Behavioral phenotyping of the MPTP mouse model of Parkinson's disease.

In mice, the systemical or intracranial application of the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) can lead to severe damage to the nigrostriatal dopaminergic system. This can result in a variety of symptoms concerning motor control resembling those in human Parkinson's disease, such as akinesia, rigidity, tremor, gait and posture disturbances. The aim of this work is to review a variety of behavioral paradigms for these and other symptoms, which have been used to characterize behavioral changes in mice after MPTP treatment. Main results are summarized, and general influential factors as well as potential problems in the experimental procedures are discussed, which should be taken into account when conducting behavioral analyses in mice with parkinsonian symptoms. Since there is reliable evidence (e.g. from strain comparisons) that the susceptibility of the nigrostriatal pathway to neurodegeneration is probably genetically influenced, relevant genes can be expected to be identified in the future. Therefore, the points discussed here will be useful not only for further applications in the MPTP mouse model, but also more generally for the behavioral characterization of future mouse models of PD, e.g. mice with a manipulation of genes relevant to the function of the basal ganglia.

Evidence for a dissociation between MPTP toxicity and tyrosinase activity based on congenic mouse strain susceptibility.

The neurotoxicity induced by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) is one of the most valuable available models for investigating critical aspects of human Parkinson's disease. In order to analyze the relevance of pigmentation for MPTP sensitivity, we compared C57Bl/6 wild-type mice with the albino mutant C57Bl/6J-Tyr(c-2J) of the same strain. These animals were treated either with systemic MPTP or with saline and were examined in behavioral tests. Seven days after treatment, the contents of dopamine and other monoamines were determined postmortem in the neostriatum and ventral striatum. Furthermore, the numbers of tyrosine hydroxylase-positive cells were counted in the substantia nigra and ventral tegmental area. Open field testing showed that rearing activity was drastically reduced as an acute effect of MPTP in both wild type and mutants; however, subsequent recovery to control levels was faster in wild-type mice. Nest building also indicated strain-dependent effects, since it was delayed only in mutants treated with MPTP. Neurochemically, MPTP led to severe neostriatal dopamine depletions, which did not differ significantly between wild-type (72.9%) and mutant mice (82.1%). Less severe dopamine depletions were also found in the ventral striatum. Histologically, a loss of tyrosine hydroxylase-labeled cells was observed only in the substantia nigra of both wild-type and mutant mice (13.3 and 21.3%, respectively), but not in the ventral tegmental area. Together, our data do not provide evidence that tyrosinase-deficient mice are less affected by MPTP treatment than the comparable wild type, thus arguing strongly against the hypothesis that enhanced MPTP sensitivity in pigmented mouse strains is caused by tyrosinase activity.

MPTP susceptibility in the mouse: behavioral, neurochemical, and histological analysis of gender and strain differences.

To investigate the impact of strain and sex in the l-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) animal model of Parkinson's disease, C57BL/6 and BALB/c mice were treated with either systemic MPTP-HCl (4 x 15 mg/kg) or saline and were examined in a number of behavioral tests. Furthermore, neostriatal and ventral striatal monoamine contents were determined, and the numbers of tyrosine hydroxylase-immunostained cells were counted in the substantia nigra and ventral tegmental area. Open-field testing showed that locomotor activity was drastically reduced as an acute effect of MPTP in both strains; however, subsequent recovery to control levels was faster in BALB/c mice than in C57BL/6. Nest building also indicated strain-dependent effects, since it was delayed only in C57BL/6 mice treated with MPTP. The other tests (grip test, pole test, rotarod, elevated plus-maze), although partly sensitive for over-all strain or gender differences, turned out not to be useful to compare MPTP effects in these two strains. Neurochemically, MPTP led to more severe neostriatal dopamine depletions in C57BL/6 (-85%) than in BALB/c mice (-58%). Histologically, a loss of tyrosine hydroxylase immunoreactivity (-25%) was observed only in the substantia nigra of C57BL/6 animals. Thus, our analysis consistently showed that the C57BL/6 mouse strain is more susceptible to MPTP than the BALB/c strain. Sex differences in MPTP sensitivity were not observed in our mice. The implications of these findings for the search for genes related to susceptibility to neurodegeneration are discussed.

Evidence for resistance to MPTP in C57BL/6 x BALB/c F1 hybrids as compared with their progenitor strains.

The neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) is capable of producing a syndrome in mice which shares major characteristics with human Parkinson's disease. There is evidence for a genetic influence on the degree of damage exerted by MPTP, since different strains of mice can dramatically differ in their response to MPTP. We produced reciprocal F1 hybrids by crossbreeding the MPTP-susceptible C57BL/6 strain with resistant BALB/c. These hybrids were compared to the parental strains using neural and behavioral measures in order to characterize the genetic transmission of MPTP-susceptibility. The F1 generation as a whole had a lower depletion of neostriatal dopamine levels than even found in BALB/c. Furthermore, there was no significant loss of tyrosine hydroxylase-positive cells in the substantia nigra and quick recovery from deficits in motor behavior in F1, herein resembling BALB/c. We suggest that several loci are involved in susceptibility to MPTP, and that the trait is under control of recessive susceptibility and/or dominant resistance alleles, which interact in F1, leading to extremely low susceptibility.

Strain-dependent recovery of open-field behavior and striatal dopamine deficiency in the mouse MPTP model of Parkinson's disease.

The neurotoxin MPTP can damage dopamine systems in the brains of rodents, cats, or monkeys, and is therefore widely used to model degenerative processes that underlie human Parkinson's disease. Here, we investigated the relationships between behavioral and neurochemical effects of systemic MPTP treatment in C57Bl/6 and Balb/c mice. Initially, different doses of MPTP were used to determine which of them might be useful to establish severe striatal dopamine depletions. These data showed that four injections of 20mg/kg at two hour intervals, were more efficient than 10 or 15mg/kg per injection. However, this dose was not usable due to its severe lethality in females. In contrast, 4x 15mg/kg had a low risk of lethality and led to substantial dopamine depletions, which were more severe in the neostriatum than the ventral striatum, and more severe in C57 than in Balb mice. In the first open field test, which was performed two hours after the last injection, this treatment led to severe behavioral inactivation in all parameters taken (distance and speed of locomotion, peripheral activity, frequency and duration of rearing). This effect was seen in both strains and gender. Thereafter, recovery differed between strains, since Balb mice, which had sustained the smaller lesions, had completely recovered on the subsequent day, whereas similar recovery took longer in C57 mice. On the fourth day, all groups appeared largely normal; however, the measure of rearing behavior still showed a deficit in C57 mice. This deficit on day 4 was correlated with neostriatal dopamine depletion; that is, the larger the lesion, the less the number and duration of rearings. Interestingly, these relationships were also observed with respect to ventral striatal dopamine damage, which was correlated with the rearing deficit not only on day 4, but also on day 1. These data will be discussed with respect to mechanisms of toxicity, functional recovery, and the function of striatal dopamine systems.