THE RETINAL DOPAMINE SYSTEM IN PARKINSONS-DISEASE

1. Parkinson's disease is a neurodegenerative disease involving mostly brain motor nuclei. It is characterized by a severe loss of dopamine (DA)-cell bodies in the substantia nigra and a severe loss of dopamine in the striatum. The retina, which is an extension of the brain, contains a population of DA neurons. It was speculated whether they were also undergoing some degeneration, since several lines of evidence support the existence of a Parkinson's related visual impairment originating in the retina. 2. To answer this question we performed a morphometric study of tyrosine hydroxylase (TH) immunoreactive cells in one eye of six Parkinsonians, correlated with biochemical assays for retinal DA in the other eye of the same patients. 3. A decreased level of retinal DA was consistently observed as compared with controls obtained in the same post-mortem delay. 4. In 50% of the cases this decreased DA level was related to a significant but discrete loss of TH immunoreactivity. 5. A more or less severe degeneration of the foveal DA innervation was observed in all patients. 6. As compared with substantia nigra, retinal DA neurons were relatively spared, which could be explained by a differential susceptibility of these neurons to degeneration. Nevertheless, the disappearance of TH immunoreactivity from processes, especially located at the fovea, indicates either a degenerative process of DA structures or a disorder in TH and/or DA synthesis which can account for the visual impairment observed in Parkinsonian patients.