Retinal Degeneration Following Chronic Administration of the Parkinsonism-Inducing Neurotoxin MPTP

During late stages, retinal degenerative disorders affecting photoreceptors progress independently from the specific disease trigger. In fact, a number of detrimental consequences occur downstream of photoreceptors, which are triggered by the loss of photoreceptors themselves. Such downstream anatomical alterations were originally thought to be compensatory events aimed to restore retinal function. At present, these phenomena are deciphered as detrimental effects and the term retinal degeneration is used to indicate the loss of cells and architecture within the inner retina as a consequence of damage to photoreceptors. In the process of testing a photoreceptor-dependent downstream spreading of neurodegeneration we applied a neurotoxin mimicking Parkinson's disease (PD), 1-methyl, 4-phenyl, 1,2,3,6-tetrahydropyridine (MPTP). Chronic MPTP administration produces degeneration within the mouse retina. This is evident by apoptosis quite circumscribed to photoreceptors, which is reminiscent of most phenotypes of retinal degeneration. Retinal pathology following plain HE histochemistry is more widespread with delamination and loss of neuronal packaging in the inner retina. The retinal damage is characterized by a marked synucleinopathy mostly within retinal ganglion cells. In contrast, dopamine-containing structures are intact while norepinephrine is significantly reduced. Despite the involvement of the retina in PD is documented, no study so far analyzed the onset of a synucleinopathy and a degenerative process mimicking what is now recognized in typical retinal degeneration. The present data provide a novel vista on the reciprocal role of the retina in neurodegenerative disorders.