Neural signatures of hyperdirect pathway activity in Parkinson's disease

Parkinson's disease (PD) is characterised by the emergence of beta frequency oscillatory synchronisation across the cortico-basal-ganglia circuit. The relationship between the anatomy of this circuit and oscillatory synchronisation within it remains unclear. We address this by combining recordings from human subthalamic nucleus (STN) and internal globus pallidus (GPi) with magnetoencephalography, tractography and computational modelling. Coherence between supplementary motor area and STN within the high (21-30 Hz) but not low (13-21 Hz) beta frequency range correlated with 'hyperdirect pathway' fibre densities between these structures. Furthermore, supplementary motor area activity drove STN activity selectively at high beta frequencies suggesting that high beta frequencies propagate from the cortex to the basal ganglia via the hyperdirect pathway. Computational modelling revealed that exaggerated high beta hyperdirect pathway activity can provoke the generation of widespread pathological synchrony at lower beta frequencies. These findings suggest a spectral signature and a pathophysiological role for the hyperdirect pathway in PD.

Localization and expression of group I metabotropic glutamate receptors in the mouse striatum, globus pallidus, and subthalamic nucleus: regulatory effects of MPTP treatment and constitutive Homer deletion

Group I metabotropic glutamate receptors (mGluRs), mGluR1 and mGluR5, regulate activity in the globus pallidus (GP) and subthalamic nucleus (STN). To test whether the localization of group I mGluRs is altered in parkinsonism, we used immunoelectron microscopy to analyze the subcellular and subsynaptic distribution of mGluR1a and mGluR5 in GP and STN of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated mice. Homer1 and Homer2 knock-out mice were used to assess the role of Homer in MPTP-induced redistribution of group I mGluRs. We also examined the effects of MPTP on the expression levels of group I mGluRs and Homer proteins in GP and striatum. MPTP treatment significantly reduced the expression levels of H1a and mGluR1a in striatum but not in GP. Although light microscopy did not reveal noticeable effects of MPTP treatment on the distribution of group I mGluRs and Homer proteins in GP and STN, specific changes in the ultrastructural localization of mGluR1a were found in MPTP-treated normal and Homer knock-out mice. An increase in the expression of presynaptic axonal and terminal mGluR1a labeling and an increased level of mGluR1a immunoreactivity in the postsynaptic specialization of putative GABAergic synapses were among the most significant effects induced by dopamine depletion. However, neither of these changes was found for mGluR5, which, in contrast, displayed complex regulatory alterations in its subsynaptic distribution in response to Homer deletion and MPTP lesion. Thus, nigrostriatal dopaminergic lesion and Homer deletion lead to changes in the trafficking of group I mGluRs in vivo that are specific to receptor subtypes and brain areas.

Involvement of sensorimotor, limbic, and associative basal ganglia domains in L-3,4-dihydroxyphenylalanine-induced dyskinesia

Dyskinesia represents a debilitating complication of L-3,4-dihydroxyphenylalanine (L-dopa) therapy for Parkinson's disease. Such motor manifestations are attributed to pathological activity in the motor parts of basal ganglia. However, because consistent funneling of information takes place between the sensorimotor, limbic, and associative basal ganglia domains, we hypothesized that nonmotor domains play a role in these manifestations. Here we report the changes in 2-deoxyglucose (2-DG) accumulation in the sensorimotor, limbic, and associative domains of basal ganglia and thalamic nuclei of four groups of nonhuman primates: normal, parkinsonian, parkinsonian chronically treated with L-dopa without exhibiting dyskinesia, and parkinsonian chronically treated with L-dopa and exhibiting overt dyskinesia. Although nondyskinetic animals display a rather normalized metabolic activity, dyskinetic animals are distinguished by significant changes in 2-DG accumulation in limbic- and associative-related structures and not simply in sensorimotor-related ones, suggesting that dyskinesia is linked to a pathological processing of limbic and cognitive information. We propose that these metabolic changes reflect the underlying neural mechanisms of not simply motor dyskinesias but also affective, motivational, and cognitive disorders associated with long-term exposure to L-dopa.

[Lesions in the pars compacta substantiae nigra and the subthalamic nucleus modify the density of muscarinic receptors in different nuclei of the basal ganglia]

INTRODUCTION

Several studies that has focused to the dopaminergic transmission in the basal ganglia in parkinsonian condition, but only a few article has taking into account the imbalance between dopaminergic and cholinergic transmission.

OBJECTIVE

To evaluate the muscarinic cholinergic receptors density in SNc and PPN in the 6-OHDA model.

MATERIALS AND METHODS

Were organized five experimental groups in correspondence to the place of the lesion: I. Non treated rats, II. 6-OHDA lesion in SNc, III. 6-OHDA lesion in SNc + quinolinic acid lesion in NST, IV. Sham operated rats, V. Quinolinic acid in STN. Were obtained coronal sections of 20 microm thickness of SNc and PPN from rats and in these sections was evaluated the muscarinic receptors density through autoradiographic technique with [3H]quinuclidinylbenzilate (QNB) (1.23 nM). The muscarinic antagonist atropine (1 microM) was utilized as non-specific union. The density was evaluated in both hemispheres and the density optical was converted in fentomolas/mg of tissue with base to values obtained from tritium standards.

RESULTS

Significant diminution of the muscarinic receptors density was found in the SNc ipsilateral to the 6-OHDA lesion from experimental groups II (t=2.76; p<0.05) and III (t=4.06; p<0.05). In the group V, was seen a significant increase of muscarinic receptor density in the SNc ipsilateral to the 6-OHDA lesion. The comparison between experimental groups evidenced significant differences among them (F=13.13; p<0.001) with a significant decrease in the density from SNc of groups II and III and significant increase in the density from SNc of group V in comparison of the others groups. In relation to PPN, muscarinic receptors density from right PPN ipsilateral to the 6-OHDA lesion, shown significant differences (F=3.93; p<0.01) between the experimental groups with a significant increase of this variable in the group II.

CONCLUSIONS

These results signal a modification of cholinergic activity after 6-OHDA lesion. The changes in the muscarinic receptors populations located in SNc and PPN could be part of different compensatory mechanisms to attempt ameliorate the imbalance between dopaminergic and cholinergic transmission that it was installed after denervation of nigrostriatal forebrain bundle. The excitotoxic lesion of STN impose a new adjust mechanism for cell from PPN, which could be expressed in the changes of muscarinic cholinergic receptors population at the level of SNc.

Neurochemical Mechanisms Induced by High Frequency Stimulation of the Subthalamic Nucleus: Increase of Extracellular Striatal Glutamate and GABA in Normal and Hemiparkinsonian Rats

High frequency stimulation (HFS) (130 Hz) of the subthalamic nucleus (STN) provides beneficial effects in patients suffering from severe parkinsonism, but the mechanisms underlying these clinical results remain to be clarified. To date, very little is known concerning the effects of STN-HFS on neurochemical transmission in the different basal ganglia nuclei and in particular the striatum. This study examines the effects of STN-HFS in intact and hemiparkinsonian rats on extracellular striatal glutamate (Glu) and GABA levels by means of intracerebral microdialysis. Unilateral STN-HFS was found to induce a significant bilateral increase of striatal Glu and GABA both in intact and in dopamine-lesioned animals. In intact rats, these increases were reversed by local administration of the D1 antagonist SCH 23390, but were potentiated by the D2 antagonist sulpiride. Potentiation was also observed after local administration of both D1 and D2 antagonists whose amplitude was similar to that measured in hemiparkinsonian rats. These data furnish the first evidence that STN-HFS influences striatal amino-acid transmission and that this influence is modulated by dopamine. They provide evidence that the effects of STN-HFS are not only restricted to the direct STN targets, but also involve adaptive changes within other structures of the basal ganglia circuitry.

Haloperidol-induced alteration in the physiological actions of group I mGlus in the subthalamic nucleus and the substantia nigra pars reticulata

Excitatory glutamatergic inputs to the subthalamic nucleus (STN), and subthalamic afferents to the substantia nigra pars reticulata (SNr) are believed to play a key role in the pathophysiology of Parkinson's disease (PD). Previously, we have shown that activation of the group I mGlus in the STN and SNr induces a direct depolarization of the neurons in these nuclei. Surprisingly, although both group I mGlus were present in the STN and SNr, mGlu5 alone mediated the DHPG-induced depolarization of the STN, and mGlu1 alone mediated the DHPG-induced depolarization of the SNr. We now report that both mGlu1 and mGlu5 are coexpressed in the same cells in both of these brain regions, and that both receptors play a role in mediating the DHPG-induced increase in intracellular calcium. Furthermore, we demonstrate that the induction of an acute PD-like state using a 16 h haloperidol treatment produces an alteration in the coupling of the group I receptors, such that post-haloperidol, DHPG-induced depolarizations are mediated by both mGlu1 and mGlu5 in the STN and SNr. Therefore, the pharmacology of the group I mGlu-mediated depolarization depends on the state of the system, and alterations in receptor coupling may be evident in pathological states such as PD.

High-frequency stimulation of the subthalamic nucleus selectively reverses dopamine denervation-induced cellular defects in the output structures of the basal ganglia in the rat

High-frequency stimulation (HFS) of the subthalamic nucleus (STN) is now recognized as an effective treatment for advanced Parkinson's disease, but the molecular basis of its effects remains unknown. This study examined the effects of unilateral STN HFS (2 hr of continuous stimulation) in intact and hemiparkinsonian awake rats on STN neuron metabolic activity and on neurotransmitter-related gene expression in the basal ganglia, by means of in situ hybridization histochemistry and immunocytochemistry. In both intact and hemiparkinsonian rats, this stimulation was found to induce c-fos protein expression but to decrease cytochrome oxidase subunit I mRNA levels in STN neurons. STN HFS did not affect the dopamine lesion-mediated overexpression of enkephalin mRNA or the decrease in substance P in the ipsilateral striatum. The lesion-induced increases in intraneuronal glutamate decarboxylase 67 kDa isoform (GAD67) mRNA levels on the lesion side were reversed by STN HFS in the substantia nigra, partially antagonized in the entopeduncular nucleus but unaffected in the globus pallidus. The stimulation did not affect neuropeptide or GAD67 mRNA levels in the side contralateral to the dopamine lesion or in intact animals. These data furnish the first evidence that STN HFS decreases the metabolic activity of STN neurons and antagonizes dopamine lesion-mediated cellular defects in the basal ganglia output structures. They provide molecular substrate to the therapeutic effects of this stimulation consistent with the current hypothesis that HFS blocks STN neuron activity. However, the differential impact of STN HFS on the effects of dopamine lesion among structures receiving direct STN inputs suggests that this stimulation may not cause simply interruption of STN outflow.

Co-localization of AMPA-type glutamate receptor immunoreactivity in neurons of the rat subthalamic nucleus

In order to determine the precise cellular localization of the alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionate (AMPA)-type glutamate receptor subunit immunoreactivity in the rat subthalamic nucleus, single and double immunofluorescence was performed. Intense level of GluR1, GluR2, GluR2/3 and GluR4 immunoreactivity was found in almost all neurons of the subthalamic nucleus. By double immunofluorescence, the subthalamic neurons in the same sections that displayed a strong immunoreactivity for GluR1 were found to display a robust GluR2 immunoreactivity and the subthalamic neurons that displayed GluR2 immunoreactivity were also found to express GluR4 immunoreactivity. The present results thus demonstrate that individual neurons of the subthalamic nucleus are likely to co-express GluR1 and GluR2, and GluR2 and GluR4 immunoreactivity. The native AMPA channels in the subthalamic neurons may, therefore, be composed of heteromeric subunits. The present results provide information of the neuroanatomical localization of AMPA receptor subunits in neurons of the subthalamic nucleus. The localization of AMPA receptor subunits may be related to functional characteristics of AMPA channels in the subthalamic neurons.

The indirect basal ganglia pathway in dopamine D(2) receptor-deficient mice

Recent pathophysiological models of basal ganglia function in Parkinson's disease predict that specific neurochemical changes in the indirect pathway would follow the lack of stimulation of D(2) dopamine receptors. Post mortem studies of the basal ganglia in genetically modified mice lacking functional copies of the D(2) dopamine receptor gene allowed us to test these predictions. When compared with their congenic N(5) wild-type siblings, mice lacking D(2) receptors show an increased expression of enkephalin messenger RNA in the striatum, and an increased activity and expression of cytochrome oxidase I in the subthalamic nucleus, as expected. In addition, D(2) receptor-deficient mice display a reduced expression of glutamate decarboxylase-67 messenger RNA in the globus pallidus, as the basal ganglia model predicts. This reduction contrasts with the lack of change or increase in glutamate decarboxylase-67 messenger RNA expression found in animals depleted of dopamine after lesions of the mesostriatal dopaminergic system. Furthermore, D(2) receptor-deficient mice show a significant decrease in substance P messenger RNA expression in the striatonigral neurons which form the direct pathway. Finally, glutamate decarboxylase-67 messenger RNA expression in the basal ganglia output nuclei was not affected by mutations in the D(2) receptor gene, a fact that could probably be related to the absence of a parkinsonian locomotor phenotype in D(2) receptor-deficient mice. In summary, these findings provide compelling evidence demonstrating that the lack of endogenous stimulation of D(2) receptors is sufficient to produce subthalamic nucleus hyperactivity, as assessed by cytochrome oxidase I histochemistry and messenger RNA expression, and strongly suggest the existence of interactions between the basal ganglia direct and indirect pathways.

Characterization of Ca(2+) channels in rat subthalamic nucleus neurons

The subthalamic nucleus (STN) plays a key role in motor control. Although previous studies have suggested that Ca(2+) conductances may be involved in regulating the activity of STN neurons, Ca(2+) channels in this region have not yet been characterized. We have therefore investigated the subtypes and functional characteristics of Ca(2+) conductances in STN neurons, in both acutely isolated and slice preparations. Acutely isolated STN cells were identified by retrograde filling with the fluorescent dye, Fluoro-Gold. In acutely isolated STN neurons, Cd(2+)-sensitive, depolarization-activated Ba(2+) currents were observed in all cells studied. The current-voltage relationship and current kinetics were characteristic of high-voltage-activated Ca(2+) channels. The steady-state voltage-dependent activation curves and inactivation curves could both be fitted with a single Boltzmann function. Currents evoked with a prolonged pulse, however, inactivated with multiple time constants, suggesting either the presence of more than one Ca(2+) channel subtype or multiple inactivation processes with a single channel type in STN neurons. Experiments using organic Ca(2+) channel blockers revealed that on average, 21% of the current was nifedipine sensitive, 52% was sensitive to omega-conotoxin GVIA, 16% was blocked by a high concentration of omega-agatoxin IVA (200 nM), and the remainder of the current (9%) was resistant to the co-application of all blockers. These currents had similar voltage dependencies, but the nifedipine-sensitive current and the resistant current activated at slightly lower voltages. omega-Agatoxin IVA at 20 nM was ineffective in blocking the current. Together, the above results suggest that acutely isolated STN neurons have all subtypes of high-voltage-activated Ca(2+) channels except for P-type, but have no low-voltage-activated channels. Although acutely isolated neurons provide a good preparation for whole cell voltage-clamp study, dendritic processes are lost during dissociation. To gain information on Ca(2+) channels in dendrites, we thus studied Ca(2+) channels of STN neurons in a slice preparation, focusing on low-voltage-activated channels. In current-clamp recordings, a slow spike was always observed following termination of an injected hyperpolarizing current. The slow spike occurred at resting membrane potentials and was sensitive to micromolar concentrations of Ni(2+), suggesting that it is a low-threshold Ca(2+) spike. Together, our results suggest that STN neurons express low-voltage-activated Ca(2+) channels and several high-voltage-activated subtypes. Our results also suggest the possibility that the low-voltage-activated channels have a preferential distribution to the dendritic processes.

Increased behavioral response to dopaminergic stimulation of the subthalamic nucleus after nigrostriatal lesions

Local infusions of the nonselective dopaminergic agonist apomorphine into the subthalamic nucleus of rats has been shown to elicit orofacial dyskinesia which can be blocked by D1 but not D2 receptor antagonists. In the present study, we show that the selective D1 agonist A77636 also induces orofacial dyskinesia when injected into the subthalamic nucleus of awake rats, thus confirming a role for D1 receptors in this effect. We also examined the dyskinesia induced by intrasubthalamic injections of apomorphine in rats with an ipsilateral lesion of the nigrostriatal pathway. The orofacial response to local administration of apomorphine (1.0 microg) into the subthalamic nucleus was markedly increased in the lesioned rats. As in control rats, the enhanced behavioral response seen in lesioned rats was blocked by peripheral administration of D1 antagonists. Although D1 receptor binding autoradiography revealed no difference in D1 receptor binding in the subthalamic nucleus on the side of the lesion compared to controls, D1 binding was higher in the subthalamic nucleus on the side of the lesion compared to the contralateral side. The increased behavioral response observed after unilateral dopamine denervation suggests that the subthalamic nucleus is tonically regulated by dopaminergic projections from the substantia nigra. Furthermore, the data suggest that subthalamic D1 receptors may be involved in the development of dyskinesia induced by dopaminergic drugs.

Dopaminergic innervation of the subthalamic nucleus in the normal state, in MPTP-treated monkeys, and in Parkinson's disease patients

The existence of a dopaminergic innervation of the subthalamic nucleus (STN) has been demonstrated in rats but has remained controversial in primates. The aim of the present study was first to demonstrate the existence of a dopaminergic innervation of the STN in monkeys using tracing methods and then to quantify the loss of dopaminergic fibers in the parkinsonian state in monkeys and humans. Following injection of Fluoro-Gold into the STN of a vervet monkey (Cercopithecus aethiops), retrogradely labeled neurons were found to be scattered in all dopaminergic areas of the mesencephalon. Injection of biotin dextran amine into dopaminergic areas A8 and A9 of two monkeys resulted in anterogradely labeled axons located throughout the whole extent of the STN. Labeled axons that also expressed tyrosine hydroxylase (TH) were reconstructed from serial sections. Some terminal axonal arborizations had profuse branching and occupied much of the STN, and others were restricted to small portions of the nucleus. In TH-immunoreactive sections, numerous sparse, fine, and varicose TH-positive fibers were observed in the STN of normal monkeys and humans. Quantification of these TH-positive fibers revealed a 51% loss of TH-positive fibers in MPTP-intoxicated monkeys and a 65% loss in Parkinson's disease patients compared with their respective controls. These findings demonstrate the existence of a dopaminergic innervation of the STN in primates. The loss of dopaminergic innervation in MPTP-intoxicated monkeys and in Parkinson's disease patients may directly affect the activity of STN neurons and could participate in the hyperactivity of the structure.

Intrasubthalamic nucleus metabotropic glutamate receptor activation: a behavioral, Fos immunohistochemical and [14C]2-deoxyglucose autoradiographic study

Metabotropic glutamate receptors are a major class of excitatory amino acid receptors. Eight metabotropic glutamate receptor subtypes have been cloned, and are classified into three groups (I, II and III) based on amino acid sequence identity, effector systems and pharmacological profile. Previous results have shown that unilateral stimulation of metabotropic glutamate receptors in the subthalamic nucleus with the non-subtype-selective metabotropic glutamate receptor agonist 1S,3R-1-amino-1,3-cyclopentane dicarboxylate results in contralateral rotation in rats and Fos expression in the subthalamic nucleus. This suggests that metabotropic glutamate receptor stimulation results in altered subthalamic nucleus activity with consequent altered basal ganglia activity on the injected side. We sought to determine the metabotropic glutamate receptor subtype(s) involved and the functional neuroanatomy underlying the rotational behavior. Unilateral intrasubthalamic nucleus injection of group II or group III metabotropic glutamate receptor agonists induced contralateral rotation. In addition to producing rotation, group II and group III metabotropic glutamate receptor agonists induce toxicity in the subthalamic nucleus and overlying thalamus. Following group II or group III subthalamic nucleus metabotropic glutamate receptor stimulation, there is Fos-like immunoreactivity in the globus pallidus, subthalamic nucleus, substantia nigra pars reticulata and entopeduncular nucleus, suggesting altered activity in subthalamic nucleus target regions. However, examination of [14C]2-deoxyglucose uptake suggests that the alterations in basal ganglia activity are different following group II versus group III metabotropic glutamate receptor stimulation, suggesting that rotation is occurring via different mechanisms. It appears that stimulation of subthalamic nucleus group II metabotropic glutamate receptors induces rotation by increasing subthalamic nucleus activity. These results suggest that group II metabotropic glutamate receptor antagonists may be useful for alleviating subthalamic nucleus overactivity in Parkinson's disease.

A light and electron microscopic study of GAT-1 in the monkey basal ganglia

The distribution of the GABA transporter GAT-1 was studied by immunocytochemistry and electron microscopy in the monkey basal ganglia. Dense staining was observed in the globus pallidus externa and interna, intermediate in the subthalamic nucleus, and substantia nigra, and light staining in the caudate nucleus and putamen. Staining was observed in axon terminals, but not cell bodies. Electron microscopy showed that the GAT-1 positive axon terminals formed symmetrical synapses, suggesting that they were the terminals of GABAergic neurons. Comparison of areas high in GAT-1 protein with that of GABA showed a good correlation between the density in neuropil staining for GAT-1, and that of GABA.