High-frequency stimulation of the subthalamic nucleus prolongs the increase in striatal dopamine induced by acute l-3,4-dihydroxyphenylalanine in dopaminergic denervated rats

Deep brain stimulation-induced neuroprotection: A critical appraisal

Over the last two decades deep brain stimulation (DBS) has become a widely used therapeutic alternative for a variety of neurological and psychiatric diseases. The extensive experience in the field of movement disorders has provided valuable knowledge and has led the path to its application to other hard-to-treat conditions. Despite the recognised symptomatic beneficial effects, its capacity to modify the course of a disease has been in constant debate. The ability to demonstrate neuroprotection relies on a thorough understanding of the functioning of both normal and pathological neural structures, as well as their stimulation induced alterations, all of which to this date remain incomplete. Consequently, there is no consensus over the definition of neuroprotection nor its means of quantification or evaluation. Additionally, neuroprotection has been indirectly addressed in most of the literature, challenging the efforts to narrow its interpretation. As such, a broad spectrum of evidence has been considered to demonstrate disease modifying interventions. This paper aims to provide a critical appraisal of the current evidence on potential neuroprotective effects of DBS in neurodegenerative brain disorders.

Discovery of novel dual adenosine A1/A2A receptor antagonists using deep learning, pharmacophore modeling and molecular docking

Adenosine receptors (ARs) have been demonstrated to be potential therapeutic targets against Parkinson's disease (PD). In the present study, we describe a multistage virtual screening approach that identifies dual adenosine A1 and A2A receptor antagonists using deep learning, pharmacophore models, and molecular docking methods. Nineteen hits from the ChemDiv library containing 1,178,506 compounds were selected and further tested by in vitro assays (cAMP functional assay and radioligand binding assay); of these hits, two compounds (C8 and C9) with 1,2,4-triazole scaffolds possessing the most potent binding affinity and antagonistic activity for A1/A2A ARs at the nanomolar level (pKi of 7.16-7.49 and pIC50 of 6.31-6.78) were identified. Further molecular dynamics (MD) simulations suggested similarly strong binding interactions of the complexes between the A1/A2A ARs and two compounds (C8 and C9). Notably, the 1,2,4-triazole derivatives (compounds C8 and C9) were identified as the most potent dual A1/A2A AR antagonists in our study and could serve as a basis for further development. The effective multistage screening approach developed in this study can be utilized to identify potent ligands for other drug targets.

Effects of subthalamic deep brain stimulation on striatal metabolic connectivity in a rat hemiparkinsonian model

Deep brain stimulation (DBS) in the subthalamic nucleus (STN) has been successfully used for the treatment of advanced Parkinson's disease, although the underlying mechanisms are complex and not well understood. There are conflicting results about the effects of STN-DBS on neuronal activity of the striatum, and its impact on functional striatal connectivity is entirely unknown. We therefore investigated how STN-DBS changes cerebral metabolic activity in general and striatal connectivity in particular. We used ipsilesional STN stimulation in a hemiparkinsonian rat model in combination with [¹⁸F]FDOPA-PET, [¹⁸F]FDG-PET and metabolic connectivity analysis. STN-DBS reversed ipsilesional hypometabolism and contralesional hypermetabolism in hemiparkinsonian rats by increasing metabolic activity in the ipsilesional ventrolateral striatum and by decreasing it in the contralesional hippocampus and brainstem. Other STN-DBS effects were subject to the magnitude of dopaminergic lesion severity measured with [¹⁸F]FDOPA-PET, e.g. activation of the infralimbic cortex was negatively correlated to lesion severity. Connectivity analysis revealed that, in healthy control animals, left and right striatum formed a bilateral functional unit connected by shared cortical afferents, which was less pronounced in hemiparkinsonian rats. The healthy striatum was metabolically connected to the ipsilesional substantia nigra in hemiparkinsonian rats only (OFF condition). STN-DBS (ON condition) established a new functional striatal network, in which interhemispheric striatal connectivity was strengthened, and both the dopamine-depleted and the healthy striatum were functionally connected to the healthy substantia nigra. We conclude that both unilateral dopamine depletion and STN-DBS affect the whole brain and alter complex interhemispheric networks.

Summary: Deep brain stimulation in the subthalamic nucleus in rats with a unilateral dopaminergic lesion established a new functional interhemispheric striatal network.

The pathophysiological mechanisms of motivational deficits in Parkinson's disease

Parkinson's disease (PD) is a progressive degenerative disorder that leads to disabling motor symptoms and a wide variety of neuropsychiatric symptoms. Apathy is the most common psychiatric disorder in the early stages of untreated PD and can be defined as a hypodopaminergic syndrome, which also includes anxiety and depression. Apathy is also considered the core feature of the parkinsonian triad (apathy, anxiety and depression) of behavioural non-motor signs, including a motivational deficit. Moreover, apathy is recognised as a distinct chronic neuropsychiatric behavioural disorder based on specific diagnostic criteria. Given the prevalence of apathy in approximately 40% of the general Parkinson's disease population, this appears to be a contributing factor to dementia in PD; also, apathy symptoms are factors that potentially contribute to morbidity, leading to a major impairment of health-related quality of life, thus stressing the importance of understanding the pathophysiology of this disease. Several studies have clearly established a prominent role for DA-mediated signals in PD apathy. However, synergistic interaction between dopaminergic impairment resulting from the neurodegenerative process and deep brain stimulation of the subthalamic nucleus may cause or exacerbate apathy. Furthermore, serotoninergic mechanism signalling is also likely to be of importance in this pathophysiology.

Striatal Glutamate and GABA after High Frequency Subthalamic Stimulation in Parkinsonian Rat

Objective

High frequency stimulation (HFS) of the subthalamic nucleus (STN) is recognized as an effective treatment of advanced Parkinson’s disease. However, the neurochemical basis of its effects remains unknown. The aim of this study is to investigate the effects of STN HFS in intact and 6-hydroxydopamine (6-OHDA)-lesioned hemiparkinsonian rat model on changes of principal neurotransmitters, glutamate, and gamma-aminobutyric acid (GABA) in the striatum.

Methods

The authors examined extracellular glutamate and GABA change in the striatum on sham group, 6-OHDA group, and 6-OHDA plus deep brain stimulation (DBS) group using microdialysis methods.

Results

High-pressure liquid chromatography was used to quantify glutamate and GABA. The results show that HFS-STN induces a significant increase of extracellular glutamate and GABA in the striatum of 6-OHDA plus DBS group compared with sham and 6-OHDA group.

Conclusion

Therefore, the clinical results of STN-HFS are not restricted to the direct STN targets but involve widespread adaptive changes within the basal ganglia.

Subthalamic nucleus deep brain stimulation on motor-symptoms of Parkinson's disease: Focus on neurochemistry

Deep brain stimulation (DBS) has become a standard therapy for Parkinson's disease (PD) and it is also currently under investigation for other neurological and psychiatric disorders. Although many scientific, clinical and ethical issues are still unresolved, DBS delivered into the subthalamic nucleus (STN) has improved the quality of life of several thousands of patients. The mechanisms underlying STN-DBS have been debated extensively in several reviews; less investigated are the biochemical consequences, which are still under scrutiny. Crucial and only partially understood, for instance, are the complex interplays occurring between STN-DBS and levodopa (LD)-centred therapy in the post-surgery follow-up. The main goal of this review is to address the question of whether an improved motor control, based on STN-DBS therapy, is also achieved through the additional modulation of other neurotransmitters, such as noradrenaline (NA) and serotonin (5-HT). A critical issue is to understand not only acute DBS-mediated effects, but also chronic changes, such as those involving cyclic nucleotides, capable of modulating circuit plasticity. The present article will discuss the neurochemical changes promoted by STN-DBS and will document the main results obtained in microdialysis studies. Furthermore, we will also examine the preliminary achievements of voltammetry applied to humans, and discuss new hypothetical investigational routes, taking into account novel players such as glia, or subcortical regions such as the pedunculopontine (PPN) area. Our further understanding of specific changes in brain chemistry promoted by STN-DBS would further disseminate its utilisation, at any stage of disease, avoiding an irreversible lesioning approach.

Subthalamic Nucleus Deep Brain Stimulation Modulate Catecholamine Levels with Significant Relations to Clinical Outcome after Surgery in Patients with Parkinson's Disease

Aims

Although subthalamic nucleus deep brain stimulation (STN-DBS) is effective in patients with advanced Parkinson’s disease (PD), its physiological mechanisms remain unclear. Because STN-DBS is effective in patients with PD whose motor symptoms are dramatically alleviated by L-3,4-dihydroxyphenylalanine (L-DOPA) treatment, the higher preoperative catecholamine levels might be related to the better clinical outcome after surgery. We aimed to examine the correlation between the preoperative catecholamine levels and postoperative clinical outcome after subthalamic nucleus deep brain stimulation. The effectiveness of STN-DBS in the patient who responded well to dopaminergic medication suggest the causal link between the dopaminergic system and STN-DBS. We also examined how catecholamine levels were modulated after subthalamic stimulation.

Methods

In total 25 patients with PD were enrolled (Mean age 66.2 ± 6.7 years, mean disease duration 11.6 ± 3.7 years). Mean levodopa equivalent doses were 1032 ± 34.6 mg before surgery. Cerebrospinal fluid and plasma catecholamine levels were measured an hour after oral administration of antiparkinsonian drugs before surgery. The mean Unified Parkinson’s Disease Rating Scale scores (UPDRS) and the Parkinson’s disease Questionnaire-39 (PDQ-39) were obtained before and after surgery. Of the 25 patients, postoperative cerebrospinal fluid and plasma were collected an hour after oral administration of antiparkinsonian drugs during on stimulation at follow up in 11 patients.

Results

Mean levodopa equivalent doses significantly decreased after surgery with improvement in motor functions and quality of life. The preoperative catecholamine levels had basically negative correlations with postoperative motor scores and quality of life, suggesting that higher preoperative catecholamine levels were related to better outcome after STN-DBS. The preoperative plasma levels of L-DOPA had significantly negative correlations with postoperative UPDRS- III score in off phase three months after STN-DBS. The preoperative cerebrospinal fluid (CSF) 3,4-dihydroxyphenylacetic acid (DOPAC) and 5-hydroxytryptamine (5-HT) levels had significantly negative correlations with postoperative UPDRS- III score in off phase one year after STN-DBS and the preoperative CSF homovanilic acid (HVA) levels had significant negative correlations with postoperative UPDRS- III score in on phase three months after STN-DBS. In PDQ-39 SI (summary index), preoperative plasma dopamine (DA) level had significantly negative correlations with postoperative PDQ-39 SI one year after STN-DBS suggesting that higher preoperative plasma DA level resulted in better quality of life (QOL) one year after STN-DBS. The stepwise multiple linear regression study revealed that higher preoperative plasma HVA levels had negative influence on the postoperative motor symptoms (i.e., increase in the score of UPDRS), whereas higher preoperative CSF L-DOPA levels had positive influence on the postoperative motor symptoms and QOL (decrease in the score of UPDRS and PDQ-39 SI) The catecholamine levels were not significantly reduced postoperatively in 11 patients despite the significant reduction in levodopa equivalent doses. Unexpectedly, CSF HVA levels significantly increased from 0.00089±0.0003 ng/μl to 0.002±0.0008 ng/μl after STN-DBS.

Conclusion

The preoperative catecholamine levels might affect the postoperative motor symptoms and quality of life. The catecholamine levels were not significantly reduced postoperatively despite the significant reduction in levodopa equivalent doses.

Mechanisms and therapeutic applications of electromagnetic therapy in Parkinson's disease

Electromagnetic therapy is a non-invasive and safe approach for the management of several pathological conditions including neurodegenerative diseases. Parkinson's disease is a neurodegenerative pathology caused by abnormal degeneration of dopaminergic neurons in the ventral tegmental area and substantia nigra pars compacta in the midbrain resulting in damage to the basal ganglia. Electromagnetic therapy has been extensively used in the clinical setting in the form of transcranial magnetic stimulation, repetitive transcranial magnetic stimulation, high-frequency transcranial magnetic stimulation and pulsed electromagnetic field therapy which can also be used in the domestic setting. In this review, we discuss the mechanisms and therapeutic applications of electromagnetic therapy to alleviate motor and non-motor deficits that characterize Parkinson's disease.

Striatal molecular signature of subchronic subthalamic nucleus high frequency stimulation in parkinsonian rat

This study addresses the molecular mechanisms underlying the action of subthalamic nucleus high frequency stimulation (STN-HFS) in the treatment of Parkinson's disease and its interaction with levodopa (L-DOPA), focusing on the striatum. Striatal gene expression profile was assessed in rats with nigral dopamine neuron lesion, either treated or not, using agilent microarrays and qPCR verification. The treatments consisted in anti-akinetic STN-HFS (5 days), chronic L-DOPA treatment inducing dyskinesia (LIDs) or the combination of the two treatments that exacerbated LIDs. STN-HFS modulated 71 striatal genes. The main biological processes associated with the differentially expressed gene products include regulation of growth, of apoptosis and of synaptic transmission, and extracellular region is a major cellular component implicated. In particular, several of these genes have been shown to support survival or differentiation of striatal or of dopaminergic neurons. These results indicate that STN HFS may induce widespread anatomo-functional rearrangements in the striatum and create a molecular environment favorable for neuroprotection and neuroplasticity. STN-HFS and L-DOPA treatment share very few common gene regulation features indicating that the molecular substrates underlying their striatal action are mostly different; among the common effects is the down-regulation of Adrb1, which encodes the adrenergic beta-1-receptor, supporting a major role of this receptor in Parkinson's disease. In addition to genes already reported to be associated with LIDs (preprodynorphin, thyrotropin-releasing hormone, metabotropic glutamate receptor 4, cannabinoid receptor 1), the comparison between DOPA and DOPA/HFS identifies immunity-related genes as potential players in L-DOPA side effects.

The role of the subthalamic nucleus in L-DOPA induced dyskinesia in 6-hydroxydopamine lesioned rats

L-DOPA is the most effective treatment for Parkinson's disease (PD), but prolonged use leads to disabling motor complications including dyskinesia. Strong evidence supports a role of the subthalamic nucleus (STN) in the pathophysiology of PD whereas its role in dyskinesia is a matter of controversy. Here, we investigated the involvement of STN in dyskinesia, using single-unit extracellular recording, behavioural and molecular approaches in hemi-parkinsonian rats rendered dyskinetic by chronic L-DOPA administration. Our results show that chronic L-DOPA treatment does not modify the abnormal STN activity induced by the 6-hydroxydopamine lesion of the nigrostriatal pathway in this model. Likewise, we observed a loss of STN responsiveness to a single L-DOPA dose both in lesioned and sham animals that received daily L-DOPA treatment. We did not find any correlation between the abnormal involuntary movement (AIM) scores and the electrophysiological parameters of STN neurons recorded 24 h or 20–120 min after the last L-DOPA injection, except for the axial subscores. Nonetheless, unilateral chemical ablation of the STN with ibotenic acid resulted in a reduction in global AIM scores and peak-severity of dyskinesia. In addition, STN lesion decreased the anti-dyskinetogenic effect of buspirone in a reciprocal manner. Striatal protein expression was altered in dyskinetic animals with increases in ΔFosB, phosphoDARPP-32, dopamine receptor (DR) D3 and DRD2/DRD1 ratio. The STN lesion attenuated the striatal molecular changes and normalized the DRD2/DRD1 ratio. Taken together, our results show that the STN plays a role, if modest, in the physiopathology of dyskinesias.

Raclopride or high-frequency stimulation of the subthalamic nucleus stops cocaine-induced motor stereotypy and restores related alterations in prefrontal basal ganglia circuits

Motor stereotypy is a key symptom of various neurological or neuropsychiatric disorders. Neuroleptics or the promising treatment using deep brain stimulation stops stereotypies but the mechanisms underlying their actions are unclear. In rat, motor stereotypies are linked to an imbalance between prefrontal and sensorimotor cortico-basal ganglia circuits. Indeed, cortico-nigral transmission was reduced in the prefrontal but not sensorimotor basal ganglia circuits and dopamine and acetylcholine release was altered in the prefrontal but not sensorimotor territory of the dorsal striatum. Furthermore, cholinergic transmission in the prefrontal territory of the dorsal striatum plays a crucial role in the arrest of motor stereotypy. Here we found that, as previously observed for raclopride, high-frequency stimulation of the subthalamic nucleus (HFS STN) rapidly stopped cocaine-induced motor stereotypies in rat. Importantly, raclopride and HFS STN exerted a strong effect on cocaine-induced alterations in prefrontal basal ganglia circuits. Raclopride restored the cholinergic transmission in the prefrontal territory of the dorsal striatum and the cortico-nigral information transmissions in the prefrontal basal ganglia circuits. HFS STN also restored the N-methyl-d-aspartic-acid-evoked release of acetylcholine and dopamine in the prefrontal territory of the dorsal striatum. However, in contrast to raclopride, HFS STN did not restore the cortico-substantia nigra pars reticulata transmissions but exerted strong inhibitory and excitatory effects on neuronal activity in the prefrontal subdivision of the substantia nigra pars reticulata. Thus, both raclopride and HFS STN stop cocaine-induced motor stereotypy, but exert different effects on the related alterations in the prefrontal basal ganglia circuits.

A combined in vivo neurochemical and electrophysiological analysis of the effect of high-frequency stimulation of the subthalamic nucleus on 5-HT transmission

Movement disability in advanced Parkinson's disease (PD) can be treated by high frequency stimulation (HFS) of the subthalamic nucleus (STN) but some patients experience psychiatric side-effects including depression, which is strongly linked to decreases in 5-hydroxytryptamine (5-HT). The current study investigated the effect of bilateral STN HFS on extracellular 5-HT in brain regions of anesthetized and freely moving rats as measured with microdialysis. Parallel in vivo electrophysiological experiments allowed a correlation of changes in extracellular 5-HT with the firing of 5-HT neurons. Bilateral STN HFS decreased (by up to 25%) extracellular levels of 5-HT in both striatum and medial prefrontal cortex of anesthetized rats. STN HFS also decreased extracellular 5-HT in the medial prefrontal cortex of freely moving rats. This decrease in extracellular 5-HT persisted after turning off the stimulation, and was present in dopamine-denervated rats. As with changes in extracellular 5-HT, in anesthetized rats STN HFS evoked a decrease in the in vivo firing of midbrain raphe 5-HT neurons that also persisted after cessation of stimulation. These data provide neurochemical evidence for an inhibition of 5-HT neurotransmission by STN HFS, which may contribute to its psychiatric side effects and guide therapeutic options.

Biodegradable microspheres loaded with an anti-Parkinson prodrug: an in vivo pharmacokinetic study

During chronic treatment with L-dopa (LD), Parkinsonian patients often experience uncontrolled motor complications due to fluctuations of the plasmatic levels of LD that result in pulsatile dopaminergic stimulation. To overcome these plasmatic fluctuations, a novel prodrug of LD, L-dopa-α-lipoic acid (LD-LA), has been proposed as a tool for achieving continuous dopaminergic stimulation. Due to slower susceptibility toward enzymatic conversion by LD-degrading enzymes (such as catechol-O-methyltransferase and monoamine oxidase), the plasma half-life of this prodrug is longer than that of LD. Moreover, the higher lipophilicity of LD-LA over LD promotes its delivery to the CNS, where the resulting levels of dopamine (DA) are kept high for a longer time than after equimolar administration of LD. To further reduce fluctuations in plasma levels of LD, LD-LA has been entrapped into biodegradable polymeric microspheres to be used as a depot system with the aim to prevent prodrug degradation and to obtain a sustained release of the intact compound. In the present work, a formulation of LD-LA loaded microspheres (characterized for drug loading, size, morphology, thermal properties, and in vitro prodrug release) has been administered subcutaneously to rats, and the resulting levels of LD and DA in plasma and striatal tissue, respectively, have been monitored. A good correlation between the in vitro release kinetics and the time range during which the formulation alters the LD/DA tissue levels in vivo was observed, suggesting that the polymeric microsphere matrix protects the loaded prodrug from chemical and enzymatic degradation and controls its release. Interestingly, LD-LA microspheres provided sustained levels of DA neurotransmitter in the striatum nucleus for up to 4 days after a single administration. In conclusion, a polymeric microsphere formulation of LD-LA is an attractive medicine for treating Parkinson's disease (PD) symptoms, avoiding motor complications.

Axonal failure during high frequency stimulation of rat subthalamic nucleus

Deep brain stimulation (DBS) has been established as an effective surgical therapy for advanced Parkinson's disease (PD) and gains increasing acceptance for otherwise intractable neuropsychiatric diseases such as major depression or obsessive–compulsive disorders. In PD, DBS targets predominantly the subthalamic nucleus (STN) and relieves motor deficits only at high frequency (>100 Hz). In contrast to the well-documented clinical efficacy of DBS, its underlying principle remains enigmatic spawning a broad and, in part, contradictory spectrum of suggested synaptic and non-synaptic mechanisms within and outside STN. Here we focused on a crucial, but largely neglected issue in this controversy, namely the axonal propagation of DBS within and away from STN. In rat brain slices preserving STN projections to substantia nigra (SN) and entopeduncular nucleus (EP, the rodent equivalent of internal globus pallidus), STN-DBS disrupted synaptic excitation onto target neurons through an unexpected failure of axonal signalling. The rapid onset and, upon termination of DBS, recovery of this effect was highly reminiscent of the time course of DBS in the clinical setting. We propose that DBS-induced suppression of axonal projections from and to STN serves to shield basal ganglia circuitry from pathological activity arising in or amplified by this nucleus.

High-frequency stimulation of the subthalamic nucleus and L-3,4-dihydroxyphenylalanine inhibit in vivo serotonin release in the prefrontal cortex and hippocampus in a rat model of Parkinson's disease

High-frequency stimulation of the subthalamic nucleus (STN-HFS) and l-3,4-dihydroxyphenylalanine (l-DOPA) medication are the most used therapeutic approaches in Parkinson's disease (PD), but their beneficial motor effects are burdened by the emergence of cognitive and depressive disorders. Although a reduced serotonergic function has been linked to the psychiatric effects of antiparkinsonian treatments, biochemical evidence supporting this hypothesis is still lacking. By using a microdialysis approach in anesthetized rats, we investigated the ability of STN-HFS (130 Hz, 30 muA, 20 min) and l-DOPA (6-12 mg/kg) to change extracellular levels of serotonin (5-HT) monitored simultaneously in the prefrontal cortex (PFC) and hippocampus (HIPP), two brain regions involved in the regulation of mood and cognition that receive a distinct 5-HT innervation. The results show that STN-HFS inhibited 5-HT levels in the PFC and HIPP of sham-lesioned and 6-hydroxydopamine (6-OHDA)-lesioned rats. The effect elicited by STN-HFS was blocked by the administration of the 5-HT(1A) agonist 8-hydroxy-N,N-dipropyl-2-aminotetralin. l-DOPA (6 and 12 mg/kg) reduced 5-HT levels in the PFC and HIPP of 6-OHDA rats. STN-HFS did not further decrease 5-HT levels induced by l-DOPA, but attenuated l-DOPA-induced dopamine release in the PFC and HIPP. These neurochemical data show that STN-HFS inhibits 5-HT release by modulating serotonergic neuron activity, while the decrease in 5-HT levels induced by l-DOPA may include its direct action inside serotonergic neurons. These results support the premise that antiparkinsonian treatments reduce central serotonergic transmission, which may favor the development of nonmotor side effects in PD.

Deep brain stimulation state of the art and novel stimulation targets

Levodopa therapy represents a major breakthrough in the treatment of Parkinson's disease (PD). As time and disease severity progresses, however, the shortcomings and adverse effects of this neurotransmitter replacement strategy become apparent and patients develop disabilities despite best medical therapy. The heightened awareness of these difficulties has given birth to a re-examination of functional neurosurgery for advanced PD. In the 20 years since the renewed interest in deep brain stimulation (DBS), approximately 60,000 patients with PD have undergone this surgery, with an annual accrual of 8000-10,000 new patients per year worldwide. Clinical studies have confirmed the beneficial effects of DBS surgery for the treatment of the cardinal motor features of PD. The likelihood of improvement, however, varies from symptom to symptom and from patient to patient. Surgery is very effective in reducing the motor fluctuations and dyskinesias--the primary reasons for patients' intolerance to medical therapy. Other problems are less or non-responsive. Further, despite the widespread use of this technology, the mechanism through which DBS alleviates symptoms is not fully understood. This review will discuss the patient population most likely to benefit from surgery, what aspects of the disease are most responsive, the current limitations of DBS, and new therapeutic targets that are being examined to address these limitations.

Long term high frequency stimulation of STN increases dopamine in the corpus striatum of hemiparkinsonian rhesus monkey

Long term subthalamic nucleus (STN) high frequency stimulation (HFS) can improve most symptoms of Parkinson's disease (PD) patients and decrease the dosage of antiparkinsonian drug such as Madopar. The mechanism of STN HFS for PD still remains elusive. We hypothesize that the level of dopamine (DA) and its metabolites in the corpus striatum is increased after long term STN HFS. The aim of this study was to examine the DA and its metabolites in the extracellular space of corpus striatum in hemiparkinsonian monkeys during long term STN HFS. Four rhesus monkeys were induced to hemiparkinsonian models by injecting 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) through right internal carotid artery. Then two of them were underwent long term right STN HFS for the subsequent microdialysis sessions. Four microdialysis probe cannulas were implanted into bilateral putamen and caudate nucleus respectively. The microdialysis probe was put into the microdialysis probe cannula of bilateral putamen and caudate nucleus. Dialysates of extracellular space in corpus striatum were collected prior to STN HFS, and subsequently 8 h, 1 week, 1 month, 2 months, 8 months and 10 months after STN HFS. The level of DA and its metabolites were determined by high performance liquid chromatography and subthalamic nucleus electrochemical detection (HPLC-ECD). HFS significantly improved PD symptoms of the monkeys. Rotation evoked by apomorphine (APO) disappeared immediately after HFS pulse generator was turned on. The levels of DA and its metabolites in putamen and caudate nucleus of electrode side increased significantly at different time points after stimulation. Long term STN HFS significantly improved symptoms of hemiparkinsonian rhesus monkey, which might be due to the increase of dopamine and/or its metabolites in corpus striatum.

In vivo microdialysis in Parkinson's research

Parkinson's disease (PD) is a progressive neurodegenerative disorder that is primarily characterized by the degeneration of dopamine (DA) neurons in the nigrostriatal system, which in turn produces profound neurochemical changes within the basal ganglia, representing the neural substrate for parkinsonian motor symptoms. The pathogenesis of the disease is still not completely understood, but environmental and genetic factors are thought to play important roles. Research into the pathogenesis and the development of new therapeutic intervention strategies that will slow or stop the progression of the disease in human has rapidly advanced by the use of neurotoxins that specifically target DA neurons. Over the years, a broad variety of experimental models of the disease has been developed and applied in diverse animal species. The two most common toxin models used employ 6-hydroxydopamine (6-OHDA) and the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine/1-methyl-4-phenilpyridinium ion (MPTP/MPP+), either given systemically or locally applied into the nigrostriatal pathway, to resemble PD features in animals. Both neurotoxins selectively and rapidly destroy catecolaminergic neurons, although with different mechanisms. Since in vivo microdialysis coupled to high-performance liquid chromatography is an established technique for studying physiological, pharmacological, and pathological changes of a wide range of low molecular weight substances in the brain extracellular fluid, here we review the most prominent animal and human data obtained by the use of this technique in PD research.

Neuronal activity in the subthalamic nucleus modulates the release of dopamine in the monkey striatum

The primate subthalamic nucleus (STN) is commonly seen as a relay nucleus between the external and internal pallidal segments, and as an input station for cortical and thalamic information into the basal ganglia. In rodents, STN activity is also known to influence neuronal activity in the dopaminergic substantia nigra pars compacta (SNc) through inhibitory and excitatory mono- and polysynaptic pathways. Although the anatomical connections between STN and SNc are not entirely the same in primates as in rodents, the electrophysiologic and microdialysis experiments presented here show directly that this functional interaction can also be demonstrated in primates. In three Rhesus monkeys, extracellular recordings from SNc during microinjections into the STN revealed that transient pharmacologic activation of the STN by the acetylcholine receptor agonist carbachol substantially increased burst firing of single nigral neurons. Transient inactivation of the STN with microinjections of the GABA-A receptor agonist muscimol had the opposite effect. While the firing rates of individual SNc neurons changed in response to the activation or inactivation of the STN, these changes were not consistent across the entire population of SNc cells. Permanent lesions of the STN, produced in two animals with the fiber-sparing neurotoxin ibotenic acid, reduced burst firing and firing rates of SNc neurons, and substantially decreased dopamine levels in the primary recipient area of SNc projections, the striatum, as measured with microdialysis. These results suggest that activity in the primate SNc is prominently influenced by neuronal discharge in the STN, which may thus alter dopamine release in the striatum.

Mechanisms and targets of deep brain stimulation in movement disorders

Chronic electrical stimulation of the brain, known as deep brain stimulation (DBS), has become a preferred surgical treatment for medication-refractory movement disorders. Despite its remarkable clinical success, the therapeutic mechanisms of DBS are still not completely understood, limiting opportunities to improve treatment efficacy and simplify selection of stimulation parameters. This review addresses three questions essential to understanding the mechanisms of DBS. 1) How does DBS affect neuronal tissue in the vicinity of the active electrode or electrodes? 2) How do these changes translate into therapeutic benefit on motor symptoms? 3) How do these effects depend on the particular site of stimulation? Early hypotheses proposed that stimulation inhibited neuronal activity at the site of stimulation, mimicking the outcome of ablative surgeries. Recent studies have challenged that view, suggesting that although somatic activity near the DBS electrode may exhibit substantial inhibition or complex modulation patterns, the output from the stimulated nucleus follows the DBS pulse train by direct axonal excitation. The intrinsic activity is thus replaced by high-frequency activity that is time-locked to the stimulus and more regular in pattern. These changes in firing pattern are thought to prevent transmission of pathologic bursting and oscillatory activity, resulting in the reduction of disease symptoms through compensatory processing of sensorimotor information. Although promising, this theory does not entirely explain why DBS improves motor symptoms at different latencies. Understanding these processes on a physiological level will be critically important if we are to reach the full potential of this powerful tool.