Ann Marie Janson

New neurons are generated from stem cells in a few regions of the adult mammalian brain. Here we provide evidence for the generation of dopaminergic projection neurons of the type that are lost in Parkinson's disease from stem cells in the adult rodent brain and show that the rate of neurogenesis is increased after a lesion. The number of new neurons generated under physiological conditions in substantia nigra pars compacta was found to be several orders of magnitude smaller than in the granular cell layer of the dentate gyrus of the hippocampus. However, if the rate of neuronal turnover is constant, the entire population of dopaminergic neurons in substantia nigra could be replaced during the lifespan of a mouse. These data indicate that neurogenesis in the adult brain is more widespread than previously thought and may have implications for our understanding of the pathogenesis and treatment of neurodegenerative disorders such as Parkinson's disease.

Presynaptic denervation is likely to play an important role in the pathophysiology of dyskinesias that develop after levodopa administration to patients with Parkinson's disease. In this study, the thresholds of nigrostriatal damage necessary for the occurrence of parkinsonism and levodopa-induced involuntary movements were compared in squirrel monkeys lesioned with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). Animals treated with a regimen of MPTP that caused parkinsonism displayed > or =95% striatal dopamine depletion, 90% reduction of striatal dopamine uptake sites, and 70% nigral neuronal loss. Levodopa administration ameliorated the parkinsonian signs of these monkeys but also induced dyskinesias. A separate group of animals was treated with a milder MPTP regimen that caused 60%-70% striatal dopamine depletion, a 50% decrease in dopamine transporter, and 40% loss of dopaminergic nigral neurons. While these monkeys displayed no behavioral signs of parkinsonism, they all became dyskinetic after levodopa administration. The priming effect of levodopa, that is, the recurrence of dyskinesias in animals previously exposed to the drug, was compared in severely versus mildly lesioned monkeys. When severely injured parkinsonian animals underwent a second cycle of levodopa treatment, they immediately and consistently developed involuntary movements. In contrast, the recurrence of dyskinesias in primed monkeys with a partial nigrostriatal lesion required several levodopa administrations and remained relatively sporadic. The data indicate that moderate nigrostriatal damage which does not induce clinical parkinsonism predisposes to levodopa-induced dyskinesias. Once dyskinesias have been induced, the severity of denervation may enhance the sensitivity to subsequent levodopa exposures.

Increasing incidence of Parkinson's disease with advancing age suggests that age-related processes predispose the nigrostriatal dopaminergic system to neurodegeneration. Several hypotheses concerning the effects of aging on nigrostriatal neurons were assessed in this study using a non-human primate model. First, we examined the possibility that the total number of dopaminergic neurons decline in the substantia nigra as a function of age. Stereological counting based on both tyrosine hydroxylase immunoreactivity (TH-ir) and neuromelanin (NM) content revealed no difference in cell number between young, middle-aged and old squirrel monkeys. We then determined whether advancing age changed the relative proportion of neurons characterized by 1) TH-ir in the absence of NM, 2) the presence of both TH-ir and NM, or 3) NM without TH-ir. Indeed, a progressive age-related depletion of TH only cells was paralleled by an increase in NM only neurons. The possibility that these changes could underlie a functional impairment of the nigrostriatal system was supported by striatal dopamine measurements showing a decrease in older monkeys. Finally, we tested the hypotheses that aging may enhance cell vulnerability to injury and that different dopaminergic subpopulations display varying degrees of susceptibility. When monkeys were exposed to the neurotoxicant 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine, cell loss was markedly more pronounced in older animals, and the ranking of vulnerability was TH only < TH/NM < NM only cells. The data indicate that, even in the absence of an overall neuronal loss, changes in the characteristics of dopaminergic cells reflect functional deficits and increased vulnerability to injury with age. NM content appears to be an important marker of these age-related effects.