Effects of neonatal administrations of 6-OHDA on brain development. III. Myelin degeneration in the mesencephalic tract of the trigeminal nerve.

In rats treated with 6-OHDA in early postnatal days, myelin degeneration was observed in large and medium-sized myelinated nerve fibers belonging to the mesencephalic tract of the trigeminal nerve. Splitting of myelin lamination at the intraperiodic line and irregular excess scrolls were frequent in these fibers, whereas the smaller sized fibers remained intact. Occasionally, naked axons with numerous neurofilaments and neurotubules appeared near the empty myelin scrolls, indicating autolysis of the once-formed myelin sheaths. Some sheaths showed decompaction through loose lamination around the axons. The perikarya of the mesencephalic neurons showed no visible damage, but dark terminals making axo-somatic synapses in the motor nucleus were regarded as degenerating features of the axon-collaterals of the mesencephalic neurons. Thus, the myelin abnormalities were partially attributed to a kind of Wallerian degeneration. A survey of the glial cells in the corpus callosum indicated no remarkable changes in the ratio of the various glial types and in the myelin structures in comparison with those of the controls, although insufficient maturation of oligodendrocytes resulted in a 5-day delay in the onset of myelinogenesis. These findings suggest that the myelin degeneration in the mesencephalic tract is a regionally specific alteration induced by 6-OHDA administrations, probably because of the abnormal accumulation of NA in this area.