MPTP-induced parkinsonism: relative changes in dopamine concentration in subregions of substantia nigra, ventral tegmental area and retrorubral field of symptomatic and asymptomatic vervet monkeys

Dysfunction of motor cortices in Parkinson's disease

The cerebral cortex has long been thought to be involved in the pathophysiology of motor symptoms of Parkinson's disease. The impaired cortical function is believed to be a direct and immediate effect of pathologically patterned basal ganglia output, mediated to the cerebral cortex by way of the ventral motor thalamus. However, recent studies in humans with Parkinson's disease and in animal models of the disease have provided strong evidence suggesting that the involvement of the cerebral cortex is much broader than merely serving as a passive conduit for subcortical disturbances. In the present review, we discuss Parkinson's disease-related changes in frontal cortical motor regions, focusing on neuropathology, plasticity, changes in neurotransmission, and altered network interactions. We will also examine recent studies exploring the cortical circuits as potential targets for neuromodulation to treat Parkinson's disease.

HIV, Tat and dopamine transmission

Human Immunodeficiency Virus (HIV) is a progressive infection that targets the immune system, affecting more than 37 million people around the world. While combinatorial antiretroviral therapy (cART) has lowered mortality rates and improved quality of life in infected individuals, the prevalence of HIV associated neurocognitive disorders is increasing and HIV associated cognitive decline remains prevalent. Recent research has suggested that HIV accessory proteins may be involved in this decline, and several studies have indicated that the HIV protein transactivator of transcription (Tat) can disrupt normal neuronal and glial function. Specifically, data indicate that Tat may directly impact dopaminergic neurotransmission, by modulating the function of the dopamine transporter and specifically damaging dopamine-rich regions of the CNS. HIV infection of the CNS has long been associated with dopaminergic dysfunction, but the mechanisms remain undefined. The specific effect(s) of Tat on dopaminergic neurotransmission may be, at least partially, a mechanism by which HIV infection directly or indirectly induces dopaminergic dysfunction. Therefore, precisely defining the specific effects of Tat on the dopaminergic system will help to elucidate the mechanisms by which HIV infection of the CNS induces neuropsychiatric, neurocognitive and neurological disorders that involve dopaminergic neurotransmission. Further, this will provide a discussion of the experiments needed to further these investigations, and may help to identify or develop new therapeutic approaches for the prevention or treatment of these disorders in HIV-infected individuals.

Restoration of Dopamine Transporter Density in the Striatum of Fetal Ventral Mesencephalon-Grafted, but not Sham-Grafted, Mptp-Treated Parkinsonian Monkeys

Transplantation of fetal dopamine neurons to the adult striatum potentially offers a means to reverse the striatal dopamine deficiency that characterizes Parkinson's disease. Many investigations in rodents have supported the hope that neural grafting may be a useful treatment for parkinsonism. However, clinical studies have generally produced more modest improvements in motor abnormalities than observed in lower species. It is possible that the number of fetal dopamine neurons that survive transplantation is insufficient to restore dopaminergic innervation of the large human striatum to a level where striking recovery is obtained. In fact, there has been no quantitative study of graft outgrowth to indicate what portion of the dopamine-depleted striatum might be reinner-vated with present techniques. Furthermore, it has been speculated that regeneration of the host dopamine system in response to the implantation surgery may play an important role in the beneficial effects of neural grafting in primates. The present study used nine 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated parkinsonian monkeys to investigate these issues. Sham implantation procedures produced no increase in either dopamine transporter density (measured by quantitative autoradiography) or tissue dopamine concentration (measured by HPLC) in the striatum of MPTP-treated monkeys. In sham-grafted and nonimplanted MPTP-treated monkeys, the striatal dopamine concentration was reduced by 99%, based on analysis of 16 sampled sites in the caudate nucleus and putamen of each monkey. No behavioral recovery was seen in the sham-grafted and nonimplanted MPTP-treated groups. In contrast, transplantation of fetal dopamine neurons to the caudate nucleus or putamen of MPTP-treated monkeys resulted in a significant elevation of dopamine transporter density and dopamine levels in the grafted striatal nucleus. Each grafted MPTP-treated monkey received ventral mesencephalon dopamine neurons from one donor harvested during putative neurogenesis. Donor ventral mesencephalon was divided equally and implanted into six sites either in the caudate nucleus or putamen. One graft site in each monkey was examined by dopamine transporter autoradiography. In sections in which graft fibers were present, a mean of one-third of the volume of the grafted nucleus was occupied by an elevated density of dopamine transporters. This increase in dopamine transporter density was defined to be at least 5-10% of the control density. However, full behavioral recovery was not observed in the grafted MPTP-treated group. These data provide no support for the hypothesis that regeneration of the host dopamine system occurs in response to a sham implantation procedure in severely parkinsonian monkeys. The current study illustrates the power of the applied techniques for delineating the relationship between the level of host dopamine depletion, the extent of graft-induced dopaminergic restoration, and behavioral recovery.

Pathophysiology of L-dopa-induced motor and non-motor complications in Parkinson's disease

Involuntary movements, or dyskinesia, represent a debilitating complication of levodopa (L-dopa) therapy for Parkinson's disease (PD). L-dopa-induced dyskinesia (LID) are ultimately experienced by the vast majority of patients. In addition, psychiatric conditions often manifested as compulsive behaviours, are emerging as a serious problem in the management of L-dopa therapy. The present review attempts to provide an overview of our current understanding of dyskinesia and other L-dopa-induced dysfunctions, a field that dramatically evolved in the past twenty years. In view of the extensive literature on LID, there appeared a critical need to re-frame the concepts, to highlight the most suitable models, to review the central nervous system (CNS) circuitry that may be involved, and to propose a pathophysiological framework was timely and necessary. An updated review to clarify our understanding of LID and other L-dopa-related side effects was therefore timely and necessary. This review should help in the development of novel therapeutic strategies aimed at preventing the generation of dyskinetic symptoms.

Drug discovery in Parkinson's disease-Update and developments in the use of cellular models

Parkinson's disease (PD) is the second most common neurodegenerative disorder and is characterized by the degeneration of dopaminergic (DA) neurons within the substantia nigra. Dopamine replacement drugs remain the most effective PD treatment but only provide temporary symptomatic relief. New therapies are urgently needed, but the search for a disease-modifying treatment and a definitive understanding of the underlying mechanisms of PD has been limited by the lack of physiologically relevant models that recapitulate the disease phenotype. The use of immortalized cell lines as in vitro model systems for drug discovery has met with limited success, since efficacy and safety too often fail to translate successfully in human clinical trials. Drug discoverers are shifting their focus to more physiologically relevant cellular models, including primary neurons and stem cells. The recent discovery of induced pluripotent stem (iPS) cell technology presents an exciting opportunity to derive human DA neurons from patients with sporadic and familial forms of PD. We anticipate that these human DA models will recapitulate key features of the PD phenotype. In parallel, high-content screening platforms, which extract information on multiple cellular features within individual neurons, provide a network-based approach that can resolve temporal and spatial relationships underlying mechanisms of neurodegeneration and drug perturbations. These emerging technologies have the potential to establish highly predictive cellular models that could bring about a desperately needed revolution in PD drug discovery.

Cell transplantation and gene therapy in Parkinson's disease

Parkinson's disease is a progressive neurodegenerative disorder affecting, in part, dopaminergic motor neurons of the ventral midbrain and their terminal projections that course to the striatum. Symptomatic strategies focused on dopamine replacement have proven effective at remediating some motor symptoms during the course of disease but ultimately fail to deliver long-term disease modification and lose effectiveness due to the emergence of side effects. Several strategies have been experimentally tested as alternatives for Parkinson's disease, including direct cell replacement and gene transfer through viral vectors. Cellular transplantation of dopamine-secreting cells was hypothesized as a substitute for pharmacotherapy to directly provide dopamine, whereas gene therapy has primarily focused on restoration of dopamine synthesis or neuroprotection and restoration of spared host dopaminergic circuitry through trophic factors as a means to enhance sustained controlled dopamine transmission. This seems now to have been verified in numerous studies in rodents and nonhuman primates, which have shown that grafts of fetal dopamine neurons or gene transfer through viral vector delivery can lead to improvements in biochemical and behavioral indices of dopamine deficiency. However, in clinical studies, the improvements in parkinsonism have been rather modest and variable and have been plagued by graft-induced dyskinesias. New developments in stem-cell transplantation and induced patient-derived cells have opened the doors for the advancement of cell-based therapeutics. In addition, viral-vector-derived therapies have been developed preclinically with excellent safety and efficacy profiles, showing promise in clinical trials thus far. Further progress and optimization of these therapies will be necessary to ensure safety and efficacy before widespread clinical use is deemed appropriate.

The 3R principle and the use of non-human primates in the study of neurodegenerative diseases: the case of Parkinson's disease

The aim of this paper is to offer an ethical perspective on the use of non-human primates in neurobiological studies, using the Parkinson's disease (PD) as an important case study. We refer, as theoretical framework, to the 3R principle, originally proposed by Russell and Burch [Russell, W.M.S., Burch, R.L., 1959. The Principles of Humane Experimental Technique. Universities Federation for Animal Welfare Wheathampstead, England (reprinted in 1992)]. Then, the use of non-human primates in the study of PD will be discussed in relation to the concepts of Replacement, Reduction, and Refinement. Replacement and Reduction result to be the more problematic concept to be applied, whereas Refinement offers relatively more opportunities of improvement. However, although in some cases the 3R principle shows its applicative limits, its value, as conceptual and inspirational tool remains extremely valuable. It suggests to the researchers a series of questions, both theoretical and methodological, which can have the results of improving the quality of life on the experimental models, the quality of the scientific data, and the public perception from the non-scientist community.

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.

Effect of intrastriatal 6-OHDA lesion on dopaminergic innervation of the rat cortex and globus pallidus

The present study examined in the rat the effect of a partial lesion of the nigrostriatal dopaminergic pathway induced by intrastriatal injection of 6-hydroxydopamine (6-OHDA), on the dopaminergic innervation of the cortex and the globus pallidus as revealed using tyrosine hydroxylase (TH) immunoreactivity. Twenty-eight days after unilateral injection of 6-OHDA into the dorsal part of the striatum, TH-positive fiber density was reduced by 41% in the dorsal and central part of the structure, and was accompanied by a retrograde loss of 33% of TH-positive neurons in the substantia nigra (SN), while the ventral tegmental area was completely spared. In the SN, TH-positive cell loss was most severe in the ventral part of the structure (-55%). In the same animals, a substantial loss of TH-positive fibers was evident in the dorsal part of the globus pallidus, and involved both thick fibers of passage and thin varicose terminal axonal branches. In the cortex, a loss of TH-positive fibers was prominent in the cingulate area, moderate in the motor area and less affected in the insular area, while the noradrenergic innervation revealed using dopamine-beta-hydroxylase immunoreactivity was preserved in all of these cortical subregions. These results demonstrate that the intrastriatal 6-OHDA lesion model in rats produces a significant loss of dopaminergic axons in extrastriatal structures including the pallidum and cortex, which may contribute to functional sequelae in this animal model of Parkinson's disease.

Tyrosine hydroxylase and dopamine transporter expression in residual dopaminergic neurons: potential contributors to spontaneous recovery from experimental Parkinsonism

1-Methyl-4-phenyl-1,2,3,6-tetrahyrdropyridine (MPTP)-exposed cats develop severe Parkinsonism that spontaneously resolves in 4-6 weeks. The present study examined the extent to which compensatory changes in tyrosine hydroxylase (TH) and dopamine transporter (DAT) gene and protein expression may underlie this behavioral recovery. In normal cats, TH and DAT protein levels were higher in the dorsal vs. ventral striatum. Expression of DAT and TH mRNA was higher in substantia nigra pars compacta (SNc) than in the ventral tegmental area (VTA). In symptomatic parkinsonian animals, DAT and TH protein levels were significantly decreased in all striatal areas studied. TH and DAT mRNA expression in residual SNc neurons were decreased a mean 32% and 38%, respectively. DAT gene expression in residual VTA neurons in symptomatic animals was decreased 30% whereas TH gene expression was unaffected. In spontaneously recovered cats, TH protein levels were significantly higher than the levels in symptomatic cats only in the ventral striatum, whereas no increase in DAT protein levels were observed in any striatal area. Residual neurons in most ventral mesencephalic regions of recovered cats had increased TH mRNA expression but not increased DAT gene expression, compared with symptomatic animals. Thus, increased TH protein and mRNA and suppression of DAT protein and mRNA expression in the striatum and ventral mesencephalon were associated with functional recovery from MPTP-induced parkinsonism.

Pattern of dopaminergic loss in the striatum of humans with MPTP induced parkinsonism

OBJECTIVES

To examine the distribution of striatal dopaminergic function in humans with parkinsonism induced by 1-methyl-4-phenyl-1, 2,3,6-tetrahydropyridine (MPTP) to determine if there is a caudate-putamen gradient as is seen in idiopathic Parkinson's disease.

METHODS

We scanned nine humans exposed to MPTP with parkinsonism ranging from minimal to severe using [(18)F]fluorodopa (FD) and high resolution PET. The results were compared with those of 10 patients with Parkinson's disease and six normal subjects.

RESULTS

In the MPTP group there was an equal degree of reduction of dopaminergic function in the caudate and putamen. This was different from the greater putaminal than caudate loss in Parkinson's disease (p<0.001).

CONCLUSIONS

Parkinson's disease is not caused by transient exposure to MPTP.

MPTP-Induced pallidal lesions in rhesus monkeys

Dopamine neurons in the substantia nigra of the midbrain are the primary neuronal population affected by 1-methyl-4-phenyl-1,2,3, 6-tetrahydropyridine (MPTP) toxicity, which produces the pathological and behavioral features of Parkinson's disease in nonhuman primates and man. We have identified another injury site in magnetic resonance imaging (MRI) brain scans in 13 of 37 rhesus monkeys taken 10-12 months after administration of this neurotoxin via the right carotid artery. Focal lesions, ranging in volume from 6.75 to 60 mm3 in the rostral globus pallidus region, were seen on the right side of the brain in these 13 animals in addition to the midbrain effects. While no significant differences were seen between globus pallidus lesioned and nonlesioned animals in the severity of MPTP-induced parkinsonian symptoms, the response to levodopa was muted in pallidal-lesioned animals. To confirm the role of neurotoxicity in producing the lesions, brain scans from an additional 12 monkeys were evaluated during the acute period following exposure to either MPTP (n = 6) or saline (n = 6). Focal lesions in the rostral globus pallidus were seen as early as 2-4 h following a carotid artery infusion in two of six MPTP recipients, but no evidence of injury was seen in saline recipients. The globus pallidus includes important components of the neural circuitry regulating motor functions. The present results indicate that in addition to midbrain dopamine neurons, a focal region of the rostral globus pallidus is selectively vulnerable to MPTP toxicity.

Recovery of striatal dopamine function after acute amphetamine- and methamphetamine-induced neurotoxicity in the vervet monkey

In six vervet monkeys, presynaptic striatal dopamine function was assessed longitudinally by [18F]fluoro-L-DOPA (FDOPA)-positron emission tomography (PET) after administration (2 x 2 mg/kg, i.m., 4 h apart) of either amphetamine (Amp), n = 3, or methamphetamine (MeAmp), n = 3. At 1-2 weeks postdrug, both Amp and MeAmp exposure effected similar decreases (60-70%) in the FDOPA influx rate constant (FDOPA Ki), an index of striatal dopamine synthesis capacity. Subsequent studies in these subjects showed that FDOPA Ki values were decreased by 45-67% at 3-6 weeks, by 25% at 10-12 weeks and by 16% in one Amp-treated subject at 32 weeks. Biochemical analysis showed that striatal dopamine concentrations were decreased by 75% at 3-4 weeks and by 55% at 10-12 weeks. These results indicate that in vervet monkey striatum, an acute Amp or MeAmp drug dosage produces extensive striatal dopamine system neurotoxicity. However, these effects were reversible; observed time-dependent recovery in both FDOPA Ki and dopamine concentrations indicates that neurochemical plasticity remains active in the adult primate striatum. At 3-4 and 10-12 weeks postdrug, the concurrent characterization of the striatal FDOPA Ki and dopamine concentrations for individual subjects showed that Ki decreases between 24 and 67% corresponded to dopamine depletions of 55-85%. These relatively larger postdrug decrements in steady-state striatal dopamine concentrations suggest that compensatory increases in dopamine synthesis capacity develop in the partially lesioned striatum. In contrast to the dopamine depletion in striatum, substantia nigra concentrations remained unchanged from referent values at both 3-4 and 10-12 weeks postdrug. Thus, the integrity of the substantia nigra could not be inferred from decreases in the striatal FDOPA Ki parameter. This disparity between striatum and substantia nigra reactivity to systemic administration of amphetamines suggests that each has unique dopamine system regulatory mechanisms.

Increased susceptibility to MPTP toxicity in middle-aged rhesus monkeys

In the present study, age-associated effects of the neurotoxin 1-methyl-4-phenyl-1,2,3,6 tetrahydropyridine (MPTP) administered via the right carotid artery were evaluated pre- and post-MPTP treatment in 26 female rhesus monkeys ranging in age from young adulthood to middle age (5 to 23 years old). A significant inverse correlation was seen between age and MPTP dose needed to produce stable, moderate parkinsonian features. Rhesus in the 5- to 9-year-old group required approximately three times the amount of MPTP as 20- to 23-year-old animals. Even though they received less MPTP, the older animals consistently displayed more severe bradykinesia, upper limb rigidity, and balance and gait abnormalities. Prior to MPTP treatment, home cage activity levels were strongly age dependent, with animals in the 10- to 19-year and 20- to 23-year groups displaying significantly less daytime activity than 5- to 9-year-old rhesus. Home cage activity levels tended to decrease in all age groups following MPTP treatment, but significant decreases were only measured in daytime activity in the 10- to 19- and 20- to 23-year age groups.

Short and long-term changes in cerebral [14C]-2-deoxyglucose uptake in the MPTP-treated marmoset: relationship to locomotor activity

The "short-term" (0.7 +/- 0.1 months post-MPTP) and "long-term" effects (36.7 +/- 4.4 months) of MPTP treatment on motor behaviour and [14C]-2DG uptake were investigated in the common marmoset. The subcutaneous administration of MPTP greatly reduced locomotor activity (-94% with respect to controls) and induced motor disability in the "short-term" MPTP-treated marmoset group. In the "long-term" MPTP group, MPTP treatment did not significantly affect locomotor activity (-27% with respect to controls) and there was partial recovery of motor disability. In the "short-term" MPTP group, there were increases in [14C]-2DG uptake in the GPl (+31 to +37%), SNc (+34 to +42%), VTA (+35%), LC (+23%), PPN (+19%) and in the VA (+19%), VL (+20%) and AM (+17%) thalamic nuclei. [14C]-2DG uptake was decreased in the STN (-15%). In the "long-term" MPTP group, [14C]-2DG uptake was increased in the GPl (+18%), SNc (+27%), VTA (+25%), PPN (+19%), ventral caudate nucleus (+18 to +23%), NAc (+22%), F.Ctx (+18%) and in the VA (+34%), VL (+28%), AV (+33%) and AM (+24%) thalamic nuclei. [14C]-2DG uptake was unchanged in the STN. The increase in metabolic activity of the surviving DA neurones and/or the reactive gliosis may account for the initial increase in [14C]-2DG uptake in the SNc and VTA. On the other hand, in the "long-term" MPTP-treated animals the increase in [14C]-2DG uptake in the SNc (though less than in the "short-term" MPTP group), ventral caudate and NAc may reflect the regenerative changes in the dopaminergic system in these areas. Despite the behavioural recovery, [14C]-2DG uptake remained elevated in the target areas for medial pallidal output (the thalamic nuclei and PPN). However, the attenuation of the changes in [14C]-2DG uptake in the GPl and STN of "long-term" MPTP-treated marmosets suggest that the striato-GPl and GPl-STN outputs closely reflect motor function in this primate model of Parkinson's disease.

Cocaine accumulates in dopamine-rich regions of primate brain after i.v. administration: comparison with mazindol distribution

Pharmacological and neurochemical evidence suggest that brain dopamine systems, and the dopamine transporter in particular, contribute significantly to the behavioral effects and reinforcing properties of cocaine. The first objective of this study was to determine whether the brain distribution of cocaine supports these conclusions. A high resolution neuroanatomical map of cocaine disposition in brain after i.v. administration was developed. [3H]Cocaine ([3H](-)-cocaine) was administered to squirrel monkeys (Saimiri sciureus) at a trace dose (0.001 mg/kg) and at doses at or above the threshold for producing behavioral effects (0.1 mg/kg, 0.3 mg/kg). After 15 min, ex vivo autoradiography revealed the highest accumulation of [3H]cocaine in dopamine-rich brain regions, including the caudate nucleus, putamen, and nucleus accumbens/olfactory tubercle. The norepinephrine-rich locus coeruleus, the hippocampus, and amygdala also accumulated large quantities of [3H]cocaine. Moderately high levels were found in the stria terminalis, medial septum, substantia nigra, and other regions. Lowest levels were found in the cerebellum. A high and positive correlation was established for the brain distribution of [3H]cocaine administered at trace or at behaviorally relevant doses (r: 0.94; P < 0.001). To determine whether radioactivity represented [3H]cocaine or its metabolic products, tissue extracts from brain regions with high levels of cocaine were subjected to thin layer chromatography using two solvent systems. In caudate-putamen, nucleus accumbens, cortex, and hippocampus, radioactivity comigrated with standard [3H]cocaine. In substantia nigra, less than 70% of the radioactivity comigrated with [3H]cocaine, suggesting that cocaine metabolites are generated more rapidly in the substantia nigra than in other brain regions. The second objective was to determine the brain distribution of mazindol, a potent norepinephrine and dopamine transport inhibitor with low abuse liability in humans. The disposition of intravenously administered [3H]mazindol in brain (0.001 mg/kg, 0.007 mg/kg) was surveyed by ex vivo autoradiography. In sharp contrast to [3H]cocaine distribution, the highest accumulation of [3H]mazindol was localized in the norepinephrine-rich pineal gland, discrete regions of the hypothalamus (paraventricular nucleus, supraoptic nucleus), and the locus coeruleus. Moderately high levels were detected in the caudate-putamen, nucleus accumbens, and other regions. The following conclusions were drawn: (1) Although dopamine-rich brain regions are principal targets of cocaine after i.v. administration to the nonhuman primate, other prominent targets of cocaine (locus coeruleus, hippocampus, and amygdala) may contribute to the acute and chronic effects of cocaine.(ABSTRACT TRUNCATED AT 400 WORDS)

Decreased tyrosine hydroxylase content in the dopaminergic neurons of MPTP-intoxicated monkeys: effect of levodopa and GM1 ganglioside therapy

Parkinson's disease is characterized by the degeneration of melanized dopaminergic neurons of the substantia nigra. The functional capacity of the surviving dopaminergic neurons is affected, as suggested by the subnormal levels of tyrosine hydroxylase messenger RNA and protein found in the remaining cells. The reduced expression of tyrosine hydroxylase may be due to either the evolving neurodegenerative process or its downregulation, possibly secondary to chronic levodopa treatment. The cellular content of tyrosine hydroxylase was determined in the mesencephalon from 16 Macaca fascicularis monkeys, using a semiquantitative immunocytochemical method. Thirteen monkeys were rendered parkinsonian by weekly intravenous injections of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) for 2 (subacute treatment) or 20 (chronic treatment) weeks. Three of the monkeys received levodopa and 3 others received GM1 ganglioside. The loss of dopaminergic neurons in the mesencephalon of the MPTP-intoxicated monkeys was severe in the substantia nigra, intermediate in cell groups A8 and A10, and almost undetectable in the central gray substance. After both subacute and chronic treatment, the cellular content of tyrosine hydroxylase was reduced by 40% in the surviving neurons of the lesioned substantia nigra, but by less in the other mesencephalic dopaminergic regions. Neuronal survival and tyrosine hydroxylase content in monkeys that had received levodopa were not significantly different. The cellular content of tyrosine hydroxylase was increased in the substantia nigra of the monkeys that received GM1 ganglioside injections. The results show that the decreased expression of tyrosine hydroxylase found in nigral dopaminergic neurons after partial degeneration of the mesostriatal dopaminergic system is not influenced by levodopa treatment and is partially reversed by GM1 ganglioside administration.

Developing a stable bilateral model of parkinsonism in rhesus monkeys

The non-human primate models of Parkinson's disease which have been developed using the neurotoxin MPTP (1-methyl-4-phenyl-1,2,3,6 tetrahydropyridine) have proven to be either unstable or variable, or to display only a limited subset of parkinsonian features. The present study examined a new two-stage lesion approach in which MPTP was administered via the carotid arteries. The first infusion through one artery produced a hemiparkinsonian state and was followed several months later by a second MPTP infusion into the contralateral carotid artery to induce bilateral parkinsonism. Animals receiving lesions were evaluated using a battery of tests which included a monkey parkinsonism rating scale, a movement time-task and continuous monitoring of home cage activity. All animals monitored showed significant decreases in activity levels of up to 95% following the second lesion. These decreased activity levels remained stable throughout the observation period of up to 12 months postlesion. In addition to the decreased home cage activity, bilaterally lesioned animals displayed bilateral parkinsonian features including akinesia, bradykinesia, rigidity, tremor and balance and gait disturbances which were stable, following an acute period of up to 45 days, for the remainder of the study. Administration of levodopa increased activity levels and reduced motor dysfunctions. Thus, a two-stage bilateral lesion approach, utilizing the neurotoxin MPTP, appears to provide a less variable and relatively stable model of bilateral Parkinson's disease in nonhuman primates. Treated animals display the cardinal features of parkinsonism and respond appropriately to the standard antiparkinsonian drug, levodopa.

Neonatal lesions of the ventral tegmental area affect monoaminergic responses to stress in the medial prefrontal cortex and other dopamine projection areas in adulthood

A mean decrease of dopamine (DA) to 20% and serotonin to 25-30% of control levels was found in the medial prefrontal cortex (mPFC) and amygdala/piriform cortex (A/PC) of adult rats with neonatal lesions of the ventral tegmental area (VTA). The metabolites were less decreased suggesting an increased activity of the remaining terminals. Moderate decreases to 30-75% were detected for DA and serotonin in the nucleus accumbens, olfactory tubercle and striatum. Footshock stress in control animals resulted in a strong increase (200% of control) in DA metabolites in mPFC and A/PC. The noradrenaline metabolite 3-methoxy-4-hydroxyphenylglycol (MHPG) in A/PC was strongly increased to 240%. When stress was given to the neonatally VTA-lesioned animals these neurochemical responses were reduced compared to the nonlesioned rats. In the case of DA in the mPFC this was clearly due to a loss of stress response in the severe lesion group where DA is depleted to less than 20% of control. The stress-induced small increases in DA metabolism in tubercle, accumbens and striatum and serotonin metabolism in the striatum (20-40%) were entirely lost, while the MHPG increase in the A/PC was blunted. The present results suggest that moderate and severe lesions of DA and serotonin alter or prevent the normal activation of these transmitter systems and even of the noradrenergic system to stress.

Distribution of cocaine recognition sites in monkey brain: I. In vitro autoradiography with [3H]CFT

The cocaine analog [3H]CFT ([3H]WIN 35,428) was used to map and characterize cocaine recognition sites in the squirrel monkey brain by quantitative autoradiography. Coronal tissue sections were incubated with 5 nM [3H]CFT to measure total binding or with [3H]CFT in the presence of 30 microM (-)-cocaine to measure nonspecific binding. High densities of [3H]CFT binding sites were present in dopamine-rich brain regions, including the caudate nucleus, putamen, nucleus accumbens, and olfactory tubercle. In each of these regions specific binding was greater than 90% of total binding. Several additional brain regions exhibited intermediate densities of [3H]CFT binding, including the substantia nigra, the zona incerta, the amygdala, and the hypothalamus. Low, though measurable levels of binding were observed in the bed nucleus of the stria terminalis, the ventral tegmental area, the medial preoptic area, the pineal, the hippocampus, and thalamic central nuclei. Near-background levels of binding were found in white matter, cortical regions, globus pallidus, and cerebellum. The pharmacological specificity of [3H]CFT binding in various brain regions was determined in competition studies using [3H]CFT and a range of concentrations of selected monoamine uptake inhibitors. In all brain regions examined, stereoselective inhibition of [3H]CFT binding was observed for the (-) over the (+) isomer of cocaine. For other drugs tested, competition experiments indicated a rank order of potency of GBR 12909 greater than or equal to CFT greater than bupropion, suggestive of binding of [3H]CFT to elements of the dopamine transport system. The results demonstrate that although densities of [3H]CFT binding sites are highest in the caudate nucleus, putamen, and nucleus accumbens/olfactory tubercle, significant levels of binding can be detected in other brain regions that may contribute to the behavioral and physiological effects of cocaine.

Subclinical damage to the nigrostriatal dopamine system by MPTP as a model of preclinical Parkinson's disease: a review

The study of subtle changes in motor and cognitive function after exposure to MPTP might serve as a guide to the very earliest stages of Parkinson's disease. Studies in nonhuman primates and man exposed to MPTP who remained asymptomatic or recovered completely are reviewed. The question of the relationship between the degree and extent of damage to the nigrostriatal dopamine system and changes in motor and cognitive (behavioral) function is addressed. What guidance they provide in the study of subclinical or preclinical Parkinson's disease is discussed.

MPTP lesions and dopaminergic drugs alter eye blink rate in African green monkeys

Eye blink rates were studied in African green monkeys following relatively specific destruction of substantia nigra and its dopamine projections with the neurotoxin, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). Monkeys treated with MPTP had a significantly lower blink rate than controls over a period from two to five and a half months after treatment. Furthermore, the degree of parkinsonism expressed in treated animals was inversely correlated with blink rate. Pharmacologic studies further supported the role of dopamine receptors in the regulation of blink rate. PHNO (4-propyl-9-hydroxynaphoxazine), a potent and highly specific D2 agonist, effective in alleviating parkinsonism, caused a significant transient increase in blink rate, while sulpiride, a D2 antagonist, caused a decrease and blocked the effect of PHNO. Apomorphine and haloperidol, although less specific, had potent and predictable effects based on their interactions with dopamine systems. Blink rate may provide a nonintrusive measure of central dopamine activity that would help to evaluate the progress of Parkinson's disease or treatments which attempt to restore dopamine function.