The Effects of Deep Brain Stimulation of the Subthalamic Nucleus on Vascular Endothelial Growth Factor, Brain-Derived Neurotrophic Factor, and Glial Cell Line-Derived Neurotrophic Factor in a Rat Model of Parkinson's Disease.

OBJECTIVE: Deep brain stimulation (DBS) of the subthalamic nucleus (STN) has evolved as a powerful therapeutic alternative for the treatment of Parkinson's disease (PD). Despite its clinical efficacy, the mechanisms of action have remained poorly understood. In addition to the immediate symptomatic effects, long-term neuroprotective effects have been suggested. Those may be mediated through neurotrophic factors (NFs) like vascular endothelial growth factor (VEGF), brain-derived neurotrophic factor (BDNF), and glial cell line-derived neurotrophic factor (GDNF). Here, the impact of DBS on the expression of NFs was analysed in a rat model of PD. METHODS: Unilateral 6-hydroxydopamine (6-OHDA) lesioned rats received DBS in the STN using an implantable microstimulation system, sham DBS in the STN, or no electrode placement. Continuous unilateral STN-DBS (current intensity 50 µA, frequency 130 Hz, and pulse width 52 µs) was conducted for 14 days. Rats were then sacrificed and brains shock frozen. Striata and motor cortices were dissected with a cryostat. Levels of VEGF, BDNF, and GDNF were analysed, both by quantitative PCR and colorimetric ELISA. RESULTS: PCR revealed a significant upregulation of only BDNF mRNA in the ipsilateral striata of the DBS group, when compared to the sham-stimulated group. There was no significant increase in VEGF mRNA or GDNF mRNA. ELISA analysis showed augmentations of BDNF, VEGF, as well as GDNF protein in the ipsilateral striata after DBS compared to sham stimulation. In the motor cortex, significant increases after DBS were observed for BDNF only, not for the other 2 NFs. CONCLUSIONS: The upregulation of trophic factors induced by STN-DBS may participate in its long-term therapeutic efficacy and potentially neuroprotective effects.

Topological Causality in Dynamical Systems.

Determination of causal relations among observables is of fundamental interest in many fields dealing with complex systems. Since nonlinear systems generically behave as wholes, classical notions of causality assuming separability of subsystems often turn out inadequate. Still lacking is a mathematically transparent measure of the magnitude of effective causal influences in cyclic systems. For deterministic systems we found that the expansions of mappings among time-delay state space reconstructions from different observables not only reflect the directed coupling strengths, but also the dependency of effective influences on the system's temporally varying state. Estimation of the expansions from pairs of time series is straightforward and used to define novel causality indices. Mathematical and numerical analysis demonstrate that they reveal the asymmetry of causal influences including their time dependence, as well as provide measures for the effective strengths of causal links in complex systems.

Stability of Neuronal Networks with Homeostatic Regulation.

Neurons are equipped with homeostatic mechanisms that counteract long-term perturbations of their average activity and thereby keep neurons in a healthy and information-rich operating regime. While homeostasis is believed to be crucial for neural function, a systematic analysis of homeostatic control has largely been lacking. The analysis presented here analyses the necessary conditions for stable homeostatic control. We consider networks of neurons with homeostasis and show that homeostatic control that is stable for single neurons, can destabilize activity in otherwise stable recurrent networks leading to strong non-abating oscillations in the activity. This instability can be prevented by slowing down the homeostatic control. The stronger the network recurrence, the slower the homeostasis has to be. Next, we consider how non-linearities in the neural activation function affect these constraints. Finally, we consider the case that homeostatic feedback is mediated via a cascade of multiple intermediate stages. Counter-intuitively, the addition of extra stages in the homeostatic control loop further destabilizes activity in single neurons and networks. Our theoretical framework for homeostasis thus reveals previously unconsidered constraints on homeostasis in biological networks, and identifies conditions that require the slow time-constants of homeostatic regulation observed experimentally.

A model for attentional information routing through coherence predicts biased competition and multistable perception.

Selective attention allows to focus on relevant information and to ignore distracting features of a visual scene. These principles of information processing are reflected in response properties of neurons in visual area V4: if a neuron is presented with two stimuli in its receptive field, and one is attended, it responds as if the nonattended stimulus was absent (biased competition). In addition, when the luminance of the two stimuli is temporally and independently varied, local field potentials are correlated with the modulation of the attended stimulus and not, or much less, correlated with the nonattended stimulus (information routing). To explain these results in one coherent framework, we present a two-layer spiking cortical network model with distance-dependent lateral connectivity and converging feed-forward connections. With oscillations arising inherently from the network structure, our model reproduces both experimental observations. Hereby, lateral interactions and shifts of relative phases between sending and receiving layers (communication through coherence) are identified as the main mechanisms underlying both biased competition as well as selective routing. Exploring the parameter space, we show that the effects are robust and prevalent over a broad range of parameters. In addition, we identify the strength of lateral inhibition in the first model layer as crucial for determining the working regime of the system: increasing lateral inhibition allows a transition from a network configuration with mixed representations to one with bistable representations of the competing stimuli. The latter is discussed as a possible neural correlate of multistable perception phenomena such as binocular rivalry.

Subthalamic beta oscillations correlate with dopaminergic degeneration in experimental parkinsonism.

Excessive beta activity has been shown in local field potential recordings from the cortico-basal ganglia loop of Parkinson's disease patients and in its various animal models. Recent evidence suggests that enhanced beta oscillations may play a central role in the pathophysiology of the disorder and that beta activity may be directly linked to the motor impairment. However, the temporal evolution of exaggerated beta oscillations during the ongoing dopaminergic neurodegeneration and its relation to the motor impairment and histological changes are still unknown. We investigated motor behavioral, in-vivo electrophysiological (subthalamic nucleus, motor cortex) and histological changes (striatum, substantia nigra compacta) 2, 5, 10 and 20-30 days after a 6-hydroxydopamine injection into the medial forebrain bundle in Wistar rats. We found strong correlations between subthalamic beta power and motor impairment. No correlation was found for beta power in the primary motor cortex. Only subthalamic but not cortical beta power was strongly correlated with the histological markers of the dopaminergic neurodegeneration. Significantly increased subthalamic beta oscillations could be detected before this increase was found in primary motor cortex. At the latest observation time point, a significantly higher percentage of long beta bursts was found. Our study is the first to show a strong relation between subthalamic beta power and the dopaminergic neurodegeneration. Thus, we provide additional evidence for an important pathophysiological role of subthalamic beta oscillations and prolonged beta bursts in Parkinson's disease.

The impact of subthalamic deep brain stimulation on nigral neuroprotection-myth or reality?

OBJECTIVE: In the present review article we summarize available clinical and preclinical evidence, if modulation of the subthalamic nucleus (STN) could be a target for neuroprotection in Parkinson's disease (PD). BACKGROUND: Chronic deep brain stimulation (DBS) of the STN has emerged as a powerful therapeutic alternative for the treatment of PD, ensuring stable symptom control for up to five years despite the progressive nature PD. MATERIALS AND METHODS: Comparative review of literature in PuBMed available up to December 2008. RESULTS: The assessment of neuroprotection has been proven difficult in the clinical situation, as medical or surgical therapeutic options that improve PD symptoms could be erroneously considered to be neuroprotective because of the difficulty of differentiating between symptomatic effects and potential neuromodulative disease-related effects of various treatment options applied in PD. The methodological limitations of clinical trials underline the importance of putative neuroprotective compounds to be tested in clinically driven preclinical studies. Thus, animal models, mimicking progressive nigrostriatal cell death, are indispensable to further advance the important issue of neuroprotection or neuromodulation following DBS. CONCLUSION: Clear clinical evidence for STN-DBS-related neuroprotection in PD is missing. However, numerous preclinical studies show (and are discussed) that silencing of the STN via lesion or DBS may exert neuromodulative effects on nigral dopamine neurons.

High-frequency stimulation of the nucleus accumbens core and shell reduces quinpirole-induced compulsive checking in rats.

Electrical deep brain stimulation (DBS) is currently studied in the treatment of therapy-refractory obsessive compulsive disorders (OCDs). The variety of targeted brain areas and the inconsistency in demonstrating anti-compulsive effects, however, highlight the need for better mapping of brain regions in which stimulation may produce beneficial effects in OCD. Such a goal may be advanced by the assessment of DBS in appropriate animal models of OCD. Currently available data on DBS of the nucleus accumbens (NAc) on OCD-like behavior in rat models of OCD are contradictory and partly in contrast to clinical data and theoretical hypotheses about how the NAc might be pathophysiologically involved in the manifestation of OCD. Consequently, the present study investigates the effects of DBS of the NAc core and shell in a quinpirole rat model of OCD. The study demonstrates that electrical modulation of NAc core and shell activity via DBS reduces quinpirole-induced compulsive checking behavior in rats. We therefore conclude that both, the NAc core and shell constitute potential target structures in the treatment of OCD.

Subthalamic stimulation increases striatal tyrosine hydroxylase phosphorylation.

Subthalamic stimulation enhances striatal tyrosine hydroxylase activity, which is regulated by phosphorylation at different serine residues. Western blotting was performed to investigate phosphorylation at the serine residues 19, 31 and 40 in striatal tissue of rats that had received subthalamic stimulation or sham stimulation for 2 h. In animals that were killed directly after stimulation, the tyrosine hydroxylase protein content was unchanged, whereas phosphorylation at the serine residue 19 was increased and phosphorylation at the serine residues 31 and 40 tended to be higher compared with controls. By contrast, tyrosine hydroxylase protein content and phosphorylation were similar in rats that were killed 24 h after stimulation. Our results suggest that subthalamic stimulation may increase tyrosine hydroxylase activity via increased phosphorylation.

Continuous high-frequency stimulation in freely moving rats: development of an implantable microstimulation system.

High-frequency stimulation (HFS) of basal ganglia and thalamic nuclei is an established treatment for various movement disorders and has recently been extended to other neuro-psychiatric conditions. Numerous experimental studies in small laboratory animals provided important insights in the mode of action of HFS. However, the interpretation of the results is often limited by the use of short-term HFS, while patients receive continuous stimulation for many years. One reason is the lack of an established model for the application of long-term HFS in small animals. Therefore, we thought to develop an implantable microstimulation system for small laboratory animals and to establish a protocol for long-term HFS by defining non-damaging stimulus parameters with respect to brain integrity. For this purpose, we designed a miniaturized, microcontroller-based, and programmable microstimulator that allows the reliable application of continuous HFS for up to 5 weeks. Chronic HFS (total stimulation time: 3 weeks) of the subthalamic nucleus with up to 100 microA (5.2 nC/phase) through monopolar electrodes comprising activated iridium did not induce significant tissue damage as assessed by various histological techniques (Nissl's, hematoxylin and eosin, Klüver-Barrera, van Gieson's staining, NeuN and GFAP-immunoreactivity). In conclusion, chronic HFS with an implantable stimulator can be successfully applied in small animals.

Survival and functional recovery of transplanted human dopaminergic neurons into hemiparkinsonian rats depend on the cannula size of the implantation instrument.

Promising therapeutic strategies for neurodegenerative diseases such as Parkinson's disease include replacement of lost striatal dopaminergic neurons by grafting of embryonic mesencephalic cells. However, the poor survival of the transplanted tissue still limits transplantation of these cells into the human brain in a larger number of patients. We addressed the question, if the diameter of the transplantation cannulas has an effect on the number of surviving transplanted human embryonic mesencephalic cells into the striatum of 6-OHDA lesioned rats. We report a significantly higher number of surviving human cells using an ultrathin micropipette compared to cannulas with wider diameters. Importantly, higher numbers of surviving cells also correlated with a behavioral recovery of the hemiparkinsonian rats.

High frequency stimulation of the subthalamic nucleus modulates neurotransmission in limbic brain regions of the rat.

Despite the benefit high frequency stimulation (HFS) of the subthalamic nucleus (STN) has on motor symptoms of Parkinson's Disease (PD), accumulating data also suggest effects of STN-HFS on non-motor behavior. This may be related to the involvement of the STN in the limbic basal ganglia-thalamocortical loops. In the present study we investigated the effect of acute STN-HFS on neurotransmission in associated structures of these pathways, i.e. the nucleus accumbens (NAc) core and shell as well as the ventral tegmental area (VTA) using in vivo microdialysis. Experiments were performed in anaesthetized naive rats and rats selectively lesioned in the substantia nigra pars compacta (SNc) or VTA. We demonstrate that: 1. STN-HFS leads to an increase in DA in the NAc, 2., these effects are more pronounced in the NAc shell than in the NAc core, 3. STN-HFS leads to a decrease in GABA in the VTA, 4. preceding lesion of the SNc does not seem to affect the effect of STN-HFS on accumbal DA transmission whereas 5. preceding lesion of the VTA seems to prohibit further detection of DA in the NAc. We conclude that STN-HFS significantly affects neurotransmission in the limbic system, which might contribute to explain the non-motor effects of STN-HFS.

Subthalamic high frequency stimulation induced rotations are differentially mediated by D1 and D2 receptors.

High frequency stimulation (HFS) of the subthalamic nucleus (STN) has clinically emerged as a promising approach in the treatment of Parkinson's disease, epilepsy, dystonia as well as compulsive and possibly other mood disorders. The underlying mechanisms are incompletely understood, but are definitely related to high frequency and likely to involve the dopamine (DA)-system. To further test this hypothesis the present study investigated the modulation of STN-HFS-induced circling by systemic and intracerebral injection of drugs acting on DA receptors in naive freely moving rats. Within this experimental setup, unilateral STN-HFS alone induced intensity-dependent circling. Systemic injections of selective D1- (SCH-23390) and D2-((-)-sulpiride) antagonists as well as the mixed D1 and D2 agonist apomorphine dose-dependently reduced STN-HFS-induced rotational behavior. Intracerebral microinjections of (-)-sulpiride but not SCH-23390 decreased circling when injected intrastriatally and increased the number of rotations when injected intranigrally (pars reticulata (SNr)). These data reveal that STN-HFS-induced contralateral circling is differentially modulated by D1 and D2 receptors. While D2 receptor-mediated effects involve the dorso-/ventrolateral striatum and the SNr, D1 receptors probably exert their actions via brain areas outside the striatum and SNr. These findings suggest the nigrostriatal DA-system to be specifically involved in the mediation of STN-HFS-induced motor effects.

High frequency stimulation of the entopeduncular nucleus has no effect on striatal dopaminergic transmission.

High frequency stimulation (HFS) of the subthalamic nucleus (STN) is thought to be superior to stimulation of the internal pallidum (GPi) in alleviating symptoms of Parkinson's disease (PD). However, preliminary controlled studies comparing the effectiveness of both targets have not found significant differences in the improvement of parkinsonian symptoms, but have shown that STN stimulation allows a dramatic decrease in dopaminergic medication. We have previously shown that STN-HFS increases striatal extracellular dopamine (DA) metabolites, but not DA, in both naive and 6-hydroxydopamine (6-OHDA)-lesioned rats, whereas stimulation of the entopeduncular nucleus (EP), the rodent equivalent of the internal pallidum, does not affect DA or metabolite levels. Intriguingly, STN-HFS increases striatal DA release after inhibition of DA reuptake or metabolism, suggesting that this observation may have been obscured in non-drug treated animals by rapid and effective DA reuptake. Since STN-HFS further enhances DA metabolism after DA reuptake inhibition or depletion it has been proposed that STN-HFS increases both, striatal DA release and metabolism, independently. Therefore, the present study assesses the impact of EP-HFS on striatal DA release and metabolism in normal rats after inhibition of DA reuptake or metabolism, using microdialysis. In summary, our data demonstrate that, contrary to STN stimulation, EP-HFS has no effect on striatal DA release and metabolism. Thus, the present study provides a partial explanation for the reported clinical differences, and experimental evidence for differential mechanisms of action between HFS of the internal pallidum and the STN, that are most likely related to differences in functional anatomy.

High-frequency stimulation of the entopeduncular nucleus improves dystonia in dtsz hamsters.

High-frequency stimulation (HFS) of the internal pallidum (GPi) has been reported to improve generalized dystonia in patients. Currently, dystonia is thought to be associated with disturbed neuronal activity of GPi neurons. Similar findings have been observed in the dtsz hamster, a model of idiopathic paroxysmal non-kinesiogenic dystonia. For this reason, we investigated the effect of bilateral HFS of the entopeduncular nucleus (EPN, rodent homologue of GPi) on the severity of dystonia. Bilateral EPN-HFS resulted in a reversible decrease of dystonia severity up to 50% when compared to both pre- and post-HFS scores, and controls. Our results underline the pathophysiological role of the EPN in the dtsz hamster and suggest the suitability of this model to further investigate mechanisms of HFS in dystonia.

The effects of electrode material, charge density and stimulation duration on the safety of high-frequency stimulation of the subthalamic nucleus in rats.

High-frequency stimulation (HFS) of deep brain structures is a powerful therapeutic tool for the treatment of various movement disorders in patients. However, the pathophysiological mechanisms of this therapeutic approach on basal ganglia network function are still largely unknown. Hitherto, experimental studies have focused on short-term stimulation. Since patients receive HFS for many years, animal studies which reproduce the conditions of long-term stimulation will be necessary to accurately investigate the effects of HFS. However, stimulation parameters of acute HFS cannot be easily transferred to long-term conditions. Accordingly, for this purpose we studied the influence of different charge densities (0, 3, 6.5, 13 and 26 microC/cm2/phase) and duration (4 h or 3 days) of subthalamic nucleus (STN)-HFS using stainless-steel and platinum-iridium (Pt/Ir) electrodes on neuronal tissue damage in rats. Our data demonstrate the advantage of Pt/Ir over stainless-steel electrodes when used in short-term HFS (frequency 130 Hz, pulse width 60 micros) and indicate that HFS using Pt/Ir-electrodes pulsed with 3 microC/cm2/phase over 3 days did not produce any relevant tissue damage in the STN.

Deep brain stimulation in dystonia.

Renewed interest in stereotaxy for dystonia followed the introduction of deep brain stimulation (DBS) in Parkinson's disease and essential tremor in the 1990s. DBS evolved from ablative surgery, which was applied with varying results in the 1950s in patients with movement disorders such as Parkinson's disease, essential tremor and dystonia. The present review summarizes the current knowledge on clinical aspects of DBS in dystonia (Dec. 2002). Excellent results have been achieved in dystonic patients carrying a mutation in the DYT1 gene with improvements up to 90 %. Similar results may also be obtained in patients with idiopathic generalized dystonia, myoclonus-dystonia syndrome, and tardive dystonia. Substantial improvement has been observed in patients with focal dystonia (for instance cervical dystonia). Patients with secondary dystonia often display a lesser and more variable degree of improvement. Long-term studies are warranted to assess both motor and neuropsychological sequelae of DBS in dystonia. Furthermore, the optimal target for different dystonic disorders remains to be determined, although the globus pallidus internus has currently emerged as the most promising target for dystonia.

Deep brain stimulation of subthalamic neurons increases striatal dopamine metabolism and induces contralateral circling in freely moving 6-hydroxydopamine-lesioned rats.

Deep brain stimulation (DBS) of the subthalamic nucleus (STN) alleviates Parkinson's disease (PD) symptoms. Although widely used, the mechanisms of action are still unknown. In an attempt to elucidate those mechanisms, we have previously demonstrated that STN-DBS increases striatal extracellular dopamine (DA) metabolites in anaesthetized rats. PD being a movement disorder, it remains to be determined whether these findings are related to any relevant motor or behavioural changes. Thus, this study investigates concomitant behavioural changes during STN-DBS and extracellular striatal DA metabolites measured using microdialysis in freely moving 6-hydroxydopamine-lesioned rats. STN-DBS induced an increase of striatal DA metabolites in awake, freely moving animals. Furthermore, we observed concomitant contralateral circling behaviour. Taken together, these results suggest that STN-DBS could disinhibit (consequently activate) substantia nigra compacta neurons via inhibition of gamma-aminobutyric acid-ergic substantia nigra reticulata neurons.

High frequency stimulation of the subthalamic nucleus leads to presynaptic GABA(B)-dependent depression of subthalamo-nigral afferents.

Patients with akinesia benefit from chronic high frequency stimulation (HFS) of the subthalamic nucleus (STN). Among the mechanisms contributing to the therapeutic success of HFS-STN might be a suppression of activity in the output region of the basal ganglia. Indeed, recordings in the substantia nigra pars reticulata (SNr) of fully adult mice revealed that HFS-STN consistently produced a reduction of compound glutamatergic excitatory postsynaptic currents at a time when the tetrodotoxin-sensitive components of the local field potentials had already recovered after the high frequency activation. These observations suggest that HFS-STN not only alters action potential conduction on the way towards the SNr but also modifies synaptic transmission within the SNr. A classical conditioning-test paradigm was then designed to better separate the causes from the indicators of synaptic depression. A bipolar platinum-iridium macroelectrode delivered conditioning HFS trains to a larger group of fibers in the STN, while a separate high-ohmic glass micropipette in the rostral SNr provided test stimuli at minimal intensity to single fibers. The conditioning-test interval was set to 100 ms, i.e. the time required to recover the excitability of subthalamo-nigral axons after HFS-STN. The continuity of STN axons passing from the conditioning to the test sites was examined by an action potential occlusion test. About two thirds of the subthalamo-nigral afferents were occlusion-negative, i.e. they were not among the fibers directly activated by the conditioning STN stimulation. Nonetheless, occlusion-negative afferents exhibited signs of presynaptic depression that could be eliminated by blocking GABA(B) receptors with CGP55845 (1 µM). Further analysis of single fiber-activated responses supported the proposal that the heterosynaptic depression of synaptic glutamate release during and after HFS-STN is mainly caused by the tonic release of GABA from co-activated striato-nigral afferents to the SNr. This mechanism would be consistent with a gain-of-function hypothesis of DBS.

Neurochemical characterization of the striatum and the nucleus accumbens in L-type Ca(v)1.3 channels knockout mice.

L-type Ca(v)1.3 channels control the autonomous pacemaking of the substantia nigra (SN) dopamine (DA) neurons, which maintains the sustained release of DA in the striatum, its target structure. The persistent engagement of L-type channels during pacemaking might lead to increased vulnerability to environmental stressors or degenerative processes, providing a mechanism for the development of Parkinson's disease (PD). Interestingly, L-type channels are not necessary for pacemaking, opening the possible use of calcium channel antagonists as neuroprotective agents for PD without disturbing normal DA function. In this study we aimed to evaluate the consequences of Ca(v)1.3 channels deletion at the neurochemical level. For this purpose, tissue concentrations of DA and their respective metabolites were measured using high performance liquid chromatography (HPLC) in the striatum and the nucleus accumbens (NAcc) of mice lacking the gene for the Ca(v)1.3 channel subunit (CACNA1D) and compared to those in wild-type mice. Striatal DA level did not differ between the two groups. In contrast, the level of serotonin, glutamate, GABA, and taurine were increased by more than 50% in the striatum of Ca(v)1.3 null mice. Neurotransmitters levels in the NAcc did not differ between the different groups. In conclusion, our results neurochemically corroborate the robustness of the nigrostriatal DA neurons in the absence of Ca(v)1.3 channels, but suggest that complete deletion of this channel affected a variety of other transmitter systems.

Development of an implantable microstimulation system for chronic DBS in rodents.

High frequency deep brain stimulation (DBS) of certain basal ganglia nuclei (e.g. subthalamic nucleus, STN) has emerged as a powerful neuromodulatory approach in the treatment of late stage Parkinson's disease patients. However, the underlying mechanisms of action are not fully understood. We have therefore established an implantable DBS device for small laboratory animals (e.g. rats) that allows the reliable and safe application of continuous DBS for at least 3 weeks. We could further show that miniaturized monopolar electrodes comprising activated iridium are suitable for continuous stimulation of small brain structures like the STN without inducing severe insertion or stimulation related injuries.