Alterations in striatal and extrastriatal D-1 and D-2 dopamine receptors in the MPTP-treated common marmoset: An autoradiographic study

In adult common marmosets (Callithrix jacchus), MPTP (1-methyl-4-phenyl-1, 2, 3, 6-tetrahydropyridine) treatment induced almost total depletion of cells in the substantia nigra pars compacts (SNc) but partial cell loss in the ventral tegmental area (VTA). There was severe depletion of [³H]-mazindol binding to dopamine (DA) uptake sites in the caudate, putamen, and SNc. The loss of [³H]-mazindol binding in the nucleus accumbens (NAc) and olfactory tubercle (OT) was less marked. [³H]-mazindol binding in the body of caudate nucleus showed a small but significant recovery with increasing post-lesion survival times. The specific binding of [³H]-SCH 23390 to D-1 DA receptor sites was increased after MPTP treatment in all subregions of both caudate and putamen but was unaltered in the NAc and OT. Substantia nigra pars reticulata (SNr), frontal cortex, and medial segment of globus pallidus (GPm) all demonstrated moderate levels of [³H]-SCH 23390 binding in control animals, which were unaffected by MPTP treatment. Specific [³H]-spiperone binding to D-2 DA receptor sites was not altered by MPTP treatment in the subregions of caudate-putamen. Moderate levels of [³H]-spiperone binding were observed in control animals in the NAc, OT, SNc, and the lateral segment of globus pallidus (GPl). [³H]-spiperone binding in the SNc and OT was partially decreased in MPTP-treated animals. The changes in specific [³H]-spiperone and [³H]-SCH 23390 binding induced by MPTP-treatment did not alter with post-lesion survival times. These results demonstrate that MPTP treatment causes greater dopaminergic denervation of the caudate-putamen than in NAc/OT. This resulted in an increase in postsynaptic D-1 DA receptor sites in the caudate-putamen but not in the NAc/OT. Also, there appeared to be loss of presynaptic D-2 DA receptic sites in the SNc and OT. In the caudate-putamen, the loss of presynaptic D-2 DA receptor sites may have masked postsynaptic D-2 DA receptor upregulation. © 1993 Wiley-Liss, Inc.     

COX-2-deficient mice are less prone to MPTP-neurotoxicity than wild-type mice

The primary lesion in Parkinson's disease is the death of dopaminergic neurons in the substantia nigra. The role of cyclooxygenase (COX)-2 in the etiology of Parkinson's disease was explored using COX-2 gene knockout mice. Mortality after injection of 1-methyl-4-phenyl-1,2,3,6 tetrahydropyridine (MPTP, a chemical known to cause parkinsonism in humans) in heterozygous COX-2-deficient mice was lower than that in wild-type mice. The number of tyrosine hydroxylase immunoreactive neurons in the substantia nigra pars compacta of MPTP-treated wild-type mice declined to a greater extent than in heterozygous mice. Inhibition of COX-2 protein expression decreased the lesion caused by MPTP and protected the dopaminergic neurons in substantia nigra pars compacta. This result suggested that inhibition of COX-2 has potential therapeutic implications.     

Excitotoxic action of NMDA agonists on nigrostriatal dopaminergic neurons: modulation by inhibition of nitric oxide synthesis

Focal infusions of N-methyl-d-aspartate (NMDA) or an endogenous NMDA agonist, quinolinic acid (QUIN), into the substantia nigra pars compacta (SNc) of adult Sprague-Dawley rats resulted in a dose-dependent depletion of ipsilateral striatal tyrosine hydroxylase (TH) activity, a biochemical marker for dopaminergic neurons. To assess the intermediary role of nitric oxide in the neurotoxicity elicited by these toxins, their action was tested in animals treated with N^ω-nitro-l-arginine methyl ester (L-NAME). Systemic injections (2 injections; 8 h apart) of L-NAME (100, 150 and 250 mg/kg) produced a dose-related inhibition of cerebellar nitric oxide synthase (NOS) activity. The time-course of cerebellar NOS inhibition following L-NAME (250 mg/kg) was rapid in onset and lasted for at least 24 h following the second injection. An L-NAME treatment regimen of 250 mg/kg, with the second injection given 24 h prior to assessment of NOS activity, produced an 87 and 91% inhibition of cerebellar and nigral NOS activity, respectively. Intranigral infusion of 40 and 60 nmol QUIN reduced ipsilateral striatal TH activity by 62 and 75%, respectively. However, 40 and 60 nmol QUIN infusions into animals pretreated with L-NAME (250 mg/kg) reduced striatal TH activity by 83 and 96%, respectively. Intranigral infusion of 15 and 30 nmol NMDA produced a 48 and 77% decrease in striatal TH activity, respectively, whereas the same doses of NMDA given to animals pretreated with L-NAME (250 mg/kg) resulted in a 59 and 88% decrease in TH activity. Thus, both QUIN and NMDA toxicity was enhanced following L-NAME pretreatment. The destruction of the nigrostriatal pathway was verified using TH immunocytochemistry of the SNc. It was also observed that a low dose of L-NAME (1.5 mg/kg), previously shown to be neuroprotective in cerebral ischemic damage, did not influence NMDA (15 nmol) neurotoxicity. The results of this study show that extensive inhibition of NOS activity enhances NMDA receptor-mediated excitoxicity.     

EGb761 protects against nigrostriatal dopaminergic neurotoxicity in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-induced Parkinsonism in mice: role of oxidative stress

1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) causes nigrostriatal dopaminergic neurotoxicity and behavioral impairment in rodents. Previous studies suggest that oxidative stress, via free radical production, is involved in MPTP-induced neurotoxicity. The MPTP-treated mouse has been the most widely used model for assessing neuroprotective agents for Parkinson's disease. It has been reported previously that EGb761 prevents dopaminergic neurotoxicity of MPTP. This compound is multifunctional via different mechanisms. Here, we report the neuroprotective effect of EGb761 against oxidative stress induced by MPTP in C57BL/6J mice. EGb761 is a patented and well-defined mixture of active compounds extracted from Ginkgo biloba leaves, with neuroprotective effects, exerted probably via its antioxidant or free radical scavenger action. MPTP administration resulted in a significant decrease in striatal dopamine levels and tyrosine hydroxylase immunostaining in the striatum and substantia nigra pars compacta. Mice receiving EGb761 had significantly attenuated MPTP-induced loss of striatal dopamine levels and tyrosine hydroxylase immunostaining in the striatum and substantia nigra pars compacta. The neuroprotective effect of EGb761 against MPTP neurotoxicity is associated with blockade of lipid peroxidation and reduction of superoxide radical production (indicated by a down-regulation of Mn-superoxide dismutase activity), both of which are indices of oxidative stress. Behavioral analyses showed that EGb761 improved MPTP-induced impairment of locomotion in a manner that correlated with enhancement of striatal dopamine levels. These findings suggest that, in mice, EGb761 attenuates MPTP-induced neurodegeneration of the nigrostriatal pathway and that an inhibitory effect against oxidative stress may be partly responsible for its observed neuroprotective effects.     

Alterations in dopamine uptake sites and D1 and D2 receptors in cats symptomatic for and recovered from experimental parkinsonism

The administration of the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) to adult cats severely disrupts the dopaminergic innervation of the striatum. Animals display a parkinson-like syndrome, consisting of akinesia, bradykinesia, postural instability, and rigidity, which spontaneously recovers by 4–6 weeks after the last administration of MPTP. In this study we used quantitative receptor autoradiography to examine changes in DA uptake sites and DA receptors in the basal ganglia of normal, and symptomatic and recovered MPTP-treated cats. Consistent with the destruction of the nigrostriatal DA pathway, there was a severe loss of DA uptake sites, labeled with [³H]-mazindol, in the caudate nucleus (64–82%), nucleus accumbens (44%), putamen (63%), and substantia nigra pars compacta (SNc, 53%) of symptomatic cats. Following behavioral recovery, there were no significant changes in DA uptake site density. Significant increases of [³H]-SCH 23390 binding to D1 DA receptors were observed in the dorsal caudate (>24%; P < 0.05) of symptomatic cats and in all regions of the caudate-putamen (>30%; P < 0.05) of recovered animals. [³H]-SCH 23390 binding in tree substantia nigra pars reticulata was half of that in the striatum and showed no changes in symptomatic or recovered animals. No alterations in the binding of [¹²⁵1]-epidepride to D2 receptors was observed in any region of the striatum in either, symptomatic or recovered animals. [¹²⁵1]-Epidepride binding in the SNc was decreased by >36% (P < 0.05) following MPTP treatment. These data show that cats made parkinsonian by MPTP exposure have a significant decrease in the number of DA reuptake sites throughout the striatum and that recovery of sensorimotor function in these animals is not correlated with an increase in the number of striatal reuptake sites. Behavioral recovery, however, does seem to be correlated with a general elevation of Dl receptors throughout the striatal complex. The present data also show that direct correlations between changes in DA receptor regulation after a large DA depleting lesion and behavioral deficits or recovery from those deficits are difficult and that the relationships between DA receptors/transporters and behavior require further study. © 1995 Wiley-Liss, Inc.     

Cerebral alterations in a MPTP-mouse model of Parkinson’s disease – an immunocytochemical study

Summary. We investigated the immunohistochemical alterations of neuronal nitric oxide synthase (nNOS), endothelial NOS (eNOS), tyrosine hydroxylase (TH), microtuble-associated protein 2a,b (MAP 2), glial fibrillary acidic protein (GFAP), parvalbumin (PV), and dopamine transporter (DAT) in the striatum and substantia nigra following the application of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) in mice. TH-, MAP 2- and DAT-immunoreactive cells were decreased gradually in the striatum and substantia nigra from 1 day up to 7 days after MPTP treatment, as well as the reduction of the striatal dopamine, DOPAC and HVA content. The number of GFAP-immunoreactive astrocytes increased gradually in the striatum and substantia nigra from 1 day up to 7 days after MPTP treatment. Striatal nNOS-immunoreactive cells were unchanged in MPTP-treated mice. In the substantia nigra, intense immunoreactivity of nNOS-positive cells increased 5hr after MPTP treatment. Thereafter, the immunoreactivity of nNOS-positive cells decreased gradually from 1 day up to 7 days after MPTP treatment. eNOS-immunopositive cells were unchanged in the striatum and substantia nigra. These results demonstrate that nNOS may play a key role in the development of MPTP neurotoxicity. Our findings also indicate that MPTP can cause the functional damage of interneurons in the substantia nigra, but not in the striatum.Received January 30, 2003; accepted May 14, 2003 Published online August 13, 2003     

Selective loss of subpopulations of ventral mesencephalic dopaminergic neurons in the monkey following exposure to MPTP

Tyrosine hydroxylase immunohistochemical examination of the mesencephalon of severely parkinsonian MPTP-treated macaque fascicularis monkeys revealed a marked loss of substantia nigra pars compacta (SNc) neurons in both medial and central portions of the nucleus with a relative sparing of neurons in the dorsal-most portions of the substantia nigra. These animals also sustained 20–65% loss of neurons in the substantia nigra pars lateralis area, ventral tegmental area (A-10), and the retrorubral area (A-8 cell group, and the parabrachialis pigmentosus region). These animals all had extreme striatal dopamine depletions. A monkey which received several small doses of MPTP and yet remained asypptomatic for a motor disorder (although it had demonstrable behavioral performance deficits) had only a loss only ventral SNc neurons, with no appreciable cells in associated ventral mesencephalic dopamine areas and no loss of striatal dopamine. These data suggest that the effects of MPTP are not as selective as originally thought and, more importantly, indicate that MPTP-induced parkinsonism in the primate may be more analogous to idiopathic Parkinson's disease, where cells other than SNc cells are affected. Furthermore, the present findings suggest that only certain mesencephalic dopamine neurons are susceptible to MPTP-induced damage. The unique characteristics of these neurons need to be elucidated.     

Significance of the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine for the etiology and therapy of idiopathic Parkinson disease

Exposure of drug addicts to MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) has caused a Parkinsonian syndrome accompanied by a selective destruction of dopamine containing neurones in the pars compacta of the substantia nigra. MPTP in the human causes a severe irreversible state that very closely resembles idiopathic Parkinson's disease both in its clinical features and response to pharmacological treatment. Interest in potential environmental agents that might play a role in the aetiology of idiopathic Parkinson's disease is likely to increase as the result of the discovery of the relatively simple molecule MPTP which is highly toxic to the substantia nigra. Until the discovery of the neurotoxicity of MPTP there was no effective animal model of Parkinson's disease. Administration of PTP to monkeys induces persistent parkinsonism which responds to classical antiparkinsonian therapy. The morphological and biochemical changes in the brains of the animals are more limited and selective than those seen in idiopathic Parkinson's disease. The model of MPTP-treated monkeys appears to provide a useful testbed for the evaluation of future treatments for the disease. The precise mechanism of MPTP toxicity has yet to be determined and may provide the clue to the mechanism of neuronal death in Parkinson's disease. After entering the brain MPTP is oxidized to MPP+ (1-methyl-4-phenylpyridine) at an extraneuronal site.(ABSTRACT TRUNCATED AT 250 WORDS)     

Unbiased morphometrical measurements show loss of pigmented nigral neurones with ageing

This study used the dissector method to evaluate pigmented nigral neuronal loss in the substantia nigra pars compacta with age. Dissector counts can be used to estimate the absolute and accurate total neurone numbers. In addition, the area and diameter of the neuronal cell body was estimated by using a computerized morphometric analysis in a single section of the substantia nigra pars compacta. Brain samples from 26 people with an age range from 17 to 90 years were studied. A significant decrease in the total number of pigmented neurones (r=-0.83, P<0.001) and their density (r=-0.83, P<0.001) with age was found in the substantia nigra pars compacta. The number of pigmented neurones counted from a single section also showed an age-dependent decline (r=-0.76, P<0.001). According to the regression equations, the total number of pigmented neurones estimated by dissector counts decreased by 9.8% per decade and the neuronal density decreased by 7.4% per decade. The area of the neuronal cell body decreased by 3.2% per decade. This latter change corresponds to an approximate 4.4% decrease per decade in neuronal volume. These findings show that both the number of pigmented neurones and their size in the substantia nigra pars compacta decreases with age. However, the reductions in the total number of pigmented neurones are more dramatic than the reduction in neurone size with ageing.     

Synaptic connections between pars compacta and pars reticulata neurones: electrophysiological evidence for functional modules within the substantia nigra

Intracellular recordings were performed in vitro from both pars compacta and pars reticulata neurones of the substantia nigra, and their postsynaptic responses to electrical stimulation within the nucleus were analysed. Intracellular staining by biocytin was used to reveal the morphology and location of the recorded neurone and its position and distance to the stimulating electrode. Inhibitory postsynaptic potentials in pars compacta neurones were evoked exclusively from a specific region of stimulation within the pars reticulata; this field could be mapped out as a cone-shaped region surrounding the apical dendrite of the cell and perpendicular to the plane of the pars compacta. Furthermore, hemitransection, prior to the experiments which eliminated the most likely source of extrinsic inhibition (the GABAergic striatonigral pathway) affected neither the generation of inhibitory postsynaptic potentials nor its topographic pattern during pars reticulata stimulation. In contrast to the response of dopaminergic pars compacta neurones, pars reticulata neurones responded to stimulation over wide areas of substantia nigra, without any clear site-specific selectivity. It is concluded that within the substantia nigra, dopaminergic neurones are arranged in functional modules such that most inhibition is derived from a highly local circuit with the collaterals of adjacent pars reticulata cells. This module is intrinsic to the substantia nigra and may represent the basic functional unit of the nucleus.     

Broad neuroprotective profile of nicotinamide in different mouse models of MPTP-induced parkinsonism

The factors contributing to substantia nigra pars compacta (SNc) dopamine (DA) neuron death and striatal DA depletion in Parkinson's disease (PD) are still poorly understood. However, mitochondrial dysfunction, cellular energy depletion and oxidative stress appear to play important roles in the pathogenesis of PD. In view of this, the current study examined the potential of nicotinamide, a form of the B-complex vitamin niacin, to protect against 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced SNc cell loss and striatal DA depletion in two mouse MPTP models that respond differently to putative neuroprotective agents. Adult male C57Bl/6 mice received nicotinamide (125, 250 or 500 mg/kg i.p.) prior to either acute (four injections in 1 day at 2-h intervals) or sub-acute (two injections per day at 4-h intervals for 5 days) MPTP administration. Striatal DA levels, changes in numbers of tyrosine hydroxylase (TH)- and cresyl violet-stained cells in the SNc at 2 and 6 weeks following the last MPTP exposure were analyzed. Nicotinamide administration resulted in a dose-dependent sparing of striatal DA levels and SNc neurons in acute MPTP-treated animals. Only the highest dose of nicotinamide had similar effects in sub-acute MPTP-treated animals. At 6 weeks after MPTP exposure, there was some spontaneous recovery of striatal DA levels in both models: neuroprotective effects were still apparent in acute but not sub-acute MPTP-treated animals. These results show neuroprotective effects of nicotinamide in different mouse Parkinson models associated with different forms of cell death and suggest that nicotinamide may have broad neuroprotective potential in PD.     

Tripchlorolide protects against MPTP-induced neurotoxicity in C57BL/6 mice

Many current studies of Parkinson's disease (PD) suggest that inflammation is involved in the neurodegenerative process. Tripchlorolide (TW397), a traditional Chinese herbal compound with anti-inflammatory and immunosuppressive properties, has been shown to protect dopaminergic neurons against, and restore their function after, the neurotoxicity induced by 1-methyl-4-phenylpyridinium ions in vitro. This study was designed to investigate the effect of TW397 in vivo in the PD model of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-lesioned C57BL/6 mice. In the animals that received vehicle-only (i.e., no TW397) treatment with MPTP i.p. injection, the survival ratios of tyrosine hydroxylase-immunoreactive (TH-IR) neurons in the substantia nigra pars compacta and TH-IR fibres in the striatum were only 59 and 13%, respectively, compared with the normal controls. Intriguingly, in conjunction with MPTP, treatment with TW397, 1 microg/kg for 16 days, once per day, dramatically improved the survival rate of the TH-IR neurons and TH-IR fibres to 80 and 43% of the control. The treatment with TW397 also significantly improved the level of dopamine in the substantia nigra and striatum to 157 and 191%, respectively, of the MPTP- plus vehicle-treated group. In addition, in MPTP-treated animals the rota-rod performances of those treated with 0.5 or 1 microg/kg TW397 were significantly improved, by approximately 2- and 3-fold, respectively, relative to vehicle-treated animals. The neuroprotective effect of TW397 was coincident with an attenuated astroglial response within the striatum. These data demonstrate a neuroprotective action of TW397 in vivo against MPTP toxicity, with important implications for the treatment of PD.     

Cell loss and class distribution of TH-I cells in the substantia nigra of the neurological mutant, weaver

Eight-week-old homozygous weaver mutant mice and littermate wildtype controls were perfused with a buffered acrolein and paraformaldehyde solution. The brains were subsequently removed, blocked and sectioned on a vibratome. Representative sections through the midbrain were incubated overnight with an antibody for tyrosine hydroxylase. Visualization of the antibody was achieved using the peroxidase-antiperoxidase technique. The immunoreactive cells in the substantia nigra were examined to determine the subclasses of cells that are affected when dopaminergic neurons are lost in these mutants. Class distributions were determined and the data were subjected to χ² analyses. The results indicate a significant loss of tyrosine hydroxylase-immunoreactive cells in both the pars compacta and pars lateralis that is dependent upon the group and the region being studied. In addition, distributional shifts within the classes of labeled neurons suggest that there is an increase in the small neurons over the expected numbers in both divisions of the substantia nigra.     

Plasticity of the nigrostriatal system in MPTP-treated mice

In order to compare the recovery capacity of the nigrostriatal system between adult and old mice, MPTP hydrochloride was administered to 48 BL/C57 male mice, which were sacrificed 24 h or 10 d after the second dose. The animals were divided into four groups, based on age (adult or old) and moment of sacrifice (24 h or 10 d). The detailed morphology of the neurons and the cellular processes of the substantia nigra pars compacta and the striatum were studied using the Golgi method. Immunostaining with a polyclonal glial fibrillary acidic protein antiserum using the peroxidase-antiperoxidase technique was performed to study the glial response. Striatal catecholamines were determined to correlate the biochemical data with the morphological changes. Significant neuronal changes of cellular processes were observed in substantia nigra pars compacta from all MPTP-treated mice, consisting of swelling and distortion of cellular bodies, discontinuous thickness, and nodulations of dendrites with baded aspect. Axons showing focal swelling and nodulations were also found in the neuropil of silver impregnated striata. Marked gliosis with reactive astrocytes in substantia nigra and striatum from all the old treated mice was found. Recovery was only observed in adult mice sacrificed 10 d after withdrawal. At this time, all the old MPTP-treated mice showed marked neuronal changes and a persistent marked gliosis. As expected, 24 h after the MPTP treatment, a marked depletion of dopamine and its metabolites was found in all the animals; at 10 d, the depletion was partially reversed in the adult group. These data correlate well with the observed morphological changes. Our results suggest that, in mice, deterioration of dendritic and axonal neuropil constitutes a significant causal factor of the MPTP neurotoxicity. These features are related to the age of the animals and the integrity of the plasticity phenomena, which appear to be altered in old mice.     

Matrix metalloproteinase-9 is elevated in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-induced parkinsonism in mice

Matrix metalloproteinases (MMPs) are proteolytic enzymes capable of degrading components of the extracellular matrix. Recent evidence has implicated MMPs in the pathogenesis of neurodegenerative diseases as Alzheimer’s disease and amyotrophic lateral sclerosis. In this study, we investigated the involvement of MMP-9 (gelatinase B) in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) mouse model of Parkinson’s disease using zymography, immunohistochemistry, and Western blot analysis. The activity of MMP-9 was upregulated at 3 h after MPTP injection in the striatum and after 24 h in the substantia nigra. Although MMP-9 expression decreased in the striatum by 72 h, it remained elevated in the substantia nigra compared to controls up to 7 d after MPTP administration. Immunohistochemistry showed that neurons and microglia are the source of MMP-9 expression after MPTP administration to mice. Treatment with a hydroxamate-based MMP inhibitor, Ro 28-2653 significantly reduced dopamine depletion and loss of tyrosine hydroxylase immunoreactive neurons in the substantia nigra pars compacta. MMP-9 expression as measured via zymography in the substantia nigra was reduced by the MMP inhibitor. These results indicate that MMP-9 is induced after MPTP application in mice and that pharmacologic inhibition of MMPs protects against MPTP neurotoxicity.     

Profiling changes in gait dynamics resulting from progressive 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-induced nigrostriatal lesioning

Current behavioral measurements for motor impairment are not consistently sensitive in rodent models of partial nigrostriatal dopamine (DA) depletion. In addition to exploratory and somatosensory behavior, motor skills that are thought to be directly translatable to human Parkinson's disease patients are assessed. However, many of these motor tests require the training and learning of particular tasks, so it cannot be determined whether impairments are due to motor or to learning deficit. Therefore, we have quantified multiple temporal and spatial indices of gait dynamics in a model of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced partial nigrostriatal lesioning using a treadmill apparatus requiring no prior training. Three days following the cessation of progressively increased MPTP administration, rearing and foot-fault behaviors showed significant deficit. Ten days after the final MPTP injection, gait dynamics were assessed and indicated differences between MPTP- and vehicle-treated animals. The major significant changes were in stride length, frequency, duration, and number of steps. Three weeks following a progressively increased dose of MPTP (administered 5 days per week over the course of 4 weeks), mice showed a 63% decrease in tyrosine hydroxylase-immunoreactive (TH-ir) nigrostriatal neurons in the substantia nigra pars compacta and a 72% decrease in TH-ir terminals in the caudate-putamen. This suggests that there is a continued effect of progressively increased MPTP on nigrostriatal DA neurons, correlated with rearing and foot-fault behaviors and further characterized by differences in temporal and spatial measurements of gait dynamics.     

Functional heterogeneity of NMDA receptors in rat substantia nigra pars compacta and reticulata neurones

The nigra substantia nigra pars compacta (SNc) and substantia pars reticulata (SNr) form two major basal ganglia components with different functional roles. SNc dopaminergic (DA) neurones are vulnerable to cell death in Parkinson’s disease, and NMDA receptor activation is a potential contributing mechanism. We have investigated the sensitivity of whole‐cell and synaptic NMDA responses to intracellular ATP and GTP application in the SNc and SNr from rats on postnatal day (P) 7 and P28. Both NMDA current density (pA/pF) and desensitization to prolonged or repeated NMDA application were greater in the SNr than in the SNc. When ATP levels were not supplemented, responses to prolonged NMDA administration desensitized in P7 SNc DA neurones but not at P28. At P28, SNr neurones desensitized more than SNc neurones, with or without added ATP. Responses to brief NMDA applications and synaptic NMDA currents were not sensitive to inclusion of ATP in the pipette solution. To investigate these differences between the SNc and SNr, NR2 subunit‐selective antagonists were tested. NMDA currents were inhibited by ifenprodil (10 μm ) and UBP141 (4 μm ), but not by Zn²⁺ (100 nm ), in both the SNr and SNc, suggesting that SNc and SNr neurones express similar receptor subunits; NR2B and NR2D, but not NR2A. The different NMDA response properties in the SNc and SNr may be caused by differences in receptor modulation and/or trafficking. The vulnerability of SNc DA neurones to cell death is not correlated with NMDA current density or receptor subtypes, but could in part be related to inadequate NMDA receptor desensitization.     

Estrogen down-regulates glial activation in male mice following 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine intoxication

Emerging evidence suggests beneficial effect of estrogen for Parkinson's disease (PD), yet the exact mechanisms implicated remain obscured. Activated glia observed in MPTP mouse model and in PD may participate in the cascade of deleterious events that ultimately leads to dopaminergic nigral neuronal death. In vitro studies demonstrate that estrogen can modify the microglial and astroglial expression of inflammatory mediator, such as cytokines and chemokines implicated in neuroinflammation and neurodegeneration. To determine whether estrogen-elicited neuroprotection in PD is mediated through glia, adult male C57Bl/6 mice were treated with 17beta-estradiol (E2) for a total of 11 days. Following 5 days of pretreatment with E2, they were injected with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) on the sixth day. The brains were collected on day 11. Immunohistochemistry and quantitative study were used to assess the number of tyrosine hydroxylase-immunoreactive (TH-IR) neurons in the substantia nigra pars compacta (SNpc) and that of activated astrocytes and activated microglia in the SNpc and the striatum. Pretreatment with E2 decreased the loss of TH-IR nigral neurons and diminished the deficit of TH-IR striatal fibers triggered by MPTP. The neuroprotective effect of E2 was coincident with an attenuation of a glial response within the nigra and the striatum. These findings suggest that the neuroprotective effects of E2 evidenced in MPTP mouse model might mediate through an inhibition of reactive glia. However, direct neuroprotective effects of E2 upon TH-IR neurons cannot be excluded.     

In vivo changes of catecholamines in hemiparkinsonian monkeys measured by microdialysis.

In monkeys, unilateral intracarotid infusion of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) produces a useful model of hemiparkinsonism. To evaluate MPTP-induced neurochemical changes in vivo, brain microdialysis was employed to measure extracellular levels of dopamine and its metabolites in the neostriatum of normal and hemiparkinsonian rhesus monkeys (Macaca mulatta). The microdialysis probes were implanted bilaterally into the caudate nucleus and putamen at coordinates determined from magnetic resonance imaging. Dopamine and its metabolites were depleted in the MPTP-lesioned side versus the unlesioned side in hemiparkinsonian monkeys. Tyrosine hydroxylase immunocytochemistry revealed a complete unilateral denervation in the caudate nucleus and putamen and a total loss of tyrosine hydroxylase-immunoreactive cells in the substantia nigra pars compacta in those monkeys. Baseline levels of amines in the neostriatum in normal monkeys were not significantly different from those in the normal (non-MPTP-treated) side in hemiparkinsonian monkeys. These data demonstrate that brain microdialysis is a valuable tool for measuring in vivo neurochemical changes in nonhuman primate brains.     

Evidence for crossed catecholaminergic nigrostriatal projections by combining wheat germ agglutinin-horseradish peroxidase retrograde transport and tyrosine hydroxylase immunocytochemistry

After unilateral injections of wheat germ agglutinin-horseradish peroxidase into the rat caudate-putamen, a few retrogradely labeled neurons were found in the contralateral pars compacta of the substantia nigra. These contralaterally projecting nigral cell bodies also immunoreacted positively to a specific tyrosine hydroxylase antiserum. We conclude that crossed catecholaminergic nigrostriatal projections may contribute to the reciprocal regulation exerted by the two nigrostriatal dopaminergic systems.