Regenerative catecholamine-containing terminals in kitten visual cortex: an ultrastructural study

Ultrastructure of catecholaminergic (CA) terminals was studied in the kitten visual cortex which had been continuously infused with 6-hydroxydopamine (6-OHDA) for one week. Two methods were used to identify CA terminals: (1) cardiac perfusion with glyoxylic acid (GA) followed by potassium permanganate (KMnO4), a fixation method which leads to the precipitation of a dense core in CA-containing synaptic vesicles; and (2) induction by 6-OHDA of terminal degeneration in glutaraldehyde-fixed material. In material which was obtained from 3-week-survival animals and treated with GA-KMnO4, the 6-OHDA-infused hemisphere contained large terminal boutons (1-3 microns in diameter) having large dense-cored vesicles. Usual CA terminal boutons with small dense-cored vesicles were found only in the opposite control hemisphere. The unusual terminal boutons with large dense-cored vesicles were interpreted as "regenerative" CA terminals. They were no longer found in animals which survived for 33 weeks. Instead, the return of usual CA terminals filled with small dense-cored vesicles was noted. The above interpretation was further supported from the results of an additional two kittens in the 3-week group which received a single injection of 6-OHDA prior to their sacrifice (in addition to the initial 6-OHDA infusion 3 weeks before). In the glutaraldehyde-fixed sections, there were found terminal boutons filled with the accumulation of abnormal electron-dense material, suggesting the presence of ("regenerative") CA terminals in the 6-OHDA-infused cortex. The present results have thus provided further evidence for the presence of "regenerative" CA terminals in a cortical area in which CA terminals have been previously destroyed by cortical 6-OHDA infusion.