The olivocerebellar system. II. Some ultrastructural correlates of inferior olive destruction in the rat.

Short- and long-term ultrastructural changes induced in rat inferior olivary nucleus (ION) and cerebellum by a single injection of 3-acetylpyridine (3-AP) were investigated. Evidence of perikaryal and dendritic alterations was already present in numerous ION neurons at 3 h after injection. All ION neurons were affected at 6 h. Complete destruction of the entire ION was achieved within 8-10 h. Time-course and cytological features of this degeneration were described. Total absence of axonal termination degeneration in the ION or at its periphery ruled out the existence of recurrent olivary axons in these locations. Climbing fiber (CF) terminal degeneration in the cerebellar cortex apparently was restricted to the molecular layer, which cast serious doubts on the existence of glomerular collaterals of CFs. Evidence of axonal terminal degeneration was observed within all cerebellar nuclei at 24 and 26 h after 3-AP treatment, but degenerating profiles were unexpectedly infrequent. Consequential to CF deafferentation, Purkinje cells (P.cells) underwent both precocious and delayed ultrastructural changes. Delayed and long-range changes involved mainly dendrites and perikarya. Axon terminals underwent precocious but prolonged alterations which were interpreted as evidence supporting enhanced synaptic activity of P. cells deprived of CFs.