A study of the synaptogenesis in the cerebellar cortex through chronic treatment and immunocytochemistry of beta-bungarotoxin.

The mechanism of synaptogenesis between dendritic spines of Purkinje cells and parallel fibers in the chicken cerebellum was studied through chronic treatment and immunocytochemistry of beta-Bungarotoxin (beta-BT). Attention was directed to the question of whether the presynaptic specializations (presynaptic vesicular grids composed of presynaptic dense projections and associated vesicles) of the parallel fibers can differentiate in the absence of the Purkinje cells. Normal cerebella from 18-day and 21-day chick embryos, incubated with beta-BT and reacted with HRP-labeled anti beta-BT guinea pig IgG, showed a positive HRP reaction on Purkinje cells but not on external and internal granule cells. Thus, in chicken cerebellum, beta-BT primarily affects Purkinje cells. When beta-BT was applied to chick embryos at 3-day intervals, beginning on the 4th day of incubation, the cerebella were markedly reduced in size and most of the Purkinje cells as well as the nerve fibers in the white matter disappeared between the 18th and the 21st day of incubation. Folia of the 21st day experimental cerebella were irregular in shape and the area of the midsagittal section was one fourth that of the controls. In the 21st day cerebella treated with beta-BT, the majority of Purkinje cells disappear. However, the external granule cells remain intact and showed the usual mitotic activity. The majority of the inner granule cells were normal. Some of the parallel fibers, which display presynaptic vesicular grids, established synaptic contact with stellate cells and the dendritic spines of the few Purkinje cells that survived the treatment. However, vast areas of the molecular layer contained neither dendrites of Purkinje cells, nor parallel fibers displaying presynaptic vesicular grids devoid of their postsynaptic counterpart. In such areas, the molecular layer consisted of parallel fibers of uniform diameter, some of which contained accumulations of vesicles, but displaying no presynaptic dense projections. This suggests that parallel fibers may not be able to completely develop the presynaptic vesicular grids in the absence of their target cells.