Functional changes of the basal ganglia circuitry in Parkinson's disease

alpha-Synuclein protects SH-SY5Y cells from dopamine toxicity

Dopaminergic human neuroblastoma SH-SY5Y cells were stably transformed to increase expression of alpha-synuclein, a Parkinson's disease-related protein. Transformed cells were more resistant to oxidative insults, showing a cytoprotective role of alpha-synuclein. The expression of redox chaperonins (DJ-1, HSP70, and 14-3-3) was evaluated by Western blotting. Expression of alpha-synuclein reduced HSP70 levels even in the presence of dopamine, with a twofold increase of DJ-1 in the absence of oxidants. DJ-1 is significantly reduced by dopamine, and even more by dopamine and Cu(II). Increased alpha-synuclein expression did not affect 14-3-3, although dopamine increased its level by 60% in wild-type cells. alpha-Synuclein not only upregulated DJ-1, but also shifted all DJ-1 forms to a single spot at pI=5.7 not observed in wild-type cells. Dopamine gradually restored the distribution of DJ-1 forms to a situation similar to wild-type cells, with the form at pI=6.1 progressively enriched under oxidative conditions.

Opioids and motor complications in Parkinson's disease

The long-term treatment of Parkinson's disease with L-dopa is often associated with the appearance of involuntary movements called L-dopa-induced dyskinesias. These debilitating side-effects are thought to result from an aberrant form of plasticity triggered by a combination of factors related to dopamine denervation and repeated L-dopa administration. In animal models of Parkinson's disease, dopamine denervation and repeated L-dopa administration are associated with an enhancement of opioid transmission in the basal ganglia. The exact role of this increased opioid activity is still under debate. It has been proposed that some of the changes in opioid transmission are directly involved in the genesis of L-dopa-induced dyskinesias. In this article, we suggest that changes in opioid transmission in the basal ganglia in response to denervation and repeated L-dopa therapy are, instead, part of compensatory mechanisms to prevent motor complications. Initially, these compensatory mechanisms might be sufficient to attenuate the parkinsonian syndrome and delay the appearance of involuntary movements. But with the progression of the disease and repeated exposure to L-dopa, these mechanisms eventually fail. These new insights could contribute to better understanding of the motor complications in Parkinson's disease and lead to the development or improvement of pharmacological strategies to prevent or reduce L-dopa-induced dyskinesias.

Chronic high-frequency stimulation of the subthalamic nucleus and L-DOPA treatment in experimental parkinsonism: effects on motor behaviour and striatal glutamate transmission

Hyperactivity of striatal glutamatergic synaptic transmission in response to dopamine depletion plays a major role in the pathogenesis of parkinsonian motor symptoms. In the present study we investigated the impact, on this hyperactivity, of chronic dyskinesiogenic L-DOPA treatment, combined or not with high-frequency stimulation (HFS) of the subthalamic nucleus (STN). In vitro patch-clamp recordings were performed from striatal spiny neurons of hemiparkinsonian rats (intranigral 6-OHDA injection). Here we show that dyskinesiogenic L-DOPA treatment exacerbated striatal glutamatergic hyperactivity induced by 6-OHDA lesion. Chronic 5-day STN HFS had the opposite effect, reducing striatal glutamatergic transmission in both parkinsonian and dyskinetic animals. Consistently, chronic HFS stimulation could progressively ameliorate motor parkinsonian signs (akinesia) but, conversely, did not improve L-DOPA-induced dyskinesia (LID). Thus, the effects of L-DOPA and HFS on corticostriatal transmission seem to be dissociated. These data show for the first time that dyskinesiogenic L-DOPA treatment and chronic STN HFS with antiakinetic effects induce opposite plastic rearrangements in the striatum. The interaction between these two treatments provides further evidence that striatal glutamatergic hyperactivity is a pathophysiological correlate of akinesia rather than LID.

Reduction in parvalbumin expression in the zona incerta after 6OHDA lesion in rats

In an effort to understand better the neurochemical changes that occur in Parkinson disease, we have examined the expression patterns of the calcium-binding protein parvalbumin in the zona incerta in parkinsonian rats. Sprague-Dawley rats had small volumes of either saline (control) or 6 hydroxydopamine (6OHDA) injected into the medial forebrain bundle, the major tract carrying dopaminergic nigrostriatal axons. After various post-lesion survival periods, ranging from 2 hrs to 84 days, rats were perfused with formaldehyde and their brains processed for routine tyrosine hydroxylase (TH) or parvalbumin immunocytochemistry. In the 3 to 84 days post-lesion cases, there was an overall 50% reduction in the number of parvalbumin(+) cells in the zona incerta on the 6OHDA-lesioned side when compared to control. In the 2 hrs post-lesion cases, there was no substantial loss of parvalbumin(+) cells in the zona incerta after 6OHDA lesion, although in these cases (unlike the longer survival periods), there was limited loss of TH(+) cells in the midbrain on the lesion side. The loss of parvalbumin(+) cells from the zona incerta was due to a loss of antigen expression rather than a loss of the cells themselves, since the number of Nissl-stained cells in the zona incerta was similar on the control and 6OHDA-lesioned sides. In summary, our results indicate that a loss of the midbrain dopaminergic cells induces a major change in parvalbumin expression within the zona incerta. This change may have key functional and clinical implications.

Iptakalim alleviated the increase of extracellular dopamine and glutamate induced by 1-methyl-4-phenylpyridinium ion in rat striatum

The present study examined the effect of iptakalim (Ipt), a novel ATP-sensitive potassium (K(ATP)) channel opener (KCO), on 1-methyl-4-phenylpyridinium ion (MPP(+))-induced dopamine (DA) and glutamate efflux in extracellular fluid of rat striatum, using microdialysis technique. Rats were implanted guide cannula in the striatum and artificial cerebrospinal fluid was infused through a microdialysis probe to detect the level of DA and glutamate in the striatum. MPP(+) significantly enhanced the extracellular levels of DA and its metabolites, DOPAC and HVA, as well as glutamate. Application of Ipt (1, 10, 100 microM) concentration-dependently suppressed DA and its metabolites efflux induced by MPP(+). Concomitantly, Ipt reduced the increase of extracellular glutamate induced by MPP(+). These results suggest that Ipt can regulate DA and glutamate efflux induced by MPP(+) in rat striatum.

Unilateral lesion of the subthalamic nucleus enhances cortical fos expression associated with focally evoked seizures in the rat

Reducing subthalamic nucleus (STN) activity has been proposed as an anti-epileptic procedure. Here we show that, on the contrary, a unilateral lesion of the STN causes slight (nonsignificant) increases in the severity of limbic seizures evoked by bicuculline infusion into the piriform cortex, associated with marked Fos expression throughout the cerebral cortex. Abolishing the STN control over the basal ganglia output may therefore play a facilitatory role on cortical activation associated with limbic seizures.

Atlas-based identification of targets for functional radiosurgery

Functional disorders of the brain, such as Parkinson's disease, dystonia, epilepsy, and neuropathic pain, may exhibit poor response to medical therapy. In such cases, surgical intervention may become necessary. Modern surgical approaches to such disorders include radio-frequency lesioning and deep brain stimulation (DBS). The subthalamic nucleus (STN) is one of the most useful stereotactic targets available: STN DBS is known to induce substantial improvement in patients with end-stage Parkinson's disease. Other targets include the Globus Pallidus pars interna (GPi) for dystonia and Parkinson's disease, and the centromedian nucleus of the thalamus (CMN) for neuropathic pain. Radiosurgery is an attractive noninvasive alternative to treat some functional brain disorders. The main technical limitation to radiosurgery is that the target can be selected only on the basis of magnetic resonance anatomy without electrophysiological confirmation. The aim of this work is to provide a method for the correct atlas-based identification of the target to be used in functional neurosurgery treatment planning. The coordinates of STN, CMN, and GPi were identified in the Talairach and Tournoux atlas and transformed to the corresponding regions of the Montreal Neurological Institute (MNI) electronic atlas. Binary masks describing the target nuclei were created. The MNI electronic atlas was deformed onto the patient magnetic resonance imaging-T1 scan by applying an affine transformation followed by a local nonrigid registration. The first transformation was based on normalized cross correlation and the second on optimization of a two-part objective function consisting of similarity criteria and weighted regularization. The obtained deformation field was then applied to the target masks. The minimum distance between the surface of an implanted electrode and the surface of the deformed mask was calculated. The validation of the method consisted of comparing the electrode-mask distance to the clinical outcome of the treatments in ten cases of bilateral DBS implants. Electrode placement may have an effect within a radius of stimulation equal to 2 mm, therefore the registration process is considered successful if error is less than 2 mm. The registrations of the MNI atlas onto the patient space succeeded in all cases. The comparison of the distance to the clinical outcome revealed good agreement: where the distance was high (at least in one implant), the clinical outcome was poor; where there was a close correlation between the structures, clinical outcome revealed an improvement of the pathological condition. In conclusion, the proposed method seems to provide a useful tool for the identification of the target nuclei for functional radiosurgery. Also, the method is applicable to other types of functional treatment.

Effects of systemic administration of iptakalim on extracellular neurotransmitter levels in the striatum of unilateral 6-hydroxydopamine-lesioned rats

The function of ATP-sensitive potassium (KATP) channels in nigrostriatal pathway in Parkinson's disease (PD) was studied by employing a novel KATP channel opener iptakalim (Ipt). Apomorphine-induced rotation behavior test and microdialysis experiment were carried out in unilateral 6-hydroxydopamine (6-OHDA) lesioned rats. Behavior test showed that systemic administration of Ipt failed to significantly alleviate apomorphine-induced rotation in unilateral 6-OHDA-lesioned PD model rats. However, using in vivo microdialysis in this PD model rats, it was found that Ipt could increase extracellular dopamine levels in the lesioned side of the striatum and decrease dopamine levels in the intact side of the striatum. Meanwhile, Ipt had no influence on glutamate levels in the intact side, but it did decrease glutamate levels in the lesioned side of the striatum of PD rats. Additionally, in primary cultured rat astrocytes, 6-OHDA decreased overall glutamate uptake activity, but this decrease was recovered and glutamate uptake activity was restored by the opening of KATP channels induced by Ipt and pinacidil. The classical KATP channel blocker glibenclamide completely abolished the effects of Ipt and pinacidil. The present study suggests that (i) the function of KATP channels in the lesioned and intact nigrostriatal pathway is different in unilateral 6-OHDA-lesioned PD model rats. (ii) KATP channels regulate extracellular neurotransmitter levels in the striatum of unilateral 6-OHDA-lesioned rats and may play neuroprotective roles due to their effects on glutamate transporters.

Subthalamic stimulation evokes complex EPSCs in the rat substantia nigra pars reticulata in vitro

The subthalamic nucleus (STN) plays an important role in movement control by exerting its excitatory influence on the substantia nigra pars reticulata (SNR), a major output structure of the basal ganglia. Moreover, excessive burst firing of SNR neurons seen in Parkinson's disease has been attributed to excessive transmission in the subthalamonigral pathway. Using the 'blind' whole-cell patch clamp recording technique in rat brain slices, we found that focal electrical stimulation of the STN evoked complex, long-duration excitatory postsynaptic currents (EPSCs) in SNR neurons. Complex EPSCs lasted 200-500 ms and consisted of an initial monosynaptic EPSC followed by a series of late EPSCs superimposed on a slow inward shift in holding current. Focal stimulation of regions outside the STN failed to evoke complex EPSCs. The late component of complex EPSCs was markedly reduced by ionotropic glutamate receptor antagonists (2-amino-5-phosphonopentanoic acid and 6-cyano-7-nitro-quinoxalone) and by a GABAA receptor agonist (isoguvacine) when these agents were applied directly to the STN using a fast-flow microapplicator. Moreover, the complex EPSC was greatly enhanced by bath application of the GABAA receptor antagonists picrotoxin or bicuculline. These data suggest that recurrent glutamate synapses in the STN generate polysynaptic, complex EPSCs that are under tonic inhibition by GABA. Because complex EPSCs are expected to generate bursts of action potentials in SNR neurons, we suggest that complex EPSCs may contribute to the pathological burst firing that is associated with the symptoms of Parkinson's disease.

Prolonged blockade of NMDA or mGluR5 glutamate receptors reduces nigrostriatal degeneration while inducing selective metabolic changes in the basal ganglia circuitry in a rodent model of Parkinson's disease

We compared the neuroprotective and metabolic effects of chronic treatment with ionotropic or metabotropic glutamate receptor antagonists, in rats bearing a unilateral nigrostriatal lesion induced by 6-hydroxydopamine (6-OHDA). The ionotropic, N-methyl-D-aspartate receptor antagonist MK-801 increased cell survival in the substantia nigra pars compacta (SNc) and corrected the metabolic hyperactivity (increased cytochrome oxidase activity) of the ipsilateral substantia nigra pars reticulata (SNr) associated with the lesion, but showed no effects on the 6-OHDA-induced hyperactivity of the subthalamic nucleus (STN). Significant-although less pronounced-protection of SNc neurons was also observed following treatment with the metabotropic glutamate receptor (mGluR5) antagonist 2-methyl-6-(phenylehtynyl)-pyridine (MPEP). As opposed to MK-801, MPEP abolished the STN metabolic hyperactivity associated with the nigrostriatal lesion, without affecting SNr activity. Specific modulation of STN hyperactivity obtained with mGluR5 blockade may, therefore, open interesting perspectives for the use of this class of compounds in the treatment of Parkinson's disease.

Selective lesion of the substantia nigra pars reticulata reduces the cortical Fos expression induced by stimulation of striatal D1-like receptors, in the rat

We investigated the effects of a selective lesion of the substantia nigra pars reticulata (SNr), obtained by stereotaxic injection of ibotenic acid, on the cortical expression of Fos protein induced by striatal infusion of dopamine, D1-like agonist SKF 38393, in Sprague-Dawley rats. The specific aim was to clarify the role of the basal ganglia output structures - SNr in particular - in the cortical activation that follows a D1-dependent activation of the striatofugal, direct pathway, in freely moving animals. The striatal, unilateral infusion of 30 mM SKF 38393 induced consistent Fos expression throughout the whole ipsilateral cerebral cortex, including motor, sensorimotor, associative, and limbic areas; such expression was dramatically reduced by excitotoxic lesion of the ipsilateral SNr. These findings confirm the prominent role of the SNr in the transmission of striatofugal signals to functionally different cortical areas.

Down-regulation of metabotropic glutamate receptor 1alpha in globus pallidus and substantia nigra of parkinsonian monkeys

Enhanced glutamatergic neurotransmission via the subthalamopallidal or subthalamonigral projection seems crucial for developing parkinsonian motor signs. In the present study, the possible changes in the expression of metabotropic glutamate receptors (mGluRs) were examined in the basal ganglia of a primate model for Parkinson's disease. When the patterns of immunohistochemical localization of mGluRs in monkeys administered systemically with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) were analysed in comparison with normal controls, we found that expression of mGluR1alpha, but not of other subtypes, was significantly reduced in the internal and external segments of the globus pallidus and the substantia nigra pars reticulata. To elucidate the functional role of mGluR1 in the control of pallidal neuron activity, extracellular unit recordings combined with intrapallidal microinjections of mGluR1-related agents were then performed in normal and parkinsonian monkeys. In normal awake conditions, the spontaneous firing rates of neurons in the pallidal complex were increased by DHPG, a selective agonist of group I mGluRs, whereas they were decreased by AIDA, a selective antagonist of group I mGluRs, or LY367385, a selective antagonist of mGluR1. These electrophysiological data strongly indicate that the excitatory mechanism of pallidal neurons by glutamate is mediated at least partly through mGluR1. The effects of the mGluR1-related agents on neuronal firing in the internal pallidal segment became rather obscure after MPTP treatment. Our results suggest that the specific down-regulation of pallidal and nigral mGluR1alpha in the parkinsonian state may exert a compensatory action to reverse the overactivity of the subthalamic nucleus-derived glutamatergic input that is generated in the disease.

Effects of rimonabant, a selective cannabinoid CB1 receptor antagonist, in a rat model of Parkinson's disease

Recent evidence suggest that the blockade of cannabinoid CB1 receptors might be beneficial to alleviate motor inhibition typical of Parkinson's disease (PD). In the present study, we have explored the motor effects of rimonabant, a selective antagonist of CB1 receptors, in a rat model of PD generated by an intracerebroventricular injection of 6-hydroxydopamine. Compared with rats subjected to unilateral injection of this toxin in the medial forebrain bundle, this model allows nigral dopaminergic neurons be symmetrically affected. Dose-response studies with 6-hydroxydopamine revealed that the application of 200 microg per animal caused hypokinetic signs (decreased ambulatory activity, increased inactivity, and reduced motor coordination), which paralleled several signs of degeneration of nigrostriatal dopaminergic neurons (dopamine depletion in the caudate-putamen, and decreased mRNA levels for tyrosine hydroxylase and superoxide dismutase-1 and -2 in the substantia nigra). In these conditions, the degree of hypokinesia and dopaminergic degeneration may be considered moderate, comparable to the disturbances occurring in early and middle stages of PD in humans, a period that might be appropriate to test the effects of rimonabant. There is also degeneration of other dopaminergic pathways out of the basal ganglia, but this does not appear to interfere significantly with the hypokinetic profile of these rats. Higher doses of 6-hydroxydopamine elevated significantly animal mortality and lower doses failed in general to reproduce motor inhibition. Like other animal models of PD, these rats exhibited an increase in the density of CB(1) receptors in the substantia nigra, which is indicative of the expected overactivity of the cannabinoid transmission in this disease and supports the potential of CB1 receptor blockade to attenuate hypokinesia associated with nigral cell death. Thus, the injection of 0.1 mg/kg of rimonabant partially attenuated the hypokinesia shown by these animals with no effects in control rats, whereas higher doses (0.5-1.0 mg/kg) were not effective. We also found that the antihypokinetic effects of low doses of rimonabant did not influence the dopamine deficits of these animals, as well as it did not modify GABA or glutamate transmission in the caudate-putamen. In summary, rimonabant may have potential antihypokinetic activity in moderate parkinsonism at low doses, but this effect is not related to changes in dopaminergic, GABAergic, or glutamatergic transmission in the striatum. Therefore, the elucidation of the neurochemical substrate involved in this effect remains a major challenge for the future.

Modifications of the iron-neuromelanin system in Parkinson's disease

Parkinson's disease is a common neurodegenerative disorder with a mainly sporadic aetiology, although a number of monogenic familiar forms are known. Most of the motor symptoms are due to selective depletion of dopaminergic, neuromelanin-containing neurones of the substantia nigra pars compacta. Neuromelanin is the dark insoluble macromolecule that confers the black (substantia nigra) or grey (locus coeruleus) colour to monoaminergic basal ganglia. In particular, nigral neurones are pigmented because of the accumulation of by-products of oxidative metabolism of the neurotransmitter dopamine. The occurrence of dopamine (and all the enzymatic machinery required for dopamine synthesis, re-uptake and disposal) and neuromelanin, and a large amount of iron ions that interact with them, makes dopaminergic nigral neurones peculiarly susceptible to oxidative stress conditions that, in turn, may become amplified by the iron-neuromelanin system itself. In this mini-review we describe biophysical evidence for iron-neuromelanin modifications that support this hypothesis. Furthermore, we discuss the formation of the covalent linkage between alpha-synuclein and neuromelanin from the early stages of the disease.

UCM707, an inhibitor of the anandamide uptake, behaves as a symptom control agent in models of Huntington's disease and multiple sclerosis, but fails to delay/arrest the progression of different motor-related disorders

To date, UCM707, (5Z,8Z,11Z,14Z)-N-(3-furylmethyl)eicosa-5,8,11,14-tetraenamide, has the highest potency and selectivity in vitro and in vivo as inhibitor of the endocannabinoid uptake. This may enable this compound to potentiate endocannabinoid transmission, with minimal side effects, in the treatment of several neurological disorders. In the present study, we examined whether the treatment with UCM707 produced beneficial effects, as other cannabinoid-related compounds have already shown, to alleviate motor deterioration or to delay/arrest neurodegeneration, in several models of neurological diseases such as Huntington's disease (HD), Parkinson's disease (PD) and multiple sclerosis (MS). UCM707 exhibited a notable anti-hyperkinetic activity in a rat model of HD generated by bilateral intrastriatal application of 3-nitropropionic acid. This effect was possibly associated with an amelioration of GABA and glutamate deficits induced by the toxin in the globus pallidus and the substantia nigra, respectively. However, UCM707 did not protect against the death of GABAergic neurons that occurs in rats with striatal atrophy generated by unilateral application of malonate, another animal model of HD, which is more useful to test neuroprotective strategies. In addition, UCM707 did not provide neuroprotection in rats with unilateral lesions of the nigrostriatal dopaminergic neurons caused by 6-hydroxydopamine, a rat model of PD. This was possibly due to the fact that UCM707 is devoid of anti-oxidant properties since another uptake inhibitor, AM404, that has these properties acted as a protective agent. Lastly, UCM707 was also unable to inhibit the development of the neurological impairment of rats with experimental autoimmune encephalomyelitis (EAE), an acute model of MS. However, UCM707, like other endocannabinoid uptake inhibitors reported previously, significantly reduced spasticity of the hindlimbs in a chronic relapsing EAE mice, a chronic model of MS. In summary, UCM707 might be a promising compound in HD to alleviate motor symptoms, which represents an important goal considering the current lack of efficient pharmacological treatments in this basal ganglia disorder. However, the compound was unable to delay neurodegeneration in this disorder and also in PD. In addition, UCM707 did not produce any neurological recovery from inflammatory attack in an EAE rat model of MS, although it retained the classic anti-spastic action shown by other uptake inhibitors in the EAE mouse model of this disease.

Prevention of levodopa-induced dyskinesias by a selective NR1A/2BN-methyl-D-aspartate receptor antagonist in parkinsonian monkeys: Implication of preproenkephalin

Enkephalin is reported to play an important role in the pathophysiology of levodopa (LD) -induced dyskinesias. The present study investigated the effect of chronic treatment with a selective NR1A/2B N-methyl-D-aspartate (NMDA) receptor antagonist, CI-1041, on the expression of preproenkephalin-A (PPE-A) in brains of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) -treated monkeys in relation to the development of LD-induced dyskinesias. Four MPTP-monkeys received LD/benserazide alone; they all developed dyskinesias. Four other MPTP-monkeys received LD/benserazide plus CI-1041; only one of them developed mild dyskinesias at the end of the fourth week of treatment. Four normal monkeys and four saline-treated MPTP monkeys were also included. MPTP-treated monkeys had extensive and similar striatal dopamine denervation. An increase of PPE-A mRNA levels assayed by in situ hybridization was observed in the lateral putamen (rostral and caudal) and caudate nucleus (rostral) of saline-treated MPTP monkeys compared to controls, whereas no change or a small increase was observed in their medial parts. Striatal PPE-A mRNA levels remained elevated in LD-treated MPTP monkeys, whereas cotreatment with CI-1041 brought them back to control values. These findings suggest that chronic blockade of striatal NR1A/2B NMDA receptors with CI-1041 normalizes PPE-A mRNA expression and prevents the development of LD-induced dyskinesias in an animal model of Parkinson disease.

Neuroprotection by rasagiline: a new therapeutic approach to Parkinson's disease?

Neuronal death in Parkinson's disease (PD) may originate from the reciprocal interactions of a restricted number of conditions, such as mitochondrial defects, oxidative stress and protein mishandling, which would favor a state of apoptotic cell death in the nigrostriatal pathway. The search for pharmacological treatments able to counteract the nigrostriatal degeneration, possibly by interfering with these phenomena, has recently raised considerable interest in rasagiline [R(+)-N-propargyl-1-aminoindan], a potent, selective, and irreversible inhibitor of monoamine oxidase B (MAO-B). Rasagiline, like selegiline, is a propargylamine, but is approximately 10 times more potent. Unlike selegiline, rasagiline is not metabolized to amphetamine and/or methamphetamine and is devoid of sympathomimetic activity. Numerous experimental studies, conducted both in vitro and in vivo, have shown that rasagiline possesses significant protective properties on neuronal populations. The pro-survival effects of the drug appear to be linked to its propargyl moiety, rather than to the inhibitory effect on MAO-B. Rasagiline's major metabolite, aminoindan--which possesses intrinsic neuroprotective activity--may also contribute to the beneficial effects of the parent compound. Rasagiline has been recently evaluated in early PD patients, with results that are consistent with slowing the progression of the disease. Therefore, the neuroprotective activity shown by the drug under experimental conditions may be reflected in the clinic, thus providing new perspectives for the treatment of PD.

A simple and fast densitometric method for the analysis of tyrosine hydroxylase immunoreactivity in the substantia nigra pars compacta and in the ventral tegmental area

Parkinson's disease is a progressive dyskinetic disorder caused by degeneration of mesencephalic dopaminergic neurons in the substantia nigra pars compacta (SNpc) and, to a lesser extent, in the ventral tegmental area (VTA). Tyrosine hydroxylase (TH) is a rate-limiting enzyme for dopamine synthesis, therefore immunohistochemistry for TH can be used as an important marker of dopaminergic cell loss in these regions. Traditionally, immunohistochemical experiments are analyzed qualitatively by optical microscopic observation or more rarely semi-quantitatively evaluated by densitometry. A common problem with such papers is the lack of a clear explanation of the algorithms and macros employed in the semi-quantitative approaches. In this paper, we describe, in detail, an easy, fast and precise protocol for the analysis of TH immunoreactivity in SNpc and VTA using one of the most popular image analysis software packages (Image Pro-Plus). We believe that this protocol will facilitate the evaluation of mesencephalic TH immunoreactivity in various available animal models of Parkinson's disease.

Effect of kynurenine 3-hydroxylase inhibition on the dyskinetic and antiparkinsonian responses to levodopa in Parkinsonian monkeys

Homeostatic interactions between dopamine and glutamate are central to the normal physiology of the basal ganglia. This relationship is altered in Parkinsonism and in levodopa-induced dyskinesias (LID), resulting in an upregulation of corticostriatal glutamatergic function. Kynurenic acid (KYNA), a tryptophan metabolite with antagonist activity at ionotropic glutamate receptors and the capability to inhibit glutamate release presynaptically, might therefore be of therapeutic value in LID. To evaluate this hypothesis, we used a pharmacological tool, the kynurenine 3-hydroxylase inhibitor Ro 61-8048, which raises KYNA levels acutely. Ro 61-8048 was tested in MPTP cynomolgus monkeys with a stable parkinsonian syndrome and reproducible dyskinesias after each dose of levodopa. Serum and CSF concentrations of KYNA and its precursor kynurenine increased dose-dependently after Ro 61-8048 administration, alone or in combination with levodopa. Coadministration of Ro 61-8048 with levodopa produced a moderate but significant reduction in the severity of dyskinesias while maintaining the motor benefit. These results suggest that elevation of KYNA levels through inhibition of kynurenine 3-hydroxylase constitutes a promising novel approach for managing LID in Parkinson's disease.

Cell survival patterns in the pedunculopontine tegmental nucleus of methyl-4-phenyl-1,2,3,6-tetrahydropyridine-treated monkeys and 6OHDA-lesioned rats: evidence for differences to idiopathic Parkinson disease patients?

We explore the patterns of cell loss in the pedunculopontine tegmental nucleus (PpT), a major locomotor and muscle tone suppression centre of the brainstem, in two animal models of Parkinson disease, namely MPTP (methyl-4-phenyl-1,2,3,6-tetrahydropyridine)-treated monkeys and 6-hydroxydopamine(6OHDA)-lesioned rats. Although there have been many studies documenting the loss of dopaminergic cells from the substantia nigra in these animal models, there has been little, if any, documentation of a loss of cells in the PpT. Results were obtained from macaque monkeys (Macaca fascicularis) and Sprague-Dawley rats. For the monkey series, animals were injected intramuscularly with MPTP (0.2 mg/kg) for 8 days consecutively and then allowed to survive for 21 days thereafter. Each monkey underwent behavioural assessment for parkinsonian symptoms. For the rat series, 6OHDA was injected into the midbrain using stereotactic coordinates. Rats were then allowed to survive for either 7, 14, 28, or 84 days thereafter. Monkey and rat brains were aldehyde-fixed and processed for routine tyrosine hydroxylase (TH; to label nigral dopaminergic cells) and nitric oxide synthase (NOs; to label PpT cholinergic cells) immunocytochemistry. In monkeys, the morphology, distribution and number of NOs(+) cells in the controls and MPTP-treated cases were very similar. In fact, in terms of number, there was only a 1% difference in the mean cell number between the controls and MPTP-treated cases. A comparable pattern was evident in 6OHDA-lesioned rats; there was no substantial difference in morphology, distribution and number of NOs(+) cells on the 6OHDA-lesioned cases when compared to the controls at each of the survival periods post-surgery. In summary, we show no loss of the large cholinergic/NOs(+) cells in the PpT in two animal models of Parkinson disease. This is in contrast to the heavy loss of these cells reported by previous findings in idiopathic Parkinson disease in patients.

Studies of ATP-sensitive potassium channels on 6-hydroxydopamine and haloperidol rat models of Parkinson's disease: implications for treating Parkinson's disease?

In the present study, we first investigated the effects of unilateral 6-hydroxydopamine (6-OHDA) lesioning of the substantia nigra pars compacta (SNc) on the expression of subunits of ATP-sensitive potassium channels (KATP channels) in the prefrontal cortex (PFC), striatum and hippocampus of adult rats by utilizing semiquantitative reverse transcription polymerase chain reaction (RT-PCR) and immunohistochemistry techniques. The results show that Kir6.2 and SUR2 expression in the PFC, Kir6.1, Kir6.2 and SUR1 expression in the striatum, and Kir6.1 and Kir6.2 expression in the hippocampus of injured side increased significantly after unilateral 6-OHDA lesioning of the SNc in rats. Afterward, we studied the effects of iptakalim (Ipt), a novel ATP-sensitive potassium channel opener (KCO), on parkinsonian symptoms, which were induced by acute injection of haloperidol. The results indicate that intraperitoneal injection of Ipt (0.125 mg/kg, 0.25 mg/kg or 0.5 mg/kg) partially alleviated haloperidol-induced catalepsy and hypolocomotion. Even though the observed effects (0.5 mg/kg) are better than those of l-3,4-dihydroxyphenylalanine (L-DOPA) (100 mg/kg), Ipt (0.25 mg/kg) failed to enhance the anti-parkinsonian actions of L-DOPA (100 mg/kg). Our results suggest that KATP channels might be involved in the pathogenesis of Parkinson's disease (PD) induced in an animal model and conceptually support the idea that KATP channels may be new therapeutic targets for PD.

Comparison of bilaterally 6-OHDA- and MPTP-lesioned rats as models of the early phase of Parkinson's disease: Histological, neurochemical, motor and memory alterations

This study compares histological, neurochemical, behavioral, motor and cognitive alterations as well as mortality of two models of Parkinson's disease in which 100 microg 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) or 6 microg 6-hydroxydopamine (6-OHDA) was bilaterally infused into the central region of the substantia nigra, compact part, of adult male Wistar rats. Both neurotoxins caused a significant loss of nigral tyrosine hydroxylase-immunostained cells and striatal dopamine depletion, but 6-OHDA caused more widespread and intense cell loss, more intense body weight loss and more mortality than MPTP. Both 6-OHDA- and MPTP-lesioned rats presented similar deficits in performing a working memory and a cued version of the Morris water maze task and few exploratory/motor alterations in the open field and catalepsy tests. However, rats presented a significant and transitory increase in locomotor activity after the MPTP lesion and a hypolocomotor behavior tended to be present after the 6-OHDA lesion. The picture of mild motor effects and robust impairment of habit learning and spatial working memory observed in MPTP-lesioned rats models the early phase of Parkinson's disease.

Intrapallidal lipopolysaccharide injection increases iron and ferritin levels in glia of the rat substantia nigra and induces locomotor deficits

Increasing evidence suggests that abnormal iron handling may be involved in the pathogenesis of Parkinson's disease. The present study investigates the role of iron and the iron-storage protein ferritin in inflammation-induced degeneration of dopaminergic neurons of the substantia nigra pars compacta. Injection of lipopolysaccharide into the globus pallidus of young and middle-aged rats substantially decreased tyrosine hydroxylase immunostaining in substantia nigra pars compacta four weeks after injection. Loss of tyrosine hydroxylase expression was accompanied by increased iron and ferritin levels in glial cells of the substantia nigra pars reticulata. Despite greater increases in nigral iron levels, ferritin induction was less pronounced in older rats, suggesting the regulation of ferritin was compromised with age. Automated movement tracking analyses showed that young rats recovered from LPS-induced locomotor deficits within four weeks, yet older rats failed to improve on measures of speed and total distance moved. Intrapallidal lipopolysaccharide injection also increased expression of alpha-synuclein and ubiquitin in tyrosine hydroxylase-positive neurons of the substantia nigra pars compacta. These results suggest that pallidal inflammation significantly increases stress on dopamine-containing neurons in the substantia nigra pars compacta. Alterations in nigral iron levels and protein handing may increase the vulnerability of nigral neurons to degenerative processes.

Functional NR2B- and NR2D-containing NMDA receptor channels in rat substantia nigra dopaminergic neurones

NMDA receptors regulate burst firing of dopaminergic neurones in the substantia nigra pars compacta (SNc) and may contribute to excitotoxic cell death in Parkinson's disease (PD). In order to investigate the subunit composition of functional NMDA receptors in identified rat SNc dopaminergic neurones, we have analysed the properties of individual NMDA receptor channels in outside-out patches. NMDA (100 nm) activated channels corresponding to four chord conductances of 18, 30, 41 and 54 pS. Direct transitions were observed between all conductance levels. Between 18 pS and 41 pS conductance levels, direct transitions were asymmetric, consistent with the presence of NR2D-containing NMDA receptors. Channel activity in response to 100 nm or 200 microm NMDA was not affected by zinc or TPEN (N,N,N',N'-tetrakis-[2-pyridylmethyl]-ethylenediamine), indicating that SNc dopaminergic neurones do not contain functional NR2A subunits. The effect of the NR2B antagonist ifenprodil was complex: 1 microm ifenprodil reduced open probability, while 10 microm reduced channel open time but had no effect on open probability of channels activated by 100 nm NMDA. When the concentration of NMDA was increased to 200 microm, ifenprodil (10 microm) produced the expected reduction in open probability. These results indicate that NR2B subunits are present in SNc dopaminergic neurones. Taken together, these findings indicate that NR2D and NR2B subunits form functional NMDA receptor channels in SNc dopaminergic neurones, and suggest that they may form a triheteromeric NMDA receptor composed of NR1/NR2B/NR2D subunits.

Cannabinoid CB1 receptors in the basal ganglia and motor response to activation or blockade of these receptors in parkin-null mice

The endocannabinoid transmission becomes overactive in the basal ganglia in Parkinson's disease (PD), as reported in patients and animal models of this disease. In the present study, we examined the status of cannabinoid CB(1) receptors in the basal ganglia of female and male Park-2 knockout mice, a genetic model of PD that progresses with no neuronal death and that may be considered representative of early and presymptomatic parkinsonian deficits. We found an increase in the density of CB(1) receptors in the substantia nigra compared to wild-type animals with no changes in other basal ganglia, although this occurred only in females. Despite this increase, the motor inhibition caused by the acute administration of the cannabinoid agonist Delta(9)-tetrahydrocannabinol to Park-2 knockout female mice was markedly of lesser magnitude compared with the response found in wild-type animals. By contrast, the administration of the CB(1) receptor antagonist SR141716 resulted in a hyperkinetic response in parkin-null mice, response that was almost absent in wild-type animals and that was accompanied by a decrease in tyrosine hydroxylase activity in the caudate-putamen. However, parkin-null male mice exhibited normal levels of CB(1) receptors in the substantia nigra and the remaining basal ganglia, with the only exception of a small decrease in the lateral part of the caudate-putamen. This was associated with an increase in mRNA levels for superoxide dismutase in this structure. In addition, the administration of Delta(9)-tetrahydrocannabinol to parkin-null male mice caused a motor inhibition that was significantly greater than in the case of their wild-type counterparts, and that was accompanied by an increase in tyrosine hydroxylase activity in the caudate-putamen. In summary, extending the data obtained in humans and animal models of basal ganglia neurodegeneration, changes in CB(1) receptors were also observed in parkin-null mice, a model of PD that may be considered representative of early stages of this disease. These changes are associated with differences in behavioral responses to cannabinoid agonists or antagonists between Park-2 knockout and wild-type mice, although parkin-null mice exhibited evident gender-dependent differences for both levels of CB(1) receptors and motor responses to agonists or antagonists.

Therapeutic potential of adenosine A2A receptor antagonists in Parkinson's disease

In the pursuit of improved treatments for Parkinson's disease (PD), the adenosine A(2A) receptor has emerged as an attractive nondopaminergic target. Based on the compelling behavioral pharmacology and selective basal ganglia expression of this G-protein-coupled receptor, its antagonists are now crossing the threshold of clinical development as adjunctive symptomatic treatment for relatively advanced PD. The antiparkinsonian potential of A(2A) antagonism has been boosted further by recent preclinical evidence that A(2A) antagonists might favorably alter the course as well as the symptoms of the disease. Convergent epidemiological and laboratory data have suggested that A(2A) blockade may confer neuroprotection against the underlying dopaminergic neuron degeneration. In addition, rodent and nonhuman primate studies have raised the possibility that A(2A) receptor activation contributes to the pathophysiology of dyskinesias-problematic motor complications of standard PD therapy--and that A(2A) antagonism might help prevent them. Realistically, despite being targeted to basal ganglia pathophysiology, A(2A) antagonists may be expected to have other beneficial and adverse effects elsewhere in the central nervous system (e.g., on mood and sleep) and in the periphery (e.g., on immune and inflammatory processes). The thoughtful design of new clinical trials of A(2A) antagonists should take into consideration these counterbalancing hopes and concerns and may do well to shift toward a broader set of disease-modifying as well as symptomatic indications in early PD.

Cannabinoid control of motor function at the basal ganglia

Classic and novel data strengthen the idea of a prominent role for the endocannabinoid signaling system in the control of movement. This finding is supported by three-fold evidence: (1) the abundance of the cannabinoid CB1 receptor subtype, but also of CB2 and vanilloid VR1 receptors, as well as of endocannabinoids in the basal ganglia and the cerebellum, the areas that control movement; (2) the demonstration of a powerful action, mostly of an inhibitory nature, of plant-derived, synthetic, and endogenous cannabinoids on motor activity, exerted by modulating the activity of various classic neurotransmitters; and (3) the occurrence of marked changes in endocannabinoid transmission in the basal ganglia of humans affected by several motor disorders, an event corroborated in animal models of these neurological diseases. This three-fold evidence has provided support to the idea that cannabinoid-based compounds, which act at key steps of the endocannabinoid transmission [receptors, transporter, fatty acid amide hydrolase (FAAH)], might be of interest because of their potential ability to alleviate motor symptoms and/or provide neuroprotection in a variety of neurological pathologies directly affecting basal ganglia structures, such as Parkinson's disease and Huntington's chorea, or indirectly, such as multiple sclerosis and Alzheimer's disease. The present chapter will review the knowledge on this issue, trying to establish future lines for research into the therapeutic potential of the endocannabinoid system in motor disorders.

Some certainty for the “zone of uncertainty”? Exploring the function of the zona incerta

The zona incerta (ZI), first described over a century ago by Auguste Forel as a "region of which nothing certain can be said," forms a collection of cells that derives from the diencephalon. To this day, we are still not certain of the precise function of this "zone of uncertainty" although many have been proposed, from controlling visceral activity to shifting attention and from influencing arousal to maintaining posture and locomotion. In this review, I shall outline the recent advances in the understanding of the structure, connectivity and functions of the ZI. I will then focus on a possible and often neglected global role for the ZI, one that links its diverse functions together. In particular, I aim to highlight the idea that the ZI forms a primal center of the diencephalon for generating direct responses (visceral, arousal, attention and/or posture-locomotion) to a given sensory (somatic and/or visceral) stimulus. With this global role in mind, I will then address recent results indicating that abnormal ZI activity manifests in clinical symptoms of Parkinson disease.

Striatal adenosine A(2A) receptor blockade increases extracellular dopamine release following l-DOPA administration in intact and dopamine-denervated rats

The influence of the selective adenosine A(2A) receptor antagonist ZM 241385 on exogenous l-DOPA-derived dopamine (DA) release in intact and dopamine-denervated rats was studied using an in vivo microdialysis in freely moving animals. Local infusion of l-DOPA (2.5 microM) produced a marked increase in striatal extracellular DA level in intact and malonate-lesioned rats. Intrastriatal perfusion of ZM 241385 (50-100 microM) had no effect on basal extracellular DA level, but enhanced dose-dependently the l-DOPA-induced DA release in intact and malonate-lesioned animals. A non-selective adenosine A(2A) receptor antagonist DMPX (100 microM), similarly to ZM 241385, accelerated conversion of l-DOPA in intact and malonate-denervated rats. This effect was not produced by the adenosine A(1) receptor antagonist, CPX (10-50 microM). However, ZM 241385 did not affect the l-DOPA-induced DA release in rats pretreated with reserpine (5 mg/kg i.p.) and alpha-methyl-p-tyrosine (AMPT, 300 mg/kg i.p.). Obtained results indicate that blockade of striatal adenosine A(2A) receptors increases the l-DOPA-derived DA release possibly by indirect mechanism exerted on DA terminals, an effect dependent on striatal tyrosine hydroxylase activity. Selective antagonists of adenosine A(2A) receptors may exert a beneficial effect at early stages of Parkinson's disease by enhancing the therapeutic efficacy of l-DOPA applied exogenously.

Variability in fundamental frequency during speech in prodromal and incipient Parkinson's disease: A longitudinal case study

Nearly two centuries ago, first observed that a particular pattern of speech changes occur in patients with idiopathic Parkinson's disease (PD). Numerous studies have documented these changes using a wide variety of acoustic measures, and yet few studies have attempted to quantify any such changes longitudinally, through the early course of the disease. Moreover, no attempt has been made to determine if speech changes are evident during the prodromal period, prior to the onset of clinically noticeable symptoms. This case-control pilot study is a first attempt to determine if changes in fundamental frequency variability during speech, an acoustic measure known to be affected later in the course of the disease, are evident during the prodromal period. A retrospective analysis of videotape footage recorded and made available by a leading national television news service. Videotape samples were obtained for a single individual (and a well-matched control subject) over an 11-year period of this individual's life (7 years prior to diagnosis of PD, and 3 years post-diagnosis). Results suggest that changes in F0 variability can be detected as early as 5 years prior to diagnosis (consistent with findings from other laboratories that have relied on cross-sectional study approaches). This pilot study supports the utility of such a design approach, and these results warrant continued effort to better understand the onset of PD and sensitivity of measurement of voice acoustical changes during the prodromal period.

Functional diversity of ventral midbrain dopamine and GABAergic neurons

Recent findings indicate that VTA and SN dopaminergic (DA) and GABAergic neurons form subpopulations that are divergent in their electrophysiological features, vulnerability to neurodegeneration, and regulation by neuropeptides. This diversity can be correlated with the anatomical organization of the VTA and SN and their inputs and outputs. In this review we describe the heterogeneity in ion channels and firing patterns, especially burst firing, in subpopulations of dopamine neurons. We go on to describe variations in vulnerability to neurotoxic damage in models of Parkinson's disease in subgroups of DA neurons and its possible relationship to developmental gene regulation, the expression of different ion channels, and the expression of different protein markers, such as the neuroprotective marker calbindin. The electrophysiological properties of subgroups of GABAergic midbrain neurons, patterns of expression of protein markers and receptors, possible involvement of GABAergic neurons in a number of processes that are usually attributed exclusively to dopaminergic neurons, and the characteristics of a subgroup of neurons that contains both dopamine and GABA are also discussed.

NMDA receptor antagonists ameliorate the stepping deficits produced by unilateral medial forebrain bundle injections of 6-OHDA in rats

RATIONALE AND OBJECTIVES

To test the hypothesis that excess glutamatergic transmission at NMDA receptors may contribute to the pathogenesis of Parkinson's disease (PD), we examined the effects of various NMDA receptor antagonists on a recently developed rat model of PD.

METHODS

Following unilateral injections of 12 microg 6-OHDA into the medial forebrain bundle of male Long Evans rats, stepping with both front paws was measured separately as the paws were dragged backwards and laterally. The effects of i.p. injections of varying doses of L-dopa, the non-competitive NMDA receptor antagonist dizocilpine [(+)-MK-801], the competitive NMDA receptor antagonist CPP, and combinations of L-dopa and NMDA receptor antagonists were then examined on stepping in three separate groups of rats.

RESULTS

The lesioned rats stepped less often with their contralateral paw than with their ipsilateral paw, and the magnitude of this stepping deficit was positively correlated with the amount of DA depletion in the ipsilateral dorsal striatum. L-dopa (1-25 mg/kg) dose dependently enhanced stepping with the contralateral paw, and 0.15-0.3 mg/kg dizocilpine and 1.5-6.25 mg/kg CPP enhanced stepping with the contralateral paw as much as did 8 mg/kg L-dopa. The combinations of L-dopa and each of the NMDA receptor antagonists did not significantly improve stepping more than either drug alone. Moreover, none of the drugs completely eliminated the stepping deficits, and high doses began to impair stepping with the ipsilateral paw by inducing turning.

CONCLUSIONS

These data indicate that deficits in contralateral stepping are a reliable and sensitive measure of akinesia in unilateral 6-OHDA-lesioned rats, and they support the hypothesis that excess glutamatergic transmission at NMDA receptors may play a role in the expression of PD symptomology.

Effects of adenosine A1, dopamine D1 and metabotropic glutamate 5 receptors-modulating agents on locomotion of the reserpinised rats

The pathophysiology of Parkinson's disease and l-3,4-dihydroxyphenylalanine (l-DOPA)-induced dyskinesia are characterised by an imbalance between activity of the direct and indirect pathways regulated by dopamine D1 and D2 receptors, respectively. In this study, we investigated the effects of treatments combining adenosine A(1) and metabotropic glutamate 5 (mGlu5) receptors modulators on locomotion induced by dopamine D1 receptor activation in the reserpine-treated rats. Administration of the adenosine A(1) receptor agonist and mGlu5 receptor antagonist resulted in the significant reduction of dopamine D1 receptor agonist-induced locomotion. The combination of adenosine A(1) receptor agonist with mGlu5 receptor antagonist had no greater effect than these compounds alone. However, the adenosine A(1) receptor antagonist attenuated the inhibitory effect of mGlu5 receptor antagonist. The data suggest that the effect of mGlu5 receptor blockade on locomotion elicited by dopamine D1 receptor stimulation involves activation of adenosine A(1) receptors. This interaction can improve our understanding of pathophysiology of L-DOPA-induced dyskinesia.

Spontaneous eye blinking in human infants: a review

Spontaneous eye blinking has been studied in clinical and neuropharmacological research in adult humans and nonhuman primates as a putative index of central dopamine system activity. One purpose of this review is to provide a general overview of the research on spontaneous eye blinking with an emphasis on the relationship between spontaneous eye blinking and central dopamine systems. We suggest that the body of research from human (adults, children, and infants) and nonhuman primates supports the continued empirical investigation of spontaneous eye blinking in human infants. A second purpose is to present approaches for empirical work to further investigate the development, correlates, and mechanisms of spontaneous eye blinking in human infants. The results of further investigation may reveal new insights into relationships between the central nervous system activity and behavior in early human development.

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.

Inhibition of ongoing responses in patients with Parkinson's disease

OBJECTIVES

We investigated the involvement of the basal ganglia in inhibiting ongoing responses in patients with Parkinson's disease (PD).

METHODS

Thirty two patients with PD and 31 orthopaedic controls performed the stop signal task, which allows an estimation of the time it takes to inhibit an ongoing reaction (stop signal reaction time, SSRT).

RESULTS

Patients with PD showed significantly longer SSRTs than the controls. This effect seemed to be independent of global cognitive impairment and severity of PD. Furthermore, in the PD patients, there was no significant relation between general slowing and inhibitory efficiency.

CONCLUSIONS

Our results provide evidence for involvement of the basal ganglia in the inhibition of ongoing responses.

Glutamate receptors and Parkinson's disease: opportunities for intervention

Parkinson's disease is a debilitating neurodegenerative movement disorder that is the result of a degeneration of dopaminergic neurons in the substantia nigra pars compacta. The resulting loss of striatal dopaminergic tone is believed to underlie a series of changes in the circuitry of the basal ganglia that ultimately lead to severe motor disturbances due to excessive basal ganglia outflow. Glutamate plays a central role in the disruption of normal basal ganglia function, and it has been hypothesised that agents acting to restore normal glutamatergic function may provide therapeutic interventions that bypass the severe motor side effects associated with current dopamine replacement strategies. Analysis of the effects of glutamate receptor ligands in the basal ganglia circuit suggests that both ionotropic and metabotropic glutamate receptors could have antiparkinsonian actions. In particular, NMDA receptor antagonists that selectively target the NR2B subunit and antagonists of the metabotropic glutamate receptor mGluR5 appear to hold promise and deserve future attention.

A unilateral 6-hydroxydopamine lesion decreases the expression of metabotropic glutamate receptors in rat substantia nigra

Loss of dopaminergic neurons in the substantia nigra pars compacta (SNc) in Parkinson's disease (PD) leads to augmentation of glutamatergic activity in the subthalamic nucleus (STN). Moreover, antagonizing excitotoxicity has yielded mostly symptomatic improvements in experimental animals of PD. Therefore, we used immunocytochemistry to examine the effect of unilateral 6-hydroxydopamine lesions of SNc on the expression of metabotropic glutamate receptors (mGluR1a, 2/3, 5, 4, 8) and tyrosine hydroxylase (TH). 6-Hydroxydopamine causes a fall in the number of mGluRs and TH in the lesioned lateral substantia nigra. Pharmacological activation of group II or III mGluRs or blockade of group I mGluRs for 1 week significantly increased the expression of the same group receptors. The alteration in the receptor expression may be a compensatory mechanism developed after mGluRs ligands neuroprotective treatment.

Transmission of the subthalamic nucleus oscillatory activity to the cortex: a computational approach

Parkinsonian tremor is most likely due to oscillatory neuronal activities of central oscillators such as the subthalamic nucleus (STN)-external segment of the globus pallidus (GPe) pacemaker within the basal ganglia (BG). Activity from the central oscillator is proposed to be transmitted via transcortical pathways to the periphery. A computational model of the BG is proposed for simulating the transmission of the STN oscillatory activity to the cortex, based closely on known anatomy and physiology of the BG. According to the results of the simulation, for transmission of the STN oscillatory activity to the cortex, the STN oscillatory activity has to be transmitted simultaneously to the thalamus via STN-internal segment of the globus pallidus (GPi)-thalamus and STN-GPe-GPi-thalamus pathways. This transmission is controlled by the various factors such as the phase between the STN and GPe oscillatory activities, the STN oscillatory activity frequency, the low-threshold calcium spike bursts of the thalamus and the GPi spontaneous activity.

Blockade of A2A receptors plus l-DOPA after nigrostriatal lesion results in GAD67 mRNA changes different from l-DOPA alone in the rat globus pallidus and substantia nigra reticulata

Studies in animal models of Parkinson's disease (PD) suggest the potential utility of adenosine A(2A) antagonists in the treatment of this disease. In the present study, unilaterally 6-hydroxydopamine (6-OHDA)-lesioned rats received chronic intermittent treatment with the adenosine A(2A) antagonist SCH58261 (5 mg/kg) plus l-DOPA (3 mg/kg) or l-DOPA (6 mg/kg) alone, at doses producing the same intensity of contralateral turning on first administration. Three days after discontinuation of treatments, GABA synthesizing enzyme glutamic acid decarboxylase (GAD67) mRNA was evaluated at cellular level in the globus pallidus (GP) and substantia nigra pars reticulata (SNr) by in situ hybridization. 6-OHDA lesion significantly increased GAD67 mRNA levels in both the GP and SNr ipsilateral to the lesion. Chronic l-DOPA (6 mg/kg), in contrast to SCH58261 plus l-DOPA (3 mg/kg), produced a sensitized contralateral turning indicative of dyskinetic potential and further increased GAD67 mRNA in the GP. In the SNr, a significant decrease in GAD67 mRNA was observed after either treatments. However, while l-DOPA (6 mg/kg) decreased SNr GAD67 mRNA below the intact side, SCH58261 plus l-DOPA (3 mg/kg) brought GAD67 mRNA to the same level of the intact SNr. l-DOPA (3 mg/kg) or SCH58261 (5 mg/kg) alone failed to modify GAD67 mRNA. Results suggest that an increase in GAD67 mRNA in GP and a decrease in SNr might underlie dyskinetic movements induced by chronic l-DOPA. In contrast, the lack of GAD67 mRNA changes in the GP and a less marked inhibition of SNr might correlate with the absence of dyskinetic potential observed after SCH58261 plus l-DOPA.

Changes in GABA(B) receptor mRNA expression in the rodent basal ganglia and thalamus following lesion of the nigrostriatal pathway

Loss of striatal dopaminergic innervation in Parkinson's disease (PD) is accompanied by widespread alterations in GABAergic activity within the basal ganglia and thalamus. Accompanying changes in GABA(B) receptor binding have been noted in some basal ganglia regions in parkinsonian primates, suggesting that plasticity of this receptor may also occur in PD. However, the molecular mechanisms underlying the changes in receptor binding and the manner and extent to which different GABA(B) receptor mRNA subunits and splice-variants are affected remain unknown. This study used in situ hybridisation to examine the full profile of changes in expression of the known rat GABA(B) receptor genes and gene variants in the basal ganglia and thalamus of rats, brought about by degeneration of the nigrostriatal tract. All of the GABA(B) mRNA species examined showed unique expression patterns throughout the basal ganglia and thalamus. In addition, all exhibited a marked loss of expression (between 46 and 80%) in the substantia nigra pars compacta of animals bearing a complete 6-hydroxydopamine-induced lesion of the nigrostriatal tract, confirming the presence of these variants in dopaminergic neurones in this region. Further analysis of autoradioagrams revealed additional changes only in GABA(B(1a)) mRNA in discrete anatomical regions. Expression of the GABA(B(1a)) variant was significantly increased in the substantia nigra pars reticulata (33+/-2%), entopeduncular nucleus (26+/-1%) and the subthalamic nucleus (16+/-1%). Since these regions all receive reduced GABAergic innervation following nigrostriatal tract lesioning, it is possible that the increased expression occurs as a compensatory measure. In conclusion, these data demonstrate that GABA(B) receptor genes exhibit regional- and subunit/variant-specific plasticity at the molecular level under parkinsonian conditions.

Activation of group III metabotropic glutamate receptors in selected regions of the basal ganglia alleviates akinesia in the reserpine-treated rat

1. This study examined whether group III metabotropic glutamate (mGlu) receptor agonists injected into the globus pallidus (GP), substantia nigra pars reticulata (SNr) or intracerebroventricularly (i.c.v.) could reverse reserpine-induced akinesia in the rat. 2. Male Sprague-Dawley rats, cannulated above the GP, SNr or third ventricle, were rendered akinetic with reserpine (5 mg kg(-1) s.c.). 18 h later, behavioural effects of the group III mGlu receptor agonists L-serine-O-phosphate (L-SOP) or L-(+)-2-amino-4-phosphonobutyric acid (L-AP4) were examined. 3. In reserpine-treated rats, unilateral injection of L-SOP (2000 and 2500 nmol in 2.5 microl) into the GP produced a significant increase in net contraversive rotations compared to vehicle, reaching a maximum of 83+/-21 rotations 120 min(-1) (n=8). Pretreatment with the group III mGlu receptor antagonist methyl-serine-O-phosphate (M-SOP; 250 nmol in 2.5 microl) inhibited the response to L-SOP (2000 nmol) by 77%. Unilateral injection of L-SOP (250-1000 nmol in 2.5 microl) into the SNr of reserpine-treated rats produced a dose-dependent increase in net contraversive rotations, reaching a maximum of 47+/-6 rotations 30 min(-1) (n=6). M-SOP (50 nmol in 2.5 microl) inhibited the response to L-SOP (500 nmol) by 78%. 4. Following i.c.v. injection, L-SOP (2000-2500 nmol in 2.5 microl) or L-AP4 (0.5-100 nmol in 2 microl) produced a dose-dependent reversal of akinesia, attaining a maximum of 45+/-17 (n=8) and 72+/-3 (n=9) arbitrary locomotor units 30 min(-1), respectively. 6. These studies indicate that injection of group III mGlu receptor agonists into the GP, SNr or cerebral ventricles reverses reserpine-induced akinesia, the mechanism for which remains to be established.

The endocannabinoid system in the basal ganglia and in the mesolimbic reward system: implications for neurological and psychiatric disorders

To date, N-arachidonoylethanolamine (anandamide) and 2-arachidonoylglycerol are the best studied endocannabinoids and are thought to act as retrograde messengers in the central nervous system (CNS). By activating presynaptic cannabinoid CB1 receptors, they can reduce glutamate release in dorsal and ventral striatum (nucleus accumbens) and alter synaptic plasticity, thereby modulating neurotransmission in the basal ganglia and in the mesolimbic reward system. In this review, we will focus on the role of the endocannabinoid system within these neuronal pathways and describe its effect on dopaminergic transmission and vice versa. The endocannabinoid system is unlikely to directly affect dopamine release, but can modify dopamine transmission trough trans-synaptic mechanisms, involving gamma-aminobutyric acid (GABA)-ergic and glutamatergic synapses, as well as by converging signal transduction cascades of the cannabinoid and dopamine receptors. The dopamine and endocannabinoid systems exert a mutual control on each other. Cannabinergic signalling may lead to release of dopamine, which can act via dopamine D1-like receptors as a negative feedback mechanism to counteract the effects of activation of the cannabinoid CB1 receptor. On the other hand, dopaminergic signalling via dopamine D2-like receptors may lead to up-regulation of cannabinergic signalling, which is likely to represent a negative feedback on dopaminergic signalling. The consequences of these interactions become evident in pathological conditions in which one of the two systems is likely to be malfunctioning. We will discuss neurological and psychiatric disorders such as Parkinson's and Huntington's disease, drug addiction and schizophrenia. Furthermore, the possible role of the endocannabinoid system in disorders not necessarily depending on the dopaminergic system, such as eating disorders and anxiety, will be described.

Failure of estrogen to protect the substantia nigra pars compacta of female rats from lesion induced by 6-hydroxydopamine

The immunostaining for tyrosine hydroxylase (TH) in the substantia nigra pars compacta (SNpc) and in the ventral tegmental area (VTA) after intranigral infusion of 6-hydroxydopamine (6-OHDA, 6 microg/side) was analyzed in ovariectomized adult female Wistar rats. Estrogen replacement for 52 days (400-microg 17-beta-estradiol capsules) did not prevent the loss of TH-immunoreactive cells induced by 6-OHDA in the SNpc. This result indicates that the neuroprotective effect of dopaminergic mesencephalic cells is not observed with long-term estrogen replacement.

The striatal GABA-ergic neurons expressing substance P receptors in the basal ganglia of mice

By using a double immunofluorescence, we have examined the distribution of striatal GABAergic neurons that expressed substance P receptor (SPR) in the basal ganglia of adult C57 mice. The distribution of GABA-immunoreactive neurons completely or partially overlapped with that of SPR-immunoreactive neurons in the striatum (i.e. the caudate-putamen), globus pallidus, ventral pallidum, and nucleus accumbens. Neurons showing both GABA- and SPR-immunoreactivities were, however, predominantly found in the caudate-putamen, and most of them were characterized by their large-sized aspiny neuronal profile. Semi-quantification indicated that only about 13% of the total GABA-immunoreactive neurons (including large and medium-sized) displayed SPR-immunoreactivity, and these double-labeled neurons constituted about 31% of the total SPR-immunoreactive cells in the striatum. Neurons double-labeled with GABA- and SPR-immunoreactivities were hardly detected in other aforementioned regions of the basal ganglia. In addition, double immunofluorescence also showed co-localization of SPR- with glutamic acid decarboxylase-immunoreactivity, but not with parvalbumin-immunoreactivity, in the striatal neurons. Taken together with previous reports, the present study has suggested that a sub-population of striatal GABA-ergic neurons, most possibly GABA-ergic interneurons, may also receive direct physiological modulation by tachykinins through SPR in the basal ganglia of mammals.

Possible roles of kainate receptors on GABAergic nerve terminals projecting to rat substantia nigra dopaminergic neurons

GABAergic afferent inputs are thought to play an important role in the control of the firing pattern of substantia nigra pars compacta (SNc) dopaminergic neurons. We report here the actions of presynaptic kainite (KA) receptors in GABAergic transmission of rat SNc dopaminergic neurons. In mechanically dissociated rat SNc dopaminergic neurons attached with native presynaptic nerve terminals, GABAergic miniature inhibitory postsynaptic currents (mIPSCs) were recorded by use of conventional whole cell patch recording mode. In the voltage-clamp condition, KA (3 microM) significantly increased GABAergic mIPSC frequency without affecting the current amplitude. This facilitatory effect of KA was not affected in the presence of 20 microM GYKI52466, a selective AMPA receptor antagonist, but was completely inhibited in the presence of 20 microM CNQX, an AMPA/KA receptor antagonist. Presynaptic KA receptors on GABAergic terminals were mainly permeable to Na+ but impermeable to Ca2+ because KA-induced facilitation of mIPSC frequency was completely suppressed in either Na+-free or Ca2+-free external solutions, and in the presence of 200 microM Cd2+, a general voltage-dependent Ca2+ channel blocker. In the slice preparation, KA increased GABAergic spontaneous mIPSC frequency, but significantly suppressed evoked IPSC (eIPSC) amplitude. However, this inhibitory action on eIPSCs was reversed by 10 microM CGP55845, a selective GABAB receptor antagonist, implicating the possible involvement of GABAB autoreceptors in KA-induced modulation of GABAergic transmission. Thus presynaptic KA receptors on GABAergic nerve terminals synapsing onto SNc neurons may play functional roles contributing the fine control of neuronal excitability and firing pattern of SNc.

Prevalence, etiology, and treatment of depression in Parkinson's disease

Parkinson's disease (PD) is primarily a disease of elderly individuals with a peak age at onset of 55 to 66 years. It is characterized by bradykinesia, rigidity, tremor, and postural instability; and affects approximately 1 million individuals in the US and is the second most common neurodegenerative disease next to Alzheimer's disease. The motor symptoms of PD are the focus of pharmacotherapy, yet the nonmotor symptoms (e.g., dementia, psychosis, anxiety, insomnia, autonomic dysfunction, and mood disturbances) can be the most disturbing, disabling, and misunderstood aspects of the disease. Depressive symptoms occur in approximately half of PD patients and are a significant cause of functional impairment for PD patients. There is accumulating evidence suggesting that depression in PD is secondary to the underlying neuroanatomical degeneration, rather than simply a reaction to the psychosocial stress and disability. The incidence of depression is correlated with changes in central serotonergic function and neurodegeneration of specific cortical and subcortical pathways. Understanding comorbid depression in PD may therefore add to the understanding of the neuroanatomical basis of melancholia.

Biosynthesis of endocannabinoids and their modes of action in neurodegenerative diseases

Endocannabinoids are thought to function as retrograde messengers, which modulate neurotransmitter release by activating presynaptic cannabinoid receptors. Anandamide and 2-arachidonoylglycerol (2-AG) are the two best studied endogenous lipids which can act as endocannabinoids. Together with the proteins responsible for their biosynthesis, inactivation and the cannabinoid receptors, these lipids constitute the endocannabinoid system. This system is proposed to be involved in various neurodegenerative diseases such as Parkinson's and Huntington's diseases as well as Multiple Sclerosis. It has been demonstrated that the endocannabinoid system can protect neurons against glutamate excitotoxicity and acute neuronal damage in both in vitro and in vivo models. In this paper we review the data concerning the involvement of the endocannabinoid system in neurodegenerative diseases in which neuronal cell death may be elicited by excitotoxicity. We focus on the biosynthesis of endocannabinoids and on their modes of action in animal models of these neurodegenerative diseases.