THE EFFECTS OF DEAFFERENTIATION OF THE CEREBELLAR CORTEX IN THE RAT.
The trilaminar structure of the mature cerebellar cortex provides a unique opportunity to examine its responses to deafferentation because the inner and outer layers possess different extrinsic afferent connections. The climbing fiber input to the ansiform lobule disappeared by 7 days following electrolytic or chemical (3-acetylpyridine) lesions. Electrolytic ablation of the basilar pontine gray resulted in the degeneration of nearly the entire population of mossy fiber rosettes within the ansiform lobule. Most glomeruli contained degenerating mossy fiber debris during the first 57 days and these terminals were cleared by 80 days following pontine ablation. Parallel fiber-deafferentation was performed using parasagittal cuts in the lateral cerebellar hemisphere. Short-term electron microscopic examinations confirmed that the only neuronal element affected was the axon of the granule cell. These axons and their terminals underwent a rapid course of electron-dense degeneration by the 5th day.;The possibility of synaptic remodeling following deafferentation of the mature cerebellum was investigated over a period ranging from 12 hours to 380 days. In the short-term cases the distribution and synaptic localization of each fiber type along with the time course for their removal was determined. At a later stage, after adequate time had passed for the removal of degenerative debris and remodeling to take place, a second lesion was made in the intact afferent system to investigate the possibility of synaptic reorganization of either the climbing or the mossy fiber system. Although both Golgi-Cox and electron microscopic methods were employed, the synaptic specificity of the cerebellar cortex was largely maintained following climbing or parallel fiber-deafferentation. Evidence is provided which suggests that the basket and parallel fiber axons and in particular the Purkinje cell dendritic tree are capable of plastic change following deafferentation. While the cerebellar cortex possesses some potential for synaptic reorganization, the primary response to deafferentation is one of transneuronal degeneration. Golgi-Cox and electron microscopic findings indicated significant losses in both the number of smooth branches and spiny branchlets of Purkinje cells. Transneuronal degeneration of molecular layer interneurons was also reported.