Short- and long-term changes in striatal and extrastriatal dopamine uptake sites in the MPTP-treated common marmoset

Progression and recovery of Parkinsonism in a chronic progressive MPTP-induction model in the marmoset without persistent molecular and cellular damage

Chronic exposure to low-dose 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) in marmoset monkeys was used to model the prodromal stage of Parkinson's disease (PD), and to investigate mechanisms underlying disease progression and recovery. Marmosets were subcutaneously injected with MPTP for a period of 12weeks, 0.5mg/kg once per week, and clinical signs of Parkinsonism, motor- and non-motor behaviors were recorded before, during and after exposure. In addition, postmortem immunohistochemistry and proteomics analysis were performed. MPTP-induced parkinsonian clinical symptoms increased in severity during exposure, and recovered after MPTP administration was ended. Postmortem analyses, after the recovery period, revealed no alteration of the number and sizes of tyrosine hydroxylase (TH)-positive dopamine (DA) neurons in the substantia nigra. Also levels of TH in putamen and caudate nucleus were unaltered, no differences were observed in DA, serotonin or nor-adrenalin levels in the caudate nucleus, and proteomics analysis revealed no global changes in protein expression in these brain areas between treatment groups. Our findings indicate that parkinsonian symptoms can occur without detectable damage at the cellular or molecular level. Moreover, we show that parkinsonian symptoms may be reversible when diagnosed and treated early.

Parameters of haematology, clinical chemistry and lipid metabolism in the common marmoset and alterations under stress conditions

Common marmosets are suitable non-human primate models for many human diseases. Standard values for blood parameters are required to evaluate physiological and pathological situations. Two studies were conducted: study I to determine standard values and study II to examine these under changed housing conditions. In study I, all parameters for clinical chemistry were similar in range for both genders with these specifics: male marmosets had significantly higher total and LDL cholesterol levels than females, whereas the mean corpuscular volume and the mean corpuscular haemoglobin were significantly lower than in females. In study II, glucose, lymphocytes and salivary cortisol were significantly lower, and faecal cortisol was increased during the change of housing conditions. In conclusion, standard values for haematology and clinical chemistry for the common marmoset were determined. Further on, parameters that are influenced by relocation stress and its importance for experimental results are described.

Extrastriatal dopaminergic circuits of the Basal Ganglia

The basal ganglia are comprised of the striatum, the external and internal segment of the globus pallidus (GPe and GPi, respectively), the subthalamic nucleus (STN), and the substantia nigra pars compacta and reticulata (SNc and SNr, respectively). Dopamine has long been identified as an important modulator of basal ganglia function in the striatum, and disturbances of striatal dopaminergic transmission have been implicated in diseases such as Parkinson's disease (PD), addiction and attention deficit hyperactivity disorder. However, recent evidence suggests that dopamine may also modulate basal ganglia function at sites outside of the striatum, and that changes in dopaminergic transmission at these sites may contribute to the symptoms of PD and other neuropsychiatric disorders. This review summarizes the current knowledge of the anatomy, functional effects and behavioral consequences of the dopaminergic innervation to the GPe, GPi, STN, and SNr. Further insights into the dopaminergic modulation of basal ganglia function at extrastriatal sites may provide us with opportunities to develop new and more specific strategies for treating disorders of basal ganglia dysfunction.

Activation of nigral and pallidal dopamine D1-like receptors modulates basal ganglia outflow in monkeys

Studies of the effects of dopamine in the basal ganglia have focused on the striatum, whereas the functions of dopamine released in the internal pallidal segment (GPi) or in the substantia nigra pars reticulata (SNr) have received less attention. Anatomic and biochemical investigations have demonstrated the presence of dopamine D1-like receptors (D1LRs) in GPi and SNr, which are primarily located on axons and axon terminals of the GABAergic striatopallidal and striatonigral afferents. Our experiments assessed the effects of D1LR ligands in GPi and SNr on local gamma-aminobutyric acid (GABA) levels and neuronal activity in these nuclei in rhesus monkeys. Microinjections of the D1LR receptor agonist SKF82958 into GPi and SNr significantly reduced discharge rates in GPi and SNr, whereas injections of the D1LR antagonist SCH23390 increased firing in the majority of GPi neurons. D1LR activation also increased bursting and oscillations in neuronal discharge in the 3- to 15-Hz band in both structures, whereas D1LR blockade had the opposite effects in GPi. Microdialysis measurements of GABA concentrations in GPi and SNr showed that the D1LR agonist increased the level of the transmitter. Both findings are compatible with the hypothesis that D1LR activation leads to GABA release from striatopallidal or striatonigral afferents, which may secondarily reduce firing of basal ganglia output neurons. The antagonist experiments suggest that a dopaminergic "tone" exists in GPi. Our results support the finding that D1LR activation may have powerful effects on GPi and SNr neurons and may mediate some of the effects of dopamine replacement therapies in Parkinson's disease.

Dieldrin exposure induces oxidative damage in the mouse nigrostriatal dopamine system

Numerous epidemiological studies have shown an association between pesticide exposure and an increased risk of developing Parkinson's disease (PD). Here, we provide evidence that the insecticide dieldrin causes specific oxidative damage in the nigrostriatal dopamine (DA) system. We report that exposure of mice to low levels of dieldrin for 30 days resulted in alterations in dopamine-handling as evidenced by a decrease in dopamine metabolites, DOPAC (31.7% decrease) and HVA (29.2% decrease) and significantly increased cysteinyl-catechol levels in the striatum. Furthermore, dieldrin resulted in a 53% decrease in total glutathione, an increase in the redox potential of glutathione, and a 90% increase in protein carbonyls. Alpha-synuclein protein expression was also significantly increased in the striatum (25% increase). Finally, dieldrin caused a significant decrease in striatal expression of the dopamine transporter as measured by (3)H-WIN 35,428 binding and (3)H-dopamine uptake. These alterations occurred in the absence of dopamine neuron loss in the substantia nigra pars compacta. These effects represent the ability of low doses of dieldrin to increase the vulnerability of nigrostriatal dopamine neurons by inducing oxidative stress and suggest that pesticide exposure may act as a promoter of PD.

1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine-induced acute transient dystonia in monkeys associated with low striatal dopamine

Unilateral intracarotid infusion of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) in baboons produces transient contralateral dystonia lasting 2-3 weeks followed by chronic hemiparkinsonism. We now extend this model to Macaca nemestrina and Macaca fascicularis. MPTP was infused unilaterally into the internal carotid artery of two M. nemestrina and 11 M. fascicularis. Effects were assessed with blinded clinical ratings of dystonia and Parkinsonism; [18F]-6-fluoro-DOPA (FDOPA) positron emission tomography; and postmortem measurements of striatal dopamine content. In two M. nemestrina, MPTP 0.4 mg/kg intracarotid produced acute dystonia within 24 h then chronic Parkinsonism starting 3 weeks later. In three M. fascicularis, MPTP 0.4 mg/kg produced acute dystonia within 3-8 h but two others died from large hemispheric infarcts within 1 day. A much lower dose, MPTP 0.1 mg/kg produced no clinical manifestations (n=1), whereas MPTP 0.25 mg/kg produced consistent transient dystonia and ipsiversive turning within 1-3 days followed by chronic Parkinsonism at 3 weeks (n=5). One week after MPTP, striatal FDOPA uptake decreased an average of 69% in M. nemestrina (0.4 mg/kg); and decreased an average of 70+/-21% in M. fascicularis (0.25 mg/kg). Striatal dopamine was reduced an average 66% in the first day (n=2) during acute dystonia, 98% at 3 days (n=1) and 99%+/-2.3% at 2-4 months (n=5). M. nemestrina had a clinical response similar to baboons whereas M. fascicularis seemed more sensitive to MPTP. These findings extend the model of MPTP-induced transient dystonia followed by chronic hemiparkinsonism to M. nemestrina and M. fascicularis and demonstrate that the early dystonic phase is accompanied by striatal dopamine deficiency.

Behavioral motor recovery in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-lesioned squirrel monkey (Saimiri sciureus): Changes in striatal dopamine and expression of tyrosine hydroxylase and dopamine transporter proteins

The neurotoxicant 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) provides an excellent opportunity to study repair and response to injury in the basal ganglia. Administration to mammals leads to the destruction of nigrostriatal dopaminergic neurons and depletion of striatal dopamine. In the squirrel monkey (Saimiri sciureus), MPTP-lesioning results in parkinsonian motor symptoms including bradykinesia, postural instability, and rigidity. Over time animals display motor behavioral recovery. To better understand this mechanism we employed a lesioning regimen of two or six subcutaneous injections of MPTP (2.0 mg/kg, free-base) to generate mild or moderate parkinsonism. Brain tissue was harvested at 6 weeks or 9 months after the last injection and analyzed for dopamine and its metabolites by high performance liquid chromatography (HPLC), and by immunohistochemical staining and Western immunoblotting for the expression of tyrosine hydroxylase (TH), dopamine transporter (DAT), and dopamine- and cAMP-responsive protein phosphatase of 32 kDa (DARPP-32), an effector molecule enriched in striatal medium spiny neurons. Several months after MPTP-lesioning, when squirrel monkeys displayed full motor behavioral recovery, striatal dopamine levels remained low with a greater return in the ventral striatum. This finding is consistent with other reports using neurotoxicant-lesioning models of the basal ganglia in rodents and other species of nonhuman primates. Elevated dopamine turnover ratio and decreased DAT expression appeared in early behavioral recovery at the 6-week time point in both mild- and moderate-parkinsonian monkeys. Tyrosine hydroxylase and DAT expression was increased in late stage recovery even within dopamine-depleted regions and supports sprouting. Altered DARPP-32 expression suggests a role of medium spiny neurons in recovery.

Timecourse of striatal re-innervation following lesions of dopaminergic SNpc neurons of the rat

Previously we described the extent of sprouting that axons of the rat substantia nigra pars compacta (SNpc) undergo to grow new synapses and re-innervate the dorsal striatum 16 weeks after partial lesions. Here we provide insights into the timing of events related to the re-innervation of the dorsal striatum by regenerating dopaminergic nigrostriatal axons over a 104-week period after partial SNpc lesioning. Density of dopamine transporter and tyrosine hydroxylase immunoreactive axonal varicosities (terminals) decreased up to 80% 4 weeks after lesioning but returned to normal by 16 weeks, unless SNpc lesions were greater than 75%. Neuronal tracer injections into the SNpc revealed a 119% increase in axon fibres (4 mm rostral to the SNpc) along the medial forebrain bundle 4 weeks after lesioning. SNpc cells underwent phenotypic changes. Four weeks after lesioning the proportion of SNpc neurons that expressed tyrosine hydroxylase fell from 90% to 38% but returned to 78% by 32 weeks. We discuss these phenotype changes in the context of neurogenesis. Significant reductions in dopamine levels in rats with medium (30-75%) lesions returned to normal by 16 weeks whereas recovery was not observed if lesions were larger than 75%. Finally, rotational behaviour of animals in response to amphetamine was examined. The clear rightward turning bias observed after 2 weeks recovered by 16 weeks in animals with medium (30-75%) lesions but was still present when lesions were larger. These studies provide insights into the processes that regulate sprouting responses in the central nervous system following injury.

Alterations in striatal neuropeptide mRNA produced by repeated administration of L-DOPA, ropinirole or bromocriptine correlate with dyskinesia induction in MPTP-treated common marmosets

Chronic administration of L-DOPA to MPTP-treated common marmosets induces marked dyskinesia while repeated administration of equivalent antiparkisonian doses of ropinirole and bromocriptine produces only mild involuntary movements. The occurrence of dyskinesia has been associated with an altered balance between the direct and indirect striatal output pathways. Using in situ hybridisation histochemistry, we now compare the effects of these drug treatments on striatal preproenkephalin-A (PPE-A) and adenosine A(2a) receptor mRNA expression as markers of the indirect pathway and striatal preprotachykinin (PPT) mRNA and preproenkephalin-B (PPE-B, prodynorphin) mRNA expression as markers of the direct pathway.The equivalent marked losses of specific [3H]mazindol binding in the striatum of all drug treatment groups confirmed the identical nature of the nigral cell loss produced by MPTP treatment. MPTP-induced destruction of the nigro-striatal pathway markedly increased the level of PPE-A mRNA in the caudate nucleus and putamen and decreased the levels of PPT and PPE-B mRNA relative to normal animals. Repeated treatment with L-DOPA for 30 days produced marked dyskinesia but had no effect on the MPTP-induced increase in PPE-A mRNA in the caudate nucleus and putamen. In contrast, L-DOPA treatment normalised the MPTP-induced decrease in the level of PPT and PPE-B mRNA. Repeated treatment with ropinirole produced little or no dyskinesia but markedly reversed the MPTP-induced elevation in PPE-A mRNA in the caudate nucleus and putamen. However, it had no effect on the decrease in PPT or PPE-B mRNA. Similarly, bromocriptine treatment which induced only mild dyskinesia attenuated the MPTP-induced elevation in PPE-A mRNA in the caudate nucleus and putamen with no effect on reduced striatal PPT or PPE-B mRNA. Neither MPTP treatment nor treatment with L-DOPA, bromocriptine or ropinirole had any effect on adenosine A(2a) receptor mRNA in the striatum. These patterns of alteration in striatal PPE-A and PPT and PPE-B mRNA produced by L-DOPA, bromocriptine and ropinirole show differential involvement of markers of the direct and indirect striatal output pathways related to improvement of locomotor activity and mirror the relative abilities of the drugs to induce dyskinesia.

Regional and subcellular compartmentation of the dopamine transporter and tyrosine hydroxylase in the rat ventral pallidum

The ventral pallidum (VP) is a major intermediary in the prefrontal cortical circuitry regulating sensorimotor gating and locomotor behavior, both of which are potently modulated by catecholamines. The VP catecholaminergic innervation is derived from midbrain dopaminergic neurons that differ in expression levels of the dopamine transporter (DAT) and from brainstem noradrenergic neurons without DAT. The preferentially low level of DAT in dopaminergic terminals in the prefrontal cortex and in striatal regions projecting more extensively to the VP medial (VPm) compared with VP lateral (VPl) compartment suggests possible region-specific differences in VP axonal distribution of DAT. To test this hypothesis, we examined the electron microscopic localization of DAT and the catecholamine-synthesizing enzyme, tyrosine hydroxylase (TH), in the VPm and VPl of rat brain. In both regions, DAT and TH were localized primarily in small unmyelinated axons and morphologically heterogeneous axon terminals. DAT-immunogold particles were few in number, but mostly located on the plasma membrane. In contrast, TH immunoreactivity was distributed in the cytoplasm of individual profiles, many of which were without detectable DAT. In comparison with TH, the mean area density of DAT-labeled axons was low throughout the VP. The mean area density of DAT-immunogold axon terminals, however, was significantly higher in VPl than in VPm, whereas that of TH-labeled axons was higher in VPm than in VPl. This dissociation suggests that, compared to the VPl, the VPm receives the greatest input from catecholaminergic afferents that are either nondopaminergic or characterized by having low levels or less terminal distributions of DAT.

Uptake of dopamine by cultured monkey amniotic epithelial cells

In this study, the ability of monkey amniotic epithelial (MAE) cells to take up dopamine was tested by incubating the cells in buffer containing unlabeled dopamine under different experimental conditions followed by assaying dopamine content using high performance liquid chromatography with electrochemical detection. Results showed the capability of MAE cells to take up dopamine in a time- and concentration-dependent fashion, and also this uptake is sodium-dependent. Further, selective dopamine transporter blockers inhibited dopamine uptake with rank order of potency that is consistent with the pharmacology of the dopamine transporter. These results suggest that MAE cells may be potential model to study dopamine uptake and release, and to explore new drugs affecting these processes.

Antiparkinsonian activity and dyskinesia risk of ropinirole and L-DOPA combination therapy in drug naïve MPTP-lesioned common marmosets (Callithrix jacchus)

De novo administration of long-acting dopamine agonists, such as ropinirole, to patients with Parkinson's disease or to 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated subhuman primates produces a lower incidence of dyskinesia than occurs with L-DOPA. This study compares the intensity of dyskinesia produced by combinations of L-DOPA and ropinirole and by these drugs alone, using the MPTP-treated common marmoset model of Parkinson's disease. The objective is to determine the optimum therapeutic strategy for the long-term control of Parkinson's disease with a minimal risk of dyskinesia. MPTP-treated marmosets received either L-DOPA alone, ropinirole alone, or one of two combinations of these drugs (either L-DOPA dominant or ropinirole dominant) daily for 28 days in doses titrated to produce a similar improvement in disability and increase in locomotion. In the group receiving L-DOPA alone, there was a trend for peak dose locomotor activity to increase and the duration of drug effect to decline over the period of the study. L-DOPA alone induced marked dyskinesia over the period of treatment, in contrast to ropinirole which produced a low intensity of involuntary movements. The L-DOPA dominant combination initially produced little dyskinesia, but this became increasingly intense as the study progressed. In contrast, the ropinirole dominant combination produced no greater intensity of dyskinesia than was produced by ropinirole alone. These data suggest that in early Parkinson's disease, the use of ropinirole alone or in combination with a low-dose L-DOPA might delay the induction of dyskinesias while improving motor performance.

GDNF reverses priming for dyskinesia in MPTP-treated, L-DOPA-primed common marmosets

Parkinson's disease (PD) is associated with a progressive loss of dopamine neurons in the substantia nigra and degeneration of dopaminergic terminals in the striatum. Although L-DOPA treatment provides the most effective symptomatic relief for PD it does not prevent the progression of the disease, and its long-term use is associated with the onset of dyskinesia. In rodent and primate studies, glial cell line-derived neurotrophic factor (GDNF) may prevent 6-OHDA- or MPTP-induced nigral degeneration and so may be beneficial in the treatment of PD. In this study, we investigate the effects of GDNF on the expression of dyskinesia in L-DOPA-primed MPTP-treated common marmosets, exhibiting dyskinesia. GDNF or saline was administered by two intraventricular injections, 4 weeks apart, to MPTP-treated, L-DOPA-treated common marmosets primed to exhibit dyskinesia. Prior to GDNF or saline administration, all animals displayed marked dyskinesia when treated with L-DOPA. GDNF administration produced a significant improvement in motor disability and, following the second injection of GDNF, a significant improvement in the locomotor activity was observed. Following the administration of L-DOPA there was a greater reversal of disability and a reduction in the intensity of L-DOPA-induced dyskinesia in GDNF-treated animals compared to saline-treated controls. However, there was no significant difference in L-DOPA's ability to increase locomotor activity between GDNF-treated and saline-treated animals. GDNF treatment caused a significant increase in the number of tyrosine hydroxylase-positive neurons in the substantia nigra, but no change in [(3)H]mazindol binding to dopamine terminals was found in the striatum of GDNF-treated animals compared to saline-treated controls. In GDNF-treated animals a small but significant reduction in enkephalin mRNA was observed in the caudate nucleus but not in the putamen or the nucleus accumbens. Substance P mRNA expression was equally reduced in the caudate nucleus and the putamen of the GDNF-treated animals but not in the nucleus accumbens. Intraventricular administration of GDNF improved MPTP-induced disability and reversed dopamine cell loss in the substantia nigra. GDNF also diminished L-DOPA-induced dyskinesia, which may relate to its ability to partly restore nigral dopaminergic transmission or to modify the activity of striatal output pathways.

Biochemical and immunohistological changes in the brain of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated mouse

We investigated neurochemically and neuropathologically the utility of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated mice as a model of Parkinson's disease. The changes in dopamine D1 and D2 receptors and dopamine uptake sites were determined by quantitative autoradiography using [3H]SCH23390, [3H]raclopride and [3H]mazindol, respectively. Dopamine and 3,4-dihydroxyphenyl acetic acid (DOPAC) contents in the striatum were measured by high-performance liquid chromatography. The distribution of nigral neurons and reactive astrocytes was determined by immunohistochemical staining with antibody against tyrosine hydroxylase (TH) and glial fibrillary acidic protein (GFAP). The mice received four intraperitoneal injections of MPTP (10 mg/kg) at 1-h intervals and then the brains were analyzed at 3 and 7 days after the treatments. No significant change in dopamine D1 receptors was observed in the striatum and substantia nigra after acute treatment with MPTP. Dopamine D2 receptors were reduced significantly in the substantia nigra only 7 days after the MPTP treatment, whereas striatum showed no significant change in the binding throughout the experiments. In contrast, dopamine uptake sites were reduced markedly in the striatum and substantia nigra 3 and 7 days after the MPTP treatment. Dopamine and DOPAC content were also reduced in the striatum 3 and 7 days after the MPTP treatment. An immunohistochemical study indicated a loss of the number of TH-positive neurons in the substantia nigra 7 days after the MPTP treatment. In contrast, numerous GFAP-positive astrocytes were evident in the striatum 7 days after the MPTP treatment. These results provide valuable information for the pathogenesis of acute stage of Parkinson's disease.

Endogenous dopaminergic tone and dopamine agonist action

Dopamine receptor agonists provide symptomatic relief in the early stages of Parkinson's disease, but with disease progression, their efficacy decreases. The reason behind this decrease in effectiveness is unknown, but maximal efficacy may be dependent on endogenous dopaminergic tone to provide stimulation of D1 and D2 receptor subtypes. Therefore, we have investigated the effects of the tyrosine hydroxylase inhibitor alpha-methyl-p-tyrosine (AMPT) on the actions of D1, D2, and D1/D2 agonists and levodopa (L-dopa) in common marmosets treated with 1 -methyl-4-phenyl-1,2,3,6-tetrahydropyridine. Administration of AMPT alone further increased motor disability and decreased locomotor activity. Administration of L-dopa reversed motor disability and increased locomotor activity, and this reversal was not affected by previous AMPT treatment. The D1 agonist A-77636 and the D2 agonist quinpirole reversed motor deficits, but these effects were markedly inhibited by previous AMPT treatment. Administration of quinpirole with A-77636 produced a reversal of motor deficits that was more resistant to AMPT pretreatment than was the effect produced by quinpirole or A-77636 alone. These data suggest that D1 and D2 receptor stimulation are required for dopamine receptor agonists to produce a maximal antiparkinsonian response. The reversal of motor deficits produced by the mixed D1/D2 agonist apomorphine was more resistant to AMPT treatment than that produced by quinpirole or A-77636. However, the motor effects of A-77636 plus quinpirole and of apomorphine were still affected by AMPT treatment. This suggests that loss of tyrosine hydroxylase activity may also alter motor activity through inhibition of endogenous L-dopa or norepinephrine synthesis, because both are also involved in the genesis of motor activity.

Striatal responses to partial dopaminergic lesion: evidence for compensatory sprouting

Dopaminergic lesions result in the acute loss of striatal dopamine content, the loss of tyrosine hydroxylase-immunoreactive fibers, upregulation of preproenkephalin mRNA expression, and compensatory changes in the synthesis and metabolism of dopamine. Despite the severe loss of fine tyrosine hydroxylase-immunoreactive fibers, larger fibers persist. We found that some tyrosine hydroxylase fiber types increase their branching and become thicker after partial lesion. To determine whether the remaining tyrosine hydroxylase fibers were degenerative or part of a compensatory response, we morphologically characterized striatal tyrosine hydroxylase fibers and compared them to silver-stained degenerative structures. Branched and large tyrosine hydroxylase fiber types were nondegenerative. Furthermore, normal preproenkephalin mRNA expression was maintained despite severe overall loss of tyrosine hydroxylase fibers in striatal regions with abundant branching, whereas preproenkephalin mRNA expression increased in severely depleted regions that lacked branched fibers, indicating that branching or sprouting was involved in the compensation for dopamine depletion and the maintenance of normal preproenkephalin expression. In support of compensatory sprouting by tyrosine hydroxylase fibers, mRNA for growth associated protein-43 was upregulated in dopaminergic midbrain cells. We conclude that an important compensatory response to partial dopaminergic depletion is the formation of new branches or sprouting.

The effects of central aromatic amino acid DOPA decarboxylase inhibition on the motor actions of L-DOPA and dopamine agonists in MPTP-treated primates

1. Endogenous L-DOPA may act as a neuromodulator contributing to the production of motor activity. We now investigate the effects of the centrally acting aromatic amino acid dopa decarboxylase (AADC) inhibitor NSD-1015 (3-hydroxybenzyl hydrazine) on the motor actions of L-DOPA and dopamine agonist drugs in MPTP treated common marmosets. 2. Pretreatment with NSD-1015 (10 - 50 mg kg(-1); i.p.) worsened baseline motor deficits in MPTP-treated common marmosets. Similarly, it abolished L-DOPA (5 - 18 mg kg(-1) s.c.) induced locomotor activity and reversal of disability. NSD-1015 pretreatment inhibited dopamine formation and elevated L-DOPA levels in plasma. 3. The increase in locomotor activity and improvement in disability produced by the administration of the D-1 agonist A-86929 (0.03 - 0. 04 mg kg(-1) s.c.) or the D-2 agonist quinpirole (0.05 - 0.3 mg kg(-1) i.p.) was abolished by NSD-1015 (25 mg kg(-1) i.p.) pretreatment. While the effects of a low dose combination of A-86929 (0.04 mg kg(-1) s.c.) and quinpirole (0.05 mg kg(-1) i.p.) were inhibited by NSD-1015 (25 mg kg(-1) i.p.), there was little effect on the action of a high dose combination of these drugs (0.08 mg kg(-1) A-86929 and 0.1 mg kg(-1) quinpirole). 4. Following central AADC inhibition with NSD-1015 (25 mg kg(-1) i.p.), locomotor behaviour induced by administration of high dose combinations of A-86929 (0.08 mg kg(-1) s.c.) and quinpirole (0.1 mg kg(-1) i.p.) was unaffected by L-DOPA (5 mg kg(-1) s.c.) pretreatment. 5. These results do not support a role for endogenous L-DOPA in spontaneous or drug induced locomotor activity. Rather, they strengthen the argument for the importance of endogenous dopaminergic tone in the motor actions of dopamine agonists.

Striatal dopaminergic correlates of stable parkinsonism and degree of recovery in old-world primates one year after MPTP treatment

Despite widespread use of the primate 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) model of Parkinson's disease, there is a paucity of data concerning the relationship between striatal dopaminergic function and behavior over time. This study examines the relationship between markers of dopamine neuron integrity and dopaminergic metabolic activity in striatal subregions with the degree of parkinsonian disability in 32 monkeys treated with MPTP one year earlier. Based on the parkinsonian summary score during the month following MPTP treatment, each monkey was assigned to one of four severity categories. We called these categories "Severe", "Moderate", "Mild" and "Asymptomatic". Monkeys in the Severe category were behaviorally stable, and loss of dopamine concentration was greater than 98% in all subregions of striatum one year after MPTP treatment. This value was not significantly different from the level of depletion, reported previously, at one to two months after MPTP in Severe monkeys, and apparently this loss of striatal dopamine is beyond the level from which effective compensations can occur. The parkinsonian disabilities in monkeys of other severity groups (Moderate, Mild, Asymptomatic) improved significantly over the year, despite having mean dopamine depletion of 75-99% in different subregions of striatum at one to two months after MPTP treatment. At one year after MPTP treatment, the mean dopamine depletions in different subregions of caudate nucleus and putamen had diminished in Asymptomatics (21-81%), Milds (35-96%), and Moderates (86-97%). Dopamine loss in nucleus accumbens was relatively spared compared with most striatal subregions, yet in Severe monkeys the decrease in this region reached 96%. In addition, at one year after MPTP treatment, there was a significant linear relationship between parkinsonian behavioral severity category and dopamine concentration, and homovanillic acid concentration and homovanillic acid/dopamine ratio in the striatum. The re-establishment of dopamine levels and homovanillic acid/dopamine ratios was most pronounced in putamen, ventromedial caudate nucleus and nucleus accumbens. Thus the small difference in striatal dopamine loss that distinguishes monkeys with widely different behavior at one to two months after MPTP increases over time. We suggest that the milder the initial loss, the greater capacity there is for regeneration or sprouting of dopamine terminals, which is reflected in marked increases in dopamine levels and modest elevations of metabolic activity (homovanillic acid/dopamine ratio). With greater initial losses, there is less capacity to increase terminal density, which is reflected later by smaller increases in striatal dopamine levels and more marked increases in metabolic activity. It appears that 5-10% of normal striatal dopamine levels is sufficient for overtly normal motor performance in non-human primates.

Tyrosine hydroxylase-immunoreactive elements in the human globus pallidus and subthalamic nucleus

In contrast to the well-established dopaminergic innervation of the neostriatum, the existence of dopaminergic innervation of the subthalamic nucleus and globus pallidus is controversial. In the present study, tyrosine hydroxylase (TH)-immunoreactive elements were observed by light microscopy after antigen retrieval in the subthalamic nucleus and in the internal and external segments of the globus pallidus in postmortem human brain. Small islands of apparent neostriatal tissue with abundant arborization of fine, TH-immunoreactive axons in the vicinity of calbindin-positive small neurons resembling neostriatal medium spiny neurons were present in the external segment of the globus pallidus. Large numbers of medium-large, TH-immunoreactive axons were observed passing above and through the subthalamic nucleus and through both pallidal segments; these are presumed to be axons of passage on their way to the neostriatum. In addition, fine, TH-immunoreactive axons with meandering courses, occasional branches, and irregular outlines, morphologically suggestive of terminal axon arborizations with varicosities, were seen in both pallidal segments, including the ventral pallidum, and the subthalamic nucleus, consistent with a catecholaminergic (probably dopaminergic) innervation of these nuclei. This finding suggests that, in Parkinson's disease and in animal models of this disorder, loss of dopaminergic innervation might contribute to abnormal neuronal activation in these three nuclei.

Neurotensin receptors and dopamine transporters: effects of MPTP lesioning and chronic dopaminergic treatments in monkeys

The effect of denervation with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) of the dopamine (DA) nigrostriatal pathway on neurotensin (NT) receptor and DA transporter (DAT) in basal ganglia of monkeys (Macaca fascicularis) was investigated. The MPTP lesion induced a marked depletion of DA (90% or more vs. control) in the caudate nucleus and putamen. The densities of NT agonist binding sites labeled with [125I]NT and the NT antagonist binding sites labeled with [3H]SR142948A decreased by half in the caudate-putamen of MPTP-monkeys. In addition, the densities of [125I]NT and [3H]SR142948A binding sites markedly decreased (-77 and -63%, respectively) in the substantia nigra of MPTP-monkeys. Levocabastine did not compete with high affinity for [125I]NT binding in the monkey cingulate cortex, suggesting that only one class of NT receptors was labelled in the monkey brain. An extensive decrease of [3H]GBR12935 DAT binding sites (-92% vs. Control) was observed in the striatum of MPTP-monkeys and an important loss of DAT mRNA(-86% vs. Control) was observed in substantia nigra. Treatments for 1 month with either the D1 agonist SKF-82958 (3 mg/kg/day) or the D2 agonist cabergoline (0.25 mg/kg/day) had no effect on the lesion-induced decrease in NT and DAT binding sites or DAT mRNA levels. The decrease of striatal NT binding sites was less than expected from the decrease of DA content in this nucleus, suggesting only partial localization of NT receptors on nigrostriatal DAergic projections. These data also suggest that under severe DA denervation, treatment with D1 or D2 DA agonists does not modulate NT receptors and DAT density.

Differential vulnerability of dopamine receptors in the mouse brain treated with MPTP

We investigated the chronological changes of dopamine D1 and D2 receptors and dopamine uptake sites in the striatum and substantia nigra of mouse brain treated with 1-methyl-4-phenyl-1,2,3, 6-tetrahydropyridine (MPTP) by quantitative autoradiography using [3H]SCH23390, [3H]raclopride and [3H]mazindol, respectively. The mice received i.p. injections of MPTP (10 mg/kg) four times at intervals of 60 min, the brains were analyzed at 6 h and 1, 3, 7 and 21 days after the last the injection. Dopamine D2 receptor binding activity was significantly decreased in the substantia nigra from 7 to 21 days after MPTP administration, whereas such binding activity was significantly increased in the medial part of the striatum at 21 days. There was no alteration of dopamine D1 receptor binding activity in either the striatum or the substantia nigra for the 21 days. The number of dopamine uptake sites gradually decreased in the striatum and the substantia nigra, starting at 6 h after MPTP administration, and the lowest levels of binding activity were observed at 3 and 7 days in the striatum (18% of the control values in the medial part and 30% in the lateral part) and at 1 day in the substantia nigra (20% of the control values). These results indicate that severe functional damage to the dopamine uptake sites occurs in the striatum and the substantia nigra, starting at an early stage after MPTP treatment. Our findings also demonstrate the compensatory up-regulation in dopamine D2 receptors, but not dopamine D1 receptors, in the striatum after MPTP treatment. Furthermore, our results support the existence of dopamine D2 receptors, but not dopamine D1 receptors, on the nigral neurons. The present findings suggest that there are differential vulnerabilities to MPTP toxicity in the nigrostriatal dopaminergic receptor systems of mouse brain.

Compensatory mechanisms in experimental and human parkinsonism: towards a dynamic approach

This paper provides an overview of the compensatory mechanisms which come into action during experimental and human parkinsonism. The intrinsic properties of the dopaminergic neurones of the substantia nigra pars compacta (SNc) which degenerate during Parkinson's disease are described in detail. It is generally considered that the nigrostriatal pathway is principally responsible for the compensatory preservation of dopaminergic function. It is also becoming clear that the morphological characteristics of dopaminergic neurones and the dual character, synaptic and asynaptic, of striatal dopaminergic innervation engender two modes of transmission, wiring and volume, and that both these modes play a role in the preservation of dopaminergic function. The plasticity of the dopamine neurones, extrinsic or intrinsic to the striatum, can thus be regarded as another compensatory mechanism. Recent anatomical and electrophysiological studies have shown that the SNc receives both glutamatergic and cholinergic inputs. The dynamic role this innervation plays in compensatory mechanisms in the course of the disease is explained and discussed. Recent developments in the field of compensatory mechanisms speak for the urgence to develop a valid chronic model of Parkinson's disease, integrating all the clinical features, even resting tremor, and illustrating the gradual evolution of nigral degeneration observed in human Parkinson's disease. Only a dynamic approach to the physiopathological study of compensatory mechanisms in the basal ganglia will be capable of elucidating these complex questions.

[123I]FP-CIT binds to the dopamine transporter as assessed by biodistribution studies in rats and SPECT studies in MPTP-lesioned monkeys

[123I]FP-CIT (N-omega-fluoropropyl-2 beta-carbomethoxy-3 beta-(4-iodophenyl)-tropane), a radioiodinated cocaine analogue, was evaluated as an agent for the in vivo labeling of dopamine (DA) transporters by biodistribution studies in rats and by single photon emission computed tomography (SPECT) studies in unilateral 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-lesioned monkeys. In rats, intravenous injection of [123I]FP-CIT resulted in high accumulation of radioactivity in the striatum. Less pronounced uptake was seen in brain areas with high densities of serotonergic uptake sites. While striatal uptake of radioactivity after injection of [123I]FP-CIT was displaced significantly by GBR12,909 but not by fluvoxamine, the opposite was observed in brain areas known to be rich of serotonin transporters. Monkeys which were unilaterally treated with neurotoxic doses of MPTP showed severe loss of striatal [123I]FP-CIT uptake at the side of treatment. The results of this study indicate that [123I]FP-CIT, although not being a selective radioligand, binds specifically to the striatal DA transporter in vivo and thus suggest that [123I]FP-CIT promises to be a suitable radioligand for SPECT imaging of DA transporters in humans.