Ultrastructure of aberrant serotonin-immunoreactive fibers in the caudate putamen complex of the aged rat.

Degeneration of neurons in the central nervous system is associated with morphological changes. Previous observations made at the light microscopical level indicated degeneration of serotonin-immunoreactive (IR) fibers in the aged rat brain. In this study, a comparison at the ultrastructural level was made between serotonin-IR normal thin and aberrant swollen varicose fibers in the caudate-putamen complex of the aged rat. Ultrastructural features such as the size and content of the thin varicose fibers resembled those in the caudate-putamen complex of the young rat as reported by others. The aberrant profiles were swollen, reaching a size of 6 microns. Their vesicles varied in size and were no longer uniformly round. Moreover, distorted mitochondria and membrane-filled vacuolelike structures were a common feature of the aberrant profiles. These changes are indicative of a degenerative process and give further evidence that, whereas many serotonergic fibers are preserved at high age, other serotonergic fibers are degenerating in the caudate-putamen complex of the aged rat.