Interactions Between MPTP-Induced and Age-Related Neuronal Death in a Murine Model of Parkinson’s Disease

Exercise protects against MPTP-induced neurotoxicity in mice

Exercise has been shown to be potently neuroprotective in several neurodegenerative models, including 1-methyl-4-phenyl-1, 2, 3, 6-tetrahydropyridine (MPTP) model of Parkinson's disease (PD). In order to determine the critical duration of exercise necessary for DA neuroprotection, mice were allowed to run for either 1, 2 or 3months prior to treatment with saline or MPTP. Quantification of DA neurons in the SNpc show that mice allowed to run unrestricted for 1 or 2months lost significant numbers of neurons following MPTP administration as compared to saline treated mice; however, 3months of exercise provided complete protection against MPTP-induced neurotoxicity. To determine the critical intensity of exercise for DA neuroprotection, mice were restricted in their running to either 1/3 or 2/3 that of the full running group for 3months prior to treatment with saline or MPTP. Quantification of DA neurons in the SNpc show that mice whose running was restricted lost significant numbers of DA neurons due to MPTP toxicity; however, the 2/3 running group demonstrated partial protection. Neurochemical analyses of DA and its metabolites DOPAC and HVA show that exercise also functionally protects neurons from MPTP-induced neurotoxicity. Proteomic analysis of SN and STR tissues indicates that 3months of exercise induces changes in proteins related to energy regulation, cellular metabolism, the cytoskeleton, and intracellular signaling events. Taken together, these data indicate that exercise potently protects DA neurons from acute MPTP toxicity, suggesting that this simple lifestyle element may also confer significant protection against developing PD in humans.

Enhanced sensitivity of dopaminergic neurons to rotenone-induced toxicity with aging

Rotenone has been reported to induce various degrees of Parkinsonism in rats. We tested whether advancing age alters the sensitivity of dopaminergic neurons to rotenone. A low, systemic dose of rotenone had no effect on young rats, but led to a 20-30% reduction of tyrosine hydroxylase-positive neurons in the substantia nigra of older rats. The effect was specific to nigral dopaminergic neurons and may be associated with the increase of glial cell activation in older rats. These data suggest that age enhances the sensitivity of dopaminergic neurons to rotenone and should be considered when assessing models of Parkinson's disease.

Age-related irreversible progressive nigrostriatal dopaminergic neurotoxicity in the paraquat and maneb model of the Parkinson's disease phenotype

While advancing age is the only unequivocally accepted risk factor for idiopathic Parkinson's disease, it has been postulated that exposure to environmental neurotoxicants combined with ageing could increase the risk for developing Parkinson's disease. The current study tested this hypothesis by exposing C57BL/6 mice that were 6 weeks, 5 months or 18 months old to the herbicide paraquat, the fungicide maneb or paraquat + maneb, a combination that produces a Parkinson's disease phenotype in young adult mice. Paraquat + maneb-induced reductions in locomotor activity and motor coordination were age dependent, with 18-month-old mice most affected and exhibiting failure to recover 24 h post-treatment. Three months post-treatment, reductions in locomotor activity and deficits in motor coordination were sustained in 5-month-old and further reduced in 18-month-old paraquat + maneb groups. Progressive reductions in dopamine metabolites and dopamine turnover were greatest in 18-month-old paraquat + maneb and paraquat groups 3 months post-treatment. Increased tyrosine hydroxylase enzyme activity compensated for striatal tyrosine hydroxylase protein and/or dopamine loss following treatment in 6-week-old and 5-month-old, but not 18-month-old paraquat and paraquat + maneb mice. Numbers of nigrostriatal dopaminergic neurons were reduced in all age groups following paraquat alone and paraquat + maneb exposure, but these losses, along with decreases in striatal tyrosine hydroxylase protein levels, were progressive in 18-month-old paraquat and paraquat + maneb groups between 2 weeks and 3 months post-exposure. Collectively, these data demonstrate enhanced sensitivity of the ageing nigrostriatal dopamine pathway to these pesticides, particularly paraquat + maneb, resulting in irreversible and progressive neurotoxicity.

Age-related changes in basic fibroblast growth factor-immunoreactive cells of rat substantia nigra

Immunohistochemistry and computer-assisted image analysis were used to examine the age-related changes in bFGF-immunoreactivity in rat substantia nigra (SN). Distribution pattern, number, size and staining intensity of bFGF-immunoreactive (bFGF-ir) cells in pars compacta and pars reticulata of 3-, 12- and 26-month-old rats were compared. The overall distribution of bFGF-immunoreactivity was similar in the three age groups, but changes in the morphological appearance of bFGF-ir somata and processes occurred in aging. The results demonstrated a significantly reduced number of bFGF-ir cells in pars compacta (by 56.87%) and pars reticulata (by 30.4%) in 26-month-old rats compared to 3-month-old rats. The reduction of the cell number did not occur smoothly and equally in the two parts of SN. The quantitative analysis clearly indicated a significant decrease in the size of bFGF-ir neurons in pars compacta (by 18.1%) and pars reticulata (by 14.15%) of 26-month-old rats compared to 3-month-old rats. Compared to 3-month-old rats, a 19.77% and 17.83% increase in the staining intensity was observed in the remaining bFGF-ir neurons of pars compacta and pars reticulata, respectively in 26-month-old rats. Since there was no correlation between the decreased size and increased staining, it is most probable that the intensification of the staining intensity of bFGF-ir neurons was a compensatory response to the cell death.

Monoamine oxidase inhibition by L-deprenyl depends on both sex and route of administration in the rat

The monoamine oxidase B (MAO-B) inhibitor L-deprenyl, widely used to treat Parkinson's disease, has frequently been studied in animal models. We have examined the effects of several variables on activity levels of MAO-A and B in rat brain and liver following chronic (3 wks) treatment with L-deprenyl. Significant effects were observed for sex (females showed lower overall MAO-B activity in the liver), dose (MAO-A and B inhibition increased with dose, with females exhibiting greater sensitivity), route of administration (subcutaneous injection was more efficient than oral dosing), and dosing interval (MAO-B was significantly inhibited when dosing interval was increased to as long as 168 hours). Our results thus indicate that the effectiveness of L-deprenyl in vivo is dependent on several factors and that these must be taken into account in studies involving the benefits or risks of this drug.

Age-related loss of knee joint afferents in mice

Previous work in our laboratory revealed markedly different rates of age-related death of four monoaminergic neuronal populations in the C57BL/6 mouse. Although dorsal root ganglion neurons (DRGns) have been reported not to suffer similar age-related death in rodents, we determined if there is age-related death of the subpopulation of DRGns innervating the knee joints of C57BL/6 mice, which are known to develop degenerative arthritis with aging. The somata of dorsal root ganglion neurons innervating the mouse knee joint (KJ-DRGns) were identified by retrograde tracing with Fluoro-Gold (FG). Lumbar ganglia were serially sectioned and the numbers of FG-labelled KJ-DRGns counted at five ages encompassing the animal's life span. Changes in size of the total population of lumbar DRGns (L-DRGns) were estimated by counting nucleated somata from every fifth toluidine blue-stained serial section from the L3 and L4 lumbar ganglia at three different ages. Using a computer-assisted video morphometric technique somal areas were measured from random sections to determine the distribution of sizes of neurons in the KJ-DRGn and general lumbar DRGn populations at different ages. Counts of FG-labelled joint afferents were 238.5 +/- 80.3 (mean +/- SD) KJ-DRGns per knee at 2 months of age, declining to 103.2 +/- 20.1 by 24 months, representing a 57% loss over the average life span of the C57 mice. The loss occurred in two phases, with a rapid rate over the first 8 months of life and a more moderate rate of loss over the remaining months. L-DRGn numbers revealed a slower overall rate of loss in comparison to the KJ-DRGn population with an average 33.7% loss over the life span of this mouse. Somal size measurements revealed that the larger sizes of KJ-DRGns were lost over the first 8 months of life, with little change in the distribution of somal sizes thereafter. The distributions of sizes of the L-DRGn population did not change significantly over the life spans of the mice. The data provides evidence that the age-related loss of KJ-DRGns is significantly greater than DRGns in general, and may be particularly apparent in the population of larger sized presumed mechanoreceptor neurons. The loss of the KJ-DRGns is approximately reciprocal to the incidence rate of knee joint osteoarthritis reported for the C57BL/6 mice.

1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) disrupts social memory/recognition processes in the male mouse

Male mice treated with MPTP or vehicle were tested for their ability to demonstrate a memory-recognition response as evaluated in a habituation-dishabituation task. Treatment with MPTP severely disrupted the male's habituation-dishabituation response profile compared to vehicle treated animals. Administration of L-DOPA at 45 min prior to behavioral testing in MPTP animals restored their performance on the habituation-dishabituation test to levels observed in vehicle treated animals. There was also a tendency for L-DOPA to produce enhanced responsiveness in vehicle treated animals. Mice treated with MPTP had significantly reduced concentrations of norepinephrine within the olfactory bulb and hippocampus. Vehicle treated mice administered L-DOPA had significantly increased dopamine concentrations within the corpus striatum. These results suggest that, in addition to its putative effects upon the nigrostriatal dopaminergic system and motor behavior, MPTP is also exerting substantial effects upon other systems. In particular, the noradrenergic system and its potential involvement with memory/recognition processes in the CD-1 mouse appears to be very sensitive to the neurotoxic effects of MPTP.

Protective effect of deprenyl against 1-methyl-4-phenylpyridinium neurotoxicity in rat striatum

Rats were treated with deprenyl for 3 weeks. Afterwards, slices of the corpus striatum were incubated with 1-methyl-4-phenylpyridinium ion (MPP+). Concentrations of dopamine and its metabolite concentrations were assayed. The effect of MPP+ in animals treated with deprenyl was smaller than in controls, indicating that deprenyl protects against MPP+. We also measured superoxide dismutase (SOD) and catalase activities and carbonyl group content of the proteins. Both activities increased in deprenyl-treated rats, and the amount of carbonyl groups was unchanged. These results suggest that the protective effect of deprenyl is independent of the monoamine oxidase-B inhibitor and of the induction of SOD and catalase activities.