knowledge about the neurobiological mechanisms of the placebo response comes from pain and Parkinson's disease, whereby the neuronal circuits involved in placebo responsiveness have been identified. In the first case, opioidergic, dopaminergic and cholecystokinergic networks have been found to be involved. In the second case, dopaminergic activation in the striatum and neuronal changes in basal ganglia have been described.

CONCLUSION

This recent research has revealed that these placebo-induced biochemical and cellular changes in a patient's brain are very similar to those induced by drugs. This new way of thinking may have profound implications both for clinical trials and for medical practice.

The clinical efficacy of L-DOPA and STN-DBS share a common marker: reduced GABA content in the motor thalamus

At odd with traditional views, effective sub-thalamic nucleus (STN) deep brain stimulation (DBS), in Parkinson's disease (PD) patients, may increase the discharge rate of the substantia nigra pars reticulata and the internal globus pallidus (GPi), in combination with increased cyclic guanosine monophosphate (cGMP) levels. How these changes affect the basal ganglia (BG) output to the motor thalamus, the crucial structure conveying motor information to cortex, is critical. Here, we determined the extracellular GABA concentration in the ventral anterior nucleus (VA) during the first delivery of STN-DBS (n=10) or following levodopa (LD) (n=8). Both DBS and subdyskinetic LD reversibly reduced (−30%) VA GABA levels. A significant correlation occurred between clinical score and GABA concentration. By contrast, only STN-DBS increased GPi cGMP levels. Hence, STN-ON and MED-ON involve partially different action mechanisms but share a common target in the VA. These findings suggest that the standard BG circuitry, in PD, needs revision as relief from akinesia may take place, during DBS, even in absence of reduced GPi excitability. However, clinical amelioration requires fast change of thalamic GABA, confirming, in line with the old model, that VA is the core player in determining thalamo-cortical transmission.

Reduced GABA Content in the Motor Thalamus during Effective Deep Brain Stimulation of the Subthalamic Nucleus

Deep brain stimulation (DBS) of the subthalamic nucleus (STN), in Parkinson's disease (PD) patients, is a well established therapeutic option, but its mechanisms of action are only partially known. In our previous study, the clinical transitions from OFF- to ON-state were not correlated with significant changes of GABA content inside GPi or substantia nigra reticulata. Here, biochemical effects of STN-DBS have been assessed in putamen (PUT), internal pallidus (GPi), and inside the antero-ventral thalamus (VA), the key station receiving pallidothalamic fibers. In 10 advanced PD patients undergoing surgery, microdialysis samples were collected before and during STN-DBS. cGMP, an index of glutamatergic transmission, was measured in GPi and PUT by radioimmunoassay, whereas GABA from VA was measured by HPLC. During clinically effective STN-DBS, we found a significant decrease in GABA extracellular concentrations in VA (−30%). Simultaneously, cGMP extracellular concentrations were enhanced in PUT (+200%) and GPi (+481%). These findings support a thalamic dis-inhibition, in turn re-establishing a more physiological corticostriatal transmission, as the source of motor improvement. They indirectly confirm the relevance of patterning (instead of mere changes of excitability) and suggest that a rigid interpretation of the standard model, at least when it indicates the hyperactive indirect pathway as key feature of hypokinetic signs, is unlikely to be correct. Finally, given the demonstration of a key role of VA in inducing clinical relief, locally administration of drugs modulating GABA transmission in thalamic nuclei could become an innovative therapeutic strategy.

Electrophysiological properties of thalamic, subthalamic and nigral neurons during the anti-parkinsonian placebo response

Placebo administration to Parkinson patients is known to induce dopamine release in the striatum and to affect the activity of subthalamic nucleus (STN) neurons. By using intraoperative single-neuron recording techniques in awake patients, here we extend our previous study on STN recording, and characterize part of the neuronal circuit which is affected by placebos. In those patients who showed a clinical placebo response, there was a decrease in firing rate in STN neurons that was associated with a decrease in the substantia nigra pars reticulata (SNr) and an increase in the ventral anterior (VA) and anterior ventral lateral (VLa) thalamus. These data show that placebo decreases STN and SNr activity whereas it increases VA/VLa activity. By contrast, placebo non-responders showed either a lack of changes in this circuit or partial changes in the STN only. Thus, changes in activity in the whole basal ganglia-VA/VLa circuit appear to be important in order to observe a clinical placebo improvement, although the involvement of other circuits, such as the direct pathway bypassing the STN, cannot be ruled out. The circuit we describe in the present study is likely to be a part of a more complex circuitry, including the striatum and the internal globus pallidus (GPi), that is modified by placebo administration. These findings indicate that a placebo treatment, which is basically characterized by verbal suggestions of benefit, can reverse the malfunction of a complex neuronal circuit, although these placebo-associated neuronal changes are short-lasting and occur only in some patients but not in others.

Multi-target strategy for Parkinsonian patients: the role of deep brain stimulation in the centromedian-parafascicularis complex

The intra-laminar (IL) thalamic complex, composed of centromedian (CM) and parafascicular (Pf) nucleus, is a strategic crossroad for the activity of the basal ganglia and is recently regaining its position has a putative neurosurgical target for Parkinsonian syndromes. The multi-target approach we have encouraged since the late nineties has allowed the combined implantation of a standard target (the subthalamic nucleus-STN or the internal pallidus-GPi) plus an innovative one (CM/Pf) in well-identified Parkinson's disease (PD) patients; hence, it is possible to study, in the same PD patients, the specific target-mediated effects on different clinical signs. Here, we focus on the potential usefulness of implanting the CM/Pf complex when required in the management of contra-lateral tremor (resistant to standard deep brain stimulation-DBS - in STN - , n=2) and disabling involuntary movements, partially responsive to GPi-DBS (n=6). When considering global UPDRS scores, CM/Pf-DBS ameliorate extra-pyramidal symptoms but not as strongly as STN (or GPi) does. Yet, CM/Pf acts very powerfully on tremor and contributes to the long-term management of l-Dopa-induced involuntary movements. The lack of cognitive deficits and psychic impairment associated with the improvement of their quality of life, in our small cohort of CM/Pf implanted patients, reinforces the notion of CM/Pf as a safe and attractive area for surgical treatment of advanced PD, possibly affecting not only motor but also associative functions.

Dopamine replacement therapy does not restore the full spectrum of normal pallidal activity in the 1-methyl-4-phenyl-1,2,3,6-tetra-hydropyridine primate model of Parkinsonism

Current physiological studies emphasize the role of neuronal oscillations and synchronization in the pathophysiology of Parkinson's disease; however, little is known about their specific roles in the neuronal substrate of dopamine replacement therapy (DRT). We investigated oscillatory activity and correlations throughout the different states of levodopa-naive parkinsonism as well as "Off-On" and dyskinetic states of DRT in the external globus pallidum (GPe) of tremulous (vervet) and rigid-akinetic (macaque) monkeys and in the internal globus pallidum (GPi) of the vervet monkey. We found that, although oscillatory activity of cells and interneuronal correlation in both pallidal segments increases after induction of parkinsonism with 1-methyl-4-phenyl-1,2,3,6-tetra-hydropyridine (MPTP) and decreases in response to DRT, important differences exist between the two pallidal segments. In the GPi, the fraction of oscillatory cells and relative power of oscillations were significantly higher than in the GPe, and the dominant frequency was within the range of 7.5-13.5 Hz compared with a range of 4.5-7.5 Hz within the GPe. The interneuronal correlations were mostly oscillatory in the GPi, whereas at least half are non-oscillatory in the GPe. We demonstrate that the tremor characteristics after exposure to DRT do not resemble those of the normal or the levodopa-naive state. Moreover, although DRT reverses the MPTP-induced neuronal changes (rate, pattern, and pairwise correlations), the balance between GPe and GPi fails to restore. We therefore suggest that this imbalance reflects additional abnormal organization of the basal ganglia networks in response to dopamine replacement and may constitute the physiological substrate of the limitations and side effects of chronic DRT.

Functional Pharmacology in Human Brain

Most neurological and psychiatric disorders involve selective or preferential impairments of neurotransmitter systems. Therefore, studies of functional transmitter pathophysiology in human brain are of unique importance in view of the development of effective, mechanism-based, therapeutic modalities. It is well known that central nervous system functional proteins, including receptors, transporters, ion channels, and enzymes, can exhibit high heterogeneity in terms of structure, function, and pharmacological profile. If the existence of types and subtypes of functional proteins amplifies the possibility of developing selective drugs, such heterogeneity certainly increases the likelihood of interspecies differences. It is therefore essential, before choosing animal models to be used in preclinical pharmacology experimentation, to establish whether functionally corresponding proteins in men and animals also display identical pharmacological profiles. Because of evidence that scaffolding proteins, trafficking between plasma membrane and intracellular pools, phosphorylation and allosteric modulators can affect the function of receptors and transporters, experiments with human clones expressed in host cells where the environment of native receptors is rarely reproduced should be interpreted with caution. Thus, the use of neurosurgically removed fresh human brain tissue samples in which receptors, transporters, ion channels, and enzymes essentially retain their natural environment represents a unique experimental approach to enlarge our understanding of human brain processes and to help in the choice of appropriate animal models. Using this experimental approach, many human brain functional proteins, in particular transmitter receptors, have been characterized in terms of localization, function, and pharmacological properties.

Spontaneous sleep modulates the firing pattern of Parkinsonian subthalamic nucleus

In Parkinson's disease, the subthalamic nucleus (STN) is a common target for functional neurosurgery. Recent investigations have suggested that physiological non-motor stimuli may dramatically alter STN firing properties. By maintaining long-lasting micro-recordings of STN single units in Parkinson's disease (PD) patients, here we show that the neurons that are responsive to passive movements are also strongly modulated by altered vigilance state (awake vs. sleep). In addition, sleep was characterized by a distinctive irregular train-like firing pattern. These findings suggest that the reduction of the somato-sensory input modifies rigidity and, hence, STN discharge mode. Further, it is suggested that specific STN electrophysiological features are potential targets for future therapeutic interventions.

Oscillatory pallidal local field potential activity inversely correlates with limb dyskinesias in Parkinson's disease

Levodopa induced dyskinesias (LIDs) are poorly understood and yet are a major cause of disability in Parkinson's disease (PD). The activity of neurons in the basal ganglia of patients with PD tends to be strongly synchronized at frequencies under 30 Hz, leading to oscillatory local field potentials (LFPs). As dopaminergic therapy acutely suppresses this synchronization, we investigated whether this suppression may contribute to LIDs. Accordingly, we sought an inverse correlation between oscillatory synchronization and dyskinesia activity across time. To this end, we recorded pallidal LFPs in two Parkinsonian subjects exhibiting LIDs following surgery for deep brain stimulation. We correlated LFP power with simultaneously recorded EMG from the dyskinetic contralateral upper limb. We found highly significant inverse correlations between the oscillatory LFP activity under 30 Hz and dyskinetic EMG (maximum r = -0.65, P < 0.001 and r = -0.33, P < 0.001 for activities over 13-30 Hz in each subject). The inverse relationship between oscillatory pallidal LFP activity and dyskinetic EMG was maintained over time periods of a few seconds and was focal. This observation links the suppression of oscillatory synchronization in the pallidum with dyskinetic muscle activity in PD.

Subthalamic stimulation activates internal pallidus: evidence from cGMP microdialysis in PD patients

Parkinson's disease patients benefit from deep brain stimulation (DBS) in subthalamic nucleus (STN), but the basis for this effect is still disputed. In this intraoperative microdialysis study, we found elevated cGMP extracellular concentrations in the internal segment of the globus pallidus, despite negligible changes in glutamate levels, during a clinically effective STN-DBS. This supports the view that a clinically beneficial effect of STN-DBS is paralleled by an augmentation (and not an inactivation) of the STN output onto the GPi.

Placebo-responsive Parkinson patients show decreased activity in single neurons of subthalamic nucleus

Placebo administration is known to affect the brain both in pain and in Parkinson disease. Here we show that placebo treatment caused reduced activity in single neurons in the subthalamic nucleus of placebo-responsive Parkinsonian patients. These changes in activity were tightly correlated with clinical improvement; no decrease in activity occurred when the clinical placebo response was absent.

D2-mediated modulation of N-type calcium currents in rat globus pallidus neurons following dopamine denervation

We have studied the effects of dopamine and the D2-like agonist quinpirole on calcium currents of neurons isolated from the striatum and the globus pallidus (GP). Experiments were performed in young adult rats, either in control conditions or following lesion of the nigrostriatal pathway by the unilateral injection of 6-hydroxydopamine (6-OHDA) in the substantia nigra. Apomorphine-driven contralateral turning, 15 days after lesioning, assessed the severity of the dopamine denervation. In addition, the loss of tyrosine hydroxylase immunohistochemistry confirmed the extent of the toxin-induced damage. In both striatal medium spiny (MS) and GP neurons of control animals dopamine and quinpirole promoted a very modest inhibition of calcium conductance. Following 6-OHDA, the inhibition was unaltered in MS (from 10 to 12%), but significantly augmented in GP neurons (21% vs. 9%). Interestingly, analogous inhibition was observed in GP neurons dissociated 20 h after reserpine treatment. Further features of the D2 response were thus studied only in neurons isolated from 6-OHDA-lesioned GP. The D2 modulation was G-protein-mediated but not strictly voltage-dependent. omega-Conotoxin-GVIA occluded the response implying the involvement of N-type calcium channels. The effect of quinpirole developed fast and was insensitive to alterations of cytosolic cAMP. The incubation in phorbol esters or OAG blocked the D2 effect, supporting the involvement of PKC. These findings suggest that postsynaptic D2-like receptors are functionally expressed on GP cell bodies and may supersensitize following dopamine-denervation. A direct D2 modulation of calcium conductance in GP may alter GP firing properties and GABA release onto pallidofugal targets.