Morphological changes induced in turtle retinal neurons by exposure to 6-hydroxydopamine and 5,6-dihydroxytryptamine.

Following intraocular injection of the dopamine neurotoxin 6-hydroxydopamine (10-50 micrograms on two successive days in a Ringer vehicle containing ascorbate and pargyline) and an incubation period of 1 to 18 days, degeneration was noted in presumptive amacrine cells in the retina of the turtle, Pseudemys scripta elegans. Injection of vehicle alone produced no effect. Affected perikarya initially showed swollen mitochondria, lysosomes and distended cisternae. At later stages the cells took on a darkened appearance. In contrast, affected amacrine processes in the inner plexiform layer became markedly distended and lost their cytoplasmic contents, resulting in empty, very swollen profiles. No degeneration was noted distal to the affected cell bodies, i.e. the affected cells were not interplexiform neurons. Cells lesioned by 6-hydroxydopamine were shown to accumulate [3H]dopamine. Intraocular administration of 5,6-dihydroxytryptamine (a single dose of 10-40 micrograms in the same vehicle) followed by 4-6 days incubation resulted in a marked darkening of certain bipolar cell axon terminals, cell bodies and Landolt's clubs. The toxic effects of 5,6-dihydroxytryptamine were blocked by zimelidine, a serotonin uptake blocker. Thus, these two neurotoxins have different targets in the turtle retina. At the highest dose tested, however, 6-hydroxydopamine did produce degenerative changes in the presumed serotonergic bipolar cell.