MitoPark mice mirror the slow progression of key symptoms and L-DOPA response in Parkinson's disease

The MitoPark mouse, in which the mitochondrial transcription factor Tfam is selectively removed in midbrain dopamine (DA) neurons, is a genetic model for Parkinson's disease (PD) that replicates the slow and progressive development of key symptoms. To further validate this model, we have extended both behavioral and biochemical analyses in these animals. We found that vertical movements decline earlier and faster than horizontal movements, possibly modeling the early occurrence of axial, postural instability in PD. L-DOPA induces different locomotor responses depending on the age: in young MitoPark mice the L-DOPA-induced motor activation is small; middle-aged MitoPark mice respond in a dose-dependent manner to L-DOPA, whereas aged MitoPark mice display a double-peaked locomotor response to a high dose of L-DOPA that includes an intermittent period of very low motor activity, similar to the 'on-off' phenomenon in PD. To correlate behavior with biochemical data, we analyzed monoamine levels in three different brain areas that are highly innervated by the DA system: striatum, anterior cortex and olfactory bulb. DA levels declined earlier and faster in striatum than in cortex; only at the latest time-point analyzed, DA levels were found to be significantly lower than control levels in the olfactory bulb. Interestingly, the ratio between homovanillic acid (HVA) and DA differed between regions over time. In striatum and olfactory bulb, the ratio increased steeply indicating increased DA turnover. In contrast, the ratio decreased over time in cortex, revealing important differences between DA cells in substantia nigra and the ventral tegmental area.