Longitudinal MR Spectroscopy, Behavioral Testing, and Disparity in Rearing Conditions Reveal Gender and Age Dependant Differences in the Pitx3(ak)/2J Mouse Model of Parkinson's Disease

Parkinson’s is a degenerative disease characterized by progressive loss of motor function. Recent evidence suggests insults to the substantia nigra prior to birth may be a factor in the development of Parkinson’s disease. In the Pitx3 ^ak /2J mouse model, over 50% of the dopaminergic neurons fail to differentiate in the substantia nigra during prenatal development. We hypothesized that prenatal loss of dopaminergic functioning would lead to an early emergence of neural deficits. To test this hypothesis, we used a longitudinal design with behavioral testing and noninvasive quantification of brain metabolites in vivo using Magnetic Resonance Spectroscopy (MRS) imaging. We assessed Pitx3 mice as juveniles and again at 8 weeks of age. The behavioral measures included the open field test, pole climb, and forced swim test. In order to control for behavioral effects possibly introduced by rearing both mutant and control mice together, additional mice from homogenous litters (e.g., all mutant, or all heterozygous) were also tested. MRS of standard brain metabolites were collected from single voxels of 2 x 2 x 2 mm ³ in the striatum using a 7T Bruker Biospin magnet, Point Resolved Spectroscopy Sequence (PRESS), and variable power RF pulses with optimized relaxation delays (VAPOR) water suppression. The resulting spectra were processed with LCModel, and are the first quantification of brain metabolites reported for the Pitx3 mouse. As expected, developmental differences in metabolites were detected between juveniles and adults. However, Pitx3 mutants also showed elevated levels of glutamate in the striatum, suggesting increased dopaminergic loss. Qualitatively different deficits during behavioral testing were also detected between juveniles and adults. Heterozygous mice reared in mixed versus homogenous litters showed gender by rearing condition differences on behavioral tests, suggesting heterozygous males and females are differentially affected by littermates during development. However, no differences were seen in Pitx3 mutants from either rearing conditions. Together these findings demonstrate the utility of a multi-level approach in longitudinal studies, and reveal for the first time intriguing patterns of development in this mouse model of Parkinson’s disease.