Ultrastructural changes after intracisternal injection of 6-hydroxydopamine in the intermediolateral nucleus of the monkey (Macaca fascicularis).

The ultrastructural changes following a single intracisternal injection of 6-OHDA were studied in the intermediolateral nucleus of the monkey spinal cord. Post-operative survival periods ranged from 20 hours to 14 days. At all stages, only boutons containing clear, round vesicles (R-boutons) and those containing dense-cored vesicles (DCV-boutons) underwent degeneration. Boutons containing flattened vesicles (F-boutons) appeared to be unaffected. Changes were seen even at 20 hours after injection and these were confined to R-boutons only. The changes included an initial swelling and crowding of the vesicles, followed by an increase in the electron density of the axoplasm. The latter change was most marked on the second post-operative day and by the third day, most of the electron-dense profiles were glia-engulfed. By the third day, also, many unmyelinated and, occasionally, myelinated axons showed accumulations of mitochondria, membrane-bound tubular profiles and electron-dense bodies. Many astrocytic processes also showed accumulation of tubular profiles. From the fifth day onwards, few degenerating R-boutons were encountered but DCV-boutons containing swollen vesicles, with or without their dense cores, were obvious. Unmyelinated and, occasionally, myelinated axons and astrocytic processes containing tubular elements still continued to be seen. By the fourteenth post-operative day, degenerating profiles were rarely observed. The probable significance of these findings has been discussed in the light of recent anatomical and biochemical studies.