Chronic continuous and intermittent L-3,4-dihydroxyphenylalanine treatments differentially affect basal ganglia function in 6-hydroxydopamine lesioned rats--an autoradiographic study using [3H]flunitrazepam

Modulation of firing activity by endogenous GABAA receptors in the globus pallidus of MPTP-treated parkinsonian mice

The globus pallidus in rodents, equivalent to the external segment of the globus pallidus in primates, plays an important role in movement regulation. Previous studies have shown abundant γ-aminobutyric acid (GABA)ergic innervation and GABAA receptors in the globus pallidus. In this study, we investigated the effects of endogenous GABAA receptors on the spontaneous firing activity of pallidal neurons in both normal and MPTP-treated mice using multi-barrel electrodes extracellular recordings in vivo. We found that in normal mice, pressure ejection of 0.1 mmol/L gabazine, a specific GABAA receptor antagonist, increased the spontaneous firing rate of globus pallidus neurons by 27.6 ± 5.6%. Furthermore, in MPTP mice (14 days after MPTP treatment), 0.1 mmol/L gabazine increased the firing rates by 51.0 ± 7.9%, significantly greater than in normal mice. These results suggest that endogenous GABAA receptors modulate the activity of globus pallidus neurons. The present findings may provide a rationale for investigations into the potential role of GABAA receptors in Parkinson's disease.

Proton magnetic resonance spectroscopy of the striatum in Parkinson's disease patients with motor response fluctuations

We have performed proton magnetic resonance spectroscopy centred on the putamen contralateral to the worst affected side in 10 patients with idiopathic Parkinson's disease (PD) and motor response fluctuations and seven age matched healthy controls. In PD, there was striking reduction in the N-acetylaspartate (NAA) and creatine and NAA/choline ratios compared to controls. This pilot study provides in vivo evidence of striatal neuronal dysfunction in PD and further studies are in progress to establish if the observed changes are due to the disease process itself or due to chronic levodopa therapy.

The pathophysiology of chorea/ballism and Parkinsonism

Recent neurobiological investigations indicate the presence of three principles of basal ganglia organization-the existence of segregated striatal output pathways, the differential regulation of these segregated outputs by dopamine, and the importance of the subthalamic nucleus-that provide insight into the pathophysiology of parkinsonism and chorea/ ballism. Choreic movement disorders result probably from diminished output from the basal ganglia, either from lesions of the subthalamic nucleus or from disordered regulation of subthalamic afferents. Parkinsonism results probably from excessive output of the basal ganglia to the thalamus. Recent data from animal experiments and human surgical studies are consistent with predictions of this model. Further articulation of this model suggests a hypothesis for the mechanism of peak dose dyskinesias. Some deficiencies of the model include its neglect of some potentially important connections within the basal ganglia, its failure to provide an adequate hypothesis for the mechanism of dystonias or tardive disorders, and an oversimplified conception of thalamic function. The model does provide a framework for future laboratory and clinical studies.

Ultrastructural localization and function of dopamine D1-like receptors in the substantia nigra pars reticulata and the internal segment of the globus pallidus of parkinsonian monkeys

The motor symptoms of Parkinson's disease (PD) are commonly attributed to striatal dopamine loss, but reduced dopamine innervation of basal ganglia output nuclei, the internal globus pallidus (GPi) and the substantia nigra pars reticulata (SNr) may also contribute to symptoms and signs of PD. Both structures express dopamine D1 and D5 receptors under normal conditions, and we have recently demonstrated that their local activation reduces neuronal discharge rates and enhances bursts and oscillatory activity in both nuclei of normal monkeys [M.A. Kliem et al. (2007)J. Neurophysiol., 89, 1489-1500]. Here, we determined the ultrastructural localization and function of D1-like receptors in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated parkinsonian monkeys. In both normal and MPTP-treated monkeys, most of the D1 and D5 receptor immunoreactivity was associated with unmyelinated axons, but we also found significant postsynaptic D5 receptor immunostaining in dendrites of GPi and SNr neurons. A significant proportion of axonal D1 immunostaining was bound to the plasma membrane in both normal and MPTP-treated monkeys. Local microinjections of the D1/D5 receptor agonist SKF82958 significantly reduced discharge rates in GPi and SNr neurons, while they increased burst firing and oscillatory activity in the 3-15-Hz band in SNr, but not in GPi, of parkinsonian monkeys. Together with our recent findings from normal monkeys, these data provide evidence that functional D1/D5 receptors are expressed in GPi and SNr in both normal and parkinsonian states, and that their activation by endogenous dopamine (under normal conditions) or dopamine receptor agonists (in parkinsonism) may regulate basal ganglia outflow.

Time-course of SKF-81297-induced increase in glutamic acid decarboxylase 65 and 67 mRNA levels in striatonigral neurons and decrease in GABA(A) receptor alpha1 subunit mRNA levels in the substantia nigra, pars reticulata, in adult rats with a unilateral 6-hydroxydopamine lesion

Striatal projection neurons use GABA as their neurotransmitter and express the rate-limiting synthesizing enzyme glutamic acid decarboxylase (GAD) and the vesicular GABA transporter vGAT. The chronic systemic administration of an agonist of dopamine D1/D5-preferring receptors is known to alter GAD mRNA levels in striatonigral neurons in intact and dopamine-depleted rats. In the present study, the effects of a single or subchronic systemic administration of the dopamine D1/D5-preferring receptor agonist SKF-81297 on GAD65, GAD67, PPD and vGAT mRNA levels in the striatum and GABA(A) receptor alpha1 subunit mRNA levels in the substantia nigra, pars reticulata, were measured in rats with a unilateral 6-hydroxydopamine (6-OHDA) lesion. After a single injection of SKF-81297, striatal GAD65 mRNA levels were significantly increased at 3 but not 72 h. In contrast, striatal GAD67 mRNA levels were increased and nigral alpha1 mRNA levels were decreased at 72 but not 3 h. Single cell analysis on double-labeled sections indicated that increased GAD or vGAT mRNA levels after acute SKF-81297 occurred in striatonigral neurons identified by their lack of preproenkephalin expression. Subchronic SKF-81297 induced significant increases in striatal GAD67, GAD65, preprodynorphin and vGAT mRNA levels and decreases in nigral alpha1 mRNA levels. In the striatum contralateral to the 6-OHDA lesion, subchronic but not acute SKF-81297 induced a significant increase in GAD65 mRNA levels. The other mRNA levels were not significantly altered. Finally, striatal GAD67 mRNA levels were negatively correlated with nigral alpha1 mRNA levels in the dopamine-depleted but not dopamine-intact side. The results suggest that different signaling pathways are involved in the modulation by dopamine D1/D5 receptors of GAD65 and GAD67 mRNA levels in striatonigral neurons. They also suggest that the down-regulation of nigral GABA(A) receptors is linked to the increase in striatal GAD67 mRNA levels in the dopamine-depleted striatum.

Pathophysiology of parkinsonism

The motor signs of Parkinson's disease are thought to result in large part from a reduction of the level of dopamine in the basal ganglia. Over the last few years, many of the functional and anatomical consequences of dopamine loss in these structures have been identified, both in the basal ganglia and in related areas in thalamus and cortex. This knowledge has contributed significantly to our understanding of the link between the degeneration of dopamine neurons in the midbrain and the development of parkinsonism. This review discusses the evidence that implicates electrophysiologic changes (including altered discharge rates, increased incidence of burst firing, interneuronal synchrony, oscillatory activity, and altered sensorimotor processing) in basal ganglia, thalamus, and cortex, in parkinsonism. From these studies, parkinsonism emerges as a complex network disorder, in which abnormal activity in groups of neurons in the basal ganglia strongly affects the excitability, oscillatory activity, synchrony and sensory responses of areas of the cerebral cortex that are involved in the planning and execution of movement, as well as in executive, limbic or sensory functions. Detailed knowledge of these changes will help us to develop more effective and specific symptomatic treatments for patients with Parkinson's disease.

GABAergic circuits in the basal ganglia and movement disorders

GABA is the major inhibitory neurotransmitter in the basal ganglia, and GABAergic pathways dominate information processing in most areas of these structures. It is therefore not surprising that abnormalities of GABAergic transmission are key elements in pathophysiologic models of movement disorders involving the basal ganglia. These include hypokinetic diseases such as Parkinson's disease, and hyperkinetic diseases, such as Huntington's disease or hemiballism. In this chapter, we will briefly review the major anatomic features of the GABAergic pathways in the basal ganglia, and then describe in greater detail the changes of GABAergic transmission, which are known to occur in movement disorders.

Unilateral 6-hydroxydopamine lesion of dopamine neurons and subchronic L-DOPA administration in the adult rat alters the expression of the vesicular GABA transporter in different subsets of striatal neurons and in the substantia nigra, pars reticulata

The loss of dopamine neurons combined or not with the subsequent administration of L-DOPA in patients with Parkinson's disease or in experimental models of the disease results in altered GABAergic signaling throughout the basal ganglia, including the striatum and the substantia nigra, pars reticulata. However, the molecular mechanisms involved in altered GABA neurotransmission remain poorly understood. In order to be released from synaptic vesicles, newly synthesized GABA is transported from the cytosol into synaptic vesicles by a vesicular GABA transporter. The objective of this study was to examine the hypothesis that expression of the vesicular GABA transporter (vGAT) is altered in the unilateral 6-hydroxydopamine model of Parkinson's disease. Our results provide evidence that a unilateral 6-hydroxydopamine lesion results in increased and decreased vGAT mRNA levels in striatopallidal and striatonigral neurons, respectively. These two subsets of neurons were identified by the co-expression or lack of co-expression of preproenkephalin, a marker of striatopallidal neurons, using double-labeling in situ hybridization histochemistry. Such changes occurred in the striatum ipsilateral to the 6-hydroxydopamine lesion and were paralleled by decreased vGAT protein levels in the substantia nigra, pars reticulate (SNr). On the other hand, the subchronic systemic administration of L-DOPA increased vGAT mRNA levels in preproenkephalin-negative neurons on the side ipsilateral and, to a lesser extent, the side contralateral to the 6-hydroxydopamine lesion. Systemic L-DOPA also increased vGAT protein levels in the ipsi- and contralateral SNr. As a whole, the results provide original evidence that vGAT expression is altered in the 6-hydroxydopamine model of Parkinson's disease. They also suggest that the behavioral effects induced by a subchronic administration of L-DOPA to 6-hydroxydopamine-lesioned rats involve an increase in the vesicular release of GABA by striatonigral neurons.

Globus pallidus internal segment

The internal segment of the globus pallidus (GP(i)) gathers many bits of information including movement-related activity from the striatum, external segment of the globus pallidus (GP(e)), and subthalamic nucleus (STN), and integrates them. The GP(i) receives rich GABAergic inputs from the striatum and GP(e), and gamma-aminobutyric acid (GABA) receptors are distributed in the GP(i) in a specific manner. Thus, inputs from the striatum and GP(e) may control GP(i) activity in a different way. The GP(i) finally conveys processed information outside the basal ganglia. Changes in GABAergic neurotransmission have been reported in movement disorders and suggested to play an important role in the pathophysiology of the symptoms.

Vesicular monoamine transporter-2 and aromatic L-amino acid decarboxylase gene therapy prevents development of motor complications in parkinsonian rats after chronic intermittent L-3,4-dihydroxyphenylalanine administration

Motor complications after chronic L-3,4-dihydroxyphenylalanine (L-DOPA) therapy occur partly because of the sensitization to dopaminergic agents resulting from pulsatile dopaminergic stimulation. The loss of presynaptic storage contributes to short duration of action by dopamine. Vesicular monoamine transporter-2 (VMAT-2) controls intraneuronal dopamine storage by packaging dopamine into synaptic vesicles, thereby allowing exocytotic release of dopamine. Using primary fibroblast doubly transduced with VMAT-2 and aromatic L-amino acid decarboxylase (AADC) genes, we previously demonstrated the beneficial effects of such double gene transduction in the production, storage, and gradual release of dopamine in vitro and in vivo. In this study, we further evaluate the effect of achieving sustained level of dopamine within the striata by VMAT-2 gene on behavioral response of parkinsonian rats after chronic intermittent L-DOPA administration. Primary fibroblast (PF) cells were genetically modified with AADC and VMAT-2 genes. We grafted primary fibroblast cells, PF with AADC (PFAADC), or doubly transduced PF with AADC and VMAT-2 (PFVMAA) (n = 6 for each group) into parkinsonian rat striata and administered L-DOPA (25 mg/kg/day) intermittently for 4 weeks. For behavioral study, we employed a model of akinesia using forepaw adjusting steps (FAS) that have been well characterized to reflect the effect of the lesion and the antiparkinsonian effect of dopaminergic drugs and transplants. The duration of FAS response to L-DOPA was sustained for a longer duration in rats grafted with PFVMAA cells than in those grafted with either control cells or cells with AADC alone. In PFVMAA-grafted animals, prolonged duration of FAS responses to L-DOPA was sustained even 6 weeks after discontinuation of 4-week intermittent L-DOPA treatment. These findings suggest that the restoration of dopamine storage capacity could enhance the efficacy of L-DOPA therapy and attenuate the motor fluctuations that result from chronic intermittent L-DOPA administration. The gene therapy expressing AADC and VMAT-2 along with systemic L-DOPA therapy could provide a novel treatment strategy to prevent motor fluctuations.

Comparative effects of acute or chronic administration of levodopa to 6-hydroxydopamine-lesioned rats on the expression of glutamic acid decarboxylase in the neostriatum and GABAA receptors subunits in the substantia nigra, pars reticulata

Current evidence suggests that behavioral sensitization to the chronic administration of levodopa (L-DOPA) to dopamine-depleted animals involves a plasticity of GABA-mediated signaling in output regions of the basal ganglia. The purpose of this study was to compare in adult rats with a unilateral 6-hydroxydopamine (6-OHDA) lesion the effects of an acute or chronic (for 3 or 7 days) injection of L-DOPA on mRNA levels encoding for glutamic acid decarboxylase (GAD65 and GAD67) in the striatum and GABA(A) receptor alpha1, beta2 and gamma2 subunits in the substantia nigra, pars reticulata (SNr), by in situ hybridization histochemistry. In addition, immunostaining levels for the alpha1 subunit were examined in the SNr. In agreement with previous studies, we found that L-DOPA administration increased GAD mRNA levels in the striatum of 6-OHDA-lesioned rats. However, the magnitude of this effect increased with the number of injections of L-DOPA. On the other hand, we found that 6-OHDA lesions resulted in increases in alpha1, beta2 and gamma2 mRNA levels in the ipsilateral SNr, which were normalized or decreased compared with the contralateral side by the acute or chronic administration of L-DOPA. In addition, alpha1 immunostaining in the SNr was significantly decreased in rats injected for 7 days but not for 3 days or acutely with L-DOPA. Our results demonstrate that a chronic administration of L-DOPA results in a progressive increase in GAD and decrease in GABA(A) receptor expression in the striatum and SNr, respectively. They provide further evidence that behavioral sensitization and dyskinesia induced by a chronic administration of L-DOPA in an experimental model of Parkinson's disease is paralleled by a plasticity of GABA-mediated signaling in the SNr.

Normalization of glutamate decarboxylase gene expression in the entopeduncular nucleus of rats with a unilateral 6-hydroxydopamine lesion correlates with increased GABAergic input following intermittent but not continuous levodopa

The expression of mRNA encoding for the 67 kilodalton isoform of glutamate decarboxylase (GAD67) was examined by in situ hybridization histochemistry in the entopeduncular nucleus (EP) of adult rats with a 6-hydroxydopamine unilaterally lesion of dopamine neurons. Our results provide original evidence that continuous or intermittent levodopa administration is equally effective at reversing the lesion-induced increase in GAD67 mRNA expression in the EP when compared with vehicle controls. To characterize the GABAergic interactions that may mediate levodopa-induced alterations in the EP, double-labeling in situ hybridization was conducted with a combination of GAD67 radioactive and preproenkephalin or preprotachykinin digoxigenin-labeled complementary RNA probes in the striatum. Levels of GAD67 mRNA labeling were significantly increased by intermittent, but not continuous levodopa. Analysis at the cellular level in a dorsal sector of the striatum revealed that GAD67 mRNA levels increased predominantly in preproenkephalin-unlabeled neuronal profiles, presumably striatal/EP neurons (+99.3%). Saturation analyses of (3)H-flunitrazepam binding to GABA(A) receptors in the EP showed that the increase in GAD67 mRNA in preproenkephalin-unlabeled neurons by intermittent levodopa paralleled a significant decrease in number of GABA(A) receptors (Bmax) in the EP ipsilateral to the lesion. Continuous levodopa failed to alter striatal GAD67 mRNA levels, or the number or affinity of GABA(A) receptors when compared with vehicle-treated controls. These results suggest the normalization of GAD gene expression in the EP by intermittent levodopa involves an increase in GABAergic inhibition by striatonigral/EP neurons of the direct pathway. Conversely, the effects of continuous levodopa on GAD mRNA levels in the EP do not appear to be mediated by GABA.

Dual effects of intermittent or continuous L-DOPA administration on gene expression in the globus pallidus and subthalamic nucleus of adult rats with a unilateral 6-OHDA lesion

Intermittent oral doses of levodopa (L-DOPA) are routinely used to treat Parkinson's disease, but with prolonged use can result in adverse motor complications, such as dyskinesia. Continuous administration of L-DOPA achieves therapeutic efficacy without producing this effect, yet the molecular mechanisms are unclear. This study examined, by in situ hybridization histochemistry, the effects of continuous or intermittent L-DOPA administration on gene expression in the globus pallidus and subthalamic nucleus of adult rats with a unilateral 6-hydroxydopamine (6-OHDA) lesion of the nigrostriatal pathway. Results were compared to 6-OHDA-treated rats receiving vehicle. Our results provide original evidence that continuous L-DOPA normalizes the 6-OHDA-lesion-induced increase in mRNA levels encoding for the 67 kDa isoform of glutamate decarboxylase in neurons of the globus pallidus and cytochrome oxidase subunit I mRNA levels in the subthalamic nucleus. The extent of normalization did not differ between the continuous and intermittent groups. In addition, intermittent L-DOPA induced an increase in the mRNA levels encoding for the 65 kDa isoform of glutamate decarboxylase in globus pallidus neurons ipsilateral to the lesion and a bilateral increase in c-fos mRNA expression in the subthalamic nucleus. These results suggest that continuous L-DOPA tends to normalize the 6-OHDA-lesion-induced alterations in cell signaling in the pallido-subthalamic loop. On the other hand, we propose that chronic intermittent L-DOPA exerts a dual effect by normalizing cell signaling in a subpopulation of neurons in the globus pallidus and subthalamic nucleus while inducing abnormal signaling in another subpopulation.

GABA(A) and mu-opioid receptor binding in the globus pallidus and endopeduncular nucleus of animals symptomatic for and recovered from experimental Parkinsonism

The expression of parkinsonian motor symptoms may be partly attributed to an increase in GABAergic neurotransmission from hyperactive GABA/enkephalinergic striatopallidal efferents. The present study measured pallidal GABA(A) and mu-opioid receptor binding in normal cats and cats symptomatic for and recovered from MPTP-induced parkinsonism. GABA(A) receptor binding was significantly decreased in the globus pallidus (GP) in symptomatic cats and returned to normal levels in spontaneously recovered cats. Mu-opioid receptor binding in the GP was significantly decreased in symptomatic cats and remained significantly decreased in recovered cats. These results suggest that GABA(A) but not mu-opioid receptor binding may correlate with the expression of parkinsonian motor deficits and may reflect increased pallidal GABA and ENK release in parkinsonian animals. Upon recovery from experimental parkinsonism, however, pallidal GABA(A) receptor binding returns to normal levels while mu-opioid receptor binding reflecting enkephalin release remains elevated.

Gamma-aminobutyric acidA and benzodiazepine receptor alterations in the rat brain after unilateral 6-hydroxydopamine lesions of the medial forebrain bundle

Gamma-aminobutyric acidA (GABA(A)) and benzodiazepine (BZ) receptors and dopamine uptake sites in 6-hydroxydopamine-treated rat brains were studied by receptor autoradiography using [3H]muscimol, [3H]flunitrazepam and [3H]mazindol binding, respectively. The rats were unilaterally lesioned in the medial forebrain bundle and the brains were analyzed at 1, 2, 4 and 8 weeks post-lesion. Degeneration of the nigrostriatal pathway after 6-hydroxydopamine treatment caused a significant loss of dopamine uptake sites in the ipsilateral striatum and substantia nigra (SN) in the lesioned animals. In the contralateral side, however, dopamine uptake sites showed no significant changes in the brain throughout the experiments. On the other hand, no significant changes in GABA(A) receptors were observed in the brain of both the ipsilateral and contralateral sides during post-lesion. In contrast, BZ receptors were observed significantly increased in the ventromedial part of striatum of the ipsilateral side from 2 to 4 weeks post-lesion. Furthermore, a transient increase in BZ receptors was found in the ipsilateral SN only at 2 weeks post-lesion. In contralateral side, most regions examined showed no significant changes in BZ receptors throughout the experiments except for a transient increase in the SN at 1 week post-lesion. These results demonstrate that 6-hydroxydopamine can cause severe functional damage in dopamine uptake sites in the nigrostriatal pathway. Our results also suggest that the change in BZ receptors is more pronounced than that in GABA(A) receptors in the brain after 6-hydroxydopamine treatment. Furthermore, our findings suggest that the increase in BZ receptors in the brain of 6-hydroxydopamine-treated model may be due to the additional disruption of the nigrostriatal dopamine system. Thus, investigations into possible changes in neurotransmitter receptors other than dopaminergic receptors appear to be important for the elucidation of pathogenesis of Parkinsons disease.

Changes in the gene expression of GABA(A) receptor alpha1 and alpha2 subunits and metabotropic glutamate receptor 5 in the basal ganglia of the rats with unilateral 6-hydroxydopamine lesion and embryonic mesencephalic grafts

By using an animal model of parkinsonism, we examined the expression of GABA(A) receptor (R) and metabotropic glutamate receptor (mGluR) 5 in the basal ganglia after transplantation with dopamine-rich tissue. The adult rats were unilaterally lesioned by the injection of 6-hydroxydopamine to their left medial forebrain bundles. At 5-10 weeks following the dopaminergic denervation, the levels of GABA(A)R in the left caudate-putamen and globus pallidus were about 20 and 16% lower than that of the right intact (control) sides, as shown by [3H]flunitrazepam binding autoradiography on the brain sections. However, the receptor density increased to around 132 and 130% of control levels in the entopeduncular nucleus and substantia nigra pars reticulata of the lesioned sides. Furthermore, in situ hybridization analysis exhibited parallel trends of changes in the levels of the GABA(A)R alpha1 and alpha2 subunit and mGluR5 mRNAs in the neurons of the brain regions with that of the proteins detected by the binding assay. A number of the rats 5 weeks postlesion were transplanted with the ventral mesencephalon of the embryonic rat into their left striata. Five weeks later, the changes in the [3H]flunitrazepam binding seemed to be recovered by approximately 50-63% on the grafted sides of the areas. Moreover, the transplantation appeared to produce a nearly complete reversal of the lesion-induced alterations in the levels of the mRNAs. Thus, the data indicate the mechanism of gene regulation for the modified expression of the receptors and could implicate the participation of the receptors in the pathogenesis of Parkinson's disease.

Dopamine-receptor stimulation: biobehavioral and biochemical consequences

The MPTP monkey is a well-characterized animal model of parkinsonism and provides an exceptional tool for the study of dyskinesias induced by dopamine-like agents. Several such agents have been tested during the past 15 years, and it has been found that the duration of action of these compounds is the most reliable variable with which to predict their dyskinesiogenic profile. It is proposed that L-dopa-induced dyskinesias represent a form of pathological learning caused by chronic pulsatile (nonphysiological) stimulation of dopamine receptors, which activates a cascade of molecular and biochemical events. These events include defective regulation of Fos proteins that belong to the deltaFosB family, increased expression of neuropeptides, and defective GABA- and glutamate-mediated neurotransmission in the output structures of the basal ganglia.

Sustained cabergoline treatment reverses levodopa-induced dyskinesias in parkinsonian monkeys

The pathophysiology of L-Dopa-induced dyskinesias (LID), a common problem after long-term use of L-dopa in the treatment of Parkinson's disease (PD), is not completely understood. Oscillations in L-Dopa concentrations in the brain are believed to be responsible, at least in part, for their pathogenesis. This study was aimed at verifying whether chronic administration of cabergoline, a long-acting dopamine D2-like receptor agonist, can reverse established LID. Four MPTP-treated cynomolgus monkeys with long-standing and stable parkinsonian syndrome and reproducible dyskinesias to L-Dopa, were used in this study. We compared the antiparkinsonian and dyskinetic responses of L-Dopa methyl ester (62.5 mg and 125 mg), given with benserazide (50 mg) (L-Dopa/benserazide), administered before and after a 6-week period during which the animals were treated only by daily administration of cabergoline (doses ranging from 0.125 to 0.185 mg/kg, subcutaneous). During cabergoline treatment, the monkeys initially showed marked dyskinesias, which were reduced significantly after 4 weeks of treatment. However, there was no tolerance to its antiparkinsonian effect. L-Dopa/benserazide given 4 days after cabergoline withdrawal produced a significant antiparkinsonian effect, but dyskinesias were dramatically reduced compared to what had been seen before chronic cabergoline treatment. The duration of the L-Dopa response was not increased after chronic administration of cabergoline. Our data suggest that sustained dopamine D2 receptor stimulation could be of value when trying to reduce or to reverse LID in patients with fluctuating advanced PD.

125I-CGP 64213 binding to GABA(B) receptors in the brain of monkeys: effect of MPTP and dopaminomimetic treatments

Much evidence indicates that abnormal GABA neurotransmission may be implicated in the pathophysiology of Parkinson's disease (PD) and dopaminomimetic-induced dyskinesias (DID). In this study, autoradiography using (125)I-CGP 64213 was performed to investigate GABA(B) receptor density in the brain of control monkeys as well as monkeys with MPTP-induced nigrostriatal depletion. Three MPTP monkeys received pulsatile administrations of the D1 dopamine (DA) receptor agonist (SKF 82958) whereas a long-acting D2 DA receptor agonist (cabergoline) was given to another three animals. SKF 82958 treatment relieved parkinsonian symptoms but two of three animals developed DID. Cabergoline induced a comparable motor benefit effect without persistent DID. (125)I-CGP 64213 binding to GABA(B) receptors was heterogeneous throughout the brain with the highest levels in the medial habenula of the thalamus. MPTP induced a decrease (-40%) of (125)I-CGP 64213 binding to GABA(B) receptors in the substantia nigra pars compacta (SNpc) and an increase (+29%) in the internal segment of the globus pallidus (GPi). This increase in the GPi was not affected by SKF 82958 but partly reversed by cabergoline. No change was seen in the striatum, the thalamus, the external segment of the globus pallidus, and the substantia nigra pars reticulata following MPTP and dopaminomimetic treatments. The changes of GABA(B) receptors observed in the SNpc and in the GPi suggest that alteration of GABA(B) receptors may play a role in the pathophysiology of PD and DID.

Effect of acute L-Dopa pretreatment on apomorphine-induced rotational behavior in a rat model of Parkinson's disease

Currently, reduction of apomorphine-induced rotational behavior in the 6-hydroxydopamine (6-OHDA) lesioned rat is the most utilized drug-induced paradigm for assessing functional efficacy in a rat model of Parkinson's disease (PD). Any clinically predictive animal model of PD should include a positive response to l-dopa, the standard pharmacotherapy for PD. However, the acute interaction between L-dopa and apomorphine has never been studied to determine if L-dopa pretreatment could reduce apomorphine-induced rotational behavior in a 6-OHDA lesioned rat. The present study was designed to explore whether, indeed, pretreatment with subrotational doses of L-dopa could inhibit apomorphine-induced rotations. The data indicate that L-dopa significantly reduced apomorphine-induced rotational behavior only at one dose (5.0 mg/kg) for 12 min. Based on these and other data, it is concluded that although the apomorphine-induced rotational paradigm may continue to be utilized as one additional indicator of efficacy in the 6-OHDA rat model of PD, it is not in itself a completely valid functional assay.

Progress in direct striatal delivery of L-dopa via gene therapy for treatment of Parkinson's disease using recombinant adeno-associated viral vectors

Viral vectors have recently been used successfully to transfer genes and express different proteins in the brain. This review discusses the requirements to consider human clinical trials in which recombinant adeno-associated virus vectors are used to transfer the genes necessary to produce l-dihydroxyphenylalanine (l-dopa) directly into the striatum of Parkinson's patients. Preclinical data that apply to the criteria defined as prerequisite for clinical trials are discussed. Thus, in animal models using recombinant adeno-associated virus vectors it has been demonstrated that l-dopa can be synthesized in the striatum after in vivo transduction. In addition, these l-dopa levels are sufficient to affect behavior in a dopamine-deficient animal model, the expression is extremely long-lasting, and the ability to transcriptionally regulate tyrosine hydroxylase has been demonstrated but not fully characterized. However, while immune responses to recombinant adeno-associated virus infection in the periphery have been studied, direct assessment of the potential immune response in the brain has not been sufficiently defined. Therefore, the rationale for delivering l-dopa directly to the striatum to treat Parkinson's disease is sound and the preclinical data are promising but all the issues surrounding this strategy are not resolved.

Chronic D1 and D2 dopaminomimetic treatment of MPTP-denervated monkeys: effects on basal ganglia GABA(A)/benzodiazepine receptor complex and GABA content

The effect of various chronic dopaminergic treatments in 1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) monkeys on the brain gamma-aminobutyric acid type A (GABA(A)) /benzodiazepine receptor complex and GABA content was investigated in order to assess the GABAergic involvement in dopaminomimetic-induced dyskinesia. Three MPTP monkeys received for one month pulsatile administrations of the D1 dopamine (DA) receptor agonist SKF 82958 whereas three others received the same dose of SKF 82958 by continuous infusion. A long acting D2 DA receptor agonist, cabergoline, was given to another three animals. Untreated MPTP as well as naive control animals were also included. Pulsatile SKF 82958 relieved parkinsonian symptoms but was also associated with dyskinesia in two of the three animals whereas animals treated continuously with SKF 82958 remained as untreated MPTP monkeys. Chronic cabergoline administration improved motor response with no persistent dyskinesia. MPTP treatment induced a decrease of 3H-flunitrazepam binding in the medial anterior part of caudate-putamen and an increase in the internal segment of globus pallidus (GPi) which was in general unchanged by pulsatile or continuous SKF 82958 administration. Throughout the striatum, binding of 3H-flunitrazepam remained reduced in MPTP monkeys treated with cabergoline but was not significantly lower than untreated MPTP monkeys. Moreover, cabergoline treatment reversed the MPTP-induced increase in 3H-flunitrazepam binding in the GPi. GABA concentrations remained unchanged in the striatum, external segment of globus pallidus and GPi following MPTP denervation. Pulsatile but not continuous SKF 82958 administration decreased putamen GABA content whereas cabergoline treatment decreased caudate GABA. No alteration in GABA levels were observed in the GPe and GPi following the experimental treatments. These results suggest that: (1) D2-like receptor stimulation with cabergoline modulates GABA(A) receptor density in striatal subregions anatomically related to associative cortical afferent and (2) the absence of dyskinesia in dopaminomimetic-treated monkeys might be associated with the reversal of the MPTP-induced upregulation of the GABA(A)/benzodiazepine receptor complex in the Gpi.

Vesicular monoamine transporter-2 and aromatic L-amino acid decarboxylase enhance dopamine delivery after L-3, 4-dihydroxyphenylalanine administration in Parkinsonian rats

Medical therapy in Parkinson's disease (PD) is limited by the short-duration response and development of dyskinesia that result from chronic L-3,4-dihydroxyphenylalanine (L-DOPA) therapy. These problems occur partly because the loss of dopamine storage sites leads to erratic dopamine delivery. Vesicular monoamine transporter-2 (VMAT-2) plays a critical role in dopamine storage by packaging dopamine into synaptic vesicles and regulating sustained release of dopamine. To restore the capacity to produce and store dopamine in parkinsonian rats, primary skin fibroblast cells (PF) were genetically modified with aromatic L-amino acid decarboxylase (AADC) and VMAT-2 genes. After incubation with L-DOPA in culture, the doubly transduced fibroblast cells (PFVMAA) produced and stored dopamine at a much higher level than the cells with either gene alone. PFVMAA cells in culture released dopamine gradually in a constitutive manner. Genetically modified fibroblast cells were grafted in parkinsonian rat striata, and L-DOPA was systemically administered. Higher dopamine levels were sustained for a longer duration in rats grafted with PFVMAA cells than in those grafted with either control cells or cells with AADC alone. These findings underscore the importance of dopamine storage capacity in determining the efficacy of L-DOPA therapy and illustrate a novel method of gene therapy combined with precursor administration to overcome the major obstacles of PD treatment.

Circling behaviour in 6-hydroxydopamine-lesioned rats given pulsed levodopa is reduced more by lesions in the entopeduncular nucleus/substantia nigra pars reticulata than in the subthalamic nucleus

Rats unilaterally lesioned with 6-hydroxydopamine to deplete striatal dopamine received daily injections of levodopa methyl ester in combination with benserazide. Delayed lesions in the subthalamic nucleus (Group 2) or entopeduncular nucleus and substantia nigra par reticulata (Group 3) were made, unilateral to the dopamine depletion. Apomorphine-induced rotation was significantly reduced in Group 2 versus sham-operated controls (P < 0.006) and in Group 3 versus Group 2 (P < 0.03). Results suggest that enhanced apomorphine-induced rotation in this model is mediated through both the striatopallidal and striatonigral pathway.

Characterization of intrastriatal recombinant adeno-associated virus-mediated gene transfer of human tyrosine hydroxylase and human GTP-cyclohydrolase I in a rat model of Parkinson's disease

To achieve local, continuous L-DOPA delivery in the striatum by gene replacement as a model for a gene therapy for Parkinson's disease, the present studies used high titer purified recombinant adeno-associated virus (rAAV) containing cDNAs encoding human tyrosine hydroxylase (hTH) or human GTP-cyclohydrolase I [GTPCHI, the rate-limiting enzyme for tetrahydrobiopterin (BH4) synthesis] or both to infect the 6-OHDA denervated rat striatum. Striatal TH and GTPCHI staining was observed 3 weeks after rAAV transduction, with little detectable perturbation of the tissue. Six months after intrastriatal rAAV transduction, TH staining was present but apparently reduced compared with the 3 week survival time. In a separate group of animals, striatal TH staining was demonstrated 1 year after rAAV transduction. Double staining studies using the neuronal marker NeuN indicated that >90% of rAAV-transduced cells expressing hTH were neurons. Microdialysis experiments indicated that only those lesioned animals that received the mixture of MD-TH and MD-GTPCHI vector displayed BH4 independent in vivo L-DOPA production (mean approximately 4-7 ng/ml). Rats that received the hTH rAAV vector alone produced measurable L-DOPA (mean approximately 1-4 ng/ml) only after receiving exogenous BH4. L-Aromatic amino acid decarboxylase blockade, but not 100 mM KCl-induced depolarization, enhanced L-DOPA overflow, and animals in the non-hTH groups (GTPCHI and alkaline phosphatase) yielded minimal L-DOPA. Although elevated L-DOPA was observed in animals that received mixed hTH and hGTPCHI rAAV vectors, there was no reduction of apomorphine-induced rotational behavior 3 weeks after intrastriatal vector injection. These data demonstrate that purified rAAV, a safe and nonpathogenic viral vector, mediates long-term striatal hTH transgene expression in neurons and can be used to successfully deliver L-DOPA to the striatum.

Effects of increasing doses of apomorphine during stereotaxic neurosurgery in Parkinson's disease: clinical score and internal globus pallidus activity. Short communication

We analysed the firing activity of internal globus pallidus cells in two Parkinson's disease patients undergoing stereotaxic surgery. Both patients showed an advanced rigid-akinetic syndrome with disabling levodopa induced dyskinesias. Apomorphine, intraoperatively administered at doses (1-2 mg) inducing a short but clear clinical improvement without involuntary movements, reduced the pallidal discharge rate by > 50% in both patients. An higher apomorphine dose (2.5 mg), tested in one hemisphere, blocked the firing activity with a time course independent from the occurrence of dyskinesias. These finding suggest that the reduction of internal pallidus excitability is one of the mechanisms underlying the efficacy of dopaminergic therapy, but also that changes in other basal ganglia stations are likely to be involved in dyskinesias.

Apomorphine priming alters the response of striatal outflow pathways to D2 agonist stimulation in 6-hydroxydopamine-lesioned rats

Chronic treatment with dopaminergic agonists is associated with response fluctuations to L-dihydroxyphenylalanine in Parkinson's disease and enhanced motor activity to D1 and D2 dopamine agonists in rats with 6-hydroxydopamine lesions of the nigrostriatal pathway. In dopamine-depleted rodents this phenomenon has been referred to as "priming" or reverse tolerance. The neurochemical changes that underlie "priming" of dopaminergic agonist responses are poorly understood. Some aspects of priming of D1 agonist-mediated rotation in the 6-hydroxydopamine-lesioned rat have been characterized, but priming of D2-agonist-dependent motor responses has been less thoroughly studied. In this study, examination of rotational behaviour and induction of Fos-like immunoreactivity were used to investigate changes in the striatal outflow systems in response to treatment with the D2 agonist quinpirole in 6-hydroxydopamine-lesioned rats that had been primed with apomorphine. Administration of apomorphine (0.5 mg/kg; three injections at three to six day intervals) permitted an otherwise inactive dose of quinpirole (0.25 mg/kg) to produce robust contralateral rotation and to induce the expression of Fos in striatal neurons belonging to the striato-nigro-entopeduncular ("direct") pathway. The increase in contralateral rotation and ipsilateral striatal Fos expression following administration of quinpirole to apomorphine-primed rats was mediated by a D2-like receptor and did not appear to be due to a change in sensitivity of D2 receptors. Apomorphine priming also enhanced the ability of quinpirole to induce Fos expression in the globus pallidus, a target of the striatopallidal ("indirect") pathway. Western blot analysis confirmed that treatment with quinpirole induced the expression of c-Fos protein with no change in the expression of 35-37,000 mol. wt Fos-related antigens in apomorphine-primed rats treated with water or quinpirole. Induction of Fos expression in the striatum generally results from blockade of D2 receptors and the striato-nigro-entopeduncular pathway preferentially expresses D1 receptors. Thus, the quinpirole-dependent induction of striatal Fos in apomorphine-primed 6-hydroxydopamine-lesioned rats represents a qualitative alteration in striatal outflow. These studies demonstrate that pretreatment of 6-hydroxydopamine-lesioned rats with apomorphine increases the activity of the "direct" and "indirect" striatal outflow pathways in response to D2 receptor stimulation. These changes have the net result of enhancing thalamocortical activity and likely underlie the enhanced contralateral rotation produced by quinpirole in apomorphine-primed rats. Changes in striatal outflow, particularly in the striato-nigro-entopeduncular pathway, may contribute to alterations in D2-dependent motor responses observed after chronic dopaminergic stimulation in the dopamine-depleted striatum.

Cabergoline, a long-acting dopamine D2-like receptor agonist, produces a sustained antiparkinsonian effect with transient dyskinesias in parkinsonian drug-naive primates

Continuous dopaminergic receptor stimulation is now considered as an interesting approach for the control of motor complications often seen in parkinsonian patients treated chronically with levodopa. Cabergoline, which is a long-acting dopamine D2-like receptor agonist, has been tried recently with good results as an adjunct in patients already on levodopa-therapy. Thus, the present study was designed to test the effects of repeated s.c. administration of cabergoline as sole therapeutic agent during a month in 3 drug-naive MPTP parkinsonian monkeys to see whether or not cabergoline, given every other day at 0.25 mg/kg, would have a sustained antiparkinsonian effect and would induce dyskinesias. The animals were rated to quantify the antiparkinsonian as well as the dyskinetic response and gross locomotor activity was monitored by photocells. The averaged locomotor response, initially greatly increased (approximately 9 times higher than after saline treatment in the same animals), decreased by approximately 50% after 2 weeks but was thereafter maintained at this level until the end of the study. The parkinsonian features were improved in a sustained manner in all monkeys and transient dyskinesias (week 1 and 2) were present in 2 of 3 monkeys. After sacrifice receptor binding assays were performed on striatal and pallidal tissues homogenates with tritiated selective ligands and compared with those of 3 normal and 3 MPTP-exposed monkeys otherwise untreated. A significant decrease in dopamine D2-like receptor density in the putamen (-36% on average vs. untreated MPTP-exposed monkeys) may be involved in the behavioral partial tolerance to antiparkinsonian effect of cabergoline and the disappearance of dyskinesias. A reversal of the supersensitivity of GABAA receptor in the internal segment of the globus pallidus (-15% on average vs. untreated MPTP-exposed monkeys) may also be implicated in this latter behavioral effect.

Unilateral 6-hydroxydopamine lesions of meso-striatal dopamine neurons and their physiological sequelae

One of the primary approaches in experimental brain research is to investigate the effects of specific destruction of its parts. Here, several neurotoxins are available which can be used to eliminate neurons of a certain neurochemical type or family. With respect to the study of dopamine neurons in the brain, especially within the basal ganglia, the neurotoxin 6-hydroxydopamine (6-OHDA) provides an important tool. The most common version of lesion induced with this toxin is the unilateral lesion placed in the area of mesencephalic dopamine somata or their ascending fibers, which leads to a lateralized loss of striatal dopamine. This approach has contributed to neuroscientific knowledge at the basic and clinical levels, since it has been used to clarify the neuroanatomy, neurochemistry, and electrophysiology of mesencephalic dopamine neurons and their relationships with the basal ganglia. Furthermore, unilateral 6-OHDA lesions have been used to investigate the role of these dopamine neurons with respect to behavior, and to examine the brain's capacity to recover from or compensate for specific neurochemical depletions. Finally, in clinically-oriented research, the lesion has been used to model aspects of Parkinson's disease, a human neurodegenerative disease which is neuronally characterized by a severe loss of the meso-striatal dopamine neurons. In the present review, which is the first of two, the lesion's effects on physiological parameters are being dealt with, including histological manifestations, effects on dopaminergic measures, other neurotransmitters (e.g. GABA, acetylcholine, glutamate), neuromodulators (e.g. neuropeptides, neurotrophins), electrophysiological activity, and measures of energy consumption. The findings are being discussed especially in relation to time after lesion and in relation to lesion severeness, that is, the differential role of total versus partial depletions of dopamine and the possible mechanisms of compensation. Finally, the advantages and possible drawbacks of such a lateralized lesion model are discussed.

Levodopa or D2 agonist induced dyskinesia in MPTP monkeys: correlation with changes in dopamine and GABAA receptors in the striatopallidal complex

Dopamine D1 and D2 receptors as well as the GABA/benzodiazepine receptor complex in the striatum and the globus pallidus (internal: GPi and external: GPe) were studied by autoradiography using [3H]SCH 23390, [3H]spiperone, and [3H]flunitrazepam ([3H]FNZ) respectively, in five groups of cynomolgus monkeys. These included (i) untreated 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-monkeys; (ii) MPTP monkeys treated chronically with levodopa injections; (iii) MPTP monkeys treated chronically with injections of the novel D2 agonist U91356A; (iv) MPTP monkeys treated chronically with U91356A delivered through an osmotic mini-pump; and (5) naive controls. Animals treated in a pulsatile mode with U91356A or levodopa injections showed progressive sensitization to their respective drug and developed choreic dyskinesia. In contrast, animals treated in a continuous mode with U91356A showed behavioral tolerance but did not develop dyskinesia. A trend for a down-regulation of putaminal D2 receptors was observed following D2 agonist stimulation with U913356A. Striatal [3H]FNZ binding was significantly decreased only in animals treated in a continuous mode with U91356A. The dopamine receptor decrease in the striatum could be implicated with the development of tolerance but cannot explain the appearance of dyskinesia. Denervation by MPTP was associated with a decrease of the GPe/GPi [3H]FNZ binding ratio which reflects an imbalance of striatal output pathways; this ratio was not reversed by any of the treatments although changes were observed in the GPe and GPi. Indeed, pulsatile U91356A treatment restored the decreased [3H]FNZ binding in the GPe near control values and levodopa showed a similar tendency. A significant increase of [3H]FNZ binding in the GPi only of dyskinetic monkeys, namely those treated with pulsatile U91356A or levodopa was seen compared to untreated MPTP or naive controls. This GABAA receptor up-regulation might lead to a supersensitive state of the GPi to gabaergic input which may be involved in the mechanism underlying the development of dopaminomimetic-induced dyskinesia.

Chronic L-DOPA treatment differentially regulates gene expression of glutamate decarboxylase, preproenkephalin and preprotachykinin in the striatum of 6-hydroxydopamine-lesioned rat

The effect of a unilateral 6-hydroxydopamine lesion of the medial forebrain bundle in rats and subsequent L-DOPA treatment for eight weeks on preproenkephalin, preprotachykinin and glutamate decarboxylase (M(r) 67,000) gene expression in the striatum was investigated by in situ hybridization. A 6-hydroxydopamine lesion of the medial forebrain bundle markedly increased the level of preproenkephalin messenger RNA (+66%) and modestly elevated the level of glutamate decarboxylase (M(r) 67,000) messenger RNA (+36%) in the denervated striatum, but caused a decrease in the level of preprotachykinin messenger RNA (-54%) relative to the intact striatum and to sham-lesioned control animals. Treatment with L-DOPA (200 mg/kg/24 h) for eight weeks reduced but did not abolish the 6-hydroxydopamine lesion-induced elevation of preproenkephalin messenger RNA and slightly reduced the elevation of glutamate decarboxylase (M(r) 67,000) messenger RNA in denervated striatum relative to intact side and control groups. However, L-DOPA treatment almost completely reversed the decrease in preprotachykinin messenger RNA caused by 6-hydroxydopamine lesioning when compared to intact side and control groups. The effect of L-DOPA on the gene expression of preproenkephalin and glutamate decarboxylase (M(r) 67,000) differs from the increase in striatal enkephalin content and glutamate decarboxylase activity previously found following L-DOPA treatment. In contrast, L-DOPA reversed the changes in preprotachykinin messenger RNA, reflecting previously reported increases in substance P content. The findings provide new evidence that chronic L-DOPA treatment differentially affects direct striatonigral and indirect striatopallidal pathways at the molecular level.